Electrochemical devices and electronic devices

Abstract

Embodiments of the present application provide an electrochemical device and an electronic device. The electrochemical device comprises a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector, a first layer and a second layer, the negative electrode current collector being located between the first layer and the second layer. The first layer comprises a first negative electrode active material, and the second layer comprises a second negative electrode active material, the first negative electrode active material comprising graphite, and the second negative electrode active material comprising graphite and a silicon-based material. The ratio of the impedance of the first layer to the impedance of the second layer is 0.7 to 0.9, and the negative electrode sheet of the present application can improve the capacity retention rate of the electrochemical device without substantially losing the energy density of the electrochemical device.

Description

Electrochemical devices and electronic devices Technical Field

[0001] This application relates to the field of electrochemical energy storage, specifically to electrochemical devices and electronic devices. Background Technology

[0002] With the development of electrochemical energy storage technology, increasingly higher demands are being placed on the energy density and cycle performance of electrochemical devices (e.g., lithium-ion batteries). Using silicon-based materials as the negative electrode active material can significantly improve the energy density of electrochemical devices; however, silicon-based materials experience volume expansion and contraction of over 300% during lithium-ion insertion / extraction, leading to capacity decay and other problems. Therefore, further improvements in this area are desired. Summary of the Invention

[0003] This application provides an electrochemical device comprising a negative electrode sheet, which includes a negative electrode current collector, a first layer, and a second layer, with the current collector located between the first and second layers. The first layer includes a first negative electrode active material, and the second layer includes a second negative electrode active material. The first negative electrode active material includes graphite, and the second negative electrode active material includes graphite and a silicon-based material. The impedance ratio of the first layer to the impedance of the second layer is between 0.7 and 0.9.

[0004] In some embodiments, the capacity ratio of the first layer to the second layer is 0.6 to 1. In some embodiments, the capacity ratio of the first layer to the second layer is 0.7 to 1. In some embodiments, the silicon-based material in the second layer comprises 6% to 40% by mass. In some embodiments, the silicon-based material in the second layer comprises 12% to 20% by mass. In some embodiments, the second layer further comprises an adhesive, the adhesive comprising 1.4% to 5% by mass. In some embodiments, the second layer further comprises carbon nanotubes, the carbon nanotubes comprising 0.3% to 0.7% by mass. In some embodiments, the second layer further comprises sodium carboxymethyl cellulose, the sodium carboxymethyl cellulose comprising 0.3% to 0.7% by mass. In some embodiments, the silicon-based material comprises at least one of silicon, silicon-carbon materials, or silicon-oxygen materials.

[0005] Embodiments of this application also provide an electronic device, including the electrochemical device described above.

[0006] This application improves the capacity retention of an electrochemical device by making the first negative electrode active material in the first layer of the negative electrode sheet include graphite, and the second negative electrode active material in the second layer include graphite and silicon-based materials, with the impedance ratio of the first layer to the second layer being 0.7 to 0.9. This results in a lower rate of charge and discharge for the second negative electrode active layer, thereby improving the capacity retention of the electrochemical device without significantly reducing the energy density of the electrochemical device. Attached Figure Description

[0007] Figure 1 shows a cross-sectional view of the negative electrode sheet along the width direction according to some embodiments. Detailed Implementation

[0008] The following embodiments are intended to enable those skilled in the art to fully understand this application, but do not limit this application in any way.

[0009] Some embodiments of this application provide an electrochemical device including a negative electrode. FIG1 shows a cross-sectional view of the negative electrode along its width according to some embodiments. In some embodiments, as shown in FIG1, the negative electrode includes a negative current collector 110, a first layer 111, and a second layer 112, with the negative current collector 110 located between the first layer 111 and the second layer 112. In some embodiments, the first layer 111 includes a first negative electrode active material, which includes graphite and substantially excludes silicon-based materials. It should be understood that when the mass percentage of silicon-based material in the first negative electrode active material is less than 0.2%, the first negative electrode active material can be considered to substantially exclude silicon-based materials. By completely separating the silicon-based material from the graphite in the first layer 111, the ineffective porosity between the graphite in the first layer 111 during cycling is reduced, thereby reducing the overall expansion of the negative electrode.

[0010] In some embodiments, the second layer 112 includes a second negative electrode active material, which comprises graphite and a silicon-based material. Therefore, this application concentrates the silicon-based material on one side of the negative electrode current collector 110. Typically, the mass percentage of silicon-based material in the layers on both sides of the negative electrode current collector is the same. In this application, it is equivalent to replacing the silicon-based material on one side with an equal amount of graphite, and replacing the graphite on the other side with an equal amount of silicon-based material.

[0011] In some embodiments, the impedance ratio of the first layer 111 to the impedance of the second layer 112 is 0.7 to 0.9. In some embodiments, the impedance of the first layer 111 and the impedance of the second layer 112 can be adjusted by the Si content, conductive agent content, binder type, coating formulation, coating quality, and compaction density of the respective coatings. Since the impedance of the first layer 111 is less than that of the second layer 112, the first layer 111 can have a larger charge / discharge rate, while the second layer 112 can have a smaller charge / discharge rate. Thus, without changing the overall charge / discharge rate of the electrochemical device, reducing the charge / discharge rate of the silicon-based second layer 112 can slow down the capacity decay rate of the negative electrode and improve the capacity retention rate of the electrochemical device.

[0012] In some embodiments, the capacity ratio of the first layer 111 to the capacity of the second layer 112 is 0.6 to 1. In some embodiments, the capacity of the first layer 111 can be adjusted by the coating quality, and the capacity in the second layer 112 can be adjusted by the coating quality and / or the Si content. By making the capacity ratio of the first layer 111 to the capacity of the second layer 112 0.6 to 1, the capacity retention of the electrochemical device can be improved.

[0013] In some embodiments, the ratio of the capacity of the first layer 111 to the capacity of the second layer 112 is 0.7 to 1. When the ratio of the capacity of the first layer 111 to the capacity of the second layer 112 is 0.7 to 1, the improvement in capacity retention of the electrochemical device is more significant.

[0014] In some embodiments, the mass percentage of silicon-based material in the second layer 112 is 6% to 40%, or the mass percentage of silicon in the second layer 112 is 4% to 20%. In some embodiments, since only silicon contributes to the capacity of the silicon-based material, when the mass percentage of silicon-based material in the second layer 112 is less than 6%, the mass percentage of silicon is very small, and the energy density gain of the electrochemical device decreases significantly; when the mass percentage of silicon-based material in the second layer 112 is greater than 40%, the mass percentage of silicon is large, and the cycling expansion of the second layer 112 increases. In some embodiments, the mass percentage of silicon-based material in the second layer 112 is 12% to 20%. When the mass percentage of silicon-based material in the second layer 112 is 12% to 20%, or the mass percentage of silicon in the second layer 112 is 4% to 10%, the improvement in the capacity retention of the electrochemical device is more significant.

[0015] In some embodiments, the silicon-based material includes at least one of silicon, silicon-oxygen materials, silicon-carbon materials, or silicon-oxygen-carbon materials. In some embodiments, the negative electrode current collector 110 may be at least one of copper foil, nickel foil, or carbon-based current collector. Both the first layer 111 and the second layer 112 may include a conductive agent, a binder, and a thickener (e.g., sodium carboxymethyl cellulose). In some embodiments, the conductive agent in the first layer 111 and the second layer 112 may include at least one of conductive carbon black, Ketjen black, sheet graphite, graphene, carbon nanotubes, or carbon fibers. In some embodiments, the binder in the first layer 111 and the second layer 112 may include at least one of carboxymethyl cellulose (CMC), polyacrylic acid, polyvinylpyrrolidone, polyaniline, polyimide, polyamide-imide, polysiloxane, styrene-butadiene rubber, epoxy resin, polyester resin, polyurethane resin, or polyfluorene. In some embodiments, the mass percentage of the first negative electrode active material in the first layer is 97% to 98%. In some embodiments, the mass percentage of the second negative electrode active material in the second layer is 93.6% to 98%. In some embodiments, the mass ratio of the first negative electrode active material, thickener, and binder in the first layer 111 may be (97 to 98): (0.2 to 0.6): (1.8 to 2.4). In some embodiments, the mass ratio of the second negative electrode active material, conductive agent, binder, and thickener in the second layer 112 may be (93.6 to 98): (0.3 to 0.7): (1.4 to 5): (0.3 to 0.7). In some embodiments, the second layer 112 further includes a binder, and the mass percentage of the binder in the second layer 112 is 1.4% to 5%. In some embodiments, the second layer 112 further includes carbon nanotubes, and the mass percentage of the carbon nanotubes in the second layer 112 is 0.3% to 0.7%. In some embodiments, the second layer 112 further includes sodium carboxymethyl cellulose, and the mass percentage of sodium carboxymethyl cellulose in the second layer 112 is 0.3% to 0.7%.

[0016] In some embodiments, the electrochemical device further includes a positive electrode and a separator, the positive electrode and the negative electrode being separated by the separator disposed therebetween.

[0017] In some embodiments, the positive electrode sheet includes a positive current collector and a positive active material layer, the positive active material layer being located on one or both sides of the positive current collector. In some embodiments, the positive current collector can be aluminum foil, or other positive current collectors commonly used in the art can be used. In some embodiments, the thickness of the positive current collector can be from 1 μm to 50 μm.

[0018] In some embodiments, the positive electrode active material layer may include a positive electrode active material, a conductive agent, and a binder. In some embodiments, the positive electrode active material may include at least one of lithium cobalt oxide, lithium iron phosphate, lithium aluminate, lithium manganese oxide, or lithium nickel cobalt manganese oxide. In some embodiments, the conductive agent in the positive electrode active material layer may include at least one of conductive carbon black, sheet graphite, graphene, or carbon nanotubes. In some embodiments, the binder in the positive electrode active material layer may include at least one of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, a styrene-acrylate copolymer, a styrene-butadiene copolymer, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethyl cellulose, polyvinyl acetate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. In some embodiments, the mass ratio of the positive electrode active material, conductive agent, and binder in the positive electrode active material layer is (80-99):(0.1-10):(0.1-10), but this is only an example, and any other suitable mass ratio may be used.

[0019] In some embodiments, the separator comprises at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, polyethylene comprises at least one selected from high-density polyethylene, low-density polyethylene, or ultra-high molecular weight polyethylene. Polyethylene and polypropylene, in particular, are effective in preventing short circuits and can improve battery stability through a turn-off effect. In some embodiments, the thickness of the separator is in the range of about 3 μm to 20 μm.

[0020] In some embodiments, the surface of the separator may further include a porous layer disposed on at least one surface of the separator. The porous layer comprises inorganic particles and a binder. The inorganic particles are selected from at least one of alumina (Al₂O₃), silicon oxide (SiO₂), magnesium oxide (MgO), titanium oxide (TiO₂), hafnium dioxide (HfO₂), tin oxide (SnO₂), cerium dioxide (CeO₂), nickel oxide (NiO), zinc oxide (ZnO), calcium oxide (CaO), zirconium oxide (ZrO₂), yttrium oxide (Y₂O₃), silicon carbide (SiC), boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. In some embodiments, the pores of the separator have a diameter in the range of about 0.01 μm to 1 μm. The binder for the porous layer is selected from at least one of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethyl cellulose, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene. The porous layer on the surface of the separator can improve the separator's heat resistance, oxidation resistance, and electrolyte wetting properties, and enhance the adhesion between the separator and the electrode.

[0021] In some embodiments, the electrochemical device includes a lithium-ion battery, but this application is not limited thereto. In some embodiments, the electrochemical device further includes an electrolyte comprising at least one of a fluoroether, a fluoroethylene carbonate, or an ether nitrile. In some embodiments, the electrolyte further includes a lithium salt comprising lithium bis(fluorosulfonyl)imide and lithium hexafluorophosphate, wherein the concentration of the lithium salt is from 1 mol / L to 2 mol / L, and the mass ratio of lithium bis(fluorosulfonyl)imide to lithium hexafluorophosphate is from 0.06 to 5. In some embodiments, the electrolyte may also include a non-aqueous solvent. The non-aqueous solvent may be a carbonate compound, a carboxylic acid ester compound, an ether compound, other organic solvents, or a combination thereof.

[0022] Carbonate compounds can be chain carbonate compounds, cyclic carbonate compounds, fluorocarbonate compounds, or combinations thereof.

[0023] Examples of chain carbonate compounds are diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), and combinations thereof. Examples of cyclic carbonate compounds are ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), vinyl ethylene carbonate (VEC), or combinations thereof. Examples of fluorinated carbonate compounds are fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, trifluoromethylethylene carbonate, or combinations thereof.

[0024] Examples of carboxylic acid ester compounds are methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanoic acid lactone, valerate lactone, mevalonic acid lactone, caprolactone, methyl formate, or combinations thereof.

[0025] Examples of ether compounds are dibutyl ether, tetraethylene dimethyl ether, diethylene dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, or combinations thereof.

[0026] Examples of other organic solvents include dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, and phosphate esters or combinations thereof.

[0027] Embodiments of this application also provide electronic devices including the electrochemical devices described above. The electronic devices in the embodiments of this application are not particularly limited and can be any electronic device known in the prior art. In some embodiments, the electronic device may include, but is not limited to, laptops, pen-based computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, drones, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0028] The following are some specific embodiments and comparative examples to better illustrate this application, wherein a lithium-ion battery is used as an example.

[0029] Comparative Example 1

[0030] Preparation of the negative electrode sheet: Artificial graphite (negative electrode active material), polyvinylidene fluoride (PVC) binder, and sodium carboxymethyl cellulose (CMC) were dissolved in deionized water at a weight ratio of 97.5:2.1:0.44 to form the first slurry. A 6 μm thick copper foil was used as the negative electrode current collector. The first slurry was coated onto one side of the negative electrode current collector at a coating weight of 0.086 mg / mm². 2 The first layer is formed by dissolving the negative electrode active materials—artificial graphite, Si, binder—polyacrylic acid, carbon nanotubes, and sodium carboxymethyl cellulose—in deionized water at a weight ratio of 59.7:36:3.5:0.4:0.4 to form the second slurry. The second slurry is then coated onto the other side of the negative electrode current collector at a coating weight of 0.035.5 mg / mm². 2 This forms the second layer. After drying, cold pressing, and slitting, the negative electrode sheet is obtained.

[0031] Positive electrode preparation: Lithium cobalt oxide (positive electrode active material), conductive agent, and polyvinylidene fluoride (PVDF) binder were dissolved in N-methylpyrrolidone (NMP) solution at a weight ratio of 97.6:1.1:1.3 to form a positive electrode slurry. An 8μm thick aluminum foil was used as the positive electrode current collector, and the positive electrode slurry was coated onto both sides of the current collector, with a coating thickness of 50μm on both sides. After drying, cold pressing, and slitting, the positive electrode sheet was obtained.

[0032] Preparation of the separator membrane: The separator membrane substrate is 8μm thick polyethylene (PE). A 2μm thick alumina ceramic layer is coated on each side of the separator membrane substrate. Finally, 2.5mg of polyvinylidene fluoride (PVDF) binder is coated on each side of the ceramic layer and then dried.

[0033] Preparation of electrolyte: Under an environment with a water content of less than 10 ppm, lithium hexafluorophosphate was mixed with a non-aqueous organic solvent (ethylene carbonate (EC): propylene carbonate (PC): polypropylene (PP): diethyl carbonate (DEC) = 1:1:1:1, mass percentage) to prepare an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0034] Preparation of lithium-ion batteries: Positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide isolation. The electrodes are then wound to form an electrode assembly. The electrode assembly is placed in an outer aluminum-plastic film package, and after dehydration at 80°C, the electrolyte is injected and the battery is sealed. Following formation, degassing, and shaping processes, a lithium-ion battery is obtained.

[0035] The difference between Comparative Example 2 and Examples 1 to 11 lies in the preparation of the negative electrode sheet, specifically the parameters of the first and second layers are different, as detailed in Tables 1 and 2.

[0036] In addition, the relevant parameters were measured using the following method in this application, and the effective area of ​​the single-layer stacked battery used in this application is 49.5*42mm. 2 .

[0037] 1) Discharge the battery to 3.0V, disassemble to obtain the positive electrode and negative electrode, soak them in dimethyl carbonate (DMC) solution for 15 minutes and then dry them for later use.

[0038] 2) In a dry, inert gas environment in a glove box, take the positive and negative electrode sheets from 1), punch them, match the positive and negative electrodes, and add a separator to form a single-layer stacked battery. Seal and inject liquid with a pressure clamp to maintain interface contact. Charge at a constant current of 0.7C to the charging cutoff voltage, then charge at a constant voltage until the battery reaches full charge. Cutoff current is 0.02C. Discharge at 0.2C DC to 3.0V to obtain the capacity of the single-layer stacked battery. Then calculate the capacity per unit area of ​​the negative electrode sheet (capacity per unit area of ​​the negative electrode sheet = capacity of the single-layer stacked battery / effective area of ​​the single-layer stacked battery). Then calculate the capacity of one side of the negative electrode sheet based on the area of ​​the active coating of the negative electrode sheet. Repeat the above method to obtain the capacity of the other side.

[0039] 3) Using the single-layer stacked cell in 2), the AC impedance method can be used to measure Rs+Rct (Rs refers to ohmic impedance, and Rct refers to electrochemical transfer impedance). Due to the influence of the area of ​​the fabricated single-layer stacked cell, the manufacturing process and other conditions, the absolute value of Rs+Rct may vary, but the ratio of Rs+Rct on both sides of the electrode can be used as the basis for current shunting on both sides of the electrode.

[0040] 4) Capacity retention rate after 500 cycles = Capacity after 500 cycles / Capacity of fresh battery. The capacity test conditions are: at 25℃, constant current charging at 0.7C to 4.5V, then constant voltage charging to 0.02C, and then DC discharge at 0.2C to 3.0V as one cycle, for a total of 500 cycles.

[0041] 5) Charge the lithium-ion battery at a constant current rate of 0.2C to 4.45V, then charge it at a constant voltage rate to 0.02C to complete the full charge of the lithium-ion battery; next, discharge it at a constant current rate of 0.2C until the voltage drops to 3.0V. Record the total capacity discharged during the discharge process as C and the voltage plateau U. Measure the actual thickness, length and width of the lithium-ion battery, and calculate the actual lithium-ion battery volume V. Energy density = C*U / V.

[0042] Tables 1 and 2 show the parameters and evaluation results for Examples 1 to 11 and Comparative Examples 1 to 4.

[0043] Table 1

[0044]

[0045] Table 2

[0046]

[0047]

[0048] Comparing Examples 1 to 3 and Comparative Examples 1 to 2, it can be seen that when the ratio of the impedance of the first layer to the impedance of the second layer is 0.7 to 0.9, the energy density loss of the lithium-ion battery is relatively small, while the capacity retention rate of the lithium-ion battery is significantly improved. Furthermore, as the silicon content of the second layer decreases, the capacity retention rate of the lithium-ion battery shows a trend of first increasing and then decreasing.

[0049] Comparing Examples 4 to 6, it can be seen that as the content of conductive agent in the second layer increases, the impedance of the second layer tends to decrease, and the capacity retention rate of the lithium-ion battery tends to decrease.

[0050] Comparing Examples 7 to 11, it can be seen that as the ratio of the capacity of the first layer to the capacity of the second layer increases, the energy density of the lithium-ion battery tends to decrease, and the capacity retention rate of the lithium-ion battery tends to increase first and then decrease.

[0051] The above description is merely a preferred embodiment of this application and an explanation of the technical principles employed. Those skilled in the art should understand that the scope of disclosure in this application is not limited to technical solutions formed by specific combinations of the above-described technical features, but should also cover other technical solutions formed by any combination of the above-described technical features or their equivalents. For example, technical solutions formed by substituting the above-described features with technical features having similar functions disclosed in this application.

Claims

1. An electrochemical device comprising: A negative electrode sheet, comprising a negative current collector, a first layer, and a second layer, wherein the negative current collector is located between the first layer and the second layer; wherein the first layer comprises a first negative electrode active material, and the second layer comprises a second negative electrode active material, wherein the first negative electrode active material comprises graphite, and the second negative electrode active material comprises graphite and silicon-based material, and the impedance ratio of the first layer to the impedance of the second layer is 0.7 to 0.9; wherein the mass percentage of silicon-based material in the first negative electrode active material is less than 0.2%, and the mass percentage of silicon-based material in the second layer is 6% to 40%.

2. The electrochemical device according to claim 1, wherein, The ratio of the capacity of the first layer to the capacity of the second layer is 0.6 to 1.

3. The electrochemical device according to claim 1, wherein, The ratio of the capacity of the first layer to the capacity of the second layer is 0.7 to 1.

4. The electrochemical device according to claim 1, wherein, The silicon-based material in the second layer has a mass percentage content of 12% to 20%.

5. The electrochemical device according to claim 1, wherein, The silicon content in the second layer is 4% to 20% by mass.

6. The electrochemical device according to claim 4, wherein, The silicon content in the second layer is 4% to 10% by mass.

7. The electrochemical device according to claim 1, wherein, The second layer also includes an adhesive, wherein the adhesive in the second layer comprises 1.4% to 5% by mass.

8. The electrochemical device according to claim 1, wherein, The second layer also includes carbon nanotubes, wherein the carbon nanotubes in the second layer have a mass percentage of 0.3% to 0.7%.

9. The electrochemical device according to claim 1, wherein, The second layer also includes sodium carboxymethyl cellulose, wherein the sodium carboxymethyl cellulose in the second layer has a mass percentage content of 0.3% to 0.7%.

10. The electrochemical device according to claim 1, wherein, The silicon-based material includes at least one of silicon, silicon-carbon materials, or silicon-oxygen materials.

11. An electronic device comprising an electrochemical device according to any one of claims 1 to 10.

Images