As a new generation of secondary battery technology, lithium-ion batteries have achieved rapid development due to their energy density advantage after replacing cadmium-nickel and hydrogen-nickel batteries, and have shown broad application prospects. However, the poor stability of their charge and discharge cycles has become a key bottleneck restricting performance improvement. This paper systematically explores the mechanism of capacity degradation in lithium-ion batteries, with a focus on analyzing core influencing factors such as overcharging, electrolyte decomposition, and self-discharge.
The mass ratio of positive and negative electrodes is closely related to battery performance. When the mass ratio is too small, the active substances of the negative electrode material cannot be fully utilized, resulting in a waste of resources; when the mass ratio is too large, the negative electrode is prone to overcharging during charging, posing a safety hazard. Therefore, only when the mass ratio of positive and negative electrodes reaches the optimal value can the battery performance be maximized.
In an ideal lithium-ion battery system, the capacity balance remains constant throughout the cycle, and the initial capacity of each cycle is relatively stable. However, the actual battery operation is far more complicated than the theory. Any side reaction involving the production and consumption of lithium ions or electrons will disrupt the original capacity balance. It is worth noting that once this equilibrium state is destroyed, it cannot be restored spontaneously, and the impact will continue to accumulate with the increase in the number of cycles. During the operation of lithium-ion batteries, in addition to the normal lithium insertion and removal redox main reaction, there are also many side reactions such as electrolyte decomposition, active material dissolution, and metal lithium deposition. The continuous occurrence of these side reactions is the important reason for the imbalance of capacity balance and the decline of battery performance.
Reason 1: Overcharging
When lithium-ion batteries are overcharged, the side reactions between the positive and negative electrodes and the electrolyte will significantly intensify the capacity loss. The specific manifestations are as follows:
1. Overcharging of graphite anode and lithium metal deposition.
When the battery is overcharged, the reduction rate of lithium ions exceeds the embedding rate, causing lithium metal (Li⁰) to deposit on the surface of the anode. The impact on capacity includes:
Active lithium loss: The deposited lithium leaves the cycling system of embedding and re-embedding, directly reducing the total amount of mobile lithium ions;
Interface side reactions: Lithium metal reacts with the electrolyte (such as solvent DMC, electrolyte LiPF₆) to form solid products such as Li₂CO₃, LiF, etc., consuming active substances and blocking electrode pores;
Internal resistance increase: Lithium dendrites grow at the negative electrode - separator interface, possibly penetrating the separator pores, increasing the resistance of ion transmission;
Electrolyte consumption: Active lithium continues to react with the electrolyte, resulting in a decrease in solvent and lithium salt concentrations, reducing the efficiency of ion conduction.
Key influencing factors:
Polarization effect: Rapid charging (high current density) intensifies the polarization of the anode, even if the ratio of active substances in the positive and negative electrodes is normal, it may trigger the growth of lithium dendrites;
Ratio imbalance: When the positive active substance is excessive (γ = m⁺/m⁻ = optimal value), the available lithium capacity of the negative electrode is insufficient, and the risk of overcharging significantly increases.
2. Positive electrode overcharge reaction
When the ratio of positive electrode active material to negative electrode active material is too low, overcharging of the positive electrode is likely to occur.
The capacity loss caused by overcharging of the positive electrode mainly results from the generation of electrochemical inert substances (such as Co3O4, Mn2O3, etc.), which disrupt the capacity balance between the electrodes and is irreversible.
3. Oxidation reaction of electrolyte during overcharging
When the voltage exceeds 4.5V, the electrolyte will oxidize and generate insoluble substances (such as Li2Co3) and gases. These insoluble substances will clog the micropores of the electrodes, hindering the migration of lithium ions and causing capacity loss during the cycling process.
Reason 2: Electrolyte decomposition (reduction)
1. Oxidation decomposition of the electrolyte on the positive electrode surface
When the voltage exceeds 4.5V (vs. Li/Li⁺), the electrolyte may undergo an oxidation reaction on the positive electrode, generating insoluble products such as Li₂CO₃ and LiF. These products will clog the electrode pores, hinder the migration of lithium ions, and lead to capacity degradation. Moreover, the oxidation reaction is accompanied by the release of gases (such as CO₂ and O₂), causing an increase in the internal pressure of the battery and creating potential safety hazards.
2. Reduction decomposition of electrolyte on the negative electrode surface
On the surface of graphite and other lithium-intercalating carbon negative electrodes, the reduction decomposition of the electrolyte is a key process affecting battery performance:
- Initial film formation:
During the first charging, the electrolyte (such as EC-based solvent) is reduced on the negative electrode surface, generating a solid electrolyte interface membrane (SEI membrane) composed of Li₂CO₃, LiOCO₂Li, LiF, etc. The ideal SEI membrane has ion conductivity (allowing Li⁺ to pass through) and electron insulation, which can prevent further electrolyte penetration reactions and stabilize the negative electrode structure;
- Irreversible capacity loss:
During the film formation process, active lithium (from the electrolyte lithium salt or the positive electrode's exfoliation of lithium) is consumed, resulting in a first charging and discharging efficiency (coulomb efficiency) lower than 100%. Typically, the first coulomb efficiency of graphite negative electrodes is 90% to 95%;
- Cycling stability:
During normal cycling, the SEI membrane maintains a dynamic balance, and only minor repairs occur when the membrane is ruptured due to electrode volume changes (such as lithium intercalation expansion). Therefore, the electrolyte consumption is controlled at a relatively low level.
3. Reduction of Electrolytes
It is generally believed to be involved in the formation of the membrane on the surface of the carbon electrode, and the type and concentration of the electrolyte will affect the performance of the carbon electrode. In some cases, the reduction of the electrolyte helps to stabilize the carbon surface and form the required passivation layer. It is generally considered that the supporting electrolyte is more prone to reduction than the solvent. The reduction products mixed in the negative electrode deposition film will affect the capacity decay of the battery.
For example, lithium hexafluorophosphate, lithium perchlorate, and lithium bis(trifluoromethanesulfonylimide) have supporting electrolytes that undergo reduction reactions on the surface of the carbon electrode. The products generated, such as LiF, ClO₃⁻, and TFSA⁻, will be embedded in the interface film. If these products can form a dense and ion-conductive film layer, it can inhibit the continuous penetration of the electrolyte. On the contrary, it may lead to capacity decay due to membrane structure problems or corrosion by by-products. Therefore, by matching the types, concentrations of the electrolyte, and adding functional substances to guide the reduction path, the quality of the interface film can be optimized to balance the capacity retention rate and interface stability.
4. Reduction of impurities
When the water content of the electrolyte is too high, water molecules undergo reduction reactions on the electrode surface, generating LiOH and Li₂O deposition layers. The related reactions include H₂O gaining electrons to form OH⁻ and H₂, OH⁻ combining with Li⁺ to form LiOH solid, and LiOH further reacting to form Li₂O and H₂. These products form a high-resistance film layer on the electrode surface, hindering the insertion of lithium ions into the graphite electrode, resulting in irreversible capacity loss. However, a small amount of water (100-300×10⁻⁶) in the solvent has no significant effect on the performance of the graphite electrode.
CO₂ in the solvent is reduced at the negative electrode to form CO gas and Li₂CO₃ solid. CO causes an increase in the internal pressure of the battery, while Li₂CO₃ increases the internal resistance of the battery. The presence of oxygen in the solvent undergoes reduction reactions to form Li₂O. Due to the small potential difference between metallic lithium and fully lithiated carbon, the reduction behavior of the electrolyte on the carbon electrode is similar to that on the metallic lithium surface. The various products generated by the reduction of impurities affect the capacity and cycle performance of the battery through pathways such as forming resistance films, generating gases, or changing interface characteristics.
Reason 3: Self-discharge
Self-discharge of lithium-ion batteries refers to the phenomenon where the battery's capacity naturally decreases over time when it is in an open-circuit state without being used. The capacity loss caused by this process can be divided into reversible and irreversible types. The former can be restored through charging, while the latter cannot be recovered as it results from permanent chemical reactions or structural damage within the battery.
Irreversible capacity loss mainly stems from the micro-cell reactions between the positive and negative electrodes and the electrolyte during charging.
For example, the lithium-manganese oxide positive electrode reacts with the solvent PC on the surface of carbon black or current collector, undergoing redox reactions, and the solvent molecules are oxidized to generate free radicals. At the same time, lithium ions are desorbed from the positive electrode material, and the active material of the negative electrode reacts with the electrolyte containing LiPF₆ and other electrolytes, with PF₆⁻ decomposing and lithium being embedded and carbon being desorbed and oxidized. These processes consume active materials and disrupt the capacity balance between the positive and negative electrodes, preventing the capacity from recovering during charging.
The self-discharge rate is affected by many factors. In the manufacturing process of the positive electrode material, the particles with more surface defects and larger specific surface area will self-discharge faster. The precision of the battery manufacturing process affects the interface contact and impurity mixing. The oxidation stability of the solvent in the electrolyte plays a leading role. For example, PC is easily oxidized while EC has stable film formation. The type of lithium salt affects the side reaction path. The temperature rise will significantly accelerate the self-discharge kinetics. The self-discharge rate at room temperature is usually 2%~5% per month, while it can rise to more than 10% per month at high temperature. Time accumulation will aggravate the deposition of byproducts. Although the diaphragm leakage current may also cause self-discharge, its rate is extremely low and has nothing to do with temperature, so it is not the main mechanism.
