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on PNNL‘s campus, the team saw that manganese oxide heated to 600 degrees had pockmarks in the nanowires that could impede the sodium ions, but the 750 degree-treated wires looked uniform and very crystalline.

But even the best-looking material is just window- dressing if it doesn‘t perform well. To find out if it lived up to its good looks, the PNNL-Wuhan team dipped the electrode material in electrolyte, the liquid containing sodium ions that will help the man- ganese oxide electrodes form a current. Then they charged and discharged the experimental battery cells repeatedly.

The team measured peak capacity at 128 milliAmp hours per gram of electrode material as the experi- mental 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 cau- sing 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% of its capacity. Material treated at 600° Celsius or 900° Celsius lost about 37% and 25%, respectively.

Even after 1,000 cycles, the capacity of the 750° Celsius-treated electrodes only dropped about 23%.

11-05 :: May/June 2011

The researchers thought the material performed very well, retaining 77% of its initial capacity.

Last, the team charged the experimental cell at dif- ferent 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 sug- gested 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 resear- chers suggest in the future they make even smaller nanowires to speed up charging and discharging. Grid batteries need fast charging so they can collect as much newly made energy coming from renewable sources as possible. And they need to discharge fast when demands shoots up as consumers turn on their air conditioners and television sets, and plug in their electric vehicles at home.

Such high performing batteries could take the heat off an already taxed electrical power grid.

Reference: Yuliang Cao, Lifen Xiao, Wei Wang, Daiwon Choi, Zimin Nie, Jianguo Yu, Laxmikant V. Saraf, Zhen- guo Yang and Jun Liu: Reversible Sodium Ion Insertion in Single Crystalline Manganese Oxide Nanowires with Long Cycle Life, In: Advanced Materials Early View, June 3, 2011, DOI 10.1002/adma.201100904: http://dx.doi.org/10.1002/adma.201100904