Modeling Polaron-Coupled Li Cation Diffusion in V₂O₅ Cathode Material

Abstract

The transport of intercalated Li cations in oxide materials comprises two aspects, ion diffusion and migration of an associated small polaron. We examined computationally these two aspects of Li transport in vanadium pentoxide (V 2 O 5 ) cathode material in a consistent fashion, using a DFT+U approach. Exploring various migration scenarios at low Li concentrations, we determined barriers of ∼0.3 eV, mostly due to polaron migration. In consequence, intercalating Li atoms, at low concentrations, migrate in the interlayer region of V 2 O 5 as quasi-particles where Li cations remain closely associated with their valence electrons, where a small polaron structure forms around the reduced vanadium center.

Transport properties of electron small polarons in a V2O5 cathode of Li-ion batteries: a computational study

“…The lowest energy site is indeed the same as reported by other DFT studies. 25,26,39,[54][55][56][57] The ionized Li + ion donates an extra electron to the lattice which localizes at the nearest V center and forms a small polaron. Note that no structural perturbation was made to obtain the polaronic structure since the Li + ion induces small reorganization of the nearby oxygen atoms.…”

“…Similar observation on structural reorganization and charge localization upon a polaron formation in V 2 O 5 was also reported in previous computational works using the DFT+U method. [25][26][27]39,42,56,71 We calculated and analyzed the PDOS of the electron-doped and the lithiated V 2 O 5 systems and compared to that of bulk V 2 O 5 . As depicted in Fig.…”

“…Once the separated charges (polarons and ions) reconcile, the Coulomb attraction between electron polarons and Li + ions enforces close proximity of ion-polaron pairs. It has been suggested that the Li + ion and its associated polaron may diffuse together in a coupled fashion, as studied in several cathode materials including Li x V 2 O 5 , [25][26][27] Li x FePO 4 , 28 Li in TiO 2 , 29,30 and Li 2 FeSiO 4 . 31 As the discharge progresses, lithiation at the cathode induces phase transformation of Li x V 2 O 5 as x increases; a (0 < x < 0.33) / 3 (0.33 < x < 0.80) / d (0.88 < x < 1.00) / g (1.0 < x < 2.0).…”

“…While studies based on the use of conventional density functional theory (DFT) with generalized gradient approximation (GGA) were successful at determining crystal structure and cell potentials of Li-intercalated V 2 O 5 , 51,52 recent studies used a variant of Kohn-Sham method, introducing an on-site Hubbard model correction (DFT+U) 53 to properly describe the localization behavior of excess electrons in the V 2 O 5 lattice. 25,26,39,[54][55][56][57] Therefore, to describe the formation of a small polaron and its transport properties, we invoked the U correction scheme to account for proper electron localizations and correct, at least in part, the self-interaction error inherent in local or semi-local exchange-correlation approximations. In particular, we calculated energy barriers of various electron migration paths in the presence and absence of a Li + ion to further explain the characteristic of electronic conduction in the V 2 O 5 cathode.…”

Phase stability of intercalated V2O5 battery cathodes elucidated through the Goldschmidt tolerance factor

“…23,24 Electronic conductivity and formation of stable localised lattice distortions (polarons) are additional factors proposed to affect mobility of intercalated ions in V 2 O 5 . [25][26][27][28][29] One key factor affecting intercalation in Mg x V 2 O 5 is the crystal structure at varying x compositions. The structure of V 2 O 5 has been investigated as a Li-ion battery cathode, and it is known to undergo multiple phase changes upon lithiation.…”