1 Key Laboratory for Thin Film and Microfabrication Technology of Ministry of Education, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
Wenhuan Zhu
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2020-05-14
2020-06-18
2020-07-31
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Abstract
Lithium selenium batteries are very promising next-generation high-energy-density batteries with the properties of high theoretical volume energy density(3253 mAh·cm-3), high electrical conductivity(1×10-3 S·m-1),and environmental friendliness, which have gradually become a research hotspot in the field of electrochemistry. However, at present, lithium selenium batteries still face many problems such as low utilization rate of active materials, low coulomb efficiency, fast capacity decay and shuttle of polyselenides intermediates. In recent years, worldwide researchers have conducted a lot of researches on these issues. For example, a variety of carbon materials, metal compounds, selenium alloys, etc. have been used for packaging modification at the positive electrode. Solid electrolyte interface methods have been used for protection at the negative electrode. We comprehensively review the latest research progress of lithium selenium batteries in cathode, anode, electrolytes, separators, binders, current collectors, etc. especially summarize the sealing of nano selenium, the preparation of solid electrolyte protective layer, the research on multifunctional separators and the application of various binders and current collectors. Finally, we prospect the future development and commercial applications of lithium selenium batteries.
Yusen Ding, Pu Zhang, Hong Li, Wenhuan Zhu, Hao Wei.
Research Status and Prospect of Li-Se Batteries.
Progress in Chemistry, 2021, 33(4): 610-632 DOI:10.7536/PC200534
With the rapid development of society and industry, on the one hand, we enjoy the benefits of development; On the other hand, due to our lack of anticipation and prevention of the negative impact of highly developed industry, it has led to three major global crises: resource shortage, environmental pollution and ecological destruction. Therefore, it is urgent to develop sustainable energy sources, such as hydrogen energy and electrochemical energy, so as to reduce the use of traditional fossil fuels.
The use of large-scale emerging sustainable energy requires appropriate and effective intermediaries for energy storage and release, and battery systems are the most widely used at this stage[1⇓~3]. With the continuous improvement of people's demand for electronic consumer products, the development of high energy density batteries has become a trend. The 2019 Nobel Prize in Chemistry was awarded to John B. Goodenough, M. Stanley Whittingham and Akira Yoshino for their significant contributions to the field of high-energy batteries such as lithium batteries. It can be seen that lithium batteries are of great significance in scientific research and daily life.
Lithium battery is a very classical battery system, which is non-toxic, low cost, high theoretical capacity and high energy density, so it has attracted much attention[4⇓⇓~7]. Our group reported the development of mesoporous covalent organic frameworks with carbon nanotubes as electrodes, and confirmed that the covalent network can enhance the stability of electrodes, and the open mesoscopic channels can promote the transport performance of ions.Carbon nanotube wires can enhance the conductivity of electrons, and the synergistic effect of these effects can provide efficient, stable cycling and high-rate performance for lithium-ion batteries[8].
In addition to lithium-ion batteries, lithium batteries generally include: lithium-sulfur batteries and lithium-selenium batteries. Gu and Lai comprehensively summarized how to systematically use one-dimensional nanomaterials to build new battery structures to make better lithium-sulfur and lithium-selenium batteries[9]. In the aspect of lithium-sulfur battery, our group has done a lot of work, including the study of the application of core-shell covalent organic framework/multi-walled carbon nanotube composites in lithium-sulfur batteries.The influence of nitrogen-doped graphite carbon based on covalent organic framework on high-performance lithium-sulfur batteries was studied, and it was found that redox graphene modified by MoS2 quantum dots could be used as the sulfur host of lithium-sulfur batteries[10][11][12]. In addition, we comprehensively summarized the latest research progress of lithium-sulfur batteries, elaborated the cathode, electrolyte, separator and anode protection of lithium-sulfur batteries in detail, and analyzed the defects and problems of lithium sulfur batteries, such as polysulfide shuttle effect, low coulombic efficiency and poor cycleability[13][11,14,15].
As one of the main substitutes for lithium-sulfur batteries, lithium-selenium batteries have attracted wide attention of researchers. Lee et al. Found that lithium selenium batteries assembled from an integrated selenium positive electrode and a lithium metal negative electrode exhibited ultra-stable performance without the use of conductive carbon, polymer binder, and separator, with a capacity fade of 0.001% per cycle over 3000 cycles at 1 C[16]. The research confirms that the lithium-selenium battery is a new type of battery with high conductivity, high theoretical volume capacity (3253 mAh·cm-3) and high theoretical mass capacity (675 mAh·g-1). Compared with the lithium-sulfur battery, its output voltage is about 0.5 V higher than that of the general lithium-sulfur battery, and the new lithium-selenium battery has better cycle performance and higher energy density[17]. However, the current research has fully proved that although lithium-selenium batteries have many advantages, some important problems (mainly focusing on the positive and negative electrodes) need to be solved urgently in order to be applied on a large scale. Zhou et al. Introduced the research progress of selenium-carbon composite cathode materials in lithium-selenium batteries, and systematically analyzed the relationship between the structure and performance of selenium-based cathode materials[18]. Combined with their research work, we believe that selenium anode faces the following problems:
(1) During the cycling process, there are dissolved and occluded polyselenides in the Li-Se battery, which leads to the continuous loss of active materials in the electrolyte, the shuttle effect, and the significant volume change of the battery during the cycling process;
(2) In the process of circulation, there is scattered selenium, which can not be recycled completely and has low utilization rate;
(3) In the specific engineering and commercial applications of lithium-selenium batteries, the overall diffusion and conductivity of the system are still lacking.
In the lithium anode, the following problems are faced:[19]
① Safety: Similar to many other metals, Li tends to deposit in the form of dendrites, which is the main cause of internal short circuit, thermal runaway and explosion of batteries. Therefore, before large-scale commercial application of lithium-selenium batteries, it is necessary to achieve internal deposition without Li dendrites.
(2) Cycleability: Lithium metal can react with the electrolyte at the interface of the electrolyte, which seriously affects the cycleability and coulombic efficiency of the battery.
In view of the above problems, researchers have proposed various solutions. With regard to the positive electrode, the usual method is to use various materials to modify and package the positive electrode and design it, such as carbon materials, metal compounds, selenium alloy electrodes, etc. For the protection of negative electrode, artificial electrolyte interface layer and nano-interface engineering are usually used. In addition to the positive and negative electrode materials, the research on other components of lithium-selenium batteries, such as electrolyte, separator, binder and current collector, is also in progress, and we will introduce them one by one later. In this paper, the recent research work on lithium-selenium batteries is reviewed from various aspects, the current research results are analyzed in detail, and the future development and application of lithium-selenium batteries are prospected. At present, there are relatively few review articles on lithium-selenium batteries at home and abroad, so this paper will play a certain role in promoting the future research on lithium-selenium batteries.
2 Lectrochemical principle of lithium selenium battery
The electrochemical reaction principle of the lithium selenium battery is as follows:
During discharge, the lithium metal anode (negative electrode) oxidizes to form lithium ions and electrons, which pass through the electrolyte and the external circuit to the selenium cathode (positive electrode). At the anode, selenium reacts with lithium ions and electrons to form lithium selenide by reduction. The charging process is the opposite. Specifically, the charging and discharging process of the lithium selenium battery is as follows:[19,20]
Discharge process:
Anode :Se+2Li++2e-→Li2Se
Negative :Li→Li++e-
Charging process:
Negative :Li++e-→Li
Anode :Li2Se→Se+2Li++2e-
In 2012, Amine and others at Argonne National Laboratory first studied the electrochemical reaction mechanism of lithium-selenium batteries[21]. This study shows that a phase transition occurs during the charging and discharging of a lithium selenium battery, as shown in Figure 1.
Several allotropes of selenium exist in the phase transition of the positive electrode of lithium selenium batteries: gray triangular selenium (containing Sen helical chain polymers), rhombic selenium (containing Se6 molecules), monoclinic Se: α-Se, β-Se, γ-Se (containing Se8 molecules), amorphous red selenium, and black glass selenium. Among the various forms of selenium, amorphous selenium is more active than crystalline selenium because it has a "dangling bond" and a higher specific surface area[17].
Because the Se lattice structure does not have enough space to accommodate the insertion of Li atoms, the volume expansion of the Se cathode will inevitably occur during the phase transition. The electrochemical performance of the lithium selenium battery can be effectively improved by dispersing the electroactive material in the whole matrix so that the electroactive material has enough space. At present, the matrix includes a variety of carbon materials with different structures, metal compounds, alloys and so on, which will be described in detail in the next chapter.
3 Cathode material
The first report on the positive electrode of lithium selenium battery is also from Amine et al. In this study, they prepared selenium/multi-walled carbon nanotubes (Se-C) composites by ball milling and high temperature treatment, and studied the electrochemical lithium storage performance[21]. After 100 cycles at 50 mA·g-1, the capacity of the Li/Se-C electrode remains about 300 mAh·g-1. This pioneering work proves that Li-Se batteries can be charged and discharged at room temperature, which has the prospect of practical application. The conclusion of this work can also be used as a standard to measure the work of future generations.
3.1 Selenium/carbon composite electrode
3.1.1 Selenium/carbon nanofiber,carbon nanotube,graphene oxide composite material
Carbon nanofibers(CNFs),carbon nanotubes(CNTs)and graphene oxide(GO)have excellent physical and chemical properties,which can meet the requirements of lithium selenium battery cathode.In this article,each of these cases will be discussed separately。
About carbon nanofibers(CNFs):Microporous carbon nanofibers/selenium(MCNFs/Se)composites can be prepared by combining selenium and microporous CNFs under vacuum and heat treatment conditions.The conductive carbon nanofibers can form a tight chemical bond with Se,which can not only promote the binding between Se and the CNFs matrix,but also reduce the dissolution of the active substance during cycling.In addition,CNFs can construct interconnected porous networks with high porosity to facilitate electrolyte permeation and accommodate its buffer volume change during the electrochemical reaction.Lithium-selenium batteries fabricated using MCNFs/Se composites have good cycle stability:the capacity remains 400 mAh·g-1after 2000 cycles at a current density of 1 C[22][17]。 Dai et al.Developed a new type of MCNFs/Se composite material.Under vacuum heat treatment,selenium particles can be easily converted into chemical chain-like substances,which can be well combined with the inner surface of MCNFs.The effect is shown in Figure 2[23]。 The results showed that the initial specific capacity of the lithium-selenium battery prepared by the composite material was 581 mAh·g-1at the current density of 0.1 C,the efficiency was more than 99%after the first cycle,and the capacity retention rate reached 80%after 11 cycles 。
In addition,Se can also be impregnated into the highly flexible microporous multi-channel carbon nanofiber(Se@MCNF)film(the film is conducive to mitigating the volume change of selenium during cycling,improving the utilization of active materials,and inhibiting the dissolution of polyselenides in electrolytes),which can also be prepared as a cathode material for Li-Se batteries[24]。
About carbon nanotubes(CNTs):The use of multi-walled carbon nanotubes(MWCNTs)based on sp2hybrid carbon can improve the conductivity of the electrode and optimize the electrochemical performance of the electrode.On carbon nanotubes,the p electrons of carbon can form delocalizedπbonds in a wide range,resulting in conjugation effect,thus improving the electrochemical performance of carbon nanotubes.In the current research,the most used is the conductivity of carbon nanotubes,when the diameter is less than 6 nm,the conductivity is good(carbon nanotubes can be regarded as a one-dimensional quantum wire with good conductivity )[25⇓⇓~28]。 Cui et al.Prepared a binderless selenium nanowire/carbon nanotube composite for the cathode of lithium-selenium batteries[29]。 After a period of charge and discharge,the selenium nanowires are converted into particles embedded in carbon nanotubes.The mass and thickness of the load can be controlled by this method.It is found that when the thickness of the load is 23μm and the selenium content is 60%,the lithium selenium battery has the best performance:the initial capacity is 537 mAh·g-1,and the capacity is 401 mAh·g-1after 500 cycles at a current density of 1 C.However,while carbon nanotubes improve the conductivity of the electrode,due to the close mixing of selenium and carbon nanotubes,the utilization rate of selenium is low and the capacity decay is fast.Scientists have studied a series of solutions:Zhang et al.Prepared MWCNTs/Se composites by solution method(using ethylenediamine as solvent)and melt impregnation method,in which selenium nanoparticles with a diameter of about 100 nm were uniformly deposited in the network of multi-walled carbon nanotubes[30]。 the charge transfer resistance of MWCNTs/Se-S is lower than that of MWCNTs/Se-M(the reference material in the citation),which indicates that the material prepared by this method has better conductivity.Bhattacharyya et al.studied a novel,simple and effective pressure-induced capillary encapsulation technique for confining selenium within carbon nanotubes[31]。 As the surface tension of molten Se is reduced,the wettability and loadability are significantly improved.In the Se/CNTs cell,more than 95%of CNTs and nearly 85%of Se are loaded,and when the area loading is very high(10 mg·cm-2),the Se-CNTs cell exhibits very stable cyclability at current densities of 0.1 to 10 C,as shown in fig.3 。
Graphene oxide:At room temperature,its carrier mobility can reach 15 000 cm2(V·s)-1,which clearly reflects that graphene oxide can be fully used in lithium selenium batteries to further improve the performance of batteries[32⇓⇓~35]。 Fan et al.introduced a method of encapsulating hollow selenium in 3D graphene,which improved the performance of the cathode of lithium-selenium batteries:hollow selenium nanospheres encapsulated in 3D graphene hydrogel were prepared by a two-step solution method,and the results of scanning electron microscopy(SEM)and transmission electron microscopy(TEM)showed that the hollow selenium nanospheres were uniformly coated with graphene shells[36]。 According to thermogravimetric analysis,the Se content in the nanocomposite is about 70%,and the nanocomposite as a positive electrode has a certain cycle stability(after 50 cycles at 0.2 C,the capacity is 343 mAh·g-1).In addition,Qian et al.Designed a graphene-encapsulated selenium/polyaniline core-shell nanowire structure(G@Se/PANI),which can improve the electrochemical performance of the battery[37]。 The G@Se/PANI nanocomposite was designed and synthesized at low temperature.The Se nanowires were well encapsulated in the PANI layer,forming a core-shell structure with a thickness of 25 nm.The Se/PANI core-shell nanowires were uniformly encapsulated in the graphene nanosheets.The capacity and cycle stability of the G@Se/ANI nanocomposite were good:the reversible discharge capacity was 567.1 mAh·g-1after 200 cycles at a current density of 0.2 C,and was 510.9 mAh·g-1at 2 C.In the study of lithium-selenium batteries,graphene can be synthesized by a variety of methods:for example,Chu et al.Synthesized graphene by the traditional Hummer method,and encapsulated selenium nanoparticles and reduced graphene oxide(Se@RGO)uniformly through a simple self-assembly process[38]。 The Se content of the Se@RGO composite is as high as 80 wt%,which is much higher than that of other selenium-based cathode materials.Due to the large capacity of Se,the high dispersion of Se nanoparticles and the high conductivity of the composite,the Se@RGO cathode has high specific capacity and rate capability in Li-Se batteries:the initial discharge capacity is 533 mAh·g-1at a current density of 0.2 C;After 200 cycles at 0.2 C and an additional 300 cycles at 1 C,the capacity can be maintained at the 265 mAh·g-1 。
3.1.2 Selenium/carbon nanoshell composite material
Among all kinds of cathode materials,carbon nanoshell structure has unique advantages,it can load more selenium;Compared with nanoparticles,nanofibers and nanosheets,they can provide enough space to buffer volume changes during charging and discharging.In addition,compared with carbon nanofibers and carbon nanotubes,they can better inhibit the dissolution of polyselenides and improve the cycle performance[39⇓⇓~42]。 Qian et al.experimentally studied a selenium/interconnected porous hollow carbon composite,which is essentially a carbon nanoshell(PHCBs porous shell)[43]。 The results show that the porous shell of PHCBs can effectively disperse and isolate most of the selenium,while the inner cavity remains hollow.The Se/PHCBs composite exhibits good cycle performance and high rate capability.The optimum loading of Se/PHCBs composite is 50 wt%Se(Se50/PHCBs),which shows 606.3 mAh·g-1reversible discharge capacity after 120 cycles at a current density of 0.1 C.As the current density increases from 0.1 C to 1 C(678 mA·g-1),Se50/PHCBs,the reversible capacity of the composite can still reach the 64%(431.9 mAh·g-1)of the theoretical capacity,and the adhesion mode is shown in Figure 4.Zhang et al.Synthesized a novel lithium-selenium battery with a 3D network structure and a double-constrained cathode.The Se0.4S0.6was encapsulated in double-layer hollow carbon spheres(DLHCs)and embedded in the material(3D G)shell in a form similar to 3D graphene.This double confinement and the chemical bonding between sulfur and selenium effectively inhibited the shuttle effect of polyselenides in the cathode[44]。 After 200 cycles,the 3DG-DLHC-Se0.4S0.6positive electrode of the lithium-selenium battery reaches a high reversible capacity of 627 mAh·g-1at 0.2 A·g-1.The capacity at 2 A·g-1is 533 mAh·g-1,with excellent long cycle stability over 500 cycles and a coulombic efficiency of almost 100% 。
3.1.3 Selenium/organic polymer composite material
the selenium cathode composite material can also be synthesized by an organic polymer so as to achieve the purpose of improving the cycle performance of the cathode material.Specifically,the encapsulation of Se using conductive polymers such as polyaniline,polypyrrole,and polyacrylonitrile inhibits the dissolution of polyselenides in C/Se electrodes because of their adsorptive functional groups such as carbonyl and carboxyl groups,unique chain structure,and excellent electrochemical properties[45]。
Qian et al.Used graphene coating to prepare selenium/polyaniline core-shell nanowires,and the key work of this study is to use polyaniline to improve the electrochemical performance of lithium-selenium batteries[37]。 Guo et al.described the role of another polymer,polyacrylonitrile(PAN)[45]。 They synthesized a carbonized polyacrylonitrile/selenium(CPAN/Se)composite by heating(600℃)polyacrylonitrile-selenium(PAN-Se)fibers and using fiber electrospinning technology.the mechanism of action of this organic polymer is as follows:Se molecules can be confined by the N-containing carbocyclic structure in the carbonized PAN-based matrix(thus chemically stable),and the mechanical stability of the conductive carbon framework attached to the selenium side chain is excellent[46]。 In addition,the CPAN/Se composite cathode has good long cycle stability and rate capability in carbonate-based electrolyte.At 0.3 C,after 500 cycles,it can reach a high capacity of nearly 600 mAh·g-1.The specific synthesis method of the composite material is shown in Figure 5.In addition,Zhang et al.Used hollow carbonized polyaniline spheres to encapsulate the selenium anode.The principle is similar to the above cases.The difference is that they used a series of methods such as self-assembly method,pyrolysis method and gas phase injection process[47]。
3.1.4 Selenium/atom doped composite material
Heteroatom doping(such as nitrogen doping)can inhibit the shuttle effect of polyselenides,improve the conductivity of carbon host materials,increase reactive sites,and improve the wettability and absorptivity of electrolytes.Therefore,atom doping materials can further improve the electrochemical performance of Li-Se batteries.At present,there are many studies in this area,mainly nitrogen doping[48⇓⇓⇓~52]。
Huang et al.Used nitrogen-doped microporous carbon to improve the performance of lithium-selenium batteries,specifically,using a fixed proportion of polypyrrole and KOH mixture,carbonized to synthesize microporous N-doped carbon[53]。 The unique microporous structure of carbonized polypyrrole provides additional active sites for lithium ion storage,which significantly improves the electrochemical performance of the composite.Huang et al.Used first-principles calculations to evaluate the effect of heteroatom doping on electrochemical performance,and further confirmed that the presence of heteroatoms in the carbon framework greatly promoted the interaction between carbon and Li2Se.At the current density of 20 C,the discharge capacity of the Se composite cathode reaches the 303 mAh·g-1.The reversible capacity of the electrode can reach 462 mAh·g-1after 150 cycles at a current density of 1 C.The synthesis process is shown in Figure 6 。
As for selenium/atom doping,another part is doping in mesoporous carbon metal-organic frameworks(MOFs)materials:MOFs materials are a kind of coordination polymers,which have been widely studied in recent years.MOFs have three-dimensional pore structure,with metal ions as connection points,supported by organic ligands,forming a three-dimensional structure,which has a wide range of applications in catalysis,energy storage and separation。
Yin et al.introduced a new material:nitrogen-doped carbon sponge(NCS)composed of microporous carbon layers,which is derived from MOFs by carbonization at high temperatures under Ar and NH3[54]。 In this study,Se was confined by impregnation into 0.4~0.55 nm micropores of NCS by melt diffusion and infiltration.This method can effectively prevent Se loss,and the mesopores between the carbon layers can absorb a sufficient amount of electrolyte to serve as a buffer space for large volume changes during the delithiation-lithiation process.In addition,nitrogen doping improves the conductivity of the carbon matrix and promotes rapid charge transfer.When used as a cathode material for Li-Se batteries,the NCS/Se-50 composite with 50 wt%Se exhibits certain cycle stability,rate capability,and extremely high coulombic efficiency.A specific capacity of 443.2 mAh·g-1and a coulombic efficiency of 99.9%can be achieved after 200 cycles 。
3.1.5 Se/porous carbon composite material
Porous carbon materials mainly include microporous carbon,mesoporous carbon and hierarchical porous carbon,etc.At present,the most widely used mesoporous carbon materials are MOFs and COFs(covalent organic frameworks).At present,they have become an important research direction in inorganic,organic,electrochemical and other fields,especially COFs materials,which have become a hot research topic in recent years。
It has been found that porous carbon materials can be used to suppress the problems that often occur in the use of lithium-selenium batteries,such as low conductivity and shuttle effect.Generally,porous carbon(including mesoporous and microporous)materials with high surface area,high conductivity and large pore volume are used to encapsulate Se molecules to improve the performance of lithium-selenium batteries[55⇓⇓⇓⇓⇓⇓⇓~63]。
(1)Microporous carbon
In contrast,microporous carbon materials have high specific surface area,large adsorption capacity and certain pore structure,so they can be used to synthesize composite electrode materials with selenium.Especially,the microporous carbon materials with small pore size(<2 nm)can effectively restrict the dissolution of polyselenides due to the volume limitation[23,64⇓⇓~67]。
Xiong et al.Introduced a nitrogen-doped microporous carbon sphere,which is used to improve the performance of lithium-selenium batteries.In the cathode material,the abundant micropores uniformly distributed in the carbon matrix have strong adsorption capacity,so they can effectively maintain the form of selenium and make it less prone to shuttle effect[51]。 In porous carbon,the unoccupied mesopores and the high conductivity of selenium are conducive to the rapid transport of lithium ions.Electrochemical performance:350 cycles at a low current density of 0.5 C;After 1600 cycles at a higher current density of 2 C,there was almost no capacity loss for both batteries,which indicated that the lithium-selenium battery had excellent cycle stability at different current densities.Dai et al.Prepared a new selenium-carbon composite by embedding Se into the microporous carbon polyhedron derived from the metal-organic framework,and the cycle performance was good[68]。 They used the method of melt diffusion to synthesize interconnected microporous carbon polyhedra with large surface area and pore volume,which can effectively restrict Se,thus inhibiting the dissolution of polyselenides in electrolytes,in which the transport of electrons and lithium ions and the ability to withstand volume changes are also enhanced.Liu et al.utilized waste coffee grounds to prepare a graded microporous activated carbon(AC)from the waste coffee grounds through a carbonization and KOH activation process.Although the synthesis process is very simple,the optimized AC(AC-700)material has a high surface area and is a 1355 m2·g-1.After 100 cycles at 0.1 C,the reversible capacity of the Se/AC-700 cathode is 655 mAh·g-1[69]。 In addition,the Se/AC-700 cathode exhibited excellent cycling performance over 400 cycles without significant capacity degradation.Liu et al.Synthesized the microporous carbon cathode MPC/Se by using the traditional PVDF without activation.The composite cathode material was synthesized at 800℃,the Se content was 50 wt%,and the reversible capacity reached 508.8 mAh·g-1after 100 cycles at 0.1 C[70]。 This work shows that commercial PVDF-derived MPC can be used as a Se carrier for high-performance Li-Se batteries through a non-activation and scalable method,with maximum cost savings。
(2)Mesoporous carbon
Mesoporous carbon is one of the most common media to improve the performance of lithium selenium battery cathode,which has many advantages,such as high specific surface area,appropriate pore size distribution(2~100 nm)and specific high-quality structure.the excellent confinement ability of mesoporous carbon,the large contact area with selenium and the shortened diffusion length of lithium ions all effectively limit the shuttle effect[71]。
Huang et al.Studied a new method of Se electrode,that is,the preparation of carbon nanospheres:the pore size of this nanosphere is 98 nm(2~100 nm),and further prepared a Se/PCNs composite material through this nanosphere[72]。 The invention has the advantages that the loading amount of the active material is high,an effective protective film is formed on the surface of the active material,and the Se/Li2Sexrich in microporous carbon structure has strong adsorption capacity.In addition,the Se loading of the composite was up to 70.5%,and the Se/PCNs cathode exhibited high volumetric capacity density and high rate capability(57%of the theoretical capacity at 20 C current density)3150 mAh·cm-3 。
As mentioned in the previous article,the current research focus on mesoporous carbon is MOFs materials and COFs materials.the first is MOFs.Su et al.Prepared MOFs-derived nitrogen-doped core-shell hierarchical porous carbon(N-CSHPC),which has interconnected mesopores/micropores and can effectively confine Se in high-performance Li-Se batteries[73]。 On the basis of double physical confinement and strong chemisorption,the battery with Se/N-CSHPC-Ⅱas positive electrode has good cycle stability.The capacity reaches 555 mAh·g-1after 150 cycles at a current density of 0.2 C,462 mAh·g-1after 200 cycles at a current density of 0.5 C,and 432 mAh·g-1after 200 cycles at 1 C.Zhang et al.prepared mesoporous carbon by simple carbonization of MOFs,and synthesized the composite by confining Se in the mesoporous carbon through melt diffusion.Se was uniformly distributed in the mesopores[74]。 The selenium is dispersed in the pores of the carbon material,and the Meso-C@Se composite cathode exhibits good cycle performance and high rate capability.The initial discharge capacity of the battery was 641 mAh·g-1,0.5 C,and after 100 cycles,the reversible capacity was 337.5 mAh·g-1.Xu et al.Prepared a mesoporous carbon matrix of MOF-Ni by hydrothermal method,which was used to encapsulate selenium and serve as a cathode material for Li-Se batteries[75]。 The results of energy dispersive X-ray analysis are shown in Figure 7 B~d:It was confirmed that the selenium was highly uniformly distributed in the composite material,the mass loading was as high as 48%,the initial discharge capacity was 599.7 mAh·g-1,after 100 cycles at a high rate of 3 C,the capacity retention was 417 mAh·g-1,and the coulombic efficiency was as high as 99.9%.Its electrochemical performance was mainly attributed to the high conductivity and the hollow structure of the carbon matrix[76]。
In addition,the direct application of COFs materials in lithium-selenium batteries is relatively small,but it is also a hot spot[8,10,77⇓⇓⇓⇓⇓⇓⇓⇓~86]。 Jiang et al.introduced the application of COFs materials in lithium-ion batteries.In their study,COFs are crystalline mesoporous polymers containing redox-active naphthalimide walls,and the triphenylene and borate bonds are electrochemically inactive[87]。
(3)hierarchical porou carbon
Mesoporous and microporous carbon materials with different structures can effectively entrap and coat selenium molecules,while hierarchical porous carbon combines the advantages of both to further improve the performance of lithium-selenium batteries。
Their main uses are as follows:mesoporous hosts are usually used for Se encapsulation,but the cycling stability is relatively poor,and electrolyte solvents and polyselenides can still shuttle through the structure.Microporous hosts with smaller pore sizes(<2 nm)can minimize the shuttling effect,but Se loading may be limited,so hierarchical porous carbon materials may be more suitable for applications in lithium selenium batteries。
Guo et al.synthesized a Se/C nanocomposite,which contains chain-like Senmolecules in the MPCS(micro-mesoporous carbon spheres)matrix.The Se/MPCS nanocomposite exhibits very good electrochemical performance in lithium selenium batteries:the reversible specific capacity is up to 660 mAh·g-1,and the rate capability is relatively high(5 C,430 mAh·g-1 )[88]。
At the end of this section,we summarize and compare several materials with better or more representative electrochemical performance in the selenium/carbon composite electrode mentioned above,as shown in Table 1。
As summarized in the previous article,the practical application of Li-Se batteries is hindered by the shuttle effect and other effects during the charging and discharging process.to solve these problems,a stable and efficient solid electrolyte interface(SEI)layer can be constructed by adding auxiliary additives To inhibit the dissolution of polyselenides and improve the cycle performance[89⇓~91]。
Yushin et al.Used fluoroethylene carbonate(FEC)as an electrolyte additive and formed an SEI coating on the Se positive electrode during the first cycle[92]。 the SEI layer is substantially impermeable to Li ions and can significantly hinder the dissolution of the polyselenide,thereby significantly enhancing the cycle stability.Figure 8 shows the core principles and steps of the study。
the overall strategy of Se-C cathode preparation(including the SEI protective layer)is as follows:porous carbon,such as spherical activated carbon(SAC),is prepared by first melting and infiltrating Se through capillary action,annealing to remove excess Se on the surface,while the Se infiltrated into the pores remains basically intact;Next,the Se-SAC composite powder is mixed with a binder and coated on a collector foil;the cell is then assembled with a solid electrolyte forming additive such as FEC.the solid electrolyte protective layer is formed during the first cycle when the positive potential becomes low enough to cause the FEC to decrease。
Yet another approach is to build a functional polyselenide absorbing intermediate layer between the positive electrode and the separator.Such an intermediate layer can prevent polyselenides from migrating to the negative electrode region by absorbing them.For example,Li et al.Introduced a graphene-coated polymer intermediate layer.This structure improves the electrochemical performance of Li-Se batteries,increases the cycle life,and has high capacity and excellent rate characteristics[34]。 graphene not only promotes electron transfer,but also prevents polyselenide from migrating to the negative electrode.With the Graphene-polymer interlayer,the pure Se cathode can provide excellent cycle performance up to 1000 cycles,and the decay rate is only 0.048%at 5 C,which is excellent。
In addition,there are several other ways to address polyselenide dissolution,such as modifying liquid electrolytes and using polymer or solid electrolytes.In general,LiNO3has been widely used as an effective electrolyte additive to form a passivation layer that inhibits redox shuttling of lithium polyselenide 。
3.3 Metal compound electrode
in lithium-sulfur batteries,S polar hosts can effectively solve the problem of polysulfide dissolution compared with non-polar carbon hosts commonly used In Li-S batteries,because of their strong interaction with polar polysulfides[93]。 By analogy,in lithium selenium batteries,many inorganic polar materials can also be effectively supported on the positive electrode of lithium selenium batteries,especially metal compounds,such as oxides and selenides of Ti,Fe,Bi,Zn and other metals.These metal compounds will be described in detail below。
As for the Ti compound,it mainly refers to the titanium dioxide-selenium(TiO2-Se)composite cathode material synthesized by TiO2,Zhang and others through a simple melt diffusion method for lithium-selenium batteries[94]。 It was found that the selenium particles and TiO2were uniformly distributed in the composite,and the capacity of the TiO2-Se composite cathode was relatively high and the cycle performance was good.At the same time,the electrochemical results showed that the initial specific discharge capacity of the TiO2-Se composite cathode was 481 mAh·g-1,and the coulombic efficiency was almost 100%after 50 cycles at 0.1 C 。
The electrochemical properties of nanocomposite Fe2O3-Se films prepared by pulsed laser deposition(PLD)were studied by cyclic voltammetry with respect to Fe2O3,Fu et al[95]。 In the study,they measured that the reversible capacity of the nanocomposite Fe2O3-Se film was about 650 mAh·g-1:,which showed that the nanocomposite Fe2O3-Se showed greater capacity and better cycle performance than the pure Fe2O3.Fe,Li2O and Li2Se were formed during the first discharge of Li-Se battery.During the second discharge,FeSe and Fe2O3were formed.In the following long cycle,the reversible oxidation/reduction of Fe2O3and the selenization/reduction of FeSe were involved,both of which contributed to the improvement of reversible capacity and cycle performance of lithium-selenium batteries 。
Metal Bi compounds are mainly used as Bi2Se3in the positive electrode of lithium selenium batteries.The Bi2Se3crystal has a layered structure,in which the interaction between the layers is Van der Waals force,while the interaction within the layers is mainly covalent bonding.Chen et al.prepared layered Bi2Se3stacked nanosheets,and electrochemical measurements showed that the initial discharge capacity of the Bi2Se3stacked nanosheets(715 mAh·g-1)was higher than that of the single-layer Bi2Se3nanosheets(421 mAh·g-1),as shown in fig.9[96]。 Karamazov et al.Synthesized Bi2Se3rectangular nanosheets with thickness of 200~500 nm by thermochemical method,and the initial discharge capacity was 726 mAh·g-1[97]。 Mao et al.Synthesized S-doped layered Bi2Se3microspheres by one-step solvothermal synthesis method,and used them as the positive electrode of the battery.The initial discharge capacity was 771.3 mAh·g-1,which had great potential for energy storage[98]。
3.4 Selenium alloy composite material electrode
the composite electrode of selenium alloy is to combine several elements into an alloy by using their respective excellent properties,for example,selenium has stable cycling ability,but its reversible capacity is low;Sulfur exhibits a high reversible capacity but a poor cyclability.Therefore,it is an effective way to improve the electrochemical performance of lithium-selenium batteries by using the advantages of Se and S(alloy)elements to atomize the heteroatoms in carbon and other hosts[99,100]。
Yu et al.constructed a selenium sulfide/double-layer hollow carbon sphere(SeS2/DLHC)composite material and designed it as a new type of Li-SeS2cell[101]。 In the composite,the SeS2is mainly encapsulated in the interlayer space of the DLHCs and has a high loading of 75%(mass percent),which can be used as an active component for lithium storage.The presence of selenium in the carbon framework not only alleviates the shuttle of polyselenides,but also improves the conductivity of the electrode.The SeS2/DLHC composite material has high specific capacity(0.2 C,930 mAh·g-1)and high rate capability(6 C,400 mAh·g-1),and has long cycle life,the capacity retention rate is 89%after 900 cycles at a current density of 1 C,and the capacity fading per cycle is only 0.012% 。
In addition,Shi et al.Have also made a breakthrough in the research of selenium alloy composite electrode.The SenS8-nmolecules are uniformly enclosed by a simple melt,and then impregnated.After 100 cycles,the battery capacity is still very high,reaching the 883 mAh·g-1,and its rate performance is shown in Figure 10[102]。 In addition,the theoretical calculation shows that the heteroatom Se2S6has higher polarizability and can combine with NMCs molecules more strongly,which further inhibits the shuttle effect of polyselenides[103]。
Germanium selenide(GeSe)can also be used as an electrode for lithium-selenium batteries.the well-designed hierarchical GeSe micro/nano structure can effectively buffer the volume change and ensure that the battery can be quickly lithiated/de-lithiated,so the lithium-selenium battery has high capacity,long cycle life and good rate capability。
Mullins et al.described in detail a slurry coated electrode with micron-sized Ge0.9Se0.1and Ge particles and studied its electrochemical performance[50]。 The Ge0.9Se0.1particles have a Li-permeable amorphous Li-Se-Ge inert phase,which can effectively buffer the large volume change in the alloying process,thereby improving the stability of the battery.The Ge0.9Se0.1showed a large reversible capacity of 800 mAh·g-1after 900 cycles at 1 C rate[104]。
4 Anode material
As an important part of lithium-selenium batteries, the stability of lithium anode also determines a series of performances of lithium-selenium batteries. At present, compared with selenium anode, the development and research of lithium anode is relatively less. However, a review recently published by Cui Yi and others shows that "lithium metal will be the most attractive'ultimate anode 'in the future after graphite carbon and silicon anode, which is a hot research topic."[19]. For half a century, the problem of metal lithium anode has not been solved. In the past five years, with the development of research tools and nanotechnology, researchers have made significant progress in the research of lithium metal anode based on these technologies, and lithium metal anode is reviving! ”
Metal lithium has the highest theoretical energy density and the lowest electrochemical potential (-3. 04 V relative to the standard hydrogen electrode), so the safety and cycle stability of the battery should be ensured before large-scale application. During the charge-discharge cycle of lithium batteries, the uneven deposition of lithium metal will form dendrites, resulting in short circuit of batteries, and the low Coulomb efficiency of lithium batteries will affect the use[19].
At present, the research of lithium-selenium battery is still in the initial stage, and the methods to protect the negative electrode are relatively limited. There are several ways to solve these problems, including electrolyte additive protection, SEI protection, negative electrode modification and so on.
4.1 Lectrolyte additive
the electrolyte and the lithium negative electrode are in direct contact,so the lithium selenium battery negative electrode is prone to side reactions.in experiments,many researchers will add some additives to the electrolyte of lithium-selenium batteries in order to improve the contact environment between the electrolyte and the lithium anode,promote the formation of a dense passivation film on the surface of the lithium anode,and improve the electrochemical performance of the battery。
in the review of Yu et al.,several methods to protect the negative electrode of the battery at present are introduced In detail,especially by improving the contact environment between the liquid electrolyte and the negative electrode,so as to improve the coulombic efficiency and cycle performance of the battery[105]。 For example,modified electrolytes such as LiNO3and PYR14TFSI can be used to perform electrochemical comparison tests on the HPCA/Se composite electrode:long-term discharge-charge tests of 300 cycles at 1 C prove that the HPCA/Se positive electrode of the LiNO3modified electrolyte has excellent cycle stability and rate capability 。
4.2 Protective layer(SEI method)
At present,the most commonly used method to stabilize SEI is to cover the surface of lithium with a protective layer(selenium powder can react with electrolyte),so as to ensure the protection of lithium anode.This requires that the protective layer has good chemical stability,is insoluble in electrolyte,does not react with lithium negative electrode and polyselenide,and has good ionic conductivity.in a word,the SEI layer can inhibit the dissolution of polyselenide in the ionic liquid electrolyte,thus improving the coulombic efficiency of lithium selenium batteries。
Huang et al.Used porous carbon nanospheres to confine selenium and produced a composite material Se/PCNs(chlorinated naphthalenes)for the positive electrode of lithium-selenium batteries[72]。 In the first lithiation process,a stable SEI passivation layer(composed of LiF,Li2O,Li2CO3and ROCO2)was formed on the surface of lithium metal,which played a role in protecting the negative electrode.Although the carbonyls of carbonate electrolytes can react with Se anions,PCNs reduce this side reaction and the SEI layer protects the LixSe from further reaction 。
In the research of Shi et al.And Guo et al.,a mixed SeSxelectrode material was developed,in which the negative protection SEI layer was also mentioned,and nitrogen doping was used[102][106]。 The Se2S6/NMC composite material is used as the positive electrode,and the mixture of LiTFSI in DOL and DME is used as the modified electrolyte,which can effectively inhibit the shuttle phenomenon due to the strong connection between Se2S6and NMCs(as shown in fig.11);As for the electrochemical performance,the lithium-selenium battery composed of SeS0.7/cyclic polyacrylonitrile(CPAN)composite materials can provide a high reversible capacity of 780 mAh·g-1,and has good stability after nearly 1200 cycles 。
4.3 Cathode modification
In order to improve the performance of the negative electrode of lithium selenium battery,the negative electrode materials can be modified.Guo et al.Proposed that the Li4Ti5O12protective layer has a high lithiation potential(1.55 V)and is considered to be one of the most promising alternatives to the graphite substrate[88]。 However,the Li4Ti5O12can only provide a lower capacity of the 175 mAh·g-1,which is less than the capacity of the graphite(372 mAh·g-1),and therefore,a new anode material with high capacity,high lithium voltage,long life and high rate should be developed.They synthesized a new Se/MPCS composite material(layered micro-mesoporous carbon spheres).In order to further study the performance of Se/MPCS-NMC battery,a long cycle test was carried out at a rate of 1 C.After 500 and 1000 cycles,the lithium-selenium battery maintained 80%and 50%of its original capacity,respectively 。
In addition,the modification of anode materials can also be improved by nano-interface engineering(this method is to use chemically stable and mechanically strong scaffolds to reinforce SEI),homogenizing lithium ion flow,minimizing volume changes through stable hosts,guiding lithium plating and protection,and preventing dendritic growth。
5 Electrolyte in Li-Se battery
Electrolyte is an important part of the battery, which plays a role in transporting ions and electrons between the positive and negative electrodes. In lithium selenide batteries, an important research goal is to solve the problem of shuttle effect (polyselenide), and the selection of appropriate electrolyte is the key to avoid the influence of polyselenide.
Due to the difference in the mutual combination of the positive and negative electrode materials and the electrolyte, there is a considerable difference in the initial discharge capacity between different batteries. In the previous research reports, although there are many organic, inorganic substances and lithium salts that can form the corresponding battery electrolyte, but the real application is not much, especially the electrolyte of lithium selenium battery is rarely reported, so in this part of this paper, a series of analysis and discussion will be made on the electrolyte of lithium selenium battery[21].
With regard to the electrolyte, lithium-selenium batteries generally need to meet the following requirements:
① Ionic conductivity: The electrolyte must have good ionic conductivity and cannot have electronic conductivity. The conductivity is within the range of the 1×10-3~2×10-3S/cm within a certain temperature range (temperature extremes do not occur).
② Lithium ion transference number: If the ion transference number of the battery is high, the degree of electrode polarization is low during charging and discharging, and the energy density and power density of the battery are high. The ideal lithium ion transference number is close to 1.
③ Stability: Generally, the electrolyte of the battery will be in direct contact with the electrode, which requires the electrolyte to have a certain chemical stability and a suitable temperature range, so as to avoid side reactions. In addition, the electrolyte must have a stable electrochemical platform to meet the electrochemical stability of the electrolyte and the unity of the electrode reaction.
5.1 Liquid electrolyte
the liquid electrolyte of lithium selenium batteries usually uses salts,alums and ethers,which can improve the performance of batteries.However,traditional electrolytes alone can not completely solve the problem of the shuttle effect of polyselenides,and traditional electrolyte solvents can even react with polyselenides,resulting in further loss of selenium.Therefore,it is necessary to develop new liquid electrolytes to replace these traditional electrolytes,or to take certain measures on the basis of traditional electrolytes to avoid the negative effects of electrolytes。
According to Guo et al.,lithium selenium batteries typically use two main types of electrolytes:ether-based electrolytes and carbonate-based ester-based electrolytes[106]。 For example,the ether-based electrolyte is typically a mixture of 1,3-dioxolane(DOL),1,2-dimethoxyethane(DME),and 1 M lithium(trifluoromethanesulfonyl)imide(LiTFSI);The carbonate-based electrolyte is usually composed of ethylene carbonate(EC),dimethyl carbonate(DMC),ethyl methyl carbonate(EMC)and a mixture of hexafluorophosphate(LiPF6)dissolved in a mixture of various carbonates 。
the method used by Ramesha et al.Is to encapsulate The selenium anode with carbon materials and prepare nitrogen-doped carbon nanotubes(N-CNT)by carbonizing polyaniline tubes[107]。 Selenium forms steric hindrance of long-chain polyselenides during electrochemical discharge,and directly forms Li2Se.In this study,Ramesha et al.Conducted comparative experiments and found that ether electrolytes correspond to higher battery capacity than carbonate-based electrolytes.After the initial cycle,the capacity of the batteries with ether and carbonate electrolytes changed little.This may be due to the redistribution of selenium on the microporous N-CNT during the initial cycle using an ether electrolyte,which controls its dissolution by polarity 。
For carbonate-based electrolytes,Huang et al.Synthesized microporous carbon spheres(MiPCS)by hydrothermal-annealing method,and compared carbonate-based electrolytes with ether electrolytes[108]。 In the carbonate-based electrolyte,only a pair of redox potential peaks can be observed,indicating that there is only a single-phase Se transition here.Se/MiPCS composite has two discharge plateaus in ether electrolyte and only one discharge plateau in carbonate electrolyte.During the cycle,the voltage in the carbonate electrolyte was 1.9 V;The voltage plateau in the ether electrolyte decreased after 20 cycles(1.95 V to 1.7 V)and continued to decrease in subsequent cycles.The capacity of Se/MiPCS composite in carbonate electrolyte was 515 mAh·g-1after 100 cycles at a current density of 0.5 C,and the retention rate was 72.8%,which was much higher than the capacity of lithium selenium battery in ether electrolyte(151 mAh·g-1,23.4% )。
As shown in fig.12,Abouimrane et al.Discussed in detail the lithiation mechanism of Li-Se batteries in carbonate-based electrolytes[109]。 It was found that Se was directly reduced to Li2Se during discharge.In the experiment,no intermediate phase was detected by X-ray diffraction and absorption spectroscopy,because the redox products Se,Li2Se and lithium selenate were insoluble in the electrolyte,so there was no shuttle effect and the performance of the battery was improved.In this study,they also made a comparison and found that Li-Se batteries performed better than Li-S batteries in carbonate-based electrolytes because of the avoidance of shuttle effects and the absence of soluble polyselenides 。
Zhang et al.described in detail two ways to improve the liquid electrolyte to improve the coulombic efficiency and cycle performance of the battery:they applied the modified electrolyte containing LiNO3and PYR14TFSI to the lithium-selenium battery[110]。 It was found that the coulombic efficiency of HPCA/Se(the composite material synthesized in this paper)anode corresponding to the PYR14TFSI modified electrolyte remained above 98%after 50 cycles.After 100 cycles,the reversible capacity of HPCA/Se with LiNO3modified electrolyte remained at 309 mAh·g-1,which was better and more stable than that of HPCA/Se with PYRth14TFSI modified electrolyte.Moreover,for the LiNO3modified electrolyte,it can still maintain the reversible capacity of the 301 mAh·g-1even at the current density of the 5 C(3375 mAh·g-1).In summary,the addition of both LiNO3and PYR14TFSI can significantly reduce the polyselenide reaction at the selenium cathode and thereby significantly reduce the shuttle effect in lithium-selenium batteries 。
Yushin et al.Synthesized mesoporous derived carbon/selenium composite cathode,and used the electrolyte system with LiNO3as electrolyte additive and LiTFSI dissolved in DOL/DME,focusing on the relationship between lithium salt concentration in the electrolyte and battery performance[111]。 All of the Se-CDC cathodes showed fairly stable cycling performance with nearly 100%coulombic efficiency after long cycling at 0.5 C in electrolytes with lithium salt concentrations of 1,3,5 and 7 M,respectively.in addition,they also found that higher electrolyte concentration can greatly improve the capacity utilization and reduce the dissolution of Se in Se-CDC composite cathode。
in addition,the pure ion solution can also be used as the electrolyte of the lithium selenium battery.Since the commercialization of lithium batteries,the electrolyte of many lithium-selenium batteries has been based on organic carbonates.Although organic carbonates have considerable advantages,they are limited by the lack of electrochemical stability,and ion mobility is also a serious problem,so scientists currently choose to use ionic electrolytes.Eftekhari et al.describe pure ionic solution electrolytes In detail[112]。 An ILs ionic liquid is mentioned in this paper,which has a series of characteristics such as non-flammable,non-toxic,low vapor pressure and so on.However,in the actual industry,the use of ionic liquids as a separate electrolyte in lithium-selenium batteries is still lack of practical feasibility,and further research is needed。
5.2 Solid electrolyte
Unlike liquid electrolytes,solid electrolytes have poor dispersibility and therefore low ionic conductivity,but one advantage of solid electrolytes is that they can significantly reduce the shuttle effect of lithium selenium batteries。
Recently,Cui Yi et al.Published an article on the electrolyte of solid-state lithium-selenium batteries in Joule:a highly compact Li6.4La3Zr1.4Ta0.6O12(LLZTO)ceramic tube is used as the electrolyte,which completely separates the positive and negative electrodes and operates at 240℃[113]。 At this temperature,the LLZTO ceramic tube has high conductivity,which is beneficial for the battery to achieve high power density.Specifically,the cell exceeds the energy density of the 500 Wh kg-1and achieves a coulombic efficiency of 99.99%.In a word,the system has low energy cost and good electrochemical stability,and is expected to be applied to large-scale energy storage and other application fields 。
Yushin et al.proposed a simple technique to form a protective solid electrolyte layer on the surface of the positive electrode[92]。 The protective layer ensures Li ion permeability and prevents polyselenide transport,thereby significantly enhancing the cycle stability of the battery and reducing the shuttle effect.Moreover,this method also forms a conformal shell that is permeable to Li(can accommodate volume changes),and this protective shell can,in principle,overcome polyselenide dissolution.This study evaluated the surface chemistry of Se-SAC-FD and Se-SAC-NR electrodes after cycling by using X-ray photoelectron spectroscopy(XPS)and electrochemical impedance spectroscopy(EIS)analysis(fig.13).Compared to the Se-SAC-NR cathode,the Se-SAC-FD cathode surface layer exhibited significantly higher C—O and C—O3bond strengths in the C1score spectrum(fig.13A).In addition,the surface of the Se-SAC-FD positive electrode shows much higher LiF intensity,indicating that the resulting solid electrolyte is chemically stable.The experimental Nyquist and Bode plots of the Li/Se-SAC cell were fitted using a simplified equivalent circuit(fig.13 C),where the Rs,RSEI,Rct,and CPE represent the solid resistance,solid electrolyte resistance,charge transfer resistance,and constant phase element,respectively.The Se-SAC-FD cell exhibited a significantly higher RSEIresistance(241.9.OMEGA.)Than Se-SAC-NR(65.6.OMEGA.),indicating that a protective SEI layer was indeed formed,as confirmed in XPS studies(fig.13B )。
Wang et al.described the use of polyacrylonitrile to stabilize the positive electrode of a SeSxand the use of a solid electrolyte to reduce the shuttle effect in lithium-selenium batteries[114]。 In this study,a carbonate-based electrolyte was used in which SEI was formed on the surface of the SeSx/CPAN electrode to further prevent dissolution of the polyselenide intermediate.The novel battery has very good long cycle stability and high rate capability,and the reversible capacity of the battery is maintained at 780 mAh·g-1after 1200 cycles at a current density of 600 mA·g-1.Moreover,even when the current density is increased to the 6 A·g-1,it can maintain 50%of the capacity at the 60 mA·g-1 。
Regarding the influence of the SEI layer on the electrochemical performance,as shown in fig.14,the SEI layer formed was very stable during subsequent charge/discharge cycles,as evidenced by the overlapping impedance curves.In addition,according to the SEM test in this paper,the rough surface of the SeS0.7/CPAN particles is due to the formation of the SEI layer.After 100 cycles in LiPF6-EC/DEC electrolyte,a clear SEI layer can be observed on the surface of the SeS0.7/CPAN electrode.Amine et al.designed a solid-electrolyte interphase(SEI),which can replace the traditional packaging method of lithium-selenium batteries and avoid the formation of polyselenides as much as possible:Se-doped S22.2Se/Ketjenblack can maintain a large reversible capacity(5 C 700 mAh·g-1)at high rates and minimize the shuttle effect[115]。
Wang et al.Published a similar paper in 2013,in which the performance of batteries was improved by mesoporous carbon and solid electrolyte[116]。 In this study,electrolytes such as carbonate/LiPF6/EC/DEC were used.In the electrochemical process,the intermediate of polyselenide was confined by mesoporous carbon,which avoided the shuttle effect of polyselenide.Although the carbonyls of carbonate electrolytes can react with Se anions,the presence of mesoporous carbon reduces this side reaction and forms a stable Se-O layer on the LixSe electrode to protect the LixSe from further reduction 。
6 Multifunctional diaphragm and positive and negative electrode separation
The separator is an important part of the lithium-selenium battery. It is located between the positive and negative electrodes inside the battery. Its main function is to prevent the transmission of electrons while ensuring the passage of lithium ions. At the same time, the performance of the separator determines the structure, internal resistance and thermal stability of the battery, thus directly affecting the capacity, cycle performance, life and safety of the battery.
At present, that preparation method of the lithium selenium battery separator mainly comprise a wet method and a dry method. Wet process and dry process have their own advantages and disadvantages: wet process has small and uniform pore size and thinner film, but it has high cost, complex process and easy to cause serious environmental pollution; The dry process is relatively simple, high added value and environmentally friendly, but the pore size and porosity are difficult to control, and the product is difficult to make thin. Lithium-selenium batteries are still in the experimental stage and are not yet mature. Through the planning and design of the separator, the purpose is to inhibit the shuttle effect of polyselenides, so as to improve the electrochemical performance of lithium-selenium batteries.
At present, there are few studies on lithium-selenium battery separators in various literatures. Li et al. Have studied a new graphene separator in detail[34]. In the study, they impregnated selenium into various porous carbon matrices, in which the use of membranes was a key step. Graphene not only facilitates the transport of electrons, but also prevents the migration of polyselenides to the negative electrode region by absorbing and trapping them. Using the graphene-polymer separator, the cycling performance of the pure Se cathode is excellent: the decay rate per cycle is as low as 0.048% in 1000 long cycles. A lithium-selenium battery with a graphene-coated polymer separator (graphene-polymer separator) is shown in Figure 15: The graphene side (black) of the separator faces the Se positive electrode and has good flexibility and mechanical stability.
With regard to the performance of the graphene separator, as shown in fig. 16, figs. 16 a and B give a comparison of the cell discharge voltage profiles of two different separators at 0.5 C rate in the first 50 cycles. Meanwhile, the change in plateau discharge capacity was quantified using different separators, respectively, as shown in figs. 16c and 16d above. Fig. 16e shows an electrochemical impedance spectroscopy (EIS) comparison plot of Li-Se cells with and without a graphene-coated separator before cycling.
Shen et al. Used Celgard 2300 polymer membrane as a separator and LiPF6 solution as an electrolyte to prepare a new porous composite material with a biochar-based (MMPBc) framework[65]. At a current density of 0.2 C, the Se/MMPBc-3 composite electrode provides a reversible capacity of 597.4 mAh·g-1 in the initial cycle. After 100 cycles, the residual capacity of the battery was as high as 517.8 mAh·g-1. At the high current density of 2 C, the reversible capacity of Se/MMPBc-3 electrode reaches the 421.0 mAh·g-1, which is still higher than the theoretical capacity of common cathode materials (such as LiFePO4,170 mAh·g-1). Chu et al. Used carbon nanotubes as a separator for lithium selenium batteries (prepared by surfactant sonication), and the CNTs interlayer between the positive and negative electrodes of Se @ RGO reduced the shuttle effect of polyselenides[38]. Therefore, Se @ RGO positive electrode has excellent electrochemical performance and has great potential in high energy and high power batteries.
In order to improve the performance of Li-Se batteries, Zhang et al inserted a conductive porous carbon separator between the positive and negative electrodes of Li-Se batteries[117]. The carbon middle layer is usually made of cellulose, and the electrolyte can permeate through this layer of carbon fibers and react with the active material. The positive electrode of the battery is a Se/TiO2 composite material prepared by a melt infiltration method; and the reversible capacity is 520 mAh·g-1 after 20 cycles at a current density of 67.5 mA·g-1, and the coulombic efficiency is nearly 99%, so the performance of the battery is superior to that of a battery without a separator. Scanning electron microscopy (SEM) analysis of the material on the cycled carbon interlayer revealed a porous microstructure (Figure 17 a, C), and the appearance of the carbon interlayer after 20 cycles (Figure 17 B, d) indicated that the corresponding material was deposited on its surface. The enlarged SEM images and elemental distribution of the recycled carbon intermediate layer are shown in figs. 17e and f, from which it can be seen that the carbon intermediate layer is covered by a layer of deposited material, indicating that the coating on the recycled carbon intermediate layer is rich in selenium.
In addition, some coatings or protective layers can be added between the positive electrode and the separator to improve the performance of lithium-selenium batteries. For example, Jiao et al. Supported 46.58 wt% selenium on carbon cloth through calcination process, which can be directly used as a self-supporting positive electrode[118]. After that, they prepared a carbonized polyacrylonitrile (PAN) nanofiber membrane by electrospinning and used it as a protective layer between the positive electrode and the separator to avoid the loss and dissolution of selenium. The intermediate layer introduced by Jiao et al enhanced the stability of the Li-Se battery, and the reversible capacity reached 590 mAh·g-1 after 1000 cycles at a current density of 0.5 C.
Wang et al. Prepared a new type of TPB-DMTP-COFs covalent organic framework separator coating for lithium-selenium batteries. The TPB-DMTP-COF material can selectively adsorb lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) in the electrolyte by forming C-H … F and O … Li hydrogen bonds, thus effectively solving the problems such as shuttle effect[119]. The battery is outstanding in energy storage and stability. At the current density of 0.5 C, the capacity reaches 844.6 mAh·g-1,SeS2 and the load capacity is 2 mg·cm-2. At the current density of the higher load 4 mg·cm-2,1 C, the capacity is 684 mAh·g-1, and even after 800 cycles, the capacity of the 416.3 mAh·g-1 is still retained, and the capacity attenuation rate per cycle is only 0.05%.
Inserting a separate carbon interlayer into the Li-Se new battery significantly improves its specific capacity and cycle stability. The conductive carbon intermediate layer reduces the charge transfer resistance of the Se/TiO2 and can trap the migrating active material during charge and discharge. According to the method, the conductivity of the lithium selenium battery is increased, the utilization rate of an active material is improved, the performance of the battery is stabilized, and the cycle life of the battery is prolonged; and in addition, the porous carbon intermediate layer separator also has the advantage of capturing and absorbing intermediate polyselenides[92,120].
As mentioned above, by changing the surface wettability of the separator and introducing a conductive carbon layer, the functions of reducing the interface resistance of the positive electrode, recovering the active selenium material, and improving the utilization rate and cycle stability of the positive electrode selenium can be effectively realized. The modification of the multifunctional separator can effectively stabilize the solid electrolyte interface of the metal lithium negative electrode (improve the contact performance of one side of the negative electrode), so that the uniform distribution of the lithium ion concentration field can be realized, and the appearance of the battery negative electrode can be maintained.
However, although some progress has been made in the multifunctional separator for lithium-selenium batteries, there is still a huge space for its follow-up research and practical application. Firstly, in view of the diffusion problem of polyselenides in lithium-selenium batteries, it is necessary to further combine electrostatic repulsion and steric hindrance to achieve more efficient polyselenide barrier and lithium ion penetration. In addition, on the positive electrode side, it is necessary to make better use of the promotion effect of the conductive layer on the positive electrode interface reaction to avoid the formation of inert precipitates.
In terms of practicality, the separator has great application potential. Through the multi-functional research and treatment of the separator, the electrochemical performance can be effectively improved without changing the battery process, but there are still the following requirements: First, the separator should have electronic insulation to ensure effective isolation between electrodes; At the same time, it should have a certain porosity to ensure high ionic conductivity and lithium ion permeability, so as to reduce the internal resistance of the battery. Secondly, the solvent of the electrolyte is an organic compound with strong polarity, and the diaphragm must be resistant to electrolyte corrosion; Diaphragm should have enough ability to absorb liquid and keep moisture, and have good wettability to electrolyte; Thirdly, it needs to have sufficient mechanical properties, including puncture strength, tensile strength, etc., but the thickness should be as small as possible to ensure spatial stability and flatness. Fourth, the thermal stability and automatic shutdown protection performance are good, especially the power battery has higher requirements for the diaphragm, usually the composite membrane can be used; Fifth, there is a need for new separators that are inexpensive, have appropriate chemical compositions, and are environmentally friendly.
With the introduction of new materials and more in-depth research on multi-functional separators, there will be broad prospects for multi-functional separators for lithium-selenium batteries.
7 Binder
The raw materials of a complete lithium battery must include active material, conductive agent, binder, solvent and additive, etc. The binder plays a role in bonding the active material and foil, active material and active material, active material and conductive agent. Although the dosage is small, the effect is remarkable.
The binder added to the lithium-selenium battery must be firmly attached to the electrode and must form a network between the selenium and the conductive carbon to ensure the integrity of the electrode. During the charge and discharge process, the binder may become an inactive site, leading to performance deterioration, so it is very important to select a functional binder with excellent performance. In some specific cases, lithium-selenium batteries can also be used without binders.
In short, lithium-selenium batteries have the following requirements for binders: first, to ensure the uniformity and safety of active material pulping; Condly, it plays a role in binding the active substance particles; Thirdly, the active material is bonded on the current collector; Fourth, maintain the adhesion between the active material and the current collector; Fifth, the formation of the SEI protective film on the surface of the carbon material (graphite) is facilitated as much as possible.
7.1 Selenium anode binder
Yu et al.studied a new type of rechargeable lithium-selenium battery with water-soluble sodium alginate(SA)as the binder[121]。 sodium alginate is a very common binder,which is a by-product of extracting iodine and mannitol from brown algae such as kelp or Sargassum.Its molecule is composed ofβ-D-mannuronic acid(M)andα-L-guluronic acid(G)in a certain order.It has the stability,solubility,viscosity and safety required by pharmaceutical excipients.At present,Sodium alginate has been widely used in the field of food industry and medicine。
Guo et al showed that the Se8molecules were confined in the pores of the ordered mesoporous carbon(CMK-3)matrix in the selenium cathode composite[122]。 they used water-soluble binder SA and assembled Li-Se batteries.in the experiment,They also used PVDF(polyvinylidene fluoride)adhesive as a control group,which is commonly used in lithium batteries,is a homopolymer material with high dielectric constant,has high viscosity in certain solvents,has good adhesion and is easy to form films.Therefore,the membrane electrode material for lithium-selenium battery prepared by using PVDF as a binder has good chemical stability,temperature stability,mechanical property and processability。
The electrochemical performances of Se/CMK-3 batteries with SA binder and PVDF binder were compared,focusing on the capacity and coulombic efficiency.For the Se/CMK-3 cathode with SA binder,the reversible capacity after 50 cycles was 600 mAh·g-1(PVDF the binder was only 300 mAh·g-1),and the coulombic efficiency was close to 100%,which confirmed its excellent cyclability.The high coulombic efficiency indicates that the shuttle effect is suppressed in the Li-Se cell.Compared with the electrode with SA binder,the Se/CMK-3 electrode with PVDF binder showed similar electrochemical behavior but poor performance 。
Wang et al.Studied a carbon-coated selenium anode,still using sodium alginate as a binder[123]。 In this study,the mass ratio of carbon/selenium composite:carbon black:sodium alginate binder was 8:1:1.After the slurry is mixed,the mixture is poured on the aluminum foil by a scraper,the aluminum foil is dried and stamped into a circular electrode with an area mass load of 1.2 mg·cm-2,and then the lithium selenium button battery is assembled.The sodium alginate binder enables carbon to physically encapsulate and chemically bond selenium,thereby improving the charge-discharge cycle stability of the battery,as shown in fig.18 。
In addition,with the continuous progress of lithium battery technology,more and more new adhesives have begun to enter people's vision,such as polyacrylic acid(PAA),polytetrafluoroethylene(PTFE),polyimide(PI)and so on.In the future,it is necessary to customize the design of binders,and customize the morphology and surface state of existing binders according to the comprehensive factors such as material surface morphology,state and functional groups,so as to continuously meet the needs of high energy density batteries。
7.2 Binderless
With the progress of science and technology,some researchers no longer use binders in lithium-selenium batteries.Shen et al.Developed a lithium-selenium battery using graphene oxide,in short,selenium was prepared on nickel foam by electrochemical replacement method,without any binder or conductive additives,and used as the positive electrode of Li-Se battery[35]。 The material provides 554 mAh·g-1capacity(82%of theoretical capacity)and retains 137 mAh·g-1capacity after 100 cycles at a current density of 0.1 C.When the graphene oxide coating is used,protection is formed on the Se surface,and the electrode performance can be further improved.In this study,the lithium-selenium battery reached a capacity of 665 mAh·g-1(98.5%of theoretical capacity)in the first cycle and maintained a capacity of 266 mAh·g-1after 100 cycles at 0.1 C 。
This study has the following advantages:first,the cost of the experiment is low,and the experimental materials used are simple and easy to obtain;Secondly,the experimental process is convenient:the amount of selenium on the foam Ni can be controlled by the reaction time,and the graphene oxide can further improve the performance of the battery;Thirdly,the most important is that it provides a new method for the preparation of selenium materials for lithium-selenium batteries:foamed Ni can be used as the matrix,and other forms of Ni materials,such as Ni nanoparticles,mesoporous films,and carbon-coated Ni particles,can also be used as the substrate.Therefore,different lithium-selenium batteries can be prepared only by changing the morphology of the Ni substrate,which will have a great impact on the research prospects of binderless lithium-selenium batteries。
Similarly,Dai et al.Also studied a lithium-selenium battery that does not require a binder:they synthesized a flexible,self-supporting Go-Se@CNT composite film positive electrode through a simple two-step assembly process,which provided 400 mAh·g-1reversible discharge capacity and maintained 315 mAh·g-1(79%retention)after 100 cycles at a current density of 0.1 C[28]。 the high utilization of Se In the composite film is due to the synergistic effect of GO and CNTs,which provides a three-dimensional conductive network not only for electron transfer and Li ion diffusion,but also limits the polyselenide.in this study,the use of binderless is an important innovation,and the study also developed a thin film electrode,which is of great significance to the research of ultra-thin flexible batteries for soft portable electronic devices。
8 Current collector
The current collector is the carrier of the positive active material, which is responsible for collecting electrons and transporting them to the active material through an external circuit. In lithium batteries, it is mainly composed of aluminum foil, stainless steel mesh, foamed nickel or copper foil (broadly speaking, it can also include tabs). The current collector is mainly used to collect the current generated by the active material of the battery so as to form a larger current output, so the current collector should be in full contact with the active material, and the internal resistance should be as small as possible. However, the current collector is easy to contact and react with the electrolyte solution, and the stability of the electrolyte and the compatibility of the current collector will affect the performance of lithium-selenium batteries[124]. When the current collector corrodes or dissolves, the contact between the electrode active material and the current collector will be reduced, which will lead to the shedding of the active material and the increase of the battery impedance, all of which can be improved by modifying the current collector[125,126].
8.1 Classification of current collector
copper current collector,copper is an excellent metal conductor with conductivity second only to silver,which has many advantages such as abundant resources,cheap and easy to obtain,good ductility and so on.However,copper is easily oxidized at higher potentials,so it is often used as a current collector for negative active materials such as graphite,silicon,tin and cobalt-tin alloy.Common copper current collectors include copper foil,copper foam,copper mesh and three-dimensional nano-copper array current collectors.aluminum current collector.Although the conductivity of aluminum is lower than that of copper,the mass of aluminum is only half that of copper when delivering the same amount of electricity.the use of aluminum current collector helps to improve the energy density of lithium-ion batteries.In addition,aluminum is cheaper than copper.in the process of battery charging and discharging,a layer of dense oxide film will be formed on the surface of aluminum foil current collector,which improves the corrosion resistance of aluminum foil and is often used as the positive current collector In lithium ion batteries.the nickel current collector has the advantages of low price,good conductivity and stability In acid and alkaline solutions,so the nickel can be used as both a positive current collector and a negative current collector.the stainless steel current collector is an alloy steel containing nickel,molybdenum,titanium,niobium,copper,iron and other elements and has good conductivity and stability;a passivation film is easily formed on the surface of the stainless steel to protect the surface from being corroded;and the stainless steel can be processed to be thinner than copper and has the advantages of low cost,simple process,mass production and the like.the carbon current collector can avoid the corrosion of electrolyte to the metal current collector when the carbon material is used as the positive or negative current collector,and has the advantages of abundant resources,easy processing,low resistivity,no harm to the environment,low price and the like.In recent years,composite current collectors,such as conductive resin,carbon-coated aluminum foil and titanium-nickel shape memory alloy,have also attracted the attention of scholars[127]。
8.2 Application of current collector
For a current collector in a lithium selenium battery,it is necessary not only to have excellent conductivity,but also to have a large space to store the active material mixture.the current collector is very important to maintain the morphology of the cathode material,and its internal structure also plays a great role in the performance of the battery.Nesper et al.studied nanofiber selenium and its polypyrrole and graphene composite cathode materials,and they used lithium metal foil current collectors[128]。 in this study,Se/PPy and Se/graphene composites were used,In which the amounts of PPy and graphene were 10 wt%and 6.5 wt%,respectively.the active material content of Se/PPy composite electrode was 67.5%,and the active material content of Se/graphene composite electrode was 70.1%.The electrode mixture was sonicated for 1 H to obtain a homogeneous slurry,which was then coated on a coin-shaped titanium current collector and dried at 100°C for 12 H,followed by vacuum drying for 6 H,after which it was ready for use(installation of the corresponding battery,etc.)。
in the lithium selenium battery,the current collector can also be used in the Li negative electrode.Guo et al.Have studied the influence of the current collector on the Li negative electrode in detail:the current collector affects the nucleation in the initial stage of Li electroplating,which is decisive for the morphology of Li in subsequent electroplating[129]。 However,the role of lithium metal anode current collectors has not been fully studied.Most current collectors used in lithium selenium batteries are planar,such as conventional copper and aluminum foils,and the initial plating of Li on the planar current collector tends to result in a non-uniform deposition of Li particles,followed By the growth of Li dendrites on the Li particles.in this study,a three-dimensional(3D)current collector with a submicron-sized framework and a porous structure could alter the plating behavior.When the porous Cu foil is used as the 3D current collector,Li grows on the sub-micron-sized Cu framework and fills the pores of the 3D current collector.by accommodate that lithium metal negative electrode in the 3D current collector,the lithium metal negative electrode can be separated from the lithium dendrite critical point,and the service life of the lithium metal positive electrode can be significantly improve。
in brief,three-dimensional porous copper foils were synthesized by planar and scalable methods,and the preparation process of 3D porous copper foils is shown In Figure 19。
the electroplating behavior of Li metal on Li current collector was studied by using 3D Cu foil,and the original Cu foil can also be tested As an example of planar current collector.on the planar current collector,Li first forms small Li dendrites(0.1~0.5μm in diameter)on the smooth surface in the nucleation step.It can be observed that the morphology of Li metal on 3D Cu foil is significantly different from that of Li metal on planar Cu.the surface of the latter is very uneven,and its height difference is beyond the measurement ability of AFM.as shown in fig.20,the morphology of the Li metal plated on the 3D current collector was in good agreement with expectations.Therefore,by containing Li metal in the 3D current collector,the growth of dendritic Li is effectively suppressed。
Through this study,it has been demonstrated that the Li anode can accommodate the reserved pores of the 3D current collector with submicron framework and high specific surface area,thereby inhibiting the growth of dendritic Li,thereby solving a series of problems caused by the dendritic growth of Li anode.According to the experiment,the lithium metal negative electrode can run for 600 hours without causing short circuit,which proves that this method can significantly improve the life and safety of lithium metal batteries.In the submicron structure of the 3D current collector,the Li anode has a high area capacity and maintains a good plating/stripping efficiency of up to 98.5%.Due to the high electroactive area of the submicron 3D structure,the Li metal can be accommodated inside the 3D current collector,and the voltage of the Li negative electrode lags and its charge transfer resistance is reduced compared to that on a planar current collector。
in addition,the porous current collector may not be limited to the porous Cu foil,and other porous structures may also be used to improve the lifetime and performance of the Li negative electrode.It is worth noting that when improving the lithium metal anode,a variety of methods should be adopted.As In this study,the 3D current collector with submicron structure can play a synergistic effect with other reasonably designed battery components(including appropriate electrolytes,additives,Li surface protective layers,modified Li metal,etc.)to improve the overall performance of lithium metal anode.the 3D current collector architecture adapted to lithium anode is the main direction of further research on lithium anode,which can promote the development of lithium metal batteries to the next generation of energy storage devices。
9 Conclusion and prospect
In this paper, the cathode and anode materials, electrolytes, multi-functional separators, binders and current collectors of lithium-selenium batteries are introduced. In this section, the lithium-selenium batteries are summarized and prospected in an all-round way, and the commercial application of the batteries is discussed.
9.1 Cathode material
In terms of cathode materials,the main purpose of lithium-selenium batteries is to improve the electronic and ionic conductivity of the cathode,thereby improving the cycle performance of the whole battery,usually using conductive materials and selenium polymerization to form a composite.At present,the common cathode materials are mainly selenium/carbon composite materials,selenium/metal compounds,selenium/conductive polymers and their combinations[23,122]。
the first is the Se/carbon composite electrode.carbon nanofibers(CNFs),carbon nanotubes(CNTs)and graphene oxide(GO)have excellent physical and chemical properties,which can meet the requirements of Se as the anode.Microporous carbon nanofibers/selenium(MCNFs/Se)composites can be prepared by combining CNFs with selenium under vacuum and heat treatment conditions.Multi-walled carbon nanotubes(MWCNTs)can improve the conductivity of the electrode and optimize the electrochemical performance of the electrode.the Se-CNTs positive electrode shows very stable cyclability and high specific capacity at a wide range of working current density.Because of its high carrier mobility at room temperature,it can improve the overall performance of the battery through coating。
Second,through the synthesis of organic polymers,the encapsulation of Se is achieved by using conductive polymers(such as polyaniline,polypyrrole and polyacrylonitrile)to alleviate the dissolution of polyselenides in C/Se electrodes。
the third is the doping of nitrogen atoms,which can promote carbon functional group-selenium bonds,reduce the shuttle effect of polyselenides,improve the conductivity of carbon,increase reactive sites,and improve the wettability and absorptivity of electrolyte.Therefore,heteroatom-doped carbon materials with high specific surface area can further improve the electrochemical performance of Li-Se batteries。
Fourth,the composite materials using porous carbon and selenium are divided into microporous carbon,Mesoporous carbon and multilayer porous carbon.In comparison,microporous carbon materials are considered to be suitable Se hosts because of their high specific surface area,large adsorption capacity and pore structure.mesoporous carbon is one of the most common media to improve the performance of lithium selenide batteries.It has many advantages,such as high specific surface area,appropriate pore size distribution and specific high-quality structure.layered porous carbon,as a supplement to the former two,does not have their disadvantages.Therefore,Layered porous carbon materials composed of carbons with different pore sizes are promising cathode hosts for lithium selenium batteries。
Fifthly,a stable and efficient SEI layer can be constructed by adding auxiliary additives to effectively inhibit the dissolution of polyselenides and promote the cycle performance。
Sixthly,metal compound electrodes are used.in lithium selenium batteries,many inorganic polar materials can also be used as effective selenium carriers In the positive electrode of lithium selenium batteries.For example,oxides and selenides of metals such as Fe,Zn,Ti,Bi and Mo can be used。
Seventhly,a selenium alloy composite material electrode is adopted to combine the respective excellent performances of several elements into an alloy,for example,selenium has stable cycle capability,but the reversible capacity is low;and sulfur exhibit a high reversible capacity but a poor cyclability.Therefore,the heteroatomization of Se and S molecules in carbon or other hosts utilizing the advantages of both Se and S is an effective way to improve the electrochemical performance of lithium-selenium batteries。
9.2 Anode material
In recent years,there are many literatures about the modification of the positive electrode of lithium-sulfur batteries,but there are few literatures about the modification of the negative electrode of lithium-selenium batteries.According to Cui Yi's recent review,the negative electrode of lithium batteries will eventually revive and become the"ultimate negative electrode".In lithium-selenium batteries,metal lithium has the largest theoretical energy density and the lowest electrochemical potential,but it still faces the following challenges:during the charge-discharge cycle,Li deposits unevenly to form dendrites(easy to cause short circuit of batteries);Low coulombic efficiency and increasing overpotential of lithium negative electrode.All these will reduce the coulombic efficiency of the battery and affect the life of the battery.In order to solve this series of problems,there are three schemes at present[13]。
The first is that the electrolyte additive protects the negative electrode.Since the electrolyte is in direct contact with the lithium negative electrode,this leads to a side reaction easily occurring at the negative electrode of the lithium selenium battery,for example,a modified electrolyte containing a LiNO3and a PYR14TFSI is applied to the lithium selenium battery,Ionic liquids,which act as"co-solvents"for electrolytes,PYR14TFSI form a protective layer on the electrode surface and inhibit the dissolution of the discharge intermediate polyselenide in the ionic liquid electrolyte during electrochemical processes 。
the second is the SEI protective layer.the most commonly used method to stabilize SEI is to cover a protective layer on the surface of lithium.in order to ensure that the protective layer can effectively protect the lithium anode,it is required that the protective layer has good chemical stability,is insoluble In electrolyte,does not react with the lithium anode and polyselenide,and has good ionic conductivity。
the third is other methods.For example,The negative electrode is modified by nano-interface engineering(using chemically stable and mechanically strong scaffolds to reinforce SEI),homogenizing lithium ion flow,guiding lithium plating and protection,and preventing dendritic growth。
9.3 Other
other important parts refer to the electrolyte,separator,binder and current collector of the battery.electrolyte is an important part of battery,including liquid electrolyte and solid electrolyte.in lithium-selenium batteries,due to the problem of shuttle effect,the key to avoid the influence of polyselenides on the utilization of batteries is to select a suitable electrolyte:In liquid electrolytes,salts,alums and ethers are usually used.However,traditional electrolyte solvents alone cannot solve the problem of polyselenide shuttling,and some researchers now use carbonate-based electrolytes.Unlike liquid electrolytes,solid electrolytes have poor dispersibility,so their ionic conductivity is low.However,one of the advantages of solid electrolytes is that they can significantly reduce the shuttle effect of lithium-selenium batteries.At present,SEI layers are mainly synthesized from polymers and Other materials。
A separator for a lithium selenium battery.the performance of the separator determines the structure,internal resistance and thermal stability of the battery,and directly affects the capacity,cycleability and safety of the battery.Therefore,the separator with excellent performance is very important to improve the comprehensive performance of the battery.At present,there are few studies on lithium-selenium battery separators in various literatures,mainly through graphene coating film to reduce the shuttle effect。
binder for lithium-selenium batteries.Bonding the active material to the foil,active material to active material,and active material to the conductive agent,although in a small amount,is critical.the Binder added to the lithium-selenium battery must be firmly attached to the electrode and must form a network between the selenium and the conductive carbon to ensure the integrity of the electrode。
the current collector is the carrier of the electrode active material,which is responsible for collecting electrons and transporting them to the active material through an external circuit.Specifically,the current collector refers to a structure or part for collecting current.the current collector on the lithium selenium battery is mainly composed of aluminum foil,stainless steel mesh,foamed nickel or copper foil,and the performance of the lithium selenium battery can be improved by modifying the current collector。
In addition,in addition to Li-Se batteries,there are Na-Se and K-Se batteries.Because Li,Na and K belong to the same group of elements,their properties are somewhat similar,which is worth comparing.Sodium metal has low cost and large volume capacity.While preparing Li-Se batteries,the same material can be used to prepare Na-Se batteries.There are some differences in performance between the two.For example,when Se@CNFs is used as the anode,the capacity of Li-Se battery is 1207 mAh·g-1,while the capacity of Na-Se battery is 494 mAh·g-1.This is due to the difference in ionic radius between Li and Na.However,Na is much more abundant than Li in nature,so Na-Se battery has a broader prospect[24,121,130]。 K-Se battery is a new type of battery,whose theoretical specific capacity is much larger than that of traditional alkali metal ion batteries,so it has attracted the attention of scientists in recent years.Xu et al.Have studied a nitrogen-doped porous material with a selenium loading of up to 60%,which effectively improves the performance of K-Se batteries[131,132]。 At the same time,they also designed a new hollow carbon matrix imitating NaCl crystal structure for effective encapsulation of Se,and the 3D structure is conducive to rapid electron transport,so its electrochemical performance is good(after 100 cycles,the capacity remains at 464 mAh·g-1).However,K-Se batteries are still in the laboratory stage due to the constraints of development cost and technology,and their large-scale industrial application still needs further research 。
9.4 Application and commercialization
lithium-selenium batteries will attract much attention in the research of next generation energy storage devices because of their high volume energy density,high conductivity and environmental friendliness.However,in order to achieve wide application and commercial development of Lithium-selenium batteries,the following problems need to be solved:
First,the problem of battery performance maintenance.the test environment and test requirements in the literature are mostly very strict:to maintain a certain temperature and humidity without significant changes.However,in actual large-scale production and application,the conditions can not be so strict,so at this stage,it is necessary to further verify the role of lithium-selenium batteries in more common non-laboratory conditions。
Second,the feasibility of battery production.in order to achieve large-scale production of lithium-selenium batteries,It is necessary to ensure that lithium-selenium batteries have a high effective selenium content,but at present it has not achieved the desired results.There is a need for improvements In the following areas:improving its overall particle-to-particle contact by rolling over the electrode;it is necessary to formulate a set of unified production standards,including the evaluation of process,preparation success rate and other parameters。
Third,the cost of battery manufacturing.This is a very critical factor for commercial application.Although the price of lithium-selenium batteries is low,whether the manufacturing process can control the cost is a key issue.Many of the steps described in the literature are inherently complex and expensive,so finding more rational production routes is also an important part of making them commercially viable。
Fourth,safety in use.If lithium-selenium batteries are to enter the market,besides cost,performance and other factors,safety is more important.Due to a series of problems such as internal defects,long-term overcharge and short circuit of lithium batteries,common lithium batteries on the market often cause spontaneous combustion or explosion.Whether lithium-selenium batteries can overcome these difficulties is an important condition to improve their safety and achieve commercialization。
9.5 Outlook
Because selenium and sulfur belong to the same group of elements,and the development of Li-S battery is relatively mature Compared with Li-Se,the research of Li-Se battery at this stage can apply the idea and framework of Li-S battery research,especially in the direction of preparation and modification of Se cathode.compared with Li-S battery,Li-Se battery has better theoretical cycle performance and higher output voltage,so it has great research potential[17]。
At present,the shuttle effect is still the most critical factor restricting the development of Li-Se/S batteries.In the positive electrode,the common method is to modify,package And design the positive electrode with various carbons(including different sizes and doping),conductive polymers,alloys,etc.Compared with Li-S batteries,Li-Se batteries can further design and develop a variety of new structures(such as one-dimensional structure,core-shell structure,etc.)and use a variety of new materials(such as covalent organic frameworks COFs,etc.)。
In the aspect of negative electrode protection,Li-Se battery needs to consider the matching of Li and Se,and can learn from Li-S battery,using electrolyte additives,coating method and lithium alloy protection method to reduce the occurrence of side reactions,so as to improve the performance of the battery。
Li-Se batteries can use solid and liquid electrolytes to reduce the shuttle effect and improve the utilization of active material.Like Li-S battery,the separator of Li-Se battery needs to inhibit the diffusion of polyselenide and improve the interface between positive and negative electrodes.A variety of new binders and current collectors can be used comprehensively,and even no binder can be used in some cases,with simple operation and low price。
in recent years,lithium-selenium batteries have attracted much attention.However,at present,the research on improving the performance of lithium-selenium batteries is still In its infancy,which requires scientists to improve the performance of lithium-selenium batteries from the perspectives of material design,assembly process adjustment,electrolyte matching and negative electrode protection,so as to give full play to the advantages of high volume energy density[133]。 Next,through further in-depth research and optimization design,while taking into account the quality and cost of lithium-selenium batteries,it is believed that in the near future,lithium-selenium batteries will be able to achieve industrialization and commercialization。
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