Structural Engineering of Tin-Based Micro/Nano Composites as Potential Anode Materials for Lithium-Ion Batteries

Abstract

Energy storage with a high energy density and long cycling life has emerged as one of the most challenging issues scientists and engineers face. Although lithium-ion batteries have revolutionised the development of energy storage, they are still limited by their low charge storage capacity determined by their electrode materials. The current commercial anode material, graphite, with a capacity of only ~370 mAh·g-1, is far from meeting the emerging requirements. Therefore, it is highly desired to explore and develop new anode materials with increased capacity for the next-generation lithium-ion batteries. Tin-based materials have been considered as one of the most promising substitutes to graphite due to their high specific capacity (992 mAh g-1 for metallic tin and 783 mAh g-1for SnO2), abundant resources, low cost, and environmental friendliness. However, tin-based anodes suffer from an extreme volume expansion up to 300% upon lithiation process. The large volume change causes severe structural collapse, rebroken and regrowth of solid electrolyte interphase (SEI), resulting in substantial deterioration of cycling performance. In addition, the low conductivity of SnO2 is another issue that hinders the charge transfer and leads to poor rate performance. Rational structure engineering is an effective strategy to stabilise structures and enhance electrical conductivities. Although some unique structures, e.g., nanotubes, nanoporous, hollow spheres, and yolk-shell structures, have been tried for tin-based materials, the performance is still not satisfactory. Another disadvantage is that most of them are synthesised with complicated and time-consuming methods. In this thesis, several novel and facile structure design approaches have been conducted to modify tin-based anode materials, aiming to improve their performance and promote their practical applications in lithium-ion batteries. A facile and universal one-step route has been developed to synthesise metal oxides-based yolk-shelled structures. Based on this approach, we synthesised a yolk-shelled SnO2@N doped C (yolk-shelled SnO2@NxC) composite as lithium-ion anode materials. The unique yolk-shelled structure with sufficient void to accommodate volume expansion, inner shell and core as active material, and NxC as the conductive framework, make the composites possess good electrochemical performance. Some other yolk-shelled metal oxides, such as yolk-shelled TiO2 and yolk-shelled Fe2O3, have also been directly synthesised through the method in this thesis for their proper applications. Although the yolk-shelled SnO2@NxC composites have solved the volume expansion issue to some extent, the charge transfer in the yolk-shelled structure is restricted due to the little point contact between core and shell. To solve this problem, we developed a novel hollow multishelled SnO2@polypyrrole (SnO2@Ppy HoMS) composite through a sequential template approach followed by an in-situ pyrrole polymerisation process. Different from the yolk-shelled structure with a dense core, the SnO2@Ppy HoMS with multiple SnO2/Ppy shells has significantly enhanced conductivity, showing an improved electrochemical performance. The higher special theoretical capacity, lower electrical resistivity and less side reactions make metallic tin more attractive than SnO2 as lithium anode materials. However, metallic tin-based anodes also suffer from the common issue of large volume expansion. The yolk-shelled structure has been proven as an effective means to prolong the cycling life in the previous section. Herein, a novel strategy of in-situ shell-to-core evolution has been proposed to synthesize yolk-shelled Sn@void@N doped C (YS-Sn@void@NxC) composite. The void space and core size in yolk-shelled composite were accurately controlled. The relationship between void space and electrochemical performance has been explored. The optimised YS-Sn@void@NxC electrode exhibits superior cycling stability with a high volumetric energy density. In addition, the facile yolk-shelled structure design strategy in this work can be extended to preparing other low melting point materials (Bi, Pb, In, Ge, etc.) for their proper applications. To further exploring the potential of metallic tin-based materials, we finally invented an innovative structure with multiple tin nanoparticles encapsulated in multishelled NxC matrix for lithium-ion battery anode material. Surprised, each NxC shell in the structure contains duplicate layers (Sn NPs@NxC HoMS-DL). The advanced structure maximises the energy storage capability of metallic tin and demonstrates an unprecedentedly excellent electrochemical performance. We have detailly studied the formation mechanism of the structure and the relationship between structure and performance through a series of advanced characterisation techniques.