CN112615008A - Positive modified material M-N-CNT for lithium-sulfur battery, preparation method thereof and battery - Google Patents

Positive modified material M-N-CNT for lithium-sulfur battery, preparation method thereof and battery Download PDF

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CN112615008A
CN112615008A CN202110251840.5A CN202110251840A CN112615008A CN 112615008 A CN112615008 A CN 112615008A CN 202110251840 A CN202110251840 A CN 202110251840A CN 112615008 A CN112615008 A CN 112615008A
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sulfur
positive electrode
lithium
battery
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高学会
张慧雯
竺图远
黄颖翀
周春燕
刘书绚
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Zhejiang Normal University CJNU
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
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    • H—ELECTRICITY
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    • H01M10/05—Accumulators with non-aqueous electrolyte
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    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
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    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
    • H—ELECTRICITY
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    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
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    • H—ELECTRICITY
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    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
    • H01M2004/028—Positive electrodes
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    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Abstract

The invention discloses a positive modified material M-N-CNT for a lithium-sulfur battery, a preparation method thereof and the battery, wherein M is Fe, Co or Ni, and the material M-N-CNT is prepared by the following method: transferring a saturated DMF solution of metal salt into a quartz boat, placing the quartz boat into one end of a tubular furnace, heating the tubular furnace to 700-1000 ℃ under Ar atmosphere, pushing the quartz boat into a heating area of the tubular furnace until the saturated solution is completely reacted, naturally cooling to room temperature, collecting a solid product, washing the solid product with aqua regia, deionized water and ethanol in sequence, and drying to obtain the positive modified material M-N-CNT; the cathode modified material prepared by the method has excellent electrochemical performance, not only has higher conductivity, but also has excellent electrocatalytic property, and can still realize high surface capacity under high sulfur loading. The method is simple, high in yield, low in cost and great in application potential.

Description

Positive modified material M-N-CNT for lithium-sulfur battery, preparation method thereof and battery
Technical Field
The invention relates to the field of lithium-sulfur batteries, in particular to a positive modified material M-N-CNT for a lithium-sulfur battery, a preparation method thereof and the battery, wherein M is Fe, Co or Ni.
Background
With the rapid development of social economy, energy shortage and environmental pollution become two major problems facing society, so that while economy is developed, emphasis is placed on using high-energy, stable and clean energy, and electrochemical materials are light in weight, large in capacity and small in volume, so that the development of scientific economy is promoted while the electrochemical materials are used as energy storage devices to provide convenience and rapidness for life of people.
The lithium-sulfur battery has high theoretical specific capacity of 1675 mAh/g and theoretical specific energy of 2600 Wh/kg, is low in cost and environment-friendly, and is considered to be one of the most potential batteries. However, lithium sulfur batteries also have some disadvantages in their own right: short lifetime, low coulombic efficiency and self-discharge consumption, etc., which seriously hamper the commercial application of lithium-sulfur batteries. Therefore, the electrochemical performance of the lithium-sulfur battery can be effectively improved by modifying the cathode material. At present, Fe is reported in the journal of energy (285 pages 47-49 in 2015) of China3C/carbon nanofiber webs. ACT @ Fe/Fe with graphene shell is reported in the journal advanced science News of the United states (2018, volume 28, 1800563, 4-7 pages)3And C, nano particles. In thatOne-dimensional porous Fe-N-C and two-dimensional graphene sheet modified diaphragm is reported in the journal of chemical engineering (333 vol.564-571 in 2018) in China, and a porous cubic composite material N-Fe is reported in the journal of inorganic salt industry (51 vol.1 in 2019, 29-32 pages) in China3C/C. Although the method effectively improves the conductivity of the sulfur positive electrode, relieves the volume expansion of sulfur in the charging and discharging processes, and effectively inhibits the dissolution of polysulfide, the preparation process is complex, the yield is low, the cost is high, the requirement of commercial large-scale production is difficult to achieve, and in general, the effective high sulfur loading and the surface capacity under the high sulfur loading are difficult to obtain.
Disclosure of Invention
The invention aims to provide a positive modified material M-N-CNT for a lithium-sulfur battery, a preparation method thereof and the battery, wherein the battery has the advantages of low cost, high specific energy, simple operation and the like, particularly the positive modified material Fe-N-CNT prepared by the method has excellent electrochemical performance, and due to a network structure formed by mutually crossing bamboo-shaped carbon nanotubes, the integral conductivity of the positive material is enhanced, the volume expansion of sulfur in the charging and discharging process can be relieved, the rapid attenuation of the capacity is effectively reduced, and Fe adsorbed on the surface of the positive modified material is adsorbed3C and Fe3The N nano particles have strong adsorption effect on polysulfide and can inhibit the dissolution of polysulfide; in addition Fe3C and Fe3N is used as a catalytic material, the catalytic conversion of polysulfide can be promoted, and the prepared cathode material not only has higher conductivity, but also has excellent electrocatalytic property, and can still realize high surface capacity under high sulfur loading.
The invention is realized by the following technical scheme:
the invention provides a positive modified material M-N-CNT for a lithium-sulfur battery, wherein M is Fe, Co or Ni, and the material is prepared by the following method: transferring a saturated DMF solution of metal salt into a quartz boat, placing the quartz boat into one end of a tubular furnace, heating the tubular furnace to 700-1000 ℃ in Ar atmosphere, pushing the quartz boat into a heating area of the tubular furnace until the saturated solution is completely reacted, naturally cooling to room temperature, collecting a solid product, washing the solid product with aqua regia, deionized water and ethanol in sequence, and drying to obtain the positive modified material M-N-CNT.
Further, the metal salt is ferrocene, cobalt acetylacetonate or nickel acetylacetonate.
Furthermore, the morphology of the M-N-CNT is that M-N, M-C nano-particles are adsorbed and distributed on bamboo-shaped CNTs.
The invention also provides a sulfur-containing anode, which is prepared by uniformly mixing the M-N-CNT and sulfur, carrying out heat treatment to obtain a sulfur-containing anode material, mixing the sulfur-containing anode material, a conductive agent and a binder, adding N-methylpyrrolidone, fully stirring, coating on a current collector, and carrying out vacuum drying.
Further, the mass ratio of the M-N-CNT to the sulfur is 2-4: 6-8.
Further, the mass ratio of the sulfur-containing cathode material to the conductive agent to the binder is 5-8: 1-3: 1-2.
Further, the heat treatment is as follows: the mixture is put into a closed container and is placed in a vacuum box at the temperature of 150-.
The invention also provides a lithium-sulfur battery which comprises the sulfur-containing positive electrode.
Further, the lithium-sulfur battery also comprises a separator, wherein the separator is provided with an M-N-CNT coating layer and is formed by coating the M-N-CNT on the separator.
The invention has the beneficial effects that:
1. according to the invention, the one-dimensional carbon nanotubes are crossed with each other to form a conductive network structure by utilizing the M-N-CNT (modified lithium-sulfur battery) which is prepared by chemical vapor deposition and is used as the positive electrode material, particularly the Fe-N-CNT, of the lithium-sulfur battery, and the generated macropores are favorable for the transmission of lithium ions and the permeation of electrolyte; strong adsorption capacity between nitrogen functional groups on the surface of the carbon nano tube and polysulfide; with single Fe3N、Fe3Compared with the carbon nano-tube, the Fe-N-CNT has higher conductivity and catalytic performance, thereby improving the electrochemical performance of the lithium-sulfur battery. Under the current density of 1.0C, the first-turn specific discharge capacity of the Fe-N-CNT-800 prepared in the embodiment 1 of the invention is 707 mAh/g, and the first-turn specific discharge capacity is after 300 cyclesThe specific capacity is 474 mAh/g.
2. The positive modified material M-N-CNT prepared by the invention has the uniform bamboo-shaped carbon nanotube appearance, has strong absorption/limitation and higher conductivity on LiPSs, can promote rapid electron/ion transmission, and ensures effective utilization of sulfur in a thick electrode; second, the CNTs adsorb distributed M-N, M-C nanoparticles (e.g., Fe)3C and Fe3N) enables the carbon nanotube material to have excellent electrocatalytic performance, can effectively inhibit shuttle effect, enhances electrochemical kinetics, and can more effectively improve the utilization rate of active substance sulfur. Thus, the high surface capacity at high sulfur loading can be achieved with the sulfur positive electrode of the invention, and the positive electrode material of the invention can accelerate the redox kinetics more efficiently with the same high sulfur loading compared to previous literature reports, with higher discharge capacity, e.g., 2.20 mA cm for Fe-N-CNT-800 of example 1 of the invention-2The sulfur loading was 13.12 mg cm-2Has stable circulation capacity and 9.10 mAh cm-2High area capacity.
3. The M-N-CNT serving as the positive electrode modified material of the lithium-sulfur battery has the advantages of extremely simple preparation method, easiness in operation, high yield and low cost, meets the requirement of industrial development, and has great application potential in the future.
Drawings
FIG. 1 is a diagram of Fe-N-CNT-800 prepared in example 1 and observed by a field emission scanning electron microscope (FE-SEM) of S-4800 type.
FIG. 2 is a graph of the morphology of the products of examples 3 and 4 observed by a field emission scanning electron microscope (FE-SEM) model S-4800, wherein a, b are Fe-N-CNT-700 prepared in example 3, and c, d are Fe-N-CNT-900 prepared in example 4.
FIG. 3 is an X-ray diffraction pattern (a) and a Raman scattering spectrum (b) of Fe-N-CNT-X prepared in examples 1, 3 and 4, wherein X is a calcination temperature, and is 800 ℃, 700 ℃ and 900 ℃, respectively.
FIG. 4 is an XPS spectrum of Fe-N-CNT-X (X is a calcination temperature, and is 800 ℃, 700 ℃, 900 ℃ respectively) prepared in examples 1, 3, 4 observed by PHI 5000C ESCA System X-ray photoelectron spectroscopy (XPS).
FIG. 5 shows the Cyclic Voltammograms (CV) of Fe-N-CNT-x + S (x is the calcination temperature, 800 deg.C, 700 deg.C, 900 deg.C) and MWCNT + S prepared in examples 1, 3, 4 as measured by CHI760E electrochemical workstation.
FIG. 6 shows the Electron Impedance Spectra (EIS) of Fe-N-CNT-x + S (x is the calcination temperature, 800 deg.C, 700 deg.C, 900 deg.C) and MWCNT + S prepared in examples 1, 3, 4 measured by CHI760E electrochemical workstation.
FIG. 7 is a constant current charge and discharge test system for observing the constant current charge and discharge performance curve (a) and the cycle stability curve and the corresponding coulombic efficiency (b-d) of the Fe-N-CNT-x + S (x is the calcination temperature, 800 ℃, 700 ℃, 900 ℃ respectively) and the MWCNT + S prepared in examples 1, 3, 4 under different multiplying factors.
FIG. 8 is an electrocatalytic performance test of the assembly of Fe-N-CNT-800 (800 ℃ C. is calcination temperature) and MWCNTs prepared in example 1 into a symmetric cell as measured by CHI760E electrochemical workstation. Where a is a Cyclic Voltammogram (CV) curve, b is an impedance plot (EIS), c is a Linear Sweep Voltammogram (LSV), and d is a Tafel curve.
Detailed Description
The following describes the preparation of the modified positive electrode material M-N-CNT-x (M is Fe, Co or Ni, x is the calcination temperature, 700 ℃, 800 ℃, 900 ℃ respectively), the corresponding sulfur positive electrode, and the influence on the electrochemical performance of the lithium sulfur battery in detail with reference to the examples and the drawings.
Example 1: preparation of modified Positive electrode Material (Fe-N-CNT-800) at a calcination temperature of 800 deg.C
20 mL of a saturated ferrocene solution in DMF was transferred to a quartz boat and then slowly placed into one end of a horizontal tube furnace. Under Ar atmosphere at 13 deg.C for min-1The tube furnace was heated to 800 ℃ at the rate of temperature rise, and then the quartz boat was slowly pushed into the tube furnace heating zone until the saturated solution was completely reacted. And naturally cooling the tube furnace to room temperature, collecting the black precipitate Fe-N-CNT, and sequentially washing with aqua regia, deionized water and ethanol. Finally, the obtained sample is dried in a vacuum drying oven at 80 ℃ for 8 h.
Scanning electron microscopy analysis is performed on the Fe-N-CNT-800 prepared in the embodiment, and the obtained electron microscopy image is shown in FIG. 1, which shows that the Fe-N-CNT-800 is composed of uniform bamboo-like Carbon Nanotubes (CNTs), the width of the CNTs is about 30 nm, and a large number of nano particles are decorated on the CNTs.
XRD crystal structure study was performed on Fe-N-CNT-800 prepared in this example, and the XRD pattern obtained is shown as a in FIG. 3, and diffraction peaks of Fe-N-CNT-800 at 2 theta =37.7 °, 42.8 °, 45.0 ° and 48.5 ° can be labeled as Fe3The (210), (211), (031) and (131) planes of the C crystal (JCPDS NO. 35-0772). Diffraction peaks at 2 θ =38.2 °, 41.2 °, 43.7 ° and 57.5 ° respectively correspond to Fe3N crystals (110), (002), (111) and (112) planes (JCPDS NO. 49-1663). Furthermore, all Fe-N-CNT-800 materials showed a distinct graphene diffraction peak at 26.4 °, corresponding to the (002) plane (JCPDS NO. 41-1487). Shown as b in FIG. 3, is the Raman scattering spectrum of the Fe-N-CNT-800 material, which further verifies the presence of the graphene shown as a in FIG. 3, wherein at 1345 cm-1(D band) and 1590 cm-1Two prominent characteristic peaks appear near (G-band), due to the vibration of the crystal boundaries and graphite sp2 stretching of the carbon. 2700 cm-1The broad two-dimensional peak at (a) indicates that the Fe-N-CNT-800 sample is composed mainly of several layers of graphene instead of single layer graphene, which is due to the moderate amount of iron-assisted catalysis and DMF templating, which facilitates graphitization of the material and produces large amounts of pyridine carbon and graphite nitrogen.
The Fe-N-CNT-800 prepared in this example was analyzed by XPS chemical composition and valence state of the elements. FIG. 4 a shows the coexistence of C, O, N, Cl and Fe elements in Fe-N-CNT-800. As shown in b of fig. 4, C1s XPS spectra mainly showed several different bonding structures C-C, C-N, C-O and O-C = O, located at 284.1, 284.8, 286.4 and 290.1 eV, respectively, and C1s XPS indicated the presence of N-containing groups on the surface of the carbon nanotubes. In the N1 s XPS spectrum (c in fig. 4), 397.6, 398.3, 399.8, 400.8 and 404.4 eV, respectively, correspond to pyridine N, Fe-N, pyrrole N, graphite N and oxidized N, indicating that the N element has been successfully doped in the CNT. Furthermore, the strong presence of pyridine N means that CNTs have good propertiesThe adsorption capacity of (2) enhances the interatomic attraction force to the LiPSs. Meanwhile, due to the electronegativity of N species, the local electronic structure of the carbon nano tube can be changed by doping N in the carbon nano tube, and the growth of metal nano particles at N-rich sites is facilitated. As can be seen from the Fe 2p XPS spectrum (d in FIG. 4), the peak appearing at 707.3 eV is attributed to Fe0It was confirmed that the Fe-N component was present in the Fe-N-CNT-800. Peaks near 709.6, 712.9, 720.0 and 725.5 eV are assigned to Fe2+ (2p3/2)，Fe3+ (2p3/2)，Fe3+Satellite Peak and Fe3+(2p1/2) These structures play an important role in immobilizing the active substance and mitigate the shuttle effect, thereby inhibiting the dissolution of the LiPS.
The Fe-N-CNT-800 and conventional MWCNT electrodes prepared in this example were subjected to symmetric cell assembly and electrocatalytic performance studies.
Preparing a pole piece: dissolving active materials (Fe-N-CNT-800 and MWCNT) and PVDF in a mass ratio of 9:1 in a proper amount of NMP to form uniformly mixed slurry, and coating the slurry on an aluminum foil (the loading amount is 2-4 mg cm)-2) Vacuum drying, and cutting into 12 mm round pieces as the positive and negative electrodes of the symmetrical battery; 50 μ L of electrolyte (1M LiTFSI and 0.2M Li)2S6Dissolved in DOL/DME at a volume ratio of 1: 1).
Assembling the symmetrical batteries: positive electrode case → electrolyte → pole piece → electrolyte → 2400 Celgard diaphragm → electrolyte → pole piece → gasket → spring piece → negative electrode case.
The catalytic performance of Fe-N-CNT-800 and MWCNT was investigated for the symmetric cell prepared in this example. The result a in fig. 8 shows that Fe-N-CNT-800 has two pairs of sharp redox peaks and the current response is significantly higher, indicating that Fe-N-CNT-800 can promote the redox reaction of the LiPS to some extent. The impedance plot (b in fig. 8) of the symmetric cell further reflects the ease of transformation of the LiPSs on the electrode surface. It can be clearly seen that the charge transfer resistance of Fe-N-CNT-800 is smaller than that of MWCNT, indicating that the electron transfer rate of Fe-N-CNT-800 is faster, accelerating the kinetics of LiPS transformation, which is consistent with CV test results. Linear Sweep Voltammetry (LSV) test of c in FIG. 8 investigated Li2S on different catalytic surfacesRedox behaviour. The initial overpotential of the Fe-N-CNT-800 electrode is-0.40V, which is lower than that of the MWCNT electrode, and shows that the kinetics of the Fe-N-CNT-800 electrode is greatly enhanced. While the corresponding Tafel plot further supports this result, with MWCNT (144 mV dec)-1) In contrast, the Tafel slope of Fe-N-CNT-800 was 117 mV dec-1(d in FIG. 8).
Example 2:
the Fe-N-CNT-800 electrode prepared in example 1 was subjected to sulfur loading, preparation of a positive electrode sheet, Li-S battery assembly, and battery performance study.
Carrying sulfur: weighing Fe-N-CNT-800 and sublimed sulfur in a mass ratio of 3:7, fully and uniformly grinding in a mortar, bottling, wrapping with aluminum foil, and annealing at 155 ℃ for 4 hours in vacuum, namely, loading sulfur on the Fe-N-CNT substrate to generate the sulfur-containing positive electrode material Fe-N-CNT + S.
Preparing a positive pole piece: uniformly grinding the sulfur-containing cathode material, the conductive agent and the binder which are prepared in the embodiment according to the mass ratio of 7: 2: 1, adding a proper amount of NMP, fully stirring for at least two times, coating a proper amount of slurry on an aluminum foil, and then carrying out vacuum drying at 60 ℃ for 12 hours; and finally cutting the pieces for later use.
Li-S battery assembly (face up): positive electrode case → electrolyte → positive electrode piece → electrolyte → diaphragm → electrolyte → lithium piece → gasket → spring piece → negative electrode case
The Cyclic Voltammetry (CV) test was performed on the Fe-N-CNT-800+ S prepared in this example, and the resulting cyclic voltammetry curve was shown in FIG. 5. As can be seen, two cathode peaks at 2.3V and 2.0V and one cathode peak at about 2.4V can be seen on the CV curve of the electrode. Reduction peak at high potential corresponds to S8Converted into long-chain LiPSs, and the reduction peak at low potential corresponds to short-chain polysulfide (Li)2S2/Li2S). While the latter peak is due to Li2S2/Li2Oxidation of S to Long-chain LiPSs and Final S8. Meanwhile, the reduction peak of the Fe-N-CNT-800 is positively shifted, which shows that the Fe-N-CNT-800 has excellent catalytic activity, can obviously enhance the electrochemical kinetics and reduce the electrochemical polarization.
Electrochemical Impedance (EIS) profiling was performed on the Fe-N-CNT-800+ S prepared in this example. FIG. 6 shows the redox kinetics of Fe-N-CNT-800 and MWCNT + S with LiPSs. In which the Fe-N-CNT-800 has a smaller charge transfer resistance compared to MWCNTs, indicating that the Fe-N-CNT-800 accelerates the LiPSs conversion reaction, which can be attributed to the presence of more active sites, such as Fe-N and Fe-C.
The rate performance analysis of the lithium-sulfur battery was performed on the Fe-N-CNT-800+ S prepared in this example, as shown in a of fig. 7, it was further confirmed that the Fe-N-CNT-800 has a higher specific capacity and a better rate performance. At 0.1C, the initial discharge specific capacity reaches 1224 mAh g-1When the current rate is increased from 0.1C to 0.2, 0.5, 1.0 and 2.0C, the charge and discharge capacities are 1224, 849, 767, 713 and 653 mAh g, respectively-1When the current density returns to 0.2C, the specific discharge capacity of the Fe-N-CNT-800 electrode is 770 mAh g-1It shows that Fe-N-CNT-800 has good durability and electrochemical activity under high current density. This also indicates that the Fe-N-CNT-800+ S cathode favors S8The conversion reaction with LiPSs is highly reversible.
The Fe-N-CNT-800+ S prepared in this example was subjected to 50 cycles (0.2C) and 250 cycles (0.5C) of cycle performance tests, respectively. FIG. 7 b shows that the initial specific discharge capacity of Fe-N-CNT-800+ S cathode at 0.2C is about 1072 mAh g-1The initial specific discharge capacity at 0.5C is about 700 mAh g-1。
The Fe-N-CNT-800+ S prepared in this example was subjected to 300 cycles of cycle performance test. The cycling performance of Fe-N-CNT-800 at 1.0C is shown in FIG. 7C, which shows that Fe-N-CNT-800 exhibits excellent cycling performance. The initial discharge specific capacity of the Fe-N-CNT-800 cathode is 707 mAh g-1After 300 times of circulation, the specific discharge capacity at 1.0C is stabilized at 474 mAh g-1。
Multifunctional sulfur cathode Fe prepared by chemical vapor deposition3C/Fe3The N @ N doped carbon nanotube (Fe-N-CNT-800) can realize high surface sulfur loading at 0.1C to meet the requirements of commercialization and large-scale production. In FIG. 7 d is the sulfur loading per unit area of Fe-N-CNT-800 of 10.57, 11.38, and 13.12 mg cm-2Constant electricity (C)Flow charge and discharge curves showing higher discharge capacities of 12.70, 13.15 and 14.26 mAh cm-2. After 50 cycles, the flour capacity is still maintained at 7.86, 8.80 and 9.10 mAh cm-2. The excellent performance of the high sulfur loaded Fe-N-CNT-800+ S cathode is attributed to the strong absorption/confinement of the LiPS and the high conductivity provided by the uniform bamboo-based CNTs, which promotes fast electron/ion transport and ensures efficient utilization of sulfur in thick electrodes.
Song et al synthesized Fe with MOF as precursor3C and nitrogen doped graphene carbon nanosheet (Fe)3C/NG), Fe was tested3The performance of the battery with C/NG under different high sulfur loading capacity shows that the sulfur loading capacity is 2, 5 and 6.5 mg/cm2When the surface volume is 0.1C times, the excellent surface volume is 3.27 mAh/cm, 4.3 mAh/cm and 6.0 mAh/cm after 50 cycles2. Li et al prepared independent C/Fe by simple phase transition technique3C film electrode, and with sulfur load directly as Li-S cell cathode. Due to C/Fe3The C membrane shows high expansibility and stackability, has rapid ion transmission micro/nano-channel and polysulfide capture network, and has S loading of 5.8 mg/cm2(four-layer film), 7.1 mg/cm2(five-layer film), the corresponding face volumes were found to be about 4.77, 5.15 mAh/cm after 100 cycles at 0.2C2. Jiang et al developed a novel self-supporting carbon nanofiber with a layered porous structure and Fe/N absorption/nucleation centers (Fe/N-HPCNF) as a high performance sulfur matrix by a simple co-spinning process. The internal porous carbon fiber structure facilitates fast Li+Electron transport and sulfur redox while maintaining a high sulfur area loading of 9 mg cm-2Can reach 6.6 mAh cm under high sulfur load-2But clearly the present invention is able to achieve higher areal capacities at higher sulfur loadings than these reports. In addition, the large amount of Fe/N heteroatoms uniformly distributed in the fiber strongly inhibits the diffusion of polysulfides by chemisorption, while regulating uniform sulfur nucleation, thereby enhancing cathode stability. To some extent, the separator modification (also a modification means of the sulfur cathode) can also be usedEffectively improve the problems of shuttle of polysulfide and the like, thereby improving the utilization of sulfur. For example, Yang et al reported that a composition containing Fe3C nanoparticles and Fe-NxA porous N-doped carbon network (NPN) with groups and interconnected for modifying the diaphragm. The strong chemical adsorption of NPCN can promote the transmission of lithium ions and electrons, and the polysulfide is anchored at the cathode of the diaphragm; fe3The C/Fe-Nx species may also provide chemisorption to capture polysulfides, Fe3C catalyzes the redox conversion of polysulfides. Fe3C/Fe−NxThe synergistic effect of @ NPCN enables the battery with the modified diaphragm to have higher capacity and good cycle performance. At a current density of 0.1C, the capacity of the battery was 683 mAh g-1Sulfur loading of 5.0 mg cm-2(ii) a After 200 times of circulation, the battery capacity can still reach 596 mAh g-1This corresponds to a capacity retention of 87%.
In conclusion, we found that the multifunctional sulfur cathode Fe-N-CNT-800 material synthesized by means of chemical vapor deposition has more excellent electrochemical performance compared with most of the documents reported previously. Firstly, the uniform bamboo-shaped carbon nano tube has strong absorption/limitation and higher conductivity to LiPSs, thereby promoting rapid electron/ion transmission and ensuring the effective utilization of sulfur in a thick electrode; second, Fe3C and Fe3N enables the carbon nanotube material to have excellent electrocatalytic performance, effectively inhibits shuttle effect, enhances electrochemical kinetics, and more effectively improves the utilization rate of active substance sulfur. Thus, high surface capacity can still be achieved at high sulfur loading. The synthesis method for preparing the cathode material of the battery not only provides a new effective strategy for realizing the catalytic conversion of polysulfide, but also is beneficial to the large-scale development of rechargeable lithium-sulfur batteries in the future.
Example 3: preparation of modified Positive electrode Material (Fe-N-CNT-700) at a calcination temperature of 700 deg.C
The procedure of preparing the positive electrode material was the same as in example 1 except that the calcination temperature was changed to 700 ℃.
Example 4: preparation of modified Positive electrode Material (Fe-N-CNT-900) at a calcination temperature of 900 deg.C
The procedure for preparing the positive electrode material was the same as in example 1, except that the calcination temperature was changed to 900 ℃.
Scanning electron microscopy analysis was performed on Fe-N-CNT-700 and Fe-N-CNT-900 prepared in examples 3 and 4. The SEM images of the obtained electron microscope pictures respectively corresponding to a, b, c and d in FIG. 2, Fe-N-CNT-700 and Fe-N-CNT-900 also show that the CNTs have uniform bamboo-like morphology, and particles are adsorbed on the surfaces of the CNTs.
XRD crystal structure studies were performed on Fe-N-CNT-700 and Fe-N-CNT-900 prepared in examples 3 and 4, and the obtained XRD patterns are shown as a and b in FIG. 3, and have characteristic peaks similar to those of Fe-N-CNT-800. Indicating that the material obtained at different calcination temperatures did not change its crystal structure.
Electrochemical performance tests were performed on Fe-N-CNT-700 and Fe-N-CNT-900 prepared in examples 3 and 4, as shown in FIGS. 5, 6 and 7, and the results showed slightly lower performance than Fe-N-CNT-800.
Example 5: preparation of modified cathode materials Co-N-CNT and Ni-N-CNT
Co-N-CNT and Ni-N-CNT the same synthetic method as Fe-N-CNT-800 in example 1 was used except that cobalt acetylacetonate and nickel acetylacetonate were used instead of ferrocene.
And adopting characterization means such as SEM scanning images, XRD spectrums, XPS spectrums and the like to illustrate the morphology structures and the components of the Co-N-CNT and the Ni-N-CNT. In addition, the results of voltammetric cycle test, rate capability, cycle capability and impedance analysis measurement show that: co (or Ni) -N-CNT can also effectively improve the reversibility, the cycling stability and the rate capability of the lithium-sulfur battery, and improve the electrochemical performance of the lithium-sulfur battery (wherein the effect of Fe-N-CNT-800 is most obvious, and simultaneously, because of Fe3C and Fe3N has excellent electrocatalytic performance (verified by a symmetrical battery), and the utilization rate of sulfur can be effectively improved even under high sulfur loading, and the structure and the components of the M-N-CNT can be further optimized and studied in depth based on the cathode material so as to meet the commercial development requirement of the lithium-sulfur battery to the maximum extent.

Claims (9)

1. A positive electrode modified material M-N-CNT for a lithium sulfur battery is characterized in that M is Fe, Co or Ni, and the material is prepared by the following method: transferring a saturated DMF solution of metal salt into a quartz boat, placing the quartz boat into one end of a tubular furnace, heating the tubular furnace to 700-1000 ℃ in Ar atmosphere, pushing the quartz boat into a heating area of the tubular furnace until the saturated solution is completely reacted, naturally cooling to room temperature, collecting a solid product, washing the solid product with aqua regia, deionized water and ethanol in sequence, and drying to obtain the positive modified material M-N-CNT.
2. The M-N-CNT for a lithium-sulfur battery as claimed in claim 1, wherein the metal salt is ferrocene, cobalt acetylacetonate or nickel acetylacetonate.
3. The positive electrode modified material M-N-CNT for lithium-sulfur battery of claim 2, wherein the morphology of the M-N-CNT is that M-N, M-C nanoparticles are adsorbed and distributed on bamboo-like CNTs.
4. The sulfur-containing positive electrode is characterized in that the sulfur-containing positive electrode is obtained by uniformly mixing the M-N-CNT in claim 3 with sulfur and performing heat treatment on the mixture to obtain a sulfur-containing positive electrode material, then mixing the sulfur-containing positive electrode material, a conductive agent and a binder, adding N-methylpyrrolidone, fully stirring, coating on a current collector, and then performing vacuum drying.
5. The sulfur-containing cathode according to claim 4, wherein the mass ratio of M-N-CNT to sulfur is 2-4: 6-8.
6. The sulfur-containing positive electrode according to claim 4, wherein the mass ratio of the sulfur-containing positive electrode material to the conductive agent to the binder is 5 to 8: 1 to 3: 1 to 2.
7. The sulfur-containing positive electrode according to claim 4, wherein the heat treatment is: the mixture is put into a closed container and is placed in a vacuum box at the temperature of 150-.
8. A lithium-sulfur battery comprising the sulfur-containing positive electrode according to any one of claims 4 to 7.
9. The lithium sulfur battery of claim 8, further comprising a separator having a coating of M-N-CNT.
CN202110251840.5A 2021-03-08 2021-03-08 Positive modified material M-N-CNT for lithium-sulfur battery, preparation method thereof and battery Pending CN112615008A (en)

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