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On March 25th, 2022, the 8th China Electric Vehicle Hundred People Conference Forum was officially held online from March 25th to 27th. Around the theme of "Welcoming the New Stage of Marketization Development of New Energy Vehicles", 14 meetings were held to focus on industry hotspots and explore development trends.

On March 27, 2022, at a high-level forum, Li Hong, a researcher at the Institute of Physics of the Chinese Academy of Sciences, delivered a speech. The following is a summary of the speech:

Li Hong, Researcher at the Institute of Physics, Chinese Academy of Sciences

Dear guests, hello everyone! I am Li Hong from the Institute of Physics, Chinese Academy of Sciences. I am very happy to have the opportunity to communicate with you at the Hundred People Association. Firstly, I would like to express my gratitude to the Hundred People Association for inviting Teacher Chen Liquan. Due to her quarantine in Shenzhen, she was unable to come to the scene to record. Today, on behalf of Teacher Chen, I would like to report to you some of our thoughts and research progress in solid-state batteries.

Advanced batteries are key supporting technologies for the development of China's dual carbon strategy and electric power strategy. They have very important applications in production, daily life, and national security. In these application fields, high energy density batteries, high power density batteries, high safety, and long-life batteries are crucial advanced technologies.

Governments in Europe, the United States, Japan, and South Korea now attach great importance to the research and industrial layout of power lithium batteries. In 2020, Europe supported the "2030 Battery Innovation Roadmap", the United States formulated the "2021-2030 National Development Blueprint for Lithium Batteries in the United States", Japan formulated the third phase of the "Innovative Battery Development for Electric Vehicles" project, especially supporting the research and development of all solid-state batteries for electric vehicles led by Toyota. South Korea also announced the "2030 Secondary Battery Industry Development Strategy", It can be said that governments around the world attach great importance to the research and industrial layout of forward-looking technology for power lithium batteries.

The widely used liquid electrolyte lithium batteries currently pose a risk of thermal runaway. According to the research results of Ouyang Minggao and Feng Xuning's team at Tsinghua University, liquid lithium batteries begin to decompose the solid electrolyte layer at lower temperatures, triggering a series of thermal runaway behaviors, leading to safety accidents in various application scenarios.

Worldwide, both basic scientific research teams and industry teams believe that the solution of replacing easily combustible liquid electrolytes with non combustible solid electrolytes to form all solid-state batteries has high safety and theoretically high energy and power density.

Solid state batteries may have advantages in the following aspects: firstly, they can be charged to a higher voltage, the positive electrode material is not prone to oxygen evolution, and the negative electrode can contain metallic lithium, which is not easy to continuously react with lithium, and is not prone to thermal runaway, gas expansion, good high-temperature stability, and support internal string. These advantages make it possible for the cells of solid-state batteries to have intrinsically safe characteristics, such as increasing the cell capacity, energy density, module integration efficiency, allowing for higher charging and discharging rates, as well as high-temperature operation, supporting thermal management of insulation and self-heating. After the battery cell is enlarged, it is also convenient to implant multiple sensors, and has an ultra long cycle life without the phenomenon of diving. Another special point is that because solid electrolytes do not have continuous side reactions, the entire material system is not sensitive to impurities, and it is more convenient to support the dry electrode process, which may simplify the formation and aging process of the later stage, and also more conveniently support the pre lithiation process, thereby improving production efficiency and significantly reducing the cost of the battery. This is a possible advantage and advantage that all solid state batteries have.

However, there are still many technical challenges for different types of all solid-state batteries, including the four major categories of all solid-state batteries. The important issue with polymer all solid-state batteries is that they can only operate at high temperatures and are not resistant to oxidation. They can only be matched with lithium iron phosphate positive electrodes, resulting in low energy density. Thin film all solid-state batteries are difficult to produce with large capacity cells and have high manufacturing costs. Sulfide all solid-state batteries have very high ion conductivity and are also a global focus of attention. However, currently, sulfide materials are air sensitive, have high costs, and have poor solid solid contact inside the positive and negative electrodes. Oxide all solid-state (batteries), electrolyte ceramic sheets are prone to brittle cracking, high interface resistance, and difficult to prepare large capacity cells. Therefore, despite the high level of global attention and intensive research and development of all solid-state batteries, there are still many technical challenges, and various technologies are constantly being developed to improve the problems faced by all solid-state batteries.

Solid state batteries have become a global research focus, including teams from multiple Japanese companies, American startups, as well as teams from Canada, South Korea, and Europe. Various countries are developing sulfide all solid-state, solid-state metal lithium batteries, and polymer solid-state batteries. Chinese startups have developed the technology route of mixed solid-liquid electrolytes, combining the advantages of easy mass production of liquid electrolytes and the safer characteristics of solid-state batteries. The choice of this technology route is different from sulfides in Japan and South Korea, as well as metal lithium negative electrodes in the United States, making it easier to mass produce and significantly improving the safety of existing liquid electrolyte lithium battery products. Moreover, the technological development of these start-up companies has reached a stage close to mass production. Therefore, we say that although Japan, South Korea, Europe, and the United States have had early research and development and industrial layout in the field of all solid-state batteries, China has taken the lead in achieving large-scale production of solid-state batteries due to its choice of the hybrid solid-liquid battery route.

In all solid-state batteries and the core of solid-state batteries, it is necessary to solve a series of key issues related to materials. For sulfide electrolytes, the most difficult and urgent task is to reduce production costs and improve air stability. The Wu Fan team from the Institute of Physics of the Chinese Academy of Sciences and the Yangtze River Delta Physics Research Center has been committed to developing stable and water stable sulfide electrolytes in the air over the past three years, and has made significant progress. The use of this water stable and air stable sulfide electrolyte has led to the development of all solid-state batteries, which have good capacity as positive electrode materials and laid a crucial foundation for the development of sulfide and all solid-state batteries.

An important point in all solid-state batteries is to address the mechanical characteristics, hoping to maintain excellent interface contact between the positive and negative electrodes during the charging and discharging process. Therefore, everyone has proposed the development of composite electrolytes. Cui Guanglei's team from the Qingdao Institute of Energy, Chinese Academy of Sciences has developed a solid-state electrolyte composed of sulfide electrolytes and PEGMEA (polyethylene glycol methyl ether acrylate) for in-situ polymerization, which has relatively high ionic conductivity and low interface resistance, effectively solving the problem of poor interface contact during the cycling process. Therefore, it has achieved good cycling performance and reduced internal resistance.

In addition to the sulfide electrolytes that have received widespread global attention, our research and development team is also actively developing lower cost, high ion conductivity, and more stable electrolytes. The team led by Ma Cheng from China University of Science and Technology has pioneered the development of low-cost halogen based solid electrolytes for lithium zirconium chloride (Li2ZrCl6) internationally, which has very important application prospects. At the same time, Tang Weiping's team from the Space Power Institute has developed the oxide electrolyte with the highest room temperature ion conductivity at present. This oxide electrolyte has excellent stability and is also stable in the air. It is also stable for metallic lithium, and the constituent elements do not contain precious or rare elements, namely lithium zirconium silicon phosphorus oxide (Li3Zr2Si2PO12), This type of material may provide important options for safer and higher performance all solid-state batteries and hybrid solid-liquid batteries in the future.

The widely used oxide solid electrolytes include garnet structured lithium lanthanum zirconium oxide (Li7La3Zr2O12). The Institute of Physics of the Chinese Academy of Sciences conducted in-depth research on the stability of this material in air and understood the proton exchange reaction in air. At the same time, the Nan Cewen team from Tsinghua University also conducted in-depth research on the behavior of lithium dendrites penetrating oxide solid electrolytes when encountering metal lithium negative electrodes. Yu Xiqian's team from the Institute of Physics of the Chinese Academy of Sciences used neutron imaging technology to conduct in-depth research on the deposition behavior of metallic lithium in a solid-state electrolyte system with three-dimensional microstructure. They found that three-dimensional pore structure can alleviate volume expansion and inhibit the growth of lithium dendrites.

In addition, it is crucial to charge the battery to a high voltage in the development of solid-state and all solid-state batteries. However, after charging to a high voltage, the positive electrode has strong oxidation ability. How can this oxidation ability be reduced while allowing electron ion transport? After 2018, the Physics Institute team has been committed to developing solutions for ultra-thin solid-state electrolyte coated positive electrodes. At present, important progress has been made in high-voltage lithium cobalt oxide for consumer electronics and ternary materials for power, and it has been proved that the cathode coated with solid electrolyte has high thermal stability and electrochemical stability, which is a very important solution for high-voltage cathode materials, which is also an original in China.

On the negative side, to further improve energy density, many domestic and foreign teams have proposed solutions for metal lithium batteries without negative electrodes. The most important thing for non negative electrode lithium metal is to prevent lithium precipitation and control the deposition form of lithium. The Suo Liumin team of the Institute of Physics has adopted a liquid metal ultra-thin coating, significantly improving the deposition efficiency of lithium and preventing the growth behavior of lithium dendrites on the negative electrode. The prototype battery cell developed has also reached over 400Wh/kg.

The core purpose of developing solid-state batteries is to improve safety, but is solid-state batteries absolutely safe? This year's article in "Joule" has attracted widespread attention and discussion within the industry. Even in high specific energy lithium metal batteries, which are all solid-state, there is a thermal runaway behavior. In fact, this thermal runaway behavior has also been proven by a series of studies conducted by the Institute of Physics of the Chinese Academy of Sciences since 2020. Not all oxidized solid electrolytes are stable when encountering metallic lithium. Materials such as LATP and LAGP can still experience thermal runaway at higher temperatures when encountering metallic lithium. However, perovskite structured lithium lanthanum titanium oxide (Li0.33La0.56TiO3) and garnet structured lithium lanthanum zirconium oxide (Li7La3Zr2O12) exhibit higher and more stable behavior towards lithium, That is to say, solid-state batteries may not have all material systems without thermal runaway on the negative side. On the positive electrode side, we found that filling solid electrolytes into the positive electrode can significantly improve the safety of the positive electrode through oxygen exchange behavior. This also proves that using solid electrolytes on the positive electrode side can improve battery safety, which is a very important understanding.

Overall, so far, the inorganic solid electrolytes and raw materials of all solid-state batteries have not been mass-produced or formed a supply chain, polymer electrolytes cannot be directly matched with high-voltage positive electrode materials, and the interface resistance and low-temperature performance of all solid-state batteries are relatively high. In addition, in the existing design of all solid-state battery cells, the impact of volume changes during the cycling process has not been fully addressed, requiring higher external pressure during testing. In addition, there is no mature mass production equipment for electrodes and cells, and the integration and application methods of the power management system for cells are also immature. The understanding of the safety of full life cycle solid-state batteries is not comprehensive, testing and evaluation are not yet complete, and a standard system has not been formed. In addition, the current cost-effectiveness of solid-state batteries is also unclear. Therefore, we say that the mass production and commercialization of all solid-state batteries still require time to further deepen understanding, optimize materials, improve battery design and production technology, and gradually move towards commercial applications.

Since the development of all solid-state batteries is very difficult, another idea is, how can we make good use of the advantages of solid-state batteries? We have proposed the development idea of developing hybrid solid-liquid electrolyte batteries that are easy to engineer. This idea was not originally proposed by us, and there are many teams at home and abroad.

We have summarized that there are various ways to introduce solid electrolytes into the cell, including coating the material surface, adding separators and electrode pores, directly introducing the intermediate solid electrolyte separator layer, and converting liquid electrolytes into solid electrolytes through chemical and electrochemical reactions. There are five non conflicting methods. These are all accumulated from long-term research in the past. The Institute of Physics started researching and developing all solid-state batteries with Professor Chen Liquan in 1976, and then transitioned to liquid electrolyte lithium batteries. Professor Huang Xuejie led the team and founded Suzhou Xingheng. At present, we continue to develop hybrid solid-liquid and all solid-state batteries based on the original foundation. In particular, a relatively original in-situ solid-state solution was proposed, which involves partially or completely converting liquid electrolytes into solid electrolytes through chemical and electrochemical reactions. If all of them are converted into solid electrolytes, it is an all solid-state battery, which is a new research path for manufacturing all solid-state batteries. Through this method, we can be compatible with various existing positive and negative electrode materials, as well as most lithium battery materials. It can solve the problem of maintaining good contact between the solid electrolyte and positive and negative electrode materials during the cycling process, and comprehensively balance the requirements of high voltage charging, safety, lithium dendrite precipitation, and volume expansion control of the battery cell.

Based on these solutions, we have developed a series of various types of batteries, including 150Wh/kg intrinsically safe solid-liquid hybrid energy storage batteries for large-scale energy storage. These cells can pass national standard safety tests and significantly outperform national standards, including higher thermal runaway temperatures, higher extreme overcharging, and tests such as short circuits and punctures.

In addition, we have developed a 270Wh/kg high specific energy hybrid solid-liquid battery for drones, which currently has significant advantages in balancing energy density and safety. It has also passed the national standard for safety testing. In addition, we have developed a portable energy storage inverter power supply based on this solution, which is significantly higher than the current energy density level of modules in the industry.

In addition, we have developed a 300Wh/kg mixed solid-liquid power lithium battery. By comparing the needle penetration of liquid and mixed solid-liquid, the high specific energy battery cell can fully pass the needle penetration test at full charge, and can also pass the 150 degree heat box. In addition, on this basis, we tested its low-temperature performance and rate characteristics, which can meet the requirements of power lithium batteries very well.

On this basis, Beijing Weilan New Energy has further developed a 360Wh/kg power lithium battery with higher specific energy, which can also pass safety tests such as puncture, overcharging, and compression, meeting the requirements of electric vehicles. We will collaborate with NIO Automotive to begin mass production and application on the ET7 model, a hybrid solid-liquid electrolyte battery based on in-situ solid-state charging with a single charge of 1000 kilometers, a battery pack of 150 kWh, and a single unit of 360Wh/kg.

In addition, we have further developed cells with higher specific energy, including 400Wh/kg hybrid solid-liquid batteries, and developed innovative integrated solutions for cells and modules that can pass shooting experiments. This is the first time this has been achieved domestically and internationally, especially for modules. We participated in the National Future Energy Storage Challenge in 2020, and a series of indicators reached international excellent or leading levels. These tests were all the results of third-party testing conducted by the Fifth Institute of Electronics.

Overall, we believe that future batteries will develop towards higher specific energy, while the entire battery cell will shift from liquid to safer mixed solid-liquid and all solid-state batteries. The major directions include: higher specific energy battery cells based on high nickel and lithium rich manganese base positive electrodes, as well as nano silicon carbon negative electrodes and lithium carbon composite negative electrodes, which can meet the requirements of passenger vehicles with a range of 1000 km and electric aircraft; And a solution based on modified lithium manganese oxide, lithium iron phosphate, nickel manganese spinel as positive electrode materials, matched with high capacity negative electrode materials, for a range of 600 kilometers of pure electric vehicles; And the solution of sodium ion battery and solid lithium iron phosphate battery for lower cost energy storage applications. This is our view on the development path of future power lithium batteries and energy storage batteries.

To achieve mass production of solid-state batteries, it is necessary to create an industrial chain. For high specific energy batteries, we need to further optimize and develop new positive electrode materials, negative electrode materials, electrolyte materials, pre lithiation materials, super binders, conductive additives, and a new generation of metal deposition current collectors. At the same time, we need to develop new processes for the front, middle, and rear stages, and implement intelligent factories to combine extreme manufacturing and minimalist manufacturing, Forming the next generation of industrial 4.0 grade solid-state lithium battery industry chain.

On January 25th of this year, with the support of Chairman Dong Yang and Secretary General Xu Yanhua, and with the support of the China Automotive Power Lithium Battery Industry Innovation Alliance, the first solid state battery branch in China was established. It is hoped that with the promotion of the branch, the development of solid state batteries in China can be promoted, and the core competitiveness of solid state lithium batteries in China can be continuously promoted and improved.

Special thanks to all colleagues from Institute of Physics, Chinese Academy of Sciences, Beijing Weilan New Energy, Tianmu Pilot Battery Materials, Tianmu Lake Advanced energy storage Research Institute, and Zhongke Haina for their help and contributions.

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