Negative active material, preparation thereof, secondary battery and device

Abstract

The invention discloses a negative electrode active material, a preparation method thereof, a secondary battery and a device, the negative electrode active material comprises first graphite and second graphite, and the ratio of OI1 of the first graphite to OI2 of the second graphite is 0.18-0.94. Wherein the first graphite comprises etched natural graphite; the second graphite comprises natural graphite which is not etched. The surface of the negative electrode active material has a gully structure, the etching depth is obvious, and the rate capability of the material can be greatly improved. Based on the improvement, the problem of poor multiplying power of the negative pole piece under the condition of relatively high surface density can be greatly improved.

Description

Technical Field

[0001] The present application relates to a negative electrode active material, and in particular, to a negative electrode active material, preparation thereof, a secondary battery and a device. Background Art

[0002] The negative electrode material is the limiting factor of the kinetic performance in lithium-ion batteries, thus affecting the performance of lithium-ion batteries in terms of rapid charging and discharging and low temperature, and easily causing the problem of low capacity retention rate and easy lithium precipitation under high-rate charging and discharging conditions. At present, the method of improving the kinetic performance of negative electrode materials is mainly through etching on the surface of negative electrode materials. For example, Chinese patents CN115241455A and CN114530597A disclose relatively typical etching materials. However, it is worth noting that the holes caused by etching disclosed in patent CN115241455A are distributed in a point-like manner and the etching depth is shallow. The increased contact area between the electrolyte and the negative electrode material is limited, and no obvious etching pore-forming effect can be observed on the surface of CN114530597A, and the rate improvement is limited.

[0003] Therefore, it is necessary to develop a novel negative electrode active material having improved rate performance. Summary of the invention

[0004] In order to solve the above technical problems, the present application provides a negative electrode active material, its preparation, secondary battery and device. The present application can greatly improve the rate performance of the material by adjusting the ratio of OI1 of etched graphite and OI2 of unetched graphite on the surface of the negative electrode active material.

[0005] The first aspect of the present application provides a negative electrode active material, which includes a first graphite and a second graphite, wherein the ratio of OI1 of the first graphite to OI2 of the second graphite is 0.10 to 0.94, wherein the first graphite includes etched natural graphite; the second graphite includes unetched natural graphite; OI1 of the first graphite refers to the ratio of the peak intensity of the 004 characteristic diffraction peak to the peak intensity of the 110 characteristic diffraction peak in the X-ray diffraction spectrum of the first graphite; OI2 of the second graphite refers to the ratio of the peak intensity of the 004 characteristic diffraction peak to the peak intensity of the 110 characteristic diffraction peak in the X-ray diffraction spectrum of the second graphite.

[0006] The second aspect of the present application provides a method for preparing a negative electrode active material, comprising the following steps: (1) mixing graphite with an alkaline solution, stirring at 60°C to 90°C for 20h to 30h to obtain a first mixture; (2) filtering the first mixture, drying at 70°C to 90°C to obtain a second mixture; (3) keeping the second mixture at 600°C to 700°C under an inert atmosphere for 1.5h to 2.5h to obtain a third mixture.

[0007] A third aspect of the present application provides a secondary battery, comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the negative electrode sheet comprises the above-mentioned negative electrode active material or the negative electrode active material prepared by the above-mentioned method.

[0008] A fourth aspect of the present application provides a device comprising the secondary battery described above.

[0009] The surface of the negative electrode active material of the present application has a gully structure and a significant etching depth, which can greatly improve the rate performance of the material. Based on the above improvements, the present application can greatly improve the rate difference problem of the negative electrode sheet when the surface density is large. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 This is a SEM image of the negative electrode active material of the present invention before etching;

[0011] Figure 2 This is a SEM image of the negative electrode active material of the present invention after etching;

[0012] Figure 3 It is a schematic structural diagram of the depth (L2) of the pores on the surface of the first graphite and the diameter (L1) of the first graphite of the present invention. DETAILED DESCRIPTION

[0013] For simplicity, this application only specifically discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form an undefined range; and any lower limit can be combined with other lower limits to form an undefined range, and any upper limit can be combined with any other upper limit to form an undefined range. In addition, each separately disclosed point or single value can itself be combined as a lower limit or upper limit with any other point or single value or with other lower limits or upper limits to form an undefined range.

[0014] Unless otherwise specified, the terms used in this application have the commonly known meanings generally understood by those skilled in the art. Unless otherwise specified, the numerical values ​​of the various parameters mentioned in this application can be measured using various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the examples of this application).

[0015] A list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single component or multiple components. Item B may contain a single component or multiple components. Item C may contain a single component or multiple components.

[0016] The term "first graphite" includes etched natural graphite.

[0017] The term "second graphite" includes natural graphite that has not been etched.

[0018] The term "OI1 of the first graphite" refers to the ratio of the peak intensity of the 004 characteristic diffraction peak to the peak intensity of the 110 characteristic diffraction peak in the X-ray diffraction spectrum of the first graphite.

[0019] The term "OI2 of the second graphite" refers to the ratio of the peak intensity of the 004 characteristic diffraction peak to the peak intensity of the 110 characteristic diffraction peak in the X-ray diffraction spectrum of the second graphite.

[0020] The term "OI of the negative electrode piece" refers to the ratio of the peak intensity of the 004 characteristic diffraction peak to the peak intensity of the 110 characteristic diffraction peak in the X-ray diffraction spectrum of the negative electrode piece.

[0021] The term "diameter of the first graphite" refers to the length of the etched first graphite in the major axis direction.

[0022] The term "depth of pores" refers to the maximum value of the depth of the grooves that the etched first graphite surface has.

[0023] The present application is further described below in conjunction with specific implementations. It should be understood that these specific implementations are only used to illustrate the present application and are not used to limit the scope of the present application.

[0024] 1. Negative electrode active materials

[0025] The present invention adjusts the conditions of alkali treatment (i.e. etching and swelling) to make the surface of the negative electrode active material have obvious etching marks, and the surface of the material presents a state of continuous etching gullies. In addition, the OI of the negative electrode active material prepared by the present invention 负极活性材料The ratio of OI1 of the first graphite to OI2 of the second graphite is relatively low, and the ratio of OI1 of the first graphite to OI2 of the second graphite is 0.10 to 0.94, which increases the contact area between the negative electrode active material and the electrolyte, expands the diffusion channel of lithium ions, and increases the 1C rate of lithium insertion by 30%, thereby effectively improving the rate performance of the negative electrode active material. If the ratio of OI1 of the first graphite to OI2 of the second graphite is too large, there will be more graphite end face material and lithium ions cannot be quickly embedded; if the ratio of OI1 of the first graphite to OI2 of the second graphite is too small, the etching degree will be relatively high and the graphite structure will be easily decomposed. Therefore, the negative electrode active material including the ratio of OI1 of the first graphite to OI2 of the second graphite of the present application exhibits excellent rate performance.

[0026] In some embodiments, the ratio of OI1 of the first graphite to OI2 of the second graphite is 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.94 or any range therebetween. In some embodiments, the ratio of OI1 of the first graphite to OI2 of the second graphite is 0.28 to 0.94.

[0027] In some embodiments, the first graphite accounts for 5% to 90% of the area of ​​the negative electrode active material. If the first graphite occupies too large an area of ​​the negative electrode active material, the etching degree of the surface of the negative electrode active material is high, which makes the natural graphite structure easy to decompose; if the first graphite occupies too small an area of ​​the negative electrode active material, the etching degree of the surface of the negative electrode active material is low (i.e., the OI value is large), and lithium ions are not easily embedded. In some embodiments, the first graphite accounts for 5%, 15%, 25%, 35%, 45%, 55%, 65%, 75%, 85%, 90% of the area of ​​the negative electrode active material or any range therebetween. In some embodiments, the first graphite accounts for 10% to 86% of the area of ​​the negative electrode active material.

[0028] In some embodiments, the ratio of the depth L2 of the alkali treatment to the diameter L1 of the negative electrode active material is 0.07 to 0.65. If the ratio of the depth of the alkali treatment to the diameter of the negative electrode active material is too large, the graphite particle structure is easily decomposed; if the ratio of the depth of the alkali treatment to the diameter of the negative electrode active material is too small, the effect produced is not obvious. In some embodiments, the ratio of the depth L2 of the alkali treatment to the diameter L1 of the negative electrode active material is 0.07, 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.65 or any range therebetween. In some embodiments, the ratio of the depth L2 of the alkali treatment to the diameter L1 of the negative electrode active material is 0.10 to 0.65. In some embodiments, the surface of the first graphite is in a grooved state; in some embodiments, the negative electrode active material includes natural graphite.

[0029] The present application also provides a method for preparing negative electrode active materials, comprising the following steps: (1) mixing graphite with an alkaline solution, stirring at 60°C to 90°C for 20h to 30h to obtain a first mixture; (2) filtering the first mixture, drying at 70°C to 90°C to obtain a second mixture; (3) keeping the second mixture at 600°C to 700°C under an inert atmosphere for 1.5h to 2.5h to obtain a third mixture. The preparation method provided in the present application can greatly reduce the OI of the negative electrode active material. 负极活性材料 , and the prepared negative electrode active material shows a groove-state etching effect, which increases the contact area with the electrolyte, improves the lithium ion diffusion capacity, and thus improves the rate.

[0030] In some embodiments, the method further comprises the following steps: (4) mixing the third mixture with concentrated sulfuric acid. In some embodiments, in step (1), the volume ratio of the alkaline solution to the mass of the graphite is 1ml:1g to 20ml:1g, wherein the concentration of the alkaline solution is 0.5mol / L to 7mol / L. In some embodiments, the volume ratio of the alkaline solution to the mass of the graphite is 1ml:1g, 5ml:1g, 10ml:1g, 15ml:1g, 20ml:1g or any range therebetween. In some embodiments, the volume ratio of the alkaline solution to the mass of the graphite is 2ml:1g to 10ml:1g. In some embodiments, the concentration of the alkaline solution is 0.5mol / L, 1mol / L, 2mol / L, 3mol / L, 4mol / L, 5mol / L, 6mol / L, 7mol / L or any range therebetween. In some embodiments, the concentration of the alkaline solution is 2mol / L to 7mol / L.

[0031] 2. Secondary battery

[0032] The secondary battery provided by the present application includes a negative electrode plate, the negative electrode plate includes a negative electrode active material layer, the negative electrode active material layer includes a negative electrode active material, and the negative electrode active material includes graphite. The negative electrode plate of the present application has better rate cycle performance because the porosity of the plate is significantly improved, thereby accelerating the diffusion of lithium ions.

[0033] In some embodiments, the porosity of the negative electrode sheet is 15.3% to 40%; in some embodiments, the OI of the negative electrode sheet is 12 to 36; in some embodiments, the surface density of the negative electrode sheet is 5 to 15 g / cm 2 .

[0034] In some embodiments, the negative electrode active material layer further comprises a binder and a conductive agent. In some embodiments, the binder includes, but is not limited to: polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylic (ester) styrene-butadiene rubber, epoxy resin or nylon, etc.

[0035] In some embodiments, the conductive agent includes, but is not limited to: carbon-based materials, metal-based materials, conductive polymers, and mixtures thereof. In some embodiments, the carbon-based material is selected from natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material is selected from metal powder, metal fiber, copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer is a polyphenylene derivative.

[0036] In some embodiments, the negative electrode plate further includes a negative electrode current collector, and the negative electrode current collector includes: copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, or any combination thereof.

[0037] In some embodiments, a separator is provided between the positive electrode plate and the negative electrode plate to prevent short circuit. The material and shape of the separator that can be used in the embodiments of the present application are not particularly limited, and it can be any technology disclosed in the prior art. In some embodiments, the separator includes a polymer or an inorganic substance formed of a material that is stable to the electrolyte of the present application.

[0038] For example, the isolation film may include a substrate layer and a surface treatment layer. The substrate layer is a non-woven fabric, a film or a composite film having a porous structure, and the material of the substrate layer includes at least one of polyethylene, polypropylene, polyethylene terephthalate or polyimide. Specifically, a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric or a polypropylene-polyethylene-polypropylene porous composite film can be selected.

[0039] A surface treatment layer is disposed on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing a polymer and an inorganic layer.

[0040] The inorganic layer includes inorganic particles and a binder, wherein the inorganic particles include at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide or barium sulfate. The binder includes at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene or polyhexafluoropropylene.

[0041] The polymer layer contains polymers, and the polymer material includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene).

[0042] In some embodiments, the secondary battery is a lithium secondary battery or a sodium secondary battery. In some embodiments, the lithium secondary battery includes, but is not limited to: a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery or a lithium ion polymer secondary battery.

[0043] In some embodiments, the secondary battery may include an outer package, which may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery may also be a soft package, such as a bag-type soft package. The material of the soft package may be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), polybutylene succinate (PBS), etc.

[0044] In some embodiments, the shape of the secondary battery is not particularly limited, and it can be cylindrical, square, or any other shape.

[0045] In some embodiments, the present application further provides a battery module. The battery module includes the above-mentioned secondary battery. The battery module of the present application uses the above-mentioned secondary battery, and therefore has at least the same advantages as the secondary battery. The number of secondary batteries contained in the battery module of the present application can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module.

[0046] In some embodiments, the present application further provides a battery pack, which includes the above-mentioned battery module. The number of battery modules included in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0047] 3. Device

[0048] The present application also provides a device, which includes at least one of the above-mentioned secondary battery, battery module or battery pack.

[0049] In some embodiments, the device includes, but is not limited to: electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, power storage systems, etc. In order to meet the device's requirements for high power and high energy density of secondary batteries, a battery pack or battery module may be used.

[0050] In other embodiments, the device may be a mobile phone, a tablet computer, a laptop computer, etc. The device is usually required to be light and thin, and a secondary battery may be used as a power source.

[0051] The test methods for the negative electrode active materials, negative electrode plates and lithium ion battery performance in the following examples and comparative examples are as follows:

[0052] 1. Determination of the area ratio of graphite to the negative electrode active material

[0053] 1) Take the sample and lay it flat on the conductive glue, use an ear-cleaning bulb to blow away the excess powder, put it into the observation chamber, and magnify it 5000 times to obtain a SEM image;

[0054] 2) Image pro plus Import the image into the image processing software, click file-open, open a picture you want to measure. Click Irregualr AOI, click count and measure object to select area ratio, and calculate the area of ​​the first graphite.

[0055] In the same way, the entire area of ​​the negative electrode active material can be obtained, and finally the area ratio can be obtained.

[0056] 2. Determination of the ratio of the depth of alkaline treatment and the diameter of the negative electrode active material

[0057] Use CP-SEM to test the cross-sectional electron microscopy of the active material particles, obtain an electron microscope SEM image at a magnification of 5000 times, mark the scale d, and use straight lines to mark the etching depth L2 and the diameter L1 of the negative electrode active material on the electron microscope image. Then, the actual values ​​of L2 and L1 can be calculated to get the ratio.

[0058] 3. Determination of OI1 of the first graphite

[0059] The processed particles were selected under 3D microscope conditions, and the X-ray diffraction spectrum was obtained by using synchrotron radiation XRD according to the general rules of X-ray diffraction analysis and the lattice parameter determination method of graphite JIS K 0131-1996 and JB / T4220-2011, and calculated according to the formula OI=C004 / C110, wherein C004 is the peak intensity of the 004 characteristic diffraction peak, and C110 is the peak intensity of the 110 characteristic diffraction peak.

[0060] 4. Determination of OI2 of Secondary Graphite

[0061] The test method is the same as the test of the OI1 value of the first graphite, except that the selected position is the unetched particle.

[0062] 5. Determination of OI of negative electrode

[0063] The X-ray powder diffractometer (X'pert PRO) was used to obtain the X-ray diffraction spectrum according to the general rules of X-ray diffraction analysis and the lattice parameter determination method of graphite JIS K 0131-1996 and JB / T4220-2011, and the OI of the negative electrode was calculated according to the formula OI=C004 / C110, wherein C004 is the peak intensity of the 004 characteristic diffraction peak, and C110 is the peak intensity of the 110 characteristic diffraction peak.

[0064] 6. Determination of the porosity of the negative electrode sheet

[0065] The porosity is measured by a mercury intrusion instrument, specifically: cut the dried electrode sample into thin strips of a certain size, and use a micrometer to measure the apparent volume of the electrode coating, apparent volume = sample coating thickness * sample length * sample width. Then the electrode is vacuum degassed, rolled and placed in the sample pool. The sample volume must be 40-70% of the effective volume of the sample tube to ensure measurement accuracy. Then use a mercury intrusion instrument to measure the pore volume of the sample, that is, the volume of mercury pressed into the sample, and the porosity = pore volume / apparent volume.

[0066] 7. Determination of the surface density of the negative electrode sheet

[0067] a) Use a fixed area sampler (1540.25mm2) to cut a small disc of the electrode; b) Use an analytical balance to weigh the cut small disc of the electrode; c) Divide the weight of the small disc by the area of ​​the small disc to obtain the surface density of the electrode including the foil, and then subtract the surface density of the foil to obtain the coating surface density of the electrode.

[0068] 8.1C / 1C 50T determination

[0069] First, charge and discharge at 0.33C / 0.33C for 1T, and the discharge capacity is recorded as C 初始 , then 1C / 1C cycle 50T, the capacity of the last cycle is recorded as C 50 , the capacity retention rate is C 50 / C 初始 .

[0070] 9.1 Determination of C lithium insertion rate

[0071] Use button battery, the first cycle is 0.05C / 0.1C, the capacity is C 0.05 Then, lithium is inserted at a rate of 1C, and the capacity of lithium insertion is recorded as C1, C1 / C0.5 The value of is recorded as the 1C lithium insertion rate.

[0072] 10. First Determination of Coulombic Efficiency

[0073] The electrochemical performance of the battery was tested using a blue electric test cabinet. At 25°C, the battery was discharged to 0.005V at 0.05C within the voltage range of 0.005V to 1.5V. After standing for 5 minutes, it was charged to 1.5V at 0.1C. The charge and discharge efficiency = initial charge capacity / 0.05C discharge capacity*100%

[0074] Examples and Comparative Examples

[0075] Example 1

[0076] The negative electrode active material, the conductive agent carbon black (SP), the thickener carboxymethyl cellulose (CMC) and the binder styrene butadiene rubber (SBR) are mixed in a weight ratio of 96:1:1:2, wherein the negative electrode active material, the thickener and the binder are first mixed in water into a pre-blend, the pre-blend having a solid content of 60% and a fineness of less than 30 μm, and then, the binder SBR is added to the pre-blend to form a negative electrode slurry, and the negative electrode slurry is coated on a copper foil to obtain a negative electrode sheet.

[0077] The battery assembly was performed using a CR2025 button cell, wherein the negative electrode sheet according to the present invention was used as the working electrode, the lithium sheet was used as the counter electrode, and the polypropylene film was used as the diaphragm; the electrolyte consisted of lithium hexafluorophosphate (LiPF6), EC, DEC and FEC. The button cell was assembled in a glove box filled with argon gas, and the assembled battery was immediately sealed by a sealing machine. After standing for 24 hours, a 0.05C constant current charge and discharge test was performed to obtain the discharge capacity of the negative electrode active material, wherein the test temperature was 25°C and the voltage range was 5mV-2V. The preparation of the negative electrode active material is as follows:

[0078] (1) Add 178 g potassium hydroxide (KOH) to 500 ml water and stir with a magnetic stirrer at a speed of 240 rpm / min for 1 h;

[0079] (2) adding 20 g of graphite to the above solution, stirring with a magnetic stirrer at a speed of 300 rpm / min, a stirring time of 24 h, and a heating temperature of 80°C;

[0080] (3) Filter the solution by suction, leaving the graphite powder, and dry it at 80°C;

[0081] (4) The dried powder was placed in a vapor deposition tube furnace (CVD furnace), heated to 650°C at 1°C / min under a nitrogen atmosphere, and kept at this temperature for 2 h;

[0082] (5) The powder after chemical vapor deposition (CVD) heat treatment is washed with deionized water and dried under neutral pH conditions;

[0083] (6) 18 g of the dried powder was added to 36 ml of concentrated sulfuric acid (mass concentration: 98%) and stirred for 2 h. The mixture was filtered and dried, and then placed in a CVD furnace and heated to 950 °C at 7 °C / min and kept at this temperature for 2 h.

[0084] (7) The powder was cooled to room temperature, washed with deionized water until the pH value was neutral, and dried at 80° C. to obtain a negative electrode active material.

[0085] Examples 2 to 3 and Comparative Example 1

[0086] Examples 2 to 3 and Comparative Example 1 are achieved by adjusting the concentration of the alkaline solution, the volume ratio of the alkaline solution to the mass ratio of graphite, etc. on the basis of Example 1. The specific adjustment measures and detailed data are shown in Table 1.

[0087] Table 1

[0088]

[0089] As can be seen from Table 1, the negative electrode active material prepared by adjusting the conditions of the alkali treatment in this application has excellent electrochemical properties. If the area ratio of the etched first graphite to the negative electrode active material is too small (such as Comparative Example 2), no obvious etching marks appear on the surface of the first graphite, that is, the purpose of shortening the diffusion distance of lithium ions is not achieved. Therefore, the lithium insertion rate is not improved. If the area ratio of the etched second graphite to the negative electrode active material is too large (such as Example 2), in the case of such a large degree of etching, although the rate performance is significantly improved, the first coulombic efficiency is significantly reduced, which will affect the capacity of the battery cell during the application of the battery cell.

[0090] Although some exemplary embodiments of the present application have been illustrated and described, the present application is not limited to the disclosed embodiments. On the contrary, those skilled in the art will recognize that some modifications and changes may be made to the described embodiments without departing from the spirit and scope of the present application as described in the appended claims.

Claims

1. A negative electrode active material comprising a first graphite and a second graphite, wherein: The ratio of OI1 of the first graphite to OI2 of the second graphite is 0.10 to 0.94, wherein, The first graphite includes etched natural graphite; The second graphite includes unetched natural graphite; OI1 of the first graphite refers to the peak intensity ratio of the 004 characteristic diffraction peak to the 110 characteristic diffraction peak in the X-ray diffraction spectrum of the first graphite; OI2 of the second graphite refers to the peak intensity ratio of the 004 characteristic diffraction peak to the 110 characteristic diffraction peak in the X-ray diffraction spectrum of the second graphite.

2. The negative electrode active material according to claim 1, characterized in that The negative electrode active material satisfies at least one of the following conditions: The ratio of OI1 of the first graphite to OI2 of the second graphite is 0.28 to 0.94; The area ratio of the first graphite to the negative electrode active material is 5% to 90%; The surface of the first graphite has pores obtained by etching.

3. The negative electrode active material according to claim 2, characterized in that: The first graphite accounts for 10% to 86% of the area of ​​the negative electrode active material; and / or The ratio of the depth L2 of the pores to the diameter L1 of the first graphite is 0.07 to 0.

65.

4. The negative electrode active material according to claim 3, characterized in that The ratio of the depth L2 of the pores to the diameter L1 of the first graphite is 0.10 to 0.

65.

5. A method for preparing the negative electrode active material according to any one of claims 1 to 4, comprising the following steps: (1) mixing graphite with an alkaline solution, and stirring at 60° C. to 90° C. for 20 h to 30 h to obtain a first mixture; (2) filtering the first mixture and drying it at 70° C. to 90° C. to obtain a second mixture; (3) The second mixture is kept at 600° C. to 700° C. for 1.5 h to 2.5 h under an inert atmosphere to obtain a third mixture.

6. The method according to claim 5, It also includes step (4): mixing the third mixture with concentrated sulfuric acid; and / or In step (1), the volume ratio of the alkaline solution to the mass ratio of the graphite is 1 ml:1 g to 20 ml:1 g, wherein: The concentration of the alkaline solution is 0.5 mol / L to 7 mol / L.

7. The method according to claim 6, characterized in that The volume ratio of the alkaline solution to the mass ratio of the graphite is 2 ml:1 g to 10 ml:1 g; and / or The concentration of the alkaline solution is 2 mol / L to 7 mol / L.

8. A secondary battery, comprising a negative electrode plate, wherein: The negative electrode sheet comprises the negative electrode active material according to any one of claims 1 to 4 or the negative electrode active material prepared by the method according to any one of claims 5 to 7.

9. The secondary battery according to claim 8, characterized in that: The secondary battery satisfies at least one of the following conditions: The porosity of the negative electrode sheet is 15.3% to 40%; OI of the negative electrode sheet 负极极片 12 to 36; The surface density of the negative electrode plate is 5g / cm 2 ~15g / cm 2 . 10 . A device comprising the secondary battery according to claim 9 .

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