The Influence and Degradation Mechanism of the Depth of Discharge on the Performance of NMC-Based Cathodes for Li-Ion Batteries

Many factors affect the degradation behavior of lithium-ion (Li-ion) batteries and one of these is the depth of discharge (DOD). As Li-ion batteries are used, a reasonable DOD can not only extend their service life (by reducing the degradation rate) but can also reduce the frequency of the re-charging. Therefore, to investigate and clarify the effect of the DOD on cathode performance, we performed cycle aging tests on coin cells considering three DODs. Furthermore, we proposed a degradation mechanism, to account for the influence of the DOD on the cathode performance, through ex-situ post-mortem analysis.
The investigated positive electrode was from a commercial cathode NMC 532, assembled with a lithium metal anode in a 2016 type coin cell. The initial discharge capacity was about 163 mAh g–1 at a 1 C rate (1C taken as 160 mAh g–1). After every certain number of cycles, the 100% DOD (2.75 – 4.3 V) capacity was measured and recorded for all DOD ranges.

Our cycle aging test experiment results (in the below Figure) show that the capacity fades faster in the higher DOD range (i.e., 3.65 – 4.3 V); the capacity of coin cells showed an initial increase due to the initial activation and a rapid decline thereafter. In contrast, the battery capacity faded slower in the two lower DOD ranges (i.e., 2.75 – 4.3 V and 3.55 – 4.3 V). The results also show that the higher DOD makes the battery more active during the initial cycles, as shown in the Figure.

We refined and analyzed the XRD results of different states of the charged cathode to calculate the change in unit cell volume in initial different DOD cycles. By calculating the Li-ion diffusion coefficient through the EIS measurements, it was found that it is larger in the higher state of charge (SOC) state, which explains the higher activity of the cathode in a higher DOD range. Furthermore, we disassembled and analyzed the coin cells, after the same numbers of equivalent full cycles. Surface microcracks of the cathode were observed by SEM, and the cathode-electrolyte interphase (CEI) film was analyzed and quantified by XPS technology.

Based on these results, we concluded that higher DODs enable the material to maintain a faster Li-ion diffusion rate and are always in a highly activated state. At the same time, it leads to a faster cathode decay. The post-mortem analysis showed the detailed mechanism of degradation. Looking at the results of this study, frequent charging, making the battery operation at a higher voltage, aggravates deterioration of the cathode.

Acknowledgments

This project has received funding from the China Scholarship Council (no. 202006370035; no. 202006220024). Furthermore, the authors would like to thank Haidi company for providing the cathode material.