Experimental investigations of ageing of commercial BEV Li-ion batteries

Sammanfattning: The rapid development of lithium-ion batteries has enabled changes in widespread applications ranging from portable electronics to electric vehicles and large scale grid energy storage. However, battery ageing remain a challenge in all these applications, and this thesis seeks to contribute to the understanding of Li-ion battery degradation. In the compiled works, results from extensive testing on discharge current frequency, variation in SOC level and temperature are reported for cells with mixed material negative electrodes (Graphite/ SiOx ). Unexpected trends where cycling in low SOC causes more rapid degradation than cycling in high SOC are observed. The capacity retention after 1200 FCE was 84.7% for cells cycled in 5-15% SOC, whereas for cells cycled in 45-55% SOC the capacity retention after 1200 FCE was 94.7%. This dependency is shown to be stronger than the temperature dependency for mixed material negative electrodes. It is also demonstrated that dynamic current profiles affect the capacity retention with a strong correlation for frequencies below 100 mHz. The ageing processes involved are analysed using various in situ electrochemical characterisation techniques and post-mortem investigations. The strong SOC dependency is shown to be largely attributed to ageing of SiOx in mixed material electrodes, with LAMSiOx contributing to a loss of 7% of overall capacity in 5-15% SOC cycling, compared to only 3% in 85-95% SOC cycling. For 0-50% SOC cycling in 45°C temperature, capacity retention of SiOx had dropped to ∼ 25 % of initial capacity, whereas graphite capacity was still in excess of 90% of initial capacity.