Understanding the High-Voltage Cycling Behavior of Ni-Rich Cathode
Authors: Liu Hanshuo , Xie Zhong , Qu Wei 
Published: December 2020

Nowadays, lithium-ion batteries (LIBs) have been widely used in the automotive industry as power sources for electric vehicles (EVs). Nevertheless, the rapid development of EVs requires LIBs to enable higher energy density, enhanced safety, lower cost and longer cycle life. Ni-rich LiNixMnyCo1-x-yO2 (NMC) layered oxides have attracted great attention due to their high specific capacity (≈ 200 mAh/g), which are promising cathode materials for high-energy-density LIBs. The capability of high-voltage operation of the cathode can largely influence the energy density of LIB. Whereas, it is reported that Ni-rich cathode materials are unstable at high-voltage cycling conditions, leading to severe performance deterioration. (1, 2)

Attempts have been made in developing synthesis and modification methods of Ni-rich cathode materials in order to improve their high-voltage performance.(3, 4) However, the design of better materials relies intensively on the deep understanding of the cycling behavior of cathode. There are a few studies looking into the degradation process of Ni-rich cathodes upon high-voltage cycling. Most of the work mainly focuses on investigating structural changes of the active material, for example, phase transformation at the surface,(2) crack generation within active particles.(5) Whereas, less attention was paid to the evolutions within the whole electrode. Herein, we study the cycling behavior of LiNi0.8Mn0.1Co0.1O2 (NMC811) cathode at high cut-off voltages and look into the structural evolutions of cathode.

In this work, Ni-rich NMC811 cathodes were cycled at different upper cut-off voltages to investigate their charge-discharge behavior under high-voltage cycling conditions. The NMC811 cathodes were assembled into coin cells with Li foil as counter electrode. The cells were cycled under different upper cut-off voltages of 4.3V, 4.6V and 4.8V with a C/10 cycling rate. Electrochemical impedance spectroscopy (EIS) measurements were conducted to analyze the ionic conduction and charge transfer at the electrolyte/particle interface of coin cells cycled at different cut-off voltages. The microstructure of pristine and cycled NMC811 cathodes were characterized in order to understand the structural evolution among different phases during high-voltage cycling.

Our results show that high cut-off voltages have a negative influence on cell performance. The specific capacity of the cells cycled at 4.6V upper cut-off voltage showed a faster decrease compared to the 4.3V cells. In addition, more severe voltage decay is observed from cells cycled at 3.0V-4.6V. Whereas, when increasing the upper cut-off voltage from 4.6V to 4.8V, the cells exhibited less pronounced voltage decay, comparing to the difference observed between 4.3V and 4.6V cells. Significant increases in the charge transfer resistance (Rct) of cells cycling under higher voltages were observed from EIS measurements, which may indicate an elevated irreversible phase transformation of the active materials under high voltage cycling.(2) The microstructural evolution of NMC811 cathodes were observed under higher cycling voltages which could contribute to the poor performance of the high-voltage cells. The different behaviors of NMC811 cathode at high-voltages were analyzed in order to understand the degradation mechanisms of NMC811 cathodes under increasing cycling voltages.

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