Phase changing cathodes are electrode materials that involve breaking and recombination of chemical bonds and generation of multiple phases. These reactions are typically multi-electron and therefore potentially high capacities can be achieved. Currently, cathode systems that are promising in terms of theoretical capacities and operation potentials are: (i) transition metal halides, such as chlorides and fluorides, and (ii) chalcogens and chalcogenides, for example the Li–S system offers a high theoretical capacity of 1,675 mAh g−1 combined with low cost (Fig. 6c)5961. Utilization of the phase changing multi-electron systems in both positive and negative electrode materials offers the opportunity for a transformative impact on EES by more than doubling the theoretical gravimetric energy density compared with currently used intercalation systems. For instance, the theoretical energy density for the Li–S system is 2,567 Wh kg−1 (or 2,199 Wh l−1 based on the sum of the volumes of Li at the beginning and Li2S at the end of discharge)61. It is important to remember that to enable multi-electron systems to operate efficiently it is essential to apply 3D nanostructuring of electrodes as discussed above to achieve reliable and fast performance of the devices with good cyclability. In the case of Li–S batteries (which suffer from high volume changes, low conductivities and dissolution of the reaction intermediates, polysulfides) an efficient strategy to boost performance involves encapsulation of the sulfur nanoparticles in 3D porous and conductive structures, that is, carbon62, MXenes6364 and so on, to provide adequate electronic transport for the reaction and to minimize the capacity losses arising from self-discharge effects.