The Achilles’ heel of batteries with alkali metal electrodes†

There is a strong drive to use Li and Na metals as anode materials for lithium and sodium batteries due to their high specific energy. However, Li and Na metals are susceptible to dendrite growth and exhibit low melting points (180.5 °C for Li and 98 °C for Na). The low melting points can lead to internal short-circuits and catastrophic failure of the battery. Here, we show that batteries using Li and Na metal electrodes are short-circuiting internally when the melting points of these metals are reached. We demonstrated this with four different solid electrolytes in lithium and sodium batteries, using symmetric-, half-, and anode-free cells and through extensive impedance measurements and direct visualisation via operando digital microscopy. The temperature required to melt these metals in batteries is often reached under various operating conditions. In light of these facts, using Li and Na metals as electrodes in commercial batteries should be reconsidered.