Stable 4 V-Class All-Solid-State Lithium Battery with Hydroborate Electrolyte and NMC811 Cathode

Solid-state batteries are expected to extend the energy and power density beyond the limits of lithium-ion batteries, and to replace their flammable liquid electrolytes with safer alternatives. Each of the most studied solid electrolyte materials has disadvantages such as limited electrochemical stability (sulfides), low ionic conductivity (polymers), mechanical stiffness (oxides) or incompatibility with metallic anodes (halides). Complex hydride electrolytes, especially hydroborates, combine many attractive characteristics such as high ionic conductivity, compatibility with alkali metal anodes and soft mechanical properties. Some of us recently reported stable 3 V and 4 V all-solid-state sodium hydroborate batteries. Lithium hydroborate solid-state batteries offer the potential to use cathodes with much higher specific discharge capacity, but have so far been limited to low-voltage cathodes such as sulfur or TiS₂ due to the limited oxidative stability of the electrolyte. Herein, we demonstrate all-solid-state lithium metal batteries employing a hydrocarbaborate solid electrolyte with a 4 V-class LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) cathode. This is achieved by tailoring the LiCB₁₁H₁₂-LiCB₉H₁₀ electrolyte mixing ratio towards high oxidative stability (~3.9 V), while maintaining high conductivity (>1.5 mS cm^-1 at room temperature). Unlike many other stable solid-state cells, we use one single electrolyte material in contact with the lithium metal, as the separator layer and in the cathode composite. Corresponding cells with lithium metal anodes display a high discharge capacity of 175 mAh g^-1 and stable cycling, demonstrating the potential of hydroborate electrolytes for 4 V-class solid-state lithium metal batteries.