High-Voltage LiNi1/2Mn3/2O4 Spinel: Cationic Order and Particle Size Distribution

Lithium nickel manganese spinel, LiNi1/2Mn3/2O4, is capable to intercalate lithium reversible at a high voltage delivering a high specific energy when used as cathode material for lithium ion batteries. In this study, the effects of cationic order and particle size distribution on the lithium intercalation in high-voltage LiNi1/2Mn3/2O4 spinel are examined by the application of diffraction and spectroscopic techniques. At 400 degrees C, nonstoichiometric LiNi1/2Mn3/2O4-delta with a disordered spinel structure and particle size distribution between between 10 and 20 nm is obtained. Li-7 NMR with ultrafast spinning rates and electron paramagnetic resonance spectroscopy show that Ni2+, Mn3+, and Mn4+ ions are nonuniformly distributed forming nanoscale domains with (Ni2+,Mn4+)-, (Ni2+,Mn4+,Mn3+)-, and (Mn3+,Mn4+) compositions, respectively, the entirely cubic spinel structure being preserved. By increasing the annealing temperature, the amount of Mn3+ decreases, and Ni2+ and Mn4+ tend to set up a long-range order. The particles start to grow above 600 degrees C, reaching submicrometer dimensions at 800 degrees C. Acid treatment of ordered submicrometer LiNi1/2Mn3/2O4 modifies the particle size distribution without change of the cationic distribution. Cationic Ni,Mn distribution affects the Li+ extraction/insertion from/into LiNi1/2Mn3/2O4, while the particle size distribution has an impact on the rate capability and on the interaction with the electrolyte. After storing of the spinels in electrolyte solutions, the nanosized particles are covered with LiF/LixPFyOz/P2O5, while Ni,MnF2/LixPFy compounds are deposited on the surface of the submicrometer particles. Ordered LiNi1/2Mn3/2O4 with a three-modal particle size distribution displays the best rate capability.