Research and development of aa battery and their current development status

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 contain metal lithium at the negative electrode, while the latter has metal lithium at the negative electrode.

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 contain metal lithium at the negative electrode, while the latter has metal lithium at the negative electrode.

Among the various new battery systems currently available, aa battery use new solid electrolytes to replace current organic electrolytes and diaphragms. They have high safety and high volume energy density. At the same time, they are widely compatible with different new high-energy-density electrode systems (such as lithium-sulfur systems, metal-air systems, etc.), which can further improve mass energy density, and are expected to become the ultimate solution for the next generation of power batteries, attracting widespread attention from many research institutions, start-ups and some car companies in Japan, the United States, Germany and other countries.

2. Advantages of solid-state lithium batteries and existing technical defects

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 battery is prone to heat up, causing electrolyte bloating, spontaneous combustion or even explosion, posing serious safety hazards. However, many inorganic solid electrolyte materials are non-flammable, non-corrosive, non-volatile, and do not have leakage problems. Compared with liquid electrolytes containing flammable solvents, polymer solid electrolytes have greatly improved battery safety.

(2) High energy density: The negative electrode of solid-state lithium batteries can use metallic lithium, and the battery energy density is expected to reach 300-400Wh/kg or even higher; its electrochemical stability window can reach more than 5V, which can match high-voltage electrode materials and further improve mass energy density; there is no liquid electrolyte and diaphragm, which reduces the weight of the battery, compresses the internal space of the battery, and improves the volume energy density; safety is improved, the battery shell and cooling system modules are simplified, and the system energy density is improved.

(3) Long cycle life: It is expected to avoid the problem of continuous formation and growth of SEI film and lithium dendrite piercing the diaphragm during the charge and discharge process of liquid electrolyte, greatly improving the cycle life and service life of metal lithium batteries.

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

(5) Improved production efficiency: No need to encapsulate liquid, support serial stacking arrangement and bipolar structure, which can reduce the invalid space in the battery pack and improve production efficiency.

(6) Possessing flexibility: All-solid-state lithium batteries can be prepared into thin-film batteries and flexible batteries. Compared with flexible liquid electrolyte lithium batteries, encapsulation 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-aa battery, the key to solving the above problems lies in the development of solid electrolyte materials and the regulation and optimization of interface properties.

3. Technical paths and research hotspots 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 electrolytes

Since polyoxyethylene (PEO) has a stronger ability to dissociate lithium salts than other polymer matrices and is stable to lithium, the current research hotspots are mainly PEO and its derivatives.

Polymer electrolytes have poor electrode wetting ability, and the active material must be transferred to the electrode surface through the pole piece to deintercalate lithium, so that the capacity of the active material in the pole piece cannot be fully utilized during the battery operation. Mixing electrolyte materials into electrode materials or replacing binders to prepare composite electrode materials, filling the gaps between electrode particles, and simulating the electrolyte wetting process is an effective way 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 is low at room temperature, so the operating temperature usually needs to be maintained at 60-85°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 positive electrodes, so it needs to be modified.

Currently, the most mature BOLLORE PEO-based electrolyte solid-state battery has been commercialized and put into a small number of urban rental cars in the UK. Its operating temperature requires 60-80°C, and the positive electrode uses LFP and LixV2O8, but 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 according to the material structure: crystalline and amorphous. Among them, the research hotspot is the LiPON type electrolyte used in thin film batteries.

The oxide battery prepared with LiPON as the electrolyte material has excellent rate performance and cycle performance, but the positive and negative electrode materials must be made into thin film electrodes by magnetron sputtering, pulsed laser deposition, chemical vapor deposition and other methods. At the same time, conductive materials cannot be added like ordinary lithium-ion battery processes, and the electrolyte cannot infiltrate the electrode, making the lithium ion and electron migration ability of the electrode 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 battery is not high, and it is not suitable for the preparation of Ah-level power battery field.

Sulfide solid electrolyte is derived from oxide solid electrolyte. Since the electronegativity of sulfur is smaller than that of oxygen, it has less binding on lithium ions, which is conducive to obtaining more freely moving lithium ions. At the same time, the radius of sulfur is larger than that of oxygen, which can form a larger lithium ion channel to improve conductivity. At present, Samsung, Panasonic, Hitachi Shipbuilding + 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, so there is still a long way to go before the all-solid-state lithium battery with sulfide electrolyte can be finally applied.

In short, inorganic solid electrolytes play the advantages of single ion conduction and high stability. They are used in all-solid-state lithium-ion batteries. They have the advantages of high thermal stability, not easy to burn and explode, environmentally friendly, high cycle stability, and strong impact resistance. At the same time, they are expected to be used in new lithium-ion batteries such as lithium-sulfur batteries and lithium-air batteries, which is the main direction of future electrolyte development.

3.2 Regulation and optimization of interface performance

Solid electrolytes have large interface impedance between electrodes and poor interface compatibility. At the same time, the volume expansion and contraction of each material during charging and discharging leads to easy interface separation. The use of lithium metal negative electrodes also has problems such as large solid-phase contact impedance, interface reaction, and low efficiency. The main directions for solving the problem are as follows:

IV. Industrialization progress of solid-state lithium batteries

4.1 Foreign giants have laid out 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 have carried out a lot of research and development work in solid-state lithium batteries. Japan has even elevated the research and development of aa battery to a national strategic level. In May 2017, the Japanese Ministry of Economy announced an investment of 1.6 billion yen to jointly develop aa battery with domestic industrial chain forces such as Toyota, Honda, Nissan, Panasonic, GS Yuasa, Toray, Asahi Kasei, Mitsui Chemicals, and Mitsubishi Chemical, hoping to achieve a range of 800 kilometers by 2030.

The EV "Bluecar" of the French company Bollore is equipped with a 30kwh metal lithium polymer battery produced by its subsidiary Batscap, which adopts the Li-PEO-LFP material system. The Paris car-sharing service "Autolib" uses about 2,900 Bluecars. This is the world's first commercial all-solid-state battery for EV. Toyota has developed an all-solid-state lithium-ion battery with an energy density of 400Wh/kg, which is planned to be commercialized in 2020; Panasonic's latest solid-state battery has a 3-4 times higher energy density; Germany's KOLIBRI battery is used in Audi A1 pure electric vehicles, but has not yet been commercialized.

In addition, several companies such as Samsung, Mitsubishi, BMW, Hyundai, and Dyson have also stepped up the layout of solid-state battery reserve research and development through independent research and development or combined mergers and acquisitions. Toyota announced that it will cooperate with Panasonic to develop aa battery; BMW announced that it will cooperate with SolidPower to develop solid-state lithium batteries; Bosch and Japan's famous GSYUASA (Yuasa) Battery Company and Mitsubishi Heavy Industries jointly established a new factory, focusing on solid-state anode lithium-ion batteries; Honda and Hitachi Zosen have developed Ah-class batteries and are expected to be mass-produced in three years.

4.2 Domestic research institutions are leading the solid-state lithium battery industry

my country started early in the basic research of solid-state lithium batteries. During the "Sixth Five-Year Plan" and "Seventh Five-Year Plan", the Chinese Academy of Sciences listed solid-state lithium batteries and fast ion conductors as key topics. At present, five research and development teams have made different progress. In addition, Peking University, China Electronics Technology Group Tianjin 18th Institute and other institutions have also launched projects to study solid-state lithium battery electrolytes.

Domestic companies that are developing solid-state lithium batteries include CATL, Guojia Xingji (Jiawei Co., Ltd.), Jiangsu Qingtao Energy, Huineng, and AVIC Lithium Battery. CATL takes sulfide electrolyte as the main research and development direction, and uses positive electrode coating to solve the interface reaction problem between positive electrode materials and solid electrolytes. At present, the polymer lithium metal solid-state battery cycle has reached more than 300 weeks, and the capacity retention rate has reached 82%. Qingtao Energy develops high-solid content all-ceramic diaphragms and inorganic solid electrolytes, and has cooperated with BAIC for pilot testing. Guojia Star uses material genome technology to determine the optimal composition of polymer solid electrolytes through high-throughput testing technology. In addition, Ganfeng Lithium, BYD, Wanxiang 123, etc. have also announced the layout of the solid-state battery field, but most companies are still in the "verbal research and development" stage.

V. Outlook of the Solid-State Lithium Battery Industry

At present, there are two research and development directions for aa battery. One is the solidification of lithium-ion batteries. Other industries have mature solutions for this direction, but grafting to lithium batteries requires secondary research and development. There are very few companies that mass-produce solid electrolytes abroad, and there is no one in China, which to a certain extent restricts the research and development progress of aa battery. The gel battery successfully developed by the Japanese laboratory has long been available in domestic universities and research institutes, but most of them remain at the level of meeting the energy ratio standard and only a few hundred cycles. In addition, the cost is very high and the yield is very low, so it cannot be mass-produced.

Another technical research and development direction is metal aa battery, the most common of which is lithium-sulfur batteries. When the electrolyte is replaced with 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 form a higher contact resistance, the battery cycle will deteriorate, and charging will not be fast. The production environment of lithium-sulfur batteries is a vacuum, and once mixed with oxygen, it 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 enterprises 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. Whether it is the production process or the surrounding environment of the production line, it requires a lot of capital investment and strict parameter control. For latecomer start-ups, the road from the laboratory to the mass production line is long, far, and expensive. Of course, in the face of its huge commercial value space, there will definitely be more excellent automakers and battery companies like BMW to 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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