Design of an Aprotic Solid-State Zn-Ion Battery

In this work we demonstrate the design of a solid-state Zn-ion battery with an aprotic solid electrolyte. The solid electrolyte is based on PVDF gelled with carbonate, similar to a solid-state Li-ion electrolyte recently reported by Khudiyev, et al.¹ This electrolyte is thermally drawable at 200 C and appropriate for use in a solid-state fiber battery. Zn-ion batteries could be an attractive alternative to Li-ion in some applications. Zn has good domestic sourcing and is more atmospherically stable than Li. Its cost is $2.99/kg as opposed to over $14/kg for Li metal. For the application of fiber batteries, Zn would allow the use of a metal wire anode, which would not be possible with Li.<br/><br/>The Zn-ion electrolyte had maximum room temperature conductivity of 1.8 mS/cm when using 0.5 M ZnTFSI. Electrolytes using Zn triflate had lower conductivity and were only semi-translucent due to incomplete dissolution of the salt. A Zn|Zn symmetric cell showed stable cycling over 500 hours, and we calculated a Zn-ion transference number of 0.41. In designing a full cell, several cathode active materials were assessed with varying success. Three different polymorphs of MnO₂ showed specific capacities far below theoretical. Of these, ��-MnO₂ had the highest capacity; ��-MnO₂ had a lower capacity; and ��-MnO₂ had virtually no reversible capacity. In contrast, Mo₆S₈ Chevrel achieved the theoretical capacity of 128.8 mAh/g that corresponded to the intercalation end member Zn₂Mo₆S₈.² However, Chevrel can only achieve full capacity at an elevated temperature of 50 C. This demonstrates the concept of a rechargeable aprotic solid-state Zn-ion battery. Previous reports of reversible Zn intercalation in ��-MnO₂ may be due to intercalation of H⁺ from residual H₂O that could be present if atmospheric control was not robust.<br/><br/><b>References</b><br/><br/>1. Khudiyev, T.; Grena, B.; Loke, G.; Hou, C.; Jang, H.; Lee, J.; Noel, G. H.; Alain, J.; Joannopoulos, J.; Xu, K.; Li, J.; Fink, Y.; Lee, J. T. "Thermally Drawn Rechargeable Battery Fiber Enables Pervasive Power," <i>Mater. Today,</i> <b>2022</b>, 52, 80–89.<br/><br/>2. Jadhav, A.L.; Juran, T.R.; Kim, M.A.; Bruck, A.M.; Hawkins, B.E.; Gallaway, J.W.; Smeu, M.; Messinger, R.J. "Reversible Electrochemical Anionic Redox in Rechargeable Multivalent-Ion Batteries," <i>Journal of the American Chemical Society</i>, 2023, 145, 15816−15826.