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Towards Improved Performance of All-Organic Nitroxide Radical Batteries with Ionic Liquids: A Theoretical Perspective Luke Wylie, Kenichi Oyaizu, Amir Karton, Masahiro Yoshizawa-Fujita, and Ekaterina I Izgorodina ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06393 • Publication Date (Web): 01 Feb 2019 Downloaded from http://pubs.acs.org on February 2, 2019
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ACS Sustainable Chemistry & Engineering
Towards Improved Performance of All-Organic Nitroxide Radical Batteries with Ionic Liquids: A Theoretical Perspective Luke Wyliea, Kenichi Oyaizub, Amir Kartonc, Masahiro Yoshizawa-Fujitad, Ekaterina I Izgorodinaa* a
School of Chemistry, Monash University, Wellington Rd, Clayton VIC 3800, Australia Department of Applied Chemistry, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan c School of Molecular Sciences, The University of Western Australia, 35 Stirling Hwy, Crawley WA 6009 d Department of Materials and Life Sciences, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan. b
Corresponding Author:
[email protected] Keywords: Nitroxide Radials, Ionic Liquids, redox potentials, organic radical batteries, explicit solvent calculations, SRS-MP2 ABSTRACT: Nitroxide radicals have previously been successfully used as electrodes in all-organic radical batteries. However, one drawback of these batteries is significantly reduced redox potentials in comparison to widely used lithium-ion batteries, making their energy producing capacity rather small for use as a primary battery. In addition, strong propensity of nitroxide radicals to engage in side reactions with traditional electrolytes based on molecular solvents give rise to a series of undesirable and irreversible by-products, thus significantly reducing the life of nitroxide batteries. Ionic liquids (ILs) have previously demonstrated their ability to reduce the reactivity of radicals through strong intermolecular interactions. In this study, we investigate the use of ionic liquids as electrolytes with the view of increasing redox potentials of nitroxide radicals. A series of imidazolium, phosphonium and pyrrolidinium-based ionic liquids coupled with widely used anions were chosen to predict redox potentials of the 2,2,6,6-tetramethyl-1-piperidinyloxy nitroxide (TEMPO) radical using stateof-the-art quantum chemical calculations using one and two ion pairs to describe ionic liquids. Some ILs showed a significant increase in the redox potential of this radical to reach as much as 5.5 eV compared to the previously measured value of 2.2 eV in aqueous media. In particular, ionic liquids were shown to stabilise the aminoxy anion, the reduced form of the nitroxide radical, which has not been achieved previously in traditional solvents. Although a simple model consisting of one and two ion pairs were used in the current study, these findings clearly demonstrate that ionic liquids have a huge potential in improving redox potentials of nitroxide radicals.
Introduction A large amount of recent research by the Nishide group has been dedicated to studying the potential of using organic radical polymers as a cathode-active material.1-2 Original polymers were based on incorporating the widely-used (2,2,6,6-tetramethylpiperidin-1-yl)oxyl radical (TEMPO) into a methacrylate backbone such as poly(2,2,6,6-tetramethyl-4-piperinidyl-N-oxyl methacrylate) (PTMA). These batteries have shown potential to be applied as a backup battery commercially when paired with a lithium anode.3 There are studies investigating the possibility of pairing nitroxide-based cathodes with organic anodes based on the phenol radical, thus providing a pathway to a fully-organic radical battery.4 The high conductivity of these radical polymers make the exchange of electrons within the radical polymer rapid.5 A current limitation of these fully organic radical batteries lies in the low discharge voltage of 1.3 V involving reduction of the radical poly(galvinoxystyrene) to an aminoxy anion
Scheme 1: Oxidation and reduction reactions in nitroxide radicals. and oxidation of the radical poly(nitronylnitroxystyrene) to an oxoammonium cation (see Scheme 1).6-7 This is somewhat resolved by using a lithium anode, which generated a redox potential of 3.51 V, thus making it competitive with currently used lithium-ion batteries with ~4 V in the discharge potential.3 This, however, is not ideal as the most appealing feature of the nitroxide battery is its po-
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tential to form a cheaper, printable and much more flexible battery that can be easily bent without a sacrifice in its performance. Recently, an exciting development of a novel styrenic nitroxide polymer, poly(5-vinyl-1,1,3,3-tetramethylisoindolin-2-yloxyl), has demonstrated the strategy to improve the redox potential to 3.7 V when combined with the lithium anode, with 6,6-tetramethylpiperidine-4yl-1-oxylvinyl ether (PTVE) also proposed as an alternative to PTMA.3, 8 The main issues in nitroxide organic batteries lie in improving the energy density of nitroxide functional groups in the polymer backbone and suppressing side reactions occurring in traditional electrolytes, usually promoting their slow decomposition.9 In particular, oxygen-based radicals are very reactive undergoing hydrogen atom transfer in traditional molecular solvents such as water.10-11 The quenching of nitroxide polymers makes radical batteries irreversible and non-rechargeable, thus severely limiting their practical use. Individual oxidation and reduction reactions of nitroxide radicals (shown in Scheme 1) have been extensively studied both experimentally and computationally to analyze the effect of chemical modifications of the radical as well as the nature of electrolytes.12,13 Theoretical oxidation potentials of TEMPO were calculated at G3(MP2)-RAD and were found to be 6.91 eV in vacuum and 0.76 eV in water accounted for through an implicit polarizable continuum solvent model.13 The reduction potential did not undergo such a drastic change. In vacuum, the potential was calculated to be 0.56 eV and in implicit water -1.48 eV. This work highlighted how the effect of water led to a significant decline of the redox potential of TEMPO from 6.15 eV in vacuum to 2.24 eV in water. The predicted potentials in implicit water agreed well with experimental data. It was also identified that for some chemical modifications of TEMPO the reduction potential was challenging to measure due to side reactions involving protonation and nucleophilic attack of the formed anion.12 Therefore, the difficulty in stabilising the anionic (i.e. reduced) form of the TEMPO radical is considered the key in improving the life cycling of nitroxide batteries. Ionic liquids (ILs) have demonstrated their potential as electrolytes in a variety of electrochemical applications.14 In previous theoretical studies, single ions common in ILs were shown to be able to interact strongly with a variety of nitroxide radicals.15 Recently, our group have identified that a nitroxide radical can interact strongly (up to -86 kJ mol-1) with ionic liquid ions.16 This finding has been supported experimentally with EPR spectra of the 3-carboxy2,2,5,5-tetramethyl-pyrroline-1-oxylnitroxide radical in ionic liquid media as well as previous studies using the TEMPO radical.17 The EPR analysis demonstrated that there was a significant degree of rotational hindrance of the radical caused by ionic liquids even at elevated temperature of 80 °C, with the rotational hindrance constant strongly correlating with the interaction energy between the radical and ionic liquid ions. The cations were found to play a critical role in the increased stabilization of nitroxide radicals. Subsequent quantum chemical calculations carried out for studying interaction between ionic
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liquid ions and carbon-based radicals also showed a very promising result in the use of ionic liquids to control radical’s reactivity and thus their propagation rate in free radical polymerisation.18 The idea of the electric field to change electroactivity of nitroxide radicals has been beautifully demonstrated through experimental work by Ciampi et al., further supported by ab initio calculations.19 In particular, the aspect of the electrostatic field was suggested to be significantly overlooked in the past, especially when radical molecules are tethered to charged surfaces.20-21 These findings clearly showcase that the electric field has a huge capacity in rendering the stability of radicals, leading to efficient electrocatalysis. Ionic liquids are also known to generate their stabilising effects through charges, thus supporting the hypothesis put forward in this paper for using ionic liquids to improve redox potentials of nitroxide radicals. Preliminary experimental studies also showed promising results in using ionic liquids as electrolytes in nitroxide batteries. For example, the used of N-butyl-Nmethylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C4mpyr][NTf2]) and 1-ethyl-3-methylimimdazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]) were used in combination with the PTMA nitroxide radical polymer as electrode-active material and a lithium anode with a varying degree of the LiNTf2 salt present.22-23 This work has demonstrated very long cycle life of the nitroxide batteries, with the performance improving with increasing temperature, predominantly due to decreased viscosity of ionic liquids. In this study, we investigate the ability of ionic liquids to increase redox potentials of the nitroxide radical, TEMPO, through the dispersion-driven stabilisation of ionic liquid ions. Redox potentials are predicted via explicit solvent calculations, thus accounting for any specific interactions occurring between the radical (and its oxidised/reduced species) and ionic liquid ions. The effect of the ions on the resulting redox potentials is analysed through interaction energies (EINT)and charge distributions. Strategies for further improving redox potentials of nitroxides are also given.
Theoretical Methodology Geometry optimizations of the TEMPO radical in the presence of ionic liquid ions (be it a single ion pair or two ion pairs) were performed with both M06-2X24 and ωB97XD25-27 using Dunning’s cc-pVDZ basis set28. For each radical-ionic liquid combination, two different configurations were studied (see Figure 1). All combinations of the TEMPO radical and single ion pairs of ionic liquid ions in Chart 1 were considered. These configurations were selected based on the lower energy structures found in our previous study on a model nitroxide radical interacting with a series of ionic liquids.16 Ionic liquid ions used in the current study are shown in Chart 1. A Conductor-like Polarizable continuum model (CPCM)29 was used for geometry optimization and ethanol as solvent to model stabilizing effects of the medium. Our previous studies showed that the use of
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Configuration 1
Configuration 2
be discussed further in the text. The radical-ionic liquid structures were used to generate initial structures of oxoammonium cations (further referred to as cations) and aminoxy anions (further referred to as anions). For both cationic and anionic structures the unrestricted HartreeFock procedure was applied during optimization. Both calculated oxidation and reduction potentials were referenced to the SHE experimental value of 4.44 eV.33 Redox potentials were calculated as the Gibbs free energy difference between the adiabatic oxidation potential (abbreviated as ∆Gox) and the adiabatic reduction potential (abbreviated as ∆Gred): 𝑅𝑒𝑑𝑜𝑥 𝑃𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 = ∆𝐺01 − ∆𝐺345
Figure 1: Examples of configurations studied for pyrrolidinium and imidazolium ionic liquids.
(1)
Within this approach the inner shell (nitroxide radical, oxoammonium cation or aminoxy anion) was calculated with the G4 composite method,34 whereas the outer shell (the whole system) was described with the SRS-MP2 method using the cc-pVTZ basis set.35-36 Further in the text this combination is referred to as G4/SRS-MP2. For the radical systems, the restricted open-shell HartreeFock procedure was used in G4 and SRS-MP2 calculations. All electronic energies for each level of theory were recalculated to Gibbs Free energies using the standard physical chemistry formulae at room temperature.37-38 To test the effect of level of theory on geometry optimizations, improved electronic energies and Gibbs free energies were calculated for both M06-2X and ωB97XD optimized structures. For each radical/anion/cation-ionic liquid combination and each DFT optimized geometry these Gibbs Free energies were Boltzmann averaged. 9BB43 ;
