Analysis of the cause analysis of lithium battery capacity attenuation

著者：Iflowpower – Fornitore di stazioni di energia portatili

In a lithium ion battery, the capacity balance is expressed as the mass ratio of the positive electrode to the negative electrode, namely:<000000>gamma;= m + / m- =δXC- /δYC + upper formula C refers the theoretical coulomb capacity of the electrode,δlittle,δY is referring to a chemical metering of lithium ions embedded in an negative electrode and a positive electrode. It can be seen from the above formula that the mass ratio of the two poles is relying on the number of coulomb capacity and its respective reversible lithium ions according to the two poles. Generally, the smaller mass ratio causes the incomplete use of the negative electrode material; the larger mass ratio may have a safety hazard due to the negative electrode being overchaired.

In short, in the most optimized quality ratio, battery performance is optimal. Related to the ideal Li-ION battery system, in its cycle period, the content quantity is not changed, and the initial capacity in each cycle is a certain value, but the actual situation is much more complicated. Any side reaction that can appear or consume lithium ions or electrons can cause a change in battery capacity balance, once the battery's capacity balance occurs, this change is irreversible, and can be accumulated by multiple cycles, and the battery performance occurs.

Serious impact. In addition, except for the oxidation retentment of the lithium ion, there is a large number of side reactions, such as electrolyte analysis, active substance dissolution, metal lithium deposition, etc. Original one: overcharge 1, graphite negative overcharge: When the battery is overcharged, the lithium ion is easily reduced in the negative surface: the deposited lithium is covered with the negative surface, blocking lithium embedding.

The discharge efficiency is reduced and capacity loss, the original: 1 can be reduced by cyclic lithium; 2 deposited metal lithium and solvent or support electrolyte to form Li2CO3, LIF or other products; 3 metal lithium is usually formed between the negative electrode and the diaphragm, possibly The pores of the blocking diaphragm increases the internal resistance of the battery;. Quick charging, too large current density, severe negative polarization, lithium deposition will be more clear. This situation is easy to occur in an occasion of the negative electrode active.

However, in the case of high charging rate, the deposition of metal lithium may occur even if the proportion of the positive and negative electrode active is normal. 2, the positive precision reaction is too low when the positive electrode active resistance is too low, and it is easy to charge. The positive transition causes the capacity loss to be due to the occurrence of electrochemical inert substances (such as CO3O4, MN2O3, etc.

), which disrupts the capacity balance between electrodes, and its capacity loss is irreversible. (1) liycoo2liycoo2→(1-y) / 3 [CO3O4 + O2 (G)] + Ylicoo2Y <0.4 Simultaneous positive electrode material analyzes oxygen in a sealed lithium ion battery to analyze the oxygen due to the absence of re-reactive reaction (such as the formation of H2O) and the combustible gas in the electrolyte analysis At the same time, the consequences will be unimaginable.

(2)λ-MnO2 lithium manganese reaction occurs in a state where the lithium manganese oxide is completely decentr:λ-Mno2→Mn2O3 + O2 (G) 3, the electrolyte is oxidized when the electrolyte is oxidized when the pressure is higher than 4.5V, and the electrolyte (e.g.

, Li2CO3) and the gas are oxidized, and these insolublements will block the micropores of the electrode. The migration of lithium ions causes capacity loss during cycle. Affecting the rate of oxidation rate: The type and surface area size of the conductive agent (carbon black, etc.

) added by the positive electrode material surface area size collector material (carbon black, etc.) in the currently used electrolytic solution, EC / DMC is considered to have the highest oxidation capacity . The electrochemical oxidation process of the solution is generally expressed as: solution→Oxidation products (gases, solutions and solid substances) + NE-any solvent oxidation can increase the concentration of the electrolyte, the electrolyte stability is lowered, and the capacity of the battery is finally.

Suppose consumes a small part of the electrolyte every time it is charged, then more electrolyte is in battery assembly. For constant containers, this means that a small amount of active substance is loaded, which will cause a decrease in initial capacity. Further, if a solid product occurs, a passivation film is formed on the surface of the electrode, which will cause the battery to increase the output voltage of the battery.

Original 2: Electrolyte (Reverting) I On the electrode analysis 1 Reducing the battery capacity, the electrolyte reduction reaction against the battery capacity and circulating life will adversely affect, and due to the reduction of the gas to increase the battery, thereby leading to safety issues. The positive electrode analysis voltage is usually greater than 4.5V (related to Li / Li +), so they are not easy to analyze in the positive.

Instead, electrolytes are more varying to analyze. 2, electrolyte is analyzed on the negative electrode: the electrolyte is not high in graphite and other pithonal carbon negatives, and it is easy to react if it is irreversible. The electrolytic solution analysis at the time of primary charge and discharge will form a passivation film on the surface of the electrode, and the passivation film can prevent further analysis of electrolyte and carbon negative electrode.

Thus, the structural stability of the carbon negative electrode is maintained. Ideally, the reduction of the electrolyte is limited to the formation stage of the passivation film, and the process no longer occurs when the cycle is stable. The reduction of the formation of the electrolyte salt of the passivation film is involved in the formation of the passivation film, which facilitates the stabilization of the passivation film, but the dissolved material that is reduced to the solvent is adversely affected by the solvent reduction product; (2) electrolyte salt reduction The concentration of the electrolytic solution was reduced, and finally caused battery capacity (LiPF6 reduction to generate LIF, LiXPF5-X, PF3O and PF3); (3) The formation of the passivation film is to consume lithium ions, which can cause the polar capacity to be imbalanced.

The entire battery is reduced. (4) If there is crack on the passivation film, the solvent molecule can be transferred to make the passivation film thickened, which not only consumes more lithium, but it is possible to block the micropores on the surface of the carbon, resulting in lithium unable to embed and discharged Resulting in irreversible capacity loss. Add some inorganic additives, such as CO2, N2O, CO, SO2, etc.

, can accelerate the formation of the passivation film, and can inhibit the symbolization and analysis of the solvent, and the addition of the crown ether organic additive has the same effect, wherein 12 crown 4 ether is best. Factors of film-forming capacity loss: (1) Type of carbon; (2) electrolyte ingredients; (3) additives in electrode or electrolyte. BLYR believes that the ion exchange reaction advances from the surface of the active material to its core, the formed new phase is buried, and the surface of the particles form a low ion and electron conductivity, so the spinel after storage.

More polarization than storage. ZHANG discovers the comparative decomposition of the AC impedance spectrum before and after the electrode material, with the new number of cycles, the resistance of the surface passivation layer has increased, and the interface capacitance is reduced. Reflecting the thickness of the passivation layer is added with the number of cycles.

The dissolution of manganese and the analysis of the electrolyte result in the formation of the passivation film, and the high temperature conditions are more conducive to these reactions. This will cause an indirect resistance of the active material particles and the increase in Li + migration resistance, thereby increasing the polarization of the battery, and the charge and discharge is not complete, and the capacity is reduced. II electrolytic solution reductant mechanism electrolyte often contains impurities such as oxygen, water, carbon dioxide, and oxidative reactions occur during battery charge and discharge process.

The reduction mechanism of the electrolyte includes solvent reduction, electrolyte reduction and impurity reduction three aspects: 1, the reduction of the solvent reduction PC and EC includes an electron reaction to the second electronic reaction process, the second electron reaction forms Li2CO3: FONG, etc., in the first During the discharge process, the electrode potential is close to O.8V (vs.

li/li +), PC / EC generates electrochemical reaction on graphite, producing CH = CHCH3 (G) / CH2 = CH2 (G) and LiCO3 (s) , Resulting in irreversible capacity loss on graphite electrodes. Aurbach et al for a wide variety of electrolyte reduction mechanism and its products on a metal lithium electrode and carbon-based electrode, found that RocO2Li and propylene occurred in an electronic reaction mechanism of PC. Roco2li is very sensitive to trace water.

Tight product is Li2CO3 and propylene, but there is no Li2CO3 in the drying case. Ein-Eliy reported that an electrolyte made of diethyl carbonate (DEC) and diomethymethane (DMC), the reaction reaction occurs in the battery, and methyl carbonate (EMC) is formed, and there is a certain loss of capacity loss. Impact.

2, the reduction reaction of the reduction electrolyte of the electrolyte is generally considered to be involved in the formation of the surface of the carbon electrode, and therefore, the types and concentrations thereof will affect the performance of the carbon electrode. In some cases, the reduction of the electrolyte contributes to the stability of the carbon surface, and can form the desired passivation layer. It is generally believed that the supporting electrolyte is easier to reduce than the solvent, and the reduction product inclusion in the negative electrode deposited film and affects the capacity attenuation of the battery.

Several reduction reactions that support electrolytes may occur as follows: 3, the water content in the Impurity reduction (1) The water content in the electrolyte will produce LiOH (S) and Li2O deposition layers, which is not conducive to lithium ion embedding, causing irreversible capacity loss: H2O + E→OH- + 1 / 2H2OH- + Li +→LiOH (s) LiOH + Li ++ E-→Li2O (S) + 1 / 2H2 produces LiOH (S) to deposit the surface of the electrode, form a large surface film having a large resistance, hindering Li + embedded graphite electrodes, resulting in irreversible capacity loss. Medium water in the solvent (100-300×10-6) There is no effect on graphite electrode performance. (2) CO2 in the solvent can be reduced on the negative electrode to form CO and LiCO3 (S): 2CO2 + 2E- + 2LI +→Li2CO3 + COCO will increase the battery in the battery, while Li2CO3 (S) increases battery resistance increases battery performance.

(3) The presence of oxygen in the solvent also forms Li2O because the potential difference between the metal lithium and the carbon of completely parallel lithium is small, and the reduction of the electrolyte on carbon is similar to the reduction in lithium. Originally 3: Self-discharge self-discharge means that the battery is naturally lost in unused state. Lithium-ion battery self-discharge results in two cases: one is reversible capacity loss; the second is the loss of irreversible capacity.

The reversible capacity loss means that the capacity of the loss can be recovered during charging, and the non-reversible capacity loss is reversed, and the positive and negative electrode may be used in micro-cell use with the electrolyte in the charging state, and lithium ion is embedded and deserted, positive and negative embedding and off. The lithium ions embedded are only related to the lithium ions of the electrolyte, and the positive and negative electrode capacity is therefore unbalanced. This part of the capacity loss cannot be recovered when charging.

Such as: Lithium manganese oxide positive electrode and solvent can generate self-discharge caused by self-discharge: solvent molecules (e.g., PC) are oxidized as microbial cells on the surface of conductive material carbon black or current fluid: the same, negative electrode active substance It may be self-discharged from the electrolytic solution to the electrolyte, and the electrolyte (such as LiPF6) is reduced by the electrolyte (such as LiPF6).

The lithium ion is removed from the negative electrode of the microcontroller as the negative electrode of the charging state: self-discharge Factors: Production process of positive electrode materials, battery production process, electrolyte properties, temperature, time. The self-discharge rate is tightly controlled by solvent oxidation rate, so the stability of the solvent affects the storage life of the battery. The oxidation of the solvent occurs in the surface of the carbon black, and the carbon black surface area can control the self-discharge rate, but for the LIMN2O4 positive electrode material, reduce the surface area of ​​the active material is also tightly, and the current collector surface faces the use of solvent oxidation can not be ignored.

The current leaked by the battery diaphragm can also cause self-discharge in the lithium ion battery, but the process is limited by the diaphragm resistance, at a very low rate, and has nothing to do with the temperature. Considering that the self-discharge rate of the battery is strongly relying strongly on the temperature, this process is not a critical mechanism in self-discharge. If the negative electrode is in the state of sufficient electricity, the contents of the battery are destroyed, which will result in permanent capacity loss.