Lithium-ion batteries are the fastest-growing secondary batteries after nickel-cadmium 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 cycles. This paper summarizes and analyzes the possible reasons for the capacity fading of Li-ion batteries, including overcharge, electrolyte decomposition and self-discharge.

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 lithium-ion batteries, 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.

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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.

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 lead to changes in battery capacity balance. Once the battery’s capacity balance state changes, this change is irreversible and can be accumulated through multiple cycles, resulting in battery performance. 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, and metallic lithium deposition.

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:

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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 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 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.

(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 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.

Reason 2: Electrolyte decomposition (reduction)

I decompose on the electrode

1. The electrolyte is decomposed on the positive electrode:

The electrolyte consists of a solvent and a supporting electrolyte. After the cathode is decomposed, insoluble products such as Li2Co3 and LiF are usually formed, which reduce the battery capacity by blocking the pores of the electrode. The electrolyte reduction reaction will have an adverse effect on the capacity and cycle life of the battery. The gas generated by the reduction can increase the internal pressure of the battery, which can lead to 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.

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 loss of battery capacity (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 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 battery, incomplete charging and discharging, 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 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 the battery to generate ethyl methyl carbonate (EMC), which is responsible for the loss of capacity. certain influence.

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 layer.

It is generally believed that the supporting electrolyte is easier to reduce than the solvent, and the reduction product is mixed in the negative electrode deposition film and affects the capacity decay of the battery. Several possible reduction reactions of supporting electrolytes are as follows:

3. Impurity reduction

(1) If the water content in the electrolyte is too high, LiOH(s) and Li2O deposits will be formed, which is not conducive to the insertion of lithium ions, resulting in irreversible capacity loss:

H2O＋e→OH-＋1/2H2

OH-＋Li+→LiOH(s)

LiOH+Li++e-→Li2O(s)+1/2H2

The generated LiOH(s) is deposited on the electrode surface, forming a surface film with high resistance, which hinders Li+ intercalation into the graphite electrode, resulting in irreversible capacity loss. A small amount of water (100-300×10-6) in the solvent has no effect on the performance of the graphite electrode.

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

2CO2+2e-+2Li+→Li2CO3+CO

CO will increase the internal pressure of the battery, and Li2CO3(s) will increase the internal resistance of the battery and affect the battery 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: Self-discharge

Self-discharge refers to the phenomenon that the battery loses its capacity naturally when it is not in use. Li-ion battery self-discharge 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 act as a microbattery with the electrolyte in the charged state, resulting in lithium ion intercalation and deintercalation, and intercalation and deintercalation of positive and negative electrodes. The embedded 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. Such as:

Lithium manganese oxide positive electrode and solvent will cause micro-battery effect and self-discharge, resulting in irreversible capacity loss:

LiyMn2O4+xLi++xe-→Liy+xMn2O4

Solvent molecules (such as PC) are oxidized on the surface of conductive material carbon black or current collector as a microbattery anode:

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 microbattery:

LiyC6→Liy-xC6+xLi+++xe-

Factors affecting self-discharge: the manufacturing process of the positive electrode material, the manufacturing process of the battery, the properties of the electrolyte, temperature, and time.