Enabling Durable and High‐Energy Aqueous Proton Batteries by Engineering a Robust Polymer‐Metal Complex Interphase on Mn ₂ O ₃ Cathode

ABSTRACT Aqueous proton batteries (APBs) have emerged as a promising candidate for next‐generation energy storage systems due to their intrinsic low cost, exceptional safety, and competitive power/energy density. However, the widely used MnO ₂ cathode in acidic electrolytes typically operates relying on a dissolution‐deposition chemistry, inherently plagued by low manganese utilization efficiency and subpar practical energy density. Herein, we report a novel Mn‐based cathode material—Mn ₂ O ₃ —for a proton battery, which demonstrates high redox potential and superior specific capacity. Nevertheless, as an Mn‐based cathode material, Mn ₂ O ₃ still suffers from dissolution and capacity decay in the long‐term cycling process. To address this challenge, an in situ polymer‐metal complex interphase engineering strategy was proposed to regulate the proton transfer kinetics and inhibit the dissolution of the Mn ₂ O ₃ electrode. In this strategy, polyacrylonitrile (PAN) was employed as both the coating matrix and the adhesive while acting as a metal ligand after cyclization. Manganese triflate (Mn(OTf) ₂ ) was introduced to catalyze the pyrolysis of cyano groups and coordinate with the generated pyridine nitrogen, driving crosslinking reactions to stabilize the polymer network and optimize ion transport pathways. Consequently, the functional PAN‐Mn ²⁺ complex interphase (C‐PMn) inhibits the transport and dissolution of Mn ²⁺ ions, enhances H ⁺ permeability, and improves the capacity, cycling stability, and coulombic efficiency of the Mn ₂ O ₃ cathode. For the first time, we reveal that the Mn ₂ O ₃ cathode stores energy through a proton insertion/extraction mechanism in H ₂ SO ₄ electrolyte, holding promise for high‐energy‐density rocking‐chair proton batteries. As‐assembled diquinoxalino [2,3‐a:2′,3′‐c] phenazine (HATN)//Mn ₂ O ₃ proton full battery exhibits a specific capacity of 239 mA h g ⁻¹ at 0.2 A g ⁻¹ (based on the cathode), an average discharge voltage of about 1 V, a good cycling stability (82% capacity retention after 500 cycles) and an energy density of 115 Wh kg ⁻¹ (based on the total mass of cathode and anode). These findings offer a promising cathode material for proton batteries and a cost‐effective approach to optimizing the cathode performance in acidic electrolytes, thereby paving the way for the development of a durable, high‐energy proton battery.