5V 영역에서 Li1-δNi0.5Mn1.5O4 양극활물질의 구조 변이 Phase Transitions in Li1-δNi0.5Mn1.5O4 during cycling at 5V

Abstract

Structural change of LiNi0.5Mn1.5O4 with two different crystallographic structures of Fdm and P4332 was studied during delithiation. Spinel LiNi0.5Mn1.5O4 with a space group of Fdm exhibits randomly distributed cations on the 16d site. On the contrary, in the case of LiNi0.5Mn1.5O4 having P4332 space group, cation ordered on the octahedral sites, Ni at 4b and Mn at 12d, resulted in lowering of its crystallographic symmetry from face-centered spinel (Fdm) to primitive cubic (P4332). LiNi0.5Mn1.5O4 cathodes with two different structures (Fdm and P4332) were synthesized by molten salt method. Mixed precursors of LiOH, LiCl, Ni(OH)2, and γ-MnOOH (2:1:3) were calcined at 900˚C for 3 h. After then, the resulting powders were thoroughly washed with distilled water to remove residual lithium salt, then dried at 110˚C. The obtained LiNi0.5Mn1.5O4 powders were oxidized by annealing at 700˚C for 48 h in air. Ex-situ XRD and TEM showed that LiNi0.5Mn1.5O4 with Fdm structure have reversible one-step phase transition from spinel to cubic phase showing the extra 002 spots. This suggests a phase transition arising from the possible migration of cations during oxidation from Ni2+ to Ni4+. In case of P4332 structure, as Li extracted from the host lattice, intermediate phase with Fdm spinel was formed. This means that the ordering of cations, Ni and Mn, are randomized by migration of cations. Further Li extraction led to the same final phase observed during delithiation of the spinel electrode. The phase transitions were reversible meaning that migration of cations is highly reversible during lithiation/delithiation.