Single Component Protection Layers for Lithium Electrodes and Their Characterization in Lithium Metal Batteries

Abstract

State-of-the art Lithium-ion batteries (LIBs) based on graphite intercalation compound (GIC) are reaching their theoretical energy density limits. Therefore, it has become prerequisite to implement active materials with higher specific energies.[1] In this direction, Lithium (Li) has reignited research interest mainly due to its high theoretical specific capacity (3860 mAh/g) and the lowest negative potential (-3.040 V vs. SHE). Many of the advantages are shadowed by its poor electrodeposition properties with the formation of high surface area lithium (HSAL) and continuous electrolyte consumption resulting in a solid electrolyte interphase (SEI) with inhomogeneous composition which still limits the practical application of Li metal electrodes in Lithium metal batteries (LMBs).[2] Overall, several published strategies can be summarized into improving the electrolyte with additives, development of 3D hosts or interface engineering by covering Li with a protective coating layer. An artificial protection layer on Lithium metal requires tuned characteristics to withstand the harsh conditions during cycling. Therefore, it is obligate for these coatings to fulfil several requirements, among many i) mechanical suppression of dendrites, ii) high Li-ion selectivity and conductivity and iii) chemical passivation against the liquid electrolytes, are mostly encountered with artificial SEI.[3],[4] In a recent study an in-situ generated lithium methyl carbonate (LMC) layer on the Li metal electrode was able to protect its surface by generating a homogeneous Li-ion flux and decreasing the electrolyte consumption process during cycling. This approach also enhanced the current distribution and effectively suppressed the lithium dendrite formation during the electrodeposition and -dissolution process. [5],[6] Herein, the LMC layer, synthesized and purified in order to exclude the presence of any halides, was coated on the surface of the Li metal electrode using an ex-situ procedure. Its characterization and the effect of LMC on the electrochemical performance of the coated Li metal electrode especially during the Li electrodeposition/dissolution process will be presented. Li metal electrodes coated with a dense LMC layer showed decreased overvoltages during the Li electrodeposition/dissolution process and reduced interfacial resistance as compared to uncoated Li metal electrodes. Beside its role as a protective layer, LMC was also employed as electrolyte additive due to its known solubility in carbonate-based electrolytes. Using ionic chromatography (IC) to investigate the electrolyte after cycling and cryo-SEM cross-section of the cycled Li metal electrodes, further characteristic properties of these LMC coating layers on Li metal electrodes will be revealed.[1] Placke, T.; Kloepsch, R.; Dühnen, S.; Winter, M., Journal of Solid State Electrochemistry 2017, 21, 1939-1964[2] Aurbach, D.; Zinigrad, E.; Cohen, Y.; Teller, H., Solid State Ionics 2002, 148, 405-416.[3] Kang, D.; Xiao, M.; Lemmon, J. P., Batteries & Supercaps 2021, 4, 445.[4] Yu, Z.; Cui, Y.; Bao Z., Cell Reports Physical Science, 2020 7, 100119.[5] Liu, H.; Zhou, H.; Lee, B., ACS Applied Materials and Interfaces 2017, 36, 30635-30642.[6] Liu, H.; Wang, X.; Zhou, H., ACS Applied Materials and Interfaces 2018, 5, 1864-1869.