Analysis of Capacity Degradation and Causes of Lithium-Ion Batteries

I. Analysis of Capacity Degradation in Lithium-Ion Batteries

The positive and negative electrodes, electrolyte, and separator are essential components of lithium-ion batteries. The capacity of a lithium-ion battery is significantly influenced by the amount of lithium intercalated into and de-intercalated from the positive and negative electrodes. Thus, maintaining a balance in the capacity of the positive and negative electrodes is crucial for ensuring optimal battery performance.

Typically, lithium-ion batteries use an electrolyte solution composed of organic solvents and electrolytes (lithium salts). This electrolyte solution must possess adequate conductivity, stability, and compatibility with the electrodes. The separator’s performance is a key factor in determining the internal resistance and interface structure of the battery, directly impacting capacity degradation. High-quality separators can significantly enhance the capacity and overall performance of lithium-ion batteries. Generally, the separator serves to isolate the positive and negative electrodes, preventing short circuits due to direct contact while allowing electrolyte ions to pass through, thereby optimizing battery efficiency.

In a lithium-ion battery, chemical reactions include not only redox reactions during lithium intercalation and de-intercalation but also side reactions such as the formation and destruction of the SEI (Solid Electrolyte Interphase) film on the negative electrode, electrolyte decomposition, and structural changes and dissolution of active materials. These side reactions are primary causes of capacity degradation.

Capacity degradation and loss during battery cycling are inevitable. To improve battery capacity and performance, researchers globally have extensively studied the mechanisms behind capacity loss. Major factors contributing to capacity degradation in lithium-ion batteries include the formation of SEI films on the electrode surfaces, metal lithium deposition, dissolution of electrode active materials, redox reactions or side reactions at the electrodes, structural changes, and phase transitions. Current research on lithium-ion battery capacity degradation and its causes is ongoing.

II. Overcharging

2.1 Negative Electrode Overcharge Reactions

Various materials can be used as active materials for the negative electrode of lithium-ion batteries, including carbon-based materials, silicon-based and tin-based materials, and lithium titanate. Different types of carbon materials exhibit varying electrochemical properties. Graphite, for example, has high electrical conductivity, an excellent layered structure, and high crystallinity, making it suitable for lithium intercalation and de-intercalation. Additionally, graphite is cost-effective and widely available, leading to its extensive use.

During the first charge and discharge cycle of a lithium-ion battery, solvent molecules decompose on the graphite surface to form a passivation film known as SEI. This reaction results in capacity loss and is irreversible. In overcharging conditions, metal lithium deposition can occur on the negative electrode’s surface, especially when there is an excess of positive electrode active material relative to the negative electrode. Metal lithium deposition may also occur under high-rate conditions.

The primary causes of capacity degradation due to metal lithium formation are: (1) a reduction in the amount of reversible lithium in the battery; (2) side reactions between metal lithium and the electrolyte or solvent, resulting in additional byproducts; and (3) metal lithium deposition primarily between the negative electrode and the separator, leading to pore blockage in the separator and increased internal resistance. The impact mechanism of capacity degradation varies depending on the type of graphite material used. Natural graphite has a higher specific surface area, making it more prone to self-discharge reactions and higher electrochemical reaction impedance compared to synthetic graphite. Factors such as the dissolution of the negative electrode’s layered structure during cycling, dispersion of conductive agents during electrode production, and increased electrochemical reaction impedance during storage also significantly contribute to capacity loss.

2.2 Positive Electrode Overcharge Reactions

Overcharging of the positive electrode mainly occurs when the proportion of positive electrode material is insufficient, leading to an imbalance in electrode capacity. This imbalance results in irreversible capacity loss and can pose safety hazards due to the accumulation of oxygen and combustible gases released from the positive electrode material and electrolyte decomposition.

2.3 Electrolyte Reactions at High Voltage

If the charging voltage of a lithium-ion battery is excessively high, it can cause the electrolyte to undergo oxidation reactions, generating byproducts that block the micro-pores of the electrodes and impede lithium ion migration, thereby causing capacity degradation during cycling. The concentration of the electrolyte and the stability of the electrolyte solution are inversely related; higher electrolyte concentration leads to lower stability, which in turn affects battery capacity. During charging, the electrolyte is partially consumed, necessitating replenishment during assembly, which reduces the battery’s active material and affects its initial capacity.

III. Decomposition of Electrolyte

The electrolyte in lithium-ion batteries, comprising electrolytes, solvents, and additives, significantly impacts the battery’s lifespan, specific capacity, high-rate charge and discharge performance, and safety. The decomposition of electrolytes and solvents in the electrolyte can lead to capacity loss in the battery. During the initial charge and discharge cycles, the formation of an SEI (Solid Electrolyte Interphase) film on the negative electrode surface due to solvent and other substances results in irreversible capacity loss, which is an unavoidable phenomenon.

If the electrolyte contains impurities such as water or hydrofluoric acid, these impurities can cause the electrolyte lithium hexafluorophosphate (LiPF6) to decompose at elevated temperatures. The decomposition products may react with the positive electrode materials, affecting the battery’s capacity. Additionally, some decomposition products may react with solvents, impacting the stability of the SEI film on the negative electrode and causing a decline in battery performance.

Furthermore, if the decomposition products of the electrolyte are not compatible with the electrolyte itself, they can block the pores of the positive electrode during migration, leading to capacity degradation. In summary, the occurrence of side reactions between the electrolyte and the positive and negative electrodes, as well as the resulting byproducts, are major factors contributing to the capacity degradation of lithium-ion batteries.

IV. Self-Discharge

In general, lithium-ion batteries experience capacity loss over time, a phenomenon known as self-discharge. Self-discharge can be categorized into reversible and irreversible capacity loss. The rate of solvent oxidation directly affects the rate of self-discharge. During charging, the active materials of the positive and negative electrodes may react with the solutes, causing imbalance and irreversible degradation in lithium ion migration capacity. Thus, reducing the surface area of active materials can slow down the rate of capacity loss, and solvent decomposition impacts the battery’s storage life.

Additionally, while less common, leakage current through the separator can also contribute to capacity loss. Persistent self-discharge can lead to metal lithium deposition, which further causes capacity degradation of the positive and negative electrodes.

V. Electrode Instability

During charging, instability in the active materials of the battery’s positive electrode can lead to reactions with the electrolyte, affecting battery capacity. Factors such as structural defects in the positive electrode material, excessive charging voltage, and the content of carbon black all significantly influence battery capacity.

5.1 Structural Phase Transition

5.1.1 LiMn₂O₄

Spinel LiMn₂O₄ is abundant and inexpensive in China, and it possesses good thermal stability, making it a major material for positive electrodes in batteries. However, LiMn₂O₄ positive electrodes experience capacity degradation during storage in high-temperature environments and during charge-discharge cycles. This degradation is primarily caused by the following factors: firstly, electrochemical reactions occur in the electrolyte under high voltage conditions, typically above 4.0V; secondly, manganese (Mn) in LiMn₂O₄ dissolves in the electrolyte, leading to disproportionation reactions that damage the crystal structure of the positive electrode material.

For lithium-ion batteries with LiMn₂O₄ as the positive electrode and carbon (C) as the negative electrode, high-pressure conditions can lead to solvent decomposition and oxidation reactions on the C negative electrode. The resulting oxidation products migrate to the positive electrode and cause dissolution reactions. The dissolved divalent manganese ions are reduced at the negative electrode and co-deposit with other impurities. Manganese oxides primarily deposit near the separator side of the negative electrode and not near the current collector, which contributes to capacity degradation. Adding inhibitors to the electrolyte can effectively suppress the dissolution of metal ions and improve battery cycle performance.

Additionally, in lithium-ion batteries with LiMn₂O₄ as the positive electrode and C as the negative electrode, the embedding and de-embedding of lithium ions during cycling can cause changes in the lattice constants of LiMn₂O₄ and phase transitions between cubic and tetragonal systems. The diffusion rate of lithium ions within the positive electrode material is lower than the rate of intercalation at the surface. When the potential reaches around 4V, lithium ions accumulate on the LiMn₂O₄ surface, leading to Jahn-Teller effects that cause structural distortions and transitions, resulting in capacity degradation.

5.1.2 LiCoO₂

LiCoO₂ is a preferred material for lithium-ion battery cathodes due to its ability to reversibly intercalate and de-intercalate lithium ions, along with its high lithium ion diffusion coefficient, reversible insertion amount, and structural stability. This material plays a crucial role in enhancing the charge and discharge currents of lithium batteries. LiCoO₂ maintains a stable structure, and its reversible lithium ion intercalation helps ensure high Coulombic efficiency and extended battery life. Studies on the capacity degradation mechanisms of the LiCoO₂ system have revealed that factors affecting capacity loss during cycling primarily include increased interface resistance at the positive electrode and loss of capacity at the negative electrode.

Furthermore, research shows that as the number of cycles increases, the contribution of capacity loss from the positive and negative electrodes to the overall battery capacity loss decreases, and the reduced mobility of active lithium ions significantly impacts overall capacity degradation. Additionally, after more than 200 cycles, the positive electrode material does not undergo phase transitions, but the layered structure of LiCoO₂ becomes less regular, leading to increased mixing of lithium and chromium ions. This makes it harder for lithium ions to be effectively de-intercalated, resulting in capacity loss. Increasing the discharge rate accelerates the mixing of lithium and chromium atoms, causing a transition from the original hexagonal to cubic crystal structure of LiCoO₂, which contributes to capacity degradation.

Moreover, studies of the LiCoO₂ system at 25°C (room temperature) and 60°C show that the discharge capacity of the battery at temperatures below 60°C is higher than that at room temperature before 150 cycles. This is due to the reduced viscosity of the electrolyte at higher temperatures, which increases the migration rate of lithium ions and improves the utilization of active lithium, resulting in higher charge and discharge capacities. However, after 300 cycles, the polarization capacity loss of the battery at 60°C is significantly higher than at room temperature, indicating that elevated temperatures exacerbate electrochemical polarization of the electrodes during cycling, leading to more severe capacity loss in the battery.

5.1.3 LiFePO₄

LiFePO₄ is widely available, cost-effective, and offers excellent stability and safety. It has a theoretical specific capacity of 170 mAh/g and its specific power and energy are comparable to those of LiCoO₂. LiFePO₄ also demonstrates good compatibility with electrolyte solutions, making it a popular choice for positive electrodes in lithium-ion batteries. The factors affecting battery capacity with this material include: (1) side reactions between the positive and negative electrodes, leading to a reduction in reversible lithium and severely disrupting the balance between the electrodes; and (2) degradation of the structure, electrode layer separation, material dissolution, and particle delamination, all of which contribute to the loss of active material and impact battery capacity.

5.2 Carbon Black Content in Positive Electrode Materials

Carbon black, being an inactive substance, does not participate in discharge reactions. However, if the amount of carbon black in the positive electrode material is too high, it can affect the strength and capacity of the positive electrode material. Therefore, it should be added in appropriate amounts. Additionally, catalytic substances generated on the surface of carbon black can enhance the rate of metal ion decomposition and effectively promote the dissolution of active materials.

Reference: Wang Kun et al., “Analysis of Capacity Degradation and Its Causes in Lithium-Ion Batteries”