Improvement of high voltage cycling behavior of surface-modified Li[Ni1/3Co1/3Mn1/3]O2 cathode by fluorine substitution for Li-ion battery

Abstract

Introduction During the past decade, lithium transition-metal oxides, Li[NixCo1-2xMnx]O2, have received a great deal of interest as a cathode material for Li-ion secondary batteries[1,2]. Among the lithium transition-metal oxides, Li[Ni1/3Co1/3Mn1/3]O2 shows the most promise as cathode material for lithium secondary batteries. In attempts to increase the reversible capacity of the cathode material, the upper cut-off voltage limit has been gradually increased. The increased upper voltage limit resulted in a moderate increase in the specific discharge capacity as expected; however, the improved discharge capacity was accompanied by unstable cycling performance when cycled up to 4.6 V [3]. Moreover, even at upper voltage limits of 4.4 - 4.5 V, capacity fading was still observed upon cycling. The origin of this capacity fading was mainly attributed to gradual decaying of electro-active Co as reported by Shaju et al. and Yoon et al. LiCoO2, a commercialized cathode material, also has shown poor electrochemical properties when the upper voltage limits was higher than 4.2 V because of its phase transition from hexagonal to monoclinic. Moreover, Co dissolution revealed that the poor electrochemical properties of LiCoO2 at high voltage are due to the dissolution of Co ions into electrolyte. To solve this problem, various metal oxides were coated on or substituted for LiCoO2. In this paper, we report the effects of partial substitution of oxygen with fluorine on the structure, electrochemical behavior, and thermal stability of Li[Ni1/3Co1/3Mn1/3]O2-zFz (where 0<z<0.5) 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. For x-ray photoelectron spectroscopy (XPS, PH 5400), monochromatic x-ray generated from Al Kα (15 kV) was used. The sputter-etching was done with Ar ion beam (1 kV, 45°) and raster area of 3 mm x 3 mm. The sputtering rate was ~1 A /min. Results and discussion Figure 1 shows X-ray diffraction (XRD) patterns of Li[Ni1/3Co1/3Mn1/3]O2-zFz with z = 0, 0.05, 0.1, 0.15, 0.2, and 0.5. The XRD patterns could be indexed by a hexagonal α-NaFeO2 structure (space group: R-3m). The Li[Ni1/3Co1/3Mn1/3]O2-zFz shows clear splits in the (006)/(102) and (018)/(110) peaks until z = 0.2, which indicates the formation of a well-developed layered structure. Meanwhile, those peaks were merged into one and hard to distinguish for z = 0.5 in Fig. 1 (f). Also, with increasing fluorine content, z, a small impurity peak was observed at around 31o in 2θ in Fig. 1 (e) and (f). The XRD patterns clearly indicate that incorporation of F did not alter the crystal structure of Li[Ni1/3Co1/3Mn1/3]O2 nor produce any secondary phases in the range of z = 0 . 0.15. By the combination of atomic absorption and IC analyses for fluorine, the detected F amounts were 0.05, 0.09, 0.14 for z = 0.05, 0.1, and 0.15, respectively.