This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Lithium ion batteries have been recognized as a critical technology to enable the advanced electric vehicles (EV) and hybrid electric vehicles (HEV). An obstacle of the traditional Li-ion batteries to the EV/HEV applications is their low peak power. Recently, numbers of phase transformation materials (such as LiFePO4, Li4Ti5O12) have emerged as promising electrode materials for the EV/HEV applications because of their superior power output. However, the mechanism for such excellent performance is still not fully understood due to the lack of accurate electroanalytical techniques to study the kinetics of Li insertion/extraction in these materials. The existing electroanalytical techniques, such as cyclic voltammetry (CV), potentiostatic intermittent titration (PITT), galvanostatic intermittent titration (GITT), and electrochemical impedance spectroscopy (EIS), rely on the classic Fickian diffusion in a solid solution phase and thus are not valid for phase transformation electrodes. The lack of electroanalytical techniques for phase transformation electrodes has delayed the exploration of high-power rechargeable batteries. The recent breakthrough in mixed-control phase transformation theory achieved by the PI provides a unique opportunity to develop new electroanalytical technologies for phase transformation electrode materials. 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. The objective of this research is to develop novel electroanalytical techniques by integrating the mixed-control phase transformation theory with GITT, PITT, EIS and CV techniques. The obtained novel electroanalytical techniques are going to be used to systematically investigate the relationships between the material properties (micro-structure, particle size and composition), phase transformation kinetics and the electrochemical performance. The existing high power electrode materials will be optimized and the next generation high power electrode materials will be developed. In the preliminary study, the PI has determined the interface mobility and the lithium diffusion coefficient in the two-phase region of two types of LiFePO4 using the mixed-control model. Determining the diffusion coefficient and the interface mobility in phase transformation electrode materials and obtaining fundamental understanding of the structure-property relationship are needed for developing next generation of high-power electrode materials. Intellectual Merit:
This research will provide techniques for electrochemical analysis of phase transformation in electrodes. The fundamental relationships between the material chemistry, structure, composition, phase transformation kinetics, and electrochemical properties of phase transformation electrodes will be investigated. New generation of high-power electrode materials will be created based on the findings of this research, which could accelerate the EV/HEV development. Although this research concentrates on electrode materials for Li-ion batteries, the electroanalytical techniques to be developed as part of this project can also be used for any ion-insertion electrodes including hydrogen, magnesium, and sodium storage materials. Broad Impacts
The knowledge created from this research could be applied to a broad range of applications, including nitriding, carburizing and carbonitriding processes in metals and alloys. This research will provide research opportunities and educational activities for both graduate and undergraduate students. Outreach activities will be extended to the public to raise the awareness of the future of energy technology. The application of high-power batteries in EV/HEVs could reduce the dependence on foreign oil, improve urban air quality, and reduce greenhouse gas emissions. The PI also plans to write scholarly reviewed articles and articles describing this research to non-specialist audiences.
