The electrochemical performance of nickel-rich NMC cathode materials in Li-ion batteries depends on the kinetic control of side reactions at NMC-electrolyte/interface such as electrolyte oxidation, surface film formation, surface structure transformation and transition metal dissolution. However, the exact mechanism i.e. interdependence, sequence and impact of these interfacial phenomena is still poorly understood. The effect of surface reconstruction from layered- to spinel- or rock salt-phase(1, 2) in NMC materials during cycling on the kinetics of charge/discharge process remains unclear. As much as the exact chemical and physical properties of such surface reconstruction layer are still largely unknown, it is likely that it occurs simultaneously with other surface reactions and phenomena, which all together are difficult to deconvolute and evaluate.

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.

Acknowledgement

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.

Reference

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