Artificial SEI Layer Combined with Single-Ion Polymer Electrolytes to Prevent Dendrite Growth in Lithium Metal Batteries

Li-metal batteries have the potential to become the next generation of batteries since they can provide an increased energy density vs. Li-ion batteries. However, the safety issue caused by the inhomogeneous Li deposition and Li dendrite growth greatly limits their commercial application. Several methods have been proposed to address this issue. On the one hand, when Li ions are released from one electrode and consumed at the other electrode, it causes a salt concentration gradient. Besides, the electrical migration of anions in the opposite direction aggravates the concentration of gradient since anions do not react at the electrodes. Fast dendritic growth then occurs when the current is high enough to fully deplete the Li+ ion at the anode during charge. Thus, suppressing concentration gradient by immobilizing the anions would in principle solve the fast dendritic growth issue. On the other hand, the Solid Electrolyte Interphase (SEI) plays a crucial role in the operation of Li metal batteries since it prevents further reduction and continuous electrolyte reaction. SEI derives from the passivation layer formed onto the surface of Li metal under dry air during lithium foil extrusion and then evolves further in contact with the electrolytes and during subsequent electrochemical Li stripping/plating. The SEI is inhomogeneous and since it contains lower resistance pathways for Li+ transport, it favors inhomogeneous deposition at any current. Thus, SEI engineering, in order to favor the homogeneity of current density and enhanced mechanical properties to confine lithium could, in principle, improve the performance of Li metal batteries. Thus, we studied the use of single-ion conducting polymers with high cationic transference numbers (tLi + close to 1) as artificial SEIs (art-SEIs) since they include immobile anions and thus prevent Li+ depletion during charge and, at the same time, improve lithium metal confinement and the homogeneity of current density. Our results show much improved homogeneity of Li deposition and cycling stability after single-ion art-SEI coating using an ether-based liquid electrolyte combined with a separator. Nevertheless, it could not enable long-term cycling as the separator does not enable homogeneous pressure and current density onto the lithium anode. Thus, the use of a single-ion conducting solid polymer electrolyte was combined with a single-ion conducting art-SEI (as illustrated in Figure 1). This strategy allows the suppression of salt concentration gradients, inhomogeneous pressure and current density, thus enables homogeneous Li deposition and long-term cycling of Li metal anode. Figure 1