Polymer-based composite particle and naked battery cell

Abstract

The invention discloses a polymer-based composite particle and a naked battery cell, and relates to the field of electrochemical energy storage. The polymer-based composite particles comprise a pore-forming agent and a high-molecular polymer; the naked battery cell comprises an anode, a cathode and a diaphragm, buffer coatings are arranged on two sides of at least one of the anode, the cathode and the diaphragm in a corner area, and the buffer coatings comprise polymer-based composite particles. The polymer-based composite particles are stacked in the corner area of the diaphragm or the pole piece to form the buffer coating, stacked pores can serve as a lithium ion transmission channel, the high elasticity of the buffer coating can effectively absorb expansion stress and inhibit the volume expansion rate, the pore-forming agent in the polymer-based composite particles volatilizes in the baking stage to form a porous framework, and the pore-forming agent in the polymer-based composite particles can effectively absorb the expansion stress and inhibit the volume expansion rate. The electrolyte can be absorbed, the electrolyte retention capacity is improved, pores can provide a certain expansion space, the lithium precipitation phenomenon at corners is reduced, high dynamic performance is maintained, and the cycling stability is improved.

Description

Technical Field

[0001] This invention relates to the field of electrochemical energy storage, and more particularly to polymer-based composite particles and bare battery cells. Background Technology

[0002] The rise of outdoor tourism has driven the development of the outdoor drone and action camera market, but it has also brought new challenges. Higher energy density, faster charging capabilities, and longer cycle life have become the mainstream directions for product battery development.

[0003] However, with the increase in cell energy density, the risk of lithium plating at corners is inevitably exacerbated, especially in ultra-thick cells. Currently, in wound cell structures, internal stress tends to concentrate at corners, where electrode expansion and compression can cause electrolyte drying, leading to lithium plating. This is particularly pronounced in high-silicon systems, where the anode expansion rate is even greater, potentially causing S-shaped deformation in flat areas. During fast charging or long-cycle charging, electrode expansion laterally compresses the electrodes at corners, causing lithium plating. This corner lithium plating further laterally compresses the inner ring, preventing stress release and resulting in S-shaped deformation. Ultimately, this reduces battery cycle life and increases safety risks. Summary of the Invention

[0004] This invention provides polymer-based composite particles and bare battery cells. By dispersing a pore-forming agent in a polymer matrix and forming polymer-based composite particles to provide a highly elastic buffering effect, these particles are placed at the corner areas of the separator or electrode to form a buffer coating. This effectively absorbs the expansion stress of the electrode. Furthermore, the polymer-based composite particles can completely volatilize after the bare battery cell is baked, leaving pores with more buffer space. This reduces the phenomenon of lithium plating at corners and effectively suppresses the volume expansion of the battery cell during cycling. Moreover, the buffer coating formed by the accumulation of polymer-based composite particles has many pores, which does not affect the transport of lithium ions, effectively delays the cycle capacity decay rate, and improves the kinetic performance.

[0005] To address the aforementioned technical problems, one objective of this invention is to provide polymer-based composite particles, comprising a pore-forming agent and a polymer, wherein the pore-forming agent is coated or embedded in the polymer, and the mass fraction of the pore-forming agent in the polymer-based composite particles is 5%-20%.

[0006] In some embodiments, the polymer comprises at least one selected from polyvinylidene fluoride, polymethyl methacrylate, aramid, polyethylene, polypropylene, polyethylene-polypropylene copolymer, polyethylene terephthalate, polyphenylene sulfide, polyimide, polystyrene, poly4-methyl-1-pentene, polyvinylidene fluoride-hexafluoropropylene, polytetrafluoroethylene, and polysulfone.

[0007] In some embodiments, the pore-forming agent includes at least one of iodine, naphthalene, and camphor.

[0008] In some embodiments, the Dv50 particle size of the polymer-based composite particles is 2-10 μm.

[0009] In some embodiments, the polymer-based composite particles are prepared by electrospinning, freezing, pulverizing and sieving a spinning solution containing a pore-forming agent and a polymer.

[0010] In some embodiments, the spinning solvent includes at least one of acetone, N,N-dimethylformamide, and N-methylpyrrolidone.

[0011] In some embodiments, the method for preparing the polymer-based composite particles includes the following steps: (1) The polymer and spinning solvent are mixed, heated and stirred to dissolve, cooled to below 40 °C, and then a pore-forming agent is added. The mixture is stirred in the dark at a temperature below 40 °C to obtain a spinning solution. (2) The spinning solution is subjected to electrospinning treatment to obtain a fiber membrane; (3) The fiber membrane is frozen with liquid nitrogen, then ground, sieved and dried to prepare polymer-based composite particles.

[0012] To solve the above-mentioned technical problems, the third objective of this invention is to provide a bare battery cell, including an anode, a cathode, and a separator. The anode, cathode, and separator are each independent and include corner regions and straight regions. At least one of the anode, cathode, and separator has a buffer coating on both sides of the corner region. The thickness of the buffer coating is less than 12 μm, and the buffer coating includes polymer-based composite particles.

[0013] In some embodiments, the anode includes an anodic active material, which comprises a silicon-based material with a mass fraction of S; When S≤10%, the Dv50 particle size of the polymer-based composite particles is 2-6μm; When 10%≤S≤40%, the Dv50 particle size of the polymer-based composite particles is 3-8μm; When 40%≤S≤60%, the Dv50 particle size of the polymer-based composite particles is 4-10μm.

[0014] In some embodiments, the thickness of the buffer coating is 5-12 μm.

[0015] In some implementations, S ≤ 60%.

[0016] In some embodiments, the silicon-based material includes porous amorphous carbon and / or porous crystalline material, as well as silicon and / or silicon oxide, wherein the silicon and / or silicon oxide is supported in the porous amorphous carbon and / or porous crystalline material.

[0017] In some implementations, the silicon-based material is silicon-carbon.

[0018] In some embodiments, the buffer coating is applied to both sides of the corner area of ​​the anode, cathode, or diaphragm by spraying or roller coating.

[0019] In some embodiments, the bare battery cell is a semi-finished product prepared after winding and before baking.

[0020] In some embodiments, the diaphragm includes a base membrane having a ceramic layer disposed on one or both sides, the ceramic layer being located between the base membrane and the buffer coating.

[0021] Compared with the prior art, the present invention has the following beneficial effects: 1. This application utilizes polymer-based composite particles to form a buffer coating in the corner area of ​​the separator or electrode, which can effectively absorb the expansion stress of the electrode. Furthermore, the pore-forming agent in the polymer-based composite particles volatilizes during the baking stage to form a porous framework, which can absorb electrolyte, increase electrolyte retention, and reserve expandable space, effectively suppressing the volume expansion of the cell during cycling and reducing the phenomenon of lithium plating at the corner. Moreover, the buffer coating formed by the accumulation of polymer-based composite particles has many pores, which does not affect the transport of lithium ions and can maintain high kinetic performance.

[0022] 2. The particle size of the polymer-based composite particles in the buffer coating of this application is adjusted according to the different silicon doping amounts of the anode. As the silicon doping amount increases, the particle size of the polymer-based composite particles gradually increases. When large-diameter polymer-based composite particles are stacked, more expandable space is reserved, and the skeleton has high support, which can effectively expand the interlayer spacing to reserve electrolyte storage space, thus improving lithium plating and volume expansion in high-silicon-doped cells. When small-diameter polymer-based composite particles are stacked, less expandable space is reserved, which can meet the needs of improving lithium plating and volume expansion in low-silicon-doped cells. At the same time, the higher the density of the buffer coating after the small-diameter polymer-based composite particles are stacked, the better the mechanical binding effect on the electrode sheet, and the better the suppression of volume expansion. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the winding structure of the bare battery cell in Embodiment 1 of the present invention; Figure 2 This is a schematic diagram of the winding structure of the bare battery cell in Embodiment 21 of the present invention; Figure 3 This is a schematic diagram of the winding structure of the bare battery cell in Embodiment 22 of the present invention; The reference numerals in the accompanying drawings are as follows: 1. Diaphragm; 2. Anode; 3. Cathode; 4. Buffer coating. Detailed Implementation

[0024] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0026] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0027] As used in this article: In these embodiments, unless otherwise specified, the portions and percentages are all by weight.

[0028] "And / or" is used to indicate that one or both of the described situations may occur, for example, A and / or B includes (A and B) and (A or B).

[0029] In the description of this invention, it should be understood that the terms "upper", "lower", "left", "right", "top", "bottom", etc., indicating orientation or positional relationship are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0030] This application provides polymer-based composite particles, including a pore-forming agent and a polymer, wherein the pore-forming agent is coated or embedded in the polymer, and the mass fraction of the pore-forming agent in the polymer-based composite particles is 5%-20%.

[0031] This application disperses a pore-forming agent in a polymer. The pore-forming agent used has a suitable volatilization temperature and leaves tiny pores in the polymer matrix, forming a porous framework. This not only does not affect the structural stability but also provides a certain buffer gap. When coated on the separator or electrode corners of the battery cell, the high elasticity of the polymer can effectively absorb expansion stress, and the gaps left by the volatilization of the pore-forming agent in the polymer-based composite particles can also absorb electrolyte, providing ample buffer space and effectively suppressing the volume expansion rate during battery cell cycling, reducing lithium plating at corners. Furthermore, by forming the polymer into spherical particles, a coating can be deposited in the corners of the separator or electrode. This particle-formed coating has high porosity, does not affect the lithium-ion transport efficiency, and maintains a high cycle capacity of the battery cell.

[0032] In some embodiments, the mass fraction of the pore-forming agent in the polymer-based composite particles is any one or a range between any two of 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, and 20%.

[0033] The mass fraction of the pore-forming agent in the polymer-based composite particles of this application is controlled within the above-mentioned range. After the pore-forming agent volatilizes, it can provide sufficient pores to absorb electrolyte and reserve compressible space. At the same time, it can also ensure the support and resilience of the porous skeleton of the polymer-based composite particles. It can open up the interlayer spacing at the corner to reserve expandable space, effectively absorb expansion stress, and reduce the risk of lithium plating.

[0034] It should be noted that the pore-forming agent content in the polymer-based composite particles was tested using the ethanol release-UV spectrophotometry method. Ethanol does not swell the polymer material; what is released is the free pore-forming agent on the particle surface / inside the micropores. The solution concentration can be read from the absorption peak, and the pore-forming agent content in the sample can be calculated. The specific steps include: (1) Standard curve: Prepare an ethanol solution containing pore-forming agent of 0-60 mg / L, scan UV-Vis, take the absorbance at λmax = 290 nm, and plot the Ac standard curve (linear range above 0.999). (2) Sample release: Weigh 30 mg of polymer-based composite particles, add 10 mL of anhydrous ethanol, shake at 25 °C in the dark for 2 h, then centrifuge and collect the supernatant, and record the volume of the supernatant as V (L). (3) Measure the A value at 290 nm in the supernatant and substitute it into the Ac standard curve to obtain the concentration c (mg / L); (4) Calculate the mass of pore-forming agent (mg) according to the following formula: c (mg / L) × V (L) × dilution factor; Pore-forming agent content (wt%) = (pore-forming agent mass / sample mass) × 100; repeated 3 times, RSD < 3%.

[0035] In some embodiments, the polymer comprises at least one selected from polyvinylidene fluoride, polymethyl methacrylate, polyethylene, polypropylene, polyethylene-polypropylene copolymer, polyethylene terephthalate, polyphenylene sulfide, polyimide, polystyrene, poly4-methyl-1-pentene, polyvinylidene fluoride-hexafluoropropylene, polytetrafluoroethylene, and polysulfone. This polymer component, similar to the membrane substrate component, can provide support and cushioning, widening the interlayer spacing at corners to allow for expansion space and reduce lithium plating.

[0036] In some embodiments, the pore-forming agent includes at least one of iodine, naphthalene, and camphor. This pore-forming agent has a suitable volatilization temperature; it can completely volatilize without residue when the baking temperature during the cell manufacturing process reaches approximately 80°C, leaving a large number of pores in the polymer-based composite particles.

[0037] In some embodiments, the average molecular weight of the polymer is 80 × 10⁻⁶. 3 -700×10 3 g / mol.

[0038] In some embodiments, the Dv50 particle size of the polymer-based composite particles is 2-10 μm.

[0039] In some embodiments, the Dv50 particle size of the polymer-based composite particles is any one or a range between any two of 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, and 10μm.

[0040] The particle size of the polymer-based composite particles in this application is controlled within the above-mentioned range. If the particle size of the polymer-based composite particles is too small, the density after stacking will be too high, which will result in too few pores reserved between the particles, thereby affecting the transport of lithium ions and reducing the kinetic performance. If the particle size of the polymer-based composite particles is too large, it will result in the buffer coating in the corner area being too thick, affecting the overall thickness of the cell.

[0041] It should be noted that the Dv50 particle size of the polymer-based composite particles was determined using laser diffraction.

[0042] In some embodiments, the polymer-based composite particles are prepared by electrospinning, freezing, pulverizing and sieving a spinning solution containing a pore-forming agent and a polymer.

[0043] In some embodiments, the method for preparing polymer-based composite particles includes the following steps: (1) Mix the polymer and spinning solvent, heat and stir to dissolve, cool to below 40 °C and add pore-forming agent, stir in the dark at a temperature below 40 °C to obtain spinning solution; (2) Electrospinning the spinning solution to obtain a fiber membrane; (3) The fiber membrane was frozen with liquid nitrogen, then ground, sieved and dried to prepare polymer-based composite particles.

[0044] In some embodiments, in step (1), the mass ratio of the polymer to the spinning solvent is (10-15):(85-90).

[0045] In some embodiments, in step (1), the heating and stirring temperature is 40-70 °C and the time is 6-24 h.

[0046] In some embodiments, in step (1), the stirring temperature is 30-40 °C and the time is 0.5-3 h.

[0047] In some embodiments, in step (2), the electrospinning voltage is 15-20 kV, the flow rate is 1-3 mL / h, the needle take-up distance is 12-18 cm, the roller speed is 100-400 rpm, the temperature of the electrospinning environment is 20-30 ℃, and the relative humidity is 30%RH-50%RH.

[0048] In some embodiments, the thickness of the fiber membrane in step (2) is 40-60 μm.

[0049] In some implementations, the liquid nitrogen freezing process in step (2) takes 3-8 minutes.

[0050] In some embodiments, in step (2), the grinding rate is 10,000-20,000 rpm and the time is 20-40 s.

[0051] In some embodiments, in step (2), the drying temperature is 30-40 °C and the time is 3-5 h.

[0052] In some embodiments, the spinning solvent includes at least one of acetone, N,N-dimethylformamide, and N-methylpyrrolidone.

[0053] In some embodiments, when the polymer is polyvinylidene fluoride, the spinning solvent is preferably acetone.

[0054] In some embodiments, when the polymer is polymethyl methacrylate, the spinning solvent is preferably N,N-dimethylformamide.

[0055] This application provides a bare battery cell, including an anode, a cathode, and a separator. The anode, cathode, and separator are each independent and include corner regions and straight regions. At least one of the anode, cathode, and separator has a buffer coating on both sides of the corner region. The thickness of the buffer coating is less than 12 μm, and the buffer coating includes polymer-based composite particles.

[0056] This application utilizes polymer-based composite particles to form a buffer coating at the corners of a separator or electrode. These polymer-based composite particles have high elasticity, effectively absorbing electrode expansion stress. Furthermore, the pore-forming agent in the polymer-based composite particles can completely evaporate during the subsequent baking stage, forming a porous framework. This not only effectively absorbs electrolyte and increases electrolyte retention, but also provides expansion space, effectively suppressing cell volume expansion during cycling and reducing corner lithium plating. Moreover, the buffer coating formed by the accumulation of polymer-based composite particles has numerous pores, which do not affect lithium-ion transport, effectively delaying the cycle capacity decay rate and improving kinetic performance.

[0057] In some embodiments, the anode includes an anodic active material, which includes a silicon-based material with a mass fraction of S; When S≤10%, the Dv50 particle size of the polymer-based composite particles is 2-6μm; When 10%≤S≤40%, the Dv50 particle size of the polymer-based composite particles is 3-8μm; When 40%≤S≤60%, the Dv50 particle size of the polymer-based composite particles is 4-10 μm.

[0058] The particle size of the polymer-based composite particles in the buffer coating of this application is adjusted according to the different silicon doping levels of the anode. As the silicon doping level increases, the particle size of the polymer-based composite particles gradually increases. This is because with the increase of silicon doping in the anode, the electrode expands more during cycling, making it more prone to electrolyte compression and drying in the corner areas. Therefore, using larger-sized polymer-based composite particles allows the buffer coating to reserve more expandable space during stacking. Furthermore, the larger-sized polymer-based composite particles have higher skeletal support, effectively widening the interlayer spacing to reserve electrolyte storage space, thereby effectively reducing lithium plating, suppressing volume expansion, and ultimately improving cycle performance. Conversely, when the silicon doping level of the electrode is lower, the particle size of the polymer-based composite particles is smaller, resulting in less expandable space reserved by the buffer coating during stacking. This meets the buffering requirements for lower electrode expansion rates. At the same time, the smaller the particle size of the polymer-based composite particles, the higher the density of the buffer coating, resulting in better mechanical confinement of the electrode and better suppression of volume expansion.

[0059] It should be noted that the testing method for silicon-based materials in anolyte active materials includes the following steps: (1) Dissolving metal and alloy phases with HCl / HNO3: Cut the electrode into 2 mm × 2 mm fragments, place them in a 250 mL glass beaker, add 10 mL of 50% HCl solution and 5 mL of 65% HNO3 solution, and heat at low temperature until the reaction stops. (2) Perchloric acid (HClO4) fumes cause silicic acid to dehydrate into SiO2·xH2O precipitate: Add 20 mL of HClO4 to the solution in step (1), place it on a hot plate to evaporate until thick white fumes are emitted for the first time (about 190 °C), keep it under reflux for 15 min to completely dehydrate the silicic acid, cool it slightly and add 50 mL of hot water + 5 mL of pulp, filter it immediately with quantitative filter paper, wash the precipitate three times each with hot HCl solution and hot water, and finally rinse it with 5 mL of hot water; (3) Filtering, washing, and calcining at 1100 °C to constant weight: Place the filter paper and the precipitate into a pre-calcined platinum crucible, carbonize at low temperature, then transfer to a muffle furnace at 600 °C, heat to 1100 °C and calcinate for 30 min, cool in a dryer for 30 min, and weigh the crude SiO2 product with mass m1. (4) Add 3-5 drops of H2SO4 to the platinum crucible to moisten it, then add 3-5 mL of HF to convert SiO2 into SiF4 and volatilize it. Place it in a fume hood and evaporate it at low temperature until it is almost dry. Gradually raise the temperature to drive out the H2SO4, and then ignite it at 1100 °C to constant weight. The mass of the residue is m2. The mass difference (m1-m2) is the mass of pure SiO2, and then the mass of Si is calculated. In some embodiments, when S≤10%, the Dv50 particle size of the polymer-based composite particles is any one of 2μm, 3μm, 4μm, 5μm, 6μm or any two of them.

[0060] In some embodiments, when 10%≤S≤40%, the Dv50 particle size of the polymer-based composite particles is any one or any two of 3μm, 4μm, 5μm, 6μm, 7μm, and 8μm.

[0061] In some embodiments, when 40%≤S≤60%, the Dv50 particle size of the polymer-based composite particles is any one or any two of 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, and 10μm.

[0062] In some implementations, the thickness of the buffer coating is 5-12 μm.

[0063] In some embodiments, the thickness of the buffer coating is any one or a range between any two of 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, and 12 μm.

[0064] This application controls the thickness of the buffer coating within the above-mentioned range, which can avoid the buffer coating being too thin and unable to effectively absorb expansion stress, while also avoiding the buffer coating being too thick and unable to meet the overall thickness requirements of the battery cell. Furthermore, controlling the thickness of the buffer coating within the above-mentioned range can not only improve the liquid retention of the battery cell, but also provide sufficient expansion space, ultimately significantly reducing the battery cell cycle volume expansion rate and lithium plating phenomenon.

[0065] It should be noted that the width of the buffer coating is measured using a two-dimensional dimension or SEM.

[0066] In some embodiments, the mass fraction S of silicon-based material in the anode active material is ≤60%.

[0067] In some embodiments, the mass fraction S of silicon-based material in the anolyte is any one or a range between any two of 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, and 60%.

[0068] In some embodiments, the buffer coating is applied to both sides of the corner area of ​​the anode, cathode, or diaphragm by spraying or roller coating.

[0069] In some implementations, the bare cell is a semi-finished product prepared after winding and before baking.

[0070] In some embodiments, the diaphragm includes a base membrane, on one or both sides of which are provided with ceramic layers, the ceramic layers being located between the base membrane and the buffer coating.

[0071] In some embodiments, the ceramic layer includes ceramic and a ceramic binder.

[0072] In some embodiments, the ceramic includes at least one of silicon dioxide, aluminum oxide, silicon oxide, calcium oxide, magnesium oxide, zinc oxide, titanium dioxide, boehmite, and ceramic fibers.

[0073] In some embodiments, the ceramic layer has a thickness of 0.5-3 μm and an areal density of 0.3-0.6 g / m³. 2 .

[0074] In some embodiments, the base membrane is a porous substrate material, including at least one of polyethylene, ultra-high molecular weight polyethylene, high-density polyethylene, polypropylene, polyethylene-polypropylene copolymer, polyethylene terephthalate, polyphenylene sulfide, cellulose, polyimide, polystyrene, poly4-methyl-1-pentene, polymethyl methacrylate, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polytetrafluoroethylene, and polysulfone.

[0075] In some embodiments, some non-limiting examples of polyolefins include polypropylene, polyethylene, and polypropylene / polyethylene copolymers.

[0076] In some embodiments, the anode includes an anode current collector and an anode active layer, the anode active layer including an anode active material, an anode conductive agent, and an anode binder.

[0077] In some implementations, the anode current collector is copper foil.

[0078] In some embodiments, the anodic active material includes silicon-based materials and carbon-based materials.

[0079] In some embodiments, the silicon-based material includes porous amorphous carbon and / or porous crystalline material, as well as silicon and / or silicon oxide, wherein the silicon and / or silicon oxide is supported in the porous amorphous carbon and / or porous crystalline material.

[0080] In some embodiments, the silicon-based material is a silicon-carbon (Si-C) composite material, a silicon-nitrogen (Si-N) composite material, or a silicon-oxygen (Si-O) composite material.

[0081] In some embodiments, the carbon-based material includes at least one of natural graphite, artificial graphite, hard carbon, soft carbon, and mesophase carbon microspheres.

[0082] In some embodiments, the cathode includes a cathode current collector and a cathode active layer, the cathode active layer including a cathode active material, a cathode conductive agent, and a cathode binder.

[0083] In some implementations, the cathode current collector is aluminum foil.

[0084] In some embodiments, the cathode active material includes, but is not limited to, LiCoO2, LiNiO2, and LiNi x Mn y O2, Li 1+ z Ni x Mn y Co 1-x-y O2, LiNi x Co y Al z At least one of O2, LiV2O5, LiTiS2, LiMoS2, LiMnO2, LiCrO2, LiMn2O4, Li2MnO3, LiFeO2, LiFePO4, and LiMnPO4, wherein each x is independent and has a value of 0.2-0.9; each y is independent and has a value of 0.1-0.45; and each z is independent and has a value of 0-0.2.

[0085] In some embodiments, the anodic conductive agent and the cathodic conductive agent are independent and include, but are not limited to, at least one of conductive carbon black, graphite, expanded graphite, graphene, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanofibers, carbon tubes, activated carbon, and mesoporous carbon.

[0086] In some embodiments, the cathode binder, anode binder, and ceramic binder are each independent and include, but are not limited to, at least one of the following: polybutyl acrylate, acrylonitrile, polyvinyl alcohol, polymethyl methacrylate, hydroxypropyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyolefin ester, polyolefin alcohol, polyacrylic acid, styrene-butadiene rubber, acrylated styrene-butadiene rubber, and epoxy resin or nylon.

[0087] In some embodiments, polyvinylidene fluoride includes, but is not limited to, at least one of vinylidene fluoride-chlorotrifluoroethylene copolymer, vinylidene fluoride-trifluoroethylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-perfluoromethyl vinyl ether copolymer, vinylidene fluoride-vinylidene fluoride copolymer, vinylidene difluoride-hexafluoropropylene copolymer, and vinylidene difluoride-tetrafluoroethylene copolymer.

[0088] This application also provides a method for preparing a bare battery cell, including the following steps: (1) Mix the cathode active material, cathode conductive agent and cathode binder, add solvent and stir evenly to obtain cathode slurry, coat the cathode slurry evenly on both sides of the cathode current collector, dry, cold press, cut into pieces and slit to obtain the cathode; (2) Mix the anode active material, anode binder, and anode conductive agent, add water and stir evenly to obtain anode slurry. Coat the anode slurry evenly on both sides of the anode current collector, dry, cold press, cut into pieces and slit to obtain the anode. (3) The polymer-based composite particles are rolled or sprayed onto both sides of the base film in the corner area to form a buffer coating and prepare a diaphragm. (4) The cathode, diaphragm and anode are stacked and assembled in sequence, and then wound to obtain a bare cell.

[0089] In some embodiments, the bare battery cell of this application can be used in an electrochemical device, which includes any device in which an electrochemical reaction occurs to convert chemical energy into electrical energy and vice versa. Specific, non-limiting examples include all types of primary batteries, secondary batteries, fuel cells, solar cells, or capacitors. In particular, the electrochemical device is a lithium secondary battery, including lithium metal secondary batteries, lithium-ion secondary batteries, lithium polymer secondary batteries, or lithium-ion polymer secondary batteries.

[0090] To further illustrate the present invention, the following detailed description is provided in conjunction with embodiments, but these should not be construed as limiting the scope of protection of the present invention. The sources of the raw materials used in the embodiments and comparative examples of this application are as follows. Unless otherwise specified, all raw materials used are commercially available, and the same raw materials were used in parallel experiments.

[0091] Example 1 A bare battery cell, which is a semi-finished product prepared after winding and before baking, includes a cathode, a separator, and an anode arranged in sequence and wound. The cathode includes a cathode current collector and a cathode active layer located on both sides of the cathode current collector. The cathode current collector is aluminum foil, and the cathode active layer includes lithium cobalt oxide, polyvinylidene fluoride, and conductive carbon in a mass ratio of 97:1.5:1.5. The anode includes an anode current collector and an anode active layer located on both sides of the anode current collector. The anode current collector is copper foil, and the anode active layer includes anode active material, styrene-butadiene rubber, conductive carbon, carboxymethyl cellulose, and polyacrylic acid in a mass ratio of 95.5:0.5:0.5:0.5:3. The anode active material includes silicon carbon with a mass fraction of S and the balance being graphite, where S is specifically 30%.

[0092] The membrane comprises a base membrane and ceramic layers disposed on both sides of the base membrane. The base membrane is an Enjie HS5-5 ultra-high molecular weight polyethylene porous membrane with a thickness of 5 μm. The ceramic layers consist of Al2O3 and polyvinylidene fluoride in a mass ratio of 90:10; the ceramic layers have a thickness of 0.5 μm and an areal density of 1 g / m³. 2 ; like Figure 1 As shown, the diaphragm includes a straight region and a corner region formed after winding. A buffer coating with a thickness of H, specifically 10 μm, is provided on the surface of the ceramic layer of the diaphragm and in the corner region. The buffer coating comprises polymer-based composite particles with a Dv50 particle size of D, specifically 6 μm. The polymer-based composite particles include a pore-forming agent with a mass fraction of Q and the balance being a high-molecular-weight polymer, specifically Q = 10%. The pore-forming agent is iodine (I2) with a particle size ≤ 0.4 mm, and the high-molecular-weight polymer is polyvinylidene fluoride (PVDF) with an average molecular weight of 440 × 10⁻⁶. 3 g / mol.

[0093] The preparation method of the above-mentioned polymer-based composite particles includes the following steps: (1) The polymer and spinning solvent are mixed at a mass ratio of 12:88. The spinning solvent is acetone. The mixture is stirred at 50°C for 12 h to completely dissolve the polymer. After cooling to below 40°C, a pore-forming agent is added. The mixture is stirred at 40°C in the dark for 1 h to obtain the spinning solution. (2) The spinning solution was subjected to electrospinning treatment. The electrospinning voltage was 18 kV, the flow rate was 1.0 mL / h, the needle take-up distance was 15 cm, the roller speed was 200 rpm, the temperature of the electrospinning environment was 25 ℃, and the relative humidity was 40%RH. A fiber membrane with a thickness of 50 μm was obtained by spinning. (3) The fiber membrane was frozen with liquid nitrogen for 5 min, and then subjected to high-speed grinding at a rate of 15000 rpm for 30 s. After sieving, it was vacuum dried at 40 ℃ for 4 h to obtain polymer-based composite particles.

[0094] The above-mentioned method for preparing bare battery cells includes the following steps: (1) Lithium cobalt oxide, conductive carbon and polyvinylidene fluoride are mixed and N-methylpyrrolidone is added. The mixture is stirred evenly under vacuum stirring to obtain a cathode slurry with a solid content of 75wt%. The cathode slurry is evenly coated on both sides of aluminum foil and dried at 125 °C for 2 h. After cold pressing, cutting and slitting, the cathode is obtained. (2) Mix the anode active material, styrene-butadiene rubber, conductive carbon, polyacrylic acid and sodium carboxymethyl cellulose, add deionized water, and stir evenly under the action of a vacuum mixer to obtain an anode slurry with a solid content of 60wt%. Coat the anode slurry evenly on both sides of the copper foil, dry it at 120 ℃ for 2 h, and obtain the anode after cold pressing, cutting and slitting. (3) After mixing Al2O3 and polyvinylidene fluoride, add water and stir evenly to prepare a ceramic slurry with a solid content of 30%. The ceramic slurry is coated on both sides of the base film and dried at 120 °C for 2 h to form a ceramic layer. Then, polymer-based composite particles are roller coated on both sides of the corner area of ​​the ceramic layer to form a buffer coating and prepare a diaphragm. (4) The cathode, diaphragm and anode are stacked and assembled in sequence, with the diaphragm in the middle of the cathode and anode to play a role in isolation, and the bare battery cell is obtained by winding.

[0095] Example 2-3 A bare battery cell is prepared in the same way as Example 1, with the same steps, reagents, equipment and process parameters. The difference is that the thickness H of the buffer coating, the Dv50 particle size D of the polymer-based composite particles, the polymer composition, the spinning solvent composition and mass ratio are different.

[0096] Examples 4-5 A bare battery cell is prepared in the same way as that in Example 1, with the same steps, reagents, equipment and process parameters. The difference is that the mass fraction Q of the pore-forming agent in the polymer-based composite particles is different.

[0097] Examples 6-7 A bare battery cell is prepared in the same way as that in Example 1, with the same steps, reagents, equipment and process parameters. The difference is that the thickness H of the buffer coating is different.

[0098] Examples 8-18 A bare battery cell is prepared in the same way as Example 1, with the same steps, reagents, equipment and process parameters. The difference is that the silicon-carbon mass fraction S and the Dv50 particle size D of the polymer-based composite particles in the anode active material are different.

[0099] Examples 19-20 A bare battery cell is prepared in the same way as that in Example 1, with the same steps, reagents, equipment and process parameters. The difference is that the thickness H of the buffer coating is different.

[0100] Example 21 A bare battery cell is prepared in the same manner as in Example 1, with each step, reagent, equipment, and process parameter being identical. The difference lies in the following: Figure 2 As shown, the diaphragm is not provided with a buffer coating on both sides, but the buffer coating is provided on both sides of the cathode corner area; in step (1), lithium cobalt oxide, conductive carbon and polyvinylidene fluoride are mixed, N-methylpyrrolidone is added, and the mixture is stirred evenly under the action of a vacuum stirrer to obtain a cathode slurry with a solid content of 75wt%. The cathode slurry is uniformly coated on both sides of the aluminum foil and dried at 125 ℃ for 2 h. Then, polymer-based composite particles are roller coated onto the corner area of ​​the cathode to form a buffer coating. After cold pressing, cutting and slitting, the cathode is obtained.

[0101] Example 22 A bare battery cell is prepared in the same manner as in Example 1, with each step, reagent, equipment, and process parameter being identical. The difference lies in the following: Figure 3 As shown, the diaphragm is not provided with a buffer coating on both sides, but the buffer coating is provided on both sides of the anode corner area; in step (2), the anode active material, styrene-butadiene rubber, conductive carbon and sodium carboxymethyl cellulose are mixed, deionized water is added, and the mixture is stirred evenly under the action of a vacuum mixer to obtain an anode slurry with a solid content of 60wt%. The anode slurry is uniformly coated on both sides of the copper foil and dried at 120 ℃ for 2 h. Then, the polymer-based composite particles are roller coated onto the corner area of ​​the anode to form a buffer coating. After cold pressing, cutting and slitting, the anode is obtained.

[0102] Comparative Example 1 A bare battery cell is prepared in the same way as Example 1, with the same steps, reagents, equipment and process parameters used in each step. The difference is that the separator does not have a buffer coating on both sides.

[0103] Comparative Example 2 A bare battery cell is prepared in the same way as that in Example 1, with the same steps, reagents, equipment and process parameters. The difference is that the content of pore-forming agent in the polymer-based composite particles is 0.

[0104] Comparative Examples 3-4 A bare battery cell is prepared in the same way as that in Example 1, with the same steps, reagents, equipment and process parameters. The difference is that the mass fraction Q of the pore-forming agent in the polymer-based composite particles is different.

[0105] Comparative Example 5 A bare battery cell is prepared in the same way as that in Example 1, with the same steps, reagents, equipment and process parameters. The difference is that the thickness H of the buffer coating is different.

[0106] Comparative Example 6 A bare battery cell is prepared in the same way as Example 1, with the same steps, reagents, equipment and process parameters. The difference is that the buffer coating is set in the corner area and straight area on both sides of the separator.

[0107] Comparative Example 7 A bare battery cell, the preparation method of which is the same as that of Example 1 in terms of each step, reagents, equipment and process parameters, except that the buffer coating includes a pore-forming agent with a mass fraction of 10% and the balance being a polymer. The pore-forming agent is iodine (I2) with a particle size ≤0.4 mm and the polymer is polyvinylidene fluoride (PVDF). In step (3) of the bare cell preparation method, Al2O3 and polyvinylidene fluoride are mixed and then water is added and stirred evenly to prepare a ceramic slurry with a solid content of 30%. The ceramic slurry is coated onto the surface of the base film and dried at 120 °C for 2 h to form a ceramic layer. The pore-forming agent and the polymer are mixed and dissolved in acetone solvent to prepare a buffer slurry with a solid content of 15%. Then, it is rolled onto both sides of the corner area of ​​the ceramic layer to form a buffer coating. After drying at 60 °C, the separator is obtained.

[0108] Comparative Example 8 A bare battery cell, the preparation method of which is the same as that in Example 1 in terms of each step, reagents, equipment and process parameters, except that the buffer coating includes 10% by mass of pore-forming agent particles and 90% by mass of polymer particles. The pore-forming agent particles are iodine (I2) with a particle size ≤0.4 mm, and the polymer particles are polyvinylidene fluoride (PVDF) with a Dv50 particle size of 6 μm. In step (3) of the bare cell preparation method, Al2O3 and polyvinylidene fluoride are mixed and then water is added and stirred evenly to prepare a ceramic slurry with a solid content of 30%. The ceramic slurry is coated onto the surface of the base film and dried at 120 °C for 2 h to form a ceramic layer. The pore-forming agent particles and polymer particles are mixed and then rolled onto both sides of the corner area of ​​the ceramic layer to form a buffer coating, thus preparing the separator.

[0109] The following table shows the following parameters for the bare battery cells in the above embodiments and comparative examples: silicon-carbon mass fraction S, buffer coating thickness H, Dv50 particle size D of polymer-based composite particles, pore-forming agent mass fraction Q of polymer-based composite particles, pore-forming agent components, polymer components, average molecular weight of polymers, spinning solvent components, and mass ratios.

[0110] Table 1 - Parameter settings of bare cells in the embodiments and comparative examples of this application Assemble the bare battery cells prepared in the above embodiments and comparative examples into an electrochemical device, specifically including the following steps: place the bare battery cells in an aluminum-plastic film packaging bag, bake them at 80 ℃ and a vacuum degree of -100 kPa until the water content of the electrode is ≤200 ppm, and then inject an electrolyte, which includes 1.4 mol / L LiPF6, 2.5% fluoroethylene carbonate by mass fraction and the balance non-aqueous solvent, which includes ethylene carbonate, dimethyl carbonate and propylene carbonate by mass ratio of 3:4:2; after vacuum sealing, standing, formation, secondary sealing, capacity testing and aging at room temperature, the battery cell is obtained.

[0111] Performance testing 1. Cyclic Test: The length, width, and height of the assembled cells in the example and comparative examples were measured before and after the cyclic test using constant pressure calipers. The cell volume change rate was calculated to obtain the initial volume of the cell as V0. The cell was then cyclically charged and discharged according to standard steps: constant current charging at 2.0 C to 4.5 V, followed by constant voltage charging to 0.05 C cutoff; constant current discharging at 0.7 C to 3.0 V cutoff. After 1000 cycles, the cell was placed in a standard environment and left to stand for 2 hours. The final volume V1 of the cell after 1000 cycles was measured and calculated. The cyclic capacity retention rate (%) was calculated according to the formulas: Cyclic capacity retention rate (%) = discharge capacity of the 1000th cycle / discharge capacity of the 1st cycle × 100%, and volume expansion rate (%) = [(V1 - V0) / V0] × 100%. The test results are shown in Table 3 below. 2. Lithium plating level: The cells that have undergone the above 700 or 1000 cycle test are disassembled and the lithium plating at the anode interface and corners is observed. The lithium plating level is evaluated according to the standards in Table 2 below. The evaluation results are shown in Table 3 below.

[0112] Table 2 - Lithium Plating Grades at Anode Interface Table 3 - Performance test results of assembled cells in the examples and comparative examples As shown in Table 3, in Examples 1-3 of this application, a highly elastic buffer coating is formed on both sides of the corner area of ​​the separator by stacking polymer-based composite particles, which can effectively absorb expansion stress. In addition, the pore-forming agent in the polymer-based composite particles volatilizes during the baking stage to form a porous skeleton, which can absorb electrolyte, increase the liquid retention in the corner area, and reserve expansion space, effectively suppressing the volume expansion of the cell during cycling and avoiding lithium plating at the corner, so as to achieve a high cycle capacity retention rate of the cell.

[0113] As shown in Table 3, compared with Example 1, Comparative Example 1 did not have a buffer coating in the corner area of ​​the separator, resulting in a lower liquid retention in the corner area. Furthermore, the electrode volume expansion rate in the corner area was high during the cycle, which easily squeezed the electrolyte and caused it to dry out, resulting in severe lithium plating. Consequently, the capacity of the cell decreased sharply during the cycle, and the cycle stability deteriorated.

[0114] As shown in Table 3, compared with Example 1, no pore-forming agent was added to the polymer-based composite particles in the buffer coating of Comparative Example 2. The absorption and retention of electrolyte by the buffer coating were reduced. Moreover, the buffer coating did not reserve enough compressible space. When the electrode expands after the cell cycle, the elastic deformation space of the buffer coating in the corner area is limited, and the electrolyte is reduced due to compression, resulting in obvious lithium plating in the corner area.

[0115] As shown in Table 3, compared to Example 1, the polymer-based composite particles in Comparative Example 3 contain too little pore-forming agent. After the battery cell is baked, the space left by the volatilized pore-forming agent cannot meet the expansion of the battery cell during cycle production, which aggravates the lithium plating phenomenon. On the other hand, the polymer-based composite particles in Comparative Example 4 contain too much pore-forming agent. After the battery cell is baked, the polymer-based composite particles form a porous skeleton. Due to the large amount of pore-forming agent, the support of the skeleton is insufficient, and the buffer coating is prone to collapse, which makes it impossible to effectively store electrolyte, thus leading to lithium plating.

[0116] As shown in Table 3, compared with Example 1, the buffer coating thickness of Comparative Example 5 is too large. During the charging and discharging process of the battery cell, the anode sheet with high silicon content expands greatly. The excessive thickness of the buffer coating will increase the distance between the anode and cathode layers at the corner, lengthen the Li+ transport path, and increase the impedance. At the same time, when most of the electrolyte is consumed in the later stage of the cycle, it may not be able to wet the internal pores of the cathode / anode sheet and the material gaps between the cathode-diaphragm-anode, resulting in electrolyte interruption and interface lithium plating.

[0117] As shown in Table 3, compared to Example 1, the buffer coating of Comparative Example 6 is set on both sides of the flat area and the corner area of ​​the separator. Although the buffer coating in the flat area can increase the liquid retention in the flat area, the flat area can expand longitudinally, and the buffer layer may increase the gap between the anode and cathode layers, resulting in an increase in cell impedance. In addition, the buffer coating may absorb too much electrolyte, which may also cause the anode and cathode sheets to deform, and the deformed area is very prone to lithium plating.

[0118] As shown in Table 3, compared to Example 1, the buffer coating of Comparative Example 7 was formed by roller coating and drying of a buffer slurry containing a pore-forming agent and polyvinylidene fluoride. Although the pore-forming agent in the buffer coating can volatilize and leave pores after baking, the pores in the buffer coating are relatively few, which affects the transport of lithium ions and leads to a decrease in cycle capacity. In contrast, the buffer coating in Example 1 of this application is formed by stacking polymer-based composite particles of a specific particle size, which has more pores and can serve as a transport channel for lithium ions, maintaining high kinetic performance and resulting in a higher cycle capacity retention rate.

[0119] As shown in Table 3, compared to Example 1, the buffer coating of Comparative Example 8 was formed by directly mixing pore-forming agent particles and polyvinylidene fluoride particles and then roller coating. Since the pore-forming agent particles were not dispersed in the polyvinylidene fluoride bulk particles through electrospinning, their adhesion was poor, resulting in powder shedding of the buffer coating. In addition, the pore-forming agent particles and polyvinylidene fluoride particles were not mixed evenly, and the pore-forming agent was prone to agglomeration. After the battery cell was baked, the agglomerated pore-forming agent volatilized, leaving behind uneven and large pores. As a result, the support strength of the buffer coating was insufficient, and it was prone to collapse, reducing the liquid retention capacity and aggravating the lithium plating phenomenon at the corner of the battery cell.

[0120] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the scope of protection of the present invention. In particular, it should be noted that 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 for those skilled in the art.

Claims

1. Polymer-based composite particles, characterized in that, The polymer-based composite particles comprise a pore-forming agent and a high molecular polymer, the pore-forming agent is coated by or embedded in the high molecular polymer, and the mass fraction of the pore-forming agent in the polymer-based composite particles is 5%-20%.

2. The polymer-based composite particle of claim 1, wherein The high molecular polymer comprises at least one of polyvinylidene fluoride, polymethyl methacrylate, polyethylene, polypropylene, polyethylene-polypropylene copolymer, polyethylene terephthalate, polyphenylene sulfide, polyimide, polystyrene, poly-4-methyl-1-pentene, polyvinylidene fluoride-hexafluoropropylene, polytetrafluoroethylene, and polysulfone. The pore-forming agent comprises at least one of iodine, naphthalene, and camphor.

3. The polymer-based composite particle of claim 1, wherein The Dv50 particle size of the polymer-based composite particles is 2-10 μm.

4. The polymeric-based composite particle of any of claims 1, wherein, The polymer-based composite particles are prepared by electrospinning, freezing, crushing, and screening of a spinning solution containing the pore-forming agent and the high molecular polymer.

5. The polymer-based composite particle of claim 4, wherein The spinning solution comprises a spinning solvent, and the spinning solvent comprises at least one of acetone, N,N-dimethylformamide, and N-methylpyrrolidone.

6. A bare cell characterized by, The battery comprises an anode, a cathode, and a separator, each of the anode, the cathode, and the separator independently comprises a corner region and a flat region, at least one of the anode, the cathode, and the separator is provided with a buffer coating on both sides of the corner region, the thickness of the buffer coating is less than or equal to 12 μm, and the buffer coating comprises the polymer-based composite particles according to any one of claims 1-5.

7. The bare cell of claim 6, wherein, The anode comprises anode active material, and the anode active material comprises a silicon-based material with a mass fraction S; When S≤10%, the Dv50 particle size of the polymer-based composite particles is 2-6 μm; When 10%≤S≤40%, the Dv50 particle size of the polymer-based composite particles is 3-8 μm; When 40%≤S≤60%, the Dv50 particle size of the polymer-based composite particles is 4-10 μm.

8. The bare cell of claim 6, wherein, The thickness of the buffer coating is 5-12 μm. And / or, S≤60%.

9. The bare cell of claim 7, wherein, The silicon-based material comprises porous amorphous carbon and / or porous crystalline material, and silicon and / or silicon oxide loaded in the porous amorphous carbon and / or porous crystalline material.

10. The bare cell of claim 6, wherein, The separator comprises a base film, and a ceramic layer is provided on one side or both sides of the base film, the ceramic layer is located between the base film and the buffer coating.

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