Cycling Non-Aqueous Lithium-Air Batteries with Dimethyl Sulfoxide and Sulfolane Co-Solvent Evaluating Influence of Sulfolane on Cell Chemistry

https://doi.org/10.1595/205651318X15233499272318 Johnson Matthey Technol. Rev., 2018, 62, (3), 332–340

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Cycling Non-Aqueous Lithium-Air Batteries with Dimethyl Sulfoxide and Sulfolane Co-Solvent Evaluating influence of sulfolane on cell chemistry

Gunwoo Kim electrolyte leads to formation of a peroxide- Department of Chemistry, University hydroxide mixture as discharge products of Cambridge, Lensfield Road, Cambridge, and the removal of both LiOH and lithium

CB2 1EW, UK; Cambridge Graphene Centre, peroxide (Li2O2) on charging from 3.2–3.6 V University of Cambridge, Cambridge, (vs. Li+/Li) is observed. In the presence of

CB3 0FA, UK sulfolane as co-solvent, a mixture of Li2O2 and LiOH is formed as major discharge products with Tao Liu, Israel Temprano slightly more LiOH formation than in the absence Department of Chemistry, University of of sulfolane. The presence of sulfolane has also Cambridge, Lensfield Road, Cambridge, CB2 significant effects on the charging behaviour, – 1EW, UK exhibiting a clearer 3 e /O2 oxygen evolution reaction profile during the entire charging process. Enrico A. Petrucco, Nathan Barrow This work provides insights into understanding the Johnson Matthey, Blounts Court Road, Sonning effects of the primary solvent on promoting LiOH Common, Reading, RG4 9NH, UK formation and decomposition in lithium iodide

(LiI) mediated non-aqueous Li-O2 batteries. Clare P. Grey* Department of Chemistry, University of 1. Introduction Cambridge, Lensfield Road, Cambridge,

CB2 1EW, UK The non-aqueous Li-O2 battery has drawn considerable scientific attention due to its higher *Email: [email protected] theoretical energy density compared to the values achieved by conventional Li-ion batteries (1–3). The

successful operation of a Li-O2 battery necessitates Despite considerable research efforts, finding the reversible formation and decomposition of a a chemically stable electrolyte mixture in the discharge product, typically Li2O2 (4–6). However, presence of reduced oxygen species remains a this process is often limited by the considerable great challenge. Previously, dimethyl sulfoxide parasitic reactions caused by highly reactive (DMSO) and sulfolane (tetramethylene sulfone reduced oxygen species, reaction intermediates

(TMS))-based electrolytes were reported (such as lithium superoxide (LiO2)) and the final to be stable for lithium air (Li-O2) battery product Li2O2 (7). These undesired reactions are applications. Recently lithium hydroxide (LiOH) accelerated at high charge overpotentials (8, 9) based chemistries have been demonstrated to and addressing such a fundamental issue remains involve supressed side reactions in water-added a major challenge hindering its commercial ether- and DMSO-based electrolytes. Herein, we implementation. One promising strategy to investigate the impact of DMSO-based electrolyte address this issue is the formation of chemically and sulfolane co-solvent on cell chemistry in the more stable discharge products, so that fewer presence of water. We found that DMSO-based side-reactions occur. Recently, LiOH formation and

decomposition has been proposed as an alternative Li-O2 batteries. A range of ex situ and operando mechanism to cycle non-aqueous Li-O2 batteries techniques were used to evaluate the influence using LiI as a redox mediator, in a water-added of sulfolane on cell chemistry, and especially ether-based electrolyte (10), and with ruthenium whether it promotes dominant and reversible LiOH catalysed cells with water-added DMSO-based chemistry. electrolyte (11). The search for chemically stable electrolytes 2. Experimental against reactive reduced oxygen species with the appropriate physicochemical properties is another Electrospun carbonised polyacrylonitrile (C-PAN) crucial research topic in this field. In this regard, electrodes were prepared using a standard recipe for a wide range of solvents such as carbonates, preparation of carbon nanofibers with an electrode ethers and sulfones have been investigated so diameter of 18 mm and the typical carbon loading far (7, 12, 13). In particular, sulfur-containing around 1.2 mg cm–2. Polyacrylonitrile (Sigma- solvents such as DMSO and sulfolane have shown Aldrich, average Mw = 150 k) was dissolved at to be promising candidates for Li-O2 batteries 7.5 wt% in dimethylformamide (DMF) and because they possess high oxidation potentials electrospun using a custom-built device onto (4.8 V and 5.6 V vs. Li+/Li, respectively), and high aluminium foil wrapped on a rotating drum boiling points (189°C and 285°C, respectively) at >1 kV cm–1 field strength. The resulting (14–16). Although initial studies have reported electrospun mat was dried in a vacuum oven at good reversibility of cells with either DMSO 50°C for 12 h, then stabilised by heating in air at or sulfolane-based electrolytes via formation 5 K min–1 up to 300°C and held for 1 h. Sections and removal of Li2O2, various issues related to were cut to size, allowing for shrinkage, and –1 cell performance were reported in later studies carbonised in flowing N2 by heating 10 K min to (13, 17). In the case of DMSO, its electrochemical 1200°C and held for 2 h. The resulting electrodes instability with Li metal anodes has been reported were dried at 110°C under vacuum for 20 h in a as a major issue. It has been suggested that Büchi oven and stored in a dry argon glove box for the addition of N-butyl-N-methylpyrrolidinium further use. bis(trifluoromethanesulfonyl) imide (Pyr14TFSI) Electrodes were assembled into cells for testing ionic liquid can mitigate this issue (18) through the in an argon glove box using EL-CELL differential formation of a stable solid electrolyte interphase electrochemical mass spectrometry (DEMS) (SEI) via TFSI– anion decompositions (19) as well hardware with 1.55 mm EL-CELL glass fibre as lowering the charging overpotential by stabilising separator, lithium metal anode (Alfa Aesar, 99.9% superoxide species (20). Similarly, sulfolane-based metals basis) and 360 µl of electrolyte. The electrolytes have shown to promote the reversible electrolyte used is a mixture of 5000 ppm water, formation and removal of Li2O2 but capacity 0.9 M Pyr14TFSI, 0.7 M LiTFSI and 0.05 M LiI in fading was observed due to the accumulation of DMSO solvent and a 1:1 DMSO:sulfolane mole ratio side-reaction products such as LiOH and lithium for the sulfolane-containing electrolyte. Added carbonate (Li2CO3) (13). water content was carefully chosen on the basis Early chemistries reported for DMSO-based of recent studies (10, 11) and the water content electrolytes were equivocally described as of 5000 ppm is sufficient to promote dominant promoting Li2O2 with significant amounts of side- LiOH formation up to the discharge capacity of reactions (17, 21). In order to increase stability 5.35 mAh. Cell temperature was controlled at the formation of LiOH as discharge product has 25°C and all cell testing was potential limited at been attempted with DMSO-based electrolytes. 2.2–3.7 V. Exhaust gas was continuously analysed

Ru-catalysed Li-O2 cells cycled with LiTFSI/DMSO in using Stanford Research Systems UGA/RGA200 the presence of added water exhibited a dominant with a heated capillary and channel electron LiOH chemistry with supressed side-reactions, multiplier (CEM) gain 200. Cell purge gas flow but the charging process undergoes dimethyl rate was 5 ml min–1 at 1.5 bar with cell charge sulfone (DMSO2) accumulation rather than oxygen performed in pure argon and cell discharge in evolution reaction (11). 20% oxygen in argon. Mass signal response was In this report, we investigate the effect of calibrated against standard concentrations to sulfolane as co-solvent in DMSO based electrolytes confirm linearity while background levels and drift containing LiI as redox mediator, LiTFSI salt with were subtracted using a linear fit before and after water and Pyr14TFSI as additives in non-aqueous the target dataset.

X-ray diffraction (XRD) measurements were carried Solid-state NMR data were acquired at 11.7 T on TM out using a Panalytical Empyrean with Cu Kα1 radiation a Bruker Avance III HD spectrometer using a (λ = 1.5406 Å). Scanning electron microscopy 2.5 mm HX probehead. A rotor synchronised Hahn- (SEM) images were recorded with a Hitachi S-5500 echo pulse sequence was used to acquire 1H and 7Li in-lens field emission electron microscope. Post- magic angle spinning (MAS) spectra with a spinning mortem electrode characterisations were performed speed of 30 kHz, with recycle delays of 150 s and with samples to avoid air exposure during transfer 20 s for 1H and 7Li, respectively. Radiofrequency (RF) and data acquisition. In the case of SEM, electrodes field strength was 100 kHz and either 256 transients were exposed to air for a maximum of 20 s prior for 1H or 128 transients for 7Li were acquired. to insertion into the high vacuum chamber. For 1H and 7Li chemical shifts were externally referenced nuclear magnetic resonance (NMR), electrodes to solid adamantane at 1.87 and Li2CO3 at 0 ppm, were scrapped and packed in an argon glovebox. respectively.

(a) (b) Capacity, mAh Capacity, mAh

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0 0.1 0.2 0.3 0.4 0.5 3.8 3.8 3.6 3.6 3.4 3.4 Potential, V 3.2 3.2 3.0 3.0 Potential, V Potential, 2.8 2.8

2.6 2.6

2.4 2.4

Discharge Charge LiOH

Li2O2

30 35 40 45 50 55 60 65 70 75 80 30 35 40 45 50 55 60 65 70 75 80

2θ, degrees 2θ, degrees

Fig. 1. Electrochemical first discharge and charge profiles of Li-O2 batteries using C-PAN electrodes cycled with 5000 ppm water-added 0.9 M Pyr14TFSI/0.7 M LiTFSI/0.05 M LiI/DMSO: (a) with; and (b) without sulfolane co-solvent, and their corresponding XRD patterns of cycled C-PAN electrodes, shown as (c) and (d), –2 –1 respectively. The cycling rates are all 0.1 mA cm (160 mA gc ). The XRD patterns of reference LiOH and Li2O2 compounds (blue and green traces and also shown as blue triangles and green diamonds on the top of the XRD patterns, respectively) are also shown for comparison

3. Results and Discussion cells DMSO or sulfolane-based electrolyte usually exceed 4.0 V (16, 22). Figures 1(a) and 1(b) show the galvanostatic The XRD patterns of electrodes after the discharge and charge profiles of the first cycle of first discharge in the presence and absence of

Li-O2 cells using the electrolyte 0.9 M Pyr14TFSI/ sulfolane are shown in Figures 1(c) and 1(d), 0.7 M LiTFSI/0.05 M LiI/DMSO with 5000 ppm water respectively. Both cases indicate the formation of added in the presence and absence of a sulfolane a mixture of LiOH and Li2O2 as discharge product. co-solvent, respectively. In both cases a similar Similarly, SEM images (Figures 2(a) and 2(c)) discharge plateau was observed at around 2.7 V, of the corresponding discharged electrodes reveal although slightly lower (2.6 V) for the sulfolane two distinct morphologies, suggesting a mixture containing cell. The charge process was noticeably of discharge products. As previously reported, different; the cell without sulfolane charged in a distinguishing LiOH and Li2O2 mixtures by single plateau from 3.4 V to 3.6 V, whereas the morphologies is possible if the grown particle size sulfolane-added cell showed a more sloped charge is large enough; electrochemically produced LiOH profile, starting at 3.2 V and finishing at 3.7 V. The typically presents various morphologies, including observed oxygen reduction voltages are consistent cones, discs, large sheets and flowers (10, 23), with the values generally reported in the literature whereas Li2O2 typically displays toroidal and (13, 22) as well as the charging voltages with that of platelet shapes (5). After charging, XRD patterns LiI-mediated oxygen evolution reaction, essentially in both cases (Figures 2(c) and 2(d)) show that – – + the redox potential of I /I3 (3.5 V versus Li/Li ) the intensity of reflections corresponding to LiOH in DMSO-based electrolytes. In the absence of and Li2O2 decreases significantly, more distinctly in the LiI mediator, the cell charging voltages in the the sulfolane-containing case, indicating that most

Fig. 2. SEM images of cycled C-PAN electrodes from Li-O2 batteries using an electrolyte of 0.9 M Pyr14TFSI/0.7 M LiTFSI/0.05M LiI/DMSO with 5000 ppm water: (a) and (b) with; and (c) and (d) without sulfolane. The states of charge are indicated and all scale bars represent 2 μm. The characteristic morphologies of LiOH and Li2O2 are highlighted by red and green circles, respectively

335 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X15233499272318 Johnson Matthey Technol. Rev., 2018, 62, (3) of the crystalline discharge products are removed. a minor one at 8.3 ppm, assigned to formate The SEM images of the charged electrodes indicate (10, 24). Other distinct resonances are present that some solid residues remain adsorbed on in the range 1–4 ppm, with the resonances at the surface of the carbon electrode as particles 2.5 ppm and 1–2 ppm attributed to DMSO and and thin films. In the sulfolane-absent case Pyr14TFSI of residual electrolyte, in agreement (Figure 2(b)), most of the discharge products with the literature (25). Despite rinsing the appear to be removed from the carbon fibres of electrodes after cell disassembly with acetonitrile C-PAN electrodes, whereas sulfolane-containing and overnight drying under vacuum, the highly cells (Figure 2(d)) display electrode surfaces viscous nature of the ionic liquid-containing more noticeably covered by particles and thin films electrolyte inevitably results in severe adsorption + after charging. of Pyr14 cations onto the C-PAN electrode after To further characterise the chemical nature cycling, showing significant contributions on the of reaction products formed during cell cycling, 1H NMR spectra. Furthermore, decomposition of 1 7 + solid-state H and Li NMR measurements Pyr14 cations by superoxide attack is known to (Figure 3) were performed on the cycled electrodes generate various 1H resonances in a 4–6 ppm characterised by XRD and SEM. In both sulfolane- range (highlighted in magenta in Figure 3(a)) absent and present cases, the 7Li NMR spectra (25). After charging, the 7Li NMR spectra of of discharged electrodes (red and orange traces, both cases (Figure 3(b)) are consistent with respectively, in Figure 3(b)) show a single broad the removal of Li-containing species from the resonance centred at 0.7 ppm, consistent with the electrodes, whereas the 1H NMR spectrum shows presence of a mixture of Li2O2 (0.4 ppm) and LiOH that the intensity of the LiOH peak is significantly (1.1 ppm). The corresponding discharged 1H NMR reduced. These observations are consistent with spectra exhibit a major resonance at –1.5 ppm, the formation and removal of a mixture of LiOH assigned to lithium hydroxide, more dominant and Li2O2 from the electrodes during cell discharge and intense in cells containing sulfolane, and and charge, respectively. The comparison of the

(a) (b) LiOH Ionic liquid

DMSO 0.7 ppm

Formate DMSO/sulfolane/Lil/5k H O Discharged 2 –4 ppm

Charged × 64

Discharged DMSO/Lil/5k H2O Charged

10 0 –10 10 0 –10 δ(1H), ppm δ(7Li), ppm Fig. 3. Solid-state: (a) 1H; and (b) 7Li MAS NMR spectra of the cycled C-PAN electrodes from cells using 5000 ppm water-added 0.9 M Pyr14TFSI/0.7 M LiTFSI/0.05 M LiI/DMSO electrolyte with and without sulfolane 1 1 + co-solvent. The magenta-shaded areas on the H spectra indicate the H shifts of Pyr14 ions and their related decomposed chemical species and residual water. A broad 1H resonance at –4 ppm is assigned to protons in the C-PAN electrode

1 – – – charged H NMR spectra of both sulfolane present 3I → I3 + 2e (i) and absent cases suggest a higher extent of – + – parasitic reactions occurring in the former case, Li2O2 + I3 → 2Li + 3I + O2 ↑ (ii) in good agreement with the SEM data. – – – In order to investigate the charging mechanism 6I → 2I3 + 4e (iii) involved in removing the LiOH and Li2O2 mixture, – + – operando DEMS experiments were performed. 4LiOH + 2I3 → 4Li + 6I + 2H2O + O2 ↑ (iv) The Faradaic efficiency profiles of the oxygen evolution reaction (OER) during galvanostatic Beyond the midpoint of charge capacity, the DEMS charging of LiI-mediated cells with and without profiles in both cases are noticeably different; in sulfolane are shown in Figures 4 and 5, the sulfolane-absent case, the oxygen evolution respectively. In both cases, oxygen evolution rate is reduced continuously as the cell voltage is observed from the beginning of charge at rises above 3.5 V. This suggests that the charging – approximately 3.4 V with an e /O2 molar ratio process deviates from the aforementioned desirable at around three for the first half of the charge oxidation reactions and side reactions involving capacity. This oxygen evolution rate is consistent less or no O2 evolution pathways may occur, such with the decomposition of equivalent amounts as the formation of iodo-oxygen species (26, 27), of Li2O2 through a two-electron and LiOH through and dimethyl sulfone (DMSO2) (11). In the case a four-electron mechanism, respectively of cells containing sulfolane the oxygen evolution (Equations (i)–(iv)): rate is maintained at an overall Faradaic efficiency

0.0 0.1 0.2 0.3 0.4 0.0 0.1 0.2 0.3 0.4 4.0 4.0 3.9 3.9 3.8 3.8 E, V + 3.7 3.7

3.6 3.6 vs . Li/Li

vs . Li/Li 3.5 3.5 3.4

3.4 + E, V 3.3 3.3 3.2 3.2 3.1 3.1 CO 0 2 evolution rate, MS signal 2 2.5×10–10 1 / O

- 2 2.0×10–10

3 1.5×10–10 4 1.0×10–10

evolution rate, e rate, evolution 5 2 O 6 5.0×10–11

0.0 0.1 0.2 0.3 0.4 0.0 0.1 0.2 0.3 0.4

Capacity, mAh Capacity, mAh Fig. 4. Operando differential electrochemical mass spectrometry measurements on charging the Li-O2 batteries on the first (left) and second (right) cycle using a 5000 ppm water-added 0.9 M Pyr14TFSI/0.7 M LiTFSI/0.05 M LiI/DMSO electrolyte with sulfolane co-solvent. The left and right axes indicate the detected oxygen (normalised) and CO2 (not normalised) evolution rates, shown as blue and red traces, respectively

0.0 0.1 0.2 0.3 0.4 0.5 0.0 0.1 0.2 0.3 0.4 0.5 4.0 4.0 3.9 3.9

3.8 3.8 E, V + 3.7 3.7 vs . Li/Li 3.6 3.6

vs . Li/Li 3.5 3.5

3.4 3.4 + E, V 3.3 3.3 3.2 3.2 3.1 3.1

0 CO

2 –10 3.0×10 2 evolution rate, MS signal

/ O 1 – –10 2 2.5×10

3 2.0×10–10 4 1.5×10–10

evolution rate, e rate, evolution 5 2 O 1.0×10–10 6

0.0 0.1 0.2 0.3 0.4 0.5 0.0 0.1 0.2 0.3 0.4 0.5 Capacity, mAh Capacity, mAh

Fig. 5. As Fig. 4., without sulfolane co-solvent (see caption for details)

– of 3 e /O2 with a slight deviation until the end of measurements. The observation of residual charge, 3.7 V. In both cases, the DEMS profiles discharge products after a full charge confirms the are reproducible for the second charge with no effect of parasitic reactions to the faradaic efficiency. significant CO2 evolution detected. The observation Further work is required to optimise the more stable – – of a 3 e /O2 OER process rather than a 2 e /O2 LiOH chemistry, and to understand the associated expected for Li2O2 decomposition, alongside OER mechanisms for the successful commercial the absence of CO2 evolution and consistency in development of rechargeable Li-O2 batteries. charging DEMS profiles might be associated with the partial removal of LiOH during the OER in cells Acknowledgements containing sulfolane as co-solvent but further work is required to understand the charging mechanisms. Gunwoo Kim and Clare P. Grey thank EUHorizon 2020 GrapheneCore1-No.696656 for research 4. Conclusions funding.

In this report, we have investigated the effect of References sulfolane as co-solvent in Li-O2 cells cycled with

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The Authors

Gunwoo Kim is a research associate in Chemistry at the University of Cambridge, UK, and Cambridge Graphene Centre, UK. He received his PhD in Chemistry at the University of Cambridge and his current research focuses on characterising graphene-based electrodes

and understanding mechanisms in metal-O2 batteries by using various spectroscopic techniques with an emphasis on solid-state NMR spectroscopy.

Tao Liu is a research associate in Chemistry at University of Cambridge, UK, and a Junior Research Fellow of Darwin College Cambridge. He received his PhD in Chemistry at University of Cambridge and his research interests include electrocatalysis and energy storage.

Israel Temprano received his PhD in Chemistry at Université Laval, Canada, and then moved to the University of Liverpool, UK, as Marie Curie Research Fellow before joining the University of Cambridge, UK, as postdoctoral research associate. His current research focuses on the development of in situ characterisation tools for the study of

Li-O2 batteries.

Enrico A. Petrucco is currently a Senior Scientist in the Battery Materials Research department at Johnson Matthey, Sonning Common, UK. He graduated with a BSc in Physics from Rensselaer Polytechnic Institute, New York, USA, in 2004 and has 18 years’ experience in the research and development of electrochemical materials. His current research interests include oxygen evolution catalysis, graphene and energy storage and conversion materials. He currently applies multidisciplinary studies to investigate next generation battery technologies.

Nathan Barrow is currently a Principal Scientist in the Advanced Characterisation department at Johnson Matthey, Sonning Common, UK. He graduated with an MPhys in 2006 from the University of Warwick, UK, where he remained to gain a PhD in solid- state NMR. In 2010 Barrow was a Knowledge Transfer Partnership associate between the University of Warwick and Johnson Matthey, helping to install and run a solid-state NMR service. His current research focuses on applying advanced characterisation to materials such as zeolites, alumina, glasses and batteries.

Clare P. Grey is the Geoffrey Moorhouse-Gibson Professor of Chemistry at Cambridge University, UK, a Fellow of Pembroke College Cambridge and a Fellow of the Royal Society. She received a BA and DPhil (1991) in Chemistry from the University of Oxford, UK. After postdoctoral fellowships in The Netherlands and at DuPont CR&D in Wilmington, DE, USA, she joined the faculty at Stony Brook University (SBU), New York, USA, as an Assistant (1994), Associate (1997) and then Full Professor (2001–2015). She moved to Cambridge in 2009, maintaining an adjunct position at SBU. Her current research interests include the use of solid-state NMR and diffraction-based methods to determine structure-function relationships in materials for energy storage (batteries and supercapacitors), conversion (fuel cells) and carbon capture.