Sodium Ion Battery Technology for High Efficiency Energy Storage
Authors: Mason Chad W. , Gocheva Irina , Hoster Harry E. 
Published: February 2020

Sodium ion battery (NIB) technology is a potential option for the large scale deployment of grid-connected energy storage. With more development, they could provide benefits over lithium ion battery technology due to abundant sodium-containing resources and different types of intercalation hosts. Like LIBs, NIBs can also offer high round trip energy efficiency – full cells can easily exceed 90% which is something few other battery technologies can achieve.(1)
Electrodes based on Natrium super ion conductor (NASICON) type materials, such as Na3V2(PO4)3& NaTi2(PO4)3, and other fast ion conductors, such as Na3V2(PO4)2F3, have been shown to be capable of enabling such efficient systems.(2-5) Yet their energy density in various full cell configurations relative to lithium systems has been shown to be insufficient to draw significant attention, despite their other inherentlyattractive properties. Based on our calculations, LIB full cells using LiFePO4/Li4Ti5O12 and Li3V2(PO4)3/Li4Ti5O12can attain about 150 and 200 mWh/g of energy density at low C-rates,respectively. To be competitive, NIBs would need to match these energy density values with similar material manufacturing costs. This will require capacities enhanced by at least 50%, high voltage cathode materials, low voltage anodes, or a combination of these.
Our group has examined the electrochemistry of NASICON materials and other phosphates in detail in order to identify what improvements could be made and how, in order to become more competitive with LIBs. Substitution into the transitional metal site was shown to be a useful approach to this problem for lithium ion electrode materials, and thus we have been applying this technique for sodium ion systems, using modern synthesis techniques.(6, 7)
Two new NASICON anode materials were developed - Na2VTi(PO4)3 & NaVNb(PO4)3, shown in the top and middle image, respectively. Through transition metal substitution, two of NASICON’s four available sodium sites can be used at lower voltages than what is otherwise possible with pristine Na1+xTi2(PO4)3 or Na3+xV2(PO4)3.
On the cathode side, iron has been investigated thoroughly for use in the NASICON structure. Fe2(SO4)3 was shown to be an interesting electrode material simply from an economic standpoint, as it is extremely low cost and can be used “off-the-shelf.”(8) Furthermore, iron substitution in place of vanadium in Na3V2(PO4)3 demonstrated that not only more capacity could be attained, but also that the V5+oxidation state could be made active in NASICON (bottom image).
In our presentation, we will discuss our findings with regard to NASICON type electrode materials, along with a few other interesting materials. We will also show some of the full cell data we have acquired recently in order that the audience better appreciates some of the important issues at hand with available configurations. Our work has demonstrated that a large number of practical variations of NASICON materials through both transition metal and polyanion group substitution are possible. We hope and believe this presentation will stimulate further thought and discussion into the general area of sodium ion battery technology and how to successfully address the pressing need for economically viable and effective storage technology for large scale applications.
References
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