To understand the mechanism of the surface reconstruction layer formation in Ni-rich NMC material during battery storage and its possible effect on the electrode and cell impedance a surface modification was deliberately induced in NMC particles artificially a priori via exposure to the electrolyte at elevated temperatures. The reaction of the electrolyte with NMC powders and the phase reconstruction of NMC surface were probed by surface sensitive techniques, including Raman, IR, SEM and X-ray absorption spectroscopy in total electron yield mode (XAS-TEY). Accompanying electrochemical testing including electrochemical impedance spectroscopy (EIS) was performed with half-cell configuration coin cells. Through comparison of the surface chemical properties and electrochemical performance between pristine NMC and treated samples, we were able to demonstrate that this artificial reconstruction layer on NMC does not cause a critical resistance in the battery. The treatment was shown to demonstrate some positive effects on the performance: i) higher capacity retention and ii)less resistance growth during electrochemical cycling. Surface chemical analysis shows that treated electrodes had reduced amount of organic film formation and less subsequent growth of reconstruction layers during electrochemical cycling compared with pristine NMC, indicating that an artificial reconstruction layer might suppress other degradation process of the battery, such as film formation and metal dissolution.
This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
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