Structural formation and charge storage mechanisms for intercalated two-dimensional carbides MXenes

Although many studies have been focused on the characterization of MXenes, surface structures and formation mechanisms in terms of their experimental processes still remain controversial. Herein, we systematically investigated the structures formed from MX intercalated with different atoms from group IA to VIIA (A = H, Li, Na, K, Mg, Al, Si, P, O, S, F, and Cl) at different sites. An effective procedure based on first-principles calculations was developed to reveal the formation mechanisms of MAX, MXA₂, MXT_x, and MXT_xA_x' structures. The competition and matching mechanisms were introduced to determine the formation probabilities of the MAX phase. The transformation processes from MAX to MXA₂ have been correlated with the energies and configurations of the transformed MX and the chemical potential of the A atom in terms of the experimental processes. The structure of MXT_x obtained using different methods has been formulated as a function of the experimental conditions and the c lattice parameter. The experimental results can be well explained based on these results. As a representative, it was proved that the capacity of Ti₃C₂T_xA_x' (A = Li) depends on the c lattice parameter and the calculated allowable value can range from 130.3 mA h g^-1 (Ti₃C₂F₂Li) to 536.8 mA h g^-1 (Ti₃C₂O₂Li₄). A higher value can be expected if the sample with a suitable c-axis value can be obtained. Energy storage mechanism should be classified into a double-layer capacitance process in the Ti₃C₂F₂ units and a redox storage mechanism in the Ti₃C₂O₂ units. The procedure can be employed to optimize the structures and compositions of the MXenes.