Influence of periodic temperature variations on calendar ageing of lithium-ion batteries

<p dir="ltr">Calendar ageing of lithium-ion batteries is usually studied under constant-temperature conditions. In real-world applications batteries are exposed to a variety of temperatures due to environmental factors. It is well understood that temperature impacts degradation rate, but there is no comprehensive study on how temperature variation during calendar ageing impacts degradation. It is also unclear if degradation from constant-temperature ageing can be used to estimate degradation in cells undergoing temperature variation.</p><p dir="ltr">This work presents a calendar ageing study on 28 lithium-ion cells that spend different durations at high-temperature (50 °C) and moderate-temperature (25 °C) storage over three years. Seven exposure levels were tested ranging from 100 % duration at 50 °C to 100 % duration at 25 °C across 100 % and 50 % state of charge (SOC). Results show greater exposure to high-temperature led to faster rates of degradation, but varied with SOC. At 100 % SOC, slower initial degradation was observed compared to 50 % SOC, before increasing with time, predominantly due to increasing loss of lithium inventory. At 50 % SOC, degradation with different exposure levels can be predicted from constant-temperature data using a proposed model that weights degradation on the square root of exposure time. The ageing dataset from this work is available open-source.</p>