UCLA

The massive combustion of fossil fuels and associated environmental problems have placed the significance of the utilization and development of renewable energy. Although renewable energy sources, such as wind, marine, solar, hydro, geothermal and biomass, can be continually replenished by nature, many of them are intermittent in nature, which request efficient energy storage systems for effective utilization. Among the various types of energy storage systems, electrochemical-energy-storage systems stands out due to their high efficiency, excellent adaptability in miscellaneous fields, low cost, and environmental benignity.As the most extensively investigated energy-storage system, lithium-ion batteries (LIBs) have been commercialized for portable electronics and electrical vehicles, because of the high energy density, long lifespan, and low maintenance cost. The capacity of currently used anode material (graphite), however, has almost achieved its theoretical capacity; developing novel anode materials with higher capacity and a sufficiently low working potential has been emerging as essential and challenging topic. Metal alloys with high gravimetric capacities and volumetric capacities are regarded as promising anode candidates in lithium-ion batteries. Unfortunately, alloyed materials usually suffer from severe volume expansion (up to 500%) and huge mechanical strain, which may lead to pulverization and drastic capacity decay. To address these issues, graphene has been used to form composites with the alloyed materials. Graphene is an allotrope of graphite with several intriguing properties, such as excellent electrical conductivity, remarkable thermal conductivity, large surface area, and robust mechanical strength. Since graphene can accommodate and buffer the volume change of alloyed materials during the cycling, and to improve the electrical conductivity and rate capability of electrodes, graphene-alloy composites have attracted much attention in recent years. In this dissertation, we have developed three types of graphene-tin (Sn) composites with designed nanostructures. For the first one, we synthesized the composites of Sn and hierarchical flower-like graphene tubes (denoted as Sn/FGT), which afforded anodes with fast-charging capability. The Sn/DGT exhibits a high reversible capacity of 742 mA h g-1, excellent rate capability (211 mA h g-1 at 8 A g-1 with 99% capacity retention when the applied current density was switched back from 8 A g-1 to 0.2 A g-1) and a long cycle life. The nano-size Sn particles, were uniformly anchored on hierarchical graphene tubes, which effectively prevented their aggregation. Such flower-like graphene tubes can serve as a highly conductive matrix, enabling efficient transfer of ions and electrons, and improving the rate performance. Second, we have designed novel composites of Sn nanoparticles confined within graphene tubes that contain a nitrogen-doped graphene inner tube and a hydrophobic graphene outer tube (denoted as Sn/DGT). The nanosized Sn particles effectively alleviated the mechanical stress during the alloying/dealloying process, leading to improved electrical conductivity. The flexible inner void space of the graphene tubes buffered the volume expansion from the Sn nanoparticles, and provided high kinetics for the diffusion of electrons and ions. The composites delivered a high reversible capacity of 918 mA h g−1 for 500 cycles, and an extraordinary rate capability with a capacity of 916, 831, 761, 642, 548, and 481 mA h g−1 at the current densities of 0.2, 0.5, 1, 2, 5, and 10 A g−1, respectively. Remarkably, Sn/DGT with a tap density around 2.76 g cm−3 showed a high volumetric capacity of 2532 mA h cm−3 and 1106 mA h cm−3 at a current density of 0.2 A g−1 and 20 A g−1, respectively. Second, we have designed novel composites of Sn nanoparticles confined within graphene tubes that contain a nitrogen-doped graphene inner tube and a hydrophobic graphene outer tube (denoted as Sn/DGT). The nanosized Sn particles effectively alleviated the mechanical stress during the alloying/dealloying process, leading to improved electrical conductivity. The flexible inner void space of the graphene tubes buffered the volume expansion from the Sn nanoparticles, and provided high kinetics for the diffusion of electrons and ions. The composites delivered a high reversible capacity of 918 mA h g−1 for 500 cycles, and an extraordinary rate capability with a capacity of 916, 831, 761, 642, 548, and 481 mA h g−1 at the current densities of 0.2, 0.5, 1, 2, 5, and 10 A g−1, respectively. Remarkably, Sn/DGT with a tap density around 2.76 g cm−3 showed a high volumetric capacity of 2532 mA h cm−3 and 1106 mA h cm−3 at a current density of 0.2 A g−1 and 20 A g−1, respectively. The work of this dissertation aims at providing possible solutions to tackle with current issues from alloy-based anodes in lithium and sodium storage, and broaden the nanostructure design of composite materials in energy storage. The high-performance anode materials are successfully developed through structural engineering of tin and tin alloy particles with graphene. The confined growth of tin or tin alloy particles within graphene scaffolds can fabricate highly conductive networks to retain the electrical contacts with active materials to enable prolonged cycling life, and facilitate the charge transport to improve the rate performance of the anodes. In addition, tin and tin alloy particles with high volumetric capacities can afford the anodes with high volumetric energy densities for lithium and sodium storage.