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23 October 2014

What Causes Lithium-Ion to Age Part 2

When charging lithium tends to gravitate to the negative electrode, or the graphite anode. When this happens the voltage potential changes. A full reset does not occur when the lithium is removed. Solid electrolyte interface (SEI), a film that consists of lithium atoms appears on top of the anode. SEI is composed of lithium carbonate and Lithium oxide. As the battery goes through these cycles the film thickens and forms an obstruction.

The positive electrode (cathode) forms a similar layer of film that is known as electrolyte oxidation. Dr. Dahn’s research has make it clear that any voltage that is more than 4.10V/cell causes this to occur. This process gets worse the longer the battery stays in these conditions. While experts have known that this happens for some time, Dr. Dahn’s system of measuring CE is more scientific and give us a better understanding of the process.

Coulombic efficiency measures changes on both the negative electrode and the positive electrode. The results of using this formula can help to rank how long a battery might last. If we were able to produce a battery with a CE of 1,000,000 the battery would last forever according to Dr. Dahn’s calculations. A fantastic CE is .9999. This is a level that some Lithium cobalt Oxides reach. When looking at CE the best Lithium-ion batteries are LTO’s that use titanate as an anode. These batteris can deliver as many as 10,000 cycles; however the are very expensive and not very economical. The energy density is also low.

Temperature and change rate can have an impact on CE readings. Electrolyte oxidation can cause the batteries to discharge on their own. At low temperatures lithium-ion looses approximately 2% each month. At higher temperatures this number can be as much as 35% when the battery is fully charges.

Over the years Lithium-ion has improved and much of the credit goes to electrolyte additives. Manufacturers are adding several additives to each cell of the battery; however the formula used has been kept tops secret. These additives help to diminish resistance, prevent erosion and improve manufacturing speed. The can also decrease gassing and improve performance at extreme temperatures. Vinylene added to the cells improves the over all SEI on anodes, and restricts electrolyte oxidation on the cathode. It also enhances the Coulombic efficiency readings. Some manufacturers use other additives. Some worry that these will react with one another. Using Dr. Dahn’s CE system it allows us to assess these reactions much more quickly. Things that used to take us year to figures out, we can find a conclusion to in just a few short weeks.

When looking at the distinction between CE and the life of a battery Dr. Dahn”s team works with battery manufacturers. Most manufacturers keep their formulas secret, so the university if only able to make their own assumptions about what is being used in the cells.
These additives help to diminish resistance, prevent erosion and improve manufacturing speed.
The team at Dalhousle University has been able to identify five batteries of interest. Table 2 gives information on the Coulombic efficiency of samples with values ranging from .9960 to .9995. The third table shows the results when the batteries were cycled until all the energy was depleted. The results show that the batteries that were ranked highest by CE lasted outlasted those with the lowest CE.

Cycle testing helps to test the wear and tear of batteries. Measuring the coulombic efficiency is beneficial in developing new batteries that will last longer. It tells us which additives are best and which are making lithium-ion batteries die more quickly. The battery that came standard in the Nissan Leaf did well in lab tests, but the scientists may have overlooked the damage that is done when the battery is kept at a high temperature.

There are four suspected reasons that contribute to battery capacity loss.

• Mechanical degradation of electrodes
• Growth of SEI on the Anode
• Electrolyte oxidation formation on the cathode
• Lithium-plating on the surface.

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