Army

Composite cathode for Li-ion batteries

Breakthrough class of high-performance cathodes developed specifically for Li-ion batteries

Energy

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This innovation could improve large-scale Li-ion batteries designed for use in mass transit vehicles (source: DOE)

Since they were first available in 1991, Li-ion batteries have become ubiquitous in portable electronics, telecom devices, and the transportation industry. They are manufactured at a high rate, and production continues to expand to meet growing demand. Yet, there are two major problems limiting the wider application of Li-ion cells: cost and safety. This is particularly so with cathodes common in today’s batteries that use cobalt and similar expensive and toxic metals.

Lithium iron phosphate (LiFePO4) was first proposed in 1997 as an alternative to the standard cathode composition, lithium cobalt oxide (LiCoO2).  A cathode featuring iron phosphate instead of cobalt is attractive for a number of reasons: better thermal stability; safer at a fully charged state; low reactivity with electrolyte; and no toxic metals. Plus, iron phosphate is prepared from raw materials far less costly than cobalt. Unfortunately, LiFePO4 has far lower electronic conductivity, which translates to weak power output. However, there has been worldwide research activity to find methods to remediate this shortcoming. The ARL response focuses on a new method of preparation of the cathode.  Their process includes compositing LiFePO4 with carbon and, if needed, doping with transition metals to tailor operating characteristics.

There are two main approaches to overcome low-power capability of LiFePO4 cathodes. One is to coat the cathode materials with an electron-conducting layer, or by doping with conductive metal cations. The other is to reduce the particle size of the material by modifying the synthesis conditions, which can be accomplished using the solution method or the solid-state reaction method. The solution method involves chemical reaction in the liquid phase. This offers some advantage in creating a homogeneous mixture, however, the process requires additional care to remove the solvent. In contrast, the solid-state reaction offers an easier and more practical approach, and it is more suitable for large-scale production.

ARL researchers optimized the process by testing several combinations of starting materials (different forms of carbon) and processing steps, which include heating, grinding, pelletizing, and reheating. While each combination yielded different performance traits, researchers confirmed that the novel composite cathode, LiFePO4-C, shows much better performance in terms of the discharge capacity and cycling stability than LiFePO4 alone.

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