Critical role of crystal water for MnV2O6·nH2O (n = 0, 2, 4) cathode materials in supercapacitors

Although interlayer engineering strategies have demonstrated significant potential in enhancing the electrochemical performance of layered manganese vanadates, the role of intercalated crystal water in regulating charge storage behavior should be clarified. Herein, several manganese vanadates (MnV 2 O 6 ·nH 2 O) with different crystal water contents are fabricated, and the storage behaviors are investigated. When the content of crystal water is limited (n = 2), the interlayer crystal water can act as a “pillar” to enhance ion diffusion kinetics, optimize the interlayer charge distribution, and alleviate structural collapse during charge/discharge cycles. However, excessive water molecules (n = 4) can form metastable coordination configurations and induce localized structural damage during repeated charge/discharge processes. As a result, MnV 2 O 6 ·2H 2 O cathode material exhibits an optimal specific capacity (1318.2 C g −1 at 1 A g −1 ) and cycle stability (84.31% capacity retention after 10,000 cycles) better than that of MnV 2 O 6 (507.9 C g −1 at 1 A g −1 , capacity retention of 61.62%) and MnV 2 O 6 ·4H 2 O (591.3 C g −1 at 1 A g −1 , capacity retention of 67.64%). Furthermore, the hybrid supercapacitor assembled with MnV 2 O 6 ·2H 2 O cathode and activated carbon anode (MnV 2 O 6 ·2H 2 O//AC) demonstrates a superior energy density of 59.12 Wh kg −1 at 690.80 W kg −1 , with 87.03% of the original capacity remaining after 10,000 cycles.