Electrochemical monitoring and mechanistic understanding of iodine film formation as a metastable intermediate during I3−/I− redox reaction in aqueous ZnI2 media

Multitechnique Detection of Lead Iodide Hybrid Perovskite Degradation Pathways under Varying Electric Fields

Although hybrid perovskite-based devices have made significant advances in terms of device performance, long-term stability remains a major challenge to widespread implementation. A unified understanding of the complexity describing the degradation of these types of materials is absent, and in this work, one common hybrid perovskite material, methylammonium lead iodide (MAPI), is used as a vehicle to show how a unified understanding can be achieved using complementary characterization techniques. This work uses low-dose in situ electron microscopes with electric fields ranging from 2.5 to 5 V/μm in a scanning electron microscope before focusing on the lower fields of 1.25 and 2.5 V/μm, where an electric threshold is identified. The results demonstrate that material loss is initiated at the MAPI grain boundaries near the negative electrode interface, where MA+ is reduced. Above the electrochemical threshold, extensive material volatilization and amorphous layer formation were detected, accompanied by significant PL quenching. High-field solid-state MAS NMR and materials modeling indicate that the MAPI decomposition process is a simultaneous combination of iodine migration, vacancy formation, and organic cation decomposition. The 1H MAS NMR data from the as-synthesized MAPI show direct evidence of preexisting iodine vacancies that induce the formation of CH3NH2, forming possible dative coordination to the lead framework positions. Subsequent data from MAPI degraded under exposure to electric fields (1.25 and 2.50 V/μm) directly demonstrate the presence of decomposition products such as NH4I, CH3I, and CH2I2 through pinhole formation at the electrochemical threshold and more widespread damage induced above this threshold. The methodology presented here can be applied to investigate other hybrid perovskite materials through direct spin coating on the corresponding substrates, deepening our understanding and providing insights for improved device stability.

Water-in-Salt Electrolyte Stabilizes Pyrazine Radical: Suppression of Its Aggregation by Interaction between Pyrazine and Li(H2O)n. J Am Chem Soc 2025;147:16812-16825. [PMID: 40327745 DOI: 10.1021/jacs.4c09561] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Stabilizing radical intermediates of redox-active organic molecules in aqueous media is crucial for advancing applications in energy storage, catalysis, and electrosynthesis. This study investigates the stabilization of protonated radical intermediates of pyrazine derivatives in water-in-salt electrolytes (WISEs) with 7-8 m LiTFSI. Strong interactions between pyrazine derivatives and Li+-coordinated water (Li(H2O)n+) in WISEs prevent molecular aggregation and protect radical intermediates from disproportionation and oxygen-induced degradation. Voltammetric results show that higher concentrations of LiTFSI enhance both the stability and redox reversibility of dimethylpyrazine (DMP) radical intermediates, with protonation identified as a key stabilizing factor. Notably, these stabilizing effects were absent in solutions containing concentrated LiCl or LiNO3. Fourier-transform infrared (FTIR) spectroscopy and molecular dynamics (MD) simulations confirmed reduced DMP aggregation in LiTFSI-based electrolytes, driven by interactions with Li(H2O)n+, while no similar solvation structure modification occurred with LiNO3. The protonated radical intermediates in LiTFSI-based WISEs exhibited greater resistance to oxygen-induced degradation compared to conventional acidic solutions. Additionally, substitution of methyl or ethyl groups on the pyrazine ring destabilized the corresponding radical intermediates in LiTFSI-based WISEs, primarily due to the alkyl inductive effect, as evidenced by electrochemical and UV-visible absorption spectroscopy. Charge-discharge tests in an H-cell further demonstrated significantly improved Coulombic efficiency of pyrazine redox reactions in LiTFSI-based WISEs compared to acidic Salt-in-Water electrolytes, underscoring the importance of radical intermediate stabilization.

Synergistic Ion Sieve and Solvation Regulation by Recyclable Clay-Based Electrolyte Membrane for Stable Zn-Iodine Battery

The high dissolution of polyiodides and unstable interface at the anode/electrolyte severely restrict the practical applications of rechargeable aqueous Zn-iodine batteries. Herein, we develop a zinc ion-based montmorillonite (ZMT) electrolyte membrane for synergizing ion sieve and solvation regulation to achieve highly stable Zn-iodine batteries. The rich M-O band and special cation-selective transport channel in ZMT locally tailor the solvation sheath around Zn2+ and therefore achieve high transference number (t+ = 0.72), benefiting for uniform and reversible deposition/stripping of Zn. Meanwhile, the mechanisms for three-step polyiodide generation and shuttle-induced Zn corrosion are highlighted by in situ characterization techniques. It is confirmed that the strong chemical adsorption between O atoms in ZMT and polyiodides species is the key to effectively inhibit the shuffle effect and side reactions. Consequently, the ZMT-based Zn-iodine battery delivers a high capacity of 0.45 mAh cm-2 at 1 mA cm-2 with a much improved Coulombic efficiency of 99.5% and outstanding capacity retention of 95% after 13 500 cycles at 10 mA cm-2. Moreover, owing to its high durability and chemical inertness and structural stability, ZMT-based electrolyte membranes can be recycled and applied in double-sided pouch cells, delivering a high areal capacity of 2.4 mAh cm-2 at 1 mA cm-2.

Chaotropic Electrolyte Enabling Wide-Temperature Metal-Free Battery

Metal-free aqueous batteries are promising candidates for grid-scale energy storage owing to their inherent safety, low cost, and cost effectiveness. The battery chemistry based on fast NH4+ diffusion kinetics avoids unfavorable generation of inactive metallic byproducts. However, their practical applications have been impeded by electrolyte instability and the intrinsic drawbacks of current electrodes. Herein, we propose an aqueous ammonium-iodine battery by using a chaotropic electrolyte, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) anode, and iodine composite (I2@CC) cathode. Experimental investigations and theoretical calculations reveal that the chaotropic electrolyte not only enhances electrolyte stability through modulating the H-bond structure but also facilitates the formation of a hydrophobic cationic sieve (HCS) on the anode, which ensures the electrolyte/electrode stability and high reversibility of the anode. Additionally, the Cl--containing electrolyte can support the consecutive I+/I0 reaction on the cathode by forming [IClx]1-x interhalogen. The as-assembled aqueous ammonium-iodine batteries (AIBs) based on NH4+ accommodation at the anode and I+/I0 redox reaction at the cathode can deliver superior electrochemical performance at room temperature and low temperature (-20 °C). This study provides a strategic insight into developing metal-free aqueous batteries with electrolyte modulation.

Cl-/Cl3- Redox Voltammetry to Recognize the Interfacial Layer on Positively Electrified Carbon in "Water-in-Salt" Electrolytes

A "Water-in-Salt" electrolyte solution (WiSE) is a promising aqueous medium for lithium-ion batteries containing highly concentrated electrolytes. For the increased kinetic overpotential of water oxidation in WiSE, the formation of an interfacial layer (IFL) on a positively electrified electrode is crucial. Nonetheless, most related studies have been restricted to theoretical approaches. In this Article, we voltammetrically study the Cl^-/Cl₃^-/Cl₂ redox reaction on Pt and glassy carbon (GC) electrodes in WiSE containing LiTFSI (WiSE_LiTFSI) and demonstrate that careful monitoring of Cl^-/Cl₃^- redox voltammetry can allow recognition of an IFL formed on a positively electrified electrode. The voltammetric wave attributed to the electro-oxidation of Cl^- on a GC electrode was negatively more shifted as the molal concentration of LiTFSI was increased from 0.5 to 6 m, while there was no shift on Pt. Also, there was voltammetric resolution into two peaks associated with Cl^-/Cl₃^- and Cl₃^-/Cl₂ on the GC electrode in WiSE_LiTFSI, while only unresolved, one redox-paired voltammograms were observed on Pt, regardless of the molal concentration of LiTFSI. These two main voltammetric features indicate the LiTFSI-induced IFL coupled with Cl^- and Cl₃^- on a GC electrode induced by an applied potential of ∼2 V versus the point of zero charge (PZC). We found other halide/halogen redox reactions did not show differentiated voltammetric behaviors in WiSE_LiTFSI, which demonstrates the uniqueness of the Cl^-/Cl₃^- redox reaction for recognizing the IFL formed on a positively charged electrode surface. Lastly, a strong interaction between the IFL and Cl species was also confirmed by XPS measurements.

Triiodide-in-Iodine Networks Stabilized by Quaternary Ammonium Cations as Accelerants for Electrode Kinetics of Iodide Oxidation in Aqueous Media

The Zn-polyiodide redox flow battery is considered to be a promising aqueous energy storage system. However, in its charging process, the electrode kinetics of I^- oxidation often suffer from an intrinsically generated iodine film (I₂-F) on the cathode of the battery. Therefore, it is critical to both understand and enhance the observed slow electrode kinetics of I^- oxidation by an electrochemically generated I₂-F. In this article, we introduced an electrogenerated N-methyl-N-ethyl pyrrolidinium iodide (MEPI)-iodine (I₂) solution, designated as MEPIS, and demonstrated that the electrode kinetics of I^- oxidation were dramatically enhanced compared to an I₂-F under conventional electrolyte conditions, such as NaI. We showed that this result mainly contributed to the fast electro-oxidation of triiodide (I₃^-), which exists in the shape of a I₃^--in-I₂ network, [I₃^-·(I₂)_n]. Raman spectroscopic and electrochemical analyses showed that the composition of electrogenerated MEPIS changed from I₃^- to [I₃^-·(I₂)_n] via I₅^- as the anodic overpotential increased. We also confirmed that I^- was electrochemically oxidized on a MEPIS-modified Pt electrode with fast electrode kinetics, which is clearly contrary to the nature of an I₂-F derived from a NaI solution as a kinetic barrier of I^- oxidation. Through stochastic MEPIS-particle impact electrochemistry and electrochemical impedance spectroscopy, we revealed that the enhanced electrode kinetics of I^- oxidation in MEPIS can be attributed to the facilitated charge transfer of I₃^- oxidation in [I₃^-·(I₂)_n]. In addition, we found that the degree of freedom of I₃^- in a quaternary ammonium-based I₂-F can also be critical to determine the kinetics of the electro-oxidation of I^-, which is that MEPIS showed more enhanced charge-transfer kinetics of I^- oxidation compared to tetrabutylammonium I₃^- due to the higher degree of freedom of I₃^-.