Lithium-sulphur (Li-S) batteries are one of the most promising energy-storage systems, with theoretical energy densities of up to 2500 Wh/kg (which is almost five times that of conventional Li-ion batteries). Sulphur itself is also an abundant, low cost and environmentally safe material.

However, Li-S batteries are relatively unstable because of sulphur’s poor conductivity. What is more, soluble reaction intermediates, namely lithium polysulphides, can diffuse from the cathode to the anode and react with the lithium itself leading to self-discharge and loss of active materials. This is known as the (unwanted) "shuttle" effect. The fact that the sulphur electrode also undergoes a huge volume change (of around 75%) during battery charge/discharge is also a problem in that it makes the electrodes unstable.

Novel sulphur-based cathode structure

Now, a team led by Guihua Yu has designed a novel sulphur-based cathode structure for high-performance Li-S batteries based on PPy-MnO2 nanotubes as efficient hosts. Thanks to PPy’s high conductivity, sulphur confined in the nanotubes can be easily accessed by electrons, so improving its conductivity. More importantly, the MnO2 can trap the polysulphide reaction intermediates in the nanotubes and prevent them from becoming dissolved in the battery electrolyte. And that is not all: the PPy-MnO2 nanotubes are also flexible, something that helps buffer the volume change of the electrode during charging/discharging.

“The S/PPy-MnO2 composite electrodes deliver an initial high capacity of 1420 mAh/g at 0.2C and retain 985 mAh/g after 200 cycles,” says team member and first author of the study Jun Zhang. “At a 1C rate, the discharge capacity stays above 500 mAh/g over 500 cycles with a decay rate of around 0.07% per cycle, which indicates good stability. (The C rate is a measure of how quickly a battery can be charged/discharged).

Towards higher energy Li-S batteries

“Our Li-S batteries with their high energy density, long-life cyclic stability and low cost might be useful in applications such as large-scale energy storage for renewable energy as well as in electric vehicles,” he tells nanotechweb.org.

The researchers, reporting their work in Nano Letters DOI: 10.1021/acs.nanolett.6b03849, say they are now busy looking into the material and structural design of Li2S-based electrodes for higher energy Li-S batteries.

“We are still concerned about using metallic lithium as the anode in batteries because of the lithium ‘dendrites’ problem. Here, ‘branches’ of lithium can grow that penetrate the separator in the battery and short-circuit it, causing overheating and even potential explosion,” explains Yu. “To avoid using metallic lithium anodes, Li2S could be used as a lithium-containing cathode that could be coupled with anodes made of graphite, silicon and tin, for example, to assemble a battery with a high energy density.

“The growth of lithium dendrites could also be suppressed by using solid-state electrolytes, but it is still difficult to synthesize such electrolytes while retaining high lithium-ion conductivity and good interfacial compatibility with electrodes,” he says. “So finding ways to stabilize and/or protect lithium metal anodes is still a very active research area in this field.”