The role of metal site vacancies in promoting Li-Mn-Ni-O layered solid solutions

The Li-Mn-Ni-O system has received much attention for potential positive electrode materials in lithium-ion batteries. Recent work mapping the phase diagrams of the entire pseudo-ternary system showed that the layered solid-solution region extends to compositions with both less and more lithium than the well-known lithium-rich layered composition line that joins Li ₂MnO₃ to LiNi_0.5Mn_0.5O₂. The part of this solid-solution region that is lithium deficient has a "bump" feature in the single-phase boundary, which could not be explained until now. The current study explores this part of the phase diagram with the use of X-ray diffraction, helium pycnometry measurements, redox titrations, and a Monte Carlo simulation. Results show that metal site vacancies are present in the structures in increasing amounts as the lithium content of the samples decreases. A Ni²⁺ ion and a vacancy can replace two Li⁺ ions in Li[Li_1/3Mn_2/3]O₂ to make the solid solution series Li[Li_(1/3)-xNi_x/2□ _x/2Mn_2/3]O₂ with 0 < x < ¹/₃. The most lithium-deficient structures contain sufficient vacancies to allow manganese to form on two-thirds ( ²/₃) of the transition-metal layer, such that the ordering of manganese on two √3 × √3 lattices yields a structure with low internal energy and sharp superlattice peaks in XRD patterns. The material with the maximum theoretical vacancy fraction that still has two-thirds of the transition-metal layer filled with manganese, Li[Ni_1/6□ _1/6Mn_2/3]O₂, was also synthesized. Both XRD and electrochemical data regarding this new material are presented.