Clean energy with a pinch of salt

Global energy consumption is projected to increase steeply over the next two decades, creating great demand for alternatives to oil. A major barrier to the wide-ranging application of renewable energy sources is continuity of supply. Solar and wind power generation, for example, can be hampered by time of day, dust, cloud, and other weather conditions. This barrier can be most effectively overcome by large-scale energy storage systems suitable for a broad range of applications. A sustainable battery could have applications in ‘smart grids’ that use technology effectively to reduce dependence on centralized power stations.

Conventional large-scale batteries use electrolytes, such as molten salt or molten sulphur, and only work at high temperatures, making them expensive and impractical. On the other hand, lead-acid batteries are cheaper and widely available, but are very corrosive and pollute the environment. While commonly used rechargeable lithium batteries have a relatively high voltage, sodium—which shares many of lithium's properties—is cheaper and more abundant.¹This low cost means that they could open up affordable green energy to the developing world.

To form a current, lithium ions travel out of the cathode and into the anode. In an imperfect analogy, the cathode and anode act like mesh filters that the ions must pass through. However, sodium ions are roughly two-and-a-half times the size of lithium ions, and a major challenge is finding a suitable ‘host material,’ with larger gaps in the mesh, to suit ions of this size.

Figure 1. Aqueous rechargeable (sodium) cell. Na⁺: Sodium ion. OH^-: Hydroxide ion. MnO₂: Manganese (IV) oxide. V: Voltage. Zn: Zinc.

We have shown that manganese (IV) oxide (MnO₂) can act as a host material for the cathode. Sodium insertion/extraction (into/from vacant sites in MnO₂) can occur reversibly in a device that uses an aqueous sodium electrolyte with an enhanced cell capacity.² The sodium intercalation occurs in the bulk of the MnO₂cathode material.³

In our experiments, we used cells with an ‘electrolytic manganese dioxide’ cathode consisting of an intergrowth of ramsdellite (γ-MnO₂, which has an orthorhombic structure and higher electrochemical activity) and pyrolusite (β-MnO₂, which is the stable form of MnO₂ at ambient conditions and has an tetragonal rutile structure and lower electrochemical activity). We used a zinc anode and an aqueous sodium hydroxide (NaOH) electrolyte. The cells were discharged/ charged galvanostatically while imposing a constant current with an eight-channel battery analyzer. We collected data using a software battery testing system, and the battery experiments were cut off at discharge and charge voltages of 1.0 and 1.8V, respectively. We carried out all electrochemical measurements at ambient temperature (25^°C).

The MnO₂/ramsdellite/pyrolusite cathode has wide, tunnel-like vacant sites suitable for accommodating the large Na⁺ ions. Its stable crystal structure makes it possible to reversibly insert/extract Na⁺ ions in MnO₂ over multiple cycles. In comparison to other cathode materials we examined (i.e., the commonly used olivine—LiMPO₄, where M is cations of manganese, iron, cobalt, or nickel—and maricite—NaMPO₄),^{4, 5} layered MnO₂ is found to be highly amenable in aqueous solutions.⁶

We used proton-induced x-ray and gamma-ray emission to evaluate the elemental concentration of elements on the surface and in the bulk of discharged and charged MnO₂samples (see Figure 2). We also analyzed structural changes using x-ray diffraction. The discharged MnO₂ samples confirmed that Na⁺ intercalation occurs in the bulk (see Figure 3). Cyclability data showed that the cell is suitable for multiple cycles with 90% efficiency.

Figure 2. X-ray diffraction pattern of MnO₂ (a) before and (b) after discharge, showing structural variations involving sodium intercalation in NaOH aqueous electrolyte. a.u.: Arbitrary units.

Figure 3. Concentration of elements present in the cathode MnO₂before and after discharge, determined by proton-induced x-ray emission.

Figure 4. Charge-discharge profiles of zinc–MnO₂cells in 5M lithium hydroxide and NaOH electrolytes.

Unlike some commonly used lithium ion electrolytes, which have caused fires on overheating (for example as has been reported in the Boeing Dreamliner airplane),⁷ aqueous electrolytes have the advantage of not being flammable. Despite this, the energy density of the NaOH electrolyte is very high (see Figure 4).

We now plan to build an aqueous hybrid cell—based on a coupling of the battery MnO₂ material (cathode) with maricite/activated carbon capacitor material (anode)—that will increase the high rate capability (i.e., the ability to deliver more power in a short period of time) to store energy generated from non-conventional energy sources.

We thank the Australian Research Council Discovery Project funding scheme (DP1092543) and the Australian Institute of Nuclear Science and Engineering (ALNGRA13067) for financial assistance and for access to ion beam analysis at the Australian Nuclear Science and Technology Organization.