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[Display omitted] • A theoretical framework for the evaluation of flexible anode materials is established. • The mechanical property of Janus MXene is comparable with that of pristine MXene. • A constant diffusion barrier on Ti 2 COF under strain is suitable for a flexible anode. Two-dimensional transition metal carbides (MXenes) have received tremendous attention because of their great promise in flexible energy storage. Although surface engineering plays an important role in regulating the properties of MXenes with the development of synthesis technology, an atomistic design of surface diversity for desired functionalities is generally limited by purely unified surface termination. Herein, the Janus MXenes consisting of different surface terminations are proposed for flexible anodes through first-principles calculations. Taking Ti 2 C TT' (T , T' = O, F, or OH) as an illustration of Janus MXenes, critical strains, ideal strengths, Li-ion diffusion barriers, equilibrium voltages, and theoretical capacities are determined and used as descriptors to evaluate the properties of flexible anodes. These Janus MXenes exhibit high critical strains and ideal strengths, indicating their mechanical flexibility. The strain-independent Li-ion diffusion barrier of Ti 2 COF suggests its fast Li-ion transport even suffering from mechanical deformation, endowing its application potential in flexible anodes. These results establish a theoretical framework for the comprehensive evaluation of flexible anode materials, providing a theoretical scheme for designing flexible anode materials. [ABSTRACT FROM AUTHOR]

[Display omitted] • Five kinds of Ti 3 C 2 T x models with various terminations ratio are particular built. • Roles of terminations are studied by electronic properties and storage performance. • Optimal termination ratio for Ti 3 C 2 T x is screened out as anode for Na-ion battery. 2D transition metal carbides MXenes stand out on energy storage application due to superior electrochemical performance and encouraging capacitances, and the energy storage properties strongly depend on the nature of terminations on the surface. Here, five different terminal ratios of Ti 3 C 2 T x (O: F: OH = 9:0:0, 6:2:1, 4:4:1, 3:5:1, 0:9:0, respectively) are designed to explore the influence of different terminations on electronic properties and Na storage capability using density functional theory, moreover the Ti 3 C 2 T x with O: F: OH = 3:5:1 has been synthesized to verify some calculation results. The results show that the Ti 3 C 2 T x MXene, with terminal ratios of 6:2:1, is thermally stable when 36Na atoms (26.67%) intercalate. Meanwhile, it also exhibits superior conductivity, lower open circuit voltages, especially the higher theoretical capacity. The theoretical capacity is more than 6 times than that of pure F termination's structure and 3 times than that of the system with the O: F: OH = 3:5:1. The presence of F can ensure good conductivity and O provide larger capacity. The finding is one to establish interlayer models of Ti 3 C 2 T x -MXene with different terminal ratios and provides a basic understanding about the influence of terminates on electronic properties and Na storage performance of Ti 3 C 2 T x (T = O, F and OH). [ABSTRACT FROM AUTHOR]