According to a report from AABC Week via Green Car Congress1, there are four phosphate formulations from which to choose for a cathode material: iron, cobalt, nickel and manganese. Manganese is essentially a voltage compromise between the other three. Swiss-based, HPL (High Power Lithium) claims a nano-structured manganese phosphate2 can offer high rate performance, yet be safe and durable.

With the promise of mass produced, advanced lithium batteries from companies such as Continental / A123Systems, Ener1, CPI, JCS (Johnson Controls-SAFT), etc. comes the anticipation that the cost will lower. One way to lower cost is to find lower costing raw materials.

Lithium-ion batteries with cobalt cathodes offer the highest energy densities. Nevertheless, cobalt-based lithium-ion has drawbacks; it offers a relatively low discharge current. A high load could overheat the pack. From a risk management perspective, it is better to switch to other cathode materials that are much cheaper, safer, and more stable.

Lithium Iron Phosphate and Lithium Manganese Oxide have emerged as substitutes. As previously noted, the former is patented as Phostech. Modified iron phosphate cathodes are the choice of A123Systems and Valence. LTC (Lithium Technology Corporation uses a Phostech iron phosphate as its cathode material; and JCS, one of the well-established large-format battery companies, recently switched to Phostech,

In the journal Nature Materials, Robert McLeod informs3, is an article about a new formulation of the iron-phosphate chemistry. In the new formulation, sodium is substituted for lithium. “This is potentially advantageous from a cost perspective; sodium is vastly more abundant.”

McLeod’s Synopsis

The basic formulation of the cathode for this battery is A2FePO4F, where A is either Li, Na or some mixture thereof (with a standard carbon anode). Most lithium-ion battery aficionados are aware that the phosphate chemistry is perhaps looked upon more favourably at the moment than nickel or manganese based ones. The substitution of the fluoride from a hydroxide (OH) is another innovation that results in a novel crystal structure.

The material seems to form favourably shaped porous crystallites with a very high surface area to volume ratio, as shown by scanning electron microscopy in the publication. The crystallites are about 200 nm across, which by my standards is still quite large (i.e. they have plenty of room to decrease it). The cells were producing ~ 3.6 V over a rather flat discharge curve, and maintained a capacity of 115 mA·hr g-1 after 50 cycles. That would correspond to a storage capacity of roughly 400 W·hr kg-1 for the battery bereft of any packaging.

Aside from the potential for replacing lithium, the authors also found little volume change when Na was lost from the NaFePO4F crystal. This implies there’s not a lot of stress on the crystal during reduction-oxidation (i.e. cycling), and hence, it may imply a high degree of reversibility (i.e. a battery fabricated from the material may be able to handle many cycles without damage). The volume change was found to be 3.7 %, compared to 6.7 % for conventional Li2FePO4OH chemistry.

Right now they appear to be suffering from a carbon coating on their material that appears to be a result of their fabrication method. This is having a negative effect on the conductivity of the material, which would impact battery efficiency.

Simulation of Lithium ion in solution, Daniel Spangberg, Uppsala Univ.

Reference:

Ellis, B.L., et al., “A multifunctional 3.5 V iron-based phosphate cathode for rechargeable batteries,” Nature Mat. 6:749 – 753 (2007).

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