Gradient composite positive electrode structure, all-solid-state lithium ion battery and preparation method

Abstract

The invention provides a gradient composite positive electrode structure, an all-solid-state lithium ion battery and a preparation method. The gradient composite positive electrode structure comprises a high ion conductivity layer and a high electron conductivity layer which are tightly attached together, and the high ion conductivity layer is made of a first positive electrode active material and a first solid electrolyte. The material of the high electron conductivity layer is composed of a second solid electrolyte, a second positive electrode active material and a conductive additive, and the ionic conductivity of the first solid electrolyte is greater than that of the second solid electrolyte. By synchronously regulating and controlling ion and electron transfer kinetics, the problem of electrochemical heterogeneity of the electrode scale is solved, and the energy density, cycle life and rate capability of the battery are improved.

Description

Technical Field

[0001] The present invention relates to the field of new energy materials and energy storage technology, and in particular to a gradient composite positive electrode structure, an all-solid-state lithium-ion battery, and a preparation method thereof. Background Art

[0002] The single-layer composite cathode (CC) of a traditional all-solid-state battery usually includes a positive electrode active material, a solid electrolyte and a conductive additive. The solid electrolyte is used to assist in the transmission of lithium ions, and the conductive additive is used to transmit electrons. Due to the use of conductive additives, the electronic conductivity of the traditional positive electrode is significantly higher than the ionic conductivity (about three orders of magnitude difference), resulting in electrochemical reactions occurring preferentially on the diaphragm side (faster ion transmission), forming a longitudinal lithium concentration gradient, causing insufficient utilization of the positive electrode active material on the current collector side, local overcharge / undercharge and chemical mechanical degradation, and charge transfer imbalance, which in turn affects the capacity and cycle durability of the solid-state battery. In addition, the existing solid-state positive electrode preparation technology only focuses on optimizing the electron transmission path (such as adding conductive carbon), and cannot simultaneously balance the ion / electron transfer, resulting in rapid capacity decay of thick positive electrodes (>100μm) (such as capacity retention <60% after 2000 cycles) and poor rate performance (5C capacity <15mAh g -1 ). Therefore, the traditional single-layer composite cathode cannot meet the requirements of electric vehicles and grid energy storage for solid-state batteries with high energy density, long cycle life and fast charge and discharge.

[0003] It should be noted that the information disclosed in the above background technology section is only used to understand the background of this application, and therefore may include information that does not constitute prior art known to ordinary technicians in this field. Summary of the Invention

[0004] The present invention provides a gradient composite positive electrode structure, an all-solid-state lithium-ion battery, and a preparation method.

[0005] The present invention adopts the following technical solutions:

[0006] In a first aspect, a gradient composite positive electrode structure is provided, which includes a high ionic conductivity layer and a high electronic conductivity layer tightly bonded together, wherein the material of the high ionic conductivity layer is composed of a first positive electrode active material and a first solid electrolyte, and the material of the high electronic conductivity layer is composed of a second solid electrolyte, a second positive electrode active material and a conductive additive, and the ionic conductivity of the first solid electrolyte is greater than the ionic conductivity of the second solid electrolyte.

[0007] Preferably, in the material of the high ionic conductivity layer, the mass fraction of the first positive electrode active material is 40% to 90%, and the balance is the first solid electrolyte; in the material of the high electronic conductivity layer, the mass fraction of the second positive electrode active material is 40% to 90%, the mass fraction of the conductive additive is 1-10%, and the balance is the second solid electrolyte.

[0008] Preferably, the ionic conductivity of the first solid electrolyte is at least 1.5 times the ionic conductivity of the second solid electrolyte.

[0009] Preferably, the particle size of the first solid electrolyte is larger than the particle size of the second solid electrolyte, so that the ionic conductivity of the first solid electrolyte is greater than the ionic conductivity of the second solid electrolyte.

[0010] Preferably, the first solid electrolyte is Li 5.5 PS 4.5 Cl 1.5 The second solid electrolyte is Li with a D50 of 600 nm. 5.5 PS 4.5 Cl 1.5 .

[0011] Preferably, the first solid electrolyte and the second solid electrolyte are each independently one of a lithium phosphorus sulfur chlorine solid electrolyte, a lithium germanium phosphorus sulfur solid electrolyte, and a halide solid electrolyte; the first positive electrode active material and the second positive electrode active material are each independently one of a nickel cobalt manganese ternary material, lithium cobalt oxide, lithium iron phosphate, lithium manganese iron phosphate, and a lithium-rich manganese-based material; the conductive additive is one of conductive carbon black, conductive carbon fiber, conductive graphite, graphene, acetylene black, and conductive carbon tube.

[0012] In a second aspect, a method for preparing the gradient composite positive electrode structure according to the first aspect is provided, comprising the following steps:

[0013] (1) uniformly mixing powders of materials for the high ionic conductivity layer and pressing them into the high ionic conductivity layer using a first pressure;

[0014] (2) mixing powders of materials for the high electronic conductivity layer uniformly and spreading the powders uniformly on the high ionic conductivity layer, and pressing the high electronic conductivity layer using a second pressure, so that the high ionic conductivity layer and the high electronic conductivity layer are tightly attached together;

[0015] Wherein, the first pressure is lower than the second pressure.

[0016] Preferably, the first pressure is 125 MPa and the second pressure is 375 MPa.

[0017] In a third aspect, an all-solid-state lithium-ion battery is provided, comprising a positive electrode current collector, a solid electrolyte membrane, the gradient composite positive electrode structure described in the first aspect, a negative electrode, and a negative electrode current collector, wherein the high ionic conductivity layer in the gradient composite positive electrode structure is adhered to one side of the solid electrolyte membrane, the high electronic conductivity layer in the gradient composite positive electrode structure is adhered to the positive electrode current collector, and the negative electrode is adhered between the other side of the solid electrolyte membrane and the negative electrode current collector.

[0018] In a fourth aspect, a method for preparing an all-solid-state lithium-ion battery is provided, comprising the following steps:

[0019] 1) Pressing a predetermined amount of a third solid electrolyte into a film at a predetermined pressure to produce a solid electrolyte membrane;

[0020] 2) uniformly mixing powders of materials for the high ionic conductivity layer, evenly spreading the powders on one side of the solid electrolyte membrane, and pressing the powders to form the high ionic conductivity layer using a first pressure;

[0021] 3) mixing powders of materials for the high electronic conductivity layer uniformly and spreading the powders uniformly on the high ionic conductivity layer, and pressing the high electronic conductivity layer using a second pressure so that the high ionic conductivity layer and the high electronic conductivity layer are tightly attached together; wherein the first pressure is less than the second pressure;

[0022] 4) Laminating a positive electrode current collector on the high electron conductivity layer, and placing a negative electrode between the other side of the solid electrolyte separator and the negative electrode current collector, and pressing to form an all-solid-state lithium-ion battery.

[0023] The present invention has the following beneficial effects: the gradient composite positive electrode structure of the present invention includes a high ionic conductivity layer and a high electronic conductivity layer that are tightly attached together, the high ionic conductivity layer does not contain a conductive additive, and the ionic conductivity of the solid electrolyte contained therein is greater than the ionic conductivity of the solid electrolyte in the high electronic conductivity layer (containing a conductive additive). Through the hierarchical structure of the two layers, the electrode ion transport is regulated, the ion transport rate is optimized, and at the same time, the electrode electron transport is regulated by whether or not the conductive additive is added, thereby optimizing the charge transfer at the electrode scale, making the electrochemical reaction at the electrode scale more uniform, improving the utilization rate of the active material and slowing down the chemical mechanical degradation. Therefore, the present invention solves the problem of electrochemical heterogeneity at the electrode scale by synchronously regulating the ion and electron transfer kinetics, and improves the battery energy density, cycle life and rate performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1a Schematic diagram of the all-solid-state lithium-ion battery in Example 1 of the present invention.

[0025] Figure 1b Schematic diagram of the all-solid-state lithium-ion battery in Comparative Example 1 of the present invention.

[0026] Figure 2a Li with a D50 of 5 μm in the embodiment of the present invention 5.5 PS 4.5 Cl 1.5 SEM image of .

[0027] Figure 2b and 2c In the embodiment of the present invention, D50 is 600nm Li 5.5 PS 4.5 Cl 1.5 SEM images at different magnifications.

[0028] Figure 2d This is an SEM image of the gradient composite positive electrode structure in Example 1 of the present invention.

[0029] Figure 2e For the present invention Figure 2d TOF-SIMS images of four areas in the image.

[0030] Figure 3 1 is a comparison chart of the battery rate performance of Example 1 of the present invention and Comparative Example 1.

[0031] Figure 4 0.5C cycle performance comparison chart of the batteries of Example 1 of the present invention and Comparative Example 1.

[0032] Figure 5 2C cycle performance comparison chart of the batteries of Example 1 of the present invention and Comparative Example 1.

[0033] Figure 6 2 is a comparison chart of the battery rate performance of Example 2 of the present invention and Comparative Example 2.

[0034] Figure 7 2 is a comparison chart of the 0.5C cycle performance of the batteries of Example 2 of the present invention and Comparative Example 2.

[0035] Figure 8 3 is a comparison chart of the battery rate performance of Example 3 of the present invention and Comparative Example 3.

[0036] Figure 9 3 is a comparison chart of the 0.5C cycle performance of the batteries of Example 3 of the present invention and Comparative Example 3. DETAILED DESCRIPTION

[0037] The following is a detailed description of the embodiments of the present invention. It should be emphasized that the following description is merely illustrative and is not intended to limit the scope and application of the present invention. In the absence of conflict, the embodiments and features in the embodiments of this application can be combined with each other.

[0038] A specific embodiment of the present invention provides a gradient composite positive electrode structure, which includes a high ionic conductivity layer and a high electronic conductivity layer tightly bonded together, wherein the material of the high ionic conductivity layer is composed of a first positive electrode active material and a first solid electrolyte, and the material of the high electronic conductivity layer is composed of a second solid electrolyte, a second positive electrode active material and a conductive additive, and the ionic conductivity of the first solid electrolyte is greater than the ionic conductivity of the second solid electrolyte.

[0039] In some embodiments, in the material of the high ionic conductivity layer, the mass fraction of the first positive electrode active material is 40% to 90%, and the balance is the first solid electrolyte; in the material of the high electronic conductivity layer, the mass fraction of the second positive electrode active material is 40% to 90%, the mass fraction of the conductive additive is 1-10%, and the balance is the second solid electrolyte.

[0040] In some embodiments, the ionic conductivity of the first solid-state electrolyte is at least 1.5 times greater than the ionic conductivity of the second solid-state electrolyte.

[0041] In some embodiments, the particle size of the first solid electrolyte is larger than the particle size of the second solid electrolyte, such that the ionic conductivity of the first solid electrolyte is greater than the ionic conductivity of the second solid electrolyte.

[0042] In some embodiments, the particle size of the first solid electrolyte is in the micrometer scale, and the particle size of the second solid electrolyte is in the nanometer scale.

[0043] In some embodiments, the first solid electrolyte is a material with an ionic conductivity of ≥5 mS / cm. More preferably, the first solid electrolyte is a Li-ion electrolyte with a D50 of 5 μm. 5.5 PS 4.5 Cl 1.5 The second solid electrolyte is Li with a D50 of 600 nm. 5.5 PS 4.5 Cl 1.5 .

[0044] In some embodiments, the first solid electrolyte and the second solid electrolyte are each independently a lithium phosphorus sulfur chlorine solid electrolyte (such as Li 5.5 PS 4.5 Cl 1.5 ), lithium germanium phosphorus sulfur solid electrolyte (such as Li 10 GeP2S 12), one of the halide solid electrolytes (such as Li3InCl6); the first positive electrode active material and the second positive electrode active material are each independently one of nickel cobalt manganese ternary material (such as NCM811, NCM523, etc.), lithium cobalt oxide (LiCoO2), lithium iron phosphate, lithium manganese iron phosphate and lithium-rich manganese-based materials; the conductive additive is one of conductive carbon black (such as brand SuperP, brand C65), conductive carbon fiber (VGCF), conductive graphite, graphene, acetylene black, and conductive carbon tube.

[0045] A specific embodiment of the present invention further provides a method for preparing the gradient composite positive electrode structure, comprising the following steps:

[0046] (1) uniformly mixing powders of materials for the high ionic conductivity layer and pressing them into the high ionic conductivity layer using a first pressure;

[0047] (2) mixing powders of materials for the high electronic conductivity layer uniformly and spreading them uniformly on the high ionic conductivity layer, and applying a second pressure to press the high electronic conductivity layer so that the high ionic conductivity layer and the high electronic conductivity layer are tightly attached together; wherein the first pressure is less than the second pressure.

[0048] The preparation method of the gradient composite positive electrode structure in the specific embodiment of the present invention is compatible with the dry electrode pressing process, does not require the modification of the existing production line, and is conducive to reducing costs.

[0049] In some embodiments, the first pressure is 125 MPa and the second pressure is 375 MPa.

[0050] A specific embodiment of the present invention also provides an all-solid-state lithium-ion battery, comprising a positive electrode current collector, a solid electrolyte separator, the gradient composite positive electrode structure, a negative electrode and a negative electrode current collector, wherein the high ionic conductivity layer in the gradient composite positive electrode structure is attached to one side of the solid electrolyte separator, the high electronic conductivity layer in the gradient composite positive electrode structure is attached to the positive electrode current collector, and the negative electrode is attached between the other side of the solid electrolyte separator and the negative electrode current collector.

[0051] A specific embodiment of the present invention further provides a method for preparing an all-solid-state lithium-ion battery, comprising the following steps:

[0052] 1) Pressing a predetermined amount of a third solid electrolyte into a film at a predetermined pressure to produce a solid electrolyte membrane;

[0053] 2) uniformly mixing powders of materials for the high ionic conductivity layer, evenly spreading the powders on one side of the solid electrolyte membrane, and pressing the powders to form the high ionic conductivity layer using a first pressure;

[0054] 3) mixing powders of materials for the high electronic conductivity layer uniformly and spreading the powders uniformly on the high ionic conductivity layer, and pressing the high electronic conductivity layer using a second pressure so that the high ionic conductivity layer and the high electronic conductivity layer are tightly attached together; wherein the first pressure is less than the second pressure;

[0055] 4) Laminating a positive electrode current collector on the high electron conductivity layer, and placing a negative electrode between the other side of the solid electrolyte separator and the negative electrode current collector, and pressing to form an all-solid-state lithium-ion battery.

[0056] The materials of the first solid electrolyte, the second solid electrolyte and the third solid electrolyte can be the same or different, preferably the same and preferably made of Li 5.5 PS 4.5 Cl 1.5 The first positive electrode active material and the second positive electrode active material may be the same or different, preferably the same and preferably a nickel-cobalt-manganese ternary material, more preferably NCM811, ie LiNi 0.8 Co 0.1 Mn 0.1 The conductive additive is preferably conductive carbon black, more preferably SuperP.

[0057] Specific embodiments of the present invention are further described below.

[0058] Example 1

[0059] The gradient composite cathode structure includes a high ion conductivity layer and a high electronic conductivity layer that are tightly bonded together. The high ion conductivity layer is made of a cathode active material (NCM811) and a first solid electrolyte (Li-ion electrolyte with a D50 of 5 μm) with a mass ratio of 65:35. 5.5 PS 4.5 Cl 1.5 , whose ion conductivity is 7.6mS / cm) and does not contain conductive additives. The material of the high electron conductivity layer is composed of the positive electrode active material (NCM811) with a mass ratio of 65:30:5, the second solid electrolyte (Li 5.5 PS 4.5 Cl 1.5 , whose ionic conductivity is 3.4mS / cm), and conductive carbon (SuperP is used in this example).

[0060] The preparation of an all-solid-state lithium-ion battery using the above gradient composite cathode structure (named CTOC in this example) includes:

[0061] (1) 100 mg of solid electrolyte (Li 5.5 PS 4.5 Cl 1.5) pressing into a film at a uniaxial pressure of 250 MPa to obtain a solid electrolyte membrane;

[0062] (2) 8 mg of the powder of the high ionic conductivity layer material was placed in a mortar and manually ground for 30 min to mix it evenly, and evenly spread on one side of the solid electrolyte separator, and pressed at a pressure of 125 MPa to form a high ionic conductivity layer;

[0063] (3) 8 mg of the powder of the high electron conductivity layer material was placed in a mortar and manually ground for 30 min to mix it evenly, and then evenly spread on the high ionic conductivity layer. The high electron conductivity layer was pressed at a pressure of 375 MPa to form the high ionic conductivity layer and the high electron conductivity layer.

[0064] (4) A positive electrode current collector (stainless steel in this example) is bonded to the high electron conductivity layer, and a negative electrode (lithium metal foil) is placed between the other side of the solid electrolyte separator and the negative electrode current collector (stainless steel in this example), and pressed into an all-solid-state lithium-ion battery.

[0065] like Figure 1a As shown, the all-solid-state lithium-ion battery includes a positive electrode current collector 1, a solid electrolyte separator 3, a gradient composite positive electrode structure 2, a negative electrode 4, and a negative electrode current collector 5. The high ionic conductivity layer in the gradient composite positive electrode structure 2 (the high ionic conductivity layer is composed of a positive electrode active material 24 and a first solid electrolyte 23) is attached to one side of the solid electrolyte separator 3, the high electronic conductivity layer (the high electronic conductivity layer is composed of a positive electrode active material 24, a second solid electrolyte 22, and a conductive carbon 21) is attached to the positive electrode current collector 1, and the negative electrode 4 is attached between the other side of the solid electrolyte separator 3 and the negative electrode current collector 5. In this gradient composite positive electrode structure 2, electron transport is fast, ion transport is also fast, which optimizes charge transfer and solves the problem of charge transfer imbalance.

[0066] like Figure 2a As shown, the D50 is 5μm Li 5.5 PS 4.5 Cl 1.5 SEM images of Figure 2b and 2c As shown, they are Li with D50 of 600nm 5.5 PS 4.5 Cl 1.5 SEM images at different magnifications, such as Figure 2d As shown, it is a SEM image of the gradient composite positive electrode structure 2. The high electron conductivity layer on the positive electrode current collector 1 side is above the dotted line, and the high ion conductivity layer on the solid electrolyte membrane 3 side is below the dotted line. Figure 2e For the gradient composite cathode structure Figure 2dThe TOF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry) images of the four regions shown in FIG. 1 show that in the gradient composite positive electrode structure, the lithium concentration distribution of the positive electrode active material particles in the longitudinal direction is uniform.

[0067] In the all-solid-state lithium-ion battery obtained in Example 1, the surface loading of the positive electrode active material is 13.2 mg cm -2 .

[0068] Comparative Example 1

[0069] The difference from Example 1 is that the positive electrode structure adopts a single-layer composite electrode (16 mg), which is composed of a positive electrode active material (NCM811) with a mass ratio of 65:30:5, a solid electrolyte (Li 5.5 PS 4.5 Cl 1.5 ), and conductive carbon (SuperP), and the same preparation method as in Example 1 was used to prepare an all-solid-state lithium-ion battery comprising the positive electrode structure (named CC) (the surface loading of the positive electrode active material was 13.2 mg cm -2 ).

[0070] like Figure 1b As shown, the all-solid-state lithium-ion battery includes a positive electrode current collector 1, a solid electrolyte separator 3, a single-layer composite electrode (composed of a positive electrode active material 24, a second solid electrolyte 22, and a conductive carbon 21), a negative electrode 4, and a negative electrode current collector 5. The single-layer composite electrode is attached between one side of the solid electrolyte separator 3 and the positive electrode current collector 1, and the negative electrode 4 is attached between the other side of the solid electrolyte separator 3 and the negative electrode current collector 5. In this single-layer composite electrode, electron transport is fast, ion transport is slow, and charge transport is unbalanced, affecting the performance of the solid-state battery.

[0071] Example 2

[0072] The difference from Example 1 is that in the preparation of the all-solid-state lithium-ion battery, the powder of the material of the high ionic conductivity layer in step (2) is 12 mg, and the powder of the material of the high electronic conductivity layer in step (3) is 12 mg. Thus, in the obtained all-solid-state lithium-ion battery, the surface loading of the positive electrode active material is 19.9 mg cm -2 .

[0073] Comparative Example 2

[0074] The difference from Example 2 is that the positive electrode structure adopts a single-layer composite electrode (24 mg), which is composed of a positive electrode active material (NCM811) with a mass ratio of 65:30:5, a solid electrolyte (Li 5.5PS 4.5 Cl 1.5 ), and conductive carbon (SuperP), and the same preparation method as in Example 1 was used to prepare an all-solid-state lithium-ion battery comprising the positive electrode structure (named CC) (the surface loading of the positive electrode active material was 19.9 mg cm -2 ).

[0075] Example 3

[0076] The difference from Example 1 is that in the preparation of the all-solid-state lithium-ion battery, the powder of the material of the high ionic conductivity layer in step (2) is 16 mg, and the powder of the material of the high electronic conductivity layer in step (3) is 16 mg. Thus, in the obtained all-solid-state lithium-ion battery, the surface loading of the positive electrode active material is 26.5 mg cm -2 .

[0077] Comparative Example 3

[0078] The difference from Example 3 is that the positive electrode structure adopts a single-layer composite electrode (32 mg), which is composed of a positive electrode active material (NCM811) with a mass ratio of 65:30:5, a solid electrolyte (Li 5.5 PS 4.5 Cl 1.5 ), and conductive carbon (SuperP), and the same preparation method as in Example 1 was used to prepare an all-solid-state lithium-ion battery comprising the positive electrode structure (named CC) (the surface loading of the positive electrode active material was 26.5 mg cm -2 ).

[0079] Hereinafter, performance tests were performed on the all-solid-state lithium-ion batteries of various embodiments and comparative examples.

[0080] like Figure 3 As shown in the figure, the rate performance comparison of the battery of Example 1 and Comparative Example 1 is shown. It can be seen that the capacity of the battery of Example 1 is significantly improved at 5C (the specific capacity of 5C discharge is 74 mAh g -1 , while Comparative Example 1 is 14 mAh g -1 ).

[0081] like Figure 4 , which is a comparison chart of the 0.5C cycle performance of the batteries of Example 1 and Comparative Example 1. It can be seen that the cycle stability of the battery of Example 1 at 0.5C is significantly improved.

[0082] like Figure 5As shown, it is a comparison chart of the 2C cycle performance of the batteries of Example 1 and Comparative Example 1. It can be seen that the cycle stability of the battery of Example 1 at 2C is significantly improved (the capacity retention rate of the battery of Example 1 after 2000 cycles at 2C rate is 82.7%, while that of the battery of Comparative Example 1 is 55.3%).

[0083] like Figure 6 , which is a comparison chart of the battery rate performance of Example 2 and Comparative Example 2, from which it can be seen that Example 2 has better rate performance.

[0084] like Figure 7 As shown, it is a comparison chart of the 0.5C cycle performance of the batteries of Example 2 and Comparative Example 2. It can be seen that Example 2 has better cycle durability.

[0085] like Figure 8 , which is a comparison chart of the battery rate performance of Example 3 and Comparative Example 3. It can be seen that Example 3 has better rate performance.

[0086] like Figure 9 As shown, it is a comparison chart of the 0.5C cycle performance of the batteries of Example 3 and Comparative Example 3. It can be seen that Example 3 has better cycle durability.

[0087] The above description further details the present invention in conjunction with specific / preferred embodiments, and the specific implementation of the present invention should not be construed as being limited to these descriptions. Persons skilled in the art will appreciate that, without departing from the spirit of the present invention, they may make various substitutions or modifications to the described embodiments, and these substitutions or modifications should be considered to fall within the scope of protection of the present invention. Throughout this specification, reference to terms such as "one embodiment," "some embodiments," "preferred embodiments," "examples," "specific examples," or "some examples" indicates that the specific features, structures, materials, or characteristics described in conjunction with such embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of these terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. Persons skilled in the art may combine and assemble the different embodiments or examples described in this specification, as well as features of different embodiments or examples, without conflicting opinions. Although the embodiments of the present invention and their advantages have been described in detail, it should be understood that various changes, substitutions, and modifications may be made herein without departing from the scope of protection of the patent application.

Claims

1. A gradient composite cathode structure, characterized in that: It includes a high ionic conductivity layer and a high electronic conductivity layer tightly bonded together, wherein the material of the high ionic conductivity layer is composed of a first positive electrode active material and a first solid electrolyte, and the material of the high electronic conductivity layer is composed of a second solid electrolyte, a second positive electrode active material and a conductive additive, and the ionic conductivity of the first solid electrolyte is greater than the ionic conductivity of the second solid electrolyte.

2. The gradient composite positive electrode structure according to claim 1, wherein: In the material of the high ion conductivity layer, the mass fraction of the first positive electrode active material is 40% to 90%, and the balance is the first solid electrolyte; In the material of the high electron conductivity layer, the mass fraction of the second positive electrode active material is 40% to 90%, the mass fraction of the conductive additive is 1-10%, and the balance is the second solid electrolyte.

3. The gradient composite positive electrode structure according to claim 1, wherein: The ionic conductivity of the first solid electrolyte is at least 1.5 times greater than the ionic conductivity of the second solid electrolyte.

4. The gradient composite positive electrode structure according to claim 1, wherein: The particle size of the first solid electrolyte is larger than the particle size of the second solid electrolyte, so that the ionic conductivity of the first solid electrolyte is larger than the ionic conductivity of the second solid electrolyte.

5. The gradient composite positive electrode structure according to claim 4, wherein: The first solid electrolyte is Li with a D50 of 5 μm 5.5 PS 4.5 Cl 1.5 The second solid electrolyte is Li with a D50 of 600 nm. 5.5 PS 4.5 Cl 1.5 .

6. The gradient composite positive electrode structure according to claim 1, wherein: The first solid electrolyte and the second solid electrolyte are each independently one of a lithium phosphorus sulfur chlorine solid electrolyte, a lithium germanium phosphorus sulfur solid electrolyte, and a halide solid electrolyte; the first positive electrode active material and the second positive electrode active material are each independently one of a nickel cobalt manganese ternary material, lithium cobalt oxide, lithium iron phosphate, lithium manganese iron phosphate, and a lithium-rich manganese-based material; the conductive additive is one of conductive carbon black, conductive carbon fiber, conductive graphite, graphene, acetylene black, and conductive carbon tubes.

7. A method for preparing a gradient composite cathode structure according to any one of claims 1 to 6, characterized in that: The steps include: (1) uniformly mixing powders of materials for the high ionic conductivity layer and pressing them into the high ionic conductivity layer using a first pressure; (2) mixing powders of materials for the high electronic conductivity layer uniformly and spreading the powders uniformly on the high ionic conductivity layer, and pressing the high electronic conductivity layer using a second pressure, so that the high ionic conductivity layer and the high electronic conductivity layer are tightly attached together; Wherein, the first pressure is lower than the second pressure.

8. The preparation method according to claim 7, wherein The first pressure is 125 MPa, and the second pressure is 375 MPa.

9. An all-solid-state lithium-ion battery, characterized in that: It includes a positive electrode collector, a solid electrolyte membrane, a gradient composite positive electrode structure according to any one of claims 1 to 6, a negative electrode and a negative electrode collector, wherein the high ionic conductivity layer in the gradient composite positive electrode structure is attached to one side of the solid electrolyte membrane, the high electronic conductivity layer in the gradient composite positive electrode structure is attached to the positive electrode collector, and the negative electrode is attached between the other side of the solid electrolyte membrane and the negative electrode collector.

10. A method for preparing an all-solid-state lithium-ion battery, characterized in that: The steps include: 1) Pressing a predetermined amount of a third solid electrolyte into a film at a predetermined pressure to produce a solid electrolyte membrane; 2) uniformly mixing powders of materials for the high ionic conductivity layer, evenly spreading the powders on one side of the solid electrolyte membrane, and pressing the powders to form the high ionic conductivity layer using a first pressure; 3) mixing powders of materials for the high electronic conductivity layer uniformly and spreading the powders uniformly on the high ionic conductivity layer, and pressing the high electronic conductivity layer using a second pressure so that the high ionic conductivity layer and the high electronic conductivity layer are tightly attached together; wherein the first pressure is less than the second pressure; 4) Laminating a positive electrode current collector on the high electron conductivity layer, and placing a negative electrode between the other side of the solid electrolyte separator and the negative electrode current collector, and pressing to form an all-solid-state lithium-ion battery.

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