All-solid-state batteries replacing the flammable liquid electrolyte by a non-flammable solid electrolyte promise significantly enhanced energy density (e.g. by enabling lithium-metal anodes and high-voltage cathodes), fast charging capabilities (e.g. by tolerating higher operating temperatures), and improved operational safety (e.g. by eliminating flammable components). These advantages render all-solid-state batteries very attractive for automotive companies. The dependency of European automotive manufacturers, which contribute substantially to Europe’s economic strength, on batteries from Asia is a major concern also for the European Commission. Consequently the European Commission considers all-solid-state batteries as a pathway to re-establish strong cell manufacturing in Europe. However, there exists currently no all-solid-state battery technology that can compete in terms of performance with today’s lithium-ion batteries.
We developed a non-flammable non-toxic solid electrolyte for all-solid-state batteries based on a novel class of inorganic borate electrolytes offering simultaneously high ionic conductivity (1 mS/cm at 25°C) and a wide electrochemical stability window (>3 V). We successfully demonstrated a 3 V-class battery cell with a sodium metal anode and NaCrO2 cathode achieving remarkable capacity retention of 85% after 250 cycles to 3.2 V. Our results bring this material class to a technology readiness level comparable to the heavily investigated oxide- and sulfide-based solid-state electrolytes investigated intensively e.g. by Samsung, Toyota, VW, etc.
Swiss National Science Foundation, Swiss State Secretariat for Education, Research, and Innovation, Polish-Swiss Research Programme
 L. Duchêne, R.-S. Kühnel, E. Stilp, E. Cuervo Reyes, A. Remhof, H. Hagemann, C. Battaglia, A stable 3 V all-solid-state sodium–ion battery based on a closo-borate electrolyte, Energy Environ. Science, 10, 2017, 2609.
 Y. Yan, R.-S. Kühnel, A. Remhof, L. Duchêne, E. Cuevo Reyes, D. Rentsch, Z. Lodziana, C. Battaglia, A lithium amide-borohydride solid-state electrolyte with lithium-ion conductivities comparable to liquid electrolytes, Adv. Energy Mater., 7, 2017, 1700294.
 E. Roedern, R.-S. Kühnel, A. Remhof, C. Battaglia, Magnesium ethylenediamine borohydride as solid-state electrolyte for magnesium batteries, Scientific Reports, 7, 2017, 46189.
Solid state batteries for tomorrow's electric cars, Empa & Fraunhofer press release, 22.2.2019 (English)