Stress response of germanium electrodes during Lithiation/delithiation cycling

An in situ study of stress evolution of germanium as a lithiumion battery electrode material is presented. Thin films of germanium are cycled in a half-cell configuration with lithium metal foil as counter/reference electrode, with 1M lithium hexafluorophosphate in ethylene carbonate, diethylene carbonate, dimethyl carbonate solution (1:1:1, wt. %) as electrolyte. Real-Time stress evolution in the germanium thinfilm electrodes during electrochemical lithiation and delithiation is measured by monitoring the substrate curvature using the multi-beam optical sensing method. Germanium thin film undergoes extensive inelastic deformation during electrochemical lithiation and delithiation similar to silicon. The peak compressive stress during lithiation in germanium was 0.6 GPa, which is almost half of the peak compressive stress of lithiated silicon. The tensile stress of lithiated Ge on the other hand was almost same as that of lithiated silicon, 1 GPa of peak stress. The stress response of the first cycle was slightly different from that of the second cycle, which is an indication of irreversible structural changes in the Ge electrode during the first cycle. The Ge thin films survived stresses as high as 1 GPa of tensile stress without developing any cracks while the Si films under same conditions showed extensive cracking, consistent with previous observations which claimed lithiated Ge to be tougher than lithiated Si.