Orbital energy level engineering: 3d high-spin Mn's d-electron mediating electronic structure of VO2 boosting highly durable aqueous ammonium ion batteries.

Abstract

Aqueous batteries have become a prospective future energy storage system because of their low coefficient of cost and stability. However, their lower energy density limits their applications. Ammonium ions (NH₄⁺) have a small hydration radius and light molar mass, and aqueous ammonium ion batteries (AAIBs) are anticipated for solving the inherent low-energy density problem of aqueous batteries. Exploring highly performing storage materials for aqueous ammonium ion batteries continues to be a research hotspot in recent years. Here, we propose a strategy to regulate the tunneling vanadium oxide' structure (VOM) based on the electron-mediated orbital-energy level synergistic strategy of the high-spin 3d transition metal (Mn) to assist AAIBs to achieve high energy density. The VOM has a capacity of up to 270 mAh g^-1 at a current density of 0.2 A g^-1, and the battery system containing poly(ammonium benzene) (PANI) (named VOM//PANI) has an energy density of up to 63.5 Wh kg^-1. At the same time, we demonstrate the chemical energy storage mechanism of hydrogen bonding and the kinetics of interfacial chemical reactions in VOM based on a series of ex-situ or in-situ tests. Density-functional theory (DFT) calculations and experiments demonstrate that the introduction of high-spin transition metals can directionally regulate and optimize the electronic structure of V, which helps to achieve efficient NH₄⁺ storage. This work offers novel concepts for the advancement of high-performance AAIBs as energy storage materials, as well as new strategies for the future large-scale grid-level applications of AAIBs.