To connect intermittent renewable energy sources (i.e., solar and wind) with variable output to the electrical grid, grid managers require electrical energy storage systems (EES) that can accommodate large amounts of energy created at the source. (Earlier post.) Lithium-ion rechargeable batteries perform well, but are too expensive for widespread use on the grid. Sodium is seen by some as a promising alternative, but the sodium-sulfur batteries currently in use run at temperatures above 300 °C, making them less energy efficient and safe than batteries that run at ambient temperatures.

Sodium-ion batteries have been discussed in the literature for some time. A battery that uses Na ions instead of Li ions is attractive because it could potentially be much less expensive and safer, and it would be more environmentally benign. A Na ion intercalation and storage mechanism is also scientifically interesting and challenging because Na ions are about 70% larger in radius than Li ions. This makes it difficult to find a suitable host material to accommodate the Na ions and allow reversible and rapid ion insertion and extraction.

—Cao et al.

Other work has shown that hard-carbon–based negative electrodes have been reported to deliver a capacity of 300 mAh g^-1 through a sodium ion insertion/extraction reaction, the authors note in their paper. However, they add, few studies have reported Na-ion battery cathode materials with decent performance.

Nanomaterials and nanotechnology have offered new opportunities to fine tune the structure and properties of well-established materials for energy applications. In particular, nanorods and nanowires have shown promising results for improving the capacity and stability of Li-ion batteries because of their short ion diffusion distance, good conductivity, and excellent stress tolerance. Here, we report the fabrication of single crystalline Na₄Mn₉O₁₈ nanowires with a high reversable capacity and exceptional cycling performance as Na-ion batteries. The Na₄Mn₉O₁₈ nanowire electrode material, after calcination at 750 °C, delivers a reversable capacity of 128 mAh g^-1 at 0.1 C (1 C corresponds to 120 mA g^-1) with an excellent initial capacity retention capability of 77% even after 1000 cycles at 0.5 C.

—Cao et al.

The nanowires were synthesized with a polymer-pyrolysis method. Na₄Mn₉O₁₈ has an orthorhombic lattice structure; the unit cell is made up of MnO₅ square pyramids and MnO₆ octahedra, which are arranged to form two types of tunnels: large S-shaped tunnels and smaller pentagon tunnels. The structure has three sodium sites: the Na site in the small tunnels is fully occupied, while the sites in the large S-shaped tunnels are only half occupied. Sodium ions in the large S-shaped tunnels are considered to be mobile and can potentially be reversibly extracted while the Na ions in the small tunnels are fixed and cannot be extracted. The Na ions in the large channels would produce a theoretical discharge capacity of 121 mAh g^-1.

The team treated the materials with temperatures ranging from 450–900 °C, then examined the materials and tested which treatment worked best. Using a scanning electron microscope, the team found that different temperatures created material of different quality. Treating the manganese oxide at 750 °C created the best crystals.

Using a transmission electron microscope at EMSL, DOE’s Environmental Molecular Sciences Laboratory on PNNL’s campus, the team saw that manganese oxide heated to 600 °C had pockmarks in the nanowires that could impede the sodium ions, but the 750 degree-treated wires looked uniform and very crystalline.

The team found that the faster they charged an experimental cell made with the material, the less electricity it could hold. This suggested to the team that the speed with which sodium ions could diffuse into the manganese oxide limited the battery cell’s capacity. To compensate for the slow sodium ions, the researchers suggest in the future they make even smaller nanowires to speed up charging and discharging.

This work was funded by the Department of Energy’s Office of Science and Office of Electricity Delivery & Energy Reliability.

Resources

• Yuliang Cao, Lifen Xiao, Wei Wang, Daiwon Choi, Zimin Nie, Jianguo Yu, Laxmikant V. Saraf, Zhenguo Yang and Jun Liu (2011) Reversible Sodium Ion Insertion in Single Crystalline Manganese Oxide Nanowires with Long Cycle Life, Advanced Materials doi: 10.1002/adma.201100904