Complex Hollow Structured Anodes for Sodium and Potassium Ion Batteries

Abstract

The development of the portable electronic devices, electrical vehicles, and smart grids boosts the development of electrical energy storage devices. Among them, lithium-ion batteries, a typical kind of rocking-chair batteries, have been considered as one of the most competitive choices. However, the limited lithium content in the Earth’s crust raises a concern that its cost might increase with the growing demand for electrical vehicles. Therefore, due to the relatively abundant content of sodium and potassium, sodium and potassium ion batteries are considered as alternatives with reduced cost to lithium-ion batteries. Nevertheless, the electrode materials for these two devices suffers the sluggish ion reaction kinetics and the large volume expansion due to the larger ion radiuses of sodium-ion and potassium-ion than one of lithium-ion. Constructing hollow structured materials with shorten ion diffusion length and large voids to alleviate volume expansion is considered as one of the best approaches to solve those issues of sodium and potassium ion batteries. However, the rational design and engineering to hollow structure according to the features of these two batteries remain rarely reported. Additionally, more insightful understandings of the superior electrochemical performance of hollow structured electrodes are also needed. Therefore, this thesis aims to offer some hollow structured electrode materials with rational design and engineering for sodium and potassium ion batteries with insightful understandings. Firstly, Chapter 2 summarizes the application and development trends of hollow structured electrode materials as anodes for sodium ion batteries. In this chapter, it points out that the future development of hollow structured electrode materials lays on the optimization of the confinement, the building units and the utilization of the inner voids. Therefore, the research efforts were mainly devoted in the rational design and synthesis of complex hollow structured anodes for sodium and potassium ion batteries in this thesis. The first aspect is about sodium and potassium titanates, a kind of conventional intercalation anodes for sodium and potassium ion batteries. In Chapter 3, the building units of hollow structured Na₂Ti₃O₇ were tuning by changing the solvothermal reaction solvents. It has been demonstrated that Na₂Ti₃O₇ hollow spheres assembled from nanosheets was with enhanced ion reaction kinetics by exhibiting a 33% higher charge capacity at the current density of 10 C than that of the ones assembled from nanoparticles. Furthermore, the as-prepared sample delivered a reversible capacity of over 60 mAh g⁻¹ after 1000 continuous cycles at the high rate of 50 C. In Chapter 4, dual-shell structured sodium and potassium titanate cubes with oxygen vacancies were achieved. Various spectroscopy approaches were employed to offer an atomic understanding of the oxygen vacancies. Additionally, it was revealed by density functional theory calculation that the superior electrochemical performance originates from the enhanced conductivity which is induced by oxygen vacancies. The second aspect of this thesis focuses on the synthesis of multi-shell structured anode materials for sodium and potassium ion batteries. Due to the large number of inward voids in hollow structured materials, hollow structured electrodes have been considered as with low volumetric energy density even though their high gravimetric energy density derived from their high reversible capacity. In Chapter 5, multi-shell structured Sb₂S₃ with high volumetric energy density and gravimetric energy density was synthesized. In the comparison of electrochemical performance, the multi-shell sample exhibited a higher reversible capacity than the one of pristine Sb₂S₃. Additionally, it also showed enhanced durability compared to its single-shell counterparts. These two points demonstrate the superiorities of multi-shell structured Sb₂S₃ to its single-shell counterpart and pristine Sb₂S₃. In Chapter 6, the dual-shell structured bismuth nanoboxes were synthesized and employed as anodes for potassium ion batteries. This as-prepared sample achieved an initial reversible capacity of over 300 mAh g⁻¹ and the reversible capacity maintained over 200 mAh g⁻¹ after 200 cycles under the current density of 1 C. More importantly, this dualshell structured bismuth was employed as a concept of proof to reveal the origin of the improved reversible capacity of nanostructured alloy anodes. Through various Operando synchrotron-based techniques, it was revealed that there are different origins of improved reversible capacity under low current density and high current density. Under the low current density, i.e. 0.2 C, the improved reversible capacity originates from the change of the electrochemical reaction path, in which the nanostructure offers enhanced capability to tolerate the volume expansion. Additionally, in the scenario of high current density, for instance, 2 C, the nanostructured alloy anodes provide higher surface area, resulting in more electrochemical surface reactions and, consequently, improved reversible capacity under high current density. To sum up, this thesis includes several examples of rational design and engineering hollow structured materials, such as Na₂Ti₃O₇ hollow spheres assembled from ultrathin nanosheets with N-doped carbon coating, dual-shell structured titanates with oxygen vacancies, multishell structured Sb₂S₃ with enhanced energy density, and dual-shell structured bismuth nanoboxes. Furthermore, some insightful understandings of the origins of their superior electrochemical performance were acquired through various physicochemical and electrochemical characterizations.