A method for testing residual stress in LiCoO2 thin film electrodes

Abstract

The present application belongs to the technical field of electrode materials, and particularly relates to a testing method for residual stress of a LiCoO2 thin film electrode. The present application provides a testing method for residual stress of a LiCoO2 thin film electrode, which comprises the following steps: fixing the LiCoO2 thin film electrode on a testing platform of an X-ray diffractometer, testing the diffraction angle θ of the (003) characteristic peak of the LiCoO2 thin film electrode at a set inclination angle ψ, and calculating the interplanar spacing d of the crystal according to the Bragg equation φψ ; and calculating the residual stress σ according to formula 1 by using the interplanar spacing d φψ and the inclination angle ψ. The testing method provided by the present application can further improve the accuracy of the residual stress test value of the thin film electrode.

Description

Technical Field

[0001] This invention belongs to the field of electrode material technology, specifically relating to a method for testing the residual stress of a LiCoO2 thin film electrode. Background Technology

[0002] Lithium cobalt oxide (LiCoO2) exhibits chemical stability and excellent electrochemical performance in air: its theoretical discharge capacity can reach 69 μAh / (cm²). 2 It has a self-discharge rate of less than 1% / month (-μm), can retain 80% of its initial discharge capacity after 2000-4000 cycles, and has a charging time of less than 1 hour.

[0003] Meanwhile, LiCoO2 can also be prepared in thin film form and used as the positive electrode of thin-film lithium batteries. However, LiCoO2 films prepared by magnetron sputtering are usually under high stress, and film stress often leads to various types of damage to the film-substrate system, such as film bulging, cracking, and even direct pulverization and detachment. In particular, for thin-film electrode materials, residual stress causing the degradation of film mechanical properties is considered one of the most important causes of battery failure. Therefore, studying the stress state of films is of great significance for optimizing key film performance.

[0004] Using traditional sin 2 The ψ method is based on Bragg-Brentano B-B diffraction geometry and a specific diffraction crystal plane. It measures the diffraction peak positions corresponding to different tilt angles (the angle ψ between the normal of the diffraction crystal plane and the normal of the sample surface) and calculates the film stress by the diffraction peak shift. However, this method has the following shortcomings: (1) The film thickness (nanometer level) is very thin relative to the substrate material (micrometer level), resulting in low film diffraction intensity and severe interference of the diffraction signal by the substrate signal; (2) Most deposited films have preferred orientation (i.e., texture), and the non-uniformity of micro-strain cannot be ignored, leading to the traditional sin 2 The interplanar spacing d obtained by the ψ method with respect to sin 2 The curve of ψ is nonlinear, and the stress fitting calculation is inaccurate; (3) The depth of this method includes the entire film thickness, and the calculation result is the average stress of the film as a whole. It is difficult to obtain the stress state at the film-substrate interface, especially for films with nanometer thickness, where the interface stress cannot be ignored, which ultimately leads to the low accuracy of the measured film stress value. Summary of the Invention

[0005] The purpose of this invention is to provide a method for testing the residual stress of LiCoO2 thin film electrodes. The testing method provided by this invention can further improve the accuracy of the residual stress test values ​​of thin film electrodes.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] This invention provides a method for testing the residual stress of a LiCoO2 thin film electrode, comprising the following steps:

[0008] The LiCoO2 thin film electrode was fixed on the test platform of an X-ray diffractometer. At a set tilt angle ψ, the diffraction angle θ of the (003) characteristic peak of the LiCoO2 thin film electrode was measured using an X-ray diffractometer. The interplanar spacing was calculated according to the Bragg equation.

[0009] The interplanar spacing The residual stress σ is obtained by calculating the tilt angle ψ according to Formula 1;

[0010]

[0011] Where d0 is the interplanar spacing of the LiCoO2 thin film under stress-free conditions, and its value is [value missing].

[0012] v is the Poisson's ratio of LiCoO2, with a value of 0.24;

[0013] E is the elastic modulus of the LiCoO2 film, in MPa;

[0014] α is the specific angle at which the X-ray passes through the LiCoO2 thin film electrode. The relationship between α, the grazing incident angle γ, and the diffraction angle θ is: α = θ - γ;

[0015] The tilt angle ψ ranges from 0° to 90°.

[0016] Preferably, the test conditions include:

[0017] A point light source with a spot diameter of 0.1–5 mm, emitting CuKα1 radiation with a wavelength λ of 0.15405 nm;

[0018] The operating current is 20-40mA, the voltage is 20-40kV, and the front slit and rear slit are set to 0.5° and 5mm respectively; the grazing incidence angle is fixed, and the value range of the grazing incidence angle is 0°-5°.

[0019] Preferably, the LiCoO2 thin film electrode includes a freshly made LiCoO2 thin film electrode or a LiCoO2 thin film electrode that has undergone charge-discharge cycle testing.

[0020] Preferably, the method for preparing the novel LiCoO2 thin film electrode includes the following steps:

[0021] The newly fabricated LiCoO2 thin film electrode is obtained by magnetron sputtering a LiCoO2 thin film on a stainless steel substrate and then annealing it.

[0022] Preferably, the conditions for magnetron sputtering include: sputtering power of 0.5–8 W / cm². 2 The target distance is 4–20 cm, and the working pressure is 0.3–5 Pa.

[0023] Preferably, the annealing temperature is 300–1000°C, and the holding time is 10–300 min;

[0024] The annealing process involves heating from room temperature to 220°C at a rate of 1–10°C / min, and then heating to the annealing temperature at a rate of 10°C / min.

[0025] Preferably, the cutoff voltage for the charge-discharge cycle test is 3.0 to 4.2V.

[0026] This invention provides a method for testing the residual stress of a LiCoO2 thin film electrode, comprising the following steps: fixing the LiCoO2 thin film electrode on the test platform of an X-ray diffractometer; at a set tilt angle ψ, using an X-ray diffractometer to test the diffraction angle θ of the (003) characteristic peak of the LiCoO2 thin film electrode; and calculating the interplanar spacing according to the Bragg equation. The interplanar spacing The residual stress σ is obtained by calculating the tilt angle ψ according to Formula 1;

[0027]

[0028] Where d0 is the interplanar spacing of the LiCoO2 thin film under stress-free conditions, and its value is [value missing]. v is the Poisson's ratio of LiCoO2, with a value of 0.24; E is the elastic modulus of the LiCoO2 thin film, in MPa; α is the specific angle at which X-rays pass through the LiCoO2 thin film electrode, and the relationship between α, the grazing incident angle γ, and the diffraction angle θ is: α = θ - γ; the tilt angle ψ ranges from 0° to 90°. This invention proposes a modified sin... 2 The ψ method employs grazing incidence X-ray diffraction (GIXRD) mode. The GIXRD method has advantages such as shallow penetration depth, minimal change in penetration depth with tilt angle ψ, insensitivity to texture effects, and no separation between positive and negative ψ curves, which can further improve the accuracy of residual stress test values ​​for thin film electrodes. Attached Figure Description

[0029] Figure 1 A schematic diagram illustrating the principle of the method for testing residual stress in LiCoO2 thin film electrodes provided by this invention;

[0030] Figure 2The charge-discharge curves are those of the LiCoO2 thin film cathodes in Examples 2-11 during the 1st, 2nd, 5th, 20th, and 50th charge-discharge cycles.

[0031] Figure 3 The images shown are characterization images of the thin film electrodes after charge-discharge tests in Examples 1 to 11, where (a) is a physical image and (b) to (l) are SEM images.

[0032] Figure 4 This is a cross-sectional SEM image of the thin-film electrode after the charge-discharge test in Example 1;

[0033] Figure 5 The images are XRD patterns of the thin-film electrodes after charge-discharge tests in Examples 1 to 11, where (b) is a magnified view of a local region (2θ = 36° to 42°);

[0034] Figure 6 The load-displacement curve of the thin-film electrode after the charge-discharge test in Example 1 is shown.

[0035] Figure 7 The results are the stress test results of the thin film electrode after charge-discharge tests in Examples 1 to 11. Detailed Implementation

[0036] This invention provides a method for testing the residual stress of a LiCoO2 thin film electrode, comprising the following steps:

[0037] The LiCoO2 thin film electrode was fixed on the test platform of an X-ray diffractometer. At a set tilt angle ψ, the diffraction angle θ of the (003) characteristic peak of the LiCoO2 thin film electrode was measured using an X-ray diffractometer. The interplanar spacing was calculated according to the Bragg equation.

[0038] The interplanar spacing The residual stress σ is obtained by calculating the tilt angle ψ according to Formula 1;

[0039]

[0040] Where d0 is the interplanar spacing of the LiCoO2 thin film under stress-free conditions, and its value is [value missing].

[0041] v is the Poisson's ratio of LiCoO2, with a value of 0.24;

[0042] E is the elastic modulus of the LiCoO2 film, in MPa;

[0043] α is the specific angle at which the X-ray passes through the LiCoO2 thin film electrode. The relationship between α, the grazing incident angle γ, and the diffraction angle θ is: α = θ - γ;

[0044] The tilt angle ψ ranges from 0° to 90°.

[0045] In this invention, the tilt angle ψ ranges from 0° to 90°, and is more preferably 0°, 24°, 35°, 45° or 53°.

[0046] In this invention, the LiCoO2 thin film electrode preferably includes a freshly made LiCoO2 thin film electrode or a LiCoO2 thin film electrode that has undergone charge-discharge cycle testing.

[0047] In this invention, the method for preparing the novel LiCoO2 thin film electrode preferably includes the following steps:

[0048] The newly fabricated LiCoO2 thin film electrode is obtained by magnetron sputtering a LiCoO2 thin film on a stainless steel substrate and then annealing it.

[0049] In this invention, the stainless steel substrate is preferably 304 stainless steel (304SS); the thickness of the stainless steel substrate is preferably 0.03 mm, and the diameter is preferably 20 mm. Before magnetron sputtering, this invention preferably includes a purification treatment of the stainless steel substrate; the purification treatment process is preferably performed by wiping with ethanol. In this invention, the purification treatment can remove surface contaminants and release residual stress.

[0050] In this invention, the target material used for magnetron sputtering is preferably a LiCoO2 target material; the purity of the LiCoO2 target material is preferably 99.9%, and the diameter is preferably 70 mm.

[0051] In this invention, the preferred conditions for magnetron sputtering include: a sputtering power of 0.5–8 W / cm². 2 The target-to-substrate distance is 4–20 cm, and the working gas pressure is 0.3–5 Pa; further preferred features include: a sputtering power of 4 W / cm². 2 The target-substrate distance is 11 cm, and the working gas pressure is 0.75 Pa. In this invention, the magnetron sputtering is preferably carried out in an atmosphere of argon or a mixture of argon and oxygen, and the gas flow ratio of argon to oxygen in the mixture of argon and oxygen is preferably 18.5:11.5.

[0052] In this invention, the thickness of the LiCoO2 film is preferably 1 mm.

[0053] In this invention, the annealing temperature is preferably 300–1000°C, more preferably 550°C; the holding time is 10–300 min, more preferably 20 min; the heating program for the annealing is preferably: heating from room temperature to 220°C at a rate of 1–10°C / min, and then heating to the annealing temperature at a rate of 10°C / min, more preferably heating from room temperature to 220°C at a rate of 3°C / min, and then heating to the annealing temperature at a rate of 10°C / min. In this invention, the annealing is preferably carried out in an oxygen atmosphere or a mixed atmosphere of oxygen and argon. This invention does not have a particular limitation on the ratio of oxygen to argon in the mixed atmosphere; any ratio well known to those skilled in the art can be used. In this invention, the annealing is preferably carried out in a tube furnace. After the annealing, this invention also preferably includes naturally cooling the obtained film to room temperature before removing it.

[0054] In this invention, the LiCoO2 thin film electrode after charge-discharge cycle testing is preferably obtained by assembling the newly made LiCoO2 thin film electrode into a button cell and then conducting charge-discharge cycle testing.

[0055] In this invention, the assembly process preferably includes:

[0056] A button cell battery is assembled in a glove box using the LiCoO2 thin film electrode as the positive electrode, a lithium metal sheet as the negative electrode, a polyethylene film as the separator, and a lithium hexafluorophosphate (LiPF6) solution as the electrolyte.

[0057] Prior to assembly, the present invention preferably includes polishing the side of the LiCoO2 thin film electrode that is not loaded with LiCoO2 film. In the present invention, polishing can remove the reddish-brown oxide layer (generally CrO and FeO) formed on the back of the stainless steel during annealing.

[0058] Prior to assembly, the present invention preferably includes polishing the lithium metal sheet. In this invention, polishing the lithium metal sheet removes surface lithium oxide, exposing metallic Li, effectively reducing the battery's internal resistance.

[0059] In this invention, the pore size of the polyethylene membrane is preferably 80 nm. In this invention, the thickness of the polyethylene membrane is preferably 30 μm. In this invention, the micropores of the polyethylene membrane are used for Li in the electrolyte. + The transmission of [materials / materials]. In this invention, the polyethylene film is preferably sourced from Linyi Jieneng New Energy Materials Co., Ltd.

[0060] In this invention, the concentration of the lithium hexafluorophosphate (LiPF6) solution is preferably 0.5 mol / L, and the solvent for the lithium hexafluorophosphate (LiPF6) solution is preferably ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 1:1. In this invention, the amount of lithium hexafluorophosphate (LiPF6) solution used in each button cell is preferably 50 mL.

[0061] The present invention does not impose any special limitations on the specific assembly process, and any process well known to those skilled in the art can be used.

[0062] In this invention, the cutoff voltage for the charge-discharge cycle test is preferably 3.0–4.2V. In this invention, the charge-discharge cycle test is preferably performed using a LAND battery testing device (LANHECo., CT2001A).

[0063] After the charge-discharge cycle test, the present invention preferably further includes disassembling the tested button battery and taking out the positive electrode to test for residual stress.

[0064] In this invention, the preferred test conditions for the diffraction angle θ include: a point light source with a spot diameter of 0.1–5 mm, CuKα1 radiation, and a wavelength λ of 0.15405 nm; an operating current of 20–40 mA and a voltage of 20–40 kV; and front and rear slits of 0.5° and 5 mm, respectively; and a fixed grazing incidence angle, wherein the grazing incidence angle ranges from 0° to 5°. More preferably, the conditions include: a point light source with a spot diameter of 1.0 mm, CuKα1 radiation, and a wavelength λ of 0.15405 nm; an operating current of 40 mA and a voltage of 40 kV; front and rear slits of 0.5° and 5 mm, respectively; and a fixed grazing incidence angle of 3°.

[0065] In this invention, the Bragg equation is preferably 2d·sinθ=λ, where the value of λ is preferably 0.15406nm.

[0066] A schematic diagram of the principle of the method for testing the residual stress of LiCoO2 thin film electrodes provided by this invention is shown below. Figure 1 As shown, where, Figure 1 (a) The sample to be tested is placed horizontally on the Euler ring, with a special point light source on the left, fixed in grazing incidence mode. During the measurement, the sample stage needs to be rotated to different angles. At each specific angle, the grazing incidence mode measurement is still performed. The measurement process is as follows: Figure 1 As shown in (b), the black circular sample in the figure is the LiCoO2 thin film electrode to be tested. It is rotated to a specific angle on the sample stage, so that the thin film sample can be along the normal S of the sample surface. z Axis rotation angle The range was selected between 0° and 90°, with a step size of 45°. The positional diagrams of the LiCoO2 thin film sample before and after rotation are shown below. Figure 1 (c) and Figure 1 As shown in (d). To ensure the x-coordinate cos 2 α·sin 2 The value of ψ is uniformly distributed, and the tilt angles ψ are successively chosen to be ψ = 0°, 24°, 35°, 45°, and 53°. At a certain rotation angle... Under the combination of the tilt angle ψ, the X-rays incident on the LiCoO2 thin film sample are as follows: Figure 1 As shown in (e), the diffraction path of X-rays after passing through the crystal structure can clearly show the key angle information α required to calculate the stress.

[0067] To further illustrate the present invention, the method for testing the residual stress of the LiCoO2 thin film electrode provided by the present invention will be described in detail below with reference to the accompanying drawings and embodiments, but these should not be construed as limiting the scope of protection of the present invention.

[0068] Example 1

[0069] The 304SS substrate (0.03 mm thick, 20 mm in diameter) was cleaned with ethanol, followed by magnetron sputtering deposition of a LiCoO2 thin film. A 70 mm diameter LiCoO2 target with 99.9% purity was used, and the sputtering power was set to 4 W / cm². 2 With a target-substrate distance of 11 cm and a working gas pressure of 0.75 Pa, the gas flow ratio of argon to oxygen was 18.5:11.5, resulting in a LiCoO2 film with a thickness of 1 mm.

[0070] The above-mentioned film was transferred to a vacuum tube furnace, and a pure oxygen atmosphere was introduced. The temperature was increased from room temperature to 220°C at a rate of 3°C / min, and then increased to 550°C at a rate of 10°C / min. After holding at this temperature for 20 min, the film was naturally cooled to room temperature and removed to obtain the LiCoO2 thin film cathode (denoted as LCO-0).

[0071] The diffraction angle θ of the (003) characteristic peak of the LiCoO2 thin film electrode, obtained by X-ray diffraction, was calculated according to the Bragg equation. The test conditions included: the X-ray generator operating current was set to 4mA, the voltage to 40kV, the front slit and rear slit were set to 0.5° and 5mm respectively, and the grazing incidence angle was fixed at 3°.

[0072] The result The residual stress σ is calculated according to Formula 1, and the diffraction angle θ is obtained. As shown in Table 1.

[0073] Example 2

[0074] Using the LiCoO2 thin film obtained in Example 1 as the positive electrode, a lithium metal sheet as the negative electrode, a polyethylene film as the separator, and a lithium hexafluorophosphate (LiPF6) solution as the electrolyte, a button cell was assembled in a glove box. The polyethylene film had a pore size of 80 nm and a thickness of 30 μm. The concentration of the lithium hexafluorophosphate (LiPF6) solution was 0.5 mol / L (the solvent was ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1). The amount of lithium hexafluorophosphate (LiPF6) solution used in each button cell was 50 mL.

[0075] The resulting button cell was charged at 6.4 μAh / cm. 2 The cell was initially charged at 4.2V. The button cell was then disassembled, and the positive terminal (denoted as LCO-1-c) was removed. Residual stress was tested according to the method described in Example 1, yielding the residual stress σ and the diffraction angle θ. As shown in Table 1.

[0076] Example 3

[0077] The residual application of the LiCoO2 thin film cathode was tested according to the method in Example 2, except that the test sample was subjected to its first discharge at a voltage of 3.0V (denoted as LCO₁⁻¹⁻d), and the discharge current density was 6.4 μAh / cm². 2 The residual stress σ and the diffraction angle θ are obtained. As shown in Table 1.

[0078] Example 4

[0079] The residual application of the LiCoO2 thin film cathode was tested according to the method in Example 2, except that the test sample was undergoing a second charge at a voltage of 4.2V (denoted as LCO-2-c), and the current density during the first charge-discharge was 6.4 μAh / cm². 2 The current density during the second charge was 12.8 μAh / cm². 2 The residual stress σ and the diffraction angle θ are obtained. As shown in Table 1.

[0080] Example 5

[0081] The residual application of the LiCoO2 thin film cathode was tested according to the method in Example 2, except that the test sample underwent a second discharge at a voltage of 3.0V (denoted as LCO₂⁻⁻d), and the current density during the first charge-discharge was 6.4 μAh / cm². 2 The current density during the second charge was 12.8 μAh / cm². 2 The residual stress σ and the diffraction angle θ are obtained. As shown in Table 1.

[0082] Example 6

[0083] The residual application of the LiCoO2 thin film cathode was tested according to the method in Example 2, except that the test sample was undergoing its 5th charge at a voltage of 4.2V (denoted as LCO-5-c), and the current density during the first charge-discharge was 6.4 μAh / cm². 2 The current density during the second charge was 12.8 μAh / cm². 2 The current density for each subsequent charge and discharge cycle is 19.2 μAh / cm². 2 The residual stress σ and the diffraction angle θ are obtained. As shown in Table 1.

[0084] Example 7

[0085] The residual application of the LiCoO2 thin film cathode was tested according to the method in Example 2, except that the test sample underwent its 5th discharge at a voltage of 3.0V (denoted as LCO-5-d), and the current density during the first charge-discharge was 6.4 μAh / cm. 2 The current density during the second charge was 12.8 μAh / cm². 2 The current density for each subsequent charge and discharge cycle was 19.2 μAh / cm². 2 The residual stress σ and the diffraction angle θ are obtained. As shown in Table 1.

[0086] Example 8

[0087] The residual application of the LiCoO2 thin film cathode was tested according to the method in Example 2, except that the test sample was undergoing its 20th charge at a voltage of 4.2V (denoted as LCO-20-c), and the current density during the first charge-discharge was 6.4 μAh / cm². 2 The current density during the second charge was 12.8 μAh / cm². 2 The current density for each subsequent charge and discharge cycle is 19.2 μAh / cm². 2 The residual stress σ and the diffraction angle θ are obtained. As shown in Table 1.

[0088] Example 9

[0089] The residual application of the LiCoO2 thin film cathode was tested according to the method in Example 2, except that the test sample was subjected to its 20th discharge at a voltage of 3.0V (denoted as LCO-20-d), and the current density during the first charge-discharge was 6.4 μAh / cm². 2The current density during the second charge was 12.8 μAh / cm². 2 The current density for each subsequent charge and discharge cycle is 19.2 μAh / cm². 2 The residual stress σ and the diffraction angle θ are obtained. As shown in Table 1.

[0090] Example 10

[0091] The residual application of the LiCoO2 thin film cathode was tested according to the method in Example 2, except that the test sample was undergoing its 50th charge at a voltage of 4.2V (denoted as LCO-50-c), and the current density during the first charge-discharge was 6.4 μAh / cm². 2 The current density during the second charge was 12.8 μAh / cm². 2 The current density for each subsequent charge and discharge cycle is 19.2 μAh / cm². 2 The residual stress σ and the diffraction angle θ are obtained. As shown in Table 1.

[0092] Example 11

[0093] The residual application of the LiCoO2 thin film cathode was tested according to the method of Example 2, except that the test sample was subjected to its 50th discharge at a voltage of 3.0V (denoted as LCO-50-d), and the current density during the first charge-discharge was 6.4 μAh / cm. 2 The current density during the second charge was 12.8 μAh / cm². 2 The current density for each subsequent charge and discharge cycle is 19.2 μAh / cm². 2 The residual stress σ and the diffraction angle θ are obtained. As shown in Table 1.

[0094] Table 1. Diffraction angles 2θ and interplanar spacings corresponding to the (003), (101), and (012) diffraction planes of LiCoO2 under different cycling conditions. Half-width at half maximum (FWHM) and grain size (D)

[0095]

[0096]

[0097] Where D = (D1 + D2 + D3) / 3 is the average grain size.

[0098] Performance testing and characterization

[0099] Test 1

[0100] Figure 2The charge-discharge curves of the LiCoO2 thin film cathodes in Examples 2-11 during the 1st, 2nd, 5th, 20th and 50th charge-discharge cycles are shown in Table 2.

[0101] Table 2. Charge-discharge cycle test results of LiCoO2 thin film cathodes in Examples 2-11.

[0102]

[0103] from Figure 2 It can be seen that the charge and discharge capacity of the LiCoO2 film decreases significantly with the increase of the number of cycles. The plateau corresponding to 3.9V decreases continuously with the increase of the number of cycles, especially when the number of cycles exceeds 20, the plateau almost disappears.

[0104] Test 2

[0105] The electrodes after charge-discharge tests in Examples 1-11 were subjected to scanning electron microscopy.

[0106] After the test, the LiCoO2 film was removed and cleaned by immersion in acetone. The surface morphology of the film was characterized by field emission scanning electron microscopy (FESEM) (Hitachi SU-8010). The working voltage was 5kV, the same magnification of 30K was selected, and the scale bar length of 1μm was marked in the lower right corner.

[0107] Figure 3 (a) is a physical image of the LiCoO2 thin film electrode. No film peeling was observed with the naked eye.

[0108] Figure 3 (b) to (l) are SEM images of the LiCoO2 film surface, respectively. The charge / discharge state and cycle number have no significant effect on the film surface morphology; all images exhibit crack morphology. These cracks are caused by thermal stress introduced during the LiCoO2 / SS annealing and cooling process due to the difference in thermal expansion coefficients between the film and the substrate, resulting in cracks with an average width of ~100 nm. Furthermore, the film surface contains two particle sizes: one approximately 30 nm in size, uniformly and densely distributed across the entire sample surface; and the other approximately 300 nm in diameter, irregularly dispersed on the sample surface. Notably, new microcracks appear on the LiCoO2 film surface after charge / discharge cycles, with crack widths approaching 30 nm. However, it is impossible to quantitatively determine from the measured SEM images that the number of microcracks increases linearly with the number of cycles. The fundamental reason for the appearance of new cracks is that the LiCoO2 undergoes charge / discharge cycles accompanied by the reaction of Li... +The extraction and insertion behavior of the film causes the expansion and contraction of the thin film lattice structure. At the same time, the film is constrained by the substrate in the in-plane direction of the film, so compressive stress is generated inside the film. The compressive stress accumulates continuously with the increase of electrochemical cycle, and eventually cracks (new surfaces) are easily formed at the grain boundary to release the accumulated stress.

[0109] at last, Figure 4 Characterized Figure 2 (b) shows the cross-sectional morphology of the LiCoO2 sample. The annealed LiCoO2 has a traditional columnar crystal structure and a film thickness of 1.09 μm, which is consistent with the thickness measurement result of 1.1 μm by the profilometer (KLA TencorP-7).

[0110] Test 3

[0111] The crystallization state of the electrodes after charge-discharge tests in Examples 1-11 was tested;

[0112] The crystallinity of the thin film was measured using a Bruker D8 Advance powder diffractometer. An incident X-ray source with a spot diameter of 1.0 mm was selected, using CuKα1 radiation at a wavelength λ of 0.15405 nm. The X-ray generator operating current was set to 40 mA and the voltage to 40 kV. The front and rear slits were set to 0.5° and 5 mm respectively. A 2θ scanning mode was used, with a scanning range of 10°–80°, a fixed step size of 0.02°, and a scanning rate of 0.2 s.

[0113] Test results are as follows Figure 5 As shown;

[0114] It can be seen that the crystal structure of the LiCoO2 film is relatively stable after different cycles and at different potential states, and the (003), (101), and (012) diffraction crystal planes can be observed. In Figure (a), the 2θ positions marked by the vertical dashed line are 19.2°, 37.3°, and 39.0°, respectively, and the diffraction peak signal represented by the symbol * comes from the stainless steel substrate. The (003) crystal plane represents the layered structure of LiCoO2, and the (104) crystal plane corresponds to the basic unit Co-O-Co in the layered structure. Figure 5 (b) Further magnification of the corresponding positions of the (101) and (012) diffraction crystal planes revealed that the intensity of the (101) diffraction crystal plane was not significant after cycling under any conditions, while the intensity of the (012) diffraction crystal plane was not significant relative to the uncycled sample when in a fully discharged state, and the intensity was close to that of the uncycled sample when in a fully charged state.

[0115] In addition, according to Figure 5The XRD diffraction information shown was used to calculate the interplanar spacing d and grain size D with reference to Bragg's equation (1) and Scherrer's formula (2). Since the XRD pattern of LiCoO2 contains multiple diffraction peaks, the average grain size is theoretically equal to the average grain size corresponding to each diffraction peak. This patent only calculates the three characteristic diffraction planes (003), (101) and (012) obtained by detection. The weighted average grain size D is shown in Table 1.

[0116] It can be seen that the value d of the LiCoO2 thin film in the charging state (4.2V) is greater than that of the LiCoO2 thin film in the discharging state (3.0V) by approximately [amount missing]. This is because LiCoO2 is accompanied by Li during the charging process. + The extraction of oxygen leads to increased electrostatic repulsion in the oxygen layer adjacent to the Li layer, causing expansion of the thin film lattice structure. The average grain size D of the samples corresponding to different cycle periods and current densities ranges from 11.4 to 17.6 nm, showing no obvious regularity. Furthermore, the grain size calculated using the Scherrer formula is less than... Figure 3 The ~30nm particle size shown in the SEM image is because the result calculated according to the Scherrer formula is the primary grain size or the distance between two crystal planes, while the electron microscope usually sees secondary particles formed by the stacking of multiple primary grains, hence the difference in size between the two.

[0117] 2d·sinθ=λ(1)

[0118] D=Kλ / βcosθ(2)

[0119] Where d is the interplanar spacing, λ is the incident X-ray wavelength, λ = 0.15406 nm, D is the grain size, K is the Scherrer constant (usually taken as 0.89), β is the full width at half maximum (FWHM) of the diffraction peak, and θ is the Bragg diffraction angle.

[0120] Test 4

[0121] The film hardness and Young's modulus of the electrodes after charge-discharge tests in Examples 1-11 were tested.

[0122] Specifically, the film hardness and Young's modulus were measured using a TI950 Tribo Indenter nanomechanical testing system (Hysitron Inc., USA).

[0123] The indentation test was performed using a triangular pyramidal diamond Berkovich indenter (cone angle 143.2°). A three-stage mechanical curve test was selected: "load 5s - hold 5s - unload 5s". The total time for a single measurement was 15s. The load increased linearly to 330μN from 0 to 5s, remained at 330μN from 5 to 10s, and decreased linearly to 0μN from 10 to 15s. The measurement was repeated at 7 different locations with an adjacent spacing of 10μm. The average hardness and Young's modulus were calculated.

[0124] To avoid the influence of substrate effects and film surface roughness, the indentation depth is generally controlled within 10% of the average film thickness. Applying a maximum load of 330 μN to the surface of a LiCoO2 film with a nominal thickness of 1.1 μm resulted in an average indentation depth of (53.28 ± 0.47) nm, meeting the requirement of being within 10%. This load was repeated for 7 measurements, and the results are as follows... Figure 6 As shown, the multiple load-displacement curves have good repeatability. The calculated Young's modulus E is equal to (30.49±0.92) GPa, and the hardness H is equal to (1.75±0.02) GPa.

[0125] Test 5

[0126] Stress tests were performed on the LiCoO2 films after each charge-discharge cycle, with the same measurement angle. Five different rotation angles ψ were selected to detect their diffraction information. As the ψ angle increases, the position of the (101) diffraction crystal plane tends to shift to higher angles. This is because the film stress causes lattice distortion and reduces the interplanar spacing (as shown in Table 1). According to the Bragg equation, the diffraction peaks shift to higher angles. First, the combination parameter cos(d-d0) / d0 and the tilt angle is plotted. 2 α·sin 2 The relationship curve of ψ is obtained, and then the slope of the curve is obtained by linear fitting, combined with... Figure 5 The test data E was used to calculate the film stress. The relationship between the film residual stress and the charge / discharge state and the number of cycles is as follows: Figure 7 As shown in the figure; the specific test results are shown in Table 3;

[0127] Table 3. Residual stress test results of LiCoO2 thin film electrodes obtained in Examples 1-11.

[0128]

[0129]

[0130] Figure 7The horizontal axis represents the number of electrochemical cycles, and the vertical axis represents the residual stress of the film. The stress of the film in the charged state (4.2V) is represented by squares, while the stress of the film in the fully discharged state (3.0V) is represented by dots. A LiCoO2 film sample that has not undergone electrochemical cycling (LCO-0) is specifically included, with an average stress of -1.55 ± 0.12 GPa. Figure 7 It is marked with a horizontal dashed line and is under compressive stress.

[0131] Figure 7 In the LiCoO2 films with fewer than 5 cycles, the stress values ​​corresponding to the discharge state are close to those of the LCO-0 sample. When the number of cycles increases from 20 to 50, the stress in the LiCoO2 film decreases from a relatively high compressive stress (-2.25 ± 0.20) GPa to a near-stress-free state (-0.01 ± 0.04) GPa. However, the LiCoO2 films in the charging state are all in a relatively high compressive stress state, but with the increase of the number of cycles, their stress change trend is similar to that of the LiCoO2 films in the discharge state. Simultaneously, at the same number of cycles, the compressive stress of the LiCoO2 film charged to 4.2V is approximately 1.63 GPa greater than the stress in the state discharged to 3.0V, meaning that the compressive stress of the LiCoO2 film in the charging state is greater than its stress value in the discharge state. Firstly, Li... x When the CoO2 thin film begins to charge, the Li in the film... + The relationship between content, phase structure, microstructure, thin film stress and external charging voltage is as follows: Li x Before charging, the theoretical stoichiometry of CoO2 is LiCoO2, belonging to the hexagonal (HI) phase structure. As the charging voltage increases from 3.0V to 3.9V, LiCoO2 undergoes Li desorption. + Formation of Li 0.9 In CoO2 materials, the film stress increases linearly because Li 0.9 Compared to LiCoO2, the interplanar spacing of CoO2 increases along the c-axis, as shown in Table 1. The interplanar spacing d(101) of the film in the charged state increases by an average of ~0.4% compared to the discharged state. Furthermore, the LiCoO2(101) crystal plane has an angle with the sample surface, and d(101) expands along the film normal (c-axis). Therefore, it must also expand along the in-plane direction (a-axis). Considering the constraint of the substrate in the horizontal direction of the film, compressive stress is generated inside the film. Secondly, Li... x CoO2 continues to charge and remains at a stable 3.9V voltage platform, Li 0.9 CoO2 further undergoes delithiation to form Li. 0.78CoO2 forms a new hexagonal (HII) phase structure, with both (HII) and (HI) phases coexisting within the film. The stable 3.9V voltage range corresponds to a constant stress range. At the end of the 3.9V charging plateau, the film stress shifts towards compressive stress because the new phase (HII) can be considered an impurity within the main phase (HI). Strain at the two-phase interface introduces additional stress. Finally, as the charging voltage increases from 3.9V to 4.2V, the transition state Li... 0.78 CoO2 gradually converts to Li 0.5 With the change in CoO2, only a single-phase (HII) structure exists, and the film stress transforms towards compressive stress, reaching a state of high compressive stress. With 50% Li... + The extraction of Li+ increases the electrostatic repulsion between adjacent oxygen layers, leading to changes in lattice size and resulting in overall crystal structure expansion, as shown in Table 1. Compared to the discharge state, the grain size calculated for the (101) crystal plane expands by ≥3% in the charging state. The (003) grains expand along the normal direction of the film, but there is no mutual compression between adjacent (003) grains. The (104) grain structure is considered to be the (003) structure "tilted" and rotated by 90°. Compared to the (003) structure, the (101) and (104) structures are tilted by 10° and 30° respectively. At this time, the electrostatic repulsion caused by the extraction of Li+ will cause these crystal plane structures to expand along the normal direction (c-axis) and the in-plane direction (a-axis) of the film. Under the constraint of the substrate, adjacent grains inside the film are squeezed, generating compressive stress. It should also be emphasized that the 3% strain is very serious for brittle ceramic cathode materials. The discharge process of LiCoO2 film is exactly the opposite of the charging process. During charging, the Li+ extracted... + Li continuously returns to embed lithium vacancies, 0.5 The CoO2 material gradually reverted to its ideal structure with the stoichiometric ratio of LiCoO2, and the overall crystal structure returned to its initial structure before electrochemical cycling, explaining the residual stress state of the LiCoO2 film in the discharge state. Furthermore, charge-discharge cycling may cause a decline in the mechanical properties of the film, and this degradation is also a contributing factor to the changes in film stress.

[0132] In summary, the effect of electrochemical cycling on the residual stress of LiCoO2 thin films was investigated using an improved grazing incidence X-ray diffraction method. The average stress of the LiCoO2 thin film without charge-discharge cycling was (-1.55 ± 0.12) GPa. At any number of cycles, the stress value corresponding to the film in the discharge state (3.0 V) was close to (-1.55 ± 0.12) GPa, while the compressive stress at 4.2 V was approximately 1.63 GPa higher than that in the discharge state. This is because the charging process from 3.0 V to 4.2 V involves the deposition of 50% of the Li... +The extraction of oxygen leads to increased electrostatic repulsion between adjacent oxygen layers, resulting in a larger lattice size and overall crystal expansion. Under the constraint of the substrate, compressive stress is generated within the film. When the number of cycles is below 5, the residual stress of the LiCoO2 film in the discharged state is close to that of the uncycled sample. However, as the number of cycles increases from 20 to 50, the compressive stress of the LiCoO2 film in the discharged state tends to decrease. This is because the accumulated stress from multiple cycles is released through the formation of cracks.

[0133] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, and not all embodiments. Other embodiments can be obtained based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for testing the residual stress of a LiCoO2 thin film electrode, characterized in that, Includes the following steps: The LiCoO2 thin film electrode was fixed on the test platform of an X-ray diffractometer. At a set tilt angle ψ, the diffraction angle θ of the (003) characteristic peak of the LiCoO2 thin film electrode was measured using an X-ray diffractometer. The interplanar spacing d was calculated according to the Bragg equation. The interplanar spacing d The residual stress σ is obtained by calculating the tilt angle ψ according to Formula 1; Official 1; Where d0 is the interplanar spacing of the LiCoO2 thin film under stress-free conditions, and its value is 2.41 Å; v is the Poisson's ratio of LiCoO2, with a value of 0.24; E is the elastic modulus of the LiCoO2 film, in MPa; α is the specific angle at which the X-ray passes through the LiCoO2 thin film electrode. The relationship between α, the grazing incident angle γ, and the diffraction angle θ is: α = θ - γ; The tilt angle ψ ranges from 0° to 90°. The test conditions include: The operating current is 20~40mA, the voltage is 20~40kV, and the front slit and rear slit are set to 0.5° and 5mm respectively; the grazing incidence angle is fixed, and the value range of the grazing incidence angle is 0°~5°; The LiCoO2 thin film electrode is either a freshly made LiCoO2 thin film electrode or a LiCoO2 thin film electrode that has undergone charge-discharge cycle testing.

2. The test method according to claim 1, characterized in that, The method for preparing the novel LiCoO2 thin film electrode includes the following steps: The newly fabricated LiCoO2 thin film electrode was obtained by magnetron sputtering of a LiCoO2 thin film on a stainless steel substrate and followed by annealing.

3. The test method according to claim 2, characterized in that, The conditions for magnetron sputtering include: sputtering power of 0.5~8W / cm. 2 The target distance is 4~20cm, and the working pressure is 0.3~5Pa.

4. The test method according to claim 2, characterized in that, The annealing treatment temperature is 300~1000℃, and the holding time is 10~300min; The annealing process involves heating from room temperature to 220°C at a rate of 1-10°C / min, and then heating to the annealing temperature at a rate of 10°C / min.

5. The test method according to claim 1, characterized in that, The cutoff voltage for the charge-discharge cycle test is 3.0~4.2V.

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