Cheaper Batteries for the Grid Based on Sodium Ions - Solar Novus Today

Cheaper Batteries for the Grid Based on Sodium Ions - Solar Novus Today

Researchers from the Department of Energy's Pacific Northwest National Laboratory (PNNL) and visiting researchers from Wuhan University in China have developed a method that improves the electrical capacity and recharging lifetime of sodium ion rechargeable batteries, which could be a cheaper alternative for large-scale uses such as storing energy on the electrical grid.

The uniform nanostructure of heat-treated manganese oxide provides tunnels for sodium ions to flow through, improving the performance of the electrodes. Credit: PNNL. To connect solar and wind energy sources to the grid requires batteries to store large amounts of energy created at the source. Lithium ion rechargeable batteries perform well but are expensive to use at the scale needed for grid applications. Sodium is the next best choice, but the sodium-sulfur batteries currently in use operate at temperatures above 300 °C, making them not as energy efficient or safe as batteries that run at ambient temperatures.

Battery developers want the best of both worlds: inexpensive sodium and the type of electrodes found in rechargeable lithium batteries. The researchers at the Department of Energy's Pacific Northwest National Laboratory used nanowires to make electrodes that work with sodium.

The electrodes in lithium batteries are made of manganese oxide. The atoms in this metal oxide form holes and tunnels that lithium ions travel through when the batteries are in use or during charging. The free movement of lithium ions allows the battery to hold electricity or release it in a current. But simply replacing the lithium ions with sodium ions is problematic because sodium ions are 70 percent bigger than lithium ions and don't fit in the crevices as well.

To find a way to make bigger holes in the manganese oxide, PNNL researchers fabricated manganese oxide nanowires. They mixed two kinds of manganese oxide atomic building blocks. They found that treating the manganese oxide at 750 °C created the best nanowires, ones that were uniform and very crystalline. They dipped the electrode material in electrolyte containing sodium ions that help the manganese oxide electrodes form a current and charged and discharged the experimental battery cells repeatedly.

They measured a peak capacity of 128 milliamp hours per gram of electrode material as the experimental battery cell discharged, and the material held up well to cycles of charging and discharging. Even after 1,000 cycles, the capacity of the 750 °C -treated electrodes only dropped about 23%.

However, the faster the battery cell was charged, the less electricity it could hold. This suggested that the speed of sodium ions diffusion into the manganese oxide limited the battery cell's capacity. The researchers suggest that using even smaller nanowires might help improve the speed of charging and discharging.