Polyacrylamide-based Hydrogel Electrolyte for Zinc-ion Batteries and Capacitors
Creators: Ma, Rujiao
Publisher: name: University of Alberta Library
Year: 2022

<p>Aqueous Zn-ion batteries (ZIBs) and Zn-ion hybrid capacitors (ZICs) have recently attracted tremendous research interest due to promising electrochemical performances, mild assembly conditions, intrinsic safety, and low-cost potential for grid-level energy storage. However, both ZIBs and ZICs are challenged with issues such as dendrite growth and side reactions on the anode side and metal dissolution and low capacity on the cathode side. While anode issues can jeopardize the cycling stability and cycle life, cathode issues lower the cell-level energy density. As such, it is urgently needed to address those issues and levitate its competitiveness among other energy storage devices. The use of hydrogel electrolytes, e.g., polyacrylamide (PAM), can tackle the above problems due to their uniform and controllable 3D polymer network for ion homogenizing, proper water absorption for ion transferring, superior adhesiveness for electrolyte-electrode interface stability, and high strength for mechanically resisting dendrites.<br>
Chapter 1 is a literature review on hydrogels for functional ZIBs, in which backgrounds of ZIBs and hydrogel electrolytes were introduced, followed by a summary of different hydrogel electrolytes applied in ZIBs, including working principles behind their suppression on dendrite growth and side reactions. Moreover, hydrogel electrolytes have shown great potential for wearable and environmental-adaptive ZIBs given their good flexibility, elasticity, and high mechanical strength. As such, more hydrogel electrolytes were introduced in the literature review based on the colorful functions they bring to ZIBs, including self-healing behavior, super toughness, tailorability, anti-freezing property, and thermo-protection behavior, etc. Also, the mechanisms behind those functions, summary, and perspectives were detailly described. <br>
In Chapter 2, hydrogel electrolytes were demonstrated to endow ZICs with decent electrochemical performance and high flexibility. Activated carbon was used as the cathode for the assembly of ZICs (Zn//AC supercapacitors). Single-network PAM hydrogel electrolyte was firstly prepared and optimized for ZICs. Then, alginate was introduced to this hydrogel electrolyte to make Alginate/PAM hydrogel electrolyte with double-crosslinked network and further improve the performance of ZICs. FTIR spectra structurally confirmed these hydrogels, and their water-absorbing ability and stretchability were tested. Zn//AC supercapacitors based on both single PAM and Zn-Alginate/PAM hydrogel electrolyte showed stable cycling life of over 5000 cycles for electrochemical performances. More importantly, hydrogels also endowed ZICs with high flexibility while maintaining good performance. The assembled flexible soft-packaged Zn//AC supercapacitor based on Zn-Alginate/PAM hydrogel electrolyte exhibited a high specific capacity of 194 mAh g-1 at 0.1 A g-1 and stable cycling performance with 77.0 % capacity retention after 1900 cycles at 1.0 A g-1. Moreover, good flexibility is demonstrated as only 5 % capacity loss under different bending angles and long resting stability of low self-discharge rate of 9.45 mV h-1.<br>
In Chapter 3, hydrogel electrolytes were further applied in ZIBs to demonstrate the capability to inhibit metal dissolution and improve cycling performance. Rather than using Zn as the anode, molybdenum oxide with hexagonal structure (h-MoO3) was synthesized, characterized, and applied as zinc-free anode materials for ZIBs. As a result, the involved battery in aqueous 3 M ZnSO4 electrolytes shows high capacities of 128 mAh g-1 and 107 mAh g-1 at 0.1 A g-1 in Zn//h-MoO3 half-cell test and h-MoO3//ZnMn2O4/Carbon (ZMC) full cell test, respectively. However, its cyclability was poor, in which after 150 cycles zero capacity is maintained for Zn//h-MoO3 half-cell and only 43 cycles for h-MoO3//ZMC full cell. By contrast, using Zn-Alginate/PAM hydrogel electrolyte containing 3 M ZnSO4 primarily enhanced cycling performances of both Zn//h-MoO3 half-cell (63.6 % after 220 cycles) and h-MoO3//ZMC full cell (91.2 % after 200 cycles) were achieved. At the same time, high capacities of 129 mAh g-1 and 102 mAh g-1 were maintained in both half-cell and full-cell, further confirming the advantage of hydrogel electrolytes for elongating cycle life in rocking-chair type ZIBs.<br>
Chapter 4 briefly summarizes recent research progress and experimental results in previous chapters, as well as the challenges and future perspectives toward hydrogel electrolytes.</p>