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A couple of days ago Jordan Giesige, who runs the informational youtube channel 'The limiting factor', released an ambitious video about Lithium-ion battery chemistry. The target audience: interested people with no background in chemistry. That video goes as far in depth as is possible within the length of that video.

My background is physics, I am active on physics.stackexchange, answering questions.

Eager to learn more I started looking for additional information. It appears there is little to no information at the level I am looking for. That is not surprising, of course, it is a rather specialized subject.

The following is an overview of how I currently understand the chemistry and electrochemistry of Lithium-ion batteries. My request is to correct me if you notice a misconception.

Rather than using the words 'anode' and 'cathode' I will simplify and I will use the names 'the graphite' and 'the cobalt' generically. That is, I'm aware there are other chemistries, such as iron phosphate chemistry, and that the cobalt is used in the form of a cobalt oxide; I'm using the name 'cobalt' generically to refer that aspect/side of the chemistry, whatever the actual implementation.


In the uncharged state there is a large population of lithium at the cobalt side, and the cobalt is in a particular oxidation state.

The electrolyte is an organic fluid, with an additive that readily forms a complex with lithium ions. That complex readily dissolves in the organic fluid. The manufacturing process introduces a population of lithium ions in the electrolyte.

(The first charging cycle (which happens at the factory), triggers an initial chemical process; that initial process is outside the scope of this question.)


Charging
When a standard charging cycle starts the following happens:
Electrons are being withdrawn from the cobalt side and are being delivered to the graphite side.

The electron depletion at the cobalt side facilitates a change in oxidation state of the Cobalt. This change in oxidation state drives release of lithium ions into the electrolyte. At the graphite side lithium ions enter the graphite structure, and obtain an electron from the graphite, from there on residing in the graphite in an intercalated state.

Maintaining charge
The electrolyte is designed to present a high energy barrier to dissolving of negative ions. Because of that energy barrier the probability of an electron migrating from the cobalt side to the graphite side is low.

Discharging
In the charged state the graphite is slightly electron enriched, the cobalt side slightly electron depleted.
As current start flowing the available electrons are attracted by the cobalt, and the cobalt changes oxidation state. The now negatively charged cobalt oxide causes lithium ions to go out of solution, joining the cobalt.

At the graphite side electrons are withdrawn, and as a consequence the graphite is withdrawing electrons from the intercalated lithium atoms. Lithium ions go out of intercalation, and dissolve in the electrolyte.


What I don't know is whether it is the case that on both sides the process of electron transfer contributes to the overall voltage of the battery cell.

It could also be, I don't know, that the maintenance of the discharge current is all due to one side, with the other side actually presenting a bit of a negative contribution.

If it is the case that one side presents a negative contribution: I don't know which side that might be.

It could be, I don't know enough about electrochemistry to guess, that during discharge the chemical process of the graphite robbing the Lithium's outer electron releases the energy that is driving the overall process.

Hence the title of this question:
The overall chemistry is like a bucket brigade, on uneven terrain. I'm interested to know which stages in the process are driving force, and which stages are being driven.




This new edit is a comment, I write it here because comment space is (for good reasons) small. This is not a request to expand the answer further. This is feedback on how the answer has helped me.

Independent of the details of the chemistry at the graphite side and at the cobalt side: if at one side more Lithium ions would be entering the electrolyte than at the other end would be leaving the electrolyte the electrolyte would become positively charged. Becoming positively charged would come at an energy cost, so that is not going to happen; something else will happen, whatever it takes that keeps the net charge of the electrolyte close to neutral.

I am aware that the direction in which a chemical reaction proceeds is also sensitive to the concentration of the reactants, and the smaller the difference in electrochemical potential the larger the contribution of sensitivity to concentration of reactants.

To my understanding: when the net reaction consists of a cascade of sub-steps then it isn't necessarily meaningful to try and attribute a property of 'being the driver of the overall reaction'.

To my understanding: since the amount of electrolyte is quite limited the rates of Lithium ions entering/leaving the electrolyte are stronly coupled, and as a consequence the rates of the reactions at the graphite side and at the cobalt side are strongly coupled.

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  • $\begingroup$ You probably mean this video How a Lithium Ion Battery Actually Works $\endgroup$
    – Poutnik
    Dec 21, 2021 at 9:44
  • $\begingroup$ While charge bilance of both electrodes must match, part of the charge exchange can be spent by parasitic reactions, like deploying metallic lithium on the "graphite" surface or forming oxygen on "cobalt". Lithium then further slowly reacts with solvent. $\endgroup$
    – Poutnik
    Dec 31, 2021 at 11:00

1 Answer 1

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Czech language has a saying ( and the movie too), literally translated as: A single hand does not clap. It fits well to fossil fuels (gas without oxygen nor vice versa does not work) and to galvanic cells too (reductant and oxidant).

Both "hands" are the source of energy, not just the fuel nor just one of reductant/oxidant. For the fuel case, carbon and hydrogen atoms of fuel molecules release energy by forming stronger bonds with oxygen. The other "hand", oxygen atoms of oxygen molecules release energy by forming stronger bonds with carbon and hydrogen.

The rechargable cells are based on general reaction

$$\ce{Ox1 + Red2 <=> Red1 + Ox2}$$

This reaction has its value of the reaction Gibbs energy $\Delta G_\mathrm{r} [\pu{kJ/mol}]$ as the maximal theoretical value of non volume (e.g. electrical) work the reaction can provide.

There is the well known equation from electronics, relating energy, charge and potential/voltage: $ \mathrm{d}E = U \cdot\mathrm{d}q$, directly applicable to galvanic cell, being a bridge between chemistry and electronics. This cell voltage is related to above by equation

$$\Delta G_\mathrm{r}=-nFU$$

where n is number of exchanged electrons (here n=1) and F is the Faraday constant.

In water based electrolytes, there is convention to define the standard electrochemical potential related to standard hydrogen electrode $\ce{H+(aq,1M)/H2(g,1 atm)/Pt}$, for the reaction

$$\ce{Ox1 + n/2 H2 -> Red1 + n H+(aq) + n e-}$$

which is not applicable for waterless systems.

In galvanic cells, the system of the overall net reaction is spatially separated to regions at the cathode and the anode.

$$\ce{Ox1 + n e- <=>[fw, charging, cathode][bw, discharging, anode] Red1 }$$

$$\ce{Red2 <=>[fw, charging, anode][bw, discharging, cathode] Ox2 + n e- }$$

For Li-ion cobalt cells:

$$\ce{CoO2(s) + e- + Li+(solv)<=>[fw, charging, cathode][bw, discharging, anode] LiCoO2(s) }$$

$$\ce{C-Li(s) <=>[fw, dicharging, anode][bw, charging, cathode] C(s) + e- + Li+(solv) }$$

Reactions are simplified. There are no distinct red/ox phases, but there is continuously changed non-stoichiometric composition of active electrode materials.

Electrolyte is then $\ce{Li+(solv) + PF6-(solv)}$ dissolved in (usually mixture of ) dialkylcarbonate solvent $\ce{R1-O-CO-O-R2}$, where R is usually methyl, ethyl or bidental ethylene.

During charging, there is transfer of lithium ions in direction $\ce{Li_xCoO2 -> LiPF6/(RO)2CO (electrolyte) -> Li_yC_z}$ and vice versa during discharging.

Each electrode material acts like a charge pump, trying to keep the particular potential of the respective electrode, at which both reduction and oxidation rates have zero net effect.

The "graphite" electrode keeps low potential, as it is eager to provide electrons to an external circuit and release lithium ions to electrolyte. The "cobalt" electrode on the other hand keeps high potential, eager to accept electrons from an external circuit and absorb lithium ions from the electrolyte. And all vice versa at discharging.

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  • $\begingroup$ The cobalt electrode is at the beginning made of $\ce{Li2Co2O3 = Li2O·2CoO}$, and not $\ce{Li_xCoO2}$ as you state. It is transformed in the charging process into $\ce{Li^+ + e- + Co2O3}$. The $\ce{Li+}$ ion passes into solution and the electron is sent to the carbon electrode. This graphite electrode is made of pure carbon before charging process. At the end of this charging process, it contains some lithium coming from the $\ce{Li+} $ moving in solution, plus the electron brought from the electric connection outside. $\endgroup$
    – Maurice
    Dec 18, 2021 at 22:14
  • $\begingroup$ @Maurice I have not described the initial state at all. At initial charging graphite electrode gets also covered by products from reaction of lithium with dialkyl carbonate..Within the operational life there is LixCoO2, where X must not be too close to 0, as it would tend to release oxygen, nor to 1, as both oxide then tend to irreversibly separate. $\endgroup$
    – Poutnik
    Dec 18, 2021 at 22:34
  • $\begingroup$ During charging, the cobalt is partially oxidized to the +4 state, with some lithium ions moving to the electrolyte, resulting in a range of compounds Li xCoO 2 with 0 < x < 1. lithium cobalt oxide $\endgroup$
    – Poutnik
    Dec 18, 2021 at 22:43
  • $\begingroup$ @Poutnik I'm not sure, but I get the impression that you read only the title of the question, and that you rather skimmed the body of the question. Anyway: as Jordan Giesige remarked: the complexity of a rechargeable battery is such that it is kind of like an organism. The chemistry is almost metabolism-like. I now feel that asking about rechargeable batteries is not a good fit for stackexchange. I feel the question needs a threaded forum, so that there is room for follow-up questions. I expect to submit this enquiry in a chemistry forum in the near future $\endgroup$
    – Cleonis
    Dec 19, 2021 at 20:24
  • $\begingroup$ @Cleonis You jump to conclusions too fast. Search also for prior Q/A on the site, as it was discussed few times You you are free to ask anywhere, but aside of that, elaborate your question more or post another question, if it is better idea. // Comparing rechargable cells to organisms is overkill Li-ion cells have just more complicated chemistry, mainly due side reactions of metallic lithium with dialkyl carbonates. $\endgroup$
    – Poutnik
    Dec 19, 2021 at 21:02

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