Influenced by the hot degree of the electric vehicle market, lithium-ion batteries, as one of the core components of electric vehicles, have been emphasized to a great extent. People are committed to developing a long life, high power, good safety lithium-ion battery. Among them, the attenuation of lithium-ion battery capacity is very worthy of everyone's attention, only a full understanding of the reasons for the attenuation of lithium-ion batteries or the mechanism, in order to be able to prescribe the right medicine to solve the problem, that lithium-ion batteries capacity why the attenuation?
Reasons for capacity degradation of lithium-ion batteries
1.Positive electrode material
LiCoO2 is one of the commonly used cathode materials (3C category is widely used, and power batteries basically carry ternary and lithium iron phosphate). As the number of cycles increases, the loss of active lithium ions contributes more to the capacity decay. After 200 cycles, LiCoO2 did not undergo a phase transition, but rather a change in the lamellar structure, leading to difficulties in Li+ de-embedding.
LiFePO4 has good structural stability, but the Fe3+ in the anode dissolves and reduces to Fe metal on the graphite anode, resulting in increased anode polarization. Generally the Fe3+ dissolution is prevented by the coating of LiFePO4 particles or the choice of electrolyte.
NCM ternary materials ① Transition metal ions in the transition metal oxide cathode material are easy to dissolve at high temperatures, thus freeing in the electrolyte or depositing on the negative side causing capacity attenuation; ② When the voltage is higher than 4.4V vs. Li+/Li, the structural change of the ternary material leads to capacity degradation; ③ Li-Ni mixed rows, leading to the blockage of Li+ channels.
The main causes of capacity degradation in LiMnO4-based lithium-ion batteries are 1. irreversible phase or structural changes, such as the Jahn-Teller aberration; and 2. dissolution of Mn in the electrolyte (presence of HF in the electrolyte), disproportionation reactions, or reduction at the anode.
2.Negative electrode materials
The generation of lithium precipitation on the anode side of graphite (part of the lithium becomes "dead lithium" or generates lithium dendrites), at low temperatures, lithium ion diffusion slows down easily leading to lithium precipitation, and lithium precipitation is also prone to occur when the N/P ratio is too low.
Repeated destruction and growth of SEI film on the anode side leads to lithium depletion and increased polarization.
The repeated process of lithium embedding/de-lithium removal in the silicon-based anode can easily lead to volume expansion and crack failure of the silicon particles. Therefore, for silicon anode, it is especially critical to find a way to inhibit its volume expansion.
3.Electrolyte
Factors in the electrolyte that contribute to capacity degradation of lithium-ion batteries include：
1. Decomposition of solvents and electrolytes (serious failure or safety problems such as gas production), for organic solvents, when the oxidation potential is greater than 5V vs. Li+/Li or reduction potential is lower than 0.8V (different electrolyte decomposition voltage is different), easy to decompose. For electrolyte (e.g. LiPF6), it is easy to decompose at higher temperature (over 55℃) due to poor stability;.
2. As the number of cycles increases, the reaction between the electrolyte and the positive and negative electrodes increases, making the mass transfer capacity weaken.
4.Diaphragm
The diaphragm can block the electrons and fulfill the transmission of ions. However, the ability of the diaphragm to transport Li+ is reduced when the diaphragm holes are blocked by decomposition products of the electrolyte, etc., or when the diaphragm shrinks at high temperatures, or when the diaphragm ages. In addition, the formation of lithium dendrites piercing the diaphragm leading to internal short circuit is the main reason for its failure.
5. Collecting fluid
The cause of capacity loss due to the collector is generally the corrosion of the collector. Copper is used as the negative collector because it is easy to oxidize at high potentials, while aluminum is used as the positive collector because it is easy to form a lithium-aluminum alloy with lithium at low potentials. Under low voltage (as low as 1.5V and below, over-discharge), copper oxidizes to Cu2+ in the electrolyte and deposits on the surface of the negative electrode, hindering the de-embedding of lithium, resulting in capacity degradation. And on the positive side, overcharging of the battery causes pitting of the aluminum collector, which leads to an increase in internal resistance and capacity degradation.
6. Charge and discharge factors
Excessive charge and discharge multipliers can lead to accelerated capacity degradation of lithium-ion batteries. An increase in the charge/discharge multiplier means that the polarization impedance of the battery increases accordingly, leading to a decrease in capacity. In addition, the diffusion-induced stress generated by charging and discharging at high multiplication rates leads to the loss of cathode active material and accelerated aging of the battery.
In the case of overcharging and overdischarging batteries, the negative electrode is prone to lithium precipitation, the positive electrode excessive lithium removal mechanism collapses, and the oxidative decomposition of the electrolyte (the occurrence of by-products and gas production) is accelerated. When the battery is over-discharged, the copper foil tends to dissolve (hindering lithium de-embedding, or directly generating copper dendrites), leading to capacity degradation or battery failure.
Charging strategy studies have shown that when the charging cut-off voltage is 4V, appropriately lowering the charging cut-off voltage (e.g., 3.95V) can improve the cycle life of the battery. It has also been shown that fast charging a battery to 100% SOC decays faster than fast charging to 80% SOC. In addition, Li et al. found that although pulsing can improve the charging efficiency, the internal resistance of the battery will rise significantly, and the loss of negative electrode active material is serious.
7.Temperature
The effect of temperature on the capacity of lithium-ion batteries is also very important. When operating at higher temperatures for extended periods of time, there is an increase in side reactions within the battery (e.g., decomposition of the electrolyte), leading to an irreversible loss of capacity. When operating at lower temperatures for extended periods of time, the total impedance of the battery increases (electrolyte conductivity decreases, SEI impedance increases, and the rate of electrochemical reactions decreases), and lithium precipitation from the battery is prone to occur.
The above is the main reason for lithium-ion battery capacity degradation, through the above introduction I believe you have an understanding of the causes of lithium-ion battery capacity degradation.
Post time: Jul-24-2023