Supporting Information

Investigation of Film Solidification and Binder

Migration during Drying of Li-Ion Battery Anodes

Stefan Jaisera*, stefan.jaiser@kit.edu

Marcus Müllerb, marcus.mueller@kit.edu

Michael Baunacha, michael.baunach@kit.edu

Werner Bauerb, werner.bauer@kit.edu

Philip Scharfera, philip.scharfer@kit.edu

Wilhelm Schabela, wilhelm.schabel@kit.edu

aInstitute of Thermal Process Engineering – Thin Film Technology, Karlsruhe Institute of

Technology, Kaiserstraße 12, D-76131, Karlsruhe, Germany.

bInstitute for Applied Materials – Ceramic Materials and Technologies; Karlsruhe Institute of

Technology, Hermann-von-Helmholtzplatz 1, D- 76344, Eggenstein-Leopoldshafen,

Germany.

Keywords. Binder migration, film solidification, drying, adhesion, lithium-ion battery.

*corresponding author: Stefan Jaiser, stefan.jaiser@kit.edu; phone: +4972160842392, fax:

+4972160845319; Institute of Thermal Process Engineering – Thin Film Technology,

Kaiserstraße 12, D-76131, Karlsruhe, Germany.

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Adhesion and pore structure. The force measured by means of 90° peel tests is interpreted

as adhesion between the porous graphite layer and the copper substrate. This takes for

granted, that adhesion to the substrate and not cohesion between constituents in the porous

layer is the weakest link. Figure A1 shows SEM micrographs of a LDR (a) and a HDR sample

(b), the corresponding adhesion forces measured were FLDR=19.9 N/m and FHDR=30.8 N/m,

respectively.

Figure A1. SEM micrographs of the copper substrate after 90° peel tests for LDR (left) and

HDR (right) reference samples.

The micrographs were taken at the edge of a pristine graphite layer to the delaminated

region to allow for comparison. They clearly show that the majority of the porous graphite

layer delaminates at the substrate interface and that adhesion and not cohesion is the main

factor contributing to the force detected by means of the 90° peel tests. However, a few

particles or particle clusters remain on the copper foil, so cohesion partly plays a (minor) role

as well. We believe that this results from the local configuration of the anode which can

strongly differ due to the broad particle size distribution of the graphite. Figure A2 shows the

cross section of a graphite layer prepared by ion beam milling (Leica EM TIC 3X). At the

interface between porous graphite layer and copper substrate both small and large pores are

present. The local adhesion is therefore not just a function of the binder concentration in the

vicinity of the substrate, but also related to the local pore structure. If the local pore structure

benefits the local accumulation of binder, a local cohesive failure can be provoked even for S2

high drying rates, when a general depletion prevails at the interface between graphite layer

and substrate. Summing up, EDS measurements at delaminated films represent a suitable

method for detection of PVDF at the interface to the substrate and the substrate-near domains.

Figure A2. Cross section of a graphite anode layer prepared by ion beam milling.

Another factor that could potentially influence adhesion is the pore structure. However, the

porosity and the pore structure were found to be affected by the drying rates used in the

presented study to a minor degree, if at all. This was asserted by mercury intrusion

porosimetry. Figure A3 compares the pore radius distribution of a LDR and HDR reference

sample. Both curves match pretty well, indicating that the drying rate plays a minor role at

most. The distributions are relatively broad, exhibiting two well separated peaks in the micron

range. Due to the separated nature of the peaks, the second peak at around 15 µm is

interpreted as the entity of surficial pores as a first estimation. The inner porosity is intruded

by mercury at higher pressures. The first intrusion happens at a pressure of about 0.3 MPa.

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With increasing pressure, a steep increase in cumulative pore volume becomes obvious. This

is a strong hint towards a well interconnected pore network. The strong increase in cumulative

pore volume in the range of pore radii between two and seven micrometer is caused by the

existence of pores that are larger than the, in the first instance, bottlenecking pore throats, and

are filled without resistance (see also the inhomogeneous pore radii prevailing in the film in

Figure A2). In summary, no impact of the drying rate on the pore structure was found.

Figure A3. Relative and cumulative pore volume over pore radius and pressure determined

by means of mercury intrusion porosimetry measurements for LDR and HDR samples.

Mixing. Polyvinylidene fluoride (PVDF; Solvay SOLEF 5130) binder was dissolved at

room temperature in N-Methyl-2-pyrrolidone (NMP, Carl Roth) for 24 hours under constant

stirring at a mass fraction of 5.55 %. Commercial graphite (SMG-A, Hitachi Chemicals,

Japan, d50=20.4 µm) and carbon black (C-NERGY C65, d50=65 nm, Imerys Graphite &

Carbon, Bodio, Switzerland) was dry-mixed for two minutes at 200 rpm in a laboratory

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dissolver (VMA Getzmann DISPERMAT CN10). The dissolver was equipped with a custom-

built mixing tool enhancing vertical transport. The double-walled dispersion container was

constantly fed with cooling water to prevent excessive warming. Following the

recommendations by Terashita [1] and Lee [2], binder solution was added stepwise to reduce

carbon black agglomeration.

In a first dilution step, the solid mass fraction was set to xS=0.57. The dispersion was

agitated for 10 minutes at 500 rpm. In a second dilution step, the solid mass content was

further decreased to the final solid content of xS=0.475. This time, the dispersion was agitated

for 30 minutes at 1500 rpm. Concurrently, vacuum #was applied to remove air introduced into

the dispersion due to the intensive stirring. The dispersion was stored in an air-tight container

for at least 18 hours. Previous experiments not included here pointed out a change of the

dispersion rheology during this time period before a stationary rheological state is reached.

Especially for a self-metered coating technique like doctor blading, constant rheological

properties were a basic requirement for production of films with a constant area loading.

Coating. Both coating and drying were conducted in a custom-built setup comprising an

impingement dryer and a temperature-controlled aluminum plate. Anode slurry was drawn up

into 10 ml syringes and stored in a preheating unit at the drying temperature adjusted in the

dryer as explained later on in this section. A doctor blade (Zehntner UA2000.60) with a

coating width of 60 mm was preheated likewise and utilized for film casting. Copper foil

(Nippon Foil Mfg. Co. Ltd.) with a thickness of 10 µm and a width of 200 mm served as

substrate. The substrate was put on a temperature-controlled aluminum plate and fixed

laterally by adhesive tape. A circumferential groove milled into the plate was connected to a

vacuum pump (KNG N035AN.18IP44). Removal of insulating air films between substrate and

aluminum plate improved coating quality as well as the heat input into the film.

Anode slurry was applied in excess to provide a sufficiently large reservoir and was

subsequently coated at 100 mm s-1 onto the copper substrate. The target area loading of the S5

dry film was 70.5±1.7 g m-², which resulted in a dry film thickness of about 78 µm. The initial

20 cm of the coating were discarded due to the entrance length and the resulting fluctuations

in mass loading that appear before a quasi-stationary coating process is obtained. Wet film

application and transfer of the wet film into the dryer were carried out simultaneously in a

single step with the elapsed time between coating and first dryer nozzle being less than a

second.

Nozzle width, distance between jet exit and substrate, spacing between two adjacent

nozzles, nozzle length and number of nozzles were 4 mm, 30 mm, 56 mm, 200 mm and 20,

respectively. The dryer was designed following the recommendations for optimized

impingement dryers given by Martin & Schlünder [3] and Polat [4].

The temperature of the dryer, the temperature-controlled aluminum plate and the preheating

unit containing the doctor blade and the slurry was held at a constant temperature of 76.5°C.

Dryer and plate temperature were either monitored by a thermocouple, one being located

inside a nozzle of the impingement dryer, while the other was attached to the aluminum plate

by adhesive aluminum tape. The local distribution of the heat and mass transfer coefficient

imposed by the impingement dryer is non-uniform. In the stagnation area below each nozzle

high transfer coefficients prevail, while they decrease with increasing lateral distance from the

nozzle. The wet film was continuously moved by an amplitude of 280 mm at a speed of

100 mm s-1. This periodic movement at relatively high speeds in combination with the small

nozzle-to-nozzle spacing allowed for a homogenization of the drying process. This is

prerequisite to prevent undesired side-effects induced by local inhomogeneity in drying rate

[5]. Although the heat and mass transfer coefficients strongly vary between maximum values

in the stagnation area below each nozzle and minimum values between two adjacent nozzles,

every discrete film element experiences the same drying curve. The experimental set-up

allowed for almost isothermal drying conditions. Most notably the improved heat input

provided by the temperature-controlled plate allowed for almost constant film temperatures S6

during drying, with a ΔT due to evaporative cooling of less than 0.5 K for the drying

conditions and materials applied in this work.

Drying curve. For low solvent loadings of X<0.1 the “constant rate period” transitions into

a “falling rate period”. In this case this does not necessarily imply a change in the

fundamental drying mechanism, such as the evolution of a diffusive limitation due to the

tortuosity within the partially dry, porous film. Even little local inhomogeneity in area weight

as a result of small deviations during coating is significant enough to cause the drying rate to

decrease due to a reduction of area actively involved in evaporation as soon as the thinnest

film domains are dry. Since this is of scientific interest in terms of interpretation of the drying

process, the possible impact of deviations in area weight is estimated in the following. As

mentioned in the coating section, the area weight fluctuated by ±1.7 g m-² around an average

value of 70.5 g m-². For two cases, the theoretical drying time was calculated from the drying

rate measured in the CRP based on the initial solvent loading of the anode dispersion. The

results are given in Table 1. The total drying time required differs by about 9% when

comparing the thinnest and thickest domains of a coating and can account for the decrease in

drying rate at low solvent contents.

Table 1. Comparison of the total drying time of two films with different area weights. Case 1

represents a coating with the lowest mass loading, Case 2 the thick counterpart.

Case 1 Case 2Mdry= 70.5-1.7g m-² 70.5+1.7g m-²mNMP=¿ 1.18g m-²s-1

xs= 0.475t= 63s 69s

t1/t2= 91.2%

References

[1] K. Terashita, K. Miyanami, Adv. Powder Technol. 13 (2002) 201–214.

[2] G.-W. Lee, J.H. Ryu, W. Han, K.H. Ahn, S.M. Oh, J. Power Sources 195 (2010) 6049–

6054.S7

[3] H. Martin, E.U. Schuender, Chem. Ing. Tech. 45 (1973) 290–294.

[4] S. Polat, Drying Technol. 11 (1993) 1147–1176.

[5] D.J. Harris, J.A. Lewis, Langmuir 24 (2008) 3681–3685.

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