Analysis of battery capacity degradation and its causes!
Jul,31,24
1、 Analysis of Capacity Decay Phenomenon in Lithium ion Batteries
The positive and negative electrodes, electrolyte, and separator are important components that make up lithium-ion batteries.
The positive and negative electrodes of lithium-ion batteries undergo lithium insertion and extraction reactions,
and the amount of lithium inserted into the positive and negative electrodes becomes the main factor affecting the capacity of lithium-ion batteries.
Therefore, it is necessary to maintain the balance of positive and negative capacity of lithium-ion batteries in order to ensure optimal performance.
Generally speaking, lithium-ion batteries commonly use electrolyte solutions composed of organic solvents and electrolytes (lithium salts),
which should have sufficient conductivity, stability, and be compatible with the electrodes.
For the separator, its performance is the main factor determining the internal resistance and interface structure of the battery,
and has a direct impact on the capacity degradation of the battery.
If the quality and performance of the separator are superior, it will significantly improve the capacity and overall performance of lithium-ion batteries.
In general, the separator plays a role in separating the positive and negative electrodes of a battery,
avoiding contact between the positive and negative electrodes and causing a short circuit in the battery.
At the same time, it can also release electrolyte ions to fully utilize the battery's effectiveness.
The chemical reactions in lithium-ion batteries not only include the oxidation-reduction reactions during the insertion and extraction of lithium ions,
but also side reactions such as the production and destruction of SEI film on the negative electrode surface,
the decomposition of electrolyte, and the structural changes and dissolution of active materials.
These side reactions are the reasons for the capacity degradation of lithium-ion batteries.
Capacity decay and loss during battery cycling are inevitable phenomena.
Therefore, in order to improve battery capacity and performance, scholars in various fields at home
and abroad have fully studied the mechanism of capacity loss in lithium batteries.
At present,
it is known that the main factors causing capacity degradation of lithium-ion batteries include the formation of SEI passivation films on the positive and negative electrode surfaces,
deposition of metallic lithium,
dissolution of electrode active materials, occurrence of redox reactions or side reactions on the anode and cathode,
structural changes, and phase changes. Currently, the research on the capacity degradation and its causes of lithium-ion batteries is still ongoing.
2、 Overcharging
2.1 Negative Overcharge Reaction
There are many types of active materials that can be used as negative electrodes for lithium-ion batteries,
mainly carbon based negative electrode materials, silicon-based, tin based negative electrode materials, lithium titanate negative electrode materials, etc.
Different types of carbon materials have different electrochemical properties,
among which graphite has the advantages of high conductivity, excellent layered structure, and high crystallinity, making it more suitable for lithium insertion and extraction.
At the same time, graphite materials are affordable and have a large inventory, so they are widely used.
When lithium-ion batteries are charged and discharged for the first time,
solvent molecules will undergo decomposition reactions on the graphite surface and form a passivation film called SEI.
This reaction will cause capacity loss of the battery and is an irreversible process.
During the overcharging process of lithium-ion batteries,
metal lithium deposition occurs on the negative electrode surface,
which can easily occur when there is an excess of positive electrode active material relative to negative electrode active material.
At the same time, under high magnification conditions, there is also a possibility of lithium metal deposition.
Generally speaking, the reasons for the capacity degradation of lithium batteries caused by the formation of metallic lithium mainly include the following aspects:
firstly, it leads to a decrease in the amount of recyclable lithium in the battery;
Secondly, lithium metal reacts with electrolytes or solvents to form other byproducts;
Thirdly, metallic lithium mainly deposits between the negative electrode and the separator, causing pore blockage in the separator and increasing the internal resistance of the battery.
The impact mechanism of capacity degradation in lithium-ion batteries varies depending on the graphite material used.
The specific surface area of natural graphite is relatively high,
therefore, the occurrence of self discharge reaction will lead to capacity loss of lithium batteries, and natural graphite, as the negative electrode of batteries,
has a higher electrochemical reaction impedance than artificial graphite.
In addition, factors such as the dissociation of the negative electrode layered structure during the cycling process,
the dispersion of the conductive agent during the electrode plate manufacturing process,
and the increase in electrochemical reaction impedance during storage are all important factors leading to capacity loss in lithium batteries.
2.2 Positive overcharge reaction
Positive electrode overcharging mainly occurs when the proportion of positive electrode material is too low,
resulting in an imbalance of electrode capacity and irreversible loss of lithium battery capacity.
In addition,
the coexistence and continuous accumulation of oxygen and flammable gases decomposed from the positive electrode material
and electrolyte may pose safety hazards to the use of lithium batteries.
2.3 Electrolyte reacts under high voltage
If the charging voltage of a lithium battery is too high, it will cause oxidation reaction of the electrolyte and generate some by-products,
blocking the micropores of the electrode and hindering the migration of lithium ions, resulting in a decrease in cycling capacity.
The trend of changes in electrolyte concentration and electrolyte stability is inversely proportional.
The higher the electrolyte concentration, the lower the electrolyte stability, which in turn affects the capacity of lithium-ion batteries.
During the charging process, the electrolyte will be consumed to a certain extent, so it needs to be replenished during assembly,
resulting in a decrease in the active materials of the battery and affecting the initial capacity of the battery.
3、 Electrolyte decomposition
Electrolytes include electrolytes, solvents, and additives,
and their properties can affect the battery's service life, specific capacity, rate charge discharge performance, and safety performance.
The decomposition of electrolytes and solvents in electrolytes can cause a loss of battery capacity.
During the first charge and discharge, the formation of SEI film on the negative electrode surface by solvents and other substances will result in irreversible capacity loss,
but this is an inevitable situation. If there are impurities such as water or hydrogen fluoride in the electrolyte,
it may cause the electrolyte LiPF6 to decompose at high temperatures, and the resulting products may react with the positive electrode material, affecting the battery capacity.
At the same time, some products may react with solvents and affect the stability of the SEI film on the negative electrode surface,
leading to a decline in the performance of lithium-ion batteries.
In addition, if the products of electrolyte decomposition are not compatible with the electrolyte,
they will block the positive electrode pores during migration, leading to a decrease in battery capacity.
Overall, the occurrence of side reactions between the electrolyte and the positive and negative electrodes of the battery,
as well as the resulting by-products, are the main factors causing battery capacity degradation.
4、 Self discharge
Lithium ion batteries generally experience capacity loss, a process known as self discharge,
which is divided into reversible capacity loss and irreversible capacity loss. The solvent oxidation rate has a direct impact on the self discharge rate.
The positive and negative active materials may react with solutes during the charging process, leading to capacity imbalance and irreversible decay of lithium ion migration.
Therefore, it can be seen that reducing the surface area of the active material can reduce the capacity loss rate,
and the decomposition of the solvent can affect the storage life of the battery.
In addition, membrane leakage can also cause capacity loss, but this possibility is relatively low.
If the self discharge phenomenon persists for a long time,
it will lead to the deposition of metallic lithium and further cause a decrease in the capacity of the positive and negative electrodes.
5、 Electrode instability
During the charging process, the active material of the positive electrode of the battery is unstable, which can cause it to react with the electrolyte and affect the battery capacity.
Among them, structural defects in the positive electrode material, high charging potential, and carbon black content are the main factors affecting battery capacity.
5.1 Structural phase transition
5.1.1 LiMn2O4
Spinel LiMn2O4 has abundant resources in China, is inexpensive, and has good thermal stability, making it the main material for battery cathodes.
The storage of LiMn2O4 positive electrode in high temperature environment
and the process of battery charging and discharging cycles can lead to a decrease in battery capacity,
mainly caused by the following factors: firstly, under high voltage conditions, the electrolyte undergoes electrochemical reactions, generally above 4.0V;
Secondly, the Mn contained in LiMn2O4 material dissolves in the electrolyte,
causing a disproportionation reaction that destroys the crystal structure of the positive electrode material.
For lithium-ion batteries with LiMn2O4 as the positive electrode and C as the negative electrode, solvent decomposition occurs under high voltage,
accompanied by the oxidation reaction of the C negative electrode.
The generated oxidation products migrate to the positive electrode and undergo dissolution reaction with the positive electrode.
The divalent manganese ions formed after dissolution will be reduced at the negative electrode and deposited together with other impurities.
And the oxide of Mn will only deposit in the direction of the negative electrode near the separator, not in the direction of the current collector,
that is, the oxide of Mn will only deposit on the surface of the SEI film,
which is why it leads to a decrease in battery capacity.
Adding inhibitors to the electrolyte can effectively inhibit the dissolution state of metal ions and improve the cycling performance of the battery.
In addition, during the charging and discharging process of lithium-ion batteries with LiMn2O4 as the positive electrode and C as the negative electrode,
the insertion and extraction of lithium ions may cause a change in the lattice constant of LiMn2O4 and a phase transition between the cubic and tetragonal crystal systems.
The diffusion rate of lithium ions inside the positive electrode material is lower than the insertion rate of lithium ions on the positive electrode surface.
When the potential is about 4V,
lithium ions aggregate on the surface of LiMn2O4 and undergo Jahn Teller effect, causing structural distortion and transformation, resulting in battery capacity degradation.
5.1.2 LiCoO2
The application of LiCoO2 in lithium-ion battery cathode materials has significant advantages, mainly reflected in the reversible insertion and extraction of lithium ions,
as well as its large lithium ion diffusion coefficient, reversible insertion amount, and degree of structural change.
Therefore, it plays an important role in improving the charging and discharging current of lithium-ion batteries.
At the same time, the material has a stable structure and good reversibility of lithium ion insertion and extraction,
which can effectively ensure the coulombic efficiency of charging and discharging and the service life of the battery.
Through the study of the capacity degradation mechanism of LiCoO2 system by relevant scholars at home and abroad,
it has been found that the factors affecting the capacity degradation changes during the cycling process of lithium batteries are mainly due to the increase
in positive electrode interface impedance and the loss of negative electrode capacity.
Meanwhile, relevant scholars have also found that the higher the number of cycles,
the lower the contribution of the capacity loss of the positive and negative electrodes compared to the capacity loss of the entire battery,
and the decrease in the migration ability of active lithium ions will have a greater impact on the overall capacity decay of the battery.
And as shown in Figure 1, after more than 200 battery cycles, the positive electrode material did not undergo phase transition,
while the regularity of LiCoO2 layered structure decreased, and the mixing phenomenon of lithium ions and chromium ions increased,
making it difficult for lithium ions to effectively deintercalate, resulting in a decrease in battery capacity.
In addition, increasing the discharge rate will promote the mixing of lithium and chromium atoms,
leading to the transformation of LiCoO2's original hexagonal crystal structure into cubic crystal structure, thereby causing capacity degradation changes in lithium-ion batteries.
Figure 1 Impedance and Capacity Variation Curve during the Cycle Process
In addition, in the LiCoO2 system,
through the study of the degradation law of battery cycling capacity at 25 ℃ (i.e. room temperature) and 60 ℃, it can be found that before 150 cycles,
the discharge capacity of batteries below 60 ℃ is higher than the capacity and rated capacity of batteries at room temperature.
This is because the viscosity of the electrolyte decreases at high temperatures,
which increases the migration rate of lithium ions and improves the utilization rate of active lithium, resulting in higher charge and discharge capacity of the battery.
After 300 cycles, the polarization capacity loss of the battery at 60 ℃ is much higher than that at room temperature.
It can be seen that the increase in temperature exacerbates the electrochemical polarization of
the electrodes during the charging and discharging process of lithium-ion batteries, making the capacity loss more severe.
5.1.3 LiFePO4
LiFePO4 has a wide range of sources, is inexpensive, and has good stability and safety performance.
It can achieve a theoretical specific capacity of 170mAh/g, and its specific power and energy are similar to LiCoO2, achieving good compatibility with electrolyte solutions.
Therefore, it is widely used as a positive electrode in lithium batteries.
The factors affecting battery capacity using this material mainly include the following two points: firstly, the reduction of recyclable lithium due to side reactions between the positive and negative electrodes, which seriously disrupts the balance between the positive and negative electrodes; Secondly, due to factors such as structural degradation, electrode layer separation, material dissolution, and particle segregation, the loss of active materials occurs, thereby affecting the battery capacity.
5.2 Carbon black content of positive electrode material
Due to its non active nature, carbon black does not participate in discharge reactions.
If the amount of carbon black in the positive electrode material is too high, it will affect the strength and capacity of the positive electrode material,
so it needs to be added as appropriate.
In addition, the transport carrier generates catalytic substances on the surface of carbon black,
which can enhance the decomposition rate of metal ions and effectively promote the dissolution of active substances.
