Graphite material, preparation method therefor, negative electrode material and secondary battery

Abstract

A graphite material, a preparation method therefor, a negative electrode material, and a secondary battery. The graphite material has a plurality of pores. Some of the plurality of pores have a pore size range of 2-120 nm. The total pore volume of some pores is in a range of 0.015 cm3 / g to 0.035 cm3 / g. Pores having a pore size of 2 nm or more and d1 or less account for 50% by volume of the total pore volume, wherein d1 is 75-96 nm. The pore structure of the graphite material has an appropriate pore size and pore volume. This pore structure can provide sufficient transmission channels having an appropriate width for lithium ions without remarkably affecting the capacity of a material and increase the number of sites at which lithium ions are intercalated or deintercalated and the intercalation or deintercalation rate of lithium ions. By further defining the pore size range of pores accounting for 50% by volume with respect to the total pore volume and by means of the cooperation of the pore volume and the pore size, side reactions of the negative electrode material during an electrochemical reaction can be reduced, thereby greatly improving the rate performance of the negative electrode material, thus achieving charging and discharging at a large current.

Description

Graphite material and preparation method thereof, negative electrode material and secondary battery

[0001] This application claims priority to the Chinese patent application filed with the State Intellectual Property Office on December 25, 2023, with application number 202311807096.8 and application name “Graphite materials and preparation methods thereof, negative electrode materials and lithium-ion batteries”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application relates to the technical field of lithium-ion batteries, and in particular to a graphite material and a preparation method thereof, a negative electrode material and a secondary battery. Background Art

[0003] Lithium-ion batteries are widely used in 3C products and power energy storage. Commercial lithium-ion batteries mainly use carbon negative electrode materials such as natural graphite, artificial graphite, soft carbon and hard carbon. Among them, natural graphite is widely used as an important negative electrode material due to its high specific capacity, good charge and discharge platform and low cost. For graphite negative electrodes, rate performance requirements are becoming increasingly important. The capacity under high current must be guaranteed for better commercial use. However, due to the lithium intercalation compound Li formed during the lithium intercalation process, the rate performance of the graphite negative electrode is relatively high. x The interlayer spacing of C6 is larger than that of natural graphite, which makes it unfavorable for the rapid deintercalation of lithium ions in the natural graphite during the charge and discharge process, thereby affecting its rate performance; in addition, since the interlayer spacing of the formed lithium-intercalated compound LixC6 is larger than that of natural graphite, the natural graphite will also undergo a volume expansion of about 10% during charge and discharge, resulting in the peeling of the graphite layer and the reduction of lithium ion cycling performance.

[0004] In order to improve the rate performance of natural graphite, researchers have developed a variety of modification methods to improve it. However, although the rate performance of the graphite produced in this way has been improved to a certain extent, the improvement is not large enough and it is still impossible to achieve high current charging and discharging.

[0005] Summary of the Invention

[0006] The present application proposes a graphite material and a preparation method thereof, a negative electrode material and a secondary battery, aiming to solve the technical problems mentioned in the above background technology.

[0007] In a first aspect, the present application provides a graphite material having a plurality of pores, wherein a portion of the pores has a pore diameter ranging from 2 nm to 120 nm, and a total pore volume of the portion of the pores is in the range of 0.015 cm 3 / g-0.035cm 3 / g, the volume proportion of pores with a pore diameter of more than 2 nm and less than d1 in the total pore volume is 50%, and d1 is 75 nm-96 nm.

[0008] In some embodiments, the pores have a pore size range of 2 nm to 120 nm.

[0009] In some embodiments, the graphite material has a layered structure including a plurality of graphite layers, and some of the pores are connected to at least some adjacent graphite layers.

[0010] In some embodiments, in the graphite material, the volume proportion of pores with a pore diameter of greater than 2 nm and less than 50 nm in the total pore volume is 13%-19%.

[0011] In some embodiments, in the graphite material, the volume proportion of pores with a pore diameter of greater than 2 nm and less than 80 nm in the total pore volume is 45%-51%.

[0012] In some embodiments, in the graphite material, the volume proportion of pores with a pore diameter of greater than 2 nm and less than 100 nm in the total pore volume is 69%-77%.

[0013] In some embodiments, the graphite material is in a spherical or spherical-like shape.

[0014] In some embodiments, the graphite material is a natural graphite material.

[0015] In some embodiments, the mass content of carbon in the graphite material is ≥99.9% based on the mass of the graphite material.

[0016] In some embodiments, the median particle size D50 of the graphite material is 5 μm-25 μm.

[0017] In some embodiments, the tap density of the graphite material is 0.5 g / cm 3 -1.2g / cm 3 .

[0018] In some embodiments, the specific surface area of ​​the graphite material is 5m 2 / g-15m 2 / g.

[0019] In a first aspect, the present application provides a method for preparing a graphite material, which comprises the steps of:

[0020] calcining a mixture of the graphite to be modified and the strong base reagent in a protective atmosphere having an oxygen volume content of 0.1% to 1% to obtain a calcined product, wherein the mass ratio of the graphite to be modified to the strong base reagent is 1:(0.1-0.5);

[0021] The calcined product is purified to obtain a graphite material having a plurality of pores, wherein a portion of the pores has a pore diameter ranging from 2 nm to 120 nm, and a total pore volume of the portion of the pores is in the range of 0.015 cm 3 / g-0.035cm 3 / g, the volume proportion of pores with a pore diameter of more than 2 nm and less than d1 in the total pore volume is 50%, and d1 is 75 nm-96 nm.

[0022] In a third aspect, the present application further proposes a negative electrode material, comprising the graphite material as described above, or comprising the graphite material prepared by the preparation method as described above.

[0023] In a fourth aspect, the present application also proposes a secondary battery comprising the negative electrode material as described above.

[0024] Compared with the existing technology, this technical solution has at least the following technical effects:

[0025] In the graphite material of the present application, since the pore structure of the graphite material has an appropriate pore diameter and pore volume, this pore structure can provide lithium ions with sufficient and appropriate width lithium ion transmission channels without significantly affecting the material capacity, increase the number of lithium ion embedding / de-embedding sites, and improve the embedding or de-embedding rate of lithium ions. By further limiting the pore diameter range of pores that account for 50% of the total pore volume, the synergistic effect of the pore volume and pore diameter can reduce the side reactions of the negative electrode material during the electrochemical reaction, thereby significantly improving the rate performance of the negative electrode material and realizing large current charging and discharging.

[0026] In the preparation method of the present application, during the calcination process, the strong alkaline reagent will etch the modified graphite to form pores. The specific reaction is: 2M x (OH) y +C+O2=M 2x O y+CO2+yH2O, and the present application controls the oxygen content in the protective atmosphere and the mass ratio of natural graphite to the strong alkaline reagent within an appropriate range, so that the pore structure of the graphite material has an appropriate pore size and pore volume. This pore structure can provide lithium ions with sufficient width and appropriate lithium ion transmission channels without significantly affecting the material capacity, increase the number of lithium ion embedding / de-embedding sites, and improve the embedding or de-embedding rate of lithium ions. By further limiting the pore size range of pores that account for 50% of the total pore volume, the synergistic effect of the pore volume and pore size can reduce the side reactions of the negative electrode material during the electrochemical reaction, thereby significantly improving the rate performance of the negative electrode material and realizing high current charging and discharging. In addition, the overall process of the preparation method of the present application is simple, the raw materials are less toxic and have a wide source. It does not need to be carried out under high temperature and high pressure, has low equipment requirements, has a high safety factor, and can prepare a large number of graphite materials with the above pore structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The present invention will be further described below with reference to the accompanying drawings and examples.

[0028] FIG1 is a diagram showing the preparation process of the graphite material of the present application;

[0029] Figure 2 (a) and Figure 2 (b) are schematic diagrams showing the principle of lithium ion embedding into the graphite materials prepared in Comparative Example 1 and Examples 1-14;

[0030] FIG3 is a schematic diagram of a secondary battery in a discharge state according to an embodiment of the present application;

[0031] FIG4 is an electron microscope image of the graphite material obtained in Example 1;

[0032] FIG5 is an electron microscope image of the graphite material obtained in Example 2;

[0033] FIG6 is an electron microscope image of the graphite material obtained in Comparative Example 1;

[0034] FIG7 is an electron microscope image of the graphite material prepared in Comparative Example 2. DETAILED DESCRIPTION

[0035] In order to better understand the technical solution of the present invention, the embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0036] It should be understood that the embodiments described are only a portion of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without creative work are within the scope of protection of the present invention.

[0037] The terms used in the embodiments of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The singular forms "a", "an", "the" and "the" used in the embodiments of the present invention and the appended claims are also intended to include plural forms unless the context clearly indicates otherwise.

[0038] It should be understood that the term "and / or" used in this article is merely a description of the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B can represent three situations: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the objects associated before and after are in an "or" relationship. In addition, in the following description, the record of "A~B" for two quantities A and B, unless otherwise specified, means "above A and below B". It should be noted that in this application, "above" and "below" both include the numbers themselves, that is, "above A and below B" includes A, B, and any value between A and B.

[0039] In a first aspect, the present application proposes a graphite material.

[0040] In the embodiment of the present application, the graphite material has a plurality of pores, and the pore diameter of some of the plurality of pores ranges from 2 nm to 120 nm, specifically 2 nm, 10 nm, 20 nm, 40 nm, 60 nm, 80 nm, 100 nm, 120 nm or any value therebetween, and the total pore volume of the plurality of pores ranges from 0.015 cm 3 / g-0.035cm 3 / g, specifically 0.015cm 3 / g, 0.025cm 3 / g, 0.035cm 3 / g or any value therebetween, the volume proportion of pores with a pore diameter of more than 2 nm and less than d1 in the total pore volume is 50%, d1 is 75 nm-96 nm, specifically 75 nm, 80 nm, 85 nm, 90 nm, 96 nm or any value therebetween.

[0041] In some embodiments, the plurality of pores have a pore size ranging from 2 nm to 120 nm.

[0042] In the graphite material of the present application, since the pore structure of the graphite material has an appropriate pore diameter and pore volume, this pore structure can provide lithium ions with sufficient and appropriate width lithium ion transmission channels without significantly affecting the material capacity, increase the number of lithium ion embedding / de-embedding sites, and improve the embedding or de-embedding rate of lithium ions. By further limiting the pore diameter range of pores that account for 50% of the total pore volume, the synergistic effect of the pore volume and pore diameter can reduce the side reactions of the negative electrode material during the electrochemical reaction, thereby significantly improving the rate performance of the negative electrode material and realizing large current charging and discharging. In some embodiments, in the graphite material, the volume proportion of pores with a pore diameter of greater than 2 nm and less than 50 nm in the total pore volume is 13%-19%, specifically 13%, 14%, 15%, 16%, 17%, 18%, 19% or any value therebetween; that is, in the graphite material, when the volume proportion of pores with a pore diameter of 50 nm and greater in the total pore volume is not less than 81%-87%, it can provide a wider transmission channel for lithium ions, which is beneficial to improving the insertion or extraction rate of lithium ions.

[0043] In some embodiments, in the graphite material, the volume proportion of pores with a pore diameter of greater than 2 nm and less than 80 nm in the total pore volume is 45%-51%, specifically 45%, 46%, 47%, 48%, 49%, 50%, 51% or any value therebetween; that is, in the graphite material, when the volume proportion of pores with a pore diameter of greater than 80 nm and less than 120 nm in the total pore volume is not less than 49%-55%, it can provide a broad transmission channel for lithium ions, which is beneficial to improving the insertion or extraction rate of lithium ions.

[0044] In some embodiments, in the graphite material, the volume proportion of pores with a pore diameter of greater than 2 nm and less than 100 nm in the total pore volume is 69%-77%, specifically 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77% or any value therebetween; that is, in the graphite material, when the volume proportion of pores with a pore diameter of greater than 100 nm and less than 120 nm in the total pore volume is not less than 23%-31%, it can provide a sufficiently wide transmission channel for lithium ions, which is beneficial to improving the insertion or extraction rate of lithium ions.

[0045] In some embodiments, the graphite material has a layered structure, including multiple stacked graphite layers, each of which is a single atomic layer, and some pores connect at least some adjacent layers of the graphite material. It will be appreciated that the pores formed in the graphite material interconnect the graphite layers, reducing the anisotropy of the graphite material and shortening the diffusion distance of lithium ions, thereby improving the rate performance of the graphite material.

[0046] In some embodiments, the graphite material is natural graphite material; compared with carbon negative electrode materials such as artificial graphite, soft carbon and hard carbon, natural graphite has the characteristics of high specific capacity, good charge and discharge platform, low cost, etc., and has greater application value.

[0047] In some embodiments, the graphite material is in a spherical or spherical-like shape.

[0048] In some embodiments, based on the mass of the graphite material, the carbon content of the graphite material is ≥ 99.9%. It should be noted that if the carbon content is less than 99.9%, it indicates that the graphite material contains a large amount of impurities and is of insufficient purity, which directly affects the electrochemical performance of the negative electrode material. If the carbon content is too low, it indicates that the graphite material contains a large amount of impurities, which can cause electrolyte failure or changes in the structure of the negative electrode material, easily leading to abnormal electrochemical performance of the negative electrode material.

[0049] In some embodiments, the median particle size D50 of the graphite material is between 5 μm and 25 μm, specifically 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, or any value therebetween. It will be appreciated that selecting a median particle size of the graphite material within this range helps reduce side reactions in subsequent reactions. It should be noted that D50 represents the particle size corresponding to the cumulative particle size distribution percentage reaching 50%.

[0050] In some embodiments, the tap density of the graphite material is 0.5 g / cm 3 -1.2g / cm 3 , specifically 0.5g / cm 3 , 0.8g / cm 3 , 1.2g / cm 3 or any value in between.

[0051] In some embodiments, the specific surface area of ​​the graphite material is 5 m 2 / g-15m 2 / g, specifically 5m 2 / g、10m 2 / g、15m 2 / g or any value therebetween. It is understandable that the specific surface area distribution of the graphite material within the above range is beneficial to reducing side reactions, reducing the consumption of active lithium on the surface of the negative electrode material having the graphite material and the consumption of the electrolyte, and is beneficial to improving the cycle performance of the negative electrode material.

[0052] Secondly, the present application also proposes a method for preparing graphite material.

[0053] Referring to FIG1 , in an embodiment of the present application, the method for preparing the graphite material includes the following steps:

[0054] S1. In a protective atmosphere having an oxygen volume content of 0.1%-1%, a mixture of graphite to be modified and a strong base agent is calcined to obtain a calcined product, wherein the mass ratio of the graphite to be modified to the strong base agent is 1: (0.1-0.5);

[0055] S2. Purifying the calcined product to obtain a graphite material having a plurality of pores, a portion of which has a pore diameter ranging from 2 nm to 120 nm, and a total pore volume of the portion of pores ranging from 0.015 cm 3 / g-0.035cm 3 / g, the volume proportion of pores with a pore diameter of more than 2 nm and less than d1 in the total pore volume is 50%, and d1 is 75 nm-96 nm.

[0056] In some embodiments, the pores of the graphite material may have a pore size of 2 nm, 10 nm, 20 nm, 40 nm, 60 nm, 80 nm, 100 nm, 120 nm, or any value therebetween.

[0057] In some embodiments, the plurality of pores have a pore size ranging from 2 nm to 120 nm.

[0058] In some embodiments, the total pore volume of the portion of pores in the graphite material (i.e., pores within the pore size range of 2 nm to 120 nm) may be 0.015 cm 3 / g, 0.025cm 3 / g, 0.035cm 3 / g or any value in between.

[0059] In some embodiments, the volume percentage of oxygen in the protective atmosphere is 0.1%, 0.2%, 0.4%, 0.6%, 0.8%, 1%, or any value therebetween.

[0060] In some embodiments, the mass ratio of natural graphite to strong alkaline agent is 1:0.1, 1:0.2, 1:0.3, 1:0.4, 1:0.5 or any value therebetween.

[0061] Please refer to FIG2 . In the preparation method of the present application, during the calcination process, the strong alkaline reagent will etch the modified graphite to form pores. The specific reaction is: 2M x (OH) y +C+O2=M 2x O y+CO2+yH2O, the pores formed on the graphite material after the modification help to connect the various graphite layers to each other, reduce the anisotropy of the graphite material, and shorten the diffusion distance of lithium ions, thereby improving the rate performance of the graphite material; and the present application controls the oxygen content in the protective atmosphere and the mass ratio of natural graphite to the strong alkaline reagent within an appropriate range, so that the pore structure of the graphite material has an appropriate pore diameter and pore volume. This pore structure can provide lithium ions with sufficient width and appropriate lithium ion transmission channels without significantly affecting the material capacity, increase the number of lithium ion insertion / deintercalation sites, and improve the insertion or extraction rate of lithium ions. By further limiting the pore diameter range of pores that account for 50% of the total pore volume, the synergistic effect of the pore volume and pore diameter can reduce the side reactions of the negative electrode material during the electrochemical reaction, thereby significantly improving the rate performance of the negative electrode material and achieving high current charging and discharging. In addition, the overall process of the preparation method of the present application is simple, the raw materials are less toxic and widely available, it does not need to be carried out under high temperature and high pressure, the equipment requirements are not high, its safety factor is high, and it can prepare a large amount of graphite materials with the above pore structure.

[0062] In some embodiments, the graphite to be modified is natural graphite. Compared with carbon negative electrode materials such as artificial graphite, soft carbon and hard carbon, natural graphite has the characteristics of high specific capacity, good charge and discharge platform, low cost, etc., and has greater application value.

[0063] In some embodiments, the graphite to be modified is spherical graphite, which can be directly purchased from the market or obtained by shaping flake graphite.

[0064] In some embodiments, the carbon content of the modified graphite is not less than 95%. Selecting a graphite with a carbon content of more than 95%, i.e., a graphite with high purity, can improve the electrochemical performance of the negative electrode material containing the graphite material.

[0065] In some embodiments, the strong base agent includes at least one of sodium hydroxide and potassium hydroxide. Of course, the strong base agent can also be selected from strong base compounds such as lithium hydroxide and rubidium hydroxide. However, considering the cost and etching effect, preferably, the strong base agent is selected from sodium hydroxide.

[0066] In some embodiments, the protective atmosphere includes one or more of nitrogen, helium, neon, and argon.

[0067] In some embodiments, the calcination temperature is 400°C-800°C, specifically 400°C, 500°C, 600°C, 700°C, 800°C or any value therebetween; by controlling the calcination temperature within this range, it is beneficial to control the etching rate of natural graphite by the strong alkaline agent within an appropriate range, and the appropriate etching rate is more conducive to controlling the pore size and total pore volume of the pores formed on the graphite within an ideal range.

[0068] In some embodiments, the calcination time is 2 h-8 h, specifically 2 h, 4 h, 6 h, 8 h or any value therebetween.

[0069] In some embodiments, the purification process comprises washing the roasted product with water.

[0070] During the etching process of modified graphite with strong alkali reagents, the strong alkali reagents will not only react with carbon, but also react with impurities in the modified graphite, such as silicon oxide, aluminum oxide, iron ions, calcium ions, magnesium ions and other impurities. Some of the new impurities generated by this reaction are insoluble or slightly soluble in water and are difficult to remove by water washing.

[0071] Furthermore, in some embodiments, the purification treatment includes washing the calcined product with water until the pH is less than 9, and then performing acid washing. The acid washing can further remove acid-soluble impurities in the calcined product that are difficult to remove by water washing.

[0072] In some embodiments, the roasted product is acid-washed with a washing liquid, and the washing liquid includes hydrochloric acid.

[0073] In some embodiments, the washing liquid further comprises nitric acid and water; during the pickling process, the mass content ratio of the roasted product to hydrochloric acid, nitric acid and water is 1:(0.3-0.9):(0-0.3):(1-2). Within this ratio range, a better washing effect can be achieved while consuming as little acid as possible.

[0074] In some embodiments, the pickling temperature is 70°C-90°C, specifically 70°C, 80°C, 90°C or any value therebetween. Within this pickling temperature range, the pickling effect is better.

[0075] In some embodiments, the pickling time is 8h-20h, specifically 8h, 10h, 12h, 14h, 16h, 18h, 20h or any value therebetween. By controlling the pickling time within the above range, it is possible to take into account the pickling efficiency while achieving a better pickling effect.

[0076] In some embodiments, the drying temperature is 60°C-100°C, specifically 60°C, 80°C, 100°C or any value therebetween.

[0077] In some embodiments, the drying time is 2 h to 8 h, specifically 2 h, 4 h, 6 h, 8 h or any value therebetween.

[0078] In some embodiments, the graphite material is in a spherical or spherical-like shape.

[0079] In some embodiments, the particle size D50 of the graphite material is 5 um-25 um, specifically 5 um, 10 um, 15 um, 20 um, 25 um or any value therebetween.

[0080] In some embodiments, the tap density of the graphite material is 0.5 g / cm 3 -1.2g / cm 3 , specifically 0.5g / cm 3 , 0.8g / cm 3 , 1.2g / cm 3 or any value in between.

[0081] In some embodiments, the specific surface area of ​​the graphite material is 5 m 2 / g-15m 2 / g, specifically 5m 2 / g、10m 2 / g、15m 2 / g or any value in between.

[0082] In some embodiments, the carbon content in the graphite material is ≥ 99.9% by mass based on the mass of the graphite material.

[0083] In a third aspect, the present application proposes a negative electrode material, comprising the graphite material of the first aspect, or comprising the graphite material prepared by the preparation method of the second aspect.

[0084] In a fourth aspect, one embodiment of the present application provides a secondary battery (such as a lithium-ion battery, a sodium-ion battery, etc.), comprising a housing, an electrode assembly, and an electrolyte / electrolyte. The electrode assembly and the electrolyte / electrolyte are both located within the housing.

[0085] The outer shell can be a packaging bag encapsulated by an encapsulation film (such as an aluminum-plastic film), such as a soft-pack battery. In other embodiments, the secondary battery can also be a steel-shell battery, an aluminum-shell battery, etc.

[0086] FIG3 shows a schematic diagram of a battery in a discharged state, i.e., during operation. As shown in FIG3 , the electrode assembly includes a positive electrode sheet 110, a negative electrode sheet 120, and a separator 130, with the separator being disposed between the positive electrode sheet and the negative electrode sheet. The electrode assembly may be a laminated structure, in which the positive electrode sheet, the separator, and the negative electrode sheet are alternately stacked in sequence. In other embodiments, the electrode assembly may also be a wound structure, in which the positive electrode sheet, the separator, and the negative electrode sheet are stacked in sequence and then wound.

[0087] positive electrode

[0088] The positive electrode sheet 110 includes a positive electrode current collector 111 and a positive electrode active layer 112 provided on at least one surface of the positive electrode current collector. The positive electrode current collector can be made of aluminum foil or nickel foil, or any composite current collector disclosed in the prior art, such as but not limited to a current collector formed by combining the aforementioned conductive foil and a polymer substrate. The positive electrode active layer contains a positive electrode active material, which includes a compound that reversibly intercalates and deintercalates metal ions. In some embodiments, the positive electrode active material may include a lithium transition metal composite oxide, a sodium transition metal composite oxide, or the like. The lithium transition metal composite oxide contains lithium and at least one element selected from cobalt, manganese, and nickel.

[0089] In some embodiments, the positive electrode active material may include but is not limited to lithium cobalt oxide (LiCoO2), lithium nickel manganese cobalt ternary material (NCM), lithium manganese oxide (LiMn2O4), lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 O4) or at least one of lithium iron phosphate (LiFePO4).

[0090] negative electrode

[0091] The negative electrode sheet 120 includes a negative electrode current collector 121 and a negative electrode active material layer 122 disposed on at least one surface of the negative electrode current collector. The negative electrode current collector can be made of at least one of copper foil, nickel foil, stainless steel foil, titanium foil, or a carbon-based current collector. It can also be any composite current collector disclosed in the prior art, such as, but not limited to, a current collector formed by combining the aforementioned conductive foil and a polymer substrate. The negative electrode active material layer includes the negative electrode material described above.

[0092] During battery operation, that is, when the battery is in a discharged state, the metal ions 140 (eg, lithium ions) in the negative electrode are released from the lattice of the negative electrode material, pass through the separator 130 via the electrolyte / electrolyte, and are embedded in the lattice of the positive electrode material.

[0093] Conversely, when the battery is charged by applying an external circuit, the oxidation of the positive electrode material causes the metal ions (such as lithium ions) in the positive electrode to be released from the lattice of the positive electrode material, pass through the isolation membrane through the electrolyte / electrolyte, and move to the negative electrode; at the same time, the negative electrode material undergoes a reduction reaction, causing the metal ions to be embedded in the lattice of the negative electrode material.

[0094] As metal ions move back and forth between the positive and negative electrodes, the battery can achieve discharge and charging processes over thousands of cycles.

[0095] The following further illustrates the embodiments of the present invention in multiple embodiments. The embodiments of the present invention are not limited to the following specific embodiments. Within the scope of protection, appropriate changes can be made to the implementation.

[0096] Example 1

[0097] Take 20 kg of low-purity spherical graphite (fixed carbon content ranges from 94 wt% to 96 wt%) and 2 kg of sodium hydroxide, mix them into material A, adjust the oxygen content of material A to 0.2% by volume under nitrogen at 800°C, and roast for 2 hours. After the roasting process is completed, a roasted product is obtained.

[0098] The calcined product was washed with deionized water until the pH was less than 9, and then acid-washed at 80°C for 8 hours in a mass ratio of calcined product: hydrochloric acid: nitric acid: water = 1:0.9:0.3:1. After acid washing, it was washed with deionized water until neutral and dried to obtain a high-purity, high-magnification spherical graphite material with appropriate porosity and volume distribution. The micromorphology of the graphite material is shown in Figure 4.

[0099] Example 2

[0100] Take 20 kg of low-purity spherical graphite (fixed carbon content ranges from 94 wt% to 96 wt%) and 4 kg of sodium hydroxide, mix them into material A, adjust the oxygen content of material A to 0.2% by volume under nitrogen at 500°C, and calcine for 4 hours. After the calcination process is completed, a calcined product is obtained.

[0101] The calcined product was washed with deionized water until the pH was less than 9, and then acid-washed at 90°C for 8 hours in a mass ratio of calcined product: hydrochloric acid: nitric acid: water = 1:0.8:0.3:1. After acid washing, it was washed with deionized water until neutral and dried to obtain a high-purity, high-magnification spherical graphite material with appropriate porosity and volume distribution. The micromorphology of the graphite material is shown in Figure 5.

[0102] Example 3

[0103] Take 20 kg of low-purity spherical graphite (fixed carbon content ranges from 94 wt% to 96 wt%) and 6 kg of sodium hydroxide, mix them into material A, adjust the oxygen content of material A to 0.4% by volume under nitrogen at 700°C, and roast for 6 hours. After the roasting process is completed, a roasted product is obtained.

[0104] The calcined product was washed with deionized water until the pH was less than 9, and then acid-washed at 70°C for 12 hours according to the mass ratio of calcined product: hydrochloric acid: nitric acid: water = 1:0.8:0.3:1. After acid washing, it was washed with deionized water until neutral and dried to obtain a high-purity and high-magnification spherical graphite material with appropriate porosity and volume distribution.

[0105] Example 4

[0106] Take 20 kg of low-purity spherical graphite (fixed carbon content ranges from 94 wt% to 96 wt%) and 8 kg of sodium hydroxide, mix them into material A, adjust the oxygen content of material A to 0.6% by volume under nitrogen at 600°C, and roast for 4 hours. After the roasting process is completed, a roasted product is obtained.

[0107] The calcined product was washed with deionized water until the pH was less than 9, and then acid-washed at 80°C for 16 hours according to the mass ratio of calcined product: hydrochloric acid: nitric acid: water = 1:0.9:0.3:1. After acid washing, it was washed with deionized water until neutral and dried to obtain a high-purity and high-magnification spherical graphite material with appropriate porosity and volume distribution.

[0108] Example 5

[0109] Take 20 kg of low-purity spherical graphite (fixed carbon content ranges from 94 wt% to 96 wt%) and 10 kg of sodium hydroxide, mix them into material A, adjust the oxygen content of material A to 0.8% by volume under nitrogen at 400°C, and roast for 8 hours. After the roasting process is completed, a roasted product is obtained.

[0110] The calcined product was washed with deionized water until the pH was less than 9, and then acid-washed at 80°C for 8 hours according to the mass ratio of calcined product: hydrochloric acid: nitric acid: water = 1:0.6:0.2:1. After acid washing, it was washed with deionized water until neutral and dried to obtain a high-purity and high-magnification spherical graphite material with appropriate porosity and volume distribution.

[0111] Example 6

[0112] Take 20 kg of low-purity spherical graphite (fixed carbon content ranges from 94 wt% to 96 wt%) and 3 kg of sodium hydroxide, mix them into material A, adjust the oxygen content of material A to 1% by volume under nitrogen at 800°C, and roast for 8 hours. After the roasting process is completed, a roasted product is obtained.

[0113] The calcined product was washed with deionized water until the pH was less than 9, and then acid-washed at 80°C for 8 hours according to the mass ratio of calcined product: hydrochloric acid: nitric acid: water = 1:0.6:0.2:1. After acid washing, it was washed with deionized water until neutral and dried to obtain a high-purity and high-magnification spherical graphite material with appropriate porosity and volume distribution.

[0114] Example 7

[0115] Take 20 kg of low-purity spherical graphite (fixed carbon content ranges from 94 wt% to 96 wt%) and 4 kg of sodium hydroxide, mix them into material A, adjust the oxygen content of material A to 0.2% by volume under nitrogen at 500°C, and roast for 6 hours. After the roasting process is completed, a roasted product is obtained.

[0116] The calcined product was washed with deionized water until the pH was less than 9, and then acid-washed at 80°C for 8 hours according to the mass ratio of calcined product: hydrochloric acid: nitric acid: water = 1:0.9:0.1:2. After acid washing, it was washed with deionized water until neutral and dried to obtain a high-purity and high-magnification spherical graphite material with appropriate porosity and volume distribution.

[0117] Example 8

[0118] Take 20 kg of low-purity spherical graphite (fixed carbon content ranges from 94 wt% to 96 wt%) and 6 kg of sodium hydroxide, mix them into material A, adjust the oxygen content of material A to 0.6% by volume under nitrogen at 700°C, and roast for 6 hours. After the roasting process is completed, a roasted product is obtained.

[0119] The calcined product was washed with deionized water until the pH was less than 9, and then acid-washed at 80°C for 8 hours according to the mass ratio of calcined product: hydrochloric acid: nitric acid: water = 1:0.9:0.1:2. After acid washing, it was washed with deionized water until neutral and dried to obtain a high-purity and high-magnification spherical graphite material with appropriate porosity and volume distribution.

[0120] Example 9

[0121] Take 20 kg of low-purity spherical graphite (fixed carbon content ranges from 94 wt% to 96 wt%) and 8 kg of sodium hydroxide, mix them into material A, adjust the oxygen content of material A to 0.8% by volume under nitrogen at 600°C, and roast for 4 hours. After the roasting process is completed, a roasted product is obtained.

[0122] The calcined product was washed with deionized water until the pH was less than 9, and then acid-washed at 80°C for 8 hours according to the mass ratio of calcined product: hydrochloric acid: nitric acid: water = 1:0.9:0.3:2. After acid washing, it was washed with deionized water until neutral and dried to obtain a high-purity and high-magnification spherical graphite material with appropriate porosity and volume distribution.

[0123] Example 10

[0124] Take 20 kg of low-purity spherical graphite (fixed carbon content ranges from 94 wt% to 96 wt%) and 10 kg of sodium hydroxide, mix them into material A, adjust the oxygen content of material A to 0.4% by volume under nitrogen at 400°C, and roast for 2 hours. After the roasting process is completed, a roasted product is obtained.

[0125] The calcined product was washed with deionized water until the pH was less than 9, and then acid-washed at 80°C for 8 hours according to the mass ratio of calcined product: hydrochloric acid: nitric acid: water = 1:0.6:0.2:2. After acid washing, it was washed with deionized water until neutral and dried to obtain a high-purity and high-magnification spherical graphite material with appropriate porosity and volume distribution.

[0126] Example 11

[0127] Take 20 kg of low-purity spherical graphite (fixed carbon content ranges from 94 wt% to 96 wt%) and 2 kg of potassium hydroxide, mix them into material A, adjust the oxygen content of material A to 0.1% by volume under nitrogen at 800°C, and roast for 2 hours. After the roasting process is completed, a roasted product is obtained.

[0128] The calcined product was washed with deionized water until the pH was less than 9, and then acid-washed at 80°C for 8 hours according to the mass ratio of calcined product: hydrochloric acid: nitric acid: water = 1:0.9:0.3:1. After acid washing, it was washed with deionized water until neutral and dried to obtain a high-purity and high-magnification spherical graphite material with appropriate porosity and volume distribution.

[0129] Example 12

[0130] Take 20 kg of low-purity spherical graphite (fixed carbon content ranges from 94 wt% to 96 wt%) and 2 kg of sodium hydroxide, mix them into material A, adjust the oxygen content of material A to 0.1% by volume under nitrogen at 800°C, and roast for 2 hours. After the roasting process is completed, a roasted product is obtained.

[0131] The calcined product was washed with deionized water until the pH was less than 9, and then acid-washed at 80°C for 8 hours in a mass ratio of calcined product: hydrochloric acid: water = 1:1.2:1. After acid washing, it was washed with deionized water until neutral and dried to obtain a high-purity and high-magnification spherical graphite material with appropriate porosity and volume distribution.

[0132] Example 13

[0133] Take 20 kg of low-purity spherical graphite (fixed carbon content ranges from 94 wt% to 96 wt%) and 6 kg of sodium hydroxide, mix them into material A, adjust the oxygen content of material A to 0.8% by volume under nitrogen at 800°C, and roast for 6 hours. After the roasting process is completed, a roasted product is obtained.

[0134] The calcined product was washed with deionized water until the pH was less than 9, and then acid-washed at 80°C for 8 hours according to the mass ratio of calcined product: hydrochloric acid: nitric acid: water = 1:0.9:0.3:1. After acid washing, it was washed with deionized water until neutral and dried to obtain a high-purity and high-magnification spherical graphite material with appropriate porosity and volume distribution.

[0135] Example 14

[0136] Take 20 kg of low-purity spherical graphite (fixed carbon content ranges from 94 wt% to 96 wt%) and 10 kg of sodium hydroxide, mix them into material A, adjust the oxygen content of material A to 0.8% by volume under nitrogen at 600°C, and roast for 6 hours. After the roasting process is completed, a roasted product is obtained.

[0137] The calcined product was washed with deionized water until the pH was less than 9, and then acid-washed at 80°C for 8 hours according to the mass ratio of calcined product: hydrochloric acid: nitric acid: water = 1:0.9:0.3:1. After acid washing, it was washed with deionized water until neutral and dried to obtain a high-purity and high-magnification spherical graphite material with appropriate porosity and volume distribution.

[0138] Comparative Example 1

[0139] 20 kg of low-purity spherical graphite (fixed carbon content ranges from 94 wt% to 96 wt%) was taken and pickled at 80°C for 8 h according to the mass ratio of graphite: hydrofluoric acid: hydrochloric acid: nitric acid: water = 1:0.2:0.45:0.15:1. After pickling, it was washed with deionized water until the pH was greater than 4. Then, it was pickled again at 80°C for 8 h according to the mass ratio of washing materials: hydrochloric acid: nitric acid: water = 1:0.45:0.15:1. After pickling, it was washed with deionized water until neutral and dried to obtain a conventional high-purity spherical graphite material. The micromorphology of the graphite material is shown in Figure 6.

[0140] Comparative Example 2

[0141] Take 20 kg of low-purity spherical graphite (fixed carbon content ranges from 94 wt% to 96 wt%) and 1 kg of sodium hydroxide, mix them into material A, adjust the oxygen content of material A to 1% by volume under nitrogen at 800°C, and roast for 8 hours. After the roasting process is completed, a roasted product is obtained.

[0142] The calcined product was washed with deionized water until the pH was less than 9, and then acid-washed at 80°C for 8 hours in a mass ratio of calcined product: hydrochloric acid: nitric acid: water = 1:0.6:0.2:1. After acid washing, it was washed with deionized water until neutral and dried to obtain a high-purity spherical graphite material with appropriate porosity and volume distribution. The micromorphology of the graphite material is shown in Figure 7.

[0143] Comparative Example 3

[0144] Take 20 kg of low-purity spherical graphite (fixed carbon content ranges from 94 wt% to 96 wt%) and 11 kg of sodium hydroxide, mix them into material A, adjust the oxygen content of material A to 0.1% by volume under nitrogen at 800°C, and roast for 2 hours. After the roasting process is completed, a roasted product is obtained.

[0145] The calcined product was washed with deionized water until the pH was less than 9, and then acid-washed at 80°C for 8 hours according to the mass ratio of calcined product: hydrochloric acid: nitric acid: water = 1:0.9:0.3:1. After acid washing, it was washed with deionized water until neutral and dried to obtain a high-purity spherical graphite material with appropriate porosity and volume distribution.

[0146] Comparative Example 4

[0147] Take 20 kg of low-purity spherical graphite (fixed carbon content ranges from 94 wt% to 96 wt%) and 10 kg of sodium hydroxide, mix them into material A, adjust the oxygen content of material A to 0.05% by volume under nitrogen at 400°C, and calcine for 8 hours. After the calcination process is completed, a calcined product is obtained.

[0148] The calcined product was washed with deionized water until the pH was less than 9, and then acid-washed at 80°C for 8 hours according to the mass ratio of calcined product: hydrochloric acid: nitric acid: water = 1:0.6:0.2:1. After acid washing, it was washed with deionized water until neutral and dried to obtain a high-purity spherical graphite material with appropriate porosity and volume distribution.

[0149] Comparative Example 5

[0150] Take 20 kg of low-purity spherical graphite (fixed carbon content ranges from 94 wt% to 96 wt%) and 2 kg of sodium hydroxide, mix them into material A, adjust the oxygen content of material A to 1.1% by volume under nitrogen at 800°C, and roast for 2 hours. After the roasting process is completed, a roasted product is obtained.

[0151] The calcined product was washed with deionized water until the pH was less than 9, and then acid-washed at 80°C for 8 hours according to the mass ratio of calcined product: hydrochloric acid: nitric acid: water = 1:0.9:0.3:1. After acid washing, it was washed with deionized water until neutral and dried to obtain a high-purity spherical graphite material with appropriate porosity and volume distribution.

[0152] Comparative Example 6

[0153] Take 20 kg of low-purity spherical graphite (fixed carbon content ranges from 94 wt% to 96 wt%) and 6 kg of sodium hydroxide, mix them into material A, adjust the oxygen content of material A to 1.1% by volume under nitrogen at 500°C, and roast for 2 hours. After the roasting process is completed, a roasted product is obtained.

[0154] The calcined product was washed with deionized water until the pH was less than 9, and then acid-washed at 80°C for 8 hours according to the mass ratio of calcined product: hydrochloric acid: nitric acid: water = 1:0.9:0.3:1. After acid washing, it was washed with deionized water until neutral and dried to obtain a high-purity spherical graphite material with appropriate porosity and volume distribution.

[0155] Test Method

[0156] (1) Observation of the surface morphology of graphite materials:

[0157] The surface morphology of the graphite material was observed using an S-4800 scanning electron microscope.

[0158] (2) Test method for median particle size of graphite materials:

[0159] The particle size volume distribution range of graphite material particles is tested by Malvern laser particle size analyzer, and the particle size distribution test method is used, where D50 represents the particle size corresponding to when the cumulative particle size distribution percentage reaches 50%.

[0160] (3) Test method for tap density of graphite materials:

[0161] Place the graphite material in the sample chamber of the tap density meter, vibrate it 1000 times, and record the sample volume at this time. The tap density can be calculated according to the mass-to-volume ratio.

[0162] (4) Test method for specific surface area of ​​graphite material:

[0163] At constant temperature and low temperature, after measuring the adsorption amount of gas on the solid surface at different relative pressures, the monolayer adsorption amount of the sample is obtained based on the Brownauer-Etter-Taylor adsorption theory and its formula (BET formula), thereby calculating the specific surface area of ​​the graphite material.

[0164] (5) Test method for pore diameter and total pore volume of graphite material:

[0165] A mercury porosimeter is used to measure the pore size d and total pore volume V of graphite materials. Mercury porosimeter typically uses a cylindrical pore model. By applying different pressures, the volume of mercury entering the pores can be measured by the change in the liquid mercury level in the dilatometer. The pore size corresponding to the pressure can be calculated using the Washburn equation, thereby obtaining data on the total pore volume and pore size.

[0166] The mercury intrusion test method is to test the pore volume corresponding to the pore size of the mesopore and macropore, which will generate a distribution curve of the pore volume corresponding to different pore sizes. The cumulative sum of the pore volumes corresponding to different pore sizes is the total pore volume. 3 / g~0.035cm 3 The total pore volume in the range of 1 / g is the cumulative addition of the pore volume and pore diameter in the range of 2-120 nm.

[0167] (6) Test method for carbon content in graphite material: muffle furnace roasting test for fixed carbon.

[0168] (7) Electrochemical rate performance test: The graphite materials prepared in Examples 1-14 and Comparative Examples 1-5 were dissolved in deionized water with carboxymethyl cellulose and styrene-butadiene rubber in a mass ratio of 96.5:1.5:1, and the solid content was controlled to be 50%. The samples were coated on a copper foil current collector and vacuum dried to obtain a negative electrode. NCM532, polyvinylidene fluoride and conductive carbon were dissolved in N-methylpyrrolidone in a mass ratio of 97:2:1, and the solid content was controlled to be 50%. The samples were coated on an aluminum foil current collector and vacuum dried to obtain a positive electrode. The battery was assembled and tested in a dew point room. After activation, charge and discharge tests were performed in the charge and discharge range of 2.8V-4.2V.

[0169] After the above tests, the graphite materials prepared in Examples 1-14 and Comparative Examples 1-6 have sample numbers S1-S14 and R1-R6 respectively. The test results of the graphite materials are shown in Table 1 below:

[0170] (It should be noted that the improvement in charging performance in Table 1 is calculated based on the 81.2% improvement in charging performance in Comparative Example 1.)

[0171] By comparing Examples 1-14 with Comparative Example 1, it can be seen that, relative to Comparative Example 1, the charging performance of Examples 1-14 is significantly improved, and the improvement is shown in Table 1 above. The improvement is not less than 3.8%, and can even be as high as 5.2%. It can also be concluded that conventional graphite (unetched) is modified (etched) using the method of the present application so that the graphite material has an appropriate pore size and pore volume. In this way, the pores on the graphite material can provide sufficient width suitable transmission channels for lithium ions, which is conducive to improving the insertion or extraction rate of lithium ions, and thus the rate performance of the graphite material can be significantly improved, and the improvement can meet the charge and discharge of large currents. In addition, by comparing Example 1 with Comparative Example 1, it can be concluded that, relative to the purification process of unmodified low-purity graphite, the purification process of the modified graphite obtained by etching with a strong alkaline reagent in the present application is simpler, and higher purity graphite can be obtained without multiple pickling, and the pickling reagent used is safer, without the need to use hydrofluoric acid-based strong corrosive reagents.

[0172] By comparing Comparative Example 2 with Example 6, Comparative Example 3 with Example 1, Comparative Example 4 with Example 5, and Comparative Example 5 with Example 1, it can be concluded that when the graphite material prepared has a plurality of pores, the pore diameter is 2nm-120nm, and the total pore volume ranges from 0.015cm 3 / g-0.035cm 3 / g, the graphite material has excellent rate performance and can meet the requirements of large current charging and discharging; and by controlling the oxygen content in the preparation process of the graphite material and the ratio of the content of the modified graphite to the strong alkaline reagent within an appropriate range, it is beneficial to form pores with a diameter range of 2nm-120nm on the graphite and a total pore volume range of 0.015cm 3 / g-0.035cm 3 / g hole.

[0173] In Comparative Example 6, the oxygen content during the calcination process was too high, resulting in oxidation of the graphite surface, an increase in graphite surface defects, and an excessively large pore size (d1>96nm), which in turn led to an increase in side reactions during the electrochemical reaction of the negative electrode material and a decrease in rate performance. Compared with Examples 1-14, it can be seen that the pore size of the pores is 2nm-120nm, and the total pore volume is in the range of 0.015cm 3 / g-0.035cm 3 / g, the volume proportion of pores with a pore diameter of more than 2nm and less than d1 in the total pore volume is 50%, among which when d1 is 75nm-96nm, the pore structure of the graphite material has an appropriate pore diameter and pore volume. This pore structure can provide lithium ions with sufficient and appropriate width lithium ion transmission channels without significantly affecting the material capacity, increase the number of lithium ion embedding / de-embedding sites, and improve the embedding or de-embedding rate of lithium ions. By further limiting the pore diameter range of pores with a volume proportion of 50% in the total pore volume, the synergistic effect of the pore volume and pore diameter can reduce the side reactions of the negative electrode material during the electrochemical reaction, thereby significantly improving the rate performance of the negative electrode material and realizing large current charging and discharging.

[0174] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A graphite material, wherein, The graphite material has a plurality of pores, and the pore diameter range of a part of the plurality of pores is 2 nm - 120 nm, and the total pore volume of the part of the pores ranges from 0.015 cm 3 / g - 0.035 cm 3 / g. The volume percentage of pores with a pore diameter of more than 2 nm and less than or equal to d1 in the total pore volume is 50%, and d1 is 75 nm - 96 nm.

2. The graphite material according to claim 1, wherein The pore diameter of the plurality of pores ranges from 2 nm to 120 nm.

3. The graphite material according to claim 1 or 2, wherein The graphite material has a layered structure, including a plurality of graphite layers, and at least some of the pores communicate with at least some adjacent graphite layers.

4. The graphite material according to claim 1 or 2, wherein In the graphite material, the volume ratio of pores with a pore diameter of more than 2 nm and less than 50 nm in the total pore volume is 13% - 19%.

5. The graphite material according to claim 1 or 2, wherein In the graphite material, the volume ratio of pores with a pore diameter of more than 2 nm and less than 80 nm in the total pore volume is 45% - 51%.

6. The graphite material according to claim 1 or 2, wherein In the graphite material, the volume ratio of pores with a pore diameter of more than 2 nm and less than 100 nm in the total pore volume is 69% - 77%.

7. The graphite material according to claim 1 or 2, wherein, Meet at least one of the following characteristics (1) - (3): (1) The graphite material is spherical or quasi-spherical in shape; (2) The graphite material is a natural graphite material; (3) Based on the mass of the graphite material, the mass content of carbon in the graphite material is ≥ 99.9%.

8. The graphite material according to claim 1 or 2, wherein The median particle size D50 of the graphite material is 5 μm - 25 μm.

9. The graphite material according to claim 1 or 2, wherein The tapped density of the graphite material is 0.5 g / cm 3 - 1.2 g / cm 3 .

10. The graphite material according to claim 1 or 2, wherein The specific surface area of the graphite material is 5 m 2 / g - 15 m 2 / g.

11. A method for preparing a graphite material, wherein, Comprise the following steps: Under a protective atmosphere with an oxygen volume content of 0.1% - 1%, calcine a mixture containing the graphite to be modified and a strong base reagent to obtain a calcined product, wherein the mass ratio of the graphite to be modified to the strong base reagent is 1:(0.1 - 0.5); Purify the calcined product to obtain a graphite material. The graphite material has a plurality of pores. The pore diameter range of a part of the plurality of pores is 2 nm - 120 nm, and the total pore volume of the part of the pores ranges from 0.015 cm 3 / g to 0.035 cm 3 / g. The volume percentage of pores with a pore diameter of more than 2 nm and less than or equal to d1 in the total pore volume is 50%, where d1 is 75 nm - 96 nm.

12. The method for preparing the graphite material according to claim 11, wherein, The pore diameter of the plurality of pores ranges from 2 nm to 120 nm.

13. A negative electrode material, wherein, Comprise the graphite material according to any one of claims 1 - 10, or comprise the graphite material prepared by the preparation method according to claim 11 or 12.

14. A secondary battery, wherein, Comprise the negative electrode material according to claim 13.

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