Highly Loaded Graphite-PLA Composite Based Filaments for Lithium

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Highly Loaded Graphite-PLA Composite Based Filaments for Lithium-Ion Battery 3D-Printing Alexis Maurel, Matthieu Courty, Benoit Fleutot, Hugues Tortajada, Kalappa Prashantha, Michel Armand, Sylvie Grugeon, Stéphane Panier, and Loic Dupont Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02062 • Publication Date (Web): 20 Oct 2018 Downloaded from http://pubs.acs.org on October 20, 2018

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Chemistry of Materials

Highly Loaded Graphite-PLA Composite Based Filaments for Lithium-Ion Battery 3D-Printing Alexis Maurel,*, †, ‡, d Matthieu Courty,†, ‡ Benoit Fleutot,†, ‡ Hugues Tortajada,d Kalappa Prashantha, q,b Michel Armand,† Sylvie Grugeon,†, ‡ Stéphane Panier,d Loic Dupont*, †, ‡ †

Laboratoire de Réactivité et de Chimie des Solides, UMR CNRS 7314, Hub de l’Énergie, Université de Picardie Jules Verne, 33 rue Saint Leu, 80039, Amiens Cedex, France

d

Laboratoire des Technologies Innovantes, LTI-EA 3899, Université de Picardie Jules Verne, 80025 Amiens, France

‡

RS2E, Réseau français sur le stockage électrochimique de l’énergie, FR CNRS 3459, 80039 Amiens Cedex, France

q

IMT Lille Douai, Institut Mines-Télécom, Polymers and Composites Technology & Mechanical Engineering Department, 941 rue Charles Bourseul C.S.10838, 59508 Douai Cedex, France b

Université de Lille, 59000 Lille, France

ABSTRACT: Actual parallel-plate architecture of lithium-ion batteries consists in lithium ion diffusion in one dimension between the electrodes. To achieve higher performances in terms of specific capacity and power, configurations enabling lithium ion diffusion in two or three dimensions is considered. With a view to build these complex 3D battery architectures avoiding the electrodes interpenetration issues, this work is focused on the Fused Deposition Modeling (FDM). In this study, the formulation and characterization of a 3D-printable graphite/polylactic-acid (PLA) filament, specially designed to be used as negative electrode in a lithium-ion battery, and to feed a conventional commercially available FDM 3D-printer is reported. The graphite active material loading in the produced filament is increased as high as possible to enhance the electrochemical performances while the addition of various amount of plasticizers such as propylene carbonate (PC), poly(ethylene glycol) dimethyl ether average Mn~2000 (PEGDME2000), poly(ethylene glycol) dimethyl ether average Mn~500 (PEGDME500) and acetyl tributyl citrate (ATBC) is investigated to provide the necessary flexibility to the filament in order to be printed. Considering the optimized plasticizer composition, an in-depth study is carried out to identify the electrical and electrochemical impact of carbon black (CSP) and carbon nanofibers (CNF) as conductive additives.

INTRODUCTION Three-dimensional energy storage systems. In parallel with the development of new renewable energy sources, electrical energy storage systems have recently emerged as a crucial issue.1 They enable the capture of energy produced at one time to deliver it at a later time when it is needed. Due to their good performances in terms of high energy density and long cycle life, different technologies of lithium batteries were developed and reported to be promising candidates among the electrochemical systems.2 Among them, lithium-ion batteries are produced by billions of units every year and are already used in a huge range of applications as electric vehicles (EVs), hybrid vehicles (HEVs), cellphones, laptops or micro-electronic and micro-electromechanical systems (MEMS).3,4 Actual technology of this type of battery consists in lithium ion diffusion in one dimension between the electrodes arranged in a parallel-plate configuration (2D). Indeed, cathode/separator/anode films are rolled or stacked.5 However, to achieve better performances, 3D battery architectures were considered in the past. This concept was demonstrated by Long et al.6 who presented different 3D

design types enabling a two-dimensional or three-dimensional diffusion of the lithium ions. Moving from 2D electrodes (plates) to a 3D architecture of electrodes, main advantage is the increase of the electrochemical active surface present on the same footprint area (specific exchange area) and consequently increase of power and specific capacity. Various scientific groups,7 already prepared independent 3D-electrodes (positive or negative). Min et al.8 developed arrays of carbon posts interdigitated with arrays of dodecylbenzenesulfonate-doped polypyrrole (PPYDBS) posts. This was done through patterning the photoresist using photolithography, pyrolyzing the patterned photoresist to form carbon electrode arrays and carbon current collectors, and electrochemically polymerizing the PPYDBS on one set of 3D carbon arrays. On the other hand, Taberna et al.9 reported an electrochemically assisted template growth of Cu nanorods onto a current collector followed by electrochemical plating of Fe3O4. However, the assembly of the final 3D apparatus by interpenetration of both electrodes and avoiding any contact

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between them in order to avoid short-circuit was reported to be very complicated due to the various irregularities on the 3D electrodes surface.7-9 Preventing these interpenetration issues of positive and negative electrode could be achieved through 3Dprinting process. In this context, recent years have witnessed an intensification of studies concerning the additive manufacturing technologies, also called 3D-printing, applied to energy storage field. Additive Manufacturing. Additive Manufacturing (AM) technologies including Fused Deposition Modeling (FDM), Liquid Deposition Modeling (LDM), Selective Laser Sintering (SLS), Stereolithography Apparatus (SLA), or Electron Beam Manufacturing (EBM), directly refer to the processes that build 3D objects by adding material layer-upon-layer as opposed to subtractive manufacturing methodologies, such as traditional machining which creates form by removing material from a block to obtain the final shape.10 Recently, FDM and LDM processes were the most explored with a view to 3D-print electrochemical storage devices such as supercapacitors or batteries. FDM technique consists in building the 3D object by depositing layers provided by a thin filament of melted thermoplastic material via a precisely calibrated nozzle, of which the displacement (XYZ-axis movement) is controlled by computer. As the width and thickness of the deposited plastic line are fine, the model is built up in many successive passes. Each successive layer fuses to the preceding layer as the thermoplastic material is heated few degrees above its melting point, then immediately cools down and solidifies after the extrusion. FDM is nowadays the most widely used 3D-printing technology to fabricate low cost, complex objects with almost no material waste. Current FDM 3D-printers layer thickness resolution usually approaches 200µm or more for applications with standard resolution demands but can be enhanced to 50µm for higher resolution purposes by modifying the machine (nozzle diameter, structural parts) and optimizing software parameters.11 It is used for rapid prototyping in a huge range of applications including medical, automotive and also aerospace sectors. On the other hand, LDM process is based on the same working principle. However, it requires a syringe instead of the nozzle as this technique uses a liquid (inks) as material source. In this case, 3D-printing is carried out at room temperature. State of the art. Concerning the LDM process, Sun et al.12 reported the first 3D-printed Li-ion microbattery. Cellulosebased inks were prepared by suspending respectively Li4Ti5O12 (LTO) and LiFePO4 (LFP) nanoparticles in solution for the preparation of negative and positive electrode. Both inks were subsequently 3D-printed through a syringe and subjected to drying and annealing at 600 ° C for 2 h in an inert atmosphere. Discharge electrochemical measurements for half-cells of LTO and LFP electrodes at 1C led to specific capacities of 131 and 160 mAh g-1 respectively, in good agreement with their respective theoretical values of 175 and 170 mAh g−1. Based on this previous work, Fu et al.13 developed graphene oxide (GO) based electrode composite inks (LTO/GO and LFP/GO) and separator PVDF-co-HFP with Al2O3 inks to 3D-print a lithiumion battery. It is worth mentioning that graphene oxide sheets were added to provide the prerequisite viscosity to bind the electrode materials together and enable 3D printing as well as to enhance the electrode’s electrical conductivity. Both electrodes were freeze-dried to remove the water solvent and solidify the 3D structures while a thermal annealing process (600 °C in Ar/H2 for 2 h) was also applied to enable the formation of

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reduced graphene oxide (rGO). Electrochemical performances of the printed LFP/GO and LTO/GO half cells led respectively to specific capacities of 164 and 185 mAh g-1 of active material at a current density of 10 mA g−1 (C/17). Recently, Liu et al.14 were able to prepare a PVDF-co-HFP/boron-nitride ink dispersed into dimethyl formamide to 3D-print a PVDF-coHFP/boron-nitride (BN) separator. They demonstrated that BNseparator can provide fast heat dispersion and a uniform thermal distribution at interface during the Li plating process enhancing the electrochemical performance of the Li-ion battery. Other groups such as Nathan-Walleser et al.15 focused their studies to prepare electrodes for supercapacitor applications. In this case, graphene oxide dispersed in isopropanol ink was printed and the electrochemical performances of the corresponding electrode demonstrated volumetric capacitance equals to 10 F cm-3 in 1M KOH. In parallel, Zhu et al.16 3D-printed a graphene aerogel using a colloidal suspension of graphene oxide as ink prepared by adding fumed silica powders and catalyst into as-prepared aqueous GO suspensions. After some post-processing steps including supercritical drying, carbonization at 1050 ºC under N2 and etching of the silica using HF acid, the graphene structure could be used in supercapacitors applications. More recently, Rocha et al.17 designed water-based thermoresponsive (polyethylene oxide - polypropylene oxide - polyethylene oxide (PEO-PPO-PEO)) inks to 3D-print graphene electrode and copper current collector for supercapacitors. After thermal treatment at 900 ºC, binder-free 3D structure was obtained, depicting a BET surface area of 193 m2 g-1, electrical conductivity values of 90 S m-1 and a capacitance value around 8 F g-1. The main limitation of the LDM is undoubtedly the necessity of carrying out post-processes such as freeze-drying and thermal annealing before its use. Moreover, viscosity of the slurry is an important parameter to take into consideration as it limits the height of the subsequently 3D-printed structure. FDM process overcomes those last issues. However, only thermoplastic polymers such as polylactic acid (PLA) and acrylonitrile-butadiene-styrene (ABS) among the most used, can be employed to produce a filament as material source for the 3Dprinter. Different groups focused their work on the FDM process. A filament based on conductive carbon black dispersed in polycaprolactone (PCL) matrix was reported by Leigh et al.18 for the fabrication of electronic sensors. Dul et al.19 developed a conductive ABS-based filament loaded with 4 wt.% graphene nanoplatelets (xGnP) in order to feed the 3D-printer. Wei et al.20 have gone further, by reporting the synthesis and characterization of PLA/graphene and ABS/graphene (loading up to 5.6 wt.%) filaments. Finally, in a recent study, Foster et al.21 used a commercially available graphene-based polylactic acid filament (graphene/PLA) to fabricate a 1mm thick 3D-printed electrode disc using an adapted FDM 3D printer. It was characterized both electrochemically and physico-chemically to be used as negative electrode in Li-ion batteries. Due to the low ratio of active material (8 wt.% graphene and 92 wt.% PLA) into the filament (103 mg of active material per cm3 of composite), relatively low discharge specific capacities of 15.8, 6.2, 2.6, 1.1 and 0.6 mAh g-1 of active material were reached at current densities of 10, 50, 70, 100 and 200 mA g−1 (C/37, C/7, C/5, C/4, C/2) respectively. Outline of the paper. The purpose of this work was to develop a highly loaded graphite-PLA composite based 3D-


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printing filament, specially designed to be used as negative electrode in a lithium-ion battery, while maintaining appropriate mechanical properties to feed a conventional FDM 3Dprinter. Through the formulation process, optimized content of plasticizer and conductive additives was investigated. Characterization of the composite electrode was described after the different elaboration stages: film, filament or 3D-printed disc. Thermal (DSC), mechanical (tensile test), microstructural (Raman spectroscopy), morphological (SEM), as well as electrical and electrochemical analysis of the composite electrodes were performed. This study acts here as a proof of concept as authors are aware of the used 3D-printer low resolution induced-limitation. Machine modifications are beyond the scope of this paper but will be addressed in future work in view to print complex 3D-electrode architectures whose design is being optimized through an in-depth modeling study.

EXPERIMENTAL SECTION Materials. Polylactic-acid (PLA 4032D) pellets were supplied by NatureWorks, USA, and used as received. Dichloromethane (DCM) was purchased from VWR Chemicals, USA. Timcal TIMREX® SLS graphite (~1.5 m2 g-1) was used as active material for the negative electrode of the lithium-ion battery. Plasticizers, propylene carbonate (PC) and poly(ethylene glycol) dimethyl ether average Mn~2000 (PEGDME2000) were purchased from Merck, Germany while poly(ethylene glycol) dimethyl ether average Mn~500 (PEGDME500) and acetyl tributyl citrate (ATBC) were purchased from Sigma-Aldrich, USA. Carbon nanofibers (CNF), (D x L 100 nm x 20-200 µm, 24 m2 g-1) and carbon black Timcal Super-P (CSP), (62 m2 g-1) were respectively supplied by Sigma-Aldrich, USA and Alfa Aesar, USA. Filaments formulation. A Filabot Original extruder (Filabot Triex LLC, USA) was fed homogeneously with 4 mm x 4 mm composite film pieces to produce regular ~1.75 mm diameter 3D-printing filaments. Temperature of the extruder was fixed typically 15 ºC higher than the melting temperature of the composite film deduced from DSC analysis. The filament coming out from the extruder nozzle was rolled manually around a spool. Prior extrusion of each sample, the extruder was cleaned with neat PLA. Printing. 11 mm diameter negative electrode discs of about 250 µm thick and elaborated 3D structures were printed by means of a Micro Delta Rework 3D-printer (eMotionTech, France). Structure of this machine, made by two folded steel blocks, highly limits the quantity of subassemblies and small parts to build it. This procures a high rigidity and good precision. Linkages and sliders, manufactured by injection molding, enable a very good alignment between the bearings leading to a better dimensional precision of the final printed object. Nozzle standard input and output diameters of respectively 1.75 mm and 0.4 mm were used. Maximum resolution in the Z direction is estimated to 0.25 mm. Temperature of the nozzle was fixed typically 15 ºC above the melting temperature of the composite filament deduced from DSC analysis. Bed temperature is fixed to 60 ºC in order to improve the adherence of the first printed layer. Thermal, mechanical, structural, electrical and electrochemical characterization. Differential Scanning Calorimetry (DSC). Thermal analyses of films, filaments and 3D-printed discs were

performed by differential scanning calorimetry (DSC) measurements on a DSC204F1 (NETZSCH-Gerätebau GmbH, Germany) machine in the temperature range from -100 to 300 ºC in argon atmosphere, using approximately 10 mg of sample of different compositions. All samples were tested at a heating rate of 10 ºC min-1. Heat flow data from the first heating process was recorded. Raman spectroscopy and Scanning Electron Microscopy (SEM). The global homogeneity of the produced composite filaments was confirmed by making use of a Raman DXR Microscope, Thermo Fisher Scientific, USA with excitation laser beam wavelength of 532 nm at a laser power of 5 mW. Fluorescence correction was applied. Scanning operations and data processing were controlled by Omnic software, Thermo Fisher Scientific, USA. Filament cross-sections were studied by previously adding samples into an epoxy resin (Struers S.A.S, France) during 4 days. The resulting resin cylinders were cut horizontally and polished. Inner material dispersion into the polymer matrix was investigated by means of a FEI Quanta200F (Thermo Fisher Scientific, USA) scanning electron microscope under high vacuum, operating at 2 kV, using secondary electrons detector. Mechanical characterization. After solvent casting, the composite films were compression molded into 2 mm thick plates using a Doloutes hydraulic press at 160 °C during 2 minutes before cooling down to room temperature. Then the samples were cut into specimens with a punch as per ISO 527 standard for further mechanical characterization. The tensile mechanical tests were performed on a Llyod LR 50 K tensile machine equipped with a 1 KN load cell. The tests were controlled by displacement with a speed of 10 mm min-1. Five samples of each composition (dumb bell shaped) were tested and the average values are reported. Electrical conductivity characterization. A MTZ-35 frequency response analyzer and an intermediate temperature system (ITS) developed by BioLogic, France, were used to perform the electrochemical impedance spectroscopy analysis. Inductive phenomenon from the cables were prevented by performing preliminarily the device calibration of the empty ITS with cables. Hence, only sample signal was considered thereafter. Composite 200 µm thick films were cut into 5.50 mm diameter discs by means of a punch. They were introduced into a controlled environment sample holder (CESH) to perform AC impedance measurements under air at various stabilized temperatures ranging from 20 ºC to 50 °C (upon heating in steps of 10 °C). A frequency range of 0.2 MHz to 1 Hz (20 points per decade and 10 measures per points) and an excitation voltage of 0.01 V were applied during the measurements. From the Nyquist and Phase-Bode plots of the complex impedance, the film electronic conductivities were obtained as well as the activation energy. Electrochemical characterization. Coin cells were assembled in an argon filled glovebox (H2O < 0.1 ppm, O2 < 0.1 ppm). Samples were used as working electrode and metallic lithium as counter/reference electrode. Fiber glass separator was supplied by Whatman, GE Healthcare, USA. 150 µL of 1M LiPF6 in ethylene carbonate and diethyl carbonate (EC-DEC 1:1 weight ratio) was used as electrolyte (LP40) and purchased from Merck KGaA, Germany. Cells were galvanostatically discharged (graphite lithiation) and charged (graphite delithiation)



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Figure 1. Main steps of the 3D-printed negative electrode disc elaboration process.

at different current densities calculated per gram of active material, between 1.5 and 0.005 V (vs Li/Li+) using a BCS-805 (BioLogic, France).

RESULTS AND DISCUSSION Slurry and film formulation. The entire fabricating process is displayed in Figure 1. Among the different composite preparation methods, the solvent method was considered as the easiest way to ensure a good homogeneity at laboratory scale. Choice of the solvent is a crucial step as it enables the active material and other additives to be incorporated evenly in the PLA matrix. Sato et al.22 demonstrated the effects of various liquid organic solvents onto polylactic-acid, observing that PLA is soluble in polar aprotic solvent but insoluble in polar protic and non-polar solvents. In this work, DCM was used as it enables a fast dissolution of PLA and its relatively low boiling point

leads to a fast evaporation of the solvent after slurry tape casting. Slurry formulation (Table 1) at increasing plasticizer concentration was done by dissolving PLA polymer matrix into dichloromethane (DCM), with a weight ratio PLA/DCM 1:10, at room temperature and under magnetic stirring for 1 hour. After complete dissolution of PLA, the desired amount of plasticizer (from 0 to 60 wt.% ratio Plasticizer/PLA) and graphite (wt.% ratio PLA/Graphite 30:70) was dispersed following the consecutive procedure: Desired amount of plasticizer was added and magnetically stirred for 30 minutes. Then, graphite was added to above mixture and again magnetically stirred for 30 minutes. Slurries were deposited onto a glass substrate by making use of a doctor-blade. Free-standing composite films were obtained after complete evaporation of the solvent at room temperature during 3 h. Samples containing conductive additives such as carbon nanofibers (CNF) and black carbon (CSP)


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Table 1. Summary of the film compositions produced at increasing plasticizer content (PC, PEGDME2000, PEGDME500 or ATBC). Sample name

Weight ratio PLA:Graphite

Weight ratio PLA:Plasticizer

Wt.% of the total composite PLA/Graphite/Plasticizer

0%Plasticizer

30:70

100:0

30/70/0

10%Plasticizer

30:70

100:10

29/68/3

20%Plasticizer

30:70

100:20

28/66/6

30%Plasticizer

30:70

100:30

27/64/9

40%Plasticizer

30:70

100:40

26/63/11

50%Plasticizer

30:70

100:50

26/61/13

60%Plasticizer

30:70

100:60

25/60/15

Figure 2. DSC curves: (a) pure PLA and PLA/graphite wt.% 30/70; impact of different plasticizers; (b) comparison between film, filament and 3D-printed disc for a same content of plasticizer 40 wt.% PEGDME500 (PEGDME500/PLA).

were prepared in the same way. However, those additives were pre-mixed with graphite in a mortar prior introduction into the slurry in order to ensure thorough mixing. Thermal characterization, filament extrusion and printability. Influence of plasticizers. Figure 2a displays DSC curves of the composite films with different compositions of PLA, graphite and plasticizer (Table 1). As reference, pure PLA pellet shows an endothermal peak corresponding to the melting (Tm) at 146 ºC and a clear glass transition temperature (Tg) at 63 ºC. By adding graphite as active material to the polymer matrix (PLA), the endothermal peak corresponding to the melting temperature of the film called 0%Plasticizer was slightly shifted to lower temperature appearing at 141 ºC and the glass transition temperature is no longer clearly visible. A small endothermal peak corresponding to the water desorption was observed around 100 ºC. The obtained composite film was noticeably brittle and therefore a plasticizer was incorporated.

Influence of a large amount (60 wt.% Plasticizer/PLA) of different plasticizers, such as PC, PEGDME2000, PEGDME500 and ATBC, was studied to highlight their effect on the thermal behavior and compatibility with the composite film (Figure 2a). Compared to 0%Plasticizer sample (only PLA/Graphite) which only shows a sharp Tm peak, the addition of a plasticizer induces a small exothermal crystallization peak (Tc) (apart from PC) around 70ºC. This behavior is typical of plasticized thermoplastics, where plasticizers may promote crystallinity due to enhanced chain mobility.23 Plasticized PLA/graphite films show Tm lower than the one without plasticizer, decreasing from 146 ºC to about 130 ºC. As far as the 60%PC film sample is concerned, a broad endothermal peak from 120 to 250 ºC can be observed. This region corresponds to the evaporation of the PC which is an important problem. Indeed, it appears that when sample is stored during few days under atmospheric conditions, it turns very brittle. Consequently, corresponding filaments obtained



afterwards will have very poor mechanical properties and are not suitable for printing. On the other hand, both DSC curves obtained for 60%PEGDME2000 and 60%PEGDME500 film samples display quite similar behavior. As expected, main difference between those two samples remains on the first endothermal peak attributed to the plasticizer fusion at 45 ºC and 4 ºC for the PEGDME2000 and PEGDME500 respectively. However, shifts about 10 ºC for crystallization and melting peaks are observed. It was observed that films containing PEGDME2000 and ATBC, stored under atmospheric conditions and room temperature (20-25 ºC), had tendency to exude the plasticizer and become fragile. This phenomenon may be caused by an impossibility of the latter to fit into the PLA matrix. Consequently, samples containing PEGDME2000 and ATBC were eliminated from the printing process. Thus, among the plasticizers studied, only PEGDME500 can be regarded as appropriate plasticizer for PLA for the 3D-printing application. The corresponding film sample 60%PEGDME500 exhibits a melting peak at a temperature equal to 128 ºC. Optimization of plasticizer content. As the produced filament corresponding to the 60%PEGDME500 sample is too flexible and soft, a deformation onto the filament due to the forces applied by the gear was observed while printing. Consequently, filament was then not able to enter the 1.75 mm diameter inlet nozzle. Hence, optimization of the plasticizer amount into the composite film was required to obtain further optimized filament with adequate mechanical properties. Samples containing x% of PEGDME500 (with x from 10 to 50 wt.% Plasticizer/PLA) were prepared. As expected for a plasticized polymer,23 by increasing the plasticizer content (Figure S1), melting temperature values slightly decreased and crystallization temperature Tc was seen to decrease from 84 ºC to 70 ºC. Films with less than 20% of plasticizer (wt.% PEGDME500/PLA) were too brittle and consequently, filaments will not be able to be printed. In the opposite, the sample containing a significant amount of plasticizer (50%PEGDME500) was too flexible. As a consequence, additional experiments such as extrusion and 3D-printing were focused on the most promising 40%PEGDME500 film sample. The DSC curves obtained for the corresponding film, filament and 3D-printed disc (Figure 2b), exhibit the same melting temperature value equals to 135 ºC. However, endothermal broad peak corresponding to the fusion of the plasticizer PEGDME500 around 6 ºC tends to decrease after extrusion and 3D-printing at 150 ºC, meaning partial evaporation of the plasticizer takes place during both steps. Fortunately, the lower plasticizer content still provides enough flexibility to the filament to feed the 3D-printer. On the other hand, after both extrusion and 3D-printing, the exothermal peak of crystallization appearing at 80 ºC becomes more intense. This phenomenon differs from what is normally expected after partial evaporation of the plasticizer as mentioned previously. This behavior may be explained by a thermally-induced rearrangement of the polymer chains during the extrusion step, promoting crystallinity. Structural characterization of the filament. The good homogeneity of the most printable filament 40%PEGDME500 was confirmed through Raman microscopy analysis of the cross section after impregnation in an epoxy resin. A non-homogeneous filament with pure cleaning PLA confined in its center can be obtained (as depicted in Figure 3a) indicating the PLA

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Figure 3. (a) Non-homogeneous filament image obtained by optical microscopy (A: Graphite, B: PLA, C: Epoxy resin); (b) 40%PEGDME500 homogeneous filament image obtained by optical microscopy; (c) Raman curves obtained for pure graphite, pure PLA and 40%PEGDME500; (d) Inner part SEM images of the 40%PEGDME500 homogeneous filament sample.

residual material used to purge the extruder machine is not thoroughly removed. An important amount of at least 60 g of material should be introduced into the extruder to finally obtain a homogeneous sample (Figure 3b). Once the extruder is cleaned, as expected for a homogeneous sample, composite filament displays characteristic peaks corresponding to both graphite and PLA (Figure 3c). Indeed, important features of the composite filament appear at 1582 cm-1, 1350 cm-1, 1620 cm-1 and at about 2700 cm-1 and correspond respectively to the G, D, D’ and G’-bands observed traditionally for graphite in accordance with the literature.24,25 The composite filament spectrum also displays very intense peaks pertaining to the PLA at 2949 cm-1 and 875 cm-1 corresponding respectively to the CH3 symmetric stretching and the C-COO stretching.26,27 Figure 3d depicts the inner part scanning electron microscopy image of the filament and confirms that graphite grains (red arrow) are really well dispersed into the PLA polymer matrix (blue arrow). Mechanical characterization and printability. Tensile mechanical tests were performed on samples containing Table 2. Mechanical properties for films of different compositions Sample name

Young’s modulus [MPa]

Tensile strength [MPa]

Elongation at break [%]

30%PEGDME500

844 ± 7

8.80 ± 0.50

4.33 ± 0.25

40%PEGDME500

780 ± 45

6.25 ± 0.43

3.70 ± 0.15

50%PEGDME500

655 ± 14

5.40 ± 0.40

3.00 ± 0.15

30%ATBC

1159 ±45

9.60 ± 0.50

12.00 ± 0.70

40%ATBC

727 ± 55

9.80 ± 0.50

7.30 ± 0.50

50%ATBC

570 ± 6

8.00 ± 0.35

4.00 ± 0.20


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Figure 4. High resolution 3D objects printed by using the 40%PEGDME500 filament as material source for the 3D-printer.

x%PEGDME500 and x%ATBC to better evaluate the plasticizer impact on a highly loaded composite film. Mechanical properties such as the Young’s modulus, tensile strength and elongation at break (Table 2), have been determined from the stress-strain curves (Figure S2). Based on experiments, material with less than 20 wt.% of plasticizers are too brittle to be tested and material with more than 60 wt.% of plasticizers is too soft to be used as filament in a FDM printer. Hence, samples with only three weight ratios of plasticizer (x = 30, 40, 50) have been tested. Compared with pure PLA mechanical properties (Young’s modulus: 3500 MPa, tensile strength: 59 MPa, elongation at break: 7%),28 the obtained properties are very low. This can be easily explained by the fact that the studied material has a high loading of charges (up to 62.5 wt.% of graphite) leading to a brittle behavior. For both investigated plasticizers, Young’s modulus decreases with the content, therefore meaning the material loses its stiffness and becomes more flexible with the addition of the plasticizer. In parallel, the tensile strength had a tendency to decrease with the plasticizer content. Those behaviors are consistent with the general expectation for a plasticized polymer.23 On the contrary, a decrease of the elongation at break was

observed whereas the elongation is expected to be higher with the addition of plasticizer. Due to the high graphite loading, the elongation property of the matrix (here PLA and plasticizer) has a small influence on the macroscopic behavior; the latter being driven by the properties of the interface between the charges and the matrix. The 40%PEGDME500 filament SEM image (Figure 3d) shows graphite grains are easily detached from the polymer matrix demonstrating a very weak interaction between these materials that leads to low elongation property. As the idealized material for a filament is to be soft enough to follow the pipe linked with the printer-extruder and resistant enough to support the stress due to the mechanical components, overall tensile results suggest an optimum plasticizer content of 40% which corroborates the 40%PEGDME500 sample as the most printable one. From this optimized plasticizer composition, printability behavior of the extruded composite filament was tested by using a commercially available 3D printer. To demonstrate its printability, high resolution complex three-dimensional structures, such as a semi-cube lattice and a “3Dbenchy” boat, were successfully 3D-printed at a temperature of 150 ºC (Figure 4). Influence of conductive additives on the composite films’ electrical conductivity. The influence of commonly used conductive additives,29-32 such as carbon nanofibers (CNF) and black carbon (CSP), on the most relevant film sample 40%PEGDME500 was investigated. For samples described in Table 3 obtained after slurry casting, with 1, 2 and 10% of conductive additives, characteristic complex impedance spectra were recorded at various temperatures from 20 ºC to 50 ºC. It is worth mentioning that due to the high volume of charges (graphite and black carbon), it was necessary to adjust the PLA/Graphite ratio (wt.% 40/60) for sample 10%CSP (weight ratio graphite/conductive additive 100:10). Porosity of this latter was found to be about 20%. The obtained Nyquist and Bode plots are consistent with an electronic conductor typical behavior as reported for the 40%PEGDME500 sample in Figure S3. Indeed, Nyquist plot depicts only Z’ real part while |Z| magnitude is constant for all frequencies. For each sample, the electrical conductivity is increasing with temperature as shown in Figure 5 and homogeneous activation energy values ranging between 0.046 eV and 0.106 eV are calculated. Panabière et al.33

Table 3. Summary of the film compositions produced at increasing conductive additives content (CNF and/or CSP) while maintaining constant PLA/PEGDME500 (Wt%. 100/40).

Sample name

Weight ratio PLA:Graphite

Weight ratio Graphite/Conductive additive

Wt.% total composite PLA/Graphite/Plasticizer/Conductive additive

40%PEGDME500

30:70

100:0

26.8/62.5/10.7/0

1%CSP

30:70

100:1

26.6/62.1/10.7/0.6

2%CSP

30:70

100:2

26.5/61.7/10.6/1.2

1%CNF

30:70

100:1

26.6/62.1/10.7/0.6

2%CNF

30:70

100:2

26.5/61.7/10.6/1.2

1%CSP+1%CNF

30:70

100:1 + 100:1

26.5/61.7/10.6/1.2

10%CSP

40:60

100:10

32.8/49.2/13.1/4.9



established that electrons transport mechanism in carbon/polymer mixtures used in battery field was tunneling with no temperature dependence (metal-like behavior). However, such temperature dependence would be observed when strong dipoles are adsorbed at the surface of the carbon particles. This behavior can be perceived through the Z’ real part resistance drop (thickness and area remaining constants) in the Nyquist plot. Indeed, conductivities were calculated from the Equation 1:

σ=

! "

$

×%

(1)

where d is the pellet thickness, A is the pellet surface area, and R is the respective resistances deduced from the Nyquist and Bode plots. From the results obtained, it is certain that conductive additives such as CNF and CSP contribute to higher conductive values compared to the 40%PEGDME500 without additives. Among them, film sample 10%CSP depicts the highest values of electrical conductivities. The small conductivity value gap observed between both 2%CSP and 10%CSP samples is justified by the aforementioned higher polymer amount contained in this latter. Electrochemical characterization. From the optimized plasticizer composition (40%PEGDME500), an in-depth study was also carried out to identify the electrochemical impact of carbon black SuperP and carbon nanofibers as conductive additives. For each film samples described in Table 3 of same dimensions (60 µm thick and 11 mm diameter, surface equals to 0.950 cm2) obtained after slurry casting, the potential profiles versus specific capacity based on the active material, specific capacity versus cycle number and differential capacity during

Page 8 of 13

the first lithiation were studied in detail at different current densities (18.6, 37.3, 74.5, 186 mA g-1 of active material corresponding to C/20, C/10, C/5, C/2) as depicted in Figure 6. Film samples 40%PEGDME500, 2%CSP, 2%CNF and 1%CNF+1%CSP respectively display an irreversible capacity during the first cycle of 21, 23, 26, 20 mAh g-1 of active material (Figure 6a and Figure S4), thus corresponding to quite similar percentage loss of 10%, 11%, 12% and 9% at a current density of 18.6 mA g-1. While £ 2% conductive additives incorporation improves the films electronic conductivity, it does not seem to have a deleterious impact on the reversible capacity loss percentages despite of the overall higher carbon surface area (+ 83% for the 2%CSP sample and + 32% for the 2%CNF) theoretically available for the electrochemically-induced passivation layer formation (also called solid electrolyte interphase, SEI).34 This phenomenon may be explained by the carbon additives isolation within the polymer matrix. The differential capacity plots (Figure 6e, f) feature a reduction peak around 0.8 V assuming that the main component reduced to form the SEI during the first lithiation is the ethylene carbonate electrolyte solvent.35 Depending on the plasticizer nature, reduction may also occur. Indeed, according to the electrochemical stability sequence, carbonate, ester and ether families are prone to reduce successively towards lower potential vs Li/Li+ and even co-intercalate into graphite as reported especially for PC and ethers.36 Concerning the PEGDME500 which is an ether used as plasticizer in this study, neither reduction nor co-intercalation are observed. It is also assumed that its low volume fraction (2% of the electrolyte) does not affect the whole electrolyte properties (solvation, conductivity). It is worth mentioning that reversible capacity loss percentage values are slightly

Figure 5. Arrhenius plots of the electrical conductivity for samples containing carbon nanofibers (CNF) and/or black carbon (CSP) as conductive additives. Sample 10%CSP contains slightly higher amount of polymer to ensure the mechanical strength.


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Figure 6. Capacity retention plots at different C-rate for: (a) 40%PEGDME500 and 2%CSP films (60µm thick) ; (b) 10%CSP film (60µm thick) and 10%CSP 3D-printed disc (250µm thick). Charge/discharge capacity profiles for: (c) 40%PEGDME500 film; (d) 10%CSP 3Dprinted disc. First lithiation and differential capacity plots for: (e) 40%PEGDME500 film; (f) 10%CSP 3D-printed disc.

high compared to commercial graphite electrodes (< 10%) due to the absence of SEI reinforcing additives such as vinylene carbonate (VC) and binder (carboxymethyl cellulose, CMC) known to act as relatively efficient passive layer in commercial cells.37 Among the tested samples with £ 2% of conductive additive incorporated, 2%CSP depicted the highest values of specific capacity retention while the film without any conductive additive displayed, as expected, the lowest ones (Figure

6a). Intermediate values between those samples were observed for 2%CNF and 1%CNF+1%CSP as depicted in Figure S4. Indeed, at a current density of 18.6 mA g-1, reversible capacity values of 184, 215, 214 and 209 mAh g-1 were respectively obtained for 40%PEGDME500, 2%CSP, 2%CNF and 1%CNF+1%CSP samples during 5 cycles. The capacity values obtained at higher current densities of 37.3 and 74.5 mA g-1 are slightly decreasing but are steadily maintained upon their respective 5 cycles. At the highest applied current density (186



mA g-1), capacity retention is not well preserved suggesting the electrode poor electronic and/or ionic conductivities. It is noteworthy that good capacity retention is still observed during 5 cycles while coming back at a lower current density of 37.3 mA g-1. Therefore, by adding £2% conductive additives, it was demonstrated that higher specific capacity is reached indicating their incorporation enables some isolated active material particles to be electronically connected to the percolating network. However, those values are still far from the theoretical capacity (372 mAh g-1) and, thus, can still be improved. An explanation could be that a small percentage of active material is still electronically and/or ionically inaccessible because of its complete confinement into the polymer matrix of PLA. The addition of a small amount of conductive additive (1 or 2 wt.%) is still not enough to approach the theoretical value. To address this issue, by adjusting the PLA/Graphite ratio (wt.% 40/60), film sample containing a higher amount of conductive additive, 10%CSP (weight ratio graphite/conductive additive 100:10), was investigated (Figure 6b and Figure S4c). It displays an irreversible capacity during the first cycle of 93 mAh g-1 of active material, thus corresponding to a percentage loss of 27% at a current density of 18.6 mA g-1 (C/20). The reversible capacity loss percentage is caused by the overall higher carbon surface area (+ 513% for the 10%CSP sample) theoretically available for the electrochemically-induced passivation layer formation. At a current density of 18.6 mA g-1 (C/20), reversible capacity value of 342 mAh g-1 of active material was obtained for 10%CSP during 5 cycles (Figure 6b), thus almost reaching the theoretical capacity of the active material. The capacity values obtained at higher current densities of 37.3 and 74.5 mA g-1 (C/10 and C/5) are slightly decreasing (respectively 333 and 322 mAh g-1 of active material) but are steadily maintained upon their respective 5 cycles. Here again, at the highest applied current density 186 mA g-1 (C/2), capacity retention is not well preserved but still maintained to reversible capacity values up to 285 mAh g-1 of active material. Good capacity retention is still observed during 5 cycles while coming back at a lower current density of 37.3 mA g-1 (C/10). While there is only a small electrical conductivity gain observed between both 2%CSP and 10%CSP samples as mentioned previously, a significant specific capacity rise is observed. This behavior reflects a good electronic and ionic percolation within the matrix. An explanation could be that carbon black (CSP), due to its high surface area, promotes the electrolyte impregnation, thus improving the ionic percolation, and allow the expanded percolating electronic network to reach the whole active material particles. Finally, as theoretical capacity was approached with the 10%CSP (773 mg of active material per cm3 of composite) film sample, a 250 µm thick 3D-printed negative electrode disc was prepared by using the optimized filament corresponding to this latter (Figure 6b, d, f). A reversible capacity of 200 mAh g1 of active material (154.6 mAh cm-3) at current density of 18.6 mA g-1 (C/20) was achieved after 6 cycles. At higher current densities of 37.3, 74.5 and 186 mA g-1 (C/10, C/5 and C/2), capacity values reached respectively 140, 49 and 15 mAh g-1 of active material (108.2, 37.9 and 11.6 mAh cm-3). Good capacity retention of about 140 mA g-1 (108.2 mAh cm-3) is still observed during 6 cycles while coming back at a lower current density of 37.3 mA g-1 (C/10). Due to the resolution of the non-modified 3D-printer, at this stage, it was not possible to obtain thinner

Page 10 of 13

3D-printed disc, thus explaining the gap between specific capacity values obtained for the film (60 µm thick) and the 3Dprinted disc (250 µm thick), but also the increasing capacity at C/20 due to an impregnation issue. To conclude, significant progress has been achieved regarding the capacity retention as compared to Foster et al.21 who used FDM technology to 3D-print a 1mm thick negative electrode disc from a commercially available filament. They were aware that the low specific capacity values reported at that time (15.8 mAh g-1 of active material) at current density of 10 mA g-1 (C/37) were unfortunately caused by the use of a commercial filament containing only 8 wt.% of graphene as active material and 92 wt.% of PLA. In this work, the unprecedented obtained reversible capacity values (200 mAh g-1 after 6 cycles at current density of 18.6 mA g-1, C/20; and 140 mAh g-1 of active material at current density of 37.3 mA g-1, C/10) for a FDM 3D-printed disc (10%CSP) is logically due to the high graphite loading reaching 49.2 wt.% and the high black carbon loading reaching 4.9 wt% of the total composite.

CONCLUSION For the first time, a 3D printable graphite/PLA filament specially designed to be used as negative electrode in a lithium-ion battery was produced to feed a FDM 3D-printer. Graphite content, used as active material into the filament, was increased as high as possible to enhance the electrochemical performances while maintaining sufficient mechanical strength to be printed. 40% of PEGDME500 was determined as best ratio of plasticizer to be added to the PLA matrix to improve its ductility and decrease its stiffness. Thus, its printability by means of a non-modified commercially available 3D-printer was demonstrated by 3D-printing high resolution complex objects such as a semi-cube lattice and a 3Dbenchy boat, very famous in the 3D-RepRap community as calibration and torture-test. Electrical and electrochemical impact of carbon black and carbon nanofibers as conductive additives were studied in parallel. All the additives containing films depicted higher values of electrical conductivity and specific capacity, suggesting their incorporation allows some isolated active material particles to be electronically connected to the percolating network. Finally, as theoretical capacity was approached with the optimized 10%CSP film sample, its corresponding filament was used as material source for the 3D-printer. Unprecedented reversible capacity for a FDM 3D-printed negative electrode disc were obtained with values reaching 200 mAh g-1 of active material at current density of 18.6 mA g-1 (C/20) after 6 cycles, and 140 mAh g-1 of active material at current density of 37.3 mA g-1 (C/10). These first results pave the way towards more performing 3D-printed electrode. The impact of SEI reinforcing additives and binder known to act as relatively efficient passive layer in commercial cells could be investigated to decrease the reversible capacity loss during the first cycle. Additionally, many researches will also be devoted to technically improve the 3D-printer resolution and develop multi-nozzles configuration in view to print complex 3D-electrode architectures whose design are being optimized through an in-depth modeling study.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. DSC curves, stress-strain plots, Nyquist and bode plot, capacity retention curves (PDF)

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected]

ACKNOWLEDGMENT This work was supported by the Fonds Européen de Développement Régional (FEDER) and the University Picardie Jules Verne (UPJV). The authors would like to acknowledge A. Jamali and the microscopy team for sharing their laboratory facilities as well as S. Cavalaglio and the prototyping unit for the material providing.

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For Table of Contents Only


Highly Loaded Graphite-PLA Composite Based Filaments for Lithium

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