The team measured peak capacity at 128 milliAmp hours per gram of electrode material as the experimental battery cell discharged. This result surpassed earlier ones taken by other researchers, one of which achieved peak capacity of 80 milliAmp hours per gram for electrodes made from manganese oxide but with a different production method. The researchers think the lower capacity is due to sodium ions causing structural changes in that manganese oxide that do not occur or occur less frequently in the heat-treated nano-sized material.
In addition to high capacity, the material held up well to cycles of charging and discharging, as would occur in consumer use. Again, the material treated at 750 Celsius performed the best: after 100 cycles of charging-discharging, it lost only 7 percent of its capacity. Material treated at 600 Celsius or 900 Celsius lost about 37 percent and 25 percent, respectively.
Even after 1,000 cycles, the capacity of the 750 Celsius-treated electrodes only dropped about 23 percent. The researchers thought the material performed very well, retaining 77 percent of its initial capacity.
Last, the team charged the experimental cell at different speeds to determine how quickly it could take up electricity. The team found that the faster they charged it, 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 -- when charged fast, the sodium ions couldn't enter the tunnels fast enough to fill them up.
To compensate for the slow sodium ions, the researchers suggest in the future they make even smaller nanowires to speed up charging and discharging. Grid ba
|Contact: Mary Beckman|
DOE/Pacific Northwest National Laboratory