Trench wall type sulfur positive electrode material and preparation method thereof

Abstract

The application discloses a kind of trench wall body type sulfur positive electrode materials and preparation method thereof, it is related to electrode material technical field.The application uses reduced graphene oxide aerogel as the carrier of active component sulfur, based on picosecond laser processing technology, trench is prepared on its surface, a kind of trench wall body type sulfur positive electrode material is obtained, unique layered pore wall structure can be realized in the electrode, can play greater inhibitory effect to lithium-sulfur battery polysulfide shuttle effect, and active substance in wall bottom can also be immersed into trench with electrolyte and realize full utilization.The positive electrode material prepared based on picosecond laser processing technology has the characteristics of excellent use performance and high stability.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrode materials, in particular to a groove wall type sulfur positive electrode material and a preparation method thereof. BACKGROUND

[0002] New energy vehicles, as the main growth engine of lithium battery applications, still have problems of battery use efficiency and endurance. The improvement speed of energy density of lithium ion batteries has obviously slowed down and gradually approaches the theoretical limit. From the perspective of technology realization or commercialization, it is a difficult problem for many car companies and battery manufacturers to overcome. Among many electrochemical energy storage systems, lithium-sulfur batteries have unique advantages in high specific energy battery design: the positive active material is based on a multi-electron reaction mechanism, with a mass energy density of 2600 Wh / kg, which is about an order of magnitude higher than commercial lithium-ion batteries. Using high-conductivity porous materials as the carrier of sulfur can improve the electrochemical performance of lithium-sulfur batteries. Because carbon materials have good electrical conductivity, low price, and are easy to make into a porous structure, they are widely used as carrier materials for active sulfur in lithium-sulfur batteries.

[0003] Currently, lithium-sulfur batteries need to achieve high active material utilization and high volumetric energy density under the harsh conditions of high mass loading, that is, the current thin electrode coating needs to be converted into a thick electrode. However, the mass energy density and volumetric energy density of sulfur positive electrodes are linearly negatively correlated with their porosity. The conventional porous electrode prepared by three-dimensional graphene (3DG) has a random pore geometry, high tortuosity, and a compacted electrode has low porosity and fewer charge transport paths, poor ion transport capacity, and low electrolyte permeability, resulting in slow sulfur conversion kinetics, low active material utilization, and therefore low total battery capacity.

[0004] High energy density and ultra-long cycle life are of great significance for lithium-sulfur batteries that pursue practicality, but it is still challenging to develop sulfur positive electrodes with ultra-thickness, ultra-high area capacity, and stable cycle performance.

[0005] In order to improve the conductivity of sulfur, low-density porous carbon materials are often introduced, resulting in a decrease in bulk density; and in order to fully infiltrate the electrolyte, low-tortuosity straight-through porous channels must usually be constructed inside the electrode. Among them, the technology of designing an oriented channel structure in a thick electrode by a template method is widely studied, however, the materials prepared by similar ice templates or other templates require a tedious process and difficult-to-control straight-through channels, ultimately leading to the easy occurrence of uneven channel morphology in the thick sulfur positive electrode. Such inclined channels will cause the local state of charge of the positive electrode to be different, thus generating a large stress in the interior of the sulfur particles, resulting in problems such as shedding and breaking of the active material, loss of connection of part of the active material with the conductive network, and capacity attenuation of the battery. On the other hand, it is also difficult to construct a rich variety of pore structures and active sites on the voids. SUMMARY

[0006] The purpose of the present application is to provide a groove wall type sulfur positive electrode material and a preparation method thereof, in order to solve the problems existing in the prior art.

[0007] To achieve the above purpose, the present application provides the following solutions:

[0008] The present application provides a preparation method of a groove wall type sulfur positive electrode material, comprising the following steps:

[0009] The sulfur-loaded reduced graphene oxide aerogel is subjected to picosecond laser processing to prepare grooves on the surface, thereby obtaining the groove wall type sulfur positive electrode material.

[0010] Further, the sulfur-loaded reduced graphene oxide aerogel is subjected to tabletting treatment, and the thickness is 0.3-0.5mm.

[0011] Further, the pressure of the tabletting treatment is 0.1Mpa.

[0012] Further, the speed of the picosecond laser processing is 1000-2000m / s, the power is 0.5-50W, and the frequency is 20-40KHz.

[0013] Further, when the picosecond laser processing is used to prepare grooves on the surface of the sulfur-loaded reduced graphene oxide aerogel, the processing distance is 60-120um, preferably 100um.

[0014] Further, the preparation method of the sulfur-loaded reduced graphene oxide aerogel comprises the following steps:

[0015] A reducing agent is added to a graphene oxide solution, and a hydrothermal reaction is carried out at 90℃, after which the hydrothermal reaction product is subjected to freeze-drying treatment, thereby obtaining a reduced graphene oxide aerogel;

[0016] The reduced graphene oxide aerogel is loaded with sulfur to obtain the reduced graphene oxide aerogel loaded with sulfur.

[0017] Further, the reducing agent is hydrazine hydrate, ascorbic acid or ethylenediamine, and the hydrothermal reaction time is 500-600 min.

[0018] Further, in the present application, the mass ratio of ascorbic acid to graphene oxide is 2:1, and the reduction of the reduced graphene oxide obtained will be reduced.

[0019] Further, in the present application, the mass ratio of sulfur to reduced graphene oxide aerogel is 7:3-8:2, and the amount of sulfur loaded will be insufficient or the conductivity will be poor.

[0020] Further, in the present application, the number of times of picosecond laser processing is greater than 2, otherwise the surface of the channel will not be smooth; and the number of times of processing is less than 8, otherwise the material will be burned through by the laser.

[0021] Further, the picosecond laser can adjust the linear speed to 1000-2000 m / s during processing, which makes the material contact with the laser for a very short time during processing, and the thermal effect of the laser is very small, which is particularly advantageous for the surface processing of light elements such as carbon and sulfur.

[0022] In the present application, the power (0.5-50 W), frequency (20-40 KHz) and processing distance (60-120 um) during picosecond processing also have a significant effect on the surface processing effect, and the processing parameters need to be controlled within a reasonable range.

[0023] The present application also provides a channel wall type sulfur positive electrode material prepared by the above preparation method.

[0024] The present application further provides the application of the above channel wall type sulfur positive electrode material in a sulfur-lithium battery.

[0025] Unlike most processes used for thick electrode research of lithium-sulfur batteries, the picosecond laser itself has low power and cannot melt the surface of the material, and is not suitable for metal composite materials that require laser thermal effects for surface treatment.

[0026] (1) In the synthesis process of reduced graphene oxide aerogel, the material with multiple pores can be obtained by freeze-drying treatment, which makes good preliminary design for the pore structure of Laser-GA@S wall.

[0027] (2) During laser processing, by reasonably controlling the processing parameters, it is possible to prepare trenches with unique orientation channels, thereby better suppressing the dissolution of lithium polysulfides and improving the stability of cathode materials.

[0028] Electrode thickness directly affects the energy density of lithium-sulfur batteries. Generally, the thicker the electrode, the higher the energy density of the battery. However, since the sulfur content in electrode materials is limited, if the electrode is too thick, the sulfur distribution in the battery will be uneven, leading to increased internal tortuosity and longer ion channels, thus reducing the utilization rate of the active material. This results in a decrease in the diffusion rate of lithium ions in the electrode, thereby affecting the battery's energy density. Therefore, excessive electrode thickness negatively impacts the battery's energy density. In general literature, the thickness of lithium-sulfur battery cathode materials prepared using the coating method is in the range of 10-100 μm. Due to interfacial effects and insufficient kinetics, thick electrodes require cumbersome template-based processes for designing thick electrode materials. Therefore, high porosity and high thickness in lithium-sulfur battery cathodes have become mutually constraining technical challenges.

[0029] This invention utilizes a self-assembled graphene aerogel self-supporting thick electrode as a carrier for the sulfur cathode. It leverages the easily tunable and richly porous three-dimensional conductive network of the graphene aerogel to load the active material sulfur. Simultaneously, it employs an ultrashort-pulse picosecond laser to process the surface of the graphene aerogel thick electrode. The ultrashort-pulse picosecond laser, characterized by its ultrashort pulse width and tunable frequency, is a laser beam with extremely high pulse energy density. Its short beam duration (10⁻⁶) further enhances its effectiveness. -12 With a thermal effect on sulfur particles attached to the graphene surface (on the order of s), low average energy, and short duration of action, it can be used to prepare uniform channels or impart porous functional properties to the surface of sulfur cathodes on self-supporting graphene materials.

[0030] This invention first utilizes the gel formed by the interaction of π-π bonds in graphene oxide (GO) under low-temperature hydrothermal environment as a three-dimensional framework, and prepares reduced graphene oxide aerogel by freeze-drying in one step; this invention uses a reducing agent to remove oxygen-containing functional groups on the surface of graphene oxide, restoring its hydrophobicity and conductivity.

[0031] Due to the presence of the hydrophobic surface of reduced graphene oxide, a hydrogel of pore structure and conductive network is formed in the reduction process, and after freeze-drying, the morphology characteristics of the product, reduced graphene oxide aerogel, have an open pore structure that is easy for molten sulfur to enter, and by directly pressing the tablet to adjust the pressure, a self-supporting sulfur-loaded thick electrode material GA@S can be obtained. The present application further adopts picosecond laser surface processing technology to design and construct a laser-processed channel on the surface of the self-supporting thick sulfur positive electrode GA@S, effectively solving the technical problems of long transmission path and ion channel of the electrode material, high tortuosity, inability of electrolyte to penetrate, and low utilization rate of the loaded active material, and the channel sides are difficult to form obvious cladding characteristics due to the small heat energy effect of the picosecond laser and the heat insulation characteristics of the porous structure of the graphene aerogel itself, and the channel surface and the inside are not formed obvious cladding characteristics, and due to the extreme laser processing process, a smooth and uniform internal surface is formed.

[0032] The thick electrode prepared by the picosecond laser processing technology has a unique layered pore wall structure inside, which makes the Laser-GA@S thick electrode have lower tortuosity and more surface pore openings, and the porous conductive network of graphene can also be seen inside the "wall body", which can greatly inhibit the shuttle effect of polysulfides in lithium-sulfur batteries, and the active material at the bottom of the wall can also be fully utilized by being immersed in the channel with electrolyte.

[0033] The present application discloses the following technical effects:

[0034] The present application uses reduced graphene oxide aerogel as a carrier of active component sulfur, and adopts picosecond laser processing technology to prepare a channel wall type sulfur positive electrode material, which can realize a unique layered pore wall structure inside the electrode, and the structure can realize a thick electrode material with lower tortuosity and more surface pore openings, and the porous conductive network of graphene is also present inside the "wall body" structure formed by the material, which can greatly inhibit the shuttle effect of polysulfides in lithium-sulfur batteries, and the active material at the bottom of the wall can also be fully utilized by being immersed in the channel with electrolyte.

[0035] The positive electrode material prepared based on the picosecond laser processing technology has excellent use performance and high stability. BRIEF DESCRIPTION OF DRAWINGS

[0036] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without any creative effort.

[0037] Figure 1 SEM images of Laser-GA@S at different resolutions for Example 1 of the present application: a) straight-through channels on the surface of Laser-GA@S; b) wall structure inside the channels of Laser-GA@S; c) pores on the wall structure of Laser-GA@S; d) SEM image of Laser-GA@S at a local magnification.

[0038] Figure 2 In the figure, a) is a microscope image of straight-through channels on the surface of Laser-GA@S prepared in Example 1 of the present application (the inset is a photo of Laser-GA@S after laser processing); b) is an SEM image of Laser-GA@S prepared in Example 1 of the present application; c) is an EDS sulfur element scanning image of Laser-GA@S prepared in Example 1 of the present application; d) is an EDS carbon element scanning image of Laser-GA@S prepared in Example 1 of the present application.

[0039] Figure 3 Performance verification of a soft-pack battery assembled with the Laser-GA@S prepared in Example 1 of the present application: a) is the 0.1C and 0.2C rate discharge curves and specific capacity comparison (the inset is a photo of the prepared soft-pack battery); b) is the cycle capacity change curve and coulombic efficiency of the Laser-GA@S soft-pack battery (the inset is a photo of thickness measurement of the Laser-GA@S cathode material with a thickness of 318 um).

[0040] Figure 4 Microscopic morphology images of Laser-GA@S cathode materials prepared in Comparative Example 1 and Comparative Example 2 of the present application; in which a) is Comparative Example 1, b) is Comparative Example 2.

[0041] Figure 5 0.1C specific capacity comparison of Laser-GA@S cathode materials prepared in Example 1 of the present application and Laser-GA@S cathode materials prepared in Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0042] Various exemplary embodiments of the present application will now be described in detail, which should be considered to be illustrative of the present application and should not be construed to limit the present application, and are understood to be a more detailed description of certain aspects, features and embodiments of the present application.

[0043] It should be understood that the terms used in the present application are merely used to describe particular embodiments and are not intended to limit the present application. In addition, for numerical ranges in the present application, it should be understood that each intermediate value between the upper limit and the lower limit of the range is specifically disclosed. Each smaller range between any stated value or stated range, and between any other stated value or stated range, is also included in the present application. The upper limit and the lower limit of these smaller ranges can be independently included or excluded from the range.

[0044] Unless otherwise indicated, all technical and scientific terms have the same meaning as those one of ordinary skill in the art of the disclosure would understand. Although preferred methods and materials are described, any method and materials similar or equivalent to those described herein can be used in the practice or testing of the present disclosure. All documents mentioned herein are incorporated by reference to disclose and describe in further detail the methods and / or materials associated with the documents. In the case of conflict between the present specification and any incorporated document, the present specification controls.

[0045] Many modifications and variations of the present disclosure described in the detailed description of the specification can be made without departing from the scope or spirit of the present disclosure, which will be apparent to those skilled in the art. Other implementations of the disclosure will be apparent to those skilled in the art from consideration of the specification and practice of the disclosure. The specification and examples of the present disclosure are illustrative only.

[0046] As used herein, "comprise", "comprising", "have", "having", "include", "including", "contain", "containing", and the like, are open-ended terms that are intended to mean including, but not limited to.

[0047] The graphene oxide (GO) powder and ascorbic acid used in the embodiments of the present disclosure are purchased from National Pharmaceutical Group Reagent Co., Ltd., and the elemental sulfur is purchased from Changrui Biological Technology Co., Ltd.

[0048] Example 1

[0049] (1) Preparation of graphene oxide (GO) dispersion

[0050] The GO powder was dispersed into deionized water for ultrasonic dispersion to obtain a GO dispersion of 3 mg / mL.

[0051] (2) Preparation of reduced graphene oxide (rGO) aerogel block material

[0052] a. Ascorbic acid was added to the above GO dispersion according to the mass ratio of ascorbic acid: GO = 2: 1, and further hydrothermal reduction was carried out at 90°C for 500 min;

[0053] b. The sample obtained in step a was freeze-dried at -50°C for 24h to obtain a reduced graphene oxide (rGO) aerogel block material (GA) with a thickness of 2cm;

[0054] (3) Preparation of GA@S positive electrode self-supporting material

[0055] The elemental sulfur was laid flat on the upper surface of the GA block material, and the elemental sulfur was melted through a muffle furnace at 155°C to obtain a GA@S positive electrode self-supporting material (the mass ratio of elemental sulfur and GA is 7:3) with a thickness of 2cm.

[0056] (4) Preparation of GA@S thick sulfur electrode

[0057] The prepared GA@S positive electrode self-supporting material was pressed into a sheet under a pressure of 0.1 MPa to prepare a GA@S thick sulfur electrode with a thickness of 0.5 mm.

[0058] (5) Preparation of laser-processed graphene aerogel (Laser-GA@S)

[0059] a. The surface channel was processed with a picosecond laser, wherein the laser processing pitch was 100 um, the laser line speed was 1000 m / s, and the entire straight-through channel was constructed within 0.5 ms. The processing times can be adjusted to 5 times according to the thickness of the GA@S thick sulfur electrode;

[0060] A picosecond fiber pulse laser with a model number of YLPP-25-3-50-R was used, and the focused spot diameter was 40 um. The laser processing parameters were as follows: wavelength was 1030 nm; laser power was 20 W; frequency was 20 khz; and pulse width was less than 15 ps.

[0061] Assembled 2032 button cell:

[0062] The prepared Laser-GA@S was cut into pieces, lithium was used as the negative electrode, Celgard separator was used, and electrolyte was selected to be 1,3-dioxolane (DOL)-dimethyl ether (DME) lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) electrolyte with a concentration of 1 mol L -1 , recorded as LiTFSI / DOL-DME (volume ratio 1:1), and 0.1 mol L -1 of LiNO3 was added to assemble a button cell.

[0063] Tested the battery performance, wherein the sulfur content in the positive electrode sheet was 60%.

[0064] Figure 1 SEM images of Laser-GA@S at different resolutions: a) straight-through grooves on the surface of Laser-GA@S; b) wall structure in the Laser-GA@S groove; c) pores on the wall structure of Laser-GA@S; and d) local enlarged SEM image of Laser-GA@S.

[0065] Figure 2 In the figure, a) is a microscope image of the straight-through grooves on the surface of Laser-GA@S (the insert is a photo of Laser-GA@S after laser processing); b) is an SEM image of Laser-GA@S; c) is an EDS sulfur element scanning image of Laser-GA@S; and d) is an EDS carbon element scanning image of Laser-GA@S.

[0066] Figure 3 Performance verification of Laser-GA@S positive electrode assembled soft package battery prepared for Example 1: a) is the discharge curve and specific capacity comparison at 0.1C and 0.2C rate (the inserted picture is the photo of the prepared soft package battery); b) is the cycle capacity change curve and coulombic efficiency of Laser-GA@S soft package battery (the inserted picture is the thickness measurement photo of Laser-GA@S positive electrode material with a thickness of 318um). It can be seen that the Laser-GA@S positive electrode material prepared under the harsh conditions of high area sulfur loading (7mg / cm 2 ), thickness greater than 300um, realizes high active material utilization rate and high area capacity, and stable cycle stability, the specific capacity and cycle performance are improved, at a rate of 0.1C, the first cycle discharge capacity reaches 1090mAh / g, and the coulombic efficiency of the first 100 cycles is maintained at 88.2%; at a rate of 0.2C, the first cycle discharge capacity is still 945mAh / g, which has potential value for commercial use.

[0067] Comparative Example 1

[0068] The difference from Example 1 is only that the laser speed is adjusted from 1000m / s to 500m / s; the power is adjusted from 20W to 15W.

[0069] Figure 4 Wherein, a) and b) are the micro-morphology diagrams of Laser-GA@S positive electrode material prepared in Comparative Example 1 under different magnifications; wherein a) is the groove SEM diagram processed by 500m / s scanning number and 15W power, it can be seen that the residual with prongs and burrs in the groove obtained by setting different scanning line speed and power of ultrafast pulsed laser, b) is the non-smooth prong structure with magnification.

[0070] Figure 5 is the 0.1C specific capacity comparison of Laser-GA@S positive electrode material prepared in Example 1 and Laser-GA@S positive electrode material prepared in Comparative Example 1. From the above data, it can be seen that the Laser-GA@S positive electrode material prepared in Example 1 has a higher specific capacity than the Laser-GA@S positive electrode material prepared in Comparative Example 1. Figure 5 It can be seen that the improper cooperation of laser speed and power in processing will lead to the failure to form a smooth wall structure, which will affect the utilization rate of active sulfur, and reduce the specific capacity of lithium-sulfur battery.

[0071] The above-described examples only describe the preferred modes of the present application, and do not limit the scope of the present application, and various modifications and improvements to the technical solutions of the present application made by those skilled in the art without departing from the design spirit of the present application shall fall within the protection scope determined by the claims of the present application.

Claims

1. A method for preparing a trench wall type sulfur cathode material, characterized in that, Includes the following steps: Picosecond laser processing was used to fabricate channels on the surface of sulfur-loaded reduced graphene oxide aerogel to obtain the trench wall type sulfur cathode material. The sulfur-loaded reduced graphene oxide aerogel is pressed into sheets with a thickness of 0.3-0.5 mm. The pressure for the tablet compression process is 0.1 MPa; The picosecond laser processing speed is 1000m / s, the wavelength is 1030nm, the power is 20W, the frequency is 20KHz, and the pulse width is less than 15ps; When using picosecond laser processing to prepare channels on the surface of sulfur-loaded reduced graphene oxide aerogel, the processing spacing is 60-120 μm; The preparation method of the sulfur-loaded reduced graphene oxide aerogel includes the following steps: A reducing agent was added to a graphene oxide solution, and a hydrothermal reaction was carried out at 90°C. The hydrothermal reaction product was then freeze-dried to obtain reduced graphene oxide aerogel. Sulfur was spread evenly on the upper surface of the reduced graphene oxide aerogel, and the sulfur was melted in a muffle furnace at 155°C to obtain a 2 cm thick sulfur-loaded reduced graphene oxide aerogel. The reducing agent is hydrazine hydrate, ascorbic acid, or ethylenediamine; the hydrothermal reaction time is 500-600 min; The mass ratio of sulfur to reduced graphene oxide aerogel is 7:3 to 8:

2.

2. The trench wall type sulfur cathode material prepared by the preparation method described in claim 1.

3. The application of the trench wall type sulfur cathode material as described in claim 2 in sulfur lithium batteries.

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