1 Introduction
With the rapid development of economy and people's increasing concern about environmental problems, the search for clean and renewable energy has become a topic of common concern to researchers around the world
[1⇓~3]. At present, two complementary electrochemical energy storage systems, lithium ion batteries (LIBs) and supercapacitors (SCs), play an important role in the market of electric vehicles and portable electronic devices. Lithium-ion batteries exhibit high energy density but low power density and limited cycle life due to the repeated deintercalation of lithium during charge and discharge. On the other hand, due to the highly reversible adsorption and desorption of ions at the electrode/electrolyte interface, supercapacitors show high power density, excellent cycle stability and fast charge-discharge capacity, but there are still problems such as unsatisfactory energy density
[4⇓~6].
In order to meet the requirements of high energy density and power density at the same time, a new type of special energy storage device, lithium-ion capacitor batteries (LICs), has gradually become the focus of research and attention of researchers. Figure 1 introduces the advantages and challenges of lithium-ion capacitor batteries
[7⇓~9].
Lithium-ion capacitor batteries, also known as lithium-ion capacitors or lithium-ion hybrid supercapacitors, are usually assembled from battery-type materials (negative electrodes) and capacitive materials (positive electrodes) in an electrolyte containing lithium salts. As early as the 1990s, Evans et al. First proposed the concept of lithium-ion capacitor, and then Amatucci et al. Reported the first lithium-ion capacitor battery system at the beginning of the 21st century, which is called "asymmetric hybrid energy storage battery". This system is constructed by using activated carbon (AC) as a capacitive positive electrode and nanostructured Li
4Ti
5O
12(LTO) as a battery-type negative electrode in an organic electrolyte based on LiPF
6[10][11]. The energy density generated by this device is 3 to 4 times that of ordinary supercapacitors. In addition, the device exhibits high power performance and excellent cycle stability, with a capacity utilization rate of 90% at 10 times current density and a capacity retention rate of about 80% of the initial capacity after 4500 charge-discharge cycles
[11]. Xia Yongyao of Fudan University and others built a "rocking chair" power storage device with activated carbon as the negative electrode and lithium intercalation material as the positive electrode, which laid a good foundation for the domestic lithium ion capacitor battery field
[12]. At present, domestic and foreign companies have produced new lithium-ion capacitor batteries with good performance. For example, JM Energy Company of foreign countries has introduced a new type of lithium ion capacitor battery with small unit. Compared with the original flat cuboid unit, the volume of this unit is reduced to less than 1/2, which realizes miniaturization and can be used in construction machinery, industrial machinery, automobiles and other fields with limited space for power supply devices. Maxwell uses a combination system of lithium-ion batteries and supercapacitors as an auxiliary technology to meet various types of grid needs. Aowei Technology Development Co., Ltd. in China has developed lithium-ion capacitor battery components ranging from 3.6 to 720 V, which are used for the starting of various vehicles and internal combustion engines, as well as the traction of light vehicles and electric buses and other fields, and has successfully established the world's first capacitor bus line put into commercial operation
[13].
In recent years, the industrialization of lithium-ion capacitor batteries has been gradually promoted at home and abroad. In order to solve the key problem of kinetic imbalance between positive and negative electrode materials, more electrode materials with excellent performance need to be developed. Many research teams have combined the related principles of lithium-ion batteries and supercapacitors with lithium-ion capacitor batteries in an attempt to make breakthroughs in the field of lithium-ion capacitor batteries. In this paper, the research progress of electrode materials for lithium ion capacitor batteries in recent years is reviewed, the advantages and disadvantages of various materials are described, and the development trend of lithium ion capacitor batteries is prospected.
2 Anode material
2.1 Transition metal oxides
Transition metal oxides (M
xO
yM=Fe,Co,Cu,Mo, etc.) Have become the research focus of electrode materials in the field of LICs because of their abundant sources, low cost, stable performance and higher specific capacitance than capacitor electrode materials
[14]. However, this kind of materials will produce large volume changes during long-term cycling, which will lead to low initial coulombic efficiency, poor rate capability and rapid capacity fading. After exploring, the researchers proposed the following solutions, on the one hand, the morphology of the material was nano-treated to form a porous nanostructure, maintain structural integrity and promote electrochemical reactions; Another aspect is the integration of carbonaceous materials into oxides to prepare nanocomposites. Under the protection of the carbonaceous material, the volume expansion phenomenon of the oxide in the cycle process can be buffered, the cycle reversibility is greatly improved, and the conductivity is improved.
As we all know, Fe
2O
3 is widely used in the field of LICs because of its low cost, non-toxicity and high theoretical specific capacitance
[15⇓⇓~18].
However, due to the large size of the primary nanoparticles and the lack of porosity, the lithium storage performance is not ideal, which affects the conductivity and cycle stability. Dong et al. Prepared a composite of fusiform α-Fe
2O
3 and reduced graphene oxide (rGO) by a simple hydrothermal method and applied it to capacitor batteries. The α-Fe
2O
3@rGO material showed high specific capacitance (SC) and good cycle stability. The specific capacitance was 455 F/G at 1 A/G, and it could still reach 87.5% of the initial capacitance after 10,000 cycles
[19]. The energy density is 73 Wh/kg at a power density of 1357 W/kg. After that, Li et al. Prepared one-dimensional mesoporous europium nanowires grown on strained steel substrate by two-step method, and used them as cathode materials for lithium batteries
[20]. The discharge capacity of the electrode is 1460 mAh/G after 200 cycles at 100 mA/G and 700 mAh/G at 800 mA/G, which is attributed to the capacitive effect of the unique mesoporous nanowire structure. These results indicate that the modified materials have potential applications in the field of LICs.
Co
3O
4 is also a commonly used oxide anode material, but the electrode has poor stability and low specific capacity. In order to improve its performance, Yin et al. Prepared GO-MOFs coated rGO/Co
3O
4 composites by co-precipitation method, and innovatively applied MOFs materials to lithium-ion capacitor batteries
[21]. The porous structure was clearly observed by SEM. This unique structure not only makes the electrode material show excellent lithium storage performance, but also has a high initial discharge capacity (1451 mAh/G at a current density of 100 mA/G), good cycle stability (capacity retention after 100 cycles is more than 96%) and excellent pseudo-capacitance performance (546 F/G). In addition, the method provides a prospect for the synthesis of other advanced electrode materials through reasonable structural design, and also makes the transition metal oxide composite materials derived from GO-MOFs become promising electrode materials in the field of next-generation LICs.
Subsequently, the researchers studied other metal oxides and found that the properties of the treated materials were greatly improved. For example, Zhao et al. Synthesized carbon-free MoO
2 nanosheets with crystalline hybrid structure through one-step solvothermal reaction. The heterogeneous crystalline structure forms a unique boundary, which can effectively improve the diffusion rate of Li
+[22]. The SC of the lithium ion capacitor battery assembled by using the composite as the negative electrode and the activated carbon as the positive electrode can reach 190 F/G at 1 A/G, and the SC is still 12 F/G after 7500 cycles at 20 A/G. Lee et al. Prepared graphite/CuO anode materials by air sintering method, and developed a high energy density LICs system with porous carbon (PC) as anode, which has a simple manufacturing process and high cost-effectiveness
[23]. When the power density is 1. 3 kW/kg, the energy density of the system can reach 212. 3 Wh/kg.These research results provide new inspiration for researchers in related fields.
2.2 Carbon Material
Carbon materials have attracted much attention in various fields of energy storage due to their low cost, high abundance and stable physical/chemical properties, and have been successfully applied to lithium-ion capacitor batteries. Carbon materials are mainly divided into graphite, hard carbon and soft carbon. Among them, graphite is considered to be an ideal anode material for high-power capacitor batteries because of its low cost, high conductivity and large theoretical capacity, and has been commercialized since 1990. However, due to the orientation of graphite, it is easy to cause the graphite layer to fall off during the charge and discharge process, which affects the high rate charge and discharge of the battery. At the same time, the pseudocapacitance effect will reduce the capacitance characteristics. Therefore, the performance can be improved by using graphene-based composites, doping and other strategies.
Graphene is the basic component of graphite materials, which has large specific surface area, excellent conductivity and high mechanical flexibility, so it shows excellent electrochemical performance in lithium batteries. Wu et al first proposed heteroatom-doped graphene composites, and the test results showed that the first reversible capacity of boron-doped graphene was as high as 1549 mAh/G, which was much higher than that of pure graphene
[24]. When the current density is increased to 25 A/G, the reversible capacity of boron-doped and nitrogen-doped graphene is 235 and 199 mAh/G, respectively, and the power density is 29. 1 and 34. 9 kW/kg, corresponding to the energy density of 226 Wh/kg and 320 Wh/kg. These results are far superior to chemically derived graphene and other carbonaceous materials. Recently, Arnaiz and others have made further breakthroughs in this field, and they have designed an ultra-fast and ultra-stable all-graphene electrode
[25]. Coffee waste powder and graphene oxide were pyrolyzed to obtain graphene-carbon composites, which were used as the negative and positive electrodes of LICs. The electrodes showed good electrochemical performance, with an unprecedented capacity retention of 98.9% after 50,000 cycles and an ultra-high power of 53.5 kW/kg. This result shows that the combination of graphene structure and matching method can realize fast and stable LICs system.
Soft carbon has high abundance, controllable crystallinity, compatibility with electrolyte, good rate capability and electrochemical stability, but it still has the problem of low diffusion rate of Li
+. It is found that the storage performance of lithium can be improved by nitrogen doping and structural design of soft carbon. By combining heteroatom control and structural design, Zhang et al. Obtained a hierarchical porous soft carbon doped with nitrogen, phosphorus and sulfur, which was used as a negative electrode and a LiVPO
4F as a positive electrode to assemble a lithium-ion capacitor battery
[26]. The discharge capacity was 770 mAh/G at 100 mA/G and 500 mAh/G after 500 cycles at 500 mA/G. This excellent performance indicates that the synergistic doping effect can be applied to traditional carbon materials to improve their electrochemical performance.
Hard carbon is synthesized from single graphene and has a turbine structure induced by sp
3, which has great advantages in chemical stability.However, the development of hard carbon is still limited by its poor rate capability and large irreversible capacity.Therefore, doping and surface modification of hard carbon can improve its performance. According to the carbonization process of low-cost bio-soap residue, Jiang et al. Prepared high-defect, N-doped hard carbon as anode without adding templates and catalysts
[27]. Due to the adoption of two strategies of N-doping and defect engineering, the electrode has high specific capacity, excellent rate capability and long cycle stability, the discharge capacity is 580.3 mAh/G at 0.05 A/G, and the electrode can be stably cycled for more than 1000 times. Electrochemical kinetics analysis and density functional theory calculation also confirmed its outstanding pseudo-capacitance performance and excellent Li
+storage capacity. As expected, the resulting LICs have excellent energy density, with a maximum energy density of 101.7 Wh/kg and an energy density of 56.3 Wh/kg even at a high power density of 12.5 kW/kg. This shows the huge energy storage potential of hard carbon anodes.
2.3 Lithium intercalation compound
Titanium-based oxides are a common class of lithium intercalation compounds, which have higher electrochemical working potential and better safety than graphite. Among them, lithium titanate Li4Ti5O12 and titanium dioxide TiO2 have been applied to LICs as high power density anode materials.
Li
4Ti
5O
12 has unique "zero strain" characteristics and excellent cycle stability, but its performance at high rates is severely limited by its extremely low electronic conductivity and Li
+ diffusion coefficient. It has been found that the conductivity of Li
4Ti
5O
12 can be significantly improved by composite or nanostructure treatment, and unexpected results can be achieved. Lee et al. Successfully prepared Li
4Ti
5O
12/ activated carbon composite anodes, and selected activated carbon cathodes to assemble LICs. The results showed that the battery had high specific capacity, superior rate capability and cycle stability
[28]. When the current increased from 0. 5 A/G to 5 A/G, the discharge capacity was 81. 9% of the initial capacity, and the capacity retention was 92. 8% after 7000 cycles at 3 A/G. Liu et al. combined the iron oxide hard template method and F127 soft template method to construct porous Li
4Ti
5O
12(PLTO) nanoparticles on the self-supporting carbon nanotube film, and prepared a new type of lithium titanate self-supporting anode CNT @ PLTO
[29]. The corresponding discharge capacities were 148 and 85 mAh/G at current densities of 0.175 and 8.75 A/G, respectively, which were significantly better than that of pure LTO. The improvement of battery performance was attributed to the particle size of the nanostructure and the transport channel through the lithium ion, which greatly improved the Li
+ diffusion coefficient. At the same time, the maximum energy density of 101. 8 Wh/kg was achieved at the power density of 436. 1 W/kg.This study provides a new idea for the realization of high-performance LICs.
Compared with the Li
4Ti
5O
12, the TiO
2 is a more ideal high-power lithium battery anode material, which has stable chemical properties, simple synthesis, low cost and environmental friendliness, and the theoretical specific capacity is at least twice that of the Li
4Ti
5O
12, and the anatase phase and bronze phase in the TiO
2 have also attracted much attention in recent years
[30]. However, in the process of lithium intercalation and deintercalation, the lattice change of TiO
2 is slightly more obvious than that of lithium titanate, which leads to the decrease of cycle performance in the process of long-term charge and discharge. Therefore, different methods have been proposed to improve the electrochemical performance of TiO
2, including carbon coating, hybrid material design, and metal and nonmetal doping. Zhou et al. Synthesized a ribbon-like TiO
2-Co
9S
8 nanocomposite by hydrothermal method, and both bronze and anatase phases were detected in the calcined product
[31]. When the current density was 100 mA/G, the first reversible capacity was 714 mAh/G, and the coulombic efficiency was 74%. During further cycling, the lithium storage capacity of the material increased and stabilized at 886 mAh/G after 100 cycles. More importantly, as the framework of the Co
9S
8 nanoparticle layer, the anatase /TiO
2 nanoribbons still have a capacity of 260 mAh/G at a high rate of 5 A/G. This shows that the material has a potential application prospect in energy storage.
In addition to Li4Ti5O12 and TiO2, other titanium-containing lithium intercalation compounds LiCrTiO4 and LiTi2(PO4)3 have been gradually applied to the anode materials of lithium ion capacitor batteries in recent years, and carbon coating, doping and material composite treatment methods are also applicable to them.
2.4 Sulfides
Sulfides (such as NiS, CoS2, MoS2, etc.) Have been used as anode materials for lithium ion capacitor batteries due to their high theoretical capacity, low cost and good electrode kinetics. However, there are still many shortcomings in practical applications. Firstly, the poor conductivity of sulfides leads to poor rate capability of batteries and slow kinetics of electrochemical reactions. Secondly, in the process of Li+ intercalation/deintercalation, serious agglomeration and volume expansion caused by repeated lithium absorption and particle breakage will lead to a rapid decline in capacity. In order to improve the above disadvantages, metal sulfides are subjected to nano treatment and material compounding, so that the electrochemical reaction efficiency and the utilization rate of active electrode materials can be effectively improved, and meanwhile, the volume expansion can be buffered and accommodated.
NiS electrode material has been successfully used in the field of lithium ion capacitor batteries, mainly because of its unique chemical and physical properties, which can provide a good transport path for ions on the electrode surface through Faraday process. Tran et al. Synthesized NiS @ NF composite electrode by electrodepositing NiS on nickel foam at room temperature
[32]. NiS @ NF has high SC and good cycle stability, with SC of 1553 F/G at 2. 35 A/G, and retains 95. 7% of its initial SC after 2000 cycles. In 2019, Chen et al. Developed a new type of 2D/3D NiS/Ni
3S
4 composite by one-pot hydrothermal method
[33]. The composite has a high capacitance of 1796 F/G at 1 A/G and a capacitance retention of 80.5% of the initial capacitance after 1000 charge-discharge cycles. The results show that 2D/3D NiS/Ni
3S
4 composites have the potential to be used as electrode materials for capacitor batteries.
CoS
2 is a promising energy storage material. In recent years, CoS
2 with different nanostructures and hafnium-based composites have been developed as electrode materials for high performance capacitor batteries. For example, Jia et al. Prepared porous four-shell hollow CoS
2 by template-directed synthesis
[34]. The SC value is 375. 2 F/G at 1 A/G, and the capacitance retention is 92. 1% after 10 000 cycles. Then Govindasamy et al. First used two-step hydrothermal method to grow NiCo
2S
4@CoS
2 nanostructures on carbon cloth. The obtained NiCo
2S
4@CoS
2@CC composite electrode had a SC as high as 1565 F/G at 1 A/G, and its SC still reached 91% of the initial SC after 8,000 cycles, showing good cycle stability
[35]. In addition, the energy density of the composite is 17 Wh/kg at a power density of 242.8 W/kg. These results indicate that the NiCo
2S
4@CoS
2@CC is a good candidate for high performance lithium ion capacitor batteries.
MoS
2 is a kind of cheap and easy to synthesize material, which has nano-flake morphology, large surface area and excellent conductivity. Because of its unique properties, MoS
2 has been widely concerned by scientists in the field of energy storage. Using SiO
2 nanospheres as templates, Gao et al. Prepared MoS
2 nanosheets through a simple hydrothermal process and assembled them into lithium-ion capacitor batteries
[36]. The maximum SC of the MoS
2 nanospheres is 683 F/G at 1 A/G, and the capacitance retention is 85. 1% after 10 000 cycles. When the energy density is 20. 42 Wh/kg, the power is 750. 31 kW/kg, showing excellent electrochemical performance. Wang et al. synthesized a continuous stepped ridge-like MoS
2/N doped carbon nanocomposite (MoS
2/NC) by chitosan-induced one-pot hydrothermal and sintering method, and the synthesis scheme is shown in Figure 2
[37]. SEM shows that the MoS
2 nanosheets with less than three layers form interconnected nanolayers and grow vertically on the two-dimensional N-doped carbon nanosheets to form a MoS
2/NC composite electrode. This unique layered nanostructure enables the electrode material to have excellent rate capability and cycle stability, with capacities of 742, 686 and 534 mAh/G after 400 cycles at current densities of 500, 1000 and 2000 mA/G. This synthesis strategy is a good choice for the construction of other transition metal disulfide and carbon nanostructures to improve their electrochemical performance.
FeS
2 is also a potential electrode material for energy storage devices because of its low price, excellent conductivity and a large number of electrochemically active sites. Sun et al. Prepared FeS
2 nano-ellipsoids for LICs by a simple microwave-assisted method
[38]. The SC of the FeS
2 electrode reaches 515 F/G and 355 F/G at current densities of 1 and 20 A/G, respectively, and the initial specific capacitance remains 90% after 5000 cycles. The high energy density of 64 Wh/kg is obtained at power density of 271.2 W/kg, which indicates that the FeS
2 nano-ellipsoid is also an ideal electrode material for high power energy storage devices.
3 Cathode material
3.1 Layered metal oxides
3.1.1 Layere LiMO2 compound (M is transition metal)
The crystal structure of the LiMO2 series cathode material belongs to a layered rock salt phase, the oxygen ions which are closely arranged are combined with the transition metal ions which are in octahedral positions to form a stable MO2 layer, and the Li+ is embedded between the layers, so that the cathode material has relatively large mass and volume theoretical capacity. The most common LiMO2 compounds include LiCoO2(LCO) and LiNiO2(LNO). The emergence of LiCoO2 fills the technical space, but it is less used in lithium-ion capacitor batteries because of its high cost, high toxicity and safety problems caused by rapid temperature rise at high rate discharge.
LiNiO2 has been widely studied by researchers as an alternative to LCO. Nickel is more abundant than cobalt and is a cheaper alternative. However, trivalent nickel is unstable at high temperatures, resulting in harsh conditions for LNO synthesis, which are strongly dependent on specific conditions. Moreover, the Jahn-Teller phase transition leads to structural changes, which affect the cycle life and structural stability. Through continuous efforts, researchers have found that Ni, Mn and Al elements can be used to partially replace the transition metal elements in the LiMO2 for doping, and the electrochemical tests show that the doped electrode materials show excellent electrochemical performance
Ohzuku et al. prepared the LiNi
0.5Co
0.5Mn
0.5O
2 material by sol-gel method for the first time. Its charge-discharge voltage platform is 3.6 ~ 4.3 V, the maximum reversible capacity is 200 mAh/G, and the capacity does not decay after 100 cycles. Its energy density is higher than that of LiCoO
2, but its Li
+ mobility is one order of magnitude lower than that of LiCoO
2, resulting in relatively poor rate capability
[39]. In order to further improve its overall performance, researchers have studied element doping on this basis, resulting in a new generation of ternary NCM/NCA cathode materials, which is the most important achievement in the research of layered LiMO
2 composite doping.
3.1.2 Spinel type LiMn2O4 compound
Spinel LiMn
2O
4(LMO) have attracted much attention in recent years due to their low cost and environmental friendliness compared with LCO and LNO. The LiMn
2O
4 belongs to the Fd3m space group and has a cubic close-packed crystal structure, so it has good rate capability. However, the Jahn-Teller effect introduced by manganese ions leads to the deterioration of cycle performance. In addition, the LiMn
2O
4 also has a significant capacity degradation at high temperatures. It was found that the cycling performance of the materials could be improved by doping and coating. Li et al. Coated a layer of porous Al
2O
3 nanoparticles on the LiMn
2O
4 by co-precipitation method. Compared with the original LMO, the Al
2O
3@LMO sample has a unique structure
[40]. Through the comparison of electrochemical performance, there is little difference between the two capacities at 25 ℃, and the cycling performance of the Al
2O
3@LMO sample is significantly improved at 55 ℃, and the capacity retention rate is 91. 9% of the initial capacity after 100 cycles. Other metal ions such as Cu
2+, Mo
2+ and Mg
2+ can also be doped into the LiMn
2O
4, and the overall performance of the battery can be improved to varying degrees.
3.1.3 Olivine-type LiFePO4 material
LiFePO4 has become one of the most mature cathode materials in olivine phosphates because of its environmental friendliness, low price, non-toxicity, abundant natural resources and good stability. The early research on LiFePO4 can be traced back to the 1980s, but it was not until it was discovered in 1997 that it gradually attracted the attention of researchers. To date, LiFePO4 have been commercially used in lithium-ion capacitor batteries.
The crystal structure of LiFePO
4 can be seen as a slightly distorted hexagonal close packing, which leads to low Li
+ diffusion coefficient and poor conductivity, and is not conducive to charging and discharging at high rates. Therefore, the conductivity of the LiFePO
4 can be improved by the carbon coating or other conductive materials so as to meet the requirements of high-current charging and discharging
[41]. Savadogo et al. Coated different amounts of citric acid as a carbon source on the LiFePO
4 cathode by a hydrothermal method. The carbon-coated LiFePO
4 sample (30 wt% citric acid) had the lowest charge transfer resistance, with a specific capacity of 167 mAh/G at 0.1 C. The result was mainly due to the increase in the specific surface area of the citric acid coating, which inhibited the growth of LiFePO
4 crystals
[42]. Wang et al studied the effect of fluorine-doped carbon (FC) modification on the electrochemical performance of LiFePO
4 cathode materials
[43]. The results showed that the optimal content of FC was about 2. 8 wt%, and the electrode with this content showed remarkable electrochemical performance, including high capacity close to the theoretical value, good cycle stability and high rate capability. After 1000 cycles at high current density of 20 C, the discharge capacity is still maintained at 100. 2 mAh/G, which proves that FC coating strategy can open up a new way to maximize the performance of LiFePO
4 cathode materials.
The LiMPO
4F is a derivative of the olivine structure. This type of cathode material combines the advantages of PO
43- (inductive effect) and F
- (high electronegativity) to provide a high operating potential
[44]. For the first time, Zhang et al. Successfully prepared LiFePO
4F nanospheres by a solid-state route combined with a chemically induced precipitation method
[45]. The initial discharge capacity of the pre-prepared sample was 110. 2 mAh/G at 0. 5 C. The stable cycling performance (94.4% capacity retention after 200 cycles) can be attributed to the fact that the uniform nano-spherical morphology is beneficial to increasing the contact area between the electrode and the electrolyte, shortening the transport path of ions, and thus improving the kinetics of lithium ions.
3.1.4 Ternary cathode material
In recent years, ternary material LiNi1-x-yCoxMnyO2 has attracted more and more attention due to its comprehensive advantages in specific capacity, operational potential, safety, cost and environmental protection, and has been successfully applied to lithium-ion capacitor batteries. The common abbreviations for this material are NCM or NAM, where the number represents the fractional part of the element (such as the NCM523=LiNi0.5Co0.2Mn0.3O2), and are actually also known as Co or Al-doped LiNiO2 and LiMnO2. Among them, the + IV oxidation state of manganese does not change during the charge-discharge cycle, so it can be used as a structural stabilizer; Ni2+/Ni3+ and Ni3+/Ni4+ have strong redox activity and contribute most of the specific capacity of the material; in addition, the presence of cobalt is conducive to the synthesis of stoichiometric compounds with low cation mixing, so the performance of the material can be adjusted by adjusting the ratio of Ni, Co and Mn. LiNi0.5Co0.5Mn0.5O2 is the first ternary cathode material synthesized by Ohzuku and others mentioned before, and other ternary cathode materials have been gradually developed in the following years, such as LiNi0.5Co0.2Mn0.3O2, LiNi0.6Co0.2Mn0.2O2, nickel-rich and lithium-rich ternary materials.
Despite the rapid development of ternary cathode materials, there are still some challenges in their application, such as high first-cycle irreversibility and rapid capacity fading. Therefore, in addition to the strict regulations on the synthesis conditions, it is necessary to further modify the composition/structure/morphology level by doping, surface coating, composite and so on, in order to greatly improve the structural stability and obtain better performance. For example, Shi et al. Synthesized spherical lithium-rich Li
1.2Mn
0.56Ni
0.16Co
0.08O
2 materials by microwave hydrothermal method, and the synthesis scheme is shown in Figure 3
[46]. The prepared material is composed of spherical particles (2 ~ 3 μm) and porous particles (150 ~ 250 nm), and is characterized by XRD and specific surface area (BET), which proves that the material has good layered structure and porous structure with large specific surface area.The specific discharge capacity was 235.6 mAh/G at the current density of 0.2 A/G, and the specific discharge capacities were 168.6 mAh/G and 131.2 mAh/G at the current densities of 1 A/G and 2 A/G, respectively.
Yang et al. Successfully synthesized a layered LiNi
0.88Co
0.095Mn
0.025O
2 cathode with primary grains arranged radially by optimizing the precursor design. During the sintering process, the Al
3+ followed the radial primary grains into the bulk by introducing nano-Al
2O
3, thus realizing the uniform distribution of Al
3+ in the whole material
[47]. When the molar mass of Al
3+ is 2%, the electrochemical performance of the electrode is the best, and the capacity retention rate of the electrode is as high as 91. 57% after 150 cycles at 1 C. High reversible capacities of 172. 3 mAh/G and 165. 7 mAh/G were obtained even at high current densities of 5 C and 10 C. In addition, from the results of the median voltage and dQ/dV curves, it can be seen that the uniform Al
3+ doping significantly suppresses the voltage deterioration, which indicates that the Al
3+ doping provides an effective and practical method to improve the rate performance and voltage stability of nickel-rich ternary materials (Fig. 4).
3.1.5 Vanadium oxide V2O5
As a layered compound for electrochemical energy storage, V
2O
5 has become one of the most promising cathode materials for LICs in recent years due to its high energy density, low cost, abundant sources and good safety performance. V
2O
5 is a typical interlayer compound, the interlayer structure of which is bonded together by van der Waals force. This layered structure makes the theoretical capacity of V
2O
5 (294 mAh/G) much higher than that of other commonly used cathode materials, but at the same time, it has poor conductivity, is easy to agglomerate and dissolve in liquid electrolyte, resulting in unsatisfactory rate and cycle performance
[48][49].
In order to increase the conductivity, the V
2O
5 can be converted into nanostructures or coupled with carbon-based composites to increase the conductivity
[50⇓~52]. Qu et al. designed and fabricated a composite cathode based on the thick film CNT/V
2O
5 and used it in combination with an organic electrolyte to prepare a capacitor cell with activated carbon as the anode
[53]. The results show that the system has excellent conductivity, high specific capacitance and large voltage window, so that the energy density is as high as 40 Wh/kg at a power density of 210 W/kg. Even at the high power density of 6300 W/kg, the energy density of the device is close to 7.0 Wh/kg. Xing et al. Synthesized a yolk shell nanostructure composed of ultra-thin nanosheets by hydrothermal method as an electrode material. The specific capacitance of the electrode was 704.17 F/G at 1 A/G, and the capacitance retention rate was 89% after 4000 cycles. At this time, the power density was 800 W/kg, and the energy density reached 29.49 Wh/kg, which greatly improved the electrochemical performance
[54]. This shows that the preparation of nanocomposites is an effective way to improve the energy density and power density of LICs.
In order to solve the agglomeration problem of vanadium-based materials, the construction of three-dimensional structure is particularly important. Yao et al. Used 3D structure to prepare rGO/V
2O
5 nanocomposite cathode
[55]. The well-crystallized V
2O
5 nanosheets can provide more electroactive sites and diffusion paths to enhance the pseudocapacitance, and the large surface area of the graphene oxide nanosheets with good conductivity can effectively promote charge transport. Thanks to its unique structure, the electrode has a capacitance of 225.6 F/G at 0.5 A/G and an energy density of 31.3 Wh/kg at 249.7 W/kg. Pandey et al. synthesized a mesoporous hybrid of V
2O
5 nanoparticles immobilized on reduced graphene oxide (rGO) by slowly hydrolyzing vanadium oxide triisopropanol using a two-step solvothermal and vacuum annealing method.The hybrid material has high surface area and meso-scale porosity, so that the electronic conductivity is obviously improved, the diffusion of electrolyte ions is accelerated, and the electric energy storage performance is synergistically improved
[56]. When used as the cathode of lithium-ion batteries, the rGO-V
2O
5 exhibited good cycle stability and discharge capacity, which reached 295,220 and 132 mAh/G at C/9,1 C and 10 C current densities, respectively. After 150 cycles at 1 C, the electrode retained about 83% of its discharge capacity, with only a 0.12% decrease in capacity per cycle (fig. 5). It can be seen that the construction of three-dimensional structure can greatly improve the energy density and cycle stability.
3.2 Graphene composite positive electrode
Graphene-based active materials have the properties of both battery materials and capacitor materials, and are considered to be ideal dual-active electrode materials for lithium-ion capacitor batteries, which can be used as both positive and negative electrodes. As mentioned above, Arnaiz and others have synthesized all-graphene capacitor batteries with excellent performance. Graphene composites can improve the electronic and ionic conductivity of electrodes by forming bicontinuous conductive networks, and then improve the performance of batteries, in which graphene oxide is the most commonly used raw material to construct porous networks.
The three-dimensional porous LiFePO
4/N/rGO composite material developed by Wang et al. Has proved that the graphene layer can protect the active material from direct contact with the electrolyte, thus reducing the side reaction and dissolution of the active material
[57]. Subsequently, Xie et al. Prepared graphene-modified LiFePO
4 composite cathode by in-situ microreactor strategy. The composite material has large specific surface area of charge adsorption, abundant active sites for Faraday reaction and ideal kinetic characteristics of electron and ion transport
[58]. As a result, it exhibits extremely fast surface dynamics-controlled lithium storage behavior. In terms of rate performance, the discharge capacity is 78 mAh/G at 200 C, and the capacity retention rate is 90% after 1000 cycles at 20 C, showing a quasi-capacitive mechanism and excellent cycle stability.
3.3 Other new cathode materials
Other new cathode materials include boric acid LiMBO
3 and silicate Li
2MSO
4. The borate LiMBO
3 has a high theoretical capacity (220 mAh/G) due to the small molecular weight of the BO
33-. In 2001, Legagneur et al. First studied the LiMBO
3 and found that the discharge capacity was only 9 mAh/G during the initial charge process, which limited the application of the material in the field of LICs
[59]. Later, Yamada et al. Proposed that the reason for the low specific capacity of the LiMBO
3 was not the material itself, but the hydrolysis of the cathode surface in contact with water in the air
[60]. Therefore, on this basis, the LiFeBO
3 is coated with carbon to avoid contact with the air, so that the specific capacity of the modified material can be as high as 190 mAh/G, which has attracted wide attention of scholars.
Silicate Li
2MSiO
4 has also attracted attention because of its high theoretical specific capacity, and the first report of using Li
2MSiO
4 as a positive electrode can be traced back to 2005, when Nyt Nyt Én et al.Two redox peaks, :2.8V(Fe
2+/Fe
3+) and 4.0~4.8 V(Fe
3+/Fe
4+), and then explored the transition mechanism between cation and anion redox activities in this material, and found that this material has a significantly high energy density
[61]. However, the transport mechanism of ions and electrons in the lattice is still an unrevealed key problem, which needs further experiments to understand.
4 Conclusion and prospect
As a new type of energy storage device, lithium-ion capacitor battery combines high energy density, high power density and long-term cycle stability in a system, which fills the gap between lithium-ion batteries and supercapacitors. Among them, the selection of electrode materials directly affects the overall performance of the capacitor battery. The anode materials for lithium ion capacitor batteries include transition metal oxides, carbon materials, lithium intercalation compounds and sulfides, which have the main problems of unstable structure, poor cycle stability and poor plasticity.The performance of anode materials is often improved by the combination of carbon coating and carbon nanofibers, as well as the addition of conductive additives (such as graphene and carbon nanotubes). The cathode material mainly comprises a layered metal oxide and graphene composite material. However, the poor cycle performance and rate capability have always limited the development of cathode materials, which can be effectively solved by ion doping, nano-treatment and material composite on this basis.
Although great progress has been made in the research of electrode materials for lithium-ion capacitor batteries, there is still a long way to go before practical commercial application. Because low energy density is still the main problem of hybrid capacitors, the following aspects can be studied in the future: 1. Combined with high conductivity materials, which can effectively improve the electron transfer kinetics; 2。 To improve the voltage window of lithium-ion capacitor batteries, it is necessary to develop new electrode materials with unique structures to obtain large specific surface area and accelerate electron conduction rate. Based on the synergistic effect, the combination of different electrode materials should be considered as an effective way to enhance the specific surface area and large potential window. 3。 Further optimization of the anode and cathode matching is of great significance for improving the overall performance of the device. The realization of high power density and high energy density of LICs is an inevitable trend in the future. Therefore, in the process of designing and preparing LICs electrodes in the future, the kinetic balance between positive and negative materials should be fully considered, and materials with low toxicity, low cost and good conductivity should be sought to realize the sustainable development and utilization of LICs.
National Natural Science Foundation of China(22075030)
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