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.