A Combined Investigation into the Effect of Ni/Mn/Co Ions on Lithium-Ion Battery Anodes Using X-Ray Photoelectron Spectroscopy and Secondary Ion Mass Spectrometry

Nickel Cobalt Manganese (NCM) cathodes utilize the combination of transition metal ions to achieve good specific capacity, voltage, and safety characteristics, and are among the best performing of industrially produced cathode materials. However, Mn is known to escape the cathode via a disproportionation reaction as Mn²⁺, and has been detected on the anode solid electrolyte interface (SEI). Furthermore, batteries incorporating Mn in the cathode are known to have a reduced cycle life due to increased impedance (1). A recent study of Lithium Manganese Oxide (LMO) cathodes reveals the Mn to stay in the 2+ oxidation state (2), but little is otherwise known about the role of Mn in the SEI layer.

To analyze the effect of the SEI on the degradation of NCM full cells, the authors previously analyzed anodes taken from commercial NCM batteries of varying degrees of degradation (3). X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) were used along with other techniques to analyze the presence of elements in the SEI as a function of depth. However, analysis of chemical species was complicated by the presence of PVDF binder, air exposure of the sample, and ion implantation/chemical decomposition due to exposure to a high energy ion beam.

In this study, the effect of temperature, cycling, and the presence of transition metal ions were studied in thin film half cells. Ni²⁺, Mn²⁺, and Co²⁺ion salts were each introduced, as these ions all dissolve into the electrolyte from NCM cathodes and have been detected in the SEI (4,5). A thin film carbon anode was used to obviate the binder, and allow clearer chemical analysis and depth profiling. XPS analysis was used to reveal chemical information about the SEI, while SIMS was used to provide a depth profile.

XPS analysis showed an increase in LiF composition with the number of charge/discharge cycles for all cells, but otherwise indicated a chemically different SEI when the salts are added. SIMS analysis showed the presence of Ni/Co/Mn ions and other species through the depth of the SEI layer. By combining these two methods, the character of the SEI with and without the transition metal ions was hypothesized.

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