Lithium-ion battery is currently the mainstream energy battery on the market, high performance also makes its future looks bright, but the lithium-ion battery short board has very obvious, in addition to the stability of the abnormal state, the stability of the charge and discharge cycle and lithium iron phosphate battery is still a gap, now we will analyze the causes of lithium-ion battery capacity failure.
Lithium-ion batteries have different embedding energy when the embedding reaction occurs 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 battery, the capacity balance is expressed as the mass ratio of positive to negative electrode
That is: γ=m+/m-=ΔxC-/ΔyC+
In the above equation, C refers to the theoretical Coulomb capacity of the electrode, and Δx and Δy refer to the stoichiometric number of lithium ions embedded in the negative electrode and the positive electrode, respectively. From the above equation, it can be seen that the required mass ratio of the two electrodes depends on the corresponding coulombic capacities of the two electrodes and their respective numbers of reversible lithium ions.
In general, a smaller mass ratio leads to incomplete utilization of the negative electrode material; a larger mass ratio may be a safety hazard due to overcharging of the negative electrode. In conclusion, the best battery performance is achieved at the most optimized mass ratio.
For an ideal Li-ion battery system, the volume balance does not change during its cycle, and the initial capacity in each cycle is a certain value, however, the actual situation is much more complicated. Any side reaction that can generate or consume lithium ions or electrons can lead to a change in the capacity balance of the battery, and once the capacity balance of the battery is changed, the change is irreversible and can accumulate through multiple cycles, with serious effects on battery performance. In lithium-ion batteries, in addition to the redox reaction that occurs when lithium ions are de-embedded, there are also a large number of side reactions, such as electrolyte decomposition, active material dissolution, lithium metal deposition, etc.
Cause 1: Overcharging
1. Overcharge reaction of graphite negative electrode.
When the battery is overcharged, lithium ions are easily reduced deposited on the surface of the cathode.
The deposited lithium covers the surface of the cathode, blocking the embedding of lithium. Resulting in lower discharge efficiency and capacity loss due to.
①reduction in the amount of recyclable lithium.
(ii) reaction of deposited lithium metal with solvent or supporting electrolyte to form Li2CO3, LiF or other products.
③ Lithium metal is usually formed between the negative electrode and the diaphragm, which may block the pores of the diaphragm to increase the internal resistance of the battery.
④ Due to the nature of lithium is very active, easy to react with the electrolyte and consume the electrolyte. This leads to lower discharge efficiency and loss of capacity.
Fast charging, excessive current density, severe polarization of the negative electrode, the deposition of lithium will be more obvious. This situation tends to occur in cases where there is an excess of positive electrode active relative to negative electrode active. However, in the case of high charging rate, even if the ratio of positive to negative active is normal, lithium metal deposition may occur.
2、Cathode overcharge reaction
When the ratio of positive active to negative active is too low, positive overcharge is likely to occur.
The capacity loss due to positive overcharge is mainly due to the generation of electrochemical inert substances (such as Co3O4, Mn2O3, etc.), which disrupts the capacity balance between electrodes, and its capacity loss is irreversible.
(1) LiyCoO2
LiyCoO2→(1-y)/3[Co3O4+O2(g)]+yLiCoO2y<0.4
Meanwhile, the oxygen generated by the decomposition of the cathode material in the sealed Li-ion battery will have unimaginable consequences due to the non-existent recombination reaction (such as the generation of H2O) and the simultaneous accumulation of flammable gases generated by the decomposition of the electrolyte.
(2) λ-MnO2
Lithium-manganese reaction occurs in the state of complete de-lithium of lithium-manganese oxide: λ-MnO2→Mn2O3+O2(g)
3、Electrolyte oxidation reaction during overcharge
When the pressure is higher than 4.5V, the electrolyte will be oxidized to produce insoluble matter (such as Li2Co3) and gas, and these insoluble matter will be blocked in the micro-pores of the electrode to prevent the migration of lithium ions and cause capacity loss during the cycle.
Factors affecting oxidation rate.
Surface area size of cathode material
Collector material
The added conductive agent (carbon black, etc.)
Type of carbon black and size of surface area
Among the more commonly used electrolytes today, EC/DMC is considered to have the highest oxidation resistance. The electrochemical oxidation process of a solution is generally expressed as: solution → oxidation products (gases, solutions and solid substances) + ne-
Oxidation of any solvent increases the electrolyte concentration, decreases the stability of the electrolyte and eventually affects the capacity of the battery. Assuming that a small amount of electrolyte is consumed during each charge, more electrolyte is needed during battery assembly. For a constant container, this means loading a smaller amount of active material, which causes a decrease in initial capacity. In addition, if solid products are produced, a passivation film will be formed on the electrode surface, which will cause an increase in cell polarization and reduce the output voltage of the cell.
Cause 2: Electrolyte decomposition (reduction)
Decomposition at the electrode
1. Decomposition of electrolyte on the positive electrode.
The electrolyte consists of solvent and supporting electrolyte. After decomposition at the positive electrode, 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, and the gas generated by the reduction will increase the internal pressure of the battery, which will lead to safety problems.
The cathode decomposition voltage is usually greater than 4.5V (relative to Li/Li+), so they are not easily decomposed at the cathode. On the contrary, electrolytes are more likely to decompose at the negative electrode.
2. Electrolyte decomposition at the negative electrode.
The electrolyte is not stable on graphite and other embedded lithium carbon negative electrodes, which can easily react to produce irreversible capacity. The decomposition of electrolyte during the first charge and discharge will form a passivation film on the electrode surface, which can separate the electrolyte from the carbon cathode and prevent further decomposition of electrolyte. Thus, the structural stability of the carbon cathode is maintained. Ideally, the reduction of the electrolyte is limited to the formation of the passivation film, and the process does not occur after the cycle is stabilized.
Passivation film formation
The reduction of the electrolyte salt is involved in the formation of the passivation film and facilitates the stabilization of the passivation film, but
(1) The insoluble matter generated by the reduction has a negative effect on the solvent reduction products.
(2) The concentration of electrolyte decreases during electrolyte salt reduction, which eventually leads to loss of cell capacity (LiF, LixPF5--x, PF3O and PF3 are generated by LiPF6 reduction).
(3) The formation of the passivation film consumes lithium ions, which leads to an imbalance in capacity between the two poles and results in a decrease in the overall cell specific capacity.
(4) If there are cracks in the passivation film, solvent molecules can penetrate and thicken the passivation film, which not only consumes more lithium, but also may block the micro-pores on the carbon surface, resulting in irreversible capacity loss due to the inability of lithium embedding and exiting. Adding some inorganic additives such as CO2, N2O, CO, SO2, etc. to the electrolyte can accelerate the formation of passivation film and inhibit the co-embedding and decomposition of solvent.
Factors of film-forming capacity loss.
(1) The type of carbon used in the process.
(2) Electrolyte composition.
(3) Additives in the electrode or electrolyte.
Blyr concluded that the ion-exchange reaction advances from the surface of the active material particle to its core, the new phase formed encapsulates the original active material, and a passivated film with lower ionic and electronic conductivity is formed on the surface of the particle, so that the spinel after storage has greater polarization than before storage.
Zhang's comparative analysis of the AC impedance spectra of the electrode materials before and after cycling revealed that the resistance of the surface passivation layer increased and the interfacial capacitance decreased as the number of cycles increased. This reflects that the thickness of the passivation layer increases with the number of cycles. The dissolution of manganese and the decomposition of electrolyte lead to the formation of passivation film, and the high temperature conditions are more favorable for these reactions. This will cause an increase in the contact resistance between the active material particles and the Li+ migration resistance, resulting in increased polarization of the cell, incomplete charging and discharging, and reduced capacity.
II The reduction mechanism of electrolyte
The electrolyte often contains impurities such as oxygen, water, carbon dioxide, etc., and the redox reaction occurs during the charging and discharging process of the battery.
The reduction mechanism of electrolyte includes solvent reduction, electrolyte reduction and impurity reduction in three aspects.
1, the reduction of solvent
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. concluded that during the first discharge, when the electrode potential approaches O.8 V (vs. Li/Li+), PC/EC undergoes an electrochemical reaction on graphite to produce CH=CHCHCH3(g)/CH2=CH2(g) and LiCO3(s), leading to irreversible capacity loss on the graphite electrode.
The reduction mechanism of various electrolytes on lithium metal and carbon-based electrodes and their products were extensively studied by Aurbach et al. It was found that the one-electron reaction mechanism of PC produces ROCO2Li and propylene. ROCO2Li is sensitive to trace water, and the main products are Li2CO3 and propylene in the presence of trace water, but no Li2CO3 is produced in dry conditions.
Reduction of DEC.
Ein-EliY reported that the electrolyte mixed by diethyl carbonate (DEC) and dimethyl carbonate (DMC) will undergo an exchange reaction in the cell to produce methyl ethyl carbonate (EMC), which has some effect on the capacity loss.
2、Reduction of electrolyte
The reduction reaction of electrolyte is usually considered to be involved in the formation of carbon electrode surface film, so its type and concentration will affect the performance of carbon electrode. In some cases, the reduction of electrolyte contributes to the stability of carbon surface and can form the desired passivation layer.
It is generally accepted that the supporting electrolyte is easier to reduce than the solvent, and the reduction products are entrapped in the deposited film of the negative electrode and affect the capacity decay of the cell. The possible reduction reactions of several support electrolytes are as follows.
3, impurity reduction
(1) High water content in the electrolyte will generate 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 generated LiOH(s) is deposited on the electrode surface, forming a highly resistive surface film that prevents Li+ from embedding in the graphite electrode, resulting in irreversible capacity loss. Trace amount of water in the solvent (100-300×10-6) has no effect on the graphite electrode performance.
(2) CO2 in the solvent can be reduced to generate CO and LiCO3(s) at 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 performance of the battery.
(3) The presence of oxygen in the solvent will also form Li2O
1/2O2+2e-+2Li+→Li2O
Because the potential difference between lithium metal and fully embedded lithium carbon is small, the reduction of electrolyte on carbon is similar to the reduction on lithium.
Reason 3: Self-discharge
Self-discharge is the phenomenon of natural loss of electric capacity of the battery in the unused state. Self-discharge of lithium-ion batteries leads to capacity loss in two cases.
One is the reversible capacity loss.
The second is the irreversible capacity loss.
Reversible capacity loss means that the lost capacity can be recovered when charging, while irreversible capacity loss is the opposite. The positive and negative electrodes may have microcellular action with the electrolyte in the charging state, lithium ion embedding and de-embedding occurs, and the lithium ions embedded and de-embedded in the positive and negative electrodes are only related to the lithium ions in the electrolyte, so the positive and negative capacity is not balanced, and this part of the capacity loss cannot be recovered when charging. For example.
Lithium-manganese oxide cathode and solvent will occur microcell action to produce self-discharge resulting in irreversible capacity loss.
LiyMn2O4+xLi++xe-→Liy+xMn2O4
Solvent molecules (e.g. PC) are oxidized on the surface of the conductive material carbon black or collector as the negative side of the microcell:
xPC→xPC-radical+xe-
Similarly, the negative active material may interact with the electrolyte in a microcell to produce self-discharge resulting in irreversible capacity loss and electrolyte (e.g. LiPF6) reduction on the conductive material.
PF5+xe-→PF5-x
Lithium carbide in the charged state is oxidized as the negative electrode of the microcell to shed lithium ions:
LiyC6→Liy-xC6+xLi+++xe-
Self-discharge influencing factors: the production process of the cathode material, the production process of the cell, the nature of the electrolyte, temperature, time.
The self-discharge rate is mainly controlled by the oxidation rate of solvent, so the stability of solvent affects the storage life of the battery.
The oxidation of solvent mainly occurs on the surface of carbon black, and reducing the surface area of carbon black can control the self-discharge rate, but for LiMn2O4 cathode material, it is equally important to reduce the surface area of the active material, and the role of the surface of the collector in the oxidation of solvent cannot be ignored.
Current leakage through the cell diaphragm can also cause self-discharge in Li-ion batteries, but the process is limited by the resistance of the diaphragm, 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 while the positive electrode is self-discharging, the internal volume balance of the battery is disrupted, which will result in permanent capacity loss.
When self-discharging for a long time or frequently, lithium may be deposited on carbon, increasing the capacity imbalance between the two poles.
Pistoia et al. compared the self-discharge rates of three major metal oxide cathodes in various different electrolytes and found that the self-discharge rates varied with the electrolyte. It was also pointed out that the self-discharge oxidation products clogged the micropores on the electrode material, making the embedding and detachment of lithium difficult and increasing the internal resistance and reducing the discharge efficiency, leading to irreversible capacity loss.
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