Functional Hydrogel Electrolytes for Aqueous Zinc-manganese Oxide Batteries

水系鋅錳電池的功能性水凝膠電解質研究

Student thesis: Doctoral Thesis

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Award date26 May 2020

Abstract

Owing to their intrinsic safety, flexibility, and cost effectiveness, hydrogel electrolytes are attracting immense attention in the field of flexible and wearable batteries. The traditional hydrogel electrolytes are dominantly based on poly(acrylamide) (PAM), polyacrylic acid, and polyvinyl alcohol matrices; sodium polyacrylate gelatin, etc. However, these hydrogel electrolytes only provide physical frameworks to support the ion transport, which could not provide considerable improvements in electrochemical performance and additional functions to satisfy the needs of diverse practical scenarios. The development of polymeric hydrogel electrolytes is still at the primary stage, and the number of available optional approaches is limited. In addition, the abundant and tunable chemistries of the hydrogels allow the introduction of novel functionalities into existing hydrogels through the engineering of polymer chains by various methods, such as modifying chain chemistries grafting functional groups, or blending polymers with different materials, thus making it is possible to fabricate unprecedented flexible batteries with additional functions. As a result, it is desirable to develop functional flexible batteries using appropriate hydrogel electrolyte materials with specific properties.

Considering the safety and the biocompatibility issues of flexible and wearable battery design, rechargeable aqueous zinc manganese dioxide batteries (Zn-MnO2 batteries ) are particularly advantageous by virtue of their inherent safety, negligible toxicity, and low flammability compared with traditional lithium ion batteries . However, one challenge identified in this field of aqueous batteries is how to surmount the freezing phenomena of these water based hydrogel electrolytes as well as the maintenance of ionic conductivity and mechanical properties while working below water freezing temperature. To address these issues, we first report an intrinsically anti-freezing Zn-MnO2 battery (AF-battery) comprising a designed freeze resistant hydrogel electrolyte which can preclude the ice crystallization of the hydrogel component and maintain a high ion conductivity even at -20 oC. Benefiting from exceptional freeze resistance, the fabricated flexible AF-battery exhibits excellent electrochemical stability and mechanical durability at subzero temperatures. Even at -20 oC , the specific capacity of the AF-battery can retain over 80% with Coulombic efficiencies approaching 100%. Moreover , the flexibility of batteries can also be well maintained even under severe mechanical stresses at subzero temperatures, such as being bent, compressed, hammered or washed in an ice bath. Thus, the creation of freeze resistant hydrogel electrolyte is testified feasible, and a designed flexible anti-freezing aqueous batteries working in extremely cold environments is thus realized.

Apart from freezing problem arised from cold temperature, aqueous batteries also suffer from the following notable defects. For instance, water molecules inevitably evaporate from hydrogels under ambient conditions, and high temperature accelerates the dehydration process, leading to deterioration of electrochemical performance. While operating under water, the hydrogel electrolyte absorbs water and swells, resulting in the loss of adhesion between electrodes and electrolyte. The exchange of solutes causes the decrease of ion concentration and depresses the device performance. These environmental effects fundamentally limit the long term stability and utilization of aqueous flexible batteries under severe conditions. Being inspired by epidermal tissue of mammalian skin, we present a biomimetic organohydrogel (BM-gel) electrolyte with extreme temperature tolerance and long term moisture lock in property. The BM-gel is synthesized in an ethylene glycol/water solvent system with a chemically elastomeric coating on the surface, which exhibits high ionic conductivity when containing different ions, such as Zn2+, Li+, H+ and Na+ ions. A rechargeable Zn-MnO2 battery is constructed with the BM-gel electrolyte, which exhibits excellent electrochemical performance over the temperature range from -20 oC to 80 oC. The specific capacity retains over 70% and Coulombic efficiencies approach ~100% in wide temperature scale. Even after a prolonged storage of 30 days without encapsulation, 84.6% capacity is retained benefiting from the superior anti-dehydration property bestowed by the thin elastomer coating.

Finally, in order to develop a new kind of hydrogel electrolyte that can facilitate the battery performance, we synthesize a zwitterionic sulfobetaine/cellulose hydrogel electrolyte using raw materials from natural plants. The intrinsic zwitterionic groups on sulfobetaine chains can provide separated ion migration channels for positive and negative ions, which largely facilitates the electrolyte ion transport. A solid-state Zn-MnO2 battery with the fabricated zwitterionic gel electrolyte exhibits a very high rate performance. Even up to 30 C, a high capacity of 74 mA h g-1 MnO2 is maintained during the charging-discharging for up to 10000 cycles. For wearable applications, the flexible solid-state batteries can be used as reliable and portable sources to power different wearable electronics, which shows their large potentials for use in next-generation flexible and wearable battery technologies.

In summary, various functional hydrogel electrolytes with anti-freezing, anti-dehydration, waterproof properties, superior ionic conductivity and exceptional mechanical durability are demonstrated in this thesis. The design principle and working mechanism have also been comprehensively studied, targeting at developing high performance of flexible aqueous Zn-MnO2 batteries. It is believed that these studies presented in the thesis can lay a solid foundation for practical applications of flexible and wearable batteries.