2D Metallic Abnormal Li₂Cl Crystals with Unique Electronic Characteristics Applied in Capacitor and Humidity Sensor

Understanding the configuration and properties of the simplest lithium-ion crystal, the Li-Cl crystal, is essential for developing Li-based electrochemical energy storage and conversion devices. Under ambient conditions, LiCl in a 1:1 stoichiometry is the only known stable form of the Li-Cl crystal, and the corresponding crystals are insulating. Here, using cryo-electron microscopy (cryo-EM) and X-ray photoelectron spectroscopy (XPS), this work reports a novel abnormal 2D Li₂Cl crystal in ultra-thin reduced graphene oxide (rGO) membranes by simply soaking the rGO membrane in unsaturated LiCl solution under ambient conditions and showed its high-resolution image. Unexpectedly, such Li₂Cl crystal presents metallic properties rather than insulating properties. Using vacuum filtration of the unsaturated LiCl solution through rGO membranes, this work prepares the rGO membranes with high content Li₂Cl, displaying a high areal capacitance of 220 mF cm⁻². Remarkably, the Li₂Cl-rGO membranes also exhibit heterostructure property and piezoelectricity. For the excess Li and unique electrical characteristics, these Li₂Cl-rGO membranes can be used as a humidity sensor. These findings point to possibilities for creating new Li-based materials and fabricating novel high-performance electronic devices, transistors, and sensors.