The crystal structure of disordered rocksalt. The red balls represent oxygen, the blue tetrahedron represents lithium in tetrahedral sites, and the green octahedron represents the lithium/vanadium shared octahedral sites. Image: Liu and Ong labs, UC San Diego.
The crystal structure of disordered rocksalt. The red balls represent oxygen, the blue tetrahedron represents lithium in tetrahedral sites, and the green octahedron represents the lithium/vanadium shared octahedral sites. Image: Liu and Ong labs, UC San Diego.

Researchers at the University of California (UC) San Diego have discovered a new anode material that allows lithium-ion batteries to be safely recharged within minutes for thousands of cycles. Known as a disordered rocksalt, the new anode material is made up of earth-abundant lithium, vanadium and oxygen atoms arranged in a similar way to ordinary kitchen table salt, but randomly.

The rocksalt is promising for commercial applications where both high energy density and high power are desired, such as electric cars, vacuum cleaners or drills. The researchers report this new material in a paper in Nature.

Currently, two materials are used as anodes in most of the commercially available lithium-ion batteries that power devices like cell phones, laptops and electric vehicles. The most common material, graphite, is extremely energy dense – a lithium ion battery with a graphite anode can power a car for hundreds of miles without needing to be recharged. However, recharging a graphite anode too quickly can result in fire and explosions due to a process called lithium metal plating. A safer alternative, lithium titanate, can be recharged rapidly but has a significantly lower energy density, which means the battery needs to be recharged more frequently.

The new disordered rocksalt anode – Li3V2O5 – sits in an important middle ground: it is safer to use than graphite, yet offers a battery with at least 71% more energy than lithium titanate.

"The capacity and energy will be a little bit lower than graphite, but it's faster, safer and has a longer life. It has a much lower voltage and therefore much improved energy density over current commercialized fast-charging lithium-titanate anodes," said Haodong Liu, a postdoctoral scholar in the laboratory of Ping Liu at UC San Diego and first author of the paper. "So with this material we can make fast-charging, safe batteries with a long life, without sacrificing too much energy density."

The researchers have formed a company called Tyfast to commercialize this discovery. The startup's first markets will be electric buses and power tools, since the characteristics of the Li3V2O5 disordered rocksalt make it ideal for use in devices where recharging can be easily scheduled. They also plan to continue developing this lithium-vanadium oxide anode material, while also optimizing other battery components to develop a commercially viable full cell.

"For a long time, the battery community has been looking for an anode material operating at a potential just above graphite to enable safe, fast charging lithium-ion batteries. This material fills an important knowledge and application gap," said Ping Liu. "We are excited for its commercial potential since the material can be a drop-in solution for today's lithium-ion battery manufacturing process."

Scientists first experimented with disordered rocksalt as a battery cathode, around six years ago. Since then, much work has been done to turn the material into an efficient cathode. According to Haodong Liu, the UC San Diego team decided to test the material as an anode based on a hunch.

"When people use it as a cathode, they have to discharge the material to 1.5 volts," he said. "But when we looked at the structure of the cathode material at 1.5 volts, we thought this material has a special structure that may be able to host more lithium ions – that means it can go to even lower voltage to work as an anode."

In the study, the team found that their disordered rocksalt anode could reversibly cycle two lithium ions at an average voltage of 0.6V. This is higher than the 0.1V of graphite, thus eliminating lithium metal plating at a high charge rate to make the battery safer, but lower than the 1.5V at which lithium titanate intercalates lithium, thus storing much more energy.

The researchers showed that the Li3V2O5 anode can undergo over 6000 charging cycles with negligible capacity decay, and can charge and discharge energy rapidly, delivering over 40% of its capacity in 20 seconds. The low voltage and high rate of energy transfer are due to a unique redistributive lithium intercalation mechanism with low energy barriers.

Zhuoying Zhu, a postdoctoral scholar in Shyue Ping Ong's Materials Virtual Lab at UC San Diego, performed theoretical calculations to understand why the disordered rocksalt Li3V2O5 anode works as well as it does. "We discovered that Li3V2O5 operates via a charging mechanism that is different from other electrode materials. The lithium ions rearrange themselves in a way that results in both low voltage as well as fast lithium diffusion," said Zhu.

"We believe there are other electrode materials waiting to be discovered that operate on a similar mechanism," added Ong.

This story is adapted from material from the University of California, San Diego, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.