Proton/Mg2+ Co‐Insertion Chemistry in Aqueous Mg‐Ion Batteries: From the Interface to the Inner

Abstract

AbstractCo‐insertion of protons happens widely and enables divalent‐ion aqueous batteries to achieve high performances. However, detailed investigations and comprehensive understandings of proton co‐insertion are scarce. Herein, we demonstrate that proton co‐insertion into tunnel materials is determined jointly by interface derivation and inner diffusion: at the interface, hdrated Mg2+ has poor insertion kinetics, and therefore accumulates and hydrolyzes to produce protons; in the tunnels, co‐inserted/lattice H2O molecules block the Mg2+ diffusion while facilitate the proton diffusion. When monoclinic vanadium dioxide (VO2(B)) anode is tested in Mg(CH3COO)2 aqueous solution, the formation of Mg‐rich solid electrolyte interphase on the VO2(B) electrode and co‐insertion of derived protons are probed; in the tunnels, the diffusion energy barrier of Mg2++H2O is 2.7 eV, while that of the protons is 0.37 eV. Thus, protons dominate the subsequent insertion and inner diffusion. As a consequence, the VO2(B) achieves a high capacity of 257.0 mAh g−1 at 1 A g−1, a high rate retention of 59.1 % from 1 to 8 A g−1, and stable cyclability of 3000 times with a capacity retention of 81.5 %. This work provides an in‐depth understanding of the proton co‐insertion and may promote the development of rechargeable aqueous batteries.