Researches On Sn-basic Alloy And Sn-basic Composite Materials For Lithium Ion Batteries Anode

Graphite materials are widely used as the anode materials for commercial lithium ion batteries due to its excellent cycling stability. However, their capacity properties are not as high as expected. Graphene, a novel carbonaceous material, has been widely investigated recently; it is considered as the basic unit of graphite. The special structure of graphene assembled by sp2 carbon atoms supports its unique physical and chemical properties. In the application field of lithium ion batteries, graphene-based materials also exhibited excellent performance. Sn could react with Li ion to form Li4.4Sn alloy that has high volume of storing lithium. The process of alloying and de-alloying is reversible. However, in long time cycling, Sn exhibited poor stability.In this work, different methods were used to synthesize Sn-based alloy and compound composite with carbonaceous materials. The structure, properties and the size of particles have been investigated to study the influence on the properties of charging-discharging, ultimately to discuss the effective routes to optimize the stability of cycling of Sn-based materials.(1) Sn-Co nanosized particles were prepared by the method of co-reductions in solution. Comparing conditions of the process of reduction, we found that the particle size of Sn-Co alloy obtained in aqueous solution with chelators was between 20～40 nm, meanwhile the alloy was pure and with good crystallization. After 25 charging-discharging cycles, the capacity of the Sn-Co alloy was 178.9 mAh/g and the rate of reserved capacity is 19.1%. Sn-Co-C composite was synthesized by ball milling method using prepared Sn-Co alloy and acetylene black. The composite that contained 16.7 wt% carbon represented the best performance, its capacity was 211.1 mAh/g after 25 charging-discharging cycles, and the rate of reserved capacity was 45.3%. The performance of the Sn-Co-C composite was much better than individual Sn-Co alloy.(2) The optimized co-reduction method was utilized to obtain the Sn-Co/Reduced Graphene Oxide (Sn-Co/RGO) composite in aqueous solution. Characterizations demonstrated the presentation of well crystalline phase of Sn and Sn-Co alloy; RGO appeared as the form of disordered carbonaceous materials which were observed in crumpled structure with low graphitization degree. The irreversible capacity loss in initial cycle is only 38.1% and the discharge capacity is still 547.6 mAh/g after 80 cycles.(3) Tin particles were loaded into carbon matrix to form Sn/C composite for improving the cycling stability meanwhile the capacity of the composites were poor due to the low content of tin. Considering to enhance the performance of the Sn/C composites that were prepared by hydrothermal treatment from the resorcinol-formaldehyde (RF) gel in which tin tetrachloride was coated to form the organic tin precursor, graphite oxide (GO) was introduced to modify the preparation. The structure of the Sn/C composite was changed from sphere to the wind-like curl. Due to the firmer binding to the carbonaceous matrix enhanced the performance of capacity and stability of cycling. The capacity decline in the modified Sn/C composite almost disappeared after 35 cycles and the reserved capacity after 90 cycles was remained above 220 mAh/g.