Thermal expansion of LiFePO ₄ , triphylite, an olivine-structured cathode material

Due to its stability and reversible lithiation, olivine structured triphylite, LiFePO4, has become an important battery cathode material. Here, we report the lattice parameters and thermal expansion coefficients of LiFePO4, measured between 89 and 823 K by X-ray powder diffraction methods. The volume thermal expansion above 200 K is α(T) = 2.63(5) × 10−5 + 4.6(26) × 10−9 T. Above ∼ 200 K, the thermal expansion of the crystal’s b-axis, the longest of the three crystallographic axes, decreases with increasing temperature and becomes the direction of least thermal expansion above ∼ 500 K. The thermal expansion of the other two crystallographic axes are approximately constant above 200 K. A Debye-Grüneisen, zero-pressure, equation of state for the unit-cell volume gives a Debye temperature of ∼150 K, with a relatively large uncertainty due to measurements not extending far below 90 K. Overall, the thermal expansion of LiFePO4 is similar in magnitude to that of silicate and phosphate olivines over the temperature range of this study.