| Although the performance of Li-ion batteries has improved dramatically in last ten years, Li-ion batteries cannot be directly used in hybrid-electric vehicles and electric vehicles, as they are limited by low pulse power, abuse tolerance, calendar and cycle life, and high cost. In this context, the newly-developed phase transformation cathode materials such as LiFePO4 make the Li-ion batteries, a promising candidate for vehicular applications, as LiFePO 4 exhibits high power density, high electrochemical and thermal stability, and relative inexpensive and less toxic nature compared to conventional insertion electrodes. However, the commercial applications of LiFePO4 in Li-ion batteries were limited due to its poor rate capability. In order to improve the rate capability of LiFePO4 cathode materials or to develop new phase transformation electrode materials with high rate capability, it is essential to understand the electrochemical kinetics and rate-controlling mechanisms involved in the charge/discharge process. To date, the shrinking core model (SCM) is the only mathematical model available in the literature, that is applicable to phase transformation electrodes. Currently, LiFePO 4 is available from different manufacturers and all of them exhibit different rate capabilities. The difference in rate capabilities of these samples cannot be explained by the shrinking core model. Also a large discrepancy can be observed between the experimental discharge curves and discharge curves obtained from shrinking core model.;In this doctoral dissertation, the reasons for the discrepancy between experimental results and the results obtained from shrinking core model are identified by experimental techniques and mathematical modeling. From these results, it is found that the diffusion is not the only controlling mechanism for the discharge process of LiFePO4 and the discrepancy between the experimental and SCM results is due to the assumptions used in the SCM. Based on these results and also by assuming that the discharge process is controlled by both diffusion and rate of phase transformation, the shrinking core model is modified. The modified SCM is validated by predicting the discharge behavior of three commercially-available LiFePO4 samples. Using the modified shrinking core model as a tool, the effects of chemical diffusion, rate of phase transformation, solid solution range, volume change, and particle size on discharge rate capability of LiFePO4 are determined. The modified SCM developed in this contribution is applicable to any phase transformation electrode such as Li4Ti5O12 in Li-ion battery and metal hydride electrode in Nickel metal hydride battery, thus making it a useful tool for practitioners in the field. |
