Tag SEI film

Causes of capacity degradation of lithium-ion power batteries

The internal factors leading to the failure of lithium-ion power batteries mainly include the performance degradation of the positive/negative electrode materials and the aging of the electrolyte decomposition membrane. The external factors include the battery temperature, the intensity of charge and discharge current, and the depth of discharge. Lithium-ion dynamics. When the battery fails, if it can reach the design life specified by the manufacturer, it is called normal failure, otherwise it is called premature failure. The main reasons leading to the premature failure of the hammer ion power battery are excessive use (excessive strength, excessive depth, overload, etc.), external short circuit, internal damage, etc. Excessive use aggravates the irreversible side reactions inside the battery, accelerates the attenuation of battery life, and may even cause fire, explosion, etc.

The ideal working state of the lithium ion power battery is that only Li⁺ intercalation and deintercalation between the positive and negative electrodes occur, and there are no other side reactions to consume Li⁺. In the actual use process, as the charge and discharge progress, the lithium-ion power battery will have metal lithium deposition, active material dissolution, electrolyte decomposition and other phenomena, resulting in irreversible loss of the capacity of the lithium-ion power battery. The main mechanisms that cause the capacity degradation of lithium-ion power batteries are:

(1) Cathode material dissolution. The cathode material will dissolve during the use of lithium-ion power batteries. This is mainly caused by factors such as structural defects of the positive electrode material and overcharging during use. With the increase in the number of battery charging and discharging, the dissolution rate of the cathode material is also increasing. The dissolution of the positive electrode material causes the formation of simple metal near the negative electrode, which increases the battery impedance and causes the capacity of the lithium-ion battery to decline.

(2) Phase change of positive/negative electrode materials. There are two types of phase changes in lithium ion battery electrode materials:

①The phase change caused by the deintercalation and intercalation of Li⁺ during the normal operation of the lithium-ion power battery. This phase change causes physical damage to the positive and negative materials and reduces the electrical contact between the internal materials of the battery.

②The over-use of overcharge and over-discharge during use will cause the phase change of the positive electrode material. This phase change changes the volume structure of the cathode material.

Both of these two phase transitions affect the propagation process of Li⁺ in the battery, which leads to the degradation of battery capacity.

(3) The electrolyte causes capacity attenuation. The decomposition of the electrolyte causes a series of irreversible reactions in the battery, which produces lithium oxides and LiOH and other deposits, which consumes the electrolyte, which leads to an increase in battery polarization, a decrease in Li⁺ concentration, and an increase in resistance to expansion.

(4) Overcharge causes capacity loss. When overcharged, Li⁺ is reduced and deposited on the negative electrode, which thickens the negative SEI film. Inert materials and oxygen are also formed near the positive electrode, which hinders the deintercalation and insertion of lithium ions and causes irreversible loss of battery capacity.

(5) Self-discharge. The self-discharge phenomenon of lithium-ion power batteries is inevitable. Only a small part of the battery capacity loss caused by self-discharge is irreversible loss, and most can be recovered by recharging. The irreversible loss caused by self-discharge is caused by the loss of Li⁺ and the blockage of the electrode pores by the oxide of the electrolyte.

(6) Formation of SEI interface film. At the beginning of the charge-discharge cycle, an irreversible reaction occurs between the negative electrode material of the lithium-ion power battery and the electrolyte, forming a solid electrolyte membrane (SEI film) on the surface of the negative electrode. Its formation and growth will consume the Li⁺ and electrolyte inside the battery, leading to a decline in the capacity of the lithium-ion power battery. The growth rate of the SEI film is closely related to the battery life, working temperature, and the specific area of ​​the negative electrode material.

(7) Current collector corrosion. In the process of charging and discharging lithium-ion power batteries, the current collector will corrode and produce a corrosive film. In the case of deep discharge, copper ions will form elemental copper deposits on the surface of the negative electrode during the charging process.These films and deposits hinder the intercalation and deintercalation of lithium ions, resulting in a decline in battery capacity.