Preparation of Electroactive Materials for High Performance Lithium-Sulfur Batteries
AuthorDirlam, Philip Thomas
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PublisherThe University of Arizona.
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EmbargoRelease after 25-Jan-2017
AbstractThis dissertation is comprised of five chapters detailing advances in the synthesis and preparation of polymers and materials and the application of these materials in lithium-sulfur batteries for next-generation energy storage technology. The research described herein discusses progress towards overcoming three critical challenges presented for optimizing Li-S battery performance, specifically, addressing the highly electrically insulating nature of elemental sulfur, extending the cycling lifetime of Li-S batteries, and enhancing the charge discharge rate capability of Li-S cathodes. The first chapter is a review highlighting the use of polymers in conventional lithium-sulfur battery cathodes. Li-S battery technology presents a grand opportunity to realize an electrochemical energy storage system with high enough capacity and energy density capable of addressing the needs presented by electrical vehicles and base load storage. Polymers are ubiquitous throughout conventional Li-S batteries and their use has been critical in overcoming the challenges presented for optimizing Li-S cathode performance towards practical implementation. The high electrical resistivity of elemental sulfur requires the incorporation of conductive additives in order to formulate it into a functional cathode. A polymer binder must be utilized to integrate the elemental sulfur as the active material with the conductive additives into an electrically conductive composite affixed to a current collector. The electrochemical action of the Li-S battery results in the electroactive sulfur species converting between high and low order lithium polysulfides as the battery is discharged and charged. These lithium polysulfides become soluble at various stages throughout this cycling process that lead to a host of complications including the loss of electroactive material and slow rate capabilities. The use of polymer coatings applied to both the electroactive material and the cathode as a whole have been successful in mitigating the dissolution of lithium polysulfides by confining the redox reactions to the cathode. Elemental sulfur is largely intractable in conventional solvents and suffers from poor chemical compatibility limiting synthetic modification. By incorporating S-S bonds into copolymeric materials the electrochemical reactivity of elemental sulfur can be maintained and allow these polymers to function as the electroactive cathode materials while enabling improved processability and properties via the comonomeric inclusions. The use of inverse vulcanization, which is the direct copolymerization of elemental sulfur, is highlighted as a facile method to prepare polymeric materials with a high content of S-S bonds for use as active cathode materials. The second chapter focuses on the synthesis and polymerization of a novel bifunctional monomer containing both a styrenic group to access free radical polymerization and a propylenedioxythiophene (ProDOT) to install conductive polymer pathways upon an orthogonal oxidative polymerization. The styrenic ProDOT monomer (ProDOT-Sty) was successfully applied to a two-step sequential polymerization where the styrenic group was first leveraged in a controlled radical polymerization (CRP) to afford well defined linear homo- and block polymer precursors with pendant electropolymerizable ProDOT moieties. Subsequent treatment of the these linear polymer precursors with an oxidant in solution enabled the oxidative polymerization of the pendant ProDOT groups to install conductive polythiophene inclusions. Although the synthesis and CRP of ProDOT-Sty was novel, the key advance in this work was successful demonstration that sequential radical and oxidative polymerizations could be carried out to install conductive polymer pathways through an otherwise nonconductive polymer matrix. The third chapter expands upon the use of ProDOT-Sty to install conductive polymer pathways through a sulfur copolymer matrix. The highly electrically insulating nature of elemental sulfur precludes its direct use as a cathode in Li-S batteries and thus the use of ProDOT-Sty in the preparation of a high sulfur content copolymer with conductive inclusions was targeted to improve electrical properties. Inverse vulcanization of elemental sulfur with ProDOT-Sty and a minimal amount of 1,3-diisopropenylbenzene (DIB) was first completed to afford a sulfur rich copolymer with electropolymerizable side chains. Subsequently, the improved processability of the sulfur copolymer was exploited to prepare thin polymer films on electrode surfaces. The poly(ProDOT-Sty-𝑐𝑜-DIB-𝑐𝑜-sulfur) (ProDIBS) films were then subjected to oxidizing conditions via an electrochemical cell to invoke electropolymerization of the ProDOT inclusions and install conductive poly(ProDOT) pathways. Evaluation of the electrical properties with electrochemical impedance spectroscopy (EIS) revealed that the charge transfer resistance was reduced from 148 kΩ to 0.4 kΩ upon installation of the conductive poly(ProDOT) corresponding to an improvement in charge conductance of more than 95%. This also represented a key advance in expanding the scope of the inverse vulcanization methodology as the first example of utilizing a comonomer with a functional side chain. The fourth chapter focuses on expanding the scope of the inverse vulcanization polymerization methodology to include aryl alkyne based comonomers and the application of new these new sulfur copolymers as active cathode materials in Li-S batteries. The early work on developing inverse vulcanization relied heavily on the use of DIB as one of the few comonomers amenable to bulk copolymerization with elemental sulfur. One of the principal limitations in comonomer selection for inverse vulcanization is the solubility of the comonomer in molten sulfur. Generally it has been observed that aromatic compounds with minimal polarity are miscible and thus common classes of comonomers such as acrylates and methacrylates are immiscible and preclude their compatibility with inverse vulcanization. It was found that aryl alkynes are a unique class of compounds that are both miscible with molten sulfur and provide reactivity with sulfur centered radicals through the unsaturated carbon-carbon triple bonds. Additionally, it was found that internal alkynes were best suited for inverse vulcanization to preclude abstraction of the somewhat acidic hydrogen from terminal alkynes. 1,4-Diphenylbutadiyne (DiPhDY) was selected as a prototypical comonomer of this class of compounds for preparing high sulfur content copolymers via inverse vulcanization. Poly(sulfur-𝑐𝑜-DiPhDY) was prepared with various compositions of S:DiPhDY and these copolymers were formulated into cathodes for electrochemical testing in Li-S batteries. The poly(S-𝑐𝑜-DiPhDY) based cathodes exhibited the best performance reported at the time for a polymeric cathode material with the figure of merit of the first inverse vulcanizate to enable a cycle lifetime of up to 1000 cycles. The fifth chapter details the preparation of composite materials composed of a sulfur or copolymeric sulfur matrix with molybdenum disulfide (MoS₂) inclusions and the use of these materials for Li-S cathodes with rapid charge/discharge rate capabilities. The higher order lithium polysulfide redox products (e.g., Li₂S₈ Li₂S₆) generated during Li-S cycling are soluble in the electrolyte solution of the battery. The rate capability of the Li-S battery is thus fundamentally limited by mass transfer as these electroactive species must diffuse back to the cathode surface in order to undergo further reduction (discharge) or oxidation (charge). In order to limit the effective diffusion length of the soluble lithium polysulfides and therefore mitigate the diffusion limited rate, composite materials with fillers capable of binding the lithium sulfides were prepared. MoS₂ was selected as the filler as simulations had indicated lithium polysulfide had a strong binding interaction with the surface of MoS₂. Furthermore, it was demonstrated for the first time that metal chalcogenides such as MoS₂ readily disperse in molten sulfur which enabled the facile preparation of the composite materials in situ. The composites were prepared by first dispersing MoS₂ in liquid sulfur or a solution of liquid sulfur and DIB below the floor temperature of S₈ (i.e.<160 °C). The dispersions were then heated above the floor temperature of S₈ to induce ring opening polymerization of the sulfur phase and afford the composites. The composites were found to be potent active cathode materials in Li-S batteries enabling extended cycle lifetimes of up to 1000 cycles with excellent capacity retention. Furthermore, the composite materials were successful in enhancing the rate capability of the Li-S cathodes where reversible capacity of >500 mAh/g was achieved at the rapid rate of 5C (i.e. a 12 min. charge or discharge time).
Degree ProgramGraduate College