Semi-Interpenetrating Polymer Network Composite Gel Electrolytes

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Semi-Interpenetrating Polymer Network Composite Gel Electrolytes Employing Vinyl-Functionalized Silica for Lithium-Oxygen Batteries with Enhanced Cycling Stability Hyun-Sik Woo, Yong-Bok Moon, Samuel Seo, Ho-Taek Lee, and Dong-Won Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15573 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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ACS Applied Materials & Interfaces

Semi-Interpenetrating Polymer Network Composite Gel Electrolytes Employing Vinyl-Functionalized Silica for Lithium-Oxygen Batteries with Enhanced Cycling Stability Hyun-Sik Woo,† Yong-Bok Moon,† Samuel Seo,‡ Ho-Taek Lee,‡ and Dong-Won Kim*,† †

Department of Chemical Engineering, Hanyang University, Seoul 04763, Republic of Korea

‡

Research & Development Division, Hyundai Motor Company, Gyeonggi-do 16082, Republic of Korea

ABSTRACT A major challenge of lithium-oxygen batteries is to develop a stable electrolyte not only to suppress solvent evaporation and lithium dendrite growth, but also to resist against the attack by superoxide anion radical formed at the positive electrode.

The present study demonstrates the

enhancement of cycling stability by addressing the above challenges through the use of threedimensional semi-interpenetrating polymer network (semi-IPN) composite gel polymer electrolyte when fabricating the lithium-oxygen cell.

The semi-IPN composite gel electrolyte synthesized from

poly(methyl methacrylate), divinylbenzene and vinyl-functionalized silica effectively encapsulated electrolyte solution and exhibited stable interfacial characteristics toward lithium electrodes.

Matrix

polymers in the semi-IPN composite gel electrolyte also retained high stability without any decomposition by superoxide anion radicals during cycling.

The lithium-oxygen cell employing semi-

IPN composite gel polymer electrolyte was shown to cycle with good capacity retention at 0.25 mAh cm-2.

The semi-IPN composite gel electrolyte is one of the promising electrolytes for the stable

lithium-oxygen battery with high energy density.

KEYWORDS: Composite gel electrolyte, Semi-interpenetrating polymer network, Vinyl-functionalized silica, Lithium-oxygen battery, Cycling stability

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INTRODUCTION Lithium-ion batteries currently used in electric vehicles have promoted the electrification of road transportation without CO2 emissions from the vehicle.

However, these lithium-ion batteries have

reached their practical limit in gravimetric energy density.

An advanced battery system with much

higher energy density should be developed to allow electric vehicles to become widespread.1 One of the promising batteries for electric vehicle with a long driving range is a lithium-oxygen battery with high theoretical energy density.2-5 The key challenge of rechargeable lithium-oxygen battery is the cell design for a semi-open electrochemical system because of the evaporation or drying out of volatile liquid electrolyte.

In this respect, solid-state and semi-solid-state electrolytes are a vital requirement. The

second challenge is to suppress the lithium dendrite growth during cycling. The third challenge is the enhancement of electrolyte stability towards reactive superoxide anion radicals produced at the positive carbon electrode.

Many researches are underway to enhance the cycle life of lithium-oxygen batteries

by not only suppressing electrolyte depletion and lithium dendrite growth during cycling, but also enhancing the stability of electrolyte against superoxide anion radicals.6-17 Recent studies demonstrate that gel polymer electrolytes are stable against lithium metal, and can effectively suppress lithium dendrite growth and hold a large amount of organic solution in the polymer matrix without leakage.18-26 Additionally, gel polymer electrolytes provide a variety of potential advantages, including good interfacial contact with electrodes, good film formability and flexibility for battery design. These are unique characteristics that make gel polymer electrolyte a desirable electrolyte system for enhancing cycle life of the lithium-oxygen batteries.

However, the mechanical strength of gel polymer electrolyte

is significantly weakened with an addition of organic solvent by plasticizing effect. Therefore, gel polymer electrolyte cannot be applied to rechargeable lithium-oxygen batteries without mechanical support such as a polypropylene separator.22,23 Moreover, the polymer materials used to prepare the gel polymer electrolyte, such as poly(vinylidene fluoride) (PVDF), polyacrylonitrile (PAN) and poly(vinyl pyrrolidone) (PVP), are unstable and may be degraded in the presence of Li2O2.24 Therefore, both the


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elaborate design and synthesis of gel polymer electrolytes are necessary to improve their mechanical stability and minimize polymer degradation by superoxide anion radicals.

Inorganic fillers have been

usually added into gel polymer electrolytes to improve their mechanical and electrochemical properties.27-30 In our earlier works, we reported that cross-linking using vinyl-functionalized SiO2 (VSiO2) particles improved the mechanical strength of gel polymer electrolyte and reduced the interfacial resistances in the lithium-ion batteries.31 On the basis of these previous studies, V-SiO2 particles can be explored as preferred inorganic cross-linking agents to prepare three-dimensional composite gel electrolyte with good mechanical and interfacial properties for lithium-oxygen batteries.

Figure 1. Schematic illustration for synthesis of the three-dimensional semi-interpenetrating polymer network (semi-IPN) composite gel polymer electrolyte.

To address the issues related to mechanical and chemical stability of gel polymer electrolytes for lithium-oxygen cells, a three-dimensional semi-interpenetrating polymer network (semi-IPN) composite


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gel electrolyte was designed and synthesized in this work.

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Poly(methyl methacrylate) (PMMA) and

cross-linked polystyrene (PS) derived from divinylbenzene (DVB) were used in synthesizing semi-IPN, because they are invulnerable to nucleophilic attack by Li2O2. A chemical cross-linking reaction was induced using both V-SiO2 and DVB as the cross-linking agents for free radical polymerization to form a three-dimensional network, as shown in Figure 1.

The semi-IPN composite gel electrolyte

effectively trapped the organic solution, suppressed electrolyte decomposition by superoxide anion radicals and exhibited stable interfacial characteristics toward the lithium electrode. Our results demonstrate that the cycling stability of a lithium-oxygen cell composed of lithium anode, semi-IPN composite gel electrolyte and porous carbon cathode was remarkably improved compared to the liquid electrolyte-based cell due to the beneficial effects of the semi-IPN composite gel polymer electrolyte.

EXPERIMENTAL SECTION Synthesis of Vinyl-Functionalized Silica Particles. V-SiO2 was synthesized by the hydrolysis and condensation reaction of vinyltrimethoxysilane (Dynasylan® VTMO, Evonik), which has vinyl and hydrolysable trimethoxysilyl groups.21,31 First, 4 ml of VTMO was dissolved in water (150 ml) by mechanically stirring.

Sol-gel reaction was then progressed at 70 oC by adding ammonium hydroxide

solution (10 ml, 28 wt.% in H2O) and nitric acid (0.2 ml) into the solution.

After the sol-gel reaction

for 4 h, the product was centrifuged and rinsed with methanol several times.

Finally, white V-SiO2

powder was obtained after drying in a vacuum oven overnight at 110 oC.

Synthesis of Semi-IPN Composite Gel Electrolyte. Gel electrolyte precursor consisting of linear polymer, cross-linking agents, initiator and liquid electrolyte was prepared as follows.

PMMA

(average molecular weight: 350,000, Sigma-Aldrich), DVB (Sigma-Aldrich), V-SiO2, 2-2’ azobis(2methylpropionitrile) (AIBN, Junsei) and liquid electrolyte were mixed in a weight ratio of 30:5:4:0.2:60.8 in anhydrous tetrahydrofuran (THF, TCI).

The liquid electrolyte used in preparing the


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gel electrolyte precursor was 1.0 M lithium bis(trifluoromethane) sulfonylimide (LiTFSI, PanaXetec Co. Ltd.) in tetra(ethylene glycol) dimethyl ether (TEGDME, Sigma-Aldrich). precursor solution was stirred well and cast onto a Teflon plate.

The gel electrolyte

After that, the cast film on a Teflon

plate was exposed to 70 oC for 12 h under inert atmosphere, and further vacuum dried at 80 oC for 24 h. The resulting semi-IPN composite gel polymer electrolyte was a free-standing flexible film, as depicted in Figure 1, and its thickness ranged from 60 to 80 µm.

Cell Assembly.

Lithium metal with a thickness of 200 µm (Honjo Metal Co. Ltd.,) was used as a

negative electrode after pressing onto a Cu foil. The slurry for the carbon positive electrode was prepared by mixing 90 wt.% Ketjen black (EC600JD) and 10 wt.% poly(vinylidene fluoride) (PVDF, Solvay) in N-methyl pyrrolidone. using a doctor-blade.

The resulting viscous slurry was coated onto a porous carbon layer

A disk-shaped electrode having an area of 1.13 cm2 was punched and vacuum-

dried for 12 h at 110 °C.

A lithium-oxygen cell was fabricated by sandwiching the semi-IPN

composite gel electrolyte between the carbon electrode and the lithium electrode.32 The semi-IPN composite gel electrolyte was not included in the carbon electrode.

For comparison, the lithium-

oxygen cell was also assembled with glass fiber separator (Whatman grade GF/D) and liquid electrolyte (1.0 M LiTFSI in TEGDME) instead of semi-IPN composite gel electrolyte.

Characterization and Measurements.

The stability of various polymers toward Li2O2 was tested in

the polymer solution containing Li2O2 powder, as previously reported.23 First, 50 mg of polymer (PAN, PVdF-HFP, PMMA, PS) was dissolved in 4 ml of N,N-dimethylformamide. N,N-dimethylacetamide was used to dissolve the polymer.

In the case of PVdF,

Then, 100 mg of Li2O2 was added to the

polymer solution and the color change was monitored during mixing the solution for 2 days.

Fourier

transform infrared (FT-IR) spectra were obtained to confirm the cross-linking reaction between DVB and V-SiO2 using a Nicolet 6700 spectrometer (Thermo Scientific).

Thermogravimetric analysis

(TGA) was conducted to compare the volatility behavior of liquid electrolyte and semi-IPN composite ACS Paragon Plus Environment

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gel electrolyte.

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DC polarization test of the symmetric Li/electrolyte/Li cells was performed in the

potential range of -1.0 to 1.0 V at constant current densities of 0.5 and 1.0 mA cm-2. The morphology of lithium electrode disassembled from the symmetric cell was analyzed using a scanning electron microscope (SEM, JEOL JSM 6701F) after washing with anhydrous THF.

Linear sweep voltammetry

was conducted using a platinum working electrode, with counter and reference electrodes of lithium metal, at a scanning rate of 1.0 m V s-1. AC impedance spectra of the electrochemical cells were obtained using a Zahner Electrik IM6 impedance analyzer.

Charge and discharge cycling test of the

lithium-oxygen cells was performed with a cut-off voltage of 2.0 to 5.0 V at constant current rates under oxygen atmosphere. Galvanostatic intermittent titration technique (GITT) experiments of the lithiumoxygen cells were conducted after 20 cycles at a constant current density of 0.25 mA cm-2. The cells were allowed to relax for 5 h after every charging and discharging at the same current density.

X-ray

photoelectron spectroscopy (XPS) (Thermo VG Scientific ESCA 2000) was conducted to identify the products formed on the lithium electrode after repeated cycling.

Raman spectra of the semi-IPN

composite gel electrolyte was obtained using a Raman spectrometer (NRS-3100) over the wavelength range of 500 to 3500 cm-1 at room temperature.

RESULTS AND DISCUSSION The chemical reactivity of various polymers was investigated in the presence of Li2O2 powder.

As

depicted in Figure S1, the color of the polymer solution was changed due to nucleophilic attack of Li2O2, except the solutions dissolving PMMA and PS, which indicates that PMMA and PS are quite stable with regard to Li2O2.

The poor leaving nature of the pendent groups (–CH3/–OCOCH3 and –

C6H5) in PMMA and PS is responsible for the stability against Li2O2 attack.23,24,33 Based on these results, PMMA was selected as a linear polymer and DVB was employed as a cross-linking agent for synthesizing the semi-IPN composite gel electrolyte for the lithium-oxygen cell.


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A cross-linking reaction between V-SiO2 and DVB in the presence of PMMA and liquid electrolyte was confirmed by FT-IR analysis.

FT-IR spectra of V-SiO2, DVB, PMMA and semi-IPN composite

gel electrolyte are shown in Figure 2.

V-SiO2 showed the broad peaks around 765 and 1100 cm-1,

corresponding to the symmetric and asymmetric stretching vibration of Si–O–Si, respectively.

The

vinyl groups in the V-SiO2 particles were observed at 1409 and 1600 cm-1,34,35 indicating the V-SiO2 particles have C=C double bonds.

In the FT-IR spectrum of DVB, two peaks that correspond to C=C

double bond could also be observed at the same positions.

The vinyl groups in V-SiO2 and DVB can

play a role as cross-linking sites to produce a three-dimensional cross-linked polymer network during cross-linking reaction at high temperature.

In the FT-IR spectrum of semi-IPN composite gel

electrolyte (Figure 2d), two peaks corresponding to the vinyl groups in V-SiO2 and DVB could not be observed after thermal curing.

This result suggests that V-SiO2 particles reacted with DVB by means

of a free-radical reaction to form the three-dimensional semi-IPN composite gel electrolyte, as schematically illustrated in Figure 1.

(a)

Transmittance (%)

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Wavenumber (cm ) Figure 2. FT-IR spectra of (a) V-SiO2, (b) DVB, (c) PMMA and (d) semi-IPN composite gel electrolyte.


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TGA can provide comparative information on evaporation behavior of volatile components in the electrolytes, and TGA results are shown in Figure S2.

The measurement was performed by monitoring

weight loss of the electrolyte at isothermal condition (100 oC) with N2 flow rate of 100 ml min-1. As depicted in Figure S2, the liquid electrolyte lost about 18.6 wt.% of its initial electrolyte weight after 60 min.

In contrast, the semi-IPN composite gel electrolyte showed only 2.7 wt.% weight loss.

These

results indicate that liquid electrolyte was effectively trapped in the three-dimensional semi-IPN composite gel electrolyte, which can minimize the evaporation of volatile solvent in the lithium-oxygen cell during semi-open operation.

Linear sweep voltammograms of liquid electrolyte and semi-IPN

composite gel electrolyte are shown in Figure S3.

As shown in figure, there is no obvious significant

reductive current until the current corresponding to the lithium deposition (Li+ + e- → Li) starts to increase at around 0 V, indicating the electrolytes are electrochemically stable at 0 V vs. Li/Li+.

Lower

reductive current corresponding to lithium plating in the semi-IPN composite gel polymer electrolyte is ascribed to its low ionic conductivity (5.6 x 10-4 S cm-1) compared to the liquid electrolyte (3.2 x 10-3 S cm-1), which arises from the restricted ionic motion in the semi-IPN composite gel polymer electrolyte. In the anodic scan, the oxidative current due to electrolyte decomposition was rapidly increased at about 4.9 V for liquid electrolyte. The anodic stability was found to increase in the semi-IPN composite gel polymer electrolyte. The interfacial characteristics of the electrolytes toward lithium electrode were investigated by the repeated plating and stripping cycling of the symmetrical Li/electrolyte/Li cells.

Figure 3 presents the

DC polarization voltage profiles of the cells during the lithium stripping and plating processes, which were obtained at 0.5 and 1.0 mA cm-2 for 2 hours.

The cell employing liquid electrolyte exhibited high

overpotential with large drift in the voltage profiles. Uneven plating and stripping of lithium in the liquid electrolyte cell may cause irregular growth of lithium dendrites on the electrode, resulting in gradual consumption of the electrolyte by continuous generation of a new solid electrolyte interphase layer.36,37 The gradual consumption of liquid electrolyte increases the resistance for the electrochemical reaction of lithium (Li ↔ Li+ + e), which causes increased polarization and unstable voltage profiles. ACS Paragon Plus Environment

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In contrast, the cell employing semi-IPN composite gel polymer electrolyte showed more stable voltage profiles and lower overpotential than those of the cell with liquid electrolyte, which resulted in more extended cycles.

From these results, it is understood that the semi-IPN composite gel polymer

electrolyte on the lithium electrode could provide a uniform current distribution to the electrolyteelectrode interface and inhibit deleterious reactions of the electrolyte solution with the lithium metal. Moreover, the intimate contact between semi-IPN composite gel electrolyte and lithium electrode could retard the lithium dendrite growth during the repeated plating and stripping cycles.

After 50 cycles, the

morphologies of lithium electrodes disassembled from the symmetric Li/electrolyte/Li cells were compared.

As depicted in Figure S4, the lithium electrode cycled in liquid electrolyte exhibited rough

and particulate dendrite morphology.

In contrast, the lithium electrode cycled with semi-IPN

composite gel electrolyte showed flat surface without any visible lithium dendrite growth.

These

results imply that the semi-IPN composite gel electrolyte mechanically suppresses lithium dendrite growth and allows uniform deposition/dissolution of lithium over the surface of the lithium electrode, which result in high cycling efficiency and good cycling stability. 1.5

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DC polarization voltage profiles of the symmetrical Li/electrolyte/Li cells with liquid

electrolyte and semi-IPN composite gel electrolyte during the lithium stripping and plating processes, which were obtained at a constant current density of (a) 0.5 mA cm-2 and (b) 1.0 mA cm-2 for 2 h within a cut-off potential of -1.0 to 1.0 V.


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When the lithium metal is in contact with organic electrolyte solution, a resistive surface layer is formed on the lithium electrode and thickened with time.

The interfacial stability of electrolytes

contacted with lithium electrode was investigated by measuring the interfacial resistances as a function of time in the Li/electrolyte/Li cell, as presented in Figure 4. composed of two semicircles.

All of the AC impedance spectra are

The semicircle appearing in the high frequency region originates from

the ionic resistance (Rf) and capacitance (Cf) of the film formed on the lithium electrode, and the semicircle in the low frequency range is associated with the charge transfer resistance (Rct) and doublelayer capacitance (Cdl), as previously reported.38-40 250 0 days 2 days 4 days 6 days 8 days 10 days

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-ZIm (Ω )

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Figure 4. Time evolution of electrochemical impedance spectra of the symmetrical Li/electrolyte/Li cells with (a) liquid electrolyte and (b) semi-IPN composite gel electrolyte at 25 oC.

As clearly seen in Figure 4a, Rf gradually increased with time in the cell employing liquid electrolyte, indicating the continuous growth of a resistive layer on the surface of lithium electrode due to the high reactivity of lithium metal toward electrolyte solution. Since the resistive layer hindered the charge transfer reaction at the electrode-electrolyte interface, Rct was also increased with time.

In contrast,

both Rf and Rct in the cell with semi-IPN composite gel electrolyte were stabilized after initial increases at an early stage.

Although the sum of the initial interfacial resistances (Rf and Rct) is much higher in


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the cell with semi-IPN composite gel electrolyte than in the liquid electrolyte cell, the interfacial resistances are maintained at lower values than in the cell employing a liquid electrolyte after 10 days. As mentioned previously, the semi-IPN composite gel polymer electrolyte could suppress the deleterious reactions between electrode and electrolyte, and thus use of a semi-IPN composite gel electrolyte is effective for enhancing the interfacial stability of the lithium electrode in the lithiumoxygen cells. The lithium-oxygen cells were cycled at 0.25 mA cm-2 (0.25 C) and 25 oC.

Figure 5a and 5b

show the cycling curves of the cells with liquid electrolyte and semi-IPN composite gel electrolyte, respectively, which were cycled within the limited capacity of 1.0 mAh cm-2. The lithium-oxygen cell employing liquid electrolyte exhibited reversible charge and discharge behavior up to the 25th cycle with a gradual increase in overpotential, and the discharge capacity faded after the 25th cycle, as shown in Figure 5c.

Such an increase in the polarization may result from the rapid evaporation of the

electrolyte solution from the carbon positive electrode, the growth of lithium dendrite on the negative electrode, and the accumulation of the side products produced on the electrodes during cycling.41,42 On the other hand, the cell with semi-IPN composite gel electrolyte showed stable cycling behavior without capacity decline up to the 60 cycles, as illustrated in Figure 5c.

Such a cycling performance is better

than the results reported for lithium-oxygen cells employing other gel polymer electrolytes.20,26,43 The performance decay at around 50 cycles in this cell is likely to be attributed to the partial evaporation of electrolyte solvent and electrode polarization due to the choking of carbon electrode on cycling.

In

order to understand the voltage hysteresis associated with overpotential, we performed GITT measurements of the lithium-oxygen cells, as previously reported.44 As depicted in Figure S5, the overpotential was increased at the end of discharge during oxygen reduction reaction (ORR). This result implies that the diffusion resistances of Li+ and O2- ions increase due to the enlargement of Li2O2 particles as the ORR progresses.

Higher overpotential in the liquid electrolyte cell can be ascribed to

more buildup of decomposition products on the porous carbon electrode. overpotential was also observed in the cell employing liquid electrolyte.

Upon charging, higher

The formation and coverage


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the side products caused by electrolyte decomposition during oxygen evolution reaction (OER) may weaken the electronic contact of Li2O2 particles and conducting agents, resulting in increase of overpotential on charging.

Large ohmic polarization caused by solvent evaporation is also contributed

to higher overpotential in the cell with liquid electrolyte. These results suggest that the voltage gap during charge and discharge cycles can be reduced by employing the semi-IPN composite gel electrolyte with enhanced stability.

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Voltage profiles of the lithium-oxygen cells assembled with (a) liquid electrolyte and (b)

semi-IPN composite gel electrolyte at 0.25 mA cm−2 and 25 oC.

(c) Discharge capacity and (d)

terminal voltage of the lithium-oxygen cells as a function of the cycle number.


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Figure 5d compares the terminal charge and discharge voltages of the lithium-oxygen cells with liquid electrolyte and semi-IPN composite gel polymer electrolyte.

It can be seen that the voltage difference

between the charge and discharge terminal voltages was decreased by replacing liquid electrolyte with semi-IPN composite gel electrolyte.

Such an improvement in the electrochemical performance of the

cell with semi-IPN composite gel polymer electrolyte can be attributed to the effective holding of organic solvent and the suppression of electrolyte decomposition during repeated cycling, as discussed earlier.

Charge and discharge curves of the lithium-oxygen cells at higher current rate (0.5 mA cm-2,

0.5C) are shown in Figure S6.

Clearly, the cell employing semi-IPN composite gel electrolyte still

showed better cycling characteristic than the cell with a liquid electrolyte at 0.5 C rate. AC impedance spectra of the lithium-oxygen cells with repeated cycling are shown in Figure 6. All the spectra gave a depressed semicircle due to different combination of interfacial resistances and capacitances, with the real axis intercept corresponding to the electrolyte resistance.45 It is notable that two cells exhibited quite different AC impedance behavior with cycling.

In the lithium-oxygen cell

with liquid electrolyte, both electrolyte resistance and interfacial resistance significantly increased with cycling.

A large increase in the electrolyte resistance is mainly attributed to the gradual loss of liquid

electrolyte during cycling, resulting from evaporation of volatile organic solvent at the positive electrode opening to the O2 atmosphere and electrolyte exhaustion due to the unwanted side reactions of liquid electrolyte.

At the same time, the interfacial resistance was also rapidly increased in the cell

with liquid electrolyte.

This result is due to the fact that the electrolyte depletion during cycling

retarded the charge transfer reaction at the electrolyte-electrode interface, as the active reaction sites in the electrode could not find lithium ions in the electrolyte.

In contrast, the cell with semi-IPN

composite gel polymer electrolyte exhibited a small increase in both electrolyte resistance and interfacial resistance with cycling.

Such a small increase can be ascribed to the partial evaporation of

organic solvent from the semi-IPN composite gel electrolyte and partial clogging of the porous carbon electrode during cycling.


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600

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Figure 6. Nyquist plots obtained by electrochemical AC impedance for the lithium-oxygen cells with (a) liquid electrolyte and (b) semi-IPN composite gel electrolyte, which were obtained at the charged state after the 1st, 10th and 20th cycle at 25 °C.

To figure out how the semi-IPN composite gel electrolyte affects the interfacial behavior of the lithium electrode, the surface composition of the lithium electrode was investigated by XPS analysis after cycling, and the resulting Li and C 1s XPS spectra are shown in Figure 7. resolved into three components, as shown in Figure 7a and 7b. corresponds to lithium metal.

The spectra could be

The highest peak at 54.9 eV

The additional two peaks appearing at 55.3 and 56.2 eV result from

Li2CO3/LiOH and LiF, respectively.46-49 Note that the peak resolution of Li2CO3 and LiOH was difficult due to their similar peak position.

It is notable that the intensities of the peaks corresponding to LiF,

Li2CO3 and LiOH are remarkably reduced in the lithium electrode employing semi-IPN composite gel polymer electrolyte.

LiF originates from the reductive decomposition of LiTFSI salt,49,50 and Li2CO3

and LiOH are by-products produced by the irreversible decomposition of liquid electrolyte.13,15 The resolved peaks of C 1s XPS spectra are also given in Figure 7c and 7d, which reveal that the amounts of lithium carbonates and lithium carboxylates were considerably reduced in the cell employing semi-IPN composite gel electrolyte.

These compounds are produced by the decomposition of TEGDME in the

presence of Li2O2 caused by H abstraction during the discharge process.51,52 Therefore, these results


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suggest that the electrolyte decomposition was significantly suppressed in the lithium-oxygen cell with semi-IPN composite gel electrolyte, which resulted in enhanced cycling stability of the lithium-oxygen cell.

Exp. data Cumulative Fit Li metal Li2CO3 and LiOH LiF

Exp. data Cumulative Fit Li metal Li2CO3 and LiOH LiF

(b)

Intensity (a.u.)

Intensity (a.u.)

(a)

Li 1s Composite gel electrolyte

Li 1s Liquid electrolyte

62

60

58

56

54

52

50

48

62

60

58

Binding energy (eV)

56

292

288

284

50

48

Exp. data Cumulative C-C / C-H Ether / C-O Carboxylate Li2CO3 CF2

Intensity (a.u.)

280

C 1s Composite gel electrolyte

296

292

288

284

280

Binding energy (eV)

Binding energy (eV)

Figure 7.

52

(d)

C 1s Liquid electrolyte

296

54

Binding energy (eV) Exp. data Cumulative C-C / C-H Ether / C-O Carboxylate Li2CO3 CF2

(c)

Intensity (a.u.)

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


Li 1s XPS spectra of the lithium electrodes cycled with (a) liquid electrolyte and (b) semi-

IPN composite gel electrolyte.

C 1s XPS spectra of the lithium electrodes cycled with (c) liquid

electrolyte and (d) semi-IPN composite gel electrolyte.

For XPS measurements, the lithium electrode

was disassembled from the lithium-oxygen cells after 20 cycles.

Raman spectroscopy was further carried out to investigate the structural stability of the polymer matrix used in preparing the semi-IPN composite gel electrolyte after cycling.

Figure S7 shows the

Raman spectra of the semi-IPN composite gel electrolyte before and after 20 cycles. The characteristic peaks corresponding to CH3 stretching, C=O stretching, CH3 bending, -OCH3 stretching and CH3 rocking in PMMA,53,54 C-N-C bending in LiTFSI salt,55 and C-C-C benzene ring bending in crossACS Paragon Plus Environment

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linked PS56 could be observed at the same position without any shifting before and after cycling. These results imply that the matrix polymers in the semi-IPN composite gel electrolyte retain high stability without any decomposition by superoxide anion radicals generated at carbon positive electrode.

CONCLUSIONS The enhancement of cycling stability of the lithium-oxygen cell by employing three-dimensional semiIPN composite gel electrolyte was demonstrated.

The semi-IPN composite gel polymer electrolyte

was synthesized by free-radical polymerization of DVB and V-SiO2 in the presence of PMMA and liquid electrolyte. The semi-IPN composite gel polymer electrolyte film showed high electrochemical stability and good interfacial characteristics toward lithium electrode.

Compared to the lithium-oxygen

cell with liquid electrolyte, the cell with the semi-IPN composite gel electrolyte exhibited enhanced cycling stability by effectively suppressing the evaporation and irreversible decomposition of electrolyte solution during repeated cycling. Consequently, the semi-IPN composite gel electrolyte in our study can be a promising electrolyte for the lithium-oxygen batteries with high energy density.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (D.-W. Kim)

ACKNOWLEDGMENTS This work was supported by the Hyundai Motor Company and the Basic Science Research Program of the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT and Future Planning (2014R1A2A2A01002154).


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ASSOCIATED CONTENT Supporting Information Photographs showing the chemical reactivity of polymers towards Li2O2 powder, TGA results of liquid electrolyte and semi-IPN composite gel electrolyte, linear sweep voltammograms of liquid electrolyte and semi-IPN composite gel electrolyte,

SEM images of lithium electrodes disassembled from the

symmetrical Li/electrolyte/Li cells, GITT curves and overpotential of lithium-oxygen cells, voltage profiles of the lithium-oxygen cells at 0.5 mA cm-2, and Raman spectra of the semi-IPN composite gel electrolytes before and after cycling.

This material is available free of charge via the Internet at

http://pubs.acs.org/.

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Semi-Interpenetrating Polymer Network Composite Gel Electrolytes

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