Method for evaluating mechanical failure of positive pole piece of polycrystalline structure in lithium ion battery

Abstract

The invention relates to a method for evaluating mechanical failure of a positive pole piece of a polycrystalline structure in a lithium ion battery, which belongs to the technical field of lithium ion batteries, and can be used for respectively evaluating the intrinsic performance of crystal grains, the stress concentration degree of a crystal boundary and the integrity of the crystal boundary of secondary particles through nanoindentation tests of different indentation areas. A multi-dimensional evaluation means is provided; according to the method, the mechanical failure degree is evaluated through quantitative mechanical property indexes, a scientific basis is provided for optimal design of the lithium ion battery positive electrode material, and the cycle life and the safety performance of the lithium ion battery can be improved. The method is used for evaluating the particle integrity and the grain boundary damage degree of the layered oxide polycrystalline positive electrode of the lithium ion battery, the technical problem that mechanical failure evaluation of the polycrystalline positive electrode of the lithium ion battery is difficult is solved, and the technical effect of accurately evaluating the grain integrity and the grain boundary cracking process after circulation is achieved.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and specifically to a method for assessing the mechanical failure of polycrystalline positive electrode sheets in lithium-ion batteries. Background Technology

[0002] In lithium-ion batteries, the cathode material is one of the key factors determining battery performance, and its mechanical integrity has a significant impact on the battery's cycle life and safety.

[0003] Currently, lithium-ion battery cathode materials mainly include layered oxides, spinel oxides, and olivine phosphates, among which layered oxides such as LiNi x Co y Mn 1-x-y O2(NCM) and LiNi x Co y Al 1-x-y O2 (NCA) is widely used due to its high specific capacity and good cycle performance. These materials typically exist in the form of polycrystalline particles, with a large number of primary particles agglomerating to form a secondary particle structure. During battery charging and discharging, the cathode material undergoes anisotropic volume changes, leading to internal stress accumulation, which may eventually cause particle cracking and structural collapse, thus affecting battery performance and lifespan.

[0004] Studies have shown that the failure of lithium-ion battery cathode materials mainly stems from two aspects: mechanical failure, represented by polycrystalline grain boundary cracking, and chemical failure, represented by surface side reactions. Among these, mechanical failure is a significant cause of battery capacity decay and increased internal resistance. Chinese patent CN112768680B mentions a technical feature where the average crushing strength of the particles exceeds 50 MPa. This indicates that the mechanical strength of the cathode material has a significant impact on battery performance, but it primarily focuses on the overall strength of the material, rather than the evaluation methods for grain boundary integrity and damage.

[0005] The existing technologies have the following problems and shortcomings: 1. There is a lack of effective methods to assess the degree of mechanical failure of polycrystalline cathode materials for lithium-ion batteries, especially quantitative assessment methods for grain boundary cracking and particle integrity. Existing technologies mainly focus on the material recycling and repair process, rather than the assessment of failure mechanisms. 2. Most existing assessment methods rely on electrochemical performance testing or macroscopic physical performance testing, which makes it difficult to directly reflect the mechanical integrity of the material's microstructure, especially the degree of damage to grain boundaries. 3. There is a lack of standardized micromechanical testing methods to quantitatively characterize the mechanical properties of cathode materials, making it difficult to accurately assess the mechanical failure of materials under different electrochemical states. 4. Existing assessment methods for polycrystalline cathode materials often ignore the relationship between primary and secondary particles, failing to comprehensively reflect the mechanical failure mechanism of the material.

[0006] Therefore, there is an urgent need to develop a method that can quantitatively assess the mechanical failure of polycrystalline cathodes in lithium-ion batteries, so as to better understand the material failure mechanism, guide battery design and material optimization, and improve the performance and lifespan of lithium-ion batteries. Summary of the Invention

[0007] In view of the above problems, this invention provides a method for evaluating the mechanical failure of polycrystalline cathode sheets in lithium-ion batteries. Through nanoindentation testing with different indentation areas, the intrinsic properties of grains, the degree of grain boundary stress concentration, and the integrity of secondary grain boundaries can be evaluated separately, providing a multi-dimensional evaluation method. This invention evaluates the degree of mechanical failure through quantitative mechanical performance indicators, providing a scientific basis for the optimized design of lithium-ion battery cathode materials and helping to improve the cycle life and safety performance of lithium-ion batteries. It is used to evaluate the integrity of layered oxide polycrystalline cathode particles and the degree of grain boundary damage in lithium-ion batteries, solving the technical problem of difficult mechanical failure evaluation of polycrystalline cathodes in lithium-ion batteries, and achieving the technical effect of accurately evaluating grain integrity and grain boundary cracking processes after cycling.

[0008] This invention provides a method for evaluating the mechanical failure of a polycrystalline cathode in a lithium-ion battery, comprising:

[0009] Step 1: Identify lithium-ion batteries in different electrochemical states; extract positive electrode sheets with polycrystalline structures from each lithium-ion battery in different electrochemical states; dry and store the extracted positive electrode sheets with polycrystalline structures.

[0010] Step 2: Pre-process each positive electrode sheet with a polycrystalline structure to obtain multiple pre-processed positive electrode sheets;

[0011] Step 3: Perform micromechanical tests on each pretreated positive electrode to obtain the mechanical performance indicators of each pretreated positive electrode.

[0012] Step 4: Based on the mechanical performance indicators of each pretreated positive electrode sheet, evaluate the degree of mechanical failure of the positive electrode sheet with polycrystalline structure under different electrochemical states.

[0013] The electrochemical states include: charge state, calendar aging state, and / or cycle decay state;

[0014] The different cyclic decay states include zero cycle, standard cycle, and intermittent cycle;

[0015] The charging states include fully discharged state, low charge state, half charge state, and high charge state.

[0016] The positive electrode sheet with a polycrystalline structure includes a positive electrode material with a polycrystalline structure and the chemical formula Li. x My O2, M is a transition metal ion, including nickel, cobalt, manganese, aluminum and other doping elements, x+y=1;

[0017] The microstructure of the positive electrode material with a polycrystalline structure is a layered structure.

[0018] Optionally, the pretreatment method is ultrasonic dispersion of positive electrode powder in solvent, argon ion surface polishing, and / or mechanical polishing with abrasive nano suspension;

[0019] The positive electrode powder is ultrasonically dispersed in solvent for single-particle strength testing; the abrasive nano suspension is mechanically polished for nanoindentation testing; and argon ion surface polishing is used for nanoindentation testing.

[0020] Optionally, the specific steps of ultrasonic dispersion of the positive electrode powder solvent include:

[0021] Active material powder was scraped from each positive electrode sheet with a polycrystalline structure.

[0022] Each active material powder was placed in a centrifuge tube containing an organic solvent to obtain multiple suspensions;

[0023] Each suspension was ultrasonically dispersed using an ultrasonic cleaner.

[0024] After ultrasonic dispersion, use a pipette to take an appropriate amount of each suspension and drop it onto a clean glass slide. Allow it to dry naturally to obtain multiple sets of dispersed cathode particle samples. Ensure that there is sufficient spacing between the particles in each set of dispersed cathode particle samples to facilitate subsequent single-particle testing.

[0025] Optionally, the specific steps of mechanical polishing with abrasive nanoparticle suspensions include:

[0026] Multiple samples were obtained by fixing each positive electrode sheet with a polycrystalline structure onto the surface of a metal disc using crystal adhesive.

[0027] Each sample surface was polished using an ion beam polishing system;

[0028] The surface of each polished sample was observed using a scanning electron microscope to confirm the surface smoothness and clear visibility of primary particles.

[0029] Optionally, the micromechanical test is a single-particle strength test and / or a nanoindentation test.

[0030] Optionally, the mechanical performance indicators are fracture toughness, fracture stress, and Young's modulus.

[0031] Optionally, the specific steps for performing single-particle strength testing on each pretreated positive electrode sheet include:

[0032] The positive electrode particles with approximately spherical shapes were observed and selected from the dispersed positive electrode particle sample using an optical microscope. They were then compressed using a nanomechanical testing instrument equipped with a flat plate indenter until the approximately spherical positive electrode particles broke.

[0033] Obtain the force-displacement curve throughout the compression process, and determine the fracture stress of the approximately spherical positive electrode particles based on the force-displacement curve.

[0034] The fracture process of the approximately spherical cathode particles was recorded using a high-speed camera, and the fracture toughness was analyzed.

[0035] For each pretreated positive electrode sheet, at least 50 particles were tested to obtain the fracture stress and fracture toughness of the particles in each pretreated positive electrode sheet.

[0036] Optionally, the specific steps for performing nanoindentation testing on the pretreated positive electrode sheet include:

[0037] Multiple pretreated positive electrode sheets were placed on the sample stage of the nanoindenter, ensuring that the sample surface was perpendicular to the glass indenter;

[0038] A load is applied at a set loading rate, causing the glass indenter to gradually press into the surface of the pretreated positive electrode sheet;

[0039] Once the predetermined maximum load is reached, maintain the load.

[0040] The load is unloaded at a set unloading rate, and the load-displacement data are recorded throughout the process.

[0041] Repeat the above process to perform indentation tests at least 50 points on each pretreated positive electrode sheet and establish load-displacement curves;

[0042] Calculate the Young's modulus of each positive electrode based on the load-displacement curve.

[0043] Optionally, the indentation test further includes: selecting 5-20% of the particle area in the positive electrode sheet to evaluate the intrinsic properties of the grains;

[0044] The degree of grain boundary stress concentration is assessed by selecting 20%-100% of the particle area in the positive electrode sheet;

[0045] The integrity of grain boundaries was assessed by selecting 100% of the particle area in the positive electrode.

[0046] Optionally, the expression for the fracture toughness is:

[0047] K IC =(P max ×f(a / h)) / (h×√h)

[0048] Where: KIC For fracture toughness, P max Where is the maximum load, h is the thickness of the positive electrode coating, a is the crack length of the positive electrode sheet, f(a / h) is the geometric correction factor, and f(.) is the correction factor function.

[0049] Compared with the prior art, the present invention has at least the following beneficial effects:

[0050] (1) The evaluation method of the present invention can effectively evaluate the mechanical failure of polycrystalline cathode of lithium-ion battery and can accurately evaluate the grain integrity and grain boundary cracking process after different cycles.

[0051] (2) This invention is applicable to layered oxide cathode materials and has a wide range of applications;

[0052] (3) This invention can evaluate the intrinsic properties of grains, the degree of stress concentration at grain boundaries and the integrity of grain boundaries by using nanoindentation tests with different indentation areas, providing a multi-dimensional evaluation method.

[0053] (4) This invention evaluates the degree of mechanical failure by quantitative mechanical performance indicators, providing a scientific basis for the optimized design of positive electrode materials for lithium-ion batteries, and helps to improve the cycle life and safety performance of lithium-ion batteries. Attached Figure Description

[0054] The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of the invention.

[0055] Figure 1 This is a schematic diagram showing the Young's modulus results of mechanical failure of the polycrystalline positive electrode sheet in a lithium-ion battery according to an embodiment of the present invention.

[0056] Figure 2 This is a schematic diagram illustrating the evaluation results of mechanical failure of polycrystalline positive electrode sheets under different cycling states in lithium-ion batteries according to embodiments of the present invention. Detailed Implementation

[0057] To better understand the above-described objectives, features, and advantages of the present invention, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other. Furthermore, the present invention can be implemented in other ways different from those described herein; therefore, the scope of protection of the present invention is not limited to the specific embodiments disclosed below.

[0058] A specific embodiment of the present invention, such as Figure 1-2 A method for evaluating the mechanical failure of polycrystalline positive electrode sheets in lithium-ion batteries is disclosed, and the specific implementation steps are as follows:

[0059] Step 1: Identify lithium-ion batteries in different electrochemical states; extract positive electrode sheets with polycrystalline structures from each lithium-ion battery in different electrochemical states; dry and store the extracted positive electrode sheets with polycrystalline structures.

[0060] Optionally, the electrochemical state includes: a charging state, a calendar aging state, and / or a cycle decay state;

[0061] Optionally, the lithium-ion batteries in different electrochemical states are lithium-ion batteries in different charging states;

[0062] Optionally, the lithium-ion batteries in different electrochemical states are lithium-ion batteries in different cycle decay states;

[0063] The different cyclic decay states include zero cycle, standard cycle, and intermittent cycle;

[0064] For example, the zero-cycle refers to a lithium-ion battery that has undergone only a standard formation process and completed four charge-discharge cycles at a low rate, after which no further cycles are performed, serving as a benchmark for mechanical strength.

[0065] The standard cycle involves charging the lithium-ion battery at a constant current rate of B to 4.25V under temperature A, and then discharging it at a constant current rate of B to 2.8V, repeating this cycle hundreds of times.

[0066] The intermittent cycle involves charging the lithium-ion battery at a constant current rate of B to 4.25V under temperature A, reaching full charge, then allowing it to rest for a period of time, followed by discharging it at a constant current rate of B to 2.8V. This "charge-rest-discharge" process is repeated hundreds of times.

[0067] For example, the calendar aging state is the state after being put on hold for a specific duration under a specific charging state;

[0068] Furthermore, the calendar aging state is 3 months of rest in a fully charged state and 1 month of rest in a 70% state of charge (SOC).

[0069] For example, the charging state includes a fully discharged state, a low-charge state, a half-charge state, and a high-charge state.

[0070] Furthermore, the state of charge in the fully discharged state is 0%, the state of charge in the low-charge state is 30%, the state of charge in the half-charge state is 60%, and the state of charge in the high-charge state is 90%.

[0071] Optionally, the specific steps for drying and preserving include:

[0072] Multiple positive electrode sheets with polycrystalline structures were extracted and transferred to a vacuum drying oven for storage, with the temperature controlled at 25-30℃.

[0073] It is understood that the electrochemical state refers to the electrochemical behavior characteristics of lithium-ion batteries under different operating conditions, state of charge (SOC), cycling stages, or environmental factors.

[0074] Furthermore, the state of charge is used to predict the particle's ability to resist anisotropic expansion or contraction;

[0075] The cyclic decay is used to analyze mechanical failure;

[0076] Optionally, the positive electrode with a polycrystalline structure comprises a positive electrode material with a polycrystalline structure and the chemical formula Li. x M y O2, M is a transition metal ion, including nickel, cobalt, manganese, aluminum and other doping elements, x+y=1;

[0077] The microstructure of the positive electrode material with a polycrystalline structure is a layered structure.

[0078] Optionally, the cathode material with a polycrystalline structure is a polycrystalline cathode of lithium metal oxide;

[0079] Optionally, the cathode material with a polycrystalline structure is a high-nickel ternary cathode material or a lithium-rich manganese-based cathode material;

[0080] Optionally, the polycrystalline particles of the lithium metal oxide are formed by the agglomeration of multiple particles through processes such as sintering, and contain a large number of grain boundaries; wherein, the diameter of the polycrystalline particles is greater than 1 μm, which is suitable for single particle strength testing, and the diameter of the primary particles is greater than 50 nm, which is suitable for nanoindentation testing.

[0081] Step 2: Pre-process each positive electrode sheet with a polycrystalline structure to obtain multiple pre-processed positive electrode sheets;

[0082] Optionally, the pretreatment method is ultrasonic dispersion of positive electrode powder in solvent, argon ion surface polishing, and / or mechanical polishing with abrasive nano-suspension.

[0083] Furthermore, ultrasonic dispersion of cathode powder solvent is used for single-particle strength testing; mechanical polishing of abrasive nano-suspension is used for nanoindentation testing; and argon ion surface polishing is used for nanoindentation testing.

[0084] Furthermore, the specific steps of ultrasonic dispersion of the positive electrode powder solvent include:

[0085] Active material powder was scraped from each positive electrode sheet with a polycrystalline structure.

[0086] Each active material powder was placed in a centrifuge tube containing an organic solvent to obtain multiple suspensions;

[0087] Each suspension was ultrasonically dispersed using an ultrasonic cleaner, with the ultrasonic power set to 50-2000W and the treatment time to 1-30 minutes.

[0088] After ultrasonic dispersion, use a pipette to take an appropriate amount of each suspension and drop it onto a clean glass slide. Allow it to dry naturally to obtain multiple sets of dispersed cathode particle samples. Ensure that there is sufficient spacing between the particles in each set of dispersed cathode particle samples to facilitate subsequent single-particle testing.

[0089] Optionally, the organic solvent is ethanol or acetone.

[0090] Optionally, the specific steps of mechanical polishing with abrasive nanoparticle suspensions include:

[0091] Multiple samples were obtained by fixing each positive electrode sheet with a polycrystalline structure onto the surface of a metal disc using crystal adhesive.

[0092] Each sample surface was polished using an ion beam polishing system;

[0093] The surface of each polished sample was observed using a scanning electron microscope to confirm the surface smoothness and clear visibility of primary particles.

[0094] The ion beam polishing parameters include: argon ion beam energy 8-10keV, polishing angle 2-10°, and polishing time 2-4 hours.

[0095] Step 3: Perform micromechanical tests on each pretreated positive electrode to obtain the mechanical performance indicators of each pretreated positive electrode.

[0096] Optionally, the micromechanical test is a single-particle strength test and / or a nanoindentation test.

[0097] Optionally, the mechanical performance indicators are fracture toughness, fracture stress, and Young's modulus.

[0098] The fracture stress includes compressive strength, fracture toughness, Young's modulus, and plastic strain.

[0099] The fracture stress is applicable to single-particle strength testing, and the Young's modulus is applicable to nanoindentation testing.

[0100] For example, the specific steps for performing single-particle strength testing on each pretreated positive electrode sheet include:

[0101] The positive electrode particles with approximately spherical shapes were observed and selected from the dispersed positive electrode particle sample using an optical microscope. They were then compressed using a nanomechanical testing instrument equipped with a flat plate indenter until the approximately spherical positive electrode particles broke.

[0102] Obtain the force-displacement curve throughout the compression process, and determine the fracture stress of the approximately spherical positive electrode particles based on the force-displacement curve.

[0103] The fracture process of the approximately spherical cathode particles was recorded using a high-speed camera, and the fracture toughness was analyzed.

[0104] For each pretreated positive electrode sheet, at least 50 particles were tested to obtain the fracture stress and fracture toughness of the particles in each pretreated positive electrode sheet.

[0105] For example, the compression rate of the flat plate indenter is 0.02-1 μm / s.

[0106] Optionally, the fracture toughness includes radial cracking or shear failure.

[0107] Optionally, the expression for the fracture toughness is:

[0108] K IC =(P max ×f(a / h)) / (h×√h)

[0109] Where: K IC For fracture toughness, P max Where is the maximum load, h is the thickness of the positive electrode coating, a is the crack length of the positive electrode sheet, f(a / h) is the geometric correction factor, and f(.) is the correction factor function.

[0110] Optionally, the specific steps for performing nanoindentation testing on the pretreated positive electrode sheet include:

[0111] Multiple pretreated positive electrode sheets were placed on the sample stage of the nanoindenter, ensuring that the sample surface was perpendicular to the glass indenter;

[0112] A load is applied at a set loading rate, causing the glass indenter to gradually press into the surface of the pretreated positive electrode sheet;

[0113] Once the predetermined maximum load is reached, maintain the load.

[0114] The load is unloaded at a set unloading rate, and the load-displacement data are recorded throughout the process.

[0115] Repeat the above process to perform indentation tests at least 50 points on each pretreated positive electrode sheet and establish load-displacement curves;

[0116] Based on the load-displacement curve, the Young's modulus of each positive electrode is calculated, and the expression is:

[0117]

[0118] Among them, E iv is the Young's modulus of the indenter. i E is the Poisson's ratio of the pressure head. r is the preset Young's modulus of the positive electrode, v is the Poisson's ratio of the positive electrode, and E is the Young's modulus of the positive electrode.

[0119] Furthermore, the indentation area of ​​the indentation test is 5%-200% of the particle area in the positive electrode sheet, the loading rate is 1-20 mN / min, the holding time is 2-20 seconds, and the unloading rate is 1-20 mN / min.

[0120] Optionally, the indentation test further includes: selecting 5-20% of the particle area in the positive electrode sheet to evaluate the intrinsic properties of the grains;

[0121] The degree of grain boundary stress concentration is assessed by selecting 20%-100% of the particle area in the positive electrode sheet;

[0122] The integrity of grain boundaries was assessed by selecting 100% of the particle area in the positive electrode.

[0123] Step 4: Based on the mechanical performance indicators of each pretreated positive electrode, evaluate the degree of mechanical failure of the positive electrode with polycrystalline structure under different electrochemical states.

[0124] Optionally, the degree of mechanical failure of the positive electrode with polycrystalline structure under different charging states can be evaluated based on the mechanical performance indicators of each pretreated positive electrode.

[0125] Optionally, the degree of mechanical failure of the positive electrode with polycrystalline structure under different cyclic decay states can be evaluated based on the mechanical performance indicators of each pretreated positive electrode.

[0126] This invention assesses fracture stress based on single-particle test results and evaluates grain boundary stress concentration based on nanoindentation tests.

[0127] Example 1

[0128] A method for evaluating the mechanical failure of polycrystalline cathodes in lithium-ion batteries, used to assess the integrity and grain boundary damage of layered oxide polycrystalline cathode particles in lithium-ion batteries, includes the following steps:

[0129] Step 1: Select three identical lithium-ion batteries and place them in different states of charge: fully discharged (SOC 0%), low charge (SOC 30%), half-charged (SOC 60%), and high charge (SOC 90%). All three lithium-ion batteries use high-nickel layered oxide as the positive electrode material.

[0130] The lithium-ion battery was disassembled in a glove box protected by inert gas to extract the positive electrode. The disassembled electrode was immediately transferred to a vacuum drying oven for storage, with the temperature controlled at 25°C to prevent changes in the state of the positive electrode.

[0131] Step 2: Preprocess each positive electrode sheet under different charging states to obtain samples suitable for micromechanical testing, which are then used for single-particle strength testing and nanoindentation testing, respectively.

[0132] For example, the pretreatment of single-particle strength test samples: 50 mg of positive electrode active material powder was scraped from each of the four positive electrode plates in four different charging states.

[0133] Each positive electrode active material powder was placed in a centrifuge tube containing 5 ml of ethanol to obtain multiple suspensions;

[0134] The suspensions were ultrasonically dispersed using an ultrasonic cleaner with an ultrasonic power of 400W and a treatment time of 15 minutes.

[0135] After ultrasonic treatment, use a pipette to take an appropriate amount of suspension and drop it onto a clean glass slide. Allow it to dry naturally to obtain a well-dispersed positive electrode particle sample, ensuring sufficient spacing between particles to facilitate single-particle strength testing.

[0136] For example, the pretreatment of nanoindentation test samples is as follows: four positive electrode sheets with different charging states are fixed to the surface of a metal disc using crystal adhesive, and the sample surface is polished using an ion beam polishing system.

[0137] The ion beam polishing parameters were set as follows: argon ion beam energy 8keV, polishing angle 10-2°, and polishing time 2 hours.

[0138] After polishing, the sample surface was observed using a scanning electron microscope to confirm the surface smoothness and the clear visibility of primary particles.

[0139] Step 3: Perform single-particle strength test and nanoindentation test on the pretreated positive electrode sample to obtain the corresponding mechanical property index;

[0140] The single-particle strength test involved using a nanomechanical testing instrument equipped with a flat indenter to compress individual positive electrode particles dispersed on a glass slide. First, particles with approximately spherical shapes were observed and selected using an optical microscope. Then, the indenter was moved downwards at a constant rate of 0.2 μm / s until the particle fractured. The force-displacement curves throughout the compression process were recorded, and the fracture force and stress of the particles were determined from the curves. For each charging state, at least 50 particles were tested to obtain statistically significant results. Simultaneously, a high-speed camera was used to record the particle fracture process and analyze the fracture mode.

[0141] The nanoindentation test involved using a nanoindenter equipped with a glass indenter to indent the polished sample surface. The maximum indentation depth was set to 50 nm, the indentation area to be 20%-100% of the primary particle area, the loading rate to 8 mN / min, and the maximum load was maintained for 8 seconds, followed by unloading at the same rate. At least 10 particles were tested.

[0142] For each test group, the load-displacement curves were analyzed, and the compressive strength, fracture toughness, Young's modulus, and plastic strain were calculated.

[0143] Step 4: Evaluate the degree of mechanical failure of polycrystalline cathode materials under different charging states based on the mechanical performance indicators of each cathode sheet under different charging states.

[0144] For example, based on the results of single-particle strength testing and nanoindentation testing, the degree of mechanical failure of NCM811 cathode materials under four different charging states was evaluated:

[0145] Single particle strength test results:

[0146] Table 1. Test results of single particles of high-nickel cathodes with different SOCs.

[0147]

[0148]

[0149] The results show that as the state of charge increases, the fracture stress of the particles first decreases and then increases. The SOC is lower at 60% charge, which is due to the performance degradation caused by anisotropic expansion stress. After the SOC exceeds 90%, the strength decreases rapidly until it is below the minimum measurement range.

[0150] Nanoindentation test results are used to assess the degree of grain boundary stress concentration, such as Figure 1 As shown:

[0151] (1) SOC 0%: Young's modulus is 147.4 GPa;

[0152] (2) SOC 30%: Young's modulus is 146.0 GPa;

[0153] (3) SOC 60%: Young's modulus is 91.7 GPa.

[0154] (4) SOC 90%: Young's modulus is 106.1 GPa.

[0155] Based on the above mechanical performance indicators, the following evaluation conclusions can be drawn:

[0156] (1) As the state of charge increases, the mechanical properties of NCM811 cathode material generally decrease, manifested as a decrease in Young's modulus, hardness and fracture toughness.

[0157] (2) During the charging process, the deintercalation of lithium ions causes lattice expansion and generates internal stress, making the material more prone to mechanical failure. Especially in the grain boundary region, the anisotropic expansion causes grain boundary stress, making it easier for grain boundary tip compression strain and slip to occur.

[0158] (3) Based on the changing trends of these mechanical performance indicators, it can be predicted that in the actual battery cycle process, the cathode material in the half-charged state is more prone to stress concentration and mechanical damage, especially grain boundary cracking and particle breakage. This will lead to an increase in the contact area between the active material and the electrolyte, accelerate side reactions, and ultimately affect the battery's calendar life.

[0159] These results provide important insights into the mechanical behavior of lithium-ion batteries under different charging states, and can be used to optimize battery charging and discharging strategies and extend battery life.

[0160] Example 2

[0161] A method for evaluating the impact of different cycle decay states on the mechanical stability of high-nickel NCM cathodes in lithium-ion batteries.

[0162] This embodiment aims to apply the evaluation method of the present invention to quantitatively compare and evaluate the degree of damage caused to the mechanical integrity of high-nickel layered oxide NCM polycrystalline cathode particles by different cycle decay states, including the following steps:

[0163] Step 1: Obtain the positive electrode from a lithium-ion battery with a specific cycle history.

[0164] In this embodiment, three stacked lithium-ion batteries produced in the same batch and using high-nickel NCM as the positive electrode material were selected and labeled as Sample A, Sample B, and Sample C, respectively. The three batteries underwent the following different treatments:

[0165] Sample A: The battery underwent only a standard formation process and completed four charge-discharge cycles at a low rate. No further cycles were performed after that. This sample served as a benchmark for mechanical strength.

[0166] Sample B: The battery was charged to 4.25V at a constant current rate of 1C at 45℃, and then discharged to 2.8V at a constant current rate of 1C, and continuously cycled for 240 cycles. Sample B was then subjected to standard cycles.

[0167] Sample C: The battery was charged at a constant current rate of 1C to 4.25V at 45℃, and after reaching full charge, it was left to stand for 6 hours, and then discharged at a constant current rate of 1C to 2.8V. This "charge-stand-discharge" process was repeated 240 times, with sample C undergoing intermittent cycling.

[0168] After processing, all three batteries were fully discharged to 2.8V to ensure consistent testing conditions. Subsequently, the batteries were disassembled in an inert atmosphere glove box filled with argon, and their respective positive electrode plates were extracted.

[0169] Step 2: Pre-treat the positive electrode to obtain a sample suitable for micromechanical testing;

[0170] In this embodiment, the same pretreatment operation is performed on the three positive electrode sheets from samples A, B, and C:

[0171] Approximately 50 mg of positive electrode active material powder was scraped from each electrode sheet.

[0172] The powder was placed in centrifuge tubes containing 5 ml of N-methylpyrrolidone (NMP) solvent to dissolve the polyvinylidene fluoride (PVDF) binder.

[0173] The suspension was ultrasonically dispersed using a probe-type ultrasonic processor with a power setting of 400W and a processing time of 10 minutes to ensure that the secondary particle agglomerates were effectively broken up.

[0174] After ultrasonic treatment, the suspension is washed with ethanol to remove dissolved binder and conductive agent residues.

[0175] The cleaned granular powder was dried in a vacuum oven at 80°C for 4 hours.

[0176] Take a small amount of dried powder, disperse it in anhydrous ethanol, drop it onto a clean silicon wafer substrate with a pipette, and allow it to evaporate and dry naturally to obtain a dispersed single positive electrode particle sample for testing.

[0177] Step 3: Use single-particle strength testing to test the dispersed individual positive electrode particle samples to obtain mechanical performance indicators;

[0178] In this embodiment, a nanomechanical testing instrument equipped with a flat-head indenter with a diameter of 50 μm is used to perform single-particle strength testing on dispersed individual positive electrode particles from samples A, B, and C.

[0179] Using the optical microscope built into the tester, we searched for and located particles on the silicon wafer substrate that were regularly shaped, approximately spherical, and had a particle size in the range of 10±2μm.

[0180] The pressure head is driven at a constant loading rate of 0.5 mN / s to compress the selected particles until the particles break significantly. A high-precision sensor records the force-displacement curve in real time during this process.

[0181] For each sample, repeat the above steps and test at least 50 valid particles to ensure that the results are statistically representative.

[0182] The fracture force of a particle is determined from the highest point of each force-displacement curve, and its fracture stress is calculated based on the contact area of ​​the particle. This index directly reflects the mechanical strength of the particle.

[0183] Step 4: Based on the mechanical performance indicators, evaluate the degree of influence of different cycle decay states on the mechanical failure of polycrystalline cathode materials.

[0184] Test results:

[0185] After statistically averaging the test data of more than 50 particles, the average fracture stress of NCM811 cathode particles under three conditions was obtained as follows: Figure 2 As shown:

[0186] Sample A (formation only): 57.1 MPa

[0187] Sample B (standard cycle): 54.0 MPa

[0188] Sample C (intermittent cycling): 48.6 MPa

[0189] Results Analysis and Evaluation:

[0190] Benchmark strength established: The fracture stress of sample A is 57.1 MPa. This value represents the initial mechanical strength of this batch of NCM811 material under almost no cyclic stress and serves as the benchmark for subsequent evaluation.

[0191] Quantification of standard cycle damage: The strength of sample B (standard cycle) decreased to 54.0 MPa, a decrease of 3.1 MPa compared to the baseline, representing a reduction of approximately 5.4%. This result, through precise mechanical data, quantitatively reveals the degree of mechanical damage accumulated by lattice anisotropic strain caused by repeated lithium-ion insertion / extraction during conventional charge-discharge cycles.

[0192] Accelerated damage effect of intermittent cycling mode: The strength of sample C (intermittent cycling) decreased significantly to 48.6 MPa, a decrease of 8.5 MPa compared to the baseline, a reduction of up to 14.9%. Compared to the standard cycling mode, its strength decreased by an additional 10%. This result strongly demonstrates that prolonged static storage under full charge and high voltage conditions will greatly accelerate the mechanical failure process of the cathode material.

[0193] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for evaluating the mechanical failure of a polycrystalline positive electrode sheet in a lithium-ion battery, characterized in that, include: Step 1: Identify lithium-ion batteries in different electrochemical states; Extracting positive electrode sheets with polycrystalline structures from various lithium-ion batteries under different electrochemical states; Multiple positive electrode sheets with polycrystalline structures were extracted and dried for preservation. Step 2: Pre-process each positive electrode sheet with a polycrystalline structure to obtain multiple pre-processed positive electrode sheets; Step 3: Perform micromechanical tests on each pretreated positive electrode to obtain the mechanical performance indicators of each pretreated positive electrode. Step 4: Based on the mechanical performance indicators of each pretreated positive electrode sheet, evaluate the degree of mechanical failure of the positive electrode sheet with polycrystalline structure under different electrochemical states. The electrochemical states include: charge state, calendar aging state, and / or cycle decay state; The different cyclic decay states include zero cycle, standard cycle, and intermittent cycle; The charging states include fully discharged state, low charge state, half charge state, and high charge state. The positive electrode sheet with a polycrystalline structure includes a positive electrode material with a polycrystalline structure and the chemical formula Li. x M y O2, M is a transition metal ion, including nickel, cobalt, manganese, aluminum and other doping elements, x+y=1; The microstructure of the positive electrode material with a polycrystalline structure is a layered structure.

2. The method for evaluating the mechanical failure of polycrystalline positive electrode sheets in lithium-ion batteries according to claim 1, characterized in that, The pretreatment method is ultrasonic dispersion of positive electrode powder in solvent, argon ion surface polishing and / or mechanical polishing with abrasive nano suspension; The positive electrode powder is ultrasonically dispersed in solvent for single-particle strength testing; the abrasive nano suspension is mechanically polished for nanoindentation testing; and argon ion surface polishing is used for nanoindentation testing.

3. The method for evaluating the mechanical failure of polycrystalline positive electrode sheets in lithium-ion batteries according to claim 2, characterized in that, The specific steps for ultrasonic dispersion of the positive electrode powder solvent include: Active material powder was scraped from each positive electrode sheet with a polycrystalline structure. Each active material powder was placed in a centrifuge tube containing an organic solvent to obtain multiple suspensions; Each suspension was ultrasonically dispersed using an ultrasonic cleaner. After ultrasonic dispersion, use a pipette to take an appropriate amount of each suspension and drop it onto a clean glass slide. Allow it to dry naturally to obtain multiple sets of dispersed cathode particle samples. Ensure that there is sufficient spacing between the particles in each set of dispersed cathode particle samples to facilitate subsequent single-particle testing.

4. The method for evaluating the mechanical failure of polycrystalline positive electrode sheets in lithium-ion batteries according to claim 2, characterized in that, The specific steps of mechanical polishing with abrasive nano suspensions include: Multiple samples were obtained by fixing each positive electrode sheet with a polycrystalline structure onto the surface of a metal disc using crystal adhesive. Each sample surface was polished using an ion beam polishing system; The surface of each polished sample was observed using a scanning electron microscope to confirm the surface smoothness and clear visibility of primary particles.

5. The method for evaluating the mechanical failure of polycrystalline positive electrode sheets in lithium-ion batteries according to claim 1, characterized in that, The micromechanical tests are single-particle strength tests and / or nanoindentation tests.

6. The method for evaluating the mechanical failure of polycrystalline positive electrode sheets in lithium-ion batteries according to claim 1, characterized in that, The mechanical properties are fracture toughness, fracture stress, and Young's modulus.

7. The method for evaluating the mechanical failure of polycrystalline positive electrode sheets in lithium-ion batteries according to claim 5, characterized in that, The specific steps for performing single-particle strength testing on each pretreated positive electrode sheet include: The positive electrode particles with approximately spherical shapes were observed and selected from the dispersed positive electrode particle sample using an optical microscope. They were then compressed using a nanomechanical testing instrument equipped with a flat plate indenter until the approximately spherical positive electrode particles broke. Obtain the force-displacement curve throughout the compression process, and determine the fracture stress of the approximately spherical positive electrode particles based on the force-displacement curve. The fracture process of the near-spherical cathode particles was recorded using a high-speed camera, and the fracture toughness was analyzed. For each pretreated positive electrode sheet, at least 50 particles were tested to obtain the fracture stress and fracture toughness of the particles in each pretreated positive electrode sheet.

8. The method for evaluating the mechanical failure of polycrystalline positive electrode sheets in lithium-ion batteries according to claim 5, characterized in that, The specific steps for performing nanoindentation testing on the pretreated positive electrode sheet include: Multiple pretreated positive electrode sheets were placed on the sample stage of the nanoindenter, ensuring that the sample surface was perpendicular to the glass indenter; A load is applied at a set loading rate, causing the glass indenter to gradually press into the surface of the pretreated positive electrode sheet; Once the predetermined maximum load is reached, maintain the load. The load is unloaded at a set unloading rate, and the load-displacement data are recorded throughout the process. Repeat the above process to perform indentation tests at least 50 points on each pretreated positive electrode sheet and establish load-displacement curves; Calculate the Young's modulus of each positive electrode based on the load-displacement curve.

9. The method for evaluating the mechanical failure of polycrystalline positive electrode sheets in lithium-ion batteries according to claim 8, characterized in that, The indentation test also includes: selecting 5-20% of the particle area in the positive electrode sheet to evaluate the intrinsic properties of the grains; The degree of grain boundary stress concentration is assessed by selecting 20%-100% of the particle area in the positive electrode sheet; The integrity of grain boundaries was assessed by selecting 100% of the particle area in the positive electrode.

10. The method for evaluating the mechanical failure of polycrystalline positive electrode sheets in lithium-ion batteries according to claim 7, characterized in that, The expression for the fracture toughness is: K IC =(P max ×f(a / h)) / (h×√h) Where: K IC For fracture toughness, P max Where is the maximum load, h is the thickness of the positive electrode coating, a is the crack length of the positive electrode sheet, f(a / h) is the geometric correction factor, and f(.) is the correction factor function.

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