CN105527581B - The discrimination method of mixed type anode material lithium ion battery key parameter and capacity attenuation mechanism - Google Patents

Abstract

The invention discloses a method for identifying key parameters of a hybrid positive electrode material lithium ion battery, which includes discharging a fully charged battery to be tested with a constant current; obtaining the positive electrode equilibrium potential curve, negative electrode equilibrium potential curve, and hybrid positive electrode The equilibrium potential curve of each component active material in the material, and the dQ/dV curve of each component active material; Calculate the estimated value of the terminal voltage of the battery based on the dQ/dV curve of the material, the constant discharge current and the initial set value of the key parameters of the battery; modify the initial set value of the key parameter until the estimated value of the battery terminal voltage and The root mean square error RMSE between the real values reaches the minimum value, and the final correction results of the key parameters are obtained. The invention also discloses an identification method for identifying the capacity fading mechanism of the lithium-ion battery with mixed positive electrode materials.

Description

Identification of Key Parameters and Capacity Fading Mechanism of Lithium-ion Batteries with Hybrid Cathode Materials method

technical field

The invention belongs to the technical field of battery management, and in particular relates to an identification method for key parameters of a lithium-ion battery with a mixed positive electrode material and an identification method for a capacity fading mechanism.

Background technique

Lithium-ion batteries have the characteristics of high power/energy density and long life, and have been widely used in electric vehicles. Lithium-ion batteries are generally composed of positive electrode materials, negative electrode materials, separators and electrolytes, among which the positive electrode materials have a great impact on the performance of the battery. Commonly used cathode active materials for lithium-ion batteries include ternary materials such as lithium iron phosphate (LFP), lithium cobalt oxide (LCO), lithium manganese oxide (LMO) and nickel-cobalt-manganese (NCM) or nickel-cobalt-aluminum (NCA). However, a single positive electrode active material cannot optimize the performance of the battery. In actual production, two different positive electrode active materials are mixed to form a hybrid positive electrode material to realize the complementary advantages of different positive electrode active materials. A method to improve the electrochemical performance of lithium-ion batteries. After a long period of testing and screening, hybrid cathode materials such as LMO-NCM (NCA) hybrid cathode materials, LFP-NCM (NCA) hybrid cathode materials, and LCO-NCM (NCA) hybrid cathode materials have obtained actual results. application.

There are many ways to define the state of health SOH of the battery, such as through capacity definition, that is, SOH=C/C ₀ , where C is the current capacity of the battery, and C ₀ is the initial capacity of the battery. At the same time, the health status of the battery can also be defined according to the internal resistance, energy/power density and other parameters of the battery. However, the estimation method of the battery state of health SOH at this stage is only to estimate how much the battery capacity has decayed, instead of analyzing the mechanism of the battery capacity decay to determine the real decay situation inside the battery.

The capacity fading mechanism of the battery may be caused by the loss of the positive active material, the loss of the negative active material, or the loss of available lithium ions. For lithium-ion batteries with mixed positive electrode materials, the loss of active materials is more complicated because the mixed positive electrode materials contain different active material components. Take the loss of LMO-NCM hybrid cathode material as an example. Both the loss of LMO active material and the loss of NCM active material will cause the loss of LMO-NCM hybrid cathode material. During the use of the battery, the two cathode active materials, LMO and NCM, may also show different decay rates.

For the research on the mechanism of battery capacity decay, it is often necessary to disassemble the battery and use XRD (X-Ray Diffraction, X-ray diffraction), SEM (Scanning Electron Microscope, scanning electron microscope) and other methods to analyze, but this is not the case in actual electric vehicles. completely impossible. Chinese patent application CN103576097A proposes a non-destructive method for estimating the state of health SOH of lithium-ion batteries, but this method is only applicable to batteries with a single positive electrode active material.

Contents of the invention

In view of this, it is indeed necessary to provide an identification method for the key parameters of lithium-ion batteries with hybrid cathode materials and an identification method for the capacity fading mechanism.

The present invention firstly provides a method for identifying key parameters of a hybrid cathode material lithium-ion battery, comprising the following steps:

S1: Discharge the fully charged battery under test with a constant current, and record the constant current discharge voltage curve during the discharge process to obtain the real value of the battery terminal voltage at different times, denoted as V(t);

S2: Obtain the equilibrium potential curve of the positive electrode, the equilibrium potential curve of the negative electrode, the equilibrium potential curve of each component active material in the mixed positive electrode material, and the charge increment (dQ/dV) curve of each component active material of the battery to be tested;

S3: According to the negative electrode equilibrium potential, the equilibrium potential of each component active material in the mixed positive electrode material, the dQ/dV curve of each component active material in the mixed positive electrode, the constant discharge current and the initial setting of the key parameters of the battery to be tested Calculate the estimated value of the terminal voltage of the battery under test, denoted as V _sim (t);

S4: Correct the initial set value of the key parameter until the root mean square error RMSE between the estimated value V _sim (t) of the terminal voltage of the battery to be tested and the real value V (t) reaches the minimum value, and the key The final correction result of the parameter.

The present invention also provides a method for identifying the capacity fading mechanism of a hybrid positive electrode material lithium-ion battery, comprising the following steps:

S1: Discharge the fully charged battery under test with a constant current, and record the constant current discharge voltage curve during the discharge process to obtain the real value of the battery terminal voltage at different times, which is recorded as V(t); and

S2: Obtain the equilibrium potential curve of the positive electrode, the equilibrium potential curve of the negative electrode, the equilibrium potential curve of each component active material in the mixed positive electrode material, and the dQ/dV curve of each component active material of the battery to be tested;

S3: According to the negative electrode equilibrium potential, the equilibrium potential of each component active material in the mixed positive electrode material, the dQ/dV curve of each component active material in the mixed positive electrode, the constant discharge current and the initial setting of the key parameters of the battery to be tested Calculate the estimated value of the terminal voltage of the battery under test, denoted as V _sim (t);

S4: Correct the initial set value of the key parameter until the root mean square error RMSE between the estimated value V _sim (t) of the terminal voltage of the battery to be tested and the real value V (t) reaches the minimum value, and the key the result of the final correction of the parameters; and

S5: Obtain the capacity fading mechanism of the battery according to the final correction results of the key parameters.

Compared with the prior art, the present invention is aimed at the lithium-ion battery of the hybrid cathode material, considering that the attenuation law of each component active material in the hybrid cathode material is different, and the mass ratio of each component active material will decrease with the decay As for the factors that change, the identification method of key parameters and capacity fading mechanism identification method of lithium-ion battery suitable for hybrid cathode materials are proposed. The outstanding advantage of this method is that it can non-destructively identify the attenuation of the active materials of each component inside the lithium-ion battery of the hybrid cathode material.

Description of drawings

Fig. 1 is a constant current discharge voltage curve after different decay cycles of a lithium ion battery using LMO-NCM hybrid material as the positive electrode active material and graphite as the negative electrode active material according to an embodiment of the present invention.

Fig. 2 is an equilibrium potential curve of a graphite negative electrode according to an embodiment of the present invention.

Fig. 3 is an equilibrium potential curve of an LMO-NCM hybrid cathode material and its components LMO and NCM according to an embodiment of the present invention.

Fig. 4 is a dQ/dV curve of an LMO-NCM hybrid cathode material and its components LMO and NCM according to an embodiment of the present invention.

5 is a schematic diagram of the internal charging and discharging mechanism of a lithium-ion battery using LMO-NCM hybrid positive electrode material as the positive electrode active material and graphite as the negative electrode active material according to an embodiment of the present invention.

6 is a schematic diagram of the distribution mechanism of lithium ions (current) inside the LMO-NCM hybrid positive electrode material according to an embodiment of the present invention.

Fig. 7 is a graph showing the variation of positive and negative electrode equilibrium potentials during discharge of a lithium-ion battery using LMO-NCM hybrid positive electrode material as the positive electrode active material and graphite as the negative electrode active material according to an embodiment of the present invention.

Fig. 8 is a comparison chart of the estimated value and the actual value of the battery terminal voltage after different decay cycles of a lithium-ion battery using LMO-NCM hybrid positive electrode material as the positive electrode active material and graphite as the negative electrode active material according to an embodiment of the present invention.

FIG. 9 is an identification result diagram of internal key parameters of a lithium-ion battery using LMO-NCM hybrid positive electrode material as the positive electrode active material and graphite as the negative electrode active material according to an embodiment of the present invention.

Detailed ways

The identification method of the key parameters and the identification method of the capacity fading mechanism of the hybrid positive electrode material lithium-ion battery of the present invention will be further described in detail below in conjunction with the accompanying drawings.

Firstly, explain some nouns involved in the description of the present invention.

The "electric charge" mentioned in the description of the present invention refers to the actual electric charge of the battery at a certain moment.

The "capacity" mentioned in the description of the present invention refers to the actual power of the battery in a fully charged state, that is, the maximum power that the battery can store.

The "active material" mentioned in the description of the present invention refers to the material that participates in the lithium ion intercalation/extraction reaction in the charge and discharge process in the positive and negative electrodes of the lithium ion battery. Commonly used positive electrode active materials include lithium iron phosphate (LFP), Lithium cobalt oxide (LCO), lithium manganese oxide (LMO), nickel-cobalt-manganese ternary material (NCM) or nickel-cobalt-aluminum ternary material (NCA), etc. Commonly used negative electrode active materials include graphite and lithium titanate.

The "mixed positive electrode material" mentioned in the description of the present invention refers to a positive electrode material that includes two or more positive electrode active materials, which is different from a positive electrode material that only contains a single positive electrode active material. Specifically, the mixed positive electrode material may include any two or more positive electrode active materials in the prior art. Common hybrid cathode materials include but are not limited to LMO-NCM hybrid cathode materials, LMO-NCA hybrid cathode materials, LFP-NCM hybrid cathode materials, LFP-NCA hybrid cathode materials, and LCO-NCM hybrid cathode materials currently on the market. type cathode material and LCO-NCA hybrid cathode material.

In the "mixed positive electrode material lithium ion battery" mentioned in the description of the present invention, the positive electrode material is the mixed positive electrode material, that is, the positive electrode includes two or more positive electrode active materials, and the negative electrode preferably includes only one negative electrode. active material.

The "equilibrium potential" mentioned in the description of the present invention refers to the electrode potential when the positive and negative electrode materials of the battery undergo slow lithium ion intercalation/extraction reactions (that is, very small current charge and discharge). quasi-balanced state. The equilibrium potential of the positive and negative electrode materials of lithium-ion batteries corresponds to the lithium ion fraction of the material one-to-one, and is generally expressed by the potential for lithium.

The "power increment" mentioned in the description of the present invention refers to the ratio of the power dQ discharged by the battery to the corresponding battery voltage drop value dV at different voltages V. For example, when the battery voltage is V, the discharge of electricity dQ causes the battery voltage to decrease by dV, and the corresponding electricity increment is dQ/dV.

The "lithium ion fraction" mentioned in the description of the present invention refers to the normalized content of lithium ions in the positive and negative electrode active materials, and the value interval is [0, 1]. Taking the negative electrode as an example, the chemical formula of the negative electrode active material is Li _x C ₆ , where x is the lithium ion fraction of the negative electrode active material. When the negative electrode active material is completely filled with lithium, x is equal to 1, and when it is completely delithiated, x is equal to 0. .

The invention provides a method for identifying key parameters of a hybrid cathode material lithium-ion battery, comprising the following steps:

S1: Discharge the fully charged battery under test with a constant current, and record the constant current discharge voltage curve during the discharge process to obtain the true value of the battery terminal voltage at any time, which is recorded as V(t);

S2: Obtain the equilibrium potential curve of the positive electrode, the equilibrium potential curve of the negative electrode, the equilibrium potential curve of each component active material in the mixed positive electrode material, and the charge increment (dQ/dV) curve of each component active material of the battery to be tested;

S3: Calculation based on the equilibrium potential of the negative electrode, the equilibrium potential of each component active material in the mixed positive electrode material, the dQ/dV curve of each component active material in the mixed positive electrode, the constant discharge current and the initial set value of the key parameters of the battery An estimate of the terminal voltage of the battery, denoted as V _sim (t);

S4: Correct the initial set values of the key parameters until the standard error RMSE between the estimated value V _sim (t) of the battery terminal voltage and the real value V(t) reaches the minimum value, and obtain the final correction result of the key parameters.

In step S1, the entire discharge time of the discharge process from the fully charged state to the discharged state is t ₁ to t _n , there are n sampling points in total, n is a positive integer, and any moment is t _k . The voltage curve may be a voltage-time curve or a voltage-electricity curve.

In step S2, the balanced potential curves of the positive and negative electrodes of the battery and the balanced potential curves of each component material of the mixed positive electrode material can be obtained by charging and discharging the half-cell with a small current (less than 0.04C), and will not be described here.

The positive electrode of the battery to be tested may include a first positive electrode active material, denoted as A, and a second positive electrode active material, denoted as B. The step S2 can respectively obtain the equilibrium potential curve and dQ/dV curve of the first positive electrode active material, and the equilibrium potential curve and dQ/dV curve of the second positive electrode active material.

The key parameters may include the capacity and lithium ion fraction of a certain component active material in the mixed positive electrode material, the capacity of the negative electrode active material, the lithium ion fraction of the negative electrode active material and the internal resistance of the battery to be tested.

The dQ/dV curve can reflect the chemical reaction process inside the battery, and can be obtained by methods such as the counting method (such as Chinese published patent application CN103698714A), and will not be repeated here. At the same voltage, the dQ/dV value of the hybrid positive electrode material is the sum of the dQ/dV value of the first positive electrode active material and the dQ/dV value of the second positive electrode active material. This is because during the charging and discharging process of the battery, the two active materials, the first positive electrode active material and the second positive electrode active material, are in a parallel connection state. During the discharge process of lithium-ion batteries, lithium ions are released from the negative electrode, pass through the electrolyte and separator, and are embedded in the positive electrode material. In the mixed positive electrode material, the two components of the first positive electrode active material and the second positive electrode active material are in the same equilibrium potential at all times, which is equivalent to the parallel connection of the two components, and lithium ions are based on the dQ/dV of the two components under the equilibrium potential. The specific gravity of the value is respectively embedded in the first positive electrode active material and the second positive electrode active material to ensure that the equilibrium potentials of the first positive electrode active material and the second positive electrode active material remain equal.

In an embodiment of the present invention, the estimated value V _sim (t _k ) of the terminal voltage of the battery under test=V _p (t _k )-V _n (x(t _k ))-I×R, wherein V _p ( t _k ) is the positive electrode equilibrium potential at time t _k , V _n (x(t _k )) is the negative electrode equilibrium potential with lithium ion fraction x(t _k ) at time t _k , R is the initial setting value of the internal resistance of the battery, I is the constant current, that is, the total amount of lithium ions embedded in the positive electrode material per unit time.

In an embodiment of the present invention, the V _p (t _k ) satisfies V _p (t _k )=V _p,A (y _A (t _k ))=V _p,B (y _B (t _k )), y _A (t _k ) is the lithium ion fraction of the first positive electrode active material at time t _k , V _p,A (y _A (t _k )) is the first positive electrode activity with lithium ion fraction y _A (t _k ) at time t _k The equilibrium potential of the material, y _B (t _k ) is the lithium ion fraction of the second positive electrode active material at time t _k , V _p,B (y _B (t _k )) is the lithium ion fraction at time t _k is y _B (t _k ) The equilibrium potential of the second positive electrode active material.

The y _A (t _k ) and x(t _k ) can respectively satisfy:

x(t _k )=x ₀ -I×t _k /C _N ;

Wherein, x(t _k ) is the lithium ion fraction of the negative electrode active material at t _k moment, x ₀ is the initial setting value of the lithium ion fraction of the negative electrode active material, C _N is the initial setting value of the capacity of the negative electrode active material, P (t _k ) is the ratio of the number of lithium ions embedded in the first positive electrode active material to the total number of lithium ions at time t _k , y _{0, A} is the initial set value of the lithium ion fraction of the first positive electrode active material, C _A is the initial set value of the capacity of the first positive electrode active material, and Δt is the sampling time interval.

The P(t _k ) can satisfy:

Wherein dQ _A /dV is the charge gain of the first positive electrode active material, dQ _B /dV is the charge gain of the second positive electrode active material, and c is the mass ratio (m _B of the second positive electrode active material to the first positive electrode active material /m _A ).

The root mean square error RMSE between the estimated value V _sim (t) of the terminal voltage of the battery under test and the real value V (t) is:

The present invention further provides a method for identifying the capacity fading mechanism of a lithium-ion battery with a hybrid positive electrode material, which includes the above steps S1 to S4, and further includes step S5: obtaining the capacity fading mechanism of the battery according to the final correction results of key parameters.

The step S5 may include obtaining the final correction result of the key parameter after the battery under test undergoes different charge and discharge cycles, so as to obtain the decay rate of the key parameter with the number of charge and discharge cycles of the battery under test. The attenuation rate is the ratio of the difference between the initial set value X ₀ of a certain key parameter and the final correction result X' to the initial set value (=(X ₀ -X')/X ₀ ×100%), According to the attenuation rate of the key parameter that decreases with the increase of the number of charge and discharge cycles, the attenuation degree of the key parameter can be analyzed, so as to determine whether the capacity attenuation of the battery is caused by the attenuation of the key parameter.

The step S5 may further include obtaining the amount Σ(Li/Li ⁺ ) of the available lithium ions inside the battery to be tested according to the final correction result of the key parameter,

Σ(Li/Li ⁺ )=x' ₀ C' _N +y' _0,A C' _A +y' _0,B C' _B ,

Wherein x' ₀ is the corrected value of the lithium ion fraction of the negative electrode active material at t ₀ , C' _N is the corrected value of the capacity of the negative electrode active material, y' _{0, A} is the lithium ion fraction of the first positive electrode active material at t ₀ , C' _A is the correction value of the capacity of the first positive electrode active material, y' _{0, B} is the correction value of the lithium ion fraction of the second positive electrode active material at t ₀ , C' _B is the second positive electrode active material The correction value of the capacity of y' _0,B is calculated according to V _p,A (y' _0,A )=V _p,B (y' _0,B )y' _0,A , C' _B =c×k ₀ ×C' _A . k ₀ is the initial capacity ratio of the first positive electrode active material and the second positive electrode active material, that is, C _A /C _B

This step S5 can further include according to the attenuation rate of the amount Σ(Li/Li ⁺ ) of available lithium ions, and the key parameters of the battery to be tested and the attenuation rate of the amount of available lithium ions obtained in the comprehensive step S5, the test can be obtained The internal decay mechanism of the battery.

The lithium-ion battery whose positive electrode material is LMO-NCM hybrid positive electrode material and the negative electrode active material is graphite is used as the battery to be tested, and the identification method of the key parameters of the lithium-ion battery of the present invention and the identification method of the capacity fading mechanism are further described in detail. .

In step S1, Fig. 1 shows the constant current discharge voltage curves of a lithium-ion battery in which the positive electrode material is LMO-NCM hybrid positive electrode material and the negative electrode active material is graphite under different attenuation conditions. In Figure 1, 0 cycle represents the constant current discharge voltage curve of the new battery, and 120 cycles, 240 cycles, and 360 cycles represent the constant current discharge voltage curves of the battery after 120, 240 and 360 durability cycles respectively. It can be seen from Figure 1 that under different attenuation conditions, the constant current discharge voltage curve of the battery is very different, and it is not a simple translation and scaling relationship. The invention aims at analyzing the capacity decay mechanism and SOH of the battery to be tested according to the constant current discharge voltage curve of the battery to be tested after a certain cycle.

In step S2, FIG. 2 is the equilibrium potential curve of the graphite negative electrode of the lithium-ion battery in this embodiment. The equilibrium potential of graphite anode varies with the internal lithium ion fraction, the closer the lithium ion fraction is to 1, the lower the potential of graphite. It should be noted that the lithium ion fraction here refers to the equivalent lithium ion fraction. When the equilibrium potential of graphite is defined as 0.05V, the lithium ion fraction is 1, and when the equilibrium potential is 1.5V, the lithium ion fraction is 0. During the charging and discharging process of the battery, lithium ions undergo intercalation/deintercalation reactions inside the negative electrode material, and the lithium ion fraction changes in the range of 0 to 1. Correspondingly, the equilibrium potential of the negative electrode also changes in the range of 0 to 1.5V.

Fig. 3 is the equilibrium potential curve of the LMO-NCM hybrid positive electrode material and its components LMO and NCM of the lithium ion battery in this embodiment. Similarly, the lithium ion fraction here refers to the equivalent lithium ion fraction. When the equilibrium potential is 3V, the lithium ion fraction is 1, and when the equilibrium potential is 4.3V, the lithium ion fraction is 0. During the charging and discharging process of the battery, lithium ions undergo intercalation/extraction reactions in the positive electrode material, and the lithium ion fraction changes in the range of 0 to 1. Correspondingly, the positive electrode equilibrium potential also changes in the range of 3 to 4.3V. It can be seen from Figure 3 that the slope of the equilibrium potential curve of the NCM material is relatively steep, while the equilibrium potential curve of the LMO material is relatively flat, and the equilibrium potential curve of the LMO-NCM hybrid cathode material is between the two.

Fig. 4 is the charge gain (dQ/dV) curve of the LMO-NCM hybrid positive electrode material and its components LMO and NCM in this embodiment. It can be seen from Figure 4 that at the same voltage, the dQ/dV value of the LMO-NCM hybrid cathode material is the sum of the dQ/dV value of LMO and the dQ/dV value of NCM. This is because the two active materials, LMO and NCM, are connected in parallel during the charging and discharging process of the battery, as shown in Figure 5. Taking discharge as an example, during the discharge process of lithium-ion batteries, lithium ions are released from the negative electrode, pass through the electrolyte and separator, and are embedded in the positive electrode material. In the hybrid positive electrode material, the two components of LMO and NCM are always at the same equilibrium potential, which is equivalent to connecting them in parallel, and lithium ions are embedded in it according to the proportion distribution of the dQ/dV value of the two at the equilibrium potential to ensure The equilibrium potentials of the LMO and NCM are kept equal. In Figure 6, the potential change of NCM is taken as an example to analyze the process of lithium ion intercalation in hybrid cathode materials. At time t _k , the fraction of lithium ions in NCM is y _NCM (t _k ), and the corresponding equilibrium potential is V _p,NCM ( y _NCM (t _k )), at this time, according to the dQ/dV value of LMO and NCM, the proportion of lithium ions embedded in the NCM material in all lithium ions (number ratio of lithium ions) can be calculated as P(t _k ):

Among them, c is the mass ratio of LMO to NCM in the hybrid cathode material, and the initial value is set to 1. At time t _k+1 , the lithium ion fraction in the NCM material increases by Δy _NCM (t _k ), and

Δy _NCM (t _k )=P(t _k )·I·Δt/C _NCM ,

Among them, I is the discharge current, that is, the total amount of lithium ions embedded in the positive electrode material per unit time, Δt is the sampling time interval, and C _NCM is the capacity of the NCM active material. Accordingly, the lithium ion fraction y _NCM (t _k+1 )=y _NCM (t _k )+Δy _NCM (t _k ) of the NCM at time t _k+1 can be obtained. By analogy, the lithium ion fraction in the NCM at each moment can be calculated.

In step S3, according to the equilibrium potential of the negative electrode, the equilibrium potential of each component active material in the mixed positive electrode material, the dQ/dV curve of each component active material in the mixed positive electrode, the constant discharge current and the initial setting of the key parameters of the battery The value calculates an estimate of the terminal voltage of the battery, denoted as V _sim (t). The key parameters include, the capacity of NCM, the lithium ion fraction of NCM, the capacity of graphite anode, the lithium ion fraction of graphite anode, and the internal resistance of the battery. Here, the initial setting value of the capacity of the NCM is C _NCM , the initial value of the lithium ion fraction of the NCM is y _0,NCM , the initial setting value of the capacity of the graphite negative electrode is C _N , and the initial value of the lithium ion fraction of the graphite negative electrode is The value is set to x ₀ , the initial value of the internal resistance of the battery is set to R, and the constant current is I. During the discharge process, at time tk, the lithium ion fraction y _NCM of the NCM, and the lithium ion fraction _x of the graphite negative electrode are calculated as:

x(t _k )＝x ₀ -I×t _k /C _N (3)

As shown in Figure 6, the terminal voltage V _sim (t _k ) of the battery is equal to the difference between the positive and negative equilibrium potentials, and the voltage loss caused by the internal resistance is subtracted at the same time. The calculation formula is as follows:

V _sim (t _k )=V _p (t _k )-V _n (x(t _k ))-I×R (4)

Among them, the equilibrium potential of the hybrid positive electrode material is equal to the equilibrium potential of NCM, as follows:

V _p (t _k ) = V _p,NCM (y _NCM (t _k )) = V _p,LMO (y _LMO (t _k ))

In step S4, assuming the entire discharge time t ₁ to t _n , there are n sampling points in total, the root mean square error RMSE between the estimated value V _sim (t) of the battery terminal voltage and the real value V (t) is calculated by the following formula get:

Given different [x ₀ , C _N , y _{0, NCM} , C _NCM , c, R], different battery terminal voltage estimates V _sim (t) can be obtained, and the corresponding root mean square error RMSE can be obtained by the formula ( 6) Calculated. The key parameters [x ₀ , C _N , y _{0, NCM} , C _NCM , c, R] are corrected by the genetic algorithm and other optimal estimation algorithms, and the optimal key parameters can be found, so that the estimated value of the battery terminal voltage V _sim The root mean square error RMSE between (t) and the true value V(t) is the smallest, as shown in Figure 7.

Fig. 8 compares the curves of the estimated value and the real value of the battery terminal voltage after different cycles of the battery. For the convenience of comparison, the corresponding positive and negative equilibrium potential curves are also drawn in the figure. In Figure 8, ① is the comparison result of the new battery, ②, ③ and ④ are the comparisons between the estimated terminal voltage and the real value of the battery after 120, 240, and 360 durability cycles respectively. It can be seen that after different times After the endurance cycle, the estimated value of the battery terminal voltage is in good agreement with the real value, and the root mean square error of the two has been kept within 6mV.

The present invention further provides a method for identifying the capacity fading mechanism of a lithium-ion battery with a hybrid positive electrode material, which includes the above steps S1 to S4, and further includes step S5: obtaining the capacity fading mechanism of the battery according to the final correction results of key parameters.

In step S5, according to the optimal correction results of the key parameters, the capacity fading mechanism of the lithium-ion battery can be obtained. Among the obtained key parameters [x ₀ , C _N , y _{0, NCM} , C _NCM , c, R], C _NCM and C _N can directly reflect the change of the capacity of NCM and graphite negative electrode, and R can reflect the change of battery internal resistance . In addition, the capacity C _LMO of LMO can be calculated by the following formula, where k ₀ is the initial capacity ratio of NCM and LMO, which can be calculated according to the dQ/dV curve.

C _LMO ＝c×k ₀ ×C _NCM (7)

The attenuation of lithium-ion batteries may also be caused by the loss of available lithium ions. According to the obtained key parameters, the amount of available lithium ions inside the battery can be calculated by the following formula:

Σ(Li/Li ⁺ )=x ₀ C _N +y _{0, NCM} C _NCM +y _{0, LMO} C _LMO (8)

Fig. 9 is the identification result of the key parameters of the internal mechanism of the battery obtained according to an embodiment of the present invention, including LMO, NCM capacity, graphite negative electrode capacity, the amount of available lithium ions and the internal resistance of the battery.

Through the examples, it can be seen that the method for identifying the attenuation mechanism of the lithium-ion battery with hybrid cathode materials proposed by the present invention can obtain the attenuation mechanism inside the battery, especially the attenuation of each component material in the hybrid cathode material, which is helpful for comprehensive Know the health status of your battery.

In addition, those skilled in the art can also make other changes within the spirit of the present invention, and these changes made according to the spirit of the present invention should be included in the scope of protection claimed by the present invention.

Claims (9)

1. A method for identifying key parameters of a hybrid cathode material lithium-ion battery, comprising the steps of: S1: Discharge the fully charged battery under test with a constant current, and record the constant current discharge voltage curve during the discharge process to obtain the real value of the battery terminal voltage at different times. The entire discharge time of the discharge process is t ₁ to t _n , there are n sampling points in total, n is a positive integer, at any moment t _k , the real value of the battery terminal voltage is recorded as V(t _k ); S2: Obtain the equilibrium potential curve of the positive electrode, the equilibrium potential curve of the negative electrode, the equilibrium potential curve of each component active material in the mixed positive electrode material, and the charge increment curve of each component active material of the battery to be tested; S3: According to the negative electrode equilibrium potential, the equilibrium potential of each component active material in the mixed positive electrode material, the charge increment curve of each component active material in the mixed positive electrode, the constant discharge current and the initial setting of the key parameters of the battery to be tested Calculate the estimated value of the terminal voltage of the battery to be tested, denoted as V _sim (t _k ), the estimated value of the battery terminal voltage V _sim (t _k ) is equal to the difference between the positive and negative equilibrium potentials while subtracting the voltage loss caused by internal resistance , wherein the key parameters of the battery to be tested include the capacity and lithium ion fraction of a certain component active material in the hybrid positive electrode material, the capacity of the negative electrode active material, the lithium ion fraction of the negative electrode active material, and the internal resistance of the battery to be tested ; S4: Correct the initial set value of the key parameter until the root mean square error RMSE between the estimated value V _sim (t _k ) of the terminal voltage of the battery to be tested and the actual value V(t _k ) reaches the minimum value, Get the final correction results of the key parameters. 2. the identification method of the key parameter of mixed positive electrode material lithium ion battery as claimed in claim 1, is characterized in that, the estimated value V _sim (t _k )=V _p (t _k )- V _n (x(t _k ))-I×R, where V _p (t _k ) is the positive electrode equilibrium potential at time t _k , V _n (x(t _k )) is the lithium ion fraction at time t _k is x(t _k ) negative electrode equilibrium potential, R is the initial setting value of the internal resistance of the battery, and I is the constant current. 3. the identification method of the key parameter of hybrid positive electrode material lithium ion battery as claimed in claim 2, is characterized in that, this battery positive electrode to be tested comprises the first positive electrode active material, denoted as A, and the second positive electrode active material, denoted as For B, the V _p (t _k ) satisfies: V _p (t _k ) = V _p,A (y _A (t _k )) = V _p,B (y _B (t _k )), where y _A (t _k ) is the lithium ion fraction of the first cathode active material at time t _k , V _p,A (y _A (t _k )) is the equilibrium potential of the first cathode active material with lithium ion fraction y _A (t _k ) at time t _k , y _B (t _k ) is the lithium ion fraction of the second positive electrode active material at time t _k , V _p,B (y _B (t _k )) is the equilibrium potential of the second cathode active material with lithium ion fraction y _B (t _k ) at time t _k . 4. the identification method of the key parameter of hybrid cathode material lithium ion battery as claimed in claim 3, is characterized in that, this y _A (t _k ) and x (t _k ) satisfy respectively: x(t _k )=x ₀ -I×t _k /C _N ; Among them, x(t _k ) is the lithium ion fraction of the negative electrode active material at time t _k , x ₀ is the initial set value of the lithium ion fraction of the negative electrode active material, C _N is the initial setting value of the capacity of the negative electrode active material, P(t _k ) is the ratio of the number of lithium ions embedded in the first positive electrode active material to the total number of lithium ions at time t _k , y _{0, A} is the initial set value of the lithium ion fraction of the first positive electrode active material, C _A is the initial setting value of the capacity of the first positive electrode active material, Δt is the sampling time interval. 5. the identification method of the key parameter of hybrid cathode material lithium ion battery as claimed in claim 4, is characterized in that, this P (t _k ) satisfies: Where dQ _A /dV is the charge gain of the first positive electrode active material, dQ _B /dV is the charge gain of the second positive electrode active material, and c is the mass ratio of the second positive electrode active material to the first positive electrode active material. 6. The identification method of the key parameter of hybrid positive electrode material lithium-ion battery as claimed in claim 1, is characterized in that, the terminal voltage estimated value V _sim (t _k ) of the battery to be tested and the real value V (t _k ) The root mean square error RMSE between is: 7. A method for identifying the capacity fading mechanism of a hybrid positive electrode material lithium-ion battery, comprising steps S1-S4 in any one of claims 1-6, and further comprising step S5: obtaining the battery's capacity according to the final correction results of key parameters Mechanism of capacity fading. 8. The method for identifying the capacity fading mechanism of a hybrid positive electrode material lithium-ion battery as claimed in claim 7, wherein the step S5 includes obtaining the final correction of the key parameter after the battery under test undergoes different charge and discharge cycles respectively. As a result, the attenuation rate of the key parameter along with the number of charge and discharge cycles of the battery to be tested can be obtained. 9. The method for identifying the capacity fading mechanism of a lithium-ion battery with a hybrid positive electrode material as claimed in claim 7, wherein the step S5 further comprises obtaining the available lithium ions inside the battery to be tested according to the final correction result of the key parameter. Quantity Σ(Li/Li ⁺ ): Σ(Li/Li ⁺ )=x' ₀ C' _N +y' _0,A C' _A +y' _0,B C' _B , Where _x'0 is the corrected value of the lithium ion fraction of the negative electrode active material, C' _N is the correction value of the capacity of the negative electrode active material, y' _{0, A} is the corrected value of the lithium ion fraction of the first positive electrode active material, C' _A is the correction value of the capacity of the first positive electrode active material, y' _{0, B} is the corrected value of the lithium ion fraction of the second positive electrode active material, C' _B is a correction value for the capacity of the second positive electrode active material.