Negating the Interfacial Resistance between Solid and Liquid Electrolytes for Next-Generation Lithium Batteries

The combination of solid and liquid electrolytes enables the development of safe and high-energy batteries where the solid electrolyte acts as a protective barrier for a high-energy lithium metal anode, while the liquid electrolyte maintains facile electrochemical reactions with the cathode. However, the contact region between the solid and liquid electrolytes is associated with a very high resistance, which severely limits the specific energy that can be practically delivered. In this work, we demonstrate a suitable approach to virtually suppress such interfacial resistance. Using a NASICON-type solid electrolyte in a variety of liquid electrolytes (ethers, DMSO, acetonitrile, ionic liquids, etc.), we show that the addition of water as electrolyte additive decreases the interfacial resistance from >100 Ω cm2 to a negligible value (<5 Ω cm2). XPS measurements reveal that the composition of the solid–liquid electrolyte interphase is very similar in wet and dry liquid electrolytes, and thus the suppression of the associated resistance is tentatively ascribed to a plasticizer or preferential ion solvation effect of water, or to a change in the interphase morphology or porosity caused by water. Our simple estimates show that the improvement in the solid–liquid electrolyte interphase resistance observed here could translate to an enhancement of 15–22% in the practical energy density of a Li–S or Li–O2 battery and improvements in the roundtrip efficiency of 21–28 percentage points.