Surface Degradation of NMC Electrodes Exposed to Air and Impact on Subsequent Cycling
Authors: Björklund Erik , Weatherup Robert S. 
Published: February 2024

To meet the ambitious international emissions commitments laid out in the Paris climate agreement making the transition of society from being dependent on fossil fuels towards increased use of renewable energy sources possible, lithium-ion batteries (LIBs) are a key enabling technology. Even though LIBs are already extensively used in portable electronics and the rapidly expanding market of hybrid- and full-electric vehicles, more extensive deployment requires longer cycle-lifetimes, enhancements in energy density, and reductions in cost. In electric vehicles one of the most common materials for the positive electrodes is based on the lithium nickel manganese cobalt oxide (LixNi1-y-zMnyCozO2, NMC) compound with different ratios of transition metals. The NMC materials are well known for consisting of reasonably abundant materials, their high energy density and good cyclebility. In order to decrease the price while also further improving the energy density, it has become more popular during the last few years to increase the nickel content, being responsible for the redox process thereby contributing to a larger capacity. By increasing the nickel content, the quantity of cobalt required will decrease further, reducing the price. However it is also well known that in materials where the nickel content is high the surface side reactions will be augmented leading to more degradation and shorter cycle life. A high nickel content will not only affect the electrochemical side reactions during cycling, it has also been found that as NMC materials are exposed to air they will react differently, with more reactions for the nickel rich materials.
It has been found that the NMC can react with H2O to form a LiOH layer and that once the layer is thick enough further side reactions would be prevented.1 It was assumed that the passivation was caused by the deintercalation of lithium, where the thin disordered rock salt layer would mitigate the Li+-water reactions through decreasing the amount of lithium deintercalation.1 Furthermore, the surface impurities formed during the air exposure have been observed to directly impact electrochemical performance during the first couple of cycles. Hatsukade et al. used isotopic labelling in order to measure the CO2 formation during the first cycles originating from the oxidative decomposition of these impurities.2 Air exposure has been shown to affect not only the first few cycles but also long term cycling as well.3 The degradation of the NMC has been found to be caused by phase transformation into rock salt and spinel structure, leading to nickel reduction and as a result formation of inter granular cracks, eventually leading to isolation of the active particles and increased charge transfer resistance.4

In this study the surface interactions at NMC electrodes during air exposure will be presented, showing both how the interactions take place as well as how the air exposure will affect the electrode-electrolyte interfaces during cycling. The NMC surface is analysed by x-ray photoelectron spectroscopy (XPS), photoemission electron microscopy (PEEM) and x-ray absorption spectroscopy (XAS).

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