Three-dimensional nano wall array negative electrode thin film material and preparation method and application thereof

Abstract

The invention discloses a three-dimensional nano wall array negative electrode film sample and a preparation method thereof. The surface of the sample is smooth and flat, the microstructure is a three-dimensional nano wall array, and the thickness of the film is about 0.3 cm. The preparation method comprises the following steps: (1) pretreatment of the Ti sheet; (2) first-step hydrothermal reaction; (3) ion exchange; and (4) second-time hydrothermal and subsequent heat treatment. The three-dimensional nano-wall negative electrode film provided by the invention has a three-dimensional nano-structure, a good crystalline state and a relatively flat surface appearance, can be characterized by using a conductive atomic force microscope under a nanoscale, has the advantages of simple process, low energy consumption, high efficiency and the like, and meets the application of a solid-state film battery negative electrode material.

Description

Technical Field

[0001] The present invention relates to the field of battery material preparation, and particularly relates to a three-dimensional nano-wall array negative electrode thin film material, a preparation method thereof, and an application thereof. Background Art

[0002] As an emerging high-efficiency energy storage device, solid-state thin-film batteries have received extensive attention in recent years. Compared with traditional liquid batteries, solid-state thin-film batteries use solid electrolytes, which can significantly improve the safety and stability of the batteries, avoid common problems such as leakage and combustion of liquid batteries, and at the same time have a longer service life and a higher energy density. Therefore, solid-state thin-film batteries are considered an important development direction for future battery technologies. The structure of a solid-state thin-film battery usually consists of a positive electrode, a negative electrode, and a solid electrolyte. The negative electrode material is one of the key factors determining the performance of the battery. It not only affects the capacity and energy density of the battery but also relates to the charge and discharge rate, cycle stability, and safety. Traditional negative electrode materials, such as graphite, have problems such as volume expansion, which may lead to degradation of material performance under high-rate charge and discharge conditions, limiting their application in high-efficiency solid-state batteries. Lithium Titanate (LTO), as a negative electrode material with high safety, long cycle life, wide voltage operating range, and excellent low-temperature performance, has gradually become an important candidate material in solid-state thin-film batteries. However, traditional LTO thin-film negative electrode materials usually adopt a planar structure. Although this structure is simple, there are many limitations in practical applications. For example, the thin film with a planar structure has a small surface area, and the ion and electron transport paths are long, which will limit the charge and discharge rate and the electrochemical reaction efficiency of the battery. Especially during high-rate charge and discharge, it is difficult to fully utilize its excellent electrochemical performance. To solve these problems, researchers have proposed three-dimensional structure thin films in recent years. The three-dimensional structure can significantly increase the specific surface area of the thin film and shorten the ion and electron transport paths, thereby improving the overall performance of the battery. [Xiong W, Xia Q, Xia H. Three-dimensional self-supported metal oxides as cathodes for microbatteries[J]. Functional Materials Letters, 2014, 07: 1430003] Recent research has shown that directly preparing a nanoarray structure (such as nanowires [Liao J Y, Xiao X C, Higgins D, Lui G, Chen Z W. Self-supported single crystalline H2Ti8O 17nanoarrays as integrated three - dimensional anodes for lithium - ion microbatteries[J].ACS Applied Materials&Interfaces,2014,6:568 - 574], nanowalls[Lei D,Zhang M,Qu B,Chen L,Wang Y,Zhang E,Xu Z,Li Q,Wang T.alpha - Fe2O3 nanowall arrays:hydrothermal preparation,growth mechanism and excellent rate performances for lithium ion batteries[J].Nanoscale,2012,4:

[0003] 3422 - 3426] and nanotube arrays[Zhong Y,Ma Y,Guo Q,Liu J,Wang Y,Yang M,Xia H.Controllable synthesis of TiO2@Fe2O3 core - shell nanotube arrays with double - wall coating as superb lithium - ion battery anodes[J].Scientific reports,2017,7(1):1 - 9] are good strategies for fabricating high - performance three - dimensional thin - film lithium - ion battery electrodes.

[0004] For the characterization of three-dimensional lithium titanate negative electrode thin film materials, conventional electrochemical characterization methods, such as cyclic voltammetry (CV) and charge-discharge tests, usually can only provide information on the average electrochemical performance of materials at the macroscopic scale and cannot deeply explore the complex behaviors of materials at the micro or nanoscale. Especially for negative electrode materials, due to the significant effects of surface and interface in nanometer thin films, the lithium-ion transport mechanism becomes more complex. To more precisely capture these changes at the micro and nanoscale, atomic force microscopy (AFM, MFP-3D Infinity, Asylum Research) and its derivative technologies such as conductive atomic force microscopy (C-AFM, Conductive Atomic Force Microscopy) are widely used in the characterization of thin film materials. Through AFM, information such as the surface roughness, morphological features, and grain distribution of the thin film can be obtained, providing a basis for in-depth study of the structural characteristics of three-dimensional lithium titanate negative electrode thin films. Among them, for the electrical property characterization of electrode materials, the C-AFM technology shows unique advantages. Based on the traditional AFM, C-AFM can measure local conductivity, current-voltage characteristics and other electrical information in real time while scanning the surface topography. For three-dimensional lithium titanate negative electrode thin films, due to the complexity of their three-dimensional structure and interface effects, C-AFM can provide electrical data with high spatial resolution, supplementing the details that cannot be provided in macroscopic electrochemical tests, and can help us comprehensively understand the electrochemical performance of materials from the micro to the macroscopic level and reveal the relationship between the lithium-ion transport behavior and electrochemical performance in thin film materials. This makes C-AFM an ideal tool for studying the electrochemical performance of electrode materials, especially the local response of three-dimensional nanostructured thin film electrodes during charge and discharge. However, the AFM and C-AFM technologies have certain requirements for the samples to be measured. First of all, the three-dimensional structure of the thin film needs to be a structure with definite height, width and depth in space. Generally, the structural units are mainly three-dimensional structures such as nanowalls and nanotubes, and the minimum unit is in the range of 1-200 nanometers, presenting an ordered three-dimensional array structure as a whole. This can ensure that during the C-AFM scanning process, the electrical responses in different regions are consistent, so as to obtain reliable conductivity data. Secondly, the three-dimensional negative electrode thin film is usually a polycrystalline nanostructured thin film. To ensure the measurement accuracy, the thickness of the thin film needs to be less than 1 micron, and the film loading is about 0.35 mg / cm 2 , and at the same time the surface roughness (Ra value) of the sample should be in the range of 20-80 nm. So that C-AFM can clearly capture the changes in surface micro-conductivity characteristics and deeply understand the relationship between the microstructure and conductivity characteristics of the material.

[0005] The existing technologies for preparing three-dimensional thin films of the negative electrode generally have the following problems: insufficient structural stability and usually complex technological processes. The three-dimensional thin films generated by traditional preparation methods often have problems of collapse or deformation, cannot meet the requirements of precise characterization, and cannot be compatible with the multi-functional characterization of AFM and C-AFM at the same time. Moreover, there is no report on three-dimensional nano-wall LTO negative electrode thin films that can be used for AFM and C-AFM characterization in the existing technologies. Summary of the Invention

[0006] Aiming at the deficiencies of the existing technologies, the first object of the present invention is to provide a three-dimensional nano-wall array negative electrode thin film sample and a preparation method thereof. The provided preparation method has the advantages of simple operation, low cost, and time-saving.

[0007] The second object of the present invention is to provide a three-dimensional nano-wall array negative electrode thin film material prepared by the above preparation method. The surface of the three-dimensional nano-wall array negative electrode thin film material is flat, the crystal grains are regular, and the size is nanoscale, meeting the sample preparation standards of the negative electrode thin film. And it can be characterized by AFM and C-AFM.

[0008] The third object of the present invention is to provide an application of a three-dimensional nano-wall array negative electrode thin film material prepared by the above preparation method, and use the three-dimensional nano-wall array negative electrode thin film material for characterization by C-AFM.

[0009] In order to achieve the above objects, the present invention adopts the following technical solutions:

[0010] A method for preparing a three-dimensional nano-wall array negative electrode thin film of the present invention includes the following steps:

[0011] Step 1 Pretreatment of the Ti sheet

[0012] Cut the Ti sheet and soak it in a solvent for washing.

[0013] Step 2 First hydrothermal reaction

[0014] Place the Ti sheet in the inner liner of the hydrothermal reaction kettle in a leaning manner, then preheat the heating instrument. After the preheating is completed, place the hydrothermal reaction kettle in an electric blast drying oven and react at 180 °C for 16 h. After the first hydrothermal reaction is completed, perform natural cooling.

[0015] Step 3 Ion exchange

[0016] Put the sample obtained after the first hydrothermal reaction into 0.5 M hydrochloric acid for 3 hours of ion exchange.

[0017] Step 4 Second hydrothermal reaction and subsequent heat treatment

[0018] Place the sample in the hydrothermal reactor lining in an inclined manner, and then preheat the heating instrument. After preheating, place the hydrothermal reactor in an electronic blast drying oven and react at 100°C for 12 hours. Cool naturally after the hydrothermal reaction. After cooling, wash the sample with a solvent and place it in a muffle furnace for the final heat treatment.

[0019] In the present invention, a hydrothermal reaction kettle with an inner lining volume of 50 ml is used.

[0020] In a preferred embodiment, the solvent is anhydrous ethanol, acetone, or deionized water.

[0021] In a preferred embodiment, the volume ratio of the solvent to the Ti flakes is (2-3):1.

[0022] Preferably, the soaking time is 5-10 minutes.

[0023] Preferably, the heating device is an electronic blast drying oven.

[0024] The inventors found that the volume of the solvent used for immersion needs to be effectively controlled. Too much solvent will cause excessive volatilization, causing the Ti material to be stained with the solution, affecting subsequent experiments. Too little immersion solution volume will result in insufficient washing of the electrode sheet. The reagents and other conditions selected by the present invention can ensure the removal of polar, non-polar and water-soluble pollutants on the Ti sheet, significantly improve the surface cleanliness of the titanium sheet, and facilitate subsequent experiments.

[0025] In the actual operation process, the sample is placed in the lining of the hydrothermal reactor in an oblique manner.

[0026] In a preferred embodiment, the sample is placed into the reactor liner using tweezers, wherein both ends of the sample need to be against the inner wall of the liner, and the angle between the sample and the bottom of the liner is approximately 40-50 degrees.

[0027] The inventors found that when both ends of the sample were against the inner wall of the lining, the sample was better fixed, and the appropriate angle allowed sufficient contact space on both sides of the sample. The solution convection generated on the upper and lower surfaces during the hydrothermal reaction also made the reaction more complete, facilitating the formation of the three-dimensional wall morphology of the sample in the later stage.

[0028] In a preferred solution, the heating instrument is a muffle furnace or an electronic blast drying oven. An electronic blast drying oven is further preferred. The electronic blast drying oven has the advantage of blowing air. The built-in fan forces air circulation, allowing the hot air in the oven to flow quickly, significantly improving the temperature uniformity, allowing the hydrothermal reactor to be heated more evenly, and the effect will be better than that of a muffle furnace.

[0029] In a preferred solution, preheating means heating the instrument to the required temperature for hydrothermal treatment before placing it into the hydrothermal reactor.

[0030] The inventors found that preheating can omit some additional reactions generated at different temperatures in the early stage, which is more conducive to the generation of the three-dimensional wall structure.

[0031] In a preferred embodiment, after the hydrothermal reaction is completed, when the heating instrument cools to about 70 °C, the hydrothermal reaction kettle is taken out and cooled to room temperature.

[0032] The inventors found that after the reaction is completed, the cooling process of the hydrothermal reaction kettle is usually relatively slow. This not only involves the cooling of the instrument to room temperature, but also requires waiting for the temperature of the reaction kettle itself to drop to room temperature. And this entire cooling process will lead to an extension of the actual experimental time and the occurrence of additional side reactions. For example, the residual heat in the instrument during slow cooling will cause uncontrollable oxidation of Ti 3 +(formation of TiO impurity phase), and Ostwald ripening and pore collapse due to thermally induced structural distortion, which may lead to partial collapse of the three-dimensional wall structure. To optimize this process, when the instrument temperature drops to about 70 °C, the hydrothermal reaction kettle is directly taken out and further cooled to room temperature. This method can effectively shorten the cooling time by 2 - 3 h and reduce the generation of thermal stress side reactions, promote the stable formation of the microscopic morphology of the sample, and thus improve the experimental efficiency and the reliability of the results.

[0033] In a preferred embodiment, the temperature of the first hydrothermal reaction is 180 °C and the time is 12 h.

[0034] The inventors found that at hydrothermal reaction temperatures of 140, 180, and 220 °C, the XRD (X-ray Diffraction) phase composition and SEM (scanning electron microscope) morphology of the thin film are different. However, the XRD of the sample at 180 °C is qualified, and the microscopic structure obtained by SEM is also closer to the three-dimensional structure. Therefore, 180 °C is selected as the temperature of the first hydrothermal reaction for subsequent optimization.

[0035] In a preferred embodiment, the volume ratio of hydrochloric acid used in ion exchange to the volume of the Ti sheet should be (2 - 3):1.

[0036] The inventors found that the volume of hydrochloric acid used for soaking needs to be effectively controlled. Excessive amount will lead to overreaction and make the Ti material contaminated with the solution, affecting subsequent experiments. Too little volume of the soaking solution will lead to insufficient reaction of the electrode sheet. The reagents and other conditions selected in the present invention can ensure that the treatment of the Ti sheet will not damage the already formed microscopic morphology, facilitating the subsequent experiments.

[0037] The present invention also provides a three-dimensional nano-wall negative electrode thin film material prepared by the above preparation method.

[0038] The three-dimensional nano-wall anode thin film material has a flat surface, regular crystal grains with a size in the nanometer range, meets the sample preparation standards for anode thin films, and can be characterized by C-AFM.

[0039] The present invention also provides an application of a three-dimensional nano-wall anode thin film material prepared by the above preparation method.

[0040] Use the three-dimensional nano-wall anode thin film for atomic force microscope characterization.

[0041] Advantages of the present invention:

[0042] (1) The present invention uses a hydrothermal method to prepare a three-dimensional nano-wall array anode thin film sample. By controlling the temperature of the first hydrothermal reaction, whether the heating instrument is preheated, whether the sample is leaned during the hydrothermal reaction, and how it is cooled after the reaction, a negative electrode thin film material with a regular three-dimensional nano-wall structure is successfully prepared. This structure significantly improves the specific surface area and porosity of the material, providing a larger charge storage space and a faster ion / electron transport channel for lithium-ion batteries. The preparation method uses conventional hydrothermal reaction, ion exchange, and heat treatment technologies, avoiding the need for complex and expensive equipment, and has the advantages of simple process and low cost.

[0043] (2) For the three-dimensional nano-wall anode material prepared by the present invention, in the two-step hydrothermal method, the effective control of the solution convection during the hydrothermal reaction makes the crystal form of the thin film tend to be perfect, the crystal morphology in the thin film is regular, and the particle size reaches the nanometer level.

[0044] (3) The present invention prepares a three-dimensional nano-wall array anode thin film sample that can be used for conductive atomic force microscopy. The prepared three-dimensional wall LTO anode thin film shows higher rate performance and cycling stability compared to other thin films. The three-dimensional nano-wall structure helps to reduce the lithium ion diffusion resistance and alleviate the volume expansion problem of the electrode material, thereby extending the service life of the battery. Description of the Drawings

[0045] Figure 1 Schematic diagrams of preparing LTO thin film materials with different process parameters;

[0046] Figure 2 SEM images of LTO thin films under different preparation conditions;

[0047] Figure 3 XRD patterns of LTO thin films at different temperatures;

[0048] Figure 4 AFM topography and 3D topography images of the coral-like thin film sample;

[0049] Figure 5 XRD patterns of LTO thin films with or without inclined preheating at 180 °C;

[0050] Figure 6 It is the SEM image of the three-dimensional nano-wall LTO thin film;

[0051] Figure 7 It is the physical image of the three-dimensional nano-wall thin film obtained by preparing the sample of the present invention;

[0052] Figure 8 It is the AFM topography image and 3D topography image of the three-dimensional nano-wall LTO thin film sample;

[0053] Figure 9 It is the electrochemical performance test of the coral-shaped and three-dimensional nano-wall LTO thin films;

[0054] Figure 10 It is the C-AFM image of the three-dimensional nano-wall LTO thin film sample; Detailed implementation mode

[0055] Test the morphological characteristics and electrochemical activity of the negative electrode LTO thin film, including the following steps:

[0056] (1) First, perform XRD and SEM tests on the prepared sample to obtain the phase composition and microscopic morphological characteristics of the sample. Then, clean and dry the thin film sample with ethanol and deionized water and assemble it into a three-electrode test system in a glove box. Among them, the platinum electrode is the counter electrode, the silver chloride is the reference electrode, and the LTO thin film sample is the working electrode. In the glove box environment, first clamp the working electrode with an electrode clip, and then pour 1M lithium hexafluorophosphate / ethylene carbonate:dimethyl carbonate LiPF6 / EC:DMC (volume ratio of 1:1) electrolyte into the electrolytic cell until it covers the three electrodes. The capacity of the electrolytic cell is 50 ml. After completion of the assembly, perform C-V, impedance, and first charge-discharge electrochemical performance tests on the assembled three-electrode system using Shanghai Chenhua Electrochemistry.

[0057] (2) Use the C-AFM module in the atomic force microscope to test the morphological characteristics and internal conductivity of the three-dimensional nano-wall thin film sample using the AC mode.

[0058] Example 1:

[0059] (1) Cut the pure Ti sheet into 1 cm x 1 cm small pieces, and clean the pure Ti sheet with ethanol, acetone, and deionized water respectively. Place the dried Ti substrate flat in the inner liner, and the solution in the inner liner is 25 mL of 1M NaOH solution.

[0060] (2) Place the reaction kettle in an electronic blast drying oven at 180 °C for hydrothermal treatment for 16 hours.

[0061] (3) Put the sample obtained after the first hydrothermal treatment into 0.5M hydrochloric acid for ion exchange for 3 hours.

[0062] (4) After that, the sample obtained after the ion exchange was placed obliquely into a polytetrafluoroethylene inner liner containing 25 ml of 2M LiOH solution. The electric blast drying oven was preheated to 100 °C, and then the reaction kettle was placed into the electric blast drying oven for hydrothermal treatment for 12 hours. The sample obtained from the second hydrothermal treatment was washed with deionized water and ethanol, and then placed into a muffle furnace for heat treatment at 550 °C for 3 hours. The final thin film sample was obtained.

[0063] Figure 1 The first line in the schematic diagram is the experimental process schematic diagram of the LTO thin film prepared in Example 1.

[0064] Figure 2 (a) and (b) are the SEM morphology map information of the LTO thin film prepared in Example 1.

[0065] Figure 3 The curve pattern shown at 180 °C is the XRD information of the LTO thin film prepared in Example 1.

[0066] Comparative Example 1:

[0067] Other conditions were the same as those in Example 1. The hydrothermal temperature in step (2) of Example 1 was modified from 180 °C to 140 °C, and the SEM morphology map of the obtained sample is as shown in Figure 2 (c) and (d). Figure 3 The XRD curve pattern shown at 140 °C is the XRD information of the LTO thin film prepared in Comparative Example 1. Due to the too low hydrothermal temperature, the thin film was in a disordered nano-sheet shape, which did not conform to the characteristics of the three-dimensional structure of the negative electrode thin film material.

[0068] Comparative Example 2:

[0069] Other conditions were the same as those in Example 1. The hydrothermal temperature in step (2) of Example 1 was changed to 220 °C, and the SEM morphology map of the obtained sample is as shown in Figure 2 (e) and (f). Figure 3 The curve pattern shown at 220 °C is the XRD information of the LTO thin film prepared in Comparative Example 2. Among them Figure 3 Due to the too high hydrothermal temperature, the thin film was in a nano-linear shape, which did not conform to the characteristics of the three-dimensional structure of the negative electrode thin film material.

[0070] Example 2:

[0071] (1) Cut the pure Ti sheet into small pieces of 1 cm x 1 cm, and clean the pure Ti sheet with ethanol, acetone and deionized water respectively. Place the dried Ti substrate flat into the inner liner, and the solution in the inner liner is 25 mL of 1M NaOH solution.

[0072] (2) Preheat the electric blast drying oven to 180 °C first, and then place the reaction kettle into the electric blast drying oven for hydrothermal treatment for 16 hours. After the first hydrothermal treatment is completed, wait for the temperature of the electric blast drying oven to drop to about 70 °C, take out the reaction kettle, and let it cool naturally outside.

[0073] (3) Put the sample obtained after the first hydrothermal treatment into 0.5 M hydrochloric acid for ion exchange for 3 hours.

[0074] (4) Then place the sample obtained after the ion exchange obliquely into a polytetrafluoroethylene inner liner containing 25 ml of 2 M LiOH solution. Preheat the electric blast drying oven to 100 °C, and then place the reaction kettle into the electric blast drying oven for hydrothermal treatment for 12 hours. After the hydrothermal treatment is completed, wait for the temperature of the electric blast drying oven to drop to about 70 °C, take out the reaction kettle, and let it cool naturally outside. The sample obtained from the second hydrothermal treatment is washed with deionized water and ethanol, and then placed in a muffle furnace for heat treatment at 550 °C for 3 hours. The final thin film sample is obtained.

[0075] Figure 1 The second line in the schematic diagram is the experimental process schematic diagram of the LTO thin film prepared in Example 1.

[0076] Figure 2 (g) and (h) are SEM characterization diagrams of the LTO thin film prepared in Example 1. Due to the preheating step, the occurrence of additional side reactions is reduced, the generation of the microstructure is optimized, and a porous semi-three-dimensional structure appears, showing a coral shape. It is defined as a semi-three-dimensional coral-shaped LTO thin film. The coral-shaped LTO thin film can be used for AFM characterization, and its image is as Figure 4 shown, in Figure 4 (a) and (b), it can be seen that the coral-shaped thin film has a three-dimensional structure and a certain height difference. Combining the information in the SEM diagram, it can be known that the three-dimensional structure of the coral-shaped thin film is not obvious and is unevenly distributed. It is not the most ideal three-dimensional structure LTO thin film. Figure 4 (c) is the Figure 4 height diagram of the cross-section line in (a). The average surface roughness (Ra value) of the thin film in the figure is about 78 nm, which meets the sample preparation standard for the three-dimensional thin film of the negative electrode.

[0077] Figure 9 (a) and (b) are the C-V and impedance test diagrams of the coral-shaped LTO thin film. It can be seen from the figure that the position where the reduction peak appears is shifted, and the potential difference between the oxidation and reduction peaks is large. The diffusion trend of the thin film electrode is correct, but it has a high impedance, poor interfacial charge transfer, and limited migration of ions or electrons.

[0078] Example 3:

[0079] A three-dimensional nano-wall array negative electrode thin film sample and its preparation method, including the following steps:

[0080] (1) Cut the pure Ti sheet into small pieces of 3 cm x 1.5 cm, and clean the pure Ti sheet with ethanol, acetone, and deionized water respectively. Place the dried Ti substrate obliquely into the inner lining, and the solution in the inner lining is 25 mL of 1M NaOH solution.

[0081] (2) Preheat the electro-drum drying oven to 180 °C first, and then place the reaction kettle into the electro-drum drying oven for hydrothermal treatment for 16 hours. After the first hydrothermal treatment, wait for the temperature of the electro-drum drying oven to drop to about 70 °C, take out the reaction kettle, and let it cool naturally outside.

[0082] (3) Put the sample obtained after the first hydrothermal treatment into 0.5M hydrochloric acid for ion exchange for 3 hours

[0083] (4) Then place the sample obtained after ion exchange obliquely into the polytetrafluoroethylene inner lining containing 25 ml of 2M LiOH solution. Preheat the electro-drum drying oven to 100 °C first, and then place the reaction kettle into the electro-drum drying oven for hydrothermal treatment for 12 hours. After the hydrothermal treatment, wait for the temperature of the electro-drum drying oven to drop to about 70 °C, take out the reaction kettle, and let it cool naturally outside. The sample obtained from the second hydrothermal treatment is washed with deionized water and ethanol, and then placed in a muffle furnace for heat treatment at 550 °C for 3 hours. Finally, the LTO negative electrode film is efficiently synthesized by the hydrothermal method, and its three-dimensional nano-wall structure is completely retained during the synthesis process.

[0084] Figure 1 The third row in the schematic diagram is the experimental process schematic diagram of the LTO film prepared in Example 3.

[0085] Figure 5 The black curve shown above is the XRD information of the LTO film prepared by preheating and leaning obliquely at 180 °C in Example 3, and the blue curve below is the XRD information of the LTO film prepared without preheating and leaning obliquely at 180 °C.

[0086] Figure 6 (a) and (b) are SEM images of the LTO film after the first hydrothermal reaction in Example 3. The sample leaning treatment optimizes the solution convection during hydrothermal treatment. The Na2Ti2O5·H2O (NTO) nano-wall array formed in-situ on the Ti sheet ensures good contact and adhesion between it and the Ti substrate, providing a good template for the subsequent preparation of the LTO nano-wall array. Figure 6(c) and (d) are SEM images of the LTO thin film prepared in Example 3 after the second hydrothermal reaction. The microstructure in the figure has an obvious three-dimensional structure in the shape of a three-dimensional wall, which is defined as a three-dimensional nano-wall LTO thin film. The thickness of the nano-wall shown by the arrow in the figure is about 150 nm, and the whole is composed of nano-particles with a size of 20-30 nm. Each nano-wall has a certain mesoporous structure, which can effectively increase its specific surface area to facilitate the increase of the contact area between the electrode and the electrolyte, thereby enhancing the film conductivity and improving the film electrochemical performance.

[0087] Figure 7 (a) and (b) are physical pictures of the three-dimensional nano-wall thin film, Figure 7 (c) and (d) are physical pictures of the complete sample before and after washing the thin film. According to this information, the loading of the thin film can be calculated to be 0.35 mg / cm 2 .

[0088] Figure 8 is the surface topography map obtained by observing the three-dimensional nano-wall LTO thin film in the AC mode of the atomic force microscope. In Figure 8 (a), the three-dimensional wall structure of the sample and the height difference between the wall structure and the surrounding pores can be clearly seen, while Figure 8 (b) The 3D topography map makes the three-dimensional wall structure more three-dimensional and obvious. The thickness of the nano-wall shown by the arrow is about 150 nm. Figure 8 (c) is Figure 8 the height map of the thin film cross-section of the red line in (a). The height of the nano-wall corresponding to the blue circle in the figure is about 160 nm. The average surface roughness (Ra value) of the thin film in the figure is about 72 nm, which meets the sample preparation standard of the three-dimensional negative electrode thin film and can be clearly characterized by AFM.

[0089] Figure 9 (c) and (d) are the electrochemical performance test diagrams of the three-dimensional nano-wall LTO thin film. The positions of the oxidation and reduction peaks in the C-V diagram appear at accurate and small potential differences, and the curve characteristics are obvious. The charge transfer resistance and the solid-phase diffusion impedance are small, the charge transfer ability is stronger, and the interface optimization effect is better. The performance of the three-dimensional nano-wall LTO thin film meets the requirements. And in the same voltage range, compared with the coral-shaped LTO thin film in Example 2, the three-dimensional nano-wall LTO thin film has a higher current response (vertical axis), and the oxidation peak and reduction peak are clearer and sharper. The current peak value remains basically the same with the increase of the number of cycles, which indicates that the three-dimensional nano-wall LTO thin film has a stronger electrochemical reaction and better electrochemical stability.

[0090] Figure 10It is the C-AFM image of the three-dimensional nanowall LTO film. The three-dimensional nanowall LTO film is superior to the coral-like LTO film in terms of macroscopic electrochemical performance. Therefore, combining the unique feature of C-AFM that can combine high-resolution topography analysis with local conductivity measurement, the in-situ C-AFM characterization of the three-dimensional nanowall LTO film was carried out in the AC mode. Figure 10 (a) and (c) are the in-situ topography images of the sample at 10 μm and 5 μm respectively, Figure 10 (b) and (d) are the surface current distribution maps at 10 μm and 5 μm. Figure 10 Although the current in (b) and (d) is small (57 pA), it can be clearly seen that at the Figure 10 three-dimensional wall structure corresponding to (a) and (c) (the black line part) Figure 10 the current in (b) and (d) is significantly higher than other parts, and the image is complete and clear, meeting the experimental expectations.

Claims

1. A method for preparing a three-dimensional nano-wall array negative electrode thin film, comprising the following steps: Step 1: Pretreatment of the Ti sheet Cut the Ti sheet and soak it in a solvent for washing. Step 2: The first hydrothermal reaction Place the Ti sheet in the inner lining of the hydrothermal reaction kettle in a leaning manner, then preheat the heating instrument. After the preheating is completed, place the hydrothermal reaction kettle in an electric blast drying oven and react at 180 °C for 16 h. After the first hydrothermal reaction ends, take it out and let it cool naturally. Step 3: Ion exchange Put the sample obtained after the first hydrothermal reaction into hydrochloric acid for ion exchange. Step 4: The second hydrothermal reaction and subsequent heat treatment Place the sample in the inner lining of the hydrothermal reaction kettle in a leaning manner, then preheat the heating instrument. After the preheating is completed, place the hydrothermal reaction kettle to start the reaction. React at 100 °C for 12 h. After the hydrothermal reaction, let it cool naturally. After the cooling ends, wash the sample with a solution and put it into a muffle furnace for the final heat treatment.

2. The method for preparing a three-dimensional nano-wall array negative electrode thin film according to claim 1, wherein: The solvent is anhydrous ethanol, acetone, deionized water, and the volume ratio of the solvent to the Ti sheet is 2 - 3:1, and the soaking time is 5 - 10 min.

3. The method for preparing a three-dimensional nano-wall array negative electrode thin film according to claim 1, characterized in that: Use tweezers to put the sample into the inner lining of the reaction kettle, and both ends of the sample need to abut against the inner wall of the lining. The angle between the sample and the bottom of the lining is approximately 50 degrees.

4. A method for preparing a three-dimensional nano-wall array negative electrode thin film according to claim 1, characterized in that: During preheating, it means that the instrument reaches the required hydrothermal temperature and then the hydrothermal reaction kettle is placed.

5. According to the method for preparing a three-dimensional nano-wall array negative electrode thin film described in claim 1, the conditions of the first hydrothermal reaction are 180 °C for 16 h, and the conditions of the second hydrothermal reaction are 100 °C for 12 h.

6. The method for preparing a three-dimensional nano-wall array negative electrode thin film according to claim 1, characterized in that: After the hydrothermal reaction ends, when the instrument cools to about 70 °C, take out the hydrothermal reaction kettle and let it cool to room temperature.

7. A method for preparing a three-dimensional nano-wall array negative electrode thin film according to claim 1, characterized in that: During ion exchange, the hydrochloric acid is 0.5 M, the soaking time is 3 h, and the volume ratio of the hydrochloric acid used to the Ti sheet is 2 - 3:

1.

8. A three-dimensional nano-wall array negative electrode thin film sample prepared by the preparation method described in claims 1 - 7.

9. Use of a high-nickel ternary cathode thin film material prepared by the preparation method according to any one of claims 1-8, characterized in that: Use the three-dimensional nano-wall LTO negative electrode thin film material for atomic force microscopy, scanning electron microscopy, and three-electrode system characterization.

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