Effect of Nanorod Aspect Ratio on Shear Thickening Electrolytes for

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Effect of Nanorod Aspect Ratio on Shear Thickening Electrolytes for Safety-enhanced Batteries Yilan Ye, Han Xiao, Kelley Reaves, Billy McCulloch, Jared F. Mike, and Jodie L. Lutkenhaus ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00457 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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ACS Applied Nano Materials

Effect of Nanorod Aspect Ratio on Shear Thickening Electrolytes for Safety-enhanced Batteries Yilan Ye1, Han Xiao2, Kelley Reaves2, Billy McCulloch2, Jared F. Mike*, 2, and Jodie L. Lutkenhaus*, 1 1

Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, United States 2

Lynntech Inc., 2501 Earl Rudder Fwy, College Station, Texas 77845, United States

KEYWORDS: Silica nanorods, aspect ratio, shear thickening electrolyte, rheology, battery

ABSTRACT: Shear thickening electrolytes are of increasing interest as they offer potentially enhanced battery safety during extreme impact. Current shear thickening electrolytes consist of a suspension of spherical nanoparticles in battery electrolyte media, but these tend to be effective only at high loadings and do not display discontinuous shear thickening. Accordingly, the present challenge is to maximize the shear thickening effect at the lowest particle loading. Here, anisotropic silica nanorods with aspect ratios ranging from 2 to 24 are explored as alternatives to

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the current paradigm of spherical nanoparticles for shear thickening electrolytes. As the aspect ratio increases, the critical volume fraction for shear thickening decreases. Discontinuous shear thickening in which solidification takes place is achieved by using nanorods of aspect ratio 24 at the relatively low volume fraction of 0.146. Interparticle attractions originating from the interaction between the nanorod’s solvation layer and the electrolyte cause hysteresis in the critical shear stress for shear thickening. Moreover, interparticle attractions increase with decreasing aspect ratio, which is attributed to the increasing surface area of the silica nanorods. Therefore, shear thickening is due to the combined effects of particle aspect ratio, interparticle attractions, hydrodynamic forces, and frictional contacts. A rheological state diagram is created for future materials design, and ballistic tests demonstrate a 37 % reduction in blunt force due to the shear thickening electrolyte.

1. INTRODUCTION Conventional liquid electrolytes are an important contributing factor to battery safety and reliability. Upon puncture or mechanical impact, the electrolyte may serve as fuel for a runaway thermal excursion, resulting in catastrophic failure. Therefore, the challenge is to design resilient battery materials that prevent or reduce mechanical failure. This is important for applications such as conformal wearable batteries, which are inserted into military body armor for ballistic protection; electric vehicle batteries, which must withstand the impact of car crashes; and batteries for prosthetic limbs, which may deform with body motion while protecting the body from dynamic impact. Shear thickening electrolyte fluids, implemented as impact-resistant electrolytes, may possibly address these challenges. However, the shear thickening electrolyte must also be ionically conductive (σ ~ 10-3 S/cm) and must facilitate normal battery operation.


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Shear thickening fluids, consisting of particles suspended in a fluid medium, dramatically increase in viscosity and become solid-like under shear. They are promising materials for energy dissipating systems, such as brakes, dampers, helmets, anti-puncture gloves, and bullet-proof fabrics.1-5 The shear-stress dependent behavior of a shear thickening fluid is due to the effects of Brownian, interparticle, hydrodynamic and frictional contact forces.6-8 At low stress, Brownian and interparticle forces dominate, and suspensions present either Newtonian or shear thinning behavior.6-7 As stress increases, hydrodynamic forces exceed Brownian forces, and particles form hydroclusters.9 These hydroclusters are highly dissipative due to a lubrication film occurring between the particles, which lead to continuous shear thickening.7-12 For dense suspensions, the lubrication film breaks down and frictional contacts between particles become dominant.8, 13-21 Therefore, discontinuous shear thickening takes place, where viscosity increases with increasing shear stress, but is independent of shear rate. When the concentration increases further, dynamic shear jamming and impact-activated solidification have been observed.22-24 For impact-resistant electrolytes, shear thickening and solid-like regimes are of interest. Microscopically, particle dimension and interparticle forces are two critical parameters to regulate shear-thickening behaviors. Concerning particle dimensions, it is reported that the critical shear stress for shear thickening decreases with the diameter of hard sphere particles.9, 14, 25

As particle anisotropy increases, shear thickening occurs at a lower loading, which can be

attributed to poor particle packing in the unaligned state. Egres et al.26-27 have investigated rodlike particles by using rheology simultaneously with small angle neutron scattering (SANS). Their research suggests that the mechanism responsible for shear thickening in anisotropic particle suspensions is analogous to the transient hydrocluster mechanism observed for spherical particle suspensions. Moreover, Brown et al.28 found that nonconvex hooked rod particles


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exhibited stronger shear thickening due to larger interparticle friction. These results highlight the promise of anisotropic nanoparticle suspensions for shear thickening fluids, but it is not clear if these same findings hold in battery electrolyte, which tends to have a high ionic strength (>0.5 M) and is aprotic. As for interparticle forces, ideally particles should be individually well-solvated and behave like hard spheres.6 The most commonly studied shear thickening systems consist of silica particles and solvents with hydroxyl groups (-OH). Raghavan et al.29 systematically studied the rheology of fumed silica suspended in various solvents. They found that for solvents bearing OH end groups, the fumed silica particles were well-solvated and shear thickening occurred; in contrast, for solvents bearing methyl end groups (–CH3), poor solvation of silica resulted in shear-thinning behavior. A similar situation applies to cornstarch mixtures. In water, the cornstarch mixture is a classical shear thickening fluid; however, in non-aqueous medium the mixture is shear thinning.30 Apart from solvent effects, heat treatment31 and pH control32 can regulate shear thickening behaviors by changing the surface chemistry as well as interparticle forces of the silica particles. In addition, sterics33-34 or electrostatic repulsion9, 25 could postpone or even eliminate shear thickening6 and strong particle-particle attractions are found to diminish shear thickening.35-36 The design of shear-thickening battery electrolytes faces several challenges. For lithium ion batteries, the majority of electrolyte solvents fall into organic esters and ethers,37 whose end groups are mainly hydrophobic, and no -OH groups exist. This negatively impacts the solvation of the silica particles. Further, shear-thickening behavior is generally observed at high particle loadings, and because silica particles are not conductive, the overall electrolyte conductivity may be reduced. Ideally, the shear thickening electrolyte should possess the desired rheology at a


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minimal silica particle loading so as to maintain good conductivity, and the electrolyte should be compatible with the cathode and anode chemistries. There have been several attempts to develop shear thickening electrolytes.38-40 For fumed silica with conventional electrolytes, Ding et al.38 reported shear thickening phenomena, but Veith et al.39 and Raghavan et al.29 only observed shear thinning. The latter observation was reasoned as being a result of the poor solvation state of fumed silica particles in the electrolyte and the dispersity.29,

39

By surface functionalization of silica with poly(methylmethacrylate)

(PMMA) brushes, Shen et al.40-41 developed shear thickening electrolytes. They observed reasonable conductivity (σ > 10-3 S/cm) at low and medium shear, and an order of magnitude drop in conductivity under shear thickening conditions, which indicated an internal shut-off mechanism. Veith et al.39 have demonstrated over 200 charge-discharge cycles with no loss in capacity for their shear thickening electrolyte. Notably, all of these examples utilized spherical particles, and anisotropic particles have not yet been investigated. Here, we investigate the shear thickening behavior of silica nanorods with aspect ratios ranging from 2 to 24 in battery electrolyte. The synthesis is shown in Scheme 1. It is hypothesized that anisotropic particles will lead to shear thickening behavior at particle loadings lower than that of spherical particles. Systematic rheological characterizations were conducted, and the influence of nanorod aspect ratio on the maximum packing density, interparticle attractions, and shear thickening was investigated. Electrochemical cycling of the electrolyte in a Li-ion battery pouch cell was performed. A ballistic test was conducted on a pouch cell battery containing the shear thickening electrolyte to demonstrate the significantly reduced impact depth.


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Scheme 1. Silica nanorods of controllable aspect ratio are synthesized from tetraethyl orthosilicate (TEOS) using sodium citrate, polyvinylpyrrolidone (PVP), ammonium hydroxide, ethanol and pentanol in an emulsion approach.

2. RESULTS AND DISCUSSION 2.1 Morphology characterization

Figure 1. Transmission electron microscopy (TEM) images of (a) AR2, (b) AR5, (c) AR8, (d) AR14, and (e) AR24 silica nanorods. AR = aspect ratio. (f) Ascending and descending shear


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sweeps for AR14 nanorods at = 0.226, as an example of the preshear protocol applied to each sample. The silica nanorods are suspended in an electrolyte consisting of EC, EMC (volume ratio 1:1) and 1 mol/L LiTFSI. The low, medium, and high shear regions are shaded pink, blue, and yellow respectively.

Figure 1 shows transmission electron microscopy (TEM) images of silica nanorods of varying aspect ratio (AR), for which a sample name of ARX corresponds to an average aspect ratio of “X”, as determined from multiple TEM images. Table 1 gives the diameter, length, and aspect ratio of AR2, AR5, AR8, AR14 and AR24 silica nanorods as determined from sampling about 50 nanorods per set. Similar to previous research,42 the silica nanorods exhibited a relatively constant diameter, but length and aspect ratio could be controlled. Table 1 also shows that the dispersity or standard deviation increases with length and AR. The synthesis conditions of these nanorods of varying AR are discussed in the Supporting Information and Table S1, and particle diameter and length distributions are shown in Figure S1. Table 1. Particle dimensions, TGA mass loss, maximum packing density, and stickiness parameters of silica nanorods of various aspect ratios Sample

Length (nm)

Diameter (nm)

Aspect ratio

TGA mass loss (wt %)

,

AR2

790±70

410±50

1.8±0.3

7

0.46

0.56

0.0020

AR5

1760±320

330±70

5.5±0.9

9

0.39

0.45

0.0024

AR8

2180±440

260±40

8.3±1.3

13

-a

-a

-a

AR14

4210±1830 290±20

14.1±3.3

15

0.32

0.31

0.0037

AR24

6150±2140 260±40

23.6±6.6

15

0.23

0.18

0.0032

a

,

Not obtained due to limited materials availability


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Thermogravimetric analysis of the silica nanorods was conducted upon heating from 120 to 500 °C, after first holding isothermally at 120 °C for 30 min. The resultant weight percentage and derivative weight as a function of temperature are shown in Figure S2. The weight losses for AR2, AR5, AR8 and AR24 nanorods are shown in Table 1. In general, the larger the aspect ratio, the larger the mass loss, with up to 15 wt% weight loss observed for AR24 nanorods. Similar results were observed by Murphy et al.,42 who attributed the mass loss to residual PVP entrapped within the porous silica network. To verify this, we conducted X-ray photoelectron spectroscopy (XPS) for AR8 nanorods to determine the atomic surface composition, Figure S3. The atomic surface composition for N, C, O, and Si was 3.27 %, 37.63 %, 35.80 %, and 23.31 %, respectively. The existence of N and C indicates the existence of PVP at the nanorod surface. It has been reported43 that PVP forms hydrogen bonds with the silica surface through participation of electron-donor centers which helps stabilize the nanoparticles.

2.2 Rheological characterization Figure 1(f) shows the ascending and descending shear sweeps for AR14 at = 0.226, as an example of the preshear protocol applied to each sample. The first ascending and descending shear sweeps aimed to erase the sample shear history. The second ascending and descending shear sweeps gave the rheological information of interest, where the low shear, medium shear, and high shear regions are shaded in pink, blue, and yellow, respectively. At low and high shear regions, reversible shear thinning and shear thickening were observed, while there existed hysteresis in the medium shear region. Note that the shear thickening was not due to secondary flow,44 since a Newtonian plateau at 3.2 mPa · s was observed for EC/EMC/LiTFSI at shear rates from 10 to 1000 s-1. The reversible shear thickening in the high shear region is due to the


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formation and disappearance of hydroclusters in response to the hydrodynamic force, which is in agreement with classical shear thickening phenomena.6, 9 The hysteresis in the medium region indicates partial recovery of the sample structure. This is a typical thixotropic phenomenon,45 in which recovery of interparticle attractions is time dependent. In the low shear region, the reversible viscosity indicates that the shear rate is slow enough for full recovery of interparticle attractions. Therefore, interparticle attractions and hydrodynamic forces dominate in the low and high shear regions, respectively, and there exists competition between interparticle attractions and hydrodynamic forces in the medium shear region.


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Figure 2. Dependence of viscosity () on shear stress () and shear rate ( ) for silica nanorod / electrolyte suspensions containing (a, b) AR2, (c, d) AR5, (e, f) AR8, (g, h) AR14, and (i, j) AR24 nanorods, respectively. Shear thickening regions are shaded in yellow. The legends describe the nanorod volume fraction . Solid and dot lines represent the ascending and descending shear sweeps, respectively.

Figure 2 shows the dependence of viscosity () on shear stress () and shear rate ( ) for silica nanorod / electrolyte suspensions of nanorods of (a, b) AR2, (c, d) AR5, (e, f) AR8, (g, h) AR14 and (i, j) AR24, respectively. Similar to the phenomena in Figure 1(f), shear thinning and hysteresis in low and medium shear regions were observed for all the suspensions. To analyze the trend of shear thinning as volume fraction increased, a power law was fitted to viscosity as a function of shear rate (Figure S4) and the power law exponent was obtained (Figure S5). Note that due to the complexity of the shear thinning behavior, only the first shear thinning regions were analyzed, while other shear thinning regions were ignored. For nanorods of various aspect ratios, roughly decreased from −0.2 to −1 as volume fraction increased, indicating growth of shear thinning and development of yield (when decreased to -1).6, 46 As for high shear regions, reversible shear thickening, shaded in yellow, was observed for nanorod suspensions of AR5, AR8, AR14 and AR24. To analyze the trend of shear thickening as volume fraction increased, a power law was fitted to viscosity as a function of stress at high shear regions (Figure S4) and the power law exponent n was obtained (Figure S6). In particular, continuous shear thickening, indicated by 0 < < 1,6, 8 occurred for AR5, AR8, AR14 and AR24 nanorods at critical volume fractions of 0.358, 0.226, 0.226, 0.107, respectively. Note that only weak shear thickening was observed for AR5 and AR8, where viscosity doubled. Discontinuous shear


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thickening, indicated by = 1,8, 27 occurred for AR14 and AR24 nanorods at critical volume fractions of 0.268 and 0.146, respectively. Apart from the power law exponent, the direction of normal force can indicate the nature of shear thickening. In particular, normal force is negative for continuous shear thickening where hydrodynamic forces cause the distortion of suspension; in contrast, normal force is positive for discontinuous shear thickening where frictional contact networks cause the dilation of suspension.17, 21 Figure S7 shows that normal force > 0 for AR14 at = 0.268 and AR24 at = 0.146 at high shear stress, which further confirmed that the nature of discontinuous shear thickening is frictional contacts, rather than hydrodynamic forces. Note that during discontinuous shear thickening, as stress further increased, samples became solid-like and fractured, and eventually ejected from the geometry. These critical volume fractions were used to map out a rheological state diagram (vide infra). These results show that as aspect ratio increases, critical volume fraction for shear thickening decreases. This is usually attributed to the lower packing density of particles for larger aspect ratios. Interestingly, we found significant differences between our system and that containing calcium carbonate nanorods of aspect ratios of 2, 4, and 7 suspended in polyethylene glycol (PEG, Mw = 200).27 The diameters of the calcium carbonate nanorods are comparable to those of our samples. No significant shear thinning or yielding behavior was observed in the calcium carbonate system, which indicated that no significant interparticle attractions existed. In our system, the yielding, shear thinning and hysteresis behaviors suggest that strong interparticle attractions exist, deviating from the classical shear thickening systems of hard particles. The source of interparticle attractions could be attributed to residual PVP at the silica surface, and will be discussed later using a sticky hard particle model.47 In addition, no shear thickening was


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observed for the AR2 nanorod / electrolyte suspension studied here, whereas the calcium carbonate AR2 nanorod / PEG suspension exhibited both continuous and discontinuous shear thickening.26-27 It is possible that the shear thickening of AR2 nanorods in the electrolyte suspension was prevented due to strong interparticle attractions, consistent with studies elsewhere,35-36 which show that interparticle interactions could diminish shear thickening. In the following, we will discuss first the maximum packing density of particles of various aspect ratios, second interparticle attractions on the basis of a sticky hard particle model, and finally the complementary effects of aspect ratio and interparticle attractions on shear thickening behaviors. The maximum packing density ( ) can be obtained by fitting relative viscosity (! ) and volume fraction () using equation 1. In particular, ! is the ratio of the suspension viscosity () to the solvent viscosity (" ). This is a semi-empirical equation widely applied to suspensions of colloidal spheres6,

48

and rods.27 Recent trials on cubic particles were not

successful.49 Note that ! and are shear rate-dependent parameters. This is because interparticle attractions and hydrodynamic forces dominate at low and high shear regions, respectively; therefore, the packing density of nanorods at different shear rates changes. In particular, ! and are written as !,# and

,#

at low shear rate, and !, and

,

at

high shear rate. ! = (1 − Interestingly,

,#

%

%&'(

)*+

(1)

could not be obtained by fitting equation 1 due to the high value of !,# ,

indicating strong interparticle attractions; notably, equation 1 does not account for these attractions. However,

,#

could be estimated as the critical volume fraction beyond which the

suspension did not flow (became jammed). Specifically, samples above were expelled from the geometry upon low shear. In contrast,

,

,#

fractured and

could be obtained by


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fitting equation 1 to the data, where interparticle attractions were destroyed by hydrodynamic forces. For suspensions that showed no shear thickening, !, was taken at 1000 s-1, and for suspensions that showed shear thickening, !, was taken as the viscosity just before shear *,/+

thickening. Figure 3(a) shows !, plotted against , from which , was extracted from the reciprocal of the slope. Table 1 lists

,

and

AR5, AR14 and AR24 nanorods, respectively. Both aspect ratio. For AR14 nanorods,

,

.

,#

,

,#

for suspensions containing AR2,

and

,#

decrease with increasing

, which shows that the influences of

interparticle attraction and hydrodynamics on packing density are comparable. For AR < 14,

,

<

,#

and interparticle attractions dominate; for AR > 14,

,

>

,# ,

and

hydrodynamic effects dominate. It has been reported that shear may increase the maximum packing density due to the alignment of rod-like particles.6 Moreover, interparticle attraction can also increase the maximum packing density by reducing interparticle distance.50

*,/+

Figure 3. (a) High shear viscosity data (!, ) from shear sweep experiments on silica nanorod / electrolyte suspensions plotted against particle volume fraction () according to equation 1. Lines represent fits to the data and are used to determine . (b) Illustration of random packing of particles with increasing aspect ratio. (c) Low shear viscosity (!,# ) at a shear rate of


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1 s *, plotted against the reduced volume fraction (/ ) for nanorod suspensions containing AR2, AR5, AR14, and AR24, respectively. Lines represent fits to the data according to equation 2. (d) Dependence of critical shear stress (3 ) on volume fraction () for nanorod suspensions of AR5, AR8, AR14, and AR24, respectively. Filled and empty symbols represent 3 during the ascending and descending shear sweeps, respectively. Data were selected from the shear thickening cases shown in yellow in Figure 2.

Philipse51 has calculated that for AR > 15 , a rod contacts 5 to 6 neighboring rods in a mechanically stable packing. For a rod of length L and diameter D, the excluded volume is of order of 56+ . As L and AR increase, the excluded volume increases, and the packing density decreases. Figure 3(b) illustrates the decreased random packing of rods with increasing AR. According to the simple equation by Philipse51, =

7.8±#.+ :;

; the theoretical maximum

packing density for AR24 nanorods was calculated as = 0.225. This value is close to the experimentally observed value of

,

= 0.23 listed in Table 1.

! () = =1 −

%

%&'(

>

*+

(1 +

,.@%A BC

)

(2)

In Figure 3(c), the low shear viscosity (!,# ) is plotted against the reduced volume fraction ( / ) for nanorod suspensions containing AR2, AR5, AR14, and AR24, respectively. Equation 2 was fit to the data to obtain the stickiness parameter (DE ). Note that equation 2 is an extension of equation 1, which combines contributions from the excluded volume interaction (the first term) and interparticle attractions (the second term).52-53 In particular, the maximum


15


packing density at low shear (

,# )

Page 16 of 35

from Table 1 was used as . Note that DE should be

properly obtained by fitting equation 2 to the zero shear viscosity. Since the zero shear viscosity is inaccessible due to lack of a Newtonian region, the viscosity at a shear rate of 1 s *, was used instead. Fits to the viscosity at a lower shear rate (e.g, 0.1 s-1) may give more reliable estimation of DE , but poorer fitting quality was observed (Table S2, Figure S8). Therefore, we report DE estimated using the viscosity at the shear rate of 1 s-1 for nanorod suspensions of AR2, AR5, AR14, and AR24, as listed in Table 1. In particular, DE is inversely related to the strength of interparticle attractions. When DE approaches infinity, ideal hard sphere behavior is obtained; when DE decreases, interparticle attractions increase, which can lead to particle aggregation, gelation, and attractive-driven glass transitions.52-53 Theoretically, suspensions of hard spheres become phase separated when DE is below ~0.1.53-55 In comparison to AR14 and AR24 nanorods, AR2 and AR5 nanorods exhibit relatively lower DE values, which indicate that attractive forces increase with decreasing aspect ratio. This is rationalized by considering that surface area increases with decreasing aspect ratio, and attractive interactions increase with surface area. However, all of these DE values are more than an order of magnitude lower than the critical value for phase separation. Experimentally, rather than phase separation, we observed gel-like structures for nanorod suspensions since their storage and loss moduli show weak dependence on frequency (Supporting Information, Figure S9). Similar gel structures have been observed by Raghavan et al.29 for fumed silica suspended in propylene carbonate and dimethyl carbonate. Most commonly, DE is obtained by fitting a sticky hard sphere model to small angle neutron scattering (SANS) data. Krishnamurthy et al.54 reported that DE obtained from equation 2 using rheology is comparable to that obtained from SANS.


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As mentioned before, the silica nanorods have residual PVP at the surface. It was found that PVP dissolved well in the EC/EMC/LiTFSI electrolytes. Therefore, it is possible that PVP serves as a solvation layer that facilitates dispersion of the silica nanorods. In contrast, we found that these nanorods dispersed poorly in tetraethylene glycol dimethyl ether (TEGDME)/TFSI and that PVP was insoluble in TEGDME. Moreover, after PVP was removed by heat treatment of the nanorods at 500 ◦C, the nanorods did not disperse well in both EC/EMC/LiTFSI and TEGDME/LiTFSI. Hydrodynamic size and zeta potential of AR2 nanorods were measured (Table S2). The zeta potential indicated that silica nanorods were stable (F = −49.6 ± 1.1mV) in H2O and were less stable in electrolyte (F = 16.2 ± 0.2mV). We speculate that nanorods of various aspect ratios exhibit similar zeta potential since they possibly possess similar surface functionalization to the similar synthetic procedures. Without PVP, the nanorods did not disperse in the electrolyte, indicating the need for this surface layer. Concerning the high ionic strength of the electrolyte (1 mol/L), it is necessary to comment on the stability of the nanorod suspensions. Silica particles have been reported to be unstable in ionic liquids due to the significantly reduced electrostatic repulsion.55 However, it was reported that shear thickening systems with high salt concentrations (> 2 mol/L) showed the same stability as that observed in the absence of salt, and the rheology in the presence of salt was qualitatively similar to that without salt.29 In our system, suspensions were stable within the timescale of experiment. Sedimentation of the silica nanorods was observed after about 24 h. However, by shaking and vortex mixing, the particles became well dispersed again. Figure 3(d) shows the dependence of critical shear stress (3 ) on volume fraction () for AR5, AR8, AR14, and AR24 nanorod suspensions, respectively, that exhibited shear thickening (Figure 2). Filled and empty symbols represent 3 during the ascending and descending shear


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sweeps, respectively. Only filled symbols are shown for the case of 3 being the same during ascending and descending shear sweeps. Since interparticle attractions prevent shear thickening and increase as aspect ratio decreases, no shear thickening was observed for AR2 nanorods, and 3 was 49 Pa for AR5 nanorods at = 0.358 , which was relatively high. For nanorod suspensions of AR14 at = 0.268 and AR24 at = 0.146, significant hysteresis in 3 was shown. In Figure 2, these two samples exhibited discontinuous shear thickening at 49 Pa and 22.6 Pa in the ascending shear sweeps, respectively. In the descending shear sweeps, the shear thickening region was reversible; however, viscosity further decreased until the stress decreased to 2.2 Pa. The lower 3 in the descending shear sweep is attributed to the delayed recovery of interparticle attractive forces, and it could be considered as the apparent critical stress for shear thickening when no attraction is involved. Note that if 3 for AR14 at = 0.268 and for AR24 at = 0.146 in the ascending shear sweep is neglected, 3 for nanorod suspensions of AR8, AR14, and AR24 fall in a narrow stress region from 2.2 Pa to 6.5 Pa, independent of aspect ratio and volume fraction. Egres et al.27 found that for calcium carbonate nanorod / PEG systems that undergo shear thickening, the nanorod hydroclusters contain aligned nanorods. As a result, 3 followed the behavior for hard sphere suspensions where the diameter of the nanorod was invoked in calculations. Since the silica nanorods here share similar diameters, 3 remains relatively constant (box in Figure 3(d)). In our system, shear thickening is a competitive result of interparticle attractions, hydrodynamic forces, and frictional contacts, which can be regulated by aspect ratio. For future materials design, 3 may be tuned by surface chemistry to regulate interparticle attractions.


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Figure 4. Rheological state diagram for silica nanorod / battery electrolyte suspensions at various aspect ratios and volume fractions. CST and DST represent continuous and discontinuous shear thickening, respectively.

Figure 4 shows a rheological state diagram for silica nanorod / battery electrolyte suspensions at various aspect ratios and volume fractions. Jamming was assigned to samples above

,#

, so the lower boundary of jamming was

,#

. Note that jamming here

specifically refer to the solid-like states of colloidal suspensions above yielding or shear thinning behavior of samples below

,# .

,# ,

in contrast to the

Shear thinning, continuous shear

thickening (CST), and discontinuous shear thickening (DST) were assigned to the samples exhibiting those behaviors described in Figure 2. The state diagram shows that shear thickening takes place as aspect ratio and volume fraction increase. For the interest of ballistic and highimpact protection, discontinuous shear thickening is most favorable and continuous shear thickening is favorable. The state diagram also suggests that nanorods of even higher aspect ratio might provide discontinuous shear thickening at an even lower volume fraction.


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In comparing with shear thickening electrolytes with nanospheres of diameter ~ 200 nm at 30 wt %, no discontinuous shear thickening was observed.41 Herein, nanorods of aspect ratio 24 at = 0.146 (~ 20 wt%) exhibited discontinuous shear thickening – a much lower volume fraction relative to nanospheres. Also, it has been reported that monodisperse nanospheres are critical for attaining shear thickening electrolytes.39 On the contrary here, we were able to attain shear thickening with a relatively disperse set of nanorods. It is possible that nanorods of large aspect ratios may exhibit dominating effects in particle packing and rheological response. However, more systematic research should be conducted to verify the speculation.

2.3 Electrochemical performance Due to materials availability, we limited our electrochemical performance evaluation to AR5 nanorods. The conductivity was found to be 3.3 mS/cm for AR5 nanorods at = 0.33 in both proprietary and EC/EMC/LiTFSI electrolyte blends, while the conductivity was 7.92 mS/cm for proprietary electrolytes without silica nanorods. The reason for the decreased conductivity may arise either from simple dilution of the conducting electrolyte medium or from the increased electrolyte viscosity due to the nanorods.56 The viscosity of the simple electrolyte was 3.2 mPa·s for EC/EMC/LiTFSI, and the viscosity of shear thickening electrolytes at low shear can be several orders of magnitude higher, depending on the nanorod volume fraction. In comparison, the conductivity of shear thickening electrolytes based upon poly(methylmethacrylate)-modified silica nanospheres was 6.1 mS/cm at 30 wt% nanospheres, as compared to 11.5 mS/cm for the electrolyte without silica.41 We next discuss the possible conductivity behavior as a function of aspect ratio, although experimental data will be necessary for future confirmation. From Table 1, we observe that for


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electrolytes containing nanorods of increasing aspect ratios, interparticle attractions decrease (i.e., DE increases) and the maximum packing density decreases. Together, these results may indicate that more void could exist, which would benefit ion transport57 with increasing aspect ratio. Therefore, we speculate that electrolytes containing nanorods of larger aspect ratio may exhibit conductivity closer to that of the native electrolyte. At the same time, increasing the aspect ratio should allow one to access shear thickening at a lower volume fraction. To demonstrate electrochemical properties, Figure 5 shows electrochemical cycling of a CR2032 full cell containing AR5 nanorods at = 0.33 in Lynntech proprietary electrolyte. The CR2032 full cell was assembled using graphite anodes and nickel manganese cobalt oxide cathodes. The cell was cycled at rates of C/10, 1C, and 2C. The cell was cathode-limited with a nominal capacity of 148 mAh/g. The cell exhibited capacity decline from 102 mAh g−1 at C/10 to 26 mAh g−1 after 120 cycles at 1C. Capacity decline of a lesser extent was also observed in prior research41 using shear thickening electrolytes containing silica spheres with and without surface treatment. The authors speculated that the steady decline in capacity over time was most likely attributed to the presence of tightly bound water on the surface of the silica nanoparticles. The residual water could lead to electrode etching over time and consequently the decline in capacity. As this was only one cell and one nanorod aspect ratio (AR5), a deeper investigation is needed.


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Figure 5. (a) Discharge capacity and (b) Coulombic efficiency for a CR2032 full cell containing AR5 nanorods at = 0.33 and Lynntech proprietary electrolyte. The CR2032 full cell was assembled using a graphite anode and a nickel manganese cobalt oxide cathode. Celgard 2500 was used as the separator membrane. The cell was cycled at rates of C/10, 1C, 2C, and 1C.

2.4 Ballistic tests


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Figure 6. Illustration of (a) the ballistic test target, which consists of a stack of armor, battery, pressure sensitive film, and polymer clay backing, from front (site of impact) to back; (b) the profile of the test target under impact; and (c) the test target after the ballistic test.

Ballistic testing was performed as a preliminary effort to gauge the effectiveness of shear thickening electrolyte to mitigate the effects of ballistic impact. As shown in Figure 6(a), the target consisted of a stack of armor, battery, pressure sensitive film, and polymer clay backing, from front (site of impact) to back. Figure 6(b) illustrates the profile of the test target under impact. Impact evaluations were performed for the batteries behind both soft (Level II) and hard (Level III+) armor plates. The battery used was a 1.5 Ah uncharged pouch cell loaded with either a standard electrolyte (EC/EMC/LiTFSI) or shear thickening electrolyte (Lynntech proprietary blend with silica nanorods). Only one battery per test was examined due to the limited availability of the silica nanorods; more tests are needed for statistically robust results. The force of the impact was evaluated using both pressure sensitive film and polymer clay. The target was shot at a distance of 50 ft, and the bullet velocity at 10 ft from the muzzle was 1332±3 ft/s. It should be noted that penetration did not occur in any of the tests as this was determined to be a good method to evaluate differences between the two electrolyte systems. Figure 6(c) illustrates a typical test target after the ballistic test.


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Figure 7. Ballistic test results of (a) soft armor (Lv II) and (b) hard armor (Lv III). The shear thickening electrolytes consisted of AR5 nanorods at = 0.358 and Lynntech proprietary electrolytes.

Ballistic testing showed a noticeable difference between batteries with and without the shear thickening electrolyte. As shown in Figure 7, the impact depth was shallower in the case of the shear thickening electrolyte in batteries behind both Level II and Level III+ armor. A clear difference can also be seen in the impact force distribution recorded in the pressure sensitive film. Considerably less pink color was present in the films where shear thickening electrolyte was used. Image processing was performed on these photos (taken at the same distance) using the calibration standard for the pressure film. This processing demonstrated that an 18.4 N force was exerted on the film when only hard armor was present. The shear thickening electrolyte plus hard armor reduced the applied force to 11.6 N, which is a 37 % reduction in the blunt force.

3. CONCLUSION


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The shear thickening of silica nanorod / electrolyte suspensions was due to the combined effects of particle aspect ratio, interparticle attractions, hydrodynamic forces, and frictional contacts. As aspect ratio increased, both the maximum packing density and the interparticle attractions decreased. As a result, shear thickening took place at lower volume fractions as aspect ratio increased, and the critical stress for shear thickening was influenced by the interparticle attractions. Discontinuous shear thickening where solidification took place has been achieved for nanorods of large aspect ratio at low loadings. A rheological state diagram was created, which suggested that nanorods of even higher aspect ratio might provide discontinuous shear thickening at an even lower volume fraction. For future materials design, interparticle attractions can be regulated by surface chemistry, which can further adjust the yielding behavior in the low shear region and the critical stress for shear thickening. The electrochemical performance of the electrolyte in a battery needs improvement, and may perhaps be addressed by examining residual water effects. It will be important to carefully balance conductivity and electrochemical performance with particle loading so as to maintain the shear-thickening behavior. This work has indicated the future design space, by way of the state diagram, and the promise of anisotropic nanomaterials in the arena of safety-enhance battery electrolytes.

4. EXPERIMENTAL DETAILS Materials.

1-Pentanol,

polyvinylpyrrolidone

(PVP, JK = 40000 g/mol),

tetraethyl

orthosilicate (TEOS), ammonium hydroxide aqueous solution (28 %), ethanol, sodium citrate dihydrate, carbonate (EC), ethyl methyl carbonate (EMC), bis(trifluoromethane)sulfonimde lithium salt (LiTFSI) were purchased from Sigma Aldrich. Ultrapure water (Milli-Q) was used.


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Synthesis. Silica nanorods of aspect ratio (AR) 2, 5, 8, 14 and 24 were synthesized, for which a sample name of ARX corresponds to an average aspect ratio of “X”. Nanorods of AR5 were synthesized using the standard protocol of Imhof et al.58 In particular, in a closed 500 ml glass laboratory bottle, 30 g of PVP was dissolved in 300 ml of 1-pentanol. Following dissolution of PVP, 30 ml ethanol, 8.4 ml ultrapure water and 3 ml of 0.18 M sodium citrate aqueous solution were added. The flask was shaken by hand for mixing. Then, 6 ml of ammonium hydroxide was added, followed by shaking. Then, 3 ml of TEOS was added. The flask was shaken again, and left to rest for 24 h. For nanorods of AR2, AR8, AR14, and AR24, parameters including TEOS volume, magnetic stirring, and batch volume were regulated, as listed in Table S1. In particular, for nanorods of AR2, the solution of sodium citrate, ammonium hydroxide, ethanol and water was stirred at 250 rpm for 8 min before adding PVP/pentanol solution and TEOS. For nanorods of AR8, AR14, and AR24, the emulsion of sodium citrate, ammonium hydroxide, ethanol, water, PVP and pentanol was stirred at 250 rpm for 15 min before adding TEOS. As for the batch volume of nanorods of AR24, the initial volume of 300 mL was distributed to 10 mL portions after magnetic stirring and adding TEOS. Silica nanorods were isolated using repetitive centrifugation.42, 58 First, the reaction mixture was centrifuged at 1500 g for 1 h, and the supernatant was removed. Then, the particles in the sediment were ground, dispersed in ethanol, and sonicated for 1 h. Next, the dispersion was centrifuged at 1500 g for 15 min, and the sediment was dispersed in ethanol in the same way as mentioned above. This procedure was repeated 2 times with ethanol, 2 times with water, and finally 2 times with ethanol at 700 g. After purification, the particles were dried under vacuum at 60 ◦C for 24 h.


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Suspension preparation. The silica nanorods were ground into powder and mixed with electrolyte containing EC, EMC (volume ratio 1:1) and 1 mol/L LiTFSI. The dispersion was sonicated for 1 h and agitated with a vortex mixer for 1 h. Then alternate sonication (1 h) and agitation (1 h) was performed to ensure complete dispersion of the silica nanorods in the electrolyte. The silica nanorod volume fraction () was calculated by = L

LM /NM

M /NM OLP /NP

, where QR ,

Q" represent the density of the electrolyte (1.24 g/mL) and the silica nanorods42 (1.82 g/mL), respectively; SR and S" represent electrolyte and silica nanorod masses, respectively. Rheology. An MCR 301 rheometer (Anton Paar Instruments, Austria) was utilized. Preshear was conducted for suspensions by applying ascending and descending shear sweeps to erase sample history and rebuild a new structure, as shown in Figure 1(f). A cone and plate geometry with 50 mm diameter and 1° cone angle was used for electrolyte (EC/EMC/LiTFSI), dilute and semidilute suspensions. In particular, the shear rate sweep was conducted with a maximum shear rate of 1000 s-1. A Newtonian plateau at 3.2 mPa · s was observed for EC/EMC/LiTFSI measured at shear rates from 10 to 1000 s-1. For concentrated samples, a parallel plate with 25 mm diameter and 500 µm gap was used due to the smaller sample volume required in comparison to the cone and plate geometry. Note that results obtained by the parallel plate were reproducible by the cone and plate geometry. A shear stress sweep was conducted and the maximum shear stress was determined by trial, beyond which the sample ejected from the geometry. To minimize evaporation (i) a solvent trap was used, (ii) the experiment was conducted at room temperature (~ 25 ◦C), and (iii) data acquisition was limited to 3 points per decade to reduce the measurement time. Suspensions were stable within the experimental time. Transmission electron microscopy (TEM). A carbon-coated copper grid (300 mesh) was used for sample preparation. Images of silica nanorods were visualized using a JEOL JEM-2010


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TEM, operating at 200 kV accelerating voltage with a LaB6 filament. A Gatan SC1000 ORIUS CCD camera was used. Particle diameter, length, and aspect ratio were determined from sampling about 50 nanorods per set. Thermogravimetric analysis (TGA). TGA was performed using a TA Instruments Q500. Temperature ramping protocols include: (1) ramp to 120 °C at 5 °C/min; (2) isothermal hold at 120 °C for 30 min to remove residual water or solvent; (3) ramp to 500 °C at 5 °C/min; (4) isothermal hold at 500 °C for 60 min. The samples were purged with nitrogen at 40 mL/min. XPS. XPS was performed using an Omicron ESCA Probe (Omicron Nanotechnology), which consisted of a Mg Kα monochromatic X-ray source (1253.6 eV). The X-ray source was operated at 150 W and the emitted photoelectrons were collected at the analyzer entrance slit normal to the sample surface. XPS survey scan was recorded at a pass energy of 100 up to 1100 eV (1.0 eV steps, 50 ms dwell time). XPS data were analyzed by CasaXPS software using Gaussian/Lorentzian peak shape and Shirley-type background correction. Hydrodynamic size and zeta potential measurements. A Zetasizer Nano ZS90 (Malvern Instruments, Ltd., Worcestershire, UK) was used to characterize the hydrodynamic size and zeta potential of the nanorods in deionized water and electrolyte (EC/EMC/LiTFSI), respectively. A disposable cuvette and capillary cell (DTS1070) were used for hydrodynamic size and zeta potential measurement, respectively. Dilute solutions of nanorods were prepared and sonicated in glass bottles, and then transferred to the cells before the measurement. Electrolyte conductivity. Electrolyte conductivity was measured using an InLab 710 conductivity cell (Mettler-Toledo). A holder for the conductivity cell was fabricated to provide an air-tight seal and prevent moisture exposure during conductivity testing. All testing was performed in an environmental chamber (Cincinnati Sub-Zero Microclimate) set to 23°C.


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Electrochemical cycling. CR2032 full cells were assembled using graphite anodes and nickel manganese cobalt oxide cathodes. Celgard 2500 was used as the separator membrane. Electrode and separator discs were cut from large sheets using a punch of the appropriate size (½ inch for cathode, ¾ inch for separator membrane, and 9/16 inch for anode). A ½ inch diameter cathode disc was deposited in the bottom of the cathode can followed by a ¾ inch diameter disc of separator. This was then wet with electrolyte, a gasket added and the 9/16 inch diameter anode placed on top. Finally, a stainless steel spacer and wave spring were added on top and the anode can seated into the gasket. The entire assembly was then crimped and tested on an Arbin battery tester at various rates (C/10, C, 2C). The cells were cathode-limited with a nominal capacity of 148 mAh/g. Ballistic testing. Ballistic testing was conducted at Cawthon Cartridge Club Shooting Complex in Navasota, Texas. Once the range was secured and clear, the testing protocol was set up. A cement block was used as a weighted backing for the test target. The battery used was a 1.5 Ah uncharged pouch cell (11 cm x 12 cm x 0.33 cm laminated pouch cell with a stack of 22 electrodes,

interdigitated separator film) loaded with either a standard

electrolyte

(EC/EMC/LiTFSI) or shear thickening electrolyte (Lynntech proprietary blend with silica nanorods). The armor was loaded into a plate carrier at the front of the stack. All materials were held flush against each other during testing. The force of the impacts was evaluated with both pressure sensitive film (Pressurex Film, 350-1400 psi, two-sheet type) and polymer clay backface signature (P-BFS) using Roma Plastilina No.1 as a standard ballistics clay. The target was shot at a distance of 50 ft using 9x19 mm NATO 124 gr FMJ rounds fired from a 13.5 inch barrel, semi-auto carbine. Bullet velocities at 10 ft from the muzzle were determined by chronograph (F-1 Master Chrony) to be 1332±3 ft/s. It should be noted that penetration did not


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occur in any of the tests as this was determined to be a good method to evaluate differences between the two electrolyte systems. The test was performed this way for safety purposes.

ASSOCIATED CONTENT Supporting Information. Synthesis methods. Particle diameter and length distributions. Thermogravimetric analysis of nanorods of various aspect ratios. XPS characterization of AR8 nanorods. Power law fits for nanorods of various aspect ratios. Power law exponents for shear thinning and shear thickening regions for nanorods of various aspect ratios. Normal force for shear thickening of AR14 and AR24 nanorods. Fitting curves to viscosity at the shear rate of 0.1 s-1 and 1 s-1 for nanorod AR2. Oscillatory shear of AR24 nanorods. Hydrodynamic size and zeta potential measurement of AR2 nanorods.

AUTHOR INFORMATION Corresponding authors *E-mail: [email protected]. *E-mail: [email protected]. ORCID Jodie Lutkenhaus: 0000-0002-2613-6016 Yilan Ye: 0000-0002-4018-7151

Funding Sources


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Air Force Office of Scientific Research for Small Business Innovation Research (Grant No. FA8650-16-P-2680) Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by Air Force Office of Scientific Research for Small Business Innovation Research (Grant No. FA8650-16-P-2680). We thank Lynntech Inc. for collaborative research and Dr. Micah Green for use of rheometer.

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TABLE OF CONTENTS


35

Effect of Nanorod Aspect Ratio on Shear Thickening Electrolytes for

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