Application Difficulties of Silicon-Based Anode Materials for Lithium Batteries

At present, the energy density of new energy vehicle power lithium-ion batteries still needs to be improved, and there is a long way to go to replace traditional fuel vehicles. The main way to improve the energy density of power lithium-ion batteries is to use new high-capacity positive and negative electrode materials. The theoretical specific capacity of silicon is as high as 4200mAh/g, which is more than 10 times that of graphite-based negative electrode materials. Therefore, it is considered by the industry as the next generation to replace graphite. Lithium battery anode material.

Silicon is the second most abundant element in the earth's crust. In theory, one silicon atom can undergo an alloying reaction with 4.4 lithium atoms to form Li4.4Si. Therefore, silicon has a very high theoretical specific capacity. In addition, the lithium intercalation potential ratio of silicon is higher. The high graphite anode can effectively avoid the formation of lithium dendrites. However, silicon is prone to a series of side reactions due to the huge volume change during the charging and discharging process:

(1) Repeated volume expansion and contraction, resulting in the accumulation of stress inside the silicon particles, and finally pulverizing the silicon material, resulting in poor electrical contact between the silicon particles in the pole piece, between the silicon particles and the conductive agent, and the cycle performance. become worse;

(2) The rupture and regeneration of the SEI film on the surface of the silicon particles consumes a large amount of lithium, and the first effect is low and the cycle is poor.

Therefore, if silicon-based anode materials want to be popularized and applied, they must be studied through modification.

The Si anode is an alloy lithium storage mechanism. The alloying/dealloying process causes huge expansion/contraction. The alloying reaction brings ultra-high specific capacity to silicon, and at the same time, it also causes drastic volume changes, so that the corresponding 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 sheet due to stress, resulting in a sharp decline in battery capacity and shortened cycle life. For a single silicon powder particle, during the lithium intercalation process, the outer layer of lithium intercalates to form amorphous LixSi, which undergoes volume expansion, and the inner layer does not expand until lithium is inserted, resulting in huge stress inside each silicon particle, causing individual silicon particles to crack, During the cycling process, new surfaces are continuously generated, which in turn leads to the continuous formation of the solid-phase electrolyte layer (SEI film), which continuously consumes lithium ions and causes the overall capacity of the battery to continue to decay.

At present, the modification applications of silicon anodes mainly focus on the composite of conductive materials, nano/porosification, development of new binders, optimization of interface stability and research on pre-lithiation technology.

1. Conductive material composite

By coating with conductive materials, mixing or constructing a good conductive network heterojunction to reduce the kinetic barrier of lithium ion migration of silicon materials, at the same time provide buffer space for silicon material expansion, and improve the electrochemical performance of silicon anodes.

The conductive materials usually introduced are Ag, conductive polymers, graphitized carbon materials, etc. The mixing and matching of silicon and graphite materials is the most potential application direction, and the current popular silicon carbon (Si/C) negative electrode material.

2. Nano-sized silicon particles

Theory and experiments prove that when the size of silicon nanoparticles is less than 150 nm, the size of silicon nanoparticles after coating is less than 380 nm or the radial width of silicon nanowires is less than 300 nm, the nano-silicon material itself can tolerate its own volume expansion, and it will not be affected after the first insertion of lithium ions. powder.

Compared with micron-scale silicon particles, nano-silicon materials show higher capacity, more stable structure and performance, and faster charge and discharge capabilities. At present, most of them are generally prepared by chemical vapor deposition (CVD), liquid reaction method, magnesium thermal reduction of silica or silicate, low temperature aluminothermic reduction, electrochemical deposition and electrochemical reduction of SiO2 and CaSiO3, etc. Forms of silicon-based nanoparticles.

3. Porous silicon material

Porosification is designed to reserve pores for the volume expansion of the silicon carbon anode material, so that the entire particle or electrode does not produce obvious structural changes. The methods for creating voids are generally: (1) preparation of hollow Si/C core-shell structure materials; (2) preparation of yolk-shell structure Si/C composite materials, which have sufficient cavities between core-shell structures and are widely used. Applied 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 materials reserves space for the volume expansion of lithium intercalation, reduces the internal stress of the particles, and delays the particle size.

Particle pulverization improves the cycle performance of silicon carbon anode materials to a certain extent.

4. New adhesive

The strong binder can effectively inhibit the pulverization of silicon particles, inhibit the cracking of silicon pole pieces, and improve the cycle stability of silicon anode materials. In addition to common CMC, PAA, PVDF binders, in the current research, attempts have been made to coat silicon materials with TiO2 to achieve the self-healing function of pole piece cracks; improve the elasticity of the binder and withstand the volume expansion of the silicon anode Shrinkage changes, releasing the resulting stress and other methods.

Five, interface stability optimization

Lithium-ion battery system is a multi-interface system, which improves the stability and bonding force of each contact interface, which has an important impact on the cycle stability and capacity performance of lithium-ion battery system. In the research, the contact interface is optimized by improving the electrolyte composition and removing the SiOx passivation layer, thereby improving the capacity and cycle stability of the silicon-based material; coating the silicon-carbon electrode with ZnO to effectively ensure the stability of the SEI film.

6. Pre-lithiation technology

The silicon anode material consumes a lot of irreversible lithium in 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 commercial graphite anodes, 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.

Currently, there are the following research directions:

(1) Silicon oxide is disproportionated at high temperature, and Si and SiO2 are generated after disproportionation. The electrochemical inertness of crystalline SiO2 can be used to improve the first effect of silicon oxide;

(2) Composite with metal materials to improve the first effect of silicon oxide anode materials;

(3) Alloy composite of silicon oxide and tin;

(4) Pre-lithiation of silicon oxide active material or silicon oxide pole piece.

Summarize

At present, the research on silicon-based anode materials can be mainly divided into two systems, the modification research with pure silicon material as the main body and the modification research with silicon oxide SiOx as the main body. and carbon coating to form a silicon-carbon composite material, so as to reduce the damage of the volume effect to the silicon particles and the SEI film.

At present, there are few domestic companies that can achieve mass production in the field of silicon-based anode materials. Betray has a first-mover advantage and has now entered the Panasonic-Tesla supply chain to achieve large-scale supply. Most of the other manufacturers are still in research and development. or small batch production stage.

The promotion and application of silicon-based anode materials is steadily advancing, but it is necessary to accelerate the industrialization and require an integration process. On the one hand, upstream material companies are required to improve the performance of their products, and on the other hand, they must study application technology and study lithium battery manufacturing together with downstream battery companies. Process improvement. It is believed that with the rapid deployment of domestic anode material manufacturers, the market penetration rate will gradually increase, the industrialization of large-scale level will be accelerated, and the market scale of silicon-based anode materials will have broad prospects.