Dong Wook Shin

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.

The high-voltage doped spinel oxides LiMn1.5Ni0.5–xMxO4 (M = Cr, Fe, and Ga; 0 ≤ x ≤ 0.08) synthesized at 900 °C have been investigated systematically before and after postannealing at 700 °C. Neutron diffraction studies reveal that the cation-ordered domain size tends to increase upon annealing at 700 °C. Time-of-flight secondary-ion mass spectroscopy data reveal that the dopant cations M = Cr, Fe, and Ga segregate preferentially to the surface, resulting in a more stable cathode–electrolyte interface and superior cyclability at both room temperature and 55 °C with conventional electrolytes. The doping with Cr and Fe stabilizes the structure with a significant disordering of the cations in the 16d sites even after postannealing at 700 °C, resulting in high rate capability due to low charge-transfer resistance and polarization loss. In contrast, the Ga-doped and undoped LiMn1.5Ni0.5O4 samples experience an increase in cation ordering upon postannealing at 700 °C, resulting in degradation in the rate capability due to an increase in the charge-transfer resistance and polarization loss.

High-voltage spinel cathodes LiMn1.5Ni0.5O4 are promising candidates for large-scale energy-storage applications such as electric vehicles. However, the widespread adoption of this high-voltage spinel cathode is hampered by severe capacity fade, particularly at elevated temperatures, resulting from aggressive formation of a thick solid-electrolyte interphase (SEI) layer through side reactions with the electrolyte at the high operating voltage, cationic ordering between Mn4+ and Ni2+ ions in the crystal lattice, and formation of a rock salt LixNi1−xO impurity phase. While these issues have been explored, the wide variation in physical and electrochemical properties with different synthesis methods is not fully understood. In this investigation, we present how the synthesis conditions of the co-precipitation method influence the microstructure and morphology through nucleation and growth of crystals in solution. The samples were prepared by two similar wet-chemical routes and were characterized by microscopy and electrochemical methods to determine the role of microstructure and morphology in the electrochemical performance. Various factors such as the degree of cation ordering between Mn4+ and Ni2+, Mn3+ content, Ni–Mn ratio in the sample, change in lattice parameter with the state of charge, and surface crystal planes were examined to develop a better understanding of the factors influencing the electrochemical performance. It is found that the surface crystal planes, the arrangement of lithium ions near the surface, and the lithium diffusion mechanism have a dominant effect on the capacity retention and rate performance.