Investigation of Thermodynamic and Structural Properties of Olivine Li- and NaFePO₄

Olivine NaFePO₄ has the potential to be a high performing and cost-effective battery cathode material. However, key differences exist between the behavior of NaFePO₄ and isostructural LiFePO₄ that are not yet fully understood. In this work, the thermodynamic and structural properties of Na_xFePO₄ and Li_xFePO₄ (0<x<1) are investigated between 300 and 800K. We use neutron powder diffraction to examine the temperature dependent structural change of Na_xFePO₄ for x=0, x=2/3, and x=1 and Li_xFePO₄ for x=1 and x=2/3. Differential scanning calorimetry provides the isobaric specific heat as a function of temperature. The relationship of structure, specific heat, and entropy will be discussed in the context of improving our understanding of changes induced by ion intercalation and deintercalation.