Lithium-ion batteries have different intercalation energies when intercalation reactions occur between the two electrodes, and in order to obtain the best performance of the battery, the capacity ratio of the two host electrodes should maintain a balanced value.
In a lithium-ion battery, the capacity balance is expressed as the mass ratio of the positive electrode to the negative electrode.
That is: γ=m+/m-=ΔxC-/ΔyC+
In the above formula, 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 electrode and the positive electrode, respectively. It can be seen from the above formula that the required mass ratio of the two poles depends on the corresponding Coulomb capacity of the two poles and the number of their respective reversible lithium ions.
Generally speaking, a smaller mass ratio leads to incomplete utilization of the negative electrode material; a larger mass ratio may cause a safety hazard due to the overcharge of the negative electrode. In short, at the optimized mass ratio, the battery performance is the best.
Regarding an ideal Li-ion battery system, the capacity balance does not change during its cycle, and the initial capacity in each cycle is a certain value, but the actual situation is much more complicated. Any side reaction that can appear or consume lithium ions or electrons may lead to changes in the battery capacity balance. Once the capacity balance state of the battery changes, this change is irreversible and can be accumulated through multiple cycles. Serious impact. In lithium-ion batteries, in addition to the redox reactions that occur when lithium ions are deintercalated, there are also a large number of side reactions, such as electrolyte decomposition, active material dissolution, metal lithium deposition, etc.
Reason 1: Overcharging
1. Overcharge reaction of graphite negative electrode:
When the battery is overcharged, lithium ions are easily reduced and deposited on the surface of the negative electrode:
The deposited lithium coats the negative electrode surface, blocking the intercalation of lithium. This results in reduced discharge efficiency and capacity loss due to:
①Reduce the amount of recyclable lithium;
②The deposited metallic lithium reacts with the solvent or supporting electrolyte to form Li2CO3, LiF or other products;
③ Metal lithium is usually formed between the negative electrode and the separator, which may block the pores of the separator and increase the internal resistance of the battery;
④ Due to the very active nature of lithium, it is easy to react with the electrolyte and consume the electrolyte, resulting in a reduction in discharge efficiency and a loss of capacity.
Fast charging, the current density is too large, the negative electrode is severely polarized, and the deposition of lithium will be more obvious. This situation is likely to occur when the positive electrode active material is excessive in relation to the negative electrode active material. However, in the case of a high charging rate, deposition of metallic lithium may occur even if the ratio of positive and negative active materials is normal.
2. Positive electrode overcharge reaction
When the ratio of the positive electrode active material to the 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 is mainly due to the appearance of electrochemically inert substances (such as Co3O4, Mn2O3, etc.), which destroys the capacity balance between electrodes, and the capacity loss is irreversible.
At the same time, the oxygen generated by the decomposition of the positive electrode material in the sealed lithium-ion battery accumulates at the same time because there is no recombination reaction (such as the generation of H2O) and the flammable gas generated by the decomposition of the electrolyte, and the consequences will be unimaginable.
The lithium-manganese reaction occurs when the lithium-manganese oxide is completely delithiated: λ-MnO2→Mn2O3+O2(g)
3. The electrolyte is oxidized when overcharged
When the pressure is higher than 4.5V, the electrolyte will oxidize to generate insolubles (such as Li2Co3) and gas. These insolubles will block the micropores of the electrode and hinder the migration of lithium ions, resulting in capacity loss during cycling.
Factors that affect the rate of oxidation:
The surface area of the positive electrode material
Current collector material
Added conductive agent (carbon black, etc.)
The type and surface area of carbon black
Among the more commonly used electrolytes, EC/DMC is considered to have the highest oxidation resistance. The electrochemical oxidation process of solution is generally expressed as: solution → oxidation products (gas, solution and solid matter) +ne-
The oxidation of any solvent will increase the electrolyte concentration, decrease the electrolyte stability, and ultimately affect the capacity of the battery. Assuming that a small amount of electrolyte is consumed each time it is charged, more electrolyte is required at the time of battery assembly. For a constant container, this means that a smaller amount of active substance is loaded, which results in a decrease in the initial capacity. In addition, if a solid product appears, a passivation film will be formed on the surface of the electrode, which will increase the polarization of the battery and reduce the output voltage of the battery.