Browsing by Author "Li, Xiaoyi"

Now showing 1 - 6 of 6
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    Self-Doped Conjugated Polymeric Binders Improve the Capacity and Mechanical Properties of V2O5 Cathodes
    (MDPI, 2019) Li, Xiaoyi; An, Hyosung; Strzalka, Joseph; Lutkenhaus, Jodie; Verduzco, Rafael
    Polymeric binders serve to stabilize the morphology of electrodes by providing adhesion and binding between the various components. Successful binders must serve multiple functions simultaneously, including providing strong adhesion, improving conductivity, and providing electrochemical stability. A tradeoff between mechanical integrity and electrochemical performance in binders for lithium-ion batteries is one of the many challenges of improving capacity and performance. In this paper, we demonstrate a self-doped conjugated polymer, poly(9,9-bis(4′-sulfonatobutyl)fluorene-alt-co-1,4-phenylene) (PFP), which not only provides mechanical robustness but also improves electrode stability at temperatures as high as 450 °C. The self-doped PFP polymer is comprised of a conjugated polyfluorene backbone with sulfonate terminated side-chains that serve to dope the conjugated polymer backbone, resulting in stable conductivity. Composite electrodes are prepared by blending PFP with V2O5 in water, followed by casting and drying. Structural characterization with X-ray diffraction and wide-angle X-ray scattering shows that PFP suppresses the crystallization of V2O5 at high temperatures (up to 450 °C), resulting in improved electrode stability during cycling and improved rate performance. This study demonstrates the potential of self-doped conjugated polymers for use as polymeric binders to enhance mechanical, structural, and electrochemical properties.
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    Side‐Chain Engineering for High‐Performance Conjugated Polymer Batteries
    (Wiley, 2021) Li, Xiaoyi; Li, Yilin; Sarang, Kasturi; Lutkenhaus, Jodie; Verduzco, Rafael
    Conjugated polymers are attractive for energy storage but typically require significant amounts of conductive additives to successfully operate with thin electrodes. Here, side‐chain engineering is used to improve the electrochemical performance of conjugated polymer electrodes. Naphthalene dicarboximide (NDI)‐based conjugated polymers with ion‐conducting ethylene glycol (EG) side chains (PNDI‐T2EG) and non‐ion‐conducting 2‐octyldodecyl side chains (PNDI‐T2) are synthesized, tested, and compared. For thick (20 µm, 1.28 mg cm−2) electrodes with a 60 wt% polymer, the PNDI‐T2EG electrodes exhibit 66% of the theoretical capacity at an ultrafast charge–discharge rate of 100C (72 s per cycle), while the PNDI‐T2 electrodes exhibit only 23% of the theoretical capacity. Electrochemical impedance spectroscopy measurements on thin (5 µm, 0.32 mg cm−2), high‐polymer‐content (80 wt%) electrodes reveal that PNDI‐T2EG exhibits much higher lithium‐ion diffusivity (DLi+ = 7.01 × 10−12 cm2 s−1) than PNDI‐T2 (DLi+ = 3.96 × 10−12 cm2 s−1). PNDI‐T2EG outperforms most previously reported materials in thick, high‐polymer‐content electrodes in terms of rate performance. The results demonstrate that the rate performance and capacity are significantly improved through the incorporation of EG side chains, and this work demonstrates a route for increasing the rate of ion transport in conjugated polymers and improving the performance and capacity of conjugated‐polymer‐based electrodes.
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    Structure Engineering of Polymers Used in Lithium-ion Battery Electrodes for Improved Performance
    (2020-08-21) Li, Xiaoyi; Verduzco, Rafael
    In this work, we focused on different applications of polymer in Lithium-ion battery electrodes, with an emphasis in structure engineering of polymers in order to provide a better understanding in the fundamental relationship between polymer structure, electrode composition, battery properties and performance. We first studied self-doped polymeric binder, PFP, in V2O5 cathodes. This fully water-processable, thermally annealed hybrid electrode shows steady cycling performance even when it is annealed at 400°C. We believe this is because the addition of 5 wt% PFP as binder helps suppress the crystallization of V2O5 xerogel and avoid the disruption of its layered structure. Then we discussed using conjugated polymer PNDI-T2EG as the active materials in electrode. We demonstrated that modification of conjugated polymer side-chains had a significant impact on the electrochemical performance of nano-composite electrodes, in particular enabling excellent performance at high charge-discharge rates. By attaching OEG side-chains, electrodes demonstrate exceptional rate performance at high charge-discharge rate, high mass loading, and high active material content. As a continued study, we look at a series of PNDI-based polymer with different ratios of OEG side-chains. All these works help to demonstrate that structure engineering of polymers is an efficient strategy when researching for the next better materials used for battery development.
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    Supramolecular block copolymer photovoltaics through ureido-pyrimidinone hydrogen bonding interactions
    (Royal Society of Chemistry, 2016) Lin, Yen-Hao; Nie, Wanyi; Tsai, Hsinhan; Li, Xiaoyi; Gupta, Gautam; Mohite, Aditya D.; Verduzco, Rafael
    A challenge in the development of bulk heterojunction organic photovoltaics (BHJ OPVs) is achieving a desirable nanoscale morphology. This is particularly true for polymer blend OPVs in which large-scale phase separation occurs during processing. Here, we present a versatile approach to control the morphology in polymer blend OPVs through incorporation of self-associating 4 2-ureido-4[1H]-pyrimidinone (UPy) endgroups onto donor and acceptor conjugated polymers. These UPy functionalized polymers associate to form supramolecular block copolymers during solution blending and film casting. Atomic force microscopy measurements show that supramolecular associations can improve film uniformity. We find that the performance of supramolecular block copolymer OPVs improves from 0.45% to 0.77% relative to the non-associating conjugated polymer blends at the same 155 °C annealing conditions. Impedance measurements reveal that UPy endgroups both increase the resistance for charge recombination and for bulk charge transport. This work represents a versatile approach to reducing large-scale phase separation in polymer–polymer blends and directing the morphology through supramolecular interactions.
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    Tannic Acid as a Small-Molecule Binder for Silicon Anodes
    (American Chemical Society, 2020) Sarang, Kasturi T.; Li, Xiaoyi; Miranda, Andrea; Terlier, Tanguy; Oh, Eun-Suok; Verduzco, Rafael; Lutkenhaus, Jodie L.
    Increasing demand for portable electronic devices, electric vehicles, and grid scale energy storage has spurred interest in developing high-capacity rechargeable lithium-ion batteries (LIBs). Silicon is an abundantly available anode material that has a theoretical gravimetric capacity of 3579 mAh/g and a low operating potential of 0–1 V vs Li/Li+. However, silicon suffers from large volume variation (>300%) during lithiation and delithiation that leads to pulverization, causing delamination from the current collector and battery failure. These issues may be improved by using a binder that hydrogen bonds with the silicon nanoparticle surface. Here, we demonstrate the use of tannic acid, a natural polyphenol, as a binder for silicon anodes in lithium-ion batteries. Whereas the vast majority of silicon anode binders are high molecular weight polymers, tannic acid is explored here as a small molecule binder with abundant hydroxyl (−OH) groups (14.8 mmol of OH/g of tannic acid). This allows for the specific evaluation of hydrogen-bonding interactions toward effective binder performance without the consideration of particle bridging that occurs otherwise with high molecular weight polymers. The resultant silicon electrodes demonstrated a capacity of 850 mAh/g for 200 cycles and a higher capacity when compared to electrodes fabricated by using high molecular weight polymers such as poly(acrylic acid), sodium alginate, and poly(vinylidene fluoride). This work demonstrates that a small molecule with high hydrogen-bonding capability can be used a binder and provides insights into the behavior of small molecule binders for silicon anodes.
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    Transformation of One-Dimensional Linear Polymers into Two-Dimensional Covalent Organic Frameworks Through Sequential Reversible and Irreversible Chemistries
    (American Chemical Society, 2021) Zhu, Dongyang; Li, Xiaoyi; Li, Yilin; Barnes, Morgan; Tseng, Chia-Ping; Khalil, Safiya; Rahman, Muhammad M.; Ajayan, Pulickel M.; Verduzco, Rafael
    Covalent organic frameworks (COFs) are crystalline porous materials linked by dynamic covalent bonds. Dynamic chemistries enable the transformation of an initially amorphous network into a porous and crystalline COF. While dynamic chemistries have been leveraged to realize transformations between different types of COFs, including transformations from two-dimensional (2D) to three-dimensional (3D) COFs and insertion of different linking groups, the transformation of linear polymers into COFs has not yet been reported. Herein, we demonstrate an approach to transform linear imine-linked polymers into ketone-linked COFs through a linker replacement strategy with triformylphloroglucinol (TPG). TPG first reacts through dynamic chemistry to replace linkers in the linear polymers and then undergoes irreversible tautomerism to produce ketone linkages. We have analyzed the time-dependent transformation from the linear polymer into COF through powder X-ray diffraction, Fourier-transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM) to understand the transition and substitution mechanisms. This work demonstrates another route to produce COFs through sequential reversible and irreversible chemistries and provides a potential approach to synthesizing COFs through the solution processing of linear polymers followed by transformation into the desired COF structure.