Increasing interest has recently been devoted to developing small, rapid, and portable electronic devices; thus, it is becoming critically important to provide matching light and flexible energy-storage systems to power them. To this end, compared with the inevitable drawbacks of being bulky, heavy, and rigid for traditional planar sandwiched structures, linear fiber-shaped lithium-ion batteries (LIB) have become increasingly important owing to their combined superiorities of miniaturization, adaptability, and weavability, the progress of which being heavily dependent on the development of new fiber-shaped electrodes. Here, we report a novel fiber battery electrode based on the most widely used LIB material, titanium oxide, which is processed into two-dimensional nanosheets and assembled into a macroscopic fiber by a scalable wet-spinning process. The titania sheets are regularly stacked and conformally hybridized in situ with reduced graphene oxide (rGO), thereby serving as efficient current collectors, which endows the novel fiber electrode with excellent integrated mechanical properties combined with superior battery performances in terms of linear densities, rate capabilities, and cyclic behaviors. The present study clearly demonstrates a new material-design paradigm toward novel fiber electrodes by assembling metal oxide nanosheets into an ordered macroscopic structure, which would represent the most-promising solution to advanced flexible energy-storage systems.

The strong interest in macroscopic graphene and/or carbon nanotube (CNT) fiber has highlighted that anisotropic nanostructured materials are ideal components for fabricating fiber assemblies. Prospectively, employing two-dimensional (2D) crystals or nanosheets of functionality-rich transition metal oxides would notably enrich the general knowledge for desirable fiber constructions and more importantly would greatly broaden the scope of functionalities. However, the fibers obtained up to now have been limited to carbon-related materials, while those made of 2D crystals of metal oxides have not been achieved, probably due to the intrinsically low mechanical stiffness of a molecular sheet of metal oxides, which is only few hundredths of that for graphene. Here, using 2D titania sheets as an illustrating example, we present the first successful fabrication of macroscopic fiber of metal oxides composed of highly aligned stacking sheets with enhanced sheet-to-sheet binding interactions. Regardless of the intrinsically weak Ti-O bond in molecular titania sheets, the optimal fiber manifested mechanical performance comparable to that documented for graphene or CNTs. This work provided important hints for devising optimized architecture in macroscopic assemblies, and the rich functionalities of titania promises fibers with limitless promise for a wealth of innovative applications.

Platelet crystals of a layered perovskite showed massive accordion-like swelling in a tetrabutylammonium hydroxide solution. The permeation of the solution induced the huge expansion of the interlayer spacing as well as the crystal thickness up to 50-fold, leading to a very high water content of >90 wt%.

Osmotic swelling behaviors in layered perovskite niobate were examined in aqueous solutions containing three types of amine-related agents including quaternary ammonium hydroxides and tertiary aminoethanol. Platelet microcrystals of a protonated layered perovskite niobate, HCaNbO·1.5HO, were found to show enormous swelling in the aqueous solutions, which was clearly recognized by the noticeable expansion of the sample volume over 100-fold. Optical microscopy observations revealed that the crystals underwent accordion-like elongation in the layer-stacking direction up to several ten-fold the initial thickness. Small-angle X-ray scattering measurements of swollen samples indicate the expansion of interlayer separation ranging from ∼20 nm to over 100 nm, which is primarily governed by the concentrations of the amine-related agents. The magnitudes of the interlayer separation were comparable to those of the macroscopic swelling. The degree of swelling was progressively suppressed with further increasing concentration, and this suppression trend was related to the amines.