With the rapid development of science and technology, lithium-ion batteries have become the mainstream energy storage technology in fields such as mobile devices and electric vehicles. However, the problem of lithium-ion battery capacity degradation has always existed, which limits the service life and performance of the battery. Why does lithium-ion battery capacity decay? Take a look.
Lithium-ion batteries have different embedding energies when an embedding reaction occurs between two electrodes, and the capacity ratio of the two host electrodes should maintain an equilibrium value in order to get the best performance from the battery.
In lithium-ion batteries, the capacity balance is expressed as the mass ratio of the positive electrode to the negative electrode, i.e.: γ = m+/m- = ΔxC-/ΔyC+
In the above equation, C refers to the theoretical coulombic capacity of the electrode, and Δx and Δy refer to the stoichiometric number of lithium ions embedded in the negative and positive electrodes, respectively. From the above equation, it can be seen that the required mass ratio of the two electrodes depends on the corresponding coulombic capacities of the two electrodes and their respective numbers of reversible lithium ions.
In general, smaller mass ratios lead to incomplete utilization of the anode material; larger mass ratios may pose a safety hazard due to overcharging of the anode. In short, the battery performance is best at the optimized mass ratio.
For an ideal Li-ion battery system, the content balance does not change during its cycling cycle and the initial capacity is a certain value in each cycle, however, the actual situation is much more complicated. Any side reaction that produces or consumes lithium ions or electrons can lead to a change in the capacity balance of the battery, and once the capacity balance of the battery has been changed, the change is irreversible and can be accumulated over many cycles, with serious impacts on battery performance. In lithium-ion batteries, in addition to the redox reaction that occurs when lithium ions are de-embedded, there are a large number of side reactions, such as electrolyte decomposition, dissolution of the active substance, lithium metal deposition, and so on.
01.Overcharging
1.Graphite anode overcharge reaction:
When the battery is overcharged, lithium ions are easily reduced and deposited on the surface of the anode:
Li++e-=li(s)
The deposited lithium is encapsulated on the surface of the negative electrode, blocking the embedding of lithium. This leads to lower discharge efficiency and capacity loss due to:
① Decrease in the amount of recyclable lithium.
② The deposited lithium metal reacts with the solvent or supporting electrolyte to form Li2CO3, LiF or other products.
③ The lithium metal is usually formed between the negative electrode and the diaphragm, which may block the pores of the diaphragm and increase the internal resistance of the battery;
④ Since lithium is very strong in nature, it can be used as an electrolyte.
Because of the active nature of lithium, it is easy to react with the electrolyte and consume the electrolyte. This leads to lower discharge efficiency and loss of capacity.
Rapid charging, excessive current density, serious polarization of the negative electrode, lithium deposition will be more obvious. This tends to occur where the positive electrode actives are in excess relative to the negative electrode actives. However, in the case of high charging rate, even if the ratio of positive and negative electrode actives is normal, the deposition of lithium metal may also occur.
2. Positive electrode overcharging reaction
When the ratio of positive electrode active material to negative electrode active material is too low, positive electrode overcharging is likely to occur.
The capacity loss caused by anode overcharging is mainly due to the generation of electrochemical inert substances (such as Co3O4, Mn2O3, etc.), which destroys the capacity balance between electrodes, and its capacity loss is irreversible.
(1)LiyCoO2
LiyCoO2→(1-y)/3[Co3O4+O2(g)]+yLiCoO2 y<0.4
Meanwhile the oxygen generated by the decomposition of the cathode material in the sealed lithium-ion battery will have unimaginable consequences due to the non-existence of the recombination reaction (e.g., the generation of H2O) accumulating at the same time with the flammable gases generated by the decomposition of the electrolyte.
(2)λ-MnO2
Lithium-manganese reaction occurs in the state of complete delithiation of lithium-manganese oxide: λ-MnO2→Mn2O3+O2(g)
3.Electrolyte oxidation reaction in overcharge
When the pressure is higher than 4.5V, the electrolyte will oxidize and generate insoluble matter (such as Li2Co3) and gas, these insoluble matter will be blocked in the electrode's microporous inside to hinder the migration of lithium ions, which will cause the loss of capacity in the cycling process.
Factors affecting oxidation rate:
Surface area of anode material
Collector material
The added conductive agent (carbon black, etc.)
Types and surface area of carbon black
Among the more commonly used electrolytes today, EC/DMC is considered to have the highest oxidation resistance. The electrochemical oxidation process of a solution is generally expressed as follows: solution → oxidation products (gases, solutions and solid substances) + ne-
Oxidation of any solvent will increase the concentration of the electrolyte, decrease the stability of the electrolyte, and ultimately affect the capacity of the battery. Assuming that a small portion of the electrolyte is consumed each time it is charged, more electrolyte will be needed during battery assembly. For a constant container, this would mean loading a smaller amount of active material, which would cause a drop in initial capacity. In addition, if solid products are produced, a passivation film will form on the electrode surface, which will cause an increase in the polarization of the cell and reduce the output voltage of the cell.
02.Electrolyte decomposition
Electrolyte includes electrolyte, solvent and additives, the nature of which will have an impact on the battery's service life, specific capacity, multiplier charging and discharging performance and safety performance. The decomposition of electrolyte and solvent in the electrolyte will cause the loss of battery capacity. In the first charge/discharge, solvents and other substances on the surface of the negative electrode to generate SEI film will form an irreversible loss of capacity, but this is the inevitable situation.
If impurities such as water or hydrogen fluoride are present in the electrolyte, the electrolyte LiPF6 may decompose at higher temperatures, and the generated products may react with the cathode material, resulting in a loss of battery capacity. At the same time, some of the products will also react with the solvent and affect the stability of the SEI film on the anode surface, which will cause the performance of lithium-ion batteries to deteriorate. In addition, if the products of electrolyte decomposition are not compatible with the electrolyte, they will block the anode pores during the migration process, which will lead to the degradation of battery capacity.
03. Self-discharge
Lithium-ion batteries in general, capacity loss phenomenon will occur, this process is called self-discharge, divided into reversible capacity loss and irreversible capacity loss. The rate of solvent oxidation has a direct impact on the rate of self-discharge, and the positive and negative active materials may react with the solute during the charging process, leading to lithium ion migration to complete the capacity imbalance and irreversible degradation, so it can be seen that the reduction of the surface area of the active material can reduce the rate of loss of capacity and the decomposition of the solvent will affect the battery storage life. In addition, diaphragm leakage can also lead to capacity loss, but this possibility is low. Self-discharge, if prolonged, can lead to lithium metal deposition and further lead to attenuation changes in the capacity of the positive and negative electrodes.
04. Electrode Instability
During the charging process, the active material of the positive electrode of the battery is unstable, which will lead to its reaction with the electrolyte and affect the battery capacity. Among them, structural defects of the cathode material, high charging potential, and carbon black content are the main factors affecting the battery capacity.
Although the problem of lithium-ion battery capacity degradation has not yet been completely solved, it is believed that in the near future, with the progress of science and technology and the development of emerging battery technologies, this problem will be solved. This will promote the development of electric vehicles, mobile devices and other industries to achieve a more long-lasting and reliable energy storage technology.
