Lightweight, high energy density Li-ion batteries with high capacity and efficiency than other batteries are widely applied in diverse areas ranging from small appliances and IT devices to power tools, energy storage systems and electric vehicles.
Let's look into Li-ion batteries inside out today.
Li-ion batteries consist of largely four main components: cathode, anode, electrolyte, and separator.
Every single component of a Li-ion battery is essential as it cannot function when one of the components is missing.
A Lithium-ion battery generates electricity through chemical reactions of lithium.
This is why, of course, lithium is inserted into the battery and that space for lithium is called “cathode”.
However, since lithium is unstable in the element form, the combination of lithium and oxygen, lithium oxide is used for cathode.
The material that intervenes the electrode reaction of the actual battery just like lithium oxide is called ”active material”.
In other words, in the cathode of a Li-ion battery, lithium oxide is used as an active material.
If you take a closer look at the cathode, you willl find a thin aluminum foil used to hold the frame of the cathode coated with a compound made up of active material, conductive additive and binder.
The active material contains lithium ions, the conductive additive is added to increase conductivity; and the binder acts as an adhesive which helps the active material and the conductive additive to settle well on the aluminum substrate.
Cathode plays an important role in determining the characteristics of the battery as the battery’s capacity and voltage are determined by active material type used for cathode. The higher amount of lithium, bigger the capacity; and the bigger potential difference between cathode and anode, higher the voltage. The potential difference is small for anode depending on their type but for cathode, the potential difference is relatively high in general. As such, the cathode plays a significant role in determining the voltage of the battery.
Just like the cathode, the anode substrate is also coated with active material. The anode’s active material performs the role of enabling electric current to flow through the external circuit while allowing reversible absorption/emission of lithium ions released from the cathode.
When the battery is being charged, lithium ions are stored in the anode and not the cathode. At this point, when the conducting wire connects the cathode to the anode (discharge state), lithium ions naturally flow back to the cathode through the electrolyte, and the electrons (e-) separated from lithium ions move along the wire generating electricity.
For anode graphite which has a stable structure is used, and the anode substrate is coated with active material, conductive additive and a binder. Thanks to graphite’s optimal qualities such as structural stability, low electrochemical reactivity, conditions for storing much lithium ions and price, the material is considered suitable to be used for anode.
When explaining about cathode and anode, it was mentioned that lithium ions move through the electrolyte and electrons move through the wire.
This is the key in enabling the use of electricity in a battery. If ions flow through the electrolyte, not only can’t we use electricity but safety will be jeopardized.
Electrolyte is the component which plays this important role.
It serves as the medium that enables the movement of only lithium ions between the cathode and anode.
For the electrolyte, materials with high ionic conductivity are mainly used so that lithium ions move back and forth easily.
The electrolyte is composed of salts, solvents and additives.
The salts are the passage for lithium ions to move, the solvents are organic liquids used to dissolve the salts, and the additives are added in small amounts for specific purposes.
Electrolyte created in this way only allows ions to move to the electrodes and doesn’t let electrons to pass.
In addition, the movement speed of lithium ions depends on the electrolyte type.
Thus, only the electrolytes that meet stringent conditions can be used.
While the cathode and anode determine the basic performance of a battery, electrolyte and separator determine the safety of a battery.
The separator functions as a physical barrier keeping cathode and anode apart.
It prevents the direct flow of electrons and carefully lets only the ions pass through the internal microscopic hole.
Therefore, it must satisfy all the physical and electrochemical conditions.
Commercialized separators we have today are synthetic resin such as polyethylene (PE) and polypropylene (PP).