A solid future for battery development

Lithium-ion batteries (LIBs) have proven to be an efficient energy storage system in terms of their energy and power density, reliability and cyclability1. Today the state-of-the-art LIBs offer volumetric and gravimetric energy densities up to 770 Wh l−1 and 260 Wh kg−1, respectively, which was not expected 10 years ago. Unfortunately, the energy density of conventional LIBs will soon reach its limit2 (Fig. 1). On the other hand, there is an everincreasing demand for batteries with even higher energy density as well as high power density (quick discharging and charging performance). Solid-state batteries (SSBs) that use solid electrolytes (SEs) instead of liquid ones could offer both high energy and high power density3. While slow kinetics is considered the Achilles’ heel of solid-state systems, a recent study has shown that SSBs may well offer high power densities4. Despite fast growing interest in SSBs, many challenges remain in both manufacturing and fundamental understanding of the technology. Here we aim to provide views on some of the most relevant questions under active discussion: which type of SEs can be used; how can we overcome the inherent mechanical stability issues of SSBs; will SSBs fulfill the expectation of safe storage systems; and will the use of SEs enable the use of lithium-metal anodes for a real jump in energy density?