| dc.description.abstract | Rapid deployment of renewable energy sources around the world is crucial to lessen energy shortage and environmental pollution. As the contribution of renewable energy rises, energy storage with different battery systems is an increasingly important solution to balance generation and loads of energy due to the intermittent nature of sustainable energy sources. Among modern battery systems, lithium-ion (Li-ion) batteries have a combination of high energy density and power density with a wide range of applications from portable electronic devices to large stationary storage systems. However, safety concerns, environmental threats, and costly manufacturing processes stemming from the implementation of toxic and flammable organic electrolytes are the key public arguments against their widespread application. Accordingly, aqueous rechargeable zinc Li-ion batteries (ARZLBs) were proposed and developed as an alternative safe, environmentally benign, and economically viable technology for electrical energy storage. Nevertheless, an ARZLB with high energy density and sufficient cycle life has not yet been introduced to the market. The limited energy density of this type of battery is mainly due to the inferior areal capacity corresponding to a low ratio of active material to inactive components in the battery. Moreover, the zinc dendrite formation on the anode surface and hydrogen evolution-induced corrosion reaction in the aqueous electrolyte restrict the cycle life of the battery. In an effort to fill this research gap, this thesis intends to ⅰ) introduce a flexible thick cathode design with superb areal capacity and active material loading for zinc/LiFePO4 battery, ⅱ) propose a novel conductive thick cathode architecture with unprecedented active material loading and areal capacity for zinc/LiMn2O4 battery, ⅲ) design a porous zinc anode with high surface area to suppress the zinc dendrite growth and prolong the cycle life of high areal-capacity zinc/LiMn2O4 battery, and ⅳ) develop a gel-type electrolyte with the ability to mitigate the hydrogen evolution-induced corrosion and further extend the cycle life.
This work begins with enhancing the areal capacity of the zinc/LiFePO4 battery. To achieve a high areal-capacity battery, the electrode was designed with a thick, flexible, and binder-less architecture called a dough-like cathode. In this innovative method, organic solvent processing is replaced by aqueous processing and thus the electrode fabrication is more straightforward, faster, cheaper, and safer than conventional electrodes. The dough-like cathodes are fabricated up to an extremely high thickness of 1000 μm with decent consistency and without any crack formation. Moreover, the electrolyte-based and binder-less electrode structure lowers the tortuosity and elevates the electronic/ionic conductivity of the cathode, leading to a near-theoretical specific capacity performance. As a result of this novel electrode design, the areal capacity and active material loading of the zinc/LiFePO4 battery manufactured with thick dough-like cathodes reach extremely high values of 10.5 mAh cm-2 and 74.5 mg cm-2, respectively. Consequently, the energy density of the zinc/LiFePO4 battery with a 700 μm thick dough-like cathode is 190% higher than that of the battery with a 100 μm conventional cathode design. Furthermore, the battery assembled with the dough-like cathode maintains 80% of its capacity compared to only 50% with the conventional cathode after 200 cycles and at the same cathode thickness.
To further improve the energy density, LiFePO4 cathode active material was replaced by LiMn2O4 that has a higher electrochemical potential. Therefore, in the next step, a newly thick cathode architecture was redesigned for LiMn2O4 cathode with three-dimensional (3D) conductive networks of carbon nanotubes (CNTs). This unique electrode structure allows the manufacturing of crack-free and reinforced thick cathodes up to 900 μm with high structural stability during battery cycling. Similar to the dough-like cathode, eco-friendly and cost-effective aqueous processing is employed for electrode manufacturing. The zinc/LiMn2O4 batteries assembled with 3D-CNT cathodes manifest an unprecedented areal capacity of 13.5 mAh cm-2, which is at least 10 times greater than that of the battery with the conventional cathode design. More importantly, the energy density of the zinc/LiMn2O4 battery with a thick 3D-CNT cathode is 140% higher than the energy density of the zinc/LiFePO4 battery with a thick dough-like cathode at the same electrode thickness.
To extend the lifespan of the zinc/LiMn2O4 battery with the thick 3D-CNT cathode over 100 cycles, a novel porous zinc paste composite (PZPC) is proposed. The PZPC provides a high surface area for zinc deposition, facilitates the homogenous current distribution, and improves the corrosion resistance of the electrode. Hence, the cycle life of the zinc/LiMn2O4 battery with the PZPC anode and a thick 3D-CNT cathode is prolonged by as much as 5 times the cycle life of the battery with the conventional zinc anode. Moreover, the PZPC promotes the rate capability and voltage efficiency of the battery due to the conductive porous structure and high electrode-electrolyte contact area.
Finally, to further extend the lifespan of the high areal-capacity zinc/LiMn2O4 battery, a low-cost gel electrolyte was developed by using fumed silica thickening agent inside the aqueous electrolyte to mitigate the destructive effect of hydrogen evolution-induced corrosion. The fumed silica gel electrolyte immobilizes the water molecules through hydrogen bonding and thus reduces the corrosion current of the zinc by at least 50% of its value in the aqueous electrolyte. Furthermore, the fumed silica gel electrolyte improves the reversibility of redox reactions on both cathode and anode sides. Consequently, the cycle life of zinc/LiMn2O4 battery with a thick 3D-CNT cathode, fumed silica gel electrolyte, and PZPC anode is extended to 600 cycles.
Taken together, these findings suggest the ability of thick 3D-CNT cathode, PZPC anode, and fumed silica gel electrolyte in promoting the areal capacity and durability of ARZLBs. The high areal-capacity and durable zinc/LiMn2O4 battery enable the manufacturing of high energy density and cost-effective ARZLBs that can compete with state-of-the-art aqueous batteries. | en |
