Low-temperature aging mechanisms of commercial graphite/LiFePO4 cells cycled with a simulated electric vehicle load profile—A post-mortem study. (December 2018)

• Record Type: Journal Article
• Authors: Rauhala, Taina; Jalkanen, Kirsi; Romann, Tavo; Lust, Enn; Omar, Noshin; Kallio, Tanja
• Is Part Of: Journal of energy storage. Volume 20(2018)
• Page Start: 344
• Page End: 356
• Publication Date: 2018-12
• Subjects: Lithium-ion battery -- Battery electric vehicle load profile -- Aging mechanism -- Post-mortem analysis -- Lithium plating -- Graphite disordering; Energy storage -- Periodicals; Energy storage -- Research -- Periodicals; 621.3126
• DOI: 10.1016/j.est.2018.10.007
• Languages: English
• ISSNs: 2352-152X

Abstract

Highlights: Post-mortem study of commercial cells aged with a simulated electric vehicle cycle. Graphite/LiFePO4 cells were aged at room temperature, 0 °C and −18 °C. Aging is seen as capacity fade, whereas cell impedance does not increase notably. Capacity fade is caused by SEI growth or Li plating depending on temperature. Signs of graphite disordering and particle cracking at low temperature were found. Abstract: Reduced cycle life is one of the issues hindering the adoption of large lithium-ion battery systems in cold-climate countries. Thus, the aging mechanisms of commercial graphite/LiFePO4 (lithium iron phosphate) cells at low temperatures (room temperature, 0 °C and−18 °C) are investigated here through an extended post-mortem analysis. The cylindrical 2.3 Ah cells were cycled with a simulated battery electric vehicle load profile, and the aged cells were then disassembled inside an argon-filled glove box. A non-cycled cell was also dismantled as a reference. Half-cell testing was utilized to evaluate the degradation of the electrochemical performance of the electrodes, whereas X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, inductively coupled plasma optical emission spectroscopy and Raman spectroscopy were used to characterize the changes in the materials properties. The full-cell performance loss was mostly seen as capacity fade whereas significant changes in the cell impedance were not observed. Depending on the cycling temperature, loss of cyclable lithium due to solid electrolyte interphase growth and/or lithium plating on the graphite electrode were observed, and they are attributed as the main mechanisms responsible for the capacity loss. Furthermore, increased disordering of the graphite electrode was observed for the cell cycled at −18 °C. The graphite disordering was hypothesized to result from diffusion-induced stress and the mechanical stress caused by severe lithium plating. In contrast, the LiFePO4 electrodes showed only minimal signs of degradation regardless of the cycling temperature.