Driven by the booming electric vehicle market, lithium-ion batteries, as one of the core components of electric vehicles, have received considerable attention. Efforts are focused on developing lithium-ion batteries with long lifespan, high power output, and good safety. Among these efforts, the capacity degradation of lithium-ion batteries is a particularly important concern. Only by fully understanding the causes and mechanisms of lithium-ion battery degradation can we effectively address this problem.
This article reviews several factors affecting the capacity decay of lithium-ion batteries from different perspectives for your reference.
1. Positive electrode material
LiCoO2 is a commonly used cathode material (widely used in 3C applications; power batteries primarily use ternary and lithium iron phosphate batteries). T. Osaka et al. used EIS to study LiCoO2 batteries and concluded that capacity decay during cycling comes from increased positive electrode impedance and loss of negative electrode capacity. Liu Wengang et al., after studying 18650-type LiCoO2 batteries, found that with increasing cycle number, the contribution of active lithium ion loss to capacity decay increases. After 200 cycles, LiCoO2 did not undergo a phase transition; instead, its layered structure changed, leading to difficulties in Li+ insertion/extraction.
LiFePO4 exhibits good structural stability, but the Fe3+ ions in the positive electrode dissolve and are reduced to Fe metal on the graphite negative electrode, leading to increased anodic polarization. Generally, coating with LiFePO4 particles or selecting the appropriate electrolyte can prevent Fe3+ dissolution.
NCM ternary materials: 1. Transition metal ions in transition metal oxide cathode materials are easily dissolved at high temperatures, thus becoming free in the electrolyte or deposited on the negative electrode side, causing capacity decay; 2. When the voltage is higher than 4.4V vs. Li+/Li, structural changes in ternary materials lead to capacity decay; 3. Li-Ni mixing leads to blockage of Li+ channels.
The main reasons for the capacity decay of LiMnO4-based lithium-ion batteries are as follows: 1. Irreversible phase transitions or structural changes, such as Jahn-Teller distortion; 2. Mn dissolves in the electrolyte (in the presence of HF in the electrolyte), undergoes disproportionation reaction, or is reduced at the negative electrode.
2. Anode materials
1. Lithium deposition on the graphite anode side (some lithium becomes "dead lithium" or forms lithium dendrites). At low temperatures, lithium ion diffusion slows down, which easily leads to lithium deposition. When the N/P ratio is too low, lithium deposition is also likely to occur. 2. Repeated damage and growth of the SEI film on the anode side leads to lithium loss and increased polarization.
Silicon- based anodes are prone to volume expansion during repeated lithium insertion/extraction processes, which can lead to cracking and failure of the silicon particles. Therefore, finding methods to suppress volume expansion is crucial for silicon anodes.
3 Electrolyte
Factors contributing to lithium-ion battery capacity decay due to electrolyte degradation include: 1. Decomposition of solvent and electrolyte (severe decomposition can lead to gas generation and other failures or safety issues). For organic solvents, decomposition is common when the oxidation potential is greater than 5V vs. Li+/Li or the reduction potential is less than 0.8V (different electrolytes have different decomposition voltages). For electrolytes (e.g., LiPF6), decomposition is more likely to occur at higher temperatures (above 55℃) due to poor stability; 2. With increasing cycle count, the reaction between the electrolyte and the positive and negative electrodes increases, resulting in reduced mass transfer capacity.
4 diaphragms
The separator can block electrons while allowing ion transport. However, when electrolyte decomposition products clog the separator pores, or when the separator shrinks at high temperatures, or when the separator ages, its ability to transport Li+ will decrease. In addition, the formation of lithium dendrites that pierce the separator and cause internal short circuits is a major cause of its failure.
5 sets of fluids
Capacity loss due to current collector corrosion is generally caused by corrosion of the current collector. Copper is easily oxidized at high potentials, so it is used as the negative electrode current collector, while aluminum easily forms a lithium-aluminum alloy with lithium at low potentials and is used as the positive electrode current collector. Under low voltage conditions (as low as 1.5V and below, over-discharge), copper oxidizes to Cu2+ in the electrolyte and deposits on the negative electrode surface, hindering lithium intercalation and deintercalation, resulting in capacity decay. On the positive electrode side, overcharging causes pitting corrosion of the aluminum current collector, leading to increased internal resistance and capacity decay.
The above analysis mainly focuses on the main components of lithium-ion batteries. Of course, the factors affecting the capacity decay of lithium-ion batteries can also be considered from the following perspectives.
6 charging and discharging factors
Charge/discharge rate: Excessive charge/discharge rates accelerate the capacity decay of lithium-ion batteries. A higher charge/discharge rate means a corresponding increase in the battery's polarization resistance, leading to a decrease in capacity. Furthermore, the diffusion-induced stress generated during high-rate charging and discharging causes loss of positive electrode active material, accelerating battery aging.
Overcharging and over-discharging : Under overcharging conditions, lithium plating is prone to occur at the negative electrode, the lithium delithiation mechanism at the positive electrode collapses due to excessive discharge, and the electrolyte oxidation and decomposition (generation of byproducts and gas) accelerates. Under over-discharging conditions, copper foil is prone to dissolution (hindering lithium insertion/extraction or directly forming copper dendrites), leading to capacity decay or battery failure.
Charging strategy research shows that when the charging cutoff voltage is 4V, appropriately lowering the charging cutoff voltage (e.g., to 3.95V) can improve the battery's cycle life. Other studies have shown that fast charging to 100% SOC results in faster battery degradation than fast charging to 80% SOC. Furthermore, Li et al. found that while pulse charging can improve charging efficiency, it significantly increases the battery's internal resistance and causes severe loss of the negative electrode active material.
7 Temperature
Temperature also has a significant impact on the capacity of lithium-ion batteries. Prolonged operation at higher temperatures increases internal side reactions (such as electrolyte decomposition), leading to irreversible capacity loss. Conversely, prolonged operation at lower temperatures increases the battery's total impedance (decreases electrolyte conductivity, increases SEI impedance, and slows electrochemical reaction rates), making lithium plating more likely.
