关键词: 金属锂电池, 锂负极, 锂合金, 薄膜, 溅射, 保护层

Abstract:

Lithium (Li) is the ideal anode material for next-generation high-energy-density Li batteries. The performance of a Li anode can be improved by coating it with a Li alloy film, the mechanism for which has not been well interpreted. Herein, a Li-Al alloy layer as a mixed conductor, which is formed by sputtering a 74 nm-thick Al film onto a Li sheet, demonstrates a different protection mechanism with respect to those of electron-conducting and ion-conducting protection layers. Scanning electron microscopy images show that Li ions are reduced on the alloy surface and spontaneously diffuse into the alloy layer because the Li⁺ concentration in the alloy layer is poor. Furthermore, the Li⁺ diffusion coefficient of the Li-Al alloy is superior to that of the bulk Li. Both factors ensure that Li is not plated on the surface of the alloy layer. A part of the diffused Li atoms is stored in the alloy layer, which considerably increases the layer thickness, and the remaining diffused Li is condensed at the interface between the alloy layer and the Li metal sheet. However, the alloy protection layer cracks after 200 cycles owing to the severe volume variation. Then, a liquid electrolyte can come in contact with Li through the cracks, and the alloy layer is gradually invalidated. This novel protection mechanism, i.e., the isolation of the reduction of Li⁺ from the nucleation/growth of Li in space, is very promising for improving the cycling performance of the Li metal anode. An ideal alloy-protection layer with high ion conductivity and excellent mechanical stability should guarantee the commercialization of Li anodes in the near future.

Key words: lithium metal battery, lithium anode, li alloy, thin film, sputtering, protective layer