Lithium-ion batteries are the fastest-developing secondary batteries after cadmium-nickel and nickel-hydrogen batteries. Its high-energy properties make its future look bright. However, lithium-ion batteries are not perfect, and their biggest problem is the stability of their charge-discharge cycle. This paper summarizes and analyzes the possible causes of capacity attenuation of Li-ion batteries, including overcharge, electrolyte decomposition, and self-discharge.

Lithium-ion batteries have different embedding energies during the embedding reaction between the two electrodes. To obtain the best performance for the battery, the capacity ratio of the two host electrodes should be maintained at a balanced value.

In lithium-ion batteries, capacity balance is expressed as the mass ratio of positive to negative terminals,

namely: γ=m+/m-=ΔxC-/ΔyC+

In the above equation, C refers to the theoretical coulomb capacity of the electrode, and Δx and Δy refer to the stoichiometric number of lithium ions embedded in the negative and positive electrodes, respectively. As can be seen from the above equation, the mass ratio required for the poles depends on the corresponding coulomb capacity of the poles and the number of reversible lithium ions each.

Generally speaking, the small mass ratio leads to the incomplete utilization of the negative electrode material; The larger mass ratio may have a safety hazard due to the negative electrode being overcharged. In short, the battery performance is the best at the optimal mass ratio.

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 produce or consume lithium ions or electrons may lead to a change in the capacity balance of the battery, once the capacity balance of the battery changes, this change is irreversible and can accumulate through multiple cycles, which has a serious impact on battery performance.

In lithium-ion batteries, in addition to the REDOX reaction that occurs when lithium ions are removed, there are also a large number of side reactions, such as electrolyte decomposition, dissolution of active substances, and metal lithium deposition.

Cause one: overcharging

When the battery is overcharged, lithium ions are easy to reduce and deposit on the negative surface:

The deposited lithium is coated on the negative surface, blocking the insertion of lithium. This results in reduced discharge efficiency and loss of capacity due to:

① Reduce the amount of recyclable lithium;

② deposited lithium metal reacts with solvents or supporting electrolytes to form Li2CO3, LiF, or other products;

③ 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;

Due to the nature of lithium is very lively, easy to react with the electrolyte and consume the electrolyte. Resulting in reduced discharge efficiency and capacity loss.

In fast charging, the current density is too large, the negative pole is severely polarized, and the deposition of lithium will be more obvious. This situation is easy to occur in the case of excess of positive active matter relative to negative active matter. However, at high charging rates, the deposition of metallic lithium may occur even if the positive and negative active matter ratio is normal.

2, positive overcharge reaction

When the ratio of positive active matter to negative active matter is too low, it is easy to occur positive overcharge.

The capacity loss caused by positive overcharge is mainly due to the production of electrochemical inert substances (such as Co3O4, Mn2O3, etc.), which destroys the capacity balance between the electrodes, and the capacity loss is irreversible.

(1) LiyCoO2

LiyCoO2→(1-y)/3[Co3O4+O2(g)]+ yLiCoO2y <0.4
At the same time, the oxygen generated by the decomposition of the positive electrode material in the sealed lithium-ion battery due to the absence of recombination reaction (such as the formation of H2O) and the flammable gas generated by the decomposition of the electrolyte accumulates, the consequences will be unimaginable.

(2) lambda-MNO2

The lithium manganese reaction occurs in a state where the lithium manganese oxide is completely de-lithium: λ-MnO2→Mn2O3+O2(g)

3, electrolyte oxidation reaction during overcharge
When the pressure is higher than 4.5V, the electrolyte will oxidize into insoluble substances (such as Li2Co3) and gases, which will block the micropores of the electrode and hinder the migration of lithium ions, resulting in the loss of capacity during the cycle.

Factors affecting oxidation rate:

The surface area of the cathode material
Collector material
The conductive agent added (carbon black, etc.)
Type and surface area of carbon black

Among the more commonly used electrolytes at present, 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 substance)+ne

The oxidation of any solvent will increase the electrolyte concentration, decrease the stability of the electrolyte, and ultimately affect the capacity of the battery. Assuming that a small percentage of the electrolyte is consumed each time it is charged, more electrolytes will be required during battery assembly. For a constant container, this means loading a smaller amount of active material, which causes a decrease in the initial capacity. In addition, if a solid product is produced, a passivation film will form on the electrode surface, which will cause the polarization of the cell to increase and reduce the output voltage of the cell.

1. The electrolyte breaks down on the positive electrode:
The electrolyte is composed of solvent and support electrolyte, usually formed after the decomposition of the positive electrode insoluble products Li2Co3 and LiF, etc., by blocking the pores of the electrode and reducing the battery capacity, electrolyte reduction reaction on the battery capacity and cycle life will have adverse effects, and due to the reduction of gas will make the battery internal pressure rise, resulting in safety problems.

The decomposition voltage of the positive electrode is usually greater than 4.5V(relative to Li+/ Li), so they are not easy to decompose on the positive electrode. In contrast, electrolytes tend to decompose more easily at the negative electrode.

2. The electrolyte decomposes on the negative electrode:

The electrolyte is not stable on graphite and another lithium-embedded carbon cathode, and it is easy to react to produce irreversible capacity.
During the initial charge and discharge, the electrolyte decomposition will form a passivation film on the electrode surface. The passivation film can separate the electrolyte from the negative carbon electrode and prevent the further decomposition of the electrolyte. Thus, the structural stability of the negative carbon electrode is maintained.
Under ideal conditions, the reduction of the electrolyte is limited to the forming stage of the passivation film, and the process no longer occurs when the circulation is stable.

Formation of passivation film

The reduction of electrolyte salts participates in the formation of passivation film and is conducive to the stabilization of passivation film, however

(1) the insoluble matter produced by reduction will hurt the product of solvent reduction;

(2) the concentration of electrolyte decreases during e electrolyte salt reduction, which eventually leads to the loss of battery capacity (LiPF6 reduction generates LiF, LixPF5-x, PF3O, and PF3);

(3) The formation of passivation film consumes lithium ions, which will cause the capacity imbalance between the poles and cause the specific capacity of the entire battery to be reduced.

(4) If there are cracks on the passivation film, the solvent molecules can penetrate and thicken the passivation film, which not only consumes more lithium but also may block the micropores on the surface of carbon, leading to the inability of lithium to be embedded and removed, resulting in irreversible capacity loss. Adding some inorganic additives in the electrolyte, such as CO2, N2O, CO, SO2, etc., can accelerate the formation of the passivated film, and can inhibit the co-embedding and decomposition of solvents, adding crown ether organic additives also have the same effect, of which 12 crown 4 ether is the best.

Factors for the loss of film capacity:

(1) the type of carbon used in the process;

(2) electrolyte composition;

(3) Additives in the electrode or electrolyte.

The ion exchange reaction advances from the surface of the active material particles to its core, and the new phase formed covers the original active material, and the passivation film with lower ionic and electronic conductivity is formed on the particle surface. Therefore, the stored spinel has greater polarization than that before storage.

Through the comparative analysis of the AC impedance spectra before and after the electrode material cycle, it is found that with the increase in the number of cycles, the resistance of the surface passivation layer increases, and the interface capacitance decreases. It shows that the thickness of the passivation layer increases with the number of cycles. Manganese dissolution and electrolyte decomposition lead to the formation of passivation film, and high-temperature conditions are more favorable for these reactions. This will cause an increase of the contact resistance and Li+ migration resistance between the particles of the active substance, so that the polarization of the battery will increase, the charge and discharge will be incomplete, and the capacity will be reduced.

II. Reduction mechanism of electrolyte

The electrolyte often contains impurities such as oxygen, water, and carbon dioxide, and REDOX reactions occur during the battery charging and discharging process.

The reduction mechanism of electrolytes includes solvent reduction, electrolyte reduction, and impurity reduction in three aspects :

1, solvent reduction

The reduction of PC and EC includes an electron reaction and a two-electron reaction process, and the two-electron reaction forms Li2CO3:
In the first discharge process, when the electrode potential is close to O.8V(vs. Li/Li+), PC/EC has an electrochemical reaction on the graphite, forming CH=CHCH3(g)/CH2=CH2(g) and LiCO3 (s), resulting in an irreversible capacity loss on the graphite electrode.

The reduction mechanism of various electrolytes on metal lithium electrodes and carbon-based electrodes and their products have been extensively studied, and it is found that the one-electron reaction mechanism of PC produces ROCO2Li and propylene. ROCO2Li is very sensitive to trace water, and the main products are Li2CO3 and propylene in the presence of trace water, but no Li2CO3 is produced in dry condition.

The electrolyte mixed with diethyl carbonate (DEC) and dimethyl carbonate (DMC) will undergo an exchange reaction in the battery to produce methyl ethyl carbonate (EMC), which has a certain impact on capacity loss.

2. Reduction of electrolytes

The reduction reaction of the electrolyte is usually considered to be involved in the formation of the surface film of the carbon electrode, so its type and concentration will affect the performance of the carbon electrode. In some cases, the reduction of the electrolyte contributes to the stability of the carbon surface, which can form the required passivation layer.

It is generally believed that the supporting electrolyte is easier to reduce than the solvent, and the reduction products are included in the negative electrode deposition film and affect the capacity attenuation of the battery. Several reduction reactions that may occur with supporting electrolytes are as follows:

3. Impurity reduction

(1) Excessive water content in the electrolyte will generate LiOH(s) and Li2O deposition layers, which is not conducive to lithium-ion embedding, resulting in irreversible capacity loss:

OH- + Li +- LiOH (s)

LiOH + Li + e﹣- Li2O + 1/2 h2 (s)

The resulting LiOH(s) deposits on the surface of the electrode, forming a highly resistible surface film that prevents Li+ from embedding into the graphite electrode, resulting in irreversible capacity loss. A trace amount of water in the solvent (100-300×10-6) does not affect the performance of the graphite electrode.

(2) CO2 in the solvent can be reduced to CO and LiCO3(s) on the negative electrode:

The co2 + 2 + 2 eli +- Li2CO3 + CO

CO will increase the internal pressure of the battery, while Li2CO3 (s) will increase the internal resistance of the battery and affect the battery's performance.

(3) The presence of oxygen in the solvent will also form Li2O1/2O2+2e-+2Li+→Li2O

Because the potential difference between lithium metal and fully Li-embedded carbon is small, the reduction of the electrolyte on carbon is similar to that on lithium.

Cause three: Self-discharge

Self-discharge refers to the phenomenon of natural loss of electrical capacity when the battery is not in use. Lithium-ion battery self-discharge leads to a capacity loss in two cases:

One is reversible capacity loss;

The other is the loss of irreversible capacity.

Reversible capacity loss refers to the loss of capacity can be recovered during charging, and irreversible capacity loss is the opposite, positive and negative electrodes in the charging state may occur with electrolyte micro battery action, lithium-ion embedment, and deembedment, positive and negative electrode embedment and de embedment of lithium ions only related to the electrolyte lithium ions, positive and negative capacity is therefore unbalanced, this part of the capacity loss can not be recovered when charging. For example:

Lithium manganese oxide positive electrode and solvent will occur micro battery action to produce self-discharge resulting in irreversible capacity loss:

LiyMn2O4+xLi ++xe-→Liy+xMn2O4

Solvent molecules (such as PC) oxidize on the surface of the conductive material carbon black or collector as the negative electrode of the microcell:

xPC→xPC free radical +xe

Similarly, the negative active substance may have micro battery interaction with the electrolyte resulting in self -discharge resulting in irreversible capacity loss, and the electrolyte (such as LiPF6) is reduced on the conductive substance:

PF5+xe- →PF5-x

Lithium carbide in the charging state is oxidized as the negative electrode of the micro battery to remove lithium ions:

LiyC6→Liy-xC6+xLi+++xe

Self-discharge influencing factors: the production process of the positive electrode material, the production process of the battery, the nature of the electrolyte, temperature, and time.The self-discharge rate is mainly controlled by the oxidation rate of the solvent, so the stability of the solvent affects the storage life of the battery.

The oxidation of the solvent mainly occurs on the surface of carbon black, reducing the surface area of carbon black can control the self-discharge rate, but for LiMn2O4 cathode materials, reducing the surface area of the active substance is equally important, and the role of the collector surface in the oxidation of the solvent can not be ignored.

Leakage of current through the battery diaphragm can also cause self-discharge in lithium-ion batteries, but this process is limited by the diaphragm resistance, occurs at a very low rate, and is independent of temperature. Given that the self-discharge rate of a battery is strongly temperature-dependent, this process is not the main mechanism of self-discharge.

If the negative terminal is in a fully charged state and the positive terminal self-discharge, the capacity balance in the battery is disrupted, resulting in a permanent capacity loss.

When it self-discharges for a long time or often, lithium may be deposited on carbon, increasing the capacity imbalance between the poles.

The self-discharge rates of three main metal oxide positive electrodes in different electrolytes were compared, and it was found that the self-discharge rates varied with different electrolytes. It is also pointed out that the oxidation products of self-discharge clog the micro holes on the electrode material, which makes it difficult to embed and remove lithium, increases the internal resistance and reduces the discharge efficiency, resulting in irreversible capacity loss.