Na₃₊ₓMₓP₁₋ₓS₄ (M = Ge⁴⁺, Ti⁴⁺, Sn⁴⁺) enables high rate all-solid-state Na-ion batteries Na₂₊₂δFe₂₋δ(SO₄)₃|Na₃₊ₓMₓP₁₋ₓS₄|Na₂Ti₃O₇

Electrolytes in current Na-ion batteries are mostly based on the same fundamental chemistry as those in Li-ion batteries – a mixture of flammable liquid cyclic and linear organic carbonates leading to the same safety concerns especially during fast charging. All-solid-state Na-ion rechargeable batteries utilizing non-flammable ceramic Na superionic conductor electrolytes are a promising alternative. Among the known sodium conducting electrolytes the cubic Na₃PS₄ phase has relatively high sodium ion conductivity exceeding 10⁻⁴ S cm⁻¹ at room temperature. Here we systematically study the doping of Na₃PS₄ with Ge⁴⁺, Ti⁴⁺, Sn⁴⁺ and optimise the processing of these phases. A maximum ionic conductivity of 2.5 × 10⁻⁴ S cm⁻¹ is achieved for Na₃.₁Sn₀.₁P₀.₉S₄. Utilising this fast Na⁺ ion conductor, a new class of all-solid-state Na₂₊₂δFe₂₋δ(SO₄)₃|Na₃₊ₓMₓP₁₋ₓS₄ (M = Ge⁴⁺, Ti⁴⁺, Sn⁴⁺) (x = 0, 0.1)|Na₂Ti₃O₇ sodium-ion secondary batteries is demonstrated that is based on earth-abundant safe materials and features high rate capability even at room temperature. All-solid-state Na₂₊₂δFe₂₋δ(SO₄)₃|Na₃₊ₓMₓP₁₋ₓS₄|Na₂Ti₃O₇ cells with the newly prepared electrolyte exhibited charge–discharge cycles at room temperature between 1.5 V and 4.0 V. At low rates the initial capacity matches the theoretical capacity of ca. 113 mA h g⁻¹. At 2C rate the first discharge capacity at room temperature is still 83 mA h per gram of Na₂₊₂δFe₂₋δ(SO₄)₃ and at 80 °C it rises to 109 mA h per gram with 80% capacity retention over 100 cycles.