(Invited) Li Intercalation into Multi-Layers Transition Metal Carbides and Carbonitrides "Mxenes"� in Li-Ion Batteries

Two-dimensional, 2D, transition metal carbides and carbonitrides, so called MXenes, consist of nearly closed packed layers of early transition metals “M” with “X” atoms (X stands for carbon or nitrogen) filling the octahedral sites with a nominal composition of M_n+1X_n where n = 1, 2, or 3. Depending on the value of n, the thickness of the M_n+1X_n change; if n = 1 the MXene layer consists of a single block of octahedra (3 atoms thick). If n = 2 the MXene layer consists of 2 octahedra blocks; if n = 3, consists of 3 blocks of octahedra. The surface of as-synthesized MXenes is terminated by a mixture of moieties viz. O, OH, and F (this mixture is referred hence after by T_x). To date, the following MXenes were successfully synthesized: Ti₂CT_x, Nb₂CT_x, V₂CT_x, (Ti_0.5,Nb_0.5)₂CT_x, Ti₃C₂T_x, (V_0.5,Cr_0.5)₃C₂T_x, Ti₃CNT_x, Ta₄C₃T_x, and Nb₄C₃T_x.

Unlike conventional 3D transition metal carbides and nitrides which are known to be inert for Li, MXenes were found to be electrochemically active as host materials for Li-ions. The latter intercalate/de-intercalate reversibly between the MXenes layers during the electrochemical cycling in Li batteries. This was confirmed by in situ XRD studies during electrochemical lithiation and de-lithiation. When they were tested as electrode materials for Li-ion batteries (LIBs), each MXene showed its own electrochemical voltage window. In principle, some MXenes could function as cathodes and some as anodes for LIBs. MXenes electrodes exhibited an excellent capability to handle high cycling rates with good capacity. This was explained by a low Li diffusion barrier in case of MXene compared to other electrode materials such as graphite. MXenes powders can be cold pressed into freestanding thick discs resulting in electrodes with high areal capacities exceeding 5 mAh/cm². Most of the research conducted –experimentally- on MXenes as electrodes for LIBs was limited to the as-synthesized MXene without further control of the surface termination. However, ab initio calculations predict that controlling the surface termination could enhance the performance. Herein the progress in the synthesis of MXenes and controlling their surface termination, in addition to their performance as electrode materials in LIBs will be discussed.