Scalable Manufacturing of Textured Na_xCoO₂ Nanosheet Thermoelectric Foils by Shear-Force-Assisted Coating

Developing scalable methods to mass produce macroscopic assemblies of 2D materials/nanosheets while retaining their nanofeatures is essential to translating their performance-enhancing 2D properties into macroscopic functionality. Here, we report a novel nanosheet processing technique to produce self-standing foils of Na_xCoO₂ nanosheets in a manner compatible with large-scale industrial manufacturing, such as roll-to-roll processing. In this approach, the nanosheets are chemically modified and coated onto a polymeric substrate using bidirectional shear force. Through a postheat treatment, the coated film is crystallized into the thermoelectric Na_xCoO₂ phase (0.63 < x < 0.71) while simultaneously burning away the polymer substrate, resulting in self-standing textured thin foils of thermoelectric Na_xCoO₂ nanosheets. Nanosheet foils up to 10 cm in lateral dimension and as thin as ∼10 μm are demonstrated. Seebeck coefficient and electrical conductivity values of the novel, shear-force-assisted coating-made foils are 64.6 μV. K^-1 and 115.2 S.cm^-1 at room temperature, respectively, resulting in significantly higher thermoelectric power factor compared to spin coating and blade coating fabricated foils, mainly owing to much higher electrical conductivity due to better nanosheet alignment and reduced porosity.