Influence of Cation Ordering and Lattice Distortion on the Charge-Discharge Behavior of LiMn1.5Ni0.5O4 Spinel between 5.0 and 2.0 V

Four LiMn1.5Ni0.5O4 spinel cathodes with different degrees of transition-metal ion ordering have been synthesized by controlling the heat treatment condition. The effect of transition-metal ion ordering on the electrochemical and structural characteristics of LiMn1.5Ni0.5O4 during cycling between 5.0 and 2.0 V has been investigated by an analysis of the X-ray diffraction (XRD) and electrochemical data. Refinement of the XRD data confirms the difference between the size of the empty octahedral sites in the ordered and disordered spinel phases. Ex situ XRD analysis reveals the following: (i) the two distinct plateaus (at similar to 2.7 and similar to 2.1 V) involving the insertion of lithium ions into the 16c octahedral sites are linked to the evolution of two tetragonal phases (T1 and T2). The T2 phase originates from the additional lattice distortion due to the smaller size of the 16c octahedral sites in the disordered phase compared to that of the lithium ion; (ii) larger structural changes occur in the disordered samples during cycling at 5.0-2.0 V due to a larger lattice distortion; (iii) the T2 phase transforms slowly to the T1 phase during the rest period without any load; and (iv) lithium ions can reversibly insert into both 8a tetrahedral and 16c octahedral sites regardless of the transition-metal ion ordering. Comparison of the cyclability data with different voltage windows reveals that the volume and c/a ratio changes associated with the cubic to T1 and T2 phase transitions cause rapid capacity fade when LiMn1.5Ni0.5O4 is cycled between 5.0 and 2.0 V. These transformations restrict its use in the entire voltage range of 5.0-2.0 V despite its impressive theoretical capacity of similar to 294 mAh/g.