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TOPIC 3: DEVICE EXPERIENCE 1 1 Three presentations were made in this session, by Esther Takeuchi of Wilson Greatbach Technologies, Inc., Nancy Dummy of Oak Ridge National Laboratory, and Robert Nowak, a consultant formerly with DARPA. Their papers are summarized below. POWER SOURCES USED FOR HUMAN IMPLANTS Esther Takeuchi discussed power sources for human implants, m~crobatteries, opportunities for nanomaterials, and alternative energy-harvesting approaches. Power sources for implantable biomedical devices must have S+ years longevity (10-12 years is desirable); they must be small; and they must be capable of providing information on their status in response to an external query. All implantable batteries in use today are Li-based primary systems, with the vast majority being Li-I2. For some applications, the battery must deliver more than 1 Ah over 5 years. If one could recharge the battery (e.g., via external RF power through the skin), batteries could be smaller (microbatteries) and device lifetimes of 10-20 years might be achievable. However, there are operational and psychological advantages to imolantable batteries that do not need to be recharged. A solicl state Li-ion battery based on Oak Ridge technology is being commercialized. It achieves a cycle life greater than 50,000 cycles, an energy density of 200 Wh/kg, and a capacity up to 0.2 rnAh/cm2. In principle, the use of nanomaterials in conventional battery systems could! enhance power and discharge rate capability. This could be useful in applications such as heart defibrilIators, which require infrequent pulses of high power. To test this approach, a coin battery cell was prepared with a nanoparticle-sized silver vanadium oxide (SVO) cathode that resulted in a two- to fivefold lower tap density and an order of magnitude (or more) improvement in surface area compares} with a conventional SVO cell. In tests, the nano-SVO system actually produced a lower gravimetric and volumetric energy density than the conventional system. While the nano SVO cathode may not have been optimized, this raises the awareness that one cannot simply insert nanomaterials into conventional designs and expect performance to improve. Takeuchi went on to discuss a number of energy-harvesting possibilities, broken out into Category 1 (systems that could be coupled with a rechargeable cell or capacitor, such as solar cells or self-winding-watch-type mechanical energy) and Category 2 (systems with self-sustaining power levels, 11 .
1 12 Summary of the Power Systems Workshop that is, those that use pH or thermal gradients, biofuel cells, or implantable nuclear devices). She concluded that any new power system for implantable `devices has to compete with existing battery sources in cost, size, reliability, and predictability. One has somewhat more flexibility (e.g., to use rechargeable systems) in applications that are not life-critical. For all options, achieving a balance of high power and high energy density presents a significant challenge. For medical applications, the system management requirements of the patient and physician are key considerations. MICRO- AND NANOSCALE RECHARGEABLE BATTERIES: STATE OF THE ART AND CURRENT RESEARCH Nancy Chutney discussed recent and projected developments in miniaturized batteries. In the near term (<3 years), the action will continue to focus on thin-fiIm, solici-state batteries made by vapor deposited} layers of electrodes and electrolyte on a substrate (overall battery widths not including substrate of about 15 ~m). The Li-LixCoO2 system appears optimal and provides both low trickle current and high pulse power for applications such as data transfer. Since these batteries must be integrated into the device, the substrate is not counted in calculation of the performance metrics. For the best cells, capacity at 25°C for a 4-pm-thick LiCoO2 cathode is around 250 ,uAh/cm2, energy density is >1 mWh/cm2, and power densities (short duration) are around} 30 mW/cm2. These battery materials are proven, have uniform current densities, and can meet the needs of MEMS devices. Known challenges include (1) battery development on thinner, more flexible substrates, (2) stacking for creation of three-climensional batteries, (3) improving yield (currently only 10 percent), and (4) packaging. At present, there is insufficient market pull to drive low-cost solutions to these problems. Over the medium term (3-5 years), Chutney foresees development of thin-fiIm batteries on very high surface area substrates (e.g., fibers) achieving a 5- to 10-fold increase in substrate surface/volume. One application would be fibers with a battery coating that could be woven into textiles, e.g., for soldier power. Prototype fiber batteries ("powerfibers") embedded in a polymeric matrix have been demonstrated that are robust under flexing, and an 1 8-ft fiber today can provide 1 mWh of energy and 90 mW peak power. By 2004, an order of magnitude improvement is expected: a 500 x 500 array of fibers is expected to be able to deliver 3 A at 3 V, with a capacity of 100 mWh. Current work includes depositing thin-film batteries (crystalline LiCoO2-based) on carbon fiber tows for use in composites. Anticipated challenges include the stability of new materials for high surface areas, the uniformity of current (with attendant concerns about stability ant! utilization after many cycles) and flaws in connectivity caused either by fabrication or stresses from cycling. Costs of the fibers remain uncertain. Over the long term (more than 5 years), batteries constructed from nanomaterials (nanofibers and nanoporous structures) will be possible. Fabrication concepts include construction of three-dimensional nanoarchitectures by concentric tube template synthesis to form electrocle fibers, individual cells fabricated within micromachined wells, and self-organized nanoarchitectures using block copolymers with oxides or disordered, mesoporous active materials. In Du~ney's view, the biggest potential for revolutionary change will come from these self-organized nanostructures, especially if they can be made substrateless. These also have the greatest potential for cost savings. Challenges include those mentioned above as well as the difficulty of incorporating a current collector into the architecture.
Topic 3: Device Experience COMMITTEE ON SOLDIER POWER/ENERGY SYSTE1\IS AND PORTABLE/COMPACT POWER SYSTEMS 1 Robert Nowack reviewed recent work by a parallel NRC committee looking at soIctier power issues (he is a member of that committee). He noted that the Pentagon has a $100 million Objective Force Warrior (OFW) program under way, now in Phase {I, which is funded at $40 million. The aim of OFW is to enable soldiers to gather and process information more readily on the battlefield. However, soldiers are already overloaded with equipment (some special operations soldiers carry 120-140 pounds of gear), even without the electronics, batteries, and computers that OFW wants to acid on. Therefore, lightweight, compact systems are at a premium, and nanotechnology couIci help reduce the weight carried into the field. With the first OFW advanced technology demonstration scheduled for FY06, the power source must be "on the shelf' today. The Army is interested in a hybrid of Zn-air and Li-ion batteries, which together can extend a mission bY about 4 hours compared with the sum of the operating times (18.5 13 =7 , ~ hours) for the two indiviclual batteries. For units that require very high power loads, such as a radio reconnaissance team, portable, lightweight devices that convert logistic fuels to electrical power are preferable to batteries. Both fuel cells and heat engines are being considered here that use fuels such as methanol, hydrocarbons, and bottled hydrogen. For fuel cells, a logistic fuel preprocessor and reformer that converts fuels to hydrogen is being developed for 3- and 10-day missions. The most appropriate power supply depends on the mission for durations of less than a day, it is hare! to match the performance of a battery. For longer times, battery weight becomes prohibitive, and energy conversion devices such as direct methanol fuel cells make more sense. One can also reduce the demand for power by utilizing more efficient chips, displays, and so on. Small two-chamber diesel engines and generators have relatively low efficiency (~15 percent) but can operate on the impure diesel fuels that are likely to be available. There are 20 or 30 programs on power from m~croelectromechanical systems (MEMS) devices in the Uniter! States, including miniaturized Wankel, turbine, and linear engines and micro fuel reformers for fuel cells. Nowak concluded with a warning that all useful soldier power systems contain enough energy to cause serious damage in the event of an accident or the impact of a bullet. Standard battery systems can be improved in various ways, but no more than a twofold increase in energy density is foreseen. Lightweight, high-efficiency energy conversion cievices that can use air as the oxidizer are the key to making use of high-energy-density, convenient liquid fuels; however, these systems need to be designed for tactical robustness for example, to withstand immersion in water. The relatively advanced state of hydrogen/air fuel cells and the very high energy density of hydrogen justify the significant DoD effort to find new ways of delivering hydrogen to the soldier. Hybrid energy systems are likely in the future, and management of power demands is essential. TOPIC 3 DISCUSSION i Once again, the discussion following this panel began with the question, What comes next after L~-ion cells? The Army is enthusiastic about C-air cells for soldier power, although these involve a molten carbonate electrolyte at high temperature. The possibility of Be-air was mentioned, but the general feeling was that the toxicity of Be would make this unlikely, except perhaps for specialized applications. Manufacturers would be unlikely to consider making Be batteries because of Environmental Protection Agency (EPA) regulations, environmental health and safety issues, ant] cleanup costs. The Army wants to get rid of nuclear materials for energy generation, though it was noted that the most efficient way to go from nuclear to electric is by way of a Stirling engine. There floes not appear to be commercial demand for energy harvesting. In medical applications, Li batteries can provide 5-7 years of service, and given the rapid! advances in the field, few patients or
1 14 Summary of the Power Systems Workshop doctors want to use a crevice more than 7 years. For the military, everything depends on the mission: how long the crevice nee(ls to function, total energy, power, and duty cycle. For doing telemetry, a lot of power is needed, ant! this might need to be supplied externally. D ARPA has funded an exploration of energy-harvesting options; biofuel cells were rater! high for current density but raiser! sustainability concerns. For most missions, it is hard to beat a battery; for others with low power requirements, the few watts one could obtain by harvesting heel-strike energy might be enough. Nonhuman sources of bioenergy might be useful for missions lasting longer than about 30 clays. There were mixer! views about whether having a commercial market for a particular power source is essential. One view was that the power source almost has to be commercial to survive. Another view was that if the government determines that the goal is worth it (e.g., the satellite programs), it can sustain the necessary infrastructures and vendors. One must find the value that justifies the investment. It was noted that there is much to be learner! from biological processes, particularly in regard} to self-organization in fabricating nanostructures. Bone is formed by collagen that self-organizes, leaving holes that line up, and minerals intercalate into the aligned! structures. One can take advantage of such thermodynamically favorable processes in nanostructured materials. 1
