A modified graphite negative electrode material and its preparation method and application

Abstract

The present invention discloses a modified graphite negative electrode material and a preparation method and application thereof. The modified graphite negative electrode material has a core-shell structure, and comprises, from the inside to the outside, a porous graphite layer, a transition metal oxide layer, and an N-doped carbon coating layer; the N-doped carbon coating layer is composed of a polymer having a structure of formula I; a transition metal oxide layer is introduced between the porous graphite and the N-doped carbon coating layer, and the transition metal oxide of the metal oxide layer can form an ion conductive network structure with high ion conductivity with the porous graphite, thereby reducing the impedance during desorption of the N active site, providing additional energy for the release of lithium ions, and overcoming the problem that the first coulomb efficiency decreases when the N content increases.

Description

Technical Field

[0001] The present invention relates to the technical field of battery negative electrode materials, and in particular to a modified graphite negative electrode material and a preparation method and application thereof. Background Art

[0002] As a negative electrode material, graphite has the characteristics of abundant resources, low price, high reversible capacity, low charge and discharge voltage platform, no voltage hysteresis, and excellent conductivity. It has attracted great attention in the application of lithium batteries. It has quickly attracted widespread attention, and there are many studies and applications on graphite materials. However, graphite materials also have some shortcomings. For example, the graphite layers are connected by van der Waals forces, the interlayer force is small, and the interlayer spacing is smaller than the crystal plane interlayer spacing of graphite intercalation compounds, which causes the graphite interlayer spacing to change during the charge and discharge process, causing the graphite flakes to peel off and pulverize, resulting in unsatisfactory electrical cycle performance; secondly, due to the structural defects of the graphite itself, there are many active groups on its surface, which makes natural graphite poorly compatible with solvents; thirdly, its tap density is relatively low. In order to overcome the above shortcomings, it is necessary to modify the surface of the graphite material.

[0003] Methods such as surface oxidation, carbon coating, and surface deposition of metals or metal oxides can effectively improve the poor compatibility of graphite with electrolytes. The spheroidization of carbon-based materials can also improve the high-current charge and discharge performance of materials. However, the method of modifying graphite negative electrode materials by forming a carbon coating on the graphite surface through high-temperature carbonization will cause a large amount of thermal decomposition and loss of conventional nitrogen sources due to long-term sintering at high temperatures. Very little nitrogen can be directly doped into the carbon coating, and high-content nitrogen doping of the carbon coating cannot be achieved, which is not conducive to the capacity improvement of negative electrode materials. If nitrogen is doped in large quantities, the first coulombic efficiency will decrease due to the obstruction of lithium ion deintercalation.

[0004] Therefore, it is necessary to provide a solution to simultaneously improve the first coulombic efficiency and capacity of graphite anode materials. Summary of the invention

[0005] In view of this, the present application provides a modified graphite negative electrode material and a preparation method and application thereof, which are used to solve the problem of how to simultaneously improve the first coulombic efficiency and capacity of the graphite negative electrode material.

[0006] In order to achieve the above technical objectives, this application adopts the following technical solutions:

[0007] In a first aspect, the present application provides a modified graphite negative electrode material, which is a core-shell structure, and includes, from the inside to the outside, a porous graphite layer, a transition metal oxide layer, and an N-doped carbon coating layer; the N-doped carbon coating layer is composed of a polymer containing a structure of formula I;

[0008]

[0009] Preferably, the transition metal oxide includes one or more of aluminum oxide and zirconium oxide.

[0010] Preferably, the thickness of the transition metal oxide layer is 5-10 nm.

[0011] Preferably, the thickness of the N-doped carbon coating layer is 1-5 nm.

[0012] In a second aspect, the present application provides a method for preparing a modified graphite negative electrode material according to any one of claims 1 to 4, characterized in that it comprises the following steps:

[0013] S1. In an inert atmosphere, porous graphite is mixed with liquid metal organic matter, and then water vapor is introduced into the reaction, and then sintered to obtain metal oxide-coated graphite;

[0014] S2. The metal oxide-coated graphite is added to the polyacrylonitrile solution and stirred to obtain a liquid-coated graphite material;

[0015] S3. The liquid-phase coated graphite material is subjected to a gradient temperature increase to perform a cyclization reaction and a dehydrogenation reaction to obtain a coated graphite material.

[0016] Preferably, the porous graphite is prepared by using a mixture of natural flake graphite, an intercalation agent and an oxidant as raw materials and calcining at 430-520° C. to obtain the porous graphite.

[0017] Preferably, the liquid metal organic is trimethylaluminum.

[0018] Preferably, in step S1, the sintering temperature is 800-1000°C.

[0019] Preferably, the gradient temperature program is: 180-220°C for reaction for 2-4h, 220-240°C for reaction for 2-4h, and 280°C for reaction for 4-6h.

[0020] In a third aspect, a modified graphite negative electrode material is provided for use in a lithium battery.

[0021] The beneficial effects of the present application are as follows: the present application introduces a transition metal oxide layer between the porous graphite and the N-doped carbon coating layer. The transition metal oxide of the metal oxide layer can form an ion conductive network structure with high ion conductivity with the porous graphite, thereby reducing the impedance during desorption of the N active site, providing additional energy for the release of lithium ions, and overcoming the problem of a decrease in the first coulombic efficiency when the N content increases. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 These are the XRD and SEM images of the modified graphite negative electrode material obtained in Example 1 of the present application. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0024] The present application provides a modified graphite negative electrode material, which is a core-shell structure, and includes a porous graphite layer, a transition metal oxide layer, and an N-doped carbon coating layer from the inside to the outside; the N-doped carbon coating layer is composed of a polymer containing a structure of formula I;

[0025]

[0026] The transition metal oxide includes one or more of aluminum oxide and zirconium oxide.

[0027] The thickness of the transition metal oxide layer is 5-10 nm.

[0028] The thickness of the N-doped carbon coating layer is 1-5 nm.

[0029] The present application provides a method for preparing a modified graphite negative electrode material, comprising the following steps:

[0030] S1. In an inert atmosphere, porous graphite is mixed with liquid metal organic matter, and then water vapor is introduced into the reaction, and then sintered to obtain metal oxide-coated graphite;

[0031] S2. The metal oxide-coated graphite is added to the polyacrylonitrile solution and stirred to obtain a liquid-coated graphite material;

[0032] S3. The liquid-phase coated graphite material is subjected to a gradient temperature increase to perform a cyclization reaction and a dehydrogenation reaction to obtain a coated graphite material.

[0033] In step S1, the solid-liquid mixing is conducive to the uniform coating of the liquid metal organic matter on the surface of the porous graphite, and then the metal liquid organic matter (trimethylaluminum) reacts with water vapor, and is sintered at high temperature and dehydrated to generate a ceramic metal oxide (Al 2 O 3 ), ceramic metal oxides are in-situ deposited on the pores and carbon layers of porous graphite to achieve better coating effects;

[0034] The specific operation of step S2 is: adding the metal oxide coated graphite to the polyacrylonitrile solution, and performing liquid phase coating by a mixer, wherein the mixer is stirred and coated at a speed of 1500 rpm for 2 hours to obtain a liquid phase coated graphite material; similarly, the subsequent N-doped carbon coating layer can be made more uniform;

[0035] The function of step S3 is to use polyacrylonitrile to form an amorphous carbon layer with high conductivity and N doping on the surface of metal oxide-coated graphite. The polyacrylonitrile (PAN) molecular chain undergoes chemical changes through gradient temperature increase during the heat treatment process. First, the comonomer in the PAN molecular chain initiates a cyclization reaction in an inert atmosphere as the temperature rises to 220°C to form a -COOR structure and become an imine / enamine tautomer. Subsequently, the temperature is raised to 240°C and a dehydrogenation reaction begins to occur, converting the imine / enamine tautomer into a conjugated structure, forming a dense conductive polymer network on the graphite surface, and forming an amorphous carbon coating layer with high conductivity and N doping.

[0036] In the present application, the N-doped carbon coating layer is obtained by reacting polyacrylonitrile. On the one hand, polyacrylonitrile (PAN) forms a conjugated structure on the graphite surface during the coating process, which is beneficial to improve the conductivity and provide a lithium ion transmission channel, and optimizes the rate charge and discharge performance of the graphite negative electrode. On the other hand, since the conjugated structure of PAN contains heteroatom N, it can be used to optimize the electronic structure of the carbon layer and improve the conductivity, and it can also be used as an adsorption site for lithium ions, which is beneficial to improve the capacity of the negative electrode material. However, as the amount of PAN coating increases, the N active sites will also increase, and the capacity will also increase accordingly, but at this time, the first coulomb efficiency decreases. The reason is that since the desorption of lithium ions on the N active sites is different from the deintercalation mechanism between the graphite layers, it needs to provide additional energy to release the lithium ions, so it has a certain resistance during the release process. Therefore, the first coulomb efficiency will decrease due to the increase in the amount of PAN used.

[0037] In order to overcome the above-mentioned defects, the present application introduces a transition metal oxide layer between the porous graphite and the N-doped carbon coating layer. The transition metal oxide of the metal oxide layer can form an ion conductive network structure with high ion conductivity with the porous graphite, thereby reducing the impedance during desorption of the N active site and providing additional energy for the release of lithium ions, so as to overcome the problem of the decrease in the first coulombic efficiency when the N content increases.

[0038] On the other hand, the present application adopts a process of first preparing a transition metal oxide layer by high-temperature sintering at 800-1000°C, and then preparing an N-doped carbon coating layer by low-temperature gradient heating (180-220°C for 2-4h, 220-240°C for 2-4h, and 280°C for 4-6h); the reason is that if high-temperature carbonization is directly performed to form a carbon coating on the graphite surface, a large amount of conventional nitrogen source will be lost by thermal decomposition, and the amount of nitrogen doping is small, which is not conducive to the improvement of capacity. The present application is coated in two stages at different temperatures, the advantages of which are, first, to increase the amount of nitrogen doping, and second, to increase the conductivity of the coating layer and optimize the rate charge and discharge performance.

[0039] The preparation method of porous graphite is as follows: using a mixed solution of natural flake graphite, an intercalating agent and an oxidant as raw materials, and calcining at 430-520° C. to obtain porous graphite.

[0040] Specifically, natural flake graphite, intercalant and oxidant are mixed at a mass ratio of 1:5:1 at 4°C until they become viscous, then dried, and calcined at 470°C for 10 hours to obtain porous graphite with an average particle size of 7 μm and an average pore diameter of 10 nm. The formation of pores can store a large amount of electrolyte, and the infiltration of electrolyte forms an ion buffer reservoir. The pore structure can shorten the distance of lithium ion diffusion and electron conduction, thereby improving the rate charge and discharge performance.

[0041] At the same time, the present application also provides an application of a modified graphite negative electrode material in a lithium battery.

[0042] Example 1

[0043] A modified graphite negative electrode material, which is a core-shell structure, and includes a porous graphite layer, a transition metal oxide layer, and an N-doped carbon coating layer from the inside to the outside; the N-doped carbon coating layer is composed of a polymer containing a structure of formula I;

[0044]

[0045] The preparation method is as follows:

[0046] S1. In N 2 Under the atmosphere, 1g of porous graphite was mixed with 0.1mol / L of trimethylaluminum liquid, and then water vapor was introduced to react, and sintered at 1000°C. The sintered ceramic deposit was deposited on the surface of the porous carbon-coated active material to obtain metal oxide-coated graphite, and the thickness of the metal oxide was 10nm.

[0047] S2. Take 1g of metal oxide-coated graphite and add it to 0.05mol / L polyacrylonitrile solution, and carry out liquid phase coating by a mixer. The liquid phase coating process is to put the precursor solution and graphite into a mixer, and the mixer is stirred and coated at a speed of 1500rmp / min for 2h to obtain a liquid-coated graphite material;

[0048] S3. The liquid-coated graphite material is subjected to a gradient temperature increase, and the heat treatment temperature program is: 180°C for cyclization reaction for 2h, 240°C for thermal stability cyclization for 2h, and then dehydrogenation at 280°C to obtain a coated graphite material, wherein the thickness of the N-doped carbon coating layer is 2nm. The identification results are as follows Figure 1 shown.

[0049] Example 2

[0050] A modified graphite negative electrode material, which is a core-shell structure, and includes a porous graphite layer, a transition metal oxide layer, and an N-doped carbon coating layer from the inside to the outside; the N-doped carbon coating layer is composed of a polymer containing a structure of formula I;

[0051]

[0052] The preparation method is as follows:

[0053] S1. In N 2 Under the atmosphere, 1g of porous graphite was mixed with 0.1mol / L of trimethylaluminum liquid, and then water vapor was introduced to react, and sintered at 1000°C. The sintered ceramic deposit was deposited on the surface of the porous carbon-coated active material to obtain metal oxide-coated graphite, and the thickness of the metal oxide was 10nm.

[0054] S2. Take 1g of metal oxide-coated graphite and add it to 0.1mol / L polyacrylonitrile solution, and carry out liquid phase coating by a mixer. The liquid phase coating process is to put the precursor solution and graphite into a mixer, and the mixer is stirred and coated at a speed of 1500rmp / min for 2h to obtain a liquid-coated graphite material;

[0055] S3. The liquid-coated graphite material is subjected to a gradient temperature increase, and the heat treatment temperature program is: 180°C for cyclization reaction for 2 hours, 240°C for thermal stability cyclization for 2 hours, and then dehydrogenation at 280°C to obtain a coated graphite material, wherein the thickness of the N-doped carbon coating layer is 5 nm.

[0056] Example 3

[0057] A modified graphite negative electrode material, which is a core-shell structure, and includes a porous graphite layer, a transition metal oxide layer, and an N-doped carbon coating layer from the inside to the outside; the N-doped carbon coating layer is composed of a polymer containing a structure of formula I;

[0058]

[0059] The preparation method is as follows:

[0060] S1. In N 2 Under the atmosphere, 1g of porous graphite was mixed with 0.05mol / L of trimethylaluminum liquid, and then water vapor was introduced to react, and sintered at 1000°C. The sintered ceramic deposit was deposited on the surface of the porous carbon-coated active material to obtain metal oxide-coated graphite, and the thickness of the metal oxide was 6nm.

[0061] S2. Take 1g of metal oxide-coated graphite and add it to 0.05mol / L polyacrylonitrile solution, and carry out liquid phase coating by a mixer. The liquid phase coating process is to put the precursor solution and graphite into a mixer, and the mixer is stirred and coated at a speed of 1500rmp / min for 2h to obtain a liquid-coated graphite material;

[0062] S3. The liquid-coated graphite material is subjected to a gradient temperature increase, and the heat treatment temperature program is: 180°C for cyclization reaction for 2 hours, 240°C for thermal stability cyclization for 2 hours, and then dehydrogenation at 280°C to obtain a coated graphite material, wherein the thickness of the N-doped carbon coating layer is 2 nm.

[0063] Example 4

[0064] A modified graphite negative electrode material, which is a core-shell structure, and includes a porous graphite layer, a transition metal oxide layer, and an N-doped carbon coating layer from the inside to the outside; the N-doped carbon coating layer is composed of a polymer containing a structure of formula I;

[0065]

[0066] The preparation method is as follows:

[0067] S1. In N 2 Under the atmosphere, 1g of porous graphite was mixed with 0.1mol / L of trimethylaluminum liquid, and then water vapor was introduced to react, and sintered at 800°C. The sintered ceramic deposit was deposited on the surface of the porous carbon-coated active material to obtain metal oxide-coated graphite, and the thickness of the metal oxide was 10nm.

[0068] S2. Take 1g of metal oxide-coated graphite and add it to 0.05mol / L polyacrylonitrile solution, and carry out liquid phase coating by a mixer. The liquid phase coating process is to put the precursor solution and graphite into a mixer, and the mixer is stirred and coated at a speed of 1500rmp / min for 2h to obtain a liquid-coated graphite material;

[0069] S3. The liquid-coated graphite material is subjected to a gradient temperature increase, and the heat treatment temperature program is: 180°C for cyclization reaction for 2 hours, 220°C for thermal stability cyclization for 2 hours, and then dehydrogenation at 280°C to obtain a coated graphite material, wherein the thickness of the N-doped carbon coating layer is 2 nm.

[0070] Comparative Example 1

[0071] A modified graphite negative electrode material, the preparation method of which is as follows:

[0072] 1 g of porous ink was added to a 0.05 mol / L polyacrylonitrile solution, and liquid-phase coating was performed by a mixer. The liquid-phase coating process was as follows: the precursor solution and graphite were placed in a mixer, and the mixer was stirred and coated at a speed of 1500 rpm for 2 hours to obtain a liquid-phase coated graphite material;

[0073] The liquid-coated graphite material is subjected to a gradient temperature increase, and the heat treatment temperature program is: 180°C for cyclization reaction for 2 hours, 240°C for thermal stability cyclization for 2 hours, and then dehydrogenation at 280°C to obtain a coated graphite material, wherein the thickness of the N-doped carbon coating layer is 2 nm.

[0074] Comparative Example 2

[0075] 1 g of porous ink was added to a 0.1 mol / L polyacrylonitrile solution, and liquid-phase coating was performed by a mixer. The liquid-phase coating process was as follows: the precursor solution and graphite were placed in a mixer, and the mixer was stirred and coated at a speed of 1500 rpm for 2 hours to obtain a liquid-phase coated graphite material;

[0076] The liquid-coated graphite material was subjected to a gradient temperature increase, and the heat treatment temperature program was: 180°C for cyclization reaction for 2 hours, 240°C for thermal stability cyclization for 2 hours, and then dehydrogenation at 280°C to obtain a coated graphite material, wherein the thickness of the N-doped carbon coating layer was 5 nm.

[0077] Comparative Example 3

[0078] A modified graphite negative electrode material, the preparation method of which is as follows:

[0079] In N 2 Under the atmosphere, 1g of porous graphite was mixed with 0.1mol / L of trimethylaluminum liquid, and then water vapor was introduced to react, and sintered at 800°C. The sintered ceramic deposit was deposited on the surface of the porous carbon-coated active material to obtain metal oxide-coated graphite, and the thickness of the metal oxide was 10nm.

[0080] 1 g of metal oxide-coated graphite was added to a 0.05 mol / L polyacrylonitrile solution and subjected to liquid-phase coating by a mixer. The liquid-phase coating process was as follows: the precursor solution and graphite were placed in a mixer, and the mixer was stirred and coated at a speed of 1500 rpm for 2 h to obtain a liquid-phase coated graphite material.

[0081] The liquid-phase coated graphite material is heated at 800° C. to obtain a coated graphite material.

[0082] Test and Evaluation

[0083] The performance of the negative electrode materials of the embodiment and the comparative example was verified by the half-cell test method. The graphite sample: conductive carbon black: PVDF (polyvinylidene fluoride) = 94.5:1.5:4 (wt%), an appropriate amount of NMP (N-methylpyrrolidone) was added to make a slurry, and then coated on the copper foil. The coated electrode was placed in a vacuum drying oven at a temperature of 110°C and vacuum dried for 4 hours for use. The electrolyte was 1 mol / L LiPF 6 +EC:DEC:DMC=1:1:1 (volume ratio), metal lithium sheet is used as counter electrode, polypropylene microporous membrane is used as separator, and they are assembled into a battery.

[0084] The capacity retention rate of each negative electrode material after 100 cycles at a fast charging rate of 2.5C was tested to evaluate its fast charging performance; the capacity and first coulombic efficiency of each negative electrode material at a rate of 0.1C were tested, and the results are shown in Table 1.

[0085] Table 1 Test results

[0086] <![CDATA[Capacity / (mAh·g -1 )]]> First coulombic efficiency / % Example 1 385 95 Example 2 411 94 Example 3 380 90 Example 4 388 92 Comparative Example 1 375 89 Comparative Example 2 409 83 Comparative Example 3 351 86

[0087] In Example 2, compared with Example 1, the concentration of polyacrylonitrile is increased, the thickness of the N-doped carbon coating layer is increased, and its capacity is improved. Under the action of the transition metal oxide, its first coulombic efficiency remains basically unchanged, indicating that the transition metal oxide of the present application can suppress the problem of reduced first coulombic efficiency due to increased N concentration; in Example 3, compared with Example 1, the concentration of trimethylaluminum is reduced, the thickness of the transition metal oxide layer is reduced, and since the nitrogen content is not much different, the two capacities are equivalent, but the first coulombic efficiency is reduced, indicating that the transition metal oxide has the effect of improving the first coulombic efficiency, and the comparative example 1 Compared with Example 1, it does not include a transition metal oxide layer. Although the N-doped carbon coating layer is the same, its first coulombic efficiency is greatly reduced. Compared with Example 2, the concentration of polyacrylonitrile is increased in Comparative Example 1, and the thickness of the N-doped carbon coating layer is increased. In the absence of the transition metal oxide layer, the capacity of Comparative Example 2 is higher but the first coulombic efficiency is lower, which is opposite to the comparison results of Example 1 and Example 2. Compared with Example 1, the temperature for preparing the transition metal oxide layer and the N-doped carbon coating layer in Comparative Example 3 is the same, resulting in a decrease in the N content and a different morphology of the N-doped carbon coating layer, thereby reducing the capacity.

[0088] The above are only preferred specific implementations of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A modified graphite negative electrode material, It is characterized in that It is a core-shell structure, which includes a porous graphite layer, a transition metal oxide layer, and an N-doped carbon coating layer from the inside to the outside; the N-doped carbon coating layer is composed of a polymer having a structure of formula I; Formula (I).

2. The modified graphite negative electrode material according to claim 1, It is characterized in that The thickness of the transition metal oxide layer is 5-10 nm.

3. The modified graphite negative electrode material according to claim 1, It is characterized in that The thickness of the N-doped carbon coating layer is 1-5 nm.

4. A method for preparing a modified graphite negative electrode material according to any one of claims 1 to 3, It is characterized in that The following steps are involved: S1. In an inert atmosphere, porous graphite is mixed with liquid metal organic matter, and then water vapor is introduced into the reaction, and then sintered to obtain metal oxide-coated graphite; S2. The metal oxide-coated graphite is added to the polyacrylonitrile solution and stirred to obtain a liquid-coated graphite material; S3. Gradual heating of the liquid-coated graphite material to perform cyclization reaction and dehydrogenation reaction to obtain the modified graphite negative electrode material.

5. The method for preparing the modified graphite negative electrode material according to claim 4, It is characterized in that The preparation method of the porous graphite is as follows: using a mixed solution of natural flake graphite, an intercalation agent and an oxidant as raw materials, and calcining at 430-520° C. to obtain the porous graphite.

6. The method for preparing the modified graphite negative electrode material according to claim 4, It is characterized in that The liquid metal organic compound is trimethylaluminum.

7. The method for preparing the modified graphite negative electrode material according to claim 4, It is characterized in that In the step S1, the sintering temperature is 800-1000°C.

8. The method for preparing the modified graphite negative electrode material according to claim 4, It is characterized in that The gradient temperature program is: 180-220° C. for reaction for 2-4 h, 220-240° C. for reaction for 2-4 h, and 280° C. for reaction for 4-6 h.

9. Use of the modified graphite negative electrode material according to any one of claims 1 to 3 in a lithium battery.

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