Revisiting the Stability of the Cr<sup>4+</sup>/Cr<sup>3+</sup> Redox Couple in Sodium Superionic Conductor Compounds
Jiansheng Zhang, Guisheng Liang, Chao Wang, Chunfu Lin, Jiajia Chen, Zhongru Zhang, Xin Zhao
Abstract
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 Na1+xTi2–xCrx(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.