How EVs Work: What They Teach @ IITM

  Author : Samyadeep 

Even if you are not particularly aware of what's going on around the world you still should have come around words like EV and Tesla float about loosely in the news.EV refers to 'Electric Vehicles', which run without oil or combustion engines, hence cause no pollution while being driven. While Tesla is yet to launch in India, EV has definitely made its mark in the automobile industry here, thanks to Tata. But how does this concern aspiring chemists? Because a largely untapped industry is taking giant strides in the most populous country in the world, and at the very core, it involves chemistry! Tesla batteries, and batteries of most other EV engines are lithium ion batteries, designed by electrochemists. And this sector will need a lot of R&D (Research and Development) in the future with the help of budding chemists. 

I am no expert in lithium ion cells (by that I mean I haven't published papers on it), but I know a bit about how they work. And by a bit I mean as much as could be learnt by paying attention to the electrochemistry lectures (and I take pride in it because we had this class at 8AM and it required immense motivation to drag myself to class and take notes while a few heads around me would gently droop as the class went on). Therefore, this article should also give you a good idea of what exactly is taught in our classes at IITM.

I would like to apologize for the slightly demanding science ahead. If I had it my way, I would never have used batteries and sticked to just oil because all you have to do is burn it. If you don't want to understand the science behind lithium ion batteries (even though I've made efforts to make it lucid for even the most uninformed person in chemistry) then head over to the final section which, I promise, contains much less science. 

The Principle

A lithium ion cell works on the principle of 'intercalation'. Intercalation is a fancy term used to describe the trapping of foreign ions inside a 'lattice', which is the framework of a crystal. In a lithium ion cell the intercalation and deintercalation of lithium ions produces current, which is then converted to kinetic energy by electromagnetic induction. 

In both the complexes lithium occupies the tetrahedral (smaller) voids and cobalt or titanium occupy the octahedral (bigger) voids of the lattice of oxide ions. Why? Because of their CFSE. Look up crystal field theory to get a detailed answer. 

But how does that work?

Let's get down to the brass tacks. A lithium ion cell consists of  a cathode made of a lithium salt and an anode made of graphite. The salt chosen is usually lithium cobaltate(LiCoO₂) or lithium titanium sulphide(LiTiS₂). Notice that in both of these salts, there are three elements, lithium, oxygen and another metal that is significantly bigger than lithium. This choice of metal is deliberate because it helps loosen the lithium ion from the crystal and slip out of its lattice. This makes it possible for lithium to escape from the cathode made of the salt and intercalate with the anode made of graphite. Now you can also guess why graphite is chosen as the anode. It has a layered structure of carbon atoms bonded together forming sheets over one another. The lithium ions can conveniently slip in between these sheets and stay there.

Charging

Charging of a lithium ion battery

You might wonder why the lithium ions take the trouble to move from their salts into the graphite lattice. They don't do it by themselves. Lithium has one electron bounded to it in its outermost shell. We charge the cell by removing these electrons from the lithium ions and transport them to the anode. The lithium ions become highly unstable and spontaneously start moving, through the electrolyte, to the anode where they meet their electrons and regain their stability. 

                                                                   

LiCoO₂ + LiC₆- xLi⁺→ Li_1-xCoO₂ +Li_1+xC₆

 

Discharging of lithium ion battery

Here x moles of lithium ions slip out of the cathode to intercalate with the graphite anode (in brown)

Discharging

 Now that the Li⁺ ions have intercalated to the anode, we have successfully stored some energy in this battery which we can now release by releasing the ions back into the cathode. Why will the Li ions take the trouble to go to the cathode? Because they are more stable there in their own lattice rather than remaining intercalated in the anode. If we connect a load (like a bulb or an engine) across the circuit to complete it, current will flow across the load and the engine of the car will start running.

Li_1-xCoO₂+Li_1+xC₆-yLi⁺→Li_1-x+yCoO2+Li_1+x-yC₆

Here y moles of lithium ions deintercalate from the graphite anode back into the cathode (in brown)

Choice of Electrolyte

The electrolyte chosen is usually a polar solvent, and the extent of polarity determines the speed of the lithium ions in the cell. The polarity must be such that the effective ionic radius of solvated lithium is high enough to prevent it from freely intercalating with the graphite layer. This is done to ensure that any fluctuations in current don't result in extremely high velocity of lithium ions. Beyond a certain speed, lithium is highly reactive and may cause the cell to catch fire. However, if the radius is too high then the ion will become too bulky and cell resistance will increase which affects the efficiency. Usually compounds like LiPF₆, LiBF₄ and LiAlSF₄ are used.

So, what does the future look like?

Hopefully greener. The ideal candidate for EVs should be Hydrogen fuel cells for their efficiency but they are way too volatile for commercial use as of now. We use Lithium ion cells because of their high energy density. For an automobile it is important to store as much energy as possible inside its battery consuming as little space as possible, and lithium ion cells are pretty good at offering just that. 

However, there are a few drawbacks to lithium ion cells, namely its cost and its tendency to catch fire. Lithium ion cells are difficult to make and operate only within a certain range of temperatures. The high cost and volatility of these cells is why we are still making traditional automobiles using combustion engines. 

Despite their drawbacks, currently lithium ion cells are still our best bet, and rigorous R&D is being carried out to make the manufacturing of these cells more sustainable and cheaper. Cobalt is a costly metal and also quite toxic. Hence we are trying to replace it with iron by introducing cells with cathode made of  LiFePO₄. Iron is much easier to mine than Cobalt, hence makes EV cells cheaper, and also makes them more resilient to fluctuations in temperature. 

We are at a crossroads of innovation in energy and probably within the next ten years we may have more affordable EVs in India which actually offer prices comparable to traditional automobiles. And for doing that getting the chemistry right is of utmost importance.