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, MXA(2), MXTx, and MXT(x)A(x') structures. The competition and matching mechanisms were introduced to determine the formation probabilities of the MAX phase. The transformation processes from MAX to MXA(2) 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 MXTx 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 Ti(3)C(2)T(x)A(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.