What is Lithium-ion Battery and How Does it Work?

Lithium-ion batteries are used in many electronic devices such as mobile phones, MP3 players, laptops, etc., due to their light weight. These are rechargeable and have high energy density as compared to many other batteries.

The amount of energy stored in the battery per unit volume is high and the energy stored in them is an electrochemical type of energy. This type is an electrical energy, which is derived from chemicals through chemical reactions. Loss of energy also occurs slowly.

The basic functioning of any electrochemical cell consists of two important elements, electrodes and electrolyte. Anode and cathode are the two electrodes that conduct electric energy (ions). Anode is connected to the negative terminal of the battery while cathode is connected to the positive terminal of the battery.

The electrodes are immersed in the electrolyte, which acts as a liquid medium for the movement of ions. The electrolyte acts as a buffer and helps in the electrochemical reactions in the battery. The movement of electrons in the electrolyte and in between the electrodes, causes electric current.

The anode and cathode are made up of carbon and lithium oxide, respectively. The electrolyte is made up of lithium salts, which are dissolved in organic solvents. The anode material is mostly graphite and cathode material could be either one of these: lithium cobalt oxide (LiCoO₂), lithium iron phosphate (LiFePO₄), or lithium manganese oxide (LiMn₂O₄).

The electrolytes that are commonly used are the salts of lithium such as lithium hexafluorophosphate (LiPF₆), lithium tetrafluoroborate (LiBF₄), lithium perchlorate (LiClO₄), etc., which are dissolved in organic solvents such as ethylene carbonate, dimethyl carbonate, and diethyl carbonate.

The electrolyte used in them is a non aqueous solution because, in aqueous solution (H₂O), lithium (highly reactive alkali metal) reacts vigorously with water to form lithium hydroxide and hydrogen gas, which is not at all desired.

During charging, the lithium ions from the cathode move to the anode and settle in the anode layer. The flow of the lithium ions is through the electrolyte. When this process takes place, the battery charges and stores the electrical energy in it.

During the process of discharging, the lithium ions move back to the cathode from the anode. When the battery is powered, the movement of the electrons flows opposite to the direction of the lithium ions, in the outer circuit. Due to this movement of electrons, electric current is produced.

Basically, potential difference and resistance is created between the electrodes, which causes electric current to flow. When the battery is powered by an external source, ionic movement will take place.

Now let us take a look at the chemical reactions taking place at the electrodes.

At the Anode: xLi⁺ + xe^- + 6C → Li_xC₆

At the Cathode: LiCoO₂ → Li_1-xCoO₂ + xLi⁺ + xe^-

Overall reaction: Li⁺ + LiCoO₂ → Li₂O + CoO

This circuit monitors the voltage and current levels in the battery.

These switches are made up of field-effect transistors, that interrupt or break off the charging and discharging process, after receiving the right control signal from the controller IC.

When the temperature of the control switch rises above a certain level, the fuse cuts off the current flow. This helps to save the battery from over current.

The current flow in the battery is controlled by the thermistor, as it can vary its resistance according to the current which passes through it.

The switch is also used in some lithium-ion batteries, instead of thermistor, to prevent overheating in the circuit.

The battery capacity i.e., the amount of charge that it can store, depends upon the surface area of the electrodes used. The number of charging and discharging cycles are more and also, the rate of charging is fast. But there is a problem of explosion due to overheating. It is advised to replace the battery often, in order to prevent them from exploding.