Lithium ion battery polarization anti-energy model and safety state determination method

Abstract

A lithium ion battery polarization anti-energy model and safety state determination method comprises the following steps: obtaining an equivalent circuit model corresponding to a measured electrochemical impedance spectrum through equivalent circuit modeling, embedding the equivalent circuit model into a vehicle-mounted battery management system, and carrying out battery model parameter identification to obtain model parameter values in real time; meanwhile, the electrochemical impedance spectrum of the battery in the actual use process is calculated by utilizing the parameter values, and polarization energy resistance modeling is carried out by taking the area enveloped by the polarization impedance and the coordinate axis of the battery in the actual use process as polarization energy resistance to obtain the polarization energy resistance of the battery at each moment in the use process; and defining the ratio of the polarization energy resistance at each moment to the polarization energy resistance of the battery in a brand new state as a polarization energy resistance state, and judging the safety state of the battery by using the polarization energy resistance state. From the perspective of the reactance of the battery, the invention provides a brand-new battery safety state judgment method, and a new thought is provided for realizing the safety management of the battery.

Description

Technical Field

[0001] The present invention is applicable to the field of electric vehicle power batteries, specifically polarization resistance observation (5) and resistance state estimation (6), which are applied to battery safety state determination (7) of electric vehicles. Background Art

[0002] In recent years, with the increasing shortage of fuel resources and the proposal of the "dual carbon" goal, electric vehicles with advantages such as energy saving, environmental protection, and low noise have been developing rapidly. As the energy supplier of electric vehicles, the safety of lithium-ion batteries affects the safety of the entire vehicle. Therefore, the safety status of lithium-ion batteries (State of safe, S os )(73) An accurate assessment of the battery safety status S is essential. os (73) Making accurate and reliable judgments is a current research hotspot.

[0003] The patent application number is CN202310446506.4, and the invention name is “A power battery safety risk warning method based on safety entropy”, which provides a battery safety status S os (73) The determination method uses a variety of battery usage data before the damage accident to calculate the probability or constant and convert it to obtain the safety entropy, and uses this entropy value to determine the battery safety status S os (73) Although it is possible to quantitatively describe the safety of the battery system, it requires calculating the probability value of each independent event, and the accuracy of each probability value will directly affect the accuracy of the battery safety status judgment.

[0004] The patent application number is CN202311012840.5, and the invention name is “A safety early warning assessment method for electrochemical energy storage batteries”, which provides a battery safety status S os (73) The evaluation method is to construct a battery safety model architecture algorithm, starting from multiple safety dimensions, using multiple factors such as battery current, voltage, temperature and other input information to form different safety event factors as model input to derive a function expression for calculating the battery safety factor, and use this function to define the battery safety status S os (73) The focus of this invention is to construct an accurate comprehensive model that takes into account multiple safety factors. Although it is reliable, it requires coupling multiple factors, which greatly increases the complexity of the calculation and the modeling process is complicated.

[0005] The patent application number is CN202310852097.8, and the invention name is “Power Battery Safety Early Warning Method and Online Monitoring Device”. It provides an SVR estimation method based on support vector regression. This method does not require the establishment of an accurate battery model. Instead, it uses the input current, voltage and other multi-dimensional signal data to accurately determine the battery safety status S through autonomous learning. os (73). However, this method requires a large amount of reference data for training, and the training data and training method have no significant impact on the battery safety status S os (73) The error in judgment has a great impact.

[0006] In summary, the existing lithium-ion battery safety status S os (73) The evaluation method has certain deficiencies and it is difficult to evaluate the battery safety status S os (73) To make accurate and simple judgments, it is necessary to improve the battery safety status judgment method on this basis to improve the efficiency of battery safety status judgment (7). Summary of the Invention

[0007] The purpose of the present invention is to propose a method for determining the battery safety state (7) based on the battery polarization resistance state, so as to solve the shortcomings of the battery safety state determination (7) method.

[0008] To achieve this object, the present invention adopts the following technical solutions:

[0009] 1. A lithium-ion battery polarization resistance model and safety state determination method, characterized by comprising an equivalent circuit model (11), model parameters (21), polarization impedance (32), diffusion impedance (41), polarization resistance (51), resistance state (61), resistance state change rate (74), and safety state (73). Based on the above modules, a battery safety state determination method (6) is constructed, comprising the following steps:

[0010] Step 1: Equivalent circuit modeling (1);

[0011] Step 2: Model parameter identification (2);

[0012] Step 3: Polarization impedance calculation (3);

[0013] Step 4: Diffusion impedance calculation (4);

[0014] Step 5: Polarization resistance observation (5);

[0015] Step 6: Estimation of resistance state (6);

[0016] The seventh step, safety state determination (7), is divided into two types: one is a safety state determination method based on the energy resistance state (71), and the other is a safety state determination method based on the energy resistance state change rate (72);

[0017] The polarization resistance state is used to determine whether the battery is in a safe area (75), a thermal runaway critical area (76), or a thermal runaway risk area (77).

[0018] 2. The equivalent circuit modeling (1) according to claim 1, characterized in that:

[0019] The first step of ECM is to determine the number of equivalent components and their corresponding frequency ranges based on the morphological characteristics of the measured battery electrochemical impedance spectrum: the measured battery electrochemical impedance spectrum is a semicircle in the medium and high frequency region and a 45° oblique line in the low frequency region; the semicircle in the medium and high frequency region represents the battery polarization impedance, which characterizes the polarization process in the battery electrode reaction and can be expressed by R p (112) and C p (113) is equivalent to the parallel network structure; the oblique line is the battery diffusion impedance, which characterizes the diffusion behavior of lithium ions and can be represented by W(114); the intersection of the impedance curve and the horizontal axis in the electrochemical impedance spectrum is the ohmic resistance R Ω (111);

[0020] The second step of ECM is to determine the battery model. According to the analysis in the first step, the medium and high frequencies are equivalent to RC parallel components, and the low frequencies are equivalent to W (114) components, plus the high frequency ohmic resistance R Ω (111) element, which can be constructed as follows Figure 3 Battery model shown;

[0021] The third step of ECM is to verify the established equivalent circuit model (ECM) (11). The equivalent circuit model (11) established based on the measured electrochemical impedance spectroscopy is fitted and analyzed using Zview software to verify the accuracy of the model.

[0022] 3. The model parameter identification (2) according to claim 1, characterized in that:

[0023] The established equivalent circuit model is embedded into the vehicle battery management system (BMS) (22) to obtain the battery model parameter values ​​in real time during actual use.

[0024] 4. The polarization impedance calculation (3) according to claim 1, characterized in that:

[0025] According to the established equivalent circuit model (11), the expression of battery impedance can be obtained as follows:

[0026]

[0027] Where, (321) is the battery ohmic impedance, (322) is the polarization resistance impedance, (323) is the polarization capacitance impedance of the battery, Z W (41) is the battery diffusion impedance;

[0028] The expression of battery ohmic impedance is as follows, and its magnitude is equal to the ohmic resistance value,

[0029]

[0030] Where R Ω (111) is the ohmic resistance;

[0031] The expression of battery polarization resistance impedance is as follows, and its magnitude is equal to the polarization resistance value.

[0032]

[0033] Where R p (112) is the polarization resistance;

[0034] The expression for the battery polarization capacitance impedance is as follows,

[0035]

[0036] Where C p (113) is the polarization resistance, and ω(341) is the angular frequency.

[0037] Battery polarization is in the medium and high frequency band, that is, ω→∞, when the internal material of the battery has no time to diffuse, that is, Z W =0, therefore, the battery impedance at this time consists of three parts: ohmic impedance, polarization resistance impedance, and polarization capacitance impedance. Substituting equations (2), (3), and (4) into equation (1), the battery polarization impedance Z can be obtained. p The expression of (32) is,

[0038]

[0039] Simplifying formula (5) we can get:

[0040]

[0041] From formula (6), the expression of the real part of the battery polarization impedance at this time is:

[0042]

[0043] At the same time, the expression of the imaginary part of the battery polarization impedance can be obtained from formula (6):

[0044]

[0045] The relationship between the real and imaginary parts of the battery polarization impedance can be established by equations (7) and (8):

[0046]

[0047] From formula (9), we can know that the polarization impedance of the battery is the circle with the center (R Ω +R p / 2,0), with a radius of R p / 2 semicircle.

[0048] 5. The diffusion impedance calculation (4) according to claim 1, characterized in that:

[0049] For the diffusion process caused by concentration gradient, the diffusion rate is related to the concentration gradient of the diffusing substance. According to Fick's first law,

[0050]

[0051] Where v(x)(416) is the diffusion velocity at x from the electrode surface, D(4161) is the diffusion coefficient, (4162) is the concentration gradient of the diffusing species at a distance x from the electrode surface.

[0052] The change in the reactant concentration near the electrode surface will cause the change in the Faraday current density. The change in the Faraday current density ΔI F (415) can be expressed as,

[0053]

[0054] Where, E(4183) is the electrode potential, C s (417) is the concentration of reactants near the electrode surface, X i (4185) is the electrode surface state variable.

[0055] Dividing both sides of Equation (11) by ΔE (4184) yields the Faraday admittance Y F The expression of (4116) is

[0056]

[0057] The Faraday admittance when the concentration change of the reactant on the electrode surface is not considered, that is, the surface process Faraday admittance of the electrode reaction is:

[0058]

[0059] Where, (4117) is the Faraday admittance of the electrode surface.

[0060] Substituting formula (13) into formula (12), we can get:

[0061]

[0062] Y F =ΔI F Substituting / ΔE into formula (14), we can obtain:

[0063]

[0064] Simplifying formula (15), we can get:

[0065]

[0066] And because Z F =1 / Y F , substituting it into formula (16) we can get the Faraday impedance expression,

[0067]

[0068] From formula (17), the battery diffusion impedance Z can be obtained W The expression is implicit,

[0069]

[0070] Substituting equation (18) into (17) can simplify the Faraday impedance expression to:

[0071]

[0072] If γ(412) is used to represent the reaction order of the reactant in the electrode reaction, then,

[0073]

[0074] Substituting formula (20) into (18), we can obtain:

[0075]

[0076] From formula (21), we only need to calculate ΔC s / ΔI F The diffusion impedance Z can be obtained W (41) is expressed explicitly. When the electrode system is subjected to a small perturbation of the electrode potential (ΔE), the Faraday current density responds within the linear range ΔI F (415), if the Faraday current is the cathode current, according to Fick's first law (10),

[0077]

[0078] Where n(413) is the stoichiometric coefficient of electron e(4132) in the electrode reaction, and F(4113) is the Faraday constant.

[0079] If the Faraday current is the anode current, then according to Fick's first law (10),

[0080]

[0081] In addition, according to Fick's second law, during the diffusion process, at a distance x, the rate of change of concentration with time is equal to the negative value of the rate of change of diffusion flux with distance at that distance, that is,

[0082]

[0083] Where, (4182) is the rate of change of concentration with time.

[0084] For a battery, when the electrode system is subjected to a sinusoidal electrode potential disturbance, that is, when ΔE=|ΔE|exp(jωt), due to the linear response, ΔI F (415) and ΔC(418) should both be signals with a frequency of ω(341), but with different phase angles. Therefore,

[0085]

[0086] In the formula (4181) is the phase angle, j = (-1) 1 / 2 .

[0087] Taking the derivative of equation (25) we can get the rate of change of concentration with time,

[0088]

[0089] Combining formula (24) and formula (26), we can get:

[0090]

[0091] The general solution of formula (27) is:

[0092]

[0093] Where k1 and k2 are constants.

[0094] For diffusion impedance, one of its boundary conditions is x = ∞, ΔC = 0, so the parameter k1 in equation (28) should be zero. Then equation (28) can be transformed into,

[0095]

[0096] Another boundary condition is x = 0, where ΔC = ΔC s Its Faraday current can be both cathode current and anode current. Taking cathode current as an example, that is, I F (414) is a negative value, and from formula (28) we can get,

[0097]

[0098] Substituting formula (30) into formula (23), we can obtain:

[0099]

[0100] Substituting equation (31) into equation (21), we can get the diffusion impedance Z W The expression of (41) is

[0101]

[0102] If the Faraday current is the anode current, then,

[0103]

[0104] Substituting formula (33) into formula (32), we can get:

[0105]

[0106] From equations (32) and (34), it can be seen that no matter whether the Faraday current is the anode current or the cathode current, Z W (41) is uniformly expressed as,

[0107]

[0108] For e x Performing Taylor series expansion, we can get:

[0109]

[0110] From formula (36), we can get e jx The Taylor series expansion of

[0111]

[0112] By j 2 =-1, formula (37) can be transformed into:

[0113]

[0114] By combining the similar terms in formula (38), we can get:

[0115]

[0116] The Taylor series expansion of sinx is,

[0117]

[0118] The Taylor series expansion of cosx is,

[0119]

[0120] From formula (39), formula (40) and formula (41), we can get:

[0121] e jx =cosx+jsinx (42)

[0122] From formula (42), we can get:

[0123]

[0124] Replacing π / 2 in equation (43) with -π / 4 yields,

[0125]

[0126] Substituting formula (44) into formula (35), we can obtain:

[0127]

[0128] If, in addition to the electrode potential E(4183) and the reactant concentration C s (417), there are no other surface state variables that affect the electrode process, then (411) is equal to the real number R p (112), which is expressed as,

[0129]

[0130] Where R(4111) is the gas constant, which is 8.314 J·K -1 ·mol -1 , T(4112) is the absolute temperature.

[0131] Substituting equation (46) into equation (45), we can get Z W The expression of (23) is,

[0132]

[0133] From equation (47), we can get that the real and imaginary parts of the impedance are both The impedance is related and the values ​​are exactly the same, so the graph on the impedance complex plane is a straight line with an inclination angle of π / 4. If the Weber constant is σ(419), then,

[0134]

[0135] Where A(4191) is the area of ​​the positive electrode material immersed in the electrolyte.

[0136] Substituting formula (48) into formula (47), we can obtain:

[0137]

[0138] Substituting equations (2) to (4) and (49) into equation (1), the impedance expression of the battery can be obtained as follows:

[0139]

[0140] From formula (50), the real part of the battery impedance can be expressed as:

[0141]

[0142] At the same time, the imaginary part of the battery impedance can be expressed as follows from formula (50):

[0143]

[0144] For the battery diffusion impedance, it is in the low-frequency region, that is, ω→0. Substituting it into equation (51), the expression of the real part of the battery diffusion impedance at this time is:

[0145] Z Re =R Ω +R p (53)

[0146] Similarly, substituting ω→0 into equation (52), we can obtain the expression of the imaginary part of the battery diffusion impedance at this time:

[0147] Z Im =2C p σ 2 (54)

[0148] According to equations (53) and (54), the relationship between the real and imaginary parts of the battery impedance is:

[0149] Z Im =Z Re -R Ω -R p +2C p σ 2 (55)

[0150] According to formula (55), the real and imaginary parts of the battery impedance are a straight line with a slope of 1. Figure 1 The straight line part in the equation corresponds to the real axis. The intersection coordinates of the straight line and the real axis (real part of impedance) are (R Ω +R p -2C p σ 2 ,0). Therefore, as long as the value of the intersection coordinates is determined, the straight line can be drawn. According to formula (55), it is necessary to find σ(419) to determine the expression of the battery electrochemical diffusion impedance spectrum. According to formula (48), if σ(238) is to be determined, it is necessary to know R(4111), T(4112), F(4113), C s The values ​​of the parameters (417), A (4191), and D (4161) are as follows: R = 8.314 J·K -1 ·mol -1 , T = 298K, F = 96487C·mol -1 , C s =96.461 g·mol -1 , D = 2 × 10 -5 cm 2 ·s -1 The only variable that needs to be calculated is the number of reaction electrons n (413), whose value needs to be determined based on the current value. The calculation steps are as follows:

[0151] The number of reaction electrons can be calculated from the current value at each moment according to the following formula:

[0152]

[0153] Where t(4134) is time and Q(4131) is charge.

[0154] The expression of the electric quantity Q(4131) is,

[0155] Q=ne (57)

[0156] Among them, e(4132) is the basic charge, and its value is 1.6×10 -19 C.

[0157] The corresponding number of reaction electrons n(413) can be obtained from equations (56) and (57). s and A = 250 cm 2 , substituting into equation (48), we can calculate the Weber impedance coefficient σ (419).

[0158] 6. The polarization resistance energy observation (5) according to claim 1, characterized in that:

[0159] By combining equations (6) and (49), we can obtain the coordinates of the intersection of the battery polarization impedance and the diffusion impedance. After Fourier transform, we can find that ω = 0 at this intersection;

[0160] The polarization impedance of the battery is located in the medium and high frequency band, and the angular frequency at the intersection with the horizontal axis is ω = ∞;

[0161] In summary, the change of battery impedance in the entire frequency range can be equivalent to the change in the area of ​​the battery polarization impedance envelope, and the change in this area will directly determine whether the battery has an internal short circuit;

[0162] Therefore, the present invention defines the area enclosed by the battery polarization impedance and the coordinate axis as the battery impedance energy (BIE) (51), which is expressed as follows:

[0163]

[0164] Where, E imp (51) is the polarization resistance, Z Im (34) is the imaginary part of the impedance, which represents the battery capacitive reactance in the polarization impedance part, Z Re (33) is the real part of the impedance, which represents the battery impedance in the polarization impedance part;

[0165] Substituting equations (7) and (8) into equation (58), we can further obtain:

[0166]

[0167] The ohmic resistance value, polarization resistance value, and polarization capacitance value during actual use are input and substituted into formula (59) to obtain the battery resistance in real time.

[0168] 7. The energy resistance state estimation (6) according to claim 1, characterized in that:

[0169] According to formula (59), the battery energy resistance E of the battery in its new state can be obtained by substituting the ohmic resistance, polarization resistance and polarization capacitance values ​​of the battery in its new state into imp0 (52),

[0170]

[0171] Where R Ω (0)(211) is the ohmic resistance of the battery in its new state, R p (0)(212) is the polarization resistance value of the battery in a new state, C p (0)(213) is the polarization capacitance value of the battery in a brand new state;

[0172] According to formula (59), the battery energy resistance K at each moment in the battery's use can be calculated by substituting the ohmic resistance, polarization resistance, and polarization capacitance values ​​at each moment in the battery's use. t ,

[0173]

[0174] Where R Ω (t)(241) is the ohmic resistance of the battery at each moment during use, R p (t)(215) is the polarization resistance value of the battery at a certain moment during use, C p (t)(216) is the polarization capacitance value of the battery at each moment during use;

[0175] The battery energy E at each moment during use impt (53) Compared with the battery resistance E in a new state imp0 The ratio of (52) is defined as the battery reactance state. Combining equations (60) and (61), the expression of the reactance state can be obtained as follows:

[0176]

[0177] Among them, S oi (t)(62) is the energy resistance state at time t.

[0178] 8. The first safety state determination method (71) according to claim 1, characterized in that:

[0179] According to formula (62), the energy resistance state S of the battery in the new state can be obtained oi (0),

[0180]

[0181] Where S oi (0)(63) is the energy resistance of the battery in its new state;

[0182] Substituting the ohmic resistance, polarization resistance, and polarization capacitance of the battery at the moment when an internal short circuit is about to occur into formula (59), the polarization resistance E of the battery at the moment when an internal short circuit is about to occur can be obtained. imps ,

[0183]

[0184] Where R Ω (s)(217) is the ohmic resistance of the battery at each moment during use, R p (s)(218) is the polarization resistance value of the battery at a certain moment during use, C p(s)(219) is the polarization capacitance value of the battery at each moment during use;

[0185] Combining equations (62) and (64), we can obtain the energy resistance state S of the battery when an internal short circuit is about to occur: OI (r),

[0186]

[0187] Where S oi (t s )(64) is the energy resistance of the battery in its new state;

[0188] During the charging process, the change of polarization resistance is more obvious, and the ohmic internal resistance hardly changes. Therefore, the battery safety status during the charging process can be determined by combining formula (62);

[0189] As the number of charge and discharge cycles increases, the SEI film becomes thicker, and therefore the polarization resistance increases. According to formula (62), when the battery is in the normal aging process, R p (t)≥R p (0), so we have: S oi (t)≥S oi (0);

[0190] For a lithium-ion battery that is about to have an internal short circuit, as the lithium dendrites continue to grow, they penetrate the diaphragm more deeply. In the process of the diaphragm being pierced, the battery polarization resistance value continues to decrease. According to formula (62), when the battery is about to have an internal short circuit, then: S oi (t s )≤S oi (t)<100%;

[0191] When the separator is punctured, the battery will have an internal short circuit. oi (t) = S ois =0, where S ois (65) is the energy resistance state of the battery when an internal short circuit occurs;

[0192] Based on the above analysis, S oi (t s ) and S ois are two critical values ​​for judging the battery safety status. The battery safety status at the current moment can be judged based on these two critical values.

[0193] When S oi When (t)≥100%, the battery is in a safe state;

[0194] When S oi (t s )≤Soi When (t) is less than 100%, the battery is in a thermal runaway risk state;

[0195] When S oi (t) = S ois =0, the battery is in thermal runaway state;

[0196] As the battery ages, its impedance increases until the battery cycle life ends. Therefore, the battery aging state can also be evaluated based on the battery reactance state at the current moment.

[0197] 9. The second safety state determination method (72) according to claim 1, characterized in that:

[0198] According to formula (62), the calculation formula for the battery resistance state change rate is established:

[0199]

[0200] Where, α E (t)(741) is the rate of change of the battery's energy resistance state at different times;

[0201] According to formula (64), the rate of change of the battery's energy resistance state at the moment when thermal runaway is about to occur can be obtained. The calculation formula is:

[0202]

[0203] Where, α E (t s )(742) is the rate of change of the reactance state of the battery when an internal short circuit is about to occur, E imps (54) is the battery resistance when the battery is about to have an internal short circuit;

[0204] Similarly, according to formula (64), the rate of change of the energy resistance state at the time of internal short circuit of the battery can be obtained, and the calculation formula is:

[0205]

[0206] Where, α E (t r )(743) is the rate of change of the reactance state of the battery at the moment of internal short circuit, E impr (55) is the battery resistance at the time of internal short circuit;

[0207] During the normal aging process of the battery, as the SEI film thickens, the polarization resistance of the battery continues to increase, and R p (t)≥R p (0), so E impt -E imp0 ≥0, then, according to formula (66), we can get αE (t)≥0; when the battery is about to have an internal short circuit, R p (s)<R p (0), then E imps -E imp0 <0, at this time, according to formula (67) we can get α E (t s )<0; When the battery is short-circuited, R p (t) = R p (r)=0,E impr -E imp0 =-E imp0 , at this time, according to formula (68) we can get α E (t r ) = -100%;

[0208] In summary, α E (t) and α E (t s ) to compare and determine the battery safety status:

[0209] When α E When (t)≥0, the battery is in a safe state;

[0210] When α E (t s )≤α E When (t)<0, the battery is in a thermal runaway risk state;

[0211] When α E (t) = α E (t r ), the battery is in thermal runaway.

[0212] 10. The safety zone (75) according to claim 1, characterized in that:

[0213] The area where the battery is in a safe state is the safe area. According to the above two battery safety state determination methods, when S oi (t)≥100% or α E When (t)≥0, the battery is in a safe state. Therefore, the battery safety area division standard can be obtained as follows:

[0214]

[0215] In summary, the area that meets the conditions of formula (69) is the safe area.

[0216] 11. The thermal runaway critical region (76) according to claim 1, characterized in that: the region where the battery is in a thermal runaway risk state is the thermal runaway critical region, and according to the above two battery safety state determination methods, when Soi (t s )≤S oi (t)<100% or α E (t s )≤α E When (t) < 0, the battery is in a thermal runaway risk state. Therefore, the battery thermal runaway critical area division standard can be obtained as follows:

[0217]

[0218] In summary, the region that satisfies the condition of formula (70) is the critical region of thermal runaway.

[0219] 12. The thermal runaway risk area (77) according to claim 1 is characterized in that: the area where the battery is in a thermal runaway state is the thermal runaway risk area, and according to the above two battery safety state determination methods, when S oi (t) = S ois =0 or α E (t) = α E (t r ), the battery is in a state of thermal runaway due to an internal short circuit. In summary, the standard for dividing the battery thermal runaway risk area is:

[0220]

[0221] Therefore, the area that meets the conditions of formula (71) is the thermal runaway risk area.

[0222] In summary, the present invention has the following advantages:

[0223] 1. This invention innovates a method for determining the safety status of a battery by using the battery energy resistance state and the rate of change of the energy resistance state, and combines the electrochemical impedance spectroscopy theory with the battery equivalent circuit model and applies it to actual vehicles. To a certain extent, it integrates the physical significance of electrochemical reactions with the simple and easy-to-operate characteristics of equivalent circuit calculations, and can more accurately simulate the dynamic characteristics of batteries.

[0224] 2. From the perspective of battery energy resistance, a new method for determining the battery safety status is proposed. The battery's energy resistance state at the moment when an internal short circuit is about to occur and the battery's energy resistance state at the moment when an internal short circuit occurs are used as two boundaries to determine the battery safety status. This provides a new theoretical basis and determination method for defining the battery safety status, and also provides new ideas for achieving battery safety management.

[0225] 3. The battery resistance is calculated using actual vehicle data, and the battery electrochemical impedance spectroscopy is used to study the battery performance during actual use, which increases the possibility of applying the battery electrochemical impedance spectroscopy method in actual vehicles. BRIEF DESCRIPTION OF THE DRAWINGS

[0226] Figure 1 A schematic diagram of battery safety status determination (7) according to a specific embodiment of the present invention;

[0227] Figure 2 Flowchart of a method for determining a battery safety state (7) according to a specific embodiment of the present invention;

[0228] Figure 3 A flowchart for modeling an equivalent circuit (1) according to a specific embodiment of the present invention;

[0229] Figure 4 A schematic diagram of a battery equivalent circuit model (11) according to a specific embodiment of the present invention;

[0230] Figure 5 Flowchart of a safety state determination method (71) based on reactance state according to a specific embodiment of the present invention;

[0231] Figure 6 Flowchart of a safety state determination method (72) based on reactance state change rate according to a specific embodiment of the present invention;

[0232] Create a description table to summarize the symbols and related concept definitions in this invention:

[0233]

[0234] DETAILED DESCRIPTION

[0235] The present invention will be further described in detail below with reference to the accompanying drawings and examples. It will be understood that the specific embodiments described herein are intended only to illustrate the present invention and are not intended to limit the present invention. It should also be noted that, for ease of description, the accompanying drawings only illustrate portions relevant to the present invention, not all structures.

[0236] The present invention is to propose a seven-step method for determining the battery safety status from the perspective of battery resistance. Figure 1As shown, an equivalent circuit model corresponding to the electrochemical impedance spectrum obtained from the experiment is obtained through equivalent circuit modeling, and the equivalent circuit model is embedded in the on-board battery management system to perform battery model parameter identification to obtain model parameter values ​​in real time. At the same time, these parameter values ​​are used to calculate polarization impedance and diffusion impedance to obtain the electrochemical impedance spectrum of the battery during actual use. The area enclosed by the battery polarization impedance and the coordinate axis during actual use is defined as polarization resistance to observe the polarization resistance, and the polarization resistance of the battery at each moment during use is obtained. At the same time, the ratio of the polarization resistance at each moment to the polarization resistance of the battery in a new state is calculated to obtain the resistance state of the battery at each moment, and further obtain the resistance state change rate of the battery at each moment. The battery resistance state and the resistance state change rate are used to determine the battery safety state.

[0237] The first step is equivalent circuit modeling (1), specifically, Figure 2 It is shown that according to the morphological characteristics of the measured battery electrochemical impedance spectrum, the number of equivalent components and their corresponding frequency ranges are determined, and the corresponding equivalent circuit model is established.

[0238] The second step is model parameter identification (2), which embeds the established equivalent circuit model into the vehicle battery management system, so that the model parameter values ​​of the battery during actual use can be obtained in real time.

[0239] The third step is polarization impedance calculation (3). According to the equivalent circuit model established in the first step, the expression of the total battery impedance can be obtained as follows:

[0240]

[0241] in, (321) is the battery ohmic impedance, (322) is the polarization resistance impedance, (323) is the polarization capacitance impedance of the battery, Z W (41) is the battery diffusion impedance;

[0242] The expression of the battery ohmic impedance (321) is as follows, and its magnitude is equal to the resistance value of the ohmic resistor (111):

[0243]

[0244] Where R Ω (111) is the ohmic resistance;

[0245] The battery polarization resistance impedance (32) is expressed as follows, and its magnitude is equal to the polarization resistance (112) resistance:

[0246]

[0247] Where R p (112) is the polarization resistance;

[0248] The expression for the battery polarization capacitance impedance (323) is as follows:

[0249]

[0250] Where C p (113) is the polarization resistance, ω(341) is the angular frequency;

[0251] Secondly, the battery polarization impedance is located in the medium and high frequency band, and within this frequency band, Z W =0, and combining equations (2), (3), and (4), the expression of battery polarization impedance can be obtained as follows:

[0252]

[0253] Simplifying formula (5) we can get:

[0254]

[0255] According to formula (6), the expressions of the real and imaginary parts of the polarization impedance can be obtained. Furthermore, by substituting the model parameters identified in the second step into formula (6), the polarization impedance of the battery during actual use can be calculated.

[0256] The fourth step is to calculate the diffusion impedance (4). According to Fick's first law and Fick's second law, the battery diffusion impedance can be expressed as:

[0257]

[0258] Where R(4111) is the gas constant, T(4112) is the absolute temperature, n(413) is the number of reaction electrons, F(4113) is the Faraday constant, and C s (417) is the molar concentration of lithium ions, A (4191) is the area of ​​the positive electrode material immersed in the electrolyte, and D (4161) is the diffusion coefficient;

[0259] According to formula (7), the real and imaginary parts of the battery diffusion impedance are both For the sake of simplicity, let Then we have:

[0260]

[0261] Where, σ(419) is the Weber coefficient;

[0262] Substituting equation (8) and equations (2), (3), and (4) into equation (1), the impedance expression of the battery in the low frequency band can be obtained as follows:

[0263]

[0264] According to formula (9), the expressions of the real and imaginary parts of the battery impedance at this time are:

[0265]

[0266] Where Z Re (33) is the real part of impedance, Z Im (34) is the imaginary part of impedance;

[0267] Since ω→0 in the low frequency band, substituting it into equation (10), we can obtain the simplified expressions of the real and imaginary parts of the battery impedance:

[0268]

[0269] The relationship between the real and imaginary parts of the battery impedance in the low frequency band can be further obtained from formula (11):

[0270] Z Im =Z Re -R Ω -R p +2C p σ 2 (12)

[0271] Further, the model parameters identified in the second step are substituted into equation (12) to calculate the diffusion impedance of the battery during actual use.

[0272] Step 5: Observation of polarization impedance (5). Since the change of battery impedance in the entire frequency range can be equivalent to the change of the area of ​​the battery polarization impedance envelope, the battery polarization impedance (BIE) (51) is represented by the area enclosed by the battery polarization impedance and the coordinate axis. The area can be solved using the integral formula, which is:

[0273]

[0274] By substituting the ohmic resistance value, polarization resistance value, and polarization capacitance value during actual use into formula (13), the polarization resistance energy at each moment during actual use can be calculated.

[0275] The sixth step is to estimate the energy resistance state (6). Substitute the identified ohmic resistance value, polarization resistance value, and capacitance value of the battery in the new state into the polarization energy resistance calculation expression (13) in the fifth step to calculate the polarization energy resistance of the battery in the new state. The calculation expression is:

[0276]

[0277] Where, E imp0 is the polarization resistance of the battery in its new state, R Ω (0)(211) is the ohmic resistance of the battery in its new state, R p (0)(212) is the polarization resistance of the battery in its new state, C p (0)(213) is the polarization capacitance of the battery in its new state;

[0278] Similarly, the ohmic resistance, polarization resistance, and polarization capacitance values ​​of the battery at each moment during use are substituted into the polarization resistance energy expression in step 5 to calculate the polarization resistance energy of the battery at each moment during use. The calculation expression is:

[0279]

[0280] Where, E impt R is the polarization resistance of the battery at each moment during use, Ω (t)(214) is the ohmic resistance of the battery at each moment during use, R p (t)(215) is the polarization resistance of the battery at a certain moment during use, C p (t)(216) is the polarization capacitance of the battery at each moment during use;

[0281] The ratio of the battery energy resistance at each moment during the use of the battery to the battery energy resistance in the new state is defined as S oi (t), and the ratio is named as the energy resistance state, and its expression is:

[0282]

[0283] Among them, S oi (t)(62) is the energy resistance state at time t;

[0284] The polarization resistance E of the battery in its new state imp0 Substituting into formula (16), the energy resistance of the battery in a brand new state can be calculated, and the calculation expression is:

[0285]

[0286] Where Soi (0)(63) is the energy resistance of the battery in its new state;

[0287] The identified battery parameter R when thermal runaway is about to occur Ω (s), R p (s), C p Substituting (s) into formula (13), the polarization resistance E of the battery at the moment of internal short circuit can be calculated imps , and substituting it into formula (16), we can further calculate the battery's energy resistance state when an internal short circuit is about to occur:

[0288]

[0289] Where S oi (t s )(64) is the energy resistance state of the battery when an internal short circuit is about to occur.

[0290] Step 7: Safety status determination (7): Two battery safety status determination methods are derived based on polarization resistance. The first method is polarization resistance state determination method, and the second method is polarization resistance state change rate determination method.

[0291] The first determination method is as follows: Figure 5 As shown, the three energy resistance states calculated by equations (16), (17), and (18) are compared to determine the state of the battery:

[0292] When S oi When (t)≥100%, the battery is in a safe state;

[0293] When S oi (t s )≤S oi When (t) is less than 100%, the battery is in a thermal runaway risk state;

[0294] When S oi (t) = S ois = 0, the battery is in thermal runaway state, S ois (65) is the energy resistance state of the battery at the moment of internal short circuit;

[0295] The second method of judgment is as follows: Figure 6 As shown, first calculate E according to formula (13) imp0 、E impt 、E imps 、E impr , define the resistance state change rate at each moment as α E (t), which is calculated as follows:

[0296]

[0297] Where, α E (t)(53) is the rate of change of the polarization resistance state of the battery at time t;

[0298] E imps Substituting into formula (19), the reactance state change rate α of the battery at the moment when an internal short circuit is about to occur can be calculated: E (t s ):

[0299]

[0300] E impr Substituting into formula (19), the reactance state change rate α of the battery at the time of internal short circuit can be calculated: E (t r ):

[0301]

[0302] By comparing α E (t) and α E (t s ), α E (t r ) to determine the current safety status of the battery:

[0303] When α E When (t)≥0, the battery is in a safe state;

[0304] When α E (t s )≤α E When (t)<0, the battery is in a thermal runaway risk state;

[0305] When α E (t) = α E (t r ), the battery is in thermal runaway.

[0306] In summary, the present invention has the following advantages:

[0307] 1. This invention innovates a method for determining the safety status of a battery by using the battery energy resistance state and the rate of change of the energy resistance state, and combines the electrochemical impedance spectroscopy theory with the battery equivalent circuit model and applies it to actual vehicles. To a certain extent, it integrates the physical significance of electrochemical reactions with the simple and easy-to-operate characteristics of equivalent circuit calculations, and can more accurately simulate the dynamic characteristics of batteries.

[0308] 2. From the perspective of battery energy resistance, a new method for determining the battery safety status is proposed. The battery's energy resistance state at the moment when an internal short circuit is about to occur and the battery's energy resistance state at the moment when an internal short circuit occurs are used as two boundaries to determine the battery safety status. This provides a new theoretical basis and determination method for defining the battery safety status, and also provides new ideas for achieving battery safety management.

[0309] 3. The battery resistance is calculated using actual vehicle data, and the battery electrochemical impedance spectroscopy is used to study the battery performance during actual use, which increases the possibility of applying the battery electrochemical impedance spectroscopy method in actual vehicles.

[0310] Obviously, those skilled in the art will appreciate that the various units or steps of the present invention described above can be implemented using a general-purpose computing device, or they can be centralized on a single computing device. Alternatively, they can be implemented using program code executable by a computer device, which can then be stored in a storage device and executed by the computing device. Alternatively, they can be fabricated into separate integrated circuit modules, or multiple modules or steps can be fabricated into a single integrated circuit module for implementation. Thus, the present invention is not limited to any specific combination of hardware and software.

[0311] The above content is a further detailed description of the present invention in combination with specific preferred embodiments. It cannot be considered that the specific embodiments of the present invention are limited to these. For ordinary technicians in the technical field to which the present invention belongs, they can make several simple deductions or substitutions without departing from the concept of the present invention, which should be regarded as belonging to the scope of protection of the present invention determined by the submitted claims.

Claims

1. A lithium-ion battery polarization resistance model and safety state determination method, characterized by: The invention includes an equivalent circuit model (11), model parameters (21), polarization impedance (32), diffusion impedance (41), polarization resistance (51), reactance state (61), reactance state change rate (74), and safety state (73). Based on the above modules, a method for polarization resistance model and safety state determination (6) of lithium-ion batteries is constructed, which includes the following steps: Step 1: Equivalent circuit modeling (1); Step 2: Model parameter identification (2) Step 3: Polarization impedance calculation (3); Step 4: Diffusion impedance calculation (4); Step 5: Observation of polarization resistance (5); Step 6: Estimation of resistance state (6); The seventh step, safety state determination (7), is divided into two types: the first is a safety state determination method based on the reactance state (71), and the second is a safety state determination method based on the reactance state change rate (72); According to the battery resistance state, it is determined whether the battery is in a safe area (75), a thermal runaway critical area (76), or a thermal runaway risk area (77).

2. The polarization resistance energy observation (5) according to claim 1, characterized in that: By combining equations (6) and (49), we can obtain the coordinates of the intersection of the battery polarization impedance and the diffusion impedance. After Fourier transform, we can find that ω = 0 at this intersection; The polarization impedance of the battery is located in the medium and high frequency band, and the angular frequency at the intersection with the horizontal axis is ω = ∞; In summary, the change of battery impedance in the entire frequency range can be equivalent to the change of the area of ​​the battery polarization impedance envelope, and the change of this area directly determines the state of the battery; Therefore, the present invention defines the area enclosed by the battery polarization impedance and the coordinate axis as the battery impedance energy (BIE) (51), which is expressed as follows: Where, E imp (51) is the polarization resistance, Z Im (34) is the imaginary part of the impedance, which represents the battery capacitive reactance in the polarization impedance part, Z Re (33) is the real part of the impedance, which represents the battery impedance in the polarization impedance part; Substituting equations (7) and (8) into equation (58), we can further obtain: As can be seen from the above formula, in actual use, the polarization resistance value can be calculated through the ohmic resistance value, polarization resistance value and polarization capacitance value.

3. The energy resistance state estimation (6) according to claim 1, characterized in that: According to formula (59), the polarization resistance E of the battery in its new state can be obtained by substituting the ohmic resistance, polarization resistance and polarization capacitance of the battery in its new state into imp0 (52), Where R Ω (0)(211) is the ohmic resistance of the battery in its new state, R p (0)(212) is the polarization resistance value of the battery in a new state, C p (0)(213) is the polarization capacitance value of the battery in a brand new state; According to formula (59), the polarization resistance E of the battery at each moment in use can be calculated by substituting the ohmic resistance, polarization resistance and polarization capacitance values ​​of the battery at each moment in use into impt : Where R Ω (t)(214) is the ohmic resistance of the battery at each moment during use, R p (t)(215) is the polarization resistance value of the battery at each time during use, C p (t)(216) is the polarization capacitance value of the battery at each moment during use; The polarization resistance E of the battery at each moment during use impt (53) and the polarization resistance E of the battery in a new state imp0 The ratio of (52) is defined as the battery polarization resistance state (61). Combining equations (60) and (61), the expression of the polarization resistance state can be obtained as follows: Among them, S oi (t)(62) is the polarization resistance energy state at time t.

4. The safety state determination method according to claim 1, wherein the first safety state determination method (71) based on reactance state is characterized by: According to formula (62), the polarization resistance state S of the battery in the new state can be obtained oi (0)(63): Where S oi (0)(63) is the polarization resistance state of the battery in a brand new state; Substituting the ohmic resistance, polarization resistance, and polarization capacitance of the battery at the moment when an internal short circuit is about to occur into formula (59), the polarization resistance of the battery at the moment when an internal short circuit is about to occur can be obtained: Where R Ω (s)(217) is the ohmic resistance of the battery at the moment when an internal short circuit is about to occur, R p (s)(218) is the polarization resistance value of the battery when an internal short circuit is about to occur, C p (s)(219) is the polarization capacitance value of the battery at the moment when an internal short circuit is about to occur; Combining equations (62) and (64), we can obtain the polarization resistance state S of the battery when an internal short circuit is about to occur: oi (t s ): Where, E imps (54) is the polarization resistance of the battery when an internal short circuit is about to occur; As the number of charge and discharge cycles increases, the SEI film of the battery continues to thicken, so the polarization resistance value continues to increase. According to formula (62), when the battery is in the normal aging process, then: S oi (t)≥S oi (0); For a lithium-ion battery that is about to have an internal short circuit, as the lithium dendrites continue to grow, the degree of their penetration into the diaphragm continues to deepen. In the process of the diaphragm being pierced, the battery polarization resistance value continues to decrease. According to formula (62), when the battery is about to have an internal short circuit, then: S oi (t s )≤S oi (t)<100%; When the separator is punctured, the battery will have an internal short circuit. ois =0, S ois It is the polarization resistance state of the battery at the moment of internal short circuit; Based on the above analysis, S oi (t s )(63) and S ois (64) are two critical values ​​for determining the battery safety status. The battery safety status at the current moment can be determined based on these two critical values: When S oi When (t)≥100%, the battery is in a safe state; When S oi (t s )≤S oi When (t) is less than 100%, the battery is in a thermal runaway risk state; When S oi (t) = S ois =0, the battery is in thermal runaway state; As the battery ages, the impedance increases, and the polarization resistance increases until the battery cycle life ends. Therefore, the battery aging state can be evaluated based on the battery polarization resistance at the current moment.

5. The safety state determination method according to claim 1, wherein the second safety state determination method (71) based on the polarization resistance energy state change rate is characterized by: According to formula (62), the calculation formula for the polarization resistance state change rate is established: Where, α E (t)(741) is the rate of change of the polarization resistance state of the battery at different times; According to formula (64), the battery energy resistance state change rate at the moment when thermal runaway is about to occur can be obtained, and the calculation formula is: Where, α E (t s )(742) is the rate of change of the battery's energy resistance state at the moment when the battery is about to have an internal short circuit; Similarly, according to formula (64), the rate of change of the energy resistance state at the time of internal short circuit of the battery can be obtained, and the calculation formula is: Where, α E (t r )(743) is the rate of change of the reactance state of the battery at the moment of internal short circuit, E impr (55) is the battery resistance at the time of internal short circuit; As the battery ages, R p (t)≥R p (0), so E impt -E imp0 ≥0, then, according to formula (66), we can get α E (t)≥0; When the battery is about to have an internal short circuit, R p (s)<R p (0), then E imps -E imp0 <0, at this time, according to formula (67) we can get α E (t s )<0; When the battery is short-circuited, R p (t) = R p (r)=0, then E impr -E imp0 =-E imp0 , at this time, according to formula (68) we can get α E (t) = α E (t r ) = -100%; In summary, α E (t) and α E (t s ) to compare and determine the battery safety status: When α E When (t)≥0, the battery is in a safe state; When α E (t s )≤α E When (t)<0, the battery is in a thermal runaway risk state; When α E (t) = α E (t r ), the battery is in thermal runaway.

6. The safety zone (75) according to claim 1, characterized in that: The area where the battery is in a safe state is the safe area. According to the above two battery safety state determination methods, when S oi (t)≥100% or α E When (t)≥0, the battery is in a safe state. Therefore, the battery safety area division standard can be obtained as follows: Therefore, the area that satisfies the condition of formula (69) is a safe area.

7. The thermal runaway critical region (76) according to claim 1, characterized in that: The area where the battery is in a thermal runaway risk state is the thermal runaway critical area. According to the above two battery safety state determination methods, it can be seen that when S oi (t s )≤S oi (t)<100% or α E (t s )≤α E When (t) < 0, the battery is in a thermal runaway risk state. Therefore, the battery thermal runaway critical area division standard can be obtained as follows: Therefore, the region that satisfies the condition of formula (70) is the critical region of thermal runaway.

8. The thermal runaway risk area (77) according to claim 1, characterized in that: The area where the battery is in thermal runaway state is the thermal runaway risk area. According to the above two battery safety state determination methods, when S oi (t) = S ois =0 or α E (t) = α E (t r ), the battery is in a state of thermal runaway due to an internal short circuit. In summary, the standard for dividing the battery thermal runaway risk area is: Therefore, the area that meets the conditions of formula (71) is the thermal runaway risk area.

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