Phase separation solid-state battery composite positive electrode and preparation and application thereof

Abstract

The invention belongs to the technical field of secondary solid-state batteries, and particularly relates to a phase separation solid-state battery composite positive electrode and preparation and application thereof. The positive electrode is composed of a positive electrode active material, a conductive agent and a phase separation ionic gel electrolyte; the phase separation ionic gel electrolyte has a bicontinuous phase structure formed by interphase penetration of a polymer enrichment phase and an ionic liquid enrichment phase. The preparation method mainly comprises the following steps: coating a current collector with positive electrode slurry, carrying out high-temperature rapid drying to preliminarily remove a surface solvent, carrying out low-temperature slow drying to induce phase separation, and finally forming the composite positive electrode. The core of the invention lies in that a continuous ion / heat transmission channel is constructed in situ through solvent selection and a two-step drying process, the two problems of low ionic conductivity and poor heat conductivity in a solid-state battery are solved at one stroke, the rate capability, cycling stability and safety of the battery are remarkably improved, and the method is particularly suitable for all-solid-state batteries with high surface capacity and high power.

Description

Technical Field

[0001] This invention belongs to the field of secondary battery solid-state battery technology, specifically relating to a phase-separated composite cathode for all-solid-state lithium-ion batteries, its preparation method, and a solid-state battery containing the cathode. Background Technology

[0002] Developing high-energy-density and high-safety solid-state batteries is key to solving the current safety risks and energy bottlenecks of lithium-ion batteries. Among them, polymer-based solid electrolytes have attracted much attention due to their good flexibility and interfacial contact with electrodes. However, polymer electrolytes have two inherent drawbacks: first, their ionic conductivity is low at room temperature; second, their thermal conductivity is low, which can easily lead to heat accumulation during battery charging and discharging, triggering the risk of thermal runaway.

[0003] Traditional composite cathodes typically employ a homogeneous structure by mechanically mixing active materials, conductive agents, and solid electrolytes. This structure results in discontinuous and non-uniform ion transport pathways, which, especially under conditions of thick electrodes and high areal capacity, leads to severe polarization, limiting the battery's rate performance and capacity. Furthermore, the discontinuous heat conduction pathways in homogeneous structures hinder rapid heat dissipation, exacerbating thermal safety hazards in the battery.

[0004] Therefore, developing a composite cathode structure that combines high ionic conductivity, high thermal conductivity, and a continuous ion / heat transport channel is crucial for advancing the practical application of high-performance solid-state batteries. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing solid-state battery composite cathodes, such as low ion conductivity and thermal conductivity, and discontinuous transport paths, and to provide a composite cathode with a dual continuous phase separation structure and its preparation method. This cathode, through component design and process control, constructs continuous ion and heat transport channels in situ, thereby simultaneously improving the battery's rate performance, cycle stability, and thermal safety.

[0006] To achieve the above objectives, this invention first provides a method for preparing a phase-separated composite cathode, the core of which lies in inducing the formation of a dual continuous phase structure through component design and process control. The method includes: dissolving a polymer binder in a first solvent to form a solution; adding a cathode active material and a conductive agent to obtain a slurry; dissolving a lithium salt and an ionic liquid in a second solvent to obtain a homogeneous solution; mixing the slurry and solution and coating it onto a current collector; and then subjecting it to a two-step drying process—first at high temperature for a short time, then at low temperature for a long time—ultimately inducing the formation of a composite cathode with an ion-enriched phase and a composite material-enriched interpenetrating structure.

[0007] Furthermore, the ionic liquid may be selected from imidazole, pyrrolidine, pyridine, quaternary ammonium salts, or quaternary phosphine salts, and its cation may include, but is not limited to, [Emim].+ [Bmim] + [Pyr13] + [Bmpyr] + [Bpy] + [N4444] + [N2222] + Anions include, but are not limited to, Cl. - ,Br - TFSI - FSI - OTf - PF6 - BF4 - The amount added is 20%-80% of the lithium salt mass. In the two-step drying process, the initial drying temperature is 80-200 ℃, and the time is 1-60 seconds; the complete drying temperature is 40-80 ℃, and the time is 2-24 hours.

[0008] A second aspect of the present invention provides a phase-separated composite cathode prepared by the above method, the core feature of which is that it has a bicontinuous phase structure comprising an ion-enriched phase and a composite material-enriched phase. The ion-enriched phase is a continuous phase containing lithium salt and ionic liquid, forming ion and heat transport channels; the composite material-enriched phase is a continuous phase containing cathode active material, conductive agent and polymer binder, forming electron transport channels and providing mechanical support.

[0009] A third aspect of the present invention provides a solid-state battery comprising the above-described phase-separated composite positive electrode as a positive electrode.

[0010] Beneficial effects

[0011] Compared with existing technologies, the phase-separated composite cathode and its preparation method provided by this invention have the following significant advantages:

[0012] 1. Formation of an optimized bicontinuous phase structure: Through a specific solvent and a two-step drying process, a bicontinuous structure in which the polymer-enriched phase and the ionic liquid-enriched phase interpenetrate each other was successfully induced and stabilized. This structure provides a low-impedance, fully covered transport network for lithium ions and heat, fundamentally solving the transport kinetics bottleneck and thermal accumulation problem in thick electrodes.

[0013] 2. Significantly improves ion transport efficiency: The ionic liquid enriched phase not only provides continuous ion channels, but its cations can also interact with lithium salt anions, effectively fixing large-sized anions, thereby significantly increasing the lithium ion transport number and suppressing concentration polarization.

[0014] 3. Excellent thermal management capability: The continuous enrichment of ionic liquids also serves as an excellent thermal channel, which greatly improves the thermal conductivity of the composite cathode. It can quickly and uniformly dissipate the Joule heat and reaction heat generated during charging and discharging, greatly improving the thermal safety and high-temperature cycle stability of the battery.

[0015] 4. Wide applicability of materials: The preparation method and phase separation mechanism described in this invention have universality. The ionic liquid is not limited to EmimCl, but can be extended to various types of anion-cation combinations (such as EmimTFSI, Pyr13TFSI, etc.); the polymer, lithium salt, active material (such as NCM, LFP), and solvent all have multiple options, providing flexibility and tunability for solid-state battery design with different performance requirements.

[0016] 5. Balancing high load and high performance: The unique dual-continuous structure ensures that the transport of ions, electrons and heat can still proceed efficiently even under high reactive material loading, thus achieving excellent rate performance and long cycle life under high areal capacity. Detailed Implementation

[0017] The present invention will be further illustrated by specific embodiments below, but the scope of protection of the present invention is not limited thereto.

[0018] Example 1: Phase-separated composite cathode based on EmimCl and PEO

[0019] (1) Dissolve 0.34 g PEO in 7 mL of anhydrous ethanol and stir at 90 °C until completely dissolved to obtain a clear solution.

[0020] (2) 9.0 g NCM811 (LiNi 0.8 Co 0.1 Mn 0.1 O2) and 0.4 g Super P were added to the solution in step (1), and the mixture was stirred at 90 °C for 2 hours to obtain a uniform slurry.

[0021] (3) Take another 3 mL of anhydrous ethanol, add 0.17 g LiTFSI and 0.085 g EmimCl (50% of the mass of LiTFSI), stir at 90 °C until completely dissolved to obtain a transparent solution.

[0022] (4) Pour the solution from step (3) into the slurry from step (2) and stir continuously at 90 °C for 12 hours to obtain a mixed slurry.

[0023] (5) The slurry was coated onto aluminum foil with a wet film thickness of 200 μm. It was first placed on a 90 ℃ hot plate for 3 seconds, and then transferred to a 50 ℃ vacuum oven for 12 hours. After complete drying, a phase-separated composite cathode was obtained.

[0024] Example 2: Phase-separated composite cathode based on Pyr13Br and PU

[0025] (1) Dissolve 1 g PU in 20 mL N,N-dimethylformamide (DMF) and stir at 60 °C to dissolve.

[0026] (2) Add 9.0 g of lithium iron phosphate (LFP) and 0.6 g of carbon nanotubes (CNT) to the solution in step (1) and stir at 60°C for 2 hours.

[0027] (3) Take another 10 mL of DMF, add 0.5 g of LiFSI and 0.4 g of Pyr13Br (80% of the mass of LiFSI), and stir to dissolve at 60 °C.

[0028] (4) Mix the solution from step (3) with the slurry from step (2) and stir at 60 °C for 10 hours.

[0029] (5) The slurry is coated on aluminum foil, treated at 150 °C for 10 seconds, and then dried at 80 °C for 12 hours to obtain a phase-separated composite cathode based on LFP.

[0030] Example 3: Phase-separated composite cathode based on BmimTFSI and PVDF-HFP

[0031] (1) Dissolve 0.5 g PVDF-HFP in 10 mL acetone and stir at 60 °C to dissolve.

[0032] (2) Add 9.0 g lithium manganese oxide and 0.5 g acetylene black to the solution in step (1) and stir at 60 °C for 2 hours.

[0033] (3) Take another 5 mL of acetone, add 0.5 g LiTFSI and 0.1 g BmimTFSI (20% of the mass of LiTFSI), and stir to dissolve at 60 °C.

[0034] (4) Mix the solution from step (3) with the slurry from step (2) and stir at 60 °C for 8 hours.

[0035] (5) The slurry is coated on aluminum foil, treated at 80 °C for 5 seconds, and then dried at 40 °C for 6 hours to obtain a phase-separated composite cathode.

[0036] Comparative Example 1: Homogeneous Cathode

[0037] The preparation process is the same as in Example 1, except that the solvent in steps (1) and (3) is replaced with NMP, and vacuum drying at 50°C is used only in step (5) to finally obtain a homogeneous positive electrode.

[0038] The positive electrodes and lithium negative electrodes obtained from each embodiment and the comparative example were used to assemble batteries with corresponding electrolytes for testing. All embodiments successfully formed a dual continuous phase structure, and their ionic conductivity, thermal conductivity, rate performance, cycle stability, and high and low temperature performance were significantly better than those of the comparative example. This indicates that the phase separation structure preparation strategy provided by this invention has good universality for different active materials, different polymer matrices, and different ionic liquids, and can effectively improve the overall performance of solid-state batteries.

[0039] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention, but the present invention is not limited to the above embodiments. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention. Attached Figure Description

[0041] Figure 1 This is a Raman image of the phase-separated electrode in Example 1, in which phase separation occurred between the polymer and the ionic liquid;

[0042] Figure 2 The thermal imaging images of Example 1 and Comparative Example 1 are shown. The electrodes were placed on a hot stage at 100 °C for comparison. Example 1 reached 100 °C within 5 seconds, while the heating rate of Comparative Example 1 was slower, indicating that the thermal conductivity of Comparative Example 1 was much lower than that of Example 1.

[0043] Figure 3 The room temperature rate performance of Example 1 and Comparative Example 1 is shown. The NCM loading in the positive electrode is 15 mg / cm³. -2 The circulation temperature is 30 ℃;

[0044] Figure 4 The cycling performance of Example 1 is shown in the range of -20 to 100 °C. The NCM loading in the positive electrode is 15 mg cm⁻¹. -2 The current density during cycling at -20 ℃ is 0.5 C, the current density during cycling at 30 ℃ is 2 C, and the current density during cycling at 100 ℃ is 10 C.

Claims

1. A method for preparing a phase-separated composite positive electrode, characterized in that, Includes the following steps: (1) Dissolve the polymer binder in the first solvent, heat and stir to form a polymer solution; (2) Add the positive electrode active material and the conductive agent to the polymer solution obtained in step (1), heat and stir to obtain a uniform active material slurry; (3) Dissolve the lithium salt and ionic liquid in a second solvent, heat and stir to obtain a lithium salt / ionic liquid solution; (4) Mix the slurry obtained in step (2) with the solution obtained in step (3), heat and stir to obtain a mixed slurry; (5) The mixed slurry is coated onto the current collector and subjected to two-step drying treatment to induce phase separation, thereby obtaining the phase-separated composite positive electrode; the two-step drying includes: firstly, treating at 80-200 ℃ for 1-60 seconds for preliminary drying, and then treating at 40-80 ℃ for 2-24 hours for low-temperature drying.

2. The preparation method according to claim 1, characterized in that, The polymer binder includes one or more of polyethylene oxide (PEO), polyurethane (PU), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP).

3. The preparation method according to claim 1, characterized in that, The lithium salt includes one or more of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium perchlorate (LiClO4).

4. The preparation method according to claim 1, characterized in that, The first solvent and the second solvent are independently selected from one or more of ethanol, acetone, tetrahydrofuran, N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), and water; and the first solvent and the second solvent are the same or different solvents.

5. The preparation method according to claim 1, characterized in that, In step (3), the amount of ionic liquid added is 20%-80% of the mass of lithium salt.

6. The preparation method according to claim 5, characterized in that, The cationic portion of the ionic liquid is selected from 1-ethyl-3-methylimidazolium ([Emim... + ), 1-Butyl-3-methylimidazolium ([Bmim) + ), 1-Methyl-1-propylpyrrolidineonium ([Pyr13]) + ), N-butyl-N-methylpyrrolidone ([Bmpyr) + ), N-butylpyridinium ([Bpy) + Tetrabutylammonium ([N4444)) + Tetraethylammonium ([N2222]) + One or more of the following are selected: The anionic portion of the ionic liquid is selected from chloride ions (Cl...). - ), bromide ions (Br) - ), bis(trifluoromethanesulfonyl)imide ion (TFSI) - ), difluorosulfonyl imide ion (FSI) - ), trifluoromethanesulfonate (OTf) - ), hexafluorophosphate (PF6) - ), tetrafluoroborate (BF4) - One or more of them.

7. The preparation method according to claim 1, characterized in that, The positive electrode active material is one or more of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium cobalt oxide, lithium manganese oxide, and lithium iron phosphate; the conductive agent is one or more of Super P, acetylene black, Ketjen black, carbon nanotubes, and graphene.

8. A phase-separated composite cathode prepared by the method according to any one of claims 1-7, characterized in that, The composite cathode has a dual continuous phase structure, comprising: an ion-enriched phase, which is a continuous phase containing the lithium salt and the ionic liquid, for providing ion transport and heat transfer channels; and a composite material-enriched phase, which is a continuous phase containing the cathode active material, the conductive agent and the polymer binder, for providing mechanical support and electron transport channels.

9. A solid-state battery, comprising a positive electrode, a negative electrode, and a solid electrolyte, characterized in that, The positive electrode is the phase-separated composite positive electrode according to claim 8.

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