Acidic-Induced Liquid Metal–Montmorillonite Self-Sintering Conductive Ink with Multilayer Sandwiched Structure for Flexible Printed Electronics

By virtue of a high conductivity comparable to rigid metals while possessing special deformability like liquid, liquid metal (LM) has presented a huge potential application in the field of flexible printed electronics. However, the typical core–shell structure for the LM ink, which aims to facilitate the stable dispersion of liquid metal nanoparticles (LMNPs), would impede their spontaneous connectivity and require extra techniques to externally rupture the organic shell layer, thus limiting the standardization for the LM ink application. Herein, we instead report an LM-based self-sintering conductive ink relying on inorganic montmorillonite (MMT) acting as an intercalated nanocomposite. In detail, we found that in the ethanol/water mixtures, hydrochloric acid (HCl) in combination with ultrasonication could induce the dissolution of the oxide shell of LMNPs and meanwhile achieve the exfoliation of MMT, subsequently self-assembling into a nanointercalated composite of LM-MMT due to the strong electrostatic interaction. When subjected to elevated temperature during curing, the MMT nanosheet would undergo a horizontally orientated stack and squeeze the LMNPs with the evaporation of the solvent, thus inducing their deformation, aggregation, and further connectivity, and finally, a conductive coating with a multilayered sandwich structure is formed. The optimized coating possessed an initially high electrical conductivity of up to 1.2 × 105 S m–1 and exhibited a unique anisotropic mechanical-electrical response, i.e., good resistance to shear deformation and sensitive response to tensile deformation. Moreover, the ink has good flowability and processability, allowing its availability on various flexible substrates with high-temperature resistance and long-term stability. Finally, LED array circuits, photosensitive circuits, and motion strain sensors were respectively fabricated on different flexible substrates as a proof of concept for the ink's processability, electrical integration capability, and future application potential.