Thermal and electrical properties of nanocrystalline superionic NaxCu1.75S (x=0.1, 0.15, 0.2, 25) compounds

Abstract

The paper presents the results of the studies of thermal properties of nanocrystalline superionic NaxCu1.75S (x=0.1, 0.15, 0.2, 25) compositions, and preliminary results of Na0.1Cu1.75S using as energy stored cathode material in Na-ion half-cell with NaPF 6 electrolyte and Na anode. The compositions contain a few copper sulfide phases: monoclinic chalcocite Cu2S , orthorhombic anilite Cu1.75S , triclinic roxbyite Cu1.74÷1.82S , also the compositions can contain monoclinic Na2Cu4S3 , orthorhombic Na2S , cubic Cu2O as inclusion phases. The sizes of powder particles lie in the range from 10 to 113 nm. Differential scanning calorimetry revealed in Na0.1Cu1.75S the endothermic thermal effects with critical temperatures near 123 ◦ C, 42 ◦ C and 442 ◦ C, caused by structural transitions in copper sulfide. Fourth endothermic peak at 323 ◦ C presumably belongs to Na2S phase. The minimum for the Fermi level at about 420 ◦ C is found with using of the e.m.f. E of the electrochemical cell of the Cu/CuBr/Na0.10Cu1.75S/Pt , which corresponds to minimum for the carrier concentration. This conclusion correlates well with the observed conductivity minimum at about 410 ◦ C. Electrode material Na0.10Cu1.75S achieved a significant specific energy density 146.5 mAh/g in half-cell assembled from the cathode active material, electrolyte (NaPF 6 in 0.5 mol PC) and Na anode.