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.
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.
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.
In 1999, D.A. Stevens and J.R. Dahn were the first to test a new anode material made of carbons, (NaxC6). 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.
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 → NaV1−xCrxPO4F
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 (%)|
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 Nax[Fe1/2Mn1/2]O2 
In 2012, researchers reported a new electrode material, P2-Na2/3[Fe1/2Mn1/2]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.
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.
- Kevin Bullis, “Sodium-Ion Cells for Cheap Energy Storage”, Technology Review, Published by MIT,Wednesday December 2, 2009, http://www.technologyreview.com/energy/24043/page1/
- "About Sodium-Sulfur (NaS) Batteries" The Energy Blog, January 18, 2006, http://thefraserdomain.typepad.com/energy/2006/01/sodiumsulfur_na.html
- Steven S. Zumdahl "Chemical Principles, 6th Edition" Cengage Learning 2009 p.495
- “Sodium Ion Batteries”, Entropy Production, October 1, 2009, http://entropyproduction.blogspot.com/2007/10/sodium-ion-batteries.html
- "The preparation of NaV1−xCrxPO4F cathode materials for sodium-ion battery"Haitao Zhuoa, Xianyou Wanga, Anping Tanga, Zhiming Liua, Sergio Gamboab and P.J. Sebastianb, Science Direct,Received 1 December 2005; revised 18 December 2005; accepted 22 December 2005. Available online 14 February 2006. http://www.sciencedirect.com/science/article/pii/S0378775306000358
- D. A. Stevens and J. R. Dahn, “High Capacity Anode Materials for Rechargeable Sodium-Ion Batteries”, Journal of The Electrochemical Society, vol.147,1271-1273 (2000)http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JESOAN000147000004001271000001&idtype=cvips&gifs=yes
- Ellis, B. L.,Makahnouk, W. R. M.; Makimura, Y.,Toghill, K.,Nazar, L. F. " A multifunctional 3.5[thinsp]V iron-based phosphate cathode for rechargeable batteries"Nature Publishing, 1476-1122 http://www.nature.com/nmat/journal/v6/n10/full/nmat2007.html
- P2-type Nax[Fe1/2Mn1/2]O2 made from earth-abundant elements for rechargeable Na batteries, Nature Materials 11, 512–517 (2012) doi:10.1038/nmat3309
- Fabrication of Na2FeP2O7 glass-ceramics for sodium ion battery, Journal of the Ceramic Society of Japan 120, 344–346 (2012) doi:10.2109/jcersj2.120.344