Voltage window-dependent electrochemical performance and reaction mechanisms of Na3V2(PO4)3 cathode for high-capacity sodium ion batteries

Na3V2(PO4)3 owns stable three-dimensional open framework and delivers a theoretical capacity of 117.6 mAh/g with a voltage plateau at 3.4 V (vs. Na+/Na). However, the voltage plateau at 1.6 V (vs. Na+/Na) corresponding to the V2+/V3+ redox couple can also deliver a theoretical capacity of 59 mAh/g. Herein, the operating voltage window of Na3V2(PO4)3 is extended to fully utilize V2+/V3+/V4+ redox couples. In 1.0–4.0 V (vs. Na+/Na), Na3V2(PO4)3 shows a high initial capacity but an inferior cycling stability, which is different from the scenario (low capacity but high stability) of 2.5–4.0 V (vs. Na+/Na). In comparison, Na3V2(PO4)3 delivers an initial capacity of 140.3 mAh/g at 200 mA/g in 1.4–4.0 V (vs. Na+/Na) and maintains a reversible capacity of 106.2 mAh/g after 600 cycles. At 20 mA/g, Na3V2(PO4)3 shows a reversible capacity of 135 mAh/g in 1.4–4.0 V (vs. Na+/Na) after 100 cycles. Meanwhile, operando X-ray diffraction is utilized to explore storage mechanisms of Na3V2(PO4)3 in different voltage ranges. The results show that the cathode reactions follow a biphasic mechanism at both 3.4 and 1.6 V (vs. Na+/Na), associated with the reversible transformation of Na3V2(PO4)3 ↔ NaV2(PO4)3 and Na3V2(PO4)3 ↔ Na4V2(PO4)3, respectively.