Negative electrode layer and preparation method thereof, negative electrode and battery

Abstract

The invention provides a negative electrode layer and a preparation method thereof, a negative electrode and a battery, and belongs to the technical field of lithium ion battery production, the negative electrode layer comprises a first graphite layer, a second graphite layer and a hard carbon film layer, and the second graphite layer is sandwiched between the first graphite layer and the hard carbon film layer; wherein the first graphite layer comprises first graphite, and the graphitization degree of the first graphite is 90%-98%; the second graphite layer comprises second graphite, and the graphitization degree of the second graphite is 85%-95%; moreover, the graphitization degree of the second graphite is smaller than that of the first graphite, according to the negative electrode layer provided by the invention, the capacity performance and the rate capability of the battery can be improved at the same time through the cooperation of the first graphite layer and the second graphite layer, and the high-temperature storage performance and the cycle performance of the battery can be further improved through the arrangement of the hard carbon film layer.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery production technology, and in particular to a negative electrode layer and its preparation method, a negative electrode, and a battery. Background Technology

[0002] Lithium-ion rechargeable batteries possess advantages such as high voltage, high capacity, long cycle life, and environmental friendliness, making them promising for applications in portable electronic devices, electric vehicles, energy storage, and defense industries. They have become a hot topic in the development and research of the new energy industry in recent years. With the rapid development of electric vehicles, higher demands have been placed on lithium-ion batteries. They should simultaneously possess good capacity and rate performance to meet the requirements of long driving range and ultra-fast charging for electric vehicles. While graphite anodes, as a traditional anode material, exhibit good cycle stability, their capacity and rate performance still need further improvement compared to other anode active materials. Therefore, a technological solution to address these issues is urgently needed in the market. Summary of the Invention

[0003] To address the aforementioned technical problems, this invention proposes a technical solution that, by employing a combination of a first graphite layer and a second graphite layer, simultaneously improves the battery's capacity and rate performance. Furthermore, by incorporating a hard carbon film layer, the battery's high-temperature storage performance and cycle life can be further enhanced. The technical solution of this invention is implemented as follows:

[0004] The first aspect of the present invention discloses a negative electrode layer, which includes a first graphite layer, a second graphite layer and a hard carbon film layer, wherein the second graphite layer is sandwiched between the first graphite layer and the hard carbon film layer.

[0005] The first graphite layer includes a first graphite, and the graphitization degree of the first graphite is 90% to 98%.

[0006] The second graphite layer includes a second graphite, the graphitization degree of which is 85% to 95%.

[0007] Furthermore, the degree of graphitization of the second graphite is less than that of the first graphite.

[0008] In one embodiment, the graphitization degree of the first graphite is 95% to 98%; and the graphitization degree of the second graphite is 90% to 95%.

[0009] In one embodiment, the particle size of the second graphite is smaller than that of the first graphite.

[0010] In one embodiment, the thicknesses d1 of the first graphite layer, d2 of the second graphite layer, and d3 of the hard carbon film layer satisfy: d3 / (d2+d1)=1:6~19.

[0011] In one embodiment, the ratio of the thickness d1 of the first graphite layer to the thickness d2 of the second graphite layer is d1 / d2 = 0.8 to 1.4.

[0012] In one embodiment, the porosity of the hard carbon film is 30% to 45%.

[0013] The second aspect of this invention discloses a method for preparing a negative electrode layer as described in the first aspect, comprising the following steps:

[0014] Preparation of the first graphite layer slurry and the second graphite layer slurry;

[0015] The first graphite layer slurry and the second graphite layer slurry are coated simultaneously using a dual-die extrusion coating machine to prepare the first graphite layer and the second graphite layer.

[0016] A hard carbon film slurry is coated on the surface of the second graphite layer away from the first graphite layer and dried to obtain the negative electrode layer.

[0017] A third aspect of the present invention discloses a negative electrode comprising a current collector and a negative electrode layer as described in the first aspect, wherein a first graphite layer is disposed close to the current collector.

[0018] A fourth aspect of the present invention discloses a battery comprising a positive electrode, a separator, and a negative electrode as described in the third aspect.

[0019] In one embodiment, the positive electrode includes a manganese-based positive electrode material.

[0020] Beneficial technical effects of the present invention:

[0021] The negative electrode layer provided by this invention, by employing a combination of a first graphite layer and a second graphite layer, can simultaneously improve the battery's capacity performance and rate performance. By setting a hard carbon film layer, it can further enhance the battery's high-temperature storage performance and cycle performance. Detailed Implementation

[0022] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0023] Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; the terminology used in the detailed description is for the purpose of describing particular embodiments only and is not intended to limit the invention; the terms “comprising” and “having” and any variations thereof in the specification and claims of this invention are intended to cover non-exclusive inclusion.

[0024] In the description of specific embodiments of the present invention, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present invention, "multiple" means two or more, unless otherwise explicitly defined.

[0025] In this invention, the reference to "embodiment" means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the invention. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described in this invention can be combined with other embodiments.

[0026] In the description of the embodiments of this invention, the term "and / or" is merely a description of the relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Additionally, in this invention, the character " / " generally indicates that the preceding and following associated objects have an "or" relationship.

[0027] Throughout this invention, numerical values ​​represent approximate measurements or limits of a range to cover minute deviations from a given value, as well as embodiments with approximately the mentioned value and embodiments with the exact mentioned value. Except for the working examples provided at the end of the detailed description, all numerical values ​​of parameters (e.g., quantities or conditions) in this specification (including the appended claims) should be understood to be modified in all cases by the term “about,” regardless of whether “about” actually appears before the numerical value. “About” indicates that the stated numerical value allows for some minor inaccuracy (approaching the exact value in some way; approximately or reasonably approaching the value; almost). If the inaccuracy provided by “about” is not otherwise understood in this general sense in the art, then “about” as used in this invention at least indicates a variation that can be produced by common methods of measuring and using such a parameter. For example, “about” may include a variation less than or equal to 5%, optionally less than or equal to 4%, optionally less than or equal to 3%, optionally less than or equal to 2%, optionally less than or equal to 1%, optionally less than or equal to 0.5%, and in some respects, optionally less than or equal to 0.1%.

[0028] Additionally, the disclosure of the range includes the disclosure of all values ​​across the entire range and the disclosure of further subdivided ranges, including the endpoints and subranges given for these ranges.

[0029] Graphite anodes, as a traditional anode material, have good cycle stability, but their capacity and rate performance still need further improvement compared to other anode active materials.

[0030] To address the above problems, this invention proposes a technical solution.

[0031] The first aspect of the present invention discloses a negative electrode layer, which includes a first graphite layer, a second graphite layer and a hard carbon film layer, wherein the second graphite layer is sandwiched between the first graphite layer and the hard carbon film layer.

[0032] The first graphite layer includes a first graphite, and the graphitization degree of the first graphite is 90% to 98%.

[0033] The second graphite layer includes a second graphite, the graphitization degree of which is 85% to 95%.

[0034] Furthermore, the degree of graphitization of the second graphite is less than that of the first graphite.

[0035] The first graphite layer has a high degree of graphitization and crystallinity, providing high reversible specific capacity and good electronic conductivity. The second graphite layer has a relatively lower degree of graphitization, resulting in a wider interlayer spacing, which improves the lithium-ion transport rate within the negative electrode. Lithium ions can be rapidly transported to the first graphite layer, and the second graphite layer also contributes to the capacity, further improving the battery's capacity performance. The hard carbon film layer can further improve the lithium-ion transport rate, facilitating the formation of a more stable SEI film on the negative electrode surface. Simultaneously, it can isolate the electrolyte from large-area contact with graphite, preventing solvent co-intercalation from damaging the graphite structure and improving the stability of the negative electrode. Furthermore, due to the significant difference in mechanical properties between the first graphite layer and the hard carbon film layer, the second graphite layer can serve as a transition layer between them, preventing structural collapse of the negative electrode.

[0036] It is understood that in this application, the interlayer spacing d002 of the first graphite and the second graphite is measured by XRD, and the degree of graphitization is calculated as ((0.344 - interlayer spacing) / (0.344 - 0.3354)) * 100%.

[0037] Specifically, the graphitization degree of the first and second graphite can be controlled by the graphitization temperature.

[0038] In some embodiments, the graphitization degree of the first graphite is 95% to 98%; and the graphitization degree of the second graphite is 90% to 95%.

[0039] In some embodiments, the first graphite has a spherical or near-spherical structure.

[0040] In some embodiments, the tap density of the first graphite is ≥1.75 g / cm³. 3The high tap density of the first graphite layer can effectively improve the energy density of the negative electrode layer.

[0041] In some embodiments, the particle size of the second graphite is smaller than that of the first graphite. Part of the second graphite can fill the pores of the first graphite layer, effectively increasing the compaction density of the negative electrode and thus improving the energy density of the battery. Furthermore, it increases the contact between the first and second graphite layers, facilitating the formation of a more uniform and denser conductive network throughout the negative electrode, reducing its internal resistance, and improving the battery's rate performance. Simultaneously, the high-layer spacing of the second graphite, combined with its small particle size, effectively shortens the lithium-ion transport path.

[0042] In some embodiments, the particle size D50 of the second graphite is 5nm to 15nm. For example, it can be selected as 5nm, 6nm, 8nm, 10nm, 12nm, 14nm, or 15nm. The above are just examples and are not limitations.

[0043] In some embodiments, the particle size D50 of the first graphite is 15nm to 25nm. For example, it can be selected as 15nm, 16nm, 17nm, 18nm, 19nm, 20nm, 21nm, 22nm, 23nm, 24nm, or 25nm. The above are just examples and are not limitations.

[0044] In some embodiments, the porosity of the hard carbon film layer is 30% to 45%. The porosity of the hard carbon film layer provides space for the wetting and storage of the electrolyte, ensuring the rapid entry and exit of lithium ions from the negative electrode, achieving high-rate charge and discharge. Exemplarily, the porosity can be selected as 30%, 32%, 34%, 36%, 38%, 40%, 43%, or 45%, but these are merely examples and not limitations. Low porosity of the hard carbon film layer reduces the ion transport rate, while high porosity leads to insufficient strength and easy detachment of the hard carbon film layer.

[0045] In some embodiments, the thicknesses d1 of the first graphite layer, d2 of the second graphite layer, and d3 of the hard carbon film layer satisfy the following ratio: d3 / (d2+d1) = 1:6 to 19. Exemplary values ​​such as 1:6, 1:8, 1:10, 1:12, 1:15, 1:17, and 1:19 can be selected. These are merely examples and not limitations. If the value of d3 / (d2+d1) is too large, the interfacial impedance is high; if it is too small, the barrier effect of the hard carbon film layer is limited.

[0046] In some embodiments, the ratio d1 / d2 of the thickness of the first graphite layer and the thickness of the second graphite layer is 0.8 to 1.4. Exemplary values ​​such as 0.8, 1.0, 1.2, and 1.4 can be selected; these are merely examples and not limitations. When the ratio of d1 / d2 is within this range, both high capacity and good rate capability can be ensured, and the interfacial polarization of the negative electrode can be effectively reduced.

[0047] In some embodiments, the first graphite layer further includes a first binder.

[0048] In some embodiments, the first adhesive has a peel strength to the copper foil >1.5 N / m to prevent structural collapse and falling off the current collector when the first graphite layer is under high pressure.

[0049] In some embodiments, the first adhesive includes one of SBR+CMC, PAA, and PI.

[0050] In some embodiments, the first graphite layer further includes a first conductive agent. This application does not limit the type of the first conductive agent; it can be any agent that can transport electrons. For example, it can be super P, acetylene black, Ketjen black, etc.

[0051] In some embodiments, the mass ratio of the first graphite, the first conductive agent, and the first binder in the first graphite layer is (92-96):(2-5):(1-3).

[0052] In some embodiments, the second graphite layer includes a second binder. This application does not limit the type of the second binder, as long as it can bond the second graphite to other substances and the first graphite layer.

[0053] In some embodiments, the second graphite layer includes a second conductive agent. The second conductive agent is preferably a combination of at least two conductive materials; for example, it can be SuperP + carbon nanotubes, SuperP + carbon nanotubes + carbon fibers, etc. This effectively compensates for the slightly poor conductivity of the second graphite.

[0054] In some embodiments, the mass ratio of the second graphite, the second binder, and the second conductive agent in the second graphite layer is (92-96):(2-6):(1.5-5.5).

[0055] In some embodiments, the hard carbon film layer also includes ceramic nanoparticles, which, for example, may be Al2O3 nanoparticles.

[0056] The second aspect of this invention discloses a method for preparing a negative electrode layer as described in the first aspect, comprising the following steps:

[0057] Preparation of the first graphite layer slurry and the second graphite layer slurry;

[0058] The first graphite layer slurry and the second graphite layer slurry are coated simultaneously using a dual-die extrusion coating machine to prepare the first graphite layer and the second graphite layer.

[0059] A hard carbon film slurry is coated on the surface of the second graphite layer away from the first graphite layer and dried to obtain the negative electrode layer.

[0060] A third aspect of the present invention discloses a negative electrode comprising a current collector and a negative electrode layer as described in the first aspect, wherein a first graphite layer is disposed close to the current collector.

[0061] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper) on the polymer substrate.

[0062] In some embodiments, the polymer material substrate includes polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), and the like.

[0063] A fourth aspect of the present invention discloses a battery comprising a positive electrode, a separator, and a negative electrode as described in the third aspect.

[0064] In some embodiments, the positive electrode includes a positive current collector and a layer of positive active material disposed on the positive current collector.

[0065] In some preferred embodiments, the positive electrode active material layer is disposed on both sides of the positive electrode current collector.

[0066] In some embodiments, the positive electrode active material layer includes a positive electrode active material, which includes a compound that can reversibly insert and deintercalate lithium ions.

[0067] In some embodiments, the positive electrode active material comprises one or more transition metal cations, such as manganese (Mn), nickel (Ni), cobalt (Co), chromium (Cr), iron (Fe), vanadium (V), and combinations thereof.

[0068] In some embodiments, the positive electrode active material is one of layered oxides, spinel, and polyanionic materials. Exemplarily, the layered oxide (e.g., rock salt layered oxide) comprises one or more lithium-based positive electrode active materials selected from: LiCoO2 (LCO), LiNi x Mn y Co1-x-y O2 (where 0 ≤ x ≤ 1 and 0 ≤ y ≤ 1), LiNi 1-x-y Co x Al y O2 (where 0 ≤ x ≤ 1 and 0 ≤ y ≤ 1), LiNi x Mn 1-x O2 (where 0 ≤ x ≤ 1), and Li 1+x MO2 (where M is one of Mn, Ni, Co, and Al and 0 ≤ x ≤ 1). Spinel contains one or more lithium-based cathode active materials selected from the following: LiMn2O4 (LMO) and LiNi. x Mn 1.5 O4. The olivine type contains one or more lithium-based positive electrode active materials, LiMPO4 (where M is at least one of Fe, Ni, Co, and Mn). The polyanionic cation contains, for example, phosphates such as LiV2(PO4). 24 And / or silicates such as LiFeSiO4.

[0069] In some embodiments, the positive electrode includes a manganese-based positive electrode material, including lithium manganese oxide (LMO), lithium nickel manganese oxide (LNMO), and lithium-rich manganese-based positive electrode materials.

[0070] In this application, the hard carbon film can effectively solve the problem of SEI damage caused by the migration of manganese ions to the negative electrode side after dissolution, resulting in the degradation of battery cycle performance.

[0071] In some embodiments, when the positive electrode includes a manganese-based positive electrode material, the thickness of the hard carbon film layer in the negative electrode is 5 μm to 15 μm, which can effectively prevent manganese ions from dissolving and damaging the SEI film of the negative electrode.

[0072] In some embodiments, the mass of the positive electrode active material accounts for 60% to 95% of the mass of the positive electrode active material layer.

[0073] In some embodiments, the positive electrode active material layer includes a binder. The binder improves the bonding between the positive electrode active material particles and also improves the bonding between the positive electrode active material layer and the positive electrode current collector.

[0074] In some embodiments, non-limiting examples of adhesives include polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon, etc.

[0075] In some embodiments, the binder accounts for 0.1% to 20% of the mass of the positive electrode active material layer.

[0076] In some embodiments, the positive electrode active material layer includes a conductive agent, thereby imparting conductivity to the electrode. The conductive agent may include any conductive material, as long as it does not cause a chemical change. Non-limiting examples of conductive materials include carbon-based materials (e.g., natural graphite, synthetic graphite, carbon black, acetylene black, Ketjen black, carbon fiber, etc.), metal-based materials (e.g., metal powders, metal fibers, etc., including, for example, copper, nickel, aluminum, silver, etc.), conductive polymers (e.g., polyphenylene derivatives), and mixtures thereof.

[0077] In some embodiments, the mass of the conductive agent accounts for 0.1% to 20% of the mass of the positive electrode active material layer.

[0078] In some embodiments, the positive electrode active material layer provided in this application further includes a fast ion conductor to improve the ionic conductivity of the positive electrode active material layer. This application does not limit the type of fast ion conductor, which can be an oxide solid electrolyte, a sulfide solid electrolyte, a halide solid electrolyte, a lithium salt, etc.

[0079] In some embodiments, the oxide solid electrolyte may comprise one or more garnet ceramics, LISICON-type oxides, NASICON-type oxides, and perovskite-type ceramics. Exemplarily, garnet ceramics include Li... 6.5 La 24 Zr 1.75 Te 0.25 O 12 、Li7La 24 Zr2O 12 Li 6.2 Ga 0.24 La 2.95 Rb 0.05 Zr2O 12 Li 6.85 La 2.9 Ca 0.1 Zr 1.75 Nb 0.25 O 12 Li 6.25 Al 0.25 La 24 Zr2O 12 Li 6.75 La 24 Zr 1.75 Nb 0.25 O 12 Li 6.75 La 24 Zr 1.75 Nb 0.25 O 12 And their combinations. LISICON-type oxides include Li 14Zn(GeO4)4, Li 24+x (P 1- x Si x O4 (where 0 < x < 1), Li 24+x Ge x V 1-x O4 (where 0 < x < 1) and their combinations. NASICON-type oxides are LiMM′(PO4). 24 M and M′ are independently selected from Al, Ge, Ti, Sn, Hf, Zr, and La. NASICON-type oxides include Li. 1+ x Al x Ge 2-x (PO4) 24 (LAGP) (where 0 ≤ x ≤ 2), Li 1+x Al x Ti 2-x (PO4) 24 (LATP) (where 0 ≤ x ≤ 2), Li 1+ x Y x Zr2-x(PO4)24(LYZP) (where 0≤x≤2), Li 1.24 Al 0.24 Ti 1.7 (PO4) 24 LiTi2(PO4) 24 LiGeTi(PO4) 24 LiGe2(PO4) 24 LiHf2(PO4) 24 And their combinations. Perovskite ceramics include Li 24.24 La 0.524 TiO 24 LiSr 1.65 Zr 1.24 Ta 1.7 O9、Li 2x-y Sr 1-x Ta y Zr 1-y O 24 (where x = 0.75y and 0.60 < y < 0.75), Li 24 / 8 Sr 7 / 16 Nb 24 / 4 Zr 1 / 4 O 24 Li 24x La (2 / 24-x) TiO 24 (where 0 < x < 0.25) and their combinations.

[0080] In some embodiments, the ionic conductivity of the oxide solid electrolyte is 10. -5 S / cm~10 -1 S / cm.

[0081] In some embodiments, the sulfide solid electrolyte includes Li2S-P2S5 and Li2S-P2S5-MS. x (where M is Si, Ge, and Sn and 0 ≤ x ≤ 2), Li 24.4 Si 0.4 P 0.6 S4, Li 10 GeP2S 11.7 O 0.24 Li 9.6 P 24 S 12 Li7P 24 S 11 Li9P 24 S9O 24 Li 10.245 Si 1.245 P 1.65 S 12 Li 9.81 Sn 0.81 P 2.19 S 12 Li 10 (Si 0.5 Ge 0.5 P2S 12 Li (Ge 0.5 Sn 0.5 P2S 12 Li 10 GeP2S 12 (LGPS), Li6PS5X (where X is Cl, Br, or I), Li7P2S8I, Li 10.245 Ge 1.245 P 1.65 S 12 Li 24.25 Ge 0.25 P 0.75 S4, Li 10 SnP2S 12 Li 10 SiP2S 12 Li 9.54 Si 1.74 P 1.44 S 11.7 Cl 0.24 , (1-x)P2S5-xLi2S (where 0.5≤x≤0.7) and their combinations.

[0082] In some embodiments, the ionic conductivity of the sulfide solid electrolyte is 10. -7 S / cm~1S / cm.

[0083] In some embodiments, the halide solid electrolyte includes Li a M b X c N d M includes one or more of the basic metal elements, such as Zr, Hf, In, Sc, Y, La, Ce, Pr, Nb, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. M also includes doped metal elements, used in conjunction with the aforementioned basic metal elements, such as one or more of Nb, Ta, Al, La, Mg, Ca, Ba, and Ag. X includes one or more of F, Cl, Br, and I. N includes one or more of O and S, and satisfies a + mb = c + nd, where m and n are the weighted valences of M and N, respectively, and 1 ≤ a ≤ 4.

[0084] In some embodiments, the halide solid electrolyte can be Li₂ZrCl₆, Li₂ZrCl₅F, or Li₂ZrCl₆. 5.5 O 0.25 At least one of Li3InCl6, Li3YCl6, Li2HfCl6, LiInBr4, Li3InBr6, Li3LaI6, Li3LuCl6, and Li3ErCl6.

[0085] In some embodiments, the lithium salt includes LiNbO3 and Li4Ti5O. 12 At least one of Li2TiO3, LiAlO2, LiTaO3, LiMoO3, Li2RuO3 or Li2WO4.

[0086] In some embodiments, the mass of the fast ion conductor accounts for 1%-20% of the mass of the positive electrode active material layer; preferably 5%-20%.

[0087] In some embodiments, the positive current collector includes a metallic material that can conduct electrons; for example, the positive current collector includes at least one of Al, Ni, tin, copper, and stainless steel.

[0088] In some embodiments, the surface of the aluminum foil can be anodized to form a nanoscale aluminum oxide layer to enhance its corrosion resistance and interfacial adhesion with the cathode material.

[0089] In some implementations, the aluminum alloy foil may contain trace amounts of titanium (Ti) or (Si) to enhance mechanical strength and high-temperature stability.

[0090] In some embodiments, the aluminum foil may also be carbon-coated aluminum foil.

[0091] In some embodiments, the separator separates the negative and positive electrodes and provides a path for lithium ions to move. Any separator can be used without particular limitation, as long as it is a commonly used separator. A separator with excellent electrolyte moisture content and low resistance to ion movement in the electrolyte is preferred. Porous polymer membranes can be used, such as porous polymer membranes made from polyolefin polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminated structures having two or more layers. Typical porous nonwoven fabrics can also be used, such as nonwoven fabrics formed from high-melting-point glass fibers, polyethylene terephthalate fibers, etc. Furthermore, coated separators containing ceramic components or polymer materials can be used to ensure heat resistance or mechanical strength, and can be selectively used in single-layer or multi-layer structures.

[0092] In some embodiments, the battery further includes an electrolyte, which includes a solvent, an electrolyte salt, and electrolyte additives.

[0093] In some embodiments, the solvent includes a non-aqueous organic solvent.

[0094] In some embodiments, the solvent includes one or more of carbonate solvents, carboxylic acid ester solvents, and aromatic hydrocarbon solvents.

[0095] In some embodiments, the carbonate solvent includes halocarbonates and / or non-halocarbonates.

[0096] In some embodiments, the halogenated carbonate includes one or more of fluoroethylene carbonate, difluoropropylene carbonate, trifluoroethyl methyl carbonate, trifluoromethyl ethylene carbonate, 4-trifluoromethyl ethylene carbonate, chloroethylene carbonate, di(2,2,2-trifluoroethyl) carbonate, or 1,1,1,3,3,3-hexafluoroisopropyl acrylate.

[0097] In some embodiments, the non-halogenated carbonate includes one or more of ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, or ethyl methyl carbonate.

[0098] In some embodiments, the carboxylic acid ester solvent includes halocarboxylic acid esters and / or non-halocarboxylic acid esters.

[0099] In some embodiments, the halocarboxylic acid ester includes one or more of propyl fluorobutyrate, propyl fluoroacetate, ethyl fluoroacetate, isopropyl fluoroacetate, butyl fluoropropionate, isopropyl fluoropropionate, ethyl fluorobutyrate, methyl fluoropropionate, ethyl fluoropropionate, or propyl fluoropropionate.

[0100] In some embodiments, the non-halogenated carboxylic acid esters include one or more of ethyl acetate, methyl acetate, propyl butyrate, propyl acetate, isopropyl acetate, butyl propionate, isopropyl propionate, ethyl butyrate, methyl propionate, ethyl propionate, or propyl propionate.

[0101] In some embodiments, the aromatic hydrocarbon solvent includes halogenated aromatic hydrocarbons and / or non-halogenated aromatic hydrocarbons.

[0102] In some embodiments, the halogenated aromatic hydrocarbon includes one or more of monofluorobenzene, difluorobenzene, 1,3,5-trifluorobenzene, trifluorotoluene, 2-fluorotoluene, or 2,4-dichlorotrifluorotoluene.

[0103] In some embodiments, the electrolyte salt includes one or more of lithium hexafluorophosphate (LiPF6), lithium hexafluoroarsenate (LiAsF6), lithium perchlorate (LiClO4), lithium bisfluorosulfonylimide (LiFSI), lithium tetrafluoroborate (LiBF4), and lithium bistrifluoromethanesulfonylimide (LiTFSI).

[0104] In some embodiments, the electrolyte salt is present in the electrolyte at a mass percentage of 0.5% to 20%, including but not limited to 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, and 20%.

[0105] In some embodiments, the electrolyte additive includes one or more of sulfonyl lactones, cyclic sulfates, phosphates, and borates.

[0106] In some embodiments, the electrolyte additive constitutes 0.1% to 5% by mass in the electrolyte, including but not limited to 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, and 5.0%.

[0107] In some embodiments, the sulfonyl lactone compound is selected from one or more of 1,3-propanesulfonyl lactone, 1,4-butanesulfonyl lactone, and propenyl-1,3-sulfonyl lactone.

[0108] In some embodiments, the cyclic sulfate compound is selected from one or more of vinyl sulfate, 4-methylvinyl sulfate, and propylene sulfate.

[0109] In some embodiments, the phosphate ester compound is selected from at least one of saturated phosphate ester compounds and unsaturated phosphate ester compounds. The saturated phosphate ester compounds include tris(trimethylsilane) phosphate; the unsaturated phosphate ester compounds include at least one of triallyl phosphate, triallyl phosphite, and hydroxyethyl methacrylate phosphate.

[0110] In some embodiments, the borate ester compound is selected from one or more of tris(trimethylsilane)borate and tris(triethylsilane)borate.

[0111] In some embodiments, the battery further includes a solid electrolyte layer, which includes a solid electrolyte, including at least one of polymer solid electrolyte, oxide solid electrolyte, sulfide solid electrolyte, halide solid electrolyte, hydride solid electrolyte, and nitride solid electrolyte.

[0112] To better understand the aforementioned objectives, features, and advantages of this application, the solution of this application will be further described below. It should be noted that, unless otherwise specified, the embodiments and features described in these embodiments can be combined with each other.

[0113] Many details are set forth in the following description in order to provide a full understanding of this application, but this application may also be implemented in other ways different from those described herein; obviously, the embodiments in this specification are only some embodiments of this application, and not all embodiments.

[0114] Example 1: A battery, comprising a positive electrode and a negative electrode.

[0115] The negative electrode includes a current collector, a negative electrode layer disposed on the current collector, and the negative electrode layer includes a first graphite layer disposed close to the current collector, an outermost hard carbon film layer, and a second graphite layer disposed between the first graphite layer and the hard carbon film layer.

[0116] The first graphite layer includes a first graphite layer, which has a spherical structure and a tap density of 1.9 g / cm³. 3 The degree of graphitization is 96%, the interlayer spacing is 0.3357 nm, and the particle size is 15 nm. The thickness d1 of the first graphite layer is 50 μm.

[0117] The second graphite layer comprises a second graphite layer with a tap density of 1.4 g / cm³. 3 The degree of graphitization is 93%, the interlayer spacing is 0.3360 nm, and the particle size is 5 nm. The thickness d2 of the second graphite layer is 45 μm.

[0118] The porosity of the hard carbon film is 45%, and the thickness d3 is 8 μm.

[0119] d1 / d2=1.11; d3 / (d1+d2)=1:11.9=0.084.

[0120] Preparation of negative electrode:

[0121] Preparation of the first graphite layer slurry: The first graphite, the binder styrene-butadiene rubber, the thickener sodium hydroxymethyl cellulose, and the conductive agent SuperP are mixed in a mass ratio of 94:2:2:2, and deionized water is added to obtain the first graphite layer slurry.

[0122] Preparation of the second graphite layer slurry: The second graphite, the binder styrene-butadiene rubber, the thickener sodium hydroxymethyl cellulose, the conductive agent SuperP, and the conductive agent carbon nanotubes are mixed in a mass ratio of 94:2:1:2:1, and deionized water is added to obtain the second graphite layer slurry.

[0123] The first graphite layer slurry and the second graphite layer slurry are coated simultaneously using a dual-die extrusion coating machine to prepare the first graphite layer and the second graphite layer.

[0124] Preparation of hard carbon film slurry: Hard carbon powder, styrene-butadiene rubber binder, sodium cellulose thickener, SuperP conductive agent, and Al2O3 nanoparticles are mixed in a mass ratio of 90:3:3:3:1, and deionized water is added to obtain hard carbon film slurry.

[0125] A hard carbon film slurry is coated on the surface of the second graphite layer away from the first graphite layer and dried to obtain the negative electrode layer.

[0126] Preparation of the positive electrode:

[0127] The positive electrode active material lithium nickel manganese oxide, the positive electrode binder polyvinylidene fluoride, and the positive electrode conductive agent super P are mixed in N-methylpyrrolidone solvent at a mass ratio of 97:2:1 and stirred for 60 minutes to form a positive electrode active material slurry. The positive electrode active material slurry is then coated onto the positive electrode current collector aluminum foil, and the positive electrode is obtained after drying, rolling, and die cutting.

[0128] Battery fabrication:

[0129] The prepared positive electrode, PE separator, and negative electrode are stacked to form a cell, and then an electrolyte is injected to form a battery.

[0130] Example 2:

[0131] The only difference between this embodiment and Embodiment 1 is that in this embodiment, the thickness of the first graphite layer d1 is 50 μm; the thickness of the second graphite layer d2 is 35 μm. d1 / d2 = 1.4; d3 / (d1+d2) = 1:10.6 = 0.094.

[0132] Example 3:

[0133] The only difference between this embodiment and Embodiment 1 is that, in this embodiment, the thickness of the first graphite layer d1 is 40 μm; the thickness of the second graphite layer d2 is 50 μm. d1 / d2=0.8; d3 / (d1+d2)=1:11.3=0.088.

[0134] Example 4:

[0135] The only difference between this embodiment and Embodiment 1 is that in this embodiment, the thickness d3 of the hard carbon film is 5 μm; d3 / (d1+d2)=1:19=0.053.

[0136] Example 5:

[0137] The only difference between this embodiment and Embodiment 1 is that in this embodiment, the thickness d3 of the hard carbon film is 15 μm; d3 / (d1+d2)=1:6=0.17.

[0138] Example 6: An electrochemical device comprising a positive electrode and a silicon-based negative electrode.

[0139] The only difference between this embodiment and Embodiment 1 is that, in this embodiment, the thickness of the first graphite layer is 45 μm; the thickness of the second graphite layer is 40 μm; the thickness of the hard carbon film layer is 10 μm; d1 / d2=1.125; d3 / (d1+d2)=1:8.5=0.118.

[0140] Example 7:

[0141] The only difference between this embodiment and Embodiment 6 is that in this embodiment, the graphitization degree of the first graphite is 98% and the graphitization degree of the second graphite is 95%.

[0142] Example 8:

[0143] The only difference between this embodiment and Embodiment 6 is that in this embodiment, the graphitization degree of the first graphite is 95% and the graphitization degree of the second graphite is 90%.

[0144] Example 9:

[0145] The only difference between this embodiment and Embodiment 1 is that in this embodiment, the graphitization degree of the first graphite is 90% and the graphitization degree of the second graphite is 85%.

[0146] Example 10

[0147] The only difference between this embodiment and Embodiment 1 is that in this embodiment, the porosity of the hard carbon film layer is 30%; the particle size of the first graphite is 25 μm, and the particle size of the second graphite is 15 μm.

[0148] Comparative Example 1:

[0149] The only difference between this comparative example and Example 1 is that no hard carbon film layer is provided.

[0150] Comparative Example 2:

[0151] The only difference between this comparative example and Example 1 is that, in this comparative example, a first graphite layer is not provided.

[0152] Comparative Example 3:

[0153] The only difference between this comparative example and Example 1 is that, in this comparative example, a second graphite layer is not provided.

[0154] Comparative Example 1:

[0155] The only difference between this comparative example and Example 1 is that the positions of the first graphite layer and the second graphite layer are interchanged in this comparative example, with the first graphite layer sandwiched between the second graphite layer and the hard carbon film layer.

[0156] Performance testing

[0157] Discharge capacity test: At 25℃, charge at a constant current of 0.33C, charge at a constant voltage until the current is ≤0.05C, let stand for 5 minutes, and then discharge at a constant current of 0.1C. Record the discharge capacity and take the average value of 3 tests.

[0158] Cyclic performance test: At 25℃, charge and discharge at 1C rate, record the capacity retention rate every 100 cycles, and calculate the final capacity retention rate after 500 cycles.

[0159] Rate performance test: Charge and discharge tests were conducted at 0.5C, 1C, 2C, 5C and 10C rates at 25℃, and the capacity retention rate at each rate was calculated (based on the 0.5C capacity).

[0160] Storage performance test: The battery was fully charged at 0.33C rate and stored at a constant temperature for 30 days at 60℃. After being removed, it was left to stand at 25℃ for 24 hours and then discharged at 0.33C rate to 2.5V. The capacity retention rate after storage was calculated.

[0161] Manganese ion leaching test: After 500 cycles, the battery was disassembled, the electrolyte was collected, and the concentration of manganese ions in the electrolyte was tested using inductively coupled plasma mass spectrometry (ICP-MS).

[0162] The test results are compiled into the table below:

[0163]

[0164]

[0165] As shown in the table above, the negative electrode structure prepared in this application can simultaneously possess good capacity performance, rate performance, cycle performance and high-temperature storage performance. When applied to manganese-based positive electrodes, it can also effectively prevent manganese ions from dissolving and migrating to the negative electrode, thus effectively improving the cycle performance of the battery.

[0166] It should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A negative electrode layer, characterized in that, The negative electrode layer includes a first graphite layer, a second graphite layer, and a hard carbon film layer, wherein the second graphite layer is sandwiched between the first graphite layer and the hard carbon film layer. Wherein, the first graphite layer includes a first graphite, and the graphitization degree of the first graphite is 90% to 98%; The second graphite layer includes a second graphite, the graphitization degree of which is 85% to 95%; Furthermore, the degree of graphitization of the second graphite is less than that of the first graphite.

2. The negative electrode layer according to claim 1, characterized in that, The graphitization degree of the first graphite is 95% to 98%; the graphitization degree of the second graphite is 90% to 95%.

3. The negative electrode layer according to claim 1, characterized in that, The particle size of the second graphite is smaller than that of the first graphite.

4. The negative electrode layer according to claim 1, characterized in that, The thicknesses d1 of the first graphite layer, d2 of the second graphite layer, and d3 of the hard carbon film layer satisfy the following: d3 / (d2+d1)=1:6~19.

5. The negative electrode layer according to claim 4, characterized in that, The ratio of the thickness d1 of the first graphite layer to the thickness d2 of the second graphite layer is d1 / d2 = 0.8 to 1.

4.

6. The negative electrode layer according to claim 1, characterized in that, The porosity of the hard carbon film is 30% to 45%.

7. The method for preparing the negative electrode layer as described in claim 1, characterized in that, Includes the following steps: Preparation of the first graphite layer slurry and the second graphite layer slurry; The first graphite layer slurry and the second graphite layer slurry are simultaneously coated using a dual-die extrusion coating machine to prepare the first graphite layer and the second graphite layer. A hard carbon film slurry is coated on the surface of the second graphite layer away from the first graphite layer and dried to obtain the negative electrode layer.

8. Negative electrode, characterized in that, The negative electrode includes a current collector and a negative electrode layer as described in claim 1, wherein the first graphite layer is disposed close to the current collector.

9. A battery, characterized in that, It includes a positive electrode, a separator, and a negative electrode as described in claim 9.

10. The battery according to claim 9, characterized in that, The cathode includes a manganese-based cathode material.

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