Kangwon National University

Understanding the dynamic lattice collapse pathways under thermal stress is pivotal for designing stable perovskite and improving the power conversion efficiency (PCE) of perovskite solar cells (PSCs). However, the lack of dynamic molecular-level characterization techniques has limited a comprehensive understanding of the degradation pathways involved. In this study, we use temperature-dependent Raman spectroscopy (25-375 °C) coupled with multimodal characterization to investigate A-site cation-dependent degradation pathways in mixed-halide perovskites, specifically FA_0.85Cs_0.15PbI₃ (FACs) and FA_0.85MA_0.15PbI₃ (FAMA). We identify three distinct stages of thermal decomposition dynamics with molecular-level precision: (1) grain boundary pre-collapse (25-175 °C), (2) halide segregation and PbI framework dynamics (200-325 °C), including PbI₂-defective domains, PbI₂ formation, and PbI₂ degradation, and (3) terminal lattice disintegration (350-375 °C). FACs preferentially stabilizes CsPbI₃ nanorods alongside amorphous β-PbO, whereas FAMA exclusively generates crystalline β-PbO without intermediate phases. Mechanistically, Cs⁺ acts as a lattice "scaffold", facilitating dynamic Pb-I-Cs coordination reconfiguration and delaying the degradation of the perovskite by more than 25 °C. In contrast, MA⁺ induces abrupt structural collapse via methylammonium volatilization, creating vacancy-propagated disintegration channels. By correlating Raman fingerprint (e.g., β-PbO at 139 cm^-1) with XRD/PL decay kinetics, we establish a predictive framework for A-site cation selection. This study not only reveals the mechanism of lattice anchoring and dynamic coordination reconstruction regulated by A-site cations at the molecular level but also clarifies the previously ambiguous Raman band assignments. These findings highlight the potential of Raman spectroscopy as a dynamic roadmap for elucidating degradation mechanisms and guiding the design of thermally resilient perovskites.

The robust respective formations of a solid electrolyte interphase (SEI) and pillar at the surfaces of hard carbon and O3-type positive electrodes are the consequences of integrating LiPF6 salt into a sodium-ion battery electrolyte that considerably strengthens both interfaces of positive and negative electrodes. The improvement of cycle performances due to the formation of highly passivating SEI on the hard carbon electrode is induced by the alternated solvation structure following the addition of Li salt, which inhibits sodium-ion and electron leakage from further electrolyte decomposition. The SEI with incorporated Li is less soluble than Na-based SEI, and the passivation ability of the initially formed SEI can thus be well preserved. Conversely, the gas evolution caused by oxygen release is reduced considerably by the marginal surface intercalation of Li ions at the surface of the O3-positive electrode. Additionally, the LiF layer that forms on the O3 surface diminishes additional deterioration of the electrolyte after formation. Compared with the fluoroethylene carbonate additive that is typically applied, a simultaneously strengthened interface yields major improvements in capacity retention.