Synthesis and Electrochemical Properties of Li[Ni1/3Co1/3Co1/3]O2-xFx via Co-precipitation

Abstract

During the past decade, the lithium transition-metal oxide Li[NixCo1-2xMnx]O2 has received a great deal of interest as rechargeable cathodes for Li secondary batteries[1]. Among them, Li[Ni1/3Co1/3Mn1/3]O2 provides the advantage over LiCoO2 system of being cost effective and thermal stability[2]. Also, Li[Ni1/3Co1/3Mn1/3]O2 comprises very promising positive electrode materials for high energy and high power lithium-ion batteries. It has been reported that Li[Ni1/3Co1/3Mn1/3]O2 can deliver a capacity of about 150 mAhg-1 in 3.5-4.2 V or 200 mAhg-1 in 3.5-5.0 V[2]. However, Li[Ni1/3Co1/3Mn1/3]O2 is known to be unstable cycling performance and their capacity fading especially when charged and discharge at high voltage (~4.5, 4.6V). Until now, the origin of this fading has been understood because neither structural nor textural modification. It appears that anion substitution appears to be a good method to modify the structural and electrochemical property[3]. In this paper we report the synthesis and electrochemical properties of Li[Ni1/3Co1/3Mn1/3]O2-xFx.

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[Ni1/3Co1/3Mn1/3]O2-xFx, Super S carbon black, and polyvinylidene fluoride (80:10:10) in N-methyl-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 4.6 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 Figure 1 shows X-ray diffraction (XRD) patterns of Li[Ni1/3Co1/3Mn1/3]O2-xFx (x=0,0.05,0.1,0.15,0.2,and 0.5) which were synthesized at 1000oC in air for 10h. All the peaks could be indexed based on a hexagonal a-NaFeO2 structure (space group: R-3m). The Li atoms are on 3a sites, the Ni, Co, and Mn atoms are randomly placed on 3b sites, and oxygen atoms are on 6c sites. For Li[Ni1/3Co1/3Mn1/3]O2-xFx (x=0.2, 0.5), the intensities for (003) was analogous to be the (104) peak. But the reflection of (018) and (110) for Li[Ni1/3Co1/3Mn1/3]O2-xFx are still distinguishable which means that the layered structure was formed. Figure 2 shows the voltage vs. capacity of Li/Li[Ni1/3Co1/3Mn1/3]O2-xFx cells with x=0, 0.05 between 2.8 and 4.6V. Initially, good initial capacities are obtained for small x=0 values. But during the cycling, specific capacity decrease from 184mAh/g to 166mAh/g. The initial discharge capacity of Li[Ni1/3Co1/3Mn1/3]O2-xFx decreased with increasing the substitution amount of fluorine. But, the lithium de-intercalation process is highly reversible. It is demonstrates that whatever the fluorine amount, the material exhibits good cycling behavior.