Graphene-sulfide composite positive electrode and preparation method and application thereof, and all-solid-state battery

Abstract

The application relates to a graphene-sulfide composite positive electrode and a preparation method and application thereof, and a full solid-state battery, and relates to the field of solid-state batteries. The graphene-sulfide composite positive electrode comprises a positive electrode active material, a solid-state electrolyte and a conductive agent, the solid-state electrolyte comprises a sulfide, and the conductive agent comprises graphene; along the thickness direction of the electrode, the reduction degree of the graphene is continuously or stepwisely decreased. The composite positive electrode provided by the application has an anisotropic gradient conductive structure, the conductive phase graphene of the conductive structure presents a spatial gradient distribution of the reduction degree: high-reduction-degree graphene close to the current collector side provides a high-speed electron channel; low-reduction-degree graphene oxide close to the electrolyte layer side acts as an electron blocking layer, so that the risk of a through short circuit caused by excessive graphene is eliminated without sacrificing the thickness of the electrode, the electrical performance such as the initial charge-discharge specific capacity is improved, and the defects of the prior art are compensated.

Description

Technical Field

[0001] This invention relates to the field of solid-state batteries, and more particularly to a graphene-sulfide composite cathode, its preparation method and application, and an all-solid-state battery. Background Technology

[0002] With the increasing global demand for longer driving ranges in new energy vehicles, developing high-energy-density lithium battery systems has become a core industry objective. High-nickel ternary materials (such as NCM811) are highly favored due to their high specific capacity (>200 mAh / g). However, the instability and flammability of liquid electrolytes under high pressure limit their further application. All-solid-state lithium batteries (ASSLBs) fundamentally solve safety issues by using non-flammable solid electrolytes and allow the use of metallic lithium anodes to significantly improve energy density. In all-solid-state battery systems, to achieve high volumetric energy density, the loading of the positive electrode must be increased, i.e., developing "thick electrodes" (typically greater than 100 μm in thickness). However, thick electrodes face significant challenges in solid-state systems: (1) Imbalance in electron / ion transport: Inside a thick electrode, electrons tend to accumulate near the current collector, while lithium ions need to travel through a long solid interface to reach the active site. Increasing the content of conductive graphene can improve electron transport, but excessively high graphene content can easily form a pervasive electron percolation network, causing electrons to directly penetrate the positive electrode layer and contact the negative electrode, resulting in micro-short circuits or self-discharge.

[0003] (2) Interfacial side reactions: When graphene with high conductivity comes into contact with sulfide electrolytes, it will catalyze the electrochemical decomposition of sulfides, especially under high voltage window. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by providing a graphene-sulfide composite cathode, its preparation method and application, and an all-solid-state battery.

[0005] In a first aspect, the present invention provides a graphene-sulfide composite cathode, wherein the graphene-sulfide composite cathode comprises a cathode active material, a solid electrolyte and a conductive agent, wherein the solid electrolyte comprises a sulfide and the conductive agent comprises graphene. Along the electrode thickness direction, the degree of reduction of the graphene decreases continuously or in a stepwise manner.

[0006] Furthermore, the graphene near the current collector side is highly reduced graphene, and the C / O ratio of the highly reduced graphene is 20:1 to 100:1, for example, it can be 20:1, 25:1, 30:1, 40:1, 50:1, 55:1, 70:1, 80:1, 90:1, 100:1 or any range between the aforementioned values.

[0007] Furthermore, the graphene near the solid electrolyte layer is low-reduction graphene or graphene oxide, and the C / O ratio of the low-reduction graphene or graphene oxide is 2:1 to 8:1, for example, it can be 2:1, 3:1, 4:1, 5:1, 5.5:1, 7:1, 8:1 or any of the aforementioned values.

[0008] Furthermore, the sulfide fills the interlayer gaps of the graphene, and the volume fraction of the sulfide gradually decreases with increasing depth.

[0009] Furthermore, the mass fraction of graphene in the graphene-sulfide composite cathode is 0.5% to 3.0%, for example, it can be 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, 1.8%, 2.0%, 2.5%, 3.0% or any of the aforementioned values; And / or, the thickness of the graphene-sulfide composite cathode is 100μm to 300μm, for example, it can be 100μm, 110μm, 115μm, 120μm, 150μm, 200μm, 240μm, 260μm, 300μm or any of the aforementioned values.

[0010] Furthermore, the positive electrode active material includes a high-nickel positive electrode active material, which includes at least one of NCM811 and NCM622; And / or, the sulfide includes a silver-germanium sulfide electrolyte, which includes Li3PS4, Li 10 GeP2S 12 At least one of Li6PS5Cl.

[0011] In a second aspect, the present invention provides a method for preparing the graphene-sulfide composite cathode described in the first aspect, comprising the following steps: A mixed slurry is prepared using graphene oxide, sulfide particles and positive electrode active material, with anhydrous diethyl ether or anhydrous xylene as the solvent. By using a stepwise coating method or an electrophoretic deposition method, the mixed slurry is layered on a current collector to construct composite layers with different graphene oxide contents. By employing single-sided laser scanning or gradient thermal treatment technology, the energy is controlled to decrease from the current collector side to the electrolyte side, allowing the graphene oxide on the current collector side to undergo sufficient thermal reduction and transformation, while the surface layer retains a high oxygen content, thus obtaining the graphene-sulfide composite cathode. Alternatively, graphene oxide with different C / O ratios can be mixed with sulfide particles and positive electrode active materials to prepare slurries, thereby obtaining graphene oxide mixed slurries with different C / O ratios. By using a stepwise coating method or an electrophoretic deposition method, graphene oxide mixed slurries with different C / O ratios are layer by layer constructed on a current collector to form graphene oxide composite layers with different C / O ratios. Subsequently, the layers are pressed and molded to obtain the graphene-sulfide composite cathode.

[0012] Furthermore, the operating conditions parameters for the single-sided laser scanning include: single-sided directional laser scanning is performed on the current collector side under inert or vacuum atmosphere, with a laser power of 8 to 22 W, a scanning rate of 80 to 350 mm / s, a spot diameter of 0.8 to 2.5 mm, and laser energy attenuating along the electrode thickness gradient. Alternatively, the working conditions parameters of the gradient heat treatment include: using single-sided heating on the current collector side to form a temperature gradient in the thickness direction, with the current collector side temperature at 220–350°C and the surface temperature at 60–120°C, a heating rate of 1.5–4°C / min, and gradient holding in an inert or vacuum atmosphere for 45–150 min.

[0013] Thirdly, the present invention provides an application of the graphene-sulfide composite cathode described in the first aspect or the graphene-sulfide composite cathode obtained by the preparation method of the graphene-sulfide composite cathode described in the second aspect in the preparation of solid-state batteries.

[0014] Fourthly, the present invention provides an all-solid-state battery, including the application of the graphene-sulfide composite cathode described in the first aspect or the graphene-sulfide composite cathode obtained by the preparation method of the graphene-sulfide composite cathode described in the second aspect in the preparation of solid-state batteries.

[0015] The technical solutions provided in the embodiments of the present invention have at least the following advantages compared with the prior art: This invention provides a graphene-sulfide composite cathode, its preparation method, and its application in all-solid-state batteries. The composite cathode provided by this invention has an anisotropic gradient conductive structure. The conductive phase graphene in this structure exhibits a spatial gradient distribution of reduction degree: high-reduction graphene near the current collector provides a high-speed electron channel; low-reduction graphene oxide near the electrolyte layer acts as an electron barrier layer. This eliminates the risk of through-circuit short circuits caused by excessive graphene without sacrificing electrode thickness, thereby improving the electrical performance such as initial charge / discharge specific capacity and overcoming the shortcomings of existing technologies. Specifically: (1) Significantly improved safety: The low conductivity layer near the electrolyte side acts as an “electron gate”, effectively cutting off the penetration path formed by the conductive agent and reducing the probability of micro short circuit.

[0016] (2) Widening of the electrochemical window: Low-reduction graphene on the surface reduces the catalytic activity of sulfide electrolytes and inhibits electrolyte decomposition under high pressure.

[0017] (3) Thick electrode dynamics optimization: The high conductivity of the bottom layer network ensures that electrons can be rapidly conducted from the active material to the current collector even when the thickness increases, reducing polarization resistance. Attached Figure Description

[0018] Figure 1 Comparison of the first charge-discharge specific capacity of the composite cathodes prepared in Example 1 and Comparative Example 1 under the test condition of 0.05C.

[0019] Figure 2 The following is a schematic diagram of the composite positive electrode with an anisotropic gradient conductive structure provided in the embodiments of the present invention; wherein, from left to right, there are current collectors, graphene with different reduction degrees (high-low), and sulfide electrolytes. Detailed Implementation

[0020] The present invention will be specifically described below through embodiments. It should be noted that these embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of the present invention. Those skilled in the art can make some non-essential improvements and adjustments based on the above description of the present invention.

[0021] Unless otherwise specified, all raw materials, reagents, instruments, and equipment used in this invention can be purchased commercially or prepared using existing methods. Furthermore, unless otherwise specified or detailed, the steps and parameters involved can be performed according to existing processing techniques or using existing equipment; these will not be elaborated upon in detail in this invention document.

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0023] Example 1 This example provides a graphene-sulfide composite cathode, such as... Figure 2 As shown, it includes a positive electrode active material, a solid electrolyte, and a conductive agent, wherein the solid electrolyte includes a sulfide, and the conductive agent includes graphene; Along the electrode thickness direction, the degree of reduction of the graphene decreases continuously or stepwise: the graphene near the current collector side is highly reduced graphene with a C / O ratio of 20:1 to 100:1, and the graphene near the solid electrolyte layer side is low-reduction graphene or graphene oxide with a C / O ratio of 2:1 to 8:1. The sulfides fill the interlayer gaps of the graphene sheets, and the volume fraction of the sulfides gradually decreases with increasing depth. This is because the graphene near the current collector has a higher reduction degree, while the proportion of graphene oxide increases near the solid electrolyte. Specifically, high-reduction graphene has a large number of oxygen-containing functional groups removed, resulting in tighter sheet stacking, smaller interlayer pore size, and a denser structure. Low-reduction graphene, with a higher graphene oxide content, is rich in polar oxygen-containing groups such as hydroxyl and epoxy groups on its surface, leading to significant electrostatic repulsion and hydrogen bonding between sheets, resulting in larger interlayer gaps, more developed pores, and stronger pore connectivity. Higher porosity allows for a higher electrolyte capacity, thus exhibiting a volume fraction gradient.

[0024] The preparation method of the above graphene-sulfide composite cathode includes the following steps: (1) Preparation of components: Weigh NCM811 positive electrode active material, sulfide electrolyte Li6PS5Cl, graphene oxide (GO, with a C / O ratio of about 3:1 in the original state), graphene oxide with a C / O ratio of 50:1 (graphene oxide with a C / O ratio of about 3:1 in the original state is ultrasonically dispersed in anhydrous toluene, and pretreated with reducing agent hydrazine to make its C / O ratio reach 50:1), and binder (specifically polyvinylidene fluoride-hexafluoropropylene copolymer).

[0025] (2) Preparation of the bottom layer slurry: 0.5 g of graphene oxide with a C / O ratio of 50:1, 80 g of NCM811 positive electrode active material, 18.0 g of sulfide electrolyte Li6PS5Cl, 2.2 g of binder, and solvent (specifically anhydrous N-methylpyrrolidone) are mixed to adjust the solid content of the slurry to 50% to obtain the bottom layer slurry.

[0026] (3) Preparation of top layer slurry: 1.0 g of graphene oxide with a C / O ratio of about 3:1 in its original state, 75.0 g of NCM811 positive electrode active material, 20.0 g of sulfide electrolyte Li6PS5Cl, 2.0 g of binder, and solvent (specifically anhydrous N-methylpyrrolidone) are mixed and the solid content of the slurry is adjusted to 50% to obtain the top layer slurry.

[0027] (4) Step-by-step coating: First, a 50 μm base coat is applied to the aluminum foil. dry Then a 100 μm top layer of slurry is applied, bringing the total thickness to 150 μm.

[0028] (5) Press molding: In an argon atmosphere, a flat press is used to cold press at a pressure of 300 MPa to fully bond the two layers together and form a graphene oxide composite layer.

[0029] Example 2 This example provides a graphene-sulfide composite cathode, which differs from Example 1 in that the initial C / O ratio of graphene oxide added to the bottom slurry reaches 100:1.

[0030] Example 3 This example provides a graphene-sulfide composite cathode, which differs from Example 1 in that the initial C / O ratio of graphene oxide added to the bottom slurry reaches 80:1.

[0031] Example 4 This example provides a graphene-sulfide composite cathode, which differs from Example 1 in that the initial C / O ratio of graphene oxide added to the bottom slurry reaches 25:1.

[0032] Example 5 This example provides a graphene-sulfide composite cathode, which differs from Example 1 in that the initial C / O ratio of graphene oxide added to the bottom slurry reaches 10:1.

[0033] Example 6 This example provides a graphene-sulfide composite cathode, which differs from Example 1 in that: Step (5) The pressing and molding process is adjusted to use single-sided laser scanning. Its working conditions include: single-sided directional laser scanning is performed on the current collector side under an inert atmosphere, with a laser power of 15 W, a scanning rate of 280 mm / s, a spot diameter of 1.5 mm, and laser energy attenuating along the electrode thickness gradient.

[0034] Example 7 This example provides a graphene-sulfide composite cathode, which differs from Example 1 in that: Step (5) The pressing and molding process is adjusted to use gradient heat treatment. The working conditions include: using single-sided heating on the current collector side to form a temperature gradient in the thickness direction, current collector side temperature 300℃, surface temperature 80℃, heating rate 2℃ / min, and gradient heat preservation for 90 min in an inert or vacuum atmosphere.

[0035] Comparative Example 1 A graphene-sulfide composite cathode differs from Example 1 in that it uses a mixed slurry containing graphene oxide, sulfide, NCM811, and binder in the original state with a C / O ratio of approximately 3:1, in the same total proportion as in Example 1, and is prepared by conventional physical rolling method to produce a graphene-sulfide composite cathode with uniform composition and the same thickness as in Example 1.

[0036] Comparative Example 2 A graphene-sulfide composite cathode differs from Example 1 in that the graphene oxide in the bottom slurry is adjusted to the original state with a C / O ratio of approximately 3:1, and the graphene oxide in the top slurry is adjusted to a C / O ratio of 50:1.

[0037] Test case In this example, the composite cathodes obtained from the above embodiments and comparative examples were assembled into an all-solid-state mold battery (the negative electrode used a 150 μm thick lithium-indium alloy, the electrolyte layer used a 500 μm thick Li6PS5Cl, and encapsulated under pressure of 0.5~1.0 MPa using a sealing machine) and their electrical performance was tested. The test results are shown in Table 1. Furthermore, the first charge-discharge specific capacity of the composite cathodes prepared in Example 1 and Comparative Example 1 under 0.05C test conditions is compared as follows: Figure 1 As shown.

[0038] Table 1. Comparison of initial charge-discharge specific capacity (0.05C) between each embodiment and the comparative example. As shown in Table 1: Within the range of 20:1 to 100:1 C / O ratio of the underlying graphene protected in this paper, the assembled battery exhibits excellent initial coulombic efficiency. However, at a ratio of 10:1 (Example 5), the initial efficiency shows a significant decline, confirming the effectiveness of the scope of protection of this invention. At the same time, through comparison of cold pressing methods, it can be seen that the cold pressing method of this invention has certain advantages and can improve the initial efficiency of the battery to a certain extent. Comparing Examples 6 and 7 with Examples 1-5, it can be found that the single-sided laser scanning and gradient heat treatment in this invention can further improve the initial efficiency of the battery. This indicates that laser energy and temperature have a positive impact on the uniformity of the distribution gradient of graphene and electrolyte on the positive electrode side. In Comparative Example 2, the degree of graphene reduction was reversed, and the initial efficiency dropped sharply. Obviously, the excessive content of sulfide solid electrolyte on the current collector side leads to a reduction in the electron transport pathway at the interface and a significant increase in interface impedance, which in turn affects the operating efficiency of the battery.

[0039] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A graphene-sulfide composite cathode, characterized in that, It includes a positive electrode active material, a solid electrolyte, and a conductive agent, wherein the solid electrolyte includes a sulfide, and the conductive agent includes graphene; Along the thickness direction of the electrode, the degree of reduction of the graphene decreases continuously or in a stepwise manner.

2. The graphene-sulfide composite cathode according to claim 1, characterized in that, The graphene near the current collector side is highly reduced graphene, and the C / O ratio of the highly reduced graphene is 20:1 to 100:

1.

3. The graphene-sulfide composite cathode according to claim 1, characterized in that, The graphene near the solid electrolyte layer is low-reduction graphene or graphene oxide, and the C / O ratio of the low-reduction graphene or graphene oxide is 2:1 to 8:

1.

4. The graphene-sulfide composite cathode according to claim 1, characterized in that, The sulfide fills the interlayer gaps of the graphene, and the volume fraction of the sulfide gradually decreases with increasing depth.

5. The graphene-sulfide composite cathode according to claim 1, characterized in that, The mass fraction of graphene in the graphene-sulfide composite cathode is 0.5% to 3.0%; And / or, the thickness of the graphene-sulfide composite cathode is 100μm~300μm.

6. The graphene-sulfide composite cathode according to claim 1, characterized in that, The positive electrode active material includes a high-nickel positive electrode active material, and the high-nickel positive electrode active material includes at least one of NCM811 and NCM622; And / or, the sulfide includes a silver-germanium sulfide electrolyte, which includes Li3PS4, Li 10 GeP2S 12 At least one of Li6PS5Cl.

7. A method for preparing the graphene-sulfide composite cathode according to any one of claims 1 to 6, characterized in that, Includes the following steps: A mixed slurry is prepared using graphene oxide, sulfide particles, and positive electrode active material; By using a stepwise coating method or an electrophoretic deposition method, the mixed slurry is layered on a current collector to construct composite layers with different graphene oxide contents. By employing single-sided laser scanning or gradient thermal treatment technology, the energy is controlled to decrease from the current collector side to the electrolyte side, allowing the graphene oxide on the current collector side to undergo sufficient thermal reduction and transformation, while the surface layer retains a high oxygen content, thus obtaining the graphene-sulfide composite cathode. Alternatively, graphene oxide with different C / O ratios can be mixed with sulfide particles and positive electrode active materials to prepare slurries, thereby obtaining graphene oxide mixed slurries with different C / O ratios. By using a stepwise coating method or an electrophoretic deposition method, graphene oxide mixed slurries with different C / O ratios are layer by layer constructed on a current collector to form graphene oxide composite layers with different C / O ratios. Subsequently, the layers are pressed and molded to obtain the graphene-sulfide composite cathode.

8. The method for preparing the graphene-sulfide composite cathode according to claim 7, characterized in that, The operating conditions parameters for the single-sided laser scanning include: single-sided directional laser scanning is performed on the current collector side under inert or vacuum atmosphere, with a laser power of 8 to 22 W, a scanning rate of 80 to 350 mm / s, a spot diameter of 0.8 to 2.5 mm, and laser energy attenuating along the electrode thickness gradient. Alternatively, the working conditions parameters of the gradient heat treatment include: using single-sided heating on the current collector side to form a temperature gradient in the thickness direction, with the current collector side temperature at 220-350℃ and the surface temperature at 60-120℃, the heating rate at 1.5-4℃ / min, and gradient holding in an inert or vacuum atmosphere for 45-150 min. Alternatively, the working conditions parameters for the compression molding include: a pressure of 250~350 MPa.

9. The application of a graphene-sulfide composite cathode according to any one of claims 1 to 6, or a graphene-sulfide composite cathode obtained by the preparation method of any one of claims 7 to 8, in the preparation of solid-state batteries.

10. An all-solid-state battery, characterized in that, The graphene-sulfide composite cathode according to any one of claims 1 to 6 or the graphene-sulfide composite cathode obtained by the preparation method of any one of claims 7 to 8 is included.

Images