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 15 + 4.6(26) ⨉ 10^-9 16 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. <br/>