Fast thin film devices boost energy storage, electronics

A team of researchers from the Max Planck Institute of Microstructure Physics, Halle (Saale) in Germany, the University of Cambridge and the University of Pennsylvania, have reported the realisation of single-crystalline T-Nb₂O₅ thin films having two-dimensional (2D) vertical ionic transport channels, resulting in a fast and colossal insulator-metal transition via Li-ion intercalation through the 2D channels. Since the 1940s, scientists have been researching the use of niobium oxide, specifically a form of niobium oxide known as T-Nb₂O₅, to create more efficient batteries. This material is known for its ability to allow lithium ions to move quickly within it. The faster these lithium ions can move, the faster a battery can be charged.

The challenge has 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 similar forms, or polymorphs, of niobium oxide. Now, in a paper published in Nature Materials, researchers have demonstrated the growth of high-quality, single-crystal thin films of T-Nb₂O₅, aligned in such a way that the lithium ions can move faster along vertical ionic transport channels.

The T-Nb₂O₅ films undergo an electrical change at an early state 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 researchers also demonstrated tuneable 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 researchers realised the growth of the single-crystalline T-Nb₂O₅ thin films and showed how Li-ion intercalation can increase their electrical conductivity. Multiple previously unknown transitions in the material’s structure were also 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 it goes from blocking electrical current to conducting it. Researchers rationalised 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.

First author Hyeon Han, from the Max Planck Institute of Microstructure Physics, said in tapping the potential of T-Nb₂O₅ to undergo colossal insulator-metal transitions, the researchers have unlocked an avenue for exploration for next-generation electronics and energy storage solutions. The researchers have discovered 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 faster. This shift enables a range of applications, from high-speed computing to energy-efficient lighting.

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