Characterization of the core-shell structured Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2 as positive material for lithium batteries

Abstract

Introduction Due to high cost of LiCoO2, a commonly used cathode material in commercial lithium battery system, much effort has been made to develop cheaper cathode materials than LiCoO2. The layered materials LiNiO2 and LiMnO2, LiNi0.5Mn0.5O2, LiNi1/3Co1/3Mn1/3O2 are candidates for alternative to LiCoO2. But they still need a breakthrough to get a goal. Recently, the microscale core-shell structured Li[(Ni0.8Co0.1Mn0.1]0.8(Ni0.5Mn0.5)0.2]O2 has been introduced in our previous paper. [1,2] This material shows a synergetic effect with good points of two materials. The one is a higher capacity for LiNi0.8Co0.1Mn0.1O2 as the core, the other is a good thermal stability for LiNi0.5Mn0.5O2 as the shell. In this study, we report a characterization of the core-shell structured Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2 and compare with chemical composition cathode (LiNi0.74Co0.08Mn0.18O2) and mixture of cathode (LiNi0.8Co0.1Mn0.1O2:LiNi0.5Mn0.5O2=8:2).

Experimental The [(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2](OH)2 and [Ni0.74Co0.08Mn0.18](OH)2 compounds were synthesized by a co-precipitation method. [1] The appropriate amounts of NiSO4·6H2O, CoSO4·7H2O and MnSO4·H2O were used as the starting materials. The concentration of the solution, pH, temperature, and stirring speed of the mixture in the reactor were controlled with care. The as-prepared powders were simply mixed with lithium sources, and then the prepared powders were calcined at 770oC 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 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 SEM images of Li[Ni0.74Co0.08Mn0.16]O2 and Li[(Ni0.8Co0.1Mn0.1]0.8(Ni0.5Mn0.5)0.2]O2 powder shown in Figure 1. which was embedded in an epoxy and ground flat to observe the core-shell structure of the powder, showed the thick shell layer approximately 1μm and the shell completely capsulated the core in Figure 1b. The initial charge-discharge curves are in Figure 2. The Li/Li[(Ni0.8Co0.1Mn0.1]0.8(Ni0.5Mn0.5)0.2]O2 cell exhibited a discharge capacity of 188mAh/g in the voltage range of 3.4-4.3V. But, the mixture of cathode Li/(LiNi0.8Co0.1Mn0.1O2:LiNi0.5Mn0.5O2=8:2) cells exhibits a lower discharge capacity.