Nanosheets of iron-based compounds : synthesis and their lithium storage properties
Date of Issue2014
School of Materials Science and Engineering
Future applications of energy storage devices require electrode materials with higher energy and power densities, better cycling stability and better rate capability. Many bulk and micrometer-sized electrode materials have failed to satisfy the high requirements. Therefore, design and fabrication of various novel nanostructures with controlled morphology, size, composition and crystallinity have attracted increasing research attention. Two-dimensional (2D) nanostructures are suitable for energy storage because of their large surface area and shorter paths for fast charge transport. Nanosheets of some single-morphology, single-component, and well-crystalline electrode materials have been developed to possess improved lithium storage properties in capacity and cycling stability compared with their bulk counterparts. In this present work, the optimizations of electrode materials are extended to fabrication and electrochemical characterization of hierarchically-constructed nanosheets, composite hierarchical nanosheets and amorphous nanosheets. The focus is to develop facial and controllable approaches for fabrication of these novel electrode materials. The hierarchical nanosheets and hybrid nanosheets are expected to benefit from the multi-functionality and advantageous of individual nanostructures or constituent components, leading to higher capacity, improved cycling stability and rate capability. The amorphization of electrode materials tends to demonstrated distinct lithium storage mechanism and kinetics compared with their crystalline counterparts. At first, the morphology advantages of hierarchically-constructed FeS nanosheets are demonstrated. The carbon coated FeS nanosheet hierarchically constructed by small nanograins is prepared via a surfactant-directed solution-based synthesis. Results indicate that hierarchical FeS particles and hexagonal-shaped FeS nanoplates could also be obtained by adjusting the experimental conditions. The hierarchical FeS nanosheet delivers promising lithium storage property of a specific capacity of 233 mAh g-1 in 100th cycle at a 10C discharge rate. It is proposed that the constituent nanograins offer large electrode-electrolyte interaction area and shorter charge transport. At the same time, the flexible nanosheets can effectively accommodate large volume change induced during the charge/discharge process the nanosheets holding nanograins can maximize the electrical connectivity. In addition, in-situ formed amorphous carbon coating further improves the cycling stability by absorbing and trapping polysulfides generated during the conversion reaction of sulfides preventing the dissolution of polysulfide from deteriorating ionic conductivity of electrolyte. Next, hybrid hierarchical nanosheets composed of Troilite FeS and Wurzite ZnS have been synthesized via the same organic solution-based synthesis. The compositional ratio of FeS and ZnS can be varied by varying the compositional ratio in precursor. Among the synthesized samples with varying ratios, sample with Fe/Zn = 7.29:1 shows best lithium storage properties over other ratios and pure FeS nanosheets in terms of capacity and rate capability. The improved lithium storage performance of hybrid hierarchical nanosheet can be attributed to: (1) the advantageous hierarchical nanosheet structure provides better stress tolerance for both conversion reaction and alloying reaction upon cycling; (2) each nanosheet are composed of fine building blocks of FeS and ZnS nanocrystals, which offers shorter diffusion path for charge transport; (3) The co-presence of the heterogeneous sulfides could behave as dispersants to each other. During repeated reversible electrochemical reactions, the heterogeneous metals reduced from the composite sulfides during lithiation are not liable to aggregate, nor are the heterogeneous sulfides formed during delithiation. Finally, the combined effects of amorphization and nanostructuring on the lithium storage performance of FeOOH are investigated. In this work, amorphous FeOOH nanosheet is prepared by surfactant-assisted oxidation of self-synthesized FeS nanosheets. The resultant amorphous FeOOH nanosheets are porous and have a high BET surface area of 223 m2 g-1. The amorphous FeOOH nanosheet demonstrates good lithium storage capacity and superior rate capability (e.g. discharge capacity of 465 mAh g-1 at a current density of 2C), which can be attributed to its high surface area, porous nanostructure and loose amorphous nature. When characterized as lithium storage electrode, it has conversion reaction with Li+. A new equivalent circuit directly modeling conversion reaction between electrode and Li+ is proposed. The conversion reaction subcircuit reproduces the hysteresis in the discharge/charge voltage profile, indicating that the lithium storage via conversion reaction in amorphous FeOOH has a thermodynamic origin rather than being limited by Li transport.