Graphene accelerates charging and extends life in lithium-ion batteries

Nov. 25 () –

A new method for coating battery cathodes lithium ion with graphene extends life and performance of these widely used rechargeable batteries.

This finding may reduce dependence on cobaltan element frequently used in lithium-ion batteries and which is difficult to obtain sustainably.

Caltech senior research scientist David Boyd has worked for the past decade to develop techniques for making graphene, a layer of carbon one atom thick which is incredibly strong and conducts electricity more easily than materials like silicon. In 2015, Boyd and his colleagues discovered that high-quality graphene could be produced at room temperature. Before this, the production of graphene required extremely high temperatures, up to 1,000 degrees Celsius.

After this advance, the search for new applications for graphene began. Recently, Boyd teamed up with Will West, a technologist at JPL (Jet Propulsion Laboratory), which Caltech manages for NASA. West specializes in electrochemistry and, in particular, the development of improved battery technologies. Boyd and West set out to see if graphene could create an improved lithium-ion battery. Now they have proven that yes.

“Demonstrating a reliable trend in battery cell performance requires consistent materials, consistent cell assembly, and careful testing under a variety of conditions,” he says. in a statement Brent Fultz, professor of Materials Science and Applied Physics at Caltech. “It’s fortunate that the team was able to do this work so reproducibly, although it took some time to be sure.”

The lithium-ion battery, first introduced to the market in 1991, has revolutionized the way we use electricity in our daily lives. From our cell phones to electric vehicles, we rely on lithium-ion batteries as a comparatively cheap, energy-efficient and, most importantly, rechargeable on-the-go power source.

Despite its successes, there is room for improvement in lithium-ion battery technology. For example, Boyd states: “Tesla engineers want a cost-effective battery that can charge quickly and run for a longer period of time between charges. “That’s called upload speed capability.”

West adds: “The more times a battery can be charged over its lifetime, the fewer batteries will have to be used. This is important because lithium-ion batteries use limited resources and disposing of lithium-ion cells safely and efficiently is a very difficult task.”

An important feature of lithium-ion batteries is their performance after many charge and use cycles. Batteries work by creating chemical energy between the two ends of the battery (the cathode and anode) and converting it into electrical energy. As the cathode and anode chemicals work over time, they may not completely recover to their original state. A common problem is the dissolution of transition metals from the cathode material, which is especially serious in cathode materials with a high manganese content, although less serious in cathode materials with a high cobalt content.

“As a result of the unwanted side reactions that occur during the cycle, the transition metals at the cathode gradually end up at the anode, where they get stuck and reduce anode performance” explains Boyd. This transition metal dissolution (TMD) is one reason why expensive cobalt-containing cathodes are used instead of inexpensive high-manganese cathodes.

Another challenge for lithium-ion batteries is that they require metals that are expensive, scarce, and not always responsibly mined. A significant amount of the world’s cobalt supply, in particular, is concentrated in the Democratic Republic of the Congo, and much of that cobalt is mined by so-called artisanal miners: self-employed workers, including children, who perform dangerous and demanding physical jobs for little or no pay.

Ways have been sought to increase battery performance while reducing or eliminating cobalt use and still preventing TMD.

Engineers knew that carbon coatings on the cathode of a lithium-ion battery could slow or stop DTM, but developing a method to apply these coatings proved difficult. “Researchers have attempted to deposit graphene directly onto the cathode material, but the process conditions normally needed to deposit graphene would destroy the cathode material” explains Boyd.

“We investigated a new technique for depositing graphene on the cathode particles called dry coating. The idea is to have a ‘host’ substance of large particles and a ‘guest’ substance of tiny particles. By mixing them under certain conditions, the system can experiment a phenomenon known as ‘ordered mixing,’ in which guest particles uniformly coat host particles.”

Dry coating technology has been used since the 1970s in the pharmaceutical industry. to prolong the shelf life of tablets by protecting them from moisture, light and air.

Dry coating of the cathode with a graphene compound was successful in the laboratory. The graphene coating dramatically reduced TMD, simultaneously doubling battery life, and allowing the batteries to operate over a somewhat wider temperature range than was previously possible. This result surprised the researchers. It was assumed that only a continuous coating could suppress TMD and that a dry coating composed of particles could not do so. Furthermore, since graphene is a form of carbon, It is widely available and environmentally friendly.