Lead-free Bismuth Based Perovskites for Integrated Electrochemical Storage and Generation

The demand for sustainable energy has driven a recent burst of interest in solar storage devices. One such device, the photo-battery (PHBAT), is capable of providing both energy storage and generation from a single device architecture. This idea has existed for many decades, since the first demonstration in the 70's and such a design should permit increased energy storage efficiency and energy density, while reducing packaging requirements, ohmic losses, size, weight, and cost. Unfortunately, in past years, the poor efficiency of these devices failed to improve upon the standard industry solution: a solar cell wired to an external battery. Recent advances in the literature suggest a promising new start for this technology. Here we present he first lead-free, bismuth-based, all-inorganic halide perovskite photo-rechargeable <i>lithium-ion</i> PHBAT. This device enables both energy storage and photovoltaic power generation over several tens of cycles, after which the anode must be regenerated by electrical charging. In that work, we used the favorable charge transport behavior, high defect tolerance and wide absorption range for colloidal nanocrystals of Cs₃Bi₂I₉ perovskites, to fabricate a photo-active electrode, and a functioning photo-battery. The champion devices were able to achieve a peak photo conversion efficiency of ~0.43 %, for the first charge and an average efficiency of ~ 0.10 % for subsequent cycles. Devices showed good performance as batteries over the first ten cycles with capacity &gt;300 mAh/g, but performance declined over the first 100 cycles. Under 1 sun and at low current, the battery could be operated with arbitrarily large <i>effective</i> capacity, demonstrated up to 900 mAh/g by essentially using the battery as a solar cell. These results compared favorably with other photo-battery devices using other materials as the active layers. We found that during the charge-discharge cycle, the lithium first intercalated into the bismuth perovskite, and, later, as the voltage decreased, formed reduced Li_xBi compounds. In addition we have found similar results are possible using Cs₂NaBiI₆ nanocrystals and 2D PEA3Bi2I9. We present these results, with high capacity and PCE of up to 0.27% and good stability. Mechanistic studies on these materials suggest future improvements are possible and give a general strategy for improving performance by perovskite engineering.