Densified sulfide all-solid-state battery composite cathode material and preparation method and application

Abstract

The application 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: under the protection of an inert atmosphere, a positive electrode active substance, a sulfide solid-state electrolyte and an additive are mixed and mechanically ground according to 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) an electrically conductive agent and a binder are added to the densified composite positive electrode blank and mixed to obtain a composite positive electrode material; the binder is compounded by nanoscale polytetrafluoroethylene and micrometer-scale polytetrafluoroethylene at a mass ratio of 1:2-1:4.

Description

Technical Field

[0001] This invention belongs to the field of all-solid-state lithium battery technology, and relates to a dense sulfide all-solid-state battery dry composite cathode material, its preparation method, and its application. Background Technology

[0002] All-solid-state lithium batteries are considered a core development direction for next-generation energy storage technology due to their high safety and energy density advantages. Among them, sulfide solid electrolytes have become the electrolyte system with the greatest industrialization potential due to their excellent room-temperature ionic conductivity. However, sulfide all-solid-state batteries still face the following key challenges in practical applications: (1) Poor interfacial chemical stability: There is serious chemical incompatibility between NCM ternary cathode active material and sulfide electrolyte. Direct contact between the two will trigger interfacial side reactions, generating high-impedance decomposition products, significantly increasing interfacial impedance and accelerating capacity decay; (2) Difficulty in dry film densification: The cathode sheet prepared by traditional dry method has problems such as low compaction density and high porosity, resulting in tortuous ion / electron transport paths, which seriously restricts the rate performance of the battery; (3) Insufficient mechanical properties of the electrode: The fiber network structure formed by conventional PTFE binder has limited strength. During battery cycling, the electrode is prone to cracking and structural damage, 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. Its high ionic conductivity can only be maintained under high pressure (>100 MPa). However, under the actual battery working pressure (<15 MPa), the deterioration of grain boundary contact leads to a sharp increase in interfacial impedance and a significant decrease in battery performance.

[0003] Currently, while wet processing can achieve relatively dense electrode structures, organic solvents can undergo irreversible reactions with sulfide electrolytes. Traditional dry processes, on the other hand, struggle to simultaneously address key issues such as densification, interface stability, and low-pressure performance. Therefore, developing a dry composite cathode fabrication technology capable of achieving high density, low porosity, and a stable interface under low-pressure conditions has become a crucial breakthrough for promoting the industrialization of sulfide-based all-solid-state batteries. Summary of the Invention

[0004] In view of this, the present invention provides a dense sulfide all-solid-state battery dry composite cathode material, its preparation method and application.

[0005] This invention first involves adding lithium phosphate during the mixing of the positive electrode active material and the sulfide electrolyte, followed by ball milling to effectively fill the grain boundary gaps and avoid direct contact between the active material and the electrolyte. Subsequently, the premix is ​​treated under high temperature and pressure using an isostatic pressing device to improve grain compatibility and form a locally molten phase, allowing Li3PO4 to form welding points between particles. Finally, a dual-scale PTFE binder is introduced (nanoscale PTFE enhances mechanical strength, and micron-scale PTFE improves film formation), resulting in a high-density sulfide-based all-solid-state battery dry-process composite positive electrode material. A composite positive electrode sheet with excellent mechanical properties and low interfacial impedance is then prepared using a fiberized hot rolling process.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] In a first aspect, the present invention provides a method for preparing a high-density sulfide all-solid-state battery dry-process composite cathode material, comprising the following steps:

[0008] (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;

[0009] (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;

[0010] (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).

[0011] In step (3), the total amount of binder added is 3-5 wt% of the total mass of the composite cathode material.

[0012] The D50 of nanoscale PTFE is 50-80nm; the D50 of microscale PTFE is 40-60μm.

[0013] 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).

[0014] 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.

[0015] 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.

[0016] The cooling rate mentioned in step (2) is 3-10 °C / min.

[0017] 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.

[0018] The positive electrode active material is NCM811; the sulfide solid electrolyte is a Li6PS5Cl type solid electrolyte (D50=3μm).

[0019] Secondly, the present invention provides a composite cathode material prepared by the preparation method described above.

[0020] Thirdly, the present invention provides a composite positive electrode sheet, wherein the composite positive electrode material is subjected to cold pressing fiberization treatment and hot pressing shaping to obtain the positive electrode sheet; wherein the cold pressing fiberization treatment is carried out at a pressure of 10-15 MPa at room temperature; the hot pressing shaping is carried out at a pressure of 10-20 MPa, a temperature of 80-100 ℃, and a time of 3-8 minutes.

[0021] The thickness of the positive electrode sheet is 100-200 μm.

[0022] Fourthly, the present invention provides a sulfide all-solid-state battery comprising the above-mentioned positive electrode sheet. The negative electrode active material is one or more of micron-sized silicon, nano-sized silicon, graphite, and lithium-silicon alloy; the sulfide electrolyte is a Li6PS5Cl type solid-state electrolyte.

[0023] The beneficial effects of this invention are as follows:

[0024] (1) Lithium phosphate interface modification: By adding lithium phosphate and ball milling, 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 interface side reactions, significantly reduce interface impedance, and improve the cycle stability of the electrode.

[0025] (2) High temperature isostatic pressing pretreatment: The isostatic pressing equipment is used to treat Li3PO4 at high temperature to form a local molten phase, which produces a "welding point" effect between particles. This not only improves the contact tightness between particles, but also builds a continuous lithium ion transport channel, so that the positive electrode can maintain high compaction density and ionic conductivity during subsequent low-pressure molding and use.

[0026] (3) Dual-scale PTFE binder system: By adopting a PTFE composite binder system with synergistic nano- and micro-scale particle sizes, the tensile strength of the electrode can be improved while maintaining its good flexibility, avoiding stress cracking of the electrode during cycling, and significantly improving the structural stability and cycle life of the electrode.

[0027] This invention successfully prepared 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 dual-scale PTFE binder system. This solves a key technical bottleneck in the dry electrode preparation of sulfide all-solid-state batteries. The process requires no complex equipment modification, is cost-controllable, and provides a practical solution for the industrialization of sulfide all-solid-state batteries. Detailed Implementation

[0028] The present invention will be described in detail below with reference to embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several adjustments and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0029] Example 1

[0030] A method for preparing a high-density sulfide all-solid-state battery dry composite positive electrode sheet, the steps of which are as follows:

[0031] (1) Raw material pretreatment: In an argon-protected glove box, NCM811 positive electrode active material, Li6PS5Cl sulfide electrolyte and Li3PO4 powder were mixed at a mass ratio of 7:2.5:0.5 and placed in a planetary ball mill (ball-to-material ratio of 1:1) and ball-milled at 400 rpm for 3 hours to obtain a uniformly mixed composite positive electrode powder.

[0032] (2) Isostatic pressing pretreatment: The above composite cathode powder is loaded into a rubber mold and placed in an isostatic pressing device. It is treated at 165℃ and 230MPa for 25 minutes, and then cooled to room temperature at a rate of 5℃ / min to obtain a densified composite cathode blank.

[0033] (3) Addition of conductive agent and binder: 2 wt% of carbon nanotube conductive agent (based on the total mass of composite cathode material) and 4 wt% (based on the total mass of composite cathode material) binder are added to the densified composite cathode blank and mixed in a mixer for 30 minutes to obtain composite cathode material; wherein the binder is a dual-scale polytetrafluoroethylene (PTFE) system; it is composed of nanoscale PTFE (D50=60nm) and microscale PTFE (D50=50μm) in a mass ratio of 1:3;

[0034] (4) Fiberization molding: The composite positive electrode material is placed in a twin-roll calender and first fiberized at room temperature and 12MPa pressure. Then it is hot-pressed at 90℃ and 15MPa for 5 minutes to obtain a positive electrode sheet with a thickness of 150μm.

[0035] Example 2

[0036] A method for preparing a high-density sulfide all-solid-state battery dry composite positive electrode sheet, the steps of which are as follows:

[0037] (1) Raw material pretreatment: In an argon-protected glove box, NCM811 positive electrode active material, Li6PS5Cl sulfide electrolyte and Li3PO4 powder were mixed at a mass ratio of 8:1.5:0.5 and placed in a planetary ball mill (ball-to-material ratio: 1:1) and ball-milled at 350 rpm for 3 hours to obtain a uniformly mixed composite positive electrode powder;

[0038] (2) Isostatic pressing pretreatment: The composite cathode powder is loaded into a rubber mold and placed in an isostatic pressing device. It is treated at 170℃ and 220MPa for 25 minutes, and then cooled to room temperature at a rate of 5℃ / min to obtain a densified composite cathode blank.

[0039] (3) Addition of conductive agent and binder: 1.5 wt% of carbon nanotube conductive agent (based on the total mass of composite cathode material) and 4.5 wt% of binder (based on the total mass of composite cathode material) are added to the densified composite cathode blank, and mixed in a mixer for 30 minutes to obtain composite cathode material; wherein the binder is dual-scale polytetrafluoroethylene (PTFE); wherein the binder is a dual-scale polytetrafluoroethylene (PTFE) system; composed of nanoscale PTFE (D50=60nm) and microscale PTFE (D50=50μm); compounded at a mass ratio of 1:4;

[0040] (4) Fiberization molding: The composite positive electrode material is placed in a twin-roll calender and first fiberized at room temperature and 10MPa pressure. Then it is hot-pressed at 95℃ and 12MPa for 5 minutes to obtain a positive electrode sheet with a thickness of 120μm.

[0041] Comparative Example 1

[0042] In step 1 of Example 1, the addition of Li3PO4 powder was omitted, and only NCM811 positive electrode active material and Li6PS5Cl sulfide electrolyte were used for ball milling pretreatment. The remaining steps were the same as in Example 1.

[0043] The impedance test result for Comparative Example 1 was 16.8 Ω, the ionic conductivity was 0.89 mS / cm, the porosity of the electrode was 12.5%, and the compaction density was 3.05 g / cm³. 3 The discharge capacity at 0.2C was 175.6 mAh / g, with an initial efficiency of 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 results for Example 1 were 8.43 Ω, ionic conductivity 2.87 mS / cm, electrode porosity 6.7%, and compaction density 3.65 g / cm³. 3 The discharge capacity at 0.2C is 198.4 mAh / g, with an initial efficiency of 86.5%, and the discharge capacity at 0.5C is 189.6 mAh / g. After 30 cycles, the capacity retention rate is 97.9%. This is because the removal of lithium phosphate means that there is no stable buffer layer between the positive electrode active material and the sulfide electrolyte surface, resulting in increased interfacial impedance, limited ion conduction, lower battery capacity utilization, side reactions during cycling, and faster capacity decay.

[0044] Comparative Example 2

[0045] In Example 1, step 2 is omitted, and the composite cathode material after ball milling is not pretreated by isostatic pressing, while the other parameters remain unchanged.

[0046] Comparative Example 2 showed an impedance of 15.7 Ω, an ionic conductivity of 1.05 mS / cm, an electrode porosity of 17.6%, and a compaction density of 2.56 g / cm³. 3 The discharge capacity at 0.2C was 178.2 mAh / g with an initial efficiency of 83.6%, and the discharge capacity at 0.5C was 154.2 mAh / g. The capacity retention rate after 30 cycles was 91.5%. It can be seen that without isostatic pressing high temperature and high pressure treatment, the impedance and porosity of the electrode are relatively high, while the ionic conductivity and compaction density are relatively low, and the capacity performance is also lower than that of the example. This is because the high temperature treatment of isostatic pressing can form a locally molten phase of Li3PO4, which generates a "welding point" effect between particles. This not only improves the contact tightness between particles, but also builds a continuous lithium-ion transport channel.

[0047] Comparative Example 3

[0048] The difference between this comparative example and the embodiment is that the adhesive consists only of micron-sized PTFE (D50=50μm); the rest is the same as in Example 1.

[0049] Comparative Example 3 showed an impedance of 12.4 Ω, an ionic conductivity of 1.74 mS / cm, an electrode porosity of 16.4%, and a compaction density of 3.12 g / cm³. 3The tensile strength was 1.25 MPa, 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 the addition of nano-sized PTFE, the electrode impedance and porosity are relatively high, while the ionic conductivity and compaction density are relatively low. In particular, its tensile strength is significantly lower than the value of 3.42 MPa in Example 1. This is because nano-sized PTFE has a stronger fiberization ability and can provide higher strength when forming a fiber film.

[0050] Comparative Example 4

[0051] The difference between this comparative example and the embodiment is that the adhesive consists only of nanoscale PTFE (D50=60nm); the rest is the same as in Example 1.

[0052] Comparative Example 4 showed an impedance of 12.8 Ω, an ionic conductivity of 1.59 mS / cm, an electrode porosity of 15.8%, and a compaction density of 2.96 g / cm³. 3 The tensile strength is 1.53 MPa, the discharge capacity at 0.2C is 182.3 mAh / g, the first-time efficiency is 83.7%, the discharge capacity at 0.5C is 173.5 mAh / g, and the capacity retention rate after 30 cycles is 92.5%. It can be seen that without the addition of micron-sized PTFE, the impedance and porosity of the electrode are relatively high, while the ionic conductivity and compaction density are relatively low. In particular, the capacity retention rate after 30 cycles is significantly lower than the value of 97.9% in Example 1. This is because micron-sized PTFE can maintain the flexibility of the electrode during the fiberization process, so that the electrode can maintain its original morphology during high-pressure rolling and battery cycling, and prevent the electrode from cracking or falling off.

[0053] Comparative Example 5

[0054] The difference between this comparative example and Example 1 is that the dual-scale polytetrafluoroethylene (PTFE) system is composed of nanoscale PTFE (particle size range D50=60nm) and microscale PTFE (particle size range 50μm) in a mass ratio of 4:1.

[0055] In Comparative Example 5, due to the excessively high content of nano-sized PTFE, continuous films could not be formed during the fiberization process; instead, scattered sheet-like films were produced. Even in subsequent rolling, complete electrode sheets could not be obtained. This is because the smaller the particle size of PTFE, the shorter the length of the fibrous strands, making it impossible to entangle and bind the positive electrode active material and electrolyte particles to form a continuous electrode sheet.

[0056] Performance test examples

[0057] Performance tests were conducted on each embodiment and comparative example using the following methods:

[0058] 1. Ionic conductivity test: The positive electrode sheets prepared in each example and comparative example were cut into small round pieces, then loaded into a pressure battery mold, and stamped under a pressure of 1 ton for 1 minute. The AC impedance was tested using an electrochemical workstation of model CHI660E at room temperature and in a normal atmospheric atmosphere, and the ionic conductivity was calculated. The results are shown in Table 1.

[0059] Table 1. Ionic conductivity test results of each embodiment and comparative example.

[0060] .

[0061] 2. Porosity Testing of the Positive Electrode: A high-performance fully automated mercury porosimeter, model MicromeriticsAutoPore V 9620, was used in the experiment. The pressure applied in the low-pressure station (LP) was approximately 0.6 to 50 PSI, and the pressure applied in the high-pressure station (HP) was 20 to 60,000 PSI. A glass cone probe was used as the sample container to apply pressure to the sample, which was 1 cm × 1 cm in size. The porosity of the positive electrode was calculated based on the volume of mercury entering the pores.

[0062] Tensile strength test of positive electrode sheet: A universal tensile testing machine was used. The sample size was 1cm×10cm. The sample was fixed in the upper and lower clamps of the tensile testing machine. The sample was stretched at a uniform speed of 0.2mm / s. The maximum tensile force value when the sample broke was recorded.

[0063] The test results of porosity, tensile strength and compaction density of the positive electrode sheet are shown in Table 2.

[0064] Table 2. Test results of porosity, tensile strength and compaction density of the positive electrode sheet.

[0065] .

[0066] 2. Full Battery Performance Test: 100 mg of Li6PS5Cl sulfide electrolyte was weighed and placed in a pressure mold, and a pressure of 1 ton was applied and held for 1 minute. Then, a small circular positive electrode sheet with a diameter of 10 mm was placed on one side of the electrolyte, and a 10 mm diameter aluminum foil was placed on the surface of the positive electrode sheet as the positive current collector. After assembly, a pressure of 1 ton was applied and held for 1 minute. Next, 30 mg of negative electrode active material (micron-sized silicon (D50=5 μm)) was weighed and spread evenly on the other side of the electrolyte. After assembly, a pressure of 1 ton was applied and held for 1 minute. Finally, a sulfide all-solid-state lithium-ion battery was obtained for testing and backup. The Xinwei Battery Testing System, model CT-4000, was used to conduct charge and discharge tests on the all-solid-state battery. The charge and discharge rate steps were 0.2C-0.5C, the voltage range was 4.3-2.5V, and the temperature was 28℃. The charge and discharge capacity of the sulfide all-solid-state battery was tested, and the test results are shown in Table 3.

[0067] Table 3. Test results of charge and discharge performance of sulfide all-solid-state batteries

[0068]

[0069] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that these are merely illustrative examples, and any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

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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