Method for analyzing overcharge type thermal runaway of lithium ion battery

Abstract

The invention discloses a lithium ion battery overcharge thermal runaway analysis method, which comprises the following steps: carrying out an adiabatic overcharge thermal runaway experiment on a lithium ion battery, and recording state parameters of the lithium ion battery in real time; and performing stage division on the adiabatic overcharge thermal runaway process of the lithium ion battery according to the change of the state parameters of the lithium ion battery, determining the states of the lithium ion battery corresponding to different stages, and evaluating the thermal runaway risk levels of the lithium ion battery at different stages. When the lithium ion battery is applied to power equipment such as a new energy automobile, analysis can be assisted by the analysis method once a thermal runaway accident occurs, so that the investigation process of the thermal runaway accident can be accelerated and simplified in an assisted manner, and the thermal runaway accident can be investigated efficiently at low cost. Meanwhile, the analysis method can assist in monitoring the charging state of the lithium ion battery so as to ensure the use safety of the lithium ion battery and reduce the occurrence of thermal runaway accidents.

Description

Technical Field

[0001] This application relates to the technical field of batteries, and particularly to an analysis method for overcharge-induced thermal runaway of lithium-ion batteries. Background Art

[0002] Lithium-ion batteries have the characteristics of high energy density, high power density, long service life, etc., and are the core components of new energy vehicles. Their safety performance, especially the core performance of thermal safety, has received extensive attention from the whole society. In recent years, thermal runaway accidents of the battery systems of new energy vehicles have emerged in an endless stream. Based on the investigation process of thermal runaway accidents, thermal runaway accidents are mainly divided into three stages: thermal runaway induction, thermal runaway occurrence, and thermal spread. Among them, thermal runaway induction includes three types: mechanical abuse, thermal abuse, and electrical abuse. Overcharging of lithium-ion batteries is a common manifestation of electrical abuse.

[0003] Overcharging of lithium-ion batteries can be caused by high-current charge and discharge, battery management system failures, inconsistent battery internal resistances, etc. Overcharging of lithium-ion batteries can cause a decrease in battery capacity or battery failures at best, and at worst, lead to thermal runaway, resulting in accidents such as fires and explosions. At present, there are various studies on the overcharging of lithium-ion batteries. For example, Ouyang et al. studied the overcharge-induced capacity decay mechanism of nickel cobalt manganese-graphite lithium-ion batteries (NCM-Gr LIBs) based on an EV-ARC calorimeter, and proposed that the reaction between lithium in the negative electrode and the electrolyte would thicken the SEI film and increase the ohmic impedance. Wang et al. charged 3Ah NCM-Gr LIBs to 2.8 / 3.8 / 4.2 / 4.6 / 5.0V, and then used an inductively coupled plasma mass spectrometer to measure the cathode material and calculate the lithium loss of the cathode material under different charging states. Ye et al. used a lithium-ion battery charge and discharge cabinet and ARC in combination to explore the energy contributions of different charging currents of 0.1 / 0.2 / 0.5 / 1.0 / 2.0C to thermal runaway, which were 82%, 84%, 80%, 60%, and 40% respectively.

[0004] However, the above studies are all based on small soft-pack batteries, and there is no research on overcharge-induced thermal runaway of prismatic aluminum shell / cylindrical battery cells commonly used in new energy vehicles with a relatively high market retention rate, so it is impossible to know the impact of overcharging on lithium-ion batteries in new energy vehicles. Summary of the Invention

[0005] To solve the defects in the above-mentioned prior art, this application aims to provide an analysis method for overcharge-induced thermal runaway of lithium-ion batteries. By analyzing the change of the state parameters of lithium-ion batteries over time during the overcharging process, the overcharge thermal runaway process of lithium-ion batteries is stage-divided, the impact on lithium-ion batteries in each stage is determined, and the risk level is evaluated to assist in the analysis of thermal runaway accidents.

[0006] The present application provides an analysis method for overcharge thermal runaway of a lithium-ion battery. The analysis method includes the following steps:

[0007] Conduct an adiabatic overcharge thermal runaway experiment on the lithium-ion battery, and record the state parameters of the lithium-ion battery in real time;

[0008] Divide the adiabatic overcharge thermal runaway process of the lithium-ion battery according to the change of the state parameters of the lithium-ion battery, determine the state of the lithium-ion battery corresponding to different stages, and evaluate the thermal runaway risk level of the lithium-ion battery in different stages.

[0009] As a preferred implementation manner, in the present application, an overcharge thermal runaway experiment is conducted on the lithium-ion battery in an adiabatic environment. At the same time, observe and record the voltage, temperature, bulging degree, mass, and explosion-proof valve state of the lithium-ion battery during the overcharge process. Then, divide the adiabatic overcharge thermal runaway process of the lithium-ion battery according to this feature, determine the state of the lithium-ion battery corresponding to different stages, and evaluate the thermal runaway risk level of the lithium-ion battery in different stages.

[0010] As a preferred implementation manner, in the present application, an overcharge thermal runaway experiment is conducted on a lithium-ion battery with 100% SOC in an adiabatic environment, and each overcharge thermal runaway experiment is based on multiple lithium-ion batteries of the same model.

[0011] As a preferred implementation manner, in the present application, an overcharge thermal runaway experiment is conducted on lithium-ion batteries with 100% SOC and different SOHs in an adiabatic environment, and each overcharge thermal runaway experiment is based on multiple lithium-ion batteries of the same model and the same SOH.

[0012] As a preferred implementation manner, in the present application, the state parameters of the lithium-ion battery include voltage. According to the change of the voltage of the lithium-ion battery with time during the overcharge thermal runaway experiment, the overcharge thermal runaway process of the lithium-ion battery is divided into six stages, namely the voltage boost stage, the rising plateau stage, the voltage drop stage, the falling plateau stage, the sharp voltage rise stage, and the thermal runaway stage.

[0013] As a preferred implementation manner, in the present application, the state parameters of the lithium-ion battery include temperature. According to the change of the temperature of the lithium-ion battery with time during the overcharge thermal runaway experiment, the overcharge thermal runaway process of the lithium-ion battery is divided into three stages, namely the stage where the temperature hardly changes, the stage where the temperature rises rapidly, and the stage where the temperature rises sharply.

[0014] As a preferred implementation manner, in the present application, the temperature of the lithium-ion battery includes the temperature of the positive electrode post, the temperature of the negative electrode post, the temperature of the explosion-proof valve, and the temperature of the battery side. The temperature of the battery side includes the temperature at the center of the large surface of the square battery and the temperature at the center of the side of the cylindrical battery.

[0015] As a preferred embodiment, in the present application, the thermal runaway risk level of the lithium-ion battery is determined according to four dimensions: the highest voltage of the rising platform, the lowest voltage of the falling platform, the highest temperature of thermal runaway, and the overcharge capacity at failure during the overcharge thermal runaway experiment, and is divided into three thermal runaway risk levels: low, medium, and high.

[0016] As a preferred embodiment, in the present application, the state parameters of the lithium-ion battery include the battery appearance. According to the change of the appearance of the lithium-ion battery over time during the overcharge thermal runaway experiment, the overcharge thermal runaway process of the lithium-ion battery is divided into four stages, which are, in sequence, the almost unchanged stage, the slightly bulging stage, the rapid expansion stage, and the valve-opening stage.

[0017] As a preferred embodiment, in the present application, the state parameters of the lithium-ion battery include the residual mass of the battery. According to the change of the mass of the lithium-ion battery over time during the overcharge thermal runaway experiment, the overcharge thermal runaway process of the lithium-ion battery is divided into two stages, which are, in sequence, the primary eruption stage and the secondary eruption stage.

[0018] The analysis method for overcharge-induced thermal runaway of the lithium-ion battery provided in the present application divides the overcharge thermal runaway process of the lithium-ion battery into stages by analyzing the change of the state parameters of the lithium-ion battery over time during the overcharge process, determines the influence of each stage on the lithium-ion battery, and simultaneously evaluates the risk level of each stage. In this way, when the lithium-ion battery is applied to power equipment such as new energy vehicles, once a thermal runaway accident occurs, this analysis method can be used to assist in the analysis to help speed up and simplify the investigation process of the thermal runaway accident, which is conducive to the efficient and low-cost investigation of the thermal runaway accident. At the same time, this analysis method can assist in monitoring the charging state of the lithium-ion battery to ensure the use safety of the lithium-ion battery and reduce the occurrence of thermal runaway accidents. Description of the Drawings

[0019] Figure 1 It is a graph showing the change of voltage and temperature over time of the lithium-ion battery in Example 1 during overcharge;

[0020] Figure 2 It is a schematic structural diagram of the lithium-ion batteries in Examples 1-5.

[0021] Reference Signs:

[0022] 1, positive terminal; 2, negative terminal; 3, explosion-proof valve; 4, large surface. Detailed Embodiments

[0023] For better understanding and implementation, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application.

[0024] In the description of the present application, it should be noted that the orientation or positional relationship indicated by the terms "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings. It is only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation to the present application.

[0025] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which this application belongs. The terms used in the description of this application herein are only for the purpose of describing specific embodiments and are not intended to limit this application.

[0026] The present application provides an analysis method for overcharge-induced thermal runaway of a lithium-ion battery, including the following steps: performing an adiabatic overcharge thermal runaway experiment on the lithium-ion battery and recording the state parameters of the lithium-ion battery in real time, then dividing the adiabatic overcharge thermal runaway process of the lithium-ion battery into stages according to the changes in the state parameters of the lithium-ion battery, determining the state of the lithium-ion battery corresponding to different stages, and evaluating the thermal runaway risk level of the lithium-ion battery in different stages.

[0027] The adiabatic overcharge thermal runaway experiment refers to overcharging the lithium-ion battery in an adiabatic environment until the lithium-ion battery undergoes thermal runaway. Conducting the overcharge thermal runaway experiment in an adiabatic environment is beneficial to accurately obtaining all the energy released by the lithium-ion battery during overcharging to ensure the accuracy of the overcharge thermal runaway experiment. The adiabatic environment can be realized by using a common adiabatic accelerating calorimeter. During the overcharging process, the voltage, temperature, etc. of the lithium-ion battery will change with time, and the internal chemical reactions of the lithium-ion battery are different. By analyzing the changes in the state parameters of the lithium-ion battery during the overcharging process, the safety state of the lithium-ion battery and its internal chemical reaction conditions can be determined, and thus the risk level of the lithium-ion battery can be judged.

[0028] Based on this, the analysis method for overcharge-induced thermal runaway of a lithium-ion battery provided by the present application divides the overcharge thermal runaway process of the lithium-ion battery into stages by analyzing the changes in the state parameters of the lithium-ion battery over time during overcharging, determines the impact on the lithium-ion battery in each stage, and evaluates the risk level of each stage. In this way, when the lithium-ion battery is applied to power equipment such as new energy vehicles, once a thermal runaway accident occurs, this analysis method can be used to assist in the analysis to help accelerate and simplify the investigation process of the thermal runaway accident, which is beneficial to investigating the thermal runaway accident efficiently and at low cost. At the same time, this analysis method can assist in monitoring the charging state of the lithium-ion battery to ensure the use safety of the lithium-ion battery and reduce the occurrence of thermal runaway accidents.

[0029] It should be noted that the overcharge thermal runaway experiments of this application mainly focus on prismatic aluminum shell batteries (cells) and cylindrical batteries (cells) commonly used in power equipment such as new energy vehicles, so as to facilitate the evaluation of the impact of overcharging on lithium-ion batteries in new energy vehicles. Of course, for other lithium-ion batteries such as soft-pack batteries and button batteries, the analysis method of this application is also applicable.

[0030] The analysis method of overcharge-induced thermal runaway of lithium-ion batteries in this application conducts overcharge thermal runaway experiments on lithium-ion batteries in an adiabatic environment, while observing and recording the voltage, temperature, swelling degree, mass, and explosion-proof valve state of the lithium-ion batteries during the overcharging process, and then dividing the adiabatic overcharge thermal runaway process of the lithium-ion batteries according to these characteristics, determining the states of the lithium-ion batteries corresponding to different stages, and evaluating the thermal runaway risk levels of the lithium-ion batteries in different stages.

[0031] During the overcharging process, the voltage, temperature, etc. of the lithium-ion battery will change with time and show obvious increases, and the chemical reactions inside the lithium-ion battery are different. Gas is generated inside the lithium-ion battery, and the internal resistance increases, which will cause the internal pressure of the lithium-ion battery to increase, resulting in battery swelling, and even cause the explosion-proof valve to be blown open. The electrolyte, high-temperature gas, etc. inside the lithium-ion battery will spray out from the explosion-proof valve. These state changes of the lithium-ion battery will all affect the safety of the use of the lithium-ion battery and are essential factors for evaluating its risk level.

[0032] Among them, this analysis method conducts overcharge thermal runaway experiments on lithium-ion batteries with 100% SOC in an adiabatic environment, and each overcharge thermal runaway experiment is based on multiple lithium-ion batteries of the same model. SOC is the current remaining charge of the lithium-ion battery, that is, the state of charge of the lithium-ion battery. When the lithium-ion battery is fully charged, its SOC is 100%. The lithium-ion batteries for the overcharge thermal runaway experiments are in a fully charged state, and on this basis, the lithium-ion batteries are continuously charged to overcharge. Moreover, selecting multiple lithium-ion batteries of the same model to conduct the same overcharge thermal runaway experiment is beneficial to establishing a model for the overcharge thermal runaway experiment process to ensure the accuracy of the evaluation.

[0033] This analysis method has conducted overcharge thermal runaway experiments on lithium-ion batteries with 100% SOC and different SOHs in an adiabatic environment, and each overcharge thermal runaway experiment is based on multiple lithium-ion batteries of the same model and the same SOH. SOH refers to the health state or service state of a lithium-ion battery, including parameters such as battery capacity, life, and performance. The SOH of a newly manufactured lithium-ion battery is 100%, and as the lithium-ion battery is continuously used, its SOH gradually decreases. The SOH of a completely scrapped lithium-ion battery is 0%. For lithium-ion batteries of the same model, the influence of different SOHs during overcharge thermal runaway experiments may be different. By conducting overcharge thermal runaway experiments on lithium-ion batteries of the same model and different SOHs under the same conditions in this application, the influence of different SOHs on the overcharge thermal runaway of lithium-ion batteries can be obtained, so as to more comprehensively analyze the influence of overcharge thermal runaway on lithium-ion batteries. Moreover, multiple lithium-ion batteries of the same model and the same SOH are selected in this application to conduct the same overcharge thermal runaway experiment, which is more conducive to establishing a model for the process of this overcharge thermal runaway experiment to ensure the accuracy of the evaluation.

[0034] Based on the above analysis method, the state parameters of the lithium-ion battery include voltage. According to the change of the voltage of the lithium-ion battery with time during the overcharge thermal runaway experiment, the overcharge thermal runaway process of the lithium-ion battery is divided into six stages, namely the voltage increase stage, the rising plateau stage, the voltage decrease stage, the falling plateau stage, the sharp voltage increase stage, and the thermal runaway stage. During the overcharge process, the voltage of the lithium-ion battery will first rise to a stable voltage, then the voltage will slightly decrease to a stable voltage, and then the voltage will increase sharply, even exceeding the measurement range of the charge and discharge cabinet, resulting in thermal runaway.

[0035] The state parameters of the lithium-ion battery include temperature. According to the change of the temperature of the lithium-ion battery with time during the overcharge thermal runaway experiment, the overcharge thermal runaway process of the lithium-ion battery is divided into three stages, namely the stage where the temperature is almost unchanged, the stage where the temperature rises rapidly, and the stage where the temperature rises sharply. During the overcharge process, the temperature of the lithium-ion battery is almost unchanged in the early stage, then there is a process where the temperature rises rapidly, and finally the temperature will rise sharply to above 600 °C, resulting in thermal runaway.

[0036] Among them, the temperature of the lithium-ion battery includes the temperature of the positive terminal, the temperature of the negative terminal, the temperature of the explosion-proof valve, and the temperature of the battery side. The temperature of the battery side includes the temperature at the center of the large surface of the square battery and the temperature at the center of the side of the cylindrical battery. In order to more accurately obtain the variation law of the temperature of the lithium-ion battery during overcharging, the positive terminal, negative terminal, explosion-proof valve, and battery side of the lithium-ion battery are all monitored for temperature, which is conducive to comprehensively understanding the temperature change of the lithium-ion battery during overcharging, so as to analyze its internal chemical reaction to determine the state and risk level of the lithium-ion battery at different times. The center area of the battery side is preferably selected, and the bulging is most obvious in this center area during thermal runaway.

[0037] The state parameters of the lithium-ion battery include the battery appearance. According to the change of the appearance of the lithium-ion battery with time during the overcharging thermal runaway experiment, the overcharging thermal runaway process of the lithium-ion battery is divided into four stages, namely, an almost unchanged stage, a slightly bulging stage, a rapidly expanding stage, and a valve-opening stage. During overcharging, due to the continuous generation of gas and increasing pressure inside the lithium-ion battery, the lithium-ion battery will bulge, and the state and risk level of the lithium-ion battery can be judged through this bulging situation.

[0038] The state parameters of the lithium-ion battery include the remaining mass of the battery. According to the change of the mass of the lithium-ion battery with time during the overcharging thermal runaway experiment, the overcharging thermal runaway process of the lithium-ion battery is divided into two stages, namely, a primary eruption stage and a secondary eruption stage. During overcharging, as the temperature of the lithium-ion battery continues to rise, the gas generation continues to increase, and the internal pressure continues to rise, which will cause the explosion-proof valve to be opened, and the electrolyte, high-temperature gas, etc. inside the lithium-ion battery will spray out from the explosion-proof valve, resulting in a decrease in the battery mass. According to the decrease in the battery mass, it can be determined whether the lithium-ion battery has experienced a primary eruption or a secondary eruption.

[0039] The acquisition of the above voltage and temperature data can be achieved through voltage sensors, temperature sensors, etc. The remaining mass of the battery can be obtained by weighing, and the battery appearance can be observed with the naked eye or known through image and video monitoring. The change data of these state parameters with time can be obtained and recorded in real time by a computer to accurately know the change law for facilitating thermal runaway analysis.

[0040] Based on the above state parameters, the thermal runaway risk level of the lithium-ion battery is determined according to four dimensions: the highest voltage of the rising platform, the lowest voltage of the falling platform, the highest temperature of thermal runaway, and the failed overcharge capacity during the overcharge thermal runaway experiment, and is divided into three thermal runaway risk levels: low, medium, and high. The highest voltage of the rising platform and the lowest voltage of the falling platform can be obtained from the voltage data, and the highest temperature of thermal runaway can be obtained from the temperature data. The failed overcharge capacity is the part of the capacity that cannot be used due to the performance degradation or damage of the lithium-ion battery caused by overcharging. The failed overcharge capacity = overcharge input charge - (battery capacity after overcharge - initial battery capacity), where the initial battery capacity is the battery capacity at 100% SOC. The failed overcharge capacity ratio = failed overcharge capacity / battery capacity after overcharge * 100%.

[0041] The following is an analysis and explanation through specific examples.

[0042] Example 1

[0043] Refer to Figure 2 , charge a new factory (100% SOH) 163Ah square NCM-Gr LIBs (3.0 - 4.35V) at 0.5C to 100% SOC, then place it in an adiabatic explosion-proof box, attach thermocouples to the center of the positive electrode post 1, negative electrode post 2, explosion-proof valve 3, and large surface 4, perform overcharging at 1.0C, observe and record the trends of voltage and temperature changing with time during the charging process, and draw a curve graph as shown in Figure 1 . At the same time, observe information such as the battery swelling situation, the start time of the explosion-proof valve, and the change in battery mass, and calculate the failed overcharge capacity and its ratio.

[0044] Example 2

[0045] The difference between this example and Example 1 is that the SOH of the NCM-Gr LIBs is 95%.

[0046] Example 3

[0047] The difference between this example and Example 1 is that the SOH of the NCM-Gr LIBs is 90%.

[0048] Example 4

[0049] The difference between this example and Example 1 is that the SOH of the NCM-Gr LIBs is 85%.

[0050] Example 5

[0051] The difference between this example and Example 1 is that the SOH of the NCM-Gr LIBs is 80%.

[0052] Table 1 shows the highest voltage of the rising platform, the lowest voltage of the falling platform, the highest temperature of thermal runaway, the overcharge capacity at failure and its proportion during the overcharge process of the lithium-ion batteries in Examples 1-5.

[0053] Table 1

[0054]

[0055] According to Figure 1 it can be known that according to the change of the voltage of the lithium-ion battery over time, the overcharge thermal runaway process of the lithium-ion battery is divided into six stages, which are the voltage boost stage, the rising platform stage, the voltage drop stage, the falling platform stage, the rapid voltage boost stage and the thermal runaway stage in sequence. According to the change of the temperature at the positive electrode terminal 1, negative electrode terminal 2, explosion-proof valve 3 and the center of the large surface 4 of the lithium-ion battery over time, the overcharge thermal runaway process of the lithium-ion battery is divided into three stages, which are the stage with almost constant temperature, the stage with accelerating temperature rise and the stage with rapid temperature rise in sequence. Specifically, ① Voltage boost stage: The voltage of the lithium-ion battery first slowly rises from 4.35 V at full charge to 5 V, and then accelerates to rise to 5.5 V, and the temperature remains almost unchanged during this process; ② Rising platform stage: After the voltage of the lithium-ion battery rises to 5.5 V, a short voltage platform appears to keep 5.5 V unchanged, and the temperature remains almost unchanged during this process; ③ Voltage drop stage: After the voltage platform, the voltage of the lithium-ion battery first slowly drops to 5.2 V, the temperature accelerates to rise, gas starts to be generated inside the lithium-ion battery, and then the voltage accelerates to drop to 5.1 V, the temperature accelerates to rise, gas is generated inside the lithium-ion battery at an accelerated rate, and the battery starts to bulge slightly; ④ Falling platform stage: After the voltage of the lithium-ion battery drops to 5.1 V, a short voltage platform appears to keep 5.1 V unchanged, the temperature accelerates to rise during this process, gas is generated inside the battery core at an accelerated rate, and the battery continues to bulge at an accelerated rate; ⑤ Rapid voltage boost stage: After the voltage platform, the voltage of the lithium-ion battery quickly rises above 10 V, even exceeding the maximum voltage of 20 V of the charge and discharge cabinet. During this process, the temperature of the lithium-ion battery rises rapidly, the explosion-proof valve is blown open, and the electrolyte, high-temperature gas, etc. inside the lithium-ion battery are ejected; ⑥ Thermal runaway stage: The voltage of the lithium-ion battery quickly drops to 0 V, the temperature quickly rises above 600 °C, and the thermal runaway temperature of Example 1 is as high as 979 °C, and the lithium-ion battery undergoes thermal runaway.

[0056] Based on the analysis of Figure 1 Example 1 can also divide the overcharge thermal runaway process of the lithium-ion battery into four stages according to the change of the appearance of the lithium-ion battery over time, which are the almost unchanged stage, the slightly bulging stage, the rapidly expanding stage and the valve-opening stage in sequence. According to the change of the mass of the lithium-ion battery over time, the overcharge thermal runaway process of the lithium-ion battery is divided into two stages, which are the first eruption stage and the second eruption stage in sequence.

[0057] Analyze the state of the lithium-ion battery according to the above stage division and evaluate the risks: In the boost stage and the rising plateau stage, the further de-lithiation of the positive electrode material of the lithium-ion battery causes the voltage of the lithium-ion battery to rise. Lithium metal is deposited on the surface of the negative electrode, and there are few side reactions. The inside of the battery remains unchanged, and the battery hardly expands. The risk level is "low". In the step-down stage and the falling plateau stage, three major chemical reactions occur inside the lithium-ion battery: further de-lithiation of the positive electrode material, decomposition of the deposited lithium on the surface of the negative electrode, and decomposition of the electrolyte. Since the decomposition rate of the deposited lithium on the surface of the negative electrode is greater than the rate of further de-lithiation of the positive electrode, the voltage shows a downward trend. At the same time, the internal pressure of the battery increases, causing the battery to expand, the internal resistance to increase sharply, and the temperature to rise rapidly. The risk level is "medium". In the sharp boost stage and the thermal runaway stage, the internal pressure of the lithium-ion battery rises sharply, the explosion-proof valve is opened, and the internal temperature of the lithium-ion battery rapidly rises to the self-heating temperature, causing the separator to melt, and the electrolyte, high-temperature gas, etc. to erupt, ultimately leading to thermal runaway of the battery. The risk level is "high".

[0058] The stage division, battery state analysis, and risk assessment of Examples 2-5 are the same as those of Example 1, except for differences in data such as voltage and temperature. Referring to Table 1, it can be seen that as the SOH decreases (100% → 80%), the highest voltage of the rising plateau also decreases, the lowest voltage of the falling plateau has no obvious pattern, the highest temperature of thermal runaway decreases, and the overcharge capacity at failure remains almost unchanged. That is, as the SOH of the lithium-ion battery decreases, the conditions for thermal runaway of the lithium-ion battery decrease, and the probability of thermal runaway of the lithium-ion battery increases.

[0059] Based on this, the method for analyzing overcharge-induced thermal runaway of the lithium-ion battery in this application divides the process from the start of overcharging to thermal runaway of the lithium-ion battery into six stages according to the change of voltage over time, lists the effects of each stage on the lithium-ion battery, divides the thermal runaway risk area, and analyzes the overcharging process of the lithium-ion battery under different SOHs. Safety risk assessment is carried out from four dimensions: the highest voltage of the rising plateau, the lowest voltage of the falling plateau, the highest temperature of thermal runaway, and the overcharge capacity at failure. In this way, when the lithium-ion battery is applied to power equipment such as new energy vehicles, once a thermal runaway accident occurs, this analysis method can be used to assist in the analysis to help speed up and simplify the investigation process of the thermal runaway accident, which is beneficial to the efficient and low-cost investigation of the thermal runaway accident. At the same time, this analysis method can assist in monitoring the charging state of the lithium-ion battery to ensure the safety of use of the lithium-ion battery and reduce the occurrence of thermal runaway accidents.

[0060] The technical means disclosed in the solution of this application are not limited to the technical means disclosed in the above embodiments, but also include technical solutions composed of any combination of the above technical features. It should be noted that for those of ordinary skill in the art in this technical field, without departing from the principle of this application, several improvements and refinements can be made, and these improvements and refinements are also regarded as the protection scope of this application.

Claims

1. A method for analyzing overcharge thermal runaway of a lithium-ion battery, characterized in that: The analytical method comprises the following steps: Conduct adiabatic overcharge thermal runaway experiments on lithium-ion batteries and record the state parameters of lithium-ion batteries in real time; The adiabatic overcharge thermal runaway process of lithium-ion batteries is divided into stages according to the changes in lithium-ion battery state parameters, the states of lithium-ion batteries corresponding to different stages are determined, and the thermal runaway risk levels of lithium-ion batteries at different stages are evaluated.

2. The method for analyzing overcharge thermal runaway of a lithium-ion battery according to claim 1, characterized in that: An overcharge thermal runaway experiment is performed on the lithium-ion battery in an adiabatic environment, and the voltage, temperature, swelling degree, mass and explosion-proof valve status of the lithium-ion battery during the overcharge process are observed and recorded. Then, the adiabatic overcharge thermal runaway process of the lithium-ion battery is divided into stages according to the characteristics, the states of the lithium-ion battery corresponding to different stages are determined, and the thermal runaway risk levels of the lithium-ion battery at different stages are evaluated.

3. The method for analyzing overcharge thermal runaway of a lithium-ion battery according to claim 2, characterized in that: An overcharge thermal runaway test is performed on the lithium-ion battery with a SOC of 100% in an adiabatic environment, and each overcharge thermal runaway test is performed based on a plurality of lithium-ion batteries of the same model.

4. The method for analyzing overcharge thermal runaway of a lithium-ion battery according to claim 3, characterized in that: An overcharge thermal runaway experiment is performed on the lithium-ion batteries with 100% SOC and different SOH in an adiabatic environment, and each overcharge thermal runaway experiment is performed on a plurality of lithium-ion batteries of the same model and the same SOH.

5. The method for analyzing overcharge thermal runaway of a lithium-ion battery according to claim 3 or 4, characterized in that: The state parameters of the lithium-ion battery include voltage. According to the change of the voltage of the lithium-ion battery over time during the overcharge thermal runaway experiment, the overcharge thermal runaway process of the lithium-ion battery is divided into six stages, namely, a voltage boost stage, a rising platform stage, a voltage drop stage, a falling platform stage, a sharp voltage boost stage and a thermal runaway stage.

6. The method for analyzing overcharge thermal runaway of a lithium-ion battery according to claim 5, characterized in that: The state parameters of the lithium-ion battery include temperature. According to the change of the temperature of the lithium-ion battery over time during the overcharge thermal runaway experiment, the overcharge thermal runaway process of the lithium-ion battery is divided into three stages, namely, a stage where the temperature is almost constant, a stage where the temperature is accelerated to rise, and a stage where the temperature rises sharply.

7. The method for analyzing overcharge thermal runaway of a lithium-ion battery according to claim 6, characterized in that: The temperature of the lithium-ion battery includes the positive pole temperature, the negative pole temperature, the explosion-proof valve temperature and the battery side temperature. The battery side temperature includes the center temperature of the large surface of the square battery and the center temperature of the side of the cylindrical battery.

8. The method for analyzing overcharge thermal runaway of a lithium-ion battery according to claim 6, characterized in that: The thermal runaway risk level of the lithium-ion battery is determined according to four dimensions: the highest voltage of the rising platform, the lowest voltage of the falling platform, the highest temperature of thermal runaway and the failure overcharge capacity during the overcharge thermal runaway experiment, and is divided into three thermal runaway risk levels: low, medium and high.

9. The method for analyzing overcharge thermal runaway of a lithium-ion battery according to claim 3 or 4, characterized in that: The state parameters of the lithium-ion battery include the appearance of the battery. According to the change of the appearance of the lithium-ion battery over time during the overcharge thermal runaway experiment, the overcharge thermal runaway process of the lithium-ion battery is divided into four stages, namely, an almost unchanged stage, a slightly swollen stage, a rapid expansion stage and a valve opening stage.

10. The method for analyzing overcharge thermal runaway of a lithium-ion battery according to claim 3 or 4, characterized in that: The state parameters of the lithium-ion battery include the residual mass of the battery. According to the change of the mass of the lithium-ion battery over time during the overcharge thermal runaway experiment, the overcharge thermal runaway process of the lithium-ion battery is divided into two stages, namely the primary eruption stage and the secondary eruption stage.

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