Redox Paradox of Vanadium in Tavorite LiVPO₄F_1–yO_y

Abstract

Vanadyl-type defects in vanadium oxy-fluoride phosphates confer interesting properties to these materials as positive electrodes in Li-ion or Na-ion batteries. The influence of defects concentration on the phase diagram and redox mechanisms in LiVPO4F1-yOy Tavorite phases has been investigated by combining operando Synchrotron X-ray powder diffraction (SXRPD) and X-ray absorption spectroscopy (XAS). Operando X-ray absorption near edge structure (XANES) reveals the activation of the V 4+ =O/V 5+ =O redox couple during the first step of the charge, and then upon further Li + extraction the activation of the V 3+ /V 4+ redox couple in fluorine-rich environments. This result, although surprising, is in full agreement with the operating voltages of the end-member phases, as LixV III,IV PO4F operates at higher voltage than LixV IV,V PO4O. The unexpected succession of these redox couples is observed for the first time in the same material. The small volume changes undergone by the materials upon electrochemical operation were proposed to be at the origin of the better performance observed for the mixed valence samples: "freezing" of the framework is observed all along the activation of the V 4+ =O/V 5+ =O redox couple in a solid solution type domain.

Titanium-based potassium-ion battery positive electrode with extraordinarily high redox potential

“…It was demonstrated that in many fluoride phosphates the fluorine-to-oxygen ratio plays a significant role in governing the electrochemical performance, which was observed and thoroughly studied for many vanadium-containing fluoride phosphates. However, the influence of the F:O ratio on the electrochemical parameters is vastly complicated because both V 3+ /V 4+ and V 4+ /V 5+ (or [V=O] 2+ /[V=O] 3+ ) redox transitions are typically electrochemically active 54 . Deviation from the stoichiometric F:O ratio thus results in changing of the electrochemical parameters (operating potential, de/intercalation mechanism, C-rates, etc.)…”

Towards Reversible High-Voltage Multi-Electron Reactions in Alkali-Ion Batteries Using Vanadium Phosphate Positive Electrode Materials

“…This strategy aimed at decreasing the difference in voltage between Li insertion and extraction reactions, conferring to the material a high energy density (i.e., 820 Wh/kg) in a reduced voltage range (i.e., 2.0–4.5 V vs. Li + /Li) with the activation of the V 3+ /V 4+ and V 4+ /V 5+ redox couples, respectively. Further investigation of the LiVPO 4 F-LiVPO 4 O tie-line has allowed several compositions to stabilize in which the competition between ionicity of the V 3+ -F bond and covalency of the V 4+ =O bond distorts the structure, freezes the framework upon Li extraction and hence allows for improved rate capabilities compared with the end-member phases [ 80 , 81 ]. Interestingly, upon Li deintercalation from these materials, the V 4+ =O/V 5+ =O redox couple is triggered first before the V 3+ /V 4+ is activated in fluorine rich environments leading to the formation of a mixed valence V 3+ -V 5+ phase at half charge [ 81 ].…”

“…Further investigation of the LiVPO 4 F-LiVPO 4 O tie-line has allowed several compositions to stabilize in which the competition between ionicity of the V 3+ -F bond and covalency of the V 4+ =O bond distorts the structure, freezes the framework upon Li extraction and hence allows for improved rate capabilities compared with the end-member phases [ 80 , 81 ]. Interestingly, upon Li deintercalation from these materials, the V 4+ =O/V 5+ =O redox couple is triggered first before the V 3+ /V 4+ is activated in fluorine rich environments leading to the formation of a mixed valence V 3+ -V 5+ phase at half charge [ 81 ]. Although surprising, this redox mechanism is in full agreement with the operating voltage of the end-member phases, the V 4+ /V 5+ redox couple being activated at 3.95 V in LiVPO 4 O and the V 3+ /V 4+ redox couple at 4.25 V in LiVPO 4 F due to the absence of vanadyl distortion in LiVPO 4 F and VPO 4 F.…”

Local atomic and electronic structure in the LiVPO4(F,O) tavorite‐type materials from solid‐state NMR combined with DFT calculations

“…The O/F clustering tendency observed here is in good agreement with the observation of microstrains and deviations from the Vegard's law for the solid solution LiVPO 4 F 1‐y O y reported in our previous study. The present study is a first step to better understand the complex redox behavior observed for the LiVPO 4 F 1‐y O y materials in Li cells with the activation of the V IV O/V V O redox couple during the first step of the charge, and then upon further Li + extraction, the activation of the V 3+ /V 4+ redox couple in fluorine‐rich environments [ 9 ] that may result from the complex electronic structure and clustering tendency observed.…”

“…[8] The concentration of vanadyl-type defects was shown to have a strong influence on the electrochemical behavior in terms of redox processes involved, energy density, cyclability, and rate capability. [8,9] A better understanding of the local structural arrangement is of prime importance for further material optimization.…”