Comparative Study of LiNi_0.5Mn_1.5O₄_-_δ and LiNi_0.5Mn_1.5O₄ Cathodes Having Two Crystallographic Structures: <i>Fd</i>3<i>m</i> and <i>P</i>4₃32

Abstract

Nonstoichiometric LiNi0.5Mn1.5O4-δ and stoichiometric LiNi0.5Mn1.5O4 cathodes with two different structures (Fd3̄m and P4332) were synthesized by a molten salt method. Rietveld refinement of the X-ray diffraction (XRD) data and a selected-area electron diffraction (SAED) study confirmed that the face-centered spinel (Fd3̄m) transformed to the primitive simple cubic (P4332) upon additional heating at 700 °C. The LiNi0.5Mn1.5O4-δ cathode having the space group of Fd3̄m showed better electrochemical behaviors than did the LiNi0.5Mn1.5O4 cathode with the space group of P4332, specifically, a lower area-specific impedance (ASI) and a higher discharge capacity during cycling at high rate. XRD and SAED showed that both stoichiometric and nonstoichiometric LiNi0.5Mn1.5O4-δ exhibited similar SAED patterns with extra 002 spots at the fully delithiated state, suggesting a structural transition arising from the possible migration of transition metal cations during Li extraction. In addition, stoichiometric LiNi0.5Mn1.5O4 (P4332) had an intermediate phase of Fd3̄m during Li extraction. Such a two-step phase transition of stoichiometric LiNi0.5Mn1.5O4 (P4332) induced low structural reversibility at a high rate (3C rate). Meanwhile, nonstoichiometric LiNi0.5Mn1.5O4-δ (Fd3̄m) exhibited good structural reversibility, as confirmed by XRD and SAED for the cycled electrode even at high rate (3C rate).