Holey graphyne anode for metal-ion batteries: suitability for potassium-ion batteries

Metal-ion batteries based on more abundant and cheaper elements than lithium are considered alternatives to Li-ion batteries. This ab initio study evaluated the performance of a recently discovered holey graphyne (HGY) with a porous layered structure composed of sp² and sp hybridized carbon atoms as an anode for Li-, Na-, K-, Mg-, Ca-, Zn-, and Al-ion batteries. HGY is the most promising anode material for K-ion batteries. For K ions, HGY exhibits a capacity of 651 mAh g^-1, a volume expansion of 31.3%, and a diffusivity of 1.1 × 10^-5 cm² s^-1 at 900 K, which are fairly good compared to those (372 mAh g^-1, 12%, and 9.2 × 10^-5 cm² s^-1) of graphite in Li-ion batteries. Moreover, the conductivity (2.38 mS cm^-1) of K ions in HGY at T = 300 K is higher than that (0.45 mS cm^-1) of Li ions in graphite. For other metal ions, HGY shows very high capacities up to 1116 mAh g^-1 and quite small volume changes of -13.3% to 8.6%, but rather low diffusivities of 4.9 × 10^-8 to 6.0 × 10^-6 cm² s^-1 at T = 900 K. The high capacity of HGY is due to the strong ion-host interaction resulting from the lower stability of HGY compared to graphite. This strong interaction suppresses the large volume expansion of HGY but hinders ion diffusion, resulting in relatively low diffusivity. Designing hybrid anodes by combining HGY with more stable materials such as graphene can further improve diffusivity, suggesting that HGY may be used in metal-ion batteries other than K-ion batteries.