Tracking Decomposition Layer Formation in Thin‐Film Si Electrodes via Thermogalvanic Profiles

Si anodes are of great interest for next‐generation Li‐ion batteries due to their exceptional energy density. One of the problems hindering the adoption of this material is the presence of electrolyte decomposition reactions that result in capacity fade and Coulombic inefficiency. This work studies the influence of the decomposition layer in Si on its electrochemical performance using thermogalvanic profiling, a non‐destructive in operando technique. This is accomplished by comparing thermogalvanic profiles of uncoated thin‐film Si to those of lithium phosphorus oxynitride (LiPON)‐coated Si, in which decomposition reactions are inhibited. Through a combination with physico‐chemical methods including scanning electron microscopy and time‐of‐flight secondary ion mass spectrometry, the thermogalvanic profiles are found to contain signatures that reflect the nature of the decomposition layer. More specifically, this decomposition layer appears to gradually develop a passivating function during the first electrochemical cycles. Thermogalvanic profiles collected at later cycles indicate that this passivating behavior is eventually lost, causing the observed capacity degradation. The identification of a passivating regime in Si is highly relevant for the development of high‐capacity Li‐ion batteries. In addition, the use of thermogalvanic profiles to track the properties of decomposition layers could be of interest for monitoring the formation or degradation of battery cells. The thermogalvanic profile of Si reflects the behavior of the formed decomposition layer. When electrolyte decomposition reactions are ongoing due to an incompletely formed decomposition layer, the profile deviates with respect to that of a decomposition‐less model system (e.g., LiPON‐coated Si). Profile convergence indicates further decomposition is temporarily halted due to the dense nature of the formed decomposition layer.