A Metal-Free Quasi-Solid-State Battery for Better Safety and Sustainability

Solid-state rechargeable batteries promise high energy, low cost, and improved safety/stability. Hence, they are considered as the new-generation battery technology for electric vehicles and grid-scale electricity storage, and expected to meet other critical needs for more compact and higher-capacity energy storage devices. On another note, current commercial batteries are mainly metal based, with metal elements in charge carriers and/or electrode materials, which poses potential economic and environmental concerns due to the heavy use of nonrenewable metals. Thus, metal-free batteries provide a more sustainable and environmental-friendly alternative to these batteries, though the relevant research is still in its infancy. Recently, rechargeable batteries based on aqueous electrolytes have shown high potential attributed to their low cost and intrinsic safety. An appealing choice is an aqueous battery using non-metal ions as charge carriers, such as ammonium ion with a lighter molecular weight (18 g/mol) and a small hydrated ionic size of 3.31 Å, leading to faster ion diffusion in the electrolyte. Furthermore, by combining with organic electrodes, nometal ammonium-ion charge carreries make it possible to realize metal-free batteries that offer affordable, eco-friendly, and sustainable alternatives to current metal-based batteries.<br/>In this work, a novel quasi-solid-state metal-free battery is assembled using a hydrogel electrolyte sandwiched between polypyrrole cathode and polyaniline anode. A hydrogel is a hydrophilic polymeric network containing large amounts of water. Herein, the quasi-solid-state hydrogel electrolyte is simply synthesized using ammonium sulfate, water, and xanthan gum that is edible and biodegradable. Concentrated salts are used to decrease the amount of free water molecules in the hydrogel electrolyte, and thus reduce hydrolysis and side reactions from water. Both the salt concentration and the polymer content in the hydrogel can be facilely tuned and optimized, to minimize side reactions and maximize ionic conductivity and mechanical strength of the electrolyte, as monitored by XRD, SEM, XPS, Raman spectroscopy, electrochemical characterizations, impedance measurements, tensile and adhesion tests, leading to superior performance of the battery. As such, this study presents the first quasi-solid-state non-metal battery with lower cost and better performance, which opens the door to future metal-free electronics that would generate long-term benefits to environment.