The electrochemical properties of Sn-Co, $Sn(Ge_2S_7)_{0.5}(MgO)_{0.5}$, and Si-C were investigated not only structural analysis but also electrochemical analysis. As usually observed in alloy materials, Sn-Co alloy was synthesized by high energy ball milling method. The starting stoichiometry was $Sn_2Co$ because $Sn_2Co$ has the highest theoretical Li storage capacity among stable Sn-Co compounds in room temperature. As the ball milling time increased, the crystallinity of $Sn_2Co$ decreased. After 16h ball milling, the structural change was not observed in XRD pattern. It was observed that the structure of $Sn_2Co$ after 16 h ball-milling was composed of $Sn_2Co$ nano crystalline surrounded by amorphous phase. As the ball milling time increased, the cycleability was improved, and came to a steady state after 16 h ball-milling. At the first cycle, the discharge capacity was about 596 mAh/g, but after 60th cycle the discharge capacity was decreased to 30 % of that of the first cycle. This means that Co worked to accommodate the Sn volume expansion during Sn reacted with Li. However, the role of Co to accommodate the volume expansion of Sn wasn`t sufficient in case of $Sn_2Co$. To make more efficient matrix structure than $Sn_2Co$, Co content was increased. SnCo was also synthesized by high energy ball milling. The phase change according to the ball milling time was similar to the $Sn_2Co$. SnCo nano crystalline was surrounded by amorphous phase, and the amount of amorphous phase was much more than that of $Sn_2Co$. The 1st cycle discharge capacity of SnCo was 485 mAh/g. This value is lower than that of $Sn_2Co$. But the cycleability was dramatically improved from 30 % to 60 %. This is because of the increase of Co amount which worked as a buffer phase. To make more efficient matrix structure, chalcogenide amorphous phase was conducted. Chalcogenide compounds make very stable phase in ambient temperature, and don`t have oxygen which makes irreversible product during Li insertion. The $Sn(Ge_2S_7)$ showed good cycleability to 17th cycle. b ＼But after 18th cycle, the capacity of $Sn(Ge_2S_7)$ was abruptly decreased. This phenomenon was also observed at the many experimental results. As Li was inserted to the active materials, Sn was electrochemically activated and small Sn particles or grains were electrochemically sintered than Sn nano particles were agglomerated. To prevent the Sn agglomeration MgO was added as an adsorbing material. So the final composition was $Sn(Ge_2S_7)_{0.5}(MgO)_{0.5}$. $Sn(Ge_2S_7)_{0.5}(MgO)_{0.5}$. showed the 1st discharge capacity as 344 mAh/g that means the volumetric capacity was 1548 mAh/l. Which is 40 % higher than that of carbon. The cycleability was very good. The capacity of 40th cycle was maintained about 93 % of initical capcity.
To make another new anode material for Li ion battery, Si-C composite was prepared by high energy ball milling method. Si-C composite showed 3 peaks at the first cycle in CV measurement. 1.7 V and 0.7 V reaction were electrolyte decomposition and SEI formation respectively. After 2nd cycle, 1.7 V peak and 0.7 V peak were disappeared. Only 0.2 V peak was observed. The discharge capacity of 1st cycle was 1020 mAh/g, and afiter 2nd cycle that was about 680 mAh/g. Coulombic efficiency was higher than 98.7 %. During Li insertion at 1st cycle, Li was intercalated to the C at first, and then reacted with Si. During Li extraction, Li was extracted from C at first, then form Si. Between Si and C, there was observes strong bonding. It suggested that during Si expansion by the reaction with Li, Si-C will contact, and electrical connection will be maintained. It was observed that Si and C were partially participated in the reaction with Li. So the volume expansion was small compared to the pure Si. From the experimental results, we can summarized the reason of good cycleability of Si-C composite. First reason is partial participation of Si and C in the reaction. Second reason is strong bonding between Si and C. And the final reason is stored stress in carbon layer due to the earlier reaction with Li than Si. The Si-C composite prepared by high energy ball milling could be suggested as a new anode active material for Li ion batteries.
