2D and layered materials that can be intercalated and exfoliated are particularly promising for incorporation into electrodes. 2D materials, such as graphene44, some transition metal oxides12, MoS2 (ref. 16) and Ti2C (MXene)45, are only a few atoms thick and therefore have a large number of immediately accessible electrochemically active sites. For many layered materials or assembled 2D structures, thermodynamic considerations are such that spontaneous intercalation takes place which leads to preferential occupation of sites. Another benefit with 2D materials is that large ions such as Na+ and K+ as well as solvated ions can be accommodated on their surface or between the layers. Finally, the 2D morphology is also convenient for flexible energy storage materials46. Although only limited research has been carried out to date, indications are that the electrochemical performance of 2D metal oxides is substantially better than the corresponding bulk material46. Metallic 2D materials such as MXenes15 and 1T transition metal dichalcogenides16 (Fig. 4c,f) are particularly attractive because they intrinsically possess good electrical conductivity along with the ability to support redox reactions from their transition metal chemistry.