Unprotected Organic Cations─The Dilemma of Highly Li-Concentrated Ionic Liquid Electrolytes.

Highly Li-concentrated electrolytes have been widely studied to harness their uniquely varying bulk and interface properties that arise from their distinctive physicochemical properties and coordination structures. Similar strategies have been applied in the realm of ionic liquid electrolytes to exploit their improved functionalities. Despite these prospects, the impact of organic cation behavior on interfacial processes remains largely underexplored compared to the widely studied anion behavior. The present study demonstrates that the weakened interactions between cations and anions engender "unprotected" organic cations in highly Li-concentrated ionic liquid electrolytes, leading to the decomposition of electrolytes during the initial charge. This decomposition behavior is manifested by the substantial irreversible capacities and inferior initial Coulombic efficiencies observed during the initial charging of graphite negative electrodes, resulting in considerable electrolyte consumption and diminished energy densities in full-cell configurations. The innate cation behavior is ascertained by examining the coordination environment of ionic liquid electrolytes with varied Li concentrations, where intricate ionic interactions between organic cations and anions are unveiled. In addition, anionic species with high Lewis basicity were introduced to reinforce the ionic interactions involving organic cations and improve the initial Coulombic efficiency. This study verifies the role of unprotected organic cations while highlighting the significance of the coordination environment in the performance of ionic liquid electrolytes.

Entropy-Driven 60 mol% Li Electrolyte for Li Metal-Free Batteries.

Highly Li-concentrated electrolytes are acknowledged for their compatibility with Li metal negative electrodes and high voltage positive electrodes to achieve high-energy Li metal batteries, showcasing stable and facile interfaces for Li deposition/dissolution and high anodic stability. This study aims to explore a highly concentrated electrolyte by adopting entropy-driven chemistry for Li metal-free (so-called anode-free) batteries. The combination of lithium bis(fluorosulfonyl)amide (LiFSA) and lithium trifluoromethanesulfonate (LiOTf) salts in a pyrrolidinium-based ionic liquid is found to significantly modify the coordination structure, resulting in an unprecedented 60 mol% Li concentration and a low solvent-to-salt ratio of 0.67:1 in the electrolyte system. This novel 60 mol% Li electrolyte demonstrates unique coordination stricture, featuring a high ratio of monodentate-anion structures and aggregates, which facilitates an enhanced Li+ transference number and improved anodic stability. Moreover, the developed electrolyte provides a facile de-coordination process and leads to the formation of an anion-based solid electrolyte interface, which enables stable Li deposition/dissolution properties and demonstrates excellent cycling stability in the Li metal-free full cell with a Li[Ni0.8Co0.1Mn0.1]O2 (NCM811) positive electrode.

Mechanism of Reductive Fluorination by PTFE-Decomposition Fluorocarbon Gases for WO3.

Reductive fluorination, which entails the substitution of O2- from oxide compounds with F- from fluoropolymers, is considered a practical approach for preparing transition-metal oxyfluorides. However, the current understanding of the fundamental reaction paths remains limited due to the analytical complexities posed by high-temperature reactions in glassware. Therefore, to expand this knowledgebase, this study investigates the reaction mechanisms behind the reductive fluorination of WO3 using polytetrafluoroethylene (PTFE) in an Ni reactor. Here, we explore varied reaction conditions (temperature, duration, and F/W ratio) to suppress the formation of carbon byproducts, minimize the dissipation of fluorine-containing tungsten (VI) compounds, and achieve a high fluorine content. The gas-solid reaction paths are analyzed using infrared spectroscopy, which revealed tetrafluoroethylene (C2F4), hexafluoropropene (C3F6), and iso-octafluoroisobutene (i-C4F8) to be the reactive components in the PTFE-decomposition gas during the reactions with WO3 at 500 °C. CO2 and CO are further identified as gaseous byproducts of the reaction evincing that the reaction is prompted by difluorocarbene (:CF2) formed after the cleavage of C═C bonds in i-C4F8, C3F6, and C2F4 upon contact with the WO3 surface. The solid-solid reaction path is established through a reaction between WO3 and WO3-xFx where solid-state diffusion of O2- and F- is discerned at 500 °C.

Generation of Elemental Fluorine through the Electrolysis of Copper Difluoride at Room Temperature.

The safe generation of F2 gas at room temperature by using simple cell configurations has been the "holy grail" of fluorine research for centuries. Thus, to address this issue, we report generation of F2 gas through the electrolysis of CuF2 in a CsF-2.45HF molten salt without the evolution of H2 gas. The CuF2 is selected through a series of thermodynamic and kinetic assessments of possible metal fluorides. Anode assessments on graphite and glass-like carbon demonstrate the effect of the absence of the anode during generation of F2 gas owing to stabilized operations at room temperature. Although the Ni anode dissolves during electrolysis in the conventional medium-temperature cell, herein, it facilitates stable electrolysis over 100 h, achieving an F2 gas purity of over 99 % with the potential to operate using one-compartment electrolysis. This work presents a safe and propitious method for the generation of high-purity F2 gas for small-scale lab and industrial applications.

Fluoride Ion Interactions in Alkali-Metal Fluoride-Diol Complexes.

The activity of F- is an important factor in the design of both inorganic and organic reactions involving fluorine compounds. The present study investigates interactions of F- with diols in alkali-metal fluoride-diol complexes. Increases in the reactivities of alkali-metal fluorides and their solubilities in alcohols is observed with increasing cation size. The difference in alkali-metal ion size produces different structural motifs for F--diol complex salts. The CsF complex salt with ethylene glycol (EG), CsF-EG, has a layered structure, whereas the Rb and K complex salts, (RbF)5-(EG)4 and (KF)5-(EG)4, form columnar structures. Comparison of the CsF complex salts with three different diols- EG, 1,3-propylene glycol (PG13), and 1,4-butylene glycol (PG14)-revealed that the diol chain length affects the bridging mode in their layered structures. EG bridges two OH oxygen atoms within the same CsF layer in CsF-EG, whereas PG13 and BG14 bridge two OH oxygen atoms in different CsF layers in (CsF)2-PG13 and CsF-BG14, respectively. The F- ion coordination environment involves interactions between alkali-metal ions and H atom(s) in the diol OH groups, where the F-···H interactions are more dominant than the F-···M+ interaction, based on Hirshfeld surface analyses. The O-H bond weakening observed by infrared spectroscopy also reflects the strengths of the F-···H interactions in these complex salts.

Sodium Ion Batteries using Ionic Liquids as Electrolytes.

Sodium ion batteries have been developed using ionic liquids as electrolytes. Sodium is superior to lithium as a raw material for mass production of large-scale batteries for energy storage due to its abundance and even distribution across the earth. Ionic liquids are non-volatile and non-flammable, which improved the safety of the batteries remarkably. In addition, operation temperatures were extended to higher values, improving the performance of the batteries by facilitating the reaction at the electrode and mass transfer. Binary systems of sodium and quaternary ammonium salts, such as 1-ethyl-3-methylimidazolium and N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)amide, were employed as electrolytes for sodium ion batteries. A series of positive and negative electrode materials were examined to be combined with these ionic liquid electrolytes. A 27 Ah full cell was fabricated employing sodium chromite (NaCrO2 ) and hard carbon as positive and negative electrode materials, respectively. The gravimetric energy density obtained for the battery was 75 Wh kg-1 and its volumetric energy density was 125 Wh L-1 . The capacity retention after 500 cycles was 87 %. Further improvement of the cell performance and energy density is expected on development of suitable electrode materials and optimization of the cell design.

Stabilization of SF5- with Glyme-Coordinated Alkali Metal Cations.

The stabilization of complex fluoroanions derived from weakly acidic parent fluorides is a significant and ongoing challenge. The [SF5]- anion is recognized as one such case, and only a limited number of [SF5]- salts are known to be stable at room temperature. In the present study, glyme-coordinated alkali metal cations (K+, Rb+, and Cs+) are employed to stabilize [SF5]-, which provides a simple synthetic route to a [SF5]- salt. The reactivities of KF and RbF with SF4 are significantly enhanced by complexation with G4, based on Raman spectroscopic analyses. A new room-temperature stable salt, [Cs(G4)2][SF5] (G4 = tetraglyme), was synthesized by stoichiometric reaction of CsF, G4, and SF4. The vibrational frequencies of [SF5]- were assigned based on quantum chemical calculations, and the shift of the G4 breathing mode accompanying coordination to metal cations was confirmed by Raman spectroscopy. Single-crystal X-ray diffraction revealed that Cs+ is completely isolated from [SF5]- by two G4 ligands and [SF5]- is disordered along the crystallographic two-fold axis. Hirshfeld surface analysis reveals that the H···H interaction between two neighboring [Cs(G4)2]+ moieties is more dominant on the Hirshfeld surface than the interaction between the H atom in glyme molecules and the F atom in [SF5]-, providing a CsCl-type structural model where the large and spherical [Cs(G4)2]+ cations contact each other and the [SF5]- anions occupy interstitial spaces in the crystal lattice. The [SF5]- anion, combined with [Cs(G4)2]+, exhibits a very limited deoxofluorinating ability toward hydroxyl groups in both neat conditions and THF solutions.

Assisted reproductive technology is effective for but does not affect the prognosis of young patients with breast cancer: Experience in a single institution.

Assisted reproductive technology (ART) helps women preserve fertility after chemotherapy for cancer treatment. We examined the long-term survival of patients with early breast cancer who did or did not receive ART, as well as post-treatment pregnancy and childbirth rates. Our study consisted of 44 young patients (≤35 years of age). Eight patients were pregnant post-treatment; however, none of these patients received ART intervention. ART intervention prevented patient omits of necessary treatment to avoid adverse events. It did not affect the prognosis of patients with breast cancer. Technical improvements are needed to increase the likelihood of pregnancy after breast cancer treatment.

Formation of a solid solution between [N(C2H5)4][BF4] and [N(C2H5)4][PF6] in crystal and plastic crystal phases.

The phase behavior of [N2222][BF4] and [N2222][PF6] (N2222+ = tetraethylammonium cation) binary systems has been investigated in the present study. Differential scanning calorimetry revealed that the crystal-to-plastic-crystal transition temperature decreases upon mixing the two salts, with a minimum at x([N2222][PF6]) = 0.4, where x([N2222][PF6]) denotes the molar fraction of [N2222][PF6]. Powder X-ray diffraction analysis indicated the formation of a solid solution with a rock-salt type structure in the plastic crystal phase at all ratios and the lattice parameter a changes according to Vegard's law. In the crystal phase, two solid solution phases based on the structures of the single salts are observed. Raman spectroscopy confirmed the changes in the solid-solid transition temperature as observed by differential scanning calorimetry. Consequently, in the resulting phase diagram, the solid solution is formed in a wide x([N2222][PF6]) range for both the crystal and plastic crystal phases.

A new sodiation-desodiation mechanism of the titania-based negative electrode for sodium-ion batteries.

TiO2 is widely investigated as a negative electrode for lithium-ion batteries. In sodium-ion batteries, however, the sodiation-desodiation mechanism of TiO2 is still unclear. Here, we report a new sodiation-desodiation mechanism for an anatase TiO2/C electrode in an ionic liquid electrolyte at 90 °C, where it shows a high reversible capacity of 278 mA h g-1. During the first charge process, TiO2 reacts with Na ions to form a Na2TiIITiIVO4 solid solution. During the first discharge process, the solid solution converts into a mixture of TiO2, Na2TiO3, and TiO, with the former two being X-ray amorphous. In the subsequent cycle, the mixture acts as the active material, reversibly reacting with Na ions to re-form the Na2TiIITiIVO4 solid solution. This mechanism, which has not been reported for Na or Li ion insertion-extraction in anatase TiO2, can help understand this promising electrode material and develop safe sodium-ion batteries with high energy density.

Generation of gas-phase zirconium fluoroanions by electrospray of an ionic liquid.

RATIONALE: New approaches for forming anions are sought that have strong abundance and no isobaric overlap, attributes that are compatible with the measurement of isotope ratios. Fluoroanions are particularly attractive because fluorine is monoisotopic, and thus will not have overlapping isobars with the isotope of interest. Since many elements do not have positive electron affinity values, they do not form stable negative atomic ions, and hence are not compatible with isotope ratio measurement using high sensitivity isotope ratio mass spectrometers such as accelerator mass spectrometers. METHODS: Zirconium fluoroanions were prepared using the fluorinating ionic liquid (IL) 1-ethyl-3-methylimidazolium fluorohydrogenate, which was used to generate abundant [ZrF5](-) ions using electrospray ionization. The IL was dissolved in acetonitrile, combined with a dilute solution of either Zr(4+) or ZrO(2+), and then electrosprayed. Mass analysis and collision-induced dissociation experiments were conducted using a time-of-flight mass spectrometer. Cluster structures were predicted using density functional theory calculations. RESULTS: The fluorohydrogenate IL solutions generated abundant [ZrF5](-) ions starting from solutions of both Zr(4+) and ZrO(2+). The mass spectra also contained IL-bearing cluster ions, whose compositions indicated the presence of [ZrF6](2-) in solution, a conclusion supported by the structural calculations. Rinsing out the zirconium-IL solution with acetonitrile decreased the IL clusters, but enhanced [ZrF5](-), which was sorbed by the polymeric electrospray supply capillary, and then released upon rinsing. This reduced the ion background in the mass spectrum. CONCLUSIONS: The fluorohydrogenate-IL solutions are a facile way to form zirconium fluoroanions in the gas phase using electrospray. The approach has potential as a source of fluoroanions for isotope ratio measurements, which would enable high-sensitivity measurement of minor zirconium isotopes without overlapping isobars caused by the charge carrier (i.e., the monoisotopic fluorine atoms).

Effects of HF content in the (FH)(n)F- anion on the formation of ionic plastic crystal phases of N-ethyl-N-methylpyrrolidinium and N,N-dimethylpyrrolidinium fluorohydrogenate salts.

Fluorohydrogenate salts based on N-ethyl-N-methylpyrrolidinium (EMPyr(FH)nF) and N,N-dimethylpyrrolidinium (DMPyr(FH)nF) cations were synthesized, and the effects of the HF content n in EMPyr(FH)nF (1.0 ≤ n ≤ 2.3) and DMPyr(FH)nF (1.0 ≤ n ≤ 2.0) on their thermal and structural properties were discussed, focusing on the characterization of ionic plastic crystal (IPC) phases. Several solid phases (IPC (I) and IPC (II) phases, and crystal phases of EMPyr(FH)1F, EMPyr(FH)2F, and EMPyr(FH)3F) are observed in the EMPyr(FH)nF system. The IPC (I) phase has an NaCl-type structure and is composed of EMPyr(+) cations and (FH)nF(-) (n = 1, 2, and 3) anions randomly occupying the anion positions in the lattice over a wide range of n values in (FH)nF(-). The melting point of EMPyr(FH)nF in the range 1.8 ≤ n ≤ 2.3 is maximal at n = 2.0, whereas it increases with a decrease in n in the range 1.0 ≤ n ≤ 1.2. Furthermore, in the range 1.3 ≤ n ≤ 1.7, the solid phase is regarded as the IPC phase (IPC (II)), and their melting points are nearly constant (260-270 K). In the DMPyr(FH)nF system, the IPC (I') phase and crystal phases of DMPyr(FH)1F and DMPyr(FH)2F were observed. Although the IPC (I') phase has an NaCl-type structure, similar to the IPC (I) phase of EMPyr(FH)nF, it has higher ordering compared to the IPC (I) phase. The melting point of DMPyr(FH)nF increases monotonously with decreasing n but disappears in the small n region where the salt decomposes below the melting point.

The structural classification of the highly disordered crystal phases of [Nn][BF4], [Nn][PF6], [Pn][BF4], and [Pn][PF6] salts (Nn(+) = tetraalkylammonium and Pn(+) = tetraalkylphosphonium).

The structures of 16 symmetric tetraalkylammonium (Nn(+)) and tetraalkylphosphonium (Pn(+)) salts ([Nn][BF4], [Nn][PF6], [Pn][BF4], and [Pn][PF6], where n = 1 to 4, and denotes the number of carbon atoms in each alkyl chain) have been investigated by X-ray diffraction in order to elucidate the effect of ion size on the disordered structure of organic salts. All the salts exhibit one or more solid-solid phase transitions in differential scanning calorimetric curves. Powder X-ray diffraction revealed that the highest temperature solid phase of these salts belongs to a crystal system with a high cubic or hexagonal symmetry. The structures are classified into 5 different types: CsCl', NaCl, NaCl', inverse NiAs, and TBPPF6. The CsCl'-type whose octant corresponds to the original CsCl unit cell is observed for [N1][PF6] owing to the orientational difference for the cation or the anion. The NaCl-type structure is observed for the N2(+) and P2(+) salts while the NaCl'-type structure is observed for [N3][PF6], where the configuration of ions is based on the NaCl-type but the four equivalent positions in the original NaCl lattice split into two sets of equivalent positions (three and one). The inverse NiAs structure is observed for [P3][PF6]. Single-crystal X-ray diffraction reveals that the disordering of ions in [P4][PF6] becomes more significant with increasing temperature. The new structure of a cubic phase, the TBPPF6-type structure, is found for the salts with long alkyl chains. The structure is roughly determined at 333 K and the ions therein are highly disordered but not rotating. The validity of the radius ratio rule is confirmed through appropriate assessment of the ion size.

Fluoride Ion in Alcohols: Isopropanol vs Hexafluoroisopropanol.

The utility of alcohol as a hydrogen bonding donor is considered a providential avenue for moderating the high basicity and reactivity of the fluoride ion, typically used with large cations. However, the practicality of alcohol-fluoride systems in reactions is hampered by the limited understanding of the pertinent interactions between the OH group and F-. Therefore, this study comparatively investigates the thermal, structural, and physical properties of the CsF-2-propanol and CsF-1,1,1,3,3,3-hexafluoro-2-propanol systems to explicate the effects of the fluoroalkyl group on the interaction of alcohols and F-. The two systems exhibit vastly different phase diagrams despite the similar saturated concentrations. A combination of spectroscopic analyses, alcohol activity coefficient measurements, and theoretical calculations reveal the fluorinated alcohol system harbors the stronger OH···F- interactions between the two systems. The diffusion coefficient and ionic conductivity measurements attribute the present results to disparate states of ion association in the two systems.

Polymorphism of alkali bis(fluorosulfonyl)amides (M[N(SO2F)2], M = Na, K, and Cs).

The polymorphic behavior of Na, K, and Cs salts of the bis(fluorosulfonyl)amide anion N(SO(2)F)(2)(-) has been investigated by means of differential scanning calorimetry (DSC), single-crystal and powder X-ray diffraction, and Raman spectroscopy. All of the polymorphs observed in the present work (three for Na[N(SO(2)F)(2)], two for K[N(SO(2)F)(2)], and two for Cs[N(SO(2)F)(2)]) are stable enough for analyses at room temperature. With increasing temperature, form II of Na[N(SO(2)F)(2)] undergoes a solid-solid phase transition to form I, whereas another form (form III) crystallizes from the melt upon cooling. The anions in forms I and II of Na[N(SO(2)F)(2)] have trans and cis conformations, respectively, at 113 K, while cis-trans disorder is observed for the anion in form I at 298 K. Form I of K[N(SO(2)F)(2)], with a melting point of 375 K, is the stable form at room temperature, whereas solidification from the molten state during DSC gives rise to form II with a melting point of 336 K. Both forms I and II of K[N(SO(2)F)(2)] have anions in the cis conformation. The difference between the two potassium polymorphs arises from their crystal packing modes. In the case of Cs[N(SO(2)F)(2)], form I melts at 387 K, whereas form II undergoes a solid-solid transition to form I at 330 K. The anion of form I in Cs[N(SO(2)F)(2)] has an oxygen/fluorine disorder that exhibits an oxygen/fluorine eclipsed conformation, even at 113 K. The powder X-ray diffraction pattern of form II matches that of the previously known Cs[N(SO(2)F)(2)] structure of the trans conformer. Vibrational frequencies observed with Raman spectroscopy do not necessarily show the same trend as those calculated for the energy-minimized cis or trans conformers in the gas phase due to packing effects.

Fluorohydrogenate cluster ions in the gas phase: electrospray ionization mass spectrometry of the [1-ethyl-3-methylimidazolium(+)][F(HF)2.3(-)] ionic liquid.

Electrospray ionization of the fluorohydrogenate ionic liquid [1-ethyl-3-methylimidazolium][F(HF)2.3] ionic liquid was conducted to understand the nature of the anionic species as they exist in the gas phase. Abundant fluorohydrogenate clusters were produced; however, the dominant anion in the clusters was [FHF(-)], and not the fluoride-bound HF dimers or trimers that are seen in solution. Density functional theory (DFT) calculations suggest that HF molecules are bound to the clusters by about 30 kcal/mol. The DFT-calculated structures of the [FHF(-)]-bearing clusters show that the favored interactions of the anions are with the methynic and acetylenic hydrogen atoms on the imidazolium cation, forming planar structures similar to those observed in the solid state. A second series of abundant negative ions was also formed that contained [SiF5(-)] together with the imidazolium cation and the fluorohydrogenate anions that originate from reaction of the spray solution with silicate surfaces.

Systematic Study of Aluminum Corrosion in Ionic Liquid Electrolytes for Sodium-Ion Batteries: Impact of Temperature and Concentration.

The development of sodium-ion batteries utilizing sulfonylamide-based electrolytes is significantly encumbered by the corrosion of the Al current collector, resulting in capacity loss and poor cycling stability. While ionic liquid electrolytes have been reported to suppress Al corrosion, a recent study found that pitting corrosion occurs even when ionic liquids are employed. This study investigates the effects of temperature and Na salt concentration on the Al corrosion behavior in different sulfonylamide-based ionic liquid electrolytes for sodium-ion batteries. In the present work, cyclic voltammetry measurements and scanning electron microscopy showed that severe Al corrosion occurred in ionic liquids at high temperatures and low salt concentrations. X-ray photoelectron spectroscopy was employed to identify the different elemental components and verify the thickness of the passivation layer formed under varied salt concentrations and temperatures. The differences in the corrosion behaviors observed under the various conditions are ascribed to the ratio of free [FSA]- to Na+-coordinating [FSA]- in the electrolyte and the stability of the newly formed passivation layer. This work aims at augmenting the understanding of Al corrosion behavior in ionic liquid electrolytes to develop advanced batteries.

Trigger of the Highly Resistive Layer Formation at the Cathode-Electrolyte Interface in All-Solid-State Lithium Batteries Using a Garnet-Type Lithium-Ion Conductor.

Interfacial materials design is critical in the development of all-solid-state lithium batteries. We must develop an electrode-electrolyte interface with low resistance and effectively utilize the energy stored in the battery system. Here, we investigated the highly resistive layer formation process at the interface of a layered cathode: LiCoO2, and a garnet-type solid-state electrolyte: Li6.4La3Zr1.4Ta0.6O12, during the cosintering process using in situ/ex situ high-temperature X-ray diffraction. The onset temperature of the reaction between a lithium-deficient LixCoO2 and Li6.4La3Zr1.4Ta0.6O12 is 60 °C, while a stoichiometric LiCoO2 does not show any reaction up to 900 °C. The chemical potential gap of lithium first triggers the lithium migration from the garnet phase to the LixCoO2 below 200 °C. The lithium-extracted garnet gradually decomposes around 200 °C and mostly disappears at 500 °C. Since the interdiffusion of the transition metal is not observed below 500 °C, the early-stage reaction product is the decomposed lithium-deficient garnet phase. Electrochemical impedance spectroscopy results showed that the highly resistive layer is formed even below 200 °C. The present work offers that the origin of the highly resistive layer formation is triggered by lithium migration at the solid-solid interface and decomposition of the lithium-deficient garnet phase. We must prevent spontaneous lithium migration at the cathode-electrolyte interface to avoid a highly resistive layer formation. Our results show that the lithium chemical potential gap should be the critical parameter for designing an ideal solid-solid interface for all-solid-state battery applications.

In Situ Orthorhombic to Amorphous Phase Transition of Nb2O5 and Its Temperature Effect on Pseudocapacitive Behavior.

Niobium pentoxide (Nb2O5) represents an exquisite class of negative electrode materials with unique pseudocapacitive kinetics that engender superior power and energy densities for advanced electrical energy storage devices. Practical energy devices are expected to maintain stable performance under real-world conditions such as temperature fluctuations. However, the intercalation pseudocapacitive behavior of Nb2O5 at elevated temperatures remains unexplored because of the scarcity of suitable electrolytes. Thus, in this study, we investigate the effect of temperature on the pseudocapacitive behavior of submicron-sized Nb2O5 in a wide potential window of 0.01-2.3 V. Furthermore, ex situ X-ray diffraction and X-ray photoelectron spectroscopy reveal the amorphization of Nb2O5 accompanied by the formation of NbO via a conversion reaction during the initial cycle. Subsequent cycles yield enhanced performance attributed to a series of reversible NbV, IV/NbIII redox reactions in the amorphous LixNb2O5 phase. Through cyclic voltammetry and symmetric cell electrochemical impedance spectroscopy, temperature elevation is noted to increase the pseudocapacitive contribution of the Nb2O5 electrode, resulting in a high rate capability of 131 mAh g-1 at 20,000 mA g-1 at 90 °C. The electrode further exhibits long-term cycling over 2000 cycles and high Coulombic efficiency ascribed to the formation of a robust, [FSA]--originated solid-electrolyte interphase during cycling.

Strategies for Harnessing High Rate and Cycle Performance from Graphite Electrodes in Potassium-Ion Batteries.

Potassium-ion batteries (PIBs) have been lauded as the next-generation energy storage systems on account of their high voltage capabilities and low costs and the high abundance of potassium resources. However, the practical utility of PIBs has been heavily encumbered by severe K metal dendrite formation, safety issues, and insufficient electrochemical performance during operations─indeed critical issues that underpin the need for functional electrolytes with high thermal stability, robust solid-electrolyte interphase (SEI)-forming capabilities, and high electrochemical performance. In a bid to establish a knowledge framework for harnessing high rate capabilities and long cycle life from graphite negative electrodes, this study presents the physical properties and electrochemical behavior of a high K+ concentration inorganic ionic liquid (IL) electrolyte, K[FSA]-Cs[FSA] (FSA- = bis(fluorosulfonyl)amide) (54:46 in mol), at an intermediate temperature of 70 °C. This IL electrolyte demonstrates an ionic conductivity of 2.54 mS cm-1 and a wide electrochemical window of 5.82 V. Charge-discharge tests performed on a graphite negative electrode manifest a high discharge capacity of 278 mAh g-1 (0.5 C) at 70 °C, a high rate capability (106 mAh g-1 at 100 C), and a long cyclability (98.7% after 450 cycles). Stable interfacial properties observed by electrochemical impedance spectroscopy during cycling are attributed to the formation of sulfide-rich all-inorganic SEI, which was examined through X-ray photoelectron spectroscopy. The performance of the IL is collated with that of an N-methyl-N-propylpyrrolidinium-based organic IL to provide insight into the synergism between the highly concentrated K+ electrolyte at intermediate temperatures and the all-inorganic SEI during electrochemical operations of the graphite negative electrode.