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Planar bismuth compounds exhibit tunable Lewis acidity and high catalytic activity for lactone polymerization.

Coloured indicators, whether solution or paper based, are often used in laboratory courses and academic/industrial research as a qualitative method to test important experimental markers. While useful, these tools present challenges to those with colour vision deficiency (CVD), who are unable to interpret the same results as their peers. What’s more, some of these tools aren’t as useful in determining important reaction specifics. This commentary presents the perspective of four individuals, three with CVD and one with trichromatic (normal) vision, on how easily coloured indicators are interpreted and how we can address any difficulties in a laboratory setting.

Geometric deformation in main group compounds can be used to elicit unique properties including strong Lewis acidity. Here we report on a family of planar bismuth (III) complexes (c.f. typically pyramidal structure for such compounds), which show a geometric Lewis acidity that can be further tuned by varying the steric and electronic features of the triamide ligand employed. The structural dynamism of the planar bismuth complexes was probed in both the solid and solution phase, revealing at least three distinct modes of intermolecular association. A modified Gutmann-Beckett method was used to assess their electrophilicity by employing trimethylphosphine sulfide in addition to triethylphosphine oxide as probes, providing insights into the preference for binding hard or soft substrates. Experimental binding studies were complemented by a computational assessment of the affinities and dissection of the latter into their intrinsic bond strength and deformation energy components. The results show comparable Lewis acidity to triarylboranes, with the added ability to bind two bases simultaneously and reduced discrimination against soft substrates. We also study the catalytic efficacy of these complexes in the ring opening polymerization of cyclic esters ε-caprolactone and rac-lactide. The polymers obtained are in the ultra-high molecular weight regime and show excellent dispersity values. The complexes retain their performance under industrially relevant conditions suggesting they may be useful as less toxic alternatives to tin catalysts in the production of medical grade materials. Collectively, these results establish planar bismuth complexes as not only a novel neutral platform for main group Lewis acidity, but also a potentially valuable one for catalysis.

Planar bismuth(III) compounds featuring a N

3-R-1,4,2-dioxazol-5-ones are a class of compounds that are increasingly finding diverse uses, including as regioselective amidation reagents and as electrolyte additives that enable long cycling lifetimes in rechargeable lithium-ion batteries. Conventional methods for their synthesis tend to be slow and time-consuming, requiring isolation and thorough drying of a hydroxamic acid intermediate, followed by a separate cyclization step with N,N′-carbonyldiimidazole. Furthermore, the cyclization is typically performed in dichloromethane, an environmentally harmful solvent. This work demonstrates a new one-pot method for the synthesis of these compounds that eliminates the need for isolation of the intermediate or the use of halogenated solvents. The reaction is mainly performed using environmentally benign ethyl acetate and a relatively small amount of N,N-dimethylformamide. The reaction proceeds readily at room temperature and requires no expensive metal catalysts to function.

Li-ion cells being developed for long lifetime applications are often subjected to storage tests at various states-of-charge and various temperatures. Storage is interrupted from time to time for reference performance tests so that cell capacity and impedance can be checked. These reference performance tests give no information about any compositional changes that may have occurred in the electrolyte. Lithium-ion differential thermal analysis applied to cells after years of storage can be used to determine if the electrolyte has changed significantly due to unwanted reactions with the electrode materials or if little to no change has occurred. Here, Li-ion differential thermal analysis is used to study electrolyte changes in a more-or-less “yes/no” manner for single crystal NMC532/graphite cells stored between 3.67 and 4.3 V at 20, 40 and 55 °C for up to five years. Such measurements can be used to give confidence about lifetime predictions. Several such cells are detailed here, with correlation between degree of cell degradation and degree of change in electrolyte composition. Relationships are shown between degradation and evolution of state of electrolyte in elevated temperature and voltage storage experiments.

We report the first application of a rigid P2N3 pincer ligand in p-block chemistry by preparing its bismuth complex. We also report the first example of bismuth complexes featuring a flexible PNP pincer ligand, which shows phase-dependent structural dynamics. Highly electrophilic, albeit thermally unstable, Bi(III) complexes of the PNP ligand were also prepared.

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.

We report the first application of a rigid P2N3 pincer ligand in p-block chemistry by preparing its bismuth complex. We also report the first example of bismuth complexes featuring a flexible PNP pincer ligand, which shows phase-dependent structural dynamics. Highly electrophilic, albeit thermally unstable, Bi(iii) complexes of the PNP ligand were also prepared.

We report the first application of a rigid P2N3 pincer ligand in p-block chemistry by preparing its bismuth complex. We also report the first example of bismuth complexes featuring a flexible PNP pincer ligand, which shows phase-dependent structural dynamics. Highly electrophilic, albeit thermally unstable, Bi(III) complexes of the PNP ligand were also prepared.

Electrolyte additives, as a small proportion of the electrolyte, greatly affect the performance of lithium-ion cells. This work performs a comparative study to reveal the difference between commercial 99.8% pure 3-phenyl-1,4,2-dioxazol-5-one (PDO) additive and lab-made 95% pure PDO in NMC622/graphite cells. In addition, a set of experiments were conducted to evaluate the performance of 99.8% pure PDO and its binary blends with vinylene carbonate (VC), 1,3,2-dioxathiolane-2,2-dioxide (DTD) or lithium difluorophosphate (LFO) in NMC811/graphite cells. 99.8% Pure PDO and 95% pure PDO show little difference in the NMC622 cells, with the latter presenting relatively better performance in the best-performing blends for long-term cycling and high-temperature storage tests. Considering all the tests including ultra high precision coulometry (UHPC) cycling, long-term cycling, and high-temperature storage, the NMC811 cells with 2%PDO+ 1%LFO outperformed the other PDO-containing cells. The PDO-based blends were confirmed to be more promising in cells with higher nickel content; that is, PDO could be a useful additive in high-nickel content cells.

We demonstrate pyridine hydroboration catalyzed by a diazaphospholene hydride precatalyst. Pyridines bearing electron-withdrawing groups in the 3-position are hydroborated efficiently. This system features low catalyst loadings, fast reaction times at ambient temperature, and tolerance of other reducible functionality. Off-cycle products of pyridine hydrophosphination were also obtained in one case from a stoichiometric reaction and structurally characterized.

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 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. Here we report an analysis of periodic trends in structure (solid, solution, and gas phase), bonding, and reactivity based on systematic variation of the central element (P, As, Sb, or Bi) with retention of a single tridentate triamide substituent. 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 the 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 as compared to bent ones, resulting in different reactivity patterns. These results provide a benchmark dataset to guide further research in this rapidly emerging area.

Li[Ni0.42Mn0.42Co0.16]O2 (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 CO2 generated at the delithiated NMC442 electrode, leading to more CO2 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 H2 results from a species created at one electrode which reacts at the other in a pouch cell.

Li[Ni0.5Mn0.3Co0.2]O2/graphite pouch cells were cycled using protocols that included 24 h spent at high voltage (≥ 4.3 V) under constant voltage or open circuit conditions to accelerate failure. Compared to traditional cycling, failure was reached up to 3.5 times faster. When this protocol was applied to cells containing low LiPF6 concentrations (≤ 0.4 M) failure was achieved up to 17.5 times faster than traditional cycling with normal LiPF6 concentrations. This represents a time improvement on the order of years and therefore can be used as a high-throughput screening method. Failure mechanisms for cells containing a range of LiPF6 concentrations undergoing these aggressive protocols were investigated using charge-discharge cycling, impedance spectroscopy (including symmetric cell analysis) and isothermal microcalorimetry. Long times at high voltage rapidly increase positive electrode impedance but do not seem to consume lithium inventory. The use of lower LiPF6 concentrations does not seem to introduce new failure mechanisms but makes cells less tolerant to positive electrode impedance growth. The utility of this method is demonstrated by screening cells with a variety of electrolyte additive combinations. Fewer than 3 months were required to distinguish cells containing 1% lithium difluorophospate as superior to cells with other additive combinations.

An effective method to reduce carbon dioxide emissions is to switch to renewables for energy generation and transportation. Since current sources of renewable energy, such as wind and solar, are intermittent, it is essential to find ways to store energy to match supply and demand. If vehicles are to be powered by renewable energy, they need portable energy storage. Currently, lithium-ion batteries are one of the most viable solutions for energy storage. Extending the lifespan of lithium-ion batteries is the goal of this research, carried out with Dr. David Hall of Dr. Jeff Dahn’s research group at Dalhousie University in late 2017. We developed and tested a chemical compound, 3-phenyl-1,4,2-dioxazol-5-one (PDO), which greatly improves the lifespan of lithium-ion batteries. One percent of this by weight in a cell’s electrolyte, along with two percent ethylene sulfate, will extend a battery’s lifespan more than three-fold over those containing conventional vinylene carbonate-containing electrolyte.

The electrolyte additive lithium difluorophosphate improves the lifetime of lithium-ion cells. This work presents the synthesis and evaluation of alternative difluorophosphate salt electrolyte additives. Ammonium difluorophosphate is readily prepared via a solid-state, benchtop reaction of ammonium fluoride and phosphorus pentoxide that requires only gentle heating to initiate. The best yield of sodium difluorophosphate (NaFO) in the present study was obtained by reacting difluorophosphoric acid and sodium carbonate in 1,2-diemethoxyethane over 3 Å molecular sieves. Tetramethylammonium difluorophosphate was prepared from NaFO via cation-exchange with tetramethylammonium chloride. NaFO is here reported to be a very good electrolyte additive, with similar performance in NMC532/graphite pouch cells as the lithium salt. The beneficial nature of both additives is attributable to the difluorophosphate anion. In contrast, ammonium and tetramethylammonium difluorophosphates are found to be poor electrolyte additives. For the former, this is suggested to be due to the formation of lithium nitride and hydrogen gas.

Finding new electrolyte additives could help create lithium-ion batteries with better performance at high voltage, allowing higher energy density. However, finding the perfect additive remains a tremendous challenge, since researchers still don’t understand how to predict their performance. A new group of dioxazolone electrolyte additives have been tested in lithium-ion batteries alone or in combination with well-known co-additives. The new additives consist of a 3-phenyl-1,4,2-dioxazol-5-one (PDO) parent structure with or without (methoxy, fluoro and nitro) functional groups on the para position of the phenyl ring. It is found that PDO (no functional group) and p-(4-nitrophenyl)-1,4,2-dioxazol-5-one (pNDO) are the best performing dioxazolones overall and show promising results.

It is very common to add additives in lithium-ion cells in order to improve their performance. However, it is not well-known what makes a good additive. The goal of this research was to study how different functional groups in the para position affect the performance of the relatively new additive PDO (3-phenyl-1,4,2-dioxazol-5-one). The PDO additive was developed following previous work by the Winter group on MDO (3-methyl-1,4,2-dioxazol-5-one)1. Each PDO-type additive was synthesized in our lab from the corresponding acyl chloride and was tested in NMC622/graphite cells. Differential capacity analysis (dQ/dV), electrochemical impedance spectroscopy (EIS), high-temperature storage, and long-term cycling tests have been used to characterize the corresponding cells. The well-known additives DTD (Ethylene Sulfate), MMDS (Methylene Methyl Disulfonate), LFO (Lithium difluorophosphate) and VC (Vinylene Carbonate) have been added as co-additives in the hope of forming binary blends with improved performance. It was found that unsubstituted and nitro-substituted PDO blends have the best long-term cycling performance, while unsubstituted and methoxy-substituted PDO blends have the best storage performance at 60°C. S. Röser, A. Lerchen, L. Ibing, X. Cao, J. Kasnatscheew, F. Glorius, M. Winter, and R. Wagner, Chem. Mater., 29, 7733–7739 (2017). Figure 1

After many charge-discharge cycles of showing little to no capacity fade, Li-ion cells can undergo rapid degradation in capacity that occurs over relatively few cycles [1]. We refer to this sudden, accelerated capacity loss as “rollover” failure. “Rollover” failure should be concerning to manufacturers and academics alike, because it can be difficult to predict when it will occur and can require years of cycling to verify. In some instances, “rollover” failure can be caused by impedance growth during cycling that eventually limits the available lithium inventory at a particular charging current due to an ohmic voltage drop. This impedance growth is followed by true loss of lithium inventory by lithium plating. We show that the impedance growth of the cell during cycling is, among many other factors, strongly tied to the concentration of salt used in the electrolyte. Using more salt (up to a reasonable limit) reduces cell impedance at all frequencies, provides better impedance control during cycling and extends the number of cycles until “rollover” failure. Ultra-High Precision Coulometry and Electrochemical Impedance Spectroscopy combined with post failure electrolyte analysis by Li-ion Differential Thermal Analysis, Gas Chromatography Mass Spectrometry and Inductively Couple Plasma Mass Spectrometry provide clues of how the cell and electrolyte change with varying salt concentration, and as the cell fails. Finally, we propose a means of testing to accelerate “rollover” failure. Cycling protocols with long constant voltage segments at the top of charge are shown to accelerate impedance growth and “rollover” failure. Using this cycling protocol on cells with electrolytes containing low salt concentrations can reduce the time to “rollover” to a few months. With traditional cycling and good cells, containing electrolytes with 1M – 1.2M salt concentrations and good electrolyte additives, this can take years. We believe that cycling with long periods at high potential, of cells with low salt concentration electrolytes is an accelerated means of testing to quickly screen electrolyte additives, electrode coatings and other cell material choices. Figure 1 shows the discharge capacity and ΔV (difference between average charge and discharge voltages) versus cycle number of cells the follow our prescribed method to accelerate “rollover”. The cells contained electrolytes with varying salt concentrations and spend 24 hours at 4.4V every second charge-discharge cycle. Figure 1 clearly shows that lifetime is increased with increased LiPF6 concentration. Similarly, Figure 1 shows that use of the electrolyte additive LiPO2F2 (LFO) extends lifetime when compared to the combination of fluoroethylene carbonate (FEC) and dioxathiolane-2,2-dioxide (DTD). Cells with longer lifetimes show better impedance control, as evidenced by ΔV. Figure 2 shows discharge capacity and ΔV versus cycle numbers for cells that are tested using typical CCCV cycling to 4.3V. The comparison between FEC and DTD versus LFO is the same as in Figure 1, but the data in Figure 2 took 8 months to collect and distinguish the two additive systems. This is eight times longer than it took to distinguish the two additive systems using cells with 0.2 M LiPF6 that were held at 4.4V for 24h every second charge, as shown in Figure 1. “Rollover” failure can be prevented by ensuring that cell impedance remains constant. Increasing LiPF6 concentrations appears to control impedance, as does avoiding extended times at high voltage. Doing the opposite results in a high throughput screening method than can quickly distinguish the lifetime benefit of small changes in cell chemistry, like electrolyte additives. [1] J. C. Burns, A. Kassam, N. N. Sinha, L. E. Downie, L. Solnickova, B. M. Way, J. R. Dahn, J. Electrochem. Soc., 160, A1451-A1456 (2013). Figure 1