New separator prepared by electron beam irradiation for high voltage lithium secondary batteries

Nuclear Instruments and Methods in Physics Research B 267 (2009) 2390–2394

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New separator prepared by electron beam irradiation for high voltage lithium secondary batteries Jun Young Lee a, Bhaskar Bhattacharya a, Young-Chang Nho b, Jung-Ki Park a,* a

Department of Chemical and Biomolecular Engineering (BK 21 Graduate Program), Korea Advanced Institute of Science and Technology (KAIST), 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea b Advanced Radiation Technology Institute, Korea Atomic Energy Research Institute (KAERI), 1266, Sinjeong-dong, Jeongup-si, Jeollabuk-do 580-185, Republic of Korea

a r t i c l e

i n f o

Article history: Received 30 January 2009 Received in revised form 6 May 2009 Available online 18 May 2009 PACS: 61.82.Pv 72.80.r 82.35.Gh 82.45.Wx 82.47.Aa

a b s t r a c t Grafted separators, for which poly(ethylene glycol) borate acrylate (PEGBA) was grafted onto polyethylene (PE) separator, were newly prepared by electron beam irradiation. The grafted separators were characterized by FT-IR, energy dispersive X-ray spectrometer (EDS). The morphological changes of the grafted separators were investigated by scanning electron microscopy (SEM). The degree of grafting was increased with irradiation doses. The ionic conductivity of the grafted separator showed the highest value of 6.24  104 S cm1 at 10 kGy. In addition, its lithium ion transference number and electrochemical stability were enhanced to 0.53 and 4.8 V, respectively owing to anion trapping effect of the grafted unit. The Li ion cells using the grafted separator showed better cycle performances than that using conventional PE separator at various C-rates and high voltage operation conditions. It is suggested that this grafted separator can be a promising candidate for high voltage operation of lithium secondary batteries. Crown Copyright Ó 2009 Published by Elsevier B.V. All rights reserved.

Keywords: Anion receptor Electron beam irradiation Graft separator High voltage Lithium rechargeable battery

1. Introduction During the past decade, rechargeable lithium batteries have been extensively studied and widely used for various potential applications including portable electronic devices [1]. With the dramatic increase on the demand of higher performance batteries in various devices such as hybrid electric vehicles, research and development of high voltage battery materials have also become evidently important [2]. As one of the battery key components, the separator plays a critical role in lithium secondary batteries. It is placed between the positive electrode and the negative electrode to prevent physical contact of the electrodes. It has to allow the rapid transport of the ionic charge carriers that are needed to complete the circuit inside the battery during charging and discharging of an electrochemical cell. Therefore, separators must be electronically good insulator and at the same time a good ionic conductor [3,4]. Although many research works have been done on the improvement of separator materials, the ideal one still remains to be devel-

* Corresponding author. Tel.: +82 42 350 3925; fax: +82 42 350 3910. E-mail address: [email protected] (J.-K. Park).

oped. The most important requirements of a good separator include the following. Structurally, the separator should have sufficient porosity to absorb liquid electrolyte for the high ionic conductivity. Since the presence of separator adds electrical resistance and takes up some fraction of the limited space inside the battery, which adversely affects battery performance, the separator is required to be thin. Essentially, it must be chemically and electrochemically stable towards the electrolyte and electrode materials, and must be mechanically strong to withstand the high tension during the battery assembly operation. The separator should be also able to shut the battery down when overheating occurs, such as the occasional short circuit, so that thermal runaway can be avoided. Therefore, selection of an appropriate separator is critical to the battery performance including energy density, power density, cycle life and safety. Polyolefin separators, which have been widely used in lithium secondary batteries, exhibit disadvantages like poor compatibility with liquid electrolytes due to their hydrophobic surface and low retention ability to hold the organic solvent with high dielectric constant [5]. Many research efforts have been focused to overcome these problems by introducing a coating layer on the polyolefin separator having inorganic fillers [6] or by modifying the polyolefin separator surface with hydrophilic materials [7,8].

0168-583X/$ - see front matter Crown Copyright Ó 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.05.003

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2. Experimental 2.1. Synthesis of the grafting material The synthesis procedure of poly(ethylene glycol) borate acrylate (PEGBA) was reported in our previous report [15]. The PEGBA as a grafting material was synthesized by the dehydrocoupling reaction of poly(ethylene glycol) acrylate (Mn = 375, Aldrich) and BH3/tetrahydrofuran (THF) complex (Aldrich), respectively. The mixture was heated to 60 °C with stirring for 24 h under nitrogen atmosphere. The solvent was evaporated with a rotary evaporator at reduced pressure. The synthesized compound was dried in a vacuum oven at 40 °C for 24 h to eliminate residual solvent and kept in the Ar filled glove box. 2.2. Preparation of the grafted separator Polyethylene separator (Asahi Kasei Co.) of 20 lm thickness, used as a polymer host, was washed with acetone and dried in vacuum oven at 50 °C for 12 h. The solution is prepared by dissolving

PE separator Grafted separator

Absorbance (a.u.)

It has been considered that above 4.4 V the conventional electrolytes currently used in Li-ion batteries usually start to decompose, which means that the stable electrochemical voltage window is limited to 4.4 V. The side reactions due to the instability of electrolytes at high voltage (>4.4 V) on the electrode surface can substantially deteriorate the cycling behavior of the cathode. To solve this problem, anion receptors have recently been considered as good additives in lithium secondary batteries, since they can form stable complexes with anions, and thereby improve electrochemical and thermal stability of anions by suppressing the decomposition reaction of anions [9,10]. They can also contribute to the increase in the lithium ion transference number and dissociation fraction of lithium salt [11]. Boron based anion receptors have been often used to enhance the battery performance [12–16]. Radiation-induced grafting has been considered an appealing method to develop ion-conducting membranes for various electrochemical applications owing to their simple processibility [17]. The choice of grafting materials is a critical factor for determining electrochemical performances of the cells. In this work, we attempted to prepare the new grafted PE separator containing anion receptor with poly(ethylene oxide) and boron units by electron beam irradiation technique. The physical, electrochemical properties of the grafted separators were studied and the cycle performances of lithium rechargeable batteries were also examined at various current rates and also at high voltage condition.

3000

2500

2000

1500

1000

-1

Wavenumber (cm ) Fig. 2. FT-IR spectra of the PE separator and the grafted separator.

PEGBA (40 wt%) and ferric chloride (0.05 wt%) as an inhibitor for polymerization of PEGBA in the mixture of methanol (Aldrich)/ decaline (Riedel-de Haën) (2:3 by volume). The PE separators were weighed and then placed in the above solution. Nitrogen was purged into the reactor for 10 min to remove oxygen which acts as a grafting inhibitor. They were irradiated using electron beam with various doses. After grafting, the separators were washed with acetone to eliminate the unreacted monomers and PEGBA derived homopolymers. The obtained grafted separators were then dried under vacuum at 50 °C for 12 h, and weighed. The degree of grafting (DG) was calculated using the following equation:

DG% ¼

Wg  W0  100 W0

ð1Þ

where W0 and Wg are weights of the bare and the grafted PE separators, respectively. 2.3. Characterization of the grafted separator FT-IR spectra were recorded in the attenuated total reflectance (ATR) mode on a Bruker Tensor 27 spectrometer with the resolution of 4 cm1 in the vibrational frequency range of 600– 4000 cm1. The bare PE separator was used as a reference material. The surface morphologies and chemical composition of the separators were investigated using a field emission scanning electron microscope (FE-SEM, FEI Sirion) attached with energy dispersive X-ray spectrometer (EDS, EDAX Inc., USA). 2.4. Electrical measurements The polymer electrolytes were prepared by soaking the separators in a liquid electrolyte (1 M LiPF6 in ethylene carbonate (EC)/ dimethylene carbonate (DMC) 1:1 by volume, Cheil Industries). To measure ionic conductivities, the polymer electrolyte films

Table 1 The degree of grafting in PE-g-PEGBA separators as a function of irradiation dose.

Fig. 1. Energy dispersive spectrometer (EDS) analysis of the grafted separator.

Sample

Degree of grafting (%)

PE separator 5 kGy 10 kGy 20 kGy 30 kGy 50 kGy

– 3.73 4.26 5.53 7.28 9.59

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Table 2 Ionic conductivities, transference numbers, and cationic conductivities of the grafted separators. Sample PE separator 5 kGy 10 kGy 20 kGy 30 kGy 50 kGy

Ionic conductivity (S cm1) 4

4.88  10 5.16  104 6.24  104 4.71  104 4.38  104 4.05  104

Transference number

Cationic conductivity (S cm1)

0.23 0.36 0.53 0.48 0.44 0.41

1.12  104 1.88  104 3.31  104 2.27  104 1.91  104 1.64  104

Fig. 3. Scanning electron microscope (SEM) images as a function of irradiation dose: (a) no irradiation, (b) 5 kGy, (c) 10 kGy, (d) 20 kGy, (e) 30 kGy, and (f) 50 kGy.

were sandwiched between two stainless steel (SS) electrodes. The ionic conductivities of polymer electrolytes were obtained from bulk resistance measured by a.c. complex impedance using a Solartron 1455 frequency response analyzer (FRA) in combination with a Solartron 1470 potentiostat/galvanostat over a frequency range of 1 Hz  1 MHz under an amplitude of 10 mV. The ionic conduc-

tivity (r) was calculated from the impedance data, using the relad , where d and A are the thickness of prepared polymer tion r ¼ RA electrolyte and the area of the electrodes, and R is bulk resistance obtained from the ac impedance spectrum. The transference number was determined by DC polarization/ AC impedance combination method [18]. A constant polarization

J.Y. Lee et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2390–2394

0.05

-2

Current density (mAcm )

PE separator Grafted separator

0.04

0.03

0.02

0.01

0.00 3.5

4.0

4.5

5.0 +

Voltage (V vs Li /Li) Fig. 4. Comparison of electrochemical stability between the PE separator and the grafted separator.

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of 10 mV was applied to the cell. The transference number of the lithium cation was calculated by the relation, tLi+ = [Is(DV–IoRo)/ Io(DV–IsRs)], where Io and Is are the currents at initial and steadystate, Ro and Rs are the interfacial resistances at initial and steady-state, respectively and could be found from the complex impedance measurements. The electrochemical stability of polymer electrolyte was determined by linear sweep voltammetry experiment performed on a stainless steel electrode as a working electrode with lithium electrode as a reference electrode at a scanning rate of 1.0 mV s1. For the battery performance tests, the prepared 2032 coin type cells were made as follows. The anode was made by mesocarbon microbead (MCMB), conducting carbon, polyvinylidene fluoride (PVdF) as a binder, and copper current collector. The cathode was made by LiCoO2, conducting carbon, PVdF, and aluminum current collector. The cells were assembled by sandwiching the membrane between graphite anode and LiCoO2 cathode, which was activated by liquid electrolytes consisting of EC/DMC (1/1 by volume) with 1 M LiPF6. All assembling of the cells were carried out in a glove box that was filled with Argon. In order to examine the cycle performances, the cells were cycled over 3.0–4.2 V with various discharge rate of 0.1C–3C (but the same charge rate: 0.1C = 0.186 mA cm2) and 3.0–4.4 V at a constant current density of 1C (=1.86 mA cm2) at room temperature using a TOSCAT3000U instrument (Toyo System Co. Ltd.).

3. Results and discussion

Fig. 5. Cycle performance of the unit cells using the PE separator and the grafted separator as a function of various discharge rates (0.1C ? 0.3C ? 0.5C ? 1C ? 2C ? 3C.)

%of the Initial discharge capacity

100

80

60

40

20 PE separator Grafted separator

0 0

50

100

150

200

No. of Cycles Fig. 6. Cycle performance of the unit cells using the PE separator and the grafted separator at high voltage (4.4 V) operating condition.

The modified PE separator was prepared by electron beam irradiation. To confirm the grafting of PEGBA onto the PE separator, the EDS spectrum analysis was done to check the presence of the particular element in the grafted separator (Fig. 1). The EDS spectra of the modified separator showed signals characteristic to boron atom, which clearly proves that the PEGBA has been grafted onto the PE separator. Fig. 2 shows the comparison of FT-IR spectra between the bare PE separator and the grafted separator. The peaks that are characteristic of C–H stretching vibrations at 2850– 3000 cm1 and C–H bending vibration at 1465 cm1 appear in both spectra, indicating that the basic structure of the PE was not altered with grafting. The additional broad peaks at 1350– 1310 cm1 for B–O stretching newly appeared for the grafted PE. Other characteristic bands of B–O and C–O stretching were also observed for the grafted separator at 1070–1040 cm1 and at 1030– 950 cm1, respectively. The peaks at 1790–1720 cm1 are also attributed to C–O stretching [19]. The degree of grafting has been calculated for different dose of the irradiation. The degree of grafting of the separator corresponding to the irradiation dose is listed in Table 1. The degree of grafting increases with increasing irradiation doses from 0 to 50 kGy. At 50 kGy, the degree of grafting of the separator is maximum and reaches about 10%. This is because the number of irradiation induced radicals, which further take part in the grafting reaction, is increased with increase of the irradiation dose. The change in the ionic conductivities and transference numbers as a function of irradiation dose is summarized in Table 2. The ionic conductivity of the grafted separator was found to increase with the increase in the irradiation dose up to 10 kGy. The highest ionic conductivity is measured to be 6.23  104 S cm1 at 10 kGy of irradiation dose. Above 10 kGy, it is observed that the ionic conductivity is decreased with increase of irradiation dose. A similar tendency was found for the behavior of the transference number. Most non-aqueous electrolytes for lithium secondary batteries are Lewis bases that interact with cations, causing a high degree of ion pairing and also the formation of triplets and higher aggregates. This makes lithium ion trans-

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ference number reduced and results in polarization losses in batteries. Up to 10 kGy, the effect of the added amount of anion receptor on ionic conductivities of the grafted separator is more important than the effect of morphology changes caused by electron beam irradiation. It is because the relative content of Lewis acid center in the grafted separator increases with increasing irradiation doses without significant changes of morphology with respect to the bare PE separator. Fig. 3 presents the surface morphologies of the bare PE separator (before irradiation) and the modified separators (at various irradiation doses, 5–50 kGy). It is found that, at higher doses above 10 kGy, the pores are substantially blocked, which leads to a decrease in ion conductivity. The surface of the grafted separator prepared with a high dose irradiation shows intermixing of the fabrics and damaged features. The cationic conductivity values, which can be calculated by multiplying ionic conductivity with transference number, also show the maximum at 10 kGy. Therefore, for further analysis, we selected the grafted separator which has been irradiated at 10 kGy. In order to evaluate the effect of Lewis acid center interacting with anions, the electrochemical stability of the grafted separator was measured by linear sweep voltammetry. Fig. 4 shows the linear sweep voltammetry curves of the cells with the bare PE and the grafted separator. The oxidation peak of the PE separator-based cell is observed around 4.3 V. This peak is attributed to the decomposition of the liquid electrolyte. For the grafted separator, the oxidation peak is found to shift towards higher values and the liquid electrolyte is found to be stable up to 4.8 V. This stability enhancement may be originated from the stabilization of anions by Lewis acid center in the grafted separator [15]. The irreversible oxidative decomposition of anions is delayed by strong complex formation between PEGBA units and anions. The cycle performance profiles of graphite/LiCoO2 cells with the grafted separator at a various discharge rate (0.1C–3C) between 3.0 and 4.2 V are shown in Fig. 5. The grafted separator-based cell shows a similar cycle performance at 0.1C discharge rate to the cell based on the bare separator. On the contrary, cycle performance becomes better for the grafted separator-based cell at higher discharge rate. Fig. 6 shows cycle performances of the unit cell at high voltage condition (3.0–4.4 V, 1C-rate of charge/discharge). After 200 cycles, the% discharge capacity of the cell based on the grafted separator is quite higher than that of the cell based on the bare PE separator. These performance improvements are attributed to the interaction of the anion receptor and anions, which suppresses degradation of cell performance during the cell operation. It will bring about sub-

stantial increase in the energy density and would extend the application to the heavy duty batteries like HEV. 4. Conclusions The grafted PE separator based on the anion receptor PEGBA could be prepared by electron beam irradiation method. It shows highest ionic conductivity around 10 kGy irradiation dose. It could also enhance electrochemical stability. The unit cell using the grafted separator exhibits improved cycle performance at various current rates and especially at high voltage operating conditions. The newly grafted separator is considered to be a good candidate for the separator of high voltage operating lithium secondary batteries. Acknowledgements This work was performed with the financial support of the Nuclear R&D Program and the Brain Korea project under the Ministry of Education, Science and Technology, Republic of Korea. And also it was supported by a grant from by the Korea Science and Engineering Foundation (KOSEF) grant (WCU program, 31-2008-00010055-0) funded by the Ministry of Education, Science and Technology, Republic of Korea. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

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