Hybrid gel polymer electrolyte for high-safety lithium-sulfur batteries

Hybrid gel polymer electrolyte for high-safety lithium-sulfur batteries

Author’s Accepted Manuscript Hybrid gel polymer electrolyte for high-safety lithium-sulfur batteries Jae-Kwang Kim www.elsevier.com PII: DOI: Refere...

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Author’s Accepted Manuscript Hybrid gel polymer electrolyte for high-safety lithium-sulfur batteries Jae-Kwang Kim

www.elsevier.com

PII: DOI: Reference:

S0167-577X(16)31668-8 http://dx.doi.org/10.1016/j.matlet.2016.10.069 MLBLUE21638

To appear in: Materials Letters Received date: 20 June 2016 Revised date: 1 October 2016 Accepted date: 16 October 2016 Cite this article as: Jae-Kwang Kim, Hybrid gel polymer electrolyte for highsafety lithium-sulfur batteries, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.10.069 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Hybrid gel polymer electrolyte for high-safety lithium-sulfur batteries

Jae-Kwang Kim*

Department of Solar & Energy Engineering, Cheongju University, Cheongju, Chungbuk 360-764, Republic of Korea *

Corresponding author. Tel.: +82 43 229 8557; fax: +82 43 229 7322 E-mail: [email protected]

Abstract A hybrid nanofiber PVDF-HFP polymer matrix with dispersed SiO2 nanoparticles was prepared by electrospinning, and 1 M lithium bis(trifluoromethysulfonyl)imide (LiTFSI)

/1-propyl-3-methylimidazolium

bis(trifluoromethysulfonyl)imide

(PMImTFSI) was incorporated in the hybrid polymer matrix to produce a polymer gel electrolyte for lithium-sulfur batteries. The PMImTFSI-based hybrid gel polymer electrolyte (ILGPE) reduced the concentration of Li+-coordinated TFSI, and the incorporation of PMImTFSI increased the α-phase content of PVDF-HFP, reducing polymer crystallinity. In addition, ILGPE showed high ionic conductivity, high oxidation potential (> 5.0 V vs. Li/Li+), and low flammability. The Li/S battery utilizing ILGPE showed a high initial discharge capacity of 1029 mAh g–1 and maintained a stable capacity of 885 mAh g–1 after 30 cycles at a current rate of 0.1 C.

Keywords: ionic liquid; polymer matrix; sulfur dissolution; lithium-sulfur battery

1

1. Introduction

Sulfur as a cathode-active material attract great attraction due to their high theoretical capacity of 1675 mAh g–1, together with the ready availability, low cost, and nontoxicity of sulfur. Although lithium/sulfur (Li/S) primary cells were known significantly earlier, the development of high-energy and high-power Li/S secondary batteries has been actively pursued since the early 1990s [1]. Numerous technical challenges had to be tackled to realize a high and stable performance of the rechargeable Li/S cells, e.g., the low conductivity of sulfur, dissolution of sulfur reduction products into the electrolyte, and deposition of solid reaction products on the cathode matrix. Ionic liquid electrolytes (ILEs), generally exhibiting negligible vapor pressure, non-flammability, high thermal stability, intrinsic ionic conductivity at room temperature, and a wide electrochemical window (good electrochemical stability in the range of 4.0–5.0 V) [2] were used to prevent the dissolution of sulfur in the electrochemical reaction [3–7]. Using ILEs improved the electrochemical properties of sulfur batteries, but stability, as evidenced by leakage, and mechanical strength were not sufficient. Polymer electrolytes (PEs) are considered the most promising electrolytes for stable batteries, offering shape versatility, flexibility, lightness, leakage prevention, and safety, allowing their commercialization in miniature electronic devices and electric vehicles. Among the PEs, gel polymer electrolytes show high ionic conductivity at room temperature due to the immobilization of a large amount of liquid electrolyte in the polymer host [8–10]. Electrospinning is particularly suitable for producing thin homogenous polymer membranes with pores in the nano- to micrometer size range. Since the electrospun membranes possess high porosity, they exhibit a relatively high electrolyte uptake, resulting in high ionic conductivity [10– 13]. To be practically useful, the membrane should be able to absorb the liquid electrolyte without leakage, be chemically compatible with sulfur electrode materials, and

adhere

well

to

the

sulfur

electrode.

PAN

(polyacrylonitrile),

PVC

(polyvinylchloride), and PVDF (polyvinylidenefluoride) were used as polymer hosts in the electrospinning process [14–19], and the poly(vinylidene fluoride-cohexafluoropropylene) P(VDF-HFP) copolymer was found to be promising, owing to its good electrochemical stability and its affinity to electrolyte solutions [20]. 2

Moreover, SiO2 ceramic particles are able to absorb polysulfide intermediates, improving the electrochemical performance of Li/S batteries [21]. In

this

study,

a

1-propyl-3-methylimidazolium

bis(trifluoromethysulfonyl)imide (PMImTFSI)-based hybrid gel polymer electrolyte (ILGPE) prepared by electrospinning was utilized in the Li/S battery, since PMImTFSI exhibits low viscosity (ca. 40 cP), high thermal stability, and low ion pairing, which can enhance charge carrier mobility [22]. Moreover, it forms a solid electrolyte interface (SEI) layer, inhibiting undesired reactions on electrodes and reducing the dissolution of sulfur. SiO2 ceramic particles were dispersed in ILGPE using PVDF-HFP as a polymer host to improve the electrochemical stability of the sulfur battery. ILGPE showed a high ionic conductivity of 1.1×10–3 S cm–1 at 20 °C.

2. Experimental

Preparation of PMImTFSI-based gel polymer electrolyte and cathode PMImTFSI ionic liquid and lithium bis(trifluoromethysulfonyl)imide (LiTFSI) were obtained from Aldrich (≥ 98% purity), stored under argon, and used as received. Nanofibrous P(VDF-HFP) membranes with 6 wt.% incorporated SiO2 nanoparticles were prepared by electrospinning according to a previously described method [13, 23]. The PMImTFSI-based hybrid gel polymer electrolyte (ILGPE) was obtained by soaking the polymer membrane in a 1 M LiTFSI/PMImTFSI solution under argon atmosphere for 30 s at room temperature. The sulfur cathode was prepared as follows. The slurry prepared by mixing 60 wt.% elemental sulfur, 30 wt.% carbon black, and 10 wt.% poly(vinylidene fluoride) (PVDF, Aldrich) binder in N-methyl-2-pyrrolidone (NMP) solvent was cast on aluminum foil and dried at 50 °C under vacuum for 6 h.

3. Results and discussion The conductivity of ILE and ILGPE between –40 and 80 °C is shown in Fig. 1a. Both electrolytes show steadily rising conductivities with increasing temperature and exhibit typical non-Arrhenius behavior. Generally, the conductivity value decreased with decreasing temperature. However, the conductivity is still high as 1.7 × 10–3 S/cm for ILE and 1.1 × 10–3 S/cm for ILGPE at 20 °C. In addition, as the temperature approaches the glass transition temperature, the conductivities become 3

equal, due to an increasing amount of amorphous phase between the ionic liquid and the nanofiber polymer membrane. Fig. 1b shows the DSC heating trance of ILE and ILGPE. ILGPE exhibits two sharp endothermic peaks at –77 and 120 °C, attributed to the glass transition of ILE and melting of the PVDF-HFP membrane; the glass transition temperature (Tg) observed was equal to that of pure ILE. However, the melting temperature of the PVDF-HFP membrane decreased from 142 °C to 120 °C [7]. This observation confirms that addition of ILE increases the amorphous range of the polymer matrix. Raman spectra of the PMImTFSI-based ionic electrolyte and ILGPE were measured between 0–3500 cm–1. Raman spectroscopy is suitable for probing the conformational changes and interactions of the ionic liquid, especially in the 720–760 cm–1 range of the C–F stretching mode, where the C1 (cisoid) and C2 (transoid) conformers of the free TFSI anion and Li-coordinated TFSI (C3) exhibit peaks at ~742 cm–1 (C1), ~746 cm–1 (C2), and ~749 cm–1 (C3) [22, 25]. Void profile fitting (Gaussian:Lorentzian

=

3:7)

corroborates

this

qualitative

estimation.

The

conformational state is sensitive to temperature and concentration of the lithium salt [22, 26]. The results of studying the interaction between the polymer membrane and ionic liquid for the two electrolytes at room temperature are shown in Figs. 2a, b. When the hybrid polymer membrane is incorporated with ILE, the concentration of Li-coordinated TFSI decreases from 0.40 to 0.44, implying that the TFSI anion interacts with the hybrid polymer membrane. This interaction increases the Li+ transference number from 0.052 to 0.064. The addition of ionic liquid to the polymer matrix can induce structural and morphological changes and improves ion mobility. The Raman spectra of polymer membranes are displayed in Fig. 2c. The band at ~800 cm–1 corresponds to the combination of CH2 rocking (γ) and CF2 stretching (ν) vibrations of the α-phase. The band at ~840 cm–1 corresponds to the out-of-plane γCH2 and ν-CF2 vibrations of the β-phase [27]. The β-phase is converted to the α-phase after addition of ILE. The decrease of the β-phase amount reduces the mechanical strength of PVDF-HFP and its crystallinity. The initial charge-discharge profiles of the Li/S cell with ILGPE probed at 0.071 mA/cm2 (0.1 C) at 30 °C are displayed in Fig. 3a. The discharge curve showed a plateau at 2.2 V, somewhat higher than in the case of other polymer electrolytes [28, 29]. This observation may be attributed to the enhancement of cell resistance by the 4

nanofibrous hybrid matrix and polysulfide stages curve is decreased by thick solid electrolyte interface (SEI) layer. The initial discharge capacities for ILE and ILGPE were 865 and 1039 mAh g–1, respectively. The discharge capacity for both electrolytes decreased with cycling, but a constant discharge capacity was maintained in subsequent cycles for ILGPE. In the case of ILE, the capacity slowly decreased to 328 mAh g–1 after 30 cycles, corresponding to 40% capacity retention. However, ILGPE delivered 885 mAh g–1 after 30 cycles, corresponding to 86% capacity retention. In Fig. 3b, the improved cycling stability of the ILGPE cell is due to the ability of the hybrid polymer matrix to adsorb polysulfides. The ILGPE Li/S cell also exhibits excellent high-rate capability (Fig. 3c). When the current densities increase from 0.1 to 0.5 and 1 C, the ILGPE cell shows good capacity retention as the specific capacities change from 1029 to 691 and 311 mAh g–1, respectively. It is noteworthy that when the rate returns to 0.1 C, the specific capacity returns to 858 mAh g–1. It is evident that the PMImTFSI-based hybrid ILGPE cell exhibits improved cyclic capacity retention and rate capability.

4. Conclusions

A PMImTFSI-based hybrid gel polymer electrolyte (ILGPE) was used to improve the safety and cycling stability of the Li/S cell. ILGPE showed a high ionic conductivity of 1.1 × 10–3 S cm–1 at 20 °C and enhanced oxidation stability up to 5.0 V. Moreover, addition of ionic liquid to the polymer membrane reduced the crystallinity of PVDFHFP by converting it to the α-phase. The ILGPE Li/S battery delivers a high initial discharge capacity and maintains a reversible capacity of 885 mAh g–1 after 30 cycles. Thus, ILGPE not only ensures high safety, but also reduces the solubility of lithium polysulfides and improves the cycling performance of the Li/S battery.

Acknowledgements I am thankful to Prof. Ahn for his help in preparing the electrospun polymer matrix.

References

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7

Figure captions

Figure 1. (a) Ionic conductivity of ILE and ILGPE as a function of temperature. (b) DSC thermograms of ILE and ILGPE for determining the glass transition temperature, and of the PVDF-HFP matrix and ILGPE for determining the melting temperature. Figure 2. Detailed Raman spectra of the 740 cm–1 band for (a) ILE and (b) ILGPE at room temperature. (c) Raman spectra of the pure PVDF-HFP matrix and ILGPE.

Figure 3. (a) Initial charge-discharge profiles, (b) cycling performances, and (c) ratecapabilities for Li/ILE/S and Li/ILGPE/S batteries (30 °C, cut-off: 1.5 to 2.8 V).

8

Figures

(a)

1M LiTFSI/PMImTFSI

Heat flow (mW)

(b)

ILGPE ILE

-100

-90

-80

-70

-60

-50

-40

Temperature (oC)

Heat flow (mW)

1M LiTFSI/PMImTFSI

ILE

ILGPE+SiO2 40

60

80

100

120

140

160

Temperature (oC)

Fig. 1. (a) Ionic conductivity of ILE and ILGPE as a function of temperature. (b) DSC thermograms of ILE and ILGPE for determining the glass transition temperature, and of the PVDF-HFP matrix and ILGPE for determining the melting temperature.

9

(a) ILE

(b) ILGPE

PVdF-HFP

Intensity (a.u)

(c)

780

pure polymer matrix ILGPE

α-phase

790

800

β -p h a s e

810

820

830

840

850

2

Raman shift (cm )

Fig. 2. Detailed Raman spectra of the 740 cm–1 band for (a) ILE and (b) ILGPE at room temperature. (c) Raman spectra of the pure PVDF-HFP matrix and ILGPE.

10

3.5

3.0

Voltage (V)

o

Li//ILPE//S cell: 0.1 C-rate at 30 C

(a)

ILE ILPE with SiO2

2.5

2.0

1.5

1.0

0

200

400

600

800

1000

1200

1400

Specific capacity (mAh/g)

Discharge capacity (mAh/g)

1400

0.1 C-rate Li/ILPE/S cell

(b)

1200

ILE ILPE with SiO2

1000 800 600 400 200 0

0

5

10

15

20

25

30

Cycle number Discharge capacity (mAh/g)

1400

o

Li/ILPE/S cell at 30 C ILE 167 mA/g (0.1C) ILPE with SiO2

(c)

1200 1000

167 mA/g (0.1C)

800 835 mA/g (0.5C)

600 400

1670 mA/g (1C)

200 0

0

10

20

30

40

Cycle number

Fig. 3. (a) Initial charge-discharge profiles, (b) cycling performances, and (c) ratecapabilities for Li/ILE/S and Li/ILGPE/S batteries (30 °C, cut-off: 1.5 to 2.8 V).

11

Graphical abstract

Discharge capacity (mAh/g)

1400

0.1 C-rate Li/ILPE/S cell

1200

ILE ILPE with SiO2

1000 800 600 400 200 0

0

5

10

15

20

25

30

Cycle number

Highlights  A hybrid nanofiber polymer matrix with dispersed SiO2 nanoparticles was prepared by electrospinning. 

ILGPE showed high ionic conductivity, high oxidation potential, and low flammability.



The Li/S battery utilizing ILGPE showed a good electrochemical performance.

12