Mn²⁺/H⁺ Co-insertion for improved voltage and cycling stability of Mn_0·54V₃O₈ cathodes in hybrid aqueous batteries

The pursuit of sustainable energy storage beyond lithium-ion batteries has intensified interest in aqueous multivalent-ion systems. Among them, manganese-based batteries stand out owing to Mn's natural abundance, high theoretical capacity, and low redox potential (−1.19 V vs. SHE), which enables higher operating voltages than Zn-based counterparts. Here, we report a pre-intercalated manganese vanadate cathode, Mn_0·54V₃O₈ (MnVO), that delivers 376 mAh g⁻¹ at 0.4 A g⁻¹ with 65.7 % capacity retention after 2000 cycles. Structural, spectroscopic, and computational analyses reveal a unique Mn²⁺/H⁺ co-insertion mechanism, in which pre-intercalated Mn ions act as structural pillars to expand interlayer spacing, stabilize the framework, and create enlarged diffusion channels. This stabilizing effect lowers the proton migration barrier (0.126 eV), enabling proton-dominant fast kinetics while maintaining lattice robustness. When paired with a Mn metal anode, MnVO achieves an operating voltage of 1.2 V, markedly higher than Zn-based analogues. Although interfacial instability from hydrogen evolution and Mn(OH)₂ formation remains a challenge, this study establishes high-degree pre-intercalation as a powerful design principle for intrinsically safe, high-voltage, and sustainable Mn-based hybrid aqueous batteries.