Comparative Study of LiNi0.5Mn1.5O4 Cathodes Having Two Crystallographic Structures: Fd3m and P4332

Abstract

LiNi0.5Mn1.5O4 cathodes with two different structures(Fd3m 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℃ for 3h. After then, the resulting powders were thoroughly washed with distilled water to remove residual lithium salt, then dried at 110℃. The obtained LiNi0.5Mn1.5O4 powders were oxidized by annealing at 700℃ for 48h in air. Rietveld refinement of X-ray diffraction (XRD) data and selected-area electron diffraction (SAED) study confirmed that face-entered spinel (Fd3m) transformed into primitive simple cubic (P4332) by additional heating at 700℃. LiNi0.5Mn1.5O4 with Fd3m and P4332 structures exhibited area specific impedance (ASI) values of ca. 55 and 75Ω-cm2 at 20-80% of SOC (state of charge). When the LiNi0.5Mn1.5O4 electrodes with Fd3m and P4332 structures were cycled at 3C-rate, capacity retentions were 91% and 84% after 50 cycle. XRD and SAED of the electrode after 50 cycles revealed that LiNi0.5Mn1.5O4 (P4332) had low reversibility between fully lithiated and delithiated structures whereas LiNi0.5Mn1.5O4 (Fd3m) showed good reversibility at high rate. Compared with one-step phase transition of the Fd3m structure, the P4332 structure had an additional intermediate phase during charge/discharge cycle, and this two-step phase transition led to deterioration of the structural reversibility at high rate.