Structural distortion and small-polaron conduction in Ga-doped LiMn2O4 cathode material

Ga-doped LiMn2O4 was synthesized via solid-state reaction. X-ray photoelectron spectroscopy and x-ray powder diffraction confirm that the samples crystallize in the Fd3¯m space group of the spinel structure, where Ga3+ partially replaces Mn3+ at 16d sites, thereby increasing the average Mn oxidation state. Rietveld refinements indicate that while the lattice parameter remains almost unchanged, the oxygen positional parameter u decreases toward 0.25 with increasing Ga content. Consequently, the M–O (M: metal ions at a 16d site) bond length and the O–M–O angle both increase. Ga substitution suppresses the trigonal distortion of the MO6 octahedron, bringing it closer to a regular octahedral geometry. Electrical conductivity and thermopower measurements demonstrate that hopping conduction in Ga-doped LiMn2O4 involves nonadiabatic small polarons, which are eg electrons on Mn3+ ions, and that Ga substitution decreases not only the carrier density but also the small-polaron hopping energy WH. The reduction of WH increases the polaron mobility and decreases the polaron mass, explaining the enhanced electrical conductivity for x ≤ 0.1 despite a reduced carrier density. Importantly, the suppression of trigonal distortion is considered to decrease the polaron binding energy WP, which arises from the interaction between charge carriers and the lattice distortion. Furthermore, the increased O–M–O angle, reflecting the suppression of trigonal distortion, is expected to enhance the hybridization between the Mn 3d and O 2p wave functions. This structural relaxation of the trigonal distortion appears to be more effective in reducing WH than the inhibition of the Jahn–Teller distortion.