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[Ni0.5–xMn1.5+x]O4 (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 Ni2+ and Mn4+ 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 Mn3+ 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 Mn3+ 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[Ni0.45Mn1.55]O4 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 Ni4+/Ni2+ redox reactions from the surface stabilizes the electrochemical performance at 55 °C, but the problem of Mn2+ dissolution resulting from surface disproportionation of Mn3+ to Mn2+ and Mn4+ remains.