Power/Performance Bits: July 25

Batteries: sodium-ion cathode; preventing dendrites; supercapacitors.


Sodium-ion cathode
Researchers at the University of Texas at Dallas and Seoul National University developed a manganese and sodium-ion-based cathode material they hope could lead to lower-cost rechargeable batteries.

In a typical lithium-ion battery, the cathode is made of lithium, cobalt, nickel and oxygen.

“Lithium is a more expensive, limited resource that must be mined from just a few areas on the globe,” said Kyeongjae Cho, professor of materials science and engineering at UT Dallas. “There are no mining issues with sodium — it can be extracted from seawater. Unfortunately, although sodium-ion batteries might be less expensive than those using lithium, sodium tends to provide 20% lower energy density than lithium.”

To address the limited energy storage capacity, the team turned to manganese. “There was great hope several years ago in using manganese oxide in lithium-ion battery cathodes to increase capacity, but unfortunately, that combination becomes unstable,” Cho said.

To increase stability, sodium replaces most of the lithium in the cathode, while manganese is used instead of the more expensive and rarer elements cobalt and nickel.

The research team’s sodium-ion design, which retains the high energy density of a lithium-ion cathode, replaces most of the lithium atoms (green) with sodium (yellow). The layered structure of the new material also incorporates manganese (purple) and oxygen (red). (Source: University of Texas at Dallas)

“Our sodium-ion material is more stable, but it still maintains the high energy capacity of lithium,” Cho said. “And we believe this is scalable, which is the whole point of our research. We want to make the material in such a way that the process is compatible with commercial mass production.”

Preventing dendrites in solid electrolytes
Researchers at MIT suggest a way to make lithium batteries safer with a new view of how dendrites, the metal whiskers that build up in a battery’s electrolyte while charging and potentially cause short circuits, form.

Rechargeable lithium-ion batteries use a liquid electrolyte between the anode and cathode. While solid electrolytes such as ceramics have been proposed as a way to improve both safety and capacity, tests have shown that such materials tend to perform somewhat erratically and are more prone to short-circuits than expected.

The problem, according to this study, is that researchers have been focusing on the wrong properties in their search for a solid electrolyte material. The prevailing idea was that the material’s firmness or squishiness (a property called shear modulus) determined whether dendrites could penetrate into the electrolyte. But the new analysis showed that it’s the smoothness of the surface that matters most.

The way dendrites form in stiff solid materials follows a completely different process than those that form in liquid electrolytes, the researchers found. On the solid surfaces, lithium from one of the electrodes begins to be deposited, through an electrochemical reaction, onto any tiny defect that exists on the electrolyte’s surface, including tiny pits, cracks, and scratches. Once the initial deposit forms on such a defect, it continues to build. The buildup extends from the dendrite’s tip, not from its base, as it forces its way into the solid, acting like a wedge as it goes and opening an ever-wider crack.

The team suggests that simply focusing on achieving smoother surfaces could eliminate or greatly reduce the problem of dendrite formation in batteries with a solid electrolyte. In addition to avoiding the flammability problem associated with liquid electrolytes, this approach could make it possible to use a solid lithium metal electrode as well. Doing so could potentially double a lithium-ion battery’s energy capacity.

Supercapacitor electrodes
Engineers at the University of Washington developed a process for manufacturing supercapacitor electrode materials more quickly and cheaply using carbon-rich aerogels combined with molybdenum disulfide or tungsten disulfide.

“In industrial applications, time is money,” said Peter Pauzauskie, assistant professor of materials science and engineering at UW. “We can make the starting materials for these electrodes in hours, rather than weeks. And that can significantly drive down the synthesis cost for making high-performance supercapacitor electrodes.”

Effective supercapacitor electrodes are synthesized from carbon-rich materials that also have a high surface area. Unlike conventional batteries, a supercapacitor stores and separates positive and negative charges directly on its surface.

To get the high surface area for an efficient electrode, the team used aerogels, wet, gel-like substances that have gone through a special treatment of drying and heating to replace their liquid components with air or another gas. These methods preserve the gel’s 3-D structure, giving it a high surface area and extremely low density.

“One gram of aerogel contains about as much surface area as one football field,” said Pauzauskie.

Slice from x-ray computed tomography image of a supercapacitor coin cell assembled with the electrode materials. The thin layers — just below the coin cell lid — are layers of electrode materials and a separator. (Source: William Kuykendall)

The aerogels were constructed from a gel-like polymer created from formaldehyde and other carbon-based molecules. While the aerogel could act as an electrode on its own, the researchers improved its performance by incorporating thin sheets (about 10 to 100 atoms thick) of either molybdenum disulfide or tungsten disulfide into the carbon-rich gel matrix. They were able to synthesize a fully-loaded wet gel in less than two hours.

After the aerogel was dried, they combined it with adhesives and another carbon-rich material to create an industrial “dough,” which the team rolled out to sheets just a few thousandths of an inch thick. They cut half-inch discs from the dough and assembled them into simple coin cell battery casings to test the material’s effectiveness as a supercapacitor electrode.

In tests, the electrodes showed a capacitance at least 127% greater than carbon-rich aerogels alone. The team expects that aerogels loaded with even thinner sheets of molybdenum disulfide or tungsten disulfide would show an even better performance, and plan to fine-tune the process.

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