Positive electrode binder, positive electrode tab, and lithium ion battery

Abstract

This invention discloses a positive electrode binder, a positive electrode sheet, and a lithium-ion battery. The positive electrode binder comprises a copolymer, which includes a first polymerization unit and a second polymerization unit. The first polymerization unit includes a silsesquioxane unit containing thiocarbonyl sulfide, which comprises a silsesquioxane structural unit and a thiocarbonyl sulfide functional group, with the silsesquioxane structural unit connected to the thiocarbonyl sulfide functional group. The second polymerization unit includes a cyano-containing multi-component copolymer unit. The positive electrode binder of this invention forms a "POSS core-branched chain of cyano-containing multi-component copolymer units," which can provide ion transport channels, improve adhesion strength and ionic conductivity, reduce battery internal resistance, and improve battery cycle stability, thereby significantly improving the overall performance of the lithium-ion battery and meeting the requirements of high-rate charge and discharge.

Description

Technical Field

[0001] This invention belongs to the field of battery material technology, specifically relating to a positive electrode binder, a positive electrode sheet, and a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries, with their advantages of high energy density, long cycle life, and no memory effect, have been widely used in portable electronic devices, electric vehicles, energy storage systems, and other fields. As a key component of lithium-ion battery electrodes, the binder's main function is to tightly bond the active material, conductive agent, and current collector, maintaining the integrity of the electrode structure. Simultaneously, it must possess a certain ion transport capacity to ensure the battery's electrochemical performance.

[0003] Currently, the main cathode binder for commercial lithium-ion batteries is polyvinylidene fluoride (PVDF), but it has the following drawbacks: the use of R142b (difluorochloroethane) in PVDF production is harmful to the environment; PVDF is a linear semi-crystalline polymer with tightly packed and rigid molecular chains, resulting in high aggregation density and a lack of dedicated ion transport channels. Lithium ions need to diffuse slowly in the gaps between polymer molecular chains, leading to low ionic conductivity and increased internal resistance of the battery; at the same time, the linear molecular chains are only bonded to the active material and current collector by weak van der Waals forces, resulting in limited interfacial adhesion strength. During long-term battery cycling, the expansion / contraction of the active material can easily cause electrode cracking and active material detachment, leading to rapid capacity decay and severe capacity decay at high rates.

[0004] Therefore, it is necessary to break through the limitations of existing linear / simple branched structures at the molecular level and develop a polymer cathode binder that can provide ion transport channels, reduce battery internal resistance, improve battery cycle stability, and thus significantly improve the overall performance of lithium-ion batteries. Summary of the Invention

[0005] To overcome the defects of existing lithium-ion battery binders, this invention provides a positive electrode binder of a branched polymer with a core of sulfur-containing carbonyl sulfide POSS. This invention also provides a positive electrode sheet containing the positive electrode binder and a lithium-ion battery.

[0006] The objective of this invention is achieved through the following technical solution.

[0007] In a first aspect, the present invention provides a positive electrode binder comprising a copolymer, the copolymer comprising a first polymerization unit and a second polymerization unit, the first polymerization unit comprising a silsesquioxane unit containing thiocarbonyl sulfide, the silsesquioxane unit containing thiocarbonyl sulfide comprising a silsesquioxane structural unit and a thiocarbonyl sulfide functional group, the silsesquioxane structural unit being connected to the thiocarbonyl sulfide functional group; the second polymerization unit comprising a cyano-containing multi-component copolymer unit.

[0008] Furthermore, the first polymerization unit has a mass percentage content of 0.5% to 5% in the copolymer.

[0009] Furthermore, the molar ratio of the silsesquioxane structural unit to the thiocarbonyl sulfur functional group is 1:(2~8).

[0010] Furthermore, the silsesquioxane structural units are derived from one or more of amino-containing silsesquioxane monomers and hydroxyl-containing silsesquioxane monomers.

[0011] Furthermore, the thiocarbonylsulfonium functional group is derived from a RAFT reagent containing a carboxyl group.

[0012] Furthermore, the cyano-containing multi-component copolymer unit includes cyano-based structural units and functional structural units containing double bonds, wherein the mass percentage of the cyano-based structural units in the cyano-containing multi-component copolymer unit is 20% to 98%.

[0013] Furthermore, the cyano structural unit is derived from a cyano monomer, which is selected from at least one of acrylonitrile, methacrylonitrile, cyanoethyl acrylate, cyanoethyl methacrylate, 2-ethyl cyanoacrylate, 2-ethyl cyanomethacrylate, N-cyanomethylacrylamide, and α-cyanostylstyrene.

[0014] Furthermore, the functional structural unit containing double bonds is derived from the functional monomer containing double bonds, and the functional monomer containing double bonds is selected from at least one of acrylate monomers, aromatic vinyl monomers, and vinyl ester monomers.

[0015] In a second aspect, the present invention provides a positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector, the positive active material layer comprising a positive binder as described in the first aspect.

[0016] Thirdly, the present invention provides a lithium-ion battery, including a positive electrode as described in the second aspect.

[0017] The positive electrode binder of this invention is a polymer structure with a branched structure formed by silsesquioxane (POSS) units containing thiocarbonyl sulfide as the core. It breaks through the limitations of existing linear / simple branched structures at the molecular level and has the following beneficial effects:

[0018] 1) The copolymer is a star-shaped polymer of POSS-type units containing sulfur-containing carbonyl sulfur. The POSS-type structural units are not easily cross-linked, thus avoiding the aggregation of POSS-type structural units.

[0019] 2) POSS-type units containing sulfur-containing carbonyl sulfide have a unique cage-like structure. The internal cavities and surface functional groups of their molecules can form continuous "ion transport channels," thereby improving ionic conductivity. Lithium ions do not need to diffuse slowly in the gaps between tightly entangled linear molecular chains as in existing PVDFs, but instead migrate directly and rapidly through the POSS cavities, reducing migration resistance. This can significantly improve the capacity retention rate of the battery at 1C rate, solving the problems of excessive internal resistance and rapid capacity decay in high-rate scenarios, and meeting the requirements of high-rate charge and discharge.

[0020] 3) The positive electrode binder of the present invention forms a three-dimensional cross-linked network of “POSS core - branched chain of cyano-containing multi-component copolymer unit”. Compared with the linear structure of existing positive electrode binders, the branched chain with POSS as the core can buffer the volume expansion of positive electrode active material during charging and discharging, disperse stress concentration, prevent electrode cracking, and significantly enhance the stability of electrode structure, thereby improving the cycle capacity retention rate.

[0021] 4) Through the functional synergistic design of "POSS core - branched chain of cyano-containing multi-component copolymer unit", the performance bottleneck of existing positive electrode binders is broken: the cyano-containing multi-component copolymer unit provides strong polar groups, which form strong van der Waals forces with the current collector and the surface of the active material, thereby generating better adhesion; the branched structure of the POSS core makes the strong polar groups uniformly distributed in three-dimensional space, avoiding the uneven local binding force caused by the aggregation of groups in the linear structure, thereby improving the bonding strength; at the same time, the POSS core itself can act as an ion transport channel and has mechanical strength. Due to the existence of the POSS channel, the conductivity is not negatively affected by the improvement of bonding strength, realizing the synergistic optimization of "adhesion-conduction" performance, thereby achieving a synergistic balance of "high conductivity-high adhesion".

[0022] 5) The positive electrode binder of the present invention has good lithium-ion conductivity, which enables the lithium-ion battery prepared by the positive electrode sheet containing the binder to have low internal resistance, thereby significantly improving the overall performance of the lithium-ion battery. Detailed Implementation

[0023] To make the technical problems solved, technical solutions, and beneficial effects of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the embodiments described herein are only some, not all, of the embodiments of this invention, and are merely illustrative and not intended to limit the invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the protection scope of this invention.

[0024] It should be noted that, in this invention, as is known to those skilled in the art of chemical synthesis, each structural unit represents the structural portion of the corresponding monomer present in the resulting polymer after the monomer participates in the polymerization reaction. The mass percentage of each structural unit is the corresponding mass percentage of the monomer providing each structural unit.

[0025] In a first aspect, the present invention provides a positive electrode binder comprising a copolymer, the copolymer comprising a first polymerization unit and a second polymerization unit, the first polymerization unit comprising a silsesquioxane unit containing thiocarbonyl sulfide, the silsesquioxane unit containing thiocarbonyl sulfide comprising a silsesquioxane (POSS) structural unit and a thiocarbonyl sulfide functional group (−C(=S)−S−), the silsesquioxane structural unit being connected to the thiocarbonyl sulfide functional group; the second polymerization unit comprising a cyano-containing multi-component copolymer unit.

[0026] POSS-type units containing sulfur-containing carbonyl sulfide possess a unique cage-like structure. Their internal cavities and surface functional groups can form continuous "ion transport channels," thus enhancing ionic conductivity. Unlike existing PVDFs that diffuse slowly through the gaps between tightly entangled linear molecular chains, lithium ions migrate directly and rapidly through the POSS cavities, reducing migration resistance. This significantly improves battery capacity retention at 1C rates, addressing the issues of excessive internal resistance and rapid capacity decay at high rates, and meeting the demands of high-rate charge and discharge.

[0027] The positive electrode binder of the present invention forms a three-dimensional cross-linked network of "POSS core - branched chain of cyano-containing multi-component copolymer unit". Compared with the linear structure of existing positive electrode binders, the branched chain with POSS as the core can buffer the volume expansion of positive electrode active material during charging and discharging, disperse stress concentration, prevent electrode cracking, and significantly enhance the stability of electrode structure, thereby improving cycle capacity retention.

[0028] By employing a functional synergistic design of "POSS core - branched chain of cyano-containing multi-component copolymer units," the performance bottleneck of existing cathode binders is broken: the cyano-containing multi-component copolymer units provide strong polar groups, forming strong van der Waals forces with the current collector and the surface of the active material, thereby generating better adhesion; the branched structure of the POSS core ensures that the strong polar groups are uniformly distributed in three-dimensional space, avoiding uneven local bonding forces caused by the aggregation of groups in linear structures, thus improving the bonding strength; at the same time, the POSS core itself can act as an ion transport channel and has mechanical strength. Due to the presence of the POSS channel, the conductivity is not negatively affected by the improvement in bonding strength, achieving synergistic optimization of "adhesion-conduction" performance, thereby achieving a synergistic balance of "high conductivity-high adhesion."

[0029] In some specific embodiments, the first polymerization unit constitutes 0.5% to 5% of the copolymer by mass. If the proportion of the first polymerization unit is too high, agglomeration is likely to occur; if it is too low, it will not achieve the desired ion transport effect. All mass percentages in this application refer to the feed ratio of the raw material monomers during preparation.

[0030] In some specific embodiments, the molar ratio of the silsesquioxane structural unit to the thiocarbonylsulfonium functional group is 1:(2~8). The POSS-type structural unit in this application has a maximum of 8 reactive groups and can be grafted with a maximum of 8 thiocarbonylsulfonium functional groups. If there are fewer than two thiocarbonylsulfonium functional groups, the synthesized binder has a non-multi-arm structure and poor mechanical properties.

[0031] In some specific embodiments, the silsesquioxane structural unit is derived from one or more of amino-containing silsesquioxane monomers (amino-containing POSS monomers) and hydroxyl-containing silsesquioxane monomers (hydroxyl-containing POSS monomers).

[0032] In some specific embodiments, the thiocarbonyl sulfur functional group is derived from a carboxyl-containing RAFT reagent. The carboxyl-containing RAFT reagent includes one or more of bis(carboxymethyl)trithiocarbonate (CAS: 6326-83-6), 4-cyanopentanoic acid dithiobenzoate (CAS: 201611-92-9), and 4-cyano-4-[(dodecylthioalkylthiocarbonyl)thioalkyl]valerate (CAS: 870196-80-8).

[0033] In some specific embodiments, the thiocarbonylsulfur-containing silsesquioxane unit is derived from thiocarbonylsulfur-containing silsesquioxane compounds (thiocarbonylsulfur-containing POSS compounds), which are obtained by reacting an amino-containing POSS monomer or a hydroxyl-containing POSS monomer with a carboxyl-containing RAFT reagent. Specifically, an amino-containing POSS monomer and a carboxyl-containing RAFT reagent are linked by an amidation reaction to form an amide bond, or a hydroxyl-containing POSS monomer and a carboxyl-containing RAFT reagent are linked by an esterification reaction to form an ester bond.

[0034] The amino-containing POSS monomers are selected from octaaminopropyl POSS or octaaminophenyl POSS, and the hydroxyl-containing POSS monomers are selected from polyhydroxy POSS, wherein the molecular cage of the POSS has 2 to 8 hydroxyl groups.

[0035] Sulphocarbonyl sulfide (PSS) compounds can achieve precise control over their structure and properties through controlled free polymerization. Unlike other polymerizable PSSs (such as vinyl PSSs), SSS compounds with sulfphocarbonyl sulfide have RAFT end groups on their surface, lack reactive carbon-carbon double bonds, and do not possess crosslinking functional groups. During polymerization, they only participate in intramolecular chain growth as dormant end groups, and there are no covalent crosslinking sites between particles. Chemical covalent bonds cannot be formed between particles; only independent chain growth of single particles occurs. Therefore, crosslinking aggregation does not occur, and the polymer structure truly achieves polymer chain growth from the PSS core outward, while avoiding possible chemical crosslinking between PSS spheres.

[0036] In some specific embodiments, the cyano-containing multi-component copolymer unit includes cyano-based structural units and functional structural units containing double bonds. The cyano-based structural units provide strongly polar groups, forming strong van der Waals forces with the current collector (aluminum foil) and the surface of the active material, thereby generating good adhesion. If only cyano-based structural units are used, the material would be too hard and brittle. The functional structural units containing double bonds, as copolymer units, are used to adjust the ratio of the two structural units, adjusting the flexibility. While providing sufficient mechanical support to prevent the electrode structure from collapsing, this avoids brittleness caused by excessively high modulus, thus synergistically improving flexibility and jointly ensuring the integrity of the electrode during long-term cycling.

[0037] In some specific embodiments, the cyano-based structural unit comprises 20% to 98% by mass in the cyano-containing multi-component copolymer unit. Too much cyano-based structural unit will result in excessive hardness and brittleness; too little will affect peel strength and ion transport, thereby impacting adhesive strength and ionic conductivity.

[0038] Preferably, the cyano-based structural unit has a mass percentage content of 55% to 90% in the cyano-containing multi-component copolymer unit.

[0039] In some specific embodiments, the cyano-based structural unit is derived from a cyano-based monomer, which is selected from at least one of acrylonitrile, methacrylonitrile, cyanoethyl acrylate, cyanoethyl methacrylate, 2-ethyl cyanoacrylate, 2-ethyl cyanomethacrylate, N-cyanomethylacrylamide, and α-cyanostylstyrene. The cyano-based monomer is preferably acrylonitrile or methacrylonitrile.

[0040] In some specific embodiments, the double-bonded functional structural unit is derived from the double-bonded functional monomer, which is selected from at least one of acrylate monomers, aromatic vinyl monomers, and vinyl ester monomers.

[0041] More specifically, acrylate monomers include, but are not limited to, methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, tert-butyl acrylate, n-pentyl acrylate, isoamyl acrylate, n-hexyl acrylate, n-octyl acrylate, isooctyl acrylate, isobornyl acrylate, phenoxyethyl acrylate, dicyclopentenyl acrylate, cyclohexyl acrylate, benzyl acrylate, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, n-pentyl methacrylate, isoamyl methacrylate, and methacrylic acid. Hexyl methacrylate, isooctyl methacrylate, isobornyl methacrylate, phenoxyethyl methacrylate, dicyclopentenyl methacrylate, cyclohexyl methacrylate, benzyl methacrylate, polyethylene glycol mono[(meth)acrylate] ester, aminoethyl methacrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, tert-butylaminoethyl methacrylate, glycidyl methacrylate, tetrahydrofuran methacrylate, ethoxylated ethylene glycol diacrylate, ethoxylated ethylene glycol dimethacrylate, allyl methacrylate, diallyl phthalate, pentaerythritol diacrylate or pentaerythritol dimethacrylate.

[0042] More specifically, aromatic vinyl monomers include, but are not limited to, styrene and substituted styrene, wherein the substituted styrene includes, but is not limited to, α-methylstyrene, vinyltoluene, 2,4-dimethylstyrene, ethylstyrene, isopropylstyrene, butylstyrene, phenylstyrene, cyclohexylstyrene, benzylstyrene, crotonylstyrene, divinylbenzene, divinyltoluene, divinyldimethylbenzene, trivinylbenzene, vinylnaphthalene, and p-tert-butylstyrene.

[0043] The vinyl ester monomers include, but are not limited to, at least one of vinyl acetate, vinyl propionate, vinyl butyrate, vinyl valerate, and neopentanoate.

[0044] Preparation method of positive electrode binder: First, prepare a POSS-type compound containing thiocarbonyl sulfur, then copolymerize it with cyano monomers and functional monomers containing double bonds in a certain proportion under an initiator via a free radical copolymerization reaction to obtain a polymer; the polymer emulsion is spray-dried to obtain positive electrode binder powder. The initiator is one or more of ammonium persulfate, potassium persulfate, and sodium persulfate, and the amount added is 0.05~0.2% of the total mass of the monomers.

[0045] Secondly, the present invention provides a positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector, the positive active material layer comprising a positive active material, a conductive agent, and a positive binder as described in the first aspect. The conductive agent includes, but is not limited to, at least one of conductive carbon black, conductive graphite, Ketjen black, acetylene black, carbon nanotubes, carbon fibers, graphene, and conductive polymers. The solvent used in the positive electrode slurry includes, but is not limited to, NMP (N-methylpyrrolidone).

[0046] The positive electrode active material is at least one of lithium cobalt oxide, lithium iron phosphate, lithium nickel cobalt aluminum oxide, or lithium nickel cobalt manganese oxide. By using the above-mentioned positive electrode active material in combination with the positive electrode binder, the cyano group in the positive electrode binder interacts polarly with the polar groups in the positive electrode active material and metal elements (such as hydroxyl groups, cobalt, iron, aluminum, manganese, etc. on the surface of the active material), thereby generating better adhesion.

[0047] Preparation method of positive electrode sheet: The positive electrode active material, conductive agent and positive electrode binder of the present invention are dispersed in the solvent NMP and stirred to form a positive electrode slurry. The above positive electrode slurry is coated on the surface of the current collector, and after drying, rolling, cutting and other processes, the positive electrode sheet is obtained.

[0048] Thirdly, the present invention provides a lithium-ion battery, including a positive electrode as described in the second aspect.

[0049] Because the POSS containing thiocarbonyl sulfur in the positive electrode binder of this invention has good lithium-ion conductivity, the lithium-ion battery prepared by the positive electrode sheet containing this binder has a low internal resistance, thereby significantly improving the overall performance of the lithium-ion battery.

[0050] The specific embodiments of the present invention will be further explained and described below through examples and comparative examples.

[0051] Unless otherwise specified, all reagents, materials, and instruments used in the following description are conventional reagents, materials, and instruments, all of which are commercially available. The reagents involved can also be synthesized using conventional synthetic methods. Unless otherwise specified, the methods in the examples are conventional methods in the art. Monomers conforming to this invention are commercially available.

[0052] Example 1

[0053] 1) Preparation of positive electrode binder:

[0054] First, a POSS-type compound containing thiocarbonyl sulfide was prepared: 0.1 mol of octaaminopropyl POSS and 0.8 mol of 4-cyano-4-[(dodecylthioalkylthiocarbonyl)thioalkyl]valeric acid were reacted via an amidation reaction in the presence of the dehydrating agent N,N'-dicyclohexylcarbodiimide (DCC) (8.8 mol) and the promoter 1-hydroxybenzotriazole (HOBt) (8.8 mol). The specific operational steps were as follows: In a reaction vessel, 4-cyano-4-[(dodecylthioalkylthiocarbonyl)thioalkyl]valeric acid and 1-hydroxybenzotriazole (HOBt) were dissolved in anhydrous dichloromethane, and nitrogen gas was introduced to create an inert protective atmosphere. Subsequently, under continuous stirring, the N,N'-dicyclohexylcarbodiimide (DCC) solution dissolved in anhydrous dichloromethane was slowly added dropwise to fully dissolve the octaaminopropyl POSS in the solution, and then gradually added dropwise to the reaction system. After all the materials were added, the ice-water bath was removed, and the reaction system was kept at room temperature with continuous stirring for 48 hours. After the reaction was completed, the mixture was filtered to remove the insoluble N,N'-dicyclohexylurea byproduct generated in the reaction; then the organic solvent in the system was removed by vacuum distillation, and the remaining crude product was further separated and purified by silica gel column chromatography, finally yielding a yellow solid product, which is a POSS compound containing thiocarbonyl sulfide.

[0055] The comonomer used to prepare the positive electrode binder includes the following components: 80 parts acrylonitrile, 18 parts tert-butyl acrylate, and 2 parts POSS compounds containing thiocarbonyl sulfide. The preparation of the positive electrode binder includes the following steps: emulsifying the comonomer with deionized water and an emulsifier, adding 0.1 parts ammonium persulfate, and stirring under heating to obtain the polymer through a free radical copolymerization reaction; then spray-drying the polymer emulsion to obtain the positive electrode binder powder.

[0056] 2) Preparation of the positive electrode sheet:

[0057] The raw materials for the positive electrode slurry include NMP and solid components; the solid components include the following components by mass percentage: NCM811 97.8%, conductive carbon black (Super .P) 2%, and the prepared positive electrode binder 1.2%, which are dispersed in NMP and stirred to obtain the positive electrode slurry.

[0058] The positive electrode slurry is coated onto carbon-coated aluminum foil, dried at 120°C, and then rolled and cut into strips of 20mm×100mm to obtain the positive electrode sheet.

[0059] 3) Preparation of lithium-ion batteries:

[0060] The prepared positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, with the separator positioned between the positive and negative electrodes to act as an separator, thus forming an electrode assembly. The electrode assembly is placed in an outer package, injected with commercially available electrolyte, and sealed. After processes such as electrolyte injection, formation, and venting, a lithium-ion secondary battery is obtained.

[0061] Example 2

[0062] This embodiment uses most of the operating steps and monomer types from Example 1 to prepare the positive electrode binder, positive electrode sheet, and lithium-ion secondary battery. The difference is that the monomer content in the positive electrode binder is 88 parts acrylonitrile, 10 parts tert-butyl acrylate, and 2 parts POSS-type compounds containing sulfur carbonyl sulfide.

[0063] Example 3

[0064] This embodiment uses most of the operating steps and monomer types from Example 1 to prepare the positive electrode binder, positive electrode sheet, and lithium-ion secondary battery. The difference is that the monomer content in the positive electrode binder is 55 parts acrylonitrile, 43 parts tert-butyl acrylate, and 2 parts POSS-type compounds containing sulfur carbonyl sulfide.

[0065] Example 4

[0066] This embodiment uses most of the operating steps and monomer types from Example 1 to prepare the positive electrode binder, positive electrode sheet, and lithium-ion secondary battery. The difference is that the monomer content in the positive electrode binder is 25 parts acrylonitrile, 73 parts lauryl methacrylate, and 2 parts POSS-type compounds containing sulfur carbonyl sulfide.

[0067] Example 5

[0068] This embodiment uses most of the operating steps and monomer types from Example 1 to prepare the positive electrode binder, positive electrode sheet, and lithium-ion secondary battery. The difference is that the monomer content in the positive electrode binder is 15 parts acrylonitrile, 83 parts lauryl methacrylate, and 2 parts POSS-type compounds containing sulfur carbonyl sulfide.

[0069] Example 6

[0070] This embodiment uses most of the operating steps and monomer types from Example 1 to prepare the positive electrode binder, positive electrode sheet, and lithium-ion secondary battery. The difference is that the monomer content in the positive electrode binder is 73 parts methacrylonitrile, 15 parts styrene, 10 parts lauryl acrylate, and 2 parts POSS-type compounds containing thiocarbonyl sulfide.

[0071] Example 7

[0072] This embodiment uses most of the operating steps and monomer types from Example 1 to prepare the positive electrode binder, positive electrode sheet, and lithium-ion secondary battery. The difference is that the monomer content in the positive electrode binder is 85 parts methacrylonitrile, 10 parts vinyl propionate, and 5 parts POSS-type compounds containing sulfur carbonyl sulfide.

[0073] Example 8

[0074] This embodiment uses most of the operating steps and monomer types from Example 1 to prepare the positive electrode binder, positive electrode sheet, and lithium-ion secondary battery. The difference is that the monomer content in the positive electrode binder is 80 parts acrylonitrile, 10 parts tert-butyl acrylate, and 10 parts POSS-type compounds containing sulfur carbonyl sulfide.

[0075] Example 9

[0076] This embodiment uses most of the operating steps and monomer types from Example 1 to prepare the positive electrode binder, positive electrode sheet, and lithium-ion secondary battery. The difference is that the monomer content in the positive electrode binder is 89.5 parts acrylonitrile, 10 parts tert-butyl acrylate, and 0.1 parts POSS-type compounds containing sulfur carbonyl sulfide.

[0077] Example 10

[0078] This embodiment uses most of the operating steps and monomer types from Example 1 to prepare the positive electrode binder, positive electrode sheet, and lithium-ion secondary battery. The difference is that the RAFT reagent containing the carboxyl group is 4-cyanopentanoic acid dithiobenzoate.

[0079] Example 11

[0080] This embodiment uses most of the operating steps and monomer types from Example 1 to prepare the positive electrode binder, positive electrode sheet, and lithium-ion secondary battery. The difference is that the POSS used to synthesize thiocarbonyl sulfide is octahydroxy POSS.

[0081] Example 12

[0082] This embodiment uses most of the operating steps and monomer types from Example 1 to prepare the positive electrode binder, positive electrode sheet, and lithium-ion secondary battery. The difference is that the molar ratio of the RAFT reagent containing carboxyl groups to octaaminopropyl POSS is 4:1.

[0083] Comparative Example 1

[0084] The positive electrode binder used in this comparative example is polyacrylonitrile; everything else is the same as in Example 1.

[0085] Comparative Example 2

[0086] The positive electrode binder used in this comparative example is PVDF; everything else is the same as in Example 1.

[0087] Comparative Example 3

[0088] This comparative example uses most of the operating steps in Example 1 to prepare the positive electrode binder, positive electrode sheet and lithium-ion battery. The difference from Example 1 is that no POSS compound containing sulfur carbonyl sulfide was added in the preparation of the positive electrode binder, and the monomers used were 80 parts acrylonitrile and 20 parts tert-butyl methacrylate.

[0089] Comparative Example 4

[0090] This comparative example uses most of the operating steps in Example 1 to prepare the positive electrode binder, positive electrode sheet, and lithium-ion battery. The difference from Example 1 is that in the preparation of the positive electrode binder, the POSS monomer used is octavinyl POSS instead of POSS containing thiocarbonyl sulfide; specifically, the monomer used is 80 parts acrylonitrile, 18 parts tert-butyl acrylate, and 2 parts octavinyl POSS.

[0091] Comparative Example 5

[0092] This comparative example uses most of the operating steps in Example 1 to prepare the positive electrode binder, positive electrode sheet and lithium-ion battery. The difference from Example 1 is that no cyano monomers are added in the preparation of the positive electrode binder. The monomers used are 20 parts styrene, 78 parts tert-butyl acrylate and 2 parts POSS containing sulfur carbonyl sulfide.

[0093] Performance testing:

[0094] To better understand the present invention, the positive electrode binder, positive electrode sheet and lithium-ion battery prepared in the above embodiments and comparative examples were tested as follows, and the test results are shown in Table 1.

[0095] [Flexibility]: The positive electrode sheets prepared in the above embodiments and comparative examples were tested according to GB / T 1731-2020 "Test Method for Coating Film Flexibility" (Extended Method for Electrode Flexibility Test); the electrode sheets were tightly wound around cylindrical needles of different diameters, and it was observed whether the electrode coating cracked or peeled off. The smallest needle diameter at which the electrode coating did not crack or peel off was recorded, in mm. The smaller the value, the better the electrode flexibility.

[0096] [Peel Strength]: The bonding strength of the positive electrode sheet was tested using a GBH-1 tensile testing machine, referring to GB / T 2792-2014 "Test Method for Peel Strength of Adhesive Tapes", employing the 180° peel method. The test specimen size was 20mm × 100mm, and the tensile rate was set to 50mm / min. The positive electrode sheet was flatly fixed on a stainless steel plate, and the electrode coating was tightly adhered to it using special adhesive tape. The coating was then peeled off from the aluminum foil substrate at a 180° angle, and the stable peel force was recorded. The peel strength was then calculated, with units of N / m.

[0097] [Internal Resistance] The DC internal resistance (DCIR) of the battery was tested using the constant current pulse discharge method. The battery was discharged at 3C for 30 seconds at 50% SOC.

[0098] [Initial Charge / Discharge Efficiency]: The battery was placed in an environment of 25℃±2℃ and charged at a constant current of 0.5C to 4.2V, then switched to constant voltage charging at 4.2V, with the cutoff current set to 0.05C. The charging capacity was recorded. After resting for 10 minutes, the battery was discharged at a constant current of 0.5C to 3.0V, and the discharge capacity was recorded. The initial charge / discharge efficiency was calculated using the formula: Initial Efficiency = (Discharge Capacity / Charge Capacity) × 100%.

[0099] [Capacity Retention Rate]: Under a constant temperature environment of 25℃±2℃, the initial discharge capacity of the pouch battery was first tested and recorded as the capacity before storage. The battery was then discharged at a constant current of 0.5C to 2.75V, and then charged at a constant current of 1C to 4.2V. The charging was then switched to constant voltage charging with a cutoff current of 0.05C. After charging was completed, the battery was discharged at a constant current of 1C to 2.75V, and the discharge capacity C1 of the first cycle was recorded. This charge-discharge cycle was repeated for 500 cycles, and the discharge capacity C of the 500th cycle was recorded. 500 Calculate the cycle capacity retention rate using the formula: Cycle capacity retention rate = (C... 500 / C1)×100%.

[0100] [Ionic Conductivity Test of Adhesive]: A stainless steel sheet / adhesive film / stainless steel sheet blocked symmetric cell was assembled in an argon-atmospheric glove box. After assembly, the cell was allowed to stand for 12 hours to eliminate internal stress. AC impedance testing was performed using a Chenhua CHI660E electrochemical workstation with a small perturbation voltage of 10mV applied. The test frequency range was set to 1Hz~1MHz. The intercept of the measured AC impedance spectrum curve with the real axis (X-axis) represents the bulk resistance R of the adhesive film. b .

[0101] The ionic conductivity at 25℃ is calculated using the following formula:

[0102] ·S)

[0103] In the formula: R is the ionic conductivity, in S·cm⁻¹; L is the thickness of the film sample, in cm; S is the effective contact area between the stainless steel sheet and the film, in cm²; b The resistance of the adhesive film is expressed in Ω.

[0104] Table 1 Test Results

[0105]

[0106] The test results in Table 1 show that:

[0107] The test results of Examples 1-7, 10 and Comparative Examples 1-2 show that: the positive electrode binder of this application forms a three-dimensional cross-linked network of "POSS core - branched chain of cyano-containing multi-component copolymer unit", which significantly improves the ionic conductivity and significantly reduces the internal resistance of the battery compared with the linear structure of existing positive electrode binders; it avoids the uneven local bonding force caused by the aggregation of groups in the linear structure, thereby improving the interfacial bonding strength; the branched chain with POSS as the core can buffer the volume expansion of the positive electrode active material during charging and discharging, disperse stress concentration, prevent electrode cracking, and significantly enhance the stability of the electrode structure, thereby improving the cycle capacity retention rate.

[0108] The test results of Example 1 and Comparative Example 3 show that, compared with the positive electrode binder obtained without the addition of POSS-type compounds containing sulfur-containing carbonyl sulfide, the positive electrode binder of this application can directly and quickly migrate through the POSS cavity, reducing migration resistance and significantly improving ionic conductivity. The battery capacity retention rate at 1C rate is significantly improved, solving the problems of excessive internal resistance and rapid capacity decay in high-rate scenarios. The branched chain with POSS as the core can buffer the volume expansion of the positive electrode active material during charging and discharging, disperse stress concentration, prevent electrode cracking, and significantly enhance the stability of the electrode structure, thereby improving the cycle capacity retention rate.

[0109] The test results of Example 1 and Comparative Example 4 show that: compared with the positive electrode binder obtained by vinyl-based POSS monomers, POSS containing sulfur carbonyl sulfur can achieve precise control of structure and performance through controlled free polymerization. The polymer structure truly realizes the growth of polymer chains from the POSS core outward, while avoiding possible chemical cross-linking between POSS structural units. In contrast, vinyl-based POSS is prone to agglomeration and hardness, and the positive electrode sheet is even harder and more brittle. Moreover, due to problems such as agglomeration and hardness, the ionic conductivity is low, the battery internal resistance is too high, and the interfacial bonding strength is limited, resulting in a significant reduction in cycle capacity retention during long-term battery cycling.

[0110] The test results of Example 1 and Comparative Example 5 show that, compared to the positive electrode binder obtained without the addition of cyano monomers, although the resulting positive electrode sheet has better flexibility, the lack of cyano groups to generate polar interactions leads to lower ionic conductivity, higher internal resistance, and reduced interfacial bonding strength. During long-term battery cycling, the electrode is prone to cracking or detachment of active material due to the volume expansion / contraction of the active material, resulting in a significant decrease in cycle capacity retention. The cyano structural units and the functional structural units containing double bonds, as multi-component copolymer units, provide sufficient mechanical support to prevent electrode structure collapse while synergistically improving flexibility, thus ensuring the integrity of the electrode during long-term cycling.

[0111] The test results of Examples 1 to 6 show that when the mass percentage of cyano-based structural units in cyano-containing multi-component copolymer units is 55% to 90%, the flexibility, adhesive strength, and rate performance reach the optimal balance.

[0112] The test results from Examples 1 and 7-9 show that when the mass percentage of the first polymerization unit in the copolymer is 0.5% to 5%, the overall performance of the positive electrode binder, positive electrode sheet, and battery is better. If the proportion of the first polymerization unit is too high, agglomeration is likely to occur, the interfacial bonding strength will decrease, the flexibility of the positive electrode sheet will be poor, and the cycle performance of the battery will be poor; if the proportion is too low, it will not play a role in ion transport, resulting in low ion conductivity and increased internal resistance.

[0113] This also explains why the overall performance of the positive electrode binder, positive electrode sheet, and battery is better when the positive electrode binder contains silsesquioxane structure of thiocarbonyl sulfur, cyano group, and functional structural unit containing double bond.

[0114] The present invention has been further described above with reference to specific embodiments. However, it should be understood that the specific description herein should not be construed as limiting the nature and scope of the present invention. Various modifications made to the above embodiments by those skilled in the art after reading this specification are all within the scope of protection of the present invention.

Claims

1. A positive electrode binder, characterized in that, The positive electrode binder comprises a copolymer with a branched structure formed by silsesquioxane units containing thiocarbonyl sulfide as the core. The copolymer comprises a first polymerization unit and a second polymerization unit. The first polymerization unit is a silsesquioxane unit containing thiocarbonyl sulfide, which includes a silsesquioxane structural unit and a thiocarbonyl sulfide functional group. The silsesquioxane structural unit is connected to the thiocarbonyl sulfide functional group. The second polymerization unit is a branched chain formed outside the core, including a cyano-containing multi-component copolymer unit. The first polymerization unit has a mass percentage content of 0.5% to 5% of the copolymer; The molar ratio of the silsesquioxane structural unit to the thiocarbonyl sulfur functional group is 1:(2~8); The cyano-containing multi-component copolymer unit includes cyano-based structural units and functional structural units containing double bonds, wherein the mass percentage of the cyano-based structural units in the cyano-containing multi-component copolymer unit is 20% to 98%.

2. The positive electrode binder according to claim 1, characterized in that, The silsesquioxane structural units are derived from one or more of amino-containing silsesquioxane monomers and hydroxyl-containing silsesquioxane monomers.

3. The positive electrode binder according to claim 1, characterized in that, The thiocarbonyl sulfur functional group is derived from a RAFT reagent containing a carboxyl group.

4. The positive electrode binder according to claim 1, characterized in that, The cyano-based structural unit is derived from cyano-based monomers, which are selected from at least one of acrylonitrile, methacrylonitrile, cyanoethyl acrylate, cyanoethyl methacrylate, 2-ethyl cyanoacrylate, 2-ethyl cyanomethacrylate, N-cyanomethylacrylamide, and α-cyanostylstyrene.

5. The positive electrode binder according to claim 1, characterized in that, The functional structural unit containing double bonds is derived from the functional monomer containing double bonds, and the functional monomer containing double bonds is selected from at least one of acrylate monomers, aromatic vinyl monomers, and vinyl ester monomers.

6. A positive electrode sheet, characterized in that, The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector, wherein the positive active material layer includes the positive binder as described in any one of claims 1 to 5.

7. A lithium-ion battery, characterized in that, Including the positive electrode sheet as described in claim 6.

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