Ally Winning, European Editor, PSD
Solid-state batteries (SSBs) look as if they will win the battle to become the technology that replaces the lithium-ion batteries we currently use, especially in applications such as electric vehicles. They have all the positive attributes of Li-ion batteries, but their solid electrolyte is safer and allows much faster charging. The one thing that has held back the adoption of SSBs has been their limited 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. The 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.
A team of scientists led by Professor Naoaki Yabuuchi of Yokohama National University, Japan, may have solved that issue by developing a new type of positive electrode material that has unprecedented stability. Their work, which was published in Nature Materials, was co-authored by Associate Professor Neeraj Sharma from UNSW Sydney, Australia, and Dr. Takuhiro Miyuki from LIBTEC, Japan.
The new electrode material used by the research team was 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 a particle size in the order of nanometers, the material offers high capacity thanks to its large quantity of lithium ions that can be reversibly inserted and extracted during the charge/discharge process. Unlike other positive electrode materials, Li8/7Ti2/7V4/7O2 has nearly the same volume when fully charged and fully discharged. This the result of two phenomena that occur when lithium ions are inserted or extracted from the crystal. On one hand, the removal of lithium ions, or ‘delithiation’, causes an increase in free volume in the crystal, which makes it shrink. On the other hand, 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,” Prof. 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.”
The research team tested the new positive electrode material in an all-solid-state cell by combining it with an appropriate solid electrolyte and a negative electrode. This cell exhibited a capacity of 300 mA.h/g with no degradation over 400 charge/discharge cycles.
“The absence of capacity fading over 400 cycles indicates the superior performance of this material compared with those reported for conventional all-solid-state cells with layered materials," A/Prof. Sharma says. "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 electric vehicles in terms of price, safety, capacity, charging speed, and lifespan.