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.
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.
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.