Rinkel Bernardine L D, Vivek J Padmanabhan, Garcia-Araez Nuria, Grey Clare P
Department of Chemistry, University of Cambridge Cambridge CB2 1EW UK
School of Chemistry, University of Southampton Southampton SO17 1BJ UK.
Energy Environ Sci. 2022 Jul 5;15(8):3416-3438. doi: 10.1039/d1ee04053g. eCollection 2022 Aug 11.
Preventing the decomposition reactions of electrolyte solutions is essential for extending the lifetime of lithium-ion batteries. However, the exact mechanism(s) for electrolyte decomposition at the positive electrode, and particularly the soluble decomposition products that form and initiate further reactions at the negative electrode, are still largely unknown. In this work, a combination of gas measurements and solution NMR was used to study decomposition reactions of the electrolyte solution at NMC (LiNi Mn Co O) and LCO (LiCoO) electrodes. A partially delithiated LFP (Li FePO) counter electrode was used to selectively identify the products formed through processes at the positive electrodes. Based on the detected soluble and gaseous products, two distinct routes with different onset potentials are proposed for the decomposition of the electrolyte solution at NMC electrodes. At low potentials (<80% state-of-charge, SOC), ethylene carbonate (EC) is dehydrogenated to form vinylene carbonate (VC) at the NMC surface, whereas at high potentials (>80% SOC), O released from the transition metal oxide chemically oxidises the electrolyte solvent (EC) to form CO, CO and HO. The formation of water this mechanism was confirmed by reacting O-labelled O with EC and characterising the reaction products H and O NMR spectroscopy. The water that is produced initiates secondary reactions, leading to the formation of the various products identified by NMR spectroscopy. Noticeably fewer decomposition products were detected in NMC/graphite cells compared to NMC/Li FePO cells, which is ascribed to the consumption of water (from the reaction of O and EC) at the graphite electrode, preventing secondary decomposition reactions. The insights on electrolyte decomposition mechanisms at the positive electrode, and the consumption of decomposition products at the negative electrode contribute to understanding the origin of capacity loss in NMC/graphite cells, and are hoped to support the development of strategies to mitigate the degradation of NMC-based cells.
防止电解质溶液的分解反应对于延长锂离子电池的寿命至关重要。然而，正极处电解质分解的确切机制，尤其是在负极形成并引发进一步反应的可溶性分解产物，在很大程度上仍然未知。在这项工作中，结合气体测量和溶液核磁共振技术研究了NMC（LiNiMnCoO）和LCO（LiCoO）电极上电解质溶液的分解反应。使用部分脱锂的LFP（LiFePO）对电极来选择性地识别通过正极处过程形成的产物。基于检测到的可溶性和气态产物，提出了NMC电极上电解质溶液分解的两条具有不同起始电位的不同途径。在低电位（<80%充电状态，SOC）下，碳酸亚乙酯（EC）在NMC表面脱氢形成碳酸亚乙烯酯（VC），而在高电位（>80%SOC）下，从过渡金属氧化物释放的O化学氧化电解质溶剂（EC）形成CO、CO和HO。通过使O标记的O与EC反应并利用H和O核磁共振光谱对反应产物进行表征，证实了该机制中H2O的形成。产生的水引发二次反应，导致通过核磁共振光谱鉴定出各种产物。与NMC/LiFePO电池相比，在NMC/石墨电池中检测到的分解产物明显更少，这归因于石墨电极处水（来自O与EC的反应）的消耗，从而防止了二次分解反应。对正极处电解质分解机制以及负极处分解产物消耗的深入了解有助于理解NMC/石墨电池中容量损失的起源，并有望支持减轻基于NMC的电池降解的策略的开发。
