Application of electrochemical simulation technology in 3.7 volt battery 18650 research

With the introduction of timetables for banning the sale of fuel vehicles in various countries, the status of new energy vehicles has become increasingly stable. As the core power source of electric vehicles, lithium-ion batteries are becoming more and more popular in the market. The production process of lithium-ion batteries involves the selection and matching of materials such as positive electrodes, electrolytes, negative electrodes, and separators, as well as the selection of pole piece design parameters. The working process of the battery involves processes such as chemical reactions, mass transfer, conduction, and heat generation. It can be seen that lithium-ion battery is a very complex system.

Using experimental methods to explore lithium-ion batteries is an effective method. Especially with the continuous advancement of characterization methods, we can get more and more information about the impact of design parameters, working conditions, etc. on battery performance. It is undeniable that in the development process of lithium-ion batteries, there are too many design parameters and the experimental tasks are heavy; the impact of each parameter on the battery performance is unclear, and the experimental design is blind to a certain extent, which sometimes even consumes time, effort and money. But it’s a thankless phenomenon. The opportunity to improve this situation is to apply battery simulation technology to batteries.

Lithium-ion battery simulation technology can use equivalent circuit models, semi-empirical models, electrochemical models, etc. Simulation technology based on electrochemical models can well solve the problems mentioned above. As a supplement to the experiment, electrochemical simulation can simulate various solutions before the experiment to eliminate the unnecessary and retain the essential; it can also simulate the charging and discharging process of the battery under different working conditions, helping researchers to understand the internal process of the battery. ;At the same time, the experimental results can also point out the shortcomings of simulation and promote the continuous development of simulation models. It can be said that simulation makes experiments even more powerful, and experiments make simulation even better.

Let’s briefly talk about the electrochemical model. The electrochemical model is mainly composed of three processes: mass transfer, conduction and electrochemical reaction. Its control equation is shown in the table below. In terms of complexity, electrochemical models include single-particle models, quasi-two-dimensional models, two-dimensional models, and three-dimensional models. Commonly used is the quasi-two-dimensional model. Based on this model, it can achieve various purposes including battery design, charge and discharge performance, battery internal resistance (polarization) analysis, etc. When predicting battery life, in order to reduce the amount of calculation, single-particle models are often used.

1. Application of simulation technology in battery design

In the battery design process, in addition to the inherent property parameters of the positive and negative electrode materials, electrolyte and separator, many design parameters also need to be considered, such as the particle size of the positive and negative electrodes (r), the thickness of the electrode piece (L), and the porosity of the electrode piece. (ε) etc. MarcDoyle et al. used simulation technology to simulate the rate of Sony's LiCoO2/EC, PC, and LiPF6/graphite batteries, and the battery rate performance obtained was very similar to the test results. The figure below is a comparison of the test results and simulation results of charge and discharge curves at different rates.

VenkatSrinivasan et al. used simulation technology to study the impact of particle size on the power density of LiFePO4 half-cells. They found that using small particle size cathode materials is beneficial to improving the power density of the battery, providing a direction for the development of high-power batteries. The author also used the discharge platform of LiFePO4 to mark the ohmic overpotential, reaction overpotential and diffusion overpotential of the battery during the constant current discharge process, and found the reason why the platform becomes a slope during high-rate discharge. At the same time, it provides a method for reducing the internal resistance of the battery. ideas.

During the battery development process, you can first use a model to understand the relationship between each design parameter and battery performance, determine the main influencing factors, and then conduct experiments based on these factors, which can greatly reduce the amount of experiments.

2. Simulation of side reactions and lithium evolution in batteries

When side reactions cannot be ignored in the model, a Bulter-Volmer formula describing the side reactions needs to be added, as shown below. Of course, if side reactions cause other changes, such as an increase in the thickness of the film on the particle surface, an increase in resistance, etc., additional considerations are required.

In the LiMn2O4 half-cell, when studying the self-discharge caused by the co-intercalation side reaction (irreversible) of the electrolyte solvent (PC) and lithium ions, the low-speed CV curve is used as the model calibration standard, and the transfer coefficient of the side reaction is used as a variable parameter. For batteries with different active material loadings, the obtained side reaction transfer coefficients are different. Side reactions in batteries are difficult to control and monitor. Using models and parameter identification to obtain physical and chemical parameters related to side reactions is sometimes an effective means.

Lithium precipitation is one of the main culprits causing battery safety and capacity degradation. Theoretically, lithium will be precipitated when the lithium potential is lower than 0V. In fact, because the reaction requires driving force, there will be a certain overpotential, causing the lithium precipitation potential of the negative electrode to deviate from 0V. In the lithium evolution model, in addition to adding a BV formula describing the lithium evolution reaction, the impact of lithium deposition on capacity and the impact of the deposition layer on the particle surface film must also be considered. Research on LiMn2O4/graphite full batteries found that N/P is an effective method to inhibit lithium evolution. The larger the particle size, the easier it is for lithium precipitation. The thicker the pole piece, the easier it is for lithium precipitation. Lithium evolution mainly occurs at the end of constant current charging. , the lithium evolution phenomenon rapidly weakens and disappears during the constant voltage stage. The figure below shows the influence of pole piece thickness, particle size, and charging cut-off voltage on the amount of lithium evolution.

In addition, simulation technology can be used to explore other side reactions, such as the decomposition of the electrolyte, the formation of the SEI film on the negative electrode, and the generation of irreversible products in the electrode.

3． Battery internal resistance

DCR and EIS are commonly used to describe battery internal resistance. Both types of internal resistance can be described by models.

The EIS test process requires the use of small disturbance signals to ensure that the system maintains a (quasi-) steady state and that the input signal and the output signal have a linear relationship. Therefore, during the modeling process, it is considered that the battery is in a steady state process and has a linear response. Based on these assumptions and the fact that impedance has real and imaginary parts, the governing equations of the electrochemical model are modified to obtain the EIS model. With the help of EIS simulation, the impact of processes such as diffusion and electrochemistry on EIS can be studied; the impact of the electrochemical activity and conductivity of electrode materials on EIS can also be studied; and the conditions of the two electrodes of the full battery can also be investigated separately. It is a very convenient method.

To put it simply, the simulation of DCR is to change the charge and discharge mode in the electrochemical model, from constant current charging or discharging to pulse charging or discharging. Andreas Nyman gave the polarization calculation formula when analyzing various polarizations of the battery in his article, and based on this calculation, he calculated the different polarities in the LiNi0.8Co0.15Al0.05O2/LiPF6, EC: EMC3:7/MAG-10 system. The proportion occupied by . Very helpful for us to understand the internal polarization of batteries. The figure below shows the decomposition results of polarization in the article.

Internal resistance is a very important performance indicator of the battery, which has an important impact on the battery's fast charging, heat production, and aging. If the ohmic internal resistance, reaction internal resistance, and diffusion internal resistance of the positive and negative electrodes and electrolyte in the battery can be clearly analyzed through the model, it will be very beneficial to improve battery performance.

4． Battery life prediction

There are many reasons for the capacity attenuation of lithium-ion batteries, such as material structure collapse, side reactions consuming lithium, SEI consuming lithium and the resulting increase in internal resistance, lithium precipitation, etc. For the convenience of calculation, only one or two attenuation causes are considered in the general model.

The single-particle model is a simplification of the quasi-two-dimensional model: it is considered that all active particles in the pole piece are the same, that is, their internal lithium ion concentration distribution and the external environment are the same.

When attributing the lifetime decay to the reduction of the electrolyte solvent, which consumes lithium and causes the negative electrode film resistance to increase, GangNing et al. quantitatively studied the effect of depth of discharge (DOD) on the discharge cut-off voltage, and the charge cut-off voltage on Li loss and internalization. The impact of resistance, as well as the impact of cycle times on capacity and internal resistance. The simulation results are consistent with our basic understanding of batteries and have the advantage of quantifying these effects.

Some researchers believe that lithium precipitation occurs in most charging and discharging processes, and the inflection point in the battery capacity decay rate (transitioning from a linear attenuation zone to a nonlinear attenuation zone) is caused by lithium precipitation. The basic idea is: in the first few cycles of the cycle, the formation of the SEI film causes the local porosity of the negative electrode near the separator to decrease, increasing the local electrolyte potential gradient, creating conditions for lithium evolution; and lithium evolution further causes the porosity to decrease. , forming a positive feedback, eventually leading to exponential decay of capacity. Based on this consideration, a lifespan model of capacity fading caused by SEI growth and lithium evolution is established. When this model predicts the life of NCM622/EC/EMC (3:7bywt.)+2%wt%VC/graphite system, although there are some small errors in the prediction of the charge and discharge curve during the cycle (as shown on the left below), the flaws are not concealed; for The capacity prediction accuracy during the cycle is high (right picture below). The model results show that the potential of the negative electrolyte gradually increases from the separator/negative end to the negative electrode/copper foil end; the electrode potential distribution is also in line with this trend. There is no lithium evolution in fresh batteries, and lithium evolution has occurred when the cycle reaches 1000 cycles. ; Lithium precipitation first occurs in the negative electrode area close to the separator. At the end of constant current charging, the negative electrode potential is the lowest, and lithium precipitation is most likely to occur.

Additionally, models can be used to estimate capacity losses. For example, it is assumed that the capacity loss mainly comes from the deviation of the positive and negative electrode SOC intervals and the loss of active materials during the charge and discharge process, and the positive and negative electrode SOC and positive and negative electrode active material contents at the beginning of discharge are used as variables, and the activity is identified through the life model and the measured discharge curve. The amount of material loss and the SOC of the positive and negative electrodes at the beginning of discharge. The contribution of positive and negative electrodes to capacity fading can be quantitatively analyzed.

The battery life is predicted through the electrochemical model. Although the model is relatively complex, it is highly accurate because it is based on the actual internal process of the battery. Using models to explore the main causes of capacity loss is faster and more convenient than disassembling and testing batteries after cycles.

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