Electrochemical devices and electronic devices

Abstract

The application provides an electrochemical device and an electronic device. The electrochemical device comprises: a positive electrode, a negative electrode and an electrolyte; the positive electrode comprises: a positive electrode active material layer; the negative electrode comprises: a negative electrode active material layer, the negative electrode active material layer comprises: a negative electrode active material, the negative electrode active material comprises: graphite, the graphitization degree of the graphite is: 88% to 93%, and the CB value is: 1.06 to 1.20, the CB value being the ratio of the capacity of the negative electrode active material layer per unit area to the capacity of the positive electrode active material layer per unit area; and the electrolyte comprises: at least one of a cyclic carbonate or a linear carbonate. The application can enable the electrochemical device to have better energy density and cycle stability.

Description

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 requirements are being placed on the cycle performance and energy density of electrochemical devices (e.g., lithium-ion batteries), with a strong expectation for further improvements. The CB (Capacity Balance, the ratio of the capacity of the negative electrode active material layer per unit area to the capacity of the positive electrode active material layer per unit area) value is an important parameter of electrochemical devices. Generally speaking, a larger CB value is beneficial for increasing the specific capacity of the positive electrode active material, thereby improving energy density. However, it may affect the stability of the positive electrode active material. Therefore, balancing the energy density and cycle performance of an electrochemical device is challenging. How to improve the specific capacity of the positive electrode active material while maintaining the cycle stability of the electrochemical device using a larger CB value is a problem that needs to be solved in this field. Summary of the Invention

[0003] This application provides an electrochemical device and an electronic device to improve electrolyte consumption, increase energy density and cycle performance of the electrochemical device.

[0004] In some embodiments, this application proposes an electrochemical device comprising: a positive electrode, a negative electrode, and an electrolyte; the positive electrode comprises: a positive electrode active material layer; the negative electrode comprises: a negative electrode active material layer, the negative electrode active material layer comprising: a negative electrode active material, the negative electrode active material comprising: graphite, the graphitization degree of the graphite being 88% to 93%; and a CB value being 1.06 to 1.20, where CB is the ratio of the capacity of the negative electrode active material layer per unit area to the capacity of the positive electrode active material layer per unit area; the electrolyte comprises: at least one of cyclic carbonate or linear carbonate. Controlling the CB value within the above range is beneficial to maximizing the specific capacity of the positive electrode active material and effectively suppressing electrolyte consumption in the electrochemical device, thus contributing to better energy density and cycle stability of the electrochemical device in this application.

[0005] In some embodiments, the electrochemical device is charged at 25°C at a constant current rate of 1C to the charging cutoff voltage, then charged at a constant voltage rate to 0.05C, and discharged at a rate of 0.5C to the discharge cutoff voltage. After 100 cycles, the percentage of electrolyte consumption is less than or equal to 10% × (1 - 5% × (CB - 1.05) / 0.01), which is beneficial to the long-cycle performance of the electrochemical device.

[0006] In some embodiments, 1.06 ≤ CB value < 1.1, 93% ≥ graphitization degree > 91.5%; or 1.1 ≤ CB value < 1.15, 91.5% ≥ graphitization degree > 90%; or 1.15 ≤ CB value ≤ 1.2, 90% ≥ graphitization degree ≥ 88%, thereby improving electrolyte consumption and cycle performance.

[0007] In some embodiments, the Raman test results of graphite are I D / I G The value is 0.1 to 0.3, which helps to improve energy density and fast charging performance.

[0008] In some embodiments, the negative electrode active material further includes a silicon-containing material, including silicon-carbon composite materials, which is beneficial to improving the capacity of the negative electrode active material layer.

[0009] In some embodiments, the specific capacity of graphite is 330 mAh / g to 350 mAh / g, which is beneficial for cycle performance and increases energy density.

[0010] In some embodiments, the specific surface area of ​​the negative electrode active material layer is: 1m² 2 / g to 25m 2 / g, thereby improving dynamic performance and fast charging performance.

[0011] In some embodiments, the graphite content in the negative electrode active material is greater than or equal to 80% by mass, thereby improving cycle performance.

[0012] In some embodiments, the sum of the mass percentages of cyclic carbonates and linear carbonates, based on the total mass of the electrolyte, is A%, and A% is 60% to 90%, which helps to reduce electrolyte consumption and improve fast charging performance.

[0013] In some embodiments, the positive electrode active material layer includes a positive electrode active material, which includes at least one of lithium cobalt oxide or ternary materials, thereby improving the specific capacity that the positive electrode active material can exert when combined with the CB value.

[0014] In some embodiments, the positive electrode active material includes a modifying element M, which includes at least one of Al or Mg; the mass percentage of the modifying element M is 0.2% to 0.5% based on the total mass of the positive electrode active material, thereby improving cycle decay and increasing energy density.

[0015] In some embodiments, the particle size Dv50 of the positive electrode active material is 12 μm to 18 μm, which is beneficial for improving cycle performance and energy density.

[0016] Embodiments of this application also provide an electronic device, including the electrochemical device described above.

[0017] An electrochemical device is proposed in the embodiments of this application, which uses graphite with a graphitization degree of 88% to 93% and is combined with cyclic carbonate or linear carbonate in the electrolyte. At the same time, the CB value is controlled to be 1.06 to 1.20. Under these conditions, the consumption of electrolyte will not increase significantly, which enables the electrochemical device to have good energy density and cycle stability. Attached Figure Description

[0018] Figure 1 A scanning electron microscope image of the positive electrode active material of the lithium-ion battery of Example 1 after 800 cycles is shown.

[0019] Figure 2 A scanning electron microscope image of the positive electrode active material of the lithium-ion battery of Comparative Example 1 after 800 cycles is shown. Detailed Implementation

[0020] 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.

[0021] For electrochemical devices (e.g., lithium-ion batteries), energy density and cycle performance are crucial indicators. Among these, the CB (Capacity Balance, the ratio of the capacity of a negative electrode active material layer per unit area to the capacity of a positive electrode active material layer per unit area) value is a significant influencing factor. Generally, a higher CB value can significantly increase the specific capacity of the positive electrode active material (the specific capacity of the positive or negative electrode active material is not the theoretical specific capacity, but rather the capacity that the positive or negative electrode active material can exert per unit mass in the electrochemical device, representing the actual specific capacity), but it may lead to a decrease in the stability of the positive electrode active material. This is mainly reflected in the increased consumption of protective additives in the electrolyte, which deteriorates cycle performance. If the consumption of protective additives in the electrolyte can be slowed down while maintaining a higher CB value, then energy density and cycle stability can be improved.

[0022] This application discloses an electrochemical device, such as a lithium-ion battery. The electrochemical device includes a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a positive electrode active material layer. The positive electrode may include a positive electrode current collector, and the positive electrode active material layer may be disposed on one or both surfaces of the positive electrode current collector. The positive electrode current collector may be made of copper foil, aluminum foil, stainless steel foil, or other current collectors commonly used in the art. In some embodiments, the thickness of the positive electrode current collector may be 3 μm to 50 μm. In some embodiments, the positive electrode active material layer may be coated only on a portion of one or both surfaces of the positive electrode current collector, thus having a certain blank area, which can be used for welding tabs. In some embodiments of this application, the positive electrode active material layer includes a positive electrode active material, which includes at least one of lithium cobalt oxide or ternary materials. The negative electrode includes a negative electrode active material layer. The negative electrode may also include a negative electrode current collector, and the negative electrode active material layer may be disposed on one or both surfaces of the negative electrode current collector. The negative electrode current collector may be made of copper foil, aluminum foil, or other materials. The negative electrode active material layer includes: negative electrode active material, which includes: graphite. The graphitization degree of graphite is 88% to 93%, for example, 89%, 90%, 91%, 92%. The graphitization degree needs to be controlled within a reasonable range. Graphite with excessively high graphitization has a relatively stable layered structure, and the path for lithium ions to insert and extract in the layered structure is stable. This accelerates the diffusion of solvent molecules and lithium ions in the electrolyte, increases the reaction rate of electrolyte decomposition and the generation of by-products at the negative electrode. Therefore, excessively high graphitization may lead to higher electrolyte consumption, affecting the electrolyte retention of the electrochemical device in the later stages of the cycle, and causing a deterioration in cycle performance. Graphite with excessively low graphitization has a more disordered structure, with irregular carbon atom arrangement, resulting in a large number of defects and grain boundaries. The presence of disordered structure and defects reduces the continuity of electron transport paths and increases the resistance to electron transport. Therefore, excessively low graphitization will lead to a deterioration in the conductivity of the negative electrode active material and a loss of fast-charging capability. In this application, the CB value of the electrochemical device is from 1.06 to 1.20, for example, it can be 1.07, 1.08, 1.09, 1.10, 1.11, 1.12, 1.13, 1.14, 1.15, 1.16, 1.17, 1.18, or 1.19. The CB value is the ratio of the capacity of the negative electrode active material layer per unit area to the capacity of the positive electrode active material layer per unit area. In addition, the CB value affects the relationship between the positive and negative electrode potentials. If the CB value is too small, the full charge potential of the positive and negative electrodes decreases, and polarization may occur under high charge and discharge rates. The risk of the negative electrode potential dropping to the lithium plating potential increases. Therefore, if the CB value is too small, lithium plating may occur during high charge and discharge rates.If the CB value is too high, the full charge potential of both the positive and negative electrodes will increase. An excessively high full charge potential in the positive electrode will reduce its structural stability, potentially leading to phenomena such as lattice expansion and layered structure disruption in the positive electrode active material. These structural changes deteriorate the high-temperature stability and cycle performance of the positive electrode active material. Therefore, an excessively high CB value may cause high-temperature gas generation during cycling, further worsening cycle performance. The electrolyte includes at least one of cyclic carbonates or linear carbonates.

[0023] In related technologies, increasing the CB value of an electrochemical device leads to increased electrolyte consumption, thereby reducing cycle performance. The inventors of this application have discovered that by using graphite with a graphitization degree of 88% to 93%, and combining it with cyclic or linear carbonates in the electrolyte, and controlling the CB value to be between 1.06 and 1.20, electrolyte consumption does not increase significantly, allowing the electrochemical device to achieve good energy density and cycle stability. Because of the higher CB value, the specific capacity of the positive electrode active material can be better utilized, increasing energy density. Due to the interaction of these three conditions, electrolyte consumption in the electrochemical device can be effectively suppressed, resulting in better cycle performance.

[0024] In some embodiments of this application, the electrochemical device is charged at 25°C at a constant current rate of 1C to the charging cutoff voltage, then charged at a constant voltage rate to 0.05C, and discharged at a rate of 0.5C to the discharge cutoff voltage. After 100 cycles, the percentage of electrolyte consumption is recorded as M%. M% is less than or equal to 10% × (1 - 5% × (CB - 1.05) / 0.01).

[0025] In some embodiments, the electrolyte consumption percentage M% is the percentage of the total mass of electrolyte consumed after 100 cycles relative to the initial total mass of the electrolyte. A lower percentage of electrolyte consumption indicates better low-consumption capability and better long-cycle performance of the electrolyte. For electrochemical devices, the percentage of electrolyte consumption can be measured as follows: First, disassemble the fully loaded electrochemical device, remove the positive and negative electrodes, cut the positive and negative electrodes into small pieces of a fixed size (e.g., 3cm × 3cm), clean them with dimethyl carbonate (DMC) and dry them. Place a polyethylene diaphragm between the positive and negative electrodes and then place them in an aluminum-plastic film. Add 0.2g of electrolyte and then seal them with the aluminum-plastic film to complete the sample preparation. Cycle the sample at 25°C for 100 cycles at a 1C rate. One cycle consists of: charging at a constant current rate of 1C to the charging cutoff voltage (the charging cutoff voltage can be 4.5V), then charging at a constant voltage until the current is less than 0.05C, and discharging at a constant current rate of 0.5C to the discharge cutoff voltage. Then, fully discharge the sample to be tested (during full discharge, discharge at a DC current of 0.1C to the discharge cutoff voltage, and then repeat three times at a DC rate of 0.01C to the discharge cutoff voltage, the discharge cutoff voltage can be 3V), weigh the sample and record the mass as M1, put the sample into a centrifuge tube, and centrifuge the electrolyte in a centrifuge; disassemble the sample and pour in an appropriate amount of DMC to completely immerse the sample, soaking for a total of 14 hours; take it out and dry it in a 60℃ oven for 2 hours; weigh the mass M2, then the total electrolyte volume after circulation M = M1 - M2; the percentage of electrolyte consumed during circulation M% = (0.2 - (M1 - M2)) / 0.2.

[0026] In some embodiments of this application, the electrolyte consumption percentage M% is specified. This is because the electrolyte consumption percentage affects cycle performance. Using a CB value of 1.05 as a standard, in order for the electrochemical device to achieve a long cycle of 1000 cycles, the electrolyte consumption percentage must satisfy M% ≤ 100 / 1000 = 10%, that is, a maximum of 10% of the electrolyte is consumed in 100 cycles. For every 0.01 increase in the CB value, the full charge potential of both the positive and negative electrodes increases. Due to the increased full charge potential of the positive electrode, the degree of delithiation of the positive electrode active material increases, and the specific capacity exerted by the positive electrode active material increases by about 0.5%. Therefore, the increase in specific capacity exerted by the positive electrode active material is (CB - 1.05) / 0.01 × 0.5%. However, for every 1% increase in specific capacity exerted by the positive electrode active material, the crystal structure stability of the positive electrode active material deteriorates by 10%. Therefore, the degree of degradation of the crystal structure of the positive electrode active material is (CB - 1.05) / 0.01 × 5%. In order to enable the electrochemical device to have long cycle capability, with the standard that the electrochemical device can achieve at least 1000 cycles, the electrolyte consumption of the electrochemical device for 100 cycles was originally at most 10%. Because the CB value is increased, the electrolyte consumption needs to be reduced proportionally. Therefore, it is necessary to control M% ≤ 10% × (1 - 5% × (CB - 1.05) / 0.01). In some embodiments, M% is controlled within the above range, which slows down the degradation during cycling and gives the battery long-cycle capability. In other embodiments, M% ≥ 50% × 10% × (1 - 5% × (CB - 1.05) / 0.01), because when M% is greater than this value, there are more active sites for lithium intercalation on the negative electrode active material layer, reducing the risk of lithium plating and improving the fast-charging performance of the electrochemical device.

[0027] In some embodiments of this application, 1.06 ≤ CB value < 1.1, 93% ≥ graphitization degree > 91.5%; or, 1.1 ≤ CB value < 1.15, 91.5% ≥ graphitization degree > 90%; or, 1.15 ≤ CB value ≤ 1.2, 90% ≥ graphitization degree ≥ 88%. In some embodiments, there is a relationship between the CB value and the graphitization degree, and the two work synergistically to improve the cycle performance of the electrochemical device. When the CB value is small, a larger graphitization degree is required, meaning there is a negative correlation between the two values. When the CB value and the graphitization degree are within the above ranges, the electrolyte consumption can be reduced more effectively, thereby improving the cycle performance.

[0028] In some embodiments of this application, the Raman test results of graphite are I D / I G The value is between 0.1 and 0.3, for example, it can be 0.2. In some embodiments, the I in the Raman test results... D / I G The value characterizes the coating of amorphous carbon. In this embodiment, I D / IG A value of not less than 0.1 indicates good coating protection of amorphous carbon, meaning the SEI (solid electrolyte interface) film will not be continuously damaged during cycling, which is beneficial for improving cycle performance. In this embodiment, I... D / I G The value is not greater than 0.3, and the disorder of the graphite structure is not too high. This helps to avoid a large number of defects and boundaries from disrupting the continuous electron transport path, which in turn helps to improve the conductivity of the negative electrode active material. This, in turn, helps to avoid the reduction of the specific capacity of the negative electrode active material due to the increase in lithium ion transport resistance. This will help to improve the energy density and fast charging performance of the electrochemical device.

[0029] In some embodiments of this application, the negative electrode active material further includes a silicon-containing material, which includes a silicon-carbon composite material. In some embodiments, the silicon-carbon composite material has a higher specific capacity, which is beneficial for improving the capacity of the negative electrode active material layer.

[0030] In some embodiments of this application, the specific capacity of graphite is between 330 mAh / g and 350 mAh / g. In some embodiments, the specific capacity of graphite affects electrolyte consumption. This application aims to improve both energy density and cycle performance; therefore, the specific capacity of graphite is not greater than 350 mAh / g, which helps to avoid increased electrolyte consumption and improve cycle performance. A specific capacity of graphite not less than 330 mAh / g is beneficial for increasing the capacity of the negative electrode active material layer and improving the energy density of the electrochemical device.

[0031] In some embodiments of this application, the graphite content in the negative electrode active material is greater than or equal to 80% by mass. In some embodiments, the graphite content in the negative electrode active material is not less than 80% by mass, for example, 85%, 90%, 95%, or 100%. The high graphite content results in the negative electrode active material exhibiting properties primarily characteristic of graphite, which, in conjunction with the degree of graphitization and the electrolyte, improves energy density and cycle performance.

[0032] In some embodiments of this application, the specific surface area of ​​the negative electrode active material layer is: 1m² 2 / g to 25m 2 / g. In some embodiments, the specific surface area of ​​the negative electrode active material layer can be 3m². 2 / g、5m 2 / g、7m 2 / g、9m 2 / g、11m 2 / g、13m 2 / g, 15m 2 / g、17m 2 / g、19m 2 / g、21m 2 / g or 23m 2 / g. In this embodiment, the specific surface area of ​​the negative electrode active material layer is no greater than 25m². 2 / g, which helps reduce electrolyte consumption and thus improves cycle performance. Meanwhile, the specific surface area of ​​the negative electrode active material layer is not less than 1m². 2 / g, which helps improve the overall dynamic performance of the negative electrode active material layer and improve fast charging performance.

[0033] It is understood that there are no particular restrictions on the method of adjusting the graphitization degree of graphite in this application, as long as it can achieve the purpose of this application, such as high-pressure treatment of graphite or addition of a catalyst. The Raman test results of graphite are shown in Figure I. D / I G There are no particular restrictions on the method of adjusting the graphite specific capacity, as long as it achieves the purpose of this application. Examples include amorphous carbon coating of graphite and prolonged mechanical ball milling. There are also no particular restrictions on the method of adjusting the specific capacity of graphite, as long as it achieves the purpose of this application. Examples include elemental doping of graphite and morphology control. It is understood that the three parameters can be adjusted simultaneously or individually. For example, the degree of graphitization of graphite can be adjusted by changing the heat treatment temperature (below 1500℃) or by subjecting graphite to mild acid treatment, thus maintaining the IL value of the Raman test results. D / I G The specific capacity and Ig of graphite remain constant. The specific capacity of graphite can be adjusted by modifying its morphology and creating pores on its surface, thus maintaining the Ig value of the Raman test results. D / I G The value and degree of graphitization remain unchanged.

[0034] In some embodiments of this application, the total mass percentage of cyclic carbonates and linear carbonates, based on the total mass of the electrolyte, is A%, and A% is 60% to 90%. In some embodiments, A% is 65%, 70%, 75%, 80%, or 85%. In some embodiments, cyclic carbonates and linear carbonates have high dielectric constants, which can effectively disperse lithium salts in the electrolyte, improve the solubility of lithium salts in the electrolyte, and form a stable electrolyte system. Moreover, the SEI film formed by cyclic carbonates and linear carbonates has good stability, which is beneficial for protecting the negative electrode active material, reducing side reactions between the negative electrode active material and the electrolyte, and improving the cycle stability of the electrochemical device. In addition, cyclic carbonates and linear carbonates are beneficial for improving the electrochemical window, and can remain stable without decomposition at a high voltage of 4.5V. Therefore, cyclic and linear carbonates in the electrolyte can reduce electrolyte consumption. A mass percentage of at least 60% is beneficial for reducing electrolyte consumption and thus improving the cycle performance of the electrochemical device, while a mass percentage of no more than 90% is beneficial for the electrolyte's conductivity and thus for fast-charging performance. In some embodiments of this application, cyclic carbonates may include, but are not limited to, one or more of: ethylene carbonate, propylene carbonate, ethylene glycol dimethyl ether carbonate, propylene glycol carbonate, butanediol carbonate, and methyl ethylene glycol carbonate. In some embodiments, linear carbonates may include, but are not limited to, one or more of: dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, and propyl methyl carbonate.

[0035] In some embodiments of this application, the positive electrode active material layer includes a positive electrode active material, which includes at least one of lithium cobalt oxide or a ternary material. In some embodiments, lithium cobalt oxide and ternary materials have high specific capacity, and the CB value has a relatively significant impact on the delithiation degree of lithium cobalt oxide and ternary materials. Using a large CB value can improve the specific capacity exerted by lithium cobalt oxide and ternary materials.

[0036] In some embodiments of this application, the positive electrode active material includes a modifying element M, which includes at least one of Al or Mg. Based on the total mass of the positive electrode active material, the mass percentage of the modifying element M is 0.2% to 0.5%, for example, 0.3% or 0.4%. In some embodiments, the modifying element M is beneficial for stabilizing the positive electrode active material after lithium removal. A mass percentage of the modifying element M of not less than 0.2% helps to prevent the electrochemical device from decaying too quickly during cycling, while a mass percentage of the modifying element M of not more than 0.5% helps to fully utilize the specific capacity of the positive electrode active material, thereby improving the energy density. In some embodiments of this application, the particle size Dv50 of the positive electrode active material is 12 μm to 18 μm. In some embodiments, Dv50 is 13 μm, 14 μm, 15 μm, 16 μm, or 17 μm. The particle size of the positive electrode active material is not less than 12μm, which helps to reduce positive electrode side reactions and improve cycle performance. The particle size of the positive electrode active material is not greater than 18μm, which helps to increase the specific capacity of the positive electrode active material and ensure energy density.

[0037] In some embodiments, the positive electrode active material layer may further include a conductive agent. In some embodiments, the conductive agent in the positive electrode active material layer may include at least one of conductive carbon black, Ketjen black, sheet graphite, graphene, carbon nanotubes, or carbon fibers. In some embodiments, the mass ratio of the positive electrode active material, conductive agent, and binder in the positive electrode active material layer may be (80 to 99):(0.1 to 10):(0.1 to 10). In some embodiments, the thickness of the positive electrode active material layer may be from 10 μm to 500 μm. It should be understood that the above descriptions are merely examples, and the positive electrode active material layer of the positive electrode may employ any other suitable material, thickness, and mass ratio.

[0038] In some embodiments, the negative electrode active material layer may further include a conductive agent. In some embodiments, the conductive agent in the negative electrode active material layer may include at least one of conductive carbon black, Ketjen black, sheet graphite, graphene, carbon nanotubes, or carbon fibers. In some embodiments, the mass ratio of the negative electrode active material, conductive agent, and binder in the negative electrode active material layer may be (80 to 98):(0.1 to 10):(0.1 to 10). It should be understood that the above description is merely an example, and any other suitable materials and mass ratios may be used. In some embodiments, the current collector of the negative electrode may be at least one of aluminum foil, copper foil, nickel foil, or carbon-based current collector.

[0039] In some embodiments, a separator is provided between the positive and negative electrodes. The separator comprises at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, polyethylene includes 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 ranges from about 3 μm to 200 μm.

[0040] In some embodiments, the surface of the separator may further include a porous layer disposed on at least one surface of the substrate 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.

[0041] In some embodiments of this application, the electrode assembly of the electrochemical device is a wound electrode assembly, a stacked electrode assembly, or a folded electrode assembly. In some embodiments, the positive and / or negative electrode of the electrochemical device can be a multilayer structure formed by winding or stacking, or a single-layer structure consisting of a single-layer positive electrode, a separator, and a single-layer negative electrode.

[0042] In some embodiments, the electrochemical device includes a lithium-ion battery, but this application is not limited thereto.

[0043] In some embodiments of this application, taking a lithium-ion battery as an example, the positive electrode, separator, and negative electrode are sequentially wound or stacked into electrode components, then encapsulated in, for example, an aluminum-plastic film, and then injected with electrolyte. Formation and encapsulation are then performed to manufacture a lithium-ion battery. The prepared lithium-ion battery is then subjected to performance testing.

[0044] Those skilled in the art will understand that the methods for preparing the electrochemical devices (e.g., lithium-ion batteries) described above are merely examples. Other methods commonly used in the art can be employed without departing from the disclosure of this application.

[0045] Embodiments of this application also provide electronic devices including the aforementioned electrochemical apparatus. 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, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium / sodium-ion capacitors, etc.

[0046] The following are some specific embodiments and comparative examples to better illustrate this application, wherein a lithium-ion battery is used as an example.

[0047] Test method:

[0048] Positive and negative electrode sampling: Under 25°C, the lithium-ion battery was discharged at a constant current of 0.5C to the discharge cutoff voltage. The lithium-ion battery was then disassembled under an argon atmosphere, and the negative and positive electrodes were immersed in dimethyl carbonate solvent for 2 hours and dried at 60°C for 1 hour to obtain the negative and positive electrodes. The discharge cutoff voltage of the lithium-ion battery in the embodiments and comparative examples of this application is 3.0V. It can be understood that when the voltage range marked on the battery packaging is 3.0V to 4.5V, the charging cutoff voltage is 4.5V and the discharging cutoff voltage is 3.0V. Unless otherwise specified, the charging cutoff voltage of the example lithium-ion battery is 4.5V and the discharging cutoff voltage is 3.0V.

[0049] Unless otherwise specified, the following test methods shall use the negative and positive electrodes obtained in the above manner for testing.

[0050] The CB value test method is as follows: Disassemble the lithium-ion battery and remove the positive and negative electrodes. Cut small circular pieces of the same area (e.g., φ14mm) from each electrode. Assemble button cells using lithium metal sheets as counter electrodes. Perform charge-discharge tests on an electrochemical workstation. The test voltage range for the negative electrode is 0.001V to 2V, and the measured reversible specific capacity is recorded as 'a'. The test voltage range for the positive electrode is 2V to 4.5V, and the measured reversible specific capacity is recorded as 'b'. The CB value of the lithium-ion battery is then calculated as a / b.

[0051] The method for testing the degree of graphitization is as follows: scrape off the negative electrode active material layer to obtain negative electrode active material layer powder. The lattice fringes of the negative electrode active material and the interlayer spacing can be observed and tested by high-resolution transmission electron microscopy. Combined with image analysis software, the degree of graphitization can be quantitatively evaluated.

[0052] Graphite I D / I G The testing method for the negative electrode active material layer is as follows: A layer of negative electrode active material is scraped off to obtain a powder. Raman spectroscopy of the powder is then performed, and the D and G peaks in the Raman spectrum are identified through instrument processing. The D peak is typically located at 1350 cm⁻¹. -1 Nearby, the G peak is typically located at 1580cm. -1 Nearby. Peak fitting software (such as Origin, GraphPad Prism, etc.) is used to perform peak fitting to accurately calculate the peak area, thus calculating I. D / I G The value is the ratio of the area of ​​peak D to the area of ​​peak G.

[0053] Method for testing the mass percentage of graphite-silicon-carbon active material: After disassembling a lithium-ion battery, scrape off powder from the negative electrode active material layer with a mass of m1. Perform a thermogravimetric analysis in air. After burning off the binder and other materials at 400℃ for 2 hours, the remaining mass is m2. After burning off the graphite at 700℃ for 2 hours, the remaining mass is m3. Therefore, the mass of graphite is m2 - m3, and the mass of silicon-carbon is m3. The mass percentage of graphite in the negative electrode active material = (m2 - m3) / m2, and the proportion of silicon-carbon in the negative electrode active material = m3 / m2.

[0054] Graphite specific capacity test:

[0055] If the negative electrode active material is pure graphite, the specific capacity test method for graphite is as follows: Disassemble the lithium-ion battery and remove the negative electrode. Weigh a φ14mm small disc from the negative electrode, the mass of which is m4. Then weigh a negative electrode current collector of the same size with the negative electrode active material layer wiped off, the mass of which is m5. The mass of the negative electrode active material layer on the small disc is m4-m5. Therefore, the mass of the negative electrode active material of the φ14mm small disc = (m4-m5)×m2 / m1. Then assemble a button cell using a lithium metal sheet as the counter electrode of the φ14mm small disc. The electrolyte in the button cell is the same as that used in Example 1. Perform charge and discharge tests in an electrochemical workstation, with a voltage range of 0.001V-2V, and measure the reversible capacity x. Then the specific capacity of graphite = x / ((m4-m5)×m2 / m1).

[0056] If the negative electrode active material is graphite mixed with silicon carbon, the specific capacity test method for graphite is as follows: Disassemble the lithium-ion battery and remove the negative electrode. Weigh a φ14mm small disc, the mass of which is m4. Then weigh a current collector of the same size with the negative electrode active material layer wiped off, the mass of which is m5. Then the mass of the negative electrode active material layer on the small disc is m4-m5. Therefore, the mass of the negative electrode active material of the φ14mm small disc = (m4-m5)×m2 / m1. Then assemble a button cell with a lithium metal sheet as the counter electrode and the φ14mm small disc. The electrolyte in the button cell is the same as the electrolyte used in Example 1. Perform charge and discharge tests in an electrochemical workstation, with a voltage range of 0.001V to 2V, and measure the reversible capacity y. Then the specific capacity of the negative electrode active material = y / ((m4-m5)×m2 / m1). Take some powder of the negative electrode active material and calcine it in air at 700°C for 2 hours. The remaining mass is the mass of pure silicon carbon material, denoted as m6. Then, disperse this pure silicon carbon with binder and conductive agent in a solvent at a ratio of 8:1:1, and then coat it onto the negative electrode current collector. Assemble a button cell using a lithium sheet as the counter electrode. The electrolyte in the button cell is the same as that used in Example 1. Charge and discharge tests are performed in an electrochemical workstation with a voltage range of 0.001V to 2V. The reversible capacity z is measured. Then, the specific capacity of pure silicon carbon = z / m6. Combining the specific capacity of the negative electrode active material = (specific capacity of graphite × percentage of graphite mass + specific capacity of silicon carbon × percentage of silicon carbon mass), we can obtain the specific capacity of graphite = (y × m1 / (m4-m5) - m3 × z / m6) / (m2-m3).

[0057] Method for testing the specific surface area of ​​the negative electrode active material layer: Disassemble the lithium-ion battery and remove the negative electrode. Heat it to 100℃ to 200℃ under vacuum to remove adsorbed moisture and gas. Then, place the treated negative electrode active material layer into a BET instrument. Measure the amount of nitrogen adsorbed by controlling the relative pressure (usually between 0.05 and 0.3). The specific surface area can be calculated based on the adsorption isotherm. Note that the mass of the tested negative electrode is recorded as m7. Therefore, the mass of the negative electrode active material layer should be calculated as (m4-m5) / m4, where m7 × ((m4-m5) / m4). This mass is used as the mass for calculating the specific surface area using BET fitting.

[0058] Method for determining the mass percentage of cyclic and linear carbonates in the electrolyte: The mass percentage of cyclic and linear carbonates in the electrolyte can be determined using gas chromatography (GC). The electrolyte to be tested is injected into the GC system, and detection is performed using a flame ionization detector (FID) or a thermal conductivity detector (TCD). The mass percentage of each component is calculated using a standard curve method, taking advantage of the differences in volatility in the gas phase and retention time on the chromatographic column.

[0059] Capacity retention test after 800 cycles at 25℃: The lithium-ion battery under test was placed at 25℃ and allowed to stand for 5 minutes. It was then charged at a constant current of 1C to 4.5V, followed by constant voltage charging at 4.5V to 0.05C. After standing for 5 minutes, it was discharged at a constant current of 0.5C to 3.0V, and then allowed to stand for 5 minutes. The capacity at this point was recorded as D0. This charge-discharge cycle was repeated 800 times, and the final discharge capacity was recorded as D1. After 800 cycles at 25℃, the capacity retention rate was D1 / D0, expressed as a percentage.

[0060] Electrolyte consumption percentage test:

[0061] First, disassemble the fully discharged lithium-ion battery, remove the positive and negative electrodes, cut them into small pieces of a fixed size (3cm × 3cm), clean them with dimethyl carbonate (DMC) and dry them. Place a polyethylene separator between the positive and negative electrodes, then place them in an aluminum-plastic film, add 0.2g of the electrolyte used in Example 1, and then seal them with the aluminum-plastic film to complete the sample preparation. Cycle the sample at 25°C for 100 cycles at a 1C rate. The specific cycle process is as follows: charge at a constant current rate of 1C to the charging cutoff voltage (charging cutoff voltage is 4.5V), then charge at a constant voltage until the current is less than 0.05C, and discharge at a constant current rate of 0.5C to the discharge cutoff voltage of 3.0V. Then, fully discharge the sample, and during full discharge, discharge at a DC current of 0.1C to the discharge cutoff voltage, then repeat three times with a DC discharge rate of 0.01C to the discharge cutoff voltage (discharge cutoff voltage is 3.0V). Weigh the sample to be tested and record the mass as M1. Place the sample to be tested into a centrifuge tube and centrifuge the electrolyte in a centrifuge. Disassemble the sample to be tested and pour in an appropriate amount of DMC to completely submerge the sample. Soak for a total of 14 hours. Take it out and dry it in a 60℃ oven for 2 hours. Weigh the mass M2. Then the total amount of electrolyte after the cycle is M = M1 - M2. The percentage of electrolyte consumed during the cycle is M% = (0.2 - (M1 - M2)) / 0.2.

[0062] 3C charging lithium plating test: First, fully charge the lithium-ion battery, then discharge it at a 0.2C rate until the voltage reaches 3.0V, and record the discharge capacity Q1. Then, determine the charging current corresponding to the 3C rate based on Q1: 3 × Q1. Perform 10 charge-discharge cycles using the 3C charging current and the 0.2C discharging current, and record the final discharge capacity Q2. The capacity loss ratio of these 10 charge-discharge cycles, x, is calculated as (Q1 - Q2) / Q1. x represents the severity of lithium plating. If x ≤ 2%, it is considered no lithium plating; if 2% < x ≤ 5%, it is considered slight lithium plating; and if x > 5%, it is considered severe lithium plating.

[0063] Example 1:

[0064] Preparation of the positive electrode: Aluminum foil is used as the current collector of the positive electrode. The positive electrode active material lithium cobalt oxide, conductive agent SuperP, and binder (PVDF, polyvinylidene fluoride) are dissolved in N-methylpyrrolidone (NMP) solution in a weight ratio of 96:2:2 to form a slurry of positive electrode active material layer. The slurry is coated on the current collector to obtain the positive electrode active material layer. After drying, cold pressing and cutting, the positive electrode is obtained.

[0065] Preparation of the negative electrode: Graphite, binder (SBR, styrene-butadiene rubber), and thickener (CMC, carboxymethyl cellulose) were dissolved in deionized water at a weight ratio of 98:1:1 to form a negative electrode slurry. A 10μm thick copper foil was used as the current collector for the negative electrode. The negative electrode slurry was coated onto the current collector, dried, and cut to obtain the negative electrode.

[0066] Preparation of the separator membrane: The separator membrane substrate is 8μm thick polyethylene (PE) as the separator membrane.

[0067] Electrolyte preparation: The solvent used is EC (ethylene carbonate), DMC (dimethyl carbonate) and EMC (ethyl methyl carbonate), with the mass ratio of EC:DMC:EMC = 30%:40%:30%; lithium salt is added to the solvent, and lithium hexafluorophosphate (LiPF6) is selected with a molar concentration of 1 mol / L, which is used as the basic electrolyte.

[0068] The preparation of lithium-ion batteries: the positive electrode, the separator, and the negative electrode are in close contact in sequence, wound up, placed in a packaging bag, electrolyte is added, and then sealed.

[0069] The differences between the various embodiments and comparative examples and Example 1 are the parameters shown in the table below; all other parameters not shown are the same. The embodiments and comparative examples added components to the base electrolyte. Examples and comparative examples 1-4 added carbonate to the base electrolyte, while comparative example 5 did not add carbonate but added methyl acetate and ethyl acetate. Specific added components are shown in the table below.

[0070] Table 1

[0071]

[0072]

[0073] Table 2

[0074]

[0075]

[0076]

[0077] The parameters and test results of each embodiment and comparative example are shown in Table 1 and Table 2.

[0078] As seen in Examples 1 to 3, Examples 9 to 11, and Comparative Examples 1 to 5, the graphitization degree in these examples was 88% to 93%, the CB value was 1.06 to 1.20, and the electrolyte contained ethylene carbonate and diethyl carbonate, respectively. Examples 1 to 3 and Examples 9 to 11 all exhibited high capacity retention after 800 cycles, and no lithium plating occurred. In contrast, Comparative Example 1 had a graphitization degree of 86%, resulting in severe lithium plating. This may be because the lower graphitization degree reduced the conductivity of graphite, thus preventing lithium ions from timely intercalating into the graphite during 3C charging, leading to lithium plating. Comparative Example 2 had a higher graphitization degree, resulting in poor capacity retention after 800 cycles. This may be because the higher graphitization degree accelerated the diffusion of solvent molecules and lithium ions in the electrolyte, increasing the rate of electrolyte decomposition and the generation of by-products at the positive and negative electrodes, leading to increased electrolyte consumption and consequently deteriorating cycle performance. Comparative Example 3 has a low CB value and experiences severe lithium plating. This is likely because its low CB value leads to polarization during high-rate charge and discharge, resulting in lithium plating. Comparative Example 4 has a high CB value and exhibits poor capacity retention after 800 cycles. This is likely because its excessively high CB value increases the full charge potential of the positive electrode, reducing the structural stability of the positive electrode active material and thus decreasing cycle performance. Comparative Example 5, lacking the addition of cyclic or chain carbonates to the electrolyte, exhibits poor cycle performance. Combining the above examples and comparative examples, it can be seen that when the graphitization degree of graphite is 88% to 93%, the CB value is 1.06 to 1.20, and the electrolyte includes at least one of cyclic or linear carbonates, the interaction of these three factors enables the electrochemical device to possess good cycle and rate performance. The absence of any one of these factors will adversely affect the performance of the electrochemical device.

[0079] As shown in Examples 4 to 6, the electrolyte consumption percentage is greater than 10% × (1 - 5% × (CB - 1.05) / 0.01). Therefore, the electrolyte consumption during cycling increases, resulting in a decrease in the cycle capacity retention rate. Furthermore, slight lithium plating occurred in Examples 4 and 5, which may be because the large electrolyte consumption affects lithium-ion transport during high-rate charge and discharge.

[0080] As shown in Examples 7 and 8, slight lithium plating occurred in Example 7, and the capacity retention after 800 cycles decreased in Example 8. This may be because the CB value and graphitization degree were poorly matched in Examples 7 and 8. The lower graphitization degree in Example 7 was detrimental to the conductivity of graphite, resulting in reduced kinetic performance and slight lithium plating. The higher graphitization degree in Example 8 affected the cycling performance.

[0081] As shown in Examples 12 to 15, Example 12 I D / I G Below 0.1, I in Example 15 D / I G The value was above 0.3. Compared to Examples 13 and 14, the cycling performance of Examples 12 and 15 was reduced, and slight lithium plating occurred in Example 15. This may be because the amorphous carbon coating in Example 12 was poor, resulting in deteriorated cycling performance. In Example 15, the graphite had more defects, which was not conducive to lithium-ion transport, deteriorated cycling performance, and caused slight lithium plating.

[0082] As shown in Examples 16 to 19, when the negative electrode active material includes graphite and silicon-carbon, the cycle performance of Examples 16 and 19 is reduced and slight lithium plating occurs compared to Examples 17 and 18. This may be because when the silicon-carbon content in the negative electrode active material increases, the interaction between graphite, CB value, and electrolyte decreases, and silicon-carbon is prone to volume expansion and breakage during cycling, which is detrimental to cycle performance and high-rate charge / discharge.

[0083] As shown in Examples 20 to 23, compared to Examples 21 and 22, slight lithium plating occurred in Example 20, and the cycle performance of Example 21 decreased. In Example 20, the specific capacity of graphite was low (the specific capacity of graphite can be affected by methods such as surface modification, pre-lithiation, and doping modification), therefore it had fewer sites to accommodate lithium ions, resulting in slight lithium plating during high-rate charge-discharge. In Example 23, the specific capacity of graphite was high, leading to increased electrolyte consumption during cycling, thus affecting cycle performance.

[0084] As shown in Examples 24 to 27, compared to Examples 25 and 26, slight lithium plating occurred and cycle performance decreased in Example 24, while cycle performance decreased in Example 27. In Example 24, the specific surface area of ​​the negative electrode active material layer was less than 1 m². 2 The negative electrode active material layer has a low specific surface area ( / g), fewer lithium-ion transport channels, and decreased kinetic performance. Therefore, its cycle performance is reduced, and lithium plating may occur under high-rate charge-discharge conditions. In Example 27, the specific surface area of ​​the negative electrode active material layer is higher than 25 μm. 2 / g (the specific surface area can be affected by internal pore formation, surface etching, etc.). Its negative electrode active material layer has a large specific surface area, which increases electrolyte consumption and thus affects cycle performance.

[0085] As shown in Examples 28 to 31, compared to Examples 29 and 30, cycle performance decreased in Example 28, and slight lithium plating occurred in Example 31. In Example 28, the lower total mass percentage of cyclic and chain carbonates in the electrolyte led to increased electrolyte consumption, thus reducing cycle performance. In Example 31, the higher total mass percentage of cyclic and chain carbonates in the electrolyte affected electrolyte conductivity, resulting in a decrease in high-rate charge / discharge performance.

[0086] As shown in Examples 32 and 33, slight lithium plating occurred in Example 33 compared to Example 32. The positive electrode active material used in Example 33 was lithium iron phosphate. Compared to lithium cobalt oxide and ternary materials, lithium iron phosphate has relatively poor high-rate charge-discharge performance, resulting in slight lithium plating.

[0087] As shown in Examples 34 and 35, the cycle performance in Example 35 is lower than that in Example 34. The modifying element in the positive electrode active material in Example 35 is Fe. Compared to Mg, Fe cannot effectively stabilize the delithiated positive electrode active material during cycling, thus negatively impacting cycle performance.

[0088] As shown in Examples 36 to 39, compared to Examples 37 and 38, the cycle performance decreased in Example 36, and slight lithium plating occurred in Example 39. In Example 36, the mass percentage of the modifying elements in the positive electrode active material was low, weakening the effect of the modifying elements in stabilizing the structure of the positive electrode active material, thus resulting in decreased cycle performance. In Example 39, the mass percentage of the modifying elements in the positive electrode active material was high, increasing defects in the positive electrode active material, reducing kinetic performance, and affecting both cycle performance and rate performance.

[0089] As shown in Examples 40 to 43, the cycle performance decreased in Examples 40 and 43 compared to Examples 41 and 42. In Example 40, the particle size of the positive electrode active material was smaller, leading to increased electrolyte consumption and side reactions, resulting in decreased cycle performance. In Example 43, the particle size of the positive electrode active material was larger, reducing the contact area between the positive electrode active material and the electrolyte, and decreasing the lithium-ion transport channels, thus reducing rate performance.

[0090] Figure 1 and Figure 2 Images of the positive electrode active materials of lithium-ion batteries from Example 1 and Comparative Example 1 after 800 cycles are shown. It can be seen that the positive electrode active material of Example 1 shows only slight cracks on its surface after cycling, with no bulk phase breakage. The positive electrode active material of Comparative Example 1 shows severe cracks on its surface and obvious bulk phase breakage after cycling.

[0091] In some embodiments of this application, by using graphite materials with a specific degree of graphitization, in conjunction with cyclic or linear carbonates in the electrolyte, electrolyte consumption at high voltage and high temperature can be improved under specific CB values, especially the consumption of positive electrode protection additives. This protects the structural stability of the positive electrode active material in the later stages of cycling, improves cycle stability while increasing energy density, and avoids a series of problems such as rapid capacity decay and gas generation during cycling.

[0092] 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, wherein, include: Positive electrode, negative electrode, and electrolyte; The positive electrode includes: a positive electrode active material layer; The negative electrode includes: a negative electrode active material layer, the negative electrode active material layer includes: a negative electrode active material, the negative electrode active material includes: graphite, the graphitization degree of the graphite is: 88% to 93%; and the CB value is: 1.06 to 1.20, the CB value is the ratio of the capacity of the negative electrode active material layer per unit area to the capacity of the positive electrode active material layer per unit area; The electrolyte includes at least one of cyclic carbonates or linear carbonates.

2. The electrochemical device according to claim 1, wherein, 1.06 ≤ CB value < 1.1, 93% ≥ the degree of graphitization > 91.5%; or, 1.1 ≤ CB value < 1.15, 91.5% ≥ the degree of graphitization > 90%; or, 1.15≤CB value≤1.2, 90%≥the degree of graphitization≥88%.

3. The electrochemical device according to claim 1, wherein, After 100 cycles, the percentage of electrolyte consumed by the electrochemical device is less than or equal to 10% × (1 - 5% × (CB - 1.05) / 0.01).

4. The electrochemical device according to any one of claims 1 to 3, wherein, The Raman test results of the graphite are I D / I G The value is between 0.1 and 0.

3.

5. The electrochemical device according to any one of claims 1 to 3, wherein, The negative electrode active material also includes: silicon-containing materials, wherein the silicon-containing materials include silicon-carbon composite materials.

6. The electrochemical device according to any one of claims 1 to 3, wherein, It satisfies at least one of the following: (a) The specific capacity of the graphite is from 330 mAh / g to 350 mAh / g; (b) The specific surface area of ​​the negative electrode active material layer is: 1m² 2 / g to 25m 2 / g; (c) The graphite content in the negative electrode active material is greater than or equal to 80% by mass.

7. The electrochemical device according to any one of claims 1 to 3, wherein, Based on the total mass of the electrolyte, the sum of the mass percentages of the cyclic carbonate and the linear carbonate is A%, where A% is 60% to 90%.

8. The electrochemical device according to any one of claims 1 to 3, wherein, The positive electrode active material layer includes: a positive electrode active material, which includes at least one of lithium cobalt oxide or ternary materials.

9. The electrochemical device according to claim 8, wherein, It satisfies at least one of the following: (d) The positive electrode active material includes: a modifying element M, wherein the modifying element M includes at least one of Al or Mg; based on the total mass of the positive electrode active material, the mass percentage of the modifying element M is 0.2% to 0.5%; (e) The particle size Dv50 of the positive electrode active material is 12 μm to 18 μm.

10. An electronic device, wherein, include: The electrochemical device as described in any one of claims 1 to 9.

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