Lithium-Nitrate-Containing Gel Polymer Electrolyte for Carbonate-Based Anode-Free Lithium Metal Batteries

Anode-free lithium metal batteries are of great interest due to their immense energy density. However, reversible lithium deposition on copper remains challenging, and extreme volume changes prevent robust solid electrolyte interface (SEI) formation. Lithium nitrate (LiNO₃) is a well-known electrolyte additive that imparts beneficial components to the SEI but has very poor solubility in carbonate electrolytes. In this work, we present a gel polymer electrolyte (GPE) that provides a reservoir of LiNO₃ to the battery contained in a polymer network of polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) and polyhedral oligomeric silsesquioxane (POSS). By incorporating POSS into the GPE, we achieve an improved ionic conductivity that reaches 1.011 mS cm^–1 at room temperature. X-ray photoelectron spectroscopy analysis shows that the GPE forms a robust SEI while simultaneously mitigating the degradation of the carbonate electrolyte. This improvement enables the GPE to deliver higher coulombic efficiency and longer cycle life in Cu||Li batteries, lasting 250 cycles at a current density of 0.5 mA cm^–2, 5 times longer than a cell with only liquid electrolyte. Furthermore, the GPE provides a smooth, spherical lithium morphology on untreated copper electrodes.