The Relationship between Ionic Conductivity and Solvation Structures of Localized High-Concentration Fluorinated Electrolytes for Lithium-Ion Batteries.

Localized high-concentration electrolytes (LHCEs) combine a diluent with a high-concentration electrolyte, offering promising properties. The ions, solvent, and diluent interact to form complex heterogeneous liquid structures, where high salt concentration clusters are embedded in the diluent. Optimizing LHCEs for desired electrolyte properties like high ionic conductivity, low viscosity, and effective solid electrolyte interphase (SEI) formation ability within the vast chemical and compositional design space requires deeper understanding and theoretical guidance. We investigated the structures and conductivities of LHCEs based on a fluorinated solvent with two different diluents at varying concentrations. 2,2,3,3-Tetrafluoropropyl trifluoroacetate (TFPTFA) enters the solvation cluster due to its stronger Li-ion interactions, whereas 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether (TFETFE) enters only at extremely high diluent concentrations. The ionic conductivity increases with decreasing diluent concentrations, with a slope change during cluster percolation. Overall, TFETFE demonstrates higher effectiveness than TFPTFA, forming higher local salt concentration clusters and resulting in higher ionic conductivity.

Electron and Ion Transport in Lithium and Lithium-Ion Battery Negative and Positive Composite Electrodes.

Electrochemical energy storage systems, specifically lithium and lithium-ion batteries, are ubiquitous in contemporary society with the widespread deployment of portable electronic devices. Emerging storage applications such as integration of renewable energy generation and expanded adoption of electric vehicles present an array of functional demands. Critical to battery function are electron and ion transport as they determine the energy output of the battery under application conditions and what portion of the total energy contained in the battery can be utilized. This review considers electron and ion transport processes for active materials as well as positive and negative composite electrodes. Length and time scales over many orders of magnitude are relevant ranging from atomic arrangements of materials and short times for electron conduction to large format batteries and many years of operation. Characterization over this diversity of scales demands multiple methods to obtain a complete view of the transport processes involved. In addition, we offer a perspective on strategies for enabling rational design of electrodes, the role of continuum modeling, and the fundamental science needed for continued advancement of electrochemical energy storage systems with improved energy density, power, and lifetime.

Multimodal electrochemistry coupled microcalorimetric and X-ray probing of the capacity fade mechanisms of Nickel rich NMC - progress and outlook.

Lithium nickel manganese cobalt oxide (NMC) is a commercially successful Li-ion battery cathode due to its high energy density; however, its delivered capacity must be intentionally limited to achieve capacity retention over extended cycling. To design next-generation NMC batteries with longer life and higher capacity the origins of high potential capacity fade must be understood. Operando hard X-ray characterization techniques are critical for this endeavor as they allow the acquisition of information about the evolution of structure, oxidation state, and coordination environment of NMC as the material (de)lithiates in a functional battery. This perspective outlines recent developments in the elucidation of capacity fade mechanisms in NMC through hard X-ray probes, surface sensitive soft X-ray characterization, and isothermal microcalorimetry. A case study on the effect of charging potential on NMC811 over extended cycling is presented to illustrate the benefits of these approaches. The results showed that charging to 4.7 V leads to higher delivered capacity, but much greater fade as compared to charging to 4.3 V. Operando XRD and SEM results indicated that particle fracture from increased structural distortions at >4.3 V was a contributor to capacity fade. Operando hard XAS revealed significant Ni and Co redox during cycling as well as a Jahn-Teller distortion at the discharged state (Ni3+); however, minimal differences were observed between the cells charged to 4.3 and 4.7 V. Additional XAS analyses using soft X-rays revealed significant surface reconstruction after cycling to 4.7 V, revealing another contribution to fade. Operando isothermal microcalorimetry (IMC) indicated that the high voltage charge to 4.7 V resulted in a doubling of the heat dissipation when compared to charging to 4.3 V. A lowered chemical-to-electrical energy conversion efficiency due to thermal energy waste was observed, providing a complementary characterization of electrochemical degradation. The work demonstrates the utility of multi-modal X-ray and microcalorimetric approaches to understand the causes of capacity fade in lithium-ion batteries with Ni-rich NMC.

Impact of Charge Voltage on Factors Influencing Capacity Fade in Layered NMC622: Multimodal X-ray and Electrochemical Characterization.

Ni-rich NMC is an attractive Li-ion battery cathode due to its combination of energy density, thermal stability, and reversibility. While higher delivered energy density can be achieved with a more positive charge voltage limit, this approach compromises sustained reversibility. Improved understanding of the local and bulk structural transformations as a function of charge voltage, and their associated impacts on capacity fade are critically needed. Through simultaneous operando synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) of cells cycled at 3-4.3 or 3-4.7 V, this study presents an in-depth investigation into the effects of voltage window on local coordination, bulk structure, and oxidation state. These measurements are complemented by ex situ X-ray fluorescence (XRF) mapping and scanning electrochemical microscopy mapping (SECM) of the negative electrode, X-ray photoelectron spectroscopy (XPS) of the positive electrode, and cell level electrochemical impedance spectroscopy (EIS). Initially, cycling between 3 and 4.7 V leads to greater delivered capacity due to greater lithium extraction, accompanied by increased structural distortion, moderately higher Ni oxidation, and substantially higher Co oxidation. Continued cycling at this high voltage results in suppressed Ni and Co redox, greater structural distortion, increased levels of transition metal dissolution, higher cell impedance, and 3× greater capacity fade.

Structural and electrochemical investigation of crystallite size controlled zinc ferrite (ZnFe2O4).

Zinc ferrite, ZnFe2O4(ZFO), is a promising electrode material for next generation Li-ion batteries because of its high theoretical capacity and low environmental impact. In this report, synthetic control of crystallite size from the nanometer to submicron scale enabled probing of the relationships between ZFO size and electrochemical behavior. A facile two-step coprecipitation and annealing preparation method was used to prepare ZFO with controlled sizes ranging ∼9 to >200 nm. Complementary synchrotron and electron microscopy techniques were used to characterize the series of materials. Increasing the annealing temperature increased crystallinity and decreased microstrain, while local structural ordering was maintained independent of crystallite size. Electrochemical characterization revealed that the smaller sized materials delivered higher capacities during initial lithiation. Larger sized particles exhibited a lack of distinct electrochemical signatures above 1.0 V, suggesting that the longer diffusion length associated with greater crystallite size causes the lithiation process to proceed via non discrete lithium insertion, cation migration, and conversion processes. Notably, larger particles exhibited enhanced electrochemical reversibility over 50 cycles, with capacity retention improving from <20% to >40% at C/2 cycling rate. This intriguing result was probed through x-ray absorption spectroscopy (XAS) and x-ray photoelectron spectroscopy (XPS) measurements of the cycled electrodes. XAS revealed that the larger crystallite size materials do not completely convert to Fe0during the first lithiation and that independent of size, delithiation results in the formation of nanocrystalline FeO and ZnO phases rather than ZnFe2O4. After 20 cycles, the larger crystallites showed reversibility between partially oxidized FeO in the charged state and Fe0in the discharged state, while the smaller crystallite size material was electrochemically inactive as Fe0. XPS analysis revealed more significant solid electrolyte interphase (SEI) formation on the cycled electrodes utilizing ZFO with smaller crystallite size. This finding suggests that excessive SEI buildup on the smaller sized, higher surface area ZFO particles contributes to their reduced electrochemical reversibility relative to the larger crystallite size materials.

Impact of sodium vanadium oxide (NaV3O8, NVO) material synthesis conditions on charge storage mechanism in Zn-ion aqueous batteries.

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 µm) 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.

Lithium vanadium oxide (Li1.1V3O8) thick porous electrodes with high rate capacity: utilization and evolution upon extended cycling elucidated via operando energy dispersive X-ray diffraction and continuum simulation.

The phase distribution of lithiated LVO in thick (∼500 µm) porous electrodes (TPEs) designed to facilitate both ion and electron transport was determined using synchrotron-based operando energy dispersive X-ray diffraction (EDXRD). Probing 3 positions in the TPE while cycling at a 1C rate revealed a homogeneous phase transition across the thickness of the electrode at the 1st and 95th cycles. Continuum modelling indicated uniform lithiation across the TPE in agreement with the EDXRD results and ascribed decreasing accessible active material to be the cause of loss in delivered capacity between the 1st and 95th cycles. The model was supported by the observation of significant particle fracture by SEM consistent with loss of electrical contact. Overall, the combination of operando EDXRD, continuum modeling, and ex situ measurements enabled a deeper understanding of lithium vanadium oxide transport properties under high rate extended cycling within a thick highly porous electrode architecture.

Unraveling the Dissolution-Mediated Reaction Mechanism of α-MnO2 Cathodes for Aqueous Zn-Ion Batteries.

Aqueous Zn/α-MnO2 batteries have attracted immense interest owing to their high energy density, low cost, and safety, making them desirable for future large-scale energy application. Despite these merits, the comprehensive understanding of their reaction mechanism has been elusive due to the limitations of standard bulk characterization. Here, via transmission electron microscopy, the dissolution-mediated reaction mechanism of a Zn/α-MnO2 system is discovered and explored in full scope to involve reversible formation of Zn4 SO4 (OH)6 ·xH2 O and "birnessite-like" Zn-MnOx phase upon cycling. Overall, α-MnO2 acts primarily as a source for cell activation through dissolution and thus is not directly involved in the Zn redox chemistry. This microscopic study offers a unique knowledge on the unconventional reaction chemistry of Zn/α-MnO2 batteries.

The effect of chemically preintercalated alkali ion on structure of layered titanates and their electrochemistry in aqueous energy storage systems.

We introduce a novel chemical preintercalation based synthesis technique based on hydrogen peroxide induced sol-gel process to obtain alkali ion containing ternary layered titanates (MTO, where M = Li, Na, K). Synthesis parameters leading to the formation of single-phase materials with homogeneous elemental distribution are reported for each of the preintercalated ion. Our analyses indicate that the interlayer spacing in the structure of the layered titanates increases with the increase of the radius of the hydrated preintercalated ion. Scanning and transmission electron microscopy imaging revealed morphological diversity: the LTO phase crystallized as nanoplates assembled in "peony-like" spherical agglomerates while NTO and KTO particles exhibited one-dimensional nanobelt or wire-like morphology, with the KTO nanobelts being shorter and more aggregated than the NTO nanobelts. Structural refinement corroborated by electron diffraction and high-resolution transmission electron microscopy revealed that the structure of the LTO phase is built by stacking Ti-O layers containing a single straight layer of connected TiO6 octahedra. The layers in NTO and KTO structures form differently and consist of double Ti-O layers with ragged arrangement of units built by TiO6 octahedra with two titanium rows. The NTO electrodes exhibited the highest electrochemical performance in cells with aqueous 1 M Na2SO4 electrolyte, followed by the KTO electrodes and then LTO electrodes, and this trend is maintained at various scan rates. The established relationships between the structure and electrochemical performance reveal that, in addition to interlayer distance and chemistry of the interlayer region, the structure of the layers can play an important role in charge storage properties of layered oxide electrodes. The double Ti-O layers in the structure of NTO and KTO phases provide a larger number of redox centers which could contribute to the superior electrochemical performance as compared to the LTO electrodes. Our findings indicate that layered materials containing double transition metal oxide layers are promising candidates for exfoliation and assembly with electronically conductive layers with the aim to create 2D heterostructures with high electrochemical performance.

Defect Control in the Synthesis of 2 D MoS2 Nanosheets: Polysulfide Trapping in Composite Sulfur Cathodes for Li-S Batteries.

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.

Reversible epitaxial electrodeposition of metals in battery anodes.

The propensity of metals to form irregular and nonplanar electrodeposits at liquid-solid interfaces has emerged as a fundamental barrier to high-energy, rechargeable batteries that use metal anodes. We report an epitaxial mechanism to regulate nucleation, growth, and reversibility of metal anodes. The crystallographic, surface texturing, and electrochemical criteria for reversible epitaxial electrodeposition of metals are defined and their effectiveness demonstrated by using zinc (Zn), a safe, low-cost, and energy-dense battery anode material. Graphene, with a low lattice mismatch for Zn, is shown to be effective in driving deposition of Zn with a locked crystallographic orientation relation. The resultant epitaxial Zn anodes achieve exceptional reversibility over thousands of cycles at moderate and high rates. Reversible electrochemical epitaxy of metals provides a general pathway toward energy-dense batteries with high reversibility.

Insights into Reactivity of Silicon Negative Electrodes: Analysis Using Isothermal Microcalorimetry.

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. Herein, we report an operando isothermal microcalorimetry (IMC) study of a model system containing lithium metal and silicon composite film electrodes during 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. Cycle 1 lithiation demonstrates the highest thermal energy output at 1509 mWh/g, compared to cycle 1 delithiation and cycle 2. 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.

Investigation of α-MnO2 Tunneled Structures as Model Cation Hosts for Energy Storage.

Future advances in energy storage systems rely on identification of appropriate target materials and deliberate synthesis of the target materials with control of their physiochemical properties in order to disentangle the contributions of distinct properties to the functional electrochemistry. This goal demands systematic inquiry using model materials that provide the opportunity for significant synthetic versatility and control. Ideally, a material family that enables direct manipulation of characteristics including composition, defects, and crystallite size while remaining within the defined structural framework would be necessary. Accomplishing this through direct synthetic methods is desirable to minimize the complicating effects of secondary processing. The structural motif most frequently used for insertion type electrodes is based on layered type structures where ion diffusion in two dimensions can be envisioned. However, lattice expansion and contraction associated with the ion movement and electron transfer as a result of repeated charge and discharge cycling can result in structural degradation and amorphization with accompanying loss of capacity. In contrast, tunnel type structures embody a more rigid framework where the inherent structural design can accommodate the presence of cations and often multiple cations. Of specific interest are manganese oxides as they can exhibit a tunneled structure, termed α-MnO2, and are an important class of nanomaterial in the fields of catalysis, adsorption-separation, and ion-exchange. The α-MnO2 structure has one-dimensional 2 × 2 tunnels formed by corner and edge sharing manganese octahedral [MnO6] units and can be readily substituted in the central tunnel by a variety of cations of varying size. Importantly, α-MnO2 materials possess a rich chemistry with significant synthetic versatility allowing deliberate synthetic control of structure, composition, crystallite size, and defect content. This Account considers the investigation of α-MnO2 tunnel type structures and their electrochemistry. Examination of the reported findings on this material family demonstrates that multiple physiochemical properties influence the electrochemistry. The retention of the parent structure during charge and discharge cycling, the material composition including the identity and content of the central cation, the surface condition including oxygen vacancies, and crystallite size have all been demonstrated to impact electrochemical function. The selection of the α-MnO2 family of materials as a model system and the ability to control the variables associated with the structural family affirm that full investigation of the mechanisms related to active materials in an electrochemical system demands concerted efforts in synthetic material property control and multimodal characterization, combined with theory and modeling. This then enables more complete understanding of the factors that must be controlled to achieve consistent and desirable outcomes.

Structural characterization and photoluminescence in the rare earth-free oxy-fluoride anti-perovskites Sr3-xBi2x/3AlO4F and Sr3-xBi2x/3GaO4F.

The activator Bi3+ has been successfully incorporated into the anti-perovskite oxy-fluoride host lattice Sr3MO4F (M = Al, Ga) to form rare earth-free phosphors of the composition Sr3-xBi2x/3AlO4F, 0 ≤ x ≤ 0.1, and Sr3-xBi2x/3GaO4F, 0 ≤ x ≤ 0.048. These phases absorb in the UV region (λex = 240-326 nm) and exhibit broad emission in the blue region of the visible spectrum (λem = 446.5-455 nm). The optimum compositions for maximum photoluminescent intensity were determined to be Sr2.976Bi0.016AlO4F and Sr2.976Bi0.016GaO4F before concentration quenching occurs. Full structural characterization based upon PXRD and NPD data were performed with DFT calculations suggesting that Bi3+ ions are preferentially incorporated on the ten coordinate Sr(1) site.