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At present, the energy density of lithium-ion batteries for new energy vehicles is still to be improved, and there is still a long way to go to replace traditional fuel vehicles. The main way to improve the energy density of power lithium ion battery is to use new high-capacity anode and cathode materials. The theoretical specific capacity of silicon is up to 4200mAh/g, which is more than 10 times of graphite anode materials. Therefore, it is considered as the next generation anode material of lithium battery instead of graphite.
Silicon is the second most abundant element in the earth's crust. Theoretically, one silicon atom can alloyed with 4.4 lithium atoms to form Li4.4Si, so silicon has a very high theoretical specific capacity. In addition, the lithium embedding potential of silicon is higher than that of graphite anode, which can effectively avoid the formation of lithium dendrites. However, silicon is prone to cause a series of side reactions due to huge volume changes in the charging and discharging process:
(1) Multiple volume expansion and contraction, resulting in the accumulation of stress inside the silicon particles, and eventually make the silicon material powder, resulting in the electrical contact between the silicon particles in the polar plate between the particles, between the silicon particles and the conductive agent is poor, poor cycling performance;
(2) The SEI film on the surface of silicon particles ruptured and regenerated, consuming a large amount of lithium, with low first-effect and poor circulation.
Therefore, silicon - based anode materials must be modified if they want to be popularized and applied.
Si negative alloy lithium storage mechanism, the process of alloying/dealloying causes huge expansion/contraction, alloying reaction brings high specific capacity to silicon,but also causes drastic volume change, so that the relative volume expansion of Li15Si4 alloy is about 300%.
For the entire electrode, the expansion and contraction of each particle will "squeeze" the surrounding particles, which will cause the electrode material to fall off the electrode due to stress, which will lead to a sharp decline in battery capacity and a shortened cycle life. For a single silicon powder particles embedded in the process of lithium, outer intercalated-li form amorphous LixSi volume expansion occurs, the inner layer is not embedded lithium is not inflation, cause huge stress is generated in each silicon particles caused by single silicon particle cracking, circulation in the process of constantly produce new surface, leading to solid electrolyte layer (SEI film) continue to form, continuously drains lithium ion, The overall battery capacity continues to decline.
At present, the application of silicon anode modification mainly focuses on conductive material composite, nano/porous, new binder development, interface stability optimization and pre-lithium technology research.
1. Conductive Material Composite
The electrochemical performance of silicon anode can be improved by coating, mixing or constructing a good conductive network heterojunction to reduce the kinetic barrier of deembedded lithium ion migration and provide buffer space for silicon material expansion.
Commonly introduced conductive materials include Ag, conductive polymer, graphitized carbon materials, etc. The mixing and matching of silicon and graphite materials is the direction with the most potential application, as well as the current hot silicon carbon (Si/C) anode materials.
2. Nano-sized Silicon Particles
Theoretical and experimental results show that when the size of silicon nanoparticles is less than 150nm, the size of coated silicon particles is less than 380nm, or the radial width of silicon nanowires is less than 300nm, the nano-silicon material can tolerate its own volume expansion and does not powder after the first insertion of lithium ions.
Compared with micron silicon particles, silicon nanomaterials show higher capacity, more stable structure and performance, and faster charging and discharging capacity. At present, generally through chemical vapor deposition method (CVD), liquid phase reaction method, magnesium thermal reduction method of silicon dioxide or silicate, low temperature thermite reduction method, electrochemical deposition method and electrochemical reduction of SiO2 and CaSiO3, etc., the preparation of various forms of silicon based nanoparticles.
3. Silicon Material Porous
The porous design reserves pores for the bulk expansion of the silicon carbon anode material, so that the whole particle or electrode does not produce significant structural changes. The general methods to create voids are :(1) preparing hollow Si/C core-shell structural materials; (2) Si/C composite with ke -shell structure was prepared. The structure with sufficient cavity between core and shell was widely used to alleviate the volume effect of high capacity anode materials. (3) Preparation of porous silicon materials (silicon sponge structure, etc.).
The porous design of silicon based material reserves space for the volume expansion of lithium embedding, reduces the internal stress of particles, and postpones particles powderization can improve the cycling performance of silicon carbon anode material to a certain extent.
The strong binder can effectively inhibit the pulverization of silicon particles, inhibit the crack of silicon electrode, and improve the cyclic stability of silicon anode materials. In addition to the common CMC, PAA and PVDF binder, TiO2 coating silicon material has been tried in the current research to realize the self-healing function of pole chip crack. To improve the elasticity of the binder, to withstand the volume expansion and contraction of the silicon anode, release the resulting stress and so on.
5. Interface Stability Optimization
Lithium ion battery system is a multi-interface system, improving the stability and bonding force of each contact interface has an important impact on the cycle stability and capacity of lithium ion battery system. By improving the composition of electrolyte and removing the SiOx passivation layer, the capacity development and cycling stability of silicon based materials were improved. The contact interface was optimized by coating ZnO on silicon carbon electrode to ensure the stability of SEI film.
6. Pre-lithiation Technology
The silicon anode material consumes a lot of irreversible lithium for the first cycle. The method of adding some lithium (metal lithium powder or LixSi) to the silicon anode in advance to supplement the irreversible lithium consumption is called pre-lithiation technology.
At present, it is commonly used to add surface-modified dry and stable metal lithium powder to achieve pre-lithiation, or to add LixSi composite additives to form a protective layer of artificial SEI film.
Compared with the 300% volume expansion rate of silicon-based anode materials, the introduction of inactive element oxygen in SiOx anode materials significantly reduces the volume expansion rate of active materials in the process of lithium deintercalation (160%, lower than 300% of silicon anodes) , while having a high reversible capacity (1400-1740mAh/g).
However, compared with the commercial graphite anode, the volume expansion of SiOx is still serious, and the electronic conductivity of SiOx is worse than that of Si. Therefore, if SiOx materials are to be put into commercial applications, the difficulties to be overcome are not small. One of the research hotspots of anode materials for ion batteries.
The electronic conductivity of silicon oxide is poor, and the most common way to apply it to the negative electrode of lithium ion battery is to compound it with carbon material. The choice of carbon source has a great influence on the performance of composite materials. Commonly used carbon sources include organic carbon sources such as phenolic resin and pitch, inorganic carbon sources such as fructose, glucose and citric acid, graphite, graphene oxide and conductive polymer materials, etc. . Among them, the two-dimensional structure of graphene is elastic, and the graphene-wrapped SiOx can achieve self-healing in the process of volume expansion and contraction. In addition to silicon oxides in particle form, one-dimensional silicon oxide materials will facilitate the diffusive transport of lithium ions and electrons.
In the application of silicon-oxygen negative electrode, although the influence of volume expansion of silicon material is smaller than that of silicon material, at the same time, due to the introduction of oxygen, the first Coulomb efficiency is reduced, so improving the first effect is a problem that needs to be solved.