Synthesis and understanding of Na₁₁Sn₂PSe₁₂ with enhanced ionic conductivity for all-solid-state Na-ion battery

All-solid-state Na-ion batteries (NIBs) that incorporate nonflammable solid-state electrolytes and an inexhaustible alkali metal offer a potential solution to the safety and cost concerns associated with conventional Li-ion batteries that use liquid electrolytes. Na-ion solid-state electrolytes (SSEs) with high ionic conductivity are the key to success for all-solid-state NIBs. Here, we report a new Na-ion SSE, Na₁₁Sn₂PSe₁₂, with a superior grain conductivity of 3.04 mS cm⁻¹ and a total ionic conductivity of 2.15 mS cm⁻¹ at 25 °C. Single-crystal X-ray diffraction, first-principles phonon calculations, and the proposed bonding energy model indicate that its superior ionic conductivity stems from the presence of a high density of dispersive Na⁺ vacancies, three-dimensional Na-ion conduction pathways, and a low bonding energy of the Na⁺ ion with its neighboring atoms. Na₁₁Sn₂PSe₁₂ is used for the first time as the electrolyte in all-solid-state Na-Sn/TiS₂ battery cell, which shows excellent rate performance and delivers a high reversible capacity of 66.2 mAh (g of TiS₂)⁻¹ after 100 cycles with cycling retention of 88.3% at a rate of 0.1 C at room temperature.