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The reason for the capacity of 6s lithium battery is a formula to understand!

6s lithium battery is the fastest growing secondary battery after nickel-cadmium and nickel-hydrogen batteries. Its high-energy properties make its future look bright. However, the 6s lithium battery is not perfect, and its biggest problem is the stability of its charge-discharge cycle. This paper summarizes and analyzes the possible reason for the capacity decline of 6s lithium battery: overcharge.

6s lithium battery capacity balance

The 6s lithium battery has different intercalation energies when the intercalation reaction occurs 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 6s lithium battery, the capacity balance is expressed as the mass ratio of the positive electrode to the negative electrode, namely: γ=m+/m-=ΔxC-/ΔyC+
In the above formula, C refers to the theoretical coulombic capacity of the 6s lithium battery electrode, and Δx and Δy refer to the stoichiometric number of lithium ions embedded in the negative electrode and the positive electrode of the lithium 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.

6s lithium battery capacity imbalance impact

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 electrode performance of 6s lithium battery is the best.
For 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 generate or consume lithium ions or electrons may cause a change in the capacity balance of the 6s lithium battery. Once the capacity balance state of the 6s lithium battery changes, the change is irreversible and can be accumulated through multiple cycles. Seriously affect the performance of 6s lithium battery. In 6s lithium 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, and metallic lithium deposition.

6s lithium battery capacity imbalance reason: overcharge

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 metal 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 is likely to occur when the positive electrode active material is excessive relative 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 positive electrode active material to negative electrode active material is too low, positive overcharge is likely to occur
The capacity loss caused by overcharge of the positive electrode is mainly due to the generation of electrochemically inert substances (such as Co3O4, Mn2O3, etc.), which destroy the capacity balance between the electrodes, and the capacity loss is irreversible.
(1) LiyCoO2
LiyCoO2→(1-y)/3[Co3O4+O2(g)]+yLiCoO2 y<0.4
At the same time, the oxygen generated by the decomposition of the positive electrode material in the sealed 6s lithium battery will accumulate at the same time due to the absence of recombination reactions (such as the generation of H2O) and the flammable gas generated by the decomposition of the electrolyte, and the consequences will be unimaginable.
(2) λ-MnO2
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 product (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 during 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 is produced, 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.

The above is the whole content of the capacity balance of 6s lithium battery brought to you today. The 6s lithium battery has different intercalation energies when the intercalation reaction occurs between the two electrodes, and in order to obtain the best performance of the battery, the two host electrodes have different intercalation energies. The capacity ratio should be kept at a balanced value; 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. At the optimized mass ratio, the electrode performance of the 6s lithium battery is the best; the main reason for the capacity imbalance of the 6s lithium battery is overcharge.
I hope today's content is helpful to you, more information will be continuously updated, see you in the next issue.

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