Novel Electroanalytical Techniques for Phase Transformation

Novel Electroanalytical Techniques for Phase Transformation

Yujie Zhu and Chunsheng Wang

Department of Chemical and Biomolecular Engineering

University of Maryland, College Park

Abstract

One major challenge of Li-ion batteries is their low power. It has been found that phase transformation materials (such as LiFePO₄, Li₄Ti₅O₁₂) have superior power densities compared to traditional LiCoO₂ and are the promising electrode materials for high power Li-ion batteries. However the mechanism of the exceptional rate performance is still a controversial issue undergoing debates, since there is no accurate electroanalytical technique to probe ion transport phenomena in the phase transformation region. Direct use of traditional electrochemical techniques, such as galvanostatic intermittent titration technique (GITT), potentiostatic intermittent titration technique (PITT), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), to the phase transformation materials will result minima of the measured diffusion coefficients in the vicinity of phase transformation regions because the traditional electroalytical techniques rely on the classic Fickian diffusion in a solid solution phase and thus are not valid for phase transformation electrodes.

Under the support of NSF and ARO, novel electroanalytical techniques (GITT. PITT, CV and EIS) for phase transformation electrodes were developed by integrating a mixed-control phase transformation theory with GITT, PITT, EIS and CV techniques. Different from the moving boundary phase transformation model assuming that the phase transformation is only controlled by Li-ion diffusion, the mixed-control phase transformation theory accounts not only the Li-ion diffusion, but also the phase-interface mobility that depends on the interface coherence, misfit strain/stress, deformations and defects. With LiFePO₄ as a specific example, the phase transformation electroanalyticl technologies (GITT, PITT and CV) were validated through the consistency of the chemical diffusion coefficient between the values calculated from traditional electrochemical methods in one-phase region and the ones obtained from mixed-control model at two-phase region, also between the values obtained from mixed-control model under different test conditions (GITT, PITT and CV). For the first time, the interface mobility of the phase transformation electrode materials (LiFePO₄ in this case) was obtained, which cannot be done by traditional electrochemical techniques or moving boundary model.

The phase trasnformation electroanalytical techniques can play a critical role in designing next generation of Li-ion battery for EV/HEV and renewable energy storage applications.