Stable and metallic two-dimensional TaC2 as an anode material for lithium-ion battery

Abstract

Relative to advanced cathode materials, anode materials have become one of the key factors to hamper the performance improvement of lithium-ion batteries (LIBs). Recently, two-dimensional (2D) transition metal carbides (e.g. MXenes) have drawn great attention due to their high Li storage ability. However, metal-rich 2D transition metal carbides as anodes usually need surface functionalization, leading to a decrease in the rate performances. Here, we propose that increasing the carbon composition in 2D Ta_xC_y is beneficial for not only eliminating surface functionalization but also greatly improving battery performance. First-principles swarm structural searches were used to explore structures and stabilities of 2D Ta_xC_y (x = 1 and y = 1–4, or x = 2 and y = 1). Besides reproducing the reported 2D TaC, TaC₂ and Ta₂C are found to be stable, and have high thermal stabilities. Metallic TaC₂ and Ta₂C provide good electronic conductivity. Intriguingly, carbon-rich TaC₂ contains carbon dimers exposed on the surface, enabling it to directly adsorb Li atoms. After adsorption of two-layer Li atoms, its structural integrity is well preserved. The resultant specific capacity, diffusion energy barrier, and open-circuit-voltage (OCV) of TaC₂ are much better than those of commercial graphite, f-Ti₃C₂ or the Ti₂C monolayer. Compared with TaC₂, TaC, and Ta₂C as anode materials, the overall performance of carbon-rich Ta_xC_y is better. Our work provides a useful strategy for designing new-type 2D transition metal materials for LIBs.