Reaction of Li with alloy thin films studied by in situ AFM

Many intermetallic materials deliver poor capacity retention when cycled vs. Li. Many authors have attributed this poor capacity retention to large volume expansions of the active material. Here we report the volume changes of continuous and patterned films of crystalline Al, crystalline Sn, amorphous Si (a- Si), and a-Si0.64Sn0.36 as they reversibly react with Li measured by in situ atomic force microscopy (AFM). Although these materials all undergo large volume expansions, the amorphous phases undergo reversible shape and volume changes. The crystalline materials do not. We attribute this difference to the homogeneous expansion and contraction that occurs in the amorphous materials. Inhomogeneous expansion occurs in the crystalline materials due to the presence of coexisting phases with different Li concentrations. Thin films of a- Si and a- Si0.64Sn0.36 show good capacity retention with cycle number. (C) 2003 The Electrochemical Society.