Penn Engineering Improves Alternative to Lithium-ion Batteries

A team led by Eric Detsi, professor in Penn Engineering’s Department of Materials Science and Engineering, has made progress toward developing alternative metal-ion batteries, a technology that currently relies heavily on lithium.

By Jacob Williamson-Rea

Lithium-ion batteries are used in smartphones, power tools, electric vehicles and more. The rechargeable, long-lasting batteries have high energy density, meaning they are relatively small and light for the amount of power they store. This has led to soaring demand for these batteries across a variety of sectors, ranging from consumer electronics to aerospace applications.

These batteries are so useful because they can be recharged hundreds of times, thanks to high cyclability of electrodes made of carbon, lithium and cobalt. But experts are worried that the surging popularity of lithium-ion batteries could become problematic, since lithium and cobalt resources are becoming more expensive due to demand, and the global cobalt market heavily depends on supplies from countries with high geopolitical risks.

As a result, researchers are seeking alternative battery technologies that do not use lithium and cobalt. The challenge is that the energy density and cyclability of lithium-ion batteries are almost unmatched by any other practical battery technology.

Eric Detsi, Stephenson Term Assistant Professor in Materials Science and Engineering (MSE), has directed research that takes a significant step toward using sodium-ion batteries as an alternative to lithium.

“We’re working to improve the cyclability of electrode materials for use in sodium-ion batteries,” says Detsi. “Lithium-ion batteries are very successful. Since they are increasingly used in so many large-scale applications, we worry that lithium and cobalt resources won’t be continuously supplied.”

The team has been exploring the potential of sodium-ion batteries, which have been the subject of intense research over the past few years owing to the abundance of sodium resources, its low extraction cost, and the fact that the chemistry of sodium is similar to that of lithium. They recently overcame a significant challenge in developing this technology, thanks to work carried out by Olivia Ruiz, an undergraduate in the MSE department at Penn, and Mark Cochrane, a master’s student in the same department.

“The fundamental challenge is that when you use sodium instead of lithium to store energy, the battery anode expands a lot compared to lithium, due to the fact that sodium atoms are much bigger than lithium atoms. As a result, the anode material will fail after just a few charging and discharging cycles,” says Detsi. “This is particularly challenging because people need small batteries for small devices, which lithium is great for, but we won’t always have it.”

Batteries convert chemical energy into electricity. Chemical energy is stored in the form of chemical bonds: at the negative electrode, lithium ions are inserted between layers of graphitic carbon; at the positive electrode, cobalt oxide is used. While you use a battery-powered device, lithium ions leave the carbon at the negative electrode and move through the electrolyte to the positive electrode’s cobalt oxide. Electrons simultaneously flow through the external circuit and into the device the battery is powering. The reverse occurs when the battery itself is being charged: lithium ions flow back to and are stored in the carbon at the negative electrode.

Unfortunately, graphitic carbon can store lithium but not sodium. The materials that can reversibly store sodium undergo a large volume expansion, up to four times their original volume. This causes the material to crack and pulverize. As a result, the battery is rendered useless within the first ten to twenty charge-discharge cycles.

Detsi and his team have developed a combination of porous and non-porous materials that minimize the volume expansion in sodium-ion battery anodes. By using porous antimony (Sb) to store sodium, in combination with magnesium fluoride (MgF2) used as a non-porous mechanical buffer to absorb the volume changes, they’ve designed a composite material which makes it possible to charge and discharge a sodium-ion battery for over 300 cycles before failure, compared to the usual ten to twenty cycles. The team has published their findings in Advanced Energy Materials.

“It’s all about improving the cyclability of electrode materials. Who does not want the battery of their smartphone or laptop to last for several years?” Detsi asks.

Salts for sodium-grade batteries are readily available and abundant, drastically more so than lithium deposits. Once the cyclability of sodium-ion batteries is improved, sodium could serve as a cost-effective supplement that would lessen the technology industry’s reliance on lithium.