- Email: [email protected]
Effects of lithium phosphorous oxynitride film coating on electrochemical performance and thermal stability of graphite anodes
Journal of Physics and Chemistry of Solids 72 (2011) 842–845
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Effects of lithium phosphorous oxynitride film coating on electrochemical performance and thermal stability of graphite anodes Yoon-Soo Park, Sung-Man Lee n Department of Advanced Materials Science and Engineering, Kangwon National University, Chuncheon, Kangwon-Do 200-701, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 October 2010 Received in revised form 18 April 2011 Accepted 24 April 2011 Available online 29 April 2011
In this study, we investigated the effects of lithium phosphorus oxynitride (LiPON) solid electrolyte thin-film deposition on the electrochemical performance and thermal stability of pristine graphite and carbon-coated graphite composite anodes. The LiPON film was deposited by radio frequency (rf) magnetron sputtering. We studied the thermal stability of the lithiated electrodes when immersed in the presence of a liquid electrolyte by differential scanning calorimetry (DSC). The LiPON thin-film coating suppressed the impedance growth during the cycling process and inhibited the reaction between the lithiated electrode and the electrolyte, thus improving the cycle performance and thermal stability of the graphite electrode. However, for the carbon-coated graphite electrode, the heat evolution below 250 1C decreased, whereas that below 300 1C increased. We attributed this phenomenon to the low thermal stability of the LiPON thin-film coating owing to an exothermic reaction between the LiPON film and the electrolyte that occurs at approximately 290 1C. & 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Thin film B. Plasma deposition C. Differential scanning calorimetry (DSC) D. Electrochemical properties
1. Introduction Lithium-ion batteries have been widely used as power sources for portable electronic devices, and their use as nextgeneration power sources for electric vehicles and energy storage systems for renewable energy is now being explored. Owing to the ever-increasing applications of lithium-ion batteries, battery safety has been an issue of concern besides the electrochemical performance. Commercially, graphite is mainly used for fabricating the anode material of lithium-ion batteries. Carbon-coated graphite has attracted attention because it offers improved electrochemical performance [1–4]. It is known that the exothermic reaction between lithiated graphite anode materials and liquid electrolytes may lead to the thermal runaway of lithium-ion batteries [5–8]. Despite this fact, the suppression of this graphite-related exothermic reaction has not been studied in adequate detail. The electrochemical properties of graphite can be improved by its surface treatment with metal and metal oxides [9,10]. It should be noted here that the surface treatment consists of coating or encapsulation of graphite particles. On the other hand, a surface modification by depositing a protective film on a surface of graphite composite electrodes has been rarely performed. In particular, the surface coating of graphite anodes with an inorganic solid electrolyte film is considered to be a promising
n
Corresponding author. Tel.: þ82 33 250 6266; fax: þ82 33 242 6256. E-mail address: [email protected] (S.-M. Lee).
0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2011.04.015
approach because this coating acts as an effective protective layer that controls the interfacial reaction of the graphite and serves as an effective ion-conducting and separating film. Among the various types of solid electrolytes, amorphous lithium phosphorous oxynitride (LiPON) finds a wide use as a solid electrolyte in thin-film batteries owing to its excellent electrochemical stability [11,12]. It has been previously reported that the initial irreversible capacity is decreased by the deposition of the LiPON film on a carbon electrode surface [13]. However, little attention has been paid to the effect of LiPON film coating on the cycling performance and thermal stability of graphite anodes. In this paper, we report the electrochemical performance and thermal stability of both graphite anode and a carbon-coated graphite anode with a LiPON thin film that is surface-deposited by sputtering.
2. Materials and methods Graphite and carbon-coated graphite anodes were fabricated using pristine and carbon-coated natural graphite (NG) spheres, respectively, provided by Carbonix, Inc., Korea. A proprietary coating method was utilized to prepare the carbon-coated spherical NG. The electrodes were prepared by coating a slurry of graphite (91 wt%), carbon black (3 wt%), and poly(vinylidene fluoride) (PVDF) binder (6 wt%) dissolved in N-methyl pyrrolidone (NMP) on a copper foil. The slurry-coated electrode was dried at 120 1C for 12 h. The LiPON thin film was deposited by rf magnetron reactive sputtering of a Li3PO4 target in a N2 gas
Y.-S. Park, S.-M. Lee / Journal of Physics and Chemistry of Solids 72 (2011) 842–845
atmosphere. The surface morphology of the pristine and LiPONcoated electrodes was observed using scanning electron microscopy (SEM). The thickness of the deposited film was estimated based on the thickness of the film deposited on a silicon wafer under similar conditions. For the evaluation of electrochemical performance, a solution of 1 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC; 1:1 by volume provided by PANAX ETEC Co., Ltd, Korea) was used as an electrolyte. A coin-type half cell was assembled with a lithium foil counter electrode. The cells were galvanostatically cycled at 0.2 mA cm 2 between 0.005 and 2.0 V at 30 1C. Electrochemical impedance spectroscopy measurements were performed on the cells in the discharged state of 2.0 V using an impedance analyzer (IM6e Zahner Elektrik). The amplitude of the AC signal was 5 mV, and the frequency ranged from 100 mHz to 1 MHz. For DSC measurements, the cells were pre-cycled three times to reach a stable capacity level, and the cycling was interrupted when the cells were charged to a fully intercalated state. Then, the charged cells were disassembled in a glove box. Disks with a diameter of 40 mm with a Cu current collector were cut from the electrode sheet without removing the electrolyte and transferred to a high-pressure stainless steel pan with a gold plated copper seal. The DSC scans were performed in Ar atmosphere using a DSC 200 F3 (NETZSCH, Germany) at a constant heating rate of 10 oC min 1. The weight of each DSC pan with the sample was found to be constant before and after the DSC measurements, indicating that there was no leakage during the experiments.
3. Results and discussion Fig. 1 shows the surface and cross section (see the inset) SEM micrographs of the pristine and LiPON-coated graphite electrodes.
843
It is observed that the deposited film homogenously covers the graphite particles. Fig. 2 shows EDAX data for the pristine and LiPON-coated electrodes. For the pristine electrode, only peaks corresponding to C and Cu are observed, whereas additional N, O, and P peaks are observed for the LiPON-coated electrodes. This indicates that the film deposited on the graphite electrode is the LiPON thin film deposited by rf magnetron reactive sputtering of the Li3PO4 target in N2 atmosphere. From the initial charge-discharge curves (not shown), we can conclude that the initial coulombic efficiency of the graphite electrode increases slightly from 87.2% to 89.5% due to the deposition of the LiPON thin film. However, in the case of the carbon-coated graphite electrode, both the pristine and the LiPON-coated electrodes have a similar coulombic efficiency of 93%.The LiPON thin-film coating on the graphite electrode prevents the graphite surface from coming in direct contact with the electrolyte and suppresses the so called solid-electrolyteinterface (SEI) film-formation reaction. As a result, this coating minimizes the initial irreversible capacity loss, most of which is attributable to side reactions such as solvent decomposition occurring on the graphite electrode surface. The cyclic voltamogram study for the untreated and LiPON-coated carbon electrodes in 1 M LiPF6/EC : DEC (1:1) shows that the reduction peaks due to EC and DEC decomposition nearly disappear for the LiPON-coated electrode, while in the case of the untreated carbon electrode, the decomposition of DEC occurs at about 2.0 V (versus Li/Liþ) and DEC decomposes at 0.7 V (versus Li/Liþ) [13]. The LiPON film also suppresses solvent co-intercalation. Therefore, the LiPON film suppresses the formation of SEI film by solvent decomposition and co-intercalation but acts as a protective and ion-conducting layer. The exact mechanism of reaction between LiPON film and electrolyte is not, however, yet clear. On the other hand, in the case of the carbon-coated graphite electrode, the deposition of the
Fig. 1. Surface and cross section (inset) FE-SEM images of (a) pristine and (b) LiPON-coated graphite electrodes.
Fig. 2. EDAX data for (a) pristine and (b) LiPON-coated electrodes.
844
Y.-S. Park, S.-M. Lee / Journal of Physics and Chemistry of Solids 72 (2011) 842–845
LiPON film has little additional effect because the active sites of the electrode such as the edge planes of natural graphite, are covered with amorphous carbon. The cycle performance of carbon-coated graphite electrode is compared with that of pristine and LiPON-coated graphite electrodes in Fig. 3. The cycling behavior of the carbon-coated graphite electrode appears to be comparable to that of LiPONcoated graphite electrodes. The cycle performance of the graphite electrode significantly improved due to the deposition of the LiPON thin film. Fig. 4 depicts the impedance spectra of the pristine and LiPON-coated graphite electrodes after 3 and 150 cycles. It can be seen that as the cycling proceeds, the increase in the electrode’s impedance is inhibited by the LiPON thin-film coating. The reduced impedance indicates charged transfer and enhanced Li migration through the surface film [14,15]. The rate performance of the pristine and 1 mm-thick LiPONcoated graphite electrodes is compared in Fig. 5. The cells are charged galvanostatically to 0.005 V at 0.2 mA cm 2 but discharged to 2.0 V at different current densities. It is seen that both electrodes exhibit a similar rate performance up to 2 C while on increasing the discharge current to 5 and 10 C, relative discharge capacity of the LiPON-coated electrode is a little bit lower than that of pristine one. This indicates that the LiPON coating film may impede the lithium-ion transport at high currents. Nevertheless, in the case of the LiPON-coated electrode, the interface with the electrolyte is very stable even after extended cycling as
illustrated by Fig. 4, which may lead to a stable rate performance for long-term cycle. Fig. 6 shows the DSC curves of the pristine and LiPON-coated graphite electrodes in a fully lithiated stage. The temperature at which the exothermic reaction started was similar for both electrodes. However, in the case of the LiPON-coated graphite electrode, the main exothermic peaks moved to a higher temperature, and therefore, the heat evolution below 300 1C was significantly reduced(2480 J/g-2010 J/g). These exothermic reactions are mainly attributed to the reaction between the intercalated lithium and electrolyte, in which the lithium must diffuse through the graphite to reach the surface of the edge planes of the graphite [16]. Therefore, it is suggested that the LiPON thin-film coating on the graphite electrode suppresses the delithiation occurring from within the structure to the surface of graphite during heating, thus enhancing the thermal stability. An 1 mm-thick LiPON thin-film deposited on a copper substrate exhibits a thermal evolution peak at around 290 1C in the presence of a liquid electrolyte as shown in Fig. 7(a). On the other hand, the main exothermic reaction of the carbon-coated graphite electrode occurs above 300 1C (Fig. 7(b)). Therefore, the main exothermic peaks of the carbon-coated graphite electrode are shifted to a lower temperature when the LiPON thin film is deposited. The heat evolution below 300 1C increases from 1020 to 1640 J g 1 when the thickness of the LiPON thin film is increased from 0.5 to 1 mm as illustrated in Fig. 7(c) and (d). It appears that the LiPON thin-film coating lowers the thermal stability of the carbon-coated graphite electrode rather than
110
100
0.5C
90
80
60
normalized Capacity (%)
Normalized Capacity (%)
100
pristine 1μm LiPON coated carbon coated
40
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2C
1C
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pristine 1 micron deposited
60
10C
50 40 30 20 10
0 0
20
40
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80 Cycle
100
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140
Fig. 3. Cycle performances of prisitine, LiPON-coated, and carbon-coated graphite electrodes.
20
20
15
15
10
10 Cycle (number)
pristine electrode 1 μm LiPON coated electrode
10 5
5 pristine electrode 1 μm LiPON coated electrode
0 0
10
20 Z' / Ω
30
40
50
15
20
Fig. 5. Rate-capabilities of pristine and LiPON-coated graphite electrode.
- Z" / Ω
- Z" / Ω
5
0
0 0
10
20
30
40
50
Z' / Ω
Fig. 4. Impedance spectra of pristine and LiPON-coated graphite electrode: after (a) three cycles and (b) 150 cycles.
Y.-S. Park, S.-M. Lee / Journal of Physics and Chemistry of Solids 72 (2011) 842–845
845
the thermal stability of pristine electrode is enhanced by the LiPON thin-film coating.
8 7
Heat flow (mW/mg)
6
pristine electrode
4. Conclusions
5 4 3 2 1 1μm LiPON coated electrode
0 -1 100
200
300 Temperature (°C)
400
500
Fig. 6. DSC curves of the pristine and LiPON-coated graphite electrodes in a fully lithiated state.
Heat flow (mW/mg)
16 14
Heat flow (mW/mg)
12 10
(d)
Acknowledgements 2
(b) 1
(c) 0 100
150 200 Temperature(°C)
250
8 6
References
(b)
0 -2
This study was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF2009-C1AAA001-0093307) and The Energy Efficiency and Resources of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (Grant no. C1006294-01-02(120090820)).
(c)
4 2
An ionic conducting LiPON thin film has been deposited on the surface of pristine graphite and carbon-coated graphite composite anodes by rf magnetron sputtering. The LiPON thin-film coating enhances lithium-ion migration through the interface between the surface of graphite particles and the electrolyte and suppresses the surface reaction between the electrode surface and the electrolyte during cycling as inferred from the cycle performance and impedance spectra data. The LiPON thin-film coating enhanced the thermal stability of the graphite electrode. However, in the case of the carbon-coated graphite electrode, the heat evolution below 250 1C decreased due to LiPON thin-film coating, whereas the heat evolution below 300 1C increased. This is attributed to the low thermal stability of the LiPON thin film.
(a) 100
200
300 Temperature (°C)
400
500
Fig. 7. DSC curves of LiPON-coated and carbon-coated graphite electrode: (a) LiPON film with electrolyte, (b) pristine carbon-coated graphite electrode, (c) 0.5 mm-thick LiPON-coated carbon-coated graphite electrode, and (d) 1 mmthick LiPON-coated carbon-coated graphite electrode.
enhancing it. Similar to the case of pristine electrode (refer to Fig. 6), the heat evolution below 200 1C, which is mainly due to the SEI layer decomposition, decreases by LiPON thin-film coating (refer to the curves in the inset of Fig. 7). It should be noted here that the main exothermic reaction of the pristine electrode starts around 250 1C, while the thermal evolution peak of LiPON thin-film appears around 290 1C. Hence,
[1] M. Yoshio, H. Wang, K. Fukuda, Y. Hara, Y. Adachi, J. Electrochem. Soc. 147 (2000) 1245. [2] H. Wang, M. Yoshio, J. Power Sources 93 (2001) 123. [3] S. Yoon, H. Kim, S.M. Oh, J. Power Sources 94 (2001) 68. [4] H.Y. Lee, J.K. Baek, S.W. Jang, S.M. Lee, S.T. Hong, K.Y. Lee, M.H. Kim, J. Power Sources 101 (2001) 206. [5] H. Maleki, G. Deng, A. Anani, J. Howard, J. Electrochem. Soc. 146 (1999) 3224. [6] D. Aurbach, J. Power Sources 89 (2000) 206. [7] K. Edstrom, A.M. Andersson, A. Bishop, L. Fransson, J. Lindgren, A. Hussenius, J. Power Sources 97-98 (2001) 87. [8] M. Yoshio, H.Y. Wang, K. Fukuda, Y. Hara, Y. Adachi, J. Electrochem. Soc. 147 (2000) 1245. [9] J. Gao, L.J. Fu, H.P. Zhang, T. Zhang, Y.P. Wu, H.Q. Wu, Electrochem.Commun. 8 (2006) 1726. [10] I.R.M. Kottegoda, Y. Kadoma, H. Ikuta, Y. Uchimoto, M. Wakihara, Electrochem. Solid-State Lett. 5 (2002) A275. [11] X. Yu, J.B. Bates, G.E. Jellison Jr., F.X. Hart, J. Electrochem. Soc. 144 (1997) 524. [12] N.J. Dudney, J. Power Sources 89 (2000) 176. [13] K.I. Chung, J.G. Park, W.S. Kim, Y.E. Sung, Y.K. Choi, J. Power Sources 112 (2002) 626. [14] D. Aurbach, M.D. Levi, J. Phys. Chem. B 101 (1997) 4630. [15] S.I. Yuasa, J. Otsuji, A. Tasaka, A. Funabiki, M. Inaba, Z. Ogumi, J. Electrochem. Soc. 145 (1998) 172. [16] Y.S. Park, S.M. Lee, Electrochim. Acta. 54 (2009) 3339.
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Effects of lithium phosphorous oxynitride film coating on electrochemical performance and thermal stability of graphite anodes Yoon-Soo Park, Sung-Man Lee n Department of Advanced Materials Science and Engineering, Kangwon National University, Chuncheon, Kangwon-Do 200-701, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 October 2010 Received in revised form 18 April 2011 Accepted 24 April 2011 Available online 29 April 2011
In this study, we investigated the effects of lithium phosphorus oxynitride (LiPON) solid electrolyte thin-film deposition on the electrochemical performance and thermal stability of pristine graphite and carbon-coated graphite composite anodes. The LiPON film was deposited by radio frequency (rf) magnetron sputtering. We studied the thermal stability of the lithiated electrodes when immersed in the presence of a liquid electrolyte by differential scanning calorimetry (DSC). The LiPON thin-film coating suppressed the impedance growth during the cycling process and inhibited the reaction between the lithiated electrode and the electrolyte, thus improving the cycle performance and thermal stability of the graphite electrode. However, for the carbon-coated graphite electrode, the heat evolution below 250 1C decreased, whereas that below 300 1C increased. We attributed this phenomenon to the low thermal stability of the LiPON thin-film coating owing to an exothermic reaction between the LiPON film and the electrolyte that occurs at approximately 290 1C. & 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Thin film B. Plasma deposition C. Differential scanning calorimetry (DSC) D. Electrochemical properties
1. Introduction Lithium-ion batteries have been widely used as power sources for portable electronic devices, and their use as nextgeneration power sources for electric vehicles and energy storage systems for renewable energy is now being explored. Owing to the ever-increasing applications of lithium-ion batteries, battery safety has been an issue of concern besides the electrochemical performance. Commercially, graphite is mainly used for fabricating the anode material of lithium-ion batteries. Carbon-coated graphite has attracted attention because it offers improved electrochemical performance [1–4]. It is known that the exothermic reaction between lithiated graphite anode materials and liquid electrolytes may lead to the thermal runaway of lithium-ion batteries [5–8]. Despite this fact, the suppression of this graphite-related exothermic reaction has not been studied in adequate detail. The electrochemical properties of graphite can be improved by its surface treatment with metal and metal oxides [9,10]. It should be noted here that the surface treatment consists of coating or encapsulation of graphite particles. On the other hand, a surface modification by depositing a protective film on a surface of graphite composite electrodes has been rarely performed. In particular, the surface coating of graphite anodes with an inorganic solid electrolyte film is considered to be a promising
n
Corresponding author. Tel.: þ82 33 250 6266; fax: þ82 33 242 6256. E-mail address: [email protected] (S.-M. Lee).
0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2011.04.015
approach because this coating acts as an effective protective layer that controls the interfacial reaction of the graphite and serves as an effective ion-conducting and separating film. Among the various types of solid electrolytes, amorphous lithium phosphorous oxynitride (LiPON) finds a wide use as a solid electrolyte in thin-film batteries owing to its excellent electrochemical stability [11,12]. It has been previously reported that the initial irreversible capacity is decreased by the deposition of the LiPON film on a carbon electrode surface [13]. However, little attention has been paid to the effect of LiPON film coating on the cycling performance and thermal stability of graphite anodes. In this paper, we report the electrochemical performance and thermal stability of both graphite anode and a carbon-coated graphite anode with a LiPON thin film that is surface-deposited by sputtering.
2. Materials and methods Graphite and carbon-coated graphite anodes were fabricated using pristine and carbon-coated natural graphite (NG) spheres, respectively, provided by Carbonix, Inc., Korea. A proprietary coating method was utilized to prepare the carbon-coated spherical NG. The electrodes were prepared by coating a slurry of graphite (91 wt%), carbon black (3 wt%), and poly(vinylidene fluoride) (PVDF) binder (6 wt%) dissolved in N-methyl pyrrolidone (NMP) on a copper foil. The slurry-coated electrode was dried at 120 1C for 12 h. The LiPON thin film was deposited by rf magnetron reactive sputtering of a Li3PO4 target in a N2 gas
Y.-S. Park, S.-M. Lee / Journal of Physics and Chemistry of Solids 72 (2011) 842–845
atmosphere. The surface morphology of the pristine and LiPONcoated electrodes was observed using scanning electron microscopy (SEM). The thickness of the deposited film was estimated based on the thickness of the film deposited on a silicon wafer under similar conditions. For the evaluation of electrochemical performance, a solution of 1 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC; 1:1 by volume provided by PANAX ETEC Co., Ltd, Korea) was used as an electrolyte. A coin-type half cell was assembled with a lithium foil counter electrode. The cells were galvanostatically cycled at 0.2 mA cm 2 between 0.005 and 2.0 V at 30 1C. Electrochemical impedance spectroscopy measurements were performed on the cells in the discharged state of 2.0 V using an impedance analyzer (IM6e Zahner Elektrik). The amplitude of the AC signal was 5 mV, and the frequency ranged from 100 mHz to 1 MHz. For DSC measurements, the cells were pre-cycled three times to reach a stable capacity level, and the cycling was interrupted when the cells were charged to a fully intercalated state. Then, the charged cells were disassembled in a glove box. Disks with a diameter of 40 mm with a Cu current collector were cut from the electrode sheet without removing the electrolyte and transferred to a high-pressure stainless steel pan with a gold plated copper seal. The DSC scans were performed in Ar atmosphere using a DSC 200 F3 (NETZSCH, Germany) at a constant heating rate of 10 oC min 1. The weight of each DSC pan with the sample was found to be constant before and after the DSC measurements, indicating that there was no leakage during the experiments.
3. Results and discussion Fig. 1 shows the surface and cross section (see the inset) SEM micrographs of the pristine and LiPON-coated graphite electrodes.
843
It is observed that the deposited film homogenously covers the graphite particles. Fig. 2 shows EDAX data for the pristine and LiPON-coated electrodes. For the pristine electrode, only peaks corresponding to C and Cu are observed, whereas additional N, O, and P peaks are observed for the LiPON-coated electrodes. This indicates that the film deposited on the graphite electrode is the LiPON thin film deposited by rf magnetron reactive sputtering of the Li3PO4 target in N2 atmosphere. From the initial charge-discharge curves (not shown), we can conclude that the initial coulombic efficiency of the graphite electrode increases slightly from 87.2% to 89.5% due to the deposition of the LiPON thin film. However, in the case of the carbon-coated graphite electrode, both the pristine and the LiPON-coated electrodes have a similar coulombic efficiency of 93%.The LiPON thin-film coating on the graphite electrode prevents the graphite surface from coming in direct contact with the electrolyte and suppresses the so called solid-electrolyteinterface (SEI) film-formation reaction. As a result, this coating minimizes the initial irreversible capacity loss, most of which is attributable to side reactions such as solvent decomposition occurring on the graphite electrode surface. The cyclic voltamogram study for the untreated and LiPON-coated carbon electrodes in 1 M LiPF6/EC : DEC (1:1) shows that the reduction peaks due to EC and DEC decomposition nearly disappear for the LiPON-coated electrode, while in the case of the untreated carbon electrode, the decomposition of DEC occurs at about 2.0 V (versus Li/Liþ) and DEC decomposes at 0.7 V (versus Li/Liþ) [13]. The LiPON film also suppresses solvent co-intercalation. Therefore, the LiPON film suppresses the formation of SEI film by solvent decomposition and co-intercalation but acts as a protective and ion-conducting layer. The exact mechanism of reaction between LiPON film and electrolyte is not, however, yet clear. On the other hand, in the case of the carbon-coated graphite electrode, the deposition of the
Fig. 1. Surface and cross section (inset) FE-SEM images of (a) pristine and (b) LiPON-coated graphite electrodes.
Fig. 2. EDAX data for (a) pristine and (b) LiPON-coated electrodes.
844
Y.-S. Park, S.-M. Lee / Journal of Physics and Chemistry of Solids 72 (2011) 842–845
LiPON film has little additional effect because the active sites of the electrode such as the edge planes of natural graphite, are covered with amorphous carbon. The cycle performance of carbon-coated graphite electrode is compared with that of pristine and LiPON-coated graphite electrodes in Fig. 3. The cycling behavior of the carbon-coated graphite electrode appears to be comparable to that of LiPONcoated graphite electrodes. The cycle performance of the graphite electrode significantly improved due to the deposition of the LiPON thin film. Fig. 4 depicts the impedance spectra of the pristine and LiPON-coated graphite electrodes after 3 and 150 cycles. It can be seen that as the cycling proceeds, the increase in the electrode’s impedance is inhibited by the LiPON thin-film coating. The reduced impedance indicates charged transfer and enhanced Li migration through the surface film [14,15]. The rate performance of the pristine and 1 mm-thick LiPONcoated graphite electrodes is compared in Fig. 5. The cells are charged galvanostatically to 0.005 V at 0.2 mA cm 2 but discharged to 2.0 V at different current densities. It is seen that both electrodes exhibit a similar rate performance up to 2 C while on increasing the discharge current to 5 and 10 C, relative discharge capacity of the LiPON-coated electrode is a little bit lower than that of pristine one. This indicates that the LiPON coating film may impede the lithium-ion transport at high currents. Nevertheless, in the case of the LiPON-coated electrode, the interface with the electrolyte is very stable even after extended cycling as
illustrated by Fig. 4, which may lead to a stable rate performance for long-term cycle. Fig. 6 shows the DSC curves of the pristine and LiPON-coated graphite electrodes in a fully lithiated stage. The temperature at which the exothermic reaction started was similar for both electrodes. However, in the case of the LiPON-coated graphite electrode, the main exothermic peaks moved to a higher temperature, and therefore, the heat evolution below 300 1C was significantly reduced(2480 J/g-2010 J/g). These exothermic reactions are mainly attributed to the reaction between the intercalated lithium and electrolyte, in which the lithium must diffuse through the graphite to reach the surface of the edge planes of the graphite [16]. Therefore, it is suggested that the LiPON thin-film coating on the graphite electrode suppresses the delithiation occurring from within the structure to the surface of graphite during heating, thus enhancing the thermal stability. An 1 mm-thick LiPON thin-film deposited on a copper substrate exhibits a thermal evolution peak at around 290 1C in the presence of a liquid electrolyte as shown in Fig. 7(a). On the other hand, the main exothermic reaction of the carbon-coated graphite electrode occurs above 300 1C (Fig. 7(b)). Therefore, the main exothermic peaks of the carbon-coated graphite electrode are shifted to a lower temperature when the LiPON thin film is deposited. The heat evolution below 300 1C increases from 1020 to 1640 J g 1 when the thickness of the LiPON thin film is increased from 0.5 to 1 mm as illustrated in Fig. 7(c) and (d). It appears that the LiPON thin-film coating lowers the thermal stability of the carbon-coated graphite electrode rather than
110
100
0.5C
90
80
60
normalized Capacity (%)
Normalized Capacity (%)
100
pristine 1μm LiPON coated carbon coated
40
20
2C
1C
80
5C
70
pristine 1 micron deposited
60
10C
50 40 30 20 10
0 0
20
40
60
80 Cycle
100
120
0
140
Fig. 3. Cycle performances of prisitine, LiPON-coated, and carbon-coated graphite electrodes.
20
20
15
15
10
10 Cycle (number)
pristine electrode 1 μm LiPON coated electrode
10 5
5 pristine electrode 1 μm LiPON coated electrode
0 0
10
20 Z' / Ω
30
40
50
15
20
Fig. 5. Rate-capabilities of pristine and LiPON-coated graphite electrode.
- Z" / Ω
- Z" / Ω
5
0
0 0
10
20
30
40
50
Z' / Ω
Fig. 4. Impedance spectra of pristine and LiPON-coated graphite electrode: after (a) three cycles and (b) 150 cycles.
Y.-S. Park, S.-M. Lee / Journal of Physics and Chemistry of Solids 72 (2011) 842–845
845
the thermal stability of pristine electrode is enhanced by the LiPON thin-film coating.
8 7
Heat flow (mW/mg)
6
pristine electrode
4. Conclusions
5 4 3 2 1 1μm LiPON coated electrode
0 -1 100
200
300 Temperature (°C)
400
500
Fig. 6. DSC curves of the pristine and LiPON-coated graphite electrodes in a fully lithiated state.
Heat flow (mW/mg)
16 14
Heat flow (mW/mg)
12 10
(d)
Acknowledgements 2
(b) 1
(c) 0 100
150 200 Temperature(°C)
250
8 6
References
(b)
0 -2
This study was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF2009-C1AAA001-0093307) and The Energy Efficiency and Resources of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (Grant no. C1006294-01-02(120090820)).
(c)
4 2
An ionic conducting LiPON thin film has been deposited on the surface of pristine graphite and carbon-coated graphite composite anodes by rf magnetron sputtering. The LiPON thin-film coating enhances lithium-ion migration through the interface between the surface of graphite particles and the electrolyte and suppresses the surface reaction between the electrode surface and the electrolyte during cycling as inferred from the cycle performance and impedance spectra data. The LiPON thin-film coating enhanced the thermal stability of the graphite electrode. However, in the case of the carbon-coated graphite electrode, the heat evolution below 250 1C decreased due to LiPON thin-film coating, whereas the heat evolution below 300 1C increased. This is attributed to the low thermal stability of the LiPON thin film.
(a) 100
200
300 Temperature (°C)
400
500
Fig. 7. DSC curves of LiPON-coated and carbon-coated graphite electrode: (a) LiPON film with electrolyte, (b) pristine carbon-coated graphite electrode, (c) 0.5 mm-thick LiPON-coated carbon-coated graphite electrode, and (d) 1 mmthick LiPON-coated carbon-coated graphite electrode.
enhancing it. Similar to the case of pristine electrode (refer to Fig. 6), the heat evolution below 200 1C, which is mainly due to the SEI layer decomposition, decreases by LiPON thin-film coating (refer to the curves in the inset of Fig. 7). It should be noted here that the main exothermic reaction of the pristine electrode starts around 250 1C, while the thermal evolution peak of LiPON thin-film appears around 290 1C. Hence,
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