Energy density and safety are two crucial parameters when evaluating lithium-metal batteries (LMBs). Herein, we present an ultralight polymer-based current collector, incorporating flame-retardant materials, designed specifically for thin lithium-metal anodes. Compared to the traditional copper current collector (8.96 mg cm$^{−2}$, 10 μm thick), the polymer-based current collector (12 μm thick) has a significantly lower areal density of 1.41 mg cm$^{−2}$, i.e., only one-sixth of the copper collector, thus enabling substantially higher energy densities. Accordingly, when employed in Li||NMC$_{622}$ full-cells, the polymer-based current collector enables a specific energy of 449 Wh kg$^{−1}$, representing a notable improvement of about 14.5% compared to cells employing a classic copper current collector. The inclusion of Al(OH)$_3$ as a flame retardant into the current collector suppresses flammability and, thereby, significantly improves the safety of the resulting LMBs.