Preparation, Thermal Stability and Electrochemical Properties of LiODFB

Transcription

1 J. Mater. Sci. Technol., 2012, 28(8), Preparation, Thermal Stability and Electrochemical Properties of LiODFB Hongming Zhou 1,2), Furong Liu 1) and Jian Li 1,2) 1) School of Materials Science and Engineering, Central South University, Changsha 483, China 2) Hunan Zhengyuan Institute for Energy Storage Materials and Devices, Changsha 483, China [Manuscript received August 30, 2011, in revised form November 18, 2011] Lithium oxalyldifluoroborate (LiODFB) was synthesized in dimethyl carbonate solvent and purified by the method of solvent-out crystallization. The structure characterization and thermal stability of LiODFB were performed by Fourier transform infrared (FTIR) spectrometry, nuclear magnetic resonance (NMR) spectrometry and thermogravimetric analyzer (TGA). LiODFB was exposed to 50% humid air at 25 C for different time, then dried at C for 12 h, and the electrochemical properties of the cells using 1 mol/l dried LiODFB in ethylene carbonate + dimethyl carbonate + ethyl(methyl)carbonate were investigated. The results showed that, pure crystallization LiODFB was obtained; it had good thermal stability with a thermal decomposition temperature of 248 C; when it was exposed to humid air, it was firstly converted into LiODFB H 2 O; with increasing exposure time, more and stronger impurity peaks in the X-ray diffraction (XRD) patterns of LiODFB were observed, and both the discharge specific capacity and the capacity retention decreased gradually. KEY WORDS: Lithium battery; Thermal stability; Lithium oxalyldifluoroborate (LiODFB); Electrochemical properties; Moisture 1. Introduction Lithium hexafluorophosphate (LiPF 6 ) is the commercially dominant electrolyte salt in the state-of-theart lithium-ion battery [1]. However, LiPF 6 has some drawbacks [2], including HF formation with traces of water [3], making the use of cheaper and environmentally more desirable cathode materials such as lithium manganese oxide spinels impossible [4,5]. In addition, the thermal stability of LiPF 6 with lithiated graphite is worse, which restricts its use in large-scale lithium ion batteries for hybrid electric vehicle (HEV) and electric vehicle (EV) applications [6]. Therefore, the development of a new alternative salt to replace LiPF 6 is attracting more and more attention now. Recently, a novel lithium salt, lithium oxalyldifluoroborate (LiODFB), has drawn intensive attention because of its significant merits [7 9]. A stable Corresponding author. Assoc. Prof., Ph.D; Tel.: ; address: csuzhm@yahoo.cn (J. Li). solid electrolyte interface (SEI) can be formed on the graphite anode to protect it from being eroded by solvent. Similar to anode, the cathode is also covered by a surface film after cycling in electrolytes due to reactions with solution components. Lithium ion batteries with LiODFB have excellent cycling performance at C. These unique characteristics make LiODFB as a promising salt for high-power applications [10,11]. Up to date, the electrochemical performances of LiODFB in ethylene carbonate (EC)/propylene carbonate (PC)/ethylmentyl carbonate (EMC) or EC/dimethyl carbonate (DMC) solvent systems with some kinds of high-power lithium-ion cells were studied, such as LiNi 0.8 Co 0.15 Al 0.05 O 2 /graphite, Li 1.1 [Ni 1/3 Mn 1/3 Co 1/3 ] 0.9 O 2 /graphite and LiFeO 4 / graphite cells [12 17]. Zhang [8] reported a thermal decomposition temperature of about 520 K for LiODFB, but its thermal decomposition process was not investigated carefully. Although LiODFB is not sensitive as good as LiPF 6 to moisture, considering the preparation, storage, transportation and usage, the elec-

2 724 H.M. Zhou et al.: J. Mater. Sci. Technol., 2012, 28(8), trochemical properties of the cells with LiODFB exposed to humid air for different time as electrolyte are needed to be investigated, which has not been reported yet. In this work, LiODFB was synthesized and purified in DMC solvent. And its structure was characterized and thermal stability was analyzed. The purified LiODFB was exposed to 50% humid air at 25 C for different time, then vacuum dried at C for 12 h, and then the dried LiODFB was dissolved in EC/DMC/EMC (EC+DMC+EMC, 1:1:1, mass ratio) mixed solvents and used as electrolyte. The performance of cells using these electrolytes was also studied. 2. Experimental 2.1 Synthesis of LiODFB BF 3 etherate (BF 3 O(CH 2 CH 3 ) 2, analytical grade, Guoyao chemical agent Co. LTD) and lithium oxalate (Li 2 C 2 O 4, analytical grade, Guoyao chemical agent Co. LTD) in a 1:1 molar ratio were added to 250 ml DMC solvent to synthesize LiODFB. The crude product was purified by solvent-out crystallization [18]. 2.2 Structure characterization and thermal stability of LiODFB 13 C and 19 F nuclear magnetic resonance (NMR) spectra of the purified LiODFB were measured on a Bruker Avance 0 nuclear magnetic resonance spectrometer. And the structure characterization of the purified LiODFB was examined through a Nicolet Fourier transform infrared spectrometer (AVATAR3, USA). Thermal stability measurements were carried out with a Perkin-Elmer thermogravimetric analyzer (TGA-7). 2.3 Experiments of LiODFB exposed to humid air The purified LiODFB was exposed to 50% humid air at 25 C in a high-low temprature testchamber (TEM-10, China) for different time, and then its weight was recorded. These exposed samples were vacuum dried for 12 h at C, 0.01 MPa in a vacuum drying chamber, and then dissolved in EC/DMC/EMC (1:1:1, mass ratio) mixed solvents and used as electrolyte. The performance of cells using these electrolytes was studied. X-ray diffraction (XRD, SIEMENS-500X, Germany) was used to analyze the phase composition of these exposed samples. 2.4 Preparation of electrolyte and electrochemical performance measurement EC, DMC and EMC solvents in battery grade were purchased from Ferro Performance Materials Company. The purified and exposed LiODFB was obtained according to the method in sections 2.1 and 2.3, respectively. The electrolyte used in this work, 1.0 mol/l purified and exposed LiODFB in EC/DMC/EMC solvents (mass ratio of EC to DMC to EMC is 1:1:1) were prepared in an Ar-filled glove box. The full cells were assembled using Celgard 20 as separator and an appropriate amount of electrolyte in an Ar-filled glove box. The LiFePO 4 cathodes consisting of 84 wt% LiFePO 4, 8 wt% carbon black and 8 wt% polyvinylidene fluoride (PVDF) binder, were fabricated by coating the slurry on aluinum foil collector. The graphite anodes, consisting of 92 wt% G, 2 wt% carbon black and 6 wt% PVDF binder, were fabricated by coating the mixing slurry on copper foil collector. The cells were charged and discharged between 2.5 and 3.6 V with 0.5 C current rate on a charge/discharge instrument (BTS0105C8, China). 3. Results and Discussion 3.1 Structure characterization of synthesized production The Fourier transform infrared (FTIR) spectrometry was used to analyze the functional groups of the purified samples. Fig. 1 shows the molecular structure of the LiODFB. Fig. 2 shows the infrared spectrum of the purified LiODFB. Fig. 2 indicates that two prominent feature peaks appear at and cm 1, which are probably assigned to C=O oscillating in phase and out of phase [19]. The absorption band observed at cm 1 is assigned to the new formed B O characteristic stretching vibration. Compared with the absorption peak of O C C stretching vibration at cm 1 in the infrared spectrum of LiBOB [19], this absorption peak shifts up to cm 1. The absorption peak of C O O stretching vibration is cm 1[12,19]. There is one medium-strong feature at cm 1 identical to the wavenumber of B O symmetric stretching mode. B O deformation vibration also appears in the fingerprint region at cm 1[20]. The broad peaks at and cm 1 are assigned to relatively uncoupled O B O and F B F stretching vibration [12]. The B F asymmetric stretching vibration is near cm 1. From Fig. 2, in addition to containing oxalate and B F characteristic functional groups, there are O B O and F B F characteristic functional groups in structures of the product, compared with the structure of the LiODFB in Fig. 1, which proves that the obtained sample in this work is LiODFB. This can be also certified by the NMR spectrum of the purified sample. Fig. 3 shows the 13 C NMR and 19 F NMR spectra of LiODFB. There is only one peak in every spectrum due to the fact that the chemical states of the two F atoms in the LiODFB molecule are the same. The chemical shift of 13 C is (referenced to TMS in CD 3 CN) and the chemical

3 H.M. Zhou et al.: J. Mater. Sci. Technol., 2012, 28(8), Weight / % o C o C o C o C Temp. / o C Exo Heat flow / mw Fig. 1 Molecular structure of LiODFB Fig. 4 TG-DTG curves of the synthesized LiODFB lization LiODFB has been obtained in this work. Transmittance / % Wavenumber / cm -1 Fig. 2 Infrared spectrum of the synthesized LiODFB 3.2 Thermal stability of LiODFB The thermogravimetri-differential thermogravimetric (TG-DTG) curves of the purified LiODFB are represented in Fig. 4. It can be observed that the TG curve of the sample shows three rapid weight loss. The first weight loss, located between and 170 C, is the result of volatilization of trace DMC and dehydration reactions of LiODFB. From the DTG curve, a large peak at 127 C and a small peak at 159 C can be observed. The large peak probably corresponds to the volatilization of DMC, and the small peak probably corresponds to the dehydration of crystallization water. The rate of loss is 12.81%. The second major weight loss, observed at C, corresponds to the decomposition reaction of LiODFB: 4LiODFB 2BF 3 +B 2 O 3 +Li 2 CO 3 +2LiF+4CO+3CO [21] 2, and a big peak at 269 C is observed. The rate of the second weight loss is 32.68%. The beginning temperature of thermal decomposition of LiODFB is 248 C, which is higher than that of LiPF 6 (214 C) [8]. The third loss of weight appears at C, corresponding to the reaction of Li 2 CO 3 +3B 2 O 3 2LiB 3 O 5 +CO [21] 2, and a small peak at 481 C is observed. The rate is 7.37%. 3.3 Experiments of LiODFB exposed to humid air Fig. 3 NMR spectrum of the synthesized LiODFBL: (a) spectrum of 19 F, (b) spectrum of 13 C shift of 19 F is (referenced to CCl 3 F in CD 3 CN), which was reported earlier in literature [12,18]. It can be concluded from the infrared spectra and the NMR spectra that fairly pure crystal- Fig. 5 shows the plots of the weight of LiODFB exposed to 50% humid air at 25 C for different time. Fig. 5 indicates that, when LiODFB is exposed to 50% humid air at 25 C, its weight increases quickly at the initial stage (0 5 h), and then increases slowly, because moisture absorption reaction occurs first quickly [17], then hydrolysis reactions take place slowly. Fig. 6 shows the XRD patterns of the synthesized LiODFB exposed to 50% humid air at 25 C for different time. It can be seen from Fig. 6, the XRD patterns of the purified LiODFB exposed at 25 C, 50% humid air for 0 and 2 h are basically the same, which

4 726 H.M. Zhou et al.: J. Mater. Sci. Technol., 2012, 28(8), Sample weight / g Exposed time / h Fig. 5 Plots of the weight of LiODFB exposed to 50% humid air at 25 C for different time Discharge capacity / ma h g (a) 0 h 2 h 3 h 4 h 48 h Cycle number Intensity / a.u. Hydrolysis product (e) (d) (c) (b) Capacity retention / % 20 0 (b) 0 h 2 h 3 h 4 h 48 h Cycle number / deg. Fig. 6 XRD patterns of the LiODFB exposed to 50% humid air at 25 C for different time: (a) unexposed, (b) 2 h, (c) 3 h, (d) 4 h, (e) 48 h may be due to the fact that, LiODFB is a chelate compound with Li + five-coordinated structure. When it is exposed to moist air, it is firstly converted into LiODFB H 2 O with more stable Li + six-coordinated structure, infinite Li H 2 O chains are formed by the combination [17], and part of O also forms hydrogen bonds with H 2 O. Fig. 6 also indicates that, with the exposed time increasing from 2 to 3 h, several impurity peaks at 23.6, 19.4 and 28.6 are observed, which may be attributed to the hydrolysis of LiODFB H 2 O [22]. With the increase of the exposed time, more and stronger impurity peaks are observed. This is because HBO 3, LiBF 4, LiBOB, LiBF(OH) 3, LiBF 2 (OH) 2 and LiBF 3 (OH) etc. are generated by the hydrolysis of LiODFB H 2 O, however, LiOOCCOOH, LiBF 4 H 2 O may be generated by the hydrolysis of LiBOB and LiBF [22] 4. As mentioned above, no HF is generated, which is different from LiPF 6 and benefits to the battery s safety performance [8]. Fig. 7 shows the electrochemical performances of the G/LiFePO 4 cells using five different electrolytes at 0.5 C discharge rate. Fig. 7 indicates that, the (a) Fig. 7 Electrochemical performances of the G/LiFePO 4 cells using five different electrolytes at 0.5 C discharge rate: (a) discharge capacity, (b) capacity retention cell with 0 h exposed LiODFB-based electrolyte has the highest initial discharge capacity (99 ma h g 1 ), then after 12 cycles, the discharge capacity is 101 ma h g 1, and the capacity retention is almost %. With the increase of the exposed time, both the discharge capacity and the capacity retention decrease gradually, and the initial discharge capacity of the cell with 2 h exposed LiODFB-based electrolyte is 85 ma h g 1. After 12 cycles, the discharge capacity is 74 ma h g 1, and the capacity retention is only 87.1%. This is due to the fact that, after being exposed for a short time, crystal water is generated and added to the electrolyte, and the crystal water (it is difficult to be dried by vacuum drier at C for 12 h) dissolved into the organic solvent affects the performance of the cell by reacting with the organic solvent and changing into alcohols, and then causes the capacity loss. After being exposed for 3 h, impurity in LiODFB-based electrolyte increases gradually. The impurity cannot dissolve into organic solvent, and reduce the content of the lithium ion, meanwhile the solid impurity is absorbed into the SEI film, which obstructs the insertion and extraction of lithium, and then the resistance is increased. According to the increase of the solid impurity, the cell turns weak quickly. When the exposed time is increased to 48 h, the electrochemical performance of the cell turns se-

5 H.M. Zhou et al.: J. Mater. Sci. Technol., 2012, 28(8), riously bad because most of LiODFB are converted into HBO 3, LiBO 2 and LiOOCCOOH etc. [22], and no lithium ions are available. 4. Conclusion LiODFB has been synthesized in DMC, and purified by the method of solvent-out crystallization. Confirmed by the infrared spectrum and the NMR spectrum, the pure crystallization products have been obtained. The thermal decomposition temperature of LiODFB is 248 C, which is higher than that of LiPF 6 (214 C). This indicates that LiODFB has good thermal stability. The TG curve of the sample shows three rapid weight loss. The first weight loss, located between and 170 C, is the result of volatilization of a small of amount of DMC and dehydration reactions of LiODFB; the second major weight loss, observed at C, corresponds to the decomposition reaction of LiODFB; the third loss of weight observed at C, corresponds to the reaction between Li 2 CO 3 and B 2 O 3. When LiODFB is exposed to moist air, it is firstly transformed into LiODFB H 2 O; with the increase of the exposed time, more and stronger impurity peaks in the XRD patterns of LiODFB are observed, and both the discharge specific capacity and the capacity retention decrease gradually. Acknowledgements This work was supported by the Science and Technology Project of Changsha, China (No. k ). REFERENCES [1 ] M. Diaw, A. Cangnes, B. Carre, P. Willmann and D. Lemordant: J. Power Sources, 2005, 146, 682. [2 ] L. Larush-Asraf, M. Biton, H. Teller and E. Zinigrad: J. Power Sources, 2007, 174, 0. [3 ] S.E. Sloop, J.K. Pugh, S. Wang, J.B. Kerr and K. Kinoshita: Electrochem. Solid-State Lett., 2001, 4, 42. [4 ] M.M. Thackeray: Prog. Solid State Chem., 1997, 25, 1. [5 ] M.S. Whittingham: Chem. Rev., 2004, 104, [6 ] S.S. Zhang: J. Power Sources, 2008, 1, 586. [7 ] S.S. Zhang: J. Power Sources, 2007, 163, 713. [8 ] S.S. Zhang: J. Electrochem. Commun., 2006, 8, [9 ] Z.H. Chen, J. Liu and K. Amine: J. Elctrochem. Soc., 2007, 10, 45. [10] V. Aravindan and P. Vickraman: Solid State Sci., 2007, 9, [11] V. Aravindan, P. Vickraman and K. Krishnaraj: Polym. Int., 2008, 57, 932. [12] H. Gao, Z. Zhang, Y. Lai, J. Li and Y. Liu: J. Cent. South Univ. Technol., 2008, 15, 830. [13] Z. Chen, Y. Qin, J. Liu and K. Amine: Electrochem. Solid-State Lett., 2009, 12, 69. [14] J. Li, K. Xie, Y. Lai, Z. Zhang, F. Li, X. Hao, X. Chen and Y. Liu: J. Power Sources, 2010, 195, [15] M. Fu, K. Huang, S. Liu, J. Liu and Y. Li: J. Power Sources, 2010, 195, 862. [16] Z. Zhang, X. Chen, F. Li, Y. Lai, J. Li, P. Liu and X. Wang: J. Power Sources, 2010, 195, [17] S. Zugmann, D. Moosbauer, M. Amereller, C. Schreiner and F. Wudy: J. Power Sources, 2011, 196, [18] T. Herzig, C. Schreiner, D. Gerhard, P. Wasserscheid and J.G. Heiner: J. Fluor. Chem., 2007, 128, 612. [19] G.V. Zhuang, K. Xu, T.R. Jow and P.N. Ross: Electrochem. Solid-State Lett., 2004, 7, 224. [20] B.T. Yu, W.H. Qiu, F.S. Li and G.X. Xu: Electrochem. Solid-State Lett., 2006, 9, 1. [21] A. Vanchiappan, V. Palanisamy and K. Kaliappa: Polym. Int., 2008, 57, 932. [22] M. Amereller, M. Multerer, C. Schreiner, J. Lodermeyer and A. Schmid: J. Chem. Eng. Data, 2009, 54, 468.