Using the Charge-Discharge Cycling of Positive Electrode Symmetric Cells to Find Electrolyte/Electrode Combinations with Minimum Reactivity

Shen, Chongheng; Xiong, Deijun; Ellis, L. D.; Gering, Kevin L.; Huang, Ling; Dahn, J. R.

November 2017 - The effects of solvents, salts, electrolyte additives and surface coatings on LiNi0.4Mn0.4Co0.2O2 (NMC442) or LiNi0.6Mn0.2Co0.2O2 (NMC622) have been probed using positive electrode Li-ion symmetric cells coupled with dV/dQ analysis. A robust symmetric cell design is presented which prevents hardware corrosion for over at least 800 hours of testing to 4.5 V vs. Li/Li+ at 40°C. Positive electrode symmetric cells using uncoated positive electrode materials and 1M LiPF6 EC:EMC 3:7 or 1M LiPF6 EMC electrolyte rapidly developed high impedance and showed poor capacity retention. However, if 1% pyridine boron trifluoride (PBF) was added to these electrolytes, cell performance was dramatically improved. Replacing LiPF6 by LiBF4 in the electrolytes above, with or without PBF, yielded positive electrode symmetric cells with good capacity retention. Two types of surface coatings were explored on NMC622 positive electrodes. Cells using surface-coated positive electrodes demonstrated better capacity retention for all electrolytes compared to cells without surface coatings. This work can be used as a guide by those attempting to find electrolyte/electrode pairs suitable for use in NMC/graphite cells that can operate with long lifetime to 4.5 V.

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