Mechanical stresses and crystallization of lithium phosphorous oxynitride-coated germanium electrodes during lithiation and delithiation

• Mechanical measurements are used to probe electrochemical processes in germanium.
• Mechanically rigid Li+-conductive coatings can suppress morphological evolution.
• Rate experiments indicate that lithium transport is limited by diffusion.
• Formation of crystalline Li15Ge4 appears as a small tensile feature in stress.
• Amorphization of crystalline Li15Ge4 reduces the energy efficiency of the cell.

The evolution of mechanical stresses during the cycling of lithium phosphorous oxynitride (LiPON) coated germanium thin film electrodes was monitored using substrate curvature measurements. By coating germanium thin films with LiPON, morphology evolution, e.g. crack and island formation, can be strongly suppressed. LiPON-coated germanium thin film electrodes can retain their planar form during cycling, resulting in a clear and reproducible stress response originating primarily from the electrochemical processes occurring during lithiation and delithiation. Together with the electrochemical data, stress measurements were used to infer mechanisms underlying the alloying of lithium with germanium. The stress signatures associated with individual phases, crystallization, and amorphization of lithium-germanium alloys are reported and discussed.

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