One dimensional (1D) nanostructures offer prospects for enhancing electrical, thermal and mechanical properties of a broad range of functional materials and composites, but their synthesis methods, such as catalyst-assisted vapor deposition, physical vapor deposition, hydrothermal synthesis, the use of sacrificial templates and others, are relatively expensive and difficult to scale. Here we demonstrate direct transformation of bulk materials into nanowires at room temperature and ambient pressure without use of catalysts, corrosive or toxic chemicals, or any external stimuli [1]. The nanowire formation proceeds via a minimization of the strain energy at the boundary of chemical transformation reaction front. We show transformation of multi-micron particles of Al alloy and Mg alloy into alkoxide nanowires of tunable dimensions, which are converted into oxide nanowires upon simple heating in air. The reported approach provides opportunities for ultra-low cost large-scale synthesis of 1D materials and membranes.

Conventional polymer separators for Li-ion batteries (LIBs) suffer from limited mechanical strength and low thermal stability, which may lead to thermal runaway and cell explosion. We produced flexible, binder-free, nonwoven fabric composed of gamma-Al₂O₃ nanowires, using a simple tape casting technique followed by a heat treatment in air. The overall morphology of the produced fabric is somewhat similar to that of a paper, where the cellulose fibers are replaced with stronger and stiffer Al₂O₃ nanowires. Owing to the fibrous nature of thus-produced free-standing films and the small diameter of the Al₂O₃ nanowires, they exhibit good flexibility. This is in contrast to anodized Al₂O₃ membranes of comparable thickness that are known to be extremely brittle and difficult to handle. Results of electrolyte wetting tests revealed significantly superior performance of Al₂O₃ paper compared to commonly used commercial olefin (polypropylene) separator or a cellulose fiber separator. The wetting rate of the Al₂O₃ separator is significantly higher owing to its polar nature, as determined by both the final wetting area and the speed of etting. Thermal stability tests demonstrate the advantage of having a flexible porous ceramic separator with operating temperatures above 800°C, which is important because of rapid heating to high temperatures that may occur in failing LIBs. In contrast, the most commonly used olefin separators typically start melting at around 120°C and oxidize at around 300°C. Finally, the strength of ceramic fibers is known to exceed that of the olefins, which should allow formation of thinner separators in automotive LIBs without sacrifice of their mechanical properties.

[1]. D Lei, J Benson, A Magasinski, G Berdichevsky, G Yushin, Transformation of bulk alloys to oxide nanowires, Science 2017, 355 (6322), 267-271.