Capacity Fade in Ni-Rich Cathodes: Additive Effects and Monitoring Surface Composition Changes
Authors: Guillot Sarah Lucienne , Usrey Monica Lee , Peña-Hueso Adrián , Zhou Liu , Kerber Brian M , Du Peng , Johnson Tobias 
Published: July 2021

Ni-rich cathodes used in lithium-ion batteries can provide a desirable increase in energy density due to their high specific capacity, such as NMC622 (LiNi0.6Co0.2Mn0.2O2) and NMC811 (LiNi0.8Co0.1Mn0.1O2). However, the higher capacity of Ni-rich cathodes comes with a downside: faster capacity fade and lower cycling stability. The source of capacity fade for these cathodes has been attributed to multiple degradation mechanisms including particle cracking[1], irreversible structural changes[2], and side reactions that catalyze electrolyte decomposition[3]. Previous studies have shown evidence that electrolyte additives that influence the cathode surface can inhibit cycling-induced degradation with high-nickel cathodes[4,5]. In this study, we investigate the cycling stability of multi-layer pouch cells with NMC622 or NMC811 cathodes and graphite anodes, and the effects of different additives on cycling performance and impedance.
Our study shows that in batteries with Ni-rich cathodes, organosilicon additives significantly improve cycling stability, as well as reduce gas generation during cycling. We use XPS to study the electrode surface compositions after different stages of cycling, and specific surface features are identified that correlate to performance and impedance over the course of cycling. NMR is used to monitor the consumption of additives from the liquid electrolyte. Through depth profiling and XPS signal attenuation, the surface layer thicknesses on the anodes and cathodes are estimated, and compared across different cycling protocols and different additives. It is proven that organosilicon additives alter the cathode and anode surface compositions, and the effect of changing the additive concentration on cycling stability and the electrode surfaces is also demonstrated. Varying structural features on the organosilicon additives, such as the degree of fluorination and type of functional group, are shown to impact the electrode interaction, impedance, and cycling stability. The impact of additives on reducing gas generation and affecting the gas composition is also quantified by the Archimedes method and gas chromatography. Overall, through this study we aim to provide enhanced understanding of the role of surface chemistry as an agent and an indicator of battery performance, and the effects of additives on surface chemistry and cycling stability.

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