Sodium-ion battery

Sodium-ion batteries are a type of reusable battery that uses sodium-ions as a way to store power in a compact system. This type of battery is still in a developmental phase but is forecasted to be a cheaper, more durable way to store energy than commonly used lithium-ion batteries. Unlike sodium-sulfur batteries, sodium ion batteries can be made portable and are able to work at room temperature (approx. 25˚C).

Energy Storage

A sodium ion battery stores energy in chemical bonds in its cathode. When the battery is charging Na⁺ ions de-intercalate and migrate towards the anode. Meanwhile charge balancing electrons pass from the cathode through the external circuit containing the charger and into the anode of the battery. During discharge the same process occurs but in the opposite direction. Once a circuit is completed electrons pass back from the anode to the cathode and the Na⁺ ions travel back to the cathode.

Benefits

Currently lithium-ion batteries are the preferred portable battery for most electronic devices and hybrid cars. Unfortunately, lithium is in short supply, whereas sodium is much more abundant. If the modern day batteries were switched over to a sodium-ion cell it would greatly lower the price of the product. Researchers like Jay Whitacre, at Carnegie Mellon University, say that the sodium cell would also be able to store more energy which would make it a better option for storing renewable energy at solar and wind farms.

Research

A normal sodium cell voltage is 3.6 volts and is able to maintain 115 mA·hr g⁻¹ after 50 cycles. Which means the battery approximately has a storage capacity of 400 W·hr kg⁻¹ Yet, sodium-ion batteries are still unable to maintain a strong charge after repeated charge and discharge. After 50 cycles most sodium-ion batteries tend to store about 50% of original capacity. Researchers are now looking at different anode and cathode materials that will allow a sodium cell to maintain its original capacity.

Na/Na_xC₆

In 1999, D.A. Stevens and J.R. Dahn were the first to test a new anode material made of carbons, (Na_xC₆). They found the average voltage on the low potential plateau was higher on the Na cells compared to the Li cells. They also showed a glucose precursor can be used to make carbon materials with a high reversible capacity.

NaV_1−xCr_xPO₄F

This type of sodium-ion battery was tested by Haitao Zhuo, Xianyou Wang, Anping Tang, Zhiming Liu, Sergio Gamboa, P J Sebastian [Journal of Power Sources (2006), Volume: 160, Issue: 1, Pages: 698-703]Haitao Zhuo, Xianyou Wang, Anping Tang, Zhiming Liu, Sergio Gamboa, P J Sebastian [Journal of Power Sources (2006), Volume: 160, Issue: 1, Pages: 698-703]. They found that through the reaction:
NaF + (1−x)VPO₄ + xCrPO₄ → NaV_1−xCr_xPO₄F
the introduction of the Cr helped the battery retain more energy through cycles of charge and discharge. The effects of Cr doping on performances of the cathode materials were analyzed in terms of the crystal structure, chargedischarge curves and cycle performances. The results showed that the as-prepared Cr-doped materials have a better cycle stability than the un-doped one, an initial reversible capacity of 83.3 mAh g1 can be obtained, and the first chargedischarge efficiency is about 90.3%. In addition, it was also observed that the reversible capacity retention of the material is still 91.4% in the 20th cycles.The chart below (from their study) shows the differences between the battery with and without Cr.

Cathode materials	The first charge capacity (mAh g−1)	The first discharge capacity (mAh g−1)	Capacity loss in the first cycle (mAh g−1)	Reversible efficiency in the first cycle (%)	The discharge capacity at the 20th (mAh g−1)	The capacity retention ratio at the 20th (%)
NaV0.92Cr0.08PO4F	83.3	75.2	8.1	90.3	68.8	91.4
NaV0.96Cr0.04PO4F	93.3	82.6	10.7	88.5	67.9	82.2
NaVPO4F	106.9	87.7	19.2	82.0	64.5	73.5

Na₂FePO₄F

In 2007 researchers B. L. Ellis, W. R. M. Makahnouk, Y. Makimura, K. Toghill, and L. F. Nazar tested Na₂FePO₄F and Li₂FePO₄F cathode materials in rechargeable batteries along with a mixture of the two cathode materials. They found the sodium iron phosphate cathode easily replaces a lithium iron phosphate in a Li cell. The combined lithium-ion and sodium-ion make up would lower the overall price of the battery.

P2-type Na_x[Fe_1/2Mn_1/2]O₂

In 2012, researchers reported a new electrode material, P2-Na_2/3[Fe_1/2Mn_1/2]O₂, that delivers 190 mAh g−1 of reversible capacity in the sodium cells with the electrochemically active Fe³⁺/Fe⁴⁺ redo, saying that these results will contribute to the development of rechargeable batteries from the earth-abundant elements operable at room temperature.

Na₂FeP₂O₇

In 2012, researchers T. Honma, N. Ito, T. Togashi, T. Komatsu succeeded to fabricate a new cathode candidate triclinic Na₂FeP₂O₇ for rechargeable sodium ion second batteries by glass-ceramics method. The precursor glass, which is same composition in Na₂FeP₂O₇, was prepared by melt-quenching method. Na₂FeP₂O₇ exhibits 2.9 V, 88 mAh/g.