Over the past two decades, lithium ion battery chemistries have enabled the practical development of many new products and technologies. Today, rechargeable Li-ion batteries are often the primary means of providing electrical power to a diverse and growing number of devices, from mobile phones to electric vehicles. Despite many advances and widespread adoption, Li-ion battery technologies have limitations. Battery chemistries based on lithium ions have a high theoretical gravimetric capacity of 3829mAh/cm3 only a modest volumetric capacity of 2044mAh/cm3 . Volumetric capacity is anticipated to be especially important in IoT devices and wearables, where thin, flexible batteries which can cover large areas are ideal. By comparison, divalent batteries based on zinc or magnesium ions have theoretical volumetric capacities of 5854mAh/cm3 and 3882mAh/cm3 respectively . In addition to somewhat a somewhat modest volumetric capacity, lithium is far less common in the earth’s crust than magnesium or zinc and possesses higher reactivity . Because of this, lithium-ion batteries are anticipated to be less environmentally friendly and cost effective than divalent metal-ion batteries for applications requiring many large battery cells. It should also be noted that over the past year, the safety of lithium-ion batteries in consumer products has been called into question after a high-profile recall involving lithium-ion batteries in smartphones.
One means of addressing the shortcomings of lithium ion-battery technologies is to develop new battery chemistries based on divalent metal ions such as magnesium. Divalent metal-ion batteries could be made of flexible materials that allow for safe operation in the event of mechanical damage. Here we present an experimental magnesium ion half-cell constructed from flexible materials. A magnesium-ion cell was chosen due to its low material cost, good theoretical volumetric capacity, simple fabrication steps, and separator-free reaction chemistry. Flexible, insertion-type anodes and cathodes were fabricated using bismuth nanotubes and tungsten disulfide respectively. A polymer-based electrolyte made of PVDF-HFP and magnesium perchlorate was chosen for its demonstrated high ionic conductivity and mechanical flexibility. Each interface of the half-cell was characterized though the use of cyclic voltammetry and Raman spectroscopy, while a complete cell was examined using a commercial battery tester. Cell fabrication, component/interface electrochemistry, electrode materials, and overall cell performance will be described in detail.
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