Shear Thickening Electrolyte Built from Sterically Stabilized Colloidal

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Shear Thickening Electrolyte Built from Sterically Stabilized Colloidal Particles Brian Shen, Beth L. Armstrong, Mathieu Doucet, Luke Heroux, James F. Browning, Michael Agamalian, Wyatt E Tenhaeff, and Gabriel M. Veith ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19441 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 4, 2018

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ACS Applied Materials & Interfaces

Shear Thickening Electrolyte Built from Sterically Stabilized Colloidal Particles Brian H. Shen,1,2 Beth L. Armstrong,1 Mathieu Doucet,3 Luke Heroux,3 James F. Browning,3 Michael Agamalian,3 Wyatt E. Tenhaeff,2 Gabriel M. Veith1,* Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge TN, 37831 of Chemical Engineering, University of Rochester, Rochester, NY 14627 3 Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge TN 37831 * Corresponding Author – [email protected] 1

2 Department

KEYWORDS: Shear thickening, electrolyte, steric stabilization, USANS, lithium ion battery ABSTRACT: We present a method to prepare shear-thickening electrolytes consisting of silica nanoparticles in conventional liquid electrolytes with limited flocculation. These electrolytes rapidly and reversibly stiffen to solid-like behaviors in the presence of external shear or high impact, which is promising for PMMA Brushes improved lithium ion battery safety, especially in electric vehicles. However, in initial chemistries the silica nanoparticles aggregate and/or sediment in solution over time. Here, we demonstrate steric ATRP stabilization of silica colloids in conventional liquid electrolyte via surface-tethered PMMA brushes, synthesized via surface-initiated atom transfer radical polymerization. The PMMA increases the magnitude of the shear thickening response, compared to the uncoated particles, from 0.311 Pa s to 2.25 Pa s. Ultra small angle Sterically Stabilized Aggregated neutron scattering revealed a reduction in aggregation of PMMASilicon Nanoparticles Silicon Nanoparticles coated silica nanoparticles compared to bare silica nanoparticles in solution under shear and at rest, suggesting good stabilization. Conductivity tests of shear thickening electrolytes (30 wt.% solids in electrolyte) at rest were performed with interdigitated electrodes positioned near the meniscus of electrolytes over the course of 24 hours to track supernatant formation. Conductivity of electrolytes with bare silica increased from 10.1 to 11.6 mS cm-1 over 24 hours due to flocculation. In contrast, conductivity of electrolytes with PMMA-coated silica remained stable at 6.1 mS cm-1 over the same time period, suggesting good colloid stability. 1.

Introduction Lithium ion batteries are continuously being optimized for demanding applications such as electric vehicles; however, significant safety concerns around the flammable alkyl carbonate electrolyte solvents in lithium ion batteries remain.1 This is especially true when considering collisions involving electric vehicles that can result in electrode shorting and lead to thermal runaway and combustion of these solvents.2 One approach to mitigating the fire hazard is the design of solid state batteries, which employ nonflammable solid ionic conductors. Ideally, the me-

chanical properties of the solid electrolyte can also impart additional mechanical strength to the cell. While many solid electrolytes have been designed with compelling ionic conductivities approaching or even surpassing conventional liquid electrolytes, the solid state nature requires the complete redesign of the electrochemical cell, and the efficient utilization of the conventional slurry-cast, porous composite electrodes is nontrivial to achieve.3-4 There is strong demand for the development of safe electrolytes that can replace conventional liquid electrolytes but do not require redesign of the whole cell.5

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Scheme 1. A) Cartoon depiction of aggregation of bare silica nanoparticles in solution. B) Scheme depicting attachment of organosilane to silica colloid surfaces (top) and grafting of poly(methyl methacrylate) (PMMA) brushes from colloid surfaces via surface-initiated activators regenerated by electron transfer atom transfer radical polymerization (ARGET-ATRP) (bottom). C) Cartoon depiction of silica nanoparticles sterically stabilized via surface tethered PMMA brushes. One such approach is the recent development of shear-thickening SAFe Impact Resistant Electrolytes (SAFIRE), which undergo a rapid, reversible rheological transition from liquid to solid-like viscosities upon application of external shear while maintaining the ionic conductivities required for high rate applications under non shear conditions.6-7 This phenomenon is known as shear thickening and is known to occur in concentrated suspensions where the force required to shear a mixture increases nonlinearly with shear rate.8 The magnitude of viscosity change is dependent on the concentration of solid particles and the shear thickening process. Continuous shear thickening (CST) occurs at lower particle concentrations (typically 30-40% in aqueous conditions) and displays a “gentle” increase in viscosity with shear.8-9 Under certain conditions the increase in viscosity becomes much larger (orders of magnitude) with further increases in particle concentration (in aqueous media).10 This increase is described as discontinuous shear thickening (DST). In both cases the origin of shear thickening is believed to be due to hydrocluster formation that occurs when particles are pushed together and to move must overcome the drag between the particles due to small lubrication gaps. However, in the case of DST, the addition of friction between the particles leads to the larger magnitude in viscosity with shear.8 It is this transition to solid-like viscosities dramatically improves the safety of lithium ion batteries during an impact, such as a car crash.7, 10 It is important to note that the conditions and molecular interactions that lead to shear thickening in non-aqueous conditions and high salt conditions, such as those in a lithium ion battery electrolyte are not well understood. Indeed, as described later the concentrations needed to obtain the DST condition are lower than in aqueous suspensions. In conventional alkyl carbonate-based electrolytes, we have observed that untreated silica colloids in suspension aggregate over time (days), even in quiescent conditions, and precipitate from solution.11-14 Significant sedimentation of solid colloids would result in an electrolyte that does not exhibit any rheological response to shear. This is an especially important consideration for

implementation in any practical electrochemical cell requiring long calendar lives, such as those found in consumer electronics and electric vehicles or in other materials such as the “soggy sand” electrolytes.14-17 We seek to reduce this sedimentation and improve dispersion stability by growing covalently-tethered polymer brushes (poly(methyl methacrylate), (PMMA)) from silica nanoparticle surfaces via activators regenerated by electron transfer atom transfer radical polymerization (ARGET-ATRP) to provide steric stabilization, as shown in Scheme 1A and 1C.18 The chemistry for functionalization of silica surfaces is shown in Scheme 1B. Steric stabilization is a well-established approach for stabilizing colloidal dispersions.19-22 Poly(methyl methacrylate) (PMMA) was chosen for the polymer compositions as its polymerization is well documented and has been used as a matrix in polymer gel electrolytes for lithium ion batteries.23-24 We investigate the rheological behavior of these electrolytes in situ to understand how these polymers affect colloid aggregation in shear. Finally, we use these electrolytes in electrochemical cells. These findings will guide design of sterically stabilizing coatings for colloids in shear thickening electrolytes that effectively reduce aggregation at rest and enable shear thickening behavior even at long calendar times. 2.Experimental Section 2.1 Silica nanoparticle synthesis Chemicals: Tetraethyl orthosilicate (≥ 99.0% purity, Sigma Aldrich), ammonium hydroxide (28% in H2O, VWR), and ethanol (200 proof, Decon Labs) were purchased and used as received. Ethanol (425mL), deionized water (50 mL), and ammonium hydroxide (50 mL) were added to a round bottom flask. The contents were chilled to 0 °C using an ice bath, after which tetraethyl orthosilicate (43 mL) was added dropwise to the round bottom flask and the contents were left to react for at least 15 hours, according to the method by Stöber.25 The resulting precipitated silica nanoparticles were rinsed in ethanol three times. To do this, the particles were collected by centrifugation (Allegra 6,

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Beckman Coulter) and the supernatant was decanted. The nanoparticles were ultrasonicated (XL2010, Heat Systems Inc.) in ethanol. This process was repeated twice more before centrifuging one final time. The particles were then dried at room temperature in vacuum overnight. The silica particles were reported to be 200 nm in diameter measured by dynamic light scattering. 2.2 Silica surface functionalization Chemicals: Toluene (anhydrous 99.8%, Sigma), silane coupling agent (3-(Trimethoxysilyl)propyl 2-bromo2-methylpropanoate, Gelest), methanol (99.8%, VWR), methyl methacrylate (99%, ≤30 ppm MEHQ inhibitor, Sigma Aldrich), deionized water, 2,2’-bypyridine (≥99%, Sigma Aldrich), copper bromide (99%, Sigma Aldrich), ascorbic acid (reagent grade, Sigma Aldrich) were purchased and used as received, except for methyl methacrylate, which was passed through an inhibitor removal column (DH-4, Scientific Polymer Products) three times. Silica nanoparticles were reacted with the silane coupling agent in toluene (5 μL per gram silica), with stirring at 500 rpm and 70 °C overnight. The silane treated silica nanoparticles were rinsed with methanol three times to remove excess silane, in a similar fashion to the process used before. PMMA chains were grown on the surface via atom transfer radical polymerization (ATRP).26 The silica nanoparticles were added to a solution of deionized water (8.0 ml), methyl methacrylate (40 ml), methanol (32 ml), copper bromide (14.70 mg), bipyridyl (103.1 mg), and ascorbic acid (116.2 mg). The nanoparticles were stirred in solution at 500 rpm for four hours at room temperature. The now functionalized nanoparticles were soaked in acetone overnight to ensure the PMMA was attached and not simply adsorbed on the silica surfaces. The particles were then rinsed in acetone three times before drying at room temperature in vacuum overnight. 2.3 Silica nanoparticle characterization Thermogravimetric analysis (TGA) was performed on PMMA-coated silica nanoparticle samples using a TA Instruments Q500 TGA. Samples were analyzed on platinum pans from 30 °C to 1000 °C in nitrogen. For comparison, commercial PMMA powder was also analyzed (600 micron, extrusion grade, Goodfellow Cambridge). Fourier transform infrared spectroscopy (FTIR) was conducted using a Bruker Alpha FT-IR spectrometer with a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) attachment. PMMA-coated silica nanoparticles were compared to bare silica nanoparticles and commercial PMMA powder. Zeta potential measurements were conducted on silica nanoparticles, bare or PMMA-coated, in dilute aqueous solutions using a zeta potential analyzer (Brookhaven Instruments, ZetaPals). To prepare these solutions at the correct dilutions (order of 10-6 wt. %) appropriate amounts of particles were added to deionized water and KNO3 (0.01M). Solutions were titrated across a range of pH (2-10) using HNO3 (1M) and HN4OH (1M). 2.4 Scanning Electron Microscopy

Bare and PMMA-coated silica nanoparticles were dispersed in ethanol and coated on to a copper grid. They were imaged using a Hitachi S-3400 scanning electron microscope with variable pressure operated between 10 and 15 kV. 2.5 Rheology Rheological measurements were conducted on shear-thickening electrolyte solutions using a rheometer (Physica MCR501, Anton Paar) fitted with a concentric cylinder geometry. Solutions were prepared by adding appropriate amounts of nanoparticles, bare of PMMAcoated, in propylene carbonate. Solutions were hand mixed as the shear-thickening behavior of the solutions prevented mixing by other techniques. Propylene carbonate was chosen instead of other conventional lithium ion battery electrolytes such as ethylene carbonate (EC) or dimethyl carbonate (DMC) for its low vapor pressure as the solutions will be exposed to air for long times. For most samples, titanium outer (50mm diameter) and inner (48mm diameter) cylinders were used. Quartz outer (50mm diameter) and inner (49mm diameter) cylinders were used for solutions to be analyzed with ultra-small-angle neutron scattering. Solutions were conditioned with a low shear rate (5 s-1) for 30 seconds to disperse agglomerated particles and were allowed to equilibrate for one minute prior to measurements. Variable rate measurements were conducted at rotation rates ranging from 0 to 100 s-1. Constant rate measurements were conducted at rotation rates where shear-thickening behavior was most prominent. All rheological measurements were conducted at room temperature. 2.6 Ultra Small-Angle Neutron Scattering (USANS) Neutron data was collected at the USANS instrument at Oak Ridge National Laboratory’s Spallation Neutron Sources (SNS). USANS was chosen as it can analyze the large 200 nm diameter silica colloids in solution. The USANS instrument is a time-of-flight triple-bounce BoseHart small-angle scattering instrument that allows measurements down to 10-6 in Q.27 The data was acquired using the 3.6 Angstrom reflection of the analyzer crystals. This provided a minimum Q of 0.0001 Ǻ-1, which is enough to cover the whole signal from our sample. The raw data was reduced using the Mantid framework to compute I(Q).28 For each sample run, a background run with only propylene carbonate was taken in the same conditions. The I(Q) distribution from the background was subtracted from the sample I(Q). The 1 wt. % data was modelled using the SasView small-angle scattering software package. 29 2.7 Electrochemistry Coin cells were assembled using graphite and LiNi1/3Mn1/3Co1/3O2 (NMC) electrodes in a balanced full cell configuration. The graphite and NMC electrode activematerial areal densities were 9 and 19.95 mg/cm2, respectively. Two separators (Gold 40, Dreamweaver) were used and 100 μL electrolyte was added (1.2M LiPF6 in 3:7 EC:DMC, BASF). The electrodes and separators were dried at 120 °C before use. Shear-thickening electrolytes for the



coin cells were prepared to a target concentration of 30% solids by weight in liquid electrolyte. At such high loadings, the shear-thickening electrolytes easily transition to almost solid like behaviors with very little shear. The electrolytes were slowly mixed by hand using a glass stirring rod. Two plastic spatulas were used to portion the electrolyte between the two separators. Electrochemical cycling was conducted using a battery cycler (Series 4000, Maccor) at a rate of C/3 from 3.0V to 4.2V at room temperature. To understand how sedimentation affects ionic conductivity, interdigitated electrodes (IDEs, cell constant 2 cm-1, model CC1.W, BVT Technologies) were submerged in solutions of 30 wt.% solids in 1.2M LiPF6 3:7 wt% EC:DMC electrolyte. Wires connected to the IDEs were threaded through the top of a butyl 20 mL scintillation vial cap and sealed with epoxy. The shear thickening electrolyte solutions were prepared by hand-mixing in Ar(g) in 20 mL scintillation vials. The vials were sealed with the modified cap with IDE. The IDEs were then connected to a potentiostat (SP300, BioLogic Science Instruments). Conductivity was measured at 25 °C at 871 kHz, 108 kHz, and 435 kHz, for electrolytes with no silica, bare silica, and PMMAcoated silica, respectively. The IDEs were positioned 1 mm from the bottom of the vial to measure the conductivity of the colloid sediment over time. 3.

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Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was performed to understand the surface composition of PMMA-coated silica nanoparticles. Figure 1b shows representative DRIFTS data collected on bare silica, PMMA-coated silica, and commercial PMMA powder for comparison. Peaks are observed in the spectrum for silica nanoparticles at 1070 and 458 cm-1, which can be attributed to Si-O-Si stretching and Si-O deformation, respectively.35 A small band is observed at 2990 cm-1 in the treated silica spectrum, which can be attributed to a C-H stretch from the methoxy and methyl groups in PMMA. The peak observed at 1728 cm-1 in both the commercial PMMA and treated silica spectra is characteristic of PMMA and is attributed to an acrylate carboxyl functional group.36 Two bands, 1390 and 754 cm-1, can be attributed to methyl group vibrations. A band is seen from 1040 1260 cm-1 and can be attributed to a -OCH3 stretching vibration. These peaks closely correspond with the spectrum obtained from analysis of commercial PMMA powder, confirming that the silica nanoparticles are covered with PMMA.

A

Results and Discussion

3.1 Characterization of Silica with SurfaceTethered PMMA Brushes To measure the amount of PMMA tethered to the silica nanoparticles, thermogravimetric analysis (TGA) was conducted on PMMA-coated silica nanoparticles and commercial PMMA for comparison. TGA data for these materials are shown in Figure 1a. Initially, loosely bound water leaves the surface from 30 to 190 °C, as evident in the decrease in weight. Subsequently, a tightly bound water layer leaves the surface beginning at 190 °C.30 A 37.03% drop in mass is observed for treated particles beginning at 285 °C and ending at approximately 400°C. This closely matches the decomposition observed for commercial PMMA beginning at 285.7 °C and ending at 391.9 °C. This suggests the loss of mass observed in functionalized silica colloids can be attributed to the decomposition of PMMA on their surfaces. The observed mass loss in treated particles was used to estimate individual chain length. Assuming each individual PMMA chain occupies 1.2 nm2 of the surface of a silica nanoparticle each chain is 78.6 nm in end-to-end length.31-32 This is close to the 10% length Wagner et al. suggested that would eliminated interparticle repulsive forces from a polymer brush layer.33-34 However, as we are operating in aprotic electrolyte systems, it is possible the polymer brushes on the surface are partially or fully collapsed on the silica particle surfaces, affecting particle aggregation behavior. Additional losses are observed for treated particles. Slight loss in weight after 400 °C may indicate the degradation of the organosilane used to tether the polymer to the silica nanoparticle surface.

B

Figure 1. A) Thermogravimetric analysis data of PMMAcoated silica colloids and commercial PMMA powder. Samples were analyzed in platinum pans in N2 from 35 to 1000 °C. B) DRIFTS spectra for treated and bare silica, as well as commercial PMMA for comparison. It is important to reiterate that the treated particles were rinsed and soaked overnight in acetone- a good solvent for PMMA. Thus, it is unlikely that there are free


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PMMA chains adsorbed on silica nanoparticle surfaces. From these data, we can conclude that the PMMA brushes are covalently bound to the silica nanoparticle surfaces and remain tethered to the silica surface in solvent. Both attributes are a benefit to shear-thickening electrolytes, as long-term stability is desirable for proper response to shear. To qualitatively evaluate differences in aggregation behavior of bare and PMMA-coated silica nanoparticles, scanning electron microscopy (SEM) was performed on dry powders. Micrographs of bare and PMMA-coated silica nanoparticles are shown in Figure 2A and Figure 2B, respectively. Bare silica nanoparticles are observed to be aggregated into small networks as expected. In contrast, PMMA-coated nanoparticles appear to remain more evenly dispersed, suggesting the PMMA surface-tethered brushes aid in sterically stabilizing the particles and reducing ag-

A

3.2. Rheological behavior The rheological behavior of the silica nanoparticles (bare and PMMA-coated) was characterized in propylene carbonate (a common solvent in lithium ion battery electrolytes), and the critical solid loadings in the colloidal dispersions for shear-thickening behavior was determined. Since the rheometer for the USANS experiment was operated in air, over 6-24 hours, we could not use a solvent that was volatile like DMC, since any evaporation would affect viscosity over the course of the experiments. Therefore, propylene carbonate (PC) was selected as solvent for its low vapor pressure and dielectric strength similar to ethylene carbonate. Dielectric strength is believed to be critical to stabilizing the silica in battery solvent.7 Experiments indicated it would take 3.8 years to evaporate a gram of PC at room temperature making it suitable for the USANS ex-

B

2 µm

2 µm

Figure 2. Micrographs of A) bare and B) PMMA-coated silica nanoparticles. gregation. periments. The USANS experiments were also performed

A

B

C

D

5 Paragon Plusrate Environment Figure 3. A) Shear stress and B) viscosity as aACS function of shear for solutions of low to moderate concentrations (0 – 15 wt%) of PMMA-coated silica colloids in propylene carbonate. C) Shear stress and D) viscosity as a function of shear rate for 30 wt% PMMA-coated silica colloids in propylene carbonate.


without LiPF6 salt to avoid the formation of HF from the reaction with atmospheric water which would etch the rheometer and dissolve the silica. Previous rheology measurements on unprotected SiO2 indicated the salt had a minor influence on rheology data7 and qualitatively adding salt to the electrolytes used in the cycling studies, discussed below, still resulted in a shear thickening electrolyte as evident by the stiffening during handling. Figure 3 shows rheology data as a function of weight loading for solutions ranging from 0 – 15 wt.% (0-10.3 vol%) PMMAcoated nanoparticles. A transition from Newtonian to nonNewtonian, shear-thickening behavior is observed between 10 - 15 wt.% loading similar to what was reported previously for uncoated materials.7 In solutions up to 10 wt. % solids loading, the viscosities remain relatively constant despite increasing shear rate and have linear rate/stress curves, suggesting Newtonian behavior. The viscosity of the 15 wt.% nearly doubles as shear rate approaches 100 s-1, as shown in Figure 3b; in addition, a subtle, rising inflection is observed in the stress/rate curve of the 15wt.% solution, as shown in Figure 3. Both properties suggest shear-thickening behavior was observed for solutions greater than 15 wt.%. Increasing particle concentration to 30 wt. % (21.8 vol%) shows prominent shear-thickening behavior. The viscosity increases an order of magnitude from 0.311 Pa s to a maximum of 2.25 Pa s. The increase in viscosity exceeds those in recently reported shear thickening electrolytes (Veith, et al.), which show improved voltage stability during an impact test.7 For example, this previous work shows a viscosity increase for 30 wt% untreated silica nanoparticles 1.2M LiPF6 3:7 EC:DMC from ca. 0.10 to 1.2 cP at a shear stress of 20 dyne cm-2. Although the viscosity reached over this range of shear stress is relatively low, the solutions with untreated particles still exhibited a sufficient safety mechanism to stabilize voltage in response to a large impact force (5.65 Joules). Over a similar range of shear stress, the viscosity of our 30 wt% dispersion increases from 250 to ca. 500 cP. We can expect that high concentration dispersions of PMMA-coated silica particles in electrolyte will exhibit a similar safety response. Additional logarithmic graphs of shear stress and viscosity as a function of shear stress are presented in Figure 4. For the 15 and 30 wt.% solutions, the slopes of viscosity versus shear rate are greater than zero, indicating shear thickening behavior. In shear stress as a function of shear rate, the slopes (α) for the 15 and 30 wt.% dispersions are 1.2 and 1.7, respectively, also an indication of shear thickening behavior. Newtonian behavior is observed when slope α=1, CST when 1 < α < 2, and DST when α > 2.8 Thus, within the range of weight fractions characterized, the specific rheological behavior observed can be described as a transition from Newtonian to CST according to this convention as mass fraction increases from 10 to 15 wt.%. CST aptly describes the mild increase in viscosity observed in the 15 wt.% solutions. However, we believe the behavior is beginning to evolve from CST to DST as mass fraction increases further (30 wt.%). A large increase in the viscosity and shear stress as a function of shear rate, characteristic of DST, is observed as the weight fraction of

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silica colloids in electrolyte increases to 30 wt.%. In addition, the viscosity and shear stress data appear to approach some upper asymptote, typical of DST. In CST, viscosity would be expected to continuously rise with shear rate, which is not observed in the data. We believe the mechanism for the observed behavior is a combination of silica colloid hydrocluster formation and interparticle friction forces. As shear rate increases, the silica colloids are tightly pressed together, increasing short-range hydrodynamics and forming large hydroclusters, increasing viscosity. Frictional stresses between close particles can lead to the large increases in shear stress observed in the 30 wt.% solutions.33

A

B

Figure 4. A) Shear stress as a function of shear rate. Shear thickening behavior is observed when the slope is greater than 1. B) Viscosity as a function of shear stress. Shear thickening behavior is observed when the slope is greater than 0. Typically, during a rheological experiment of a shear thickening slurry, particle sediment can be redistributed into the bulk using a starch pasting cell fixture. Neutron measurement limitations forced use of a concentric cylinder attachment instead, and thus sedimentation was prone to occur more quickly within the timeframe of the experiment. However, this was fortuitous, as this al-


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lowed us to compare the sedimentation rate or stability of shear-thickening electrolytes containing bare or PMMAcoated silica nanoparticles. To qualitatively compare the long-term stability of shear-thickening fluid under constant shear, two 30 wt.% solutions were prepared using either treated or bare silica nanoparticles. The treated particle solution was sheared at 50 s-1, and the bare particle solution was sheared at 16.7 s-1. The rates were chosen based on regimes where the rate of viscosity increase was highest. Within these regimes shear thickening response was realized in both bare silica and PMMA-coated silica based solutions. Beyond these regimes, viscosity decline is observed, attributed either to particle sedimentation due to rheometer attachment limitations or particle rearrangement under high shear. The solutions were sheared for 24 hours to determine whether the particles remain evenly dispersed in solution or aggregate and precipitate from solution. Viscosity data as a function of time is shown in Figure 5. It is observed that the decline in viscosity of the solution containing treated silica particles under constant shear is less severe than that of solutions with bare silica. After 24 hours, electrolyte viscosity containing bare silica colloids reaches 51.7% of initial viscosity, compared with 76.7% in electrolyte with PMMA-coated silica colloids. Moreover, viscosity of the dispersion containing PMMA-coated nanoparticles appears to stabilize at around 15 hours with a flatter slope, whereas the viscosity of the bare nanoparticle dispersion appears to continue to decay after 24 hours. This confirms that polymer treatment of silica nanoparticles helped to stabilize the particle dispersion.

PMMA-coated and bare particles in solution. For bare particles, the isoelectric point is moderately low, pH (~3), followed by > 40 mV increase in the magnitude of the potential at higher pH. Initially, the surface hydroxyls remain stable in solution. However, as pH increases, the presence of hydroxides interfere with silica surface silanols, forming a mixture of Si-OH/Si-O- termination groups. More ionized silanols initiate bridging between particles, leading to increased aggregation.12, 37-39 For PMMA-coated particles with PMMA, similar zeta potential behavior is observed at low pH. However, as pH increases, the zeta potential is observed to be lower in magnitude compared to that of bare silica nanoparticles. This suggests that the silica surfaces are more neutral as pH approaches 6-8, and the plane of shear is further from the nanoparticle surfaces.40-41 There appears to be a deviation at pH 7 for the PMMA sample which was reproducible in our measurements with an error of ~2 mV. The origin of this deviation is unknown at this time, but we note there is a wide variability of zeta potentials reported for PMMA in the literature.42 This data indicates the presence of PMMA on the silica surface, and confirms that the PMMA is sterically stabilizing the particles, reducing aggregation at higher pH. It is important to note that the zeta potential for PMMA-coated nanoparticles at greater pH (6, >8), though smaller in magnitude than that of bare nanoparticles, is still rather high. This suggests that despite the large brush network covering the surface of PMMA-coated nanoparticles, a sufficient amount of surface is exposed to initiate some aggregation, as mentioned above. Because the PMMA initiation sites rely on siloxane bond chemistry, it is unlikely that the entire particle surface is covered in brushes, supported by the loss of tightly bound water in the TGA results as well as evidence of some surface charge in the zeta potential results. This is desirable, as shear-thickening behavior can be maintained. These results suggest that the PMMA brushes are effective in reducing aggregation and improving nanoparticle stability/dispersion, but not so much as to prevent shearthickening behavior altogether. Finally, the zeta potential data from aqueous silica colloid suspensions provides a general understanding of the effects of the polymercoating on surface charge and aggregation. Specifically, the PMMA coating generally decreases the magnitude of the surface charge, enhancing non-DLVO stability. However, the data may not be fully representative of the silica colloid surfaces in aprotic solvents, such as lithium ion battery electrolytes. Regardless, the behavior observed is improved stabilization of silica colloids in such conditions.

Figure 5. Viscosity of silica nanoparticle dispersion in propylene carbonate measured as a function of time. 3.2.1 Zeta Potential Additional characterization methods were used to further understand interfacial interactions. Zeta potentials were measured to understand how the polymer treatment affecting particle-particle interactions at rest. Dilute (1 wt.%) solutions of silica nanoparticles, PMMAcoated or bare, in aqueous solutions were analyzed for zeta potential. Figure 6 shows zeta potential data for



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gle sphere, Rg is related to the radius of the sphere, R, by 3⁄5 .

Table 1. Guinier-Porod fit parameters for the bare and PMMA-treated USANS data sets.

Figure 6. Zeta potential data for dilute solutions of silica colloids (bare or PMMA-coated) in deionized water. Solutions were titrated using 1M HNO3 and 1M HN4OH. Dashed lines are included only to show trends. 3.2.2 Ultra small angle neutron scattering Ultra small angle neutron scattering (USANS) was performed on solutions with PMMA-coated and bare particles to understand the degree to which the respective particles aggregate in quiescent media and under shear. USANS measures scattering length density variations for large structures in solution. Specifically, while zeta potential and rheology reveals general aggregation trends, USANS provides average aggregate size and surface features. Initially, low loading solutions (1 wt.% concentration in propylene carbonate) were analyzed using bare and PMMA-coated or bare silica nanoparticles. The sample cuvettes were rotated to promote good mixing. Figure 7 shows USANS data for these solutions of PMMA-coated and bare particles. The solid lines show a representative fit of the data. A Guinier-Porod model was used to fit the data. This model parametrizes the scattering intensity as a function of a radius of gyration of an object Rg, a unitless dimensionality parameter s, and a unitless Porod parameter d which relates to the object’s inhomogeneity43. The fit parameters are shown in Table 1. The fluctuations for Q > 10^-3 are due to statistics. As we get in a Q range where very few neutrons are detected, the statistical error on the resulting intensity is large. The error bars on the points are shown on the figures. The fitted curves agree with the data points within errors, and the overall chi squared values are close to unity. The radius of gyration is defined as follows:

Rg [Å]

s

d

χ2

1 wt% bare SiO2 1 wt% PMMA/SiO2

2680 ±130 2060±230

0.99±0.05 1.0±0.1

4.9±0.4 3.4±0.4

1.0 0.9

15 wt% bare SiO2 15 wt% bare SiO2 sheared 15 wt% PMMA / SiO2 15 wt% PMMA/ SiO2 sheared

3060±60 3540±90 3310±140 3650±160

0.00±0.02 0.00±0.03 0.08±0.03 0.10±0.03

6.8±1.4 5.6±0.6 5.0±0.7 4.0±0.2

1.1 0.7 0.5 0.8

A greater intensity is observed in 1 wt. % solutions with bare particles in the range of Q ~ 10-4 than those with PMMA-coated particles. This implies that bare solids have a greater diameter than PMMA-coated solids. Originally this was thought counterintuitive, as the PMMA-coated particles should have a greater volume with the addition of PMMA brushes, resulting in a greater intensity. It is more likely, however, that particles will not be isolated and small agglomerates will form in solution. Thus, the greater intensity in the bare particle data suggests that the aggregate size of bare particles in solution is greater than that of PMMA-coated particles. This is consistent with rheological data and zeta potential data, and shows that the PMMA treatments aids in stabilizing particles in equilibrium. From the fits of the USANS data the 1 wt% samples have Rg values consistent with a single silica particle in solution (average diameter about 650 nm based on SEM data). At the higher 15 wt% concentrations the Rg values are 40-130 nm larger indicating aggregation. This aggregation is likely a combination of 1, 2 and 3 member SiO2 clusters resulting in a larger average diameter. With the application of shear the Rg increases by another 40-50 nm indicating the increased clustering of the silica with shear though again averaged over a range of small aggregates (i.e. 2, 3 SiO2 units).

1

where V is the volume of the object considered. Depending on whether we have aggregation in our system, V will either be the volume of our nanoparticle or the volume of the aggregate. Since aggregates can be formed of any number of nanoparticles, the measured value of Rg will be an average over all aggregate types in our system. For a sin-


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Figure 7. Markers show USANS data for 1 wt.% bare and PMMA-coated silica particles in propylene carbonate. Fitting is represented by solid lines. USANS investigations of higher concentration (15 wt. %) shear-thickening electrolytes at rest and under shear using a rheometer were also performed. Use of the concentric cylinder rheometer attachments due to USANS sampling requirements encouraged sedimentation of bare silica nanoparticles quickly, so we opted to limit solids concentrations to 15 wt.%. USANS was performed on the solutions first at rest, then under shear (60 s-1) for the duration of the procedure. Figure 8 shows the USANS data collected for solutions containing PMMA-coated or bare particles, at rest and under shear. It is interesting to note that the intensity observed for both higher solids content solutions is similar (ca. 1.1∙10-3 cm-1), while, the difference in intensity between the low solids concentration solutions (Figure 7) is observed to be much larger. For smaller aggregates at 1 wt.%, the coating will yield a larger relative change in volume due to relatively more surface area available to the coating, which will result in a larger intensity change. Because of the configuration of the concentric cylinder attachment, at 15 wt.% the colloids in solution will be initially rearranged into aggregates, and thus the relative intensities observed should be similar.

Figure 8. A) USANS data of bare silica nanoparticles for at rest (black) and under shear (gold). B) USANS data of 15% PMMA-coated silica nanoparticles for at rest (blue) and under shear (green) The intensity increases slightly under shear in solutions with bare particles (from ca. 1.1 to 2.2*10-3 cm-1) and PMMA-coated particles (from ca. 1.1 to 1.6*10-3 cm-1), as was expected in Figure 8. This is attributed to an increase in particle aggregation as generally observed in sheared dilatants.44-45 Fit results are shown in Table 1. The increase in intensity is smaller in sheared solutions with PMMA-coated particles than that of solutions with bare particles. Likely the aggregation of particles in solution with PMMA-coated particles is smaller than that of bare particles, further confirming that the PMMA brushes act to sterically stabilize the silica nanoparticle surfaces without completely negating shear-thickening effects. While aggregation behavior is lesser with PMMA-coated particles, the shear imparted by vehicle collisions will elicit a much greater shear-thickening response than at the shear rates used experimentally. The USANS results also suggest lower tendency to aggregate, which will yield more evenly dispersed particles in solution for longer periods of time. For many electrochemical applications, this is a great benefit, as the solutions would exhibit shear-thickening behaviors even after long calendar times.



3.3 Stability During Electrochemical Cycling To verify the viability of these colloidal dispersions as shear thickening electrolytes, their influence on the cycling performance of lithium ion cell was assessed. Electrochemical full cells consisting of graphite anodes and LiNi1/3Mn1/3Co1/3O2 (NMC) were cycled galvanostatically at a C/3 rate (based on the capacity of the cathode). Figure 9 shows cycling data for coin cells with shear thickening electrolytes containing either 30 wt% bare silica or PMMAcoated silica colloids. The 30 wt% samples were selected to reflect the maximum viscosity of the samples which should limit cycling performance due to ionic conductivity. Coin cells with neat electrolyte containing no silica were cycled for comparison. In electrochemical cells using conventional ethylene carbonate (EC) and dimethyl carbonate (DMC) solvent based lithium ion battery electrolytes (1.2M LiPF6 3:7 wt% EC:DMC). The EC/DMC system was investigated, instead of the PC based system used for rheology, because PC does not form a stable SEI on graphite. This change will not affect rheological behavior as noted in our previous report. 7 For cells without silica nanoparticles, a steady reversible charge capacity of 141 mAh g-1 was observed over 100 cycles. The coulombic efficiency stabilizes to 99.90 ± 0.12% (excluding first three cycles) which is due to a stable SEI and indicates less capacity loss to continual degradation of the electrolyte. The cycling behavior observed is consistent with other studies of NMC, and was used as a baseline to compare the cells employing the shear thickening electrolytes.46 CV and charge discharge profiles are shown in Supplemental Figure 1. The deviation in capacity at 20 cycles was due to a loss in temperature control. Capacity is observed to be lower in cells with shear-thickening electrolyte containing PMMA-coated silica nanoparticles starting with 140 mAh g-1 reversible capacity and ending with 122 mAh g-1. The increase in capacity is attributed to less SEI formation and consumption of Li as well as the reduced ionic conductivity of the shear thickening electrolytes. Average coulombic efficiency (excluding first cycle) was 99.78 ± 0.21% The additional slight loss in capacity can be attributed to the presence of electrically insulating PMMA brushes on the surface of the silica nanoparticles, which are non-ionically conductive. The similar decline in capacity can also be attributed exposed silica nanoparticle surfaces, introducing water into the

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electrolyte. Cells with electrolyte containing bare silica nanoparticles have the lowest capacities starting with a discharge capacity of 126 mAh g-1 and ending at 108 mAh g-1 after 100 cycles. The loss in capacity can be explained by the introduction of non-ionically conducting silica nanoparticles, hindering ion mobility, as well as the NernstEinstein equation, which has been previously used to support the reported observation that ionic conductivity is inversely proportional to viscosity, which in turn is proportional to concentration.47-48 Previous comparisons of conventional and shear-thickening electrolytes show that ionic conductivity is lower in the dispersions.7 Average coulombic efficiency (excluding first cycle) was 99.85 ± 0.32%. The steady decline in capacity over time is most likely attributed to the presence of tightly bound water on the surface of the silica nanoparticles. Although the separators were dried above 100 °C overnight, there is a complex of hydroxyl and water groups that can only be removed at temperatures exceeding 190 °C. While the particles could be dried beyond such temperatures, the separators they were cast on would suffer degradation. The small amount of water on the surface may be auto-catalyzing the LiPF6 in the electrolyte to form HF over time. Electrode etching over time may explain the decline in capacity.49 To understand how sedimentation affects conductivity over time, interdigitated electrodes (IDEs) were positioned 1 mm below the bottom of the meniscus of shear thickening electrolytes (30 wt.% solids, 1.2M LiPF6 3:7 EC:DMC) in 20 ml vials to track supernatant formation as a function of time. The electrolytes were left to rest at 25 °C for 24 hours. Electrolyte conductivity data as a function of time is given in Figure 10. The conductivity of electrolyte with no silica was initially 11.5 mS cm-1 and 11.6 mS cm-1 after 24 hours. Initial conductivities of electrolytes with particles with bare and PMMA-coated silica nanoparticles were lower, measured at 10.1 and 5.4 mS cm-1, respectively. This was expected as the 30 wt.% solids in electrolyte solution will have a higher viscosity. After 24 hours of rest, conductivities measured for the bare and PMMA coated silica based electrolytes were 11.6 and 6.1 mS cm-1, respectively. Sedimentation is expected to lead to an increase in electrolyte conductivity measured near the IDEs. As phase separation occurs and a supernatant forms, the decreased solid content during sedimentation increases the fraction of conductive liquid phase and decreases the

10 Figure 9. Cycling data for coin cells using shear thickening electrolytes, which were 30 wt.% bare or PMMAcoated silica colloids in 1.2M LiPF6 3:7 wt% EC:DMC . Cells electrolyte containing no silica colloids were ACS Paragon Pluswith Environment used for comparison. Graphite anodes were cycled against NMC cathodes at a cycling rate of C/3 from 3 to 4.2V

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local viscosity near the IDEs, both of which result in increased conductivity.47 Conductivity data revealed that the conductivity of electrolyte with no silica changed little over 24 hours, as expected. Minor fluctuations in conductivity are mostly likely due to temperature changes. Also, as expected, addition of solid non-ionically conducting particles reduced initial conductivity slightly, again due to increased viscosity and reduced conductive phase near the IDEs. However, the change in conductivity of electrolytes with bare and PMMA-coated silica nanoparticles differs greatly over 24 hours as sedimentation is allowed to occur. The conductivity of electrolytes with bare silica nanoparticles increases drastically over four hours, suggesting that a supernatant forms quickly, rapidly decreasing local viscosity near the IDE surface. The solution visually appeared to be phase separated. Beyond four hours the conductivity remained similar to that of the neat electrolyte, most likely due to near-complete sedimentation. The conductivity of electrolytes with PMMA coated silica nanoparticles increases shortly after the initial measurement, most likely due to the electrolyte settling to equilibrium. However, after this period, the conductivity remains comparable to the conductivity of the neat electrolyte and stable, an indication that the composition of the dispersion near the interdigitated electrodes is constant. This confirms that the PMMA coating aids in sterically stabilizing the particles and preventing sedimentation.

measurements. Additionally, our results suggest that introduction of highly concentrated (30 wt. %) shearthickening electrolytes to lithium ion batteries do not severely adversely affect overall cell capacity. Conductivity measured near the meniscus of electrolytes with PMMAcoated silica remains stable at 6.1 mS cm-1 for 24 hours at rest, whereas the conductivity of electrolyte with bare silica increases from 10.1 to 11.6 mS cm-1 over the same time period as phase separation and supernatant formation occurs. This finding confirms the PMMA-coating successfully sterically stabilizes the particles in solution at rest. Electrochemical cycling of full lithium ion cells (NMC vs graphite) also confirms that the PMMA-coated silica colloid electrolytes are viable for use in lithium ion batteries, with 86.5% capacity of cells with conventional electrolytes after 100 cycles. Several considerations must be made going forward for proper implementation of shear-thickening electrolytes in lithium ion batteries. PMMA-coated silicon nanoparticles should be dried sufficiently to minimize the presence of water in electrolyte. Drop casting is currently an acceptable technique for small-scale cell assembly. However, an alternative casting method that accommodates the shear-thickening behavior will be necessary for fabrication of larger format cells. Future research will examine the effect of additional parameters on dispersion stabilization and electrochemical performance, such as the size and composition of the colloids and polymer brushes.

AUTHOR INFORMATION Corresponding Author *Gabriel M. Veith, [email protected] Author Contributions The manuscript was written through contributions of all authors. BHS fabricated samples, performed electrochemistry, USANS data collection, rheology. BLA lead the zeta potential measurement and rheology. MD, LH, JFB, MA planned and implemented the USANS study. WET supervised the ATRP synthesis. GMV lead the experimental effort. All authors have given approval to the final version of the manuscript.

Acknowledgments Figure 10. Conductivity of 30 wt% solids in electrolyte (1.2M LIPF6 3:7 EC:DMC) as a function of time. At rest, particle sedimentation occurs, affecting conductivity. Interdigitated electrodes were used (cell constant 2 cm-1) to track supernatant formation. 4. Conclusion In this work, we demonstrate the surface functionalization of silica colloids with PMMA brushes via ATRP for steric stabilization in alkyl carbonate electrolytes used in lithium ion batteries. Improved stabilization of silica nanoparticle colloids was confirmed by in situ ultra small angle neutron scattering, rheology, and conductivity

This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists, Office of Science Graduate Student Research (SCGSR) program (BHS). The SCGSR program is administered by the Oak Ridge Institute for Science and Education for the DOE under contract number DE-SC0014664. This work was supported by the National Science Foundation under IGERT award #DGE-0966089 (BHS). A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. Small angle scattering measurements were done using the USANS instrument and the Spallation Neutron Source a DOE Office of Science

User Facility operated by the Oak Ridge National Laboratory. GMV and BLA were supported by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy,



under Award Number DE-AR-0869-1617. The authors thank Tracie Lowe for taking the scanning electron micrograph images. The authors particularly thank Ping Liu, Susan Babinec and Julian Sculley for their helpful discussions. This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

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PMMA Brushes

ATRP

Aggregated Silicon Nanoparticles

Sterically Stabilized Silicon Nanoparticles

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ACS Paragon Plus Environment

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Shear Thickening Electrolyte Built from Sterically Stabilized Colloidal

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