Communication—High-Capacity Hard Carbons Enabled by a Sodium Carborane Solid Electrolyte for Sodium-Ion Batteries

High-energy-density sodium-ion batteries, which are promising alternatives to lithium-ion batteries, require high-capacity hard carbons as negative electrodes. Conventional sodium-ion batteries use carbonate liquid electrolytes that decompose at hard-carbon surfaces, which limits the use of high-surface-area hard carbons. Herein, we report that high-surface-area hard carbons synthesized using magnesium oxide templates exhibit high capacities when combined with a sodium carborane solid electrolyte. Unlike carbonate liquid electrolytes, the solid electrolyte is highly stable and enables reversible electrochemical reactions in hard carbons to deliver high capacities of approximately 500 mA h g −1 .