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Nitrile functionalized disiloxanes with dissolved LiTFSI as lithium ion electrolytes with high thermal and electrochemical stability
Journal of Power Sources 274 (2015) 629e635
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Nitrile functionalized disiloxanes with dissolved LiTFSI as lithium ion electrolytes with high thermal and electrochemical stability Benjamin Pohl, Martin M. Hiller, Sarah M. Seidel, Mariano Grünebaum, € fer* Hans-Dieter Wiemho €lische Wilhelms-Universita €t Münster, Corrensstr. 28/30, 48149 Münster, Germany Institute of Inorganic and Analytical Chemistry, Westfa
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
Dinitrile functionalized disiloxanes/ LiTFSI electrolytes show the inhibition of aluminum pitting corrosion. These electrolytes are characterized by non-inflammability and enhanced thermal stability. The ionic conductivity of 1 mS cm1 at 30 C is the highest up to now among siloxane based electrolytes. High electrochemical stability up to 5.6 V versus Li/Liþ.
a r t i c l e i n f o
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Article history: Received 9 July 2014 Received in revised form 19 September 2014 Accepted 6 October 2014 Available online 20 October 2014
Liquid disiloxanes functionalized with terminal nitrile groups are introduced as alternative non-volatile solvents for lithium-ion battery electrolytes in combination with LiTFSI as lithium salt. Two series of disiloxanes were investigated differing with respect to the attachment of the nitrile containing side group to silicon, i.e. via a SieC or a SieO bond. Total conductivities up to 1 mS cm1 at 30 C were measured by impedance spectroscopy. Electrochemical characterization was done on half cells using LiFePO4 cathodes by cyclic voltammetry and constant current cycling. Attractive issues and advantages of the investigated LiTFSI containing disiloxanes in comparison to current electrolyte solvents are: a) In spite of the presence of LiTFSI, the aluminum pitting corrosion is suppressed, b) the electrochemical stability window is extended on the cathode side up to 5.6 V vs. Li/Liþ, for a LiTFSI concentration of 0.7 mol kg1, c) the reported nitrile functionalized disiloxanes show excellent thermal stability with a boiling point up to 106 C (0.1 mbar), a rather low glass transition temperature of 107 C, while no melting/crystallization was observed. © 2014 Elsevier B.V. All rights reserved.
Keywords: Disiloxanes Dinitrile Aluminum corrosion inhibition Electrolyte stability Lithium ion battery
1. Introduction Nowadays, there is an increasing demand for high safety electrolytes due to the growing interest in lithium-ion batteries for mobile applications as well as for high voltage cells with higher energy density. Common liquid electrolytes based on organic
* Corresponding author. Tel.: þ49 251 83 33115; fax: þ49 251 83 36705. €fer). E-mail address: [email protected] (H.-D. Wiemho http://dx.doi.org/10.1016/j.jpowsour.2014.10.080 0378-7753/© 2014 Elsevier B.V. All rights reserved.
carbonate solvents show high ionic conductivity and electrochemical stability up to 4.2 V, but their drawbacks are a considerable vapor pressure at elevated temperature and a high flammability [1]. Alternative aprotic solvents should exhibit high polarity and an extended electrochemical stability window. Acetonitrile as an electrolyte solvent shows high ionic conductivity [2] and a very good electrochemical stability up to higher voltages [3], but it cannot substitute conventional electrolyte solvents due to its low boiling point and considerable toxicity. Disiloxanes, on the other
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hand, are liquids with low viscosity and high chemical and thermal stability. They can easily be modified chemically by introducing a broad variety of different side groups The applicability of modified disiloxanes as solvents for liquid electrolytes has been successfully shown already in the past [4e10]. Furthermore, disiloxanes exhibit rather low flammability and, as crystallization is most often suppressed, low glass transition temperatures (and hence a wide liquid range) because of the low rotation energies of the SieOeSi backbone [11]. Disiloxanes were extensively investigated as electrolyte component by Yoon et al. [12]. Lateron, Zhang et al. reported ionic conductivities up to s ¼ 5.5$104 S cm1 for disiloxane derivatives functionalized with ethylene oxide side chains at both silicon atoms [4]. Such the oxygen donor atoms of ether side chains are known for their ability to strongly coordinate lithium cations [13] hence supporting the dissociation of lithium-salts. On the other hand, the enhanced coordination lowers the lithium cation mobility which leads to lower lithium ion transference numbers and therefore a low steady state lithium ion transport in corresponding electrolytes. Furthermore, the electrochemical stability of oligoether functionalized siloxanes is limited by electrochemical oxidation of the ether groups starting at about 4.2e4.5 V vs. Li/Liþ [8,14]. Therefore, the functionalization of disiloxanes by polar groups with enhanced oxidation stability may lead to improvements. In this context, terminal nitrile groups, i.e. eCN, are an interesting alternative as they combine a high polarity, very good chemical stability and a moderate interaction with lithium ions [15]. The latter favors a higher electrical mobility of lithium ions which is also favorable for a higher lithium ion transference number. But enhancing the thermal stability of a lithium electrolyte does not only concern the solvent, but also the lithium salt. Replacement of the conventional LiPF6 by Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) would achieve a considerable advantage in terms of thermal stability of the salt. But LiTFSI is still not used in rechargeable lithium ion batteries with conventional substituted carbonate solvents due to its corrosiveness for aluminum current collectors at potentials of 3.7 V vs. Li/Liþ and higher [16,17] which drastically limits the cycle life of the 4 V lithium ion batteries [18]. A solution to suppress the corrosive effect of LiTFSI was demonstrated by Di Censo et al. who used an organic nitrile functionalized solvent and showed that this in combination with LiTFSI and 4 V cathode materials yields high cycle life with much reduced aluminum pitting corrosion [19]. €mer et al. recently pointed out that nitriles prevent aluminum Kra corrosion provided one takes care to use nitrile solvents with high purity [20]. Organic dinitriles like adiponitrile and glutaronitrile were investigated by Abu-Lebdeh et al. [23,24] and showed promising results, too, but exhibit a high melting point and are therefore unpractical for battery electrolytes at ambient temperatures. Polymer electrolytes based on highly cross-linked polysiloxanes functionalized nitrile side chains have already been studied, but suffer from low conductivities [21,22]. The concept of this work is to investigate nitrile functionalized disiloxanes as electrolyte solvents in combination with LiTFSI as alternative lithium ion electrolyte with enhanced safety. Nitrile functionalized disiloxanes were expected to show a good electrochemical stability, good ionic conductivity and suppressed aluminum corrosion. Further advantages of disiloxanes are properties like excellent thermochemical stability, environmental friendliness, low-price, and good synthetic accessibility, in many cases starting from commercially well available precursors. In this work, the focus was laid on two different nitrilefunctionalized disiloxane solvents, i.e. 1,3-bis(cyanopropyl)tetramethyldisiloxane (TmdSx-CN) [25] and 1,3-bis(cyanoethoxyl)tetramethyldisiloxane (TmdSx-OPN) [26] (Scheme 1).
Scheme 1. Novel nitrile-disiloxanes solvent, with the nitrile group bonded to the siloxane with a carbon- (TmdSx-CN) or an oxygen-atom (TmdSx-OPN).
2. Experimental 2.1. Materials and NMR characterization 1,3-Bis(cyanopropyl)tetramethyldisiloxane (95%) was purchased from ABCR and distilled at least two times (106 C, 1$103 mbar). The clear solution was subsequently dried over molecular sieve (3 Å) prior to use. 1,3dichlorotetramethyldisiloxane (97%) and was purchased from ABCR, 3-Hydroxypropiononitrile and triethylamine from Acros. Pentane, THF and toluene (VWR) were dried over sodium/benzophenone. All reagents were purified by distillation prior to use. NaH (Sigmal Aldrich) 60% dispersion in mineral oil was rinsed three times with pentane before using. Vinylene carbonate (99%, Sigma Aldrich) was used as received. Lithium trifluoromethanesulfonimide (LiTFSI, 99.95%, Aldrich) was dried before using. The purity was checked by NMR, All NMR measurements were carried out on a Bruker AV(III)400 spectrometer using deuterated chloroform (CDCl3, 99.8%, euriso-top) as reference solvent. 2.2. Synthesis 1,3-Bis(oxypropanenitrile)tetramethyldisiloxane (TmdSx-OPN) was synthesized according to the following procedure (Scheme 2). 3-Hydroxypropionitrile (7.5 g, 106.0 mmol, 2.2 eq.), triethylamine (25.6 g, 252.5 mmol, 5.4 eq.) and THF (100 mL) were added to a dry 500 mL Schlenk tube, Then, 1,3-Dichloro-1,1,3,3tetramethyldisiloxane (9.6 g, 47.0 mmol, 1.0 eq.) was added drop wise under ice cooling and vigorous stirring. The reaction solution was allowed to warm to room temperature and was then stirred overnight. The white precipitate was filtered and THF was removed by rotary evaporation. The crude product was dissolved in chloroform and washed with water, until the aqueous solution was neutral. The solution was dried over MgSO4 and the solvent was then removed under reduced pressure. The product was dried at 103 mbar for three days and subsequently stored over molecular sieve (4 Å). 1 H NMR (400 MHz, CDCl3) d/ppm ¼ 3.88 (t, J ¼ 6.2 Hz, 4H), 2.56 (t, J ¼ 6.2 Hz, 4H), 0.15 (s, 12H). 13 C NMR (101 MHz, CDCl3) d/ppm ¼ 118.04, 57.47, 21.48, 1.31. 29 Si Dept24.1 NMR (79 MHz, CDCl3) d/ppm ¼ 9.95. 2.3. Preparation of the electrolytes and electrodes LiTFSI was dried at 150 C and high vacuum (5$107 mbar) for at least one week. In a glovebox, the lithium salt was dissolved directly into the disiloxane without additional solvents. Sodium carboxymethyl-cellulose (Na-CMC, WALOCEL™ CRT 2000 PPA 12, Dow Wolff Cellulosics), lithium iron phosphate (LiFePO4, P2, Süd-Chemie) and carbon black (Super P®, Timcal) were used as received. The preparation of electrodes was performed using the doctor blade method. First a slurry was prepared by dissolving 0.15 g NaeCMC as a binder in 3 mL water (Millipore, 2 h stirring). Then, 300 mg carbon black was added to the as-obtained binder solution followed by 15 h stirring. Afterward, 2.6 g LFP P2 and 3 mL water were added and stirred, until the suspension was
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Scheme 2. Synthesis of TmdSx-OPN using 1,3-dichlorotetramethyldisiloxane and 3-hydroxypropionitrile as starting material.
homogeneous. Finally the slurry was dispersed with a high energy stirrer (T 18 ULTRA-TURRAX®, IKA) for 1 h at 5000 rpm. Aluminum foil was etched in 5 wt% KOH (60 C, 60 s, d ¼ 20 mm), cleaned with purified water and then directly coated with the slurry using a doctor blade (wet film ¼ 120 mm, dry film ¼ 22e24 mm). The sheets were dried in an oven at 80 C for 15 h, stamped into discs of 12 mm diameter and dried for 48 h (120 C at 103 mbar). The electrode composition was 85 wt% LFP P2, 10 wt% Super P and 5 wt% NaeCMC and had a load of ~2.85 mg cm2 ≙ ~2.42 mg cm2 LFP. 2.4. Cell assembly and measurement The cycling experiments were carried out in 3-electrode-Swagelok®-cells (CE, RE ¼ Li, WE ¼ LiFePO4 abbreviated as LFP in the following) which were assembled under Argon atmosphere. A series 4000 battery tester (Maccor) and a potentiostat (Autolab PGSTAT302N, Metrohm) were used. Lithium foil (99.99%, Rockwood Lithium) was stamped into discs (d ¼ 0.5 mm, ø ¼ 12 mm) and used as counter- and reference electrode (CE, RE). The cells were built with four separator fleeces FS 2190 (d z 1.200 mm, ø ¼ 13 mm, Freudenberg) containing 80 mL liquid electrolyte. The various electrolyte compositions are specified in the following with the reported results. The charge capacity and charge/discharge rates during constant current cycling (CC) was related to the LFPcontent of the electrode taking a specific capacity of 170 mAh g1 into account. The cells were cycled between 2.8 and 4.2 V vs. Li/Liþ. The potential of the cathode was monitored using the lithium reference electrode. The first 5 cycles were recorded at 0.1C to obtain a stable cell response; all further charge- and dischargerates were done at 1C.
highest conductivities were observed for a solution with a LiTFSI concentration of 0.7 mol kg1 (CN:Li ¼ 11:1) over the whole temperature range from 20 C up to 90 C. TmdSx-OPN at the same concentration of 0.7 mol kg1 LiTFSI achieves an ionic conductivity of 1.0 mS cm1 at 30 C and 3.3 mS cm1 at 90 C. Here, the concentration differences of the ionic conductivity become less at higher temperatures (Fig. 1b). In general, solutions of LiTFSI in TmdSx-OPN show a higher ionic conductivity than in TmdSx-CN. This is correlated with the corresponding inverse changes of the viscosity (see Fig. 3). It may be attributed to the different connectivity of the nitrile group to the Si atoms in the disiloxanes modifying the intramolecular side group mobility. Zhang et al. reported similar observations for disiloxanes functionalized by oligoether side groups [4]. It should be noted that the achieved ionic conductivities for the nitrile functionalized disiloxanes are clearly higher than for most of the previously published values on siloxane based solvents, e.g. disiloxanes with ethylene oxide side chains (Fig. 2) [4,5]. The most reasonable explanation is the weaker coordination of the lithium cation to the nitrile group as compared to the O-donor of oligoether side chains, as discussed by Hou et al. [15].
3. Results and discussion 3.1. Ionic conductivity Conductivity cell preparations were done by filling the electrolyte into the volume enclosed by an O-ring (made of PEEK ¼ polyether ether ketone) which was sandwiched between two stainless steel electrodes with a surface area of 1 cm2. Ionic conductivities were determined from frequency dependent AC impedance (frequency response analyzer: Agilent E 4980 A) in the frequency range from 20 Hz to 2 MHz and with an amplitude of 40 mV. The impedance was recorded in the temperature range from 20 C to 90 C controlled by a thermostat (Julabo FP 45) under nitrogen atmosphere. Concentrations are given as molalities or expressed by the number of nitrile groups per lithium cation. LiTFSI shows high solubility in both investigated dinitrile functionalized disiloxanes. The concentration dependence was measured in the range from molalities of 0.2 up to 1.3 mol kg1 LiTFSI (not all results are shown here). Maximum conductivities were found in the middle of this range. At higher salt concentrations, the ionic conductivities decreased, in particular at low temperatures, in parallel to the steep increase of the viscosity. For TmdSx-CN, ionic conductivities around 4$105 S cm1 were obtained at 20 C for LiTFSI concentrations 1 mol kg1 increasing to a maximum of 2.3 mS cm1 at 90 C (Fig. 1a). The
Fig. 1. Arrhenius plot of the temperature dependent ionic conductivities of LiTFSI containing electrolytes with the solvents a) TmdSx-CN and b) TmdSx-OPN.
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B. Pohl et al. / Journal of Power Sources 274 (2015) 629e635 Table 1 Ionic conductivities at room temperature of nitrile-disiloxanes compared to oligoether-disiloxanes, FG ¼ functional group, CN ¼ nitrile group and EO ¼ ethylene oxide unit. (*) Data of the two disiloxanes TmdSx-PEG and TmdSx-OPEG taken from Ref. [4] (see meaning of PEG and OPEG in the text). (**) Pure solvent. Electrolyte Disiloxane TmdSx-CN TmdSx-OPN Disiloxane* TmdSx-PEG TmdSx-OPEG
FG:Li CN:Li 10:1 11:1 EO:Li 20:1 20:1
mol kg1
h/mPa s**
s/(104 S cm1)
9.2 7.0
5.9 9.0
6.0 4.1
3.4 5.5
T ¼ 25 C 0.7 0.7 T ¼ 25 C
3.3. Thermal stability
Fig. 2. Comparison of Arrhenius plots of the ionic conductivities of an oligoether functionalized disiloxane TmdSx-PEG (synthesized in this work according to 2a from Ref. [4] with PEG ¼ CH2CH2CH2O(CH2CH2O)3CH3) and both dinitrile disiloxanes from this work.
Table 1 gives an overview of corresponding ionic conductivities and viscosities of the nitrile disiloxanes and in addition of two disiloxanes from the literature denoted as TmdSx-PEG and TmdSxOPEG where PEG stands for two side chains of the type CH2CH2CH2O(CH2CH2O)3CH3 and OPEG for two side chains of the type O(CH2CH2O)3CH3 (replacing the two nitrile containing side chains of TmdSx-CN).
3.2. Viscosity The temperature dependent viscosities were measured using an Anton Paar Physica MCR 301 rheometer inside a dry room. All measurements were done from 20 C up to 90 C in 10 C steps. Fig. 3 illustrates the measured viscosities of both dinitrile functionalized disiloxanes. It is evident that the viscosity of the samples increases gradually with decreasing temperature. TmdSx-OPN has a lower viscosity than TmdSx-CN especially at very low temperatures which is also accompanied by higher ionic conductivities. A possible explanation could be a higher flexibility of the SieO binding of the side chain. Nevertheless, due to the very low glass transition temperatures, reasonable conductivities and viscosities were observed for both disiloxanes even down to temperatures of 20 C.
Thermal analysis based on differential scanning calorimetry (DSC) was carried out on a Netzsch DSC 204 calorimeter in the temperature range 150 C to 250 C (heating rate 10 K min1, nitrogen atmosphere). All samples were prepared under dry conditions in an argon filled glovebox using aluminum crucibles. Both investigated dinitrile functionalized disiloxanes show no crystallisation/melting transition in the DSC analysis, but very low glass transition temperatures down to 108 C which linearly increase with the LiTFSI concentration as listed in Table 2. At the highest salt concentrations, the glass transition temperatures are still as low as 85 C. No noticeable difference was found between TmdSx-CN and TmdSx-OPN. 3.4. Electrochemical stability The electrochemical stability range was measured by cyclic voltammetry using a potentiostat (Autolab PGSTAT302N, Metrohm). The cathodic and anodic electrode potential ranges were studied separately with different working electrode materials (WE). Stability with respect to cathodic reduction was measured in the range of electrode potentials from þ2.5 V down to 0.5 V (vs. Li/ Liþ) with Cu as working electrode (area ¼ 0.785 mm2). Stability versus anodic oxidation was studied using a Pt working electrode (same area as Cu WE) in the electrode potential range þ2.5 V up to 7 V. A lithium metal electrode was taken as reference electrode (RE), the counter electrode (CE) was made from a coaxial copper electrode (area of CE ¼ 8.635 mm2). All measurements were done at room temperature under argon atmosphere with a scan rate of 1 mV s1. The first cycles of the cyclic voltammograms in the anodic and cathodic domains for both dinitrile functionalized disiloxanes are presented in Fig. 4. A strongly increasing anodic current density is observed marking an irreversible oxidation of the electrolyte starting in case of TmdSx-OPN at about 5.6 V vs. Li/Liþ and at 5.2 V for TmdSx-CN (cf. Fig. 4a and b). Note for comparison that for oligoether functionalized disiloxanes Zhang et al. reported electrochemical stabilities up to 4.5e4.7 V vs. Li/Liþ [4]. Hence, the stability against anodic oxidation is considerably increased with
Table 2 Glass transition temperatures Tg of dinitrile-disiloxanes in dependency of the LiTFSI salt concentration. TmdSx-OPN
Fig. 3. Temperature dependent viscosities of both dinitrile disiloxanes (without a lithium salt).
TmdSx-CN
mol kg1
CN:Li
Tg
mol kg1
CN:Li
Tg
0 0.35 0.7 1.0 1.3
1:0 21:1 10:1 7:1 6:1
108 102 96 89 83
0 0.35 0.7 1.0 1.3
1:0 22:1 10:1 7:1 6:1
107 102 95 90 85
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dinitrile functionalized disiloxane solvents which is clearly due to the different choice of the side chain. Both dinitrile based electrolytes show the usual plating/stripping peaks of metallic lithium in the low electrode potential range around 0 V on a copper working electrode (cf Fig. 4a and b). However, it should be noted that the presence of vinyl carbonate (VC) in the electrolyte was necessary to achieve a stable SEI and a good coulombic efficiency. Without VC additive, the lithium stripping was suppressed. This is due to the reactivity between the bare lithium metal surfaces with the nitrile groups accompanied by polymerization products [27e29]. Without the presence of the VC additive, the resulting SEI is not stable enough to protect the electrolyte from continuing decomposition at the lithium anode. 3.5. Aluminum corrosion test
Fig. 4. Cyclic voltammetry measurements of a) TmdSx-CN and b) TmdSx-OPN based electrolytes with a scan rate of 1 mV s1 at 22 C.
With conventional carbonate based solvents, lithium electrolytes with LiTFSI salt cause a pitting corrosion at the aluminum current collector on the cathode side at potentials around 3.7 V vs. Li/Liþ [16,17]. Therefore, the stability of aluminum in contact to the investigated electrolytes was studied by cyclic voltammetry in the high electrode potential range using aluminum as the working electrode. Fig. 5 shows the first six cycles of electrolytes with both dinitrile functionalized disiloxane solvents (Fig. 5a and b). Fig. 5c shows a corresponding experiment with the same salt using a reference electrolyte with standard solvent. It is evident that the current densities are distinctly higher for the EC/DMC-LiTFSI reference electrolyte, and remain high even for the second and the following cycles. On the contrary, the current densities of TmdSxCN and TmdSx-OPN based electrolytes with comparable LiTFSI concentrations are very low. At 5.5 V vs. Li/Liþ, for instance,
Fig. 5. Cyclovoltammetry measurement of a) 1 mol kg1 LiTFSI in TmdSx-CN b) 1 mol kg1 LiTFSI in TmdSx-OPN and for comparison b) 1 M LiTFSI in EC/DMC ¼ 3:7 using aluminum as working electrode.
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currents of only 14 mA cm2 (see Fig. 5a) and 5.6 mA cm2 (see Fig. 5b) were attained in the first cycle, being two to three orders of magnitude lower as compared to the EC/DMC based reference electrolyte and further decaying to negligible values in subsequent cycles. Photos from scanning electron microscopy (SEM) in Fig. 6 illustrate the difference in corrosive activity between the dinitrile based solvents and the EC/DMC reference electrolyte. Fig. 6c) shows the rinsed aluminum electrode after the measurement with the TmdSx-CN based electrolyte. The aluminum surface is still planar, only weak traces from polishing are visible. On the other hand, the rinsed aluminum electrode in Fig. 6d) which was in contact to the EC/DMC based reference electrolyte is completely damaged due to intensive aluminum pitting corrosion. Both dinitrile functionalized disiloxane solvents in combination with LiTFSI seem to cause the formation of a protective layer on aluminum preventing further corrosion. Possible explanations may be an insolubility of Al-TFSI complexes in the disiloxane solvent or the formation of a product layer formed by a reductive condensation of the nitrile group induced by highly reactive Alþ as discussed by Di Censo et al. [19]. The detected surface film on the Al electrode is shown in Fig. 6a and b. The entire surface film on the electrode contains sulfur from TFSI and silicon from the disiloxane as analyzed by EDX mapping. In the lower part of Fig. 6a, the surface film was partly removed so that the aluminum current collector becomes visible there again.
3.6. Constant current cycling A last series of experiments concerned the cycling stability with the developed alternative electrolytes. Fig. 7a shows a charge/ discharge test using the TmdSx-CN/LiTFSI electrolyte in a half cell with a LiFePO4 cathode (LFP) coated on an aluminum current collector. The initial cycles (1e5) show the performance of the electrolyte/electrode couple at a charge/discharge rate of 0.1C and a charge capacity of 140 mAh g1 which is 82% of the theoretical
Fig. 7. Average values of the charge-, discharge performance at 1C of a) TmdSxCN þ 2 wt% vinylene carbonate (as SEI additive) with 1 mol kg1 LiTFSI and b) 1 M LiTFSI in EC/DMC ¼ 3:7 as reference on LiFePO4 electrodes at 22 C. Each fifth cycle is presented.
capacity of LFP. The following constant current cycling was continued with a rate of 1C. These subsequent cycles exhibit a stable reversible capacity of 110 mA g1 for 230 cycles. Then the
Fig. 6. a) EDX mapping of the unrinsed Al electrode after CV measurement using TmdSx-CN and 1 mol kg1 LiTFSI as electrolyte (colors encode the distribution of the three elements S - red, Al - blue, Si - green); b) corresponding SEM image to a); c) rinsed Al electrode without visible Al pitting corrosion after CV from TmdSx-CN/LiTFSI electrolyte; d) rinsed aluminum electrode with clearly visible Al pitting corrosion after CV measurement using the reference electrolyte 1 M LiTFSI in EC/DMC ¼ 3:7.
B. Pohl et al. / Journal of Power Sources 274 (2015) 629e635
capacity slowly decreases and reaches 90 mAh g1 after 595 cycles. This is still 82% of the initial charge capacity. The coulombic efficiencies during the cycling with TmdSx-CN/LiTFSI averaged over all cycles amounted to about 99.9%. Fig. 7b shows the capacity fading of analogous cells with common 1 M LiTFSI in EC/DMC ¼ 3:7 as electrolyte. Compared to the dinitrile disiloxane electrolyte, the initial capacity is higher, but the end of life is already reached after 177 cycles which again demonstrates the corrosive action of LiTFSI based electrolytes with conventional solvents.
References
4. Conclusion
[9] [10]
One can conclude that dinitrile functionalized disiloxanes with LiTFSI as lithium salt present an attractive approach towards electrolytes with enhanced thermal and electrochemical stability and, in particular, with enhanced stability of the aluminum current collectors against corrosion at high electrode potentials, a wellknown drawback of LiTFSI when used in conventional carbonate based solvents. The investigated disiloxane based solvents with dinitrile functionalization offer potential use for high voltage cells. Conventional SEI stabilizing additives can be used to protect the nitrile solvents on the anode side. The total conductivities are among the best values measured so far in disiloxane based solvents. In particular, the system denoted here as TmdSx-CN is interesting due to its commercial availability.
[11]
Acknowledgments The authors acknowledge the support of the initial synthetic work by the Deutsche Forschungsgemeinschaft DFG within the research initiative “Functional materials and material analysis to high performance lithium ion batteries (PAK 177)” and the support of extended electrolyte configuration and cell tests in a second phase by the Federal Ministry of Education and Research (BMBF) within the project MEET Hi-EnD (High energy density batteries, 03X4634A). The authors also thank Timo Hopp for recording the SEM images.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Nitrile functionalized disiloxanes with dissolved LiTFSI as lithium ion electrolytes with high thermal and electrochemical stability Benjamin Pohl, Martin M. Hiller, Sarah M. Seidel, Mariano Grünebaum, € fer* Hans-Dieter Wiemho €lische Wilhelms-Universita €t Münster, Corrensstr. 28/30, 48149 Münster, Germany Institute of Inorganic and Analytical Chemistry, Westfa
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
Dinitrile functionalized disiloxanes/ LiTFSI electrolytes show the inhibition of aluminum pitting corrosion. These electrolytes are characterized by non-inflammability and enhanced thermal stability. The ionic conductivity of 1 mS cm1 at 30 C is the highest up to now among siloxane based electrolytes. High electrochemical stability up to 5.6 V versus Li/Liþ.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 July 2014 Received in revised form 19 September 2014 Accepted 6 October 2014 Available online 20 October 2014
Liquid disiloxanes functionalized with terminal nitrile groups are introduced as alternative non-volatile solvents for lithium-ion battery electrolytes in combination with LiTFSI as lithium salt. Two series of disiloxanes were investigated differing with respect to the attachment of the nitrile containing side group to silicon, i.e. via a SieC or a SieO bond. Total conductivities up to 1 mS cm1 at 30 C were measured by impedance spectroscopy. Electrochemical characterization was done on half cells using LiFePO4 cathodes by cyclic voltammetry and constant current cycling. Attractive issues and advantages of the investigated LiTFSI containing disiloxanes in comparison to current electrolyte solvents are: a) In spite of the presence of LiTFSI, the aluminum pitting corrosion is suppressed, b) the electrochemical stability window is extended on the cathode side up to 5.6 V vs. Li/Liþ, for a LiTFSI concentration of 0.7 mol kg1, c) the reported nitrile functionalized disiloxanes show excellent thermal stability with a boiling point up to 106 C (0.1 mbar), a rather low glass transition temperature of 107 C, while no melting/crystallization was observed. © 2014 Elsevier B.V. All rights reserved.
Keywords: Disiloxanes Dinitrile Aluminum corrosion inhibition Electrolyte stability Lithium ion battery
1. Introduction Nowadays, there is an increasing demand for high safety electrolytes due to the growing interest in lithium-ion batteries for mobile applications as well as for high voltage cells with higher energy density. Common liquid electrolytes based on organic
* Corresponding author. Tel.: þ49 251 83 33115; fax: þ49 251 83 36705. €fer). E-mail address: [email protected] (H.-D. Wiemho http://dx.doi.org/10.1016/j.jpowsour.2014.10.080 0378-7753/© 2014 Elsevier B.V. All rights reserved.
carbonate solvents show high ionic conductivity and electrochemical stability up to 4.2 V, but their drawbacks are a considerable vapor pressure at elevated temperature and a high flammability [1]. Alternative aprotic solvents should exhibit high polarity and an extended electrochemical stability window. Acetonitrile as an electrolyte solvent shows high ionic conductivity [2] and a very good electrochemical stability up to higher voltages [3], but it cannot substitute conventional electrolyte solvents due to its low boiling point and considerable toxicity. Disiloxanes, on the other
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hand, are liquids with low viscosity and high chemical and thermal stability. They can easily be modified chemically by introducing a broad variety of different side groups The applicability of modified disiloxanes as solvents for liquid electrolytes has been successfully shown already in the past [4e10]. Furthermore, disiloxanes exhibit rather low flammability and, as crystallization is most often suppressed, low glass transition temperatures (and hence a wide liquid range) because of the low rotation energies of the SieOeSi backbone [11]. Disiloxanes were extensively investigated as electrolyte component by Yoon et al. [12]. Lateron, Zhang et al. reported ionic conductivities up to s ¼ 5.5$104 S cm1 for disiloxane derivatives functionalized with ethylene oxide side chains at both silicon atoms [4]. Such the oxygen donor atoms of ether side chains are known for their ability to strongly coordinate lithium cations [13] hence supporting the dissociation of lithium-salts. On the other hand, the enhanced coordination lowers the lithium cation mobility which leads to lower lithium ion transference numbers and therefore a low steady state lithium ion transport in corresponding electrolytes. Furthermore, the electrochemical stability of oligoether functionalized siloxanes is limited by electrochemical oxidation of the ether groups starting at about 4.2e4.5 V vs. Li/Liþ [8,14]. Therefore, the functionalization of disiloxanes by polar groups with enhanced oxidation stability may lead to improvements. In this context, terminal nitrile groups, i.e. eCN, are an interesting alternative as they combine a high polarity, very good chemical stability and a moderate interaction with lithium ions [15]. The latter favors a higher electrical mobility of lithium ions which is also favorable for a higher lithium ion transference number. But enhancing the thermal stability of a lithium electrolyte does not only concern the solvent, but also the lithium salt. Replacement of the conventional LiPF6 by Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) would achieve a considerable advantage in terms of thermal stability of the salt. But LiTFSI is still not used in rechargeable lithium ion batteries with conventional substituted carbonate solvents due to its corrosiveness for aluminum current collectors at potentials of 3.7 V vs. Li/Liþ and higher [16,17] which drastically limits the cycle life of the 4 V lithium ion batteries [18]. A solution to suppress the corrosive effect of LiTFSI was demonstrated by Di Censo et al. who used an organic nitrile functionalized solvent and showed that this in combination with LiTFSI and 4 V cathode materials yields high cycle life with much reduced aluminum pitting corrosion [19]. €mer et al. recently pointed out that nitriles prevent aluminum Kra corrosion provided one takes care to use nitrile solvents with high purity [20]. Organic dinitriles like adiponitrile and glutaronitrile were investigated by Abu-Lebdeh et al. [23,24] and showed promising results, too, but exhibit a high melting point and are therefore unpractical for battery electrolytes at ambient temperatures. Polymer electrolytes based on highly cross-linked polysiloxanes functionalized nitrile side chains have already been studied, but suffer from low conductivities [21,22]. The concept of this work is to investigate nitrile functionalized disiloxanes as electrolyte solvents in combination with LiTFSI as alternative lithium ion electrolyte with enhanced safety. Nitrile functionalized disiloxanes were expected to show a good electrochemical stability, good ionic conductivity and suppressed aluminum corrosion. Further advantages of disiloxanes are properties like excellent thermochemical stability, environmental friendliness, low-price, and good synthetic accessibility, in many cases starting from commercially well available precursors. In this work, the focus was laid on two different nitrilefunctionalized disiloxane solvents, i.e. 1,3-bis(cyanopropyl)tetramethyldisiloxane (TmdSx-CN) [25] and 1,3-bis(cyanoethoxyl)tetramethyldisiloxane (TmdSx-OPN) [26] (Scheme 1).
Scheme 1. Novel nitrile-disiloxanes solvent, with the nitrile group bonded to the siloxane with a carbon- (TmdSx-CN) or an oxygen-atom (TmdSx-OPN).
2. Experimental 2.1. Materials and NMR characterization 1,3-Bis(cyanopropyl)tetramethyldisiloxane (95%) was purchased from ABCR and distilled at least two times (106 C, 1$103 mbar). The clear solution was subsequently dried over molecular sieve (3 Å) prior to use. 1,3dichlorotetramethyldisiloxane (97%) and was purchased from ABCR, 3-Hydroxypropiononitrile and triethylamine from Acros. Pentane, THF and toluene (VWR) were dried over sodium/benzophenone. All reagents were purified by distillation prior to use. NaH (Sigmal Aldrich) 60% dispersion in mineral oil was rinsed three times with pentane before using. Vinylene carbonate (99%, Sigma Aldrich) was used as received. Lithium trifluoromethanesulfonimide (LiTFSI, 99.95%, Aldrich) was dried before using. The purity was checked by NMR, All NMR measurements were carried out on a Bruker AV(III)400 spectrometer using deuterated chloroform (CDCl3, 99.8%, euriso-top) as reference solvent. 2.2. Synthesis 1,3-Bis(oxypropanenitrile)tetramethyldisiloxane (TmdSx-OPN) was synthesized according to the following procedure (Scheme 2). 3-Hydroxypropionitrile (7.5 g, 106.0 mmol, 2.2 eq.), triethylamine (25.6 g, 252.5 mmol, 5.4 eq.) and THF (100 mL) were added to a dry 500 mL Schlenk tube, Then, 1,3-Dichloro-1,1,3,3tetramethyldisiloxane (9.6 g, 47.0 mmol, 1.0 eq.) was added drop wise under ice cooling and vigorous stirring. The reaction solution was allowed to warm to room temperature and was then stirred overnight. The white precipitate was filtered and THF was removed by rotary evaporation. The crude product was dissolved in chloroform and washed with water, until the aqueous solution was neutral. The solution was dried over MgSO4 and the solvent was then removed under reduced pressure. The product was dried at 103 mbar for three days and subsequently stored over molecular sieve (4 Å). 1 H NMR (400 MHz, CDCl3) d/ppm ¼ 3.88 (t, J ¼ 6.2 Hz, 4H), 2.56 (t, J ¼ 6.2 Hz, 4H), 0.15 (s, 12H). 13 C NMR (101 MHz, CDCl3) d/ppm ¼ 118.04, 57.47, 21.48, 1.31. 29 Si Dept24.1 NMR (79 MHz, CDCl3) d/ppm ¼ 9.95. 2.3. Preparation of the electrolytes and electrodes LiTFSI was dried at 150 C and high vacuum (5$107 mbar) for at least one week. In a glovebox, the lithium salt was dissolved directly into the disiloxane without additional solvents. Sodium carboxymethyl-cellulose (Na-CMC, WALOCEL™ CRT 2000 PPA 12, Dow Wolff Cellulosics), lithium iron phosphate (LiFePO4, P2, Süd-Chemie) and carbon black (Super P®, Timcal) were used as received. The preparation of electrodes was performed using the doctor blade method. First a slurry was prepared by dissolving 0.15 g NaeCMC as a binder in 3 mL water (Millipore, 2 h stirring). Then, 300 mg carbon black was added to the as-obtained binder solution followed by 15 h stirring. Afterward, 2.6 g LFP P2 and 3 mL water were added and stirred, until the suspension was
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Scheme 2. Synthesis of TmdSx-OPN using 1,3-dichlorotetramethyldisiloxane and 3-hydroxypropionitrile as starting material.
homogeneous. Finally the slurry was dispersed with a high energy stirrer (T 18 ULTRA-TURRAX®, IKA) for 1 h at 5000 rpm. Aluminum foil was etched in 5 wt% KOH (60 C, 60 s, d ¼ 20 mm), cleaned with purified water and then directly coated with the slurry using a doctor blade (wet film ¼ 120 mm, dry film ¼ 22e24 mm). The sheets were dried in an oven at 80 C for 15 h, stamped into discs of 12 mm diameter and dried for 48 h (120 C at 103 mbar). The electrode composition was 85 wt% LFP P2, 10 wt% Super P and 5 wt% NaeCMC and had a load of ~2.85 mg cm2 ≙ ~2.42 mg cm2 LFP. 2.4. Cell assembly and measurement The cycling experiments were carried out in 3-electrode-Swagelok®-cells (CE, RE ¼ Li, WE ¼ LiFePO4 abbreviated as LFP in the following) which were assembled under Argon atmosphere. A series 4000 battery tester (Maccor) and a potentiostat (Autolab PGSTAT302N, Metrohm) were used. Lithium foil (99.99%, Rockwood Lithium) was stamped into discs (d ¼ 0.5 mm, ø ¼ 12 mm) and used as counter- and reference electrode (CE, RE). The cells were built with four separator fleeces FS 2190 (d z 1.200 mm, ø ¼ 13 mm, Freudenberg) containing 80 mL liquid electrolyte. The various electrolyte compositions are specified in the following with the reported results. The charge capacity and charge/discharge rates during constant current cycling (CC) was related to the LFPcontent of the electrode taking a specific capacity of 170 mAh g1 into account. The cells were cycled between 2.8 and 4.2 V vs. Li/Liþ. The potential of the cathode was monitored using the lithium reference electrode. The first 5 cycles were recorded at 0.1C to obtain a stable cell response; all further charge- and dischargerates were done at 1C.
highest conductivities were observed for a solution with a LiTFSI concentration of 0.7 mol kg1 (CN:Li ¼ 11:1) over the whole temperature range from 20 C up to 90 C. TmdSx-OPN at the same concentration of 0.7 mol kg1 LiTFSI achieves an ionic conductivity of 1.0 mS cm1 at 30 C and 3.3 mS cm1 at 90 C. Here, the concentration differences of the ionic conductivity become less at higher temperatures (Fig. 1b). In general, solutions of LiTFSI in TmdSx-OPN show a higher ionic conductivity than in TmdSx-CN. This is correlated with the corresponding inverse changes of the viscosity (see Fig. 3). It may be attributed to the different connectivity of the nitrile group to the Si atoms in the disiloxanes modifying the intramolecular side group mobility. Zhang et al. reported similar observations for disiloxanes functionalized by oligoether side groups [4]. It should be noted that the achieved ionic conductivities for the nitrile functionalized disiloxanes are clearly higher than for most of the previously published values on siloxane based solvents, e.g. disiloxanes with ethylene oxide side chains (Fig. 2) [4,5]. The most reasonable explanation is the weaker coordination of the lithium cation to the nitrile group as compared to the O-donor of oligoether side chains, as discussed by Hou et al. [15].
3. Results and discussion 3.1. Ionic conductivity Conductivity cell preparations were done by filling the electrolyte into the volume enclosed by an O-ring (made of PEEK ¼ polyether ether ketone) which was sandwiched between two stainless steel electrodes with a surface area of 1 cm2. Ionic conductivities were determined from frequency dependent AC impedance (frequency response analyzer: Agilent E 4980 A) in the frequency range from 20 Hz to 2 MHz and with an amplitude of 40 mV. The impedance was recorded in the temperature range from 20 C to 90 C controlled by a thermostat (Julabo FP 45) under nitrogen atmosphere. Concentrations are given as molalities or expressed by the number of nitrile groups per lithium cation. LiTFSI shows high solubility in both investigated dinitrile functionalized disiloxanes. The concentration dependence was measured in the range from molalities of 0.2 up to 1.3 mol kg1 LiTFSI (not all results are shown here). Maximum conductivities were found in the middle of this range. At higher salt concentrations, the ionic conductivities decreased, in particular at low temperatures, in parallel to the steep increase of the viscosity. For TmdSx-CN, ionic conductivities around 4$105 S cm1 were obtained at 20 C for LiTFSI concentrations 1 mol kg1 increasing to a maximum of 2.3 mS cm1 at 90 C (Fig. 1a). The
Fig. 1. Arrhenius plot of the temperature dependent ionic conductivities of LiTFSI containing electrolytes with the solvents a) TmdSx-CN and b) TmdSx-OPN.
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B. Pohl et al. / Journal of Power Sources 274 (2015) 629e635 Table 1 Ionic conductivities at room temperature of nitrile-disiloxanes compared to oligoether-disiloxanes, FG ¼ functional group, CN ¼ nitrile group and EO ¼ ethylene oxide unit. (*) Data of the two disiloxanes TmdSx-PEG and TmdSx-OPEG taken from Ref. [4] (see meaning of PEG and OPEG in the text). (**) Pure solvent. Electrolyte Disiloxane TmdSx-CN TmdSx-OPN Disiloxane* TmdSx-PEG TmdSx-OPEG
FG:Li CN:Li 10:1 11:1 EO:Li 20:1 20:1
mol kg1
h/mPa s**
s/(104 S cm1)
9.2 7.0
5.9 9.0
6.0 4.1
3.4 5.5
T ¼ 25 C 0.7 0.7 T ¼ 25 C
3.3. Thermal stability
Fig. 2. Comparison of Arrhenius plots of the ionic conductivities of an oligoether functionalized disiloxane TmdSx-PEG (synthesized in this work according to 2a from Ref. [4] with PEG ¼ CH2CH2CH2O(CH2CH2O)3CH3) and both dinitrile disiloxanes from this work.
Table 1 gives an overview of corresponding ionic conductivities and viscosities of the nitrile disiloxanes and in addition of two disiloxanes from the literature denoted as TmdSx-PEG and TmdSxOPEG where PEG stands for two side chains of the type CH2CH2CH2O(CH2CH2O)3CH3 and OPEG for two side chains of the type O(CH2CH2O)3CH3 (replacing the two nitrile containing side chains of TmdSx-CN).
3.2. Viscosity The temperature dependent viscosities were measured using an Anton Paar Physica MCR 301 rheometer inside a dry room. All measurements were done from 20 C up to 90 C in 10 C steps. Fig. 3 illustrates the measured viscosities of both dinitrile functionalized disiloxanes. It is evident that the viscosity of the samples increases gradually with decreasing temperature. TmdSx-OPN has a lower viscosity than TmdSx-CN especially at very low temperatures which is also accompanied by higher ionic conductivities. A possible explanation could be a higher flexibility of the SieO binding of the side chain. Nevertheless, due to the very low glass transition temperatures, reasonable conductivities and viscosities were observed for both disiloxanes even down to temperatures of 20 C.
Thermal analysis based on differential scanning calorimetry (DSC) was carried out on a Netzsch DSC 204 calorimeter in the temperature range 150 C to 250 C (heating rate 10 K min1, nitrogen atmosphere). All samples were prepared under dry conditions in an argon filled glovebox using aluminum crucibles. Both investigated dinitrile functionalized disiloxanes show no crystallisation/melting transition in the DSC analysis, but very low glass transition temperatures down to 108 C which linearly increase with the LiTFSI concentration as listed in Table 2. At the highest salt concentrations, the glass transition temperatures are still as low as 85 C. No noticeable difference was found between TmdSx-CN and TmdSx-OPN. 3.4. Electrochemical stability The electrochemical stability range was measured by cyclic voltammetry using a potentiostat (Autolab PGSTAT302N, Metrohm). The cathodic and anodic electrode potential ranges were studied separately with different working electrode materials (WE). Stability with respect to cathodic reduction was measured in the range of electrode potentials from þ2.5 V down to 0.5 V (vs. Li/ Liþ) with Cu as working electrode (area ¼ 0.785 mm2). Stability versus anodic oxidation was studied using a Pt working electrode (same area as Cu WE) in the electrode potential range þ2.5 V up to 7 V. A lithium metal electrode was taken as reference electrode (RE), the counter electrode (CE) was made from a coaxial copper electrode (area of CE ¼ 8.635 mm2). All measurements were done at room temperature under argon atmosphere with a scan rate of 1 mV s1. The first cycles of the cyclic voltammograms in the anodic and cathodic domains for both dinitrile functionalized disiloxanes are presented in Fig. 4. A strongly increasing anodic current density is observed marking an irreversible oxidation of the electrolyte starting in case of TmdSx-OPN at about 5.6 V vs. Li/Liþ and at 5.2 V for TmdSx-CN (cf. Fig. 4a and b). Note for comparison that for oligoether functionalized disiloxanes Zhang et al. reported electrochemical stabilities up to 4.5e4.7 V vs. Li/Liþ [4]. Hence, the stability against anodic oxidation is considerably increased with
Table 2 Glass transition temperatures Tg of dinitrile-disiloxanes in dependency of the LiTFSI salt concentration. TmdSx-OPN
Fig. 3. Temperature dependent viscosities of both dinitrile disiloxanes (without a lithium salt).
TmdSx-CN
mol kg1
CN:Li
Tg
mol kg1
CN:Li
Tg
0 0.35 0.7 1.0 1.3
1:0 21:1 10:1 7:1 6:1
108 102 96 89 83
0 0.35 0.7 1.0 1.3
1:0 22:1 10:1 7:1 6:1
107 102 95 90 85
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dinitrile functionalized disiloxane solvents which is clearly due to the different choice of the side chain. Both dinitrile based electrolytes show the usual plating/stripping peaks of metallic lithium in the low electrode potential range around 0 V on a copper working electrode (cf Fig. 4a and b). However, it should be noted that the presence of vinyl carbonate (VC) in the electrolyte was necessary to achieve a stable SEI and a good coulombic efficiency. Without VC additive, the lithium stripping was suppressed. This is due to the reactivity between the bare lithium metal surfaces with the nitrile groups accompanied by polymerization products [27e29]. Without the presence of the VC additive, the resulting SEI is not stable enough to protect the electrolyte from continuing decomposition at the lithium anode. 3.5. Aluminum corrosion test
Fig. 4. Cyclic voltammetry measurements of a) TmdSx-CN and b) TmdSx-OPN based electrolytes with a scan rate of 1 mV s1 at 22 C.
With conventional carbonate based solvents, lithium electrolytes with LiTFSI salt cause a pitting corrosion at the aluminum current collector on the cathode side at potentials around 3.7 V vs. Li/Liþ [16,17]. Therefore, the stability of aluminum in contact to the investigated electrolytes was studied by cyclic voltammetry in the high electrode potential range using aluminum as the working electrode. Fig. 5 shows the first six cycles of electrolytes with both dinitrile functionalized disiloxane solvents (Fig. 5a and b). Fig. 5c shows a corresponding experiment with the same salt using a reference electrolyte with standard solvent. It is evident that the current densities are distinctly higher for the EC/DMC-LiTFSI reference electrolyte, and remain high even for the second and the following cycles. On the contrary, the current densities of TmdSxCN and TmdSx-OPN based electrolytes with comparable LiTFSI concentrations are very low. At 5.5 V vs. Li/Liþ, for instance,
Fig. 5. Cyclovoltammetry measurement of a) 1 mol kg1 LiTFSI in TmdSx-CN b) 1 mol kg1 LiTFSI in TmdSx-OPN and for comparison b) 1 M LiTFSI in EC/DMC ¼ 3:7 using aluminum as working electrode.
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currents of only 14 mA cm2 (see Fig. 5a) and 5.6 mA cm2 (see Fig. 5b) were attained in the first cycle, being two to three orders of magnitude lower as compared to the EC/DMC based reference electrolyte and further decaying to negligible values in subsequent cycles. Photos from scanning electron microscopy (SEM) in Fig. 6 illustrate the difference in corrosive activity between the dinitrile based solvents and the EC/DMC reference electrolyte. Fig. 6c) shows the rinsed aluminum electrode after the measurement with the TmdSx-CN based electrolyte. The aluminum surface is still planar, only weak traces from polishing are visible. On the other hand, the rinsed aluminum electrode in Fig. 6d) which was in contact to the EC/DMC based reference electrolyte is completely damaged due to intensive aluminum pitting corrosion. Both dinitrile functionalized disiloxane solvents in combination with LiTFSI seem to cause the formation of a protective layer on aluminum preventing further corrosion. Possible explanations may be an insolubility of Al-TFSI complexes in the disiloxane solvent or the formation of a product layer formed by a reductive condensation of the nitrile group induced by highly reactive Alþ as discussed by Di Censo et al. [19]. The detected surface film on the Al electrode is shown in Fig. 6a and b. The entire surface film on the electrode contains sulfur from TFSI and silicon from the disiloxane as analyzed by EDX mapping. In the lower part of Fig. 6a, the surface film was partly removed so that the aluminum current collector becomes visible there again.
3.6. Constant current cycling A last series of experiments concerned the cycling stability with the developed alternative electrolytes. Fig. 7a shows a charge/ discharge test using the TmdSx-CN/LiTFSI electrolyte in a half cell with a LiFePO4 cathode (LFP) coated on an aluminum current collector. The initial cycles (1e5) show the performance of the electrolyte/electrode couple at a charge/discharge rate of 0.1C and a charge capacity of 140 mAh g1 which is 82% of the theoretical
Fig. 7. Average values of the charge-, discharge performance at 1C of a) TmdSxCN þ 2 wt% vinylene carbonate (as SEI additive) with 1 mol kg1 LiTFSI and b) 1 M LiTFSI in EC/DMC ¼ 3:7 as reference on LiFePO4 electrodes at 22 C. Each fifth cycle is presented.
capacity of LFP. The following constant current cycling was continued with a rate of 1C. These subsequent cycles exhibit a stable reversible capacity of 110 mA g1 for 230 cycles. Then the
Fig. 6. a) EDX mapping of the unrinsed Al electrode after CV measurement using TmdSx-CN and 1 mol kg1 LiTFSI as electrolyte (colors encode the distribution of the three elements S - red, Al - blue, Si - green); b) corresponding SEM image to a); c) rinsed Al electrode without visible Al pitting corrosion after CV from TmdSx-CN/LiTFSI electrolyte; d) rinsed aluminum electrode with clearly visible Al pitting corrosion after CV measurement using the reference electrolyte 1 M LiTFSI in EC/DMC ¼ 3:7.
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capacity slowly decreases and reaches 90 mAh g1 after 595 cycles. This is still 82% of the initial charge capacity. The coulombic efficiencies during the cycling with TmdSx-CN/LiTFSI averaged over all cycles amounted to about 99.9%. Fig. 7b shows the capacity fading of analogous cells with common 1 M LiTFSI in EC/DMC ¼ 3:7 as electrolyte. Compared to the dinitrile disiloxane electrolyte, the initial capacity is higher, but the end of life is already reached after 177 cycles which again demonstrates the corrosive action of LiTFSI based electrolytes with conventional solvents.
References
4. Conclusion
[9] [10]
One can conclude that dinitrile functionalized disiloxanes with LiTFSI as lithium salt present an attractive approach towards electrolytes with enhanced thermal and electrochemical stability and, in particular, with enhanced stability of the aluminum current collectors against corrosion at high electrode potentials, a wellknown drawback of LiTFSI when used in conventional carbonate based solvents. The investigated disiloxane based solvents with dinitrile functionalization offer potential use for high voltage cells. Conventional SEI stabilizing additives can be used to protect the nitrile solvents on the anode side. The total conductivities are among the best values measured so far in disiloxane based solvents. In particular, the system denoted here as TmdSx-CN is interesting due to its commercial availability.
[11]
Acknowledgments The authors acknowledge the support of the initial synthetic work by the Deutsche Forschungsgemeinschaft DFG within the research initiative “Functional materials and material analysis to high performance lithium ion batteries (PAK 177)” and the support of extended electrolyte configuration and cell tests in a second phase by the Federal Ministry of Education and Research (BMBF) within the project MEET Hi-EnD (High energy density batteries, 03X4634A). The authors also thank Timo Hopp for recording the SEM images.
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