Synergy of structural engineering and VO2 self-transformation enables ultra-high areal capacity cathodes for zinc-ion batteries

The development of cathodes with high areal capacities is a prerequisite for realizing the practical application of zinc-ion batteries (ZIBs). However, the slow ion (de)intercalation kinetics and the highly tortuous ion transport channels in the traditionally blade-coated electrodes limit the practical areal capacities of the ZIB cathodes below 3 mAh cm-2. In this work, multi-stage pore structures, including through-holes perpendicular to the electrode and submicron-scale channels along the radial direction of the printed filaments, are obtained through polyvinylidene difluoride (PVDF) phase separation assisted 3D printing procedure. In this novel electrode material, the porous network enables fast electron/ion transport and improves the accessibility of active materials for thick electrodes with high active material loadings. More importantly, a phase self-transformation of VO2 to a high oxidation state of V5O12∙6H2O is achieved by regulating the charging voltage, which significantly enhances the zinc storage capacity and provides smoother channels for fast ion (de)intercalation. With these unique features, the 3D printed VO2 cathode with a mass loading of 8.9 mg cm-2 demonstrates a high specific capacity of 478.1 mAh g-1 at 1 A g-1 and a stable life cycle for 1000 cycles at 4 A g-1. When the mass loading is increased to 29.6 mg cm-2, the electrode is still able to deliver an ultra-high areal capacity of 16.8 mAh cm-2. The strategies adopted in this work greatly boost up the performance of VO2 cathodes, especially the high areal capacity, by combining structural engineering and material activity regulation.