Strain engineering of LiFePO4 cathodes: Effects on voltage, energy density, and electronic structure for lithium-ion batteries

Abstract

Improving cathodes in commercial battery applications is one of the current trends aimed at achieving optimal performance for energy storage and reuse. In this study, triaxial tensile/compressive strains were applied to LiFePO4 and FePO4 structures, which are the main components of the cathode in lithium batteries. The goal is to control the open-circuit voltage (OCV) and energy density (Ed) under the influence of these strains, as well as to study the structural, electronic, and electrochemical properties. The study is based on density functional theory (DFT) and uses the generalized gradient approximation (GGA) developed by Perdew-Wang 1991 (PW91). The results show that the voltage of the undeformed systems is 3.92 V, and the energy density is 666.83 Wh/kg for the undeformed systems. Under a maximum tensile strain of ε = +6 % applied to LiFePO4, the voltage decreases to 3.32 V, and the energy density drops to 564.42 Wh/kg. In contrast, for FePO4, subjected to the same strain, the voltage decreases to 2.99 V and the energy density to 507.71 Wh/kg. When the LiFePO4 system is subjected to a maximum compressive strain of ε = 􀀀 6 %, the voltage and energy density decrease to 3.08 V and 523.28 Wh/kg, respectively. Similarly, when these strains are applied to FePO4, the voltage drops to 2.82 V and the energy density to 478.46 Wh/kg. On the other hand, the interatomic distance of the studied features, as well as the volume and electronic charge density, vary depending on the intensity of the applied strains. The analysis of the total density of states showed that the characteristics of LiFePO4 and FePO4, in the absence of strain, behave as semiconductors, with a band gap of 2.265 eV and 1.831 eV, respectively. However, these characteristics undergo modifications under the effect of the applied strains.