Iron offers a number of advantages for use in a lithium-ion cathode material, including cost and safety and iron sulphide cathodes have been of interest for decades—a 1975 study by engineers from Rockwell International projected energy densities of 77 Wh/lb (169 Wh/kg) for lithium-iron sulfide batteries. QinetiQ has also been working on lithium-ion/iron sulphide cells for a number of years. For use as a cathode material, the iron sulphide must be lithiated. QinetiQ has developed new synthesis methods for lithiated iron sulphide that are quicker and easier than other methods.

To prove the potential of the alternative cell chemistry, the RED-LION project replaced the battery pack used in the Efficient-C prototype HEV with one using new cells capable of demonstrating suitable material cycle life, capacity, specific energy, rate capability and safety.

Custom cells were manufactured and packaged within a bespoke battery pack incorporating a number of novel features. Ricardo designed the overall battery pack to directly replace the existing unit, developing a new battery management system to manage the iron sulphide chemistry.

The prototype cells, designed and manufactured wholly by QinetiQ, have been shown to deliver a cycle life of more than 1,000 cycles at a limited depth of discharge. The high charge/discharge rate capability shown by the particular cell design would make it suited to both HEVs and low-range Plug-in Hybrid Electric Vehicles (PHEVs). Comparing the performance of the prototype high rate cells with the cells originally used in the Efficient-C HEV, a 20% reduction in weight, on a Wh/kg basis, was achieved.

Furthermore, QinetiQ estimates that significant cost savings are possible in comparison with current commercial production Li-Ion chemistries through cheaper raw materials and a more energy-efficient patented manufacturing process.

The new battery management system, developed by Ricardo based on a bespoke architecture, is fully adaptable to a range of cell chemistries and battery architectures. Ricardo has fully integrated its system into a battery pack containing the QinetiQ cells. The pack includes a number of innovations to improve performance and reduce cost, which are the subject of three Ricardo patent applications.

The project has demonstrated the potential of the cell chemistry within an HEV environment. While the current cell is most suited to HEVs and low-range PHEVs due to its high charge/discharge rate capability, the inherently high energy density exhibited by the chemistry combined with other anticipated improvements, would make it a strong candidate for all-electric vehicles, not least due to its highly attractive cost versus energy-density benefit, according to the partners.

Resources

• A. G. Ritchie, P. G. Bowles and D. P. Scattergood (2004) Lithium-ion/iron sulphide rechargeable batteries. Journal of Power Sources Volume 136, Issue 2, 1 Pages 276-280 doi: doi:10.1016/j.jpowsour.2004.03.043

• Herédy, Laszlo A. et al. (1975) Metal Sulfide Electrodes for Secondary Lithium Batteries. New Uses of Sulfur, Chapter 13, pp 203–215 doi: 10.1021/ba-1975-0140.ch013