CN114914560A

CN114914560A - Method for determining proportion of conductive agent in double-layer distributed lithium ion battery pole piece

Info

Publication number: CN114914560A
Application number: CN202210343561.6A
Authority: CN
Inventors: 康健强; 宋海钦; 王菁; 朱国荣
Original assignee: Wuhan University of Technology WUT
Current assignee: Wuhan University of Technology WUT
Priority date: 2022-04-02
Filing date: 2022-04-02
Publication date: 2022-08-16

Abstract

The invention discloses a method for determining the proportion of a conductive agent in a double-layer distributed lithium ion battery pole piece, which comprises the steps of manufacturing a lithium iron phosphate positive pole piece with the double-layer distributed conductive agent, manufacturing the lithium iron phosphate positive pole piece into a battery, and carrying out formation activation, constant multiplying power cycle test, multiplying power performance test, alternating current impedance test and volt-ampere cycle test on the battery, so that the difference between battery samples manufactured by pole pieces with different conductive distributions can be obtained, and the lithium ion diffusion coefficient of the battery can be obtained through calculation. According to the invention, through the test, the weight evaluation method is used, and various performances are comprehensively considered, so that the excellent distribution ratio of the gradient conductive agent is obtained.

Description

Method for determining proportion of conductive agent in double-layer distributed lithium ion battery pole piece

Technical Field

The invention belongs to the field of lithium ion batteries, and particularly relates to a method for determining the proportion of a conductive agent in a double-layer lithium ion battery pole piece.

Background

Currently, lithium ion batteries have been widely used in various consumer electronics, electric vehicles, and energy storage systems, because they have advantages of high energy density, long cycle life, no memory effect, etc. compared with batteries of other systems. Compared with other types of batteries, such as a ternary system lithium ion battery, the lithium iron phosphate battery has more excellent safety and cycle performance, is more applied in the field of commercial vehicles, but has lower energy density, and how to improve the energy density is a hot problem of research.

The components of a lithium ion battery generally include: the battery comprises a positive electrode material, a positive electrode current collector, a negative electrode material, a negative electrode current collector, a diaphragm, electrolyte, an external coating part (a cylindrical steel shell, a square aluminum shell and an aluminum-plastic composite film) and the like, wherein the type and the proportion of the positive electrode material and the negative electrode material have great influence on the final capacity of the battery. For materials of various systems, the self-releasable capacity of the materials is limited by the physical and chemical properties of the materials, and the energy density of the battery cannot be infinitely increased, so that currently, the improvement of the energy density of the battery can only be achieved by adding other components into an electrode active material to improve the conductivity and ion diffusion capacity of the active material so as to increase the energy released by the electrode material.

The patent CN 109585779 a discloses a method for preparing a lithium ion battery positive electrode plate, wherein the active material of the lithium ion battery positive electrode plate is distributed in a double-layer structure, and the double-layer structure is composed of a current collector aluminum foil, a first active material coating and a second active material coating, the two active materials are different in component, and the thickness of the first active material coating is greater than that of the first active material coating; similarly, the negative electrode tab also has such a structure. The battery pole piece prepared by the battery pole piece can improve the power density of the battery under the condition of not remarkably reducing the energy density of the battery, thereby giving consideration to both the energy density and the power density of the battery. However, the method cannot determine an optimal component ratio, and the specific effect of improving the battery performance of the pole piece structure is not obvious in quantification, so that the superiority of the pole piece is not enough to be evaluated only from the energy density and the power density.

The patent CN 112436103 a discloses a double-layer structure pole piece, and a preparation method and an application thereof, the lithium ion battery pole piece is also a double-layer structure, and the peeling force between the pole piece active material and the current collector can be improved by adjusting the proportion of the binder in the upper and lower layer active materials, so as to reduce the internal resistance and improve the cycle performance of the battery. However, the method still cannot determine an optimal component ratio, and a better judgment method is not proposed.

Therefore, for the lithium ion battery pole piece with the double-layer structure, certain differences exist among the components of the layers, the differences can cause the performance differences of the battery in all aspects, the superiority of the structure of a certain component is not enough to be judged only from a certain aspect, and the comprehensive evaluation needs to be carried out through a scientific experimental test method and a calculation method.

Disclosure of Invention

Aiming at the defects in the prior art, the invention aims to provide a method for determining the proportion of a conductive agent in a double-layer lithium ion battery pole piece, which can obtain the difference between battery samples manufactured by pole pieces with different conductive distributions, and can also calculate the lithium ion diffusion coefficient of a battery to obtain a more excellent gradient conductive agent distribution ratio.

The purpose of the invention is realized by the following technical scheme:

a method for determining the proportion of a conductive agent in a double-layer distributed lithium ion battery pole piece comprises the following steps:

(1) manufacturing lithium ion battery pole pieces with different double-layer distributions into batteries, activating the batteries, performing constant-rate cyclic test, gradient-rate performance test, alternating-current impedance test and cyclic voltammetry test on the batteries, extracting characteristic parameters from each test result, normalizing, adding a weight as an evaluation score under the test result, and finally integrating each score to obtain a total score under the ratio;

(2) taking the ratio of the conductive agents of the lithium ion battery pole pieces distributed in different double layers as an independent variable x, taking the corresponding total score as a dependent variable, fitting to obtain a fitting curve, wherein the numerical value of an x axis corresponding to the maximum value of the curve is an optimal ratio, and the optimal ratio is used for determining the ratio of the conductive agents when the battery pole pieces are prepared;

the characteristic parameters comprise a characteristic parameter 1, a characteristic parameter 2, a characteristic parameter 3, a characteristic parameter 4 and a characteristic parameter 5, wherein the characteristic parameter 1 is the maximum capacity value of low-rate charge and discharge in the formation process, the characteristic parameter 2 is the variance of a curve of a constant-rate cyclic test, the characteristic parameter 3 is the variance of a curve of a gradient rate performance test, the characteristic parameter 4 is a diffusion coefficient measured by an alternating current impedance test, and the characteristic parameter 5 is the redox peak width of the curve of the cyclic voltammetry test.

Preferably, in the step (2), a MATLAB fitting toolbox is adopted for fitting, and a fitting curve is obtained after fitting.

Preferably, the lithium ion battery pole pieces distributed in different double layers are manufactured into the button battery in the step (1).

Preferably, during formation activation, the current multiplying power of constant-voltage charging and constant-current discharging is set to be 0.1C, the cut-off multiplying power is set to be 0.05C, the charging and discharging voltage interval is set to be 2-3.65V, and the specific multiplying current is obtained by weighing and calculating according to actually manufactured battery pole pieces.

Preferably, in the constant-rate cycle test, the current rate is set to be 0.5C, and the number of times of cyclic charge and discharge is 100.

Preferably, in the gradient multiplying power performance test, the current multiplying power is set to 7 working conditions of 0.3C, 0.5C, 0.8C, 1C, 1.5C, 2C and 0.3C, and the next multiplying power working condition is entered after 10 cycles are performed at each multiplying power.

Preferably, in the alternating current impedance test process, an electrochemical workstation is used for testing, the scanning speed is set to be 0.5mV/s, the scanning voltage interval is 2.5-4.2V, and the scanning frequency is 10 kHz-10 mHz.

Preferably, in the cyclic voltammetry test, the scanning speed is set to be 0.5mV/s, and the scanning voltage interval is 2.5-4.2V.

Preferably, the environmental temperature of the constant multiplying power cycle test, the gradient multiplying power performance test, the alternating current impedance test and the cyclic voltammetry test is 25 ℃.

Preferably, the active material of the positive electrode material of the button cell is at least one of lithium iron phosphate, lithium cobaltate, lithium manganate and lithium nickel cobalt manganese oxide; the negative electrode material of the button cell is graphite, the positive current collector of the button cell is aluminum foil, and the negative current collector of the button cell is copper foil.

During preparation of the positive electrode material of the button battery, polyvinylidene fluoride (PVDF) is used as a binder, N-methyl pyrrolidone (NMP) is used as a solvent, and at least one of acetylene black, a carbon nano tube and graphene is used as a conductive agent; the button cell comprises a positive electrode active material, a conductive agent, a binder and a solvent, wherein the positive electrode active material accounts for 80-95 wt% of the mass fraction of the positive electrode material, the conductive agent accounts for 2.5-12 wt% of the mass fraction of the positive electrode material, the binder accounts for 2.5-8 wt% of the mass fraction of the positive electrode material, and the mass of the solvent is 3 times that of the solute.

Preferably, the weight of the characteristic parameter 1 is 0.15; the weight of the characteristic parameter 2 is 0.2; the weight of the characteristic parameter 3 is 0.2; the weight of the characteristic parameter 4 is 0.05; the weight of the characteristic parameter 5 is 0.2; the battery is a button battery.

Compared with the prior art, the invention has the beneficial effects that:

under the constant temperature condition, the capacity, the aging characteristic, the impedance characteristic and the volt-ampere characteristic of the battery manufactured by electrode plates with different component proportions can be obtained through low-rate charge and discharge, constant-rate charge and discharge cycle test, rate performance test, alternating current impedance test and volt-ampere cycle test in the formation process, and the diffusion coefficient of lithium ions is obtained through calculation, so that the optimal comprehensive performance manufactured by the double-layer structure battery electrode plate with different components is comprehensively judged.

Drawings

Fig. 1 is a schematic structural diagram of a two-layer electrode material layer of sample 1, where a is a first electrode material layer and B is a second electrode material layer.

FIG. 2 is a graph showing the relationship between voltage and specific capacity of battery samples 1-5 during formation.

FIG. 3 is a cyclic voltammetry curve of battery samples 1-5, with a scan rate of 0.5mV/s and a scan voltage interval of 2.5-4.2V.

FIG. 4 is a graph of the aging characteristics of battery samples 1-5 in a constant rate charge-discharge cycle test.

FIG. 5 is a graph of aging characteristics of battery samples 1-5 in a cycle test under gradient magnification.

FIG. 6 is an AC impedance spectrum of battery samples 1-5 after formation at a current magnification of 0.1C.

Fig. 7 is a first-order RC equivalent circuit with weber impedance.

FIG. 8 shows the real part of impedance Z _re And omega ^-0.5 Actual and fitted linear relationship.

FIG. 9 is a graph showing the performance combination scores and the total score fit of the battery samples 1-5.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

The preparation process of the positive plate of the button cell is as follows:

(1) preparing lithium ion battery positive plates with different double-layer distributions: mixing lithium iron phosphate powder, acetylene black, PVDF and NMP to prepare slurry A and B with different conductive agent contents, wherein the mass ratio of the slurry A to the slurry B in samples 1-5 is 399.5:390.5, 399:391, 397:393, 395:395 and 393:397 respectively. Fixing an aluminum foil on a glass plate, uniformly coating the prepared slurry A on the aluminum foil, putting the electrode plate into a vacuum constant temperature box, and carrying out vacuum drying under the vacuum drying condition of 80 ℃ for 12 hours to obtain a first electrode material layer; uniformly coating the prepared slurry B on the first electrode material layer, putting the electrode plate into a vacuum constant temperature box again, and carrying out vacuum drying under the vacuum drying condition of 80 ℃ for 12 hours to obtain a second electrode material layer; and adjusting the gap between two press rolls of the roll squeezer, feeding the pole piece into the roll squeezer for rolling for multiple times, wherein the adjustment range of the gap between the press rolls is 40-60 microns, and finally obtaining the pole piece with the thickness of 42 microns, wherein the thickness of the aluminum foil is 26 microns, and the thickness of the electrode material is 16 microns. Here, 5 positive plate samples were prepared, wherein the mixture ratios of the slurry a and the slurry B were respectively:

sample 1:

slurry A: the mass part ratio of the lithium iron phosphate powder to the acetylene black to the PVDF to the NMP is 80:9.5: 10:300, respectively;

slurry B: the mass part ratio of the lithium iron phosphate powder to the acetylene black to the PVDF to the NMP is 80:0.5: 10:300, respectively;

sample 2:

slurry A: the mass part ratio of the lithium iron phosphate powder to the acetylene black to the PVDF to the NMP is 80:9: 10:300, respectively;

slurry B: the mass part ratio of the lithium iron phosphate powder to the acetylene black to the PVDF to the NMP is 80:1: 10: 300.

sample 3:

slurry A: the mass part ratio of the lithium iron phosphate powder to the acetylene black to the PVDF to the NMP is 80:7: 10:300, respectively;

slurry B: the mass part ratio of the lithium iron phosphate powder to the acetylene black to the PVDF to the NMP is 80:3: 10: 300.

sample 4:

slurry A: the mass part ratio of the lithium iron phosphate powder to the acetylene black to the PVDF to the NMP is 80:5: 10:300, respectively;

slurry B: the mass part ratio of the lithium iron phosphate powder to the acetylene black to the PVDF to the NMP is 80:5: 10: 300.

sample 5:

slurry A: the mass part ratio of the lithium iron phosphate powder to the acetylene black to the PVDF to the NMP is 80:3: 10:300, respectively;

slurry B: the mass part ratio of the lithium iron phosphate powder to the acetylene black to the PVDF to the NMP is 80:7: 10: 300.

(2) preparing a button cell: manufacturing the manufactured lithium ion battery positive plates with different double-layer distribution and a metal lithium plate into a 2032 type button battery, wherein an electrolyte consists of 1.0mol/L LiPF6/EC + DEC (the volume ratio is 1:1), and the injection volume is 60 mu L which is respectively marked as a battery sample 1-5; samples 1-5 correspond to battery samples 1-5, respectively.

Multiplying current magnitude determination

Because the battery is manufactured in a laboratory, the actual capacity of the battery is unknown, and the charging and discharging multiplying power current of the battery cannot be determined, the multiplying power of the current in the formation process is calculated according to an empirical formula, wherein the formula is as follows:

in the formula I _1C For the current of the battery with 1C multiplying power charging and discharging under theoretical condition, alpha is the active material proportion, phi is the coefficient of converting the theoretical specific capacity into the actual specific capacity, the value is set as 0.9 according to the reference, M is the mass of the coating material on the positive pole piece, and Q is _a The theoretical specific capacity of the lithium iron phosphate material is 170mAh/g, and t is a time constant and is 1 h.

The multiplying power current in the formation process is calculated by the formula, after formation, the actual capacity of the battery can be obtained, and the subsequent charge and discharge test can determine the charge and discharge multiplying power current of the battery according to the capacity after formation.

Battery test equipment: the CT4008 series battery test equipment of New Wien company is used for battery formation and cyclic charge-discharge tests; CHI660E electrochemical workstation was used for battery ac impedance analysis.

Fig. 1 is a schematic structural diagram of a two-layer electrode material layer of sample 1, and the structures of samples 2 to 5 are similar to those of sample 1, and are not repeated here.

After the battery samples 1-5 are subjected to formation activation, the optimal gradient conductive agent distribution ratio is comprehensively evaluated and determined by performing constant current cycle test, rate capability test, alternating current impedance test and cyclic voltammetry test on the battery.

FIG. 2 is a graph showing a relationship between voltage and specific capacity during formation of battery samples 1-5, wherein A (9.5%) + B (0.5%) corresponds to battery sample 1, and the current is 0.1C; a (9%) + B (1%) corresponds to cell sample 2; a (7%) + B (3%) corresponds to cell sample 3; a (5%) + B (5%) corresponds to cell sample 4; a (3%) + B (7%) corresponds to cell sample 5. Fig. 2 shows that the formation effect of each sample under the condition of small rate of 0.1C can be seen, the theoretical specific capacity of the battery can be accurately obtained through small rate discharge, the theoretical specific capacity of the lithium iron phosphate material is 170mAh/g, it can be seen that the specific capacities finally reflected by different battery samples are different, the parameter is large, the score is higher, and the specific capacities and scores of the battery samples 1-5 are shown in table 1.

TABLE 1 specific capacities and scores for Battery samples 1-5

A in Table 1 ₁ And a ₂ The calculation method of (2) is as follows:

a ₂ ＝0.15×a ₁ (2)

wherein a is the specific capacity of the battery during the formation process during the low-rate charge and discharge, and a _max Is a maximum value of a _min Is a minimum value of a ₁ Is a value after normalization ₂ The score after considering the weight value.

FIG. 3 is a plot of cyclic voltammetry for cell samples 1-5, with a sweep rate set at 0.5mV/s and a sweep voltage interval of 2.5-4.2V, where A (9.5%) + B (0.5%) corresponds to cell sample 1; a (9%) + B (1%) corresponds to cell sample 2; a (7%) + B (3%) corresponds to cell sample 3; a (5%) + B (5%) corresponds to cell sample 4; a (3%) + B (7%) corresponds to cell sample 5. It is apparent from fig. 3 that each sample has a reduction peak and an oxidation peak in the charging and discharging process, which represents the process of lithium ion extraction and intercalation, and if the widths of the oxidation peak and the reduction peak are smaller, the reversibility of the battery is better, the polarization effect is smaller, the polarization internal resistance is smaller, and the battery performance is better, therefore, the smaller the parameter is, the higher the score is.

TABLE 2 redox peak widths and scores for cell samples 1-5

The calculation methods of b1, b2 and b3 in table 2 are as follows:

b ₂ ＝|b ₁ -1| (4)

b ₃ ＝0.2×b ₂ (5)

wherein b is the redox peak width of the cell, b _max Is the maximum width, b _min Is a minimum width, b ₁ For the values after normalization, since the smaller the width of the redox peak, the better the battery performance, it is necessary to apply b ₁ Is converted from formula (4) to b ₂ ，b ₃ The score after considering the weight value.

FIG. 4 is a graph of the aging characteristics of battery samples 1-5 in a constant rate charge-discharge cycle test, wherein A (9.5%) + B (0.5%) corresponds to battery sample 1; a (9%) + B (1%) corresponds to cell sample 2; a (7%) + B (3%) corresponds to cell sample 3; a (5%) + B (5%) corresponds to cell sample 4; a (3%) + B (7%) corresponds to cell sample 5. It is apparent from fig. 4 that the capacity of the battery slightly increases during the initial front-stage cycle in the 0.5C rate cycle charging and discharging process of each sample, because the current rate is small and the battery active material is not completely activated during the formation process; in the later period of the cycle, different battery samples have different aging rates because the battery samples have different polarization and ohmic internal resistance, so that the aging speeds are different, the fluctuation amplitudes of the curves are different, the sample with large fluctuation amplitude shows that the capacity failure is faster, the performance is worse, and the fluctuation amplitude can select the curve variance as a characteristic evaluation index.

TABLE 3 constant Rate aging Curve variance and score for Battery samples 1-5

The calculation methods of b1, b2 and b3 in table 3 are as follows:

c ₂ ＝|c ₁ -1| (7)

c ₃ ＝0.2×c ₂ (8)

wherein c is the variance of the constant rate aging curve of the battery, c _max Is the maximum value of the variance, c _min Is the minimum of the variance, c ₁ For the values after normalization, c needs to be set as smaller variance indicates better battery capacity retention and better battery performance ₁ Is converted from formula (7) to c ₂ ，c ₃ The score after considering the weight value.

FIG. 5 is a graph of the aging characteristics of battery samples 1-5 under a cycling test at a gradient rate, where A (9.5%) + B (0.5%) corresponds to battery sample 1; a (9%) + B (1%) corresponds to cell sample 2; a (7%) + B (3%) corresponds to cell sample 3; a (5%) + B (5%) corresponds to cell sample 4; a (3%) + B (7%) corresponded to cell sample 5, with current multiplying factors set to 7 conditions of 0.3C, 0.5C, 0.8C, 1C, 1.5C, 2C, and 0.3C, and the next multiplying factor condition was entered after 10 cycles at each multiplying factor. From fig. 5, it can be seen that the cycle performance of each sample under the gradient multiplying power is not much different in the capacity decrease range of each sample under the low multiplying power condition; when the multiplying power is suddenly increased, the sample with lower conductive agent content in the first electrode material layer is dropped by a significantly larger amount, and the variation range of the curve is large, so that the variance is extracted as the characteristic of the curve, and the smaller the parameter, the better.

TABLE 4 gradient rate aging curve variance and score for Battery samples 1-5

The calculation methods for d1, d2, and d3 in table 4 are as follows:

d ₂ ＝|d ₁ -1| (10)

d ₃ ＝0.2×d ₂ (11)

wherein d is the variance of the aging curve of the gradient multiplying power of the battery, d _max Is the maximum value of the variance, d _min Is the minimum of the variance, d ₁ For the values after normalization, d is required to be set as smaller variance means better battery capacity retention and better battery performance ₁ Is converted from the formula (10) to d ₂ ，d ₃ The score after considering the weight value.

FIG. 6 is an AC impedance spectrum of battery samples 1-5 after formation at a current magnification of 0.1C, wherein A (9.5%) + B (0.5%) corresponds to battery sample 1; a (9%) + B (1%) corresponds to cell sample 2; a (7%) + B (3%) corresponds to cell sample 3; a (5%) + B (5%) corresponds to cell sample 4; a (3%) + B (7%) corresponds to the cell sample 5, and the electrochemical workstation is used for testing, wherein the scanning speed is set to be 0.5mV/s, the scanning voltage interval is 2.5-4.2V, and the scanning frequency is 10 kHz-10 mHz. Fig. 6 shows the ac impedance characteristics of each sample, the intercept between the curve and the horizontal axis is the ohmic impedance of the battery corresponding to the high frequency region, the middle frequency region can be regarded as a semicircle approximately, and represents the charge transfer impedance of the battery, and the part of the low frequency region that is an oblique line of approximately 45 ° represents the weber impedance of lithium ion diffusion. As can be seen from the graph, the difference in ac impedance characteristics between the samples is mainly shown in the middle frequency region, and from the battery sample 1 to the battery sample 5, the charge transfer impedance of the battery gradually increases, and the transfer resistance of the lithium ions increases.

An alternating current impedance diagram is fitted through an equation (12) by selecting a lithium ion battery equivalent circuit (shown in fig. 7) of a first-order RC loop with weber impedance, a relation diagram obtained through fitting is shown in fig. 8, a weber coefficient sigma can be obtained through calculation through the equation (12), and the calculation method is as follows:

Z _re ＝R _O +R _CT +σω ^-0.5 (12)

in the formula, Z _re Is the real part of the impedance, R _O Is ohmic impedance, R _CT The charge transfer impedance, omega, is the impedance angular frequency, and sigma is the weber coefficient.

The lithium ion diffusion coefficient D of the battery can be obtained through further calculation, and the calculation formula is as follows:

wherein D is a diffusion coefficient of lithium ions, R is an ideal gas constant, T is an ambient absolute temperature, A is a surface contact area of a pole piece, n is an electron transfer number, F is a Faraday constant, and c is a concentration of lithium ions.

The larger the diffusion coefficient, the smaller the resistance in the cell transfer process, and the larger the weber coefficient σ and the diffusion coefficient D of each sample, the higher the fraction, see table 5.

TABLE 5 Weber coefficient σ and diffusion coefficient D and score of cell samples 1-5

E and e in Table 5 ₁ The calculation method of (2) is as follows:

e ₁ ＝0.05×e (15)

wherein e is the value after diffusion coefficient normalization, e ₁ The score after considering the weight value.

And (3) integrating the scores of the five indexes, and calculating according to an equation (16) to obtain a total score:

f＝a ₂ +b ₃ +c ₃ +d ₃ +e ₁ (16)

the overall performance scoring results for each battery sample are shown in table 6.

TABLE 6 summary of the comprehensive Performance scores of the various battery samples

And (3) using the data in the table 6, taking the ratio of the conductive agents of the samples as an independent variable x, converting the data in the table 6 into the data in the table 7, fitting the data in the table 7 by using a MATLAB fitting tool box, fitting the data, and solving the maximum value of the curve, wherein the value of the corresponding x axis is the optimal ratio, namely the ratio of the conductive agents in the slurry A and the slurry B.

Table 7 fitting input data

Ratio x of A to B conductive agent	19	9	2.333	1	0.429
						Total fraction f (x)	0.71895	0.7825	0.6508	0.29345	0

The fit formula is shown below:

in the formula, p ₁ 、p ₂ 、p ₃ And q is ₁ The undetermined coefficient is obtained by automatic calculation of MATLAB fitting tool box, and related parameters, error index coefficient and variance SSE of fitting, root mean square error RMSE and goodness of fitting R ² As shown in table 8:

TABLE 8 fitting results

Parameter/index	p ₁	p ₂	p ₃	q ₁	SSE	RMSE	R ²
								Numerical value	-0.01606	1.073	-0.4682	0.8238	0.0028	0.0504	0.9943

As shown in fig. 9, from the fitting results, the optimal ratio was found to be 8.39, i.e., a (8.94%) + B (1.06%), with a score of 0.836, which corresponds to: slurry A: the mass part ratio of the lithium iron phosphate powder to the acetylene black to the PVDF to the NMP is 80:8.94:10: 300; slurry B: the mass part ratio of the lithium iron phosphate powder to the acetylene black to the PVDF to the NMP is 80:1.06:10: 300.

And manufacturing electrode plates corresponding to the proportion of the conductive agent according to the obtained result, manufacturing a battery, and performing the electrical property test in the same way to obtain a score of 0.830 and a percentage error of 1.08%.

Through the test experiment method and the calculation method, the cyclic characteristics, the volt-ampere characteristics, the alternating current impedance characteristics and the like of different samples can be obtained, so that the performance difference among different samples can be contrasted and analyzed, and researchers are helped to screen out the battery pole piece with the double-layer structural formula, which is excellent in impedance performance, cyclic performance and high specific capacity.

In this embodiment, the battery pole piece with gradient conductive agent distribution is used as an example to verify the feasibility of the method, and the method is also applicable to battery pole pieces with double-layer structure with other component distribution ratios, so researchers in the field should understand that certain modifications, changes or substitutions are made in the technical method of the present invention and fall within the protection scope of the present invention.

Claims

1. A method for determining the proportion of a conductive agent in a double-layer distributed lithium ion battery pole piece is characterized by comprising the following steps:

(1) manufacturing lithium ion battery pole pieces distributed in different double layers into a battery, activating the battery, performing constant-rate cycle test, gradient rate performance test, alternating-current impedance test and cyclic voltammetry test on the battery, extracting characteristic parameters from each test result, normalizing, adding a weight as an evaluation score under the test result, and finally integrating all the scores into a total score under the ratio;

2. The method for determining the proportion of the conductive agent in the double-layer lithium ion battery pole piece is characterized in that in the formation activation, the current multiplying power of constant voltage charging and constant current discharging is set to be 0.1C, the cut-off multiplying power is set to be 0.05C, and the charging and discharging voltage interval is set to be 2-3.65V.

3. The method for determining the proportion of the conductive agent in the double-layer lithium ion battery pole piece according to claim 1, wherein in the constant-rate cycle test, the current rate is set to 0.5C, and the number of cycles of charge and discharge is 100.

4. The method for determining the proportion of the conductive agent in the double-layer lithium ion battery pole piece is characterized in that in the gradient multiplying power performance test, the current multiplying power is set to be 7 working conditions of 0.3C, 0.5C, 0.8C, 1C, 1.5C, 2C and 0.3C, and the next multiplying power working condition is started after 10 cycles are carried out at each multiplying power.

5. The method for determining the proportion of the conductive agent in the double-layer lithium ion battery pole piece is characterized in that an electrochemical workstation is used for testing in the alternating current impedance testing process, the scanning speed is set to be 0.5mV/s, the scanning voltage interval is 2.5-4.2V, and the scanning frequency is 10 kHz-10 mHz.

6. The method for determining the proportion of the conductive agent in the double-layer lithium ion battery pole piece according to any one of claims 1 to 3, wherein in the cyclic voltammetry test, the scanning speed is set to be 0.5mV/s, and the scanning voltage interval is 2.5 to 4.2V.

7. The method for determining the proportion of the conductive agent in the double-layer lithium ion battery pole piece according to claim 6, wherein the weight of the characteristic parameter 1 is 0.15; the weight of the characteristic parameter 2 is 0.2; the weight of the characteristic parameter 3 is 0.2; the weight of the characteristic parameter 4 is 0.05; the weight of the characteristic parameter 5 is 0.2;

and (3) in the step (2), fitting by adopting an MATLAB fitting tool box to obtain a fitting curve after fitting.

8. The method for determining the proportion of the conductive agent in the double-layer lithium ion battery pole piece according to any one of claims 1 to 3, wherein the ambient temperature of the constant-rate cyclic test, the gradient-rate performance test, the alternating-current impedance test and the cyclic voltammetry test is 25 ℃.

9. The method for determining the proportion of the conductive agent in the double-layer lithium ion battery pole piece according to any one of claim 7, wherein the positive electrode material active material of the battery is at least one of lithium iron phosphate, lithium cobaltate, lithium manganate and lithium nickel cobalt manganate; the negative electrode material of the battery is graphite, the positive current collector of the battery is aluminum foil, and the negative current collector of the battery is copper foil.

10. The method for determining the proportion of the conductive agent in the double-layer lithium ion battery pole piece according to claim 9, wherein the positive electrode material of the battery is prepared by using polyvinylidene fluoride as a binder, N-methyl pyrrolidone as a solvent, and at least one of acetylene black, carbon nanotubes and graphene as a conductive agent.