It is not new for OEMs to invest in batteries, and Volkswagen CEO Murren also said that they are already planning the next generation of electric vehicle batteries, solid-state batteries with a range of more than 1,000 kilometers, which will be mass-produced in 2025.
Toyota has made new progress on solid-state battery patents, while announcing that it will launch a new electric vehicle with an all-solid-state battery in 2022. CATL, a leading power battery company in my country, has also set an example. It has carried out related research and development work in the direction of polymer and sulfide-based solid-state batteries and made preliminary progress, and proposed a preliminary process route for large-scale production.
Solid-state batteries have the core advantages of taking into account both safety and high energy density. The energy density of liquid electrolyte batteries can be as high as 300 watt-hours per kilogram, which can meet the goals put forward by the state in the "Action Plan for Promoting the Development of the Power Battery Industry", but it is considered impossible to exceed 500 watt-hours per kilogram. The safety of solid-state batteries is shown in that they can inhibit lithium dendrites, are not easy to burn, are not easy to explode, have no electrolyte leakage, and do not cause side reactions at high temperatures.
In liquid electrolytes, the problems of spontaneous chemical side reactions, lithium dendrite growth, unstable interfacial films, and large volume changes faced by metal lithium anodes are still difficult to solve simultaneously. Many research teams have proposed to use solid electrolytes to replace liquid electrolytes in whole or in part to solve the main technical challenges faced by batteries that use or contain metallic lithium anodes.
Japan's NEDO formulated a research and development plan as early as 2008, and planned to achieve mass production of solid-state batteries in 2030, including solid-state metal lithium, solid-state lithium-sulfur and solid-state lithium-air batteries. Solid-state batteries are divided into three categories in terms of electrolyte morphology, one is pure polymers, such as polyethylene oxide; one is oxides or sulfides of inorganic solid electrolytes; the third is a combination of polymers and inorganics. The most difficult problem for these three solid electrolytes is that in lithium-ion batteries or future metal lithium batteries, after repeated volume expansion and contraction of the positive electrode, the contact with the solid electrolyte phase will gradually deteriorate. For solid-state batteries, it is how to maintain low electronic and ionic impedance during cycling. If there is no better way, a small amount of liquid can also be added to these three types of electrolytes to solve the problem of electrical contact deterioration during cycling. This type of electrolyte can be called a mixed solid-liquid electrolyte, which means that the cell also contains a solid electrolyte. and liquid electrolytes.
In terms of solid electrolyte materials, many classes have been developed internationally, mainly including oxides, sulfides, hydrides, halogens, phosphate films and polymers. There are three mainstream electrolyte materials:
The first is the oxide solid electrolyte, which uses an inorganic ceramic electrolyte to replace the liquid electrolyte, mainly to solve the problem of filling and contact on the positive side, which may require very complex surface coating technology.
For sulfide electrolytes, the ionic conductivity is very high, and it is also necessary to solve the problem of increasing resistance on the positive electrode side, and at the same time solve the problems of poor chemical stability and generation of hydrogen sulfide during preparation, storage, and service.
For thin-film electrolytes, although the ionic conductivity is very low, it is possible to prepare and use devices by reducing the sheet resistance by thinning them. However, it is still very challenging to make large-capacity batteries with large-area stacks.
At present, the large-capacity solid-state batteries that have been commercialized are mainly polymer solid-state batteries, which are polyethylene oxide-based solid-state electrolytes. The polymer electrolyte and the 30-micron-thick lithium iron phosphate positive electrode can be cycled more than 1,000 times at 1/3C. The working temperature is between 60 and 85 degrees. The battery pack needs to have a heating and heat preservation function.