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A self-passivating cathode/electrolyte interface achieves stable, room-temperature long-term cycling of 4 V-class Na3(VOPO4)2F|Na4(CB11H12)2(B12H12)|Na all-solid-state sodium batteries with the highest reported discharge cell voltage and cathode-based specific energy.

We report a robust methodology based on linear sweep voltammetry to determine experimentally the electrochemical oxidative stability of hydroborate-based solid-state electrolytes for all-solid-state batteries. To accelerate kinetics and improve the sensitivity to decomposition, we explore different solid-state electrolyte/carbon composites and employ a low scan rate of 10 μV s −1. Using LiBH 4 as a model system, we show that proper selection of the conductive carbon and its ratio in the composite are important for an accurate determination of the intrinsic oxidative stability. This method is robust with respect to the choice of the current collector material and the ionic conductivity of the solid-state

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 NaB 11 H 14 nido-borates as a new building block for solid electrolytes that can be synthesized via a costeffective low-temperature method using cheap NaBH 4 as a chemical feedstock. We employ 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-NaB 11 H 14 with closo-Na 2 B 12 H 12 into cubic unit cells. Importantly, Na 2 B 12 H 12 can also be synthesized costeffectively from NaBH 4. 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 mV to 2.5 mV after 400 cycles at 50 A/cm 2. We also discuss how our results can be translated to render lithium solid electrolytes economically viable.