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The integration of solid-polymer electrolytes into all-solid-state lithium batteries is highly desirable to overcome the limitations of current battery configurations that have a low energy density and severe safety concerns. Polyacrylonitrile is an appealing matrix for solid-polymer electrolytes; however, the practical utilization of such polymer electrolytes in all-solid-state cells is impeded by inferior ionic conductivity and instability against a lithium-metal anode. In this work, we show that a polymer-in-salt electrolyte based on polyacrylonitrile with a lithium salt as the major component exhibits a wide electrochemically stable window, a high ionic conductivity, and an increased lithium-ion transference number. The growth of dendrites from the lithium-metal anode was suppressed effectively by the polymer-in-salt electrolyte to increase the safety features of the batteries. In addition, we found that a stable interphase was formed between the lithium-metal anode and the polymer-in-salt electrolyte to restrain the uncontrolled parasitic reactions, and we demonstrated an all-solid-state battery configuration with a LiFePO4 cathode and the polymer-in-salt electrolyte, which exhibited a superior cycling stability and rate capability.

A sodium‐ion battery operating at room temperature is of great interest for large‐scale stationary energy storage because of its intrinsic cost advantage. However, the development of a high capacity cathode with high energy density remains a great challenge. In this work, sodium super ionic conductor‐structured Na3V2−xCrx(PO4)3 is achieved through the sol–gel method; Na3V1.5Cr0.5(PO4)3 is demonstrated to have a capacity of 150 mAh g−1 with reversible three‐electron redox reactions after insertion of a Na+, consistent with the redox couples of V2+/3+, V3+/4+, and V4+/5+. Moreover, a symmetric sodium‐ion full cell utilizing Na3V1.5Cr0.5(PO4)3 as both the cathode and anode exhibits an excellent rate capability and cyclability with a capacity of 70 mAh g−1 at 1 A g−1. Ex situ X‐ray diffraction analysis and in situ impedance measurements are performed to reveal the sodium storage mechanism and the structural evolution during cycling.

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