Depletion of Electrolyte Salt during Calendaric Aging at High State-of-Charge in Li-Ion Batteries
Authors: Hartmann Louis , Wallisch Lea , Beiersdorfer Anton , Reuter Lennart , Adam Alexander , Goldbach Daniel , Teufl Tobias , Lamp Peter , Gasteiger Hubert A. , Wandt Johannes 
Published: September 2023

Batteries using Ni-rich cathode active materials are the most common cells in the automotive sector. They offer a high energy density at competitive prices which makes them attractive in a wide-range of applications.[1,2] However, a higher Ni-content results in more challenges for battery lifetime in comparison to low-Ni materials. These challenges are aggravated at elevated temperatures and high SOCs, stemming from detrimental side-reaction, such as parasitic heat and oxygen release.[3–6] In this study, we investigated an aging effect which is relevant in a wide-range of cell formats ranging from small single layer pouch cells to automotive cells. Upon storing cells at a high SOC at elevated temperatures (30 days, 45 °C) a significant drop in rate-performance was visible afterwards. At the same time, no active Li-loss and no significant increase in 10 second DCIR resistance at 50%SOC was seen. We propose a mechanism of LiPF6-loss during storage as the main reason for the worsened rate-performance.
By conducting ion chromatography (IC) and NMR analysis of electrolytes from aged cells, it was found that cells stored at an upper cut-off potential of 4.2 to 4.4 V lose a significant fraction of LiPF6 after a 30-day storage at 45 °C. In addition to these storage experiments, cells with fresh electrolyte, but lower electrolyte salt concentrations (1 M LiPF6 to 0.2 M LiPF6) were built, with the goal of using them as a calibration curve to map the worsened rate performance after storage onto a corresponding electrolyte salt concentration without the need of a destructive analysis of the cells. As seen in Figure 1, the rate performance of freshly built single layer pouch cell is highly dependent on the electrolyte salt concentration. While all cells show good discharge capacities at C/10, at higher discharge rates the lower concentration cells show a worsened performance. 1 M LP572 cells show excellent rate-capabilities up to a rate of 2C and a 0.2 M cell already has a significant drop in discharge capacity at C/2. Using OEMS, electrochemical impedance measurements and theoretical models we further elucidate the mechanism of LiPF6-loss at elevated temperatures and high SOCs.

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Figure 1: Calibration discharge rate-test for identical single layer pouch cells with varying conductive salt concentration of 1 M (black line and dots), 0.8 M (blue line and dots), 0.6 M (green line and dots), 0.4 M (yellow line and dots), 0.2 M (purple line and dots) LiPF6. 300 µL LP572 was used, and the charge was done at either C/10 or C/3 (CCCV) while the discharge was done at C/10, C/3, C/2, 1C, 2C, 3C, 4C, 5C.

Figure 1