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

Jiansheng Zhang; Guisheng Liang; Chao Wang; Chunfu Lin; Jiajia Chen; Zhongru Zhang; Xiu Song Zhao

doi:10.1021/acsami.0c07702

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

ACS Appl Mater Interfaces. 2020 Jun 24;12(25):28313-28319. doi: 10.1021/acsami.0c07702. Epub 2020 Jun 11.

Authors

Jiansheng Zhang¹, Guisheng Liang¹, Chao Wang¹, Chunfu Lin¹, Jiajia Chen², Zhongru Zhang², Xiu Song Zhao^{1

3}

Affiliations

¹ Institute of Materials for Energy and Environment, College of Materials Science and Engineering, Qingdao University, Qingdao 266071, P.R. China.
² State Key Laboratory for Physical Chemistry of Solid Surface, Collaborative Innovation Centre of Chemistry for Energy Materials, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, P.R. China.
³ School of Chemical Engineering, The University of Queensland, St Lucia, QLD 4072, Australia.

PMID: 32464048
DOI: 10.1021/acsami.0c07702

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 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.

Keywords: Cr4+/Cr3+ redox couple; NASICON-type compound; Na2TiCr(PO4)3; high-voltage cathode; sodium-ion battery.