In this article, we show that succinonitrile-based lithium metal batteries (SN-LMB) can cycle over 1000 times at 1 C and at room temperature with a coulombic efficiency (CE) of ~99.8% when low amounts of fluoroethylene carbonate (FEC) are added into SN. FEC loadings as low as 5 and 10 wt% (of SN and Li salt) are sufficient to overcome the polymerization of SN with the lithium metal (LM) because FEC facilitates the formation of a protective LiF layer. We also show that the separator type significantly influences the SN-LMB cycle life. Using relatively thick separators with wide pores, SN-LMB have experienced an early failure due to the growth of lithium dendrites. Conversily, thinner separators with narrower pores have enabled SN-LMB to cycle 1000 times with an average CE of ~99.8% and a capacity drop of 0.03 mAh g−1 cycle−1, due to the slow growth of an inactive Li layer on the LM surface. Our results highlight the important role that the separators play on the lifetime of SN-LMB and elucidate the role of inactive lithium formation on the capacity decay of SN-LMB. In light of the long cycle life of the developed batteries, this work motivates future development of SN-LMBs.