Synthesis And Modification Of SnO₂Anode Material For Lithium Ion Batteries

Lithium-ion battery was regarded as one of promising energy storage devicesrencently. The performance of lithium-ion batteries largely depends on Li+storagecapacities of active material electrode. Improvements in electrode materials couldmotivate efforts to making the structure more flexibility and stability, improving therevesible capacity, making much safer and Li+insertion-removal stagingelectrochemical process facile. To date, graphite has been widely used as thecommercially dominant anode material in lithium-ion batteries due to its low cost,high yield, negligible volume expansion (9%) on cycling and excellent capacityretention. But the theoretical specific capacity of graphite was limited to372mAh g-1and graphite suffered safety issues. Tin oxide (SnO2) could be proposed as one ofpromising substitutes for the graphite anode owing to its high theoretical specificlithium storage capacity of781mAh g-1and low potential of lithium ion intercalation.However, the practical application of SnO2has been restrained by its cyclingperformance. The large volume change (about300%) and severe agglomerationduring the charge and discharge processes lead to electrode pulverization andelectrical disconnection, resulting in limitation of cycle life. Many attempts havemitigated these limitations. In this paper, two kinds of effective methods, includingoptimizing the morphology of SnO2and making SnO2-based composites, were usedto modify SnO2. The electrochemical performances of two materials as anodeelectrodes were also studied. The conclusion can be summarized as following:1. We report an effective templating methodology to prepare corrugate-likeSnO2by using cellulosic substances (cotton fibers) as sacrificing template. SnCl2wasused as precursor. Cotton fibers naturally have many functionalized groups likehydroxyl, which can attach to stannous ions by electrostatic attraction. SnO2nanoparticles can be grown by spontaneous oxidation during hydrolysis processwithout reductant and surfactant. After5h at550℃, the template can be removedand the corrugated SnO2can be obtained. The structure and morphology of sampleuniformly were confirmed by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). This corrugate-likeSnO2was assembled by SnO2nanoparticles with the diameter about16-21nm. Suchan assembly mode of SnO2is highly beneficial to the electrochemical performanceimprovement. The novel anode material possesses good capacity retention of614mAh g-1up to the50th cycle at a current density of70mA g-1, showing a stablecyclability. This electrode also exhibits excellent rate capability, with a reversiblecapacity of405mAh g-1at a current density of3A g-1. For comparision, thecorrugated SnO2significantly improve the electrochemical performance. On the onehand, corrugated structure can store much liquid electrolyte, which is benefit toclosely contact with active materials, offer fast diffusion channel for Li+andeffectively alleviate the volume expansion. On the other hand, SnO2nanoparticles canshorten the diffusion path for Li+and enhance structural stablility. Electrochemicalmeasurements also prove that the corrugated SnO2anode electrochemicalperformance in terms of improved cyclability, better reversibility and high elelctricconductivity.2. SnO2-filled carbon nanotubes (SnO2@OCNTs) composites anode materialswere prepared by a facile one-step wet chemical method. Potassium hydroxide wasused to chemically active CNTs, which aimed to open the tips and introduce manyfunctionalized groups with negative charges on the exterior walls. The solutioncontaining Sn2+was filled into the empty channels of CNTs due to the capillaryforces and surface tension. The ultrafine SnO2nanoparticles (4-6nm) were grew byspontaneous oxidation without any additional chemical agents so that no impurity canbe introduced. SnO2@OCNTs composites were characterized by XRD, SEM andTEM. The results indicate that most interconnected SnO2nanoparticles are filled intoOCNTs successfully. Electrochemical measurements indicate that the SnO2@OCNTsnovel engineered anode significantly enhance electrochemical performance in termsof an high initial capacity of848mAh g-1at the current density of30mA g-1, goodcapacity retention (83.5%of original capacity after300cycles at the current densityof70mA g-1) and excellent rate performance (398mAh g-1at3000mA g-1).Undoubtedly, the outstanding electrochemical performance is rooted in its uniquemorphology. Firstly, SnO2nanoparticles can short the diffusion path of Li+. Secondly, OCNTs provide an excellent electronic conductivity to facilitate the electron transportfor the composites. Thirdly, SnO2nanoparticles uniformly are located on the externalwalls of the OCNTs and introduced into the channels of the OCNTs. So this uniquearchitecture effectively alleviates aggregation of in-situ formed Sn nanoparticlesduring cycles. Finally, OCNTs acts as a buffer layer to contain stresses induced byvolume change during charging/discharging.