Lithium ion secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a lithium ion secondary battery which comprises a positive plate, the positive plate comprises a positive active material layer, and the positive active material layer comprises a lithium iron phosphate material and a solid electrolyte material; the chemical formula of the solid electrolyte material is LixAlyTiz (PO4) 3, x is more than 1 and less than or equal to 1.5, y is more than 0 and less than or equal to 0.5, and z is more than or equal to 1.5 and less than 2; the lithium iron phosphate material comprises a doping element M, wherein M comprises V and / or Mn; on the basis of the total weight of the positive electrode active material layer, the content c1 of the element Fe is 16%-35%, the content c2 of the element M is 0.5%-3%, and the content c3 of the element Ti is 0.05%-5%; the ratio of c3 to c1 is 1: (3-310). The lithium ion secondary battery provided by the invention has excellent cycle performance, rate capability, low-temperature performance and high-temperature safety performance.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and specifically to a lithium-ion secondary battery. Background Technology

[0002] Lithium-ion batteries, as a type of rechargeable battery, have advantages such as high energy density, long cycle life, and no pollution, and are widely used in mobile phones, laptops, electric vehicles, and other fields. Lithium iron phosphate (LFP), as the cathode material for lithium-ion batteries, has advantages such as structural stability, low cost, and long cycle life, making it an ideal cathode material for lithium-ion batteries. However, LFP cathode materials suffer from low electronic conductivity and lithium-ion diffusion rate, resulting in poor rate performance and low-temperature performance. Furthermore, charging at high temperatures can lead to the decomposition and breakage of LFP particles, affecting the battery's cycle life and safety, thus limiting its market application.

[0003] Therefore, there is an urgent need to invent a lithium-ion secondary battery that can take into account excellent rate performance, low-temperature performance, cycle performance and high-temperature safety performance. Summary of the Invention

[0004] The purpose of this invention is to overcome the aforementioned problems in the prior art and provide a lithium-ion secondary battery. The lithium-ion secondary battery of this invention (hereinafter referred to as the battery) has excellent cycle performance, rate performance, low-temperature performance, and high-temperature safety performance.

[0005] The inventors of this invention, through extensive research, discovered that when the positive electrode active material layer contains lithium iron phosphate material doped with V and / or Mn and a solid electrolyte material, the rate performance, low-temperature performance, and cycle performance of the battery can be effectively improved. The reason is as follows:

[0006] First, solid-state electrolyte materials (LATP), as lithium-ion conductors, possess high ionic conductivity and excellent lithium-ion conductivity. Adding them in an appropriate proportion to the positive electrode active material layer can form localized high-ionic-conductivity regions. By filling the pores between lithium iron phosphate materials or adhering to the surface, a continuous and stable lithium-ion transport network is ultimately formed, significantly reducing the transport resistance of lithium ions in the positive electrode active material layer. This improves the overall lithium-ion diffusion rate of the battery, thereby enhancing its rate performance and low-temperature performance. Furthermore, because lithium iron phosphate undergoes volume changes during charge and discharge processes such as delithiation or lithium insertion, leading to structural stress accumulation and even particle breakage, an appropriate proportion of solid-state electrolyte material acts as a rigid structural material, suppressing the volume expansion and structural collapse of the lithium iron phosphate material. Moreover, the solid-state electrolyte material has a high lattice structure matching degree with lithium iron phosphate. Blending an appropriate proportion of solid-state electrolyte material can form a low-impedance interface, reducing interfacial polarization and improving interfacial stability, thus enhancing the battery's cycle performance.

[0007] Secondly, doping lithium iron phosphate with V and / or Mn can improve the voltage plateau of lithium iron phosphate and enhance the stability of the material under high voltage and high temperature conditions. Specifically, V doping can optimize the electronic structure of lithium iron phosphate, reduce its band gap, thereby improving the material's conductivity and accelerating electron transport within the material, thus improving the battery's rate performance. Simultaneously, V doping can broaden the migration channels of lithium ions in lithium iron phosphate, lower the energy barrier for lithium ion migration, reduce the diffusion resistance of lithium ions in the material, and accelerate the diffusion rate, enabling them to more quickly extract from the cathode material and embed into the anode material, thereby further improving the battery's rate performance. Furthermore, Mn has a similar ionic radius and some chemical properties to Fe; therefore, Mn doping not only does not affect… In addition to the original structure, the crystal structure of lithium iron phosphate materials can be optimized to make them more stable, reducing structural changes and volume expansion during charging and discharging, thereby improving the cycle stability of the material. At the same time, the redox potential of Mn is higher than that of Fe. After doping lithium iron phosphate with Mn, the electrode-electrolyte interface can be stabilized, which significantly improves the average voltage platform of the battery during charging and discharging. This suppresses the oxidative decomposition of the electrolyte under high voltage, reduces the increase in interfacial impedance and gas generation, and can effectively reduce the risk of thermal runaway under high temperature and high voltage conditions, thereby improving the cycle stability and safety performance of the battery.

[0008] Furthermore, this invention regulates the mass content of Ti and Fe elements in the positive electrode active material layer, so that when the mass content ratio meets a specific range, it can further improve the rate performance and cycle stability of the battery. When the mass content ratio of Ti to Fe is too small (e.g., less than 1:310), the excessive Fe content will accelerate the side reactions of the material during charging and discharging, leading to loss of active material and even gradual structural collapse. At the same time, the excessively low Ti content reduces the stability of the crystal structure of the material, making it more prone to irreversible changes during cycling, resulting in poorer cycle stability and shorter cycle life. In addition, when the Ti content is insufficient, it cannot effectively improve the lithium-ion diffusion capability of lithium iron phosphate, while the excessively high Fe content leads to crystal structure distortion, hindering the diffusion and migration of lithium ions, increasing battery polarization, increasing the voltage drop of the battery during high-rate discharge, and reducing the rate performance of the battery. When the mass ratio of Ti to Fe is too high (e.g., greater than 1:3), the low content of iron, an important element in lithium iron phosphate that participates in chemical reactions, will lead to a reduction in the active material of the battery and a decrease in the battery's discharge capacity. At the same time, an excessively high Ti content will lead to changes in the crystal structure and an increase in defects, which will hinder the diffusion and transport of lithium ions, reduce the number of lithium ions that can participate in the reaction during the charging and discharging process, and affect the battery's rate performance.

[0009] Based on this, the inventors of this invention propose the following solution:

[0010] This invention provides a lithium-ion secondary battery, comprising a positive electrode; the positive electrode comprises a positive electrode active material layer, the positive electrode active material layer comprising lithium iron phosphate material and a solid electrolyte material; the solid electrolyte material has the chemical formula Li. x Al y Ti z (PO4)3, wherein 1 < x ≤ 1.5, 0 < y ≤ 0.5, 1.5 ≤ z < 2; the lithium iron phosphate material includes doping element M, wherein M includes V and / or Mn; based on the total weight of the positive electrode active material layer, the content of element Fe c1 is 16%-35%, the content of element M c2 is 0.5%-3%, and the content of element Ti c3 is 0.05%-5%; c3:c1 is 1:(3-310).

[0011] By employing the above technical solution, the present invention has at least the following advantages compared with the prior art:

[0012] The lithium-ion secondary battery of the present invention has excellent cycle performance, rate performance, low temperature performance and high temperature safety performance.

[0013] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. Detailed Implementation

[0014] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0015] This invention provides a lithium-ion secondary battery, comprising a positive electrode; the positive electrode comprises a positive electrode active material layer, the positive electrode active material layer comprising lithium iron phosphate material and a solid electrolyte material; the solid electrolyte material has the chemical formula Li. x Al y Ti z(PO4)3, wherein 1 < x ≤ 1.5 (e.g., 1.1, 1.2, 1.3, 1.4, or 1.5), 0 < y ≤ 0.5 (e.g., 0.1, 0.2, 0.3, 0.4, or 0.5), and 1.5 ≤ z < 2 (e.g., 1.5, 1.6, 1.7, 1.8, or 1.9); the lithium iron phosphate material includes a dopant element M, wherein M includes V and / or Mn; based on the total weight of the positive electrode active material layer, the Fe content c1 is 16%-35% (e.g., 16%, 18%, 20%, 2%). The content of element M is 2%, 24%, 26%, 28%, 30%, 32%, or 35%, and the content of element Ti is 0.5%-3% (e.g., 0.5%, 1%, 1.5%, 2%, 2.5%, or 3%). The content of element Ti is 0.05%-5% (e.g., 0.05%, 0.1%, 1%, 2%, 3%, 4%, or 5%). The ratio of c3 to c1 is 1:(3-310), for example, 1:3, 1:50, 1:100, 1:150, 1:200, 1:250, 1:300, or 1:310.

[0016] In one instance, c1 is 20%-30%.

[0017] In one instance, c2 is 1%–2.5%.

[0018] In one instance, c3 is 0.1%–3%.

[0019] In one instance, c3:c1 is 1:(10-200).

[0020] In this invention, the mass content of elements Fe, M and Ti in the positive electrode active material layer can be obtained by conventional means in the art, such as discharging the battery to 0% SOC, disassembling and removing the positive electrode sheet, polishing its cross-section with an argon ion mill, and then testing it with an energy dispersive spectrometer (EDS).

[0021] In this invention, the lithium-ion secondary battery further includes an electrolyte comprising ethylene carbonate and fluoroethylene carbonate; based on the total weight of the electrolyte, the content of ethylene carbonate (c4) is 5%-30% (e.g., 5%, 10%, 15%, 20%, 25%, or 30%), and the content of fluoroethylene carbonate (c5) is 0.1%-15% (e.g., 0.1%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 12%, or 15%).

[0022] In one instance, 5.1% ≤ c4 + c5 ≤ 30%.

[0023] Ethylene carbonate (EC) possesses a high dielectric constant, which can effectively improve the electrolyte's solubility for lithium salts and its ionic conductivity. However, EC is prone to oxidative decomposition at the positive electrode because: EC molecules coordinate with lithium ions in the electrolyte to form a solvated structure and migrate with the lithium ions to the active sites on the positive electrode surface; the active hydrogens on the two methylene groups in the EC molecule are oxidized by TM-O2 on the positive electrode surface, leading to the oxidative decomposition of the EC molecule and the generation of gaseous byproducts, which affects battery performance; Fluorinated ethylene carbonate (FEC), as a negative electrode film-forming additive, can undergo a reduction reaction at the negative electrode, decomposing and generating an organic-inorganic solid electrolyte interphase (SEI) film containing fluorine functional groups, protecting the negative electrode and preventing other solvents from reducing, decomposing, or embedding into the negative electrode. Since EC is associated with gas generation during the oxidation and decomposition of the positive electrode and FEC is associated with the reduction of the negative electrode, in order to further improve battery safety and cycle life, this invention controls the total mass ratio of EC and FEC in the electrolyte to below 30%, which helps to reduce the total gas generation of the battery under high temperature or overcharge conditions, suppress problems such as battery casing swelling and electrode interface peeling, reduce the risk of thermal runaway, and improve the cycle life of the battery.

[0024] In this invention, the mass content of ethylene carbonate and fluoroethylene carbonate in the electrolyte can be obtained by conventional means in the art, such as by gas chromatography (GC) or gas chromatography-mass spectrometry (GCMS).

[0025] In this invention, the electrolyte further includes sulfur-containing compounds, including 1,3-propanesulfonate lactone, vinyl sulfate, etc. At least one of them.

[0026] In one example, the content of the sulfur-containing compound c6 is 0.1%-5% (e.g., 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5% or 5%) based on the total weight of the electrolyte.

[0027] In one instance, c6 is 0.1%–3%.

[0028] Sulfur-containing compounds are further added to the electrolyte, and their content is regulated to improve gas production in the battery under high-temperature conditions. The sulfur-containing compounds contain sulfur, which can oxidize and form a film at the positive electrode, creating a sulfur-containing interfacial film with good thermal stability. Simultaneously, sulfur can complex with metal elements on the positive electrode surface, reducing the surface oxidizability and thus minimizing the oxidation of the electrolyte by the positive electrode. When the sulfur-containing compound content in the electrolyte is too low (e.g., below 0.1%), it leads to continuous electrolyte decomposition, continuous loss of active lithium, accelerated irreversible capacity decay, and reduced battery life. Furthermore, insufficient sulfur content is insufficient to form a complete and effective protective film on the electrode surface, leading to continuous side reactions on the electrode surface, releasing heat and generating gas, exacerbating battery expansion. When the sulfur-containing compound content in the electrolyte is too high (e.g., above 5%), excess sulfur reacts with the electrolyte decomposition product HF to generate corrosive substances, accelerating the corrosion of the current collector, posing a safety risk of internal short circuits, and reducing the battery's cycle life.

[0029] In this invention, the mass content of sulfur-containing compounds in the electrolyte can be obtained by conventional means in the art, such as by GC or GCMS testing.

[0030] In this invention, the lithium-ion secondary battery further includes a separator, the separator comprising a substrate layer and a coating located on at least one side surface of the substrate layer.

[0031] In one example, the substrate layer comprises polypropylene and / or polyethylene.

[0032] In one example, the thickness h of the substrate layer is 5 μm to 16 μm (e.g., 5 μm, 7 μm, 9 μm, 11 μm, 13 μm or 16 μm).

[0033] In one instance, h is 7μm-12μm.

[0034] By limiting the thickness of the separator substrate layer to a suitable range, the mechanical strength and thermal stability of the separator can be effectively improved. When the substrate layer is too thick (e.g., >16μm), it leads to an extended ion migration path and increased internal resistance of the battery, affecting the rate performance and low-temperature performance of the battery. When the substrate layer is too thin (e.g., <5μm), the mechanical strength of the separator is insufficient, and it cannot effectively resist the external impact and deformation of the battery during charge and discharge cycles, which can easily cause the risk of short circuit between the positive and negative electrodes.

[0035] In this invention, the thickness h of the substrate layer can be measured using conventional methods in the art. For example, after discharging the battery to 0% SOC, the separator is disassembled, soaked in DMC solvent for 12 hours, and then rinsed with DMC to remove lithium salts adhering to the negative electrode. After drying, the substrate layer on the separator is peeled off from the negative electrode current collector using a metal scraper, and the substrate layer is collected as a test sample. The substrate layer is cut using an argon ion milling machine with a CP laser, observed using SEM, and at least 5 sites are randomly selected on the substrate layer. The thickness of the substrate layer at each site is measured, and the average value is taken.

[0036] In one example, the coating comprises organic particles selected from one or more of melamine cyanurate, 1,3,5-triazine-2,4,6-triamine, melamine thiocyanate, 2,3-dicyanopyrazine, and symmetrical triaminotriazine.

[0037] In one instance, the coating comprises melamine cyanurate.

[0038] In one example, the organic particles have a particle size Dv50 of 0.2 μm to 1.2 μm (e.g., 0.2 μm, 0.4 μm, 0.6 μm, 0.8 μm, 1 μm or 1.2 μm) and a Dv90 of 0.7 μm to 5 μm (e.g., 0.7 μm, 1 μm, 2 μm, 3 μm, 4 μm or 5 μm).

[0039] In one example, the organic particles have a particle size Dv50 of 0.4 μm-0.9 μm and a Dv90 of 1.5 μm-4 μm.

[0040] In one example, the organic particle content in the coating is 10%-95% by mass (e.g., 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or 95%).

[0041] In one example, the organic particle content in the coating is 50%-90% by mass.

[0042] Organic particles in the separator coating, such as melamine cyanurate, combined with a positive electrode containing LATP, contribute to the battery's flame-retardant properties. LATP is sensitive to acidic substances; when the battery's internal temperature rises, the melamine cyanurate particles decompose and release acidic substances. LATP reacts with these acidic substances, releasing heat and triggering the flame-retardant properties of the organic particles in the separator early, preventing thermal runaway. Separators containing melamine cyanurate possess multiple flame-retardant mechanisms, including phase change endothermic reaction, decomposition endothermic reaction, and inert gas release, significantly delaying the thermal runaway trigger time and effectively improving the battery's high-temperature safety performance and extending its cycle life.

[0043] In this invention, the particle sizes Dv50 and Dv90 of the organic particles in the coating can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, the separator is disassembled, soaked in dimethyl carbonate (DMC) solvent for 12 hours, rinsed with DMC solvent to remove lithium salts attached to the separator, dried, and the organic particles in the separator coating are collected as test samples and tested with a particle size analyzer.

[0044] In this invention, the mass content of organic particles in the coating can be obtained through thermogravimetric analysis (TGA). For example, the temperature of the weight loss peak of melamine cyanurate in the separator is 350℃-700℃. After discharging the battery to 0% SOC, the separator is disassembled and removed. It is then soaked in dimethyl carbonate (DMC) solvent for 12 hours, followed by rinsing with DMC solvent to remove lithium salts adhering to the separator. After drying, the separator is collected as a test sample. Using a thermogravimetric analyzer (e.g., a TGA 550 thermogravimetric analyzer), the sample amount is 5mg-15mg. Under a nitrogen atmosphere, the temperature is increased from room temperature (25℃) to 700℃ at a rate of 10℃ / min. The change in sample mass with temperature is recorded to obtain the thermogravimetric curve (TGA curve). Based on the original sample amount and the weight loss in each temperature range, the weight loss percentage in each temperature range can be calculated. The weight loss percentage can be calculated using the following formula: Weight loss percentage = (weight loss / initial mass) × 100%. The mass content of organic particles in the coating at the corresponding temperature was calculated.

[0045] In one example, the coating also includes an adhesive selected from polyvinylidene fluoride (PVDF) and / or polymethyl methacrylate (PMMA).

[0046] In this invention, the porosity ε1 of the positive electrode is 30%-50% (e.g., 30%, 35%, 40%, 45%, 48% or 50%); and the porosity ε2 of the separator is 26%-64% (e.g., 26%, 30%, 36%, 40%, 45%, 50%, 55%, 60% or 64%).

[0047] In one instance, ε1 was 34%–44%; ε2 was 40%–58%.

[0048] This invention regulates the porosity of the positive electrode and the separator within a specific range to ensure efficient lithium-ion transport between the positive electrode and the separator. When the porosity of the positive electrode is too small (e.g., <30%), the wettability of the electrolyte in the positive electrode deteriorates, the lithium-ion transport channel narrows, the lithium-ion diffusion resistance increases, and the internal resistance of the battery increases. Conversely, when the porosity of the positive electrode is too large (e.g., >50%), the excessive porosity reduces the compaction density of the positive electrode active material, lengthening the diffusion path of lithium ions inside the positive electrode and reducing the lithium-ion transport efficiency. It also leads to excessive electrolyte wetting, triggering numerous side reactions, increasing irreversible lithium loss, and affecting the cycle stability of the battery. When the porosity of the separator is too high (e.g., >64%), the separator's ability to adsorb electrolyte decreases, resulting in uneven distribution of electrolyte within the separator. This further exacerbates the unevenness of the lithium-ion concentration gradient, triggering side reactions that consume more lithium-ions and electrolyte, reducing the battery's reversible capacity and leading to battery life degradation. Conversely, when the porosity of the separator is too low (e.g., <26%), the resistance to lithium-ion transport within the separator is too high. During high-rate discharge, lithium-ions cannot be transported between the positive and negative electrodes in a timely manner, resulting in a decrease in the battery's rate performance.

[0049] In this invention, the porosity ε1 of the positive electrode or the porosity ε2 of the separator can be obtained by conventional means in the art, such as by measuring the porosimetry using a Micron AutoPore V 9600 mercury porosimeter.

[0050] In this invention, the lithium-ion secondary battery further includes a negative electrode sheet, the negative electrode sheet including a negative electrode active material layer, the negative electrode active material layer including graphite and hard carbon.

[0051] In one example, the average particle size d1 of the graphite is 5 μm to 20 μm (e.g., 5 μm, 10 μm, 15 μm or 20 μm).

[0052] In one example, the average particle size d2 of the hard carbon is 1 μm-12 μm (e.g., 1 μm, 3 μm, 5 μm, 7 μm, 9 μm or 12 μm).

[0053] In one instance, d1 is 8μm-18.8μm.

[0054] In one instance, d2 is 2μm-9μm.

[0055] The negative electrode active material of this invention is graphite mixed with hard carbon. Compared with graphite, hard carbon has a larger interlayer spacing, which can construct a fast lithium-ion transport channel and reduce concentration polarization. At the same time, its disordered carbon network structure can improve conductivity, enabling the negative electrode to have both low polarization and high rate performance. Large-particle-size graphite has a smaller specific surface area, reducing the contact area with the electrolyte. Under high-temperature conditions, this helps to reduce the occurrence of side reactions, reduce lithium-ion loss, and improve the high-temperature safety performance of the battery. However, when the average particle size of graphite is too large (e.g., >20μm), the lithium-ion diffusion path is prolonged, the proportion of point contact between particles increases, and the conductive network becomes sparse, leading to a decrease in the rate performance of the battery and an increase in polarization potential. Conversely, when the average particle size of graphite is too small (e.g., <5μm), due to the excessively large specific surface area, side reactions are prone to occur at high temperatures, leading to a deterioration in the high-temperature safety performance of the battery. When the average particle size of hard carbon is too large (e.g., >12μm), it will hinder the migration channels of lithium ions in graphite, causing more capacity to be irreversibly embedded in hard carbon, resulting in severe capacity loss during high-temperature storage of the battery. When the average particle size of hard carbon is too small (e.g., <1μm), its large specific surface area during the initial charge and discharge process causes excessive charge to be consumed in forming the SEI film, resulting in a large amount of irreversible capacity loss, which seriously affects the storage performance of lithium-ion batteries.

[0056] In this invention, the average particle size of the graphite or hard carbon can be obtained by conventional methods in the art, such as SEM. At least 20 graphite or hard carbon particles are selected in the electron microscope image, and the particle size of each graphite or hard carbon particle is measured and the average value is taken. If the graphite or hard carbon appears as a regular circle in the microscope image, then the particle size is the diameter of that circle; if the graphite or hard carbon appears as a non-regular circle in the microscope image, then the particle size is the average value of the dimensions measured from multiple different angles.

[0057] In this invention, the positive electrode active material layer comprises a positive electrode material, a positive electrode conductive agent, and a positive electrode binder. The positive electrode material comprises the lithium iron phosphate material and the solid electrolyte material. The positive electrode conductive agent may include conductive agents conventionally used in the art, such as at least one selected from vapor-grown carbon fiber (VGCF), single-walled / multi-walled carbon nanotubes, conductive carbon black, Ketjen black, and acetylene black. The positive electrode binder may include binders conventionally used in the art, such as at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), polyimide (PI), styrene-butadiene rubber (SBR), polyisobutylene (PIB), and carboxymethyl cellulose (CMC).

[0058] In this invention, based on the total weight of the positive electrode active material layer, the content of the positive electrode material can be 80-99.8% by weight, the content of the positive electrode binder can be 0.1-10% by weight, and the content of the positive electrode conductive agent can be 0.1-10% by weight.

[0059] In this invention, the negative electrode active material layer comprises a negative electrode material, a negative electrode conductive agent, and a negative electrode binder. The negative electrode material may include negative electrode materials conventionally used in the art, such as at least one selected from graphite, soft carbon, hard carbon, and silicon carbide. The negative electrode conductive agent may include conductive agents conventionally used in the art, such as at least one selected from vapor-grown carbon fiber (VGCF), single-walled / multi-walled carbon nanotubes, conductive carbon black (such as Super-P), Ketjen black, and acetylene black. The negative electrode binder may include binders conventionally used in the art, such as at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), polyimide (PI), styrene-butadiene rubber (SBR), polyisobutylene (PIB), and sodium carboxymethyl cellulose.

[0060] In this invention, based on the total weight of the negative electrode active material layer, the content of the negative electrode material can be 80-99.8% by weight, the content of the negative electrode binder can be 0.1-10% by weight, and the content of the negative electrode conductive agent can be 0.1-10% by weight.

[0061] In this invention, the electrolyte may include at least one of carbonate solvents, carboxylic acid ester solvents, and nitrile additives. The carbonate solvents may include cyclic carbonate solvents and chain carbonate solvents. The cyclic carbonate solvents may include cyclic carbonates conventionally used in the art, such as at least one of propylene carbonate (PC) and butylene carbonate (BC). The chain carbonate solvents may include chain carbonates conventionally used in the art, such as at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl ethyl carbonate (EMC), methyl propyl carbonate (MPC), and ethyl propyl carbonate (EPC). The carboxylic acid ester solvents may include carboxylic acid esters conventionally used in the art, such as at least one of methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), and ethyl butyrate (EB). The nitrile additives may include at least one of succinic anionyl nitrile, glutaronitrile, adiponitrile, heptanonitrile, octanoic anionyl nitrile, sebaconitrile, 1,3,6-hexanetrionitrile (HTCN), and glycerol trionitrile.

[0062] It should be noted that the numerical designations such as "first" and "second" in this invention are only used to distinguish different substances or methods of use, and do not represent a difference in order.

[0063] The present invention will be described in detail below through embodiments. The embodiments described herein are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0064] In the following examples, unless otherwise specified, all materials used are commercially available analytical grade.

[0065] Example 1

[0066] The battery is prepared according to the following method:

[0067] (1) Preparation of positive electrode sheet

[0068] Lithium iron phosphate (doped with V and Mn in a 1:1 mass ratio), Li 1.3 Al 0.3 Ti 1.7 (PO4)3, PVDF, and conductive carbon black were mixed in a mass ratio of 88.5:5:3.5:3. N-methylpyrrolidone was added, and the mixture was stirred under vacuum until homogeneous to obtain a positive electrode slurry with a solid content of 58 wt%. This positive electrode slurry was uniformly coated on both sides of an aluminum foil, and after baking, rolling, and slitting, a positive electrode sheet was obtained. The composition of the positive electrode sheet was: c1 = 25%, c2 = 1.5%, c3 = 1.06%, and c3:c1 = 1:23.58. The porosity ε1 of the positive electrode sheet was 38%.

[0069] (2) Preparation of negative electrode sheet

[0070] Graphite, hard carbon, SBR, and conductive carbon black were mixed uniformly in a mass ratio of 80.75:14.25:3:2. Ethylene carbonate (1% by mass of the total materials) was added, followed by deionized water to obtain a negative electrode slurry with a solid content of 44%. This slurry was uniformly coated onto copper foil, dried, rolled, die-cut, and sheet-formed to obtain the negative electrode sheet. The average particle size d1 of the graphite was 13.5 μm, and the average particle size d2 of the hard carbon was 5.5 μm.

[0071] (3) Battery fabrication

[0072] The positive electrode sheet, separator (a polyethylene film with a thickness of 9.5 μm and a coating with a thickness of 2 μm on the surface of the polyethylene film) and the negative electrode sheet prepared in step (2) are stacked in sequence to ensure that the separator is between the positive and negative electrode sheets to play a role in isolation. Then, the bare battery is obtained by stacking and winding. After hot pressing the bare battery, aluminum tabs and copper-plated nickel tabs are welded on. After encapsulation, it is vacuum baked at 95°C for 24 hours. The electrolyte (lithium hexafluorophosphate dissolved in a mixed solution of DMC / EMC / DEC (mass ratio 1:1:1), EC, FEC and 1,3-propanesulfonate lactone are added, wherein the concentration of lithium hexafluorophosphate is 1 mol / L) is injected into the dried bare battery. After formation, secondary sealing, sorting and OCV testing, a lithium-ion battery is obtained.

[0073] Among them, c4 is 8%, c5 is 4%, and c6 is 1.5%; the particle size of melamine cyanurate in the coating is Dv50 of 0.6μm and Dv90 of 2.5μm, and the mass content of melamine cyanurate in the coating is 75%; the porosity ε2 of the diaphragm is 48%.

[0074] Example 2

[0075] The battery is prepared according to the following method:

[0076] (1) Preparation of positive electrode sheet

[0077] Lithium iron phosphate (doped with V and Mn in a 1:1 mass ratio), Li 1.3 Al 0.3 Ti 1.7 (PO4)3, PVDF, and conductive carbon black were mixed in a mass ratio of 93:0.5:3.5:3. N-methylpyrrolidone was added, and the mixture was stirred under vacuum until homogeneous to obtain a positive electrode slurry with a solid content of 58 wt%. This positive electrode slurry was uniformly coated on both sides of an aluminum foil, and after baking, rolling, and slitting, a positive electrode sheet was obtained. The composition of the positive electrode sheet was: c1 = 20%, c2 = 1%, c3 = 0.106%, and c3:c1 = 1:188.68. The porosity ε1 of the positive electrode sheet was 34%.

[0078] (2) Preparation of negative electrode sheet

[0079] Graphite, hard carbon, SBR, and conductive carbon black were mixed uniformly in a mass ratio of 80.75:14.25:3:2. Ethylene carbonate (1% by mass of the total materials) was added, followed by deionized water, to obtain a negative electrode slurry with a solid content of 44%. This slurry was uniformly coated onto copper foil, dried, rolled, die-cut, and sheet-made to obtain the negative electrode sheet. The average particle size d1 of the graphite was 8 μm, and the average particle size d2 of the hard carbon was 2 μm.

[0080] (3) Battery fabrication

[0081] The positive electrode sheet, separator (a 7.2 μm thick polypropylene film and a 2 μm thick coating on the surface of the polypropylene film) and negative electrode sheet prepared in step (2) are stacked in sequence to ensure that the separator is between the positive and negative electrode sheets to play a role in isolation. Then, the bare battery is obtained by stacking and winding. After hot pressing the bare battery, aluminum tabs and copper-plated nickel tabs are welded on. After encapsulation, it is vacuum baked at 95°C for 24 hours. The electrolyte (lithium hexafluorophosphate dissolved in a mixed solution of DMC / EMC / DEC (mass ratio 1:1:1), EC, FEC and ethylene sulfate are added, wherein the concentration of lithium hexafluorophosphate is 1 mol / L) is injected into the dried bare battery. After formation, secondary sealing, sorting and OCV testing, a lithium-ion battery is obtained.

[0082] Among them, c4 is 6%, c5 is 2%, and c6 is 1%; the particle size of melamine cyanurate in the coating is Dv50 of 0.4μm and Dv90 of 1.5μm, and the mass content of melamine cyanurate in the coating is 65%; the porosity ε2 of the diaphragm is 40%.

[0083] Example 3

[0084] The battery is prepared according to the following method:

[0085] (1) Preparation of positive electrode sheet

[0086] Lithium iron phosphate (doped with V and Mn in a 1:1 mass ratio), Li 1.3 Al 0.3 Ti 1.7 (PO4)3, PVDF, and conductive carbon black were mixed in a mass ratio of 83.5:10:3.5:3. N-methylpyrrolidone was added, and the mixture was stirred under vacuum until homogeneous to obtain a positive electrode slurry with a solid content of 58 wt%. This positive electrode slurry was uniformly coated on both sides of an aluminum foil, and after baking, rolling, and slitting, a positive electrode sheet was obtained. The composition of the positive electrode sheet was: c1 = 30%, c2 = 2.5%, c3 = 2.12%, and c3:c1 = 1:14.15. The porosity ε1 of the positive electrode sheet was 44%.

[0087] (2) Preparation of negative electrode sheet

[0088] Graphite, hard carbon, SBR, and conductive carbon black were mixed uniformly in a mass ratio of 80.75:14.25:3:2. Ethylene carbonate (1% by mass of the total materials) was added, followed by deionized water, to obtain a negative electrode slurry with a solid content of 44%. This slurry was uniformly coated onto copper foil, dried, rolled, die-cut, and sheeted to obtain the negative electrode sheet. The average particle size d1 of the graphite was 18.8 μm, and the average particle size d2 of the hard carbon was 9 μm.

[0089] (3) Battery fabrication

[0090] The positive electrode sheet, separator (a polyethylene film with a thickness of 11.6 μm and a coating with a thickness of 2 μm on the surface of the polyethylene film), and negative electrode sheet prepared in step (1) are stacked in sequence to ensure that the separator is between the positive and negative electrodes to provide isolation. Then, a bare battery is obtained by stacking and winding. After hot pressing, aluminum tabs and copper-plated nickel tabs are welded onto the bare battery. After encapsulation, it is vacuum baked at 95°C for 24 hours. The electrolyte (lithium hexafluorophosphate dissolved in a mixed solution of DMC / EMC / DEC (mass ratio 1:1:1)) is added, along with EC, FEC, and... The lithium hexafluorophosphate (LiPF6) concentration of 1 mol / L is injected into the dried bare battery, and after formation, secondary sealing, sorting, OCV testing and other processes, a lithium-ion battery is obtained.

[0091] Among them, c4 is 13%, c5 is 7%, and c6 is 2.5%; the particle size of melamine cyanurate in the coating is Dv50 of 0.9 μm and Dv90 of 3.8 μm, and the mass content of melamine cyanurate in the coating is 86%; the porosity ε2 of the diaphragm is 58%.

[0092] Example 4 group

[0093] Used to verify the impact of changes to "c3:c1".

[0094] This set of embodiments is based on Embodiment 1, except that the content ratio of solid electrolyte material in the positive electrode active material layer and the mass content c1 of element Fe are changed.

[0095] Example 4a: Lithium iron phosphate doped with V, the content of solid electrolyte material in the positive electrode active material layer is 0.5%, c1 is 30%, c3 is 0.106%, and c3:c1 is 1:283.02.

[0096] Example 4b: Lithium iron phosphate doped with Mn, the content of solid electrolyte material in the positive electrode active material layer is 15%, c1 is 20%, c3 is 3.18%, and c3:c1 is 1:6.29.

[0097] In Example 4c, the content of solid electrolyte material in the positive electrode active material layer is 0.5%, c1 is 32%, c3 is 0.106%, and c3:c1 is 1:301.89.

[0098] In Example 4d, the content of solid electrolyte material in the positive electrode active material layer was 20%, c1 was 16%, c3 was 4.24%, and c3:c1 was 1:3.77.

[0099] Example 5 group

[0100] Used to verify the impact of the changes to "c4+c5".

[0101] This set of embodiments is based on Embodiment 1, except that the mass content of ethylene carbonate c4 and the mass content of fluoroethylene carbonate c5 in the electrolyte are changed.

[0102] In Example 5a, c4 is 3%, c4 is 0.1%, and c4+c5 is 3.1%.

[0103] In Example 5b, c4 is 28%, c4 is 7%, and c4+c5 is 35%.

[0104] Example 6 group

[0105] This was used to verify the effects of changes in "sulfur-containing compounds in the electrolyte".

[0106] This set of embodiments is based on Embodiment 1, except that the type and mass content (c6) of sulfur-containing compounds in the electrolyte are changed.

[0107] In Example 6a, c6 is 0.

[0108] In Examples 6b and 6c, the value is 5%.

[0109] Example 7 group

[0110] This was used to verify the impact of changes to the "substrate layer of the diaphragm".

[0111] This set of embodiments is based on Embodiment 1, except that the thickness h of the substrate layer of the diaphragm is changed.

[0112] Example 7a, h is 5 μm.

[0113] Example 7b, h is 18 μm.

[0114] Example 8 group

[0115] This was used to verify the effects of changes to the "diaphragm coating".

[0116] This set of embodiments is based on Embodiment 1, except that the mass content of organic particles melamine cyanurate in the diaphragm coating is changed.

[0117] Example 8a: The mass content of melamine cyanurate in the coating is 0%.

[0118] Example 8b: The coating contains 10% melamine cyanurate by mass.

[0119] Example 8c: The coating contains 95% melamine cyanurate by mass.

[0120] Example 9 group

[0121] This was used to verify the impact of changes in the porosity of the positive electrode.

[0122] This set of embodiments is based on Embodiment 1, except that the porosity ε1 of the positive electrode is changed.

[0123] Example 9a, ε1 is 30%.

[0124] Example 9b, ε1 is 50%.

[0125] Example 10 group

[0126] Used to verify the effects of changes in the "porosity of the membrane".

[0127] This set of embodiments is based on Embodiment 1, except that the porosity ε2 of the diaphragm is changed.

[0128] Example 10a, ε2 is 30%.

[0129] Example 10b, ε2 is 64%.

[0130] Example 11 group

[0131] This was used to verify the impact of changes in the "average particle size d1 of graphite".

[0132] This set of embodiments is based on Embodiment 1, except that the average particle size d1 of graphite is changed.

[0133] Example 11a, d1 is 5.2 μm.

[0134] Example 11b, d1 is 21.5 μm.

[0135] Example 12 group

[0136] This was used to verify the impact of changes in the "average particle size d2 of hard carbon".

[0137] This set of embodiments is based on Embodiment 1, except that the average particle size d2 of the hard carbon is changed.

[0138] Example 12a, d2 is 1 μm.

[0139] Example 12b, d2 is 15μm.

[0140] Example 12c: The negative electrode active material layer does not contain hard carbon.

[0141] Comparative Example 1

[0142] The same procedure was followed as in Example 1, except that the lithium iron phosphate material did not contain the dopant element M.

[0143] Comparative Example 2

[0144] The procedure was carried out in accordance with Example 1, except that the positive electrode active material layer did not contain solid electrolyte material.

[0145] Comparative Example 3 Groups

[0146] This comparative example is based on Example 1, except that the content ratio of solid electrolyte material in the positive electrode active material layer and the mass content c1 of element Fe are changed.

[0147] In Comparative Example 3a, the content of solid electrolyte material in the positive electrode active material layer is 30%, c1 is 10%, c3 is 6.36%, and c3:c1 is 1:1.6.

[0148] In Comparative Example 3b, the content of solid electrolyte material in the positive electrode active material layer is 0.14%, c1 is 38%, c3 is 0.03%, and the c3:c1 ratio is 1:1266.7.

[0149] Test case

[0150] (1) Loop test

[0151] The lithium-ion batteries prepared in the examples and comparative examples were subjected to cycle tests. The specific test methods are as follows:

[0152] Discharge the battery at 55°C with a standard 1C constant current until the discharge cutoff voltage is 2.2V, and let it rest for 30 minutes; then charge it with a standard 1C constant current and constant voltage until the charging cutoff voltage is 3.65V, with a cutoff current of 0.05C, and let it rest for 30 minutes; discharge it with a standard 1C constant current until the discharge cutoff voltage is 2.2V, and let it rest for 30 minutes; repeat the above full charge and discharge steps until the capacity decays to 80% and the cutoff voltage is reached. The number of repeats is the cycle number, which is used to evaluate the cycle performance of the battery after aging. The results are recorded in Table 1.

[0153] (2) Cold start test

[0154] The lithium-ion batteries prepared in the examples and comparative examples were subjected to cold start tests. The specific test methods are as follows:

[0155] Under an environment of (25±2)℃, discharge at 1C standard constant current to the discharge cutoff voltage of 2.2V, and let stand for 30 minutes; then charge at 1C standard constant current and constant voltage to the charging cutoff voltage of 3.65V, with a cutoff current of 0.05C, and let stand for 30 minutes; discharge at 1C standard constant current to the discharge cutoff voltage of 2.2V to obtain the actual battery capacity C0; let stand for 30 minutes; charge at 1C standard constant current and constant voltage to the charging cutoff voltage of 3.65V, with a cutoff current of 0.05C, and discharge at 1C for 30 minutes, which is 50% SOC; after standing at (25±2)℃ for 2 hours, place the battery in a -30℃ constant temperature chamber and maintain the constant temperature for 4 hours; test the terminal voltage value of 10C constant current discharge for 2 seconds, which is the battery cold start terminal voltage, in V, and record the results in Table 1;

[0156] (3) Discharge rate test

[0157] The lithium-ion batteries prepared in the examples and comparative examples were subjected to discharge rate tests. The specific test methods are as follows:

[0158] Under an environment of (25±2)℃, discharge at a standard constant current of 1C to the discharge cutoff voltage of 2.2V, and let stand for 30 minutes; then charge at a standard constant current and constant voltage of 1C to the charging cutoff voltage of 3.65V, with a cutoff current of 0.05C, and let stand for 30 minutes; discharge at a standard constant current of 1C to the discharge cutoff voltage of 2.2V to obtain the actual battery capacity C0, and let stand for 30 minutes; charge at a standard constant current and constant voltage of 1C to the charging cutoff voltage of 3.65V, with a cutoff current of 0.05C, and discharge at 1C for 30 minutes, which is 50% SOC; after standing at (25±2)℃ for 2 hours, discharge at a certain rate at a constant current for 10 seconds to the discharge cutoff voltage of 2.2V, which is the discharge rate of the battery, and the results are recorded in Table 1;

[0159] (4) Thermal shock test

[0160] The batteries prepared in the examples and comparative examples were subjected to thermal shock tests. The specific test methods are as follows:

[0161] At 25℃, the battery was discharged to 2.2V at a given current of 0.2C; left to stand for 5 minutes; then charged to 3.65V at a charging current of 0.2C. When the battery voltage reached 3.65V, it was switched to constant voltage charging at 3.65V until the charging current was less than or equal to the given cutoff current of 0.05C. The battery state before the test was recorded by taking pictures. After the battery was left to stand for 1 hour, the cells were placed in an oven. The oven temperature was increased to 132±2℃ at a rate of 5±2℃ / min and maintained for 30 minutes before stopping. The passing standard was that the battery did not catch fire or explode. Ten batteries were selected for each group for the experiment, and the results are recorded in Table 1.

[0162] Table 1

[0163]

[0164]

[0165] As can be seen from Table 1, the battery of the present invention has better high-temperature safety performance compared with the comparative example, and has a higher discharge rate, which significantly improves the cold start voltage and cycle number of the battery.

[0166] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A lithium-ion secondary battery, characterized in that, The lithium-ion secondary battery includes a positive electrode sheet; the positive electrode sheet includes a positive electrode active material layer, and the positive electrode active material layer includes lithium iron phosphate material and solid electrolyte material; The chemical formula of the solid electrolyte material is Li x Al y Ti z (PO4)3, wherein 1 < x ≤ 1.5, 0 < y ≤ 0.5, 1.5 ≤ z < 2; the lithium iron phosphate material includes a dopant element M, wherein M includes V and / or Mn; Based on the total weight of the positive electrode active material layer, the content of element Fe (c1) is 16%-35%, the content of element M (c2) is 0.5%-3%, and the content of element Ti (c3) is 0.05%-5%; the ratio of c3 to c1 is 1:(3-310).

2. The lithium-ion secondary battery according to claim 1, wherein, c1 is 20%-30%; And / or, c2 is 1%-2.5%; And / or, c3 is 0.1%-3%; And / or, c3:c1 is 1:(10-200).

3. The lithium-ion secondary battery according to claim 1 or 2, wherein, The lithium-ion secondary battery also includes an electrolyte, which comprises ethylene carbonate and fluoroethylene carbonate; based on the total weight of the electrolyte, the content of ethylene carbonate (c4) is 5%-30%, and the content of fluoroethylene carbonate (c5) is 0.1%-15%; 5.1% ≤ c4 + c5 ≤ 30%.

4. The lithium-ion secondary battery according to claim 3, wherein, The electrolyte also includes sulfur-containing compounds; Preferably, the sulfur-containing compound includes 1,3-propanesulfonate lactone, vinyl sulfate, etc. At least one of them; Preferably, the content of the sulfur-containing compound c6 is 0.1%-5% based on the total weight of the electrolyte.

5. The lithium-ion secondary battery according to claim 1 or 2, wherein, The lithium-ion secondary battery further includes a separator, the separator comprising a substrate layer and a coating located on at least one surface of the substrate layer; Preferably, the substrate layer comprises polypropylene and / or polyethylene; Preferably, the thickness h of the substrate layer is 5μm-16μm; more preferably, h is 7μm-12μm.

6. The lithium-ion secondary battery according to claim 5, wherein, The coating comprises organic particles selected from one or more of melamine cyanurate, 1,3,5-triazine-2,4,6-triamine, melamine thiocyanate, 2,3-dicyanopyrazine, and symmetrical triaminotriazine. Preferably, the organic particles have a particle size Dv50 of 0.2 μm-1.2 μm and a Dv90 of 0.7 μm-5 μm; more preferably, the organic particles have a particle size Dv50 of 0.4 μm-0.9 μm and a Dv90 of 1.5 μm-4 μm. Preferably, the organic particles in the coating contain 10%-95% by mass; more preferably, 50%-90%.

7. The lithium-ion secondary battery according to claim 5, wherein, The porosity ε1 of the positive electrode is 30%-50%; And / or, the porosity ε2 of the diaphragm is 26%-64%.

8. The lithium-ion secondary battery according to claim 7, wherein, ε1 is 34%-44%; And / or, ε2 is 40%-58%.

9. The lithium-ion secondary battery according to claim 1 or 2, wherein, The lithium-ion secondary battery also includes a negative electrode sheet, which includes a negative electrode active material layer, and the negative electrode active material layer includes graphite and hard carbon. Preferably, the average particle size d1 of the graphite is 5μm-20μm; Preferably, the average particle size d2 of the hard carbon is 1μm-12μm.

10. The lithium-ion secondary battery according to claim 9, wherein, d1 ranges from 8 μm to 18.8 μm; And / or, d2 is 2μm-9μm.

Images