Existence of path-dependence in the LiFePO4 electrode

This paper explores two unique features of the lithium iron phosphate (LiFePO4) electrode that provide insight into the electrochemical behavior of this system. First, we show the existence of an asymmetric behavior between charge and discharge, whereby the utilization on charge is considerably larger than that on discharge under current densities where transport limitations are important. Second, we show the existence of a path-dependence in this system whereby the high-rate electrochemical behavior of the electrode at a particular state of charge (SOC) depends on the path by which the electrode was brought to that SOC. We qualitatively explain both these features using a shrinking-core model to account for the juxtaposition of the two phases. The path-dependence reported in this paper could have implications in batteries used in hybrid-electric-vehicles as the power capability of this chemistry will depend on its cycling history, thereby complicating predictions of power. In addition, the data reported here emphasizes the importance of ensuring consistency in defining the SOC in experiments on this electrode. (c) 2006 The Electrochemical Society.