Ab initio identification of the Li-rich phase in LiFePO₄

Abstract

A recent discovery of anionic redox activity in Li-rich layered compounds opens a new direction for the design of high-capacity cathode materials for lithium-ion batteries. Here using extensive ab initio calculations, the thermodynamic existence of the Li-rich phase in LiFePO4 to form Li1+xFe1-xPO4 with x not exceeding 12.5% has been proved. Anionic redox activity and structural stability during delithiation are further investigated. Interestingly, it is found that Li1+xFe1-xPO4 cannot be delithiated completely and thus cannot achieve extra capacity by anionic redox activity, because the local oxygen-ion redox will cause the fracture of the rigid framework formed by phosphate tetrahedral polyanions. Although an extra capacity cannot be realized, the excess Li-ions at Fe sites can enhance the Li-ion diffusivity along the adjacent [010] channel and contribute to the shift from 1D to 2D/3D diffusion. This study provides a fresh perspective on olivine-type LiFePO4 and offers some important clues on designing Li-rich cathode materials with high energy density.

Density functional theory study of substituent effects on gas‐phase heterolytic Fe–O and Fe–S bond energies of m‐G‐C6H4OFe(CO)2(η5‐C5H5) and m‐G‐C6H4SFe(CO)2(η5‐C5H5)

“…Plenty of studies have showed that DFT methods are becoming very useful, [35] and the Becke three-parameter functional with the Lee, Yang, and Parr correlation functional (B3LYP) [24] method is the most promising approach because it is very fast and sufficiently accurate for most problems. [2][3][4][5][6][7][8][9]34,[36][37][38][39][40][41][42] The B3LYP procedure is known to provide reliable geometries and frequencies. [36] Cedeño and Weitz [43] considered that the Becke88-Perdew86 exchange-correlation functional (BP86) [24,44] was the most reliable method for calculating the Fe-allyl bond enthalpy in both HFe(CO) 3 (η 3 -C 3 H 5 ) and IFe(CO) 3 (η 3 -C 3 H 5 ).…”

“…Nevertheless, the range of such data is somewhat limited, with emphasis found on weaker bonds in coordinatively saturated complexes. [1][2][3][4][5][6][7][8][9][10] Accurate quantum chemical calculations can give valuable information. The main advantage of using a theoretical method to study trends of bond strengths is that a systematically selected set of molecules can be studied without additional problems such as the appearance of instabilities.…”

“…[25] Thermodynamic studies to so important FpR complexes are notably scarce. [3][4][5][6][7][8][9][25][26][27] In our laboratory, (meta-substituted phenoxy)dicarbonyl(η 5 -cyclopentadienyl) iron [m-G-C 6 H 4 O(η 5 -C 5 H 5 )Fe(CO) 2 , abbreviated as m-G-C 6 H 4 OFp (1), where G = NO 2 , CN, COMe, CO 2 Me, CF 3 , Br, Cl, F, H, Me, MeO, and NMe 2 ] and m-G-C 6 H 4 SFp (2) complexes were studied as models for the Fp-O and Fp-S bond heterolyses ( Fig. 1) using density functional theory (DFT) methods used in previous work.…”

“…1) using density functional theory (DFT) methods used in previous work. [3][4][5][6][7][8][9] Methods described by Venimadhavan et al, [28] Bordwell et al, [29] and Brauman [30] for determining enthalpies of heterolysis (ΔH het ) for the cleavage of N-H, C-H, O-H or C-S bonds were extended to the study of Fe-C, Fe-N, Fe-O, and Fe-S bonds [3][4][5][6][7][8][9]20,23,31,32] in solution. However, the directly accurate experimental thermodynamic data were not available.…”

“…The methods are widely applicable. [3][4][5][6][7][8][9]34] The calculated substituent effects [ΔΔH het (Fe-O)'s and ΔΔH het (Fe-S)'s] and charge effects of the heterolytic Fe-O and Fe-S bond energies of series 1 and 2 are also investigated in detail. We expect to obtain a number of interesting results from the study.…”