introduce

In early August this year, the "Fifth Symposium on Innovative Research and Development of Silicon-Based Anode Materials for High Energy Density Lithium-ion Batteries" was successfully held from Chengdu to Changzhou after several twists and turns. Representatives from universities, scientific research institutes, and upstream, midstream, and downstream enterprises of silicon-based anode materials gathered together to participate in the event. This conference has two main themes: (1) The latest research progress of silicon-based anode materials; (2) The industrialization path of silicon-carbon anode materials. This paper systematically summarizes some important viewpoints and research progress of this meeting.

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In 2020, the State Council issued the "New Energy Vehicle Industry Development Plan (2021-2035)", and in 2021, the "Carbon Peak Action Plan Before 2030" and "Made in China 2025" pointed out that by 2025, the power ratio of the energy of a single battery will reach /kg, and the energy density will reach /kg in 2030. To achieve this goal, the current best path is to replace traditional graphite anode materials with silicon-carbon anodes and combine them with (ultra) high-nickel cathode materials.

Si has an ultra-high specific capacity (more than 10 times that of commercial graphite) and is abundant in the earth's crust, which reduces its application cost. In addition, the lithium intercalation potential of Si is 0.4 V vs Li/Li+, which prevents the formation of lithium dendrites under overcharge conditions, so the silicon-based anode is also safer. However, there are problems such as poor conductivity and large volume expansion.

Figure 1 Failure mechanism and modification strategy of silicon anode (JW Choi, et al. Nat. Rev.., 2016)

In academia, the problems existing in silicon-carbon anodes are mainly dealt with from three aspects: parameterization, compounding, and system optimization. Professor Wei Fei used fluidized bed technology to coat ceramics on the surface of Si through structural design as a selectively permeable layer, which can improve the cycle stability of silicon-carbon anodes. Here, Professor Wei Fei specifically mentioned that the chemical reaction between Si and the electrolyte will seriously affect the stability of the interface, and incomplete carbon coating will lead to Si leakage and act as a catalyst to accelerate the occurrence of side reactions. Life.

Figure 2 Interfacial reaction of carbon layer and ceramic layer coating (CH Yu, et al. Nano Lett., 2020)

Cui Guanglei, a researcher at Qingdao Institute of Energy Research, Chinese Academy of Sciences, has improved the electrochemical performance of Si-negative electrodes through multi-dimensional comprehensive research strategies, including modification and doping coating of Si itself, pre-lithiation, electrolyte additives, binders, etc., Covers almost all silicon anode modification strategies, including the application of silicon-based solid-state batteries in deep-sea energy space stations. Professor Song Jiangxuan from Xi'an Jiaotong University is focusing on the development and utilization of silicon anode binders. Starting from molecular design, the adhesive integrates multiple functions (high adhesion, self-healing, stress dissipation, electrical conductivity, artificial SEI, etc.), and expands the laboratory's 10L reactor to 1-3 tons of the reactor. The adaptation of the system is mainly concentrated in the research of universities, but it is also involved in the enterprise. It is mentioned in the report that the functionalized polymer-modified silicon anode can inhibit the erosion of the electrolyte.

Figure 3 Optimization strategy of silicon carbon anode binder (Z. Chen, et al. ACS Appl. ., 2020)

In terms of Si parameterization, this meeting also mentioned Si nanowires in addition to commonly used Si nanoparticles. Compared with granular silicon electrode materials, silicon nanowire electrode materials can effectively alleviate the volume expansion of silicon, and the larger specific surface area is conducive to the penetration of electrolytes, shortening the distance of electron transmission and ion diffusion. In this field, Dr. Wang Cen from () said that they have developed silicon nanowires with an initial efficiency of 93% and a reversible specific capacity of 1400 mAh/g for the first time, which can replace graphite in any proportion to prepare high-energy-density batteries. Such batteries do not have an advantage in price competition. To this end, Chengdu Tuomei Applied Technology Researcher said that he has developed low-cost, commercially available silicon nanowires, which are currently being produced at the kilogram level in a pilot production line. At the same time, the Institute of Process Technology of the Chinese Academy of Sciences used photovoltaic silicon waste to realize the one-step preparation of silicon nanowire/ carbon electrode materials from waste silicon powder for the first time, forming a common technology. Scope cleaning of waste silicon powder to prepare silicon nanowires.

At present, there are two routes for silicon carbon anodes, Si and SiO. Unlike the previous four sessions, most industrial players at this session turned their attention to SiO. Although the first Coulombic efficiency of SiO is low, it can be solved by pre-lithiation and pre-magnesium.

The industrialization of silicon-carbon anode materials is also the main topic of this meeting. Wang Lin from Tianjin Lishen shared the work that their company has done in the face of the practical application of silicon carbon materials. They pointed out that the key indicators affecting the application of silicon carbon materials include silicon crystal size, coating treatment, composite structure, post-processing, etc. For silicon-carbon anode materials, Lishen mainly explores the influence on the performance of silicon-carbon anodes from the aspects of carbon coating, heat treatment temperature, pre-magnetization, and pre-lithiation. Although pre-magnesium or pre-lithiation is beneficial to improve the first effect, due to the formation of a large number of inert substances and the growth of silicon crystal size, the gram capacity of silicon-carbon materials decreases, and the cycle performance decreases. Alkali improves the comprehensive performance of silicon-carbon negative electrode materials have a great impact. For pure silicon-based materials, Wang Lin pointed out that the higher the degree of graphitization of graphite, the softer the material texture, which is more conducive to maintaining the structural stability of the silicon-carbon negative electrode during the cycle, and at the same time building a long-distance conductive network to ensure the effective transmission of electrons... Transport, and obtain silicon carbon anode materials with excellent cycle performance.

Dr. Shenzhen Bike Linjian has developed a new preparation process, which can effectively reduce the resistance of silicon carbon electrode sheets. Silicon carbon anode materials also have excellent cycle stability at high temperatures. Wanxiang 123 Zhang Xiaozhu pointed out that compared with silicon-oxygen materials, silicon-carbon materials have smaller DCR at different SOCs, better fast charging performance, and low-temperature performance, but larger expansion and poorer high-temperature performance. How to match the appropriate graphite material has a great influence on the material's performance. At the same time, the adjustment of the binder and electrolyte of the electrode support system can also synergistically buffer volume expansion, and improve battery gas production and thermal stability, thereby improving the cycle and reducing DCR.

Graphite plays a vital role in the application of silicon anodes in high-energy-density batteries, which means that graphite raw materials and subsequent preparation processes have an important impact on the performance of silicon-based batteries. Dr. Li Hongsheng from Funeng Technology focused on the influence of the binder formula and homogenization process on the dispersion of Si particles, analyzed in detail some problems existing in the mass production process of silicon-carbon anodes, and developed a /Kg silicon-carbon anode electrode. At present, the core products are being industrialized.

As a leading domestic anode material company, Dr. Pang Chunlei of Shenzhen Beiterui expounded the countermeasure optimization and low expansion technology research on the core issues of silicon-based materials from the aspects of high first-efficiency technology development, high-magnification structure design, and interface stabilization technology. In terms of the industrialization process, Dr. Pang pointed out that how to complete product integration, energy saving, and intelligent production is still a difficult problem in industrialization. Dr. Ding Jun of Shida Shenghua New Energy pointed out that the current SiO-based anode industry chain is mature, the performance is improving year by year, and the cost is gradually decreasing. Various pre-lithiation technologies are progressing rapidly and are expected to become mainstream products. In general, the industrialization and integration of silicon-based anode materials is difficult, but it has attracted much attention.

Figure 4. Sila is building a factory in Washington State, USA, and importing its silicon anode material Mercedes-Benz G-car

Finally, Duan Yueting from the Sila China team was almost the only speaker in the industry present who focused on the nano-silicon route. He presented the first presentation of Sila USA's work in product development and performance optimization. The former Tesla No. 7 employee and the Georgia Tech professor joined forces to open the door to the optimization design and industrial application of nano-silicon.  Sila currently holds more than 200 patents, has a valuation of more than $1 billion, and has completed mass production plans in the United States. With the help of the Ningde era, in 2025, the Mercedes-Benz G series electric vehicles will provide positive solutions. Last year's 4.0 sports bracelet was the first time that Sila applied its nano-silicon composite electrode in the 3C field. The performance was improved by 17%, and the expansion rate was controlled at 6%, which is enough to prove the excellent material and the control ability of mass production stability.

meeting minutes

The industrialization of silicon anodes has entered a period of real acceleration. In the next three years, most of the planned production capacity will be released, and silicon carbide, the cathode material, is unstoppable. The fast charging of power batteries and the improvement of energy density will be visible. However, how to control the material performance and price stability, while constantly overcoming the shortcomings of the silicon anode itself, especially how to improve the efficiency of scientific research and technology conversion between academia and industry, will be the silicon anode material. Challenges and opportunities for academics and engineers.

about the author

Li Jianbin, a lecturer, is currently working in the innovation team of new energy materials and power batteries at Changzhou University (team leader: Professor Ren Yurong), and his research direction is lithium-ion batteries and sodium-ion battery materials and devices.

Liu Wenjing, Ph.D., is studying in the research group, mainly engaged in the preparation and performance research of silicon-carbon anode materials for lithium-ion batteries.

Project team leader Meng Xinghua, working in, is mainly engaged in the industrialization of silicon-based anode materials.