Boosting the Cycle Performance of Iron Trifluoride Based Solid State Batteries at Elevated Temperatures by Engineering the Cathode Solid Electrolyte Interface.

Iron trifluoride (FeF3) is attracting tremendous interest due to its lower cost and the possibility to enable higher energy density in lithium-ion batteries. However, its cycle performance deteriorates rapidly in less than 50 cycles at elevated temperatures due to cracking of the unstable cathode solid electrolyte interface (CEI) followed by active materials dissolution in liquid electrolyte. Herein, by engineering the salt composition, the Fe3O4-type CEI with the doping of boron (B) atoms in a polymer electrolyte at 60 °C is successfully stabilized. The cycle life of the well-designed FeF3-based composite cathode exceeds an unprecedented 1000 cycles and utilizes up to 70% of its theoretical capacities. Advanced electron microscopy combined with density functional theory (DFT) calculations reveal that the B in lithium salt migrates into the cathode and promotes the formation of an elastic and mechanic robust boron-contained CEI (BOR-CEI) during cycling, by which the durability of the CEI to frequent cyclic large volume changes is significantly enhanced. To this end, the notorious active materials dissolution is largely prohibited, resulting in a superior cycle life. The results suggest that engineering the CEI such as tuning its composition is a viable approach to achieving FeF3 cathode-based batteries with enhanced performance.

Modulating luminescence through anion variation in lead-free Cs2NaInX6 (X = Cl, Br, and I) perovskites: a first-principles study.

The A2B+B'3+X6-type lead-free halide perovskite Cs2NaInCl6 has demonstrated limited luminescence performance attributed to parity-forbidden transitions in its intrinsic form. While extensive exploration has been dedicated to partial cation substitution in Cs2NaInCl6, there is a noticeable gap in understanding the impact of anion composition on this material. In this study, we investigated the influence of anion composition on the luminescence performance of Cs2NaInX6 using first-principles calculations. We first conducted calculations on Cs2NaInX6 in its intrinsic state and on Cs2NaInCl6 with cation substitution to establish the reliability of the transition dipole moment (TDM) as a luminescence descriptor in this system. Following this, we systematically assessed the formation energies, octahedral distortions, and luminescence properties of Cs2NaInX6 with diverse anion compositions. Despite sharing similar stability, closely aligned with the experimentally accessible Cs2NaInCl6, all mixed halide structures exhibited significant octahedral distortions. Additionally, most of these structures displayed considerably enhanced TDM compared to their single halide counterparts. Notably, the structures Cs2NaInX4X'2-b and Cs2NaInX3X'3-b demonstrated superior luminescence performance compared to other structures. The absorption spectra calculated for selected structures revealed the enhancement of their photo-absorbance in the presence of iodine, particularly in the low energy region. This observation provides additional evidence that light absorbance in different energy regions can be effectively regulated in this way. Finally, we also investigated other optical properties that impact luminescence performances, such as the energy loss spectrum L(ω), the reflectivity spectrum R(ω) and the refractivity index n(ω). The findings offer insights into optimizing the luminescence performance of lead-free halide perovskites through anion composition variation.