Rate Induced Solubility and Suppression of the First-Order Phase Transition in Olivine LiFePO4

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 Li_1-bFePO₄ (LFP) and lithium-poor heterosite Li_aFePO₄ (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 transition^1-3 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 Li_x∼0.6FePO₄ phase in addition to the thermodynamically stable LiFePO₄ and FePO₄ phases rather than the predicted solid-solution transformation^1-3.

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 Li_xFePO₄ 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.

1                     Malik, R., Zhou, F. & Ceder, G. Kinetics of non-equilibrium lithium incorporation in LiFePO(4). Nature Materials 10, 587-590, doi:10.1038/nmat3065 (2012).

2                     Bai, P., Cogswell, D. A. & Bazant, M. Z. Suppression of Phase Separation in LiFePO4 Nanoparticles During Battery Discharge. Nano Letters 11, 4890-4896, doi:10.1021/nl202764f (2011).

3                     Cogswell, D. A. & Bazant, M. Z. Coherency Strain and the Kinetics of Phase Separation in LiFePO4 Nanoparticles. Acs Nano 6, 2215-2225, doi:10.1021/nn204177u (2012).

4                     Orikasa, Y. et al. Direct Observation of a Metastable Crystal Phase of LixFePO4 under Electrochemical Phase Transition. Journal of the American Chemical Society 135, 5497-5500, doi:10.1021/ja312527x (2013).

5                     Orikasa, Y. et al. Transient Phase Change in Two Phase Reaction between LiFePO4 and FePO4 under Battery Operation. Chemistry of Materials 25, 1032-1039, doi:10.1021/cm303411t (2013).