Flexible sodium-ion based energy storage devices: Recent progress and challenges

Flexible energy storage devices are gaining considerable attentions due to their great potentials in the emerging flexible electronics market, ranging from roll-up displays, bendable mobile phones, conformable health-monitoring skin sensors to implantable medical devices. The development of reliable and flexible electrodes with low cost, high performance, superior stability and safety remains a great challenge to meet the requirements for many practical applications. On the other hand, structural design can also enhance their flexibility and electrochemical performance. On account of the low cost and easily accessible sodium resources, in the present review we mainly focus on recent progress in flexible energy storage devices with sodium-ions as the charge carriers. We begin with a brief introduction of flexible energy storage systems and the current development of flexible sodium-ion based energy storage devices. Then, a more detailed overview of recent progress on pliable materials in flexible sodium-ion batteries (SIBs) is presented, including carbon-based substrates materials (e.g., graphene, carbon cloths, carbon nanofibers (CNFs) and carbon nanotubes (CNTs)), metal-based materials (e.g., stainless steel, copper (Cu) and titanium (Ti) foils), other flexible substrate materials and electrolytes. This is followed by detailed discussion of the advancements in other flexible analogous sodium-based energy systems, such as the fiber-shaped flexible SIBs, sodium-ion capacitors (SICs), Na-S batteries and Na-Se batteries. Finally, future prospects and challenges are examined and possible solutions are brought up to inspire the future flexible sodium-based energy storage devices.