Batteries are an attractive grid energy storage technology, but a reliable battery system with the functionalities required for a grid such as high power capability, high safety and low cost remains elusive. Here, we report a solid electrolyte-based molten lithium battery constructed with a molten lithium anode, a molten Sn–Pb or Bi–Pb alloy cathode and a garnet-type Li6.4La3Zr1.4Ta0.6O12 (LLZTO) solid electrolyte tube. We show that the assembled Li||LLZTO||Sn–Pb and Li||LLZTO||Bi–Pb cells can stably cycle at an intermediate temperature of 240 °C for about one month at current densities of 50 mA cm−2 and 100 mA cm−2 respectively, with almost no capacity decay and an average Coulombic efficiency of 99.98%. Furthermore, the cells demonstrate high power capability with current densities up to 300 mA cm−2 (90 mW cm−2) for Li||LLZTO||Sn–Pb and 500 mA cm−2 (175 mW cm−2) for Li||LLZTO||Bi–Pb. Our design offers prospects for grid energy storage with intermediate temperature operations, high safety margin and low capital and maintenance costs.