H_d-Graphene: A Hexagon-Deficient Carbon-Based Anode for Metal-Ion Batteries with High Charge/Discharge Rates

Although two-dimensional (2D) hexagon-deficient graphene allotropes as anode materials have been studied in the field of metal-ion batteries (MIBs), high-performance hexagon-deficient carbon allotrope anodes for MIBs are still rare. Here, a 2D hexagon-deficient planar carbon allotrope Hd-graphene, with excellent in-plane stiffness and metallicity, is designed by employing first-principles calculations. Hd-graphene, which consists of pentagons and heptagons as well as a small number of hexagons and squares, is 0.6 eV/atom energetically more stable than the well-known hexagon-free pentagraphene. Hd-graphene can be seen as a high-performance candidate anode for MIBs because its higher maximum theoretical capacities (1395.83, 1116.67, and 1116.67 mA h/g for Li, Na, and K ions, respectively) are approximately 3.7 times that of the well-known commercial anode graphite (372 mA h/g). Moreover, compared to graphite, Hd-graphene has lower average open-circuit voltages of 0.03, 0.11, and 0.09 V for Li-, Na-, and K-ion batteries, respectively. Most importantly, Hd-graphene possesses extremely low diffusion energy barriers of 0.21, 0.14, and 0.09 eV for Li, Na, and K ions, respectively, which ensure high charge/discharge rate capacities. Our work not only proposes a promising high-performance anode material for MIBs but also provides a guide for designing carbon-based anode materials.