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Novel electrolyte mixtures based on dimethyl sulfone, ethylene carbonate and LiPF6 for lithium-ion batteries
Journal of Power Sources 298 (2015) 322e330
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Novel electrolyte mixtures based on dimethyl sulfone, ethylene carbonate and LiPF6 for lithium-ion batteries Andreas Hofmann a, *, Thomas Hanemann a, b a
Karlsruher Institut für Technologie (KIT), Institut für Angewandte Materialien e Werkstoffkunde (IAM-WK), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany b €t Freiburg, Institut für Mikrosystemtechnik, Georges-Ko €hler-Allee 102, 79110 Freiburg, Germany Universita
h i g h l i g h t s Novel electrolytes based on ethylene carbonate (EC) and dimethyl sulfone (DMSN). Enhanced electrolyte safety based on high flash points. Electrochemical characterization and cell testing of EC/DMSN-based electrolytes. Use of LiPF6, LiBOB and LiDFOB as conducting salts. Investigation of programmed current chronopotentiometry related to Li mobility.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 April 2015 Received in revised form 17 August 2015 Accepted 19 August 2015 Available online xxx
In this study, novel electrolyte mixtures for Li-ion cells are presented which are composed of ethylene carbonate/dimethyl sulfone (80:20 wt./wt.) as a solvent mixture and LiPF6, lithium bis(oxalato)borate and lithium difluoro(oxalato)borate as conducting salts. The main advantages of the solvent mixture are high flash points of >140 C which enhance the intrinsic safety of Li-ion cells while maintaining good cell performance above 0e5 C. The movability of the lithium ions in the electrolyte is investigated via programmed current derivative chronopotentiometry. It is found that pure electrolyte properties cannot necessarily predict the electrolyte behavior in real Li-ion cells but the complex interplay between electrolytes, electrode materials and separators has to be taken into account. Using the newly developed electrolytes, it is possible to achieve C-rates up to 1.5C with >80% of the initial specific discharge capacity (25 C). Within 200 cycles during one month in cell tests (CjjNMC) it is proven that the retention of the specific capacity is >98% of the third discharge cycle in dependence of the conducting salt. © 2015 Elsevier B.V. All rights reserved.
Keywords: Li-ion battery Electrolytes Safety Programmed current chronopotentiometry
1. Introduction The electrolyte formulation for nowadays Li-ion battery electrolytes consists mostly of carbonate solvents, lithium conducting salts and various additives [1,2]. The carbonate solvents include cyclic organic carbonates like ethylene carbonate (EC) or propylene carbonate (PC) and linear carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) or diethyl carbonate (DEC). Unfortunately, linear carbonates (DMC, DEC, EMC) exhibit low boiling and flash points which makes these mixtures and electrolytes highly inflammable. Besides, low boiling points of linear
* Corresponding author. E-mail address: [email protected] (A. Hofmann). http://dx.doi.org/10.1016/j.jpowsour.2015.08.071 0378-7753/© 2015 Elsevier B.V. All rights reserved.
carbonates cause a significant increase in cell pressure based on gaseous solvents at moderate temperatures (<100 C). Additionally, during a fire incident highly toxic fluorine-containing decomposition products can be formed [3e5]. Therefore, high-cost safety devices are necessary in order to ensure the battery safety of a lithium-ion battery during an accident. These devices however lower the gravimetric energy density of the battery pack. Nevertheless, there still remains a risk during unexpected events due to the high energy stored in a lithium ion cell. Intensive research is therefore done to evaluate novel safe solvent and electrolyte mixtures which retain low temperature applicability (<0 C) and cell performance, especially at elevated discharge rates. Some classes of novel solvents which are studied in the last years include sulfolane [6e10], sulfones [11e15], sulfoxides [16], nitriles [17,18], ionic liquids [19e28], esters [29e31], ethers
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[32e35], silicon-based solvents [17,36e39], phosphorus-based solvents [40e42], g-butyrolactone [43e45] or the omit of linear carbonates. Despite of single properties which outperform carbonate mixtures by far, only very few electrolyte formulations have the potential to replace standardly used organic carbonate mixtures in industrial scale so far [46]. Dimethyl sulfone (abbreviated as DMSN to distinguish from dimethyl sulfoxide (DMSO) and dimethyl sulfite (DMS)) exhibits a melting point of ~110 C and is not-miscible with solvents used in Li-ion batteries (sulfolane, DMC, DEC, PC) at common mixing ratios. Due to this, it is expected that it is not suitable as electrolyte component for Li-ion batteries. However, it was recently shown that mixtures composed of EC and DMSN form an eutectic mixture and thus could be an interesting solvent mixture for Li-ions batteries [47,48]. Nowadays, LiPF6 is standardly used as conducting salt in spite of its toxicity and decomposability due to its most balanced properties compared to other lithium salts. Various lithium containing salts had however been investigated with excellent properties in terms of solid electrolyte interphase (SEI), cycle and temperature stability which can be used as additives in electrolyte formulations. Two of these compounds, lithium bis(oxalato)borate (LiBOB) and lithium (difluorooxalato)borate (LiDFOB), are very popular for an effective SEI protection on the surface of graphite and for improving the overall electrolyte properties [49e53]. Therefore, it is expected that these salts can improve new solvent mixtures. In chronopotentiometry techniques, a programmed current function is imposed to an electrode and its potential is measured subsequently as a function of time. In 1960, Reinmuth proposed the technique of linear current scan potentiometry where the current varying linearly with time and the variation of electrode potential is observed [54]. Various electrode reactions can be detected and investigated in dependence of the nature of electrolyte and electrode. However, for a quantitative analysis of such electrode processes, the reactions have to fulfil requirements based on Fick's laws [55]. The aim of this study is to evaluate the usability of novel mixtures composed of EC and DMSN as electrolyte solvents and LiPF6, LiBOB and LiDFOB as conducting salts for Li-ion based cells which can be used at moderate C-rates up to 2C and at temperatures of 0e70 C. The electrolytes are studied based on their electrochemical properties and in the cell system of graphite as anode and LiNi1/3Mn1/3Co1/3O2 (NMC) as cathode material with no further additives. It is investigated if the method of programmed current derivative chronopotentiometry could be helpful in predicting the electrolyte performance. 2. Experimental 2.1. Materials Ethylene carbonate (SigmaeAldrich, 99%, anhydrous) and dimethyl sulfone (Alfa Aesar, 99%) were dried by using a coulommetric Karl-Fischer titrator, which consists of a 831 KF Coulometer and a 860 KF Thermoprep oven from Metrohm. The compounds were placed into the Thermoprep oven (120 C) and a continuous gas flow (dried air) was bubbled through the liquid. The water content of the gas flow was detected in the Coulometer (final drift of <1 mg min1). After drying, the water content of the solvents was determined to be less than 25 ppm. Subsequently, the liquid was degassed with dry argon and placed into the glove box. Dimethyl carbonate (SigmaeAldrich, anhydrous), Lithium hexafluorophosphate (SigmaeAldrich, 99.99%, battery grade, anhydrous), lithium bis(oxalato)borate (Chemetall), lithium difluorooxalato borate (ABCR) and lithium foil (Alfa Aesar,
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thickness ¼ 0.75 mm) were used as received. ELM-0 (1.0 M LiPF6 in EC/DMC; battery grade) was purchased from SigmaeAldrich. The preparation of the electrolytes was performed in an argon-filled glove box (MBraun GmbH) with oxygen and water levels below 0.5 ppm. 2.2. Density measurements The density of the electrolyte mixtures was obtained by repeated measurement of the mass of 75.0 ± 0.2 ml of an electrolyte mixture at room temperature inside the glovebox at 25 C. 2.3. Conductivity measurements The ionic conductivity of the mixtures was measured by the standard complex impedance method, using a Zahner Zennium IM6 electrochemical workstation in the frequency range from 1 kHz to 1 MHz. A 1.6 ml closed cell (850 ml solvent) from RHD instruments is used for the measurements. In the phase minimum of 0 , the ! impedance value Z was used as ohmic resistance ! for calculating the specific conductivity k according to k ¼ C= Z with the cell constant of C ~17 cm1. The exact cell constant (error of 5 measurements <1%) was received by measuring a standard solution (1.413 mS cm1 at 25 C, Hanna instruments, HI 70031). The RHD test cell was filled inside the glove box to exclude water contamination. 2.4. Cyclovoltammetry measurements The cyclovoltammogramms were measured at a Zahner XPOT potentiostat (software: PPSeries, Potentiostat XPot Zahner elektrik 6.4). The potential range was 3e6 V vs. Li/Liþ with platinum as working electrode. The cells were measured in 3-electrode configuration (EE-Cells manufactured by EL-Cell GmbH) with reference and working electrodes composed of lithium. The scan speed applied for all CV tests was 5 mV/s. 2.5. Chronovoltammetry The AljLi anodic oxidation experiments were performed in two electrode configuration (Swagelok® type cells) with Al (d ¼ 12 mm) as working electrode, Li (d ¼ 12 mm) as reference and counter electrode, and a glass fiber separator (Whatman, GF/B, d ¼ 13 mm) in between. The measurements are performed by using a X-POT potentiostat from Zahner-Elektrik. The maximum deviation between same measurements was in the order of ±1.5 mA cm2. 2.6. Viscosity measurements Dynamic viscosity was measured using a Malvern Gemini HR Nano rotational rheometer with 40/1 cone geometry and a gap of 30 mm via a temperature sweep (20e80 C) at a shear rate of 100 s1. These experiments were performed by using a solvent evaporation protecting cover in air. 2.7. DSC measurements The mixtures were investigated by differential scanning calorimetry (DSC) regarding their phase behavior (Netzsch DSC 204 F1 Phoenix®, Al crucible closed). The measurements were carried out from 150 to 100 C at a heating rate of 10 and 20 K/min in argon atmosphere.
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2.8. Flash points The flash points were measured with a Grabner miniflash instrument according to the norm ASTM D6450 (certified reference material: decahydronaphthalene). 2.9. Cell tests In this study, common coin cells (type: CR 2032, Hohsen Corp.) were used with a coin cell crimper from BT Innovations. The cells were assembled in an argon-filled glove box according to standard procedures. Precisely, a graphite anode (Ø ¼ 15 mm), a NMC cathode (Ø ¼ 14 mm), and a glass fiber separator (Whatman®, GF/B and QMA 450; Ø ¼ 16 mm) were used inside a coin type cell with one spring and one stainless steel spacer. Calandered electrodes based on graphite and NMC with a content of approximately 90% of active material were provided in industrial cooperation. 2.10. Galvanostatic polarization Programmed current derivative chronopotentiometry was performed in two electrode Swagelok cells at Zahner Zennium IM6 electrochemical workstation. Lithium metal (foil, Alfa Aesar, Ø ¼ 12 mm) is used as working and reference/counter electrode. It is observed that almost identically values of Imax are detected whether 4 or 6 layers of glass fiber separators (Whatman, GF-B, Ø ¼ 13 mm) were used in the cell (33.5 ± 0.2 mA, test electrolyte: EC/EMC þ 1 M LiPF6). Two layers result in slightly decreased values, thus 4 layers are used for the study. 75 ml of electrolyte solution is used for each separator layer. The cells were prepolarized at b ¼ 1 mA s1 up to 0.7 V and afterwards (after relaxation of the potential) measured at b ¼ 100 mA s1 up to 10 V. 3. Results and discussion 3.1. Electrolyte mixtures In this study, selected electrolyte formulations based on dimethyl sulfone (DMSN), ethylene carbonate (EC) and lithium conducting salts lithium bis(oxalato)borate (LiBOB), lithium (difluorooxalato)borate (LiDFOB) and lithium hexafluorophosphate (LiPF6) are investigated. The chemical formulas of the compounds are depicted in Fig. 1 and the composition of the electrolyte mixtures is listed in Table 1. All concentrations are related to the total mass of the mixture in order that 0.75 mol kg1 corresponds to ~1 mol dm3. 3.2. Physical and chemical properties An overview about fundamental electrochemical and physicochemical data is provided in Table 2. In terms of improved cell safety, the extraordinary high flash point of 142 C of the pure solvent mixture has to be mentioned (Table 2) which was measured according to the standard norm ASTM D6450. It is expected that the addition of conducting salts as LiPF6, LiBOB and LiDFOB will not
affect the flashpoint significantly because it is mainly determined by solvents [18,48]. Compared to ELM-0, the density values of the electrolytes based on EC/DMSN (ELM-1 e ELM-4) are in the order of 1.38 ± 0.02 g cm3 which is slightly increased compared to mixture ELM-0. This gives rise to a decrease in the gravimetric energy density of the battery, if other factors are in the same order of magnitude. However, the intrinsic improve of safety by using lowflammable solvents influences the whole safety concept which can affect the battery weight, too. Differential scanning calorimetry (DSC) measurements of the solvent mixtures and the electrolytes are shown in Fig. 2. All measurements are done in closed Al crucials under argon atmosphere. A shift to lower melting and crystallizing temperatures (approximately DΤ ¼ 10e20 C) is observed from pure solvent mixtures (LM-1, LM-2) to salt concentrated electrolytes. DSC measurements can however reveal quite low crystallizing temperatures which result from supercooled meltings and consequently overestimate temperature working conditions for Li battery cells [56]. Thus, small seed crystals which are in battery cells in terms of electrode or separator materials can result in higher crystallizing temperatures close to the melting temperature. It is observed that electrolyte ELM-4 can be used in cell testings (see below) down to 0 Ce5 C in spite of an onset in the crystallizing temperature at 12 C. Here, the onset of the melting point (þ1.6 C) is a better measure than the crystallizing temperature.
3.3. Viscosity and conductivity measurements Temperature-dependent viscosity and conductivity measurements of the electrolytes are shown in Fig. 3. The ionic conductivity is decreased by a factor of ~2 (20 C) compared to mixture ELM0 regardless of whether pure LiPF6 or mixtures of LiPF6, LiBOB and LiDFOB are used as conducting salts. The dynamic viscosity of the novel electrolytes ELM-1 e ELM-4 is enhanced by a factor of ~2.5 (20 C) compared to ELM-0. All in all the commercial electrolyte ELM-0 exhibits higher ionic conductivities and lower viscosities caused by the low viscous DMC. Viscosity values of <10 mPa s at 25 C of the novel electrolytes are lower than sulfolane (SL) based electrolytes (e.g. 1 mol kg1 LiTFSI in SL: ~50 mPa s [57]; 1 M LiPF6 in SL: ~31 mPa s [58]) or ionic liquid based electrolytes (e.g. 0.5 M LiTFSI in trimethyl-n-hexylammonium-TFSI: ~300 mPa s [59]; 0.75 M LiTFSI in trimethyl-n-butylammonium-TFSI: 360 mPa s [60]; 1 M LiTFSI in 1-ethyl-3-methylimidazolium-TFSI: ~200 mPa s [61]; 1 M LiPF6 in N-methyl-N-propylpyrrolidinium-TFSI/EC (1:1 wt.): ~20 mPa s [62]; 1 mol kg1 LiTFSI in sulfolane/diethylmethyl-(2-methoxy)-ethylammonium TFSI (2:1 wt.): 67 mPa s [57]) and gives rise to a promising cell performance in Li-ion cells. The dynamic viscosity values are in the same order of magnitude compared to pure propylene carbonate (PC)-based electrolytes (e.g. 0.7 M LiBOB in PC: ~7.3 mPa s [63]; 1 M LiClO4 in PC: 8.1 mPa s [64]). Temperature-dependent viscosity data reveal a more temperaturedependent behaviour of electrolyte ELM-1 e ELM-4 compared to mixture ELM-0. This can be quantified by calculating the quotient h20-80 ¼ h(20 C)/h(80 C) which yields h20-80 ¼ 2.5 (ELM-0) and h2080 ¼ 5.8 ± 0.2 (ELM-1 e ELM-4). All observations lead to the assumption that the formation of small ionic clusters or molecular
Fig. 1. Chemical structures of EC, DMSN, LiBOB, LiDFOB and LiPF6.
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Table 1 Composition of the electrolyte formulations. Electrolyte
Solvent
Ratio wt./wt.
LiPF6 mol kg1
LiBOB mol kg1
LiDFOB mol kg1
[Liþ] mol kg1
LM1 LM2 ELM-0 ELM-1 ELM-2 ELM-3 ELM-4
EC/DMC EC/DMSN EC/DMC EC/DMSN EC/DMSN EC/DMSN EC/DMSN
50:50 20:80 50:50 80:20 80:20 80:20 80:20
e e 0.787 0.787 0.65 0.65 0.65
e e e e 0.1
e e e e e 0.1 0.05
e e 0.787 0.787 0.75 0.75 0.75
0.05
Table 2 Physicochemical properties of electrolyte mixtures. TK crystallizing temperature; Tm melting point; fp. flash point; d density; h viscosity; k conductivity. Electrolyte
LM-1
LM-2
ELM-0
ELM-1
ELM-2
ELM-3
ELM-4
mp./ C (DSC) Freezing Point/ C (DSC) fp./ C d/g cm3 (25 C) (±0.01 g cm3) h/mPa s (20 C) (±0.5 mPa s) k/mS cm1 (20 C) (±0.05 mS cm1) Imax/mA at b ¼ 100 mA s1 (±0.7 mA)
0.6 32.7 24 1.20 1.7 e e
36.1 9.1 142 1.32 4.4 e e
19.4 56.1 e 1.27 4.4 10.67 34.5
16.0 26.1 e 1.40 11.4 5.95 28.4
18.1 12.3 e 1.39 11.0 5.38 28.4
18.6 19.1 e 1.36 11.4 5.95 27.5
18.8 17.3 e 1.37 11.4 5.76 27.7
Fig. 2. Differential scanning calorimetry data of solvent and electrolyte mixtures Fig. 2a: Heating-up curves in argon atmosphere at 10 K min1 (closed Al cap). Fig. 2b: Cooling down curves at 10 K min1 (closed Al cap).
aggregates is more pronounced in the novel electrolyte formulations compared to less viscous mixture ELM-0. A correlation between conductivity and viscosity data is taken into account within the Walden rule which relates the fluidity h1 to the limiting molar conductivity L0m [65]. Applying an additional parameter a, the so called fractional Walden rule (Equation (1)) is obtained [48,66e69]. For linear fittings of electrolyte mixtures, the limiting molar conductivity L0m was replaced by the molar conductivity Lcs m which consists of the amount of substance of the conducting salts (cs).
log L0m ¼ log C 0 þ a log h1
(1)
The Walden plots are depicted in Fig. SI-1 (supporting information) in the temperature range of 20e80 C and the fitting parameters are listed in Table 3. The results show an excellent matching of the data within a Walden fitting procedure, where the parameter a is reduced in case of the novel electrolytes (a z 0.65) compared to ELM-0 (a ¼ 0.96). This is in accordance with the results which are received for EC/DMSC mixtures with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) or LiBF4 as conducting salts [48]. A negative deviation of the parameter a from 1 is indicative that the conductivity is determined not only from the viscosity [66], but rather from ion clustering and the interaction of ionic species
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Fig. 3. Temperature dependent viscosity h (t ¼ 100 s1) (Fig. 3a) and conductivity k (Fig. 3b) data of the electrolyte mixtures.
with an external field [70]. In accordance to Lee et al., the electrolytes can be classified as solvent-co-solvent systems [58]. It can be concluded that the conductivity and rheology measurements provide an indication of ionic solvation (aggregates/clusters) in the electrolyte mixture. The addition of up to 13 mol-% of both or either LiBOB and LiDFOB to the conducting salt LiPF6 has no significant effect on the viscosity and the conductivity measurements. 3.4. Lithium movement The measure of Li transference numbers which is the fraction of the total current carried by a selected ion in an electrolyte can be achieved by different methods including pulsed field gradient nuclear magnetic resonance, Bruce-Vincent/potentiostatic polarization, galvanostatic polarization, moving boundary method and electromotive force method. Significant differences among these methods are found on account of different assumptions and experimental conditions during the measurement [1,71,72]. Additionally, prerequisites are necessary which restrict the electrolyte formulations. In the end, the behavior of the electrolyte in the Liion cell and in particular the Li-cell performance can be predicted only in a very limiting manner based on the knowledge of Li transference numbers. Therefore, another technique is investigated here to prove the accessible cell performance enabled by the electrolyte. A measure of the lithium mobility in the electrolyte is investigated with programmed current derivative chronopotentiometry [55,73]. A current I respectively current density j that increases linearly with time t (I(t) ¼ b·t) is imposed to LijLi (Ø ¼ 12 mm) Swagelok-cells with several layers of glass fibre separators. Principally, a current corresponds to an electrode reaction which is enabled by a movement of Li-ions from one Li electrode to the other Li electrode. On account of solvation effects, lithium dissolution, lithium plating or interactions with separator material in the electrolyte which hamper the movement of the Li-ions, the observed potential increases with ascending current. Above a certain applied current limit (Imax) the voltage increases in a
dramatic fashion because no more Liþ ions can be delivered by the electrolyte. That way, Imax corresponds to the maximum accessible lithium ion flux under applied current conditions. Certainly, this assumption is only valid if other effects like lithium plating/dissolution are not rate-determining. It was proven by using one-sided smaller electrode areas that higher current densities can be received which indicates that Li dissolution and Li platting are not rate determining reactions (see supporting information). Ideally, at the beginning of the experiment the ionic polarization inside the cell should be in the same order of magnitude meaning a prepolarization at 1 mA s1 is performed up to identical potential differences (0.7 V vs. Li/Liþ). In case of lithium as active electrode material, the electrode area A varies within the experiment because of lithium dendritic growth. Additionally, a continuous decrease of the LieLi distance due to dendritic Li deposition affects the electric field inside the cell. Nevertheless, polarization experiments reveal that these effects are small compared to the voltage jump due to discontinued Liþ flux (see supporting information). However, these effects significantly complicate theoretical derivations based on Fick's law [55] in case of lithium electrodes, though. Finally, it is investigated, if there are differences from EC-DMC electrolyte and if
Table 3 Results of Walden fitting procedures. Sample
cc.s./mol dm3
Slope
ELM-0 ELM-1 ELM-2 ELM-3 ELM-4
1.008 1.102 1.042 1.020 1.028
0.96 0.64 0.66 0.64 0.64
± ± ± ± ±
R2 0.03 0.02 0.01 0.01 0.01
0.997 0.998 0.999 0.999 0.999
Fig. 4. Potential (vs. Li/Liþ) versus current during programmed-current chronopotentiometry (working electrode: lithium, counter/reference electrode: lithium, four-layer glass fiber separators GF/B; b ¼ 100 mA s1).
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a qualitative comparison between the electrolytes is possible. The diagrams of the potential response versus current are shown in Fig. 4. For each electrolyte mixture, three measurements are performed which result in comparable voltage responses. The results of the measurements are listed in Table 2, too. The novel electrolytes result in comparable values of Imax ¼ 28 ± 1 mA which is significantly reduced compared to EC/DMC þ 1 M LiPF6 (Imax ¼ 35.5 ± 0.5 mA). Principally, two plateaus in the potential can be observed which represent electrode reactions. It is found that the value of Imax is dependent on the current rate b and should be mentioned within a series of experiments. Here, b ¼ 100 mA s1 is chosen based on a balance between dendritic growth and reproducibility. Based on the experiments it can be expected that the performance of the novel electrolyte mixtures in Li-ion-cells is inferior to ELM-0. Additionally, one would expect similar performance of ELM-n (n ¼ 1e4) values in Li-ion-cells in case of a rate determining by the Li mobility. 3.5. Electrochemical stability The electrochemical stability of the electrolytes is investigated against platinum working electrodes within a potential range of 3e6 V vs. Li/Liþ. It should be mentioned that one cannot predict the oxidative stability against electrode materials based on the measurement against platinum electrodes but an estimation about the principal electrochemical stability can be obtained. EC as well as DMSN should withstand high oxidative potentials up to 4.5 V vs. Li/ Liþ which have to be fulfilled in Li-ion cells [47,58]. In Fig. 5, the current density responses of the novel electrolytes are shown in dependence on the potential. The first and the third potential sweep are both depicted for comparison. It can be observed that the electrolyte mixtures exhibit a significant increase (>50 mA cm2) in current density response starting at 5.5 V vs. Li/Liþ. A significant enhanced current density response is observed for LiBOB or LiDFOB containing samples ELM-3, ELM-4 and ELM-5. However, an excellent oxidative stability is obtained in a potential range up to 5 V vs. Li/Liþ for all electrolytes which makes them prospective candidates for high voltage electrolyte applications. In Li-ion cells, the electrolyte is in permanent contact with aluminum. Therefore it is a necessity that the protection of aluminum is sufficient at operating cell potentials. The oxidative dissolution stability of battery grade aluminum is investigated in chronoamperometry measurements. The potential is set to 4.3 V vs. Li/Liþ and 4.7 V vs. Liþ, respectively, for a time period up to 24 h to evaluate the current density response. Both diagrams are plotted in Fig. 6. It can be observed that an excellent stability of Al is obtained up to 4.7 V vs. Li/Liþ. It should be noted, that the error of the measurements is in the order of 1.5 mA cm1 based on the Swagelok type cell design. Very low current density responses can be caused by little electrolyte decomposition, impurities, leakage current and different formation of surface layers onto Al and Li. The findings are verified by optical analyses via optical microscopy of the Al disc which give no rise to any Al dissolution/corrosion (Fig. SI-5, supporting information). The excellent protection of aluminum by addition of LiBOB or LiDFOB [49,57,74,75] is confirmed in order that the aluminum inside the cell should resist high potentials up to 4.7 V vs Li/Liþ when the electrolytes are used. 3.6. Cell cycling tests The electrolytes are investigated in cell tests with graphite and NMC (LiNi1/3Mn1/3Co1/3O2) as electrode materials and glass fiber separators. In Fig. 7, the first cycles are compared with respect to formation (Fig. 7a) and capacity loss (Fig. 7b). It can be seen that LiBOB or LiDFOB containing electrolytes exhibit a plateau around
Fig. 5. Cyclovoltammetry of Ptjj Li cells (working electrode: Pt, counter electrode: lithium, reference electrode: Li, separator: glass fibre separator, 1 mV s1). The first (1, black line and dotted line) as well as the third (3, reddish dashed/dotted and fine dotted line) CV spectra are shown for comparison. The scale of the oxalato-free mixtures (ELM-0 and ELM-1) were adjusted for a better comparison.
2e2.2 V vs. Li/Liþ which can be attributed to a reduction of oxalate molecular moieties [49,52,76]. In Fig. 7b it can be seen that the combination of both salts (LiBOB and LiDFOB) in the EC/DMSN mixture reduces significantly the capacity loss during the first cycle and increases the available specific capacity to ~150 mAh g1 which is in the same order of magnitude compared to ELM-0 (EC/ DMC þ 1 M LiPF6). In the third cycle, almost 100% in capacity retention (discharge to charge cycle) is received in all EC/DMSN electrolytes. It can be concluded that the novel electrolytes exhibit good cycling properties within the first cycles. In Fig. 7c, the cycling up to 200 cycles is depicted and the electrolyte ELM-0 is compared within the first 100 cycles. It is observed that different cell performances can be seen especially at relatively fast discharging rates. This is in contrast to the findings of the programmed current chronopotentiometry measurements (Fig. 4) which reveal a similar lithium transport behavior. Therefore it is expected that the Liþ transport is heavily influenced from the boundary layer of the electrodes (solid electrolyte interphase, SEI) which is different for each mixtures based on different SEI compositions. These effects are supposed to be much more pronounced and rate determinant in the cell than the
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Fig. 6. Chronoamperograms of AljjLi cells to evaluate the aluminum anodic dissolution limit of the electrolytes. Fig. 6a: Applied potential is 4.3 V vs. Li/Liþ. Fig. 6b: Applied potential is 4.7 V vs. Li/Liþ.
Fig. 7. NMCjjC full cell tests with electrolytes and glass fiber separators. In Fig. 7a, the first cell charging is depicted (same behavior is observed within three cells with same electrolyte). The charging is done at 0.05C. In Fig. 7b, a comparison in charging (c) and discharging (dc) specific capacity is shown within the first three cycles. The deviation of three cells with same electrolytes is <2.5 mAh g1. First cycle at 0.05/0.1C, second and third cycle at 0.1/0.2C (charge/discharge) (coin cells CR 2032; CC charging only). Fig. 7c: Measurement of the specific discharge capacity of NMCjC coin cells. The charging/discharging characteristic is mentioned as charging/discharging C-rate. Starting from 100 cycles, CCCVcharging is used (Ie ¼ C/15). All tests (Fig. 7aec) are done at 25 ± 1 C.
transport of the lithium through the electrolyte and separator. However, only glass fiber separator is studied so far, thus other kind of separators may result in stronger or weaker interactions to Liions. The best performance in cell tests is obtained with LiBOB/ LiDFOB mixed electrolytes compared to ELM-0. The best capacity
retention after 200 cycles is delivered by electrolyte ELM-2 (>97% compared to the fifth cycle directly after the formation), whereas the performance is significantly reduced compared to LiDFOB containing electrolytes at fast discharge rates. Additionally it is observed that the fading in capacity retention is most pronounced
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when the lithium concentration decreases and the charging rate is increased. When the charging rate exceeds ~1.5C, a significant fading is found for the electrolytes. For all electrolytes same electrode compounds and separator materials are used. It is investigated that the electrolytes can be used also at decreased temperatures. Therefore, the best electrolyte ELM-4 is investigated in discharge cycles at selected temperatures to quantify the temperature-dependence of the cell performance (Fig. 8). It can be observed that the discharge capacity decreases at lower temperature. In this case, the movement of the lithium ions inside the cell (in the electrolyte as well as into the electrode material/SEI) is reduced. At 1C discharge rate, almost 50% of the initial capacity (at 25 C) can be used at 5 C. Below 5 C it is observed that the discharge capacity decreases very rapidly. It is expected that the electrolyte mixture becomes highly viscous below 5 C based on the melting point of the mixture. It should be noted that the freezing point in DSC studies is detected to be 17 C. However, a supercooled melting is presumably detected based on the lack of seed crystals in the DSC vials in contrast to the lithium ion cells. It is observed that the decrease in discharge capacity is much less pronounced at discharge rates of 0.5C. Almost the full capacity (>90%) can be extracted at 5e10 C. However, at 0.5C a disruption in discharge capacity is found below 5 C, too. It should be mentioned that fast (>1C) as well as moderate (e.g. 0.5C) charging at low temperatures (<10 C) increases significantly the risk of dendritic lithium deposition onto graphite.
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surface effects and conditions (e.g. SEI layer) come into play. The performance of the electrolytes is quantified in full cell tests against graphite and NMC material whereas an excellent discharge capacity performance is obtained up to a discharge rate of 2C at room temperature for these flame-resistant electrolyte mixtures. It is shown that the electrolytes can be used down to ~5 C where still >90% of discharge capacity can be used at 0.5C. Based on the properties found in this study, the electrolyte mixture could be of high interest also for other applications like supercaps or other Li-based technologies, too. Since only one component compared to standardly used electrolyte mixtures (linear/cyclic carbonates þ LiPF6) is replaced in the novel electrolyte formulations, these mixtures might be of interest also for individual applications in industrial purpose (e.g. in-house energy storage systems in combination with autonomous energy harvesting devices). Acknowledgements AH acknowledges support by Deutsche Forschungsgemeinschaft (Sachbeihilfe, HO 5266/1-1). We thank Chemetall for providing LiBOB and Matthias Migeot for performing selected electrochemical measurements. We acknowledge Kerstin Prengel for proofreading. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.08.071.
4. Conclusion References In this study, novel electrolyte mixtures based on EC/DMSN (80:20 wt./wt.) and LiPF6 are presented with focus on usability for Li-ion cells. LiBOB and LiDFOB are investigated as conducting salt additives to improve the cell performance. The electrolytes exhibit excellent oxidative stabilities which is proved against platinum and aluminum up to 4.7 V vs. Li/Liþ. It could be demonstrated that programmed current derivative chronopotentiometry measurements can be used to qualitatively compare different electrolytes with respect to the lithium ion movability. However it is shown that the mobility in the electrolyte can differ from real cells when various
Fig. 8. Temperature-dependent performance of electrolyte ELM-4 in NMCjC cells during discharge at 1C (black squares). Three individual cells are charged at 0.2C at 25 C and discharged at selected temperatures (blue circle). The measurements are performed after 200 cycles (cell tests according to Fig. 7c). The deviation between the measurements are <5%. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Novel electrolyte mixtures based on dimethyl sulfone, ethylene carbonate and LiPF6 for lithium-ion batteries Andreas Hofmann a, *, Thomas Hanemann a, b a
Karlsruher Institut für Technologie (KIT), Institut für Angewandte Materialien e Werkstoffkunde (IAM-WK), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany b €t Freiburg, Institut für Mikrosystemtechnik, Georges-Ko €hler-Allee 102, 79110 Freiburg, Germany Universita
h i g h l i g h t s Novel electrolytes based on ethylene carbonate (EC) and dimethyl sulfone (DMSN). Enhanced electrolyte safety based on high flash points. Electrochemical characterization and cell testing of EC/DMSN-based electrolytes. Use of LiPF6, LiBOB and LiDFOB as conducting salts. Investigation of programmed current chronopotentiometry related to Li mobility.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 April 2015 Received in revised form 17 August 2015 Accepted 19 August 2015 Available online xxx
In this study, novel electrolyte mixtures for Li-ion cells are presented which are composed of ethylene carbonate/dimethyl sulfone (80:20 wt./wt.) as a solvent mixture and LiPF6, lithium bis(oxalato)borate and lithium difluoro(oxalato)borate as conducting salts. The main advantages of the solvent mixture are high flash points of >140 C which enhance the intrinsic safety of Li-ion cells while maintaining good cell performance above 0e5 C. The movability of the lithium ions in the electrolyte is investigated via programmed current derivative chronopotentiometry. It is found that pure electrolyte properties cannot necessarily predict the electrolyte behavior in real Li-ion cells but the complex interplay between electrolytes, electrode materials and separators has to be taken into account. Using the newly developed electrolytes, it is possible to achieve C-rates up to 1.5C with >80% of the initial specific discharge capacity (25 C). Within 200 cycles during one month in cell tests (CjjNMC) it is proven that the retention of the specific capacity is >98% of the third discharge cycle in dependence of the conducting salt. © 2015 Elsevier B.V. All rights reserved.
Keywords: Li-ion battery Electrolytes Safety Programmed current chronopotentiometry
1. Introduction The electrolyte formulation for nowadays Li-ion battery electrolytes consists mostly of carbonate solvents, lithium conducting salts and various additives [1,2]. The carbonate solvents include cyclic organic carbonates like ethylene carbonate (EC) or propylene carbonate (PC) and linear carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) or diethyl carbonate (DEC). Unfortunately, linear carbonates (DMC, DEC, EMC) exhibit low boiling and flash points which makes these mixtures and electrolytes highly inflammable. Besides, low boiling points of linear
* Corresponding author. E-mail address: [email protected] (A. Hofmann). http://dx.doi.org/10.1016/j.jpowsour.2015.08.071 0378-7753/© 2015 Elsevier B.V. All rights reserved.
carbonates cause a significant increase in cell pressure based on gaseous solvents at moderate temperatures (<100 C). Additionally, during a fire incident highly toxic fluorine-containing decomposition products can be formed [3e5]. Therefore, high-cost safety devices are necessary in order to ensure the battery safety of a lithium-ion battery during an accident. These devices however lower the gravimetric energy density of the battery pack. Nevertheless, there still remains a risk during unexpected events due to the high energy stored in a lithium ion cell. Intensive research is therefore done to evaluate novel safe solvent and electrolyte mixtures which retain low temperature applicability (<0 C) and cell performance, especially at elevated discharge rates. Some classes of novel solvents which are studied in the last years include sulfolane [6e10], sulfones [11e15], sulfoxides [16], nitriles [17,18], ionic liquids [19e28], esters [29e31], ethers
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[32e35], silicon-based solvents [17,36e39], phosphorus-based solvents [40e42], g-butyrolactone [43e45] or the omit of linear carbonates. Despite of single properties which outperform carbonate mixtures by far, only very few electrolyte formulations have the potential to replace standardly used organic carbonate mixtures in industrial scale so far [46]. Dimethyl sulfone (abbreviated as DMSN to distinguish from dimethyl sulfoxide (DMSO) and dimethyl sulfite (DMS)) exhibits a melting point of ~110 C and is not-miscible with solvents used in Li-ion batteries (sulfolane, DMC, DEC, PC) at common mixing ratios. Due to this, it is expected that it is not suitable as electrolyte component for Li-ion batteries. However, it was recently shown that mixtures composed of EC and DMSN form an eutectic mixture and thus could be an interesting solvent mixture for Li-ions batteries [47,48]. Nowadays, LiPF6 is standardly used as conducting salt in spite of its toxicity and decomposability due to its most balanced properties compared to other lithium salts. Various lithium containing salts had however been investigated with excellent properties in terms of solid electrolyte interphase (SEI), cycle and temperature stability which can be used as additives in electrolyte formulations. Two of these compounds, lithium bis(oxalato)borate (LiBOB) and lithium (difluorooxalato)borate (LiDFOB), are very popular for an effective SEI protection on the surface of graphite and for improving the overall electrolyte properties [49e53]. Therefore, it is expected that these salts can improve new solvent mixtures. In chronopotentiometry techniques, a programmed current function is imposed to an electrode and its potential is measured subsequently as a function of time. In 1960, Reinmuth proposed the technique of linear current scan potentiometry where the current varying linearly with time and the variation of electrode potential is observed [54]. Various electrode reactions can be detected and investigated in dependence of the nature of electrolyte and electrode. However, for a quantitative analysis of such electrode processes, the reactions have to fulfil requirements based on Fick's laws [55]. The aim of this study is to evaluate the usability of novel mixtures composed of EC and DMSN as electrolyte solvents and LiPF6, LiBOB and LiDFOB as conducting salts for Li-ion based cells which can be used at moderate C-rates up to 2C and at temperatures of 0e70 C. The electrolytes are studied based on their electrochemical properties and in the cell system of graphite as anode and LiNi1/3Mn1/3Co1/3O2 (NMC) as cathode material with no further additives. It is investigated if the method of programmed current derivative chronopotentiometry could be helpful in predicting the electrolyte performance. 2. Experimental 2.1. Materials Ethylene carbonate (SigmaeAldrich, 99%, anhydrous) and dimethyl sulfone (Alfa Aesar, 99%) were dried by using a coulommetric Karl-Fischer titrator, which consists of a 831 KF Coulometer and a 860 KF Thermoprep oven from Metrohm. The compounds were placed into the Thermoprep oven (120 C) and a continuous gas flow (dried air) was bubbled through the liquid. The water content of the gas flow was detected in the Coulometer (final drift of <1 mg min1). After drying, the water content of the solvents was determined to be less than 25 ppm. Subsequently, the liquid was degassed with dry argon and placed into the glove box. Dimethyl carbonate (SigmaeAldrich, anhydrous), Lithium hexafluorophosphate (SigmaeAldrich, 99.99%, battery grade, anhydrous), lithium bis(oxalato)borate (Chemetall), lithium difluorooxalato borate (ABCR) and lithium foil (Alfa Aesar,
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thickness ¼ 0.75 mm) were used as received. ELM-0 (1.0 M LiPF6 in EC/DMC; battery grade) was purchased from SigmaeAldrich. The preparation of the electrolytes was performed in an argon-filled glove box (MBraun GmbH) with oxygen and water levels below 0.5 ppm. 2.2. Density measurements The density of the electrolyte mixtures was obtained by repeated measurement of the mass of 75.0 ± 0.2 ml of an electrolyte mixture at room temperature inside the glovebox at 25 C. 2.3. Conductivity measurements The ionic conductivity of the mixtures was measured by the standard complex impedance method, using a Zahner Zennium IM6 electrochemical workstation in the frequency range from 1 kHz to 1 MHz. A 1.6 ml closed cell (850 ml solvent) from RHD instruments is used for the measurements. In the phase minimum of 0 , the ! impedance value Z was used as ohmic resistance ! for calculating the specific conductivity k according to k ¼ C= Z with the cell constant of C ~17 cm1. The exact cell constant (error of 5 measurements <1%) was received by measuring a standard solution (1.413 mS cm1 at 25 C, Hanna instruments, HI 70031). The RHD test cell was filled inside the glove box to exclude water contamination. 2.4. Cyclovoltammetry measurements The cyclovoltammogramms were measured at a Zahner XPOT potentiostat (software: PPSeries, Potentiostat XPot Zahner elektrik 6.4). The potential range was 3e6 V vs. Li/Liþ with platinum as working electrode. The cells were measured in 3-electrode configuration (EE-Cells manufactured by EL-Cell GmbH) with reference and working electrodes composed of lithium. The scan speed applied for all CV tests was 5 mV/s. 2.5. Chronovoltammetry The AljLi anodic oxidation experiments were performed in two electrode configuration (Swagelok® type cells) with Al (d ¼ 12 mm) as working electrode, Li (d ¼ 12 mm) as reference and counter electrode, and a glass fiber separator (Whatman, GF/B, d ¼ 13 mm) in between. The measurements are performed by using a X-POT potentiostat from Zahner-Elektrik. The maximum deviation between same measurements was in the order of ±1.5 mA cm2. 2.6. Viscosity measurements Dynamic viscosity was measured using a Malvern Gemini HR Nano rotational rheometer with 40/1 cone geometry and a gap of 30 mm via a temperature sweep (20e80 C) at a shear rate of 100 s1. These experiments were performed by using a solvent evaporation protecting cover in air. 2.7. DSC measurements The mixtures were investigated by differential scanning calorimetry (DSC) regarding their phase behavior (Netzsch DSC 204 F1 Phoenix®, Al crucible closed). The measurements were carried out from 150 to 100 C at a heating rate of 10 and 20 K/min in argon atmosphere.
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2.8. Flash points The flash points were measured with a Grabner miniflash instrument according to the norm ASTM D6450 (certified reference material: decahydronaphthalene). 2.9. Cell tests In this study, common coin cells (type: CR 2032, Hohsen Corp.) were used with a coin cell crimper from BT Innovations. The cells were assembled in an argon-filled glove box according to standard procedures. Precisely, a graphite anode (Ø ¼ 15 mm), a NMC cathode (Ø ¼ 14 mm), and a glass fiber separator (Whatman®, GF/B and QMA 450; Ø ¼ 16 mm) were used inside a coin type cell with one spring and one stainless steel spacer. Calandered electrodes based on graphite and NMC with a content of approximately 90% of active material were provided in industrial cooperation. 2.10. Galvanostatic polarization Programmed current derivative chronopotentiometry was performed in two electrode Swagelok cells at Zahner Zennium IM6 electrochemical workstation. Lithium metal (foil, Alfa Aesar, Ø ¼ 12 mm) is used as working and reference/counter electrode. It is observed that almost identically values of Imax are detected whether 4 or 6 layers of glass fiber separators (Whatman, GF-B, Ø ¼ 13 mm) were used in the cell (33.5 ± 0.2 mA, test electrolyte: EC/EMC þ 1 M LiPF6). Two layers result in slightly decreased values, thus 4 layers are used for the study. 75 ml of electrolyte solution is used for each separator layer. The cells were prepolarized at b ¼ 1 mA s1 up to 0.7 V and afterwards (after relaxation of the potential) measured at b ¼ 100 mA s1 up to 10 V. 3. Results and discussion 3.1. Electrolyte mixtures In this study, selected electrolyte formulations based on dimethyl sulfone (DMSN), ethylene carbonate (EC) and lithium conducting salts lithium bis(oxalato)borate (LiBOB), lithium (difluorooxalato)borate (LiDFOB) and lithium hexafluorophosphate (LiPF6) are investigated. The chemical formulas of the compounds are depicted in Fig. 1 and the composition of the electrolyte mixtures is listed in Table 1. All concentrations are related to the total mass of the mixture in order that 0.75 mol kg1 corresponds to ~1 mol dm3. 3.2. Physical and chemical properties An overview about fundamental electrochemical and physicochemical data is provided in Table 2. In terms of improved cell safety, the extraordinary high flash point of 142 C of the pure solvent mixture has to be mentioned (Table 2) which was measured according to the standard norm ASTM D6450. It is expected that the addition of conducting salts as LiPF6, LiBOB and LiDFOB will not
affect the flashpoint significantly because it is mainly determined by solvents [18,48]. Compared to ELM-0, the density values of the electrolytes based on EC/DMSN (ELM-1 e ELM-4) are in the order of 1.38 ± 0.02 g cm3 which is slightly increased compared to mixture ELM-0. This gives rise to a decrease in the gravimetric energy density of the battery, if other factors are in the same order of magnitude. However, the intrinsic improve of safety by using lowflammable solvents influences the whole safety concept which can affect the battery weight, too. Differential scanning calorimetry (DSC) measurements of the solvent mixtures and the electrolytes are shown in Fig. 2. All measurements are done in closed Al crucials under argon atmosphere. A shift to lower melting and crystallizing temperatures (approximately DΤ ¼ 10e20 C) is observed from pure solvent mixtures (LM-1, LM-2) to salt concentrated electrolytes. DSC measurements can however reveal quite low crystallizing temperatures which result from supercooled meltings and consequently overestimate temperature working conditions for Li battery cells [56]. Thus, small seed crystals which are in battery cells in terms of electrode or separator materials can result in higher crystallizing temperatures close to the melting temperature. It is observed that electrolyte ELM-4 can be used in cell testings (see below) down to 0 Ce5 C in spite of an onset in the crystallizing temperature at 12 C. Here, the onset of the melting point (þ1.6 C) is a better measure than the crystallizing temperature.
3.3. Viscosity and conductivity measurements Temperature-dependent viscosity and conductivity measurements of the electrolytes are shown in Fig. 3. The ionic conductivity is decreased by a factor of ~2 (20 C) compared to mixture ELM0 regardless of whether pure LiPF6 or mixtures of LiPF6, LiBOB and LiDFOB are used as conducting salts. The dynamic viscosity of the novel electrolytes ELM-1 e ELM-4 is enhanced by a factor of ~2.5 (20 C) compared to ELM-0. All in all the commercial electrolyte ELM-0 exhibits higher ionic conductivities and lower viscosities caused by the low viscous DMC. Viscosity values of <10 mPa s at 25 C of the novel electrolytes are lower than sulfolane (SL) based electrolytes (e.g. 1 mol kg1 LiTFSI in SL: ~50 mPa s [57]; 1 M LiPF6 in SL: ~31 mPa s [58]) or ionic liquid based electrolytes (e.g. 0.5 M LiTFSI in trimethyl-n-hexylammonium-TFSI: ~300 mPa s [59]; 0.75 M LiTFSI in trimethyl-n-butylammonium-TFSI: 360 mPa s [60]; 1 M LiTFSI in 1-ethyl-3-methylimidazolium-TFSI: ~200 mPa s [61]; 1 M LiPF6 in N-methyl-N-propylpyrrolidinium-TFSI/EC (1:1 wt.): ~20 mPa s [62]; 1 mol kg1 LiTFSI in sulfolane/diethylmethyl-(2-methoxy)-ethylammonium TFSI (2:1 wt.): 67 mPa s [57]) and gives rise to a promising cell performance in Li-ion cells. The dynamic viscosity values are in the same order of magnitude compared to pure propylene carbonate (PC)-based electrolytes (e.g. 0.7 M LiBOB in PC: ~7.3 mPa s [63]; 1 M LiClO4 in PC: 8.1 mPa s [64]). Temperature-dependent viscosity data reveal a more temperaturedependent behaviour of electrolyte ELM-1 e ELM-4 compared to mixture ELM-0. This can be quantified by calculating the quotient h20-80 ¼ h(20 C)/h(80 C) which yields h20-80 ¼ 2.5 (ELM-0) and h2080 ¼ 5.8 ± 0.2 (ELM-1 e ELM-4). All observations lead to the assumption that the formation of small ionic clusters or molecular
Fig. 1. Chemical structures of EC, DMSN, LiBOB, LiDFOB and LiPF6.
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Table 1 Composition of the electrolyte formulations. Electrolyte
Solvent
Ratio wt./wt.
LiPF6 mol kg1
LiBOB mol kg1
LiDFOB mol kg1
[Liþ] mol kg1
LM1 LM2 ELM-0 ELM-1 ELM-2 ELM-3 ELM-4
EC/DMC EC/DMSN EC/DMC EC/DMSN EC/DMSN EC/DMSN EC/DMSN
50:50 20:80 50:50 80:20 80:20 80:20 80:20
e e 0.787 0.787 0.65 0.65 0.65
e e e e 0.1
e e e e e 0.1 0.05
e e 0.787 0.787 0.75 0.75 0.75
0.05
Table 2 Physicochemical properties of electrolyte mixtures. TK crystallizing temperature; Tm melting point; fp. flash point; d density; h viscosity; k conductivity. Electrolyte
LM-1
LM-2
ELM-0
ELM-1
ELM-2
ELM-3
ELM-4
mp./ C (DSC) Freezing Point/ C (DSC) fp./ C d/g cm3 (25 C) (±0.01 g cm3) h/mPa s (20 C) (±0.5 mPa s) k/mS cm1 (20 C) (±0.05 mS cm1) Imax/mA at b ¼ 100 mA s1 (±0.7 mA)
0.6 32.7 24 1.20 1.7 e e
36.1 9.1 142 1.32 4.4 e e
19.4 56.1 e 1.27 4.4 10.67 34.5
16.0 26.1 e 1.40 11.4 5.95 28.4
18.1 12.3 e 1.39 11.0 5.38 28.4
18.6 19.1 e 1.36 11.4 5.95 27.5
18.8 17.3 e 1.37 11.4 5.76 27.7
Fig. 2. Differential scanning calorimetry data of solvent and electrolyte mixtures Fig. 2a: Heating-up curves in argon atmosphere at 10 K min1 (closed Al cap). Fig. 2b: Cooling down curves at 10 K min1 (closed Al cap).
aggregates is more pronounced in the novel electrolyte formulations compared to less viscous mixture ELM-0. A correlation between conductivity and viscosity data is taken into account within the Walden rule which relates the fluidity h1 to the limiting molar conductivity L0m [65]. Applying an additional parameter a, the so called fractional Walden rule (Equation (1)) is obtained [48,66e69]. For linear fittings of electrolyte mixtures, the limiting molar conductivity L0m was replaced by the molar conductivity Lcs m which consists of the amount of substance of the conducting salts (cs).
log L0m ¼ log C 0 þ a log h1
(1)
The Walden plots are depicted in Fig. SI-1 (supporting information) in the temperature range of 20e80 C and the fitting parameters are listed in Table 3. The results show an excellent matching of the data within a Walden fitting procedure, where the parameter a is reduced in case of the novel electrolytes (a z 0.65) compared to ELM-0 (a ¼ 0.96). This is in accordance with the results which are received for EC/DMSC mixtures with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) or LiBF4 as conducting salts [48]. A negative deviation of the parameter a from 1 is indicative that the conductivity is determined not only from the viscosity [66], but rather from ion clustering and the interaction of ionic species
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Fig. 3. Temperature dependent viscosity h (t ¼ 100 s1) (Fig. 3a) and conductivity k (Fig. 3b) data of the electrolyte mixtures.
with an external field [70]. In accordance to Lee et al., the electrolytes can be classified as solvent-co-solvent systems [58]. It can be concluded that the conductivity and rheology measurements provide an indication of ionic solvation (aggregates/clusters) in the electrolyte mixture. The addition of up to 13 mol-% of both or either LiBOB and LiDFOB to the conducting salt LiPF6 has no significant effect on the viscosity and the conductivity measurements. 3.4. Lithium movement The measure of Li transference numbers which is the fraction of the total current carried by a selected ion in an electrolyte can be achieved by different methods including pulsed field gradient nuclear magnetic resonance, Bruce-Vincent/potentiostatic polarization, galvanostatic polarization, moving boundary method and electromotive force method. Significant differences among these methods are found on account of different assumptions and experimental conditions during the measurement [1,71,72]. Additionally, prerequisites are necessary which restrict the electrolyte formulations. In the end, the behavior of the electrolyte in the Liion cell and in particular the Li-cell performance can be predicted only in a very limiting manner based on the knowledge of Li transference numbers. Therefore, another technique is investigated here to prove the accessible cell performance enabled by the electrolyte. A measure of the lithium mobility in the electrolyte is investigated with programmed current derivative chronopotentiometry [55,73]. A current I respectively current density j that increases linearly with time t (I(t) ¼ b·t) is imposed to LijLi (Ø ¼ 12 mm) Swagelok-cells with several layers of glass fibre separators. Principally, a current corresponds to an electrode reaction which is enabled by a movement of Li-ions from one Li electrode to the other Li electrode. On account of solvation effects, lithium dissolution, lithium plating or interactions with separator material in the electrolyte which hamper the movement of the Li-ions, the observed potential increases with ascending current. Above a certain applied current limit (Imax) the voltage increases in a
dramatic fashion because no more Liþ ions can be delivered by the electrolyte. That way, Imax corresponds to the maximum accessible lithium ion flux under applied current conditions. Certainly, this assumption is only valid if other effects like lithium plating/dissolution are not rate-determining. It was proven by using one-sided smaller electrode areas that higher current densities can be received which indicates that Li dissolution and Li platting are not rate determining reactions (see supporting information). Ideally, at the beginning of the experiment the ionic polarization inside the cell should be in the same order of magnitude meaning a prepolarization at 1 mA s1 is performed up to identical potential differences (0.7 V vs. Li/Liþ). In case of lithium as active electrode material, the electrode area A varies within the experiment because of lithium dendritic growth. Additionally, a continuous decrease of the LieLi distance due to dendritic Li deposition affects the electric field inside the cell. Nevertheless, polarization experiments reveal that these effects are small compared to the voltage jump due to discontinued Liþ flux (see supporting information). However, these effects significantly complicate theoretical derivations based on Fick's law [55] in case of lithium electrodes, though. Finally, it is investigated, if there are differences from EC-DMC electrolyte and if
Table 3 Results of Walden fitting procedures. Sample
cc.s./mol dm3
Slope
ELM-0 ELM-1 ELM-2 ELM-3 ELM-4
1.008 1.102 1.042 1.020 1.028
0.96 0.64 0.66 0.64 0.64
± ± ± ± ±
R2 0.03 0.02 0.01 0.01 0.01
0.997 0.998 0.999 0.999 0.999
Fig. 4. Potential (vs. Li/Liþ) versus current during programmed-current chronopotentiometry (working electrode: lithium, counter/reference electrode: lithium, four-layer glass fiber separators GF/B; b ¼ 100 mA s1).
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a qualitative comparison between the electrolytes is possible. The diagrams of the potential response versus current are shown in Fig. 4. For each electrolyte mixture, three measurements are performed which result in comparable voltage responses. The results of the measurements are listed in Table 2, too. The novel electrolytes result in comparable values of Imax ¼ 28 ± 1 mA which is significantly reduced compared to EC/DMC þ 1 M LiPF6 (Imax ¼ 35.5 ± 0.5 mA). Principally, two plateaus in the potential can be observed which represent electrode reactions. It is found that the value of Imax is dependent on the current rate b and should be mentioned within a series of experiments. Here, b ¼ 100 mA s1 is chosen based on a balance between dendritic growth and reproducibility. Based on the experiments it can be expected that the performance of the novel electrolyte mixtures in Li-ion-cells is inferior to ELM-0. Additionally, one would expect similar performance of ELM-n (n ¼ 1e4) values in Li-ion-cells in case of a rate determining by the Li mobility. 3.5. Electrochemical stability The electrochemical stability of the electrolytes is investigated against platinum working electrodes within a potential range of 3e6 V vs. Li/Liþ. It should be mentioned that one cannot predict the oxidative stability against electrode materials based on the measurement against platinum electrodes but an estimation about the principal electrochemical stability can be obtained. EC as well as DMSN should withstand high oxidative potentials up to 4.5 V vs. Li/ Liþ which have to be fulfilled in Li-ion cells [47,58]. In Fig. 5, the current density responses of the novel electrolytes are shown in dependence on the potential. The first and the third potential sweep are both depicted for comparison. It can be observed that the electrolyte mixtures exhibit a significant increase (>50 mA cm2) in current density response starting at 5.5 V vs. Li/Liþ. A significant enhanced current density response is observed for LiBOB or LiDFOB containing samples ELM-3, ELM-4 and ELM-5. However, an excellent oxidative stability is obtained in a potential range up to 5 V vs. Li/Liþ for all electrolytes which makes them prospective candidates for high voltage electrolyte applications. In Li-ion cells, the electrolyte is in permanent contact with aluminum. Therefore it is a necessity that the protection of aluminum is sufficient at operating cell potentials. The oxidative dissolution stability of battery grade aluminum is investigated in chronoamperometry measurements. The potential is set to 4.3 V vs. Li/Liþ and 4.7 V vs. Liþ, respectively, for a time period up to 24 h to evaluate the current density response. Both diagrams are plotted in Fig. 6. It can be observed that an excellent stability of Al is obtained up to 4.7 V vs. Li/Liþ. It should be noted, that the error of the measurements is in the order of 1.5 mA cm1 based on the Swagelok type cell design. Very low current density responses can be caused by little electrolyte decomposition, impurities, leakage current and different formation of surface layers onto Al and Li. The findings are verified by optical analyses via optical microscopy of the Al disc which give no rise to any Al dissolution/corrosion (Fig. SI-5, supporting information). The excellent protection of aluminum by addition of LiBOB or LiDFOB [49,57,74,75] is confirmed in order that the aluminum inside the cell should resist high potentials up to 4.7 V vs Li/Liþ when the electrolytes are used. 3.6. Cell cycling tests The electrolytes are investigated in cell tests with graphite and NMC (LiNi1/3Mn1/3Co1/3O2) as electrode materials and glass fiber separators. In Fig. 7, the first cycles are compared with respect to formation (Fig. 7a) and capacity loss (Fig. 7b). It can be seen that LiBOB or LiDFOB containing electrolytes exhibit a plateau around
Fig. 5. Cyclovoltammetry of Ptjj Li cells (working electrode: Pt, counter electrode: lithium, reference electrode: Li, separator: glass fibre separator, 1 mV s1). The first (1, black line and dotted line) as well as the third (3, reddish dashed/dotted and fine dotted line) CV spectra are shown for comparison. The scale of the oxalato-free mixtures (ELM-0 and ELM-1) were adjusted for a better comparison.
2e2.2 V vs. Li/Liþ which can be attributed to a reduction of oxalate molecular moieties [49,52,76]. In Fig. 7b it can be seen that the combination of both salts (LiBOB and LiDFOB) in the EC/DMSN mixture reduces significantly the capacity loss during the first cycle and increases the available specific capacity to ~150 mAh g1 which is in the same order of magnitude compared to ELM-0 (EC/ DMC þ 1 M LiPF6). In the third cycle, almost 100% in capacity retention (discharge to charge cycle) is received in all EC/DMSN electrolytes. It can be concluded that the novel electrolytes exhibit good cycling properties within the first cycles. In Fig. 7c, the cycling up to 200 cycles is depicted and the electrolyte ELM-0 is compared within the first 100 cycles. It is observed that different cell performances can be seen especially at relatively fast discharging rates. This is in contrast to the findings of the programmed current chronopotentiometry measurements (Fig. 4) which reveal a similar lithium transport behavior. Therefore it is expected that the Liþ transport is heavily influenced from the boundary layer of the electrodes (solid electrolyte interphase, SEI) which is different for each mixtures based on different SEI compositions. These effects are supposed to be much more pronounced and rate determinant in the cell than the
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Fig. 6. Chronoamperograms of AljjLi cells to evaluate the aluminum anodic dissolution limit of the electrolytes. Fig. 6a: Applied potential is 4.3 V vs. Li/Liþ. Fig. 6b: Applied potential is 4.7 V vs. Li/Liþ.
Fig. 7. NMCjjC full cell tests with electrolytes and glass fiber separators. In Fig. 7a, the first cell charging is depicted (same behavior is observed within three cells with same electrolyte). The charging is done at 0.05C. In Fig. 7b, a comparison in charging (c) and discharging (dc) specific capacity is shown within the first three cycles. The deviation of three cells with same electrolytes is <2.5 mAh g1. First cycle at 0.05/0.1C, second and third cycle at 0.1/0.2C (charge/discharge) (coin cells CR 2032; CC charging only). Fig. 7c: Measurement of the specific discharge capacity of NMCjC coin cells. The charging/discharging characteristic is mentioned as charging/discharging C-rate. Starting from 100 cycles, CCCVcharging is used (Ie ¼ C/15). All tests (Fig. 7aec) are done at 25 ± 1 C.
transport of the lithium through the electrolyte and separator. However, only glass fiber separator is studied so far, thus other kind of separators may result in stronger or weaker interactions to Liions. The best performance in cell tests is obtained with LiBOB/ LiDFOB mixed electrolytes compared to ELM-0. The best capacity
retention after 200 cycles is delivered by electrolyte ELM-2 (>97% compared to the fifth cycle directly after the formation), whereas the performance is significantly reduced compared to LiDFOB containing electrolytes at fast discharge rates. Additionally it is observed that the fading in capacity retention is most pronounced
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when the lithium concentration decreases and the charging rate is increased. When the charging rate exceeds ~1.5C, a significant fading is found for the electrolytes. For all electrolytes same electrode compounds and separator materials are used. It is investigated that the electrolytes can be used also at decreased temperatures. Therefore, the best electrolyte ELM-4 is investigated in discharge cycles at selected temperatures to quantify the temperature-dependence of the cell performance (Fig. 8). It can be observed that the discharge capacity decreases at lower temperature. In this case, the movement of the lithium ions inside the cell (in the electrolyte as well as into the electrode material/SEI) is reduced. At 1C discharge rate, almost 50% of the initial capacity (at 25 C) can be used at 5 C. Below 5 C it is observed that the discharge capacity decreases very rapidly. It is expected that the electrolyte mixture becomes highly viscous below 5 C based on the melting point of the mixture. It should be noted that the freezing point in DSC studies is detected to be 17 C. However, a supercooled melting is presumably detected based on the lack of seed crystals in the DSC vials in contrast to the lithium ion cells. It is observed that the decrease in discharge capacity is much less pronounced at discharge rates of 0.5C. Almost the full capacity (>90%) can be extracted at 5e10 C. However, at 0.5C a disruption in discharge capacity is found below 5 C, too. It should be mentioned that fast (>1C) as well as moderate (e.g. 0.5C) charging at low temperatures (<10 C) increases significantly the risk of dendritic lithium deposition onto graphite.
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surface effects and conditions (e.g. SEI layer) come into play. The performance of the electrolytes is quantified in full cell tests against graphite and NMC material whereas an excellent discharge capacity performance is obtained up to a discharge rate of 2C at room temperature for these flame-resistant electrolyte mixtures. It is shown that the electrolytes can be used down to ~5 C where still >90% of discharge capacity can be used at 0.5C. Based on the properties found in this study, the electrolyte mixture could be of high interest also for other applications like supercaps or other Li-based technologies, too. Since only one component compared to standardly used electrolyte mixtures (linear/cyclic carbonates þ LiPF6) is replaced in the novel electrolyte formulations, these mixtures might be of interest also for individual applications in industrial purpose (e.g. in-house energy storage systems in combination with autonomous energy harvesting devices). Acknowledgements AH acknowledges support by Deutsche Forschungsgemeinschaft (Sachbeihilfe, HO 5266/1-1). We thank Chemetall for providing LiBOB and Matthias Migeot for performing selected electrochemical measurements. We acknowledge Kerstin Prengel for proofreading. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.08.071.
4. Conclusion References In this study, novel electrolyte mixtures based on EC/DMSN (80:20 wt./wt.) and LiPF6 are presented with focus on usability for Li-ion cells. LiBOB and LiDFOB are investigated as conducting salt additives to improve the cell performance. The electrolytes exhibit excellent oxidative stabilities which is proved against platinum and aluminum up to 4.7 V vs. Li/Liþ. It could be demonstrated that programmed current derivative chronopotentiometry measurements can be used to qualitatively compare different electrolytes with respect to the lithium ion movability. However it is shown that the mobility in the electrolyte can differ from real cells when various
Fig. 8. Temperature-dependent performance of electrolyte ELM-4 in NMCjC cells during discharge at 1C (black squares). Three individual cells are charged at 0.2C at 25 C and discharged at selected temperatures (blue circle). The measurements are performed after 200 cycles (cell tests according to Fig. 7c). The deviation between the measurements are <5%. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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