Understanding the Improved High-Temperature Cycling Stability of a LiNi_0.5Mn_0.3Co_0.2O₂/Graphite Cell with Vinylene Carbonate: A Comprehensive Analysis Approach Utilizing LC-MS and DART-MS

An analysis approach utilizing liquid chromatography mass spectrometry (LC-MS) and direct analysis in real time mass spectrometry (DART-MS) is applied to discern the influence of vinylene carbonate (VC) on the cycling performance of a LiNi_0.5Mn_0.3Co_0.2O₂ (NMC)/graphite cell at elevated temperature. The VC-containing cell exhibits much improved cycling performance against the elevated temperature rather than the VC-free one. On the basis of the LC-MS results, more decomposition compounds including carbonate oligomers and organophosphates are present in the electrolyte without VC after the initial cycling, while they are less identifiable in the VC-containing electrolyte. On the other hand, the DART-MS results show that a thermally resistant film, which is primarily composed of the cyclic organophosphates, favors forming on the graphite anode surface of the VC-containing cell rather than the VC-free one, preventing further electrolyte decomposition. Moreover, a reaction scheme is proposed to reasonably explain the formation of the decomposition compounds, in which it is understood that VC can trap the free alkoxide anions and meanwhile allow more EC to react with POF₃ during the initial cycling, thus reducing the decomposition compounds in the electrolyte and facilitating the formation of thermally stable organophosphates. Consequently, the cell’s cycling performance is improved at elevated temperature.