Powerelectronics 5422 Clean Energy Think 0

Research Sponsored by ARPA-E Seeks Next-Generation EV Power Technology

May 16, 2017
The Advanced Research Projects Agency-Energy invests in transformational ideas to create America’s future energy technologies. ARPA-E focuses exclusively on early-stage technologies that could fundamentally change the way we generate, use, and store energy.

ARPA-E seeks innovative ideas from academia, private industry, national labs, start-up companies, and small businesses—providing project teams with an average award of $2 million to $3 million over several years. Every project team receives hands-on guidance to meet ambitious technical milestones that push the boundaries of energy innovation. Its unique Technology-to-Market program also empowers project teams with business insight and strategies to accelerate the adoption of their potentially game-changing technologies. ARPA-E, in the Department of Energy (DoE), is modeled after the successful Defense Advanced Research Projects Agency (DARPA) in the Department of Defense (DoD).

Electric vehicles (EVs) are a major research effort for the ARPA-E as it relates to the generation, use, and storage of energy. One ARPA-E research program is called RANGE, or "Robust Affordable Next Generation Energy” storage systems. It is tasked with developing electrochemical energy storage technologies that will accelerate the widespread adoption of electric vehicles by improving their driving range, cost, and safety.

Central to the RANGE system-level approach is the use of robust design principles for energy storage systems. Robust design is defined as electrochemical energy storage chemistries and/or architectures (i.e., physical designs) that avoid thermal runaway and are immune to catastrophic failure regardless of manufacturing quality or abuse conditions. In addition, this program seeks multifunctional energy storage designs that use these robust storage systems to simultaneously serve other functions on a vehicle (for example, in the frame, body, and/or crumple zone), thus further reducing an energy storage system’s effective weight when normalized to the entire electric vehicle weight. Following is information obtained from the RANGE Program Overview.

ARPA-E points out that the U.S. transportation sector represents about 27% of all greenhouse gas emissions (GHG). However, even with today’s U.S. electric power generation mix of fossil, nuclear, and renewable energy sources, it is estimated that on a well-to-wheel basis an all-electric vehicle will generate 25% less GHG emissions than a conventional gasoline powered vehicle, with even lower emissions predicted by increased penetration of renewable energy sources.

Despite a compelling national imperative, electric vehicles have achieved little market penetration in the U.S., with EV sales representing less than 1% of approximately 14 million vehicles sold in the U.S. This is primarily due to the high initial purchase cost of EVs relative to gasoline-powered vehicles.  Also, the EV payback period for the average consumer is over seven years even after accounting for government subsidies.

The incremental cost of an electric vehicle compared with an internal combustion engine (ICE) vehicle, is dominated by the energy storage system. The widely used EV grade lithium-ion battery pack costs 400-800 $/kWh, so the EV’s price is directly related to the battery pack’s cost and size as well as the vehicle’s range.

ICE vehicles show no clear correlation between price and range largely because, unlike batteries, the size of the fuel tank and the amount of fuel contribute little to vehicle price or weight.

RANGE Program

RANGE program research efforts are devoted to increasing the specific energy of lithium-ion battery packs in order to extend EV range and reducing their cost (assuming the cost of materials and manufacturing do not significantly vary for higher specific energy systems). For long-range EVs, battery pack weight is a significant portion of the vehicle weight (1/4 to 1/3). A recent study showed that for every 10% in vehicle weight reduction, there is about a 5% reduction in energy consumption.

Besides efforts focused on cell-level innovations, battery pack-level research has also received increasing attention. In 2012, ARPA-E initiated the AMPED (Advanced Management and Protection for Energy Storage Devices) program, which focuses on improving lithium-ion battery cell utilization and state-of-health prognosis by developing advanced controls, models and sensors. If successful, the AMPED program will improve the usable specific energy of lithium-ion battery packs without changing their chemistry.

Traditional research approaches to enhancing electric vehicle energy storage systems have focused primarily on increasing cell-level specific energy density. But the cost and performance advantages of high specific energy cells are often offset by more demanding pack-level and system-level engineering requirements. Pack refers to the space (volume) associated with energy storage. To obtain high pack-level specific energies, you need to maximize both the cell performance and the packing factor. However, there is an inherent tradeoff between these two features. High specific energy batteries often employ high-voltage redox couples and flammable organic electrolytes. In these systems, the total combustible energy in a lithium-ion cell can be an order of magnitude greater than the stored electrochemical energy. Out of concerns for thermal runaway, thermal and electric control and management systems are employed, but these reduce the packing factor.

Higher specific energy cells often demand extensive mechanical protection to avoid intrusion and deformation. Consequently, packing factors tend to decrease as battery cell-level specific energy increases, resulting in additional vehicle-level structure and weight. In fact, it is common practice to design the battery pack as if it were a fuel tank (i.e., isolate the energy storage component from the rest of the vehicle) rather than as one component in a larger system. Additional protective structures significantly reduce the effective specific energy of the energy storage system. Despite these conservative system designs, several incidents of electric vehicle fires have raised concerns for consumers and illustrate the challenges of addressing abuse tolerances with ad hoc system-level engineering solutions. Consequently, concerns over battery cell life and thermal runaway require thermal and mechanical protection that can lead to lithium-ion battery packs costing >80% more than the cells, while the specific energy decreases by more than 40%.

High specific energy cells with flammable materials also suffer from increased materials and manufacturing costs resulting from limited robustness. The demanding high-voltage environments of high specific energy electrode couples require the use of costly, ultra-pure electrolytes and electrode materials. In addition, highly precise and reproducible manufacturing processes are critical to minimize defects that can lead to cell failure. Both the material and manufacturing aspects contribute significantly to the high cost of lithium-ion batteries.

 Lower-cost, longer-range EVs focus on energy storage systems with improved vehicle-level specific energy, rather than only cell or pack-level improvements. This approach requires the development of robust energy storage chemistries and architecture. Some examples include:

·         Development of an electrochemical energy storage chemistry that utilizes non-combustible aqueous or solid state electrolytes.

·         Use of a redox flow battery architecture that is inherently more robust due to the physical separation (storage) of its active components far from the cell electrodes.

·         Design of a mechanism that allows a battery to automatically fail in open circuit when placed under abuse conditions.

Robust designs have the potential to transform EV design and create new pathways to dramatically lower cost by:

·         Reducing the demands on system-level engineering and its associated weight and cost.

·         Liberating the energy storage system from the need for vehicle impact protection, which allows the energy storage to be positioned anywhere on the vehicle, thereby freeing-up the EV design.

·         Enabling multiple functions, such as assisting vehicle crash energy management and carrying structural load.

Multifunctional energy storage designs help reduce the effective weight of an EV energy storage system. In Fig. 1, the weight distribution of an EV powered by a traditional lithium-ion battery is compared to one powered by a robust, multifunctional battery. If the vehicle-level specific energy requirements are assumed to be the same, the reduction in protection/control overhead with the multifunctional battery relaxes the need for a high cell-level specific energy. This design freedom may allow the use of battery chemistries with lower theoretical specific energy that are not considered viable today. If these chemistries have inherently lower cost structures than lithium-ion batteries, a new set of EV energy storage technology solutions becomes possible.

To connect performance and cost requirements for robust battery systems with vehicle range and system costs, ARPA-E performed an analysis to set specific energy system and cost targets that would ultimately lead to electric vehicles at cost parity with ICE vehicles. Current performance requirements for lithium-ion batteries are usually defined at the pack level in industry technology development roadmaps such as those defined by USABC (United States Advanced Battery Consortium). Analysis performed for this program shows that a cost to manufacture target of 100 $/kWh and a specific energy of 150 Wh/kg on a pack level should enable a long range EV (> 240 miles) with competitive vehicle purchase price (<$30,000). For energy storage systems with multifunctional capabilities, these targets and metrics will still apply but require further clarification.

ARPA-E defines a new metric termed “effective specific energy.” If the energy storage system reduces the need for a structural part, the weight of the structural part is subtracted from the storage system weight. The difference is then used to calculate the specific energy. This new metric thus embodies the impact of synergistic interplay between energy storage and vehicle structure. Vehicles of the same weight equipped with batteries of the same “effective specific energy” will have the same vehicle level specific energy. Although a similar cost benefit is expected, the magnitude varies greatly (on the order of > 100%) with the actual vehicle platform. Internal ARPA-E analysis shows a cost benefit of > 25 $/kWh due to weight reductions by employing multifunctional design. Consequently, ARPA-E expects that a cost target of 125 $/kWh for a multifunctional energy storage system will enable achievement of the program goals of a >240 mile range at <$30,000 vehicle price.

Energy storage chemistries and architectures with robust design features include but are not limited to:

·         High specific energy aqueous batteries. Areas of particular interest are approaches to novel high specific energy cathode/anode redox couples; materials and device designs for long life metal-air systems; ultrahigh capacity negative electrode materials to replace La-Ni alloys in nickel metal hydride batteries; and organic and inorganic redox couples, including their hybrids.

·         Ceramic and other solid electrolyte batteries. Areas of particular interests are high-conductivity inorganic electrolytes for lithium and other alkaline metal ion systems; and solid-state and hybrid battery designs and low-cost manufacturing processes.

·         Other batteries completely without or with negligible combustible or flammable materials.

·         Materials and architectures that eliminate the possibility of thermal runaway.

·         Robust design architectures. Examples include flow cells and electrically rechargeable fuel cells, fail open circuited designs, non-propagating system architectures, and designs resulting in reductions in individual storage unit sizes and energy contents.

·         Hybridization of different energy storage chemistries and architectures to offer improved robustness including mechanical abuse tolerance.

Innovative designs optimally utilize energy storage systems to contribute to vehicle structural performance. Examples of technical approaches include but are not limited to:

·         Energy storage systems that assist vehicle impact energy management. Areas of particular interest are material, cell, pack, and system designs that act synergistically with the rest of the vehicle structure to manage mechanical impact. Energy absorption mechanisms may include deformation, disintegration, and disengagement by design.

·         Energy storage systems that act as structural members. In this case, the energy storage system may directly replace other structural members of the vehicle in the load path.

·         Energy storage systems that serve other vehicle functions not listed above.

A fully integrated energy storage unit should provide1 kWh or greater for EV energy storage systems. This involves several factors:

·         Cost to manufacture. To attain near cost parity between the EV and a gasoline powered vehicle, the cost to manufacture the energy storage system needs to be less than 125 $/kWh if they are multifunctional and 100 $/kWh if they are not. This cost includes the cost of materials as well as the cost of labor and facilities for manufacturing. It does not include profits for either the energy storage system maker or the automotive OEM.

·         Effective specific energy. The specific energy is calculated using the total usable energy measured at a C/3 rate divided by the mass of the whole energy storage system, including any control, thermal management system (hardware and fluid if used), and enclosure. A C/3 rate is defined as a current level to discharge the battery in 3 hours.

·         Effective energy density. Energy density is defined as the total usable energy measured at C/3 rate divided by the volume of the energy storage system. The dimensions of the system are defined by the outer boundaries.

·         Robustness. The primary criteria for the energy storage system is to pass a crush test to 50% of original dimension or at a 1 g-force of 1,000 times of battery mass without maximum temperature on any point of the inner or outer surfaces reaching either the flash point of any volatile component (both original and generated during operation) or the melting point of any solid component. This target is essentially to eliminate the possibility of thermal runaway and vehicle fire.

·         Cycle life at 80% DOD. Cycle life of  >1,000 is required to ensure that the energy storage system will last the life of the vehicle. The long-term goal is 1,000 cycles.

·         Calendar life. A calendar life of  >10 years is required to ensure that the energy storage system will last the lifetime of the vehicle. This is especially important for multifunctional designs where replacement might be difficult.

·         Effective specific power. The energy storage system should deliver power capabilities at 300 W/kg for 30 s and when discharged to 80% of DOD.

·         Operating temperature. The energy storage system should be operational at temperatures > -30°C. The energy storage system should not impose additional thermal management burdens on the rest of the vehicle systems. The outer surface of the energy storage system should not exceed 52°C.

ARPA-E Proposals

ARPA-E does not accept unsolicited proposals. Instead, ARPA-E broadly solicits energy-related research proposals using periodic Open Funding Opportunity Announcements (Open FOAs). Open FOAs are generally issued every two to three years. In addition, ARPA-E accepts, on a rolling basis, proposals to the Innovative Development In Energy-Related Applied Science (IDEAS) Funding Opportunity Announcement for single-phase efforts of up to 12 months and up to $500,000. To receive announcements about ARPA-E’s new Open FOAs and other program-specific FOAs, join its newsletter mailing list, available here. More information on the IDEAS FOA is available here. Do not contact ARPA-E about unsolicited proposals.


To join the conversation, and become an exclusive member of Electronic Design, create an account today!