Green and sustainable metal-air batteries for powering flexible wearable electronics: current status and future prospects

The use of eco-unfriendly materials in wearable electronic devices poses a serious threat to the environment. It is therefore crucial to develop flexible, wearable devices that are environmentally friendly and safe for human skin. To power such devices, miniaturized power sources are needed, such as metal-air batteries (MABs) that have excellent power density and longevity. However, traditional MABs are built on rigid, non-flexible platforms and use a large amount of electrolyte, which is not sustainable or suitable for flexible electronics. Green wearable MABs can be created by using biocompatible and biodegradable battery components, such as electrolytes, electrodes, and flexible platforms, or by reducing the electrolyte volume. In this review, we critically examine a range of cost-effective, downsized, green, and sustainable wearable MABs that use non-toxic and abundant natural materials and can withstand bending, twisting, stretching, and folding. The various components, their green and sustainable aspects and the synthesis approaches of the key air-cathode have been demonstrated in detail. The electrochemical performance of various green MABs, as well as the obstacles to their commercialization has also been discussed. Naturally occurring materials can serve as green alternatives to synthesize and fabricate green wearable metal-air batteries.