Comparison of Aqueous and NMP Processed LiNi0.6Mn0.2Co0.2O2 Cathodes - Microstructure, Electrochemical Performance and Aging Effects

摘要

In current industry-oriented manufacturing processes for lithium ion battery electrodes, the non-aqueous solvent N-methyl pyrrolidone (NMP) is widely used to dissolve the polyvinylidene difluoride (PVDF) binder and produce electrode pastes. Due to the high toxicity of NMP, aqueous anode processing was established as an alternative with the binder system of carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR), allowing for a more ecologically friendly and economic process. However, for cathode production, non-aqueous processing is still state of the art due to a performance reduction induced by aqueous processing. The current study compares conventionally NMP processed LiNi0.6Mn0.2Co0.2O2 (NMC622) cathodes and the aqueous processed ones in terms of microstructure, electrochemical performance and aging effects. The aqueous electrode paste shows a high pH value (> 12), which confirms a reaction between the aqueous binder solution and the NMC622 surface, also known as lithium-proton exchange. Furthermore, uniform cavities (? 50-100 μm) can be observed on the aqueous processed electrodes, which indicates gas formation due to an Aluminum current collector decomposition. Compared to NMP processed cathodes, the aqueous processed ones show a capacity drop of at least 10 mAh/g, which is even more pronounced at higher C-rates. Surface sensitive techniques (X-ray photoelectron spectroscopy, scanning electron microscopy), also structural (X-ray diffraction) and compositional (inductively coupled plasma-optical emission spectrometry) investigations are applied to examine the degradation effects behind, which lays a good foundation to further develop an aqueous processing for NMC622 cathodes.