Organic Spacer Engineering Enables Three-Dimensional Ion Migration in Layered Halide Perovskite Photocathodes for Photo-rechargeable Batteries

Photo-rechargeable batteries, which integrate energy conversion and storage in a single device, significantly enhance solar energy utilization. Halide perovskites, especially the two-dimensional (2D) counterparts, have emerged as promising photocathodes in photo-rechargeable Li-ion batteries. However, the photocathodes suffer from the lack of three-dimensional (3D) Li-ion migration and charge transport due to the presence of organic spacer cations separating the inorganic layers, which act as a barrier between them. In this work, using density functional theory, we rationalize the Li-ion migration during charging and discharging in lead-iodide-based 2D perovskites with n-butyl-, cyclohexyl ethyl-, phenyl ethyl-, and pyrene ethyl-ammonium spacer cations. Our results show that the Li-ion migration along the organic spacer cations can be improved by choosing a cation with less dense packing. However, during the charging process under illumination, we find that the large hole polaron creates a positively charged polaronic medium that increases the Li-ion migration barrier along the horizontal direction. Interestingly, if the organic spacer cation does not contribute to the electronic band edges of the material, the Li-ion migration barrier along the spacer cations becomes easier due to the lower migration barrier; however, the barrier significantly increases for electroactive spacer cations. Furthermore, our calculations show that an optimal balance between the spacer’s length and electroactive nature can significantly lower the effective carrier mass along the vertical direction, enabling 3D charge transport in layered metal halide perovskites (MHPs). Thus, our study demonstrates that selecting an organic spacer cation with less dense packing, short length, and non-electroactive nature enhances the 3D Li-ion migration and charge transport. Our study opens the door to spacer-cation engineering for the development of 2D MHP-based photo-rechargeable batteries.