Negative active material and preparation method thereof, battery, battery pack and electronic equipment

Abstract

The invention provides a negative active material and a preparation method thereof, a battery, a battery pack and electronic equipment. The negative electrode active material comprises a hard carbon body, a transition region and a carbon coating layer; the transition area is located on at least part of the outer surface of the hard carbon body, and the carbon coating layer is located on at least part of the outer surface of the transition area; and the transition region is a mixed phase of the hard carbon body and the carbon coating layer. The negative electrode active material has excellent dynamic performance, side reaction is not easy to occur between the negative electrode active material and electrolyte when the negative electrode active material is applied to the battery, and the rate capability and the cycle performance of the battery can be improved by the negative electrode active material.

Description

Technical Field

[0001] The present invention relates to the field of electrochemical technology, and in particular to a negative electrode active material and a preparation method thereof, a battery, a battery pack and an electronic device. Background Art

[0002] With the rapid development of the new energy vehicle industry, demand for lithium batteries has also increased. However, global lithium reserves are limited and unevenly distributed. Consequently, the development of new, high-performance, and low-cost alternative energy storage devices has attracted considerable attention. Sodium-ion batteries (Na-ion batteries) share similar operating principles with Li-ion batteries. Furthermore, Na is much more abundant in the Earth's crust than Li, making them a promising alternative to Li-ion batteries.

[0003] Anode active materials are a key factor affecting the performance of sodium-ion batteries. Currently, hard carbon materials are commonly used as anode active materials for sodium-ion batteries. However, existing hard carbon materials exhibit sluggish sodium storage kinetics, which is detrimental to the battery's rate performance. Furthermore, during the battery's charge and discharge processes, the hard carbon materials experience intense side reactions with the electrolyte, which negatively impacts the battery's cycling performance. Summary of the Invention

[0004] In view of this, the present invention provides a negative electrode active material, which has excellent kinetic properties and is not prone to side reactions with the electrolyte when used in a battery. The negative electrode active material can improve the rate performance and cycle performance of the battery.

[0005] The present invention provides a method for preparing the above-mentioned negative electrode active material. The preparation method has a simple process and can successfully prepare the above-mentioned negative electrode active material.

[0006] The present invention also provides a battery, which includes the above-mentioned negative electrode active material, so that the battery has higher rate performance and cycle performance.

[0007] The present invention also provides a battery pack comprising the above-mentioned battery. The battery pack has excellent rate performance and cycle performance and is suitable for wide promotion and application.

[0008] The present invention also provides an electronic device, which includes the above-mentioned battery, and therefore has the advantages of excellent fast charging performance and long battery life.

[0009] In detail, in a first aspect, the present invention provides a negative electrode active material comprising a hard carbon body, a transition region, and a carbon coating layer;

[0010] The transition zone is located on at least a portion of the outer surface of the hard carbon body, and the carbon coating layer is located on at least a portion of the outer surface of the transition zone;

[0011] The transition zone is a mixed phase of the hard carbon body and the carbon coating layer.

[0012] The negative electrode active material as described above, wherein the interlayer spacings among the hard carbon body, the transition region, and the carbon coating layer gradually increase or decrease.

[0013] The negative electrode active material as described above, wherein the interlayer spacing of the transition region gradually increases or decreases in a direction away from the hard carbon body.

[0014] The negative electrode active material as described above, wherein the negative electrode active material includes at least one of the following characteristics:

[0015] a. The interlayer spacing of the hard carbon body is 0.362-0.386 nm;

[0016] b. The interlayer spacing of the carbon coating layer is 0.35-0.4 nm;

[0017] c. The thickness of the carbon coating layer is 1-6 nm;

[0018] d. The thickness of the transition zone is 2-5 nm.

[0019] The negative electrode active material as described above, wherein the surface of the negative electrode active material has N regions, N ≥ 2;

[0020] In the Raman spectra of the N regions, the intensity of peak D is d The intensity of the G peak I g The ratio of satisfies the normal distribution;

[0021] In the Raman spectrum, the D peak is at a wavelength of 1345 cm -1 -1355cm -1 The characteristic peak of G peak is at a wavelength of 1575cm -1 -1585cm -1 characteristic peaks.

[0022] The negative electrode active material as described above, wherein, in the Raman spectra of the N regions,

[0023] The intensity of peak D I d The intensity of the G peak I g The range of the ratio is ≤1.2; and / or the intensity of the D peak is I d The intensity of the G peak I g The standard deviation of the ratio is ≤0.2.

[0024] The negative electrode active material as described above, wherein the negative electrode active material includes at least one of the following characteristics:

[0025] a. The specific surface area of ​​the negative electrode active material is ≤5m 2 / g;

[0026] b. The D50 of the negative electrode active material is 4-7 μm;

[0027] c. The true density of the negative electrode active material is 1.7-2.1 g / m 3 ;

[0028] d. The powder conductivity of the negative electrode active material is ≥1.5S / mm.

[0029] In a second aspect, the present invention provides a method for preparing the negative electrode active material as described above, characterized in that it comprises:

[0030] The hard carbon precursor material is subjected to pre-carbonization treatment, activation treatment and carbonization treatment in sequence to obtain a hard carbon body, wherein the specific surface area of ​​the hard carbon body is 5-100m 2 / g;

[0031] The negative electrode active material is obtained by calcining a raw material system including a hard carbon body and a coating agent.

[0032] The preparation method as described above, wherein the carbonization treatment is carried out at a temperature of 1000-1600°C and a time of 2-8 hours;

[0033] And / or, during the calcination process, the temperature is 850-1200° C. and the time is 2-8 hours.

[0034] In the preparation method as described above, the mass percentage of the coating agent in the raw material system is 1-10%.

[0035] In a third aspect, the present invention provides a battery comprising the negative electrode active material as described above.

[0036] The battery as described above, wherein the battery includes at least one of the following features:

[0037] a. In the charge and discharge curve of the battery, the slope area ratio is 20-80%;

[0038] b. In the diffusion coefficient curve of the battery, the diffusion coefficient in the slope area is 10 -8 -10 -6 cm 2 / s; and / or, the diffusion coefficient of the plateau region is 10 -11 -10 -9 cm / s; preferably, the diffusion coefficient of the slope region is 10 -7 -10 -6 cm 2 / s; and / or, the diffusion coefficient of the plateau region is 10-10 -10 -9 cm 2 / s.

[0039] In a fourth aspect, the present invention provides a battery pack comprising the battery described above.

[0040] In a fifth aspect, the present invention provides an electronic device comprising the battery or the battery pack as described above.

[0041] The negative electrode active material provided by the present invention includes a hard carbon body, a transition zone and a carbon coating layer in order from the inside out, and the transition zone is a mixed phase of the hard carbon body and the carbon coating layer. Since the transition zone between the hard carbon body and the carbon coating layer is a mixed phase of the carbon coating layer and the hard carbon body, the interfacial stress between the hard carbon body and the carbon coating layer can be reduced, the stability of the negative electrode active material and the ion migration rate can be improved, and the cycle performance and rate performance of the battery can be improved; and the transition zone and the carbon coating layer can protect the surface of the hard carbon body, reduce the defect exposure points on the surface of the hard carbon body, effectively prevent the electrolyte from directly contacting the hard carbon body, reduce the probability of oxidative decomposition reaction between the electrolyte and the hard carbon body, and improve the cycle performance of the battery. Therefore, when the negative electrode active material of the present invention is applied to a battery, it can improve the rate performance and cycle performance of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 TEM image of the negative electrode active material of Example 1;

[0043] Figure 2 TEM image of the negative electrode active material of Example 2;

[0044] Figure 3 TEM image of the negative electrode active material of Example 6;

[0045] Figure 4 TEM image of the negative electrode active material of Comparative Example 1;

[0046] Figure 5 XRD patterns of the negative electrode active materials in Examples 1 and 2 of the present invention;

[0047] Figure 6 I of the negative electrode active material of Example 1, Example 2, Example 7 and Comparative Example 1 d / I g Data distribution curve;

[0048] Figure 7 1 is the ICI diffusion coefficient curve of the negative electrode active materials of Example 1, Example 2 and Comparative Example 1.

[0049] Reference numerals:

[0050] 1: Carbon coating;

[0051] 2: transition zone;

[0052] 3: Hard carbon body. DETAILED DESCRIPTION

[0053] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0054] Compared with lithium-ion batteries, the sodium ions in sodium-ion batteries have a larger ionic radius When graphite is used as the negative electrode active material to prepare sodium ion batteries, it is difficult for sodium ions to be effectively embedded / extracted from the graphite, which is not conducive to the electrochemical performance of sodium ion batteries. Compared with graphite, hard carbon materials have a higher specific capacity (250-350mAh / g) and are considered to be the most commercially promising negative electrode active materials for sodium ion batteries. However, on the one hand, the existing hard carbon materials have a disordered carbon layer structure inside, and the chaotic ion transport path causes sodium ions to have a high diffusion energy barrier (>0.5eV) inside the hard carbon material. Under high-rate charge and discharge conditions (>5C), sodium ion batteries containing hard carbon materials have a serious concentration polarization phenomenon caused by the obstruction of ion transport, which is not conducive to the capacity retention rate of sodium ion batteries; on the other hand, there are a large number of active defect sites on the surface of hard carbon materials (such as sp 3 These active defect sites will continuously catalyze the decomposition of the electrolyte during the cycle, resulting in repeated rupture / reconstruction of the solid electrolyte interface (SEI) film, which is not conducive to the cycle performance of sodium ion batteries.

[0055] The existing technology mainly improves the electrochemical performance of hard carbon materials through the following methods: 1. Constructing a porous structure inside the hard carbon material through a template method or activation treatment method to shorten the transmission path of sodium ions. However, the disordered distribution of the pore structure of the hard carbon material will aggravate the penetration of the electrolyte, thereby reducing the initial charge and discharge efficiency of the battery; 2. Forming a dense coating layer on the surface of the hard carbon material, and using the coating layer to passivate the active sites on the surface of the hard carbon material, but the dense coating layer will hinder the three-dimensional diffusion channel of sodium ions; 3. Introducing high-capacity phases such as metal sulfides into the hard carbon material. Although this improves the theoretical capacity of the hard carbon material, it increases the interfacial stress, which is not conducive to the cycle stability of the hard carbon material.

[0056] In view of this, the present invention provides a negative electrode active material, which can optimize the transmission path of sodium ions through interface design and performance regulation, avoid side reactions between the negative electrode active material and the electrolyte, and simultaneously improve the rate performance and cycle performance of the battery.

[0057] A first aspect of the present invention provides a negative electrode active material comprising a hard carbon body, a transition region, and a carbon coating layer;

[0058] The transition zone is located on at least a portion of the outer surface of the hard carbon body, and the carbon coating layer is located on at least a portion of the outer surface of the transition zone;

[0059] The transition zone is a mixed phase of hard carbon body and carbon coating layer.

[0060] It is understood that the transition region may be located on a portion of the surface of the hard carbon body or on the entire surface of the hard carbon body; and the carbon coating layer may be located on a portion of the surface of the transition region or on the entire surface of the transition region. The negative electrode active material of the present invention comprises, from the inside out, a hard carbon body, a transition region, and a carbon coating layer.

[0061] In the present invention, interlayer spacing refers to the interlayer spacing between the hard carbon bulk, transition region, and carbon coating. In some embodiments, the negative electrode active material can be tested using high-resolution TEM to determine the interlayer spacing of each component of the negative electrode active material based on the TEM image. Alternatively, each component of the negative electrode active material can be tested using XRD and, based on the XRD image, high-precision XRD calculations can be performed to determine the interlayer spacing of the negative electrode active material.

[0062] In the negative electrode active material of the present invention, since the transition zone between the hard carbon body and the carbon coating layer is a mixed phase of the carbon coating layer and the hard carbon body, the interfacial stress between the hard carbon body and the carbon coating layer can be reduced, the stability of the negative electrode active material and the ion migration ability can be improved, and the cycle performance and rate performance of the battery can be improved. Compared with the negative electrode active material prepared by using alloys or graphite to form a coating layer on the surface of the hard carbon body in the prior art, the negative electrode active material of the present invention has better stability. At the same time, the carbon coating layer can serve as a protective layer, which can reduce the exposed sites of defects on the surface of the hard carbon body, effectively prevent direct contact between the electrolyte and the hard carbon body, reduce the oxidative decomposition reaction of the electrolyte, and improve the cycle performance of the battery. In summary, the negative electrode active material of the present invention can improve the cycle performance and rate performance of the battery when applied to the battery.

[0063] In some embodiments of the present invention, the interlayer spacings of the hard carbon body, the transition region, and the carbon coating layer gradually increase or decrease.

[0064] The negative electrode active material of the present invention has a gradually increasing or decreasing interlayer spacing from the inner layer to the outer layer. In the present invention, gradually increasing can include increasing in a geometric progression, increasing in an arithmetic progression, or increasing irregularly; gradually decreasing can include decreasing in a geometric progression, decreasing in an arithmetic progression, or decreasing irregularly.

[0065] In some embodiments, in the negative electrode active material, the interlayer spacing of the hard carbon body is uniform, the interlayer spacing of the transition zone is uniform, the interlayer spacing of the carbon coating layer is uniform, and the interlayer spacing between the hard carbon body, the transition zone and the carbon coating layer gradually increases or gradually decreases; in other embodiments, in the negative electrode active material, the interlayer spacing of the hard carbon body, the transition zone and the carbon coating layer itself gradually increases or gradually decreases, and the interlayer spacing between the parts gradually increases or gradually decreases (the interlayer spacing of the hard carbon body gradually increases or gradually decreases, the interlayer spacing of the transition zone gradually increases or gradually decreases, the interlayer spacing of the carbon coating layer gradually increases or gradually decreases, and the interlayer spacing of the hard carbon body, the transition zone and the carbon coating layer gradually increases or gradually decreases).

[0066] Since the interlayer spacing of the negative electrode active material gradually increases or decreases from the inside to the outside, it can match the differentiated transmission requirements of sodium ions in the bulk phase and near-surface area of ​​the negative electrode active material, improve the interface performance between the negative electrode active material and the electrolyte, reduce the interface resistance between the negative electrode active material and the electrolyte, reduce the embedding barrier of sodium ions, optimize the transmission path of sodium ions, thereby reducing the transmission obstacles of sodium ions in the negative electrode active material, improving the transmission efficiency and speed of sodium ions, improving the conductivity of the negative electrode active material, and improving the rate performance of the battery.

[0067] Furthermore, in the direction away from the hard carbon body, the interlayer spacing of the transition zone gradually increases or decreases. When the obtained negative electrode active material is used in a battery, the rate performance and cycle performance of the battery can be further improved.

[0068] The present invention can further improve the kinetic performance and stability of the negative electrode active material by selecting the interlayer spacing of the hard carbon body and the interlayer spacing of the carbon coating. In some embodiments of the present invention, the interlayer spacing of the hard carbon body is 0.362-0.386 nm; and / or the interlayer spacing of the carbon coating is 0.35-0.4 nm.

[0069] The inventors also discovered that when the thickness of the carbon coating layer is 1-6 nm and / or the thickness of the transition zone is 2-5 nm, the capacity and kinetic performance of the negative electrode active material can be maintained, thereby improving the capacity and rate performance of the battery. In some embodiments, the thickness of the carbon coating layer and the transition zone can be measured using TEM.

[0070] In some embodiments of the present invention, the surface of the negative electrode active material has N regions, N ≥ 2;

[0071] In the Raman spectra of N regions, the intensity of peak D is d The intensity of the G peak I g The ratio of satisfies the normal distribution.

[0072] In the present invention, the N regions may be continuous or discontinuous. In the Raman spectrum, the D peak is located at a wavelength of 1345 cm -1 -1355cm -1 (1350cm -1 around), originating from sp in defects or disordered structures 2 Breathing vibration mode of hybrid carbon, used to characterize defects; the G peak is located at a wavelength of 1575 cm -1 -1585cm -1 (1580cm -1 Left and right), corresponding to sp 2 The in-plane vibration mode of hybrid carbon atoms (stretching vibration of C-C bonds) reflects the graphitized ordered structure in the material. When the intensity of the D peak in the Raman spectrum of N regions is d The intensity of the G peak I g When the ratio of satisfies a quasi-normal distribution, it indicates that in the negative electrode active material, the transition zone and the carbon coating layer have fully coated the hard carbon body, which helps to further reduce the probability of side reactions between the hard carbon body and the electrolyte, thereby improving the cycle performance of the battery.

[0073] Furthermore, when the intensity of peak D in the Raman spectrum of N regions is d The intensity of the G peak I g The range of the ratio is ≤1.2; and / or the intensity of the D peak is I d The intensity of the G peak I g When the standard deviation of the ratio is ≤0.2, it indicates that the transition zone and the carbon coating layer cover the hard carbon body more uniformly, which can further improve the stability of the negative electrode active material and thus improve the cycle performance of the battery.

[0074] In some embodiments, Raman spectroscopy can be performed on 50 areas of the surface of the negative electrode active material at random, and Raman spectra of the 50 areas are obtained respectively, and the intensity I of the D peak in each Raman spectrum is calculated. d The intensity of the G peak I g The ratio (I d / I g ), then get 50 I d / I g The distribution curve of 50 I d / I g The range and standard deviation.

[0075] In some embodiments of the present invention, when the specific surface area of ​​the negative electrode active material is ≤5m 2 / g, the transition zone and the carbon coating layer fully and evenly coat the hard carbon body, which can further improve the stability of the negative electrode active material.

[0076] When the D50 of the negative electrode active material is 4-7 μm, in the negative electrode active material, the transition region and the carbon coating layer fully and uniformly coat the hard carbon body, and the negative electrode active material is not easy to agglomerate during use.

[0077] In some embodiments of the present invention, the true density of the negative electrode active material is 1.7-2.1 g / m 3 .

[0078] The true density is the solid mass per unit volume of the negative electrode active material in an absolutely dense state, excluding internal pores and interparticle spaces. A true density within the above range indicates that the hard carbon matrix in the negative electrode active material is fully and uniformly coated, which helps improve the negative electrode active material's cycling performance.

[0079] When the powder conductivity of the negative electrode active material is ≥1.5S / mm, the negative electrode active material can further improve the rate performance of the battery when applied to the battery.

[0080] The second aspect of the present invention provides a method for preparing the negative electrode active material of the first aspect, comprising:

[0081] The hard carbon precursor material is subjected to pre-carbonization treatment, activation treatment and carbonization treatment in sequence to obtain a multi-hard carbon material, and the specific surface area of ​​the hard carbon body is 5-100m 2 / g;

[0082] A raw material system including a hard carbon body and a coating agent is calcined to obtain a negative electrode active material.

[0083] Specifically, a hard carbon precursor material is pre-carbonized to obtain a pre-carbonized powder, and then the pre-carbonized powder is activated to make the pre-carbonized powder have more pores, thereby forming a porous carbon material, and the porous carbon material is carbonized to obtain a hard carbon body, and the specific surface area of ​​the hard carbon body is 5-100m 2 / g;

[0084] The hard carbon body and the coating agent are mixed to form a raw material system, and the raw material system is calcined. During the calcination process, the coating agent will be coated on the surface of the hard carbon body. Since the hard carbon body has a special specific surface area, a negative electrode active material including a transition zone and a carbon coating layer can be formed.

[0085] In some embodiments, the specific surface area of ​​the porous carbon material may be 200-1000 m 2 / g;

[0086] The preparation method of the present invention can prepare the negative electrode active material of the first aspect, and the preparation method is simple to operate and suitable for wide promotion and application.

[0087] In some embodiments, the hard carbon precursor material may be washed with deionized water, crushed, sieved, acid-washed, and dried to obtain a purified hard carbon precursor material.

[0088] In the present invention, the hard carbon precursor material is a hard carbon precursor material commonly used in the art. For example, the hard carbon precursor material may include at least one of biomass (one or more of coconut shells, walnut shells, apricot shells, oil tea shells, cellulose, lignin, straw, wood, and sugarcane bagasse), resin (one or more of aldehyde resin, epoxy resin, etc.), and coal (one or more of anthracite, bituminous coal, lignite, etc.).

[0089] The pre-carbonization treatment is a commonly used pre-carbonization treatment in this field. For example, the hard carbon precursor material can be pre-carbonized under a nitrogen or other inert gas protective atmosphere to obtain a pre-carbonized product. After the pre-carbonized product is cooled to room temperature, it is crushed and purified by acid washing to obtain a pre-carbonized powder. In the pre-carbonization treatment, the temperature is 300-800°C and the time is 1-10 hours. Furthermore, in the pre-carbonization treatment, the temperature is 400-600°C and the time is 4-8 hours. The acid solution used for acid washing and purification is one or more of HCl, H2SO4, and HF, with a concentration of 1-4 mol / L; the D50 of the pre-carbonized product after crushing is 3-10 μm.

[0090] The activation treatment is a commonly used activation treatment in the art. For example, the pre-carbonized powder can be activated under an atmosphere of CO2 or water vapor to obtain a porous carbon material; wherein, during the activation treatment, the temperature is 700-1100°C and the time is 1-5 hours. In some embodiments, the specific surface area of ​​the porous carbon material is 200-1000m 2 / g.

[0091] The carbonization treatment can be carried out in a tube furnace under the protection of inert gas.

[0092] In some embodiments, the protective gas for the pre-carbonization treatment, the activation treatment, and the carbonization treatment may be argon or nitrogen, or a mixture thereof.

[0093] In the present invention, the coating agent can be at least one of a solid coating agent (oil-based asphalt, solid resin, etc.), a liquid coating agent (liquid polymer), and acetylene gas. The present invention can use at least one of gas phase spray coating and CVD coating for coating.

[0094] The present invention can further control the specific surface area of ​​the hard carbon body by selecting specific parameters of the carbonization process, thereby ensuring the subsequent calcination process. For example, during the carbonization process, the temperature is 1000-1600°C and the time is 2-8 hours.

[0095] The inventors found in their research that when the temperature during the calcination process is 850-1200°C and the time is 2-8 hours, the efficiency of the calcination process can be improved while saving energy consumption, thereby obtaining a negative electrode active material.

[0096] In some embodiments, the temperature of the reaction system can be increased to the temperature required for carbonization treatment at a heating rate of 1-10°C / min, thereby performing carbonization treatment, or the temperature of the reaction system can be increased to the temperature required for calcination treatment at a heating rate of 1-10°C / min, thereby performing calcination treatment.

[0097] The present invention can also improve the capacity and ion transport performance of the negative electrode active material by controlling the thickness of the coating layer and transition zone in the negative electrode active layer by selecting the coating agent content in the raw material system. For example, the weight percentage of the coating agent in the raw material system is 1-10%.

[0098] A third aspect of the present invention provides a battery comprising the negative electrode active material of the first aspect.

[0099] It can be understood that the battery of the present invention further includes a positive electrode sheet, a separator, an electrolyte and an outer packaging.

[0100] In the present invention, the negative electrode active material of the first aspect can be used to prepare a negative electrode sheet, and then the negative electrode sheet and the separator and positive electrode sheet commonly used in the field are stacked or stacked and then wound to form an electrode assembly. The electrode assembly is placed in an outer package, and the electrolyte is injected into the outer package. After sealing and formation, a battery is formed.

[0101] Since the battery of the present invention includes the negative electrode active material of the first aspect, the battery has excellent rate performance and cycle performance.

[0102] Furthermore, in the battery's charge-discharge curve, when the slope region ratio is 20-80%, the battery has better kinetic performance. Wherein, the battery is a sodium ion battery, and the slope region refers to a slope region greater than 0.1V.

[0103] In the diffusion coefficient curve of the battery, the diffusion coefficient in the slope area is 10 -8 -10 -6 cm 2 / s; and / or, the diffusion coefficient of the plateau region is 10 -11 -10-9 cm / s, the obtained battery has both better rate performance and cycle performance. Among them, the diffusion coefficient curve of the battery can be obtained by intermittent current interruption technology testing. Further, when the diffusion coefficient of the slope area is 10 -7 -10 -6 cm 2 / s; and / or, the diffusion coefficient of the plateau region is 10 -10 -10 -9 cm 2 / s, the obtained battery has both the best rate performance and cycle performance.

[0104] A fourth aspect of the present invention provides a battery pack comprising the battery of the third aspect.

[0105] It will be understood that the battery pack of the present invention may include one or more of the above-mentioned batteries. When the battery pack includes multiple batteries, the multiple batteries may be connected in series, in parallel, or in a mixed connection. For example, the battery pack of the present invention may include two or more of the above-mentioned batteries connected in series, in parallel, or in a mixed connection. In the battery pack of the present invention, any two batteries may be connected by one or more of ultrasonic welding, resistance welding, and laser welding.

[0106] Since the battery pack of the present invention includes the above-mentioned battery, it also has the advantages of high rate performance and long service life.

[0107] A fifth aspect of the present invention provides an electronic device comprising the battery according to the third aspect or the battery pack according to the fourth aspect.

[0108] It should be noted that the electronic device can be any conventional device that requires electricity, including but not limited to computers, electric vehicles, air conditioners, refrigerators, washing machines, microwave ovens, printers, fax machines, etc. Because the electronic device includes the battery of the third aspect or the battery pack of the fourth aspect, the electronic device has better battery life and fast charging performance.

[0109] The present invention will be further described below with reference to specific embodiments:

[0110] In the examples, the raw materials used are all conventional commercial products, and materials such as resins (such as phenolic resins, etc.), biomass (such as coconut shells, bamboo, etc.), coal (such as bituminous coal, anthracite, etc.), hydrochloric acid, sulfuric acid, and hydrofluoric acid are available from Sinopharm Chemical Reagent Co., Ltd.; the equipment used are all conventional equipment, and the test methods are all conventional methods.

[0111] The XRD data of the samples prepared in the present invention were obtained by detection using a Smartlab X-ray diffractometer from Rigaku Corporation of Japan; the scanning electron microscope images and element distribution images were obtained by detection using a German Zeiss Supra-55 field emission scanning electron microscope; and the transmission electron microscope images were obtained by detection using a Hitachi field emission transmission electron microscope.

[0112] Example 1

[0113] The negative electrode active material of this embodiment is prepared by a method comprising the following steps:

[0114] S1. Select coconut shell as a hard carbon precursor material, wash, crush, sieve, acid-wash, and dry the hard carbon precursor material to obtain a purified hard carbon precursor material with a D50 of 3-5 μm;

[0115] S2. Pre-carbonizing the purified hard carbon precursor material under nitrogen protection to obtain a pre-carbonized product, taking out the pre-carbonized product and grinding it to obtain a pre-carbonized powder;

[0116] In the pre-carbonization treatment, the temperature was 400 °C and the time was 8 h;

[0117] S3. activating the pre-carbonized powder under the condition of introducing water vapor to obtain a porous carbon material;

[0118] The specific surface area of ​​porous carbon materials is 300m 2 / g, activation treatment, temperature 800 ° C, time 2h;

[0119] S4. Carbonizing the porous carbon material under a protective gas atmosphere to obtain a hard carbon body;

[0120] The specific surface area of ​​the hard carbon body is 30m 2 / g, in the carbonization treatment, the temperature was 1400 °C and the time was 8 h;

[0121] S5. Under a protective gas atmosphere, the hard carbon body and the asphalt coating agent are uniformly mixed, placed in a tube furnace, and calcined to obtain a negative electrode active material, named N1;

[0122] Wherein, based on the total mass of the hard carbon body and the coating agent, the mass percentage of the coating agent is 5%; during the calcination treatment, the temperature is 1000° C. and the time is 3 hours.

[0123] Example 2

[0124] The preparation method of the negative electrode active material of this embodiment is basically the same as that of Example 1, except that:

[0125] In S5 , during the calcination treatment, the temperature is 1100° C. and the time is 2 h to obtain the negative electrode active material N2.

[0126] Example 3

[0127] The preparation method of the negative electrode active material of this embodiment is basically the same as that of Example 1, except that:

[0128] In S1, the hard carbon precursor material is bituminous coal material;

[0129] In S4, during the carbonization treatment, the temperature was 1430°C, and the specific surface area of ​​the hard carbon body was 26m 2 / g;

[0130] The negative electrode active material N3 is obtained.

[0131] Example 4

[0132] The preparation method of the negative electrode active material of this embodiment is basically the same as that of Example 1, except that:

[0133] In S1, the hard carbon precursor material is a phenolic resin material;

[0134] In S4, during the carbonization treatment, the temperature was 1420°C, and the specific surface area of ​​the hard carbon body was 22m 2 / g;

[0135] The negative electrode active material N4 is obtained.

[0136] Example 5

[0137] The preparation method of the negative electrode active material of this embodiment is basically the same as that of Example 1, except that:

[0138] In S5, based on the total mass of the hard carbon body and the coating agent, the mass percentage of the coating agent is 12%, thereby obtaining a negative electrode active material N5.

[0139] Example 6

[0140] The preparation method of the negative electrode active material of this embodiment is basically the same as that of Example 1, except that:

[0141] In S4, during the carbonization treatment, the temperature was 1630°C, and the specific surface area of ​​the hard carbon body was 17m 2 / g;

[0142] The negative electrode active material N6 was obtained.

[0143] Example 7

[0144] The preparation method of the negative electrode active material of this embodiment is basically the same as that of Example 1, except that:

[0145] In S5, based on the total mass of the hard carbon body and the coating agent, the mass percentage of the coating agent is 0.8%, the temperature is 850° C., and the time is 2 h, thereby obtaining the negative electrode active material N7.

[0146] Comparative Example 1

[0147] The preparation method of the negative electrode active material of this comparative example is basically the same as that of Example 1, except that:

[0148] Excluding S5, the hard carbon body obtained using S4 is the negative electrode active material NS1.

[0149] Comparative Example 2

[0150] The preparation method of the negative electrode active material of this comparative example is basically the same as that of Example 1, except that:

[0151] In S3, the temperature is 600 °C, the time is 1 h, and the specific surface area of ​​the porous carbon material is 80 m 2 / g;

[0152] In S4, the carbonization time is 12 h, the temperature is 1630 ° C, and the specific surface area of ​​the hard carbon body is 4.5 m 2 / g.

[0153] Performance Testing

[0154] The following performance tests were performed on the negative electrode active materials in the examples and comparative examples, and the results are shown in Tables 1 and 2.

[0155] 1) High-resolution transmission electron microscopy (HRTEM) testing

[0156] A small amount of negative electrode active material is ultrasonically dispersed in anhydrous ethanol, and then a small amount is pipetted onto a carbon film or microgrid. After natural drying, the test is performed to obtain a TEM image of the negative electrode active material. Based on the TEM image of the negative electrode active material, the thickness of each part of the negative electrode active material and the interlayer spacing are obtained; the instruments used are Hitachi-7650 (Japan, 80kV) and HRTEM, JEOL JEM-3000F.

[0157] Figure 1 TEM image of the negative electrode active material of Example 1; Figure 2 TEM image of the negative electrode active material of Example 2; Figure 3 TEM image of the negative electrode active material of Example 6; Figure 4 TEM image of the negative electrode active material of Comparative Example 1. Figure 1-4It can be seen that the negative electrode active material of the embodiment of the present invention includes a hard carbon body 3, a transition zone 2 and a carbon coating layer 1 from the inside to the outside, and the sum of the thicknesses of the transition zone 2 and the carbon coating layer 1 in Example 1 is 8.6 nm, the interlayer spacing of the carbon coating layer 1 in Example 1 is 0.364 nm, and the interlayer spacing of the hard carbon body 3 is 0.382 nm. The interlayer spacing of the negative electrode active material gradually decreases from the inside to the outside; the sum of the thicknesses of the transition zone 2 and the carbon coating layer 1 in Example 2 is 3.5 nm, the interlayer spacing of the carbon coating layer 1 is 0.354 nm, and the interlayer spacing of the hard carbon body 3 is 0.385 nm. The interlayer spacing of the negative electrode active material gradually decreases from the inside to the outside; in Example 6, the interlayer spacing of the carbon coating layer 1 is 0.365 nm, and the interlayer spacing of the hard carbon body 3 is 0.354 nm. From the inside to the outside, the interlayer spacing of the negative electrode active material gradually increases, and the interlayer spacing of the transition zone 2 also gradually increases, while the negative electrode active material in Comparative Example 1 only has the hard carbon body 3.

[0158] 2) XRD test

[0159] XRD tests were performed on the negative electrode active materials in the examples and comparative examples to obtain XRD patterns of the negative electrode active materials. Based on the XRD patterns, the interlayer spacing d value was calculated using the Bragg equation (2dsinθ=nλ). Figure 5 The XRD patterns of the negative electrode active materials in Examples 1 and 2 of the present invention are shown in FIG. Figure 5 The interlayer spacing of the carbon coating layer in Example 1 and Example 2 can be obtained, thereby verifying the interlayer spacing obtained by TEM calculation.

[0160] 3) Raman spectroscopy test

[0161] Randomly select 50 positions (N=50) of the negative electrode active material, obtain the Raman spectrum curve of the negative electrode active material, and obtain the I d / I g Data from 50 I d / I g The range and standard deviation of the Raman spectrum are calculated by testing 50 positions. d / I g The difference between the maximum and minimum values ​​of the data, the standard deviation is the statistical 50 positions I d / I g The standard deviation of the data.

[0162] Figure 6 : The Id / Ig data distribution curve of the negative electrode active materials of Example 1, Example 2, Example 7 and Comparative Example 1. Figure 6 It can be seen that compared with the comparative example, the d / I gThe data distribution is more concentrated, forming a normal distribution curve. The ranges of Example 1, Example 2, Example 4, and Comparative Example 1 are 0.8, 0.6, 1.2, and 1.6, respectively, and the standard deviations are 0.18, 0.09, 0.21, and 0.32, respectively, indicating that the surface of the negative electrode active material in the examples is more uniform.

[0163] 4) Specific surface area test

[0164] According to the provisions of GB / T 19587, the specific surface area of ​​the negative electrode active material was measured using a Quantachrome NOVA2000e surface area analyzer.

[0165] 5) Particle size test

[0166] According to Appendix A of GB / T 24533 2019, the particle size of the negative electrode active material was detected using a Malvern laser particle size analyzer.

[0167] 6) Powder conductivity

[0168] According to GBT30835-2014, an automated powder resistivity tester was used to test resistivity-pressure, conductivity-pressure and other parameters using the four-probe method, and the powder conductivity at 30 MPa was finally read.

[0169] 7) Electrochemical sodium storage performance test and ICI test

[0170] A negative electrode sheet was prepared using the negative electrode active materials obtained in the embodiment and the comparative example, respectively. The negative electrode sheet included a copper foil and a negative electrode active layer located on at least one surface of the copper foil. The negative electrode active layer included a negative electrode active material, carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), and conductive carbon black. The mass ratio of the negative electrode active material, carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), and conductive carbon black (SP) was 100:1.5:2.5:1.

[0171] The negative electrode sheet, glass fiber separator, sodium positive electrode and electrolyte were assembled into a button half-cell in a glove box filled with high-purity argon, wherein the electrolyte was NaPF6 electrolyte with a concentration of NaPF6 of 1M, the solvent included PC and EMC, and the volume ratio of PC and EMC was 1:1.

[0172] Electrochemical performance was measured on a Wuhan Blue Electric CT2001A battery tester with the charge and discharge voltage range controlled at 0–2 V and the charge and discharge rate at 0.1 A / g. The first-cycle sodium insertion and removal capacities were recorded, and the first-cycle coulombic efficiency was calculated (first-cycle coulombic efficiency = first-cycle sodium removal capacity / first-cycle sodium insertion capacity).

[0173] At the same time, the above button half-cell was used to perform ICI testing and calculate the diffusion coefficient of the negative electrode material.

[0174] Figure 7 : ICI diffusion coefficient curves of the negative electrode active materials of Example 1, Example 2 and Comparative Example 1. Figure 7 It can be seen that the diffusion coefficient of the negative electrode active material of the embodiment is significantly better than that of the comparative example, with the diffusion coefficient in the slope region increased by 1-2 orders of magnitude, and the diffusion coefficient in the plateau region also significantly increased by 1 order of magnitude, which fully demonstrates that the negative electrode active material of the present invention is beneficial to the sodium ion diffusion kinetics.

[0175] 8) Rate performance

[0176] The rate performance was tested using the button half-cell in 7), with the charge and discharge voltage range controlled at 0-2 V and the charge and discharge rate at 0.1 A / g. The first-cycle sodium insertion capacity and sodium removal capacity were recorded. In the second-cycle test, charge and discharge tests were performed at rates of 0.2C, 0.5C, and 1C, respectively, to obtain the corresponding sodium removal capacity, which was compared with the first-cycle sodium removal capacity to calculate the capacity retention rate.

[0177] Table 1

[0178]

[0179] Table 2

[0180]

[0181] As can be seen from Table 1, the negative electrode active materials of the embodiments of the present invention, when used in batteries, can improve the battery's capacity, first efficiency, and rate performance. In particular, as can be seen from Example 1 and Comparative Example 1, the inclusion of a transition region and a carbon coating in the negative electrode active material can improve the battery's capacity, first efficiency, and rate performance. As can be seen from Example 2 and Comparative Example 2, the negative electrode active material including a transition region has even better capacity, first efficiency, and rate performance.

[0182] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A negative electrode active material, characterized in that including a hard carbon body, a transition zone and a carbon coating layer; The transition zone is located on at least a portion of the outer surface of the hard carbon body, and the carbon coating layer is located on at least a portion of the outer surface of the transition zone; The transition zone is a mixed phase of the hard carbon body and the carbon coating layer.

2. The negative electrode active material according to claim 1, characterized in that The interlayer spacings among the hard carbon body, the transition zone, and the carbon coating layer gradually increase or decrease.

3. The negative electrode active material according to claim 1 or 2, characterized in that In a direction away from the hard carbon body, the interlayer spacing of the transition zone gradually increases or decreases.

4. The negative electrode active material according to any one of claims 1 to 3, characterized in that The negative electrode active material includes at least one of the following characteristics: a. The interlayer spacing of the hard carbon body is 0.362-0.386 nm; b. The interlayer spacing of the carbon coating layer is 0.35-0.4 nm; c. The thickness of the carbon coating layer is 1-6 nm; d. The thickness of the transition zone is 2-5 nm.

5. The negative electrode active material according to any one of claims 1 to 4, characterized in that: The surface of the negative electrode active material has N regions, N ≥ 2; In the Raman spectra of the N regions, the intensity of peak D is d The intensity of the G peak I g The ratio of satisfies the normal distribution; In the Raman spectrum, the D peak is at a wavelength of 1345 cm -1 -1355cm -1 The characteristic peak of G peak is at a wavelength of 1575cm -1 -1585cm -1 characteristic peaks.

6. The negative electrode active material according to claim 5, characterized in that In the Raman spectra of the N regions, The intensity of peak D I d The intensity of the G peak I g The range of the ratio is ≤1.2; and / or the intensity of the D peak is I d The intensity of the G peak I g The standard deviation of the ratio is ≤0.

2.

7. The negative electrode active material according to any one of claims 1 to 6, characterized in that: The negative electrode active material includes at least one of the following characteristics: a. The specific surface area of ​​the negative electrode active material is ≤5m 2 / g; b. The D50 of the negative electrode active material is 4-7 μm; c. The true density of the negative electrode active material is 1.7-2.1 g / m 3 ; d. The powder conductivity of the negative electrode active material is ≥1.5S / mm.

8. A method for preparing the negative electrode active material according to any one of claims 1 to 7, characterized in that: include: The hard carbon precursor material is subjected to pre-carbonization treatment, activation treatment and carbonization treatment in sequence to obtain a hard carbon body, wherein the specific surface area of ​​the hard carbon body is 5-100m 2 / g; The negative electrode active material is obtained by calcining a raw material system including a hard carbon body and a coating agent.

9. The preparation method according to claim 8, characterized in that In the carbonization treatment, the temperature is 1000-1600°C and the time is 2-8h; And / or, during the calcination process, the temperature is 850-1200° C. and the time is 2-8 hours.

10. The preparation method according to any one of claims 8 to 9, characterized in that: In the raw material system, the mass percentage of the coating agent is 1-10%.

11. A battery, characterized in that: The negative electrode active material comprises the negative electrode active material according to any one of claims 1 to 7.

12. The battery according to claim 11, characterized in that The battery includes at least one of the following features: a. In the charge and discharge curve of the battery, the slope area ratio is 20-80%; b. In the diffusion coefficient curve of the battery, the diffusion coefficient in the slope area is 10 -8 -10 -6 cm 2 / s; and / or, the diffusion coefficient of the platform region is 10 -11 -10 -9 cm / s; preferably, the diffusion coefficient of the slope region is 10 -7 -10 -6 cm 2 / s; and / or, the diffusion coefficient of the platform region is 10 -10 -10 -9 cm 2 / s.

13. A battery pack, characterized in that: A battery comprising the battery according to claim 11 or 12.

14. An electronic device, characterized in that: The method comprises the battery according to claim 11 or 12 or the battery pack according to claim 13.

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