The paper presents post-mortem analysis of commercial LiFePO4 battery cells, which are aged at 55 °C and − 20 °C using dynamic current profiles and different depth of discharges (DOD). Post-mortem analysis focuses on the structure of the electrodes using atomic force microscopy (AFM) and scanning electron microscopy (SEM) and the chemical composition changes using energy dispersive X-ray spectroscopy (SEM-EDX) and X-ray photoelectron spectroscopy (XPS). The results show that ageing at lower DOD results in higher capacity fading compared to higher DOD cycling. The anode surface aged at 55 °C forms a dense cover on the graphite flakes, while at the anode surface aged at − 20 °C lithium plating and LiF crystals are observed. As expected, Fe dissolution from the cathode and deposition on the anode are observed for the ageing performed at 55 °C, while Fe dissolution and deposition are not observed at − 20 °C. Using atomic force microscopy (AFM), the surface conductivity is examined, which shows only minor degradation for the cathodes aged at − 20 °C. The cathodes aged at 55 °C exhibit micrometer size agglomerates of nanometer particles on the cathode surface. The results indicate that cycling at higher SOC ranges is more detrimental and low temperature cycling mainly affects the anode by the formation of plated Li.
Graphic abstract
This paper uses several techniques to monitor the ageing of commercial LiFePO 4 cells, which are cycled at 55 °C and −20 °C at various depths of discharge. Ageing at lower depth of discharge leads to higher capacity fading, as compared to higher depth of discharge. The highest capacity fading is observed using 50% depth of discharge for cycling at 55 °C, while the lowest capacity fading is observed for the cells aged at 100% depth of discharge when cycled at −20 °C. Using incremental capacity analysis and differential voltage analysis the capacity fading is monitored and underlying ageing mechanisms are described. The loss of lithium inventory and the loss of active material, especially on the cathode side, are the major degradation mechanisms for the cells. The first incremental capacity analysis peak of the discharge process can be used in our case to predict remaining life and cell capacity.
Hydrogen production by water electrolysis can decouple energy production from actual demand by storing electricity from renewable energies as hydrogen and using it when needed. In July 2021, the EU emphasized the importance of action and resolved the ambitious targets to reduce net emissions by at least 55% by 2030 compared to 1990 and to be the first climate-neutral continent by 2050. [1] To this purpose, the share of renewable energies must be increased to 40% [2] and hydrogen is to be used particularly in sectors such as industry or transport, where emissions are difficult to reduce. [3] Therefore, large-scale electrolysis implementation is necessary for the conversion of surplus electricity from renewable energies sources like wind or sun to green hydrogen which significantly promotes the reduction in global CO 2 emissions. [4] In recent years, various companies are currently involved in the large-scale installation of electrolysis plants, for example, a recently installed 10 MW electrolysis plant at a Shell refinery which started operation in July 2021. [5] However, with the scaling of electrolysis plants, the cost aspect is becoming increasingly important in order to be able to offer green hydrogen at competitive prices. The urgently needed cost reduction of electrolysis technologies can
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