Revisiting the Stability of the Cr⁴⁺/Cr³⁺ Redox Couple in Sodium Superionic Conductor Compounds

In this work, we revisited the stability of the Cr⁴⁺/Cr³⁺ redox couple in a sodium superionic conductor (NASICON)-type compound, Na₂TiCr(PO₄)₃. Experimental results showed that the Na₂TiCr(PO₄)₃ compound exhibited a specific capacity of 49.9 mA h g^-1 at 20 mA g^-1, about 80% of its theoretical capacity of 62.2 mA h g^-1 with one Na⁺ insertion/deinsertion per formula Na₂TiCr(PO₄)₃. The redox couple was found to be stable against cycling with some 90.3% capacity retention after 300 cycles within the voltage range between 2.5 and 4.7 V. With a wider voltage range between 2.5 and 5.0 V, the capacity retention was about 76.6% after 1000 cycles, indicating the redox couple is stable against overvoltage. In addition, the effect of Ti/Cr ratio on the reversibility of the redox couple was studied by varying x in Na_1+xTi_2-xCr_x(PO₄)₃ (where x = 0.6, 0.8, 1.0, 1.2, 1.4, 2.0). It was confirmed that x = 1 is optimal for balancing the electrode stability and the capacity. The obtained optimal content of Cr in the compound provides useful guidance for designing new Cr-based NASICON-type cathode materials. Furthermore, in situ X-ray diffraction (XRD) analysis of compound Na₂TiCr(PO₄)₃ indicated a two-phase sodium-ion storage mechanism, which is different from the previously reported one-phase mechanism. Rietveld refinement XRD analysis showed a small volume change of the compound during cycling (about 2.6%), indicating good structural stability.