Design Of Porous Carbon-Modified Hybrid Anodes And Its Electrochemical Performance For Lithium Ion Batteries

To develop power lithium ion batteries (LIBs) for electric vehicles, it is essential to fulfill the requirements of electrode materials for long cycle stability, high reversible capacity, good safety and fast charge/discharge capability. However, how to controllably synthesize a suitable structure for improved performance according the electrochemical characteristics of active materials, especially their cycle stability, remains a big challenge. Considering the unique merits of porous carbons, herein, my research works have focused on designing and synthesizing a series of porous carbon-modified metal oxide/sulfide nanohybrids with greatly enhanced electrochemical cycle performance as anode materials for advanced LIBs. Furthermore, the correlation between the electrochemical properties, reaction mechanism and their structural features have been fundamentally investigated. Specifically, the works include the following parts:(1) Aiming to buffer large volume expansion (250%) and improve electrical conductivity of SnO2anodes, ultrafine SnO2nanoparticles with size between4-5nm were uniformly encapsulated into ordered tubular mesoporous carbon, which possesses thin carbon walls (-2nm) and high pore volume (2.16cm3g-1). The filling ratios of pore channels in carbon matrix were tailored in a large range of7-27%to efficiently utilize the pore space and adequately accommodate the volume change of SnO2. Even when the loading of SnO2was up to80wt%, the nanoparticles were still highly dispersed in the mesopore channels of carbon matrix, and no bulky aggregated were visible. The tubular hybrid exhibited an ultrahigh reversible capacity of1039mAh g-1after100cycles with a high capacity retention of106%, and the stable capacity value was much higher than the conventional theoretical capacity of SnO2(782mAh g-1). The extra reversible capacity came from partial reversiblity of conversion reaction (oxidation of Sn to SnO2).(2) To further reduce volume expansion of SnO2intrinsically, we tried to introduce ZnO having a relatively low volume change of103%into SnO2for the fabrication of ZnSnO3with decreased volume expansion of～191%. Thus, a core-shell carbon-coated ZnSnO3(ZnSnO3@C) anode material has been designed, in which amorphous ZnSnO3nanocubes (size:-37nm) with a well-developed mesoporous structure are coated with a continuous and thin carbon layer. The core-shell nanocubes cross-linked with each other to form a continuous conductive framework and interconnected porous channels with macropores of74nm width. The amorphous ZnSnO3@C exhibited greatly enhanced integration of alloying and conversion reaction processes (reversible transformation of Li4.4Sn and LiZn alloys to pristine), thus achieving a high reversible capacity. Electrochemical results showed that the reversible capacity of such amorphous ZnSnO3@C reached a high value of1060mA h g’1after100cycles with a capacity retention of93%.(3) Considering of the electrode/electrolyte interface stability issues, e.g., transition metal oxide Fe2O3, which suffers a severe capacity fading due to the instability of interfacial solid electrolyte interface (SEI) film. Herein, a nano-engineering strategy has been successfully developed for fabricating a novel multifunctional nanohybrid composed by highly dispersed nanosized Fe2O3particles, tubular mesoporous carbon host and conductive polypyrrole (PPy) sealing layer. Fe2O3particles were well-dispersed within the carbon matrix and PPy is spatially and selectively coated onto the external surface and the pore entrances of the Fe2O3@C, thereby bridging the hybrid particles together into a larger unit. As an anode material for Li-ion batteries, the PPy-coated Fe2O3@C hybrid exhibited stable cycle performance with97%capacity retention after100cycles. Additionally, the PPy-coated Fe2O3@C also possessed fast electrode reaction kinetics, high Fe2O3use efficiency, and large volumetric capacity.(4) An effective strategy has been developed for the in situ generation of a Cu2S/tubular mesoporous carbon hybrid by exploiting the charge-discharge processes of a S/C hybrid on a copper-foil current collector. By studying the reaction mechanism were explored. It is discovered that the dissolved polysulfide ions (from the reaction of pristine sulfur and inserted Li+) were firmly anchored by the copper ions released from the copper foil, which formed insoluble CuxS intermediates, and further converted into final Cu2S product in the mesopore channels of the carbon matrix. The highly dispersed Ou2S in tubular mesoporous carbon displayed excellent lithium storage properties with extremely stable cycle performance of270mAh g-1at0.2C for300cycles and excellent rate capability of225mAh g-1at10C.