Tracking Lithium Ions via Widefield Fluorescence Microscopy for

Jul 12, 2017 - resolution can provide an important diagnostic tool for under- ... KEYWORDS: lithium ion sensing, fluorescence microscopy, lithium ion ...
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Tracking Lithium Ions via Widefield Fluorescence Microscopy for Battery Diagnostics Nicolas A. Padilla,†,# Morgan T. Rea,†,# Michael Foy,† Sunil P. Upadhyay,† Kyle A. Desrochers,† Tyler Derus,† Kassandra A. Knapper,† Nathanael H. Hunter, Sharla Wood,† Daniel A. Hinton,† Andrew C. Cavell,† Alvaro G. Masias,‡ and Randall H. Goldsmith*,† †

Department of Chemistry, The University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States ‡ Ford Motor Company, 2101 Village Road, Dearborn, Michigan 48121, United States S Supporting Information *

ABSTRACT: Direct tracking of lithium ions with time and spatial resolution can provide an important diagnostic tool for understanding mechanisms in lithium ion batteries. A fluorescent indicator of lithium ions, 2-(2-hydroxyphenyl)naphthoxazole, was synthesized and used for real-time tracking of lithium ions via widefield fluorescence microscopy. The fluorophore can be excited with visible light and was shown to enable quantitative determination of the lithium ion diffusion constant in a microfluidic model system for a plasticized polymer electrolyte lithium battery. The use of widefield fluorescence microscopy for in situ tracking of lithium ions in batteries is discussed. KEYWORDS: lithium ion sensing, fluorescence microscopy, lithium ion batteries, microfluidics, widefield imaging, in situ imaging

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perform in situ explorations of mechanisms of delithiation.10 However, extremely thin samples were required, the method is capable of quickly damaging and altering the structure of the system under study,9 and highly specialized equipment is required. Neutron scattering has provided an in operando means of providing structural information on lithium occupation of electrodes even in unmodified commercial batteries,11−14 while neutron tomography offers three-dimensional imaging of lithium distributions,11 albeit at limited spatial resolution15 (tens of μm) and also requires specialized equipment. Confocal Raman microscopy was used to image flowing lithium in a label-free manner,16 but discernible spectral features were only available at concentrations approaching 1 M. NMR is particularly well-suited to revealing chemical properties of lithium ions,17−19 and when using specialized toroid core NMR spectrometers, they can even provide limited spatial resolution.20 Though NMR can generally provide useful information about the chemical environment of lithium atoms it cannot give the necessary spatial resolution to reveal the micro- and nanostructure of ion transport pathways. Consequently, there is an outstanding need for a complementary analytical method that is capable of providing high resolution spatial information about distributions of lithium ions which offers high time

ithium ion batteries have emerged as a leading technology due to their high energy density, high power, long cycle life, and low rates of self-discharge. However, battery aging can result in a slow decrease of battery performance, frequently manifested as capacity and voltage fade and resistance rise.1 In other cases, battery failure can be much more acute, and constitutes a significant safety hazard.2 Many of these failure modes are due to lithium ions traversing paths outside of device design specifications. Consequently, a variety of methods have been deployed to characterize the physical and chemical properties of lithium ions.3 However, a common theme is indirect mapping of the avenues taken by the lithium ions using another physical observable as a proxy since the lithium ions themselves are difficult to image. For example, scanning probe microscopy (SPM) identified intercalation points of lithium ions into the cathode by measuring the swelling of the cathode.4,5 Similarly, X-ray tomography has allowed the three-dimensional mapping of electrode structure.6−8 Though the resulting 3D maps have provided the basis for simulations of lithium ion transport,6 they fail to probe the Li ions themselves, but instead probe the assumed tunnels that the ions travel. Electron energy loss spectroscopy (EELS) is one of the few techniques that can directly probe spatial distributions of lithium ions due to lithium’s high ionization cross section, and was used to correlate high local lithium concentrations with distortions in the graphite electrode’s lattice as indicated by TEM9 and © XXXX American Chemical Society

Received: February 10, 2017 Accepted: June 14, 2017

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well as results in less autofluorescence from the sample, leading to improved signal-to-background. Here, we report the use of 2-(2-hydroxyphenyl)naphthoxazole (HPNO), 1, as a sensitive indicator of lithium ions using a visible pump excitation (Figure 1A). Further, we show that 1 can be used to report spatial and temporal information on lithium ions in a chemical environment similar to what would be encountered in a battery. As a demonstration of how imaging lithium flow can enable quantitative determination of lithium ion transport properties, we develop a simple diffusion model and determine the lithium ion diffusion constant under these conditions using a microfluidic channel.

resolution and is applicable to current lithium ion battery technology. Fluorescence microscopy can satisfy this need once certain technical obstacles are overcome, and additionally provide an observational tool relying on relatively accessible equipment. Fluorescence microscopy is one of the most powerful modes of probing structures and mechanisms in biological systems, due to the low background of the technique.21 In particular, widefield fluorescence microscopy allows illumination of a large excitation area, enabling rapid, simultaneous resolution of multiple features and tracking with high specificity and time resolution that can be varied from hundreds of frames per second to frames per hour depending on dynamics of interest. Spatial resolution down to hundreds of nanometers can be easily achieved, and even down to tens of nanometers with modern super-resolution techniques.21 Fluorescence microscopy is also an enabling mechanistic tool for materials research, allowing the tracking of chemical species in time and space in a variety of systems, even down to the level of individual molecules.22−30 The ability to perform analysis in situ derives from the nondestructive nature of fluorescence microscopy. However, because lithium is not intrinsically fluorescent, a suitable sensor molecule must be found. A variety of small molecule probes show altered fluorescence when bound to lithium. Intercalation of the lithium ion by a crown ether31 or metallocrown32 results in the shutting off of a nonradiative excited-state decay pathway, leading to a fluorescent species. However, the quenching of fluorescence before ion binding in these methods is typically incomplete, leading to high backgrounds that are incommensurate with detection of small lithium ion concentrations. Optode systems can enable fluorescent or photoacoustic imaging of lithium in biological environments, but rely on mechanisms based on local pH changes and differential hydrophobicity that may not operate under aprotic nonbiological conditions. 33 The 2-(2hydroxyphenyl)benzoxazole (HPBO) family34,35 of molecules (Figure 1A) shows large changes to their absorption spectra

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EXPERIMENTAL METHODS

Fluorophore Synthesis. The synthetic scheme is shown in Figure 1C. An oven-dried 50 mL flask was charged with salicylic acid (785 mg, 5.69 mmol), 3-amino-2-naphthol (539 mg, 3.39 mmol), and 10 g of polyphosphoric acid. The reaction flask was then heated in an oil bath to 135 °C for 4.5 h, and the product was extracted into ethyl acetate and purified via column chromatography. 1H NMR (400 MHz, CDCl3): δ 8.17 (1H, s), 8.10 (1H, dd, J = 1.7 Hz, J = 7.8 Hz), 8.03− 7.97 (3H, overlapping), 7.54−7.47 (3H, overlapping), 7.16 (1H, d, J = 8.6 Hz,), 7.05 (1H, t, J = 7.5 Hz). 13C NMR (100 MHz, CDCl3): δ 164.9, 159.4, 148.0, 139.8, 134.2, 131.7, 131.6, 128.5, 128.0, 127.6, 125.7, 125.0, 119.7, 117.5, 116.5, 110.3, 110.0, 106.5. MS-ESI+ (m/z): [M + H]+ calcd for C23H15N2O3237.27; found, 237.26. Widefield Imaging. All imaging was performed with a Nikon Eclipse Ti inverted microscope with a 405 nm pump laser (World Star Tech, 0.2 W/cm2). Emission light was filtered by a dichroic filter (Semrock) and a 470/100 nm bandpass filter (Semrock) then collected by an EM-CCD Camera (Andor iXon Ultra 897). Microfluidic Channel Fabrication and Loading. Poly(dimethylsiloxane) (PDMS) microfluidic devices were fabricated via soft lithography following published procedures using CU-8 3050 photoresist.38 Negative photomasks were patterned by CAD/Art Services Inc., Bandon, OR, USA. The channel had a width of 400 μm and height of 50 μm and two reservoirs. After channel production, the device was cut in a plane perpendicular to the channel, through the reservoir, to provide an access port. Other access methods to insert LiCl involving syringes resulted in significant back or forward flow that interfered with diffusion kinetics. To load the microfluidic channel, the modified reservoir was filled with a small crystal mass of LiCl salt while the other reservoir was filled slowly via syringe with a mixture of propylene carbonate (PC), triethylamine (TEA) at a 25:1 v/v ratio of PC to TEA, with a poly(ethylene oxide) (PEO) electrolyte at 17 mg/mL and saturated HPNO (13 mM of HPNO). Lithium chloride was chosen as opposed to other lithium salts due to its air and moisture stability, though at the cost of less complete dissociation39 and limited application in commercial batteries. Diffusion Constant Measurements. Time-lapse images were acquired over a timespan of approximately 3 h. Exposure time was 30 ms and images were taken every minute, with a shutter synchronized to the camera to minimize excitation time and consequent photobleaching. All image analysis was performed with the National Institutes of Health’s open source image processing program, ImageJ.

Figure 1. (A) HPBO family of fluorescent indicators for lithium ions. (B) Scheme for lithium binding of 1. (C) Synthesis of 1. Full synthetic details can be found in the Experimental Methods.

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and quantum yield of fluorescence upon binding lithium ions, operates in a turn-on manner, and is selective to lithium ions over other metal cations.34,36,37 HPBO molecules bind lithium in a 2:1 stoichiometry, with the lithium ion maintaining a tetrahedral coordination geometry (Figure 1B).34,35 However, the UV excitation required for HPBO can result in high rates of fluorophore photobleaching and may also cause damage to the battery system under observation. A visible pump, on the other hand, reduces the probability of photobleaching and damage, as

RESULTS AND DISCUSSION The synthesis of 1 followed literature procedures (Figure 1C) with polyphosphoric acid as a dehydrating reagent to induce the formation of the napthoxazole ring.40 The absorption of 1 shows a significant red-shift as compared to HPBO,34,35 which only includes a benzoxazole ring, consistent with the larger conjugation length and consequently smaller HOMO−LUMO gap (Figure 2A). Further tuning of the optical properties is B

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Figure 2. (A) UV−vis spectra of 1 without lithium (black) and with saturated LiCl (red) in PC. (B) Fluorescence spectra of 1 in PC with increasing lithium concentration: dashed black (0 M), purple (2.9 × 10−7 M), dark blue (2.9 × 10−6 M), cyan (5.8 × 10−6 M), light green (2.9 × 10−5 M), dark green (5.8 × 10−5 M), orange (2.9 × 10−4 M), red (5.8 × 10−4 M). (C) Integrated fluorescence signal as a function of lithium ion concentration (blue square) and sodium ion concentration (orange diamond, NaCl). 1 showes a response to lithium ions with a dynamic range between 10−6 and 10−2 M whereas no response is seen to sodium ions. Molecule 1 was at 1.00 mM, and PEO was at 17 mg/mL in 25:1 v/v ratio of PC to TEA for the above experiments.

possible by extension of the π-system in geometries guided by the nodal planes of the ground and excited states.37 The steady-state photophysical properties that make 1 an effective reporter of the presence of lithium ions are conspicuously demonstrated in Figure 2. Molecule 1 shows a new spectral feature in the visible region after exposure to LiCl, and upon excitation of this new spectral feature with a visible pump wavelength, an increasing amount of fluorescence with increasing lithium concentration is observed, similar to the HPBO precursor molecule,34,35 but with a visible pump wavelength (Figure 2B). A measurable response is obtained even at μM concentrations of LiCl, with saturation beginning to occur at hundreds of μM in PC (Figure 2C). Over the same range, 1 is seen to be unresponsive to sodium ions. To evaluate 1 as a means to track lithium ions in an analog of a battery, solution conditions were selected to include reagents typically found in a plasticized polymer electrolyte lithium battery, including a polar aprotic solvent, propylene carbonate (PC), a polymer electrolyte, poly(ethylene oxide) (PEO), along with a small amount of organic base, triethylamine (TEA), required to deprotonate the HPNO. The battery mimic solution was saturated with 1 at 13 mM (see SI) for all imaging experiments. To assess the ability of 1 to image lithium ion trajectories, we designed a compact PDMS microfluidic cell that would allow us to quantitatively determine the diffusion constant of lithium (Figure 3). A small crystal mass of solid LiCl was introduced into the reservoir and the working solution was injected into the channel via the opposite reservoir (Figure 3) to make contact with the solid. In this manner, any flow caused by the injection is separated from the lithium diffusion since this flow dissipates quickly, before the solid dissolves. Several millimeters of the microfluidic channel were then imaged over time to record the flow of lithium ions as indicated by the increasing fluorescence (Figure 4A). A time-lapse imaging mode was used to minimize the influence of photobleaching. Supporting experiments imaging solutions of known concentrations of LiCl in the PDMS channels under comparable imaging conditions suggest the concentration of Li+ in Figure 4 ranges up to approximately 0.4 mM. To aid in quantitative extraction of the diffusion constant, a finite-difference time domain simulation of the diffusion of lithium through the microfluidic channel was performed using

Figure 3. (A) Schematic of a PDMS microfluidic channel immediately after a LiCl crystal (red cube) is placed at one end. (B) Using widefield fluorescence microscopy, the flow of lithium ions can be tracked as lithium ions bind to 1, making it fluorescent under excitation.

the Comsol Multiphysics package. These simulations were used to compare different experimental assumptions regarding transport, including the role of finite or infinite source and sink boundary conditions (Figure 5). Critically, the initial onset of fluorescence is model-independent, allowing non-Fickian effects from sample geometry to be ignored. In this limit, transport is well-described by the simplest model of onedimensional diffusion derived from Fick’s second law with a finite source and no sink (constant total concentration), which has a convenient analytical form (eq 1), where C(x,t) is the temporally and spatially dependent concentration, x is the distance from the origin, M is the initial concentration of C

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the distance between a particular ROI and the ion source. However, the relative spacing between the ROI’s can be determined exactly via the known magnification. Thus, simultaneous fitting of fluorescence−time traces for all seven ROI’s (Figure 4B) with known spacing, and a shared center distance from the source allows the distance between the center of the ROI packet and the source to be made into a free fitting parameter to be extracted from the fit. In this manner, the simultaneous fit provided significant constraints for determining the absolute distance and the diffusion constant. Using this approach, a diffusion constant of (2.11 ± 0.7) × 10−10 m2/s was determined. This value is near the diffusion constant value of 0.69 × 10−10 m2/s determined via NMR spectroscopy for LiPF6 in PC without PEO electrolyte,42 though the different counterion and presence of PEO are expected to play a role. Small deviations from the model at early times are likely due to geometric effects beyond the simple model of eq 1. The analysis above assumes that all shifting of fluorescence intensity is a result of lithium ion diffusion. However, diffusion of the fluorescent indicator 1 may also be factor. The diffusion constant of 1 under experimental conditions can be estimated from a measure of the solution viscosity, η (including PEO), 8.02 cP, the diffusion constant of a similar sized dye,43 fluorescein, in water at 0.91 cP, 4.25 × 10−10 m2/s, and the Stokes−Einstein Equation44

D=

Figure 4. (A) Widefield images of the illuminated channel at different time points showing lithium ion diffusion. (B) Fluorescence intensity at a series of rectangular ROI’s (see inset) as a function of time, with satisfactory fits from eq 1.

lithium ions at the point source, and D is the diffusion constant.41 2 M e−x /4Dt 4πDt

(2)

where kb is Boltzmann’s constant, T is temperature, r is the dye radius assuming a spherical approximation, and η is the solution viscosity. Extrapolation of the diffusion constant to the PEO/ PC solution suggests a value of 4.82 × 10−10 m2/s, roughly a factor of 4 lower than the lithium ion diffusion coefficient. However, this value should be regarded as an upper limit, as dilute polymer solutions are known to significantly reduce diffusion of tracer dye molecules.45 In particular, solutions of PEO have been observed to reduce the diffusion of dye molecules by over an order of magnitude at PEO concentrations of 1 mg/mL,46 a concentration significantly smaller than the 17 mg/mL used in our experiment. Thus, dye motion is believed to be negligible compared to the motion of the smaller lithium ion. In future experiments, a derivative of 1 can be conjugated to PEO to completely eliminate dye diffusion or target the dye to specific parts of a battery analog. These experiments are in progress. While the proof-of-concept experiments described here show that 1 can be used as a molecular proxy to sense, image, and track lithium ions in the nonaqueous media used in a lithium polymer battery, additional modifications will be required to close the gap in realism between our test system and a real, functional battery cell. One major modification will be the addition of electrodes. A transparent electrodesuch as ITO will be required if imaging is desired. Inclusion of electrodes will enable simultaneous imaging and impedance spectroscopy, allowing electrochemical features to be corroborated with the formation or disappearance of features in images. Use of impedance spectroscopy will also be crucial to ascertain if the inclusion of 1 or the requisite base in small amounts affects electrochemical properties. Electrochemical stability of 1 will need to be investigated as well, though the use of anchored derivatives, as described above, can limit mass transport if electrochemical degradation is an issue. Addition of these

Figure 5. Finite difference time domain calculations of diffusion with various boundary conditions. Finite source refers to an impulsive amount of diffuser added, while infinite source refers to a steady rate of diffuser added. Sink refers to the presence of an absorbing boundary condition at the opposite channel terminus.

C(x , t ) =

k bT 6πηr

(1)

One of the main advantages of a widefield imaging technique for imaging lithium ion concentrations over confocal techniques16 is that images of the entire active area can be rapidly acquired at high frame rate. Thus, we were able to easily extract the change in lithium ion concentration along multiple region of interest (ROI) lines in the image (Figure 4B). Each ROI was normalized to the excitation intensity to account for spatial inhomogeneities in the excitation profile. A critical limitation in the use of eq 1 is the uncertainty in determining D

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endorsement. This article does not provide financial, safety, medical, consumer product, or public policy advice or recommendation. Readers should independently replicate all experiments, calculations, and results. The views and opinions expressed are of the authors and do not necessarily reflect those of Ford. This disclaimer may not be removed, altered, superseded or modified without prior Ford permission.

elements will allow 1 and future derivatives to provide the basis for a new in situ imaging and sensing tool to understand mechanisms in lithium ion batteries, and is currently underway. In these situations, deviations in observed behavior from eq 1 may require inclusion of non-Fickian effects that emerge in more chemically complex environments, but as shown above, the use of widefield imaging provides the significant constraints needed to test more elaborate transport models.

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ABBREVIATIONS PEO, poly(ethylene oxide); PC, propylene carbonate; HPBO, 2-(2-hydroxyphenyl)benzoxazole; HPNO, 2-(2hydroxyphenyl)naphthoxazole

CONCLUSION HPNO, molecule 1, was shown to form a new visible spectral feature upon exposure to lithium ions in PC, and to yield a turn-on fluorescence signal. Using a microfluidic channel, molecule 1 was further shown to allow imaging of lithium ion diffusion. Inclusion of a simple diffusive transport model and a series of constraints allowed quantitative determination of the lithium ion diffusion constant under these conditions and was in reasonable agreement with a previous measure of lithium ion diffusion. After incorporation of additional components to allow imaging of a functional lithium ion battery, we anticipate fluorescence imaging to become an important new tool for revealing lithium ion flows in batteries.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssensors.7b00087. Details of sample preparation, device fabrication, Comsol simulations (PDF) Representation of imaging experiments (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Alvaro G. Masias: 0000-0001-9354-1970 Randall H. Goldsmith: 0000-0001-9083-8592 Author Contributions #

N.A.P. and M.T.R. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a University Research Project (URP) grant from Ford Motor Company (URP 2014−7102R) and by the National Science Foundation (CHE-1254936). We thank the University of Wisconsin-Madison Ronald E McNair Postbaccalaureate Achievement Program and staff and the NSF Research Experience for Undergraduates (REU) program at UW Madison. We thank M. Stillwell for help producing microfluidics. We thank Prof. M. Zanni for loan of laser and Prof. S. Cavagnero for use of a viscometer. We thank Dr. S. Peczonczyk, Dr. S. Simko, K. Lupo, and J. Ng for helpful conversations. While this article is believed to contain correct information, Ford Motor Company (Ford) does not expressly or impliedly warrant, nor assume any responsibility, for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, nor represent that its use would not infringe the rights of third parties. Reference to any commercial product or process does not constitute its E

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DOI: 10.1021/acssensors.7b00087 ACS Sens. XXXX, XXX, XXX−XXX