Organic electronics are receiving increasing attention for the development of flexible devices, due to the generally easier processability and higher flexibility of organic, compared to inorganic, materials. However, although many suitable organic materials possess some measure of elasticity, flexible electronic devices are often prone to cracking or breaking over time, resulting in poor durability and reliability. As a result, the burgeoning field of self-healing materials for electrochemical devices has begun to attract attention. Self-healing polymers can be designed to recover their original mechanical properties after physical damage, increasing their useful lifetime. Therefore, development of self-healing polymers is highly desirable to accommodate increasing demand for flexible devices. For decades, soft gels, elastomers, and hybrid organic-inorganic systems with self-healing properties have been designed based on reversible interactions and dynamic covalent bonds. However, previous approaches typically sacrifice stiffness to achieve self-healing properties, mostly resulting in materials with tensile moduli less than 1 MPa. These soft and deformable materials are less likely to maintain reliable and stable pathways for electron or ion movement in devices. Additionally, considering different application conditions, how to modulate these self-healing and mechanical properties in response to various environment cues, such as temperature, pH, or light is vital for improving long-term mechanical integrity as well. In particularly, tunability based on temperature is widely useful for initiating self-healing ability and controlling stiffness for flexible electronics, which will widen their ultimate potential applications. Herein, we report a supramolecular polymer system based on pi-pi interaction between naphthalenediimide (NDI) and pyrene (Py) derivatives, which possesses high stiffness (Young’s modulus > 69 MPa) while also retaining mild self-healing temperature (< 50 o C) and inherent ionic conductivity (> 10 -6 S/cm at 50°C when doped with LiTFSI). Additionally, we demonstrate the tunability of the pi-pi interaction modes by doping small molecule additives into the polymer, which can modulate the interaction strength and crosslinking density in the system. The self-healing temperature and Young’s modulus are tunable in a wide range (30-60 o C, 69-219 MPa) to satisfy different potential applications, such as artificial skin and wearable batteries. We demonstrate our polymer possesses decent stretchability (100% strain without cracking), flexibility, and ability to heal cracking at human body temperature, which shows application potential for wearable electronics. Compared to softer self-healing polymers reported previously, our system not only transcends the currently-available regime of mechanical properties in self-healing systems, but also provides a general strategy for tuning self-healing and mechanical properties in supramolecular polymers.
High-capacity battery technology has become a dire need in an increasingly energy-hungry world, especially if mass transportation without fossil fuels is to be realized. Lithium-sulfur batteries are an intriguing solution to this need, stemming from their theoretical energy density 5x that of current lithium-ion systems, as well as the relative abundance of their active materials. However, commercialization of lithium-sulfur batteries has yet to be effectively realized due to a multitude of intrinsic challenges with this chemistry, ranging from dendrite growth on the lithium metal anode to dissolution of polysulfide intermediates in the electrolyte, resulting in capacity loss, redox shuttling, and eventual cell failure. Ionic liquid electrolytes have recently emerged as a popular method for addressing these challenges, since they generally have a far lower solubility for lithium polysulfides than organic electrolytes, and also tend to suppress lithium dendrite growth, while still maintaining relatively high ionic conductivity. So-called solvate ionic liquids (SILs), based on stoichiometric complexes of tetraglyme with lithium salts, have proven particularly effective at addressing lithium-sulfur performance challenges, since the transference number of Li + in these solvents tends to be 0.5 or higher, unlike ternary ionic liquids containing organic cations in addition to Li + . SILs can also be diluted with low-basicity solvents in order to modulate various properties without changing their ionic-liquid-like character. Combining organic electrolytes, or even ionic liquids, with solid particles or polymers to form free-standing gel electrolytes has also proven popular for improving lithium-sulfur performance. Immobilized electrolytes are not only useful to prevent leakage in larger cells, but can also produce effects such as increased lithium dendrite resistance, trapping or blocking of dissolved lithium polysulfides, and elastic buffering of mechanical stress from electrode volume change during operation. However, introduction of a solid matrix almost always lowers ionic conductivity significantly, which negatively affects performance at moderate or high current density. In this work, we demonstrate ionogel electrolytes based on SILs and polymerizable methacrylate groups with varying functionality. This materials platform allows us to achieve very high conductivity for an ionogel (>1 mS/cm), while also addressing the unique challenges of lithium-sulfur chemistry through rational molecular design of polymer structure. Our strategy allows us to adjust electrolyte properties such as lithium transference number and polysulfide affinity for optimum battery performance, while requiring no elaborate fabrication steps or costly chemical reagents. We discuss both the properties of, and rationale behind, initial gel designs, as well as the implications for lithium-sulfur battery performance and creative designs we plan to pursue in the near future. Figure 1
Vehicle electrification is a critical application of lithium-ion batteries (LIBs), and it is essential to develop LIBs that can operate at sub-ambient temperatures with satisfying performance. Conventional LIBs have performance deficits at low temperatures which hinder their use in extreme environments. One approach to address this problem is to rationally engineer the electrode/electrolyte interface with electrolyte additives to improve the electrochemical kinetics at sub-ambient temperatures. In this work, silicic acid (SiAc) is incorporated into standard LIB electrolyte as an additive to enhance the capacity and energy density of LIBs at temperatures down to −20 °C. Full-cell impedance analysis and X-ray photoelectron spectroscopy of cycled electrodes point towards an additive-induced change in surface chemistry which alters the charge transfer process. It is proposed that the SiAc additive participated in the formation of solid electrolyte interphase (SEI) and lowered the activation energy of the interface impedance, assisting lithium ion transport across the interface at lower temperatures.
Recent developments in silicon anode binders derived from various biomass sources, with a focus on polymer properties and their effect on battery performance.
Freestanding gel electrolytes based on Li(G4)TFSI/PEG are demonstrated with enhanced lithium transport and stripping/plating performance due to unique chemical interactions.
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