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A tetraethylene glycol dimethylether-lithium bis(oxalate)borate (TEGDME-LiBOB) electrolyte for advanced lithium ion batteries
Electrochemistry Communications 14 (2012) 43–46
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
A tetraethylene glycol dimethylether-lithium bis(oxalate)borate (TEGDME-LiBOB) electrolyte for advanced lithium ion batteries Dong-Ju Lee a, Jusef Hassoun b,⁎, Stefania Panero b, Yang-Kook Sun a, c,⁎⁎, Bruno Scrosati a, b a b c
Department of WCU Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea Department of Chemistry, University of Rome Sapienza, 00185, Rome, Italy Department of Chemical Engineering, Hanyang University, Seoul 133-791, Republic of Korea
a r t i c l e
i n f o
Article history: Received 4 October 2011 Received in revised form 30 October 2011 Accepted 31 October 2011 Available online 7 November 2011 Keywords: Tetraethylene glycol dimethylether TEGDME Lithium bis(oxalate)borate LiBOB Electrolyte Lithium ion battery
a b s t r a c t Tetraethylene glycol dimethylether-lithium bis(oxalate)borate (TEGDME-LiBOB) electrolyte is here studied. Electrochemical impedance spectroscopy (EIS) measurements demonstrate that the electrolyte has conductivity higher than 10− 3 S cm − 1 at room temperature and about 10 − 2 S cm − 1 at 60 °C, while thermogravimetry indicates a stability extending up to 180 °C. Sweep voltammetry of the electrolyte shows anodic stability extending over 4.6 V vs. Li and cathodic peak at about 1.5 V vs. Li/Li+, caused by a decomposition of LiBOB salt, and following prevented by using a pre-treated Sn-C anode. Furthermore, LiFePO4 electrode is successfully used as cathode in a lithium cell using the TEGDME-LiBOB electrolyte. The promising electrochemical results, the low cost and the very high safety level candidate the electrolyte here reported as a valid alternative to the conventional electrolyte based on fluorinated salts presently used in the lithium ion battery field. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Lithium ion batteries are presently attracting large demand as an energy storage system for portable devices and hybrid electric vehicles (HEV), plugged-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs) due to their high energy density and excellent cycle life. However, further improvements are required both in terms of a safety and of cost, in particular in the field of transportation system [1]. For this reasons, new cell configurations involving safe and low cost materials are strongly needed to reach the market targets. Olivine lithium iron phosphate (LiFePO4) is one of the most promising cathode materials due to its low cost, high safety and environmental compatibility [2] and lithium-tin alloying composites are excellent alternatives to the conventional anodes [3]. Moreover, conventional electrolytes based on carbonate solvents (e. g. DMC, DEC, EMC, et al.) and fluorinated salts (e.g. LiPF6) should be replaced, this is due to their high flammability, reactivity and toxicity [4]. The poly (ethylene) glycols, PEGs, are generally considered to be inert and possess a low order of toxicity in animals and humans [5]. In addition, end capped linear glycols, such as tetra ethylene glycol dimethylether (TEGDME), have good solvating power for the lithium salts, suitable conductivity and low flammability, thus they can be
⁎ Correspondence to: J. Hassoun, Tel.: + 39 06 49913530; fax: + 39 06 491769. ⁎⁎ Correspondence to: Y.-K. Sun, Tel.: + 82 2 2220 0524; fax: + 82 2 2282 7329. E-mail addresses: [email protected] (J. Hassoun), [email protected] (Y.-K. Sun). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.10.027
proposed as alternative systems [6]. Finally, new lithium salts, having higher safety level compared with standard fluorinated ones, are necessary. Lithium bis(oxalate) borate (LiBOB) salt has been proposed both as electrolyte salt and as electrolyte additive [7] and it has been shown that this salt has higher stability and higher safety, with only slightly lower rate capability, compared with LiPF6 [8]. Here, we characterized a tetraethylene glycol dimethyletherlithium bis(oxalate)borate (TEGDME-LiBOB) solution as new electrolyte for advanced lithium ion battery. Electrochemical impedance spectroscopy measurements, linear sweep voltammetry and electrochemical cycling tests carried out on lithium cells using both tincarbon alloying anode and LiFePO4 cathode, demonstrated the validity of our TEGDME-LiBOB electrolyte as new, safe and low cost electrolyte for lithium ion battery. 2. Experimental The tetraethylene glycol dimethylether-lithium bis(oxalate)borate (TEGDME-LiBOB) electrolyte was prepared from LiBOB salt (Chemtall GmBH) and TEGDME solvent(Acros, extra pure 99%). The LiBOB salt and TEGDME solvent were mixed in molar ratio of 1:4 in argon filled glove with water content lower than 1 ppm. Blocking electrode cells and Lithium–Lithium symmetrical Swagelok cells have been used for the ionic conductivity and the lithium interphase stability measurements, respectively. The error bar of the ionic conductivity is calculated by meaning of error distribution analysis taking into account the errors associated with the parameters involved in its determination, i.e. the cell constant and the electrolyte resistance.
44
D.-J. Lee et al. / Electrochemistry Communications 14 (2012) 43–46
Electrochemical impedance spectroscopy (EIS) test was carried out using VersaSTAT MC (Princeton applied research) with amplitude of 10 mV in a frequency range of 100 kHz–100 mHz. Thermo gravimetric analysis (TGA) of the electrolyte has been performed in a 25–500 °C temperature range, under nitrogen atmosphere, with a scan rate of 10 °C/min, by using a Mettler–Toledo (TGA/SDTA 851) instrument. The Sn-C and the LiFePO4 powders were prepared according to processes well described in our previous papers [9,10]. Each active material powder was blended with super P carbon (Timcal) and polyvinylidene fluoride (Solvey), with 80:10:10 weight ratio, respectively, in N-methyl-2-pyrrolidone (Aldrich). The slurry was cast on aluminium and copper foils with weight density of 4 and 2 mg cm − 2, for the LiFePO4 and the Sn-C respectively, and dried under vacuum at 110 °C. The Sn-C electrode was pre-treated using lithium foil with appropriate time and pressure, as described in a previous paper [11]. The prepared electrodes were assembled in a Swagelok cell using Whatman TM glass filter as a separator and Li foil as counter electrode then the cells were used for galvanostatic test using Maccor Series 4000 Battery Test System (Maccor, Inc.). Cyclic voltammetry (CV) was performed using a three electrode cell with lithium metal counter and reference electrode at a scan rate of 0.5 mV/s and using a Biologic VMP3 instrument.
3. Result and discussion The ionic conductivity and the electrolyte weight change in function of the temperature are reported in Fig. 1. The Arrhenius plot of Fig. 1A, performed by heating from room temperature to 100 °C and by cooling
a
T / oC
a
111
84
back, evidences an increase of the ionic conductivity of the TEGDMELiBOB electrolyte from 10− 3 S cm− 1 to 10− 2 S cm− 1, respectively, and an excellent overlapping of the heating and the cooling plot. This behavior indicates a good lithium transport properties and an excellent thermal stability of the studied electrolyte. In addition, Fig. 1B, reporting the weight change of the electrolyte by heating under argon atmosphere, shows a first weight loss centered at about 180 °C, in correspondence with the TEGDME evaporation, followed by a second and smaller change at about 340 °C due to the LiBOB salt decomposition. The thermo-gravimetric profiles, in combination with the Arrhenius plots, indicate that the electrolyte can be safely used in a wide range of temperature. The lithium interphase stability of the studied electrolyte together with its electrochemical stability window are reported in Fig. 2 showing the time evolution of the overall resistance and, in inset, the corresponding impedance spectra of a symmetrical Li/LiBOB-TEGDME/Li cell under open circuit conditions (A) and the voltage/current profile of the electrolyte (B). The Fig. 2a evidences how the Li-interphase resistance increases up to a value of the order of 1.6 kΩ to finally stabilize at that level. This behavior is associated with the growing of a solid electrolyte interface (SEI) layer at the surface of the lithium metal with increased resistance until complete formation and consequent stabilization of the resistance value. Furthermore, Fig. 2b indicates that the electrolyte is stable up to 4.6 V vs. Li in the anodic region, while the cathodic scan is affected by the expected LiBOB salt decomposition at about 1.5 V, as reported by our previous paper studying the LiBOB solution in standard, carbonate based, electrolyte [12] and by literature papers [7,8]. Accordingly, at the lower voltage value, i.e. below 0.9 V vs. Li, film forming and insertion of the lithium in the carbon are observed. As we demonstrated in a previous paper
39
60
2.0
21
1x10-1
1x10-2
1.2 0.8 0.4
1x10-3
1
0 0
Heating Cooling
1 2 Zre / kΩ
0.0 0
2
-4
1x10
day 0 day 1 day 2 day 5 day 8 day 13
2 -Zim / kΩ
Resistance / kΩ
1.6
4
6
8
10
12
14
Time / Day
2.6
2.8
3.0
3.2
3.4
b
1000/T / K-1
1.5
b
1.0 100
Anodic 0.5
TG
60 o
40
340 C LiBOB decomposition
i / mA cm-2
Weight / %
80
o
180 C TGDME evaporation
Cathodic
0.0 -0.5 -1.0
20
-1.5
0
Differential 100
0
1
2
3
4
5
Potential vs. Li/Li+ / V 200
300
400
Temperature / oC Fig. 1. (a) Arrhenius plot of ionic conductivity of TEGDME-LiBOB electrolyte, (b) TGA of the electrolyte.
Fig. 2. (a) Time evolution of the overall resistance and, in inset, the corresponding impedance spectra of a symmetrical Li/TEGDME-LiBOB/Li cell under open circuit conditions, (b) Current–voltage curves of a cell using Super-P as working electrode and Li foil as counter electrode in TEGDME-LiBOB electrolyte. Room temperature.
D.-J. Lee et al. / Electrochemistry Communications 14 (2012) 43–46
LiFePO4 ¼ Li þ FePO4
ð1Þ
Sn C þ 4:4Li ¼ Li4:4 Sn C
ð2Þ
a
4.0
First cycle
Voltage / V
3.5
LiFePO
3.0 2.0 1.5
Sn-C
1.0 0.0
First cycle
0
200
300
400
b 750 600 450
SnC
300
LiFePO 4
150 0
15
30
45
60
75
Cycles Fig. 4. Voltage profiles (a) and cycling behavior (b) of the room temperature galvanostatic tests performed on lithium cells using the LiFePO4 and the pre-treated Sn-C electrode in the TEGDME-LiBOB electrolyte, at current density of 50 mA g− 1 and of 150 mA g− 1, respectively.
4. Conclusion The full characterization of a new, low cost and very safe electrolyte formed by the dissolution of a lthium bis(oxalate)borate (LiBOB) salt in a tetraethylene glycol dimethylether (TEGDME) has been reported. We demonstrated that the electrolyte has conductivity ranging between 10− 3 and 10 − 2 S cm− 1 and a thermal stability as high as 180 °C. Furthermore, test performed in lithium cells evidenced wide electrochemical stability window and good cycling response when the solution is utilized as electrolyte media in combination with LiFePO4 and pretreated Sn-C electrodes, so that the TEGDME-LiBOB solution has been proposed as high performance electrolyte for lithium ion batteries.
Acknowledgements This work was in part performed within the Project “REALIST” (Rechargeable, advanced, nano structured lithium batteries with high energy storage) sponsored by the Italian Institute of Technology (IIT), in part by the Italian National Research Council (CNR) and in part within the WCU (World Class University) program through the National Research Foundation of Korea funded by Ministry of Education, Science and Technology (R31-2008-000-10092), and by the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 20114010203150).
3 1
i / mA cm-2
100
Specific capacity / mAh g-1
0
Furthermore, the cycling behavior of the Fig. 4B evidences capacities of the order of 400 mAh g − 1 and of 140 mAh g − 1 for the cell using Sn-C and LiFePO4 electrode, respectively. The cell using LiFePO4 cathode shows excellent stability, while that using Sn-C shows slight decay, since it is more affected by the increasing of the cell resistance associated with the lithium electrode side (see Fig. 2A and related discussion). It is also observed in Fig. 4A that the specific capacity of the first charge of the LiFePO4 cathode (150 mAhg − 1) is higher than that of the discharge (126 mAh g − 1). It is assumed that this initial irreversibility is due to a series of side reactions involving electrolyte oxidation with solid electrolyte interface (SEI) formation, as well as to possible initial reorganization of the olivine structure. However, the overall behavior of the half cells well matches with that observed in standard electrolyte, this confirming the good properties of the new electrolyte developed in this work. Further work, with promising results, is in progress in our laboratory with the aim to investigate the behavior of full lithium ion cells using the Sn-C anode together with the LiFePO4 cathode in the new TEGDME-LiBOB electrolyte, and the results will be reported in a future paper.
4
2.5
0.5
Capacity / mAh g-1
[12] the decomposition of the LiBOB salt at about 1.5 V vs. Li is promoted by the presence of the carbon in the anode side, so that this parasite process severely obstacles the use of this salt in lithium ion battery using carbon-containing anodes instead of metal lithium. Recently, we successfully employed pre-treated Sn-C composite anode in LiBOB based electrolyte using standard carbonate solvent with no evidences of salt decomposition in the 1.5 V vs. Li voltage region [12]. Similar approach has been reported in this work, using the new TEGDME-LiBOB electrolyte in view of its possible application in advanced lithium ion battery. Fig. 3, reporting the cyclic voltammetry profiles of a lithium cell using the pre-treated Sn-C electrode in TEGDME-LiBOB electrolyte, clearly shows the absence of the peak associated to the salt decomposition at about 1.5 V vs. Li, in addition to a remarkable reversibility of the Li-alloying/de-alloying process with merged peaks centered at about 0.6 V vs. Li [9,12]. Finally, in order to further investigate the behavior of the new electrolyte, half lithium cells using it together with LiFePO4 and pretreated Sn-C electrodes have been galvanostatically cycled at current density of 50 mA g -1 and of 150 mA g -1, respectively, and the results are reported in Fig. 4. The reversible voltage profiles of Fig. 4A show the flat profile of the LiFePO4 electrode centered at about 3.5 vs. Li and the sloppy profile of the Sn-C electrode centered at about 0.6 vs. Li, reflecting, respectively, the electrochemical processes:
45
0 -1 -3 -4 0.0
0.6
1.2
1.8
2.4
3.0
Potential vs. Li/Li+ / V Fig. 3. Cyclic voltammetry profiles of the lithium cell using the pre-treated Sn-C as working electrode and Li foil as counter and reference electrode in TEGDME-LiBOB electrolyte.
References [1] J.M. Tarascon, M. Armand, Nature 414 (2001) 359. [2] J. Hassoun, A. Fernicola, M.A. Navarra, B. Scrosati, Journal of Power Sources 195 (2010) 574. [3] N. Li, C.R. Martin, Journal of the Electrochemical Society 148 (2001) A164. [4] K. Xu, Chemical Reviews 104 (2004) 4303.
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D.-J. Lee et al. / Electrochemistry Communications 14 (2012) 43–46
[5] V.O. Sheftel, Indirect Food Additives and Polymers: Migration and Toxicology, Lewis, 2000, p. 1114. [6] D. Aurbach, E. Granot, Electrochimica Acta 42 (1997) 697. [7] Z.H. Chen, W.Q. Lu, J. Liu, K. Amine, Electrochimica Acta 51 (2006) 3322. [8] M. Wachtler, M. Wohlfahrt-Mehrens, S. Ströbele, J.-C. Panitz, U. Wietelmann, Journal of Applied Electrochemistry 36 (2006) 1199. [9] J. Hassoun, G. Derrien, S. Panero, B. Scrosati, Advanced Materials 20 (2008) 3169.
[10] S.-W. Oh, S.-T. Myung, S.-M. Oh, K.-H. Oh, K. Amine, B. Scrosati, Y.-K. Sun, Advanced Materials 22 (2010) 4842. [11] J. Hassoun, K.-S. Lee, Y.-K. Sun, B. Scrosati, Journal of the American Chemical Society 133 (2011) 3139. [12] J. Hassoun, M. Wachtler, M. Wohlfaht-Mehrens, B. Scrosati, Journal of Power Sources 196 (2011) 349.
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
A tetraethylene glycol dimethylether-lithium bis(oxalate)borate (TEGDME-LiBOB) electrolyte for advanced lithium ion batteries Dong-Ju Lee a, Jusef Hassoun b,⁎, Stefania Panero b, Yang-Kook Sun a, c,⁎⁎, Bruno Scrosati a, b a b c
Department of WCU Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea Department of Chemistry, University of Rome Sapienza, 00185, Rome, Italy Department of Chemical Engineering, Hanyang University, Seoul 133-791, Republic of Korea
a r t i c l e
i n f o
Article history: Received 4 October 2011 Received in revised form 30 October 2011 Accepted 31 October 2011 Available online 7 November 2011 Keywords: Tetraethylene glycol dimethylether TEGDME Lithium bis(oxalate)borate LiBOB Electrolyte Lithium ion battery
a b s t r a c t Tetraethylene glycol dimethylether-lithium bis(oxalate)borate (TEGDME-LiBOB) electrolyte is here studied. Electrochemical impedance spectroscopy (EIS) measurements demonstrate that the electrolyte has conductivity higher than 10− 3 S cm − 1 at room temperature and about 10 − 2 S cm − 1 at 60 °C, while thermogravimetry indicates a stability extending up to 180 °C. Sweep voltammetry of the electrolyte shows anodic stability extending over 4.6 V vs. Li and cathodic peak at about 1.5 V vs. Li/Li+, caused by a decomposition of LiBOB salt, and following prevented by using a pre-treated Sn-C anode. Furthermore, LiFePO4 electrode is successfully used as cathode in a lithium cell using the TEGDME-LiBOB electrolyte. The promising electrochemical results, the low cost and the very high safety level candidate the electrolyte here reported as a valid alternative to the conventional electrolyte based on fluorinated salts presently used in the lithium ion battery field. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Lithium ion batteries are presently attracting large demand as an energy storage system for portable devices and hybrid electric vehicles (HEV), plugged-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs) due to their high energy density and excellent cycle life. However, further improvements are required both in terms of a safety and of cost, in particular in the field of transportation system [1]. For this reasons, new cell configurations involving safe and low cost materials are strongly needed to reach the market targets. Olivine lithium iron phosphate (LiFePO4) is one of the most promising cathode materials due to its low cost, high safety and environmental compatibility [2] and lithium-tin alloying composites are excellent alternatives to the conventional anodes [3]. Moreover, conventional electrolytes based on carbonate solvents (e. g. DMC, DEC, EMC, et al.) and fluorinated salts (e.g. LiPF6) should be replaced, this is due to their high flammability, reactivity and toxicity [4]. The poly (ethylene) glycols, PEGs, are generally considered to be inert and possess a low order of toxicity in animals and humans [5]. In addition, end capped linear glycols, such as tetra ethylene glycol dimethylether (TEGDME), have good solvating power for the lithium salts, suitable conductivity and low flammability, thus they can be
⁎ Correspondence to: J. Hassoun, Tel.: + 39 06 49913530; fax: + 39 06 491769. ⁎⁎ Correspondence to: Y.-K. Sun, Tel.: + 82 2 2220 0524; fax: + 82 2 2282 7329. E-mail addresses: [email protected] (J. Hassoun), [email protected] (Y.-K. Sun). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.10.027
proposed as alternative systems [6]. Finally, new lithium salts, having higher safety level compared with standard fluorinated ones, are necessary. Lithium bis(oxalate) borate (LiBOB) salt has been proposed both as electrolyte salt and as electrolyte additive [7] and it has been shown that this salt has higher stability and higher safety, with only slightly lower rate capability, compared with LiPF6 [8]. Here, we characterized a tetraethylene glycol dimethyletherlithium bis(oxalate)borate (TEGDME-LiBOB) solution as new electrolyte for advanced lithium ion battery. Electrochemical impedance spectroscopy measurements, linear sweep voltammetry and electrochemical cycling tests carried out on lithium cells using both tincarbon alloying anode and LiFePO4 cathode, demonstrated the validity of our TEGDME-LiBOB electrolyte as new, safe and low cost electrolyte for lithium ion battery. 2. Experimental The tetraethylene glycol dimethylether-lithium bis(oxalate)borate (TEGDME-LiBOB) electrolyte was prepared from LiBOB salt (Chemtall GmBH) and TEGDME solvent(Acros, extra pure 99%). The LiBOB salt and TEGDME solvent were mixed in molar ratio of 1:4 in argon filled glove with water content lower than 1 ppm. Blocking electrode cells and Lithium–Lithium symmetrical Swagelok cells have been used for the ionic conductivity and the lithium interphase stability measurements, respectively. The error bar of the ionic conductivity is calculated by meaning of error distribution analysis taking into account the errors associated with the parameters involved in its determination, i.e. the cell constant and the electrolyte resistance.
44
D.-J. Lee et al. / Electrochemistry Communications 14 (2012) 43–46
Electrochemical impedance spectroscopy (EIS) test was carried out using VersaSTAT MC (Princeton applied research) with amplitude of 10 mV in a frequency range of 100 kHz–100 mHz. Thermo gravimetric analysis (TGA) of the electrolyte has been performed in a 25–500 °C temperature range, under nitrogen atmosphere, with a scan rate of 10 °C/min, by using a Mettler–Toledo (TGA/SDTA 851) instrument. The Sn-C and the LiFePO4 powders were prepared according to processes well described in our previous papers [9,10]. Each active material powder was blended with super P carbon (Timcal) and polyvinylidene fluoride (Solvey), with 80:10:10 weight ratio, respectively, in N-methyl-2-pyrrolidone (Aldrich). The slurry was cast on aluminium and copper foils with weight density of 4 and 2 mg cm − 2, for the LiFePO4 and the Sn-C respectively, and dried under vacuum at 110 °C. The Sn-C electrode was pre-treated using lithium foil with appropriate time and pressure, as described in a previous paper [11]. The prepared electrodes were assembled in a Swagelok cell using Whatman TM glass filter as a separator and Li foil as counter electrode then the cells were used for galvanostatic test using Maccor Series 4000 Battery Test System (Maccor, Inc.). Cyclic voltammetry (CV) was performed using a three electrode cell with lithium metal counter and reference electrode at a scan rate of 0.5 mV/s and using a Biologic VMP3 instrument.
3. Result and discussion The ionic conductivity and the electrolyte weight change in function of the temperature are reported in Fig. 1. The Arrhenius plot of Fig. 1A, performed by heating from room temperature to 100 °C and by cooling
a
T / oC
a
111
84
back, evidences an increase of the ionic conductivity of the TEGDMELiBOB electrolyte from 10− 3 S cm− 1 to 10− 2 S cm− 1, respectively, and an excellent overlapping of the heating and the cooling plot. This behavior indicates a good lithium transport properties and an excellent thermal stability of the studied electrolyte. In addition, Fig. 1B, reporting the weight change of the electrolyte by heating under argon atmosphere, shows a first weight loss centered at about 180 °C, in correspondence with the TEGDME evaporation, followed by a second and smaller change at about 340 °C due to the LiBOB salt decomposition. The thermo-gravimetric profiles, in combination with the Arrhenius plots, indicate that the electrolyte can be safely used in a wide range of temperature. The lithium interphase stability of the studied electrolyte together with its electrochemical stability window are reported in Fig. 2 showing the time evolution of the overall resistance and, in inset, the corresponding impedance spectra of a symmetrical Li/LiBOB-TEGDME/Li cell under open circuit conditions (A) and the voltage/current profile of the electrolyte (B). The Fig. 2a evidences how the Li-interphase resistance increases up to a value of the order of 1.6 kΩ to finally stabilize at that level. This behavior is associated with the growing of a solid electrolyte interface (SEI) layer at the surface of the lithium metal with increased resistance until complete formation and consequent stabilization of the resistance value. Furthermore, Fig. 2b indicates that the electrolyte is stable up to 4.6 V vs. Li in the anodic region, while the cathodic scan is affected by the expected LiBOB salt decomposition at about 1.5 V, as reported by our previous paper studying the LiBOB solution in standard, carbonate based, electrolyte [12] and by literature papers [7,8]. Accordingly, at the lower voltage value, i.e. below 0.9 V vs. Li, film forming and insertion of the lithium in the carbon are observed. As we demonstrated in a previous paper
39
60
2.0
21
1x10-1
1x10-2
1.2 0.8 0.4
1x10-3
1
0 0
Heating Cooling
1 2 Zre / kΩ
0.0 0
2
-4
1x10
day 0 day 1 day 2 day 5 day 8 day 13
2 -Zim / kΩ
Resistance / kΩ
1.6
4
6
8
10
12
14
Time / Day
2.6
2.8
3.0
3.2
3.4
b
1000/T / K-1
1.5
b
1.0 100
Anodic 0.5
TG
60 o
40
340 C LiBOB decomposition
i / mA cm-2
Weight / %
80
o
180 C TGDME evaporation
Cathodic
0.0 -0.5 -1.0
20
-1.5
0
Differential 100
0
1
2
3
4
5
Potential vs. Li/Li+ / V 200
300
400
Temperature / oC Fig. 1. (a) Arrhenius plot of ionic conductivity of TEGDME-LiBOB electrolyte, (b) TGA of the electrolyte.
Fig. 2. (a) Time evolution of the overall resistance and, in inset, the corresponding impedance spectra of a symmetrical Li/TEGDME-LiBOB/Li cell under open circuit conditions, (b) Current–voltage curves of a cell using Super-P as working electrode and Li foil as counter electrode in TEGDME-LiBOB electrolyte. Room temperature.
D.-J. Lee et al. / Electrochemistry Communications 14 (2012) 43–46
LiFePO4 ¼ Li þ FePO4
ð1Þ
Sn C þ 4:4Li ¼ Li4:4 Sn C
ð2Þ
a
4.0
First cycle
Voltage / V
3.5
LiFePO
3.0 2.0 1.5
Sn-C
1.0 0.0
First cycle
0
200
300
400
b 750 600 450
SnC
300
LiFePO 4
150 0
15
30
45
60
75
Cycles Fig. 4. Voltage profiles (a) and cycling behavior (b) of the room temperature galvanostatic tests performed on lithium cells using the LiFePO4 and the pre-treated Sn-C electrode in the TEGDME-LiBOB electrolyte, at current density of 50 mA g− 1 and of 150 mA g− 1, respectively.
4. Conclusion The full characterization of a new, low cost and very safe electrolyte formed by the dissolution of a lthium bis(oxalate)borate (LiBOB) salt in a tetraethylene glycol dimethylether (TEGDME) has been reported. We demonstrated that the electrolyte has conductivity ranging between 10− 3 and 10 − 2 S cm− 1 and a thermal stability as high as 180 °C. Furthermore, test performed in lithium cells evidenced wide electrochemical stability window and good cycling response when the solution is utilized as electrolyte media in combination with LiFePO4 and pretreated Sn-C electrodes, so that the TEGDME-LiBOB solution has been proposed as high performance electrolyte for lithium ion batteries.
Acknowledgements This work was in part performed within the Project “REALIST” (Rechargeable, advanced, nano structured lithium batteries with high energy storage) sponsored by the Italian Institute of Technology (IIT), in part by the Italian National Research Council (CNR) and in part within the WCU (World Class University) program through the National Research Foundation of Korea funded by Ministry of Education, Science and Technology (R31-2008-000-10092), and by the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 20114010203150).
3 1
i / mA cm-2
100
Specific capacity / mAh g-1
0
Furthermore, the cycling behavior of the Fig. 4B evidences capacities of the order of 400 mAh g − 1 and of 140 mAh g − 1 for the cell using Sn-C and LiFePO4 electrode, respectively. The cell using LiFePO4 cathode shows excellent stability, while that using Sn-C shows slight decay, since it is more affected by the increasing of the cell resistance associated with the lithium electrode side (see Fig. 2A and related discussion). It is also observed in Fig. 4A that the specific capacity of the first charge of the LiFePO4 cathode (150 mAhg − 1) is higher than that of the discharge (126 mAh g − 1). It is assumed that this initial irreversibility is due to a series of side reactions involving electrolyte oxidation with solid electrolyte interface (SEI) formation, as well as to possible initial reorganization of the olivine structure. However, the overall behavior of the half cells well matches with that observed in standard electrolyte, this confirming the good properties of the new electrolyte developed in this work. Further work, with promising results, is in progress in our laboratory with the aim to investigate the behavior of full lithium ion cells using the Sn-C anode together with the LiFePO4 cathode in the new TEGDME-LiBOB electrolyte, and the results will be reported in a future paper.
4
2.5
0.5
Capacity / mAh g-1
[12] the decomposition of the LiBOB salt at about 1.5 V vs. Li is promoted by the presence of the carbon in the anode side, so that this parasite process severely obstacles the use of this salt in lithium ion battery using carbon-containing anodes instead of metal lithium. Recently, we successfully employed pre-treated Sn-C composite anode in LiBOB based electrolyte using standard carbonate solvent with no evidences of salt decomposition in the 1.5 V vs. Li voltage region [12]. Similar approach has been reported in this work, using the new TEGDME-LiBOB electrolyte in view of its possible application in advanced lithium ion battery. Fig. 3, reporting the cyclic voltammetry profiles of a lithium cell using the pre-treated Sn-C electrode in TEGDME-LiBOB electrolyte, clearly shows the absence of the peak associated to the salt decomposition at about 1.5 V vs. Li, in addition to a remarkable reversibility of the Li-alloying/de-alloying process with merged peaks centered at about 0.6 V vs. Li [9,12]. Finally, in order to further investigate the behavior of the new electrolyte, half lithium cells using it together with LiFePO4 and pretreated Sn-C electrodes have been galvanostatically cycled at current density of 50 mA g -1 and of 150 mA g -1, respectively, and the results are reported in Fig. 4. The reversible voltage profiles of Fig. 4A show the flat profile of the LiFePO4 electrode centered at about 3.5 vs. Li and the sloppy profile of the Sn-C electrode centered at about 0.6 vs. Li, reflecting, respectively, the electrochemical processes:
45
0 -1 -3 -4 0.0
0.6
1.2
1.8
2.4
3.0
Potential vs. Li/Li+ / V Fig. 3. Cyclic voltammetry profiles of the lithium cell using the pre-treated Sn-C as working electrode and Li foil as counter and reference electrode in TEGDME-LiBOB electrolyte.
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