Lithium-ion batteries

When you walk around having a chat to your friends on your mobile phone, or watch a show on your tablet, or do your homework on your laptop in a cosy cafe, have you ever stopped to wonder about the amazing revolution in chemistry that allows you to do these "every day" things?

Batteries that are small, that can store enough electrical energy so that they can be used continuously for hours, can be quickly recharged, and can be discharged and recharged many, many times, are a very recent development. Without these batteries your life-style would be a lot less mobile!

What makes these batteries so special?

Find out in the December 2019 issue of AUS-e-NEWS

https://www.ausetute.com.au/ausenews.html

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 300^oC (572^oF), 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 750^oC (1382^oF) 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.

Reference
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.

Hydrogen from Water Splitting

The production of hydrogen as an alternative fuel to current fossil fuels relies on the creation of a suitably cheap and efficient way to split water using the power of sunlight. Monash University scientists in Australia, working with UC Davis scientists in the USA, have found that a manganese mineral known as birnessite can be used as a catalyst to speed up the splitting of water into hydrogen and oxygen gases.

Birnessite, a soft, black mineral formed from precipitation reactions in lakes, oceans and groundwater, is predominantly an oxide of manganese, but calcium, potassium and sodium are also present in smaller amounts.
The formula for birnessite is (Na_0.3Ca_0.1K_0.1)(Mn⁴⁺,Mn³⁺)₂O₄ · 1.5 H₂O
As a catalyst for the water splitting reaction, the manganese in the birnessite cycles between oxidation states. First, when a voltage is applied manganese (II) is oxidized to manganese (IV). Then in sunlight, birnessite goes back to the manganese (II) state.

The water splitting reaction has two steps:

1. Two molecules of water are oxidized to form one molecule of oxygen gas, four protons and four electrons.
   
2. The protons and electrons combine to form two molecules of hydrogen gas

Reference:
Rosalie K. Hocking, Robin Brimblecombe, Lan-Yun Chang, Archana Singh, Mun Hon Cheah, Chris Glover, William H. Casey, Leone Spiccia. Water-oxidation catalysis by manganese in a geochemical-like cycle. Nature Chemistry, 2011; DOI: 10.1038/nchem.1049

Further Reading
Oxidation States (Numbers)
Oxidation and Reduction
Balancing Half Equations
Electrolysis - Electrolytic Cells
Percentage Composition

Study Questions:

1. What is meant by the term oxidation state (or oxidation number)?
2. What is the oxidation state (or oxidation number) for each of the following:
   
   • Mn³⁺
   • Mn⁴⁺
   • manganese (II)
   • manganese (IV)
     
3. Write equations to represent each of the following:
   
   • The oxidation of manganese (II) to manganese (IV)
   • The reduction of manganese (IV) to manganese (II)
4. For each reaction in question 3 above, identify:
   • the oxidant
   • the reductant
     
5. Write an equation to represent the first step in the water splitting reaction.
6. Write an equation to represent the second step in the water splitting reaction.
7. Use the equations in question 5 and 6 above to write an overall reaction for the water splitting reaction.
8. For each equation in questions 5 and 6,
   • label the reaction as an oxidation or reduction reaction
   • identify the oxidizing agent(s)
   • identify the reducing agent(s)
9. In the formula of birnessite, (Na_0.3Ca_0.1K_0.1)(Mn⁴⁺,Mn³⁺)₂O₄ · 1.5 H₂O, what does the 1.5 H₂O mean?
   
10. Calculate the percentage composition of birnessite.
    
    

Looking Inside Lithium Ion Batteries

Lithium-ion batteries are used to power electronic devices such as mobile phones (cell phones) and are widely used because of their low weight, high energy density and recharging ability. If scientists could see the batteries working at the nanoscale, observing the functionality of the batteries at the level of a single grain or an extended defect, they could determine what makes one battery work and another one fail.
Department of Energy's Oak Ridge National Laboratory (ORNL) scientists have developed a new type of scanning probe microscopy called electrochemical strain microscopy (ESM) to examine the movement of lithium ions through a battery's cathode material. They showed that the lithium ion flow could concentrate along grain boundaries, leading to cracking and battery failure.

Reference:
N. Balke, S. Jesse, A. N. Morozovska, E. Eliseev, D. W. Chung, Y. Kim, L. Adamczyk, R. E. García, N. Dudney, S. V. Kalinin. Nanoscale mapping of ion diffusion in a lithium-ion battery cathode. Nature Nanotechnology, 2010; DOI: 10.1038/nnano.2010.174

Further Reading:
Batteries
Galvanic Cells
Oxidation and Reduction

Study Questions

1. What is the difference between a battery and an electrochemical (galvanic or voltaic) cell?
2. Is the lithium-ion battery described in the article an example of a primary or secondary cell? Explain your answer.
3. Draw a sketch of a galvanic (voltaic) cell. Label the anode, cathode, and electrolyte. Clearly show the direction of electron flow through the cell.
4. Explain how the galvanic (voltaic) cell above could be recharged.
5. In the lithium-ion battery in the article above, will lithium ions be produced at the anode or the cathode while the battery is being discharged?
6. Describe the movement of lithium ions in the lithium-ion battery described above during the process of recharging the battery.