A lithium-ion battery and an electrical device

Abstract

This invention provides a lithium-ion battery and an electrical device thereof. The lithium-ion battery includes at least an electrolyte, a separator, and a negative electrode. The electrolyte includes additives, including vinylene carbonate and / or fluoroethylene carbonate. The electrolyte, separator, and negative electrode satisfy the following relationship: 1.25 × 10⁻⁶ ‑3 ≤(a×b) / c≤0.4. The lithium-ion battery provided by this invention achieves a comprehensive improvement in the cycle performance and rate performance of the lithium-ion battery by comprehensively controlling the total mass percentage of additives in the electrolyte (a), the liquid absorption thickness change rate of the separator (b), and the rebound rate of the negative electrode (c).

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically relating to a lithium-ion battery and an electrical device. Background Technology

[0002] With the rapid development of electric vehicles and high-end consumer electronics, the high-rate performance of lithium-ion batteries has become one of the key indicators for evaluating their performance. In lithium-ion battery systems, the electrolyte, as the medium for lithium-ion transport, is typically a complex mixture of organic solvents, lithium salts, and various functional additives. However, under high-rate conditions, the lithium-ion flux in the electrolyte system increases dramatically, triggering strong electrochemical polarization and concentration gradients, leading to electrolyte composition imbalance. At the negative electrode interface, localized enrichment or depletion of components can cause uneven formation of the solid electrolyte interphase (SEI) film, resulting in localized high impedance, hindering lithium-ion intercalation kinetics, and thus causing lithium plating reactions. This not only irreversibly consumes active lithium but also induces lithium dendrite growth, severely damaging the battery's long-term lifespan.

[0003] It is noteworthy that for high-energy-density batteries, the industry commonly employs a manufacturing process involving secondary or even multiple electrolyte injections. The first electrolyte injection is the initial injection after the cell has dried, used to wet the electrodes, participate in formation, and form the SEI film. The second electrolyte injection is a supplementary injection after formation, used to compensate for losses, improve wetting, and optimize performance. However, in practice, the electrolytes injected at each stage often differ in composition concentration and additive ratios, further exacerbating the component concentration gradient. This initial inhomogeneity is amplified at high rates, causing localized electrolyte composition mismatch and deterioration of interface stability, thus making it easier to induce high-resistance sites and lithium plating, severely limiting the battery's long-term lifespan.

[0004] Therefore, overcoming the contradiction between rate performance and cycle life of lithium-ion batteries has become a key technical challenge. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the purpose of this invention is to provide a lithium-ion battery and power device that improves the rate performance and cycle life of the battery.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a lithium-ion battery, the lithium-ion battery comprising at least an electrolyte, a separator, and a negative electrode; the electrolyte includes additives, the additives including vinylene carbonate and / or fluoroethylene carbonate; the electrolyte, separator, and negative electrode satisfy the following relationship:

[0008] 1.25×10 -3≤(a×b) / c≤0.4;

[0009] Where a is the total mass percentage of the additives in the electrolyte, b is the change rate of the liquid absorption thickness of the diaphragm, and c is the rebound rate of the negative electrode.

[0010] In lithium-ion batteries, vinylene carbonate and / or fluoroethylene carbonate serve as film-forming additives in the electrolyte. Vinylene carbonate preferentially decomposes on the negative electrode surface to form a Li₂CO₃-based organic layer, while the fluorine atoms of fluoroethylene carbonate induce the formation of a LiF-containing inorganic layer, resulting in a dense and stable solid electrolyte interphase (SEI) film. The total mass percentage 'a' of additives in the electrolyte indirectly reflects the additive content in the secondary electrolyte. By controlling the total mass percentage 'a' of additives in the electrolyte, the additives in the secondary electrolyte can help compensate for the consumption of the initial film-forming additives and repair the solid electrolyte interphase film damaged during battery cycling. Simultaneously, it can optimize electrode interface impedance, suppress side reactions and gas generation, thereby improving the battery's cycle stability. However, a higher additive content in the secondary electrolyte can prevent uniform mixing with the primary electrolyte, leading to increased film impedance during long-term or high-temperature storage, difficulty in lithium-ion intercalation, and impact on rate performance. By controlling the membrane swelling rate *b*, the amount of electrolyte absorbed by the membrane after the first electrolyte injection is reduced, leaving more free electrolyte for easier mixing with the second injection and improving the mixing uniformity of the electrolyte. Simultaneously, by controlling the negative electrode rebound rate *c*, the volume expansion of lithium ions inserted into the negative electrode promotes the uniformity of electrolyte mixing. By comprehensively controlling the total mass percentage of additives in the electrolyte *a*, the membrane's liquid absorption thickness change rate *b*, and the negative electrode rebound rate *c*, the overall battery cycle performance and rate performance are improved. A value that is too large for the formula (a×b) / c results in poor rate performance; a value that is too small results in poor cycle performance.

[0011] In a second aspect, the present invention provides an electrical device comprising the lithium-ion battery described in the first aspect.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0013] The lithium-ion battery provided by this invention achieves a comprehensive improvement in the cycle performance and rate performance of the lithium-ion battery by comprehensively controlling the total mass percentage of additives in the electrolyte (a), the liquid absorption thickness change rate of the separator (b), and the rebound rate of the negative electrode (c). Detailed Implementation

[0014] This invention provides a lithium-ion battery, which includes at least an electrolyte, a separator, and a negative electrode; the electrolyte includes additives, including vinylene carbonate and / or fluoroethylene carbonate; the electrolyte, separator, and negative electrode satisfy the following relationship:

[0015] 1.25×10 -3 ≤(a×b) / c≤0.4;

[0016] Where a is the total mass percentage of the additives in the electrolyte; b is the change rate of the liquid absorption thickness of the diaphragm; and c is the rebound rate of the negative electrode.

[0017] In this invention, the range of (a×b) / c is 1.25×10 -3 -0.4, for example, can be 0.00125, 0.005, 0.01, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35 or 0.4, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0018] In lithium-ion batteries, vinylene carbonate and / or fluoroethylene carbonate serve as film-forming additives in the electrolyte. Vinylene carbonate preferentially decomposes on the negative electrode surface to form a Li₂CO₃-based organic layer, while the fluorine atoms of fluoroethylene carbonate induce the formation of a LiF-containing inorganic layer, resulting in a dense and stable solid electrolyte interphase (SEI) film. The total mass percentage 'a' of additives in the electrolyte indirectly reflects the additive content in the secondary electrolyte. By controlling the total mass percentage 'a' of additives in the electrolyte, the additives in the secondary electrolyte can compensate for the consumption of the initial film-forming additives and continuously repair the solid electrolyte interphase film damaged during battery cycling. Simultaneously, it can optimize electrode interface impedance, suppress side reactions and gas generation, thereby improving the battery's cycle stability. However, the higher additive content in the secondary electrolyte prevents uniform mixing with the primary electrolyte, leading to increased film impedance during long-term or high-temperature storage, difficulty in lithium-ion intercalation, and impact on rate performance. By controlling the membrane swelling rate *b*, the amount of electrolyte absorbed by the membrane after the first electrolyte injection is reduced, leaving more free electrolyte for easier mixing with the second injection and improving the mixing uniformity of the electrolyte. Simultaneously, by controlling the negative electrode rebound rate *c*, the volume expansion of lithium ions inserted into the negative electrode promotes the uniformity of electrolyte mixing. By comprehensively controlling the total mass percentage of additives in the electrolyte *a*, the membrane's liquid absorption thickness change rate *b*, and the negative electrode rebound rate *c*, the overall battery cycle performance and rate performance are improved. If the formula (a×b) / c is too large, the battery's rate performance is poor; if the formula (a×b) / c is too small, the battery's cycle performance is poor.

[0019] The liquid absorption thickness change rate of the separator reflects its liquid absorption capacity and is determined using the following test method: The battery is discharged at 0.33C to the lower limit voltage of 2.5V, the separator is disassembled, soaked in dimethyl carbonate, dried, and its thickness is measured as d1. The separator is then soaked in an organic solvent and its thickness is measured as d2. The liquid absorption thickness change rate is calculated as: (d2-d1) / d1×100%. To facilitate measurement and reduce errors, in the actual measurement process, the separator is folded into 10 layers and measured using a micrometer, with the thickness measured as 10 layers. The organic solvent is selected according to the type of organic solvent in the electrolyte. The separator is soaked in the organic solvent for 4 hours.

[0020] By controlling the thickness of the coating in the diaphragm and the porosity of the diaphragm, the volume of electrolyte absorbed and stored in the diaphragm can be adjusted, thereby controlling the rate of change of the electrolyte absorption thickness of the diaphragm.

[0021] The rebound rate of the negative electrode is characterized by the change in the thickness of the negative electrode material layer under the conditions of full charge (100% SOC) and discharge (0% SOC). It can reflect the volume change of the negative electrode due to the deintercalation / intercalation of lithium ions during the charging and discharging process of the battery.

[0022] The rebound rate of the negative electrode sheet was determined using the following test method: The lithium-ion battery was discharged at 0.33C to the lower limit voltage of 2.5V. The negative electrode sheet was disassembled and its thickness was measured as d1, which is the thickness of the negative electrode sheet in the discharge state (0% SOC). The negative electrode sheet was then assembled with lithium metal as the working electrode to form a coin cell. The electrolyte was lithium hexafluorophosphate with a concentration of 1mol / L. The assembled half-cell was charged to the charging cutoff voltage of 2.0V. The negative electrode sheet was then immediately disassembled and its thickness was measured as d2, which is the thickness of the negative electrode sheet in the 100% SOC state. After scraping off the negative electrode material layer on the surface of the negative electrode sheet, its thickness was measured as d0, which is the thickness of the current collector of the negative electrode sheet. The rebound rate of the negative electrode sheet was then calculated as: [(d2-d0)-(d1-d0)] / (d1-d0)×100%. To reduce errors, at least three points are measured on the negative electrode sheet during each thickness measurement, and the average value of multiple measurement points is taken as the thickness of the negative electrode sheet.

[0023] By controlling the compaction density and porosity of the negative electrode material layer, the micropore structure and network structure in the negative electrode sheet can be adjusted, thereby controlling the volume expansion space of the negative electrode material and thus the rebound rate of the negative electrode sheet.

[0024] Preferably, the electrolyte, separator, and negative electrode satisfy the following condition: 3.3 × 10⁻⁶. -3≤(a×b) / c≤0.03, for example, it can be 0.0033, 0.005, 0.008, 0.01, 0.015, 0.02, 0.025 or 0.03, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0025] Preferably, the total mass percentage of additives in the electrolyte is 1%-21%, for example, it can be 1%, 3%, 5%, 8%, 10%, 12%, 15%, 18%, 20% or 21%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable. More preferably, it is 2%-8%.

[0026] In the electrolyte, the additives vinylene carbonate and / or fluoroethylene carbonate can preferentially decompose on the surface of the negative electrode to form a stable solid electrolyte interphase (SEI) film. By controlling the total mass percentage 'a' of the additives in the electrolyte, it is ensured that the additives in the secondary electrolyte can help compensate for the consumption of the initial film-forming additives and repair the solid electrolyte interphase film damaged during battery cycling. At the same time, it can optimize the electrode interface impedance, promote the formation of a uniform and stable SEI film, and ensure the long-term cycle life of the battery.

[0027] Preferably, the liquid absorption thickness change rate of the diaphragm is 4%-20%, for example, it can be 4%, 5%, 8%, 10%, 12%, 15%, 18% or 20%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, and it is more preferably 5%-10%.

[0028] By controlling the rate of change of the absorbent thickness of the separator, it is helpful to provide a free environment for the electrolyte components, improve their concentration gradient, and thus ensure the cycle life of the battery; on the other hand, it ensures electrolyte wetting and improves ion conduction, thereby ensuring the rate performance of the battery.

[0029] Preferably, the rebound rate of the negative electrode is 9%-40%, for example, it can be 9%, 10%, 15%, 20%, 25%, 30%, 35% or 40%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, and it is more preferably 20%-35%.

[0030] Controlling the rebound rate of the negative electrode and utilizing the volume change of the negative electrode during the lithium-ion deintercalation / intercalation process can improve the local electrolyte composition imbalance of the negative electrode and improve the lithium plating caused by local high impedance, thereby improving the cycle life of the battery. On the other hand, it can ensure the smooth deintercalation / intercalation of lithium ions on the surface of the negative electrode and ensure the rate performance of the battery.

[0031] Preferably, the electrolyte further includes a second additive, which includes 1,3-propanesulfonate lactone.

[0032] Adding 1,3-propanesulfonate lactone to the electrolyte can reduce and decompose it on the surface of the negative electrode to form a sulfur-containing, thermally stable SEI film component. This component then forms a composite SEI film with vinylene carbonate and / or fluorovinyl carbonate, improving the flexibility and mechanical strength of the SEI film and enhancing the structural stability of the negative electrode.

[0033] Preferably, the electrolyte further includes lithium salt.

[0034] Preferably, the lithium salt includes at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, or lithium bis(fluorosulfonyl)imide.

[0035] By adding lithium hexafluorophosphate, lithium tetrafluoroborate, or lithium bis(fluorosulfonyl)imide as lithium salts, the ionic conductivity of the electrolyte can be guaranteed. In particular, the use of compound components can ensure the stability of the lithium salt composition and improve the battery rate.

[0036] Preferably, the diaphragm includes a base membrane and a coating disposed on at least one surface of the base membrane.

[0037] Preferably, the diaphragm is made of polyethylene and / or polypropylene.

[0038] Preferably, the total thickness of the coating is 1-5 μm, for example, it can be 1 μm, 1.5 μm, 2 μm, 2.5 μm or 3 μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0039] Preferably, the porosity of the diaphragm is 25%-40%, for example, it can be 25%, 28%, 30%, 32%, 35%, 38% or 40%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0040] By controlling the thickness of the coating and the porosity of the diaphragm, the volume of electrolyte absorbed and stored in the diaphragm can be adjusted, thereby affecting the rate of change of the electrolyte absorption thickness of the diaphragm.

[0041] Preferably, the material of the coating includes organic coatings and / or inorganic coatings.

[0042] Preferably, the inorganic coating material includes alumina and / or boehmite.

[0043] Preferably, the material of the organic coating includes at least one of polyvinylidene fluoride, styrene-butadiene rubber, sodium carboxymethyl cellulose, or polyacrylic acid.

[0044] Preferably, the diaphragm surface is provided with an adhesive layer, the thickness of which is 1-3 μm.

[0045] Preferably, the material of the adhesive layer includes polyvinylidene fluoride and / or styrene-butadiene rubber.

[0046] Preferably, the negative electrode includes a current collector and a negative electrode material layer coated on at least one side of the current collector.

[0047] Preferably, the compaction density of the negative electrode material layer is 1.5-1.7 g / cm³. 3 For example, it could be 1.5g / cm³ 3 1.55g / cm 3 1.6g / cm 3 1.65g / cm 3 Or 1.7g / cm 3 However, this does not limit the listed values; other unlisted values ​​within the range are also applicable.

[0048] Preferably, the porosity of the negative electrode material layer is 25%-45%, for example, it can be 25%, 30%, 35%, 40% or 45%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0049] By controlling the compaction density and porosity of the negative electrode material layer, the micropore structure and network structure in the negative electrode sheet can be adjusted, thereby controlling the volume expansion space of the negative electrode material and thus the rebound rate of the negative electrode sheet.

[0050] In this invention, the lithium-ion battery is prepared using the following method:

[0051] (1) Preparation of positive electrode sheet: According to the mass percentage of the positive electrode material layer, 90%-99% of positive electrode active material, 0.5%-5% of conductive agent, 0.5%-5% of binder and 0%-2% of dispersant are mixed evenly, and solvent is added to obtain positive electrode slurry. The positive electrode slurry is coated on at least one surface of the positive electrode current collector, dried, rolled and cut to obtain positive electrode sheet.

[0052] In the above-mentioned positive electrode sheet, the positive electrode active material is selected from at least one of lithium cobalt oxide, nickel-cobalt ternary materials, lithium manganese oxide, lithium iron phosphate, lithium nickel manganese oxide, or lithium manganese iron phosphate.

[0053] Specifically, the general chemical formula of nickel-cobalt ternary materials can be represented as: Li a Ni b Co c M1 d M2 e O f R g, where 0.75 ≤ a ≤ 1.2, 0 < b < 1, 0 < c < 1, 0 < d < 1, b + c + d = 1, 0 ≤ e ≤ 0.2, 1 ≤ f ≤ 2.5, 0 ≤ g ≤ 1, f + g ≤ 3; M1 can be Mn and / or Al, M2 is selected from at least one of B, Zr, Ti, Si, Mo, La, Ce, Te, Nb, Ta, Si, Al, W, Sr, V, Y, Mg, Co or Li, and the content of M2 in the nickel-cobalt-manganese ternary material is 300 - 30000 ppm; R includes but is not limited to at least one of N, F, S or Cl.

[0054] Specifically, lithium nickel manganese oxide is a cathode active material with a spinel structure and has the advantages of high voltage characteristics. Its chemical general formula can be expressed as: Li 1+x Ni y M z Mn 2-x-y-z O 4-k , where -0.1 ≤ x ≤ 0.2, 0.4 ≤ y ≤ 0.6, 0 ≤ z ≤ 0.2, 0 ≤ k ≤ 0.1, M is a doping element, and M includes but is not limited to at least one of Cr, Mo, Nb, Ru, P, S, Ta, W or Ti.

[0055] Specifically, lithium iron manganese phosphate is a cathode active material with an olivine structure and a hexagonal close-packed structure at the same time. Its chemical general formula can be expressed as: Li a G b Fe x Mn y M 1-x-y P 1-m Q m O 4-n R n , where 0.9 ≤ a ≤ 1.1, 0 ≤ b ≤ 0.1, 0.001 ≤ x ≤ 0.999, 0.001 ≤ y ≤ 0.999, 0 ≤ 1 - x - y ≤ 0.1, 0 ≤ m ≤ 0.1, 0 ≤ n ≤ 0.1; M represents a doping element at the manganese site and / or iron site, and M includes but is not limited to at least one of Co, Mg, Zn, Ca, Ti, V, Ni or Cr; G represents a doping element at the lithium site, and G includes but is not limited to at least one of Zn, Al, Na, K, Mg, Nb, Mo or W; Q represents a doping element at the phosphorus site, and Q includes but is not limited to at least one of B, S, Si or N; R represents a doping element at the oxygen site, and R includes but is not limited to at least one of S, F, Cl or Br.

[0056] Specifically, lithium iron phosphate is a cathode active material with an olivine-type crystal structure and has the advantages of low cost and high safety. Its chemical general formula can be expressed as: LiFe 1-x M x PO y Qz Where x≤0.1, 3.85≤y≤4, 0≤z≤0.05; M is a doping element, including but not limited to at least one of Mn, Ni, Co, Cr, Cu, Bi or Sb, and the content of M in lithium iron phosphate is 500-5000ppm.

[0057] In the above positive electrode, the conductive agent is selected from at least one of superconducting carbon (SP), acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers.

[0058] In the above-mentioned positive electrode sheet, the binder is selected from at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene-propylene terpolymer, ethylene-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, or fluorinated acrylate resin.

[0059] In the aforementioned positive electrode sheet, the dispersant is selected from at least one of acrylic acid, acrylate, polyether ester, phosphate ester, small molecule alkanolamine, polyurethane, modified styrene / maleic anhydride, hydrogenated nitrile butadiene rubber (HNBR), polyvinylpyrrolidone (PVP), or polyethylene glycol (PEG). When the positive electrode active material is selected from lithium iron phosphate and / or lithium manganese iron phosphate, a dispersant is used.

[0060] In the above-mentioned positive electrode, the positive current collector can be a metal foil or a composite current collector.

[0061] Specifically, the metal foil can be aluminum or an aluminum alloy.

[0062] Specifically, the composite current collector includes an intermediate high-molecular layer and metal layers disposed on both sides of the polymer layer; wherein, the polymer layer includes polymer materials, including at least one of polyamide (PA), polyterephthalate, polyimide (PI), polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), aramid, acrylonitrile-butadiene-styrene copolymer (ABS), polybutylene terephthalate (PET), poly(p-phenylene terephthalamide) (PPTA), polypropylene (PPE), polyoxymethylene (POM), epoxy resin, phenolic resin, polytetrafluoroethylene (PTEE), polyvinylidene fluoride (PVDF), silicone rubber, polycarbonate (PC), polyvinyl alcohol (PVA), polyethylene glycol (PEG), cellulose, starch, protein, its derivatives, its crosslinks or copolymers; the metal layer includes at least one of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver or silver alloy.

[0063] (2) Preparation of negative electrode sheet: According to the mass percentage composition of the negative electrode material layer, 95%-98% of the negative electrode active material, 0.2%-1.5% of the conductive agent and 1%-3% of the binder are mixed, and a solvent is added to obtain a negative electrode slurry. The negative electrode slurry is coated on at least one surface of the negative electrode current collector, dried, rolled, and slit to obtain the negative electrode sheet. In the negative electrode sheet, the compaction density of the negative electrode material layer is 1.5-1.7 g / cm³. 3 The porosity is 25%-40%.

[0064] In the aforementioned negative electrode sheet, the negative electrode active material is selected from carbon materials, silicon-based materials, or lithium titanate (Li4Ti5O). 12 At least one of the following.

[0065] Specifically, the carbon material is selected from at least one of natural graphite, artificial graphite, mesophase carbon microspheres, hard carbon, or soft carbon.

[0066] Specifically, the silicon-based material is selected from at least one of elemental silicon, silicon oxide, silicon carbide, or silicon alloy.

[0067] In the aforementioned negative electrode sheet, the conductive agent is selected from at least one of superconducting carbon (SP), conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers.

[0068] In the aforementioned negative electrode sheet, the binder is selected from at least one of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, waterborne acrylic resin, polyvinyl alcohol (PVA), sodium alginate (SA), or sodium carboxymethyl methacrylate (CMC). The waterborne acrylic resin may be at least one of polyacrylic acid (PAA), polymethacrylic acid (PMAA), sodium polyacrylate (PAAS), or polyacrylamide (PAM).

[0069] In the aforementioned negative electrode sheet, the negative electrode current collector is selected from metal foil or composite current collector.

[0070] Specifically, the metal foil can be copper or a copper alloy.

[0071] Specifically, the composite current collector includes an intermediate high-molecular layer and metal layers disposed on both sides of the polymer layer; wherein, the polymer layer includes polymer materials, including at least one of polyamide (PA), polyterephthalate, polyimide (PI), polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), aramid, acrylonitrile-butadiene-styrene copolymer (ABS), polybutylene terephthalate (PET), poly(p-phenylene terephthalamide) (PPTA), polypropylene (PPE), polyoxymethylene (POM), epoxy resin, phenolic resin, polytetrafluoroethylene (PTEE), polyvinylidene fluoride (PVDF), silicone rubber, polycarbonate (PC), polyvinyl alcohol (PVA), polyethylene glycol (PEG), cellulose, starch, protein, its derivatives, its crosslinks or copolymers; the metal layer includes at least one of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver or silver alloy.

[0072] (3) Electrolyte preparation: The electrolyte includes a first-phase electrolyte and a second-phase electrolyte. According to the mass percentage of the composition of the first-phase electrolyte, 20%-30% ethylene carbonate, 30%-40% methyl ethyl carbonate and 20%-30% dimethyl carbonate are mixed to obtain an organic solvent. 10%-18% of dried lithium salt is dissolved in the organic solvent, and then 1%-10% of additives are added to obtain the first-phase electrolyte. According to the mass percentage of the composition of the second-phase electrolyte, 20%-28% ethylene carbonate, 25%-35% methyl ethyl carbonate and 20%-28% dimethyl carbonate are mixed to obtain an organic solvent. 10%-18% of dried lithium salt is dissolved in the organic solvent, and then 1%-21% of additives are added to obtain the second-phase electrolyte.

[0073] (4) Separator: The separator is disposed between the positive electrode and the negative electrode to separate the positive electrode and the negative electrode and prevent short circuit between the positive electrode and the negative electrode. The separator includes a substrate and a coating disposed on at least one surface of the substrate. The total thickness of the coating is 1-5 μm, and the porosity of the separator is 25%-40%.

[0074] (5) Stack the positive electrode, separator and negative electrode in sequence so that the separator is between the positive and negative electrode sheets to play a role in isolation. Then wind them to obtain the bare cell. Place the bare cell in the outer packaging shell, dry it and inject one injection of electrolyte. After vacuum sealing, standing and formation, inject two injections of electrolyte. The first injection of electrolyte and the second injection of electrolyte are injected according to the mass percentage of electrolyte of 80%-90% and 10%-20% respectively. Shape it to obtain a lithium-ion battery.

[0075] Steps (1) to (4) are not in any particular order.

[0076] The technical solution of the present invention will be further illustrated below through specific embodiments.

[0077] Example 1

[0078] (1) Preparation of positive electrode sheet: According to the mass percentage composition of the positive electrode material layer, 97.42% lithium iron phosphate, 1.84% PVDF, 0.02% conductive agent (SP and SWCNT formed in a mass ratio of 36:1) and 0.72% dispersant are mixed evenly and dispersed in N-methylpyrrolidone to obtain a positive electrode slurry. The positive electrode slurry is coated onto aluminum foil to obtain a double-sided coated positive electrode sheet. The double-sided coated positive electrode sheet is then rolled and cut to obtain the positive electrode sheet. The compaction density of the positive electrode material layer in the obtained positive electrode sheet is 2.58 g / cm³. 3 .

[0079] (2) Preparation of negative electrode sheet: According to the mass percentage composition of the negative electrode material layer, 96.85% graphite, 0.6% conductive agent SP, 0.5% binder CMC, 1% binder SBR, and 1.05% binder PAA were mixed evenly and dispersed in deionized water to obtain a negative electrode slurry. The negative electrode slurry was coated onto copper foil to obtain a double-sided coated electrode sheet, which was then rolled and cut to obtain the negative electrode sheet. The compaction density of the negative electrode material layer in the obtained negative electrode sheet was 1.61 g / cm³. 3 The porosity is 38.5%.

[0080] (3) Electrolyte preparation: The electrolyte includes a first-phase electrolyte and a second-phase electrolyte. According to the mass percentage of the composition of the first-phase electrolyte, 25.0% ethylene carbonate, 33.3% methyl ethyl carbonate and 24.2% dimethyl carbonate are mixed to obtain an organic solvent. 14.5% of dried lithium hexafluorophosphate is dissolved in the organic solvent, and then 3.0% of additive vinylene carbonate is added to obtain the first-phase electrolyte. According to the mass percentage of the composition of the second-phase electrolyte, 21.0% ethylene carbonate, 28.0% methyl ethyl carbonate and 20.0% dimethyl carbonate are mixed to obtain an organic solvent. 11.50% of dried lithium hexafluorophosphate is dissolved in the organic solvent, and then 19.50% of additive vinylene carbonate is added to obtain the second-phase electrolyte.

[0081] (4) Separator: The substrate of the separator is polypropylene (PP) separator, and the surface of the substrate is coated with alumina (Al2O3) with a thickness of 1.5μm. The porosity of the separator is 28.0%.

[0082] (5) Stack the positive electrode, separator and negative electrode in sequence, so that the separator is between the positive electrode and the negative electrode to play a role in isolation. Then wind them to obtain the bare cell. Place the bare cell in the outer packaging shell, dry it and inject one injection of electrolyte, perform vacuum sealing and formation. After formation, inject two injections of electrolyte. The first injection of electrolyte and the second injection of electrolyte are injected according to the mass percentage of electrolyte of 80% and 20% respectively. After shaping, the lithium-ion battery is obtained.

[0083] Example 2

[0084] The difference between this embodiment and Embodiment 1 is as follows: in the preparation step of the negative electrode sheet, the compaction density and porosity of the negative electrode material layer are changed; in the preparation step of the electrolyte, in the second electrolyte, the type and mass percentage of additives are changed, and a second additive, 1,3-propanesulfonate lactone, is added at a mass percentage of 2.5 wt% of the electrolyte; and a separator with different coating thickness and porosity is selected. The rest of the preparation process is consistent with Embodiment 1.

[0085] Example 3-17

[0086] The difference between Examples 3-17 and Example 1 is that: in the preparation step of the negative electrode sheet, the compaction density and porosity of the negative electrode material layer are changed; in the preparation step of the electrolyte, the type and mass percentage of additives are changed in the second electrolyte injection; and membranes with different coating thicknesses and porosities are selected, while the rest of the preparation process is consistent with Example 1.

[0087] Comparative Examples 1-4

[0088] The differences between Comparative Examples 1-4 and Example 1 are as follows: in the preparation step of the negative electrode sheet, the compaction density and porosity of the negative electrode material layer are changed; in the preparation step of the electrolyte, the type and mass percentage of additives are changed in the second electrolyte injection; and membranes with different coating thicknesses and porosities are selected, while the rest of the preparation process is consistent with Example 1.

[0089] In the above embodiments and comparative examples, the change in the mass percentage of additives in the two-electrode electrolytes was replaced by an equal mass of the organic solvent ethylene carbonate, so as to ensure that the composition of the two-electrode electrolytes meets 100%.

[0090] The lithium-ion batteries obtained in the examples and comparative examples were tested, including: the types and mass percentages of additives in the electrolyte, wherein vinylene carbonate is represented as VC and fluoroethylene carbonate is represented as FEC; the coating thickness, porosity, and liquid absorption thickness change rate of the separator; and the compaction density, porosity, and rebound rate of the negative electrode material layer in the negative electrode sheet. The results are shown in Table 1.

[0091] The testing method is as follows:

[0092] (1) Compacted density of the negative electrode material layer:

[0093] Discharge the battery at 0.33C to a cutoff voltage of 2.5V, disassemble to obtain the negative electrode sheet, soak the negative electrode sheet in dimethyl carbonate (DMC) solution for 4 hours, and air dry. Use a punching machine to punch the pretreated negative electrode sheet into circular pieces of fixed area, and record the area of ​​the circular pieces as S0. Take three circular pieces as parallel samples, and weigh the mass of the three circular pieces using an electronic balance. Take the average value and record it as M1. Use a micrometer to measure the thickness of the negative electrode material layer in the three circular pieces. The thickness of the negative electrode material layer is the total thickness of the negative electrode sheet minus the thickness of the current collector. Take the average value and record it as H. Finally, add an appropriate amount of deionized water to each of the three circular pieces, and gently wipe the coating on the circular pieces with lint-free paper to expose the copper foil. Let it stand at room temperature (or dry) for 10 minutes. After the copper foil is dry, weigh the three copper foil pieces, take the average value and record it as M0. Calculate the compaction density A of the negative electrode sheet according to the following formula: A=(M1-M0) / (H×S0).

[0094] (2) Porosity of the negative electrode material layer:

[0095] The battery was discharged at 0.33C to a cutoff voltage of 2.5V. After disassembly, the negative electrode sheet was obtained. Using an electrode sheet punching machine, the negative electrode sheet was cut into round pieces with a diameter of 19mm. Simultaneously, the thickness of the negative electrode sheet and the current collector were measured using a micrometer. The mass was measured using a balance with an accuracy of 0.0001g, and the volume of the cut negative electrode sheet was calculated. The negative electrode sheet was then immersed in a sealed container containing hexadecane for 1 hour. The negative electrode sheet was removed with tweezers and placed on lint-free paper to absorb dryness until a constant weight was reached (generally, 1 hour is sufficient). The mass was then measured again. The porosity was calculated using the formula: Porosity = (Mass after immersion - Mass before immersion) / (Density × Volume), where the density was 0.7734g / cm³. 3 .

[0096] (3) Rebound rate of the negative electrode:

[0097] The battery was discharged at 0.33C to a cutoff voltage of 2.5V. After disassembly, the negative electrode was obtained, cleaned and dried with DMC, and four test points were taken on the surface of the negative electrode. The thickness of the negative electrode at each point was measured using a multimeter, and the average value was taken to obtain the discharge state (0%). The thickness of the negative electrode sheet at 100% SOC is d1. Using this negative electrode sheet as the working electrode and lithium metal as the reference electrode, a coin cell half-cell is assembled in a glove box. The half-cell is charged to the cutoff voltage, and then charged at a constant voltage until the current is ≤0.05C. After charging, the battery is immediately disassembled in the glove box. The negative electrode sheet is cleaned and dried with DMC. Four test points are taken on the surface of the negative electrode sheet, and the thickness of the negative electrode sheet at each point is measured using a multimeter. The average value is taken to obtain the thickness of the negative electrode sheet at 100% SOC as d2. The negative electrode material layer on the surface of the negative electrode sheet is scraped off, and four test points are taken on its surface again. The thickness at each point is measured using a multimeter, and the average value is taken to obtain the thickness of the current collector as d0. Based on the above thickness values, the rebound rate of the negative electrode sheet is calculated.

[0098] (4) Types and amounts of additives:

[0099] ① Electrolyte Collection: The secondary battery under test is discharged using a battery charging / discharging device under the following conditions: current 0.3C, cutoff voltage. After recording the battery number / barcode, the battery is disassembled and the electrolyte collected in a glove box (H2O≤0.1ppm, O2≤0.1ppm). There are three methods for collecting the electrolyte: After removing the battery cover, if there is free electrolyte, collect it into a 5mL sample tube using a pipette and seal it with sealing tape to prevent leakage; if there is no free electrolyte, it can be... Pressurize continuously using a hydraulic press (Beijing Heng'ao Technology Co., Ltd.'s FY-30 hydraulic press) until free electrolyte appears. Collect the electrolyte into a sample tube and seal it. Add an appropriate amount of dichloromethane extractant to the battery and record the dichloromethane content. After adding dichloromethane, put the battery into an aluminum-plastic bag and seal it with a heat sealer. Transfer it to an ultrasonic oscillator and oscillate for 12 hours to allow the electrolyte in the electrode to mix thoroughly with the dichloromethane. Then, use a pipette to draw the mixture of dichloromethane and electrolyte into a 5mL sample tube and seal the sample tube with sealing tape.

[0100] ② The collected electrolyte sample was injected into an Agilent Intuvo 9000 gas chromatograph-mass spectrometer using a microsyringe for testing to obtain GC-MS chromatograms. Additives such as vinylene carbonate or fluoroethylene carbonate were dissolved in EMC solvent to prepare solutions of different concentrations, and these solutions were injected into the Agilent Intuvo 9000 gas chromatograph-mass spectrometer to obtain standard GC-MS chromatograms. The GC-MS chromatogram of the electrolyte to be tested was compared with the standard GC-MS chromatogram to confirm the presence of the corresponding components. The content of the corresponding components was then determined based on the peak area of ​​the corresponding components in the electrolyte to be tested.

[0101] (5) Coating thickness of the diaphragm:

[0102] After discharging the battery to a cutoff voltage of 2.5V at 0.33C, the separator was disassembled, cleaned, and dried. Then, a cross-sectional sample of the separator was cut to make an SEM sample. An SEM image of the separator cross-section was obtained under SEM testing. Five different positions were selected on the separator cross-section along the transverse direction. Using the measurement tool of the SEM software, the distance from the outer surface of the coating to the interface between the coating and the base film was measured in a direction perpendicular to the surface of the base film. The average value of the measurements at each position was taken as the coating thickness of the separator.

[0103] (6) Porosity of the diaphragm:

[0104] Discharge the battery at 0.33C to a cutoff voltage of 2.5V, disassemble and remove the separator, clean and dry it, and weigh the dry membrane (m0). Then, completely immerse the separator in anhydrous ethanol. The solvent should be a component with good wettability to the separator, generally anhydrous ethanol, hexadecane, or n-butanol. After immersion for a period of time, quickly remove the separator, gently wipe the surface with filter paper to remove the solvent, and weigh the wet membrane (m). Calculate the porosity of the separator. The calculation formula is as follows:

[0105]

[0106] Where ε is the porosity of the membrane; ρ and ρ0 are the densities of the membrane and the solvent anhydrous ethanol, respectively, in g / cm³. 3 m and m0 represent the masses of the wet film and dry film, respectively, in grams.

[0107] (7) Change rate of diaphragm thickness due to liquid absorption:

[0108] The battery was discharged at 0.33C to a cutoff voltage of 2.5V. The separator was then removed and soaked in DMC solution for 1 hour, followed by drying at 25°C for 1 hour. The dried separator was then cut into 500mm x 500mm pieces and folded into 10 layers. The thickness was measured as d1 using a micrometer. The separator was then soaked in an organic solvent (ethylene carbonate, dimethyl carbonate, and diethyl carbonate in a mass ratio of 2:2:1) for 4 hours. The separator was then removed and folded into 10 layers again, and its thickness was measured as d2 using a micrometer. The liquid absorption thickness change rate was calculated.

[0109] Table 1

[0110]

[0111] The test results of the total mass percentage of additives in the electrolyte, the change rate of liquid absorption thickness of the separator, and the rebound rate of the negative electrode in the lithium-ion batteries obtained in each embodiment and comparative example are shown in Table 2. The values ​​of the calculation formula (a×b) / c are also shown in Table 2.

[0112] Performance testing

[0113] The lithium-ion batteries obtained in the examples and comparative examples were calibrated according to the following steps: The test subject was placed in a 25°C temperature chamber (the specific test temperature was adjusted according to the case), and the test subject was operated as follows: charged at 0.33C to the upper limit voltage of 3.65V, and then charged at constant voltage to the cutoff current of 0.05C; left to stand for 30 minutes, and discharged at 0.33C to the lower limit voltage of 2.5V; the above operation was repeated 3 times, and the discharge capacity of the third cycle was taken as the calibrated capacity of the battery.

[0114] After volume determination, the following performance tests were performed:

[0115] (1) Peak power

[0116] The testing method is as follows:

[0117] ① Let the battery rest at 25±2℃ until it reaches thermal equilibrium (i.e., the battery temperature is the same as the ambient temperature).

[0118] ② Discharge at a constant current of 0.33C to the cutoff voltage (2.5V), and let stand for 30 minutes;

[0119] ③Then charge at a constant current of 0.33C to 3.65V, then charge at a constant voltage until the current is ≤0.05C, and let stand for 30 minutes;

[0120] ④ Then discharge at a constant current of 0.33C to 2.5V and let stand for 30 minutes;

[0121] ⑤ Repeat steps ③ and ④ 3 times;

[0122] ⑥ Charge at a constant current of 0.33C to 3.65V, then charge at a constant voltage until the current is ≤0.05C, and let stand for 30 minutes;

[0123] ⑦ Then, discharge at a constant current of 0.33C to adjust the load to the target SOC (90% SOC).

[0124] ⑧ Let the battery rest at the target temperature (25°C) until thermal equilibrium is reached, and record the voltage V0 before discharge;

[0125] ⑨ Discharge at the maximum current I for 30 seconds until the discharge cutoff voltage V is reached. 30 Calculate the peak power using the following formula: W=V 30 ×I.

[0126] (2) Cycle life

[0127] The testing method is as follows:

[0128] The lithium-ion battery was cycled according to the following procedure: constant current charging at 1C rate to 3.65V, constant voltage charging until the current drops to 0.05C, rest for 30 minutes, discharge at 1C rate to 2.5V, and rest for 30 minutes; cycled according to the above procedure until the capacity of the lithium-ion battery was less than 80% of the initial capacity, and the number of cycles was recorded.

[0129] Table 2

[0130]

[0131] As can be seen from the test results in Table 2, referring to Examples 1-17, by coordinating and controlling the total mass percentage of additives in the electrolyte (a), the liquid absorption thickness change rate of the separator (b), and the rebound rate of the negative electrode (c), the ratio (a×b) / c of ​​the three factors was kept within a certain range, ensuring the rate performance and cycle life of the lithium-ion battery. However, in Comparative Examples 1-4, when the ratio of the three factors exceeded 1.25×10... -3 When the concentration is in the range of ~0.4, it can be seen that the rate performance and cycle performance of lithium-ion batteries both show a significant decline. This is because by coordinating and controlling the ratio of the three components, the electrolyte concentration gradient and component imbalance can be effectively improved, the ion transport rate can be enhanced, and a uniform and stable SEI film can be formed, promoting the uniform deintercalation / intercalation of lithium ions in the negative electrode, thereby improving the rate performance and cycle performance of lithium-ion batteries. Furthermore, in Comparative Examples 3-4, if the mass percentage of the additives vinylene carbonate and / or fluoroethylene carbonate in the electrolyte (a), the liquid absorption thickness change rate of the separator (b), and the rebound rate of the negative electrode (c) are not effectively controlled, the battery performance shows further deterioration.

[0132] Furthermore, in Examples 1-18, the lithium-ion batteries provided in Examples 1-5 have superior rate performance and cycle life compared to other examples. Comparing the results of Examples 1-8, in lithium-ion batteries, when the mass percentage of additives vinylene carbonate and / or fluoroethylene carbonate is too high, an excessively thick SEI film is formed on the negative electrode, increasing interfacial impedance and leading to a decrease in battery rate performance. Conversely, when the mass percentage is too low, a sufficient SEI film cannot be formed, resulting in a decrease in battery cycle life. When the liquid absorption thickness change rate of the separator is too high, the separator thickness is too high, increasing the ion transport path and reducing battery rate performance. Conversely, when the mass percentage is too low, it is difficult to maintain electrolyte stability during long-term cycling, easily causing dry areas, increasing side reactions, inducing lithium plating, and reducing battery cycle life. When the rebound rate of the negative electrode is too high, the volume change of the negative electrode is too large, reducing the stability of the electrode structure, easily causing SEI film rupture and active material peeling, reducing battery cycle life. Conversely, when the mass percentage is too low, local lithium ions cannot be effectively conducted on the negative electrode, the negative electrode material layer is too dense, and the contact with the electrolyte is poor, resulting in a decrease in battery rate performance. Comparing the results of Examples 9-17, a higher value in the ratio of a×b to c results in an excessively thick SEI film and separator, reducing the lithium-ion transport rate and worsening the battery's rate performance. Conversely, a lower value results in insufficient SEI film formation, excessive volume change of the negative electrode, poorer electrode structural stability, and a shorter battery cycle life. This demonstrates that precisely coordinating and controlling the numerical relationship between the three factors can simultaneously ensure both the rate performance and cycle performance of lithium-ion batteries.

[0133] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the 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 lithium-ion battery, characterized by, The lithium ion battery at least comprises an electrolyte, a separator and a negative electrode sheet; the electrolyte comprises an additive, the additive comprises vinylene carbonate and / or fluoroethylene carbonate; the electrolyte, the separator and the negative electrode sheet satisfy the following relationship: 1.25×10 -3 ≤(a×b) / c≤0.4; Wherein, a is the total mass percentage content of the additive in the electrolyte; b is the liquid absorption thickness change rate of the separator; c is the rebound rate of the negative electrode sheet.

2. The lithium-ion battery of claim 1, wherein, The electrolyte, the separator and the negative electrode sheet satisfy: 3.3 x 10 -3 ≤ (a x b) / c ≤ 0.

03.

3. The lithium-ion battery of claim 1, wherein, The total mass percentage content of the additive in the electrolyte is 1%-21%; And / or, the liquid absorption thickness change rate of the separator is 4%-20%; And / or, the rebound rate of the negative electrode sheet is 9%-40%.

4. The lithium-ion battery of claim 3, wherein, The total mass percentage content of the additive in the electrolyte is 2%-8%; And / or, the liquid absorption thickness change rate of the separator is 5%-10%; And / or, the rebound rate of the negative electrode sheet is 20%-35%.

5. The lithium-ion battery of claim 1, wherein, The electrolyte further comprises a second additive, and the second additive comprises 1,3-propane sulfone lactone.

6. The lithium-ion battery of claim 1, wherein, The electrolyte further comprises a lithium salt; and the lithium salt comprises at least one of lithium hexafluorophosphate, lithium tetrafluoroborate or lithium bisfluorosulfonylimide.

7. The lithium-ion battery of claim 1, wherein, The porosity of the separator is 25%-40%.

8. The lithium-ion battery of claim 1, wherein, The separator comprises a base film and a coating layer arranged on at least one side surface of the base film, and the total thickness of the coating layer is 1-5μm.

9. The lithium-ion battery of claim 8, wherein, The material of the coating layer comprises an organic coating layer and / or an inorganic coating layer; The material of the inorganic coating layer comprises alumina and / or boehmite; The material of the organic coating layer comprises at least one of polyvinylidene fluoride, styrene butadiene rubber, sodium carboxymethyl cellulose or polyacrylic acid.

10. The lithium-ion battery of claim 8, wherein, The surface of the separator is provided with a bonding layer, and the thickness of the bonding layer is 1-3μm; and the material of the bonding layer comprises polyvinylidene fluoride and / or styrene butadiene rubber.

11. The lithium-ion battery of claim 1, wherein, The negative electrode sheet comprises a current collector and a negative electrode material layer coated on at least one side surface of the current collector; The compaction density of the negative electrode material layer is 1.5-1.7 / cm 3 , and / or the porosity of the negative electrode material layer is 25%-45%.

12. An electrical device, characterized by The electric device comprises the lithium ion battery according to any one of claims 1-11.

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