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Tri-(4-methoxythphenyl) phosphate: A new electrolyte additive with both fire-retardancy and overcharge protection for Li-ion batteries
Electrochimica Acta 53 (2008) 8265–8268
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Tri-(4-methoxythphenyl) phosphate: A new electrolyte additive with both fire-retardancy and overcharge protection for Li-ion batteries J.K. Feng, Y.L. Cao, X.P. Ai, H.X. Yang ∗ Department of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China
a r t i c l e
i n f o
Article history: Received 22 January 2008 Received in revised form 9 May 2008 Accepted 9 May 2008 Available online 18 May 2008 Keywords: Tri-(4-methoxythphenyl) phosphate Electrolyte additive Fire retardant Overcharge protection Lithium-ion batteries
a b s t r a c t A novel compound, tri-(4-methoxythphenyl) phosphate, was synthesized and investigated as a safety electrolyte additive for lithium-ion batteries. It was found that this additive could lower the flammability of the electrolyte, and thereby enhance the thermal stability of the Li-ion battery. Moreover, this molecule can also be polymerized at 4.35 V (vs. Li/Li+ ) to form a conducting polymer, which can protect the batteries from voltage runaway at overcharge by internal bypassing the overcharging current in the batteries. Thus, it is possible to use this electrolyte additive to provide both overcharge protection and flame retardancy for lithium-ion batteries without much influence on the battery performance. © 2008 Published by Elsevier Ltd.
1. Introduction Li-ion batteries are now extensively applied in various portable electronics and are also considered as promising power sources for high rate applications such as electric tools and electric vehicles. A key obstacle for high rate and high capacity applications of Liion batteries is due to safety concerns. Though occasionally, the firing and explosive accidents of the batteries are still ceaselessly recorded, most of which were found to happen in overcharged or overheated status of the Li-ion batteries [1,2]. To solve this problem, considerable effort has been focused on development of nonflammable electrolytes [3–12] and overcharge protection additives [13–19]. A number of fire-retardant organic phosphates have been investigated as electrolyte additives to prevent the batteries from firing at abusive conditions [3–12]. Several polymerizable molecules were suggested as safety additives, which form conducting [13,14,19] or isolating polymers [15] in the batteries at overcharge to bypass or interrupt the internal current flow. If an additive can have the functions of both fire-retardancy and overcharge protection, it would provide more reliable protection for Li-ion batteries at wider storage and working circumstances. In this study, we synthesized a new phosphate ether (tri-4methoxythphenyl phosphate, TMPP) and test this compound as a safety additive for Li-ion batteries. It is found that this electrolyte
∗ Tel.: +86 27 87884476; fax: +86 27 87884476. E-mail address: [email protected] (H.X. Yang). 0013-4686/$ – see front matter © 2008 Published by Elsevier Ltd. doi:10.1016/j.electacta.2008.05.024
additive can depress the ignitability of the electrolyte and polymerize at overcharged potential of +4.35 V (vs. Li/Li+ ) to form a conductive polymer bypassing the reaction current. The thermal stability and electrochemical compatibility of the electrolyte in the presence of the additive are also discussed. 2. Experimental 2.1. Synthesis and characterizations TMPP was synthesized by an etherifying reaction (shown in Fig. 1), similarly to the etherization of phosphites reported in Ref. [20]. A typical experimental procedure is to place 0.3 mol of 4methoxythphenol, 0.36 mol of triethylamine (Et3 N) in 200 ml of dehydrated tetrahydrofuran (THF) in a three-necked 500 ml flask, and then add 0.1 mol of POCl3 in the flask. The reaction mixture was stirred at room temperature for 48 h; the resultant triethylammonium chloride was filtered off through a filter tipped cannula. The solid product was filtrated and washed several times with THF and then dried to give the oily product p-methoxythphenyl-phosphate in ∼70% yield, and then the product was washed with 5% HCl solution and 5% NaOH solution successively, finally dried in a vacuum oven using P2 O5 as drier. All the chemicals were of analytical grade and used without further purification except otherwise noted. THF was distilled over Mg power before use. The FT-IR spectra of TMPP were recorded on a NICOLET AVATAR360 FT-IR spectrometer with KBr pellets. Elemental analysis of TMPP was performed on VarioEL III instrument. 1 H NMR spectra
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Fig. 1. Synthetic reaction of TMPP.
of TMPP were recorded on a Mercury VX-300 (300 Hz) apparatus with tetramethylsilane (TMS) as internal standard and CDCl3 as solvent. The pure TMPP compound so synthesized is a nonflammable oily liquid. When ignited, this compound gave only a white smoke, but did not burn to produce flame. To examine the nonflammability of the TMPP electrolyte, we measured the self-extinguishing time (SET) for pre-weighed samples of the electrolyte solutions, in a similar method as described in Ref. [7]. The typical procedure for SET measurements is to use the fiberglass balls (∼10 mm in diameter) to absorb 0.5 g electrolyte and then burn the fiberglass to record the burning time of the electrolyte. Each test was repeated four times and the burning times were averaged for the electrolyte samples containing different amount of TMPP.
Fig. 2 shows the combustibility of the 1 M LiClO4 EC + DEC electrolyte at various content of TMPP additive. It can be seen that with increasing TMPP content from 0 to 15 v%, the burning time of the electrolyte decreases from 38 to 22 s/g, suggesting the electrolyte transforming from a flammable to a fire-retarded solution [7]. Phosphate compounds are known as flame retardants that can function in the vapor phase by a radical mechanism [6]. This may account for the incombustibility of the organic carbonate solvent in the addition of TMMP. However, the ionic conductivity of the electrolyte is decreased lineally with the increase in the TMPP content, due to the low solvation and high viscosity of TMPP. To get a compromise between the ionic conductivity and nonflammability of the electrolyte, we selected 10% TMPP content as a suitable addition to test the electrochemical performances and safety behaviors of the electrolyte.
2.2. Electrochemical measurements 3.2. Electrochemical behaviors of TMPP The electrochemical behaviors of TMPP were examined by cyclic voltammetry (CV) using a Pt microdisk electrode (0.1 mm in diameter) as working electrode and recorded on a two-electrode cell using a larger lithium sheet as both counter electrode and reference electrode. The data acquisition and analysis were carried out by a CHI 660A electrochemical workstation (Shanghai, China). The electrochemical compatibility of the TMPP-containing electrolytes was examined by charge–discharge experiments of laboratory Li-LiFePO4 and graphite-Li cells. The positive LiFePO4 electrode was consisted of 80% LiFePO4 powder, 12% acetylene black and 8% PTFE (wt.%) and the negative graphite electrode is consist 90% graphite, 2% carbon black and 8%PTFE. The base electrolyte was 1 M LiClO4 /EC-DMC (1:1, v/v) purchased from Guotaihuarong Chemical, Co. Ltd. China. The charge–discharge measurements were carried out using a computer-controlled programmable battery charger (BTS-0518001 type, Shenzhen, China). The conductivity of the electrolyte with and without TMPP was measured and calibrated with reference to the conductivity of 0.1 M KCl solution, using a conductivity measuring meter (DDS-307, Shanghai, China).
Fig. 3 compares the CV curves of a Pt electrode in 1 M LiClO4 + EC/DMC electrolyte with or without addition of 10% TMPP. In the presence of TMPP, a large oxidation current arose at positive potential of 4.35 V (vs. Li/Li+ ), obviously due to the electrochemical oxidation of TMPP molecules as shown as an inset in Fig. 3. When the potential scan was reversed and successively cycled, a number of small CV bands appeared at 1.5–3.4 V, possibly contributed from both the redox reactions of the electrolyte components and the oxidation product of the TMPP additive. As an aromatic molecule, the electrochemical oxidation of TMPP molecules proceeded most likely through an oxidative polymerization mechanism at the very high potential of 4.35 V and the resulting polymer product is mostly
3. Results and discussion 3.1. Physical properties and characterization of TMPP TMPP is a nonflammable oily liquid with a chemical formula (C6 H5 OCH3 )3 PO4 . The molecular structures of TMPP were confirmed by FT-IR bands (3003, 2959, 2837, 1595, 1504, 1465, 1442, 1295, 1034, 966, 832, 721, 701, and 638 cm−1 ), 1 H NMR (CDCl3 , 300 M) ı: 6.7–7.2 (12H, s), 3.77 (9H, s), and MS (M+ , 416.2). These data are well agreed with those reported in Ref. [21].
Fig. 2. Flammability and ionic conductivity of 1 M LiClO4 EC + DMC (1:1, v/v%) at different content of TMPP additive.
J.K. Feng et al. / Electrochimica Acta 53 (2008) 8265–8268
Fig. 3. The CVs of 10% TMPP in 1 M LiClO4 EC/DMC (1:1, v/v) measured at a scan rate: 10 mV/s. The suggested polymerization reaction of TMPP is inserted in the figure.
electronically conductive, as it was reported in the case of electrochemical polymerization of dialkoxybenzene compounds [22]. This suggestion was confirmed by the formation of a layer of dark solid deposit on the overcharged surfaces of LiCoO2 or LiFePO4 cathode in the electrolyte containing TMPP additive and by the electrochemical activity of the polymerized product of TMPP. Since the oxidation potential (4.35 V) of TMPP is higher than the complete charging potential of 4 V Li-ion batteries and quite lower than the decomposition potentials of organic electrolytes currently used, we can also use TMPP molecules as a safety additive for overcharge protection of Li-ion batteries. Once the Li-ion batteries are overcharged to rise up the charging voltage to 4.35 V, the TMPP molecules begin to polymerize at the cathode electrode, which forms an internal electronic wiring in the batteries and protect the charging voltage from runaway [13,22]. Fig. 4 shows the effect of TMPP additive on the overcharge performance of Graphite/LiFePO4 cells. In the absence of TMPP additive, the charging voltage of the cells rises up sharply to above 5 V, at which the oxidative decomposition of electrolyte takes place to sustain the charging current. Since the electrolyte decomposition is exothermic and can produce a large number of flammable small organic molecules, the prolonged overcharge is no doubt a direct cause for hazardous behaviors of the cells. However, when the cells contain 10% TMPP in the electrolyte, the cell voltage goes up firstly and then stabilizes at 4.3 V even at 500% overcharge, due to the electrochemical oxidation of TMPP molecules. Because the electrochemical polymerization of TMPP takes place below the oxidation potential of the electrolyte,
Fig. 4. Charge–discharge curves of the graphite/LiFePO4 cells: (a) with and (b) without addition of 10 v% TMPP.
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the decomposition of electrolyte is thus avoided, protecting the cell from voltage runaway. The stabilized voltage plateau at 4.3 V is obviously due to the constant oxidization of TMPP. As mentioned above, this oxidation reaction can proceed through an oxidative polymerization of TMPP molecules to produce a conductive polymer deposit on the cathode surface. Once the polymer grows increasingly to reach the anode, it must form an electronic bridge between the two electrodes, which partially bypasses the charging current and thereby keep the charging voltage at the acceptable region. A general problem for organic phosphates as electrolyte additives is their electrochemical incompatibility with graphite anode due to the electrochemical decomposition or co-intercalation of phosphates on the graphite surface, which produces an incomplete SEI film and leads to a very low columbic efficiency of the graphite anode at cycling [3]. However, we found that TMPP is quite compatible with graphite anode, showing no significant influences on the charge and discharge performance of graphite. Fig. 5 compares the CV curves and charge–discharge profiles of graphite in 1 M LiClO4 + EC/DMC electrolyte with or without addition of 10% TMPP. It can be seen from the CV curves in Fig. 5 that the graphite anode exhibited well-defined and very reversible anodic and cathodic bands at the potential region of 0.5–0 V, characterizing the normal insertion and de-insertion reaction of Li+ ions into/from the graphite. Correspondingly, the graphite anode showed a well-defined charging plateau at ∼0.15 V and a discharging voltage at ∼0.25 V, resembling very well with those obtained from pure carbonate electrolytes. Though the initial coulombic efficiency of the graphite was only ∼68% in the presence of TMPP, it is comparable to that (66%) observed in the absence of TMPP in our parallel tests, suggesting that there was no negative impact of TMPP on the coulombic efficiency of the graphite. This low initial efficiency in our laboratory cells is possibly due to the fact that the graphite was poorly selected and the test cells were poorly assembled with flooded electrolyte. Nevertheless, the charge–discharge efficiency of the graphite anode rose up quickly to >95% since the second cycle and the reversible capacity of the graphite remained very stable at 300 mAh/g in the subsequent cycling. These data demonstrates that TMPP as an electrolyte additive does not cause remarkable detrimental effects on the electrochemical performances of graphite-based Li-ion batteries. This better electrochemical compatibility of TMPP additive than other organic phosphates is possibly due to its large molecular volume and more stably conjugated -electronic structure, which limits the co-intercalation of the additive molecules at graphite.
Fig. 5. The CVs (a) and charge–discharge curves (b) of graphite in 1 M LiClO4 EC/DMC (1:1, v/v) with or without 10% TMPP. The columbic efficiency of the graphite at first ten cycles is shown in the inset (c).
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4. Conclusions Tri-(4-methoxythphenyl) phosphate was synthesized and tested as a safety electrolyte additive for lithium-ion batteries. Experiments demonstrated that this molecule have a strong ability to lower the flammability of the organic carbonate electrolyte and therefore greatly enhance the thermal stability of the Li-ion batteries when used as additive in the electrolyte. Besides, this molecule can also be polymerized at 4.35 V (vs. Li/Li+ ) to form a conducting polymer, which can be used to protect the batteries from voltage runaway at overcharge. Since this electrolyte additive has not much influence on the electrodes performance, it is possible to use this additive for both overcharge protection and flame retardancy of lithium-ion batteries. Acknowledgement This research was financially supported by the National 973 Program of China (No. 2002CB211800). References [1] D.D. MacNeil, J.R. Dahn, J. Electrochem. Soc. 149 (2002) A912. [2] J.R. Dahn, E.W. Fuller, M. Obrovac, U.V. Sacken, Solid State Ionics 69 (1994) 265.
[3] X. Wang, C. Yamada, H. Naito, G. Segami, K. Kibe, J. Electrochem. Soc. 153 (2006) A135. [4] Y.E. Hyung, D.R. Vissers, K. Amine, J. Power Sources 119–121 (2003) 383. [5] S.S. Zhang, K. Xu, T.R. Jow, J. Power Sources 113 (2003) 166. [6] K. Xu, M.S. Ding, S.S. Zhang, J.L. Allen, T.R. Jow, J. Electrochem. Soc. 149 (2002) A622. [7] M. Morita, Y. Niida, N. Yoshimoto, K. Adachi, J. Power Sources 146 (2005) 427. [8] X.L. Yao, S. Xie, C.H. Chen, Q.S. Wang, J.H. Sun, Y.L. Li, S.X. Lu, J. Power Sources 144 (2005) 170. [9] H.F. Xiang, H.Y. Xu, Z.Z. Wang, C.H. Chen, J. Power Sources 173 (2007) 562. [10] E.G. Shim, T.H. Nam, J.G. Kim, H.S. Kim, S.I. Moon, J. Power Sources 172 (2007) 919. [11] Q.S. Wang, J.H. Sun, Mater. Lett. 61 (2007) 3338. [12] H. Ota, A. Kominato, W.J. Chun, E. Yasukawa, S. Kasuya, J. Power Sources 119–121 (2003) 393. [13] L.F. Xiao, X.P. Ai, Y.L. Cao, H.X. Yang, Electrochim. Acta 49 (2004) 4189. [14] S. Tobishima, Y. Ogino, Y. Watanabe, J. Appl. Electrochem. 33 (2003) 142. [15] X.M. Feng, X.P. Ai, H.X. Yang, J. Appl. Electrochem. 34 (2004) 1199. [16] L.F. Xiao, X.P. Ai, Y.L. Cao, Y.D. Wang, H.X. Yang, Electrochem. Commun. 7 (2005) 589. [17] Z.H. Chen, K. Amine, Electrochem. Commun. 9 (2007) 703. [18] L.M. Moshuchak, M. Bulinski, W.M. Lamanna, R.L. Wang, J.R. Dahn, Electrochem. Commun. 9 (2007) 1497. [19] Y. Watanabe, Y. Yamazaki, K. Yasuda, H. Morimoto, S. Tobishima, J. Power Sources 2 (2006) 1375. [20] J. Hernández, F. Goycoolea, D. Zepeda-Rivera, et al., Tetrahedron 62 (2006) 2520. [21] A.D. Sagar, N.A. Shinde, B.P. Bandgar, Organic Prep. Procedures Int. 32 (2000) 269. [22] B. Fabre, P. Marrec, J. Simonet, J. Electrochem. Soc. 145 (1998) A4110.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Tri-(4-methoxythphenyl) phosphate: A new electrolyte additive with both fire-retardancy and overcharge protection for Li-ion batteries J.K. Feng, Y.L. Cao, X.P. Ai, H.X. Yang ∗ Department of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China
a r t i c l e
i n f o
Article history: Received 22 January 2008 Received in revised form 9 May 2008 Accepted 9 May 2008 Available online 18 May 2008 Keywords: Tri-(4-methoxythphenyl) phosphate Electrolyte additive Fire retardant Overcharge protection Lithium-ion batteries
a b s t r a c t A novel compound, tri-(4-methoxythphenyl) phosphate, was synthesized and investigated as a safety electrolyte additive for lithium-ion batteries. It was found that this additive could lower the flammability of the electrolyte, and thereby enhance the thermal stability of the Li-ion battery. Moreover, this molecule can also be polymerized at 4.35 V (vs. Li/Li+ ) to form a conducting polymer, which can protect the batteries from voltage runaway at overcharge by internal bypassing the overcharging current in the batteries. Thus, it is possible to use this electrolyte additive to provide both overcharge protection and flame retardancy for lithium-ion batteries without much influence on the battery performance. © 2008 Published by Elsevier Ltd.
1. Introduction Li-ion batteries are now extensively applied in various portable electronics and are also considered as promising power sources for high rate applications such as electric tools and electric vehicles. A key obstacle for high rate and high capacity applications of Liion batteries is due to safety concerns. Though occasionally, the firing and explosive accidents of the batteries are still ceaselessly recorded, most of which were found to happen in overcharged or overheated status of the Li-ion batteries [1,2]. To solve this problem, considerable effort has been focused on development of nonflammable electrolytes [3–12] and overcharge protection additives [13–19]. A number of fire-retardant organic phosphates have been investigated as electrolyte additives to prevent the batteries from firing at abusive conditions [3–12]. Several polymerizable molecules were suggested as safety additives, which form conducting [13,14,19] or isolating polymers [15] in the batteries at overcharge to bypass or interrupt the internal current flow. If an additive can have the functions of both fire-retardancy and overcharge protection, it would provide more reliable protection for Li-ion batteries at wider storage and working circumstances. In this study, we synthesized a new phosphate ether (tri-4methoxythphenyl phosphate, TMPP) and test this compound as a safety additive for Li-ion batteries. It is found that this electrolyte
∗ Tel.: +86 27 87884476; fax: +86 27 87884476. E-mail address: [email protected] (H.X. Yang). 0013-4686/$ – see front matter © 2008 Published by Elsevier Ltd. doi:10.1016/j.electacta.2008.05.024
additive can depress the ignitability of the electrolyte and polymerize at overcharged potential of +4.35 V (vs. Li/Li+ ) to form a conductive polymer bypassing the reaction current. The thermal stability and electrochemical compatibility of the electrolyte in the presence of the additive are also discussed. 2. Experimental 2.1. Synthesis and characterizations TMPP was synthesized by an etherifying reaction (shown in Fig. 1), similarly to the etherization of phosphites reported in Ref. [20]. A typical experimental procedure is to place 0.3 mol of 4methoxythphenol, 0.36 mol of triethylamine (Et3 N) in 200 ml of dehydrated tetrahydrofuran (THF) in a three-necked 500 ml flask, and then add 0.1 mol of POCl3 in the flask. The reaction mixture was stirred at room temperature for 48 h; the resultant triethylammonium chloride was filtered off through a filter tipped cannula. The solid product was filtrated and washed several times with THF and then dried to give the oily product p-methoxythphenyl-phosphate in ∼70% yield, and then the product was washed with 5% HCl solution and 5% NaOH solution successively, finally dried in a vacuum oven using P2 O5 as drier. All the chemicals were of analytical grade and used without further purification except otherwise noted. THF was distilled over Mg power before use. The FT-IR spectra of TMPP were recorded on a NICOLET AVATAR360 FT-IR spectrometer with KBr pellets. Elemental analysis of TMPP was performed on VarioEL III instrument. 1 H NMR spectra
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Fig. 1. Synthetic reaction of TMPP.
of TMPP were recorded on a Mercury VX-300 (300 Hz) apparatus with tetramethylsilane (TMS) as internal standard and CDCl3 as solvent. The pure TMPP compound so synthesized is a nonflammable oily liquid. When ignited, this compound gave only a white smoke, but did not burn to produce flame. To examine the nonflammability of the TMPP electrolyte, we measured the self-extinguishing time (SET) for pre-weighed samples of the electrolyte solutions, in a similar method as described in Ref. [7]. The typical procedure for SET measurements is to use the fiberglass balls (∼10 mm in diameter) to absorb 0.5 g electrolyte and then burn the fiberglass to record the burning time of the electrolyte. Each test was repeated four times and the burning times were averaged for the electrolyte samples containing different amount of TMPP.
Fig. 2 shows the combustibility of the 1 M LiClO4 EC + DEC electrolyte at various content of TMPP additive. It can be seen that with increasing TMPP content from 0 to 15 v%, the burning time of the electrolyte decreases from 38 to 22 s/g, suggesting the electrolyte transforming from a flammable to a fire-retarded solution [7]. Phosphate compounds are known as flame retardants that can function in the vapor phase by a radical mechanism [6]. This may account for the incombustibility of the organic carbonate solvent in the addition of TMMP. However, the ionic conductivity of the electrolyte is decreased lineally with the increase in the TMPP content, due to the low solvation and high viscosity of TMPP. To get a compromise between the ionic conductivity and nonflammability of the electrolyte, we selected 10% TMPP content as a suitable addition to test the electrochemical performances and safety behaviors of the electrolyte.
2.2. Electrochemical measurements 3.2. Electrochemical behaviors of TMPP The electrochemical behaviors of TMPP were examined by cyclic voltammetry (CV) using a Pt microdisk electrode (0.1 mm in diameter) as working electrode and recorded on a two-electrode cell using a larger lithium sheet as both counter electrode and reference electrode. The data acquisition and analysis were carried out by a CHI 660A electrochemical workstation (Shanghai, China). The electrochemical compatibility of the TMPP-containing electrolytes was examined by charge–discharge experiments of laboratory Li-LiFePO4 and graphite-Li cells. The positive LiFePO4 electrode was consisted of 80% LiFePO4 powder, 12% acetylene black and 8% PTFE (wt.%) and the negative graphite electrode is consist 90% graphite, 2% carbon black and 8%PTFE. The base electrolyte was 1 M LiClO4 /EC-DMC (1:1, v/v) purchased from Guotaihuarong Chemical, Co. Ltd. China. The charge–discharge measurements were carried out using a computer-controlled programmable battery charger (BTS-0518001 type, Shenzhen, China). The conductivity of the electrolyte with and without TMPP was measured and calibrated with reference to the conductivity of 0.1 M KCl solution, using a conductivity measuring meter (DDS-307, Shanghai, China).
Fig. 3 compares the CV curves of a Pt electrode in 1 M LiClO4 + EC/DMC electrolyte with or without addition of 10% TMPP. In the presence of TMPP, a large oxidation current arose at positive potential of 4.35 V (vs. Li/Li+ ), obviously due to the electrochemical oxidation of TMPP molecules as shown as an inset in Fig. 3. When the potential scan was reversed and successively cycled, a number of small CV bands appeared at 1.5–3.4 V, possibly contributed from both the redox reactions of the electrolyte components and the oxidation product of the TMPP additive. As an aromatic molecule, the electrochemical oxidation of TMPP molecules proceeded most likely through an oxidative polymerization mechanism at the very high potential of 4.35 V and the resulting polymer product is mostly
3. Results and discussion 3.1. Physical properties and characterization of TMPP TMPP is a nonflammable oily liquid with a chemical formula (C6 H5 OCH3 )3 PO4 . The molecular structures of TMPP were confirmed by FT-IR bands (3003, 2959, 2837, 1595, 1504, 1465, 1442, 1295, 1034, 966, 832, 721, 701, and 638 cm−1 ), 1 H NMR (CDCl3 , 300 M) ı: 6.7–7.2 (12H, s), 3.77 (9H, s), and MS (M+ , 416.2). These data are well agreed with those reported in Ref. [21].
Fig. 2. Flammability and ionic conductivity of 1 M LiClO4 EC + DMC (1:1, v/v%) at different content of TMPP additive.
J.K. Feng et al. / Electrochimica Acta 53 (2008) 8265–8268
Fig. 3. The CVs of 10% TMPP in 1 M LiClO4 EC/DMC (1:1, v/v) measured at a scan rate: 10 mV/s. The suggested polymerization reaction of TMPP is inserted in the figure.
electronically conductive, as it was reported in the case of electrochemical polymerization of dialkoxybenzene compounds [22]. This suggestion was confirmed by the formation of a layer of dark solid deposit on the overcharged surfaces of LiCoO2 or LiFePO4 cathode in the electrolyte containing TMPP additive and by the electrochemical activity of the polymerized product of TMPP. Since the oxidation potential (4.35 V) of TMPP is higher than the complete charging potential of 4 V Li-ion batteries and quite lower than the decomposition potentials of organic electrolytes currently used, we can also use TMPP molecules as a safety additive for overcharge protection of Li-ion batteries. Once the Li-ion batteries are overcharged to rise up the charging voltage to 4.35 V, the TMPP molecules begin to polymerize at the cathode electrode, which forms an internal electronic wiring in the batteries and protect the charging voltage from runaway [13,22]. Fig. 4 shows the effect of TMPP additive on the overcharge performance of Graphite/LiFePO4 cells. In the absence of TMPP additive, the charging voltage of the cells rises up sharply to above 5 V, at which the oxidative decomposition of electrolyte takes place to sustain the charging current. Since the electrolyte decomposition is exothermic and can produce a large number of flammable small organic molecules, the prolonged overcharge is no doubt a direct cause for hazardous behaviors of the cells. However, when the cells contain 10% TMPP in the electrolyte, the cell voltage goes up firstly and then stabilizes at 4.3 V even at 500% overcharge, due to the electrochemical oxidation of TMPP molecules. Because the electrochemical polymerization of TMPP takes place below the oxidation potential of the electrolyte,
Fig. 4. Charge–discharge curves of the graphite/LiFePO4 cells: (a) with and (b) without addition of 10 v% TMPP.
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the decomposition of electrolyte is thus avoided, protecting the cell from voltage runaway. The stabilized voltage plateau at 4.3 V is obviously due to the constant oxidization of TMPP. As mentioned above, this oxidation reaction can proceed through an oxidative polymerization of TMPP molecules to produce a conductive polymer deposit on the cathode surface. Once the polymer grows increasingly to reach the anode, it must form an electronic bridge between the two electrodes, which partially bypasses the charging current and thereby keep the charging voltage at the acceptable region. A general problem for organic phosphates as electrolyte additives is their electrochemical incompatibility with graphite anode due to the electrochemical decomposition or co-intercalation of phosphates on the graphite surface, which produces an incomplete SEI film and leads to a very low columbic efficiency of the graphite anode at cycling [3]. However, we found that TMPP is quite compatible with graphite anode, showing no significant influences on the charge and discharge performance of graphite. Fig. 5 compares the CV curves and charge–discharge profiles of graphite in 1 M LiClO4 + EC/DMC electrolyte with or without addition of 10% TMPP. It can be seen from the CV curves in Fig. 5 that the graphite anode exhibited well-defined and very reversible anodic and cathodic bands at the potential region of 0.5–0 V, characterizing the normal insertion and de-insertion reaction of Li+ ions into/from the graphite. Correspondingly, the graphite anode showed a well-defined charging plateau at ∼0.15 V and a discharging voltage at ∼0.25 V, resembling very well with those obtained from pure carbonate electrolytes. Though the initial coulombic efficiency of the graphite was only ∼68% in the presence of TMPP, it is comparable to that (66%) observed in the absence of TMPP in our parallel tests, suggesting that there was no negative impact of TMPP on the coulombic efficiency of the graphite. This low initial efficiency in our laboratory cells is possibly due to the fact that the graphite was poorly selected and the test cells were poorly assembled with flooded electrolyte. Nevertheless, the charge–discharge efficiency of the graphite anode rose up quickly to >95% since the second cycle and the reversible capacity of the graphite remained very stable at 300 mAh/g in the subsequent cycling. These data demonstrates that TMPP as an electrolyte additive does not cause remarkable detrimental effects on the electrochemical performances of graphite-based Li-ion batteries. This better electrochemical compatibility of TMPP additive than other organic phosphates is possibly due to its large molecular volume and more stably conjugated -electronic structure, which limits the co-intercalation of the additive molecules at graphite.
Fig. 5. The CVs (a) and charge–discharge curves (b) of graphite in 1 M LiClO4 EC/DMC (1:1, v/v) with or without 10% TMPP. The columbic efficiency of the graphite at first ten cycles is shown in the inset (c).
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4. Conclusions Tri-(4-methoxythphenyl) phosphate was synthesized and tested as a safety electrolyte additive for lithium-ion batteries. Experiments demonstrated that this molecule have a strong ability to lower the flammability of the organic carbonate electrolyte and therefore greatly enhance the thermal stability of the Li-ion batteries when used as additive in the electrolyte. Besides, this molecule can also be polymerized at 4.35 V (vs. Li/Li+ ) to form a conducting polymer, which can be used to protect the batteries from voltage runaway at overcharge. Since this electrolyte additive has not much influence on the electrodes performance, it is possible to use this additive for both overcharge protection and flame retardancy of lithium-ion batteries. Acknowledgement This research was financially supported by the National 973 Program of China (No. 2002CB211800). References [1] D.D. MacNeil, J.R. Dahn, J. Electrochem. Soc. 149 (2002) A912. [2] J.R. Dahn, E.W. Fuller, M. Obrovac, U.V. Sacken, Solid State Ionics 69 (1994) 265.
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