Electronic , Structural , and Electrochemical Properties of LiNi x Cu y Mn 2 x y O 4 ( 0 < x < 0 . 5 , 0 < y < 0 . 5 ) High - Voltage Spinel Materials

LiMn2O4 spinel is an attractive compound as a cathode material in lithium-ion batteries, due to its economical, environment, and safety advantages over LiCoO2. LiMn2O4 adopts the spinel structure with the space group Fd3 hm, in which the Li and Mn occupy the 8a tetrahedral and 16d octahedral sites of the cubic close-packed oxygen ions framework, respectively. However, LiMn2O4 tends to exhibit capacity fade in the 4 V region, particularly at elevated temperatures. Factors such as Mn dissolution into the electrolyte and the development of microstrains during cycling have been suggested to be the main sources of capacity fade. The poor cycling performance could be improved by partial substitution of Mn with other metals, an approach to making the LiMxMn2 xO4 (M =Co, Mg, Cr, Ni, Fe, Al, Ti, Cu, Zn, etc.) electrode material. 5 It has been found that a higher voltage plateau (>4 V) accompanies some transition metal doping. Among all LiMxMn2 xO4 materials, LiNi0.5Mn1.5O4 is an attractive high voltage cathode material because it offers a flat voltage plateau at 4.7 V and demonstrates a reversible capacity >135 mAh/g. Several strategies were developed to improve the rate capability of LiNi0.5Mn1.5O4. Arrebola et al. obtained excellent rate properties by homogeneously mixing nanoand micro-size LiNi0.5Mn1.5O4 powders, 8 or by adding polyethyleneglycol (PEG) to increase the crystallinity and decrease the strain. The rate capability was also affected by the atomistic structure of the spinel material, as proposed by Kunduraci et al. It is found that nonstoichiometric LiNi0.5Mn1.5O4 δ (Fd3 hm) exhibited better structural reversibility at high rate as compared to stoichiometric LiNi0.5Mn1.5O4 (P4332). The oxygen deficiency leads to the presence of a small amount of Mn3þ in the pristine materials, which leads to better electronic conductivities in the material. The Cu-doped spinel materials were investigated by a few research groups. 14 Although the Cu-rich spinel electrodes provide lower discharge capacity than Ni-rich spinel electrodes, it is more stable during electrochemical cycling. Moreover, LiCuxMn2 xO4 has a higher electronic conductivity than do other spinel LiMxMn2 xO4 materials (Cr, Fe, Co, Ni), because Cu can participate in the