Li [Ni 0.42 Mn 0.42 Co 0.16 ]O 2 (NMC442)/graphite pouch cells with an ethylene carbonate-containing or a fluorinated electrolyte were used to prepare charged electrodes for studies using "pouch bags". Sealed pouch bags containing either lithiated graphite or delithiated NMC442 electrodes taken from pouch cells, and also "sister" pouch cells, were subjected to 500 h storage at elevated temperature. The electrodes recovered from the pouch bags and pouch cells after storage were studied using electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy while the gases generated were quantified using gas chromatography. The fluorinated electrolyte suppressed impedance growth of the positive electrode during storage but caused a large initial negative electrode impedance compared to the carbonate electrolyte. The solid electrolyte interface (SEI) formed by the fluorinated electrolyte at the graphite electrode hinders the consumption of CO 2 generated at the delithiated NMC442 electrode, leading to more CO 2 in pouch cells with fluorinated electrolyte than in cells with carbonate electrolyte. Hydrogen gas was only observed in pouch cells after storage and not in pouch bags which contained either a single negative electrode plus electrolyte or a single positive electrode plus electrolyte, suggesting the H 2 results from a species created at one electrode which reacts at the other in a pouch cell. , one of the layered NMC series, has attracted attention due to its high working potential, high specific capacity and excellent safety. 11-14This material can be operated to up to 4.7 V without any substantial structural change.11,12 Its discharge specific capacity increases almost linearly with the upper cutoff potential from 4.1 V to 4.7 V. At 4.7 V, a reversible specific capacity of ∼207 mAh/g can be obtained from this material. In addition. NMC442 shows higher onset temperatures for exothermic reactions with electrolyte compared to other NMC materials such as LiNi 0. 16-27 A similar approach was employed to improve NMC442/graphite cell performance at high voltage as well.28-39 For example, pyridine boron trifluoride (PBF), [28][29][30] triallyl phosphate (TAP), 31 and a blend of additives containing prop-1-ene-1,3-sultone (PES), 1,3,2-dioxathiolane-2,2-dioxide (DTD) + tris-(trimethyl-silyl) phosphite (TTSPi) [32][33][34][35][36] have been used to improve high voltage performance of NMC442/graphite cells to some extent. Sulfone-based electrolytes can improve high voltage performance of NMC442/graphite cells as well. 38 Compared to the options of additives in conventional EC-based electrolyte and sulfone-based electrolyte, fluorinated electrolytes may be the best option for improving NMC442/graphite cell cycle life when cells are operated to 4.5 V and above.39 This is because the cell impedance can be controlled when fluorinated electrolytes are used while it increases dramatically with carbonate solvents operated above 4.5 V.Recently, a pouch cell and pouch bag method has been put forward to study the inte...

Physical properties of LiPF 6 in ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) electrolytes were studied by conductivity measurements, Fourier transform infrared spectroscopy (FT-IR) and differential thermal analysis (DTA). Conductivity measurements show that the addition of additive levels of FEC to EMC electrolyte can dramatically increase the conductivity of EC-free EMC electrolytes at low salt concentrations below 0.4 M. FT-IR results show that the added FEC hinders ion pair formation by competing with EMC to dissociate LiPF 6 resulting in increased conductivity in EMC electrolytes. Conductivity measurements show that the conductivity of DMC electrolytes decreases significantly below 0 • C due to the high melting point of DMC. Differential thermal analysis was used to determine the LiPF 6 -DMC phase diagram which then can be used to explain the conductivity results. The results presented here identify avenues by which EC-free electrolytes can be improved for use in practical Li-ion cells. Li-ion battery packs for electrified vehicles and grid energy storage require high energy density and long lifetime cells.1-3 One approach to increase the energy density of Li-ion cells is to adopt high potential positive electrodes. Layered Li[Ni x Mn y Co 1-x-y ]O 2 materials have been extensively investigated since they are cheaper alternatives to LCO and since they can function at high potentials. 4-9However, it is a great challenge to cycle high voltage NMC cells (above 4.3V) well. The use of new solvent blends and the introduction of electrolyte additives are two common ways to improve the lifetime of high voltage NMC Li-ion cells. 24-32 Therefore, it is important to study the physical properties of EC-free electrolytes to optimize the performance of cells where EC-free electrolytes are used.Differential scanning calorimetry (DSC) 29,32,33 and differential thermal analysis (DTA) 34 can be used to examine the phase diagrams of solvent blends and of electrolytes. Ding et al. showed that supercooling and superheating during DSC measurements for phase diagram determination could be dramatically reduced if carbon black and/or electrode powders were present in the samples during measurement. 33Recently Day et al. demonstrated that the DTA technique can be applied to an entire Li-ion cell and is a non-invasive in-situ method to probe the state of the liquid electrolyte within intact Li-ion cells. 34The DTA measurements provide information about the amount of liquid electrolyte remaining in the cell and about the electrolyte composition. The interpretation of the DTA results requires information about the phase diagram of the electrolyte of interest. However, the phase diagrams of LiPF 6 :ethyl methyl carbonate and LiPF 6 :dimethyl carbonate electrolytes have not yet been reported.In this report, the conductivity of LiPF 6 :EMC electrolytes with various salt concentrations were measured over a wide range of temperatures. Fourier transform infrared spectroscopy (FT-IR) was used to study the interactions between t...

The first use of phosphenium cations in asymmetric catalysis is reported. A diazaphosphenium triflate, prepared in two or three steps on a multigram scale from commercially available materials, catalyzes the hydroboration or hydrosilylation of cyclic imines with enantiomeric ratios of up to 97:3. Catalyst loadings are as low as 0.2 mol %. Twenty-two aryl/heteroaryl pyrrolidines and piperidines were prepared using this method. Imines containing functional groups such as thiophenes or pyridyl rings that can challenge transition-metal catalysts were reduced employing these systems.

The use of pincer ligands to access non‐VSEPR geometries at main‐group centers is an emerging strategy for eliciting new stoichiometric and catalytic reactivity. As part of this effort, several different tridentate trianionic substituents have to date been employed at a range of different central elements, providing a patchwork dataset that precludes rigorous structure–function correlation. An analysis of periodic trends in structure (solid, solution, and computation), bonding, and reactivity based on systematic variation of the central element (P, As, Sb, or Bi) with retention of a single tridentate triamide substituent is reported herein. In this homologous series, the central element can adopt either a bent or planar geometry. The tendency to adopt planar geometries increases descending the group with the phosphorus triamide (1) and its arsenic congener (2) exhibiting bent conformations, and the antimony (3) and bismuth (4) analogues exhibiting a predominantly planar structure in solution. This trend has been rationalized using an energy decomposition analysis. A rare phase‐dependent dynamic covalent dimerization was observed for 3 and the associated thermodynamic parameters were established quantitatively. Planar geometries were found to engender lower LUMO energies and smaller band gaps than bent ones, resulting in different reactivity patterns. These results provide a benchmark dataset to guide further research in this rapidly emerging area.

The effects of electrolyte additives on gas evolution, gas consumption and impedance growth at elevated temperature have been studied using Li [Ni 0.42 Mn 0.42 Co 0.16 ]O 2 (NMC442)/graphite pouch cells and pouch bags containing delithiated NMC442 or lithiated graphite electrodes plus electrolyte. It was found that there was much more gas, mostly CO 2 , generated in pouch bags containing charged positive electrodes than pouch cells. It was found that the impedance of the charged positive electrodes stored in pouch bags increased dramatically, while those stored in pouch cells did not. The two observations show that there are interactions between positive and negative electrodes that limit gas evolution and reduce impedance growth in Li-ion cells. To verify this, CO 2 intentionally added to pouch bags containing lithiated graphite and electrolyte was consumed. The use of several electrolyte additives, known to affect gassing and high voltage cycling did not substantially alter these conclusions. XPS studies were used to eliminate some possible mechanisms responsible for these phenomena. Lithium-ion battery packs for electrified vehicles and grid energy storage need longer lived cells, operating over a wide range of temperatures. A pouch-type lithium-ion cell, cycled at elevated temperature, may experience volume expansion due to gas production, leading to rapid capacity fade.1-6 When Li-ion cells are charged above 4.2 V, reactions between the electrolyte and the positive electrode can cause charge-transfer impedance (R ct , electron transfer and diffusion of lithium ions through the SEI) growth at the positive electrodeelectrolyte interface, depletion of liquid electrolyte, capacity loss and eventual cell failure.7-12 The use of electrolyte additives is an effective way to suppress gas evolution and impedance growth for cells cycled at high voltage and high temperature. [13][14][15][16][17][18][19] Xia et al. and Li et al. found that prop-1-ene-1,3-sultone (PES) suppressed gas evolution when cells were cycled to an upper cutoff voltage of 4.2 V at elevated temperature.13,14 Nie et al. found that pyridine boron trifluoride (PBF) also suppressed gassing at high temperature, when cells were cycled to 4.4. [16][17][18] Vinylene carbonate (VC) is a popular additive which is widely used in industry and has been extensively studied. [20][21][22][23][24] This additive can cause severe gas evolution at high temperature and high voltage if residual VC remains in the electrolyte after the formation cycle. 26-28 However, impedance growth for NMC442/graphite cells containing these additives or additive blends cannot yet be controlled as the cutoff voltage increases above 4.4 V.27 Figure 1 shows that the impedance of NMC442/graphite cells increases dramatically as the cycle number increases. Symmetric cells made from electrodes harvested from pouch cells tested above 4.4 V showed that the impedance growth originates from the positive electrode ( Figure 1b) rather than from the negative electrode (Figure 1c). Detailed informa...