Unraveling Processing-Structure-Electrical Conductivity Relationships of NaCrO2 Cathodes for Na-Ion Batteries

Abstract

The electrical conductivities of NaCrO2 powders synthesized from Na2CO3 and Cr2O3 reactants with and without mechanical activation at room temperature before the high temperature reaction are investigated in detail. The effect of holding time at the 900°C reaction on the electrical conductivity is studied as well. It is found that the impurities in the powders and structural defects of the resulting NaCrO2 crystals strongly depend on the synthesis conditions. The impurities and specially the crystal defects, in turn, influence the electrical conductivity of NaCrO2 powders substantially. In particular, the electrical conductivity of NaCrO2 is found to be mainly controlled by the degree of misplacement of Cr+3 ions in the Na sites of the O3-NaCrO2 layered structure. The bulk ionic conductivity is determined to be 9.8 × 10−7 S/cm when there is only 0.34% misplacement of Cr+3 ions in the Na sites, but drops to 1.7 × 10−7 S/cm when the misplacement increases to 9.97%. Such a strong correlation of more misplacement with lower electrical conductivity is attributed to blocking of Na ion transport on the Na plane by the misplaced Cr+3 ions in the O3-NaCrO2 layered structure.