Tuesday, June 7, 2011

Sodium ion Batteries

To connect solar and wind energy sources to the electrical grid, grid managers require batteries that can store large amounts of energy created at the source. Lithium ion rechargeable batteries, common in consumer electronics and electric vehicles, perform well, but are too expensive for widespread use on the grid because many batteries will be needed, and they will likely need to be large. Sodium is the next best choice, but the sodium-sulfur batteries currently in use run at temperatures above 300oC (572oF), making them less energy efficient and safe than batteries that run at ambient temperatures.

The electrodes in lithium rechargeable batteries that interest scientists are made of manganese oxide. The atoms in this metal oxide form many holes and tunnels that lithium ions travel through when batteries are being charged or are in use. The free movement of lithium ions allows the battery to hold electricity or release it in a current. But there are problems with simply replacing the lithium ions with sodium ions because sodium ions are 70 percent bigger than lithium ions and don't fit in the crevices as well. So, the scientists needed to find a way to make the holes in the manganese oxide larger. They have done this by mixing different kinds of manganese oxides at different temperatures. The manganese oxide formed after mixing at 750oC (1382oF) created a material that had the best crystals, high capacity, and held up well to cycles of charging and discharging. Unfortunately, the scientists found that the faster they charged the sodium ion batteries, the less electricity these batteries could hold, and grid batteries need fast charging so they can collect as much newly made energy coming from renewable sources as possible.

Yuliang Cao, Lifen Xiao, Wei Wang, Daiwon Choi, Zimin Nie, Jianguo Yu, Laxmikant V. Saraf, Zhenguo Yang, Jun Liu. Reversible Sodium Ion Insertion in Single Crystalline Manganese Oxide Nanowires with Long Cycle Life. Advanced Materials, 2011; DOI: 10.1002/adma.201100904

Further Reading
Oxidation and Reduction
Oxidation State (oxidation number)
Batteries and Fuel Cells
Trends in Atomic Radius

Study Questions
  1. Explain what is meant when an electrochemist refers to
    • oxidation
    • reduction
    • redox

  2. What do Chemists mean when they refer to
    • electrochemical cell
    • battery
    • primary cell
    • secondary cell

  3. Write a balanced half-equation for each of the following:
    • oxidation of sodium atoms
    • oxidation of lithium atoms

  4. The following compounds have been used as positive electrodes in lithium ion batteries:
    • LiCoO2
    • LiMn2O4
    • LiNiO2
    • LiFePO4
    • Li2FePO4F
    For each of the compounds above, determine the oxidation state (oxidation number) of the transition metal in the compound.

  5. Explain why sodium ions are larger than lithium ions.
  6. Why is the size of the metal ion important in the functioning of the rechargeable batteries being discussed?
  7. Could potassium ions be used instead of lithium ions in the rechargeable batteries under discussion? Explain your answer.
  8. Could beryllium ions be used instead of lithium ions in the rechargeable batteries under discussion? Explain your answer.

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