Li half-cells were prepared as described in the Experimental Section and SI. Briefly, each cell contained a Li anode and a current collector (either a stainless steel (SS) spacer or Cu foil), separated by either gel-2 or by a traditional (Celgard) separator soaked in electrolyte (LHCE control). Figure compares the Li half-cell CE over 50 cycles for the former (black trace) and latter (red and blue traces) cases, at a current density of 1.0 mA cm -2 (= 1 C) and a capacity of 1.0 mAh cm -2 . We note that an SS current collector was chosen for gel-2, rather than Cu foil (the conventional choice for Li half-cells), because it provided a flatter surface for gel deposition. Comparison of the average Li half-cell CE over 50 cycles for the red (98.3 ± 0.9%) and blue (98.5 ± 0.6%) traces indicates that no significant artifacts are introduced by this choice.