Researchers from the UK’s WMG, University of Warwick, have found that dew points of around -45oC are the best for storing and processing the materials for making high-nickel batteries such as NMC 811.
It is the first time the researchers have given a definite temperature for ensuring the best battery performance in high-Ni cathode materials, which are prone to reactivity and instability when exposed to humidity.
The team found that after 28 days of ambient moisture exposure the H2O and CO2 reacts with the Li+ ions in the battery cell, resulting in the formation of lithium carbonate and hydroxide species on the surface of NMC 811 cells.
This contributes to the inferior ionic and electronic conductivity, and the electrical isolation of the active particles so they can no longer reversibly store lithium ions to convey charge.
SEM analysis by the team confirmed the inter-granular porosity and micro-cracks on these aggregate particles, following the 28 days of ambient exposure.
Researchers exposed NMC 811 cells to different temperatures and humidities, then measured the material’s performance and degradation in a battery over a 28 day period, analysing them using a combination of physical, chemical and electrochemical testing.
Testing included high-resolution microscopy to identify the morphological and chemical changes that occurred at the micron and sub-micron scale during the batteries charging and discharging.
The storage conditions included vacuum oven-dried, as exposed (to humidity) and a control measure.
The findings were explained in the paper ‘The effects of Ambient Storage Conditions on the Structural and Electrochemical Properties of NMC 811 Cathodes for Li-ion batteries,’ published in the peer-reviewed journal Electrochimica Acta.
Dr Mel Loveridge from WMG at the University of Warwick said: “Whilst moisture is well known to be problematic here, we set about to determine the optimal storage conditions that are required to mitigate unwanted, premature degradation in battery performance.
“Such measures are critical to improve processing capability, and ultimately maintain performance levels. This is also of relevance to other Ni-rich systems e.g. NCA materials.”
