High-Voltage Cr⁴⁺/Cr³⁺ Redox Couple in Polyanion Compounds

Abstract

Higher-voltage operation is crucial to increase the energy density of rechargeable batteries. Here we explored the Cr 4+ /Cr 3+ redox couple in a polyanion framework for a high-voltage sodium battery cathode. An archetypal NASICON-phase Na 3 Cr 2 (PO 4 ) 3 exhibited a reversible redox activity at ca. 4.5 V versus Na/Na + , offering further scope for searching new high-voltage materials based on Cr 4+ /Cr 3+ redox couple.

A High‐Energy NASICON‐Type Cathode Material for Na‐Ion Batteries

“…Cr is active from somewhere between 3.9 and 4.3 V to 4.5 V but not at lower voltage. While it is possible that within a small voltage region near 4.3 V, Mn and Cr redox could be both active, as the calculated voltages of Mn 3+/4+ and Cr 3+/4+ redox are extremely similar (Figure ), it has been experimentally reported that Cr redox in NASICON framework tends to be higher than Mn redox . Therefore, it is likely that the Mn redox contributes to the first two plateaus in the charge profile (Figure 3a) of NMCP, whereas the Cr redox mainly contributes to the third.…”

“…Upon further charge to 4.3 V, another peak at higher energy appears and continues to grow before returning to the pristine spectrum after one cycle. This prominent pre‐edge peak results from the 1s–3d quadrupole transition, indicating the existence of Cr 4+ . As Cr 4+ with the d 2 electron configuration usually has a distorted octahedral symmetry, the probability of Cr–3d and O–2p orbital mixing increases, thus increasing the probability of a 1s–3d transition; this transition is forbidden when Cr is present as Cr 3+ which has a symmetric octahedral coordination .…”

“…[3] Despite the high specific capacity of layered compounds, [4] they often NASICON-type compounds with different metal cation combinations (V 2 , MnV, MnTi, MnZr, VCr, VGa, NiV, FeV, etc.) have been synthesized and studied, [15,16,[19][20][21][22][23][24] giving rise to a much more diverse chemical space and tunable electrochemical pro perties in contrast to NVPF. Though NASICONtype compounds have a large Na reservoir with potentially up to four extractable Na (i.e., a theoretical capacity of ≈220 mAh g −1 ), reversible extraction/insertion of more than 2Na can rarely be achieved.…”

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

“…This superior cycling stability and rate performance illustrates that an appropriate amount of Cr 3+ substitution for V 3+ is favorable for increasing both the energy density and the structural stability during cycling. As is known, an excessive oxidation can induce highly toxic Cr 6+ , which is recognized as a possible risk . However, a careful regulation of the operation potential window can stabilize the valance of Cr, which has made O3‐type NaCrO 2 a promising cathode for sodium‐ion batteries.…”

“…As is known, an excessive oxidation can induce highly toxic Cr 6+ , which is recognized as a possible risk. [20] However, a careful regulation of the operation potential window can stabilize the valance of Cr, which has made O3-type NaCrO 2 a promising cathode for sodium-ion batteries.…”