Extended Data Fig. 3: Galvanostatic cycling of the In/InLi|LBPSI|In/InLi symmetric cell, the three-electrode sulfur-containing cell and long-term cycling of the ASSLSB at extreme charging rates.

a,b, The galvanostatic cycling of the In/InLi|LBPSI|In/InLi symmetric cell (In/InLi foil as both electrodes) with current increasing from 0.26 mA cm⁻² to 31.8 mA cm⁻² at 25 °C (the Li plating/stripping time is 30 min for measurements with current density ≤ 3.8 mA cm⁻² and 5 min for those with current density ≥ 6.4 mA cm⁻²) and enlarged section at high currents (reaching the voltage limit) (b). The symmetric cell can indeed operate at higher currents without soft or hard short-circuit at the areal capacity used, but in fact suffers from much greater overpotential than at low currents; this indicates that the In/InLi electrode shows limited reaction kinetics. This result offers some qualitative hints on the overall poor reaction kinetics for In/InLi electrodes. Such a high overpotential at the anode, along with the ohmic resistance from the SE, contributes to the high overpotential of full cells at high currents. c–f, Three-electrode cell measurement to examine the origin of the high overpotential of full cells at high currents. c, The scheme of the three-electrode cell, with a thin Li wire placed in the electrolyte layer close to the In/InLi counter electrode as a reference electrode. d–f, Discharge–charge voltage profiles at the current density of 0.08 mA cm⁻² (d), 8.3 mA cm⁻² (e) and 15.0 mA cm⁻² (f) (sulfur loading of 1.0 mg cm⁻²; 25 °C). At a very low current density of 0.08 mA cm⁻², the potential of the In/InLi electrode stabilizes at around 0.61 V. As the current density increases to 8.3 mA cm⁻², the potential of the In/InLi electrode increases to about 0.75 V (on discharge) and reduces to about 0.46 V (on charge), resulting in a discharging overpotential of 0.14 V and charging overpotential of 0.15 V. On further increasing of the current to 15.0 mA cm⁻², the potential of the In/InLi electrode increases to about 0.90 V (on discharge) and the potential decreases to about 0.31 V (on charge), resulting in a discharging overpotential of 0.29 V and charging overpotential of 0.30 V. Therefore, the reaction overpotential of the In/InLi electrode is high and increases greatly with the current density. Furthermore, there is a large increase of the ohmic resistance when the current is increased from 0.08 mA cm⁻² to 8.3 mA cm⁻². The ohmic resistance is qualitatively recognized by the abrupt voltage change (IR drop) on supplying of the charging/discharging currents. The increased ohmic resistance is the result of the thick layer of the electrolyte separator between the sulfur working electrode and the Li reference electrode, and the moderate ionic conductivity of the SE (2.4 mS cm⁻¹). The high overpotential of the cell at high charging rates is largely because of the In/InLi anode and IR ohmic polarization⁵⁶. g–j, The actual discharge–charge voltage profiles by using the potential versus In/InLi counter electrode of the ASSLSBs fitted with the LBPSI and Li_5.5PS_4.5Cl_1.5 electrolyte at different current densities at 30 °C (g,h) and 60 °C (i,j). k,l, Cycling performance (k) and discharge–charge voltage profiles (l) of ASSLSB fitted with LBPSI electrolyte at a charging rate of 100C and a discharging rate of 2C at 60 °C. The cell was subjected to an activation process by cycling at 1–80C (charging rates) for several cycles. The cell sustains 5,000 cycles with a capacity retention greater than 80.4% (sulfur loading: 1.0 mg cm⁻²). It is clear that the cell experienced marginal voltage degradation over the cycling. The fluctuation in Coulomb efficiency is because of the limited resolution for data acquisition from the equipment with such a short duration of charging.