[Image] A battery cycler in Arumugam Manthiram’s lab at the University of Texas at Austin for testing multiple coin cells at the same time. Manthiram and his graduate students Sanjay Nanda and Amruth Bhargav believe they found a way to ensure stable lithium deposition in lithium-sulfur batteries. Credit: University of Texas at Austin
When it comes to batteries, lithium-ion technology is the mainstay of the field.
First introduced commercially in the 1990s, lithium-ion batteries made possible development of portable electronics, long-range electric cars, and storage of energy from renewable sources. No wonder three lithium-ion battery pioneers won the Nobel Prize in Chemistry last year.
However, like any good technology, lithium-ion batteries still face challenges, including limited capacity and energy density. So improving this technology—and developing entirely new types of batteries—is an important field of research, especially as society comes to rely more on energy generated from renewable sources.
One type of battery receiving a lot of attention is lithium-sulfur (Li-S) batteries. These batteries hold great potential for enabling the next generation of high-energy-density rechargeable batteries due to sulfur and lithium’s large gravimetric capacities (1,675 mA·h·g-1 and 3,861 mA·h·g-1, respectively).
Unfortunately, the theoretical cell capacity of Li-S batteries is just that—theoretical. To understand why, let’s take a look at Li-S battery structure and chemistry.
Li-S battery: Polysulfides and unstable lithium deposition
Li-S batteries consist of a sulfur cathode, a lithium metal anode, and a liquid electrolyte.
While there are several structural and chemical factors limiting Li-S batteries from reaching their full potential, two features in particular cause difficulties—polysulfides and unstable lithium deposition.
The polysulfide “shuttle” effect
Polysulfides are sulfides that contain two or more atoms of sulfur in the molecule. During charge and discharge of Li-S batteries, lithium sulphide (Li2S) and lithium polysulfides (Li2Sn, 3 ≤ n ≤ 8) form at the sulfur cathode.
While sulfur and Li2S are relatively insoluble in most electrolytes, many polysulfides are not. So when intermediate polysulfides are formed, they diffuse away from the sulfur cathode to the lithium metal anode—a process called the polysulfide “shuttle” effect—and lead to irreversible loss of the sulfur cathode and deposition of sulfur-containing species on the anode.
Unstable lithium deposition
In an ideal battery, ions would travel freely between the cathode and anode during charge and discharge cycles. However, in lithium-based batteries, lithium ions tend to leave and return to the surface of the anode unevenly, causing growth of dendritic or mossy formations. Lithium ions are “trapped” in these formations, meaning fewer ions are flowing between the two electrodes—causing the battery’s efficiency to degrade.
In conventional lithium-ion batteries, anodes made of graphite are used because they can store lithium ions during charge, preventing uneven deposition. However, Li-S batteries use lithium metal anodes—so the risk of lithium dendritic or mossy formation is ever-present.
To compensate for lithium ions “lost” to dendritic or mossy formations, currently a large excess of lithium metal and liquid electrolyte is necessary in Li-S batteries.
“However, the excess lithium and electrolyte employed are effectively dead weight in the battery as they do not contribute to the capacity of the cell. Hence, they reduce the system-level energy density (user time for a given battery weight or size) of the battery,” Arumugam Manthiram, ACerS Fellow and Cockrell Family Regents Chair in Engineering and Director of the Texas Materials Institute at the University of Texas at Austin, explains in an email.
Manthiram says a large volume of literature on combating the polysulfide “shuttle” effect has been built up over the last decade. “In comparison, there have been only a few attempts at stabilizing the lithium metal anode in Li-S batteries,” he says.
Stabilizing the lithium metal anode with Te0
In a new study that Manthiram conducted with graduate students Sanjay Nanda and Amruth Bhargav, they explored a way to stabilize the lithium metal anode and prevent unstable lithium deposition—by adding elemental tellurium to the sulfur cathode.
How does modifying the cathode improve stability of the anode? The reasoning circles back to the polysulfides mentioned above—and the solid-electrolyte interphase (SEI) layer.
The SEI is a layer between the liquid electrolyte and anode in lithium-based batteries. It forms when the liquid electrolyte reacts with lithium in the anode.
The SEI helps ensure stable lithium deposition by serving as a protective layer that prevents further reactions from occurring, such as the reactions that cause dendritic and mossy formations. However, if the SEI layer grows too thick or uneven, it will block the flow of lithium ions in and out of the anode—and could promote dendritic and mossy formations as well.
In Li-S batteries, when the polysulfides migrate from the cathode to anode, they react with the lithium anode to form a SEI made of Li2S. Li2S is an insulating material, so the SEI blocks lithium ion flow.
To modify the SEI so that lithium ions can flow through it, Manthiram, Nanda, and Bhargav knew they needed to modify the polysulfides involved in creating the SEI. And that’s where the elemental tellurium comes in.
“Tellurium sits two positions below sulfur in the periodic table. Hence, its chemistry shares many similarities with sulfur, and it can be easily incorporated into sulfur compounds,” Manthiram explains in an email. “At the same time, tellurium is more metallic than sulfur, and it shares many properties with the metalloid class of elements. For these reasons, we anticipated tellurium having a very interesting role to play in Li-S batteries and possibly forming a more desirable SEI layer on the lithium surface.”
They found that when elemental tellurium was added to the cathode, it combined with the polysulfides to form a soluble polytellurosulfide species (Li2TexSy). When these polytellurosulfide molecules migrated from the cathode to anode, they formed a novel bilayer SEI structure consisting of Li2TeS3 and Li2Te.
Unlike Li2S, Li2TeS3 is a semiconductor and a better lithium ion conductor. This property means lithium ions could flow through the new SEI, leading not only to better conductivity but a smoother, planar deposition of lithium—and less chance of dendritic and mossy formations.
The researchers have filed a provisional patent application for the technology, but Manthiram says the research is far from over.
“We are currently running experiments to see if our strategy can be extended to elements other than tellurium as well to stabilize lithium deposition. The initial results have been very promising,” he says. “We are also combining different strategies for improving the electrochemical performance of Li-S batteries to further improve the energy density and cycle life.”
The paper, published in Joule, is “Anode-free, lean-electrolyte lithium-sulfur batteries enabled by tellurium-stabilized lithium deposition” (DOI: 10.1016/j.joule.2020.03.020).