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2D Raman correlation analysis of formation mechanism of passivating film on overcharged LiCoO2 electrode with additive system
Journal of Molecular Structure xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
2D Raman correlation analysis of formation mechanism of passivating film on overcharged LiCoO2 electrode with additive system Yeonju Park a,1, Su Hyun Shin a,1, Sung Man Lee b, Sung Phil Kim c, Hyun Chul Choi c,⇑, Young Mee Jung a,⇑ a
Department of Chemistry, and Institute for Molecular Science and Fusion Technology, Kangwon National University, Chunchon 200-701, Republic of Korea Department of Advanced Materials Science & Engineering, Kangwon National University, Chunchon 200-701, Republic of Korea c Department of Chemistry, Chonnam National University, Gwangju 500-757, Republic of Korea b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Effect of vinylene carbonate (VC) on
the electrochemical performance of the LiCoO2 cathode. Solid electrolyte interface (SEI) film formation on the LiCoO2 cathode under overcharge conditions. Reduced capacity fading in VCcontaining electrolyte. Electrochemical reaction kinetics in cathode/electrolyte interface with and without VC.
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Lithium-ion battery 2D correlation spectroscopy Solid electrolyte interface Vinylene carbonate Raman spectroscopy
a b s t r a c t The effect of vinylene carbonate (VC) as solid electrolyte interface (SEI)-forming additive on the electrochemical performance of the LiCoO2 cathode was investigated by galvanostatic charge–discharge testing as well as Raman and 2D correlation spectroscopy. It was found that VC-containing electrolyte has a positive effect on capacity fading. An analysis of the 2D Raman correlation spectra suggested that even though the same SEI components (i.e., Co3O4 and Li2O) are produced on the cathode surface, the electrochemical reaction kinetics in the cathode/electrolyte interface differ according to the non-use or use of VC: in the latter case, formation of the SEI components is delayed. Ó 2014 Published by Elsevier B.V.
1. Introduction The lithium-ion (Li-ion) battery has the highest energy density among commercial rechargeable batteries. Commercial rechargeable Li-ion batteries use a lithium transition metal oxide as the cathode, a carbonaceous material as the anode, and an organic
⇑ Corresponding authors. Tel.: +82 62 530 3491; fax: +82 62 530 3389 (H.C. Choi). Tel.: +82 33 250 8495; fax: +82 33 259 5667 (Y.M. Jung). E-mail addresses: [email protected] (H.C. Choi), [email protected] (Y.M. Jung). 1 These authors contributed equally to this work.
electrolyte based on a solution of lithium salt in a mixture of two or more organic solvents. During the initial charge–discharge process, the solid electrolyte interface (SEI) film is formed on the electrode surface due to the decomposition reaction of electrolyte [1,2]. This film, allowing lithium ion transfer while preventing electron transfer, is a key to battery operation stability. Its composition and morphology, moreover, affect faradaic efficiency, cycle life, and irreversible capacity loss. Recently, many researchers have focused on the SEI as a route to Li-ion battery performance and safety improvement via the use of ionic liquids [3,4] or electrolyte additives [5–10]. Among them, vinylene carbonate (VC) is widely applied as an additive to organic
http://dx.doi.org/10.1016/j.molstruc.2014.01.083 0022-2860/Ó 2014 Published by Elsevier B.V.
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electrolyte. Enhancement of both cell performance and thermal stability by use of VC, specifically by formation of the stable SEI film on the anode surface prior to the decomposition of the other electrolytes, has been reported in a number of papers [11,12]. However, VC’s positive effects on the anode are not similar to that on the cathode. Eom et al. reported that the residual VC after formation of SEI on the anode was decomposed on the cathode during elevated temperature storage, resulting in formation of poly(VC) and increased CO2 gas generation [13]. Although the adverse effects of VC in high-voltage or Ni-based cathode materials have been reported [7,14,15], detailed studies on the effects of VC are still lacking. In this study, VC was added to 1.0 M LiPF6/ethylene carbonate– diethyl carbonate (EC:DEC = 1:1 by volume) electrolyte as an additive. Its effects on the LiCoO2 cathode/electrolyte interfacial reaction during the initial charging process were studied by comparing the electrochemical behavior of batteries with and without VC. At the same time, the produced SEI films were characterized, and their effects on electrochemical performance were investigated by Raman and 2D Raman correlation spectroscopy. 2. Experimental The electrochemical behavior of LiCoO2 was investigated using coin cells (2016 type). Slurries consisting of 95 wt% LiCoO2 (Nippon Chemical Co.) powder, 3 wt% acetylene black, and 2 wt% polyvinylidene fluoride (PVdF) dissolved in 1-methyl-2-pyrrolidinone were prepared. Cathodes were made by coating the slurry onto an aluminum foil substrate. Using these electrodes, test cells were fabricated with metallic Li anodes and polypropylene separators (Celgard 2400) in a glove box filled with Ar gas. The electrolytes were 1.0 M solution of LiPF6 in ethylene carbonate–diethyl carbonate (EC:DEC = 1:1 by volume) with or without 1.0 wt% VC (PANAX ETEC Co., Korea). For a cell performance evaluation, test cells were aged for 4 h at 40 °C in a vacuum oven. They were then galvanostatically discharged and charged (constant current density: 0.2 C) within the 3.0–4.5 V range at room temperature using a WBCS 3000 battery tester system (Won A Tech Co., Korea). The cut-off voltage ranges of each cell were 3.0–4.2, 3.0–4.4, and 3.0–4.5 V. For the purposes of the evaluation, the term charge referred to lithium extraction from LiCoO2, and the term discharge, to lithium insertion into delithiated LiCoO2. Raman spectra were obtained ex situ at room temperature using a Jobin Yvon/HORIBA LabRam ARAMIS Raman spectrometer equipped with integral BX 41 confocal microscopy. Radiation from an air-cooled frequency-doubled Nd:Yag laser (532 nm) was used as the excitation source. Raman scattering was detected with 180° geometry using a multi-channel air-cooled (70 °C) chargecoupled device (CCD) camera (1024 256 pixels). The synchronous and asynchronous 2D Raman correlation spectra were obtained using the MATLAB R2010a (The Mathworks Inc., Natick, MA). The red and blue lines represent positive and negative cross peaks, respectively. 3. Results and discussion Fig. 1 presents the charge–discharge profiles of the LiCoO2/Li cell in VC-free and VC-containing electrolytes, respectively, during the initial cycle. In the VC-free electrolyte, the irreversible capacity of the LiCoO2/Li cell at 4.2 and 4.5 V was 6 and 15 mA h g1, respectively (see Fig. 1(a) and (b)); in the VC-containing electrolyte, the irreversible capacity decreased to 5 and 6 mA h g1 at 4.2 and 4.5 V, respectively (see Fig. 1(c) and (d)). Irreversible capacity loss generally is correlated with SEI formation due to decomposition of organic electrolytes during the electrochemical reaction. This
indicates that in the present experimentation, as typically, the SEI film structure (i.e., composition, thickness, and morphology) was altered by the VC additive. Fig. 2 plots the Raman spectra of delithiated Li1xCoO2 in VCfree and VC-containing electrolytes within the 4.2–4.5 V cut-off range. For comparison, the Raman spectrum of the pristine LiCoO2 also is shown. The pristine LiCoO2 exhibited two strong bands at around 470 and 582 cm1, originating from the Eg (OCoO bending) and A1g (CoO stretching) modes, respectively, for the hexagonal LiCoO2 structure [16,17]. In the VC-free electrolyte, these intense features dramatically weakened upon charging, the band positions shifting to 467 and 603 cm1, respectively (see Fig. 2(a)). In the delithiated Li1xCoO2, the two bands were shifted to 509 and 670 cm1, respectively, due to the formation of surface compounds. Recently, Markevich et al. suggested that LiCoO2, when stored in LiPF6 solution, undergoes partial decomposition according to the surface reaction [18] þ
3LiCoO2 þ Li þ e ! Co3 O4 þ 4Li2 O: The band positions observed in the present study (509 and 670 cm1), being quite consistent with those of Li2O and Co3O4 reported in the literature [19,20], therefore were assigned to Li2O and Co3O4, respectively. The intensities of these bands were diminished in the course of further Li+ extraction. In the VC-containing electrolyte, the spectral features were very similar to those in the VC-free electrolyte, indicating that the SEI components are similar without and with the VC additive. Upon charging, however, the intensities of four bands at 662, 505, 598, and 461 cm1 were less altered than in the VC-free electrolyte system. This Raman signal intensity difference suggests, given its strong correlation with sample thickness, that in the VC-free electrolyte, the SEI film thickness gradually increased with increasing cut-off voltage. And certainly, formation of new SEI film would consume Li+ ions, leading to an increase of irreversible capacity as manifest in charge–discharge curves. The observed Raman results indicate, in other words, that a more stable SEI film is formed on the LiCoO2 electrode in VC-containing electrolyte. To better understand the SEI film formation mechanism, we applied 2D correlation analysis to the Raman spectra of the charged LiCoO2 electrode in the VC-free and VC-containing electrolytes. 2D correlation spectroscopy has various advantages, such as spectral resolution enhancement, unambiguous band assignments, and accurate determination of sequential order of spectral intensity changes [21,22]. 2D correlation spectroscopy is very powerful analytical technique to investigate electrochemical reactions for electrode materials for Li-ion batteries [16,19,23–28]. The 2D correlation spectra were calculated from the dynamic spectra obtained from an external-perturbation-based measurement of the initial spectra. In a synchronous 2D correlation spectrum, the intensities of the auto peaks located at the diagonal positions represent the overall susceptibility of the corresponding spectral regions to intensity change as the result of application of external perturbation to the system; the intensities of the cross peaks located at the off-diagonal positions, meanwhile, indicate simultaneous or coincidental changes of spectral intensities observed for two different spectral variables (m1 and m2). By contrast, an asynchronous 2D correlation spectrum consisting only of cross peaks provides information useful to the interpretation of the chemical/ physical interaction mechanisms. The intensity of an asynchronous 2D correlation spectrum represents sequential, or successive, changes of spectral intensity measured at m1 and m2. Accordingly, an asynchronous cross peak develops only if the intensities of two spectral features become out of phase with each other (i.e., if one feature is delayed or accelerated). If the signs of synchronous and asynchronous cross peaks are the same, the intensity change
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Fig. 1. Charge–discharge profiles of LiCoO2/Li cell in (a and b) VC-free and (c and d) VC-containing electrolytes at different cut-off voltages.
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at m1 occurs before m2; but if the signs of synchronous and asynchronous cross peaks are different, the intensity change at m1 occurs after m2 [21,22]. Figs. 3 and 4 present the synchronous and asynchronous 2D correlation spectra of LiCoO2 electrodes without and with the VC additive, respectively. For the VC-free electrolyte, three peaks, at 469 (Eg mode of LiCoO2), 511 (formation of Li2O), and 672 cm1 (formation of Co3O4) in the synchronous 2D correlation spectrum (see Fig. 3(a)), were resolved to six peaks, at 456 and 469, 500 and 514, and 660 and 672 cm1, respectively, in the asynchronous 2D correlation spectrum (see Fig. 3(b)). From the analysis of the 2D correlation spectra of the charged LiCoO2 electrode in the VC-free electrolyte, we deduced the following sequence of spectral events with increasing voltage: 672 ? 469 ? 514 ? 500 ? 456 ? 660 cm1. This suggests that, initially, Co3O4 is produced by the surface reaction in the cathode/electrolyte interface, that, next, the Li2O component is formed on the cathode surface, and that finally, hexagonal
LiCoO2 structural distortion accompanying the surface change is caused by the further extraction of Li+ ions that occurs with increasing charge voltage. In the VC-containing electrolyte, four peaks, at 478 (Eg mode of LiCoO2), 509 (formation of Li2O), 673 (formation of Co3O4), and 686 cm1 (formation of Co2O3) in the synchronous 2D correlation spectrum (see Fig. 4(a)), were obtained [29]. Two peaks, at 478 and 509 cm1, were resolved to four peaks, at 467 and 477 and 506 and 514 cm1, respectively, in the asynchronous 2D correlation spectrum (see Fig. 4(b)). Analysis of the 2D correlation spectra showed the following sequence of spectral events with increasing voltage: 477 ? 673 ? 506 ? 514 ? 686 ? 467 cm1. This indicates that, initially, structural distortion of the LiCoO2 electrode occurs, that, subsequently, the Co3O4 and Li2O components are formed on the cathode, and that finally, Co2O3 is formed on the cathode surface with increasing charge voltage. Overall, the 2D correlation analysis results suggest that, even though the same SEI components are produced on the cathode surface, the
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Y. Park et al. / Journal of Molecular Structure xxx (2014) xxx–xxx
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Fig. 3. (a) Synchronous and (b) asynchronous 2D Raman correlation spectra of LiCoO2 electrodes in VC-free electrolyte at different cut-off voltages. The red and blue lines represent the positive and negative cross peaks, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. (a) Synchronous and (b) asynchronous 2D Raman correlation spectra of LiCoO2 electrodes in VC-containing electrolyte during overcharge process. The red and blue lines represent the positive and negative cross peaks, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
electrochemical reaction kinetics in the cathode/electrolyte interface differ according to non-use or use of VC: in the latter case, formation of the SEI components (i.e., Co3O4 and Li2O) is delayed. 4. Conclusions The mechanism of SEI film formation on the cathode surface is instrumental to the proper understanding of the electrochemical behavior of the Li-ion battery. In the present work, we studied the electrochemical reaction of the LiCoO2/Li cell in VC-free and VC-containing electrolytes and characterized the SEI film composition and properties by Raman and 2D correlation spectroscopy. The
electrochemical results showed that VC reduces the irreversible capacity loss incurred in the charge–discharge process. In summary, we suggest, based on the analysis of the 2D correlation Raman spectra, that even though the same SEI components are produced on the cathode surface, the electrochemical reaction kinetics in the cathode/electrolyte interface differ: crucially, formation of the SEI components (i.e., Co3O4 and Li2O) is delayed by use of VC. Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korea government (MEST) (No.
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2009-0087013). The authors thank the Central Laboratory of Kangwon National University for the measurements of Raman spectra. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
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[14] H.M. Jung, S.-H. Park, J.J. Jeon, Y. Choi, S. Yoon, J.-J. Cho, S. Oh, Y.-K. Han, H. Lee, J. Mat. Chem. A 1 (2013) 11975–11981. [15] H. Lee, S. Choi, S. Choi, H.-J. Kim, Y. Choi, S. Yoon, J.-J. Cho, Electrochem. Commun. 9 (2007) 801–806. [16] Y. Park, N.H. Kim, J.Y. Kim, I.-Y. Eom, Y.U. Jeong, M.S. Kim, S.M. Lee, H.C. Choi, Y.M. Jung, Vib. Spectrosc. 53 (2010) 60–63. [17] M. Inaba, Y. Iriyama, Z. Ogumi, Y. Todzuka, A. Tasaka, J. Raman Spectrosc. 28 (1997) 613–617. [18] E. Markevich, G. Salitra, D. Aurbach, Electrochem. Commun. 7 (2005) 1298– 1304. [19] H.C. Choi, Y.M. Jung, I. Noda, S.B. Kim, J. Phys. Chem. B 107 (2003) 5806– 5811. [20] J.S.V. Sluytman, W.C. West, J.F. Whitacre, F.M. Alamgir, S.G. Greenbaum, Elechrochim. Acta 51 (2006) 3001–3007. [21] I. Noda, Y. Ozaki, Two-dimensional Correlation Spectroscopy: Applications in Vibrational Spectroscopy, John Wiley & Sons Inc., New York, 2004. [22] I. Noda, Appl. Spectrosc. 47 (1993) 1329–1336. [23] Y. Park, N.H. Kim, H.C. Choi, S.M. Lee, H. Hwang, Y.U. Jeong, Y.M. Jung, Vib. Spectrosc. 60 (2012) 226–230. [24] L. Aldon, A. Perea, J. Power Sources 196 (2011) 1342–1348. [25] Y. Park, N.H. Kim, J.M. Kim, Y.C. Kim, Y.U. Jeong, S.M. Lee, H.C. Choi, Y.M. Jung, Appl. Spectrosc. 65 (2011) 320–325 (6). [26] Y. Park, N.H. Kim, S.B. Cho, J.M. Kim, G.C. Kim, M.S. Kim, S.M. Lee, H.C. Choi, Y.M. Jung, J. Mol. Struct. 974 (2010) 139–143. [27] H.C. Choi, J. Mol. Struct. 799 (2006) 91–95. [28] H.C. Choi, Y.M. Jung, S.B. Kim, Appl. Spectrosc. 57 (2003) 984–990. [29] Z. Wang, H. Dong, L. Chen, Y. Mo, X. Huang, Solid State Ionics 175 (2004) 239– 242.
Please cite this article in press as: Y. Park et al., J. Mol. Struct. (2014), http://dx.doi.org/10.1016/j.molstruc.2014.01.083
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
2D Raman correlation analysis of formation mechanism of passivating film on overcharged LiCoO2 electrode with additive system Yeonju Park a,1, Su Hyun Shin a,1, Sung Man Lee b, Sung Phil Kim c, Hyun Chul Choi c,⇑, Young Mee Jung a,⇑ a
Department of Chemistry, and Institute for Molecular Science and Fusion Technology, Kangwon National University, Chunchon 200-701, Republic of Korea Department of Advanced Materials Science & Engineering, Kangwon National University, Chunchon 200-701, Republic of Korea c Department of Chemistry, Chonnam National University, Gwangju 500-757, Republic of Korea b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Effect of vinylene carbonate (VC) on
the electrochemical performance of the LiCoO2 cathode. Solid electrolyte interface (SEI) film formation on the LiCoO2 cathode under overcharge conditions. Reduced capacity fading in VCcontaining electrolyte. Electrochemical reaction kinetics in cathode/electrolyte interface with and without VC.
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Lithium-ion battery 2D correlation spectroscopy Solid electrolyte interface Vinylene carbonate Raman spectroscopy
a b s t r a c t The effect of vinylene carbonate (VC) as solid electrolyte interface (SEI)-forming additive on the electrochemical performance of the LiCoO2 cathode was investigated by galvanostatic charge–discharge testing as well as Raman and 2D correlation spectroscopy. It was found that VC-containing electrolyte has a positive effect on capacity fading. An analysis of the 2D Raman correlation spectra suggested that even though the same SEI components (i.e., Co3O4 and Li2O) are produced on the cathode surface, the electrochemical reaction kinetics in the cathode/electrolyte interface differ according to the non-use or use of VC: in the latter case, formation of the SEI components is delayed. Ó 2014 Published by Elsevier B.V.
1. Introduction The lithium-ion (Li-ion) battery has the highest energy density among commercial rechargeable batteries. Commercial rechargeable Li-ion batteries use a lithium transition metal oxide as the cathode, a carbonaceous material as the anode, and an organic
⇑ Corresponding authors. Tel.: +82 62 530 3491; fax: +82 62 530 3389 (H.C. Choi). Tel.: +82 33 250 8495; fax: +82 33 259 5667 (Y.M. Jung). E-mail addresses: [email protected] (H.C. Choi), [email protected] (Y.M. Jung). 1 These authors contributed equally to this work.
electrolyte based on a solution of lithium salt in a mixture of two or more organic solvents. During the initial charge–discharge process, the solid electrolyte interface (SEI) film is formed on the electrode surface due to the decomposition reaction of electrolyte [1,2]. This film, allowing lithium ion transfer while preventing electron transfer, is a key to battery operation stability. Its composition and morphology, moreover, affect faradaic efficiency, cycle life, and irreversible capacity loss. Recently, many researchers have focused on the SEI as a route to Li-ion battery performance and safety improvement via the use of ionic liquids [3,4] or electrolyte additives [5–10]. Among them, vinylene carbonate (VC) is widely applied as an additive to organic
http://dx.doi.org/10.1016/j.molstruc.2014.01.083 0022-2860/Ó 2014 Published by Elsevier B.V.
Please cite this article in press as: Y. Park et al., J. Mol. Struct. (2014), http://dx.doi.org/10.1016/j.molstruc.2014.01.083
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electrolyte. Enhancement of both cell performance and thermal stability by use of VC, specifically by formation of the stable SEI film on the anode surface prior to the decomposition of the other electrolytes, has been reported in a number of papers [11,12]. However, VC’s positive effects on the anode are not similar to that on the cathode. Eom et al. reported that the residual VC after formation of SEI on the anode was decomposed on the cathode during elevated temperature storage, resulting in formation of poly(VC) and increased CO2 gas generation [13]. Although the adverse effects of VC in high-voltage or Ni-based cathode materials have been reported [7,14,15], detailed studies on the effects of VC are still lacking. In this study, VC was added to 1.0 M LiPF6/ethylene carbonate– diethyl carbonate (EC:DEC = 1:1 by volume) electrolyte as an additive. Its effects on the LiCoO2 cathode/electrolyte interfacial reaction during the initial charging process were studied by comparing the electrochemical behavior of batteries with and without VC. At the same time, the produced SEI films were characterized, and their effects on electrochemical performance were investigated by Raman and 2D Raman correlation spectroscopy. 2. Experimental The electrochemical behavior of LiCoO2 was investigated using coin cells (2016 type). Slurries consisting of 95 wt% LiCoO2 (Nippon Chemical Co.) powder, 3 wt% acetylene black, and 2 wt% polyvinylidene fluoride (PVdF) dissolved in 1-methyl-2-pyrrolidinone were prepared. Cathodes were made by coating the slurry onto an aluminum foil substrate. Using these electrodes, test cells were fabricated with metallic Li anodes and polypropylene separators (Celgard 2400) in a glove box filled with Ar gas. The electrolytes were 1.0 M solution of LiPF6 in ethylene carbonate–diethyl carbonate (EC:DEC = 1:1 by volume) with or without 1.0 wt% VC (PANAX ETEC Co., Korea). For a cell performance evaluation, test cells were aged for 4 h at 40 °C in a vacuum oven. They were then galvanostatically discharged and charged (constant current density: 0.2 C) within the 3.0–4.5 V range at room temperature using a WBCS 3000 battery tester system (Won A Tech Co., Korea). The cut-off voltage ranges of each cell were 3.0–4.2, 3.0–4.4, and 3.0–4.5 V. For the purposes of the evaluation, the term charge referred to lithium extraction from LiCoO2, and the term discharge, to lithium insertion into delithiated LiCoO2. Raman spectra were obtained ex situ at room temperature using a Jobin Yvon/HORIBA LabRam ARAMIS Raman spectrometer equipped with integral BX 41 confocal microscopy. Radiation from an air-cooled frequency-doubled Nd:Yag laser (532 nm) was used as the excitation source. Raman scattering was detected with 180° geometry using a multi-channel air-cooled (70 °C) chargecoupled device (CCD) camera (1024 256 pixels). The synchronous and asynchronous 2D Raman correlation spectra were obtained using the MATLAB R2010a (The Mathworks Inc., Natick, MA). The red and blue lines represent positive and negative cross peaks, respectively. 3. Results and discussion Fig. 1 presents the charge–discharge profiles of the LiCoO2/Li cell in VC-free and VC-containing electrolytes, respectively, during the initial cycle. In the VC-free electrolyte, the irreversible capacity of the LiCoO2/Li cell at 4.2 and 4.5 V was 6 and 15 mA h g1, respectively (see Fig. 1(a) and (b)); in the VC-containing electrolyte, the irreversible capacity decreased to 5 and 6 mA h g1 at 4.2 and 4.5 V, respectively (see Fig. 1(c) and (d)). Irreversible capacity loss generally is correlated with SEI formation due to decomposition of organic electrolytes during the electrochemical reaction. This
indicates that in the present experimentation, as typically, the SEI film structure (i.e., composition, thickness, and morphology) was altered by the VC additive. Fig. 2 plots the Raman spectra of delithiated Li1xCoO2 in VCfree and VC-containing electrolytes within the 4.2–4.5 V cut-off range. For comparison, the Raman spectrum of the pristine LiCoO2 also is shown. The pristine LiCoO2 exhibited two strong bands at around 470 and 582 cm1, originating from the Eg (OCoO bending) and A1g (CoO stretching) modes, respectively, for the hexagonal LiCoO2 structure [16,17]. In the VC-free electrolyte, these intense features dramatically weakened upon charging, the band positions shifting to 467 and 603 cm1, respectively (see Fig. 2(a)). In the delithiated Li1xCoO2, the two bands were shifted to 509 and 670 cm1, respectively, due to the formation of surface compounds. Recently, Markevich et al. suggested that LiCoO2, when stored in LiPF6 solution, undergoes partial decomposition according to the surface reaction [18] þ
3LiCoO2 þ Li þ e ! Co3 O4 þ 4Li2 O: The band positions observed in the present study (509 and 670 cm1), being quite consistent with those of Li2O and Co3O4 reported in the literature [19,20], therefore were assigned to Li2O and Co3O4, respectively. The intensities of these bands were diminished in the course of further Li+ extraction. In the VC-containing electrolyte, the spectral features were very similar to those in the VC-free electrolyte, indicating that the SEI components are similar without and with the VC additive. Upon charging, however, the intensities of four bands at 662, 505, 598, and 461 cm1 were less altered than in the VC-free electrolyte system. This Raman signal intensity difference suggests, given its strong correlation with sample thickness, that in the VC-free electrolyte, the SEI film thickness gradually increased with increasing cut-off voltage. And certainly, formation of new SEI film would consume Li+ ions, leading to an increase of irreversible capacity as manifest in charge–discharge curves. The observed Raman results indicate, in other words, that a more stable SEI film is formed on the LiCoO2 electrode in VC-containing electrolyte. To better understand the SEI film formation mechanism, we applied 2D correlation analysis to the Raman spectra of the charged LiCoO2 electrode in the VC-free and VC-containing electrolytes. 2D correlation spectroscopy has various advantages, such as spectral resolution enhancement, unambiguous band assignments, and accurate determination of sequential order of spectral intensity changes [21,22]. 2D correlation spectroscopy is very powerful analytical technique to investigate electrochemical reactions for electrode materials for Li-ion batteries [16,19,23–28]. The 2D correlation spectra were calculated from the dynamic spectra obtained from an external-perturbation-based measurement of the initial spectra. In a synchronous 2D correlation spectrum, the intensities of the auto peaks located at the diagonal positions represent the overall susceptibility of the corresponding spectral regions to intensity change as the result of application of external perturbation to the system; the intensities of the cross peaks located at the off-diagonal positions, meanwhile, indicate simultaneous or coincidental changes of spectral intensities observed for two different spectral variables (m1 and m2). By contrast, an asynchronous 2D correlation spectrum consisting only of cross peaks provides information useful to the interpretation of the chemical/ physical interaction mechanisms. The intensity of an asynchronous 2D correlation spectrum represents sequential, or successive, changes of spectral intensity measured at m1 and m2. Accordingly, an asynchronous cross peak develops only if the intensities of two spectral features become out of phase with each other (i.e., if one feature is delayed or accelerated). If the signs of synchronous and asynchronous cross peaks are the same, the intensity change
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Fig. 1. Charge–discharge profiles of LiCoO2/Li cell in (a and b) VC-free and (c and d) VC-containing electrolytes at different cut-off voltages.
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at m1 occurs before m2; but if the signs of synchronous and asynchronous cross peaks are different, the intensity change at m1 occurs after m2 [21,22]. Figs. 3 and 4 present the synchronous and asynchronous 2D correlation spectra of LiCoO2 electrodes without and with the VC additive, respectively. For the VC-free electrolyte, three peaks, at 469 (Eg mode of LiCoO2), 511 (formation of Li2O), and 672 cm1 (formation of Co3O4) in the synchronous 2D correlation spectrum (see Fig. 3(a)), were resolved to six peaks, at 456 and 469, 500 and 514, and 660 and 672 cm1, respectively, in the asynchronous 2D correlation spectrum (see Fig. 3(b)). From the analysis of the 2D correlation spectra of the charged LiCoO2 electrode in the VC-free electrolyte, we deduced the following sequence of spectral events with increasing voltage: 672 ? 469 ? 514 ? 500 ? 456 ? 660 cm1. This suggests that, initially, Co3O4 is produced by the surface reaction in the cathode/electrolyte interface, that, next, the Li2O component is formed on the cathode surface, and that finally, hexagonal
LiCoO2 structural distortion accompanying the surface change is caused by the further extraction of Li+ ions that occurs with increasing charge voltage. In the VC-containing electrolyte, four peaks, at 478 (Eg mode of LiCoO2), 509 (formation of Li2O), 673 (formation of Co3O4), and 686 cm1 (formation of Co2O3) in the synchronous 2D correlation spectrum (see Fig. 4(a)), were obtained [29]. Two peaks, at 478 and 509 cm1, were resolved to four peaks, at 467 and 477 and 506 and 514 cm1, respectively, in the asynchronous 2D correlation spectrum (see Fig. 4(b)). Analysis of the 2D correlation spectra showed the following sequence of spectral events with increasing voltage: 477 ? 673 ? 506 ? 514 ? 686 ? 467 cm1. This indicates that, initially, structural distortion of the LiCoO2 electrode occurs, that, subsequently, the Co3O4 and Li2O components are formed on the cathode, and that finally, Co2O3 is formed on the cathode surface with increasing charge voltage. Overall, the 2D correlation analysis results suggest that, even though the same SEI components are produced on the cathode surface, the
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Fig. 3. (a) Synchronous and (b) asynchronous 2D Raman correlation spectra of LiCoO2 electrodes in VC-free electrolyte at different cut-off voltages. The red and blue lines represent the positive and negative cross peaks, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. (a) Synchronous and (b) asynchronous 2D Raman correlation spectra of LiCoO2 electrodes in VC-containing electrolyte during overcharge process. The red and blue lines represent the positive and negative cross peaks, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
electrochemical reaction kinetics in the cathode/electrolyte interface differ according to non-use or use of VC: in the latter case, formation of the SEI components (i.e., Co3O4 and Li2O) is delayed. 4. Conclusions The mechanism of SEI film formation on the cathode surface is instrumental to the proper understanding of the electrochemical behavior of the Li-ion battery. In the present work, we studied the electrochemical reaction of the LiCoO2/Li cell in VC-free and VC-containing electrolytes and characterized the SEI film composition and properties by Raman and 2D correlation spectroscopy. The
electrochemical results showed that VC reduces the irreversible capacity loss incurred in the charge–discharge process. In summary, we suggest, based on the analysis of the 2D correlation Raman spectra, that even though the same SEI components are produced on the cathode surface, the electrochemical reaction kinetics in the cathode/electrolyte interface differ: crucially, formation of the SEI components (i.e., Co3O4 and Li2O) is delayed by use of VC. Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korea government (MEST) (No.
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