Electrochemical and magnetic studies of Cr- or Co-substituted Li-Mn-Ni spinel oxides

LiMn_1.5−xM_2xNi_0.5−xO₄ (M = Co or Cr, 0.0 ≤ x ≤ 0.2) compounds were synthesized by the sol-gel method. It is found from X-ray diffraction analysis that single spinel phases are obtained for both Cr³⁺- and Co³⁺-substitution, but a small cation mixing between the 8a and 16d sites is found only in the Co³⁺-substitution case. All of the compounds exhibit M-T curves characteristic of ferromagnetic materials. As the substitution degree is raised, the saturation magnetization at 4.2 K decreases and the Curie temperature shifts lower, irrespective of the kind of the substituent. That is, the cation substitution yields a reduction of the ferromagnetism. Additionally, both Cr³⁺- and Co³⁺-substitution have a large influence on the electrochemical properties. The Cr³⁺-substitution induces a rise in the average operating potential without a loss of redox capacity, while both the average operating potential and the redox capacity decrease with an increase in the Co³⁺-substitution degree.