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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.

1. The working principle of lithium ion power battery

At present, lithium-ion power batteries have been widely used in electric vehicles, and the research on their performance has become a hot spot in the industry.

Lithium-ion power battery is a high-performance secondary battery composed of four parts: positive electrode, negative electrode, separator and electrolyte: the positive electrode undergoes a reduction reaction when the battery is discharged, and most transition metal oxides such as LCoO₂、Li_x2NiO₂、Li_xMn₂O₄ are used; there are many negative electrodes. Using carbon materials, oxidation reaction occurs during discharge; the diaphragm provides electronic isolation for the positive and negative electrodes; the electrolyte is generally an organic solution such as LiAsF₆, which is a transport medium for ion movement. When charging, Li⁺ is deintercalated from the positive electrode through the electrolyte and inserted into the negative electrode. At the same time, the electronic compensation charge is supplied from the external circuit to the carbon negative electrode to maintain the electric balance of the negative electrode. On the contrary, Li⁺ is deintercalated from the negative electrode and inserted into the positive electrode through the electrolyte. It can be seen that the lithium-ion power battery uses Li⁺ to reciprocally intercalate and de-intercalate between the positive and negative electrodes for charging and discharging. It is a lithium ion concentration difference battery. Its general working principle is shown in Figure 2-1. The electrode reaction expression is shown in the formula diagram (1-1), formula (1-2), and formula (1-3).

Figure 2-1 The working principle of lithium-ion power battery

Formula diagram

According to the different cathode materials, lithium-ion power batteries mainly include lithium manganese oxide batteries, lithium cobalt oxide batteries, lithium iron phosphate batteries, and ternary material lithium-ion batteries; according to different electrolyte materials, lithium-ion power batteries are divided into liquid lithium-ion There are two major categories of batteries and polymer lithium-ion batteries. Compared with power batteries of other structures, the main advantages of lithium-ion power batteries are:

(1) The working voltage is high. According to the different cathode materials, the operating voltage range of lithium-ion power batteries is 3.2~3.7V, which is about three times the operating voltage of other types of batteries such as nickel-cadmium batteries.

(2) Higher than energy. The theoretical specific energy of lithium-ion power batteries is as high as 200Wh/kg, and the actual specific energy is higher than 140Wh/kg, which is about twice that of nickel-hydrogen batteries.

(3) High capacity and energy conversion efficiency

(4) Long storage and cycle life. In a suitable environment, the lithium-ion power battery can be stored for more than 5 years, the number of deep-cycle charge and discharge can reach more than 1,000 times, and the cycle life is up to 10,000 times at low depth of discharge. The life characteristics are much better than other types of batteries.

(5) The self-discharge is small. When the ambient temperature is (20±5)℃, the monthly self-discharge rate of lithium-ion batteries is only 5%-9%, which greatly alleviates the problem of power loss caused by self-discharge when traditional secondary batteries are placed.

(6) No memory effect

(7) Wide operating temperature range. Lithium-ion power battery can work in the temperature range of -20~60℃. However, we should try our best to provide a suitable working temperature for the lithium-ion power battery, because the working environment of high temperature (≥40℃) and low temperature (≤0℃) will damage the electrical performance of the lithium-ion power battery and accelerate the lithium-ion power battery. The life of the decay.

(8) High environmental protection, lithium ion power battery does not contain cadmium, lead, mercury and other harmful substances, has low environmental pollution, and is a true green battery

The excellent electrical performance of lithium-ion power batteries lays the foundation for their application in electric vehicles and accelerates the research and development of new energy vehicles. However, it is not a perfect car power battery. Its main disadvantages are: large internal resistance, and large internal resistance causes the lithium-ion power battery to rapidly decrease its energy at high power output; due to the charging and discharging of the lithium-ion power battery It has a wide range and requires special protection circuits to prevent overcharge and overdischarge of the battery; poor compatibility with ordinary batteries, which is mainly due to the large difference in voltage between batteries; poor overcharge and overdischarge resistance.