Dissolution–Redeposition Mechanism of the MnO₂ Cathode in Aqueous Zinc-Ion Batteries

A dissolution–redeposition reaction mechanism of the MnO2 cathode is directly visualized in rechargeable aqueous zinc-ion batteries via in situ Raman microscopy. MnO2 is reduced to Mn3+ during the discharge process, followed by a disproportionation reaction to form Mn2+ and Mn4+. The dissolved Mn2+ plays an important role in the battery chemistry. During the following charge process, the redeposition of Mn2+ forms a species with high Zn-content on the surface of the MnO2 cathode in the high-potential window. Moreover, an effective method that allows in operando observation of Jahn–Teller distortion of manganese is provided for the first time. This method uses in situ Raman microscopy to reveal the correlation between Jahn–Teller distortion and Mn–O bond length change

Activating ZnV₂O₄ by an Electrochemical Oxidation Strategy for Enhanced Energy Storage in Zinc-Ion Batteries

Rechargeable aqueous zinc-ion batteries (RAZIBs) are recognized as promising energy storage systems to meet the ever-growing demand for grid-scale applications. Developing reliable cathode materials with superior electrochemical performance plays a decisive role in this field. In this work, an electrochemical oxidation strategy is employed to successfully activate the electrochemical activity of ZnV2O4 spinel oxide. Operating at high potentials up to 2.0 V enables the capacity activation process efficiently, in which the specific capacity increases from 86 to 232 mAh g–1 (corresponding to 170% capacity enhancement) after 50 cycles at 2 A g–1. On the contrary, ZnV2O4 operating in the potential window of 0.4–1.6 V only delivers 87 mAh g–1 after 50 cycles, whereas negligible capacity (–1) is obtained in the case of 0.4–1.3 V. As characterized by X-ray diffraction (XRD), Raman microscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and in situ pH measurements, the underlying mechanism is unraveled as a hydrolysis reaction coupled with the dissolution–recrystallization process, leading to the formation of high-valent Zn0.06V2O5·1.07H2O with a localized layered structure. The activated cathode demonstrates facilitated ion transport kinetics, reduced charge transfer resistance, and high electrochemical reversibility in RAZIBs. Benefiting from these features, stable cycle stability is achieved, that is, a reversible capacity of 138 mAh g–1 (83% capacity retention) can be retained after 2000 cycles at 4 A g–1. This work sheds light on activating low-valent vanadium-based oxides for practical application in RAZIBs, opening an avenue for developing cathode materials for aqueous batteries

Quantitative Resolution of Complex Stoichiometric Changes during Electrochemical Cycling by Density Functional Theory-Assisted Electrochemical Quartz Crystal Microbalance

The capability to simultaneously measure changes of mass and charge of electro-active materials during a redox process makes Electrochemical Quartz Crystal Microbalance (EQCM) a powerful technique to monitor stoichiometric changes during reversible electrochemical processes. In principle, quantitative resolution of the stoichiometry of the electro-active sample during electrochemical cycling can be obtained by solving the system of equations for the EQCM mass and charge balance. Such a system of equations couples the measured changes in mass and charge through the stoichiometry of the redox process. Unfortunately, whenever more than two chemically inequivalent species are involved in the redox process, the system of equations is mathematically undetermined, having more variables (stoichiometric coefficients) than equations. In these cases, current best practice is the arbitrary reduction of the number of variables in the mass and charge balance equation, using chemical intuition to set some of the stoichiometric coefficients to fixed values. For layered ion-intercalation host materials, widespread practical approximations are the use of arbitrarily defined solvation numbers for the intercalating ions or the neglect of ion-induced displacement of structural solvent inside the host. Here, we propose an alternative approach based on the use of Density Functional Theory (DFT) to sample and screen, on an energy basis, the whole unreduced spectrum of stoichiometric coefficients compatible with EQCM measurements, leading to DFT energy-assisted resolution of stoichiometric changes during cycling. We illustrate the approach by taking nickel hydroxide Ni­(OH)2 as a case system and studying its ion intercalation-driven phase transformations in the presence of different LiOH, NaOH, and KOH electrolytes. Quantitative resolution of the Ni­(OH)2 stoichiometry during electrochemical cycling unambiguously reveals ion intercalation to displace structural water from the layered host, promoting electrochemical degradation and aging of the material. The process is found to be strongly dependent on the size of the electrolyte cation, with larger cations displacing larger amounts of structural water and resulting in faster degradation rates

Polypropylene Carbonate-Based Adaptive Buffer Layer for Stable Interfaces of Solid Polymer Lithium Metal Batteries

Solid polymer electrolytes (SPEs) have the potential to enhance the safety and energy density of lithium batteries. However, poor interfacial contact between the lithium metal anode and SPE leads to high interfacial resistance and low specific capacity of the battery. In this work, we present a novel strategy to improve this solid–solid interface problem and maintain good interfacial contact during battery cycling by introducing an adaptive buffer layer (ABL) between the Li metal anode and SPE. The ABL consists of low molecular-weight polypropylene carbonate , poly­(ethylene oxide) (PEO), and lithium salt. Rheological experiments indicate that ABL is viscoelastic and that it flows with a higher viscosity compared to PEO-only SPE. ABL also has higher ionic conductivity than PEO-only SPE. In the presence of ABL, the interface resistance of the Li/ABL/SPE/LiFePO4 battery only increased 20% after 150 cycles, whereas that of the battery without ABL increased by 117%. In addition, because ABL makes a good solid–solid interface contact between the Li metal anode and SPE, the battery with ABL delivered an initial discharge specific capacity of >110 mA·h/g, which is nearly twice that of the battery without ABL, which is 60 mA·h/g. Moreover, ABL is able to maintain electrode–electrolyte interfacial contact during battery cycling, which stabilizes the battery Coulombic efficiency