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 M 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]O-2 to make the solid solution series Li[Li(1/3)-xNix/2 square Mn-x/2(2/3)]O-2 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 root 3 x root 3 lattices yields a structure with IOW 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 square Mn-1/6(2/3)]O-2 was also synthesized. Both XRD and electrochemical data regarding this new material are presented.