Tuning the Stability of Organic Active Materials for Nonaqueous Redox

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Tuning the Stability of Organic Active Materials for Non-Aqueous Redox Flow Batteries via Reversible, Electrochemically-Mediated Li Coordination +

Emily V. Carino, Jakub Staszak-Jirkovsky, Rajeev S. Assary, Larry A Curtiss, Nenad Markovic, and Fikile R. Brushett Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04053 • Publication Date (Web): 24 Mar 2016 Downloaded from http://pubs.acs.org on April 7, 2016

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Chemistry of Materials

Prepared as an Article for publication in Chem. Mater.

Tuning the Stability of Organic Active Materials for Non-Aqueous Redox Flow Batteries via Reversible, Electrochemically-Mediated Li+ Coordination Emily V. Carinoa,b, Jakub Staszak-Jirkovskya,c‡, Rajeev S. Assarya,c, Larry A. Curtissa,c, Nenad Markovica,c,, Fikile R. Brushett a,b,* a

Joint Center for Energy Storage Research Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge MA, 02139 c Materials Science Division, Argonne National Laboratory, Argonne IL 60439 ‡ current address: Machavert Pharmaceuticals LLC, Aurora CO, 80045 *corresponding author: [email protected] b

ACS Paragon Plus Environment 1


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Abstract We describe an electrochemically-mediated interaction between Li+ and a promising active material for non-aqueous RFBs, 1,2,3,4-tetrahydro-6,7-dimethoxy-1,1,4,4tetramethylnaphthalene (TDT), and the impact of this structural interaction on material stability during voltammetric cycling. TDT could be an advantageous organic positive electrolyte material for non-aqueous RFBs due to its high oxidation potential, 4.21 V vs. Li/Li+, and solubility of at least 1.0 M in select electrolytes. Although results from voltammetry suggest TDT displays Nernstian reversibility in many non-aqueous electrolyte solutions, bulk electrolysis reveals significant degradation in all electrolytes studied, the extent of which depends on the electrolyte solution composition. Results of subtractively normalized in situ fourier transform infrared spectroscopy (SNIFTIRS) confirm that TDT undergoes reversible structural changes during cyclic voltammetry in propylene carbonate and 1,2-dimethoxyethane solutions containing Li+ electrolytes, but irreversible degradation occurs when TBA+ replaces Li+ as the electrolyte cation in these solutions. By combining the results from SNIFTIRS experiments with calculations from density functional theory, solution-phase active species structure and potentialdependent interactions can be determined. We find that Li+ coordinates to the Lewis-basic methoxy groups of neutral TDT and, upon electrochemical oxidation, this complex dissociates into the radical cation TDT•+ and Li+. The improved cycling stability in the presence of Li+ relative to TBA+, suggests that the structural interaction reported herein may be advantageous to the design of energy storage materials based on organic molecules.


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Introduction Redox flow batteries (RFBs) are electrochemical systems well-suited for multi-hour energy storage at the low system costs needed for economically viable grid storage.1, 2 Whereas the majority of RFBs presently rely on water-soluble species as the charge-storage material, transitioning to non-aqueous chemistries offers the opportunity to increase cell voltage via wider electrochemical stability windows of electrolyte solutions, and to leverage new electrochemical couples which are incompatible with aqueous electrolyte solutions due to either low solubility, chemical instability, or redox potentials outside of the stability window.3–6 If realized together, these advantages lead to higher energy density, smaller system footprint, and lower costs of energy. Recently, efforts deploying electroactive organic molecules as storage materials in RFBs have gained traction because properties such as redox potential, solubility, diffusivity, and cycling stability can be tuned by modifying the molecular structure, which, in turn, can lead to enhanced performance.4 –12 For example, Wang and co-workers demonstrated that adding an anionic substituent group to ferrocene enhances solubility from 0.04 M to 0.85 M in carbonatebased electrolyte solutions, which leads to marked improvements in cell energy densities when coupled with a Li-graphite anode.4 Identifying and employing active materials with higher solubility and redox potentials will enable further improvements in energy density. For emerging materials, how modifying molecular structure to enhance one property subsequently impacts other properties is often unclear. Of particular concern are interactions between the active material, electrolyte, and solvent, as these have proven effects on both electrochemical and solution properties, as well as stability.7,13–15 In some cases these interactions can be advantageous, as demonstrated by Hernandez-Burgos and co-workers, who reported that coordination of carbonyl-based organic molecules to Mg2+ resulted in increased energy density due to increasing of the redox potential.7



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Several promising classes of active materials for the positive electrolyte in RFBs are based on redox shuttles for Li+-ion batteries, an electroactive organic species added to batteries to provide overcharge protection, as these materials have established cycling stability and reasonable gravimetric capacity.5,16,17 Of particular note are derivatives of di-tertbutyl- di-noxybenzene (DDB), a redox shuttle displaying a reversible redox potential ca. 3.90 V vs Li/Li+, which have recently been developed and explored as candidate positive electrolytes.5,6,14,16,18 Lessons learned from the DDB platform led to the design, synthesis, and evaluation of 1,2,3,4tetrahydro-6,7-dimethoxy-1,1,4,4-tetramethylnaphthalene (TDT), a redox shuttle which undergoes reversible electrochemical oxidation to a radical cation TDT•+ at potentials above 4.15 V vs Li/Li+ (see our comment on reported reference electrode potentials in the Experimental Methods section).19,20 Unlike ferrocene, a molecule with well-established properties and stability in many nonaqueous solutions, the behavior of emerging redox-active organic molecules, such as TDT, in a given environment is less clear. Redox-active organic molecules can undergo a plethora of parasitic reactions,21 and elucidating these often complex reaction pathways is a time- and material-intensive endeavor. Moreover, newer materials, especially those which are not yet commercially available, may not be readily synthesized in large enough quantities to enable comprehensive testing, and therefore improved scientific methodologies for analyzing structural changes pursuant to electrochemical reactions are desirable. A handful of ex situ and in situ methods have been applied to analyze the stability of families of redox-active organic molecules and estimate trends in cycling performance.13,17,22 However, these approaches only partially describe complex interactions occurring in a real electrochemical cell at different stages of polarization. Some in situ methods, such as UV-Vis spectroelectrochemistry, are useful under


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dilute conditions, but may not be a suitable probe for the high-concentration environment of RFBs. Electron paramagnetic resonance spectroscopy provides insights into how radicals interact with their environment, but can only interrogate solution components displaying paramagnetism.13 Subtractively normalized in situ fourier transform spectroscopy (SNIFTIRS) is an operando spectroscopic method suitable to characterizing organic materials within complex reaction mixtures employed in RFBs, such as those described herein.23–26 Because SNIFTIRS spectra reveal changes in composition at the electrode surface and within the first few microns of solution during electrochemical cycling, and these changes can be correlated to both potential and time, this method is a powerful analytical tool for elucidating electrochemical reaction mechanisms. Calculations from first principles, such as density functional theory (DFT), are another tool for accelerating the discovery and development of new materials.27 DFT is often applied in combination with electrochemical studies to predict the effects of molecular structure and electrolyte interactions on redox potential,7,10,15 but the fidelity of these calculations presumes the modeled structure is a good approximation of the active material within the reaction mixture. Further, uncertainty in calculations of redox potential, as high as 0.2 V, leads to difficulty discriminating between models of similar energies. When combined with both spectroscopic and electrochemical data, however, DFT calculations augment insights about structure that are difficult to unambiguously determine from electrochemistry data alone.23 The purpose of this study is to determine how electrochemical cycling modulates the structure of TDT and its interactions with electrolyte, with the aim of understanding how these types of interactions impact cycling stability. TDT can be reversibly cycled in a wide variety of electrolytes in propylene carbonate (PC) and 1,2-dimethoxyethane (DME) using cyclic voltammetry (CV). However, evidence of irreversible decomposition is apparent when large



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volumes of TDT are electrochemically cycled using bulk electrolysis, and the extent of capacity loss differs with electrolyte. Interestingly, the presence of Li+ as the electrolyte cation resulted in higher coulombic efficiencies, denoting improved stability in these solutions. Comparing the results of SNIFTIRS with DFT simulations reveals that neutral TDT molecules sequester Li+ in a methoxyether complex, and these coordinated Li+ ions are subsequently released upon electrochemically oxidizing TDT to the radical cation, TDT•+ (Scheme 1). Coordination of Li+ to neutral TDT enhances the stability of the solution during cycling by deterring parasitic attack by TDT•+ on the Lewis-basic veratrole group.

Scheme 1. Reversible, electrochemically-mediated Li+ coordination


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Experimental Methods Chemicals and materials. Propylene carbonate (PC, 99.7% anhydrous), 1,2-dimethoxyethane (DME, 99.5% anhydrous), dichloromethane (DCM, >99.8%), lithium perchlorate (LiClO4, battery grade, 99.99% trace-metal basis), tetrabutylammonium perchlorate (TBAClO4, electrochemical grade, >99.99%), lithium bis(trifluoromethane)sulfonimide (LiTFSI, 99.95%), and ferrocene (98%) were purchased from Sigma-Aldrich (St. Louis, Missouri). Electrolyte salts were dried overnight at 60 °C under vacuum and immediately transferred to the glovebox prior to use. Solvents were dried using activated aluminum oxide powder (basic, Sigma-Aldrich). Lithium foil (Li, 99.9% metals basis, Alfa Aesar) was packaged under Argon and opened inside of the glove box. TDT (99.9%, battery grade) was synthesized19 and generously provided by the Materials Engineering Research Facility at Argonne National Laboratory. All materials were stored and prepared for analysis in a positive-pressure, Ar-filled glove box (MBraun, Exeter, New Hampshire) with oxygen and moisture content below 1 ppm. Following drying, the water content of electrolyte solutions used for experiments was < 25 ppm as measured by Karl-Fischer coulometry (Mettler-Toledo C20 coulometric titrator). Electrochemical characterization. Electrochemical characterization was performed in the glovebox using a CH760E potentiostat (CH Instruments, Austin, Texas). The glovebox temperature was about 29 °C. Where noted, a correction to the uncompensated resistance was applied during the measurement. A glassy carbon working electrode (GC, 3 mm diameter) and a gold working electrode (Au, 2 mm diameter) were purchased from CH Instruments. A carbon fiber ultramicroelectrode (UME, 11 µm) was purchased from BASi (West Lafayette, Indiana). We used a Vycor®-fritted reference electrode consisting of a Li metal foil immersed in a solution of 1.0 M LiClO4 in PC to assure the accurate comparison of potentials measured within the



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different solutions. A Au coil served as the counter electrode in CV experiments. The CV experiments were carried out in glass vials in which the electrodes were inserted through a Teflon cap. Constant potential bulk electrolysis experiments were conducted in an commercially available bulk electrolysis cell (BASi ) consisting of a reticulated vitreous carbon (RVC) working electrode, a fritted counter electrode, and a Vycor®-fritted reference electrode consisting of a Li metal foil immersed in a solution of 1.0 M LiClO4 in PC. A Li metal foil served as the counter electrode for all bulk electrolysis experiments in which Li+-containing electrolyte solutions were used. A Au coil served as the counter electrode for the experiments in TBA+ containing electrolyte solutions. A consistent solution volume of 25 mL and a stir rate of 1200 rpm were used for all bulk electrolysis experiments. The reference potential scale for all measurements reported herein was calibrated to a ferrocene standard (standard reduction potential, Eo = 0.40 vs. SHE) (Supporting Information, Figure S1) and all potentials are reported versus Eo for Li/Li+ (-3.04 vs. SHE). We note that, commonly, potential measurements made using Li metal foil in Li+-containing electrolyte solution are reported versus the measured potential of Li/Li+, but these values are not always corrected to the standard potential (Eo) the Li/Li+ redox couple, as done here. Correcting the reference potential scale was necessary for these studies to assure that experimental results could be compared to the calculations from DFT theory. Li metal foil electrodes used without calibrating the potential scale to a known standard reference couple are quasi-reference electrodes whose potential may deviate from the Eo of Li/Li+ by more than 0.1 V (Supporting Information, Figure S1). This difference should be kept in mind when comparing these results to other reports from the literature.


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SNIFTIRS. A Thermo Nicolet 8700 FTIR spectrometer with a liquid nitrogen cooled MCT high D* detector was used in the SNIFTIRS measurements. The experiments were carried out in specialized sealed cell to keep moisture out.25 Reflectance conditions were set up using a Au working electrode (5 mm diameter, Pine instruments, Grove City, Pennsylvania) pressed against a 60° CaF2 prism to form a thin electrolyte layer of ca. 10 µm between the working electrode and prism. P-polarized light was used in the measurements by employing a polarization filter to enhance signal from the interface. A Pike optical box (Pike Technologies, Madison, Wisconsin) was used to control the incidence beam angle close to 63°. Electrochemical measurements made using a Au working electrode showed that the electrochemical properties of TDT were not sensitive to the choice of working electrodes used in the studies reported herein. The potential of the cell was controlled against a Ag/Ag+ quasi-reference electrode (BASi), and a Au wire served as the counter electrode. The potential of the quasi-reference electrode was calibrated to ferrocene prior to use in the SNIFTIRS experiments. A correction to the uncompensated resistance was applied during the measurement to assure accurate correlation of spectra with electrochemical potential. Prior to sweeping the potential and recording in situ spectra, a reference spectrum was recorded at 4.10 V vs. Li/Li+, a potential below the measured onset of TDT electrooxidation. Next, spectra were recorded while cycling the potential between 4.10 V and 4.40 V at a sweep rate of 0.002 V/s. Eight interferograms were collected and averaged to form one single spectrum every 7 s. These spectra were then subtractively normalized with respect to the reference spectrum to produce a series of difference spectra. Computational methods. All calculations presented in the paper were performed using the 28

Gaussian 09 software. The B3LYP/6-31+G(d) level of theory was used to compute the structure and energeties of all species in the gas phase. The same level of theory was used to



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calculate zero point energies, free energy corrections (298 K, 1 atm pressure) and solvation energies. The SMD2 model was used to compute the solvation free energy by a single point energy calculation on the gas phase optimized geometry using water as the dielectric medium for selected molecules. We find that this is an effective approximation for computing free energies of redox active species in solution and this level of theory is reasonably accurate for the 29

computation of reduction potentials.

The Gibbs free energy, ∆G, (at 298 K) of molecule ‘M’

and its corresponding electrooxidized cation, ‘M+’, in the solution is computed as the sum of the free energy in the gas phase (∆Ggas) and the solvation free energy (∆Gsolv) using the thermodynamic cycle shown Scheme 2.

Scheme 2. Thermodynamic cycle involved in the oxidation process of an arbitrary molecule, M.

The solution phase free energy change for oxidation process (∆Gox) can be computed from Equation 1.

Eq. 1.

∆‫ܩ‬௢௫ = ∆‫ܩ‬௚௔௦ + (∆‫ܩ‬௦௢௟௩ (‫ܯ‬ା ሻ − ∆‫ܩ‬௦௢௟௩ (‫ܯ‬ሻሻ

Next, ∆Gox is used to calculate the solution phase redox potential, Eox (Equation 2)


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Eq. 2.

‫ܧ‬௢௫ =

ି∆ீ೚ೣ ௡ி

− 1.24

Here, n is the number of electrons involved in the redox reaction and F is the Faraday constant. The constant ‘-1.24 V’ represents the difference between the standard hydrogen electrode (SHE, -4.28 V)30 and Li/Li+ redox couple (-3.04 V) and is required to convert the free energy changes to reduction potential with respect to Li/Li+ reference electrode, a commonly used experimental convention.31–33 Further details regarding the computation of electrochemical potential can be found elsewhere.34–41

Results & Discussion Electrochemical properties of TDT in different electrolyte solutions. The electrochemical properties of redox-active organic molecules can be influenced by explicit interactions with electrolyte and solvent. 7,15 Of particular concern for organic RFB applications, in which the active material must undergo hundreds of cycles or more, is the stability of the active species, which are susceptible to parasitic reactions such as nucleophilic or electrophilic attack, or scavenging by trace water, during charging and discharging.21 These processes are also sensitive towards solution composition,21 thus optimizing the stability of emerging RFB active materials requires characterizing them in a range of different electrolyte solutions. We analyzed the electrochemical properties TDT in solutions containing LiTFSI, LiClO4 and TBAClO4 dissolved in PC and DME using CV with UME, and bulk electrolysis. These salts were selected because the anions exhibit superior stability against degradation via thermolysis or hydrolysis, are stable against electrochemical oxidation up to at least 5 V in many aprotic solvents, and can be



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considered ideal for electroanalytical characterization.42 The solubility of TDT in PC with 1.0 M electrolyte is only around 0.4 M, however, we found that TDT concentrations as high as 1.0 M within 1.0 M electrolyte can be achieved by using DME as the solvent.

Figure 1. UME voltammetry of TDT in various electrolyte solutions. a) 0.01 M TDT in 0.1 M electrolyte in PC; b) 0.01 M TDT in 0.1 M electrolyte in DME; c) varying concentrations of TDT in 1.0 M LiTFSI in DME. The sweep rate was 0.01 V/s.


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Table 1. Electrochemical analysis of reversibility

solvent PC PC PC DME DME DME DME DME DME DME notes:

electrolyte LiTFSI LiClO4 TBAClO4 LiTFSI LiClO4 TBAClO4 LiTFSI LiTFSI LiTFSI LiTFSI

[TDT] , [electrolyte] (M)a 0.01 , 0.10 0.01 , 0.10 0.01 , 0.10 0.01 , 0.10 0.01 , 0.10 0.01 , 0.10 0.05 , 1.00 0.10 , 1.00 0.50 , 1.00 1.00 , 1.00

|E3/4 – E1/4| (mV) 58 58 59 63 69 74 59 60 62 64

[TDT] , [electrolyte] (M)b 0.005 , 1.00 0.005 , 1.00 0.005 , 1.00 0.005 , 1.00 0.005 , 1.00 0.005 , 1.00

CE (%) 88 68 49 85 84

Tuning the Stability of Organic Active Materials for Nonaqueous Redox

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