Solid state batteries are the next new direction of energy, and major companies around the world have made technological breakthroughs. However, it is still difficult to mass produce them in 2024, as there are issues such as energy density, safety, and cost that cannot be balanced, and complex preparation processes that are difficult to mass produce.
All solid state batteries are batteries with completely solid electrode and electrolyte materials, which have advantages such as high safety, high energy density, and long lifespan. They are considered the most likely new generation battery technology to replace traditional liquid state batteries. Major economies such as China, Japan, South Korea, the European Union, and the United States have all introduced relevant plans and policies to promote the development of all solid state batteries. Major car companies, battery manufacturers, and startups around the world are actively expanding their research and development of all solid state batteries, and have made significant progress. However, there are still many challenges that need to be overcome, and industrialization still takes time.
Energy density, safety, and cost cannot be balanced
At present, the main positive electrode materials include lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, and ternary materials. China's positive electrode materials are mainly lithium iron phosphate and ternary materials, and Japan's Toyota, Nissan, and South Korea's Samsung SDI, which have made rapid industrialization progress, have all adopted high nickel ternary positive electrodes. The electrochemical window of all solid electrolyte is wide, and positive electrode materials with higher voltage platforms can be used. The theoretical capacity of lithium rich manganese based cathode can reach 320 milliampere hours/gram, with a voltage platform of 3.7-4.6 volts, which is significantly higher than traditional low nickel ternary and lithium iron phosphate cathode materials. It is considered an ideal cathode material for all solid state batteries.
However, positive electrode materials still face challenges in technological development that cannot balance energy density, safety performance, and cost. Insufficient solid solid interface contact between the positive electrode material and the solid electrolyte can lead to an increase in charge impedance during charging and discharging, affecting battery performance. Coating, spraying and other technologies can improve interface issues, but complex operations and high production costs can hinder the industrialization of all solid state batteries.
Negative electrode materials mainly include graphite, silicon carbon, silicon oxide, lithium titanate, etc. Lithium metal has a high specific capacity (3861 milliampere hours/g), low electrochemical potential (-3.04 volts, relative to standard hydrogen electrodes), and a small density (0.534 grams/cubic centimeter), making it an ideal negative electrode material for the next generation of high specific energy and rechargeable batteries. However, the high activity of lithium metal and the high diffusion energy barrier of lithium ions in its surface passivation layer can promote the formation of lithium dendrites, which can cause short circuits and cause battery failure. Researchers need to fully understand the mechanism of solid-state dendrite formation and growth, and address related issues. In addition, the difficulty of all solid state lithium metal batteries lies in increasing the number of battery cycles. At present, a 500 watt hour/kilogram lithium metal battery only has about a few dozen cycles.
Each of the four types of solid electrolytes has its own advantages and disadvantages
The commonly used solid electrolytes include four types: sulfides, oxides, polymers, and halides, each with its own advantages and disadvantages.
Sulfide electrolytes have high conductivity, good mechanical properties, and thermal stability. Compared to oxide electrolytes, sulfide electrolytes have better compatibility with electrodes and moderate processing costs, making them a type of electrolyte material with better comprehensive performance. They are also a technology field that major economies such as China, Japan, South Korea, and Europe and America are generally concerned about and actively layout. The poor stability of water and oxygen is the most prominent problem of sulfide solid electrolytes. When exposed to a water oxygen environment, it will produce harmful gas hydrogen sulfide, causing damage to the electrolyte structure and reduced electrochemical performance. As a result, its synthesis, storage, transportation, and post-treatment processes heavily rely on inert atmospheres or drying rooms, which not only increases the complexity of environmental control but also increases production costs. Therefore, the development of new materials, material coating, and composite with polymers are key areas for addressing water oxygen stability issues.
Solid oxide electrolytes have good chemical stability and high thermal stability, and many companies choose this type of technology route. However, many Chinese enterprises choose semi-solid oxide electrolytes, and many leading solid-state battery companies such as Huineng Technology, Qingtao Energy, and Ganfeng Lithium are vigorously promoting the technology of oxide solid-liquid hybrid batteries. This is because compared to all solid state battery technology, semi-solid state battery technology is compatible with the process equipment of traditional liquid state batteries and is easier to mass produce. Semi solid state batteries (with a liquid electrolyte mass ratio of less than 10%) and quasi solid state batteries (with a liquid electrolyte mass ratio of less than 5%) can serve as transition routes for all solid state batteries. The high interface impedance is the biggest obstacle to the development of oxide solid-state batteries. Construction of interface modification layers is often used to improve interface wettability, or techniques such as discharge plasma sintering and hot pressing are used to densify solid electrolytes, thereby reducing interface impedance.
Polymer solid electrolytes have the characteristics of small mass, good elasticity, easy processing, and low cost, and are the main technical route chosen by some European and American enterprises in the early stages. Europe was the earliest region to promote the industrialization of polymer all solid state batteries. Bollore, a French company, achieved the installation and application of polymer solid-state batteries in thousands of vehicles in 2011. In 2023, American startup Factory Energy announced the official production of a 200 megawatt hour polymer solid-state lithium battery pilot line and sent samples for testing to Stellantis Automotive Company. Low electrical conductivity, thermal stability, and safety at room temperature are the main problems of polymer solid electrolytes. The main reason for the low conductivity at room temperature is that the ion transport of polymers mainly occurs in the amorphous region, which has a high crystallinity at room temperature but a softening temperature higher than 60 degrees Celsius. In addition, when the temperature exceeds 400 degrees Celsius, the polymer solid electrolyte will decompose and burn, posing significant safety hazards. Composite with inorganic fillers, cross-linking modification or blending modification, and the introduction of flame retardants can solve the above problems.
As an emerging type of inorganic solid electrolyte material, halide solid electrolytes have advantages over sulfide solid electrolytes in terms of low cost, environmental friendliness, and good stability of high-voltage positive electrodes. Further solutions are needed to address issues such as low conductivity of halide solid electrolytes, poor compatibility of positive electrode materials, and poor stability in air/humid environments.
The preparation process is complex and difficult to mass produce
All solid state batteries have certain advantages in terms of safety and reliability, but their preparation process is more complex. The electrolyte film-forming process is a key process in the manufacturing of solid-state batteries, which can be divided into wet process and dry process depending on whether solvents are used. Currently, all solid state batteries can to some extent continue to use wet processes, with a compatibility rate of about 50-60% with the existing industry chain, and a lower compatibility rate with dry processes. According to different carriers, wet process can be divided into mold supported film formation, positive electrode supported film formation, and skeleton supported film formation. This process first pours the solid electrolyte solution onto the mold, then evaporates the solvent to obtain a solid electrolyte film. The thickness of the film is controlled by adjusting the volume and concentration of the solution.
Solid state batteries still face significant challenges in engineering manufacturing and mass production. One is to achieve complete densification of the solid-state battery structure and improve interface issues, which requires special high-temperature and high-pressure (hundreds of megapascals) processes. However, there is currently no relevant equipment in China that can meet the requirements of high temperature and high pressure, and there is an urgent need for development and optimization. The second is that solid-state battery manufacturing has high requirements for the process environment. For example, in terms of humidity, solid-state batteries need to be much higher than the humidity requirements of existing liquid batteries in order to avoid the release of hydrogen sulfide gas in contact with moisture. Therefore, there are still significant difficulties in large-scale production. Thirdly, the consistency of solid-state battery technology is required to be higher, and large-scale production requires strong engineering capabilities and manufacturing experience, resulting in higher production costs. Therefore, further cost reduction is needed.