Novel electroanalytical techniques and in situ high resolution transmission electron microscopy investigation for phas transformation electrode materials

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Zhu, Yujie
Wang, Chunsheng
Lithium ion (Li-ion) batteries are currently being used to power an increasingly diverse range of applications, and have been recognized as a critical enabling technology to make electric vehicle/hybrid electric vehicle (EV/HEV) a success. It has been found that phase transformation electrode materials (such as LiFePO4) are the promising electrode materials for high power Li-ion batteries. However the mechanism of the exceptional rate performance is still undergoing debates, since there is no accurate analysis method to study ion transport phenomena in the phase transformation regions. The analysis methods of current electrochemical techniques, including galvanostatic intermittent titration technique (GITT), potentiostatic intermittent titration technique (PITT), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS), can only be used to analyze the ion transport in solid solution electrode materials, because they were developed mainly based on Fick's second law of diffusion without any consideration of the inter-phase boundary movement in phase transformation electrode materials. Motivated by the increasing demand for accurate analysis methods for electroanalytical techniques and recent advances in the theory of phase transformation, a mixed-control phase transformation model is proposed by us. The mixed-control model accounts not only the ion diffusion, but also the phase boundary mobility that depends on the interface coherence, misfit strain/stress, deformations and defects. With LiFePO4 as a specific example, we study the potential hysteresis and strain accommodation energy. By integrating the mixed-control model with GITT, PITT and CV, we determine the Li-ion diffusion coefficient and interface mobility of LiFePO4 electrodes in two-phase region. For the first time, the interface mobility of LiFePO4 is obtained. The electrochemical lithiation of FePO4 particles is investigated by in situ high resolution transmission electron microscopy (HRTEM), and the anisotropic lithiation mechanism is directly observed. For the first time, a sharp (010) phase boundary between LiFePO4 and FePO4 is observed, which migrates along the [010] direction during lithiation. Furthermore, our in situ HRTEM observations revealed misfit dislocation populations on the (010) phase boundary, overthrowing previous model assumption of fully coherent phase boundary. These misfit dislocations provide a mechanism for long-term lithium ion battery electrode fatigue and failure, due to repeated coherency loss.