Activation energy estimation and safety assessment method for lithium ion battery in self heat release stage

Abstract

The invention belongs to the technical field of lithium ion battery safety evaluation, and particularly relates to an activation energy estimation and safety evaluation method for a lithium ion battery in a self heat release stage. Estimating the activation energy of the lithium ion battery in the self heat release stage according to # imgabs0 #; wherein R represents the ideal gas constant, kappa total represents the total reaction rate of the lithium ion battery in the self-heat-release stage, n represents the number of side reactions, Ax represents the pre-exponential factor of the side reaction x, Ea and x represent the activation energy of the side reaction x, and T represents the average reaction temperature during diaphragm decomposition. If the activation energy of the lithium ion battery in the self-heat-release stage is greater than the set threshold value, thermal runaway is not easy to occur; and if the activation energy in the self-heat-release stage is less than or equal to the set threshold value, thermal runaway is easy to occur. The method provides an important basis for judging the difficulty degree of thermal runaway, can be used as a safety and stability evaluation means before the battery leaves a factory, and does not need to damage the battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of safety assessment of lithium-ion batteries, and specifically relates to a method for estimating the activation energy and safety assessment of the self-heating stage of lithium-ion batteries. Background Art

[0002] Lithium-ion batteries have become key power supply components for new energy vehicles, intelligent devices, and new energy storage systems due to their high energy density and long cycle life. However, under conditions of thermal abuse (such as high-temperature environments), electrical abuse (overcharging / overdischarging), or mechanical abuse (crushing / puncturing), the highly active material system inside them may trigger a thermal runaway process dominated by self-heating reactions. The thermal runaway process involves multiple chain reactions such as the decomposition of electrode materials and the melting of separators, and its core kinetic characteristics can be characterized by the key kinetic parameter of activation energy. Activation energy reflects the energy barrier that needs to be overcome for a material system to undergo a specific chemical reaction, and directly determines the reaction rate and the severity of thermal runaway. Therefore, the activation energy of a lithium-ion battery can evaluate the ease of occurrence of its thermal runaway.

[0003] There are multiple side reactions during the self-heating stage of lithium-ion batteries, and there is a coupling relationship between the side reactions. It is very difficult to obtain the activation energy of the self-heating stage. Currently, existing methods generally conduct thermal runaway experiments on batteries through adiabatic accelerated calorimetry experiments, obtain the temperature-time curve of the battery during the thermal runaway process, and combine the heating rate during the experiment to obtain the activation energy of the battery when thermal runaway occurs. This method is destructive to the battery and can only obtain the activation energy when thermal runaway occurs, but cannot obtain the activation energy of the self-heating stage. Summary of the Invention

[0004] Aiming at the deficiencies of the existing technology, the technical problem to be solved by the present invention is to provide a method for estimating the activation energy and safety assessment of the self-heating stage of lithium-ion batteries.

[0005] The present invention adopts the following technical solutions to solve the above technical problems:

[0006] A method for estimating the activation energy of the self-heating stage of a lithium-ion battery, characterized in that the method includes the following steps:

[0007] Discharge the fully charged lithium-ion battery in an inert atmosphere, and obtain the positive and negative electrode plates and the separator; clean and dry the positive and negative electrode plates and the separator with DMC; separate the positive and negative electrode materials from the positive and negative electrode plates, and prepare the positive and negative electrode materials into powders;

[0008] Mix the positive electrode material with the electrolyte and the negative electrode material with the electrolyte evenly according to the ratio in the lithium-ion battery to obtain samples A and B, and record the separator as sample C. Conduct DSC experiments on samples A, B, and C to obtain the heat flow curves of each sample at different heating rates;

[0009] The peak temperatures of different side reactions are obtained from the heat flow curves of various samples, and the peak temperatures are substituted into the Kissinger equation in Equation (1) for fitting to obtain the activation energy and pre-exponential factor of each side reaction;

[0010]

[0011] In the formula, β k represents the heating rate k, T k,x represents the peak temperature of side reaction x at the heating rate k, A x represents the pre-exponential factor of side reaction x, E a,x represents the activation energy of side reaction x, m represents the total number of heating rates, and R represents the ideal gas constant;

[0012] The reaction rates of each side reaction follow the Arrhenius formula, so there is:

[0013]

[0014] In the formula, κ x represents the reaction rate of side reaction x, and T represents the average reaction temperature during diaphragm decomposition;

[0015] The total reaction rate in the self-heating stage of the lithium-ion battery is approximately the linear superposition of the reaction rates of each side reaction, so there is:

[0016]

[0017] In the formula, κ total represents the total reaction rate in the self-heating stage of the lithium-ion battery, and n represents the number of side reactions;

[0018] Taking the logarithm of Equation (3) and differentiating with respect to T -1 gives:

[0019]

[0020] The calculation formula for the activation energy in the self-heating stage of the lithium-ion battery is:

[0021]

[0022] In the formula, E a,total represents the activation energy in the self-heating stage of the lithium-ion battery.

[0023] Furthermore, the side reactions in the self-heating stage of the lithium-ion battery include the decomposition of the CEI film, the decomposition of the cathode material, the decomposition of the SEI film, and the decomposition of the diaphragm.

[0024] The present invention also provides a method for evaluating the safety of a lithium-ion battery, characterized in that if the activation energy of the lithium-ion battery in the self-heating stage is greater than a set threshold value, thermal runaway is not likely to occur; if the activation energy in the self-heating stage is less than or equal to the set threshold value, thermal runaway is likely to occur.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] The present invention can estimate the activation energy of a lithium-ion battery in the self-heating stage, providing an important basis for judging the ease of occurrence of thermal runaway, and can be used as a means for evaluating the safety and stability before the battery leaves the factory. This method is simple to implement, does not require damaging the battery, and is applicable to various types of lithium-ion batteries. Description of the Drawings

[0027] Figure 1 is the heat flow curve of a brand-new battery, (a) is the heat flow curve of the positive electrode material and the electrolyte, (b) is the heat flow curve of the negative electrode material and the electrolyte, (c) is the heat flow curve of the separator;

[0028] Figure 2 is the heat flow curve of a high-temperature calendar-aged battery, (a) is the heat flow curve of the positive electrode material and the electrolyte, (b) is the heat flow curve of the negative electrode material and the electrolyte, (c) is the heat flow curve of the separator;

[0029] Figure 3 is the fitting result of the activation energy of different side reactions of a brand-new battery, (a) is the fitting of the activation energy of CEI film decomposition, (b) is the fitting of the activation energy of positive electrode material decomposition, (c) is the fitting of the activation energy of SEI film decomposition, (d) is the fitting of the activation energy of separator decomposition;

[0030] Figure 4 is the fitting result of the activation energy of different side reactions of a high-temperature calendar-aged battery, (a) is the fitting of the activation energy of CEI film decomposition, (b) is the fitting of the activation energy of positive electrode material decomposition, (c) is the fitting of the activation energy of SEI film decomposition, (d) is the fitting of the activation energy of separator decomposition;

[0031] Figure 5 is the experimental and fitting results of the activation energy of the self-heating stage of a brand-new battery, (a) is the ARC experimental result, (b) is the fitting result;

[0032] Figure 6 is the experimental and fitting results of the activation energy of the self-heating stage of a high-temperature calendar-aged battery, (a) is the ARC experimental result, (b) is the fitting result. Detailed Embodiments

[0033] Specific embodiments are given below in conjunction with the drawings. The specific embodiments are only used to introduce the technical solutions of the present invention in detail and do not limit the protection scope of this application.

[0034] The present invention provides a method for estimating the activation energy of the self-heating stage of a lithium-ion battery, including the following steps:

[0035] Disassemble a fully charged lithium-ion battery in an inert atmosphere to obtain the positive and negative electrode plates and the separator; wash the positive and negative electrode plates and the separator with DMC, and then dry them after washing; separate the positive and negative electrode materials from the current collectors, and prepare the positive and negative electrode materials into powders;

[0036] Mix the positive electrode material with the electrolyte and the negative electrode material with the electrolyte evenly according to the proportion in the lithium-ion battery to obtain samples A and B, and denote the separator as sample C. Conduct DSC (Differential Scanning Calorimetry) experiments on samples A, B, and C to obtain the heat flow curves of each sample at different heating rates;

[0037] Obtain the peak temperatures of different side reactions according to the heat flow curves of each sample, substitute the peak temperatures into the Kissinger equation for fitting to obtain the activation energy and pre-exponential factor of each side reaction; The Kissinger equation is as follows:

[0038]

[0039] In the formula, β k represents the heating rate k, with the unit of °C / min; T k,x represents the peak temperature of side reaction x at the heating rate k, with the unit of °C; A x represents the pre-exponential factor of side reaction x, with the unit of s -1 ; E a,x represents the activation energy of side reaction x, in J / mol; m represents the total number of heating rates; R represents the ideal gas constant, with a value of 8.314 J / mol·K;

[0040] The reaction rates of each side reaction all follow the Arrhenius formula, so there is:

[0041]

[0042] In the formula, κ x represents the reaction rate of side reaction x; T represents the average reaction temperature during the decomposition of the separator, with the unit of °C;

[0043] The total reaction rate of the self-heating stage of the lithium-ion battery is approximately the linear superposition of the reaction rates of each side reaction, so there is:

[0044]

[0045] In the formula, κ total represents the total reaction rate of the self-heating stage of the lithium-ion battery, and n represents the number of side reactions;

[0046] Taking the logarithm of Equation (3) and differentiating with respect to T -1 gives:

[0047]

[0048] According to the definition of the Arrhenius formula, the calculation formula for the activation energy in the self-heat release stage of a lithium-ion battery is:

[0049]

[0050] where E a,total represents the activation energy in the self-heat release stage of the lithium-ion battery.

[0051] For a method for evaluating the safety of a lithium-ion battery, if the activation energy in the self-heat release stage is greater than the set threshold, the lithium-ion battery is not likely to undergo thermal runaway; if the activation energy in the self-heat release stage is less than or equal to the set threshold, the lithium-ion battery is likely to undergo thermal runaway.

[0052] Example

[0053] In this example, a commercial 0.7 Ah soft-pack NCM811 / graphite lithium-ion battery is used as an example, and experiments are carried out using a brand-new battery and a high-temperature calendaring-aged battery (high-temperature calendaring aging for 90 days at 55 °C), but it is not limited to such lithium-ion batteries and working conditions.

[0054] The brand-new battery and the high-temperature calendaring-aged battery are respectively charged to full charge in the constant current-constant voltage mode, and then disassembled in an inert atmosphere in a glove box to obtain the positive and negative electrode sheets and the separator; the positive and negative electrode sheets and the separator are rinsed with DMC for 2 hours, and then dried in the glove box for 2 hours to eliminate the residual LiPF 6 ; scrape the positive and negative electrode materials from the current collector, and grind the positive and negative electrode materials to prepare powders;

[0055] According to the ratio in the battery, mix the positive electrode material and the electrolyte in equal proportion, denoted as sample A; mix the negative electrode material and the electrolyte in equal proportion, denoted as sample B; the separator is denoted as sample C; use a Shimadzu DSC-60 differential scanning calorimeter to perform DSC experiments on each sample of the brand-new battery and the high-temperature calendaring-aged battery, and set the heating rate to 5 °C / min, 10 °C / min, 15 °C / min, 20 °C / min to obtain the heat flow curves of different samples; see the heat flow curves of the brand-new battery and the high-temperature calendaring-aged battery in Figure 1 and 2 , where (a), (b), and (c) are the heat flow curves of the positive electrode material and the electrolyte, the negative electrode material and the electrolyte, and the separator, respectively.

[0056] Since the safety performance of lithium-ion batteries mainly depends on whether they are prone to thermal runaway, during the self-heating process, as the separator decomposes, reactions occur between the positive and negative electrode materials, resulting in internal short circuits and causing thermal runaway. Therefore, to evaluate the safety of lithium-ion batteries, it is only necessary to fit the side reactions during the self-heating stage. In the reaction between the positive electrode material and the electrolyte, there are mainly two side reactions: the decomposition of the CEI film and the decomposition of the positive electrode material. The first reaction peak of the heat flow curve of the positive electrode material and the electrolyte is the peak temperature of the CEI film decomposition, and the second reaction peak is the peak temperature of the positive electrode material decomposition; in the reaction between the negative electrode material and the electrolyte, there is mainly one side reaction: the decomposition of the SEI film. The first reaction peak of the heat flow curve of the negative electrode material and the electrolyte is the peak temperature of the SEI film decomposition; the separator reaction mainly includes one side reaction: the decomposition of the separator. The reaction peak of the separator heat flow curve is the peak temperature of the separator decomposition;

[0057] Substitute the peak temperature into the Kissinger equation to fit the activation energy of each side reaction, Figure 3 、 4 which are the activation energy fitting straight lines of the brand-new battery and the high-temperature calendar-aged battery respectively. The fitting results are shown in Table 1.

[0058] Table 1 Activation energy fitting results

[0059]

[0060] Calculate the total activation energy E a,total,1 and E a,total,2 of the self-heating stage of the brand-new battery and the high-temperature calendar-aged battery, which are 9019.60 J / mol and 13435.98 J / mol respectively.

[0061] Verify the method of the present invention through an adiabatic accelerating calorimetry (ARC) experiment. Use the brand-new batteries and high-temperature calendar-aged batteries of the same batch for the ARC experiment. The adiabatic accelerating calorimeter is set to the "heating-waiting-searching" mode, the heating step temperature is set to 5 °C, and the detection sensitivity of the ARC is set to 0.02 °C / min. That is, when the battery undergoes self-heating and the ARC detects that the self-heating rate of the battery is greater than 0.02 °C / min, the ARC starts to track the sample temperature and enters the adiabatic stage. At this time, there is no heat exchange process between the battery and the outside world. Fit the ARC experiment results through the Kissinger equation to obtain the total activation energy E' a,total,1 and E' a,total,2 of the self-heating stage of the brand-new battery and the high-temperature calendar-aged battery, which are 9312.68 and 14125.49 J / mol respectively. Figure 5 、 6 are the experimental results and fitting results of the activation energy of the self-heating stage of the brand-new battery and the high-temperature calendar-aged battery respectively.

[0062] The relative errors between the fitted values and the experimental values of the activation energy during the self-heating stage of the brand-new battery and the high-temperature calendar-aged battery are respectively:

[0063]

[0064] For both types of batteries, the fitting error of the activation energy during the self-heating stage is less than 5%, indicating that the method of the present invention is reliable. Therefore, this method can be used to estimate the activation energy of the lithium-ion battery during the self-heating stage and evaluate the safety and stability of the battery.

[0065] Matters not described in the present invention are applicable to the prior art.

Claims

1. A method for estimating the activation energy of a lithium-ion battery during the self-heating stage, characterized in that: The method includes the following: Disassemble the fully charged lithium-ion battery in an inert atmosphere to obtain the positive and negative electrode sheets and the separator; clean and dry the positive and negative electrode sheets and the separator with DMC; separate the positive and negative electrode materials from the positive and negative electrode sheets, and prepare the positive and negative electrode materials into powders; The positive electrode material and the electrolyte, and the negative electrode material and the electrolyte are uniformly mixed according to the ratio in the lithium-ion battery to obtain samples A and B, and the separator is recorded as sample C. DSC experiments are performed on samples A, B, and C to obtain heat flow curves of each sample at different heating rates; The peak temperatures of different side reactions were obtained according to the heat flow curves of each sample, and the peak temperatures were substituted into the Kissinger equation of formula (1) for fitting to obtain the activation energy and pre-exponential factor of each side reaction; In the formula, β k represents the heating rate k, T k,x represents the peak temperature of the side reaction x at the heating rate k, A x represents the pre-exponential factor of the side reaction x, E a,x represents the activation energy of the side reaction x, m represents the total number of heating rates, and R represents the ideal gas constant; The reaction rates of each side reaction follow the Arrhenius formula, so: In the formula, κ x represents the reaction rate of the side reaction x, and T represents the average reaction temperature when the diaphragm decomposes; The total reaction rate of the self-heating stage of lithium-ion batteries is approximately the linear superposition of the rates of each side reaction, so: In the formula, κ total It represents the total reaction rate of the self-heating stage of lithium-ion batteries, and n represents the number of side reactions; Take the logarithm of equation (3) and calculate T -1 Taking the derivative, we have: The activation energy calculation formula for the self-heating stage of lithium-ion batteries is: In the formula, E a,total Represents the activation energy of the self-heating stage of lithium-ion batteries.

2. The method for estimating activation energy of a lithium-ion battery in a self-heating stage according to claim 1, characterized in that: The side reactions in the self-exothermic stage of lithium-ion batteries include CEI film decomposition, cathode material decomposition, SEI film decomposition and diaphragm decomposition.

3. A lithium-ion battery safety assessment method, characterized in that: If the activation energy of the lithium-ion battery in the self-heating stage is greater than the set threshold, thermal runaway is not likely to occur; if the activation energy in the self-heating stage is less than or equal to the set threshold, thermal runaway is likely to occur.

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