A Proton‐Intercalation Pathway Realizes Long‐Life Manganese‐Ion Hybrid Batteries With Layered KV ₃ O ₈

Rechargeable aqueous manganese-based batteries offer a low-cost, high-safety, and promising solution to meet the growing demand for large-scale energy storage systems. However, since aqueous manganese-ion batteries (AMIBs) are still in the early stages of development, discovering high-performance cathode materials is critical for enabling future commercialization. In this study, we introduce monoclinic KV₃O₈ as a highly stable and unprecedented cathode material for AMIBs. KV₃O₈ has a layered structure with an interlayer spacing of approximately 7.63 Å (d₀₀₁), which facilitates the reversible intercalation and deintercalation of cations. This structural feature ensures excellent long-term cycling stability (88.0% capacity retention after 3600 cycles) and outstanding rate capability. By integrating diffusion path and barrier calculations with X-ray photoelectron spectroscopy, ex situ X-ray absorption spectroscopy, ex situ X-ray diffraction, Fourier-transform infrared spectroscopy, and Raman spectroscopy, we identify both Mn²⁺ ions and protons as active charge carriers. Furthermore, the formation of a Mn(OH)₂ layer on the cathode surface during discharge suggests that protons predominantly govern the charge storage mechanism. This study provides critical insights into the design of advanced manganese ion cathode materials and represents a significant step toward the practical realization of AMIBs.