While sulfide solid electrolytes such as Na₁₁Sn₂PS₁₂ can allow fast transport of Na⁺ ions, their utilization in solid sodium ion batteries is rather unsuccessful since they are not electrochemically compatible to both high-voltage cathodes and sodium metal anode. In this work, we devise an effective approach toward realizing solid sodium ion batteries, using the Na₁₁Sn₂PS₁₂ electrolyte and slurry-coated NASICON-type Na₃MnTi(PO₄)₃@C as high-voltage cathode, highly beneficial for low processing cost and high content/loading of active cathode matter. We report that through significantly improved integrity of electrolyte-cathode interface, such solid sodium ion batteries can deliver outstanding cycling and rate performance, with a charge voltage resilience up to 4.1 V, a high cathode discharge capacity of 128.7 mAh g⁻¹ against the Na₃MnTi(PO₄)₃@C in cathode is achieved at 0.05 C, and capacity retention ratio of 82% with a rate of 0.1 C is realized after prolonged cycling at room temperature. Besides, we demonstrate that such a solid sodium ion battery can even perform at a sub-zero Celsius temperature of −10°C, when the conventional control cell using liquid electrolyte completely fail to function. This work is to offer a dependable avenue in engineering next generation of safe solid ion batteries based on highly sustainable and much cheaper material resources.

While argyrodite sulfides are getting more and more attention as highly promising solid-state electrolytes (SSEs) for solid batteries, they also suffer from the typical sulfide setbacks such as poor electrochemical compatibility with Li anode and high-voltage cathodes and serious sensitivity to humid air, which hinders their practical applications. Herein, we have devised an effective strategy to overcome these challenging shortcomings through modification of chalcogen chemistry under the guidance of theoretical modeling. The resultant Li_6.25PS₄O_1.25Cl_0.75 delivered excellent electrochemical compatibility with both pure Li anode and high-voltage LiCoO₂ cathode, without compromising the superb ionic conductivity of the pristine sulfide. Furthermore, the current SSE also exhibited highly improved stability to oxygen and humidity, with further advantage being more insulating to electrons. The remarkably enhanced compatibility with electrodes is attributed to in situ formation of helpful electrolyte–electrode interphases. The formation of in situ anode–electrolyte interphase (AEI) enabled stable Li plating/stripping in the Li|Li_6.25PS₄O_1.25Cl_0.75|Li symmetric cells at a high current density up to 1 mA cm⁻² over 200 h and 2 mA cm⁻² for another 100 h. The in situ amorphous nano-film cathode–electrolyte interphase (CEI) facilitated protection of the SSE from decomposition at elevated voltage. Consequently, the synergistic effect of AEI and CEI helped the LiCoO₂|Li_6.25PS₄O_1.25Cl_0.75|Li full-battery cell to achieve markedly better cycling stability than that using the pristine Li₆PS₅Cl as SSE, at a high area loading of the active cathode material (4 mg cm⁻²) in type-2032 coin cells. This work is to add a desirable SSE in the argyrodite sulfide family, so that high-performance solid battery cells could be fabricated without the usual need of strict control of the ambient atmosphere.