Compact sulfide all-solid-state battery composite positive electrode material and preparation method and application thereof

Abstract

The invention belongs to the technical field of all-solid-state lithium batteries, and particularly relates to a densified sulfide all-solid-state battery composite positive electrode material and a preparation method and application thereof. The preparation method of the composite positive electrode material comprises the following steps: (1) raw material pretreatment: mixing a positive electrode active substance, a sulfide solid electrolyte and an additive according to a predetermined mass ratio under the protection of an inert atmosphere, and carrying out mechanical grinding to obtain composite positive electrode powder; the additive is Li3PO4 (lithium phosphate); (2) isostatic pressing pretreatment: performing hot isostatic pressing treatment on the composite positive electrode powder, and then controlling the cooling rate to cool to room temperature to obtain a densified composite positive electrode blank; (3) adding a conductive agent and a binder into the densified composite positive electrode blank, and mixing; the composite positive electrode material is obtained; the binder is formed by compounding nano-scale polytetrafluoroethylene and micron-scale polytetrafluoroethylene according to a mass ratio of 1: 2-1: 4.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of all-solid-state lithium batteries, and relates to a densified sulfide all-solid-state battery dry method composite positive electrode material and a preparation method and application thereof. BACKGROUND

[0002] All-solid-state lithium batteries are considered as the core development direction of the next generation of energy storage technology due to their high safety and energy density advantages. Among them, sulfide solid electrolyte has become the most potential electrolyte system due to its excellent room temperature ionic conductivity. However, sulfide all-solid-state batteries still face the following key challenges in practical application: (1) poor interface chemical stability: there is serious chemical incompatibility between NCM ternary positive electrode active material and sulfide electrolyte, and direct contact between the two will cause interface side reactions, generate high impedance decomposition products, significantly increase interface impedance and accelerate capacity decay; (2) difficulty in densification by dry film forming: the positive electrode sheet prepared by traditional dry method has low compaction density and high porosity, which causes the ion / electron transport path to be tortuous, seriously restricting the rate performance of the battery; (3) insufficient mechanical properties of the electrode sheet: the fiber network structure formed by the conventional PTFE binder has limited strength, and the electrode sheet is prone to cracking and structure damage during battery cycling, which seriously affects the structural integrity of the electrode and the cycle life of the battery; (4) prominent pressure sensitivity: sulfide electrolyte has significant pressure dependence, and its high ionic conductivity can only be maintained under high pressure (>100 MPa), while under the actual battery working pressure (<15 MPa), the interface impedance increases sharply due to the deterioration of the grain boundary contact, and the battery performance decreases significantly.

[0003] At present, although the wet preparation process can obtain a relatively dense electrode structure, the organic solvent will react irreversibly with the sulfide electrolyte; and the traditional dry process is difficult to solve the key problems such as densification, interface stability and low pressure performance at the same time. Therefore, developing a dry method composite positive electrode preparation technology that can realize high density, low porosity and stable interface under low pressure conditions has become a key breakthrough for promoting the industrialization of sulfide all-solid-state batteries. SUMMARY

[0004] In view of this, the application provides a densified sulfide all-solid-state battery dry method composite positive electrode material and a preparation method and application thereof.

[0005] The application firstly fills the gap between the grain boundaries effectively by adding lithium phosphate and ball milling during the mixing of the positive active material and the sulfide electrolyte, thereby avoiding the direct contact between the active material and the electrolyte; then the pre-mixture is treated at high temperature and high pressure by an isostatic pressing device to improve the grain compatibility and form a local molten phase, so that Li3PO4 forms a welding point between the particles; finally, the high-density sulfide all-solid-state battery dry composite positive electrode material is prepared by introducing a double-scale PTFE binder (nanoscale PTFE improves the mechanical strength, and micrometer PTFE improves the film-forming property). The composite positive electrode sheet with excellent mechanical properties and low interface impedance is prepared through the fiberization hot roller pressing process.

[0006] To achieve the above object, the application provides the following technical solutions. In a first aspect, the application provides a preparation method of a high-density sulfide all-solid-state battery dry composite positive electrode material, comprising the following steps: (1) Raw material pretreatment: under the protection of an inert atmosphere, a positive active material, a sulfide solid-state electrolyte and an additive are mixed and mechanically ground in a predetermined mass ratio to obtain a composite positive electrode powder; the additive is Li3PO4; (2) Isostatic pressing pretreatment: the composite positive electrode powder is subjected to hot isostatic pressing treatment, and then the cooling rate is controlled to cool to room temperature to obtain a densified composite positive electrode blank; (3) Adding a conductive agent and a binder to the densified composite positive electrode blank and mixing to obtain a composite positive electrode material; the binder is compounded by nanoscale polytetrafluoroethylene and micrometer polytetrafluoroethylene in a mass ratio of (1:2)-(1:4).

[0007] In step (3), the total amount of the binder added is 3-5 wt% of the total mass of the composite positive electrode material.

[0008] The D50 of the nanoscale PTFE is 50-80 nm, and the D50 of the micrometer PTFE is 40-60 microns.

[0009] In step (1), the mass ratio of the positive active material, the sulfide solid-state electrolyte and the additive is (7-9):(3-1):(0.5-2).

[0010] In step (1), the mechanical grinding is planetary ball milling, the ball-to-material ratio is (0.8-1.2):1, the rotation speed is 300-500 rpm, and the grinding time is 2-4 hours.

[0011] In step (2), the pressure of the hot isostatic pressing treatment is 200-250 MPa, the temperature is 160-180 ℃, and the treatment time is 30-60 minutes.

[0012] In step (2), the cooling rate is 3-10 ℃ / min.

[0013] The conductive agent in step (3) is one or more of conductive graphite, carbon nanotube, conductive carbon fiber, and acetylene conductive carbon black; and the addition amount is 1-3 wt% of the total mass of the composite cathode material.

[0014] The cathode active material is NCM811; and the sulfide solid electrolyte is a Li6PS5Cl type solid electrolyte (D50=3 μm).

[0015] In a second aspect, the application provides a composite cathode material prepared by the preparation method described above.

[0016] In a third aspect, the application provides a composite cathode sheet, wherein the composite cathode material is subjected to cold pressing and fiberization treatment and hot pressing and setting to obtain the cathode sheet; the pressure of the cold pressing and fiberization treatment is 10-15 MPa, and the treatment is carried out at room temperature; the pressure of the hot pressing and setting is 10-20 MPa, the temperature is 80-100 ℃, and the time is 3-8 minutes.

[0017] The thickness of the cathode sheet is 100-200 μm.

[0018] In a fourth aspect, the application provides a sulfide full solid-state battery comprising the cathode sheet described above, wherein the negative electrode active material is one or more of micron silicon, nano silicon, graphite, and lithium-silicon alloy; and the sulfide electrolyte is a Li6PS5Cl type solid electrolyte.

[0019] The application has the following beneficial effects: (1) Lithium phosphate interface modification: by adding lithium phosphate and performing ball milling treatment, a stable interface buffer layer is formed between the NCM active material and the sulfide electrolyte, which can effectively block the direct contact between the two, reduce the interface side reaction, significantly reduce the interface impedance, and improve the cycle stability of the electrode; (2) High-temperature isostatic pressing pretreatment: high-temperature treatment with an isostatic pressing device makes Li3PO4 form a local molten phase, which produces a "welding point" effect between the particles, which not only improves the contact tightness between the particles, but also constructs a continuous lithium ion transmission channel, so that the cathode sheet can still maintain high compaction density and ionic conductivity during subsequent low-pressure forming and use; (3) Dual-scale PTFE binder system: by using a PTFE composite binder system with synergistic nano- and micron-sized particles, the tensile strength of the sheet can be improved while maintaining good flexibility of the sheet, avoiding stress cracking of the sheet during the cycle process, and significantly improving the structural stability and cycle life of the electrode; The application successfully prepares a composite positive electrode sheet with high density, low porosity, high ionic conductivity and excellent mechanical properties through the synergistic effect of lithium phosphate interface modification, high-temperature isostatic pressing pretreatment and a double-scale PTFE binder system, solving the key technical bottleneck in the preparation of sulfide full solid-state battery dry electrodes. The process does not require complex equipment modification and the cost is controllable, providing a practical solution for the industrialization of sulfide full solid-state batteries. DETAILED DESCRIPTION

[0020] The application will be described in detail below with reference to examples. The following examples will help those skilled in the art to further understand the application, but do not limit the application in any form. It should be pointed out that those skilled in the art can make several adjustments and improvements without departing from the concept of the application. These all belong to the protection scope of the application.

[0021] Example 1 A preparation method of a high-density sulfide full solid-state battery dry composite positive electrode sheet, the steps of which are as follows: (1) Raw material pretreatment: In an argon-protected glove box, NCM811 positive active material, Li6PS5Cl sulfide electrolyte and Li3PO4 powder were mixed in a mass ratio of 7:2.5:0.5, placed in a planetary ball mill (ball-to-material ratio of 1:1), and ball milled at a speed of 400 rpm for 3 hours to obtain a uniformly mixed composite positive electrode powder; (2) Isostatic pressing pretreatment: The composite positive electrode powder was loaded into a rubber mold and placed in an isostatic pressing device, and treated at 165℃ and 230MPa for 25 minutes, and then cooled to room temperature at a rate of 5℃ / min; a densified composite positive electrode blank was obtained; (3) Additive of conductive agent and binder: 2wt% of carbon nanotube conductive agent (based on the total mass of the composite positive electrode material) and 4wt% (based on the total mass of the composite positive electrode material) of binder were added to the densified composite positive electrode blank and mixed in a mixer for 30 minutes; a composite positive electrode material was obtained; wherein the binder is a double-scale polytetrafluoroethylene (PTFE) system; which is compounded by nano-PTFE (D50=60nm) and micro-PTFE (D50=50μm) in a mass ratio of 1:3; (4) Fiberization forming: The composite positive electrode material was placed in a double-roller calender, first subjected to fiberization treatment at room temperature and a pressure of 12MPa, and then hot-pressed at 90℃ and a pressure of 15MPa for 5 minutes to obtain a positive electrode sheet with a thickness of 150μm.

[0022] Example 2 A preparation method of a high-density sulfide full solid-state battery dry composite positive electrode sheet, the steps of which are as follows: (1) Raw material pretreatment: In an argon-protected glove box, NCM811 positive active material, Li6PS5Cl sulfide electrolyte and Li3PO4 powder were mixed in a mass ratio of 8:1.5:0.5, placed in a planetary ball mill (ball-to-material ratio: 1:1), and ball-milled at a speed of 350 rpm for 3 hours to obtain a uniformly mixed composite positive electrode powder; (2) Isostatic pressing pretreatment: The composite positive electrode powder was loaded into a rubber mold and placed in an isostatic pressing device for treatment at 170°C and 220 MPa for 25 minutes, and then cooled to room temperature at a rate of 5°C / min; a densified composite positive electrode blank was obtained; (3) Add conductive agent and binder: Add 1.5wt% carbon nanotube conductive agent (based on the total mass of the composite positive electrode material) and 4.5wt% binder (based on the total mass of the composite positive electrode material) to the densified composite positive electrode blank, and mix in a mixer for 30 minutes; obtain a composite positive electrode material; wherein the binder is a double-scale polytetrafluoroethylene (PTFE); wherein the binder is a double-scale polytetrafluoroethylene (PTFE) system; composed of nano-PTFE (D50=60nm) and micro-PTFE (D50=50μm) in a mass ratio of 1:4; (4) Fiberization forming: The composite positive electrode material was placed in a double-roller calender, first subjected to fiberization treatment at room temperature and a pressure of 10 MPa, and then hot-pressed at 95°C and 12 MPa for 5 minutes to obtain a positive electrode sheet with a thickness of 120μm.

[0023] Comparative Example 1 In step 1 of Example 1, the addition of Li3PO4 powder was omitted, and only NCM811 positive active material and Li6PS5Cl sulfide electrolyte were used for ball milling pretreatment, and the remaining steps were the same as Example 1. The impedance test of Comparative Example 1 was 16.8Ω, the ionic conductivity was 0.89mS / cm, the porosity of the electrode sheet was 12.5%, the compaction density was 3.05g / cm 3 , the discharge capacity at 0.2C was 175.6 mAh / g, the initial efficiency was 82.2%, the discharge capacity at 0.5C was 168.3 mAh / g, and the capacity retention rate after 30 cycles was 89.4%; the impedance test of Example 1 was 8.43Ω, the ionic conductivity was 2.87mS / cm, the porosity of the electrode sheet was 6.7%, the compaction density was 3.65g / cm 3, the discharge capacity at 0.2C was 198.4 mAh / g, the initial efficiency was 86.5%, the discharge capacity at 0.5C was 189.6 mAh / g, and the capacity retention rate after 30 cycles was 97.9%; this is because the addition of lithium phosphate is cancelled, there is no stable buffer layer on the surface of the positive active material and the sulfide electrolyte, which leads to an increase in interface impedance, limited ion conduction, and low capacity of the battery. In the process of cycling, side reactions occur, and the capacity decays rapidly.

[0024] Comparative Example 2 In Example 1, the operation of Step 2 was cancelled, and the composite positive electrode material after ball milling without isostatic pressure pretreatment was used, and the remaining parameters were unchanged.

[0025] The impedance test of Comparative Example 2 was 15.7Ω, the ionic conductivity was 1.05mS / cm, the porosity of the electrode sheet was 17.6%, and the compacted density was 2.56g / cm 3 , the discharge capacity at 0.2C was 178.2 mAh / g, the initial efficiency was 83.6%, the discharge capacity at 0.5C was 154.2 mAh / g, and the capacity retention rate after 30 cycles was 91.5%; it can be seen that without using isostatic pressure high temperature and high pressure treatment, the impedance and porosity of the electrode sheet are relatively high, the ionic conductivity and compacted density are relatively low, and the capacity is also lower than that of the example, because the high temperature treatment of isostatic pressure can make Li3PO4 form a local molten phase, and "welding point" effect is generated between the particles, which not only can improve the contact tightness between the particles, but also can build a continuous lithium ion transmission channel.

[0026] Comparative Example 3 The difference between this comparative example and the example is that the binder only includes micron-sized PTFE (D50=50μm); the rest is the same as Example 1.

[0027] The impedance test of Comparative Example 3 was 12.4Ω, the ionic conductivity was 1.74mS / cm, the porosity of the electrode sheet was 16.4%, and the compacted density was 3.12g / cm 3 , the tensile strength was 1.25MPa, the discharge capacity at 0.2C was 180.4 mAh / g, the initial efficiency was 82.4%, the discharge capacity at 0.5C was 172.1 mAh / g, and the capacity retention rate after 30 cycles was 95.4%; it can be seen that without adding nano-sized PTFE, the impedance and porosity of the electrode sheet are relatively high, the ionic conductivity and compacted density are relatively low, and especially the tensile strength is significantly lower than the value 3.42MPa in Example 1; this is because nano-sized PTFE has stronger fiberization ability, and can provide higher strength when fiberizing into a film.

[0028] Comparative Example 4 The difference between the present comparative example and Example 1 is that the binder only includes nano-sized PTFE (D50 = 60 nm).

[0029] The impedance test of Comparative Example 4 is 12.8 Ω, the ionic conductivity is 1.59 mS / cm, the porosity of the electrode sheet is 15.8%, the compacted density is 2.96 g / cm 3 , the tensile strength is 1.53 MPa, the discharge capacity at 0.2 C is 182.3 mAh / g, the first efficiency is 83.7%, the discharge capacity at 0.5 C is 173.5 mAh / g, and the capacity retention rate after 30 cycles is 92.5%; it can be seen that when no micron-sized PTFE is added, the impedance and porosity of the electrode sheet are relatively high, the ionic conductivity and compacted density are relatively low, and especially the capacity retention rate after 30 cycles is significantly lower than the value 97.9% in Example 1; this is because micron-sized PTFE can maintain the flexibility of the electrode sheet during fiberization, so that the electrode sheet can maintain its original morphology during high-pressure roller forming and battery cycling, and is not prone to cracking or falling off.

[0030] Comparative Example 5 The difference between the present comparative example and Example 1 is that the dual-scale polytetrafluoroethylene (PTFE) system is compounded by nano-sized PTFE (particle size range D50 = 60 nm) and micron-sized PTFE (particle size range 50 μm) in a mass ratio of 4:1.

[0031] In Comparative Example 5, due to the high content of nano-sized PTFE, it is not possible to form a continuous film during fiberization and molding, but only scattered sheet-shaped film materials, and even after rolling, a complete electrode sheet cannot be obtained. This is because the smaller the particle size of PTFE, the smaller the length of the fiberized silk, and it is not possible to entangle and bind the positive active material and electrolyte particles to form a continuous electrode sheet.

[0032] Performance test example The performance of each example and comparative example is tested as follows: 1. Ionic conductivity test: cut the positive electrode sheet prepared in each example and comparative example into small round pieces, then load into a pressure cell mold, and punch under 1 ton of pressure, and keep pressure for 1 minute. The electrochemical workstation with specification model CHI660E is used to perform AC impedance test at room temperature under normal atmosphere, and the ionic conductivity is calculated, and the results are shown in Table 1.

[0033] Table 1: Ionic conductivity test results of each example and comparative example .

[0034] 2. Porosity test of the positive electrode tab: The experiment uses a high-performance automatic mercury intrusion porosimeter, model number Micromeritics AutoPore V 9620, with a pressure of about 0.6 to 50 PSI applied in the low pressure station (LP) and a pressure of 20 to 60,000 PSI applied in the high pressure station (HP). A cone penetrometer made of glass is used as the sample container, and the pressure is applied to the sample, which has a size of 1 cm x 1 cm. The porosity of the positive electrode tab is calculated based on the volume of the mercury intrusion into the pores.

[0035] Tensile strength test of the positive electrode tab: A universal tensile testing machine is used, with a sample size of 1 cm x 10 cm. The sample is fixed in the upper and lower clamps of the tensile testing machine, and the sample is stretched at a constant speed of 0.2 mm / s. The maximum tensile force value at the time of sample fracture is recorded.

[0036] The test results of the porosity, tensile strength, and compaction density of the positive electrode tab are shown in Table 2.

[0037] Table 2: Test results of the porosity, tensile strength, and compaction density of the positive electrode tab .

[0038] 2. Full battery performance test: 100 mg of Li6PS5Cl sulfide electrolyte is weighed and placed in a pressure mold, and a pressure of 1 ton is applied for 1 min. Then a 10 mm diameter positive electrode tab disc is placed on one side of the electrolyte, and a 10 mm diameter aluminum foil is placed on the surface of the positive electrode tab as the positive electrode current collector. After assembly, a pressure of 1 ton is applied for 1 min. Then 30 mg of negative electrode active material (micron silicon (D50 = 5 um)) is weighed and laid on the other side of the electrolyte. After assembly, a pressure of 1 ton is applied for 1 min. Finally, a sulfide full solid-state lithium ion battery is obtained, ready for testing. A new battery test system, model number CT-4000, is used to test the full solid-state battery. The full solid-state battery is tested for charge and discharge using a 0.2C-0.5C rate charge and discharge process, with a voltage range of 4.3-2.5V and a temperature of 28°C. The charge and discharge capacity of the sulfide full solid-state battery is tested, and the test results are shown in Table 3.

[0039] Table 3: Test results of the charge and discharge performance of the sulfide full solid-state battery

[0040] The above specific embodiments further illustrate the purpose, technical solutions, and beneficial effects of the present application. It should be understood that these are only examples, and any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application should be included within the protection scope of the present application.

Claims

1. A method for preparing a dense sulfide all-solid-state battery composite cathode material, characterized in that, Includes the following steps: (1) Raw material pretreatment: Under the protection of an inert atmosphere, the positive electrode active material, sulfide solid electrolyte and additives are mixed in a predetermined mass ratio and mechanically ground to obtain composite positive electrode powder; the additive is Li3PO4; (2) Isostatic pressing pretreatment: The composite cathode powder is subjected to hot isostatic pressing treatment, and then cooled to room temperature by controlling the cooling rate to obtain a densified composite cathode blank; (3) Add conductive agent and binder to the densified composite cathode blank and mix them to obtain composite cathode material; the binder is composed of nano-sized polytetrafluoroethylene and micro-sized polytetrafluoroethylene in a mass ratio of 1:2-1:

4.

2. The preparation method according to claim 1, characterized in that, In step (3), the amount of binder added is 3-5 wt% of the total mass of the composite cathode material powder.

3. The preparation method according to claim 1 or 2, characterized in that, The mass ratio of the positive electrode active material, sulfide solid electrolyte, and additives in step (1) is 7-9:3-1:0.5-2.

4. The preparation method according to claim 1, characterized in that, The mechanical grinding described in step (1) is a planetary ball mill with a ball-to-material ratio of 0.8-1.2:1, a rotation speed of 300-500 rpm, and a grinding time of 2-4 hours.

5. The preparation method according to claim 1, characterized in that, The hot isostatic pressing process in step (2) is performed at a pressure of 200-250 MPa, a temperature of 160-180 ℃, and a processing time of 30-60 minutes; the cooling rate is 3-10 ℃ / min.

6. The preparation method according to claim 1, characterized in that, The conductive agent mentioned in step (3) is one or more of conductive graphite, carbon nanotubes, conductive carbon fibers, and acetylene conductive carbon black; its addition amount is 1-3 wt% of the total mass of the composite cathode material.

7. The preparation method according to claim 1, characterized in that, The positive electrode active material is NCM811; the sulfide solid electrolyte is a Li6PS5Cl type solid electrolyte.

8. A composite cathode material prepared by any one of claims 1-7.

9. A composite positive electrode sheet, characterized in that, The composite positive electrode material of claim 8 is subjected to cold pressing fiberization and hot pressing shaping to obtain the positive electrode sheet; wherein the pressure of cold pressing fiberization is 10-15 MPa and is carried out at room temperature; the pressure of hot pressing shaping is 10-20 MPa, the temperature is 80-100 ℃, and the time is 3-8 minutes.

10. A sulfide all-solid-state battery comprising the composite positive electrode sheet of claim 9.

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