Enhanced electrochemical performance of NASICON-type sodium ion cathode based on charge balance theory

The variable components and relatively high Na+ intercalation voltage makes sodium ion superconductors (NASICON) as ideal cathode materials for secondary sodium-ion batteries. Na3V2(PO4)3, as one of the most typical NASICON-type cathode, needs urgent improvement to enable its wide application. Based on charge balance theory, via utilizing Al3+and SiO44− to replace V3+ and PO43− in Na3V2(PO4)3, respectively, Na3+xV1.5Al0.5(PO4)3-x(SiO4)x (0 ≤ x ≤ 0.5) cathode materials were prepared in this work. In comparison with the initial state, the operation potential was apparently promoted because of the reversible access of V4+/V5+ redox couple at 4.0 V vs. Na+/Na; the higher specific capacity can be achieved owing to much more conductive Na+ involved in the redox reaction of V species. Furthermore, the structural stability and diffusion kinetics is dramatically enhanced, which are further validated by ex-situ XRD and GITT analysis. Specifically, Na3.3V1.5Al0.5(PO4)2.7(SiO4)0.3 can offer highly reversible capacities of 127.3 and 181.5 mAh g−1 within the region of 2.5–4.4 V and 1.4–4.4 V, respectively, making itself a promising sodium ion cathode material. Na-ion full cells paired with Na3.3V1.5Al0.5(PO4)2.7(SiO4)0.3 cathode and hard carbon anode are assembled, rendering a specific capacity of 154.3 mAh g−1 at 0.5 C based on the mass of cathode.