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

Co-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 H₂ O molecules block the Mg²⁺ diffusion while facilitate the proton diffusion. When monoclinic vanadium dioxide (VO₂ (B)) anode is tested in Mg(CH₃ COO)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⁺ +H₂ O 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.