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Influence of the Oil on the Structure and Electrochemical Performance of Emulsion-Templated Tin/Carbon Anodes for Lithium Ion Batteries Yuzi Zhang, Yue Pan, Yingnan Dong, Brett L Lucht, and Arijit Bose Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01404 • Publication Date (Web): 07 Aug 2017 Downloaded from http://pubs.acs.org on August 8, 2017
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Langmuir
Influence of the Oil on the Structure and Electrochemical Performance of Emulsion-Templated Tin/Carbon Anodes for Lithium Ion Batteries
Yuzi Zhang1, Yue Pan2, Yingnan Dong2, Brett L. Lucht2, Arijit Bose1,* 1
Department of Chemical Engineering, 2 Department of Chemistry, University of Rhode Island, Kingston, Rhode Island, 02881
* Corresponding author: Arijit Bose,
[email protected], 401-874-2804
Key words: tin/carbon anode, emulsion-templating, oil
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Abstract Tin (Sn) is a useful anode material for lithium ion batteries (LIBs) because of its high theoretical capacity. We fabricated oil-in-water emulsion-templated tin nanoparticle/carbon black (SnNP/CB) anodes with octane, hexadecane, 1-chlorohexadecane and 1-bromohexadecane as the oil phases. Emulsion creaming, the oil vapor pressure and the emulsion droplet size distribution all affect drying and thus the morphology of the dried emulsion. This morphology has a direct impact on the electrochemical performance of the anode. SnNP/CB anodes prepared with hexadecane showed very few cracks, and had the highest capacities and capacity retention. The combination of low vapor pressure, creaming, that forced the emulsion droplets into a close packed arrangement on the surface of the continuous water phase, and the small droplets, allowed for gentle evaporation of the liquids during drying. This led to lower differential stresses on the sample, and reduced cracking. For octane, the vapor pressure was high, the droplet sizes were large for 1-cholorohexadecane, and there was no creaming for 1-bromohexadecane. All of these factors contributed to cracking of the anode surface during drying, and reduced the electrochemical performance. Choosing an oil with balanced properties is important for obtaining the best cell performance for emulsion-templated anodes for LIBs.
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Introduction Lithium ion batteries (LIBs) with high energy density and long cycle life are important for advanced electronic devices and electric vehicles.1, 2, 3, 4, 5, 6 Among a variety of anode materials for LIBs, tin (Sn) is one of the most promising candidates due to its high theoretical specific capacity of 994 mAh/g. Because Sn has a density of 7.3g/cm3, it also has a high volumetric energy density of ~7260 mAh/cm3.7, 8 However, Sn undergoes a volume change of around 300% during lithiation/delithiation, resulting in fracture and pulverization, contact loss with conductive carbon and the current collector, as well as the continuous formation of a solid electrolyte interphase (SEI) that consumes lithium ions.9 All these mechanisms contribute to capacity fading.10, 11, 12 Sn nanoparticles (SnNPs),13, 14 porous Sn nanostructures,15, 16 core-shell Sncarbonaceous configurations,7, 17, 18, 19 free-standing composites of carbon foam with SnO2 nanoparticles,20 SnNPs in a 3-D nanoporous carbon network derived acid etching of a metalorganic framework,21 and Sn-graphene nanocomposites have been used for making Sn-based anodes.22, 23 These configurations are designed to accommodate volume changes of Sn during lithiation/delithiation, leading to superior cycle performance.
Previously, we reported an emulsion-templated approach for preparing silicon (Si)-based anodes for LIBs.24 Si nanoparticles (SiNPs) were confined in octane droplets stabilized by carbon black (CB), which also formed a porous CB cage surrounding the SiNPs. There was enough available room within these cages to allow for volume expansion and contraction of the SiNPs during lithiation/delithiation without transmitting these strains to the surrounding CB network. Spatial variations of stress during lithiation and delithiation are small over the dimensions of the
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nanoparticles, thus reducing pulverization. This Si/CB anode exhibited excellent cycle performance with capacity retention of 67% after 50 cycles at a cycle rate of 0.1C.
The oil phase is an important component in this emulsion-based strategy. It can have different densities, affecting creaming in the emulsion. The oils can also have significant differences in vapor pressure.25, 26 Because of a complex interplay between the oil-water interfacial tension, the transfer rate of stabilizing particles to the oil-water interface during emulsion formation, the viscosities of the oil and the aqueous suspension, different oils can also lead to distinct emulsion droplet size distributions even when the energy imparted for creating the emulsions are identical. All of these factors can impact the drying rates at a fixed temperature. Drying can induce differential stresses in the sample, which leads to cracks in the dried anode. The formation of these defects can have a large effect on the cell performance.27 In this paper, we chose four oils with different physical properties, and showed that the morphology and the electrochemical performance of SnNP based anodes are strongly influenced by the selection of the oil. Experiments Preparation of anodes SnNPs (average diameter ~60-80 nm) were purchased from US Research Nanomaterials Incorporation. A para-amino benzoic acid-terminated CB suspension in water at pH 7.5 and 15wt% CB loading was provided by Cabot Corporation. The CB particles are fractal, have a nominal diameter of 120-150nm, and a specific surface area of ~200 m2/g. At neutral pH, the carboxyl groups on CB are deprotonated. These CB particles are highly hydrophilic and are stably suspended in water. The oils used in this work were octane (99%), hexadecane (99%), 1chlorohexadecane (95%) and 1-bromohexadecane (97%), purchased from Sigma Aldrich. All
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oils were used as received. Their densities at 25°C and vapor pressures at 50°C are shown in Table I. SnNP has a density of 7.3 g/cm3, making it challenging to disperse in the oil because of potential sedimentation. Brownian motion will dominate over sedimentation so long as D4
