Exploring Highly Reversible 1.5-Electron Reactions (V³⁺/V⁴⁺/V⁵⁺) in Na₃VCr(PO₄)₃ Cathode for Sodium-Ion Batteries

Abstract

The development of highly reversible multielectron reaction per redox center in sodium super ionic conductor-structured cathode materials is desired to improve the energy density of sodium-ion batteries. Here, we investigated more than one-electron storage of Na in NaVCr(PO). Combining a series of advanced characterization techniques such as ex situ V solid-state nuclear magnetic resonance, X-ray absorption near-edge structure, and in situ X-ray diffraction, we reveal that V/V and V/V redox couples in the materials can be accessed, leading to a 1.5-electron reaction. It is also found that a light change on the local electronic and structural states or phase change could be observed after the first cycle, resulting in the fast capacity fade at room temperature. We also showed that the irreversibility of the phase changes could be largely suppressed at low temperature, thus leading to a much improved electrochemical performance.

Three Electron Reversible Redox Reaction in Sodium Vanadium Chromium Phosphate as a High‐Energy‐Density Cathode for Sodium‐Ion Batteries

“…In the other three analogues, two distinct couples of redox reaction peaks at ≈3.65/3.25 and ≈4.1/3.9 V were observed, corresponding to the reversible transition of V 3+ / 4+ and V 4+ / 5+ , respectively. The substitution of V 3+ by Cr 3+ in Na 3 V 2 (PO 4 ) 3 activates the redox reaction of V 4+ /V 5+ to boost the energy density, in accord with previous studies …”

“…With specific regard to the Na 3 VCr(PO 4 ) 3 electrode, its S ‐shaped profile during the first cycle is evidently different from the dominant potential plateau for the other three kinds of electrode. The observed S ‐shaped curve of Na 3 VCr(PO 4 ) 3 electrode is consistent with previous reports showing irreversibility of the phase change during initial cycles and fast capacity fade at room temperature . Therefore, an optimized Cr doping can achieve a high‐energy‐density cathode with stable cycling performance.…”

“…The valence states of V and Cr were investigated by X‐ray photoelectron spectroscopy (Figure S5, Supporting Information). In accordance with the as‐designed composition, the valance states of V and Cr are V 3+ and Cr 3+ in Na 3 V 1.5 Cr 0.5 (PO 4 ) 3 …”

Rational Architecture Design Enables Superior Na Storage in Greener NASICON‐Na4MnV(PO4)3 Cathode

“…[8][9][10] In this context, great attention has been paid on the polyanion-type materials due to their robust crystal framework with high-level thermal stability, and the moderate capacity with tunable high redox potential, which can achieve high energy density. [25] In addition, introducing F into Na 3 V 2 (PO 4 ) 3 has been proven effective and the resulting Na 3 V 2 (PO 4 ) 2 F 3 could exhibit an average potential at 3.7-3.8 V, [26][27][28][29] apparently surpassing Na 3 V 2 (PO 4 ) 3 . [14][15][16][17][18] Among diverse NASICON-structured compounds, Na 3 V 2 (PO 4 ) 3 is a hotspot, which can deliver a highly reversible capacity over 110 mAh g −1 , and an energy density of over 370 Wh kg −1 as a result of a flat voltage plateau located at 3.3-3.4 V. [19] Although massive work has been reported to promote the development of Na 3 V 2 (PO 4 ) 3 by nanosizing and/or optimizing its poor electronic conductivity, [20][21][22][23] in order to meet the demand of practical applications of Na 3 V 2 (PO 4 ) 3 , improving its operating voltage to reach a higher energy density is urgent.…”

Towards High‐Performance Aqueous Sodium‐Ion Batteries: Stabilizing the Solid/Liquid Interface for NASICON‐Type Na2VTi(PO4)3 using Concentrated Electrolytes

“…[15,16] The V 4 + /V 3 + and Ti 4 + /Ti 3 + redox processes in vanadium-and titanium-based insertion hosts with the NASI-CON structure occur within the electrochemical stability window of water. [20][21][22][23][24][25] In consequence,a mphoteric insertion hosts that can be used as cathode and anode materials are being developed. Specifically,N aTi 2 (PO 4 ) 3 is of high interest because it is redox active at the lower end of the aqueous electrolyte stabilityw indow,w hicht hus enables enhanced energy densities.…”