Theoretical Evaluation of Behaviours and Properties of LiFePO4 as Li-ion Battery Cathode Material: a DFT Study

Cathode material of Li-ion batteries is responsible for performances and behaviours of these high energy storage devices. In this study, olivine, LiFePO4 (LFP), with Pnma space group (orthorhombic) is evaluated by density functional theory (DFT) using Wien2k program. Calculations were performed by different methods, i. e. LSDA, PBE-GGA, LSDA+U, GGA+U, modified Becke-Johnson (mBJ) and PBE-Fock-a (a Hybrid Functionals method, named HF). Assessments showed structural stability after extraction of one Li per formula. The relaxed structure using LSDA (GGA) calculations was underestimated (overestimated) regarding experimental structure data.  According to the calculations, the most shrinkage after Li extraction is occurred in ab plane, which could cause uniform domino-cascade Li extraction in c direction. This phenomenon would lead to constant voltage in charging/discharging. Considering experimental reaction voltage value is 3.2 V, the closest theoretical calculated voltage values belonged to GGA+U, GGA and mBJ methods. According to calculated density of states (DOS) diagrams, lithiated structure (LiFePO4) is N-type semiconductor and delithiated structure (FePO4) is P-type semiconductor. Inversely-biased-diode phenomenon turns this material as low rate capable in comparison by the oxide cathode materials. However, based on the DOS configurations, its rate capability is better than many of polyanion cathodes. Extraction of Li (turning Fe ionic state from II to III) leads to conductivity enhancement. The empty 3d-Fe orbitals are responsible for this phenomenon.