The fact behind why Lithium-ion batteries are the future Power Source in Automotive Industries?
Let’s take a lithium-ion cell used in a DSLR camera and explore its internal parts. You can see that electrons will flow between the sheets when we connect a load across the battery. This natural electron flow means that the electrons were stored in an unstable condition before the load was connected.
We need to better understand this basic principle to understand the technological advancements happening in lithium-ion batteries. Here, the unstable electrons are stored in a kind of container called graphite with a higher electrochemical potential, and before storing the electrons should be separated from the atomic structure of an element. There is a metal called lithium which has a high tendency to lose electrons from its outer shell and due to this lithium is very reactive in nature.
However, as part of a metal oxide, lithium atoms are very stable. Let’s use an external power source. The positive side of the power source attracts the electrons. We use an electrolyte too, which blocks any electron flow through it, so instead, they flow through the external circuit and get trapped between the graphite layers. Similarly, the negative side of the power source attracts lithium-ions and they also get trapped in the graphite layers. Eventually, the lithium-ions are stored with a higher electrochemical potential. As soon as we remove the power source and connect it to a load, all the electrons in the graphite will flow through the load and hence we can get electricity from this.
There are three good characteristics for an energy source. Low cost, high energy density, and longer life. Let’s explore how the lithium-ion cell fares in these three aspects and what are the future trends. Let’s first consider the cost factor of lithium-ion batteries. The capital cost required to set up a lithium-ion based technology is way higher than its counterparts. However, when we compare the running cost of electric cars to gasoline cars, electric cars run at 1/3 of the price of gasoline cars.
The main reason for the high capital cost is the presence of nickel and cobalt in the metal oxide compound. Moreover, battery makers use these two metals in greater quantities than lithium. Due to this reason, the cost of a lithium-ion battery is almost six times that of a lead-acid one and three times that of a nickel-metal hydride battery.
However, the good news is that the cost per kilowatt-hour of lithium-ion battery technology has been dropping at a rapid rate over the last few years so that in the future it might overcome the capital cost hurdle. Lithium-ion batteries provide much higher energy density than any other battery technologies but are greatly inferior to gasoline’s energy density. The most crucial part which affects the energy density is the storage medium of lithium-ions and electrons.
Scientists are now trying a breakthrough technology by replacing the storage medium graphite with silicon. With this technique itis possible to multiply the energy density by almost 4.4 times. However, silicon causes an unacceptable level of volume expansion and compression during each cycle. To take advantage of the high-energy-density of silicon but to avoid its negative effects, some manufacturers have started using 5% silicon mixed in with the graphite. Now let’s get it to the most crucial part, the life of lithium-ion batteries.
The lithium-ion batteries of your old laptops used to die in one year. However, now they are easily giving three to four years of life. How do lithium-ion batteries die? To get to know it a better way, we need to understand the mechanism behind the death of a lithium-ion battery. Generally, the lithium-ion battery fails after a few years even if you don’t use it. This capacity loss is not abrupt. In fact, the process is electroless which means it does not require any electricity flow.
As per the operation discussed earlier, when the lithium ions are flowing through the electrolyte, they are covered with a coating called a solvent molecule. During the very first charge, the lithium-ions along with the solvent molecules react with the graphite and form an SEI layer. The SEI layer is a blessing in disguise because it allows the lithium-ions to pass through it.
The SEI layer helps to avoid direct contact between the electrons and the electrolyte, thus saving the electrolyte from degradation. Even though the SEI layer tries to prevent the electrons from entering it, a small number of electrons in the graphite can still tunnel through it. Due to some proportions of the SEI layer, the solvent molecules present in the electrolyte can easily enter into it. The solvent molecule reacts and forms an SEI layer again. Here we can observe that the SEI layer becomes thicker than before and simultaneously the electrolyte is consumed. It is interesting to note that the degradation process of your lithium-ion battery is a very slow process when there is an open circuit.
This process of lithium-ion cell death that we discussed above will be accelerated many times during actual operation. Let’s see how. This is because the movements of the lithium-ions bring more solvent molecules, thus the thickening of the SEI layer is accelerated. This process consumes active lithium-ions and electrolyte and that’s why the life of the battery is significantly shortened depending on the number of cycles. From this discussion, it can be seen that the SEI plays a dual role in battery performance. On the one hand, it protects the electrolyte from degradation and will support the main working of the battery.
While on the other hand, it consumes cyclable lithium-ions and electrolytes inside the cell which leads to the death of the battery. However, the longevity of the battery can be scalable up to a certain limit with the help of an electrolyte additive. This is like a secret sauce in a recipe that slows down the degradation process and helps to improve the battery life. Currently, Tesla batteries last for around 3,000 cycles or around seven years and researchers are putting their best efforts into extending this to 10,000 cycles, which is equivalent to25 years of battery life.
This is because factors like cost, life cycles, and energy density vary depending upon the type of application. The discussions so far have clearly explained how lithium-ion batteries are getting better in terms of energy density and longevity.
A recent innovation in lithium-ion battery technology has given a big boost to the safety of lithium-ion batteries. This technology uses an aqueous electrolyte with halogen intercalation. In this technique, the addition of the helper halogen to the metal oxide side increases the mobility of the lithium-ions. As the electrolyte is a pinch of salt in water type, it can resolve the issues of flammability and it also increases the mobility of the lithium-ions.