Extended Data Fig. 6: Behaviors regarding constriction susceptibility for Si, Mg₂Si, and Cu in experiment, and computational prediction of such behaviors for more materials.

(a) Change of XRD FWHM of Si powder in a solid layer pressed with Li metal solid layer at 400 MPa and held at 50 MPa for 40 hours (solid press), compared with that of such solid-pressed Si-Li interface after dropping liquid electrolyte (LE) and holding for 20 hours without external pressure. (b) Time evolution of XRD FWHM change for solid-pressed Si with Li metal after 10 min, 1 h, 20 h, and 40 h. (c) XRD peaks of Si after solid-pressing with Li metal and holding for 0 min, 10 min, and 40 hours. (d) XRD peaks of solid-pressed Si after dropping LE for 10 min, 20 min, and 30 hours. (e1-e3) Illustration of the time evolution of different XRD peaks in (c), which were used to plot ΔFWHM in (b). (e4) The time evolution of a XRD peak in (d) at 10 min and 20 h. The peak almost disappears after 30 hours. (f) Voltage v.s. lithiation composition per atom for materials with voltage between 0-1 V, and with capacity between 0-10000 mAh/g. (g) Two elements coappearance between the dashed lines in Fig. 3f. Only compounds with bandgap less than 1.5 eV are counted. (h) Electrochemical profiles for discharging Li toward Mg₂Si in liquid-state and solid-state battery systems at 0.2 mA cm^–2 at room temperature. Mg₂Si is on the boundary but to the far right of Si in Fig. 3f. The lithiation capacity originally located above 0 V in the liquid electrolyte battery is largely suppressed in the solid-state battery, which is replaced by the lithium plating capacity below 0 V. This suggests that due to the much lower lithiation voltage and much higher constriction-susceptibility, Mg₂Si intrinsically prefers Li plating, or with a much less active alloying preference than Si. Thus, lithium plating from Mg₂Si will benefit less from the homogeneous current density distribution from the surface lithiation sites than Si. (i) Cyclig performance at 1 C-rate at 55 °C of battery with a structure of (Cu particle and graphite mixture)-SEs-NMC83 (cathode loading = 15 mg/cm²).