A lithium-ion battery is a secondary battery (rechargeable battery) that mainly relies on the intercalation and deintercalation of Li+ back and forth between two electrodes. With the continuous development of downstream industries such as new energy vehicles, the production scale of lithium-ion batteries is expanding.

This article takes lithium cobalt oxide as an example to explain the principle of lithium ion batteries

Working principle

  1. Positive structure

      LiCoO2 + conductive agent + binder (PVDF) + current collector (aluminum foil)

  1. Negative structure

     Graphite + Conductive Agent + Thickener (CMC) + Binder (SBR) + Current Collector (Copper Foil)

  1. Working principle

  • Charging process

When a power supply charges the battery, the electrons on the positive electrode run from the external circuit to the negative electrode, and the positive lithium ions "Li+" enter the electrolyte from the positive electrode, and "swim" to the negative electrode through the curved holes in the diaphragm. Combined with the electrons that have already run over.

The reactions that take place on the positive electrode are:

LiCoO2→Li1-xCoO2+xLi++xe-

The reaction that takes place on the negative electrode is:

6C+xLi++xe-→LixC6

  •  Battery discharge process

Discharge includes constant current discharge and constant resistance discharge. Constant current discharge is actually adding a variable resistance to the external circuit that can change with the voltage. The essence of constant resistance discharge is to add a resistance to the positive and negative electrodes of the battery to allow electrons to pass through.

It can be seen from this that as long as the electrons on the negative electrode cannot run from the negative electrode to the positive electrode, the battery will not discharge.

Both electrons and Li+ move at the same time, in the same direction but different paths.

During discharge, electrons run from the negative electrode to the positive electrode through the electronic conductor, and lithium ions Li+ enter the electrolyte from the negative electrode, and "swim" to the positive electrode through the curved holes in the diaphragm, where they are combined with the electrons that have already run over.

  • Charge and discharge characteristics

The positive electrode of the cell adopts LiCoO2, LiNiO2, LiMn2O2, of which LiCoO2 is a crystal form with a very stable layer structure, but when xLi+ are removed from LiCoO2, its structure may change, but whether it changes depends on the value of x.

Through research, it is found that when x > 0.5, the structure of Li1-xCoO2 is extremely unstable, and the crystal collapse will occur, and its external appearance is the overwhelming termination of the cell. Therefore, the value of x in Li(1-x)CoO2 should be controlled by limiting the charging voltage during the use of the cell.

Generally, the charging voltage is not greater than 4.2V and x is less than 0.5. At this time, the crystal form of Li(1-x)CoOis still stable.

The negative electrode C6 itself has its own characteristics. After the first formation, Li in the positive electrode LiCoOis charged into the negative electrode C6. When discharging, Li returns to the positive electrode LiCoO2, but after the formation, a part of Li must remain in the negative electrode Ccenter. , in order to ensure the normal insertion of Li in the next charge and discharge, otherwise the overwhelm of the cell is very short. In order to ensure that a part of Li remains in the negative electrode C6, it is generally realized by limiting the lower limit voltage of discharge: the upper limit voltage of safe charging is ≤4.2V, the lower limit of discharge Voltage≥2.5V

 

The principle of the memory effect is crystallization, a reaction that hardly occurs in lithium batteries.

However, the capacity of lithium-ion batteries will still decrease after multiple charging and discharging, and the reasons are complex and diverse. It is mainly the change of the positive and negative materials themselves. From the molecular level, the hole structure that accommodates lithium ions on the positive and negative electrodes will gradually collapse and block; from a chemical point of view, it is the active passivation of the positive and negative materials, resulting in stable side reactions and other compounds. Physically, the positive electrode material will gradually peel off, which ultimately reduces the number of lithium ions in the battery that can freely move during charging and discharging.

 

Overcharging and over-discharging will cause permanent damage to the positive and negative electrodes of lithium-ion batteries.

From the molecular level, it can be intuitively understood that over-discharging will cause the negative electrode carbon to excessively release lithium ions and cause its lamellar structure to collapse.

Overcharging will force so many lithium ions into the negative carbon structure that some of them can no longer be released.

 

Unsuitable temperature will trigger other chemical reactions inside the lithium-ion battery to generate compounds that we do not want to see, so many lithium-ion batteries are provided with protective temperature-controlled separators or electrolyte additives between the positive and negative electrodes. When the temperature of the battery reaches a certain level, the pores of the composite membrane are closed or the electrolyte is denatured, the internal resistance of the battery increases until the circuit is disconnected, and the battery does not heat up any more, ensuring that the battery charging temperature is normal.