LiNi_0.5Co_0.2Mn_0.3O₂ (NCM 523) is a promising cathode material for lithium-ion batteries due to its higher capacity, stability, and lower cost compared to LiCoO₂, the other commonly used layered oxide.  However, the high operating voltage of NCM523 (> 4.3V vs. Li/Li⁺) makes conventionally used electrolytes unstable. Their continuous electrochemical degradation at the electrodes surface adds to impedance rise, which is a major contributor to rapid capacity fade. High temperature conditions can accelerate these processes. Despite its practical and academic interest, there is no coherent model of cell degradation mechanisms responsible for the observed impedance changes.

In this work, lithium-ion cells consisting of an NCM523 cathode and a Li₄Ti₅O₁₂ (LTO) anode were prepared and subsequently aged at different temperatures and for different times after galvanostatically charging to a final hold potential of 4.6V vs. Li/Li⁺. Impedance increase and oxidation current were monitored as a function of aging time, hold voltage, temperature, and electrolyte additive concentrations. After aging, the cells were disassembled and chemical changes in the NCM523 electrode are characterized using a combination of techniques (including XPS, IR, and LC-MS) and correlated to the previously measured oxidation current and impedance increase.

 

Acknowledgement

The submitted issue has been created by the University of Chicago as Operator of Argonne National Laboratory (“Argonne”) under Contract No. W-31-109-Eng-38 with the U.S. Department of Energy.

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