CN114824225A

CN114824225A - Composite negative electrode material doped with nano graphite and preparation method thereof

Info

Publication number: CN114824225A
Application number: CN202210519968.XA
Authority: CN
Inventors: 郑正; 成洪宽; 王之锴; 黄素珍; 张威振
Original assignee: Fuhuan Qingyun Technology Zhejiang Co ltd; Fudan University
Current assignee: Fuhuan Qingyun Technology Zhejiang Co ltd; Fudan University
Priority date: 2022-05-12
Filing date: 2022-05-12
Publication date: 2022-07-29
Also published as: LU502408B1

Abstract

The invention discloses a composite negative electrode material doped with nano graphite and a preparation method thereof, belonging to the technical field of battery material preparation. The preparation method comprises the following steps: (1) doping nano graphite into spartina alterniflora powder, performing primary pyrolysis, cooling and performing secondary pyrolysis to obtain a mixed carbon material; (2) and sequentially carrying out alkali treatment and acid treatment on the mixed carbon material to obtain the nano graphite doped composite negative electrode material. The preparation method is simple and convenient, the nanometer graphite-doped spartina alterniflora source hard carbon material (composite negative electrode material) is prepared by the preparation method, the hard carbon material has good first coulombic efficiency, and a battery prepared by taking the hard carbon material as a negative electrode has excellent sodium storage performance.

Description

Composite negative electrode material doped with nano graphite and preparation method thereof

Technical Field

The invention relates to the technical field of battery material preparation, in particular to a composite cathode material doped with nano graphite and a preparation method thereof.

Background

Compared with the traditional graphite material, the nano graphite material not only can complete the mechanism of sodium storage between graphite layers, but also can reversibly intercalate and deintercalate sodium ions in defects, holes, active sites and the like at the edge of the graphite, so that the nano graphite material has more sodium storage sites than the traditional graphite. Because the nano graphite has larger specific surface area, the negative electrode material doped with the nano graphite can be better infiltrated by electrolyte generally, the contact area is larger, the transfer resistance of sodium ions is lower in the charging and discharging process, and the nano graphite can play a more outstanding role in the aspects of battery multiplying power and large-current long-cycle performance. However, the nano graphite material has a certain disadvantage that the nano graphite inevitably contacts with the electrolyte in a larger area in the first circulation process due to a larger specific surface area, the area of the generated SEI membrane is larger, and the first irreversible capacity is higher than that of the SEI membrane. On the other hand, the nano graphite material has a stacking and agglomeration phenomenon, and the aggregation of a large amount of nano graphite can cause the increase of the local graphite degree, the specific surface area and the number of active sites are lower, so that the sodium storage performance of the material is influenced.

Disclosure of Invention

The invention aims to provide a composite anode material doped with nano graphite and a preparation method thereof, and aims to solve the problems in the prior art.

In order to achieve the purpose, the invention provides the following scheme:

one of the technical schemes of the invention is as follows: a composite cathode material doped with nano graphite and a preparation method thereof comprise the following steps:

(1) doping nano graphite into spartina alterniflora powder, carrying out primary pyrolysis, cooling and secondary pyrolysis to obtain a mixed carbon material;

(2) and sequentially carrying out alkali treatment and acid treatment on the mixed carbon material to obtain the nano graphite doped composite negative electrode material (G-SALC).

Further, the preparation of the spartina alterniflora powder specifically comprises: chopping leaves and stalks of Spartina Alterniflora (SALC), drying for 4-8 h at 140-160 ℃, and crushing to obtain spartina alterniflora powder.

Further, the doping amount of the nano graphite (G) in the spartina alterniflora powder is 0-10 wt%, and the doping amount value is not 0%.

Further, the primary pyrolysis conditions are: in an argon atmosphere, the temperature is 550-650 ℃, and the time is 18-22 min; the temperature of the secondary pyrolysis is 1100-1300 ℃, and the time is 2.5-3.5 h.

The primary pyrolysis serves to remove tar and impurities.

Further, the alkali treatment specifically includes: adding the mixed carbon material into 8-12% by mass of potassium hydroxide solution, and heating at 55-65 ℃ for 50-70 min.

Further, the acid treatment specifically comprises: adding the mixed carbon material into hydrochloric acid with the concentration of 2.8-3.2 mol/L, and heating for 50-70 min at the temperature of 55-65 ℃.

The second technical scheme of the invention is as follows: a composite cathode material doped with nano-graphite prepared by the preparation method.

The third technical scheme of the invention is as follows: an application of the composite cathode material doped with the nano graphite in electrode preparation.

The invention discloses the following technical effects:

the preparation method is simple and convenient, and the nanometer graphite doped spartina alterniflora source hard carbon material (composite negative electrode material) is prepared by the preparation method and has more pores. The electrochemical performance measurement shows that the concentration of the active carbon is 20mA g ^-1 Under the condition of the current, the first charge-discharge capacity of the G-SALC2 can reach 200mAh G ^-1 The first coulombic efficiency was 67%. While in the long cycle, 200mA g ^-1 Under the current of (2)The circle can still maintain 108mAh g ^-1 The capacity retention of (2) was 88%. In the rate test, 20mA g ^-1 、50mA g ^-1 、100mA g ^-1 、200mA g ^-1 And 500mA g ^-1 Under the condition of changing current, the multiplying power capacity of G-SALC2 (the composite negative electrode material doped with the nano graphite) is 297, 199, 129, 78 and 67mAh G respectively ^-1 . The capacity of the G-SALC is measured by using variable-speed cyclic voltammetry, the capacity of the G-SALC is jointly controlled by diffusion capacity and surface capacity, and the ratio of pseudocapacitance to the battery capacity is higher and higher with the increase of sweeping speed, which shows that the G-SALC has excellent electrochemical performance and battery capacity under the conditions of high current and high multiplying power.

The Spartina Alterniflora (SALC) is a worldwide invasive species, and the raw material is used for preparing the negative electrode material, so that the convenient, low-cost and large-scale industrial negative electrode material can be provided, and meanwhile, a new solution and thought can be brought for eliminating and solving the invasive species, and the purpose of killing two birds with one stone is achieved.

The invention improves the capacity, long cycle performance and rate capability of the SALC by doping different amounts of nano graphite. The specific capacity of the nano-graphite modified SALC reaches 297mAh g through measurement ^-1 And 200mA g in a long cycle ^-1 Still has 108mAh g under the condition of large current ^-1 The capacity of (2) and the capacity retention rate of 88 percent indicate that the G-SALC is a high-performance anode material.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings without creative efforts.

FIG. 1 is a schematic diagram of a synthesis process for preparing a composite anode material doped with nano-graphite according to an embodiment of the present invention;

FIG. 2 is a scanning electron micrograph of the negative electrode materials prepared in examples 1 to 4 of the present invention, wherein (a) is example 4, (b) is example 1, (c) is example 2, and (d) is example 3;

FIG. 3 is a high-magnification scanning electron microscope image of the negative electrode material prepared in examples 1-4 of the present invention, wherein (a) is example 4, (b) is example 1, (c) is example 2, and (d) is example 3;

FIG. 4 is a transmission electron micrograph of the negative electrode materials prepared in examples 1 to 4 of the present invention, wherein (a) is example 4, (b) is example 1, (c) is example 2, and (d) is example 3;

FIG. 5 is a high-power transmission electron microscope image of the negative electrode materials prepared in examples 1-4 of the present invention, wherein (a) is example 4, (b) is example 1, (c) is example 2, and (d) is example 3;

fig. 6 is a performance characterization diagram of the negative electrode materials prepared in examples 1 to 4, wherein (a) is XRD, (b) is raman spectrum, (c) is nitrogen adsorption-desorption curve isotherm, and (d) is pore size distribution;

FIG. 7 shows the carbon composition of the negative electrode materials prepared in examples 1 to 4 of the present invention, wherein (a) is example 4, (b) is example 1, (c) is example 2, and (d) is example 3;

FIG. 8 is an electrochemical representation of the negative electrode of a sodium cell, wherein (a) is 20mA g ^-1 The first charge-discharge curve is shown in the specification, (b) the slope and platform capacity of the first charge-discharge is shown in the specification, and (c) the first charge-discharge capacity is 50mA g ^-1 Short cycle performance of (d) rate cycle performance of (e) 200mA g ^-1 Long cycle performance under conditions;

FIG. 9 is a cyclic voltammogram of the negative electrode materials prepared in examples 1-4 of the present invention, wherein (a) is example 4, (b) is example 1, (c) is example 2, and (d) is example 3;

FIG. 10 is a graph showing the analysis of the mechanism of sodium storage behavior of the anode material (G-SALC2) prepared in example 1 of the present invention, wherein (a) is the cyclic voltammogram at different scan rates, (b) is the relationship between peak current and log scan rate, and (c) is 1mV s ^-1 The pseudo-capacitance contribution of (d) is the ratio of the capacitance contribution and the diffusion contribution at different scan speeds;

FIG. 11 is an EIS spectrum and an equivalent circuit diagram of the negative electrode material prepared in examples 1 to 4 of the present invention after the first cycle.

Detailed Description

Reference will now be made in detail to various exemplary embodiments of the invention, the detailed description should not be construed as limiting the invention but as a more detailed description of certain aspects, features and embodiments of the invention.

It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. Further, for numerical ranges in this disclosure, it is understood that each intervening value, between the upper and lower limit of that range, is also specifically disclosed. Every smaller range between any stated value or intervening value in a stated range and any other stated or intervening value in that stated range is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included or excluded in the range.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and materials in connection with which they pertain. In case of conflict with any incorporated document, the present specification will control.

It will be apparent to those skilled in the art that various modifications and variations can be made in the specific embodiments of the present disclosure without departing from the scope or spirit of the disclosure. Other embodiments will be apparent to those skilled in the art from consideration of the specification. The specification and examples are exemplary only.

As used herein, the terms "comprising," "including," "having," "containing," and the like are open-ended terms that mean including, but not limited to.

Example 1

A preparation method of a composite negative electrode material doped with nano graphite comprises the following steps:

(1) cleaning leaves and stems of spartina alterniflora, cutting into strips with the length of 0.5-1.0 cm, then putting the strips into an oven, drying for 4 hours at 150 ℃, and crushing to obtain spartina alterniflora powder with the particle size of 100 meshes.

(2) Mixing nano graphite with spartina alterniflora powder to ensure that the doping amount of the nano graphite is 2 wt%, pyrolyzing the nano graphite for 20min at 600 ℃ in an argon protective atmosphere (to remove tar and impurities in a sample), cooling the nano graphite to room temperature, putting the nano graphite into a tubular furnace for pyrolysis again, wherein the pyrolysis temperature is 1200 ℃, the time is 3 hours, and cooling the nano graphite to obtain the mixed carbon material.

(3) Grinding the mixed carbon material and sieving the ground mixed carbon material with a 100-mesh sieve to obtain mixed carbon material powder; adding the mixed carbon material powder into a potassium hydroxide solution with the mass fraction of 10%, stirring for 15min, covering with a preservative film, putting into an oven, heating at 60 ℃ for 1h, filtering, adding into hydrochloric acid with the concentration of 3mol/L, stirring for 15min, covering with the preservative film, putting into the oven, heating at 60 ℃ for 1h, filtering, adding into ethanol, stirring for 15min, covering with the preservative film, putting into the oven, heating at 40 ℃ for 1h, filtering, washing with deionized water until the solution is neutral, and drying to obtain the nano-graphite doped composite negative electrode material (G-SALC 2).

Example 2

The difference from example 1 is that the amount of nanographite doped in step (2) is 5 wt%.

And obtaining the composite cathode material (G-SALC5) doped with the nano graphite.

Example 3

The difference from example 1 is that the amount of nanographite doped in step (2) is 10 wt%.

And obtaining the nano graphite doped composite anode material (G-SALC 10).

Example 4

(1) Cleaning leaves and stems of spartina alterniflora, cutting into strips with the length of 0.5-1.0 cm, then putting into an oven, drying for 4h at 150 ℃, crushing and sieving to obtain spartina alterniflora powder with the particle size of 100 meshes.

(2) Pyrolyzing spartina alterniflora powder at 600 ℃ for 20min under the protection of argon, cooling to room temperature, putting the cooled spartina alterniflora powder into a tubular furnace for pyrolysis again, wherein the pyrolysis temperature is 1200 ℃, and the time is 3 hours, and cooling to obtain the mixed carbon material.

(3) Grinding and sieving the mixed carbon material to obtain mixed carbon material powder; adding the mixed carbon material powder into a potassium hydroxide solution with the mass fraction of 10%, stirring for 15min, covering with a preservative film, putting into an oven, heating at 60 ℃ for 1h, filtering, adding into hydrochloric acid with the concentration of 3mol/L, stirring for 15min, covering with the preservative film, putting into the oven, heating at 60 ℃ for 1h, filtering, adding into ethanol, stirring for 15min, covering with the preservative film, putting into the oven, heating at 40 ℃ for 1h, filtering, washing with deionized water until the solution is neutral, and drying to obtain the cathode material (G-SALC 0).

Example 5

The difference from example 1 is that, in step (1), the leaves of Spartina alterniflora are used, the tissue structure in the leaves is more complicated, the high-hardness carbon material is easy to form, and the slope capacity in the sodium storage capacity is more.

Example 6

The difference from example 1 is that the stalk of Spartina alterniflora used in step (1) has a larger specific surface area and thus has more additional sodium storage sites and thus has more total sodium storage capacity.

Example 7

As in example 1, the mixing of the nano-graphite and the Spartina alterniflora powder must be completed before pyrolysis in the operation step, and high temperature is a necessary condition for the combination of the two.

Example 8

(1) Cleaning leaves and stems of spartina alterniflora, cutting into strips with the length of 0.5-1.0 cm, then putting the strips into an oven, drying for 8 hours at the temperature of 140 ℃, and crushing to obtain spartina alterniflora powder with the particle size of 100 meshes.

(2) Mixing nano graphite with spartina alterniflora powder to ensure that the doping amount of the nano graphite is 2 wt%, pyrolyzing at 550 ℃ for 22min (to remove tar and impurities in a sample) in an argon protective atmosphere, cooling to room temperature, putting into a tubular furnace for pyrolysis again, wherein the pyrolysis temperature is 1100 ℃, and the time is 3.5h, and cooling to obtain the mixed carbon material.

(3) Grinding the mixed carbon material and sieving the ground mixed carbon material with a 100-mesh sieve to obtain mixed carbon material powder; adding the mixed carbon material powder into a potassium hydroxide solution with the mass fraction of 8%, stirring for 15min, covering a preservative film, putting the mixture into an oven, heating for 70min at 55 ℃, filtering, adding the mixture into hydrochloric acid with the concentration of 2.8mol/L, stirring for 15min, covering the preservative film, putting the mixture into the oven, heating for 70min at 55 ℃, filtering, adding the mixture into ethanol, stirring for 15min, covering the mixture, putting the mixture into the oven, heating for 1h at 40 ℃, filtering, washing with deionized water until the solution is neutral, and drying to obtain the nano-graphite-doped composite negative electrode material (the performance is equivalent to that of example 1).

Example 9

(1) Cleaning leaves and stems of spartina alterniflora, cutting into strips with the length of 0.5-1.0 cm, then putting into an oven, drying for 4 hours at 160 ℃, and crushing to the particle size of 100 meshes to obtain spartina alterniflora powder.

(2) Mixing nano graphite and spartina alterniflora powder to ensure that the doping amount of the nano graphite is 2 wt%, pyrolyzing the nano graphite for 18min at 650 ℃ in an argon protective atmosphere (to remove tar and impurities in a sample), cooling the nano graphite to room temperature, putting the nano graphite into a tubular furnace for pyrolysis again, wherein the pyrolysis temperature is 1300 ℃, and the time is 2.5h, and cooling the nano graphite to obtain the mixed carbon material.

(3) Grinding the mixed carbon material and sieving the ground mixed carbon material with a 100-mesh sieve to obtain mixed carbon material powder; adding the mixed carbon material powder into a potassium hydroxide solution with the mass fraction of 12%, stirring for 15min, covering with a preservative film, placing in an oven, heating at 65 ℃ for 50min, filtering, adding into hydrochloric acid with the concentration of 3.2mol/L, stirring for 15min, covering with the preservative film, placing in the oven, heating at 65 ℃ for 50min, filtering, adding into ethanol, stirring for 15min, covering with the preservative film, placing in the oven, heating at 40 ℃ for 1h, filtering, washing with deionized water until the solution is neutral, and drying to obtain the nano-graphite-doped composite anode material (the performance is equivalent to that of example 1).

Comparative example 1

The difference from example 1 is that the mass ratio of nano graphite to spartina alterniflora powder in step (2) is 1: 100.

Comparative example 2

The difference from example 1 is that the mass ratio of nano graphite to spartina alterniflora powder in step (2) is 15: 100.

Comparative example 3

The difference from example 1 is that the mass ratio of the nano graphite to the spartina alterniflora powder in step (2) in step (1) is 30:100

Effect example 1

The anode materials prepared in examples 1-4 were characterized, and the results are shown in FIGS. 2-6 and Table 1.

The characterization method is as follows:

(1) x-ray diffraction (XRD)

XRD is a method for researching the crystal structure and phase structure of a substance by utilizing the phenomenon that X-rays generate diffraction in the process of penetrating through an object. Generally, when the crystallization condition in a subject is good, the diffraction peak is strong and sharp. However, when the crystallization condition in the object is poor, the diffraction peak shows a broad peak. The technology is widely applied to researches in a series of fields such as analysis of a crystal structure of a sample, stress analysis, thin film preparation and the like. The instrument used in the present invention is Bruker Advance D8 with parameters: the radiation source is CuKa, the diffraction wavelength lambda of the Ka is 0.15406nm, the tube voltage is 40kV, and the tube current is 35 mA. In practical applications, the bragg formula is used to calculate the lattice spacing, which is given by:

2dsinθ＝nλ

where d is the layer spacing, θ is the angle between the incident X-ray and the crystal plane, n is the order of reflection, and λ is the wavelength of the incident light (λ 0.15406 nm).

(2) Raman spectroscopy

Raman spectroscopy is a Raman scattering spectroscopy analysis technique named Raman by scientists in india, and molecules show scattering spectra with different wavelengths under the irradiation of incident light, so that information about the vibration and rotation of the molecules can be obtained, and based on the information, related information about the structure and properties of the molecules can be obtained.

The Raman spectrometer used in the invention is HORIBA XPlora Plus, Japan, the excitation wavelength is 532nm, and the scanning range is 600-2000cm ^-1 In the meantime.

(3) Specific surface area and pore size distribution

The specific surface area and pore size distribution (BET) are mainly determined by the adsorption of the solid surface itself to gas, and the specific surface area, pore size distribution and pore size volume of the material are determined by using the apparatus of the invention, model ASAP2020, degassing at 200 deg.C for 2h, and measuring at-195.850 deg.C.

(4) Scanning electron microscope

Scanning Electron Microscope (SEM) is a very effective method for studying the microscopic surface of a substance, and the principle is a microscopic technique that by focusing an electron beam on the surface of an object, the electron and the atom on the surface of the object have a certain effect, thereby generating various information carrying the appearance and structure of the object, and generating an image of the surface of the object by detecting and capturing the information. The image structure of the SEM can well reflect the microscopic morphology and the material distribution of the material, and is favorable for intuitively and effectively knowing the microscopic morphology of the material. The scanning electron microscope used in the present invention was Zeiss Sigma 300, manufactured by Japan, at a magnification of 10X to 1,000,000X.

(5) Transmission electron microscope

Transmission Electron Microscopy (TEM) is another important microscopic technique, and its principle mainly relies on transmission electron to penetrate through the sample to image, and the generated image is received and amplified by a detector to finally generate a transmission image, and in order to ensure the definition and accuracy of the image, the sample is generally required to be prepared into an ultra-thin slice of 100nm to ensure the transmission effect. Due to the fact that the electron wavelength used by the transmission electron microscope is short, the generated image has higher resolution and further achieves higher magnification, details on a structure similar to a graphite sheet layer or a fine structure are captured, and the disorder and the graphitization degree of the carbon material are deduced according to the information.

The transmission electron microscope used in the invention is jeol2100f, and the main application is to observe the information such as the microstructure and the atomic layer arrangement of the spartina alterniflora source hard carbon material. The acceleration voltage was 100 kV.

(6) X-ray photoelectron spectroscopy (XPS)

The principle is that valence electrons or inner layer electrons in atoms are excited in the process of radiating the surface of a sample by X rays, so that the energy of the photoelectrons is obtained, spectral energy spectrum data is collected by a detector, and relevant data such as content, proportion, valence and the like of elements in the sample can be obtained through software analysis based on the spectral energy spectrum data.

The X-ray photoelectron spectroscopy instrument used in the invention is Thermo Scientific K-Alpha, and an excitation source thereof is as follows: al K α ray (hv of 1486.6eV), vacuum of the analysis cell better than 5.0E-7mBar, operating voltage: 12kV, filament current: 6mA, step size 0.05 eV.

Fig. 2 is a scanning electron microscope image of the negative electrode material prepared in examples 1 to 4 of the present invention, wherein (a) is example 4, (b) is example 1, (c) is example 2, and (d) is example 3.

Fig. 3 is a high-magnification scanning electron microscope image of the negative electrode material prepared in examples 1 to 4 of the present invention, wherein (a) is example 4, (b) is example 1, (c) is example 2, and (d) is example 3.

As can be seen from fig. 2, the surface of the composite negative electrode material doped with nano-graphite has more similar wrinkles and very small particles with very small particle size compared to the surface of the Spartina Alterniflora (SALC) material prepared in example 4, and the substance attached to the surface of the main structure should be doped nano-graphite. As can be seen from fig. 2(a), the surface of the Spartina Alterniflora (SALC) material is relatively clean and regular, does not have any impurities attached, is an original hard carbon structure, and has a large number of clear micro pores; as can be seen in fig. 2(b), lumps and wrinkle-like structures begin to develop outside the original hard carbon structure, indicating that the nanographite has successfully attached to the surface of the SALC, forming a G-SALC material; as can be seen from fig. 2(c), a large amount of nano-graphite is attached to the surface of the carbon material, so that the performance of the material is effectively modified, and the attached graphite also has a significant agglomeration phenomenon; as can be seen from fig. 2(d), the amount of graphite attached to the surface of the SALC is maximized, a large amount of granular nano-graphite with uneven surfaces is attached to the surface of the SALC, and meanwhile, a large amount of nano-graphite is also distributed in the space outside the material, so that part of the nano-graphite is significantly agglomerated to form relatively large graphite blocks.

Fig. 4 is a transmission electron micrograph of the negative electrode material prepared in examples 1 to 4 of the present invention, wherein (a) is example 4, (b) is example 1, (c) is example 2, and (d) is example 3.

Fig. 5 is a high power transmission electron micrograph of the negative electrode material prepared in examples 1 to 4 of the present invention, wherein (a) is example 4, (b) is example 1, (c) is example 2, and (d) is example 3.

As can be seen from fig. 4, compared to the SEM picture, the Transmission Electron Microscope (TEM) can clearly see that the surface of the composite anode material doped with nano-graphite contains a large number of graphitized sheets, and most of the sheets are disordered and formed by disordered, twisted and amorphous carbon fragments. As can be seen from fig. 4(a), the disordered graphite in example 4(G-SALC0) still occupies the major portion, and fewer more regular graphitized sheets appear; as can be seen from fig. 4(b), the graphitized sheets are significantly increased in example 1(G-SALC2), a number of sheet-like sheets exist in the G-SALC2 in a spiral-like structure, and nanographite has been doped into the hard carbon structure. As can be seen from fig. 4(c), in example 2(G-SALC5), there are more distinct graphite sheets, and not only are the graphite sheets increased in number, but also the area occupied by the graphite sheets and the number of single-particle sheets are increased, and the nano-graphite is increased in G-SALC. As can be seen from fig. 4(d), the degree of graphitization was highest in example 3(G-SALC10), a large number of graphite sheets could be found not only easily, but also it could be found that graphitized sheets occupied most of the surface area and graphitized sheets occupied most of the surface area, which is consistent with the phenomenon that nano-graphite highly adheres to the surface of hard carbon material in SEM images, and the interlayer spacing of G-SALC was 0.341nm, sufficiently indicating that nano-graphite has been doped into the SALC material.

XRD properties of the anode materials prepared in examples 1-4 are shown in Table 1, and XRD characterization patterns are shown in 6.

Fig. 6 is a performance characterization diagram of the negative electrode materials prepared in examples 1 to 4, wherein (a) is XRD, (b) is raman spectrum, (c) is nitrogen adsorption-desorption curve isotherm, and (d) is pore size distribution.

TABLE 1 XRD properties of G-SALC

Samples	d ₀₀₂ (nm)	L _c (nm)	L _a (nm)	N _i
					G-SALC0	0.358	1.13	2.48	3.15
G-SALC2	0.356	1.17	2.54	3.28
					G-SALC5	0.353	1.34	2.83	3.80
G-SALC10	0.347	1.54	3.41	4.45

As can be seen in fig. 6(a), broad peaks and sharp peaks appear at the 24 and 26 degree positions, corresponding to amorphous carbon and attached nanographite, respectively, in typical hard carbon materials. Broad peak at 43 degree position and sp in G-SALC ² The hybrid carbon is involved. Meanwhile, with the increasing amount of doped nano graphite, the original (002) peak of the hard carbon material is reduced, the graphite characteristic peak generated at 26.5 degrees is stronger, the strength of the graphite characteristic peak is in direct proportion to the content of the nano graphite, and the content increase and disorder reduction of the nano graphite in the sample are proved. The interlayer spacing of the four groups of samples was 0.358nm (G-SALC0), 0.356nm (G-SALC2), 0.354nm (G-SALC5), and 0.347nm (G-SALC10), respectively, as calculated by the Bragg formula. Decreased interlayer spacing means an increase in ordered graphite sheet layers. Meanwhile, as shown in table 1, the average layer thickness (Lc) and the planar width (La) of the negative electrode material were also increased as the doped nano graphite material was increased.

As can be seen from FIG. 6(b), the negative electrode material prepared by the invention has two strong peaks, which are respectively located at 1345cm ^-1 D peak of (2) and a peak position of 1590cm ^-1 G peak of (2). Wherein the occurrence of the D peak and sp ³ Defects of amorphous carbon caused by hybridization, and G peak and sp ² The relative proportions of the hybridized graphitic carbon, and thus the D and G peaks, expressed as Id/Ig, are parameters that help to study the degree of amorphous carbon and the degree of misordering in the sample, and are 1.05, 0.83, 0.74 and 0.47, respectively, demonstrating the reduced degree of disorder and the increased degree of graphitization of G-SALC. Meanwhile, with the increase of doped nano graphite, the G peak gradually gets close from a wide and slow peak to a thin and sharp peak, which shows the increase of the content of graphite microcrystals.

As can be seen from FIG. 6(c), the specific surface areas of G-SALC0, G-SALC2, G-SALC5 and G-SALC10 were 51.4, 22.23, 25.27 and 38.23m, respectively ² g ^-1 The nitrogen adsorption-desorption amount is obviously increased (related to the spartina alterniflora hard carbon structure) in the range of the relative pressure between 0.8 and 1, and a closed annular curve exists in the range of the relative pressure between 0.5 and 0.9, wherein the curve is □ type lag loop, □ type lag loopThe appearance of the line means that a large number of minute pores consisting of sheets and carbon blocks are present in the G-SALC sample, and the presence of the large number thereof leads to a hysteresis curve.

As can be seen from fig. 6(d), the pores of about 2nm of the added nano-graphite negative electrode material are mostly filled with nano-graphite, and the pore content decreases with the increase of the nano-graphite content, which indicates that the nano-graphite largely replaces part of the micro-pore positions.

Effect example 2

Measuring the carbon element composition of the negative electrode material prepared in the examples 1 to 4;

the determination method comprises the following steps: the results of the measurement were obtained by X-ray photoelectron spectroscopy (XPS) and are shown in FIG. 7.

the solid lines in fig. 7 represent actual test data and the dashed lines represent data fit values.

As can be seen from FIG. 7, the peak C1s is decoupled, the peak C1s is located at about 285eV, and the peak C1s can be decomposed into four peaks, each sp, by fitting the peak C1s ² ，sp ³ The four main peaks of C ═ O, O ═ C — O, located at 284.6, 285.6, 286.7eV respectively, where sp is ² Peaks are generally produced from graphitized carbon, and sp ³ Peaks are generated by amorphous carbon, and the graphitization degree of the sample can be verified through researching the relative proportion of the peaks and the amorphous carbon. Sp is to be ³ Has an area of I _sp3 And sp ² Has an area of I _sp2 Ratio I of G-SALC0, G-SALC2, G-SALC5 and G-SALC10 _sp3 /I _sp2 0.140, 0.137, 0.127 and 0.084, respectively. With increasing doped graphite content, I _sp3 /I _sp2 The ratio of (A) to (B) is gradually reduced, which shows that the content of amorphous carbon in the G-SALC sample is reduced, while the content of graphitized carbon is gradually increased, which shows that the nano graphite is successfully doped into the SALC sample, and the doping amount is continuously increased in order.

Effect example 3

Preparation of electrodes and assembly of batteries

(1) Preparation of the electrodes

Like the negative electrode of a lithium battery, the preparation of the electrode of a sodium battery is a critical step in the whole battery test, and the performance of the battery is directly influenced by the quality of the electrode. The negative electrode material, the binder and the conductive carbon black prepared in the embodiments 1-4 are added into an agate grinding body according to the mass ratio of 8:1:1, grinding and mixing are carried out for 15min, and then deionized water is added to continue grinding for 25min until uniform viscous liquid is obtained.

Coating the viscosity-adjusting liquid on a copper foil by a coating machine with a certain thickness, wherein the loading range of the active substance in the electrode is 1.2-1.8 mg cm ^-3 ,. And (3) putting the coated copper foil into a vacuum drying oven, and drying at the temperature of 100 ℃ for more than 1 h. Then, the pole piece is pressed by a tablet press, and is dried for 12 hours in a vacuum drying oven at the temperature of 100 ℃ to obtain the negative electrode material (active substance) on the carbon negative electrode piece, wherein the load range of the negative electrode material (active substance) is 1.2-1.8 mg cm ^-3 And weighing and recording the carbon cathode plate.

(2) Assembly of a battery

The cell assembly was carried out entirely in a glove box filled with argon, with oxygen and hydrogen contents below 1 ppm. The sodium ion battery used in the invention is a 2032 type button cell assembly, wherein the counter electrode is metallic sodium, and the used diaphragm is a fibrous thin film Whatman GF/D. The electrolyte solution contains 1mol L ^-1 NaClO (NaClO) ₄ The volume ratio of (1) to (1) of the EC and PC mixed liquid, and 5% of fluoroethylene carbonate added thereto. The button cell is well installed according to the sequence of the negative electrode shell, the carbon negative electrode plate, the glass fiber diaphragm, the positive electrode plate, the gasket, the spring and the positive electrode shell, the installed cell is placed into an encapsulation machine for encapsulation, and standing is carried out for 12 hours at room temperature and then testing is waited.

(3) Constant current charge and discharge test

The constant current charge and discharge test is the most common means for detecting the performance of the battery, and the battery tester used in the invention is the model of Shenzhen New Wilson CT-4000 and 5V 10 mA. The voltage range of the battery is 0.01/3.0V, and the test environment is room temperature.

In the cycle test, the current density used in the present invention is 50mA g each ^-1 、200mA g ^-1 The current density of (1). The current density used in the present invention in the rate test was 20mA g ^-1 、50mA g ^-1 、100mA g ^-1 、200mA g ^-1 、500mA g ^-1 The current density of (1).

(4) Cyclic voltammetry/AC impedance test

Cyclic voltammetry is a commonly used effective method for researching an electrochemical system, and mainly depends on that different electrochemical reactions occur in alternation between electrodes at different sweep rates and voltage variation ranges, and the properties of the tested electrochemical system are analyzed through an obtained current-voltage diagram. Generally, in the process from high potential to low potential, Na corresponds to ⁺ The reduction peak of the negative electrode inserted is formed, and conversely, in the process from low potential to high potential, Na corresponds to ⁺ Formation of the liberated oxidation peak. A series of information about diffusion coefficient, capacity and the like of the electrode material can be obtained by observing the peak height, peak area, half-peak width and the like of the cyclic voltammogram.

The alternating current impedance test is a commonly used electrochemical test means at present, a sinusoidal alternating current voltage with a certain frequency is added into an electrochemical system, the electrochemical system feeds back an alternating current signal, and simulation calculation can be performed according to the information of the current and the voltage, so that the relationship between the impedance and the frequency of the electrochemical system can be obtained.

The instrument for testing the alternating current impedance is CHI 760E, the frequency range is 0.01-100000 Hz, and the amplitude is 10 mV.

FIG. 8 is an electrochemical representation of the negative electrode of a sodium cell, wherein (a) is 20mA g ^-1 The first charge-discharge curve is shown in the specification, (b) the slope and platform capacity of the first charge-discharge is shown in the specification, and (c) the first charge-discharge capacity is 50mA g ^-1 Short cycle performance of (d) rate cycle performance of (e) 200mA g ^-1 Long cycle performance under conditions.

As can be seen from FIG. 8(a), at 20mA g ^-1 Under the current density of (3), and under the voltage condition of 0.01-3V, carrying out constant-current charging and discharging. Amplification of G-SALC0, G-SALC2, G-SALC5 and G-SALC10The capacitances are 354, 439, 377 and 371mAh g ^-1 And a charge capacity of 208, 297, 261, 238mAh g ^-1 Coulombic efficiencies were 58%, 67%, 69% and 64%. As with the CV first cycle, part of the irreversible capacity comes from the consumption of the SEI film formation process, and the reasons for the first low coulombic efficiency also include the process of irreversible reaction by trapping part of sodium ions by defects.

According to adsorption-intercalation theory, the capacity at the high voltage plateau is mainly due to the capacity contributed by sites such as carbon layer edge defects and pores, while the capacity contributed in the low voltage plateau region is mostly due to intercalation completed by graphite crystallites. As can be seen from fig. 8(b), as the amount of doped graphite increases, more capacity contribution comes from the plateau region. This result is consistent not only with the theoretical explanation, but also fully demonstrates that the doped nanographite has fully combined with the SALC hard carbon material, demonstrating the presence of G-SALC.

As can be seen from FIG. 8(c), 50mA g ^-1 In the short circulation of the current density of (1), the initial specific capacities of G-SALC0, G-SALC2, G-SALC5 and G-SALC10 are 132, 198, 177 and 166mAh G respectively ^-1 The capacity retention after the end of the cycle was 94%, 95%, 105%, and 98%. Although the specific capacity of the G-SALC0 is slightly fluctuated in the circulation process, the good circulation capacity and capacity retention rate are still maintained, and the specific capacity of the G-SALC0 is 132mAh G after 100 cycles of circulation ^-1 The capacity retention rate is 95%, and the capacity of the G-SALC2 after 100 cycles is 188mAh G ^-1 The capacity retention rate was 95%. The specific capacity of the G-SALC5 circulating for 100 circles is 177mAh G ^-1 And the specific capacity of the G-SALC10 for 100 cycles is 163mAh G ^-1 Compared with the negative electrode material in the prior art, the G-SALC has good capacity retention capacity and cycling stability, and is a disordered structure and graphite microcrystals formed by adopting a porous and tough spartina alterniflora source as a raw material and performing pyrolysis at a proper temperature when a primary skeleton of the G-SALC is built.

As can be seen from FIG. 8(d), the capacity of G-SALC2 was 20mA G in the rate test ^-1 、50mA g ^-1 、100mA g ^-1 And 200mA g ^-1 And 500mA g ^-1 Respectively, the current capacities of (1) are 297, 199, 129, 78 and 67mAh g ^-1 When the current returns to 20mA g ^-1 The capacity can be restored to 273mA g ^-1 . The capacity of the G-SALC slowly rises along with the increase of the cycle number, the capacity even exceeds the initial current, the capacity retention rate reaches 103 percent, the battery performance is activated by multiple cycles, and the capacity performance is improved. And, when the amount of nano-graphite is smaller, the capacity of the material is improved, mainly because the doped nano-graphite provides additional intercalation capacity for the G-SALC. However, as the degree of graphitization increases, the growth of pseudo graphite results in a decrease in interlayer spacing and sodium ion diffusion channels, resulting in a more difficult process for the insertion of sodium ions in G-SALC. At the same time, a further increase in the number of graphite layers and surface area also means that intercalation of sodium ions in the pseudo-graphite structure is more difficult.

As can be seen from FIG. 8(e), the initial capacity of G-SALC2 was 123mAh G ^-1 After 1000 cycles, the final capacity is 108mAh g ^-1 The capacity retention rate was 88%. In this process, the capacity of the G-SALC fluctuates, rises, or falls throughout the charge and discharge process, but remains relatively stable overall. Therefore, the G-SALC still maintains good capacity retention capacity under high current, and the G-SALC has quite excellent material stability and shows excellent structural stability.

FIG. 9 shows cyclic voltammograms of negative electrode materials prepared in examples 1 to 4 of the present invention, wherein (a) shows example 4, (b) shows example 1, (c) shows example 2, and (d) shows example 3.

As can be seen in FIG. 9, the G-SALC material has a scan rate of 0.1mV s ^-1 The scanning results of the first 3 circles under the condition that the voltage range is 0.01-3V are as above. The strong reduction peak appearing at the position close to 0V corresponds to the process of sodium ion intercalation, and the phenomenon that the G-SALC has a more obvious reduction peak generated between 0 and 1V in the first scanning process can be seen to disappear in the subsequent cyclic voltammetry result, which proves that the generation result of the strong reduction peak is greatly related to the first irreversible capacity. The irreversible capacity comes from two aspects, namely, the process that the electrolyte reacts on the surface of the electrode to generate an SEI filmOn the other hand, it is also related to the result of irreversible reaction of sodium ions with the electrode surface and defects. The smaller irreversible reduction peak and also the G-SALC demonstrate less side reactions at the phase interface during SEI film formation, indicating that G-SALC possesses higher first coulombic efficiency and first efficiency performance, compared to other documents. The high coincidence of subsequent CV curves also demonstrates the better cycling stability of the material.

FIG. 10 is a graph showing the analysis of the sodium storage behavior mechanism of the negative electrode material (G-SALC2) prepared in example 1 of the present invention, (a) is a cyclic voltammogram at different scan rates, (b) is a logarithmic curve of peak current versus scan rate, and (c) is 1mV s ^-1 The pseudocapacitance contribution of (d) is the ratio of the capacitance contribution and the diffusion contribution at different scan rates.

As can be seen from FIG. 10(a), with the increase of the sweep rate, G-SALC2 shows a broad reduction peak around 0.2-0.3V, which should be related to the mechanism of sodium ion intercalation under high current conditions. To analyze the sodium storage properties of G-SALC, the relationship between peak current i and scan current v is used to determine i ═ av ^b The capacity mechanism is analyzed by the equation, where i is the peak current, v represents the scan rate, and a is the correlation constant. If b is 0.5, the battery is controlled by the diffusion behavior, if the value of b is between 0.5 and 1, the sodium battery is controlled by the diffusion behavior and the capacitance behavior together, and if b is equal to 1, the battery is controlled by the capacitance behavior. When the peak current calculation of the oxidation state and the reduction state is carried out by drawing the relation of curves of log (i) and log (v), respectively (fig. 10(b)), the b value of the G-SALC2 is 0.65 and 0.75 respectively, and compared with the prior SALC material, the b value of the G-SALC material is obviously improved, which proves that the battery capacity is improved after the surface control capacity is increased after the nano graphite is doped, and the battery capacity is further improved.

According to adsorption-intercalation theory, the total capacitive charge contribution at a fixed rate can be quantified by decoupling the specific contributions of the diffusion control charge and the capacitive charge anywhere and adding up at the end. The formula is i (V) ═ k ₁ v+k ₂ v ^0.5 Wherein i (V) represents a current at a certain voltage, k ₁ v is the surface control capacitance, k ₂ v ^0.5 Is the capacity for diffusion control. We benefit fromThis was used to quantitatively analyze the contribution of the G-SALC2 pseudocapacitance to the cell. The dark portion in FIG. 10(c) shows G-SALC2 at a scan rate of 1mV s ^-1 The pseudocapacitance is found to be 0.1, 0.2, 0.5, 1, 1.5 and 2mV s by calculation ^-1 Under the condition, the capacity contribution reaches 30%, 33%, 41%, 59% and 66%. With the increase of the scanning speed, the sodium storage process of charging and discharging in the G-SALC increasingly depends on the surface contribution capacitance, which shows that the G-SALC is very important in multiplying power performance and large-current long cycle.

Electrochemical impedance spectroscopy was also used for performance testing of G-SALC, where the amplitude of the sine wave was 10mv amplitude and the frequency ranged from 100000Hz to 0.01 Hz. As can be seen from fig. 11, the spectrum of the G-SALC is composed of two parts, a semicircle of the high frequency region and a diagonal line of the low frequency region. The semi-circle of the high frequency region represents the charge transfer resistance, and the diagonal line of the low frequency region represents the ion diffusion resistance inside the negative electrode material. R of G-SALC0, G-SALC2, G-SALC5 and G-SALC10 after fitting _ct 536, 452, 476 and 638 Ω, respectively. Compared with other G-SALC series samples, G-SALC2 has relatively lower electron transfer resistance.

By the formula

The ion diffusion coefficient of sodium ions in the cathode material G-SALC can be calculated, wherein R is a gas constant, T is an absolute temperature, N is the number of electrons transferred in the reaction, A is an electrode active area, F is a Faraday constant, C is a sodium ion phase density, and A is _W At Z' and omega ^-1/2 Slope after plotting. The calculated diffusion coefficients for G-SALC0, G-SALC2, G-SALC5 and G-SALC10 were 2.18X 10 ^–13 m ² s ^-1 、2.36×10 ^-13 m ² s ^-1 、3.85×10 ^-13 、1.05×10 ^-13 m ² s ^-1 . Wherein G-D of SALC2 _Na The height of the upper part is higher than that of the lower part,and the charge transfer resistance is low, which proves that the capacity and ion transmission performance of the properly doped nano graphite G-SALC are improved greatly.

The preparation method is simple and convenient, and the prepared spartina alterniflora source hard carbon material (composite negative electrode material) doped with nano graphite has excellent performance. The structural morphology and the morphological characteristics of the G-SALC are characterized by means of BET, XRD, SEM, TEM, Raman and the like. The prepared G-SALC successfully completes the process of doping the nano graphite into the SALC carbon material, and forms a hard carbon material structure with more pores and doped nano graphite. Then the measurement of the electrochemical performance of the G-SALC is completed by using the methods of charging and discharging, cyclic voltammetry and EIS, and the measurement is carried out at 20mA G ^-1 Under the condition of the current, the first charge-discharge capacity of the G-SALC2 can reach 200mAh G ^-1 The first coulombic efficiency was 67%. While in the long cycle, 200mA g ^-1 Can still maintain 108mAh g in one thousand cycles under the condition of the current ^-1 The capacity retention of (2) was 88%. In the rate test, 20mA g ^-1 、50mA g ^-1 、100mA g ^-1 、200mA g ^-1 And 500mA g ^-1 The multiplying power capacity of the G-SALC2 is 297, 199, 129, 78 and 67mAh G respectively under the condition of changing current ^-1 . In the further process of measuring the battery capacity by using the variable-speed cyclic voltammetry, the capacity of the G-SALC is jointly controlled by diffusion capacity and surface capacity, and the pseudocapacitance has higher and higher proportion in the battery capacity along with the increase of the sweep rate, which shows that the G-SALC has excellent electrochemical performance and capacity performance under the conditions of high current and high multiplying power.

Effect example 4

The electrical properties of the composite negative electrode material prepared from the starting materials were measured, and the preparation method was the same as in example 1, and the results are shown in table 2.

TABLE 2

The above-described embodiments are merely illustrative of the preferred embodiments of the present invention, and do not limit the scope of the present invention, and various modifications and improvements of the technical solutions of the present invention can be made by those skilled in the art without departing from the spirit of the present invention, and the technical solutions of the present invention are within the scope of the present invention defined by the claims.

Claims

1. A preparation method of a composite anode material doped with nano graphite is characterized by comprising the following steps:

(1) doping nano graphite into spartina alterniflora powder, performing primary pyrolysis, cooling and performing secondary pyrolysis to obtain a mixed carbon material;

(2) and sequentially carrying out alkali treatment and acid treatment on the mixed carbon material to obtain the nano graphite doped composite negative electrode material.

2. The method according to claim 1, wherein the preparation of the spartina alterniflora powder specifically comprises: chopping leaves and stalks of the spartina alterniflora, drying for 4-8 h at 140-160 ℃, and crushing to obtain the spartina alterniflora powder.

3. The preparation method according to claim 1, wherein the amount of the nano-graphite doped in the spartina alterniflora powder is 0-10 wt%, and the doping amount is not 0%.

4. The preparation method according to claim 1, wherein the primary pyrolysis conditions are: in an argon atmosphere, the temperature is 550-650 ℃, and the time is 18-22 min; the temperature of the secondary pyrolysis is 1100-1300 ℃, and the time is 2.5-3.5 h.

5. The method according to claim 1, wherein the alkali treatment specifically comprises: adding the mixed carbon material into 8-12% by mass of potassium hydroxide solution, and heating at 55-65 ℃ for 50-70 min.

6. The method according to claim 1, wherein the acid treatment specifically comprises: adding the mixed carbon material into hydrochloric acid with the concentration of 2.8-3.2 mol/L, and heating for 50-70 min at the temperature of 55-65 ℃.

7. The composite anode material doped with the nano graphite prepared by the preparation method of any one of claims 1 to 6.

8. Use of the nanographite-doped composite anode material according to claim 7 in the preparation of an electrode.