A method for preparing a lithium-ion battery

Abstract

The present invention discloses a method for preparing a lithium-ion battery, comprising the following steps: preparing a plurality of positive and negative sub-layer electrodes; obtaining the actual DCR parameters of each positive sub-layer electrode; screening at least two positive sub-layer electrodes based on the target DCR parameters of the electrode sheet of the target lithium-ion battery and combining them into a positive composite electrode; screening at least two negative sub-layer electrodes based on the voltage drop of the positive composite electrode and combining them into a negative composite electrode; and assembling the positive composite electrode with the negative composite electrode and a separator into a battery. Based on the DCR decomposition theory, the present invention designs a sub-layer electrode with controllable DCR components. After determining the order requirements and design goals, a strategy is quickly formulated to combine the multiple sub-layer electrodes into a composite electrode and assemble them into a battery. This can achieve quantitative control of the battery's electrochemical performance, facilitate battery modeling and simulation, and enable digital management of the power system at the subsequent application end, thereby shortening the R&D cycle and reducing R&D costs.

Description

Technical Field

[0001] The present invention relates to the field of lithium ion batteries, and in particular to a method for preparing a lithium ion battery. Background Art

[0002] Lithium-ion batteries, due to their high energy density, long cycle life, and relatively stable electrochemical properties, have become the preferred energy storage solution for electronic devices, electric vehicles, and large-scale energy storage systems. As these applications continue to demand higher battery performance, such as higher energy density, longer cycle life, and faster charging capabilities, the requirements for battery design and manufacturing are becoming more stringent and diverse.

[0003] However, the current design and development process for lithium-ion batteries faces a number of challenges. This is primarily due to the storage issues associated with pre-production battery products, as performance degradation can occur during prolonged storage, increasing maintenance and safety costs. Consequently, existing technologies typically design products based on demand after receiving an order. This process involves extensive experimental optimization and testing verification, often taking months or even years to complete the transition from concept to product, resulting in high R&D costs and long development cycles. Summary of the Invention

[0004] The embodiment of the present invention provides a method for preparing a lithium-ion battery to solve the problems of high R&D cost and long R&D cycle in the design process of existing lithium-ion batteries.

[0005] Specifically, the present invention provides a method for preparing a lithium ion battery, comprising the following steps:

[0006] preparing a plurality of sub-layer electrodes, wherein the plurality of sub-layer electrodes are classified into positive sub-layer electrodes and negative sub-layer electrodes;

[0007] Obtaining the actual DCR parameter of each of the positive sub-layer electrodes; screening out at least two of the positive sub-layer electrodes according to the target DCR parameter of the electrode sheet of the target lithium-ion battery; combining the screened positive sub-layer electrodes into a positive composite electrode; wherein the actual DCR parameter of the positive composite electrode is the same as the target DCR parameter of the positive electrode sheet of the target lithium-ion battery; screening out at least two of the negative sub-layer electrodes according to the voltage drop of the positive composite electrode; combining the screened negative sub-layer electrodes into a negative composite electrode; or,

[0008] Obtaining an actual DCR parameter of each of the negative sub-layer electrodes; screening at least two of the negative sub-layer electrodes according to the target DCR parameter of the electrode sheet of the target lithium-ion battery; combining the screened negative sub-layer electrodes into a negative composite electrode; wherein the actual DCR parameter of the negative composite electrode is the same as the target DCR parameter of the negative electrode sheet of the target lithium-ion battery; screening at least two of the positive sub-layer electrodes according to the voltage drop of the negative composite electrode; combining the screened positive sub-layer electrodes into a negative composite electrode;

[0009] The voltage drop ΔU of the positive composite electrode pos The voltage drop ΔU of the negative composite electrode neg same;

[0010] The positive composite electrode, the negative composite electrode and a separator are assembled into a lithium ion battery.

[0011] Optionally, ΔU neg =I neg1 *R neg1 +I neg2 *R neg2 +...+I negm *R negm ;

[0012] ΔU pos =I pos1 *R pos1 +I pos2 *R pos2 +...+I posn *R posn ;

[0013] Among them, I neg is the current passing through the negative sub-layer electrode, I neg1 is the current passing through the first negative sub-layer electrode in the negative composite electrode, I neg2 is the current passing through the second negative electrode sub-layer electrode in the negative composite electrode; I negm is the current passing through the mth negative sub-layer electrode in the negative composite electrode; R neg is the resistance of the negative sub-layer electrode, R neg1 is the resistance of the first negative sub-layer electrode in the negative composite electrode, R neg2 is the resistance of the second negative sub-layer electrode in the negative composite electrode, R negm is the resistance of the mth negative sub-layer electrode in the negative composite electrode;

[0014] I pos is the current passing through the positive sub-layer electrode, I pos1 I is the current passing through the first positive sub-layer electrode in the positive composite electrode; pos2is the current passing through the second positive sub-layer electrode in the positive composite electrode, I posn is the current passing through the nth positive sub-layer electrode in the positive composite electrode; R pos is the resistance of the positive sub-layer electrode, R pos1 is the resistance of the first positive sub-layer electrode in the positive composite electrode, R pos2 is the resistance of the second positive sub-layer electrode in the positive composite electrode, R posn is the resistance of the nth positive sub-layer electrode in the positive composite electrode.

[0015] Optionally, the DCR parameters include one or more of porosity, tortuosity, effective electrochemical area, electrochemical reaction constant, and lithium ion solid phase diffusion coefficient.

[0016] Optionally, the porosity of the actual DCR parameter is calculated by the following formula:

[0017]

[0018]

[0019]

[0020] Where ε is the porosity, is the compaction density of the electrode, is the electrode quality, is the average density of the electrode, is the electrode area, is the thickness of the electrode;

[0021] is the mass ratio of the components in the electrode, is the mass proportion of the first component in the electrode, Is the mass proportion of the second component in the electrode... is the mass proportion of the nth component in the electrode; is the true density of the components in the electrode, is the true density of the first component in the electrode, is the true density of the second component in the electrode... is the true density of the nth component in the electrode.

[0022] Optionally, the tortuosity of the actual DCR parameter is calculated by any one of the following formulas:

[0023]

[0024]

[0025] Where τ is the tortuosity, ε is the porosity, is the ionic internal resistance, Acc is the current collector area, L is the electrode thickness, and K0 is the intrinsic conductivity of the electrolyte;

[0026] γ is the correction factor, the value range of γ is 1.5~2.0, ε is the porosity, and α is the Bruggeman index.

[0027] Optionally, the electrochemical reaction constant of the actual DCR parameter is calculated by the following formula:

[0028]

[0029]

[0030]

[0031] Where: k is the electrochemical reaction constant, is the maximum concentration of lithium ions in the solid phase, is the lithium ion solid phase concentration of the positive electrode material at 50% SOC, is the lithium ion concentration of the electrolyte, i 0 is the exchange current density of the positive electrode material, R is the ideal gas constant, T is the temperature, n is the reaction order, and F is the Faraday constant.

[0032] Optionally, the lithium ion solid phase diffusion coefficient of the actual DCR parameter is calculated by the following formula:

[0033]

[0034] in, is the peak current; n is the number of electron transfers in the redox process; A is the electrode area immersed in the solution; D is the solid phase diffusion coefficient of lithium ions; is the scanning rate; is the concentration of lithium ions participating in the redox reaction.

[0035] Optionally, the positive electrode sheet includes a positive electrode current collector and the positive electrode composite electrode arranged on the surface of the positive electrode current collector, and the material of the positive electrode composite electrode includes one or more of lithium cobalt oxide, lithium iron phosphate, spinel lithium manganese oxide, ternary positive electrode material, and lithium manganese-rich positive electrode material.

[0036] Optionally, the positive sub-layer electrode is prepared by the following steps:

[0037] The SWCNT dispersion and lithium cobalt oxide are mixed uniformly in a mass ratio of 1:0.3-0.5 and dispersed into a slurry; the slurry is filtered onto a filter membrane, and the positive sub-layer electrode is obtained by freeze drying and roller pressing;

[0038] Based on 100% by weight of the SWCNT dispersion, the content of the single-walled carbon nanotubes is 0.2-0.6 wt %, the content of the carboxymethyl cellulose is 0.4-0.8 wt %, and the balance is deionized water.

[0039] The beneficial effects of the present invention are:

[0040] Based on the DCR decomposition theory, the present invention designs a sub-layer electrode with controllable DCR components. After determining the order requirements and design goals, a strategy is quickly formulated to combine multiple sub-layer electrodes into a composite electrode and assemble them into a battery. This can achieve quantitative control of the battery's electrochemical performance, facilitate battery modeling and simulation, and realize digital management of the power system at the subsequent application end, thereby shortening the R&D cycle and reducing R&D costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments of the present invention. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0042] Figure 1 is a schematic flow chart of a method for preparing a lithium-ion battery in accordance with a first embodiment of the present invention;

[0043] Figure 2 is a schematic flow chart of a method for preparing a lithium-ion battery in accordance with a first embodiment of the present invention;

[0044] Figure 3 1 is a schematic diagram of a method for preparing a lithium-ion battery in accordance with a first embodiment of the present invention.

[0045] In the figure: 100, positive composite electrode, 110, positive sub-layer electrode, 200, negative composite electrode, 300, separator. DETAILED DESCRIPTION

[0046] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0047] In the description of the present invention, it should be understood that the terms "longitudinal", "radial", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are intended only to facilitate the description of the present invention and simplify the description. They do not indicate or imply that the devices or elements referred to must have a specific orientation, be constructed and operate in a specific orientation, and therefore should not be understood as limiting the present invention. In the description of the present invention, unless otherwise specified, "plurality" means two or more.

[0048] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "mounted," "connected," and "connected" should be understood in a broad sense. For example, they may refer to fixed, detachable, or integral connections; mechanical or electrical connections; direct or indirect connections through an intermediate medium; and internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on the specific circumstances.

[0049] Figure 1 is a schematic flow chart of a method for preparing a lithium-ion battery in an embodiment of the present invention, such as Figure 1 As shown, and reference Figure 2 、 Figure 3 , an embodiment of the present invention provides a method for preparing a lithium ion battery, comprising the following steps:

[0050] Obtaining DCR design parameters of multiple electrode materials, and preparing the multiple electrode materials into multiple sub-layer electrodes with different thicknesses by roller pressing;

[0051] Prepare a plurality of sub-layer electrodes, wherein the plurality of sub-layer electrodes are classified into positive sub-layer electrodes 110 and negative sub-layer electrodes;

[0052] Obtaining the actual DCR parameter of each of the positive sub-layer electrodes; screening out at least two of the positive sub-layer electrodes 110 according to the target DCR parameter of the electrode sheet of the target lithium-ion battery; combining the screened positive sub-layer electrodes 110 into a positive composite electrode 100; screening out at least two of the negative sub-layer electrodes according to the voltage drop of the positive composite electrode 100; combining the screened negative sub-layer electrodes into a negative composite electrode 200; wherein the actual DCR parameter of the positive composite electrode 100 is the same as the target DCR parameter of the positive electrode sheet of the target lithium-ion battery; the voltage drop ΔU of the positive composite electrode 100 pos The voltage drop ΔU of the negative composite electrode 200 neg same.

[0053] The positive composite electrode 100 is assembled with the negative electrode and the separator 300 to form a battery.

[0054] It should be noted that the release of lithium ions from the positive electrode and the insertion of lithium ions into the negative electrode of lithium-ion batteries will encounter obstacles. The obstacle under DC working conditions is the direct current internal resistance (DCR).

[0055] Based on the DCR decomposition theory, the present invention designs a sub-layer electrode with controllable DCR components. After determining the order requirements and design goals, a strategy is quickly formulated to combine multiple sub-layer electrodes into a composite electrode and assemble them into a battery. This allows for quantitative control of the battery's electrochemical performance, facilitates battery modeling and simulation, and enables digital management of the power system at the subsequent application end, thereby shortening the R&D cycle and reducing R&D costs.

[0056] Since DC internal resistance can be broken down into ohmic resistance, electrochemical reaction resistance, and diffusion resistance, if the three internal resistances encountered during the process of lithium ion extraction from the positive electrode and insertion into the negative electrode are not properly matched, this will result in different lithium ion migration rates between the positive and negative electrode regions. In theory, the negative electrode should have sufficient capacity to accommodate the transferred lithium ions. However, due to a mismatch in the kinetics of the positive and negative electrodes, this difference in lithium ion migration rates can cause localized "blockage" of lithium ions in the negative electrode region. Lithium ions that have not yet fully inserted into the negative electrode material crystals will still undergo reduction reactions, resulting in localized lithium deposition. Therefore, the kinetic mismatch between the positive and negative electrodes can cause localized overcharge and overdischarge in the battery, thereby affecting its cycle life. Excessively loose kinetic matching can compromise both the volumetric and gravimetric energy densities of the battery. Furthermore, the thickness of the sublayer electrode is controlled by roller pressing. A smaller thickness results in lower porosity, greater tortuosity, and higher ohmic internal resistance, while the electrochemical reaction internal resistance remains unchanged and the diffusion internal resistance increases. That is, the ohmic internal resistance is negatively correlated with the thickness, and thus the actual DCR parameter of the composite electrode can be made the same as the target DCR parameter of the target lithium-ion battery electrode by making the thickness of the composite electrode the same as the thickness of the target lithium-ion battery electrode.

[0057] Based on the above reasons, the present invention follows the principle that the overpotential caused by the positive electrode DCR is equal to the overpotential caused by the negative electrode DCR, so that the voltage drop ΔU of the negative electrode composite electrode 200 formed by the negative electrode sub-layer electrode is neg and the voltage drop ΔU of the positive composite electrode 100 posThe same can achieve quantitative control of the kinetics of the battery's electrochemical sub-processes, making it easier to achieve kinetic matching of the different electrochemical processes at the positive and negative electrodes. While ensuring energy density, it also reduces the possibility of battery performance degradation and capacity drop due to local failure, thus achieving a balance between high energy density and battery life.

[0058] like Figure 2 As shown, in an alternative embodiment of the present invention, the preparation method includes the following steps:

[0059] Obtaining DCR design parameters of multiple electrode materials, and preparing the multiple electrode materials into multiple sub-layer electrodes with different thicknesses by roller pressing;

[0060] Prepare a plurality of sub-layer electrodes, wherein the plurality of sub-layer electrodes are classified into positive sub-layer electrodes 110 and negative sub-layer electrodes;

[0061] Obtaining the actual DCR parameter of each of the negative sub-layer electrodes; screening at least two of the negative sub-layer electrodes according to the target DCR parameter of the electrode sheet of the target lithium-ion battery, and combining the screened negative sub-layer electrodes into a negative composite electrode 200; screening at least two of the positive sub-layer electrodes 110 according to the voltage drop of the negative composite electrode; combining the screened positive sub-layer electrodes 110 into a positive composite electrode 100; wherein the actual DCR parameter of the positive composite electrode 100 is the same as the target DCR parameter of the positive electrode sheet of the target lithium-ion battery; the voltage drop ΔU of the positive composite electrode 100 pos The voltage drop ΔU of the negative composite electrode 200 neg same.

[0062] The positive composite electrode 100 is assembled with the negative electrode and the separator 300 to form a battery.

[0063] Specifically, ΔU neg =I neg *R neg *m;ΔU pos =I pos *R pos *n;

[0064] Where m is the number of negative electrode layers, I neg is the current passing through the negative sub-layer electrode, R neg is the resistance of the negative sub-layer electrode; n is the number of positive sub-layer electrodes, I pos is the current passing through the positive sub-layer electrode, R pos is the resistance of the positive sub-layer electrode.

[0065] In one embodiment of the present invention, the negative electrode sheet includes a negative electrode current collector and the negative electrode composite electrode provided on the surface of the negative electrode current collector, and the materials of the negative electrode composite electrode include a negative electrode active material, a conductive agent, a binder, and a solvent. The active material includes one or more of a carbon-based negative electrode material, lithium titanate, a silicon-based negative electrode material, a tin-based negative electrode material, and a transition metal oxide negative electrode. The conductive agent includes one or more of conductive carbon black, Ketjen black, carbon nanotubes, and graphene. The binder includes one or more of polytetrafluoroethylene, carboxymethyl cellulose, and styrene-butadiene rubber. The solvent includes one or more of ethylene glycol, glycerol, polyethylene glycol, dimethyl sulfoxide, N-methylpyrrolidone, deionized water, alcohol, and N-methylpyrrolidone (NMP).

[0066] The positive electrode sheet includes a positive current collector and a positive composite electrode disposed on the surface of the positive current collector. The positive composite electrode is composed of a positive electrode active material, a conductive agent, and a binder. The positive electrode active material may include one or more of lithium cobalt oxide, lithium iron phosphate, spinel lithium manganese oxide, ternary positive electrode materials (Li(NiCoMn)O2), and lithium-manganese-rich positive electrode materials (xLi2MnO3•(1–x)LiTMO2, where TM = Ni, Mn, Co, etc.). The thickness of the electrode sheet ranges from 1μm to 50μm.

[0067] In one embodiment of the present invention, the actual DCR parameter includes one or more of porosity, tortuosity, effective electrochemical area, electrochemical reaction constant, and lithium ion solid phase diffusion coefficient. The porosity of the actual DCR parameter is calculated by the following formula:

[0068]

[0069]

[0070]

[0071] Where ε is the porosity, is the compaction density of the electrode, is the electrode quality, is the average density of the electrode, is the electrode area, is the thickness of the electrode;

[0072] is the mass ratio of the components in the electrode, is the mass proportion of the first component in the electrode, Is the mass proportion of the second component in the electrode... is the mass proportion of the nth component in the electrode;

[0073] is the true density of the components in the electrode, is the true density of the first component in the electrode, is the true density of the second component in the electrode... is the true density of the nth component in the electrode.

[0074] The tortuosity of the actual DCR parameter is calculated by either of the following two formulas:

[0075]

[0076]

[0077] Where τ is the tortuosity, ε is the porosity, is the ionic internal resistance, Acc is the current collector area, L is the electrode thickness, and K0 is the intrinsic conductivity of the electrolyte. τ is the tortuosity, γ is the correction factor (γ ranges from 1.5 to 2.0), ε is the porosity, and α is the Bruggeman exponent. Furthermore, the two formulas are mutually verifiable; only when the tortuosity measured by the two methods is the same should that tortuosity be used to ensure calculation accuracy.

[0078] The electrochemical reaction constant of the actual DCR parameter is calculated by the following formula:

[0079]

[0080]

[0081]

[0082] Where: k is the electrochemical reaction constant, is the maximum solid phase concentration of lithium ions in the positive electrode material, is the lithium ion solid phase concentration of the positive electrode material at 50% SOC, is the lithium ion concentration of the electrolyte, i 0 is the exchange current density of the positive electrode material, R is the ideal gas constant, T is the temperature, n is the reaction order, and F is the Faraday constant.

[0083] The lithium ion solid phase diffusion coefficient of the actual DCR parameter is calculated by the following formula:

[0084]

[0085] in, is the peak current; n is the number of electron transfers in the redox process; A is the electrode area immersed in the solution; D is the solid phase diffusion coefficient of lithium ions; is the scanning rate; is the concentration of lithium ions participating in the redox reaction.

[0086] Specifically, the positive sub-layer electrode is prepared by the following steps:

[0087] A SWCNT (single-walled carbon nanotube) dispersion and lithium cobalt oxide are uniformly mixed in a mass ratio of 1:0.3-0.5 and dispersed into a slurry. The slurry is filtered onto a filter membrane, freeze-dried, and roller-pressed to produce a sub-layer electrode. Furthermore, based on 100% by mass of the SWCNT dispersion, the single-walled carbon nanotube content is 0.2-0.6 wt%, the carboxymethyl cellulose content is 0.4-0.8 wt%, and the balance is deionized water.

[0088] The present invention is further illustrated by the following examples. However, the present invention is not limited to the scope of the examples. Experimental methods in the following examples without specific conditions are based on conventional methods and conditions, or are selected according to the product specifications.

[0089] Example 1

[0090] Taking the positive electrode material as high voltage 4.48V lithium cobalt oxide (LCO-4.48V) as an example, the specific information of lithium cobalt oxide is shown in Table 1.

[0091] Table 1 Detailed information of lithium cobalt oxide

[0092]

[0093] (1) Preparation of positive sub-layer electrode

[0094] 0.4 wt% SWCNTs were dispersed in water to obtain a SWCNT dispersion, stabilized by 0.6 wt% carboxymethyl cellulose. LCO-4.48V and the SWCNT dispersion (0.4 wt% SWCNTs dispersed in water, stabilized by 0.6 wt% carboxymethyl cellulose) were mixed at a mass ratio of 0.39:1 and dispersed using a high-speed shear emulsifier to form a slurry. Deionized water was added to adjust the viscosity to obtain a slurry. The slurry was vacuum filtered onto a water-based filter membrane (mixed cellulose ester membrane, 0.45 μm thick) to form an emulsion. The emulsion was then freeze-dried for 24 hours and roller-pressed to form a positive sub-layer electrode. Positive sub-layer electrodes of various thicknesses were prepared using the aforementioned procedures. These electrodes exhibited different DCR electrochemical parameters. Two groups of sub-layer electrodes with thicknesses of 82 μm (A) and 100 μm (B) were selected for experiments.

[0095] (2) Preparation of negative sublayer electrode

[0096] The negative electrode active material graphite, conductive carbon black, binder sodium carboxymethyl cellulose, and binder styrene-butadiene rubber were thoroughly mixed in a suitable amount of deionized water at a mass ratio of 97.8%:0.4%:0.8%:1.0% to form a uniform negative electrode slurry. This negative electrode slurry was then vacuum filtered onto a water-based filter membrane (mixed cellulose ester membrane, 0.45 μm thick) to form an emulsion. The emulsion was then freeze-dried for 24 hours and roller-pressed to form a negative electrode sublayer electrode with a roller compaction density of 1.75 g / cm³. The negative electrode sheets were then cut into strips. Experiments were conducted on several negative electrode sublayer electrodes of varying thickness.

[0097] (3) Taking the positive electrode as an example, obtain the actual DCR parameters of the positive electrode.

[0098] By assembling a symmetrical battery, the electrochemical impedance spectroscopy (EIS) of the symmetrical battery can be tested and the ionic internal resistance (Rion) of the electrode can be obtained. The tortuosity of the electrode can then be calculated using the following formula (1) or (2), and the porosity can be calculated using the following formulas (3)-(5).

[0099] (1)

[0100] (2)

[0101] (3)

[0102] (4)

[0103] (5)

[0104] Where τ is the tortuosity, ε is the porosity, is the ionic internal resistance, Acc is the current collector area, L is the electrode thickness, K0 is the intrinsic conductivity of the electrolyte, γ is the correction factor, the value range of γ is 1.5~2.0, ε is the porosity, α is the Bruggeman index, is the compaction density of the electrode, is the electrode quality, is the average density of the electrode, is the electrode area, is the thickness of the electrode. is the mass ratio of the components in the electrode; is the true density of the components in the electrode. It is an active substance (LCO-4.48V), For single-walled carbon nanotubes, The relevant data of the battery before aging are shown in Table 2.

[0105] Table 2. Relevant parameters of sub-layer electrodes.

[0106]

[0107] Obtaining key parameters of electrochemical reaction internal resistance:

[0108] By testing the EIS of the positive electrode at different temperatures, the electrochemical reaction impedance R at different temperatures is obtained. ct The exchange current density is obtained according to formula (6), and the electrochemical reaction rate constant is calculated by formula (7). The electrochemical reaction activation energy is calculated by the relationship between the electrochemical reaction rate constant and temperature in formula (8).

[0109] (6)

[0110] (7)

[0111] (8)

[0112] in:: is the maximum solid phase concentration of lithium ions in lithium cobalt oxide, is the lithium ion solid phase concentration of lithium cobalt oxide at 50% SOC, is the lithium ion concentration of the electrolyte, i 0 is the exchange current density of lithium cobalt oxide electrode particles, k is the electrochemical reaction constant, is the activation energy of the electrochemical reaction, R is the ideal gas constant, T is the temperature, n is the reaction order, F is the Faraday constant, T1 and T2 are arbitrary temperature values, T1 ≠ T2, k1 is the electrochemical reaction constant at T1, and k2 is the electrochemical reaction constant at T2. The specific parameters are shown in Table 3.

[0113] Table 3 Electrochemical reaction parameters of lithium cobalt oxide (50% SOC)

[0114]

[0115] Obtaining the solid-phase diffusion coefficient of lithium ions:

[0116] Based on the Randles-Sevchik equation:

[0117]

[0118] At room temperature, the following formula is used:

[0119]

[0120] in, is the magnitude of the peak current; n is the number of electron transfers in the redox process; A is the electrode area immersed in the solution; D is the diffusion coefficient in the electrode; is the scanning rate; is the concentration of lithium ions participating in the redox reaction.

[0121] The area of ​​the electrode immersed in the solution can generally be considered as its geometric area. The radius of the electrode is 6 mm, and the area of ​​the electrode is 0.000113 m 2 Since the diffusion of lithium in the electrode material is a very slow process, the scan rate should be selected below 1mV / s. In this experiment, (0.1mV / s, 0.2mV / s, 0.3mV / s, 0.5mV / s, 0.8mV / s) were selected. is the concentration of lithium ions participating in the redox reaction. The corresponding oxidized or reduced charge can be obtained by integrating the corresponding electrode peak. Since it is a single-electron reaction, the charge is the amount of substance of the corresponding lithium ion. Then, the concentration of lithium ions participating in the reaction can be calculated by dividing it by the volume of the electrolyte. Plot the peak current at different scan rates against the square root of the scan rate. According to I p -v 1 / 2 The parameters related to the formula are shown in Table 4.

[0122] (9)

[0123] (10)

[0124] Table 4 Relevant parameters in the lithium ion diffusion process.

[0125]

[0126] By obtaining the above key electrochemical parameters, the DCR of the lithium cobalt oxide electrode can be calculated. The relevant parameters of the A and B sub-layer electrodes are shown in Table 5:

[0127] Table 5 DCR related parameters of the two sub-layer electrodes

[0128]

[0129] (4) Composite positive sub-layer electrodes with different DCR sizes

[0130] Different positive electrode sub-layer electrodes are combined into a double-layer positive electrode composite electrode by hot pressing.

[0131] (5) The negative electrode sheet is tested according to the method in step (3) and combined into a negative composite electrode.

[0132] (6) The positive electrode composite electrode, the negative electrode composite electrode, and the separator are assembled into a lithium-ion battery, and the cycle performance test is performed. Among them, the positive electrode sub-layer electrode close to the current collector (hereinafter referred to as the "near current collector layer") in the positive electrode composite electrode is A, and the positive electrode sub-layer electrode close to the separator (hereinafter referred to as the "near separator layer") is B.

[0133] The differences between Examples 2-4 and Comparative Examples 1-3 and Example 1 are shown in Table 6.

[0134] Table 6 Composite electrode structures of Examples 1-4

[0135]

[0136] The cycling performance of the lithium-ion batteries of Examples 1-4 and the lithium-ion batteries of Comparative Examples 1-3 was tested, and the cycling performance comparison is shown in Table 7.

[0137] Table 7

[0138]

[0139] From Table 7 we can see that:

[0140] Example 1 is the best example. Compared with Comparative Example 1, Example 3 with Comparative Example 2, and Example 4 with Comparative Example 3, the cycle performance of the composite electrode is not inferior to the cycle performance of the electrode integrally formed in the prior art, indicating that the composite electrode composed of multiple sub-layer electrodes does not reduce its cycle performance. At the same time, the present invention designs a sub-layer electrode with controllable DCR components based on the DCR decomposition theory, prepares multiple sub-layer electrodes with different parameters in advance, and after determining the order requirements and design goals, quickly formulates a strategy to combine multiple sub-layer electrodes into a composite electrode and assemble them into a battery. This can achieve quantitative control of the electrochemical performance of the battery, facilitate battery modeling and simulation, and realize digital management of the power system of the subsequent application end, thereby shortening the R&D cycle and reducing R&D costs.

[0141] Moreover, compared with Comparative Example 1, when the thickness is the same, the cycle performance of the double-layer composite electrode of Example 1 is improved, indicating that the cycle performance of the composite electrode formed by reasonable arrangement of the sub-layer electrodes can be improved, while shortening the R&D cycle and reducing the R&D cost, it can also improve the cycle performance and obtain an electrode structure with excellent cycle performance.

[0142] The embodiments described above are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention, and should all be included in the scope of protection of the present invention.

Claims

1. A method for preparing a lithium ion battery, characterized in that: The following steps are involved: preparing a plurality of sub-layer electrodes, wherein the plurality of sub-layer electrodes are classified into positive sub-layer electrodes and negative sub-layer electrodes; Obtaining the actual DCR parameter of each of the positive sub-layer electrodes; screening out at least two of the positive sub-layer electrodes according to the target DCR parameter of the electrode sheet of the target lithium-ion battery; combining the screened positive sub-layer electrodes into a positive composite electrode; wherein the actual DCR parameter of the positive composite electrode is the same as the target DCR parameter of the positive electrode sheet of the target lithium-ion battery; screening out at least two of the negative sub-layer electrodes according to the voltage drop of the positive composite electrode; combining the screened negative sub-layer electrodes into a negative composite electrode; or, Obtaining an actual DCR parameter of each of the negative sub-layer electrodes; screening at least two of the negative sub-layer electrodes according to the target DCR parameter of the electrode sheet of the target lithium-ion battery; combining the screened negative sub-layer electrodes into a negative composite electrode; wherein the actual DCR parameter of the negative composite electrode is the same as the target DCR parameter of the negative electrode sheet of the target lithium-ion battery; screening at least two of the positive sub-layer electrodes according to the voltage drop of the negative composite electrode; combining the screened positive sub-layer electrodes into a negative composite electrode; The voltage drop ΔU of the positive composite electrode pos The voltage drop ΔU of the negative composite electrode neg same; Assembling the positive composite electrode, the negative composite electrode and a separator into a lithium ion battery; The DCR parameters include one or more of porosity, tortuosity, effective electrochemical area, electrochemical reaction constant, and lithium ion solid phase diffusion coefficient.

2. The preparation method according to claim 1, characterized in that ΔU neg =I neg1 *R neg1 +I neg2 *R neg2 +...+I negm *R negm ; ΔU pos =I pos1 *R pos1 +I pos2 *R pos2 +...+I posn *R posn ; Among them, I neg is the current passing through the negative sub-layer electrode, I neg1 is the current passing through the first negative sub-layer electrode in the negative composite electrode, I neg2 is the current passing through the second negative electrode sub-layer electrode in the negative composite electrode; I negm is the current passing through the mth negative sub-layer electrode in the negative composite electrode; R neg is the resistance of the negative sub-layer electrode, R neg1 is the resistance of the first negative sub-layer electrode in the negative composite electrode, R neg2 is the resistance of the second negative sub-layer electrode in the negative composite electrode, R negm is the resistance of the mth negative sub-layer electrode in the negative composite electrode; I pos is the current passing through the positive sub-layer electrode, I pos1 I is the current passing through the first positive sub-layer electrode in the positive composite electrode; pos2 is the current passing through the second positive sub-layer electrode in the positive composite electrode, I posn is the current passing through the nth positive sub-layer electrode in the positive composite electrode; R pos is the resistance of the positive sub-layer electrode, R pos1 is the resistance of the first positive sub-layer electrode in the positive composite electrode, R pos2 is the resistance of the second positive sub-layer electrode in the positive composite electrode, R posn is the resistance of the nth positive sub-layer electrode in the positive composite electrode.

3. The preparation method according to claim 1, characterized in that The porosity of the actual DCR parameter is calculated by the following formula: Where ε is the porosity, is the compaction density of the electrode, is the electrode quality, is the average density of the electrode, is the electrode area, is the thickness of the electrode; is the mass ratio of the components in the electrode, is the mass proportion of the first component in the electrode, Is the mass proportion of the second component in the electrode... is the mass proportion of the nth component in the electrode; is the true density of the components in the electrode, is the true density of the first component in the electrode, is the true density of the second component in the electrode... is the true density of the nth component in the electrode.

4. The preparation method according to claim 1, characterized in that The tortuosity of the actual DCR parameter is calculated by the following formula: Where τ is the tortuosity, ε is the porosity, is the ionic internal resistance, Acc is the current collector area, L is the electrode thickness, and K0 is the intrinsic conductivity of the electrolyte.

5. The preparation method according to claim 1, characterized in that The tortuosity of the actual DCR parameter is calculated by the following formula: Where τ is the tortuosity, γ is the correction factor, γ ranges from 1.5 to 2.0, ε is the porosity, and α is the Bruggeman index.

6. The preparation method according to claim 1, characterized in that The electrochemical reaction constant of the actual DCR parameter is calculated by the following formula: Where: k is the electrochemical reaction constant, is the maximum concentration of lithium ions in the solid phase, is the lithium ion solid phase concentration of the positive electrode material at 50% SOC, is the lithium ion concentration of the electrolyte, i 0 is the exchange current density of the positive electrode material, R is the ideal gas constant, T is the temperature, n is the reaction order, and F is the Faraday constant.

7. The preparation method according to claim 1, characterized in that The lithium ion solid phase diffusion coefficient of the actual DCR parameter is calculated by the following formula: in, is the peak current; n is the number of electron transfers in the redox process; A is the electrode area immersed in the solution; D is the solid phase diffusion coefficient of lithium ions; is the scanning rate; is the concentration of lithium ions participating in the redox reaction.

8. The preparation method according to claim 1, characterized in that The positive electrode sheet includes a positive electrode current collector and the positive electrode composite electrode arranged on the surface of the positive electrode current collector. The material of the positive electrode composite electrode includes one or more of lithium cobalt oxide, lithium iron phosphate, spinel lithium manganese oxide, ternary positive electrode material, and lithium manganese-rich positive electrode material.

9. The preparation method according to claim 8, characterized in that The positive sub-layer electrode is prepared by the following steps: The SWCNT dispersion and lithium cobalt oxide are mixed uniformly in a mass ratio of 1:0.3-0.5 and dispersed into a slurry; the slurry is filtered onto a filter membrane, and the positive sub-layer electrode is obtained by freeze drying and roller pressing; Based on 100% by weight of the SWCNT dispersion, the content of the single-walled carbon nanotubes is 0.2-0.6 wt %, the content of the carboxymethyl cellulose is 0.4-0.8 wt %, and the balance is deionized water.

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