New electrode material ‘ideal’ for solid-state EV batteries

Scientists have developed a positive electrode material that doesn’t diminish after repeated charge/discharge cycles, saying it’s “ideal” for solid-state batteries.

Repeated charge/discharge cycles cause volume changes in electrodes, damaging their interface with the electrolyte and gradually reducing battery performance, so scientists developed a new positive electrode material that is almost dimensionally invariant which could pave the way to developing long-life, high-capacity solid-state batteries.

Most EVs tend to use lithium-ion batteries, which the scientists say don’t deliver the necessary performance and durability at a reasonable price.

Solid-state batteries are developing as an alternative, improving operational safety since these batteries won’t spill toxic liquids when punctured, and they can be charged much more quickly.

Until now, an unsolved problem in solid-state batteries limited their durability.

When lithium ions are inserted into or extracted from the electrodes of the battery, the crystalline structure of the material changes, making the electrode expand or shrink. These repeated changes in volume damage the interface between the electrodes and the solid electrolyte and cause irreversible alterations in the crystal chemistry of the electrodes.

So a team of scientists led by professor Naoaki Yabuuchi of Yokohama National University, Japan, investigated a new type of positive electrode material with unprecedented stability in solid-state batteries.

Their work, which was published in Nature Materials, was co-authored by associate professor Neeraj Sharma from the University of New South Wales Sydney, and Dr Takuhiro Miyuki from Japan’s Libtec.

The material is Li8/7Ti2/7V4/7O2, a binary system composed of optimised portions of lithium titanate (Li2TiO3) and lithium vanadium dioxide (LiVO2). When ball-milled down to an appropriate particle size in nanometres, this material offers high capacity thanks to its large quantity of lithium ions that can be reversibly inserted and extracted during the charge/discharge process.

It has nearly the same volume when fully charged and fully discharged.

Researchers found this is the result of a fine balance between two independent phenomena that occur when lithium ions are inserted or extracted from the crystal.

The removal of lithium ions, or ‘delithiation’, causes an increase in free volume in the crystal, which makes it shrink, while some vanadium ions migrate from their original position to the spaces left behind by the lithium ions, acquiring a higher oxidation state in the process. This causes a repulsive interaction with oxygen, which in turn produces an expansion of the crystal lattice.

“When shrinkage and expansion are well balanced, dimensional stability is retained while the battery is charged or discharged, i.e. during cycling,” Yabuuchi says.

“We anticipate that a truly dimensionally invariable material – one that retains its volume upon electrochemical cycling – could be developed by further optimising the chemical composition of the electrolyte.”

Yabuuchi says the development of long-life and high-performance solid-state batteries would solve some of EV problems, adding it may be possible to fully charge an EV in five minutes.

Testing of the solid-state cell by combining it with an appropriate solid electrolyte and a negative electrode showed a remarkable capacity of 300mA.h/g with no degradation over 400 charge/discharge cycles.

“The absence of capacity fading over 400 cycles clearly indicates the superior performance of this material compared with those reported for conventional all-solid-state cells with layered materials,” Sharma adds.

“This finding could drastically reduce battery costs. The development of practical high-performance solid-state batteries can also lead to the development of advanced electric vehicles.”

By further refining dimensionally invariant electrode materials, it may soon be possible to manufacture batteries that are good enough for EVs in price, safety, capacity, charging speed, and lifespan, the scientists add.