Journal
JOURNAL OF THE ELECTROCHEMICAL SOCIETY
Volume 157, Issue 4, Pages A499-A507Publisher
ELECTROCHEMICAL SOC INC
DOI: 10.1149/1.3294790
Keywords
ageing; crystal morphology; electrochemical analysis; electrochemical electrodes; lithium compounds; secondary cells; X-ray diffraction
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The performance loss of lithium-ion batteries with lithium iron phosphate positive chemistry was analyzed using electrochemical characterization techniques such as galvanostatic charge-discharge at different rates, ac impedance, and hybrid pulse power characterization measurements. Differentiation analysis of the discharge profiles as well as in situ reference electrode measurement revealed loss of lithium as well as degradation of the carbon negative; the cell capacity, however, was limited by the amount of active lithium. Destructive physical analyses and ex situ electrochemical analyses were performed at test completion on selected cells. While no change in positive morphology and performance was detected, significant cracking and delamination of the carbon negative was observed. In addition, X-ray diffraction analysis confirmed the changes in the crystal structure of the graphite during cycling. The degradation of the carbon negative is consistent with the observations from the electrochemical analysis. Ex situ electrochemical analysis confirmed that active lithium controlled cell capacity and its loss with cycling directly correlated with cell degradation. The relationship between carbon negative degradation and loss of active lithium is discussed in the context of a consistent overall mechanism.
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