It is essential to improve the energy density of the state-of-the-art lithium battery technology in order to facilitate the extensively application of electric vehicles (EVs). Owing to their outstanding thermostability and high energy density, Ni-rich layered materials LiNixMnyCozO2 (NMC) are one of the most promising cathode materials for EV batteries. Yet, the instability of conventional electrolyte, which comprises lithium hexafluorophosophate (LiPF6) dissolved in a mixture of ethylene carbonate (EC) and linear carbonates, hinders the application of these new materials. This instability was attributed to the low anodic stability of EC, resulting in severe electrolyte decomposition and triggering serious transition metal dissolution. However, it is too early to conclude that the anodic instability of electrolyte solvents solely determines the effectiveness of the electrolyte system without performing an in-depth study of the structure-activity relationship between the solvents and the interfacial parasitic reactions. In this presentation, we successfully introduce the molecular pair analysis and linear free-energy relationship (LFER) study as powerful physical organic analytical techniques to probe the underlying fading mechanism of NMC materials. The molecular pair study clearly shows the major decaying mechanism of the NMC/graphite cell cycled at a high voltage is solvation-driven, but there is no correlation between the solvating power of an electrolyte solvent and the determining step in the decaying mechanism of a LNMO/graphite cell. This result was corroborated by the establishment of LFER between the relative solvating power of the electrolyte solvents and the extent of cathode parasitic reactions in the NMC cell cycled at high potential (≥ 4.4 V). Yet, the same LFER cannot be found when the NMC cell is cycled at a relatively low cutoff voltage (≤ 4.3 V). Therefore, apart from the anodic stability, the overall solvating power of the electrolyte is also important in designing a reliable high-voltage electrolyte system.

Quantitative structure-activity relationship between electrolytes and the decaying reactions of lithium layered oxide materials

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