Foreword
Lithium-ion batteries are the fastest-growing secondary batteries after cadmium-nickel and nickel-hydrogen batteries. Its high-energy features 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 cycle. This paper summarizes and analyzes the possible causes of capacity decay of lithium-ion batteries, including overcharge, electrolyte decomposition and self-discharge.
Essential cause
Lithium-ion batteries have different embedding energies when intercalating between two electrodes, and in order to obtain the best performance of the battery, the capacity ratio of the two host electrodes should be kept at an equilibrium value.
In a lithium ion 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 Coulomb 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 equation that the mass ratio required for the two poles depends on the corresponding Coulomb capacity of the two poles and the number of their respective reversible lithium ions.
In general, a smaller mass ratio results in incomplete utilization of the negative electrode material; a larger mass ratio may present a safety hazard due to overcharge of the negative electrode. In short, the battery performance is best at the optimized mass ratio.
For an ideal Li-ion battery system, the amount 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 produces or consumes lithium ions or electrons can cause a change in battery capacity balance. Once the battery's capacity balance changes, the change is irreversible and can be accumulated over multiple cycles to produce battery performance. Serious impact. In lithium ion batteries, in addition to the redox reaction occurring during lithium ion deintercalation, there are a large number of side reactions such as electrolyte decomposition, active material dissolution, metal lithium deposition, etc.
First, 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. This leads to reduced discharge efficiency and capacity loss due to:
➤ 1 The amount of lithium that can be recycled is reduced;
沉积 2 deposited lithium metal reacts with a solvent or supporting electrolyte to form Li 2 CO 3 , 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 to increase the internal resistance of the battery;
➤ 4 Since the nature of lithium is very active, it is easy to react with the electrolyte to consume the electrolyte, resulting in a decrease in discharge efficiency and a loss in capacity.
Fast charging, excessive current density, severe polarization of the negative electrode, and lithium deposition will be more pronounced. This situation is liable to occur in the case where 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 the positive and negative active materials is normal.
2. Positive overcharge reaction
When the ratio of the positive electrode active material to the negative electrode active material is too low, positive electrode overcharge easily occurs.
The capacity loss caused by the overcharge of the positive electrode is mainly due to the generation of electrochemically inert substances (such as Co3O4, Mn2O3, etc.), which destroys the capacity balance between the electrodes, and the capacity loss is irreversible.
➤ 1LiyCoO2
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 is accumulated because the recombination reaction (such as the formation of H2O) and the flammable gas generated by the decomposition of the electrolyte are simultaneously accumulated, and the consequences are unimaginable.
➤ 2λ-MnO2
The lithium manganese reaction occurs in the state where lithium manganese oxide is completely delithiated: λ-MnO2→Mn2O3+O2(g)
3. Oxidation reaction of electrolyte in overcharge
When the pressure is higher than 4.5V, the electrolyte will oxidize to form insoluble matter (such as Li2Co3) and gas. These insoluble materials will block the pores in the electrode and hinder the migration of lithium ions, resulting in capacity loss during the cycle. Welcome to pay attention to the Lithium Battery Alliance. President.
Factors affecting oxidation rate:
➤ 1 positive electrode material surface area size
➤ 2 collector material
➤ 3 added conductive agent (carbon black, etc.)
➤ 4 carbon black type and surface area
Among the more commonly used electrolytes, EC/DMC is considered to have the highest oxidation resistance. The electrochemical oxidation process of a solution is generally expressed as: solution → oxidation products (gas, solution and solid matter) + ne-
Oxidation of any solvent will increase the electrolyte concentration and decrease the stability of the electrolyte, ultimately affecting the capacity of the battery. Assuming that a small portion of the electrolyte is consumed each time it is charged, more electrolyte is needed when the battery is assembled. For a constant container, this means loading a smaller amount of active material, which causes a drop in initial capacity. In addition, if a solid product is produced, a passivation film is formed on the surface of the electrode, which causes an increase in polarization of the battery and lowers the output voltage of the battery.
Second, the electrolyte decomposition (reduction)
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 decomposition of the positive electrode, insoluble products such as Li2Co3 and LiF are usually formed, and the battery capacity is reduced by blocking the pores of the electrode. The electrolyte reduction reaction adversely affects the capacity and cycle life of the battery, and Reducing the gas creates an increase in the internal pressure of the battery, which leads to safety problems.
The positive electrode decomposition voltage is usually greater than 4.5 V (vs. 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 is decomposed on the negative electrode:
The electrolyte is not stable on graphite and other lithium-intercalated carbon anodes, and is easily reacted to produce irreversible capacity. The electrolyte decomposition at the initial charge and discharge forms 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. Thereby maintaining the structural stability of the carbon negative electrode. The reduction of the electrolyte under ideal conditions is limited to the formation phase of the passivation film, which does not occur after the cycle is stabilized.
➤ 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) Insoluble matter produced by reduction may adversely affect the solvent-reduced product;
(2) The concentration of the electrolyte decreases when the electrolyte salt is reduced, eventually leading to 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 causes an imbalance in capacity between the two electrodes, resulting in a decrease in the specific capacity of the entire battery.
(4) If there is a crack on the passivation film, the solvent molecules can penetrate and thicken the passivation film, which not only consumes more lithium, but also may block micropores on the carbon surface, resulting in lithium being unable to be embedded and ejected. , causing irreversible capacity loss. Adding some inorganic additives such as CO2, N2O, CO, SO2, etc. in the electrolyte can accelerate the formation of the passivation film and inhibit the co-intercalation and decomposition of the solvent. The same effect can be obtained by adding a crown ether organic additive. 12 crown 4 ether is best.
➤filming capacity loss factor
(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 particles to its core, and the new phase is formed to embed the original active material. The surface of the particle forms a passivation film with low ion and electron conductivity, so the spinel after storage It has greater polarization than before storage.
By comparing and analyzing the AC impedance spectra before and after the cycle of the electrode material, Zhang found that as the number of cycles increases, the resistance of the surface passivation layer increases and the interface capacitance decreases. It is reflected 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 favorable for the progress of these reactions. This causes an increase in the contact resistance between the active material particles and the Li+ migration resistance, so that the polarization of the battery is increased, the charge and discharge are incomplete, and the capacity is reduced.
2. Reduction mechanism of electrolyte
The electrolyte often contains impurities such as oxygen, water, carbon dioxide, etc., and an oxidation-reduction reaction occurs during charging and discharging of the battery.
The reduction mechanism of the electrolyte includes solvent reduction, electrolyte reduction and impurity reduction:
➤1, solvent reduction
The reduction of PC and EC involves an electronic reaction and a two-electron reaction process, and the two-electron reaction forms Li2CO3:
Fong et al. believe that during the first discharge, when the electrode potential is close to O.8V (vs.Li/Li+), PC/EC electrochemically reacts on the graphite to form CH=CHCH3(g)/CH2=CH2 ( g) and LiCO3(s), resulting in 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 an electron reaction mechanism of PC produces ROCO2Li and propylene. ROCO2Li is sensitive to traces of water. The main products are Li2CO3 and propylene in the presence of trace amounts of water, but no Li2CO3 is produced in the dry state.
DEC restore:
Ein-Eli Y reported that an electrolyte composed of a mixture of diethyl carbonate (DEC) and dimethyl carbonate (DMC) exchanges in a battery to form ethyl methyl carbonate (EMC), which causes a loss of capacity. Certain influence.
➤2, reduction of electrolytes
The reduction reaction of the electrolyte is generally considered to be involved in the formation of a film on the surface of the carbon electrode, and thus the type and concentration thereof will affect the performance of the carbon electrode. In some cases, the reduction of the electrolyte contributes to the stabilization of the carbon surface and forms the desired passivation layer.
It is generally believed that the supporting electrolyte is easier to reduce than the solvent, and the reducing product is contained in the negative electrode deposited film to affect the capacity decay of the battery. The reduction reactions that may occur in several supporting electrolytes are as follows:
➤3, impurity reduction
(1) If the water content in the electrolyte is too high, LiOH(s) and Li2O deposit layers will be formed, which is not conducive to lithium ion intercalation, resulting in irreversible capacity loss:
H2O+e→OH-+1/2H2
OH-+Li+→LiOH(s)
LiOH+Li++e-→Li2O(s)+1/2H2
The formation of LiOH(s) deposits on the surface of the electrode, forming a surface film with a large electrical resistance, hindering the insertion of Li+ into the graphite electrode, resulting in irreversible capacity loss. Trace water (100-300×10-6) in the solvent had no effect on the performance of the graphite electrode.
(2) CO2 in the solvent can be reduced on the negative electrode to form CO and LiCO3(s):
2CO2+2e-+2Li+→Li2CO3+CO
CO causes the internal pressure of the battery to rise, while Li2CO3(s) causes the internal resistance of the battery to increase to affect the battery performance.
(3) The presence of oxygen in the solvent also forms Li2O
1/2O2+2e-+2Li+→Li2O
Since the potential difference between metallic lithium and fully lithium intercalated carbon is small, the reduction of the electrolyte on carbon is similar to the reduction on lithium.
Third, self-discharge
Self-discharge refers to the phenomenon that the battery naturally loses its capacity when it is not in use. Lithium-ion battery self-discharge leads to capacity loss in two cases:
One is the loss of reversible capacity;
The second is the loss of irreversible capacity.
Reversible capacity loss means that the lost capacity can be recovered during charging, while the irreversible capacity loss is reversed. In the charged state, the positive and negative electrodes may interact with the electrolyte to generate micro-cells, lithium ion insertion and deintercalation, and positive and negative electrode insertion and removal. The embedded lithium ions are only related to the lithium ion of the electrolyte, and the positive and negative electrodes are therefore unbalanced, and this capacity loss cannot be recovered during charging. Such as:
Lithium manganese oxide positive electrode and solvent will occur micro-cell action resulting in self-discharge resulting in irreversible capacity loss:
LiyMn2O4+xLi++xe-→Liy+xMn2O4
Solvent molecules (such as PC) are oxidized as microbattery anodes on the surface of the conductive material carbon black or current collector:
xPC→xPC-free radical+xe-
Similarly, the negative active material may react with the electrolyte to generate a microbattery to cause self-discharge to cause irreversible capacity loss, and the electrolyte (such as LiPF6) is reduced on the conductive material:
PF5+xe-→PF5-x
Lithium carbide in a charged state is oxidized by removing lithium ions as a negative electrode of the microbattery:
LiyC6→Liy-xC6+xLi+++xe-
Factors influencing self-discharge: the manufacturing process of the positive electrode material, the manufacturing process of the battery, the nature 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 battery.
The oxidation of the solvent mainly occurs on the surface of the carbon black. The surface area of ​​the carbon black can be controlled to control the self-discharge rate. However, for the LiMn2O4 cathode material, it is equally important to reduce the surface area of ​​the active material, and the effect of the surface of the collector on the oxidation of the solvent cannot be ignored. .
The current leaking through the battery separator can also cause self-discharge in a lithium-ion battery, but this process is limited by the diaphragm resistance, occurs at a very low rate, and is independent of temperature. Considering that the self-discharge rate of the battery is strongly dependent on 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 is self-discharged, the battery content balance is destroyed, resulting in permanent capacity loss.
When prolonged or often self-discharged, lithium may deposit on carbon, increasing the degree of capacity imbalance between the two poles.
Pistoia et al. compared the self-discharge rates of three main metal oxide positive electrodes in various electrolytes and found that the self-discharge rate varies with electrolyte. It is also pointed out that the self-discharged oxidation product blocks the micropores on the electrode material, making it difficult to intercalate and extract lithium and to increase the internal resistance and discharge efficiency, resulting in irreversible capacity loss.
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