Fig. 6: Operando Li-driven stress monitoring in a symmetrical InLi_x | LPS | InLi_x cell by an FBG located at the interface between the cathode and the solid-state electrolyte.

a Scheme of the modified Swagelok cell with the implemented optical fiber and the external force sensor. The corresponding X-, Y-, and Z-axis are detailed in the different views. For the sake of simplicity, the “longitudinal” and “transversal” axis is used with respect to the optical fiber, and the “axial” axis is only used with respect to the cell. The direction of the Li⁺ ions during charge/discharge is detailed in the scheme. b Time-resolved voltage (top), and external cycling pressure (bottom) for the aforementioned ASSB cycled at C/30 (5.83 mA g⁻¹) and 25 °C for three consecutive cycles, at an externally applied pressure of 2.7 MPa, and e 21 MPa. c, f 2D stack view of the collected spectra by the FBG sensor, with the corresponding galvanostatic charge/discharge cycle when the externally applied pressure is 2.7 and 21 MPa, respectively. The charge and discharge processes are plotted in red and blue, respectively. d Galvanostatic cycle (top), and operando stress evolution obtained internally by the FBG sensor and with the experimental calibration of the sensor (bottom) when the externally applied pressure is 2.7 MPa, and g 21 MPa. The points at the beginning/middle/end of the charge/discharge are indicated by colored dots, also marked in the corresponding FBG spectra in (c). Note that due to the location of the FBG sensor in the positive electrode, the relative stress is normalized (Δσ = 0 MPa) at the beginning of the discharge to compare positive stress variations.