A three-dimensional elastic conductive positive electrode frame structure material for all-solid-state lithium-sulfur batteries and its preparation method

Abstract

A three-dimensional elastic conductive positive electrode framework structure material for an all-solid-state lithium-sulfur battery and a preparation method thereof belong to the technical field of all-solid-state lithium-sulfur batteries. The present invention uses natural linear polymer sodium alginate, two-dimensional nano-highly conductive material MXene, Li6PS5Cl and sulfur powder to form a strong and elastic three-dimensional cross-linked network in the positive electrode by ball milling. This elastic three-dimensional network can effectively bear the stress generated during the cycle and alleviate the volume change caused by the reaction. In addition, by reducing the ball milling time and completely abandoning the traditional conductive auxiliary carbon material, the adverse effects of sulfide electrolyte decomposition on cycle performance are effectively avoided. Therefore, the present invention can achieve excellent electron and ion transmission performance, while avoiding the effects of electrolyte decomposition on the increase of electrode impedance and the deterioration of cycle performance, and achieves long-term and stable cycle performance under the condition of 40% sulfur mass content.

Description

Technical Field

[0001] The present invention belongs to the technical field of all-solid-state lithium-sulfur batteries, and specifically relates to a three-dimensional elastic conductive positive electrode frame structure material for an all-solid-state lithium-sulfur battery and a preparation method thereof. Background Art

[0002] With the rapid development of electronic devices, people's requirements for battery energy density are constantly increasing. Lithium-sulfur batteries have a high theoretical specific capacity (1675 mAh g -1 ), abundant reserves, and environmental friendliness are considered potential candidates for the next generation of energy storage solutions. However, the organic electrolytes used in traditional lithium-sulfur batteries have many challenges, such as poor cycle life caused by the shuttle effect, rapid capacity decay, and safety issues such as puncturing the diaphragm due to the growth of dendrites of the metal lithium negative electrode, causing short circuits. In recent years, the rapid development of inorganic sulfide solid electrolytes has led to the development of a new type of electrolyte with high ionic conductivity (10 -2 ~10 -3 S cm -1 ), good mechanical properties and high safety performance have attracted widespread attention. All-solid-state lithium-sulfur batteries are expected to eliminate many of the problems caused by liquid electrolytes while achieving high energy density and long cycle life.

[0003] Although all-solid-state lithium-sulfur batteries have the ability to fundamentally avoid the shuttle effect and improve battery safety, there are still some challenges. + +16e - =8Li2S, a huge volume change (~80%) will occur during the lithiation process of sulfur. This huge volume change will disconnect the previously constructed ion / electron pathways, making some sulfur unable to undergo redox reactions and becoming isolated "dead sulfur". At the same time, the continuous accumulation of stress during the cycle causes the electrode structure to be destroyed, which has an adverse effect on capacity and cycle stability. In addition, due to the electronic / ionic insulating properties of sulfur, a large amount of conductive additives (electrolytes, electronic conductor carbon) are usually required in the positive electrode. These large amounts of inactive substances make the sulfur content in the positive electrode generally less than 40%, reducing the energy density of the battery. It has also been proven that the large amount of highly conductive carbon materials added to the positive electrode promotes the decomposition of the electrolyte. The decomposition of the electrolyte provides additional first-cycle capacity and produces low-conductivity insulating substances. As the cycle accumulates, it causes a large interfacial resistance, which makes the cycle performance worse.

[0004] Previously, people have explored many strategies to construct a reasonable cathode structure, increase contact, improve sulfur conversion kinetics, alleviate the huge volume change of sulfur, and stabilize the cathode structure. However, the low energy density caused by the excessively high content of inactive substances limits their application. At the same time, because the pre-prepared structure is rigid, the stress accumulated during the reaction cannot be released, and there is still the possibility of destroying the pre-prepared electron / ion pathway. Taking these factors into consideration, it is very important to develop a cathode structure that is elastic, can continuously accommodate the stress during the reaction, and ensure sufficient charge transfer. Summary of the Invention

[0005] The present invention aims to provide a three-dimensional elastic conductive cathode framework material for all-solid-state lithium-sulfur batteries and its preparation method. This invention utilizes ball milling to construct a three-dimensional cross-linked elastic conductive cathode, primarily addressing the issues of large volume changes, structural instability, and slow reaction speed in solid-state lithium-sulfur battery cathodes.

[0006] In the present invention, we use natural linear polymer sodium alginate (SA), two-dimensional nano-high conductive material MXene (Ti3AlC2), Li6PS5Cl and sulfur powder (S8) to form a strong and elastic three-dimensional cross-linked network in the positive electrode by ball milling. The high heat and high shear force brought by ball milling can gradually melt the large SA particles into long fibers and construct a three-dimensional elastic network between the layered MXene material, Li6PS5Cl and sulfur powder. This elastic three-dimensional network can effectively bear the stress generated during the cycle and alleviate the volume change caused by the reaction. The results of the in-situ pressure test show that after the three-dimensional elastic network is constructed, the stress change in the sulfur positive electrode is significantly reduced by 47%. In addition, by reducing the ball milling time and completely abandoning the traditional conductive additive carbon material, we effectively avoid the adverse effects of sulfide electrolyte decomposition on cycle performance. Therefore, the composite sulfur positive electrode with a carefully designed three-dimensional elastic network can achieve excellent electron and ion transport performance, while avoiding the effect of electrolyte decomposition on the increase of electrode impedance and the deterioration of cycle performance, and achieves long-term and stable cycle performance under the condition of 40% sulfur mass content (corresponding to Figure 5 ).

[0007] The present invention provides a method for preparing a three-dimensional elastic conductive positive electrode framework structure material for an all-solid-state lithium-sulfur battery, wherein the positive electrode material is obtained by two-step ball milling, and the specific steps are as follows:

[0008] (1) Weigh 0.05-0.2 g of sulfur powder, MXene (Ti3AlC2), and sodium alginate (SA) in a mass ratio of 35-45: 15-25: 0.5-2 and place them in a stainless steel ball milling jar. Add stainless steel balls, seal the jar, and transfer it to a ball mill for ball milling.

[0009] (2) Under argon protection, 0.05-0.2 g of the material obtained after ball milling in step (1) and Li6PS5Cl electrolyte were weighed in a mass ratio of 6:3-5, placed in a stainless steel ball milling jar, added with stainless steel balls, sealed, and then transferred to a ball mill for ball milling to obtain a three-dimensional elastic conductive positive electrode framework structure material for all-solid-state lithium-sulfur batteries.

[0010] Furthermore, in step (1) and step (2), the ball-to-material ratio is 15-25:1;

[0011] Furthermore, in step (1), the ball milling speed is 400-600 rpm, and the ball milling time is 5-10 hours;

[0012] Furthermore, in step (2), the ball milling speed is 400-600 rpm, and the ball milling time is 1-3 h.

[0013] The three-dimensional elastic conductive positive electrode frame structure material of an all-solid-state lithium-sulfur battery described in the present invention is prepared by the above-mentioned preparation method.

[0014] This study developed a novel framework material for the positive electrode of an all-solid-state lithium-sulfur battery. This framework was successfully used to load the active sulfur and solid-state sulfide electrolyte. This method demonstrates a degree of universality. The cycling stability of the all-solid-state lithium-sulfur battery constructed with this elastic conductive framework was significantly improved.

[0015] The beneficial effects of the present invention are:

[0016] 1) Developed a novel cathode framework material for solid-state lithium-sulfur batteries. This cathode framework material, constructed through a two-step ball milling process, exhibits both elasticity and conductivity. It can serve as a carrier for elemental sulfur and solid sulfide electrolytes. By constructing an elastic framework and controlling the ball milling time, the sulfur cathode structure and composition are stabilized.

[0017] 2) The main reason for the instability of the positive electrode of solid-state lithium-sulfur batteries is that the sulfur conversion reaction process is accompanied by huge volume changes. As the charging and discharging process continues, the stress and strain caused by the volume change continue to accumulate and cannot be released. Finally, the stress causes the positive electrode structure to be destroyed, the electron and ion pathways to be disconnected, and the cycle stability is destroyed. Therefore, the stability and practical application of solid-state lithium-sulfur batteries are largely restricted and affected by the stability of the sulfur positive electrode structure. The positive electrode constructed by the present invention forms a fibrous elastic network during the ball milling process of polymer SA, MXene and Li6PS5Cl, which can buffer the stress and strain caused by the volume change of sulfur. Through the in-situ pressure testing device, it was found that the elastic network effectively reduced the pressure change by 47%, thereby maintaining the structural stability of the positive electrode, thereby achieving long-cycle stability of the battery;

[0018] 3) By introducing a small amount of highly conductive layered MXene material, the use of a large amount of conductive carbon material is avoided, which leads to a reduction in the sulfur content in the positive electrode and achieves a high sulfur mass content of 40% in the positive electrode. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 X-ray diffraction (XRD) patterns of the positive electrode materials prepared in Example 1 and Example 2;

[0020] Figure 2 Surface electron micrographs of the positive electrode materials prepared in Example 1 and Example 2;

[0021] Figure 3 Fourier transform infrared spectra (FTIR) of the cathode materials prepared in Example 1 and Example 2;

[0022] Figure 4 The first cycle charge and discharge curves of the positive electrode materials prepared in Example 1 and Example 2;

[0023] Figure 5 Long cycle performance curves of the positive electrode materials prepared in Example 1 and Example 2;

[0024] Figure 6 The in-situ pressure change diagram of the positive electrode materials prepared in Example 1 and Example 2. DETAILED DESCRIPTION

[0025] Example 1:

[0026] The positive electrode material is obtained by two-step ball milling. The specific steps are as follows:

[0027] (1) S8, MXene (Ti3AlC2) and sodium alginate (SA) were weighed in a mass ratio of 40:19:1 and placed in a stainless steel ball mill. Stainless steel balls were added to achieve a ball-to-material ratio of 20:1. The ball mill was sealed and transferred to a ball mill. The milling speed was set at 510 rpm and the milling time was 8 h.

[0028] (2) Under argon protection in a glove box, 0.1 g of the material obtained after ball milling in step (1) and Li6PS5Cl electrolyte were weighed in a mass ratio of 6:4, placed in a stainless steel ball milling jar, and stainless steel balls were added. The ball-to-material ratio was 20:1; the ball milling jar was sealed and transferred to a ball mill. The ball milling speed was 510 rpm and the ball milling time was 2 h. After the ball milling was completed, a three-dimensional elastic conductive positive electrode framework structure material for an all-solid-state lithium-sulfur battery was obtained.

[0029] The specific assembly steps of all-solid-state lithium-sulfur batteries are as follows:

[0030] (1) Weigh 80 mg of Li6PS5Cl electrolyte powder into a solid-state battery mold with an inner diameter of 10 mm. Gently shake the powder to evenly spread it, and then apply a pressure of 240 MPa to form a dense electrolyte sheet with a thickness of about 600 μm.

[0031] (2) Weigh 3 mg of cathode framework material powder and evenly spread it on one end of the electrolyte sheet. Apply a pressure of 480 MPa to make it fit tightly on the electrolyte sheet.

[0032] (3) Place a lithium-indium alloy with a diameter of 8 mm at the other end of the electrolyte sheet, ensuring that the positive electrode frame structure material and the lithium-indium alloy do not contact each other, and apply a pressure of 240 MPa to make it fully contact with the electrolyte sheet.

[0033] Preparation of lithium-indium alloy: Apply pressure to the lithium sheet to cut and thin it, and then apply pressure to the indium sheet on both sides to form a metal sheet. Use an 8 mm punch to punch holes to prepare the lithium-indium alloy.

[0034] (4) Place the battery mold in a pressurized fixture, apply a pressure of 240 MPa and fix the pressure with screws, so that the battery can be electrochemically tested while maintaining a certain pressure. The preparation of the above materials and battery assembly need to be carried out under the protection of an argon atmosphere.

[0035] Example 2:

[0036] The positive electrode material is obtained by two-step ball milling. The specific steps are as follows:

[0037] In a glove box, 0.1 g of S8 and Super P were weighed together in a 4:2 mass ratio and placed in a stainless steel ball mill, resulting in a 20:1 ball-to-material ratio. The milling speed was 510 rpm for 8 hours. In the glove box, under argon protection, 0.1 g of the resulting material and Li6PS5Cl electrolyte were ball milled together in a 6:4 mass ratio in a stainless steel ball mill, using a 20:1 ball-to-material ratio at 510 rpm for 2 hours. This resulted in the cathode material.

[0038] The all-solid-state battery assembly process is the same as that in Example 1.

[0039] like Figure 1 As shown in the figure, it can be seen from the XRD patterns that the introduction of polymer SA and MXene does not have much influence on the crystal structure of the active material sulfur and Li6PS5Cl electrolyte. Both cathode materials show the diffraction peaks of S8 and Li6PS5Cl electrolyte.

[0040] like Figure 2As shown, Figure (a) and Figure (b) correspond to Example 1 and Example 2, respectively. From the surface electron microscope image, it can be seen that both positive electrode materials form a three-dimensional elastic network structure;

[0041] like Figure 3 As shown, compared with Example 2, Example 1 has a -1 The vibration peak corresponding to OH at 0 shifts to a lower wavenumber. This is because the OH groups in SA interact with the electronegative functional groups (-F, -O) on the MXene surface to form hydrogen bonds, thereby reducing the energy of OH. This result shows that the polymer SA and MXene achieve a close composite by forming hydrogen bonds, which is conducive to the formation of an elastic network.

[0042] like Figure 4 As shown in Figure 1, the cathode material prepared in Example 1 achieved a capacity of 750.6 mAh g at 0.1 C. -1 The specific capacity of the positive electrode material prepared in Example 2 is lower, and the polarization voltage is larger;

[0043] like Figure 5 As shown in Figure 1, the capacity of the positive electrode material prepared in Example 1 increased to 603 mAh g during the activation process of 100 cycles. -1 (corresponding to the data on the left vertical axis); during the activation process of 500 cycles, the capacity was maintained at 630 mAh g -1 , while the coulombic efficiency is basically maintained at 100% (corresponding to the data on the right vertical axis); the specific capacity of the positive electrode material prepared in Example 2 is very low, and the coulombic efficiency is also unstable during the cycle;

[0044] like Figure 6 As shown, Figure (a) is a curve of pressure change over time, and Figure (b) is a bar graph of pressure change; by comparing the pressure change amplitudes of the two positive electrode materials of Example 1 and Example 2 during the cycle, it can be seen that the maximum pressure change of the positive electrode material prepared in Example 2 with a rigid structure is 0.17 kPa, while the pressure change of the positive electrode material prepared in Example 1 is only 0.09 kPa, and the pressure change of the positive electrode material prepared in Example 1 is reduced by 47% compared with that in Example 2; this significant suppression of pressure change indicates the key role of the elastic cross-linked network in buffering stress changes and stabilizing the positive electrode structure.

Claims

1. A method for preparing a three-dimensional elastic conductive positive electrode framework structure material for an all-solid-state lithium-sulfur battery, characterized by: The steps are as follows: (1) Weigh 0.05-0.2 g of sulfur powder, MXene, and sodium alginate in a mass ratio of 35-45: 15-25: 0.5-2 and place them in a stainless steel ball milling jar. Add stainless steel balls, seal the jar, and transfer it to a ball mill for ball milling. (2) Under argon protection, 0.05-0.2 g of the material obtained after ball milling in step (1) and Li6PS5Cl electrolyte were weighed in a mass ratio of 6:3-5 and placed in a stainless steel ball milling jar. After adding stainless steel balls and sealing, the jar was transferred to a ball mill for ball milling to obtain a three-dimensional elastic conductive positive electrode framework structure material for all-solid-state lithium-sulfur batteries.

2. The method for preparing a three-dimensional elastic conductive positive electrode framework structure material for an all-solid-state lithium-sulfur battery according to claim 1, characterized in that: In step (1) and step (2), the ball-to-material ratio is 15-25:

1.

3. The method for preparing a three-dimensional elastic conductive positive electrode framework structure material for an all-solid-state lithium-sulfur battery according to claim 1, characterized in that: In step (1), the ball milling speed is 400-600 rpm, and the ball milling time is 5-10 hours.

4. The method for preparing a three-dimensional elastic conductive positive electrode framework structure material for an all-solid-state lithium-sulfur battery according to claim 1, characterized in that: In step (2), the ball milling speed is 400-600 rpm, and the ball milling time is 1-3 h.

5. A three-dimensional elastic conductive positive electrode framework structure material for an all-solid-state lithium-sulfur battery, characterized by: The invention is prepared by the preparation method according to any one of claims 1 to 4.

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