Figure 1: Theoretical and (estimated) practical energy densities of different rechargeable batteries: Pb–acid – lead acid, NiMH – nickel metal hydride, Na-ion – estimate derived from data for Li-ion assuming a slightly lower cell voltage, Li-ion – average over different types, HT-Na/S8 – high temperature sodium–sulfur battery, Li/S8 and Na/S8 – lithium–sulfur and sodium–sulfur battery assuming Li2S and Na2S as discharge products, Li/O2 and Na/O2 – lithium–oxygen battery (theoretical values include the weight of oxygen and depend on the stoichiometry of the assumed discharge product, i.e., oxide, peroxide or superoxide). Note that the values for practical energy densities can largely vary depending on the battery design (size, high power, high energy, single cell or battery) and the state of development. All values for practical energy densities refer to the cell level (except Pb–acid, 12 V). The values for the Li/S8 and Li/O2 batteries were taken from the literature (cited within the main text) and are used to estimate the energy densities for the Na/S8 and Na/O2 cells. Of the above technologies, only the lead acid, NiMH, Li-ion and high temperature Na/S8 technologies have been commercialized to date.