Design of an aluminium ion battery with a graphyne host: lowest volume expansion, high stability and low diffusion...

Commercialization of aluminium ion battery (AIB) requires limited volume expansion of the host cathode materials after AlCl₄ intercalation, lower activation barrier, high theoretical specific capacity (TSC), cyclic durability and thermodynamic stability. Most of the carbon and non-carbon based cathode hosts explored so far failed to address the issue of volume expansion and there is a lack of clarity about thermodynamic stability. In this work, we employed multipronged first principles computational approaches on α- and γ-graphyne (GY) and showed that α-GY as a promising cathode host addresses each of the above concerns. Both α and γ-GYs provide ample space to accommodate more number of AlCl₄ molecules leading to a high TSC of 186 mA h g^-1 and open circuit voltages of 2.18 and 2.22 V, respectively. The absence of bond dissociation of AlCl₄ and deformation of GY sheets at 300 and 600 K, as revealed by ab initio molecular dynamics (AIMD) simulation, indicates the stability of α- and γ-GY with adsorbed AlCl₄. α-GY after intercalation shows a volume expansion of 186% which is the lowest among the cathode materials studied so far. The negligible expansion energy per unit surface area (∼0.003 eV Å^-2) ensures the reversibility and hence cyclic durability of α-GY. Although the γ-GY shows a volume expansion of 249%, it is still promising. The NEB based diffusion study on monolayer and bilayer GY estimates the activation barriers to be (0.26, 0.06 eV) and (0.42, 0.16 eV) for α and γ phases, respectively. These values are either comparable to or lower than those of earlier reported cathode hosts.