Effect of the addition of carbon black and carbon nanotube to FeS2 cathode on the electrochemical performance of thermal battery

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JIEC-1782; No. of Pages 6 Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx

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Effect of the addition of carbon black and carbon nanotube to FeS2 cathode on the electrochemical performance of thermal battery Yusong Choi a,b, Sungbaek Cho a, Young-Seak Lee b,* a b

Agency for Defense Development, Yuseong P.O. Box 35-41, Daejeon 305-600, Korea Department of Fine Chemical Engineering and Applied Chemistry, Chungnam National University, Daejeon 305-764, Korea

A R T I C L E I N F O

Article history: Received 6 November 2013 Accepted 17 December 2013 Available online xxx Keywords: Thermal battery Iron-disulfide Carbon black Carbon nanotube Functional group Conductive network

A B S T R A C T

Effect of the addition of conductive carbonaceous materials to the FeS2 (pyrite) cathode on electrochemical performance of thermal battery is investigated by adding carbon blacks (CBs) or multi-walled carbon nanotubes (MWCNTs) which has conductive network structures with various amounts from 0.1 to 1 wt.%, compared to the amount of pure FeS2. Among the samples prepared with various amounts of CB or MWCNT addition, the 1 wt.% CB-added sample exhibits the highest electrochemical properties. These results suggest that the improvement in the electrochemical performance of thermal batteries can be achieved by the addition of the conductive carbonaceous materials to pyrite electrode. ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

1. Introduction Due to their excellent mechanical robustness, reliability, and long shelf life, thermal batteries have been used as the primary or assistant power sources for guided weapons [1]. Thermal batteries are activated by the melting of a solid electrolyte to a molten salt at the high temperature of 500 8C. Li–Si alloys and pyrite (FeS2) are the typical anode and cathode materials for thermal batteries [2,3], because pyrite is naturally abundant and Li–Si is inexpensive and has a high electromotive force (emf), compared to the other anode materials. However, the pyrite cathodes have the high internal resistance issue which reduces the electrochemical performance of Li–Si/FeS2 thermal batteries [4,5]. Many research efforts have been made to reduce the internal resistances of the electrode materials in thermal batteries, including the use of new electrode materials and the new fabrication methods, such as plasma spray method [6,7]. Due to the high cost of these new materials and fabrication processes, these new approaches have not yet been commercialized. Numerous researchers have reported that the internal resistance of lithium and Ni-MH batteries can be reduced by the introduction of conductive carbonaceous materials, such as carbon

* Corresponding author. Tel.: +82428217007. E-mail addresses: [email protected], [email protected] (Y.-S. Lee).

blacks (CBs) and carbon nanotubes (CNTs) [8–14]. Until now, there is no research result to improve the electrochemical performance of Li–Si/FeS2 thermal batteries by the addition of conductive materials to the electrode. Therefore, this study aims to investigate the potential use of the conductive carbonaceous additives for the pyrite cathode for thermal batteries. CBs and CNTs were selected as the conductive carbonaceous additives and added to the pyrite cathode with various amounts from 0.1 to 1 wt.%, compared to the amount of pure FeS2. The effects of the CBs and MWCNTs addition on the electrochemical properties of Li–Si/FeS2 thermal batteries are investigated using the single cell discharge test, scanning electron microscope (SEM), electrochemical impedance spectroscopy (EIS) and lumped element modeling analysis. 2. Experimental 2.1. Sample preparation All of the raw materials were dried at 100 8C for 12 h under vacuum condition before use. Pyrite cathode was prepared with 73.5 wt.% FeS2, 1.5 wt.% Li2O, and 0.11.0 wt.% of CBs or MWCNTs, compared to FeS2, and the balance weight of LiCl–KCl eutectic salts. The pyrite cathode electrode was prepared using the following procedure. LiCl–KCl eutectic salts mixture (LiCl 45%–KCl 55%) was made by melting. The eutectic salt was mixed with the MgO particles (300 nm, 99%, Scora, France), an electrolyte binder, and

1226-086X/$ – see front matter ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.12.052

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then melted followed by grinding. And then FeS2 particles (average size of 98 mm, 99%, LinYi, China) were mixed with Li2O (Sigma– Aldrich, USA), a voltage regulator, and then melted followed by grinding and sieving (100 + 325 mesh). A conductive additive, either CBs (Super–P, TIMCAL, Swiss) or MWCNTs (CM-250, Hanwha Chemical Co., Ltd, South Korea) was added to this final mixing process. The amounts of CB or MWCNT added to the pyrite cathode pellet were 0.1, 0.25, 0.50, 0.75, and 1.0 wt.%, compared to the pure pyrite, respectively. The net pyrite weight was the same for each pyrite cathode. The pyrite cathode pellets (pristine, CB- or MWCNT-added) were made by pressing with a density of 3 g cm3. The anode was fabricated with following ratios and methods. The anode was prepared with the mixture of 75 wt.% of the commercial Li-Si (99%, Li 44–Si 56 wt.%, GRINM, China) and 25 wt.% of LiCl–KCl eutectic salts. Then, the anode was prepared by melting the mixture followed by grinding and pressing. The electrode preparations were performed at room temperature in a dry room with a dew point less than 52 8C. 2.2. Electrochemical testing and EIS measurement The morphologies of the three types of pyrite cathodes, pristine, CB- or MWCNT-added samples, were observed using a SEM (XL30FEG, Philips, Netherland). Crystallographic properties were also measured on three types of pyrite cathodes with an X-ray diffractometer (XRD, D8-Advance, Bruker, Germany, CuKa radiation, l = 1.54 A˚). The single cell discharge test was performed at the consecutive current pulses using a current density of 0.52 A cm2 for 3 s, followed by a subsequent current density of 0 A cm2 at for 1 s. Single cell was discharged in the hydraulic press, which can heat up to 500 8C. The total polarization values of the pristine, 0.1 wt.% CB- or 0.1 wt.% MWCNT- added single cells were calculated from the single cell discharge results using the following equation, as reported by Fujiwara et al. [15]. Rt ¼ ðV oc  V cc Þ=I

(1)

where, Rt, Voc, Vcc and I are the total polarization, the opencircuit voltage (OCV), the closed-circuit voltage (CCV) and the applied current, respectively. In order to measure the voltage variation as a function of current density, the discharge tests for the pristine and at the range of 0.25  1.0 wt.% CB- or MWCNT-added samples were performed with increasing the applying current for every 5 s. EIS (Solartron 1260, USA) measurement was performed using a single cell by applying a small perturbation voltage of 10 mV in the frequency range of 100 kHz to 100 MHz [16]. Lumped equivalent circuit modeling and simulation were conducted with impedance analysis results of 0.25  1.0 wt.% CBor MWCNT-added. An equivalent circuit model applied in this study is as follows. Parameter evaluation on this equivalent circuit model was conducted using a Zsimpwin software by a iteration method. 3. Results and discussion 3.1. Characteristics of the electrode powder The SEM images of the pristine, 0.1 wt.% CB- or MWCNT-added samples are shown in Fig. 1. In the microstructure of pristine in Fig. 1(a), pyrite particles, designated by an arrow, shows the general crystalline shapes as reported by Cheong et al. [17]. The LiCl–KCl eutectic salts are well adhered onto the pyrite particles, which can improve the electrochemical properties by providing

Fig. 1. SEM images of (a) pristine, (b) 0.1 wt.% CB-added, and (c) 0.1 wt.% MWCNTadded cathode samples.

ionic path inside cathode (flooded type electrode). In the prestine’s microstructure, there is no significant crack and void. The fractured surface morphologies of the 0.1 wt.% CB-added (Fig. 1(b)) and MWCNT-added (Fig. 1(c)) samples are similar to that of the pristine sample. In Fig. 1(b,c), the CB and MWCNT added to the pyrite cathode are not identified. However, in SEM analysis, it can be concluded that a sound structure of CB- or MWCNT-added cathode can be easily fabricated by the traditional pyrite cathode fabrication process. In order to investigate further about CB or MWCNT on pyrite cathode pellet, XRD and Energy Dispersive X-ray Spectroscopy (EDS) analysis are performed. The XRD profiles of the pristine, 0.1 wt.% CB- or MWCNT-added samples and the EDS spectra of the 0.1 wt.% CB-added pyrite cathode are shown in Fig. 2. In Fig. 2,

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Fig. 2. XRD profiles of the pristine, 0.1 wt.% CB-added, and 0.1 wt.% MWCNT-added cathode samples, and energy dispersive X-ray spectra of the CB-added pyrite cathode.

diffraction peaks for the single phase of FeS2 [18], eutectic salt, and MgO particles are identified as each symbol. The pattern of the 0.1 wt.% MWCNT-added sample shows the sharp diffraction peak at approximately 268, which corresponds to the (0 0 2) crystallographic plane of MWCNTs [8]. Due to the amorphous properties of CBs (Super P), the pattern of the CB-added sample exhibits no diffraction peaks near 268. Further characterization with EDS on the CB-added sample is performed to 0.1 wt.% CB-added sample. A carbon peak is detected in the EDS spectra of the 0.1 wt.% CB-added sample. This result suggests more certainly that the CBs or

MWCNTs can be well incorporated inside of the pyrite cathodes with the conventional fabrication process. 3.2. Electrochemical properties The single cell discharge results of the pristine, 0.1 wt.% CB- or MWCNT-added samples are shown in Fig. 3. The single cell pulse discharge results exhibit three voltage plateaus that originate from the phase changes of FeS2 as follows [3]: Plateau1 : FeS2 ! Li3 Fe2 S4 ðZ-phaseÞ Plateau2 : Li3 Fe2 S4 ! Li2þx Fe1x S2 þ Fe1y S Plateau3 : Li2 FeS2 ! Fe þ Li2 S

Fig. 3. Single cell discharge results of (a) pristine, (b) 0.1 wt.% CB-added, and (c) 0.1 wt.% MWCNT-added pyrite samples.

At the beginning of discharge, the OCVs of the pristine, 0.1 wt.% CB- or MWCNT-added samples are approximately 1.9 V. The 0.1 wt.% CB- or MWCNT-added sample exhibits a higher voltage than the pristine sample over the entire voltage range and the voltage of the MWCNT-added sample is higher than that of the CBadded sample over the same range. It is reported that the Z-phase plateau region is mostly used for Li–Si/FeS2 thermal battery because of strict voltage range regulation of the electronic devices. The Z-phase plateau is finished about 1.58 V in Fig. 3. The increase of discharge time and higher voltage performances in the CB- or MWCNT-added samples are noticeable, compared to the pristine. Compared to the 0.1 wt.% CB-added sample in single cell discharge results, MWCNT-added sample shows higher electrochemical properties, voltage and discharge time, than those of CB-added. As shown in Fig. 4, the total polarization of pristine is higher during the entire range of discharge than the CB- or MWCNT-added

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0.020 Pristine CB added MWCNT added

Total polarization (Ω)

0.018 0.016 0.014 0.012 0.010 0.008 0

50

100

150

200

250

300

Time (sec) Fig. 4. Total polarization values of the pristine, 0.1 wt.%CB-added, and 0.1 wt.% MWCNT-added single cells.

samples. Total polarization result is in accordance with the single cell discharge result. It is found that both CBs and MWCNTs are effective conductive additives for decreasing the internal resistance of pyrite cathode. In addition, this result implies that at a small amount of CBs or MWCNTs addition to the pyrite cathode can exhibit higher effect on enhancing electrochemical properties than the prestine. Nyquist plots of the samples with CB and MWCNT-added at the range of 0.25  1.0 wt.% are shown in Fig. 5. In Fig. 5, the intercept -0.020

(a) CB 0.25 wt.% CB 0.50 wt.% CB 0.75 wt.% CB 1.00 wt.%

Im Z (Ω)

-0.015

-0.010

-0.005

0.000 0.00

0.01

0.02

0.03

0.04

0.05

0.06

Re Z (Ω) -0.020

(b)

Im Z (Ω)

-0.015

-0.010

MWCNT 0.25 wt.% MWCNT 0.50 wt.% MWCNT 0.75 wt.% MWCNT 1.00 wt.%

-0.005

0.000 0.00

0.01

0.02

0.03

0.04

0.05

0.06

Re Z (Ω) Fig. 5. Nyquist plots for (a) CB-added and (b) MWCNT-added pyrite samples from 0.25 to 1.0 wt.%.

Fig. 6. The crumbled cathode of more than 1 wt.% CB-added samples.

on the real impedance axis (Re Z) represents the total resistance of the electrolyte, whereas the semi-circle over the high- and medium-frequency ranges and the slope of the line over the low-frequency range represent the impedance of the chargetransfer process and the solid-state diffusion process of lithium ions inside the pyrite particles, respectively [19–21]. Though the semi-circle is not clear, the diameter of the semicircle is decreased with increasing the amount of CB. Similarly, the diameter of the semi-circle is decreased with increasing the amount of the MWCNT. Due to MWCNT’s favorable electron conductivity characteristics compared to CB’s [22,23], the MWCNT-added sample produces a smaller semi-circle than the CB-added sample, which suggesting that charge transfer in the MWCNT samples is more effective than the CB-added samples. At a 1.0 wt.% addition, both the CB- and MWCNT-added samples produces smaller semi-circles compared with those produced by samples that contained 0.25  0.75 wt.%. In this study, 1.0 wt.% is the maximum amount, which can be fabricated the mechanically treatable CB- or MWCNT-added cathodes. The cathodes, which are added over 1.0 wt.% of CB and MWCNT, is easily crumbled, as shown in Fig. 6. The variations of the voltage as a function of current density of the CB- or MWCNT-added samples at the amount of 0.25  1.0 wt.% are shown in Fig. 7. Fig. 7(a) shows that the polarization of the CB-added samples decreases with increasing the amount of CB. Similarly, the polarization of the MWCNT-added samples decreases with increasing the amount of MWCNT. Consequently, the 1.0 wt.% MWCNT-added sample exhibits the highest discharge performance compared to those samples containing 0.25  0.75 wt.%. Interestingly, the higher OCVs of 2.2 V for the CB- or MWCNTadded samples are found. It is reported that higher voltage at open circuit can be caused by the presence of impurity in electrode and the oxidation layers of FeS2 surface. However, as shown in Fig. 2, there is no significant evidence of impurity by addition of carbonaceous materials. Moreover, Li2O, a voltage regulator, is added during the fabrication. Therefore, further study is needed for the higher OCVs for this result. 3.3. Lumped-element modeling analysis Simplified and lumped modeling and simulation results for thermal battery were reported by several researchers [24–27]. However, their models are based on many approximations and

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0.030

2.2

CB CNT

(a) 2.0

0.020

Rct ( Ω )

1.8

Voltage (V)

0.025

CB 0.25 wt.% CB 0.50 wt.% CB 0.75 wt.% CB 1.00 wt.%

0.015

1.6

0.010

1.4 0.005

1.2 0.000

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.25

0.50

0.75

1.00

Weight %

-2

Current Density (mA cm ) Fig. 8. Charge transfer resistance of CB- and MWCNT-added cells.

2.2

(b)

MWCNT 0.25 wt.% MWCNT 0.50 wt.% MWCNT 0.75 wt.% MWCNT 1.00 wt.%

Voltage (V)

2.0 1.8 1.6

sample showed the lowest charge transfer resistance. Whereas, at the range of 0.25  0.5 wt.%, the MWCNT-added samples show more effect on reducing the charge transfer resistance than the CBadded samples. However, over 0.5 wt.% addition, CB is more effective than MWCNT. The exchange current density (jo) is calculated with the charge transfer resistance (Rct) by using the equation as follows [29]. jo ¼

1.4 1.2 0.0

0.5

1.0

1.5

2.0

2.5

3.0

-2

Current Density (mA cm ) Fig. 7. Current density vs. V for (a) CB-added and (b) MWCNT-added pyrite samples from 0.25 to 1.0 wt.%.

referred to the reported parameters. There are no reports on the test-based model and simulation for thermal batteries. Lumped-element modeling and simulation are conducted based on the impedance measurement results of CB- or MWCNT-added samples, as shown in Fig. 5. Using the lumpedequivalent circuit model, the each element parameter of samples is obtained using a Zsimpwin desktop version of software. In the lumped-equivalent circuit model, Re is an electrolyte resistance of single cell. Cd and Rct are a capacitance and a charge transfer resistance at the interface of electrode and electrolyte, respectively. Rw and L are a Warburg resistance and an inductance of electrode and current collector [28]. The simulated element parameters imply that which elements are mostly influencing to the electrochemical properties in CB- or MWCNT-added sample. As a result of parameter evaluation, it is found that the most influencing parameter to the electrochemical properties is the charge transfer resistance (Rct). By the addition of conductive additives to the pyrite cathode, the charge transfer resistance is decreased with increasing conductive additives, as shown in Fig. 8. The charge transfer resistance of CB- or MWCNT-added sample is decreased with increasing the amount of CB or MWCNT, except the 0.75 wt.% samples. This result is ascribed to the effect on reducing the charge transfer resistance of CBs and MWCNTs caused by forming conductivity networks and their high electric conductivity. Among the various amounts of samples, 1 wt.% CB-added

RT nFRct A

(2)

where, R and T are a gas constant and a temperature (K), respectively. n is a number of electrons involved in the electrochemical reaction and F is a Faraday constant. A is an electrode area involved in electrochemical reaction. Table 1 represents the exchange current density as a function of conductive additive amounts. As the increase of CB from 0.25 to 1.0 wt.% the exchange current density increase from 0.2275 A cm2 to 0.8306 A cm2. However, as the increase of MWCNT from 0.25 to 1.0 wt.%, the exchange current density increases from 0.3682 A cm2 (0.2275 A cm2 for 0.25 wt.% CB-added sample), to 0.6796 A cm2. 2. Concerned with these results, it can be concluded that MWCNTs has higher electric conductivity than that of CBs [22,23]. Therefore, a small amount (less than 0.5 wt.%) of MWCNT to pyrite can reveal the higher charge transfer rate and exchange current density compared to the same amount of CBs. However, the lower charge transfer rate of MWCNTs as increasing the amount (over 0.5 wt.%) of MWCNTs, compared to CB, is may be attributed to the agglomeration of MWCNTs. It seems that further study is needed for the lower charge transfer rate as increasing MWCNTs amount compared to CBs over the amount of 0.5 wt.%. Consequently, excellent performance of the pyrite cathode has resulted from the addition of electro-conductive additives, CBs and MWCNTs, which reduce the charge transfer resistance by forming conductive network structures between the pyrite particles. Table 1 Exchange current density change of CB- or MWCNT-added cells. Wt.% of CB or MWCNT

0.25 0.5 0.75 1.0

Exchange current density (jo, A cm2) CB

MWCNT

0.2275 0.3042 0.2482 0.8306

0.3682 0.4261 0.1903 0.6796

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4. Conclusions The changes in electrochemical performance of thermal battery induced by the addition of conductive carbonaceous materials was investigated using CBs and MWCNTs additives to the pyrite cathode with the various amount from 0.1 to 1 wt.%, compared to the amount of pure pyrite. Among the various amounts of CB- or MWCNT-added samples, the 1 wt.% CB-added sample exhibits the highest electrochemical properties. Within the range of 0.1  0.5 wt.%, the MWCNT-added samples show higher electrochemical properties than those of CBadded samples. However, over 0.5 wt.% of addition, CB-added samples exhibit higher electrochemical properties than those of MWCNT-added samples. These results suggest that the improvement in the electrochemical performance of thermal batteries can be achieved by the addition of the conductive carbonaceous materials to pyrite electrode. References [1] [2] [3] [4] [5] [6]

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