Preparation method and application of cathode material of cathode-free lithium-sulfur battery

Abstract

The invention discloses a preparation method and application of a cathode material of a cathode-free lithium-sulfur battery, and belongs to the field of lithium-sulfur batteries. The preparation method comprises the following steps: putting phenanthroline, hydroxide, tellurium salt, tin salt, powdered sulfur and lithium sulfide into a closed device for grinding and mixing to obtain a precursor; and carrying out high-temperature calcination in an inert atmosphere to obtain the cathode material of the cathode-free lithium-sulfur battery. According to the invention, tellurium sulfide and tin disulfide are used as electrocatalysts to accelerate the oxidation-reduction reaction of lithium sulfide, so that the utilization rate of lithium sulfide is improved. More importantly, an insoluble intermediate can be formed in the conversion process of lithium sulfide by introducing tellurium and tin atoms, so that the shuttle effect problem is fundamentally solved. The preparation method is simple, the problem of shuttle effect of polysulfide can be effectively solved, and the electrochemical performance of the lithium-sulfur battery is further optimized.

Description

Technical Field

[0001] The present invention belongs to the field of lithium-sulfur batteries, and particularly relates to a method for preparing a cathode material for a lithium-sulfur battery without a negative electrode and its application. Background Art

[0002] With the development of energy storage technologies, lithium-sulfur batteries are increasingly regarded as a promising candidate for future energy storage solutions due to their cost-effectiveness, environmental friendliness, and excellent energy density. However, the lithium metal negative electrode currently used in lithium-sulfur batteries faces several challenges, which mainly include: First, the use of a lithium metal negative electrode may reduce the overall energy density of the battery because the lithium metal negative electrode itself is heavy and large in volume; Second, in order to ensure the stability and cycling performance of the battery during charge and discharge, an amount of lithium metal exceeding the actual demand is often required. This excessive use not only increases the material cost but also may pose safety hazards. Therefore, developing new negative electrode materials or improving existing technologies to enhance battery performance and reduce costs has become an important direction in lithium-sulfur battery research. The use of a lithium-sulfur battery with a non-negative electrode structure can not only provide an energy storage solution with high energy density and cost-effectiveness but also enhance the safety of the battery. This innovative lithium-sulfur battery technology without a negative electrode has the potential to become a breakthrough technology in the future energy storage field.

[0003] However, the electrochemical activity of Li2S as a cathode active material is not ideal, mainly because of its large particle size, poor electrical conductivity, and slow reaction kinetics. To address the above problems, a relatively effective method is to composite Li2S with carbon materials for use as a cathode material for lithium-sulfur batteries. For example, the Chinese patent document with the publication number CN117317131A discloses a flexible self-supporting cathode for a lithium-sulfur battery without a negative electrode and its preparation method. This technology directly integrates the active material Li2S on a conductive substrate formed by a polyacrylonitrile fiber membrane containing carbon nanotubes, which is beneficial to the electron and ion transfer of the active material. However, its electrochemical reaction still follows the "dissolution / deposition" mechanism, which leads to the dissolution of polysulfide (LiPS) intermediates in the electrolyte, thereby increasing the risk of the shuttle effect and resulting in poor battery performance. Summary of the Invention

[0004] The object of the present invention is to provide a method for preparing a cathode material for a lithium-sulfur battery without a negative electrode and its application. The preparation method is simple and adopts a solid-solid conversion strategy, which can effectively solve the problem of the shuttle effect of polysulfides, thereby optimizing the electrochemical performance of lithium-sulfur batteries.

[0005] To solve the above technical problems, the present invention provides the following technical solutions: The present invention provides a method for preparing a cathode material for a lithium-sulfur battery without a negative electrode, comprising the following steps: (1) Put phenanthroline, hydroxide, tellurium salt, tin salt, sulfur powder and lithium sulfide into a closed device for grinding and mixing to obtain a precursor; (2) Place the precursor obtained in step (1) in a calcination device and perform high-temperature calcination under an inert atmosphere to obtain a cathode material for a lithium-sulfur battery without a negative electrode.

[0006] Preferably, in step (1), the hydroxide is one of sodium hydroxide or potassium hydroxide, the tellurium salt is one of sodium tellurate or sodium tellurite, and the tin salt is one of sodium stannate, tin oxide or stannous chloride; the molar ratio of the tin salt to the tellurium salt, sulfur powder, phenanthroline, hydroxide and lithium sulfide is 1:1-1.5:3-5:4-6:6-10:8-15; the grinding speed is 600-1000 rpm and the grinding time is 12-48 h.

[0007] Preferably, in step (2), the inert atmosphere is nitrogen or argon, the heating rate during calcination is controlled at 2-5 °C / min, the high-temperature calcination temperature is 600-1000 °C, and the time is 2-6 h.

[0008] The present invention also provides the application of the prepared cathode material for a lithium-sulfur battery without a negative electrode to the cathode of a lithium-sulfur battery without a negative electrode.

[0009] The beneficial effects of the present invention are as follows: 1. The preparation method of the cathode material for a lithium-sulfur battery without a negative electrode provided by the present invention first mixes a variety of metal salts and organic ligands in a closed device for grinding to ensure that each component is fully and evenly mixed, improving the conductivity and electrochemical activity of the material; then the prepared precursor is placed in a calcination device and subjected to high-temperature calcination under an inert atmosphere to promote the phase change and crystallization of the material and form the required cathode material structure. The cathode material of the lithium-sulfur battery of the present invention can be used in a lithium-sulfur battery without a negative electrode. The design without a negative electrode avoids the growth of lithium dendrites caused by the use of a traditional lithium metal negative electrode, which helps to improve the safety and stability of the battery; the preparation method is simple and only requires two main steps of mixing and grinding the precursor and high-temperature calcination to obtain a cathode material with strong conductivity and stability.

[0010] 2. The cathode material for a lithium-sulfur battery without a negative electrode prepared by the method of the present invention introduces tellurium sulfide and tin disulfide as electrocatalysts, and realizes a synergistic effect with an appropriate proportion, significantly improving the battery performance. The synergistic effect of tellurium sulfide and tin disulfide increases the number of active sites, accelerates the redox reaction rate of lithium sulfide, and further improves the utilization rate of lithium sulfide and the charge-discharge efficiency of the lithium-sulfur battery; this synergistic effect not only enhances the chemisorption effect and effectively inhibits the peeling of intermediates, but also improves the overall conductivity of the sulfur electrode due to the good conductivity of tellurium sulfide and tin disulfide. More importantly, the introduction of tellurium and tin atoms can make lithium sulfide form Li that is insoluble in the electrolyte during the formation of intermediatesx Te y In (1-y) S z The formation of the intermediate (solid-solid transformation) avoids the dissolution and shuttling of the intermediate. This synergistic effect fundamentally solves the shuttling effect problem and provides an important guarantee for the stability and safety of lithium-sulfur batteries.

[0011] 3. In the preparation method of the cathode material of the anode-free lithium-sulfur battery provided by the present invention, carbon materials with a specific pore structure and high specific surface area are generated during the calcination process of o-phenanthroline at high temperature, providing a buffer space for the volume expansion of the cathode material, reducing the damage to the electrode structure, and at the same time increasing the loading of lithium sulfide and catalytic components, restricting the peeling of the intermediate, thereby improving the cycle stability of the battery. It should be noted that if the carbon-sulfur ratio is too low, it is not sufficient to enhance the electron conduction efficiency of the sulfur cathode or accommodate more active substances, while if the carbon-sulfur ratio is too high, the energy density of the battery will decrease due to the increase of non-active materials in the battery.

[0012] 4. The cathode material of the anode-free lithium-sulfur battery prepared by the method of the present invention shows outstanding performance in the application of lithium-sulfur batteries. At a charge-discharge rate of 0.2C, the initial specific capacity is as high as 1167.0 - 1179.8 mAh / g, and after 100 cycle tests, the capacity retention rate is 85.3% - 88.1%; while under the condition of 1C, the initial specific capacity is as high as 831.8 - 852.9 mAh / g, and the capacity retention rate is 78.5% - 85.9% in 300 cycle tests. Description of the Drawings

[0013] Figure 1 Scanning electron microscope (SEM) image of the cathode material (Li2S - TeS / SnS2) of the anode-free lithium-sulfur battery prepared in Example 1; Figure 2 X-ray diffraction (XRD) pattern of Li2S - TeS / SnS2 prepared in Example 1; Figure 3 Cycling stability of Li2S - TeS / SnS2 prepared in Example 1 at 0.2C; Figure 4 Cycling stability of Li2S - TeS / SnS2 prepared in Example 1 at 1C; Figure 5 Discharge curve of the material prepared with the publication number CN116936777A; Figure 6 Discharge curve of Li2S - TeS / SnS2 prepared in Example 1. Detailed Description of the Invention

[0014] The present invention is described in detail below with reference to the embodiments and comparative examples, but the present invention is not limited thereto. Example 1

[0015] (1) 1 mmol of tin chloride, o-phenanthroline, sodium hydroxide, sodium tellurate, sulfur powder and lithium sulfide are placed in a sealed tank for grinding and mixing to obtain a precursor; the molar ratio of tin chloride to sodium tellurate, sulfur powder, o-phenanthroline, sodium hydroxide and lithium sulfide is 1:1:5:6:7:15; the grinding speed is 600 rpm and the grinding time is 48 hours. ; The molar ratio of tin salt to tellurium salt, sulfur powder, o-phenanthroline, hydroxide and lithium sulfide is 1:1-1.5:3-5:4-6:6-10:8-15; the grinding speed is 600-1000 rpm and the grinding time is 12-48 hours.

[0016] (2) The precursor was loaded into a porcelain boat and placed in a calcination furnace. The temperature was raised to 1000°C at a heating rate of 5°C / min under a nitrogen atmosphere and kept at that temperature for 2 h to obtain the positive electrode material Li2S-TeS / SnS2 for a negative electrode-free lithium-sulfur battery. Example 2

[0017] (1) 2 mol of sodium stannate, 1,1-phenanthroline, potassium hydroxide, sodium tellurite, sulfur powder and lithium sulfide are placed in a sealed tank and ground and mixed to obtain a precursor; the molar ratio of sodium stannate to sodium tellurite, sulfur powder, 1,1-phenanthroline, potassium hydroxide and lithium sulfide is 1:1.5:3:4:6:8; the grinding speed is 800 rpm and the grinding time is 36 h.

[0018] (2) The precursor was loaded into a porcelain boat and placed in a calcination furnace. The temperature was raised to 600°C at a heating rate of 3°C / min under an argon atmosphere and kept at this temperature for 6 h to obtain the positive electrode material Li2S-TeS / SnS2 for a negative electrode-free lithium-sulfur battery. Example 3

[0019] (1) 5 mmol of tin oxide, 1,1-phenanthroline, potassium hydroxide, sodium tellurite, sulfur powder and lithium sulfide were placed in a sealed tank and ground and mixed to obtain a precursor; the molar ratio of tin oxide to sodium tellurite, sulfur powder, 1,1-phenanthroline, potassium hydroxide and lithium sulfide was 1:1.2:4:5:10:10; the grinding speed was 1000 rpm and the grinding time was 12 h.

[0020] (2) The precursor was loaded into a porcelain boat and placed in a calcination furnace. The temperature was raised to 800°C at a heating rate of 2°C / min under a nitrogen atmosphere and kept at that temperature for 4 h to obtain the positive electrode material Li2S-TeS / SnS2 for a negative electrode-free lithium-sulfur battery. Comparative Example 1

[0021] The molar ratio of tin salt to tellurium salt in this comparative example is 1:1-1.5, and the other steps are consistent with the process of Example 1. Comparative Example 2

[0022] In this comparative example, only tellurium salt was added, and the others were the same as those in Example 1. Comparative Example 3

[0023] In this comparative example, carbon black was added instead of o-phenanthroline, and the others were the same as those in Example 1. Comparative Example 4

[0024] In this comparative example, the molar ratio of sulfur powder to o-phenanthroline was 5:1, and the others were the same as those in Example 1. Comparative Example 5

[0025] The non-aqueous lithium-sulfur battery designed in the Chinese patent document with the publication number CN117317131A has an initial discharge capacity of over 800 mAh / g at 0.5C.

[0026] Performance test: The cathode material Li2S-TeS / SnS2 of the non-aqueous lithium-sulfur battery obtained above was mixed with N-methylpyrrolidone to form a uniform slurry, and uniformly coated on an aluminum foil as the cathode of the lithium-sulfur battery. Celgard 2300 was used as the separator and nickel foil as the anode, and assembled into a CR2032 coin cell with an electrolyte (a mixed solution of 1.0 mol / L LiTFSI and 0.2 mol / L LiNO3 in 1,3-dioxolane and 1,2-dimethoxyethane (w / w, 1 / 1)), and the E / Li2S ratio was controlled at 7 μL / mg. On a CT-4008-5A 6V system, the battery was cycled at a fixed potential range (1.7 - 2.8 V vs. Li + / Li). The test current densities were 0.2C and 1C, and the test results are shown in Table 1:

[0027] Figure 1 The SEM image of the cathode material of the non-aqueous lithium-sulfur battery prepared in Example 1 is shown. It can be clearly observed from the figure that the cathode material exhibits a distinct pore structure, which provides sufficient space for the loading of Li2S-TeS / SnS2, helping to reduce the material damage caused by volume expansion. In addition, the carbon material itself has excellent electrical conductivity, which helps to improve the electron transfer rate and enhance the charge-discharge efficiency of the battery. The porous characteristics of the carbon material effectively inhibit the peeling of intermediates, thus significantly improving the cycle stability of the battery. In addition, there is no obvious aggregation of Li2S-TeS / SnS2 on the material surface, indicating that Li2S-TeS / SnS2 is well distributed in the carbon host.

[0028] The cathode material obtained in Example 1 was tested by XRD, as Figure 2 shown. FromFigure 2 It can be observed that a broad peak appears at approximately 25°, which can be attributed to the diffraction of the (002) crystal plane of graphite carbon. In addition, the XRD patterns of SnS2 and Li2S confirm their pure phases and crystal structures. However, no signal of Te is detected in the XRD patterns, which may be because Te forms C-Te or Te-S bonds in the carbon material and thus does not exist in the form of an independent phase.

[0029] The prepared cathode material Li2S-TeS / SnS2 for the lithium-sulfur battery without a negative electrode was assembled into a lithium-sulfur battery in a glove box and subjected to electrochemical tests at a current density of 0.2C and 1C under the condition of 1.7 - 2.8V, as Figure 3 and Figure 4 shown. The Li2S-TeS / SnS2 prepared in Example 1 had an initial specific capacity as high as 1179.8 mAh / g at a charge-discharge rate of 0.2C. After 100 cycles of testing, the capacity retention rate was 88.1%; while at 1C, the initial specific capacity was 852.9 mAh / g, and the capacity retention rate reached 85.9% after 300 cycles, showing excellent charge-discharge performance and cycle stability. The same tests were conducted on the materials of Example 2 and Example 3, and the results were similar to those of Example 1 (Table 1). At a current density of 0.2C, the difference in the discharge specific capacity of the samples in each example did not exceed 13 mAh / g, and the difference in the capacity retention rate did not exceed 2.8%; while at a current density of 1C, the difference in the discharge specific capacity of the samples in each example did not exceed 22 mAh / g, and the difference in the capacity retention rate did not exceed 7.4%.

[0030] For Comparative Example 1, the introduction of the two electrocatalysts did not show excellent performance but was worse than that of a single electrocatalyst (Comparative Example 2), which reveals that the uncoordinated ratio of TeS and SnS2 seriously affects the synergistic effect between them. Further, for Comparative Example 2, since its cathode material contains only a single catalyst, the electrochemical oxidation-reduction reaction rate of lithium sulfide is insufficient, and the chemisorption ability is also weak. This situation not only leads to the loss of lithium sulfide but also has an obvious negative impact on the cycle reversibility and capacity retention of the battery. On the contrary, the examples show excellent performance. This is because the dual electrocatalyst composed of TeS and SnS2 increases the number of active sites through synergistic effects, accelerates the oxidation-reduction reaction rate of lithium sulfide, and thus improves the utilization rate of lithium sulfide and the charge-discharge efficiency of the lithium-sulfur battery; this synergistic effect not only enhances the chemisorption effect and effectively inhibits the peeling of intermediates but also improves the overall conductivity of the sulfur electrode due to the good conductivity of tellurium sulfide and tin disulfide; more importantly, the introduction of tellurium and tin atoms promotes the formation of Li x Te y In (1-y) S zThe generation of such intermediates, thus avoiding the dissolution and shuttling of intermediates. This synergistic effect fundamentally solves the shuttling effect problem. The above improvements significantly enhance the charge-discharge capacity and cycle stability of the battery. Specific data are shown in Table 1.

[0031] Compared with Comparative Example 3, after calcination treatment, the phenanthroline in this example can form a carbonaceous material with a specific pore structure and high specific surface area. This material provides an effective buffer space for the volume expansion of the battery during charge and discharge, effectively reducing the damage to the electrode structure. In addition, the high specific surface area of this carbon material helps to increase the loading amount of lithium sulfide and catalytic components, thereby enhancing the cycle stability of the battery. However, it should be noted that insufficient dosage of phenanthroline will lead to too low carbon-sulfur ratio, which not only cannot effectively enhance the electron conduction efficiency of the sulfur cathode, but also cannot provide enough support space for more active substances, resulting in the occurrence of the shuttling effect (Comparative Example 4).

[0032] Compared with Comparative Example 5, the lithium-sulfur battery cathode designed in the present invention shows outstanding advantages in performance. This significant improvement is mainly due to the innovative introduction of TeS and SnS2 in the present invention, which makes the electrochemical reaction inside the battery more efficient and rapid, and effectively improves the utilization rate of lithium sulfide.

[0033] Figure 6 The discharge curve of Li2S-TeS / SnS2 in shows a standard single-slope curve, because the introduction of central tellurium and tin atom connectors ensures Li x Te y In (1-y) S z The formation of intermediates eliminates the formation of long-chain polysulfides, thus forming a "solid-solid" phase reaction mechanism. This method can fundamentally solve the problem of the "solid-liquid-solid" phase redox reaction mechanism.

[0034] To verify the "solid-solid" phase reaction mechanism proposed in the present invention, we made a distinction based on the characteristics of the discharge curve in electrochemical behavior: the traditional "solid-liquid-solid" phase reaction mechanism is manifested as a curve with two obvious fluctuation segments, while in contrast, the "solid-solid" phase reaction mechanism shows a smooth and monotonous slope curve. For this purpose, we compared and analyzed the discharge curve of the present invention with the research with the publication number CN116936777A. As Figure 5As shown, in this comparative study, solid sulfur first undergoes a reduction reaction to form highly soluble long-chain Li2S8, and then Li2S8 is further reduced to soluble Li2S6 / Li2S4. This process belongs to the category of liquid-liquid single-phase reactions. Finally, soluble Li2S4 is further reduced to insoluble Li2S2 and Li2S, and this process exhibits typical two fluctuating curves, namely the characteristics of the "solid-liquid-solid" phase reaction mechanism. However, in Figure 6 In the Li2S-TeS / SnS2 discharge curve shown, a single-slope curve was observed. This phenomenon is attributed to the introduction of central tellurium and tin atom connectors, which ensure the formation of Li x Te y In (1-y) S z intermediates, thus effectively inhibiting the formation of long-chain polysulfide lithium and prompting the reaction to follow the "solid-solid" phase reaction mechanism. This method provides a new approach to solving the fundamental problems in the "solid-liquid-solid" phase redox reaction mechanism.

[0035] The specific embodiments described above further elaborate on the present invention, but these descriptions should not be construed as limitations on the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a cathode material for a lithium-sulfur battery without an anode, characterized in that: The steps include: (1) placing o-phenanthroline, hydroxide, tellurium salt, tin salt, sulfur powder and lithium sulfide in a closed device for grinding and mixing to obtain a precursor; (2) placing the precursor obtained in step (1) in a calcination device and calcining it at a high temperature under an inert atmosphere to obtain a positive electrode material for a negative electrode-free lithium-sulfur battery.

2. The method for preparing the cathode material of a non-negative electrode lithium-sulfur battery according to claim 1, characterized in that: In the step (1), the hydroxide is sodium hydroxide or potassium hydroxide, the tellurium salt is one of sodium tellurate and sodium tellurite, and the tin salt is one of sodium stannate, tin oxide, and tin chloride; the molar ratio of the tin salt to the tellurium salt, sulfur powder, o-phenanthroline, hydroxide, and lithium sulfide is 1:1-1.5:3-5:4-6:6-10:8-15; the grinding speed is 600-1000 rpm, and the grinding time is 12-48 hours.

3. The method for preparing the cathode material of a non-negative electrode lithium-sulfur battery according to claim 1, characterized in that: In the step (2), the inert atmosphere is nitrogen or argon, the high temperature calcination is controlled at a heating rate of 2 to 5°C / min, a temperature of 600 to 1000°C, and a time of 2 to 6 hours.

4. An application of a cathode material for a non-negative-electrode lithium-sulfur battery prepared by the preparation method according to any one of claims 1 to 3, characterized in that: The cathode material of lithium-sulfur battery without negative electrode is used for the cathode of lithium-sulfur battery.

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