Lithium ion battery with low-temperature and fast-charging performance and preparation method thereof

Abstract

The invention discloses a lithium ion battery with low-temperature and fast-charging performance and a preparation method thereof, and belongs to the technical field of lithium ion batteries. The lithium ion battery provided by the invention comprises a positive pole piece, a negative pole piece, a diaphragm and an electrolyte, wherein the negative pole piece comprises a negative current collector, a first coating and a second coating; the second coating is positioned on the surface of the first coating; the first coating comprises first graphite, a first conductive agent and a first binder, and the particle size D50 of the first graphite is 14-20 [mu] m; the second coating comprises second graphite, a second conductive agent and a second binder, and the particle size D50 of the second graphite is 9-15 [mu] m; the electrolyte comprises ethylene carbonate, ethyl acetate, lithium hexafluorophosphate and vinylene carbonate, and the mass ratio of ethylene carbonate to ethyl acetate is (3-4): (6-7). The lithium ion battery provided by the invention has good capacity retention rate, energy retention rate and fast charging performance in a low-temperature environment, and is relatively low in cost.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a lithium ion battery with low temperature and fast charging performance and a preparation method thereof. BACKGROUND

[0002] The information disclosed in the background of the present application is only intended to increase the understanding of the overall background of the present application and should not necessarily be regarded as acknowledging or implying in any form that the information constitutes prior art known to those skilled in the art.

[0003] Since the commercialization of lithium ion batteries, they have been widely used in high-tech fields such as energy storage, new energy vehicles, and modern communication due to their long life, high safety, and high energy density. Among them, lithium iron phosphate lithium ion batteries have become the mainstream choice for the electrification of passenger cars. However, existing lithium iron phosphate batteries still face significant performance bottlenecks in low temperature environments and fast charging scenarios, and there is a difficult contradiction between performance improvement and cost control.

[0004] Under low temperature conditions, the discharge capacity and energy density of the battery are greatly reduced, which seriously affects the user experience. This phenomenon is mainly due to the increase in the viscosity of the electrolyte and the significant decrease in the ionic conductivity at low temperatures, as well as the contraction of the electrode material lattice, the slowing down of the lithium ion diffusion rate in the active material, and the increase in the impedance of the electrode interface film, ultimately causing the charge transfer kinetics to deteriorate. More importantly, the rate of lithium ion insertion into the negative electrode graphite during low temperature charging lags behind the rate of extraction, which easily causes the formation of lithium dendrites on the negative electrode surface, not only leading to irreversible capacity loss, but also possibly puncturing the separator and causing a short circuit, posing a safety hazard.

[0005] In the fast charging scenario, large current charging easily causes heat accumulation in the battery, and fast charging also increases the risk of lithium precipitation in the negative electrode, shortening the cycle life of the battery. To alleviate the above problems, existing solutions often rely on high-cost materials (such as silicon-based negative electrodes, special electrolyte additives, etc.) or complex structural designs. These measures not only increase the cost of raw materials and manufacturing, but also reduce the feasibility of mass production due to the complexity of the processing technology, forming a "performance improvement-cost explosion" technical dilemma.

[0006] Therefore, how to simultaneously improve the low temperature performance and fast charging capability of lithium ion batteries while controlling costs has become a key problem that the current industry urgently needs to break through. SUMMARY

[0007] Therefore, the present application provides a lithium ion battery with low temperature and fast charging performance and a preparation method thereof. The present application realizes the improvement of the low temperature performance and fast charging capability of the lithium ion battery through the dual optimization of the negative electrode sheet design and the electrolyte, and has low cost.

[0008] In a first aspect, the present application provides a lithium ion battery with low temperature and fast charging performance, comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte. The negative electrode sheet comprises a negative electrode current collector, a first coating layer and a second coating layer; the first coating layer is located on the surface of the negative electrode current collector, and the second coating layer is located on the surface of the first coating layer; the first coating layer comprises first graphite, a first conductive agent and a first binder, and the particle size D50 of the first graphite is 14-20 μm; the second coating layer comprises second graphite, a second conductive agent and a second binder, and the second graphite is fast charging graphite with a particle size D50 of 9-15 μm; the particle size D50 of the first graphite is greater than that of the second graphite. The electrolyte comprises ethylene carbonate, ethyl acetate, lithium hexafluorophosphate and vinylene carbonate, and the mass ratio of the ethylene carbonate and the ethyl acetate is (3-4) : (6-7).

[0009] Preferably, the first conductive agent and the second conductive agent are each independently selected from one or more of acetylene black, Super P (SP), Ketjen black (KB), carbon nanotubes (CNTs), graphene or carbon nanofibers.

[0010] Preferably, the first binder and the second binder are each independently selected from one or more of sodium carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR) or polyacrylic acid (PAA).

[0011] Preferably, the mass ratio of the first graphite, the first conductive agent and the first binder is (95-97) : (0.5-1.5) : (2.0-4.0), and the mass ratio of the second graphite, the second conductive agent and the second binder is (95-97) : (0.5-1.5) : (2.0-4.0).

[0012] Preferably, the ratio of the area densities of the first coating layer and the second coating layer is 1 : (0.8-1.2).

[0013] Preferably, the area densities of the first coating layer and the second coating layer are 80-150 g / m 2 .

[0014] Preferably, in the electrolyte, the concentration of lithium hexafluorophosphate is 0.5-1.5 mol / L, and the content of vinylene carbonate is 1-3 wt%.

[0015] Preferably, the positive electrode sheet uses lithium iron phosphate as a positive electrode active material, and the area density of the positive electrode sheet is 350-460 g / m 2 .

[0016] In a second aspect, the present application provides a preparation method of the lithium ion battery with low temperature and fast charging performance, comprising the following steps: The prepared positive electrode sheet, negative electrode sheet and electrolyte are sequentially subjected to lamination, liquid injection, formation and capacity distribution, to obtain the lithium ion battery with low temperature and fast charging performance.

[0017] Preferably, the preparation method of the negative electrode sheet comprises: mixing the first graphite, the first conductive agent and the first binder, and then adding water to prepare a first negative electrode slurry; mixing the second graphite, the second conductive agent and the second binder, and then adding water to prepare a second negative electrode slurry; coating on the negative electrode current collector by using a double-layer coating method; and drying and rolling to obtain the negative electrode sheet.

[0018] In a third aspect, the present application provides a use electric device comprising the lithium ion battery or the lithium ion battery prepared by the preparation method.

[0019] Compared with the prior art, the present application has the following beneficial effects: The present application realizes the effective consideration of the low temperature performance and the fast charging performance of the lithium ion battery through the synergistic effect of the double-layer coating design of the negative electrode sheet and the specific electrolyte formula. The second coating layer, as the surface layer, uses fast-charging graphite as the active material, which can directly shorten the diffusion path of lithium ions on the electrode surface, improve the charge transfer rate, and provide core support for the fast charging performance. The first coating layer, as the bottom layer, uses large-particle-size graphite as the active material, and forms a gradient particle size structure with the second coating layer, thereby optimizing the porosity distribution and ion transmission channel, and reducing the concentration polarization during high-rate charging and discharging. At the same time, the complex system of ethylene carbonate and ethyl acetate in the electrolyte improves the ionic conductivity, the vinylene carbonate additive preferentially forms a stable SEI film on the surface of the fast-charging graphite in the surface layer, reduces the interface impedance, and significantly improves the capacity retention rate, energy retention rate and fast charging performance of the battery in a low temperature environment, and the cost is relatively low. DETAILED DESCRIPTION

[0020] It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present application belongs.

[0021] The present application provides a lithium ion battery with low temperature and fast charging performance, comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte. The negative pole piece comprises a negative pole current collector, a first coating layer and a second coating layer; the first coating layer is located on the surface of the negative pole current collector, and the second coating layer is located on the surface of the first coating layer; the first coating layer comprises first graphite, a first conductive agent and a first binder, and the particle size D50 of the first graphite is 14-20 mu m; the second coating layer comprises second graphite, a second conductive agent and a second binder, and the second graphite is fast-charging graphite, and the particle size D50 of the second graphite is 9-15 mu m; the particle size D50 of the first graphite is greater than the particle size D50 of the second graphite. The electrolyte comprises ethylene carbonate, ethyl acetate, lithium hexafluorophosphate and vinylene carbonate, and the mass ratio of the ethylene carbonate and the ethyl acetate is (3-4):(6-7).

[0022] The application builds a lithium ion battery system considering low-temperature performance, fast-charging performance and cost control through the synergistic design of the double-layer coating structure of the negative pole piece and the specific electrolyte formula. The negative pole piece adopts the gradient coating design of "bottom layer large particle size graphite + surface layer fast-charging graphite", the particle size D50 of the fast-charging graphite of the second coating layer (surface layer) is controlled to be 9-15 mu m, the smaller particle size can shorten the diffusion path of lithium ions in the active material particles, reduce the kinetic resistance in the charge transfer process, and provide a structural basis for high-rate fast-charging; and the large particle size graphite of the first coating layer (bottom layer) can improve the capacity and the compaction density of the electrode by optimizing the particle accumulation mode, reduce the overall volume expansion risk of the battery, and reduce the amount of the surface layer fast-charging graphite to control the material cost. The gradient pore network formed by the double-layer structure can not only enhance the electrolyte wettability through the high specific surface area of the small particle size graphite of the surface layer, but also maintain the electrode structure stability through the skeleton support effect of the large particle size graphite of the bottom layer, so as to avoid the problems of the separation and cracking of the pole piece caused by stress concentration in the fast-charging process.

[0023] The ethylene carbonate (EC) and ethyl acetate (EA) in the electrolyte system are compounded according to a mass ratio of (3-4):(6-7), forming a synergistic effect of 'high dielectric constant-low viscosity': EC, as a high dielectric constant solvent, can effectively dissolve lithium hexafluorophosphate (LiPF6) and dissociate high-concentration lithium ions, providing sufficient carriers for ion transmission; and EA, as a low-viscosity solvent, can significantly reduce the overall viscosity of the electrolyte and improve the lithium ion migration rate, especially in a low-temperature environment, the low freezing point of EA can avoid the ion conduction obstruction caused by the sharp increase in electrolyte viscosity. The optimization of the ratio of the two makes the ionic conductivity of the electrolyte at-20 DEG C lower than that of the traditional electrolyte by more than 80%, reducing the ohmic resistance in the ion transmission process, and solving the contradiction between the 'high viscosity at low temperature' and 'insufficient ion concentration at room temperature' of the traditional electrolyte. At the same time, the vinylene carbonate (VC) additive in the electrolyte will preferentially undergo a reduction reaction on the surface of the surface fast-charging graphite, forming a dense SEI film rich in Li2CO3 and organic lithium salt. The film layer has high lithium ion conductivity and low electron conductivity, and can not only inhibit the intercalation of solvent molecules into the graphite interlayer, but also reduce the side reaction consumption of lithium ions at the interface, significantly enhancing the cycle stability during fast charging.

[0024] In the present application, the synergistic effect between the double-layer design of the negative electrode sheet and the electrolyte is mainly reflected in the following aspects: the short diffusion path of the surface fast-charging graphite and the high ion migration rate of the low-viscosity electrolyte match each other, together reducing the concentration polarization during high-rate charging and discharging, so that the battery can still maintain a high capacity output under 5C fast charging conditions, and the highest capacity retention rate can reach more than 95%. The structural support of the bottom layer of large-particle-size graphite combined with the high lithium salt solubility of EC ensures the volume stability of the electrode during long-term cycling, and the stable SEI film formed by VC improves the cycle life of the battery. The high proportion of EA used in the electrolyte reduces the dependence on high-cost solvents such as propylene carbonate, and the use of large-particle-size graphite instead of high-cost fast-charging graphite (the surface density ratio can be controlled within a certain range according to different performance requirements), which reduces the cost of battery materials. The above design of the present application breaks the triangular relationship of 'low-temperature performance-fast charging performance-cost' in traditional lithium-ion batteries, and realizes the optimization of comprehensive performance through multi-scale structure and interface regulation.

[0025] In the present application, the first conductive agent and the second conductive agent are each independently selected from one or more of acetylene black, Super P (SP), Ketjen black (KB), carbon nanotubes (CNTs), graphene or carbon nanofibers. The conductive agent is used to build an efficient electron transmission network.

[0026] In the present application, the first binder and the second binder are each independently selected from one or more of sodium carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR) or polyacrylic acid (PAA). The above-mentioned binders use water as a solvent, avoiding the interference of residual organic solvents of oily binders on the formation of SEI films, and forming a uniformly distributed three-dimensional network through water evaporation during drying, and the polar groups on the molecular chains can form hydrogen bonding with the surface of graphite, achieving electrode structure stability at a lower additive amount, while reducing the obstruction of lithium ion diffusion channels. This structure makes the electrolyte more easily infiltrate the electrode pores, especially at low temperatures of-20℃, the ion transmission path in the electrode is more unobstructed, the concentration polarization is alleviated, and the low viscosity ethyl acetate in the electrolyte synergistically improves the low temperature discharge performance.

[0027] In the present application, the mass ratio of the first graphite, the first conductive agent and the first binder is (95-97):(0.5-1.0):(2.0-3.5); the mass ratio of the second graphite, the second conductive agent and the second binder is (95-97):(0.5-1.0):(2.0-3.5). The above-mentioned ratio design balances the energy density and electrode functionality, the high proportion of active materials maximizes the number of lithium storage sites, directly improving the energy density of the battery; a small amount of conductive agent can form a conductive path through the electrode; the binder needs to ensure the structural integrity of the electrode after rolling, and on the other hand, it needs to avoid excessive addition and block the ion diffusion channel.

[0028] In the present application, the first graphite is preferably a primary particle graphite with a large particle size, and the second graphite is a secondary particle graphite with a small particle size, i.e. after secondary coating treatment. The present application does not make special restrictions on the source of the first graphite and the second graphite.

[0029] In the present application, the area density ratio of the first coating and the second coating is 1:(0.8-1.2). If the surface layer (second coating) is too thick, although the fast charging performance can be enhanced, the structural support of the large particle size graphite in the bottom layer is weakened, resulting in a decrease in the overall compaction density of the electrode; if the bottom layer coating is too thick, the proportion of the surface layer fast charging graphite is insufficient, and the lithium ion diffusion path cannot be effectively shortened. A reasonable area density ratio can form a "gradient pore structure", the large particle size graphite in the bottom layer provides mechanical strength through close packing, and the small particle size graphite in the surface layer forms a loose porous structure, accelerating the electrolyte infiltration and ion transmission, and the two synergistically improve the battery performance.

[0030] In the present application, the area density of the first coating and the second coating is 80-150 g / m 2The surface density is preferably the same.

[0031] In the electrolyte, the concentration of lithium hexafluorophosphate is 0.5-1.5 mol / L, for example, 0.6 mol / L, 0.8 mol / L, 1.0 mol / L, 1.2 mol / L, 1.4 mol / L, etc. Too low concentration will result in insufficient ion carriers, and too high concentration will increase ion association degree, resulting in decreased ion migration rate. The content of vinylene carbonate (VC) is 1-3 wt%, for example, 1.2 wt%, 1.5 wt%, 2.0 wt%, 2.5 wt%, 2.8 wt%, etc. In the present application, the content of VC refers to the mass fraction of the electrolyte.

[0032] In the present application, the positive electrode sheet uses lithium iron phosphate as the positive active material, which has a wide source of raw materials and low cost, and its olivine-type crystal structure has strong thermal stability. The surface density of the positive electrode sheet is 350-460 g / m 2 , which is balanced with the negative electrode capacity.

[0033] The selection of the separator in the present application is not specially limited, and the commonly used separator materials in the art can be used, such as polyethylene separator, polypropylene separator, multi-layer composite separator, non-woven fabric separator, ceramic coating separator, polymer coating separator, etc. The person skilled in the art can select according to the actual needs.

[0034] The selection of the negative current collector in the present application is not specially limited, and copper foil, nickel foil, stainless steel foil, metal-polymer composite foil, etc. can be selected. The present application preferably uses copper foil as the negative current collector.

[0035] The present application also provides a preparation method of the above-mentioned lithium ion battery with low temperature and fast charging performance, comprising the following steps: Preparation of the positive electrode sheet, the negative electrode sheet and the electrolyte, the positive electrode sheet, the separator and the negative electrode sheet are sequentially subjected to lamination, liquid injection, formation and capacity distribution, to obtain the lithium ion battery with low temperature and fast charging performance.

[0036] In the present application, the preparation method of the negative electrode sheet is as follows: the first graphite, the first conductive agent and the first binder are mixed, then water is added to prepare a first negative electrode slurry; the second graphite, the second conductive agent and the second binder are mixed, then water is added to prepare a second negative electrode slurry; the double-layer coating method is used for coating on the negative current collector, and then drying and rolling to obtain the negative electrode sheet.

[0037] The application does not make special restrictions on the preparation method of the positive electrode sheet, and the application preferably mixes lithium iron phosphate, a binder and a conductive agent, then adds a solvent to prepare a positive electrode slurry, and then coats the positive electrode slurry on a positive electrode current collector, dries and rolls to obtain the positive electrode sheet. The application does not make special restrictions on the binder, conductive agent and solvent of the positive electrode sheet, and materials commonly used in the art can be used.

[0038] The application does not make special restrictions on the preparation method of the electrolyte, and the application preferably mixes ethylene carbonate and ethyl acetate, then adds lithium hexafluorophosphate, and then adds vinylene carbonate after the lithium hexafluorophosphate is completely dissolved. The electrolyte is added during the liquid injection process.

[0039] The application does not make special restrictions on the specific lamination, liquid injection, formation and capacity distribution processes, and the preparation method of the lithium ion battery commonly used in the art can be used.

[0040] In an embodiment of the application, a plurality of batteries can be assembled together to form a battery module, and the battery module contains two or more batteries, and the specific number depends on the application of the battery module and the parameters of the single battery module.

[0041] In an embodiment of the application, two or more of the above battery modules can be assembled into a battery pack, and the number of battery modules contained in the battery pack depends on the application of the battery pack and the parameters of the single battery module.

[0042] The application also provides a power consumption device comprising the above lithium ion battery or the lithium ion battery prepared by the above preparation method, or the above battery module or battery pack.

[0043] The power consumption device includes but is not limited to mobile digital devices (such as mobile phones, notebook computers, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0044] The technical solutions of the application will be further described below in combination with specific embodiments. The application does not make special restrictions on the sources of reagents used in the following embodiments, and commercially available products well known to those skilled in the art can be used.

[0045] Example 1 The embodiment provides a lithium ion battery and a preparation method thereof, which take into account low temperature and fast charging performance.

[0046] (1) Preparation of the positive electrode sheet: The lithium phosphate positive electrode material, SP, and PVDF are mixed and dispersed in a mass ratio of 95:2:3, NMP is used as a solvent, and the mixture is uniformly stirred to reach a viscosity of about 6000 mPa·s to prepare a positive electrode slurry. The positive electrode slurry has a surface density of 375 g / m 2 The positive electrode slurry is coated on an aluminum foil current collector, dried, and then rolled and die-cut to obtain a positive electrode sheet.

[0047] (2) Preparation of a negative electrode sheet: Large-particle-size artificial graphite with a particle size D50 of 16 μm, SP, CMC, and SBR are mixed in a mass ratio of 95.6:1.0:1.4:2.0, and then water is added to prepare a first negative electrode slurry with a viscosity of about 4000 mPa·s. Fast-charging graphite with a particle size D50 of 10 μm, SP, CMC, and SBR are mixed in a mass ratio of 95.6:1.0:1.4:2.0, and then water is added to prepare a second negative electrode slurry with a viscosity of about 4000 mPa·s. A negative electrode sheet is prepared on the surface of a copper foil current collector by a double-layer coating technique. The first negative electrode slurry forms a first coating layer, and the second negative electrode slurry forms a second coating layer. The first coating layer is on the surface of the copper foil current collector, and the second coating layer is on the surface of the first coating layer. The surface density of the first coating layer and the second coating layer is both 88 g / m 2 .

[0048] (3) Preparation of an electrolyte: Ethylene carbonate (EC) and ethyl acetate (EA) are mixed in a mass ratio of 3:7, and then lithium hexafluorophosphate is added to control its concentration to 1.0 mol / L. After the lithium hexafluorophosphate is completely dissolved, 2 wt% of vinylene carbonate is added and mixed to obtain the electrolyte.

[0049] (4) Laminating: The positive electrode sheet obtained in step (1), the negative electrode sheet obtained in step (2), and a polyethylene separator are laminated to prepare a 120 Ah square cell.

[0050] (5) Liquid injection, formation, and capacity grading: After the processes of baking, liquid injection (the injected electrolyte is the electrolyte of step (3)), high-temperature standing, formation, high-temperature aging, and capacity grading, a lithium ion battery with a capacity of 120 Ah is obtained.

[0051] Example 2 The present embodiment provides a lithium ion battery with low-temperature and fast-charging performance and a preparation method thereof.

[0052] (1) Preparation of a positive electrode sheet: The lithium phosphate positive electrode material, SP, and PVDF are mixed and dispersed in a mass ratio of 95:2:3, NMP is used as a solvent, and the mixture is uniformly stirred to reach a viscosity of about 6000 mPa·s to prepare a positive electrode slurry. The positive electrode slurry has a surface density of 375 g / m 2Coated on the aluminum foil current collector, after drying, rolling, die cutting, the positive electrode sheet is obtained.

[0053] (2) Preparation of the negative electrode sheet: The large particle size artificial graphite with a particle size D50 of 18 μm, SP, CMC and SBR are mixed in a mass ratio of 95.6:1.0:1.4:2.0, and then water is added to prepare a first negative electrode slurry with a viscosity of about 4000 mPa·s; the fast-charging graphite with a particle size D50 of 12 μm, SP, CMC and SBR are mixed in a mass ratio of 95.6:1.0:1.4:2.0, and then water is added to prepare a second negative electrode slurry with a viscosity of about 4000 mPa·s. The negative electrode sheet is prepared on the surface of the copper foil current collector by double-layer coating technology, the first negative electrode slurry forms the first coating layer, and the second negative electrode slurry forms the second coating layer, the first coating layer is located on the surface of the copper foil current collector, and the second coating layer is located on the surface of the first coating layer. The area densities of the first coating layer and the second coating layer are both 88 g / m 2 .

[0054] (3) Preparation of the electrolyte: Vinyl carbonate and ethyl acetate are mixed in a mass ratio of 4:6, then lithium hexafluorophosphate is added, and the concentration thereof is controlled to be 1.0 mol / L; after the lithium hexafluorophosphate is completely dissolved, 2 wt% of vinylene carbonate is added, and the mixture is uniformly mixed to obtain the electrolyte.

[0055] (4) Lamination: the positive electrode sheet obtained in step (1), the negative electrode sheet obtained in step (2) and the polyethylene separator are subjected to a lamination process to prepare a 120 Ah square shell battery cell.

[0056] (5) Liquid injection, formation and capacity grading: after the processes of baking, liquid injection (the injected electrolyte is the electrolyte of step (3)), high-temperature standing, formation, high-temperature aging and capacity grading, a lithium ion battery with a capacity of 120 Ah is obtained.

[0057] Example 3 Compared with Example 1, the difference between the present example and Example 1 is that the area density of the positive electrode sheet of the present example is 390 g / m 2 , and the area densities of the first coating layer and the second coating layer in the negative electrode sheet are both 92.75 g / m 2 .

[0058] Example 4 Compared with Example 1, the difference between the present example and Example 1 is that the area density of the positive electrode sheet of the present example is 410 g / m 2 , and the area densities of the first coating layer and the second coating layer in the negative electrode sheet are both 97.51 g / m 2 .

[0059] Example 5 The difference between the present embodiment and embodiment 1 is that the areal density of the positive electrode tab of the present embodiment is 430 g / m 2 The areal density of the first coating and the second coating in the negative electrode tab is 102.27 g / m 2 .

[0060] Example 6 The difference between the present embodiment and embodiment 1 is that the areal density of the positive electrode tab of the present embodiment is 450 g / m 2 The areal density of the first coating and the second coating in the negative electrode tab is 107.02 g / m 2 .

[0061] Example 7 The difference between the present embodiment and embodiment 1 is that the areal density of the positive electrode tab of the present embodiment is 470 g / m 2 The areal density of the first coating and the second coating in the negative electrode tab is 111.78 g / m 2 .

[0062] Comparative Example 1 The difference between the present comparative example and embodiment 1 is that the preparation process of the electrolyte of the present comparative example is as follows: Ethylene carbonate (EC), methyl ethyl carbonate (EMC) and dimethyl carbonate (DMC) are mixed in a mass ratio of 1:1:1, then lithium hexafluorophosphate is added, and the concentration is controlled to be 1.0 mol / L; after the lithium hexafluorophosphate is completely dissolved, 2wt% of vinylene carbonate is added, and mixed to obtain the electrolyte.

[0063] Comparative Example 2 The difference between the present comparative example and embodiment 1 is that the negative electrode tab of the present comparative example is single-layer, and the fast-charging graphite with a particle size D50 of 10 μm is used as the negative electrode active material. The preparation method of the negative electrode tab of the present embodiment is as follows: The fast-charging graphite with a particle size D50 of 10 μm, SP, CMC and SBR are mixed in a mass ratio of 95.6:1.0:1.4:2.0, then water is added to prepare a negative electrode slurry with a viscosity of about 4000 mPa·s, and the negative electrode slurry is coated on the surface of the copper foil current collector, and the areal density of the coating is 176 g / m 2 .

[0064] Comparative Example 3 The difference between the present comparative example and embodiment 1 is that the negative electrode tab of the present comparative example is single-layer, and the large-particle-size graphite with a particle size D50 of 16 μm is used as the negative electrode active material. The preparation method of the negative electrode tab of the present embodiment is as follows: A large particle size graphite with a D50 of 16 μm, SP, CMC, and SBR were mixed in a mass ratio of 95.6 : 1.0 : 1.4 : 2.0, and then water was added to prepare a negative electrode slurry with a viscosity of about 4000 mPa-s, and the negative electrode slurry was coated on the surface of a copper foil current collector with a surface density of 176 g / m 2 .

[0065] Comparative Example 4 The difference between this comparative example and Example 1 is that the mass ratio of ethylene carbonate (EC) and ethyl acetate (EA) in this comparative example is 1:9.

[0066] Comparative Example 5 The difference between this comparative example and Example 1 is that the mass ratio of ethylene carbonate (EC) and ethyl acetate (EA) in this comparative example is 2:8.

[0067] Comparative Example 6 The difference between this comparative example and Example 1 is that the mass ratio of ethylene carbonate (EC) and ethyl acetate (EA) in this comparative example is 5:5.

[0068] Comparative Example 7 The difference between this comparative example and Example 1 is that the mass ratio of ethylene carbonate (EC) and ethyl acetate (EA) in this comparative example is 6:4.

[0069] Comparative Example 8 The difference between this comparative example and Example 1 is that the mass ratio of ethylene carbonate (EC) and ethyl acetate (EA) in this comparative example is 7:3.

[0070] Comparative Example 9 The difference between this comparative example and Example 1 is that the mass ratio of ethylene carbonate (EC) and ethyl acetate (EA) in this comparative example is 8:2.

[0071] Comparative Example 10 The difference between this comparative example and Example 1 is that the mass ratio of ethylene carbonate (EC) and ethyl acetate (EA) in this comparative example is 9:1.

[0072] Test Example The lithium ion batteries of Examples 1-7 and Comparative Examples 1-10 were tested for low-temperature discharge (temperatures of 25°C and -20°C, respectively, at a charge-discharge rate of 1C), direct current internal resistance DCR (temperatures of 25°C and -20°C, respectively, at 50% SOC, 2C 10s), and fast-charging performance (at a charge-discharge rate of 5C), and the results are summarized in Tables 1 and 2.

[0073] Table 1: Performance data for lithium ion batteries of Examples 1-7 and Comparative Examples 1-10 at room temperature (25°C)

[0074] Table 2. Low temperature (-20℃) performance data of lithium ion batteries of Examples 1-7 and Comparative Examples 1-10

[0075] As can be seen from Table 1, in Examples 1-7, the low temperature and fast charging performance of Example 3 is optimal, and compared with the conventional electrolyte of Comparative Example 1, the lithium ion battery using the low temperature electrolyte of the application has a low temperature discharge capacity and energy increased by more than 10%, mainly due to the compounding of ethylene carbonate (EC) and ethyl acetate (EA) in the electrolyte system, forming a synergistic effect of "high dielectric constant-low viscosity"; in addition, it can be found from the comparative examples that by changing the double-layer coating to single-layer coating, single-layer fast-charging graphite (second coating layer in double-layer coating) and single-layer large-particle graphite (first coating layer in double-layer coating), the low temperature discharge performance can be improved, mainly because compared with single-layer coating, the gradient pore network formed by this double-layer structure can not only enhance the electrolyte wettability through the high specific surface area of the small particle size graphite on the surface layer, but also maintain the stability of the electrode structure through the skeleton support effect of the large particle size graphite on the bottom layer, and the two synergistically improve the battery performance.

[0076] The preferred embodiments of the application have been described above with the purpose of not limiting the application, and for those skilled in the art, the application can have various changes and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the application shall be included in the protection scope of the application.

Claims

1. A lithium-ion battery that balances low-temperature performance and fast-charging capabilities, characterized in that, Includes positive electrode, negative electrode, separator and electrolyte; The negative electrode sheet includes a negative current collector, a first coating, and a second coating. The first coating is located on the surface of the negative current collector, and the second coating is located on the surface of the first coating. The first coating includes a first graphite, a first conductive agent, and a first binder, wherein the particle size D50 of the first graphite is 14~20μm. The second coating includes a second graphite, a second conductive agent, and a second binder, wherein the second graphite is fast-charging graphite with a particle size D50 of 9~15μm. The particle size D50 of the first graphite is larger than that of the second graphite. The electrolyte comprises ethylene carbonate, ethyl acetate, lithium hexafluorophosphate and vinylene carbonate, wherein the mass ratio of ethylene carbonate to ethyl acetate is (3~4):(6~7).

2. The lithium-ion battery as described in claim 1, which combines low-temperature performance and fast-charging performance, is characterized in that, The first conductive agent and the second conductive agent are each independently selected from one or more of acetylene black, Super P, Ketjen black, carbon nanotubes, graphene or carbon nanofibers.

3. The lithium-ion battery as described in claim 1, which combines low-temperature performance and fast-charging performance, is characterized in that... The first adhesive and the second adhesive are each independently selected from one or more of sodium carboxymethyl cellulose, styrene-butadiene rubber, or polyacrylic acid.

4. The lithium-ion battery as described in claim 1, which combines low-temperature performance and fast-charging performance, is characterized in that... The mass ratio of the first graphite, the first conductive agent, and the first binder is (95~97): (0.5~1.5): (2.0~4.0); the mass ratio of the second graphite, the second conductive agent, and the second binder is (95~97): (0.5~1.5): (2.0~4.0).

5. The lithium-ion battery as described in claim 1, which combines low-temperature performance and fast-charging performance, is characterized in that... The areal density ratio of the first coating to the second coating is 1:(0.8~1.2); the areal density of the first coating to the second coating is 80~150 g / m³. 2 .

6. The lithium-ion battery as described in claim 1, which combines low-temperature performance and fast-charging performance, is characterized in that... The electrolyte contains lithium hexafluorophosphate at a concentration of 0.5-1.5 mol / L and vinylene carbonate at a content of 1-3 wt%.

7. The lithium-ion battery as described in claim 1, which combines low-temperature performance and fast-charging performance, is characterized in that... The positive electrode uses lithium iron phosphate as the positive electrode active material, and the areal density of the positive electrode is 350~460 g / m³. 2 .

8. The method for preparing a lithium-ion battery that combines low-temperature performance and fast-charging performance as described in any one of claims 1 to 7, characterized in that, Includes the following steps: A positive electrode, a negative electrode, and an electrolyte are prepared. The prepared positive and negative electrode sheets are then sequentially stacked, injected with electrolyte, formed, and tested for capacity to obtain the lithium-ion battery that combines low-temperature and fast-charging performance.

9. The preparation method according to claim 8, characterized in that, The method for preparing the negative electrode sheet is as follows: first graphite, first conductive agent and first binder are mixed and water is added to form a first negative electrode slurry; second graphite, second conductive agent and second binder are mixed and water is added to form a second negative electrode slurry; the negative electrode sheet is coated on the negative electrode current collector by a double-layer coating method; and then dried and rolled to obtain the negative electrode sheet.

10. An electrical appliance, characterized in that, This includes the lithium-ion battery that combines low-temperature and fast-charging performance as described in any one of claims 1 to 7, or the lithium-ion battery that combines low-temperature and fast-charging performance prepared by the preparation method described in claim 8 or 9.

Images