Developing a high-voltage electrolyte based on conjuncto-hydroborates for solid-state sodium batteries

Abstract

Developing solid electrolytes with a wide electrochemical window and high ionic conductivity is key to the development of all-solid-state batteries with high energy and power density. A novel conjuncto-hydroborate of Na₂B₂₀H₁₈ is reported here as a promising solid electrolyte with electrochemical stability up to 5.1 V vs. Na⁺/Na and a Na-ion conductivity of 2.5 × 10⁻⁶ S cm⁻¹ at 25 °C. Furthermore, encouraging performance has been realized using the mixed-anion electrolyte of Na₂B₂₀H₁₈/Na₂B₁₂H₁₂, with a fast ionic conductivity of 2.8 × 10⁻⁴ S cm⁻¹, wide electrochemical window beyond 4.8 V vs. Na⁺/Na, superior sodium-ion transference number of 0.976 at 25 °C, and excellent interfacial compatibility with the electrodes. The corresponding Na symmetrical cell could stay stable at 40 mV and a high current density of 0.1 mA cm⁻² for 100 h at 25 °C. Meanwhile, the all-solid-state battery of Na|Na₂B₂₀H₁₈–4Na₂B₁₂H₁₂|TiS₂ exhibited a high capacity retention of 82% after 100 cycles at 0.1C.