American Institute of Aeronautics and Astronautics1

Integrated Power for Microsystems

Charles D.E. Lakeman,* Patrick F. Fleig,† and Jenniffer L. Degreeff‡

TPL Inc, 3921 Academy Parkway North, NE, Albuquerque, NM 87109

Wireless microsystems, such as sensors for structural health monitoring and r.f. id devices for asset tracking, need efficient, integrated energy sources with long-life, high power density, and small size. Most approaches to integrating power on a small scale fall far short of the necessary energy or power densities. Our approach fully exploits the third dimension to maximize the available energy and power densities. Both microbatteries and microsupercapacitors can be integrated on chip and deliver a unique, low-cost solution to integrated power for a variety of stand-alone microsystems. In this paper we present the performance of zinc-air, lithium and lithium ion microbatteries, and both aqueous and organic electrolyte microsupercapacitors. Furthermore, we also discuss size, power, endurance and cost trade-offs for design of power systems optimized for mission requirements. The analysis shows that electrochemical devices are capable of delivering low-cost power for many microsystems with mission durations of several weeks to several months. We also show that our patented microsupercapacitors efficiently deliver high power pulses, help stabilize the supply voltage under load and minimize the overall power supply volume. These novel devices are also critical to enabling “energy harvesting” devices to deliver small size power supplies for long duration missions.

NomenclatureESR = equivalent series resistance (Ω)C = capacitance (mF)C* = capacitance density (mF/mm3)P = power (mW)P* = power density (mW/mm3)E = stored energy (mJ)E* = energy density (J/mm3)V = voltage (V)∆V = voltage fluctuation under load (V)

I. Introduction

EMS, and microsystems in general, are revolutionizing complex space, defense and civilian engineering systems by enabling miniaturized accelerometers, gyroscopes, r.f. id beacons and numerous other innovative

sensors and actuators.1, 2, 3 Furthermore, the advancement of wireless communication technologies opens the possibility of completely wireless systems, eliminating the cost, weight and potential for failure of conventional wiring.4 For many applications space and weight are a premium; however, while sensors, processors and communications devices have followed a continuous path of miniaturization, similar developments in power technologies have not kept up. Even the smallest commercially available battery dwarfs these tiny devices, and many miniaturized power systems currently being developed are unable to deliver useful capacities or power levels. Therefore, there is a need for efficient, integrated energy sources with long-life, high power density, and small size.

There are many approaches to integrating power in small volume/small footprint packages ranging from simple, low cost batteries to advanced, complex engineered systems such as combustion engines, miniature turbines, various

* Program Manager, Micropower Technologies† Advanced Scientist, Micropower Technologies‡ Staff Scientist, Micropower Technologies

M

CANEUS 2004--Conference on Micro-Nano-Technologies1 - 5 November 2004, Monterey, California

AIAA 2004-6733

Copyright © 2004 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

American Institute of Aeronautics and Astronautics2

energy harvesting technologies (solar, thermal, impact, etc.), micro-fuel cells, and radioisotope generators (so-called “mini- or micro-nukes”). Although proponents of each system promote their solution as the answer to small power challenges, as in the macro-scale world, there will be no “universal” micropower source suitable for use in all microsystems. Power supply engineering for microsystems will involve striking a balance between numerous factors including total energy budget, desired lifetime, availability and quality of ambient energy sources (solar, thermal, vibration, etc.), transient power demands, size, reliability, shelf life, and, of course, cost. Therefore, an intelligent approach to engineering power for microsystems will involve appropriate combinations of electrochemical and energy harvesting devices.

Wireless sensors essentially perform two basic functions: they collect data and communicate or transmit the data to some remote monitoring system. The power demands for these two functions are significantly different. The former usually occurs continually (steady state), and draws microwatts (µW) of power or less; wireless data transmission, on the other hand, can require several milliwatts (mW) or more, but usually with very short duty cycles (e.g., <<1%). In order to accommodate these two disparate power needs, designers usually specify a battery or other power source capable of handling the largest power demand. In this situation, the power source is oversized for most of the operational lifetime and capabilities of the system.

TPL has designed and built integrated micropower supplies based on patented volumetric microbatteries and microsupercapacitors that fully exploit the third dimension to maximize the energy and power densities for a given footprint. Our 3-D microbatteries have high energy density for a given footprint, while our microsupercapacitors efficiently deliver high power pulses and help stabilize the supply voltage under load. Both devices can be integrated on chip or onto printed circuit boards (PCB) and deliver a unique, low-cost solution to integrated power for a variety of stand-alone microsystems. In this paper we describe the performance of microbatteries of various chemistries, as well as different microsupercapacitors. We demonstrate that combining both devices to create a MEMSElectrochemical Power Supply (MEPS) uniquely meets both steady state and transient power needs in a minimum volume system. We also discuss the size, power, endurance and cost trade-offs for design of power systems optimized for mission requirements. Our analysis shows that electrochemical devices are capable of delivering low-cost power for many microsystems with mission durations of several weeks up to several months. We also show that energy harvesting devices alone (e.g., solar, thermal or vibration) are unable to efficiently meet the range of power demands for wireless microsystems. Our microsupercapacitors provide a unique combination of energy storage capability and high power density that enable miniaturized energy harvesting-based power supplies for long duration missions.

II. Device Performance

A. Microbatteries Figure 1a) is a photograph of our batteries (supercapacitors have a similar configuration) with a penny and the

smallest available hearing aid battery for comparison. Figures 1b) and 1c) show the performance of the 0.1cm3 and 2mm3 zinc-air (Zn-air) cells, respectively. The smallest cells illustrated in Figure 1 have an inner diameter of 1.8mm, and are 600µm tall. Primary lithium (Li) and secondary Li-ion batteries have similar configurations. Under a constant current drain of 128µA, these devices deliver a capacity to 1V of ~2mAhr and can last nearly 16 hours. By comparison thin film batteries with similar footprint (1.9mm x 1.3mm) delivered~3µAhr in 10 seconds.5 Figure 1 also shows that our microdevices deliver the desirable flat voltage discharge curve that is characteristic of Zn-air batteries. The performance of our different microbattery chemistries is summarized in Table 1.

Voltage (V) E* (J/mm3) E* (Whr/L) P* (mW/mm3) Cycles

TPL Zn-air 1.4 4.6 – 5.2 1300-1440 0.088

COTS Zn-air6 1.4 4.5 – 5.1 1250-1400 0.006

TPL Li† 3.0 1.1 – 2.0 305-550 0.006

COTS Li6 3.0 1.38-2.01 380-560 0.01

TPL Li-ion† 4.1 0.71 200 0.008 10

COTS Li-ion6 3.7-4.2 1.01 280 0.560 >1000

Table 1 Performance of Microbatteries († preliminary data)

American Institute of Aeronautics and Astronautics3

B. Micro-Supercapacitors. Figure 2 shows a schematic representation of microsupercapacitor design and operation. Like batteries,

supercapacitors are electrochemical devices; however, rather than generating a voltage from a chemical reaction, supercapacitors store energy by separating charged species in an electrolyte (Figure 2). As with COTS batteries, COTS supercapacitors are far too bulky for integration at the volume and length scales needed for stand-alone, wireless microsystems. Our patented MEMS supercapacitor can be integrated on-chip and with energy generation devices (batteries and energy harvesting) to deliver a clean, efficient energy delivery system. Supercapacitor electrochemistry is well understood and scales readily to cubic millimeter volumes7. In supercapacitors, the useful electric charge is stored in thin electric double layers at each electrode/electrolyte interface. Thus, a design goal for supercapacitors is to maximize surface area in a given volume in order to maximize the stored energy. Our 3-D approach, employing with high surface area carbon electrodes, provides a large electrolyte-electrode interfacial area and generates high capacitance (and therefore energy and power) densities (Table 2).

The aqueous electrolyte is a 30wt% KOH solution, while the non-aqueous electrolyte consists of a 1.0M solution of tetraethylammonium tetra fluoroborate (TEATFB) in propylene carbonate (PC). It should be noted that while supercapacitors that employ organic electrolytes can operate at higher voltages (i.e., 2.5 - 3V) and can, therefore, store more energy (E = ½CV2),

Figure 2 Schematic illustration of electrochemical supercapacitors

Carbon aerogelelectrode Electrolyte

Separator film

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Figure 1 a) TPL’s micro electrochemical devices can be integrated on various substrates. Hearing aid battery and penny for scale b) Performance of 0.1cm3 cells; c) Performance of 2mm3 cells.

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American Institute of Aeronautics and Astronautics4

the larger ionic size leads to decreased capacitance density, and the electrolytes’ higher resistivity contributes to higher equivalent series resistance (ESR) values. Therefore, an important design consideration is minimization of the ESR. So far, we have produced aqueous capacitors with ESR values as low as 1.8 .

As-constructed cells were charged using a constant current of 5mA until they reach the target voltage (1.0V for aqueous, 2.5V for organic) at which point, they were charged at a constant voltage until the current falls to 100µA. The capacitors were then discharged into different loads to evaluate their power delivery capabilities and the time to self-discharge (open-circuit) to 80% of the charging voltage. This charge-discharge routine is cycled to evaluate capacitance stability under load.

Figure 3 compares TPL’s 560mF microsupercapacitors with a commercial 470mF device and Table 2 summarizes the properties of both aqueous and organic electrolyte devices. These data show that, so far, aqueous devices have superior capacitance, ESR, power and self-discharge behavior compared with organic devices. However, because of the higher operating voltage, the energy stored in the organic devices (22mJ/mm3) is higher than aqueous (9.2mJ/ mm3).

Capacitance (mF) ESR (Ω)Power (mW)C* (mF/mm3)P* (mW/mm3)Aqueous (1.0V) 589±7 2.9±0.3 30.5±1.7 18.4±0.2 0.905±0.005

Organic (2.5V) 241±17 32±4 14.5±0.8 7.15±0.5 0.430±0.023

Table 2 Properties of TPL Microsupercapacitors

C. Hybrid Power SuppliesA combination of a supercapacitor and a battery in parallel delivers power in a volume-efficient package. Under

high loads, most of the energy is delivered by the supercapacitor, because of the difference in internal resistance of the two devices. Using a supercapacitor reduces the voltage drop across the battery under load, and extends battery life.

The performance of a battery-supercapacitor hybrid system was compared to the performance of a battery on its own (Figure 4). Two pulse profiles were used: 1mW and 10mW for 1 second each. Figure 4a illustrates how, after only a few cycles, the voltage drop of the battery alone under load starts increasing, and would, eventually, fall below a useful operating level. The hybrid, which included the supercapacitor, showed less of a voltage drop during each cycle. Under more severe conditions (Figure 4b) the benefits of the supercapacitor are more noticeable. Delivering a 1 second 10mW pulse, the hybrid voltage dropped to only about 1V, while voltage on the battery alone

Figure 4 Supercapacitor reduces voltage drop under load

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Figure 3 TPL’s microsupercapacitors (~560mF, 1.0V, ESR~2 ). COTS 470mF, 2.5V supercapacitor and penny shown for scale.

fully assembled

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American Institute of Aeronautics and Astronautics5

dropped to below 0.3V. The supercapacitor delivers energy more efficiently than the battery, which can be seen as less of a voltage drop under peak load.

We have also demonstrated the feasibility of combining an energy harvesting technology with microsupercapacitors using solar cells - readily available COTS devices. As-received 1cm x 1cm solar cells from Solar World, Colorado Springs, CO (www.solar-world.com) generated ~0.5V and delivered ~25mA (equivalent to 0.238mW/mm3). Cells were connected in series to deliver ~1.5V. These cells were tested under a profile of 1s constant power pulse loads starting at 1mW, 5mW, and increasing in 5mW increments up to 100mW. In one configuration, the solar cells were tested alone and compared with the performance of a system in which the solar cell was used to charge a 470mF supercapacitor that was used to deliver high power pulses. Figure 5 shows that supercapacitors clearly improve the power delivery capabilities of this system.

III. Micro-Power Supply AnalysisThe critical challenge for delivering power in a small size package is the trade-off between size, peak power

capability, lifetime and cost. Batteries provide a convenient, low cost portable means of storing and delivering electrical energy; however, they face well-known limitations in both capacity (lifetime) and specific power (power per unit volume, P*, W/mm3), and in a small volume, both are substantially compromised. Energy harvesting devices such as solar cells, or other systems currently in development (e.g., vibration, r.f., thermal), can provide long lifetimes (assuming a constant, high quality, source of energy); however, these devices are more expensive than batteries and suffer from low specific power values (typically significantly less than 1mW/mm3). Fuel cells are the third approach being developed for providing power on a small scale. While these devices can provide high energy density, can be easily recharged, and the reactors can deliver relatively high specific power values, they are inevitably encumbered by peripheral equipment such as fuel reformers or heat sinks that drastically reduce the overall energy density and specific power.

In the macro-world in which we live, energy sources are usually designed to be large enough to meet the highest anticipated power demand. This is an inefficient approach that results in an energy source that is oversized for much of the operation of the system it is powering. Moreover, for miniaturized systems with strict volume, footprint and weight constraints, this is a fundamentally impractical approach.

TPL’s micro-supercapacitors provide an elegant solution to this problem.

A. The Challenge for Small PowerAll small power sources are limited in their specific power (P*, mW/mm3). Table 3 compares reported specific

power (P*) and maximum specific power (P*max) for different energy sources and the corresponding volume that would be needed to deliver 500mW (v500mW) transient pulse. These data clearly show that no energy source on its own (i.e., without a supercapacitor) is able to meet transient high power needs in a small size package.

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Figure 5 Performance of COTS solar cells under constant power loads. a) solar cells alone; b) combined with a 470mF supercapacitor.

a) b)

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P*(mW/mm3)

P*max

(mW/mm3)v500mW

(mm3)

COTS Zn-Air6 0.006 0.053 9,434COTS Li-ion6 0.056 0.560 892Solar Cell (sunny) 8 0.094 0.236 224P3 (Washington State University)9 0.003 0.035 14,285Solar Cell (cloudy) 8 0.001 0.0024 208,333Radioisotope (Qynergy BEC) 0.0003 0.001 500,000Solar Cell (indoors) 8 0.00047 0.0001 5,000,000

We have developed a model to describe the endurance-size-power trade-offs in MEMS-scale power supplies. This model can serve as a useful design tool to select the optimum configuration power supply for a given application, and demonstrates the benefit of microsupercapacitors for delivering transient power pulses. The user-defined variables are the volumetric packing efficiency (i.e., active material volume), choice of supercapacitor type (aqueous operating at 1V or organic operating at 2.5 - 3.0V) and power, duty cycle, operating voltage and tolerable voltage fluctuation (∆V) for different operations (e.g., sleep, sense, CPU, transmit, etc.). The calculation determines the size of the capacitor needed to meet the required power, duration, and voltage fluctuation needs, and the volume of the energy source needed to deliver sufficient power to recharge the capacitor during the sleep cycle. Figure 6 shows the results of these calculations for power supplies capable of delivering one 500mW pulse for up to 1s every 15 minutes. The white region indicates the regime in which the total power supply size is larger than 100mm3. These data show that generators with higher power densities (e.g., solar cells on sunny days) allow longer peak power operation for a given size or can realize a given performance in a smaller volume than those with low power densities.

This analysis did not consider combustion-based systems. These systems, while they exploit the very high energy density of (e.g.) hydrocarbon fuels, are inherently limited by the need for a system to deliver a consistent fuel supply as well as advanced thermal management to safely remove waste heat. These challenges can, of course, be overcome; however, the plumbing and additional system engineering adds considerably to their overall size and

Table 3 Power density values for energy sources

Figure 6 Total volume of power supplies which include a microsupercapacitor capable of delivering one 500mW pulse for up to 1s.

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complexity. For the purpose of this analysis, it was difficult to quantify the additional size and weight contributions of these auxiliary systems, and so accurate values for P* were unavailable. Nevertheless, it is unlikely that they can be miniaturized to meet the target 0.1cm3 size.

There are similar challenges with micro-fuel cells. Most types are intended for hydrogen fuel, as protons are the simplest ion to send through the electrolyte. As hydrogen is not readily available, other fuels (e.g., hydrocarbons, methanol or formic acid, or natural gas) can be re-formed with steam at high temperature (600°C) to yield hydrogen and CO. These reformers again add engineering complexity and require extensive insulation for both safety and efficient operation. Furthermore, reported data for micro-fuel cells indicate maximum peak power densities on the order of 50mW/cm2 but with a duration of less than 100ms.10

IV. ConclusionWe have designed, built, and characterized volumetric micropower devices (microbatteries and

microsupercapacitors) that can be used to provide power for volume- and weight constrained microsystems for space, defense, homeland security and commercial applications. Fully exploiting the 3rd dimension maximizes the stored energy and delivered power for a given footprint. These devices provide a low-cost power supply for microsystems with limited lifetime missions. Our microsupercapacitors also uniquely enable minimum volume power supplies based on energy harvesting technology.

AcknowledgmentsThis work has been funded by the National Science Foundation, Air Force Research Lab, DARPA, Missile

Defense Agency and NASA through SBIR grants and contracts.

References1Gates, B., “The disappearing computer,” The Economist, Special Issue: The World in 2003, December 2002, p. 99. 2Rabaey, J. M., Ammer, M. J., da Silva, J. L., Patel, D., Roundy S., 2000. PicoRadio Supports Ad Hoc Ultra-Low Power

Wireless Networking. IEEE Computer, Vol. 33, No. 7, p. 42-48.3 Collins, J., “Hitachi Unveils Integrated RFID Chip,” RFiD Journal, Sept. 4, 2003.4“Industrial Wireless Technology for the 21st Century,” US Department of Energy, December 20025Ryan, D., LaFollette, R.M., Salmon, L., "Microscopic Batteries for Micro ElectroMechanical Systems (MEMS),"

Proceedings of 32nd IECEC, 97-8, 97136, Honolulu, HI, August (1997).6Linden, D.B., Reddy, T., Batteries Handbook, 3rd ed., McGraw-Hill, New York, 2002, Chaps 13, 14, 35, 38.7Conway, B.E., Electrochemical Supercapacitors, Kluwer Academic/Plenum Publishers, 1999.8Roundy, S., Wright, P.K., Rabaey, J., “A study of low level vibrations as a power source for wireless sensor nodes,”

Computer Communications, Vol. 26 (2003) 1131–1144.9Whalen, S., Thompson, M., Bahr, D., Richards, C., and Richards, R., “Design, fabrication and testing of the P3 microheat

engine,” Sensors and Actuators A Vol. 104 (2003) 290–29810Adams, B., Ha, S., Zhu, Y., Larsen, R., Shannon, M., Wieckowski, A., Masel, R., “Formic Acid Fuel Cells: New

Possibilities for Portable Power,” Proceedings of the 41st Power Sources Conference, Army Research Lab., 2004, 428 – 430.