Strain accommodation and potential hysteresis of LiFePO4 cathodes during lithium ion insertion/extraction

Equilibrium potential hysteresis of electrode materials refers to the phenomenon that discharge equilibrium potential is lower than charge equilibrium potential. It is induced by the strain accommodation energy of phase transformation. Taking LiFePO4 as an example, the equilibrium potential hysteresis and accommodation energy of two LiFePO4 with different particle sizes were characterized using galvanostatic intermittent titration technique (GITT), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) methods. The LiFePO4 sample with 40 nm particle size has lower accommodation energy than that of 100 nm LiFePO4, which is due to a narrower miscibility gap and smaller potential hysteresis of 40 nm LiFePO4 comparing to 100 nm LiFePO4. The phase transformation of LiFePO4 occurs only at potential levels above delithiation (charge) equilibrium potential and below lithiation (discharge) equilibrium potential. No phase transformation occurs when charge/discharge is limited in the potential hysteresis range, i.e. LiFePO4 behaves like a solid solution in the potential hysteresis range. A reliable EIS can only be obtained when it is performed at the center of potential hysteresis and the potential amplitude does not exceed the range of equilibrium potential hysteresis. The analysis on strain accommodation and potential hysteresis of LiFePO4 cathodes during Li ion insertion/extraction is also suitable for other phase transformation electrodes.