An Update on the EETD Research Program on Batteries for Advanced Transportation Technologies
The demand for so-called hybrid-electric vehicles (HEVs), which are powered by a battery and combustion engine (or fuel cell) working in tandem, has surged during the past few years. Automakers are responding to this demand by offering more and more models with hybrid power sources. This is good news from both the environmental and energy points of view because HEVs have fewer exhaust emissions and better fuel economy than equivalent vehicles powered only by combustion engines.
The HEV's current market success is a result of its combined battery and combustion-engine configuration, which allows it to avoid many of the problems of the once-promising pure electric vehicle (EV), powered only by a battery. Environmental Energy Technologies Division (EETD) electrochemists are intimately involved in the future of both types of vehicles through the Batteries for Advanced Transportation Technologies (BATT) Program, which focuses on developing advanced high-energy rechargeable batteries for EVs as well as high-power rechargeable batteries for HEVs.
EVs vs. HEVS
The development and commercialization of battery-powered EVs has proven extremely challenging because EV batteries must simultaneously meet multiple, demanding requirements: high energy per unit mass (to provide acceptable driving range between battery recharges), high power (to provide acceptable vehicle acceleration and to receive energy generated during vehicle deceleration, a.k.a. regenerative braking), long lifetime, low cost, resistance to abuse and operating temperature extremes, absolute safety, and minimal environmental impact. Despite years of intensive worldwide research, no EV battery has been developed that can meet all of these goals. Perhaps the most vexing barrier is cost. There are three components to this problem: 1) advanced batteries cost between $150 and $300 or more per kilowatt hour (kWh) of stored energy, 2) a battery-powered EV must store at least 30 kWh of energy, and 3) battery lifetimes are typically shorter than five years. This combination of limitations means that there is little demand for EVs and therefore little incentive for automakers to produce and market them.
The HEV has succeeded because it circumvents many of the EV's problems. A key advantage of the HEV is that it uses a small, e.g., one-kWh-capacity battery compared the 30 kWh or more required for an EV. This small battery significantly reduces overall vehicle cost (as detailed in "Lithium Batteries for Hybrid-Electric Vehicles", EETD Newsletter, Winter 2000). HEVs also avoid the recharging challenge faced by EVs because the combustion engine charges the battery while the vehicle is driven. So, although long-term research on EV batteries continues in part because of the attractiveness of EVs' zero emissions, the HEV design is currently the most viable for the mass market.
Battery Designs and Chemistries
Because HEV batteries do not need to be as large as EV batteries, HEVs can use versions of the same products that are found in today's consumer electronic devices (e.g., laptop computers, cellular telephones, power tools). The key difference between batteries used in HEVs and those used in other consumer electronics is that HEV batteries are designed to maximize power output per unit mass/volume, for example by using thin electrodes and heavy metallic electronic conductors. In contrast, consumer (and EV) batteries are generally designed to maximize energy content per unit mass/volume, for example by using thick electrodes and lighter electronic conductors.
Today's most common HEV battery is nickel-metal hydride and uses a nickel oxide positive electrode, aqueous alkaline electrolyte, and a hydride negative electrode. The nickel-metal hydride battery meets or approaches many of requirements listed above for the EV and is sufficiently robust for HEV applications. However, the nickel-metal hydride battery has limitations such as marginal energy and power characteristics and poor ability to accept charging currents at elevated temperatures, so automakers are searching intensively for a next generation battery system with improved performance.
Because the lithium-ion battery has emerged as the system of choice for many consumer electronics applications, it has also attracted major interest as the likely battery of the future for HEVs. Present-day lithium-ion batteries designed for consumer electronics use a carbon-based negative electrode, an organic liquid electrolyte with dissolved lithium salt, and a cobalt oxide positive electrode. However, although the cost of lithium-ion batteries has dropped significantly during the past few years, it remains expensive for HEV applications, so there is strong incentive to develop less-expensive cell components (e.g., to replace at least some of the cobalt with another metal). Lithium-ion batteries also pose a potential safety problem because of the instability and flammability of the organic electrolyte. This problem can be overcome for small consumer batteries that contain only a few Wh of stored energy, but it is a major concern for HEV batteries that contain about one kWh of stored energy.
The BATT Program
Lawrence Berkeley National Laboratory (Berkeley Lab) has had a long-standing "Lead Laboratory" role in the battery research area of the U.S. Department of Energy FreedomCAR and Vehicle Technology Program (http://berc.lbl.gov/BATT/BATT.html). BATT Program research addresses the fundamental chemical and mechanical instabilities that have impeded the development of EV batteries that have acceptable cost, lifetime, ruggedness, and safety. This work supports the development of both advanced high-energy rechargeable batteries for EVs and high-power rechargeable batteries for HEVs. BATT Program researchers assemble advanced components into battery cells, determine failure modes, synthesize and evaluate new materials, carry out advanced diagnostics, and develop sophisticated electrochemical models to improve our understanding of the complex processes that affect battery performance and life.
Baseline Battery Chemistries
An essential feature of BATT research is the selection of battery chemistries to serve as "baseline" systems against which new materials and designs can be compared. There are currently three BATT Program baseline cell chemistries; all are variants of the lithium-ion battery and include low-cost natural graphite as the negative electrode and an organic electrolyte with dissolved lithium salt. The major difference among the three is the material used for the low-cost positive electrode, usually called the cathode. The three cathode materials in the baseline systems are: mixed transition metal oxide, LiNi1/3Mn1/3Co1/3O2, which provides high voltage and high energy; lithium iron phosphate, LiFePO4, which provides lower voltage but excellent stability; and manganese oxide spinel, LiMn2O4, which provides high voltage and high power.
In a project led by Kathryn Striebel, baseline materials are incorporated into standardized cells and tested using a consistent protocol to determine capacity, energy, power, and lifetime characteristics (see "Testing New Battery Materials in Standard Cells," EETD Newsletter, Fall 2003). Striebel and her colleagues benchmark BATT Program baseline cells against established battery performance goals, evaluate new materials, and then disassemble the cells and components for detailed characterization to determine failure modes (see Figure 1). They also test advanced cell designs suggested by BATT modeling groups, led by John Newman and Venkat Srinivasan.
One critical concern is developing approaches to protect batteries from overcharging. Because HEV and EV electric motors operate at 300 volts or higher and individual lithium-ion cells deliver only three or four volts each, many cells must be arranged in series to provide the needed voltage. In contrast to batteries with an aqueous electrolyte (e.g., the nickel-metal hydride batteries currently used in HEVs), which can be designed to accept limited overcharging, batteries with a non-aqueous electrolyte cannot easily be made to accept overcharging. This is a particularly serious problem for HEVs and EVs because if even one cell in the series is slightly weaker than the others, it will become overcharged unless corrective measures are taken. Overcharging of a lithium-ion battery cell is extremely dangerous because it not only heats the cell but also destabilizes cell components, and both processes can result in cell leakage, fire, and explosions. Unfortunately, established corrective measures for consumer lithium-ion batteries (e.g., individual electronic control of each cell) are expensive and cumbersome. To address this problem in a practical manner, Tom Richardson and his BATT colleagues are developing novel electro-active conducting polymers that can protect against overcharge in lithium-ion cells.
Present-day lithium-ion batteries, which employ various forms of carbon as anode materials, suffer from safety, cycle-life, and storage-life problems. BATT Program researchers are studying non-carbonaceous anodes as possible alternatives to address these problems. Low-cost metal alloys and carbons with acceptable capacity, rate, cyclability, and calendar life are under investigation (see Figure 2). The interaction of these alloys with the organic electrolyte is complex and poorly understood but is believed to be a critical process that destabilizes and thereby limits the lifetime of alloy anodes. Philip Ross and Vera Zhuang [of the Berkeley Lab Materials Sciences Division (MSD)] and colleagues are using advanced characterization techniques to improve our understanding of how these alloy components interact with the electrolyte. This research has increased our understanding of model electrodes (i.e., very smooth thin films of a single anode component), and researchers are close to applying this knowledge to explain alloy behavior.
BATT Program electrolyte research, led by Nitash Balsara (MSD) and John Kerr, focuses primarily on polymer electrolytes, which are safer than liquid organic electrolytes and will not flow or leak from cells. Prior efforts (including major, multi-year, multi-million-dollar projects by industrial consortia) to develop practical lithium batteries based on polymer electrolytes have been hampered by the polymer electrolyte's low ionic conductivity and the critical problem of lithium metal dendrites, which are tree-like branched structures that grow across the electrolyte and short the electrodes. Resolution of these two problems is recognized as an extremely difficult task. BATT Program scientists searching for polymer electrolytes with improved properties are studying composite polymer-particle mixtures, diblock polymers with aligned cylindrical structures, oligomeric anions, and other unusual assemblies.
New Cathode Materials
The identification of novel cathodes is critical because of the high cost and environmental problems of the cobalt oxide materials used in today's lithium-ion batteries. An important BATT goal is to develop a high-rate stable manganese-based cathode. Although manganese is a low-cost constituent, manganese-based cathodes tend to lose capacity at unacceptable rates. BATT research is directed at understanding the reasons for this deterioration in capacity as well as evaluating novel forms of manganese-based cathodes that exhibit high capacity (greater than 150 ampere-hour per kilogram of oxide) and acceptable stability.
Two of the baseline cathode materials mentioned above are examples. The mixed transition metal oxide LiNi1/3Mn1/3Co1/3O2 has a layered structure into which a large amount lithium can be inserted, and it has the potential for higher coulombic capacity and higher energy than cobalt oxide, at a lower cost. The manganese oxide spinel LiMn2O4 has a three-dimensional structure that offers an easy path for inserting lithium ions and thereby supports high ion-transport rates and high battery power. Marca Doeff (MSD) and Tom Richardson are leading the Berkeley Lab effort to develop improved cathode materials for the BATT Program.
Diagnostics and Modeling
Advanced diagnostics are essential to investigating the processes that limit battery life and performance. Unfortunately, it is either impossible or very difficult to use most of the common analytical tools to characterize the specific processes that affect battery life and performance. The reasons for this are varied and complex, e.g.:
- battery charge and discharge involve numerous simultaneous and inter-related phenomena, including ion transport, electron transport, molecular transport, chemical reactions, electrochemical reactions;
- degradation processes may constitute only a small fraction of the electrochemical process and may require many months or years to become apparent;
- there is no accepted "accelerated life test" for batteries;
- battery electrochemical processes require juxtaposition of multiple phases, including an electrolyte phase that usually obscures the electrode surface or renders it invisible to many experimental probes.
Notwithstanding these difficult problems, Robert Kostecki, Phil Ross, and Tom Devine are using powerful analytic methods, including advanced spectroscopy and microscopy, to characterize materials and cell components and determine the morphological, structural, and compositional changes in electrode materials that accompany cell cycling. Their experimental results have:
- provided new insight into unwanted side reactions that take place along with the main electrochemical reactions in rechargeable batteries,
- revealed unexpected physical changes that take place on electrode surfaces as BATT baseline cells degrade, and
- shed light on corrosion processes on the metal foils used as substrates for cathodes in lithium-ion batteries.
Sophisticated mathematical modeling is required to understand the complex phenomena in batteries. As noted previously, mathematical models are being used to optimize the design of BATT Program baseline cells (see Figure 3). Comprehensive stability analysis is also being designed to determine the conditions under which lithium-dendrite growth can be inhibited or eliminated by mechanical means. This type of modeling can be extremely cost effective because it relies on rapid computer simulations rather than slow and expensive experimental approaches to address fundamental problems.
The BATT Program has changed immensely during the past several years, incorporating the latest experimental and modeling tools to address fundamental battery problems and keeping pace with the rapid introduction of the lithium-ion battery. The BATT Program has played and will continue to play a leading role in battery research and is currently poised to help introduce the lithium-ion "superbattery" to the HEV market. The ongoing work of BATT researchers is essential to a future with cleaner vehicles and less dependence on petroleum.
For more information, contact:
- Frank McLarnon
- (510) 486-4636; fax (510) 486-4260
This research is supported by the U.S. Department of Energy's Office of FreedomCAR and Vehicle Technologies.