Synthesis and electrochemical properties of Li(Ni1/3Co1/3Mn1/3)O2 cathode materials prepared by ultrasonic spray pyrolysis method

Abstract

The layered lithium metal oxides, LiMO2 (M=Co, Ni, Mn) and the LiMn2O4 are most widely studied 4 V class cathode materials for lithium secondary batteries with high energy density. The LiCoO2 has been commercialized but has still some problems, such as its high cost, moderate, and toxicity. The LiNiO2 and LiMnO2 have been extensively studied as possible alternatives to LiCoO2. However, LiNiO2 still has some severe problems, such as difficulty in the synthesis of stoichiometric LiNiO2, capacity decay due to the formation of NiO2 phases and thermal instability1. LiMnO2 is not thermodynamically stable as the layered structure. Manganese layered oxide are transformed to spinel phase during cycling2. Accordingly, these materials have some problems for practical applications. Recently, Ohzuku et al. reported that the concept of Li[Ni1/2Mn1/2]O2 and Li[Ni1/3Co1/3Mn1/3]O2 were prepared by co-precipitation of nickel, manganese double hydroxide and nickel, cobalt, manganese triple hydroxide3,4. In this work, the layered Li[Ni1/3Co1/3Mn1/3]O2 powders have been synthesized by ultrasonic spray pyrolysis method. We investigate the effects of synthesis condition, such as polymeric agents, calcination temperature and metal doping. Figure 1 show X-ray diffraction (XRD) pattern of the Li[Ni1/3Co1/3Mn1/3]O2 powders with miller indices indicated. All of the peaks can be indexed based on a hexagonal α-NaFeO2 structure with space group R_3m, which is characteristic of the layered LiCoO2 and LiNiO2 structure. The lattice parameters of are determined to be a = 2.86 A and c = 14.25 A calculated by Rietveld refinement from the X-ray diffraction data. Furthermore, we found that the prepared samples have a spherical morphology with an average particle size is approximately 2-3 ㎛. The discharge capacity vs. number of cycle of Li/Li[Ni1/3Co1/3Mn1/3]O2 cell cycled at various voltage ranges are shown in Figure 2. The cycling was carried out at a constant current density of 0.2 mA/cm2 (20 mA/g). The discharge capacity increases linearly the increase of the upper cut-off voltage range. The Li[Ni1/3Co1/3Mn1/3]O2 electrode delivers a discharge capacity of 180 mAh/g in the voltage range of 2.8 - 4.5 V.