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Combining structure and battery (power) functions in a single material entity permits improvements in system performance not possible through independent subsystem optimizations. The design of composite multifunctional materials for optimal system performance involves selection of constituents, material architecture, and interface connections. This overview focuses primarily on plastic lithium-ion structure-battery materials. Three main topic areas are considered: rules and tools for analysis and design of multifunctional materials; multifunctional structure-battery material systems; and structure-battery in the Defense Advanced Resources Projects Agency Wasp micro-air vehicle.

INTRODUCTION

Multifunctional materials systems are capable of performing multiple “primary” functions simultaneously or sequentially in time and are specifically developed to improve system performance through a reduction of redundancy between subsystem materials and functions.¹ The constituents in multifunctional materials are often disparate in nature with widely differing property values. This has led to the need for new analysis tools and design methodologies for relating constituent properties and cross-section architecture to system level performance. The feasibility of a multifunctional material design depends on the internal and external interfacing capabilities and physical/chemical compatibility of the desired combination of subsystem functions. The question of which materials and functions to join in a multifunctional material system is best answered by considering the targeted system performance metric expressed in terms of various subsystem design parameters.

The combination of structure and battery for use in electric-propelled unmanned air vehicles (UAVs) is a good example of how subsystem materials/ functions can be identified for possible integration in a multifunctional material system. For electric-propelled UAVs, an important system performance metric is flight endurance time, which is explicitly related to the available battery energy, subsystem weights, and aerodynamic parameters, as shown in Equation 1.^2,3 (All equations are shown in Table I.)

In Equation 1, E_B is the nominal stored battery energy, and η_B is an efficiency factor that accounts for the influence of the current draw rate, temperature, etc., on the amount of energy that can be extracted from the battery. Aircraft structure, battery, propulsion, and payload subsystem weights are represented by W_S, W_B, W_PR, and W_PL, respectively. Aerodynamic parameters include air density, ρ, wing platform area, S, and the lift and drag coefficients, C_L and C_D. Finally, η_ρ is the motor/propeller efficiency, which equals the thrust power available from the motor/propeller combination divided by the electrical input power to the motor.

MULTIFUNCTIONAL MATERIAL DESIGN

An important design issue is the efficacy of multifunctionality in improving system performance. To quantify this, one must have a system metric(s) that can be applied to quantify and compare the system performance of candidate multifunctional and unifunctional system designs. Note that the optimal multifunctional system design may be very different from the optimal unifunctional system design. Any proposed design must be capable of successfully performing/completing the intended mission/task/purpose, but otherwise, there may be significant design freedom that can be used to achieve a multifunctional advantage. For example, fixed-wing unmanned air vehicles serve as sensor platforms and must accomplish certain missions subject to restrictions on the vehicle’s size, weight, and endurance. A variety of configurations (e.g., classical wing-tail, flying wing, etc.) may be capable of accomplishing the mission while also meeting the imposed requirements. Each configuration will have certain advantages and disadvantages in any given multifunctional/unifunctional implementation. “Rules and Tools” are the meth-odology for developing new multi-functional materials. The authors’ experiences in developing structure-battery materials have led to several rules that can help in guiding the material designer to achieve maximal gains in system performance through multi-functionality. Analysis tools have been developed that are useful for ranking the structural performance of multifunctional composite materials. Material-architecture indices can be plotted against other functional properties to assess multifunctional performance. Rules 1. Add New Functionality to the Material with the More Complex Function New structure-plus multifunctional material systems can be created by adding functionality to an existing material. That is, new functionality can be added to an existing structural material or structure function can be added to “non-structural” materials with other primary functionalities. The best multifunctional performance can be obtained by adding the new functionality to the material whose primary function arises from more physically/chemically complex phenomena. Determining which function is more complex, structure or other, is not always easy and usually depends on the system particulars. 2. Target Unifunctional Subsystem Materials/Components Operating in the Mid-to-Lower Functional Performance Regimes for Replacement by Multifunctional Materials/Components In many cases, the structural properties of a multifunctional structure-plus material may not achieve the high levels possible with an optimized unifunctional structural material. One must intelligently select the regions of unifunctional structure for replacement by structure-plus materials in order to realize the best possible gains from multifunctionality. For example, replacement of material subjected to lower structural loads (e.g., secondary structure) may achieve the largest system performance gains. Multifunctional design must focus on system performance as the primary objective. This sometimes results in a decrease in sub-system performance. 3. Implement Multifunctionality in the Conceptual Stage of System Design Multifunctionality can be more effective in improving system performance when it is implemented during the system’s conceptual design stage due to greater flexibility in integrating sub-system materials and functions at this point in the process. Tools The Materials Selection approach developed by Ashby et al.6,7 is a useful method for ranking the performance of materials and cross-section shape with respect to system-related objectives like mass, volume, or cost. Modifications are needed, however, for non-homogenized composite materials because of the interrelated way that the material and the cross-section geometry variables mix in the basic mechanics expressions for stresses and deflections.8,9 Many multifunctional materials (systems) are composite in nature. Their functional capabilities depend on the constituent compositions, functionalities, interactions, etc. and on their arrangement (architecture) within the system. The authors have developed a new methodology for deriving performance indices that can be used to rank composite material design configurations relative to a user-defined objective (e.g., minimize mass) under a variety of loading and deflection constraint conditions.8 These new performance metrics for composite materials are denoted as material-architecture indices (MA). The methodology is an extension of the Ashby methods wherein modulus-weighted cross-section parameters are employed in the mechanical analysis allowing for the direct use of standard "mechanics of materials" equations. Consider the selection of constituents and the cross-section arrangement for a composite beam of minimum mass with maximal bending stiffness. The system (beam) mass, m, is inversely proportional to the MA index, as shown in Equation 3,8 where Equation 4 is the material-architecture index for a composite beam in bending. In Equation 4, ER is an arbitrary reference modulus, ρi are the cross-section component’s densities, Ai are the cross-section areas, and I*zz is the modulus-weighted second moment of inertia of area about the bending axis.8,9 The composite shape factor, ΦB*e := 4p I*zz/A2, accounts for the effect of the cross-section component modulus values, geometries, and arrangements on the bending performance. The MA indices are independent from the load-related variables and reduce to the classical Ashby material-shape indices when there is only one material constituent. There are multiple design (free) variables in the expression for MA that lead to the need for more sophisticated material-optimization procedures. The additional degrees of freedom provide an opportunity, however, to achieve superior material performance through the arrangement of the cross-section architecture, especially in multifunctional composites where large constituent property differences can exist.

Equation 1 shows that combining the battery with one of the other subsystems can provide an increase in flight time by increasing the available energy and/or decreasing the vehicle weight. Empirical data on the weight fractions of various aircraft subsystems show that structure and fuel (battery) each contribute 20–40% to the total weight of the aircraft (UAV).³ This supports the notion of achieving increases in UAV flight time by combining structure with battery in a multifunctional material. The use of a multifunctional structure-battery in a UAV influences the flight endurance time through changes in available energy and the structure and battery weights. The normalized change in flight time endurance, as illustrated by Equation 2,³ shows that decreases in vehicle weight are one-and-a-half times more effective in increasing endurance than increases in the stored battery energy capacity.

Other electric energy storage devices such as fuel cells, super/ultracapacitors, or capacitors may also be usefully employed in multifunctional structure energy roles.⁴ Each of these devices has different power-energy capabilities (Figure 1). Some low-power systems (<<1 W) such as remote or embedded sensors may be difficult to access and may be required to function over extended periods of time (e.g., years). Primary battery cells are used, but have a limited life. Hard-wiring to an external power supply, where possible, solves the life problem but may introduce unacceptable structural, manufacturing, and reliability compromises. Integrated energy harvesting devices like piezo-vibration generators are being investigated for this purpose. Mid-range power applications (~1–1,000 Ws), such as electric UAVs, have stringent weight restrictions and mission times on the order of hours. High-specific-energy batteries (polymer lithium-ion cells) are currently serving this need as unifunctional energy sources or in the form of a multifunctional structure-battery. There is also significant interest in developing fuel cells for these medium-power applications. High-power applications (>1,000 Ws) generally involve large systems and vehicles with weight and size restrictions, but they have large structures that may be integrated with an energy storage capability (structure-capacitor).

This overview focuses primarily on plastic lithium-ion structure-battery materials and their use in the Defense Advanced Research Projects Agency (DARPA) Wasp micro-air vehicle (MAV). Three main topic areas are considered: rules and tools for analysis and design of multifunctional materials; multifunctional structure-battery material systems; and structure-battery in the Wasp MAV. See the sidebar for details on multifunctional material design.

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MULTIFUNCTIONAL STRUCTURE-BATTERY MATERIAL SYSTEMS

The authors believe that adding structure function to an existing battery system is the best strategy for creating structure-battery materials due to the complexity of the energy storage process relative to that of elastic deformation (Rule #1 in the sidebar). Figure 2 shows the energy storage capacity for various primary (one-time-use) and secondary (rechargeable) battery cells normalized by volume and mass. For rechargeable cells, the lithium batteries stand out with their high energy storage capacities. The polymer lithium-ion intercalation cells (Li-Ion (S)) are particularly desirable because of their layered construction, soft packaging, safe-failure modes, and widespread commercial availability.

Polymer Lithium-ion Cells
Polymer lithium-ion cells¹⁰ consist of energy storing bicell layers (Figure 3) that are stacked and vacuum-sealed in a flexible laminate packaging. The anode and cathode layers consist of particles of lithium intercalation materials bonded together to a metal current collector foil or grid by small volume fractions of polymer “glue.” The proportion of active materials is maximized to obtain the best energy-storage capacity, and the thickness of each layer is optimized to achieve the desired power-delivery performance. Thinner layers allow for higher power delivery rates because of the decrease in distance the ions must diffuse during the discharge process; however, the energy-storage capacity also decreases. The anode and cathode layers are separated by a thin layer of microporous polyolefin that allows for ion passage but not electron transfer. The individual layers can be cut to arbitrary shapes, stacked, and then bonded together to form complex three-dimensional geometries that can be conformally integrated as system structure. Chemically compatible structural materials can also be added to enhance mechanical performance and facilitate external connections. The current collecting foils are joined to the positive and negative electrode terminals that exit the cell. A lithium-containing electrolyte is added in a zero-humidity environment and absorbed by the active layers so that there is no free liquid. The entire stacked assembly is then vacuum-packed in a heat-sealing laminate with only the electrode terminals exiting. The packaging laminate always includes one or more aluminum foil layers to prevent moisture ingress to the highly hygroscopic and reactive electrolyte. Some commercial and custom plastic lithium-ion cells are shown in Figure 4.

The open-circuit potential of the cells at full charge is 4.2 V. This rapidly drops at the start of discharge then slowly decreases until the end of discharge at 3.0 V. These lithium-ion cells have to be recharged using a specific charging protocol and must not be discharged below ~3.0 V to avoid irreversible damage to their energy-storage capabilities. Cells in the ~100–4,000 mAh capacity range are readily available commercially and can be combined in serial and parallel configurations to get a wide range of voltages and capacities.

Electrical energy storage devices are performance rated by their “Ragone” data¹⁰ that shows specific energy delivery capacity versus specific power discharge rate. Figure 1 is an example of a Ragone plot. In general, the quicker the energy is discharged, the less that can be delivered. The best energy storage performance is that which provides the most energy at the largest power discharge rates.

Figure 5 shows Ragone data for the custom and commercial cells shown in Figure 4. The tests are conducted on freshly charged cells (starting voltage ~4.2 V). Constant current discharge is maintained until the terminal voltage reaches 3.0 V. Incrementally measured current-voltage data pairs are used to calculate power, which is averaged over the test period to obtain the power discharge rate. The commercial Thunder Power cells show the best performance, followed closely by E-Tec and then Kokam cells. The Kokam custom cells also show very good performance. Note that the measured energy capacity at the higher discharge rates (currents greater than two times the nominal cell capacity) can be underestimated due to significant voltage drops at the cell terminals.

The mechanical performance of plastic lithium-ion cells is governed by the material properties of constituent layers as well as their arrangement and method of construction. Figure 6 shows several cells in their disassembled state. The Philips Lithylene™ cell uses a patented construction that effectively rivets the layers together in the thickness direction. The E-Tec cells use a continuous roll construction, and the ThunderPower and Kokam cells use stacked-layer construction. Achieving good mechanical bending stiffness requires the prevention of layer sliding by shear. Based on the cell constructions shown in Figure 6, the Philips cells are expected to exhibit the best bending stiffness followed by the E-Tec and then Kokam and ThunderPower cells. Experimentation to verify this ranking is needed.

One difficulty in experimentally characterizing the mechanical performance of structure-battery materials is developing test protocols that are meaningful from an applications point of view. That is, experimental data should be available that is applicable to the way the material might be loaded in service. This raises the question: how might these cells be used as a mechanical structure? The commercial rectangular cells may serve as the webs for beams in bending and as shear panels in lightweight truss structures. The applications for custom cells are more diverse as they can be more easily integrated into a variety of structural locations.

Qidwai et al.¹¹ have made a preliminary examination of the multifunctional potential of commercial cells acting as shear panels and spar caps. Shear experiments were performed to quantify the mechanical behavior of the commercial cells shown in Figure 4. The free packaging at the side edges of the cells was bonded to a rigid four-bar shear fixture using pressure-sensitive adhesive or epoxy (Figure 7). Achieving good bonds between packaging and the test fixture was difficult. The pressure-sensitive adhesive tended to shear excessively under load and the epoxy tended to peel off from the packaging. Polypropylene, polyethylene, and nylon are often used as the outer layer of the soft battery packaging laminate materials, and these materials can be hard to bond to without special surface treatments. Figure 8 shows the load-deformation behavior for the four-bar shear test. The epoxy-bonded cells sustained larger loads relative to their adhesive counterparts. However, progressive debonding was observed in all tests. The load-displacement response was likely dominated by the bond and not the battery cell or packaging material. The Thunder Power cells showed the best performance followed by Kokam and E-Tec cells. The results correlate with the size of the bonded tab areas (side packaging length times tab width). Some of the cells exhibited local wrinkling of the packaging, but no loss of vacuum or change in open-circuit voltage was experienced.

Structure-Battery Design Example
The previously described analysis tools can be used to rank the multifunctional performance of polymer lithium-ion structure-battery configurations (Figure 9). The basic design consists of one or more bicell layers that are stacked and packed with and without mechanical reinforcement. The “control” configuration consists of the bicell layers and packaging with no mechanical reinforcement. A second configuration incorporates a carbon fiber cloth layer between the packaging and the bicell core stack. The third incorporates a carbon-epoxy layer on the outside of the packaging. The reinforcement layers are approximately 0.2 mm thick, about one-half of the thickness of one bicell layer. ⁸

The multifunctional performance of the structure-battery beams is shown in Figure 10, which is a plot of MA for bending stiffness versus specific energy. Both MA and specific energy are inversely proportional to the mass given a specified limit on the bending deflection and a required energy storage capacity. The best multifunctional performance (lowest mass in this case) corresponds to the upper right-hand region of the plot. The carbon-epoxy reinforced structure-battery has the best stiffness per unit mass, and the plain packaging has the best energy capacity per unit mass. Specific energy increases with number of bicell layers in all configurations due to the increasing fraction of energy-storage materials in the laminate. Energy capacity approaches that of the bicell alone as the number of bicell layers increases (~165 Wh/kg in this analysis).

The selection of an appropriate material design configuration will depend on system requirements related to bending stiffness and energy storage plus possible constraints (e.g., beam thickness). The carbon-epoxy reinforced structure-battery is best for systems with high mechanical performance requirements. The plain-packaged structure-battery is best for light-to-moderately loaded systems with high energy requirements and restrictions on beam thickness. The carbon-epoxy structure-battery is probably the best overall multifunctional performer. An important advantage of this configuration is that mechanical reinforcement is external to the battery cell and can be done after the battery is fabricated. The maximum curing temperature, however, must be kept less than 80–90°C to avoid damaging the cell.

STRUCTURE-BATTERY IN THE WASP MICRO-AIR VEHICLE

A multifunctional structure-battery is used in the Wasp micro-air vehicle developed under DARPA sponsorship.¹² The Wasp is a radio-controlled “flying-wing” aircraft being developed for military reconnaissance and surveillance missions by AeroVironment, Inc. of Simi Valley, California. The original prototype, shown in Figure 11, has a wingspan of ~32 cm and weighs 171 g with 98 g of polymer lithium-ion structure-battery in the wing. It achieved a record-setting endurance time of 1 h 47 min. on one charge. The four embedded structure-battery cells were fabricated by Telcordia Technologies of Red Bank, New Jersey and were combined in a two parallel-two-series configuration giving 1.8 Ah capacity at ~7.5 V with an average power draw rate during steady-level flight of 7.6 W. The specific energy of the cells was 136 Wh/kg.

A multidisciplinary design optimization (MDO) analysis based on Equation 1 was used to assess the influence of the combination of structure-battery on the flight endurance time.¹² Three equal-weight designs were considered in the analysis: the original Wasp shown in Figure 11; a notional Wasp with structure-battery in the upper wing skin only to minimize packaging weight; and a notional unifunctional Wasp with commercial Kokam polymer lithium-ion cells. The efficiency of the motor/propeller was taken to be constant in all three designs, and the battery output voltages and total aircraft weight was the same for all three vehicles. All of the avionics and shape/geometry parameters were held constant in the analysis. For the unifunctional design, 20 g of weight was added to account for wing skin structure and battery containment/carriage; the weight of the Kokam cells was therefore reduced by 20 g to maintain constant overall vehicle weight.

The results of the analysis are shown in Figure 12. The predicted endurance for the notional multifunctional Wasp (II) is the largest at 126 min. The notional unifunctional Wasp (Kokam) had a predicted endurance of 115 min. An important consideration is the different battery-specific energies assigned to each of the three aircraft. These values were assigned based on actual measurements. Equation 2 can be used to modify the MDO results of Figure 12 so that they correspond to analyses performed with identical battery-specific energies. When the battery-specific energies are equalized, a 26% increase in flight endurance time is predicted for the multifunctional Wasp.¹² The predicted increase in flight endurance time for the multifunctional design is independent of battery-specific energy values but does depend on the other variables in Equation 1 (e.g., subsystem weights, aerodynamics, and propeller/motor efficiencies). The important point is that the multifunctional advantage is independent of the specific energy values, assuming they are the same for both designs, but does depend on other variables in the design implementation.

Current versions of the Wasp are now utilizing the new Kokam custom cells, shown in Figure 4. These long, narrow rectangular cells are sheathed in a thin fiberglass blanket and reside interior to the wing at the leading edge. This new design allows for easy replacement of the cells. The most recent Wasp endurance flights achieved 129 min. of sustained flight, including two unplanned takeoffs and landings. The MDO code predicted 145 min. of flight.

TECHNOLOGICAL BOTTLENECKS AND FUTURE CHALLENGES

Perhaps the biggest bottleneck in developing multifunctional structure-battery applications is the difficulty in procuring custom cells. There are, at present, only a few manufacturers of polymer lithium-ion batteries willing to even consider making custom cells. Commercial battery production runs involve large numbers of cells (e.g., thousands to millions). Small lot custom runs (<10–100) can be very costly. The custom cells purchased for this study have cost several thousand dollars per cell. This is due to the fact that they are made in very small lots in the laboratory by hand. Larger fabrication quantities using automated processing would certainly lower the cost.

Improving and optimizing the multifunctional performance of structure-battery materials is a long-range challenge. From a structural point of view, the critical issues are, first, achievement of good load transfer from the structure through the structure-battery packaging to the active energy-storage materials inside of the packaging, and, second, possible mechanical reinforcement schemes. Packaging is required for all battery cells, and load transfer through the packaging is a problem. The active electrode materials are particles that are held together by some sort of glue, which places severe limitations on their mechanical performance potential. With these types of materials, the best mechanical performance comes through providing volumetric constraint (e.g., like the core material in a skinned wing beam). Incorporating mechanical reinforcement layers with through-the thickness“riveting” similar to the Philips Lithylene technology would be a useful approach in enhancing the mechanical performance of these materials.

From an electrical point of view, the technology must move toward higher specific energies with corresponding increases in specific power. This is an ongoing focus within the energy storage research community. There is active research on hybrid technologies¹³ that combine battery and supercapacitor electrodes to achieve improvements in performance. The authors are aware of research on developing three-dimensional lithium-ion battery cells with networked electrodes that have improved power delivery capabilities. The nature of these materials may make them particularly useful as core materials for beams and components. There is also ongoing research with carbon-carbon composite anode materials that may offer much better mechanical benefits.¹⁴

Finally, there is a need to develop procedures for assessing multifunctional efficacy. To make this determination requires performance estimates for equivalent multifunctional and unifunctional designs. Ideally, the comparison should be between system performance data and/or estimates for the “best” of each design type. Obtaining these estimates/data without going through two separate design and build stages is the challenge. Even the most simple of systems can have multiple objectives and complex subsystem interactions. Fortunately, the electric UAV systems have a quantifiable system metric (flight endurance time), which when coupled with advanced MDO software and databases, can be used to make accurate endurance predictions for developing and refining multifunctional designs.

ACKNOWLEDGEMENTS

Support of this work by the DARPA Defense Sciences Office under the Synthetic Multifunctional Materials Program and by the Naval Research Laboratory under the Core Research Program is gratefully acknowledged.

REFERENCES