Comparative Study of LiNi0.5Mn1.5O4-(delta)and LiNi0.5Mn1.5O4 Cathodes Having Two Crystallographic Structures: Fdm and P4332

Nonstoichiometric LiNi0.5Mn1.5O4- and stoichiometric LiNi0.5Mn1.5O4 cathodes with two different structures (Fdm 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 (Fdm) transformed to the primitive simple cubic (P4332) upon additional heating at 700 C. The LiNi0.5Mn1.5O4- cathode having the space group of Fdm 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- 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 Fdm 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-(delta)(Fdm) exhibited good structural reversibility, as confirmed by XRD and SAED for the cycled electrode even at high rate (3C rate).