Enhanced Lithium Oxygen Battery Using a Glyme Electrolyte and

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An Enhanced Lithium Oxygen Battery using a Glyme Electrolyte and Carbon Nanotubes Lorenzo Carbone, Paolo Tomislav Moro, Mallory Gobet, Stephen Munoz, Matthew Devany, Steven G. Greenbaum, and Jusef Hassoun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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ACS Applied Materials & Interfaces

An Enhanced Lithium Oxygen Battery using a Glyme Electrolyte and Carbon Nanotubes Lorenzo Carbone a, Paolo Tomislav Moro b, Mallory Gobet c, Stephen Munoz,c,d Matthew Devany e, Steven G. Greenbaum c,*, and Jusef Hassoun b,f* a

b

Chemistry Department, Sapienza University of Rome, Piazzale Aldo Moro, 5, 00185, Rome, Italy Department of Chemical and Pharmaceutical Sciences, University of Ferrara, Via Fossato di

Mortara, 44121, Ferrara, Italy c

Department of Physics & Astronomy, Hunter College of the City University of New York, New

York, New York 10065, United States d

Ph.D. Program in Physics, City University of New York, New York, NY 10016 United States

e

Department of Chemistry and Biochemistry, Hunter College of the City University of New York,

New York, New York 10065, United States f

National Interuniversity Consortium of Materials Science and Technology (INSTM) University of

Ferrara Research Unit, University of Ferrara, Via Fossato di Mortara, 17, 44121, Ferrara, Italy

*

Corresponding Authors: [email protected], [email protected]

Keywords Lithium Oxygen; MWCNTs; Glyme Electrolyte; Reaction Mechanism; Long Cycle Life Abstract The lithium oxygen battery has a theoretical energy density potentially meeting the challenging requirements of electric vehicles. However, safety concerns and short lifespan hinder its application in practical systems. In this work we show a cell configuration, including a multi-walled carbon nanotubes electrode (MWCNTs) and a lowly flammable glyme electrolyte, capable of hundreds of cycles without signs of decay. Nuclear magnetic resonance and electrochemical tests ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

confirm the suitability of the electrolyte in a practical battery, while morphological and structural aspects revealed by electron microscopy and X-ray diffraction demonstrate the reversible formation and dissolution of lithium peroxide during the electrochemical process. The enhanced cycle life of the cell and the high safety of the electrolyte suggest the lithium oxygen battery herein reported as a viable system for the next generation of high-energy applications.

Introduction Recent advances in lithium ion batteries and vehicle manufacturing technology have increased the driving range of full electric cars (EVs) to 200 km per charge 1, while a new commercial EV from Tesla, Inc. claims a range of 350 km.2 The latter number appears presently to be the limit ascribed to the state-of-the-art lithium ion battery, which is composed of a carbon-based anode, lithium transition-metal oxide cathode, and organic carbonate-based electrolyte.3 Further increase in the driving range may be achieved only by switching to a completely different chemistry, such as that associated with lithium sulfur or lithium oxygen based batteries.4,5 Lithium sulfur technology, with a theoretical energy density of about 3500 Wh kg-1, is a strong candidate to reach this challenging goal,6 while the lithium oxygen battery is indicated to be even more energetic.7 However, the Li/O2 cell is still far from application due to several problems affecting its safety and cycle life, and an electrochemical process still not fully clarified.8,9 The relevant safety issues are mainly associated with the use of lithium metal in the cell, and reactivity and flammability of the organic electrolytes commonly employed in the lithium ion battery.10 Furthermore, cycle life and reaction mechanism are intimately connected, and strongly associated with the electrode/electrolyte interface,11 electrode nature,12 and the operating condition of the cell including the cell geometry, the use of new separators and protections at the lithium metal side.13 The lithium oxygen electrochemical process involves the formation of lithium peroxide (Li2O2), reactive intermediates such as lithium superoxide (LiO2), and radicals,14,15 with a stability mainly determined by the electrolyte, the cathode support16,17 for the electrochemical reaction and the ACS Paragon Plus Environment

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current rate18 applied to the lithium oxygen cell.19 In particular, different types of aprotic organic solvents, such as ethers or glyme, sulfoxides, phosphates, nitriles, and ionic liquids actually affect the formation of discharge products in Li/O2 batteries.20 Among the various solvents, literature paper focusing on XRD, XPS and GC/MS measurements, indicated that the glyme leads to significant formation of Li2O220, while other paper suggested the addition of ionic liquid to the glyme for improving the electrolyte conductivity and enhancing the kinetics of both the O2 reduction and oxidation.21 The dominant side reactions at the oxygen electrode have been indicated to involve electrolyte decomposition to form mainly Li2CO3 accompanied by lithium carboxylates depending on the interplay between the electrode and the electrolyte.22 Furthermore, a fundamental study of the influence of solvents on the oxygen reduction reaction (ORR) in non-aqueous electrolytes in four different solvents, including dimethyl sulfoxide (DMSO), acetonitrile (MeCN), dimethoxyethane (DME), and tetraethylene glycol dimethyl ether (TEGDME), differing by the donor numbers (DN), determined that the solvent and the supporting electrolyte cations in the solution act in concert to influence the nature of reduction products and reversibility.23 The study concluded that high DN solvents provide increased stability for the complex [Li+(solvent)n---O2-], due to the modulated Lewis acidity of the hard acid, leading to distinct O2/O2- reversible couple, while low DN solvents lead to fast O2- decomposition or electrochemical reduction to O22-.23 The formation of this strongly nucleophilic specie decomposes the common organic carbonate-based electrolytes (characterized by the electrophilic carbonyl bond) during the discharge processes causing fast capacity fading until cell failure,24 while end-capped glyme, based on ether bond with chemical formula CH3–(OCH2CH2)n–OCH3 appear more suitable with promising properties in terms of stability, ionic conductivity,25 and solvation ability for lithium salt dissociation.26 The glyme electrolyte stability may be further improved by including ionic liquids20 or suitable additives23 in the electrolyte for enhancing the solid electrolyte interface (SEI) characteristics. Ether- and glymebased electrolytes,27 and high surface area carbons16 at the cathode side have demonstrated the most promising characteristics for efficient lithium oxygen batteries, with ACS Paragon Plus Environment


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satisfactory cycle life, capacity higher than 500 mAh g-1 relative to the carbon mass, and an operating voltage of about 2.8V during the discharge process.28 In this work we describe an enhanced system, formed by the combination of a safe electrolyte solution of lithium bistrifluoromethanesulfonimidate (CF3SO2NLiSO2CF3, LiTFSI) salt in tetra ethylene glycol dimethyl ether solvent (TEGDME), and multi-walled carbon nanotubes (MWCNTs) as the reaction host for the lithium oxygen reaction. The cell shows an outstanding cycle life, extended up to 250 cycles, due to the optimal characteristics of the electrolyte as revealed by NMR and electrochemistry, in particular the reversible formation and dissolution of Li2O2 clearly detected by XRD, SEM and TEM on the MWCNTs surface. Although further efforts are required for practical employment, the cell herein studied is proposed as a valid candidate for powering next-generation EVs with extended driving range.

Experimental Section Electrolyte preparation The electrolyte was prepared by mixing Tetra ethylene glycol dimethyl ether (CH3O(CH2CH2O)4CH3)

following

named

by

TEGDME

and

lithium

bistrifluoromethanesulfonimidate (CF3SO2NLiSO2CF3), herein named LiTFSI. Both chemicals purchased from Sigma Aldrich. Molecular sieves of 3Å from Sigma Aldrich were dried under vacuum at 250°C for 48 hours and used to reduce the water content of the TEGDME below 10 ppm. The water content was measured by Karl Fischer titration instrument (831 Karl Fisher Coulometer, Metrohm). Before mixing the TEGDME and the lithium salt in 1 mol kg-1 ratio, the LiTFSI was dried under vacuum at 80°C for 24 hours. Electrolyte characterization The thermal properties were evaluated by differential scanning calorimetry (DSC) using the Mattler Toledo DSC821 instrument. The TEGDME-LiTFSI electrolyte was sealed in aluminum ACS Paragon Plus Environment

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crucible under argon atmosphere before the DSC analysis. The measurement was performed by cooling down from -30°C to -90 °C with a rate of -5°C min-1. The lithium stripping deposition test was performed with constant current rate of 0.1 mA cm-2 and step limit of 1 hour using a symmetrical lithium/lithium 2032 coin-cell and a MACCOR 4000 series Battery Test System. The lithium/electrolyte interface resistance was measured using the latter cell configuration by electrochemical impedance spectroscopy in the frequency range of 0.1MHz - 0.1Hz with a signal amplitude of 10 mV with a VersaSTAT MC Princeton Applied Research-AMETEK potentiostat. The impedance spectra were associated to the R(RQ)nQ equivalent circuit, where R is the resistance and Q a constant phase element (CPE), and analyzed using the nonlinear least square (NLLSQ) fit by Boukamp Software,29,30 only the results with a chi-square χ2 lower than 10-4 were accepted. The voltammetry tests were carried out in Swedglock T-cell with lithium metal as reference and counter electrode and a working electrode formed by depositing super P (Timcal) on Cu or Al support for the cathodic and the anodic scan, respectively. The cyclic voltammetry (CV) for the cathodic region was performed in the potential range 0V - 2V vs. Li/Li+, while the linear sweep voltammetry (LSV) for the anodic region was performed up to 5V vs. Li/Li+, both with a scan rate of 0.1 mV sec-1. The measurements were carried out by VersaSTAT MC Princeton Applied Research-AMETEK potentiostat. The self-diffusion coefficient of the 1H 7Li and

19

F nuclei were measured by a Bruker 400

Avance III NMR spectrometer suppressing the convection effect by PFG double-stimulated echo sequence.31,32 The data were collected every 10°C from 20°C to 80°C with a gradient strength g of 1-45 G cm-1, gradient pulse of 1-2 ms and a diffusion delay ∆ of 100-200 ms. The error on the selfdiffusion coefficient is about 3−5%.31 The lithium transport number (t+) was calculated by equation (1):33 (1)

ACS Paragon Plus Environment


where DLi is the lithium self-diffusion coefficient and DF is the fluorine self-diffusion coefficient of the TFSI- anion. The ionic conductivity of the electrolyte (δNMR) was calculated from the selfdiffusion coefficient by the Nernst-Einstein equation (2):34 (2) where F is the Faraday constant (96485 Coulomb), [C] represents the solution concentration (mol cm-3), R is the ideal-gas constant (8.314472 J K-1 mol-1). Furthermore, the ionic conductivity was measured by impedance spectroscopy (δEIS) in a symmetrical SS/SS 2032-coin cell with a Teflon O-ring separator in the temperature range of 20°C 80°C, using a signal amplitude of 10 mV in the frequency range of 1MHz and 0.1Hz by employing a VersaSTAT MC Princeton Applied Research-AMETEK potentiostat. The conductivity values δEIS and δNMR were combined in equation 3 to obtain the ionic association degree ( ):35 (3)

Cathode preparation and cell assembly Multi walled carbon nanotubes from Sigma Aldrich, following named MWCNTs, were annealed under nitrogen flow at 750°C for 12 hours. The active material (named N2@MWCNTs) was mixed separately with polyvinylidinedifluoride (PVdF, Solvay) binder in a 80:20 mass ratio and dispersed in N-methylpirrolidone (NMP, Aldrich). The slurry was cast on a gas diffusion layer (TGP-H-030 carbon paper, Toray) by Doctor blade. The active material ratio on the current collector was of about 0.8 mg cm-2. The electrode foils were punched in disks, dried overnight at 100°C and finally introduced in argon filled glove box with oxygen and moisture content lower than 1 ppm. Lithium oxygen cells were then assembled by combining lithium metal, the N2@MWCNTs, and the TEGDME-LiTFSI in a top-meshed 2032 coin-cell inserted into a glass tube filled subsequently by pure oxygen.


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Cathode and cell characterization The powders structure was characterized by X ray diffraction using a Bruker instrument equipped with a Cu Kα source and a graphite monochromator, in the 2θ/θ scanning mode. The morphology of the samples was evaluated by SEM (Zeiss EVO 40) with a thermionic electron gun equipped by LaB6 crystal. Lithium oxygen cell was studied by galvanostatic test using voltage cutoff performed with a constant current of 100 mA g-1 in a 1.5V - 4.5V range. Galvanostatic cycling tests were also carried out with a current rate of 100 mA g-1 and limiting the capacity to 500, 1000 and 2000 mAh g-1. The current density and specific capacity was referred to the carbon weight into the electrode, while the surface capacity was determined with respect to the electrode geometric area.

Results and discussions Several studies have focused on the characterization of glyme-based electrolytes using various salts, concentrations, and experimental setups.18,36–38 In this work we shed light on the performance of a TEGDME-LiTFSI electrolyte (1 mol kg-1)39–41 by combining calorimetry, electrochemistry and NMR. The electrolyte concentration adopted in this work (i.e., 1 mol kg-1) has been optimized by our group in a previous comparative study on the role of the lithium salt in the performance of lithium–oxygen battery,39 while the combination of TEGDME and LiTFSI was suggested as efficient electrolyte by our group as well as by other literature works.40,41 A direct injection test, performed in order to evaluate the flammability of the TEGDMELiTFSI electrolyte, reveals the absence of fire evolution upon the direct exposure to the flame prolonged over 20 seconds, thus suggesting the low flammability of the electrolyte under the experimental setup adopted in our work. Despite the test indicates possible use of the electrolyte in a relatively safe lithium oxygen battery, further experiments with a more suitable setup, such as a



direct test in cell or more effective exposure to aggressive conditions, are certainly required to full proof the electrolyte safety level.

Figure 1 Flammability test of the TEGDME-LiTFSI

The DSC plot of Fig. 2A indicates that the electrolyte freezes below -50°C, hence permitting use at relatively low temperature. The lithium stripping/deposition profile in a symmetrical Li cell in Fig. 2B evidences a very low polarization (limited to about 20 mV during the whole test) demonstrating stability of the electrolyte. Further proof of the electrolyte stability is shown in Fig 2C on the lithium/electrolyte interface impedance trend, and, in the inset, the corresponding Nyquist plots, as well as by the linear sweep voltammetry of the Li/electrolyte/SP cell in the anodic region (Fig. 2D), and the CV of the cell in the cathodic region between 0 V and 2 V (Fig. 2D inset). The electrolyte reveals an impedance ranging from 150 to 300 ohm due to solid electrolyte interphase (SEI) film formation,42 and an applicability extending from 0 V to 4.7 V vs. Li/Li+, i.e., a region in which only SEI film formation at about 0.8V,43 reversible insertion of lithium into amorphous carbon at about 1 V, 44,45 and lithium deposition/stripping around 0V 46 can be observed.


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Figure 2. Characteristics of the TEGDME-LiTFSI electrolyte (1 mol kg-1). (A) Differential Scanning Calorimetry (DSC) performed between -30°C and -90°C under argon. (B) Lithium stripping/deposition voltage profile of a symmetrical lithium/lithium cell performed with a constant current of 0.1 mA cm-2 and a step limit of 1 h. (C) Time evolution of the lithium/electrolyte interface resistance, and in inset corresponding Nyquist plots, investigated by impedance spectroscopy in symmetrical lithium with a signal amplitude of 10 mV in a 0.1 MHz - 0.1Hz frequency range. (D) Electrochemical stability window of the electrolyte determined by linear sweep voltammetry (LSV) up to 5V in the anodic region and by cyclic voltammetry (CV) between 0 and 2 V vs. Li+/Li in the cathodic region (inset), using a Swagelok T-cell with lithium as counter and reference electrode and Super P as working electrode, scan rate of 0.1mV s-1. Pulsed-field gradient (PFG) NMR data reported in Fig. 3A show the self-diffusion coefficients of the 1H, 7Li and 19F nuclei (tracking the TEGDME solvent, the lithium cation, and the TFSI anion, respectively) in the temperature range between 20°C and 80°C. The data reveal faster mobility of the solvent than the ions. The NMR data allows the determination of the apparent electrolyte conductivity, which is compared in Fig. 3B with the conductivity determined by



impedance spectroscopy, to find the ionic association degree (Fig. 3C). The lithium transference number (Fig. 3D) between 20°C and 80°C can also be calculated (see experimental section for the details).47,48 Temperature / °C

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Figure 3. Species mobility in the TEGDME-LiTFSI electrolyte (1 mol kg-1). (A) self-diffusion coefficients of the 1H, 7Li and

19

F nuclei carried out by NMR in the temperature range of 20°C -

80°C. (B) Comparison of the conductivity values calculated from the self-diffusion coefficients and determined by electrochemical impedance spectroscopy (EIS). (C) Ionic association degree of the lithium salt determined by combining NMR and EIS data. (D) Lithium transport number determined using the NMR data in the temperature range 20°C - 80°C. All the equations used for the determination of the parameters of the figure are reported in the experimental section. Increasing temperature increases both conductivity and ionic association degree, from 3 10-3 S cm1

to 10-2 S cm-1 (Fig. 3B), and from 0.40 to 0.55 (Fig. 3C), respectively. The apparent lithium

transport number appears only slightly affected by the temperature increase, with a value of about ACS Paragon Plus Environment

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0.45 over the whole temperature range despite the increase in ion association at elevated temperature. It should be mentioned that an apparent transport number close to 0.5 is partly a consequence of ion association.26,49 In summary, the TEGDME-LiTFSI electrolyte shows an ionic conductivity higher than 3 10-3 S cm-1 in the whole range of temperature, stable overvoltage upon cycling of 0.02 V, low lithium/electrolyte interface resistance, and wide electrochemical stability window extended up to 4.5V, which are characteristics comparable to the those of the carbonate electrolytes commonly used in lithium ion battery.50,51 In addition, bonuses such as the relatively low volatility, hence the low flammability,52 and the modest reactivity with the nucleophile products compared to other electrolytes such as those based on DMSO, MeCN, and DME,23-26 make this electrolyte particularly suitable for lithium oxygen battery. Therefore, the solution is subsequently used in a Li/O2 cell employing multi walled carbon nanotubes (MWCNTs) at the cathode side. Prior to use as the electrode support for the Li/O2 cell, the MWCNTs were treated under nitrogen atmosphere at 750 °C (see experimental section) in order to remove possible undesired functional groups.53,54 The XRD patterns of Fig. 4A reveals the graphitic nature of the MWCNTs, and show only minor structural changes due to the thermal treatment. More relevant effects of annealing are observed for the MWCNTs morphology, as revealed by SEM (Fig. 4B, E) and TEM (C, D, F and G) at various magnification. The images, in particular the TEM, suggest that annealing is effective in removing impurities from the nanotubes (Fig. 4F and C), increasing the electrode surface (see also low magnification SEM in Figure S1 in SI), and enabling cavities and free spaces (Fig. 4G and D) which may act as the host and the catalyst for the Li/O2 electrochemical process.22,55



Figure 4. (A) XRD patterns of the bare multi walled carbon nanotubes (MWCNTs) and of the same material after the annealing process under nitrogen atmosphere (N2@MWCNTs). (B, E) SEM images of the bare and the N2-annealed MWCNTs. (C, D, F, G) TEM images at various magnifications of the bare and the N2-annealed MWCNTs.

The TEGDME-LiTFSI electrolyte and the nitrogen annealed MWCNTs electrode (N2@MWCNTs) were combined and studied in a lithium oxygen cell. The cell was fully discharged and charged, the voltage profile collected (Fig. 5A), and the reaction products studied in terms of structure by ex-situ XRD (Fig. 5B) and morphology by ex-situ SEM (Fig. 5C-F). The voltage profile of Fig. 5A shows that the N2@MWCNTs electrode reaches a capacity of about 16 Ah g-1 relative to the carbon mass (a value of about 9 mA cm-2 as referred to the electrode geometric area, see Fig. S2 in SI). It is worth mentioning that the cell was cycled using the voltage cutoff of 1.5V4.5V, which is a cycling mode aimed to detect the full charge discharge products instead of optimizing cell stability, as confirmed by the performance decay over the course of the test (Fig. S3 in the SI). Lithium oxygen cell stability may be achievable only by limiting the discharge capacity during cycling in order to avoid a decrease of electrode conductivity by an excessive Li2O2 accumulation at the electrode surface, as reported by previous works56 and fully confirmed hereafter. The XRD patterns of the N2@MWCNTs electrode collected from the lithium oxygen cells at different states of charge reported in Fig. 5B at the OCV (marked by T1 in Fig. 5A), fully discharged state (marked by T2 in Fig. 5A), and fully charged state (marked by T3 in Fig. 5A), clearly show the formation and complete dissolution of lithium peroxide during the galvanostatic ACS Paragon Plus Environment

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test, without any residual product This is in line with the high efficiency approaching 100 % demonstrated by the galvanostatic test of Fig. 5A. Furthermore, the corresponding SEM images (Fig. 5C-F) show the formation of particles of a size of about 50-100 µm (Fig. 5D, E), and suggest an electrodeposition mechanism of Li2O2 at the N2@MWCNTs surface rather than a solution mechanism involving the formation of the reaction products at the electrode/electrolyte interface, generally observed in cells using ionic liquid electrolytes.57–66 According to the surface mechanism, the nucleation and growth of the Li2O2 crystallites evolve over the sites of the carbon support surface by a direct electrodeposition which is strongly affected by the current rate applied to the lithium oxygen cell leading generally to a micrometric non uniform deposit.57–62 Instead, solution mechanism occurs when the OOR proceeds within the solution at the electrode/electrolyte interface, rather than at the electrode surface, which may cause the precipitation of Li2O2 crystallites when the concentration of this specie exceed the solubility limit of the electrolyte solvent, with consequent formation of small particle with a homogeneous distribution over the electrode.63–66 Therefore, the micrometric particle size of the lithium peroxide, and their distribution at the electrode surface observed in figure 5(D–E) suggests for our cell a surface mechanism. The SEM shown in Fig. 5F evidences a complete dissolution of the products upon full charge, thus further suggesting the reversibility of the electrochemical process.



(E)

Figure 5. (A) Galvanostatic voltage profile of the lithium oxygen battery using the TEGDMELiTFSI (1mol kg-1) electrolyte and the N2@MWCNTs electrode in a 2032 top-meshed coin cell. Current rate of 100 mA g-1. Voltage limit of 1.5V-4.5V. (B) XRD pattern of the cell at different state of the charge, marked by T1 (OCV), T2 (fully discharged), and T3 (fully charged). SEM images of the N2@MWCNTs electrode at the OCV (C), at the discharged ((D and E) and at the charged state (F). The lithium oxygen battery was then cycled using a capacity limited to 500 mAh g-1 relative to carbon mass in order to demonstrate superior cycle life. Fig. 6 shows that the cell under this condition may achieve 250 cycles of continuous charge and discharge without any signs of polarization increase (Fig. 6A) or capacity fading (Fig. 6B). The cell operates at about 2.7 V and, therefore, holds a theoretical energy density of about 1350 Wh kg-1 considering the carbon mass. The energy content of the cell may be increased by raising the capacity limit up to 1000 or even 2000 mAh g-1, as suggested by the voltage profiles reported in Fig. S3 and S4, respectively, in the SI section.66 Taking into account the cell voltage and the delivered capacity, under the various regimes, we can estimate the energy density according to a normalization factors (N) of 1/3, typically considered in lithium-ion battery,67 or a more restrictive factor of 1/5 which may be more


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appropriate for the lithium oxygen battery, in order to include anode and electrolyte masses, and inactive elements weight such as current collectors, and cell case (see Table S1 in SI). The table S1 reveals a minimum value of the energy density of 270 Wh kg-1 for the lithium oxygen cell limited to 500 mAh g-1 considering a normalization factor of 1/5, which approaches the energy density of the lithium ion battery based on LiCoO2 cathode and graphite anode reported for comparison in the same table. Remarkably, the maximum value of the energy density for the Li/O2 battery with capacity limit of 2000 mAh g-1 and considering the less restrictive normalization factor of 1/3 exceeds 1700 Wh kg-1. These performances well suggest the suitability of the lithium oxygen cell studied in this work as a high performance energy storage system. However, we may point out that an excessive capacity limit may lead to a decay of the cell performances due to the electrode insulation, as indeed evidenced by Fig. S3 in SI.



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Figure 6 (A) Galvanostatic voltage profile, and (B) corresponding cycling trend of the lithium oxygen battery using the TEGDME-LiTFSI (1mol kg-1) electrolyte and the N2@MWCNTs electrode in a 2032 top-meshed coin cell. Current rate of 100 mA g-1. Capacity limit capacity 500 mAhg-1. Current density and specific capacity are referred to the carbon weight.


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In summary, we reported new insight on the characteristics of the TEGDME-LiTFSI electrolyte by combining PFG-NMR and electrochemical analyses in order to shed light on very important ion mobility properties of the charge carries within the glyme media, such as self-diffusion coefficients, practical and apparent conductivity, ionic association degree of the lithium salt, and lithium transport number, which are crucial parameters allowing the electrolyte operation in efficient battery. Diffraction patterns, and electron microscopy evidenced in detail the structure and morphology of multi walled carbon nanotubes before and after the annealing process under nitrogen atmosphere, which actually allow the operation of the electrode in lithium oxygen cell with enhanced characteristics. Hence, we have originally shown a lithium oxygen cell using a carbon nanotubes electrode with a cycle life as long as 250 cycles. Conclusion The cycling performance and stability of the lithium oxygen cell may be ascribed to the nature of the electrolyte due to the formation of strongly nucleophilic species (in particular O2-2) during the redox process, which affect cells using common organic carbonate-based electrolytes characterized by the electrophilic carbonyl bond,24 while increased cycling stability is obtained in end-capped glyme (i.e., CH3–(OCH2CH2)n–OCH3) based on the less electrophile ether bond, and more stable molecules.25 Literature papers reported lithium oxygen cells using an electrolyte formed by dissolving LiPF6 in propylene carbonate (PC) with stability limited to less than 10 cycles,24 while the stability was extended over 50 cycles using LiClO4 in TEGDME,68 and over 150 cycles using an electrolyte based on TFSI and TEGDME,28,69 as summarized by recent review on lithium oxygen cell.5 In this work the lithium oxygen cell using nitrogen-annealed MWCNTs electrode and TEGDME-LiTFSI electrolyte revealed remarkable cycle life, extended up to 250 cycles, and excellent characteristics suitable for advanced high energy storage. This outstanding performance has been attributed to the excellent ion mobility and stability of the electrolyte, as revealed by NMR and EIS, as well as to an extremely reversible electrochemical process promoted by the optimal



morphology of the MWCNTs evidenced by SEM and TEM. Formation and dissolution of Li2O2 during reduction and oxidation, respectively, have been clearly observed by X-ray diffraction and electron microscopy. Micrometric crystalline peroxide is stably supported by the nanotube aggregates at the electrode surface, with no signs of electrode/electrolyte degradation or cell performance decay. The results obtained in this study indicate this cell as a stable, long life and relatively safe battery for application in modern EVs. The practical energy density of the cell is estimated to range from 270 to 1700 Wh kg-1, depending on the capacity limit adopted for cycling, however the study reveals that excessive formation of reaction product may lead to electrode insulation, cell degradation and capacity decay. Acknowledgements This work was supported by the grant "Fondo di Ateneo per la Ricerca Locale FAR 2017" and the collaboration project “Accordo di Collaborazione Quadro 2015” between University of Ferrara (Department of Chemical and Pharmaceutical Sciences) and Sapienza University of Rome (Department of Chemistry). This research was also partially supported by the RISE Program at Hunter College, grant #: GM060665. The authors thank Daniela Palmeri (Electronic Microscopy Centre, Department of Chemical and Pharmaceutical Sciences, University of Ferrara, Italy) for performing electron microscopy images. The NMR measurements at Hunter College were supported by a grant from the U.S. Office of Naval Research, and the NMR Facility is supported in part by a National Institutes of Health RCMI infrastructure grant (MD007599). Supporting Information Low magnification SEM image of carbon nanotubes after the annealing process under nitrogen atmosphere (Fig. S1). Additional characteristics of the lithium oxygen cell using TEGDME-LiTFSI electrolyte and N2@MWCNTs electrode. Voltage profile of the cell cycled at current of 100 mA g-1 and cutoff of 1.5V-4.5V referred to the carbon weight and to the electrode geometric area (Fig. S2). Voltage profiles (Fig. S3a) and cycling trend (Fig. S3b) of the cell cycled at current of 100 mA g-1 ACS Paragon Plus Environment

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and cutoff of 1.5V-4.5V. Voltage profiles of the cell cycled at current rate of 100 mA g-1 and a limit capacity at 1000 mAh g-1 (Fig. S4a) and 2000 mAh g-1 (Fig. S4b). Estimated energy density for the lithium oxygen battery studied in this work (Table S1). References (1)

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Enhanced Lithium Oxygen Battery Using a Glyme Electrolyte and

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