Magnesium-Ion Storage Capability of MXenes

Rechargeable magnesium-ion batteries (MIBs) with Mg metal anodes have been attracting attention due to their potential safety, low cost, and high theoretical energy densities. Nevertheless, developing a high-energy-density MIB with long cycle life and reasonable rate capability is still a huge challenge due to the lack of high-performance cathodes beyond the Chevrel phases. Here, we investigate the mechanism of Mg-ion uptake and storage by MXenes, that have been theoretically predicted to be promising candidates for MIB cathodes. Flexible and conductive 3D macroporous Ti 3 C 2 T x MXene films were fabricated and tested as MIB cathodes after the incorporation of Mg ions from a Mg 2+ -containing electrolyte. The 3D MXene cathode exhibited promising cycling stability, accompanied by good rate performance. A 3D Mg 0.21 Ti 3 C 2 T x MXene electrode delivered, at 0.5, 1, and 5 C, capacities of ∼210, ∼140, and ∼55 mA h g –1, respectively. A reversible intercalation charge-storage mechanism was demonstrated and a possible redox reaction mechanism proposed. Considering the large family of 2D transition metal carbides and nitrides, with over 30 different MXenes synthesized to date, this work suggests the availability of a variety of high-rate cathode candidates for MIBs.