Understanding the Effects of Uniform Stack Pressure in Extreme Fast Charging Lithium-Ion Batteries Via Operando X-Ray Diffraction

Abstract

Enabling fast charging in lithium-ion batteries (LIBs) is critical for electric vehicle development, the U.S. goal being to charge to 80% of full capacity in 15 minutes [1]. This is a challenge due to significant capacity fade occurring at charge rates of 6C (charge in 60/6 = 10 minutes) and above. Fast charge rates induce irreversible lithium (Li) plating that adversely impacts battery performance and safety. External stack pressure can improve electrical contact and minimize delamination; further, an understanding of the impact of pressure on capacity fade can provide insight into the degradation mechanisms [2,3]. In this work, we investigate the extreme fast charging (XFC) regime, focusing on cell degradation and parasitic Li plating using single-layer pouch cells with graphite anodes, LiNi0.5Mn0.3Co0.2O2 (NMC532) cathodes, and 1.2 M LiPF6 in 3:7 weight % ethylene carbonate and diethyl carbonate (Gen2 electrolyte). To study the effect of pressure, electrode thickness and material capacity were varied. Cells were cycled under set uniform stack pressure configurations at 6C up to 140 times and tested between 10 – 150 psi. We have characterized fast charging cells under uniform pressure with spatially resolved operando X-ray diffraction (XRD) from high energy, high flux synchrotron radiation to study Li plating, parasitic reactions, and Li intercalation into graphite. Through spatially resolved operando XRD, we have mapped the intensity of LixC6 and Li metal peaks across the electrode to correlate peak intensity as a function of pressure and electrode thickness. This was complemented with ex situ X-ray photoelectron spectroscopy to study the composition of parasitic reaction products, corrosion and to correlate heterogeneity and local Li plating. Our results show an improvement in battery performance with increased uniform pressure. A reduction in Li plating was observed at elevated uniform pressures and Li plating appears more prevalent on the thicker electrodes. We believe that at increased pressure the reduced capacity fade arises from improved wettability, where the electrolyte acts as a lubricant increasing active sights, and a reduction in polarization across the thickness of the electrode leading to more homogenous reaction conditions. We emphasize the importance of electrode stack pressure in XFC batteries by quantifying the degradation mechanisms that cause capacity fade to enable better battery engineering.References 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=85C. Cao et al. J. Electrochem. Soc. 129, 040540 (2022).J. Cannarella and C. B. Arnold, J. Power Source, 245, 745-751 (2014).