A negative electrode active material and its preparation method, a negative electrode sheet, a secondary battery, and an electrical device thereof.

Abstract

This invention discloses a negative electrode active material and its preparation method, a negative electrode sheet, a secondary battery, and an electrical device, relating to the field of solid-state battery negative electrode material technology. The negative electrode active material includes a host material, which comprises porous microcrystalline graphite and a first lithium-ion conductive material. The first lithium-ion conductive material is at least partially filled within the pores of the porous microcrystalline graphite. The porous microcrystalline graphite has a first pore and a second pore, with the first pore having a diameter of 1-10 nm and the second pore having a diameter of 10-80 nm. The ratio of the volume Vse of the first lithium-ion conductive material within the pores of the porous microcrystalline graphite to the total pore volume Vp of the porous microcrystalline graphite is 0.8-1.2. By controlling Vse / Vp to be 0.8-1.2, the density of the first lithium-ion conductive material within the pores is improved, thereby increasing the ionic and electronic conductivity of the negative electrode active material, mitigating its volume expansion, and ultimately improving the cycle performance of the battery.

Description

Technical Field

[0001] This invention relates to the field of solid-state battery anode material technology, and particularly to an anode active material and its preparation method, anode sheet, secondary battery, and electrical device. Background Technology

[0002] Solid-state batteries are a type of secondary battery system that uses a solid electrolyte instead of the traditional liquid or gel electrolyte. Compared to traditional liquid batteries, solid-state batteries offer higher safety and energy density.

[0003] However, in solid-state batteries, the traditional natural microcrystalline graphite anode will continuously expand in volume during the lithium intercalation process. The huge mechanical stress caused by the expansion can easily lead to the breakage, delamination, or even microcracks of the natural microcrystalline graphite particles, ultimately affecting the cycle performance of the battery. Summary of the Invention

[0004] The main objective of this invention is to propose a negative electrode active material and its preparation method, a negative electrode sheet, a secondary battery, and an electrical device, aiming to solve the problem of poor cycle performance of solid-state batteries in the prior art.

[0005] To achieve the above objectives, the present invention proposes a negative electrode active material, which includes a host material comprising porous microcrystalline graphite and a first lithium-ion conductive material, wherein the first lithium-ion conductive material at least partially fills the pores of the porous microcrystalline graphite. The porous microcrystalline graphite includes at least a first pore and a second pore, wherein the pore diameter of the first pore is 1-10 nm and the pore diameter of the second pore is 10-80 nm. The ratio of the volume Vse of the first lithium-ion conductive material in the pores of the porous microcrystalline graphite to the total pore volume Vp of the second pores in the porous microcrystalline graphite is 0.8 to 1.2.

[0006] In one embodiment, the first lithium-ion conductive material includes first lithium-ion conductive particles, and the ratio of the average pore size dav of the porous microcrystalline graphite to the average particle size Dsmall avg of the first lithium-ion conductive particles is 2 to 10.

[0007] In one embodiment, the volume Vse of the first lithium-ion conductive material within the pores of the porous microcrystalline graphite is 0.19~0.53 cm³. 3 / g; and / or, The total pore volume Vp of the second pore in the porous microcrystalline graphite is 0.19~0.48 cm³. 3 / g; and / or, The average pore size (dav) of the porous microcrystalline graphite is 22-95 nm; and / or, The particle size of the first lithium-ion conductive particle is 10~40 nm; and / or, The average particle size Dsmall avg of the first lithium-ion conductive particles is 10~40 nm; and / or, The total pore volume of the first pore accounts for 10-20% of the total pore volume of the porous microcrystalline graphite, and the total pore volume of the second pore accounts for 40-60% of the total pore volume of the porous microcrystalline graphite.

[0008] In one embodiment, the negative electrode active material further includes an interface buffer layer, the interface buffer layer at least partially covering the host material, and the interface buffer layer includes a second lithium-ion conductive material.

[0009] In one embodiment, the ratio of the thickness δ of the interface buffer layer to the average pore diameter dav of the porous microcrystalline graphite is ≤0.2.

[0010] In one embodiment, the ratio of the thickness δ of the interface buffer layer to the average pore size dav of the porous microcrystalline graphite is 0.08 to 0.13; and / or, The thickness δ of the interface buffer layer is 3~20nm; and / or, The first lithium-ion conductive material and / or the second lithium-ion conductive material comprises at least one of oxide solid electrolyte, sulfide solid electrolyte, and halide solid electrolyte, wherein the oxide solid electrolyte comprises at least one of LLZO, LATP, LLTO, LBO, LNbO, LiZrO, and LiAlO, and the sulfide solid electrolyte comprises LiGPS, LiSiPS, LGPS, Li6PS5Cl, Li6PS5Br, and Li 10 GeP2S 12 The halide solid electrolyte comprises at least one of Li3InCl6, Li2ZrCl6, Li3YBr6, Li3YCl6, LiTaF6, and LiNbF6.

[0011] The present invention also provides a method for preparing a negative electrode active material, the method comprising the following steps: Provide a porous microcrystalline graphite; The porous microcrystalline graphite is mixed with at least a first lithium-ion conductive material and mechanically fused to obtain the negative electrode active material.

[0012] In one embodiment, the step of mixing porous microcrystalline graphite with at least a first lithium-ion conductive material and performing a mechanical fusion treatment to obtain the negative electrode active material includes: Porous microcrystalline graphite and a first lithium-ion conductive material are mixed and mechanically fused to obtain the main material; A second lithium-ion conductor material is deposited in situ on the surface of the main material to form an interface buffer layer, thereby obtaining the negative electrode active material.

[0013] In one embodiment, the step of mixing porous microcrystalline graphite with at least a first lithium-ion conductive material and performing a mechanical fusion process to obtain the negative electrode active material includes: Porous microcrystalline graphite, a first lithium-ion conductive material, and a third lithium-ion conductive material are mixed and mechanically fused to obtain the negative electrode active material. The particle size of the third lithium-ion conductive material is larger than the average pore size of the porous microcrystalline graphite.

[0014] In one embodiment, the step of mixing porous microcrystalline graphite, a first lithium-ion conductive material, and a third lithium-ion conductive material and mechanically fusing them to obtain the negative electrode active material includes: The first lithium-ion conductive material and the third lithium-ion conductive material are mixed to obtain mixed particles; The porous microcrystalline graphite and the mixed particles are mixed and mechanically fused to obtain the negative electrode active material.

[0015] In one embodiment, in the step of mixing the first lithium-ion conductive material and the third lithium-ion conductive material to obtain mixed particles: The mixing includes ball milling, wherein the ball milling speed is 150~250 rpm; and / or, the ball milling time is 3~5 hours; and / or, The particle size of the third lithium-ion conductive material is 1-5 μm; and / or, The mass ratio of the first lithium-ion conductive material to the third lithium-ion conductive material is (65~80):(20~35).

[0016] In one embodiment, the step of mixing the porous microcrystalline graphite and the mixed particles and performing a mechanical fusion process to obtain the negative electrode active material includes: The mechanical fusion process is performed at a rotation speed of 2500~3500 rpm; and / or, The temperature of the mechanical fusion treatment is 75~85℃; and / or, The mechanical fusion time is 25-35 minutes; and / or, The mass ratio of the porous microcrystalline graphite to the mixed particles is 100:(15~40).

[0017] In one embodiment, the step of mixing porous microcrystalline graphite with at least a first lithium-ion conductive material and performing a mechanical fusion treatment to obtain the negative electrode active material includes: Porous microcrystalline graphite and a first lithium-ion conductive material are mixed and mechanically fused to obtain an intermediate. The intermediate is subjected to rinsing, centrifugation, and drying processes in sequence to obtain the negative electrode active material.

[0018] In one embodiment, the step of providing a porous microcrystalline graphite includes: Natural microcrystalline graphite is mixed with an alkaline substance and activated under an inert atmosphere to obtain porous microcrystalline graphite.

[0019] In one embodiment, the alkaline substance includes at least one of potassium hydroxide and sodium hydroxide; and / or, The mass ratio of the natural microcrystalline graphite to the alkaline substance is 1:3~6; and / or, The activation reaction is performed at a temperature of 700~850℃; and / or, The activation reaction takes 1.8 to 2.2 hours.

[0020] The present invention also provides a negative electrode sheet, wherein the negative electrode sheet comprises the aforementioned negative electrode active material or a negative electrode active material prepared by the aforementioned method for preparing negative electrode active material.

[0021] The present invention also provides a secondary battery, the secondary battery comprising the aforementioned negative electrode sheet.

[0022] The present invention also provides an electrical device, which includes the aforementioned secondary battery.

[0023] In the technical solution of this invention, a first lithium-ion conductive material is filled into the pores of porous microcrystalline graphite. The filling amount of the first lithium-ion conductive material in all pores and the total pore volume of the second pore are controlled to satisfy Vse / Vp = 0.8~1.2. This ensures that the first lithium-ion conductive material is densely packed, at least in the second pores. After filling, the second pores and even some of the first pores on the surface of the porous microcrystalline graphite are also compacted (i.e., the pores are partially sealed). This dense filling method allows the first lithium-ion conductive material filled in the pores to be in close contact, thereby reducing pore impedance and improving ion and electron conduction efficiency. Furthermore, the first lithium-ion conductive material in the pores can act as an interlayer support framework for the porous microcrystalline graphite. This support framework can effectively absorb and buffer the huge volume expansion stress generated by the deposition of lithium metal in the pores, thereby effectively alleviating the volume expansion of the porous microcrystalline graphite, reducing the expansion rate of the negative electrode and the battery, and preventing the porous microcrystalline graphite structure from cracking or pulverizing due to excessive stress. Ultimately, this improves the structural stability and cycle performance of the battery. In addition, partial densification of the orifice can prevent the first lithium-ion conductive material inside the pore from being squeezed out of the pore when the porous microcrystalline graphite is continuously subjected to stress, and can also ensure that there are certain pores inside the pore to accommodate lithium metal deposition. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0025] Figure 1 This is an external morphology diagram of the porous microcrystalline graphite in Embodiment 1 of the present invention; Figure 2 This is a morphological diagram of the negative electrode active material particles in Example 1 of the present invention.

[0026] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially. Furthermore, the meaning of "and / or" throughout the text includes three parallel solutions; for example, "A and / or B" includes solution A, or solution B, or a solution where both A and B are satisfied simultaneously. In addition, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] In solid-state batteries, the traditional natural microcrystalline graphite anode undergoes continuous volume expansion during lithium intercalation. The enormous mechanical stress caused by this expansion can easily lead to the breakage, delamination, or even microcracks of the natural microcrystalline graphite particles, ultimately affecting the cycle life of the battery.

[0029] In view of this, the present invention provides a negative electrode active material, the negative electrode active material comprising a host material, the host material comprising porous microcrystalline graphite and a first lithium-ion conductive material, wherein the first lithium-ion conductive material is at least partially filled in the pores of the porous microcrystalline graphite; the pores in the porous microcrystalline graphite include at least a first pore and a second pore, the pore diameter of the first pore is 1-10 nm, and the pore diameter of the second pore is 10-80 nm; the ratio of the volume Vse of the first lithium-ion conductive material in the pores of the porous microcrystalline graphite to the total pore volume Vp of the second pores in the porous microcrystalline graphite is 0.8~1.2.

[0030] In the technical solution of this invention, porous microcrystalline graphite actually has three types of pores: a first pore, a second pore, and a third pore. These three types of pores are interconnected. The third pore has the largest diameter, so its material transport resistance is the lowest. Although the first and second pores have small diameters, resulting in large steric hindrance, once the first lithium-ion conductive material enters these two types of pores, it will be anchored in the pores due to capillary attraction. Therefore, during filling, the first lithium-ion conductive material will first enter the first and second pores through the large pores for filling or directly enter these two types of pores for filling. Since the second pore has a larger diameter and lower resistance, it is easier to be completely filled than the first pore. This invention fills the pores of porous microcrystalline graphite with a first lithium-ion conductive material, and controls the filling amount of the first lithium-ion conductive material in all pores and the total pore volume of the second pore to satisfy Vse / Vp=0.8~1.2. This ensures that the first lithium-ion conductive material is densely packed, at least in the second pores. After filling, the second pores and even some of the first pores on the surface of the porous microcrystalline graphite are also compacted (i.e., the pores are partially sealed). This dense filling method allows the first lithium-ion conductive material filled in the pores to be in close contact, thereby reducing the pore impedance and improving the ion and electron conduction efficiency. On the other hand, the first lithium-ion conductive material in the pores can act as an interlayer support framework for the porous microcrystalline graphite. This support framework can effectively absorb and buffer the huge volume expansion stress generated by the deposition of lithium metal in the pores, thereby effectively alleviating the volume expansion of the porous microcrystalline graphite, reducing the expansion rate of the negative electrode and the battery, and preventing the porous microcrystalline graphite structure from cracking or pulverizing due to excessive stress. Ultimately, this improves the structural stability and cycle performance of the battery. In addition, partial densification of the orifice can prevent the first lithium-ion conductive material inside the pore from being squeezed out of the pore when the porous microcrystalline graphite is continuously subjected to stress, and can also ensure that there are certain pores inside the pore to accommodate lithium metal deposition.

[0031] In this paper, "Volume Vse of the first lithium-ion conductive material within the pores of porous microcrystalline graphite" refers to the volume of the first lithium-ion conductive material within the pores of the porous microcrystalline graphite. If the first lithium ion is embedded at the pore opening, the portion embedded within the pore is included in Vse, while the portion outside the pore is not included in Vse. Vse can be determined by combining liquid nitrogen adsorption-desorption testing with BJH (Barrett-Joyner-Halenda method) pore size distribution testing and particle filling volume conversion methods. Specifically, the effective filling volume of the first lithium-ion conductive material within the pores is calculated by testing the difference in total pore volume of the porous microcrystalline graphite before and after filling with the first lithium-ion conductive material.

[0032] In this paper, "total pore volume Vp of the second pore in porous microcrystalline graphite" has a meaning known in the art and can be tested using methods known in the art. For example, the liquid nitrogen low-temperature adsorption-desorption method (BET test) is used for characterization, and based on the BET specific surface area, the total pore volume of the second pore in the sample is calculated according to the BJH model.

[0033] It should be noted that when Vse / Vp > 1.2, the first lithium-ion conductive material will be excessively squeezed into the pores, which can easily cause the porous microcrystalline graphite to crack, leading to the peeling of its layers and structural damage. When Vse / Vp < 0.8, the volume ratio of particles filling the pores is relatively low, and the particles are relatively loose. They are easily squeezed out of the pores and enter the graphite surface during the continuous lithium deposition stress of the porous graphite microcrystals, thus causing the negative electrode sheet to crack.

[0034] For example, Vse / Vp can be a numerical range consisting of any two values ​​of 0.8, 0.85, 0.9, 0.95, 1.0, 1.05, 1.1, 1.15, 1.2 or above; the pore size distribution range of the first pore is 1~5nm, and the pore size of a single first pore can be 1nm, 2nm, 3nm, 4nm or 5nm; the pore size distribution range of the second pore is 10~80nm, and the pore size of a single second pore can be 10 nm, 20nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm or 80 nm.

[0035] Preferably, the volume Vse of the first lithium-ion conductive material within the pores of the porous microcrystalline graphite is 0.19~0.53 cm³. 3 / g; and / or, the total pore volume Vp of the second pore in the porous microcrystalline graphite is 0.19~0.48 cm³. 3 / g. That is, Vse can be any range of two values ​​from 0.19, 0.30, 0.38, 0.46, 0.53, or higher, and Vp can be 0.19 cm. 3 / g, 0.25 cm 3 / g, 0.35 cm 3 / g, 0.45 cm 3 / g, 0.48 cm 3 By controlling the values ​​of Vse and Vp within the range of any two values ​​above / g, the expansion rate of the negative electrode and the battery as a whole can be kept low, while the ionic conductivity and electronic conductivity of the battery are kept high, resulting in better long-term cycle performance of the battery.

[0036] Preferably, the total pore volume of the first pore accounts for 10-20% of the total pore volume of the porous microcrystalline graphite, and the total pore volume of the second pore accounts for 40-60% of the total pore volume of the porous microcrystalline graphite. That is, the total pore volume of the first pore can account for 10%, 15%, or 20% of the total pore volume of the porous microcrystalline graphite; the total pore volume of the second pore can account for 40%, 50%, or 60% of the total pore volume of the porous microcrystalline graphite; and the remaining pore volume is the volume of the third pore. Since lithium metal is first deposited in micropores (pore size less than 2nm) and then in mesopores (pore size 2~50nm), controlling the pore size and volume ratio of the first and second pores within the above range can ensure that there is sufficient lithium storage space in both types of pores and that there is sufficient first lithium-ion conductive material in at least the second pore. This ensures that a relatively dense conductive and ion-conducting network is formed inside the negative electrode active material, and that the filled first lithium-ion conductive material has a good buffering effect on the volume expansion of graphite. In addition, there is sufficient lithium storage space left in the graphite. Ultimately, the material has both good structural stability and low expansion rate, and the long-term cycle stability of the material is improved.

[0037] In some embodiments, the first lithium-ion conductive material includes first lithium-ion conductive particles, and the ratio of the average pore size dav of the porous microcrystalline graphite to the average particle size Dsmall avg of the first lithium-ion conductive particles is 2 to 10.

[0038] In the technical solution of this invention, controlling the value of dav / Dsmall avg within the range of 2 to 10 ensures a good match between the pore size of the porous microcrystalline graphite and the size of the first lithium-ion conductive particles. This facilitates the smooth entry of the first lithium-ion conductive particles into the pores for dense filling, thereby ensuring a relatively tight filling of each second pore and part of the first pore. This results in a denser ion and electron conductive network within the pores, thus ensuring the structural stability and long cycle life of the negative electrode active material. Therefore, dav / Dsmall avg is mainly used to evaluate the density of the filling within the pores.

[0039] In this paper, "average pore size dav of porous microcrystalline graphite" has a meaning known in the art and can be tested using methods known in the art, such as BET testing combined with the BJH model to calculate the average pore size; "average particle size Dsmall avg of the first lithium-ion conductive particle" has a meaning known in the art and can be tested using methods known in the art, such as dynamic light scattering particle size analyzer (DLS) testing.

[0040] It should be noted that when dav / Dsmall avg > 10, the particle size of the first lithium-ion conductive particles is too small compared to the pore size of the porous microcrystalline graphite. In this case, a single second pore will be filled with a large number of small particles. The small particles are in close contact with each other, and the remaining pores are small. That is, the buffer space left for lithium metal deposition is too small, which will lead to the expansion and deterioration of the porous microcrystalline graphite, resulting in the cracking or pulverization of the porous microcrystalline graphite structure. When dav / Dsmall avg < 2, the particle size of the first lithium-ion conductive particles is relatively large compared to the porous microcrystalline graphite. In this case, the steric hindrance of the second pore opening is too large. The first lithium-ion conductive particles cannot enter the pore channel smoothly and can only block the pore opening, causing pore filling failure and loss of internal ion pathways, thereby deteriorating the ionic conductivity, electronic conductivity and expansion rate of the battery.

[0041] For example, dav / Dsmall avg can be a range of values ​​consisting of any two values ​​from 2, 3, 4, 5, 6, 7, 8, 9, 10 or more.

[0042] Preferably, the average pore size dav of the porous microcrystalline graphite is 22-95 nm; and / or, the average particle size Dsmall avg of the first lithium-ion conductive particles is 10-40 nm. That is, dav can be any two values ​​of 22 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 95 nm or above; Dsmall avg can be any two values ​​of 10 nm, 18 nm, 25 nm, 32 nm, 40 nm or above. Controlling the values ​​of dav and Dsmall avg within the above ranges can ensure high electronic conductivity and ionic conductivity of the battery, and low expansion rate of the battery and negative electrode.

[0043] In some embodiments, the particle size of the first lithium-ion conductive particles is 10-40 nm. That is, the particle size distribution range of the first lithium-ion conductive particles is 10-40 nm. This can be because each first lithium-ion conductive particle has the same particle size, or it can be a mixture of particles of different sizes within this particle size range. The latter is preferred, as the mixture can use small particles to fill the pores between large particles, thereby further reducing impedance and battery expansion rate. It should be noted that the smaller the particle size of the first lithium-ion conductive particles, the greater their surface energy, and the easier it is for the particles to agglomerate. When the particle size of the first lithium-ion conductive particles is less than 10 nm, the particles will agglomerate before entering the pores, blocking the pores and creating a bridging effect, resulting in ineffective internal filling. Conversely, when the particle size of the first lithium-ion conductive particles is greater than 10 nm, the particles are too large to enter the micropores, also resulting in poor filling effect.

[0044] In some embodiments, the negative electrode active material further includes an interface buffer layer, the interface buffer layer at least partially covering the host material, and the interface buffer layer includes a second lithium-ion conductive material.

[0045] In the technical solution of the present invention, coating at least a portion of the main material surface with a second lithium-ion conductive material means that an interface buffer layer is jointly coated on at least a portion of the surface of the porous microcrystalline graphite and at least a portion of the surface of the first lithium-ion conductive material filling the outermost part of the pores. The second lithium-ion conductive material in the interface buffer layer is a dense layer. This interface buffer layer can react with lithium metal to form a solid electrolyte interphase (SEI) film, thereby reducing the consumption of solid electrolyte. Moreover, as a dense and flexible material, the interface buffer layer can also constrain the expansion of the porous microcrystalline graphite material, further alleviating the expansion rate of the porous microcrystalline graphite.

[0046] Specifically, after filling porous microcrystalline graphite with first lithium-ion conductive particles, these particles often remain on the surface of the host material. Without an interface buffer layer, these surface particles gradually shift due to repeated expansion of the host material during long-term cycling, creating larger gaps between them and the host material surface. Eventually, they detach from the host material surface, causing cracking and pulverization of the negative electrode sheet, thus affecting cycle life. When an interface buffer layer is provided, although the first lithium-ion conductive particles still remain on the surface of the host material, these residual particles are bound by the interface buffer layer and are less likely to detach from the negative electrode active material, thereby improving the cycle life of the negative electrode active material. When providing an interface buffer layer, the host material is preferably a host material from which surface residual particles have been removed.

[0047] In some embodiments, the ratio of the thickness δ of the interface buffer layer to the average pore size dav of the porous microcrystalline graphite is ≤0.2.

[0048] In the technical solution of this invention, controlling the value of δ / dav to ≤0.2 ensures that the interface buffer layer does not completely seal the pores of the porous microcrystalline graphite, thereby ensuring a low expansion rate and long cycle life of the negative electrode active material. For example, δ / dav can be 0.2, 0.15, 0.1, 0.05, or even lower. When δ / dav > 0.2, the residual voids at the pores of the porous microcrystalline graphite are small, reducing the internal residual lithium metal deposition space. Furthermore, the lack of internal and external stress output pathways worsens the material's expansion rate, exacerbates the collapse of the porous microcrystalline graphite structure, and significantly reduces the battery's cycle performance (the capacity retention rate is below 80% after 300 cycles, and the capacity retention rate continues to deteriorate with increasing δ / dav).

[0049] Preferably, the ratio of the thickness δ of the interface buffer layer to the average pore size dav of the porous microcrystalline graphite is 0.08 to 0.13. When the value of δ / dav is within this range, the expansion rate of the negative electrode and the battery is small, and the cycle capacity retention rate of the battery is high, maintaining more than 90% after 300 cycles.

[0050] In this paper, the "thickness δ of the interface buffer layer" has a meaning known in the art and can be tested using methods known in the art, such as statistically analyzing the average thickness of the nanoscale interface buffer layer by transmission electron microscopy (TEM) cross-sectional morphology testing.

[0051] Preferably, the thickness δ of the interface buffer layer is 3~20nm. That is, δ can be any two values ​​of 3nm, 6nm, 9nm, 12nm, 15nm, 18nm, 20nm or more.

[0052] In some embodiments, the first lithium-ion conductive material and / or the second lithium-ion conductive material comprises at least one of an oxide solid electrolyte, a sulfide solid electrolyte, and a halide solid electrolyte, wherein the oxide solid electrolyte comprises LLZO (Li7La3Zr2O). 12 Lithium lanthanum zirconium oxide), LATP (Li 1+x Al x Ti 2-x (PO4)3, where x = 0.2~0.5, at least one of lithium titanium aluminum phosphate, LLTO (lithium lanthanum titanium oxide), LBO (LiB3O5, lithium triborate), LNbO (LiNbO3, lithium niobate), LiZrO (Li2ZrO3, lithium zirconate), and LiAlO (LiAlO2, lithium aluminate), wherein the sulfide solid electrolyte includes LiGPS (lithium germanium phosphorus sulfide), LiSiPS (lithium silicon phosphorus sulfide, for example Li 10 SiP2S 12 ), LGPS, Li6PS5Cl, Li6PS5Br, Li10 GeP2S 12 The halide solid electrolyte comprises at least one of Li3InCl6, Li2ZrCl6, Li3YBr6, Li3YCl6, LiTaF6, and LiNbF6. Selecting the above-mentioned lithium-ion conductive materials ensures high ionic conductivity of the negative electrode active material.

[0053] The present invention also provides a method for preparing a negative electrode active material, the method comprising the following steps: Step S1: Provide a porous microcrystalline graphite; Step S2: Mix porous microcrystalline graphite with at least the first lithium-ion conductive material and perform mechanical fusion treatment to obtain the negative electrode active material.

[0054] In the technical solution of the present invention, in step S2, porous microcrystalline graphite is mixed with at least a first lithium-ion conductive material and then subjected to mechanical fusion treatment. During the mechanical fusion treatment, the first lithium-ion conductive material is plastically deformed by externally applied mechanical force, thereby being pressed into, embedded in, or uniformly distributed in the pores of the porous microcrystalline graphite. After the first lithium-ion conductive material is filled and formed in the pores, it will form a strong mechanical interlocking force and shear bonding strength, effectively avoiding the common problems of hollowing and peeling in traditional VC mixed filling systems. This not only improves the filling density of the pores by the first lithium-ion conductive material, but also improves the structural stability of the porous microcrystalline graphite.

[0055] In some embodiments, step S2 includes: Step S2A1: Mix porous microcrystalline graphite and the first lithium-ion conductive material, and perform mechanical fusion treatment to obtain the main material; Step S2A2: Deposit a second lithium-ion conductor material in situ on the surface of the main material to form an interface buffer layer, thereby obtaining the negative electrode active material.

[0056] In the technical solution of the present invention, after mechanical fusion, the surface of the obtained host material can also form an interface buffer layer by in-situ deposition. This formation method is simple and can ensure that a relatively dense interface buffer layer is formed on the surface of the host material, further reducing the expansion rate of the negative electrode active material.

[0057] Preferably, in step S2, the in-situ deposition method includes at least one of atomic layer deposition, electrophoretic deposition, and magnetron sputtering. Atomic layer deposition involves alternately introducing a gaseous precursor of a second lithium-ion conductor material, causing a self-limiting surface chemical reaction on the surface of the host material, thereby achieving layer-by-layer film growth. Electrophoretic deposition utilizes an external electric field to directionally migrate charged second lithium-ion conductor material nanoparticles suspended in a liquid medium towards the electrode (host material) and deposit them into a film. Magnetron sputtering uses a magnetic field to confine plasma in a vacuum environment, bombarding a second lithium-ion conductor material target with high-energy ions, causing target atoms to sputter out and deposit on the substrate. All of the above deposition methods can form a relatively dense interface buffer layer on the surface of the host material.

[0058] In some embodiments, step S2 includes: Step S2B1: Mix porous microcrystalline graphite, a first lithium-ion conductive material, and a third lithium-ion conductive material, and mechanically fuse them to obtain the negative electrode active material; the particle size of the third lithium-ion conductive material is larger than the average pore size of the porous microcrystalline graphite.

[0059] In the technical solution of this invention, a third lithium-ion conductive material can be added during the mechanical mixing process of porous microcrystalline graphite and the first lithium-ion conductive material. Since the particle size of the third lithium-ion conductive material is larger than the average pore size of the porous microcrystalline graphite, it hardly fills the mesopores and micropores. Instead, it provides a force point for the first lithium-ion conductive material with a smaller particle size, thereby continuously pushing the first lithium-ion conductive material to quickly enter the pores (mainly mesopores). This filling method can reduce the first lithium-ion conductive material from preferentially entering the pores of porous microcrystalline graphite, rather than accumulating and bridging at the pore openings, thus interfering with and blocking the pore openings. This filling method ultimately improves the effectiveness of the first lithium-ion conductive material filling.

[0060] In some embodiments, step S2B1 includes: Step S2B11: Mix the first lithium-ion conductive material and the third lithium-ion conductive material to obtain mixed particles; Step S2B12: Mix the porous microcrystalline graphite and the mixed particles, and mechanically fuse them to obtain the negative electrode active material.

[0061] In the technical solution of the present invention, a first lithium-ion conductive material with a smaller particle size and a third lithium-ion conductive material with a larger particle size can be mixed to obtain mixed particles, so as to ensure that a large amount of first lithium-ion conductive material is evenly distributed around the third lithium-ion conductive material, thereby further improving the filling efficiency and shortening the mechanical fusion time.

[0062] In some embodiments, in step S2B11: the mixing includes ball milling, the ball milling speed is 150~250 rpm; and / or, the ball milling time is 3~5 h; and / or, the particle size of the third lithium-ion conductive material is 1-5 μm; and / or, the mass ratio of the first lithium-ion conductive material to the third lithium-ion conductive material is (65~80):(20~35). That is, the ball milling speed can be 150 rpm, 200 rpm, or 250 rpm, the ball milling time can be 3 h, 4 h, or 5 h, the particle size of the third lithium-ion conductive material can be 1 μm, 3 μm, or 5 μm, and the mass ratio of the first lithium-ion conductive material to the third lithium-ion conductive material can be 65:35, 70:30, or 20:80. Controlling the above conditions within a suitable range ensures that the first lithium-ion conductive material is quickly and fully filled into the pores of the porous microcrystalline graphite.

[0063] In some embodiments, in step S2B12: the rotation speed of the mechanical fusion process is 2500~3500 rpm; and / or, the temperature of the mechanical fusion process is 75~85℃; and / or, the mechanical fusion time is 25~35 min; and / or, the mass ratio of the porous microcrystalline graphite to the mixed particles is 100:(15~40). That is, the rotation speed of the mechanical fusion process can be 2500 rpm, 3000 rpm or 3500 rpm, the temperature of the mechanical fusion process can be 75℃, 80℃ or 85℃, the mechanical fusion time can be 25 min, 30 min or 35 min, and the mass ratio of porous microcrystalline graphite to mixed particles can be 100:15, 100:20, 100:25, 100:30 or 100:40. Controlling the above conditions within a suitable range can ensure that the first lithium-ion conductive material is quickly and fully filled into the pores of the porous microcrystalline graphite, and that the filling density between the first lithium-ion conductive materials in the pores and the filling density of the pores are good.

[0064] In some embodiments, step S2 includes: Step S2C1: Mix porous microcrystalline graphite and the first lithium-ion conductive material, and perform mechanical fusion treatment to obtain an intermediate; Step S2C2: The intermediate is sequentially rinsed, centrifuged, and dried to obtain the negative electrode active material.

[0065] In the technical solution of the present invention, after mechanical fusion treatment, the obtained intermediate can be rinsed and centrifuged to remove the first lithium-ion conductive material remaining on the surface of the intermediate, so as to ensure that the subsequent interface buffer layer deposition is relatively dense or that the surface of the negative electrode active material is relatively flat.

[0066] In some embodiments, in step S2C2: the rinsing agent includes at least one of anhydrous ethanol, isopropanol, anhydrous methanol, and acetone; and / or, the centrifugation speed is 1800~2200 rpm; and / or, the drying process includes vacuum drying, and the vacuum drying temperature is 55~65°C. That is, the centrifugation speed can be 1800 rpm, 2000 rpm, or 2200 rpm, and the vacuum drying temperature can be 55°C, 60°C, or 65°C. Controlling the type of rinsing agent, the centrifugation speed, and the vacuum drying temperature within the above ranges ensures that the residual amount of the first lithium-ion conductor material and the rinsing agent on the surface of the main material is low.

[0067] In some embodiments, step S1 includes: Step S11: Mix natural microcrystalline graphite with an alkaline substance and carry out an activation reaction under an inert atmosphere to obtain porous microcrystalline graphite.

[0068] In the technical solution of this invention, the alkaline substance can be a solid alkaline substance or an alkaline solution containing an alkaline substance. Taking a solid alkaline substance as an example, at the activation reaction temperature, the solid alkaline substance transforms into a molten state, i.e., molten alkali. Molten alkali has extremely strong oxidizing properties and will undergo a redox reaction with carbon atoms in natural microcrystalline graphite. The molten alkali will etch into the interior of the porous microcrystalline graphite, consuming part of the carbon skeleton and generating gaseous products such as carbonates, carbon monoxide, or carbon dioxide, which then escape. As carbon atoms are continuously etched, the originally dense interior of the natural microcrystalline graphite is hollowed out, thereby generating a large number of micropores and mesopore structures in situ. This etching usually occurs preferentially at the grain end faces and internal defects, ultimately forming a well-developed three-dimensional interconnected network of pores.

[0069] In some embodiments, in step S11: the alkaline substance includes at least one of potassium hydroxide and sodium hydroxide; and / or, the mass ratio of the natural microcrystalline graphite to the alkaline substance is 1:3~6; and / or, the activation reaction temperature is 700~850℃; and / or, the activation reaction time is 1.8~2.2h. That is, the mass ratio of natural microcrystalline graphite to the alkaline substance can be 1:3, 1:4 or 1:6, the activation reaction temperature can be 850℃, 850℃ or 850℃, and the activation reaction time can be 2.2h, 2.2h or 2.2h. Controlling the above conditions within a suitable range can ensure that a suitable proportion of micropores and mesopores are formed inside and on the surface of the natural microcrystalline graphite.

[0070] The present invention also provides a negative electrode sheet, which comprises the aforementioned negative electrode active material or a negative electrode active material prepared by the aforementioned method for preparing negative electrode active materials. Therefore, this negative electrode sheet has all the beneficial effects of the aforementioned negative electrode active material or the aforementioned method for preparing negative electrode active materials, which will not be elaborated here.

[0071] The present invention also provides a secondary battery comprising the aforementioned negative electrode. Therefore, this secondary battery possesses all the beneficial effects of the aforementioned negative electrode, which will not be elaborated further here.

[0072] The present invention also provides an electrical device comprising the aforementioned secondary battery. Therefore, this electrical device possesses all the beneficial effects of the aforementioned secondary battery, which will not be elaborated upon here.

[0073] The technical solution of the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings. It should be understood that the following embodiments are only used to explain the present invention and are not intended to limit the present invention.

[0074] Example 1 A negative electrode active material includes a host material comprising porous microcrystalline graphite and first lithium-ion conductor material particles LLZO, wherein the first lithium-ion conductor material particles at least fill the pores of the porous microcrystalline graphite. The average pore size (dav) of the porous microcrystalline graphite is 38 nm, and the total pore volume (Vp) of the second pore is 0.38 cm³. 3 / g, the porous microcrystalline graphite has pores including a first pore, a second pore, and a third pore. The pore size of the first pore is 1~10nm, and the total pore volume of the first pore accounts for 16% (0.11 cm³) of the total pore volume of the porous microcrystalline graphite. 3 / g); the pore size of the second pore is 10~80nm, and the total pore volume Vp of the second pore accounts for 55% (0.38 cm³) of the total pore volume of the porous microcrystalline graphite. 3 / g), the remainder being the third pore; the volume Vse of the first lithium-ion conductive material particles filling the porous microcrystalline graphite is 0.38 cm³. 3 / g, the average particle size Dsmall avg of the first lithium-ion conductive material particles is 18nm (the particle size is also 18nm), that is, Vse / Vp=1.0, R=dav / Dsmall avg=2; This negative electrode active material is prepared through the following steps: Step 1: Preparation of porous microcrystalline graphite Natural microcrystalline graphite was uniformly mixed with KOH at an alkali-to-carbon ratio of 4.5:1 and activated at 800℃ for 2 hours under an inert atmosphere. The activated product was then acid-washed with a dilute hydrochloric acid solution (5% hydrochloric acid by mass) to remove impurities, and repeatedly washed with deionized water until the washing solution was neutral. The activated product was then vacuum-dried at 120℃ for 12 hours to obtain porous microcrystalline graphite. Step 2: Mechanical Fusion Filling 10g of porous microcrystalline graphite and 3.8g of first lithium-ion conductive material particles LLZO were put into a mechanical fusion device and mechanically fused at 3000rpm and 80℃ for 30min to achieve dense filling and obtain the negative electrode active material.

[0075] Example 2 The difference between Example 2 and Example 1 is that: Step 2 is as follows: 10g of porous microcrystalline graphite and 3.8g of first lithium-ion conductive material particles LLZO are put into a mechanical fusion device and mechanically fused at 3000rpm and 80℃ for 30min to achieve dense filling and obtain an intermediate; the intermediate is washed with anhydrous ethanol and centrifuged at 2000rpm. This process is repeated 3 times until the mass change after drying of the washing liquid is <0.1% to ensure that the ineffective free powder on the surface is completely removed. The intermediate is then vacuum dried at 60℃ to obtain the negative electrode active material.

[0076] Example 3 The difference between Example 3 and Example 1 is that: Step 2 is as follows: 10g of porous microcrystalline graphite, 3.8g of first lithium-ion conductive material particles LLZO and 1.63g of third lithium-ion conductive material particles LLZO are put into a mechanical fusion device and mechanically fused at 3000rpm and 80℃ for 30min to achieve dense filling and obtain the negative electrode active material. The particle size of the third lithium-ion conductive material is 3 μm, and the mass ratio of the first lithium-ion conductor material particles to the third lithium-ion conductor material particles is 7:3.

[0077] Example 4 The difference between Example 4 and Example 3 is that: Step 2 is as follows: Mix 3.8g of the first lithium-ion conductive material particles LLZO and 1.63g of the third lithium-ion conductive material particles LLZO, and put them into a ball mill at 200rpm for 4h to obtain mixed particles; then mechanically fuse the mixed particles with 10g of porous microcrystalline graphite at 3000rpm and 80℃ for 30min to achieve dense filling and obtain the negative electrode active material.

[0078] Example 5 The difference between Example 5 and Example 4 is that: In step 2, after dense filling, an intermediate is obtained. The intermediate is washed with anhydrous ethanol and centrifuged at 2000 rpm. This process is repeated 3 times until the mass change after drying of the washing liquid is <0.1% to ensure that the surface ineffective free powder is completely removed. The intermediate is then vacuum dried at 60℃ to obtain the negative electrode active material.

[0079] Example 6 The difference between Example 6 and Example 1 is that: Vse / Vp=0.8.

[0080] Example 7 The difference between Example 7 and Example 1 is that: Vse / Vp=1.2.

[0081] Example 8 The difference between Example 8 and Example 1 is that: R=1.2.

[0082] Example 9 The difference between Example 9 and Example 1 is that: R=3.2.

[0083] Example 10 The difference between Example 10 and Example 1 is that: R=10.

[0084] Example 11 The difference between Example 11 and Example 1 is that: R=10.5.

[0085] Example 12 The difference between Example 12 and Example 1 is that: In step 2, after the dense filling is obtained as the host material, an atomic layer deposition process and a second lithium-ion conductive material are deposited on the surface of the host material to form an interface buffer layer. The deposition conditions are: deposition temperature 180℃, precursor pulse time 0.1s, purge time 10s, and 80 cycles of deposition. After the deposition is completed, the negative electrode active material is obtained. In the negative electrode active material, the thickness of the interface buffer layer is δ, which is 5 nm, and δ / dav = 0.13.

[0086] Example 13 The difference between Example 13 and Example 12 is that: δ / dav=0.04.

[0087] Example 14 The difference between Example 14 and Example 12 is as follows: δ / dav=0.08.

[0088] Example 15 The difference between Example 15 and Example 12 is that: δ / dav=0.2.

[0089] Example 16 The difference between Example 16 and Example 12 is that: δ / dav=0.29.

[0090] Example 17 The difference between Example 17 and Example 12 is that: δ / dav=0.32.

[0091] Example 18 The difference between Example 18 and Example 12 is as follows: δ / dav=0.42.

[0092] Example 19 The difference between Example 19 and Example 12 is that: Replace LLZO with LATP of equal particle size and mass.

[0093] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that: In step 2, the mechanical fusion filling method is replaced with VC mixing (that is, mixing is carried out using a Vertical Centrifugal high-efficiency powder mixer). The conditions for VC mixing are: 3000 rpm, 30 min, and room temperature 25℃.

[0094] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that: Vse / Vp=0.47.

[0095] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that: Vse / Vp=1.53.

[0096] The parameters for Examples 1, 6-19 and Comparative Examples 1-3 are shown in Table 1.

[0097] Table 1

[0098] Performance testing The external morphology of the porous microcrystalline graphite and negative electrode active material particles in Example 1 was observed, and the results are as follows: Figure 1 and Figure 2 As shown.

[0099] Depend on Figure 1 It is known that porous microcrystalline graphite itself has a large number of pores; from Figure 2 It can be seen that the white powder in the negative electrode active material particles is the first lithium-ion conductive material filling the pores of porous microcrystalline graphite.

[0100] The negative electrode active materials of Examples 1-19 and Comparative Examples 1-3 were prepared into all-solid-state batteries. The specific steps were as follows: the negative electrode active material, styrene-butadiene rubber binder, and carbon black conductive agent were mixed in a mass ratio of 95:3:2 to prepare a negative electrode slurry. The negative electrode slurry was uniformly coated on a copper foil current collector, vacuum dried at 80°C, and rolled to obtain a negative electrode sheet. The positive electrode sheet (the active material is high-nickel ternary positive electrode material NCM811, i.e., LiNi) was then used to prepare the negative electrode sheet. 0.8 Co 0.1 Mn 0.1 An all-solid-state battery was obtained by pressing O2, a solid electrolyte (LLZO), and a negative electrode sheet. The following tests were then conducted: (1) Ionic conductivity: The electrochemical impedance spectroscopy (EIS) method was used for testing. The specific steps were as follows: the negative electrode active material was pressed into a dense thin sheet, and platinum electrodes were clamped at both ends. The room temperature electrochemical impedance spectroscopy was tested on an electrochemical workstation with a test frequency range of 10 Hz. - 2 Hz~10 6 Hz, the bulk ionic resistance R of the material is calculated based on the impedance fitting results, and the lithium-ion conductivity is calculated according to the formula σ=L / (R·S) based on the sample thickness L and cross-sectional area S, where σ is the lithium-ion conductivity; (2) Electronic conductivity: The four-probe method was used for testing. The specific steps were: pressing the negative electrode active material into a sheet, using a four-probe resistance tester to test the sheet resistance of the sample surface at room temperature, and calculating the electronic conductivity of the material in combination with the sample size parameters. (3) Capacity retention rate: The constant current charge and discharge test was carried out using the Blue Battery Test System at 25°C. The specific steps were as follows: the all-solid-state battery was charged and discharged at a rate of 0.5C. The initial discharge capacity and the discharge capacity after 300 cycles were recorded. The battery capacity retention rate was calculated by "capacity after cycle / initial capacity × 100%". (4) Battery expansion rate: Using a high-precision thickness micrometer, the overall thickness of the battery in the fully charged state and the initial uncharged state after 100 cycles was measured respectively. The overall expansion rate of the all-solid-state battery was calculated by “(full charge thickness - initial thickness) / initial thickness × 100%”. (5) Expansion rate of negative electrode sheet: After disassembling the all-solid-state battery after 100 cycles, the negative electrode sheet was taken out, cleaned and dried, and the thickness of the electrode sheet was tested. The expansion rate of the negative electrode sheet was calculated by comparing it with the initial thickness of the negative electrode sheet when the all-solid-state battery was prepared by "(thickness after 100 cycles - initial thickness) / initial thickness × 100%".

[0101] The test results are shown in Table 2.

[0102] Table 2 shows the performance tests of all-solid-state batteries corresponding to Examples 1-19 and Comparative Examples 1-3.

[0103] From Table 1 and Table 2, we can see that: A comparison of Example 1 and Comparative Example 1 reveals that, in the first step of creating pores in natural microcrystalline graphite, followed by filling the pores of the porous microcrystalline graphite with VC mixing, the VC mixing is merely a conventional physical mixing of powders without the effect of high-pressure plastic injection into the pores. Only a small amount of first lithium-ion conductive material particles are physically adsorbed and attached to the graphite surface and pore openings. This filling method has low filling efficiency because the pore openings are not sealed, the particles inside the pores are dispersed and loosely packed, resulting in high impedance and limited improvement on battery expansion. Furthermore, during battery cycling, the particles inside the pores are easily affected by the internal stress and escape from the pores to the outside. These particles will detach as the porous microcrystalline graphite expands in volume, causing the electrode to crack and thus affecting the battery cycle life. Comparing Example 1 with Comparative Examples 2 and 3, it was found that Comparative Example 2 had a Vse / Vp ratio of 0.48, indicating insufficient filling of the first lithium-ion conductive material particles within the pores of the porous microcrystalline graphite. The particles were loosely packed, failing to construct a continuous and complete ion-conducting network. This resulted in localized breaks in the conductive pathways within the pores, a significant increase in ion transport impedance, and insufficient support from the first lithium-ion conductive material particles for the graphite framework. Consequently, the volume expansion of the porous microcrystalline graphite during charging and discharging could not be effectively buffered, leading to a significant increase in electrode expansion rate. This resulted in a tendency for the structure to loosen and fail during cycling, resulting in poor battery cycle performance. In Comparative Example 3, the Vse / Vp ratio was 1.52. This indicated an excessive amount of first lithium-ion conductive material particles forcibly occupying the graphite pores, expanding and tearing the graphite channel framework, causing the porous microcrystalline graphite sheets to peel off and the microstructure to break down. This not only failed to improve conductivity but also... The pulverization and shedding of the negative electrode active material during cycling leads to a significant acceleration in battery capacity decay and a significant reduction in capacity retention. This indicates that a Vse / Vp ratio between 0.8 and 1.2 is the optimal range for balancing the structural stability and conductivity of the negative electrode active material. A Vse / Vp ratio that is too low or too high will deteriorate the cycling performance. It should be noted that compared with Comparative Example 3 and Example 7, the Vse / Vp ratio of Comparative Example 3 is significantly higher, and its ionic conductivity, electronic conductivity, and electrode expansion rate are all significantly worse. In particular, the electronic conductivity drops significantly. This is because when the Vse / Vp ratio reaches 1.53, the excessive filling of the first lithium-ion conductive material particles (close to the electronic insulator) will cause the porous microcrystalline graphite structure to partially collapse and will also appear on the surface of the porous microcrystalline graphite, ultimately cutting off the electron transport between the porous microcrystalline graphite particles. A comparison of Examples 1 and 2 reveals that Example 2, after mechanical fusion filling, adds multiple rinsing, centrifugation, and drying processes, which effectively removes free and non-embedded LLZO powder from the graphite surface, retaining only the effective first lithium-ion conductive material particles that are mechanically interlocked and densely filled within the pores. Compared to Example 1, Example 2 avoids the problems of surface free powder agglomeration and detachment, further reduces the electrode interface impedance, optimizes electrode interface stability, and shows no powder detachment or electrode cracking during battery cycling. The capacity retention rate is slightly improved, and the electrode and battery expansion rates are slightly reduced, proving that the post-processing purification process can further optimize the overall material performance and improve product stability. A comparison of Examples 1 and Examples 3, 4, and 5 reveals that, compared to the direct filling method using a single-size first lithium-ion conductive material particle in Example 1, Examples 3, 4, and 5 introduce a large-size third lithium-ion conductive material particle. In this composite system with different particle sizes, the large particles can act as force fulcrums, assisting in the compression and pushing of small-size conductive particles to quickly embed into the pores of porous microcrystalline graphite during mechanical fusion, significantly improving the density and uniformity of the filling within the pores. Example 4 pre-blends the large and small particles through ball milling to achieve uniform particle dispersion, further optimizing the filling effect. Example 5 combines pre-treatment of large and small particle blending with post-treatment purification processes, achieving both high filling efficiency and high purity of the powder on the material surface. Compared to Example 1, the porous microcrystalline graphite in Examples 3-5 has a denser conductive network within the pores, better mechanical support, and even a cleaner surface, thus significantly improving ionic conductivity, better suppressing material volume expansion, and ultimately significantly improving the battery's cycle life and capacity retention. Comparing Examples 1 with Examples 6 and 7, it was found that compared with Example 1: In Example 6, the filling amount of the first lithium-ion conductive material particles was lower, the interparticle gaps within the pores were slightly larger, and the ion transport efficiency and structural support effect were slightly inferior to Example 1, while the expansion rate was slightly increased; In Example 7, the filling amount of the first lithium-ion conductive material particles was higher, the pores were more densely filled, and the ion conduction pathway was more complete. However, the slightly excessive filler not only slightly increased the internal stress of the structure, but also increased the tortuosity of lithium-ion transport (due to the denser pore filling), ultimately resulting in a lower conductivity and expansion rate than Example 1, and the cycle stability of the battery was also lower than that of Example 1. A comparison of Examples 1 and Examples 8-11 revealed that the ratios of Examples 1, 8, 9, and 10 were within the protection range of 2-10, enabling conductive particles to smoothly enter and densely fill the pores, resulting in a complete conductive network and good structural stability. Only slight differences in conductivity and expansion suppression occurred due to variations in the ratios, with Example 1 being the optimal group, followed by Example 9. However, Example 11 had an excessively high R value (10.5), resulting in an excessively large pore size relative to the particle size, an excessive number of particles filling the pores, small interparticle spacing, and insufficient remaining buffer space. This prevented effective stress release during lithium deposition, exacerbating the expansion of the porous microcrystalline graphite and ultimately leading to an increased electrode expansion rate and a decrease in cycle stability to near the critical value (80%). This demonstrates that controlling dav / Dsmall avg within the range of 2-10 is crucial for ensuring the quality of pore filling and balancing conductivity and structural buffer performance. A comparison of Examples 1 and Examples 12-18 reveals that, compared to Example 1 without an interface buffer layer, Examples 12-18 construct a nanoscale dense interface buffer layer on the surface of the main material. This effectively binds residual conductive particles on the surface, preventing particle shedding during cycling and significantly suppressing the expansion of the electrode and battery. The preferred δ / dav is 0.08-0.13, resulting in a high battery capacity retention rate, maintained above 90%. Examples 12-15 have δ / dav ≤ 0.2, indicating a moderate interface buffer layer thickness. This achieves interface protection without blocking pores or ion transport channels, resulting in good overall battery performance. However, Examples 16-18 have δ / dav > 0.2, indicating an excessively thick interface buffer layer. This partially blocks the pores of the porous microcrystalline graphite, encroaching on lithium storage and stress buffer space, leading to impaired ion transport, reduced expansion suppression, and gradual performance degradation, even reducing the capacity retention rate to around 60%. A comparison of Examples 12 and 19 reveals that Example 12 uses an LLZO oxide electrolyte, while Example 19 uses an LATP oxide electrolyte with the same particle size and mass. Both mainstream oxide fast ion conductors are compatible with the filling system and interface modification system of this invention, and can construct stable ion-conducting networks and interface structures, resulting in batteries with excellent cycle stability and low expansion characteristics. Among them, the room temperature ion conductivity of LLZO is slightly higher than that of LATP, which makes the rate performance and ion conduction efficiency of Example 12 slightly better than those of Example 19. This also proves that the technical solution of this invention is not limited to a single electrolyte material, but is compatible with a variety of lithium-ion conductive materials, with a wide range of protection and strong process versatility.

[0104] The above are merely preferred embodiments of the present invention and do not limit the patent scope of the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the patent protection scope of the present invention.

Claims

1. A negative electrode active material, characterized in that, The negative electrode active material includes a host material, which comprises porous microcrystalline graphite and a first lithium-ion conductive material, wherein the first lithium-ion conductive material at least partially fills the pores of the porous microcrystalline graphite. The porous microcrystalline graphite includes at least a first pore and a second pore, wherein the pore diameter of the first pore is 1-10 nm and the pore diameter of the second pore is 10-80 nm. The ratio of the volume Vse of the first lithium-ion conductive material in the pores of the porous microcrystalline graphite to the total pore volume Vp of the second pores in the porous microcrystalline graphite is 0.8 to 1.

2.

2. The negative electrode active material as described in claim 1, characterized in that, The first lithium-ion conductive material includes first lithium-ion conductive particles, and the ratio of the average pore size dav of the porous microcrystalline graphite to the average particle size Dsmall avg of the first lithium-ion conductive particles is 2 to 10.

3. The negative electrode active material as described in claim 2, characterized in that, The volume Vse of the first lithium-ion conductive material within the pores of the porous microcrystalline graphite is 0.19~0.53 cm³. 3 / g; and / or, The total pore volume Vp of the second pore in the porous microcrystalline graphite is 0.19~0.48 cm³. 3 / g; and / or, The average pore size (dav) of the porous microcrystalline graphite is 22-95 nm; and / or, The particle size of the first lithium-ion conductive particle is 10~40 nm; and / or, The average particle size Dsmall avg of the first lithium-ion conductive particles is 10~40 nm; and / or, The total pore volume of the first pore accounts for 10-20% of the total pore volume of the porous microcrystalline graphite, and the total pore volume of the second pore accounts for 40-60% of the total pore volume of the porous microcrystalline graphite.

4. The negative electrode active material as described in claim 1, characterized in that, The negative electrode active material further includes an interface buffer layer, which at least partially covers the main material, and the interface buffer layer includes a second lithium-ion conductive material.

5. The negative electrode active material as described in claim 4, characterized in that, The ratio of the thickness δ of the interface buffer layer to the average pore diameter dav of the porous microcrystalline graphite is ≤0.

2.

6. The negative electrode active material as described in claim 5, characterized in that, The ratio of the thickness δ of the interface buffer layer to the average pore size dav of the porous microcrystalline graphite is 0.08~0.13; and / or, The thickness δ of the interface buffer layer is 3~20nm; and / or, The first lithium-ion conductive material and / or the second lithium-ion conductive material comprises at least one of oxide solid electrolyte, sulfide solid electrolyte, and halide solid electrolyte, wherein the oxide solid electrolyte comprises at least one of LLZO, LATP, LLTO, LBO, LNbO, LiZrO, and LiAlO, and the sulfide solid electrolyte comprises LiGPS, LiSiPS, LGPS, Li6PS5Cl, Li6PS5Br, and Li 10 GeP2S 12 The halide solid electrolyte comprises at least one of Li3InCl6, Li2ZrCl6, Li3YBr6, Li3YCl6, LiTaF6, and LiNbF6.

7. A method for preparing a negative electrode active material as described in any one of claims 1 to 6, characterized in that, The preparation method of the negative electrode active material includes the following steps: Provide a porous microcrystalline graphite; The porous microcrystalline graphite is mixed with at least a first lithium-ion conductive material and mechanically fused to obtain the negative electrode active material.

8. The method for preparing the negative electrode active material as described in claim 7, characterized in that, The step of mixing porous microcrystalline graphite with at least a first lithium-ion conductive material and performing a mechanical fusion process to obtain the negative electrode active material includes: Porous microcrystalline graphite and a first lithium-ion conductive material are mixed and mechanically fused to obtain the main material; A second lithium-ion conductor material is deposited in situ on the surface of the main material to form an interface buffer layer, thereby obtaining the negative electrode active material.

9. The method for preparing the negative electrode active material as described in claim 7, characterized in that, The step of mixing porous microcrystalline graphite with at least a first lithium-ion conductive material and performing mechanical fusion treatment to obtain the negative electrode active material includes: Porous microcrystalline graphite, a first lithium-ion conductive material, and a third lithium-ion conductive material are mixed and mechanically fused to obtain the negative electrode active material. The particle size of the third lithium-ion conductive material is larger than the average pore size of the porous microcrystalline graphite.

10. The method for preparing the negative electrode active material as described in claim 9, characterized in that, The step of mixing porous microcrystalline graphite, a first lithium-ion conductive material, and a third lithium-ion conductive material and mechanically fusing them to obtain the negative electrode active material includes: The first lithium-ion conductive material and the third lithium-ion conductive material are mixed to obtain mixed particles; The porous microcrystalline graphite and the mixed particles are mixed and mechanically fused to obtain the negative electrode active material.

11. The method for preparing the negative electrode active material according to claim 10, characterized in that, In the step of mixing the first lithium-ion conductive material and the third lithium-ion conductive material to obtain mixed particles: The mixing includes ball milling, wherein the ball milling speed is 150~250 rpm; and / or, the ball milling time is 3~5 hours; and / or, The particle size of the third lithium-ion conductive material is 1-5 μm; and / or, The mass ratio of the first lithium-ion conductive material to the third lithium-ion conductive material is (65~80):(20~35).

12. The method for preparing the negative electrode active material as described in claim 10, characterized in that, In the step of mixing the porous microcrystalline graphite and the mixed particles and performing mechanical fusion treatment to obtain the negative electrode active material: The mechanical fusion process is performed at a rotation speed of 2500~3500 rpm; and / or, The temperature of the mechanical fusion treatment is 75~85℃; and / or, The mechanical fusion time is 25-35 minutes; and / or, The mass ratio of the porous microcrystalline graphite to the mixed particles is 100:(15~40).

13. The method for preparing the negative electrode active material as described in claim 7, characterized in that, The step of mixing porous microcrystalline graphite with at least a first lithium-ion conductive material and performing a mechanical fusion process to obtain the negative electrode active material includes: Porous microcrystalline graphite and a first lithium-ion conductive material are mixed and mechanically fused to obtain an intermediate. The intermediate is subjected to rinsing, centrifugation, and drying processes in sequence to obtain the negative electrode active material.

14. The method for preparing the negative electrode active material as described in claim 7, characterized in that, The step of providing a porous microcrystalline graphite includes: Natural microcrystalline graphite is mixed with an alkaline substance and activated under an inert atmosphere to obtain porous microcrystalline graphite.

15. The method for preparing the negative electrode active material as described in claim 14, characterized in that, Alkaline substances include at least one of potassium hydroxide and sodium hydroxide; and / or, The mass ratio of the natural microcrystalline graphite to the alkaline substance is 1:3~6; and / or, The activation reaction is performed at a temperature of 700~850℃; and / or, The activation reaction takes 1.8 to 2.2 hours.

16. A negative electrode sheet, characterized in that, The negative electrode sheet comprises the negative electrode active material as described in any one of claims 1 to 6 or the negative electrode active material prepared by the preparation method of the negative electrode active material as described in any one of claims 7 to 15.

17. A secondary battery, characterized in that, The secondary battery includes the negative electrode sheet as described in claim 16.

18. An electrical appliance, characterized in that, The electrical device includes the secondary battery as described in claim 17.

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