|Abstract:||Next generation applications such as wearable electronics, roll-up displays, tactile communications, and advanced energy storage require solid electrolytes that are flexible, highly conductive, and stable (mechanically, electrochemically, and environmentally) in their operating environments. However, the current challenge of the field is to achieve solid electrolytes with high stability and "liquid-like" ionic conductivity. In this dissertation, two model systems of network ionic polymers (nIPs) are carefully designed and investigated on how molecular structures affect their functional properties such as ionic conductivity, glass transition, and modulus. A highly conductive yet self-standing solid electrolyte was also designed to serve as the electrolyte material for dopant-free soft actuators and high temperature and voltage stable solid-state supercapacitors.
The ammonium nIPs were synthesized by step-growth polymerization between trifunctional amine and dibromo species to form networks with ammonium cation fixed on the polymer backbone. The Br- anions were later exchanged to bis(trifluoromethanesulfonyl)imide (TFSI) and BF4 mobile anions. In this system, we have systematically demonstrated that the extent of ion exchange has a significant impact on the conductivity and thermal stability of the final material, with minimal changes in the glass transition temperature (Tg). In addition, when the linker of the ammonium nIPs is varied precisely, we observe an odd-even effect in the Tg; specifically, the Tgs follow a zig-zag trend as the linker switches between odd and even linker lengths. This effect causes the room temperature conductivity of these nIPs to vary by 1 to 2 orders of magnitude.
The second model system consists of fixed TFSI like anion and Li+ mobile cation. Network structures such as crosslinking density, polymer backbone, and crosslinker length have a significant impact on the polymer dynamic (represented by Tg) and further affect the ionic conductivity of the Li+ conducting nIPs. At low crosslinking density (<8%), decoupling was observed where the modulus is increased by 8 times without affecting the ionic conductivity as the network Tgs stabilize at low crosslinking density. Other molecular parameters such as ion concentration and coordination environment are also shown to play a role in the ion conduction capability through more complex pathways.
To improve the existing ionic polymer soft actuator and demonstrate the unique advantage of nIP electrolyte, an imidazolium-based nIP with TFSI mobile anion was designed and synthesized. This class of nIP electrolyte is self-standing with a modulus at the MPa range and conductivities comparable to linear low Tg ionic polymers (10-5 S/cm). When integrated with single-wall carbon nanotube (SWCNT) electrodes, a dopant-free soft actuator was fabricated with bending strain (0.9%) comparable to current state-of-the-art small molecule doped systems. Compared to the traditional salt solution doped ionic polymer-metal composite actuator, prolonged cyclability was observed due to the high electrochemical stability (3V) and no leakage of the electrolyte materials. Polar ethylene oxide polymer matrix and lower crosslinking density were shown to increase the conductivity and lower the modulus of the electrolyte, which ultimately contributed to the high performance of the device.
When the flexible electrolyte was combined with flexible reduced graphene oxide (r-GO) polymer composite electrodes, a flexible, solid-state supercapacitor was fabricated. The high electrochemical and thermal stability of the nIP allowed the device to be charged to 3V at temperatures as high as 120 ˚C. These operating conditions are not possible with traditional salt solution electrolytes. The device shows an excellent specific capacitance of 300 F/g with respect to the mass of active materials. Furthermore, due to the mechanical flexibility of all the components, the device sustains performance when deformed to 180 degrees.