Solid-state batteries keep on attracting tremendous attention and investment with the maturing technologies and closeness to mass production. Even with the influence of COVID-19, the potential market size is expected to grow to over $8 billion by 2031, according to IDTechEx
The mature lithium-ion battery technology has its intrinsic limitations
Since their commercialization in 1991, lithium-ion batteries have received worldwide success. However, this cannot hide their intrinsic limitations in terms of safety, performance, and form factor.
Most current lithium-ion technologies employ liquid electrolytes, with lithium salts such as LiPF6, LiBF4, or LiClO4 in an organic solvent. However, the solid electrolyte interface, which is caused as a result of the decomposition of the electrolyte at the negative electrode, limits the effective conductance. Furthermore, liquid electrolytes need membranes to separate the cathode and anode, as well as an impermeable casing to avoid leakage. Therefore, the size and design freedom for these batteries are constrained. In addition, liquid electrolytes have safety and health issues as they use flammable and corrosive liquids. Samsung’s Firegate has particularly highlighted the risks that even large companies incur when flammable liquid electrolytes are used, as well as the dozens of electric vehicle fire accidents caused by battery combustion.
Current high-end lithium-ion batteries can reach an energy density of over 700 Wh/L at cell level, with a maximum driving range of about 500 Km for electric vehicles. The high-nickel-cathode materials being improved may further push the energy density but the characteristics of the active materials may draw a threshold.
Solid-state batteries can make a difference
Solid-state batteries replace organic liquid electrolytes with solid-state counterparts, enabling safer, long-lasting batteries. In addition, solid-state electrolytes can be compatible with high voltage-cathode materials and high-capacity lithium metal anode. It is possible to push the energy density beyond 1,000 Wh/L.
Better safety means less safety monitoring electronics in the battery modules/packs. Therefore, even the initial generations of solid-state batteries may have similar or even less energy density than conventional lithium-ion batteries. The energy available in the battery pack can be comparable or even higher than the latter.
With the larger electrochemical window that the solid electrolytes can provide, high voltage cathode materials can be used. In addition, high-energy-density lithium metal anode can further push the energy density beyond 1,000 Wh/L. These features can further make the solid-state battery a game-changer.
Within solid-state batteries, there are various technology approaches. IDTechEx has identified solid-state battery technologies including solid polymer electrolyte, as well as other 8 inorganic electrolyte types such as LISICON-like, argyrodites, garnet, NASICON-like, Perovskite, LiPON, Li-Hydride, and Li-Halide. In general, oxide, sulfide, and polymer systems have become popular options in next-generation development. Sulfide electrolytes have advantages of high ionic conductivity, even better than liquid electrolytes, low processing temperature, wide electrochemical stability window, etc. Many features make them appealing, being considered by many as the ultimate option. However, the difficulty of manufacturing and the toxic by-product hydrogen sulfide generated in the process make the commercialization relatively slow. Polymer systems are easy to fabricate, and they are closest to commercialization, while the relatively high operating temperature, low anti-oxide potential, and worse stability indicate challenges. Oxide systems are stable and safe, while the higher interface resistance and high processing temperature show some difficulties in general.
The race between polymer, oxide, and sulfide systems is unclear so far and it is common to see battery companies trying multiple approaches. Complex technology approaches make the decision difficult.
What is next?
There have been numerous press releases on the progress of solid-state batteries and electric vehicle prototypes equipped with solid-state batteries. It seems the focus on solid-state batteries will mainly be battery cells. However, this will only be part of the future focus. Another important trend is the cell to pack (CTP) approach.
CTP is not a new concept and it can be found in designs based on lithium-ion batteries already, such as BYD’s blade battery and CATL’s CTP designs. The CTP concept will become more important due to the better safety of solid-state battery cells. Better battery safety means a more flexible pack design and fewer electronics/components used in the battery module/packs. For instance, the bipolar design enables higher energy density. It is possible that initial generations of solid-state batteries may not equip with high-voltage, high-capacity cathode, and lithium metal anode, thus leading to energy densities less than commercial lithium-ion batteries. It is also expected that the initial costs of solid-state battery cells are higher than lithium-ion counterparts. However, with higher energy density at pack level with flexible design and fewer materials/components used, it is possible that solid-state battery packs can be comparable to or even better than lithium-ion batteries.