Negative active material, method of preparing the same, and device including the same

Abstract

The application relates to a negative electrode active material, a preparation method thereof and a device containing the same. The negative electrode active material comprises silicon-based particles, a coating layer and piezoelectric particles; the coating layer is coated on at least part of the surface of the silicon-based particles, and the coating layer comprises a conductive carbon material; and the piezoelectric particles are dispersed in the coating layer. When the negative electrode active material of the application is applied to a secondary battery, the silicon-based particles may expand in volume, thereby giving the piezoelectric particles in the coating layer a compressive stress; the piezoelectric particles are excited to generate a piezoelectric effect under the action of an internal electric field of the secondary battery and the compressive stress, thereby generating a reverse electric field, homogenizing the distribution of metal ions, and making the metal ions uniformly insert into the negative electrode active material, reducing the loss of the metal ions, and thus ensuring the energy density of the secondary battery; and the energy barrier of the metal ions when migrating to the interior of the negative electrode active material via the piezoelectric particles is relatively low, and the migration speed is relatively fast, so that the charging performance can be improved.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to negative electrode active materials, methods for preparing the same, and apparatus comprising the same. Background Technology

[0002] Rechargeable batteries possess advantages such as small size, high energy density, high safety, low self-discharge, and long lifespan, and are widely used in various fields including energy storage, communications, electric vehicles, and aerospace. As the application scope of batteries becomes increasingly wide, the requirements for battery performance are also becoming more stringent, such as the requirement for fast charging capabilities.

[0003] The performance of the negative electrode active material in the battery has a certain limiting effect on the fast charging capability, so it is urgent to improve the performance of the negative electrode active material. Summary of the Invention

[0004] This application provides a negative electrode active material, a method for preparing the same, and an apparatus comprising the same, which can improve the fast-charging performance of the negative electrode active material.

[0005] In a first aspect, this application proposes a negative electrode active material. The negative electrode active material includes silicon-based particles, a coating layer, and piezoelectric particles; the coating layer covers at least a portion of the surface of the silicon-based particles, and the coating layer includes a conductive carbon material; the piezoelectric particles are dispersed in the coating layer.

[0006] Therefore, when the negative electrode active material of this application is applied to a secondary battery, the silicon-based particles may expand in volume during charging, thereby applying compressive stress to the piezoelectric particles in the coating layer. Under the action of the internal electric field and compressive stress of the secondary battery, the piezoelectric particles are excited by the piezoelectric effect, thereby generating a reverse electric field, which homogenizes the distribution of metal ions, so that the metal ions are uniformly embedded in the negative electrode active material, reducing the loss of metal ions and thus ensuring the energy density of the secondary battery. Moreover, the energy barrier when metal ions migrate to the interior of the negative electrode active material through the piezoelectric particles is relatively low and the migration speed is relatively fast, thus improving the charging performance.

[0007] In any embodiment, the dielectric constant of the piezoelectric particles is greater than that of the conductive carbon material; optionally, the dielectric constant ε of the piezoelectric particles satisfies: 100 ≤ ε ≤ 100000. When the dielectric constant of the piezoelectric particles is within the above range, their effect in reducing the migration barrier is better.

[0008] In any embodiment, the piezoelectric particles are partially exposed on the surface of the coating layer opposite to the silicon-based particles. The exposed surface allows the piezoelectric particles to directly contact the electrolyte, thereby further facilitating the reduction of the energy barrier for lithium-ion migration.

[0009] In any embodiment, a portion of the piezoelectric particle protrudes from the surface of the coating layer away from the silicon-based particle; thereby, the piezoelectric particle has a larger exposed surface area, increasing the contact area with the electrolyte, and can reduce the kinetic energy barrier of the desolvation process with a smaller amount.

[0010] In any embodiment, the piezoelectric particle penetrates the coating layer along its thickness direction. One end of the piezoelectric particle is in contact with the electrolyte, and the other end is in contact with the silicon-based particle. The surface of the piezoelectric particle in contact with the electrolyte is positively charged, and the surface in contact with the silicon-based particle is negatively charged. This can better reduce the kinetic energy barrier of the desolvation process, increase the speed at which metal ions reach the surface of the silicon-based particle, and reduce the resistance to metal ion insertion into the negative electrode. Therefore, the negative electrode active material of this application has good kinetic performance and can withstand high-rate charging, thereby improving the fast charging capability of the secondary battery.

[0011] In any embodiment, the average thickness of the coating layer is H nm; the volume average particle size Dv50 of the piezoelectric particles is d1 nm, wherein H and d1 satisfy the following condition: 0.25 ≤ H / d1 ≤ 1; optionally, 0.25 ≤ H / d1 ≤ 0.5. When H / d1 is within the above range, it is beneficial for the secondary battery to have both high fast charging capability and high energy density.

[0012] In any embodiment, the volume average particle size Dv50 of the piezoelectric particles is d1 nm; the volume average particle size Dv50 of the silicon-based particles is d2 μm, wherein d1 and d2 satisfy the following condition: 0 < d1 / d2 ≤ 50; optionally, 0 < d1 / d2 ≤ 20. When d1 / d2 is within the above range, on the one hand, the piezoelectric effect of the piezoelectric particles 103 can be fully utilized and the kinetic energy barrier of the desolvation process can be reduced; on the other hand, the negative electrode active material 10 of this application can be guaranteed to have a high compaction density, thereby enabling the secondary battery to simultaneously have high fast charging capability and high energy density.

[0013] In any embodiment, the volume average particle size Dv50 of the piezoelectric particles is d1 nm, 0 < d1 ≤ 200; optionally, 0 < d1 ≤ 100. When the volume average particle size Dv50 of the piezoelectric particles is within the above range, the specific surface area of ​​the piezoelectric particles is relatively moderate, so fewer piezoelectric particles can be used with the same specific surface area, thereby reducing the energy density loss of the secondary battery.

[0014] In any embodiment, the volume average particle size Dv50 of the silicon-based particles is d2μm, where 2≤d2≤10; optionally, 3≤d2≤8. When the volume average particle size of the silicon-based particles meets the above range, it can ensure that the negative electrode active material has a high compaction density and can provide sufficient bonding surface for the piezoelectric particles, thereby facilitating the piezoelectric effect of the piezoelectric particles.

[0015] In any embodiment, the average thickness of the coating layer is H nm, where 0 < H ≤ 100; optionally, 0 < H ≤ 50. When the thickness of the coating layer is within the above range, it can suppress the excessive expansion of silicon-based particles to a certain extent, thereby improving the overall stability of the negative electrode active material; and the coating layer can fully exert its role as a medium for binding piezoelectric particles to silicon-based particles, ensuring the bonding strength between piezoelectric particles and silicon-based particles.

[0016] In any embodiment, based on the total mass of the negative electrode active material, the mass content of the silicon-based particles is a1; based on the total mass of the negative electrode active material, the mass content of carbon in the coating layer is a2, wherein a1 and a2 satisfy the following condition: 0.02 ≤ a2 / a1 ≤ 0.1; optionally, 0.02 ≤ a2 / a1 ≤ 0.05. When a2 / a1 satisfies the above range, it is beneficial for the negative electrode active material to have high fast charging capability while also having high specific capacity, high initial coulombic effect, and high compaction density, thereby enabling the secondary battery to simultaneously have high fast charging capability, high energy density, and high cycle capacity retention.

[0017] In any embodiment, based on the total mass of the negative electrode active material, the mass content of the silicon-based particles is a1; based on the total mass of the negative electrode active material, the mass content of the piezoelectric particles is a3, wherein a1 and a3 satisfy the following condition: 0.005 ≤ a3 / a1 ≤ 0.1; optionally, 0.01 ≤ a3 / a1 ≤ 0.03. When the mass content of the silicon-based particles and the mass content of the piezoelectric particles satisfy the above ranges, the silicon-based particles can apply compressive stress to the piezoelectric particles at various locations when the volume of the silicon-based particles expands, thereby stimulating the piezoelectric particles at various locations to generate a piezoelectric effect, causing metal ions to migrate uniformly towards the interior of the silicon-based particles, which is beneficial for the secondary battery to have high and fast charging energy; and the proportion of piezoelectric particles will not be too high, thereby ensuring the high energy density of the secondary battery.

[0018] In any embodiment, the piezoelectric particles comprise one or more of barium titanate, lead titanate, lithium niobate, lead zirconate titanate, lead metaniobate, and lead barium lithium niobate. The above-mentioned types of piezoelectric particles can effectively exert the piezoelectric effect and reduce the energy barrier for metal ion migration processes.

[0019] In any embodiment, the conductive carbon material in the coating layer comprises amorphous carbon; optionally, the conductive carbon material comprises hard carbon. The amorphous carbon layers have a larger interlayer spacing and do not cause volume shrinkage or expansion during lithium-ion extraction and insertion, resulting in a more stable crystal structure. This enables the negative electrode active material to possess good kinetic performance and withstand high-rate charging, thereby improving the fast-charging capability of the secondary battery.

[0020] In any embodiment, the silicon-based particles comprise silicon and oxygen. Based on the total molar amount of the elements contained in the silicon-based particles, the molar content of silicon in the silicon-based particles is M1, and based on the total molar amount of the elements contained in the silicon-based particles, the molar content of oxygen in the silicon-based particles is M2, wherein 0.5 ≤ M1 / M2 ≤ 2. When the silicon-based particles meet the above ranges, the silicon-based particles are more conducive to the utilization of silicon capacity, thereby improving the capacity of the secondary battery.

[0021] In any embodiment, the silicon-based particles further include a dopant element M, which includes one or more of Fe, Ti, Ni, Zr, and Co; optionally, based on the total mass of the silicon-based particles, the mass content of the dopant element M is 1% to 5%. When the molar content of the dopant element M meets the above range, it is beneficial for the dopant element to ensure the structural stability of the silicon-based particles 101.

[0022] In any embodiment, the silicon-based particles include one or more of elemental silicon, silicon oxide, silicon carbide, and ferrosilicon alloy.

[0023] In any embodiment, the negative electrode active material satisfies at least one of the following conditions (1) to (3):

[0024] (1) The volume average particle size Dv50 of the negative electrode active material is d0μm, 3≤d0≤8;

[0025] (2) The BET specific surface area of ​​the negative electrode active material is S0 m2 / g, 1≤S0≤3;

[0026] (3) The compaction density of the negative electrode active material under a force of 20000N is P0 g / cm3, 1.1≤P0≤1.4.

[0027] By adjusting the volume average particle size (Dv50) of the negative electrode active material within a suitable range, it is beneficial to achieve better ion and electron transport performance, as well as fast charging performance, while also maintaining a higher powder compaction density. By adjusting the specific surface area of ​​the negative electrode active material within a suitable range, interfacial side reactions between the negative electrode and the electrolyte can be reduced, while also ensuring that the negative electrode exhibits appropriate electrochemical reactivity, thereby enabling the secondary battery to have a higher fast charging capability. Finally, by adjusting the powder compaction density of the negative electrode active material within a suitable range, the negative electrode film layer can achieve a higher compaction density, thus resulting in a higher energy density for the secondary battery.

[0028] Secondly, this application provides a method for preparing a negative electrode active material, comprising S10, providing silicon-based particles, a carbon source, and piezoelectric particles; S20, uniformly mixing the silicon-based particles, the carbon source, and the piezoelectric particles, and forming a coating layer comprising conductive carbon material on at least a portion of the surface of the silicon-based particles by carbonization sintering treatment, wherein the piezoelectric particles are dispersed in the coating layer.

[0029] In any embodiment, in S20, the carbonization sintering temperature is 900°C to 1500°C; and / or, in S20, the carbonization sintering time is 1 h to 8 h.

[0030] Thirdly, this application provides a negative electrode sheet, which includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, wherein the negative electrode film layer includes a negative electrode active material of any embodiment of the first aspect of this application or a negative electrode active material prepared by a method of any embodiment of the second aspect of this application.

[0031] Fourthly, this application provides a secondary battery that includes the negative electrode sheet of the third aspect of this application.

[0032] Fifthly, this application provides a battery module that includes the secondary battery of the fourth aspect of this application.

[0033] Sixthly, this application provides a battery pack that includes the secondary battery of the fourth aspect of this application.

[0034] In a seventh aspect, this application provides an electrical device, which includes a secondary battery according to the fourth aspect, a battery module according to the fifth aspect, or a battery pack according to the sixth aspect. Attached Figure Description

[0035] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments of this application will be briefly described below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the drawings without creative effort.

[0036] Figure 1 This is a schematic diagram of one embodiment of the negative electrode active material of this application.

[0037] Figure 2 This is a schematic diagram of one embodiment of the secondary battery of this application.

[0038] Figure 3 yes Figure 2 An exploded view of the implementation method of the secondary battery.

[0039] Figure 4This is a schematic diagram of one embodiment of the battery module of this application.

[0040] Figure 5 This is a schematic diagram of one embodiment of the battery pack of this application.

[0041] Figure 6 yes Figure 5 An exploded view of an embodiment of the battery pack shown.

[0042] Figure 7 This is a schematic diagram of one embodiment of an electrical device that uses a secondary battery as a power source, as described in this application.

[0043] The accompanying drawings may not be drawn to scale.

[0044] The annotations in the attached figures are explained as follows:

[0045] 1. Battery pack; 2. Upper casing; 3. Lower casing; 4. Battery module;

[0046] 5. Secondary battery; 51. Housing; 52. Electrode assembly; 53. Cover plate;

[0047] 6. Electrical appliances;

[0048] 10. Negative electrode active material; 101. Silicon-based particles; 102. Coating layer; 103. Piezoelectric particles. Detailed Implementation

[0049] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of negative electrode active materials, methods for preparing the same, and apparatus comprising the same. However, unnecessary details may be omitted. For example, detailed descriptions of well-known facts and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0050] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0051] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0052] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0053] Unless otherwise specified, all steps of this application may be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0054] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0055] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0056] In this application, the terms "multiple" or "various" refer to two or more kinds.

[0057] In this application, "about" a certain value represents a range, specifically a range of ±10% of that value.

[0058] In this application, the secondary battery may include lithium-ion batteries, lithium-sulfur batteries, sodium-lithium-ion batteries, sodium-ion batteries, or magnesium-ion batteries, etc., and the embodiments of this application are not limited to this.

[0059] A secondary battery consists of an electrode assembly and an electrolyte. The electrode assembly comprises a positive electrode, a negative electrode, and a separator. The secondary battery primarily functions by the movement of metal ions between the positive and negative electrodes. The positive electrode includes a positive active material, and the negative electrode includes a negative active material. In this paper, the metal ions can be lithium ions, sodium ions, etc.; the charging process will be explained using lithium ions as an example.

[0060] During the charging process of a secondary battery, the electrode dynamics process typically includes the following steps.

[0061] (1) Lithium ion extraction step: Lithium ions are extracted from the positive electrode active material and migrate into the electrolyte phase; (2) Liquid phase mass transfer step in the electrolyte phase: Solvated lithium ions in the electrolyte diffuse and transfer to the surface of the negative electrode active material; (3) Surface conversion step: During the first charge, solvated lithium ions are adsorbed on the surface of the negative electrode active material and react to form a solid electrolyte interface (SEI) film. During subsequent charging, solvated lithium ions are adsorbed on the surface of the SEI film. After the desolvation process, lithium ions reach the surface of the negative electrode active material; (4) Charge exchange step: Lithium ions gain electrons from the surface of the negative electrode active material and form lithium intercalation products; (5) Solid phase mass transfer step of lithium intercalation products: Lithium intercalation products diffuse from the surface of the negative electrode active material to the interior in a solid phase, completing the charging process.

[0062] As the charging rate increases, lithium ions are rapidly extracted from the positive electrode active material, and the current density and lithium ion concentration distribution in the electrolyte phase may be uneven. This causes lithium ions in the electrolyte phase to tend to accumulate on the surface of the negative electrode active material in certain areas. The lithium ions extracted from the positive electrode active material cannot be equally embedded in the negative electrode active material. These unembedded lithium ions gain electrons on the surface of the negative electrode active material, forming silvery-white metallic lithium, known as "lithium dendrites." The formation of lithium dendrites not only degrades the performance of the secondary battery, such as shortening cycle life, but in severe cases, their sharp morphology can pierce the separator, leading to internal short circuits and potentially catastrophic consequences such as combustion or explosion. Simultaneously, the continuously deposited metallic lithium can detach from the surface of the negative electrode active material, forming "dead lithium" that cannot participate in the reaction, resulting in a decrease in the energy density of the secondary battery.

[0063] In view of this, the inventors improved the negative electrode active material and proposed a fast-charging negative electrode active material, which enables metal ions such as lithium ions to be uniformly embedded in the negative electrode active material, thereby improving the problems of "lithium dendrites" and "dead lithium" and improving the fast-charging performance of the negative electrode active material.

[0064] Negative electrode active materials

[0065] The first aspect of this application proposes a negative electrode active material.

[0066] like Figure 1 As shown, the negative electrode active material 10 includes silicon-based particles 101, a coating layer 102, and piezoelectric particles 103; the coating layer 102 covers at least a portion of the surface of the silicon-based particles 101, and the coating layer 102 includes a conductive carbon material; the piezoelectric particles 103 are dispersed in the coating layer 102.

[0067] Although the mechanism is not yet clear, the inventors have found that the negative electrode active material 10 of this application has good kinetic performance, can effectively alleviate the metal precipitation phenomenon, and can improve the fast charging capability of the secondary battery.

[0068] When silicon-based particles 101 are used as negative electrode active material 10 in a secondary battery, during the charging and discharging process of the secondary battery, as metal ions such as lithium ions are inserted or extracted, the crystal phase of silicon-based particles 101 may change, thereby causing silicon-based particles 101 to exhibit volume changes; specifically, during the charging process of the secondary battery, silicon-based particles 101 may expand.

[0069] The piezoelectric particles 103 exhibit a piezoelectric effect, which refers to the polarization of the particles and the generation of charges on their surfaces when subjected to external forces. In this application, during the charging process of the secondary battery, an internal electric field exists between the positive and negative electrode plates, and the silicon-based particles 101 may undergo volume expansion. The expanded silicon-based particles 101 can exert a certain compressive stress on the piezoelectric particles 103. Under the presence of the internal electric field and compressive stress, the piezoelectric particles 103 may be excited to generate a piezoelectric effect, causing the positive and negative charge centers inside the piezoelectric particles 103 to shift and generate a reverse electric field. This smooths out the unevenly distributed concentration of metal ions, homogenizes the concentration distribution of metal ions on the surface of the negative electrode active material 10 and at the electrolyte interface, and thus allows metal ions to be uniformly embedded in the negative electrode active material 10. This reduces the risk of metal precipitation due to local enrichment of metal ions and reduces metal ion loss, thereby ensuring the energy density of the secondary battery. Furthermore, the energy barrier for metal ions migrating from the piezoelectric particles 103 to the interior of the negative electrode active material 10 is relatively low, and the migration speed is relatively fast, thus improving the charging performance.

[0070] In the solid-phase mass transfer step of the lithium intercalation product of this application, metal ions need to diffuse from the surface of the negative electrode active material 10 to the interior to complete the charging process. During this process, the migration of metal ions includes at least two pathways: one pathway is the migration via amorphous carbon to the silicon-based particles 101; the other pathway is the migration via piezoelectric particles 103 to the silicon-based particles 101.

[0071] In some embodiments, the dielectric constant of the piezoelectric particle 103 is greater than that of amorphous carbon.

[0072] The piezoelectric particles 103 have a relatively high dielectric constant. When metal ions migrate from the piezoelectric particles 103 to the silicon matrix particles, the energy barrier required for their migration is relatively low, and the migration rate is fast, which can improve the fast charging performance of the negative electrode active material 10.

[0073] Optionally, the dielectric constant ε of the piezoelectric particles 103 satisfies: 100 ≤ ε ≤ 100000. When the dielectric constant of the piezoelectric particles 103 is within the above range, the effect of reducing the migration energy barrier is better. Exemplarily, the dielectric constant of the piezoelectric particles 103 can be 100 to 50000, 100 to 25000, 100 to 10000, 100 to 5000, 100 to 4000, 100 to 3000, 100 to 2000, 100 to 1000, 100 to 500, 150 to 50000, 150 to 25000, 150 to the 10000, 150 to 5000, 150 to 4000, 150 to 3000, 150 to 2000, 150 to 1000, 150 to 500, 200 to 50000, 200 to 25000, 200 to 10000, 200 to 5000, 200 to 4000, 200 to 3000, 2 to 2000 or 200 to 1000.

[0074] In this application, the dielectric constant of the piezoelectric particles 103 refers to the dielectric constant at room temperature (25 ± 5 °C), which has the meaning well known in the art and can be tested by instruments and methods known in the art. For example, after preparing the piezoelectric particles into circular specimens, the capacitance C can be measured by an LCR tester and calculated according to the formula: dielectric constant ε = (C × d) / (ε0 × A). C represents the capacitance, with the unit of farad (F); d represents the thickness of the specimen, with the unit of cm; A represents the area of the specimen, with the unit of cm 2 ; ε0 represents the vacuum permittivity, ε0 = 8.854 × 10 -14 F / cm. In this application, the test conditions can be 1KHz, 1.0V, 25 ± 5 °C. The test standard can be based on GB / T 11297.11 - 2015. When preparing the specimen, reference can be made to the Chinese patent application CN114217139A.

[0075] In some embodiments, the piezoelectric particles 103 are insoluble in water and have a relatively high Curie temperature. For example, the Curie temperature is usually above 100 °C; further, the Curie temperature can be 100 °C to 500 °C.

[0076] In some embodiments, the volume - average particle size Dv50 of the piezoelectric particles 103 is d1 nm, where 0 < d1 ≤ 200; optionally, 0 < d1 ≤ 180, 0 < d1 ≤ 160, 0 < d1 ≤ १५०, 0 < d1 ≤ 120, 0 < d1 ≤ 100, 0 < d1 ≤ 80, 0 < d1 ≤ 60, 0 < d1 ≤ 40. When the volume - average particle size Dv50 of the piezoelectric particles 103 is within the above range, the specific surface area of the piezoelectric particles 103 is relatively moderate. Thus, fewer piezoelectric particles 103 can be used at the same specific surface area, thereby reducing the energy density loss of the secondary battery.

[0077] In the present application, the volume average particle size Dv50 of the material has the meaning well-known in the art, which represents the particle size corresponding to when the cumulative volume distribution percentage of the material reaches 50%, and can be measured by the instruments and methods well-known in the art. For example, it can be conveniently measured by referring to GB / T 19077-2016 Laser diffraction method for particle size distribution and using a laser particle size analyzer, such as the Mastersizer 2000E laser particle size analyzer of Malvern Instruments Limited, UK.

[0078] In some embodiments, based on the total mass of the negative electrode active material 10, the mass content of the piezoelectric particles 103 is a3, and 0.005 ≤ a3 ≤ 0.1. When the mass content a3 of the piezoelectric particles 103 is within the above range, the piezoelectric effect of the piezoelectric particles 103 can be fully exerted, thereby further homogenizing the metal ions between the negative electrode active material 10 and the electrolyte interface.

[0079] In some embodiments, optionally, the piezoelectric particles 103 include a combination of one or more selected from perovskite structure oxides, tungsten bronze type compounds, bismuth oxide type layered structure compounds, lithium niobate (LiNbO3), and lithium tantalate (LiTaO3). More optionally, the piezoelectric particles 103 are selected from perovskite structure oxides.

[0080] Optionally, the perovskite structure oxide has the molecular formula Ba 1-x A x Ti 1-y B y O3. A includes a combination of one or more selected from Pb, Sr, Ca, K, Na, and Cd, B includes a combination of one or more selected from Sn, Hf, Zr, Ce, Nb, and Th, 0 ≤ x ≤ 1, 0 ≤ y ≤ 1. For example, the perovskite structure oxide can include a combination of one or more selected from BaTiO3, Ba 1-x1 Sr x1 TiO3 (0 ≤ x1 ≤ 1), SrTiO3, PbTiO3, PbZr y1 Ti 1-y1 O3 (0 ≤ y1 ≤ 1), BaZr y2 Ti 1-y2 O3 (0 < y2 < 1), KNbO3, NaNbO3. Further, the piezoelectric particles 103 include one or more of barium titanate, lead titanate, lithium niobate, lead zirconate titanate, lead metaniobate, and lithium barium lead niobate. The piezoelectric particles 103 of the above types can effectively exert the piezoelectric effect and reduce the energy barrier in the process of metal ion migration.

[0081] Optionally, the tungsten bronze type compound may have the molecular formula M zWO3. M comprises a combination of one or more selected from Na, K, Rb, and Cs, where 0 < z < 1. For example, the tungsten bronze-type compound may comprise a combination of one or more selected from Na z1 WO3(0 < z1 < 1), K z2 WO3(0 < z2 < 1).

[0082] Optionally, the bismuth oxide-type layered structure compound has the molecular formula (Bi2O2)(C n-1 D n O 3n+1 ). C comprises a combination of one or more selected from Na, K, Ba, Sr, Pb, Ca, Ln, and Bi, D comprises a combination of one or more selected from Zr, Cr, Nb, Ta, Mo, W, Fe, Ti, and V, and 2 ≤ n ≤ 5. For example, the bismuth oxide-type layered structure compound may be one or more combinations of SrBi2Nb2O9, SrBi2Ta2O9, SrBi2Nb2O9, Bi4Ti3O 12 .

[0083] The coating layer 102 is coated on the surface of the silicon-based particle 101, which can provide a certain protection for the silicon-based particle 101; and can act as a medium to bind the piezoelectric particle 103 to the silicon-based particle 101. The coating layer 102 includes a conductive carbon material. The content of locally ordered graphite microcrystals in the conductive carbon material is less, and the crystallinity is lower. Macroscopically, it presents a transition-state carbon material in an approximately amorphous form (or without a fixed shape and periodic structural rules), which can be obtained by carbonization and sintering treatment of a carbon source (such as asphalt, resin, biomass materials, etc.).

[0084] The conductive carbon material in the coating layer 102 can provide capacity for the negative electrode active material 10. The layer spacing of the conductive carbon material is relatively large, and it basically does not cause volume shrinkage and expansion effects during the process of metal ion extraction and insertion. Therefore, its crystal structure is more stable, enabling the negative electrode active material 10 to have good kinetic performance and withstand high-rate charging, thereby improving the fast charging ability of the secondary battery.

[0085] Optionally, the conductive carbon material includes amorphous carbon. Amorphous carbon refers to a transitional carbon material with a low degree of graphitization and crystallization, exhibiting an approximately amorphous morphology (or lacking a fixed shape and periodic structural regularity). It can be obtained through carbonization and sintering treatment with a carbon source (such as pitch, resin, biomass materials, etc.). Amorphous carbon has a larger interlayer spacing and does not cause volume shrinkage or expansion effects during lithium-ion extraction and insertion, resulting in a more stable crystal structure. This enables the negative electrode active material 10 to possess good kinetic performance and withstand high-rate charging, thereby improving the fast-charging capability of the secondary battery. Further, amorphous carbon includes soft carbon, hard carbon, or a combination thereof. Even further, the conductive carbon material includes hard carbon, which can further improve the fast-charging capability of the secondary battery.

[0086] In some embodiments, the average thickness of the coating layer 102 is H nm, where 0 < H ≤ 100; optionally, 0 < H ≤ 50. When the thickness of the coating layer 102 is within the above range, it can suppress the excessive expansion of the silicon-based particles 101 to a certain extent, thereby improving the overall stability of the negative electrode active material 10; and the coating layer 102 can fully exert its dielectric function in binding the piezoelectric particles 103 to the silicon-based particles 101, ensuring the bonding strength between the piezoelectric particles 103 and the silicon-based particles 101.

[0087] The average thickness H of the coating layer 102 can be tested using instruments and methods known in the art. For example, a transmission electron microscope can be used to obtain a TEM image, and then the thickness at multiple (e.g., more than 30) different locations can be measured on the TEM image, and the average value can be taken as the average thickness of the coating layer 102.

[0088] In some embodiments, based on the total mass of the negative electrode active material 10, the mass content of carbon in the coating layer 102 is a2, where 0.02 ≤ a2 ≤ 0.1. When the mass content of carbon in the coating layer 102 is within the above range, it is more beneficial for carbon to increase the overall capacity of the negative electrode active material 10.

[0089] In this application, a coating layer 102 coats silicon-based particles 101. The structure of the coating layer 102 is determined by the external shape of the silicon-based particles 101. For example, if the silicon-based particles 101 are spherical, then the coating layer 102 can be spherical, and its outer surface, i.e., the surface of the coating layer 102 facing away from the silicon-based particles 101, is a spherical surface. Of course, the silicon-based particles 101 may have an irregular geometric structure, and the coating layer 102 may also exhibit an irregular geometric structure. There are various positional relationships between the piezoelectric particles 103 and the coating layer 102, which will be described in detail below.

[0090] In some embodiments, the piezoelectric particles 103 can be completely embedded in the coating layer 102, and the surface of the piezoelectric particles 103 can be regarded as being covered by the coating layer 102. In this case, the piezoelectric particles 103 may not be in direct contact with the electrolyte, but the piezoelectric effect of the piezoelectric particles 103 can still be guaranteed.

[0091] In other embodiments, piezoelectric particles 103 are partially exposed on the surface of the coating layer 102 away from the silicon-based particles 101. The piezoelectric particles 103 have exposed surfaces that allow them to directly contact the electrolyte, thereby further facilitating the reduction of the energy barrier for lithium-ion migration. Specifically, the piezoelectric particles 103 have spontaneous polarization intensity, which can be reversed when the external electric field reverses. This reduces the kinetic energy barrier of the desolvation process of solvated metal ions, increases the speed at which metal ions reach the surface of the silicon-based particles 101, and reduces the resistance to metal ion insertion into the negative electrode. As a result, the negative electrode active material 10 of this application has good kinetic performance and can withstand high-rate charging, thereby improving the fast charging capability of the secondary battery.

[0092] As examples, a portion of the piezoelectric particle 103 protrudes from the surface of the coating layer 102 away from the silicon-based particle 101; thus, the piezoelectric particle 103 has a larger exposed surface area and an increased contact area with the electrolyte, which can reduce the kinetic energy barrier of the desolvation process with a smaller amount.

[0093] As another example, the surface of the piezoelectric particle 103 is flush with the surface of the coating layer 102 facing away from the silicon-based particle 101. The piezoelectric particle 103 is an inactive material and cannot contribute to capacity; its small volume ensures the volume occupied by the active material (e.g., the silicon-based particle 101 and the conductive carbon material), thereby guaranteeing the overall energy density of the negative electrode active material 10. Taking the coating layer 102 as a spherical shape as an example, its surface facing away from the silicon-based particle 101 is a spherical surface. The fact that the surface of the piezoelectric particle 103 is flush with the surface of the coating layer 102 means that the surfaces of the piezoelectric particle 103 and the coating layer 102 are both located on the same spherical surface. Of course, the silicon-based particle 101 may have an irregular geometric structure, and the coating layer 102 may also exhibit an irregular geometric structure.

[0094] As another example, the surface of the coating layer 102 protrudes from the surface of the silicon-based particles 101 and the surface of the piezoelectric particles 103. While reducing the kinetic energy barrier of the metal ion desolvation process, the piezoelectric particles 103 can also reduce the volume occupied by the piezoelectric particles 103, thereby ensuring the overall energy density of the negative electrode active material 10.

[0095] Furthermore, the piezoelectric particle 103 penetrates the coating layer 102 along its thickness direction. One end of the piezoelectric particle 103 is in contact with the electrolyte, and the other end is in contact with the silicon-based particle 101. The surface of the piezoelectric particle 103 in contact with the electrolyte is positively charged, and the surface in contact with the silicon-based particle 101 is negatively charged. This can better reduce the kinetic energy barrier of the desolvation process, increase the speed at which metal ions reach the surface of the silicon-based particle 101, and reduce the resistance to metal ion insertion into the negative electrode. Therefore, the negative electrode active material 10 of this application has good kinetic performance and can withstand high-rate charging, thereby improving the fast charging capability of the secondary battery.

[0096] In some embodiments, the average thickness of the coating layer is H nm; the volume average particle size Dv50 of the piezoelectric particle 103 is d1 nm, wherein H and d1 satisfy the following condition: 0.25 ≤ H / d1 ≤ 1; optionally, 0.25 ≤ H / d1 ≤ 0.5.

[0097] When H / d1 is less than or equal to 1, some piezoelectric particles 103 can be exposed on the surface of the coating layer 102, thereby increasing the contact area with the electrolyte. This allows for the reduction of the kinetic energy barrier in the desolvation process with a smaller amount of material, and also helps to ensure the energy density of the secondary battery. When H / d1 is greater than or equal to 0.25, the contact area between the piezoelectric particles 103 and the electrolyte is larger, which better enables the reduction of the kinetic energy barrier in the desolvation process. Therefore, when H / d1 is within the above range, it is beneficial for the secondary battery to have both high fast charging capability and high energy density. Optionally, 0.25≤H / d1≤0.9, 0.25≤H / d1≤0.8, 0.25≤H / d1≤0.7, 0.25≤H / d1≤0.6, 0.25≤H / d1≤0.5, 0.25≤H / d1≤0.4, 0.30≤H / d1≤0.9, 0.30≤H / d1≤0.8, and 0.30≤H / d1≤0. 7, 0.30≤H / d1≤0.6, 0.30≤H / d1≤0.5, 0.30≤H / d1≤0.4, 0.35≤H / d1≤0.9, 0.35≤H / d1≤0.8, 0.35≤H / d1≤0.7, 0.35≤H / d1≤0.6, 0.35≤H / d1≤0.5 or 0.35≤H / d1≤0.4.

[0098] In some embodiments, the silicon-based particles may comprise silicon and oxygen. Based on the total molar amount of the elements comprised in the silicon-based particles 101, the molar content of silicon in the silicon-based particles 101 is M1; based on the total molar amount of the elements comprised in the silicon-based particles 101, the molar content of oxygen in the silicon-based particles 101 is M2, where 0.5 ≤ M1 / M2 ≤ 2. Specifically, the silicon-based particles 101 may comprise a composite of elemental silicon and silicon dioxide. In this application, the total molar amount of the elements comprised in the silicon-based particles 101 refers to the sum of the molar amounts of all elements in the silicon-based particles 101.

[0099] When the silicon-based particles 101 meet the above-mentioned ranges, the silicon-based particles 101 are more conducive to the utilization of silicon capacity, thereby improving the capacity of the secondary battery. For example, 0.5≤M1 / M2≤1.5, 0.5≤M1 / M2≤1.2, 0.5≤M1 / M2≤1.0, 0.5≤M1 / M2≤0.8, 0.5≤M1 / M2≤0.6, 0.6≤M1 / M2≤1.5, 0.8≤M1 / M2≤1.5, 1.0≤M1 / M2≤1.5, or 1.2≤M1 / M2≤1.5.

[0100] Optionally, the silicon-based particles 101 further include a dopant element M, which includes one or more of Fe, Ti, Ni, Zr, and Co. By doping the silicon-based particles 101 with the dopant element M, the dopant element can occupy local sites in the crystal phase structure, thereby ensuring the structural stability of the silicon-based particles 101 to a certain extent and reducing the risk of excessive expansion of the silicon-based particles 101 leading to the negative electrode active material 10 when the silicon-based particles 101 undergo volume expansion.

[0101] Optionally, based on the total mass of the silicon-based particles 101, the mass content of the dopant element M is 1% to 5%. When the molar content of the dopant element M meets the above range, it is beneficial for the dopant element to ensure the structural stability of the silicon-based particles 101. For example, the mass content of the dopant element M can be 1%, 2%, 3%, 4%, or 5%; or a range of any two of the above values.

[0102] As an example, silicon-based particles 101 may include one or more of elemental silicon, silicon oxide, silicon carbide, and ferrosilicon alloy.

[0103] In some embodiments, based on the total mass of the negative electrode active material 10, the mass content of silicon-based particles 101 is a1, where 0.85 < a1 ≤ 0.97.

[0104] Since the capacity of silicon-based particles 101 is relatively high, when the mass content of silicon-based particles 101 is within the above range, the capacity of the negative electrode active material 10 can be guaranteed, thereby improving the capacity performance of the secondary battery.

[0105] In some embodiments, the volume average particle size Dv50 of the silicon-based particles 101 is d2μm, where 2≤d2≤10; optionally, 3≤d2≤8. The silicon-based particles 101 can be composed of primary particles, secondary particles, or a mixture of primary and secondary particles. When the volume average particle size of the silicon-based particles 101 meets the above range, it ensures that the negative electrode active material 10 has a high compaction density and provides sufficient bonding surface for the piezoelectric particles 103, thereby facilitating the piezoelectric effect of the piezoelectric particles 103.

[0106] In some embodiments, the volume average particle size Dv50 of the piezoelectric particles is d1 nm; the volume average particle size Dv50 of the silicon-based particles 101 is d2 μm, wherein d1 and d2 satisfy the following condition: 0 < d1 / d2 ≤ 50; optionally, 0 < d1 / d2 ≤ 20.

[0107] When d1 / d2 is within the aforementioned range, on the one hand, the piezoelectric effect of the piezoelectric particles 103 can be fully utilized and the kinetic energy barrier of the desolvation process can be reduced; on the other hand, the negative electrode active material 10 of this application can be guaranteed to have a high compaction density, thereby enabling the secondary battery to simultaneously possess high fast charging capability and high energy density. Furthermore, when d1 / d2 is within a suitable range, the preparation difficulty of the negative electrode active material 10 of this application can be reduced, thereby lowering production costs.

[0108] In some embodiments, based on the total mass of the negative electrode active material 10, the mass content of silicon-based particles 101 is a1; based on the total mass of the negative electrode active material 10, the mass content of carbon elements in the coating layer 102 is a2, wherein a1 and a2 satisfy the following condition: 0.02≤a2 / a1≤0.1; optionally, 0.02≤a2 / a1≤0.05.

[0109] When a2 / a1 meets the above range, it is beneficial for the negative electrode active material 10 to have high fast charging capability while also having high specific capacity, high initial coulombic effect and high compaction density. As a result, the secondary battery can simultaneously have high fast charging capability, high energy density and high cycle capacity retention.

[0110] Compared to silicon-based particles 101, the coating layer 102 has a moderate thickness and a moderate content of conductive carbon material. Since the conductive carbon material has fewer pores and a smaller specific surface area, it reduces the occurrence of interfacial side reactions with the electrolyte. Simultaneously, the conductive carbon material has fewer surface morphology defects, making the negative electrode active material 10 easier to compact. Furthermore, the stable surface structure of the negative electrode active material 10 ensures its initial coulombic efficiency and capacity, which is beneficial for the capacity utilization and cycle performance improvement of the secondary battery. The moderate thickness of the coating layer 102 also facilitates the rapid insertion and extraction of metal ions, thereby enhancing the fast-charging capability of the secondary battery.

[0111] In some embodiments, based on the total mass of the negative electrode active material 10, the mass content of silicon-based particles 101 is a1; based on the total mass of the negative electrode active material 10, the mass content of piezoelectric particles 103 is a3, wherein a1 and a3 satisfy the following condition: 0.005≤a3 / a1≤0.1; optionally, 0.01≤a3 / a1≤0.03.

[0112] When the mass content of silicon-based particles 101 and the mass content of piezoelectric particles 103 meet the above ranges, the silicon-based particles 101 can apply compressive stress to the piezoelectric particles 103 at various locations when the volume of silicon-based particles 101 expands, thereby stimulating the piezoelectric effect of the piezoelectric particles 103 at various locations, causing metal ions to migrate uniformly toward the interior of silicon-based particles 101, which is beneficial for the secondary battery to have high and fast charging energy; and the proportion of piezoelectric particles 103 will not be too high, thereby ensuring the high energy density of the secondary battery.

[0113] In some embodiments, the volume average particle size Dv50 of the negative electrode active material 10 is d0 μm, where 3 ≤ d0 ≤ 8.

[0114] By adjusting the volume average particle size Dv50 of the negative electrode active material 10 to a suitable range, it is beneficial for the negative electrode active material 10 to have better ion transport and electron transport performance as well as fast charging performance, while also having a higher powder compaction density.

[0115] In some embodiments, the BET specific surface area of ​​the negative electrode active material 10 is S0 m². 2 / g, 1≤S0≤4.

[0116] By adjusting the BET specific surface area of ​​the negative electrode active material 10 to a suitable range, the interfacial side reactions between the negative electrode sheet using it and the electrolyte can be reduced, and the negative electrode sheet using it can also have suitable electrochemical reaction activity, thereby enabling the secondary battery to have a higher fast charging capability.

[0117] In this application, the specific surface area of ​​a material has a meaning known in the art and can be tested using instruments and methods known in the art. For example, it can be tested using the nitrogen adsorption specific surface area analysis method according to GB / T 19587-2017 and calculated using the BET (Brunauer Emmett Teller) method. The nitrogen adsorption specific surface area analysis can be performed using the Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, Inc., USA.

[0118] In some embodiments, the powder compaction density of the negative electrode active material 10 under a force of 20000N is P0 g / cm³. 3 , 1.1≤P0≤1.4.

[0119] By adjusting the powder compaction density of the negative electrode active material 10 within a suitable range, the negative electrode film can achieve a higher compaction density, thereby resulting in a higher energy density for the secondary battery. Furthermore, by adjusting the powder compaction density of the negative electrode active material 10 within a suitable range, the negative electrode film can also maintain a stronger pore structure during cycling, leading to better electrolyte wettability of the negative electrode sheet and improved cycle performance of the secondary battery.

[0120] In this application, the powder compaction density of the material has a meaning known in the art and can be tested using instruments and methods known in the art. For example, it can be tested using an electronic pressure testing machine (e.g., UTM7305 type) in accordance with standard GB / T24533-2009. An exemplary test method is as follows: Weigh 1g of material and add it to a mold with a bottom area of ​​1.327cm2, pressurize it to 2000kg (equivalent to 20000N), hold the pressure for 30s, then release the pressure, hold for 10s, and then record and calculate the powder compaction density of the material under a force of 20000N.

[0121] It should be noted that the various parameter tests for the negative electrode active material 10 mentioned above can be performed by sampling before coating or by sampling from the cold-pressed negative electrode film layer. When the negative electrode active material 10 test sample is taken from the cold-pressed negative electrode film layer, as an example, sampling can be performed as follows: Randomly select a cold-pressed negative electrode film layer and sample the negative electrode active material 10 (for example, a blade can be used to scrape the powder); place the collected negative electrode active material 10 powder in deionized water, then filter and dry it; then sinter the dried negative electrode active material 10 at a certain temperature and time (e.g., 400℃, 2h) to remove the binder and conductive agent, thus obtaining the negative electrode active material 10 test sample.

[0122] Methods for preparing negative electrode active materials

[0123] A second aspect of this application provides a method for preparing a negative electrode active material, which can be used to prepare the negative electrode active material of the first aspect of this application. The method includes the following steps:

[0124] S10 provides silicon-based particles, carbon sources, and piezoelectric particles;

[0125] S20, silicon-based particles, carbon source and piezoelectric particles are uniformly mixed, and a coating layer including conductive carbon material is formed on at least part of the surface of silicon-based particles by carbonization sintering treatment, wherein the piezoelectric particles are dispersed in the coating layer.

[0126] A carbon source is a compound that can form conductive carbon materials. The carbon source is selected from organic and / or inorganic carbon sources. Optionally, the carbon source is an organic carbon source.

[0127] In some embodiments, the carbon source includes one or more combinations selected from bitumen, resin, and biomass materials. As an example, bitumen includes one or more combinations selected from coal tar pitch and petroleum asphalt, optionally petroleum asphalt. As an example, resin includes one or more combinations selected from phenolic resin and epoxy resin. As an example, biomass materials refer to materials derived from living organisms such as animals, plants, and microorganisms, mainly composed of organic polymers, and chemically composed primarily of carbon, hydrogen, and oxygen; for example, they can be polysaccharides (such as starch, sucrose polymers, glucose polymers, cellulose, etc.). The above-mentioned carbon sources have good fluidity, which is beneficial for fully coating the surface of silicon-based particles.

[0128] In some embodiments, a carbon source is coated onto the surface of silicon-based particles, specifically using a solid-phase coating method or a liquid-phase coating method. In the solid-phase coating method, the carbon source is decomposed under high-temperature conditions to form a coating layer containing conductive carbon material, which then adheres to the surface of the silicon-based particles. In the liquid-phase coating method, a liquid carbon source is mixed with silicon-based particles and piezoelectric particles and then coated, followed by carbonization treatment.

[0129] In some embodiments, the carbonization sintering temperature in S20 is 900°C to 1500°C, and more preferably 1000°C to 1300°C.

[0130] In some embodiments, the carbonization sintering time in S20 is 1 h to 8 h.

[0131] In S20, by controlling the carbonization sintering temperature and carbonization sintering time within the above range, the carbon source can be carbonized, and a coating layer containing conductive carbon material can be formed on at least a portion of the surface of the silicon-based particles. At the same time, the coating layer can also have a suitable thickness, and the piezoelectric particles can be uniformly dispersed in the coating layer.

[0132] Negative electrode sheet

[0133] A third aspect of this application provides a negative electrode sheet, the negative electrode sheet comprising a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer comprising the negative electrode active material of the first aspect of this application or a negative electrode active material prepared according to the method of the second aspect of this application. For example, the negative current collector has two surfaces opposite to each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative current collector.

[0134] In some embodiments, the negative electrode film may further comprise other negative electrode active materials known in the art for use in secondary batteries. As examples, other negative electrode active materials include one or more combinations selected from natural graphite, artificial graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. Silicon-based materials may include one or more combinations selected from elemental silicon, silicon oxide, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may include one or more combinations selected from elemental tin, tin oxide, and tin alloys.

[0135] In some embodiments, the negative electrode film layer may optionally include a negative electrode conductive agent. This application does not impose any particular limitation on the type of negative electrode conductive agent. As an example, the negative electrode conductive agent may include one or more combinations selected from superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, the mass percentage of the negative electrode conductive agent is less than 5% based on the total mass of the negative electrode film layer.

[0136] In some embodiments, the negative electrode film layer may optionally include a negative electrode binder. This application does not impose any particular limitation on the type of negative electrode binder. As an example, the negative electrode binder may include one or more combinations selected from styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, waterborne acrylic resins (e.g., polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS). In some embodiments, the mass percentage of the negative electrode binder is less than 5% based on the total mass of the negative electrode film layer.

[0137] In some embodiments, the negative electrode film may optionally include other additives. As an example, other additives may include thickeners, such as sodium carboxymethyl cellulose (CMC-Na), PTC thermistor materials, etc. In some embodiments, the mass percentage of other additives is less than 2% based on the total mass of the negative electrode film.

[0138] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. Examples of metal foils include copper foil or copper alloy foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. Examples of the metal material include one or more combinations selected from copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. The polymer substrate may include one or more combinations selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0139] The negative electrode film is typically formed by coating a negative electrode slurry onto a negative electrode current collector, followed by drying and cold pressing. The negative electrode slurry is usually formed by dispersing the negative electrode active material, optional conductive agent, optional binder, and other optional additives in a solvent and stirring until homogeneous. The solvent can be N-methylpyrrolidone (NMP) or deionized water, but is not limited to these.

[0140] The negative electrode sheet does not exclude other additional functional layers besides the negative electrode film layer. For example, in some embodiments, the negative electrode sheet of this application further includes a conductive undercoat layer (e.g., composed of a conductive agent and an adhesive) sandwiched between the negative electrode current collector and the negative electrode film layer and disposed on the surface of the negative electrode current collector. In other embodiments, the negative electrode sheet of this application further includes a protective layer covering the surface of the negative electrode film layer.

[0141] Secondary batteries

[0142] The fourth aspect of this application provides a secondary battery that includes the negative electrode sheet of the third aspect of this application.

[0143] A secondary battery, also known as a rechargeable battery or accumulator, is a battery that can be recharged after discharge to reactivate its active materials and continue to be used. A secondary battery includes an electrode assembly and an electrolyte. The electrode assembly typically includes a positive electrode, a negative electrode, and a separator. The separator is positioned between the positive and negative electrodes, primarily preventing short circuits between them while allowing metal ions to pass through. The electrolyte, located between the positive and negative electrodes, conducts metal ions. The secondary battery described in this application can be a lithium-ion secondary battery, a sodium-ion battery, etc., and particularly, a lithium-ion secondary battery.

[0144] [Negative electrode plate]

[0145] The negative electrode used in the secondary battery of this application is the negative electrode of any embodiment of the third aspect of this application.

[0146] [Positive electrode plate]

[0147] In some embodiments, the positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector. For example, the positive current collector has two surfaces opposite each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0148] The positive electrode film includes a positive electrode active material, which may be a positive electrode active material known in the art for use in secondary batteries. For example, the positive electrode active material may include one or more combinations selected from lithium transition metal oxides, lithium-containing phosphates with an olivine structure, and their respective modified compounds. Examples of lithium transition metal oxides may include one or more combinations selected from lithium cobalt oxides, lithium nickel oxides, lithium manganese oxides, lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, lithium nickel cobalt manganese oxides, lithium nickel cobalt aluminum oxides, and their respective modified compounds. Examples of lithium-containing phosphates with an olivine structure may include one or more combinations selected from lithium iron phosphate, lithium iron phosphate and carbon composites, lithium manganese phosphate, lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, lithium manganese iron phosphate and carbon composites, and their respective modified compounds.

[0149] In some embodiments, to further improve the energy density of the secondary battery, the positive electrode active material may include one or more combinations of lithium transition metal oxides and their modified compounds as shown in Formula 1.

[0150] Li a Ni b Co c M d O e A f Formula 1

[0151] In Equation 1, 0.8≤a≤1.2, 0.5≤b<1, 0<c<1, 0<d<1, 1≤e≤2, 0≤f≤1, M includes one or more combinations selected from Mn, Al, Zr, Zn, Cu, Cr, Mg, Fe, V, Ti and B, and A includes one or more combinations selected from N, F, S and Cl.

[0152] In this application, the modified compounds of the above-mentioned positive electrode active materials can be those that have been doped or surface-coated to modify the positive electrode active materials.

[0153] In some embodiments, the positive electrode film may optionally include a positive electrode conductive agent. This application does not impose any particular limitation on the type of positive electrode conductive agent. As an example, the positive electrode conductive agent includes one or more combinations selected from superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, the mass percentage of the positive electrode conductive agent is less than 5% based on the total mass of the positive electrode film.

[0154] In some embodiments, the positive electrode film layer may optionally include a positive electrode binder. This application does not impose any particular limitation on the type of positive electrode binder. As an example, the positive electrode binder may include one or more combinations selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resins. In some embodiments, the mass percentage of the positive electrode binder is less than 5% based on the total mass of the positive electrode film layer.

[0155] In some embodiments, the positive current collector may be a metal foil or a composite current collector. Examples of metal foils include aluminum foil or aluminum alloy foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. Examples of the metal material include one or more combinations selected from aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. The polymer substrate may include one or more combinations selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0156] The positive electrode film is typically formed by coating a positive electrode slurry onto a positive electrode current collector, followed by drying and cold pressing. The positive electrode slurry is usually formed by dispersing the positive electrode active material, optional conductive agent, optional binder, and any other components in a solvent and stirring until homogeneous. The solvent can be N-methylpyrrolidone (NMP), but is not limited to it.

[0157] Electrolyte

[0158] The electrolyte used in this application may be a known electrolyte for secondary batteries. The electrolyte includes lithium salts and organic solvents.

[0159] As an example, lithium salts may include one or more combinations selected from lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP).

[0160] As an example, the organic solvent may include one or more combinations selected from ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butyl ester carbonate (BC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0161] [Isolation membrane]

[0162] This application does not impose any particular restrictions on the type of separator membrane; any known porous separator membrane with good chemical and mechanical stability can be selected.

[0163] In some embodiments, the material of the separator may include one or more combinations selected from glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of each layer may be the same or different.

[0164] In some embodiments, the positive electrode, the separator, and the negative electrode can be fabricated into an electrode assembly by a winding process or a stacking process.

[0165] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0166] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic, such as one or more combinations of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0167] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. Figure 2 This is an example of a square-structured secondary battery 5.

[0168] In some embodiments, such as Figure 3As shown, the outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 is used to cover the opening to close the receiving cavity. Positive electrode sheets, negative electrode sheets, and a separator may be formed into an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is encapsulated in the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be adjusted according to requirements.

[0169] The method for preparing the secondary battery described in this application is well known. In some embodiments, a positive electrode, a separator, a negative electrode, and an electrolyte can be assembled to form a secondary battery. As an example, the positive electrode, separator, and negative electrode can be formed into an electrode assembly through a winding or stacking process. The electrode assembly is then placed in an outer packaging, dried, and injected with an electrolyte. After vacuum sealing, settling, formation, and shaping, a secondary battery is obtained.

[0170] In some embodiments of this application, the secondary battery according to this application can be assembled into a battery module. The number of secondary batteries contained in the battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module.

[0171] Figure 4 This is a schematic diagram of battery module 4 as an example. Figure 4 As shown, in battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0172] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0173] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0174] Figure 5 and Figure 6 This is a schematic diagram of battery pack 1 as an example. Figure 5 and Figure 6 As shown, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3. The upper body 2 covers the lower body 3, forming a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0175] Electrical appliances

[0176] The fifth aspect of this application provides an electrical device, which includes at least one of the secondary battery, battery module, and battery pack described in this application. The secondary battery, battery module, and battery pack can be used as a power source for the electrical device or as an energy storage unit of the electrical device. The electrical device can be, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0177] Electrical devices can be equipped with secondary batteries, battery modules, or battery packs depending on their usage requirements.

[0178] Figure 7 This is a schematic diagram of an example electrical device. The electrical device 6 is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of this electrical device, a battery pack 1 or a battery module can be used.

[0179] Another example of an electrical device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use rechargeable batteries as their power source.

[0180] Example

[0181] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0182] Example 1

[0183] 1. Preparation of negative electrode sheet

[0184] 1.1 Preparation of negative electrode active materials

[0185] Silicon-oxygen aggregate powder was selected as the silicon-based particle.

[0186] The obtained silicon-based particles were uniformly mixed with carbon source petroleum asphalt and piezoelectric particles BaTiO3, and then subjected to carbonization and sintering treatment in a track kiln. The highest temperature zone T1 was approximately 1150℃, and the highest temperature zone running time t1 was approximately 4 hours, in order to form a coating layer containing conductive carbon material on at least a portion of the surface of the silicon-based particles, thereby obtaining the negative electrode active material. BaTiO3 was dispersed in the coating layer.

[0187] 1.2 Preparation of negative electrode sheet

[0188] A copper foil with a thickness of 10 μm was used as the negative electrode current collector.

[0189] The prepared negative electrode active material, styrene-butadiene rubber (SBR) binder, sodium carboxymethyl cellulose (CMC-Na) thickener, and carbon black conductive agent were mixed thoroughly in an appropriate amount of deionized water at a mass ratio of 96.8:1.2:1.2:0.8 to form a uniform negative electrode slurry. The negative electrode slurry was then uniformly coated onto the surface of a copper foil current collector. After drying and cold pressing, the negative electrode sheet was obtained. The coating amount was 0.162 kg / cm². 2 The compacted density is 1.65 g / cm³. 3 .

[0190] 2. Preparation of the positive electrode sheet

[0191] Aluminum foil with a thickness of 6μm was used as the positive electrode current collector.

[0192] LiNi, the positive electrode active material 0.5 Co 0.2 Mn 0.3 O2 (NCM523), conductive agent carbon black, and binder polyvinylidene fluoride (PVDF) were thoroughly mixed in an appropriate amount of solvent N-methylpyrrolidone (NMP) at a mass ratio of 96.2:1.8:2 to form a uniform positive electrode slurry. The positive electrode slurry was then uniformly coated onto the surface of the positive electrode current collector aluminum foil. After drying and cold pressing, the positive electrode sheet was obtained. The coating amount was 0.256 kg / cm². 2 The compacted density is 3.4 g / cm³. 3 .

[0193] 3. Preparation of electrolyte

[0194] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Then, fully dried LiPF6 was dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0195] 4. Preparation of the separating membrane

[0196] Porous polyethylene film is used as the separator.

[0197] 5. Preparation of secondary batteries

[0198] The positive electrode, separator, and negative electrode are stacked and wound in sequence to obtain an electrode assembly. The electrode assembly is placed in an outer packaging, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a secondary battery is obtained.

[0199] Examples 2 to 5

[0200] The secondary batteries of Examples 2 to 5 were prepared in a similar manner to those of Example 1, except that the particle size d1 of the piezoelectric particles in the "Preparation of Negative Electrode Active Material" was adjusted.

[0201] Comparative Example 1

[0202] The secondary battery of Comparative Example 1 was prepared in a similar manner to that of Example 1, except that conventional uncoated silicon-based particles were used as the negative electrode active material.

[0203] Comparative Example 2

[0204] The secondary battery of Comparative Example 2 was prepared using a method similar to that of Example 1, except that the negative electrode active material was prepared using the following method, and the piezoelectric particles were not dispersed in the coating layer of Comparative Example 2; specifically as follows:

[0205] The obtained silicon-based particles are uniformly mixed with carbon source petroleum asphalt and then subjected to carbonization and sintering treatment in a track kiln. The highest temperature zone T1 is about 1150°C and the highest temperature zone running time t1 is about 4 hours, so as to form a coating layer containing conductive carbon material on at least part of the surface of the silicon-based particles, thereby obtaining the negative electrode active material.

[0206] Comparative Example 3

[0207] The secondary battery of Comparative Example 3 was prepared using a method similar to that of Example 1, except that the negative electrode active material and the negative electrode sheet were prepared using the following method: In Comparative Example 3, the piezoelectric particles were not dispersed in the coating layer; instead, the piezoelectric particles were mixed with the negative electrode active material during the preparation of the negative electrode slurry. Specifically:

[0208] The obtained silicon-based particles are mixed with carbon source petroleum asphalt and then carbonized and sintered in a track kiln. The highest temperature zone is about 1150°C and the highest temperature zone runs for about 4 hours to form a conductive carbon material coating layer on at least a portion of the surface of the graphite particles, thus obtaining the negative electrode active material.

[0209] The prepared negative electrode active material, piezoelectric particles BaTiO3, binder styrene-butadiene rubber (SBR), thickener sodium carboxymethyl cellulose (CMC-Na), and conductive agent carbon black were mixed thoroughly in an appropriate amount of deionized water at a mass ratio of 93.9:2.9:1.2:1.2:0.8 to form a uniform negative electrode slurry. The negative electrode slurry was uniformly coated on the surface of the negative electrode current collector copper foil, and after drying and cold pressing, the negative electrode sheet was obtained.

[0210] Comparative Example 4

[0211] The secondary battery of Comparative Example 4 was prepared using a method similar to that of Example 1, except that the negative electrode active material was prepared using the following method: Comparative Example 4 directly used piezoelectric particles to coat silicon-based particles; specifically as follows:

[0212] The obtained silicon-based particles and piezoelectric particles BaTiO3 were ball-milled and mixed uniformly at a mass ratio of 97:3.

[0213] The parameters for the embodiments and comparative examples are shown in Tables 1 and 2.

[0214] Table 1

[0215]

[0216]

[0217] Table 2

[0218] project H / d1 d1 / d2 a3 / a1 a2 / a1 α1:α2 Example 1 0.36 23.4 0.03 0.02 1.50 Example 2 0.16 53.2 0.03 0.02 1.50 Example 3 0.25 34.0 0.03 0.02 1.50 Example 4 0.50 17.0 0.03 0.02 1.50 Example 5 0.80 10.6 0.03 0.02 1.50 Example 6 0.36 23.4 0.005 0.02 0.25 Example 7 0.36 23.4 0.01 0.02 0.50 Example 8 0.36 23.4 0.02 0.02 1.00 Example 9 0.36 23.4 0.05 0.02 2.50 Example 10 0.36 23.4 0.10 0.02 5.00 Example 11 0.20 23.4 0.03 0.01 3.00 Example 12 0.53 23.4 0.03 0.03 1.00 Example 13 0.82 23.4 0.03 0.05 0.60 Example 14 1.64 23.4 0.03 0.10 0.30 Comparative Example 1 / / / / / Comparative Example 2 / / / 0.02 / Comparative Example 3 / / 0.03 0.02 / Comparative Example 4 / / 0.03 / /

[0219] In Table 2, α1:α2 refers to (a3 / a1) / (a2 / a1) = a3 / a2.

[0220] Table 3

[0221]

[0222]

[0223] Test section

[0224] 1. Testing of negative electrode active materials

[0225] 1.1 Volume average particle size Dv50 test

[0226] A certain amount of the prepared negative electrode active material sample was taken and the volume average particle size Dv50 was measured using a Mastersizer 2000E laser particle size analyzer. The testing standard was based on GB / T 19077-2016.

[0227] A certain amount of silicon-based particles were taken as a test sample, and the volume average particle size (Dv50) was measured using a Mastersizer 2000E laser particle size analyzer. The testing standard was based on GB / T 19077-2016.

[0228] A certain amount of piezoelectric particles were taken as test samples, and the volume average particle size (Dv50) was measured using a Mastersizer 2000E laser particle size analyzer. The test standard was based on GB / T 19077-2016.

[0229] 1.2BET Specific Surface Area Test

[0230] A certain amount of the prepared negative electrode active material sample was taken, and the specific surface area was measured using a Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, USA. The specific surface area was calculated using the BET (Brunauer-Emmett-Teller) method. The testing standard was based on GB / T 19587-2017.

[0231] 1.3 Powder compaction density test

[0232] A certain amount of the negative electrode active material sample prepared above was taken and placed in a UTM7305 electronic pressure testing machine with a bottom area of ​​1.327 cm². 2 In the mold, pressure was applied to 2000 kg (equivalent to 20000 N), held for 30 seconds, then depressurized and held for 10 seconds. The compacted density of the negative electrode active material under a force of 20000 N was then recorded and calculated. The test standard was based on GB / T24533-2009.

[0233] 1.4 Element Content Test

[0234] Determination of carbon / metals / other nonmetals:

[0235] The carbon / metal / other non-metallic element content was obtained using inductively coupled plasma emission spectra (ICP) obtained from an Agilent ICP-OES730, and then the carbon / metal / other non-metallic element content was calculated from the ICP results.

[0236] How to determine the molecular formula of piezoelectric particles:

[0237] A certain amount of the prepared negative electrode active material sample was added to a Bruker AXS D8-focus X-ray diffractometer (Germany) to obtain the X-ray diffraction pattern of the sample. The phase composition of the sample was then qualitatively determined by comparing the pattern with that of a standard substance. The testing standard referenced was JIS K0131-1996.

[0238] 1.5 Average thickness test of coating layer

[0239] Take a certain amount of the negative electrode active material sample prepared above, cut a thin slice of about 100 nm from the middle of a single particle, and then perform transmission electron microscopy analysis on the thin slice to obtain TEM image. Then measure the thickness at multiple (e.g., more than 30) different positions on the TEM image and take the average value as the average thickness of the coating layer.

[0240] Furthermore, the TEM image shows a clear grain boundary between the surface carbon coating layer and the internal silicon-based material.

[0241] 1.6 Initial Capacity Test

[0242] The negative electrode active material, conductive carbon black, and binder polyvinylidene fluoride (PVDF) prepared above were mixed evenly with solvent N-methylpyrrolidone (NMP) at a mass ratio of 91.6:1.8:6.6 to form a slurry. The prepared slurry was coated onto copper foil and dried in an oven for later use. Then, a lithium metal sheet was used as the counter electrode, a polyethylene (PE) film was used as the separator, a few drops of the same electrolyte as the above secondary battery were added, and the CR2430 coin cell was assembled in an argon-protected glove box.

[0243] After the obtained coin cells were allowed to stand for 12 hours, they were discharged at 25°C with a constant current of 0.05C to 0.005V, allowed to stand for 10 minutes, and then discharged again with a constant current of 50μA to 0.005V. After standing for 10 minutes, they were discharged again with a constant current of 10μA to 0.005V. Then, they were charged at a constant current of 0.1C to 2V, and the charging capacity was recorded. The ratio of the charging capacity to the mass of the negative electrode active material is the initial specific capacity of the negative electrode active material.

[0244] 2. Performance testing of secondary batteries

[0245] 2.1 Fast charging performance test of secondary batteries

[0246] At 25°C, the prepared secondary battery was charged at a constant current of 0.33C to the charging cutoff voltage of 4.4V, then charged at a constant voltage to a current of 0.05C, left to stand for 5 minutes, and then discharged at a constant current of 0.33C to the discharge cutoff voltage of 2.8V. Its actual capacity was recorded as C0.

[0247] Then, the secondary battery was sequentially charged at a constant current of 0.5C0, 1C0, 1.5C0, 2C0, 2.5C0, 3C0, 3.5C0, 4C0, and 4.5C0 until the full battery charging cutoff voltage of 4.4V or the negative terminal cutoff potential of 0V (whichever comes first). After each charging, it was discharged at 1C0 until the full battery discharge cutoff voltage of 2.8V. The state of charge (SOC) was recorded at different charging rates until 10%, 20%, 30%...80%. By plotting the negative electrode potential corresponding to the state of charge (SOC), rate-negative electrode potential curves are generated for different SOC states. Linear fitting yields the charging rate corresponding to a negative electrode potential of 0V at each SOC state. This charging rate is the charging window for that SOC state, denoted as C10%SOC, C20%SOC, C30%SOC, C40%SOC, C50%SOC, C60%SOC, C70%SOC, and C80%SOC. The charging time T from 10% SOC to 80% SOC is calculated using the formula (60 / C20%SOC + 60 / C30%SOC + 60 / C40%SOC + 60 / C50%SOC + 60 / C60%SOC + 60 / C70%SOC + 60 / C80%SOC) × 10%. A shorter charging time T indicates better fast-charging performance of the secondary battery.

[0248] 2.2 Cycle performance test of secondary batteries

[0249] At 25°C, the prepared secondary battery was charged at a constant current of 0.33C to the charging cutoff voltage of 4.4V, then charged at a constant voltage to a current of 0.05C, allowed to stand for 5 minutes, and then discharged at a constant current of 0.33C to the discharge cutoff voltage of 2.8V. Its initial capacity was recorded as C0. Then, charging was performed according to the strategy shown in Table 4, followed by discharging at 0.33C. The discharge capacity Cn for each cycle was recorded until the cycle capacity retention rate (i.e., Cn / C0×100%) reached 80%, and the number of cycles was recorded. A higher number of cycles indicates better cycle performance of the secondary battery.

[0250] Table 4

[0251] State of charge (SOC) of a secondary battery Charging rate (C) 0～10％ 0.33 10％～20％ 5.2 20％～30％ 4.5 30％～40％ 4.2 40％～50％ 3.3 50％～60％ 2.6 60％～70％ 2.0 70％～80％ 1.5 80％～100％ 0.33

[0252] 2.3 Secondary battery 4C charging resistor

[0253] At 25°C, the prepared secondary battery was discharged to 50% capacity, allowed to stand for 30 minutes, and the voltage value V1 was recorded. It was then charged at a 4C rate with a current A0 for 10 seconds, and the voltage value V2 at the end of charging was recorded. The charging resistance was calculated as: R = (V2 - V1) / A0. In this application, the kinetic performance of the secondary battery was evaluated using the 4C charging resistance at 25°C.

[0254] 2.4 First Coulomb Efficiency Test of Secondary Batteries

[0255] In the process of preparing the secondary batteries described above, the following tests were performed: the unformed secondary batteries were first charged to 3.4V at 25℃ and 0.02C, then charged to 3.8V at 0.1C, then charged to 4.35V at a constant current of 0.5C, and then charged to 0.05C at a constant voltage of 4.35V. After resting for 10 minutes, they were discharged at a constant current of 0.5C with a cutoff voltage of 3.0V (5 batteries per group). After obtaining the first-cycle charging capacity and the first-cycle discharging capacity of the secondary batteries, the first coulombic efficiency of the secondary batteries was calculated according to the following formula.

[0256] First-week coulombic efficiency (%) = (First-week discharge capacity / First-week charge capacity) × 100%

[0257] Test Results

[0258] The effect of this application on improving the fast charging performance of secondary batteries is shown in Table 5.

[0259] Table 5

[0260]

[0261]

[0262] Table 5 shows that Comparative Example 1, which did not have piezoelectric particles coated, had relatively poor overall performance of its secondary battery. Comparative Example 2 used carbon coating on the silicon-based material, which effectively mitigated the volume expansion of the silicon-based material and improved the performance of the secondary battery to some extent. Comparative Example 3, based on Comparative Example 2, mixed piezoelectric particles and negative electrode active material as the negative electrode slurry, but it did not significantly improve the performance of the secondary battery. Comparative Example 4 directly mixed silicon-based particles and piezoelectric particles as the negative electrode active material; however, because the volume expansion of the silicon-based particles could not be well suppressed, the overall performance of the secondary battery was poor.

[0263] Compared to the comparative examples, the embodiments of this application have a coating layer on the surface of silicon-based particles, and piezoelectric particles are dispersed in the coating layer, which can effectively improve the performance of the secondary battery. Specifically, the particle size d1 of the piezoelectric particles in Examples 1 to 5 is different, which causes the ratio of at least one of H / d1 and d1 / d2 to change. When 0.25≤H / d1≤1 and 0<d1 / d2≤50 are satisfied, especially when 0.25≤H / d1≤0.5 and 0<d1 / d2≤20 are satisfied (Example 4), the fast charging performance of the secondary battery is better. In Examples 6 to 10, a3 / a1 is different. When 0.005≤a3 / a1≤0.1 are satisfied, especially when 0.01≤a3 / a1≤0.03, the fast charging performance of the secondary battery is better. In Examples 11 to 14, a2 / a1 are different. When 0.02≤a2 / a1≤0.1, especially when 0.02≤a2 / a1≤0.05, the fast charging performance of the secondary battery is better.

[0264] Although this application has been described with reference to preferred embodiments, various modifications can be made thereto and components can be replaced with equivalents without departing from the scope of this application. In particular, the technical features mentioned in the various embodiments can be combined in any manner, provided there is no structural conflict. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A negative electrode active material, comprising: Silicon-based particles; A coating layer is applied to at least a portion of the surface of the silicon-based particles, the coating layer comprising a conductive carbon material; as well as Piezoelectric particles are dispersed in the coating layer; The piezoelectric particle penetrates the coating layer along the thickness direction of the coating layer, with one end of the piezoelectric particle in contact with the electrolyte and the other end in contact with the silicon-based particles; The average thickness of the coating layer is H nm; the volume average particle size Dv50 of the piezoelectric particles is d1 nm, wherein H and d1 satisfy the following condition: 0.25 ≤ H / d1 < 1.

2. The negative electrode active material according to claim 1, characterized in that, The dielectric constant of the piezoelectric particles is greater than that of the conductive carbon material; The dielectric constant ε of the piezoelectric particles satisfies: 100≤ε≤100000.

3. The negative electrode active material according to claim 1, characterized in that, The average thickness of the coating layer is H nm; The volume average particle size Dv50 of the piezoelectric particles is d1 nm. Among them, H and d1 satisfy the condition: 0.25≤H / d1≤0.

5.

4. The negative electrode active material according to claim 1, characterized in that, The volume average particle size Dv50 of the piezoelectric particles is d1 nm. The volume average particle size Dv50 of the silicon-based particles is d2 μm. Among them, d1 and d2 satisfy the condition: 0 < d1 / d2 ≤ 50.

5. The negative electrode active material according to claim 4, characterized in that, 0 < d1 / d2 ≤ 20.

6. The negative electrode active material according to claim 1, characterized in that, The volume average particle size Dv50 of the piezoelectric particles is d1 nm, 0 < d1 ≤ 200; and / or The volume average particle size Dv50 of the silicon-based particles is d2μm, where 2≤d2≤10; and / or The average thickness of the coating layer is H nm, where 0 < H ≤ 100.

7. The negative electrode active material according to claim 6, characterized in that, 0 < d1 ≤ 100; and / or 3≤d2≤8; and / or 0＜H≤50。 8. The negative electrode active material according to claim 1, characterized in that, Based on the total mass of the negative electrode active material, the mass content of the silicon-based particles is a1; Based on the total mass of the negative electrode active material, the mass content of carbon in the coating layer is a2. Among them, a1 and a2 satisfy the condition: 0.02≤a2 / a1≤0.

1.

9. The negative electrode active material according to claim 8, characterized in that, 0.02≤a2 / a1≤0.

05.

10. The negative electrode active material according to claim 1, characterized in that, Based on the total mass of the negative electrode active material, the mass content of the silicon-based particles is a1; Based on the total mass of the negative electrode active material, the mass content of the piezoelectric particles is a3. Among them, a1 and a3 satisfy the condition: 0.005≤a3 / a1≤0.

1.

11. The negative electrode active material according to claim 10, characterized in that, 0.01≤a3 / a1≤0.

03.

12. The negative electrode active material according to claim 1, characterized in that, The piezoelectric particles include one or more of barium titanate, lead titanate, lithium niobate, lead zirconate titanate, lead metaniobate, and lead barium lithium niobate; and / or The conductive carbon material in the coating layer includes amorphous carbon.

13. The negative electrode active material according to claim 12, characterized in that, The conductive carbon material includes hard carbon.

14. The negative electrode active material according to claim 1, characterized in that, The silicon-based particles include silicon and oxygen elements; Based on the total molar amount of elements contained in the silicon-based particles, the molar content of silicon in the silicon-based particles is M1. Based on the total molar amount of elements contained in the silicon-based particles, the molar content of oxygen in the silicon-based particles is M2. Where 0.5≤M1 / M2≤2.

15. The negative electrode active material according to claim 14, characterized in that, The silicon-based particles also include a doping element M, which includes one or more of Fe, Ti, Ni, Zr and Co. Based on the total mass of the silicon-based particles, the mass content of the dopant element M is 1% to 5%.

16. The negative electrode active material according to claim 14, characterized in that, The silicon-based particles include one or more of elemental silicon, silicon oxide, silicon carbide, and ferrosilicon alloy.

17. The negative electrode active material according to any one of claims 1 to 16, characterized in that, The negative electrode active material satisfies at least one of the following conditions (1) to (3): (1) The volume average particle size Dv50 of the negative electrode active material is d0 μm, 3≤d0≤8; (2) The BET specific surface area of ​​the negative electrode active material is S0 m 2 / g, 1≤S0≤3; (3) The compacted density of the negative electrode active material under a force of 20000N is P0 g / cm³. 3 , 1.1≤P0≤1.

4.

18. A method for preparing a negative electrode active material, characterized in that, include: S10 provides silicon-based particles, carbon sources, and piezoelectric particles; S20, the silicon-based particles, the carbon source, and the piezoelectric particles are uniformly mixed, and a coating layer including conductive carbon material is formed on at least a portion of the surface of the silicon-based particles by carbonization sintering treatment, wherein the piezoelectric particles are dispersed in the coating layer; Optionally, the dielectric constant of the piezoelectric particles is greater than the dielectric constant of the conductive carbon material.

19. The method according to claim 18, characterized in that, In S20, the carbonization and sintering temperature is 900°C to 1500°C; and / or, In S20, the carbonization and sintering time is 1 hour to 8 hours.

20. A negative electrode sheet, characterized in that, include: Negative electrode current collector; A negative electrode film layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode film layer comprises the negative electrode active material according to any one of claims 1 to 17 or the negative electrode active material prepared according to the method of claim 18 or 19.

21. A secondary battery, characterized in that, Includes the negative electrode sheet according to claim 20.

22. An electrical appliance, characterized in that, Includes the secondary battery according to claim 21.

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