Comparative study of LiNi0.5Mn1.5O4 cathodes having two crystallographic structures: Fdm and P4332

Abstract

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. Rietveld refinement of X-ray diffraction (XRD) data and selected-area electron diffraction (SAED) study confirmed that face-centered spinel (Fdm) transformed into primitive simple cubic (P4332) by additional heating at 700˚C. LiNi0.5Mn1.5O4 with Fdm structure showed better electrochemical behaviors than the cathode with P4332 structure. LiNi0.5Mn1.5O4 with Fdm and P4332 structures exhibited area specific impedance (ASI) values of c.a. 55 and 75 Ω-cm2 at 20-80 % of SOC(state of charge). When the LiNi0.5Mn1.5O4 electrodes with Fdm and P4332 structures were cycled at 3 C-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(Fdm) showed good reversibility at high rate. Compared with one-step phase transition of the Fdm 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.