Flexible batteries to power our flexible electronics

Dr Evangelia Founta, Research Fellow in Flexible Electronics, University of Southampton

Dr Founta gave a fascinating talk on  how flexible devices can be used in in consumer electronics like smart watches, and in wearables such as everyday healthcare sensors, to monitor our health and habits. These devices may also be used in implantable electronics used for diagnostics.

Batteries were first developed by Alessandro Volta in 1800. They now fall into two groups: those that cannot be recharged (primary) and those that are rechargeable (secondary). The most well-known rechargeable battery is lith1um-ion which was invented between 1975 – 1985, and first commercialised by Sony in 1991. It consists of a sandwich with the layers being (a) aluminium metal (b) lithium cobalt oxide (c) separator membrane and electrolyte (d) graphite and (e) copper metal.  During charging the lithium ions flow from positive to negative through the membrane, and during discharge they flow in the opposite direction. The electricity is produced by the electrons flowing in the opposite direction through the electrical circuit.

Batteries are based on redux reactions – a combination of reduction and oxidation. Redux reactions are also seen in photosynthesis, and when iron rusts into iron oxide.  While lithium-ion batteries can survive multiple charge/discharge cycles there are tips to improving their life: a) don’t stress it too much with 0 to 100% cycles – charging between 20% and 80% is better, (b) don’t subject them to extreme heat or cold and (c) don’t store uncharged for long periods.

There is high demand for lithium to make batteries with most of it sourced from China which does most of the refining. Alternatives to lithium include aluminium, sodium zinc and magnesium.

The limiting factor for flexible electronics is how to make the battery flexible. Several strategies can be used: (a) replace the rigid base with a flexible material like plastic, fabric, paper or fibre, (b) by using smarter mechanical designs, and (c) using different fabrication patterns. One approach has been to combine the graphite with cellulose. Other materials that are being evaluated are gelatin, gold leaf, activated charcoal, riboflavin and beeswax.

Returning to flexible electronics, Eva gave examples of usage: (a) wearable displays and in smart clothing (b) flexible solar cells for spacecraft and drones and (c) devices that can be implanted in the human body.  Her team is also researching how to make memory devices at nanoscale – the simplest example being two metals separated by a gap of 10 – 15nm which mimics the arrangement of brain synapses.  One arrangement is to use aluminium and gold as the two metals separated by a nanometre gap.  It works like a binary switch either allowing current to flow when there is low resistance between the two metals or preventing current if there is high resistance.

There is a bright future for flexible electronics to make them soft (flexible), smart (with advanced capabilities) and sustainable (more environmentally friendly).

Charles Emes