Many materials which can reversibly alloy with lithium, especially Si, have been reported to be one of the most promising candidates which can be substituted for graphite, due to their much higher theoretical capacity than graphite. However, their commercial use has been hindered by significant volume variation during charge and discharge, which leads to a rapid decay of the mechanical stability and cycle life of the electrode. Therefore, this work was focused on the accommodation of the volume change of Si-based anode material. In order to counteract the mechanical degradation, ($NiSi_2 / Si$) dual phase alloy where submicron-sized Si particles were uniformly distributed within the $NiSi_2$ alloy matrix, was made by arc-melting followed by high energy ball-milling for 30 minutes. The composite of ($NiSi_2 /Si$) alloy and graphite was formed by high energy ball-milling for 30 minutes with various ratios of alloy and graphite.
The charge / discharge test of the composite anode material showed the drastically improved cycleability as well as high reversible capacity. In the case of the composite material composed of 60 wt.% of alloy and 40 wt.% of graphite, the discharge and charge capacity of the 1st cycle were 1274 mAh/g, and 962 mAh/g respectively, which represents the initial coulombic efficiency of 75.5 %. After 2nd cycle, the reversible capacity was 780 mAh/g with the coulombic efficiency of 98 % until 50 cycle. From the XRD analysis during charge and discharge, it could be found that during discharge which means the lithium insertion into the composite electrode, (002) peak of the graphite shifted to lower angle while the intensity of Si peak decreased, and during charge (002) peak of the graphite moved to higher angle of its original position while the intensity of Si peak didn`t recovered owing to the amorphization of Si. In both cases, the peak of $NiSi_2$ remained invariant which confirmed that $NiSi_2$ phase played its role of alloy matrix without reaction with lithium. However, the reason for the greatly improved cycleability might not be attributed only to the $NiSi_2$ alloy matrix. To investigate the role of graphite, the simply mixed powder of 30 min. ball-milled alloy and 30 min. ball-milled graphite, was tested as active material. It showed the gradual decrease of reversible capacity during cycling, which confirmed that the improved cycleability was not the results of the simple mixing of graphite and alloy particles. HR-TEM analysis showed the existence of disordered carbon layer between graphite and alloy particle. To ascertain whether the bonding between them, FT-IR analysis was carried out. From the FT-IR data, three peaks were observed after ball milling. One of the peaks was identified as the bonding of Si-C, but the identification of the others wasn`t clear. As a result, the bonding formed after ball-milling between alloy and graphite was confirmed.
In conclusion, $NiSi_2$ alloy matrix, alloy-C bonding, and porous structure enabled the composite to accommodate the volume change efficiently. In addition, The composite material showed high Si utilization up to 60 % of its theoretical capacity due to small size of alloy particle and $NiSi_2$ alloy matrix having fast Li ion transport property and high electronic conductivity.
