Why Did The Capacity Of The Lithium Battery Decline, And Finally Someone Explained！

Lithium-ion batteries are the fastest-growing secondary batteries after nickel-cadmium and nickel-hydrogen batteries. Its high-energy characteristics make its future look bright. However, the lithium ion battery is not perfect, and its biggest problem is the stability of its charge and discharge cycles. This paper summarizes and analyzes the possible causes of capacity decay of lithium-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 get 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 represents 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 coulomb capacity of the electrode, and Δx and Δy refer to the stoichiometric numbers of lithium ions embedded in the negative electrode and the positive electrode, respectively. It can be seen from the above formula that the mass ratio required by 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 negative electrode materials; a larger mass ratio may cause safety hazards due to overcharge of the negative electrode. In short, the battery performance is the best at the optimized mass ratio.

For an ideal Li-ion battery system, the balance of volume 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 a change in battery capacity balance. Once the battery capacity balance state changes, this change is irreversible and can be accumulated through multiple cycles to produce battery performance. Serious impact. In lithium-ion batteries, 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 metal lithium deposition.

Reason one: Overcharge

1. Overcharge reaction of graphite anode:

When the battery is overcharged, lithium ions are easily reduced and deposited on the surface of the negative electrode:

The deposited lithium coats the surface of the negative electrode, blocking the insertion of lithium. The causes of reduced discharge efficiency and capacity loss are:

①The amount of recyclable lithium is reduced;

② The 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 active nature of lithium, it is easy to react with the electrolyte and consume the electrolyte. This leads to 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 relative to the negative electrode active material. However, in the case of a high charging rate, even if the ratio of the positive and negative electrode active materials is normal, the deposition of metallic lithium may occur.

2. Positive charge overcharge reaction

When the ratio of the positive electrode active material to the negative electrode active material is too low, overcharge of the positive electrode easily occurs.

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 disrupts the capacity balance between the electrodes, and its 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 cathode material in the sealed lithium ion 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.

(2) λ-MnO2

The lithium manganese reaction occurs when the lithium manganese oxide is completely delithiated: λ-MnO2→Mn2O3+O2(g)

         

3. The oxidation reaction of the electrolyte when it is overcharged

When the pressure is higher than 4.5V, the electrolyte will oxidize to form insolubles (such as Li2Co3) and gas. These insolubles will block the pores of the electrode and hinder the migration of lithium ions, resulting in capacity loss during cycling.

                 

Factors affecting the oxidation rate:

Cathode material surface area

                    

Current collector material

                              

The added conductive agent (carbon black, etc.)

                   

Types 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 the solution is generally expressed as: solution → oxidation product (gas, solution and solid matter) +ne-

The oxidation of any solvent will increase the electrolyte concentration and decrease the stability of the electrolyte, which will ultimately affect the capacity of the battery. Assuming that a small portion of the electrolyte is consumed each time it is charged, then more electrolyte is needed when the battery is assembled. For a constant container, this means charging a smaller amount of active material, which will cause 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 battery to increase and lower the output voltage of the battery.

Reason two: electrolyte decomposition (reduction)

I Decompose on the electrode

1. The electrolyte decomposes on the positive electrode:

The electrolyte is composed of a solvent and a supporting electrolyte. After the positive electrode is decomposed, insoluble products Li2Co3 and LiF are usually formed, which reduces the battery capacity by blocking the pores of the electrode. The gas generated by the reduction will increase the internal pressure of the battery, 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 easily decomposed on the positive electrode. On the contrary, the electrolyte is more easily decomposed at the negative electrode.

2. The electrolyte decomposes on the negative electrode:

The electrolyte is not stable on graphite and other lithium-embedded carbon anodes, and it is easy to react to produce irreversible capacity. The decomposition of the electrolyte during the initial charge and discharge will form a passivation film on the surface of the electrode. The passivation film can separate the electrolyte from the carbon anode and prevent further decomposition of the electrolyte. Thus maintaining the structural stability of the carbon anode. Under ideal conditions, the reduction of the electrolyte is limited to the formation of the passivation film, and this process no longer occurs when the cycle is stable.

Passivation film formation

The reduction of the electrolyte salt 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 reduced product of the solvent;

(2) The concentration of the electrolyte decreases when the electrolyte salt is reduced, which ultimately leads to a loss of battery capacity (LiPF6 is reduced to LiF, LixPF5-x, PF3O and PF3)

              

(3) The formation of a passivation film consumes lithium ions, which will lead to an imbalance in the capacity between the two electrodes and cause a decrease in the specific capacity of the entire battery.

(4) If there are cracks in 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 carbon surface, resulting in the inability of lithium to insert and extract , Causing 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 can inhibit the co-embedding and decomposition of the solvent. Adding crown ether organic additives has the same effect. 12 crown 4 ether is the best.

Factors of film-forming capacity loss:

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

                           

(2) Composition of electrolyte;

                                       

(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, and the new phase formed embeds the original active material, and a passive film with low ion and electron conductivity is formed on the particle surface, so the spinel after storage It has greater polarization than before storage.

Zhang analyzed the AC impedance spectroscopy before and after cycling of the electrode material and found that with the increase of the number of cycles, the resistance of the surface passivation layer increases and the interface capacitance decreases. 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 a passivation film, and high temperature conditions are more conducive to the progress of these reactions. This will cause an increase in contact resistance and Li+ migration resistance between active material particles, thereby increasing the polarization of the battery, incomplete charging and discharging, and decreasing capacity. ;

                                                

II Reduction mechanism of electrolyte

The electrolyte often contains impurities such as oxygen, water, carbon dioxide, etc., and a redox reaction occurs during the charging and discharging of the battery.

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

1. Reduction of solvent

The reduction of PC and EC includes a one-electron reaction and a two-electron reaction process. The two-electron reaction forms Li2CO3:

Fong et al. believe that in the first discharge process, when the electrode potential is close to 0.8V (vs. Li/Li+), PC/EC reacts electrochemically on graphite, generating CH=CHCH3(g)/CH2=CH2( g) and LiCO3 (s), leading to irreversible capacity loss on the graphite electrode.

Aurbach et al. conducted extensive research on the reduction mechanism and products of various electrolytes on metal lithium 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 in the presence of trace water are Li2CO3 and propylene, but no Li2CO3 is produced under dry conditions.

DEC reduction:

                  

Ein-Eli Y reported that an electrolyte composed of mixed diethyl carbonate (DEC) and dimethyl carbonate (DMC) will undergo an exchange reaction in the battery to produce ethyl methyl carbonate (EMC), which will cause capacity loss Certain influence.

2. Reduction of electrolyte

The reduction reaction of the electrolyte is usually 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 helps to stabilize the carbon surface and can form the required 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 deposited film and affects the capacity decay of the battery. The possible reduction reactions of several 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 lithium ion insertion and causes irreversible capacity loss:

H2O＋e→OH-＋1/2H2

OH-＋Li+→LiOH(s)

LiOH＋Li+＋e-→Li2O(s)＋1/2H2

LiOH(s) is generated and deposited on the surface of the electrode to form a surface film with high resistance, which prevents Li+ from embedding into the graphite electrode, resulting in irreversible capacity loss. Trace water (100-300×10-6) in the solvent has no effect on the performance of the graphite electrode.

          

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

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 lithium-intercalated carbon is small, the reduction of electrolyte on carbon is similar to that on lithium.

Reason three: self-discharge

Self-discharge refers to the phenomenon of natural loss of capacity when the battery is not in use. There are two cases of capacity loss caused by self-discharge of lithium-ion batteries:

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, but irreversible capacity loss is the opposite. The positive and negative electrodes may have a microbattery action with the electrolyte in the charged state, lithium ion insertion and extraction, positive and negative electrode insertion and removal The intercalated lithium ions are only related to the lithium ions of the electrolyte, the positive and negative electrode capacities are therefore unbalanced, and this part of the capacity loss cannot be recovered during charging. Such as:

The lithium manganese oxide positive electrode and the solvent will act as a micro-battery, resulting in self-discharge and irreversible capacity loss:

LiyMn2O4+xLi++xe-→Liy+xMn2O4

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

xPC→xPC-free radical+xe-

Similarly, the negative electrode active material may interact with the electrolyte in a microbattery to cause self-discharge and cause irreversible capacity loss. The electrolyte (such as LiPF6) is reduced on the conductive material:

PF5+xe- →PF5-x

                                                        

Lithium carbide in the charged state is used as the negative electrode of the micro battery to remove lithium ions and be oxidized:

LiyC6→Liy-xC6+xLi+++xe-

Factors affecting self-discharge: the manufacturing process of the positive electrode material, the manufacturing process of the battery, the nature of the electrolyte, the temperature, and the 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 the carbon black. Reducing the surface area of the carbon black can control the self-discharge rate, but for LiMn2O4 cathode materials, reducing the surface area of the active material is also important, and the role of the collector surface on the oxidation of the solvent cannot be ignored. .

Current leaking through the battery separator can also cause self-discharge in lithium-ion batteries, 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 battery strongly depends on the temperature, 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 battery's internal volume balance is destroyed, which will result in permanent capacity loss.

                             

During long time or frequent self-discharge, lithium may be deposited on carbon, increasing the degree of capacity imbalance between the two poles.

Pistoia et al. compared the self-discharge rates of the three main metal oxide positive electrodes in various electrolytes, and found that the self-discharge rate differs with the electrolyte. It is also pointed out that the self-discharged oxidation products block the micropores in the electrode material, making lithium insertion and extraction difficult and increase the internal resistance and decrease the discharge efficiency, resulting in irreversible capacity loss.