Lithium ion battery and electric device

Abstract

The invention relates to a lithium ion battery and an electric device. The lithium ion battery comprises: a negative pole piece, which comprises a negative current collector and a negative active coating located on the negative current collector, the negative active coating contains silicon carbon particles, a carbon material and a binder, the specific surface area of the silicon carbon particles is S m < 2 > / g, and the silicon carbon particles account for the mass fraction W < 1 >% of the negative active coating; the electrolyte comprises a lithium salt, a solvent and an additive, the additive comprises fluoroethylene carbonate, and the fluoroethylene carbonate accounts for W2% of the mass fraction of the electrolyte; wherein 1 < = S < = 15, 1 < = W1 < = 20, 1 < = W2 < = 30, and 1 < = S * W1 / W2 < = 10. According to the scheme provided by the invention, the stability of the electrode interface film can be improved, the expansion of the silicon negative electrode is inhibited, and the cycle performance and the service life of the battery are improved.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and in particular to a lithium-ion battery and an electrical device. Background Art

[0002] Lithium-ion batteries are a common energy storage / conversion system that can not only store renewable energy such as wind, hydro, and solar energy, but also store electrical energy when there is excess electricity.

[0003] Lithium-ion batteries mainly rely on the movement of lithium ions between the positive and negative electrodes to achieve charge and discharge cycles. Among the negative electrode materials of lithium-ion batteries, silicon materials have great potential in battery negative electrode materials due to their ultra-high theoretical specific capacity, suitable operating voltage, abundant reserves and good safety.

[0004] However, compared with graphite and lithium titanate, the volume change of single silicon during the charge and discharge process is as high as 300%. The large volume change can easily cause the electrode particles to pulverize and the active material to fall off. In addition, Si has low electrical conductivity, which leads to a slow electrode reaction kinetics process, limiting its specific capacity and reducing rate performance. With the continuous research on silicon-based materials, it is found that the electrochemical performance of silicon-carbon composite materials has been improved, but silicon-carbon composite materials still undergo violent volume expansion when lithium is inserted, resulting in damage to the electrode structure and affecting the cycle life of the battery. Summary of the invention

[0005] In order to solve or partially solve the problems existing in the related art, the present application provides a lithium-ion battery and an electrical device, which can form a stable SEI film at the negative electrode interface, inhibit the expansion of the silicon negative electrode material, and improve the cycle performance and service life of the silicon negative electrode battery.

[0006] The first aspect of the present application provides a lithium ion battery, comprising:

[0007] The negative electrode sheet comprises a negative electrode current collector and a negative electrode active coating on the negative electrode current collector, wherein the negative electrode active coating contains silicon carbon particles, carbon material and a binder, and the specific surface area of ​​the silicon carbon particles is Sm 2 / g, the mass fraction of the silicon-carbon particles in the negative electrode active coating is W1%;

[0008] An electrolyte, comprising a lithium salt, a solvent and an additive, wherein the additive comprises fluoroethylene carbonate, and the mass fraction of the fluoroethylene carbonate in the electrolyte is W2%;

[0009] Among them, 1≤S≤15, 1≤W1≤20, 1≤W2≤30, 1≤S*W1 / W2≤10.

[0010] As an important component of lithium-ion batteries, electrolyte plays a role in transmitting ions and conducting current between the positive and negative electrodes of the battery. This application improves the formula composition of the electrolyte and adjusts the relationship between the content of the electrolyte additive fluoroethylene carbonate and the negative electrode silicon element, so that fluoroethylene carbonate can form a sufficient and stable SEI film on the surface of silicon-carbon particles. On the one hand, it can limit the expansion of silicon elements and protect silicon-carbon particles and negative electrode sheets; on the other hand, the SEI film can effectively prevent the side reactions between the oxidant in the electrolyte and the negative electrode of the battery, especially the side reactions during high-temperature cycles, thereby improving the stability of the electrolyte and the cycle performance of lithium-ion batteries.

[0011] Furthermore, in the negative electrode plate of the lithium-ion battery, when the specific surface area and mass ratio of the silicon-carbon particles are within the above range, significant volume expansion is not likely to occur when lithium is inserted, thereby avoiding structural rupture or crushing of the electrode material during the cycle. The SEI film has good stability and can reduce irreversible lithium ion loss, thereby improving the battery energy density and cycle stability.

[0012] In some embodiments, the lithium ion battery further satisfies: 5.4≤S≤8.0, 3≤W1≤10, 10.5≤W2≤15.8, 1.6≤S*W1 / W2≤5.5.

[0013] In some embodiments, in the Raman test spectrum of the silicon-carbon particles, at (1350±50) cm -1 The position has a characteristic peak Id at (1575±50)cm -1 The position has a characteristic peak Ig, and Id / Ig is 0.8 to 1.2; preferably 0.95 to 1.05.

[0014] At (1350±50)cm -1 Position and (1575±50)cm -1 When the graphitization degree of silicon-carbon particles with characteristic peaks at the position is combined with carbon materials and applied to the negative electrode, the silicon element and the carbon matrix have good uniformity and stability, which can significantly improve the energy density of the battery while reducing the use of silicon-based materials and saving costs. At the same time, it can inhibit the volume expansion of silicon during the cycle to a certain extent, protect the stability of the negative electrode structure, reduce the probability of reduced coulomb efficiency and rapid capacity attenuation, and improve the cycle stability of the battery. In particular, when the Raman spectrum peak intensity ratio of the D peak and the G peak of the carbon matrix of the silicon-carbon particles is within a suitable range, it can reduce the over-embedding caused by the insertion of lithium ions, reduce the lithium plating phenomenon of the negative electrode, and thus extend the service life of the battery cell.

[0015] In some embodiments, the mass fraction of silicon in the silicon-carbon particles in the negative electrode active coating is 1% to 15%, preferably 2.5 to 5%.

[0016] In some embodiments, the mass ratio of the silicon-carbon particles to the carbon material is (15-18):1, preferably 16:1.

[0017] In some embodiments, the silicon-carbon particles have a carbon content of 50 wt% to 80 wt%, a silicon content of 10 wt% to 70 wt%, and an oxygen content of less than 10 wt%.

[0018] On the one hand, by adjusting the composition between silicon-carbon materials and carbon materials with a certain degree of graphitization, the silicon-based material and the carbon-based material are evenly distributed in the negative electrode interface on the surface of the negative electrode plate. The combination of carbon-based materials and carbon materials in the silicon-carbon particles can effectively alleviate the volume change of silicon during the charging and discharging process, improve the structural stability of the electrode material, and enable the silicon negative electrode material to provide channels for lithium ion embedding and extraction from all directions, thereby achieving faster charging speed and improving the fast charging performance of the battery.

[0019] On the other hand, by using silicon-carbon particles that meet the above conditions in the negative electrode material, it is possible to achieve the effect of saving the use of silicon elements to a greater extent while improving the battery energy density. When the proportion of silicon elements is within the above range, on the one hand, it can effectively save costs, and on the other hand, the carbon matrix can be used to effectively improve the problem of silicon volume expansion, so that when the silicon-carbon particles used in the lithium-ion battery are used in combination with the carbon material, and then the use of reasonable fluoroethylene carbonate is used, it can effectively inhibit the expansion of the silicon negative electrode during the battery charging and discharging process, and improve the cycle performance of the silicon negative electrode battery.

[0020] In some embodiments, the carbon material includes at least one of amorphous carbon, SuperP, Ketjen black, acetylene black, carbon nanotubes, carbon nanofibers, graphene, artificial graphite, natural graphite, and mesophase carbon microbeads;

[0021] The binder includes at least one of lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, styrene-butadiene rubber, styrene-acrylic emulsion, lithium polyacrylate, sodium polyacrylate, potassium polyacrylate, lithium alginate, sodium alginate and potassium alginate.

[0022] In some embodiments, the solvent includes a carboxylate compound and a carbonate compound;

[0023] The carboxylate compound comprises at least one of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, n-propyl propionate, isopropyl propionate, n-butyl propionate, isobutyl propionate, n-pentyl propionate, isopentyl propionate, ethyl butyrate, n-propyl butyrate, propyl isobutyrate, n-pentyl butyrate, n-pentyl isobutyrate, n-butyl butyrate, isobutyl isobutyrate, n-pentyl valerate, butyl acetate, methyl formate, ethyl formate, ethyl propionate, propyl propionate, methyl butyrate, butyl butyrate, methyl isovalerate, propyl hexanoate and isobutyl acetate;

[0024] The carbonate compound includes at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate and dimethyl carbonate.

[0025] In some embodiments, the mass fraction of the carboxylate compound in the electrolyte is 5% to 20%, preferably 8% to 15%; further preferably, the mass ratio of the carboxylate compound to fluoroethylene carbonate is 1:(1 to 3), more preferably 1:(2 to 2.5).

[0026] By using a solvent containing both carboxylate compounds and carbonate compounds, the carbonate compound solvent and fluoroethylene carbonate can form a stable solid electrolyte interface at the electrode interface, prevent the side reaction between the oxidant in the electrolyte and the negative electrode of the battery, protect the stability of the electrolyte and the electrode material, and improve the capacity retention rate and energy density of the battery; while the carboxylate compound can reduce the viscosity of the electrolyte, improve the ionic conductivity of the electrolyte, and improve the low-temperature performance of the battery. In particular, when the mass ratio between the carboxylate compound solvent and the fluoroethylene carbonate additive is within a suitable range, the two can be used in combination to synergistically enhance the effect, which can not only avoid the problem that the carboxylate compound is easy to produce gas and the side reaction of the lithium graphite on the negative electrode that leads to the deterioration of the high-temperature cycle and rate performance of the battery, but also ensure that the battery has good low-temperature performance and ion transmission performance, and improve the fast charging performance of the battery.

[0027] In some embodiments, the lithium salt includes at least one of LiPF6, LiBF4, LiClO4, LiB(C6H5), LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, LiSiF6, LiBOB and LiDFOB.

[0028] In some embodiments, the concentration of the lithium salt is 0.9 mol / L to 1.3 mol / L.

[0029] A second aspect of the present application provides an electrical device, comprising the lithium-ion battery as described in the first aspect of the present application.

[0030] The technical solution provided by this application may have the following beneficial effects:

[0031] The present application scheme adjusts the relationship between the content of fluoroethylene carbonate added in the electrolyte and the specific surface area and content of silicon-carbon particles in the negative electrode plate, so that the additive fluoroethylene carbonate can form a SEI film with good stability and density at the negative electrode interface, prevent the side reaction between the oxidant in the electrolyte and the negative electrode of the battery, reduce the decomposition of the electrolyte, reduce gas production, and inhibit the expansion of the negative electrode silicon-carbon particles, thereby protecting the stability of the electrolyte and electrode interface, thereby improving the cycle performance of the silicon negative electrode battery and extending the service life of the battery.

[0032] Furthermore, a reasonable combination of silicon-carbon particles and carbon materials is used in the negative electrode plate, and the composition of the additives fluoroethylene carbonate and carboxylic acid ester compound solvent in the electrolyte is improved to enhance the overall stability of the electrolyte, improve the conductivity of the electrolyte, and enhance the kinetic performance of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art are briefly introduced below.

[0034] Figure 1 This is the Raman spectrum of the silicon-carbon particles in Example 1 of the present application. DETAILED DESCRIPTION

[0035] To make the present invention easy to understand, the present invention will be described in detail below. However, before describing the present invention in detail, it should be understood that the present invention is not limited to the specific embodiments described. It should also be understood that the terms used herein are only for describing specific embodiments and are not intended to be limiting.

[0036] The terms used in this application are for the purpose of describing specific embodiments only and are not intended to limit this application. The singular forms of "a", "said" and "the" used in this application and the appended claims are also intended to include plural forms unless the context clearly indicates other meanings. It should also be understood that the term "and / or" used herein refers to and includes any or all possible combinations of one or more associated listed items.

[0037] Where a numerical range is provided, it is to be understood that each intermediate value between the upper and lower limits of the range and any other provisions or intermediate values ​​in the specified range is encompassed within the present invention. The upper and lower limits of these smaller ranges may be independently included in the smaller ranges and are also encompassed within the present invention, subject to any explicitly excluded limits in the specified range. Where a specified range includes one or two limits, the scope excluding any or both of those included limits is also encompassed within the present invention. In the description of the present application, the meaning of "multiple" is two or more, unless otherwise clearly and specifically limited.

[0038] Unless otherwise defined, all terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the invention belongs. Although any methods and materials or equivalent methods and materials described herein can also be used in the practice or testing of the present invention, preferred methods and materials are now described.

[0039] Compared with graphite and lithium titanate, the volume change of single silicon during the charge and discharge process is as high as 300%. The large volume change can easily cause the electrode particles to pulverize and the active material to fall off. In addition, Si has low electrical conductivity, which leads to a slow electrode reaction kinetics process, limiting its specific capacity and reducing rate performance. With the continuous research on silicon-based materials, it is found that the electrochemical performance of silicon-carbon composite materials has been improved, but silicon-carbon composite materials still undergo violent volume expansion when lithium is inserted, resulting in damage to the electrode structure and affecting the cycle life of the battery.

[0040] In view of the above problems, an embodiment of the present application provides a lithium-ion battery that can inhibit the expansion of silicon negative electrode materials, improve the cycle performance of silicon negative electrode batteries, and extend the service life of the battery.

[0041] The lithium-ion battery of the embodiment of the present application comprises a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the negative electrode sheet comprises a negative electrode current collector and a negative electrode active coating on the negative electrode current collector, wherein the negative electrode active coating comprises silicon-carbon particles, carbon material, a conductive agent and an adhesive.

[0042] Among them, the specific surface area of ​​silicon carbon particles is Sm 2 / g, the mass fraction of silicon-carbon particles in the negative electrode active coating is W1%, and the silicon-carbon particles satisfy: 1≤S≤15, 1≤W1≤20. The specific surface area of ​​the silicon-carbon particles can be, for example, 1.0m 2 / g, 3.0m 2 / g, 5.4m 2 / g, 6.0m 2 / g, 8.0m 2 / g, 10.0m 2 / g, 14.2m 2 / g, 15.0m 2 / g, etc., preferably 5.4m 2 / g~8.0m 2 The mass fraction of silicon-carbon particles in the negative electrode active coating layer may be, for example, 1%, 3%, 5%, 8%, 10%, 12%, 15%, 20%, etc., preferably 3% to 10%.

[0043] The specific surface area of ​​the silicon-carbon particles described in the present application can be measured according to a test method known in the art without limitation. For example, the specific surface area of ​​the silicon-carbon particles can be measured in the following manner:

[0044] 1. Test: Weigh 1.5g to 3.5g of powder sample and put it into the test sample tube of the specific surface area and porosity analyzer (model TriStarII 3020), and then degas at 200℃ for 120min before testing;

[0045] 2. Calculation: At a constant temperature (e.g. 200°C), after measuring the amount of gas adsorbed on the solid surface at different relative pressures, the monolayer adsorption of the sample is obtained based on the Brown-Noll-Et-Taylor adsorption theory and its formula (BET formula), thereby calculating the specific surface area of ​​the negative electrode material.

[0046] The BET formula is:

[0047]

[0048] In the formula, P represents the nitrogen partial pressure; P0 represents the saturated vapor pressure of nitrogen at liquid nitrogen temperature; V represents the actual adsorption amount of nitrogen on the sample surface; V m represents the saturated adsorption capacity of nitrogen monolayer; c represents a constant related to the adsorption capacity of the sample; (c-1) / (V m C) represents the slope; 1 / V m C represents the intercept.

[0049] The total surface area St is: St = (V m *N*Acs / M), the specific surface area S is: S=St / m', where m' represents the sample mass and Acs represents the average area occupied by each N2 molecule.

[0050] In some specific embodiments, the silicon-carbon particles can be characterized by Raman testing. In the Raman test spectrum of the silicon-carbon particles, at (1350±50) cm -1 Position and (1575±50)cm -1 The position has a characteristic peak.

[0051] The Raman test described in this application can be performed according to the test methods known in the art, and is not limited here. The Raman spectrum (Raman) test equipment used in this application can be a Jobin Yvon LabRAM HR spectrometer, with a light source of 532nm and a test range of 0cm -1 Up to 4000cm -1 , 100μm×100μm.

[0052] It can be understood that the carbon matrix of silicon carbon particles is (1350±50)cm -1 The characteristic peak observed at (1575±50) cm -1 The characteristic peak observed at the position represents the change of carbon atoms in the silicon-carbon particles when they vibrate in the plane, and can also be called G band or G peak. The silicon-carbon particles can be observed to have D peak and G peak, that is, the defect density and structural quality of the carbon matrix in the silicon-carbon particles can be known by the intensity ratio of D peak and G peak Id / Ig. Due to the influence of differences in the instruments and equipment used, test errors or other relevant test factors, the test results will fluctuate slightly. Therefore, this application does not limit the values ​​of D peak and G peak at specific positions. For example, D peak can refer to the carbon atom at 1350cm -1 The peak intensity of the Raman spectrum at the position can also refer to the carbon atom at 1400 cm -1 The peak intensity of the Raman spectrum at the position, or at (1350±50)cm -1 Any other value that fluctuates within the allowable error range; the G peak can refer to the carbon atom at 1575cm -1 The peak intensity of the Raman spectrum at 1580 cm -1 The peak intensity of the Raman spectrum at the position, or at (1575±50)cm -1 Any other value that fluctuates within the permissible error range.

[0053] In some specific embodiments, the Id / Ig of the silicon-carbon particles is 0.8 to 1.2, preferably 0.95 to 1.05, wherein Ig represents the silicon-carbon particles at (1350±50) cm -1 The Raman spectrum peak intensity at position Ig represents the peak intensity of silicon-carbon particles at (1575±50) cm -1 The Raman spectrum peak intensity at the position. In the embodiment of the present application, the value of Id / Ig can be obtained by counting the average value of 100 Id / Ig values ​​of silicon-carbon particles. The carbon matrix is ​​evenly distributed in the silicon-carbon particles with the above characteristic peaks, and then evenly distributed with the silicon matrix material; and when its graphitization degree is mixed with carbon material as a negative electrode material, it can reduce the over-embedding caused by the insertion of lithium ions, reduce the phenomenon of negative electrode lithium plating, and thus extend the service life of the battery cell.

[0054] In some specific embodiments, the carbon content of the silicon-carbon particles is 50 wt% to 80 wt%, the silicon content is 10 wt% to 70 wt%, and the oxygen content is less than 10 wt%. In the present application, the carbon content, silicon content and oxygen content in the silicon-carbon particles refer to the mass fractions of carbon, silicon and oxygen, respectively, which can be measured by instruments and methods known in the art, for example, the inductively coupled plasma (ICP) method can be used to test the mass percentage of the silicon-carbon particles.

[0055] In some specific embodiments, the mass fraction of silicon element in the silicon-carbon particles in the negative electrode active coating is 1% to 15%, for example, it can be 1%, 1.5%, 5%, 8%, 10%, 12%, 15%, etc.; preferably it is 2.5% to 5%.

[0056] The carbon material includes at least one of amorphous carbon, SuperP, Ketjen black, acetylene black, carbon nanotubes, carbon nanofibers, graphene, artificial graphite, natural graphite and mesophase carbon microspheres.

[0057] In some specific embodiments, the mass ratio of silicon-carbon particles to carbon material is (15-18):1, preferably 16:1.

[0058] In some specific embodiments, the conductive agent includes at least one of conductive carbon black, acetylene black, Ketjen black, carbon dots, carbon nanotubes, graphene, carbon nanofibers, and superconducting carbon.

[0059] In some specific embodiments, the adhesive includes at least one of lithium carboxymethyl cellulose (CMC-Li), sodium carboxymethyl cellulose (CMC-Na), potassium carboxymethyl cellulose (CMC-K), styrene-butadiene rubber (SBR), styrene acrylic emulsion, lithium polyacrylate (PAA-Li), sodium polyacrylate (PAA-Na), potassium polyacrylate (PAA-K), lithium alginate (ALG-Li), sodium alginate (ALG-Na) and potassium alginate (ALG-K).

[0060] In some specific embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode active coating on the positive electrode current collector, wherein the positive electrode active coating contains a positive electrode active material, a conductive agent, and a binder. The positive electrode active material includes but is not limited to lithium iron phosphate, lithium manganese iron phosphate, lithium cobalt oxide, ternary LiNi x Co y Mn zOne or more of O2 materials (where x+y+z=1, x≥y). Conductive agents include but are not limited to at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers. Binders include but are not limited to at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer and fluorine-containing acrylate resin.

[0061] In some specific embodiments, the electrolyte includes a lithium salt, a solvent, and an additive.

[0062] Wherein, the additive includes fluoroethylene carbonate, and the mass fraction of fluoroethylene carbonate in the electrolyte is W2%, satisfying: 1≤W2≤30. The mass fraction of fluoroethylene carbonate can be, for example, 1%, 5%, 10%, 15%, 20%, 25%, 30%, etc., preferably 10.5% to 15.8%.

[0063] Furthermore, in the lithium-ion battery, the relationship between the fluoroethylene carbonate and the negative electrode plate satisfies: 1≤S*W1 / W2≤10, preferably 1.6≤S*W1 / W2≤5.5.

[0064] In some specific embodiments, the lithium salt includes at least one of LiPF6, LiBF4, LiClO4, LiB(C6H5), LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, LiSiF6, LiBOB and LiDFOB.

[0065] Furthermore, the concentration of the lithium salt is 0.9 mol / L to 1.3 mol / L.

[0066] In some specific embodiments, the solvent includes a carboxylate compound and a carbonate compound. The carboxylate compound may include at least one of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, n-propyl propionate, isopropyl propionate, n-butyl propionate, isobutyl propionate, n-pentyl propionate, isopentyl propionate, ethyl butyrate, n-propyl butyrate, propyl isobutyrate, n-pentyl butyrate, n-pentyl isobutyrate, n-butyl butyrate, isobutyl isobutyrate, n-pentyl valerate, butyl acetate, methyl formate, ethyl formate, ethyl propionate, propyl propionate, methyl butyrate, butyl butyrate, methyl isovalerate, propyl caproate and isobutyl acetate; preferably butyl butyrate, n-propyl butyrate, propyl isobutyrate, n-pentyl butyrate, n-pentyl isobutyrate, n-butyl butyrate or isobutyl isobutyrate.

[0067] Furthermore, in the electrolyte, the mass fraction of the carboxylate compound in the electrolyte is 5% to 20%, for example, 5%, 10%, 12%, 15%, 18%, 20%, etc., preferably 8% to 15%. Further preferably, the mass ratio of the carboxylate compound to the fluoroethylene carbonate is 1:(1 to 3), more preferably 1:(2 to 2.5).

[0068] The carbonate compound may include at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, and dimethyl carbonate.

[0069] In some specific embodiments, the lithium-ion battery further includes a diaphragm. The diaphragm is disposed between the positive electrode plate and the negative electrode plate to serve as an isolation. The diaphragm described in the present application can be arbitrarily selected from a well-known porous structure diaphragm with good chemical stability and mechanical stability. The material of the diaphragm can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The diaphragm can be a single-layer film or a multi-layer composite film, without particular limitation. When the diaphragm is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0070] In the lithium-ion battery of the embodiment of the present application, during the battery charging and discharging process, active lithium ions are embedded or extracted back and forth between the positive electrode sheet and the negative electrode sheet, and the electrolyte plays the role of conducting ions between the positive electrode sheet and the negative electrode sheet. By improving the formula composition of the electrolyte and the composition of the negative electrode sheet material, adjusting the content of the additive fluoroethylene carbonate in the electrolyte, and the relationship between the specific surface area and content of the silicon-carbon particles in the silicon negative electrode, a sufficiently stable SEI film can be formed on the surface of the silicon-carbon particles of the negative electrode sheet during the battery formation and circulation process, so as to limit the expansion of the silicon negative electrode during the circulation process, protect the negative electrode sheet, thereby improving the battery's cycle performance and extending the battery's service life.

[0071] The present application also provides an electrical device or various energy storage systems using batteries as energy storage elements. Electrical devices include but are not limited to mobile phones, tablets, computers, electric toys, electric tools, battery cars, electric cars, ships, spacecraft, etc.

[0072] In order to make the present invention easier to understand, the present invention will be further described in detail below in conjunction with examples, which are merely illustrative and are not intended to limit the scope of application of the present invention. The raw materials or components used in the present invention can be obtained by commercial routes or conventional methods unless otherwise specified.

[0073] Example 1

[0074] (1) Preparation of electrolyte

[0075] In an environment with a water content of less than 10ppm, propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC) are mixed in a mass ratio of 1:3:6, and 8% butyl butyrate is added based on the total mass of the electrolyte; then lithium hexafluorophosphate (LiPF6) is added to the solvent, dissolved and mixed evenly, and then fluoroethylene carbonate (FEC) is added to obtain an electrolyte. Among them, the molar concentration of LiPF6 in the electrolyte is 1.15mol / L, and the mass concentration of FEC in the electrolyte is 10.1%.

[0076] (2) Preparation of positive electrode sheet

[0077] The positive electrode active material lithium cobalt oxide, conductive carbon black, and polyvinylidene fluoride PVDF are mixed in a mass ratio of 95:2.5:2.5, fully stirred and mixed in N-methylpyrrolidone solvent, and prepared into a slurry with a solid content of 75%, and stirred evenly to obtain a positive electrode slurry. The positive electrode slurry is evenly coated on one surface of an aluminum foil with a thickness of 12 μm, dried at 90°C, and cold pressed to obtain a positive electrode sheet with a positive electrode active material layer thickness of 110 μm, and then the above steps are repeated on the other surface of the positive electrode sheet to obtain a positive electrode sheet coated with a positive electrode active material layer on both sides. The positive electrode sheet is cut into a specification of 76 mm × 851 mm and welded to the pole ear for standby use.

[0078] (3) Preparation of negative electrode sheet

[0079] Silicon carbon particles, graphite particles, conductive carbon black, adhesive styrene butadiene rubber and sodium carboxymethyl cellulose are mixed in a mass ratio of 80:5:5:10 to obtain a mixed material, N-methyl pyrrolidone (NMP) is added as a solvent, a slurry with a solid content of 50% is prepared, and it is stirred evenly. The slurry is evenly coated on one surface of a negative electrode current collector copper foil with a thickness of 10 μm, and dried at 110°C to obtain a negative electrode sheet with a single-sided coating of a negative electrode material layer with a coating thickness of 150 μm. Repeat the above steps on the other surface of the negative electrode sheet to obtain a negative electrode sheet with a negative electrode material layer coated on both sides. Then, the negative electrode sheet is cut into a specification of 74 mm × 867 mm for standby use.

[0080] (4) Preparation of lithium-ion batteries

[0081] PE porous polymer film is used as the isolation membrane.

[0082] The positive electrode sheet, separator and negative electrode sheet are stacked in order, so that the separator is in the middle of the positive and negative electrodes to play a role of isolation, and then the stacked electrode sheet and separator are wound to obtain an electrode assembly. The electrode assembly is placed in the aluminum-plastic film that has been punched and formed, and the moisture is removed at 80°C, and the prepared electrolyte is injected. After vacuum packaging, standing, formation, shaping and other processes, a lithium-ion battery is obtained.

[0083] Examples 2-7 and Comparative Examples 1-5 adopt the same method as Example 1, and the differences are shown in Table 1.

[0084] Lithium-ion battery performance test:

[0085] (1) Cyclic performance test

[0086] In a constant temperature box at (45±2)℃ and (25±2)℃, the lithium-ion battery was charged to 4.5V at 0.7C constant current and constant voltage, and charged to 0.025C at constant voltage. After standing for 5 minutes, it was discharged to 3V at 0.5C. The capacity obtained in this step was taken as the initial capacity, and a cycle test was performed using 0.7C charging / 0.5C discharging. The capacity at each step was compared with the initial capacity to obtain the capacity decay curve.

[0087] The room temperature cycle performance of the battery is recorded as the number of cycles at 25°C at which the capacity retention rate is 90%, and the high temperature cycle performance of the battery is recorded as the number of cycles at 45°C at which the capacity retention rate is 80%. The cycle performance of the materials is compared by comparing the number of cycles in the above two cases.

[0088] There are 5 batteries in each group, and the calculated room temperature cycle capacity retention rate and high temperature cycle capacity retention rate are averaged and recorded in Table 1.

[0089] (2) Lithium-ion battery full charge expansion rate test

[0090] Use a screw micrometer to test the thickness of the lithium-ion battery when it is half-charged, that is, 50% state of charge (SOC). When it is cycled to 300 cycles at 25°C and 200 cycles at 45°C, the lithium-ion battery is fully charged, that is, 100% SOC. Use a screw micrometer to test the thickness of the lithium-ion battery at this time. By comparing it with the thickness of the lithium-ion battery at the initial half-charge, the expansion rate of the fully charged lithium-ion battery at this time can be obtained.

[0091] There are 5 batteries in each group, and the calculated room temperature cycle expansion rate and high temperature cycle expansion rate are averaged and recorded in Table 1.

[0092] Table 1

[0093]

[0094] By the contrast of embodiment 1 and comparative example 1,2 display: when fluoroethylene carbonate is too high relative to silicon-carbon particle content, battery core is easy to produce gas in room temperature and high temperature cycle process, affects electrolyte and electrode interface stability, and especially easily forms too thick SEI film during long-term circulation under high temperature conditions, can not effectively prevent the solvent molecules in electrolyte from being co-embedded, affects the cycle stability, service life and charge-discharge performance of battery.When fluoroethylene carbonate is too low relative to silicon-carbon particle content, it is difficult to form stable and dense SEI film in battery cycle process, therefore it is difficult to suppress the expansion of silicon-carbon particle, negative pole piece expansion rate is high, can cause irreversible lithium ion loss, side reaction increases, affects energy density and cycle performance of battery.

[0095] Therefore, when the electrolyte contains fluoroethylene carbonate, the negative electrode contains silicon-carbon particles, and the relationship between the two satisfies 1≤S*W1 / W2≤10, fluoroethylene carbonate FEC, as a film-forming additive, can react with lithium salts to form a dense and high elastic modulus SEI film, so that the surface of the silicon-carbon particles has a stable SEI film, which limits the expansion of the silicon-carbon particles, protects the silicon-carbon particles and the negative electrode interface, and improves the fast charging performance to a certain extent. In terms of electrical performance, it can also improve the capacity retention rate of long cycles and reduce the expansion growth during the cycle.

[0096] A comparison of Examples 1-3 shows that when the specific surface area of ​​the silicon-carbon particles in the negative electrode plate increases, the content of fluoroethylene carbonate also needs to increase accordingly to meet the requirements of forming a stable and dense SEI film on the surface of the silicon-carbon particles in the negative electrode plate, thereby ensuring that the battery has good cycle performance in both normal temperature cycles and high temperature cycles, and effectively inhibiting the expansion of negative electrode silicon.

[0097] Comparison of Example 1 and Examples 4-6 shows that: when the content of silicon-carbon particles in the negative electrode plate continues to increase, the content of fluoroethylene carbonate also needs to increase accordingly to meet the requirements of forming a stable and dense SEI film on the surface of the silicon-carbon particles in the negative electrode plate, thereby ensuring that the battery has good cycle performance in both normal temperature cycles and high temperature cycles and effectively inhibiting the expansion of negative electrode silicon; at the same time, the content of silicon-carbon particles in the negative electrode plate cannot be too high. As the content of silicon-carbon particles continues to increase, when the silicon content in the negative electrode plate is too high, significant volume expansion will occur during lithium insertion, which may cause structural rupture and crushing of the electrode material. At this time, the SEI film formed by fluoroethylene carbonate will also be destroyed, increasing irreversible lithium ion loss, affecting the energy density, cycle stability and service life of the battery.

[0098] The comparison of Examples 6 and 7 and Comparative Examples 3 and 4 shows that no matter how the specific surface area of ​​silicon-carbon particles changes, as the silicon content and the corresponding fluoroethylene carbonate content increase, the battery expansion performance will also deteriorate accordingly, and when the silicon content in the negative electrode plate is too high, it will further affect the cycle performance and safety performance of the battery. When the fluoroethylene carbonate content in the electrolyte exceeds 30%, an overly thick SEI film will be formed on the surface of the silicon-carbon particles, which will sharply deteriorate the conductivity of the silicon-carbon particles, and the cycle capacity retention rate will also be seriously deteriorated, and the battery core will not be able to be used normally. At the same time, when the mass ratio of the carboxylic acid ester compound and the fluoroethylene carbonate is within the range of 1: (2 to 2.5), the two are more effective in improving the electrolyte performance. The relative fluoroethylene carbonate content of the carboxylic acid ester compound is too low, which will not be conducive to alleviating the volume expansion of the battery cycle process, and will also produce a large amount of gas, which will deteriorate the battery cycle stability and cause the battery to expand rapidly.

[0099] Comparison between Example 1 and Comparative Example 5 shows that when no carboxylate compound solvent is added to the electrolyte, the battery cell dynamics deteriorates significantly, the cycle retention rate decreases, and lithium deposition occurs during normal temperature cycling, resulting in increased thickness expansion.

[0100] Figure 1 is the Raman spectrum of the silicon-carbon particles of Example 1 of the present application. Figure 1 It can be seen that at (1350±50)cm -1 and (1575±50)cm -1 There are obvious characteristic peaks at the position.

[0101] It should be noted that the embodiments described above are only used to explain the present application and do not constitute any limitation to the present application. The present application is described with reference to typical embodiments, but it should be understood that the words used therein are descriptive and explanatory words, rather than restrictive words. The present application may be modified as specified within the scope of the claims of the present application, and the present application may be revised without departing from the scope and spirit of the present application. Although the present application described therein relates to specific methods, materials and embodiments, it does not mean that the present application is limited to the specific examples disclosed therein. On the contrary, the present application can be extended to all other methods and applications with the same functions.

Claims

1. A lithium ion battery, characterized in that: include: The negative electrode sheet comprises a negative electrode current collector and a negative electrode active coating on the negative electrode current collector, wherein the negative electrode active coating contains silicon carbon particles, carbon material and a binder, and the specific surface area of ​​the silicon carbon particles is Sm 2 / g, the mass fraction of the silicon-carbon particles in the negative electrode active coating is W1%; An electrolyte, comprising a lithium salt, a solvent and an additive, wherein the additive comprises fluoroethylene carbonate, and the mass fraction of the fluoroethylene carbonate in the electrolyte is W2%; Among them, 1≤S≤15, 1≤W1≤20, 1≤W2≤30, 1≤S*W1 / W2≤10.

2. The lithium-ion battery according to claim 1, characterized in that The lithium ion battery further satisfies: 5.4≤S≤8.0, 3≤W1≤10, 10.5≤W2≤15.8, 1.6≤S*W1 / W2≤5.

5.

3. The lithium-ion battery according to claim 1, characterized in that In the Raman test spectrum of the silicon-carbon particles, (1350±50)cm -1 The position has a characteristic peak Id, (1575±50)cm -1 The position has a characteristic peak Ig, and Id / Ig is 0.8 to 1.2; preferably 0.95 to 1.

05.

4. The lithium-ion battery according to claim 3, characterized in that: The mass fraction of silicon in the silicon-carbon particles to the negative electrode active coating is 1% to 15%, preferably 2.5 to 5%.

5. The lithium-ion battery according to claim 3, characterized in that: The mass ratio of the silicon-carbon particles to the carbon material is (15-18):1, preferably 16:

1.

6. The lithium ion battery according to claim 3, characterized in that: The silicon-carbon particles have a carbon content of 50 wt% to 80 wt%, a silicon content of 10 wt% to 70 wt%, and an oxygen content of less than 10 wt%.

7. The lithium-ion battery according to claim 1, characterized in that: The carbon material includes at least one of amorphous carbon, SuperP, Ketjen black, acetylene black, carbon nanotubes, carbon nanofibers, graphene, artificial graphite, natural graphite and mesophase carbon microspheres; The binder includes at least one of lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, styrene-butadiene rubber, styrene-acrylic emulsion, lithium polyacrylate, sodium polyacrylate, potassium polyacrylate, lithium alginate, sodium alginate and potassium alginate.

8. The lithium-ion battery according to any one of claims 1 to 7, characterized in that: The solvent includes a carboxylate compound and a carbonate compound; The carboxylate compound comprises at least one of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, n-propyl propionate, isopropyl propionate, n-butyl propionate, isobutyl propionate, n-pentyl propionate, isopentyl propionate, ethyl butyrate, n-propyl butyrate, propyl isobutyrate, n-pentyl butyrate, n-pentyl isobutyrate, n-butyl butyrate, isobutyl isobutyrate, n-pentyl valerate, butyl acetate, methyl formate, ethyl formate, ethyl propionate, propyl propionate, methyl butyrate, butyl butyrate, methyl isovalerate, propyl hexanoate and isobutyl acetate; The carbonate compound includes at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate and dimethyl carbonate; And / or, the lithium salt includes at least one of LiPF6, LiBF4, LiClO4, LiB(C6H5), LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, LiSiF6, LiBOB and LiDFOB; preferably, the concentration of the lithium salt is 0.9 mol / L to 1.3 mol / L.

9. The lithium-ion battery according to claim 8, characterized in that: The mass fraction of the carboxylate compound in the electrolyte is 5% to 20%, preferably 8% to 15%; further preferably, the mass ratio of the carboxylate compound to fluoroethylene carbonate is 1:(1 to 3), more preferably 1:(2 to 2.5).

10. An electrical device, characterized in that: Comprising a lithium ion battery as claimed in any one of claims 1 to 9.

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