Exploring the Efficiency of Lithium Stripping/Plating in Solid Polymer Anode-Free Lithium Cells

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Traditional Li-ion batteries (LIBs) are widely used in various electronic devices due to their advantages in energy storage applications. However, finding ways to push the energy densities even higher would be highly desirable for the further rapid development of electric vehicles and electromobility. Li metal batteries and anode-free batteries (AFBs) – where the Li metal is plated in situ from Li+ ions taken from the cathode – have received extensive attention and research due to their very high specific capacity.[1] However, Li ions are unevenly deposited on the surface of anodes to form Li dendrite during the charging and discharging process, which severely deteriorates their electrochemical performance and battery safety.[2] To solve this problem, many researchers choose solid polymer electrolytes (SPEs) instead of liquid electrolytes in the batteries, because of the stability of SPEs towards Li metal.Long-term cycling of AFBs requires high-efficiency Li metal stripping and plating, which in a solid polymer cell is highly dependent on the polymer electrolyte system used, so finding appropriate host materials and salts to for the SPEs is very necessary. Poly(ethylene oxide) (PEO)-based SPEs are one of the most popular host materials for SPEs, due to their excellent chemical stability. Poly(ε-caprolactone) (PCL)-based SPEs have higher ionic conductivity and can be cycled at room temperature.[3] Poly(trimethylene carbonate) (PTMC)-based SPEs have high lithium transference number and better mechanical properties.[4] Lithium bis(trifluoromethanesulphonyl)imide (LiTFSI) is one of the most popular salts used for SPEs. Recently, lithium difluoro(oxalate)borate (LiDFOB) and lithium bis(oxalato)borate (LiBOB) have been considered a promising alternative as electrolyte because they can passivate electrode surfaces and protect electrode materials.[5] Combinations of these materials are all potentially relvant for application in AFBs.In this work, three different polymer host materials, PEO, PTMC, and PCL, were explored as polymer electrolytes together with LiTFSI salt for their performance in anode-less cell setups, testing the plating/striping coulombic efficiency (CE) in Cu || Li cells. The results reveal clear issues for the CE of cycling in the PEO:LiTFSI system, while low-concentration PTMC:LiTFSI instead shows much more promising results. Changing the LiTFSI salt for LiBOB or LiDFOB also improves the CE of the cells.[1] Beyene, T. T., Jote, B. A., Wondimkun, Z. T., Olbassa, B. W., Huang, C. J., Thirumalraj, B., Wang C. H., Su W. N., Dai H., Hwang B. J., Effects of concentrated salt and resting protocol on solid electrolyte interface formation for improved cycle stability of anode-free lithium metal batteries, ACS applied materials & interfaces, 2019, 11(35), 31962.[2] Huang W. Z., Zhao C. Z., Wu P., Yuan H., Feng W. E., Liu Z. Y., Lu Y., Sun S., Fu Z. H., Hu J. K., Yang S. J., Anode‐Free Solid‐State Lithium Batteries: A Review, Advanced Energy Materials, 2022, 12(26), 2201044.[3] Chiu, C. Y., Chen, H. W., Kuo, S. W., Huang, C. F., Chang, F. C., Investigating the effect of miscibility on the ionic conductivity of LiClO4/PEO/PCL ternary blends. Macromolecules, 2004, 37(22), 8424.[4] Li Z., Mogensen R., Mindemark J., Bowden T., Brandell D., Tominaga Y., Ion‐conductive and thermal properties of a synergistic poly (ethylene carbonate)/poly (trimethylene carbonate) blend electrolyte. Macromolecular rapid communications, 2018, 39(14):1800146.[5] Shkrob, I. A., Zhu, Y., Marin, T. W., Abraham, D. P., Mechanistic Insight into the Protective Action of Bis (oxalato) borate and Difluoro (oxalate) borate Anions in Li-Ion Batteries, The Journal of Physical Chemistry C, 2013, 117(45), 23750.