VO2 Polymorphic Cathodes and Dual-Salt Aqueous Electrolyte for Zn-Ion Batteries

Three VO 2 polymorphic cathodes, namely monoclinic VO 2 (B), monoclinic VO 2 (M), and tetragonal VO 2 (A), are synthesized and their morphologies and electrochemical performances are examined for Zn-ion battery (ZIB) applications. It is found that VO 2 (B) outperforms VO 2 (M) and VO 2 (A) in terms of the specific capacity and rate capability due to its relatively large number of charge storage sites, high electronic conductivity, and wide structural tunnels that can facilitate Zn 2+ transport. Further, a cost-effective ZnSO 4 -based aqueous electrolyte with various LiTFSI concentrations (3−7 m) is formulated. To obtain superior Zn//VO 2 battery performance, the optimal ZnSO 4 /LiTFSI ratio in the electrolyte is explored. Wide-angle X-ray scattering (WAXS) is used to examine the solvation structures of various electrolytes to find the correlation between the solution coordination status and electrolyte properties (such as the electrochemical potential window, ionic conductivity, and Zn(OH) 2 ) 3 (ZnSO 4 )· x H 2 O byproduct formation tendency). According to the WAXS data, the incorporation of LiTFSI greatly alters the solution structures and 1 m ZnSO 4 /5 m LiTFSI electrolyte has reduced water activity. Thus, the anodic limit (that related to oxygen evolution from the decomposition of water) of the electrolyte is extended, and the Zn(OH) 2 ) 3 (ZnSO 4 )· x H 2 O byproduct formation tendency is suppressed. The thinner byproduct layer, which reduces the blocking of ion/proton intercalation reactions, is responsible for the superior charge-discharge properties of the VO 2 (B) electrode obtained in the 1 m ZnSO 4 /5 m LiTFSI dual-salt electrolyte. The unique solution structure of this electrolyte also alleviates the hydrogen evolution side reaction, leads to the flat and compact Zn deposits, and reduces the amount of dead Zn on the electrode after cycling. The VO 2 (B) electrode has great dimensional reversibility and stability during cycling in the 1 m ZnSO 4 /5 m LiTFSI electrolyte, as verified via operando transmission X-ray microscopy (TXM). The proposed electrode/electrolyte design strategies are expected to be applicable for further development of ZIBs.