Don’t break out the widow’s weeds just yet, but it looks like momentum is building for energy storage to move past the lithium-ion phase and get into the more powerful territory of lithium-sulfur technology. In the latest development, a multinational research team has figured out how to overcome a major obstacle in the path of lithium-sulfur energy storage, by using graphene as a “bridge” between different components.
In theory, lithium sulfur (Li-S) batteries possess far greater energy density than the familiar lithium-ion (Li-ion), so breaking the technology out of the lab and into commercial development could have huge clean tech implications for EV battery range and energy storage for solar and wind sources, among other applications.
Lithium-Sulfur Energy Storage
Sulfur is super-cheap, which is mainly why researchers are interested in developing energy storage devices incorporating the material.
Sulfur also has some bonus attributes compared to conventional Li-ion battery technology, such as a high tolerance for overcharging, relatively light weight, and low toxicity.
However, sulfur opens up a can of worms. One problem is that in conventional Li-S batteries, the liquid electrolytes acts by dissolving the Li-S compound, which necessarily makes for a short-lived battery.
In other words, sulfur is brittle.
The University of Arizona has also been tinkering around with Li-S energy storage, and has come up with a process for converting waste sulfur from industrial processes into a lightweight, solid compound with potential use in EV batteries.
So, What’s Wrong With Lithium-Ion?
The latest development was published earlier this month by researchers from the University of Cambridge and the Bejing Institute of Technology, in the AIP journal APL Materials, where you can find it under the title, “Graphene-wrapped sulfur/metal organic framework-derived microporous carbon composite for lithium sulfur batteries.”
According to the research team, the established specific energy density of Li-ion batteries weighs in at 130-220 Wh kg – 1, which looks pretty good but not when you consider that at least in theory, Li-S batteries can get up into the 2,600 range.
As described in the paper, Li-ion batteries are typically hampered by the use of a graphite anode and a cobalt oxide cathode. In contrast, Li-S batteries use a pure lithium anode and a sulfur cathode (for those of you new to the topic, the anode and cathode are the two parts of the battery that collect and discharge the current, which is stored in the electrolyte).
The team points out that even though Li-S technology is still in the prototype state, samples are already developed that achieve an energy density in the 150-220 range.
The Graphene Solution For Sulfur
As for the sulfur degradation problem, here’s how the team sums it up:
…S is lost by dissolving in the solvent and further by shuttling away from the cathode and even more by reacting with the Li anode…The shuttle mechanism has been directly implicated as the cause for low S utilization following the initial discharge, which is exacerbated in subsequent charge-discharge cycle.
That’s not all. The team also identifies other obstacles stemming from the use of sulfur, including poor conductivity.
One promising solution is the use of porous carbon as a protective “host” for sulfur in the cathod, specifically carbonized metal-organic frameworks (MOFs).
The team took the MOF ball and ran with it, as explained by Cambridge’s Kai Xi:
Our carbon scaffold acts as a physical barrier to confine the active materials within its porous structure. This leads to improved cycling stability and high efficiency.
This is where graphene comes in. Typical MOFs have a relatively low capacity, and previous research has shown that a nice shot of graphene can overcome that obstacle.
The graphene energy storage solution, as described in the team’s press materials, consists of wrapping the sulfur-carbon unit in graphene sheets:
Fast charge-transfer kinetics are made possible by an interconnected graphene network with high electrical conductivity, according to the team. Their work shows that the composite structure of a porous scaffold with conductive connections is a promising electrode structure design for rechargeable batteries.
So, now what? The next step will be to develop new electrolytes and tweak the lithium “protection layers” for increased efficiency.
They better act fast. Just last year, researchers from the Lawrence Berkeley National Laboratory announced the development of a Li-S battery based on a new material they’re calling sulfur-graphene oxide.
They’ve already demonstrated a Li-S battery that has double the specific energy of Li-ion, and it exhibits minimal decay even after 1,500 charge-discharge cycles.
The Berkeley team is eyeballing the new technology to develop a low cost electric vehicle battery in the 300 mile range, so stay tuned.