Sodium-ion Batteries

In a new issue of the journal Nature Materials, there is an article about a new formulation of the lithium-iron-phosphate chemistry, but with sodium substituted for lithium. This is potentially advantageous from a cost perspective, especially as concerns have been raised as to whether or not their are sufficient proven reserves of lithium metal to convert the entire world's vehicle fleet to lithium-ion batteries. Sodium is vastly more abundant.

The citation is, B.L. Ellis et al., "A multifunctional 3.5 V iron-based phosphate cathode for rechargeable batteries," Nature Mat. 6:749 - 753 (2007). Nature only provides the first paragraph to the public, so I will try to provide a 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 maganese 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.

Overall, an interesting development that points to plenty of room left for chemistry advances in the area of battery technology. There is also a lot of room for this material in terms of its material science, being brand new and hardly optimized for performance.