Forming, today's article may be longer, I think this process is worthy of detailed explanation, because the quality of this process directly determines the quality of the battery, the complexity of this process, many have not yet been explored I am more inclined to the actual production process. If there is any improper theory, please correct me.

  By charging the battery after high temperature aging for the first time, the battery is activated, and the SEI film is formed on the surface of the negative electrode (in fact, the positive electrode also has the SEI film, but the effect is not obvious, we generally do not discuss it), it is okay to know what the SEI film is, you can It is understood that a film on the surface of the electrode material is formed by consuming lithium ions by the electrolyte, and is capable of blocking a thin film through which electrons and solvent molecules pass.

The influence of the formation on the performance of the battery is particularly important. In particular, suitable formation parameters have a great influence on the interface state of the pole piece. In actual production, the voltage point at which the pole piece is decomposed is usually determined by a different voltage. In addition, if CVC (constant current and constant voltage) is used for charging, it is generally charged at 0.1C, 0.2C or 0.3C. Here, C refers to the charge and discharge rate. Simple understanding 1C means that a battery is charged or discharged in one hour. 0.2C takes five hours to fill or finish. The overall voltage consistency is good. When CC (constant current) charging is used, the two sides are affected by temperature, but the capacity is not obvious after volume sharing. The abnormal battery can be screened by combining the voltage and the formation time double screening standard to prevent the defective product from flowing into the back.

Using a popular stick figure to express the charging process is: One day, lithium ions and electrons are discussed with the army, occupying the negative pole together. After observing the terrain and fully studying the strategy, it was decided that the two-way lithium-ion belt army would advance from the front and the electronic belt would be copied from the army side. As the commander ordered (charging), lithium ions and electrons rushed to the negative pole. After a fierce battle, they took the negative pole, stayed inside and ate and drank, waiting for a command to go back.

(1) Chemical gasification into gas production: When the battery electrolyte is 1 mol/L LiPF6-EC ~ DMC ~ EMC (three volume ratio 1:1:1), the formation voltage is less than 2.5 V, resulting The gas is mainly H2 and CO2; when the formation voltage is 2.5 V, the EC in the electrolyte begins to decompose, and the voltage is in the range of 3.0 to 3.5 V. Due to the reductive decomposition of EC, the gas generated is mainly C2H4; When the voltage is greater than 3.0 V, due to the decomposition of DMC and EMC in the electrolyte, in addition to the production of C2H4 gas, alkane gases such as CH4 and C2H6 also begin to appear; after the voltage is higher than 3.8 V, DMC and EMC The reductive decomposition becomes the main reaction. In addition, when the formation voltage is between 3.0 and 3.5 V, the amount of gas generated during the formation is the largest; after the voltage is greater than 3.5 V, since the SEI layer on the surface of the negative electrode of the battery is substantially formed, The reductive decomposition reaction of the electrolyte solvent is suppressed, and the amount of gas generated is also rapidly decreased.

(1) The main reason for the gas generated during the formation of the battery is that the SEI layer is formed on the surface of the electrolyte and the electrode at the initial discharge, and the solvent system of the electrolyte is decomposed to generate a hydrocarbon gas, and the type of the gas is related to the composition of the electrolyte.

(2) During the storage phase, a small number of batteries are inflated. The reason for the gas generation may be: First, due to poor battery sealing performance, external moisture and air infiltration, resulting in a significant increase in CO2 in the gas, and at the same time a considerable amount of O2 and N2. At the same time, the infiltration of water leads to the generation of HF, which will destroy the SEI layer. The second is that the SEI layer formed for the first time is unstable. The SEI layer is destroyed during the storage stage. In order to repair the SEI layer, the gas is released again, mainly based on hydrocarbon gas. .

(2) SEI film 1. Formation mechanism of SEI When the lithium ion battery is charged and discharged for the first time, a small amount of polar aprotic solvent in the electrolyte undergoes a reduction reaction after partial electrons are obtained, and reacts with lithium ions to form a thickness of about 100-120 nm. The interface film, this film is SEI. The SEI is usually formed at a solid-liquid phase interface between the electrode material and the electrolyte. When the lithium ion battery begins to charge, lithium ions are removed from the positive active material, enter the electrolyte and penetrate the membrane and then enter the electrolyte, and finally embedded in the layered void of the negative carbon material, and the lithium ion completes a complete deintercalation behavior. At this time, electrons exit from the positive electrode along the outer end circuit and enter the negative carbon material. Oxidation-reduction reaction occurs between the solvent and the lithium ion in the electron and the electrolyte. The solvent molecule receives electrons and combines with lithium ions to form SEI and generates gases such as H2, CO, and CH2. As the thickness of the SEI increases, until the electrons cannot penetrate, a passivation layer is formed, which suppresses the continuation of the redox reaction.

2. What is the composition of SEI? The thickness of SEI is about 100-120 nm, and its composition varies with electrolyte composition. It is generally composed of Li2O, LiF, LiCl, Li2CO3, LiCO2-R, alkoxide and non-conductive polymer. It is a multi-layer structure close to the electrolyte. One side is porous and the side close to the electrode is dense. 3. What is the impact of SEI on lithium batteries?

  The role of SEI is to be analyzed from its own characteristics.

Its characteristic is: 1SEI is an interface layer between the electrode material and the electrolyte, which separates the two. 2 has the characteristics of a solid electrolyte; 3Li+ can pass smoothly (good conductor of lithium ion), but electrons cannot pass. SEI has an important influence on the performance of carbon negative lithium ion batteries.

First, the SEI is completed between the first charge and discharge, and the consumption of lithium ions is accompanied by the consumption of lithium ions. The lithium ion is consumed to increase the irreversible capacity of the battery, thereby reducing the charge and discharge efficiency (Coulomb efficiency) of the electrode material.

Second, the SEI film has an organic solvent insolubility and can be stably present in an organic electrolyte solution. PC exists in some electrolytes, PC is easy to be embedded in the anode material to cause damage to the electrode material, and if a suitable admixture can be added to the electrolyte to promote the formation of SEI, the co-insertion of solvent molecules can be effectively prevented, and the solvent is avoided. The molecular co-intercalation causes damage to the electrode material, thereby greatly improving the cycle performance and service life of the electrode.

Third, SEI allows lithium ions to pass through and prohibits the passage of electrons. On the one hand, it ensures the continuous charging and discharging cycle of the rocking chair, and on the other hand, it hinders the further consumption of lithium ions and improves the service life of the battery.

4. Influencing factors of SEI formation The formation of SEI is mainly affected by electrolyte (Li salt, solvent, admixture, etc.), chemical formation (first charge and discharge) current, temperature and other factors.

First, the influence of electrolyte composition. The difference in Li salt and solvent composition results in different SEI components, and the stability of the product is different. Second, the impact of the formation of current. When the charging current is large, the high-potential inorganic component is formed first, followed by the insertion of lithium ions, and finally the formation of organic components. When the formation current is small, the organic component of the SEI film begins to form rapidly. Third, the lithium ion battery formed at -20 ° C formed SEI dense and has a low impedance, which is very beneficial to the battery life.

Excessive temperatures can reduce the stability of the SEI and affect battery cycle life. In addition, the thickness of the SEI is also affected by the type of negative electrode material. 5. Reaction of SEI under thermal runaway of lithium battery SEI consists of two layers of materials. The inner layer is mainly composed of Li2CO3, and the outer layer is mainly composed of lithium alkyl carbonate such as (CH2OCOLi)2. When the internal temperature of the battery is 80-120 ° C, the outer layer gradually decomposes, releasing heat to generate gas, and the reaction equation is as follows. In the SEI pyrolysis reaction, the reaction temperature and exotherm are related to the type of lithium salt, the composition of the solvent, the active material of the negative electrode, and the number of cycles of the battery. (CH2OCOLi)2→ Li2CO3+CH2=CH2+1/2O2+CO2Li+(CH2OCOLi)2→ 2Li2CO3+CH2=CH26, SEI film of positive electrode The latest research shows that there is also film formation at the solid-liquid interface between the positive electrode material and the electrolyte. The thickness of the film is much thinner than that of the negative SEI film, which is about 1-2 nanometers. Since the potential of the positive electrode material is high, the reduction product of the organic electrolyte is unstable, and the inorganic product such as LiF can be stably present, and becomes a main component of the positive SEI film.

(III) SOC Estimation The estimation of SOC (state of charge) state of charge mainly includes discharge experiment method, ampere-hour measurement method, open circuit voltage method, internal resistance method, linear model method, Kalman filter method and neural network method. 1 Discharge test method The target battery is subjected to continuous constant current discharge until the battery cut-off voltage, and the time used for the discharge process is multiplied by the magnitude of the discharge current, that is, the remaining capacity of the battery. The method is relatively simple and reliable, and the result is relatively accurate, and is effective for different types of batteries.

There are two main disadvantages. First, the test process takes a lot of time. Second, the battery needs to be removed from the electric car when using this method. Therefore, this method cannot be used to calculate the battery under working conditions. 2 An hour measurement method (current integration method, ampere-hour integration method) The discharge capacity of the battery at different currents is equivalent to the discharge capacity at a specific current. The main idea is the peukert equation (not understood). The method is relatively simple, focusing only on the external features of the system. In the electricity estimation, only the amount of electricity flowing in and out is concerned. However, this method does not obtain the relationship between the SOC and the discharge amount from the inside of the battery, but only records the charge and discharge electric quantity, which causes the SOC accumulation error, and the result is low in accuracy, and the method cannot determine the initial value of the battery. 3 Open circuit voltage method According to the relationship between the open circuit voltage of the battery and the lithium ion concentration inside the battery, the one-to-one correspondence between him and the SOC is indirectly fitted. It is necessary to discharge a fully charged battery at a fixed discharge rate (generally 1c), and a relationship curve between ocv and SOC is obtained according to the discharge process. In actual operation, the SOC is determined by voltage and is valid for all batteries.

However, there are two drawbacks. First, the target battery must be allowed to stand for more than 1 hour before measuring the ocv, so that the internal electrolyte of the battery is evenly distributed to obtain a stable terminal voltage. Second, the battery is at different temperatures and different lifetimes, although The open circuit voltage is the same, but the actual SOC difference may be large. The long-term use of this method does not guarantee complete accuracy. It is therefore not suitable for batteries in operation. 4 Internal resistance method Exciting the battery with alternating current of different frequencies, measuring the internal AC resistance of the battery, and obtaining the SOC estimation value through the established calculation model. The state of charge obtained by this side reflects the SOC value of the battery under a certain constant current discharge condition. Since there is no one-to-one correspondence between the SOC and the internal resistance of the battery, it is impossible to accurately model with a mathematical model. Therefore, this method is rarely used with electric vehicles. 5 Linear model method This method establishes a linear model based on the SOC's variation, current, voltage and SOC value at the previous time point. This model is used in the case of low current, SOC ramping, initial conditions for measurement error and error, Has a strong robustness. Theoretically, it can be applied to different stages of different batteries, but it is currently only applied to lead-acid batteries. Since the relationship between changed SOC and current and voltage is not universal, the applicability on other batteries and the estimation effect of variable current conditions are Further research is needed. 6 Kalman Filtering Method Based on the An-Time integration method, the main idea is to make the optimal estimation of the state of the dynamic system in the sense of minimum variance. The method is applied to battery SOC estimation, and the optimal estimation of complex systems can be made according to the principle of minimum mean square error. Predict-measure-correction mode to eliminate interference and deviation.

However, there are two disadvantages. First, the accuracy of SOC estimation depends largely on the accuracy of the battery model. If the model is inaccurate, the estimation result is not necessarily reliable. Second, the algorithm involved in this method is very complicated. Great, the calculation cycle required is long, and a single piece with high computing power is required.

7 The neural network method simulates the new algorithm of the human brain and its neurons for processing nonlinear systems. It is not necessary to study the internal structure of the battery in depth, and only a large number of input and output samples corresponding to its working characteristics are extracted from the target battery in advance, and The input to the system established using this method can obtain the SOC value in operation. The method is simple in post processing, reduces errors, and acquires dynamic parameters in real time.

However, the pre-work volume of this method is large, and it is necessary to extract a large amount of comprehensive target sample data to train the system. The input training data and training methods will greatly affect the estimation accuracy of SOC, and the long-term use accuracy will decrease. less. In practical applications, the correction of adding some influence factors based on the integration method of the time is adopted, and the disadvantage is that the error is large. The future is mainly improved from four aspects. First of all. Through a large number of experiments, a wealth of data is created, so that data can be checked; secondly, relying on hardware technology to improve measurement accuracy such as current and voltage; third, introducing an accurate battery model to more realistically characterize the dynamics of the battery during use. Characteristics; Finally, a variety of algorithms are integrated to complement each other, minimizing errors in different states and improving their estimation accuracy.