Chalcogen-based Composite Materials for Sodium Ion Batteries


Student thesis: Doctoral Thesis

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Awarding Institution
Award date14 Aug 2019


The developed technologies for harvesting energy from renewables have created a fast-growing demand for electrical energy storage devices. Lithium ion batteries (LIBs) are presently the dominant type for mobile electronics and electric vehicles. However, they are still not a perfect choice due to the elemental scarcity and the consequent inevitable future rising price curve for lithium. Sodium ion batteries (SIBs), by contrast, offer an advantage of lower cost and comparable performance with LIB, as affordability is considered over power capacity for stationary applications. Commercialization of SIB is well underway following a decade of fruitful academic research. One important aspect of SIB is to ensure proper design and practical realization of electrode materials serving as sodium ion hosts. In this thesis, chalcogen-based electrode materials will be explored, as they show appealing capability in various respects including efficiency, capacity, kinetics and lifespan.

In the first study on selenides, carbon supported nickel selenide (Ni0.85Se/C) hollow nanowires are prepared from carbon coated selenium (Se) nanowires via a self-templating hydrothermal method, by first dissolving Se in the Se/C nanowires in hydrazine, allowing it to diffuse out of the carbon layer, and then reacting with nickel ions into Ni0.85Se nanoplates on the outer surface of the carbon. Ni0.85Se/C hollow nanowires exhibit greatly enhanced cycle stability and rate capability as compared to bare Ni0.85Se nanoparticles, with a reversible capacity around 390 mAh g-1 (the theoretical capacity is 416 mAh g-1) at the rate of 0.2 C and 97% capacity retention after 100 cycles. When the current rate is raised to 5 C, they still deliver a capacity of 219 mAh g-1.

Emerging sodium-selenium batteries suffer from volume expansion of Se cathode and shuttling effects of soluble intermediates. Confining Se within carbon matrix is the most adopted strategy to address these two issues, which is generally realized via a melt-infusion method. In the second study focused on Se cathodes, a vacuum calcination approach is developed to fabricate selenium/carbon composites, which does not require intensive mixing and durable heating such as in commonly used melt-infusion methods of loading Se into carbon hosts. Starting from carbon-coated Se wires prepared via a wet-chemical reaction, selenium/carbon tubes are fabricated by a straightforward calcination process. The calcination is conducted in a confined space to produce the insulating carbon shell under vacuum, and Se melts but remains a constituting part of the composite. Paired with sodium metal anode, the resultant selenium/carbon tubes deliver a high reversible capacity of 601 and 509 mAh g-1 at 0.2 and 2 C normalized by the mass of Se, which corresponds to energy and power densities of 860 and 667 Wh kg-1 at 193 and 1770 W kg-1, respectively. Such capacity and rate performance surpass most typical cathode materials for lithium or sodium (ion) batteries, according to the comparative literature analysis. 

In the third study, we developed a vapor-infiltration method to fabricate selenium/carbon composites which are advantageous over the melt-infusion route in terms of several aspects: it relieves the requirement of intensive mechanical mixing and simplifies the ratio optimization between Se and carbon; it avoids Se aggregation and makes it possible to utilize all the surface and pores of carbon host. Utilizing this method, we fabricated a selenium/graphene composite from thermally reduced graphene oxide with a Se loading equal to 71wt%, thus approaching the record value. The obtained composite achieved the highest reported to date initial coulombic efficiency (ICE) of 88% among various Se cathodes, with superior rate and cycle performance (410 and 367 mA h g-1 at 0.1 and 1 A g-1 respectively; capacity decay <10% after 800 cycles at 2 A g-1) enabled by the supporting graphene framework and the use of the ether electrolyte. 

Se-based materials for sodium (ion) batteries are currently attracting extensive attentions owing to their fast kinetics and excellent cyclability; at the same time, achieving high Se content, which is crucial to maintain the competitive edge over other kinds of electrode materials, still remains a challenge. In the fourth study, a confined annealing method is developed which allows direct conversion of pristine metal-organic frameworks (MOFs) into selenium/selenide/carbon composites. It is a simultaneous process of carbonization, selenization and Se vapor deposition, and the combination of elemental Se and selenide results in a record-high Se content of 76 wt%, enhanced capacity and rate capability (490 and 384 mAh g-1 at 0.1 and 2.0 A g-1) exceeding most documented Se-based materials. The produced composites also exhibit excellent cycle stability (no decay for 700 cycles at 2 A g-1), which are correlated to dominant capacitive charge transport mode and the MOF-derived robust structure.

As conversion-type anode materials for SIBs, metal sulfides have a lower theoretical capacity but generally deliver a much higher reversible capacity in practice than their oxide counterparts; the origin of this phenomenon has not been fundamentally understood yet. In the fifth study focused on sulfides, according to a performed reaction enthalpy analysis, the sulfidation reaction with sulfur is anticipated to be feasible for a wide variety of metal oxides. As a practice of the sulfidation strategy, less anode-active tin oxide/carbon (SnO2/C) composites are converted into highly anode-active tin sulfide/carbon (SnS2/C) materials by confined annealing with sulfur powder. For anode applications in sodium ion batteries, SnS2/C delivers a reversible capacity of 770 mAh g-1 which is more than double the value of SnO2/C (360 mAh g-1). The observed superiority of SnS2/C anodes is correlated with the narrower bandgap, higher conversion potential that allows for a larger polarization, lower sodium ion diffusion barrier in the crystalline lattice, and more energetically favorable conversion process on the surface of SnS2. In particular, a modelling strategy for the surface conversion reactions is demonstrated, which could potentially be considered for kinetics study of various conversion-type anode materials.

In summary, we introduced novel synthetic methods for a variety of chalcogen-based electrode materials, and their operating mechanisms and sodium storage properties are discussed. In particular, the confined annealing technique of realizing chalcogen deposition and chalcogenidation is an easy and versatile approach to fabricate various structured chalcogen-based materials. For optimization of battery performance, we highlighted the importance of interfaces between electrode and electrolyte. On one hand, conversion-type electrode materials that suffer from drastic volume expansion upon sodiation should be rationally designed to avoid frequent destruction and generation of solid-electrolyte interface films; on the other hand, the selection of suitable electrolyte is critical for improving initial coulombic efficiency and extending cycle life, and the ether electrolyte that enables high ICE and excellent cycle stability appears as one preferred choice for chalcogen-based materials.