Graphite electrode lithium precipitation detection method and device based on differential open circuit voltage

Abstract

The application discloses a graphite electrode lithium precipitation detection method and device based on differential open-circuit voltage, and the method comprises the following steps: assembling a lithium ion battery of a graphite ultrathin non-porous electrode as a detection sample; gradually increasing the depth of discharge during the cycle test of the detection sample; collecting the differential open-circuit voltage of the detection sample at each depth of discharge; and when the differential open-circuit voltage curve fluctuates, a lithium precipitation peak appears; for the detection sample with the lithium precipitation peak, voltage sampling is performed in the first t seconds of the standing period after the end of discharging, a voltage curve is obtained, a difference between an intersection point of a straight line formed by fitting a later part of the voltage curve after reducing the unit scale of the abscissa and the ordinate and a starting point of the voltage curve is taken as a sum of an overpotential of a lithium metal electrode and an ohmic overpotential of electron conduction, lithium precipitation overpotential is obtained in combination with the battery voltage, and lithium precipitation is triggered when the lithium precipitation overpotential is less than or equal to zero. The application can quickly and accurately detect whether the graphite electrode triggers lithium precipitation.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and more specifically, relates to a method and apparatus for detecting lithium plating on graphite electrodes based on a differential open circuit voltage. Background Technology

[0002] High-energy-density lithium-ion batteries are widely used in electric vehicles and other fields, and achieving fast charging without affecting performance and cycle life is considered one of the important directions for the development of electric vehicles. The charging rate of lithium-ion batteries is mainly limited by the lithium plating side reaction on the graphite surface of the negative electrode, and the deposited metallic lithium can accelerate battery aging and even cause safety accidents. For example, continuously growing lithium can easily form dendrites and may puncture the separator, causing a short circuit and leading to thermal runaway of the battery pack. Furthermore, partial lithium plating can lead to irreversible loss of active lithium and failure of lithiated graphite, rapidly reducing the battery's cycle capacity. To develop effective strategies to suppress lithium plating, it is necessary to accurately determine lithium plating during fast charging.

[0003] Currently, there are two methods for determining the triggering mechanism of lithium plating during fast charging of batteries: concentration (diffusion) control or overpotential control. The concentration determination method assumes that the slow Li-dilation in graphite crystals... + Diffusion, accompanied by a phase transition, cannot cope with the rapid interfacial ion intercalation under fast charging, thus causing a steep Li-phase gradient to form from the center to the surface of the graphite particles. + Concentration gradient. The overpotential control criterion is based on the deposition / dissolution kinetics of lithium metal electrodes. Under fast-charging conditions that deviate significantly from thermodynamic equilibrium, the lithium plating reaction on the graphite electrode is considered to be driven by the kinetic overpotential of the reaction (also known as the lithium plating overpotential), i.e., the criterion is determined when the lithium plating overpotential becomes negative.

[0004] The challenges of the two methods mentioned above are: (1) spatially averaged potential measurements cannot capture lithium plating that typically originates at local locations on porous electrodes (at the membrane / electrode interface); and (2) the coupling of multiple overpotential processes makes it difficult to verify the correlation between lithium plating and a single overpotential condition. In summary, the multi-process coupling of porous electrodes poses a challenge to the determination of lithium plating on graphite electrodes.

[0005] It is evident that existing lithium plating detection technologies for graphite electrodes in lithium-ion batteries suffer from the technical problem of being unable to accurately detect lithium plating triggered by graphite electrodes due to the inability to accurately determine the cause of lithium plating. Summary of the Invention

[0006] To address the aforementioned deficiencies or improvement needs of existing technologies, this invention provides a method and apparatus for detecting lithium plating on graphite electrodes based on micro-switched open-circuit voltage. This solves the technical problem in existing lithium-ion battery lithium plating detection technologies where the inability to accurately determine the cause of lithium plating leads to inaccurate detection of graphite electrode-triggered lithium plating.

[0007] To achieve the above objectives, according to one aspect of the present invention, a method for detecting lithium plating on a graphite electrode based on a differential open-circuit voltage is provided, comprising the following steps:

[0008] (1) A lithium-ion battery assembled with a graphite ultrathin non-porous electrode with a thickness of 20μm-50μm was used as a test sample. When the test sample was subjected to cyclic testing, the discharge depth was gradually increased. The micro-discharge voltage of the test sample at each discharge depth was collected to form a micro-discharge voltage curve at each discharge depth. When the micro-discharge voltage curve fluctuates, it indicates that a lithium plating peak will appear at the discharge depth corresponding to the micro-discharge voltage curve.

[0009] (2) For the test sample with lithium plating peak, voltage sampling is performed in the first t seconds of the static period after discharge to obtain the voltage curve, where the value of t is in the range of 1-10. The unit scale of the horizontal axis in the voltage curve is reduced by 150-200 times, and then reduced by 150-200 times. The difference between the intersection of the straight line formed by fitting the latter part of the voltage curve and the vertical axis and the starting point of the voltage curve is taken as the sum of the overpotential of the lithium metal electrode and the ohmic overpotential of electron conduction in the lithium-ion battery.

[0010] (3) The sum of the lithium-ion battery voltage, the overpotential of the lithium metal electrode in the lithium-ion battery and the ohmic overpotential of electron conduction is used as the lithium plating overpotential. The magnitude of the lithium plating overpotential is detected. When the lithium plating overpotential is less than or equal to zero, the graphite ultrathin non-porous electrode triggers lithium plating.

[0011] In this invention, the starting point of the voltage curve is the point at which the discharge ends and the circuit begins to settle. The vertical axis is the straight line perpendicular to the horizontal axis at the starting point.

[0012] Furthermore, the method also includes:

[0013] The difference between the intersection of the straight line formed by fitting the latter part of the voltage curve after the unit scale of the horizontal axis in the voltage curve is reduced by 150-200 times for the first time and the intersection point in step (2) is taken as the overpotential of the lithium-ion intercalation reaction.

[0014] The equilibrium potential of the lithium-ion intercalation reaction on the graphite surface is obtained by subtracting the lithium-ion battery voltage, the overpotential of the lithium metal electrode in the lithium-ion battery, and the ohmic overpotential of electron conduction. This equilibrium potential is used to determine the lithium-ion concentration on the surface of the graphite particles.

[0015] Furthermore, the ultrathin, non-porous graphite electrode is prepared by the following method:

[0016] The active graphite particles, binder, conductive agent and solvent are mixed evenly to obtain the electrode slurry, and the solid content in the electrode slurry is between 15wt% and 30wt%.

[0017] Electrode paste is coated onto copper foil with a coating thickness of 20μm-50μm to obtain an ultrathin, non-porous graphite electrode.

[0018] Furthermore, the thickness of the graphite ultrathin non-porous electrode is 25μm-35μm.

[0019] Furthermore, the value of t ranges from 1 to 5.

[0020] According to another aspect of the present invention, a graphite electrode lithium plating detection device based on micro-switched voltage is provided, comprising: a lithium-ion battery, an electrochemical workstation, a battery charge-discharge test device, and a processor;

[0021] The lithium-ion battery is formed by assembling an ultrathin, non-porous graphite electrode with a thickness of 20μm-50μm with a lithium metal electrode.

[0022] The battery charge and discharge test equipment is electrically connected to the lithium-ion battery and is used to control the depth of discharge to gradually increase during the cycle test of the lithium-ion battery.

[0023] The electrochemical workstation is electrically connected to the lithium-ion battery and is used to collect the micro-switching voltage of the lithium-ion battery at each discharge depth to form a micro-switching voltage curve for each discharge depth.

[0024] The processor communicates with the electrochemical workstation to monitor the micro-switching voltage curve. When the micro-switching voltage curve fluctuates, it indicates that a lithium plating peak will appear at the discharge depth corresponding to the micro-switching voltage curve.

[0025] The battery charge and discharge test equipment is also used to sample the voltage of lithium-ion batteries that exhibit lithium plating peaks during the first t seconds of the resting period after discharge to obtain a voltage curve, where the value of t ranges from 1 to 10.

[0026] The processor communicates with the battery charge / discharge testing equipment to first reduce the unit scale of the horizontal axis in the voltage curve by 150-200 times, then reduce it by another 150-200 times. The difference between the intersection point of the straight line formed by fitting the latter part of the voltage curve and the vertical axis and the starting point of the voltage curve is used as the sum of the overpotential of the lithium metal electrode and the ohmic overpotential of electron conduction in the lithium-ion battery. The sum of the lithium-ion battery voltage, the overpotential of the lithium metal electrode and the ohmic overpotential of electron conduction in the lithium-ion battery is used as the lithium plating overpotential. The magnitude of the lithium plating overpotential is detected. When the lithium plating overpotential is less than or equal to zero, the graphite ultrathin non-porous electrode triggers lithium plating.

[0027] Furthermore, the processor is also used to take the difference between the intersection point of the straight line formed by fitting the latter part of the voltage curve after the unit scale of the horizontal axis is reduced by 150-200 times for the first time and the intersection point obtained by the second reduction of the horizontal axis as the overpotential of the lithium-ion intercalation reaction; and to subtract the overpotential of the lithium-ion intercalation reaction from the sum of the lithium-ion battery voltage, the overpotential of the lithium metal electrode in the lithium-ion battery and the ohmic overpotential of electron conduction to obtain the equilibrium potential of the lithium-ion intercalation reaction on the graphite surface, which is used to determine the lithium-ion concentration on the surface of the graphite particles.

[0028] Furthermore, the ultrathin, non-porous graphite electrode in the lithium-ion battery is prepared by the following method:

[0029] The active graphite particles, binder, conductive agent and solvent are mixed evenly to obtain the electrode slurry, and the solid content in the electrode slurry is between 15wt% and 30wt%.

[0030] Electrode paste is coated onto copper foil with a coating thickness of 20μm-50μm to obtain an ultrathin, non-porous graphite electrode.

[0031] According to another aspect of the present invention, a computer-readable storage medium is provided, the computer-readable storage medium storing a computer program that, when executed by a processor, implements a method for detecting lithium plating on a graphite electrode based on a differential open-circuit voltage.

[0032] In summary, compared with the prior art, the above-described technical solutions conceived by this invention can achieve the following beneficial effects:

[0033] (1) Existing graphite electrodes are typically 1 mm thick and have a porous structure. Such electrodes cannot capture lithium plating, which usually originates at localized locations (the separator / electrode interface), during lithium plating detection. Furthermore, the coupling of overpotentials across multiple processes makes it difficult to verify the correlation between lithium plating and a single overpotential condition. This invention utilizes a lithium-ion battery assembled with an ultrathin, non-porous graphite electrode with a thickness of 20 μm-50 μm. This eliminates the overpotential difference along the thickness direction of the porous electrode. Time-resolved potential response allows for the separation of individual kinetic processes and their overpotential magnitudes. The region of rapid potential increase corresponds to the sum of the overpotential of the lithium metal electrode and the Ohm overpotential of electron conduction, while the region of slower potential increase corresponds to the interface Li… + The intercalation reaction overpotential, with the region of slowest potential rise corresponding to the solid-phase diffusion overpotential within the graphite particles. Lithium plating overpotential and Li on the graphite electrode surface. +Whether concentration and other factors are triggering conditions for graphite lithium plating is controversial. However, this invention, using lithium-ion batteries assembled with ultrathin, non-porous graphite electrodes with a thickness of 20μm-50μm, found that lithium plating is triggered by the ultrathin, non-porous graphite electrode when the lithium plating overpotential is less than or equal to zero. The instantaneous saturation of Li+ concentration on the graphite particle surface at the end of discharge is not a necessary condition for triggering graphite lithium plating. Therefore, when detecting lithium plating in lithium-ion batteries, it is only necessary to monitor the sum of the overpotential of the lithium metal electrode and the ohmic overpotential of electron conduction. The lithium-ion battery voltage, the sum of the overpotential of the lithium metal electrode and the ohmic overpotential of electron conduction, are used as the lithium plating overpotential. The magnitude of the lithium plating overpotential is then measured to determine whether the ultrathin, non-porous graphite electrode triggers lithium plating. This invention first determines whether a graphite electrode has undergone lithium plating by checking whether a lithium plating peak appears in the micro-switched voltage curve. Since the triggering conditions for graphite lithium plating have been accurately determined through preliminary experiments, the lithium plating overpotential can be monitored separately after lithium plating is determined to exist. When the lithium plating overpotential is less than or equal to zero, the ultrathin non-porous graphite electrode triggers lithium plating, thereby enabling rapid and accurate detection of triggered lithium plating.

[0034] (2) The detection method of the present invention also provides a method for rapidly and accurately determining the lithium ion concentration on the surface of graphite particles. The lithium-ion battery assembled by the present invention using ultrathin non-porous graphite electrodes with a thickness of 20μm-50μm can eliminate the overpotential difference in the thickness direction of the porous electrode. The individual kinetic process and its overpotential can be separated by the time-resolved potential response. The region where the potential rises slowly corresponds to the overpotential of the Li+ intercalation reaction at the interface. Therefore, the lithium-ion intercalation reaction overpotential is obtained by subtracting the lithium-ion intercalation reaction overpotential from the sum of the lithium-ion battery voltage, the overpotential of the lithium metal electrode in the lithium-ion battery and the ohmic overpotential of electron conduction. This is used to determine the lithium ion concentration on the surface of graphite particles.

[0035] (3) The present invention also provides a method for preparing an ultrathin non-porous graphite electrode. The solid content in the electrode slurry represents the percentage of the total mass of graphite active particles, binder and conductive agent in the electrode slurry to the mass of the solvent. The existing porous thick graphite electrode has a solid content of 50wt%-70wt% in the electrode slurry during preparation, while the solid content in the electrode slurry of the present invention is between 15wt%-30wt%. Moreover, the coating thickness is small during preparation of the present invention, with only a single layer of active particles sparsely distributed, which can obtain an ultrathin non-porous graphite electrode, thereby supporting the subsequent detection of lithium plating on graphite electrodes.

[0036] (4) When the thickness of the graphite ultrathin non-porous electrode is 25μm-35μm, the time-resolved potential response can more quickly and accurately separate individual kinetic processes and their overpotentials. When the value of t is in the range of 1-5, the sampling data can be reduced, the time required for subsequent voltage curve fitting can be shortened, and the region of rapid potential rise can be obtained quickly and accurately, thus obtaining the sum of the overpotential of the lithium metal electrode and the ohmic overpotential of electron conduction in the lithium-ion battery.

[0037] (5) The lithium plating detection device designed in this invention can quickly and accurately determine the triggering of lithium plating using a lithium-ion battery, an electrochemical workstation, a battery charge-discharge test device, and a processor. The lithium-ion battery is formed by assembling a 20μm-50μm thick graphite ultrathin non-porous electrode with a lithium metal electrode. The lithium-ion battery assembled with the graphite ultrathin non-porous electrode can eliminate the overpotential difference in the thickness direction of the porous electrode, and the individual kinetic process and its overpotential can be separated through time-resolved potential response. Both the electrochemical workstation and the battery charge-discharge testing equipment are common devices, indicating that the detection device can be set up quickly and simply. The electrochemical workstation collects the micro-splitter voltage, and the battery charge-discharge testing equipment samples the voltage. The data to be collected is also easy to obtain. Finally, the processor processes the collected data and separates the sum of the overpotential of the lithium metal electrode in the lithium-ion battery and the ohmic overpotential of electron conduction. The lithium-ion battery voltage, the sum of the overpotential of the lithium metal electrode in the lithium-ion battery and the ohmic overpotential of electron conduction are used as the lithium plating overpotential. By detecting the magnitude of the lithium plating overpotential, the triggering of lithium plating can be quickly and accurately determined. Attached Figure Description

[0038] Figure 1 This is a flowchart of a graphite electrode lithium plating detection method based on micro-switching voltage provided in an embodiment of the present invention;

[0039] Figure 2 A flowchart illustrating the fabrication process of the graphite ultrathin non-porous electrode provided in an embodiment of the present invention;

[0040] Figure 3 SEM image of the graphite ultrathin non-porous electrode provided in an embodiment of the present invention;

[0041] Figure 4 A flowchart illustrating the lithium plating test process for graphite||Li half-cells provided in this embodiment of the invention;

[0042] Figure 5 The micro-open circuit voltage curve of the graphite||Li half cell under 3C rate in the resting section provided in the embodiment of the present invention;

[0043] Figure 6 Time-resolved potential measurement of graphite||Li half-cell provided in this embodiment of the invention;

[0044] Figure 7 The lithium plating overpotential of the graphite||Li half-cell provided in this embodiment of the invention during the discharge stage at 3C rate.

[0045] Figure 8 This invention provides graphite||Li half-cells at different rate conditions for embodiments of the invention. Variation of DOD with different depths of discharge;

[0046] Figure 9 The voltage curves of the graphite||Li half-cell in the 3C discharge and resting stages provided in the embodiments of the present invention are compared with the equilibrium potential of the lithiated graphite.

[0047] Figure 10 The Li content on the surface of graphite particles during the instantaneous discharge from 3C to different depths of discharge (DOD) of the graphite||Li half-cell provided in the embodiments of the present invention + concentration;

[0048] Figure 11 The graphite||Li half-cell provided in this embodiment of the invention discharges at different rates to the critical DOD moment of lithium plating, where Li on the surface of the graphite particles... + concentration. Detailed Implementation

[0049] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0050] like Figure 1 As shown, a method for detecting lithium plating on a graphite electrode based on differential open-circuit voltage includes the following steps:

[0051] (1) A lithium-ion battery assembled with a graphite ultrathin non-porous electrode with a thickness of 20μm-50μm was used as a test sample. When the test sample was subjected to cyclic testing, the discharge depth was gradually increased. The micro-discharge voltage of the test sample at each discharge depth was collected to form a micro-discharge voltage curve at each discharge depth. When the micro-discharge voltage curve fluctuates, it indicates that a lithium plating peak will appear at the discharge depth corresponding to the micro-discharge voltage curve.

[0052] (2) For the test sample with lithium plating peak, voltage sampling is performed in the first t seconds of the static period after discharge to obtain the voltage curve, where the value of t is in the range of 1-10. The unit scale of the horizontal axis in the voltage curve is reduced by 150-200 times, and then reduced by 150-200 times. The difference between the intersection of the straight line formed by fitting the latter part of the voltage curve and the vertical axis and the starting point of the voltage curve is taken as the sum of the overpotential of the lithium metal electrode and the ohmic overpotential of electron conduction in the lithium-ion battery.

[0053] (3) The sum of the lithium-ion battery voltage, the overpotential of the lithium metal electrode in the lithium-ion battery and the ohmic overpotential of electron conduction is used as the lithium plating overpotential. The magnitude of the lithium plating overpotential is detected. When the lithium plating overpotential is less than or equal to zero, the graphite ultrathin non-porous electrode triggers lithium plating.

[0054] like Figure 2 As shown, the ultrathin, non-porous graphite electrode is prepared by the following method:

[0055] Graphite active particles, binder, and conductive agent are added to a solvent and continuously stirred until uniformly mixed to obtain an electrode slurry. The solid content in the electrode slurry is between 15wt% and 30wt%.

[0056] Electrode paste is coated onto copper foil to obtain a wet electrode sheet with a coating thickness of 20μm-50μm. The wet electrode sheet is then dried to obtain an ultrathin, non-porous graphite electrode.

[0057] Specifically, in the stirring process, the binder is first added to the solvent and mixed, and then magnetically stirred at a speed of 300-500 r / min for 20-50 min to obtain a uniform gel-like solution. Then, the conductive agent and active material are added in sequence and mixed, and magnetically stirred at a speed of 400-700 r / min for 5-8 h.

[0058] The graphite active particles are natural graphite, artificial graphite, or mesophase carbon microspheres used in the preparation of graphite electrodes for conventional lithium-ion batteries. The binder is sodium carboxymethyl cellulose and styrene-butadiene rubber, the conductive agent is conductive carbon black, and the solvent is deionized water. Coating is performed manually or by plate coating. Heating and drying are carried out by infrared radiation drying or vacuum drying at a temperature of 60℃ to 100℃ for 5 to 10 hours. The mass ratio of graphite active particles, binder, and conductive agent is (18-22):(6-8):(2-4).

[0059] The aforementioned ultrathin, non-porous graphite electrode was assembled into a graphite-Li half-cell in an argon-atmosphere glove box. In this graphite-Li half-cell structure, the positive electrode is the ultrathin graphite electrode, the negative electrode is a lithium sheet, and a porous polymer membrane separates the positive and negative electrodes. Electrolyte is added to completely fill the pores of the membrane and wet the positive and negative electrodes. (In conventional lithium-ion battery use, the full cell uses the graphite electrode as the negative electrode. However, in measuring and testing electrochemical characteristics, a half-cell is typically used for testing. Therefore, the electrode to be measured is the positive electrode, i.e., the graphite electrode, and the negative electrode is usually a lithium sheet.)

[0060] As a further preferred embodiment, the lithium metal and the separator are commercially available lithium sheets and separators commonly used in conventional lithium-ion battery assemblies, and the electrolyte is an aqueous electrolyte or an organic electrolyte commonly used in conventional lithium-ion battery assemblies.

[0061] During the experiment, voltage sampling was performed at 1ms intervals during the first 10 seconds of the static period after discharge, and at 0.5s intervals during the remaining static periods to obtain the voltage curve. (By sampling only the first t seconds during the detection process, the region of rapid potential rise can be quickly separated.)

[0062] Based on the time difference in the potential response, individual kinetic processes and their overpotential quantities are isolated. Specifically, the region of rapid potential rise corresponds to the sum of the overpotential of the lithium metal electrode and the Ohm overpotential of electron conduction. The region where the potential rises slowly corresponds to the overpotential of the Li+ intercalation reaction at the interface. The region where the potential rises most slowly corresponds to the solid-phase diffusion overpotential within the graphite particles. These three parameters represent: the overpotential of lithium dissolution / deposition reaction on the lithium metal electrode + the Ohm overpotential generated by the contact resistance between the graphite ultrathin electrode and the current collector + Li + The overpotential generated by diffusion in the SEI film on the lithium metal surface and the graphite electrode surface.

[0063] As a further preferred embodiment, the steps for determining the graphite lithium plating trigger condition are as follows:

[0064] (1) Lithium plating overpotential Determining the relationship with lithium plating in graphite

[0065] The lithium plating overpotential at the appearance of the lithium plating peak was detected, and the lithium plating sites at different rate of reaction were compared. The lithium plating overpotential can be calculated using equation (1) and is used to monitor the overpotential of lithium dissolution / deposition reaction on the graphite ultrathin electrode. And verify its relationship with graphite lithium plating. The superscripts gr and Li represent the spatial positions of the graphite electrode and the lithium metal electrode, respectively, and V... cell This represents the voltage of a graphite||Li half-cell.

[0066]

[0067] (2) Li on the surface of graphite particles + Determining the relationship between concentration and lithium deposition in graphite

[0068] Calculate the depth of discharge (DOD) at different discharge rates and the Li content on the graphite particle surface at the instant of discharge to the critical DOD for lithium plating. + Concentration. Li on graphite surface + Equilibrium potential E of intercalation reaction eq，surf The Li content on the surface of graphite particles can be calculated using equation (2). + Concentration (based on the relationship between equilibrium potential and the stoichiometric coefficient of lithium graphite).

[0069]

[0070] This invention details the process of preparing an ultrathin, non-porous graphite electrode and the experimental process of determining the triggering conditions for graphite lithium plating using this electrode, through Example 1.

[0071] Example 1

[0072] In Example 1 of this invention, a graphite ultrathin electrode was prepared using natural graphite (NG), a commonly used electrode material. The binder was sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR), the conductive agent was conductive carbon black (Super-P), and the solvent was deionized water. The graphite ultrathin electrode was assembled into a half-cell coin cell for lithium plating testing based on the differential open-circuit voltage method and time-resolved potential measurement. The electrolyte was 1M LiPF6 EC / EMC / DMC 1∶1∶1 1.0% VC, the separator was the commercially available Celgard 2400 separator, the counter electrode was a lithium sheet, the current collector was copper foil, and the testing equipment was a LANHE M340A. The preparation of the graphite ultrathin non-porous electrode and the determination of the graphite lithium plating trigger conditions included the following steps:

[0073] Step 1: The process of preparing an ultrathin, non-porous graphite electrode is as follows: Figure 2As shown, CMC was first added to deionized water and stirred at 500 rpm for 20 min to prepare a transparent colloid. Then, Super-P was added and stirred at 500 rpm for 15 min. Next, NG was added and stirred at 700 rpm for 5 h. Finally, SBR was added and stirred at 500 rpm for 1 h to obtain the electrode slurry. The specific component mass ratio was (NG∶Super-P∶CMC∶SBR=20∶3∶3∶4). The slurry was then coated onto a copper foil (12 μm) with a 30 μm gap using an infrared drying plate coater. Subsequently, the wet electrode was infrared-dried at 80℃ for 10 h to obtain an ultrathin graphite electrode. The structure was then observed using a scanning electron microscope to confirm its ultrathin electrode structure. Figure 3 As shown, the graphite electrode prepared by this invention has a non-porous structure with only a single layer of sparsely distributed active particles.

[0074] Step 2: The prepared graphite ultrathin electrode was cut into circular pieces using a slicer to serve as the positive electrode, and then assembled into a 2032 coin cell (graphite||Li) in an argon-atmospheric glove box. A lithium sheet was used as the counter electrode, Celgard 2400 was used as the separator, and 80 μl of electrolyte was added.

[0075] Step 3: Mount the battery onto the LANHE testing system and perform graphite lithium deposition testing and time-resolved potential measurement of the graphite||Li half-cell using the differential open-circuit voltage method in a constant temperature chamber at 25°C. The specific procedure is as follows:

[0076] (1) Graphite lithium plating test based on differential open-circuit voltage method

[0077] First, the graphite||Li half-cell was cycled three times to complete the formation step. In each formation cycle, the graphite||Li half-cell was first discharged to 0.005V at a C / 10 rate, allowed to rest for 30 minutes, and then charged to 1.5V at a C / 5 rate. In subsequent tests, the discharge capacity of the third cycle of the formation step was used as the rated capacity of the graphite||Li half-cell. Then, seven pre-cycle tests were performed: the depth of discharge (DOD) increased by 5% in each cycle, the discharge rate was 1C, and the charge rate and cutoff voltage were the same as in the formation. Subsequently, lithium plating tests were performed at specific discharge rates (1C, 2C, 3C, 4C, and 6C), also for seven cycles, with all test conditions except for the discharge rate being consistent with the pre-cycle tests. During the first 10 seconds of the rest period after discharge, voltage sampling was performed at 1ms intervals, and at 0.5s intervals for the remaining periods. The test setup procedure is shown in Table 1. An example of the lithium plating test voltage / current curve at 2C rate is shown in the figure. Figure 4 As shown.

[0078] Furthermore, the widely used micro-open-circuit voltage method was employed to detect lithium deposition on the graphite ultrathin electrode during the discharge of the graphite||Li half-cell. The open-circuit voltage curve after discharge interruption was analyzed to determine whether lithium deposition had occurred. Taking the lithium deposition test of the graphite||Li half-cell at 3C rate as an example, the open-circuit voltage curves of the resting period after 7 discharge cycles are shown below. Figure 5 As shown, in Example 1, a lithium plating peak was first detected in the micro-switching voltage curve when the discharge reached 35% DOD.

[0079] Table 1 Lithium plating test procedure and test condition settings

[0080] serial number Test steps Test parameters Deadline 1 form 1-1 Discharge C / 10 0.005V 1-2 Let stand 30min 1-3 Charge C / 5 1.5V 1-4 Let stand 30min 1-5 Repeat (1-1)-(1-4) 3 times 2 Pre-cycle 2-1 Discharge 1C Specific DOD 2-2 Let stand 30min 2-3 Charge C / 5 1.5V 2-4 Let stand 30min 3 Lithium plating test 3-1 Discharge 1C, 2C, 3C, 4C Specific DOD 3-2 Let stand 30min 3-3 Charge C / 5 1.5V 3-4 Let stand 30min

[0081] In Table 1, number 1 corresponds to formation, and 1-1, 1-2, 1-3, 1-4, and 1-5 correspond to discharge, rest, charge, rest, and repeat (1-1)-(1-4) during the formation process, respectively. In Table 1, number 2 corresponds to pre-cycling, and 2-1, 2-2, 2-3, and 2-4 correspond to discharge, rest, charge, and rest during the pre-cycling process, respectively. In Table 1, number 3 corresponds to lithium plating test, and 3-1, 3-2, 3-3, and 3-4 correspond to discharge, rest, charge, and rest during the pre-cycling process, respectively.

[0082] (2) Time-resolved potential measurement

[0083] Based on high-speed voltage sampling performed during the 10-second resting period after each discharge (lithiation) in the lithium plating test, the voltage resting period of the graphite||Li half-cell can be divided into three regions, such as... Figure 6 As shown. The region of rapid potential increase corresponds to the sum of the overpotential of the lithium metal electrode and the Ohm overpotential of electron conduction. The region where the potential rises slowly corresponds to the overpotential of the Li+ intercalation reaction at the interface. The region where the potential rises most slowly corresponds to the solid-phase diffusion overpotential within the graphite particles. and The dividing point is marked as P1. and The dividing point is marked as P2. Figure 6 First, the unit scale of the horizontal axis of the voltage curve was reduced from 800 to 5, a reduction of 160 times, and then from 5 to 0.03, a reduction of 167 times. Figure 6 Since it is an experimental process, the starting point of the voltage curve is not the starting point of the static period. At this time, a perpendicular line is drawn from the point where the discharge ends and the static period begins. The point where the straight line formed by fitting the slowly changing part of the voltage curve intersects with this perpendicular line is taken as the intersection point. The difference is also the difference between the intersection point and the point where the discharge ends and the static period begins.

[0084] Step 4: Based on Step 3, analyze and determine the lithium plating overpotential. The relationship between the Li+ concentration on the surface of graphite particles and lithium deposition in graphite:

[0085] (1) Lithium plating overpotential Determining the relationship with lithium plating in graphite

[0086] According to equation (1), the lithium plating overpotential of the graphite ultrathin electrode The measured battery voltage V cell and overpotential Calculate (i.e., point P1). First, calculate the lithium plating overpotential corresponding to the first appearance of the lithium plating peak during the lithium plating test discharge, such as... Figure 7 As shown, in Example 1, the discharge rate at 3C reaches 35% DOD, at which the lithium plating peak first appears. (-0.0105V) touches the 0V line and monotonically decreases below 0V. Further comparison of the lithium plating overpotential with depth of discharge (DOD) at different discharge rates revealed that most lithium plating points are below 0V, and the critical points are all near 0V. Figure 8 As shown (each cell was used only for lithium plating testing at a single rate). This phenomenon reveals the overpotential. The relationship with lithium plating supports the overpotential-triggered view, namely: when Lithium plating is triggered when the voltage is ≤0V.

[0087] (2) Li on the surface of graphite particles + Determining the relationship between concentration and lithium deposition in graphite

[0088] The Li on the surface of the graphite particles at the instant of discharge termination is estimated using equation (2). + The concentration was analyzed and its relationship with lithium plating was examined. For example... Figure 9 As shown, firstly, by comparing the voltage at point P2 with that of lithium graphite Li... x The equilibrium potential of C6 determines the value of Li. x The stoichiometry x of the C6 surface (considered as normalized Li) + (Concentration). Furthermore, the Li content on the graphite surface at the instant of discharge termination was compared with increasing DOD at different discharge rates. + Concentration, such as Figure 10 As shown, in Example 1, Li + The concentration shows an increasing trend, and in the range above the critical DOD (35%) for lithium deposition (shaded area in the figure), the instantaneous surface Li... + The concentration is nearing saturation (x > 0.98). Further comparison of the concentration values ​​at the critical lithium deposition DOD at different expansion rates is shown below. Figure 11 As shown, in Example 1, the surface Li at the critical DOD increases with magnification. + The concentration is decreasing slowly; at 6C, the surface Li... +The concentration dropped to 0.88, far from saturation. Therefore, it can be concluded that the Li concentration on the surface of the graphite particles instantaneously after the discharge ended... + Concentration saturation is not a necessary condition for triggering lithium deposition in graphite.

[0089] Embodiment 1 of the present invention found through experiments that: in the embodiment when Lithium plating is triggered by time; the Li on the surface of the graphite particles instantly appears after the discharge ends. + Concentration saturation is not a necessary condition for triggering graphite lithium plating. This resolves the controversy in the prior art regarding whether both the lithium plating overpotential and the Li+ concentration on the graphite electrode surface are triggering conditions for graphite lithium plating.

[0090] The graphite lithium plating test based on the differential open-circuit voltage (dOCV) proposed in this invention can clarify the triggering mechanism of graphite lithium plating on the negative electrode under fast charging and reveal the limiting process that leads to lithium plating. It inspires an optimized approach to suppress fast-charging lithium plating by regulating various limiting processes, thereby bringing about a significant improvement in the performance of lithium-ion batteries.

[0091] Once the triggering conditions for graphite lithium plating are accurately determined, targeted detection of graphite lithium plating triggers can be performed. Based on this, effective strategies to suppress graphite lithium plating can be formulated to improve the efficiency and performance of graphite fast charging.

[0092] Example 2

[0093] (1) A lithium-ion battery assembled with a 20μm thick graphite ultrathin non-porous electrode was used as a test sample. When the test sample was cycled, the discharge depth was gradually increased. The micro-discharge voltage of the test sample at each discharge depth was collected to form a micro-discharge voltage curve at each discharge depth. When the micro-discharge voltage curve fluctuated, it indicated that a lithium plating peak would appear at the discharge depth corresponding to the micro-discharge voltage curve.

[0094] (2) For the test sample with lithium plating peak, voltage sampling is performed in the first 10 seconds of the static period after discharge to obtain the voltage curve. The unit scale of the horizontal axis in the voltage curve is reduced by 150 times and then reduced by 150 times. The difference between the intersection of the straight line formed by fitting the latter part of the voltage curve and the vertical axis and the starting point of the voltage curve is taken as the sum of the overpotential of the lithium metal electrode and the ohmic overpotential of electron conduction in the lithium-ion battery.

[0095] (3) The sum of the lithium-ion battery voltage, the overpotential of the lithium metal electrode in the lithium-ion battery and the ohmic overpotential of electron conduction is used as the lithium plating overpotential. The magnitude of the lithium plating overpotential is detected. When the lithium plating overpotential is less than or equal to zero, the graphite ultrathin non-porous electrode triggers lithium plating.

[0096] Example 3

[0097] (1) A lithium-ion battery assembled with a graphite ultrathin non-porous electrode with a thickness of 25μm was used as a test sample. When the test sample was subjected to cyclic testing, the discharge depth was gradually increased. The micro-discharge voltage of the test sample at each discharge depth was collected to form a micro-discharge voltage curve at each discharge depth. When the micro-discharge voltage curve fluctuates, it indicates that a lithium plating peak will appear at the discharge depth corresponding to the micro-discharge voltage curve.

[0098] (2) For the test sample with lithium plating peak, voltage sampling is performed in the first 5 seconds of the static period after discharge to obtain the voltage curve. The unit scale of the horizontal axis in the voltage curve is reduced by 170 times and then by 180 times. The difference between the intersection of the straight line formed by fitting the latter part of the voltage curve and the vertical axis and the starting point of the voltage curve is taken as the sum of the overpotential of the lithium metal electrode and the ohmic overpotential of electron conduction in the lithium-ion battery.

[0099] (3) The sum of the lithium-ion battery voltage, the overpotential of the lithium metal electrode in the lithium-ion battery and the ohmic overpotential of electron conduction is used as the lithium plating overpotential. The magnitude of the lithium plating overpotential is detected. When the lithium plating overpotential is less than or equal to zero, the graphite ultrathin non-porous electrode triggers lithium plating.

[0100] Example 4

[0101] (1) A lithium-ion battery assembled with a 30μm thick graphite ultrathin non-porous electrode was used as a test sample. When the test sample was cycled, the discharge depth was gradually increased. The micro-discharge voltage of the test sample at each discharge depth was collected to form a micro-discharge voltage curve at each discharge depth. When the micro-discharge voltage curve fluctuated, it indicated that a lithium plating peak would appear at the discharge depth corresponding to the micro-discharge voltage curve.

[0102] (2) For the test sample with lithium plating peak, voltage sampling is performed in the first second of the static period after discharge to obtain voltage curve. The unit scale of the horizontal axis in the voltage curve is reduced by 200 times and then by 190 times. The difference between the intersection of the straight line formed by fitting the latter part of the voltage curve and the vertical axis and the starting point of the voltage curve is taken as the sum of the overpotential of the lithium metal electrode and the ohmic overpotential of electron conduction in the lithium-ion battery.

[0103] (3) The sum of the lithium-ion battery voltage, the overpotential of the lithium metal electrode in the lithium-ion battery and the ohmic overpotential of electron conduction is used as the lithium plating overpotential. The magnitude of the lithium plating overpotential is detected. When the lithium plating overpotential is less than or equal to zero, the graphite ultrathin non-porous electrode triggers lithium plating.

[0104] Example 5

[0105] (1) A lithium-ion battery assembled with a 35μm thick graphite ultrathin non-porous electrode was used as a test sample. When the test sample was cycled, the discharge depth was gradually increased. The micro-discharge voltage of the test sample at each discharge depth was collected to form a micro-discharge voltage curve at each discharge depth. When the micro-discharge voltage curve fluctuated, it indicated that a lithium plating peak would appear at the discharge depth corresponding to the micro-discharge voltage curve.

[0106] (2) For the test sample with lithium plating peak, voltage sampling is performed in the first 2 seconds of the static period after discharge to obtain the voltage curve. The unit scale of the horizontal axis in the voltage curve is reduced by 200 times and then reduced by 200 times. The difference between the intersection of the straight line formed by fitting the latter part of the voltage curve and the vertical axis and the starting point of the voltage curve is taken as the sum of the overpotential of the lithium metal electrode and the ohmic overpotential of electron conduction in the lithium-ion battery.

[0107] (3) The sum of the lithium-ion battery voltage, the overpotential of the lithium metal electrode in the lithium-ion battery and the ohmic overpotential of electron conduction is used as the lithium plating overpotential. The magnitude of the lithium plating overpotential is detected. When the lithium plating overpotential is less than or equal to zero, the graphite ultrathin non-porous electrode triggers lithium plating.

[0108] Example 6

[0109] (1) A lithium-ion battery assembled with a graphite ultrathin non-porous electrode with a thickness of 50 μm was used as a test sample. When the test sample was subjected to cyclic testing, the discharge depth was gradually increased. The micro-discharge voltage of the test sample at each discharge depth was collected to form a micro-discharge voltage curve at each discharge depth. When the micro-discharge voltage curve fluctuates, it indicates that a lithium plating peak will appear at the discharge depth corresponding to the micro-discharge voltage curve.

[0110] (2) For the test sample with lithium plating peak, voltage sampling is performed in the first 8 seconds of the static period after discharge to obtain the voltage curve. The unit scale of the horizontal axis in the voltage curve is reduced by 200 times and then reduced by 200 times. The difference between the intersection of the straight line formed by fitting the latter part of the voltage curve and the vertical axis and the starting point of the voltage curve is taken as the sum of the overpotential of the lithium metal electrode and the ohmic overpotential of electron conduction in the lithium-ion battery.

[0111] (3) The sum of the lithium-ion battery voltage, the overpotential of the lithium metal electrode in the lithium-ion battery and the ohmic overpotential of electron conduction is used as the lithium plating overpotential. The magnitude of the lithium plating overpotential is detected. When the lithium plating overpotential is less than or equal to zero, the graphite ultrathin non-porous electrode triggers lithium plating.

[0112] Through the lithium plating detection process in Examples 2-6, it was found that when the thickness of the graphite ultrathin non-porous electrode is 25μm-35μm, the time-resolved potential response can more quickly and accurately separate individual kinetic processes and their overpotential amounts. When the value of t is in the range of 1-5, the sampling data can be reduced, shortening the time required for subsequent voltage curve fitting. At the same time, the region of rapid potential rise can be obtained quickly and accurately, yielding the sum of the overpotential of the lithium metal electrode and the ohmic overpotential of electron conduction in the lithium-ion battery.

[0113] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for detecting lithium plating on a graphite electrode based on differential open-circuit voltage, characterized in that, Includes the following steps: (1) A lithium-ion battery assembled with a graphite ultrathin non-porous electrode with a thickness of 20μm-50μm was used as a test sample. When the test sample was subjected to cyclic testing, the discharge depth was gradually increased. The micro-discharge voltage of the test sample at each discharge depth was collected to form a micro-discharge voltage curve at each discharge depth. When the micro-discharge voltage curve fluctuates, it indicates that a lithium plating peak will appear at the discharge depth corresponding to the micro-discharge voltage curve. (2) For the test sample with lithium plating peak, voltage sampling is performed in the first t seconds of the static period after discharge to obtain the voltage curve, where the value of t is in the range of 1-10. The unit scale of the horizontal axis in the voltage curve is reduced by 150-200 times, and then reduced by 150-200 times. The difference between the intersection of the straight line formed by fitting the latter part of the voltage curve and the vertical axis and the starting point of the voltage curve is taken as the sum of the overpotential of the lithium metal electrode and the ohmic overpotential of electron conduction in the lithium-ion battery. (3) The sum of the lithium-ion battery voltage, the overpotential of the lithium metal electrode in the lithium-ion battery and the ohmic overpotential of electron conduction is used as the lithium plating overpotential. The magnitude of the lithium plating overpotential is detected. When the lithium plating overpotential is less than or equal to zero, the graphite ultrathin non-porous electrode triggers lithium plating.

2. The method for detecting lithium plating on a graphite electrode based on differential open-circuit voltage as described in claim 1, characterized in that, The method further includes: The difference between the intersection of the straight line formed by fitting the latter part of the voltage curve after the unit scale of the horizontal axis in the voltage curve is reduced by 150-200 times for the first time and the intersection point in step (2) is taken as the overpotential of the lithium-ion intercalation reaction. The equilibrium potential of the lithium-ion intercalation reaction on the graphite surface is obtained by subtracting the lithium-ion battery voltage, the overpotential of the lithium metal electrode in the lithium-ion battery, and the ohmic overpotential of electron conduction. This equilibrium potential is used to determine the lithium-ion concentration on the surface of the graphite particles.

3. A method for detecting lithium plating on a graphite electrode based on a differential open-circuit voltage as described in claim 1 or 2, characterized in that, The ultrathin, non-porous graphite electrode is prepared by the following method: The active graphite particles, binder, conductive agent and solvent are mixed evenly to obtain the electrode slurry, and the solid content in the electrode slurry is between 15wt% and 30wt%. Electrode paste is coated onto copper foil with a coating thickness of 20μm-50μm to obtain an ultrathin, non-porous graphite electrode.

4. A method for detecting lithium plating on a graphite electrode based on a differential open-circuit voltage as described in claim 1 or 2, characterized in that, The thickness of the graphite ultrathin non-porous electrode is 25μm-35μm.

5. A method for detecting lithium plating on a graphite electrode based on a differential open-circuit voltage as described in claim 1 or 2, characterized in that, The value of t is in the range of 1-5.

6. A graphite electrode lithium plating detection device based on differential open-circuit voltage, characterized in that, include: Lithium-ion batteries, electrochemical workstations, battery charge / discharge testing equipment, and processors; The lithium-ion battery is formed by assembling an ultrathin, non-porous graphite electrode with a thickness of 20μm-50μm with a lithium metal electrode. The battery charge and discharge test equipment is electrically connected to the lithium-ion battery and is used to control the depth of discharge to gradually increase during the cycle test of the lithium-ion battery. The electrochemical workstation is electrically connected to the lithium-ion battery and is used to collect the micro-switching voltage of the lithium-ion battery at each discharge depth to form a micro-switching voltage curve for each discharge depth. The processor communicates with the electrochemical workstation to monitor the micro-switching voltage curve. When the micro-switching voltage curve fluctuates, it indicates that a lithium plating peak will appear at the discharge depth corresponding to the micro-switching voltage curve. The battery charge and discharge test equipment is also used to sample the voltage of lithium-ion batteries that exhibit lithium plating peaks during the first t seconds of the resting period after discharge to obtain a voltage curve, where the value of t ranges from 1 to 10. The processor communicates with the battery charge / discharge testing equipment to first reduce the unit scale of the horizontal axis in the voltage curve by 150-200 times, then reduce it by another 150-200 times. The difference between the intersection point of the straight line formed by fitting the latter part of the voltage curve and the vertical axis and the starting point of the voltage curve is used as the sum of the overpotential of the lithium metal electrode and the ohmic overpotential of electron conduction in the lithium-ion battery. The sum of the lithium-ion battery voltage, the overpotential of the lithium metal electrode and the ohmic overpotential of electron conduction in the lithium-ion battery is used as the lithium plating overpotential. The magnitude of the lithium plating overpotential is detected. When the lithium plating overpotential is less than or equal to zero, the graphite ultrathin non-porous electrode triggers lithium plating.

7. The graphite electrode lithium plating detection device based on differential open-circuit voltage as described in claim 6, characterized in that, The processor is further configured to take the difference between the intersection of the straight line formed by fitting the latter part of the voltage curve after the unit scale of the horizontal axis is reduced by 150-200 times for the first time and the intersection of the straight line with the vertical axis and the intersection obtained by the second reduction of the horizontal axis as the overpotential of the lithium-ion intercalation reaction; and to subtract the overpotential of the lithium-ion intercalation reaction from the sum of the lithium-ion battery voltage, the overpotential of the lithium metal electrode in the lithium-ion battery and the ohmic overpotential of electron conduction to obtain the equilibrium potential of the lithium-ion intercalation reaction on the graphite surface, which is used to determine the lithium-ion concentration on the surface of the graphite particles.

8. A computer-readable storage medium storing a computer program, characterized in that, When the computer program is executed by the processor, it implements a method for detecting lithium plating on a graphite electrode based on a micro-switching voltage as described in any one of claims 1 to 5.

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