Synthesis and Electrochemical properties of Li[Ni0.8Co0.1Mn0.1]O2-XFX via co-precipitation

초록

Introduction LiNiO2 is a promising positive electrode due to its advantages over LiCoO2. LiNiO2 is less expensive, less toxic, and has a higher capacity than LiCoO2. However, it has several problems, such as a difficult synthesis, low thermal stability and poor cycle life. To overcome these problems, several cations have been substituted for Ni. Recently, Li[Ni0.8Co0.1Mn0.1]O2 has been introduced in our previous paper. Compared to LiNiO2, this material showed excellent cycling performance and thermal stability resulting from improved structural stability.[1] Anion doping also resulted in the good cycling performances.[2] Sun et al.[3] reported that fluorine doping at the oxygen site of Li[Ni1/3Co1/3Mn1/3]O2 has beneficial effects of improving the capacity retention and thermal stability at highly oxidized state, probably due to the formation of improved structural stability by the F doping, In this study, we report the synthesis of Li[Ni0.8Co0.1Mn0.1]O2-XFX via co-precipitation and electrochemical properties. Experimental The [Ni0.8Co0.1Mn0.1](OH)2 compounds were synthesized by co-precipitation. An aqueous solution of NiSO4·6H2O, CoSO4·7H2O and MnSO4·H2O were used as the starting materials and NaOH solution and desired amount of NH4OH solution (aq) as a chelating agent were also used. The obtained [Ni0.8Co0.1Mn0.1](OH)2 powders were dried in 110oC for 12hr to remove adsorbed water. Finally, powders were simply mixed with lithium sources, and then the mixed powders were calcined at 750oC in a box furnace Li[Ni0.8Co0.1Mn0.1]O2-xFx was prepared by heating a reaction mixture of the dehydrated [Ni0.8Co0.1Mn0.1](OH)2 and LiOH·H2O and NH4F at 770°C for 15 h in a box furnace. The morphology of the powders was observed using a scanning microscope (SEM, JSM 6400, JEOL, Japan). Powder X-ray diffraction (Rigaku, Rint-2000) using Cu Kα radiation was used to identify crystalline phase of the prepared powders. Cell tests were done using the 2032 coin-type cell with Li-metal as the negative electrode. The cells were charged and discharged between 3.0 and 4.3 V by applying a constant current of 40mA g-1 at 25oC Results and discussion Figure 1a shows XRD pattern of Li[Ni0.8Co0.1Mn0.1]O2 and Li[Ni0.8Co0.1Mn0.1]O1.96F0.04. The diffraction pattern of the two materials show a well-ordered layer structure having a R3m space group. Figure 1b shows discharge capacities as a function of cycle number for the two electrodes. Although the Li[Ni0.8Co0.1Mn0.1]O1.96F0.04 material delivered a somewhat lower capacity, the cycling characteristics was much improved compared to Li[Ni0.8Co0.1Mn0.1]O2.