Boosting the\nZinc-Ion Storage Capability of NH<sub>4</sub>V<sub>3</sub>O<sub>8</sub> via Cation-Defect Engineering

Ammonium vanadate (NVO) is regarded as one of the most\npromising\ncathodes for aqueous zinc-ion batteries (AZIBs) by virtue of its favorable\ntheoretical capacity and comparatively stable layered structure. Nevertheless,\nthe crowded NH<sub>4</sub><sup>+</sup> cation in the interlayer would\npartially occupy the transfer routes of Zn<sup>2+</sup>, and the strong\nelectrostatic interaction contributed by the excessive NH<sub>4</sub><sup>+</sup> would further lower the mobility of Zn<sup>2+</sup>,\nthus resulting in the sluggish kinetics of Zn<sup>2+</sup> and inferior\nrate performances. Herein, cation-modulated engineering is proposed\nand achieved via a facile thermal-treatment process. By modulating\nthe number of NH<sub>4</sub><sup>+</sup> cations, the interlayer spacing\nof NH<sub>4</sub>V<sub>3</sub>O<sub>8</sub> is significantly broadened\nand the migration barrier of Zn<sup>2+</sup> is effectively reduced.\nAs a result, the proposed NH<sub>4</sub>V<sub>3</sub>O<sub>8</sub> cathode with moderate NH<sub>4</sub><sup>+</sup> removal exhibits\nthe favorable capacity of 375 mAh g<sup>–1</sup> at 2 A g<sup>–1</sup>, while ∼363 mAh g<sup>–1</sup> could\nbe maintained after 1000 cycles, corresponding to a superior capacity\nretention of ∼97%, suggesting the significantly boosted electrochemical\nproperties contributed by the cation-modulated engineering. Moreover,\nthe related ex-situ characterizations substantiate the Zn<sup>2+</sup>/H<sub>2</sub>O co-intercalation mechanism of the proposed NVO cathode.\nThis work sheds light on the potential of the cation-modulation strategy\non accelerating the kinetics of zinc-ions and improving the electrochemical\nproperties of ammonium vanadate-based cathodes and further broadens\nthe application potentials of vanadium-based cathodes in rechargeable\nAZIBs.