Large-scale energy storage systems are needed to deal with intermittent electricity production of solar and wind. While high-energy Li-ion batteries (LIBs) are expected to contribute in part to the solution, the high cost and low stability prohibit wide application in this area, the researchers observe.

Recently, attention has been refocused on room-temperature Na-ion batteries (NIBs) as a low-cost alternative technology as compared to LIBs. The abundance and low cost of Na in the earth will become advantageous when a large amount of material is demanded for renewable energy solutions. Moreover, because rechargeable Na batteries share many similarities with LIBs, opportunities for fast-advancing NIB research can be found in state-of-the-art LIB technologies.

...Despite the current rigorous search for high-performance NIB electrode materials, their overall electrochemical performance remains inferior to Li chemistry. The less negative redox potential of Na⁺/Na as compared to Li⁺/Li (−2.71 and −3.04 V vs standard hydrogen electrode) reduces the operating voltage, leading to a generally lower energy density. The larger Na⁺ ion as compared to the Li⁺ ion (1.02 vs 0.59 Å) causes greater change in the host structure upon insertion or removal, which often results in poorer cycle life or even sluggish diffusion. Such fundamental demerits must be overcome to advance NIBs and their wide application as an alternative to LIBs.

In this study exploring a new NIB cathode, we attempted to counterbalance these limitations by searching for a material chemistry with a high redox potential and a rigid open framework that is less sensitive to the volume change from the guest ion insertion. In this regard, we focused on polyanion-based open crystal frameworks with vanadium redox couples.

—Park et al.

Other studies recently have shown that vanadium redox can exhibit high voltage in a Na cell. Further, the authors had earlier demonstrated that multi-electron transfer of a vanadium redox couple is possible in certain polyanion frameworks maintaining its high potential. In a prior study, they developed a new Li-ion battery electrode—Li_1.1Na_0.4VPO_4.8F_0.7 by tailoring the polyanion group of the crystal and expanding the redox range to V3.8+/V5+ in the high-voltage region. This tuned redox couple provided an additional 0.2 electrons per formula unit, as compared to the one-electron transfer reaction of V⁴⁺/V⁵⁺.

In their new paper, the researchers show that Na_1.5VPO_4.8F_0.7—the precursor of Li_1.1Na_0.4VPO_4.8F_0.7—can function as an excellent cathode for rechargeable sodium-ion batteries with a high energy density.

The study found that the material exceeds the advanced P2-type layered oxide with the highest energy density to date. The researchers discovered that most electrochemical properties of the material—such as its power capability and long- term cycle life—outperform those of existing cathodes for NIBs and even rival those of LIB cathodes such as olivine LiFePO₄ and spinel LiMn₂O₄.

The open framework of Na_1.5VPO_4.8F_0.7 allowed fast conduction of Na⁺ ions through two-dimensional diffusional pathways, which resulted in excellent rate capability. We believe that our study of this novel Na_1.5VPO_4.8F_0.7 cathode with fascinating electrochemical properties can provoke stimulating discussions about the next-generation NIB and its potential new applications.

—Park et al.

Resources

• Young-Uk Park, Dong-Hwa Seo, Hyung-Soon Kwon, Byoungkook Kim, Jongsoon Kim, Haegyeom Kim, Inkyung Kim, Han-Ill Yoo, and Kisuk Kang (2013) A New High-Energy Cathode for a Na-Ion Battery with Ultrahigh Stability. Journal of the American Chemical Society doi: 10.1021/ja406016j