Beyond flexible-Li-ion battery systems for soft electronics

Wearable devices, one of the most innovative technological breakthroughs of the consumer electronics sector, promises to develop even more in the near future. Therefore, the research focus has shifted to developing corresponding robust energy storage sources for these devices, the seamless integration of these storage sources and their ability to perform well even under persistent dynamic mechanical stresses. The Li-ion rechargeable battery system has been the gold standard so far for energy storage, owing to its excellent energy and power densities and being an already mature technology. However, Li based devices have drawbacks also: toxicity, flammability, environmental issues, and limited natural abundance tend to offset the said advantages and so alternatives must be studied. It is here that non-Li chemistries, such as Na, K, Ca, Al, Mg, Zn-ion based battery systems can play a leading role due to their earth abundance, advantageous economics and low toxicity. This article, therefore, provides a strong rationale for the implementation of these non-Li systems for energy storage in flexible systems, also giving a brief techno-economic analysis. The manuscript also details the electrode and battery design state-of-the-art, followed by an in-depth review of the notable works for each of these systems, including the type of electrodes used and their novel synthesis procedure, electrochemical performances and other critical aspects regarding their optimization. A brief discussion at the end puts into perspective some of the critical areas needing further improvement.