Microwave-Assisted Synthesis of Silver Vanadium Phosphorus Oxide, Ag₂VO₂PO₄: Crystallite Size Control and Impact on Electrochemistry

Silver vanadium phosphorus oxide, Ag₂VO₂PO₄, is a promising cathode material for Li batteries due in part to its large capacity and high current capability. Herein, a new synthesis of Ag₂VO₂PO₄ based on microwave heating is presented, where the reaction time is reduced by approximately 100× relative to other reported methods, and the crystallite size is controlled via synthesis temperature, showing a linear correlation of crystallite size with temperature. Notably, under galvanostatic reduction, the Ag₂VO₂PO₄ sample with the smallest crystallite size delivers the highest capacity and shows the highest loaded voltage. Further, pulse discharge tests show a significant resistance decrease during the initial discharge coincident with the formation of Ag metal. Thus, the magnitude of the resistance decrease observed during pulse tests depends on the Ag₂VO₂PO₄ crystallite size, with the largest resistance decrease observed for the smallest crystallite size. Additional electrochemical measurements indicate a quasi-reversible redox reaction involving Li⁺ insertion/deinsertion, with capacity fade due to structural changes associated with the discharge/charge process. In summary, this work demonstrates a faster synthetic approach for bimetallic polyanionic materials which also provides the opportunity for tuning of electrochemical properties through control of material physical properties such as crystallite size

Battery Relevant Electrochemistry of Ag₇Fe₃(P₂O₇)₄: Contrasting Contributions from the Redox Chemistries of Ag⁺ and Fe³⁺

Ag₇Fe₃(P₂O₇)₄ is an example of an electrochemical displacement material which contains two different electrochemically active metal cations, where one cation (Ag⁺) forms metallic silver nanoparticles external to the crystals of Ag₇Fe₃(P₂O₇)₄ via an electrochemical reduction displacement reaction, while the other cation (Fe³⁺) is electrochemically reduced with the retention of iron cations within the anion structural framework concomitant with lithium insertion. These contrasting redox chemistries within one pure cathode material enable high rate capability and reversibility when Ag₇Fe₃(P₂O₇)₄ is employed as cathode material in a lithium ion battery (LIB). Further, pyrophosphate materials are thermally and electrically stable, desirable attributes for cathode materials in LIBs. In this paper, a bimetallic pyrophosphate material Ag₇Fe₃(P₂O₇)₄ is synthesized and confirmed to be a single phase by Rietveld refinement. Electrochemistry of Ag₇Fe₃(P₂O₇)₄ is reported for the first time in the context of lithium based batteries using cyclic voltammetry and galvanostatic discharge–charge cycling. The reduction displacement reaction and the lithium (de)­insertion processes are investigated using <i>ex situ</i> X-ray absorption spectroscopy and X-ray diffraction of electrochemically reduced and oxidized Ag₇Fe₃(P₂O₇)₄. Ag₇Fe₃(P₂O₇)₄ exhibits good reversibility at the iron centers indicated by ∼80% capacity retention over 100 cycles following the initial formation cycle and excellent rate capability exhibited by ∼70% capacity retention upon a 4-fold increase in current

Synthetic Control of Composition and Crystallite Size of Silver Hollandite, Ag_<i>x</i>Mn₈O₁₆: Impact on Electrochemistry

Synthetic control of the silver content in silver hollandite, Ag_<i>x</i>Mn₈O₁₆, where the silver content ranges from 1.0 ≤ <i>x</i> ≤ 1.8 is demonstrated. This level of compositional control was enabled by the development of a lower temperature reflux based synthesis compared to the more commonly reported hydrothermal approach. Notably, the synthetic variance of the silver content was accompanied by a concomitant variance in crystallite size as well as surface area and particle size. To verify the retention of the hollandite structure, the first Rietveld analysis of silver hollandite was conducted on samples of varying composition. The impacts of silver content, crystallite size, surface area, and particle size on electrochemical reversibility were examined under cyclic voltammetry and battery testing

Synthetic Control of Composition and Crystallite Size of Silver Hollandite, Ag_<i>x</i>Mn₈O₁₆: Impact on Electrochemistry

Synthetic control of the silver content in silver hollandite, Ag_<i>x</i>Mn₈O₁₆, where the silver content ranges from 1.0 ≤ <i>x</i> ≤ 1.8 is demonstrated. This level of compositional control was enabled by the development of a lower temperature reflux based synthesis compared to the more commonly reported hydrothermal approach. Notably, the synthetic variance of the silver content was accompanied by a concomitant variance in crystallite size as well as surface area and particle size. To verify the retention of the hollandite structure, the first Rietveld analysis of silver hollandite was conducted on samples of varying composition. The impacts of silver content, crystallite size, surface area, and particle size on electrochemical reversibility were examined under cyclic voltammetry and battery testing

Ionic Liquid Hybrid Electrolytes for Lithium-Ion Batteries: A Key Role of the Separator–Electrolyte Interface in Battery Electrochemistry

Batteries are multicomponent systems where the theoretical voltage and stoichiometric electron transfer are defined by the electrochemically active anode and cathode materials. While the electrolyte may not be considered in stoichiometric electron-transfer calculations, it can be a critical factor determining the deliverable energy content of a battery, depending also on the use conditions. The development of ionic liquid (IL)-based electrolytes has been a research area of recent reports by other researchers, due, in part, to opportunities for an expanded high-voltage operating window and improved safety through the reduction of flammable solvent content. The study reported here encompasses a systematic investigation of the physical properties of IL-based hybrid electrolytes including quantitative characterization of the electrolyte–separator interface via contact-angle measurements. An inverse trend in the conductivity and wetting properties was observed for a series of IL-based electrolyte candidates. Test-cell measurements were undertaken to evaluate the electrolyte performance in the presence of functioning anode and cathode materials, where several promising IL-based hybrid electrolytes with performance comparable to that of conventional carbonate electrolytes were identified. The study revealed that the contact angle influenced the performance more significantly than the conductivity because the cells containing IL–tetrafluoroborate-based electrolytes with higher conductivity but poorer wetting showed significantly decreased performance relative to the cells containing IL–bis­(trifluoromethanesulfonyl)­imide electrolytes with lower conductivity but improved wetting properties. This work contributes to the development of new IL battery-based electrolyte systems with the potential to improve the deliverable energy content as well as safety of lithium-ion battery systems

Lithiation of Magnetite (Fe₃O₄): Analysis Using Isothermal Microcalorimetry and Operando X‑ray Absorption Spectroscopy

Conversion electrodes, such as magnetite (Fe₃O₄), offer high theoretical capacities (>900 mAh/g) because of multiple electron transfer per metal center. Capacity retention for conversion electrodes has been a challenge in part because of the formation of an insulating surface electrolyte interphase (SEI). This study provides the first detailed analysis of the lithiation of Fe₃O₄ using isothermal microcalorimetry (IMC). The measured heat flow was compared with heat contributions predicted from heats of formation for the Faradaic reaction, cell polarization, and entropic contributions. The total measured energy output of the cell (7260 J/g Fe₃O₄) exceeded the heat of reaction predicted for full lithiation of Fe₃O₄ (5508 J/g). During initial lithiation (3.0–0.86 V), the heat flow was successfully modeled using polarization and entropic contributions. Heat flow at lower voltage (0.86–0.03 V) exceeded the predicted values for iron oxide reduction, consistent with heat generation attributable to electrolyte decomposition and surface electrolyte interphase (SEI). Operando X-ray absorption spectroscopy (XAS) indicated that the oxidation state of the Fe centers deviated from predicted values beginning at ∼0.86 V, supportive of SEI onset in this voltage range. Thus, these combined results from electrochemistry, IMC, and XAS indicate parasitic reactions consistent with SEI formation at a moderate voltage and illustrate an approach for deconvoluting Faradaic and non-Faradaic contributions to heat, which should be broadly applicable to the study of energy-storage materials and systems

Investigation of Solid Electrolyte Interphase Layer Formation and Electrochemical Reversibility of Magnetite, Fe₃O₄, Electrodes: A Combined X‑ray Absorption Spectroscopy and X‑ray Photoelectron Spectroscopy Study

Magnetite (Fe₃O₄) is a promising electrode material for the next generation of Li-ion batteries with multiple electron transfers per metal center and a theoretical capacity of 924 mA h/g. However, multiple phase conversions during (de)­lithiation of Fe₃O₄ and formation of a solid electrolyte interphase (SEI) contribute to capacity fade. In this study, X-ray absorption spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to determine the surface chemistry, redox chemistry, and the impact on the electrochemical reversibility in the presence and absence of fluoroethylene carbonate (FEC) solvent. With FEC, improved capacity retention and enhanced reversibility are observed. In contrast, electrodes cycled with no FEC exhibit decreased reversibility where the active material remains as reduced Fe⁰. XPS results reveal LiF and lower quantities of oxygen-containing species, especially carbonates at the electrode surface tested in FEC. The improvement in electrochemical reversibility with FEC is attributed to the formation of a solid electrolyte interphase which forms prior to initiation of the conversion reaction limiting SEI growth on the reduced products, Fe⁰ and Li₂O. In contrast, ethylene carbonate-based carbonate electrolyte forms SEI at a potential where the formation of Fe⁰ and Li₂O has already initiated, resulting in SEI formation on Fe⁰ nanograins

Synthesis and Characterization of CuFe₂O₄ Nano/Submicron Wire–Carbon Nanotube Composites as Binder-free Anodes for Li-Ion Batteries

A series of one-dimensional CuFe₂O₄ (CFO) nano/submicron wires possessing different diameters, crystal phases, and crystal sizes have been successfully generated using a facile template-assisted coprecipitation reaction at room temperature, followed by a short postannealing process. The diameter and crystal structure of the resulting CuFe₂O₄ (CFO) wires were judiciously tuned by varying the pore size of the template and the postannealing temperature, respectively. Carbon nanotubes (CNTs) were incorporated to generate CFO-CNT binder-free anodes, and multiple characterization techniques were employed with the goal of delineating the relationships between electrochemical behavior and the properties of both the CFO wires (crystal phase, wire diameter, crystal size) and the electrode architecture (binder-free vs conventionally prepared approaches). The study reveals several notable findings. First, the crystal phase (cubic or tetragonal) did not influence the electrochemical behavior in this CFO system. Second, regarding crystallite size and wire diameter, CFO wires with larger crystallite sizes exhibit improved cycling stability, whereas wires possessing smaller diameters exhibit higher capacities. Finally, the electrochemical behavior is strongly influenced by the electrode architecture, with CFO-CNT binder-free electrodes demonstrating significantly higher capacities and cycling stability compared to conventionally prepared coatings. The mechanism(s) associated with the high capacities under low current density but limited electrochemical reversibility of CFO electrodes under high current density were probed via X-ray absorption spectroscopy mapping with submicron spatial resolution for the first time. Results suggest that the capacity of the binder-free electrodes under high rate is limited by the irreversible formation of Cu⁰, as well as limited reduction of Fe³⁺ to Fe²⁺, not Fe⁰. The results (1) shed fundamental insight into the reversibility of CuFe₂O₄ materials cycled at high current density and (2) demonstrate that a synergistic effort to control both active material morphology and electrode architecture is an effective strategy for optimizing electrochemical behavior

Reversible Electrochemical Lithium-Ion Insertion into the Rhenium Cluster Chalcogenide–Halide Re₆Se₈Cl₂

The cluster-based material Re₆Se₈Cl₂ is a two-dimensional ternary material with cluster–cluster bonding across the <i>a</i> and <i>b</i> axes capable of multiple electron transfer accompanied by ion insertion across the <i>c</i> axis. The Li/Re₆Se₈Cl₂ system showed reversible electron transfer from 1 to 3 electron equivalents (ee) at high current densities (88 mA/g). Upon cycling to 4 ee, there was evidence of capacity degradation over 50 cycles associated with the formation of an organic solid–electrolyte interface (between 1.45 and 1 V vs Li/Li⁺). This investigation highlights the ability of cluster-based materials with two-dimensional cluster bonding to be used in applications such as energy storage, showing structural stability and high rate capability

Electron/Ion Transport Enhancer in High Capacity Li-Ion Battery Anodes

Magnetite (Fe₃O₄) was used as a model high capacity metal oxide active material to demonstrate advantages derived from consideration of both electron and ion transport in the design of composite battery electrodes. The conjugated polymer, poly­[3-(potassium-4-butanoate) thiophene] (PPBT), was introduced as a binder component, while polyethylene glycol (PEG) was coated onto the surface of Fe₃O₄ nanoparticles. The introduction of PEG reduced aggregate size, enabled effective dispersion of the active materials and facilitated ionic conduction. As a binder for the composite electrode, PPBT underwent electrochemical doping which enabled the formation of effective electrical bridges between the carbon and Fe₃O₄ components, allowing for more efficient electron transport. Additionally, the PPBT carboxylic moieties effect a porous structure, and stable electrode performance. The methodical consideration of both enhanced electron and ion transport by introducing a carboxylated PPBT binder and PEG surface treatment leads to effectively reduced electrode resistance, which improved cycle life performance and rate capabilities