Abstract
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
Highlights
Energy storage devices are receiving more and more attention due to the shift from fossil to renewable energy sources for the power grid and the electrification of the1 3 Vol.:(0123456789)Journal of Applied Electrochemistry (2020) 50:1101–1117 mobility sector
For the cells aged at 100% depth of discharges (DOD), the capacity fading starts to level during the ageing
The P2p surface spectra points likewise to a higher LixPFz concentration on the anodes aged at − 20 °C compared to 55 °C, since the peaks around 138 eV are only observed in the low temperature ageing
Summary
Journal of Applied Electrochemistry (2020) 50:1101–1117 mobility sector. The focus is especially on lithium ion batteries, due to their relatively high energy density, low selfdischarge, falling costs and scalability in battery packs. Wu et al analysed aged LiCoO2 cathode surfaces and observed an increase in grain size and surface roughness and a decrease of the surface potential and material stiffness They attributed these changes to the loss of lithium, the fracturing and agglomeration of particles and the generation of a surface layer from electrolyte decomposition products [23]. Several studies performed post-mortem analyses of commercial LiFePO4 cells using a variety of ageing parameters, such as different temperatures and current loads and analysed the electrodes afterwards [9, 10, 13, 14, 24, 25] These studies show, that cycling at elevate temperatures promotes the dissolution of Fe from the cathode and the capacity fading is mainly caused by loss of lithium inventory due to the generation of surface layers and loss of active material. Unknown degradation mechanisms with respect to LiF deposition, decrease in surface conductivity and the occurrence of higher amount of oxygenated species for the ageing at − 20 °C compared to 55 °C are reported
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