Fig. 1. Characterization of the home-made PI film. Cross-section SEM image (a) and the top views (b) of the PI film; The cross-section view of the composite separator (c); The tensile strength test of the three kinds of film after tailoring the same size (d); Thermostability experiment at 200 °C for 10 min (e), Wettability examine of the PP separator, PI membrane and PI with coating to the common electrolyte.

Fig. 2. Characterization of the Li anode interface conductivity and phase change. CV (cyclic voltammetry) curves (a) and the EIS measurement at different rest time (b,c) of the symmetric cells with the different separator; (d) The ion conductivity of the Li anode interface layer based on the EIS result in (b, c); (e) XRD analysis of the cycled Li anode in a symmetric cell with the composite separator.

Fig. 3. The in-situ observation of Li ion nucleation on the current collector without/with the coating. Li ion nucleation on the current collector without the coating at starting (a), 15 min (b) and 30 min (c); Li ion nucleation on the current collector with the coating at starting (d), 15 min (e) and 30 min (f); Electric field gradient simulation at the Li electrode surface without (g) and with (h) the coating by COMSOL Multiphysics. The scale bar is 100 μm.

Fig. 4. Electrochemical performance of the asymmetric and the symmetric cells. The voltage-capacity curves of the first three cycles of the control (a) and the modified cell (b); CE-Cycle profiles of the two kinds of cell in 200 cycles (c); voltage-Cycle curves of the control (d) and the modified (e) cell. The voltage-time curves of the control (f) and modified (g) cell at 1.0 mA cm-2 with the capacity of 1.0 mAh cm-2; The first (h) and the 130th (i) cycle voltage-capacity curve of the symmetric cells; The plating overpotential-cycle graph (j) and the rate capability of the two kinds of cells (k).

Fig. 5. SEM images of the Li anode after 100 cycles with the capacity of 1 mAh cm-2 at 1 mA cm-2. The cross-section views of the pure Li electrode at low (a) and high (b) magnification; The EDS analysis of pure Li (c); The cross-section views of the Li electrode with the composite separator used at low (d) and high (e) magnification, and the corresponding EDS analysis.

Fig. 6. Illustration of Li ions plating behavior with the common PP separator (a) and (b) the PI with the coating.

Fig. 7. Electrochemical performance of the full cell with the NCA cathode. The capacity-cycle curve of the pouch cell with PP and PI@coating separator (a) and the corresponded typical charge/discharge profiles in (b) and (c); The thermostability of the cell with the different separator (d) and electrochemical test at 0.1C after the thermostability treatment for cell with PI@coating (e); The cycling capability with Li-free anode in the coin cell (d).