Rechargeable Zinc Batteries and Capacitors via Electrodes and Electrolyte Engineering
Creators: Xu, Zhixiao
Publisher: name: University of Alberta Library
Year: 2023

<p>With advantages of high capacity, water compatibility, suitable redox potential, high safety, and low cost, Zn-based aqueous energy storage devices are promising for grid-level energy storage. However, Zn metal reversibility is challenged with dendritic growth, low Coulombic efficiency (CE), metal corrosion, and hydrogen evolution. That is why excessive amount of Zn has often been used for cells, which however also reduces cell-level energy densities. Besides challenges to the anode, cathode materials suffer from metal dissolution and poor conductivity in batteries or limited capacitance in capacitors. Overall, achieving high depth of discharge (DOD) in Zn, low negative-to-positive (N/P) ratios, high loading cathodes, lean electrolyte, and wide-temperature operation is essential for the commercialization of rechargeable Zn batteries and capacitors. However, the development of these devices is still in the early stages. Furthermore, sustainable development requires recycling/upcycling of spent zinc batteries. To that end, this thesis aims to achieve practical Zn batteries and supercapacitors through electrode, electrolyte, and binder engineering, including i) making of ultrafast, long-life, high-loading, and wide-temperature zinc metal capacitors, ii) construction of a 3D hierarchical zincophilic carbon host for efficient zinc plating/stripping, iii) design of ultrafast, durable, and high-loading Zn-metal-free organic anodes, and iv) upcycling of primary alkaline batteries into secondary Zn-MnO2 batteries. <br>
The first work described in this thesis focuses on optimizing the electrolyte and binder towards high-performance zinc metal capacitors. As the benchmark, commercial activated carbon with hierarchical pores and large surface area was rationally selected as the cathode material. An aqueous binder enhances electrode-electrolyte wettability enabling high-mass-loading electrode, and concentrated electrolytes give high Zn stripping/plating efficiency, high ionic conductivity as well as suppressed hydrogen bonding interaction in water realizing ultralow freezing temperature. The optimal combination unlocks unprecedented zinc metal capacitors with a large capacitance of 436 F g−1, ultrahigh rate up to 200 Ag−1, ultralong cycles (0.3 million), ultrahigh loadings (10 mg cm−2) in lean electrolyte (8.8 µL mg−1), and wide-temperature operation (-60 to 60 °C). <br>
To further increase cell-level energy densities, a 3D carbon host was constructed to regulate highly efficient Zn plating/stripping under a high DOD. Through theoretical calculation, monomer selection, polymer assembly, and carbon fabrication, an oxygen- and nitrogen-codoped carbon superstructure was synthesized as an efficient host for high-DOD Zn metal anodes. With microscale 3D hierarchical structures, microcrystalline graphitic layers, and zincophilic heteroatom dopants, a flower-shaped carbon (Cflower) host could guide Zn nucleation and growth in a heteroepitaxial mode, affording horizontal plating with high CE and long life. As a demonstration, the Cflower-hosted Zn anode was paired with both battery and supercapacitor cathodes and delivered high performance, outclassing hostless Zn-based devices. <br>
It is still challenging to achieve high rates and ultra-long cycles in Cflower-hosted Zn metal anodes. Alternatively, an ultrafast, stable, and high-loading polymer anode was developed for aqueous Zn-ion batteries and capacitors (ZIBs and ZICs) by engineering both the electrode and electrolyte. The anode polymer was rationally prepared to have a suitable electronic structure and a large π-conjugated structure, whereas the electrolyte was manufactured based on the superiority of triflate anions over sulfate anions, as analyzed and confirmed via experiments and simulations. This dual engineering results in an optimal polymer anode with a low discharge potential, near-theoretical capacity, ultrahigh-loading capability (≈50 mg cm−2), ultrafast rate (100 A g−1), and ultralong lifespan (one million cycles). When the polymer anode is coupled with cathodes for both ZIB and ZIC applications, corresponding devices demonstrate ultrahigh power densities and ultralong lifespans, surpassing those of Zn-metal-based devices.<br>
To achieve sustainable development, spent alkaline batteries were upcycled into rechargeable Zn metal batteries through a simple thermal treatment of electrode waste. The regenerated Zn powder anode showed super-zincophilicity and low overpotentials even under fast rates (8 mA cm−2) and high DOD (50 %), which can be ascribed to coating of hydroxyl-rich organic layer with abundant nucleation sites as well as high orientation of favorable (002) plane and induced horizontal plating behavior. The regenerated cathode composed of MnO2 and MnOOH had enhanced capacity in comparison to pristine ones. Under a low N/P capacity ratio of 3.8 and high loading of ~10 mg cm−2, the regenerated electrodes were paired to fabricate zinc metal batteries which demonstrate high energy and power densities (94 Wh kg−1, 1349 W kg−1), holding potential for practical applications. </p>