Faster thin film devices for energy storage and electronics

Since the 1940s, scientists have been exploring the use of niobium oxide, specifically a form of niobium oxide known as T-Nb₂O₅, to create more efficient batteries. This unique material is known for its ability to allow lithium ions, the tiny charged particles that make batteries work, to move quickly within it. The faster these lithium ions can move, the faster a battery can be charged.

The challenge, however, has always been to grow this niobium oxide material into thin, flat layers, or 'films' that are of high enough quality to be used in practical applications. This problem stems from the complex structure of T-Nb₂O₅ and the existence of many similar forms, or polymorphs, of niobium oxide.

Now, in a paper published in Nature Materials, researchers from the Max Planck Institute of Microstructure Physics, University of Cambridge and the University of Pennsylvania have successfully demonstrated the growth of high-quality, single-crystal thin films of T-Nb₂O₅, aligned in such a way that the lithium ions can move even faster along vertical ionic transport channels.

The T-Nb₂O₅ films undergo a significant electrical change at an early stage of Li insertion into the initially insulating films. This is a dramatic shift -- the resistivity of the material decreases by a factor of 100 billion. The research team further demonstrate tunable and low voltage operation of thin film devices by altering the chemical composition of the 'gate' electrode, a component that controls the flow of ions in a device, further extending the potential applications.

The Max Planck Institute of Microstructure Physics group realized the growth of the single-crystalline T-Nb₂O₅ thin films, and showed how Li-ion intercalation can dramatically increase their electrical conductivity. Together with the University of Cambridge group multiple previously unknown transitions in the material's structure were discovered as the concentration of lithium ions was changed. These transitions change the electronic properties of the material, allowing it to switch from being an insulator to a metal, meaning that it goes from blocking electric current to conducting it. Researchers from the University of Pennsylvania rationalized the multiple phase transitions they observed, as well as, how these phases might be related to the concentration of lithium ions and their arrangement within the crystal structure.

These results could only have been successful through synergies between the three international groups with diverse specialties: thin films from the Max Planck Institute of Microstructure Physics, batteries from the University of Cambridge, and theory from the University of Pennsylvania.

"In tapping into the potential of T-Nb₂O₅ to undergo colossal insulator-metal transitions, we have unlocked an exciting avenue for exploration for next-generation electronics and energy storage solutions," says first author Hyeon Han of the Max Planck Institute of Microstructure Physics.

"What we have done is find a way to move lithium ions in a way that doesn't disrupt the crystal structure of the T-Nb₂O₅ thin films, which means the ions can move significantly faster.," says Andrew Rappe of the University of Pennsylvania. "This dramatic shift enables a range of potential applications, from high-speed computing to energy-efficient lighting and more."

Clare P. Grey of University of Cambridge comments that "The ability to control the orientation of these films allows us to explore anisotropic transport in this technologically-important class of materials, which is fundamental to our understanding of how these materials operate."

"This research is a testament to the power of an interdisciplinary experiment-theory collaboration and an insatiable scientific curiosity," says Stuart S. P. Parkin, of Max Planck Institute of Microstructure Physics. "Our understanding of T-Nb₂O₅ and similar complex materials has been substantially enhanced, leading we hope to a more sustainable and efficient future, by taking advantage of the very interesting field of iontronics that goes beyond today's charge-based electronics."