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A Method for Controlled Oxide and Carbon Coating of Silicon Nanoparticles as Anode in Lithium-Ion Batteries

Orderud Skare, Marte
Master thesis
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URI
http://hdl.handle.net/11250/2454365
Date
2017
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  • Institutt for materialteknologi [1604]
Abstract
Silicon has emerged as a promising alternative to graphite in lithium-ion batteries.

Silicon can, theoretically, provide up to ten times higher capacity due to its high

alloying capabilities with lithium. This improvement in initial capacity is however

followed by capacity fade during the early cycles, which is undesirable for commercial

applications. The low cycle life is in turn due to the high alloying with

lithium, as this leads to a large volume expansion and subsequent contraction of

the material.

In this work, a method for coating silicon has been investigated to eventually be

able to buffer the volume change associated with cycling. Double core-shell particles

were fabricated by wet chemical methods, and can serve as a base for creating

yolk-shell particles in the future. Silicon nanoparticles from a Free Space Reactor

with diameters between 50 nm and 1 ¹m have successfully been coated with

a 50-80nm thick oxide layer by the use of tetraethyl orthosilicate, designed to be a

sacrificial layer to be etched away at a later state. Following the oxide layer, a thin

carbon-coating has been applied by a resorcinol-formaldehyde resin and pyrolysis,

which interconnects the oxide coated particles in a web-like morphology. The

coating steps have been thoroughly characterized by electron microscopy, energy

dispersive spectroscopy, x-ray diffraction, thermogravimetric analysis, dynamic

light scattering and nitrogen adsorption. Problems related to the upscaling of this

process were revealed during carbon-coating, as it did not give sufficient control

and reproducibility with coating thickness and morphology for the chosen starting

material.

Li-ion battery half-cells were prepared with the reference silicon and electrochemically

cycled, and one half-cell exhibited a superior lifetime (up to 1,000 cycles)

when cycled at a reversible capacity of 700 mAh/g, while others could retain 1,200 mAh/g for only 150 cycles. The cycling of both amorphous and crystalline silicon was done, but no lifetime-dependence on crystallinity was found. Crystalline silicon anodes with a loading of 0.4 mg/cm2 displayed an irreversible capacity loss (ICL) in the first cycle of 8-9%, and a reversible capacity of 1,750 mAh/g for 120 cycles, while a higher loading of 0.5 mg/cm2

displayed the same ICL but a capacity of 2,500 mAh/g for only 50 cycles. Amorphous

silicon anodes with a loading of 0.5 mg/cm2 displayed a low ICL of 1%,

and a reversible capacity of 2,000 mAh/g for 70 cycles. The lifetimes are clearly

more dependent upon loading and cell fabrication, than of initial crystallinity of

the material.

In sum, this work has provided a starting point for the fabrication of yolk-shell particles

by using Si nanoparticles from an FSR. A homogeneous coating of oxide has

been applied, and a carbon coating was observed for a few samples. Electrochemical

results from the cycling of pure silicon showed a high dependence on loading

and cell fabrication, and it is therefore imperative to keep the electrode fabrication

and cell assembly as similar as possible for comparison purposes.
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NTNU

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