Mechanical stresses and morphology evolution in germanium thin film electrodes during lithiation and delithiation

摘要

Germanium and silicon can be used to reversibly store large amounts of lithium, but as a result, suffer from significant volumetric and structural changes during cycling. The mechanical stresses associated with these changes were investigated using in situ stress measurements on thin film electrodes. Results for germanium, with its superior transport properties, are compared to silicon, which is structurally similar. The nominal stresses developed in amorphous lithium germanium (a-LixGe) were found to be roughly 30% lower than in a-LixSi. When the cycling rate was increased, the germanium electrode showed a smaller loss in capacity than silicon. Crystalline Li15Ge4 was observed to form below 100 mV and resulted in a distinct tensile bump in nominal stress. During extended cycling, the maximum mechanical stress signal of the film electrodes irreversibly decreased without an apparent loss in capacity. In contrast to silicon films, which typically fracture and lose capacity during cycling, germanium films were capable of reorganizing into three-dimensional structures, thereby improving their mechanical response while minimizing electrochemical energy loss. The reduced nominal flow stresses observed in a-LixGe and their weak dependence on charge discharge rates correlated with the reduced rate sensitivity found in germanium electrodes as compared to silicon. (C) 2015 Elsevier B.V. All rights reserved.