Synergistic Regulation of δ-MnO2 Cathode via Crystal Engineering and pH Buffering for Long-Cycle Aqueous Zinc-Ion Batteries

Aqueous zinc-ion batteries (ZIBs) have emerged as a promising candidate for large-scale energy storage due to their inherent safety, low cost, and environmental friendliness. However, manganese dioxide (MnO2)-based cathodes, which are widely studied for ZIBs owing to their high theoretical capacity and low cost, face severe capacity fading issues that hinder the commercialization of ZIBs. This performance degradation mainly stems from the weak van der Waals forces between MnO2 layers leading to structural collapse during repeated Zn2+ insertion and extraction; it is also exacerbated by irreversible Mn dissolution via Mn3+ disproportionation that depletes active materials, and further aggravated by dynamic electrolyte pH fluctuations promoting insulating zinc hydroxide sulfate (ZHS) formation to block ion diffusion channels. To address these interconnected challenges, in this study, a synergistic strategy was developed combining crystal engineering and pH buffer regulation. We synthesized three MnO2 polymorphs (α-, δ-, γ-MnO2), identified δ-MnO2 with flower-like microspheres as optimal, and introduced sodium dihydrogen phosphate (NaH2PO4) as a pH buffer (stabilizing pH at 2.8 ± 0.2). The modified electrolyte improved δ-MnO2 wettability (contact angle of 17.8° in NaH2PO4-modified electrolyte vs. 26.1° in base electrolyte) and reduced charge transfer resistance (Rct = 78.17 Ω), enabling the optimized cathode to retain 117.25 mAh g−1 (82.16% retention) after 2500 cycles at 1 A g−1. This work provides an effective strategy for stable MnO2-based ZIBs, promoting their application in renewable energy storage.