Synthesis and Electrochemical properties of LiNi0.5Mn1.5O4-xFx via Ultrasonic spray pyrolysis

Abstract

Introduction Presently commercialized lithium-ion batteries use layer structured LiCoO2 cathodes. Although LiCoO2 cathodes exhibit excellent performance characteristics, an intensive search for new cathode materials has been carried out for many years because of the high cost and toxicity of LiCoO2. One of the most attractive cathode materials is the spinel LiMn2O4.[1-4] LiMn2O4 with spinel structure has received much attention for their low cost, low toxicity, and relatively high energy density. Despite such advantages, the significant capacity fading during cycling, especially at high temperature, hindered the commercial use of the LiMn2O4 as positive materials for lithium secondary battery. Although the origin of the poor cycling performance has not been fully understood, several possible mechanisms were suggested [2,5], including Jahn-Teller distortion, Mn dissolution at high temperature originated from the Mn3+/4+ redox [6,7], change in crystal lattice arrangement with cycling[8], and so on. To improve the cycle performance in 4V range, several research groups have investigated the properties of manganese-substituted spinels LiMyMn2-y O4 (M=Al, Cr, Ga, Ti, Ge, Fe, Co, Zn,Ni, Mg) [3,9-14]. Anion doping also resulted in the good cycling performances. Sun et al. reported that S doping at the oxygen site of LiAl0.24Mn1.76O3.97S0.03 has a beneficial effect to maintain the high capacity in the 3 V region due probably to the formation of flexible structure by the S doping, compared to the conventional spinel LiMn2O4 [15-17]. Amatucci et al [18]. observed that the formation of oxyfluoride, LiAl0.2 Mn1.8O3.8F0.2, resulted in improvements in capacity and its retention during 300 cycles. They explained that the good characteristics may be rooted in the resistance of fluorides to attack by HF found in the electrolyte. In this work, we have attempted to synthesize F-1 ion doped Li[Ni0.5Mn1.5]O4-xFx (0≤x≤0.1) materials. The structural and electrochemical properties of the prepared materials were characterized. Here, we would like to first introduce new fluorine doped 5 V spinel Li[Ni0.5Mn1.5]O4-xFx cathode materials prepared by the ultrasonic spray pyrolysis method. Experimental Powder X-ray diffraction measurement using Cu Kα radiation was employed to identify the crystalline phase of the synthesized materials. The as-prepared powders were observed using a scanning electron microscope (SEM, JSM 6400, JEOL, Japan). The cathodes were prepared by blending Li[Ni0.5Mn1.5]O4-xFx, Super S carbon black, and polyvinylidene fluoride (80:10:10) in Nmethyl- 2-pyrrolidone. The cell was assembled in an argon-filled dry box and tested at a current density of 20 mA g-1 at 30 oC. For differential scanning calorimetry experiments the coin cells were charged to 5.0 V at 20 mA g-1. The samples were analyzed in the DSC using a temperature scan rate of 2 oC min-1. Results and discussion LiNi0.5Mn1.5O4-xFx powders were synthesized in the range of x=0-1.0 by the ultrasonic spray pyrolysis method. Figure 1 shows X-ray diffraction patterns of various fluorine doping contents for Li[Ni0.5Mn1.5]O4-x Fx cathode materials. All samples can be indexed based on a cubic spinel structure with a space group of Fd 3 _ m. Therefore, we assumed that the lithium ions occupied the tetrahedral (8a) sites ; Ni, Mn are located at the octahedral (16d) sites; and O2- and F-1 ions are located (32e) sites.