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A novel facile, fast, and efficient microwave-assisted method was developed to synthesize a number of diverse nanostructured motifs (ranging from nanorods to nanoflowers) of VS4 along with its asso...

Sodium vanadate (Na1+xV3O8 or NVO) has recently attracted significant interest as a potential cathode material for an aqueous Zn ion battery for its unique pillared framework facilitating Zn ion migration. Here, we performed a detailed study on reaction mechanisms of hydrated Na2V6O16·2H2O slabs and nonhydrated Na1.25V3O8 nanorods using transmission electron microscopy. Our initial observation reveals that the thin (30–50 nm) Na2V6O16·2H2O system successfully undergoes discharge with Zn ion insertion into the structure while thick (120–170 nm) Na1.25V3O8 allows Zn ion insertion only at the surface, signifying the importance of both the presence of water and the nanostructure thickness in determining the reaction mechanism of NVO. More in-depth analysis of these two systems revealed the irreversible formation of the stable byproduct phase Zn3Nax(OH2)V2O7 (ZNVO), which likely evolved through a Zn-ion redox reaction, contributing to overall cell performance. Eventually, the entire discharge/charge process ap...

The syntheses of FeS2 and Fe3S4 nanomaterials were optimized using a novel facile, surfactant-free, and microwave-assisted, one-pot synthesis method, run under ambient and reasonably mild reaction conditions. Synthetic parameters, such as metal precursor salt identity, reaction time, reaction temperature, metal:sulfur molar ratios, and solvent combinations, were all systematically investigated and optimized. A series of FeS2 (pyrite) samples was initially fabricated using thioacetamide (TAA) as the sulfur precursor to generate a distinctive, uniform octahedra-based morphology. Switching the sulfur precursor from TAA to L-cysteine resulted in a corresponding transformation in not only chemical composition from FeS2 to an iron thiospinel structure, Fe3S4 (otherwise known as greigite), but also an associated morphological evolution from octahedra to nanosheet aggregates. The study of these materials has enabled crucial insights into the formation mechanisms of these materials under a relatively non-conventional microwave-assisted setting. Furthermore, in separate experiments, multi-walled carbon nanotubes (MWNTs) and graphene were added in with underlying metal sulfide species to create conductive Fe–S/MWNT composites and Fe–S/graphene composites, respectively. The method of addition of either MWNTs or graphene was also explored, wherein an ‘ex-situ’ synthetic procedure was found to be the least disruptive means of attachment and immobilization onto iron sulfide co-reagents as a means of preserving the latter’s inherent composition and morphology. The redox acidity for the parent material and associated composites demonstrates the utility of our as-developed synthetic methods for creating motifs relevant for electrochemical applications, such as energy storage.

One of the inherent challenges with Li-S batteries is polysulfide dissolution, in which soluble polysulfide species can contribute to the active material loss from the cathode and undergo shuttling reactions inhibiting the ability to effectively charge the battery. Prior theoretical studies have proposed the possible benefit of defective 2 D MoS2 materials as polysulfide trapping agents. Herein the synthesis and thorough characterization of hydrothermally prepared MoS2 nanosheets that vary in layer number, morphology, lateral size, and defect content are reported. The materials were incorporated into composite sulfur-based cathodes and studied in Li-S batteries with environmentally benign ether-based electrolytes. Through directed synthesis of the MoS2 additive, the relationship between synthetically induced defects in 2 D MoS2 materials and resultant electrochemistry was elucidated and described.

This study demonstrates that thermal regeneration is an effective approach to convert degraded phases to functioning electroactive materials, restore functional delivered capacity and recover material crystallinity while retaining the integrity of the parent electrode. V2O5 nanowires were synthesized through a facile hydrothermal method and used to fabricate V2O5/carbon nanotube (CNT) binder free electrodes. Discharge of the V2O5–CNT electrodes coupled with operando energy dispersive X-ray diffraction shows no evidence of phase segregation throughout the 150 μm thick binder free electrodes indicating full utilization of a thick electrode. When V2O5 is highly electrochemically lithiated (x > 2 in LixV2O5), irreversible phase transformation to ω-LixV2O5 was observed, accompanied by a capacity decrease of ∼40% over 100 cycles. A simple thermal treatment of the entire electrode results in a delivered capacity equal to or higher than the original value. Both phase conversion and an increase in material crystallinity as a result of thermal treatment are observed where structural analysis indicates the formation of Li1V3O8. The electrode design approach with thick electrodes and functional thermal regeneration may provide a broader choice of electroactive materials through decreasing the environmental burden by extending the lifetime of energy storage systems.

Vanadium dioxides exist as a variety of polymorphs, each with differing structural and electrochemical capabilities. The monoclinic to rutile transition is an interesting system for study as the transition temperature is easily accessible at moderate temperature and corresponds to an increase in electrical conductivity by 2 orders of magnitude. The transition from monoclinic to rutile is characterized structurally herein using synchrotron-based X-ray diffraction and related to lithium-ion electrochemistry using electrochemical impedance spectroscopy and intermittent pulsatile galvanostatic discharge tests. The experimental results indicate a decrease in ohmic resistance for lithium-based cells tested under higher temperatures. Complementary density functional theory calculations described the experimentally measured intercalation voltages and identified a possible Li-induced LixVO2(M) to LixVO2(R) phase transition during the discharging process rationalizing the favorable impact on the function of a lithi...

Understanding the current response at an electrode from suspended solid particles in an electrolyte is crucial for developing materials to be used in semi-solid electrodes for energy storage applications. Here, an analytical model is proposed to predict and understand the current response from non-disintegrable solid particles at a rotating disk electrode. The current is shown to be limited by a combination of ion diffusion within the solid particle and the mean residence time of the particle at the rotating disk electrode. This results in a relationship between current and angular frequency of I ∝ ω 3 / 4 , instead of the classical I ∝ ω 1 / 2 predicted by Levich theory. Specifically, the current response of Li4Ti5O12 (LTO) microparticles suspended in a non-aqueous electrolyte of lithium hexafluorophosphate (LiPF6) in ethylene carbonate: diethyl carbonate (EC:DEC) was determined experimentally and compared favorably with predictions from the proposed analytical model using fitting parameters consistent with the experimental conditions.

Silicon offers high theoretical capacity as a negative electrode material for lithium-ion batteries; however, high irreversible capacity upon initial cycling and poor cycle life have limited commercial adoption. In this work, an operando isothermal microcalorimetry (IMC) study of a model system containing lithium metal and silicon composite film electrodes was reported on the first two cycles of (de)lithiation. The total heat flow data are analyzed in terms of polarization, entropic, and parasitic heat flow contributions to quantify and determine the onset of parasitic reactions. These parasitic reactions, which include solid−electrolyte interphase formation, contribute to electrochemical irreversibility. To complement the calorimetry, operando X-ray diffraction is used to track the phase evolution of silicon. During cycle 1 lithiation, crystalline Si undergoes transformation to amorphous lithiated silicon and ultimately to crystalline Li15Si4. The solid-state amorphization process is correlated to a decrease in entropic heat flow, suggesting that heat associated with the amorphization contributes significantly to the entropic heat flow term. This study effectively uses IMC to probe the parasitic reactions that occur during lithiation of a silicon electrode, demonstrating an approach that can be broadly applied to quantify parasitic reactions in other complex systems.

Vanadium dioxides exist as different polymorphs, each with unique electrochemical properties. Herein, we investigate the monoclinic to rutile transition of vanadium dioxide using different temperatures. The transition from monoclinic to rutile is characterized using synchrotron-based X-ray diffraction and electrochemical performance is performed using electrochemical impedance spectroscopy and intermittent pulsatile galvanostatic discharge tests in a lithium-ion environment. The experimental results indicate a decrease in ohmic resistance when lithium-ion cells are tested at higher temperatures. Density functional theory calculations also identified a possible LixVO2(M) to LixVO2(R) phase transition during the discharging process. Since the monoclinic to rutile transition corresponds to an increase of electrical conductivity by 2 orders of magnitude this can favorably impact the function of a lithium-based electrochemical cell.

The electrochemical charge storage of sodium vanadate (NaV3O8 or NVO) cathodes in aqueous Zn-ion batteries has been hypothesized to be influenced by the inclusion of structural water for facilitating ion transfer in the material. Materials properties considered important (morphology, crystallite and particle size, surface area) are systematically studied herein through investigation of two NVO materials, NaV3O8· 0.34H2O [NVO(300)] and NaV3O8·0.05H2O [NVO(500)], with different water content, acicular morphologies with different size and surface area achieved via post-synthesis heat treatment. The electrochemistry of the two materials was evaluated in aqueous Zn-ion cells with 2 M ZnSO4 electrolyte using cyclic voltammetry, galvanostatic cycling, and rate capability testing. The thinner NVO(300) nanobelts (0.13 mm) demonstrate greater specific capacities and higher effective diffusion coefficients relative to the thicker NVO(500) nanorods. Notably however, while cells containing NVO(500) deliver lower specific capacity, they demonstrate enhanced capacity retention with cycling. The structural changes accompanying oxidation and reduction are elucidated via ex situ X-ray diffraction, transmission electron microscopy, and operando V K-edge X-ray absorption spectroscopy (XAS), where NVO material properties are shown to influence the ion insertion. Operando XAS verified that electron transfer corresponds directly to change in vanadium oxidation state, affirming vanadium redox as the governing electrochemical process.