Synthesis, Structure, and Electrochemical Properties of LiFeV₂O₇

Abstract

The structure of a novel compound, LiFeV 2 O 7 , has been determined from single-crystal X-ray diffraction data. The phase crystallizes in the noncentrosymmetric monoclinic Cc space group. The structure can be described as a layered type compound alternating (Li,Fe)−O sheets and V−O chains that are perpendicular to the [101] direction. Within the (Li,Fe)−O sheets, "hexagonal" holes are formed and assembled into tunnels running parallel to the [201] direction and hosting the vanadium atoms. Original (V 4 O 14 ) 8− strings are observed within the structure in association with well-known (V 2 O 7 ) 4− pyrovanadate units. Both units alternate parallel to the [−101] direction. LiFeV 2 O 7 displays a reversible insertion− deinsertion mechanism for Li + ions. The theoretical capacity for the insertion of one Li + into LiFeV 2 O 7 reaches 97 mAh/g. When the compound is cycled between 3.50 and 2.35 V versus Li + /Li, the electrochemical curve displays an initial capacity of 100 mAh/g, with 85% of this capacity retained after 60 cycles. No evidence of the formation of Fe 4+ upon oxidation to a high voltage was observed. LiFeV 2 O 7 represents the first reported phase in the Li 2 O−Fe 2 O 3 −V 2 O 5 ternary diagram with electrochemical activities.

Structural Complexity and Evolving Lithium-Ion Dynamics within the Cathode Material LiFeV2O7 Revealed by Diffraction and Solid-State NMR

“…It can be confused as a polyanionic framework due to the presence of VO 4 ; however, vanadium also participates in the redox process, whereas the polyanions should be electrochemically inert according to the definition of polyanionic structures. In the previous study, it was hypothesized that both V 5+ and Fe 3+ could be electrochemically reduced in the lithiation process. , A similar behavior was observed for the other phases such as LiFeVO 4 , Fe 4 (V 2 O 7 ) 3 , and Fe 2 V 4 O 13 . , Based on 57 Fe Mössbauer spectroscopy and X-ray diffraction results, Denis et al showed that for LiFeVO 4 both Fe 3+ and V 5+ are reducing during the process of lithiation. In the previous study, LiFeV 2 O 7 (LFVO) achieved around 100 mAh/g on the first cycle from 3.50 to 2.35 V, with a retention of 85% of this capacity after 60 cycles .…”

“…In the previous study, it was hypothesized that both V 5+ and Fe 3+ could be electrochemically reduced in the lithiation process. , A similar behavior was observed for the other phases such as LiFeVO 4 , Fe 4 (V 2 O 7 ) 3 , and Fe 2 V 4 O 13 . , Based on 57 Fe Mössbauer spectroscopy and X-ray diffraction results, Denis et al showed that for LiFeVO 4 both Fe 3+ and V 5+ are reducing during the process of lithiation. In the previous study, LiFeV 2 O 7 (LFVO) achieved around 100 mAh/g on the first cycle from 3.50 to 2.35 V, with a retention of 85% of this capacity after 60 cycles . The promising behavior of this new structure type as a positive electrode encourages us to investigate the Li dynamics of the phase.…”

“…This prompted further investigation, beginning with deconvolution that showed each peak split into two in relatively comparable intensity ratios. In total, the spectrum presents at least seven signals with shifts of 291, 223, 218, 151, 146, 116, and 107 ppm, revealing more subtle details than available from the original single crystal . Deconvolution of the signals enables the determination of relative site occupancies, revealing the relationship among sites.…”

“…The previous diffraction data set of LFVO was determined by X-ray diffraction from a single crystal that was grown using the LiVO 3 flux technique. , The iron vanadate structure crystallizes in the monoclinic Cc space group (noncentrosymmetric) and consists of extended V-O chains that create layers between which Li and Fe-O units alternate. The lattice parameters are a = 13.4630(2) Å, b = 8.18820(10) Å, c = 14.3860(2) Å, and β = 95.5469(10) Å.…”

“…In this paper, we are interested in LiFeV 2 O 7 (LFVO), which was first reported by Benabed et al and has three unique Li sites in a 1:1:1 ratio in the 4, 5, and 6 coordination. It is a complex three-dimensional structure that was considered by the author as a layered compound alternating (Li, Fe) and V sheets, perpendicular to the [101] direction.…”