A new sodium-based battery can store the same amount of energy as a state-of-the-art lithium ion at a substantially lower cost.
As a warming world moves from fossil fuels toward renewable solar and wind energy, industrial forecasts predict an insatiable need for battery farms to store power and provide electricity.
Chemical engineer Zhenan Bao and materials scientists Yi Cui and William Chueh of Stanford University aren’t the first researchers to design a sodium ion battery. But they believe their approach has the price and performance characteristics to create a sodium ion battery that costs less than 80 percent of a lithium ion battery with the same storage capacity.
$150 a ton
“Nothing may ever surpass lithium in performance,” Bao says. “But lithium is so rare and costly that we need to develop high-performance but low-cost batteries based on abundant elements like sodium.”
With materials constituting about one-quarter of a battery’s price, the cost of lithium—about $15,000 a ton to mine and refine—looms large. Researchers say that’s why they are basing the new battery on widely available sodium-based electrode material that costs just $150 a ton.
The sodium-based electrode has a chemical makeup common to all salts: It has a positively charged ion—sodium—joined to a negatively charged ion. In table salt, chloride is the positive partner, but in the battery a sodium ion binds to a compound known as myo-inositol.
Unlike the chloride in table salt, myo-inositol isn’t a household word—but it is a household product, found in baby formula and derived from rice bran or from a liquid byproduct of the process used to mill corn. Crucial to the idea of lowering the cost of battery materials, myo-inositol is an abundant organic compound familiar to industry.
Charge-recharge cycle
The sodium salt makes up the cathode, the pole of the battery that stores electrons. The battery’s internal chemistry shuttles those electrons toward the anode, which in this case is made up of phosphorous. The more efficiently the cathode shuttles those electrons toward and backward versus the anode, the better the battery works.
For the prototype, postdoctoral scholar Min Ah Lee and the team improved how sodium and myo-inositol enable that electron flow, significantly boosting the performance of this sodium ion battery over previous attempts.
Researchers focused mainly on the favorable cost-performance comparisons between their sodium ion battery and state of the art lithium. In the future they’ll have to look at volumetric energy density—how big does a sodium ion battery have to be to store the same energy as a lithium ion system.
Researchers also optimized the battery’s charge-recharge cycle—how efficiently the battery stores electricity coming in from a rooftop solar array, for instance, and how effectively it delivers that power to, say, run the house lights at night.
To better understand the atomic-level forces at play during the process, postdoctoral scholar Jihyun Hong and graduate student Kipil Lim worked with Chueh and Michael Toney, a scientist with the SLAC National Accelerator Laboratory, to study precisely how the sodium ions attach and detach from the cathode, an insight that helped improve the overall battery design and performance.
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The researchers say the Nature Energy paper demonstrates that sodium-based batteries can be cost-effective alternatives to lithium-based batteries. Having already optimized the cathode and charging cycle, Cui says they next plan to focus on tweaking the battery’s anode.
“This is already a good design, but we are confident that it can be improved by further optimizing the phosphorus anode.”
The US Department of Energy’s Advanced Battery Materials Research (BMR) Program funded the work. X-ray measurements took place at the Stanford Synchrotron Radiation Laboratory (SSRL), a national user facility that Stanford operates on behalf of the US Department of Energy, Office of Basic Energy Sciences.
Source: Stanford University
