Understanding the Role of Commercial Separators and Their Reactivity toward LiPF_6 on the Failure Mechanism of High-Voltage NCM523 Graphite Lithium Ion Cells

摘要

NCM523 || graphite lithium ion cells operated at 4.5 V are prone to an early "rollover" failure, due to electrode cross-talk, that is, transition metal (TM = Mn, Ni, and Co) dissolution from NCM523 and deposition at graphite, subsequent formation of Li metal dendrites, and, in the worst case, generation of (micro-)short-circuits by dendrites growing to the cathode. Here, the impact of different separators on the high-voltage performance of NCM523 || graphite cells is elucidated focusing on the separators' structural properties (e.g., membrane vs fiber) and their reactivity toward LiPF6 (e.g., ceramic-coated separators). First, the separator architecture has a major impact on cycle life. Fiber-structured separators can prevent the "rollover" failure by a more homogeneous deposition of TMs and formation of Li metal dendrites, thus, hindering penetration of dendrites to the cathode. In contrast, porous membrane-structured separators cannot prevent the cell failure due to inhomogeneous TM deposits/Li metal dendrites. Second, it is demonstrated that different types of ceramic-coated separators (Boehmite (gamma-AlO(OH)) vs alpha-Al2O3) exhibit different reactivities toward LiPF6. While alpha-Al2O3 shows a minor reactivity toward LiPF6, the gamma-AlO(OH) coating leads to in situ formation of the beneficial difluorophosphate anion in high amounts due the high reactivity toward LiPF6 decomposition, which significantly improves cycle life.