ethyleneâstyrene): A New Polymer Binder for

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Poly(Styrene-Butene/Ethylene-Styrene): a New Polymer Binder for High Performance Printable Lithium-Ion Battery Electrodes Renato Ferreira Gonçalves, Juliana Oliveira, Marco P. Silva, Pedro Costa, Loic Hilliou, Maria Manuela Silva, Carlos M. Costa, and Senentxu Lanceros-Mendez ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00528 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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ACS Applied Energy Materials

Poly(Styrene-Butene/Ethylene-Styrene): a New Polymer Binder for High Performance Printable Lithium-Ion Battery Electrodes

Renato Gonçalvesa,#, Juliana Oliveiraa#, Marco P. Silvaa,b, Pedro Costaa,c, Loic Hilliouc, Maria Manuela Silvad, Carlos M. Costaa,d,*, Senentxu Lanceros-Méndeze,f,*

a

Centro de Física, Universidade do Minho, 4710-057 Braga, Portugal

b

C-MAST – Center for Mechanical and Aerospace Science and Technologies, Universidade da Beira Interior, Rua Marquês d’Ávila e Bolama, 6201-001 Covilhã, Portugal

c

IPC – Institute for Polymers and Composites, Universidade do Minho, Campus de Azurém, 4800-058 Guimarães, Portugal

d

Centro de Química, Universidade do Minho, 4710-057 Braga, Portugal

e

BCMaterials, Basque Center for Materials, Applications and Nanostructures, UPV/EHU Science Park, 48940 Leioa, Spain f

IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Spain

#equal contribution

KEWYORDS: SEBS; binder; electrodes; lithium-ion batteries; printed batteries _____________________________ *

Corresponding Authors

C.M. Costa ([email protected]), S. Lanceros-Méndez ([email protected])

1 ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT: Printed electrodes were developed by screen-printing from ink formulation developed from a green solvent, a new polymer binder and a graphite (anode) or C-LiFePO4 (cathode). The morphological and electrochemical properties of the electrodes were investigated and compared to electrodes prepared with poly(vinylidene fluoride), PVDF, as a binder. The developed ink formulation shows similar processability and apparent viscosity of 0.09 and 0.07 Pa.s at 100 s-1 for the cathode and the anode inks, respectively. The morphology for both electrodes is homogeneous and in relation to the electrochemical performance, the discharge capacity value for the cathode film with poly(styrene-butene/ethylene-styrene) (SEBS) and PVDF binders is 137 mAh.g-1 and 53 mAh.g-1 at C/5 and 113 mAh.g-1 and 29 mAh.g-1 at 5C, respectively. The SEBS binder provide better mechanical stability and a more effective electronic conductive network. For the anode, the discharge capacity value with SEBS after 48 cycles is 177 mAh.g-1 with a capacity retention of 78%. Thus, it is concluded that SEBS as a polymer binder, shows adequate cycling performance and the developed inks for the electrodes films are suitable for printed lithium-ion batteries.


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1. INTRODUCTION Batteries are the most commonly used energy storage systems in portable electronic devices and there are increasingly demanded in electric vehicles based on their efficiency, autonomy and energy density 1-2. In the batteries market, lithium-ion batteries are more used than other batteries types, such as lead acid or nickel-metal hydride, once they show high power and energy, lightweight and the small size, which are the key attributes for many applications 3-4. Typically, the improvement in the performance of lithium-ion batteries is related to the development of higher performance materials for its components: electrodes and separators

5-6

. The electrodes are designated by anode and cathode, and they are

responsible for the capacity value and life cycle of the battery 7. Regardless of the type of electrode, these are composed by two types of particles, identified as active material and conductive additive, and a polymer binder 3, 8. In relation to the active materials, carbonaceous materials such as graphite or carbon nanotubes 9, and lithium titanate (Li4Ti5O12)

10

are the most used for the anode

electrode. Lithium cobalt oxide (LiCoO2) 11, lithium manganese oxide (LiMn2O4) 12 and lithium iron phosphate (LiFePO4)

13

are the most commonly used active materials for

the cathode electrode. Further, poly(vinylidene fluoride) (PVDF) and carbon black are most commonly used polymer binder and conductive additive, respectively, for both electrodes 7. The inks for the electrodes preparation, typically consist in an homogeneous dispersion of the active material and conductive additive into a polymer binder solution, being essential the control of the rheological properties of the slurries 14-15. The preparation of the electrode suspensions affects the performance of the battery. The polymer has two main functions: guarantee the adhesion/cohesion between the electrode



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and the current collector and to bind together all constituents, ensuring the network structure of the active material and conductive additive, while maintaining the mechanical stability, flexibility and the electrical pathways of the electrode 7. In addition to their functions, the properties of the polymer binder are the insolubility in the electrolyte solution, excellent chemical and electrochemical stability and ease of use in the form of inks 6. Typically, there is no polymer that satisfies all these properties simultaneously, but the most commonly reported polymer in the literature is PVDF as it shows excellent electrochemical stability, bondability and good absorbing electrolyte capability to allow the transport of Li to the surface of the active material. Others polymer binders are the carboxymethyl cellulose (CMC), as it is soluble in water, inexpensive and environmentally friendly, improving the ratio of active material in a cell due to the reduction of binder content and rapid drying rate during electrode fabrication, and the styrene-butadiene rubber copolymer SBR / CMC as it improves adhesion strength leading to suitable life cycle and better binder distribution 7, 16. The polymer is usually not suitable for all the different active materials used in the electrodes. However, a single polymer binder that can be applied to both electrodes would be interesting, in particular for printed batteries 17 which are a suitable alternative to conventional batteries in many applications such as RFID tags, smart cards and toys, and remote sensors 18, due to flexibility, low cost and simple integration into devices. Considering that SBR leads to the most flexible electrodes and shows improved binding capacity at small polymer content, and that this polymer is being replaced by poly(styrene-butene/ethylene-styrene) (SEBS) for some applications, the focus of this work is the development of an anode and a cathode ink formulation based on graphite


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and carbon coated LiFePO4, respectively, with SEBS as polymer binder for the fabrication of printed batteries. SEBS is a thermoplastic elastomer obtained through the hydrogenation of the SBS copolymers that results in a predominantly polyethylene (PE) mid-block. SEBS shows a triblock structure where polystyrene blocks are placed in both ends and the ethylenebutylene (EB) rubber phase in middle

19-20

. It is also relevant to notice that the inks are

produced based on cyclopentyl methyl ether (CPME), a "green solvent" with low explosive nature, low vaporization energy and water-immiscibility 21. The rheological properties of the inks were analyzed as well as the microstructure and the electrochemical performance of the developed electrodes.



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2. EXPERIMENTAL 2.1. Materials C-LiFePO4 (LFP) was provided by Phostech Lithium and carbon black (Super P-C45) and graphite particles (Timrex SLG3) were provided from Timcal Graphite & Carbon. The styrene–ethylene/butylene–styrene copolymer (SEBS, Calprene CH-6110 with a ratio of ethylene-butylene/styrene of 70/30) and the poly(vinylidene fluoride) (PVDF, Solef 5130) were supplied by Dynasol and Solvay, respectively. The solvents cyclopentyl methyl ether (CPME) and N,N’-dimethylpropyleneurea (DMPU) and the electrolyte solution (1 M LiPF6 in ethylene carbonate-diethyl carbonate (EC-DEC, 1:1 vol)) were purchased from Carlo Erba, LaborSpirit and Solvionic, respectively.

2.2. Rheology 2.2.1. Polymer binder solution SEBS solutions with concentration between 9.4 wt% to 22.5 wt% were loaded into the Couette geometry of a stress controlled rotational rheometer (Anton Paar, MCR300). The gap between cylinder and cup was 0.5 mm. The shearing geometry was tapered with water to prevent solvent evaporation. For each sample, two flow curves were measured by first sweeping up the shear rate, immediately followed by a second sweeping down of the shear rates. 1 s measurement time was allowed before reading the stress values at larger shear rates, whereas 30 seconds were left for the sample to equilibrate at lower shear rates before reading the stress values. All tests were performed at 25 ºC. A similar experimental protocol was followed to measure the flow curves of electrode inks.


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2.2.2. Inks preparation LFP or graphite, Super P-C45 and the polymers binder, i.e, SEBS and PVDF, were mixed in CPME and DMPU 22, respectively, in a weight ratio (%) of 80:10:10. In order to obtain a good dispersion of the solid material, the inks were produced by the addition of small amounts of dry blended solid material to the polymer binder solution. After the addition was complete, the dispersion was kept under magnetic stirring for 2 h at 1000 rpm. After this step, the electrode slurry was placed inside an ultrasonic bath for 1 h and again placed under magnetic stirring for 30 minutes.

2.3. Electrode preparation The electrodes were fabricated using a manual screen-printing machine, fabricated in stainless steel. The screen printer has a substrate holder that can be adjusted on the x and y axis. Further, it has a frame to add and hold steady the screen mesh that can be adjusted in the z axis to guarantee that the required distance to the substrate is obtained. The used polyester mesh count was 65 threads/cm, the thread diameter was 52 µm, and the square-edged mesh opening was 102 µm. The squeegee orientation angle was set to 45º relative to the print substrate. A force of 17 N was applied to the mesh at 100 mm distance to the aluminum or copper substrate for cathode and anode electrode, respectively Finally, the electrode films were dried in air atmosphere at 60 ºC in a conventional oven, ED 23 Binder, for 20 minutes. The active mass loading, thickness and average porosity 23 of the cathode electrodes are 0.95 ± 0.27 mg.cm-2, 19 ± 3 µm and 70 ± 4 %, respectively. For the anode electrodes,



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these parameters have the following values: 5.75 ± 0.63 mg.cm-2, 74 ± 16 µm and 48 ± 6 %, respectively.

2.4. Electrodes characterization A home-made “bend tester” was used to evaluate the cohesion and flexibility of the electrode films as well as their adhesion to the current collector composed by a series of metal rods of decreasing diameter from 10.0 mm to 1.5 mm. Three measurements were performed for each rod 23. The morphology of the electrode films was obtained by scanning electron microscopy (SEM) (NanoSEM - FEI Nova 200 and EDAX - Pegasus X4 M) with an accelerating voltage of 10 kV. The value of the swelling was determined immersing the electrodes in the electrolyte solution (1M LiPF6 in EC/DEC (1:1vol)) at room temperature for 48 hours and applying equation 1: Swelling

(% ) =

m − m0 × 100 m0

(1)

where m0 and m are the mass of the dry and wet electrode films, respectively.

2.5. Cells manufacturing and testing Li/C-LiFePO4 or Li/Graphite half-cells of Swagelok type with two electrodes were assembled in a home-made argon-filled glove box. Whatman glass microfiber filters (grade GF/A) (10 mm diameter) were used as separators after soaking in electrolyte solution. Finally, the separator was placed between metallic lithium (8 mm diameter) and the prepared LFP or graphite printed electrode film (8 mm diameter).


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The half-cells were cycled at room temperature in the voltage range from 2.5 V to 4.2 V at different current densities (C/5 at 5C, C = 170 mA.g-1) for Li/LFP half-cells and 0.01 V to 1.5 V at the same current density (C/5, C = 370 mA.g-1) for Li/graphite half-cells using a Landt CT2001A instrument. For Li/LFP half-cells, electrochemical impedance spectroscopy (EIS) was performed in an Autolab PGSTAT12 instrument in the frequency range from 1 MHz to 10 mHz with an amplitude of 10 mV. The cyclic voltamogram (CV) was measured also with the same equipment in the range from 2.5 V to 4.2 V at a scan rate of 0.1 mV/sec. For each electrochemical test, a minimum of 6 batteries were fabricated and tested.



3. RESULTS AND DISCUSSION 3.1. Rheological properties of the inks 3.1.1.

Polymer binder solution

The flow curves of SEBS solutions in CPME are presented in figure 1 for selected SEBS concentrations. A Newtonian behavior is found for all solutions but the most concentrated one at 22.5 wt%. The concentration dependence of the specific viscosity

η SPE =

η0 − η s of the SEBS solutions is plotted in the inset of figure 1. η0 is the ηs

Newtonian viscosity of the SEBS solution, whereas ηS is the viscosity of CPME. The double logarithmic plot in the inset shows that ηSPE exhibits a power law dependence with the SEBS concentration.

0

10

-1

10

3

10

ηSPE (Pa.s)

η (Pa.s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2

10 -2

10

10

15 20 25 concentration (wt / wt)

1

10

30

2

-1

γ (s )

10

22.5 wt% 19.7 wt% 17.3 wt% 15.4 wt% 13.8 wt% 12.4 wt% 11.2 wt% 10.2 wt% 9.4 wt%

3

10

Figure 1. Flow curves, shear viscosity η as a function of the shear rate ߛሶ , of SEBS solutions in CPME for various SEBS concentrations as indicated in the legend. Inset:


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concentration dependence of the specific viscosity ηSPE of SEBS solutions. The line is a power law fit to the data.

A power law fit to the data shows that the exponent of the power law is 5.07 ± 0.07. This exponent is much larger than those predicted from scaling concepts in polymer solutions 24. This comes as no surprise as block copolymers are known to form micelles in specific solvents. CPME is most probably a selective solvent for PS blocks, thus forcing EB blocks to collapse in the core of micelles decorated by swollen PS end blocks in CPME

25

. Thus, the SEBS solutions in CPME are actually suspensions of

micellar objects. As SEBS concentration increases above the critical one for micelles formation, the latter self-assemble enroute to gelation, which results in a steep increase of solution viscosity. Overall, the rheological data in Figure 1 indicate that, for the range of tested concentrations, interacting SEBS micelles are flowing in CPME. Thus, one expects that these polymer solutions show film forming properties. In addition, the shear viscosity values displayed in Figure 1 are approaching those obtained elsewhere for PVDF in NMP

26

, indicating that similar polymer concentrations can be used to formulate the

binder in the cathode and anode inks. As a result, inks with SEBS were prepared with 1g of solid (active material, i.e, LFP or graphite, Super P-C45 and binder in proportion 80:10:10) for 1.75 ml of CPME solvent. With respect to PVDF-based inks, this is composed of 1 g of solid material (again in proportion 80:10:10 for active material, Super P-C45 and polymer, respectively) and 2.25 ml of DMPU solvent



3.1.2. Cathode and anode inks The flow curves presented in Figure 2a) were obtained with two samples of cathode inks, possessing the same formulation but studied in different days, using different flow curves protocols. Essentially, data in Figure 2a) show good reproducibility between 10 s-1 and 1500 s-1. Further, stress values obtained by sweeping up the shear rates overlap the stress values measured by sweeping down the shear rates, in the high shear rate regime. 3

τ = τy+ Kγ 2

10

1

10

b)

n

τ = τy+ Kγ

sweep up sweep down duplicate up duplicate down

2

10

τ (Pa)

10

10

a)

n

τ (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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sweep up sweep down aged up aged down

1

10

0

0

10 -1

10 0 10

10

1

-1 10

γ (s )

2

10

3

10

-1 1

10

2

-1

10

10

γ (s )

Figure 2. a) Flow curves measured on two samples of cathode inks (80% of LFP, 10% of Super-P C45 and 10% of polymer binder) formulated in separate days and studied with two different shearing protocols: sweeping up (open symbols) and down (solid symbols) between 10 s-1 and 1500 s-1 in 20 data points (black), or sweeping up and down between 0.1 s-1 and 1500 s-1 in 35 data points (red) and b) Flow curves measured with a fresh anode ink (80% of graphite, 10% of Super-P C45 and 10% of polymer binder) (black) by sweeping up (open symbols) then down (solid symbols) the shear 12 ACS Paragon Plus Environment

3

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rates. Data in red where obtained using an identical protocol but after allowing the sheared sample during the first two sweeps to rest 1 hour before application of a new shear. Lines are Herschel-Bulkley fits to the data, with adjusted parameters presented in Table 1

Thus, within the evaluated experimental conditions, the cathode ink does not present any thixotropic behavior. Data can be rationalized by a Herschel-Bulkley law which is the constitutive equation describing yield stress materials, namely . n

(1)

τ =τy + Kγ .

where τy is the yield stress beyond which the material flows with a shear rate γ in a non-Newtonian fashion, characterized by a thinning index n and a constant K.

The values of yield stress and thinning index obtained from the fit of the Herschel Bulkley equation to the data displayed in Figure 2a) are gathered in Table 1. Whereas excellent reproducibility is observed for the flow regime (both K and n parameters), some scattering related to the experimental difficulty inherent to the determination of the yield stress 27 is noted for parameter τy. Indeed, when the cathode ink is tested with longer experimental flow curves (the duplicated experiments were conducted by sweeping a larger range of shear rate and the test was accordingly longer), larger yield stresses are measured. However, the non-thixotropic behavior of the cathode ink is evident as yield stress values are identical when measured by sweeping up or down the shear rates.



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Table 1. Values of yield stress, thinning index n and constant K obtained from the fitting of the Herschel –Bulkley equation to the data displayed in Figure 1. CATHODE

τy (Pa)

K

n

Sweep up

1.43 ± 0.39

0.39 ± 0.06

0.60 ± 0.02

Sweep down

1.81 ± 0.50

0.38 ± 0.08

0.60 ± 0.03

Duplicate up

3.27 ± 0.54

0.30 ± 0.08

0.62 ± 0.03

Duplicate down

2.46 ± 0.40

0.40 ± 0.08

0.59 ± 0.02

Sweep up fresh

3.35 ± 0.51

0.024 ± 0.004

1.16 ± 0.02

Sweep up aged

3.52 ± 0.53

0.015 ± 0.003

1.20 ± 0.02

ANODE

The rheological conclusion for the cathode ink is that the material is a solid that flows when a stress larger than 2.24 ± 0.40 Pa (statistical mean and standard error of parameter τy displayed in table) is applied. The non-Newtonian flow behavior is characterized by a power law dependence with the shear rate, with a power law index of 0.60 ± 0.01. The flow curves measured with the anode ink by sweeping up the shear rates exhibit a qualitatively similar behavior as the cathode ink, as can be seen in Figure 2b). The qualitative similarity is mirrored in the fact that a Herschel-Bulkley fit to the data shows a satisfactory quality. Values of the fitting parameters gathered in Table 1 indicate that the anode ink shows a slightly larger yield stress than the cathode ink, and that the flow behavior after yielding is basically Newtonian (n is close to 1). However, in contrast to the cathode ink, the material in Figure 2b) shows some degree of thixotropy. When sweeping down the shear rates, no yield stress can be measured as torque values measured at lower shear rates fall below the limit of sensitivity of the 14 ACS Paragon Plus Environment

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rheometer. As such, stresses measured at lower shear rates do not plateau to a value corresponding to the yield stress. To measure the latter, the samples needs to age during some time. This is shown in the data (red symbols in Figure 2b) measured 1 hour after having sweeping down the shear rates. These data overlay on the first set of data, thus showing that 1hour structural recovery in the ink is enough for measuring yield stress. This structural recovery is responsible for the thixotropy evident in Figure 2b). As far as knife coating is concerned, the flow curves in Figure 2b) indicate that the apparent viscosity exhibited by the anode ink at high shear rates, nearly matches the apparent viscosity measured with the cathode ink (shear viscosities between 0.09 Pa.s and 0.07 Pa.s are measured for the cathode and the anode inks, respectively, for shear rates around 100 s-1). Thus, for the printing process used in this study, both inks show similar processability with equivalent apparent shear viscosities, whereas these are rheologically different materials with yield stress behavior at low shear rates (yield stress values being larger for the anode) and different flow behavior at larger shear rates (shear thinning versus Newtonian). Differences at larger shear rates may relate to the anisotropy of the particles used in the cathode ink formulation (see Figure 3c below) which can explain particles flow-induced alignment and subsequent shear thinning.

3.2. Morphological images, conductivity and swelling properties

The SEBS formulation used in this work presents adhesive properties

28

that are very

important in the electrodes, once, a low adhesion strength leads to homogeneities in current density in the electrode volume,

29

influenced by the physical/chemical surface

interactions between polymer binder, active material and conductive additive 30-31.



Page 16 of 37

Regardless the rod diameter used for the “bend tester”, no cracks and no delamination were detected on the surface of the electrode films (both to anode and cathode) or on the surface of the current collector. The electrodes morphology, measured by SEM are shown in Figure 3 and 4 for the cathode and anode, respectively, and is very important once it influences battery performance. For low magnification (Figure 3a), a three-dimensional (3-D) interconnected structure without aggregates and with a homogeneous distribution of the different components was observed in the SEM images of the cathode film. Moreover, it is observed that the presence of voids is distributed along the surface of the electrode, allowing the easy access of Li+ and facilitating the intercalation kinetics, even with higher current densities, increasing the electrode/electrolyte interface area 32. Comparing Figure 3a) with the back-scattered electron (BSE) image (Figure 3b), it is observed that the conductive additive and binder are homogenously distributed, as identified by the brighter regions representing the active materials and the dark regions corresponding to the polymer binder and conductive additive, once the contrast of the BSE image is determined by the atomic number of the materials in which the active materials have relatively heavier elements. Finally, Figure 3c) shows the active material particles and their non-spherical shape with rod-like structure of the different sizes below 2 µm (Figure 3c). The shape and size of the active material can lead to improved battery performance due to high surface-tovolume ratio contact area between the electrolyte and the electrode 32.


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Figure 3. a) and c) Secondary electron and b) and d) back-scattered electron SEM surface images of the cathode with SEBS as polymer binder for low and high magnifications.

It is also to notice (Figure 3d) that the active material particles are coated with SEBS, the SEBS covering all active material particles resulting in enhanced mechanical and electrical connectivity between the active particles. The cross-section SEM images for the cathode SEBS film also report the good homogeneity of the particles and the polymer (data not showed).



Page 18 of 37

Further, this good homogeneity was demonstrated by the DC-electrical conductivity value (~ 8.4 S m-1, see supplementary information) once this value is higher when compared to the ionic conductivity of the electrolyte solution (σi = 0.82 S m-1) 7 and the electrical conductivity of the active material (3.7 × 10-7 S m-1) 33. The swelling of the cathodes with the SEBS polymer determined by equation 1 is ~ 31 ± 3 %. The swelling value is slightly higher in comparison to the one for PVDF reported in the literature

34-35

. Considering the swelling value, it is observed that SEBS is easily

swollen by the electrolyte, with a swelling value 40%

36

, limit value reported in the

literature for the breaking of the conductive electrode network without affecting battery performance. The morphology of the anode electrode was also analyzed by SEM images and the cross-section and surface images are shown in Figure 4a) and b), respectively.

Figure 4. a) Cross-section and b) surface SEM images showing the morphology of the anode films prepared with SEBS as polymer binder.

Figure 4a) shows that the morphology of the anode electrode is homogeneous along the thickness of the films, without aggregates and with the graphite particles distributed along the thickness of the film. Further, the presence of voids is detected between the 18 ACS Paragon Plus Environment

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graphite particles, which is very important for the interaction between the electrode and the electrolyte solution (figure 4a). Figure 4b) shows the surface morphology of the anode with graphite particles with a flake geometry and an average particle size varying between 10 µm to 15 µm.

3.3. Electrochemical properties 3.3.1. Cathodic half-cell The cathodic performance of the LFP printed film with SEBS as polymer binder was measured at C/5 and 5C in the voltage range between 2.5 V and 4.2 V as it is shown in Figure 5. The fifth cycle of the charge–discharge curves obtained at different C-rates is shown in Figure 5a). It is observed that the charge-discharge curves are characterized by a flat plateau of about 3.2 -3.7 V which decreases with increasing of the C-rate. This typical flat voltage plateau at around 3.4 V indicates the presence of the two-phase Fe2+/ Fe3+ redox reaction between FePO4- and LiFePO4 37.

b)

C/5

140

C/5

-1

C/5 C/2 C 2C 5C

3,6 3,3 3,0

Capacity / mAh.g

+

C/2

3,9

120 100

60 5C

60

40

60

80

100

120

50

40

40

20

30

Charge Discharge

0

20

70

2C

80

0

0

90 80

C

2,7 2,4

100

140

5

20

10

15

20

25

30

Cycle number

-1

Capacity / mAh.g


Coulombic efficiency / %

160

a)

4,2

Voltage / V vs Li/Li

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1

120 90 100

C 80

80 60

2C

70

40 20

60

Charge Discharge

50

0 0

20

40

60

80

Discharge capacity / mAh.g

100


c)

140

Capacity / mAh.g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-1


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d)

C/5

140

C/2

120 C

PVDF SEBS

100 2C

80 60

5C

40 20 0 0

2

100

4

6

8

10

12

14

Cycle number

Cycle number

Figure 5. a) Fifth charge-discharge cycle profiles of the cathode with SEBS polymer binder at scan rates from C/5 to 5C at room temperature. b) Rate performance of the cathode during the charge-discharge process and the corresponding coulombic efficiency. c) Charge- discharge capacity values as a function of the cycles number when cycled at C and 2C. d) Comparison between the discharge profile at different Crates for the cathodes with SEBS and with PVDF as polymer binder.

Further, the capacity decreases with the increase of C-rate due to electrode polarization, related to a large ohmic drop and activation overpotential indicating low active material utilization and transport limitations in the LiFePO4 solid particles 38. Figure 5b shows the rate capabilities of the cathode at different C-rates in the chargedischarge process. The capacity value is stable for both cycling processes and the cycle number presents an excellent capacity value between 137 mAh g-1 at C/5 and 52 mAh g1

at 5C. Figure 5b also shows an excellent capacity value after rate performance in the

recovering cycle proving the reduced capacity fading at C/5. Thus, the rate performance of the printed cathode (Figure 5b) shows that the SEBS polymer binder supports good electronic connection between current collector and


16

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electrode and excellent mechanical stability, leading to a coulombic efficiency (CE) of ~100 %. Figure 5c shows the cycle stability of the cathode for more than 50 cycles in the charge and discharge processes, at C and 2C-rate. Independently of the cycling number and scan rate, the cathode sample shows good stability and high capacity retention over several cycles. At C-rate, it is observed a decrease of the capacity value due to electrolyte penetration in detached areas of the active material giving rise to capacity loss 39. For example, the initial discharge capacity at 2C is 62 mAh g-1 and after 50 cycles this value is 54 mAh g-1, representing 87% of capacity retention. Further, the CE is around 99% for all cycles and both C-rates. In addition, a rate capability test of the cathode prepared with SEBS as polymer binder was compared with a cathode prepared with the same components and contents but with PVDF as polymer binder (Figure 5d). The processing conditions, the main characteristics and the performance of this cathode can be found in 40. Figure 5d shows that the cathode with SEBS presents a best cell performance for all Crates and cycle number comparing with the cathode with PVDF as polymer binder. Thus, the discharge capacity for the cathode film with SEBS and PVDF binders is 137 mAh g-1 and 53 mAh g-1 at C/5 and 113 mAh g-1 and 29 mAh g-1 at 5C, respectively. These results can be explained by the different morphology of the cathodes prepared with PVDF and SEBS as polymer binders after cycling, as schematically represented in Figure 6 a) and b), respectively.



Page 22 of 37

Figure 6. Schematic representation of the cathode morphology prepared with PVDF a) and SEBS b) as polymer binders.

During

cycling

performance,

mechanical

stress

occurs

during

lithium

intercalation/deintercalation due to the expansion/shrinkage of the active material Comparing the thermoplastic PVDF, with a higher Young modulus wettability

43

with the thermoplastic elastomer SEBS

44

42

41

.

and lower

in the presence of the active

material, the PVDF is more prone to suffer detachment of the active particles to the polymer binder as well as degradation of the interface with the current collector, leading also to loss of efficiency in the global electron transport network. Considering the electrochemical results (Figure 5) and the excellent mechanical properties, the SEBS binder provides a more effective conductivity network with a more stable interface structure than the PVDF binder as it is represented in Figure 6, leading to an improvement of the cyclability. This fact is explained by the styrenic domains in each end of triblock polymer, “glue together” several particles simultaneously acting as crosslink molecule with flexibility and adhesive properties. The effect of the SEBS as polymer binder on the electrochemical characteristics of the cathode was analysed through EIS and CV curves (Figure 7). 22 ACS Paragon Plus Environment

Page 23 of 37

The Nyquist plots of the cathode before, after cycling and after cycling + 3 cycles CV are shown in Figure 7a).

-4

2x10

16000

a)

14000

-4

2x10

b)

~0.15V

-4

1x10

Current / A

12000 10000

-Z'' / Ω

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8000 6000 4000

0 -5

-5x10

-4

-1x10

Before cycling After cycling After cycling + 3 cycles CV Fit

2000

-5

5x10

-4

-2x10

0

2,5

0

1000

2000

3000

4000

5000

Z' / Ω

3,0

3,5

4,0 +

Potential / V vs Li/Li

Figure 7. EIS curve before, after cycling, after cycling + 3 cycles CV and the corresponding equivalent circuit a) and CV curves b) for the cathode prepared with SEBS as polymer binder.

The Nyquist plots of Figure 7a) are characterized by a depressed semicircle at high frequencies and a straight line at low frequencies. At high frequencies, the observed semicircle represents the sum contribution of the electrolyte resistance (Re), surface film resistance (Rf), representing the migration resistance of Li-ion through the solid electrolyte interface (SEI) film on the cathode surface, and charge-transfer resistance (Rct). The straight line represents the semi-infinite diffusion (Warburg element, W) in the low frequency region 45. It should be noted that Warburg line is present in all states, except for the before cycling state, where the line is not definitely patterned at 45 °, since the SEI was not yet fully formed. The electrochemical phenomena occurring at the cathode can be modeled by the equivalent electric circuit as is showed in the Figure 7a). It is observed a good agreement between the fitting of the equivalent electrical circuit model and the 23 ACS Paragon Plus Environment

4,5


Page 24 of 37

experimental data (Figure 7a). The obtained values for R1, R2

SEI

and R3

CT

for the

cathode sample before, after cycling and after cycling + 3 cycles CV are represented in Table 2.

Table 2. Resistance values obtained from the equivalent circuit representing the cathode sample before, after cycling and after cycling + 3 cycles CV. Cycling

R1 / ± 2 Ω

R2, SEI / ± 2 Ω

R3, CT / ± 2 Ω

RTotal / Ω

Before

2

120

3869

3991

After

3

286

3926

4215

After 3CV

4

383

5013

5400

Also, it is observed an adequate evolution of the values of resistances between different states, derived from the normal phenomena of formation of the solid interface and electrolyte. Impregnation of the electrolyte in the cathode pores is also observed, being the relative value to the total resistance of the cathode sample is 5400 Ω, after the electrochemical impedance spectroscopy test was fully performed. The straight line at low frequency represents a typical Warburg behavior, which corresponds to the lithium ions diffusion of in the active cathode material

46

, the Li+

diffusion coefficient (DLi+) of the cathode sample before and after cycling was calculated according to 47: D Li =

R 2T 2 2 A 2 n 4 F 4 C 2σ W2

Z ' = R1 + R 2 , SEI + R3,ct + σ W W −1 / 2

(2) (3)


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where R is the gas constant, T is the absolute temperature, A is the surface area of the cathode, n is the number of electrons per molecule during oxidization, F is the Faraday constant, C is the concentration of Li+, σW is the Warburg factor, R1 is the electrolyte resistance, R2,SEI is the resistance of the SEI, R3,ct is the charge transfer resistance and W is the angular frequency.

The Li+ diffusion coefficient (DLi) calculated from equations 2 and 3 and Figure 8a is 3.5× 10-16 cm2.s-1, 2.4× 10-16 cm2.s-1 and 3.3× 10-16 cm2.s-1 for before, after cycling and after cycling + 3 cycles CV, respectively. These diffusion coefficients are similar to others reported in the literature for same active material 26, 48 and these values of lithium ion diffusion coefficients refers to a good kinetics of the diffusion process of lithium ions during cycling process 49. Finally, the CV curves of the cathodes are presented in Figure 7b. An oxidation and reduction peak are observed at around 3.50 V and 3.35 V associated to deintercalation and intercalation process of the lithium ions, respectively

50

. Moreover, the cathode

electrode with SEBS as a binder shows a smaller voltage difference (0.15 V) between the oxidation and reduction peak potentials, when compared to the cathodes with PVDF as a polymer binder (0.21 V) 40. Thus, the cathodes prepared with SEBS polymer binder show better reversibility, lower polarization and reduced particle agglomerates size 51-52. Table 3 shows the electrode composition and electrochemical performance of the cathode with SEBS polymer binder developed in this work with cathodes prepared with other polymer binders reported in the literature for the same active material.

Table 3. Electrode composition and electrochemical performance of the cathode electrode prepared with SEBS polymer binder developed in this work comparing with



Page 26 of 37

cathodes prepared with others polymer binders reported in the literature for the same active material, LFP. Composition

Capacity

Capacity

value /

retention

mAh.g-1

/%

C-rate and Binder type

LFP:carbon:binde

Ref

cycler number r /% PVDF

80:10:10

C; 50

129

101

34

PVDF

75:20:5

5C; 800

~110

92

53

PVDF-TrFE

80:10:10

C; 50

94.3

89

34

PVDF-HFP

80:10:10

C; 50

75.5

84

34

PTFE

90:5:5

C/2; 100

146.5

97.5

54

LA 132/133

90:5:5

5C; 50

120.5

98

55

90:6:4

C; 1000

150

99

52

CMC

88:7:5

C;1000

105

75

56

PAA

90:0:10

0.2mA.g-1; 50

~135

98

57

PAN-PMMA

75:20:5

C/2; 45

110

-----

58

Lignin

80:11:9

C/3; 50

112

80

59

SEBS

80:10:10

2C; 50

54

87

PEDOT:PSS/CCTS binder

This work

Each polymer binder reported in the Table 2 shows specific advantages as it is the case of CMC that is water soluble or PVDF, with good electrochemical stability from 0 to 5 V and higher electrolyte uptake, but cathodes prepared with SEBS shows similar electrochemical properties than the best polymer binders reported in the literature, with


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the advantage of the interesting adhesive properties and elastomeric mechanical properties, allowing flexibility and even stretchability. Thus, Table 2 shows that cathodes prepared with SEBS are characterized by high capacity retention at high scan-rate (2C) with high polymer binder amount (10 wt%), in comparison to other cathode compositions, demonstrating the SEBS as an innovate polymer binder material that could be used in Lithium-ion batteries.

3.3.2. Anodic half-cell Considering the excellent electrochemical performance of the cathodes prepared with SEBS as polymer binder (section 3.3.1) and to demonstrate that this polymer binder can be applied in electrodes with different active materials. Graphite has been selected as active materials due to their use in large scale lithium-ion battery applications and the electrochemical behaviour (charge-discharge curves) of anodes prepared with SEBS binder and graphite as active material (80%) is shown in Figure 8. Figure 8a shows the voltage profile from 0.02 V to 1.5 V vs Li/Li+ of the anode electrode for the 1st, 10th, 20th, 30th, 40th and 48th cycles at C/5. Independently of the cycling number, the typical voltage plateaus is observed and corresponds to the phase transitions during Li-ion intercalation/deintercalation in n-stages, representing LiC18, LiC12 and LiC6 60. For the first cycle, an additional capacity value is observed attributed to irreversible processes, but keeping stable until the end of the cycles, both in shape and values (Figure 8a).



320

a)

b)

100

240

Capacity / mAh.g )

+

160 1,0

0,5

-1

1º cycle 10º cycle 20º cycle 30º cycle 40º cycle 48º cycle

0,0 0

100

200

Capacity / mAh.g

300

50

80

Charge Discharge

0 0

10

20

30

0 50

40

240 c)

100

180 120

50

60

-1

0

0 0

4

8

12

16

Cycle number Figure 8. Charge-discharge profile for the anode with SEBS binder and b)-c) Chargedischarge capacity value and coulombic efficiency for the anode with SEBS and PVDF binder, respectively as a function of the number of cycles at C/5.

Figure 8b) shows the capacity value for the charge and discharge processes during 48 cycles for the anodes prepared with SEBS as polymer binder, the charge-discharge values for the same electrodes with PVDF as polymer binder being shown in Figure 8c). The discharge capacity value of SEBS binder after 48 cycles is 177 mAh.g-1 with a capacity retention of 78%. Thus, when compared to the discharge capacity of electrodes with PVDF as a binder (147.2 mAh.g-1, figure 8c), the SEBS based ones binder show higher electrochemical values indicated better cyclability. Considering the results presented in this work, it was concluded that the SEBS polymer is a suitable binder for lithium-ion cathode and anode films produced by printing techniques, being a suitable material for the development of printed lithium-ion batteries.



1,5

Voltage / V vs Li/Li

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 37

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4. CONCLUSIONS A new anode and cathode film based on graphite and C-LiFePO4, respectively, with a new polymer binder were produced by screen-printing technique after the development of inks prepared with a “green solvent” CPME in which have low degree of acute toxicity. Thus, poly(styrene-butene/ethylene-styrene), SEBS, is proposed as a binder for printed electrodes and it’s electrochemical performance was evaluated. Both inks, for anode and cathode, showed similar processability with equivalent apparent shear viscosities, the apparent viscosity for the anode ink at high shear rates nearly matching the apparent viscosity of the cathode ink, i.e, 0.09 Pa.s and 0.07 Pa.s for the cathode and the anode inks, respectively, at 100 s-1. Independently of the electrode type, both electrodes show a homogeneous distribution of the different components. The printed cathode showed a high delivery capacity between 137 mAh.g-1 at C/5 and 52 mAh.g-1 at 5C, independently of the cycle number. The printed cathode with SEBS binder showed excellent battery performance and provides a more effective conductive network with a more stable interface structure than the PVDF polymer binder, commonly used for cathode and anode preparation. For the anode, the discharge capacity after 48 cycles is 177 mAh.g-1 with a capacity retention of 78%. Thus, it is shown that SEBS can be suitably used as polymer binder for anode and cathode films with good cycle performance comparing with PVDF as polymer binder and the inks developed for anode and cathode films with SEBS polymer and green solvent “CPME” are suitable for printed lithium-ion batteries.



Page 30 of 37

ASSOCIATED CONTENT Supporting Information: Electrical conductivity descriptions

AUTHOR INFORMATION Corresponding Author [email protected] (C. M. Costa), [email protected] (S. Lanceros-Méndez)

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UID/FIS/04650/2013 and UID/QUI/0686/2016. The authors thank FEDER funds through the COMPETE 2020 Programme and National Funds through FCT under the projects PTDC/CTMENE/5387/2014 and UID/CTM/50025/2013 and grants SFRH/BD/98219/2013 (J.O.) 30 ACS Paragon Plus Environment

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and SFRH/BPD/112547/2015 (C.M.C). The authors acknowledge funding by the Spanish Ministry of Economy and Competitiveness (MINECO) through the project MAT2016-76039-C4-3-R (AEI/FEDER, UE) and from the Basque Government Industry Department under the ELKARTEK and HAZITEK program. The authors thank Solvay, Dynasol, Timcal and Phostech for kindly supplying the high quality materials.

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