1. Overview of solid-state lithium batteries All-solid-state lithium batteries are lithium batteries that use solid electrode materials and solid electrolyte materials and do not contain any liquid. They mainly include all-solid-state lithium-ion batteries and all-solid-state metal lithium batteries. The difference is that the former does not have a negative electrode Contains metallic lithium, and the latter is metallic lithium as the negative electrode. Among the various current new battery systems, solid-state batteries use new solid-state electrolytes to replace the current organic electrolytes and separators. They have high safety, high volume energy density, and are compatible with different new high-specific-energy electrode systems (such as lithium-sulfur systems, metal- Air system, etc.) has wide adaptability and can further improve the mass energy density, which is expected to become the ultimate solution for the next generation of power batteries, attracting widespread attention from many research institutions, start-up companies and some car companies in Japan, the United States, Germany, etc.

　　2. Advantages of solid-state lithium batteries and current technical shortcomings Compared with traditional lithium-ion batteries, solid-state lithium batteries have significant advantages:

　　(1) High safety performance: Traditional lithium-ion batteries use organic liquid electrolytes. Under abnormal conditions such as overcharging and internal short circuits, the batteries are prone to heat, causing the electrolyte to swell, spontaneously ignite or even explode, posing serious safety risks. Many inorganic solid electrolyte materials are non-flammable, non-corrosive, non-volatile, and have no leakage problems. Compared with liquid electrolytes containing flammable solvents, polymer solid electrolytes have greatly improved battery safety.

　　(2) High energy density: Metal lithium can be used as the negative electrode of solid-state lithium batteries, and the battery energy density is expected to reach 300-400Wh/kg or even higher; its electrochemical stability window can reach above 5V, and can be matched with high-voltage electrode materials to further improve quality Energy density; there is no liquid electrolyte and separator, which reduces the weight of the battery, compresses the internal space of the battery, and improves the volumetric energy density; safety is improved, and the battery shell and cooling system module are simplified, improving the system energy density.

　　(3) Long cycle life: It is expected to avoid the problem of the liquid electrolyte continuing to form and grow the SEI film during the charge and discharge process and the problem of lithium dendrites piercing the separator, greatly improving the cycleability and service life of metal lithium batteries.

　　(4) Wide operating temperature range: Solid-state lithium batteries have excellent acupuncture and high-temperature stability. If all inorganic solid electrolytes are used, the maximum operating temperature is expected to reach 300°C, thereby avoiding the possibility of causing positive and negative electrode materials to react with the electrolyte at high temperatures. thermal runaway.

　　(5) Improved production efficiency: There is no need to encapsulate liquid, and it supports serial stacking arrangement and bipolar mechanism, which can reduce the ineffective space in the battery pack and improve production efficiency.

　　(6) Advantages of flexibility: All-solid-state lithium batteries can be prepared into thin-film batteries and flexible batteries. Compared with flexible liquid electrolyte lithium batteries, packaging is easier and safer. In the future, they can be used in smart wearables and implantable medical devices. Although all-solid-state lithium batteries have obvious advantages in many aspects, there are also some urgent problems that need to be solved: For the research and development of all-solid-state batteries, the core to solve the above problems lies in the development of solid-state electrolyte materials and the control and optimization of interface properties.

　　3. Technical paths and research hot spots of solid-state lithium batteries 3.1 Technical paths of solid electrolyte materials The performance of electrolyte materials largely determines the power density, cycle stability, safety performance, high and low temperature performance and service life of the battery. Common solid electrolytes can be divided into two categories: polymer electrolytes and inorganic electrolytes. Polymer solid electrolyte Because polyoxyethylene (PEO) has a stronger ability to dissociate lithium salts than other polymer matrices and is stable to lithium, the current research focus is on PEO and its derivatives. The polymer electrolyte has poor ability to wet the electrode. The active material must be transferred to the electrode surface through the pole piece to deintercalate lithium. As a result, the capacity of the active material in the pole piece cannot be fully exerted during battery operation. The electrolyte material is mixed into the electrode material or replaces the adhesive. The binder is prepared into composite electrode materials to fill the gaps between electrode particles and simulate the electrolyte wetting process. It is an effective method to improve the lithium ion migration ability in the pole piece and the battery capacity. Due to the high crystallinity of PEO-based electrolytes, the conductivity at room temperature is low, so the operating temperature usually needs to be maintained at 60 to 85°C, and the battery system needs to be equipped with a special thermal management system. In addition, PEO has a narrow electrochemical window and is difficult to match with high-energy-density cathodes, so it needs to be modified. At present, BOLLORE's PEO-based electrolyte solid-state battery, which is the most mature, has been put into commercial use and has been put into urban rental cars in a small amount in the UK. Its operating temperature is required to be 60~80°C, and the positive electrode uses LFP and LixV2O8. However, the current Pack energy density is only 100Wh/kg. Inorganic solid electrolytes Inorganic solid electrolytes mainly include oxides and sulfides. Oxide solid electrolytes can be divided into two categories: crystalline and amorphous according to their material structure. The research focus is LiPON electrolyte used in thin film batteries. Oxide batteries prepared with LiPON as electrolyte material have excellent rate performance and cycle performance. However, the positive and negative electrode materials must be made into thin film electrodes using methods such as magnetron sputtering, pulsed laser deposition, and chemical vapor deposition. At the same time, they cannot be used like ordinary lithium. The ion battery process also adds conductive materials, and the electrolyte cannot wet the electrode, making the electrode's lithium ion and electron migration ability poor. Only when the positive and negative electrode layers are ultra-thin can the battery resistance be reduced. Therefore, the single cell capacity of inorganic LiPON thin-film solid-state lithium batteries is not high and is not suitable for the field of preparing Ah-level power batteries. The sulfide solid electrolyte is derived from the oxide solid electrolyte. Since the electronegativity of the sulfur element is smaller than that of the oxygen element, it has less constraints on lithium ions, which is beneficial to obtaining more freely moving lithium ions.

　　At the same time, the radius of sulfur element is larger than that of oxygen element, which can form a larger lithium ion channel to improve conductivity. Currently, Samsung, Panasonic, Hitachi Zosen + Honda, and Sony are all conducting research and development of sulfide inorganic solid electrolytes. However, the challenges brought by air sensitivity, easy oxidation, high interface resistance, and high cost are not easy to be completely solved in the short term. Therefore, there is still a long way to go before the final application of all-solid-state lithium batteries with sulfide electrolytes. In short, inorganic solid electrolytes take advantage of single ion conduction and high stability and are used in all-solid-state lithium-ion batteries. They have the advantages of high thermal stability, resistance to combustion and explosion, environmental friendliness, high cycle stability, and strong impact resistance. It is also expected to be used in new lithium-ion batteries such as lithium-sulfur batteries and lithium-air batteries, which is the main direction of electrolyte development in the future. 3.2 Regulation and Optimization of Interface Performance The solid electrolyte has a large interface impedance with the electrodes and poor interface compatibility. At the same time, the volume expansion and contraction of each material during the charge and discharge process causes the interface to easily separate and other problems. The use of lithium metal negative electrodes also has problems such as large solid-phase contact resistance, interface reactions, and low efficiency. The main directions for solutions at present are as follows: 4. Industrialization progress of solid-state lithium batteries 4.1 Foreign giants have laid out the solid-state lithium battery industry. In order to make lithium batteries have higher energy density and better safety, foreign lithium-ion battery manufacturers and research institutes are located A lot of research and development work has been carried out in solid-state lithium batteries. Japan has elevated the research and development of solid-state batteries to a national strategic level. In May 2017, the Ministry of Economy of Japan announced an investment of 1.6 billion yen, joining forces with Toyota, Honda, Nissan, Panasonic, GS Yuasa, Toray, Asahi Kasei, Mitsui Chemicals, and Mitsubishi Chemical and other top domestic industry chain forces are jointly developing solid-state batteries, hoping to achieve the 800-kilometer battery life goal by 2030. The EV "Bluecar" of the French Bollore company is equipped with a 30kwh metal lithium polymer battery produced by its subsidiary Batscap, using the Li-PEO-LFP material system. About 2,900 Bluecars are used by the Paris car sharing service "Autolib", which is the first time in the world Commercial all-solid-state battery for EV. Toyota has developed an all-solid-state lithium-ion battery with an energy density of 400Wh/kg, and plans to commercialize it in 2020; Panasonic’s latest solid-state battery has a relatively increased energy density of 3 to 4 times; German KOLIBRI batteries are used in Audi A1 pure electric vehicles. It has not yet been commercially applied.

　　In addition, several companies such as Samsung, Mitsubishi, BMW, Hyundai, and Dyson have also stepped up their efforts to develop solid-state battery reserves through independent research and development or combined mergers and acquisitions. Toyota announced that it is cooperating with Panasonic to develop solid-state batteries; BMW announced that it is cooperating with SolidPower to develop solid-state lithium batteries; Bosch has jointly established a new factory with Japan's famous GSYUASA (Yuasa) Battery Company and Mitsubishi Heavy Industries, focusing on solid-state anode lithium-ion batteries; Honda and The organization established by Hitachi Zosen has developed Ah-class batteries and is expected to mass-produce them in three years. 4.2 Domestic research institutions take the lead in getting involved in the solid-state lithium battery industry. Basic research on solid-state lithium batteries in my country started early. During the "Sixth Five-Year Plan" and "Seventh Five-Year Plan", in addition, Peking University, China Electronics Technology Group Tianjin 18th Institute and other institutes also established projects to conduct research on solid lithium electrolytes. Domestic companies that are developing solid-state lithium batteries include CATL, Guojia Interstellar (Jiawei Co., Ltd.), Jiangsu Qingtao Energy, Taiwan Huineng, AVIC Lithium Battery, etc. CATL focuses on sulfide electrolytes as its main research and development direction, and uses cathode coating to solve the interface reaction problem between cathode materials and solid electrolytes. Currently, polymer lithium metal solid-state batteries have a cycle cycle of more than 300 cycles and a capacity retention rate of 82%. Qingtao Energy has developed high-solid-content all-ceramic separators and inorganic solid electrolytes, and has currently cooperated with BAIC for pilot testing. Guojia Interstellar uses material genome technology to determine the optimal composition of polymer solid electrolytes through high-throughput testing technology. In addition, companies such as Ganfeng Lithium, BYD, and Wanxiang 123 have also announced their deployment in the solid-state battery field, but most companies are still in the "verbal research and development" stage.

　　5. Prospects for the Solid-State Lithium Battery Industry Currently, there are two research and development directions for solid-state batteries. One is the solidification of lithium-ion batteries. Other industries have mature solutions in this direction, but grafting it to lithium batteries requires secondary research and development. There are very few companies that mass-produce solid-state electrolytes abroad, and none in China, which to a certain extent restricts the progress of solid-state battery research and development. Domestic universities and scientific research institutes have already had samples of gel-state batteries successfully developed by Japanese laboratories, but most of them are stuck at the level where the energy ratio meets the standard and the cycle is only a few hundred times. In addition, the cost is very high, and the yield rate is very low and cannot be measured. Produce. Another technology research and development direction is metal solid-state batteries, the most common of which is lithium-sulfur batteries. When the electrolyte is replaced by a solid, the lithium battery system transforms from the solid-liquid interface of the electrode material-electrolyte to the solid-solid interface of the electrode material-solid electrolyte. There is no wettability between the solids, and the interface is prone to higher contact resistance. The battery cycleability will become worse and charging will not be fast. The production environment of lithium-sulfur batteries is a vacuum. Once mixed with oxygen, they will explode, which brings great challenges to equipment companies. As one of the future battery technology directions to replace traditional lithium batteries, all-solid-state lithium batteries have attracted many domestic and foreign research institutions and companies to conduct research and development. However, there is still a long way to go in terms of solid electrolyte materials, interface performance optimization, electrode material selection, cost, and technology. There is still a long way to go. Both the production process and the surrounding environment of the production line require a large amount of capital investment and strict parameter control. For backward start-up companies, it is a long way from the laboratory to the mass production line. Far from expensive. Of course, in the face of its huge commercial value space, more excellent car manufacturers and battery companies like BMW will definitely invest in it. I believe that with the promotion and deepening of R&D technology, the pace of solid-state battery industrialization will gradually accelerate.

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