Structural Mapping the Fate of Lithium and Electrode Heterogeneity Under Uniform Stack Pressure in Extreme Fast Charging Lithium-Ion Batteries

Enabling fast charging in lithium-ion batteries (LIBs) is crucial for the development of electric vehicles, with the U.S. goal targeting 80% of full capacity within 15 minutes [1]. However, achieving this goal is challenging due to significant capacity fade at high charge rates, particularly at 6C (full charge in 10 minutes) and above. Extreme fast charging (XFC) induces irreversible lithium (Li) plating, which negatively impacts battery performance and safety. One way to mitigate accelerated degradation associated with fast charging is to apply uniform elevated stack pressure [2]. Stack pressure is known to improve electrical contact and prevent delamination, while increased stack pressure has also been observed to enhance electrochemical performance by reducing Li plating [2-4]. In this study, we quantify the effects of uniform stack pressure on lithium plating and provide structural insights into the degradation mechanisms responsible for capacity fade during fast charging.
We employed spatially resolved operando X-ray diffraction (XRD) using high-energy, high-flux synchrotron radiation to investigate lithium plating, and lithium intercalation into graphite under uniform stack pressure (10 to 125 psi). We quantify loss of lithium inventory (LLI) from irreversible Li plating and trapped Li as LiC₆, demonstrating a significant reduction in detectable irreversible Li plating with increased uniform pressure. Additionally, we quantified the loss of active material in the anode (LAM-NE), which showed low variation with increased pressure, as the applied pressure is insufficient to affect the anode crystal structure. By assessing lithium occupancy across both the anode and cathode, as well as lithium plating, we accounted for the fate of lithium in XFC full-cell LIBs. Moreover, we observed a decrease in heterogeneity in lithium distribution across the electrodes as a function of applied pressure. Our results suggest that the reduction in lithium plating and heterogeneity is driven by a more homogeneous stress field within the cell, which reduces the formation of local pressure gradients and large variations in the electric field where lithium plating is most likely to occur. These findings provide valuable insights into how mechanical pressure can be optimized to extend battery life and enhance safety in fast-charging applications.

References
1. USABC Goals – Lithium Electrode Based Cell and Manufacturing for Automotive Traction Applications, USABC (United States Advanced Battery Consortium), http://www.uscar.org/guest/article_view.php?articles_id=85
2. C. Cao et al. J. Electrochem. Soc. 129, 040540 (2022).
3. J. Cannarella and C. B. Arnold, J. Power Source, 245, 745-751 (2014).
4. X. Zhang et al. J. Electrochem. Soc., 166 (15) A3639-A3652 (2019)