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

Abstract

Vanadium phosphate positive electrode materials attract great interest in the field of Alkali-ion (Li, Na and K-ion) batteries due to their ability to store several electrons per transition metal. These multi-electron reactions (from V2+ to V5+) combined with the high voltage of corresponding redox couples (e.g., 4.0 V vs. for V3+/V4+ in Na3V2(PO4)2F3) could allow the achievement the 1 kWh/kg milestone at the positive electrode level in Alkali-ion batteries. However, a massive divergence in the voltage reported for the V3+/V4+ and V4+/V5+ redox couples as a function of crystal structure is noticed. Moreover, vanadium phosphates that operate at high V3+/V4+ voltages are usually unable to reversibly exchange several electrons in a narrow enough voltage range. Here, through the review of redox mechanisms and structural evolutions upon electrochemical operation of selected widely studied materials, we identify the crystallographic origin of this trend: the distribution of PO4 groups around vanadium octahedra, that allows or prevents the formation of the vanadyl distortion (O…V4+=O or O…V5+=O). While the vanadyl entity massively lowers the voltage of the V3+/V4+ and V4+/V5+ couples, it considerably improves the reversibility of these redox reactions. Therefore, anionic substitutions, mainly O2− by F−, have been identified as a strategy allowing for combining the beneficial effect of the vanadyl distortion on the reversibility with the high voltage of vanadium redox couples in fluorine rich environments.

Validating the Electronic Structure of Vanadium Phosphate Cathode Materials

“…Structural representations of these alkali vanadium phosphates (AVPs) are shown in Figure a–c. Unique Li, Na, and K sites are represented within their respective open frameworks of corner- and edge-sharing PO 4 tetrahedra and VO 6 octahedra, which enable rapid alkali-ion diffusion compared to competing battery cathode materials Figure d–f shows the powder X-ray diffraction (XRD) patterns for these bulk samples.…”

Redox Paradox of Vanadium in Tavorite LiVPO4F1–yOy

“…This phase transition seems to present some similarities to the one observed at the Li 0.67 VPO 4 F composition for LiVPO 4 F. Indeed, whatever the oxidation state of vanadium in the starting material, the phase transition appears at a V 3+ /V 4+ ratio approximately equal to 2. Nevertheless, even using a bright SXRPD source, we were not able to identify the superstructure peaks observed in the Li 0.67 VPO 4 F phase for the Li x VPO 4 ­F 1– y O y samples. Its limitation on the local fluorine-rich domains makes it much more difficult to detect.…”

In Honor of John Bannister Goodenough, an Outstanding Visionary

“…(3) High-voltage multi-electron reactions in alkali-ion batteries using vanadium phosphate positive electrodes are described by Edouard Boivin, Jean-Noël Chotard, Christian Masquelier and Laurence Croguennec [9]. They clearly showed how the vanadyl distortion affects the reversibility of the intercalation/de-intercalation mechanism, the voltage of the battery and the number of exchange electrons per transition metal, and how these parameters can be modified by controlling the local chemical environment of vanadium, i.e., by an appropriate change in the composition (e.g., fluorine/oxygen exchange) of the material.…”

“…As an example of cost-effective electrocatalysts with effective bifunctional activity for both oxygen reduction and oxygen evolution, they discussed a hybrid catalyst, Co 3 O 4 -infiltrated La 0.5 Sr 0.5 MnO 3-δ , to demonstrate that hybrid catalysts are a promising approach for oxygen electrocatalysts for renewable and sustainable energy devices. (9) In the last contribution [15], Jan L. Allen, Bria A. Crear, Rishav Choudhury, Michael J. Wang, Dat T. Tran, Lin Ma, Philip M. Piccoli, Jeff Sakamoto and Jeff Wolfenstine examined Li-stuffed spinels with a high conductivity of Li as a material for a fully structured all solid battery. Their work provides a useful concept for enabling a single-phase fully solid electrode without interphase impedance.…”