A fast and accurate lifetime estimation of a lithium-ion batteries is decisive for planning a sustainable technological future. For this purpose, batteries are aged using the highest possible current rates in accelerated cyclic aging tests excluding test pauses. This strategy, however, can lead to electrolyte-motion-induced salt inhomogeneity (EMSI) [1], which disrupts the homogeneity of lithium distribution (HLD) within the anode and skews aging predictions. This study presents findings on how EMSI and HLD impacts capacity and resistance measurements.
During accelerated cyclic aging, we observed that capacity declines during cycling and recovers to a high extend when cells are idle. This is called apparent aging in addition to potential anode overhang effect. Our used test strategy for this observation is in Figure 1a. We observed this phenomenon using differential voltage analysis (DVA) in cylindrical [2], compressed prismatic [3], and now compressed pouch cells [4]. In these cases, the jelly-roll confinement and cycling currents ensured the proper conditions for electrolyte pumping effect causing EMSI. In these cases, we investigated the effects of pressure, temperature, and current rates on the apparent aging effect.
For the first part of our study, five pouch cells were compressed as shown in Figure 1b. By varying holder stiffness through different battery pads (Figure 1b, right), we ensured a different amount of electrolyte displacement during cycling. Cells with higher holder stiffness showed greater recoverable capacity loss and more pronounced inhomogeneity of lithium distribution. Figure 1c illustrates this, with aging trends dividing the cells into two groups: Group 1 (low holder stiffness) and Group 2 (high holder stiffness). This approach allows us to characterize the effects of holder stiffness and offset pressure on apparent aging.
In the second part, we examined the influence of current rates and temperature. Here, 14 cells were compressed to 651 kPa (7 cells) and 321 kPa (7 cells) with a stiffness of 11.12 MPa/mm, while varying current rates according to the test matrix in Figure 2a. Contrary to common belief, cells with the highest C-rates did not exhibit the highest inhomogeneity of lithium distribution and with this, the highest rate of capacity loss. As observed, the capacity loss during cycling is recovered during resting, suggesting that cell temperature plays a significant role in EMSI during accelerated aging tests, which will be elaborated in our presentation in detail.
This investigation clarifies how factors like holder stiffness, C-rate, and temperature impact EMSI and lifetime estimation in accelerated cyclic aging. While higher holder stiffness and C-rate encourage EMSI, increased temperature mitigates it.
This research was conducted at the Technische Hochschule Ingolstadt as part of the “Beschleunigte Alterungstests und Lebensdauerprognosen” project, funded by BMBF from 2021 to May 2024.”
1. Solchenbach et al., DOI 10.26434/chemrxiv-2024-xdx2q
2. Morales Torricos et al., DOI 10.3390/batteries9040230
3. Lewerenz et al., DOI /10.1016/j.est.2019.01.004
4. Morales Torricos et al., DOI 10.2139/ssrn.4980038