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The use of the flipped classroom approach in higher education STEM courses has rapidly increased over the past decade, and it appears this type of learning environment will play an important role i...

The substitution of sterically bulky groups at precise locations along the periphery of fused-ring aromatic systems is demonstrated to increase electrochemical oxidation potentials by preventing relaxation events in the oxidized state. Phenothiazines, which undergo significant geometric relaxation upon oxidation, are used as fused-ring models to showcase that electron-donating methyl groups, which would generally be expected to lower oxidation potential, can lead to increased oxidation potentials when used as the steric drivers. Reduction events remain inaccessible through this molecular design route, a critical characteristic for electrochemical systems where high oxidation potentials are required and in which reductive decomposition must be prevented, as in high-voltage lithium-ion batteries. This study reveals a new avenue to alter the redox characteristics of fused-ring systems that find wide use as electroactive elements across a number of developing technologies.

4-Deoxypentenosides (4-DPs) are versatile synthons for rare or higher-order pyranosides, and they provide an entry for structural diversification at the C5 position. Previous studies have shown that 4-DPs undergo stereocontrolled DMDO oxidation; subsequent epoxide ring-openings with various nucleophiles can proceed with both anti or syn selectivity. Here, we report the synthesis of α- and β-linked 4'-deoxypentenosyl (4'-DP) disaccharides, and we investigate their post-glycosylational C5' additions using the DMDO oxidation/ring-opening sequence. The α-linked 4'-DP disaccharides were synthesized by coupling thiophenyl 4-DP donors with glycosyl acceptors using BSP/Tf2O activation, whereas β-linked 4'-DP disaccharides were generated by the decarboxylative elimination of glucuronyl disaccharides under microwave conditions. Both α- and β-linked 4'-DP disaccharides could be epoxidized with high stereoselectivity using DMDO. In some cases, the α-epoxypentenosides could be successfully converted into terminal l-iduronic acids via the syn addition of 2-furylzinc bromide. These studies support a novel approach to oligosaccharide synthesis, in which the stereochemical configuration of the terminal 4'-DP unit is established at a post-glycosylative stage.

A new family of polyfluoroalkyl-substituted ethylene carbonates is synthesized and tested as additives in lithium-ion cells containing EC:EMC + LiPF 6 -based electrolyte. The influence of these compounds is investigated in Li 1.2 Ni 0.15 Mn 0.55 Co 0.1 O 2 //graphite cells via a combination of galvanostatic cycling and electrochemical impedance spectroscopy (EIS) tests. Among the four additives studied in this work (4-(trifluoromethyl)-1,3-dioxolan-2-one (TFM-EC), 4-(perfluorobutyl)-1,3-dioxolan-2-one (PFB-EC), 4-(perfluorohexyl)-1,3-dioxolan-2-one (PFH-EC), and 4-(perfluorooctyl)-1,3-dioxolan-2-one (PFO-EC)), small amounts (0.5 wt%) of PFO-EC is found to be most effective in lessening cell performance degradation during extended cycling. Linear sweep voltammetry (LSV), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy are used to further characterize the effects of PFO-EC on the positive and negative electrodes. LSV data from the electrolyte, and XPS analyses of electrodes harvested after cycling, suggest that PFO-EC is oxidized on the cathode forming surface films that slow electrode/cell impedance rise. Differential capacity (dQ/dV) plots from graphite//Li cells suggest that PFO-EC is involved in solid electrolyte interphase (SEI) formation. Raman data from anodes after cycling suggest that structural disordering of graphite is reduced by the addition of PFO-EC, which may explain the improved cell capacity retention.

Commercial redox flow batteries (RFBs) contain highly acidic and corrosive electrolytes, a cause for concern with widespread use to due concerns about safety and environmental contamination. Replacing these electrolytes with non-aqueous equivalents allows for a safer storage medium. Additionally, utilization of organic electroactive materials may lead to more scalable technologies that do not rely on mined materials such as vanadium and lithium. Furthermore, non-aqueous electrolytes could enable a 2- to 3-fold increase in operating voltage due to the wider operating voltage of non-aqueous systems. Despite the promise for a safer, scalable, higher energy system, the number of organic compounds reported in non-aqueous flow battery systems has been limited to a few classes of compounds, many of which suffer from instability and/or insolubility, especially in charged states. Our recent efforts have focused on the development of organic electron donors and acceptors with stable oxidized and reduced states. This presentation will focus on design strategies utilized to increase molecular stability and solubility in all relevant states of oxidation while keeping syntheses short, high yielding, and scalable. In particular, we highlight the design, synthesis, and electrochemical analysis of new phenothiazine and napthoquinone derivatives designed to serve as one- or two-electron donors. We show that tactical placement of substituents leads to improved stability and increased solubility. Additionally, a new approach to modification of redox potentials using strategic substituent placement will be presented.

Redox flow batteries (RFBs) are promising candidates for grid storage, with a few large-scale systems currently in operation. However, current systems have not met the stringent cost and/or safety requirements needed for widespread implementation. Replacing vanadium with organic compounds may lower materials cost, and utilizing non-aqueous (aprotic) electrolyte solvents, in place of water, could enable a 2- to 3-fold increase in operating voltage. Both features make non-aqueous RFBs candidates for large-scale stationary storage. Currently a limited number of organic compounds have been reported as stable electron donors and acceptors, with even fewer materials being studied as small molecule two-electron donors and/or two-electron acceptors. Yet if the amount of charged stored within an individual molecule were raised without significantly increasing the molecular weight, then electrolyte capacity could be increased proportionally, assuming solubility of neutral and charged species is retained. Our recent efforts have focused on the development of highly soluble electron donors and acceptors with stable oxidized and reduced states. This presentation will focus on design strategies utilized to increase molecular stability in all relevant states of charge as well as solubility. In particular, we highlight the design, synthesis, and electrochemical analysis of phenothiazine and napthoquinone derivatives designed to serve as two-electron donors and two-electron acceptors, respectively. We show that tactical placement of substituents leads to improved stability of doubly oxidized and doubly reduced species, whilst retaining atom economy and that high solubility.

The redox potentials of pi-conjugated organic molecules and polymers are essential factors that determine in part their performance in energy collection and storage applications. In these systems, the control of redox potentials usually involves substitution with electron-donating and/or electron-withdrawing groups, making use of the well-known Hammett constants as predictors of the extent of the change of redox potentials. An additional, less utilized route exploits strain-induced disruptions of π-conjugated frameworks by either (1) introducing bulky substituents to manipulate the dihedral torsions among the π-conjugated moieties or (2) to instill curvature into pi-conjugated networks. In each of these strained examples, the π-conjugated networks are strained in both the ground (neutral) and ionized (oxidized or reduced) electronic states. Taking these studies as inspiration, we sought to develop molecular design principles that impart strain in π-conjugated molecules in only one electronic state, enabling a new method to synthetically control redox potentials. This presentation will include on our approaches to raising oxidation potentials, including calculations, synthesis, electrochemical analysis, and – in some cases – cycling in overcharging lithium-ion batteries. A key highlight is the use of electron-donating alkyl groups to raise oxidation potentials – the opposite effect to what Hammett constants predict.

Many of the electrochemical energy-storage systems under consideration for large-scale power storage and allocation rely on stable electroactive materials to shuttle and/or store charge. Here we seek to design a new class of organic compounds, based on the electroactive molecule phenothiazine, for use as (a) redox-shuttle additives to mitigate excess charge in overcharging lithium-ion batteries, and (b) catholyte materials for use in redox flow batteries. This presentation will include the results of density functional theory calculations that are used to model the physicochemical effects of substituent identity and placement on phenothiazine by introducing electron-donating and/or -withdrawing substituents at strategic positions around the heterocyclic core. Substituent crowding is shown to effect changes in oxidation potential at odds with those anticipated from Hammett constants, as well as changes in energetically optimal conformations, consistent with more-restricted relaxation pathways afforded by the additional steric strain. A subset of the compounds under consideration were subsequently selected for synthesis and electrochemical analysis. The results of this testing suggest that, unlike prior methods of increasing oxidation potentials using electron-withdrawing groups, strategic placement of substituents can be exploited to raise oxidation potentials without raising reduction potentials, thereby preventing access to reduction decomposition pathways. The results of this analysis reveal strategies for designing and tuning the properties of new electroactive compounds for energy-storage applications and beyond.

Only about 3% of the electrical energy consumed today is supplied by intermittent energy sources such as solar and wind power. To decrease our reliance on CO2-producing fossil fuels, which currently supply almost 70% of electricity, it is necessary to smooth out the intermittency of renewable energy production. The development of low-cost electrical energy storage (EES) systems could lead to significant integration with the electrical grid. Redox flow batteries (RFBs) are of great interest as a low-cost solution, and systems as large as 20 MWh (check value) have been employed for commercial applications. The commercialization of vanadium-based RFBs has been enabled by the combination of low cost, long lifetime, and high efficiency, and stands out due to the unique ability to scale storage capacity independently of the reactor area. Despite their advantages, current RFBs are limited in charging voltage to ca. 1.5 V due to the electrochemical instability of water above this potential. The replacement of the acidic, aqueous electrolytes with aprotic organic equivalents could allow for higher charging voltages – up to 4-5 V, depending on the identity of the electrolyte solvent.1 Compared to aqueous RFBs, non-aqueous RFBs are still in their infancy due to limited stability and/or solubility of electro-active materials, membrane crossover, and the cost of materials. Recently the performance of organic compounds has been tested in flow systems or stationary mimics. More recently, reports of non-aqueous RFBs containing organic electro-active materials have surfaced. N-oxidanyl amines (e.g. TEMPO), dialkoxybenzenes, and phenothiazines serve as electron-donating electro-active materials, while phthalimide, anthroquinones, quinoxilanes, fluorenone, and viologen act as electron-accepting counterparts. Many of the electron donors have been used as electron-transfer catalysts in other energy storage and collection applications, including redox shuttles for overcharge protection of lithium-ion batteries (LIBs), electron-transfer agents in lithium-air batteries, and redox mediators in dye-sensitized solar cells, among others. Our recent work in protecting LIBs from overcharge has led to the development of several phenothiazine derivatives that exhibit high stability in the neutral and singly oxidized (radical cation) forms,2-6 and some examples exhibit high solubility in organic electrolytes – the combination of which inspired us to evaluate these materials as electron-donating electro-active materials for non-aqueous RFBs.7 Here we will share our recent results in the evaluation of highly soluble phenothiazine derivatives using a variety of electrochemical techniques and – in some cases – in flow batteries. Two highly soluble (miscible) derivatives – one of which is a liquid at room temperature – can be prepared in large scales in one step from commercially available phenothiazine. The persistency of the singly oxidized state has enabled us to synthesize and isolate radical cation salts, which can be used in symmetric flow tests. Additionally, we have found that appropriate identity and positioning of substituents around the periphery of the phenothiazine ring has led to a reversible second oxidation event as measured by cyclic voltammetry. Cycling tests reveal that the dication of these molecules is more stable than versions that do not contain substituents and give an atom economy as low as 150 g/mol e-. References: Darling, R. M. et al., Energy. Environ. Sci., 2014, 7, 3459-3477. Kaur, A. P. et al., J. Mater. Chem. A, 2016 , 4, 5410-5414. Kaur, A. P. et al., J. Electrochem. Soc., 2016, 163, A1-A7. Kaur, A. P. et al. , J. Mater. Chem. A, 2014, 2, 18190–18193. Ergun, S. et al., J. Phys. Chem. C, 2014, 118, 14824–14832. Ergun, S. et al., Chem. Commun., 2014, 50, 5339–5341. Kaur, A. P. et al., Energy Tech., 2015, 3, 476–480.

The development of robust electro-active materials is critical for a variety of electrochemical energy storage applications. Phenothiazines, phenoxazines, carbazoles, and triphenylamines are among some of the numerous electron-donating aromatic compounds that have been studied for numerous applications in organic electronics, photovoltaics, and electrochemically-mediated synthesis. Their use as electrolyte components and electrode materials has only been studied by a few groups, yet these materials are easy to functionalize, enabling tuning their electronic properties, solubility, and stability. My research group initially explored derivatives of these compounds as redox shuttles for overcharge protection of lithium-ion batteries. With systematic variations in structure, our efforts combining analytical techniques with density functional theory calculations have enabled us to identify decomposition products and to predict reasonable mechanisms that lead to failure in neutral and oxidized states. Our development of robust derivatives that survived for thousands of hours of overcharge was a result of the design rules that came from these studies. Recently we have expanded our use of these compounds as catholyte components for non-aqueous redox flow batteries. In one to three steps from commercially available materials, we have prepared highly soluble phenothiazine derivatives (solubility of 1-2 M in organic electrolytes) that can serve as one- and/or two-electron donors with long lifetimes in the charged states. This presentation will focus on the synthesis, spectroscopic, and electrochemical analysis of these materials as electron donors for redox flow batteries.

Harnessing silicon (Si) as the anode material in lithium-ion batteries offers the possibility of a nearly ten-fold capacity increase over traditional graphite anodes. However, the 300% volume expansion and contraction upon charging and discharging hinders its practical use due to a significant loss in capacity with cycling. Unique geometric arrangements of Si particles, as well as modifications of polymer binder, have resulted in advances in capacity retention over time, but many modifications are cost-prohibitive for producing a commercially viable electrode. Our strategy to minimize the loss in capacity retention of this high-capacity anode material involves the utilization of a viscoelastic cross-linked polysiloxane polymer binders. This presentation will discuss the mechanical properties and cycling characteristics of the electrodes prepared with this binder and comparable polymers. A highlight of our progress is a retention of charge capacity over >100 cycles. In addition to the retention of capacity, we highlight the low cost and simple synthesis of this polymer binder.

Phenothiazines have seen widespread use as stable electron-donating organic compounds with generally stable oxidized states, making them an amenable core for functionalization toward electrochemical energy storage applications. With phenothiazine itself as a starting material, functionalization of the 3, 7, and 10 positions is facile, providing options to modify redox potentials and improve stability in both the neutral and singly oxidized (radical cation) states. In one to three steps, we can create highly soluble, stable derivatives that are suitable for applications requiring the donation of one or two electrons. We have observed thousands of hours of overcharge protection in lithium-ion batteries and more recently have moved into testing these species as the electro-active materials for non-aqueous redox flow batteries. This presentation will focus on the synthesis and characterization of new derivatives, highlighting new results in which we found that a liquid phenothiazine that can be prepared in one step from commercially-available starting materials can serve as an electron donor and solvent, dissolving some lithium salts to concentrations of at least 1 M.

The performance of aromatic compounds as redox shuttles for overcharge protection in lithium-ion batteries is variable and difficult to predict. Redox shuttles can decompose in battery electrolyte in their neutral and oxidized forms, both of which are present during overcharge protection. The reasons for differing stability in compounds with only slight structural differences is often unclear, and the exploration of decomposition of redox shuttles has been severely limited, restricting our ability to design improved versions of redox shuttles. To better understand the stability and reactivity of redox shuttles (also relevant to the improvement of positive electrode materials in non-aqueous redox flow batteries) our research has focused on measuring the stability of neutral and oxidized forms of redox shuttle candidates as well as using a variety of spectroscopic methods to analyze the byproducts of decomposition, both from radical cations generated in model solvents and electrolytes from postmortem analysis of failed batteries. Figure 1

In designing robust, effective redox shuttles for overcharge protection in lithium-ion batteries, a variety of factors are necessary to consider and evaluate. The stability and reactivity of the radical cation form of a redox shuttle candidate are critical to extensive overcharge performance. Radical cations are subject to a variety of reactions because of their unpaired electron and positive charge, making them subject to nucleophilic attack and dimerization, among other reactions including disproportionation or premature reduction. Equally as important is the stability of the neutral form of the redox shuttle, which may decompose through reaction with electrolyte, SEI, and/or be reduced at the anode/electrolyte interface. Neither stability nor reactivity is easy to predict, although an understanding of organic chemistry can lead to some general guidelines for the design of less reactive species. Density functional theory (DFT) calculations can be used to predict energies of reaction and have been employed by Curtiss, Dahn, and our group. DFT calculations can also be used to compute oxidation and reduction potentials, helping to identify promising candidates before synthesizing materials. This presentation focuses on our work using DFT calculations to help us identify promising redox shuttles and to gain a better understanding of their reactivity and includes experimental data in support of some of the proposed mechanisms of decomposition.

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