CN113517443A

CN113517443A - Preparation method of polyacrylonitrile/iron disulfide composite positive electrode material for lithium secondary battery

Info

Publication number: CN113517443A
Application number: CN202110682069.7A
Authority: CN
Inventors: 赵彦硕; 王丽平
Original assignee: Tianmu Lake Institute of Advanced Energy Storage Technologies Co Ltd
Current assignee: Tianmu Lake Institute of Advanced Energy Storage Technologies Co Ltd
Priority date: 2021-06-19
Filing date: 2021-06-19
Publication date: 2021-10-19
Anticipated expiration: 2041-06-19
Also published as: CN113517443B

Abstract

The invention belongs to the technical field of lithium iron disulfide batteries, and particularly relates to a preparation method of a polyacrylonitrile/iron disulfide composite positive electrode material for a lithium secondary battery₂Adding the mixture into polyacrylonitrile dispersion liquid for full dispersion, and then removing the solvent to obtain PAN @ FeS₂Precursor, to PAN @ FeS₂Tabletting the precursor, heating to cyclize PAN, further heating for heat treatment, naturally cooling, and pulverizing to obtain PAN @ FeS₂And (3) compounding the positive electrode material.

Description

Preparation method of polyacrylonitrile/iron disulfide composite positive electrode material for lithium secondary battery

Technical Field

The invention belongs to the technical field of lithium iron disulfide batteries, and particularly relates to a preparation method of a polyacrylonitrile/iron disulfide composite positive electrode material for a lithium secondary battery.

Background

With the rapid development of lithium ion batteries, the demand of people for high energy density lithium ion batteries is more urgent. Energy density is a critical parameter of batteries, and for large-scale energy storage, high endurance electric vehicles, advanced portable electronic devices, and many other applications, it is necessary to increase energy density to over 300Wh/kg for the purpose of extending driving range and reducing cost. Currently, the specific energy of a lithium ion battery with a high-nickel ternary cathode material (NCM811 system) matched with a silicon-carbon cathode material can reach about 300Wh/kg, and the aim of 2020 formulated by the national action compendium is preliminarily achieved. However, the specific energy of 300Wh/kg is already approaching the limits of the existing material systems. Recently, the conversion type cathode material has received much attention from researchers because it can accommodate more ions and electrons. Meanwhile, the lithium metal cathode has two main characteristics: the standard electrode potential was-3.04V, the lowest of all negative electrode materials; the theoretical specific capacity is as high as 3860 mAh/g. Thus, metallic lithium has the potential to be the most desirable negative electrode material. Thus, regression studies of lithium metal batteries based on conversion reactions without lithium cathode materials are a significant way to explore new high specific energy batteries.

Pyrite (FeS)₂) Rich reserves and low price. FeS₂Complete conversion to metallic Fe and alkali metal compound (Li)₂S or Na₂S) can reach 894mAh/g, so the material is always a good choice for high-capacity electrode materials. Li/FeS₂Primary batteries have been produced with well established technology, but room temperature Li/FeS was developed₂Secondary batteries have been a problem of concern to researchers. Development of Li/FeS₂There are several problems encountered in the process of secondary batteries, and FeS is encountered in the electrolyte aspect₂Incompatibility with carbonate solvents and "shuttle effect" in ether solvents, the material itself also faces severe volume expansion during charging and discharging and atom agglomeration due to the absence of a releasable structure. The electrochemical performance of the battery is optimized by changing the electrolyte or the self structure of the material, and the Li/FeS is difficult to solve at one time₂Secondary batteries face a number of problems.

Disclosure of Invention

In order to solve the technical problems, the invention provides a simple and rapid preparation method of polyacrylonitrile/iron disulfide composite cathode material for a lithium secondary battery, which has easy commercialization of equipment requirements, and comprises the following steps:

(1) fully dispersing polyacrylonitrile in a solvent to obtain polyacrylonitrile dispersion liquid,

the solvent is preferably absolute ethyl alcohol,

(2) FeS is prepared₂Adding the polyacrylonitrile solution into the polyacrylonitrile dispersion liquid obtained in the step (1), fully dispersing, and removing the solvent to obtain PAN @ FeS₂A precursor of the compound (I) is prepared,

FeS added in the step (2)₂The mass ratio of the polyacrylonitrile to the polyacrylonitrile in the step (1) is 7: 3,

FeS₂and polyacrylonitrile are fully ground before being added,

in the operation of removing the solvent, the obtained mixed system is heated and stirred to ensure that most of the solvent is heated and evaporated, then the mixed system is placed in an oven to be fully dried,

(3) for PAN @ FeS obtained in step (2)₂The precursor is tabletted to form tablets,

tabletting under the pressure of 4 MPa-8 MPa,

(4) PAN @ FeS subjected to tabletting in the step (3)₂The precursor is firstly heated to lead PAN to be cyclized, then is further heated for heat treatment, is naturally cooled and then is crushed, thus obtaining PAN @ FeS₂The composite anode material is prepared by compounding an anode material,

wherein, firstly, the temperature is raised to 260-310 ℃ through the temperature raising rate of 2-10 ℃/min in the air atmosphere, the temperature is kept for 1-2 hours to enable the PAN to generate cyclization (the cyclization of the polyacrylonitrile needs to react with oxygen to realize dehydrogenation, so the temperature needs to be carried out in the air or oxygen atmosphere), then the temperature is raised to 370-400 ℃ through the temperature raising rate of 2-10 ℃/min in the protective gas atmosphere, the temperature is kept for 1 hour (if the polyacrylonitrile directly reacts in the air under the high temperature condition, the polyacrylonitrile can be rapidly carbonized, so the temperature needs to be carried out in the protective atmosphere),

taking into account FeS₂The reaction of desulfurization easily occurs at an excessively high temperature, resulting in a phase change of the raw material or a large loss of sulfur element, and at the same time, PAN is also largely lost in weight at an excessively high temperature, meaning that PAN is largely carbonized at an excessively high temperature, so that the heat treatment temperature is defined as not more than 400 ℃.

The lithium iron disulfide battery anode material PAN @ FeS prepared by the invention₂Middle, FeS₂Can be stably embedded in continuous matrix formed by PAN, and inhibits FeS by high elasticity and constraint effect of PAN₂Volume expansion in electrochemical process, inhibit Fe⁰And Li₂S is agglomerated, and meanwhile, the structure after PAN cyclization can absorb S simple substance generated in the electrochemical process, so that the shuttle effect of the lithium iron disulfide battery matched with ether electrolyte is inhibited, the irreversible loss of active substances caused by side reaction when the lithium iron disulfide battery is matched with ester electrolyte is avoided,

the coating modified material PAN is non-toxic and harmless and has low cost, and the prepared product PAN @ FeS is₂The anode material is matched with a metal lithium battery, has good cycle stability, is very suitable for serving as the anode material of the lithium iron disulfide battery, is simple to operate, has easily obtained raw materials, and is easy for large-scale industrial production.

Drawings

Figure 1 is a thermogravimetric analysis profile of a feedstock PAN in the present application,

FIG. 2 shows X-ray diffraction patterns of the positive electrode materials prepared in comparative example 1(a), comparative example 2(b) and example 1(c) of the present application,

FIG. 3 is a SEM micrograph of the positive electrode materials prepared in comparative example 1(a), comparative example 2(b) and example 1(c) of the present application,

FIG. 4 is a scanning electron microscope microscopic morphology image of the surface of the electrode sheet of the positive electrode material prepared in comparative example 1(a), comparative example 2(b), and example 1(c) of the present application,

FIG. 5 is an SEM micrograph and section element distribution of sections of electrode sheets of the positive electrode materials prepared in comparative example 1(a), comparative example 2(b) and example 1(c) of the present application,

fig. 6 is a graph showing cycle performance and coulombic efficiency of the positive electrode material batteries prepared in comparative example 1(a), comparative example 2(b), example 1(C), comparative example 3(d), and comparative example 1(e) of the present application, with a charge and discharge rate of 0.1C,

fig. 7 is an ac impedance spectrum of the positive electrode material batteries prepared in comparative example 1(a), comparative example 2(b), and example 1(c) of the present application.

Detailed Description

Transferring the pure iron disulfide powder into a vacuum oven to dry for 1h at 100 ℃, and fully grinding to characterize the structural characteristics, as shown in (a) of the attached figure 2 and the attached figure 3.

Comparative example 1

Taking the iron disulfide powder as a positive electrode active material, a conductive agent acetylene black and a binder PVDF according to the weight ratio of 8: 1: 1 stirring and mixing the mixture fully in enough NMP to obtain anode slurry, coating the anode slurry on an anode current collector aluminum foil with the thickness of 20 mu m, and transferring the anode current collector aluminum foil to a vacuum oven at the temperature of 80 ℃ for drying for 12 hours to obtain the lithium battery anode, as shown in attached figures 4 and 5.

The lithium battery positive electrode, the metal lithium counter electrode, the commercial lithium hexafluorophosphate electrolyte and the PP/PE/PP diaphragm (Celgard 2400) of the comparative example were assembled into a button cell in a glove box filled with argon gas, and the electrochemical properties of the button cell were characterized.

Comparative example 2

The precursor was not subjected to a sheeting operation, the remainder of the operation being the same as in example 1, which follows:

dispersing 1.2g of polyacrylonitrile powder (Mw is 250,000) in 30mL of anhydrous ethanol, magnetically stirring for 30 minutes until the polyacrylonitrile powder is dissolved, adding 2.8g of the iron disulfide powder, magnetically stirring for 3 hours, heating and stirring at 75 ℃ to evaporate most of the anhydrous ethanol in the mixed system by heating, drying the mixed system in a vacuum oven at 100 ℃ for 1 hour to remove residual anhydrous ethanol in the mixture after heating and stirring are finished, and obtaining the dry @ PAN FeS₂A precursor; converting PAN @ FeS₂Directly placing the precursor in a quartz boat, transferring the quartz boat into a tube furnace, heating to 270 ℃ at a heating rate of 5 ℃/min in the air atmosphere, preserving heat for 1h, then heating to 400 ℃ at the same heating rate in the nitrogen atmosphere, preserving heat for 1h, naturally cooling to room temperature (25 ℃, the same below), opening the tube furnace, and taking out the product PAN @ FeS₂The structural characteristics of the material are characterized after the material is fully ground, as shown in attached figures 2 and 3;

PAN @ FeS prepared in the comparative example₂The positive electrode active material, conductive agent acetylene black and adhesive PVDF are mixed according to the weight ratio of 8: 1: 1 stirring and mixing the mixture fully in sufficient NMP to obtain anode slurry, and coating the anode slurry on a cathode with the thickness of 20 mu mAnd (3) collecting the anode fluid on an aluminum foil, and transferring the anode fluid to a vacuum oven at the temperature of 80 ℃ for drying for 12 hours to obtain the lithium battery anode, as shown in attached figures 4 and 5.

Example 1

Dispersing 1.2g of polyacrylonitrile powder (Mw is 250,000) in 30mL of anhydrous ethanol, magnetically stirring for 30 minutes until the polyacrylonitrile powder is dissolved, adding 2.8g of the iron disulfide powder, magnetically stirring for 3 hours, heating and stirring at 75 ℃ to evaporate most of the anhydrous ethanol in the mixed system by heating, drying the mixed system in a vacuum oven at 100 ℃ for 1 hour to remove residual anhydrous ethanol in the mixture after heating and stirring are finished, and obtaining the dry @ PAN FeS₂A precursor; 4g of dried PAN @ FeS was compressed using an oil pressure tablet press₂Pressing the precursor into a wafer (diameter is 15mm) with the thickness of 6.5mm in a mould under the pressure of 5MPa, and then pressing PAN @ FeS₂Placing the precursor wafer in a quartz boat, transferring the quartz boat to a tube furnace, heating to 270 ℃ at a heating rate of 5 ℃/min in the air atmosphere, preserving heat for 1h, then heating to 400 ℃ at the same heating rate in the nitrogen atmosphere, preserving heat for 1h, naturally cooling to room temperature, opening the tube furnace, and taking out the product PAN FeS₂The structural characteristics of the material are characterized after the material is fully ground, as shown in attached figures 2 and 3;

PAN @ FeS prepared in this example₂The positive electrode active material, conductive agent acetylene black and adhesive PVDF are mixed according to the weight ratio of 8: 1: 1 stirring and mixing the mixture fully in enough NMP to obtain anode slurry, coating the anode slurry on an anode current collector aluminum foil with the thickness of 20 mu m, and transferring the anode current collector aluminum foil to a vacuum oven at the temperature of 80 ℃ for drying for 12 hours to obtain the lithium battery anode, as shown in attached figures 4 and 5.

The lithium battery positive electrode, the metallic lithium counter electrode, the commercial lithium hexafluorophosphate electrolyte and the PP/PE/PP diaphragm (Celgard 2400) of the example were assembled into a button cell in a glove box filled with argon gas, and the electrochemical properties were characterized.

Comparative example 3

Unpaired PAN @ FeS₂The precursor was subjected to cyclization treatment, and the rest of the operation was the same as in comparative example 2 described above:

dispersing 1.2g of polyacrylonitrile powder (Mw is 250,000) in 30mL of anhydrous ethanol, magnetically stirring for 30 minutes until the polyacrylonitrile powder is dissolved, adding 2.8g of the iron disulfide powder, magnetically stirring for 3 hours, heating and stirring at 75 ℃ to evaporate most of the anhydrous ethanol in the mixed system by heating, drying the mixed system in a vacuum oven at 100 ℃ for 1 hour to remove residual anhydrous ethanol in the mixture after heating and stirring are finished, and obtaining the dry @ PAN FeS₂A precursor of the compound (I) is prepared,

converting PAN @ FeS₂Directly placing the precursor in a quartz boat, transferring the quartz boat into a tube furnace, heating to 400 ℃ at a heating rate of 5 ℃/min in a nitrogen atmosphere, preserving heat for 1h, naturally cooling to room temperature, opening the tube furnace, and taking out a product PAN @ FeS₂Fully grinding;

PAN @ FeS prepared in the comparative example₂The positive electrode active material, conductive agent acetylene black and adhesive PVDF are mixed according to the weight ratio of 8: 1: 1 stirring and mixing the mixture fully in sufficient NMP to obtain anode slurry, coating the anode slurry on an anode current collector aluminum foil with the thickness of 20 mu m, and transferring the anode slurry to a vacuum oven at the temperature of 80 ℃ for drying for 12 hours to obtain the lithium battery anode.

Comparative example 1

Unpaired PAN @ FeS₂The precursor was subjected to cyclization treatment, and the rest of the procedure was the same as in example 1:

dispersing 1.2g of polyacrylonitrile powder (Mw is 250,000) in 30mL of absolute ethyl alcohol, magnetically stirring for 30 minutes until the polyacrylonitrile powder is dissolved, adding 2.8g of the iron disulfide powder, magnetically stirring for 3 hours, heating and stirring at 75 ℃ to evaporate most of absolute ethyl alcohol in a mixed system by heating, after heating and stirring are finished, drying the mixed system in a vacuum oven at 100 ℃ for 1 hour to remove residual absolute ethyl alcohol in the mixture, and obtaining the dry @ PAN FeS₂A precursor of the compound (I) is prepared,

4g of dried PAN @ FeS was compressed using an oil pressure tablet press₂The precursor is pressed into a wafer (diameter is 15mm) with the thickness of 6.5mm in a mould under the pressure of 5MPa, and then PAN @ FeS is added₂Placing the precursor wafer in a quartz boat, transferring the quartz boat to a tube furnace, heating to 400 ℃ at a heating rate of 5 ℃/min in a nitrogen atmosphere, preserving heat for 1h, naturally cooling to room temperature, opening the tube furnace, and taking out a product PAN @ FeS₂Fully grinding;

PAN @ FeS prepared in this example₂The positive electrode active material, conductive agent acetylene black and adhesive PVDF are mixed according to the weight ratio of 8: 1: 1 stirring and mixing the mixture fully in sufficient NMP to obtain anode slurry, coating the anode slurry on an anode current collector aluminum foil with the thickness of 20 mu m, and transferring the anode slurry to a vacuum oven at the temperature of 80 ℃ for drying for 12 hours to obtain the lithium battery anode.

As can be seen from fig. 2: prepared PAN @ FeS₂The sample was pure phase, and since PAN was amorphous structure, the appearance of impurity peak was hardly observed, and the peak position was hardly shifted. At the same time, since the heat treatment temperature is relative to FeS₂Since the active material is relatively high, the X-ray diffraction intensity of the sample after heat treatment is slightly reduced, and the degree of crystallization is slightly reduced.

As can be seen in fig. 3: b, the cyclized PAN granules are tightly coated on FeS₂Compacting around large particles, cyclizing and heat treating, and coating FeS₂PAN on a large particle is no longer a small particle and has become a continuous polymer matrix, indicating that PAN inter-links to form a continuous matrix, while FeS₂The active particles are embedded in a continuous PAN matrix, so that the structure is more stable, the continuous matrix is favorable for the rapid migration of lithium ions and electrons in the material, and FeS can be inhibited₂Fe in electrochemical processes⁰And (4) agglomeration.

As can be seen in fig. 4: FeS in a₂The particles, the conductive agent and the adhesive are uniformly distributed on the current collector, and FeS in b₂The particles are connected by PAN matrix, but the connection does not form an effective whole completely, the structural gap is more, and some FeS in the figure₂The particles appear to be isolated, c after compaction the porosity on the PAN matrix is smaller, FeS₂There is little isolation of the active particles and the particle surface is covered by PAN. Indicating the PAN @ FeS obtained by the preparation of example 1₂The composite material has a more stable structure, and the stable structure can lead to excellent electrochemical performance.

FeS in b, as seen by comparison of SEM micrographs in the leftmost column of FIG. 5₂The connection between the particles is not tight, the PAN does not form a stable continuous matrix, the longitudinal distribution is also in a loose state as a whole, and after compaction in c, FeS is hardly observed in the whole cross section₂Particles presenting only a continuous matrix, FeS by surface and profile analysis₂The particles should be embedded therein;

from the comparison between the remaining cross-sectional element profiles of FIG. 5, the C, S, Fe elements in the sheet were more uniformly distributed in a, and the S and Fe elements near the surface of the sheet were relatively more distributed, indicating that bare FeS₂The problem that slurry formed by the particles and PVDF as a binder cannot be completely uniform when preparing the electrode slice is solved, the distribution of the element C, S, Fe on the section of the electrode slice in the step b is consistent, the element content of a region S, Fe with more C element distribution is lower, and the element content of a region S, Fe with less C element distribution is obviously increased on the section of the electrode slice in the step C. This is because the PAN after heat treatment in b is coated only in the granular state on FeS₂On the granules, continuous matrix is not formed among the PAN, so C, S is consistent with Fe element distribution, and in c, continuous PAN matrix is formed through tabletting and then heat treatment, so FeS₂The particles are distributed in a manner embedded in the PAN.

As can be seen in fig. 6: PAN @ FeS with post-compaction heat treatment to form a continuous matrix₂The electrochemical performance of the composite material is most excellent, and the first-circle discharge capacity is as high as 830mAh/g (by FeS)₂Measured), the coulombic efficiency of the lithium iron phosphate also tends to be stable after the activation of the first few circles, and the reversible capacity is 700mAh/g (measured as FeS) after the cyclic charge and discharge of 100 circles₂Meters) and its decay tends to be slower;

in the case where neither tabletting was performed, the cycle capacity of comparative example 3(d) was lower than that of comparative example 2(b) because polyacrylonitrile in comparative example 3(d) was not subjected to cyclization treatment, and a cyclized structure adsorbing the S simple substance generated in the electrochemical process was not formed;

under the condition that neither is subjected to cyclization treatment, the cycle capacity of the tabletted comparative example 1(e) is lower than that of the non-tabletted comparative example 3(d), because the cyclization of polyacrylonitrile can form a structure for fixing elemental sulfur and also form a certain conductive network structure in the material, and the comparative examples 1(e) and 3(d) do not have the conductive network structure because the material is not cyclized, namely, the material does not have the conductive network structure, so that the conductivity of the material is deteriorated after the material is compacted, and the polarization and the internal resistance of the electrode material are increased after the conductivity of the electrode material is reduced, thereby influencing the electrochemical platform and the capacity exertion;

in example 1(c), the electrode material having the conductive network structure inside after cyclization was subjected to tabletting treatment, so that the conductive network structure was better bonded to the active material of the electrode material, the capacity of the active material was more efficiently exhibited, and the cycle capacity was still sufficiently improved,

in conclusion, the tabletting operation of the electrode material and the cyclization structure are closely related and influenced, and the scheme can obviously achieve the effects of improving the advantages and avoiding the disadvantages by carrying out the tabletting operation on the basis of the cyclization of the polyacrylonitrile.

As can be seen in fig. 7: PAN @ FeS obtained by heat treatment after compaction₂The composite material has the advantages of best conductivity and minimum charge transfer resistance, and the electronic conductivity and the ionic conductivity of the battery are remarkably improved.

Claims

1. A preparation method of polyacrylonitrile/iron disulfide composite cathode material for a lithium secondary battery is characterized by comprising the following steps: the preparation method comprises the following steps of,

(4) PAN @ FeS subjected to tabletting in the step (3)₂The precursor is firstly heated to lead PAN to be cyclized, then is further heated for heat treatment, is naturally cooled and then is crushed, thus obtaining PAN @ FeS₂And (3) compounding the positive electrode material.

2. The PAN @ FeS for a lithium secondary battery according to claim 1₂The preparation method of the composite anode material is characterized by comprising the following steps: FeS added in the step (2)₂The mass ratio of the polyacrylonitrile to the polyacrylonitrile in the step (1) is 7: 3.

3. the method for preparing a polyacrylonitrile/iron disulfide composite positive electrode material for a lithium secondary battery as claimed in claim 1, wherein: and (3) removing the solvent in the step (2), heating and stirring the obtained mixed system to evaporate most of the solvent, and then placing the mixed system in an oven for fully drying.

4. The method for preparing a polyacrylonitrile/iron disulfide composite positive electrode material for a lithium secondary battery as claimed in claim 3, characterized in that: in the step (2), the obtained polyacrylonitrile dispersion liquid is magnetically stirred for more than 3 hours before being heated and stirred.

5. The method for preparing a polyacrylonitrile/iron disulfide composite positive electrode material for a lithium secondary battery as claimed in claim 1, wherein: in the step (3), tabletting is carried out under the pressure of 4 MPa-8 MPa.

6. The method for preparing a polyacrylonitrile/iron disulfide composite positive electrode material for a lithium secondary battery as claimed in claim 1, wherein: in the step (4), the temperature is raised to 260-310 ℃ in the air atmosphere at a heating rate of 2-10 ℃/min, and the temperature is maintained for 1-2 hours to enable the PAN to be cyclized.

7. The method for preparing a polyacrylonitrile/iron disulfide composite positive electrode material for a lithium secondary battery as claimed in claim 1, wherein: in the step (4), after cyclization of PAN occurs, the temperature is raised to 370-400 ℃ in a protective gas atmosphere at a temperature rise rate of 2-10 ℃/min and is kept for 1 hour.

8. The method for preparing a polyacrylonitrile/iron disulfide composite positive electrode material for a lithium secondary battery as claimed in claim 1, wherein: the solvent in the step (1) is absolute ethyl alcohol.