Chemical Crossover Accelerates Degradation of Lithium Electrode in High Energy Density Rechargeable Lithium-Oxygen Batteries

There has been considerable interest in developing rechargeable lithium oxygen batteries (LOBs) that have the potential to significantly exceed the energy density limit of a Li ion battery (approximately 300 Wh/kg). Although 500 Wh/kg LOB has been demonstrated and exhibited stable discharge/charge cycle at room temperature condition¹, their cyclability remains less than 10 cycle. For realizing the LOB with practically high energy density and long cycle life, the deep understanding of complicated reaction in LOBs has been highly demanded. Especially, the problems specifically associated with the limited electrolyte (“lean-electrolyte”) conditions need to be taken into account in LOBs.<br/>Although the importance of investigating negative lithium electrodes in lean-electrolyte systems has been recognised in recent years², specific problems associated with the negative lithium electrode in a LOB has not been investigated so far. In particular, chemical crossover from the positive electrode to the negative electrode is a crucial factor that needs to be taken into consideration. Actually, recent studies have shown that LiOH is formed on the surface of a lithium-metal electrode through reaction with H₂O in the electrolyte^3,4. This H₂O is generated by the decomposition of the tetraethylene glycol dimethyl ether (TEGDME) solvent at the positive oxygen electrode, which then migrates to the negative electrode side to react with the lithium-metal electrode. Consequently, problems related to chemical crossover are prominent in lean-electrolyte LOBs, where the effective water concentration in the electrolyte is much higher than that in an excess electrolyte system.<br/>Despite the importance of investigating the reaction of negative lithium electrodes in LOB with appropriate technological parameters, mechanistic details remains unclear due to the technical difficulty for applying non-destructive analytical methods. In the present study, we investigated the reaction of a negative lithium electrode in a lean-electrolyte LOB operating under high areal capacity conditions. The use of advanced non-destructive analytical methods, including three-electrode electrochemical techniques and an in-situ analytical setup revealed that the reaction efficiency of the lithium electrode mostly decreases through chemical crossover from the positive oxygen electrode. In particular, the H₂O generated as a side-product at the positive oxygen electrode crosses over to the negative electrode side and reacts with the lithium-metal electrode to form a porous LiOH/Li₂CO₃ layer on the electrode surface. LiOH and Li₂CO₃ accumulates, and the porous layer thickens through repeated discharge/charge cycling, which supresses efficient Li-ion transport through the layer. In addition, the solvent decomposes rather than lithium peroxide during the final stage of charging at the positive oxygen electrode, and the generated CO₂ crosses over to the negative electrode side where it is further electrochemically reduced. This complicated LOB chemistry results in a significant decrease in reaction efficiency at the negative electrode. Importantly, the degradation behaviour observed for the lithium-metal electrode, which is due to chemical crossover from the positive electrode, is a common problem for next-generation rechargeable battery systems that use lithium electrodes. Hence, the methodology demonstrated in the present study, which combines non-destructive analytical techniques, effectively clarified the complicated hidden reaction mechanism associated with lithium-metal-based rechargeable batteries that operate under lean electrolyte conditions.<br/><br/><b>References</b><br/>[1] S. Matsuda, et al. Cell Reports Phys. Sci. 2021, 2, 100506.<br/>[2] S. Chen, et al. Joule, 2019, 3, 1094.<br/>[3] J. L. Shui, et al. Nat. Commun. 2013, 4, 1.<br/>[4] F. Sun, et al. ACS Energy Lett. 2019, 4, 306.