Studies on Non-Destructive State-of-Health Estimation for Lithium-Ion Batteries
Authors: Plathong Peerada , Miyahara Yuto , Miyazaki Kohei , Lee Changhee , Abe Takeshi 
Published: December 2024

Introduction
Recently, lithium-ion batteries (LIBs) have been used as an energy storage technology for electric vehicles and energy storage systems due to their high energy, power density and wide operating temperature range. However, degradation inevitably causes capacity and power decay. For the safer use of LIBs, state of health (SOH), which describes the condition of a battery compared to its initial condition, needs to be monitored to avoid electrolyte leakage and micro-short circuiting that lead to battery failure. In general, SOH can be estimated by identifying changes of parameters such as voltage, capacity, and impedance. Also, thickness swelling caused by side reactions such as SEI growth, lithium plating, or gas generation can be used as a degradation parameter to estimate SOH.[1] Applying mechanical pressure to LIB cells, which could externally modify the contribution of the side reactions, is a well-known effective approach to decrease irreversible thickness swelling.[1] On the other hand, the impact of electrolyte additives, which could internally modify the contribution of the side reactions, on the swelling behaviors has not been well-understood.
In this study, non-destructive techniques including electrochemical impedance spectroscopy (EIS) combined with thickness measurement were employed to investigate the effects of electrolyte additives on the degradation behaviors of the cycled LIBs, which could be related with the SOH. Also, post-mortem analysis of the cycled LIBs was performed to further investigate the degradation degree of positive/negative electrodes separately.
Experimental
Laminated LIB cells with nominal capacity of 380 mAh were used for accelerated degradation tests. The positive and negative electrodes were LiNi0.5Co0.2Mn0.3O2 (NCM) and graphite, and the electrolyte was 1 mol dm−3 LiPF6/ethylene carbonate–diethyl carbonate (EC–DEC, 3:7 by volume) containing vinylene carbonate–fluoroethylene carbonate (VC–FEC, 1 wt% each), or without additive. At the beginning, low-rate charge–discharge cycles at a current density of 76 mA were performed at 25 ℃. EIS with a frequency range from 1 MHz to 1 mHz was carried out, followed by the measurement of the cell thickness. Then, high-rate charge–discharge cycles at a current density of 760 mA were performed at 60 ℃. Low-rate capacities of the degraded cells were measured every 100 cycles of high-rate charge–discharge.
Results and discussion
The discharge capacities of the LIB cells roughly showed a decrease with charge–discharge cycles and dropped to around 80-85% of their initial value after 512 cycles. Compared with the LIB cells without additives, those with VC + FEC exhibited better high-rate and low-rate durability.
In figure 1, Nyquist plots of the fresh full cell had two semicircles. The one at lower frequency was dependent upon temperature and voltage which was corresponded to charge transfer resistance.[2] From results of the symmetric cells, the semicircle at higher frequency should be assigned to the contact resistance between the current collector and the NCM electrode. There was also a semicircle of SEI resistance observed in graphite symmetric cells after degradation cycles, although it overlapped with the larger contact resistance in the full cell due to their similar time constants.
With the increased number of the high-rate charge–discharge cycles, the semicircle at higher frequency which mainly came from the contact resistance increased, indicating the degradation of the LIB cells. This value significantly increased with the decrease in the low-rate discharge capacity, suggesting that the high-frequency semicircle could be used as a parameter for SOH estimation. Also, the increasing behavior of the contact resistance was suppressed with the presence of VC + FEC additives, indicating that suitable additive could suppress the increase of the contact resistance at the positive electrode and/or could be important for the formation of robust SEI at the negative electrode.

References

[1] P. Mohtat, S. Lee, J. B. Siegeland, and A. G. Stefanopoulou, J. Electrochem. Soc. 168 100520 (2021).
[2] A. S. Keefe, S. Buteau, I. G. Hill, and J. R. Dahn, J. Electrochem.
Soc.
166 A3272 (2019).

Figure 1