Effect of cation size on alkali acetate-based ‘water-in-bisalt’ electrolyte and its application in aqueous rechargeable lithium battery

Abstract In this study, the properties of aqueous electrolytes based on alkali metal (Li, Na, K and Cs) acetates have been systematically investigated in order to figure out the effects of cations on the stability windows of corresponding electrolytes. At the same concentration of 7 m (molar-salt in kg-solvent), it is discovered that the stability windows of KOAc and CsOAc-based aqueous electrolytes are wider than those of LiOAc and NaOAc-based ones. This is due to enhanced polymer-like chains and weakened hydrogen bonds for the aqueous electrolytes with cations of larger size and higher mass. It is found that saturated 45 m CsOAc ‘water-in-salt’ electrolyte (WiSE) has an electrochemical stability window of 3.4 V, which is further mixed with 7 m LiOAc to form a mixed cation ‘water-in-bisalt’ electrolyte (WiBSE), prepared from a hydrated second salt (LiOAc•2H2O), delivering a stability window of 2.95 V. CR2032 coin cells are assembled based on the 45 m CsOAc + 7 m LiOAc WiBSE, with a VO2 nanobelt anode and LiNi0.5Mn1.5O4 hollow microsphere cathode, which deliver an operating voltage of 2.5 V. It exhibits a maximum specific discharge capacity of 68.1 mAh•g − 1 and relevant energy density of 114.8 Wh•kg−1 (calculated on the total mass of cathode and anode) under 1 C rate. At 4 C rate, the coin cell has a coulombic efficiency (CE) of about 99% reaching near 100% at certain cycles, and a desired capacity retention of 86.5% during the 1200 cycles. The crystal structure of the VO2 prepared in this study has been refined by GSAS+EXPGUI and it was found the lattice parameters are smaller than the conventional VO2 (B) phase although they share the same symmetry.