“LITHIUM-ION IS SUBJECT TO A LOT OF DEVELOPMENT for next-generation energy storage,” noted Pete Savagian, GM Hybrid Powertrain Systems engineering director. “Several technology options are being presented, all by serious players trying to capture what they think is a growing business, but not all of these technologies are fully proven, nor is there evidence from all of them of sufficient capacity to service all hybrid vehicles. The suppliers are working to establish credibility.”
Savagian said GM (www.gm.com) has many different batteries under test, though most of its testing is done by the United States Advanced Battery Consortium (www.uscar.org), which includes the U.S. Department of Energy (www.doe.gov) as well as Daimler-Chrysler (www.daimlerchrysler.com), Ford (www.ford.com) and General Motors.
Although the size and cost of NiMH batteries could bear improvement, according to Savagian, especially considering the run-up in the price of nickel within the past few years, he said NiMH batteries are sufficient for near-term hybrid vehicle requirements due to the proven durability of NiMH and current suppliers' high-quality production capabilities. NiMH battery makers include Cobasys (www.cobasys.com), Matsushita Battery Industrial Co. Ltd. (http://panasonic.co.jp/mbi/en/) Panasonic EV Energy (www.peve.panasonic.co.jp/) and Sanyo Electric Co. Ltd. (www.global-sanyo.com).
Battery makers Johnson Controls (www.johnsoncontrols.com) and Saft (www.saftbatteries.com) have joined forces to supply NiMH and Li-ion batteries for current and future-generation hybrid vehicles (Figure 1). Last summer, the USABC awarded Johnson Controls-Saft Advanced Power Solutions a 24-month contract to develop Li-ion batteries for hybrid vehicles. The firms will supply cell modules that can be tested for abuse tolerance, pulse power, calendar life, and cycle life.
In September, Johnson Controls-Saft Advanced Power Solutions was awarded a Li-ion battery development contract from a major vehicle manufacturer.
The first Li-ion-equipped vehicles may go into production as early as 2008, according to Dave Hermance, executive engineer for advanced technology vehicles at Toyota's Technical Center (www.toyota.com). Migration from NiMH to Li-ion is likely to occur over a decade, as happened in consumer electronics applications. “The issue is durability,” he said. “A consumer product may be designed to last three years, but a battery in a hybrid vehicle needs to last for the life of the car.” The Toyota Prius II battery stack (Figure 2) consists of 38 prismatic NiMH modules connected in series.
Honda (www.honda.com) currently uses NiMH batteries in all of its hybrid vehicles. It won't discuss the battery technology planned for a new hybrid model to be introduced in 2009.
The battery pack in Honda's Civic Hybrid stores electricity in a bank of 132 1.2 V NiMH cells that stores up to 158 V of electrical energy for the integrated motor assist (IMA) motor compared to 144 in previous versions. A new Panasonic dual-module casing reduces weight from previous hybrid battery packs and also allows it to increase efficiency of the electrical flow. The 12% smaller volume of the battery pack accommodates more cargo space.
Energy for the IMA motor is stored in a bank of 120 1.2 V NiMH cells that stores up to 144 V of electrical energy for the IMA motor, as in previous versions. A new Sanyo dual-module casing reduces weight from previous hybrid battery packs and also increases the efficiency of the electrical flow.
Honda has used ultracapacitors on its FCX fuel cell vehicle; however, its next-generation FCX will use Li-ion batteries for the same purpose because Li-ion batteries are lighter and more compact, and offer high output).
Li-ion technology is emerging as the primary candidate to replace NiMH in part because some required developments are supported by the growing dominance of Li-ion in consumer technology, according to Prabhakar Patil, chief executive officer of Compact Power Inc. (www.compactpower.com), a subsidiary of LG Chemicals (www.lgchem.com).
Patil noted that the weight advantage of Li-ion over NiMH materials gives Li-ion batteries a 2x to 3x advantage in energy and power density. “This is significant, as consumers are willing to pay a premium for longer usage time or lighter weight,” he said, adding “Li-ion batterycost on a $/kWh basis is already comparable to Ni-MH and is on track to go further down.”
Patil explained that when Sony (www.sony.com) commercialized Li-ion batteries in 1991, it introduced a graphite carbon as the anode material, pairing it with a metal oxide cathode (LiCoO2). “In Li-ion batteries, lithium ions leave the metal oxide cathode during charge and intercalate into the graphite carbon anode. The reverse happens on discharge,” he said.
Battery firms are focusing their research effort on development of new cathode materials, according to Patil, with a goal of replacing LiCoO2, which is costly because it contains cobalt and also has less than desirable abuse tolerance, especially on overcharge and internal shorting.
“The focus now is on Ni-based, layered cathodes that have better thermal stability than LiCoO2, but still have drawbacks based on price, abuse tolerance and low-temperature performance,” Patil said, adding that LiMn2O4 and LiFePO4 are robust with respect to thermal runaways on overcharge and internal shorts. “A combination of abuse-resistant cathode and chemistry, selection of appropriate voltage windows, and hardware and software system-level protection mechanisms can be used to make Li-ion batteries abuse-tolerant.”
Valence Technology Inc. (www.valence.com) offers U-Charge large-format Li-ion batteries based on the firm's Saphion technology, which is said to eliminate many of the safety issues associated with traditional oxide-based lithium-ion battery technologies. The batteries are said to deliver greater energy density and better reliability at a lower cost of ownership than lead-acid batteries, and to offer better cycle and battery life compared to NiMH nickel batteries of the same power.
Valence's U-Charge RT power systems feature built-in battery management electronics for volt-age, temperature and state-of-charge monitoring and cell balancing, and they also include internal disconnects, which makes them extremely fault tolerant since each battery system can protect itself from potentially damaging conditions. The batteries are capable of peak power rates of 500 to 1700 continuous Watts, and as many as 30 of these battery systems can be connected in series.
Valence's Saphion I technology uses natural, phosphate-based cathode material in place of the less stable and more costly cobalt-oxide, used in other Li-ion batteries.
GM's Savagian noted that nanotechnology firms' focus on advanced treatment of the electrode in Li-ion batteries, along with other chemistry. Changes in the electrode composition allow high rates of charge and a high power-to-energy ratio, which has potential for further reduction in battery mass.
Nanoexa (www.nanoexa.com) and Decktron (www.decktron.com) are partnering to develop and commercialize lithium battery technology originally developed at the U.S. Department of Energy's Argonne National Laboratory (www.anl.gov). The firms plan to develop batteries with increased power output, storage capacity, safety and lifetime for HEV and other applications.
Under the FreedomCAR Partnership (www.eere.energy.gov), Argonne has conducted R&D to help industrial battery developers lower cost and increase the lifetime and inherent safety of high-power lithium batteries.
Traditional Li-ion technology uses active electrode materials with particles that range in size from 5 microns to 20 microns, but A123Systems (www.a123systems.com), a nanotechnology firm, offers non-combustible active materials composed of particles smaller than 100 nm. The firm claims that faster kinetics in its next-generation Li-ion storage electrode provide higher power than possible from other chemistries; its electrode and cell designs offer high thermal conductivity and low impedance compared with other batteries of similar size, and its electrolyte system operates over a wider temperature range. The firm said batteries made from its nanoscale electrode material are 80% lighter and offer higher charge/discharge rates (charging to high capacity in five minutes or less) and longer cycle life compared to NiMH batteries.
According to A123Systems, conventional Li-ion cells extract only half their lithium content when they reach their upper cut-off voltage. In contrast, A123 materials are designed to ensure that all lithium is extracted from the cathode when the battery is fully charged, thus safety issues related to overcharging are eliminated.
Roy Graham, senior vice president, commercial development, at Altair Nanotechnologies Inc. (www.altairnano.com) said lithium-ion batteries currently use graphite for the negative electrode and most use lithium cobalt oxide for the positive electrode. Altairnano uses a patented nano-titanate material as the negative electrode in its NanoSafe batteries. Graham explained that when the highly reactive graphite is replaced with nano-titanate materials, no interaction takes place with the electrolyte, thus the battery is inherently safe.
Graham said that in current-generation Li-ion batteries, lithium ions deposit inside the graphite particles during charge; however, the rate at which the lithium ions can deposit is limited by the electrochemical properties of the graphite, and if they cannot enter the graphite particles they can collect (plate) as lithium metal on the negative electrode's surface. “This can occur if the ions are deposited too rapidly on the graphite electrode as might be the case if the battery is charged too quickly. If this plating occurs, the battery will severely degrade in performance and in extreme cases, will short, causing overheating and thermal runaway,” Graham said. “The time required to charge a Li-ion battery is restricted by the ion incorporation rate,” he continued. “This results in charge times measured in hours.” He added that charge rate can be affected by external factors such as temperature. “At low temperatures, the lithium-ion incorporation rate is significantly less than at room temperature so charging at these temperatures may take much longer. Since the charge rate is governed by fundamental properties of the materials the only option is to change the materials and chemistry of the battery.”
Graham said Altairnano's use of nano-titanate material for the negative electrode allows lithium to be deposited — and batteries charged — at high rates, thus no plating occurs, even at extremely cold temperatures. “In laboratory tests, a cell can be charged to more than 80% capacity in about one minute,” he reported. “The technology also increases battery discharge rates, which is important when bursts of power are needed, such as a freeway electric vehicle accelerating rapidly.”
Altairnano's first customer for its NanoSafe battery is Phoenix Motorcars (www.phoenixmotorcars.com), which will use the batteries in sport utility trucks. The trucks will also include 90 kW electric drive systems from Enova Systems (www.enovasys).
ABOUT THE AUTHOR
John Day writes about automotive electronics and other technology. He holds a BA degree in liberal arts from Northeastern University and an MA in Journalism from Penn State. He can be reached by e-mail at [email protected].