Development of Flexible Ionic Liquid-based Ionic Conductors for Energy Harvesting and Sensing


Student thesis: Doctoral Thesis

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Award date13 Sept 2023


The fast development of advanced electronics pursues the evolution of intelligent ionic conductors. Ionic liquid (IL), molten salt at or near room temperature which composed of organic cations and organic/inorganic anions, is rising a dazzling star material to construct flexible ionic conductors due to its attractive properties including high thermal stability, negligible vapor pressure, nonflammability, high ionic conductivity, and easy recovery. Although the design feasibility enables the development of various flexible IL-based ionic conductors such as hydrogels, ionogels, and liquid-free ionic conductors, they still suffer from the limitation of the trade-off between the ionic conductivity and mechanical properties (e.g., stretchability, elasticity, strength, and modulus). In addition, the difficulty also lies in realization of the selective ion transport within such flexible ionic conductors for specific applications such as sensing and energy harvesting. In this research, I prepared a series of flexible ionic conductors based on intrinsically ionic conductive IL and poly(ionic liquid) (PIL) which showed promising properties including high stretchability, high mechanical strength, high toughness, high ionic conductivity, water-responsiveness, and selective ion transport. Specifically, there are three parts in this thesis.

In the first part, a microphase-separated bicontinuous polyvinylidene fluoride copolymer-based ionogels was fabricated which can simultaneously achieve skin-like mechanical properties and self-powered sensing ability. By tailoring the bicontinuous structure, the ionogel exhibited high elasticity, ultrastretchability, high toughness, and a Young’s modulus similar to that of human skin. The ionogel-based ionic skin (I-skin) can respond to a wide range of strains with high sensitivity as resistive sensor and piezoionic sensor. Importantly, the I-skin shows a highly reproducible electrical response over 1000 uninterrupted cycles. The strain sensing of the I-skin is driven by the selective ion transport induced by the different cationic and anionic mobility which is mediated by ion-polymer interactions.

In the second part, a series of liquid-free PILs with reversible and dynamic hydration/dehydration property were fabricated by rational design of anions. Both experimental studies and theoretical simulations revealed that there is numerous ion transport nanochannel within PIL membranes induced by cascade and reversible hydrogen-bonding during the hydration/dehydration process. The PIL membrane shows anion selectivity due to the abundant positive surface charges. Moreover, the membrane also demonstrated an adaptive ion transport behavior in different liquid environments. When integrates with an osmotic energy generator, the PIL membrane yields a high-power density of 24.5 W/m2 under a 500-fold salinity gradient, superior to those of start-of-art membranes.

In the third part, mechanically strong cellulose supramolecule gel polymer electrolyte (CSE GPE) was fabricated for high-performance lithium metal battery. Specifically, cellulose nanofibrils (CNFs) are assembled with PIL through electrostatic coacervation between negatively charged CNFs and positively charged PIL, which offers numerous ion transport channels. Consequently, the relatively fast lithium-ion transport within the CSE GPE enabled a high ionic conductivity of 0.65 mS cm-1 and a high lithium transference number of 0.7. Moreover, benefiting from the robust CNFs network and layered structure, CSE GPE showed high mechanical strength of 39 MPa, which is favorable for suppressing the lithium dendrite to enhance the safety of the battery. With high ionic conductivity and interfacial stability, batteries using CSE GPE displayed a long cycle life of 700 cycles and high-rate performance at 5 C. Besides, flexible and safety pouch cells are also demonstrated to harvest the advantages of CSE GPE.

Overall, I prepared various flexible IL-based ionic conductors which show desired mechanical properties, ionic conductivity, and intelligent responsive characteristics. The ionic conductors can achieve selective ion transport which enables their applications in sensing, osmotic energy harvesting and lithium metal battery.