Surface Treatment of Lithium Nickel Manganese Oxide Cathodes (NMx) for Electric Car Batteries

Sammanfattning

Todays lithium-ion batteries (LIBs) rely heavily on the supply of nickel, manganese and cobalt metals (NMC) for usage in cathode materials, but this dependence on cobalt presents significant challenges, such as price volatility and ethical concerns associated with mining, particularly as most cobalt originates from the Democratic Republic of Congo and is processed in China. This necessitates the wests demands to explore cobalt-free alternatives like nickel-manganese oxide (NMx). While promising, NMx suffers from among other things limited long-term cycle stability. This thesis aimed to improve the long-term cycle stability of NMx by applying a protective cobalt-based surface coating using a solid-state synthesis method. The goal was to significantly reduce the overall cobalt content compared to traditional NMC. NMx was mixed with varying amounts of cobalt(II) hydroxide and heat treated. The resulting materials were then characterized using techniques such as SEM/EDX, XRD, XPS, and tested electrochemically via galvanostatic cycling. Characterization, specifically XPS depth profiling, indicated that a uniform and protective cobalt surface layer was not successfully formed on the NMx particles with the method used. Cobalt concentration remained relatively constant with sputtering depth, contrary to expected results of a successful coating. Unexpected cobalt-rich spherical particles were also observed via SEM/EDX. Consistent with the lack of a formed coating, electrochemical testing showed no significant improvement in long-term cycle stability compared to the uncoated NMx material. All samples exhibited a similar substantial capacity drop after approximately 80 cycles. The conclusion is that the solid-state method did not yield the desired protective coating, potentially due to the precursor particle size. Future work could investigate alternative methods, such as a wet chemical approach involving dissolution and precipitation, to achieve better cobalt distribution and coating formation.