Study of Heterogeneous Aging Inside Different Pouch Cell Formats with Equal Chemistry (NMC 622/ Graphite + Silicon) and Operando Monitoring of the Aging Mechanisms

Lithium-ion batteries (LIBs) present a major challenge for the coming decade as they could be massively implemented in the EVs in order to replace combustion engines. Although they have been extensively studied, some issues remain, particularly with regard to fast charging and the resulting aging mechanisms. When it comes to the choice of electrode materials, compromises must be made between high energy density, safety and fast charging capability. In order to get performant batteries in these three topics, a better understanding of cell behavior at all scales is needed.<br/><br/>The objective of our study is to understand the impact of the cell format on the development of non-uniform aging [1] by establishing links between different scales: materials, electrodes and cell. There are currently several cell formats on the market ranging from a few mAh to several Ah, packaged in flexible or hard cases and with the electrodes stacked or wound. In order to control the different parameters that can influence the aging, we chose to manufacture the electrodes (NMC 622 (LiNi0.6Mn0.2Co0.2O2) / Graphite + 10% SiOx) and the cells in our lab. By doing this we were able to know the precise chemistry of the electrodes and electrolyte, and to assemble them in the formats we desired. Two different formats were studied: one with a single layer pair of electrodes of 30 mAh and another one, a multi-layer stack of 500 mAh. Having the same chemistry allows for comparison of aging behaviors [2] with minimal influencing parameters, except for cell design. The cells experienced the same protocols at different C-rates and different temperatures and were stopped at different state of health (SoH).<br/><br/>We monitored electrochemical responses of both positive and negative electrodes independently with a reference electrode, allowing the operando follow-up of the aging behaviors. One of the main concerns with LIBs aging is the growth of lithium plating, which is the metallic deposition of lithium ions on the surface of the negative electrode. The growth of lithium dendrites is a major safety issue because they can pass through the separator and create an internal short-circuit in the cell, causing a thermal runaway that can set the battery on fire. Therefore, following the negative electrode potential is crucial to determine the conditions of apparition of lithium plating. We noticed that the depth of discharge of the previous cycle highly influences the next charging profile of the negative electrode. That could bring an explanation about the sudden decrease in capacity observed frequently after the checkup cycles at low current in discharge performed regularly during the cycling test and used to evaluate the effective capacity.<br/><br/>We performed post-mortem analyses in order to correlate electrochemical response of each electrode, capacity fade, and local heterogeneous degradation mechanisms. Using visual inspection, surface analysis techniques such as EDS and XPS, impedance spectroscopy and XRD mapping, we tracked lithiation and degradation heterogeneities at different scale levels: material, electrode and cell.<br/><br/>References<br/><br/>[1] Beck, et al. « Inhomogeneities and Cell-to-Cell Variations in Lithium-Ion Batteries, a Review ». Energies 14, no 11 (2021): 3276. [2] Li, X. et al. « Degradation mechanisms of high capacity 18650 cells containing Si-graphite anode and nickel-rich NMC cathode ». Electrochimica Acta 297 (2019): 1109-20.