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Integration of energy storage functionalities into fiber reinforced spacecraft structures
Journal Pre-proof Integration of energy storage functionalities into fiber reinforced spacecraft structures Benjamin Grzesik, Guangyue Liao, Daniel Vogt, Linus Froböse, Arno Kwade, Stefan Linke, Enrico Stoll PII:
S0094-5765(19)31315-3
DOI:
https://doi.org/10.1016/j.actaastro.2019.10.009
Reference:
AA 7694
To appear in:
Acta Astronautica
Received Date: 1 March 2019 Revised Date:
24 June 2019
Accepted Date: 6 October 2019
Please cite this article as: B. Grzesik, G. Liao, D. Vogt, L. Froböse, A. Kwade, S. Linke, E. Stoll, Integration of energy storage functionalities into fiber reinforced spacecraft structures, Acta Astronautica, https://doi.org/10.1016/j.actaastro.2019.10.009. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd on behalf of IAA.
Integration of energy storage functionalities into fiber reinforced spacecraft structures Benjamin Grzesika*, Guangyue Liaob, Daniel Vogt ab, Linus Froböseb, Arno Kwadeb, Stefan Linkea Enrico Stolla a
Institute of Space Systems (IRAS), Technische Universität Braunschweig, Hermann-Blenk-Str. 23, 38108 Braunschweig, Germany, [email protected] b Institute for Particle Technology (IPAT), Technische Universität Braunschweig, Volkmaroder Str. 5, 38104 Braunschweig, Germany, [email protected] * Corresponding Author Abstract One of the common challenges of spaceflight and e-mobility is the energy storage. The operational ranges of spacecraft or electric cars as well as operational usability is strongly dependent on the capacity of the energy storage, which is usually constrained by the mass and volume of the vehicle. For example, the battery mass of large communication satellites ranges from 6 % to 9 % of the dry mass. This amount increases for smaller satellites like CubeSats up to 13 % without accounting wiring harnesses and subsystem volume. Thus, a system can benefit from a reduction in mass and volume by combining multifunctional use of the components and materials. In the presented research, energy storage is integrated into lightweight carbon fiber materials. Carbon fibers have a distinct mass advantage compared to metal structures. In addition, they have very low thermal expansions that can reduce thermal stresses during the operation of a satellite. Fiber composites or laminates consist of two components, the fibers and the matrix material. In laminates, the typically resin based matrix content is 30 to 40 % of the component volume and could be substituted with novel solid state battery materials. Latter have the advantage that they have nearly similar physical properties as usually used resins and are also able to store energy electrochemically. Thus, about 25 % of the volume can be used for electrochemical energy storage without compromising structure integrity. To reach such electrochemical functions the host structures have to be infiltrated with anode, cathode, and separator materials. Using the developed recipes and a component thickness of 5 millimeters, an energy amount per component area of 1130 Wh/m² can be reached depending on the applied battery active material. This article will give an overview of how fiber composite materials, which are increasingly being used in lightweight construction processes, can be combined with energy storage materials to be used in spacecraft structures. The goal of the research on this topic at the Technische Universtät Braunschweig is a structure battery for a spacecraft. However the article addresses as a first step the characterization and manufacturing of structural battery composite negative half-cells from carbon fiber reinforced PEO/LiTFSI. An estimation of the expected performance is carried out. The initial material evaluation and the manufacturing process as well as mechanical and electrical result will be discussed. Keywords: Energy storage, carbon fiber, satellite technologies, power subsystem, multifunctional structure, half-cells Acronyms/Abbreviations Constant Phase Element (CPE), Carbon Fiber Reinforced Plastic (CFRP), Carbon Fiber (CF) Differential scanning calorimetry (DSC), Glass Fiber Reinforced Plastic (FRP), Glass Fiber (GF),
Lithium-Ion-Batteries (LIB), Lithium-bis(trifluormethylsulfonyl)imid (LiTFSI), Porous Electrolye Interface (PEI), Polyethylenoxid (PEO), Solid Electrolye Interface (SEI), Ultra-Highmodulus Carbon Fiber (UHM-CF)
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1. Introduction The last transition in battery technology for spacecraft applications, in which NiCd where replaced by NiH2, occurred in the mid-80s [1]. Satellite power systems have progressively shifted to lithium-ion batteries. In all transitions the key parameter, the specific energy (Wh/kg), was approximately doubled. Within the next 20 years, a doubling may be again necessary due to the increase of payload power. The commonly used battery formats for satellites are cylindrical 18650 lithium ion (LIB) batteries with a form factor of 18.4 mm x 65.1 mm [2] or flat lithium polymer batteries. From the view point of energy density, lithium-ion batteries (120-140 Wh/kg) have a great advantage over Ni-Cd batteries (~ 30 Wh/kg), so the total mass of the satellite system can be reduced. With the development in the consumer electronics market and the progressive miniaturization, the stringent requirements for the mass and volume of batteries for space applications can be fulfilled much easier. Nevertheless, significantly new battery technologies for small satellites are not expected in the near future [1]. However, in the area of power storage there are several efforts for improving the storage capability by enhancing the Li-Technology with e.g. carbon nanotubes that can significantly increases available energy density and can lead to the next doubling in specific energy or to develop multifunctional structural battery systems [3]. To develop advanced aerospace systems and products with better performances, the saving of mass and volume becomes indispensible. This results from the direct proportional relation between these two factors and the occurring project costs, which is clearly proposed in a parametric cost models given by U. S. Air Force [4], [5]. One of the most promising approaches for the mass and volume saving are multifunctional structures with integrated structural, thermal, sensing, actuation or power systems. The aim of this technology is to maximize the ratio of volume and mass of the functional subsystems to the total packaging volume and mass of the satellites [6]. Structural
energy devices are realized by integrating energy storage devices into the satellites’ structures. Thus, the enclosure of energy storage devices as well as connectors become unnecessary. Furthermore, as the structural energy devices are capable of bearing mechanical loads, a part of structure materials could be substituted. Therefore, not only the inside space, but also the total mass and volume of the satellite is reduced [7]. This article will give an overview of multifunctional energy storage, state of the art of structural batteries and focus then on first step towards the research goal of a structural battery on the characterization and manufacturing of structural battery composite negative half-cells from carbon fiber reinforced PEO/LiTFSI.
2. Multifunctional Energy Storage Composites The concept of structural energy storage devices has already been developed for capacitors, supercapacitors, batteries, and fuel cells. To evaluate the multifunctional design in the subsystem, Wetzel [8] proposed a method, which describes the relationship between mass and functionalities: (1) = + 1 − ∙
Here, is the mass of total system, which equals to sum of structure mass ( ) and storage device‘s mass ( ). is defined as structural mass efficiency, which represents the structural property of the multifunctional materials compared with conventional fiber reinforced composites ( = 1). On the other
Figure 1: system mass reduction by multifunctionalization according to Wetzel [8].
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hand, the conventional energy storage device possesses no structural functions, therefore the
value is considered as 0. Similarly, energy mass efficiency factor is 1 for the storage device as it should provide full energy density, while the value for composites is 0, because they delivery no energy. If the energy storage device could carry the loads and substitute a part of the structural function, the total system mass is acquired by equation (1). Figure 1 shows an example for a mini satellite, with a structure mass of 100 kg and battery mass of 10 kg. In the ideal situation, the energy storage system delivers all energy and has complete structural property,
= 1 and = 1 , then = . However, in accordance to [8] reality it’s more reasonable that a structural storage device reaches values between
= 0.75 and = 0.5 . In general, the system has benefits from this multi-functionality when the energy delivery and structural property could be acquired in the multifunctional materials simultaneously in a sufficient high value. 2.1 State of the Art of Structural Batteries Significant researches about the structural batteries for satellite applications have been carried out in the past decades [9], [10], [11], including the conception development, demonstrator manufacture, component design and performance characterization. Different methods have been used for the structure integration of energy storage devices from macro scale to nanoscale. The first approach of the integration is embedding the commercial battery cells, as they have considerable mechanical properties, into structural composites for satellites [12], [13], [14], [15]. However, these cells need hard and relative heavy cases and be bolted to the rest of the structure. Thus, this integration will add parasitic mass and requires a substantial volume allocation [7]. Therefore, some researchers used polymer electrolyte based thinfilm all-solid-state batteries to make the integration [16], [17]. Clark [18] manufactured a complete power system, which consists of thin photovoltaic cell, lithium polymer thin-film batteries and power management electronics on a
polyamide substrate. This power system could be directly bonded to the outer skin of a satellite. Theoretically it could produce a power of 20 W/kg for a 28 V power requirement. Marcelli [19] proposed a way of integration in which the thinfilm batteries were combined with honeycomb core. As there is large amount of unused surface area inside the honeycomb structure, the thin-film batteries could be deposited onto aluminum or titanium aluminide surface for honeycomb cores. No additional volume is required, however the fabrication techniques is inconvenient for the scale-up manufacture. The results of their work shows a rapidly and significantly degradation of the cell capacity. A more efficient way for the structural integration is to use battery components also as structural elements. First working structural batteries were reported by Ekstedt et al. [20], which confirms the concept of using carbon fibres as anode’s current collector and provide mechanical performance simultaneously. A charge capacity of 116 Wh kg-1 and stiffness of 35 GPa were acquired in the fabricated structural battery. In the most recent study, Johannisson et al. [21] has demonstrated a structural battery with carbon fibre lamina vacuum infused with a Bisphenol A dimethacrylate based solid electrolyte. The capacity of 200 Wh kg-1 and Young’s modulus of 52 GPa have been acquired, which represent a great improvement of the multifunctionality. The solid electrolytes as matrix should be multifunctional, as they not only provide the properties for the ion migration, but also deliver a good mechanical performance. Y. Yu [11] reported a promising electrolyte with Young’s modulus of 400 MPa and the ionic conductivity of 0.08 mS/cm, which is synthesized by blending LiTF2N/EMIMTF2N solution in bisphenol-A type epoxy resin. Inorganic particles have been reported to act as filler materials in polymers for composite electrolytes, usually used ceramic nanoparticles are SiO2, TiO2, ZrO2 or Al2O3 [22]. Snyder [23] showed a structural battery, which uses carbon fabric as anode, LiFePO4 cathode and a vinyl ester random copolymer as solid electrolyte and the laminates were fabricated using vacuum assisted resin transfer molding technique [24]. The sample
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showed a tensile specific stiffness of 3.6 GPa g-1, however the ionic conductivity is low. Another promising work in the field of structural batteries is reported by Liu et al [9] who develop structural batteries, based on a carbon fiber reinforced PVDF composite, with tunable mechanical properties showing a flexural modulus of 3.1 GPa and an energy density of 35 Wh kg−1. Yu et al [11] have reported a structural battery, based on carbon fiber reinforced plastic composites, yielding a specific capacity of 12 mAh/g and a tensile modulus of 80 GPa. Ihrner et al [25] has reported a novel type of multifunctional solid-state electrolyte, which is manufactured by combining liquid electrolyte phase with a stiff vinyl ester based thermoset matrix. The as acquired materials possess an ionic conductivity of 1.5 10-4 S cm-1 and a corresponding storage modulus of 750 MPa. The highest integration level is called power fibers [26] which is realized by depositing electrolyte and cathode materials directly on a single fiber of carbon, glass, silicon carbide, or a metal, which are used as current collectors. The fabricated fibers could subsequently be produced as a woven fabric, composites, or highly compact batteries. These batteries could acquire high rate capability up to 50 C and over 2000 cycles at 100 % depth of discharge for a 50 C discharge rate. Hagberg et al. [27] characterized the electrochemical properties of the PAN-based carbon fibers, which possess not only high modulus, but also high capacity of ca. 250–350 mAh/g and very high columbic efficiencies—for some >99.9 % after 10 cycles. Fredi et al. [28] systematically studied the PAN-based carbon fibre microstructure influencing on lithium insertion mechanism and mechanical performance. They provide a new aspect to manufacture carbon fibres with desired properties, which is valuable for the developments of new generation of the structural batteries. Potential Using of Structural Batteries in Satellites During a typical Low-Earth-Orbit (LEO) Mission profile solar energy is not always or not sufficient available so on-board energy needs to be stored. A typical electrical power system of a spacecraft
consist of a primary energy source (e.g. a solar energy), power management and energy storage (e.g. Lithium-based secondary batteries) providing rechargeable power on-demand (Figure 2). One of the main driver for satellite design are weight and volume limitations, so there is a need for advanced power generation and storage technology which are currently >29% efficient solar cells and lithium-ion batteries.
Figure 2: Block diagram of a typical satellite power sub system according to [35].
As mentioned in the introduction LIB Batteries are currently the commonly used battery technology in satellite applications and can contribute a significant amount of dry mass to the spacecraft design Figure 3. Such proportions can range from a fraction of a percent in launcher systems such as the Ariane 5, to the single digit percentage in Earth Observation (~3.7 % Small Satellites ~150 kg class) or in communications (~7.5 % Small GEO), even up to double digit CubeSats (~13 %) 1.33 Kg CubeSat) [29], [30].
Figure 3: Energy density of battery technologies that are relevant for space application. Credit: NASA.
So there is a clear potential for savings, considering that the payload to dry mass ratio for Earth observation and science missions is around 12 % to 20 %. A typical approach in battery sizing
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practice is to consider degradation of the cells (Li-ion) and define a necessary end of life (EOL) power demand and a failure margin. The following formulas and assumptions are based on information’s provided during the first ESA Academy's CubeSats Hands-On Training Week 2018. For a typical LEO mission, the remaining LIB capacity of 30% at EOL is considered with a maximum Depth of Discharge (DoD) of also 30%. A failure margin Sfail of one additional string estimated with +5 % extra cells are used. Now the required battery energy Ebat can be calculated with an assumed discharge efficiency ηdis of 98 %: =
! ∙"#$% &&∙' $(
= 3.6 ∙ +,-+
(3)
The content of PEO (600 kg/mol, DOW) and LiTFSI (3M) depends on the ratio of Li+ and O-, namely:
89
67 89
PEO/LiTFSI as a typical solid electrolyte for all-solid-state batteries is firstly manufactured to thin films for the electrochemical characterization and mechanical tests. It is afterwards coated on the both sides of carbon fiber fabrics for the mechanical tests. Considering an enhancement of the mechanical properties, the epoxy resin blended with lithium salt and PEO is potentially utilized as solid electrolyte. The mechanical tests are executed of CFRPs which are manufactured by carbon fiber fabrics being impregnated with this novel kind of electrolytes. 3.1 Manufacture of Solid State Electrolyte
c.f. Figure 4) is considered: = ∙ ;, ∙ ;7<- ∙ .[
3. Methods, Materials and Experiments Experiments
(2)
With the so estimated required battery energy of 3.6 times the actual power demand of the s/c the battery mass can be assessed. Here a 5 % maturity margin of established technology, 25 % harnessing, enclosing etc. and the energy density 67 67 . (e.g../012345 = 133 ,.: "13 = 155 also 89
not or only insignificantly increase. [21]
] (4)
As mentioned, integrating energy storing functionality in the structure harnessing and cell mass and overall battery mass can be significantly reduced. So multifunctional structures are very promising for usage in satellite applications. The construction of a functionalized fiber composite structure, as conducted in this research, however, provides a full-fledged battery and differs from the existing concepts also in that way that the active battery materials are incorporated directly into the matrix of the individual fiber composite layers at the micro- and nanoscale level. Whereby the volume and mass of the component should
?=
-@$A -BC
=
,@$DEFG IH @$DEFG ,JKB IH JKB
The both dry powders were manually blended in glass bottle, then the acetonitrile (≤10 ppm H2O Carl Roth) was added according to the concentration of 40 g/l (L M /OP<-Q
Page 5 of 11 Figure 4: State of the Art Small Satellite Energy Storage Density [3]
(5)
µm. After drying, the film was pulled out from the Teflon foil. To coat the PEO/LiTFSI on the carbon fiber fabrics, the solution was coated firstly on the Teflon foil. Before drying, the carbon fiber fabrics were laid on the wet film. Then the coated fabrics were pulled out, then turned over for another side following with drying process in 40 °C. The fabrics were then hot pressed in a hydraulic press in 80 °C under 500 N/cm2 for 1 h. The impregnation of epoxy resin based solid electrolyte is realized by hand lamination. Here, a mold (in this case plate) is chemically prepared to afterwards impregnate fabric layers (carbon fiber and / or glass fiber) evenly by hand with the solution mixed with hardeners. A vacuum film is then stretched over the fiber composite (vacuum bag) so that the CFRP can harden under vacuum conditions and or be tempered for further curing.
metal foils (thickness of 500 µm) were used as electrodes with a diameter of 12 mm. The coil cells were then laid in a battery holder (CR 2032, Gamry Instrument, UK) which was connected with an electrochemical workstation (Zennium, Zahner-Elektrik GmbH & CoKG) and subsequently put in a chamber oven (Binder GmbH, Germany). The impedance was measured under 5 mV amplitude in a frequency range of 4 MHz - 0.1 Hz in temperatures of 20 °C, 40 °C, 60 °C, and 80 °C respectively. Before the tests, coil cells were thermal treated in 80 °C for 72 h to acquire a stable solid-electrolyte-interface. The equivalent circuit was analyzed by software ZVIEW and the ionic conductivity σ was calculated by the equation
=
R S
(6)
∙T
3.3 Thermal Analyses Differential scanning calorimetry (DSC) used to characterize the thermal properties of the PEO/LiTFSI samples (1-6 mg from the film), is executed with a differential scanning calorimeter (Typ DSC 3, METTLER TOLEDO). The samples were at first heated from 20 °C to 100 °C of 10 °C/min, then stayed in 100 °C for 3 min. It was followed with a cooling process from 100 °C to -80 °C in rate of 10 °C, then the samples were heated second time to 100 °C in rate of 10 °C/min. 3.4 Ionic Conductivity Measurements To make the electrochemical impedance spectroscopy for ionic conductivity measurements, the samples were cut to round form with diameter of 16 mm and then made into
σ - Ionic conductivity, S/cm l - Thickness of sample, cm Re - Electrolyte resistance, Ohm A – Surface area of lithium metal electrode, cm2 3.5 Mechanical tests Tensile test is used to examine the mechanical performance of the PEO/LiTFSI film and correlated CFRP which was executed in a static material test machine (RetroLine, ZwickRoell AG, Germany). The film samples were cut into form shown in Figure 6 and the measured filed is 50 mm in the length direction. On the other hand, the PEO/LiTFSI based CFRP was cut into 15×70 mm in fiber direction of 0/90° an -45/45°. The test began when a pre-load reached to 10 N. The test speed was 1 mm/min. It was ended when the
Figure 5: Schematic and components of coil cell used for impedance tests.
coil cells as shown in Figure 5, in which the lithium
stress decreased to 80 % of the maximum stress.
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4. Results and Discussions
Figure 6: Schematic of a sample for mechanical tests.
For the samples with epoxy resin based electrolyte a combinations of standardized samples (200 mm x 200 mm x 2 mm) are prepared to assesses the mechanical and processing properties for a future interlaminar shear strength in tension and bending test (ILSS). Those samples (Figure 6) where preliminary investigated within a simple bending testbed where the samples could have positioned and load can be applied through a piston. It is positioned in the center of the sample and can be loaded with standardized weights of 50, 100, 200, 300, 500, 1000 g and has a self-weight of 65 g. The load is measured with force meter that can measure a maximal equivalent bowing under load of 10 mm.
Figure 8 shows the measurement results of DSC for the PEO/LiTFSI (r = 0, pure PEO film). An enthalpy peak in the first heating process indicates a melting temperature of 65 °C for pure PEO film, in which the crystalline structure transfers to amorphous structure. An exothermal peak of 45 °C appears during cooling which is related to the crystallization of the PEO molecular chain indicating the formation of crystalline structure from amorphous structure. The melting temperature slightly decreases to 64 °C in the second heating process, because the crystal structure in PEO molecule chains increase after second thermal cycling. Similar phenomenon has been acquired for PEO/LiTFSI film with ? 0.01 0.06 , the result of melting temperature, crystallization temperature, enthalpy of melting as well as enthalpy of crystallization are shown in Table 1. It is obvious that all of the four parameters decrease with an increase of lithium salt content. It implies an insertion of anions (TFSI-) dissociate within the PEO molecular chains impedes the formation of crystalline structure furthermore decreases the melting temperature. Especially, the crystallization temperature of samples with r = 0.05 and 0.06 are already lower than room temperature (20 °C), that means the film remain in large amount of amorphous structure, which usually have a benefit of ionic transport [32].
Figure 7: Different reference samples manufactured in hand laminate process.
Page 7 of 11 Figure 8: DSC curve in heating and cooling process of PEO film.
The electrochemical characteristics is demonstrated by electrochemical impedance spectroscopy, the Nyquist-plots of PEO/LiTFSI ? 0.01 are shown in Figure 9.
electron transformation between electrolyte and lithium metal. VW and VW P are constant phase elements which exhibit a deviation from ideal capacitor. The ionic conductivities in various temperatures of PEO film with different LiTFSI content are calculated and demonstrated in Figure 10. It is clear to see that higher the lithium salt content higher the ionic conductivity in the temperature range of 20-40 °C, as the conductivity 1.5×10-7 S/cm in 20 °C of PEO/LiTFSI ? 0.01 increases to 7.1×10-6 S/cm for PEO/LiTFSI ? 0.06. It is interesting to see that when temperature increases above 60 °C, the ionic conductivity increases not furthermore when ? X 0.03. The highest conductivity reaches 6.1×10-4 S/cm for PEO/LiTFSI ? 0.06 at 80 °C. As
Figure 9: Nyquist plots of PEO/LiTFSI r=0.01 in temperatures of 20/40/60/80 °C.
Two semicircles are observed for all samples, which could be simulated with two RC circuits connected in series as the equivalent circuit shown in Figure 9.
Figure 12: Ionic conductivity dependency on temperature of PEO/LiTFSI films with different r values.
Figure 10: Equivalent circuit extracted from Nyquist plots in Figure 9.
The U measured in high frequency range indicates the electrolyte resistance, and UP acquired in lower frequency range is contact resistance, which is usually caused by ion and
the samples with high ? value have more amorphous structure in molecular chains under melting temperature and additionally possess more lithium ions, both influence factors result in
Table 1: DSC results of PEO/LiTFSI samples with different r values.
Figure 11: E-modulus of the electrolytes function Pageas8aof 11 of r. The error bars correspond to a standard deviation.
a higher ionic conductivity. When the measurement temperature raises over the melting temperature, all samples are in amorphous structure, the concentration of lithium ions is the only influence factor. Because PEO host could only provide limited positions for the ion transport, there comes finally a saturation of the Li+ doping. Figure 13 shows the Stress-strain diagram of PEO/LiTFSI films with ? value from 0 - 0.06, and Figure 11 exhibits the correlated E-modulus and maximum stress. In the examined strain range a linear regression was performed according to the standard for tensile testing of polymers and the modulus of elasticity is determined as per DIN ISO 527-1:2012 [33] for elongations between 0.05% and 0.25%. A regression line is determined from the corresponding measurement points where the slope of the regression line corresponds to the modulus of elasticity of the sample tested. It is clear that higher the lithium salt concentration worse the mechanical performance. The E-modulus of pure PEO is 450 MPa, which decreases to 10 MPa when ? value reaches to 0.06. It could be attribute to the dominant amorphous structure in PEO/LiTFSI with high ? values. To compare the mechanical properties with ionic conductivity, the optimized materials proportion is acquired in PEO/LiTFSI? 0.03, that has a relative high ionic conductivity of 6.1×10-4 S/cm in 80 °C and the E-modulus of 116 MPa.
Figure 13: Stress-strain curves of PEO/LiTFSI films with different r values.
Figure 14 and Figure 15 show the Stress-strain value of the CFRP in 0/90° and in -45/45° respectively. The influence of lithium salt on mechanical properties of CFRP 0/90° is not dramatically, while the most of the loads are carried by the carbon fibers, thus the E-modulus and maximum strength to some content represent only the properties of the carbon fibers. When the -45/45° sample is exposed to shear stress, the matrix determines the material behavior because the fibers cannot absorb any load at all. A flatter stress-strain curve is visible for increasing strains and a plateau is formed. In the area of the plateau, a re-orientation of the fibers occurs so that orientation of the fibers no longer corresponds to -45/45° and strain hardening effect is observed.
Figure 14: Stress-strain curves of PEO/LiTFSI coated carbon fiber fabrics in 0/90° direction.
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This work is supported (support code 50RP1720) by the Federal Ministry for Economic Affairs and Energy on the basis of a decision by the German Bundestag. The authors would like to also thank the INNOspace Masters DLR challenge. List of references
Figure 15: Stress-strain curves of PEO/LiTFSI coated carbon fiber fabrics in -45/45° direction.
5. Conclusion & Outlook The goal of the presented research is the integration of battery materials into fiber composite structures on a microscale level to reducing the mass and volume of a spacecraft. In this work, a solvent-based approach for impregnating carbon fibers with a solid electrolyte was investigated. The potential of using typical polymer solid electrolyte, namely polyethylene oxide combined with lithium salt LiTFSI is firstly investigated. Results show that the mechanical properties of electrolytes decrease with increased LiTFSI concentration. It is confirmed by the DSC measurements as the addition of LiTFSI increases the amorphous structure inside the PEO host, which leads to a weakness of the mechanical performance. The electrolyte infiltrated carbon fiber fabrics exhibit an excellent mechanical performance when fiber is in direction of tensile force. The ionic conductivity, which is important for the battery performance, increases obviously with higher lithium salt content. An optimized multifunctional properties are acquired with PEO/LiTFSI r=0.03 (Li+/ O-). For a final characterization of the mechanical properties ILSS tests are going to be conducted. Finally, for the proof of concept of the structural battery all layers of the electrolyte, anode and cathode needs to be integrated and tested. Acknowledgements
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[16] R. Raffaellea, J. Harris, D. Hehemann, D. Scheiman, G. Rybicki and A. Hepp, "A facile route to thin-film solid state lithium microelectronic batteries,," Journal of Power Sources, no. 89, p. 52–55, 2000. [17] R. P. Raffaelle, A. F. Hepp, G. A. Landis and D. J. Hoffman, "Mission applicability assessment of integrated power components and systems," Research and Applications, no. 10(6), pp. 391-397, 2002. [18] C. Clark and J. Summers, "Innovative Flexible Lightweight Thin-film Power Generation and Storage for Space Applications in," Proceedings of the 35th Intersociety Energy Con-version Engineering Conference, vol. Vol. 1, p. 692–698.], 2000. [19] D. Marcelli, J. Summers and B. Neudecker, "LiBaCore II:Power Storage in Primary Structure," in StructuralDynamics and Materials Conference, Denver, CO, USA, 2002. [20] S. Ekstedt, M. Wysocki and L. Asp, "Structural batteries made from fibre reinforced composite," s, Plastics, Rubber and Composites: Macromolecular Engineering, vol. 39, no. 3/4/5, pp. 148-150, 2010. [21] W. Johannisson , . N. Ihrner, D. Zenkert , M. Johansson, D. Carlstedt, . L. Asp and F. Sieland , "Multifunctional performance of a carbon fiber UD lamina electrode for structural batteries.," Composites Science and Technology, no. 168, pp. 81-87, 2018. [22] V. Aravindan and P. Vickraman, "A novel gel electrolyte with lithium difluoro (oxalato) borate salt and Sb2O3 nanoparticles for lithium ion batteries," Solid State Sci, p. 1069–1073, 2007. [23] J. F. Snyder, R. H. Carter, K. Xu, E. I. Wong, P. A. Nguyen, E. H. Hgo and E. D. Wetzel, "Multifunctional structural composite batteries for US army applications," in Proc. of the 2006 Army Science Conf., Orlando, FL, USA, 2007. [24] E. L. Wong, D. M. Baechle, K. Xu, R. H. Carter, J. F. Snyder and E. D. Wetzel, "Design and Processing of Structural Composite Batteries," in Proc. of the SAMPE , Baltimore, MD, USA, 2007. [25] N. Ihrner, W. Johannisson, F. Sieland and D. Zenkert , "Structural Lithium Ion Battery Electrolytes via Reaction Induced Phase-Separation," Journal of Materials Chemistry A, pp. 5:25652-25659, 2017.
[26] B. J. Neudecker, M. H. Benson and B. K. Em, "Power Fibers: Thin-Film Batteries on Fiber Substrates," ReportNo.: ADA511230. Defense Technical InformationCenter (DTIC),, 2003. [27] J. Hagberg, S. Leijonmarck and G. Lindbergh, " High precision coulometry of commercial PAN-based carbon fibres as electrodes in structural batteries," J. Electrochem. Soc. , no. 163, p. A1790–6, 2016. [28] G. Fredi, S. Jeschke, A. Boulaoued, J. Wallenstein, M. Rashidi , F. Liu , R. Harnden , D. Zenkert, J. Hagberg, P. Johansson , L. Stievano and L. Asp, "Graphitic microstructure and performance of carbon fibre Li-ion battery electrodes," in Multifunctional Materials, 2018. [29] D. Z. Genc and C. Thwaite, "Proba-1 And Mars Express: An ABSL Lithium-Ion Legacy," in 9th European Space Power Conference, Saint Raphael, France, 2011. [30] D. Curzon, C. Thwaite and R. Buckle, "Lithium-Ion Battery Performance On Board Geostationary And Lunar Orbiter NASA Missions- SDO And LRO," in Proceedings of the 9th European Space Power Conference, Saint Raphael, France, 2011. [31] R & G Faserverbundwerkstoffe GmbH, "R & G Faserverbundwerkstoffe/Composite Technology," 19 03 2018. [Online]. Available: http://www.ezentrumbilder.de/rg/pdf/si_en_100135.pdf. [Accessed 16 04 2019]. [32] S. Cheng, D. M. Smith and C. Y. Li, "Macromolecules," no. 47, p. 3978–398, 2014. [33] DIN ISO 527-1:2012, "Plastics - Determination of tensile properties - Part 1: General principles (ISO 5271:2012); German version EN ISO 527-1:2012," Beuth-Verlag, Berlin, 2012. [34] N. Shirshova, H. Qian, M. S., P. Shaffer, J. H. G. Steinke, E. S. Greenhalgh, P. T. Curtis, A. Kucernak and A. Bismarck, "Structural composite supercapacitors," Composites Part A: Applied Science and Manufacturing, no. 46, pp. 96-107, 2013. [35] E. Messerschmid and S. Fasoulas, "Energieversorgungsanlagen," in Raumfahrtsysteme, Germany, Springer-Verlag GmbH, 2017, p. 299.
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Integration of energy storage functionalities into fiber reinforced spacecraft structures Benjamin Grzesika*, Guangyue Liaob, Daniel Vogt ab, Linus Froböseb, Arno Kwadeb, Stefan Linkea Enrico Stolla a
Institute of Space Systems (IRAS), Technische Universität Braunschweig, Hermann-Blenk-Str. 23, 38108 Braunschweig, Germany, [email protected] b Institute for Particle Technology (IPAT), Technische Universität Braunschweig, Volkmaroder Str. 5, 38104 Braunschweig, Germany, [email protected] * Corresponding Author Highlights
• • • • •
We integrate energy storage functionality into the spacecraft structure Carbon fiber can be impregnated with resin blended with Li-Salt (LiTFSI) and PEO Infiltrated CFRP has excellent mechanical performance in direction of tensile force LiTFSI in the electrolyte is used to increase ionic conductivity Best multifunctional properties are acquired with PEO/LiTFSI r=0.03 (Li+/O-)
Keywords: Energy storage, carbon fiber, satellite technologies, power subsystem, multifunctional structure
Page 1 of 1
S0094-5765(19)31315-3
DOI:
https://doi.org/10.1016/j.actaastro.2019.10.009
Reference:
AA 7694
To appear in:
Acta Astronautica
Received Date: 1 March 2019 Revised Date:
24 June 2019
Accepted Date: 6 October 2019
Please cite this article as: B. Grzesik, G. Liao, D. Vogt, L. Froböse, A. Kwade, S. Linke, E. Stoll, Integration of energy storage functionalities into fiber reinforced spacecraft structures, Acta Astronautica, https://doi.org/10.1016/j.actaastro.2019.10.009. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd on behalf of IAA.
Integration of energy storage functionalities into fiber reinforced spacecraft structures Benjamin Grzesika*, Guangyue Liaob, Daniel Vogt ab, Linus Froböseb, Arno Kwadeb, Stefan Linkea Enrico Stolla a
Institute of Space Systems (IRAS), Technische Universität Braunschweig, Hermann-Blenk-Str. 23, 38108 Braunschweig, Germany, [email protected] b Institute for Particle Technology (IPAT), Technische Universität Braunschweig, Volkmaroder Str. 5, 38104 Braunschweig, Germany, [email protected] * Corresponding Author Abstract One of the common challenges of spaceflight and e-mobility is the energy storage. The operational ranges of spacecraft or electric cars as well as operational usability is strongly dependent on the capacity of the energy storage, which is usually constrained by the mass and volume of the vehicle. For example, the battery mass of large communication satellites ranges from 6 % to 9 % of the dry mass. This amount increases for smaller satellites like CubeSats up to 13 % without accounting wiring harnesses and subsystem volume. Thus, a system can benefit from a reduction in mass and volume by combining multifunctional use of the components and materials. In the presented research, energy storage is integrated into lightweight carbon fiber materials. Carbon fibers have a distinct mass advantage compared to metal structures. In addition, they have very low thermal expansions that can reduce thermal stresses during the operation of a satellite. Fiber composites or laminates consist of two components, the fibers and the matrix material. In laminates, the typically resin based matrix content is 30 to 40 % of the component volume and could be substituted with novel solid state battery materials. Latter have the advantage that they have nearly similar physical properties as usually used resins and are also able to store energy electrochemically. Thus, about 25 % of the volume can be used for electrochemical energy storage without compromising structure integrity. To reach such electrochemical functions the host structures have to be infiltrated with anode, cathode, and separator materials. Using the developed recipes and a component thickness of 5 millimeters, an energy amount per component area of 1130 Wh/m² can be reached depending on the applied battery active material. This article will give an overview of how fiber composite materials, which are increasingly being used in lightweight construction processes, can be combined with energy storage materials to be used in spacecraft structures. The goal of the research on this topic at the Technische Universtät Braunschweig is a structure battery for a spacecraft. However the article addresses as a first step the characterization and manufacturing of structural battery composite negative half-cells from carbon fiber reinforced PEO/LiTFSI. An estimation of the expected performance is carried out. The initial material evaluation and the manufacturing process as well as mechanical and electrical result will be discussed. Keywords: Energy storage, carbon fiber, satellite technologies, power subsystem, multifunctional structure, half-cells Acronyms/Abbreviations Constant Phase Element (CPE), Carbon Fiber Reinforced Plastic (CFRP), Carbon Fiber (CF) Differential scanning calorimetry (DSC), Glass Fiber Reinforced Plastic (FRP), Glass Fiber (GF),
Lithium-Ion-Batteries (LIB), Lithium-bis(trifluormethylsulfonyl)imid (LiTFSI), Porous Electrolye Interface (PEI), Polyethylenoxid (PEO), Solid Electrolye Interface (SEI), Ultra-Highmodulus Carbon Fiber (UHM-CF)
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1. Introduction The last transition in battery technology for spacecraft applications, in which NiCd where replaced by NiH2, occurred in the mid-80s [1]. Satellite power systems have progressively shifted to lithium-ion batteries. In all transitions the key parameter, the specific energy (Wh/kg), was approximately doubled. Within the next 20 years, a doubling may be again necessary due to the increase of payload power. The commonly used battery formats for satellites are cylindrical 18650 lithium ion (LIB) batteries with a form factor of 18.4 mm x 65.1 mm [2] or flat lithium polymer batteries. From the view point of energy density, lithium-ion batteries (120-140 Wh/kg) have a great advantage over Ni-Cd batteries (~ 30 Wh/kg), so the total mass of the satellite system can be reduced. With the development in the consumer electronics market and the progressive miniaturization, the stringent requirements for the mass and volume of batteries for space applications can be fulfilled much easier. Nevertheless, significantly new battery technologies for small satellites are not expected in the near future [1]. However, in the area of power storage there are several efforts for improving the storage capability by enhancing the Li-Technology with e.g. carbon nanotubes that can significantly increases available energy density and can lead to the next doubling in specific energy or to develop multifunctional structural battery systems [3]. To develop advanced aerospace systems and products with better performances, the saving of mass and volume becomes indispensible. This results from the direct proportional relation between these two factors and the occurring project costs, which is clearly proposed in a parametric cost models given by U. S. Air Force [4], [5]. One of the most promising approaches for the mass and volume saving are multifunctional structures with integrated structural, thermal, sensing, actuation or power systems. The aim of this technology is to maximize the ratio of volume and mass of the functional subsystems to the total packaging volume and mass of the satellites [6]. Structural
energy devices are realized by integrating energy storage devices into the satellites’ structures. Thus, the enclosure of energy storage devices as well as connectors become unnecessary. Furthermore, as the structural energy devices are capable of bearing mechanical loads, a part of structure materials could be substituted. Therefore, not only the inside space, but also the total mass and volume of the satellite is reduced [7]. This article will give an overview of multifunctional energy storage, state of the art of structural batteries and focus then on first step towards the research goal of a structural battery on the characterization and manufacturing of structural battery composite negative half-cells from carbon fiber reinforced PEO/LiTFSI.
2. Multifunctional Energy Storage Composites The concept of structural energy storage devices has already been developed for capacitors, supercapacitors, batteries, and fuel cells. To evaluate the multifunctional design in the subsystem, Wetzel [8] proposed a method, which describes the relationship between mass and functionalities: (1) = + 1 − ∙
Here, is the mass of total system, which equals to sum of structure mass ( ) and storage device‘s mass ( ). is defined as structural mass efficiency, which represents the structural property of the multifunctional materials compared with conventional fiber reinforced composites ( = 1). On the other
Figure 1: system mass reduction by multifunctionalization according to Wetzel [8].
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hand, the conventional energy storage device possesses no structural functions, therefore the
value is considered as 0. Similarly, energy mass efficiency factor is 1 for the storage device as it should provide full energy density, while the value for composites is 0, because they delivery no energy. If the energy storage device could carry the loads and substitute a part of the structural function, the total system mass is acquired by equation (1). Figure 1 shows an example for a mini satellite, with a structure mass of 100 kg and battery mass of 10 kg. In the ideal situation, the energy storage system delivers all energy and has complete structural property,
= 1 and = 1 , then = . However, in accordance to [8] reality it’s more reasonable that a structural storage device reaches values between
= 0.75 and = 0.5 . In general, the system has benefits from this multi-functionality when the energy delivery and structural property could be acquired in the multifunctional materials simultaneously in a sufficient high value. 2.1 State of the Art of Structural Batteries Significant researches about the structural batteries for satellite applications have been carried out in the past decades [9], [10], [11], including the conception development, demonstrator manufacture, component design and performance characterization. Different methods have been used for the structure integration of energy storage devices from macro scale to nanoscale. The first approach of the integration is embedding the commercial battery cells, as they have considerable mechanical properties, into structural composites for satellites [12], [13], [14], [15]. However, these cells need hard and relative heavy cases and be bolted to the rest of the structure. Thus, this integration will add parasitic mass and requires a substantial volume allocation [7]. Therefore, some researchers used polymer electrolyte based thinfilm all-solid-state batteries to make the integration [16], [17]. Clark [18] manufactured a complete power system, which consists of thin photovoltaic cell, lithium polymer thin-film batteries and power management electronics on a
polyamide substrate. This power system could be directly bonded to the outer skin of a satellite. Theoretically it could produce a power of 20 W/kg for a 28 V power requirement. Marcelli [19] proposed a way of integration in which the thinfilm batteries were combined with honeycomb core. As there is large amount of unused surface area inside the honeycomb structure, the thin-film batteries could be deposited onto aluminum or titanium aluminide surface for honeycomb cores. No additional volume is required, however the fabrication techniques is inconvenient for the scale-up manufacture. The results of their work shows a rapidly and significantly degradation of the cell capacity. A more efficient way for the structural integration is to use battery components also as structural elements. First working structural batteries were reported by Ekstedt et al. [20], which confirms the concept of using carbon fibres as anode’s current collector and provide mechanical performance simultaneously. A charge capacity of 116 Wh kg-1 and stiffness of 35 GPa were acquired in the fabricated structural battery. In the most recent study, Johannisson et al. [21] has demonstrated a structural battery with carbon fibre lamina vacuum infused with a Bisphenol A dimethacrylate based solid electrolyte. The capacity of 200 Wh kg-1 and Young’s modulus of 52 GPa have been acquired, which represent a great improvement of the multifunctionality. The solid electrolytes as matrix should be multifunctional, as they not only provide the properties for the ion migration, but also deliver a good mechanical performance. Y. Yu [11] reported a promising electrolyte with Young’s modulus of 400 MPa and the ionic conductivity of 0.08 mS/cm, which is synthesized by blending LiTF2N/EMIMTF2N solution in bisphenol-A type epoxy resin. Inorganic particles have been reported to act as filler materials in polymers for composite electrolytes, usually used ceramic nanoparticles are SiO2, TiO2, ZrO2 or Al2O3 [22]. Snyder [23] showed a structural battery, which uses carbon fabric as anode, LiFePO4 cathode and a vinyl ester random copolymer as solid electrolyte and the laminates were fabricated using vacuum assisted resin transfer molding technique [24]. The sample
Page 3 of 11
showed a tensile specific stiffness of 3.6 GPa g-1, however the ionic conductivity is low. Another promising work in the field of structural batteries is reported by Liu et al [9] who develop structural batteries, based on a carbon fiber reinforced PVDF composite, with tunable mechanical properties showing a flexural modulus of 3.1 GPa and an energy density of 35 Wh kg−1. Yu et al [11] have reported a structural battery, based on carbon fiber reinforced plastic composites, yielding a specific capacity of 12 mAh/g and a tensile modulus of 80 GPa. Ihrner et al [25] has reported a novel type of multifunctional solid-state electrolyte, which is manufactured by combining liquid electrolyte phase with a stiff vinyl ester based thermoset matrix. The as acquired materials possess an ionic conductivity of 1.5 10-4 S cm-1 and a corresponding storage modulus of 750 MPa. The highest integration level is called power fibers [26] which is realized by depositing electrolyte and cathode materials directly on a single fiber of carbon, glass, silicon carbide, or a metal, which are used as current collectors. The fabricated fibers could subsequently be produced as a woven fabric, composites, or highly compact batteries. These batteries could acquire high rate capability up to 50 C and over 2000 cycles at 100 % depth of discharge for a 50 C discharge rate. Hagberg et al. [27] characterized the electrochemical properties of the PAN-based carbon fibers, which possess not only high modulus, but also high capacity of ca. 250–350 mAh/g and very high columbic efficiencies—for some >99.9 % after 10 cycles. Fredi et al. [28] systematically studied the PAN-based carbon fibre microstructure influencing on lithium insertion mechanism and mechanical performance. They provide a new aspect to manufacture carbon fibres with desired properties, which is valuable for the developments of new generation of the structural batteries. Potential Using of Structural Batteries in Satellites During a typical Low-Earth-Orbit (LEO) Mission profile solar energy is not always or not sufficient available so on-board energy needs to be stored. A typical electrical power system of a spacecraft
consist of a primary energy source (e.g. a solar energy), power management and energy storage (e.g. Lithium-based secondary batteries) providing rechargeable power on-demand (Figure 2). One of the main driver for satellite design are weight and volume limitations, so there is a need for advanced power generation and storage technology which are currently >29% efficient solar cells and lithium-ion batteries.
Figure 2: Block diagram of a typical satellite power sub system according to [35].
As mentioned in the introduction LIB Batteries are currently the commonly used battery technology in satellite applications and can contribute a significant amount of dry mass to the spacecraft design Figure 3. Such proportions can range from a fraction of a percent in launcher systems such as the Ariane 5, to the single digit percentage in Earth Observation (~3.7 % Small Satellites ~150 kg class) or in communications (~7.5 % Small GEO), even up to double digit CubeSats (~13 %) 1.33 Kg CubeSat) [29], [30].
Figure 3: Energy density of battery technologies that are relevant for space application. Credit: NASA.
So there is a clear potential for savings, considering that the payload to dry mass ratio for Earth observation and science missions is around 12 % to 20 %. A typical approach in battery sizing
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practice is to consider degradation of the cells (Li-ion) and define a necessary end of life (EOL) power demand and a failure margin. The following formulas and assumptions are based on information’s provided during the first ESA Academy's CubeSats Hands-On Training Week 2018. For a typical LEO mission, the remaining LIB capacity of 30% at EOL is considered with a maximum Depth of Discharge (DoD) of also 30%. A failure margin Sfail of one additional string estimated with +5 % extra cells are used. Now the required battery energy Ebat can be calculated with an assumed discharge efficiency ηdis of 98 %: =
! ∙"#$% &&∙' $(
= 3.6 ∙ +,-+
(3)
The content of PEO (600 kg/mol, DOW) and LiTFSI (3M) depends on the ratio of Li+ and O-, namely:
89
67 89
PEO/LiTFSI as a typical solid electrolyte for all-solid-state batteries is firstly manufactured to thin films for the electrochemical characterization and mechanical tests. It is afterwards coated on the both sides of carbon fiber fabrics for the mechanical tests. Considering an enhancement of the mechanical properties, the epoxy resin blended with lithium salt and PEO is potentially utilized as solid electrolyte. The mechanical tests are executed of CFRPs which are manufactured by carbon fiber fabrics being impregnated with this novel kind of electrolytes. 3.1 Manufacture of Solid State Electrolyte
c.f. Figure 4) is considered: = ∙ ;, ∙ ;7<- ∙ .[
3. Methods, Materials and Experiments Experiments
(2)
With the so estimated required battery energy of 3.6 times the actual power demand of the s/c the battery mass can be assessed. Here a 5 % maturity margin of established technology, 25 % harnessing, enclosing etc. and the energy density 67 67 . (e.g../012345 = 133 ,.: "13 = 155 also 89
not or only insignificantly increase. [21]
] (4)
As mentioned, integrating energy storing functionality in the structure harnessing and cell mass and overall battery mass can be significantly reduced. So multifunctional structures are very promising for usage in satellite applications. The construction of a functionalized fiber composite structure, as conducted in this research, however, provides a full-fledged battery and differs from the existing concepts also in that way that the active battery materials are incorporated directly into the matrix of the individual fiber composite layers at the micro- and nanoscale level. Whereby the volume and mass of the component should
?=
-@$A -BC
=
,@$DEFG IH @$DEFG ,JKB IH JKB
The both dry powders were manually blended in glass bottle, then the acetonitrile (≤10 ppm H2O Carl Roth) was added according to the concentration of 40 g/l (L M /OP<-Q
Page 5 of 11 Figure 4: State of the Art Small Satellite Energy Storage Density [3]
(5)
µm. After drying, the film was pulled out from the Teflon foil. To coat the PEO/LiTFSI on the carbon fiber fabrics, the solution was coated firstly on the Teflon foil. Before drying, the carbon fiber fabrics were laid on the wet film. Then the coated fabrics were pulled out, then turned over for another side following with drying process in 40 °C. The fabrics were then hot pressed in a hydraulic press in 80 °C under 500 N/cm2 for 1 h. The impregnation of epoxy resin based solid electrolyte is realized by hand lamination. Here, a mold (in this case plate) is chemically prepared to afterwards impregnate fabric layers (carbon fiber and / or glass fiber) evenly by hand with the solution mixed with hardeners. A vacuum film is then stretched over the fiber composite (vacuum bag) so that the CFRP can harden under vacuum conditions and or be tempered for further curing.
metal foils (thickness of 500 µm) were used as electrodes with a diameter of 12 mm. The coil cells were then laid in a battery holder (CR 2032, Gamry Instrument, UK) which was connected with an electrochemical workstation (Zennium, Zahner-Elektrik GmbH & CoKG) and subsequently put in a chamber oven (Binder GmbH, Germany). The impedance was measured under 5 mV amplitude in a frequency range of 4 MHz - 0.1 Hz in temperatures of 20 °C, 40 °C, 60 °C, and 80 °C respectively. Before the tests, coil cells were thermal treated in 80 °C for 72 h to acquire a stable solid-electrolyte-interface. The equivalent circuit was analyzed by software ZVIEW and the ionic conductivity σ was calculated by the equation
=
R S
(6)
∙T
3.3 Thermal Analyses Differential scanning calorimetry (DSC) used to characterize the thermal properties of the PEO/LiTFSI samples (1-6 mg from the film), is executed with a differential scanning calorimeter (Typ DSC 3, METTLER TOLEDO). The samples were at first heated from 20 °C to 100 °C of 10 °C/min, then stayed in 100 °C for 3 min. It was followed with a cooling process from 100 °C to -80 °C in rate of 10 °C, then the samples were heated second time to 100 °C in rate of 10 °C/min. 3.4 Ionic Conductivity Measurements To make the electrochemical impedance spectroscopy for ionic conductivity measurements, the samples were cut to round form with diameter of 16 mm and then made into
σ - Ionic conductivity, S/cm l - Thickness of sample, cm Re - Electrolyte resistance, Ohm A – Surface area of lithium metal electrode, cm2 3.5 Mechanical tests Tensile test is used to examine the mechanical performance of the PEO/LiTFSI film and correlated CFRP which was executed in a static material test machine (RetroLine, ZwickRoell AG, Germany). The film samples were cut into form shown in Figure 6 and the measured filed is 50 mm in the length direction. On the other hand, the PEO/LiTFSI based CFRP was cut into 15×70 mm in fiber direction of 0/90° an -45/45°. The test began when a pre-load reached to 10 N. The test speed was 1 mm/min. It was ended when the
Figure 5: Schematic and components of coil cell used for impedance tests.
coil cells as shown in Figure 5, in which the lithium
stress decreased to 80 % of the maximum stress.
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4. Results and Discussions
Figure 6: Schematic of a sample for mechanical tests.
For the samples with epoxy resin based electrolyte a combinations of standardized samples (200 mm x 200 mm x 2 mm) are prepared to assesses the mechanical and processing properties for a future interlaminar shear strength in tension and bending test (ILSS). Those samples (Figure 6) where preliminary investigated within a simple bending testbed where the samples could have positioned and load can be applied through a piston. It is positioned in the center of the sample and can be loaded with standardized weights of 50, 100, 200, 300, 500, 1000 g and has a self-weight of 65 g. The load is measured with force meter that can measure a maximal equivalent bowing under load of 10 mm.
Figure 8 shows the measurement results of DSC for the PEO/LiTFSI (r = 0, pure PEO film). An enthalpy peak in the first heating process indicates a melting temperature of 65 °C for pure PEO film, in which the crystalline structure transfers to amorphous structure. An exothermal peak of 45 °C appears during cooling which is related to the crystallization of the PEO molecular chain indicating the formation of crystalline structure from amorphous structure. The melting temperature slightly decreases to 64 °C in the second heating process, because the crystal structure in PEO molecule chains increase after second thermal cycling. Similar phenomenon has been acquired for PEO/LiTFSI film with ? 0.01 0.06 , the result of melting temperature, crystallization temperature, enthalpy of melting as well as enthalpy of crystallization are shown in Table 1. It is obvious that all of the four parameters decrease with an increase of lithium salt content. It implies an insertion of anions (TFSI-) dissociate within the PEO molecular chains impedes the formation of crystalline structure furthermore decreases the melting temperature. Especially, the crystallization temperature of samples with r = 0.05 and 0.06 are already lower than room temperature (20 °C), that means the film remain in large amount of amorphous structure, which usually have a benefit of ionic transport [32].
Figure 7: Different reference samples manufactured in hand laminate process.
Page 7 of 11 Figure 8: DSC curve in heating and cooling process of PEO film.
The electrochemical characteristics is demonstrated by electrochemical impedance spectroscopy, the Nyquist-plots of PEO/LiTFSI ? 0.01 are shown in Figure 9.
electron transformation between electrolyte and lithium metal. VW and VW P are constant phase elements which exhibit a deviation from ideal capacitor. The ionic conductivities in various temperatures of PEO film with different LiTFSI content are calculated and demonstrated in Figure 10. It is clear to see that higher the lithium salt content higher the ionic conductivity in the temperature range of 20-40 °C, as the conductivity 1.5×10-7 S/cm in 20 °C of PEO/LiTFSI ? 0.01 increases to 7.1×10-6 S/cm for PEO/LiTFSI ? 0.06. It is interesting to see that when temperature increases above 60 °C, the ionic conductivity increases not furthermore when ? X 0.03. The highest conductivity reaches 6.1×10-4 S/cm for PEO/LiTFSI ? 0.06 at 80 °C. As
Figure 9: Nyquist plots of PEO/LiTFSI r=0.01 in temperatures of 20/40/60/80 °C.
Two semicircles are observed for all samples, which could be simulated with two RC circuits connected in series as the equivalent circuit shown in Figure 9.
Figure 12: Ionic conductivity dependency on temperature of PEO/LiTFSI films with different r values.
Figure 10: Equivalent circuit extracted from Nyquist plots in Figure 9.
The U measured in high frequency range indicates the electrolyte resistance, and UP acquired in lower frequency range is contact resistance, which is usually caused by ion and
the samples with high ? value have more amorphous structure in molecular chains under melting temperature and additionally possess more lithium ions, both influence factors result in
Table 1: DSC results of PEO/LiTFSI samples with different r values.
Figure 11: E-modulus of the electrolytes function Pageas8aof 11 of r. The error bars correspond to a standard deviation.
a higher ionic conductivity. When the measurement temperature raises over the melting temperature, all samples are in amorphous structure, the concentration of lithium ions is the only influence factor. Because PEO host could only provide limited positions for the ion transport, there comes finally a saturation of the Li+ doping. Figure 13 shows the Stress-strain diagram of PEO/LiTFSI films with ? value from 0 - 0.06, and Figure 11 exhibits the correlated E-modulus and maximum stress. In the examined strain range a linear regression was performed according to the standard for tensile testing of polymers and the modulus of elasticity is determined as per DIN ISO 527-1:2012 [33] for elongations between 0.05% and 0.25%. A regression line is determined from the corresponding measurement points where the slope of the regression line corresponds to the modulus of elasticity of the sample tested. It is clear that higher the lithium salt concentration worse the mechanical performance. The E-modulus of pure PEO is 450 MPa, which decreases to 10 MPa when ? value reaches to 0.06. It could be attribute to the dominant amorphous structure in PEO/LiTFSI with high ? values. To compare the mechanical properties with ionic conductivity, the optimized materials proportion is acquired in PEO/LiTFSI? 0.03, that has a relative high ionic conductivity of 6.1×10-4 S/cm in 80 °C and the E-modulus of 116 MPa.
Figure 13: Stress-strain curves of PEO/LiTFSI films with different r values.
Figure 14 and Figure 15 show the Stress-strain value of the CFRP in 0/90° and in -45/45° respectively. The influence of lithium salt on mechanical properties of CFRP 0/90° is not dramatically, while the most of the loads are carried by the carbon fibers, thus the E-modulus and maximum strength to some content represent only the properties of the carbon fibers. When the -45/45° sample is exposed to shear stress, the matrix determines the material behavior because the fibers cannot absorb any load at all. A flatter stress-strain curve is visible for increasing strains and a plateau is formed. In the area of the plateau, a re-orientation of the fibers occurs so that orientation of the fibers no longer corresponds to -45/45° and strain hardening effect is observed.
Figure 14: Stress-strain curves of PEO/LiTFSI coated carbon fiber fabrics in 0/90° direction.
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This work is supported (support code 50RP1720) by the Federal Ministry for Economic Affairs and Energy on the basis of a decision by the German Bundestag. The authors would like to also thank the INNOspace Masters DLR challenge. List of references
Figure 15: Stress-strain curves of PEO/LiTFSI coated carbon fiber fabrics in -45/45° direction.
5. Conclusion & Outlook The goal of the presented research is the integration of battery materials into fiber composite structures on a microscale level to reducing the mass and volume of a spacecraft. In this work, a solvent-based approach for impregnating carbon fibers with a solid electrolyte was investigated. The potential of using typical polymer solid electrolyte, namely polyethylene oxide combined with lithium salt LiTFSI is firstly investigated. Results show that the mechanical properties of electrolytes decrease with increased LiTFSI concentration. It is confirmed by the DSC measurements as the addition of LiTFSI increases the amorphous structure inside the PEO host, which leads to a weakness of the mechanical performance. The electrolyte infiltrated carbon fiber fabrics exhibit an excellent mechanical performance when fiber is in direction of tensile force. The ionic conductivity, which is important for the battery performance, increases obviously with higher lithium salt content. An optimized multifunctional properties are acquired with PEO/LiTFSI r=0.03 (Li+/ O-). For a final characterization of the mechanical properties ILSS tests are going to be conducted. Finally, for the proof of concept of the structural battery all layers of the electrolyte, anode and cathode needs to be integrated and tested. Acknowledgements
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Integration of energy storage functionalities into fiber reinforced spacecraft structures Benjamin Grzesika*, Guangyue Liaob, Daniel Vogt ab, Linus Froböseb, Arno Kwadeb, Stefan Linkea Enrico Stolla a
Institute of Space Systems (IRAS), Technische Universität Braunschweig, Hermann-Blenk-Str. 23, 38108 Braunschweig, Germany, [email protected] b Institute for Particle Technology (IPAT), Technische Universität Braunschweig, Volkmaroder Str. 5, 38104 Braunschweig, Germany, [email protected] * Corresponding Author Highlights
• • • • •
We integrate energy storage functionality into the spacecraft structure Carbon fiber can be impregnated with resin blended with Li-Salt (LiTFSI) and PEO Infiltrated CFRP has excellent mechanical performance in direction of tensile force LiTFSI in the electrolyte is used to increase ionic conductivity Best multifunctional properties are acquired with PEO/LiTFSI r=0.03 (Li+/O-)
Keywords: Energy storage, carbon fiber, satellite technologies, power subsystem, multifunctional structure
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