Influence of Cycling Profile, Depth of Discharge and Temperature on Commercial LFP/C Cell Ageing: Cell Level Analysis with ICA, DVA and OCV Measurements

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.

Influence of cycling profile, depth of discharge and temperature on commercial LFP/C cell ageing: post-mortem material analysis of structure, morphology and chemical composition

“…Next to the cell capacity, the power capabilities, which would be influenced by a resistance increase of the cells, is not affected by the observed reduction of surface conductivity. This observation was reported before by the authors in Figure S11 [26], which depicts the evolution of the internal cell resistance of the electrodes (called R total ). For the cells aged at − 20 °C and the cell aged at 55 °C with 100% DOD using AP2, the resistance stays nearly constant throughout the ageing.…”

“…The cycling and ageing protocol is as well reported in [26] and the main points will be stated here.…”

“…The deposition of transition metals is in addition hindering the Li intercalation into the graphite anode by clogging anode pores [14,43,44]. The increase of the surface layer related resistance was analyzed using EIS in [26]. It was observed, that the cells aged at 55 °C exhibit a surface layer related increase of resistance between 12 and 27%.…”

“…The appearance of negative adhesion is based on the existence of negatively charged LFP particles [Li (1-x) FePO4] x− , due to the loss of lithium inventory over cycling, while applying a positive bias voltage. The loss of lithium inventory is next to the loss of active material the main influence for the capacity decay over aging (see [26] for more information on the degradation of the cells). The nanometer particles forming the agglomerates must be covered with a thicker, isolating surface layer or exhibit a degradation of the conductive carbon coating.…”

Comparison of fresh and aged lithium iron phosphate cathodes using a tailored electrochemical strain microscopy technique

“…The dissolution of iron leads to Fe-deficient inactive phases. The aged cathode showed a higher Fe content on the cathode surface and lower Fe content in the cross-section (Fe mass content fresh: 32.1 ± 0.3% and aged: 27.8 ± 0.8%), indicating iron dissolution from the bulk material [71].…”