Evaluation method for fatigue fracture failure of lithium negative electrode and application of evaluation method

Abstract

The invention discloses a lithium negative electrode fatigue fracture failure evaluation method which comprises the following steps: firstly, preparing a lithium negative electrode into a standard sample piece, testing to obtain a stress-strain curve, and obtaining the yield strength of the lithium negative electrode according to the curve; and testing and calculating to obtain the fracture toughness of the lithium negative electrode. And finally, monitoring the crack length on the surface of the lithium negative electrode, and calculating to obtain the crack instability extension critical length ac of the lithium negative electrode. Whether cracks appear on the surface of the lithium negative electrode or not is judged according to the relation between the stress borne by the lithium negative electrode in the electrochemical cycle process and the yield strength of the lithium negative electrode; and judging whether the crack on the surface of the lithium negative electrode expands or not according to the relationship between the crack length on the surface of the lithium negative electrode in the electrochemical cycle process and the crack instability expansion critical length. Based on the evaluation method, the invention further provides a plurality of methods for improving the safety of the lithium negative electrode, and the cycling stability of the lithium negative electrode is remarkably improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of secondary batteries, in particular to a lithium negative electrode fatigue fracture failure evaluation method and application thereof. BACKGROUND

[0002] Energy is the cornerstone of human survival and development. Large-scale use of fossil energy has led to increasingly serious problems of energy depletion and environmental pollution. Developing new, efficient and clean energy conversion, storage technology and energy utilization method has become the key to solving these problems and realizing the sustainable development of human society. Chemical power supply is an important technical approach to alleviating energy, resource and environmental crisis problems and plays an important role in the social energy system. Among numerous chemical power supplies, lithium ion batteries, which are characterized by high energy and efficiency and green environmental protection, are increasingly attracting people's attention. New energy storage, electric vehicles, smart grids and other applications have put forward higher requirements for the energy density, cycle life, power density, safety, cost and environmental friendliness of lithium ion batteries.

[0003] Lithium metal has extremely high mass specific capacity (3860 mAh / g) and volume specific capacity (2061 mAh / cm 3 ), and is one of the materials with the highest theoretical specific capacity. At the same time, the standard electrode potential of Li + / Li is -3.04 V, which is the lowest among all redox couples. Therefore, metal lithium is a battery negative electrode material with great application prospect. However, the problem of metal lithium as the negative electrode material of lithium secondary batteries, lithium-sulfur batteries and lithium-air batteries is also very obvious, which is that the formation of lithium dendrites during lithium deposition / dissolution will pierce the separator, leading to serious safety problems.

[0004] Many studies have been devoted to exploring how to inhibit the generation of lithium dendrites, but previous studies have mainly focused on the mechanical properties of electrolytes or explained the lithium metal negative electrode dendrite growth from the electrochemical point of view, and have not proposed to analyze from the perspective of lithium negative electrode mechanical properties and fatigue fracture behavior. More importantly, since lithium metal is soft and difficult to process, and is sensitive to air, there is currently no method to accurately and reliably test the fracture toughness of lithium metal negative electrode, and there is no characterization of its fatigue fracture behavior. SUMMARY

[0005] In view of the deficiencies of the prior art, the present application discloses a lithium negative electrode fatigue fracture failure evaluation method, thereby accurately evaluating the fatigue fracture behavior of the lithium electrode during the cycle process, and based on the evaluation method, various methods for improving the safety of the lithium negative electrode are proposed, which significantly improves the cycle stability of the lithium negative electrode.

[0006] The specific technical solutions are as follows:

[0007] An evaluation method of lithium negative electrode fatigue fracture failure, comprising:

[0008] Step 1, the lithium negative electrode is made into a standard test piece, a stress-strain curve is obtained by using a scanning electron microscope in-situ tensile test device, and the yield strength of the lithium negative electrode is obtained according to the stress-strain curve;

[0009] Step 2, a force displacement curve is obtained by using a scanning electron microscope in-situ tensile test device, the strain energy of the lithium negative electrode is obtained, and the fracture toughness K of the lithium negative electrode is obtained according to the following formula (1) IC ;

[0010] (1)

[0011] In formula (1), J represents a line integral of the mechanical properties of the crack tip described by the stress and displacement field, which can be calculated according to the following formula (2); E is the Young's modulus of the lithium negative electrode, and v is the Poisson's ratio of the lithium negative electrode;

[0012] (2)

[0013] In formula (2), B is the thickness of the lithium negative electrode, W is half the width of the lithium negative electrode, a is the maximum length of the crack in the lithium negative electrode, U is the strain energy of the lithium negative electrode, and N is a constant related to the value of a / W, and the value is 2.32;

[0014] Step 3, the crack length of the surface of the lithium negative electrode is monitored by using a scanning electron microscope in-situ tensile test device, and the critical length a of crack unstable propagation of the lithium negative electrode is obtained according to the following formula (3) c ;

[0015] (3)

[0016] In formula (3), K IC is the fracture toughness of the lithium negative electrode, and E is the Young's modulus of the lithium negative electrode.

[0017] In the present application, another calculation method is also proposed for obtaining the fracture toughness K IC of step 2, that is, using an inert atmosphere hardness tester, testing the lithium negative electrode by using a diamond indenter, and obtaining the fracture toughness K IC of the lithium negative electrode based on the Lawn-Evans-Marshall formula:

[0018] (4)

[0019] In formula (4), δ represents an empirical parameter related to the shape of the indenter, generally taking a value of 0.016±0.004; E represents the Young's modulus of the lithium negative electrode, H represents the Vickers hardness of the lithium negative electrode, and F mrepresents the indentation load, and c represents the maximum length of the radial crack in the lithium anode.

[0020] The calculated K values ​​for both schemes were obtained. IC The values ​​are on the same order of magnitude and are close in magnitude, which verifies the accuracy of the two calculation schemes.

[0021] During the experiment, the inventors discovered that during electrochemical cycling, lithium metal undergoes alternating electroplating / stripping on its surface, causing volume expansion and contraction, and introducing tensile and compressive stresses. This periodic alternating stress loading on the lithium anode surface leads to fatigue, similar to the fatigue phenomenon that occurs in structural materials subjected to purely mechanical alternating loads, further resulting in crack formation within the material. The alternating tensile and compressive stresses experienced by the lithium anode during electrochemical cycling can also lead to fatigue and crack formation. After crack formation, due to electron accumulation at the crack site, the potential at the crack will preferentially reach the lithium anode electrodeposition potential compared to smooth areas. Therefore, preferential deposition of lithium occurs at the crack site, ultimately leading to dendrite growth. Based on the above research results, this invention discloses for the first time a method for calculating the fracture toughness of lithium anodes and provides an evaluation method for fatigue fracture failure of lithium anodes.

[0022] The lithium anode used in this invention is selected from lithium metal anodes or lithium alloy anodes;

[0023] The lithium alloy is selected from one or more of the following: magnesium, aluminum, silicon, calcium, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium, germanium, arsenic, selenium, strontium, yttrium, zirconium, niobium, molybdenum, rhodium, palladium, silver, cadmium, indium, tin, antimony, tellurium, barium, iridium, platinum, gold, thallium, lead, and bismuth, combined with lithium.

[0024] The standard sample has a length of 30-60 mm, a width of 1-10 mm, and a thickness of 0.5-5.0 mm.

[0025] In step 1:

[0026] In the stress-strain curve, the stress corresponding to a strain of 0.2% is the yield strength of the lithium anode.

[0027] Tensile fatigue simulation tests and in-situ stress tests during actual electrochemical cycling processes have demonstrated that:

[0028] When the stress experienced by the lithium anode during electrochemical cycling is less than the yield strength of the lithium anode, no cracks will appear on the surface of the lithium anode.

[0029] When the stress experienced by the lithium anode during electrochemical cycling is greater than or equal to the yield strength of the lithium anode, cracks appear on the surface of the lithium anode.

[0030] In step 3:

[0031] Tensile fatigue simulation tests and in-situ stress tests during actual electrochemical cycling processes have demonstrated that:

[0032] When the crack length on the surface of the lithium anode is less than the critical length α for crack instability propagation during electrochemical cycling... c At that time, the cracks on the surface of the lithium anode do not propagate;

[0033] When the crack length on the surface of the lithium anode is greater than or equal to the critical length a for crack instability propagation during electrochemical cycling... c At that time, the cracks on the surface of the lithium anode rapidly expand, causing the lithium anode to break and fail.

[0034] Experiments have confirmed that this evaluation method is universally applicable, suitable not only for liquid batteries but also for solid-state batteries.

[0035] Based on the above evaluation methods, it is clear that to improve the fatigue fracture failure of lithium anodes, a series of fatigue-relieving technical modifications can be made to the lithium anode, including alleviating fatigue stress during electrochemical cycling, improving the intrinsic mechanical properties of the lithium anode, and improving the mechanical properties of the lithium anode-electrolyte interface. Therefore, this invention also discloses several methods for improving the safety of lithium anodes, including:

[0036] I. After assembling the lithium anode, cathode, electrolyte, and separator into a battery, electrochemical cycling is carried out using the charging, resting I, discharging, and resting II methods.

[0037] The time for resting I and resting II were independently selected from 2 to 120 minutes.

[0038] Preferably, the time for resting I and resting II is independently selected from 3 to 60 minutes; further, it is independently selected from 15 to 60 minutes, and more preferably 30 minutes.

[0039] This method improves the safety of lithium anodes by mitigating fatigue stress during electrochemical cycling through improved charging and discharging processes. Tests have shown that it can significantly improve the cycle stability of batteries.

[0040] 2. The lithium anode used is prepared from lithium metal powder and / or lithium alloy powder, or lithium metal sheet and / or lithium alloy sheet;

[0041] The particle size of lithium metal powder and lithium alloy powder is independently selected from 10 nm to 10 μm;

[0042] The grain size of the lithium metal sheet and the lithium alloy sheet are independently selected from 10 nm to 10 μm.

[0043] The method is to improve the fatigue fracture failure of the lithium negative electrode and improve the safety of the lithium negative electrode by fine grain strengthening, by refining the particle size of lithium powder (including lithium metal powder or lithium alloy powder) or the grain size of lithium sheet (including lithium metal sheet or lithium alloy sheet), to increase the yield strength and fracture toughness of the lithium negative electrode, and the test shows that the method can significantly improve the cycle stability of the battery.

[0044] Specifically, the lithium powder or lithium sheet is pressed on the current collector to form a lithium negative electrode.

[0045] III. Assembling the lithium negative electrode, the positive electrode, the electrolyte and the separator to obtain a battery, and the electrolyte used includes a lithium salt, an additive and a solvent;

[0046] The additive is selected from one or more of phthalocyanine compounds, fluorine-containing additives, nitrogen-containing additives, sulfur-containing additives, boron-containing additives, phosphorus-containing additives and silicon-containing additives, and the above additives can form an electrode-electrolyte interface (SEI) film rich in inorganic layers on the surface of the lithium negative electrode in situ;

[0047] Preferably:

[0048] The phthalocyanine compound is selected from one or more of phthalocyanine (Pc), copper phthalocyanine (CuPc), manganese phthalocyanine (MnPc), iron phthalocyanine (FePc), cobalt phthalocyanine (CoPc), nickel phthalocyanine (NiPc) and zinc phthalocyanine (ZnPc);

[0049] The fluorine-containing additive is selected from fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), lithium difluoro(oxalato)borate (LiDFOB) and the like.

[0050] The nitrogen-containing additive is selected from new silicon-nitrogen film-forming additives, lithium nitrate and the like.

[0051] The sulfur-containing additive is selected from Li2S6, Li2S8 and the like.

[0052] The boron-containing additive is selected from lithium bis(oxalato)borate (LiBOB) and the like.

[0053] The phosphorus-containing additive is selected from tris(trimethylsilyl) phosphite (TMSPi) and the like.

[0054] The silicon-containing additive is selected from trimethylsilyl triflate (TMSOTf), trimethylchlorosilane (TMSCl) and the like.

[0055] The lithium salt is not particular and is selected from conventional types in the art, such as lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium fluorohydroxysulfonate (LiC(SO2CF3)3) and the like.

[0056] The solvent is not special, selected from the conventional category in the art, such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), methyl ethyl carbonate (EMC) and the like.

[0057] Preferably, the additive is added in an amount of 0.1-10wt% based on the total mass of the electrolyte.

[0058] The method is to form an inorganic-rich SEI film by adding a special additive to the electrolyte to improve the mechanical properties of the lithium negative electrode and the electrolyte interface, thereby improving the lithium negative electrode fatigue fracture failure and improving the safety of the lithium negative electrode. The test shows that the method can significantly improve the cycle stability of the battery.

[0059] Four, a polymer layer and / or an inorganic layer is coated on the surface of the lithium negative electrode;

[0060] The polymer layer or the inorganic layer has high yield strength and / or high fracture toughness.

[0061] The preparation of the coating layer is obtained by conventional technical means in the art, such as coating, impregnation, chemical synthesis or physical spherulite, etc.

[0062] The polymer layer is selected from one or more of a polymethyl methacrylate layer, an aramid layer, and a carbon fiber reinforced polymer layer.

[0063] The carbon fiber reinforced polymer layer includes a carbon fiber reinforced epoxy resin layer, a carbon fiber reinforced polyester layer, a carbon fiber reinforced nylon layer, a carbon fiber reinforced polyethylene layer, etc.

[0064] For example, the polymer coating layer is prepared by impregnation, the lithium negative electrode is immersed in a polymer solution, and after drying, a polymer layer is coated on the surface of the lithium negative electrode.

[0065] According to the type of the polymer layer to be coated, a solvent in which the polymer layer can be dissolved is selected and mixed to prepare a polymer solution. The concentration of the polymer solution is not particularly limited. For example, the polymer layer to be coated is a PMMA layer, and THF is selected as the solvent.

[0066] The inorganic material in the inorganic layer is selected from one or more of a fluoride, an oxide, a sulfide, a nitride, a phosphate, and a silicate. The fluoride is selected from LiF, AlF3, MgF2, CaF2, LaF3, etc.

[0067] The oxide is selected from Al2O3, SiO2, TiO2, ZrO2, Li2O, etc.

[0068] The sulfide is selected from Li2S, Li2SO3, Li3PS4, Li7P3S 11 , etc.

[0069] The nitrides are selected from Li3N, BN, AlN, Si3N4, etc.

[0070] The phosphates are selected from Li3PO4, etc.

[0071] The silicates such as Li2SiO3 and Li4SiO4.

[0072] This method significantly enhances the yield strength and fracture toughness of the lithium anode by pre-depositing an artificial interface film on the surface of the lithium anode, thereby improving the fatigue fracture failure of the lithium anode and enhancing its safety. Tests have shown that this method can significantly improve the cycle stability of the battery.

[0073] Compared with the prior art, the present invention has the following beneficial effects:

[0074] This invention proposes for the first time an evaluation method for the mechanical properties, specifically fatigue fracture behavior, of lithium anodes, and applies it to the fatigue fracture and dendrite growth behavior of lithium anodes during electrochemical cycling. Based on this evaluation method, a series of technical modifications can be made to lithium anodes to alleviate fatigue, including improving the intrinsic mechanical properties of lithium metal anodes, improving the mechanical properties of the lithium metal anode-electrolyte interface, and alleviating fatigue stress during electrochemical cycling. These methods can suppress fatigue decay and dendrite growth in lithium anodes, significantly improve their cycling stability, and promote their industrial application. Attached Figure Description

[0075] Figure 1 A scanning electron microscope image of the lithium metal anode standard sample prepared in Example 1;

[0076] Figure 2 Stress-strain curves of the standard lithium metal anode sample prepared in Example 1;

[0077] Figure 3 Force curves from an atomic force microscope for the lithium metal anode standard sample prepared in Example 1;

[0078] Figure 4 Force-displacement curve of the standard lithium metal anode sample prepared in Example 1;

[0079] Figure 5 The fatigue load curves of the lithium metal anode standard specimen prepared in Example 1 after 100 fatigue loading cycles within a stress range of 0.28~0.30MPa are obtained.

[0080] Figure 6 In order to be in Figure 5 In-situ scanning electron microscope images of the lithium metal anode standard sample prepared in Example 1 during fatigue loading process;

[0081] Figure 7 The fatigue load curves of the lithium metal anode standard specimen prepared in Example 1 after 100 fatigue loading cycles within a stress range of 0.31~0.33MPa are obtained.

[0082] Figure 8 In order to be in Figure 7 In-situ scanning electron microscope images of the lithium metal anode standard sample prepared in Example 1 during fatigue loading process;

[0083] Figure 9 The fatigue load curves of the lithium metal anode standard specimen prepared in Example 1 after 100 fatigue loading cycles within a stress range of 0.35~0.37MPa are obtained.

[0084] Figure 10 In order to be in Figure 9 In-situ scanning electron microscope images of the lithium metal anode standard sample prepared in Example 1 during fatigue loading process;

[0085] Figure 11 The lithium metal anode standard sample prepared in Example 1 was tested in a liquid battery at 1 mA / cm. -2 In-situ stress monitoring curves during cycling at current density;

[0086] Figure 12 The lithium metal anode standard sample prepared in Example 1 was tested in a liquid battery at 5 mA / cm. -2 In-situ stress monitoring curves during cycling at current density;

[0087] Figure 13 The lithium metal anode standard sample prepared in Example 1 was tested in a solid-state battery at 0.1 mA / cm. -2 In-situ stress monitoring curves during cycling at current density;

[0088] Figure 14 The lithium metal anode standard sample prepared in Example 1 was tested in a solid-state battery at 1 mA / cm. -2 In-situ stress monitoring curves during cycling at current density;

[0089] Figure 15 The lithium metal anode standard sample prepared in Example 1 was tested in a liquid battery at 1 mA / cm. 2 Quasi-in-situ scanning electron microscope images after 150 cycles;

[0090] Figure 16 The lithium metal anode standard sample prepared in Example 1 was tested in a liquid battery at 1 mA / cm. 2 Electrochemical cyclic voltage curves under these conditions;

[0091] Figure 17 The lithium metal anode standard sample prepared in Example 1 was tested in a liquid battery at 5 mA / cm. 2 Quasi-in-situ scanning electron microscope images after 150 cycles;

[0092] Figure 18 The lithium metal anode standard sample prepared in Example 1 was tested in a liquid battery at 5 mA / cm. 2 Electrochemical cyclic voltage curves under these conditions;

[0093] Figure 19 The lithium metal anode standard sample prepared in Example 1 showed an A / cm ratio of 0.1 mA in a solid-state battery. 2 Quasi-in-situ scanning electron microscope images after 80 cycles;

[0094] Figure 20 The lithium metal anode standard sample prepared in Example 1 showed an A / cm ratio of 0.1 mA in a solid-state battery. 2 Electrochemical cyclic voltage curves under these conditions;

[0095] Figure 21 The lithium metal anode standard sample prepared in Example 1 was tested in a solid-state battery at 1 mA / cm. 2 Quasi-in-situ scanning electron microscope images after 80 cycles;

[0096] Figure 22 The lithium metal anode standard sample prepared in Example 1 was tested in a solid-state battery at 1 mA / cm. 2 Electrochemical cyclic voltage curves under these conditions;

[0097] Figure 23 This is a scanning electron microscope image of the indentation of the lithium metal anode in Example 2. Detailed Implementation

[0098] To make the technical problems, solutions, and advantages of this invention easier to understand, a detailed description will be provided below with reference to the accompanying drawings and specific embodiments. However, it should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention. Unless otherwise specified, the raw materials, reagents, and equipment involved herein can be purchased or obtained by known methods.

[0099] Example 1

[0100] Step 1: In an argon-atmospheric glove box, the lithium strip was cut into standard sample sizes: 46 mm in length, 2.5 mm in width, and 1.8 mm in thickness. The lithium metal anode standard sample was placed on an in-situ tensile testing stage using a scanning electron microscope. Subsequently, the lithium metal anode standard sample was subjected to in-situ tensile testing at a tensile speed of 5 μm / s to obtain stress-strain and force-displacement curves. The force curve was then obtained using atomic force microscopy.

[0101] Figure 1 This is a scanning electron microscope image of the lithium metal anode standard sample prepared in step one of this embodiment. Its dimensions are consistent with the design dimensions.

[0102] Figure 2 The stress-strain curve obtained in step one of this embodiment is shown. According to the definition of yield strength corresponding to 0.2% strain, the yield strength of the lithium metal anode is 0.31 MPa.

[0103] Figure 3 The image shows the atomic force microscopy force curve obtained in step one of this embodiment. Based on the fitting results, the Young's modulus E of the lithium metal anode is 2.00 GPa.

[0104] Figure 4 The force-displacement curve obtained in step one of this embodiment is used. Calculations show that the strain energy before fracture is 1.4656 mJ, the crack length a is 0.625 mm, a / W is 0.5, N is 2.32, and J is calculated to be 3022 Pa·m according to formula (2); Poisson's ratio ν is 0.38, and the fracture toughness K of the lithium metal anode is calculated according to formula (1). IC 2.66 MPa 1 / 2 This is the first report internationally on the fracture toughness of lithium metal anodes.

[0105] Step 2: The crack length on the surface of the lithium metal anode is monitored using an in-situ tensile testing device with a scanning electron microscope, and the critical length for crack instability propagation of the lithium metal anode is 10.61 μm according to formula (3).

[0106] To further test the accuracy of the above evaluation methods, a tensile fatigue test was conducted in sequence, followed by in-situ stress testing using a lithium metal anode during electrochemical cycling. Details are as follows:

[0107] 1. In an argon-atmospheric glove box, the lithium metal anode standard specimen prepared in step one of this embodiment is placed on an in-situ tensile testing stage using a scanning electron microscope. Tensile-tensile fatigue tests are performed on the standard specimen, selecting three different load stress amplitudes: 0.28~0.30MPa, 0.31~0.33MPa, and 0.35~0.37MPa. The morphological changes of the standard specimen during 100 cycles of fatigue loading under different stress amplitudes are observed in situ.

[0108] Figure 5 The fatigue load curve is obtained after the lithium metal anode standard sample prepared in step one of this embodiment has undergone 100 fatigue loading cycles within a stress range of 0.28~0.30MPa. Figure 6 In order to be in Figure 5 During the fatigue loading process, in-situ scanning electron microscope images of the standard lithium metal anode specimens showed that, under a stress range of 0.28~0.30 MPa, the lithium metal surface remained smooth and flat as fatigue loading progressed.

[0109] Figure 7 The fatigue load curve is obtained after the lithium metal anode standard sample prepared in step one of this embodiment has undergone 100 fatigue loading cycles within a stress range of 0.31~0.33MPa. Figure 8 In order to be in Figure 7 During the fatigue loading process, in-situ scanning electron microscope images of the standard lithium metal anode specimens revealed that, under a stress range of 0.31~0.33MPa, cracks gradually appeared on the lithium metal surface as fatigue loading progressed. When the crack length exceeded 10.61μm, the cracks propagated.

[0110] Figure 9 The fatigue load curve is obtained after the lithium metal anode standard sample prepared in step one of this embodiment has undergone 100 fatigue loading cycles within a stress range of 0.35~0.37MPa. Figure 10 In order to be in Figure 9 During the fatigue loading process, in-situ scanning electron microscope images of the standard lithium metal anode specimens revealed that, under a stress range of 0.35~0.37 MPa, cracks rapidly initiated on the lithium metal surface as fatigue loading progressed, and the cracks propagated when the crack length exceeded 10.61 μm.

[0111] 2. In a glove box under an argon atmosphere, at 2 mol L... –1A symmetrical battery was assembled using LiPF6 / EC+DEC+DMC (volume ratio 1:1:1) as the electrolyte and Celgard-2400 as the separator, in the order of lithium sheet, separator, liquid electrolyte, and lithium sheet, for electrochemical testing of liquid batteries. A symmetrical battery was also assembled using LiBH4 as the solid electrolyte, in the order of lithium sheet, solid electrolyte, and lithium sheet, for electrochemical testing of solid batteries. A pressure sensor was installed at the bottom of the battery to record the stress changes during charging and discharging in real time; this stress represents the fatigue stress experienced by lithium metal during electrochemical cycling.

[0112] Figure 11 The lithium metal anode standard sample prepared in step one of this embodiment was tested in a liquid battery at 1 mA / cm. -2 In-situ stress monitoring curves during cycling at current density; Figure 12 The lithium metal anode standard sample prepared in step one of this embodiment was subjected to a 5 mA cm⁻¹ test in a liquid battery. -2 In-situ stress monitoring curves during cycling at current density; Figure 13 The lithium metal anode standard sample prepared in step one of this embodiment was used in a solid-state battery at 0.1 mA / cm. -2 In-situ stress monitoring curves during cycling at current density; Figure 14 The lithium metal anode standard sample prepared in step one of this embodiment was used in a solid-state battery at 1 mA / cm. -2 In-situ stress monitoring curves during cyclic operation at current density; based on Figures 11-14 According to stress monitoring results, in liquid batteries, when the current density is greater than or equal to 5 mA / cm², -2 At this point, the stress experienced by the lithium sheet during cycling exceeds its yield strength; in solid-state batteries, when the current density is greater than or equal to 1 mA / cm², the stress exceeds its yield strength. -2 At that time, the stress experienced by the lithium sheet during cycling will exceed its yield strength.

[0113] Furthermore, the morphological changes of the lithium metal anode standard sample during cycling in different batteries and at different current densities were observed using a quasi-in-situ scanning electron microscope.

[0114] Figure 15 A standard lithium metal anode sample in a liquid battery with an A / cm ratio of 1 mA. 2 Quasi-in-situ scanning electron microscopy images after 150 cycles showed no crack formation on the lithium metal surface throughout the entire cycle. Further testing under these conditions determined the stable cycle life of the standard lithium metal anode sample to be 253 cycles. Figure 16 The electrochemical cyclic voltage curve is shown.

[0115] Figure 17 A standard lithium metal anode sample was tested in a liquid battery at 5 mA / cm.2 Quasi-in-situ scanning electron microscope images after 150 cycles show that, as the cycling progresses, fatigue cracks appear on the surface of the lithium metal after 50 cycles, but the crack length does not exceed the critical length for unstable propagation, and unstable propagation does not occur. After 100 cycles, the fatigue crack length exceeds the critical length for unstable propagation, and propagation occurs. After 150 cycles, dendrites grow from the cracks. Further testing showed that under these conditions, the stable cycle count of the standard lithium metal anode sample was 132 cycles. Figure 18 The electrochemical cyclic voltage curve is shown.

[0116] Figure 19 The standard sample of lithium metal anode in a solid-state battery is 0.1 mA / cm. 2 Quasi-in-situ scanning electron microscopy images after 80 cycles showed no crack formation on the lithium metal surface throughout the entire cycle. Further testing under these conditions determined the stable cycle life of the standard lithium metal anode sample to be 250 cycles. Figure 20 The electrochemical cyclic voltage curve is shown.

[0117] Figure 21 A standard lithium metal anode sample in a solid-state battery with an A / cm ratio of 1 mA. 2 Quasi-in-situ scanning electron microscope images after 80 cycles show that, as the cycling progresses, fatigue cracks appear on the surface of the lithium metal after 20 cycles, but the crack length does not exceed the critical length for unstable propagation, and unstable propagation does not occur. After 40 cycles, the fatigue crack exceeds the critical length for unstable propagation and propagates. After 80 cycles, dendrites grow from the crack. Further testing showed that the stable cycle count of the standard lithium metal anode sample under these conditions was 80 cycles. Figure 22 The electrochemical cyclic voltage curve is shown.

[0118] Example 2

[0119] The evaluation method is similar to that in Example 1, except that the fracture toughness K of the lithium metal anode is calculated. IC The hardness test method used is as follows:

[0120] The lithium metal anode was cut into circular pieces with a diameter of 14 mm and a thickness of 5 mm for hardness testing.

[0121] Figure 23 This is a scanning electron microscope image of the indentation of the lithium metal anode in this embodiment. According to the test results of the hardness tester, the Vickers hardness H = 0.93 MPa, and the indenter load F... m =9.8N, the maximum radial crack length is 211μm, and combined with formula (4), the fracture toughness of the lithium metal anode is obtained as 2.37MPa. 1 / 2 .

[0122] The fracture toughness values ​​obtained by the two methods are similar, indicating the accuracy of the fracture toughness evaluation method for lithium metal anodes in this invention.

[0123] Application Example 1

[0124] In a glove box under an argon atmosphere, at 2 mol L... –1 LiPF6 / EC+DEC+DMC (volume ratio 1:1:1) was used as the electrolyte, and Celgard–2400 was used as the separator. Symmetrical cells were assembled in the order of lithium sheet, separator, liquid electrolyte, and lithium sheet for electrochemical testing of liquid batteries. LiBH4 was used as the solid electrolyte, and symmetrical cells were assembled in the order of lithium sheet, solid electrolyte, and lithium sheet for electrochemical testing of solid batteries.

[0125] Different current densities (1 mA / cm²) are used when charging and discharging liquid batteries. 2 5mA / cm 2 Test:

[0126] 1mA / cm 2 After charging at the current density for 1 hour, the circuit is left to stand for a period of time, then discharged for 1 hour, and then left to stand for a period of time again to complete one cycle. This process is repeated. For simplicity, the time after charging and the time after discharging are the same. However, in actual applications, they can be the same or different.

[0127] 5mA / cm 2 After charging at the current density for 0.2 hours, the battery is left to stand for a period of time, then discharged for 0.2 hours, and then left to stand for a period of time again to complete one cycle. This process is repeated. For simplicity, the time after charging and the time after discharging are the same. However, in actual applications, they can be the same or different.

[0128] When charging and discharging solid-state batteries, different current densities (0.1 mA / cm²) are used. 2 1mA / cm 2 Test:

[0129] 0.1mA / cm 2 After charging at the current density for 1 hour, the circuit is left to stand for a period of time, then discharged for 1 hour, and then left to stand for a period of time again to complete one cycle. This process is repeated. For simplicity, the time after charging and the time after discharging are the same. However, in actual applications, they can be the same or different.

[0130] 1mA / cm 2 After charging at the current density for 0.1 hours, the battery is left to stand for a period of time, then discharged for 0.1 hours, and then left to stand for a period of time again to complete one cycle. This process is repeated. For simplicity, the time after charging and the time after discharging are the same. However, in actual applications, they can be the same or different.

[0131] Table 1 below lists the number of stable cycles of the batteries obtained for different resting times.

[0132] Table 1

[0133]

[0134] Application Example 2

[0135] In a glove box under an argon atmosphere, at 2 mol L... –1 Using LiPF6 / EC+DEC+DMC (volume ratio 1:1:1) as the electrolyte and Celgard–2400 as the separator, lithium powder was pressed onto nickel foam at 150 MPa for 1 min to form a lithium powder electrode. The median diameter of the lithium powder used was 50 nm. A symmetrical battery was assembled in the order of lithium powder electrode, separator, liquid electrolyte, and lithium powder electrode for electrochemical testing of liquid batteries. A symmetrical battery was also assembled in the order of LiBH4 as the solid electrolyte for electrochemical testing of solid batteries.

[0136] Tests showed that the liquid battery assembled in this application example operates at 1 mA / cm². 2 The stable cycle life at the current density is 800 cycles. The solid-state battery assembled in this application example operates at 0.1 mA / cm². 2 The stable cycle life at current density is 600 cycles, which is significantly better than liquid and solid batteries assembled from pure lithium sheets.

[0137] Application Example 3

[0138] In a glove box under an argon atmosphere, at 2 mol L... –1 In a LiPF6 / EC+DEC+DMC electrolyte (volume ratio 1:1:1), 2wt% manganese phthalocyanine was added. A symmetrical battery was assembled using Celgard–2400 as the separator, in the order of lithium sheet, separator, liquid electrolyte, and lithium sheet, for electrochemical testing of liquid batteries.

[0139] Tests showed that the liquid battery assembled in this application example operates at 1 mA / cm². 2 The stable cycle life at current density is 600 cycles, which is significantly better than that of liquid batteries assembled from lithium sheets without electrolyte additives.

[0140] Application Example 4

[0141] (1) The lithium sheet was completely immersed in a polymethyl methacrylate (PMMA) / tetrahydrofuran (THF) solution, left to stand for 2 hours, and then removed. After drying to remove the solvent, a PMMA film was uniformly coated on the surface of the lithium sheet as an artificial SEI film. According to the evaluation method in Example 1, the yield strength of the lithium metal anode coated with the PMMA film was tested to be 5580 MPa, which is significantly better than the 0.31 MPa of the pure lithium sheet, and the fracture toughness was 10 MPa. 1 / 2 It is significantly higher than the 2.66 MPa of pure lithium sheets. 1 / 2 .

[0142] (2) In a glove box under an argon atmosphere, at 2 mol L –1 A symmetrical battery was assembled using LiPF6 / EC+DEC+DMC (volume ratio 1:1:1) as the electrolyte and Celgard–2400 as the separator, in the following order: lithium sheet coated with PMMA membrane, separator and liquid electrolyte, and lithium sheet coated with PMMA membrane, for electrochemical testing of liquid batteries. A symmetrical battery was also assembled using LiBH4 as the solid electrolyte, in the following order: lithium sheet coated with PMMA membrane, solid electrolyte, and lithium sheet coated with PMMA membrane, for electrochemical testing of solid batteries.

[0143] Tests showed that the liquid battery assembled in this application example operates at 5 mA / cm². 2 The stable cycle life at current density is 600 cycles. The solid-state battery assembled in this application example achieves this at 1 mA / cm². 2 The stable cycle count at current density is 900 cycles, which is significantly better than liquid and solid batteries assembled from pure lithium sheets.

[0144] The above-described embodiments are preferred embodiments, but the scope of protection of the present invention is not limited thereto. Those skilled in the art can easily understand the spirit of the present invention based on the above embodiments and make different extensions and variations, but as long as they do not depart from the spirit of the present invention, they are all within the scope of protection of the present invention.

Claims

1. A method for evaluating fatigue fracture failure of a lithium negative electrode, characterized by, Comprising: Step 1, the lithium negative electrode is made into a standard test piece, and a stress-strain curve is obtained by using a scanning electron microscope in-situ tensile test device; according to the stress-strain curve, the yield strength of the lithium negative electrode is obtained; Step 2, the force displacement curve is obtained by using a scanning electron microscope in-situ tensile testing device, the strain energy of the lithium negative electrode is obtained, and the fracture toughness K of the lithium negative electrode is obtained according to the following formula (1) IC ; （1）； In formula (1), J represents the line integral of the stress and displacement field describing the mechanical properties of the crack tip, which can be calculated according to formula (2) below; E is the Young's modulus of the lithium negative electrode, and v is the Poisson's ratio of the lithium negative electrode; （2）； In formula (2), B is the thickness of the lithium negative electrode, W is half the width of the lithium negative electrode, a is the maximum length of the crack in the lithium negative electrode, U is the strain energy of the lithium negative electrode, and N is a constant related to the value of a / W, which is 2.32; Step 3, the crack length of the lithium negative electrode surface is monitored by using a scanning electron microscope in-situ tensile test device, and the crack unstable propagation critical length a of the lithium negative electrode is obtained according to formula (3) below c ; （3）； In formula (3), K IC is the fracture toughness of the lithium negative electrode, and E is the Young's modulus of the lithium negative electrode.

2. A method for evaluating fatigue fracture failure of a lithium negative electrode, characterized by, Comprising: Step 1, the lithium negative electrode is made into a standard test piece, and a stress-strain curve is obtained by using a scanning electron microscope in-situ tensile test device; according to the stress-strain curve, the yield strength of the lithium negative electrode is obtained; Step 2, testing the lithium negative electrode by a diamond indenter using an inert atmosphere hardness tester, obtaining the fracture toughness K of the lithium negative electrode based on the Lawn-Evans-Marshall formula IC : （4）； In formula (4), δ represents an empirical parameter related to the shape of the indenter, and generally takes a value of 0.016±0.004; E represents the Young's modulus of the lithium negative electrode; H represents the Vickers hardness of the lithium negative electrode; F m represents the indentation load, and c represents the maximum length of the radial crack in the lithium negative electrode. Step 3, the crack length of the lithium negative electrode surface is monitored by using a scanning electron microscope in-situ tensile test device, and the crack unstable propagation critical length a of the lithium negative electrode is obtained according to formula (3) below c ; （3）； In formula (3), K IC is the fracture toughness of the lithium negative electrode, and E is the Young's modulus of the lithium negative electrode.

3. The method for evaluating lithium negative electrode fatigue fracture failure according to claim 1 or 2, characterized by, In step 1: The lithium negative electrode is selected from lithium metal negative electrode or lithium alloy negative electrode; The lithium alloy is selected from one or more of magnesium, aluminum, silicon, calcium, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium, germanium, arsenic, selenium, strontium, yttrium, zirconium, niobium, molybdenum, rhodium, palladium, silver, cadmium, indium, tin, antimony, tellurium, barium, iridium, platinum, gold, thallium, lead, bismuth and lithium to form an alloy; The length of the standard test piece is 30-60mm, the width is 1-10mm, and the thickness is 0.5-5.0mm.

4. The method for evaluating lithium negative electrode fatigue fracture failure according to claim 1 or 2, characterized by, In step 1: In the stress-strain curve, the stress corresponding to a strain of 0.2% is the yield strength of the lithium negative electrode; When the stress received by the lithium negative electrode during electrochemical cycling is less than the yield strength of the lithium negative electrode, no cracks will appear on the surface of the lithium negative electrode; When the stress received by the lithium negative electrode during electrochemical cycling is greater than or equal to the yield strength of the lithium negative electrode, cracks appear on the surface of the lithium negative electrode.

5. The method for evaluating lithium negative electrode fatigue fracture failure according to claim 1 or 2, characterized by, In step 3: When the crack length on the surface of the lithium negative electrode during electrochemical cycling is less than the crack instability propagation critical length, the crack on the surface of the lithium negative electrode does not propagate; When the crack length on the surface of the lithium negative electrode during electrochemical cycling is greater than or equal to the crack instability propagation critical length, the crack on the surface of the lithium negative electrode rapidly propagates, and the lithium negative electrode is broken and fails.

6. A method for improving safety of a lithium negative electrode according to the evaluation method of any one of claims 1 to 5, characterized by, After the lithium negative electrode, the positive electrode, the electrolyte and the separator are assembled to obtain a battery, the method of charging, standing I, discharging and standing II is used for electrochemical cycling; The time of standing I and standing II is independently selected from 2-120min.

7. A method for improving safety of a lithium negative electrode according to the evaluation method of any one of claims 1 to 5, characterized by, The lithium negative electrode, the positive electrode, the electrolyte and the separator are assembled to obtain a battery, and the lithium negative electrode is prepared from lithium metal powder and / or lithium alloy powder, or lithium metal sheet and / or lithium alloy sheet; The particle size of the lithium metal powder and the lithium alloy powder is independently selected from 10nm-10μm; The grain size of the lithium metal sheet and the lithium alloy sheet is independently selected from 10nm-10μm.

8. A method for improving safety of a lithium negative electrode according to the evaluation method of any one of claims 1 to 5, characterized by, The lithium negative electrode, the positive electrode, the electrolyte and the separator are assembled to obtain a battery, and the electrolyte comprises a lithium salt, an additive and a solvent; The additive is selected from one or more of phthalocyanine compounds, fluorine-containing additives, nitrogen-containing additives, sulfur-containing additives, boron-containing additives, phosphorus-containing additives, silicon-containing additives.

9. A method for improving safety of a lithium negative electrode according to the evaluation method of any one of claims 1 to 5, characterized by, A polymer layer and / or an inorganic layer are coated on the surface of the lithium negative electrode; The polymer layer and the inorganic layer both have high yield strength and / or high fracture toughness.

10. The method of claim 9, wherein: the polymer layer is selected from one or more of a polymethyl methacrylate layer, an aramid layer, a carbon fiber reinforced polymer layer; the inorganic material in the inorganic layer is selected from one or more of a fluoride, an oxide, a sulfide, a nitride, a phosphate, a silicate.

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