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Explain in detail the reasons for the capacity decay of 2s 5600 lipo battery

2s 5600 lipo battery is the fastest growing secondary 2s 5600 lipo battery after nickel cadmium and nickel hydrogen 2s 5600 lipo battery. Its high-energy properties make its future look bright. However, the 2s 5600 lipo battery is not perfect, its biggest problem is the stability of its charge-discharge cycle.
CNHL summarizes and analyzes the possible reasons for the capacity decline of 2s 5600 lipo battery, including overcharge, electrolyte decomposition and self-discharge.
The 2s 5600 lipo battery has different intercalation energies when the intercalation reaction occurs between the two electrodes, and in order to obtain the best performance of the 2s 5600 lipo battery, the capacity ratio of the two host electrodes should maintain a balance value.
In the 2s 5600 lipo 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 2s 5600 lipo battery has the best performance.
For an ideal Li-ion2s 5600 lipo 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 2s 5600 lipo battery. Once the capacity balance of the 2s 5600 lipo battery is changed, the change is irreversible and can be done through multiple cycles Cumulatively, it has a serious impact on the performance of the 2s 5600 lipo battery. In the 2s 5600 lipo battery, in addition to the redox reaction that occurs 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.

Reason 1: 2s 5600 lipo battery is overcharged

1. Overcharge reaction of graphite negative electrode:
When the 2s 5600 lipo battery is overcharged, lithium ions are easily reduced and deposited on the negative electrode surface:
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 2s 5600 lipo 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.
For overcharging of lithium batteries, please refer to the following: Lipo battery 4s charging and discharging principle, be sure to store it well!
2. Positive electrode overcharge reaction
When the ratio of positive electrode active material to negative electrode active material is too low, positive electrode 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 2s 5600 lipo 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.
(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 be oxidized to generate insolubles (such as Li2Co3) and gases. 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 2s 5600 lipo battery. Assuming that a small amount of electrolyte is consumed with each charge, more electrolyte is required when the 2s 5600 lipo battery is assembled. 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 cause the polarization of the 2s 5600 lipo battery to increase and reduce the output voltage of the 2s 5600 lipo battery.

Reason 2: 2s 5600 lipo battery electrolyte decomposition (reduction)

I decompose on the electrode
1. The electrolyte is decomposed on the positive electrode:
The electrolyte is composed of a solvent and a supporting electrolyte. After the decomposition of the positive electrode, insoluble products such as Li2Co3 and LiF are usually formed, which reduce the capacity of the 2s 5600 lipo battery by blocking the pores of the electrode. The reduction reaction of the electrolyte will affect the capacity and cycle life of the 2s 5600 lipo battery. It has adverse effects, and the gas generated by the reduction will increase the internal pressure of the 2s 5600 lipo battery, resulting in safety problems.
The positive electrode decomposition voltage is usually greater than 4.5V (vs. Li/Li+), so they do not easily decompose on the positive electrode. On the contrary, the electrolyte is more easily decomposed at the negative electrode.
The following article about lithium battery electrolyte has a detailed introduction, and interested partners can refer to:
Cnhl 6s lipo battery electrolyte, practical function and classic system construction
2. The electrolyte is decomposed on the negative electrode:
The electrolyte is not stable on graphite and other lithium-inserted carbon anodes, and it is easy to react to generate irreversible capacity. During the initial charge and discharge, the decomposition of the electrolyte will form a passivation film on the surface of the electrode, and the passivation film can separate the electrolyte from the carbon negative electrode to prevent further decomposition of the electrolyte. Thus, the structural stability of the carbon anode is maintained. Under ideal conditions, the reduction of the electrolyte is limited to the passivation film formation stage, and this process does not occur when the cycle is stable.
Formation of passivation film
The reduction of electrolyte salts participates in the formation of the passivation film, which is beneficial to the stabilization of the passivation film, but
(1) The insoluble matter produced by the reduction will have an adverse effect on the solvent reduction product;
(2) The concentration of the electrolyte decreases when the electrolyte salt is reduced, which eventually leads to the capacity loss of the 2s 5600 lipo battery (LiPF6 is reduced to form LiF, LixPF5-x, PF3O and PF3);
(3) The formation of the passivation film consumes lithium ions, which will cause the capacity imbalance between the two electrodes to reduce the specific capacity of the entire 2s 5600 lipo battery.
(4) If there are cracks on the passivation film, solvent molecules can penetrate and thicken the passivation film, which not only consumes more lithium, but also may block the micropores on the carbon surface, resulting in the inability of lithium to be inserted and extracted. , resulting in irreversible capacity loss. Adding some inorganic additives to the electrolyte, such as CO2, N2O, CO, SO2, etc., can accelerate the formation of the passivation film and inhibit the co-insertion and decomposition of the solvent. The addition of crown ether organic additives also has the same effect. 12 crowns and 4 ethers are the best.
Factors for film capacity loss:
(1) The type of carbon used in the process;
(2) Electrolyte composition;
(3) Additives in electrodes or electrolytes.
Blyr believes that the ion exchange reaction advances from the surface of the active material particle to its core, the new phase formed bury the original active material, and a passive film with low ionic and electronic conductivity is formed on the surface of the particle, so the spinel after storage Greater polarization than before storage.
Zhang found that the resistance of the surface passivation layer increased and the interfacial capacitance decreased with the increase of the number of cycles. It reflects that the thickness of the passivation layer increases with the number of cycles. The dissolution of manganese and the decomposition of the electrolyte lead to the formation of passivation films, and high temperature conditions are more conducive to the progress of these reactions. This will increase the contact resistance between the active material particles and the Li+ migration resistance, thereby increasing the polarization of the 2s 5600 lipo battery, incomplete charge and discharge, and reduced capacity.

II Reduction Mechanism of Electrolyte
The electrolyte often contains oxygen, water, carbon dioxide and other impurities, and redox reactions occur during the charging and discharging process of the 2s 5600 lipo battery.
The reduction mechanism of the electrolyte includes three aspects: solvent reduction, electrolyte reduction and impurity reduction:
1. Solvent reduction
The reduction of PC and EC includes one-electron reaction and two-electron reaction process, and the two-electron reaction forms Li2CO3:
Fong et al. believed that during the first discharge process, when the electrode potential was close to 0.8V (vs. Li/Li+), the electrochemical reaction of PC/EC occurred on graphite to generate CH=CHCH3(g)/CH2=CH2( g) and LiCO3(s), leading to irreversible capacity loss on graphite electrodes.
Aurbach et al. conducted extensive research on the reduction mechanism and products of various electrolytes on lithium metal electrodes and carbon-based electrodes, and found that the one-electron reaction mechanism of PC produces ROCO2Li and propylene. ROCO2Li is very sensitive to trace water. The main products are Li2CO3 and propylene in the presence of trace water, but no Li2CO3 is produced under dry conditions.
Restoration of DEC:

Ein-Eli Y reported that the electrolyte mixed with diethyl carbonate (DEC) and dimethyl carbonate (DMC) will undergo an exchange reaction in a 2s 5600 lipo battery to generate ethyl methyl carbonate (EMC), which Capacity loss has some impact.
2. Electrolyte reduction
The reduction reaction of the electrolyte is generally considered to be involved in the formation of the carbon electrode surface film, so its type and concentration will affect the performance of the carbon electrode. In some cases, the reduction of the electrolyte contributes to the stabilization of the carbon surface, which can form the desired passivation layery performance.
(3) The presence of oxygen in the solvent will also form Li2O
1/2O2+2e-+2Li+→Li2O

Because the potential difference between metallic lithium and fully intercalated carbon is small, the reduction of the electrolyte on carbon is similar to the reduction on lithium.

Reason 3: 2s 5600 lipo battery self-discharge

Self-discharge refers to the phenomenon that the capacity of the 2s 5600 lipo battery is naturally lost when it is not in use. 2s 5600 lipo battery self-discharge (the following article about lipo battery self-discharge has a detailed introduction: lipo battery 3s self-discharge dry goods!) leads to capacity loss in two cases:
One is the reversible capacity loss;
The second is the loss of irreversible capacity.
Reversible capacity loss means that the lost capacity can be recovered during charging, while irreversible capacity loss is the opposite. The positive and negative electrodes may interact with the electrolyte for a micro-2s 5600 lipo battery in the charged state, resulting in lithium ion intercalation and deintercalation. The intercalated and deintercalated lithium ions are only related to the lithium ions of the electrolyte, so the capacity of the positive and negative electrodes is unbalanced, and this part of the capacity loss cannot be recovered during charging. like:
The lithium manganese oxide positive electrode and the solvent will have a micro-2s 5600 lipo battery effect, resulting in self-discharge and irreversible capacity loss:
LiyMn2O4+xLi++xe-→Liy+xMn2O4
Solvent molecules (such as PC) are oxidized as micro 2s 5600 lipo battery negative electrode on the surface of conductive material carbon black or current collector:
xPC→xPC-radical+xe-
Similarly, the negative active material may interact with the electrolyte to cause self-discharge and cause irreversible capacity loss, and the electrolyte (such as LiPF6) is reduced on the conductive material:
PF5+xe-→PF5-x
Lithium carbide in the charged state is oxidized by removing lithium ions as the negative electrode of the micro 2s 5600 lipo battery:
LiyC6→Liy-xC6+xLi+++xe-
Factors affecting self-discharge: the manufacturing process of the positive electrode material, the manufacturing process of the 2s 5600 lipo battery, the properties of the electrolyte, temperature, and time.
The self-discharge rate is mainly controlled by the solvent oxidation rate, so the stability of the solvent affects the storage life of the 2s 5600 lipo battery.
The oxidation of the solvent mainly occurs on the surface of carbon black, and reducing the surface area of ​​carbon black can control the self-discharge rate, but for LiMn2O4 cathode materials, it is equally important to reduce the surface area of ​​active materials, and the role of the surface of the current collector on solvent oxidation cannot be ignored. .
Current leakage through the 2s 5600 lipo battery separator can also cause self-discharge in the Li-ion 2s 5600 lipo battery, but this process is limited by the resistance of the separator, occurs at a very low rate and is independent of temperature. Considering that the self-discharge rate of the 2s 5600 lipo battery is strongly temperature-dependent, this process is not the main mechanism in self-discharge.
If the negative electrode is in a fully charged state and the positive electrode self-discharges, the capacity balance in the 2s 5600 lipo battery will be destroyed, resulting in permanent capacity loss.

During prolonged or frequent self-discharge, lithium may deposit on the carbon, increasing the capacity imbalance between the electrodes.
Pistoia et al. compared the self-discharge rates of three main metal oxide cathodes in various electrolytes and found that the self-discharge rates varied with different electrolytes. It is pointed out that the self-discharged oxidation products block the micropores on the electrode material, making the intercalation and extraction of lithium difficult, increasing the internal resistance and reducing the discharge efficiency, resulting in irreversible capacity loss.
For more information on lithium batteries, please click below:
5600mah 2s lipo battery modeling basics 

 

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