Rechargeable lithium metal batteries are secondary lithium metal batteries. They have metallic lithium as a negative electrode. The high specific capacity of lithium metal (3,860 mAh g−1), very low redox potential (−3.040 V versus standard hydrogen electrode) and low density (0.59 g cm−3) make it the ideal negative material for high energy density battery technologies.[1] Rechargeable lithium metal batteries can have a long run time due to the high charge density of lithium. Several companies and many academic research groups are currently researching and developing rechargeable lithium metal batteries as they are considered a leading pathway for development beyond lithium-ion batteries.[2] Some rechargeable lithium metal batteries employ a liquid electrolyte and some employ a solid-state electrolyte.