Ca2+ pre-intercalated bilayered vanadium oxide for high-performance aqueous Mg-ion batteries
Qiang Fu, Xiaoyu Wu, Xianlin Luo, Ziming Ding, Sylvio Indris, Angelina Sarapulova, Zhen Meng, Morgane Desmau, Zhengqi Wang, Weibo Hua, Christian Kübel, Björn Schwarz, Michael Knapp, Helmut Ehrenberg, Yingjin Wei, Sonia Dsoke
Abstract
The “oxygen-rich” Ca2+ pre-intercalated bilayered vanadium oxide (CaVOnH) was synthesized via hydrothermal method and determined as a monoclinic structure with reasonable lattice parameters. CaVOnH achieves a first discharge capacity of 273 mAh g−1 with capacity retention of 91% at 50 mA g−1 in 0.8 m Mg(TFSI)2-85%PEG-15%H2O (polyethylene glycol, PEG), but limited rate capability due to the low ionic conductivity of electrolyte. Dimethyl sulfoxide (DMSO) is used as a co-solvent to tune the physical-chemical properties of aqueous Mg-ion electrolyte (AME), resulting in the reorganization of Mg2+ solvation and hydrogen bond network. The AME containing DMSO shows improved ionic conductivity, low viscosity, and high Mg2+ diffusion coefficient and allows CaVOnH and V2O5 to achieve a much-improved rate capability and capacity. Moreover, the reaction mechanism and reversibility of CaVOnH are elucidated by combining in operando and ex situ techniques. The results demonstrate that CaVOnH undergoes 2-phase reaction and solid solution, the variation of oxidation state and the local environment of vanadium, and reversible formation/decomposition of MgF2 cathode electrolyte interface during Mg2+ (de)intercalation, where MgF2 originated from the decomposition of TFSI−.