Lithium-ion battery, How does it work?

- A portable power source has become the lifesaver of the modern technological world, especially the

lithium

-ion

battery

. Imagine a world where all cars are powered by induction motors and not internal combustion engines. Induction motors are far superior to IC motors in almost all engineering aspects, as well as being more robust and economical. Another major disadvantage of IC motors is that they only produce usable torque in a narrow band of motor RPM. Considering all these factors, induction motors are definitely the perfect choice for a car. However, the power supply for an induction motor is the real obstacle to achieving a major induction motor revolution in the automotive industry.

Let's explore how Tesla, with the help of

lithium

-ion cells, solved this problem and why lithium-ion cells will be even better in the future. Let's take a Tesla cell out of the

battery

pack and break it down. You can see different layers of chemical compounds inside. Tesla's lithium-ion battery

work

s based on an interesting concept associated with metals called electrochemical potential. Electrochemical potential is the tendency of a metal to lose electrons. In fact, the 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.

Based on these values, lithium has the greatest 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 lithium-ion battery in 1991. It was again based on the same concept of electrochemical potential. Lithium, which has the greatest tendency to lose electrons, was used in lithium-ion cells. Lithium has only one electron in its outer shell and always wants to lose it. For this reason, pure lithium is a very reactive metal.

It even reacts with water and air. The trick to how a lithium-ion battery

work

s 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. Suppose we have somehow separated a lithium atom from this metal oxide. This lithium atom is very unstable and will instantly form a lithium ion and an electron. However, lithium, as part of the metal oxide, is much more stable than this state. If you can provide two different paths for the flow of electrons and lithium ions between lithium and metal oxide, the lithium atom will automatically reach the metal oxide part.

During this process, we have produced electricity from the flow of electrons through a path. From these discussions it is clear that we can produce electricity from this lithium metal oxide, if we first separate the lithium atoms from the lithium metal oxide and second, guide the lost electrons from said lithium atoms through an external circuit . Let's see how lithium-ion cells achieve these two goals. A practical lithium-ion cell also uses an electrolyte and graphite. Graphite has a layered structure. These layers are loosely bonded, so the separated lithium ions can be stored there very easily. The electrolyte between the graphite and the metal oxide acts as a protection that only allows lithium ions to pass through.

Now let's see what happens when you connect a power source through 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 since they cannot pass through the electrolyte and reach the graphite layer. Meanwhile, the positively charged lithium ions will be attracted to the negative terminal and flow through the electrolyte. Lithium ions also reach the space of the graphite layer and are 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 the electrons released from the metal oxide.

As we discussed, this is an unstable state, as if you were perched on the top of a hill. As soon as the power source is removed and a load is connected, the lithium ions want to return to their stable state as part of the metal oxide. Because of this tendency, lithium ions move through the electrolyte and electrons through the charge, as if sliding downhill. Thus we obtain an electric current through the charge. Please note that graphite plays no role in the chemical reaction of lithium-ion cells. Graphite is just a storage medium for lithium ions. If the internal temperature of the cell increases due to any abnormal condition, the liquid electrolyte will dry out and there will be a short circuit between the anode and cathode and this may cause a fire or explosion.

To avoid this situation, an insulating layer is placed between the electrodes, called a separator. The separator is permeable to lithium ions due to its microporosity. In a practical cell, copper and aluminum sheets are coated with graphite and metal oxide. The sheets act as current collectors here and the positive and negative tabs can be easily removed from the current collectors. An organic lithium salt acts as the electrolyte and is coated on the separator sheet. These three sheets are wound in the cylinder around a central steel core, making the cell more compact. A standard Tesla cell has a voltage between three and 4.2 volts.

Many of these Tesla cells are connected in series and parallel to form a module. 16 of these modules are connected in series to form a battery pack in the Tesla car. Lithium-ion cells produce a lot of heat during operation and high temperature will decrease the performance of the cells. A battery management system is used to manage the temperature, state of charge, voltage protection and cell health monitoring of such a large number of cells. Glycol-based cooling technology is used in Tesla's battery pack. The BMS adjusts to the glycol flow rate to maintain optimal battery temperature. Voltage protection is another crucial task of the BMS.

For example, in these three cells, during charging a cell with a higher capacity will charge more than the rest. To solve this problem, the BMS uses something called cellular balance. In cell balancing, all cells can be charged and discharged equally, thus protecting them from overvoltage and undervoltage. This is where Tesla surpasses Nissan's battery technology. The Nissan Leaf has a major battery cooling problem due to the large size of its cells and the lack of an active cooling method. The small multi-cell design has one more advantage. During high power demand situations, the discharge voltage will be divided equally between 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 stress and would eventually suffer a premature death. By using many small cylindrical cells, whose manufacturing technology is already well established, Tesla clearly made a winning decision. There is a magical phenomenon that occurs inside lithium-ion cells during their first charge that saves them from sudden death. Let's see what it is. The electrons in the graphite layer are a major problem. The electrolyte will degrade if 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 first charge the cell, as explained above, lithium ions move through the electrolyte. Here, on this journey, the solvent molecules of the electrolyte cover the lithium ions. When they reach the graphite, the lithium ions, together 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. Avoid any direct contact between the electrons and the electrolyte, thus preventing the electrolyte from degrading. In this overall process of forming the SEI layer, it will consume 5% of the lithium. The remaining 95% of the lithium contributes to the main operation of the battery.

Although the SEI layer was an accidental discovery, with more than two decades of research and development, scientists have optimized the thickness and chemistry of the SEI layer to achieve maximum cell performance. It is surprising to discover that the electronic devices we used about two decades ago did not use lithium-ion batteries. With its astonishing growth speed, the lithium-ion battery market is expected to become a $90 billion annual industry within a few years. The number of charge and discharge cycles of a lithium-ion battery currently achieved is about 3,000. Great minds around the world are doing their best to increase this to 10,000 cycles.

That means you wouldn't have to worry about replacing your car battery for 25 years. Millions of dollars have already been invested in research to replace graphite as a storage medium with silicon. If this is successful, the energy density of the lithium-ion cell will increase more than five times. We hope this video has given you a clear conceptual understanding about lithium-ion cells and their future. If you want to learn more about lithium-ion cells used in mobile phones, watch the video made by Branch Education. And please, don't forget to support us at patron.com. Thank you.