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 Ti3C2Tx MXene films were fabricated and tested as MIB cathodes after the incorporation of Mg ions from a Mg2+-containing electrolyte. The 3D MXene cathode exhibited promising cycling stability, accompanied by good rate performance. A 3D Mg0.21Ti3C2Tx 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