Innovation in lithium-ion batteries

During the oil crisis of the 1970s, a chemist at Exxon named M. Stanley Whittingham, working on a new type of rechargeable battery, discovered that lithium ions could slip inside the gaps in a layered material called titanium disulfide. He created the world’s first lithium-ion battery, which sported a titanium disulfide cathode and a lithium anode. The device was lightweight and had excellent energy storage capacity, but it frequently short-circuited. John B. Goodenough, then at the University of Oxford, and Akira Yoshino at Asahi Kasei refined the design by switching out the titanium disulfide for cobalt oxide and the lithium metal for petroleum coke. The trio won the 2019 Nobel Prize in Chemistry for the work.

Just 50 years after Whittingham’s original invention, lithium-ion batteries have come to power an enormous swath of our world. Our cell phones, laptops, power tools, and electric vehicles all rely on this technology, and demand is now expanding to larger-scale energy storage for electricity generated by solar cells and wind turbines.

Researchers continue to improve on Li-ion batteries’ four key components: the cathode, anode, electrolytes through which lithium ions travel, and separators that keep positive and negative electrodes apart. But the need for higher capacity and safer and cheaper power sources is growing, as is the recognition of environmental harms caused by mining metals required for Li-ion batteries.

Spurred by this reality, researchers are exploring a wide array of chemistries and materials for batteries beyond lithium ion. One approach gathering momentum is sodium-ion batteries, which originally gained interest in the 1980s, before lithium ion became the dominant technology.

“There’s a whole new chemistry and electrochemistry that has to be developed for sodium,” says Linda Nazar, a chemist at the University of Waterloo. For now, this technology’s relatively low energy density makes stationary energy storage the most obvious application, but that could change as new research brings improvements, she says. Researchers are investigating other ions, such as magnesium and sulfur, as well as changes in cathode, anode, and electrolyte materials, but most of these approaches have a way to go before commercialization.

Whichever battery chemistries and materials end up on top, the industry will have to address the issue of battery recycling, which today is barely done. “As people get beyond the growing pains of things like electrifying cars, we face hard questions about where those spent batteries go,” says Lynden A. Archer, a chemical engineer at Cornell University. The next wave of power storage technology “could be the next big thing for society,” he says—but for it to be successful, governments, scientists, and policymakers will have to create frameworks for greening the industry.

Cathode

When lithium-ion batteries were first commercialized by Sony in 1991 for use in personal electronic devices, the cathodes were made of lithium cobalt oxide. Over the next 15 years, as the batteries’ use expanded to applications that consumed more energy, researchers added nickel and manganese to boost energy density. Today, most lithium-ion batteries have cathodes made from some combination of cobalt, nickel, and manganese, but mining these transition metals is becoming ethically, environmentally, and financially untenable. Companies are increasingly adopting cathodes made of lithium iron phosphate, which consists of cheaper, earth-abundant materials that are more sustainable to obtain. Other possibilities for revamping the cathode are also in development.

Anode

Early versions of lithium-ion batteries used anodes made of lithium metal, which has excellent energy storage capacity but some drawbacks. Among them is a propensity to develop metallic projections called dendrites that can short-circuit and potentially explode the battery. Commercial lithium-ion batteries therefore mainly use graphite, which is much safer but degrades over time. Over the past few years, the search for better anode materials has gained increased attention. One approach is to replace some of the graphite with silicon, which has a much higher energy density. Silicon swells significantly as it charges, so researchers have had to find ways to tame this property. Efforts to use lithium metal more safely are also underway, though some doubt they will be able to surmount multiple barriers to commercialization.

Electrolytes

Traditionally, electrolytes—which transport lithium ions between the cathode and the anode—have consisted of a lithium salt dissolved in an organic solvent. These materials are flammable and tend to have low viscosity and high volatility, making them susceptible to exploding when heated. Researchers are exploring several avenues for creating safer, nonflammable solutions. Some electrolytes now use additives that coat anodes to prevent potential explosions. And aqueous electrolyte solutions that are cheaper and safer and that don’t hurt batteries’ performance are finally on the horizon. More recently, developers of solid-state battery technology have turned to solid electrolytes typically made from a polymer, ceramic, or sulfide.

Separator

Humble slips of polymer with an outsize job, separators in lithium-ion batteries are tasked with keeping the cathode and anode from making contact. These polymers insulate the electrodes while remaining permeable to the lithium ions that pass between them. Separators also play a crucial role in safety: they must be defect-free to avoid sparking a runaway reaction, and their pores are designed to melt closed if one occurs. As uses for lithium-ion batteries have grown and their energy density requirements have risen for applications like electric vehicles, researchers have sought lighter and thinner yet durable materials made of polymers or ceramic. Today, separators can be as thin as 7 µm and come in various shapes and sizes depending on the design of the batteries for which they are intended.

Alla Katsnelson is a freelance science writer and editor in Southampton, Massachusetts.