Electrophoretic Deposition of Sulfur Host Nanomaterials as a Binder-Free Approach to Fabricate 3D Cathode Structures for Li-S Batteries

無粘結劑電泳沉積納米材料方法製成的3D陰極結構Li-S電池

Student thesis: Doctoral Thesis

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Award date21 Sep 2020

Abstract

The rapid development of portable electronic devices and electric vehicles has triggered a growing demand for batteries with high energy and power density. Compared with lithium-ion batteries, which have dominated the market of portable electronic equipment for decades, lithium-sulfur batteries (LSBs) have emerged as a promising alternative for its high theoretical specific capacity of 1675 mAh g−1 and theoretical specific energy of 2600 Wh kg−1. Moreover, sulfur is naturally abundant, environmentally benign, and industrially produced in extensive quantities. In spite of all these advantages, LSBs are still facing serious fundamental challenges for industrial application including the insulating nature of sulfur and lithium sulfides (Li2S2/Li2S), huge volume expansion of ∼80% during the charge-discharge process, and the shuttling of the dissolved LiPSs between anode and cathode, which will lead to high cell polarization, sluggish reaction kinetics, loss of active material, and low sulfur utilization.

Encapsulation of sulfur in porous carbon nanostructures (C/S composites) with high surface area and high porosity per unit mass have been extensively studied during the last two decades as an effective approach to tackle high electronic resistance of the sulfur cathode as well as to improve the physical impediment of LiPSs. In this context, different strategies such as tape casting, pasting, and wet powder spraying have been adopted to apply the binder-containing C/S composite on the surface of the conductive current collector. However, since the binder tends to be electronically and ionically nonconductive and does not contribute to the capacity, the addition of binder (typically 10-20 wt%) can reduce the energy density of the electrode and undermine the electrochemical performance of the cell by decreasing the number of active reaction sites and blocking the pores. Therefore, binder-free cathodes are preferred in order to approach the theoretical energy density of Li-S cells. However, most binder-free methods entail the growth of carbon nanomaterials by complicated and expensive chemical procedures such as CVD or solution-based chemical deposition routes. In the present study, electrophoretic deposition (EPD) was employed as a binder-free approach to manipulate nanoparticles suspended in a colloidal suspension, using an external DC potential, to form homogeneous layers of sulfur host material as cathode in Li-S batteries.

In the present study, the EPD method was employed to modify the surface of carbon fiber paper (CFP), as 3D current collector, by depositing crack-free and uniform layers of conventional carbon-based nanomaterials, including CNT and carbon black, along with noble metal catalyst nanoparticles. As demonstrated by the cycling tests, the EPD:CFP/CNT/S delivered a reversible discharge capacity of 935 mAh g-1 after 100 cycles, which is attributed to the electric-field induced homogeneous distribution of CNTs across the carbon fiber network. A comparison between the electrochemical performance of the CNT films prepared by EPD and the conventional slurry-casting method revealed that the EPD:CFP/CNT/S delivered up to 23% higher capacities than Cast:CFP/CNT/S, while displaying lower polarization and higher coulombic efficiency. Furthermore, layer-by-layer architectures were successfully applied by EPD using carbon black particles, where a specific capacity of 1033 mAh g-1 was delivered by EPD:CFP/CNT/KB/S after 100 cycles. In addition, the rate performance of CFP/S was improved through modifying the CFP surface with EPD-CNT, EPD-CNT/SP and EPD-CNT/KB layers. However, the best rate capability was obtained in the presence of noble metal electrocatalysts in EPD:CFP/CNT-Pt/S which was fabricated via the co-deposition of CNT and Pt nanoparticles from a CNT-Pt/acetone suspension. The obtained results were firmly supported by cyclic voltammetry and impedance analysis, showcasing the capability of EPD as a binder-free, time-effective, and scalable method in preparing efficient nanocomposite cathode structures from a wide variety of host materials for Li-S batteries.

Having exhibited the effectiveness of EPD to fabricate 3D cathode structures from carbon nanomaterials, we turned our focus to the electrophoretic deposition of polar-natured metal oxides as the most widely used inorganic catalyst materials to improve the affinity of the cathode surface to adsorb migrating long-chain polysulfides. The obtained results revealed that the electrophoretically deposited TiO2 nanoparticles improved the cycling performance of EPD:CFP/CNT/TiO2/S electrodes by acting as preferential polysulfide-anchoring sites on the surface of the EPD-CNT layer. In terms of the undesirable activation phenomenon, a semi-quantitative analysis was carried out to compare different samples, revealing that applying an EPD-TiO2 layer on the EPD:CFP/CNT cathode can effectively mitigate this effect by decreasing the activation factor. In the next step, TiO2 nanoparticles were directly deposited on CFP to serve as an independent sulfur host layer without the introduction of any conductive carbon materials. The results obtained from the electrochemical performance of the EPD:CFP/TiO2/S cathodes with different TiO2 areal loadings showed that the T4 sample containing 3.8 mg cm-2 of TiO2 exhibited the best combination of specific capacity, capacity retention, overpotential, and rate capability. The deterioration of these parameters at TiO2 areal loadings above 4.83 mg cm-2 was attributed to the formation of cracks during the drying step. In general, the charge-discharge behavior and rate performance results of the EPD:CFP/TiO2/S strongly support the idea that the TiO2 surface is directly engaged in the charge transfer process during the conversion reactions of LiPSs. It means the entrapped polysulfides do not necessarily have to diffuse all the way to the underlying conductive carbon fibers to undergo the redox reactions.

To further study the contribution of the EPD-TiO2 layer to the redox reaction across the electrode, SEM microstructural studies of EPD:CFP/TiO2/S were carried out at the end of the charging step, revealing the formation of flake-shaped sulfur precipitates at certain areas across cathode. The sulfur flakes formed during the charging process were found to nucleate at the CF/TiO2 interface and follow a preferential growth path away from the CF surface into the TiO2 matrix. The successful precipitation of sulfur on the EPD-TiO2 matrix far from the conductive carbon surface indicates the involvemnt of TiO2 nanoprticles, well connected to each other by EPD, in the charge transfer process during the electrode re-dox reactions. Also, the EIS studies revealed a drastic increase in the impedance of EPD:CFP/TiO2/S after the first charging, being attributed to the redistribution of the pristine vapor-deposited fulfur into flower-like structures formed by the accumulation of sulfur flakes upon the first charge. This remarkable increase in impedance was correlated with the 10% drop in the specific capacity and 21% increase in overpotential after the first cycle. In addition, the evolution of the Nyquist plots during the first charge revealed that the charge-transfer resistance increses as a function of charging time. The increment of the charge-transfer at the initial stages of charging is attributed to the nucleation of sulfur flakes at the CF/TiO2 interface, while the drastic increase at the intermediate and final stages is ascribed to the growth of the insulating sulfur flakes across the EPD-TiO2 matrix. Finally, in a comparison with EPD:CFP/MnO2/S, which undergoes severe pulverization upon charging, we demonstrated that the sulfur flake formation in EPD:CFP/TiO2/S protects the cathode film from agglomeration over charging by providing preferential nucleation sites and growth paths for sulfur precipitates.