Anion-Regulated Deposition-Dissolution Chemistry in Acidic Aqueous Indium─MnO₂ Batteries

Acidic aqueous metal─MnO₂ rechargeable batteries enable high-capacity energy storage via a two-electron MnO₂/Mn²⁺ redox chemistry but suffer from severe anode corrosion and poor reversibility in acidic environments. Here, we report an acidic In─MnO₂ battery (IMB) system that employs anion-regulated interfacial chemistry to enable reversible deposition-dissolution reactions at both electrodes. The In³⁺/In redox couple offers a favorable potential that mitigates acidic corrosion while matching the MnO₂ two-electron redox. Systematic investigation of anion effects reveals that concentrated Cl^- anions restructure the In³⁺ solvation, forming stable chloro-complexes that lower dissociation energy and promote uniform Indium (In) deposition. Conversely, SO₄ ^2--rich electrolytes impede In plating but enhance MnO₂ crystallization and MnO₂/Mn²⁺ reversibility. Through tailored Cl^-/SO₄ ^2- ratios and an anion-decoupled dual-electrolyte configuration, the battery operates at ∼1.7 V with 72.3% energy efficiency (EE) and over 1500 cycles at 5 mA cm^-2. Furthermore, we demonstrate an electrode-less IMB using a Bi substrate at the anode, where in situ alloying-driven In deposition enables over 2000 stable cycles at 4 mA cm^-2 (2 mAh cm^-2), delivering a compelling energy density of 484.5 Wh kg^-1 and a high cumulative areal capacity of 4120 mAh cm^-2. This work establishes a design paradigm coupling anion-regulated interfacial chemistry with substrate engineering for high-performance acidic IMB.