Phase Composition and Dynamical Studies of Lithium Iron Phosphate

Abstract

The olivine phase of lithium iron phosphate (LiₓFePO₄) is a promising cathode material for lithium-ion batteries. Some of its advantages are that it is nontoxic, highly stable, and inexpensive, but its low intrinsic electrical conductivity is a major disadvantage. LiₓFePO₄ has generally been described as a two-phase system as lithium is removed from or inserted into the material. However, the mechanism of lithium removal and the system's phase composition is still not fully understood, and is an area of interest. The two low-temperature phases, heterosite and triphylite, have previously been shown to transform to a single-phase disordered solid solution at temperatures above 200°C. Here, the phase diagram for LiₓFePO₄ has been determined for different lithium concentrations and temperatures. This disordered phase is stable at relatively low temperatures. The proposed phase diagram resembles a eutectoid system, with eutectoid point at around x = 0.6 and 200°C. The kinetics of mixing and unmixing transformations, including the hysteresis between heating and cooling, will be shown. The enthalpy of this transition is at least 700 J/mol. Further thermostability studies of the material up to temperatures of 800°C will also be discussed. Solid solution regions have also been indicated near the end compositions of x = 0 and 1 at room temperature. Measurements of the entropy (ΔS(x)) and enthalpy (ΔH(x)) of lithiation were performed, indicating the ranges of solid solution regions to be x 0.85. In addition, the entropy of lithiation in between (0.05 The topic of dynamics in LiₓFePO₄ is also very relevant, especially since this material is now touted as an important high-rate capability cathode. The electronic and ionic conductivity of the new disordered solid-solution phase has generated widespread interest. The local electronic structure around iron ions in Li0.6FePO₄ was studied by 57Fe Mossbauer spectrometry at temperatures from 25 to 240°C. The equilibrium two-phase triphylite plus heterosite material was compared to a disordered solid solution that was obtained by quenching from a high temperature. Substantial electronic relaxations were found in the disordered solid solution compared to the two-phase material at temperatures of 130°C and above. Fluctuations in the electric field gradient and the isomer shift showed activation energies of 335 +- 25 meV and 600 +- 100 meV, respectively. It is suggested that these spectral relaxations are caused by the motions of Li⁺ ions. The activation energies from the isomer shift can be related to the material conductivity, giving values of 10-6 to 10-4 S/cm; 3 to 5 orders of magnitude higher than the measured value of 10-9 for fully lithiated LiFePO₄. A slight relaxation at 180°C in 10% of the two-phase material can be attributed to defects in the heterosite and triphylite phases. Overall, the disordered solid-solution phase shows faster electronic dynamics than the two-phase material. Additional studies on the dynamics of the disordered solid solution, along with attempts to stabilize this phase at lower temperatures, should be a topic of further work.