Identifying a suitable electrode material with desirable electrochemical properties remains a primary challenge for rechargeable Al-ion batteries. Recently an ultrafast rechargeable Al-ion battery was reported with high charge/discharge rate, (relatively) high discharge voltage and high capacity that uses a graphite-based cathode. Using calculations from first-principles, we have investigated the staging mechanism of AlCl intercalation into bulk graphite and evaluated the stability, specific capacity and voltage profile of AlCl intercalated compounds. Ab initio molecular dynamics is performed to investigate the thermal stability of AlCl intercalated graphite structures. Our voltage profiles show that the first AlCl intercalation step could be a more sluggish step than the successive intercalation steps. However, the diffusion of AlCl is very fast in the expanded graphite host layers with a diffusion barrier of ∼0.01 eV, which justifies the ultrafast charging rate of a graphite based Al-ion battery. And such an AlCl intercalated battery provides an average voltage of 2.01-2.3 V with a maximum specific capacity of 69.62 mA h g, which is excellent for anion intercalated batteries. Our density of states and Bader charge analysis shows that the AlCl intercalation into the bulk graphite is a charging process. Hence, we believe that our present study will be helpful in understanding the staging mechanism of AlCl intercalation into graphite-like layered electrodes for Al-ion batteries, thus encouraging further experimental work.

Many of the best-performing perovskite photovoltaic devices make use of 2D/3D interfaces, which improve efficiency and stability – but it remains unclear how the conversion of 3D-to-2D perovskite occurs and how these interfaces are assembled. Here, we use in situ Grazing-Incidence Wide-Angle X-Ray Scattering to resolve 2D/3D interface formation during spin-coating. We observe progressive dimensional reduction from 3D to n = 3 → 2 → 1 when we expose (MAPbBr3)0.05(FAPbI3)0.95 perovskites to vinylbenzylammonium ligand cations. Density functional theory simulations suggest ligands incorporate sequentially into the 3D lattice, driven by phenyl ring stacking, progressively bisecting the 3D perovskite into lower-dimensional fragments to form stable interfaces. Slowing the 2D/3D transformation with higher concentrations of antisolvent yields thinner 2D layers formed conformally onto 3D grains, improving carrier extraction and device efficiency (20% 3D-only, 22% 2D/3D). Controlling this progressive dimensional reduction has potential to further improve the performance of 2D/3D perovskite photovoltaics.

The origin of the long carrier lifetime in lead halide perovskites is still under debate, and, among different hypotheses, the formation of large polarons preventing the recombination of charge couples is one of the most fascinating. Using state-of-the art ab initio calculations, we report a systematic study of the polaron formation process in metal halide perovskites, focusing on the influence of the chemical composition of the perovskite on the polaron properties. We examine variations in A-site cations (FA, MA, Cs, and Cs–MA), B-site cations (Pb, Sn, and Pb–Sn), and X-site anions (Br, I). Our study confirms that stronger structural distortions occur for Cs than for MA and FA, with the effect of different A-site cations being almost additive. For the same A cation, bromide features stronger distortions than iodide perovskites. The pure Sn phase has an almost double polaron stabilization energy compared with the pure Pb phase. Surprisingly, the trend of polaron stabilization energy is nonmonotonic in mixed Sn–Pb perovskites, with a maximum for small Sn percentages. Polaron formation is found to be promoted by bond asymmetry, ranging from small to large polarons in mixed Sn–Pb perovskites depending on the relative Sn percentage.