Cycling Behavior of Thin Film LiAl Electrodes with Liquid and Solid Electrolytes

Abstract

An Al foil was partially converted to by electrodeposition onto one side. The remaining unalloyed Al acted as a substrate for the film. Cycling was carried out with the film in contact with either the liquid propylene (PC) or the solid . After initial substantial capacity losses, coulombic efficiencies were 97 and 99% with the PC and PEO electrolytes, respectively. There was an additional loss with PEO (2 μA cm−2) due to a combination of oxygen and water vapor ingression and Li diffusion into the steel current collector. The loss in PC occurred during active operation, there being negligible loss on standing. Lithium incorporation into the film occurred readily due to the high diffusion coefficient of Li in . Stripping of the Li caused some cracking of the film as a result of the large molar volume change in forming from , but adhesion of the film to the Al substrate was strong. Literature values for the diffusion coefficient of Li in are low (10−16 and 10−22 cm2s−1 at 140° and 25°C, respectively), indicating that the growth of a few monolayers of the α phase on the surface of the could prevent further discharge. A model is proposed whereby the at the interface with the electrolyte becomes “super depleted” on discharge, but not enough to nucleate . Thus a diffusion gradient is set up in the film, causing the to grow from the α/β interface in a needle‐like morphology towards the electrolyte. Such a model is compatible with the substantial capacity losses during initial cycles.