Two-Dimensional Carbon-Based Auxetic Materials for Broad-Spectrum Metal-Ion Battery Anodes.

Auxetic materials possess special applications due to their unique negative Poisson's ratios (NPRs). As a classic 2D carbon material, the NPR of graphene is still deliberated. Introducing the NPR in graphene would increase its extraordinary properties, and the NPR together with other properties would bring more significant applications for graphene. In this Letter, on the basis of first-principles calculations, we reconfigure the structure of graphene, and, as an example, we propose a new 2D planar carbon allotrope, xgraphene, which is constructed by 5-6-7 carbon rings. Our theoretical calculations indicate that xgraphene has an NPR and constitutes a broad spectrum of metal ion battery anodes with high performance. Its maximum storage capacities are 930/1302/744/1488 mAh/g for Li/Na/K/Ca-ion batteries. It has low metal-ion diffusion energy barriers (≤0.49 eV) and low average open-circuit voltages (≤0.53 V). Our density functional theory results also showed that it is intrinsically metallic and possesses dynamic, thermal, and mechanical stabilities. Its intrinsic NPR, which stems from the weakness of coupling of carbon-carbon bonds, is found upon loading the uniaxial strain along the armchair direction. This work not only opens up a new direction for the design of the next-generation broad-spectrum energy-storage materials with low cost and high performance but also offers a class application for auxetic materials.