- [Narrator] 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
definitely 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
the electrochemical potential. Electrochemical potential is the tendency of a metal to lose electrons. In fact, 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 firstly separate out 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 obviously 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 micro porosity. 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 easy taken out from the current collectors. An organic salt of lithium
acts as the electrolyte and it is coated on to
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 clearly made a winning decision. There is a magical
phenomenon which 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. We hope this video provided you with a clear conceptual understanding about lithium-ion cells and their future. If you would like to learn more about the lithium-ion cells
used in mobile phones, please have a look at the
video made by Branch Education. And please, don't forget to
support us at patron.com. Thank you.
Great video!
Very interesting!