“Single‐phase” all‐solid‐state battery, based on Na3−xV2−xZrx(PO4)3 as the cathode, anode, and electrolyte, is reported. It successfully undergoes room temperature charge–discharge reactions based on the V3+/V2+ (anode) and V3+/V4+ (cathode) redox systems with insertion reaction. The interfacial resistance is essentially zero at the anode and cathode.

To realize an ideal interface between electrode and electrolyte, a single‐phase all‐solid‐state lithium‐ion battery is studied using Li3V2(PO4)3‐based NASICON‐type material as the cathode, anode, and electrolyte. A dense Li3‐xV2‐xAlx(PO4)3 pellet with the Pt current collectors on both the front and back surfaces successfully demonstrates charge‐discharge reactions based on the V3+/V2+ (anode) and V3+/V4+ (cathode) redox systems at 373 K. Al substitutional doping for V brings the suppression of the electronic conductivity and increase in the ionic conductivity. As a result, Al‐doped material (Li3V1.6Al0.4(PO4)3) exhibites the best charge‐discharge performance in the Li3V2‐xAlx(PO4)3 system (x=0 ‐ 0.6) because of its high ionic conductivity and low electronic conductivity. The concept of a single‐phase battery with a single material offers a new approach to avoid side reactions during cell fabrication of oxide‐based all‐solid‐state lithium‐ion batteries.