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 Li2MnO3 to LiNi0.5Mn0.5O2. 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[Li1/3Mn2/3]O2 to make the solid solution series Li[Li(1/3)–x Ni x/2□ x/2Mn2/3]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[Ni1/6□1/6Mn2/3]O2, was also synthesized. Both XRD and electrochemical data regarding this new material are presented.