Ultrathin, Compacted Gel Polymer Electrolytes Enable High‐Energy and Stable‐Cycling 4 V Lithium‐Metal Batteries

Abstract Batteries with lithium metal as anodes and gel polymer membranes as electrolytes are promising, because of their high energy densities and improved safety. To achieve metal battery energy densities that are comparable to liquid‐electrolyte batteries, ultrathin and lightweight gel electrolytes with wide electrochemical stability windows are desired. However, it is challenging to make gel polymer electrolyte membranes with comparable thicknesses to commercial polymer electrolyte separators (<20 μm), because of the porous configuration and increased risk of short‐circuiting cells. Here, we report on a 10 μm‐thick compacted gel polymer electrolyte (TCGPE), demonstrated to have a robust chemical interaction between SiO 2 nanoparticles and poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐HFP) chains that are activated via an in situ sol‐gel reaction. The developed TCGPE exhibits an extremely wide electrochemical stability window of 5.5 V vs. Li/Li + and prevents the batteries from short‐circuiting even at a high current density of 1.0 mA cm −2 . Moreover, Li/TCGPE/LiNi 0.5 Co 0.2 Mn 0.3 O 2 shows an excellent capacity retention of 99.8 % after 180 cycles at 1 C rate.