Worldwide Lithium Battery
Development Progress at the
41st Power Sources Conference

by Henry Oman
Consulting Engineer
Seattle, Washington

Figure 1: This 14.8-volt, 76 ampere-hour lithium-ion ModPack battery pack is an assembly of 1.9 ampere-hour cells and weighs 8.4kg.

Lithium-ion batteries are being developed all over the world for applications ranging from deep-ocean submarines to spacecraft that will explore remote regions in the solar system. Progress in development for these applications was reported by the authors of 70 technical papers presented at the 41st Power Sources Conference held in a new venue, the Adams Mark Hotel in Philadelphia, Pennsylvania, June 14-17, 2004. A record 660 people attended, and 44 exhibiting companies were represented at this biennial event sponsored by units of the U.S. Department of Defense and other government agencies. In this article we summarize from the proceedings of this conference the most important lithium-ion battery progress reported there.

High-power lithium-ion battery development has progressed in two directions. The first involves assembling the presently available type 18650 lithium-ion cells into series strings connected into parallel modules that produce the required current and have the required ampere-hour energy content. Then these modules are connected in series to form the battery that delivers the required energy with a depth of discharge that results in the required life in charge/discharge cycles. The advantages of this approach are (1) the 18650 cells are in mass production and have high reliability, (2) lifetimes up to 50,000 charge/discharge cycles have been demonstrated, and (3) the cost of cells is decreasing as more firms start producing them. However, a battery of this type requires sophisticated internal controls to prevent any possible single failure from disabling it.

The alternative battery development approach is to make larger lithium-ion cells so that the new battery will have fewer cells than a lead-acid, nickel-cadmium, or nickel-metal-hydride battery with the same terminal voltage. This requires designing and testing a new cell to confirm its performance in terms of charge/discharge cycles.

Successes in both of these basic approaches were reported in 17 papers presented at the conference. The text that follows cites the highlights of these presentations.


Adapting Lithium Batteries for Propelling EVs

America’s petroleum production peaked in 1975, and we now import over half of the petroleum that we consume. The world’s oil production is predicted to peak in 2005, and already many nations are intensely developing alternatives to gasoline-fueled vehicles.

Now available are new products that can revive the use of electric vehicles. An example of the usefulness of lithium-ion cells is in the results of the 2003 Michelin Challenge Bibendum electric car race. In it, 100 electric vehicles competed for the lowest energy consumption in 100 miles of travel at a speed of at least 45 miles per hour. The two-passenger tZero car, driven by Martin Eberhard, won this race by covering the 100-mile distance in two hours (1). His car consumed 21.7kWh from its lithium-ion battery. The energy in a gallon of gasoline is 33.8kWh. Therefore, tZero’s energy consumption at a speed of 50 miles per hour was equivalent to 155 miles per gallon of gasoline. Eberhard, when driving to the site of the Bibendum race from Los Angeles, traveled 260 miles before he had to stop to recharge his lithium-ion battery.

Rapidly Configurable Lithium-ion Battery Modules

At the 41st Power Sources Conference Stephen S. Eaves showed how large and rapidly-configurable lithium-ion batteries can be built from small commercial 18650 Li-ion cells by using the proprietary “Massively Parallel Modular Architecture” (2). He illustrated his technology with the ModPack lithium-ion 14.8-volt, 76Ahr module shown in Figure 1. Within this ModPack the lithium cells are combined in series to achieve specified capacity and voltage. Multiple modules are connected in series to obtain desired pack voltage. Embedded electronic circuitry reliably balances the capacities of the individual cells in the module, and a supervisory battery management system enables the user to properly interface the battery to the application bus. The module control circuitry has the ability to disable a section of a module that contains a failed cell with only a minor loss in module capacity, thus providing a high level of reliability.

Discharge characteristics of the ModPack 15-76 module are shown in Figure 2. The ModPack’s discharge voltages at temperatures down to -20°C are plotted in Figure 3. Eaves used test data for predicting in Figure 4 the battery’s capacity after 500 charge/discharge cycles.

Two ModPack 15-64 lithium-ion battery modules were installed on an electric bicycle manufactured by eGo Vehicles Corp. of Providence, Rhode Island. This bicycle, when lead-acid batteries propelled it, had traveled a distance of 20 miles on one battery charge in a Tour De Sol competition organized by the Northeast Sustainable Energy Association. Replacing the lead-acid battery with ModPack lithium-ion batteries increased the bicycle’s speed from 25 mph to 31 mph, and its range from 20 miles to 56 miles (Figure 5).

Batteries for Hybrid Electric Vehicles

Battery-powered electric vehicles for long-distance travel must have battery-recharging services along the travel route or be powered by batteries that are very heavy and costly. An alternative is a hybrid electric vehicle (HEV) with a low-power high-efficiency engine that runs at its most-efficient speed most of the time, plus batteries that absorb braking energy and deliver acceleration energy. Presently available commercial hybrid cars deliver over 60 miles per gallon of gasoline consumed. However, the battery pack must be sized to deliver the required power and energy and still have a required lifetime. Lead-acid batteries, if completely discharged in every charge/discharge cycle can fail within 100 cycles.

J. M. Miller observed that lithium-ion batteries are superior to NiCd and NiMH batteries in energy density, but have been excluded from the highest power-density applications due to rate and safety limitations (3). T/J Technologies has developed new nanostructured composite electrodes based on bulk energy storage in metal oxide anodes and/or metal phosphate cathodes treated with a conductivity enhancing process. Up to 40% of the C/10 capacity (140mAh per gram) is retained at charge/discharge rates of >100 C. The key design features include: no lithium deposition at high charge rates, higher tolerance to overcharge, improved thermal stability and electrolyte oxidation resistance, excellent cycle life, and potentially low cost for high volume applications.

Miller showed a capacity versus charge/discharge cycles for Li-ion cells that used the new composite cathode and MCMB anode (Figure 6). With a charge and discharge rate of C/2 there was virtually no loss in capacity during the 150-cycle test. Based on these properties, these batteries are being targeted for future combat and HEV applications.

High-Power Lithium-ion Cells for Electric Vehicles

An electric vehicle needs a battery with high energy density so that energy does not have to be expended to accelerate a heavy energy source, such as a lead-acid battery, and haul it over hills and for long distances. Today’s lithium cell is the lightest of commercially available power sources, but its weight would be higher if it has to carry high peak loads.

For autonomous undersea vehicles and unmanned vehicles Saft America had developed its VLE series of cells that provide a specific energy of 195Wh/kg and an energy density of 300Wh/l. However, a HEV needs regenerative power, and power for cold cranking of the HEV engine, plus high-power pulses. It also needs to deliver a long calendar life and safely respond to abuse.

Saft America has now completed the development of a new high-power lithium cell in a cost-share program with the Partnership for a New Generation of Vehicles (PNGV) and the U.S. Department of Energy. N. Raman described the results of this work (4). The cell specifications for this very high-power 4Ahr cell are summarized for operation at 25°C in Figure 7. When discharged at 500 amperes into a load, it delivered 82% of its rated capacity and its temperature did not rise above 40°C. Tests showed that the cells can meet cold-cranking requirements at -30°C, and can provide the required power even after 30 days of storage at 25°C.

Very High Power for Aircraft and
Directed Energy Applications

New technology enables military units to survive enemy attacks. For example, “Active Armor” can sense a launched rocket, compute its flight path, and dispatch a missile that explodes when it gets beside the rocket. Other novel technologies are electric gun, solid-state heat capacity laser systems, electro-thermal chemicals, and pulse power systems. These units require quick availability of electric power at an unanticipated moment.

Kamen Nechev, from Saft America, described the SSHCL, a directed energy weapon program under way at the Lawrence Livermore Laboratory (5). This program offers a new approach to countering mortars, artillery, cruise missiles, and ATGMs. Saft had built a power system for a demonstration laser that requires 3MW of power. The system is made up of a number of 24-cell modules in which each module delivers about 100kW of pulse power. The stationery prototype systems use Saft standard high-power cells. The production SSHCL could be mounted on a hybrid HMMWV which produces low noise, has minimum thermal signature, and could easily be transported to any point in the world in transport aircraft.

The steps in Saft’s development of the required very-high-power lithium-ion cell for this service are summarized in Figure 8. In this table the VL8P and VL16P are the standard high-power cells that Saft offers commercially. The VL12P cell has been designed to meet the specific requirements of the automotive market. It is an intermediate between Saft’s high-power and very-high-power cell classifications. For the high-power cells the power is measured at 50% state-of-charge. For the very-high-power cells it is measured at 100% state-of-charge.

Low and high temperature performance of Li-ion cells are driven by two different mechanisms. The low-temperature performance is controlled by the inability of technology to deliver required power and energy at some limiting temperatures. The main reasons for this are severely reduced ionic conductivity of the organic electrolyte and impeded Li diffusion in both active materials. At elevated temperatures the stability of the SEI layer is critical, so the selection of solvents in the electrolyte responsible for SEI formation is most important.

Saft’s concentrated effort in developing low temperature electrolytes has produced excellent electrolyte performance and also long life at elevated temperature. Performance of a VL4V cell at a 1C rate at -60°C temperature is shown in Figure 9. The cell delivered 60% of its capacity. The cell must also withstand temperatures of 71°C without any impact on its performance.

Routes to Higher Energy-Storage Capacity

For the United Kingdom’s Bowman “battlefield” communications system the emphasis is placed on lithium-cell lifetime in charge/discharge service, and low- temperature performance. Tony Jeffery of the U.K. firm, AGM Batteries Ltd., described how they might increase the energy-storage capacity of their ICR36550 cell to make it useful for service in the United States where great emphasis is placed on cell capacity (6). More capacity could be obtained by putting more active material within the available cell volume. This would be a simple task, but it must not affect other key factors such as cycle life, and low-temperature and rate performance. The key variables in this development were cathode formulation, LiCoO2 grade, anode formulation, separator properties, and electrolyte formulation.

A matrix of potential cell designs was formulated, and a selected group of these cells was manufactured and evaluated. The final adopted cell design differs from the standard ICR36550 cell in the details shown in Figure 10. Its improved performance is summarized in Figure 11. The cycle life and rate performance of this new cell significantly exceeded expectations. An in-cell electronic protection system has been developed, and the K2 cells are now in commercial manufacture. A follow-on program to develop a 7Ah cell is under way.

In evaluating further cell improvements Jeffery observed that during its development years the capacity of the18650 lithium-ion cells has grown to a plateau of 2500mAh. This means that lithium ion cells must contain twice as much lithium as can be safely used, and this limits the maximum available energy density. Consequently AGM and its partners have started developing a technology based on the use of stabilized lithium metal powder (SLMP), which can be used in conjunction with:

1. Existing LiCoO2 system to increase capacity by 19% and energy density by 16%.

2. Non-lithiated cathode materials to increase capacity by 75% and energy density by 25%.

3. New anode materials can be optimized for cell performance rather than low irreversible capacity.

The projected performance of these cells based on SLMP is summarized in Figure 12.

Lithium-ion and Zinc-Air Hybrid Power Sources

A recently developed fuel cell runs on powdered zinc plus oxygen from the air. It was tested in powering buses that ran through hundreds of miles through the Alps in Europe. The zinc powder can be fed into the cell until the cell is fully loaded with zinc oxide from its electric-power producing reaction. Recharging requires refilling the zinc-powder hopper, pumping out the zinc-oxide loaded electrolyte and replacing it with fresh electrolyte.

The zinc-air fuel cell, although very efficient, would have to be big enough to supply the maximum power required by its vehicle when accelerating or climbing steep hills. The vehicle’s power-system weight could be reduced if a lithium-ion battery supplied the required peak power. Fuel-cell power could recharge the battery during non-peak-power periods. Arek Suszko described an evaluation of this option that was made at the U.S. Army Research, Development, and Engineering Command at Fort Monmouth, New Jersey (7). The objective was to reduce the weight that a soldier has to carry in performing his electric-energy consuming assignment

An important discovery that he reported is that the zinc-air “battery” did not have the capability at states of charge of less than 50% for carrying the peak load. However, its energy reserve exceeded 90 watthours. It could still recharge the lithium battery during low load periods.

Suszko described a technique called “sweep voltammetry” for indicating the expected performance of each component of a hybrid system. This allows the system designer to predict the range at which the system will perform optimally, and the point at which it cannot perform the required task.

Additional Battery and Fuel Cell Information

Lithium-ion batteries, other batteries, and fuel cells were topics of many other papers that were presented at the 41st Power Sources Conference. Rather than summarize all the lithium-battery papers, I chose to summarize those that showed clearly that lithium-ion battery performance is improving and its useful lifetime is growing. It is also obvious that well-funded and intense research effort is directed toward making lithium-powered electric vehicles a practical substitute for vehicles powered by petroleum-based fuels. The price of petroleum products is going to rise after the world’s petroleum production peaks next year.

Those who need more lithium-battery data can find it in the “Proceedings of the 41st Power Sources Conference, June 14 to 17, 2004.” I will summarize in subsequent reports the important fuel cell and other battery developments reported at the conference.

References

Unless otherwise indicated, the following references are from the “Proceedings of the 41st Power Sources Conference, June 14 to 17, 2004.” This hardback book is available from the 41st Power Sources Conference, c/o Ralph Nadell, Palisades Convention Management, 411 Lafayette Street, Suite 201, New York, NY 10003, phone: (800) 350-0111 or email: rnadell@pcm411.com.

1. “tZero Earns Highest Grade at 2003 Michelin Challenge Bibendom,” Current Events, Nov-Dec 2003, Electric Auto Association.

2. Eaves, Stephen S., “Large, Low Cost, Rapidly Configurable Lithium-ion Battery Modules Constructed from Small Commercial Cells,” pages 328 to 330.

3. Miller, J. M. and associates, “Ultra-High-Rate Batteries Based on Nanostructured Electrode Materials,” page 393 to 396. 4. Raman, N., and associates, “Development of High Power Li-Ion Battery Technology for Hybrid Electric Vehicle (HEV) Applications,” pages 435-437.

5. Nechev, Kamen, “Very High Power Lithium Ion for Aircraft and Directed Energy Applications,” pages 274-277.

6. Jeffery, Tony, and Hinde, Jason, “The Development of High Energy Density Lithium-ion Cells,” pages 438- 441.

7. Suszko, Arek, and associates, “The Making of a Hybrid System: Performance Matrix of Zn-Air/Lithium-Ion Hybrid Variants,” pages 496-499.