Lithium ion batteries are the fastest developing secondary clean renewable energy source after cadmium nickel and hydrogen nickel batteries. They have advantages such as high working voltage, light weight, and high energy density, and are widely used in electric tools, digital cameras, mobile phones, laptops, and other fields. The future development trend is promising. The high-energy properties of lithium-ion batteries make their future look bright. However, lithium-ion batteries are not perfect either, and their biggest problem is the stability of their charging and discharging cycles. This article summarizes and analyzes the possible causes of capacity degradation in lithium-ion batteries, including overcharging, electrolyte decomposition, and self discharge.
Lithium ion batteries have different embedding energies when embedding reactions occur between two electrodes, and in order to achieve optimal battery performance, the capacity ratio of these two host electrodes should maintain an equilibrium value.
In lithium-ion batteries, capacity balance is expressed as the mass ratio of the positive electrode to the negative electrode,
Namely: γ= M+/m-= Δ XC-/ Δ YC+
In the above equation, C refers to the theoretical Coulomb capacity of the electrode, Δ X Δ Y refers to the stoichiometric number of lithium ions embedded in the negative and positive electrodes, respectively. From the above equation, it can be seen that the mass ratio required for the two poles depends on the corresponding Coulomb capacity and the number of reversible lithium ions in each pole.
Generally speaking, a smaller mass ratio leads to incomplete utilization of negative electrode materials; A larger mass ratio may pose safety hazards due to overcharging of the negative electrode. In short, at the optimal quality ratio, the battery performance is optimal.
For an ideal Li ion battery system, the capacity balance does not change during its cycle, and the initial capacity in each cycle is a certain value. However, the actual situation is much more complex. Any side reaction that can generate or consume lithium ions or electrons may cause a change in battery capacity balance. Once the capacity balance state of the battery changes, this change is irreversible and can accumulate through multiple cycles, seriously affecting battery performance. In lithium-ion batteries, in addition to the oxidation-reduction reaction that occurs during lithium-ion deintercalation, there are also a large number of side reactions, such as electrolyte decomposition, dissolution of active substances, and deposition of metallic lithium
1、 Overcharging
1. Overcharge reaction of graphite negative electrode:
During overcharging, lithium-ion batteries are prone to reduction and deposition on the negative electrode surface:
The deposited lithium is coated on the negative electrode surface, blocking the insertion of lithium. The reasons for the decrease in discharge efficiency and capacity loss include:
① Reduced recyclable lithium content;
② The deposited metallic lithium reacts with solvents or supporting electrolytes to form Li2CO3, LiF or other products;
③ Lithium metal 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 easily reacts with the electrolyte and consumes it, resulting in a decrease in discharge efficiency and loss of capacity.
Fast charging, high current density, severe polarization of the negative electrode, and more pronounced lithium deposition. This situation is prone to occur in situations where there is an excess of positive electrode activity relative to negative electrode activity. However, in high charging rates, even if the proportion of positive and negative active materials is normal, the deposition of metallic lithium may still occur.
2. Positive electrode overcharge reaction
When the proportion of positive electrode active material to negative electrode active material in lithium-ion batteries is too low, it is easy to cause positive electrode overcharging.
The capacity loss caused by overcharging of the positive electrode is mainly due to the generation of electrochemical inert substances (such as Co3O4, Mn2O3, etc.), which disrupt 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 a sealed lithium-ion battery will accumulate simultaneously with the combustible gas generated by the decomposition of the electrolyte due to the absence of recombination reactions (such as generating H2O), and the consequences will be unimaginable.
② λ- MnO2
The lithium manganese reaction occurs in the state of complete delithiation of lithium manganese oxide: λ- MnO2 → Mn2O3+O2 (g)
3. Electrolyte oxidation reaction during overcharging
When the pressure exceeds 4.5V, the electrolyte will oxidize to form insoluble substances (such as Li2Co3) and gases, which will block the micropores of the electrode and hinder the migration of lithium ions, causing capacity loss during the cycling process.
Factors affecting oxidation rate:
① Surface area size of positive electrode material
② Collector material
③ Conductive agents added (such as carbon black)
④ Types and surface area size of carbon black
Among the commonly used electrolytes, EC/DMC is considered to have the highest oxidation resistance. The electrochemical oxidation process of a solution is generally expressed as follows: solution → oxidation products (gas, solution, and solid substances)+ne-
The oxidation of any solvent will increase the electrolyte concentration, decrease the stability of the electrolyte, and ultimately affect the capacity of the battery. Assuming that a small amount of electrolyte is consumed during each charging, more electrolyte is required during battery assembly. For a constant container, this means loading a smaller amount of active substance, which can cause a decrease in initial capacity. In addition, if solid products are produced, a passivation film will form on the electrode surface, which will cause an increase in battery polarization and reduce the output voltage of the battery
2、 Electrolyte decomposition (reduction)
1. Decompose on the electrode
① Electrolyte decomposes on the positive electrode:
The electrolyte is composed of a solvent and a supporting electrolyte. After the positive electrode decomposes, insoluble products such as Li2Co3 and LiF are usually formed, which reduce the battery capacity by blocking the pores of the electrode. The reduction reaction of the electrolyte can have adverse effects on the battery capacity and cycle life, and the gas produced by reduction can increase the internal pressure of the battery, leading to safety issues.
The positive electrode decomposition voltage is usually greater than 4.5V (relative to Li/Li+), so they are not easily decomposed on the positive electrode. On the contrary, electrolytes are more prone to decomposition at the negative electrode.
② Electrolyte decomposes on the negative electrode:
The electrolyte has low stability on graphite and other lithium embedded carbon negative electrodes, and is prone to react to produce irreversible capacity. During the initial charge and discharge, the electrolyte decomposes and forms a passivation film on the electrode surface. The passivation film can separate the electrolyte from the carbon negative electrode and prevent further decomposition of the electrolyte. To maintain the structural stability of the carbon negative electrode. Under ideal conditions, the reduction of the electrolyte is limited to the formation stage of the passivation film, and this process no longer occurs when the cycle stabilizes.
Formation of passivation film
The reduction of electrolyte salts participates in the formation of passivation films, which is beneficial for the stabilization of passivation films. However
(1) The insoluble substances produced by reduction will have adverse effects on the solvent reduction products;
(2) The decrease in electrolyte concentration during electrolyte salt reduction ultimately leads to battery capacity loss (LiPF6 reduction generates LiF, LixPF5-x, PF3O, and PF3);
(3) The formation of passivation film requires the consumption of lithium ions, which can lead to an imbalance in the capacity between the two electrodes and result in a decrease in the specific capacity of the entire battery.
(4) If there are cracks on the passivation film, solvent molecules can penetrate, making the passivation film thicker. This not only consumes more lithium, but also may block the micropores on the carbon surface, causing lithium to be unable to be embedded and removed, resulting in irreversible capacity loss. Adding some inorganic additives such as CO2, N2O, CO, SO2, etc. to the electrolyte can accelerate the formation of passivation film and inhibit solvent co embedding and decomposition. The addition of crown ether organic additives also has the same effect, with 12 crown 4 ethers being the best.
Factors affecting membrane capacity loss
(1) The type of carbon used in the process;
(2) Electrolyte composition;
(3) Additives in electrodes or electrolytes.
The ion exchange reaction advances from the surface of the active substance particles towards their core, forming a new phase that encapsulates the original active substance. The particle surface forms a passive film with lower ion and electron conductivity, so the stored spinel has greater polarization than before storage.
By comparing and analyzing the AC impedance spectra of electrode materials before and after cycling, it was found that as the number of cycles increases, the resistance of the surface passivation layer increases and the interface capacitance decreases. 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 high temperature conditions are more conducive to the progress of these reactions. This will cause an increase in the contact resistance between active material particles and Li+migration resistance, resulting in increased polarization of the battery, incomplete charging and discharging, and reduced capacity.
2. The reduction mechanism of electrolyte
Electrolytes often contain impurities such as oxygen, water, and carbon dioxide, which undergo redox reactions during battery charging and discharging processes.
The reduction mechanism of electrolyte includes three aspects: solvent reduction, electrolyte reduction, and impurity reduction:
① Reduction of solvents
The reduction of PC and EC involves both one electron reaction and two electron reaction processes, with the two electron reaction forming Li2CO3:
During the first discharge process, when the electrode potential approaches O.8V (vs. Li/Li+), PC/EC undergoes an electrochemical reaction on graphite, generating CH=CHCH3 (g)/CH2=CH2 (g) and LiCO3 (s), resulting in irreversible capacity loss on the graphite electrode.
Extensive research has been conducted on the reduction mechanisms and products of various electrolytes on metal lithium electrodes and carbon based electrodes, and it has been found that the one electron reaction mechanism of PC produces ROCO2Li and propylene. ROCO2Li is sensitive to trace amounts of water, and the main products in the presence of trace amounts of water are Li2CO3 and propylene, but no Li2CO3 is produced in dry conditions.
DEC restoration:
According to Ein Eli Y, an electrolyte composed of a mixture of diethyl carbonate (DEC) and dimethyl carbonate (DMC) undergoes an exchange reaction in the battery, generating methyl ethyl carbonate (EMC), which has a certain impact on capacity loss.
② Reduction of electrolytes
The reduction reaction of electrolyte is usually considered to be involved in the formation of the facial mask on the surface of the carbon electrode, so its type and concentration will affect the performance of the carbon electrode. In some cases, the reduction of electrolytes helps stabilize the carbon surface and can form the required passivation layer.
It is generally believed that supporting electrolytes are easier to reduce than solvents, and the reduction products are mixed in the negative electrode deposition film, which affects the capacity degradation of the battery. Several possible reduction reactions that may occur with supporting electrolytes are as follows:
③ Impurity reduction
(1) Excessive water content in the electrolyte can generate LiOH (s) and Li2O deposition layers, which are not conducive to lithium ion insertion and cause 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 electrode surface, forming a surface facial mask with high resistance, which prevents Li+from embedding into the graphite electrode, thus causing irreversible capacity loss. The trace amount of water (100-300 × 10-6) in the solvent has no effect on the performance of graphite electrodes.
(2) CO2 in the solvent can be reduced to generate 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 and affect its performance.
(3) The presence of oxygen in solvents can also form Li2O
1/2O2+2e -+2Li+→ Li2O
Because the potential difference between metallic lithium and carbon completely embedded with lithium is small, the reduction of electrolyte on carbon is similar to that on lithium.
3、 Self discharge
Self discharge refers to the phenomenon of natural loss of capacitance in lithium-ion batteries when they are not in use. There are two types of capacity loss caused by self discharge of lithium-ion batteries:
One is reversible capacity loss;
The second is the loss of irreversible capacity.
Reversible capacity loss refers to the loss of capacity that can be restored during charging, while irreversible capacity loss is the opposite. The positive and negative electrodes may interact with the electrolyte in a micro battery state during charging, causing lithium ion insertion and removal. The lithium ions embedded and removed by the positive and negative electrodes are only related to the lithium ions in the electrolyte, and the capacity of the positive and negative electrodes is therefore unbalanced. This part of capacity loss cannot be restored during charging. For example:
Lithium manganese oxide positive electrode and solvent will undergo micro battery interaction, resulting in self discharge and irreversible capacity loss:
LiyMn2O4+xLi++xe - → Liy+xMn2O4
Solvent molecules (such as PC) act as negative electrodes for micro battery oxidation on conductive materials such as carbon black or collector surfaces:
XPC → xPC radical+xe-
Similarly, the negative electrode active material may interact with the electrolyte in a micro battery, resulting in self discharge and irreversible capacity loss. Electrolytes (such as LiPF6) can be reduced on conductive materials:
PF5+xe - → PF5- x
Lithium carbide in the charging state is oxidized as the negative electrode of a micro battery to remove lithium ions:
LiyC6 → Liy xC6+xLi++xe-
The influencing factors of self released films include the production process of positive electrode materials, the production process of batteries, the properties of electrolytes, 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 solvents mainly occurs on the surface of carbon black, and reducing the surface area of carbon black can control the self discharge rate. However, for LiMn2O4 cathode materials, reducing the surface area of active substances is equally important, and the role of collector surface in solvent oxidation cannot be ignored.
The leakage of current through the battery separator can also cause self discharge in lithium-ion batteries, but this process is limited by the resistance of the separator and occurs at an extremely low rate, independent of temperature. Considering that the self discharge rate of batteries strongly depends 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 self discharges, the capacity balance inside the battery will be disrupted, resulting in permanent capacity loss.
When self discharging for a long time or frequently, lithium may deposit on carbon, increasing the degree of capacity imbalance between the two electrodes.
Comparing the self discharge rates of three main metal oxide positive electrodes in various electrolytes, it was found that the self discharge rate varies with different electrolytes. And it is pointed out that the oxidation products of self discharge block the micropores on the electrode material, making it difficult to embed and remove lithium, increasing internal resistance and reducing discharge efficiency, resulting in irreversible capacity loss.