Beneficial rheological properties of lithium-ion battery cathode slurries from elevated mixing and coating temperatures

Journal of Energy Storage 26 (2019) 100994

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Beneficial rheological properties of lithium-ion battery cathode slurries from elevated mixing and coating temperatures W. Blake Hawleya,b, Jianlin Lia,b,

T

⁎

a

Oak Ridge National Laboratory, Energy and Transportation Science Division, 1 Bethel Valley Road, Oak Ridge, TN 37831, United States University of Tennessee, Bredesen Center for Interdisciplinary Research and Graduate Education, 418 Greve Hall, 821 Volunteer Blvd., Knoxville, TN 37996, United States

b

A R T I C LE I N FO

A B S T R A C T

Keywords: Viscosity Slot-die coating Sedimentation Bridging flocculation Gelation

It is imperative that lithium-ion battery manufacturers implement strategies to expedite production without sacrificing quality due to rising consumer demand. Cathode coating is commonly performed at the industrial scale with a slot-die coater. In slot-die coating, substrate velocity is maximized and imperfections (such as air entrainment and thickness variations) are minimized by reducing the viscosity of the material being coated. A simple, scalable method of reducing the viscosity of the cathode slurry is to increase its temperature, though it is dire that this heat does not cause irreversible gelation or otherwise deteriorate the slurry constituents. Cathode slurries were prepared at different mixing temperatures between 25 °C and 75 °C and their flow behavior was studied at their mixing temperature. At practical shear rates, the slurry coated at 60 °C was 23% less viscous than that coated at 25 °C, meaning the critical coating speed could be increased by roughly 14% at 60 °C. Between 25 °C and 60 °C, the slurries’ yield stress and equilibrium storage modulus increased monotonically, providing the additional benefit of higher sedimentation resistance of the active materials. To examine the influence of temperature on coating morphology and electrochemical performance, slurries were prepared and coated at 25 °C and 60 °C. Micrographs revealed no superficial differences between coatings. The electrode coated at 60 °C demonstrated comparable capacity retention during long-term cycling and high-rate discharge testing when compared to the electrode coated at 25 °C. The results of this study indicate that warmer mixing and coating operations serve to maximize cathode productivity, particularly if advancements can be made in industrial-scale electrode drying.

1. Introduction

resist sedimentation, both of which can devastatingly affect an electrode's capability. Slurry viscosity is of relevance in the coating stage, for which the state-of-the-art technology is the slot-die coater due to its versatile and high-speed capabilities. Materials that flow too easily tend to trickle during coating which results in a nonuniform coating layer [4], while materials that are too viscous will take longer to coat and may reduce the effectiveness of the vacuum pressure [5–9]. The maximum (or critical) coating speed that a slot-die coater can achieve is defined as the speed just below the onset of dynamic wetting failure. Dynamic wetting is the process by which the slurry replaces air as the fluid at the substrate surface. Dynamic wetting failure refers to the formation of air bubbles in the slurry and is noticed when the dynamic contact line between the upper meniscus and substrate breaks into a

Improving the energy density of lithium-ion batteries (LIBs) relies on not only synthesizing high energy density electrode materials but also developing novel electrode processing and manufacturing techniques to reduce the percentage of inactive components [1,2]. Slurry processing is critical in obtaining high performance electrodes and reducing scrap rate. Conventional LIB cathodes are produced through a slurry homogenization operation where active materials are blended with other components, such as carbon black as a conductive additive, polyvinylidene fluoride (PVDF) as a binder, and N-methyl-2-pyrrolidone (NMP) as a solvent [3]. Appropriate slurry processing is required to control viscosity and

This manuscript has been authored in part by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE 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). ⁎ Corresponding author at: Oak Ridge National Laboratory, Energy and Transportation Science Division, 1 Bethel Valley Road, Oak Ridge, TN 37831, United States. E-mail address: [email protected] (J. Li). https://doi.org/10.1016/j.est.2019.100994 Received 15 August 2019; Received in revised form 13 September 2019; Accepted 1 October 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.

Journal of Energy Storage 26 (2019) 100994

W.B. Hawley and J. Li

a slurry during the mixing and coating stages provides advantages both at the macroscopic and microscopic scale. To the authors’ knowledge, these possibilities have not been investigated in the literature, nor has the possibility that extraordinary heat application may have a destructive effect on slurry properties. It is the aim of this study to examine the effect of temperature on the overall processability of the cathode slurry, as well as the influence this modified rheology could have on coating morphology and electrochemical performance. Further, the potential benefits of a slurry at elevated temperatures are discussed from a manufacturing perspective within the context of the entire fabrication process.

sawtooth pattern [10]. Slurry viscosity is a function of mixing method and solid loading [2]. It is of interest to the manufacturer to maximize the solid loading of a slurry in order to reduce the amount of solvent needed, since NMP is a hazardous chemical that requires an energy-intensive recycling stage [11–13]. However, precaution must be taken as to not overload the slurry such that it is too viscous. A simple, scalable way to manipulate a fluid's viscosity is to change its temperature. A warmer material's comprising molecules will flow more easily due to an increase in their kinetic energy, thus resulting in a lower bulk viscosity. Therefore, increasing the temperature of a cathode slurry could provide the benefits of a higher maximum coating speed and a more effective vacuum pressure. Conversely, the manufacturer could choose to increase the solid loading and, as a result, minimize solvent use and recovery. Another pitfall to avoid in slurry formulation is the gravitational settling of active material particles. An electrode slurry can be considered a colloidal suspension; this is because colloidal particles constitute a size range whose low end is large enough for the solvent to be approximated as a continuum due to the size discrepancy between the colloidal particles and the molecules comprising the suspending medium. However, even the largest colloidal particles are still small enough to be subjected to Brownian (FB), or thermal, motions (see Eq. (1)). The omnipresent force of gravity (Fg) also acts on these particles (see Eq. (2)) [14].

FB =

kB Tabs r

(1)

Fg =

4 (ρ − ρm ) πr 3g 3 p

(2)

2. Experimental 2.1. Slurry preparation In this study, cathode slurries consisted of NMC532 (LiNi0.5Mn0.3Co0.2O2, d50 = 9.4 µm, Toda America) as an active material, carbon black (Li-100, Denka) as a conductive additive, and 8 wt % of Solvay 5130 PVDF (MW 1.0–1.2 × 106 g mol−1) binder dissolved in NMP solvent. The solid components (NMC532, carbon black, and PVDF) were mixed in the mass ratio of 90:5:5, with a solid mass fraction ϕ = 0.45 in the slurry. A hydrodynamic shear mixer (Model 50, Netzsch) in a dry room fume hood (T ≈ 22 °C) was used to prepare the slurry. The slurry was homogenized at 4000 rpm for 30 min, then the bottle was removed from the mixer. A metal spatula was used to scrape powder residue from the bottle wall, which was promptly deposited into the center of the slurry. The slurry was mixed again at 4000 rpm for 15 min to achieve a reproducible, homogeneous, and practical slurry consistency. For heated mixing, a metal cup filled with tap water was placed on a hot plate under the mixing head. Once the temperature of the water bath reached the target, the 250 mL bottle was set in the water bath. Both the plastic bottle and metal cup were fixed in position using a ring stand. The temperature of the water bath was monitored throughout homogenization and did not fluctuate more than ± 2 °C from the desired temperature. The following temperatures were tested: 25 °C, 40 °C, 50 °C, 60 °C, and 75 °C. These temperatures are convenient in that their lower bound is substantially higher than the glass transition temperature of PVDF [20], their upper bound is lower than the boiling point of NMP [21], and they are still cool enough to be attainable from an energy input and cost perspective.

In Eqs. (1) and (2), kB is the Boltzmann constant (1.381 × 10−23 J K−1), Tabs is the absolute temperature in K, r is the radius of the particle, ρp and ρm are the densities of the particle and the medium, respectively, and g is the acceleration due to gravity (9.81 m s−2). Rudimentary calculations reveal that for most colloidal particles in most solvents, Brownian forces dominate. This is, however, not the case for many active material-sized particles. Since the density of active materials, especially for cathodes, is normally much greater than the medium (e.g. NMP), sedimentation is expected to take place, unless stirring is applied continuously prior to coating. Conforming to Eqs. (1) and (2), effective solutions to minimize sedimentation would be: (i) selecting a solvent with a matching density to the active materials, thus causing Fg to approach zero; ii) reducing active particle size; (iii) increasing solid content, thereby densifying the medium; and (iv) increasing temperature. The first two options involve changes in slurry components which would affect manufacturing protocols and electrode performance, and the third choice will inflate the viscosity of the slurry and thus shrink the coating window. The fourth tactic is the approach studied here, specifically to quantify the improvement in sedimentation resistance and to determine if increasing the temperature will be malignant to slurry component properties. In addition, Bauer and Notzel argued that the formation of a bridging flocculation network was among the optimal choices for slurry stabilization [15]. In this scheme, the ends of a polymer strand adsorb to the surface of two separate active materials, forming a weakly coagulated state that dissuades the particles from aggregating. The adsorption behavior between binder and active materials is complex but has been characterized for some electrode systems, such as graphite anodes, in the literature [16,17]. With a sufficiently large volume fraction of high-molecular weight polymer, the percolation threshold can be reached and a volume-spanning bridging network can be formed [18,19]. If adequate connectivity is established, the slurry exhibits gellike features. Specifically, the dispersion can carry stresses without disrupting the connectivity between particles. The preceding discussion implies that increasing the temperature of

2.2. Rheological measurements Once prepared, the slurry was transferred to the rheometer (Discovery HR-3, TA Instruments). The rheometer was pre-heated to the corresponding mixing temperature of the slurry. A SmartSwap™ concentric cylinder geometry (bob diameter = 28.05 mm, bob length = 42.01 mm) was used to acquire six measurements that were displayed in TRIOS software. First, the sample was pre-sheared at 5 s−1 for 1 min to set the initial state of the sample. Second, a flow ramp test was performed from shear rates 5 × 10−3–3500 s−1 for 8 min. The resulting stress vs. strain data was plotted and the apparent yield stress was calculated as the average stress in the plateau region in the curve. This data was also used to fit the Herschel–Bulkley model to determine additional flow properties of the slurry. Third, an oscillation time sweep was performed at ω = 100 rad s−1 and at γ = 0.1% for 15 min in order to rebuild the soft solid structure of the slurry and minimize hysteresis effects in subsequent trials. For this reason, the strain selected was well within the linear viscoelastic regime (LVR) as determined by preliminary testing. Fourth, an oscillation amplitude sweep was conducted at ω = 100 rad s−1 from γ = 0.01–10%. This allowed for precise characterization of the LVR of each slurry and provided information about the strength of the structure. Fifth, another oscillation time sweep was implemented, using the same parameters as the previous time sweep, except this procedure lasted only 5 min since the amplitude 2

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W.B. Hawley and J. Li

sweep was not as destructive to the slurry structure. Last, an oscillation frequency sweep was run at γ = 0.1% from ω = 100–0.1 rad s−1 to provide further information on the structure of each slurry. 2.3. Scanning electron microscopy (SEM) Once prepared, slurries mixed at 25 °C and 60 °C were doctor-blade coated onto an aluminum current collector foil at a wet thickness of 100 μm. The coatings were dried in a dry room with the wet coating sitting on a heated bed at 115 °C with exhaust at 100 °C overnight. Once dry, the coatings were examined using a Zeiss MERLIN™ FE-SEM. The SEM electron high tension (EHT) was 1 kV and the working distance was 5.5 mm. 2.4. Electrochemical characterization Half-cell coin cells were constructed using 14-mm punches from the dried coatings. Punches were left in a vacuum oven at 120 °C overnight to remove moisture. Punches were weighed and transferred to a glove box backfilled with argon. The areal capacity of cathodes was roughly 0.9 mAh cm−2. The electrolyte used was 1.2 M LiPF6 in 3:7 wt% ethylene carbonate (EC)/ethylmethyl carbonate (EMC). Coin cells were assembled in the following order: metal spring, metal spacer, lithium chip, three drops of electrolyte, Celgard 2325 separator, three drops of electrolyte, cathode punch, and metal spacer. Due to the relative thinness of the cathode punches (∼40 μm including Al current collector), two spacers were used to ensure compression in the cells. Coin cells were pressed and manually dried to remove any leaking electrolyte. Two protocols were used to evaluate battery performance: a cycle life test and a rate capability test. In both tests, the cells were cycled between 3.0–4.3 V. For cycle life testing, there were three cycles of charge and discharge at C/10 to form the cell. Afterwards, the cell was cycled 200 times, charging and discharging at C/3. The cell was allowed to rest for 10 min after charge and discharge. In the rate capability test, the cell was charged at C/3 and discharged for three cycles at the following C-rates: C/10, C/5, C/3, C/2, 1C, 2C, 3C, and 5C. After charging, the potential was held for 2 h. After charge and discharge, the cell was allowed to rest for 10 min. Three coin cells were constructed and cycled according to both protocols for both coatings, except for the cycle life analysis of the 60 °C slurry, for which only two cells were constructed and tested.

Fig. 1. Yield stress determination of tested cathode slurries. Data collected at higher shear rates is inconsequential to the determination of yield stress and is omitted.

the slurry, interparticle separation distances would become vanishingly small and this connectivity would likely be achieved regardless of the more energetic particles. With the yield stress determined, the Herschel–Bulkley model (Eq. (3)) may be implemented to quantify the degree of shear thinning in the slurry.

σ = σ0 + Kγ˙ n

(3)

In Eq. (3), σ represents the stress measured at a given shear rate in Pa, σ0 is the yield stress in Pa, K is the consistency index in Pa•sn, γ˙ is the shear rate in s − 1, and n is the dimensionless flow index. Only data collected at shear rates above 5 s−1 were considered in the modeling (hence the dashed purple in Fig. 2), since this is the range of interest in practical coating operations. The consistency index sets the scale of the viscous contribution as the shear rate increases; for fluids with equivalent flow index values, the consistency index will scale with viscosity [22]. The flow index describes the Newtonian or non-Newtonian behavior of the suspension. It is observed that the consistency index is greatest for the 40 °C and 50 °C samples; this is because they show a greater degree of shear thinning than the 25 °C slurry and are more viscous than the 60 °C slurry. The 75 °C slurry has a substantially lower consistency index, since it exhibits less shear thinning and is less

3. Results and discussion 3.1. Flow ramp testing As the level of bridging within the sample proliferates, it is predicted that so do the yield stress, degree of shear thinning, and equilibrium storage modulus (G’) within the material's LVR [14]. All five samples exhibit a plateau in shear stress at shear rates on the order of 1.0 s−1, thus verifying the existence of a yield stress in all of the slurries over the temperature range tested (see Fig. 1). The numerical value for the yield stress was obtained by averaging the first five data points of this flat region. It is observed that the yield stress increases at all temperatures from 25 °C to 60 °C, though at 75 °C, the yield stress declines to the level of the 25 °C slurry. As the temperature intensifies, two consequences are proposed: an increase in the segmental motion of the polymer (i.e. reptation) and an increase in the kinetic energy of the particles. It can be concluded that the former effect dominates up to 60 °C, with more particle linkages and polymer entanglements being established due to binder adsorption to the particle surface. These connections cause the slurry to have a slightly greater resistance to flow at shear rates before the yield stress, as is seen. At 75 °C, the reduction in yield stress could be attributed to the particles’ heightened kinetic energy causing them to tear some of the polymer bridges. If the concentration of particles were escalated in

Fig. 2. Dynamic viscosity profile of tested cathode slurries. The dashed purple line indicates the start of data points considered in Herschel–Bulkley modeling. 3

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Table 1 Calculated Herschel–Bulkley parameters for tested cathode slurries. Sample

Yield Stress, σ0 (Pa)

Consistency index, K (Pa·sn)

Flow index, n

R2

25 °C 40 °C 50 °C 60 °C 75 °C

27.4 28.6 30.1 31.0 27.7

5.1 5.7 5.6 5.1 3.0

0.68 0.65 0.64 0.64 0.70

0.995 0.999 0.997 0.996 0.993

Table 2 Comparison of gelation parameters for the frequency sweep. Sample

Gelation parameter (whole range)

Gelation parameter (moderate-to-high)

R2 (whole range)

R2 (moderateto-high)

25 °C 40 °C 50 °C 60 °C 75 °C

0.34 0.22 0.23 0.17 0.13

0.37 0.31 0.33 0.27 0.20

0.985 0.934 0.909 0.884 0.903

0.999 0.999 0.998 0.995 0.988

viscous than the other samples. All of the slurries had n < 1, agreeing with the shear thinning behavior signified in Fig. 2. It follows that as n approaches 0, the degree of shear thinning intensifies. The flow index is lowest at temperatures between 40 °C and 60 °C, complimenting the claim asserting that binder linkages are quickly degraded. The 25 °C and 75 °C slurries do not have as many linkages to disrupt, due to a lack of binder motions and excessively energetic particles, respectively. It follows that their flow indices are higher (Table 1). The gap in viscosity between the 25 °C slurry and the others widens through shear rates of about 500 s−1 (dubbed the high shear viscosity, or HSV), where the percentage difference flattens (see Fig. 3a). Since modern coating operations seek to achieve higher speeds, which correspond to higher shear rates (i.e. several hundred s−1), the benefit of increasing temperature up to 75 °C is demonstrated. Another benefit is the increase in the low shear viscosity (LSV) at temperatures between 40 °C and 60 °C. A higher LSV leads to a coating with sharper edge contours, meaning there will be less cutoff waste during processing [23]. These results also suggest that the fraction of active materials could be raised in the slurry, since the marginal change in viscosity can be counterbalanced by heating the slurry. The inflation in concentration of solids necessitates a reduction in the amount of solvent, thus leading to shorter, less energy intense, lower cost, and less hazardous drying operations [13]. The capillary number (Ca) is a dimensionless number used in fluid dynamics that is defined as the ratio between the viscous forces and surface tension forces during flow. In slot-die coating, coating thickness and speed are functions of Ca [24,25]. The critical coating speed (Vcrit) can be substituted for velocity in the definition of Ca to yield the critical capillary number (Cacrit).

Cacrit =

μV crit T

Fig. 3. (a) Percent difference in viscosity between the room temperature slurry (25 °C) and the slurries at elevated temperatures. (b) Percent difference in critical coating speed between the room temperature slurry (25 °C) and the slurries at elevated temperatures, as calculated by Eqs. (4)–(7).

V crit =

T=

(6)

k (Tcrit − T ) V 2/3

(7) −7

−1

−2/3

), Tcrit In Eq. (7), k is the Eötvös constant (2.1 × 10 J K mol is the critical temperature (taken here as the critical temperature of NMP, 721.7 K [28]), and V is the molar volume. With the dependence of surface tension on temperature factored in, it is possible to determine the percent increase in maximum coating speed (see Fig. 3b). At practical coating speeds, increasing the temperature of the slurry to 60 °C would increase the maximum coating speed (before dynamic wetting failure) by roughly 14%. These calculations also reveal that the viscosity reduction observed by increasing the temperature from 60 °C to 75 °C does not actually prompt a further improvement in maximum coating speed. This analysis demonstrates that heated coating can increase coating speed without sacrificing coating quality, but consideration of the whole electrode fabrication process reveals that advancements will need to be made in the drying step to fully realize them. Presently, the evaporation stage is the rate-limiting step in electrode manufacturing, lasting generally 1–2 min [29,30] resulting from a concise drying zone. Further shortening of the drying time is limited by the persistence of the

(4) −1

In Eq. (4), T is the surface tension in N m . Vandre et al. expressed Cacrit in terms of viscosity and a dimensionless constant β that depends largely on coating gap (see Eq. (5)) [9,25,26], which is the distance from the bottom of the die to the surface of the substrate. This expression can be substituted into Eq. (4) and rearranged to give Vcrit in terms of β, surface tension, and viscosity (see Eq. (6)). However, it is a poor assumption to suggest that surface tension is independent of temperature; a good approximation for the dependence of surface tension on temperature is the Eötvös equation (see Eq. (7)) [27].

Cacrit = βμ0.258

βT μ0.742

(5) 4

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W.B. Hawley and J. Li

higher temperature, a reduction in retention time in electrode drying could be observed [13]. Thus, the manufacturing speed and the production in capacity per time will be increased. 3.2. Oscillatory shear measurements To further investigate the nature of the bridging flocculation, the microstructure of the slurry was probed using oscillatory shear procedures. An amplitude sweep was used to characterize the LVR of the sample. The LVR allows for precise determination of where the breakdown of the polymer network begins and, subsequently, deduction of the strength of the initial structure. In all samples at low strain, the storage modulus exceeds the loss modulus (G’’), indicating an elasticdominant response (see Fig. 4a). At strains in the range of 2 to 6%, the magnitude of G’ falls below that of G’’, meaning only a small amount of strain must be induced to make the slurry a viscous-dominant material. Fig. 4b recasts this data in terms of phase angle, δ, which is evaluated according to Eq. (8).

G″ δ = tan−1 ⎛ ⎞ ⎝ G′ ⎠

(8)

G′ increases with temperature up to 60 °C; this conveys that the slurry is capable of bearing more stress, thus supporting the idea that more polymer bridges are formed at warmer temperatures. There is a slight decline in G′ at 75 °C, which suggests that some bridges may not be maintained due to extraordinarily energetic particles. This is consistent with the observation of a decreased yield stress at 75 °C. Similarly, G′′ decreases as the temperature is raised, with the most significant drop occurring between 60 °C and 75 °C. This decline in G″ is the expected result, since PVDF is above its glass transition temperature and less energy is dissipated in the glassy region. The decline in G′′ is more pronounced than that in G′ at 75 °C, as indicated by the lower phase angle (compared to the 60 °C slurry). The strain at which the LVR ends is called the critical strain (γc) and is calculated by the point at which G′ decreases below 95% of its initial value (see Fig. 4c). It should be noted that this cutoff criterion is arbitrary; some studies choose to select the position at which G′ reaches 90% of its initial value for γc determination. In general, γc decreases with increasing temperature. Since the particles are more energetic at high temperatures, it is logical that less strain would be required to disrupt the soft solid structure. The other oscillatory shear procedure utilized was the frequency sweep (see Fig. 5). For all samples tested, G′ > G′′ over the entire frequency range, meaning the slurries are more solid-like than liquidlike and there is no sol-gel transition as was observed in the amplitude

Fig. 4. Amplitude sweep data presented in terms of modulus (a) and phase angle (b) at ω = 100 rad s−1. The normalized storage modulus is also plotted as a function of strain (c). The dashed purple line in (b) demarcates the tan(δ) = 1 line, indicating a transition from a solid-like material to a liquid-like material. The purple line in (c) is indicative of the point at which the slurry may no longer be considered to be in its LVR.

migration of binder [31–35] and conductive additive [36,37] to the coating surface, since this upward diffusion occurs more rapidly at elevated temperatures. However, the lower viscosity at elevated mixing temperatures can still enable higher throughput by increasing solid contents in the slurry. Essentially, if two slurries have identical viscosities, but one has a greater solid content due to being processed at a

Fig. 5. Frequency sweep data presented in terms of modulus at γ = 0.1%. 5

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W.B. Hawley and J. Li

sweep. At low frequency, G′ rises exclusively with temperature. As the frequency surges, however, the dependency of G′ on frequency varies considerably. This can be quantified as follows:

G′ = αω J

(9)

In Eq. (9), α is a constant and J is the dimensionless gelation parameter. A value of J closer to zero is indicative of a more solid-like structure. It is worthwhile to divide the frequency sweep into two ranges: low-frequency (0.1 ≤ ω ≤ 1 rad s−1) and moderate-to-highfrequency (1 < ω ≤ 100 rad s−1). In the low-frequency region, G′ is a slight function of ω (J = 0.188) at 25 °C, but is virtually independent of ω at every other temperature (J < 0.05). This plateauing behavior is typical of materials with established network structures. In the moderate-to-high-frequency range, the rate of G′ evolution declines with temperature (see Table 2) since motions are quick and relaxations are localized and far less sensitive to structure, which could explain why the values of G′ seem to converge at high-ω. Another feature of the frequency sweep is the growing instability of G′′ with temperature in the low-frequency range. This appears to be a product of enhanced particle motions with temperature, which also bears influence on the decline in yield stress and equilibrium G′ between 60 °C and 75 °C, as discussed previously. The viscoelastic properties of the slurry will also have an impact on its processability. It has been found that modest slurry elasticity can widen the operational window [38–41]. Fluid elasticity can be calculated according to the Weissenberg number (Wi) in Eq. (10):

Wi =

λVsub h

(10)

where λ is the relaxation time of the sample, Vsub is the velocity of the substrate, and h is the coating gap. Wi is the ratio of the viscous force to the elastic force; below Wi = 0.1, the fluid is deemed to have low elasticity and thus can stabilize the coating operation, while above this threshold the fluid has high elasticity and causes an increase in the minimum thickness [38]. Since structural relaxation times decrease with an increase in temperature [42], it can be reasonably deduced that a higher coating speed would be achievable at elevated temperatures. 3.3. SEM imaging While heating of slurries provides the manufacturing benefits described, it is imperative to ensure that heated mixing and coating supplies cathodes that are rate capable and demonstrate good cyclability. Microscopy images of the surfaces of the coatings reveal active material clusters with diameters on the order of 10 μm surrounded by a conductive additive/binder network for both mixing conditions (see

Fig. 7. (a) Average discharge capacity as a function of cycle number for slurries mixed at 25 °C and 60 °C according to the cycle life protocol. Error bars show the standard deviation of the discharge capacity for the coin cells tested. (b) Voltage curve for a representative 25 °C and 60 °C coin cell showing the 1st, 50th, and 200th cycles. (c) Average discharge capacity as a function of C-rate for slurries mixed at 25 °C and 60 °C according to the rate capability protocol. Error bars show the standard deviation of the discharge capacity for a total of nine measurements (three coin cells cycled at each C-rate three times).

Fig. 6). Active materials spacings are similar and there is adsorption of the conductive additive/binder network on the surface of the active materials, thus providing mechanical strength to the coating as well as enhanced electronic conductivity. Both electrodes demonstrate a relatively large, randomly distributed porosity, which could be optimized with calendering. Carbon black agglomeration could be minimized by varying other mixing parameters, such as the intensity, time, or

Fig. 6. (a,b) SEM images of electrodes coated at 25 °C. (c,d) SEM images of electrodes coated at 60 °C. 6

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Declaration of Competing Interest

instrument of mixing, though this analysis lies outside of the scope of the present work.

We declare that there is no conflict of interest in the submission. 3.4. Electrochemical characterization

Acknowledgments

The cycle life of the two coatings is essentially equal through 100 cycles (see Fig. 7a), which means the binder preserves the mechanical integrity of the cathode during charge and discharge equally for both mixing and coating protocols. The coulombic efficiency of both coatings for the first cycle is about 94.6% and increases to nearly 99% for subsequent cycles, with some instability observed beyond 100 cycles for the 25 °C coating. These results support the idea that electrochemical performance is not harmed (and, in fact, may be improved slightly, particularly as cycling persists beyond 100 cycles) when temperature manipulation is implemented to expedite production and lower the probability of imperfections. Similarly, there is no appreciable difference in the capacity retention of these coatings at any C-rate (see Fig. 7b). The slurry mixed at 60 °C did deliver a slightly higher average discharge capacity at each C-rate, which could be attributed to minute differences in the active material content of the electrodes and cell to cell variation in coin cells. It should be emphasized that improved rheological properties do not necessarily correlate to improved electrochemical performance [2,43]. For instance, Bitsch et al. demonstrated an increase of two orders of magnitude in the LSV for aqueous slurries through the addition of a cosolvent [23]. However, the electrodes prepared with the co-solvent did not perform noticeably different than those prepared without it. Hintennach and Novak observed that longer surfactant chain lengths could better disperse titanium dioxide particles, but that the slurry viscosity that provided the best performance was unique to each surfactant [44]. The results here agree with the assertion that improved slurry rheology does not always lead to improved battery performance and that consideration of the subsequent drying process is necessary, as discussed previously. Nevertheless, it is demonstrated that mixing slurries at temperatures up to 60 °C does not have a negative impact on electrochemical performance while simultaneously enabling higher throughput and lower energy consumption during electrode drying, as discussed previously.

This research at Oak Ridge National Laboratory (ORNL), managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) under contract DE-AC05-00OR22725, was sponsored by the Office of Energy Efficiency and Renewable Energy (EERE) Vehicle Technologies Office (VTO) (Deputy Director: David Howell; Applied Battery Research (ABR) Program Manager: Peter Faguy). The authors thank Dr. Yangyang Wang (ORNL) for fruitful discussion and Dr. Mengya Li (ORNL) for assistance with SEM. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.est.2019.100994. References [1] J. Li, Z. Du, R.E. Ruther, S.J. An, L.A. David, K. Hays, M. Wood, N.D. Phillip, Y. Sheng, C. Mao, S. Kalnaus, C. Daniel, D.L. Wood, Toward low-cost, high-energy density, and high-power density lithium ion batteries, JOM 69 (2017) 1484–1496, https://doi.org/10.1007/s11837-017-2404-9. [2] W.B. Hawley, J. Li, Electrode manufacturing for lithium-ion batteries-Analysis of current and next generation processing, Energy Storage 25 (2019) 100862, , https://doi.org/10.1016/j.est.2019.100862. [3] J. Li, B. Armstrong, J. Kiggans, C. Daniel, D. Wood III, Optimization of LiFePO4 nanoparticle suspensions with polyethyleneimine for aqueous processing, Langmuir 28 (2012) 3783–3790, https://doi.org/10.1021/la205157d. [4] J.C. Flynn, C. Marsh, Development and experimental results of continuous coating technology for lithium-ion electrodes, Thirteenth Annual Battery Conference on Applications and Advances, 1998. [5] M. Schmitt, M. Baunach, L. Wengeler, K. Peters, P. Junges, P. Scharfer, W. Schabel, Slot-die processing of lithium-ion battery electrodes – coating window characterization, Chem. Eng. Process 68 (2013) 32–37, https://doi.org/10.1016/j.cep.2012. 10.011. [6] R. Burley, B.S. Kennedy, An experimental study of air entrainment at a solid/liquid/ gas interface, Chem Eng. Sci. 31 (1976) 901–911, https://doi.org/10.1016/00092509(76)87040-6. [7] E.B. Gutoff, C.D. Kendrick, Dynamic contact angles, AIChE 28 (1982) 459–466, https://doi.org/10.1002/aic.690280314. [8] O. Cohu, H. Benkreira, Air entrainment in angled dip coating, Chem Eng. Sci. 53 (1998) 533–540, https://doi.org/10.1016/S0009-2509(97)00323-0. [9] K.Y. Lee, L.D. Liu, T.J. Liu, Minimum wet thickness in extrusion slot coating, Chem Eng. Sci. 47 (1992) 1703–1713, https://doi.org/10.1016/0009-2509(92)85018-7. [10] X. Ding, J. Liu, T.A.L. Harris, A review of the operating limits in slot die coating processes, AIChE 62 (2016) 2508–2524, https://doi.org/10.1002/aic.15268. [11] D.L. Wood, J. Li, C. Daniel, Prospects for reducing the processing cost of lithium ion batteries, Power Sources 275 (2015) 234–242, https://doi.org/10.1016/j.jpowsour. 2014.11.019. [12] S. Ahmed, P.A. Nelson, K.G. Gallagher, D.W. Dees, Energy impact of cathode drying and solvent recovery during lithium-ion battery manufacturing, J. Power Sources 322 (2016) 169–178, https://doi.org/10.1016/j.jpowsour.2016.04.102. [13] D.L. Wood, J.D. Quass, J. Li, S. Ahmed, D. Ventola, C. Daniel, Technical and economic analysis of solvent-based lithium-ion electrode drying with water and NMP, Drying Technol. 36 (2018) 234–244, https://doi.org/10.1080/07373937.2017. 1319855. [14] J. Mewis, N. Wagner, Colloidal Suspension Rheology, first ed., Cambridge University Press, Cambridge, 2012. [15] W. Bauer, D. Notzel, Rheological properties and stability of NMP based cathode slurries for lithium ion batteries, Ceram. Int. 40 (2014) 4591–4598, https://doi.org/ 10.1016/j.ceramint.2013.08.137. [16] M. Yoo, C. Frank, S. Mori, Interaction of poly(vinylidene fluoride) with graphite particles. 1. Surface morphology of a composite film and its relation to processing parameters, Chem. Mater. 15 (2003) 850–861, https://doi.org/10.1021/ cm0209970. [17] M. Yoo, C. Frank, S. Mori, S. Yamaguchi, Interaction of poly(vinylidene fluoride) with graphite particles. 2. Effect of solvent evaporation kinetics and chemical properties of PVDF on the surface morphology of a composite film and its relation to electrochemical performance, Chem. Mater. 16 (2004) 1945–1953, https://doi.org/ 10.1021/cm0304593. [18] J. Swenson, M. Smalley, H. Hatharasinghe, Mechanism and strength of polymer bridging flocculation, Phys. Rev. Lett. 81 (1998) 5840–5843, https://doi.org/10. 1103/PhysRevLett.81.5840. [19] W. Russel, D. Saville, W. Schowalter, Colloidal Dispersions, Cambridge University Press, Cambridge, 1989.

4. Conclusions In this work, increasing the temperature of cathode slurry mixing and coating over the range of 25 °C–60 °C has been demonstrated to (i) monotonically reduce the HSV of the slurry, (ii) monotonically increase the LSV of the slurry, and (iii) monotonically increase the yield stress and equilibrium storage modulus of the slurry. The first benefit permits faster, more defect-free operation of slot-die coaters, the second benefit reduces waste generation during processing, and the third benefit aids in sedimentation resistance. Increasing the temperature from 60 °C to 75 °C provided only the first benefit, likely due to a rise in the particles’ kinetic energy beyond the threshold where binder linkages can contain their motion at rest or very low shear. Heating the slurry from 25 °C to 60 °C was shown to reduce the HSV by 23%, which could mean an improvement in the maximum coating speed of 14%. It was further shown that while the HSV of the 75 °C slurry was lower than that of the 60 °C slurry, it does not actually permit faster coating operations, due to the dependence of surface tension on temperature. While maximizing coating speed will require improvement in electrode drying kinetics, an improvement in coating speed may still be observed by heating if the solid content of cathode slurries is increased. With an increased solid content, drying times and energy input required to recycle the solvent may be reduced. In addition, it was demonstrated that moderate temperature increases in the slurry due to heat generation during mixing will not negatively impact the electrode performance. 7

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