A first-principles study of bulk and surface Sn-doped LiFePO₄ : The role of intermediate valence component in the multivalent doping

Abstract

Doping can be employed to enhance the electrical conductivity and electrochemical performance of LiFePO4, a promising material for Li‐ion batteries. However, the microscopic mechanism of doping is not fully understood. In this study, ab initio density functional theory (DFT) with the generalized gradient approximation (GGA) + U calculations was performed on both bulk and surface Sn‐doped LiFePO4. Our results indicate that surface doping is preferred over bulk or subsurface doping because it shows a lower doping energy and surface energy. The doping effect appears to be local, and the effect of the Li vacancy (VLi) distribution was examined. The multivalent Sn doping may facilitate the formation of an Fe2+/Fe3+ complex with the involvement of an effective intermediate Sn3+ component, which complements the existing charge transfer model for LiFePO4. The effective Sn3+–Fe3+/2VLi complex may exist on the LiFePO4 surfaces, providing possible surface design schemes to control charge transfer. The results suggest that the Sn dopant could modulate band gap and local charge transfer, and improve the electrochemical performance at the last stage of the charging process with no capacity loss. However, an optimized doping concentration may exist for electrochemically inactive doping with an unfavorable doping energy.

Lithium Clustering during the Lithiation/Delithiation Process in LiFePO4 Olivine-Structured Materials

“…Here, we use the first-principles density functional theory (DFT) GGA + U method to study the configuration of the lithium atoms because the method gives small error especially for transition metals with active localization of d or f orbitals compared with conventional GGA and linear discriminant analysis (LDA) methods. − In the GGA + U framework, the onsite Coulomb term U , and the exchange term J can be merged into a single effective parameter ( U – J ), and thus, we only need to take U as an effective parameter that can be numerically fitted by a self-consistent ab initio calculation . For the olivine FePO 4 and LiFePO 4 system, the value of U = 4.3 eV for Fe well describes the ground state properties and is widely used in the literature. − All of the calculations are performed by the Quantum Espresso package, with supercells (2 a × b × 4 c ) of eight-unit cells of Li x FePO 4 . The kinetic energy of 600 eV has been used as the cutoff energy for the plane-wave basis, and reciprocal-space k-point meshes of 1 × 3 × 1 have been used to ensure that the total energies converge within five meV per supercell.…”

First‐Principles Study on the Electronic Properties and Mechanical Stabilities of Anion‐Cation Multiple‐Doped LiFePO4

“…To enhance the electrochemical properties of LiFePO 4 , a great deal of research has focused on three approaches: coating, [14–17] particle size minimization, [18,19] and doping [20–23] . Among these three approaches, doping has an advantage over the others in enhancing the intrinsic conductivity without loss of energy density due to the introduction of inert materials or voids during the process [24] . First principles can help accelerate the design and development of new energy storage materials and therefore have a clear advantage in creating and optimizing new storage and conversion materials [25] .…”

Site Selection of Niobium-Doped LiMn0.6Fe0.4PO4 and Effect on Electrochemical Properties

“…It has a particular guiding effect on ion doping. [19][20][21][22][23][24][25] However, d electrons and f electrons in transition metal elements have a strong interaction, resulting in large deviations in the calculation results. This deviation is due to the Kohn-Sham orbital is not a molecular orbital but is usually a fractional occupied state.…”