Core-Shell Sn@Sioc Nanocomposite Synthesized Via Spray Pyrolysis for LIBs

Abstract

Lithium-ion batteries (LIBs) have become promising energy storage devices for the consumer electronics and electric vehicles due to their high energy and power density. Anode materials such as Si, Sn and Ge which are alloying with Li+ ions have been attracted as alternatives to conventional graphite due to their very high theoretical capacities. Among the possible candidates, Sn has been thought to be a great anode material that shows high theoretical Li ion storage capacity of 994mAh g-1 and high electrical conductivity. However, the major drawback of tin based anode is their poor cycling stability because of large volme changes (260% for Li22Sn5) during lithiation resulting in pulverization and loss of electrical contact within the electrode. Especially, the repetitive formation and rupture of solid electrolyte interface (SEI) layer continuously deplete the electrolyte. Nanocrystallization can effectively decrease the absolute volume change of every single particle and mitigated the strain improving the cycling stability of Sn anodes. However, preparation methods of nano-Sn have some negative factors, such as the high cost, complex process and reaggregation after cycling. In order to take full advantage of nano-Sn, many studies introduce suitable matrix. These matrix acts as a buffer agents and avoids side reactions by preventing the direct contact of Sn and electrolyte. The common one is carbon matrix works as a conducting medium and enhances the rate performance of electrode. Another one is active metals including Ge-Sn, Sb-Sn and Ag-Sn that can contribute to the overall capacity of the electrodes and long cycling life. Yet these comes with loss of charge storage capacity or large volume expansion. The quest for suitable matrix with high Li ion storage, low volume expansion and sufficient electronic conductivity is important. In this study, silicon oxycarbide (SiOC) was adapted for Sn based anodes. The SiOC consists of silica tetrahedral SiO2, SiOC glass phase and free carbon. This matrix features a high charge storage capacity (~800mAh g-1), low voulme expansion (~7%) and sufficient mechanical strength contributed by SiO2 domains to accommodate the high volume expansion without capacity fading of Sn anode. In particular, the SiOC can suppress the aggreation of metallic Sn at high temperature remaining nano size. First, we obtained the Sn@SiOC nanocomposite with uniformly coated SiOC layer containing metallic Sn nanoparticle by spray pyrolysis and a subsequent carbonization process. The spray pyrolysis is scalable and facile method to synthesize various nanostructure functional materials via one-pot process. Also it can be easily scaled up for mass production. During the spray pyrolysis process, the tin(II) acetate and diphenylsilanediol (DPSD), the starting materials of Sn and SiOC, can be easily vaporized due to its low boiling point. The tin acetate nucleates first due to lower boiling point than DPSD and formates many clusters. Subsequently, DPSD vapors can be deposited onto the Sn clusters via aerosol assisted chemical vapor depsition (AACVD) mechanism. The deposited DPSD vapors lead to formation of coating layer by nucleation, growth and coagulation. Then, the spray pyrolyzed Sn@SiOC nanocomposite thermally treated at the inert atmosphere for growth of carbon network. This approach yields unifomly coated SiOC matrix with Sn nanoparticles of sizes on the order of 40-50nm. Anodes of the Sn@SiOC nanocomposite demonstrate high capacities and better stability against volume change during lithiation and delithiation cycling.