On the conversion of NDP energy spectra into depth concentration profiles for thin-films all-solid-state batteries

A three-step numerical procedure has been developed, which facilitates the conversion of NDP energy spectra into lithium concentration depth profiles for thin-film Li-ion batteries. The procedure is based on Monte Carlo modeling of the energy loss of charged particles (ions) in the solid media, using the publically available SRIM/TRIM software. For the energy-to-depth conversion, the battery stack has been split into finite volume elements. Each finite volume element becomes a source of ions according to the employed nuclear reaction. Ions loos energy when they move across the battery stack towards the detector. The as-obtained simulated spectra have been compared with the experimentally measured spectra. The thicknesses of the battery stack layers were estimated by minimizing the deviation between the simulated and measured spectra. Subsequently, a relation between the average energy of detected ions and the depth of the corresponding finite volume element, yielding a calibration function, was used to relate that particular part of the spectra with the depth of its source. At the final stage, a Bayesian estimator was used to find the distribution of lithium at a particular depth. The developed procedure was applied to a practically relevant case study of Si immobilization in the LPO electrolyte of all-solid-state thin-film batteries. It is shown that the lithium immobilization process in the LPO electrolyte is responsible for the battery degradation process.