Zinc-Ion Conducting Nonaqueous Polymer Electrolyte for Zinc-Metal Batteries through UV-Light Induced Cross-Linking Polymerization (2020)

Recently, rechargeable zinc-metal batteries (ZMBs) employing a metallic zinc (Zn) anode (negative electrode) coupled with a suitable cathode (positive electrode), and a zinc-ion conducting electrolyte are receiving tremendous attention as a post-lithium battery technology among the electrochemists. ( 1 ) Mostly, the electrolyte of choice in ZMBs are aqueous liquid electrolytes with few available reports on aqueous gel polymer electrolytes. The use of aqueous electrolytes often restricts the operation of the cell to potentials below 2V vs. Zn|Zn 2+ . ( 2 ) To improve the operating voltage of the ZMB cells, nonaqueous electrolytes can be employed. Despite few available reports suggesting acetonitrile and carbonate solvent-based nonaqueous liquid electrolytes for ZMBs, nonaqueous zinc-ion conducting polymer electrolytes (ZIPs) are rarely explored in ZMB full-cells. ( 3 ) During the evolution of nonaqueous ZMB technology, a transition from nonaqueous liquid electrolytes to solid/semi-solid-state ZIPs is highly desirable, which can leverage the prospects of futuristic safe and flexible ZMBs. In this context, we are reporting a simple and scalable process for the preparation of cross-linked ZIPs by ultraviolet (UV)-light induced cross-linking polymerization. The reactive mixture consisting of mono- and di-functional acrylate monomers, propylene carbonate solvent as the active plasticizer, zinc trifluoromethanesulfonate (Zn(OTf) 2 ) as the conducting salt, and a photo-initiator, on UV curing produces mechanically stable ZIP films. ( 4 ) Depending on the concentration of zinc-salt in the ZIP, the physicochemical properties were found to be varied. For instance, the glass transition temperature, as well as the compressibility of the ZIPs, increased with an increase in zinc-salt content. The optimized ZIP with ≈ 20 wt.% Zn-salt displayed the highest ionic conductivity value in the order of 10 -3 S/cm at room temperature. The same ZIP exhibited an oxidation stability beyond 2.4 V vs . Zn/Zn 2+ , which was higher than ZIPs with low zinc-salt content. Besides, the same ZIP exhibited a stable plating/stripping profile for approximately 8 days in Zn|ZIP|Zn symmetric cell indicating excellent interfacial compatibility against Zn metal. For fabricating ZMB full-cell using the optimized ZIP, vanadyl phosphate (VOPO 4 . 2H 2 O) was used as the cathode. The VOPO 4 . 2H 2 O|ZIP|Zn cell was fabricated by the direct generation of ZIP over the cathode electrode (the in situ polymerizationprocess) ( 5 ) . The in situ process helps in achieving an improved electrode|electrolyte interface compared to the ZIP film-based ZMB cell. On cycling, the in situ fabricated ZMB cell exhibited a specific capacity of ≈ 80 mAh g -1 at a current density of 0.50 A/g. At 0.20 A/g, the ZMB cell retained 63% of the initial capacity over 50 continuous galvanostatic charge-discharge cycles. Considering the rarely explored nonaqueous zinc electrochemistry in polymer electrolytes, this report is important, improving the prospects of zinc-based post-lithium battery technology. References G. Fang, J. Zhou, A. Pan and S. Liang, ACS Energy Letters , 3 , 2480 (2018). M. Ghosh, V. Vijayakumar and S. Kurungot, Energy Technology , 7 , 1900442 (2019). Y. Zhang, Z. Chen, H. Qiu, W. Yang, Z. Zhao, J. Zhao and G. Cui, NPG Asia Materials , 12 , 1 (2020). V. Vijayakumar, D. Diddens, A. Heuer, S. Kurungot, M. Winter and J. R. Nair, ACS Applied Materials &amp; Interfaces , 12 , 567 (2020). V. Vijayakumar, B. Anothumakkool, S. B. Nair, M. V. Badiger and S. Kurungot, Journal of Materials Chemistry A , 5 , 8461 (2017). Figure 1