Li-iron-sulfide chemistry for batteries studied

  • 01-Feb-2010 09:27 EST
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Ricardo and QinetiQ collaborated on a project to build this prototype 2-kW hybrid vehicle battery pack that incorporates innovative lithium-ion cells that use a potentially lower-cost Li-iron-sulfide cathode formulation.

As lithium-ion battery technology for hybrid and electric vehicle propulsion continues to evolve, the automotive industry is increasingly confronting its high price tag—some three to five times what would be desirable.

As such, the battery system often represents about one-third of the incremental manufacturing cost of a typical hybrid vehicle, as well as a considerable addition to its mass.

Among those working to make Li-ion more affordable is a British collaboration between the U.K. defense technology company QinetiQ and Warwickshire-based automotive technology specialist Ricardo. As part of a two-year, partly government-funded effort, researchers and engineers at the two organizations have built a prototype Li-ion battery pack containing cells using a lesser known electrochemistry that offers potential improvements in costs, safety, and performance. The team is now working to perfect the innovative Li-iron-sulfide batteries.

The Reduced Cost Li-Ion (Red Lion) project aims to demonstrate an alternative, potentially cheaper chemistry that features Li-iron-sulfide cathodes. The battery is controlled by a uniquely flexible battery-management system that can handle a range of cell types and configurations. The joint research project has received $3.2 million from the U.K. Department for Transport under the U.K. Energy Saving Trust Low Carbon R&D program.

“Red Lion is the result of a previous collaboration to build the Efficient-C low-carbon vehicle,” said Corin Wren, Chief Engineer for Hybrid Electric Vehicles at Ricardo. Introduced in May 2006, the parallel hybrid-electric diesel demonstrator (based on a Citroën Berlingo Multispace family car) got the equivalent of more than 63 mpg. “But we didn’t address the cost of the technology in that project,” Wren noted.

If British armed forces are going “to move into hybrid-electric propulsion for logistical vehicles, we need to improve the capacity of batteries while reducing costs,” said Steve Farmer, QinetiQ’s Manager for Transportable Power. The firm, which has a strong track record in delivering high-energy Li-ion battery technology to military customers, was formed in 2001 when the former U.K. military research organization called the Defence Evaluation and Research Agency was split up.

As the Efficiency-C program progressed, QinetiQ researchers identified “this mostly overlooked chemistry” and determined that it offered promise for better cost efficiency, cycle life, capacity, specific energy, rate capability, and safety. Cathodes made of Li-iron sulfide, for example, feature two lithium ions per sulfide group (or unit cell)—twice the theoretical energy density of many conventional counterparts.

Internal studies aimed at improving cell performance had succeeded in lengthening operating lifetime to 1000 charge/discharge cycles and demonstrating greater capacity, Farmer said, “so we proposed to undertake with Ricardo an effort to build better batteries for hybrid and electric vehicles.” QinetiQ built the necessary cells on its pilot production line while Ricardo developed the compact battery pack and the sophisticated control hardware and software needed to safely and optimally manage a variety of battery chemistries and architectures.

Peter Miller, Ricardo’s Director of Electronic and Electrical Engineering, cited several drawbacks of competing Li-ion formulations that needed to be addressed to make hybrid technology more viable. Current technology is still at the early stage of development—"not production-ready”—with lingering cost and safety issues, he said.

Miller rated the performance of the new Li-iron-sulfide cell chemistry to be “in the middle of the capacity range.” Although the prototype battery technology has so far undergone only limited testing and considerably more work will be necessary, researchers have demonstrated 50% faster discharge rates. The pack weighs 20% less than the 2-kW pack used on the Efficient-C diesel-hybrid.

Theoretically, specialists will be able to optimize the chemistry to provide high energy production at a somewhat lower power, but that has yet to be explored, said Miller. Although the current cell design is most suited to hybrids and low-range plug-in or range-extended electric vehicles because of its high charge/discharge rate capability, the inherently high energy density exhibited by the chemistry, combined with other anticipated improvements, would make it a candidate for all-electric vehicles by boosting its range capability.

The “mostly overlooked” battery formulation has four key selling points, Farmer explained: better performance; fewer safety concerns (the chemistry is not susceptible to overcharging and thermal runaway, so less automated cell management is needed); lower material costs (the iron sulfide replaces more costly, heavier metals such as cobalt, nickel, and manganese in existing systems); and less processing cost (iron-sulfide cathodes have low resistance to lithium transport so they do not require extra processing). The inexpensive Li-on phosphate materials used in cathodes in batteries from makers such as A123 Systems need modifications to make them run at high power.

Using patented manufacturing techniques, QinetiQ built the early preproduction high-rate cells—the laminated-pouch variety—on an in-house pilot manufacturing line, said Gary Mepsted, Technical Manager for Power: “Of course, there’s never a point in battery development where you stop and say ‘That’s it.’ So we’re now working with researchers at local universities to improve the cell’s energy density and extend cycle life by doing things such as optimizing electrolyte additives and cathode composition.”

Key to the project, Mepsted continued, is the intelligent battery-management system (BMS) that was developed by their partners at Ricardo. The flexible BMS—“a device that can often cost as much as the battery," Mepsted said—can self-tune to different battery chemistries and architectures. It automatically tests a battery and calibrates its operational parameters to the results.

“The BMS does a series of tests, such as evaluating voltage vs. energy, detecting any faults, and adapting the operating parameters to the climate conditions, drive cycle, manufacturing variability, and other issues, said Ricardo’s Miller.

Li-iron-sulfide cells, for example, have a lower voltage than similar cells and exhibit a significant amount of hysteresis when charging or discharging. “Part of the reason for the universal aspect of the new BMS,” he said, “is because it was developed in parallel with the new cells.”

Ricardo engineers hope to test the operational performance of the new batteries in a hybrid vehicle in the future and are today using the flexible BMS on a still-proprietary “straight EV.”

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