Li metal thickness has been considered a key factor in determining the electrochemical performance of Li metal anodes. The use of thin Li metal anodes is a prerequisite for increasing the energy density of Li secondary batteries intended for emerging large-scale electrical applications, such as electric vehicles and energy storage systems. To utilize thin (20 μm thick) Li metal anodes in Li metal secondary batteries, we investigated the synergistic effect of a functional additive (Li nitrate, LiNO3) and a dual-salt electrolyte (DSE) system composed of Li bis(fluorosulfonyl)imide (LiTFSI) and Li bis(oxalate)borate (LiBOB). By controlling the amount of LiNO3 in DSE, we found that DSE containing 0.05 M LiNO3 (DSE–0.05 M LiNO3) significantly improved the electrochemical performance of Li metal anodes. DSE–0.05 M LiNO3 increased the cycling performance by 146.3% [under the conditions of a 1C rate (2.0 mA cm–2), DSE alone maintained 80% of the initial discharge capacity up to the 205th cycle, whereas DSE–0.05 M LiNO3 maintained 80% up to the 300th cycle] and increased the rate capability by 128.2% compared with DSE alone [the rate capability of DSE–0.05 M LiNO3 = 50.4 mAh g–1, and DSE = 39.3 mAh g–1 under 7C rate conditions (14.0 mA cm–2)]. After analyzing the Li metal surface using scanning electron microscopy and X-ray photoelectron spectroscopy, we were able to infer that the stabilized solid electrolyte interphase layer formed by the combination of LiNO3 and the dual salt resulted in a uniform Li deposition during repeated Li plating/stripping processes.