X-Ray Computed Tomography Evidence for Minimal Li-Metal Expansion during High-Rate Discharge in High-Energy Li|NMC811 Pouch Cells
Authors: Dutta Arghya , Mizuki Emiko , Tomori Yuka , Matsuda Shoichi 
Published: December 2024

Lithium (Li) metal batteries (LMBs) hold great promise in energy storage, offering high energy densities surpassing 350 Wh kg⁻¹ at the cell level.[1] However, their widespread adoption faces significant hurdles, including inefficient Li plating/stripping, low Coulombic efficiency, short cycle life, and dendrite formation. To address these challenges and enable the practical implementation of high-energy-density LMBs, a comprehensive understanding of degradation mechanisms affecting Li metal electrodes is crucial. Yet, the intricate degradation mechanisms of Li metal electrodes remain elusive, particularly regarding the impact of charge/discharge rates, which are essential for various practical applications. While several studies have extensively explored the influence of charging rates on LMB performance, the effect of discharging rates remains unclear.[2] Additionally, in high-energy-density cell designs, the Li metal electrode is prone to damage during disassembly, highlighting the importance of non-destructive techniques for accurate degradation assessment.

This study investigates the impact of discharging rates on the degradation of Li metal electrodes in pouch-type cells with cell-level energy densities exceeding 350 Wh kg⁻¹.[3] Leveraging X-ray computed tomography (XCT) analysis as a non-destructive technique, three-dimensional structural information was obtained without sample damage. XCT analysis revealed substantial volume expansion of the Li electrode during charge/discharge cycles, especially under high-rate charging conditions. The repeated cycles induce unwanted side reactions between Li metal and the organic electrolyte, forming an electrically insulating solid-electrolyte interphase (SEI) layer on the electrode surface. Additionally, trapping of the limited amount of electrolyte (2 g Ah⁻¹) within the electrode pores exacerbates impedance growth and uneven current distribution. Interestingly, XCT observations, as shown in Figure 1, indicate suppressed volume expansion of Li during high-rate discharge, resulting in reduced impedance increase and improved cycle life. However, discharge rates beyond a threshold lead to declining capacity retention due to inefficient Li kinetics, amplifying discharge overpotential and prematurely reaching cut-off conditions. This study identifies an intermediate discharge rate offering superior cycle life, elucidating the interplay between Li electrode stability and kinetic constraints in high-energy-density LMBs under ultra-high discharge current rates. These insights hold substantial implications for advancing practical applications of high-power and high-energy LMBs.

References

[1] Niu, C.; Lee, H.; Chen, S.; Li, Q.; Du, J.; Xu, W.; Zhang, J.-G.; Whittingham, M. S.; Xiao, J.; Liu, J. High-Energy Lithium Metal Pouch Cells with Limited Anode Swelling and Long Stable Cycles. Nat
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2019, 4, 551–559.

[2] Bai, P.; Li, J.; Brushett, F. R.; Bazant, M. Z. Transition of Lithium Growth Mechanisms in Liquid Electrolytes. Energy Environ
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[3] A. Dutta, E. Mizuki, S. Matsuda, Batteries & Supercaps. 2023, 6, e20230030.

Figure 1. Cross-section XCT images of Li|NMC811 pouch cells. (a) After 50th cycle at 0.4 mA cm⁻² discharge and (b) 1.6 mA cm⁻² discharge under 0.4 mA cm⁻² charge condition and a limited capacity of 4 mAh cm⁻².

Figure 1