Structural Changes in a High-Energy Density VO₂F Cathode upon Heating and Li Cycling

Abstract

Structural changes in VO 2 F, which allow twoelectron transfer during electrochemical Li cycling, were investigated. This compound adopts a rhombohedral structure, space group R3̅ c, with O and F sharing one site, and was synthesized by high-energy ball-milling. The thermal stability of VO 2 F, which is related to the battery safety, is studied by in situ XRD upon heating and by thermal gravimetric analysis. VO 2 F is found to be stable up to 160 °C under inert atmosphere; above this temperature, it converts into vanadium oxide with fluorine loss. The structure evolution upon lithium cycling was studied by ex situ X-ray diffraction and absorption techniques. The results show that lithiation of VO 2 F goes through a solid-solution reaction, and the rhombohedral structure is preserved if no more than one lithium ion is intercalated. Upon a second Li insertion, an irreversible transition to a rock-salt structure occurs. We show using first-principles calculations that this irreversible transformation can be explained by an asymmetric energetic preference between the rhombohedral and rock-salt forms of Li x VO 2 F, which result in large thermodynamic driving forces to convert to the rock-salt structure at x > 1 and relatively small thermodynamic driving forces to convert back to the rhombohedral structure when delithiating to x < 1.

Perovskite-related ReO3-type structures

“…This VO 2 F material also underwent an irreversible phase change if cycled to lower voltages, which has been shown by multiple groups to be a disordered rock-salt phase. [143][144][145] The most recent of those reports clarified the performance of the ball-milled VO 2 F material, 145 and described that it delivers 276 mA h g −1 (1.04 mol Li) and 206 mA h g −1 (0.8 mol Li) at C/20 in its first and second cycles, respectively, with 97.5% capacity maintenance after 100 cycles. If taken to the irreversible rock-salt phase, 406 mA h g −1 (1.54 mol Li, C/60) capacity was achieved, but with only 60% capacity maintenance after 50 cycles.…”

Site-Selective Halogenation of Polyoxovanadate Clusters: Atomically Precise Models for Electronic Effects of Anion Doping in VO2

“…The wide range of oxidation states accessible to vanadium has resulted in the isolation of a variety of solid-state oxides (e.g., V 2 O 5 , VO 2 , V 2 O 3 , VO, etc. ), many of which exhibit intriguing optical, electrical, thermal, and magnetic properties. − In particular, vanadium dioxide (VO 2 ) has benefitted from extensive research, given its near-room-temperature metal-to-insulator phase transition. This distinct property has resulted in the implementation of VO 2 films in energy-saving smart window technologies. − …”

ACS Applied Energy Materials

“…They are also in line with the 2019 Nobel Prize in Chemistry that was awarded to John B. Goodenough, M. Stanley Whittingham, and Akira Yoshino for the development of lithium-ion batteries that created a rechargeable world . Two of the Prize winners have disseminated their results in ACS Applied Energy Materials . − …”

“…In summary, this past year has witnessed enhanced attention to energy and energy applications as reflected by publications in ACS Applied Energy Materials , the Nobel Prize in Chemistry, and a decade of funding in U.S. Department of Energy supported EFRCs. The Nobel Committee’s selection of three scientists who advanced lithium-ion battery technologies brings renewed focus to this important application. − , The 10 years of successful energy advances realized by the EFRCs may inspire other countries to focus on team-based research. − I look forward to seeing these and other advances published here in ACS Applied Energy Materials in 2020.…”

“…The Nobel Committee's selection of three scientists who advanced lithium-ion battery technologies brings renewed focus to this important application. [6][7][8]29 The 10 years of successful energy advances realized by the EFRCs may inspire other countries to focus on team-based research. 1−4 I look forward to seeing these and other advances published here in ACS Applied Energy Materials in 2020.…”