Rate Induced Solubility and Suppression of the First-Order Phase Transition in Olivine LiFePO4
Authors: Zhang Xiaoyu , Wagemaker Marnix , van Dijk Niels H. , van Hulzen Martijn , Singh Deepak P. , Brownrigg Alex , Wright Jonathan P. 
Published: February 2020

Electrode phase transitions are of large practical importance in Li-ion batteries determining the voltage profile and as a decisive factor in the transition kinetics. In particular in LiFePO4 the impact of defects, particle size, and charge rate on the phase transition in LiFePO4, has been studied intensively. The end-member phases, lithium-rich triphilyte Li1-
bFePO4 (LFP) and lithium-poor heterosite LiaFePO4 (FP), demonstrate narrow solid-solution domains however both phase field modelling and DFT predict that the first-order phase transition can be suppressed by applying high rates leading to a solid-solution transition1-3 giving a rationale for the intrinsically fast kinetics of the LiFePO4 material Recent in-situ high-rate diffraction studies by Orikasa et al.4
,
5 revealed a metastable Lix
∼
0.6FePO4 phase in addition to the thermodynamically stable LiFePO4 and FePO4 phases rather than the predicted solid-solution transformation1-3.
To determine the phase transition mechanism dependence on the dis(charge) rate we performed an in-situ synchrotron diffraction study with (dis)charge rates ranging from a very low rate of C/5 up to ultra-high rates of 60C. At C/5 charging diffraction shows the established first-order phase transition that occurs upon delithiating LiFePO4. With increasing charging rate the LiFePO4 and FePO4reflections increasingly shift indicating lattice parameters associated with increasing solubility in both end members. In addition with increasing charge rates considerable diffracted intensity is observed between the Bragg reflections indicating that a fraction of the material undergoes a solid solution transformation bypassing the first-order phase transition.
At the early stages of charging the metastable phase reported by Orisaka et al.4
,
5 is observed which can be explained based on the phase diagram of LiFePO4. The distribution in environments observed at high rates, including the charged FePO4 the distribution of LixFePO4 phases and the non-reacted LiFePO4 at high rates indicates that the transformation moves through the electrode as a transformation wave, revealing that improvement of electrode performance should focus on optimization the ionic/electronic transport in LiFePO4electrodes rather than on lowering nucleation barriers. These results challenge theorists to capture the observed non-equilibrium thermodynamics of this material.
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