A composite positive electrode for an all-solid-state lithium-sulfur battery and a preparation method thereof

Abstract

The present invention provides a composite positive electrode for an all-solid-state lithium-sulfur battery and a method for preparing the same, belonging to the technical field of positive electrode materials for lithium-sulfur batteries. The composite positive electrode comprises a sulfur / carbon composite material with a gradient surface modification of phosphorus polysulfide and a solid electrolyte. The composite material comprises porous carbon, sulfur, and phosphorus polysulfide, with the sulfur loaded on the outer surface and / or in the pores of the porous carbon, and a phosphorus polysulfide layer with a gradient distribution of a specific structure on the surface of the sulfur / carbon composite. The present invention utilizes the sulfur / carbon composite material with a gradient surface modification of phosphorus polysulfide as a positive electrode material for an all-solid-state lithium-sulfur battery, effectively increasing the active sulfur content and specific capacity of the composite positive electrode layer. The composite positive electrode of the present invention can have an active sulfur content exceeding 50 wt%, a specific capacity calculated based on elemental sulfur exceeding 1300 mAh / g, and a specific capacity based on the total mass of the composite positive electrode exceeding 650 mAh / g. Furthermore, the composite positive electrode exhibits excellent cycle and rate performance.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-sulfur battery positive electrode materials, and in particular to a composite positive electrode of an all-solid-state lithium-sulfur battery and a preparation method thereof. Background Art

[0002] To address energy and environmental challenges, the development of new energy sources has risen to a strategic level in national development. Lithium-ion batteries are widely used in various fields. Energy density and safety are particularly challenging for lithium-ion battery development, particularly in electric vehicles and grid energy storage. However, current lithium-ion batteries based on intercalation-type cathodes and anodes (such as NCM811 cathodes and graphite anodes) have an energy density of approximately 280 Wh / kg, approaching their theoretical limit. To further increase energy density, replacing low-capacity intercalation-type materials with high-capacity conversion-type electrode materials is an important approach. Among these, lithium-sulfur batteries (LiS) are one of the most competitive options. These batteries, relying on the conversion reaction between sulfur and lithium, offer a theoretical capacity of 1675 mAh / g and a theoretical energy density of 2600 Wh / kg. Sulfur, with its abundant reserves, environmental friendliness, and low cost, is considered a promising next-generation high-energy battery technology. However, since sulfur and its reduction product lithium sulfide are both ionic / electronic insulators, the reaction kinetics are slow, and the nucleophilic lithium polysulfide intermediates are easy to attack and destroy the carbonate-based electrolyte of commercial lithium-ion batteries. Therefore, traditional liquid lithium-sulfur batteries have to use ether-based electrolytes and use the dissolution-precipitation process of lithium polysulfide intermediates to improve the reaction kinetics of the sulfur positive electrode, but this also brings about the "shuttle effect". The "shuttle effect" refers to the long-chain lithium polysulfide intermediates (Li2S x , 4≤x≤8) dissolve in the ether-based electrolyte, then migrate through the separator to the negative electrode and react with the active lithium, leading to rapid loss of active sulfur, low coulombic efficiency, high self-discharge rate, and poor long-term cycling stability. Therefore, liquid electrolytes and sulfur cathodes present difficult compatibility issues.

[0003] Replacing liquid organic electrolytes with solid-state inorganic electrolytes can physically eliminate polysulfide dissolution, avoid the shuttle effect, and reduce the safety risks associated with volatiles and flammable solvents in liquid electrolyte batteries. This holds promise for the construction of high-energy-density, highly safe all-solid-state lithium-sulfur batteries. Among various solid-state electrolytes, sulfide electrolytes are the most widely used in all-solid-state lithium-sulfur batteries, boasting ionic conductivity comparable to or even higher than that of liquid electrolytes and excellent interfacial wettability with electrode active materials. Sulfide solid electrolytes also exhibit excellent interfacial compatibility with elemental sulfur and its reduction product, lithium sulfide. While using sulfide solid electrolytes effectively avoids the shuttle effect, all-solid-state lithium-sulfur batteries suffer from inherently sluggish pure solid-phase kinetics and high solid-solid interfacial impedance. Furthermore, solid electrolytes lack fluidity, requiring the addition of an ionic conductor (solid electrolyte) and an electronic conductor (conductive carbon) comparable in volume to the active material in the composite sulfur cathode. These three components must be mixed at the nanoscale to form an effective three-phase conductive interface: active material / electronic conductor / ionic conductor. Therefore, the content of active material sulfur or lithium sulfide in the positive electrode composite is usually less than 30 wt%, and the actual specific capacity calculated based on the positive electrode composite is less than 500 mAh / g, which greatly sacrifices the high specific capacity and high specific energy characteristics of the sulfur positive electrode.

[0004] Patent CN115763787A discloses an all-solid-state lithium-sulfur cathode material with a core-shell structure, whose core layer is lithium sulfide and whose shell layer is a lithium-containing transition metal sulfide. This material can effectively reduce the loss of active material caused by volume changes in lithium sulfide. Because the shell material is a transition metal sulfide, it has a certain conductivity, which can improve the battery's cycle performance and rate capability. However, the coating process of this method is too cumbersome and not suitable for industrial production. The lithium ion conductivity of the shell is also moderate, which prevents the full utilization of active sulfur, resulting in limited improvement in battery performance. Patent CN104701542B discloses a conductive polymer / sulfur / carbon composite cathode material. This composite cathode material is obtained by in-situ polymerization of conductive polymer monomers to form a corresponding conductive polymer coated on the surface of an elemental sulfur / carbon material mixture, followed by high-temperature treatment. This composite cathode material can effectively improve the ionic and electronic conductivity of sulfur. At the same time, the polymer coating and winding of the sulfur / carbon material can increase the utilization rate of active sulfur in the positive electrode, improving cycle performance and rate capability. However, this method is complex and time-consuming, and the large amount of organic reagents used can cause environmental pollution. Furthermore, its performance improvement is limited. Patent CN111799459A discloses a method for preparing a sulfur composite cathode. This composite cathode material is obtained by ball-milling conductive carbon and a sulfide solid electrolyte to produce a mixed conductor powder. The sulfur element is then evaporated at high temperature and uniformly deposited within the mixed conductor powder, followed by cooling to room temperature. This sulfur composite cathode material has a small sulfur particle size, a large specific surface area, and close contact with the mixed conductor powder, which can increase the utilization of active sulfur in the cathode and improve cycle performance. However, the active sulfur content of the sulfur composite cathode obtained by this method is only 15% to 30%, resulting in an electrode-based specific capacity of less than 400 mAh / g. This results in a low actual energy density for the battery, making it difficult to meet commercial requirements. Patent CN106784690A discloses a sulfur / conductive carbon / solid electrolyte composite cathode material. This composite cathode material is formed by reacting amorphous elemental sulfur with thiamine, depositing sulfur particles on the surface of a conductive carbon material to form a carbon-sulfur composite material. The carbon-sulfur composite material, sulfide solid electrolyte, and conductive carbon material are then ball-milled together. This composite cathode material has the advantages of high-rate discharge specific capacity, stable cycling performance, and high safety performance. However, the active material content in this composite cathode material is still relatively low, with sulfur accounting for no more than 40% by mass of the composite cathode material, resulting in a low actual energy density for the battery.

[0005] Therefore, how to effectively improve the sulfur cathode reaction kinetics and sulfur utilization while maintaining a high content of active substances (sulfur or lithium sulfide) in the positive electrode while having good cycle stability and rate performance is the key to the practical application of all-solid-state lithium-sulfur batteries. Summary of the Invention

[0006] In view of the above shortcomings of the prior art, one of the objectives of the present invention is to provide a composite positive electrode for an all-solid-state sulfur-lithium battery. The present invention uses a sulfur / carbon composite material with a gradient surface modification of phosphorus polysulfide as the positive electrode material of an all-solid-state lithium-sulfur battery, which can ensure a high sulfur content of active material while having good cycle stability and rate performance.

[0007] To achieve the above purpose, the specific technical solutions of the present invention are as follows:

[0008] A composite positive electrode for an all-solid-state sulfur-lithium battery, comprising a sulfur / carbon composite material with a phosphorus polysulfide surface gradient modification and a solid electrolyte; the sulfur / carbon composite material with a phosphorus polysulfide surface gradient modification comprises porous carbon, sulfur, and phosphorus polysulfide, wherein the sulfur is loaded on the outer surface and / or in the pores of the porous carbon, and the porous carbon surface has a gradient-distributed phosphorus polysulfide layer.

[0009] Preferably, the method for preparing the sulfur / carbon composite material with gradient surface modification of phosphorus polysulfide comprises the following steps:

[0010] S1. Sulfur is loaded on a porous carbon matrix to obtain a sulfur / carbon composite material;

[0011] S2. The sulfur / carbon composite material prepared in step S1 was ball-milled with red phosphorus powder to obtain a uniform mixture of sulfur / carbon and red phosphorus;

[0012] S3. The uniform mixture of sulfur / carbon and red phosphorus prepared in step S2 is heated at 112-180°C to allow elemental sulfur and red phosphorus near the sulfur / carbon surface to undergo an interfacial reaction, thereby obtaining a sulfur / carbon composite material with a gradient surface modification of phosphorus polysulfide.

[0013] The present invention provides a sulfur / carbon composite material with a gradient-modified phosphorus polysulfide surface in a composite positive electrode, comprising porous carbon, sulfur, and phosphorus polysulfide, wherein the sulfur is loaded on the outer surface and / or in the pores of the porous carbon, and the surface layer has a gradient-distributed phosphorus polysulfide layer. The present invention heat-treats the sulfur / carbon composite material and phosphorus to cause an interfacial reaction between the sulfur / carbon composite material and phosphorus, thereby gradiently distributing phosphorus polysulfide P4S with a specific structure on and near the S / C surface. n (n = 10, 16, 22 ...) layers, the structure of the polysulfide phosphorus includes chain sulfur-PS x-P- and double-bonded sulfur S=P. During the first charge process, the phosphorus polysulfide with this structure can be fully and irreversibly lithiated into ion-conducting Li3PS4. Since Li3PS4 exists on the surface of the sulfur / carbon particles and extends to the interior to form a gradient distribution, it can significantly improve the effective contact between sulfur and the solid electrolyte and provide an ion path for lithium ions to diffuse into the interior of the sulfur / carbon material, thereby ensuring that the positive electrode containing high electrode active material (elemental sulfur content> 50 wt%) can be cycled stably at a high rate. When the sulfur / carbon composite material with gradient surface modification of phosphorus polysulfide is used in an all-solid-state sulfur-lithium battery, it can form a good interface contact with the solid electrolyte, effectively reducing the interface resistance and making the battery exhibit good overall performance.

[0014] Preferably, the mass ratio of sulfur, porous carbon and red phosphorus is (70-80): (20-30): (6-20).

[0015] Preferably, the porous carbon is one or more of Ketjen black, activated carbon, porous carbon fiber, and mesoporous carbon.

[0016] Further preferably, the porous carbon is Ketjen black.

[0017] Preferably, the specific operation of the heat treatment is: placing the uniform mixture of sulfur / carbon and red phosphorus obtained by ball milling in a closed container and heating for 12 hours, the heating temperature being higher than the melting point of elemental sulfur (112°C) and lower than the vaporization temperature of elemental sulfur (180°C).

[0018] More preferably, the temperature of the heating treatment is 160°C.

[0019] Preferably, the mass ratio of the sulfur / carbon composite material with gradient surface modification of phosphorus polysulfide to the solid electrolyte is (1.1-2.4):1.

[0020] Preferably, the solid electrolyte is a sulfide solid electrolyte.

[0021] Preferably, the sulfide solid electrolyte is Li6PS5Cl, Li 5.4 PS 4.4 Cl 1.6 He Li 5.3 PSCl 4.3 Br 0.7 One or more of .

[0022] Preferably, the sulfur content of the active material in the composite positive electrode is 35.0 wt% to 52.5 wt%.

[0023] The present invention also provides a method for preparing the composite positive electrode of the all-solid-state sulfur-lithium battery, the method comprising the steps of: thoroughly mixing the sulfur / carbon composite material with gradient surface modification of phosphorus polysulfide and a solid electrolyte to obtain the composite positive electrode of the all-solid-state sulfur-lithium battery.

[0024] Compared with the prior art, the present invention is beneficial in that:

[0025] 1. The composite cathode provided by the present invention can improve interfacial contact and ion conduction. The active material sulfur content in the composite cathode can be higher than 50 wt%, and the specific capacity calculated based on elemental sulfur can exceed 1300 mAh / g, and the specific capacity based on the total mass of the composite cathode can exceed 650 mAh / g.

[0026] 2. The composite positive electrode of the present invention and the lithium-indium hard alloy negative electrode are made into an all-solid-state battery, and the composite positive electrode thereof performs excellently in terms of cycle stability and rate performance.

[0027] 3. The raw materials of the composite positive electrode provided by the present invention are cheap and readily available, the cost is low, and the preparation process is simple and convenient, and is easy to mass produce. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] For the convenience of description, porous carbon is referred to as C; sulfur / carbon composite material is referred to as S / C; phosphorus polysulfide is referred to as P4S n ; The sulfur / carbon composite material with gradient surface modification of phosphorus polysulfide is referred to as S / C@P.

[0029] Figure 1 Schematic diagram of the synthesis mechanism of S / C@P at the particle level;

[0030] Figure 2 is the XPS spectrum of S / C@P; Figure 2 a is the XPS spectra of S / C and S / C@P; Figure 2 b is the XPS spectra of P and S / C@P; Figure 2 c is the S 2p XPS spectrum of S / C@P during the first discharge process;

[0031] Figure 3 is the electrochemical performance test result of the composite positive electrode; among them, Figure 3 a is the first cycle charge and discharge curves of the composite cathode S / C-Li6PS5Cl, S / C@P(4%)-Li6PS5Cl and S / C@P(10%)-Li6PS5Cl; Figure 3 b is the rate performance test diagram of the composite cathode S / C-Li6PS5Cl, S / C@P(4%)-Li6PS5Cl and S / C@P(10%)-Li6PS5Cl; Figure 3c is the CV test curve of the composite cathode S / C@P(4%)-Li6PS5Cl; Figure 3 d is the sulfur loading of 1 mg / cm 2 , the long-term cycling performance curves of the composite cathodes S / C-Li6PS5Cl, S / C@P(4%)-Li6PS5Cl and S / C@P(10%)-Li6PS5Cl at a rate of 0.15C and a temperature of 25 ℃; Figure 3 e is the sulfur loading of 1 mg / cm 2 , long-term cycling performance curve of the composite cathode S / C@P(4%)-Li6PS5Cl at a rate of 0.5 C and a temperature of 60 ℃;

[0032] Figure 4 The electrochemical performance test results of the composite positive electrode; Figure 4 a is the composite cathode S / C-Li 5.3 PS 4.3 ClBr 0.7 , S / C@P(4.1%)-Li6PS5Cl, S / C@P(4.1%)-Li 5.4 PS 4.4 Cl 1.6 and S / C@P(4.1%)- Li 5.3 PS 4.3 ClBr 0.7 The first cycle charge and discharge curve; Figure 4 b is the composite cathode S / C@P(4.1%)-Li 5.3 PSCl 4.3 Br 0.7 Rate performance test chart; Figure 4 c is the composite cathode S / C@P(4.1%)-Li 5.4 PS 4.4 Cl 1.6 Rate performance test chart; Figure 4 d is the composite cathode S / C-Li 5.3 PS 4.3 ClBr 0.7 , S / C@P(4.1%)-Li6PS5Cl, S / C@P(4.1%)-Li 5.4 PS 4.4 Cl 1.6 、S / C@P(4.1%)-Li 5.3 PS 4.3 ClBr 0.7 and S / C / P(4.1%)-Li 5.3 PS 4.3 ClBr 0.7 Long-term cycling performance diagram. DETAILED DESCRIPTION

[0033] The technical solutions of the present invention are described clearly and completely below. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. All other embodiments obtained by persons of ordinary skill in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0034] The present invention provides a composite positive electrode for an all-solid-state sulfur-lithium battery, comprising a sulfur / carbon composite material with a phosphorus polysulfide surface gradient modification and a solid electrolyte; the sulfur / carbon composite material with a phosphorus polysulfide surface gradient modification comprises porous carbon, sulfur, and phosphorus polysulfide, wherein the sulfur is loaded on the outer surface and / or in the pores of the porous carbon, and the porous carbon surface layer has a gradient-distributed phosphorus polysulfide layer.

[0035] The inventive concept of the present invention is to solve the key problem of sulfur and its reduction product lithium sulfide in the sulfur positive electrode of all-solid-state lithium-sulfur batteries, which is that sulfur and its reduction product lithium sulfide are electronic / ionic insulators and have difficulty forming effective carrier channels with the solid electrolyte, resulting in low sulfur utilization and sluggish reaction kinetics when the positive electrode contains a high proportion of sulfur as the electrode active material. By heating the sulfur / carbon composite material and phosphorus, the sulfur / carbon composite material and phosphorus undergo an interfacial reaction, generating a gradient-distributed phosphorus polysulfide P4S with a specific structure on and near the surface of the sulfur / carbon composite material. n (n = 10, 16, 22...) layers are prepared to obtain S / C@P, which is then combined with sulfide electrolyte as the positive electrode of all-solid-state lithium-sulfur battery. After assembling into a battery, during the first cycle of charge and discharge, P4S n With Li + It reacts fully and is irreversibly converted into the Li3PS4 layer, thereby reducing the interface resistance and providing an ion path for lithium ions to conduct into the interior of the sulfur / carbon material, greatly improving the interface reaction kinetics process, thereby improving the cycle life, energy density and rate performance of the all-solid-state battery.

[0036] The present invention can be realized by Figure 1 More intuitive expression, Figure 1 The preparation process of S / C@P and its working principle as a cathode material are shown. From the figure, we can see that P generates a gradient distribution of P4S on the surface of S / C and near the surface. n , and in the charge and discharge cycle process with Li + The reaction generates Li3PS4 ion conductors, which reduces the interface resistance and provides an ion path for lithium ions to conduct into the interior of the sulfur / carbon material, effectively improving the sulfur utilization rate and kinetic performance. nThe reaction depth of the layer is an important parameter affecting its performance as a positive electrode material. After a large number of experimental studies, the inventors found that the mass ratio of S / C to P, heating temperature and time, etc. will have a significant impact on the structure of phosphorus polysulfide, and thus affect the performance of S / C@P.

[0037] The preparation method of the sulfur / carbon composite material with gradient surface modification of phosphorus polysulfide comprises the following steps:

[0038] S1. Sulfur is loaded on a porous carbon matrix to obtain a sulfur / carbon composite material;

[0039] S2. The sulfur / carbon composite material prepared in step S1 was ball-milled with red phosphorus powder to obtain a uniform mixture of sulfur / carbon and red phosphorus;

[0040] S3. The uniform mixture of sulfur / carbon and red phosphorus prepared in step S2 is heated at 112-180°C to allow elemental sulfur and red phosphorus near the sulfur / carbon surface to undergo an interfacial reaction, thereby obtaining a sulfur / carbon composite material with a gradient surface modification of phosphorus polysulfide.

[0041] In some examples, in the sulfur / carbon composite material with gradient surface modification of phosphorus polysulfide, the mass ratio of sulfur, porous carbon, and phosphorus is (70~80): (20~30): (6~20).

[0042] In some examples, the porous carbon is one or more of Ketjen black, activated carbon, porous carbon fiber, and mesoporous carbon;

[0043] In some examples, the mass ratio of the sulfur / carbon composite material with gradient surface modification of phosphorus polysulfide to the solid electrolyte is (1.1~2.4):1.

[0044] The solid electrolyte is a sulfide solid electrolyte.

[0045] In some examples, the sulfide solid electrolyte is Li6PS5Cl, Li 5.4 PS 4.4 Cl 1.6 He Li 5.3 PSCl 4.3 Br 0.7 One or more of .

[0046] In some examples, the active material sulfur content in the composite positive electrode is 35.0 wt% to 52.5 wt%.

[0047] In the following specific embodiment, S / C is prepared using the conventional sulfur vapor deposition method: sulfur powder and porous carbon are mixed in a predetermined mass ratio, then heated in an inert atmosphere to vaporize the sulfur powder. After cooling, sulfur vapor is deposited on the outer and inner surfaces of the porous carbon to form the S / C composite. The specific operation of the heat treatment is as follows: the uniform mixture of sulfur / carbon and red phosphorus obtained by ball milling is placed in a sealed container and heated for 12 hours at a temperature above the melting point of elemental sulfur (112°C) and below the vaporization temperature of elemental sulfur (180°C).

[0048] Example 1

[0049] A composite positive electrode for an all-solid-state lithium sulfur battery, the preparation steps are as follows:

[0050] (1) Sulfur powder and Ketjen black were weighed in a mass ratio of 70:30, sealed in a glass container under an inert atmosphere, and then placed in a muffle furnace and heated to 300 °C at a heating rate of 5 °C / min and kept at this temperature for 12 h. After cooling to room temperature, the container was opened to obtain S / C;

[0051] (2) In an inert atmosphere glove box, weigh S / C and red phosphorus in a mass ratio of 100:8 into a ball mill jar, seal it, and ball mill to obtain a uniform mixture of S / C and P;

[0052] (3) The homogeneous mixture of S / C and P was placed in a sealed container, heated to 160 °C, and kept warm for 12 h to undergo interfacial reaction to obtain S / C@P.

[0053] (4) The S / C@P and Li6PS5Cl obtained above were placed in a ball mill at a mass ratio of 108:92. After sealing, the mixture was ball milled to obtain a composite positive electrode for an all-solid-state sulfur-lithium battery, which was labeled as S / C@P(4%)-Li6PS5Cl. The red phosphorus content in the composite positive electrode was 4 wt%, and the active material S content was 35 wt%.

[0054] Example 2

[0055] A composite positive electrode for an all-solid-state lithium sulfur battery, the preparation steps are as follows:

[0056] (1) Sulfur powder and Ketjen black were weighed in a mass ratio of 70:30, sealed in a glass container under an inert atmosphere, and then placed in a muffle furnace and heated to 300 °C at a heating rate of 5 °C / min and kept at this temperature for 12 h. After cooling to room temperature, the container was opened to obtain S / C;

[0057] (2) In an inert atmosphere glove box, weigh S / C and red phosphorus in a mass ratio of 100:20 in a ball mill jar, seal it, and then ball mill to obtain a uniform mixture of S / C and P;

[0058] (3) The homogeneous mixture of S / C and P was placed in a sealed container, heated to 160 °C, and kept warm for 12 h to undergo interfacial reaction to obtain S / C@P.

[0059] (4) The S / C@P and Li6PS5Cl obtained above were placed in a ball mill at a mass ratio of 120:80. After sealing, the mixture was ball milled to obtain a composite positive electrode for an all-solid-state sulfur-lithium battery, which was labeled as S / C@P(10%)-Li6PS5Cl. The red phosphorus content in the composite positive electrode was 10 wt%, and the active material S content was 35 wt%.

[0060] Example 3

[0061] A composite positive electrode for an all-solid-state lithium sulfur battery, the preparation steps are as follows:

[0062] (1) Sulfur powder and Ketjen black were weighed in a mass ratio of 80:20, sealed in a glass container under an inert atmosphere, and then placed in a muffle furnace and heated to 300 °C at a heating rate of 5 °C / min and kept at this temperature for 12 h. After cooling to room temperature, the container was opened to obtain S / C;

[0063] (2) In an inert atmosphere glove box, weigh S / C and red phosphorus in a mass ratio of 100:6.2 in a ball mill jar, seal it, and then ball mill to obtain a uniform mixture of S / C and P;

[0064] (3) The homogeneous mixture of S / C and P was placed in a sealed container, heated to 160 °C, and kept warm for 12 h to undergo interfacial reaction to obtain S / C@P.

[0065] (4) Combine the S / C@P obtained above with Li 5.3 PS 4.3 ClBr 0.7 The composite cathode of the all-solid-state sulfur-lithium battery was obtained by ball milling according to the mass ratio of 106.2: 46.28, and then sealed and ball milled to obtain the composite cathode of the all-solid-state sulfur-lithium battery, which was marked as S / C@P(4.1%)-Li 5.3 PS 4.3 ClBr 0.7 The red phosphorus incorporation in the composite positive electrode is 4.1 wt%, and the active material S content is 52.5 wt%.

[0066] Example 4

[0067] A composite positive electrode for an all-solid-state lithium sulfur battery, the preparation steps are as follows:

[0068] (1) Sulfur powder and Ketjen black were weighed in a mass ratio of 80:20, sealed in a glass container under an inert atmosphere, and then placed in a muffle furnace and heated to 300 °C at a heating rate of 5 °C / min and kept at this temperature for 12 h. After cooling to room temperature, the container was opened to obtain S / C;

[0069] (2) In an inert atmosphere glove box, weigh the S / C composite and red phosphorus in a ball milling jar at a mass ratio of 100:7.4, seal the jar, and then ball mill to obtain a uniform mixture of S / C and P;

[0070] (3) The homogeneous mixture of S / C and P was placed in a sealed container, heated to 160 °C, and kept warm for 12 h to undergo interfacial reaction to obtain S / C@P.

[0071] (4) Combine the S / C@P obtained above with Li 5.3 PS 4.3 ClBr 0.7 The composite cathode of the all-solid-state sulfur-lithium battery was obtained by ball milling according to the mass ratio of 107.4: 46.28, sealed, and marked as S / C@P(4.8%)-Li 5.3 PS 4.3 ClBr 0.7 The red phosphorus incorporation in the composite positive electrode is 4.8 wt%, and the active material S content is 52.1 wt%.

[0072] Example 5

[0073] A composite positive electrode for an all-solid-state lithium sulfur battery, the preparation steps are as follows:

[0074] (1) Sulfur powder and Ketjen black were weighed in a mass ratio of 80:20, sealed in a glass container under an inert atmosphere, and then placed in a muffle furnace and heated to 300 °C at a heating rate of 5 °C / min and kept at this temperature for 12 h. After cooling to room temperature, the container was opened to obtain S / C;

[0075] (2) In an inert atmosphere glove box, weigh the S / C composite and red phosphorus in a ball milling jar at a mass ratio of 100:6.2, seal the jar, and then ball mill to obtain a uniform mixture of S / C and P;

[0076] (3) The homogeneous mixture of S / C and P was placed in a sealed container, heated to 160 °C, and kept warm for 12 h to undergo interfacial reaction to obtain S / C@P.

[0077] (4) Combine the S / C@P obtained above with Li 5.4 PS 4.4 Cl 1.6 The composite cathode of the all-solid-state sulfur-lithium battery was obtained by ball milling according to the mass ratio of 106.2: 46.28, and then sealed and ball milled to obtain the composite cathode of the all-solid-state sulfur-lithium battery, which was marked as S / C@P(4.1%)-Li 5.4 PS 4.4 Cl 1.6The red phosphorus incorporation in the composite positive electrode is 4.1 wt%, and the active material S content is 52.5 wt%.

[0078] Example 6

[0079] A composite positive electrode for an all-solid-state lithium sulfur battery, the preparation steps are as follows:

[0080] (1) Sulfur powder and Ketjen black were weighed in a mass ratio of 80:20, sealed in a glass container under an inert atmosphere, and then placed in a muffle furnace and heated to 300 °C at a heating rate of 5 °C / min and kept at this temperature for 12 h. After cooling to room temperature, the container was opened to obtain S / C;

[0081] (2) In an inert atmosphere glove box, weigh the S / C composite and red phosphorus in a ball milling jar at a mass ratio of 100:6.2, seal the jar, and then ball mill to obtain a uniform mixture of S / C and P;

[0082] (3) The homogeneous mixture of S / C and P was placed in a sealed container, heated to 160 °C, and kept warm for 12 h to undergo interfacial reaction to obtain S / C@P.

[0083] (4) The S / C@P and Li6PS5Cl obtained above were placed in a ball mill at a mass ratio of 106.2:46.28. After sealing, the composite positive electrode of the all-solid-state sulfur-lithium battery was obtained by ball milling, which was marked as S / C@P(4.1%)-Li6PS5Cl. The red phosphorus content in the composite positive electrode was 4.1 wt%, and the active material S content was 52.5 wt%.

[0084] Comparative Example 1

[0085] A composite positive electrode for an all-solid-state lithium sulfur battery, the preparation steps are as follows:

[0086] (1) Sulfur powder and Ketjen black were weighed in a mass ratio of 70:30, sealed in a glass container under an inert atmosphere, and then placed in a muffle furnace and heated to 300 °C at a heating rate of 5 °C / min and kept at this temperature for 12 h. After cooling to room temperature, the container was opened to obtain S / C;

[0087] (2) The S / C and Li6PS5Cl obtained above were placed in a ball mill at a mass ratio of 100:100, sealed, and ball milled to obtain a composite positive electrode for an all-solid-state sulfur-lithium battery, labeled as S / C-Li6PS5Cl. The red phosphorus content in the composite positive electrode was 0 wt%, and the active material S content was 35 wt%.

[0088] Comparative Example 2

[0089] A composite positive electrode for an all-solid-state lithium sulfur battery, the preparation steps are as follows:

[0090] (1) Sulfur powder and Ketjen black were weighed in a mass ratio of 80:20, sealed in a glass container under an inert atmosphere, and then placed in a muffle furnace and heated to 300 °C at a heating rate of 5 °C / min and kept at this temperature for 12 h. After cooling to room temperature, the container was opened to obtain S / C;

[0091] (2) The S / C obtained above is mixed with Li 5.3 PS 4.3 ClBr 0.7 The composite cathode of the all-solid-state sulfur-lithium battery was obtained by ball milling according to a mass ratio of 100:52.4, sealed, and marked as S / C-Li. 5.3 PS 4.3 ClBr 0.7 The red phosphorus incorporation in the composite positive electrode is 0 wt%, and the active material S content is 52.5 wt%.

[0092] Comparative Example 3

[0093] A composite positive electrode for an all-solid-state lithium sulfur battery, the preparation steps are as follows:

[0094] (1) Sulfur powder and Ketjen black were weighed in a mass ratio of 80:20, sealed in a glass container under an inert atmosphere, and then placed in a muffle furnace and heated to 300 °C at a heating rate of 5 °C / min and kept at this temperature for 12 h. After cooling to room temperature, the container was opened to obtain S / C;

[0095] (2) In an inert atmosphere glove box, S / C and red phosphorus were weighed in a ball mill at a mass ratio of 100:6.2, and a uniform mixture of S / C and P was obtained by ball milling, which was labeled as S / C / P.

[0096] (3) Combine the S / C / P obtained above with Li 5.3 PS 4.3 ClBr 0.7 The composite cathode of the all-solid-state sulfur-lithium battery was obtained by ball milling according to the mass ratio of 106.2: 46.28, which was sealed and marked as S / C / P (4.1%)-Li 5.3 PS 4.3 ClBr 0.7 The red phosphorus incorporation in the composite positive electrode is 4.1 wt%, and the active material S content is 52.5 wt%.

[0097] The S / C@P prepared in Example 1 was characterized and tested. The results of S 2p XPS tests on S / C and S / C@P are shown in the figure. Figure 2 a; the results of P 2p XPS test on P and S / C@P are shown in Figure 2 As shown in b, it can be seen from the figure that obvious P4S can be observed in the spectrum of S / C@P.n Typical chemical bonds (chain sulfur-PS x -P- and double bond sulfur S=P), indicating that P forms P4S on the surface and near the surface of S / C n (n = 10, 16, 22...). The results of semi-in-situ XPS tests of the S / C@P cathode after discharging to different potentials are shown in the figure below. Figure 2 As shown in c, by comparing with the spectrum of commercially available Li3PS4, it is confirmed that P4S n During the first cycle of charge and discharge, it will be fully and irreversibly converted into the solid electrolyte Li3PS4.

[0098] The composite positive electrode of the present invention and the lithium-indium hard alloy negative electrode are made into an all-solid-state battery, and its electrochemical performance is tested. Figure 3 The electrochemical performance test results of the composite cathode of the present invention are shown, and the test objects are: composite cathode S / C-Li6PS5Cl (Comparative Example 1), S / C@P(4%)-Li6PS5Cl (Example 1) and S / C@P(10%)-Li6PS5Cl (Example 2). From the first cycle charge and discharge curve (see Figure 3 As can be seen from a), S / C@P(4%)-Li6PS5Cl has an extremely high first cycle charge capacity of nearly 1700 mAh / g (specific capacity calculation is based on the active material S, the same below). At the same time, its rate performance was tested (see Figure 3 b) As can be seen from the figure, compared with S / C-Li6PS5Cl and S / C@P(10%)-Li6PS5Cl, S / C@P(4%)-Li6PS5Cl shows better rate performance. The three are more obviously different in terms of cycle stability (see Figure 3 d) The sulfur loading on the positive electrode is 1 mg / cm 2 Under the conditions of 25 ℃ and low current (0.15 C), S / C@P(4%)-Li6PS5Cl also showed the best cycle stability, with a capacity retention rate of nearly 100% after 400 cycles. In addition, when the positive electrode sulfur loading was 1 mg / cm 2 and 60 ℃, the S / C@P(4%)-Li6PS5Cl cathode can stably cycle for more than 600 cycles under high current conditions (2 C) (see Figure 3 e).

[0099] Figure 4 The electrochemical performance results of composite cathodes made of different sulfide solid electrolytes. The test objects are: composite cathode S / C-Li 5.3 PS 4.3 ClBr 0.7 (Comparative Example 2), S / C@P(4.1%)-Li6PS5Cl (Example 6), S / C@P(4.1%)-Li5.4 PS 4.4 Cl 1.6 (Example 5), S / C@P(4.1%)-Li 5.3 PS 4.3 ClBr 0.7 (Example 3) and S / C / P (4.1%)-Li 5.3 PS 4.3 ClBr 0.7 (Comparative Example 3). From the first cycle charge and discharge curve, we can see (see Figure 4 a), S / C@P(4.1%)-Li 5.3 PS 4.3 ClBr 0.7 It has the best first cycle charge capacity of nearly 1600 mAh / g. At the same time, it is tested for rate performance (see Figure 4 b and 4c), it can be seen that S / C@P(4.1%)- Li 5.3 PS 4.3 ClBr 0.7 and S / C@P(4.1%)-Li 5.4 PS 4.4 Cl 1.6 In terms of cycle stability, under the condition of high active material content (elemental sulfur content in the positive electrode> 50wt%), S / C@P(4.1%)-Li 5.4 PS 4.4 Cl 1.6 and S / C@P(4.1%)-Li 5.3 PS 4.3 ClBr 0.7 It also showed the best cycle stability; in contrast, the positive electrode S / C-Li without red phosphorus doping 5.3 PS 4.3 ClBr 0.7 (Comparative Example 2), or the positive electrode S / C / P(4.1%)-Li doped with red phosphorus but not heated for interface reaction 5.3 PS 4.3 ClBr 0.7 (Comparative Example 3), all showed large polarization, low specific capacity and unstable cycle performance.

[0100] In summary, the present invention heat-treats the sulfur / carbon composite material and phosphorus, causing an interfacial reaction between the sulfur / carbon composite and phosphorus. This results in the formation of a gradient-distributed phosphorus polysulfide layer with a specific structure on and near the S / C surface. This phosphorus polysulfide is fully and irreversibly lithiated to ion-conductive Li3PS4 during the initial charge process. Because the Li3PS4 is present on the surface of the sulfur / carbon particles and extends into the interior to form a gradient distribution, it significantly improves the effective contact between the sulfur and the solid electrolyte and provides an ion path for lithium ions to diffuse into the sulfur / carbon material, thereby ensuring high-rate and stable cycling of positive electrodes containing high electrode active material (elemental sulfur content > 50 wt%).

[0101] The above specific embodiments describe the implementation of the present invention in detail, but the present invention is not limited to the specific details of the above embodiments. Within the scope of the claims and technical concept of the present invention, various simple modifications and changes can be made to the technical solution of the present invention, and these simple modifications all fall within the scope of protection of the present invention.

Claims

1. A composite positive electrode for an all-solid-state lithium sulfur battery, characterized in that: The composite positive electrode comprises a sulfur / carbon composite material with a gradient surface modification of phosphorus polysulfide and a solid electrolyte; the sulfur / carbon composite material with a gradient surface modification of phosphorus polysulfide comprises porous carbon, sulfur, and phosphorus polysulfide, wherein the sulfur is loaded on the outer surface and / or in the pores of the porous carbon, and the porous carbon surface layer has a gradient distributed phosphorus polysulfide layer; The preparation method of the sulfur / carbon composite material with gradient surface modification of phosphorus polysulfide comprises the following steps: S1. Sulfur is loaded on a porous carbon matrix to obtain a sulfur / carbon composite material; S2. The sulfur / carbon composite material prepared in step S1 was ball-milled with red phosphorus powder to obtain a uniform mixture of sulfur / carbon and red phosphorus; S3. The homogeneous mixture of sulfur / carbon and red phosphorus prepared in step S2 is heated at 112-180°C to allow elemental sulfur and red phosphorus to react at the interface near the sulfur / carbon surface to obtain a sulfur / carbon composite material with a gradient surface modification of phosphorus polysulfide; The mass ratio of the sulfur, porous carbon and red phosphorus is (70-80): (20-30): (6-20).

2. The composite positive electrode of an all-solid-state sulfur-lithium battery according to claim 1, characterized in that: The porous carbon is one or more of Ketjen black, activated carbon, porous carbon fiber, and mesoporous carbon.

3. The composite positive electrode of an all-solid-state sulfur-lithium battery according to claim 1, characterized in that: The specific operation of the heat treatment is: placing the uniform mixture of sulfur / carbon and red phosphorus obtained by ball milling in a closed container and heating it for 12 hours. The heating temperature is higher than the melting point of elemental sulfur (112°C) and lower than the vaporization temperature of elemental sulfur (180°C).

4. The composite positive electrode of an all-solid-state sulfur-lithium battery according to claim 1, characterized in that: The mass ratio of the sulfur / carbon composite material with gradient surface modification of phosphorus polysulfide to the solid electrolyte is (1.1-2.4):

1.

5. The composite positive electrode of an all-solid-state sulfur-lithium battery according to claim 1, characterized in that: The solid electrolyte is a sulfide solid electrolyte.

6. The composite positive electrode of an all-solid-state sulfur-lithium battery according to claim 5, characterized in that: The sulfide solid electrolyte is one or more of Li6PS5Cl, Li5.4PS4.4Cl1.6 and Li5.3PSCl4.3Br0.

7.

7. The composite positive electrode of an all-solid-state sulfur-lithium battery according to claim 1, characterized in that: The active material sulfur content in the composite positive electrode is 35.0 wt% to 52.5 wt%.

8. The method for preparing a composite positive electrode for an all-solid-state lithium sulfur battery according to any one of claims 1 to 7, wherein: The steps include: fully mixing the sulfur / carbon composite material with gradient surface modification of phosphorus polysulfide and the solid electrolyte to obtain a composite positive electrode of an all-solid-state sulfur-lithium battery.

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