Lithium-Ion Batteries are the bedrock of our transition to sustainable energy, and as we benefit from economies of scale the cost of battery has gone down drastically. In this post I try my best to dissect the basics of how a lithium-ion battery works and improve my own understanding in the process.
A lithium-ion cell consists of the following major components:
cathode– positive electrode usually made of Lithium Oxide (with a current collector made from aluminium).
anode- negative electrode usually made of a graphite structure (with a current collector made from copper).
electrolyte– The lithium salt solution carrying the ions between the electrodes
Seperator: as the name implies this is a mechanically rigid but porous material usually some polymer, that keeps the cathode and anode at separated. (If the cathode and anode come in contact it will result in a short circuit where electrons rapidly flow between the path of least resistance).
There are 2 important types of chemical reactions to understand: oxidation and reduction which happen at the anode and cathode respectively.
**A useful tip for remembering the cathode and anode symbols (Red Cat, An Ox – where reduction (gain in electrons) happens at the Cathode and Oxidation (loss of electrons) happens at the Anode. **
The fundamental reason why lithium is used is because it has only one valence electron that makes it highly reactive but it forms a stable oxide. A more technical definition of this property would be lithium’s high electrochemical potential which means it loses electrons very easily (the graphite structure that makes the anode has a low electrochemical potential), and if we can separate the ion and electron flow then we can generate current. This is exactly what lithium-ion batteries do.
If a power source is connected to the cell, then the electrons in the lithium oxide (cathode) will be attracted to the positive terminal of the battery whereas the lithium ions will be attracted to the negative terminal. While charging the electrons will flow from the external circuit from the cathode to the anode. But why do electrons not flow through the electrolyte if it is conductive? The simple answer is because in a real battery the elements of a simple cell are tightly packed and separated with membranes that have a very high resistance to electrons but enable ion flow.
The lithium ions flow through the electrolyte (which serves as an ion conductor) to get intercalated in the graphite layers of the anode (intercalation simply means inserting the lithium ion in the graphite lattice). This is a very unstable state, similar to a ball on the top of the hill (the battery now has a high potential because it is fully charged).The issue with fast charging is the increase in the internal resistance of the battery. During fast charging the lithium ions move faster and they don’t have the time to be gradually intercalated in the graphite sheets (the graphite structure itself gets distorted due to this), instead lithium ions stick to the surface of the anode and react with other chemicals to become metallic. This loss of lithium ions contributes to battery degradation.
When the external power source is replaced with a load, electrons will flow through again from the external circuit (we have now generated current through a load! ) And lithium ions will flow back to the cathode through the electrolyte and get reduced (gain electron) to come back to the stable oxide state. This is the process of discharging a cell.
Another point to the original question I posed about electrons not flowing through the electrolyte is that when the lithium ions first pass through the electrolyte they react with the solvent and the graphite to produce a protective SEI layer on the electrolyte that prevents electrons from making direct contact with the electrolyte solution (which can damage the electrolyte). While the SEI layer is protective and essential for the cell to function, over time the layer consumes more lithium ions hence reducing the total amount of available lithium for reaction in the battery.
When discussing batteries there are some important terminologies that can seem like jargon but if you use the ‘water in a damn’ analogy they become very easy to understand.
The unit of a battery pack is a single cell which is assembled into segments which are then connected and cased to make a battery pack (accumulator). There are several important specs about this battery pack that can be used to understand its performance.
It is important to understand what Voltage means in context of the battery pack. This is analogous to the height of the water level held in a damn. The Open Circuit voltage is the potential difference between the terminals without a load (the state of charge of the battery is based on the open circuit voltage but not directly proportional to it). And the terminal voltage is measured with a load connected and it varies with the state of charge and the charge or discharge current (the rate at which gains or loses charge). The nominal voltage can be defined as the typical operating voltage of the battery.
Another spec that is important to understand is the capacity of the battery (analogous to the amount of water stored in a damn). The capacity can be expressed in units of charge – Ah. The charge of the battery gives the amount of current that can be discharged over a certain period of time (and of course the current in analogous to the rate of water flow). The storage capability of a battery pack is the amount of energy that can be stored and it is expressed as units of energy- kWh. Depending on the context both units can be used to express the capacity of the battery.
To monitor and optimize the battery pack’s functioning the electric drives system has a BMS (battery management system). To make it easy to remember its purpose just think of these things, it monitors the state of the cells (retrieving data about the voltage and temperature) and it uses that to maintain cell performance for the particular application while protecting cells from damage and prolonging the life of the battery.
Akshat Jain 