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Nov 2, 2013 - Herein we demonstrate the combination of in situ 1D imaging and slice-selective NMR diffusion measurements as a tool for the spatially a...
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Slice-Selective NMR Diffusion Measurements: A Robust and Reliable Tool for In Situ Characterization of Ion-Transport Properties in Lithium-Ion Battery Electrolytes Sergey A. Krachkovskiy,† Allen. D. Pauric,† Ion C. Halalay,‡ and Gillian R. Goward*,† †

Department of Chemistry, McMaster University, 80 Main Street West, Hamilton, Ontario L8S 4L8, Canada General Motors Global R&D, 30500 Mound Road, Warren, Michigan 48090-9055, United States

‡

S Supporting Information *

ABSTRACT: The main impediments to the widespread acceptance of electric drive vehicles are the cost, energy-storage capacity, and durability of portable electrical energy sources and, in particular, batteries. In situ experimental techniques that can accurately detect and monitor performance degradation mechanisms on the nanoscale, including the identities of short-lived chemical species and changes in materials properties as a function of cycling rate, temperature, or time, are not widely used. Herein we demonstrate the combination of in situ 1D imaging and slice-selective NMR diffusion measurements as a tool for the spatially and temporally resolved determination of lithium diffusivities in a conventional liquid electrolyte (1.0 M lithium bis(trifluoromethanesulfonyl)imide solution in propylene carbonate) under application of a constant electrical current. All experiments were carried out using standard NMR equipment, so the proposed technique can be easily implemented in any modern R&D facility. SECTION: Energy Conversion and Storage; Energy and Charge Transport

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that this dependence is nonlinear, which leads to changes in the average value of diffusion coefficients measured by NMR. Complementary to the eNMR studies of electrolyte performance, there have been several successful implementations of in situ NMR and magnetic resonance imaging to monitor Li-insertion mechanisms during cycling of lithium-ion electrode materials.10−12 Also, recently using in situ 7Li NMR imaging, Klett et al. have demonstrated the formation of the concentration gradient in lithium ion battery electrolyte upon the application of an electric field.13 Moreover, the salt diffusivity and Li+ transport number were obtained from fitting these data. While ion pairing is a known limitation of the investigation of diffusion coefficients by NMR, in the case of eNMR, the neutral ion pairs will not be affected by the applied potential and thus will not contribute to the time-dependent concentration gradient. The main advantage of the imaging approach in comparison with the direct eNMR technique is that it does not require application of large electric potential to detect the signal evolution, mitigating electrolyte degradation on the surface of the electrodes. Moreover, an interesting outcome of the results shown below is the observation of a gradient of diffusion coefficients, which directly reflects the impact of the Li+ transference numbers on the ion dynamics. Therefore, this technique appears promising for characterization of diffusional parameters of electrolytes under

he design and optimization of nonaqueous lithium ion battery electrolytes requires methods for measuring all relevant diffusional parameters, including diffusion constants and ionic transference numbers. Low diffusivities are often associated with a high dynamic viscosity and are indicative of a low lithium-ion conductivity, while a low lithium transference number leads to the formation of a concentration gradient, which in turn results in an electromotive force that acts in opposition to the desired current direction. Pulsed-field gradient nuclear magnetic resonance spectroscopy (PFG NMR) is the premiere tool for the study of self-diffusion and transport of mobile species in condensed matter.1,2 The main advantage of the NMR, compared with electrochemical methods, is its possibility to provide ion-specific information on the transport parameters. Furthermore, NMR experiments in the presence of an electric field (electrophoretic NMR, or ENMR) create the possibility to also determine ionic mobilities and transference numbers.3,4 ENMR was successfully used to determine ion migration parameters in common Li-ion battery electrolytes.5,6 However, in previous studies, the data were analyzed under the assumption that the diffusion parameters are constant during the time when an electric field is applied. Meanwhile, taking into account the fact that in typical lithium electrolyte solution the cation transference number t+ ranges from 0.3 to 0.4,7 one can expect the formation of a significant concentration gradient across a Li-ion cell. Concentration dependence of Li + diffusivities was previously reported,8,9 and it is noteworthy © 2013 American Chemical Society

Received: September 30, 2013 Accepted: November 2, 2013 Published: November 2, 2013 3940

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application of low voltage potential. However, the analysis was carried out again under the assumption that the parameters are independent of concentration. Here we combine NMR imaging with slice-selective diffusion measurements and show that differences in ion diffusivities at different points along the electric field direction inside the sample are significant enough to be taken into account in mass transport models. The ability to measure migration parameters simultaneously with the formation of a concentration gradient is highly valuable for the analysis of electrolytes with multicomponent transport because for such systems it is impossible to prepare a set of samples with all possible concentrations of components that may arise under the operating conditions of an electrochemical cell. Our experiments were carried out with a conventional 5 mm NMR probe. This positions our approach as a simple and robust method for comparing candidate electrolytes and for validating electrochemical models describing battery performance. A set of four calibration samples consisting of 0.2, 0.8, 1.2, and 1.8 M solutions of lithium bis(trifluoromethanesulfonyl)imide lithium salt (LiTFSI) in propylene carbonate (PC, Sigma Aldrich) was prepared for determining the concentration dependence of the cation and anion diffusion coefficients. The 1.0 M LiTFSI/PC solution was chosen as a test electrolyte for the proposed method, consisting of in situ 1D 7Li NMR imaging and slice-selective NMR diffusion measurements. The self-diffusion coefficients of the ions were measured using standard PFG NMR technique14 for the set of LiTFSI in PC calibration solutions. Diffusivities of the lithium cation (red) and the TFSI anion (blue) are plotted versus the salt concentration in Figure 1. They are monotonically decreasing

Table 1. Concentration Dependence of Self-Diffusion Coefficients and Lithium Transport Numbers DLi+ × 10−10 m2 s−1 DTFSI− × 10−10 m2 s−1 τ+

0.2 M

0.8 M

1.2 M

1.8 M

1.73 2.79 0.38

0.91 1.45 0.39

0.67 1.06 0.39

0.39 0.56 0.41

transport, which represents transference numbers only in the case of negligible ion pairing, as previously described by Stolwijk et al.16 Nevertheless, the emphasis here is on the NMR observation of time-dependent concentration changes and their impact on the resulting diffusion coefficients. Therefore, for the present purpose, an estimation of transference number by eq 1 is sufficient to illustrate the main reason for concentration gradient formation. The following trend is apparent: the higher the salt concentration, the stronger the ion pairing and, therefore, the more similar the self-diffusion coefficients will be.17 In the extreme case (at high lithium concentration) when DLi+ = DTFSI−, τ+ is equal to 0.5. However, over the concentration range examined in our experiment, the lithium transport number is close to 0.4 and increases only a little with salt concentration. These results are in good correlation with the data published previously by Aihara et al.9 At the same time, diffusivities are much more sensitive to the environment. For example, the difference between DLi+ (0.8 M) and DLi+ (1.2 M) is more than 30%, which is 10 times larger than the experimental error of the NMR diffusion measurement. Therefore, while the use of a constant transference number in models of mass transport in lithium ion battery is an adequate approximation, it is necessary to apply a more rigorous approach for the self-diffusion coefficient, which includes its dependence on salt concentration. For that purpose, we suggest using the combination of NMR imaging, slice-selective NMR, and PFG NMR to determine the spatial distribution and temporal evolution of the lithium ion concentration and diffusion coefficient in an electrolyte solution of an electrochemical cell. The self-diffusion coefficient of the lithium ion measured in a 1.2 M LiTFSI solution in the absence of an applied electric field for slice sizes of 20, 4, and 2 mm yielded values of 0.67, 0.65, and 0.65 × 10−10 m2 s−1, respectively. A reduced sensitivity, and therefore an increased experimental error, is the cost for the decreasing the slice thickness. One can, however, see that all diffusivity values deviate no more than 3% from each other, which is within the estimated error for conventional PFGmeasurements. Hence a 2 mm slice size was chosen as optimal for the present in situ experiments because it is rather small, thus allowing for a good spatial resolution of the diffusion coefficient while also providing an adequate signal/noise ratio. Using a total of 16 scans, we were able to obtain a reliable set of data with only 5 min of NMR data acquisition time for one diffusion coefficient, which demonstrates the efficiency of the method. Electrophoretic NMR experiments were carried out using the DC-NMR cell constructed according to the design by Hallberg et al.18 and shown in Figure 2. We used a standard 5 mm NMR tube with two insulated Pt lead wires inside it. A bare length at the end of one of the insulated Pt lead wires, formed into a loop perpendicular to the tube axis at the top of the tube, served as one electrode. A piece of metallic lithium, placed at the bottom of the NMR tube and connected to the other Pt wire, was the

Figure 1. Self-diffusion coefficients of the lithium cation (red) and the TFSI anion (blue) versus salt concentration.

with increasing salt concentration. Although the lithium ion is much smaller than the TFSI− anion (van der Waals radii of 0.076 and 0.326 nm, respectively) the order of the coefficients is DLi < DTFSI over the examined concentration range because the solvation shell changes the hydrodynamic radii of the charged species. For example, Ganesh et al. reported that Li+ in LiPF6/PC solutions is tetrahedrally coordinated by four PC molecules.15 DLi τ+ = DLi + DTFSI (1) Lithium transport numbers were calculated from the ion diffusion coefficients according to eq 1 and are listed in Table 1. Note that this is only a parameter representative of mass 3941

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due to TFSI−. During the experiment Li+ ions accumulate near the metallic lithium electrode (at z = −10) while, at the same time, their concentration near the platinum electrode (at z = +10) decreases. This creates a concentration gradient inside the cell in the presence of an applied electric field. As expected, the image obtained without an applied current (light purple line in Figure 3, corresponding to t = 0) is practically rectangular. Small deviations can be explained by the inhomogeneity of the radio-frequency field strength and sensitivity issues.20 The shape of the concentration profile clearly changes systematically with the time. As one can see, the most significant changes in lithium concentration happen closest to the electrodes, while in the central part of the cell concentration is fairly stable (more shallow gradient, slower change over time). This observation is consistent with the Maxwell−Stefan equation.21 Therefore, processes occurring in the slices next to the electrodes are the most significant ones to describe and understand to elucidate issues related to battery performance. For that purpose, we have chosen two slices near the electrodes. Using the obtained images as guides, the frequency offset for the slice-selective pulses was chosen such that it saturated an area at a distance of 2 mm from each electrode to avoid uncertainties caused by magnetic field disturbances at the electrode−electrolyte interfaces (Figure 4).

Figure 2. The electrophoretic sample cell based on a conventional 5 mm NMR tube.

other electrode. A constant current of 100 μA, with a direction chosen to strip Li+ ions from the Li electrode and deposit them on the Pt electrode, was applied to the sample cell using an AUTOLAB PGSTAT30 instrument. The applied electric potential was varied between 0.45 and 0.55 V to maintain a constant current during the experiment. (See the Supporting Information for more details regarding the cell design and experimental conditions.) It is important to note that under the small applied current any heating effects are assumed to be negligible, as has been previously reported.19 The eight 7Li NMR images collected under the application of the constant current of 100 μA are shown in Figure 3. Two

Figure 4. 1D 7Li NMR image acquired after passing a 100 μA current for 21 h. The positions of the two 2 mm slices in which Li+ diffusion coefficients were measured are also indicated.

For the image measured at 15 h and the slices selected as shown above, average lithium concentrations on the Li and Pt electrode sides of the cell were 1.07 and 0.87 M, respectively. For the image at 21 h, the same slices now have Li+ concentrations of 1.16 and 0.80 M. The Li self-diffusion coefficients measured by slice-selective NMR under those conditions are shown in Figure 5 and Table 2. As expected from the measurements of diffusion coefficients in reference samples (Figure 1), the difference in diffusivities increases from 10 to 30% as the difference between lithium salt concentrations at opposite ends of the cell increases. This difference is substantially larger than the experimental error, estimated as 3%. In addition, these data fit perfectly on the curve obtained from the analysis of the calibration solutions (Figure 5). As well, the correlation to the calibration curves indicates that any heating effects are negligible in this experimental design. Therefore, for an accurate characterization of electrolytetransport properties, one can either use the slice-selective in situ NMR technique presented in this paper or perform diffusion experiments on many samples with different salt concentrations. While the method presented here is preferable in terms of efficient use of experiment time alone, it is especially powerful in the case of multicomponent mixtures, where concentration gradients for several components are building up

Figure 3. 1D 7Li NMR images acquired every 3 h with a constant 100 μA current applied to the cell. The image corresponding to t = 0 was acquired before the application of an electrical potential to the cell.

main processes can be identified. The first is dendritic lithium deposition at the platinum electrode (located at z = +10 in Figure 3). As a result, the total free electrolyte volume within the RF coil (i.e., not contained inside the extended dendritic Li aggregate deposited at the Pt electrode) is reduced, and the gradient near the Pt electrode shifts continuously toward the Li electrode. Dendritic Li deposition is the limiting factor that prevents us from applying higher currents and the development of a steeper concentration gradient. Future experiments will be carried out with nonblocking electrodes made of Li-ion battery anode and cathode materials to eliminate Li deposition at the electrode surface and better reproduce conditions extant in a Li-ion battery. The second process, which is visible from the NMR imaging, is the development of a concentration gradient in the electrolyte along the electric field (axial, or z) direction. Table 1 shows that for the set of calibration samples, only ∼39% of the current is carried by the lithium cations in the 1 M LiTFSI/PC solution, while the remaining fraction of ∼61% is 3942

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Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (905)-525-9140, x24176. Fax: (905)-522-2509. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Prof. Ken Jeffrey, for helpful discussions, and acknowledge funding through the NSERC APC program, and also GM of Canada.

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Figure 5. Self-diffusion coefficient of the lithium cation versus salt concentration obtained for the calibration samples (thick trend line) and two slices next to each electrode inside the cell after applying a constant current of 100 μA for 15 (green) and 21 (blue) hours. Data from the slice adjacent to the Li electrode are indicated in open circles, noted DLi+@Li, whereas data from the slice adjacent to the Pt electrode are indicated in filled circles, noted DLi+@Pt. Data for the calibration samples (measured separately, without applied field) are from Figure 1.

(1) Hahn, E. Spin Echoes. Phys. Rev. 1950, 80 (4), 580−594. (2) Stejskal, E. O. Use of Spin Echoes in a Pulsed Magnetic-Field Gradient to Study Anisotropic, Restricted Diffusion and Flow. J. Chem. Phys. 1965, 43 (10), 3597−3603. (3) Holz, M.; Lucas, O.; Muller, C. NMR in the Presence of an Electric-Current - Simultaneous Measurements of Ionic Mobilities, Transference Numbers, and Self-Diffusion Coefficients Using an Nmr Pulsed-Gradient Experiment. J. Magn. Reson. 1984, 58 (2), 294−305. (4) Dai, H. L.; Zawodzinski, T. A. Determination of Lithium Ion Transference Numbers by Electrophoretic Nuclear Magnetic Resonance. J. Electrochem. Soc. 1996, 143 (6), L107−L109. (5) Hayamizu, K.; Seki, S.; Miyashiro, H.; Kobayashi, Y. Direct In Situ Observation of Dynamic Transport for Electrolyte Components by NMR Combined with Electrochemical Measurements. J. Phys. Chem. B 2006, 110 (45), 22302−22305. (6) Hayamizu, K.; Aihara, Y. Correlating the Ionic Drift under Pt/Pt Electrodes for Ionic Liquids Measured by Low-Voltage Electrophoretic NMR with Chronoamperometry. J. Phys. Chem. Lett. 2010, 1 (14), 2055−2058. (7) Gores, H. J.; Barthel, J.; Zugmann, S.; Moosbauer, D.; Amereller, M.; Hartl, R.; Maurer, A. Handbook of Batter Materials, 2nd ed.; Daniel, C., Ed.; WileyVCH: Weinheim, Germany, 2011. (8) Takeuchi, M.; Kameda, Y.; Umebayashi, Y.; Ogawa, S.; Sonoda, T.; Ishiguro, S.-i.; Fujita, M.; Sano, M. Ion−Ion Interactions of LiPF6 and LiBF4 in Propylene Carbonate Solutions. J. Mol. Liq. 2009, 148 (2−3), 99−108. (9) Aihara, Y.; Sugimoto, K.; Price, W. S.; Hayamizu, K. Ionic Conduction and Self-Diffusion Near Infinitesimal Concentration in Lithium Salt-Organic Solvent Electrolytes. J. Chem. Phys. 2000, 113 (5), 1981−1991. (10) Letellier, M.; Chevallier, F.; Clinard, C.; Frackowiak, E.; Rouzaud, J.-N.; Béguin, F.; Morcrette, M.; Tarascon, J.-M. The First In Situ 7Li Nuclear Magnetic Resonance Study of Lithium Insertion in Hard-Carbon Anode Materials for Li-Ion Batteries. J. Chem. Phys. 2003, 118 (13), 6038−6045. (11) Chandrashekar, S.; Trease, N. M.; Chang, H. J.; Du, L. S.; Grey, C. P.; Jerschow, A. 7Li MRI of Li Batteries Reveals Location of Microstructural Lithium. Nat. Mater. 2012, 11 (4), 311−315. (12) Gerald, R. E.; Sanchez, J.; Johnson, C. S.; Klingler, R. J.; Rathke, J. W. In Situ Nuclear Magnetic Resonance Investigations of Lithium Ions in Carbon Electrode Materials Using a Novel Detector. J. Phys.: Condens. Matter 2001, 13 (36), 8269−8285. (13) Klett, M.; Giesecke, M.; Nyman, A.; Hallberg, F.; Lindstrom, R. W.; Lindbergh, G.; Furo, I. Quantifying Mass Transport During Polarization in a Li Ion Battery Electrolyte by In Situ 7Li NMR Imaging. J. Am. Chem. Soc. 2012, 134 (36), 14654−14657. (14) Gibbs, S. J.; Johnson, C. S. A PFG NMR Experiment for Accurate Diffusion and Flow Studies in the Presence of Eddy Currents. J. Magn. Reson. 1991, 93 (2), 395−402. (15) Ganesh, P.; Jiang, D. E.; Kent, P. R. Accurate Static and Dynamic Properties of Liquid Electrolytes for Li-Ion Batteries from Ab Initio Molecular Dynamics. J. Phys. Chem. B 2011, 115 (12), 3085− 3090.

Table 2. Li Self-Diffusion Coefficients Measured near Platinum and Lithium Electrodes after Application of Constant Current for 15 and 21 h 15 h 21 h

DLi+@Pt × 10−11 [m2/S]

DLi+@Li × 10−11 [m2/S]

8.5 ± 0.3 9.2 ± 0.3

7.7 ± 0.2 7.1 ± 0.2

under conditions akin to existing in a Li-ion cell. Our results reveal the consequence of the time-dependent evolution of concentration gradients, which directly reflect the impact of the Li+ transference numbers on the ion dynamics and which will be a valuable result for modeling of transport properties. To summarize, we have demonstrated that the values of selfdiffusion coefficients of ions in an electrolyte solution strongly depend on the salt concentration. At the same time, the salt concentration inside a battery is neither stationary nor homogeneous during its use; rather, it is a function of current density, time, and distance from the electrodes. Therefore, to achieve a correct and accurate estimation of electrolyte transport properties, including ionic transport numbers, one needs information about the spatial and temporal dependence of the diffusivity of ions in the electrolyte solution. We have demonstrated that a combination of in situ 1D NMR imaging and slice-selective NMR techniques can be successfully used for this purpose. All experiments were carried out with a conventional NMR probe with the standard 5 mm NMR tube. We chose a 1 M solution of LiTFSI in PC as a test sample and have performed our analysis under conditions that resemble those existing in electrolytes for Li-ion batteries. We have established that the concentration-dependent results correlate with measured changes in the diffusion coefficient as a function of the concentration gradient. We envisage that this approach will become a powerful tool for the investigation of new electrolyte systems and for validating battery models.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

Sample pretreatment, cell design, NMR experiments, and data evaluation. This material is available free of charge via the Internet at http://pubs.acs.org. 3943

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(16) Stolwijk, N. A.; Kosters, J.; Wiencierz, M.; Schoenhoff, M. On the Extraction of Ion Association Data and Transference Numbers from Ionic Diffusivity and Conductivity Data in Polymer Electrolytes. Electrochim. Acta 2013, 102, 451−458. (17) Pregosin, P. S. NMR Spectroscopy and Ion Pairing: Measuring and Understanding How Ions Interact. Pure Appl. Chem. 2009, 81 (4), 615−633. (18) Hallberg, F.; Furo, I.; Yushmanov, P. V.; Stilbs, P. Sensitive and Robust Electrophoretic NMR: Instrumentation and Experiments. J. Magn. Reson. 2008, 192 (1), 69−77. (19) Pettersson, E.; Furo, I.; Stilbs, P. On Experimental Aspects of Electrophoretic NMR. Concepts Magn. Reson. 2004, 22A (2), 61−68. (20) Hoult, D. I. The NMR Receiver: A Description and Analysis of Design. Prog. Nucl. Magn. Reson. Spectrosc. 1978, 12 (1), 41−77. (21) Nyman, A.; Behm, M.; Lindbergh, G. Electrochemical Characterisation and Modelling of the Mass Transport Phenomena in LiPF6−EC−EMC Electrolyte. Electrochim. Acta 2008, 53 (22), 6356−6365.

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