Role of Oxygen Vacancies on the Performance of Li[Ni0.5–xMn1.5+x]O4 (x = 0, 0.05, and 0.08) Spinel Cathodes for Lithium-Ion Batteries

Investigation of the high-voltage Li[Ni 0.5– x Mn 1.5+ x ]O 4 ( x = 0, 0.05, 0.08) spinels prepared at temperatures of T ≤ 900 °C and given different thermal treatments has shown that the solubility limit for oxygen vacancies in the disordered spinel phase is small at 600 °C. With x = 0, long-range ordering of Ni 2+ and Mn 4+ and elimination of all oxygen vacancies occurs after an anneal at 700 °C. Above 700 °C, a reversible transition from spinel to rock-salt is initiated, to accommodate oxygen loss. A sample quenched from 900 °C into liquid nitrogen traps some rock-salt second phase; the volume fraction of rock-salt phase decreases with oxygen uptake to 600 °C. However, upon slow cooling (1 °C min –1 ) from 900 °C, the particles have time to eliminate most of the rock-salt phase by 700 °C; upon further cooling below 700 °C, the spinel phase and the oxygen gain are retained. However, the spinel phase retains oxygen vacancies and attendant Mn 3+ with only short-range order of Ni and Mn. The rock-salt phase lowers sharply the electrochemical capacity of the quenched sample; but retention of Mn 3+ in the slow-cooled sample improves the electrochemical performance relative to that of an oxygen-stoichiometric spinel formed by annealing at 700 °C. The Mn-rich Li[Ni 0.45 Mn 1.55 ]O 4 sample annealed at 700 °C exhibits a segregation of a long-range-ordered spinel phase and a Ni-deficient spinel phase having a larger fraction near the particle surface. Removal of the Ni 4+ /Ni 2+ redox reactions from the surface stabilizes the electrochemical performance at 55 °C, but the problem of Mn 2+ dissolution resulting from surface disproportionation of Mn 3+ to Mn 2+ and Mn 4+ remains.