Solid-phase diffusion coefficient measuring method based on single-layer particle electrode

Abstract

The invention discloses a solid-phase diffusion coefficient measurement method based on a single-layer particle electrode, and belongs to the technical field of lithium ion batteries. Comprising the following steps: A1, counting the number of particles on a scanning image through image processing software, and calculating the effective reaction area of active particles and an electrolyte interface; a2, assembling the single-layer particle electrode serving as a positive electrode and a lithium metal negative electrode into a button cell; a3, applying a constant current to the button cell at a first multiplying power within a preset time period to carry out a periodic charge-discharge pulse test; step A4, extracting solid-phase diffusion overpotential according to the speed difference of voltage response in different time periods; and step A5, calculating to obtain the solid-phase diffusion coefficient. The technical scheme has the beneficial effects that the solid-phase diffusion overpotential can be accurately extracted, and the measurement accuracy of the solid-phase diffusion coefficient is improved; the single-layer particle electrode structure is convenient for counting the number of particles in effective reaction, and the influence of electron conduction and liquid-phase ion transmission impedance on measurement is eliminated.

Description

Technical Field

[0001] The invention relates to the technical field of lithium ion batteries, and in particular to a solid phase diffusion coefficient measurement method based on a single-layer particle electrode. Background Art

[0002] The lithium ion embedding / de-embedding reaction in the battery electrode and its solid-phase diffusion in the active particles is a relatively slow kinetic process, which produces concentration polarization. The lithium diffusion rate will affect the reaction rate of the electrode. The higher the solid-phase diffusion coefficient, the better the rate performance of the electrode. This parameter mainly depends on factors such as the crystal structure, ion spacing, porosity and migration energy barrier of lithium ions in the crystal of the active material. Different active materials have different solid-phase diffusion characteristics. Therefore, the study of the diffusion kinetic behavior of lithium ions in actual active materials and the measurement of the diffusion coefficient are important basic scientific issues.

[0003] In the prior art, the most commonly used method for measuring the solid-phase diffusion coefficient is to prepare a conventional composite electrode for constant current intermittent titration technique (GITT) testing. Due to the complexity of the microstructure and components of conventional composite electrodes and the influence on the lithium metal of the electrode, the voltage curve measured by GITT is the result of complex coupling of multiple kinetic processes, including various overpotential polarizations. Even if a small current pulse is taken and IR DROP (voltage drop) is concerned, it is impossible to accurately extract the solid-phase diffusion overpotential for solid-phase diffusion coefficient calculation. In addition, the effective reaction area of ​​the electrode / electrolyte can only be obtained by inverse deduction of the mass of the active particles and the intrinsic parameters due to the accumulation of the active particles, the binder, and the conductive agent, which is prone to errors. Summary of the invention

[0004] The purpose of the present invention is to provide a method for measuring the solid-phase diffusion coefficient based on a single-layer particle electrode to solve the above technical problems;

[0005] A method for measuring solid-phase diffusion coefficient based on a single-layer particle electrode, comprising:

[0006] Step A1, using a field emission scanning electron microscope in a high vacuum mode to obtain a scanning image of a single-layer particle electrode, counting the number of particles on the scanning image and calculating the effective reaction area of ​​the interface between the active particles and the electrolyte through image processing software;

[0007] Step A2, assembling the single-layer particle electrode as a positive electrode and a lithium metal negative electrode into a button battery, and adding an electrolyte to the button battery;

[0008] Step A3, using an electrochemical workstation or a battery charge and discharge test device to apply a constant current to the button battery at a first rate within a preset time period to perform a periodic charge and discharge pulse test, perform open circuit voltage measurement during the relaxation period, and record the change in the potential of the button battery during the application of the current and the change in the steady-state potential of the relaxation period before and after the application of the current;

[0009] Step A4, extracting the solid phase diffusion overpotential according to the rate difference of voltage response in different time periods;

[0010] Step A5, calculating the solid phase diffusion coefficient based on the effective reaction area and the solid phase diffusion overpotential.

[0011] Preferably, the calculation formula for the effective reaction area in step A1 is:

[0012]

[0013] Wherein, S represents the effective reaction area, BEF represents the specific surface area of ​​the active particles, and S e Indicates the electrode area, N sem represents the number of particles on the scanned image, S sem represents the area of ​​the scanned image, D 50 Represents the average equivalent diameter of active particles.

[0014] Preferably, in step A2, the electrolyte infiltrates the single-layer particle electrode, the lithium metal negative electrode, and the diaphragm gap between the electrodes.

[0015] Preferably, in step A3, the first ratio is 0.02C or 0.05C or 0.1C, the preset time period is 20 min to 60 min, and the relaxation period is 2 h to 5 h.

[0016] Preferably, the calculation formula of the solid phase diffusion coefficient in step A5 is:

[0017]

[0018] Among them, D s represents the solid phase diffusion coefficient, τ represents the time of a single constant current pulse, m B represents the mass of active material, V m represents the molar volume of the active material, M B represents the molar mass of the active material, S represents the effective reaction area, ΔE s Indicates the change in steady-state potential before and after a pulse, ΔE t represents the solid phase diffusion overpotential.

[0019] Preferably, the method for preparing the single-layer particle electrode in step A1 comprises:

[0020] Step B1, adding a binder to a solvent, and magnetically stirring at a first speed within a first time period to obtain a colloidal solution;

[0021] Step B2, adding a conductive agent to the colloidal solution, and magnetically stirring at a second speed within a second time period to obtain a conductive solution;

[0022] Step B3, adding the active material to the conductive solution, and magnetically stirring at a third speed within a third time period to obtain an electrode slurry;

[0023] Step B4, coating the electrode slurry onto a current collector, and then drying the slurry to obtain the single-layer particle electrode.

[0024] Preferably, the solvent in step B1 is N-methylpyrrolidone for the positive electrode or deionized water for the negative electrode;

[0025] The binder is polyvinylidene fluoride for the positive electrode or methyl cellulose and styrene-butadiene rubber for the negative electrode;

[0026] The first time period is 20 minutes to 40 minutes, and the first rotation speed is 500 r / min to 700 r / min.

[0027] Preferably, the conductive agent in step B2 is conductive carbon black;

[0028] The second time period is 20 minutes to 30 minutes, and the second rotation speed is 400 r / min to 600 r / min.

[0029] Preferably, the active material in step B3 is lithium cobalt oxide or lithium iron phosphate or lithium manganese oxide or lithium nickel cobalt manganese oxide or graphite or silicon-based negative electrode;

[0030] The solid content of the electrode slurry is 15wt% to 30wt%;

[0031] The third time period is 4 hours to 8 hours, and the third rotation speed is 400 r / min to 600 r / min.

[0032] Preferably, the current collector in step B4 is aluminum foil for the positive electrode or copper foil for the negative electrode;

[0033] The coating gap of the electrode slurry is larger than the maximum particle size, and the maximum particle size is 20 μm to 50 μm;

[0034] The active particle packing density coefficient on the current collector is less than 0.2, and the calculation formula of the active particle packing density coefficient on the current collector is:

[0035]

[0036] Among them, Y A represents the active particle packing density coefficient on the current collector, n A Denotes bulk density, D 50 represents the average equivalent diameter of the active particles, l represents the coating gap of the electrode slurry, m ap represents the mass of active particles in the electrode slurry, ρ ap represents the physical density of active particles, and V represents the total volume of the electrode slurry.

[0037] The beneficial effects of the present invention are as follows: in a constant current intermittent pulse test, the solid-phase diffusion overpotential can be accurately extracted according to the rate difference of the voltage response in different time periods, thereby improving the accuracy of the solid-phase diffusion coefficient measurement; the single-layer particle electrode structure facilitates the counting of the number of particles with effective reactions, directly obtains the effective reaction area of ​​the interface between the active particles and the electrolyte, and eliminates the influence of electron conduction and liquid-phase ion transfer impedance on the measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a step diagram of the solid phase diffusion coefficient measurement method based on a single-layer particle electrode of the present invention;

[0039] Figure 2 is a step diagram of a method for preparing a single-layer particle electrode of the present invention;

[0040] Figure 3 It is a flow chart of preparing a single-layer particle electrode according to a preferred embodiment of the present invention;

[0041] Figure 4 is a scanning electron microscope image of a single-layer particle electrode involved in a preferred embodiment of the present invention;

[0042] Figure 5 is a scanning electron microscope image of a single-layer particle electrode according to another preferred embodiment of the present invention;

[0043] Figure 6 It is a constant current intermittent pulse test voltage-time diagram of the present invention;

[0044] Figure 7 It is a single pulse voltage analysis diagram of the present invention;

[0045] Figure 8 It is a single pulse local voltage analysis diagram of the present invention;

[0046] Fig. 9 It is the result diagram of the solid phase diffusion coefficient of NCM523 of the present invention;

[0047] Fig.10It is a result diagram of the solid phase diffusion coefficient of natural graphite of the present invention. DETAILED DESCRIPTION

[0048] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0049] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0050] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, but they are not intended to limit the present invention.

[0051] A solid phase diffusion coefficient measurement method based on a single-layer particle electrode, such as Figure 1 As shown, including,

[0052] Step A1, using a field emission scanning electron microscope (SEM) in a high vacuum mode to obtain a scanning image of a single-layer particle electrode, and using image processing software to count the number of particles on the scanning image and calculate the effective reaction area of ​​the interface between the active particles and the electrolyte;

[0053] Step A2, assembling a button battery with the single-layer particle electrode as a positive electrode and a lithium metal negative electrode, and adding an electrolyte to the button battery;

[0054] Step A3, using an electrochemical workstation or a battery charge and discharge test device to apply a constant current to the button battery at a first rate within a preset time period to perform a periodic charge and discharge pulse test, measuring the open circuit voltage during the relaxation period, and recording the change in the button battery potential during the application of the current and the change in the steady-state potential in the relaxation period before and after the application of the current;

[0055] Step A4, extracting the solid phase diffusion overpotential according to the rate difference of voltage response in different time periods;

[0056] Step A5, calculating the solid phase diffusion coefficient based on the effective reaction area and the solid phase diffusion overpotential.

[0057] Specifically, the present invention provides a method for measuring the solid-phase diffusion coefficient based on a single-layer particle electrode. The single-layer particle electrode structure has only a single layer of particles sparsely distributed on a metal current collector. The number of particles that react effectively can be directly counted through a scanning electron microscope image to obtain the effective reaction area of ​​the interface between the active particles and the electrolyte. Compared with the conventional composite electrode structure, there is no contact resistance between particles, only the particles are in contact with the current collector, and there is no complex pore network to hinder the transmission of liquid-phase lithium ions, which can eliminate the influence of electronic conduction and liquid-phase ion transmission impedance on the measurement. Combining a single-layer particle electrode with fast potential measurement, in a constant current intermittent pulse test, the solid-phase diffusion overpotential can be accurately extracted based on the rate difference of the voltage response in different time periods, which provides a basis for the accurate measurement of the solid-phase diffusion coefficient and the analysis of the solid-phase diffusion dynamics.

[0058] Specifically, the present invention first designs an ultra-thin electrode structure (i.e., a single-layer particle electrode) with only a single layer and particles sparsely distributed on the surface of the current collector, which can directly and accurately calculate the effective reaction area of ​​the electrode-electrolyte interface and greatly weaken the contribution of liquid-phase ion transport and electron conduction kinetics to the voltage; based on the time domain scale difference between the active material interface reaction kinetics and the solid-phase diffusion kinetics, the solid-phase diffusion overpotential is accurately extracted by high-speed potential acquisition, thereby accurately measuring the solid-phase diffusion coefficient of the active material.

[0059] Among them, step A1 uses a field emission scanning electron microscope (SEM) in a high vacuum mode to observe the microstructure morphology of the surface of the single-layer ultra-thin electrode current collector, and uses ImageJ software to count the number of particles on the SEM image to calculate the effective reaction area of ​​the interface between the active particles and the electrolyte.

[0060] The lithium metal in step A2 is a common commercial lithium sheet, and there is a polymer porous separator between the electrodes, and the electrolyte can completely infiltrate the pores of all electrodes and separators. The separator and electrolyte are consistent with the materials used in conventional lithium-ion batteries.

[0061] Step A3 performs open circuit voltage measurement during relaxation, observes the change of battery potential in the current application period and the change of steady-state potential in the relaxation period before and after current application, and obtains the change trend of solid phase diffusion coefficient with battery lithium concentration.

[0062] The IR DROP stage of the single pulse rapid decline in step A4 corresponds to the particle-electrolyte interface reaction process, the electronic conduction process at the interface between the active particles and the current collector, and the kinetic process at the lithium metal interface. The stage of slow voltage change corresponds to the solid phase diffusion process in the active particles.

[0063] In a preferred embodiment, the calculation formula for the effective reaction area in step A1 is:

[0064]

[0065] Where S represents the effective reaction area, BET represents the specific surface area of ​​the active particles, and S e Indicates the electrode area, N sem Indicates the number of particles on the scanned image, S sem Represents the area of ​​the scanned image, D 50 Represents the average equivalent diameter of active particles.

[0066] In a preferred embodiment, in step A2, the electrolyte infiltrates the single-layer particle electrode and the lithium metal negative electrode as well as the gaps in the separator between the electrodes.

[0067] Specifically, an electrolyte with good wettability and high ionic conductivity is selected to ensure that it can form a uniform wetting layer between the electrode and the separator. The concentration and viscosity of the electrolyte need to be adjusted to a suitable range to ensure a good wetting effect.

[0068] By rationally selecting the electrolyte, controlling the wetting process, and optimizing the matching of the electrolyte permeability and the electrode structure, accurate measurement of the electrode material performance can be achieved, ensuring that the electrolyte can completely penetrate into the gap between the electrode and the diaphragm, thereby obtaining an accurate solid-phase diffusion coefficient, which reflects the actual performance and electrochemical behavior of the electrode.

[0069] In a preferred embodiment, in step A3, the first ratio is 0.02C or 0.05C or 0.1C, the preset time period is 20 min to 60 min, and the relaxation period is 2 h to 5 h.

[0070] Specifically, selecting a rate of 0.02C, 0.05C or 0.1C can ensure that the battery charge and discharge process is carried out at an appropriate rate during the test, which helps to accurately reflect the solid phase diffusion behavior in the battery material. Too high or too low a rate may lead to errors.

[0071] Setting a test period of 20 to 60 minutes allows the battery to reach a steady state during the charge and discharge cycle, ensuring accurate and stable measurement results. If the time is too short, the complete diffusion characteristics may not be captured, and if the time is too long, resources may be wasted.

[0072] Setting a relaxation period of 2 to 5 hours allows the battery to fully recover to a stable state after charge and discharge, thereby accurately measuring the open circuit voltage of the battery and obtaining reliable steady-state potential data, which helps to eliminate transient effects caused by current action in a short period of time.

[0073] The open circuit voltage measurement during the relaxation period is used to record the potential change of the battery, thereby reflecting the effect of solid phase diffusion on the potential and helping to analyze the diffusion behavior of lithium ions in battery materials.

[0074] By recording the potential changes during the application of current and the relaxation phase, the solid-phase diffusion coefficient in the battery material can be obtained through data analysis, revealing the relationship between battery performance and material properties.

[0075] In a preferred embodiment, the calculation formula of the solid phase diffusion coefficient in step A5 is:

[0076]

[0077] Among them, D s represents the solid phase diffusion coefficient, τ represents the time of a single constant current pulse, m B represents the mass of active material, V m represents the molar volume of the active material, M B represents the molar mass of the active material, S represents the effective reaction area, ΔE s Indicates the change in steady-state potential before and after a pulse, ΔE t represents the solid phase diffusion overpotential, which is the total transient potential change during the applied current pulse minus the potential of the IR DROP segment.

[0078] In a preferred embodiment, the method for preparing the single-layer particle electrode in step A1 is as follows: Figure 2 ,include,

[0079] Step B1, adding a binder to a solvent, and magnetically stirring at a first speed within a first time period to obtain a colloidal solution;

[0080] Step B2, adding a conductive agent to the colloidal solution, and magnetically stirring at a second speed within a second time period to obtain a conductive solution;

[0081] Step B3, adding active material to the conductive solution, and magnetically stirring at a third speed for a third time period to obtain an electrode slurry;

[0082] Step B4, coating the electrode slurry onto the current collector, and then drying it to obtain a single-layer particle electrode.

[0083] Specifically, by gradually adding binders, conductive agents and active materials and using different stirring speeds at different stages, the uniformity and stability of the electrode slurry can be effectively controlled, ensuring that the conductive agent and active materials are evenly dispersed, and optimizing the electrochemical properties of the electrode, including improving the capacity and conductivity of the battery.

[0084] Using a binder to form a colloidal solution and strictly controlling the stirring conditions during the preparation process helps to form a good electrode structure, enhance the mechanical strength and stability of the electrode, improve the battery's ability to withstand deformation and stress during charging and discharging, and extend the battery's life.

[0085] The coating and drying process can make the active material particles of the single-layer particle electrode evenly distributed, thereby achieving a lower packing density coefficient, effectively improving the ion and electron conductivity of the electrode and improving the overall efficiency of the battery.

[0086] The preparation method has clear steps and refers to the standards of conventional lithium-ion battery electrode preparation processes, indicating that the method is feasible and mature, and can be easily connected to existing production processes, reducing the need for process adjustments and equipment modifications. It is suitable for different types of electrode materials and battery systems, including lithium-ion batteries and other types of batteries. By adjusting the types and proportions of solvents, binders, conductive agents and active materials, the performance requirements of different battery systems can be optimized.

[0087] In a preferred embodiment, the solvent in step B1 is N-methylpyrrolidone for the positive electrode or deionized water for the negative electrode;

[0088] The binder is polyvinylidene fluoride for the positive electrode or methyl cellulose and styrene-butadiene rubber for the negative electrode;

[0089] The first time period is 20 minutes to 40 minutes, and the first rotation speed is 500 r / min to 700 r / min.

[0090] Specifically, N-methylpyrrolidone (NMP) is used for the positive electrode and deionized water is used for the negative electrode. These solvents can effectively dissolve the corresponding binders to ensure uniform electrode slurry preparation. NMP can dissolve polyvinylidene fluoride (PVDF) to form a stable bonding layer in the positive electrode, while deionized water can dissolve methyl cellulose and styrene butadiene rubber (in the negative electrode), which helps to form a good slurry structure.

[0091] The binder can be fully dissolved and evenly dispersed in the solvent to form a colloidal solution, which helps to form a good slurry matrix.

[0092] In a preferred embodiment, the conductive agent in step B2 is conductive carbon black;

[0093] The second time period is 20 minutes to 30 minutes, and the second rotation speed is 400 r / min to 600 r / min.

[0094] Specifically, the addition of the conductive agent and the stirring conditions in step B2 make the conductive agent uniformly dispersed in the colloidal solution to form a good conductive network. The optimized stirring speed ensures that the conductive agent is fully dispersed and avoids aggregation.

[0095] In a preferred embodiment, the active material in step B3 is lithium cobalt oxide or lithium iron phosphate or lithium manganese oxide or lithium nickel cobalt manganese oxide or graphite or silicon-based negative electrode;

[0096] The solid content of the electrode slurry is 15wt% to 30wt%;

[0097] The third time period is 4h to 8h, and the third rotation speed is 400r / min to 600r / min.

[0098] Specifically, after adding the active material, stirring is continued to ensure uniform mixing of the active material with the conductive agent and the binder. This gradual addition and stirring process prevents uneven distribution between materials and ensures the uniformity and stability of the electrode slurry.

[0099] In a preferred embodiment, the current collector in step B4 is an aluminum foil for the positive electrode or a copper foil for the negative electrode;

[0100] The coating gap of the electrode slurry is larger than the maximum particle size, and the maximum particle size is 20 μm to 50 μm;

[0101] The active particle packing density coefficient on the current collector is less than 0.2. The calculation formula of the active particle packing density coefficient on the current collector is:

[0102]

[0103] Among them, Y A represents the active particle packing density coefficient on the current collector, n A Denotes bulk density, D 50 represents the average equivalent diameter of active particles, l represents the coating gap of electrode slurry, m ap represents the mass of active particles in the electrode slurry, ρ ap represents the physical density of active particles, and V represents the total volume of the electrode slurry.

[0104] Specifically, the packing density of the active particles distributed on the current collector must satisfy the packing density coefficient Y A <0.2, so as to achieve a sufficiently sparse distribution to form a single-layer particle coating structure. The coating and drying methods are the same as the conventional lithium-ion battery electrode preparation process.

[0105] The electrode slurry coated on the current collector is dried to remove the solvent and solidify the electrode slurry. The temperature and time control during this process ensure the structural integrity and bonding strength of the electrode, so that the final single-layer particle electrode has good mechanical and electrochemical properties.

[0106] In the first embodiment, the following is a process of preparing a single-layer ultra-thin electrode and a process of calculating a solid-phase diffusion coefficient by performing a constant current intermittent pulse test measurement based on the electrode:

[0107] Step 1, reference Figure 3The commonly used ternary material NCM523 was used to prepare a single-layer ultra-thin electrode. The binder was polyvinylidene fluoride (PVDF), the conductive agent was conductive carbon black (Super-P), the solvent was N-methylpyrrolidone (NMP), the mass ratio of NCM523, Super-P, PVDF and NMP was 6:1:2:40, and the speed of the magnetic stirrer was 500r / min. The specific process is as follows:

[0108] First, PVDF binder was added to NMP solvent and stirred for 30 min to form a uniform colloidal solution, and then Super-P was added to the colloidal solution and stirred for 20 min. Finally, NCM523 was added and stirred for 5 h to obtain electrode slurry.

[0109] Then, the electrode slurry was coated on the aluminum foil using an infrared drying flat plate coating machine, and the coating gap was adjusted to 30 μm.

[0110] Finally, the electrode was dried by infrared drying at 80°C for 5 h to obtain a single-layer ultra-thin electrode.

[0111] Step 2: The prepared single-layer ultra-thin electrode is cut into small discs (12 mm in diameter) by a slicer and assembled into 2032-type button cells (NCM523||Li) in a glove box filled with argon atmosphere.

[0112] A lithium sheet (15 mm in diameter) was used as the counter electrode, a Celgard 2500 (18 mm in diameter) was used as the separator, and 80 μl of electrolyte (1M LiPF6 EC / EMC / DMC 1:1:1 1.0% VC) was added.

[0113] Step 3: Obtain the SEM image of the surface of the NCM523 single-layer ultra-thin electrode to count the number of active particles in the effective reaction and further calculate the effective reaction area of ​​the electrode / electrolyte. The specific process is as follows:

[0114] First, the single-layer ultra-thin electrode sheet was magnified 25 times to select representative active particle distribution areas, which were evenly divided into four areas: 1, 2, 3, and 4. Figure 4 .

[0115] The four areas were magnified to 150 times and SEM images were taken. Figure 5 Then, the number of particles on the image was counted using ImageJ software, and four electrode pieces were cut from each group of electrodes as parallel samples for statistical calculation.

[0116] Finally, the calculation formula of the effective reaction area in step A1 is:

[0117]

[0118] Where S represents the effective reaction area, BET represents the specific surface area of ​​the active particles, and S e Indicates the electrode area, N sem Indicates the number of particles on the scanned image, S sem Represents the area of ​​the scanned image, D 50 Represents the average equivalent diameter of active particles.

[0119] To calculate the effective reaction area of ​​the active particle / electrolyte interface.

[0120] Step 4: Clamp the button cell into the LANHE test system (LANHE M340A) and perform a constant current intermittent pulse test in a constant temperature room at 25°C. The specific procedure is as follows:

[0121] First, the battery was subjected to a constant current discharge pulse at a rate of 0.1C for 30 minutes and then left to rest for 3 hours, and this process was repeated until a cut-off voltage of 3V was reached.

[0122] Then, the battery is charged at a constant current pulse rate of 0.1C for 30 minutes and then left to stand for 3 hours. This process is repeated until the cut-off voltage reaches 4.2V. Figure 6 The test procedure uses a 30s sampling interval, and only uses a 1ms high-frequency sampling interval 10s before each pulse to accurately extract the solid phase diffusion overpotential. For single pulse process, please refer to Figure 7 , ΔEs is the change in steady-state potential before and after a pulse, and ΔEt is the total transient potential change during the application of the current pulse minus the potential of the IR DROP segment.

[0123] Step 5: Based on the high-speed voltage acquisition section in step 4, the solid phase diffusion overpotential ΔEt is accurately extracted according to the rate difference of voltage response in different time periods. Figure 8 , the potential response curve first shows the stage from the relaxed equilibrium potential to the rapid potential drop (IR DROP) stage (t = 2s to t≈2.2s) on a time scale of 5s to 10s, corresponding to the particle-electrolyte interface reaction process ΔEct, the electronic conduction process ΔEe at the interface between the active particle and the current collector, and the kinetic process ΔELi at the lithium metal interface; then the potential slowly drops, which corresponds to the lithium ion diffusion kinetic process ΔEsp inside the active particle. The curve of the slow potential drop stage is linearly extrapolated and intersects with the vertical line at t = 2s at point P. The difference between the instantaneous potential at the end of a single discharge pulse and the potential at point P is the solid phase diffusion overpotential ΔEsp (= ΔEt) at this lithium concentration.

[0124] Step 6: Based on the effective reaction area and solid phase diffusion overpotential of the active particle / electrolyte interface accurately calculated in steps 3 and 5, the solid phase diffusion coefficient is calculated by the formula in step A5:

[0125]

[0126] Among them, D s represents the solid phase diffusion coefficient, τ represents the time of a single constant current pulse, m B represents the mass of active material, V m represents the molar volume of the active material, M B represents the molar mass of the active material, S represents the effective reaction area, ΔE s Indicates the change in steady-state potential before and after a pulse, ΔE t represents the solid phase diffusion overpotential, which is the total transient potential change during the applied current pulse minus the potential of the IR DROP segment.

[0127] To accurately calculate the solid phase diffusion coefficient at different lithium concentrations on the surface of active particles, please refer to the calculation results Fig. 9 .

[0128] In the second embodiment, the following is a process of preparing a single-layer ultra-thin electrode and a process of calculating a solid-phase diffusion coefficient by performing a constant current intermittent pulse test measurement based on the electrode:

[0129] Step 1: Prepare a single-layer ultra-thin electrode using natural graphite as the negative electrode material, the binder is sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR), the conductive agent is conductive carbon black (Super-P), the solvent is deionized water, the mass ratio of natural graphite, CMC, SBR, Super-P and deionized water is 20:3:4:3:200, and the speed of the magnetic stirrer is 500r / min. The specific process is as follows:

[0130] First, CMC was added to deionized water and stirred for 30 minutes to form a uniform colloidal solution, and then Super-P was added to the colloidal solution and stirred for 20 minutes, followed by adding natural graphite powder and stirring for 20 minutes, and finally SBR was added to the colloidal solution and stirred for 5 hours to obtain electrode slurry.

[0131] Then, the electrode slurry was coated on the aluminum foil using an infrared drying flat plate coating machine, and the coating gap was adjusted to 30 μm.

[0132] Finally, the electrode was dried by infrared drying at 80°C for 5 h to obtain a single-layer ultra-thin electrode. The prepared single-layer ultra-thin electrode was cut into small discs (12 mm in diameter) by a slicer and assembled into a 2032-type button cell (graphite||Li) in a glove box filled with argon atmosphere.

[0133] Step 2: Count the number of active particles in effective reaction by obtaining SEM images of the surface of the graphite single-layer ultra-thin electrode plate and calculate the effective reaction area of ​​the electrode / electrolyte.

[0134] Step three, the button cell was clamped into the LANHE test system (LANHE M340A) and a constant current intermittent pulse test was performed in a constant temperature chamber at 25°C. Based on the high-speed voltage acquisition section, the solid-phase diffusion overpotential ΔEt was accurately extracted according to the rate difference of voltage response in different time periods.

[0135] Step 4: Based on the accurate calculated effective reaction area and solid diffusion overpotential ΔEt of the active particle / electrolyte interface, the solid diffusion coefficient of natural graphite material at different lithium concentrations on the active particle surface is accurately calculated using the solid diffusion coefficient calculation formula in step A5. The calculation results can be found in Fig.10 .

[0136] In summary, the present application provides a method for measuring the solid-phase diffusion coefficient based on a single-layer particle electrode, which can simply, reliably, directly and accurately calculate the effective reaction area between the active particles and the electrolyte, and accurately extract the solid-phase diffusion overpotential, thereby achieving accurate measurement of the solid-phase diffusion coefficient of the active material and providing guidance for the optimal design of active materials for lithium-ion batteries.

[0137] The above description is only a preferred embodiment of the present invention, and does not limit the implementation mode and protection scope of the present invention. For those skilled in the art, it should be aware that all solutions obtained by equivalent substitutions and obvious changes made using the description and illustrations of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for measuring solid-phase diffusion coefficient based on a single-layer particle electrode, characterized in that: include, Step A1, using a field emission scanning electron microscope in a high vacuum mode to obtain a scanning image of a single-layer particle electrode, counting the number of particles on the scanning image and calculating the effective reaction area of ​​the interface between the active particles and the electrolyte through image processing software; Step A2, assembling the single-layer particle electrode as a positive electrode and a lithium metal negative electrode into a button battery, and adding an electrolyte to the button battery; Step A3, using an electrochemical workstation or a battery charge and discharge test device to apply a constant current to the button battery at a first rate within a preset time period to perform a periodic charge and discharge pulse test, perform open circuit voltage measurement during the relaxation period, and record the change in the potential of the button battery during the application of the current and the change in the steady-state potential of the relaxation period before and after the application of the current; Step A4, extracting the solid phase diffusion overpotential according to the rate difference of voltage response in different time periods; Step A5, calculating the solid phase diffusion coefficient based on the effective reaction area and the solid phase diffusion overpotential.

2. The method for measuring solid-phase diffusion coefficient based on a single-layer particle electrode according to claim 1, characterized in that: The calculation formula for the effective reaction area in step A1 is: Wherein, S represents the effective reaction area, BET represents the specific surface area of ​​the active particles, and S e Indicates the electrode area, N sem represents the number of particles on the scanned image, S sem represents the area of ​​the scanned image, D 50 Represents the average equivalent diameter of active particles.

3. The method for measuring solid-phase diffusion coefficient based on a single-layer particle electrode according to claim 1, characterized in that: In step A2, the electrolyte infiltrates the single-layer particle electrode, the lithium metal negative electrode, and the diaphragm gap between the electrodes.

4. The method for measuring solid-phase diffusion coefficient based on a single-layer particle electrode according to claim 1, characterized in that: In step A3, the first ratio is 0.02C or 0.05C or 0.1C, the preset time period is 20min to 60min, and the relaxation period is 2h to 5h.

5. The method for measuring solid-phase diffusion coefficient based on a single-layer particle electrode according to claim 1, characterized in that: The calculation formula for the solid phase diffusion coefficient in step A5 is: Among them, D s represents the solid phase diffusion coefficient, τ represents the time of a single constant current pulse, m B represents the mass of active material, V m represents the molar volume of the active material, M B represents the molar mass of the active material, S represents the effective reaction area, ΔE s Indicates the change in steady-state potential before and after a pulse, ΔE t represents the solid phase diffusion overpotential.

6. The method for measuring solid-phase diffusion coefficient based on a single-layer particle electrode according to claim 5, characterized in that: The method for preparing the single-layer particle electrode in step A1 comprises: Step B1, adding a binder to a solvent, and magnetically stirring at a first speed within a first time period to obtain a colloidal solution; Step B2, adding a conductive agent to the colloidal solution, and magnetically stirring at a second speed within a second time period to obtain a conductive solution; Step B3, adding the active material to the conductive solution, and magnetically stirring at a third speed within a third time period to obtain an electrode slurry; Step B4, coating the electrode slurry onto a current collector, and then drying the slurry to obtain the single-layer particle electrode.

7. The method for measuring solid-phase diffusion coefficient based on a single-layer particle electrode according to claim 6, characterized in that: The solvent in step B1 is N-methylpyrrolidone for the positive electrode or deionized water for the negative electrode; The binder is polyvinylidene fluoride for the positive electrode or methyl cellulose and styrene-butadiene rubber for the negative electrode; The first time period is 20 minutes to 40 minutes, and the first rotation speed is 500 r / min to 700 r / min.

8. The method for measuring solid-phase diffusion coefficient based on a single-layer particle electrode according to claim 6, characterized in that: The conductive agent in step B2 is conductive carbon black; The second time period is 20 minutes to 30 minutes, and the second rotation speed is 400 r / min to 600 r / min.

9. The method for measuring solid-phase diffusion coefficient based on a single-layer particle electrode according to claim 6, characterized in that: The active material in step B3 is lithium cobalt oxide or lithium iron phosphate or lithium manganese oxide or lithium nickel cobalt manganese oxide or graphite or silicon-based negative electrode; The solid content of the electrode slurry is 15wt% to 30wt%; The third time period is 4 hours to 8 hours, and the third rotation speed is 400 r / min to 600 r / min.

10. The method for measuring solid-phase diffusion coefficient based on a single-layer particle electrode according to claim 6, characterized in that: The current collector in step B4 is aluminum foil for the positive electrode or copper foil for the negative electrode; The coating gap of the electrode slurry is larger than the maximum particle size, and the maximum particle size is 20 μm to 50 μm; The active particle packing density coefficient on the current collector is less than 0.2, and the calculation formula of the active particle packing density coefficient on the current collector is: Among them, Y A represents the active particle packing density coefficient on the current collector, n A Denotes bulk density, D 50 represents the average equivalent diameter of the active particles, l represents the coating gap of the electrode slurry, m ap represents the mass of active particles in the electrode slurry, ρ ap represents the physical density of active particles, and V represents the total volume of the electrode slurry.

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