22Dec 2014

You have got to be saying, “Wait!”, this is non sense. That perennial Periodic Table that we all took in high-school chemistry said that Lithium had an atomic number of 3, yes three, and Gold had an atomic number of 79. Gold is 28 times heavier than Lithium…are we preaching alchemy?

First, let me just say that this not a metaphor for lithium bringing financial rewards. Lithium is fairly abundant in the Earth’s crust in the form of lithium salt deposits, and second, I have yet to see a battery manufacturer that is reaping huge financial profits. Financial margins in battery manufacturing are dismally challenged, and the financial markets have not rewarded such performers. Dare I say A123, an old and no-more darling of Wall Street?

This is much simpler. You see, graphite turns into a pretty golden color when lithium diffuses and intercalates inside its carbon matrix. Think of the graphite as swiss cheese. Naturally, this is not evident to most of us since the battery is actually sealed and we don’t see what happens inside. But should you develop a transparent battery, then its colors shine. This is precisely what researchers from Michigan did, and while they were at it, they took some nice photographs of the graphite as it turned from black to gold.

Source: S. J. Harris et al., Chemical Physics Letters, 2010 (pages 265 – 274)

The first photograph in the sequence on the far left shows half of a small battery as we look through a transparent window. The battery is discharged, in other words, the graphite is empty of lithium ions. As the battery is slowly charged, lithium ions move from the cathode, the other electrode, with the direction of motion upwards in the screen. The lithium ions are physically intercalating themselves inside the carbon matrix. In the process, the graphite electrode swells to accommodate the physical presence of these lithium ions. This and the chemical bonds that the lithium forms with the carbon change the nature of the material, and consequently its optical properties. The graphite slowly turns from black to reddish and ultimately to gold color. Adding more lithium ions to the carbon ultimately results in the lithium depositing on the surface of the carbon as metallic lithium which has a silver-like appearance.

Now, take a deep look at the third photograph from the left. The bottom of the graphite electrode is red, indicating that it is beginning to fill up with lithium ions, yet the top of the electrode is still black.   This is called diffusion: Put a little red dye in a glass of water and see how it slowly diffuses into the surrounding water. The diffusion of lithium ions creates a steep gradient across the thin electrode (only 0.8 mm thick). Consequently, this puts enormous mechanical stresses across the graphite and is one of several causes of battery failure. This gradient, and consequent battery degradation, is the result of how you charge the battery, and in particular CCCV and its variants such as step charging.  You see, CCCV has no idea what may be the diffusion characteristics of the lithium ions. Charging at faster rates only makes this situation worse unless more clever charging algorithms are incorporated to mitigate this and other degradation effects.