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Designing flexible 2D transition metal carbides with strain-controllable lithium storage

Significance The discovery of MXenes opens an opportunity on flexible energy storage. We explored systematically several factors including metal species, layer thicknesses, functional group, strain, and Li concentration on the mechanical and electrochemical properties of 2D transition metal carbides (TMCs). Taking the electrode polarization into account, we found several critical factors that govern the ionic mobility on the surface of 2D TMCs. Under multiaxial loadings, the electrical conductivity, high ionic mobility, low equilibrium voltage with good stability, excellent flexibility, and high theoretical capacity offered bare 2D TMCs the potential to be ideal flexible anode materials, whereas the surface functionalization degraded the transport mobility and increased the equilibrium voltage. General rules are proposed to identify the optimal candidate based on a combined analysis of these critical parameters. Efficient flexible energy storage systems have received tremendous attention due to their enormous potential applications in self-powering portable electronic devices, including roll-up displays, electronic paper, and “smart” garments outfitted with piezoelectric patches to harvest energy from body movement. Unfortunately, the further development of these technologies faces great challenges due to a lack of ideal electrode materials with the right electrochemical behavior and mechanical properties. MXenes, which exhibit outstanding mechanical properties, hydrophilic surfaces, and high conductivities, have been identified as promising electrode material candidates. In this work, taking 2D transition metal carbides (TMCs) as representatives, we systematically explored several influencing factors, including transition metal species, layer thickness, functional group, and strain on their mechanical properties (e.g., stiffness, flexibility, and strength) and their electrochemical properties (e.g., ionic mobility, equilibrium voltage, and theoretical capacity). Considering potential charge-transfer polarization, we employed a charged electrode model to simulate ionic mobility and found that ionic mobility has a unique dependence on the surface atomic configuration influenced by bond length, valence electron number, functional groups, and strain. Under multiaxial loadings, electrical conductivity, high ionic mobility, low equilibrium voltage with good stability, excellent flexibility, and high theoretical capacity indicate that the bare 2D TMCs have potential to be ideal flexible anode materials, whereas the surface functionalization degrades the transport mobility and increases the equilibrium voltage due to bonding between the nonmetals and Li. These results provide valuable insights for experimental explorations of flexible anode candidates based on 2D TMCs.