A portable power supply has become the lifeline
of the modern technological world,
especially the lithium-ion battery.
Imagine a world where all cars are driven
by induction motors and not internal combustion engines.
Induction motors are far superior to IC engines
in almost all engineering aspects,
as well as being more robust and cheaper.
Another huge disadvantage of IC engines
is that they only produce usable torque
in a narrow band of engine RPM.
Considering all of these factors,
induction motors are the perfect choice
for an automobile.
However, the power supply for an induction motor
is the real bottleneck in achieving
a major induction motor revolution
in the automobile industry.
Let's explore how Tesla, with the help of lithium-ion cells,
solved this issue and why lithium-ion cells are going
to become even better in the future.
Let's take a Tesla cell out
from the battery pack and break it down.
You can see different layers
of chemical compounds inside it.
Tesla's lithium-ion battery works on an interesting concept
associated with metals called electrochemical potential.
The electrochemical potential is the tendency
of a metal to lose electrons.
The very first cell, developed by Alessandro Volta
more than 200 years ago, was based
on the concept of electrochemical potential.
A general electrochemical series is shown here.
According to these values, lithium has the highest tendency
to lose electrons and fluorine has the least tendency
to lose electrons.
Volta took two metals
with different electrochemical potentials,
in this case, zinc, and silver,
and created an external flow of electricity.
Sony made the first commercial model
of a lithium-ion battery in 1991.
It was again based on the same concept
of electrochemical potential.
Lithium, which has the highest tendency to lose electrons,
was used in lithium-ion cells.
Lithium has only one electron in its outer shell
and always wants to lose this electron.
Due to this reason, pure lithium is a highly reactive metal.
It even reacts with water and air.
The trick of a lithium-ion battery operation is the fact
that lithium, in its pure form, is a reactive metal.
But when lithium is part of a metal oxide,
it is quite stable.
Assume that somehow we have separated a lithium atom
from this metal oxide.
This lithium atom is highly unstable
and will instantly form a lithium-ion and an electron.
However, lithium, as a part of metal oxide,
is much more stable than this state.
If you can provide two different paths
for the electron and lithium-ion flow
between the lithium and the metal oxide,
the lithium atom will automatically reach
the metal oxide part.
During this process, we have produced electricity
from the electron flow through the one path.
From these discussions, it is clear
that we can produce electricity
from this lithium metal oxide,
if we first separate lithium atoms
from the lithium metal oxide, and secondly,
guide the electrons lost from such lithium atoms
through an external circuit.
Let's see how lithium-ion cells
achieve these two objectives.
A practical lithium-ion cell
also uses an electrolyte and graphite.
Graphite has a layered structure.
These layers are loosely bonded
so that the separated lithium-ions
can be stored very easily there.
The electrolyte between the graphite and the metal oxide
acts as a guard which allows only lithium-ions through.
Now let's see what happens when you connect a power source
across this arrangement.
The positive side of the power source will attract
and remove electrons from the lithium atoms
of the metal oxide.
These electrons flow through the external circuit
as they cannot flow through the electrolyte and reach the graphite layer.
In the meantime, the positively charged lithium-ions
will be attracted towards the negative terminal
and will flow through the electrolyte.
lithium-ions also reach the graphite layer space
and get trapped there.
Once all the lithium atoms reach the graphite sheet,
the cell is fully charged.
Thus we have achieved the first objective
which is the lithium-ions and electrons detached
from the metal oxide.
As we discussed, this is an unstable state,
as if being perched on top of a hill.
As soon as the power source is removed,
and a load is connected, the lithium-ions want to go back
to their stable state as a part of the metal oxide.
Due to this tendency, the lithium-ions move
through the electrolyte and electrons via the load,
just like sliding down a hill.
Thus we get an electrical current through the load.
Please note that that graphite does not have a role
in the chemical reaction of the lithium-ion cells.
Graphite is just a storage medium for lithium-ions.
If the internal temperature of the cell rises due
to some abnormal condition,
the liquid electrolyte will dry up
and there will be a short circuit between the anode
and cathode and this can lead to a fire or an explosion.
To avoid such a situation, an insulating layer,
called the separator, is placed between the electrodes.
The separator is permeable for the lithium-ions
because of its microporosity.
In a practical cell, the graphite and metal oxide are coated
onto copper and aluminum foils.
The foils act as current collectors here
and the positive and negative tabs can be easily taken out
from the current collectors.
An organic salt of lithium acts as the electrolyte
and it is coated onto the separator sheet.
All these three sheets are wound onto the cylinder
around a central steel core,
thus making the cell more compact.
A standard Tesla cell has a voltage
of between three and 4.2 volts.
Many such Tesla cells are connected in series
and in a parallel fashion to form a module.
16 such modules are connected in series
to form a battery pack in the Tesla car.
Lithium-ion cells produce a lot of heat during the operation
and the high temperature will decay the cells' performance.
A battery management system is used
to manage the temperature, state of charge,
voltage protection, and cell health monitoring
of such a huge number of cells.
Glycol-based cooling technology is used
in the Tesla battery pack.
The BMS adjusts to the glycol flow rate
to maintain the optimum battery temperature.
Voltage protection is another crucial job of the BMS.
For example, in these three cells, during charging
a higher capacity cell will be charged more than the rest.
To solve this problem,
the BMS uses something called cell balancing.
In cell balancing, all the cells are allowed to charge
and discharge equally, thus protecting them
from over and under-voltage.
This is where Tesla scores over Nissan battery technology.
The Nissan Leaf has a huge battery cooling issue due
to the big size of its cells and the absence
of an active cooling method.
The small multiple cell design has one more advantage.
During high power demand situations,
the discharge strain will be divided equally
among each of the cells.
Instead of many small cells
if we had used a single giant cell,
it would have been put under a lot of strain,
and eventually, it would suffer premature death. By using many small cylindrical cells,
the manufacturing technology of which
is already well established,
Tesla made a winning decision.
There is a magical phenomenon that happens
within lithium-ion cells during their very first charge
that saves the lithium-ion cells from sudden death.
Let's see what it is.
The electrons in the graphite layer are a major problem.
The electrolyte will be degraded
if the electrons come into contact with it.
However, the electrons never come into contact
with the electrolyte due to an accidental discovery,
the solid electrolyte interface.
When you charge the cell for the first time,
as explained above, the lithium-ions move
through the electrolyte.
Here, in this journey, solvent molecules
in the electrolyte cover the lithium-ions.
When they reach the graphite, the lithium-ions,
along with the solvent molecules,
react with the graphite and form a layer there
called the SEI layer.
The formation of this SEI layer is a blessing in disguise.
It prevents any direct contact
between the electrons and the electrolyte,
thus saving the electrolyte from degradation.
In this overall process of the formation of the SEI layer,
it will consume 5% of the lithium.
The remaining 95% of the lithium contributes
to the main working of the battery.
Even though the SEI layer was an accidental discovery,
with over two decades of research and development,
scientists have optimized the thickness and chemistry
of the SEI layer for maximum cell performance.
It is amazing to find out that those electronic gadgets
we used around two decades back
did not use lithium-ion batteries.
With its amazing speed of growth,
the lithium-ion battery market is expected
to become a $90 billion annual industry within a few years.
The currently achieved number of charge-discharge cycles
of a lithium-ion battery is around 3,000.
Great minds across the globe are putting their best efforts
into increasing this to 10,000 cycles.
That means you would not have to worry
about replacing the battery in your car for 25 years.
Millions of dollars have already been invested in research
into replacing the storage medium graphite with silicon.
If this is successful, the energy density
of the lithium-ion cell will then increase
by more than five times.
about lithium-ion cells and their future.
If you would like to learn more about
the lithium-ion cells used in mobile phones,
Thank you.
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