Surface and Bulk Stabilization of Silicon Anodes with Mixed-Multivalent Additives: Ca(TFSI)2 and Mg(TFSI)2.

Silicon is drawing attention as an emerging anode material for the next generation of lithium-ion batteries due to its higher capacity compared with commercial graphite. However, silicon anions formed during lithiation are highly reactive with binder and electrolyte components, creating an unstable SEI layer and limiting the calendar life of silicon anodes. The reactivity of lithium silicide and the formation of an unstable SEI layer are mitigated by utilizing a mixture of Ca and Mg multivalent cations as an electrolyte additive for Si anodes to improve their calendar life. The effect of mixed salts on the bulk and surface of the silicon anodes was studied by multiple structural characterization techniques. Ca and Mg ions in the electrolyte formed relatively thermodynamically stable quaternary Li-Ca-Mg-Si Zintl phases in an in situ fashion and a more stable and denser SEI layer on the Si particles. These in turn protect silicon particles against side reactions with electrolytes in a coin cell. The full cell with the mixed cation electrolyte demonstrates enhanced calendar life performance with lower measured current and current leakage in comparison to that of the baseline electrolyte due to reduced side reactions. Electron microscopy, HR-XRD, and solid-state NMR results showed that electrodes with mixed cations tended to have less cracking on the electrode surface, and the presence of mixed cations enhances cation migration and formation of quaternary Zintl phases stabilizing the bulk and forming a more stable SEI in comparison to baseline electrolyte and electrolyte with single multivalent cations.

Structural and reactive evolution of oxidatively grafted Pd catalysts on MnO2 for the low-temperature oxidation of CO.

Isolated Pd atoms supported on high surface area MnO2, prepared by the oxidative grafting of (bis(tricyclohexylphosphine-palladium(0)), catalyze (>50 turnovers, 17 h) the low temperature (≤325 K) oxidation of CO (7.7 kPa O2, 2.6 kPa CO) with results of in situ/operando and ex situ spectroscopic characterization signifying a synergistic role of Pd and MnO2 in facilitating redox turnovers.

A Carboranyl Electrolyte Enabling Highly Reversible Sodium Metal Anodes via a "Fluorine-Free" SEI.

Realization of practical sodium metal batteries (SMBs) is hindered due to lack of compatible electrolyte components, dendrite propagation, and poor understanding of anodic interphasial chemistries. Chemically robust liquid electrolytes that facilitate both favorable sodium metal deposition and a stable solid-electrolyte interphase (SEI) are ideal to enable sodium metal and anode-free cells. Herein we present advanced characterization of a novel fluorine-free electrolyte utilizing the [HCB11 H11 ]1- anion. Symmetrical Na cells operated with this electrolyte exhibit a remarkably low overpotential of 0.032 V at a current density of 2.0 mA cm-2 and a high coulombic efficiency of 99.5 % in half-cell configurations. Surface characterization of electrodes post-operation reveals the absence of dendritic sodium nucleation and a surprisingly stable fluorine-free SEI. Furthermore, weak ion-pairing is identified as key towards the successful development of fluorine-free sodium electrolytes.

Tunable Solid Acid Catalyst Thin Films Prepared by Atomic Layer Deposition.

Solid acid catalysts, including zeolites and amorphous silica-aluminas (ASAs), are industrially important materials widely used in the fuel and petrochemical industries. The versatility of zeolites is due to the Brønsted acidity of the bridging hydroxyl and shape selectivity that can be tailored during and after synthesis. This is in contrast to amorphous silica-alumina, where tailoring acidity is a major challenge as the Brønsted acid structure in ASA is still debated. In both cases, however, the pore size and acidity cannot be tuned independently, and this is particularly limiting in the application of biomass conversion, where zeolite pores are too small for the molecules of interest. Herein, we present a method using atomic layer deposition (ALD) to prepare thin films of solid acid materials where the ratio of Brønsted to Lewis acid sites can be tuned precisely. This capability, combined with the sub-nm pore size control afforded by ALD yields a powerful and flexible method for synthesizing solid acid catalysts inside virtually any mesoporous host. We demonstrate the utility of these materials in two acid-catalyzed reactions relevant to biomass conversion: (1) Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reaction and dehydration of fructose and (2) cascade reaction of glucose to 5-hydroxymethylfurfural. Finally, we propose a plausible structure for the Brønsted acid sites in our materials based on infrared spectroscopy and solid-state nuclear magnetic resonance measurements and density functional theory calculations and argue that this same structure might apply to conventional ASAs as well.

Serum estradiol level predicts acute kidney injury in medical intensive care unit patients.

Previous studies have shown that serum estradiol (E2) levels can predict mortality in intensive care unit patients. Our study investigated the predictive role of admission estradiol level on patient mortality and development of acute kidney injury in medical intensive care unit patients with a wide range of diagnoses. We conducted a prospective cohort study using serum samples from hospitalized patients in medical, cardiac, and pulmonary intensive care units at the Ege University Hospital within 6 months. Serum estradiol levels from 118 adult patients were collected within 48 h of hospitalization. Receiver operating curves and multiple logistic regression analyses were performed to investigate its relationship with acute kidney injury development and mortality. Serum estradiol levels were significantly higher in non-survivor patients than in survivor patients [85 (19-560) pg/mL vs. 32 (3-262) pg/mL, p < 0.001]. Admission estradiol levels were significantly higher in patients with AKI on admission than in patients with chronic kidney disease (p = 0.002) and normal renal function (p = 0.017). Serum E2 levels were higher in patients with renal deterioration during follow-up than patients with stable renal functions [62 (11-560) pg/mL vs. 38 (3-456) pg/mL, p = 0.004]. An admission estradiol level of 52.5 pg/mL predicted follow-up renal deterioration with 63% sensitivity and 74% specificity. A combined (APACHE II-E) score using APACHE II and serum estradiol level predicted overall mortality with 66% sensitivity and 82% specificity. Admission estradiol level is a good marker to predict the development of acute kidney injury and mortality in medical intensive care unit patients.

Probing the Reactivity of the Active Material of a Li-Ion Silicon Anode with Common Battery Solvents.

Calculations and modeling have shown that replacing the traditional graphite anode with silicon can greatly improve the energy density of lithium-ion batteries. However, the large volume change of silicon particles and high reactivity of lithiated silicon when in contact with the electrolyte lead to rapid capacity fading during charging/discharging processes. In this report, we use specific lithium silicides (LS) as model compounds to systematically study the reaction between lithiated Si and different electrolyte solvents, which provides a powerful platform to deconvolute and evaluate the degradation of various organic solvents in contact with the active lithiated Si-electrode surface after lithiation. Nuclear Magnetic Resonance (NMR) characterization results show that a cyclic carbonate such as ethylene carbonate is chemically less stable than a linear carbonate such as ethylmethyl carbonate, fluoroethylene carbonate, and triglyme as they are found to be more stable when mixed with LS model compounds. Guided by the experimental results, two ethylene carbonate (EC)-free electrolytes are studied, and the electrochemical results show improvements with graphite-free Si electrodes relative to the traditional ethylene-carbonate-based electrolytes. More importantly, the study contributes to our understanding of the significant fundamental chemical and electrochemical stability differences between silicon and traditional graphite lithium-ion battery (LIB) anodes and suggests a focused development of electrolytes with specific chemical stability vs lithiated silicon which can passivate the surface more effectively.

Using Mixed Salt Electrolytes to Stabilize Silicon Anodes for Lithium-Ion Batteries via in Situ Formation of Li-M-Si Ternaries (M = Mg, Zn, Al, Ca).

Replacing traditional graphite anode by Si anode can greatly improve the energy density of lithium-ion batteries. However, the large volume expansion and the formation of highly reactive lithium silicides during charging cause the continuous lithium and electrolyte consumption as well as the fast decay of Si anodes. In this work, by adding 0.1 M M(TFSI)x (M = Mg, Zn, Al and Ca) as a second salt into the electrolyte, we stabilize the anode chemistry through the in situ formation of Li-M-Si ternary phases during the charging process. First, lithium silicides and magnesium lithium silicides were synthesized as model compounds to investigate the influence of metal doping on the reactivity of lithiated Si. Using solid-state nuclear magnetic resonance spectroscopy, we show that Mg doping can dramatically suppress the chemical reactions between the lithium silicide compounds and common electrolyte solvents. New mixed salt electrolytes were prepared containing M(TFSI)x as a second salt to LiPF6 and tested in commercially relevant electrodes, which show higher capacity, superior cyclability, and higher Coulombic efficiencies in both half-cell and full-cell configurations (except for Zn) when compared with standard electrolytes. Post-electrochemistry characterizations demonstrate that adding M salts leads to the co-insertion of M cations along with Li into Si during the lithiation process, stabilizing silicon anions by forming more stable Li-M-Si ternaries, which fundamentally changes the traditional Li-Si binary chemistry while minimally affecting silicon electrochemical profiles and theoretical capacities. This study opens a new and simple way to stabilize silicon anodes to enable widespread application of Si anodes for lithium-ion batteries.

Electrophilic Organoiridium(III) Pincer Complexes on Sulfated Zirconia for Hydrocarbon Activation and Functionalization.

Single-site supported organometallic catalysts bring together the favorable aspects of homogeneous and heterogeneous catalysis while offering opportunities to investigate the impact of metal-support interactions on reactivity. We report a ( dmPhebox)Ir(III) ( dmPhebox = 2,6-bis(4,4-dimethyloxazolinyl)-3,5-dimethylphenyl) complex chemisorbed on sulfated zirconia, the molecular precursor for which was previously applied to hydrocarbon functionalization. Spectroscopic methods such as diffuse reflectance infrared Fourier transformation spectroscopy (DRIFTS), dynamic nuclear polarization (DNP)-enhanced solid-state nuclear magnetic resonance (SSNMR) spectroscopy, and X-ray absorption spectroscopy (XAS) were used to characterize the supported species. Tetrabutylammonium acetate was found to remove the organometallic species from the surface, enabling solution-phase analytical techniques in conjunction with traditional surface methods. Cationic character was imparted to the iridium center by its grafting onto sulfated zirconia, imbuing high levels of activity in electrophilic C-H bond functionalization reactions such as the stoichiometric dehydrogenation of alkanes, with density functional theory (DFT) calculations showing a lower barrier for ß-H elimination. Catalytic hydrogenation of olefins was also facilitated by the sulfated zirconia-supported ( dmPhebox)Ir(III) complex, while the homologous complex on silica was inactive under comparable conditions.

Effect of Passivating Shells on the Chemistry and Electrode Properties of LiMn2O4 Nanocrystal Heterostructures.

Building a stable chemical environment at the cathode/electrolyte interface is directly linked to the durability of Li-ion batteries with high energy density. Recently, colloidal chemistry methods have enabled the design of core-shell nanocrystals of Li1+ xMn2- xO4, an important battery cathode, with passivating shells rich in Al3+ through a colloidal synthetic route. These heterostructures combine the presence of redox-inactive ions on the surface to minimize undesired reactions, with the coverage of each individual particle in an epitaxial manner. Although they improve electrode performance, the exact chemistry and structure of the shell as well as the precise effect of the ratio between the shell and the active core remain to be elucidated. Correlation of these parameters to electrode properties would serve to tailor the heterostructure design toward complete shutdown of undesired reactions. These knowledge gaps are the target of this study. Li1+ xMn2- xO4 nanocrystals with Al3+-rich shells of different thicknesses were synthesized. Multimodal characterization comprehensively revealed the elemental distribution, electronic state, and crystallinity in the heterostructures, which confirmed the potential of this approach to finely tune passivating layers. All of the modified nanocrystals improved the capacity retention while retaining charge storage compared to the bare counterpart, even under harsh conditions.

Nanocrystal heterostructures of LiCoO2 with conformal passivating shells.

Stabilization of electrode-electrolyte interfaces is required to increase the energy stored in battery electrodes. Introducing redox-inactive ions on the electrode surface minimizes deleterious side reactions without affecting the bulk properties. A synthetic challenge exists to grow such layers conformally at each primary particle, to fully passivate interfaces that are buried in the final electrode architecture. The development of methods of sequential colloidal growth of complex oxides and overlayers, enabled by surfactant interactions, would provide novel means to advance toward this goal. Here, nanocrystals composed of LiCoO2, a commercially relevant material for high energy devices, were grown with a shell enriched in Al3+, deposited conformally through a one-pot colloidal synthetic method. The effects of synthetic conditions on the composition of the Al-rich shell and the corresponding electrochemical performance were investigated. The modified nanocrystals showed enhanced electrochemical properties, while maintaining carrier transport.

From Coating to Dopant: How the Transition Metal Composition Affects Alumina Coatings on Ni-Rich Cathodes.

Surface alumina coatings have been shown to be an effective way to improve the stability and cyclability of cathode materials. However, a detailed understanding of the relationship between the surface coatings and the bulk layered oxides is needed to better define the critical cathode-electrolyte interface. In this paper, we systematically studied the effect of the composition of Ni-rich LiNixMnyCo1-x-yO2 (NMC) on the surface alumina coatings. Changing cathode composition from LiNi0.5Mn0.3Co0.2O2 (NMC532) to LiNi0.6Mn0.2Co0.2O2 (NMC622) and LiNi0.8Mn0.1Co0.1O2 (NMC811) was found to facilitate the diffusion of surface alumina into the bulk after high-temperature annealing. By use of a variety of spectroscopic techniques, Al was seen to have a high bulk compatibility with higher Ni/Co content, and low bulk compatibility was associated with Mn in the transition metal layer. It was also noted that the cathode composition affected the observed morphology and surface chemistry of the coated material, which has an effect on electrochemical cycling. The presence of a high surface Li concentration and strong alumina diffusion into the bulk led to a smoother surface coating on NMC811 with no excess alumina aggregated on the surface. Structural characterization of pristine NMC particles also suggests surface Co segregation, which may act to mediate the diffusion of the Al from the surface to the bulk. The diffusion of Al into the bulk was found to be detrimental to the protection function of surface coatings leading to poor overall cyclability, indicating the importance of compatibility between surface coatings and bulk oxides on the electrochemical performance of coated cathode materials. These results are important in developing a better coating method for synthesis of next-generation cathode materials for lithium-ion batteries.

Understanding the Role of Temperature and Cathode Composition on Interface and Bulk: Optimizing Aluminum Oxide Coatings for Li-Ion Cathodes.

Surface coating of cathode materials with Al2O3 has been shown to be a promising method for cathode stabilization and improved cycling performance at high operating voltages. However, a detailed understanding on how coating process and cathode composition change the chemical composition, morphology, and distribution of coating within the cathode interface and bulk lattice is still missing. In this study, we use a wet-chemical method to synthesize a series of Al2O3-coated LiNi0.5Co0.2Mn0.3O2 and LiCoO2 cathodes treated under various annealing temperatures and a combination of structural characterization techniques to understand the composition, homogeneity, and morphology of the coating layer and the bulk cathode. Nuclear magnetic resonance and electron microscopy results reveal that the nature of the interface is highly dependent on the annealing temperature and cathode composition. For Al2O3-coated LiNi0.5Co0.2Mn0.3O2, higher annealing temperature leads to more homogeneous and more closely attached coating on cathode materials, corresponding to better electrochemical performance. Lower Al2O3 coating content is found to be helpful to further improve the initial capacity and cyclability, which can greatly outperform the pristine cathode material. For Al2O3-coated LiCoO2, the incorporation of Al into the cathode lattice is observed after annealing at high temperatures, implying the transformation from "surface coatings" to "dopants", which is not observed for LiNi0.5Co0.2Mn0.3O2. As a result, Al2O3-coated LiCoO2 annealed at higher temperature shows similar initial capacity but lower retention compared to that annealed at a lower temperature, due to the intercalation of surface alumina into the bulk layered structure forming a solid solution.

Exploring Lithium-Cobalt-Nickel Oxide Spinel Electrodes for ≥3.5 V Li-Ion Cells.

Recent reports have indicated that a manganese oxide spinel component, when embedded in a relatively small concentration in layered xLi2MnO3·(1-x)LiMO2 (M = Ni, Mn, or Co) electrode systems, can act as a stabilizer that increases their capacity, rate capability, cycle life, and first-cycle efficiency. These findings prompted us to explore the possibility of exploiting lithiated cobalt oxide spinel stabilizers by taking advantage of (1) the low mobility of cobalt ions relative to that of manganese and nickel ions in close-packed oxides and (2) their higher potential (∼3.6 V vs Li0) relative to manganese oxide spinels (∼2.9 V vs Li0) for the spinel-to-lithiated spinel electrochemical reaction. In particular, we revisited the structural and electrochemical properties of lithiated spinels in the LiCo1-xNixO2 (0 ≤ x ≤ 0.2) system, first reported almost 25 years ago, by means of high-resolution (synchrotron) X-ray diffraction, transmission electron microscopy, nuclear magnetic resonance spectroscopy, electrochemical cell tests, and theoretical calculations. The results provide a deeper understanding of the complexity of intergrown layered/lithiated spinel LiCo1-xNixO2 structures when prepared in air between 400 and 800 °C and the impact of structural variations on their electrochemical behavior. These structures, when used in low concentrations, offer the possibility of improving the cycling stability, energy, and power of high energy (≥3.5 V) lithium-ion cells.

Direct Observation of Lattice Aluminum Environments in Li Ion Cathodes LiNi1-y-zCoyAlzO2 and Al-Doped LiNixMnyCozO2 via (27)Al MAS NMR Spectroscopy.

Direct observations of local lattice aluminum environments have been a major challenge for aluminum-bearing Li ion battery materials, such as LiNi1-y-zCoyAlzO2 (NCA) and aluminum-doped LiNixMnyCozO2 (NMC). (27)Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy is the only structural probe currently available that can qualitatively and quantitatively characterize lattice and nonlattice (i.e., surface, coatings, segregation, secondary phase etc.) aluminum coordination and provide information that helps discern its effect in the lattice. In the present study, we use NMR to gain new insights into transition metal (TM)-O-Al coordination and evolution of lattice aluminum sites upon cycling. With the aid of first-principles DFT calculations, we show direct evidence of lattice Al sites, nonpreferential Ni/Co-O-Al ordering in NCA, and the lack of bulk lattice aluminum in aluminum-"doped" NMC. Aluminum coordination of the paramagnetic (lattice) and diamagnetic (nonlattice) nature is investigated for Al-doped NMC and NCA. For the latter, the evolution of the lattice site(s) upon cycling is also studied. A clear reordering of lattice aluminum environments due to nickel migration is observed in NCA upon extended cycling.

Re-entrant lithium local environments and defect driven electrochemistry of Li- and Mn-rich Li-ion battery cathodes.

Direct observations of structure-electrochemical activity relationships continue to be a key challenge in secondary battery research. (6)Li magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy is the only structural probe currently available that can quantitatively characterize local lithium environments on the subnanometer scale that dominates the free energy for site occupation in lithium-ion (Li-ion) intercalation materials. In the present study, we use this local probe to gain new insights into the complex electrochemical behavior of activated 0.5(6)Li2MnO3·0.5(6)LiMn(0.5)Ni(0.5)O2, lithium- and manganese-rich transition-metal (TM) oxide intercalation electrodes. We show direct evidence of path-dependent lithium site occupation, correlated to structural reorganization of the metal oxide and the electrochemical hysteresis, during lithium insertion and extraction. We report new (6)Li resonances centered at ∼1600 ppm that are assigned to LiMn6-TM(tet) sites, specifically, a hyperfine shift related to a small fraction of re-entrant tetrahedral TMs (Mn(tet)), located above or below lithium layers, coordinated to LiMn6 units. The intensity of the TM layer lithium sites correlated with tetrahedral TMs loses intensity after cycling, indicating limited reversibility of TM migrations upon cycling. These findings reveal that defect sites, even in dilute concentrations, can have a profound effect on the overall electrochemical behavior.

Searching for microporous, strongly basic catalysts: experimental and calculated 29Si NMR spectra of heavily nitrogen-doped Y zeolites.

Nitrogen substituted zeolites with high crystallinity and microporosity are obtained by nitrogen substitution for oxygen in zeolite Y. The substitution reaction is performed under ammonia flow by varying the temperature and reaction time. We examine the effect of aluminum content and charge-compensating cation (H(+)/Na(+)/NH(4)(+)) on the degree of nitrogen substitution and on the preference for substitution of Si-O-Al vs Si-O-Si linkages in the FAU zeolite structure. Silicon-29 magic angle spinning (MAS) nuclear magnetic resonance (NMR) and (1)H/(29)Si cross-polarization MAS NMR spectroscopy have been used to probe the different local environments of the nitrogen-substituted zeolites. Experimental data are compared to simulated NMR spectra obtained by constructing a compendium (>100) of zeolite clusters with and without nitrogen, and by performing quantum calculations of chemical shifts for the NMR-active nuclei in each cluster. The simulated NMR spectra, which assume peak intensities predicted by statistical analysis, agree remarkably well with the experimental data. The results show that high levels of nitrogen substitution can be achieved while maintaining porosity, particularly for NaY and low-aluminum HY materials, without significant loss in crystallinity. Experiments performed at lower temperatures (750-800 degrees C) show a preference for substitution at Si-OH-Al sites. No preference is seen for reactions performed at higher temperatures and longer reaction times (e.g., 850 degrees C and 48 h).

Spectroscopic signatures of nitrogen-substituted zeolites.

Nanoporous acid catalysts such as zeolites form the backbone of catalytic technologies for refining petroleum. With the promise of a biomass economy, new catalyst systems will have to be discovered, making shape-selective base catalysts especially important because of the high oxygen content in biomass-derived feedstocks. Strongly basic zeolites are attractive candidates, but such materials are notoriously difficult to make due to the strong inherent acidity of aluminosilicates. Several research groups have endeavored to produce strongly basic zeolites by treating zeolites with amines, but to date there is no compelling evidence that nitrogen is incorporated into zeolite frameworks. In this communication, we detail synthesis, NMR spectroscopy, and quantum mechanical calculations showing that nitrogen adds onto both surface and interior sites while preserving the framework structure of zeolites. This finding is crucial for the rational design of new biomass-refinement catalysts, allowing 50 years of zeolite science to be brought to bear on the catalytic synthesis of biofuels.

Speciation of chromium in the presence of copper and zinc and their combined toxicity.

Speciation of chromium has attracted a great deal of interest in view of the toxic properties of Cr(6+) compared with the much less toxic Cr(3+). A freshwater plant, duckweed which is uniquely suitable for aquatic toxicity tests is utilized to study the growth responses and accumulation of chromium in the presence of copper and zinc. Speciation analysis was carried out by using differently activated alumina. Metal ion concentrations in the plant were determined by FAA spectrometry. Results of the present study revealed that the oxidation state of chromium as well as its concentration in media controlled the growth and the metal accumulation in duckweed. Regarding this, Lemna minor (common duckweed species) accumulated Cr(3+) in smaller quantities and it resulted in relatively larger growth responses, whereas Cr(6+) was accumulated in larger quantities and it yielded relatively smaller growth responses. Different types of metal interactions among chromium, copper, and zinc ions were evident. These interactions were classified as synergistic, antagonistic, and additive and they were strictly dependent on the oxidation state of the chromium ion.