Investigation of Na3V2(PO4)2O2F as a sodium ion battery cathode material: Influences of morphology and voltage window

Na-ion batteries (NIBs) offer a low-cost solution for energy storage applications. However, the cathode materials of NIBs are still limited by their low energy density, exhibiting either small capacity or low operation voltage. In this work we explored a high-voltage cathode material, vanadium fluorophosphate Na3V2(PO4)2O2F. We prepared the material in both nano-size and micro-size by a hydrothermal method, adopting ethylene glycol and water as the solvent, respectively. The morphology and physiochemical properties of the nano-sized and micro-sized Na3V2(PO4)2O2F are systematically characterized and the battery cycling performances are examined in different voltage windows. Interestingly, we find that the battery capacity and rate capability are highly dependent on the cathode morphology while the cyclability of the electrode is mainly affected by the operation voltage window. The nano-sized Na3V2(PO4)2O2F can achieve an energy density of 459 Wh kg−1 with good cyclability within a voltage window of 3–4.5 V while the micro-sized Na3V2(PO4)2O2F delivers a much poorer electrochemical performance due to its coarse morphology. By widening the operation voltage window to 1–4.5 V, an energy density of 660 Wh kg−1 can be obtained from the nano-sized Na3V2(PO4)2O2F electrode at the initial cycles, but it decreases significantly after tens of cycles. We analyzed the rate capability of Na3V2(PO4)2O2F by considering multi Na+ ion intercalation reactions and found the large voltage window induced additional Na+ ion intercalation reaction occurs through a much slower kinetic process as compared to other reactions activated in the narrower voltage window (e.g., 3–4.5 V). The model of the intercalation reactions as well as the possible reasons of the degraded cycling performance in wide operation voltage windows are discussed based on the battery charging-discharging behavior, electrochemical impedance analysis, ex-situ morphology measurements and previously established material models. The findings validate the potential of Na3V2(PO4)2O2F as a high energy density cathode material for NIB, but also call for new crystal structure design of Na3V2(PO4)2O2F to achieve high energy density and good cyclability at the same time.