Galvanostatic Intermittent Titration Technique for Phase-Transformation Electrodes

A novel galvanostatic intermittent titration technique (GITT) and a novel potentiostatic intermittent titration technique (PITT) for phase-transformation electrodes were developed by integrating mixed control phase-transformation theory with traditional GITT and PITT methods. The contribution of the strain accommodation energy to the thermodynamic driving force for phase transformation was assessed. These novel GITT and PITT methods can be used to determine the true ion diffusion coefficient and the interface mobility of phase-transformation electrodes in the two-phase region. To demonstrate the utility of this method, the lithium ion diffusion coefficient and the interface mobility of two LiFePO4 samples with different particle sizes were obtained in the two-phase region. The lithium ion diffusion coefficient in the two-phase region as measured using phase-transformation GITT was on the order of 10−13 cm2/s in the β phase (Li1−yFePO4) and 10−12 cm2/s in the α phase (LixFeO4), which is similar to the diffusion coefficients in the single β and single α phase regions determined using traditional GITT and PITT. This similarity with the diffusion-coefficient-validated phase-transformation GITT and PITT is expected since traditional GITT/PITT is reliable in the single-phase region. The interface mobility of the LiFePO4 (about 10−15 m mol/J s) increases with decreasing particle size. The interface mobility of the LiFePO4/FePO4 during electrochemical discharge at room temperature is comparable to that of the martensite−austenite transformation in an Fe−C alloy with a semicoherence interface at 350 °C.