Towards Quantifying Capacity Losses Due to Solid Electrolyte Interphase Evolution in Silicon Thin Film Batteries

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To better understand failure mechanisms in lithium-ion batteries (LIBs), it is imperative to quantify the capacity fading contributions from active material loss and solid electrolyte interphase (SEI) growth. However, these two contributions are indistinguishable in traditional electrochemical measurements. In this study, we used a model system in combination with (1) precision measurements of the overall Coulombic efficiency via electrochemical experiments and (2) X-ray reflectivity (XRR) measurements of the active material losses. The model system consisted of a 515 Å thick amorphous silicon (a-Si) thin film on silicon carbide in a half-cell geometry using a carbonate electrolyte with LiPF6 salt. Schematics of the cells used in operando XRR and precision electrochemistry measurements are shown in Fig. (a) and (b) respectively.Specifically, we used surface/interface sensitive operando XRR to monitor the a-Si/LixSi thickness during (de)lithiation in order to quantify the capacity fading due to active material loss. Combined with electrochemical analysis, we quantified the capacity loss originating in SEI growth during each cycle (Fig. (c)). We have found that SEI continues to grow with cycling, and the SEI thickness is dependent on the time spent at low potentials (Fig. (d)). The continued SEI growth results in increasing overpotentials due to increased SEI resistance, which leads to a lower extent of lithiation when the cut-off voltage is reached. Finally, we extracted a proportionality constant for SEI growth following a parabolic growth law. Our results are in fair agreement with the capacity losses observed in full cell systems.We envision that the proposed methodology can be used as a qualitative measure of the ion transport properties of the SEI. The model-based approach allows for quantitative determination of lithium ion loss mechanisms in LIBs by separately tracking lithium ions within the active materials and will aid in developing ways to reduce parasitic losses. Figure 1