Functional Polymers with Tailored Molecular Linkers for High-Capacity Lithium Batteries

Abstract

Along with the development of portable electronic devices and electric vehicles (EV), rechargeable batteries have been permeated into our daily lives and people now desire to experience lighter and long-lasting batteries beyond existing lithium-ion batteries (LIBs). To satisfy these demands, the development of polymeric materials on electrodes, electrolyte, separator, binder, and even electrode-electrolyte interphases have been being accelerated by numerous researchers owing to the polymer nature of tunability, versatility, and processability. In this thesis, the functional polymers with tailored molecular linkers for high-capacity batteries are described in the perspective of synthesis and electrochemical properties. In Chapter 1, brief introduction of Li-S batteries and the use of organosulfur polymers as an alternative cathode material instead of elemental sulfur are firstly given. Since elemental sulfur has severe disadvantages such as inherent limited electrical conductivity and dissolution issue of lithiated sulfur species, the evolution and development of organosulfur polymers with specific functionalities have been regarded as one of the promising candidates to overcome such limitations. Then, the extended roles of the polymer binder are introduced in this part. Apart from the typical role of intimate connection between active materials and conducting materials, the polymer binders with additional functionalities have broadened the scope of polymer binder. In the last part, the electrode-electrolyte interphase layers on anode materials are introduced. This part highlights the impact of artificial layer formed by tailored additive on the battery performances. In Chapter 2, I introduce synthesis and application of organosulfur polymer cathodes for high-energy lithium-sulfur batteries. In the first part of this chapter, I elucidate the investigation on sulfur-rich polymers with functional linkers to enhance electrical and electrochemical properties. The rational molecular designs of linker molecules via computational method (DFT calculation) were conducted prior to the synthesis of monomers and copolymers. The lithium-sulfur batteries based on synthesized sulfur-rich copolymers delivered high discharge capacity of 1346 mAh⋅g-1 at 0.1C, high-rate performance of 833 mAh⋅g-1 at 10C. The long cyclability was demonstrated by long lifetime of 500 cycles with a low capacity decay of 0.052% per cycle. These outstanding performances are realized via the linker-controlled synthesis of sulfur-rich polymers with the possible tuning of charge transfer properties and effective suppression of the polysulfide-shuttle via chemical interactions. In particular, the use of tetra(allyloxy)-1,4-benzoquinone linker enables the conductivity enhancement by a factor of 450 and lithium diffusion kinetics improvement by two orders of magnitude during redox reactions, compared with elemental sulfur cathodes. I conclude that the importance of this study is fundamental limitations of typical organosulfur cathode materials were overcome by introducing tailored functional linker molecule. In the second part of chapter 2, I explain a facile and rapid one-pot synthesis of hierarchically ordered sulfur microparticles which enables high-energy, flexible lithium-sulfur batteries. The spontaneous formation of wrinkles and pores on the particles facilitates electrolyte access and alleviates mechanical stress during battery cycling. The large surface area created by the ordered sulfur domains enhances lithium diffusion coefficients and facilitates polysulfide conversion kinetics in the Li-S cells, leading to a high discharge capacity of 1529 mAh⋅g-1 at 0.05 C and excellent rate performance showing 721 mAh⋅g-1 at 7 C. Owing to the inherent electrode porosity imparted by SVPA microparticles, a high areal capacity of ~5 mAh⋅cm-2 was obtained at increased cathode loadings. Abundant phosphonate moieties at the surfaces and interfaces of particles act as effective chemical anchors for lithium polysulfides, thereby mitigating the shuttle effects, which can prolong cycle lives at various C rates. The impressive properties of the SVPA microparticles are demonstrated for Li-S pouch cells, which exhibit stable operation under various deformations and highlight the SVPA microparticles as a promising candidate for next-generation Li-S batteries for wearable electronics. In Chapter 3, synthesis of multifunctional polymer binders and the application of lithium-sulfur batteries are discussed. The design of poly(ethylene oxide)-b-poly(4-vinyl catechol) (PEO-b-P4VC) block copolymer binders imparts improved lithium transport as well as mussel-inspired adhesive properties. Furthermore, effective interchain hydrogen-bonding interactions between P4VC and PEO blocks enables high elastic properties with an improved electrochemical stability window of up to 5.4 V. In terms of shuttle effect in Li-S batteries, the strong chemical affinity of abundant catechol moieties in polymer binder to polysulfide redox intermediates leads to stable battery performance with high capacity of >1000 mAh⋅g-1 even after 150 cycles at 0.5C. In Chapter 4, I introduce molecular design and synthesis of electrolyte additives for lithium batteries featuring an Si–graphite composite (SGC) anode to enhance rate performance and cycle stability. The key strategy involves the formation of a hybrid artificial solid-electrolyte interphase (SEI) on the anode via a synergistic combination of fluoroethylene carbonate (FEC) and dilithium vinylphosphonate (VPLi). VPLi was preferentially reduced (over carbonated electrolytes) on the electrode surface to form a crosslinked, Li-rich polymeric SEI layer with single-ion conducting characteristics. Coexisting FEC modulated the chemical composition and thickness of the SEI by promoting the formation of a thin LiF layer, resulting in polyVPLi-LiF hybrid artificial interphases. The Li-SGC cell with the FEC/VPLi coadditive demonstrated a specific capacity of 386 mAh·g-1 at 0.1C with a capacity retention of 95.3% for 100 cycles and enhanced rate capability of up to 4C. It is attributable to the chemical and mechanical robustness of the hybrid SEI layer, which mitigates the precipitation of decomposed electrolytes. Furthermore, the FEC/VPLi coadditive in full cells fabricated with an Ni-rich NCA cathode and SGC anode resulted in a stable cycling performance at 0.1 C for 200 cycles, with reduced gas evolution.