Imagine an electric car capable of traveling 600, 700 or even 1,000 miles on a single charge. That far exceeds the range of the electric vehicles with the longest range on the US market, according to the magazine Car and Driverand doubles the official range of the long-range, rear-wheel drive Tesla Model 3, which has a maximum approved range of 584 kilometers.
Current electric vehicles use lithium-ion batteries, which are also present in smartphones, laptops and even large-scale energy storage systems connected to the electrical grid.
These batteries, a standard for decades, have been perfected by generations of scientists and are now near their physical limits. Even with the best materials and most optimized designs, the amount of energy that can be stored in a lithium-ion battery is limited.
I am a materials engineer and I study these batteries in search of alternatives with better performance, greater environmental sustainability and lower cost. One promising design uses sulfur, which could significantly increase battery capacity, although there are still significant hurdles before it can become widely used.
Lithium-sulfur vs. lithium-ion
Every battery has three basic components: a positively charged region, called the cathode; a negatively charged region, called the anode; and a substance called electrolyte in between, through which charged atoms—also known as ions—move between the cathode and anode.
In a lithium-ion battery, the cathode is made of a metal oxide that usually contains metals such as nickel, manganese, and cobalt bonded with oxygen. The materials are arranged in layers, with lithium ions physically sandwiched between them. During charging, lithium ions break away from the cathode material and travel through the electrolyte to the anode.
The anode is usually made of graphite, which is also made up of layers with space between them for lithium ions. During discharge, lithium ions leave the graphite layers, travel back through the electrolyte and reinsert into the sheet structure of the cathode, recombining with the metal oxide to release electricity that powers cars and smartphones.
In a lithium-sulfur battery, the lithium ions still move back and forth, but the chemistry is different. Its cathode is made of sulfur embedded in a carbon matrix that conducts electricity, and the anode is composed primarily of lithium, rather than layers of graphite with lithium between them.
During discharge, lithium ions move from the anode, through the electrolyte, to the cathode, where—rather than sliding between the layers of the cathode—they chemically convert the sulfur in sequential steps into a series of compounds called lithium sulfides. During charging, lithium ions separate from the sulfur compounds, leave the cathode, and return to the anode.
The charging and discharging process of lithium-sulfur batteries is a chemical conversion reaction that involves more electrons than the same process in lithium-ion batteries. This means that a lithium-sulfur battery can theoretically store much more energy than a lithium-ion battery of the same size.
Sulfur is cheap and available in abundance around the world, meaning battery makers don’t need to rely on scarce metals like nickel and cobalt, which are unevenly distributed on Earth and often come from regions like the Democratic Republic of the Congo, where workplace safety regulations and fair labor practices are limited.
These advantages could lead to batteries with much higher capacity, cheaper and more sustainable to produce.
Also read: A $250 million plan to extract lithium for batteries from the Great Salt Lake
Why aren’t lithium-sulfur batteries widely used?
The main obstacle to the mass production and use of sulfur batteries is their durability. A good lithium-ion battery, like those in an electric vehicle, can withstand thousands of discharge and recharge cycles before its capacity begins to decrease. This is equivalent to thousands of car trips.
But lithium-sulfur batteries tend to lose capacity much more quickly, sometimes after less than 100 cycles. That doesn’t represent a lot of travel.
The reason lies in chemistry. During the chemical reactions that store and release energy in a lithium-sulfur battery, some of the lithium sulfide compounds dissolve in the battery’s liquid electrolyte.
When this occurs, those amounts of sulfur and lithium are no longer used in the remaining reactions. This effect, known as “transport,” means that with each discharge and recharge cycle there are fewer elements available to release and store energy.
Over the past two decades, research has resulted in improved designs. Previous versions of these batteries lost much of their capacity after a few dozen discharge and recharge cycles, and even the best laboratory prototypes struggled to make it past a few hundred.
The new prototypes retain more than 80% of their initial capacity even after thousands of cycles. This improvement is due to the redesign of key parts of the battery and the adjustment of the chemical components: special electrolytes help prevent lithium sulfides from dissolving and displacing.
The electrodes were also improved, using materials such as porous carbon that physically trap intermediate lithium sulfides, preventing them from moving away from the cathode. This facilitates discharge and recharge reactions with fewer losses, making them more efficient and prolonging the useful life of the battery.
The road ahead
Lithium-sulfur batteries are no longer fragile laboratory curiosities, but there are still significant challenges before they can become serious candidates for real-world energy storage.
In terms of safety, lithium-sulfur batteries have a less volatile cathode than lithium-ion batteries, but research continues into other aspects of safety.
Another problem is that the more energy a lithium-sulfur battery stores, the fewer charging cycles it can withstand. This is because the chemical reactions involved are more intense with increasing energy.
This tradeoff may not represent a major obstacle to the use of these batteries in drones or in grid-level energy storage, where ultra-high energy densities are less critical.
However, for electric vehicles, which require both high energy capacity and long service life, battery scientists and researchers have yet to find a workable balance. This means that the basis for the next generation of lithium-sulfur batteries will likely take a few years to develop.
*Golareh Jalilvand is an assistant professor of Chemical Engineering at the University of South Carolina
This article was originally published in The Conversation
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