Ozone-Based Atomic Layer Deposition of Crystalline V₂O₅ Films for High Performance Electrochemical Energy Storage

Abstract

A new atomic layer deposition (ALD) process for V 2 O 5 using ozone (O 3 ) as oxidant has been developed that resulted in crystalline V 2 O 5 thin films which are single-phase and orthorhombic on various substrates (silicon, Au-coated stainless steel, and anodic aluminum oxide (AAO)) without any thermal post-treatment. Within a fairly narrow temperature window (170−185°C), this low temperature process yields a growth rate of ∼0.27 Å/cycle on Si. It presents good uniformity on planar substrates. Excellent conformality enables deposition into high aspect ratio (AR) nanopores (AR > 100), as needed for fabrication of three-dimensional (3D) nanostructures for next generation electrochemical energy storage devices. V 2 O 5 films obtained using O 3 -based ALD showed superior electrochemical performance in lithium cells, with initial specific discharge capacity of 142 mAh/g in the potential range of 2.6−4.0 V, as well as excellent rate capability and cycling stability. These benefits are attributed primarily to the crystallinity of the material and to fast transport through the thin active storage layers used. KEYWORDS: atomic layer deposition, vanadium oxide, ozone, electrochemical energy storage ■ INTRODUCTIONElectrical energy storage is a key challenge for effective use of conventional and renewable energy sources. Applications include electric vehicles, residential energy systems based on renewables, management of distributed and grid level large scale power systems, and portable electronic devices. 1,2 Electrochemical devices for reversible charge storage, both Li ion batteries and supercapacitors, are needed with high power and high energy density. Three-dimensional (3D) nanostructures offer large surface area, enabling active storage material in the nanostructures to be spatially close to electrolyte, reducing diffusion time for Li transport to fully utilize the active material and thus achieving higher power. However, high density nanostructure arrays, aimed at maintaining high gravimetric and volumetric energy density, require materials synthesis methods capable of conformally coating nanostructures in high aspect ratio (AR) 3D geometries. 3−5 Conformal thin film coating methods are required to achieve mechanically stable, binder-free, and high surface area electrodes. 6,7 Atomic layer deposition (ALD) is a unique thin-film deposition technique which exploits self-limited reactions, leading to monolayer thickness control and unprecedented uniformity and conformality in even the most stringent high AR nanostructures. It is therefore a very promising technique for fabrication of nanoscale heterostructured 3D energy storage devices as we have shown in previous work. 7,8 Cathode materials typically limit the energy density of electrochemical storage devices since they have much lower specific capacities compared with anode materials. 2 Among well-known cathode materials, V 2 O 5 offers relatively high specific capacity (147 mAh/g at 2.6−4.0 V; 294 mAh/g at 2.0− 4.0 V), fast lithiation, and better safety, which has...

Atomic Layer Deposition of Bismuth Vanadates for Solar Energy Materials

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