The essential reason is that the lithium ion battery has different embedding energies when the embedding reaction occurs between the two electrodes, and in order to obtain the 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 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, at the optimum quality ratio, battery performance**.
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 in reality the 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 the lithium ion battery, in addition to the redox reaction occurring during lithium ion deintercalation, there are also a large number of side reactions such as electrolyte decomposition, active material dissolution, metal lithium deposition, and the like, as shown in FIG. Arora et al. [3] compared these capacity decay processes with the discharge curves of the half-cells, so that we can clearly see the possibility of capacity decay and its causes when the battery is operating, as shown in Figure 2.
First, overcharge
1. Overcharge reaction of graphite anode:
When the battery is overcharged, lithium ions are easily deposited on the surface of the negative electrode: Li++e→Li(s), and the deposited lithium is coated on the surface of the negative electrode to block the intercalation of lithium. This leads to reduced discharge efficiency and capacity loss due to:
1 The amount of recyclable lithium is reduced;
2 deposited lithium metal reacts with a solvent or supporting electrolyte to form Li2CO3, LiF or other products;
3 Metallic 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 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 the positive and negative active materials is normal.
2. Positive Electrode Overcharge Reaction When the ratio of the positive electrode active material to the negative electrode active material is too low, the positive electrode overcharge is likely to occur.
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.
(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 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.
(2) λ-MnO2
The lithium manganese reaction occurs when the 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 substances will block in the micropores of the electrode and hinder the migration of lithium ions. Capacity loss during the cycle.
Factors affecting oxidation rate:
Positive electrode material surface area
Collector material
Conductive agent added (carbon black, etc.)
Type and surface area of ​​carbon black
Among the more commonly used electrolytes, EC/DMC is considered to have ** 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 cell polarization and lowers the output voltage of the battery.
Second, the 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 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 (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 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.
The formation of the passivation film reduces the formation of the electrolyte salt and participates in the formation of the passivation film, which is advantageous for the stabilization of the passivation film, but (1) the insoluble matter produced by the reduction adversely affects the solvent reduction product;
(2) The concentration of the electrolyte decreases when the electrolyte salt is reduced, eventually resulting in loss of battery capacity (LiPF6 reduction leads to 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 and Sx2- to the electrolyte can accelerate the formation of the passivation film and inhibit the co-intercalation and decomposition of the solvent. The addition of the crown ether organic additive also has the same effect. , which has 12 crowns of 4 ether**.
Factors contributing to film loss capacity:
(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.
II Reduction Mechanism of Electrolyte The electrolyte often contains impurities such as oxygen, water and carbon dioxide, 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 in the ** discharge process, 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. The reduction reaction of the reducing electrolyte of the electrolyte is generally considered to be involved in the formation of the surface film 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:
last step:
3, impurity reduction (1) too high water content in the electrolyte will produce LiOH (s) and Li2O deposition layer, which is not conducive to lithium ion embedding, 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 to produce CO and LiCO3(s) on the negative electrode:
2CO2+2e+2Li+→Li2CO3+CO
CO will increase the internal pressure of the battery, while Li2CO3(s) will increase the internal resistance of the battery 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.
3. 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 reversible capacity loss; the other is irreversible capacity loss. 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 affecting self-discharge:
a manufacturing process of a positive electrode material;
Battery manufacturing process;
The nature of the electrolyte;
temperature;
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.
4. Electrode Instability The positive active material will decompose the oxidizing electrolyte under charge and cause capacity loss. Factors affecting the dissolution of the positive electrode material are:
Structural defects of the positive active material;
The charging potential is too high;
The content of carbon black in the positive electrode material.
1. Structural change (phase change)
The structural change of the electrode during the charge and discharge cycle is the most important factor.
(1) There are two different structural changes in the process of lithium manganese oxide during charge and discharge: one is the phase change that occurs when the stoichiometry is constant; the other is the change in the amount of lithium insertion and deintercalation during charge and discharge. Phase change. For example, lithium manganese oxide is completely charged and delithiated to form λ-MnO2, which may undergo phase change under the condition of constant stoichiometry, and is converted into inactive β-MnO2 by ε-MnO2; when lithium manganese oxide is deeply discharged, it is excessively embedded. In lithium, the original cubic spinel undergoes lattice distortion and transforms into a tetragonal spinel, which causes Jahn-Teller distortion, the z-axis elongation of the unit cell is 15%, and the x and Y axis shrinks by 6%. After such multiple cycles, the positive electrode material will be powdered. LiMn2O4 is prone to capacity decay under deep circulation.
Xia et al. believe that in the low voltage region, the voltage curve is “S†shaped, which corresponds to a single cubic crystal structure, while the “L†shape of the high voltage region indicates the existence of two phases, which is due to the instability of the two-phase structure. Sexuality leads to a reduction in capacity. The authors also found that the positive electrode material with oxygen deficiency is prone to phase change during cycling, and capacity decay occurs at both 4.0 and 4.2V. The sample with anaerobic defects is less prone to phase change during cycling and only occurs during overdischarge. Capacity attenuation.
The preparation of lithium-rich LiMn2O4, such that lithium occupies part of the manganese, increases the average valence of manganese, which can greatly reduce the Jahn-Telle deformation. The synthesis of lithium-rich or oxygen-rich compounds can also improve the high temperature electrochemical performance of LiMn2O4. The charge and discharge performance of the doped spinel LiMn2-xMxO4 (M=Ni, Co, Fe, etc.) is significantly improved. The reason is: 1 due to the small atomic radius of Ni, Co, Fe, etc., the unit cell of LiMn2-xMxO4 prepared by them is relatively small, and the deformation of the structure subjected to lithium deintercalation is small, so during charging and discharging Its structure is easier to maintain and its cycle performance is better. 2 Because Co and Ni are all less than 3, the substitution of Mn by Co, Ni, etc. will increase the average valence state of manganese and reduce the content of Mn3+, thereby avoiding the presence of more (50%) Mn3+ in deep discharge. Causes the deformation of the structure to the tetrahedron. Mn3+ is an internal cause of the Jahn-Teller effect.
(2) Lithium cobalt oxide is hexagonal crystal under fully charged state, and 50% of theoretical capacity is discharged to form a new phase monoclinic crystal, so LiyCoO2 is usually at 0.5. (3) Lithium nickel oxide in the process of charge and discharge cycle involving rhombohedral and monoclinic crystals of LiyNiO2 usually at 0.3 The negative electrode carbon also involves a change in the crystal phase during lithium intercalation and delithiation, but it is generally believed that the phase transition of the negative active material does not cause a decrease in the capacity of the battery.
Note: Phase transitions are also observed for LiCoO2 and lithium nickel oxide electrodes, but have little to do with capacity loss [38]. LiyNiO2 electrodes typically circulate between y = 0.3 and 0.9, while LiyCoO2 circulates between y = 0.5 and 1.0 to avoid significant phase transitions during cycling.
2. The factors that affect the dissolution of the positive electrode material by the positive electrode dissolution are:
1 Structural defects of the positive active material
Defects in oxygen atoms in the LiMn2O4 and LiNi2O4 structures can weaken the bond energy between the metal atoms and the oxygen atoms, resulting in the dissolution of manganese and nickel. The Mn2+ and Ni2+ dissolved in the electrolyte are finally deposited on the negative electrode in a simple form, which not only consumes the positive electrode active material, but also blocks the micropores on the negative electrode, making it difficult to insert and remove lithium in the negative electrode, resulting in capacity loss. According to the literature, after the manganese in the positive electrode LiMn2O4 dissolves into the electrolyte, 25% of Mn2+ is deposited on the surface of the negative electrode. The use of high purity LiFP6 and low specific surface LiMn2O4 can reduce the dissolution of Mn2+.
2 charging potential is too high;
3 Content of carbon black in the positive electrode material The catalytic property of the electrolytic solution on the surface of the carbon black causes the metal ion dissolution rate to increase, thereby adversely affecting the dissolution of the positive electrode material.
4 catalytic oxidation reduction
Robertson et al. believe that catalytic redox results in the removal of manganese from the LiMn2O4 cathode material to form Li2MnO3 and Li2Mn4O9. Among them, Li2MnO3 is electrochemically inert, and Li2Mn4O9 has no charge and discharge capacity near 4v, which causes irreversible capacity loss. In addition, the disproportionation of Mn3+ dissolves into a cation-deficient spinel phase, which destroys the crystal lattice and blocks the Li+ diffusion channel.
But for a long time, the mechanism by which spinel electrodes dissolve has been controversial. More classical is the mechanism proposed by Hunter for dissolution by disproportionation reaction. The total reaction is as follows:
4H++2LiMn3+Mn4+O4→3λ-MnO2+Mn2++2Li++2H2O (2)
This is a dissolution process of spinel due to the presence of acid. The essence of this mechanism is that the oxidized state of Mn3+ is unstable, and disproportionation reaction occurs to form Mn2+ and Mn4+. Mn2+ can enter the solution, deposit Mn(s) on the negative electrode, or react with Li+ to react with the oxidation product of the electrolyte to form a passivation film containing lithium and manganese on the surface of the electrode. Both of these conditions increase the internal resistance of the battery, resulting in a loss of capacity.
In the fully discharged state, the lithium manganese oxide is a spinel LiMn2O4 in which trivalent manganese accounts for half of the total manganese. Trivalent manganese undergoes deuteration reaction under acidic conditions:
Mn3+ (solid)→Mn2+ (dissolved)+Mn4+ (solid)
The acid comes from the action of the electrolyte and the impurity water in the electrolyte:
LiPF6+H2O→POF3+2HF+LiF
Inoue believes that the decomposition of LiPF6 at high temperature causes the lattice defect of LiMn2O4 due to the dissolution of manganese, and the disorder of structure causes the diffusion of Li+ to be hindered. They ruled out the damage to the SEI film caused by the deposition of Mn2+ on the negative electrode and its effect on battery performance.
It is believed that no matter how the dissolution of Mn occurs, what is important is the process of protonation, which is the reason for the capacity decay.
The dissolution of manganese by HF is the direct cause of the capacity decay of LiMn2O4. The HF impurities contained in the F-containing electrolyte itself, the HF formed by the oxidation of the solvent and the HF formed by the F-combination, and the moisture impurities in the electro-hydraulic or the water adsorbed by the electrode material cause the dissolution of the spinel due to the HF generated by the decomposition of the electrolyte. It is worth mentioning that LiMn2O4 has a catalytic effect on the decomposition reaction of the electrolyte, so that the dissolution reaction of manganese is autocatalytic. The dissolution reaction of manganese is kinetically controlled, and the dissolution rate is accelerated above 40 ° C, and the higher the temperature, the more severe the dissolution loss of manganese.
γ=m+/m-=ΔxC-/ΔyC+
In the 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, at the optimum quality ratio, battery performance**.
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 in reality the 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 the lithium ion battery, in addition to the redox reaction occurring during lithium ion deintercalation, there are also a large number of side reactions such as electrolyte decomposition, active material dissolution, metal lithium deposition, and the like, as shown in FIG. Arora et al. [3] compared these capacity decay processes with the discharge curves of the half-cells, so that we can clearly see the possibility of capacity decay and its causes when the battery is operating, as shown in Figure 2.
First, overcharge
1. Overcharge reaction of graphite anode:
When the battery is overcharged, lithium ions are easily deposited on the surface of the negative electrode: Li++e→Li(s), and the deposited lithium is coated on the surface of the negative electrode to block the intercalation of lithium. This leads to reduced discharge efficiency and capacity loss due to:
1 The amount of recyclable lithium is reduced;
2 deposited lithium metal reacts with a solvent or supporting electrolyte to form Li2CO3, LiF or other products;
3 Metallic 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 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 the positive and negative active materials is normal.
2. Positive Electrode Overcharge Reaction When the ratio of the positive electrode active material to the negative electrode active material is too low, the positive electrode overcharge is likely to occur.
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.
(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 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.
(2) λ-MnO2
The lithium manganese reaction occurs when the 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 substances will block in the micropores of the electrode and hinder the migration of lithium ions. Capacity loss during the cycle.
Factors affecting oxidation rate:
Positive electrode material surface area
Collector material
Conductive agent added (carbon black, etc.)
Type and surface area of ​​carbon black
Among the more commonly used electrolytes, EC/DMC is considered to have ** 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 cell polarization and lowers the output voltage of the battery.
Second, the 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 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 (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 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.
The formation of the passivation film reduces the formation of the electrolyte salt and participates in the formation of the passivation film, which is advantageous for the stabilization of the passivation film, but (1) the insoluble matter produced by the reduction adversely affects the solvent reduction product;
(2) The concentration of the electrolyte decreases when the electrolyte salt is reduced, eventually resulting in loss of battery capacity (LiPF6 reduction leads to 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 and Sx2- to the electrolyte can accelerate the formation of the passivation film and inhibit the co-intercalation and decomposition of the solvent. The addition of the crown ether organic additive also has the same effect. , which has 12 crowns of 4 ether**.
Factors contributing to film loss capacity:
(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.
II Reduction Mechanism of Electrolyte The electrolyte often contains impurities such as oxygen, water and carbon dioxide, 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 in the ** discharge process, 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. The reduction reaction of the reducing electrolyte of the electrolyte is generally considered to be involved in the formation of the surface film 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:
last step:
3, impurity reduction (1) too high water content in the electrolyte will produce LiOH (s) and Li2O deposition layer, which is not conducive to lithium ion embedding, 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 to produce CO and LiCO3(s) on the negative electrode:
2CO2+2e+2Li+→Li2CO3+CO
CO will increase the internal pressure of the battery, while Li2CO3(s) will increase the internal resistance of the battery 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.
3. 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 reversible capacity loss; the other is irreversible capacity loss. 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 affecting self-discharge:
a manufacturing process of a positive electrode material;
Battery manufacturing process;
The nature of the electrolyte;
temperature;
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.
4. Electrode Instability The positive active material will decompose the oxidizing electrolyte under charge and cause capacity loss. Factors affecting the dissolution of the positive electrode material are:
Structural defects of the positive active material;
The charging potential is too high;
The content of carbon black in the positive electrode material.
1. Structural change (phase change)
The structural change of the electrode during the charge and discharge cycle is the most important factor.
(1) There are two different structural changes in the process of lithium manganese oxide during charge and discharge: one is the phase change that occurs when the stoichiometry is constant; the other is the change in the amount of lithium insertion and deintercalation during charge and discharge. Phase change. For example, lithium manganese oxide is completely charged and delithiated to form λ-MnO2, which may undergo phase change under the condition of constant stoichiometry, and is converted into inactive β-MnO2 by ε-MnO2; when lithium manganese oxide is deeply discharged, it is excessively embedded. In lithium, the original cubic spinel undergoes lattice distortion and transforms into a tetragonal spinel, which causes Jahn-Teller distortion, the z-axis elongation of the unit cell is 15%, and the x and Y axis shrinks by 6%. After such multiple cycles, the positive electrode material will be powdered. LiMn2O4 is prone to capacity decay under deep circulation.
Xia et al. believe that in the low voltage region, the voltage curve is “S†shaped, which corresponds to a single cubic crystal structure, while the “L†shape of the high voltage region indicates the existence of two phases, which is due to the instability of the two-phase structure. Sexuality leads to a reduction in capacity. The authors also found that the positive electrode material with oxygen deficiency is prone to phase change during cycling, and capacity decay occurs at both 4.0 and 4.2V. The sample with anaerobic defects is less prone to phase change during cycling and only occurs during overdischarge. Capacity attenuation.
The preparation of lithium-rich LiMn2O4, such that lithium occupies part of the manganese, increases the average valence of manganese, which can greatly reduce the Jahn-Telle deformation. The synthesis of lithium-rich or oxygen-rich compounds can also improve the high temperature electrochemical performance of LiMn2O4. The charge and discharge performance of the doped spinel LiMn2-xMxO4 (M=Ni, Co, Fe, etc.) is significantly improved. The reason is: 1 due to the small atomic radius of Ni, Co, Fe, etc., the unit cell of LiMn2-xMxO4 prepared by them is relatively small, and the deformation of the structure subjected to lithium deintercalation is small, so during charging and discharging Its structure is easier to maintain and its cycle performance is better. 2 Because Co and Ni are all less than 3, the substitution of Mn by Co, Ni, etc. will increase the average valence state of manganese and reduce the content of Mn3+, thereby avoiding the presence of more (50%) Mn3+ in deep discharge. Causes the deformation of the structure to the tetrahedron. Mn3+ is an internal cause of the Jahn-Teller effect.
(2) Lithium cobalt oxide is hexagonal crystal under fully charged state, and 50% of theoretical capacity is discharged to form a new phase monoclinic crystal, so LiyCoO2 is usually at 0.5.
Note: Phase transitions are also observed for LiCoO2 and lithium nickel oxide electrodes, but have little to do with capacity loss [38]. LiyNiO2 electrodes typically circulate between y = 0.3 and 0.9, while LiyCoO2 circulates between y = 0.5 and 1.0 to avoid significant phase transitions during cycling.
2. The factors that affect the dissolution of the positive electrode material by the positive electrode dissolution are:
1 Structural defects of the positive active material
Defects in oxygen atoms in the LiMn2O4 and LiNi2O4 structures can weaken the bond energy between the metal atoms and the oxygen atoms, resulting in the dissolution of manganese and nickel. The Mn2+ and Ni2+ dissolved in the electrolyte are finally deposited on the negative electrode in a simple form, which not only consumes the positive electrode active material, but also blocks the micropores on the negative electrode, making it difficult to insert and remove lithium in the negative electrode, resulting in capacity loss. According to the literature, after the manganese in the positive electrode LiMn2O4 dissolves into the electrolyte, 25% of Mn2+ is deposited on the surface of the negative electrode. The use of high purity LiFP6 and low specific surface LiMn2O4 can reduce the dissolution of Mn2+.
2 charging potential is too high;
3 Content of carbon black in the positive electrode material The catalytic property of the electrolytic solution on the surface of the carbon black causes the metal ion dissolution rate to increase, thereby adversely affecting the dissolution of the positive electrode material.
4 catalytic oxidation reduction
Robertson et al. believe that catalytic redox results in the removal of manganese from the LiMn2O4 cathode material to form Li2MnO3 and Li2Mn4O9. Among them, Li2MnO3 is electrochemically inert, and Li2Mn4O9 has no charge and discharge capacity near 4v, which causes irreversible capacity loss. In addition, the disproportionation of Mn3+ dissolves into a cation-deficient spinel phase, which destroys the crystal lattice and blocks the Li+ diffusion channel.
But for a long time, the mechanism by which spinel electrodes dissolve has been controversial. More classical is the mechanism proposed by Hunter for dissolution by disproportionation reaction. The total reaction is as follows:
4H++2LiMn3+Mn4+O4→3λ-MnO2+Mn2++2Li++2H2O (2)
This is a dissolution process of spinel due to the presence of acid. The essence of this mechanism is that the oxidized state of Mn3+ is unstable, and disproportionation reaction occurs to form Mn2+ and Mn4+. Mn2+ can enter the solution, deposit Mn(s) on the negative electrode, or react with Li+ to react with the oxidation product of the electrolyte to form a passivation film containing lithium and manganese on the surface of the electrode. Both of these conditions increase the internal resistance of the battery, resulting in a loss of capacity.
In the fully discharged state, the lithium manganese oxide is a spinel LiMn2O4 in which trivalent manganese accounts for half of the total manganese. Trivalent manganese undergoes deuteration reaction under acidic conditions:
Mn3+ (solid)→Mn2+ (dissolved)+Mn4+ (solid)
The acid comes from the action of the electrolyte and the impurity water in the electrolyte:
LiPF6+H2O→POF3+2HF+LiF
Inoue believes that the decomposition of LiPF6 at high temperature causes the lattice defect of LiMn2O4 due to the dissolution of manganese, and the disorder of structure causes the diffusion of Li+ to be hindered. They ruled out the damage to the SEI film caused by the deposition of Mn2+ on the negative electrode and its effect on battery performance.
It is believed that no matter how the dissolution of Mn occurs, what is important is the process of protonation, which is the reason for the capacity decay.
The dissolution of manganese by HF is the direct cause of the capacity decay of LiMn2O4. The HF impurities contained in the F-containing electrolyte itself, the HF formed by the oxidation of the solvent and the HF formed by the F-combination, and the moisture impurities in the electro-hydraulic or the water adsorbed by the electrode material cause the dissolution of the spinel due to the HF generated by the decomposition of the electrolyte. It is worth mentioning that LiMn2O4 has a catalytic effect on the decomposition reaction of the electrolyte, so that the dissolution reaction of manganese is autocatalytic. The dissolution reaction of manganese is kinetically controlled, and the dissolution rate is accelerated above 40 ° C, and the higher the temperature, the more severe the dissolution loss of manganese.
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