Synthesis and Characterization of Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2 with the Microscale Core-Shell Structure as the Positive Electrode Material for Lithium Batteries

Abstract

Introduction Ni-rich Li[Ni1-xMx]O2 (M = metals) have attracted much interest as lithium storage materials for rechargeable lithium batteries because of their low cost and high capacity. However, it has several problems, such as the structural instability and low thermal stability. These problems have hindered their commercial use in a lithium battery system, though they have much higher specific capacity than LiCoO2 in the same voltage range (3.0-4.3 V). Recently, a lot of attention has been paid to layered Li[Ni0.5Mn0.5]O2 as an alternative to LiCoO2 and LiNiO2- based cathode materials for Lithium battery system. This material has good thermal and structural stabilities, Due to its small amount of active nickel Ni2+ and a unique role of Mn4+ in stabilizing the structure during cycling. But it has a low capacity and rate capability relative to the Ni-rich Li[Ni1- xMx]O2. We expected that the two materials described above would have synergetic effects, that is, a high capacity for Li[Ni0.8Co0.1Mn0.1]O2 and a good thermal stability for Li[Ni0.5Mn0.5]O2. A high capacity was delivered from the Li[Ni0.8Co0.1Mn0.1]O2 core, and a high thermal stability was achieved by the Li[Ni0.5Mn0.5] O2 shell. In this study, we report the synthesis of Core-Shell Structured Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2, that is, Li[Ni0.8Co0.1Mn0.1]O2 as the core and a Li[Ni0.5Mn0.5]O2 as the shell and electrochemical properties.

Experimental [Ni0.8Co0.1Mn0.1](OH)2 and [(Ni0.8Co0.1Mn0.1)1- x(Ni0.5Mn0.5)x](OH)2 compounds were synthesized by the coprecipitation method. The appropriate amounts of NiSO4·6H2O, CoSO4·7H2O and MnSO4·H2O (cationic ratio of Ni : Co : Mn = 8: 1: 1) were used as the starting materials for [Ni0.8Co0.1Mn0.1](OH) 2 and NaOH solution and desired amount of NH4OH solution (aq) as a chelating agent were also used. To synthesize the core-shell structured [(Ni0.8Co0.1Mn0.1)1-x(Ni0.5Mn0.5)x](OH)2, the resulting spherical [Ni0.8Co0.1Mn0.1](OH)2 was continuously reacted with NiSO4·6H2O and MnSO4·H2O (cationic ratio of Ni : Mn = 1 : 1). These [Ni0.8Co0.1Mn0.1](OH)2 and [(Ni0.8Co0.1Mn0.1)1- x(Ni0.5Mn0.5)x](OH)2 compounds were thoroughly mixed with LiOH·H2O and heated at 750 °C for 12 h and 770 °C for 20 h in a box furnace. The chemical compositions of the resulting powders were analyzed by atomic absorption spectroscopy (AAS, Vario 6, Analyticjena). The morphology of the powders was observed using a scanning microscope (SEM, JSM 6400, JEOL, Japan). Cell tests were done using the 2032 coin-type cell with Limetal 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 show that the synthesized Li[(Ni0.8Co0.1Mn0.1]0.8(Ni0.5Mn0.5)0.2]O2 material had a spherical morphology and the average particle size was estimated to about 15 μm in diameter. SEM image of Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2 shown in Figure 1b which was partially crushed in an agate mortar to observe the core-shell structure of the powder, show the thick shell layer approximately 1-1.5μm and the shell completely capsulated the core.