Hydroxyl Defects in LiFePO₄ Cathode Material: DFT+U and an Experimental Study

Abstract

Lithium iron phosphate, LiFePO4, a widely used cathode material in commercial Li-ion batteries, unveils a complex defect structure, which is still being deciphered. Using a combined computational and experimental approach comprising density functional theory (DFT)+U and molecular dynamics calculations and X-ray and neutron diffraction, we provide a comprehensive characterization of various OH point defects in LiFePO4, including their formation, dynamics, and localization in the interstitial space and at Li, Fe, and P sites. It is demonstrated that one, two, and four (five) OH groups can effectively stabilize Li, Fe, and P vacancies, respectively. The presence of D (H) at both Li and P sites for hydrothermally synthesized deuterium-enriched LiFePO4 is confirmed by joint X-ray and neutron powder diffraction structure refinement at 5 K that also reveals a strong deficiency of P of 6%. The P occupancy decrease is explained by the formation of hydrogarnet-like P/4H and P/5H defects, which have the lowest formation energies among all considered OH defects. Molecular dynamics simulation shows a rich structural diversity of these defects, with OH groups pointing both inside and outside vacant P tetrahedra creating numerous energetically close conformers, which hinders their explicit localization with diffraction-based methods solely. The discovered conformers include structural water molecules, which are only by 0.04 eV/atom H higher in energy than separate OH defects.

A Li-rich strategy towards advanced Mn-doped triphylite cathodes for Li-ion batteries

“…The LFP crystal structure is formally represented as a hexagonal oxygen close-packing where Li is located in edgesharing octahedra (the M1 site) and Fe is positioned in cornersharing octahedra (the M2 site). 5 This structure easily accommodates various point defects: lithium (V Li ) and iron (V Fe ) vacancies, Fe Li and Li Fe antisite pairs, hydroxyl defects associated with the P substitution, or a combination of these, [7][8][9][10] with all of them drastically influencing the electrochemical performance. 11 The antisite defects are the most abundant due to their low formation energy.…”

Thermodynamics as a Driving Factor of LiCoO2 Grain Growth on Nanocrystalline Ta-LLZO Thin Films for All-Solid-State Batteries

“…To estimate the change of the oxygen chemical potential (Δμ­(O)) corresponding to the conditions of LCO deposition at a finite temperature T (K) and oxygen partial pressure p (atm) we used the following equation where Δ H (O 2 , T , JANAF) and Δ S (O 2 , T , JANAF) are the change of both enthalpy and entropy according to the temperature and obtained from the JANAF thermochemical table, and RT ln p (O 2 ) is the contribution of O 2 partial pressure. Note that we used the JANAF database implemented as a python-based package Thermochem ().…”

Effective Nitrogen Incorporation for High‐Potential Anthracene Cathodes with Conjugated Frameworks

“…explored anionic redox chemistry in a Co‐free and Li‐rich layered cathode material, Li 1.2 Ni 0.2 Mn 0.6 O 2 . [ 4d ] They reported that cationic Ni 2+ oxidation, followed by a series of oxygen‐related oxidation and/or extraction steps resulted in a charge capacity of 0.97e – /formula unit. Despite the promised effectiveness of these inorganic cathodes, their intrinsic characteristics do not allow for fast charging/discharging cycles and the use of these materials is also not sustainable.…”

“…[ 3 ] In an effort to improve the energy‐storage capability of Li‐ion batteries, inorganic cathode materials have been widely studied, from conventional transition metal oxides (i.e., LiCoO 2 , LiFePO 4 , and LiMn 2 O 4 ) and mixed‐metal oxides (i.e., Ni‐Mn‐Co oxides, namely NMC) to Li‐rich inorganic cathodes with an anionic redox process. [ 4 ] For instance, Luo et al. explored anionic redox chemistry in a Co‐free and Li‐rich layered cathode material, Li 1.2 Ni 0.2 Mn 0.6 O 2 .…”