Complicated chemical processes take place in a battery during charging. Lithium ions move through electrolyte from the cathode to the anode, electrons pass through the wires and complete the circuit. Differences in potential and concentration occur. It all sounds very complicated. But it doesn’t have to be that difficult. This article gives a simple overview of how a lithium battery works – with lots of pictures and without chemical formulas or math.
Lithium-ion batteries are available in various designs: Round cell, coin cell, prismatic and in pouch-bag format. However, the internal structure is always the same: A cell consists of:
- Anode active material
- Cathode active material
- Separator
- Electrolyte
- Current collector foil
- and housing
Anode active material and cathode active material are applied to copper or aluminum foils. The separator foil is packed between the anode foil and the cathode foil to prevent a short circuit. In the case of a round cell, e.g. type 18650, the foil stack is then rolled up into a cylinder. This can be seen in the picture below. The foil composite is then packed into a metal housing and the electrolyte is added. In the final step, the housing is sealed tightly.
What happens when a battery is charged?
When the battery is charged, lithium ions migrate from the cathode to the anode and are deposited in the material there. The lithium ions do not migrate voluntarily, this is forced by the fact that a higher voltage is applied to the cell than it has at the moment.
The migration of the lithium ions from the cathode to the anode continues until the open-circuit voltage of the cell corresponds to the applied external voltage – i.e. equilibrium is reached. Exactly what a voltage curve looks like depends on the charging speed, the type of charging process and, in particular, the exact chemical composition. A typical charging curve of a lithium-ion cell looks like this:
So what happens on a chemical level?
The exact chemical reactions that take place in a cell depend on various factors such as the exact chemical composition, the mechanical structure and the temperature. In simplified terms, however, the charging process can be divided into 5 steps:
1. Solid-state diffusion at the cathode
The cathode can be imagined as a collection of many individual particles that contain the lithium ions. The particles are surrounded by electrolyte, in which there is a conducting salt that is responsible for transporting the ions. When discharged, the lithium ions are chemically stable in the cathode. As soon as a charging voltage is applied, the lithium begins to move from the inside of the particle to its outer edge. This process is called solid-state diffusion.
2. Boundary layer reaction Cathode
Where the cathode active material and electrolyte come into contact, a chemical reaction takes place in which an electron is released and a lithium ion is immersed in the electrolyte. The electrolyte consists largely of conductive salt, which does not form a chemical bond with the lithium, but is grouped around the lithium and forms a kind of shell. This shell helps the lithium to move through the electrolyte.
3. Electrons and lithium-ion migration
The emitted electron now travels almost without loss of time via the negative pole via the connected circuit to the anode and there to the boundary layer between the anode active material and the electrolyte. Like the cathode, the anode consists of many particles in which lithium ions can be deposited.
As soon as the electron reaches the anode, it reacts with lithium ions from the conducting salt. Lithium ions are dissolved in the electrolyte, so there are normally enough reaction partners available for this process. The lithium that is newly immersed in the electrolyte from the cathode migrates through the separator to the anode side. The separator is ion-permeable and prevents the cathode and anode from coming into direct contact with each other.
4. Solid state diffusion anode
A further chemical reaction with the lithium ion takes place on the surface of the anode particles . The lithium ion then migrates into the interior of the graphite particles and is then deposited in the intermediate layers of the graphite. This process is called intercalation.
5. Li+ depletion and surplus
If a battery is only charged very slowly and only low charging currents flow, the electrolyte is almost evenly distributed and there is an even distribution of lithium ions and conductive salt on both the anode side and the cathode side.
The situation is different with fast charging. The lithium is released very quickly from the cathode particle into the electrolyte. The electron reaches the anode almost without delay and reacts there with the lithium from the conducting salt. The boundary layer reaction also takes place very quickly and the lithium penetrates the graphite particle.
The subsequent lithium in the electrolyte is not as fast. The Li movement there is very slow, so that concentration differences in the electrolyte are not balanced out. This means that there is a local surplus of lithium ions on the cathode side, while there is a lack of lithium ions on the anode side.
This concentration gradient leads to an overvoltage at the anode-electrolyte interface. This overvoltage has several negative effects:
- Solvent can decompose and settle on the anode surface or outgas.
- The few remaining Li ions tend to deposit as metallic lithium (Li plating), which favors the formation of Li dendrites. These are spear-shaped deposits that can penetrate the separator and lead to a short circuit.
For this reason, fast charging is generally bad for batteries. The lower the charging current, the longer the service life is achieved.
Thanks to Dr. Manuel Kuder as well as Dr. Katarina Cicvaric for their support for this article.
