A core-shell material, a composite diaphragm and a preparation method thereof and a lithium ion battery

Abstract

This invention provides a core-shell material, a composite separator, a method for preparing the same, and a lithium-ion battery. The core-shell material comprises a polymer shell and an inactivating agent core; the inactivating agent core comprises a hydroxide with HCO3 as the anion. ‑ The coating material comprises any one of inorganic compounds and organic compounds with a conjugation degree greater than 1. The core-shell material coating of this application contains a polymer core and a deactivating agent core. The polymer shell rapidly melts or decomposes at 100–140°C, releasing the deactivating agent core. This core material can react with PF5, a secondary product of LiPF6, to generate H2O at high temperatures, and can also rapidly chemically react with the lithium-intercalated negative electrode at high temperatures, or with stilbene, which has the characteristic of reacting with a fully charged negative electrode, thereby causing the negative electrode to fail and ensuring the safety of the high-energy-density battery cell at high temperatures.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and more specifically, to a core-shell material, a composite separator and its preparation method, and a lithium-ion battery. Background Technology

[0002] The continuous improvement in the driving range of new energy vehicles has made high-energy-density power cells the primary choice for power batteries. Correspondingly, high energy density leads to higher thermal instability. The separator is a key component of lithium-ion batteries, playing a crucial role in blocking the electron conductivity between the positive and negative electrodes while allowing electrolyte ions to pass freely. Simultaneously, the separator is one of the initial barriers in the event of thermal runaway within the battery cell.

[0003] The safety of power batteries is extremely complex. A negative electrode in a certain state of charge is highly unstable at high temperatures. With the decomposition of the SEI film and continuous reaction with the electrolyte, flammable gases are constantly generated, posing a significant safety hazard to the battery cell. Therefore, treating the negative electrode in a state of charge at high temperatures is one of the measures to improve safety.

[0004] Currently, the use of inorganic ceramic electrolytes with high thermal stability and high ionic conductivity as coatings has become a research hotspot. Separators using inorganic ceramic solid electrolytes as coatings have good physical properties, but they cannot achieve self-deactivation by chemically reacting with electrode materials under high-temperature conditions in lithium-ion batteries. Summary of the Invention

[0005] The main objective of this invention is to provide a core-shell material, a composite separator and its preparation method, and a lithium-ion battery, in order to solve the problem that the separator using inorganic ceramic solid electrolyte as a coating in the prior art has poor safety under high temperature conditions in lithium-ion batteries.

[0006] To achieve the above objectives, according to one aspect of the present invention, a core-shell material is provided, comprising a polymer shell and an inactivating agent core; the inactivating agent core comprises a hydroxide, the anion of which is HCO3. - Any one of inorganic compounds or organic compounds with a conjugation degree greater than 1.

[0007] Furthermore, the thickness of the polymer shell is 0.5 to 1 μm, and the particle size of the core-shell material is preferably 0.1 to 20 μm.

[0008] Furthermore, the melting temperature or decomposition temperature of the polymer shell is 100-140°C. Preferably, the polymer shell includes one or more polymers such as polyethylene, polystyrene, polypropylene, polyethylene terephthalate, melamine-formaldehyde resin, polyfluoroethylene carbonate, polyvinyl chloride, polycarbonate, and polyvinylidene fluoride. More preferably, the polymer shell is polyethylene.

[0009] Furthermore, the hydroxide includes one or more of sodium hydroxide, lithium hydroxide, and potassium hydroxide; preferably, the anion is HCO3-. - The inorganic compounds include one or more of sodium bicarbonate, lithium bicarbonate, potassium bicarbonate, and ammonium bicarbonate. Preferably, the organic compounds with a conjugation degree greater than 1 include cis-stilbene derivatives. More preferably, the organic compounds with a conjugation degree greater than 1 include one or more of cis-stilbene, 2-methylstilbene, and 4-methylstilbene.

[0010] Furthermore, the inactivating agent core is selected from one or more of sodium bicarbonate, sodium hydroxide, ammonium bicarbonate, and stilbene.

[0011] Furthermore, the mass ratio of the polymer shell in the inactivator core and the core-shell material is 0.1 to 99.9%:1, preferably 20% to 85%.

[0012] To achieve the above objectives, according to one aspect of the present invention, a composite separator is provided, the composite separator comprising a base membrane and a core-shell material coating coated on at least one surface of the base membrane; the core-shell material coating comprising the core-shell material as described above; preferably the base membrane is a polyethylene membrane and / or a polypropylene membrane; preferably the core-shell material coating is coated on the surface of the base membrane facing the negative electrode; preferably the thickness of the core-shell material coating is 0.2 to 20 μm.

[0013] According to another aspect of the present invention, a method for preparing the above-mentioned composite membrane is provided, the method comprising: mixing an adhesive, a stabilizer, a solvent, and a core-shell material to obtain a suspension; coating the suspension onto the surface of a base membrane and drying it to obtain the composite membrane.

[0014] Furthermore, the adhesive is polyvinylidene fluoride; the stabilizer is preferably at least one of carboxymethyl cellulose, sodium alginate, sodium polyacrylate, and polyamide; the solvent is preferably at least one of N-methylpyrrolidone, acetonitrile, tetrahydrofuran, and deionized water.

[0015] According to another aspect of the present invention, a lithium-ion battery is provided, the lithium-ion battery comprising the above-described composite separator.

[0016] Applying the technical solution of this invention, the core-shell material of this application comprises a polymer shell and an inactivating agent core, which can effectively improve the safety performance of the battery cell at high temperatures. Specifically, the polymer shell rapidly melts or decomposes at 100–140°C, releasing the inactivating agent core, with hydroxides and anions being HCO3. -When inorganic compounds are used as core materials, they can react with PF5, a byproduct of LiPF6, to generate H2O at high temperatures. This H2O then undergoes a rapid chemical reaction with the lithium-intercalated anode at high temperatures, with the reaction of LiPF6 generating byproducts at high temperatures being LiPF6 = LiF + PF5. This effectively disables the anode, thus ensuring the safety of high-energy-density cells at high temperatures. Conversely, when organic compounds with a conjugation degree greater than 1 are used as core materials, they can undergo addition reactions with LiCx or LiSix alloys at high temperatures, thereby deactivating the lithium-intercalated anode and achieving disabling, thus ensuring the safety of high-energy-density cells at high temperatures. Detailed Implementation

[0017] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0018] As analyzed in the background section of this application, a negative electrode in a certain state of charge is extremely unstable at high temperatures. With the decomposition of the SEI film and continuous reaction with the electrolyte, flammable gases are constantly generated, posing a significant safety hazard to the battery cell. Using inorganic ceramic electrolytes with high thermal stability and high ionic conductivity as coating layers has become a research hotspot. While membranes using inorganic ceramic solid electrolytes as coatings have good physical properties, they cannot achieve self-deactivation by chemically reacting with the electrode materials at high temperatures in lithium-ion batteries. To address this issue, this application provides a core-shell material, a composite membrane, its preparation method, and a lithium-ion battery.

[0019] In one typical embodiment of this application, a core-shell material is provided, the coating of which includes a polymer shell and an inactivating agent core; the inactivating agent core includes hydroxide, with HCO3 as the anion. - Any one of inorganic compounds or organic compounds with a conjugation degree greater than 1.

[0020] The core-shell material of this application comprises a polymer shell and an inactivating agent core, which can effectively improve the safety performance of the battery cell at high temperatures. Specifically, the polymer shell rapidly melts or decomposes at 100–140°C, releasing the inactivating agent core, with hydroxides and anions being HCO3-. -When inorganic compounds are used as core materials, they can react with PF5, a byproduct of LiPF6, to generate H2O at high temperatures. This H2O then undergoes a rapid chemical reaction with the lithium-intercalated anode at high temperatures, with the reaction of LiPF6 generating byproducts at high temperatures being LiPF6 = LiF + PF5. This effectively disables the anode, thus ensuring the safety of the high-energy-density battery cell at high temperatures. Conversely, when organic compounds with a conjugation degree greater than 1 are used as core materials, they can undergo addition reactions with LiCx or LiSix alloys at high temperatures, thereby deactivating the lithium-intercalated anode and ensuring its safety at high temperatures. If organic compounds with a conjugation degree less than 1 are used, they cannot react with the anode, compromising the safety of the battery cell.

[0021] In some embodiments, the steps for preparing the core-shell material according to this application include:

[0022] (1) Add the polymer shell powder, emulsifier (the mass of the emulsifier is 20-30% of the polymer shell powder) and solvent to a three-necked flask, stir evenly, and obtain a homogeneous polymer solution.

[0023] (2) Add the inactivating agent to the above polymer solution, heat to 70-80℃ and keep warm, and stir continuously to obtain uniform spherical particles;

[0024] (3) Add anhydrous ethanol to the above spherical particles, and then wash away excess solvent and inactivating agent to obtain a mixture;

[0025] (4) The mixture is dried at low temperature to obtain the core-shell material described in this application.

[0026] The emulsifier mentioned above can be one or more of nonionic or ionic emulsifiers such as Span80, SDS, Span60, and Tween60, and the solvent can be xylene.

[0027] In some embodiments, the thickness of the polymer shell is preferably 0.5–1 μm. The particle size of the core-shell material is preferably 0.1–20 μm. If the polymer shell is too thick, the inactivator core cannot be released quickly at high temperatures, leading to thermal runaway; if the polymer shell is too thin, it is prone to melting or decomposition during battery cycling, resulting in deactivation of the battery negative electrode and affecting the battery's electrochemical performance.

[0028] To enable the polymer casing to melt or decompose rapidly at battery operating temperatures, a polymer material with a melting or decomposition temperature between 100 and 140°C is preferred. In some embodiments, the polymer casing includes one or more polymers such as polyethylene, polystyrene, polypropylene, polyethylene terephthalate, melamine-formaldehyde resin, polyfluoroethylene carbonate, polyvinyl chloride, polycarbonate, and polyvinylidene fluoride. Preferably, the polymer casing is polyethylene.

[0029] In some embodiments, in order to enable the inactivator core to react with PF5, the hydroxide preferably includes one or more of sodium hydroxide, lithium hydroxide, and potassium hydroxide, and the anion is preferably HCO3-. - The inorganic compound includes one or more of sodium bicarbonate, lithium bicarbonate, potassium bicarbonate, and ammonium bicarbonate. Preferably, the organic compound with a conjugation degree greater than 1 includes cis-stilbene derivatives. More preferably, the organic compound with a conjugation degree greater than 1 includes one or more of cis-stilbene, 2-methylstilbene, and 4-methylstilbene. Preferably, the core of the inactivating agent is one or more of sodium bicarbonate, sodium hydroxide, ammonium bicarbonate, and stilbene.

[0030] In some embodiments, the mass ratio of the polymer shell in the inactivator core and core-shell material is 0.1% to 99.9%. Preferably, the mass ratio of the polymer shell in the inactivator core and core-shell material is 20% to 85%. Too much polymer shell will prevent the inactivator from being released. Too little polymer shell will result in reduced shell uniformity, an excessively thin shell, and reduced mechanical strength.

[0031] In another typical embodiment of this application, a composite membrane is provided, which includes a base membrane and a core-shell material coating coated on at least one surface of the base membrane, the core-shell material coating including the aforementioned core-shell material.

[0032] Composite separators coated with core-shell materials can effectively improve the safety performance of battery cells at high temperatures. Since stabilizers and binders are required during the preparation of the composite separator in this application, a small amount of stabilizers and binders will remain in the coating after drying, but the content is extremely small and negligible.

[0033] This application does not impose any particular limitation on the type of base film; any base film commonly used in the art can be applied to this application. In some embodiments, the base film is a polyethylene film and / or a polypropylene film.

[0034] In some embodiments, the core-shell material coating is applied to the surface of the base film facing the negative electrode. When the core-shell material of this application is applied to the surface of the base film facing the negative electrode, the deactivating agent in the core-shell material reacts with the fully charged negative electrode as follows: PF5 + 8NaOH = 5NaF + Na3PO4 + 4H2O; PF5 + 8NaHCO3 = 5NaF + Na3PO4 + 8CO2 + 4H2O; H2O + Li = H2 + Li2O, thereby causing the negative electrode to deactivate, thus ensuring the safety of the high-energy-density battery cell at high temperatures.

[0035] In order to ensure that the core-shell material coating can prevent battery short circuits and avoid the core-shell material coating being too thick and affecting electron transport, the thickness of the core-shell material coating is preferably 0.2 to 20 μm.

[0036] In another typical embodiment of this application, a method for preparing the above-mentioned composite membrane is provided. The method includes: mixing an adhesive, a stabilizer, a solvent, and a core-shell material to obtain a suspension; coating the suspension onto the surface of a base membrane and drying it to obtain the composite membrane.

[0037] The preparation method described in this application is simple and the conditions are mild. The composite separator obtained using the preparation method described in this application can effectively improve the safety performance of the battery cell at high temperatures.

[0038] This application does not impose any particular limitation on the types of stabilizers, adhesives, and solvents; commonly used stabilizers, adhesives, and solvents in the art can all be applied to this application. In some embodiments, the adhesive is polyvinylidene fluoride; the preferred stabilizer is at least one of carboxymethyl cellulose, sodium alginate, sodium polyacrylate, and polyamide; the preferred solvent is at least one of N-methylpyrrolidone, acetonitrile, tetrahydrofuran, and deionized water.

[0039] In some embodiments, in order to uniformly coat the core-shell material onto the surface of the base film and achieve the target thickness of the core-shell material coating, the mass ratio of the adhesive, stabilizer, core-shell material, and solvent is 0.25:0.25:9.5:25.

[0040] This application does not impose particular limitations on the drying method or temperature, as long as the solvent in the diaphragm can be removed. In some embodiments, the drying can be vacuum drying, spray drying, or drying in an oven. Preferably, the drying temperature is 80–100°C.

[0041] In another typical embodiment of this application, a lithium-ion battery is provided, which includes the above-described composite separator.

[0042] Lithium-ion batteries incorporating the aforementioned composite separators exhibit better safety and reduce the likelihood of fire and explosion.

[0043] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0044] Example 1

[0045] (1) Take a core-shell material with a D50 of 0.2 μm (PE shell and sodium bicarbonate core, with a mass ratio of PE shell to sodium bicarbonate of 8:2), polyvinylidene fluoride (PVDF), polyamide, and N-methylpyrrolidone (NMP) and mix them in a mass ratio of 9:0.5:0.5:25. After dispersing the four materials in a planetary ball mill for 3 hours, a stable slurry is obtained.

[0046] (2) The above-mentioned stable slurry is coated on one surface of a polyethylene film by a micro-concave roller, and after drying at 80°C, a composite diaphragm is obtained with a coating thickness of 3μm.

[0047] Example 2

[0048] (1) Take a core-shell material with a D50 of 0.2 μm (PE shell and sodium hydroxide core, with a mass ratio of PE shell to sodium bicarbonate of 7:3), polyvinylidene fluoride (PVDF), sodium polyacrylate and deionized water, with a mass ratio of 9.5:0.25:0.25:25. After dispersing the four materials in a planetary ball mill for 3 hours, a stable slurry is obtained.

[0049] (2) A stable slurry is coated onto one surface of a polyethylene film using a micro-grooved roller. After drying at 80°C, a composite diaphragm is obtained with a coating thickness of 5 μm.

[0050] Example 3

[0051] (1) Take a core-shell material with a D50 of 0.2 μm (polyethylene shell, ammonium bicarbonate as the core, and the mass ratio of PE shell to sodium bicarbonate is 1:1), polyvinylidene fluoride (PVDF), sodium alginate, and deionized water, with a mass ratio of 9.5:0.25:0.25:25. After dispersing the four materials in a planetary ball mill for 3 hours, a stable slurry is obtained.

[0052] (2) A stable slurry is coated onto one surface of a polyethylene film using a micro-concave roller. The coating thickness is 5 μm. After drying at 80°C, a composite diaphragm is obtained.

[0053] Example 4

[0054] Unlike Example 1, the outer shell of the core-shell material is polycarbonate, and the core is potassium hydroxide.

[0055] Example 5

[0056] Unlike Example 1, the outer shell of the core-shell material is melamine-formaldehyde resin, and the core is lithium bicarbonate.

[0057] Example 6

[0058] Unlike Example 1, the quality of the core-shell material was controlled so that the thickness of the core-shell material coating was 20 μm.

[0059] Example 7

[0060] Unlike Example 1, the quality of the core-shell material was controlled so that the thickness of the core-shell material coating was 0.1 μm.

[0061] Example 8

[0062] Unlike Example 1, the quality of the core-shell material was controlled so that the thickness of the core-shell material coating was 25 μm.

[0063] Example 9

[0064] Unlike Example 1, the mass ratio of PE shell to sodium bicarbonate is 1:1.

[0065] Example 10

[0066] Unlike Example 1, the core is cis-stilbene.

[0067] Example 11

[0068] Unlike Example 2, the core is cis-stilbene.

[0069] Comparative Example 1

[0070] Choose a polyethylene-based membrane, specifically a polypropylene-polyethylene-polypropylene three-layer membrane structure.

[0071] Comparative Example 2

[0072] (1) PE, polyvinylidene fluoride (PVDF), polyamide and N-methylpyrrolidone (NMP) were mixed in a mass ratio of 9:0.5:0.5:25. The mixture was dispersed in a planetary ball mill for 3 hours to obtain a stable slurry.

[0073] (2) The above-mentioned stable slurry is coated on one surface of a polyethylene film by a micro-concave roller. The coating thickness is 3μm. After drying at 80℃, a composite diaphragm is obtained.

[0074] Comparative Example 3

[0075] (1) Sodium bicarbonate as coating, polyvinylidene fluoride (PVDF), polyamide and N-methylpyrrolidone (NMP) were mixed in a mass ratio of 9:0.5:0.5:25. The four were dispersed in a planetary ball mill for 3 hours to obtain a stable slurry.

[0076] (2) The above-mentioned stable slurry is coated on one surface of a polyethylene film by a micro-concave roller. The coating thickness is 3μm. After drying at 80℃, a composite diaphragm is obtained.

[0077] Comparative Example 4

[0078] (1) Take the core and shell materials (PE shell and silicon dioxide as the core, with a mass ratio of PE shell to silicon dioxide of 8:2), polyvinylidene fluoride (PVDF), polyamide, and N-methylpyrrolidone (NMP) and mix them in a mass ratio of 9:0.5:0.5:25. After dispersing the four materials in a planetary ball mill for 3 hours, a stable slurry is obtained.

[0079] (2) The above-mentioned stable slurry is coated on one surface of a polyethylene film by a micro-concave roller. The coating thickness is 3μm. After drying at 80℃, a composite diaphragm is obtained.

[0080] The composite separators obtained in the above embodiments and comparative examples were assembled with positive and negative electrode sheets to form a 20Ah soft-pack battery cell. In this case, the surface of the composite separator containing the coating layer was arranged facing the negative electrode. After being fully charged, a hot box test was performed, and the test results are shown in Table 1.

[0081] The hot box test conditions are as follows:

[0082] (1) First stage: Charge the soft-pack battery cells to 4.25V;

[0083] (2) Second stage: Heat to 60℃ at a rate of 2℃ / min and hold for 1 hour;

[0084] (3) Third stage: Heat to 130℃ at a rate of 2℃ / min and hold for 1 hour;

[0085] (4) Fourth stage: Heat to 150℃ at a rate of 2℃ / min and keep warm for 1 hour.

[0086] 20Ah pouch cell positive and negative terminals:

[0087] Positive electrode: NCM811 is the positive electrode, conductive agent is conductive carbon black, and binder is PVDF;

[0088] Negative electrode: Graphite is used as the negative electrode, the conductive agent is a composite of single-walled carbon nanotubes and conductive carbon, and the binder is a composite of CMC and SBR.

[0089] The electrolyte is LiPF6 (1 mol / L), and the solvent ratio is EC:DEC:EMC:DMC = 1:1:1:1 (volume ratio).

[0090] Table 1

[0091]

[0092]

[0093] As shown in Table 1, after being kept at 60℃ for 1 hour, the voltage of the battery cell did not change, indicating that the coating of the core and shell material did not affect the normal use of the battery cell. After being kept at 130℃ for 1 hour, the voltage of the battery cell assembled from the composite separator dropped to about 1.2V (this voltage is close to the relative voltage between the empty negative electrode and the fully charged positive electrode), while the battery cell in Comparative Example 1 still maintained a normal voltage, indicating that at this time, the inactivating agent in the core and shell material had been released and reacted with PF5 to generate H2O, which then reacted chemically with the fully charged negative electrode, causing it to become ineffective. At 150℃, due to the thermal shrinkage of the ordinary separator, the positive and negative electrodes were short-circuited, causing the battery cell to short-circuit and release a large amount of heat, resulting in thermal runaway of the battery cell, and the voltage dropped to 0V. However, the composite separator containing the core and shell material did not experience thermal runaway behavior such as fire or explosion because the negative electrode had failed.

[0094] In Example 8, the 25µm coating indicates a thicker polymer coating. The polymer melts in the third stage, causing a partial open circuit and a significant voltage drop.

[0095] The core of Comparative Example 4 is made of ceramic material, which is an inert substance and will not react directly or indirectly with the fully charged negative electrode. Therefore, it is impossible to ensure the safety of the battery cell at high temperatures by causing the negative electrode to fail.

[0096] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects: By coating the base film surface with a core-shell material coating, the safety performance of the battery cell at high temperatures can be effectively improved. The core-shell material coating contains a polymer core and a deactivating agent core. The polymer shell rapidly melts or decomposes at 100–140°C, releasing the deactivating agent core. This core material can react with PF5, a secondary product of LiPF6, to generate H2O at high temperatures, and then rapidly chemically react with the lithium-intercalated negative electrode at high temperatures. Alternatively, it can react with stilbene, which has the characteristic of reacting with a fully charged negative electrode, thereby causing the negative electrode to fail and ensuring the safety of the high-energy-density battery cell at high temperatures.

[0097] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., 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 composite separator, characterized by, The composite membrane includes a base membrane and a core-shell material coating applied to at least one surface of the base membrane; the thickness of the core-shell material coating is 3~5μm; the core-shell material coating is applied to the surface of the base membrane facing the negative electrode. The core-shell material coating includes a core-shell material; the core-shell material includes a polymer shell and an inactivator core; the mass ratio of the inactivator core and the polymer shell in the core-shell material is 2:8 to 3:7; The inactivating agent core is an organic compound with a conjugation degree greater than 1; the organic compound with a conjugation degree greater than 1 is selected from one or more of cis-stilbene, 2-methylstilbene, and 4-methylstilbene; The melting temperature of the polymer shell is 100~140℃, and the polymer shell is made of polyethylene.

2. The composite separator of claim 1, wherein The thickness of the polymer shell is 0.5~1 μm.

3. The composite separator of claim 1, wherein The core-shell material has a particle size of 0.1~20 μm.

4. The composite diaphragm according to claim 1, characterized in that, The base film is a polyethylene film and / or a polypropylene film.

5. A method for preparing a composite separator according to any one of claims 1 to 4, characterized in that, The preparation method includes: The binder, stabilizer, solvent, and core-shell material are mixed to obtain a suspension; The suspension is coated onto the surface of the base membrane and dried to obtain the composite membrane.

6. The method for preparing the composite diaphragm according to claim 5, characterized in that, The adhesive is polyvinylidene fluoride; the stabilizer is at least one of carboxymethyl cellulose, sodium alginate, sodium polyacrylate, and polyamide; and the solvent is at least one of N-methylpyrrolidone, acetonitrile, tetrahydrofuran, and deionized water.

7. A lithium-ion battery, characterized in that, The lithium-ion battery includes the composite separator as described in any one of claims 1 to 4.

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