Electrochemical and Electronic Devices

Abstract

The present invention provides an electrochemical device and an electronic device. The electrochemical device includes a negative electrode piece, the negative electrode piece includes a negative electrode material layer, the negative electrode material layer includes a negative electrode active material, and the negative electrode active material includes graphite. In the electrochemical device, d1 and d2 satisfy (d1 / d2-1)×100%≦0.56%, 3.3600≦d1≦3.3720, d1 Å is the lattice spacing of the (002) plane of the negative electrode active material obtained by baking the negative electrode material layer, measured by an X-ray diffraction device, and d2 Å is the lattice spacing of the (002) plane of the negative electrode active material in the negative electrode material layer, measured by an X-ray diffraction device. In the electrochemical device provided by the present invention, the negative electrode piece has a low expansion rate during cycling, so the electrochemical device also has a low expansion rate during cycling.

Description

[Technical Field]

[0001] This invention claims priority to a Chinese patent application filed with the China Patent Office on October 18, 2021, bearing application number 202111209294.5 and titled "Electrochemical Device and Electronic Device," the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to the field of electrochemical technology, and more particularly to electrochemical and electronic devices. [Background technology]

[0003] Lithium-ion batteries have advantages such as high energy storage density, high open-circuit voltage, low self-discharge rate, long cycle life, and good safety, and are widely used in various fields such as portable electrical energy storage, electronic devices, and electric vehicles.

[0004] As lithium-ion battery technology continues to develop, the charging speed of lithium-ion batteries is becoming increasingly faster. However, due to the inherent properties of lithium-ion battery materials, expansion can occur during cycling, particularly in the negative electrode strips, which have a high expansion rate. A high expansion rate can cause deformation of the lithium-ion battery and even lead to cycle failure of the lithium-ion battery. Furthermore, a high cycle expansion rate can easily cause deformation in the final electronic product or shorten the battery life, and can also pose a safety risk. Therefore, how to reduce the expansion rate of lithium-ion batteries during cycling is an issue that needs to be resolved urgently. Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention aims to provide electrochemical and electronic devices that reduce the expansion rate during cycling of the electrochemical devices. [Means for solving the problem]

[0006] In a first aspect of the present invention, there is provided an electrochemical device comprising a negative electrode piece, the negative electrode piece comprising a negative electrode material layer, the negative electrode material layer comprising a negative electrode active material, the negative electrode active material comprising graphite, wherein d1 and d2 in the electrochemical device satisfy (d1 / d2-1)×100%≦0.56%, 3.3600≦d1≦3.3720, where d1Å is the lattice spacing of the (002) plane of the negative electrode active material obtained by baking the negative electrode material layer, and is measured by an X-ray diffractometer, and d2Å is the lattice spacing of the (002) plane of the negative electrode active material in the negative electrode material layer, and is measured by an X-ray diffractometer. By controlling the lattice spacing d1 of the (002) plane of the negative electrode active material powder in the electrochemical device and the lattice spacing d2 of the (002) plane of the negative electrode active material in the negative electrode material layer to satisfy the above relationship and to satisfy the above range, the degree of expansion of the negative electrode active material during cycling is reduced, and the expansion rate of the negative electrode piece during cycling can be effectively reduced, thereby reducing the expansion rate of the electrochemical device during cycling.

[0007] In one embodiment of the present invention, 3.3460≦d2≦3.3700. By making d2 fall within this range, the expansion rate of the negative electrode piece during cycling can be effectively reduced, and thus the expansion rate of the electrochemical device can be reduced.

[0008] In one embodiment of the present invention, the negative electrode active material includes primary particles and secondary particles formed by aggregation of the primary particles. By controlling the particle types in the negative electrode active material to satisfy the above-mentioned limitations, it is possible to alleviate stress generated during the lithium absorption / desorption process, improve particle crushing due to stress generated by lithium absorption / desorption, and improve the electrochemical performance of the electrochemical device.

[0009] In one embodiment of the present invention, the negative electrode active material satisfies at least one of the following conditions based on the number of particles of the negative electrode active material: (a) the percentage of primary particles is 1% to 50% by number, and (b) the percentage of secondary particles is 50% to 99% by number. By controlling the content of the primary particles and / or secondary particles to fall within the above range, particle crushing during the lithium absorption / desorption process can be effectively reduced, and the energy density of the electrochemical device can be improved.

[0010] In one embodiment of the present invention, amorphous carbon is present on the surface of the negative electrode active material, which is advantageous in reducing stress during the lithium absorption / desorption process, thereby reducing particle fracture and reducing the expansion rate during cycling of the electrochemical device.

[0011] In one embodiment of the present invention, the mass percentage of amorphous carbon is 0.1% to 5% based on the mass of the negative electrode active material.

[0012] In one embodiment of the present invention, when the density of the negative electrode material layer is X g / cc and the porosity of the negative electrode material layer is Y, X and Y satisfy Y + 53.47X ≥ 125. Controlling the density and porosity of the negative electrode material layer to satisfy the above formula is advantageous for the transport of lithium ions, and can improve the electrochemical performance of the electrochemical device.

[0013] In one embodiment of the present invention, the negative electrode piece satisfies at least one of the following: (c) the density of the negative electrode material layer is 1.4 g / cc to 1.75 g / cc; (d) the electrical conductivity of the negative electrode material layer is 15 S / cm or higher; (e) the porosity of the negative electrode material layer is 25% to 50%; and (f) the graphite contains at least one type selected from the group consisting of artificial graphite and natural graphite. By controlling the density of the negative electrode material layer within the above range, the expansion rate of the negative electrode piece during cycling can be reduced. By controlling the electrical conductivity of the negative electrode material layer within the above range, it is advantageous for the bonding of lithium ions and electrons and can effectively improve the lithium deposition phenomenon in the negative electrode piece. By controlling the porosity of the negative electrode material layer within the above range, it is possible to improve the electrochemical performance of the electrochemical device. By controlling the type of graphite within the above range, it is advantageous for improving the electrochemical performance of the electrochemical device.

[0014] In one embodiment of the present invention, the electrochemical device satisfies L1-L2≦135 μm, where L1 is the thickness of the electrochemical device when fully charged and L2 is the thickness of the electrochemical device when fully discharged.

[0015] In one embodiment of the present invention, the electrochemical device further includes a positive electrode piece, the positive electrode piece including a positive electrode material layer, where d3 μm is the thickness of the positive electrode material layer and d4 μm is the thickness of the negative electrode material layer, and the relationship d4 / d3 satisfies 1.2≦d4 / d3≦1.6. Controlling the thickness relationship between the positive electrode material layer and the negative electrode material layer to fall within this range is advantageous for the absorption and desorption of lithium ions in the positive electrode and the negative electrode, and can reduce the expansion rate during cycling of the electrochemical device.

[0016] In one embodiment of the present invention, the density of the positive electrode material layer is 3.85 g / cc to 4.25 g / cc.

[0017] In one embodiment of the present invention, the electrochemical device further includes an electrolyte solution, and the electrolyte solution includes a carboxylic acid ester, and the carboxylic acid ester includes at least one selected from the group consisting of ethyl acetate, propyl acetate, propyl propionate, and ethyl propionate.

[0018] In one embodiment of the present invention, the mass percentage of the carboxylic acid ester is 5% to 55% based on the mass of the electrolyte solution.

[0019] In one embodiment of the present invention, the electrolyte further comprises fluoroethylene carbonate and 1,3-propane sultone.

[0020] In one embodiment of the present invention, the mass percentage of fluoroethylene carbonate is greater than the mass percentage of 1,3-propane sultone, based on the mass of the electrolyte.

[0021] In one embodiment of the present invention, the mass percentage of fluoroethylene carbonate is 1% to 10% based on the mass of the electrolyte solution.

[0022] In one embodiment of the present invention, the mass percentage of 1,3-propane sultone is 0.1% to 4% based on the mass of the electrolyte.

[0023] In one embodiment of the present invention, the electrolytic solution further contains lithium difluorophosphate, and the mass percentage of lithium difluorophosphate is 0.001% to 0.9% based on the mass of the electrolytic solution.

[0024] In one embodiment of the present invention, the electrolyte solution comprises 1,3-propane sultone and lithium difluorophosphate, and the mass percentage of the 1,3-propane sultone is greater than the mass percentage of the lithium difluorophosphate.

[0025] In a second aspect of the present invention, there is provided an electronic device comprising an electrochemical device according to an embodiment of the present invention.

[0026] Other aspects and advantages of embodiments of the invention will be set forth in part in the description that follows, and in part will be illustrated by, or may be learned by, the practice of embodiments of the invention. [Brief explanation of the drawings]

[0027] In order to more clearly explain the embodiments of the present invention and the technical solutions of the prior art, the drawings necessary for use in the embodiments and the prior art will be briefly described below. Obviously, the drawings described below are only some embodiments of the present invention, and those skilled in the art can obtain other drawings based on these drawings without requiring creative work.

[0028] [Figure 1] FIG. 1 is an X-ray diffraction spectrum of the negative electrode active material in Example 1 of the present invention.

[0029] [Figure 2] FIG. 2 is a scanning electron microscope image of negative electrode active material particles containing a mixture of primary particles and secondary particles in an example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0030] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be described in more detail below with reference to the drawings and examples. Of course, the described examples are only some of the examples of the present invention, and are not all of the examples. All other examples that can be obtained by those skilled in the art based on the examples of the present invention without requiring creative work fall within the scope of protection of the present invention.

[0031] In the specific embodiment of the present invention, the present invention will be described using a lithium ion battery as an example of an electrochemical device, but the electrochemical device of the present invention is not limited to only a lithium ion battery.

[0032] In a first aspect of the present invention, there is provided an electrochemical device comprising a negative electrode piece, the negative electrode piece comprising a negative electrode material layer, the negative electrode material layer comprising a negative electrode active material, the negative electrode active material comprising graphite, wherein d1 and d2 satisfy (d1 / d2-1)×100%≦0.56% and 3.3600≦d1≦3.3720, wherein d1Å is the lattice spacing of the (002) plane of the negative electrode active material obtained by baking the negative electrode material layer, as measured by an X-ray diffraction apparatus, and d2Å is the lattice spacing of the (002) plane of the negative electrode active material in the negative electrode material layer, as measured by the X-ray diffraction apparatus. For example, the value of (d1 / d2-1)×100% may be 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.56%, or any range therebetween, such as 0.05%≦(d1 / d2-1)×100%≦0.56% or (d1 / d2-1)×100%≦0.4%. The value of d1 may be 3.3600, 3.3620, 3.3640, 3.3660, 3.3680, 3.3700, 3.3720, or any range therebetween, such as 3.3640≦d1≦3.3720. Here, d2 refers to the lattice spacing of the (002) plane of the negative electrode active material in a negative electrode piece processed at a pressure of 9.8 KN to 980 KN. In the present invention, the magnitude of the force applied during the pressing process (e.g., cold pressing) is determined based on the weight of the negative electrode material layer per unit area and the thickness of the negative electrode material layer, and the magnitude of d2 is controlled by controlling the magnitude of the force applied during the pressing process. For example, when the pressure is controlled to 88.2 KN, the value of d2 is 3.354. The magnitude of d2 can also be controlled in combination with controlling the pressing time. For example, the cold pressing speed may be controlled to 8 m / min to 20 m / min to achieve the objectives of the present invention. In the present invention, the aforementioned calcination treatment refers to calcination at 400°C for 4 hours in an air atmosphere.

[0033] Without being limited by any theory, the present invention proposes that if the value of (d1 / d2-1) × 100% is too large (e.g., greater than 0.56%), i.e., if the lattice spacing of the (002) plane of the negative electrode active material in the pressed negative electrode pieces changes significantly compared to the lattice spacing of the (002) plane of the negative electrode active material powder, the stress change occurring during the lithium absorption / desorption process will also be large, increasing the expansion rate of the negative electrode active material during the lithium absorption / desorption process and further causing particle fracture, which will affect the cycle performance of the electrochemical device. d1 reflects the lattice spacing of the (002) plane of the negative electrode active material before pressing. If d1 is too small (e.g., less than 3.3600 Å), there will be more contact between particles during the lithium absorption / desorption process, which will increase the stress change and increase the expansion rate of the negative electrode pieces, thereby increasing the expansion rate during the cycle of the electrochemical device. If d1 is too large (e.g., greater than 3.3720 Å), the relationship (d1 / d2-1)×100%≦0.56% cannot be satisfied, which may result in an increased expansion rate during cycling of the electrochemical device. Therefore, when the lattice spacing d1 of the (002) plane of the negative active material in the electrochemical device and the lattice spacing d2 of the (002) plane of the negative active material in the negative electrode layer satisfy the above relationship and d1 is within the above range, the expansion rate of the negative electrode piece during cycling can be effectively reduced, thereby reducing the expansion rate of the electrochemical device during cycling and the low-temperature impedance of the electrochemical device.

[0034] In one embodiment of the present invention, the lattice spacing d2 of the (002) plane of the negative electrode active material in the negative electrode layer satisfies the relationship 3.3460≦d2≦3.3700. For example, d2 may be 3.3460, 3.3500, 3.3540, 3.3580, 3.3600, 3.3620, 3.3640, 3.3660, 3.3680, or 3.3700, or any range therebetween. Without being limited by any theory, when d2 is within the above range, the expansion rate of the negative electrode piece during cycling can be effectively reduced, thereby reducing the expansion rate of the electrochemical device during cycling. When the negative electrode piece is subjected to a pressing process (e.g., cold pressing), the magnitude of the pressing pressure has an effect on d2. For example, as the pressure increases, d2 first decreases and then stabilizes.

[0035] In one embodiment of the present invention, the negative electrode active material comprises primary particles and secondary particles formed by aggregation of the primary particles. When the negative electrode active material of the present invention comprises the above primary particles and secondary particles, it is advantageous for alleviating stress generated during the lithium absorption / desorption process, reducing particle fracture due to stress generated during the lithium absorption / desorption process, and improving the electrochemical performance of the electrochemical device. Figure 2 shows a scanning electron microscope photograph of the negative electrode active material in one embodiment of the present invention. In Figure 2, the negative electrode active material comprises primary particles and secondary particles. It can be seen from Figure 2 that the primary particles have a different appearance from the secondary particles and that the particle diameter of the primary particles is smaller than the particle diameter of the secondary particles.

[0036] In the present invention, primary particles refer to particles obtained by graphitizing particles having a certain particle size distribution obtained by crushing and separating precursor particles. For example, the primary particles may include graphite primary particles, and the secondary particles may include graphite secondary particles. In the present invention, the particle size distribution is not particularly limited as long as the objective of the present invention is achieved. For example, the particle size distribution is 2 μm to 100 μm. In the present invention, the aggregation method by which primary particles aggregate to form secondary particles is not particularly limited as long as the objective of the present invention is achieved. For example, the secondary particles are obtained by crushing primary particles, mixing them with a certain ratio of asphalt, and graphitizing them by heat treatment using a granulator so that the particles are stuck together via the asphalt. Here, the mass ratio of the primary particles to asphalt is not particularly limited as long as the objective of the present invention is achieved. For example, the mass ratio of the primary particles to asphalt is 95:5 to 80:20. The heat treatment temperature is not particularly limited as long as the objective of the present invention is achieved. For example, the heat treatment temperature is 400°C to 600°C.

[0037] In one embodiment of the present invention, the negative electrode active material satisfies at least one of the following conditions based on the number of particles of the negative electrode active material: (a) the percentage of primary particles is 1% to 50%; and (b) the percentage of secondary particles is 50% to 99%. For example, the percentage of primary particles may be 1%, 10%, 20%, 30%, 40%, 50%, or any range therebetween. The percentage of secondary particles may be 50%, 60%, 70%, 80%, 90%, 99%, or any range therebetween. Without being limited by any theory, controlling the content of primary particles and the content of secondary particles to fall within the above ranges can effectively alleviate the problem of particle breakage during the lithium absorption / desorption process and improve the energy density of the electrochemical device.

[0038] In one embodiment of the present invention, amorphous carbon is present on the surface of the negative electrode active material. Amorphous carbon may be present on a portion of the surface of the negative electrode active material particles, or on the entire surface of the negative electrode active material particles. Without being limited by any theory, amorphous carbon can form a proximal shell layer on the surface of the negative electrode active material, thereby creating a binding effect on the negative electrode active material and reducing the volumetric expansion of the negative electrode active material. This reduces contact between particles and also reduces stress generated during the lithium absorption / desorption process. This reduces particle fracture and reduces the expansion rate during the cycling of the electrochemical device. At the same time, if the proximal shell layer formed by amorphous carbon has holes, the negative electrode active material can easily react with the electrolyte in the electrochemical device to form a more stable solid electrolyte interface (SEI), effectively preventing by-products from entering the negative electrode material layer, reducing particle fracture due to the influence of by-products, and further alleviating the expansion problem during the cycling of the electrochemical device.

[0039] In one embodiment of the present invention, the mass percentage of amorphous carbon is 0.1% to 5%, preferably 1% to 3%, based on the total mass of the negative electrode active material. For example, the mass percentage of amorphous carbon may be 0.1%, 0.3%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 4%, or 5%, or any range therebetween. Without being limited by any theory, a too low mass percentage of amorphous carbon (e.g., below 0.1%) is less effective in improving the performance of the negative electrode active material. As the mass percentage of amorphous carbon increases, the short-range disorder formed by the amorphous carbon is unfavorable for electron transport, resulting in a certain decrease in the electrical conductivity of the negative electrode material layer. However, the short-range disorder formed by the amorphous carbon is favorable for lithium ion transport, resulting in a lower DC impedance and improved lithium deposition in electrochemical devices. Furthermore, when the degree of disorder of the amorphous carbon increases with the type of amorphous carbon, it further affects electron transport but improves lithium ion transport. When the degree of disorder of the amorphous carbon decreases, it improves electron transport but affects lithium ion transport. If the mass percentage of amorphous carbon is too high (e.g., higher than 5%), electrons cannot be transported normally, resulting in a deterioration of DC impedance and serious lithium deposition in electrochemical devices. Therefore, controlling the mass percentage of amorphous carbon within the above range is advantageous for rapid lithium ion absorption / desorption, generates a stable SEI, easily prevents the accumulation of by-products, reduces the expansion rate of electrochemical devices, and improves lithium deposition in electrochemical devices.

[0040] In the present invention, amorphous carbon can be obtained by carbonizing an organic material. The type of organic material is not particularly limited as long as the object of the present invention can be achieved. For example, the organic material may include at least one selected from the group consisting of citric acid, asphalt, heavy oil, furfural resin, epoxy resin, and phenolic resin, but is not limited thereto.

[0041] In one embodiment of the present invention, when the density of the negative electrode material layer is X g / cc and the porosity of the negative electrode material layer is Y, X and Y satisfy the relationship Y + 53.47X ≥ 125. Without being limited by any theory, controlling the density and porosity of the negative electrode material layer to satisfy the above formula is advantageous for the transport of lithium ions and can improve the electrochemical performance of the electrochemical device. In the present invention, the density of the negative electrode material layer refers to the ratio of the coating weight of the negative electrode active material per unit area in the negative electrode material layer to the thickness of the negative electrode material layer.

[0042] In one embodiment of the present invention, the negative electrode piece satisfies at least one of the following: (c) the density of the negative electrode material layer is 1.4 g / cc to 1.75 g / cc, (d) the electrical conductivity of the negative electrode material layer is 15 S / cm or more, (e) the porosity of the negative electrode material layer is 25% to 50%, and (f) the graphite contains at least one type selected from the group consisting of artificial graphite and natural graphite. For example, the density of the negative electrode material layer may be 1.4 g / cc, 1.45 g / cc, 1.5 g / cc, 1.55 g / cc, 1.6 g / cc, 1.65 g / cc, 1.7 g / cc, 1.75 g / cc, or any range therebetween. The electrical conductivity of the negative electrode layer may be 15 S / cm, 25 S / cm, 35 S / cm, 45 S / cm, 55 S / cm, 65 S / cm, 75 S / cm, or 85 S / cm, or any range therebetween. The porosity of the negative electrode layer may be 25%, 30%, 35%, 40%, 45%, or 50%, or any range therebetween. In the present invention, the density of the negative electrode layer refers to the density when treated at a pressure of 9.8 KN to 980 KN, and the porosity of the negative electrode layer refers to the porosity of the negative electrode layer in a negative electrode piece obtained by fully discharging and disassembling an electrochemical device after 200 or fewer charge-discharge cycles.

[0043] Without being limited by any theory, the present invention contemplates that a negative electrode layer with a density that is too low (e.g., less than 1.4 g / cc) is detrimental to improving the energy density of a lithium-ion battery. A negative electrode layer with a density that is too high (e.g., greater than 1.75 g / cc) increases particle-to-particle contact, which in turn increases the stress generated during the lithium absorption / desorption process, resulting in an increase in the expansion rate of the negative electrode piece during cycling. Therefore, when the density of the negative electrode layer is within the above range, the expansion rate of the negative electrode piece during cycling can be reduced, thereby reducing the expansion rate of the electrochemical device during cycling and providing a high energy density. When the electrical conductivity of the negative electrode layer is too low (e.g., less than 15 S / cm), electrons cannot be transported normally, resulting in a deterioration in DC impedance and a serious lithium deposition phenomenon in the electrochemical device. Therefore, when the electrical conductivity of the negative electrode layer is within the above range, it is favorable for the bonding of lithium ions and electrons, effectively improving the lithium deposition phenomenon in the electrochemical device. If the porosity of the negative electrode layer is too small (e.g., less than 25%), it is unfavorable for the transport of lithium ions, affecting the electrochemical performance of the electrochemical device. If the porosity of the negative electrode layer is too large (e.g., greater than 50%), it affects the contact between the negative electrode active material particles in the negative electrode layer, reducing the mechanical performance of the negative electrode layer and likely affecting the electrical performance of the electrochemical device during cycling. If the type of graphite is within the above range, it is advantageous for improving the electrochemical performance of the electrochemical device.

[0044] In one embodiment of the present invention, the electrochemical device satisfies L1-L2≦135 μm, where L1 is the thickness of the electrochemical device when fully charged and L2 is the thickness of the electrochemical device when fully discharged. The fact that the electrochemical device satisfies L1-L2≦135 μm explains the low expansion coefficient of the electrochemical device provided by the present invention. Here, L1 is the thickness of the electrochemical device including one electrode assembly when fully charged, and L2 is the thickness of the electrochemical device including one electrode assembly when fully discharged. In the present invention, the voltages at full charge and full discharge can be selected according to the specific electrochemical device. For example, the full charge voltage can be 4.45 V, and the full discharge voltage can be 3.0 V. The electrode assembly referred to in the present invention can include a positive electrode piece, a negative electrode piece, and a separator. Those skilled in the art should understand that the above description is merely an example and does not limit the scope of protection of the present invention.

[0045] In one embodiment of the present invention, the electrochemical device further includes a positive electrode piece, the positive electrode piece including a positive electrode material layer, and where the thickness of the positive electrode material layer is d3 μm and the thickness of the negative electrode material layer is d4 μm, the ratio d4 / d3 satisfies 1.2≦d4 / d3≦1.6. For example, the value of d4 / d3 may be 1.2, 1.3, 1.4, 1.5, 1.6, or any range therebetween. The density of the positive electrode material layer is 3.85 g / cc to 4.25 g / cc. For example, the density of the positive electrode material layer may be 3.85 g / cc, 3.9 g / cc, 3.95 g / cc, 4 g / cc, 4.05 g / cc, 4.1 g / cc, 4.15 g / cc, 4.2 g / cc, 4.25 g / cc, or any range therebetween. Without being limited by any theory, it has been found that when the thickness relationship between the positive electrode material layer and the negative electrode material layer and the density range of the positive electrode material layer are within the above ranges, it is advantageous for the absorption and desorption of lithium ions in the positive electrode and the negative electrode, and the expansion rate during the cycling process of the electrochemical device can be reduced. When the positive electrode piece or the negative electrode piece is subjected to a pressing process (e.g., cold pressing), it has been found that the magnitude of the pressing pressure affects the thickness and density of the positive electrode material layer or the negative electrode material layer. For the same thickness, the greater the density of the positive electrode material layer and the negative electrode material layer, the greater the pressing pressure.

[0046] In the present invention, the thickness d3 of the positive electrode material layer and the thickness d4 of the negative electrode material layer are not particularly limited as long as the object of the present invention can be achieved. For example, the thickness d3 of the positive electrode material layer is 30 μm to 120 μm, and the thickness d4 of the negative electrode material layer is 30 μm to 120 μm. In the present invention, the thickness d3 of the positive electrode material layer and the thickness d4 of the negative electrode material layer are the thicknesses of the positive electrode material layer provided on one side of the positive electrode current collector and the negative electrode material layer provided on one side of the negative electrode current collector, respectively.

[0047] In one embodiment of the present invention, the thickness of the negative electrode pieces obtained by fully discharging and disassembling the electrochemical device is defined as MMC0, and the thickness of the negative electrode pieces obtained by fully discharging and disassembling the electrochemical device after 50 charge-discharge cycles is defined as MMC. x In this case, (MMC x -MMC0)MMC0 × 100%≦6.5%. For example, (MMC x The value of -MMC0)MMC0 x 100% may be 1%, 2%, 3%, 4%, 5%, 6%, 6.5%, or any range therebetween. The electrochemical device provided by the present invention exhibits a small expansion rate of the negative electrode piece after multiple cycles, which indicates that the expansion rate of the electrochemical device is also small.

[0048] In one embodiment of the present invention, W1 denotes the full width at half maximum of the (002) diffraction peak of the negative electrode active material in the negative electrode pieces obtained by fully discharging and disassembling the electrochemical device, and W2 denotes the full width at half maximum of the (002) diffraction peak of the negative electrode active material in the negative electrode pieces obtained by fully discharging and disassembling the electrochemical device after 50 charge-discharge cycles. W1 / W2≦1.06. For example, the value of W1 / W2 may be 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, or any range therebetween. Without being limited by theory, if the value of W1 / W2 is too large (e.g., greater than 1.06), the density change of the negative electrode material layer after multiple cycles of the electrochemical device will be too large. This indicates that the volume change of the negative electrode pieces during the lithium absorption / desorption process is large, i.e., the expansion rate during the cycling of the electrochemical device is large. Therefore, when the value of W1 / W2 satisfies the above range, the expansion rate during the cycle process of the electrochemical device is small.

[0049] The graphite preparation process of the present invention is not particularly limited and is well known to those skilled in the art. For example, the graphite preparation process may include, but is not limited to, grinding a graphite precursor to a certain particle size using a grinding device, then placing the ground particles in a classifier to obtain particles having a certain particle size range, and then graphitizing them in a graphitization furnace to obtain the desired graphite. In the present invention, the particle size and type of the graphite precursor are not particularly limited as long as the objectives of the present invention are achieved. For example, the average particle size of the graphite precursor is 5 μm to 15 μm, and the graphite precursor may include at least one selected from the group consisting of petroleum coke, needle coke, and sponge coke, but are not limited to these. In the present invention, the graphitization temperature, graphitization time, and graphitization equipment are not particularly limited as long as the objectives of the present invention are achieved. For example, the graphitization temperature is 2500°C to 3200°C, the graphitization time is 18 to 30 days, and the graphitization equipment may be, but is not limited to, an Acheson furnace, an LWG furnace, or a continuous graphitization furnace.

[0050] In the present invention, amorphous carbon may be present on the surface of the negative electrode active material. The method for preparing such a negative electrode active material having amorphous carbon on its surface is not particularly limited and is well known to those skilled in the art. For example, the method for preparing such a negative electrode active material having amorphous carbon on its surface may include, but is not limited to, mixing the negative electrode active material with an amorphous carbon precursor and carbonizing the mixture in a carbonization furnace to obtain a negative electrode active material having amorphous carbon on its surface. Here, the mass percentage of the amorphous carbon precursor is 0.5% to 15% based on the total mass of the negative electrode active material and the amorphous carbon precursor. It can be seen that as the content of the amorphous carbon precursor added during the preparation process increases, the amorphous carbon content in the prepared negative electrode active material first increases and then stabilizes. In the present invention, the carbonization temperature and carbonization time are not particularly limited as long as the object of the present invention can be achieved. For example, the carbonization temperature is 500°C to 1300°C, and the carbonization time is 1 to 5 days. In the present invention, the type of amorphous carbon precursor is not particularly limited as long as the object of the present invention can be achieved. For example, the amorphous carbon precursor may contain at least one selected from the group consisting of citric acid, asphalt, heavy oil, furfural resin, epoxy resin, and phenol resin, but is not limited thereto.

[0051] In the present invention, the negative electrode piece further includes a negative electrode current collector. Here, the negative electrode current collector is not particularly limited as long as it can achieve the objectives of the present invention. For example, the negative electrode current collector may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, and composite current collectors. In the present invention, the thickness of the negative electrode current collector layer is not particularly limited as long as it can achieve the objectives of the present invention. For example, the thickness of the negative electrode current collector layer is 4 μm to 12 μm.

[0052] In the present invention, the negative electrode material layer may further contain a conductive agent. The conductive agent is not particularly limited as long as it can achieve the object of the present invention. For example, the conductive agent may contain at least one selected from the group consisting of conductive carbon black (SuperP), carbon nanotubes (CNTs), carbon fiber, flake graphite, Ketjen black, and graphene. The carbon nanotubes may contain at least one selected from the group consisting of single-walled carbon nanotubes and multi-walled carbon nanotubes. The carbon fiber may contain at least one selected from the group consisting of vapor-grown carbon fiber (VGCF) and carbon nanofiber.

[0053] In the present invention, the negative electrode material layer may further contain a binder. In the present invention, the binder is not particularly limited as long as the object of the present invention can be achieved. For example, the binder may contain at least one selected from the group consisting of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, polyimide, polyamide-imide, styrene-butadiene rubber, and polyvinylidene fluoride.

[0054] The negative electrode piece may optionally further include a conductive layer, which is located between the negative electrode current collector and the negative electrode material layer. The composition of the conductive layer is not particularly limited and may be any conductive layer commonly used in this field. For example, the conductive layer may include the conductive agent and the binder.

[0055] The positive electrode piece of the present invention further includes a positive electrode current collector. The positive electrode current collector is not particularly limited as long as it can achieve the objectives of the present invention. For example, the positive electrode current collector may include aluminum foil, aluminum alloy foil, or a composite current collector. In the present invention, the thickness of the positive electrode current collector is not particularly limited as long as it can achieve the objectives of the present invention. For example, the thickness of the positive electrode current collector is 8 μm to 12 μm.

[0056] In the present invention, the positive electrode layer includes a positive electrode active material. The positive electrode active material is not particularly limited as long as it achieves the objectives of the present invention. For example, the positive electrode active material may include at least one selected from the group consisting of composite oxides of lithium and transition metal elements. The transition metal element is not particularly limited as long as it achieves the objectives of the present invention. For example, the transition metal element may include at least one selected from the group consisting of nickel, manganese, cobalt, and iron. Specifically, the positive electrode active material may include at least one selected from the group consisting of lithium nickel cobalt manganese oxide (811, 622, 523, 111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide, lithium manganese oxide, lithium iron manganese phosphate, and lithium titanate.

[0057] The positive electrode piece may further include a conductive layer, which is located between the positive electrode current collector and the positive electrode material layer. The composition of the conductive layer is not particularly limited and may be any conductive layer commonly used in this field. For example, the conductive layer may include the conductive agent and the binder.

[0058] The electrochemical device of the present invention further includes a separator. In the present invention, the separator is not particularly limited as long as the objectives of the present invention can be achieved. For example, the separator material may include, but is not limited to, at least one selected from the group consisting of polyethylene (PE), polypropylene (PP), polytetrafluoroethylene-based polyolefins (PO), polyesters (e.g., polyethylene terephthalate (PET)), cellulose, polyimide (PI), polyamide (PA), spandex, and aramid. The separator type may include, but is not limited to, at least one selected from the group consisting of woven films, nonwoven films, microporous membranes, composite films, separator paper, rolled films, and spun films. The separator of the present invention may have a porous structure. The pore size is not particularly limited as long as the objectives of the present invention can be achieved. For example, the pore size is 0.01 μm to 1 μm. In the present invention, the separator thickness is not particularly limited as long as the objectives of the present invention can be achieved. For example, the thickness of the separator is 5 μm to 500 μm.

[0059] For example, the separator may include a substrate layer and a surface treatment layer. The substrate layer may be a porous nonwoven fabric, film, or composite film, and the material of the substrate layer may include at least one material selected from the group consisting of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Optionally, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite film may be used. Optionally, a surface treatment layer is provided on at least one surface of the substrate layer, and the surface treatment layer may be a polymer layer or an inorganic layer, or may be a layer formed by mixing a polymer and an inorganic material.

[0060] For example, the inorganic layer contains inorganic particles and a binder. The inorganic particles are not particularly limited and may be, for example, at least one selected from the group consisting of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. The binder is not particularly limited and may be, for example, at least one selected from the group consisting of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The polymer layer contains a polymer. The polymer material includes at least one selected from the group consisting of polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride, and poly(vinylidene fluoride-hexafluoropropylene).

[0061] The electrochemical device of the present invention further comprises an electrolyte solution, and the electrolyte solution comprises a carboxylic acid ester, and the carboxylic acid ester comprises at least one selected from the group consisting of ethyl acetate, propyl acetate, propyl propionate, and ethyl propionate.

[0062] In one embodiment of the present invention, the mass percentage of the carboxylic acid ester is 5% to 55% based on the mass of the electrolyte. For example, the mass percentage of the carboxylic acid ester may be 5%, 8%, 10%, 15%, 30%, 40%, 50%, 55%, or any range therebetween. The electrolyte contains the above carboxylic acid ester, and when (d1 / d2-1)×100%≦0.56% and 3.3600≦d1≦3.3720, it is advantageous for the electrolyte to penetrate the negative electrode pieces. Furthermore, when the content of the carboxylic acid ester is within the above range, the rate performance of the electrochemical device can be further improved.

[0063] In one embodiment of the present invention, the electrolyte further comprises fluoroethylene carbonate (FEC) and 1,3-propane sultone (PS). When the electrolyte comprises fluoroethylene carbonate and 1,3-propane sultone, and the ratios (d1 / d2-1)×100%≦0.56% and 3.3600≦d1≦3.3720, the cooperation of the fluoroethylene carbonate and 1,3-propane sultone can form a protective film on the surface of the negative electrode active material. During the cycling of the electrochemical device, a new protective film can be formed on the crushed negative electrode active material particles, reducing the occurrence of side reactions, reducing the amount of gas produced by the electrochemical device, improving the cycling performance of the electrochemical device, and reducing the impedance of the electrochemical device.

[0064] In one embodiment of the present invention, the mass percentage of fluoroethylene carbonate is greater than the mass percentage of 1,3-propane sultone, based on the mass of the electrolyte. When the mass percentage of fluoroethylene carbonate is greater than the mass percentage of 1,3-propane sultone, the composition and structure of the protective film formed on the surface of the negative electrode active material particles are better, and the electrochemical device can have better cycle performance and rate performance.

[0065] In one embodiment of the present invention, the mass percentage of fluoroethylene carbonate is 1% to 10% based on the mass of the electrolyte solution.

[0066] In one embodiment of the present invention, the mass percentage of 1,3-propane sultone is 0.1% to 4% based on the mass of the electrolyte.

[0067] In one embodiment of the present invention, the electrolyte further contains lithium difluorophosphate, and the mass percentage of lithium difluorophosphate is 0.001% to 0.9%, preferably 0.01% to 0.9%, and more preferably 0.1% to 0.8%, based on the mass of the electrolyte. When the electrolyte contains lithium difluorophosphate, the diversity of the protective film components on the surface of the negative electrode active material particles is increased, and the impedance of the protective film is reduced, thereby improving the cycle performance and rate performance of the electrochemical device and reducing the impedance of the electrochemical device.

[0068] In one embodiment of the present invention, the electrolyte solution comprises 1,3-propane sultone and lithium difluorophosphate, and the mass percentage of the 1,3-propane sultone is greater than the mass percentage of the lithium difluorophosphate.

[0069] In one embodiment of the present invention, the electrolyte includes fluoroethylene carbonate, 1,3-propane sultone, and lithium difluorophosphate.

[0070] The electrolyte of the electrochemical device of the present invention includes a lithium salt and a nonaqueous solvent. In some embodiments of the present invention, the lithium salt may include at least one selected from the group consisting of LiPF, LiBF, LiClO, LiB(CH), LiCHSO, LiCFSO, LiN(SOCF), LiC(SOCF), LiSiF, LiBOB, and lithium difluoroborate. For example, LiPF can be used as the lithium salt. In the present invention, the mass percentage of the lithium salt in the electrolyte is not particularly limited and can be selected according to actual needs as long as the objectives of the present invention can be achieved.

[0071] The non-aqueous solvent may be dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinyl ethylene carbonate (VEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, trifluoromethylethylene carbonate, and combinations thereof. In the present invention, the mass percentage of the non-aqueous solvent in the electrolyte is not particularly limited, and can be selected according to actual needs as long as the object of the present invention can be achieved.

[0072] The preparation process for the electrochemical device is not particularly limited and may be a preparation process well known to those skilled in the art. For example, the electrochemical device may be produced by stacking a positive electrode piece, a separator, and a negative electrode piece in that order, rolling or folding them as necessary, placing them in a case, injecting an electrolyte into the case, and sealing it. Furthermore, to prevent pressure buildup inside the electrochemical device and overcharging and discharging, an overcurrent protection element, lead plates, etc. may be placed in the case as needed.

[0073] In a second aspect of the present invention, there is provided an electronic device comprising an electrochemical device according to the above embodiments of the present invention.

[0074] The electronic device of the present invention is not particularly limited and may be used in any electronic device known in the art. In some embodiments, the electronic device may include, but is not limited to, a laptop computer, a pen-input computer, a mobile computer, an electronic book player, a mobile phone, a portable facsimile machine, a portable copier, a portable printer, a stereo headset, a video recorder, an LCD television, a portable vacuum cleaner, a portable CD player, a mini CD, a walkie-talkie, an electronic notebook, a calculator, a memory card, a portable tape recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, a bicycle assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flashlight, a camera, a large household storage battery, and a lithium ion capacitor.

[0075] The present invention provides an electrochemical device and an electronic device, the electrochemical device comprising a negative electrode piece, the negative electrode piece comprising a negative electrode material layer, the negative electrode material layer comprising a negative electrode active material, wherein the lattice spacing d1 of the (002) plane of the negative electrode active material and the lattice spacing d2 of the (002) plane of the negative electrode active material in the negative electrode material layer satisfy (d1 / d2-1)×100%≦0.56% and 3.3600nm≦d1≦3.3720nm, thereby effectively reducing the expansion rate of the negative electrode piece during cycling of the electrochemical device, and thereby reducing the expansion rate of the electrochemical device during cycling.

[0076] Example Measurement method and equipment: Measurement of lattice spacing and full width at half maximum (FWHM) of (002) plane: The lattice spacing of the (002) plane of the negative electrode active material or the negative electrode material layer was measured using a Bruker D8 ADVANCE X-ray diffractometer (XRD). The measurement target was CuKα, the voltage was 40 kV, the current was 40 mA, the scanning angle range was 5° to 80°, the scanning step size was 0.00836°, and the time per step was 0.3 s.

[0077] The lattice spacing is λ / 2 / sin θ, where θ is the angle at the position of the maximum peak intensity of the (002) peak, and λ is the wavelength.

[0078] FWHM is the full width at 50% between the minimum and maximum peak intensity points of the (002) diffraction peak.

[0079] 1 shows the X-ray diffraction spectrum of the negative electrode active material in Example 1 of the present invention. In the spectrum, the diffraction peak is the diffraction peak of the (002) plane of the negative electrode active material, with a corresponding θ value of 26.4 and a full width at half maximum of 0.285.

[0080] The lattice spacing d1 of the negative electrode active material powder was measured as follows. The lithium-ion battery was discharged to a voltage of 3.0 V and disassembled to obtain anode pieces. The anode material layer was peeled from the anode current collector and placed in a box furnace in an air atmosphere and fired at 400°C for 4 hours to obtain anode active material. The lattice spacing and full width at half maximum of the anode active material were measured using the above-mentioned measurement methods.

[0081] The lattice spacing d2 of the negative electrode active material in the negative electrode material layer was measured as follows. The discharge process for full discharge was as follows: The battery was discharged at 0.5 C to the theoretical minimum voltage of the lithium-ion battery, with a voltage change of ≦0.05 V, and then allowed to stand for 30 minutes. The battery was then discharged at 0.2 C to the minimum voltage, and then allowed to stand for 30 minutes. The battery was then disassembled to obtain negative electrode pieces. The negative electrode layer was then peeled from the negative electrode current collector, and the lattice spacing and full width at half maximum of the negative electrode active material in the negative electrode layer were measured using the methods described above.

[0082] Determination of amorphous carbon content: The mass of amorphous carbon on the surface of the negative electrode active material was measured using a thermogravimetric analyzer (model STA449F3-QMS403C, manufactured by NETZSCH GmbH, Germany). The specific measurement method was as follows. An appropriate amount of the negative electrode active material containing amorphous carbon was placed in a crucible and heated in an air atmosphere from 30°C to 1200°C at a heating rate of 5°C / min to determine the activation reaction temperature of the negative electrode active material (i.e., the reaction temperature T1 at which the mass loss of the negative electrode active material begins). After determining the activation reaction temperature, an appropriate amount of the negative electrode active material containing amorphous carbon was weighed and placed in a crucible. The temperature was then raised in a nitrogen gas atmosphere from 30°C to T1 at a heating rate of 5°C / min. The mass at this temperature was recorded as m0. At this temperature, the atmosphere was switched to air, and the sample mass measured by the thermogravimetric analyzer was recorded as m1 until it no longer changed. The reaction time required was usually 12 hours or less, preferably 6 hours. The resulting mass loss was the mass of amorphous carbon, m=m0-m1.

[0083] Measurement of porosity of the negative electrode material layer: The lithium ion battery was discharged until the voltage reached a fully discharged state, and then disassembled to obtain negative electrode pieces. The negative electrode material layer was prepared into a complete disk. Each sample had a volume of approximately 0.35 cm. 3 For each example or comparative example, 30 samples were measured. The porosity of the negative electrode material layer was measured based on the standard "GB / T24586-2009 Measurement of apparent density, true density and porosity of iron ore."

[0084] Measurement of electrical conductivity of the negative electrode material layer: The electrical conductivity of the negative electrode layer was measured by a four-probe membrane impedance measurement method. The measurement process involved pressing the tip of a probe against the negative electrode layer to be measured, and measuring the electrical conductivity of the negative electrode layer.

[0085] Measurement of Lithium-ion Liquid Phase Transfer Impedance (Rion): The lithium-ion battery was connected to an electrochemical measurement system (Bio-Logic VMP3B, manufactured by Bio-Logic, France) to measure the lithium-ion liquid-phase transfer impedance. The measurement frequency range was 30 mHz to 50 kHz, and the amplitude was 5 mV. Data was collected and the lithium-ion liquid-phase transfer impedance value was obtained.

[0086] Measurement of 2C discharge capacity retention rate of lithium-ion batteries: The lithium-ion battery was left standing at 25°C for 5 minutes, then charged at a constant current of 0.7C to 4.45V, then charged at a constant voltage of 0.05C from 4.45V, left standing for 5 minutes, and then discharged at a constant current of 0.5C to 3.0V, and left standing for 5 minutes. The above charge / discharge process was repeated, discharging at 0.1C and recording the 0.1C discharge capacity of the lithium-ion battery, and then discharging at 2C and recording the 2C discharge capacity of the lithium-ion battery. The 2C discharge capacity retention of the lithium-ion battery was calculated using the following formula.

number

[0087] Judgment on lithium deposition phenomenon in lithium-ion batteries: The lithium-ion batteries were left standing at 25°C for 5 minutes. According to the settings of the Examples and Comparative Examples, the lithium-ion batteries of the Examples and Comparative Examples were charged at a constant current of 2C to 4.45V, then charged at a constant voltage of 0.05C from 4.45V, and then left standing for 5 minutes. They were then discharged at a constant current of 0.5C to a fully discharged state (3.0V), and then left standing for 5 minutes. The above charge / discharge process was repeated 10 times. The batteries were fully charged and disassembled under dry conditions, and the state of the negative electrode pieces was photographed.

[0088] The lithium-ion batteries were left standing at 0°C for 5 minutes. According to the settings of the example and comparative example, the lithium-ion batteries of the example and comparative example were charged at a constant current of 0.8 C to 4.45 V, then charged at a constant voltage of 0.05 C from 4.45 V, and then left standing for 5 minutes. They were then discharged at a constant current of 0.5 C to a fully discharged state (3.0 V), and then left standing for 5 minutes. The above charge / discharge process was repeated 10 times. The batteries were fully charged and disassembled under dry conditions, and the state of the negative electrode pieces was photographed.

[0089] The degree of lithium deposition in lithium-ion batteries was judged according to the following criteria.

[0090] When the disassembled negative electrode pieces were golden overall with only a small amount of gray observed, and the area of ​​the gray areas was less than 2% of the total area of ​​the negative electrode pieces, it was determined that no lithium had been deposited.

[0091] When most of the disassembled negative electrode pieces were gold in color, gray areas were observed, and the area of ​​the gray areas accounted for 2% to 20% of the total area of ​​the negative electrode pieces, it was determined that slight lithium deposition had occurred.

[0092] When the disassembled negative electrode pieces were gray overall with some gold observed and the area of ​​the gray region accounted for 20% to 60% of the total area of ​​the negative electrode pieces, it was determined that lithium deposition had occurred.

[0093] When the disassembled negative electrode pieces were gray overall and the area of ​​the gray region was more than 60% of the total area of ​​the negative electrode pieces, it was determined that there was severe lithium deposition.

[0094] Measurement of cycle swelling rate of lithium-ion batteries: The lithium-ion battery was left standing at 25°C for 5 minutes, then charged at a constant current of 0.7C until it reached a fully charged state (4.45V), then charged at a constant voltage of 0.05C from the fully charged state, and then left standing for 5 minutes. The thickness of the lithium-ion battery was measured at three locations using the MMC (Maximum Material Condition) measurement method, the average value was calculated, and this was designated as MMC0. The lithium-ion battery was then discharged at a constant current of 0.5C until it reached a fully discharged state, and then left standing for 5 minutes. The above charge-discharge cycle was repeated 50 times, and the thickness of the lithium-ion battery was measured at three locations for each charge-discharge cycle, the average value was calculated, and this was designated as MMC0. x (x is the number of cycles).

[0095] The cycle expansion rate of the lithium ion battery at 25°C was calculated using the following formula.

number

[0096] The MMC measurement method was as follows: a micrometer measuring device (Mitutoyo Corporation, model: MDC-25SX) was used to measure the thickness of the negative electrode tab of the lithium-ion battery, and measurements were taken at three different positions for each sample. The average value was taken and recorded as the MMC thickness.

[0097] Measuring the DC Impedance (DCR) of a Li-ion Battery: At 25°C, the lithium-ion battery was charged at a constant current of 1.5 C to 4.45 V, then at a constant voltage of 0.05 C from 4.45 V, and then allowed to stand for 30 minutes. It was discharged at 0.1 C for 10 seconds, and the voltage value was recorded as U1. It was discharged at 1 C for 360 seconds, and the voltage value was recorded as U2. This charge-discharge procedure was repeated five times.

[0098] The DC impedance R of the lithium-ion battery at 25°C was calculated using the following formula. Here, "1C" is the current value that completely discharges the capacity of the lithium-ion battery in 1 hour.

number

[0099] The DC impedance of the lithium ion battery at 0°C was measured in the same manner as described above, except that the operating temperature was 0°C.

[0100] In the present invention, unless otherwise specified, DCR refers to the direct current impedance when the state of charge (SOC) of the lithium ion battery is 10%.

[0101] Example 1-1 <Preparation of negative electrode active material> The graphite precursor (needle coke) was placed in a jaw crusher and pulverized to particles with an average particle diameter Dv50 of 1 mm. The pulverized particles (needle coke) were then pulverized and classified using a jet mill to obtain particles with an average particle diameter Dv50 of 5 μm to 12 μm and Dv99≦30 μm. The pulverized particles were placed in an LWG furnace and graphitized at a graphitization temperature of 3000°C for 20 days to obtain primary particles of the negative electrode active material.

[0102] Preparation of secondary particles: In the above process, particles with a particle size Dv50 of 8 μm were obtained by classifying the pulverized precursor, and these particles were mixed with asphalt at a mass ratio of 10:1. Then, the mixture was placed in a graphitization furnace and graphitized at a graphitization temperature of 3000°C for 20 days to obtain secondary particles of negative electrode active material.

[0103] The graphitized primary particles and the graphitized secondary particles were mixed in a ratio of 3:7 to obtain a composite negative electrode active material of Example 1-1.

[0104] <Preparation of negative electrode pieces> The negative electrode active material obtained above, styrene butadiene rubber, and sodium carboxymethyl cellulose were mixed in a mass ratio of 95:2:3 with deionized water as a solvent and thoroughly stirred to prepare a slurry with a solids content of 45 wt%. The slurry was then uniformly mixed. The slurry was uniformly applied to both surfaces of a negative electrode current collector copper foil, which had previously been coated with a 1.5 μm-thick carbon black coating layer. The negative electrode current collector had a thickness of 10 μm. The resultant was dried at 110°C to obtain negative electrode pieces coated with negative electrode material layers on both sides. The coating layer thickness on one side of the negative electrode material layer was 90 μm. The negative electrode pieces were then pressed at a pressure of 100 kN. The pressing speed was 14 m / min, the density of the negative electrode material layer was 1.40 g / cc, and the average thickness d4 μm of the pressed negative electrode material layer on one side was 76.5 μm. The negative electrode pieces were cut into 74 mm x 867 mm pieces for use in the following steps. The density of the negative electrode material layer was controlled by controlling the magnitude of the pressure applied during the pressing process, that is, the density of the negative electrode material layer can reflect the magnitude of the pressure applied during the pressing process.

[0105] <Preparation of positive electrode piece> Lithium cobalt oxide (positive electrode active material), acetylene black, and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 96:2:2 and N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry with a solids content of 50 wt%. The slurry was then uniformly mixed. The slurry was uniformly applied to both surfaces of a 10 μm-thick aluminum foil positive electrode current collector, dried at 90°C, and cold-pressed to obtain a positive electrode piece coated with the positive electrode active material on both sides. The thickness of the coating layer on one side of the positive electrode material layer was 80 μm. The pressing process was then performed with a pressure of 100 kN. The density of the negative electrode material layer was 3.95 g / cc, and the average thickness (d3 μm) of the pressed negative electrode material layer on one side was 52 μm. The positive electrode piece was cut to a size of 74 mm x 867 mm and tabs were welded for use in the following steps.

[0106] <Preparation of electrolyte> Under an argon atmosphere with a water content of less than 10 ppm, organic solvents ethylene carbonate (EC), diethyl carbonate (DEC), and propylene carbonate (PC) were mixed in a mass ratio of EC:DEC:PC = 3:4:3, and lithium hexafluorophosphate (LiPF6) was dissolved in the organic solvent and mixed uniformly. Fluoroethylene carbonate and succinonitrile were then added and dissolved to obtain an electrolyte. Based on the mass of the electrolyte, the mass percentages of LiPF6, fluoroethylene carbonate, and succinonitrile were 12.5%, 3%, and 2%, respectively, and the remainder was organic solvent.

[0107] <Preparation of separator> A 7 μm thick polyethylene (PE) film (manufactured by Celgard) was used.

[0108] <Preparation of lithium-ion batteries> The positive electrode pieces, separator, and negative electrode pieces obtained above were stacked in order, with the separator interposed between the positive electrode pieces and the negative electrode pieces to serve as insulators, and then wound up to obtain an electrode assembly. The electrode assembly was placed in an aluminum plastic film packaging bag and dried, after which an electrolyte solution was injected, and the assembly was subjected to processes such as vacuum sealing, standing, formation, degassing, and trimming to obtain a lithium-ion battery.

[0109] In Examples 1-2 to 1-4, the same procedures as in Example 1-1 were carried out, except that the pressure applied to the negative electrode pieces during the pressing process was adjusted so that d2 and the density of the negative electrode material layer were as shown in Table 1.

[0110] In Example 1-5, the negative electrode active material obtained in the preparation step of <Preparation of negative electrode active material> was mixed with asphalt, which was an amorphous carbon precursor, and then placed in a carbonization furnace and carbonized at 800°C for 8 hours to obtain a negative electrode active material containing amorphous carbon. The mass percentage of amorphous carbon was 0.5%. The same procedure as in Example 1-1 was followed, except that in the preparation step of <Preparation of negative electrode pieces>, the graphite negative electrode active material was replaced with a negative electrode active material containing amorphous carbon.

[0111] In Examples 1-6 to 1-8, the same procedures as in Example 1-5 were carried out except that the magnitude of the pressure applied to the negative electrode pieces in the pressing treatment was adjusted so that d2 was as shown in Table 1.

[0112] In Examples 1-9 to 1-16, the same procedures as in Example 1-5 were carried out except that the average particle size of the graphite precursor and the graphitization temperature were adjusted and the magnitude of the pressure applied to the negative electrode pieces in the press treatment was adjusted so that d1 and d2 were as shown in Table 1.

[0113] In Examples 2-1 to 2-14, the same procedures as in Example 1-6 were carried out except that the type of amorphous carbon precursor and the mass percentage of amorphous carbon were adjusted according to Table 2.

[0114] In Example 3-1, the graphite obtained in the preparation step of <Preparation of Negative Electrode Active Material> in Example 1-1 was used as primary particles. The primary particles were mixed with asphalt in a mass ratio of 10:1 and then carbonized in a carbonization furnace to obtain carbonized primary particles. Here, the carbonization temperature was 1000°C, and the mixture was kept at 1000°C for 8 hours. The carbonized primary particles were mixed with the secondary particles prepared in Example 1-1 in a mass ratio of 3:7 to obtain a composite negative electrode active material. In the preparation step of <Preparation of Negative Electrode Pieces>, when preparing the negative electrode material layer, the coating layer on one side of the composite negative electrode active material had a thickness of 90 μm. In the preparation step of <Preparation of Positive Electrode Pieces>, when preparing the positive electrode material layer, the coating layer thickness of the positive electrode material layer on one side of the positive electrode active material was 80 μm, and after pressing the positive electrode piece and the negative electrode piece, the average thickness d4 μm of the negative electrode material layer on the pressed side was 62.5 μm, and the average thickness d3 μm of the positive electrode material layer on the pressed side was 52 μm.Otherwise, the same procedures as in Example 1-1 were carried out.

[0115] In Examples 3-2 and 3-3, the same procedures as in Example 3-1 were carried out except that the magnitude of the pressure applied to the negative electrode pieces during the pressing process was adjusted so that the density of the negative electrode material layer, the density of the positive electrode material layer, the value of d3, the value of d4, and the value of d4 / d3 were as shown in Table 3, and the percentage of the number of primary particles and the percentage of the number of secondary particles were adjusted so that they were as shown in Table 3.

[0116] In Examples 3-4 to 3-12, the same procedures as in Example 3-1 were carried out except that the magnitude of the pressure applied to the positive and negative electrode pieces during the press treatment was adjusted so that the density of the negative electrode material layer, the density of the positive electrode material layer, the value of d3, the value of d4, and the value of d4 / d3 were as shown in Table 3, and the percentage of the number of primary particles and the percentage of the number of secondary particles were adjusted so as to be as shown in Table 3.

[0117] In Examples 4-1 to 4-20, the same procedures as in Example 1-5 were carried out except that the preparation parameters of the electrolytic solution were adjusted to be as shown in Table 4.

[0118] In Comparative Example 1-1, the same procedures as in Example 1-1 were carried out, except that the average particle size of the graphite precursor and the graphitization temperature were adjusted according to Table 1 so that d1 was as shown in Table 1.

[0119] In Comparative Example 1-2, the same procedures as in Example 1-8 were carried out, except that the average particle size of the graphite precursor and the graphitization temperature were adjusted according to Table 1 so that d1 was as shown in Table 1.

[0120] The preparation parameters and performance measurements for each example and comparative example are shown in Tables 1 to 4.

[0121] [Table 1]

[0122] [Table 2]

[0123] [Table 3]

[0124] [Table 4]

[0125] According to Table 1, as can be seen from Examples 1-1 to 1-16 and Comparative Examples 1 to 2, when the value of (d1 / d2-1)×100% and the value of d1 are within the ranges of the present invention, the volume change during cycling of the lithium ion battery can be reduced, that is, the expansion rate during cycling of the lithium ion battery can be reduced, and the lithium ion battery obtained by the present invention has excellent expansion performance.

[0126] As can be seen from Examples 1-1 to 1-4, when d1 remains constant, as d2 increases, the density of the negative electrode material layer tends to gradually decrease, and the volume change during the cycling process of the lithium-ion battery tends to decrease; that is, the expansion rate during the cycling process of the lithium-ion battery decreases. As can be seen from Examples 1-5, 1-9, and 1-13, when the density of the negative electrode material layer remains constant, as d1 increases, d2 increases, and the volume change during the cycling process of the lithium-ion battery tends to decrease. As can be seen from the above, controlling d1 and d2 to fall within the ranges of the present invention is advantageous for improving the expansion performance of the lithium-ion battery.

[0127] As can be seen from Example 1 and Comparative Example 1, when the value of (d1 / d2-1) × 100% is within the range of the present invention but d1 is not within the range of the present invention, the resulting lithium-ion battery exhibits a slight improvement in lithium deposition, but still experiences a large volume change during cycling, i.e., a large expansion rate during cycling. As can be seen from Examples 1-8 and Comparative Example 2, when the values ​​of d1 and (d1 / d2-1) × 100% are not within the range of the present invention, the resulting lithium-ion battery exhibits a large volume change during cycling and an increased degree of lithium deposition, i.e., a large expansion rate during cycling, resulting in poor cycling performance. This indicates that lithium-ion batteries with excellent expansion and cycling performance can only be obtained when the values ​​of (d1 / d2-1) × 100% and d1 simultaneously satisfy the ranges of the present invention.

[0128] According to Table 2, as can be seen from Examples 1-6 and 2-1 to 2-14, when amorphous carbon is present on the surface of the negative electrode active material, the resulting lithium ion battery simultaneously has low impedance and high capacity retention, and the lithium deposition phenomenon is improved.

[0129] As can be seen from Examples 1-6 and 2-1 to 2-5, when the mass percentage of amorphous carbon is 0.1% to 5%, the electrical conductivity of the negative electrode piece is reduced to a certain extent, DC impedance is reduced, and the lithium deposition phenomenon and capacity retention rate of the lithium ion battery are improved. Comparing Examples 2-4 and 2-5, when the amorphous carbon content is 5%, the electrical conductivity of the negative electrode material layer is significantly reduced and the capacity retention rate is also affected to a certain extent. As can be seen from Examples 1-6 and 2-1 to 2-14, amorphous carbon can be prepared using different precursors, and all of them can improve the lithium deposition phenomenon and capacity retention rate of the lithium ion battery.

[0130] According to Table 3, as can be seen from Examples 3-1 to 3-10, when the percentage of the number of primary particles and the percentage of the number of secondary particles are within the ranges of the present invention, the liquid phase transfer impedance of the obtained lithium ion battery is small, which is favorable for the bonding between lithium ions and electrons, and therefore the electrochemical performance of the lithium ion battery can be improved.

[0131] As can be seen from Examples 3-1 to 3-12, when the value of d4 / d3 and the density of the negative electrode material layer are within the ranges of the present invention, the liquid phase transfer impedance of the obtained lithium ion battery is small, which is advantageous for improving the electrochemical performance of the lithium ion battery.

[0132] As can be seen from Examples 3-1 to 3-3, when the percentage of the number of primary particles and the percentage of the number of secondary particles remain unchanged, as the value of d4 / d3 increases, the liquid phase transfer impedance tends to decrease, and the porosity of the negative electrode pieces after 50 cycles tends to increase. By controlling d4 / d3 to fall within the range of the present invention, negative electrode pieces with appropriate porosity can be obtained, which is advantageous for improving the electrochemical performance and kinetic performance of lithium ion batteries.

[0133] As can be seen from Examples 3-4 to 3-6, when the density of the negative electrode material layer, the density of the positive electrode material layer, and the value of d4 / d3 remain unchanged, as the percentage of the number of primary particles increases, the value of W1 / W2 increases, and the porosity of the negative electrode pieces after 50 cycles tends to decrease, which is beneficial to improving the electrochemical and kinetic performance of lithium-ion batteries.

[0134] As can be seen from Examples 3-2, 3-4, and 3-6, when the value of d4 / d3 remains constant, the liquid phase transfer impedance increases slightly as the percentage of primary particles increases, and the porosity of the negative electrode pieces after 50 cycles tends to increase first and then decrease. In other words, by controlling the percentage of primary particles and / or the percentage of secondary particles to fall within the range of the present invention, it is possible to obtain negative electrode pieces with a suitable porosity while still maintaining a low liquid phase transfer impedance, which is beneficial to improving the electrochemical and kinetic performance of lithium-ion batteries.

[0135] As can be seen from Examples 3-1 to 3-3 and 3-11, when the density and d4 / d3 values ​​of the negative electrode layer are not within the ranges of the present invention, the liquid phase transfer impedance of the lithium ion battery increases sharply, affecting the electrochemical performance of the lithium ion battery. As can be seen from Examples 3-1 to 3-3 and 3-12, when the density and d4 / d3 values ​​of the negative electrode layer are not within the ranges of the present invention, the porosity of the negative electrode piece increases sharply after 50 cycles, also affecting the electrochemical performance of the lithium ion battery. As can be seen from the above, controlling the density and d4 / d3 values ​​of the negative electrode layer to fall within the ranges of the present invention can effectively improve the liquid phase transfer impedance of the lithium ion battery, further improving the electrochemical and kinetic performance of the lithium ion battery.

[0136] The components of an electrolyte solution generally affect the performance of a lithium-ion battery. As can be seen from Table 4 and Examples 1-5 and 4-1 to 4-20, when an electrolyte solution contains a carboxylic acid ester and the content of the carboxylic acid ester is within an appropriate range, the 2C capacity retention rate of the lithium-ion battery can be improved. When the electrolyte solution further contains PS or lithium difluorophosphate, the impedance and lithium deposition phenomenon of the lithium-ion battery can be further improved.

[0137] The above is merely a preferred embodiment of the present invention, and is not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made without departing from the spirit and principles of the present invention are within the scope of protection of the present invention.

Claims

1. 1. An electrochemical device comprising: the electrochemical device includes a negative electrode piece, the negative electrode piece includes a negative electrode material layer, the negative electrode material layer includes a negative electrode active material, and the negative electrode active material includes graphite; The negative electrode active material is d １ and d ２ But (d １ / d ２ −1) × 100%≦0.56%, 3.3600≦d １ ≦3.3720, d １ Å is the lattice spacing of the (002) plane of the negative electrode active material obtained by baking the negative electrode material layer, and is measured with an X-ray diffraction device; d ２ Å is the lattice spacing of the (002) plane of the negative electrode active material in the negative electrode material layer, and is measured with an X-ray diffraction device; the negative electrode active material includes primary particles and secondary particles formed by aggregation of the primary particles, Based on the number of particles of the negative electrode active material, the negative electrode active material is (a) the number percentage of the primary particles is 1% to 50%; (b) the number percentage of the secondary particles is 50% to 99%; An electrochemical device that satisfies at least one of the above.

2. The above d ２ is 3.3460≦d ２ 2. The electrochemical device of claim 1, wherein the electrochemical device satisfies ≦3.3700.

3. Amorphous carbon is present on the surface of the negative electrode active material, 2. The electrochemical device according to claim 1, wherein the mass percentage of the amorphous carbon is 0.1% to 5% based on the mass of the negative electrode active material.

4. 2. The electrochemical device according to claim 1, wherein, when the density of the negative electrode material layer is X g / cc and the porosity of the negative electrode material layer is Y, X and Y satisfy Y+53.47X≧125.

5. The negative electrode piece is (c) the density of the negative electrode material layer is 1.4 g / cc to 1.75 g / cc; (d) the negative electrode layer has an electrical conductivity of 15 S / cm or more; (e) the porosity of the negative electrode material layer is 25% to 50%; (f) the graphite comprises at least one selected from the group consisting of artificial graphite and natural graphite; At least one of the following is satisfied: The electrochemical device of claim 1 .

6. The electrochemical device is １ -L ２ ≦135 μm, and L １ is the thickness of the electrochemical device when fully charged, and L ２ 10. The electrochemical device of claim 1, wherein: is the thickness of the electrochemical device when fully discharged.

7. The electrochemical device further includes a positive electrode piece, the positive electrode piece including a positive electrode material layer, and the thickness of the positive electrode material layer is d ３ μm, and the thickness of the negative electrode material layer is d ４ When μm is used, 1.2≦d ４ / d ３ 10. The electrochemical device of claim 1, wherein the ρ is ≦1.

6.

8. The electrochemical device further includes an electrolyte solution, the electrolyte solution comprising: (i) the electrolytic solution contains a carboxylic acid ester, the carboxylic acid ester containing at least one selected from the group consisting of ethyl acetate, propyl acetate, propyl propionate, and ethyl propionate, and the mass percentage of the carboxylic acid ester is 5% to 55% based on the mass of the electrolytic solution; (ii) the electrolytic solution further contains fluoroethylene carbonate and 1,3-propane sultone, and the mass percentage of the fluoroethylene carbonate is greater than the mass percentage of the 1,3-propane sultone, based on the mass of the electrolytic solution; (iii) the mass percentage of fluoroethylene carbonate is 1% to 10% and the mass percentage of 1,3-propane sultone is 0.1% to 4% based on the mass of the electrolyte; (iv) the electrolyte solution further contains lithium difluorophosphate, and the mass percentage of the lithium difluorophosphate is 0.001% to 0.9% based on the mass of the electrolyte solution; At least one of the following is satisfied: The electrochemical device of claim 1 .

9. An electronic device comprising the electrochemical device according to any one of claims 1 to 8.

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