Phase behavior and phase transformation kinetics during electrochemical cycling of lithium transition metal olivine compounds

Abstract

Olivine LiMPO4 (M = Fe, Mn, Co, Ni) compounds have received most attention from the battery research community as the cathodes for Li-ion batteries because of several advantages, including a high theoretical capacity, 170 mAh/g, and flat discharge potential (with respect to Li/Li+) of 3.45 V, 4.1V, 4.8V, and 5.1V, respectively, for Fe, Mn, Co, and Ni. Among these, LiFePO4 has received the most attention for its likelihood to provide low price, good cycling stability, thermal stability, and low-toxicity. It is being utilized in a new generation of Li-ion batteries for high power applications such as power tools and electric vehicles. However, LiFePO4 cathodes also have several drawbacks, such as low electronic conductivity, and slow Li-ion transport during the LiFePO4/FePO4 two-phase transformation during the charge-discharge process. This results initially in poor rate capability and making the practical utility of these compounds unclear. Numerous studies have attributed the rate capability of olivines purely to chemical diffusion limitations. Many efforts have been devoted to improving the conductivity and the rate performance of LiFePO4 cathodes. Since this class of olivines undergoes a first-order phase transition upon electrochemical cycling, in order to improve rate capability, an equally important goal is to maximize the rate of phase transformation. In this work, the impact of phase behavior and phase transformation on electrochemical properties such as voltage profile, cycle life, and rate capability of olivine compounds was studied in several aspects.

(cont.) We found that: (1) the phase diagram of LilxFePO4 is size and composition-dependent; (2) elastic misfit between the triphylite and heterosite phases during electrochemical cycling plays a significant and previously unrecognized role in determining the rate capability and cycle life of olivine compounds; (3) the phase transformation path of nanoscale olivines Li1-xMPO4 (M = Mn, Fe) is much more complex than their conventional coarse grained counterparts. Upon electrochemical cycling, a fraction (increasing with increasing size) of the delithiated LiyMPO4 that is formed is partially amorphous or metastable. Finally, (4) aliovalent cation substitution is an effective and controllable way to improve electrochemical properties, especially rate capability, of the Li1-xFePO4 olivine compounds.