Inhibiting irreversible Zn2+/H+ co-insertion chemistry in aqueous zinc-MoOx batteries for enhanced capacity stability

Rechargeable aqueous Zn-MoO_x batteries are promising energy storage devices with high theoretical specific capacity and low cost. However, MoO₃ cathodes suffer drastic capacity decay during the initial discharging/charging process in conventional electrolytes, resulting in a short cycle life and challenging the development of Zn-MoO_x batteries. Here we comprehensively investigate the dissolution mechanism of MoO₃ cathodes and innovatively introduce a polymer to inhibit the irreversible processes. Our findings reveal that this capacity decay originates from the irreversible Zn²⁺/H⁺ co-intercalation/extraction process in aqueous electrolytes. Even worse, during Zn²⁺ intercalation, the formed Zn_xMoO_3−x intermediate phase with lower valence states (Mo⁵⁺/Mo⁴⁺) experiences severe dissolution in aqueous environments. To address these challenges, we developed a first instance of coating a polyaniline (PANI) shell around the MoO₃ nanorod effectively inhibiting these irreversible processes and protecting structural integrity during long-term cycling. Detailed structural analysis and theoretical calculations indicate that =N– groups in PANI@MoO_3−x simultaneously weaken H⁺ adsorption and enhance Zn²⁺ adsorption, which endowed the PANI@MoO_3−x cathode with reversible Zn²⁺/H⁺ intercalation/extraction. Consequently, the obtained PANI@MoO_3−x cathode delivers an excellent discharge capacity of 316.86 mA h g⁻¹ at 0.1 A g⁻¹ and prolonged cycling stability of 75.49% capacity retention after 1000 cycles at 5 A g⁻¹. This work addresses the critical issues associated with MoO₃ cathodes and significantly advances the understanding of competitive multi-ion energy storage mechanisms in aqueous Zn-MoO₃ batteries.