Scientists develop long-life electrode for solid-state batteries

Electric cars are regarded as the best bet to replace conventional cars with a more environmentally friendly alternative; however, electric cars will most likely run on lithium-ion batteries, which don’t deliver the necessary performance and durability at a reasonable price. Solid-state batteries (SSBs) have gained traction among researchers as a viable alternative. While conventional lithium-ion batteries contain a liquid electrolyte in which lithium ions flow during the charge/discharge process, SSBs are made entirely from solid materials. Besides providing an improvement in operational stability (as they won’t spill toxic liquids when punctured), SSBs can also be charged faster.

However, 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.

A team of scientists led by Professor Naoaki Yabuuchi of Yokohama National University, Japan, have investigated a new type of positive electrode material with enhanced stability in SSBs. The research, which was published in Nature Materials, was co-authored by Associate Professor Neeraj Sharma from UNSW Sydney and Dr Takuhiro Miyuki from LIBTEC, Japan.

The researchers focused on Li_8/7Ti_2/7V_4/7O₂, 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 the order of nanometres, this material offers high capacity due to its large quantity of lithium ions that can be reversibly inserted and extracted during the charge/discharge process.

Unlike other positive electrode materials, Li_8/7Ti_2/7V_4/7O₂ has nearly the same volume when fully charged and fully discharged. The researchers analysed the origin of this property and concluded that it is the result of a fine balance between two independent 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, ie, during cycling. 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 said.

The research team tested this 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 notable capacity of 300 mA·h/g with no degradation over 400 charge/discharge cycles. According to Sharma, 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. “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,” Sharma said.

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.

“The development of long-life and high-performance solid-state batteries would solve some of the problems of electric vehicles. In the future, for instance, it may be possible to fully charge an electric vehicle in as little as five minutes,” Yabuuchi said.

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