Lithium iron phosphate coated carbon fiber electrodes for structural lithium ion batteries

Lithium iron phosphate coated carbon fiber electrodes for structural lithium ion batteries

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Composites Science and Technology xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

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Lithium iron phosphate coated carbon fiber electrodes for structural lithium ion batteries Johan Hagberga,∗, Henry A. Maplesd, Kayne S.P. Alvimd, Johanna Xub, Wilhelm Johannissonc, Alexander Bismarckd,e, Dan Zenkertc, Göran Lindbergha a

Applied Electrochemistry, Department of Chemical Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden Polymeric Composite Materials, Department of Engineering Sciences and Mathematics, Luleå University of Technology, SE-97187, Luleå, Sweden c Lightweight Structures, Department of Aeronautical and Vehicle Engineering, KTH Royal Institute of Technology, SE-100 44, Stockholm, Sweden d Polymer and Composite Engineering (PaCE) Group, Institute of Materials Chemistry and Research, Faculty of Chemistry, University of Vienna, Währinger Straße 42, A1090 Vienna, Austria e Polymer and Composite Engineering (PaCE) Group, Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK b

A R T I C LE I N FO

A B S T R A C T

Keywords: Electrophoretic deposition Structural positive electrode Carbon fibers Lithium-ion battery Multifunctional battery

A structural lithium ion battery is a material that can carry load and simultaneously be used to store electrical energy. We describe a path to manufacture structural positive electrodes via electrophoretic deposition (EPD) of LiFePO4 (LFP), carbon black and polyvinylidene fluoride (PVDF) onto carbon fibers. The carbon fibers act as load-bearers as well as current collectors. The quality of the coating was studied using scanning electron microscopy and energy dispersive X-ray spectroscopy. The active electrode material (LFP particles), conductive additive (carbon black) and binder (PVDF) were found to be well dispersed on the surface of the carbon fibers. Electrochemical characterization revealed a specific capacity of around 60–110 mAh g−1 with good rate performance and high coulombic efficiency. The cell was stable during cycling, with a capacity retention of around 0.5 after 1000 cycles, which indicates that the coating remained well adhered to the fibers. To investigate the adhesion of the coating, the carbon fibers were made into composite laminae in epoxy resin, and then tested using 3-point bending and double cantilever beam (DCB) tests. The former showed a small difference between coated and uncoated carbon fibers, suggesting good adhesion. The latter showed a critical strain energy release rate of ∼200–600 J m−2 for coated carbon fibers and ∼500 J m−2 for uncoated fibers, which also indicates good adhesion. This study shows that EPD can be used to produce viable structural positive electrodes.

1. Introduction Lithium-ion batteries (LIB) are the dominant battery technology for electric and hybrid electric vehicles. One of the major challenges when transitioning from internal combustion engines to battery powered drive lines, in vehicles, is the much lower energy density of batteries compared to petrol and diesel. A lot of research is therefore focused on increasing the gravimetric and volumetric energy density of lithium ion batteries [1]. A possible route to reduce the weight of electrical vehicles is to replace heavy battery packs with structural batteries. These are materials that can carry load, while storing and delivering electrical energy [2]. Integrating a structural battery can lower the weight of a vehicle by allowing the structure to become part of the energy storage system. In recent years there has been considerable effort to realize this concept. There are three main component in a structural battery: The positive



electrode/current collector, negative electrode/current collector and a solid electrolyte/separator. Many studies have focused on using carbon fibers as negative electrodes, because of their mechanical properties and their ability to intercalate lithium [2–7]. Several other studies have investigated replacing the composite matrix with a solid polymer electrolyte. These multifunctional materials have the added functionality of ion conductivity [2,8,9]. However, there have been few studies investigating potential positive electrodes for use in structural batteries. Liu et al. [10] manufactured a structural battery using carbon nanofibers and lithium cobalt oxide dispersed in polyvinylidene fluoride (PVDF) as a positive electrode. However, the carbon nanofibers were not used as current collectors and they were mixed in the electrolyte in chopped form, an additional aluminum current collector was used. Wang et al. [11] demonstrated a structural positive electrode based on carbon nanotubes and cobalt fluoride. They achieved fairly stable cycling performance and mechanical properties, however, a large

Corresponding author. E-mail address: [email protected] (J. Hagberg).

https://doi.org/10.1016/j.compscitech.2018.04.041 Received 24 January 2018; Received in revised form 27 March 2018; Accepted 30 April 2018 0266-3538/ © 2018 Published by Elsevier Ltd.

Please cite this article as: Hagberg, J., Composites Science and Technology (2018), https://doi.org/10.1016/j.compscitech.2018.04.041

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compositions are henceforth given as weight ratios of LFP:CB:PVDF. Note that the compositions used throughout the manuscript refers to the bath compositions which might differ from the composition in the coating itself, rigorous stirring by ultrasonication is used to minimize the difference. Unsized polyacrylonitrile-based carbon fibers (Hexcel Hextow AS4, Hexcel Cambridge UK) having a nominal diameter of 7.1 μm were used as substrate for fabrication of the structural electrodes. The coating bath contained I2 (purity ≥ 99.8%, Sigma-Aldrich Co.) and the non-ionic surfactant polyethylene glycol p-(1,1,3,3-tetramethylbutyl)-phenyl ether (Triton X-100, Sigma-Aldrich Co.) dissolved in acetone (purity ≥ 99%, Sigma-Aldrich Co.). The composition of the impregnation bath used for EPD is given in Table 1.

disadvantage is the price of carbon nanotubes. Both Liu et al. [10] and Wang et al. [11] used nanocomposites in some form but the mechanical performance of these materials does not match those of continuous carbon fiber reinforced polymers (both the ultimate tensile strength and the modulus is several orders of magnitude lower). This study aims at bridging this gap, by manufacturing coated positive structural carbon fiber electrodes. In addition to using carbon fibers as negative electrodes in structural batteries, some studies have investigated replacing metallic current collectors with carbon fibers [12–15]. In addition to the mechanical properties, carbon fibers have been shown to be more electrochemically stable than aluminum [13]. One technique that has been used to coat carbon fibers and other substrates is electrophoretic deposition (EPD). EPD has been used to deposit nanofibers or nanotubes onto carbon fibers, which resulted in improved interfacial properties (adhesion, shear strength and fracture toughness) between coated carbon fibers and a matrix [16,17]. EPD has also been used to deposit electrode materials onto other substrates such as graphite, titanium, aluminum, copper, carbon cloths or nickel [18–20]. There are numerous other deposition techniques to deposit thin or thick films on various surfaces, planar or more complicated porous 3D substrates. Some examples are chemical vapor deposition [21], pulsed laser deposition [22], atomic layer deposition [21,23], or sputtering [24]. EPD was chosen due to its low cost, low environmental impact, scalability, controllability, high deposition rate and relative simplicity [25], combined with the possibility to coat individual carbon fibers. In this study we present a structural positive electrode consisting of lithium iron phosphate (LFP) coated carbon fibers. The carbon fibers are continuous, self-standing tows acting as current collectors and will provide mechanical stiffness and strength. Under optimal conditions, the fibers are coated individually. The morphology and composition of the electrodes have been characterized as well as the adhesion of the coating to the carbon fibers and a composite matrix. Electrochemical characterization was carried out to test capacity, rate performance, cycle life and the presence of side reactions. The results show that the electrochemical properties are good with a reasonable capacity of around 60–110 mAh g−1, good rate performance (0.7 retention at 2C compared to 0.1C) and high coulombic efficiency (99.8%). The mechanical tests reveal that the adhesion of the coating is sufficient for the electrodes to be used in a load carrying composite material.

2.2. Electrophoretic deposition The different ratios of LFP, CB and PVDF (Table 1) were coated onto carbon fibers via EPD. A schematic of the EPD setup is shown in Fig. 1. The working electrode (WE) consisted of a 25 cm long bundle of about 3000 carbon fibers, separated from a spread 12k tow, which was pulled over a glass tube framework. Two platinum rods (length 10 cm, diameter 0.2 cm), serving as counter electrodes (CE), were fixed parallel to the carbon fibers at a distance of 3.5 cm. The electrodes were submerged in to an EPD bath containing an excess of the coating materials listed in Table 1. It has been shown that when adding I2 to acetone, protons form through the following chemical reaction: [26,27].

CH3 COCH3 ⇋ CH3 C (OH ) CH2 CH3 C (OH ) CH2 + I2 → CH3 COCH2 I + H+ + I − These protons adsorb on the surface of the particles dispersed in the bath, causing them to be positively charged. The charged particles are accelerated in the electric field produced between the electrodes causing them to move towards the negative carbon fiber electrode (CF). The surfactant (Triton X-100) was added to aid the dispersion of the particles and avoid the formation of aggregates. The carbon fiber tow working electrode (WE) was connected to the negative output and the counter electrode (CE) to the positive output of an adjustable bench power supply delivering 61–65 V (EA-PS 3065-05 B, Elektro-Automatik, Germany). Before EPD the bath was ultrasonicated for at least 20 min using an ultrasonic processor (UP100H, Hielscher Ultrasonics GmbH). EPD was performed for 300 s. The same EPD procedure was used for all samples.

2. Materials and methods

2.3. Material characterization

2.1. Materials

Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) was used to investigate the quality and composition of the EPD coating deposited onto the carbon fibers. SEM images of the samples before and after cycling were collected using a Zeiss Supra 55 VP FESEM at an acceleration voltage of 5 kV. EDX with elemental mapping was performed using the same instrument operating at an acceleration voltage of 20 kV. The cross-section and high magnification SEM images were collected using a Hitachi S-4800 SEM operated at an acceleration voltage of 2 kV.

LiFePO4 (LFP, Life Power P2) with a particle size of 100–300 nm was supplied by Phosphate Lithium. LFP was carbon coated (2–3 wt.%) and had a specific capacity of 150 mAh g−1. Carbon black (CB, Super-P) was provided by Imerys Graphite & Carbon and had a particle size of 10–100 nm. PVDF (Kynar® 711) used as binder was kindly supplied by ARKEMA Innovative Chemistry (Serquigny, France). Different mixtures of LFP, CB and PVDF were coated onto carbon fibers (see Table 1). The Table 1 Coating and bath compositions. Coating

2.4. Electrochemical characterization For electrochemical cycling experiments, a pouch cell design was used. The EPD coated carbon fibers were dried in a vacuum oven overnight at 60 °C prior to cell assembly in a glove box with argon atmosphere (< 1 ppm H2O and O2). To ensure electrical contact an aluminum current collector was attached to the end of the carbon fiber tows on an uncoated part with silver glue, and then sealed inside the pouch outside of the active cell volume to ensure the electrolyte was not contaminated. A schematic of the pouch cell design is presented in Fig. S1. A half-cell setup was used with lithium foil as the common counter

EPD Bath

LFP (wt. %)

CB (wt. %)

PVDF (wt. %)

I2 (mg)

Triton X-100 (ml)

Acetone (ml)

92 90 88 88 86

4 6 8 6 6

4 4 4 6 8

182 182 182 182 182

1.2 1.2 1.2 1.2 1.2

300 300 300 300 300

2

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Fig. 1. Schematic of the setup used for the EPD process.

Fig. 2. Schematics of DCB test, a) Schematics of test specimen with piano hinges, with top diagram showing the specimen mid-plane and the location of the LFP coated CF, b) DCB test definition, c) Load vs. displacement curves for multiple loading-unloading steps.

and reference electrode, a 250 μm Whatman glass microfiber filter was used as separator and BASF 1.0 M LiPF6 in ethylene carbonate (EC): diethyl carbonate (DEC) (1:1 by weight) was used as electrolyte. Galvanostatic cycling at different rates was carried out using a potentiostat (VMP3 Biologic, Seyssinet-Pariset, France). High precision coulometry was performed with an in-house constructed cycling setup similar to the one used by Dahn et al. [28] utilizing Keithley 220 current sources and high precision resistors connected to a multimeter (Keithley 2700/ 2701) to measure the current. The cells were placed in temperature controlled chambers at 25 ± 0.1 °C. This setup allows for the determination of the coulombic efficiency (CE) with an accuracy of ± 0.02% for C-rates below 0.1C. All cycling was carried out between 2.8 and 3.8 V vs. Li/Li+. The C-rate was defined according to the specified specific capacity of the LFP of 150 mAh g−1, 1C = 150 mA g−1, the current used to fully charge in one hour.

of the laminae was back calculated from the measurement. UTS2 was taken as the point where the load-displacement curve diverged from linearity. EPD coated carbon fibers (88:6:6 composition) were used and compared to a control sample consisting of uncoated carbon fibers from the same tow. These were impregnated with epoxy resin (Huntsman RenLam LY113) by vacuum assisted infusion. The resulting laminae (cured thickness of approximately 50 μm) were then glued using Reichhold Dion 9102 resin to the substrate film and tested using a Deben 300 N three-point bending rig. 2.5.2. Double cantilever beam test To further study the adhesion of the coating, double cantilever beam (DCB) tests were carried out to determine the mode I interlaminar fracture toughness. The tests were carried out in accordance to ASTM D5528, with the exception that multiple loading-unloading steps were used to achieve stable crack propagation (see Fig. 2c). The DCB specimen, shown schematically in Fig. 2a, is a 22-ply carbon fiber/epoxy prepreg (HexPly® M10/38%/UD300/CHS, ABIC Kemi, Sweden) stacked in 0° direction, with the coated bundles placed in the specimen mid-plane, also in 0° direction. A pre-crack of 40 mm was created using a 7 μm thick PTFE-film insert placed next to the coated part of the fiber bundle. In Table 2, the macroscopic dimensions of the DCB specimens are presented. A hydraulic universal testing machine (Instron 3366) with a 10 kN load cell was used to perform the tests under displacement control at room temperature, with a crosshead speed of 2 mm/min during loading and 10 mm/min during unloading. The energy release rate is defined as G = -dV/dA, where V is the total potential energy and dA is the area of the new surface created by crack propagation. For a linear elastic body of constant width b, the critical strain energy release rate (SERR) for

2.5. Characterization of coating to fiber adhesion 2.5.1. Three-point bending The transverse modulus (E2) and transverse ultimate tensile strength (UTS2) of a unidirectional composite are dependent on the matrix properties and the adhesion of the matrix to the fibers. In order to examine the adhesion of the EPD coating to the carbon fibers, it was necessary to compare the transverse properties of a unidirectional composite manufactured with coated and uncoated fibers. Using the same matrix this comparison provided a qualitative indication of the coating adhesion to the carbon fibers. As the unidirectional laminae were too thin, the samples were glued to a 130 μm thick polyethylene terephthalate (PET) substrate film, with the sample oriented so the bending translates to transverse tension in the laminae. From the known bending stiffness of the substrate film, E2 3

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before and after cycling are presented in Fig. 3. No apparent difference between the various coating compositions could be seen (see Fig. S2) and, therefore, only SEM images of 88:6:6 are shown in Fig. 3. The EPD coating is porous containing micro-sized pores; this is beneficial in terms of electrolyte penetration into the coating material. Well dispersed sub-micrometer sized LFP particles can be seen throughout the coating. No apparent difference is seen before and after electrochemical cycling (see below), which indicates that the adhesion of the coating to the carbon fibers is sufficiently high even when inserting/extracting lithium in the LFP. The coating thickness varies between carbon fibers from around 1 μm to several micrometers, while some carbon fibers remained almost completely uncoated. The reason for this inhomogeneity is that EPD is very sensitive to the distance between the counter electrode (Pt wire) and the substrate (carbon fibers). This varies in the setup as some carbon fibers within the bundle are closer to the counter electrode than others. The carbon fibers closest to the counter electrode are more heavily coated. This might affect the electrochemical performance since the current distribution becomes uneven and the particles furthest away from fibers might lose contact. However, commercial electrodes are usually around 20–30 μm thick with a current collector attached to one side, so a thick coating is not a significant problem as long as enough conductive additive is present. In terms of mechanical performance, a thin, even coating is preferred to guarantee consistent interfacial properties between the EPD coated carbon fibers and the matrix. To maximize performance some optimization will be needed. Fig. 4 shows SEM cross-sections of the EPD coated carbon fibers (composition 90:6:4) and a high magnification image of the coating. Some areas of excessive coating were observed, as seen in Fig. 4a. This might lead to poor mechanical performance and loss of electrochemical contact to the active material (LFP), leading to a capacity loss. With good coating conditions however (optimal distance between carbon fibers and Pt counter electrode during EPD, well spread fibers, etc.), the carbon fibers were coated individually (Fig. 4b and c). The coating homogeneously encases the fibers. This is beneficial in terms of

Table 2 DCB specimen dimensions. Specimen

Length, L (mm)

Width, b (mm)

Thickness, h (mm)

DCB-1 DCB-2 DCB-3

184 184 191

8.8 13.3 10.4

5.26 5.64 5.68

each crack length is defined as:

GIC =

2 Pmax ∂c 2b ∂a

(1)

where Pmax is the load at onset of the delamination for each crack length, b the specimen width, a the measured crack length, and c the specimen compliance for each crack length:

c=

Δ P

(2)

where Δ is the load point vertical displacement, and P the recorded load, see Fig. 2. The compliance described as a function of crack length was calibrated using a power function:

c = Aan

(3)

where coefficients A and n for the particular specimen are obtained from the compliance (c) versus crack length (a) curve. After completion of the tests, fully separated half beams of two DCB specimens (with different characteristic G1C values) were chosen for fractographic analysis. Fracture surfaces of the coated bundle were Au sputter-coated and examined using a JEOL-IT300LV SEM. 3. Results and discussion 3.1. Material characterization SEM images of carbon fibers EPD coated with composition 88:6:6

Fig. 3. SEM images of carbon fibers EPD coated with the composition 88:6:6. a) and c) before cycling, b) and d) after cycling. Magnifications a) 1000x, b) 1000x, c) 7000x, d) 4000x. The coating is observed as a porous mass around the carbon fibers with a varying thickness. No apparent difference before and after electrochemical cycling could be seen. 4

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Fig. 4. Cross-section and high magnification SEM images of EPD coated carbon fibers with the composition 90:6:4. Magnifications a) 0.8 kx, b) 0.8 kx, c) 1.5 kx, d) 30 kx. Excessive coating is seen in a) and a homogenous coating in b) and c), d) reveals well dispersed LFP and CB particles.

and iron (Fe) EDX map indicates a homogenous distribution of LFP across the coating. The fluorine (F), stemming from the PVDF, is mainly present around the particles, which is expected for a binder. The distribution of carbon black was not able to be determined because of the overlapping signal from the carbon fiber substrate.

electrochemical performance since a short electrical contact distance between active particles and the carbon fiber current collector is maintained. The high magnification SEM image (Fig. 4d) reveals the presence of both larger (sub micrometer) LFP particles and smaller (10–100 nm) carbon black particles in the coating. The particles are relatively well dispersed throughout the coating which is attributed to the use of surfactant (Triton X-100) and the ultrasonication step prior to EPD. Fig. 5 shows the EDX map of a carbon fiber EPD coated with the composition 92:4:4. Only this representative example is shown but similar results was observed for the other compositions. The phosphor (P)

3.2. Electrochemical characterization Fig. 6 shows typical charging/discharging voltage profiles for different C-rates for a carbon fiber electrode EPD coated with the composition 90:6:4. The charging/discharging voltage profiles looked

Fig. 5. EDX map of carbon fibers EPD coated with the 92:4:4 composition. A homogenous distribution of P and Fe, related to the presence of LFP, is seen. F, related to the presence of PVDF, is coagulated around the particles, as expected for a binder. 5

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Fig. 6. Voltage profile for different charging rates for carbon fibers EPD coated with the 90:6:4 composition.

similar for all tested coating compositions. Voltage profiles with a plateau around 3.4 V vs. Li/Li+ are typical for LFP [29]. The polarization is low, the charge/discharge overpotential is 39 mV at 0.1C, 51 mV at 0.2C, 94 mV at 1C and 142 mV at 2C. The capacity retention is also high at around 0.85 at 1C and 0.75 at 2C compared to the 0.1C capacity. This is attributed to the short distance between the carbon fiber current collectors and the active particles because individual fibers were coated. This also shows that the electrical conductivity of the carbon fibers is high enough even for high power applications. The conductivity of the carbon fibers used in this study is 588 S cm−1, which is typical for PAN-based carbon fibers [4]. The rate performance might be even further improved by using carbon fibers with a higher electrical conductivity. The low polarization also indicates a homogenous distribution of the carbon black particles (conductive additive) throughout the coating providing a conducting pathway between the carbon fibers and dispersed LFP particles. According to the manufacturer the specified capacity for the carbon coated LFP we used is 150 mAh g−1. For our samples (Fig. 6) the capacity reached was 62 mAh g−1, which means around 41% of the active material could be utilized. This varies for carbon fiber electrodes with different compositions, as discussed below, but the trend is that around half the active material could be utilized. This is attributed to loss of electrical contact between a fraction of LFP particles and the carbon fibers in areas of excessive coating thickness (see Fig. 4a). Table 3 shows the average specific capacities of the carbon fibers EPD coated with various compositions cycled at 0.1C. There are no clear trends and the values vary between around 60 and 110 mAh g−1. The amount of coating varies between 30 and 50 wt.%. This is relatively low compared to commercial batteries; a low current collector weight is better for a higher energy density. However, here the carbon fibers should perform multiple functions simultaneously; they should carry mechanical load and act as current collectors. An excessive amount of active coating would lower the fiber volume fraction in a composite

Fig. 7. Rate performance of structural carbon fiber electrodes EPD coated with different LFP:carbon black:PVDF binder compositions (see legends). a) Varying the amount of carbon black and b) that of PVDF binder in the functional carbon fiber coating.

structural battery, which is detrimental for its mechanical properties. Balancing the mechanical properties and the energy storage capabilities is an optimization question, i.e. a higher coating weight fraction would lead to a higher energy density but poorer mechanical properties, and vice versa. It is interesting to note that the composition with the lowest amount of additive (4 wt.% carbon black and PVDF) had the highest capacity. This is attributed to the short electrical contact distance between the carbon fibers and the network of LFP particles dispersed throughout the coating and indicates that as little as 4 wt.% of additive is sufficient to bind the LFP particles to the carbon fibers while guaranteeing sufficient electrical contact. This is beneficial in terms of gravimetric energy density since a high loading of active material can be incorporated into the coating, counterbalancing the relatively high carbon fiber:coating ratio. The rate performance for structural battery electrodes of different bath compositions is shown in Fig. 7. A retention of 1 is defined from an initial slow cycle (around 0.1C) to determine the maximum capacity of that particular sample. This was done to ensure that the true C-rate was used when varying the current since the capacity between samples had some variation (a retention of 1 does not mean a capacity of 150 mAh g−1, see Table 3 for capacity data). Some points show a

Table 3 Average specific capacities of the EPD coated CF electrodes based on the weight fraction of the LFP in the coating for the different compositions and the coating wt.% of total electrode weight. *Not enough samples for standard deviation. Composition

Average specific capacity at 0.1C (mAh g−1)

Coating wt.%

92:4:4 90:6:4 88:8:4 88:6:6 86:6:8

108 ± 20.5 62 ± 6.8 90 ± 5.7 66* 75 ± 8.5

32 31 45 50 41

6

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retention above 1, which could mean that more active material is made available after some cycling compared to what was determined with the initial slow cycle. Varying the amount of carbon black from 6 to 8 wt.% (Fig. 7a) does not affect the rate performance. The capacity retention is around 0.85 at 1C and 0.75 at 2C. For a very low amount (4 wt.%) of carbon black the rate performance is slightly worse with a retention of around 0.7 at 2C, but this is still acceptable as 2C is a very high current. At 0.1C all EPD coated carbon fiber electrodes have a retention close to 1, in this case the 90:6:4 composition have a slightly better retention. This indicates that the internal resistance in the electrodes is low, even for as low amounts as 4 wt.% carbon black conductive additive, which provides the electrically conducting network throughout the coating. This can be compared to commercial electrodes which contain a carbon black loading of around 10 wt.%, which is often needed for full utilization of the active material [30]. This is because the electrical conductivity of LFP is inherently low, and especially for sub micrometer sized particles a substantial amount of conducting additive is needed, even when the LFP particles are carbon coated. Fig. 7b shows the rate performance of coated CF electrodes when varying the amount of PVDF binder in the bath. Increasing the amount of PVDF binder to more than 4 wt.% affected the rate performance negatively. At 2C the retention is around 0.75 for the coating containing 4 wt.% PVDF, around 0.70 for 6 wt.% and around 0.65 for 8 wt.% PVDF binder loading. When reapplying 0.1C, the capacity retention was close to 1 for the fibers coated with compositions 90:6:4 and 88:6:8, but the retention declined quickly for fibers coated with the 86:6:8 composition, indicating that the cycling is not stable for a high PVDF content in the coating. The reason for the decreased rate performance for functional fiber coatings containing a PVDF binder content in excess of 4 wt. % could be that Li-ion diffusion pathways in the porous coating were more tortuous or that LFP particles became isolated from electrically conductive paths. The fast fade for the 86:6:8 sample could mean that there are additional problems for a high amount of PVDF content, such as isolating particles in the EPD process, hindering acceleration and proper deposition of the coating, making a coating with poor adhesion. In terms of the electrochemical performance, there is no benefit in increasing the PVDF binder content in the coatings above 4 wt.%. The best rate performance had the functional carbon fiber coating with composition 90:6:4. This is a relatively low additive content, which is beneficial for the gravimetric energy density. The differences in rate performance between different coating compositions however are not large and since the fibers coated with composition 92:4:4 exhibited a higher capacity (Table 3), that could be a better composition in terms of overall electrochemical performance. The big advantage of the EPD process to coat carbon fibers is that every fiber is individually coated and, therefore, only a rather low amount of PVDF is required to bind the LPF particles to the fibers. A 3D network structural battery electrode with additional benefits compared to traditional layer by layer designs is achieved, such as short electrical contact distances between active particles and current collector and the possibility for the electrodes to be used in a carbon fiber reinforced composite (see below). The long-term cycling performance, 1000 cycles at 1C for a carbon fiber electrode EPD coated with the composition 90:6:4, is shown in Fig. 8a. The capacity is steadily decreasing and the capacity retention for this particular coating was 62% after 500 cycles, which dropped to 47% after 1000 cycles as compared to the first cycle. The observed degradation is higher than for commercial cells [31] but could be an effect of the pouch cells used. Reasons for the declining capacity could be slow ingress of air/moisture in to the pouch, for high C-rates the thick electrode separator used (a 250 μm thick glass microfiber filter compared to around 30 μm thick polymer separator in commercial cells [32]) could also affect the performance. Other factors could be electrolyte degradation or dendrite formation on the lithium foil counter electrode leading to an increased cell resistance. The latter is a known problem for half-cells when cycling at higher rates [33]. The few points with lower capacity at around 700 and 820 cycles could be

Fig. 8. a) Long term cycling at 1C for a carbon fiber electrode with the composition 90:6:4; b) SEM image of the electrode after 1000 cycles, magnification 900x.

measurement errors since the channel was switched around at this times. A SEM image of the electrode after the long-term cycling is presented in Fig. 8b. The coating seems to be still adhered to the fibers. However, since the coating thickness varied across samples, even before cycling, it cannot be excluded that some of the poorly adhered coating (in areas of excessive coating) is lost from the fibers during cycling. This would lead to a capacity decline as well. Overall the cycling seems relatively stable over 1000 cycles indicating that the functional coating maintains electrical contact with the carbon fiber current collectors. The coulombic efficiency (CE) of the carbon fiber battery electrode coated with composition 90:6:4, determined by high precision coulometry, is shown in Fig. 9. The CE remained stable at around 99.8%, which is comparable to commercial LFP/graphite batteries [34], indicating that few side reactions occur (from impurities, reactions with solvent etc.), which is beneficial for the battery lifetime. 3.3. Adhesion of the functional coating to carbon fibers Well impregnated composites were successfully fabricated with the EPD coated carbon fibers to test the adhesion of the coating. Two testing methods were used, three-point bending using a vacuum infused laminae and double cantilever beam tests (see experimental section for details). 3.3.1. Three point bending The mechanical performance of a composite using the EPD coated carbon fibers is dependent on the adhesion of the coating. The 7

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Fig. 9. CE for a 90:6:4 sample cycled at 0.1C. Fig. 10. Typical critical energy release rates for composites containing original carbon fibers with and without functional coating.

transverse modulus (approximately 2.4 GPa) and ultimate transverse tensile strength (approximately 25 MPa) determined by the three-point bending test were statistically the same for both composite laminae (with coating and without coating). This result indicated that the functional coating did adhere sufficiently well to the carbon fibers to allow for mechanical load transfer through the interphase. Important to note is that the modulus and ultimate tensile strength would be at least one order of magnitude higher in fiber direction since it would be fiber dominated. We are looking mainly at the carbon fiber to coating adhesion so that is why the properties in the transverse direction is measured.

Furthermore, G1C measured at the end of the tested specimen, containing uncoated unsized CF with the same epoxy matrix (see Figs. 2 and 10) had values in the same range as the coated part of the DCB-1 specimen. Hence, it can be concluded that the quality of the interfaces in the EPD coated CF composite is equivalent to the adhesion in the untreated CF/epoxy composite, if the functional coating layer is thin. This was supported by the fractographic analysis of the composite specimen containing the uncoated carbon fibers (Fig. 11c) showing an interfacial failure with a very smooth fracture surface, indicating a brittle fracture at the carbon fiber/epoxy interface. It is also consistent with the results obtained by the three-point bending tests, showing similar modulus and transverse strength of the composites containing coated and bare, unsized carbon fibers.

3.3.2. Double cantilever beam test To further evaluate the adhesion, DCB tests were used. From this you gain information about the critical energy release rate which is related to the energy needed for failure propagation. The tests were performed with the EPD coated carbon fibers embedded in CF-epoxy prepreg (see experimental section). To get accurate results a good impregnation is important. Optical microscopy showed that the impregnation was good, with the epoxy matrix surrounding the LFP coated fiber bundle, which was dry prior to manufacturing the composite specimens. The critical energy release rate GIC calculated using Eqs. (1)–(3), which is a measure of the materials resistance to delamination growth, is presented in Fig. 10. The measured G1C in the coated region for all DCB specimen ranges between 185 J m−2 to 566 J m−2. Further, G1C was stable with increasing crack length, for specimen DCB-1 and DCB-2, at around 500 J m−2 and 200 J m−2, respectively. Post mortem analysis was conducted to study the underlying mechanisms behind the differences in G1C. SEM images of the fracture surface show that these two extreme values can be related to fracture at different interfaces, since the carbon fiber imprints, shown in Fig. 11, have different appearances regarding surface smoothness. The variations in measured fracture toughness can be attributed to the inherent challenges of our laboratory EPD coating method, which yields relatively large variations in coating thickness between bundles and of the fibers in each bundle. In specimen DCB-2, with the lowest average G1C, a separation between CF and coating can be seen. Fig. 11b clearly shows that the failure occurred at the interface between carbon fiber/coating. For specimen DCB-1, Fig. 11a, where the imprints from the carbon fiber in the fracture surface show some LFP coating particles still embedded into the epoxy matrix, the measured critical SERR values were much higher. One possible explanation for the increased SERR is that the failure surface is alternating inside the coating and at the carbon fiber/coating interface. This phenomenon is believed to occur in thinner coatings.

4. Conclusions Structural positive electrodes were successfully manufactured by depositing a functional LFP/carbon black/PVDF coating onto structural carbon fibers using electrophoretic deposition. The electrochemical evaluation of the EPD-coated carbon fibers demonstrated that they perform well as positive electrodes, with a specific capacity around 60–110 mAh g−1, good rate performance with a capacity retention around 0.75 at 2C compared to 0.1C and with a high coulombic efficiency of 99.8%. Furthermore, the coating is unaffected by continuous cycling. Three-point bending tests suggest that the coating adheres sufficiently well to the carbon fibers to allow mechanical load transfer though the interface. Furthermore, the resistance to delamination in the LFP coated CF/epoxy composite materials has been characterized by SERR in DCB Mode I loading conditions. Post-test fractographic analysis show that differences in measured SERR values can be related to fractures at different sites in the material. Further studies are needed in order to determine the influence of the coating thickness on the G1C. The DCB and 3-point bend tests indicate that the adhesion between coating and CF is sufficient to function well in a composite material and is promising for structural battery applications. We have shown that carbon fibers EPD-coated with LFP, PVDF and carbon black, would function well as positive electrodes in structural batteries. The next step would be to manufacture a full structural battery, with the coated carbon fiber positive electrodes and pristine carbon fiber negative electrodes held within a structural solid electrolyte matrix. This is an important step towards the goal of realizing structural batteries. 8

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