Comparative study of LiNi0.5Mn1.5O4-δ and LiNi0.5Mn1.5O4 cathodes having two crystallographic structures:: Fd(3)over-barm and P4332

Nonstoichiometric LiNi0.5Mn1.5O4-delta and stoichiometric LiNi0.5Mn1.5O4 cathodes with two different structures (Fd (3) over barm and P4(3)32) 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 (Fd (3) over barm) transformed to the primitive simple cubic (P4(3)32) upon additional heating at 700 degreesC. The LiNi0.5Mn1.5O4-delta cathode having the space group of Fd (3) over barm showed better electrochemical behaviors than did the LiNi0.5Mn1.5O4 cathode with the space group of P4(3)32, 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-delta 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 (P4(3)32) had an intermediate phase of Fd (3) over barm during Li extraction. Such a two-step phase transition of stoichiometric LiNi0.5Mn1.5O4 (P4(3)32) induced low structural reversibility at a high rate (3C rate). Meanwhile, nonstoichiometric LiNi0.5Mn1.5O4-delta (Fd (3) over barm) exhibited good structural reversibility, as confirmed by XRD and SAED for the cycled electrode even at high rate (3C rate).