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Design and Preparation of Electrode Nanomaterials with “Yolk-Shell”Structure for Lithium/Sodium-Ion/Lithium-Sulfur Batteries

Fangyuan Li , Junhao Li , Yujie Wu , Kaixiang Shi , Quanbing Liu , Hongjie Peng

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (6) : 1369 -1383.

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Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (6) : 1369-1383 DOI: 10.7536/PC210733
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Design and Preparation of Electrode Nanomaterials with “Yolk-Shell”Structure for Lithium/Sodium-Ion/Lithium-Sulfur Batteries
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Abstract

“Yolk-shell” nanomaterials with adjustable “yolk”, "shell" and "cavity" structures are regarded as "nanoreactors" and have outstanding performance in the application fields of catalysis and energy storage. Especially for electrochemical energy storage and conversion, this type of material has a considerable specific surface area and a special core-shell structure, which could alleviate the volume change of the electrode, provide fast ions/electron transport channels, enhance the adsorption of intermediates, and strengthen the conversion reactions during the charge/discharge process. It can significantly improve electrode stability, rate and cycling performance, which is a relatively ideal electrode material. This article focuses on the application of “yolk-shell” nanostructured electrodes in the field of secondary batteries including lithium-ion batteries, sodium-ion batteries, and lithium-sulfur batteries and summarizes the design and synthesis strategies of this type of nanostructured electrodes, including template method, Ostwald ripening, Galvanic replacement, and Kirkendall effect, presents the advantages and disadvantages of various methods for electrochemical applications, and finally discusses prospects of yolk-shell structure in research and applications of lithium/sodium-based and lithium-sulfur secondary batteries.

Contents

1 Introduction

2 Template methods

2.1 SiO2as a hard template

2.2 Metal oxide as a hard template

2.3 Carbon as a hard template

2.4 Soft templates

3 Self-assemble methods

3.1 Ostwald ripening to synthesis of yolk-shell structure

3.2 Galvanic replacement to the synthesis of yolk-shell structure

3.3 Kirkendall effect on synthesis of yolk-shell structure

4 The applications of yolk-shell structure in batteries

4.1 Yolk-shell in lithium-ion batteries

4.2 Yolk-shell in sodium-ion batteries

4.3 Yolk-shell in lithium-sulfur batteries

5 Conclusion and outlook

Graphical abstract

Keywords

Li-ion battery / sodium-ion battery / Li-S battery / synthesis yolk-shell structure / nano-electrode

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Fangyuan Li, Junhao Li, Yujie Wu, Kaixiang Shi, Quanbing Liu, Hongjie Peng. Design and Preparation of Electrode Nanomaterials with “Yolk-Shell”Structure for Lithium/Sodium-Ion/Lithium-Sulfur Batteries. Progress in Chemistry, 2022, 34(6): 1369-1383 DOI:10.7536/PC210733

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1 Introduction

Yolk-eggshell nanomaterials have the advantages of large specific surface area, controllable interface composition, precise active component loading and good stability, which can be regarded as nanoreactors and have broad application prospects in the fields of energy storage/conversion and catalysis[1]. The nano electrode with that structure has the follow advantages: (1) the large 'cavity' structure can be use as a container of an electrochemical active center on the one hand, and can also provide a volume buffer 'cavity' for the electrode material dure charging and discharging on the other hand, so that the deformation and the mechanical stress in the reaction process are slowed down, and the stability of the electrochemical process is ensured[2]; (2) the adjustable thickness and pore structure of the conductive "eggshell" provide a fast ion/electron transport channel for the electrochemical reaction, and realize the screening and confinement of ions or reactants[3,4]; (3) The "yolk" active center can be designed to strengthen the catalytic conversion and improve the electrode reaction kinetics[5]; (4) The controllable core-shell spacing is convenient to control the diffusion rate and reaction rate of reactants[6,7]. In view of this, we summarize the design and preparation methods of YS-structured nanoelectrodes, which can provide a theoretical basis for the rational design of high-performance electrode materials.
The common preparation methods of YS-structured nanoelectrodes include template method and self-assembly method, in which the template method is divided into hard template method and soft template method, which is a "bottom-up" method[8][9]. The hard template method generally has three steps: synthesizing an active substance with a special morphology, changing the surface charge and polarity of the substance, synthesizing a template on the surface of the material by using electrostatic attraction or intermolecular force, and then synthesizing a "shell" substance on the surface of the template in a similar way, and finally selectively removing the hard template to obtain a YS structure[10]. The soft template method generally uses emulsified droplets, vesicles, micelles, bubbles, etc. As the template, which only requires the outer coating process without removing the soft template, providing more possibilities for adjusting the structure of the inner and outer layers, so it has a more complex layered structure[11]. The self-assembly method is to directly synthesize the YS structure without a template, which generally includes two steps: the first is the synthesis of the precursor material, and the key points are: (1) precisely controlling the chemical conditions of the system, such as the composition of the solvent, the type of the precursor, the concentration of the precursor, the adsorption time, and the pH value; (2) accurately controlling physical conditions such as precursor concentration, calcination temperature, time and rate, and gas atmosphere[12][13][14]; The second is the conversion of the precursor material, which precisely controls the removal rate of the template and the crystallization rate to control the YS structure[15]. In this paper, the recent progress of YS structure and its latest applications in lithium-ion batteries, sodium-ion batteries and lithium-sulfur batteries are summarized from the synthesis strategies and practical applications of YS structure, and the structure-activity relationship between the structure and performance of electrochemical energy storage is emphasized, and the development direction of YS structure in these fields is prospected.

2 Template method

The hard template method uses the outer electrostatic force or adsorption of the target material to synthesize the template, synthesizes the shell material on the outer layer of the template, and then removes the template to obtain the YS structure. The principle of template removal is based on the property difference between the template and the active material. While removing the hard template, it is also necessary to ensure the stable existence of the active material in the etching environment. Common methods for template removal include chemical etching and thermal decomposition in a specific atmosphere[16,17][18]. In view of the similarity between the YS structure and the prepared hollow structure, it is also a common synthesis method to obtain the YS structure by etching part of the core. Different from the hard template method, the soft template uses emulsion, vesicle, micelle, bubble to form micelle system or reverse micelle system, and the YS structure is synthesized by conversion without etching step[19].

2.1 Fabrication of YS structure by SiO2 hard template method

Silica (SiO2) has a high stability, which can play a role in maintaining the morphology and supporting the shell material in the reaction, and can be simply removed by using hydrofluoric acid (HF) and strong alkali such as NaOH, so it is very suitable as a hard template to synthesize YS structure. SiO2 can be easily prepared by St Stöber method, mainly by hydrolysis of tetraethyl orthosilicate (TEOS) in water and alcohol solution, and ammonia gas produced by adding ammonia water will affect the appearance of particles.When ammonia is formed, the particles are spherical, the type of alcohol affects the particle size and uniformity, the hydrolysis rate of methanol is the fastest, the particles are the smallest, the particle size of higher alcohols tends to be wider, and the concentration of water, alcohol and ester affects the particle size[20]. The stacking method (Figure 1) is mainly used as a hard template to construct the YS structure by using the SiO2. The thickness of the SiO2 can be well adjusted by changing the amount of the precursor TEOS, and the synthesized SiO2 is negatively charged and attaches to the surface of the SiO2 by electrostatically attracting the positively charged ion layer[21]. In addition, the sol-gel method can be used to prepare SiO2 layers on the surface of inorganic simple substances and metal oxides, and this simple preparation strategy has been widely used[22,23][24⇓~26]. Yu et al. Hydrolyzed TEOS to generate SiO2, used electrostatic force to synthesize FeOx@ cavity @ carbon structure, and changed the amount of TEOS to prepare 22 and 25 nm thick SiO2 layers, respectively, to achieve the controllable adjustment of the "yolk" and "eggshell" gap, thus realizing the volume buffer effect of the battery in the charge-discharge process, so that the battery has a longer cycle performance[27]. The preparation of YS-structure by SiO2 template method is not only suitable for non-metallic simple substance and metal oxide, but also suitable for metal simple substance[8]. Li et al. Synthesized a SiO2 layer on the surface of gold by sol-gel method, and then deposited methane gas as a carbon source on the surface by chemical vapor deposition (CVD) method, and the SiO2 between gold and carbon was etched by HF to obtain the YS structure[28]. This scheme not only provides a preparation route for growing graphene on oxide surface by CVD without metal catalyst, but also invents a new method for preparing carbon-containing layer.
Although SiO2 have simple preparation, stable properties and high abundance in nature, their performance is difficult to play when they are used as battery electrode materials due to the defects of low electronic conductivity. Silicon (Si), which is also an electrode material for lithium-ion batteries, has a theoretical specific capacity of 4200 mAh·g-1, and the conversion between SiO2 and Si is often used to prepare electrode materials with higher performance. Aiming at the problem of volume expansion of Si as anode material for lithium-ion batteries, Cui's research group covered Si nanoparticles with a layer of SiO2, then coated dopamine on the surface of the SiO2, and removed the SiO2 with HF after carbonization to obtain Si @ cavity @ carbon YS structure, which effectively alleviated the volume expansion effect of silicon anode material during charge-discharge process[29]. Dou et al. Used phenolic resin as a carbon source and added surfactant cetyltrimethylammonium bromide as a pore-forming agent to prepare a silicon @ cavity @ carbon structure. The shell carbon has abundant mesoporous distribution, which accelerates the rate of ion transfer, thereby improving the rate performance of lithium-ion batteries[4]. The main component of sand is SiO2, and Si can be obtained from sand by thermal reduction of active metal. Zhou et al. Used phenolic resin as carbon source to convert SiO2 in sand into Si nanoparticles by magnesiothermic reduction or aluminothermic reduction, and synthesized Si @ cavity @ carbon structure by SiO2 hard template method[23]. Sand has a large content in nature, and this route has the function of turning waste into treasure. In addition, the hard template method can easily change the type of "shell" to achieve the diversity of carbon layer structure. Commonly used carbon sources include oleic acid, phenolic resin, glucose, propylene, polydopamine, etc., in which propylene is deposited on the surface of the template by CVD method under the conditions of high temperature, argon and hydrogen as carrier gases[30][22][31][32][21]. Glucose is a good carbon precursor. Under the hydrothermal reaction, glucose polymerizes on the surface of the template, and the pores generated after carbonization are very dense, generally micropores, which can be adjusted by adding surfactant cetyltrimethylammonium bromide (CTAB). The surface of the generated template is provided with a hydroxyl group, and the carboxyl group of the oleic acid is saponified with the hydroxyl group to generate an ester substance, which is converted into a carbon shell in the process of high temperature treatment; Phenolic resin and polydopamine are polymerized on the surface of the template to produce resin and macromolecular substances, which are decomposed into carbon at high temperature, thus forming a carbon "shell".

2.2 Preparation of YS structure by metal oxide hard template method

In addition to SiO2, amphoteric or basic oxides such as alumina (Al2O3) can also serve as hard templates. The SiO2 is unstable in strong alkaline solution, and other oxides stable under this condition can make up for this shortcoming when they are used as hard templates. These oxides can be removed by the commonly used dilute acid solution, which also avoids the environmental pollution caused by the HF used for etching the SiO2. Mo et al. Synthesized a Al2O3 layer on the surface of germanium by atomic deposition, covered the surface of the Al2O3 with polypyrrole, then carbonized, and finally etched off the Al2O3 with acid to obtain the YS structure[33]. A controllable Al2O3 layer is used as a sacrificial template to synthesize a controllable morphology with uniform shape. Li et al. Used SiO2 nanoparticles as a Si source, stirred the particles with aluminum salt and formic acid to generate a Al2O3 layer on the surface of the particles to maintain their shape, and then covered the surface with a layer of carbon by ethylene CVD, reduced the SiO2 to Si by magnesiothermic reduction, and removed the by-product magnesium oxide (MgO) with dilute hydrochloric acid to obtain a Si @ cavity @ carbon YS structure[17]. The benefit of this method is to take advantage of the stability of Al2O3 in magnesiothermic reduction to selectively reduce SiO2 to Si, thus maintaining a homogeneous YS structure.

2.3 Preparation of YS structure by carbon hard template method

The sp2 hybridized carbon has excellent electrical conductivity with gain electron conduction. The precursor of carbon is coated on the surface of the material, and the morphology of the precursor can be maintained during calcination. In addition, the use of carbon as a shell can improve the conductivity of electrode materials, thereby improving the electrochemical performance.
After the carbon source is calcined and converted into a carbon layer, the carbon layer has certain rigidity, can provide certain mechanical strength in the synthesis process of the material, keeps better appearance, has certain acid and alkali resistance, and can be suitable for more various synthesis acid and alkali environments. The inner material of the carbon layer can be etched or pyrolyzed to obtain the YS structure. For example, after the nanospheres are synthesized, the outer layer of the microspheres is wrapped by a template to maintain the shape of the microspheres, and then a good spherical YS structure can be formed by partial etching. Li et al. Wrapped a layer of phenolic resin on the surface of SiO2, calcined and carbonized the phenolic resin into a carbon layer, and then partially etched and reduced the core of SiO2 to obtain the Si @ cavity @ carbon YS structure (Fig. 2)[34]. When Pan et al. Synthesized the Si @ cavity @ carbon structure, dopamine was used to wrap the Si nanospheres and maintain the morphology of the microspheres, and NaOH was used to partially etch the Si to form the YS structure[35]. Liu et al. Used SiO2 to coat the surface of phenolic resin, and the added triblock polymer F127 and surfactant FC-4 were crosslinked in the phenolic resin layer[36]. During the calcination process, the phenolic resin shrinks and is converted into a carbon core when being heated, mesopores are formed in the carbon core due to the introduction of the surfactant, an outer layer SiO2 is used as a hard template to generate a microporous carbon shell layer, and finally a structure of mesoporous carbon @ cavity @ microporous carbon is formed to be used as a positive sulfur carrier of the lithium-sulfur battery. Mesoporous carbon is beneficial to ion migration, microporous carbon restricts the shuttle of polysulfides, and physical confinement improves the cycle performance of batteries. It should be noted that the etchant should not damage the carbon layer during partial etching, otherwise the YS structure cannot be maintained.
The YS structure can also be prepared by pyrolysis of the polymer. Polyethyleneimine is a commonly used organic template agent, which can gel under acidic conditions and react with hydroxyl groups to crosslink and polymerize, and its boiling point is only 250 ℃. Polyethyleneimine can be removed by controlling the calcination temperature, leaving a "cavity" to form a YS structure. Mi et al. First aminated the surface of Si nanoparticles with polyethyleneimine, and coated the hydroxyl-rich sodium alginate on the surface of polyethyleneimine by electrostatic force to form a Si @ polyethylenenimine @ sodium alginate structure[37]. After calcination, polyethyleneimine is thermally decomposed, while sodium alginate is carbonized to form a conductive mesoporous network, and finally a Si @ cavity @ carbon structure is formed. Similarly, Huang et al. Used polystyrene (PS) as the pyrolysis template and polyaniline (PAni) as the carbon source to synthesize the Si @ cavity @ carbon structure (Fig. 3). Instead of using HF etching, they used PS as the carbon source to generate the inner carbon shell and make holes at the same time, and PAni as the carbon resource to generate the outer carbon shell[18]. As the pyrolysis properties of PS and PAni are significantly different during calcination, PS is first converted to carbon in one step, leaving voids and a thin layer of carbon as the inner shell, while PAni is then carbonized to form the outer carbon shell.

2.4 The soft template is used to synthesize the YS structure.

Although the hard template method has good controllability for the synthesis of the structure, size and composition of YS-type nanomaterials, the steps are complicated, and the reagents used to remove the template are not environmentally friendly. Compared with the hard template method, the soft template method does not require the use of environmentally unfriendly chemical reagents, and generally uses emulsions, vesicles, micelles, bubbles, etc. As templates to regulate the interfacial properties by forming micellar systems or reverse micellar systems. Emulsion, micelle, surfactant, etc. contain amphoteric groups, and hydrophilic groups and lipophilic groups are connected end to end, which can be used to insert other materials between hydrophilic or lipophilic groups to synthesize layered structures of different substances, and then synthesize YS structures on this basis.
Metal alkoxides are usually highly reactive to water molecules in aqueous solution and can be rapidly hydrolyzed into different forms of hydroxides and oxides. Li et al. Hydrolyzed vanadium triisopropoxide by the solvothermal reaction of isopropanol and water to produce vanadium pentoxide (V2O5) of about 50 nm. Due to the relative hydrophobicity of isopropanol, the surface of V2O5 was coated with a hydrophobic layer and aggregated into "yolk" in water, so it sandwiched a water layer with the secondary hydrolysis product of vanadium triisopropoxide to form a YS structure[38]. The method does not need a surfactant or a hard template to control the uniformity of the morphology, and does not need to add an etchant, thereby improving the synthesis efficiency. Lee et al. Dispersed iron oxide and ammonium polyacrylate in ethanol and acetonitrile solution, acrylic acid coated iron oxide particles, titanium dioxide was coated on acrylate by hydrolysis of n-butyl titanate, and ammonium acrylate was dissolved in water and removed to obtain YS structure[39]. This scheme is simple and convenient, but the morphology of the synthesized YS structure is not uniform.
Surfactants have both hydrophilic and hydrophobic groups, and can be used to modify metal and non-metal materials to control the morphology. Yamaki et al. Used polyvinylpyrrolidone (PVP) to modify Si nanoparticles, and added pyrrole as a carbon source and "bridge" to connect PVP and ferric chloride[40]. During the calcination process, PVP was heated and decomposed, and the pyrrole was converted into a carbon layer to prepare the YS structure. Xia et al. Used zinc acetate and manganese acetate dissolved in PVP-ethylene glycol solution as precursors, and then the solution was heated and refluxed to react to produce zinc manganese glycolate precursor, and then the obtained precursor was calcined in a tube furnace to form YS-type zinc manganate (ZnMnO3) microspheres[41]. Hong et al. added PVP to the mixed solution of zinc (Zn) salt and tin (Sn) salt to make the precursor mixture microspherical. During the heating treatment, Zn and Sn were subjected to ion exchange, Sn migrated from the outside to the inside, and Zn volatilized due to its low boiling point (907 ℃), finally forming a YS structure of Sn @ cavity @ carbon (Fig. 4)[42].
Generally speaking, compared with the hard template method, the soft template method does not need corrosive reagents such as strong acid and strong alkali to etch, and is more suitable for environmentally friendly mass production.However, it needs to add a proper amount of surfactant to form uniform micelles, and the choice of surfactant is not universal for different material preparation systems, so it also has some limitations.
The synthesis of YS structure by template method can clearly reflect the synthetic material thinking of researchers. By changing the surface groups, the "yolk", "eggshell" and "cavity" can be regulated to achieve the effect of layer-by-layer connection, which is easy to understand. However, template methods often involve rational design, synthesis and removal of templates (removal is mainly for hard template methods), and mass production faces certain challenges, so more convenient synthesis methods need to be developed.

3 Self-templating method

The self-templating method directly uses the thermodynamic or kinetic property differences of precursors in some simple physicochemical processes to directly synthesize YS structures without additional templates. This method is widely used in practice, and it is suitable for mass production because it can significantly reduce production consumption. Many self-templating methods have been developed based on different principles, including the Ostwald ripening method, the Galvanic replacement method, the Kirkendall effect, etc.

3.1 Preparation of YS structure by Ostwald ripening

Ostwald ripening is a phenomenon in which the multiphase structure in a solid solution changes with time[44]. In nanosynthesis, small crystals or sol particles are dissolved and then redeposited on the surface of larger crystals or sol particles. The crystals in the middle of nanoparticles are small in size and low in concentration, and void structures are easily formed on their surfaces. The method is simple in synthesis and easy to scale up. Jiang et al. Used vanadium ions to coordinate with ethylene glycol to form vanadium glycolate microspheres, polymerized nanoparticles by hydrogen bonding and Van der Waals force interaction, and prolonged the reaction time to increase the particles coated on the microspheres.Larger size microspheres, as well as vanadium oxide, dissolve to form a shell in a specific region below the surface layer, while the internal core remains basically intact, eventually forming a YS structure of a compact solid microsphere core (Fig. 5)[43].
It is also one of the commonly used methods to synthesize nanoparticle spheres by solvothermal method and then prepare YS structure by calcination[45]. In the heating process of solvothermal and calcination, the difference of internal and external temperatures of the material during heating leads to the difference of kinetic rates of internal and external crystallization processes: the temperature of the outermost layer is high, which is easy to crystallize; The temperature of the inner layer is low, the heat distribution is uneven, and the YS structure is formed by contraction. Qiu et al. Used hydrothermal reaction to obtain cobalt disulfide nanoparticles in the initial reaction stage (5 H), and the particles tended to aggregate to form smooth spheres driven by the spontaneous decrease of surface energy[46]. When the reaction time reaches 10 H, the solid phase spheres begin to disperse and a shell is formed on the surface of the solid phase core, and the core material is partially dissolved by the high surface energy. As dissolution intensifies, larger and larger cavities are created between the core and the shell. When the reaction time was extended to 16 H, the perinuclear space was preferentially dissolved to form the YS structure. Similar to the phenomenon of uneven temperature distribution from the inside to the outside of the solvothermal precursor material, the precursor material can also be synthesized by changing the calcination temperature rate to form a temperature gradient during the calcination process. Liu's group changed the calcination temperature rate of the precursor cobalt salt in air to synthesize the multi-shell YS structure of cobaltosic oxide (Co3O4)[47].
Crystallized materials are more conducive to the intercalation and deintercalation of lithium ions due to their structural stability, and the crystallization of metal precursors through the calcination process can improve the electrochemical performance of batteries. In addition to changing the calcination rate, the calcination atmosphere and procedure also affect the morphology of the final electrode material. Zhang et al. Used glucose as carbon source to synthesize carbon sphere template by hydrothermal method, adsorbed manganese, cobalt and iron ions, crystallized in nitrogen atmosphere, and then calcined in air atmosphere to form multi-shell YS structure[48]. If the crystallization is carried out directly and simultaneously in air without the preceding nitrogen atmosphere crystallization step, the single shell structure will be formed directly. Therefore, it is very important to control the calcination step, select the appropriate atmosphere, temperature and heating time to control the structure and morphology by this method.

3.2 Preparation of YS structure by electrochemical replacement

Electrochemical replacement is an effective self-templating method for the fabrication of hollow nanostructures[50]. It can controllably adjust the size, shape and element composition of the structure, especially for metal elements. The preparation mechanism is as follows: two metals with different electrochemical potential differences are selected, one is used as a reducing agent (anode), the other is used as an oxidizing agent (cathode), the anode metal first forms a nanostructure, and then is oxidized and dissolved by a metal ion with a high reduction potential, and the metal ion is reduced and electroplated to the outer surface of the anode template. The anode metal is usually used as a template to control the shape and pore size so as to achieve the effect of morphology control. When Guo et al. Synthesized nickel oxide (NiO) microspheres, they first used nickel (Ni) salt and trimethylbenzene to prepare a Ni organic skeleton sphere under microwave irradiation, then introduced copper (Cu) salt, and converted it into a Cu/Ni mixed organic skeleton sphere by controlling the cation exchange between Cu and Ni[51]. Cu ions replace part of Ni ions on the outer surface and inner core of the microspheres, and the subsequent thermal decomposition of the organic ligands in air forms a CuO @ cavity @ NiO structure. There is an element gradient in the YS structure (high concentration of Cu on the outer surface and high concentration of Ni on the inner surface), and lithium intercalation into the two metal oxides in sequence can exert higher capacity than CuO or NiO alone. Similarly, Dai et al. Synthesized Ni organic framework structure with Ni salt and trimesic acid, introduced Sn salt to cause ion exchange, and prepared Ni-Sn-P @ carbon nanostructure through cation exchange reaction, CVD and phosphating process in turn. The microspheres with dual electrochemical active components can accelerate the transfer of electrons and ions, relieve mechanical stress, and improve the stability of the electrode during repeated cycles[14]. Liu et al. First synthesized tin dioxide (SnO2) hollow structure by hydrothermal method, and then covered its surface with a layer of carbon precursor for confinement and protection to form Sn @ cavity @ carbon YS structure by reduction in hydrogen atmosphere, and further obtained Sb @ cavity @ carbon by electrochemical replacement reaction between Sn and antimony (Sb) (Fig. 6)[49]. Encapsulating Sb in carbon nanocages, the YS internal void allows sufficient expansion of Sb nanoparticles, thereby maintaining structural integrity and a stable interfacial film with good lithium-ion and sodium-ion storage properties.
Electrochemical replacement often uses metals and their oxides to prepare YS structures. In the process, the formation of the shell and core is regulated by controlling the kinetics of the replacement reaction, which is difficult to operate and requires precise control of the reaction conditions to improve the repetition rate. For the YS structure preparation of bimetals, it is difficult to flexibly change the core-shell composition of the YS structure, and there is a huge challenge to selectively change the composition of the shell and core.

3.3 Preparation of yolk and eggshell structure by Kekendall effect

Kekendall effect is a classical phenomenon in metallurgy, which describes the movement of metal boundary layer caused by the difference of diffusion rate of different metal atoms, and the movement of boundary layer can be used to effectively prepare YS structure[53]. Cho et al. First mixed Sn salt and PVP to prepare microspheres, which were calcined at 180 ℃ in air for 1 H to obtain Sn@SnO2 hollow structure, and then adopted different calcination temperatures to present three different structures: firstly, the outer Sn layer of microspheres was oxidized to SnO by reaction with oxygen, and the SnO was oxidized to SnO2,Sn, SnO, and SnO2 with different densities during calcination, resulting in a gap between the innermost Sn layer and the SnO2/SnO to form a Sn@Void@SnO/SnO2YS structure (Fig. 7)[52]. Xie et al. Mixed Zn salt and Cu salt precursors, changed the concentration ratio of Cu salt, and thermally decomposed them in argon atmosphere. When the concentration of Cu salt was low, the structure synthesized was YS structure. On the contrary, when the concentration of Cu was high, the morphology was ZnO hollow structure with Cu nanorods attached to the surface[54]. Similarly, Yin et al. And Li et al. Also obtained spinel Co3O4 and manganese cobaltate YS structure by adjusting the calcination temperature rate[55][56]. The number of shell layers can be effectively controlled by annealing the Co complex precursor at a controlled heating rate. The results show that the YS structure can be formed at a high heating rate (10 ℃/min). Low heating rate (5 ℃/min) can form few-shell structure. When the heating rate is further reduced to 1 ℃/min, the hollow structure of multi-shell Co3O4 can be obtained. The Kekendall effect is suitable for the preparation of YS structures composed of metals and their oxides, similar to other self-templating methods, which has the disadvantage of not being able to precisely control the composition of the shell and core.
Flexible use of the self-template method can draw inferences about other cases from one instance. Lu et al. Used Ostwald ripening to synthesize the core-shell structure of Co3O4@Co3O4, which was used as a precursor to soak in a Cu ion solution, and then partially exchanged Co with Cu by electrochemical replacement, and finally calcined to obtain the Co3O4YS structure containing Cu[57]. To sum up, the self-templating method includes two steps: one is the synthesis of nanomaterial template, and the other is the transformation of template to hollow structure. During the template synthesis of nanomaterials, the kinetic temperature, time and reagent concentration are precisely controlled, so as to effectively adjust the kinetic rate of the conversion reaction or crystallization process, thereby changing the molecular distribution and crystallization process to prepare the YS morphology[58]. The self-template synthesis process is relatively simple, high in reproducibility, low in production cost, and strong in adjustability of shell thickness and particle uniformity. However, the disadvantage is that the self-template method is usually used to construct the overall composition of the internal hollow structure and the external shell at the same time, which can not accurately adjust the shell and the inner core separately, that is, the thickness of the "cavity" can not be accurately controlled, and the composition of the "yolk" and "eggshell" can not be separately regulated.
The yolk-eggshell structure has the following advantages: the designable "yolk" active center can be used as the catalytic active center; The unique "cavity" structure and the controllable core-shell spacing provide a container for the active center; The outermost layer with controllable pore size protects the active center, so that the YS individual can remain independent and fully contacted. The comparison of the YS structure to the hollow structure is mainly due to its controllable "cavity" size. (1) from the angle of a template method, the SiO2, the Al2O3 and the like which are used as a sacrificial template lean against the shell from the core inwards to the shell outwards, react under the action of chemical reagents such as HF/NaOH, H2SO4 and the like to etch off the template, and the size of a synthetic cavity is regulated and controlled by regulating the thickness of the synthetic sacrificial template[59]; (2) regulate that size of the cavity by partial etching, wherein the partial etch is applicable when the core is in contact with the shell, and regulating the etching time and the concentration of the etchant to etch the core part to form the cavity, thereby regulating the size of the synthetic cavity[60,61]; (3) For the soft template and self-template methods, on the one hand, they can be regulated by adding surfactants such as PVP and CTAB, which will gather on the surface of the precursor in the solvent to wrap the surface tightly, and the larger the amount of surfactants added, the larger the radius of the wrapping, thus regulating the size of the "cavity"; (4) The size of the "cavity" is controlled by kinetics, and the transformation from the YS structure to the hollow structure can be controlled by temperature rate, temperature gradient, calcination atmosphere, calcination time, etc[62][63][64][65].
The theoretical specific capacity >4000 mAh·g-1 of silicon has a good application prospect in lithium-ion batteries, but the traditional silicon has the disadvantages of nearly four times volume change in the charge-discharge process and poor conductivity, and the YS structure can improve the shortcomings of the traditional silicon as an electrode material.The cavity of the YS structure plays a role in buffering the volume expansion effect of the Si electrode during charging and discharging, and the carbon layer shell plays a role in enhancing the conductivity to make up for the poor conductivity of Si, so the YS structure has great application potential in the Si electrode. For the application of YS structure in the field of lithium metal anode, Lou et al. Used peanut-like iron oxide as a sacrificial template, polymerized p-phenylenediamine (PDA) on the outer surface of the iron oxide, carbonized the iron oxide, etched it with hydrochloric acid, grew a layer of MoS2 on the remaining peanut-like hollow carbon shell through hydrothermal reaction, and finally melted sulfur into the peanut core to form a C@MoS2/S by hot melting method[66]. The C@MoS2 nanostructure in the material can be used as a good support for repeatedly electroplating lithium metal and stripping lithium metal, and in the process of electroplating lithium metal and stripping lithium metal,Sulfur can be gradually released to form lithium polysulfide, and as an effective additive, sulfur can promote the formation of solid electrolyte interface layer doped with lithium-based crystals, which together inhibit the growth of lithium crystals. The YS structure adjusts the "core" and "shell" components, which can not only inhibit the growth of lithium dendrites, but also accelerate the migration of lithium ions, so that the lithium metal anode has a good cycle performance, so the YS structure has great application potential in the field of lithium metal anode.

4 Application of YS-structure in secondary battery

The charge/discharge mechanism of lithium/sodium ion batteries originates from the intercalation and deintercalation of lithium/sodium ions in the positive and negative electrodes. There is a difference in the density of the electrode material in the de/intercalation state during the charge/discharge process, which leads to the volume effect. For example, the volume expansion effect of Si anode can reach 300% before and after charging and discharging of lithium-ion battery[29]. When the YS structure is used as an electrode material of a lithium/sodium ion battery, the shells of the materials are in mutual contact, so that the shells have good conductivity, the electron transmission rate is improved, and the charging and discharging performance of the battery is improved; The "cavity" between the core and the shell can alleviate the volume expansion effect, maintain the stability of the morphology of the electrode material, prevent the material from caking and breaking, and improve the cycle performance of the battery. Carbon is generally used as the shell of the YS structure in the construction of the YS structure[67]; Oxides, sulfides, phosphides, selenides, etc., can serve as the inner core of the YS structure[26][68,69][25][70].
Lithium-sulfur battery is different from lithium/sodium-ion battery. Its cathode active material sulfur will produce polysulfide during charge and discharge, and polysulfide will dissolve in organic electrolyte.It shuttles to the lithium negative electrode side and reacts with metal lithium, resulting in the loss of sulfur on the negative electrode side, self-discharge and efficiency decline of the battery, which is the shuttle effect of lithium-sulfur batteries[71]. When the YS structure is used as a positive sulfur carrier material for lithium-sulfur batteries, the shell not only has good conductivity, but also can inhibit the shuttle of polysulfides through physical confinement. At the same time, the rationally designed "yolk" can catalyze the conversion of polysulfides and combine with the "cavity" to store sulfur to form an efficient nanoreactor. The double synergistic effect of "yolk" and "eggshell" can improve the electrochemical performance of lithium-sulfur batteries.

4.1 Application of YS-structure in lithium-ion battery

In the past 30 years, despite the great success of new electrode materials used in lithium-ion batteries, there are still many difficulties to be overcome to achieve more advanced lithium-ion batteries with the increasing requirements for fast charge-discharge ability, reliable safety and long cycle life of batteries. The YS structure has high specific surface area and abundant buffer space, which can effectively alleviate the volume effect of charge-discharge electrode materials, accelerate the electron/ion transfer at the interface, and improve the cycle and rate performance.
Carbon shell plays an important role in YS nanostructured electrode materials. Li et al. First synthesized a uniform precursor nickel disulfide (NiS2) cube by a solvothermal method, and then coated a layer of ultrafine SiO2 nano-coating on the surface of the NiS2 cube as a sacrificial template to create a buffer space, and then coated the phenol-formaldehyde resin on the surface of the SiO2 by polycondensation reaction.The NiSx@SiO2@ carbon was obtained after carbonization at 600 ℃ in nitrogen, and the uniform NiSx@ cavity @ carbon YS structure was successfully prepared after etching SiO2 with HF, which was used as the cathode material for lithium-ion batteries to maintain a capacity of 460 mAh·g-1 after 2000 cycles at a current density of 1 A·g-1[72]. Yang et al. Synthesized ferric oxide (Fe2O3) nanocubes by hydrothermal method, coated a layer of dopamine hydrochloride on the surface of the cube, calcined at 550 ° C in argon atmosphere, followed by etching away part of the Fe2O3 with hydrochloric acid to leave some gaps, and finally phosphating the material[25]. The carbon in the outer layer keeps the structure of YS well from being destroyed by phosphating, and promotes the electron transport during the charge-discharge process. As a cathode material for lithium-ion batteries, it maintains a capacity of 450 mAh·g-1 after 400 cycles at a current density of 500 mA·g-1. With the increase of research depth, it is found that the simple carbon layer can not provide enough rigidity to cope with the volume expansion of the core (such as Si), so the growth of carbon nanotubes (CNTs) in the "cavity", on the one hand, further improves the conductivity of the "core" and the "shell", on the other hand, buffers the volume expansion force of the core and leads to the fragmentation of the YS structure. Zhao et al. Synthesized a layer of calcium carbonate (CaCO3) on the surface of elemental Si by co-precipitation method as a sacrificial template to make the "cavity" of YS, and synthesized a layer of SiO2 on the outer layer of the template before etching the template with hydrochloric acid to prevent the collapse of the morphology of the material after template etching and high temperature treatment[73]. Fe was used as a catalyst for the growth of carbon nanotubes (CNTs) under the treatment of CVD Ar/H2 mixture, and the SiO2 was also reduced to Si at a high temperature of 1000 ℃, and finally the Si @ Void @ C structure with "cavity" filled with CNTs connecting core Si and outer C was synthesized. As a full cell (LiFePO4 as cathode, Si @ Void @ C as anode), the material reached a 150 mAh·g-1 at a current density of 50 mA·g-1, and retained 89. 7% of its capacity after 300 cycles in the voltage window of 2. 0-4. 0 V.
The size of the "cavity" is an important parameter to alleviate the volume effect of the YS structure, and adjusting the calcination temperature is one of the most commonly used methods to control the "cavity". For example, the theoretical gram capacity of manganese oxide (MnO) is about 756 mAh·g-1, which is higher than that of commercial lithium-ion battery electrode materials, but its volume change during charge and discharge is about 130%, and the conductivity of the oxide is not good, so its application in batteries is limited. In view of this, Chen et al. Polymerized a layer of phenolic resin film on the surface of the precursor manganese carbonate (MnCO3), and during the calcination process, with the protection of the resin film, the surface MnCO3 could maintain its morphology when it was converted into MnO shell, and the inner MnCO3 contracted into MnO core when it was converted, which together formed a YS structure (Fig. 8)[74]. This work also explored the influence of different temperatures on the thickness of the "cavity" in the range of 450 ~ 750 ℃, and found that the higher the temperature, the smaller the wall thickness. The morphology and charge-discharge performance of the materials were investigated. Compared with other morphologies, the materials with YS structure had outstanding performance at high rate, and the discharge capacity remained at 888 mAh·g-1 after 400 cycles at a current density of 2. 64 C. Wu et al. Hydrothermally synthesized SnO2@ carbon by using mesoporous hollow carbon spheres as templates. In hydrogen and argon atmosphere, SnO2 was partially reduced to polynuclear YS structure at 500 ℃, and SnO2 was completely reduced to Sn @ cavity @ carbon YS structure at 600 ℃[67]. Compared with the pure Sn electrode, the structure as the negative electrode of the lithium ion battery has a discharge capacity of 500 cycles 620 mAh·g-1 under 768 mAh·g-1,0.5 C at 0. 2 C for 100 cycles, which shows good cycle stability. In addition to controlling the calcination temperature, the "cavity" can also be controlled by adjusting the amount of the template. Yu et al. Synthesized SiO2 layers with different thicknesses of 22 and 25 nm, respectively, by changing the amount of TEOS, thus adjusting the cavity size between the "yolk" and the "eggshell" in the FeOx@ cavity @ carbon structure, giving full play to the role of volume expansion of its buffer material, and thus enabling it to have excellent cycle performance[27]. Similarly, the "cavity" structure can also be regulated by controlling the reaction atmosphere. During the synthesis of LiMn1.5Ni0.5O4, Park et al. Used capillary attraction to guide the metal precursor ethanol solution to penetrate into the micropores and mesopores of carbonaceous microspheres through a repeated "drip-drying" process[75]. Next, the temperature can be gradually raised in the atmosphere, and the carbon can be slowly converted into CO2 gas, so that the phenomenon of core contraction caused by sudden combustion can be avoided, and the microspheres with compact structure and high crystallinity can be formed; and the YS microspheres can also be generated under high oxygen flow by directly and rapidly raising the temperature to trigger the combustion of the carbon template to cause the contraction of the core part. The latter can maintain a capacity of 113 mAh·g-1 after 1200 cycles at a high rate current density of 10 C.
YS-structured materials are selected as electrode materials for lithium-ion batteries. With the intercalation of lithium ions into the materials to exert lithium storage capacity, these materials are designed as "yolk" cores, which are the core of lithium storage performance. The "cavity" of the YS structure can provide a certain buffer space to alleviate the volume effect generated during the cycling process of the lithium-ion battery, and at the same time, the size and structure of the "cavity" can be adjusted to cope with different volume expansion effects and balance the long cycle performance and unit volume capacity of the electrode material; The conductivity of electrode materials for lithium-ion batteries is also very important. The YS structure usually uses carbon as the shell and has a certain degree of rigidity, which not only keeps the morphology of lithium-ion batteries during cycling, but also improves the electron/lithium-ion transport speed and gives full play to the electrochemical performance of materials.

4.2 Application of YS-structure in sodium-ion battery

Although lithium-ion batteries have been widely used, they still face the problems of shortage of lithium resources, high cost of cathode material Co and poor rate performance of graphite anode. The charging and discharging principle of the sodium ion battery is similar to that of the lithium ion battery, and the sodium resource is abundant in storage, the cost is low, and the field of the rechargeable battery can be widened. However, the radius of sodium ion (2.0404 Å) is larger than that of lithium ion (1.5252 Å), and the electrode materials for sodium-ion batteries usually need to withstand greater volume change stress. Correspondingly, the YS structure has greater application prospects in sodium-ion batteries by playing its role in alleviating the volume effect[76].
For example, the theoretical specific capacity of iron disulfide (FeS2) used in the positive electrode of sodium-ion battery reaches 894 mAh·g-1. However, the structure is easily damaged due to the large volume expansion after sodiation, resulting in rapid capacity decay, which greatly hinders its practical application. Researchers can solve this problem by adjusting the morphology on the nanoscale and using the YS structure. Liu et al. Hydrothermally synthesized a Fe2O3 with a cubic morphology and coated it with a layer of polydopamine.After calcination, the iron oxide was partially etched by hydrochloric acid to form a YS structure, and finally the iron oxide was sulfurized by sulfur powder to obtain FeS2@ cavity @ carbon, which retained a stable capacity of 330 mAh·g-1 after 800 cycles at a current density of 2 A·g-1[77].
Man et al. Used the Fe-based organic framework as a precursor, coated it with phenolic resin, and used the shrinkage of the Fe-based organic framework during carbonization to form YS, which was then sulfurized by thioacetamide. The developed FeS2@ cavity @ carbon material showed a high specific capacity of 616 mAh·g-1 after 100 cycles at a current density of 0.1 A·g-1, and at the same time, it had excellent rate performance[78]. As a typical two-dimensional sodium storage material, tin diselenide (SnSe2) has a high theoretical capacity (756 mAh·g-1) and a hexagonal layered structure. However, its sodium storage mechanism is complex, including intercalation, conversion and alloying/dealloying processes, and its volume expansion is about 420%, which makes it a promising and challenging anode material for sodium-ion batteries.
Similar to the application in lithium-ion batteries, the YS structure with carbon shell can also effectively improve the electronic conductivity and shorten the ion transfer distance, thereby improving the rate performance. Zhang et al. Used nitric acid to change the surface groups of carbon spheres to adsorb iron phosphate (FePO4), and during the carbonization process, the nanospheres were gradually converted to YS structure[79]. The conductivity of FePO4 is poor, and the conductivity can be increased by using carbon as a template. The YS structure is beneficial to alleviate the volume stress of sodium ion intercalation and deintercalation. After 1000 cycles at 100 mA·g-1 current density, the specific capacity of FePO4 is 97.1 mAh·g-1, and the specific capacity retention rate is 91.3%. Compared with FePO4 two-dimensional nanosheets (maintained at 114 mAh·g-1 after 100 cycles at 20 mA·g-1 current density) and FePO4The tubular structure (18 mA·g-1) was cycled for 70 cycles at a current density to maintain the best charge-discharge current and cycle performance at the 157 mAh·g-1),YS structure.
As mentioned above, as a 2D material, SnSe2 has a high theoretical capacity and a hexagonal layered structure, but it has a complex Na storage mechanism. Xiao et al. Used SiO2 as a hard template to polymerize and add a layer of SnO2. Glucose was added to improve the conductivity of the shell. In the process of etching the SiO2 with hydrofluoric acid and selenization, the inner material was heated and contracted.The SnSe2@Se-C YS material is obtained, and the Se — C bond is regulated and controlled by changing the Se doping amount, so that the rapid and efficient sodium ion storage is allowed, and the capacity of 406 mAh·g-1 is maintained after 2000 cycles at a large current density of 5 A·g-1[80]. Sun et al. Constructed homogeneous monodisperse YS NiS2/CuS hollow microspheres in situ by sulfidation and ion exchange of Ni Zn-MOF, which have a hairy surface and a particle size of 1 – 2 mm, benefiting from the huge internal vacancy between the outer part of the core and the inner part of the shell, with a strong cooperative coupling effect and abundant Na insertion active sites in the heterointerface.The NiS2/CuS electrode retained a capacity of 410.9 mAh·g-1 after 750 cycles at a current density of 2 A·g-1, and a remarkable rate capability,Over 4200 cycles at a current density of 20 A·g-1 also had a capacity of 283.4 mAh·g-1, thus demonstrating the high energy storage potential of the NiS2/CuS electrode in sodium-ion batteries[81].
The simultaneous regulation of "cavity" and "eggshell" can realize the optimization of electrode materials. Yang et al. adjusted the "cavity" of the bismuth (Bi) @ void @ carbon YS structure by adjusting the amount of TEOS, and adjusted the "eggshell" by changing the amount of phenolic resin, and prepared "cavities" with void volume of 1 ∶ 10 ∶ 20 and diameters of 200, 310, and 380 nm, respectively, to optimize the void structure[82]. The optimized voids can not only buffer the volume change of Bi nanospheres, but also maintain the high volumetric energy density of the cell. At an ultra-high current density of 100 A·g-1 (Fig. 9), Bi @ void @ carbon has an excellent capacity 173 mAh·g-1 and a long cycle life (maintaining a capacity of 198 mAh·g-1 at 10 000 cycles at a current of 20 A·g-1).
The mechanism of sodium-ion batteries is similar to that of lithium-ion batteries, and the engineering technology of lithium-ion batteries can also be used in sodium-ion batteries, which makes the field of sodium ion batteries progress rapidly. However, because the graphite anode of commercial lithium-ion batteries can not achieve the intercalation and deintercalation of sodium ions well, there are many ways to develop effective sodium ion anodes, including alloying anodes represented by Bi, Sn and Sb. Because these alloyed negative electrodes are usually affected by serious volume effect, the YS structure, with its unique "cavity" structure, can not only shorten the transport distance between ions, but also buffer the volume effect of charge and discharge, resulting in excellent cycle performance and rate performance of sodium-ion batteries.

4.3 Application of YS-structure in lithium-sulfur battery

The action principle of lithium-sulfur battery is completely different from that of lithium-ion battery. Its active substance is elemental sulfur (S), which stores energy through electrochemical reaction with metal lithium. The electronic and ionic conductivities of S and the discharge product lithium sulfide (Li2S) are relatively poor, and the electronic insulation between the two substances limits the solid-solid conversion of S and Li2S. The soluble intermediate lithium polysulfide becomes the key, and its dissolution in the ether electrolyte makes it easier to transfer electrons with the current collector than the insulating solid, thus making the electrochemical reaction between S and Li2S possible[83]. As the S carrier of lithium-sulfur battery, the YS structure can meet four requirements: (1) because the conductivity of S/Li2S is very low, carbon is needed to enhance the conductivity, and carbon or carbonitride is generally selected as the shell to increase the conductivity of the material; (2) Because soluble lithium polysulfide can induce "shuttle effect", resulting in capacity loss, the shell can also be made of materials with strong adsorption capacity for lithium polysulfide; And (4) the existence of the cavity can also relieve about 80% of volume expansion in the process of fully lithiating S into Li2S,The electrode carrier is not broken, the integrity is maintained, and the cavity also has a certain space limiting effect, so that the cycle stability of the lithium-sulfur battery is improved.
The size of the "cavity" in the structure can be controlled by adjusting the etching degree by using the SiO2 as a template. Wang et al. Successfully designed YS-structured SiO2@ void @ carbon nanomaterials as the host of S[84]. The porous shell not only improves the conductivity of the S cathode, but also effectively restricts the shuttle of lithium polysulfides through physical confinement. The SiO2 nucleus may further limit polysulfide leaching by chemical binding; In addition, the huge internal space allows higher S loading and also provides enough space to accommodate the volume expansion, so the sulfur loading is about 1.5 mg·cm-2, and the initial specific capacity of the material is 1200 mAh·g-1 at 0.2 C and 728 mAh·g-1 at 3 C. Liu et al. Used SiO2 to coat the surface of phenolic resin, added triblock polymer F127 and surfactant FC-4 to crosslink the phenolic resin layer, and calcined to remove the SiO2 to form a mesoporous carbon @ void @ microporous carbon structure[36]. When the YS-structured carbon material is applied to lithium-sulfur batteries, the micropores in the shell layer confine S in the gap, while the mesopores in the core layer increase the loading of S and alleviate the volume effect during charge and discharge. When the sulfur content is as high as 76 wt%, the initial capacity at 0.5 C reaches 1106 mAh·g-1, and after 1000 cycles, it still maintains good performance with only 0.05% attenuation per cycle.
The structure of YS can be effectively regulated by partial conversion reduction. Using tin oxalate, PVP and sucrose as precursors, Hong et al. Prepared SnO2/ carbon @ carbon core-shell microspheres by one-step spray pyrolysis (SP) method. The carbon in the shell and core was formed by the carbonization of PVP and sucrose, respectively. Further, the "cavity" was created by removing the SnO2 with Se in a reducing atmosphere, and a multi-yolk structure was obtained[7]. At a current density of 0.1 A·g-1, the first cycle discharge capacity reaches 1368 mAh·g-1; The discharge capacity of the YS after 150th cycle was 602 mAh·g-1 at a current density of 0.5 A·g-1. Compared with the homologous system, the YS structure shows better rate capability than the hollow structure (Fig. 10). Similarly, the reduction method was used to remove the template, and Park et al. Used SP technology to synthesize multi-shell iron oxide as a sacrificial template, and added pitch, which could not only be used as the precursor of mesoporous carbon, but also as a support to maintain the morphology of the material during the reduction of iron oxide to iron. Finally, the reduced iron was removed by dilute hydrochloric acid to leave the pore size, and the sulfur loading was about 1.4 mg·cm-2[85]. After 1000 cycles at a current density of 2.0 A·g-1, the capacity was still 1018 mAh·g-1. After that, his research group used a similar method to prepare multi-shelled YS-structured Co3O4-MgO microspheres with uniform composition. After post-treatment of Co3O4-MgO microspheres with dicyanic acid under Ar atmosphere, Co3O4 was reduced, in which the thermally stable MgO matrix of the shell limited the size of the reduced metallic Co nanocatalyst. After increasing the reaction temperature to 800 ° C, carbon nanotubes were grown using metallic Co nanocrystals as catalyst[86]. With a sulfur loading of about 1.84 mg·cm-2 and a current density of 1 C, it has a high discharge capacity of 700 mAh·g-1 after 400 cycles.
In addition to the regulation of shell carbon structure, the functional design of "yolk" can also realize the efficient utilization of YS structure in lithium-sulfur battery cathode. For example, molybdenum disulfide (MoS2) is a three-atom layer accumulated by weak van der Waals forces and has abundant active sites, which can adsorb and accelerate the conversion of lithium polysulfides, suppress the shuttle effect, and achieve more stable long-period cycling. However, as a carrier material, MoS2 is limited by charge transfer resistance and poor conductivity, resulting in insufficient utilization of active materials. Therefore, combining MoS2 with other materials to form heterojunction is a good way to overcome the shortcomings of MoS2 by changing the energy gap of materials and then changing the conductivity of MoS2 and the adsorption and conversion ability of polysulfides. Jin et al. Used polyacrylic acid to synthesize Mo-containing precursor microspheres, added Ni element by hydrolysis, and formed MoS2/Ni3S2YS heterostructure due to Kekendall effect in the process of sulfurization, which has abundant active sites and can effectively adsorb polysulfides and promote the conversion of polysulfides[87]. As a cathode support for lithium-sulfur batteries, it has excellent electrochemical performance, with a sulfur loading of about 1.2 mg·cm-2, an initial discharge capacity of 1445 mAh·g-1 at 0.1 C, and a capacity of 739 mAh·g-1 after 1000 cycles at 1 C. Under the condition of high load sulfur 5.2 mg·cm-2, the discharge specific capacity can reach the 760 mAh·g-1 after 100 cycles at 0. 5 C.
The template method or self-template method can be used to easily prepare the YS structure and change the composition of the "yolk", for example, oxides can be prepared first, and then treated to easily prepare carbides, sulfides, nitrides and phosphides. Liu et al. Used a lamination method to polymerize a layer of dopamine on the outside of iron oxide, etched the iron oxide into a polynuclear YS structure with hydrochloric acid, and reduced it with potassium permanganate to generate a layer of manganese oxide on the surface of dopamine for protection to enhance the adsorption of polysulfides, catalyze the conversion of polysulfides, improve the utilization rate of sulfur, and reduce polarization[88]. Due to the strong chemical bonding energy and effective catalytic activity of the polar Fe2N core for polysulfides, Sun et al. Used a similar method to prepare the Fe2N@ interstitial @ carbon YS structure by ammonia treatment of iron oxide nitride[89]. The S/Fe2N@Void@C electrode has a high specific capacity, superior rate capability, and long-term cycling stability, with a sulfur loading of about 4.2 mg·cm-2, an initial discharge capacity of 1029 mAh·g-1 at 0.5 C, and an average capacity fading rate of only 0.17% after 100 cycles. Shao et al. First synthesized TiO2 carbon nanofiber by electrospinning technology, then synthesized a layer of SiO2 on the outer layer of the fiber as a hard template to protect the TiO2 from chemical reaction and the morphology of the nanofiber from collapse in the subsequent treatment, and then prepared the TiO2 and RF layer on the outer surface of the SiO2 by sol-gel method.In the process of calcination under ammonia atmosphere, the outer layer of TiO2 was converted into TiN, and RF was converted into carbon, and finally the TiO2-CNFs@Void@TiN@C material was prepared. Under the condition of sulfur loading of 1.5~2 mg·cm-2 and current density of 1 C, the 675 mAh·g-1 was maintained after 1000 cycles, and the average attenuation per cycle was 0. 054%[90]. Zhao et al. Synthesized Zn/C materials by hydrothermal method, carbonized them at low heating rate (1℃min-1) under argon atmosphere, and finally converted them into ZnO YS materials due to Ostwald ripening during calcination in a muffle furnace[91]. When the sulfur loading is 3 mg·cm-2 and the current density is 0.5 C, the average attenuation per cycle is 0.021% after 500 cycles. The 996.3 mAh·g-1 was maintained after 100 cycles at a current density of 1 C even under high sulfur loading 6 mg·cm-2 conditions.
As a cathode carrier in lithium-sulfur batteries, the "cavity" can be highly loaded with sulfur, so the ion migration rate under the condition of high sulfur loading can be optimized by adjusting the size of the "cavity". The "yolk" can be designed as a catalytic site to fully contact with polysulfides and promote the conversion of polysulfides; The "eggshell" has a certain degree of rigidity, and the shuttle effect of physically confined polysulfides not only maintains the stability of charge-discharge cycling materials, but also promotes electron conduction. Therefore, the YS structure can improve the cycle performance, rate performance and so on of lithium-sulfur batteries to a certain extent.

5 Conclusion and prospect

The YS structure has a mobile core and a void between the core particle and the shell, giving it unique advantages over conventional core-shell nanoparticles. To date, various synthesis methods have been developed and used to prepare YS structures of different sizes, compositions, and geometries. However, there are still major challenges in its synthesis. The hard template method can controllably adjust the composition of the shell and core, so that the shell and core of YS can perform their respective functions and play different roles. However, the use of hard templates involves cumbersome steps, and requires the use of corrosive reagents to etch the template, which will pollute the environment. The soft template method does not require the use of an etchant, but it requires the addition of a surfactant to maintain the stability of micelles, vesicles, etc., which is difficult to achieve uniform control in actual production. The self-templating method can easily synthesize the YS structure and facilitate scale-up production, but it is very difficult to precisely control the shell, core and "cavity".
The YS structure has controllable size, shape, structure and composition, and the inner core and the outer shell can be designed into substances with different functions, and the inner core and the outer shell structure can be regulated to play different roles to meet various complex needs. For the regulation of shell, core and cavity, the template method is the most representative, which can controllably regulate the morphology of YS and provide a new direction for the development of high-performance battery electrode materials and other fields.

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Funding

National Natural Science Foundation of China(21975056)

National Natural Science Foundation of China(U1801257)

National Natural Science Foundation of China(52002079)

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