Thermophysical Properties of LiFePO4: DFT+U Computations Combined with a Thermodynamically Self-Consistent (TSC) Method
Authors: Seifitokaldani Ali , Gheribi Aïmen E , Dollé Mickeal , Chartrand Patrice 
Published: February 2020

Introduction

LiFePO4 (LFP) was proposed by Padhi et al. 1 in 1997 as a promising cathode material for Li-ion batteries and very soon became the center of attention for researchers and industries in energy fields. Nowadays, it is commercialized and available in mass production through different methods. A lot of improvements in terms of material and production process have been achieved since its nascence. Nevertheless, our thermodynamic knowledge about LFP is limited to handful experimental works 2 on heat capacity and few computational studies 3 to estimate elastic properties at ground state condition. Enlarging our knowledge about thermophysical properties of LFP such as elastic properties, Debye temperature, heat capacity and also thermal expansion at higher temperatures is a crucial key to expand the thermodynamic information, and consequently to answer the arising questions about phase equilibrium in production process.

Methodology

Here, we present the abovementioned thermophysical properties of LFP in a wide range of temperature by Density Functional Theory (DFT) and a new Thermodynamically Self-Consistent (
TSC) method 4. The TSC method is, in summary, an extension of the quasi-harmonic approximation (QHA) satisfying the Maxwell relations and thus ensuring thermodynamic consistency. For future, we assay the effect of vacancy and also elemental substitution on thermophysical and electrochemical properties of LFP.

Computational Details. We use Vienna ab initio Simulation Package (VASP) 5 to address our aim. Projected Augmented Wave (PAW) approach 6 and the General Gradient Approximation (GGA) of Perdew, Burke and Ernzerhof (PBE) 7 are used to represent the electron-ion core and electron-electron interactions, respectively. The Hubbard U correction to the GGA (GGA+U) equal to 5.3 eV for Fe in oxide systems is employed to address a correct band structure and binding energies. Our results indicate that, a cut-off energy of 520 eV and 3×4×5 Γ-centered k-points grid in first Brillouin zone with a Gaussian smearing parameter σ of 0.02 eV ensure the accuracy in energy of the system to be more than 0.01 meV.

Acknowledgement. The authors would like to thank the Natural Science and Engineering Research Council of Canada (NSERC) for the financial support awarded to this project as part of the Automotive Partnership Canada (APC) program.

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