The R&D of power batteries has been advancing fast in the past few years. In addition to lithium-ion batteries — ternary lithium-ion batteries and lithium iron phosphate batteries — as the mainstream applications, sodium-ion batteries, lithium-manganese-ferro-phosphate (LMFP) batteries, lithium-rich manganese-based batteries,, silicon-based anode (mostly carbon-based anodes at present and solid-state batteries are all options for next-generation battery technology, of which solid-state batteries are the current R&D focus.
Since the rise of solid-state battery startups in 2018, the technology has not progressed very well in the past four years. On the one hand, the performance of traditional lithium-ion batteries continues to improve, and the production process is becoming more mature and stable. On the other hand, the target for battery energy density is generally set at 500 wh/kg by 2030 worldwide (currently, the most powerful ternary lithium-ion battery only reaches 250 wh/kg, and the average commercially available EV battery does not reach 200 wh/kg), so technologies that make solid-state batteries commercially viable continue to raise the bar. Not until a balance is struck between the manufacturing cost and the energy density will solid-state batteries be able to compete well with other products. There are three main technology roadmaps for the types of solid-state electrolytes now…
- Polymers. High cost and low conductivity have an impact on the charge and discharge rates. Most European and American companies adopt this roadmap.
- Oxides. Performance does not vary greatly from indicator to indicator, but conductivity is also relatively low. This is the roadmap mostly used by Chinese companies.
- Sulfides. Higher conductivity, lower cost, but the technical threshold is the highest. Japanese and Korean companies tend to adopt this roadmap.
Overall, although solid-state batteries have high stability, a long lifespan, and high energy density, the conductivity of solid electrolytes is about 10 times lower than that of liquid electrolytes, so charging and discharging efficiency is a challenge. Moreover, because of the high impedance of the contact interface with the electrode, the long-term chemical reaction may cause side effects and affect the overall performance of the battery. In theory, the energy density of solid-state battery cells can reach 900 wh/kg, but how to improve the charging and discharging efficiency, at least close to the level of lithium-ion batteries? Last year, Harvard University published a paper on a "BLT sandwich-like" battery structure that compresses the time to less than 20 minutes for EVs to be charged from zero to full.
- Lithium, instead of graphite, is used as the anode to increase battery capacity.
- The solid-state electrolyte is divided into two layers that differ in chemical composition to solve the problem of lithium dendrites formed on the anode penetrating the separators and causing a short circuit.
- A graphite coating is used for the anode, blocking direct contact from the first layer of solid-state electrolytes to provide thermal insulation when the voltage rises.
Of course, the research is still in the laboratory stage. There is a long way to go before the design is scaled up to the commercial battery. Overall, solid-state battery startups in Europe, the U.S., and Japan will equip finished vehicles with their R&D results for a trial run after 2025 and start mass production around 2028. But one fact remains unchanged: Solid-state batteries are not likely to revolutionize the EV industry if they can't compete with lithium-ion batteries in the cost, and if the energy density of the battery pack (not just the cell) does not reach 500 wh/kg or more.