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 positive electrode active material layer comprises a positive electrode active material; the positive electrode active material comprises at least one of lithium cobaltate or a ternary material, the positive electrode active material comprises a modified element M, the modified element M comprises at least one of Al or Mg; the mass percentage of the modified element M is X% based on the total mass of the positive electrode active material, X% is 0.1% to 0.4%; the resistivity of the positive electrode active material layer is 10 ‑4 Ω·cm to 10 ‑2 Ω·cm. The electrolyte comprises at least one of a cyclic carbonate or a linear carbonate. The electrochemical device provided by the application can improve the energy density while improving the electrolyte consumption of the electrochemical device and being beneficial to the cycle performance.

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 energy density of electrochemical devices (e.g., lithium-ion batteries). To improve energy density, high-voltage (charging cut-off voltage of 4.5V and above) lithium cobalt oxide systems are often used as positive electrode active materials. However, under high-voltage systems, the increase in positive electrode side reactions accelerates electrolyte consumption and deteriorates cycle performance. 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] This application provides an electrochemical device, comprising: a positive electrode, a negative electrode, and an electrolyte;

[0005] The positive electrode comprises: a positive electrode active material layer, which in turn comprises: a positive electrode active material; the positive electrode active material comprises at least one of lithium cobalt oxide or a ternary material, and the positive electrode active material comprises a modifying element M, which comprises 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 X%, and X% is 0.1% to 0.4%; the resistivity of the positive electrode active material layer is 10 Nm. -4 Ω·cm to 10 -2 The electrolyte comprises at least one of cyclic carbonates or linear carbonates. When the mass percentage of the modifying element M is within the above range, it is beneficial to reduce the active sites in the positive electrode active material layer, which is conducive to the utilization of the specific capacity of the positive electrode active material. When the resistivity of the positive electrode active material layer is within the above range, it is beneficial to reduce the electron transport and ion diffusion performance on the positive electrode side, thereby reducing the electrolyte consumption of the electrochemical device. The electrochemical device of this application can improve the electrolyte consumption of the electrochemical device while increasing the energy density, and is also beneficial to the cycle performance.

[0006] 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 - 10% × (0.5 - X) / 0.05), which is beneficial for the electrochemical device to have long cycle performance.

[0007] In some embodiments, the specific surface area of ​​the positive electrode active material layer is Am. 2 / g, 0.2≤A≤2.0, to ensure that the electrochemical device has good rate performance and does not accelerate the consumption of electrolyte in the electrochemical device.

[0008] In some embodiments, the bulk porosity of the positive electrode active material layer is B, 20% ≤ B ≤ 40%, which is beneficial to the ion transport performance of the positive electrode active material layer, while also improving the energy density and avoiding accelerated electrolyte consumption of the electrochemical device.

[0009] In some embodiments, 0.8 ≤ A / B ≤ 8, and optionally, 1 ≤ A / B ≤ 5. By controlling the value of A / B, lithium ions can be distributed more evenly, which helps to reduce concentration polarization and avoid the continuous occurrence of side reactions in local areas, thereby improving the electrolyte consumption of the electrochemical device.

[0010] In some embodiments, the pH value of the positive electrode active material layer is C, 8≤C≤12, which is beneficial to the structural stability and electrochemical performance of the positive electrode active material, thereby improving the performance degradation of the positive electrode active material during cycling.

[0011] In some embodiments, the positive electrode active material layer includes a binder; the mass percentage of the binder in the positive electrode active material layer is D, based on the total mass of the positive electrode active material layer, where 1% ≤ D ≤ 2%, thereby improving the adhesion of the positive electrode active material layer while benefiting the rate performance of the electrochemical device.

[0012] In some embodiments, 500 ≤ C / D ≤ 1000, which helps to reduce the impedance degradation of the positive electrode active material layer and improve the electrolyte consumption of the electrochemical device.

[0013] In some embodiments, the particle size D of the positive electrode active material V The size of 50 is 12μm to 18μm, which is beneficial for the positive electrode active material to exert a high specific capacity and to have good cycle performance.

[0014] In some embodiments, the negative electrode active material layer includes: a negative electrode active material; the negative electrode active material includes: graphite, or the negative electrode active material includes: graphite and silicon-containing materials, thereby adapting to the positive electrode and electrolyte in the embodiments of this application, which is beneficial to reducing the electrolyte consumption of the electrochemical device.

[0015] In some embodiments, the graphitization degree of graphite is 88% to 93%, which is beneficial to improving the cycle performance and rate performance of the electrochemical device.

[0016] In some embodiments, the Raman test results of graphite are I D / I G The value is 0.1 to 0.3, which is beneficial to improving the cycle performance and fast-charging performance of electrochemical devices.

[0017] In some embodiments, the graphite content in the negative electrode active material is greater than or equal to 80% by mass, thereby being compatible with the positive electrode active material and the electrolyte, which helps to reduce the electrolyte consumption of the electrochemical device.

[0018] In some embodiments, the specific capacity of graphite is 330 mAh / g to 350 mAh / g, which is beneficial to both the energy density of the electrochemical device and the reduction of electrolyte consumption.

[0019] In some embodiments, the specific surface area of ​​the negative electrode active material is: 1m² 2 / g to 25m 2 / g, which helps improve the cycle performance and fast-charging performance of electrochemical devices.

[0020] Embodiments of this application also provide an electronic device, including the electrochemical device described above.

[0021] In this embodiment, by reducing the mass percentage of the modifying elements in the lithium cobalt oxide system, controlling the resistivity of the positive electrode active material layer, and using a suitable electrolyte, the interaction of these three factors not only improves the energy density of the electrochemical device but also helps to reduce electrolyte consumption and improve cycle performance. Detailed Implementation

[0022] 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.

[0023] For electrochemical devices (e.g., lithium-ion batteries), energy density is one of their key performance indicators. To improve the energy density of an electrochemical device, increasing the charging cut-off voltage is a direct method to increase the specific capacity of the positive electrode active material. Therefore, high-voltage (i.e., charging cut-off voltage reaching 4.5V or higher, such as 4.55V) lithium cobalt oxide systems are often used as positive electrode active materials. However, while the specific capacity of the positive electrode active material is increased under high-voltage systems (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, which is the actual specific capacity), its structural stability deteriorates, and its capacity decays severely during cycling. This is mainly because the increased side reactions on the positive electrode accelerate the consumption of the electrolyte in the electrochemical device. Increased electrolyte consumption leads to the loss of effective protection for the positive electrode active material, resulting in structural damage. Coating or doping the positive electrode active material can improve its stability, but this also reduces its specific capacity. Therefore, without reducing the specific capacity exerted by the positive electrode active material, improving the capacity decay during the cycling process is crucial for improving the performance of electrochemical devices.

[0024] Some embodiments of this application provide an electrochemical device including a positive electrode, a negative electrode, and an electrolyte. The electrochemical device may be a lithium-ion battery.

[0025] The positive electrode includes a positive electrode active material layer. The positive electrode may also 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, or stainless steel foil; of course, other current collectors commonly used in the art may also be used. In some embodiments, the thickness of the positive electrode current collector may be from 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. The positive electrode active material layer includes a positive electrode active material; the positive electrode active material includes at least one of lithium cobalt oxide or ternary materials (lithium nickel cobalt manganese oxide). The positive electrode active material includes a modifying element M, which includes at least one of Al or Mg, and may also include other elements; based on the total mass of the positive electrode active material, the mass percentage of the modifying element M is X%, where X% is from 0.1% to 0.4%, and X% may specifically be 0.1%, 0.2%, 0.3%, or 0.4%. In some embodiments of this application, the mass percentage of the modifying element is low. The modifying element plays a role in stabilizing the structure of the positive electrode active material layer, but it does not provide energy itself. The modifying element reduces the active sites in the positive electrode active material layer. By reducing the mass percentage of the modifying element in the positive electrode active material, the specific capacity of the positive electrode active material is improved. If the mass percentage of the modifying element is high, it is detrimental to the specific capacity of the positive electrode active material. Reducing the mass percentage of the modifying element can improve the specific capacity of the positive electrode active material. Furthermore, if the mass percentage of the modifying element is too high, it is detrimental to the specific capacity and energy density of the positive electrode active material. Increased lattice distortion in the positive electrode active material is detrimental to lithium-ion insertion and extraction, and may also hinder lithium-ion diffusion and kinetic performance due to uneven distribution within the lattice, thus negatively impacting the cycle performance of the positive electrode active material. If the mass percentage of the modified element is too small, it will not be conducive to the structural stability and conductivity of the positive electrode active material, and the surface side reactions may increase. During the cycling process of the electrochemical device, the capacity of the positive electrode active material will decay faster, which will also be detrimental to the cycle performance.

[0026] In this application, the resistivity of the positive electrode active material layer is: 10 -4 Ω·cm to 10 -2 Ω·cm, for example, 1×10 -3 Ω·cm, 2×10 -3 Ω·cm, 3×10 -3 Ω·cm, 4×10 -3 Ω·cm, 5×10 -3 Ω·cm, 6×10 -3 Ω·cm, 7×10 -3 Ω·cm, 8×10 -3 Ω·cm or 9×10-3 Ω·cm. In some embodiments, ion diffusion and electron transport need to be compatible in the electrochemical device. Modified elements can influence the lattice through charge compensation, which is beneficial to improving the ionic conductivity of the positive electrode active material. In this embodiment, the mass percentage of modified elements in the positive electrode active material is relatively low. Although this is beneficial to improving the specific capacity of the positive electrode active material, it is not beneficial to the ionic conductivity of the positive electrode active material. If the resistivity of the positive electrode active material layer is too high, electron transport will be hindered, and the local potential of the positive electrode will be too high. The high local potential will lead to electrolyte decomposition, thereby increasing electrolyte consumption. However, if the resistivity of the positive electrode active material layer is too low, electron transport will be too fast, resulting in an excessively high local lithium ion concentration at the negative electrode. This will easily lead to an increase in side reactions between the negative electrode and the electrolyte, thereby increasing electrolyte consumption. Therefore, controlling the resistivity of the positive electrode active material layer within the above range is beneficial to reducing the electron transport and ion diffusion performance on the positive electrode side, thereby reducing the electrolyte consumption of the electrochemical device, which is beneficial to improving energy density and cycle performance.

[0027] In this application, the electrolyte includes at least one of cyclic carbonates or linear carbonates. The type of electrolyte needs to be compatible with the type of positive electrode active material, the type and mass percentage of modifying elements. In this embodiment, the positive electrode active material is either lithium cobalt oxide or a ternary material, and a very low mass percentage of modifying elements is used, including Al or Mg. When using this positive electrode active material, the inventors found that the cyclic carbonates and linear carbonates in the electrolyte are well compatible with it, which is beneficial to improve the electrochemical window, that is, to have higher oxidation stability and reduction stability, to maintain the stability of the electrolyte and prevent decomposition, to reduce side reactions caused by the rupture and repair of the SEI (solid electrolyte interface) film and CEI (chemical-electrochemical interface) film during cycling, and also to disperse lithium salts, improve the conductivity of the electrolyte, reduce polarization, and thus reduce side reactions. Cyclic and linear carbonates significantly improve the side reactions occurring in the negative electrode active material during the early stages of cycling, possibly due to their superior film-forming properties. In the later stages of cycling, they improve the side reactions occurring in the positive electrode active material, possibly due to their inherent oxidation resistance after polarization accumulation. Therefore, cyclic and linear carbonates can mitigate the side reactions of both positive and negative electrode active materials during cycling, thus improving electrolyte consumption. In some embodiments, cyclic carbonates include, but are not limited to, at least one 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 include, but are not limited to, at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, and propyl methyl carbonate.

[0028] In the embodiments of this application, for the positive electrode active material of the lithium cobalt oxide system, by employing a modifying element with a mass percentage of 0.1% to 0.4%, the specific capacity of the positive electrode active material can be better improved, which is beneficial to increasing the energy density. Considering that ion diffusion and electron transport need to be compatible, in some embodiments of this application, the resistivity of the positive electrode active material layer is controlled at 10. -4 Ω·cm to 10 -2The Ω·cm value helps reduce electrolyte consumption. Furthermore, the inventors discovered that for the modified element-containing positive electrode active material used in this application, the inclusion of cyclic or linear carbonates in the electrolyte can significantly reduce electrolyte consumption in the electrochemical device. Specifically, in the embodiments of this application, by reducing the mass percentage of the modified element in the lithium cobalt oxide system, controlling the resistivity of the positive electrode active material layer, and using a suitable electrolyte, these three factors interact to better improve the specific capacity of the positive electrode active material. Simultaneously, because electrolyte consumption is reduced, the electrolyte continuously protects the positive electrode active material during high-voltage (charging cutoff voltage of 4.5V and above) cycling, maintaining structural stability. This improves cycle decay without reducing the specific capacity, thus achieving the effect of increasing the energy density of the electrochemical device while simultaneously improving cycle performance.

[0029] In some embodiments of this application, when the mass percentage of the modifying element is 0.1% to 0.25%, the resistivity of the positive electrode active material layer is 10⁻² Ω·cm to 10⁻² Ω·cm. -3 Ω·cm; when the mass percentage of the modifying element is 0.25% to 0.4%, the resistivity of the positive electrode active material layer is 10 Ω·cm. -3 Ω·cm to 10 -4 Ω·cm. In some embodiments, the mass percentage of the modifying element affects the conductivity of the positive electrode active material. A higher mass percentage results in better ionic conductivity of the positive electrode active material itself. Since ion diffusion and electron transport need to be compatible, it is necessary to improve the conductivity of the positive electrode active material layer to match the ionic conductivity of the positive electrode active material. Therefore, as the mass percentage of the modifying element in the positive electrode active material increases, the resistivity of the positive electrode active material layer needs to decrease accordingly; the two are inversely related. By setting the correlation between the mass percentage of the modifying element and the resistivity of the positive electrode active material layer, electrolyte consumption can be better mitigated, thereby improving cycle stability.

[0030] It is understood that this application does not impose any particular restrictions on the method of adjusting the mass percentage content of the modified elements, as long as the application can be realized. For example, the modified elements can be uniformly distributed in the precursor of the positive electrode active material through co-precipitation, sol-gel method, solid-phase reaction method, etc., and then calcined under appropriate atmosphere, temperature and time to increase the mass percentage content of the modified elements. It is understood that there are no particular restrictions on the method of adjusting the resistivity of the positive electrode active material layer, as long as the application can be realized. For example, surface coating, changing the type and content of conductive agent, and changing the material morphology can be used simultaneously. As the mass percentage content of the modified elements increases, the resistivity of the positive electrode active material can be reduced by increasing the proportion of conductive agent, optimizing the morphology of the active material, and performing carbon coating or metal oxide coating.

[0031] In some embodiments of this application, the electrochemical device is charged at a constant current of 1C to the charging cutoff voltage at 25°C, then charged at a constant voltage to 0.05C, and discharged at a rate of 0.5C to the discharge cutoff voltage. After 100 cycles, the percentage of electrolyte consumption M% is less than or equal to 10% × (1 - 10% × (0.5 - X) / 0.05).

[0032] In some embodiments, the electrolyte consumption percentage 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 electrolyte, and record the mass of the added electrolyte as 0.2g. Then seal the aluminum-plastic film to complete the sample preparation. Cycle the sample at 25℃ at a 1C rate for 100 cycles. The specific process of one cycle is to charge at a constant current rate of 1C to the charging cutoff voltage (the charging cutoff voltage can be 4.5V), then charge at a constant voltage until the current is less than 0.05C, and then discharge at a constant current rate of 0.5C to the discharge cutoff voltage (the discharge cutoff voltage can be 3V). Then, fill the sample to be tested with a full load (when fully loaded, discharge with a DC current of 0.1C to the discharge cutoff voltage, and then repeat three times with a DC discharge rate of 0.01C to the discharge cutoff voltage), weigh the sample and record the mass as M1, put the sample to be tested into a centrifuge tube, and centrifuge the electrolyte in a centrifuge; disassemble the sample to be tested, pour in an appropriate amount of DMC to completely immerse the sample to be tested, and soak for a total of 14 hours; take out the sample to be tested and dry it in a 60℃ oven for 2 hours, and then weigh it with a 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.

[0033] 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 modified element mass percentage of 0.5% in the positive electrode active material as a standard, to ensure the electrochemical device can achieve 1000 cycles, the electrolyte consumption percentage M% must satisfy M% ≤ 100 / 1000 = 10%, meaning a maximum of 10% electrolyte is consumed per 100 cycles. For every 0.05% decrease in the modified element mass percentage, the specific capacity of the positive electrode active material increases by approximately 1%. However, for every 1% increase in specific capacity, the crystal structure stability of the positive electrode active material deteriorates by 10%. To improve the long-cycle capability of the electrochemical device, electrolyte consumption needs to be reduced proportionally; therefore, M% needs to be controlled to ≤ 10% × (1 - 10% × (0.5 - X) / 0.05).

[0034] In some embodiments of this application, the specific surface area of ​​the positive electrode active material layer is Am. 2 / g, 0.2≤A≤2.0. In some embodiments, A can be 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, or 1.8. The specific surface area affects the contact area between the positive electrode active material layer and the electrolyte, and thus affects the reaction rate between the positive electrode active material layer and the electrolyte. Controlling the specific surface area of ​​the positive electrode active material layer within the above range is beneficial to reducing electrolyte consumption and improving the rate performance of the electrochemical device. In some embodiments, the specific surface area of ​​the positive electrode active material layer can be adjusted by laser drilling the positive electrode active material layer or by adjusting the contact surface properties of the positive electrode active material layer during cold pressing. The specific surface area of ​​the positive electrode active material layer is related to whether the positive electrode active material layer undergoes special surface treatment, and is not highly correlated with the parameters of the positive electrode active material itself or the pressure during cold pressing.

[0035] In some embodiments of this application, the bulk porosity B of the positive electrode active material layer is 20% ≤ B ≤ 40%. In some embodiments, B can be 20%, 25%, 30%, 35%, or 40%. If the porosity of the positive electrode active material layer is too high, there will be too many internal pores, which will accelerate electrolyte consumption and reduce the volumetric energy density of the positive electrode active material layer. If the porosity of the positive electrode active material layer is too low, there will be too few internal pores, which is not conducive to ion transport in the positive electrode active material layer. In some embodiments, the porosity of the positive electrode active material layer is strongly related to the material parameters of the positive electrode active material and the pressure during cold pressing. The porosity of the positive electrode active material layer can be adjusted by changing the pressure during cold pressing, the tap density of the positive electrode active material, and the type and content of binder in the positive electrode active material layer. In some embodiments of this application, 0.8 ≤ A / B ≤ 8. In some embodiments, the specific surface area and bulk porosity of the positive electrode active material layer need to satisfy a certain relationship. This helps to avoid uneven electrolyte distribution within the positive electrode active material layer, thereby preventing persistent side reactions in localized areas of the positive electrode active material layer that would accelerate electrolyte consumption. A / B can be 0.8, 1, 2, 3, 4, 5, 6, 7, or 8. In this embodiment, A / B is not too high, which helps to prevent lithium ions from accumulating in the pores on the surface of the positive electrode active material layer. If this happens, due to concentration polarization, surface side reactions of the positive electrode active material layer will continue, accelerating electrolyte consumption. In this embodiment, A / B is not too low, which helps to prevent the interconnection between particles in the positive electrode active material layer, requiring the formation of a large liquid film. If this occurs, electrolyte consumption will worsen. Therefore, in this embodiment, controlling both according to the above relationship can better improve electrolyte consumption.

[0036] It is understood that there are no particular restrictions on the method of adjusting the specific surface area of ​​the positive electrode active material layer, as long as it achieves the purpose of this application. For example, it can be adjusted by laser drilling, mechanical pore creation, etc. It is also understood that there are no particular restrictions on the method of adjusting the bulk porosity of the positive electrode active material layer, as long as it achieves the purpose of this application. For example, it can be adjusted by adjusting the pressing process, adjusting the cold pressing pressure, etc. Furthermore, it is understood that the A / B ratio adjustment can be achieved by keeping the specific surface area of ​​the positive electrode active material layer constant, while adjusting the type and content of the binder, adjusting the coating speed and pressure, etc., to adjust the bulk porosity of the positive electrode active material layer.

[0037] In some embodiments of this application, the pH value of the positive electrode active material layer is C, where 8 ≤ C ≤ 12. In some embodiments, C can be 8, 9, 10, 11, or 12. In some embodiments, the pH value of the positive electrode active material layer is not too low. An acidic environment is avoided, as it may accelerate the dissolution of cobalt in the positive electrode active material under high voltage, thereby leading to a deterioration in the cycle life of the electrochemical device. In some embodiments, the pH value of the positive electrode active material layer is not too high. If the pH is too high, it may cause structural changes in the positive electrode active material, such as lattice distortion or phase transition. Such structural changes will affect the electrochemical performance of the positive electrode active material and accelerate the capacity decay of the positive electrode active material during cycling.

[0038] In some embodiments, the positive electrode active material layer includes a binder; based on the total mass of the positive electrode active material layer, the mass percentage of the binder in the positive electrode active material layer is D, where 1% ≤ D ≤ 2%, for example, 1.2%, 1.4%, 1.6%, or 1.8%. In some embodiments, the binder in the positive electrode active material layer can be one or more of polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), polyvinyl alcohol (PVA), and polyimide (PI). A binder mass percentage that is not too low helps to prevent desorption between the positive electrode active material layer and the positive electrode current collector; a binder mass percentage that is not too high helps to prevent deterioration of the conductivity of the positive electrode active material layer, thereby facilitating high-rate charge and discharge.

[0039] In some embodiments of this application, 500 ≤ C / D ≤ 1000. In some embodiments, the pH value of the positive electrode active material layer and the binder content in the positive electrode active material layer satisfy a certain relationship. This is because increasing the pH value of the positive electrode active material layer can help reduce the dissolution of cobalt in the positive electrode active material, but increasing the pH value of the positive electrode active material layer has a certain destructive effect on the binder. Therefore, as the pH value of the positive electrode active material layer increases, simultaneously increasing the mass percentage content of the binder is beneficial to the overall bonding stability of the positive electrode active material layer on the positive electrode current collector. The C / D ratio satisfies the range 500 ≤ C / D ≤ 1000. More specifically, C / D can be 500, 600, 650, 700, 750, 800, 900, or 1000. In some embodiments, the C / D ratio is not too high. This helps to avoid deterioration of the adhesion stability of the positive electrode active material layer on the positive electrode current collector, and helps to avoid local detachment of the positive electrode active material layer on the positive electrode current collector, thereby avoiding an increase in the contact area with the electrolyte and reducing electrolyte consumption. In some embodiments, the C / D ratio is not too low. This helps to avoid deterioration of the impedance of the positive electrode active material layer, and helps to avoid increased polarization of the electrochemical device, thereby avoiding accelerated cobalt dissolution from the positive electrode active material due to increased polarization, and thus helping to reduce side reactions of the positive electrode active material layer and electrolyte consumption. Therefore, when the two are controlled according to the above ratio, electrolyte consumption can be reduced more effectively.

[0040] In some embodiments of this application, the particle size D of the positive electrode active material V The particle size Dv50 ranges from 12 μm to 18 μm. The Dv50 of the positive electrode active material directly affects its specific capacity and cycle performance. When the particle size Dv50 of the positive electrode active material meets the above range, it is beneficial to improve the specific capacity and structural stability of the positive electrode active material, thereby improving its cycle performance, which in turn helps to increase the energy density and cycle performance of the electrochemical device.

[0041] In some embodiments of this application, the negative electrode active material layer includes: a negative electrode active material; the negative electrode active material includes: graphite, or the negative electrode active material includes: graphite and a silicon-containing material, wherein the silicon-containing material may be a silicon-carbon composite. In some embodiments, the graphitization degree of graphite is 88% to 93%, for example, it may be 88%, 89%, 90%, 91%, 92%, or 93%. In some embodiments, the graphitization degree affects the electrolyte consumption on the negative electrode side and the conductivity of the negative electrode active material layer. In this embodiment, the graphitization degree is not too high, which is beneficial to reducing the electrolyte consumption of the electrochemical device and improving the cycle performance of the electrochemical device. Moreover, in this embodiment, the graphitization degree is not too low, which is beneficial to avoiding the deterioration of the conductivity of graphite, thereby improving the fast-charging performance of the electrochemical device.

[0042] 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, 0.1, 0.2, or 0.3. In some embodiments, the I value of the Raman test result is... D / I G The value represents the amorphous carbon coating condition, I D / I G A value that is not too low indicates that the amorphous carbon in this embodiment provides good coating protection. Therefore, the SEI (solid electrolyte interface) film of the negative electrode active material layer is not easily damaged during cycling, which helps to reduce side reactions with the electrolyte and improves cycling performance. In some embodiments, I D / I G The value is not too high, which is beneficial to improving the conductivity of graphite and increasing the specific capacity, thereby benefiting fast charging performance and energy density.

[0043] In some embodiments of this application, the graphite content in the negative electrode active material is greater than or equal to 80%, for example, 85%, 90%, or 95%. In some embodiments, the negative electrode active material is mainly composed of graphite, which exhibits the characteristics of graphite and is beneficial for its compatibility with the positive electrode active material.

[0044] In some embodiments of this application, the specific capacity of graphite is from 330 mAh / g to 350 mAh / g, for example, 335 mAh / g, 340 mAh / g, and 345 mAh / g. In some embodiments, the specific capacity of graphite is not excessively high, which is beneficial for reducing electrolyte consumption and improving the cycle performance of the electrochemical device. The specific capacity of graphite is not excessively low, which is beneficial for increasing the overall energy density of the electrochemical device. It is understood that there are no particular limitations on the method of adjusting the graphitization degree of graphite, as long as it achieves the purpose of this application, such as high-voltage treatment or the 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 GThe 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, while maintaining the Ig value of the Raman test results. D / I G The value and degree of graphitization remain unchanged.

[0045] 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. A lower specific surface area of ​​the negative electrode active material layer helps reduce electrolyte consumption and thus improves cycle performance. Conversely, a lower specific surface area helps improve the kinetic performance of the negative electrode active material and enhances fast-charging performance.

[0046] In some embodiments of this application, 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%. In some embodiments, A% is 65%, 70%, 75%, 80%, or 85%. In some embodiments, the cyclic carbonates and linear carbonates can reduce electrolyte consumption, and their mass percentages are not too low, which helps to avoid the deterioration of electrolyte consumption and thus improve cycle performance. Conversely, their mass percentages are not too high, which helps to improve electrolyte conductivity and thus improve fast-charging performance.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] In some embodiments of this application, the electrolyte includes lithium salts. This application does not impose any particular limitation on the lithium salt, as long as it achieves the purpose of this application. For example, the electrolyte salt may include, but is not limited to, at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (C4BLiO8, abbreviated as LiBOB), or lithium difluoroborate. This application does not impose any particular limitation on the content of the electrolyte salt in the electrolyte, as long as it achieves the purpose of this application.

[0052] 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.

[0053] In some embodiments, the electrochemical device includes a lithium-ion battery, but this application is not limited thereto.

[0054] In some embodiments, the electrochemical device further includes a housing for accommodating the positive electrode, diaphragm, negative electrode, and electrolyte, as well as other components known in the field of electrochemical devices. This application does not limit the scope of these other components. This application does not impose any particular limitation on the housing; it can be a housing known in the art, as long as it achieves the purpose of this application. For example, the housing can be a rigid housing or a flexible housing. The material of the rigid housing can be metal; this application does not limit the type of metal and can use known metal rigid housings, as long as they achieve the purpose of this application. The flexible housing can be a metal-plastic film, such as aluminum-plastic film, steel-plastic film, etc.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] Example

[0059] The following uses an electrochemical device, specifically a lithium-ion battery, as an example to illustrate the implementation methods of this application in more detail through examples and comparative examples. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" refer to mass measurements. Test methods:

[0060] Test methods and equipment:

[0061] 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 positive or negative electrode sheets were soaked in dimethyl carbonate solvent for 2 hours and dried at 60°C for 1 hour to obtain the negative or positive electrode. The discharge cutoff voltage of the lithium-ion battery in the embodiments and comparative examples of this application is 3.0V. It is 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.

[0062] Unless otherwise specified, the following test methods shall use the positive or negative electrode obtained in the above manner for testing.

[0063] Mass percentage test of modified elements: Take the positive electrode active material powder and perform thermogravimetric analysis in air at 400℃ for 2 hours to remove the binder and conductive agent, leaving the remaining positive electrode active material powder. Inductively coupled plasma optical emission spectroscopy (ICP-OES) can be used to test the mass percentage of doped elements such as Al, Mg, Ti, Nb, Zn, and W. Take the positive electrode active material powder in the testing instrument, and excite it with plasma to emit characteristic spectra. By detecting these spectra, the type and content of the elements can be determined.

[0064] Resistivity test of positive electrode active material layer: A positive electrode active material layer with a length and width of not less than 20 mm and a thickness of not less than 1 mm is selected as the test sample. The four-wire two-terminal method is used to determine the fixed resistance by measuring the voltage across the resistor of the test sample and the current flowing through it. The conductivity is calculated by combining the height and bottom area of ​​the test resistor.

[0065] Electrolyte consumption percentage test at 25℃ for 100 cycles: First, the fully discharged lithium-ion battery was disassembled, and the positive and negative electrodes were removed. The positive and negative electrodes were cut into small pieces of a fixed size (3cm×3cm), cleaned with dimethyl carbonate (DMC), and dried. A polyethylene separator was placed between the positive and negative electrodes and then placed in an aluminum-plastic film. 0.2g of the electrolyte used in Example 1 was added, and then the aluminum-plastic film was sealed to complete the sample preparation. The sample was cycled 100 times at 25℃ at a 1C rate. The specific process of one cycle was as follows: charging at a constant current rate of 1C to the charging cutoff voltage (charging cutoff voltage is 4.5V), then constant voltage charging until the current is less than 0.05C, and discharging at a constant current rate of 0.5C to the discharge cutoff voltage of 3.0V. Then, fully discharge the sample to be tested at a DC current of 0.1C until the discharge cutoff voltage (3.0V). Repeat this process three times at a DC current of 0.01C until the discharge cutoff voltage. Weigh the sample and record the mass as M1. Place the sample in a centrifuge tube and centrifuge the electrolyte. Disassemble the sample and pour in an appropriate amount of DMC to completely submerge it. Soak for a total of 14 hours. Remove the sample and dry it in a 60℃ oven for 2 hours. Weigh the sample and record the mass as M2. The total electrolyte volume after circulation is M = M1 - M2. The percentage of electrolyte consumed during circulation is M% = (0.2 - (M1 - M2)) / 0.2.

[0066] Bulk porosity testing: The bulk porosity of the positive electrode active material layer is tested using the mercury intrusion method. The positive electrode active material layer is dried to remove adsorbed water and volatiles. Then, the layer is placed in a mercury intrusion apparatus, and the pressure is gradually increased. The amount of mercury intrusion is recorded, and the amount of mercury entering the pores of the positive electrode active material layer at different pressures is measured. The pore volume and porosity are then calculated. It is important to note that the mass effect of the positive electrode current collector must be subtracted before calculating the porosity.

[0067] pH test of positive electrode active material layer: After the lithium-ion battery is fully discharged, the positive electrode is removed. The electrolyte on the surface of the positive electrode is cleaned with dimethyl carbonate (DMC). After drying, a portion of the active material layer powder is taken and added to an appropriate amount of deionized water. After ensuring full dispersion, the mixture of sample and water is stirred thoroughly with a glass rod or stirrer to prepare a uniform suspension. Then, a pH meter is inserted into the suspension to test the pH value.

[0068] Test of the mass percentage of binder in the positive electrode active material layer: After disassembling the lithium-ion battery, scrape off the positive electrode active material layer powder with a mass of a1, and perform a thermogravimetric test in air. Burn off the binder at 400℃ for 2H. After leaving only the active material and conductive agent, the mass of the remaining material is a2. Then the mass percentage of binder = (a1-a2) / a1.

[0069] Positive electrode active material particle size D V Test for 50: After disassembling the lithium-ion battery, scrape off the positive electrode active material layer. First, separate the positive electrode active material from the binder and conductive agent by mechanical grinding and sieving. Then, take about 0.02g of positive electrode active material as the test powder sample in a 50ml clean beaker, add about 20ml of deionized water, and then add a few drops of 1% surfactant to completely disperse the test powder sample in the water. Sonicate in a 120W ultrasonic cleaner for 5 minutes, and use a MasterSizer2000 to test the particle size distribution. The cumulative 50% diameter Dv50 in the volume baseline distribution obtained by laser scattering particle size analyzer is measured.

[0070] The degree of graphitization of graphite can be tested by observing the negative electrode active material powder with a high-resolution transmission electron microscope. Combined with image analysis software, the degree of graphitization can be quantitatively assessed. (It should be noted that high-resolution transmission electron microscopy identifies lattice fringes and reads the interlayer spacing, and then the degree of graphitization is quantified by software processing and analysis. Separation of graphite and silicon is not required; their respective fringes can be read.)

[0071] Raman spectroscopy of graphite:

[0072] The lithium-ion battery was discharged to 3.0V at 0.5C and then disassembled to obtain the negative electrode. The negative electrode was immersed in DMC solution for 4 hours and then dried. An area of ​​100μm × 100μm was selected on the active material layer of the negative electrode. The graphite particles within this area were scanned using a laser confocal Raman spectroscopy system (Raman, HR Evolution, HORIBA Scientific Instruments Division). The d-peaks and g-peaks of all graphite particles within this area were obtained. The data were processed using LabSpec software to obtain the peak intensities of the d-peak and g-peak for each graphite particle, which are Id, Ig ... d and I g The D peak is usually located at 1350 cm⁻¹ -1 Nearby, the G peak is typically located at 1580cm. -1 Nearby. The laser wavelength of the Raman spectrometer is in the range of 532 nm to 785 nm. d / I g The value is the I of all graphite particles measured within this range. d and I g The average of the ratios.

[0073] Testing the percentage of graphite by mass:

[0074] After disassembling a lithium-ion battery, a layer of negative electrode active material with a mass of m1 is scraped off. A thermogravimetric analysis (TGA) is performed in air. After burning off the binder and other materials at 400℃ for 2 hours, the remaining mass is m2. After burning off the binder and other materials 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 is (m2 - m3) / m2, and the mass percentage of silicon carbon in the negative electrode active material is m3 / m2.

[0075] Graphite specific capacity testing: If the negative electrode active material is pure graphite, the specific capacity testing method is as follows: Disassemble the lithium-ion battery and remove the negative electrode. Weigh... The mass of the small disc is m4. After wiping away the negative electrode current collector, the mass of the negative electrode current collector is weighed as m5. Therefore, the mass of the negative electrode active material layer is m4 - m5. The mass of the negative electrode active material in the small disc = (m4-m5)×m2 / m1, then use The small discs are used to assemble button batteries with lithium metal sheets as the counter electrode. The electrolyte in the button batteries 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 x is measured. Then the graphite capacity = x / ((m4-m5)×m2 / m1).

[0076] 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 small disc has a mass of m4. A current collector of the same size, with its negative electrode active material layer removed, has a mass of m5. Therefore, the mass of the negative electrode active material layer on the small disc is m4 - m5. The mass of the negative electrode active material of the small disc is (m4-m5)×m2 / m1, and then a lithium metal sheet is used as the counter electrode. Small discs are used to assemble button batteries. The electrolyte in the button batteries is the same as that used in Example 1. Charge-discharge tests are performed in an electrochemical workstation with a voltage range of 0.001V to 2V. The reversible capacity y is measured. Then, the specific capacity of the negative electrode active material is y / ((m4-m5)×m2 / m1). Take some negative electrode active material powder and calcine it in air at 700℃ for 2 hours. The remaining mass is the mass of pure silicon carbon material, recorded 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 battery with a lithium sheet as the counter electrode. The electrolyte in the button battery is the same as the electrolyte 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 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).

[0077] 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 in a BET instrument. Measure the amount of nitrogen adsorbed by controlling the relative pressure (usually between 0.05-0.3). The specific surface area can be calculated based on the adsorption isotherm. Note that the mass of the negative electrode used for the BET test 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 should be used as the mass for calculating the specific surface area using the BET fitting method.

[0078] Method for testing the specific surface area of ​​the positive electrode active material layer: Disassemble the lithium-ion battery and remove the positive electrode. Heat it to 100℃ to 200℃ under vacuum to remove adsorbed moisture and gas from the surface. Then, place the treated positive electrode in a BET instrument. Measure the amount of nitrogen adsorbed by controlling the relative pressure (usually between 0.05-0.3). The specific surface area can be calculated based on the adsorption isotherm. Note that the BET test is conducted on the positive electrode. The mass of the positive electrode active material layer needs to be determined based on its proportion of the positive electrode active material layer. The mass of the positive electrode active material layer is used as the mass for calculating the specific surface area using BET fitting.

[0079] 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.

[0080] Volumetric energy density test: The lithium-ion battery was placed in a 25°C constant temperature chamber and left to stand for 30 minutes to allow it to reach a constant temperature. The lithium-ion battery that had reached a constant temperature was then charged at a constant current of 0.5C to a voltage of 4.5V, and then charged at a constant voltage of 4.5V to a current of 0.05C. It was then discharged at 0.5C to a voltage of 3.0V, and the discharge energy was recorded.

[0081] Volumetric energy density = Discharge energy / (Length × Width × Thickness of lithium-ion battery).

[0082] Example 1:

[0083] Preparation of the positive electrode:

[0084] Using aluminum foil as the current collector for the positive electrode, lithium cobalt oxide, conductive agent Super P, and binder (PVDF, polyvinylidene fluoride) are dissolved in N-methylpyrrolidone (NMP) solution in a weight ratio of 96:2:2 to form a slurry for the positive electrode active material layer. This slurry is coated onto the current collector to obtain the positive electrode active material layer. After drying, cold pressing, and cutting, the positive electrode is obtained.

[0085] Preparation of the negative electrode:

[0086] Graphite, binder (SBR, styrene-butadiene rubber), and thickener (CMC, carboxymethyl cellulose) were dissolved in deionized water in 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.

[0087] Electrolyte preparation:

[0088] An organic solvent was prepared by mixing ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), propyl propionate (PP), and ethylene carbonate (VC) in a mass ratio of 20:30:20:28:2 under an environment with a water content of less than 10 ppm. Then, the electrolyte salt LiPF6 was added to the organic solvent and mixed thoroughly to obtain the basic electrolyte. The concentration of the electrolyte salt in the electrolyte solution was 1 mol / L.

[0089] Preparation of the separating membrane:

[0090] The separator film substrate is 8μm thick polyethylene (PE) as the separator film.

[0091] Preparation of lithium-ion batteries:

[0092] The prepared positive electrode, separator, and negative electrode are stacked in sequence to obtain an electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag and dehydrated at 80°C. The electrolyte prepared above is then injected, and the lithium-ion battery is obtained through vacuum sealing, settling, formation, degassing, and edge trimming. The upper limit of the formation voltage is 4.5V, the formation temperature is 70°C, and the formation settling time is 2 hours.

[0093] The preparation processes of each embodiment and comparative example differ from those of Example 1 only in Tables 1 and 2 below. Parameters for preparation processes not shown in the tables are the same as in Example 1. Each embodiment and comparative example uses a basic electrolyte or adds additives to the basic electrolyte. The added additives constitute 70% of the electrolyte by mass. The additives added in Examples 1 to 6 are carbonates, the specific types of which are shown in the tables below. Comparative Example 7 did not add carbonates, but instead added methyl acetate and ethyl acetate.

[0094] Table 1

[0095]

[0096]

[0097]

[0098] Table 2

[0099]

[0100]

[0101] As can be seen from Examples 1 to 10 and Comparative Examples 1 to 7, the positive electrode active material in Examples 1 to 10 includes lithium cobalt oxide or ternary materials, the modifying element M includes Al or Mg, X% is 0.1% to 0.4%, and the resistivity of the positive electrode active material layer is 10. -4 Ω·cm to 10 -2Ω·cm. The examples shown higher electrolyte consumption after 100 cycles, higher capacity retention after 800 cycles, and higher energy density. In contrast, Comparative Example 1 used lithium iron phosphate as the positive electrode active material, resulting in a lower volumetric energy density. Comparative Example 2 used Ti as the modifying element, resulting in lower cycle capacity retention and volumetric energy density. Comparative Example 3 had a lower content of the modifying element, resulting in lower cycle capacity retention. Comparative Example 4 had a higher content of the modifying element, resulting in lower volumetric energy density. Comparative Example 5 had a lower resistivity of the positive electrode active material layer, resulting in lower cycle capacity retention and volumetric energy density. Comparative Example 6 had a higher resistivity of the positive electrode active material layer, resulting in lower cycle capacity retention and volumetric energy density. Comparative Example 7 had no cyclic or chain carbonates in the electrolyte, resulting in lower cycle capacity retention. As can be seen, when lithium cobalt oxide or ternary materials are used as the positive electrode active material, and the modifying element M includes at least one of Al or Mg, with a mass percentage content of 0.1% to 0.4%, the resistivity of the positive electrode active material layer is 10 Ω·cm. -4 Ω·cm to 10 -2 When the electrolyte contains at least one of cyclic carbonate or linear carbonate, the electrochemical device can have good cycle performance and volumetric energy density. The absence of either one will have an adverse effect on the performance of the electrochemical device.

[0102] As shown in Examples 11 to 14, the cycle capacity retention and volumetric energy density in Examples 11 and 14 are lower than those in Examples 12 and 13. This may be because the specific surface area of ​​the positive electrode active material layer in Example 11 is too low, which affects lithium-ion transport, while the specific surface area of ​​the positive electrode active material layer in Example 11 is too large, which affects electrolyte consumption.

[0103] As shown in Examples 15 to 18, the cycle capacity retention and volumetric energy density in Examples 15 and 18 are lower than those in Examples 16 and 17. This may be because the bulk porosity of the positive electrode active material layer in Example 15 is lower, which is not conducive to ion transport in the positive electrode active material layer. In Example 18, the bulk porosity of the positive electrode active material layer is higher, which accelerates electrolyte consumption, affects cycle capacity retention, and reduces volumetric energy density.

[0104] As shown in Examples 19 to 22, the cycle capacity retention and volumetric energy density in Examples 21 and 22 are lower than those in Examples 19 and 20. This may be because the A / B ratio is lower in Example 21, and the interconnection between particles in the positive electrode active material layer requires the formation of a large amount of liquid film, which worsens electrolyte consumption. In Example 22, the A / B ratio is higher, and lithium ions in the electrolyte are enriched in the pores on the surface of the positive electrode active material layer, and surface side reactions of the positive electrode active material layer continue to occur, accelerating electrolyte consumption.

[0105] As shown in Examples 23 to 26, the cycle capacity retention rates in Examples 23 and 26 are lower than those in Examples 24 and 25. This may be because the pH value of the positive electrode active material layer in Example 23 is lower, which accelerates the dissolution of cobalt in the positive electrode active material under high voltage, thereby leading to a deterioration in the cycle performance of the electrochemical device. In Example 6, the pH value of the positive electrode active material layer is higher, which causes changes in the structure of the positive electrode active material and affects the cycle performance.

[0106] As shown in Examples 27 to 30, the cycle capacity retention rates in Examples 27 and 30 are lower than those in Examples 28 and 29. This may be because the binder content of the positive electrode active material layer in Example 27 is lower. Although this increases the proportion of positive electrode active material, the positive electrode active material layer is prone to detachment from the positive electrode current collector during cycling. In Example 30, the binder content of the positive electrode active material layer is higher, which reduces the proportion of positive electrode active material, decreases the volumetric energy density, and reduces conductivity, thus affecting cycle performance.

[0107] As shown in Examples 31 to 34, the cycle capacity retention and volumetric energy density in Examples 33 and 34 are lower than those in Examples 31 and 32. This may be because the C / D value is higher in Example 33, leading to a deterioration in the adhesion stability of the positive electrode active material layer on the positive electrode current collector. This may cause the positive electrode active material layer to partially detach from the current collector, increasing the contact area with the electrolyte and accelerating electrolyte consumption. In Example 32, the C / D value is lower, resulting in a deterioration in the impedance of the positive electrode active material layer, increased polarization of the electrochemical device, accelerated cobalt dissolution from the positive electrode active material, and increased side reactions of the positive electrode active material layer and electrolyte consumption.

[0108] As shown in Examples 35 to 38, the cycle capacity retention rate in Example 35 is lower than that in Examples 36 and 37, and the volumetric energy density in Example 38 is lower than that in Examples 36 and 37. This may be because the Dv50 of the positive electrode active material in Example 35 is smaller, increasing the contact area between the positive electrode active material and the electrolyte, thus accelerating electrolyte consumption. In Example 38, the Dv50 of the positive electrode active material is higher, increasing the lithium ion extraction pathway and reducing the specific capacity that can be utilized.

[0109] As shown in Examples 39 to 42, the cycle capacity retention rate in Examples 39 and 42 is lower than that in Examples 40 and 41. This may be because the silicon-carbon content is higher in Examples 39 and 42. Silicon-carbon expands more significantly in volume during cycling than graphite. The protective layer on the surface of the negative electrode active material is easily damaged, increasing the side reactions with the electrolyte and thus affecting the cycle performance.

[0110] As shown in Examples 43 to 46, the volumetric energy density in Example 43 is lower than that in Examples 44 and 45, and the cycle capacity retention rate in Example 46 is lower than that in Examples 44 and 45. This may be because the degree of graphitization is lower in Example 43, resulting in a reduction in the capacity that graphite can utilize. In Example 46, the degree of graphitization is higher, leading to increased electrolyte consumption during cycling, which in turn affects the cycling performance.

[0111] As shown in Examples 47 to 50, the cycle capacity retention rate in Example 47 is lower than that in Examples 48 and 49, and the volumetric energy density in Example 50 is lower than that in Examples 48 and 49. This may be because in Example 47, I D / I G The low SEI value indicates poor coating protection due to amorphous carbon, which may continuously damage the SEI film during cycling, increasing side reactions with the electrolyte and reducing cycling performance. In Example 49, I... D / I G A higher conductivity value reduces the conductivity of graphite, which is detrimental to the specific capacity that graphite can achieve.

[0112] As shown in Examples 51 to 54, the cycle capacity retention rate in Example 51 is lower than that in Examples 52 and 53, and the volumetric energy density in Example 54 is lower than that in Examples 52 and 53. This may be because the specific capacity of graphite in Example 51 is higher, which increases electrolyte consumption and reduces cycle performance. The specific capacity of graphite in Example 54 is lower, resulting in a decrease in volumetric energy density.

[0113] As shown in Examples 55 to 58, the cycle capacity retention and volumetric energy density in Example 55 are lower than those in Examples 56 and 57, and the cycle capacity retention in Example 58 is lower than that in Examples 56 and 57. This may be because the specific surface area of ​​the negative electrode active material layer is lower in Example 55, resulting in reduced kinetic performance of the negative electrode and consequently decreased volumetric energy density and cycle performance. In Example 58, the specific surface area of ​​the negative electrode active material layer is higher, leading to increased electrolyte consumption, which is detrimental to cycle performance.

[0114] 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 comprises: a positive electrode active material layer, the positive electrode active material layer comprising: a positive electrode active material; the positive electrode active material comprises at least one of lithium cobalt oxide or a ternary material, the positive electrode active material comprises a modifying element M, the modifying element M comprising 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 X%, where X is 0.1 to 0.4; the resistivity of the positive electrode active material layer is 10⁻⁶. -4 Ω·cm to 10 -2 Ω·cm. The electrolyte includes at least one of cyclic carbonates or linear carbonates.

2. The electrochemical device according to claim 1, wherein, It satisfies at least one of the following: (a) The specific surface area of ​​the positive electrode active material layer is Am2 / g, 0.2≤A≤2.0; (b) The bulk porosity of the positive electrode active material layer is B, 20% ≤ B ≤ 40%.

3. The electrochemical device according to claim 2, wherein, 0.8≤A / B≤8.

4. The electrochemical device according to claim 1, wherein, It satisfies at least one of the following: (c) The pH value of the positive electrode active material layer is C, 8≤C≤12; (d) The positive electrode active material layer includes: a binder; based on the total mass of the positive electrode active material layer, the mass percentage of the binder in the positive electrode active material layer is D, where 1% ≤ D ≤ 2%.

5. The electrochemical device according to claim 4, wherein, 500≤C / D≤1000.

6. The electrochemical device according to claim 1, wherein, After the electrochemical device has undergone 100 cycles, the percentage of electrolyte consumed is less than or equal to 10% × (1 - 10% × (0.5 - X) / 0.05).

7. The electrochemical device according to any one of claims 1 to 6, wherein, The particle size D of the positive electrode active material V 50 is: 12μm to 18μm.

8. The electrochemical device according to any one of claims 1 to 6, wherein, The negative electrode active material layer includes: a negative electrode active material; The negative electrode active material includes graphite; or, the negative electrode active material includes graphite and silicon-containing materials.

9. The electrochemical device according to claim 8, wherein, It satisfies at least one of the following: (e) The graphitization degree of the graphite is 88% to 93%; (f) The Raman test results of the graphite D / I G The value ranges from 0.1 to 0.

3. (g) The graphite content in the negative electrode active material is greater than or equal to 80% by mass; (h) The specific capacity of the graphite is 330 mAh / g to 350 mAh / g; (i) The specific surface area of ​​the negative electrode active material layer is: 1m 2 / g to 25m 2 / g.

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

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