Dual-Salt aqueous electrolyte for enhancing Charge-Storage properties of VO2 polymorphic cathodes for Zn-Ion batteries

Abstract

Three VO2 polymorphs, namely monoclinic VO2 (B), monoclinic VO2 (M), and tetragonal VO2 (A), are synthesized, and their microstructures and electrochemical properties are studied for application in Zn-ion batteries (ZIBs). A cost-effective ZnSO4-based aqueous electrolyte with various concentrations (3 − 7 m) of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) additive is developed. The optimal ZnSO4/LiTFSI ratio in the electrolyte for superior Zn//VO2 battery properties is examined. The coordination structures of various electrolytes are investigated using wide-angle X-ray scattering. The incorporation of LiTFSI impairs the hydrogen-bonded network of H2O molecules and causes solvated-TFSI and TFSI‧‧‧TFSI structures to form, altering the electrolyte properties (such as electrochemical stability window, ionic conductivity, and Zn(OH)2)3(ZnSO4)·xH2O byproduct formation tendency). It is found that an excessive LiTFSI concentration leads to the formation of ion aggregates in the electrolyte and thus deterioration of electrode specific capacity and rate capability. Operando transmission X-ray microscopy observations confirm the high dimensional stability of the VO2 cathode during charging and discharging; the length variation of the VO2 (B) rods is approximately 10 %. The electrolyte composition also affects the Zn2+/Zn redox behavior at the anode side. The incorporation of LiTFSI into the electrolyte affects the Zn plating/stripping Coulombic efficiency, morphology of the deposited Zn, dead Zn amount, and anode cycle life. The constructed Zn//VO2 (B) cell with 1 m ZnSO4/5 m LiTFSI dual-salt electrolyte shows 90 % capacity retention after 1000 cycles. The proposed electrode material and electrolyte composition have great potential for applications in high-performance and high-stability rechargeable ZIBs.