Rachel Carter’s research is driven by the need to keep sailors and soldiers safe. In a submarine or a fighter jet, even the tiniest risk of fire bears huge consequences. That makes today’s lithium-ion batteries, which carry a small risk of overheating and catching fire, a hazard.

Based at the US Naval Research Laboratory (NRL), Carter investigates ways to enhance the safety of emerging battery technologies. She is also developing next-generation battery chemistries that are inherently safe and that avoid expensive materials coming from complicated, fraught international supply chains. “We need newer types of batteries to alleviate the demand on critical materials,” she says. “Using materials that are globally abundant, nontoxic, and friendly to humans and the environment is a really exciting proposition.”

Growing up, Carter was inspired by her grandfather, a mechanical and materials engineering professor at Vanderbilt University who studied specialty ceramics and glasses, such as those used on space shuttles. “It was pretty cool stuff,” she says. “He also had a woodshop in his home, and we used to build things together.”

Carter’s love of math and her creative side drew her to the problem-solving aspect of engineering. She chose to study mechanical engineering at Vanderbilt as an undergraduate, becoming the first woman in her family to pursue a scientific degree.

Her initiation into the battery field came while applying for graduate school. Cary Pint at Vanderbilt talked to her about the pressing need for better battery technologies. She decided to pursue her PhD with Pint, studying sulfur-based cathodes for lithium-ion batteries with an eye toward replacing the expensive, complex metal oxide cathodes used in today’s batteries. The simpler sulfur electrodes would make recycling batteries easier, she says. Plus, sulfur is a plentiful, nontoxic material: “We have sulfur in our bodies; it’s all around us.”

During her PhD, Carter also demonstrated the first room-temperature sodium-sulfur battery. These batteries, which use molten sodium and sulfur electrodes, typically operate at 300 °C.

In 2017, she started her postdoctoral fellowship under materials engineer Corey T. Love at the NRL. There, she returned to “old school” lithium-ion batteries, which she says the Navy hardly uses because of the risk involved. She collaborated with researchers in the NRL’s spacecraft engineering division to evaluate how battery packaging could incorporate cooling technologies used on missiles to avoid overheating.

Carter became a full-time NRL research engineer in 2019. She was tasked with translating her expertise on the thermal safety of lithium-ion batteries to newer sodium-ion battery technology. Not to be confused with molten sodium–sulfur batteries, sodium-ion batteries work much like lithium ion, except they move sodium ions between two electrodes to charge and discharge. They can store less energy per weight than lithium can, but their higher safety and faster charging time have attracted significant R&D, and they are now at a commercialization threshold. At least two Chinese companies plan to launch sodium-ion batteries in electric vehicles this year, and a handful of companies in the US and Europe are close behind.

Carter is evaluating the safety and performance of sodium-ion batteries from manufacturers in three countries. This work involves a slew of tests that include heating to blistering temperatures, crushing, puncturing with nails, and overcharging to see how the battery responds.

Now considered the NRL’s top expert on sodium- ion batteries, she is leading a project funded by the US Department of Transportation (DOT) that will guide regulations on the transport of these batteries. Because the DOT is one of the regulatory bodies that guide the United Nations’ regulations on the transport of hazardous goods, “there’s a global impact of the measurements that Rachel is doing right now,” says Love, who is now her colleague.

As she helps shape policy, Carter hasn’t lost sight of scientific research. She continues to push forward the development of advanced room-temperature sodium-sulfur and lithium-sulfur batteries. Carter and her colleagues conduct in-depth studies of the fundamental reactions in these batteries to inform the design of better electrodes and electrolytes.

“One of her strengths is that she sees the big picture,” Love says. “In the US, we’re not geologically blessed with current battery materials. Rachel is developing advanced energy storage materials that are more supply-chain resilient and green in processing. That will have a big impact in the foreseeable future.”