by Oliver J Gross
Senior Battery Engineer
Valence Technology Inc.

Lithium-ion (Li-ion) battery technology has been in the marketplace for fifteen years, but it has not been widely accepted into applications other than the portable electronics. Lead acid (PbA), Nickel Cadmium (NiCd) and Nickel Metal Hydride (NiMH) have remained the dominant battery chemistries for applications in the utility, motive and consumer markets. Batteries used in these markets are typically higher energy than those used for portable electronics devices, often in the 250Wh – 50kWh range. Li-ion is attractive to these markets, due to its high volumetric and gravimetric energy density, long cycle life and excellent storage characteristics, but only if particular issues associated with traditional Li-ion implementation can be addressed.

Concerns over both cost and safety have precluded the use of Li-ion in many larger applications. Traditional Li-ion cells utilize lithiated metal oxides for their cathode active material, with lithium cobalt oxide (LCO) and variations thereof being the most popular. The cost, environmental sensitivity and toxicity of cobalt present a concern for the use of LCO. Metal oxide-based cathode materials are also prone to decomposition under abuse conditions, whereby they release oxygen into the cell, leading to thermal runaway of the cell.

In recent years, a new class of lithiated metal phosphate (LMP) cathode materials has emerged. The metal (M) can be a transition metal or mixture of transition metal and non-transition metal. This material exhibits significantly greater thermal stability than lithiated metal oxides and does not liberate oxygen on decomposition. This means it is less prone to thermal runaway under abusive conditions. The most common form of LMP also does not contain any environmentally sensitive materials.

The physical and thermal properties of LMP lend themselves well to consideration for use in larger Li-ion cells and large packs made with LMP cells. The synthesis method for LMP also makes use of low-cost precursors, leading to a significant reduction in materials cost over other metal oxide cathode materials.

The differences between LMP and metal oxide Li-ion cells need to be taken into account in order to best utilize LMP cells in an end application. Cells made with the more common LMP cathodes will operate at a lower average voltage than with metal oxides (3.2-3.3V vs. 3.6-3.8V), and the overall cell energy densities will be lower than for metal oxides, when considering traditional Li-ion cell sizes (i.e.: 18mm x 65mm cylindrical and 6mm x 34mm x 48mm prismatic). Today’s and most commonly used LMP cathode material is therefore not normally considered practical for cells used in small portable electronic applications.

In order to fabricate larger cells and battery packs, consideration must be made for the reactivity of the materials used. Combinations of large arrays of small cells or the use of large cell designs can both lead to thermal runaway issues, where an exotherm in one cell can heat an adjacent cell to the point that it too undergoes rapid self-heating. In the case of lithium metal oxide-based cathode cell designs, the safety induced trade-off leads to cell energy densities lower than those achieved in the smaller cell designs. Designs using LMP cathodes do not require such a significant compromise in energy density, due to the higher stability of the cathode. The result is a LMP battery that is not significantly lower in energy density than one made with metal oxide-based cathodes, yet is much safer and more environmentally friendly.

Valence Technology was the first to commercialize, under the Saphion® brand, lithium-ion cells using phosphate-based cathodes. Along with thin profile polymer cells, Valence has focused on implementing LMP cathodes into conventional cell construction, and scaling batteries into large formats and higher voltage applications.