What I know:

Coming from a physics background and a frequenter of more glib stack exchange sites, I only recently found out that intercalation is a thing when studying Lithium-Ion Batteries.

As I understand it, intercalation happens when a material, such as graphite, has "layers" which have a small charge. Oppositely charged particles become attracted to these layers and diffuse their way between the sheets. This is common in graphite, such as in this image from Wikipedia:

Potassium in Graphite

My understanding is that coulomb forces are responsible for forming this structure. The purple ions will not move out because they experience a coulomb attraction to the black sheets of atoms shown above.

I also remember from undergraduate and grade school classes that ionic bonds are also a thing. Once again, wikipedia has a great graphic showing what is going on in ionic bonding:

Ionic Bonding

Some chemical species looses a charge, one gains a charge, and the bond is formed by coulomb forces between the two.

Careful reading of my reasoning notes that these two situations arise because of coulomb forces shoving things together. This has lead me to a simple question: what is the difference between the attraction in intercalation and ionic bonding? I've asked around, and nothing seems to settle well with me.

The Explanations So Far

I have asked some people more knowledgeable in chemistry than me what the difference is, and they've not satisfied this question, as shown by the following italicized text. These explanations are:

  • Ionic bonds require the transfer of electrons, whereas intercalation does not. This explanation implies that two ions who became so in another way do not form ionic bonds with each other, in spite of their clear attraction to one another.
  • Ionic bonds are "evenly matched," whereas intercalated ions and their sheets are not. My problem with this is that the same mechanism still attracts them, so why call them something different? What about Tin(IV)Chloride, which matches 1 tin with 4 chlorines and intercalation of, say, $\text{KC}_8$?
  • The difference in the strength/length of the bonds is the difference. We physicists tend not to have different names simply because of magnitude; gravity makes things fall to each other, no matter the magnitude!
  • It's a "paper difference" only; intercalated cations are just as bonded to their negatively charged sheets as any other ion in other bonds. Paper differences are terrible, for many, many reasons!
  • Intercalation is a subset of ionic bonds. I'm actually okay with this, except it should be mentioned more clearly.

For the sake of clarity, my questions are: What is the difference between ionic bonding and the attraction in intercalation? If I've already hit a correct/best explanation, what about my reservations?

  • $\begingroup$ I'd also create the "intercalation" tag, but that's above my current rep here. $\endgroup$ – PipperChip Sep 28 '16 at 5:38
  • $\begingroup$ "Ionic bonds require the transfer of electrons" - nope... not quite. Ionic bonds can and will exist between any pair of oppositely charged species, and it doesn't matter how they came about. $\endgroup$ – orthocresol Sep 28 '16 at 6:18
  • $\begingroup$ And, very simplistically speaking, an ionic bond is a force of attraction between these cations and anions. Intercalation refers to the act of inserting a certain species into a compound that already exists. For example, you can intercalate $\ce{Li+}$ ions into $\ce{TiS2}$. Once you have shoved these $\ce{Li+}$ ions in (never mind how you get them in), then they may experience ionic bonds of their own, with $\ce{S^2-}$ ions. $\endgroup$ – orthocresol Sep 28 '16 at 6:30

To start, the animated graphic for NaF doesn't really show what is happening in a crystal of NaF. In the solid salt a particular $\ce{Na^+}$ cation is not associated with a particular $\ce{F^-}$ anion. Rather there is a regular 3D structure. So the lattice energy has to be summed over all ions in the ccrystal. The summation is given by the Madelung constant.

The image shown in the intercalation would belie the actual arrangement. Think of the graphite units as having various shapes/sizes. So a stack won't always have say 80 carbon atoms in each layer but may have 40 carbon atoms in one layer, 120 in the next and 80 in the third.

So an intercalation material won't have the nice 3D structure that an ionic bonded salt would have, even though there might be some stoichiometry for the intercalation material.


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