What chemical compound exists at the cathode of a lithium-ion battery when the lithium is not there?

The cathode is usually described as $\ce{LiCoO2}$, so does it become $\ce{CoO2}$?

$\ce{CoO2}$ puts cobalt in an oxidation state of +4, which is possible but rare and unstable. I think $\ce{CoO2}$ doesn't even have a Wikipedia page.

  • $\begingroup$ CoO2 exists: cobalt.atomistry.com/cobalt_dioxide.html $\endgroup$ Jun 2 '20 at 16:00
  • 1
    $\begingroup$ Not an expert on this, but I think this is one of the reasons you don't ever want to overcharge a LiCoO2 battery... $\endgroup$
    – orthocresol
    Jun 2 '20 at 16:12
  • $\begingroup$ It is quite complicated. See sciencedirect.com/science/article/pii/S0364591613000035 for some of the thermodynamics and crystal structures. $\endgroup$
    – Jon Custer
    Jun 2 '20 at 20:31
  • $\begingroup$ @JonCuster That statement is a bit over the top, as is the paper you link as a first read. The general principles of the Li ion battery have been textbook knowledge for quite a few years now. $\endgroup$
    – Karl
    Jun 2 '20 at 20:36
  • 1
    $\begingroup$ @Karl - but you did a good job at it! $\endgroup$
    – Jon Custer
    Jun 2 '20 at 23:47

The general textbook formula for the Li ion battery is


, and the simplest/earliest type has $y=1$, i.e. cobalt-only. These "mixed (lithium transition metal) oxides" have a layered structure (see e.g. wikipedia), where you can relatively easily electrochemically remove (and later reinsert) a part $a$ of the Li atoms, without destroying the whole structure. You cannot increase $a$ to much more than 0.1 - 0.3 or so, depending on the exact formulation/grain size/quality/temperature/etc., before the structure does break down.

Even the "uncharged" $a=0$ Li-cobalt(III) oxide gives off oxygen (!) if heated above 180°C. (It is produced by tempering at ~800°C in a pure oxygen atmosphere.) With $a>0$, this happens earlier already, and basically always leads to a runaway destruction of your whole battery setup. With higher contents of Ni and Mn, this danger becomes less pronounced. I´m not sure the Co-only variant was ever commercialised. (?)

Simply speaking, a cobalt-rich battery blows up when overcharged, and in ones with more nickel and manganese the layered structure still breaks down, and you get somewhat stable Ni/Mn(III/IV) species (and still quite a lot of overheating/general destruction). In any case this happens long, long before you reach the ideal $\ce{CoO2}$ (or generally $\ce{MO2}$) stochiometry.

If you look closely (e.g. via XRD) at the crystal structure during charging, you seemingly find that there are a number of intermediate deformed structures, and even more mixed/disordered states in between. And that of course becomes even more complicated (or washed out) with a (more or less) random arrangement of different transition metals in the structure, which will depend also on the specific synthesis route.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.