Nido-Borate/Closo-Borate Mixed-Anion Electrolytes for All-Solid-State Batteries

Closo-borates and carba-closo-borates recently emerged as a promising class of solid electrolytes for all-solid-state batteries but are too expensive. Here, we introduce NaB11H14nido-borates as a new building block for solid electrolytes that can be synthesized via a cost-effective low-temperature method using cheap NaBH4 as a chemical feedstock. We employ two-dimensional (2D) nuclear magnetic resonance techniques to analyze the structure of this new building block. High ionic conductivity of up to 4 mS/cm at 20 °C is achieved by combining nido-NaB11H14 with closo-Na2B12H12 into cubic unit cells. Importantly, Na2B12H12 can also be synthesized cost-effectively from NaBH4. Crystal structures are solved and refined by synchrotron X-ray diffraction and density functional theory. Cyclic voltammetry reveals electrochemical stability from 0.15 to 2.6 V vs Na/Na+. Galvanostatic cycling in symmetrical cells with sodium metal electrodes shows only a small overpotential increase from 2 to 2.5 mV after 400 cycles at 50 μA/cm2. We also discuss how our results can be translated to render lithium solid electrolytes economically viable.