Hybrid solid electrolyte-liquid electrolyte systems for (almost) solid-state batteries: Why, how, and where to?

All-solid-state batteries (SSBs) offer an alternative to current state of the art lithium-ion batteries, promising improved safety and higher energy densities due to the incorporation of non-flammable solid electrolytes and Li metal as an anode material. Despite this, SSBs face numerous issues, including the tendency for the solid electrolytes to decompose upon contact with anode and cathode materials as well as during cycling. In addition, poor particle on particle contact can result in sluggish transport of lithium ions to and from the solid electrolytes. One potential solution is by combining the solid electrolyte with a liquid electrolyte to form a hybrid solid–liquid electrolyte system. By using a liquid electrolyte with a wide electrochemical stability window and good wetting properties some of the problems with solid electrolytes in SSBs may be overcome. However, due to the reactive nature of solid electrolytes, a new interphase known as the solid liquid electrolyte interphase (SLEI) forms. This SLEI may be resistive and therefore increase the total impedance of the cell, thus making certain liquid/solid electrolyte combinations unsuitable for use in ASSBs. In this review we discuss the recent history of these systems, look into the ionic transport model and focus on how the chemical stability of the solid electrolyte with respect to the liquid electrolyte is a vital factor in the formation of a stable SLEI. In the case of salt-in-solvent systems the stability of the solid electrolyte is driven by the chemical nature of the solvent, therefore we also discuss what solvent properties-such as dielectric constant or donor number-may have an effect on the degree of decomposition of the solid electrolyte used