Single-crystal silicon membranes with high lithium conductivity and application in lithium-air batteries

Silicon has become an intriguing anode material in lithium ion secondary batteries because of its theoretical high gravimetric and volumetric capacity for lithiation (i.e., 4200 and 1750 mA h cm ^-3 , respectively). ^[1-6] To approach the theoretical limits, the conductivity of lithium in silicon should be high because the diffusion rate of lithium in and out of silicon determines the discharge/charge speed and influences the battery performance. ^[7-9] For example, enormous efforts have been devoted to utilizing nanostructured silicon to improve the electrochemical performance of lithium-ion batteries since decreasing the lateral dimensions of silicon is expected to increase the diffusion rate of lithium and reduce the diffusion time for efficiently exchanging lithium ions between electrolytes and anodes. ^[4,10-20] Moreover, the large surface-to-volume ratios of silicon nanostructures can facilitate accommodation of their large volume expansion/contraction during lithiation/delithiation. ^[2,4,12-14] However, reducing the size of silicon structures is not always helpful for applications that require selective diffusion of lithium ions across a continuous membrane, such as for a lithium ion selective electrode (Li-ISE). ^[21-24] This kind of application motivated us to study the diffusion behavior of lithium in extended, large-area silicon membranes, in particular, single-crystalline silicon membranes with continuous lattices. Herein we report the precise measurement of lithium conductivity in single-crystalline silicon membranes and in-situ monitoring of the structural variation of the membranes under continuous current flows. The results reveal that the singlecrystalline silicon membranes exhibit exceptional lithium conductivity as high as - 10 ^-6 S cm ^-1 and their single crystallinity remains even with a current density of 1 mA cm ^-2 . The high conductivity and structural stability makes the single-crystalline silicon membranes promising in lithium-air batteries, Li-ISEs, and detectors.