Do you remeber the days when you needed to wait half a day for you phone to charge. I often used to plug in my phone to charger before I fell asleep. So, my phone used to charge until I unlugged it the next day after I woke up(wow !!!!!! almost 8 hours, alas if only I had realized sooner how I was abusing my dear batteries). But with the advent of fast chargers, that is not the case, at present, my phone only takes about 1 and half hour to full charge.
When I charge my first phone that came with a fast charger, I realized that it blazed through the 0 to 80 percent mark in about 30 min but after that it turned into a complete sloth and took nearly double the time to reach 100 percent. At first, I just shook if off as a fluke but the pattern kept repeating so, I concluded to have either a faulty charger or a faulty phone. I even thought the whole ‘fast charging’ was just a hoax and that my phone’s battery charge display was designed to dupe me. I kept cursing my phone until my friend complained that he had the same issue with his fast charging phone. (ah!! nothing makes you happier more than a friend who gives you company at failure). After some googling, the problem seemed commonplace to all the ‘fast charge’ phones, the phones that had Liion batteries in them. This article describes my journey to find the reason behind this mysterious behaviour in Liion batteries.
But to know the reason you have to dig deeper into the world of battery chemistry. In a nutshell, battery chemistry defines how a battery cell comprises of precisely selected chemicals which react together to make battery into a energy reserve. Even in that respect Li-ion cells are peculiar as the main chemical reaction responsible for storing energy is more of a redistribution of Lithium and no chemical changes takes place as a direct consequence of the Lithium exchange. This phenomenon is termed as Intercalation.
Redox reaction vs Intercalation
The above picture depicts the most basic cell that we learned in our high school chemistry. On discharge,
ions dissolve into the aqueous electrolyte in the negative electrode whereas in the positive electrode Cu++ ions are consumed by electroplating copper in copper electrode. This produces a temporary deficiency of SO in the negative-electrode region and a surplus in the positive-electrode region. As Zn dissolves it produces electrons or charge which then, travels through the external circuit toward the negative electrode producing work and the current flows. This loss of electron known as oxidation coupled with simultaneous gain of electron known as reduction, is the redox reaction that forms the whole basis of working of the daniel cell. Similar process takes place during charging of the cell.
Notice here that the Cu and Zn electrodes are both changing their form chemically by the process of electroplating, such chemical change will not take place in Li cells atleast, not intentionally.
In Li-ion cells such redox reaction does not take place instead a much less chemically simpler process called intercalation takes. As the electrodes in the Li-ion cells do not have go through the constant process of degradation and accumulation of metal through electroplating, the electrodes last much longer, thus Lithium battery have longer lifespan than the conventional redox-type battery.
The image shows the simpler version of the construction of a Li cell but is a very good represention of the chemistry inside the cell. The green plates here represent a layered structure of the graphite negative electrode and the lithium cobalt oxide crystal structure in purple as the positive electrode. The black dots here represent the Li atoms. Lithium here is stored inside these lattice layers inside the electrodes but does not take in any type of chemical reaction there. The electrode act much like a sponge that absorbs the water molecule and stores it but doesn’t change the form of water in any way. This is the process of intercalation.
Intercalation involves insertion of lithium ions into crystalline lattice of host electrode without changing its crystal structure. The electrodes are selected such that they have open crystalline structure which allows insertion or extraction of lithium ions in vacant spaces and also must have ability to accept compensation electrons. Within the electrode, the lithium atom’s electron is loosely shared with neighboring atoms. The lithium is not tightly bonded in one place, it is actually quite free to move around.
During discharge, the negative electrode gives up an electron to the external circuit and Li exits the surface of the electrode and moves toward the positive electrode through the electrolyte to balance the charge. Li diffuses outward from center of negative-electrode particles to equalize concentrations, replenishing Li at particle surface (over time). This process of redistribution of Li inside the electrode crystal structure is called diffusion which is a slow process. Diffusion is responsible for the slow rise in the voltage of the li cell when it rests after discharge.
The seperator is a porous membrane that prohibits the contact between negative and positive electrode but allows the flow of Li. Li+ joins with electron and enters the surface of the positive electrode and similar diffusion takes place from the outward toward the inward direction in the electrode to balance the amount of lithium concentration.
The image above shows the diffusion of ink in water. Almost a similar process takes place inside an electrode only difference being that Li is diffused instead of ink.
The voltage measured by the voltmeter when we attach to the negative and positive terminal of the cell is the potential difference between the negative and positive electrode surface, rather than the whole electrode. The graphite or any other crystalline electrode as shown in the above picture is powder like subsctance that is incapable of attaching to the external circuit for uniform conduction of electron. Instead, we attach a conductor like thin copper plates to the graphite negative electrode that will conduct the electricity and attach to the negative terminal.
In the image we see that the negative electrode is attached to both sides of orange copper foil and the potential measured is directly proportional to the concentration of lithium at surface instead of the surplus lithium concentration in the electrode.
Charging a Li-ion cell
As shown in the above figure, the cells are often charged at first with either a constant current or constant power upto the 80-90% mark thus, the voltage keeps on rising upto that mark linearly (4.15V). During charge the lithium atom enter at first to the surface of the negative electrode so, the surface potential reaches to max potential of 4.15V. If we increase the voltage anymore the Lithium itself begins to take part in unwanted chemical reaction which causes production of heat and then more reaction until the cell itself begins to burn. Thus, we keep the voltage contant for remaining 10-20 percent of the chargin process.
Initially due to the voltage increase, the Li atoms are pushed inside the cell but after the voltage remains constant the Lithium only moves inward only due to diffusion which is a very slow process. Furthermore, as the lithium concentration is higher at later stages of charging, the movement of lithium inside electrode is hindered much like in a traffic jam in the highway thus, the total accumulation of lithium to the total capacity i.e the 100% charging takes a long time.