High-temperature transport properties of lithium manganese spinels substituted with Ni and Ti

Abstract

Spinel Li1.05Mn2O4−δ substituted with Ni and Ti were fabricated via a solid-state reaction technique. The Rietveld refinement of synchrotron X-ray powder diffraction data confirmed an Fd3‾m space group of spinel structure for all synthesized materials. Variation in the lattice parameter and M-O distances (M: transition metal at 16d site) due to the dopant content indicated that Ni substitution enhanced the overlap in Mn 3d wave functions and Ti substitution induced structural distortion. Electrical conductivity measurements based on a four-probe method were conducted at high temperatures, demonstrating that both the Ni- and Ti-substituted samples exhibited the same non-adiabatic small-polaron hopping mechanism as that of the non-substituted LiMn2O4. Thermopower measurements were performed at high temperatures via a steady-state technique. All samples with and without substitution were characterized as n-type semiconductors and their Seebeck coefficients were thermally activated. The electrical conductivities and the Seebeck coefficients decreased with increasing Ni content, whereas they were less affected by Ti substitution. The activation energies of electrical conductivity Eσ and thermopower ES were derived from the experimental results and were used to calculate the hopping energies, WH. Ni substitution initially reduced WH but substantially increased WH when Ni ​= ​0.5. In contrast, Ti substitution had a major effect on only ES. Variation in carrier density and structural distortion following the substitution for Mn exerted a large influence on the electrical conductivity and Seebeck coefficients of the doped materials.