Method for calculating liquid retention capacity of long-circulation winding lithium ion battery

Abstract

The invention discloses a method for calculating the liquid retention capacity of a long-circulation winding lithium ion battery. According to the method, the DFN model is used for deducing the relation between the consumption rate of the electrolyte and the number of cycles, and compared with previous linear processing, the electrolyte consumption model is more accurate, and more development time is saved.

Description

Technical Field

[0001] The invention relates to the technical field of lithium ion batteries, and in particular to a method for calculating the liquid retention amount of a long-cycle winding lithium ion battery. Background Art

[0002] The electrolyte in lithium-ion batteries is responsible for conducting Li + If the amount of liquid injected is too little, the positive and negative active materials cannot be fully infiltrated, resulting in black spots and lithium precipitation, which is not only not conducive to the battery capacity, but also causes safety problems. In addition, the consumption of electrolyte during the cycle of lithium-ion batteries is one of the main reasons for the cycle dive. When the amount of liquid injected is too much, it not only increases the difficulty of the manufacturing process and the manufacturing cost, but also leads to problems such as decreased battery energy density and excessive life. Therefore, determining the appropriate amount of liquid injection is particularly important for the balance between performance and cost of lithium-ion batteries.

[0003] At present, the minimum injection volume is first determined according to the porosity of the positive and negative electrodes and the diaphragm, and then the minimum injection volume is obtained according to the electrolyte density. The residual capacity under the number of cycles is linearly fitted with the remaining electrolyte to obtain a linear relationship between the number of cycles and the remaining electrolyte, and finally the required electrolyte retention amount is obtained. However, in the actual cycle process of lithium-ion batteries, the SEI film formed at the negative electrode gradually becomes thicker, and the consumption rate of the electrolyte gradually slows down. This treatment result will evaluate the consumption of the electrolyte too quickly, and the minimum injection volume value obtained is higher than the actual value. Secondly, in order to obtain approximate linear consumption of electrolyte during the implementation of this method, a longer number of cycles is required (the more cycles, the more approximate the later cycle consumption). Therefore, time and accuracy are difficult to be compatible in the implementation of this method. Summary of the invention

[0004] The present invention aims to solve one of the technical problems in the related art at least to a certain extent.

[0005] To this end, an embodiment of the present invention provides a method for calculating the liquid retention amount of a long-cycle wound lithium-ion battery.

[0006] The present invention proposes a method for calculating the liquid retention amount of a long-cycle winding lithium-ion battery, comprising:

[0007] (a) According to the DFN model,

[0008] The reaction current density of the SEI film is: Among them, j SEI The unit is A / m 2 ; F is the Faraday constant, unit is C / mol; a n is the specific surface area per unit volume, with the unit of 1 / nm; is the diffusion coefficient of the electrolyte, in cm 2 / s;c EC is the concentration of the solvent in mol / m 3 ; δ SEI,out is the thickness of the SEI film, in nm;

[0009] The current density flowing into the SEI membrane and the current density flowing out of the SEI membrane are,

[0010] j SEI,in =α·j SEI

[0011] j SEI,out =(1-α)·j SEI (Equation 2), where α is the proportional parameter;

[0012] According to the ratio of different solvents, the growth rate of SEI film is obtained.

[0013] Among them, M SEI is the molar mass of the SEI film, in g / mol; ρ SEI is the density of the SEI film, in g / m 3 ;

[0014] (b) According to (Formula 1), (Formula 2) and (Formula 3),

[0015] Among them, t is the quantitative time unit; δ SEI is the thickness of the SEI film, in nm; D Sol is the diffusion coefficient of the solvent in SEI, c Sol Volume fraction of solvent, M SEI is the molar mass of the SEI film, in g / mol; ρ SEI is the density of the SEI film, in g / m 3 ;

[0016] (c) According to (Equation 4), the thickness of the SEI film and the time t are obtained 0.5 The consumption rate of electrolyte is proportional to t 0.5 Inversely proportional, when a certain number of cycles are repeated, m y1 =m y -a*n 0.5 (Formula 5), ​​where m y1 is the amount of free electrolyte remaining after disassembly, m y is the injection amount of free electrolyte, n is the number of cycles;

[0017] (d) According to (Equation 5), m = m y +m min , where m is the total injection volume, m min It is the minimum threshold for the battery cell to experience cycle diving.

[0018] Furthermore, by disassembling the battery cells at different cycle numbers, the amount of free electrolyte corresponding to the cycle number is obtained, and then the value a is obtained by fitting (Formula 5).

[0019] Furthermore, the m min It is obtained by calculating the porosity and free space utilization in the positive electrode, negative electrode and separator.

[0020] Furthermore, the number of cycles n is proportional to time.

[0021] Compared with the prior art, the beneficial effects of the present invention are:

[0022] The present invention uses the DFN model to derive the relationship between the consumption rate of the electrolyte and the number of cycles. Compared with the previous linear processing, the electrolyte consumption model is more accurate and saves development time. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the following description of the embodiments in conjunction with the accompanying drawings, in which:

[0024] Figure 1 This is the fitting diagram of Example 1 of the present invention. DETAILED DESCRIPTION

[0025] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and should not be construed as limiting the present invention.

[0026] The method for calculating the liquid retention amount of a long-cycle wound lithium-ion battery of the present invention is described in detail below.

[0027] The method for calculating the liquid retention amount of a long-cycle wound lithium-ion battery proposed by the present invention comprises the following steps:

[0028] (a) During the cycle of lithium batteries, the electrolyte is continuously consumed. In addition to part of it decomposing and producing gas, most of it forms the SEI film, which leads to the thickening of the electrode. Therefore, the consumption rate of the electrolyte is related to the thickening of the SEI film.

[0029] According to the DFN model,

[0030] The reaction current density of the SEI film is:

[0031] Among them, j SEI The unit is A / m 2 ; F is the Faraday constant, unit is C / mol; a n is the specific surface area per unit volume, with the unit of 1 / nm; is the diffusion coefficient of the electrolyte, in cm 2 / s;c EC is the concentration of the solvent in mol / m 3 ; δ SEI,out is the thickness of the SEI film, in nm.

[0032] The current density flowing into the SEI membrane and the current density flowing out of the SEI membrane are,

[0033] j SEI,in =α·j SEI

[0034] j SEI,out =(1-α)·j SEI (Formula 2),

[0035] Among them, α is the scale parameter.

[0036] According to the ratio of different solvents, the growth rate of SEI film is obtained.

[0037]

[0038] Among them, M SEI is the molar mass of the SEI film, in g / mol; ρ SEI is the density of the SEI film, in g / m 3 .

[0039] (b) According to (Formula 1), (Formula 2) and (Formula 3),

[0040]

[0041] Among them, t is the quantitative time unit; δ SEI is the thickness of the SEI film, in nm; D Sol is the diffusion coefficient of the solvent in SEI, c Sol Volume fraction of solvent, M SEI is the molar mass of the SEI film, in g / mol; ρ SEI is the density of the SEI film, in g / m 3 .

[0042] (c) According to (Equation 4), the thickness of the SEI film and the time t are obtained 0.5The consumption rate of electrolyte is proportional to t 0.5 Inversely proportional, when a certain number of cycles are repeated, m y1 =m y -a*n 0.5 (Formula 5),

[0043] Among them, m y1 is the amount of free electrolyte remaining after disassembly, m y is the injection amount of free electrolyte, and n is the number of cycles.

[0044] a is a constant related to A in (Equation 4), and the number of cycles n is proportional to time. Specifically, assuming that the charge and discharge is at a power of 0.5P, the time for one cycle is 4h. When charging and discharging for n cycles, the time used is t=4n, that is, t^0.5=4n^0.5, 4 is a constant, so it can be incorporated into the constant A. The value of A can be obtained by fitting, and the value of a can be obtained by fitting. By disassembling the battery cells under different cycle numbers, the amount of free electrolyte corresponding to the number of cycles is obtained, and then the value of a is obtained by fitting (Equation 5).

[0045] The amount of liquid injected is consistent, and the electrolyte that can be poured out is called free electrolyte. The free electrolyte poured out of a fresh battery cell is m y , the electrolyte poured out after a certain cycle of n times is m y1 .

[0046] (d) According to (Equation 5), m = m y +m min , where m is the total injection volume, i.e., the volume of liquid to be retained, m min is the minimum threshold of the battery cell cycle diving, m y is the injection amount of free electrolyte.

[0047] Example 1

[0048] Select 280Ah cells with different cycle numbers at 25°C. The cycle numbers of the cells are 1, 141, 142, 624, 624, 1234, 1332, and 1627. Disassemble all cells and record the amount of free electrolyte, which is 131g, 115.5g, 106g, 88.5g, 105g, 72g, 69.5g, and 67g. Plot the amount of free electrolyte corresponding to different cycles in the data processing software, such as Figure 1 As shown, the data processing software can be Excel, Origin, etc.

[0049] Using the fitting software, the data of the amount of free electrolyte were fitted using (Formula 5), ​​and the fitted formula was m y1=131.293–1.61492*n^0.5, substitute n=0, 1, 2, 3… (n is the number of cycles, an integer) into the above formula to get a series of m y1 The values ​​obtained are plotted to obtain a fitting curve (dashed line) for the disassembly data, which is the relationship between the number of cycles and the remaining free electrolyte.

[0050] The consumption of electrolyte is 1.61492*n^0.5. Similarly, by substituting n=0, 1, 2, 3... (n is the number of cycles, an integer) into the above formula, we can obtain the relationship between the number of battery cell cycles and the total consumption of electrolyte (solid line).

[0051] The porosity and free space utilization rate in the positive electrode, negative electrode and separator were calculated using the existing method to obtain m min and m max According to the above fitting dotted line, we can get m y , according to m min and m y The liquid retention volume under the corresponding design cycle number is obtained. Among them, the maximum liquid injection volume m max According to the volume V of the positive electrode 1 , the porosity of the positive electrode δ 1 , the volume of the negative electrode V 2 , the porosity of the negative electrode δ 2 , the volume of the diaphragm V 3 , the porosity of the diaphragm δ 3 , the volume of the free space V 4 , the density ρ of the electrolyte is obtained, and the specific calculation formula is m max =(V 1 δ 1 +V 2 δ 2 +V 3 δ 3 +V 4 )*ρ; similar (minimum injection volume does not include free space)m min =(V 1 δ 1 +V 2 δ 2 +V 3 δ 3 )*ρ, this calculation method is an existing method and will not be repeated here.

[0052] According to the solid line fitted above, the consumption of electrolyte under the corresponding design cycle number is obtained. When the electrolyte consumption in the cycle is not enough to support the cycle design value, that is, m y When the consumption of electrolyte is less than the value m, the increase y , but according to the increased m yThe liquid retention volume under the designed number of cycles is no more than m max .

[0053] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms may be for different embodiments or examples. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, unless they are contradictory.

[0054] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In the description of the present invention, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0055] Although the embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the claims and their equivalents.

Claims

1. A method for calculating the liquid retention of a long-cycle wound lithium-ion battery, characterized in that: The following steps are involved: (a) According to the DFN model, The reaction current density of the SEI film is: Among them, j SEI The unit is A / m 2 ; F is the Faraday constant, unit is C / mol; a n is the specific surface area per unit volume, with the unit of 1 / nm; is the diffusion coefficient of the electrolyte, in cm 2 / s;c EC is the concentration of the solvent in mol / m 3 ; δ SEI,out is the thickness of the SEI film, in nm; The current density flowing into the SEI membrane and the current density flowing out of the SEI membrane are, Among them, α is the scale parameter; According to the ratio of different solvents, the growth rate of SEI film is obtained. Among them, M SEI is the molar mass of the SEI film, in g / mol; ρ SEI is the density of the SEI film, in g / m 3 ; (b) According to (Formula 1), (Formula 2) and (Formula 3), Among them, t is the quantitative time unit; δ SEI is the thickness of the SEI film, in nm; D Sol is the diffusion coefficient of the solvent in SEI, c Sol Volume fraction of solvent, M SEI is the molar mass of the SEI film, in g / mol; ρ SEI is the density of the SEI film, in g / m 3 ; (c) According to (Equation 4), the thickness of the SEI film and the time t are obtained 0.5 The consumption rate of electrolyte is proportional to t 0.5 Inversely proportional, when a certain number of cycles are repeated, m yl =m y –a*n 0.5 (Formula 5), Among them, m y1 is the amount of free electrolyte remaining after disassembly, m y is the injection amount of free electrolyte, n is the number of cycles; (d) According to (Equation 5), m = m y +m min , where m is the total injection volume, m min It is the minimum threshold for the battery cell to experience cycle diving.

2. The method according to claim 1, characterized in that By disassembling the battery cells at different cycle numbers, the amount of free electrolyte corresponding to the cycle number is obtained, and then the value a is obtained by fitting (Equation 5).

3. The method according to claim 1, characterized in that The m min It is obtained by calculating the porosity and free space utilization in the positive electrode, negative electrode and separator.

4. The method according to claim 1, characterized in that The number of cycles n is proportional to time.

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