Lithium-sulfur batteries (Li-S) can hold as much as five times the energy per unit mass that lithium-ion (Li-ion) batteries can. However, Li-S batteries suffer from the propensity for polysulfides to pass through the cathode, foul the electrolyte, then pass through to the other electrode, depleting it of sulfur after just a few charge-discharge cycles. This phenomenon is known as the “shuttle effect.”
Now researchers at the University of Texas at Austin have developed an electrode structure for a Li-S battery that makes use of coaxial polypyrrole-manganese dioxide (PPy-MnO2) nanotubes. This novel electrode combats the shuttle effect by essentially encapsulating the electrodes with the nanotubes.
The nanotubes, which are highly conductive, also counteract the poor conductivity of the sulfur and the lithium sulfide, which typically act more like insulators. The high conductivity of the PPy-MnO2 nanotubes makes it possible for the electrons to gain easier access to the sulfur confined within the nanotubes.
Another key benefit gained by adding the nanotubes is their flexibility, which allows them to accommodate the huge volume changes that can occur in the electrode (as much as 75 percent) during charge-discharge cycles.
In research described in the journal Nano Letters, the batteries exhibited 98.6 percent Coulombic efficiency—the ratio of the output of charge to the input of charge—as well as good cyclic stability and good charge and discharge rates.
The researchers report that PPy-MnO2 composite electrodes have an initial high rate of 1420 milliampere-hours per gram (mAh/g) at 0.2 C and deliver 985 mAh/g after 200 cycles. These figures compare rather favorably to a typical Li-ion battery with graphite on the anode; those top out at around 380 mAh/g.
And after 500 cycles, the battery discharge rate stays above 500 mAh/g, with a decay rate of around 0.07 percent per cycle—an indication of good battery stability.
Other research efforts have looked to encapsulate Li-S electrodes with graphene to prevent the polysulfides from mixing with the electrolyte and migrating to the opposite electrode. But the University of Texas researchers contend that these other efforts to encapsulate the electrode either can trap the polysulfides or aid the conductivity of the electrode, but they can’t really achieve both.
“Although graphene provides high conductivity for sulfur, it cannot effectively trap lithium polysulfides, as evidenced in previous research that shows very low Coulombic efficiency (< 90%),” explained Guihua Yu, who led the research, in an e-mail interview with IEEE Spectrum. Yu adds:
“In our PPy-MnO2 coaxial nanotubes, PPy provides high conductivity for sulfur that is confined in the tubular structure. And more importantly, MnO2 can greatly trap the reaction intermediate polysulfides in the nanotubes, because of its strong adsorption ability to polysulfides. The flexibility of PPy is also beneficial to buffer the volume change of the electrode. As a result, shuttle effect and self-discharging are eliminated.”
While the electrode structure provides high utilization of sulfur and excellent cyclic stability, the areal mass loading of sulfur on electrode still needs improvement, says Yu. “Conventionally, sulfur-based electrode needs a lot of conductive additives, which reduces the overall energy density of a battery,” he explained. “It is important to fabricate electrodes with a higher mass ratio of sulfur to further increase the overall energy density of the battery.”
In future research, Yu and his colleagues will address the issue of combining sulfur with metallic lithium in Li-S batteries. This is a major safety concern because lithium dendrites can penetrate the separator and short-circuit the battery.
To avoid using a metallic lithium anode, Yu will be looking at the use of lithium sulfide (Li2S) for the cathode, which can be coupled with anodes such as graphite, silicon, and tin to assemble a battery with high energy density and reasonable safety. “We are trying now to see if we can work out a novel structural design to make Li2S-based electrodes work for high energy Li–S batteries,” he added.
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.