First-Principles Study of 2D Haeckelite C<sub>7</sub>N as a High-Capacity Anode for Post-Lithium-Ion Batteries
Zahra Hajiahmadi, S. Alireza Ghasemi, Thomas D. Kühne, S. Shahab Naghavi
Abstract
Post-Li batteries based on Na, K, Ca, and Mg offer compelling alternatives to Li ones, whose resources are scarce and unevenly distributed in the earth’s crust. However, the development of these forthcoming batteries is currently thwarted by the lack of cost-effective, nontoxic, and highly efficient anode materials. To tackle this challenge, we employed a comprehensive structure search using the minima hopping method, followed by density functional theory (DFT) calculations. Our search led to an as-yet-unobserved metallic C 7 N monolayer with a Haeckelite structure. This unique structure features a network of sp 2 -nitrogen-containing heptagon and pentagon rings that are arranged in a way that enforces metallic characteristics. As such, C 7 N outperforms widely known 2D anode materials such as graphene, MoS 2, and black phosphorus because of its superior storage capacity, lower diffusion barriers, and higher open-circuit voltages. It achieves a remarkable storage capacity of 1366 mA h g –1 for Na/K and an impressive 2730 mA h g –1 for Ca. Detailed analyses of charge, elastic constants, and molecular dynamics simulations demonstrate that the C 7 N possesses a strong yet flexible covalent network with a volume change of 2–4% during full charge and discharge cycles, ensuring long-term stability and reliability. The robust network of C 7 N also allows it to maintain a flat and thermally stable surface at full metal coverage and high temperatures. These findings open up avenues for exploring the Haeckelite carbon-nitride family as a promising candidate for next-generation battery technologies.