Inspired by the human intestinal lining, scientists including one of Indian origin have designed a next-generation smartphone battery with five times greater energy density than current lithium-sulphur batteries. The new design overcomes one of the key technical problems hindering the commercial development of lithium-sulphur batteries, by preventing the degradation of the battery caused by the loss of material within it.
Working with collaborators at the Beijing Institute of Technology in China, researchers based in Dr Vasant Kumar's team at University of Cambridge in the UK developed and tested a lightweight nanostructured material which resembles villi, the finger-like protrusions which line the small intestine. In the human body, villi are used to absorb the products of digestion and increase the surface area over which this process can take place.
In the new lithium-sulphur battery, a layer of material with a villi-like structure, made from tiny zinc oxide wires, is placed on the surface of one of the battery's electrodes. This can trap fragments of the active material when they break off, keeping them electrochemically accessible and allowing the material to be reused.
"It's a tiny thing, this layer, but it's important. This gets us a long way through the bottleneck which is preventing the development of better batteries," said study co-author Dr Paul Coxon from Cambridge. A typical lithium-ion battery is made of three separate components: An anode (negative electrode), a cathode (positive electrode) and an electrolyte in the middle.
The most common materials for the anode and cathode are graphite and lithium cobalt oxide respectively, and both have layered structures. Positively-charged lithium ions move back and forth from the cathode, through the electrolyte and into the anode. The crystal structure of the electrode materials determines how much energy can be squeezed into the battery. For example, due to the atomic structure of carbon, each carbon atom can take on six lithium ions, limiting the maximum capacity of the battery.
Sulphur and lithium react differently, via a multi-electron transfer mechanism meaning that elemental sulphur can offer a much higher theoretical capacity, resulting in a lithium-sulphur battery with much higher energy density. However, when the battery discharges, the lithium and sulphur interact and the ring-like sulphur molecules transform into chain-like structures, known as a poly-sulphides.
As the battery undergoes several charge-discharge cycles, bits of the poly-sulphide can go into the electrolyte, so that over time the battery gradually loses active material.
The researchers have created a functional layer which lies on top of the cathode and fixes the active material to a conductive framework so the active material can be reused. The research was published in the journal Advanced Functional Materials.