Lithium supplementation materials and their preparation methods, positive electrode sheets and secondary batteries

Abstract

A lithium replenishment material, its preparation method, a positive electrode, and a secondary battery are disclosed. The lithium replenishment material includes a core containing a lithium-sulfur compound and an encapsulation layer covering the outer surface of the core. The encapsulation layer has channels, through which the core communicates with the external space, wherein 2 ≤ n ≤ 8. This application addresses the lithium replenishment problem by providing appropriately sized channels in the encapsulation layer, allowing a suitable amount of high-valence lithium polysulfides generated from the lithium sulfide reaction to enter the electrolyte through these channels and further diffuse to the negative electrode, reacting with lithium deposited at the negative electrode.

Description

Technical Field

[0001] This application relates to the field of secondary battery technology, specifically to a lithium replenishment material and its preparation method, a positive electrode sheet, and a secondary battery. Background Technology

[0002] With the development of new energy vehicles, the demand for energy density in lithium-ion batteries is increasing. Adding lithium-replenishing materials to the positive electrode is an effective way to improve energy density. However, because the addition of these materials increases the amount of active lithium in the battery, the risk of lithium plating may be greater. Therefore, how to improve energy density while minimizing lithium plating has become a crucial issue. Summary of the Invention

[0003] The purpose of this application is to provide a lithium supplement material and its preparation method, a positive electrode sheet, and a secondary battery.

[0004] This application provides the following technical solution:

[0005] In a first aspect, this application provides a lithium supplement material, comprising a core containing a lithium-sulfur compound and an encapsulation layer covering the outer surface of the core. The encapsulation layer has channels through which the core communicates with the external space. The channels are used to allow lithium polysulfide (Li2S) to pass through them. n The formula is as follows: 2 ≤ n ≤ 8. Specifically, the core is an amorphous lithium-containing compound, which has the advantages of lower polarization and a lower voltage plateau during charging. This effectively reduces the activation barrier of the lithium replenishment material and significantly improves the lithium-ion insertion / extraction efficiency, thereby achieving a higher specific capacity for lithium replenishment. The encapsulation layer not only improves the stability of the lithium replenishment material but also effectively improves the electronic and ionic conductivity of the lithium-containing compound in the core, increasing lithium extraction during charging. It also provides some isolation from water and oxygen, further enhancing the stability of the lithium replenishment material and achieving a stable lithium replenishment effect. Furthermore, when using a lithium-sulfur compound as the core, lithium polysulfides are formed in the core after lithium ions are extracted. By designing channels, a suitable amount of lithium polysulfides can also be extracted to the outside of the lithium replenishment material, dissolve in the electrolyte, diffuse to the negative electrode, and react with the metallic lithium deposited at the negative electrode, thereby alleviating or eliminating the lithium plating problem in lithium batteries.

[0006] Reaction equation: mLi2S n +2(nm)Li=n Li2S m (n>m)

[0007] Li2S n Li2S is a high-priced intermediate product formed during the charging process. m It is a low-valence intermediate product that is produced after the high-valence intermediate product migrates to the negative electrode and reacts with lithium metal, where n>m.

[0008] In one possible implementation, the core formula of the lithium-sulfur compound includes Li x S y , where 1≤x≤2, 1≤y≤6.

[0009] In one possible implementation, the lithium-sulfur compound includes at least one of Li2S, Li2S2, Li2S4, and Li2S6.

[0010] In one possible implementation, the encapsulation layer includes at least one of an isolation encapsulation layer, an ion conductor encapsulation layer, and an electronic conductor encapsulation layer. These encapsulation layers can effectively improve the electronic and ion conductivity of the lithium-replenishing material in the core, promoting lithium extraction during charging; they can also provide some moisture isolation, improving the stability of the positive electrode lithium-replenishing additive and achieving a stable lithium-replenishing effect. Furthermore, they can ensure the stability, uniform dispersion, and good processing performance of the positive electrode lithium-replenishing additive in the electrode active slurry and active layer.

[0011] In one possible implementation, when the encapsulation layer includes an isolation encapsulation layer, the material of the isolation encapsulation layer includes at least one of ceramic, polymer, or carbon material.

[0012] In one possible implementation, when the encapsulation layer includes the ion conductor encapsulation layer, the material of the ion conductor encapsulation layer may include at least one of perovskite, NASICON, garnet, or polymer solid electrolytes.

[0013] In one possible implementation, when the encapsulation layer includes the electronic conductor encapsulation layer, the material of the electronic conductor encapsulation layer includes at least one of carbon material, conductive polymer, or conductive oxide.

[0014] In one possible implementation, the cross-sectional profile of the channel is circular, with a diameter d of 0.5 nm ≤ d ≤ 5 nm. Specifically, the encapsulation layer may include an inner surface and an outer surface, wherein the inner surface is connected to the core, and the outer surface faces away from the inner surface and is exposed to the external space. The channel extends from the inner surface in a meandering or straight line, and the internal shape of the channel can be cylindrical, i.e., the cross-sectional profile of the channel is circular. Of course, since the channel is a micro-nano structure on the lithium-supplementing material, due to different fabrication processes, the cross-section of the channel can also be other shapes, such as irregular polygons or curves, etc., without specific limitations. When the cross-section of the channel is circular, its diameter d is 0.5 nm-5 nm. The advantage of designing the channel diameter within this range is that it ensures that an appropriate amount of lithium polysulfide can pass through while maintaining the stability of lithium-sulfur compounds in air. Understandably, while lithium polysulfide can pass through, external moisture may also enter the interior through the channel, thereby affecting the structural stability of the lithium-supplementing material, so it is necessary to reasonably control the channel size. When the pore diameter is smaller than the aforementioned range, the extraction efficiency of lithium polysulfides is too low, failing to effectively eliminate lithium plating. When the pore diameter is larger than the aforementioned range, water vapor in the air can more easily enter the core through the pores, reacting with the lithium sulfides in the core. Since lithium sulfides are extremely sensitive to water, this generates hydrogen sulfide gas, thus negating the protective function of the encapsulation layer. Simultaneously, excessively large pores allow excess lithium polysulfides to migrate from the interior into the electrolyte and further to the negative electrode. The loss of a large amount of incompletely delithiated lithium polysulfides to the negative electrode results in a very low specific capacity of lithium sulfides as a lithium replenishment material, rendering it ineffective. Therefore, only a suitable pore size can simultaneously achieve the dual functions of lithium replenishment and prevention of lithium plating at the negative electrode while ensuring the interfacial stability of lithium sulfides.

[0015] In one possible implementation, the channel is a straight channel penetrating the encapsulation layer. The advantage of a straight channel is that the extraction path of lithium polysulfides is the shortest, resulting in the highest extraction efficiency.

[0016] In one possible implementation, the volume of the channel accounts for 0.01%-5% of the volume of the encapsulation layer. Understandably, the volume of the channel refers to the space occupied by the channel on the encapsulation layer. Maintaining a suitable volume ratio for the channel can prevent excessive or overly dense channels from entering the core through them.

[0017] In one possible implementation, the core particle size is 10 nm-2 μm. Understandably, smaller core particle sizes result in a larger active specific surface area, which is beneficial for lithium-ion insertion and extraction. When the core particle size is smaller than this range, the overall size of the lithium replenishment material is easily reduced, leading to significant particle agglomeration. When the core particle size is larger than this range, the specific surface area of ​​the core decreases, reducing the lithium-ion insertion and extraction efficiency, thus resulting in poor lithium replenishment performance.

[0018] In one possible implementation, the thickness of the encapsulation layer is 3nm-50nm. Understandably, the thickness of the encapsulation layer simultaneously ensures both the specific capacity and electronic conductivity environment of the lithium replenishment material. When the encapsulation layer thickness is less than the aforementioned range, the encapsulation layer does not completely cover the core, which is detrimental to building a good electronic conductivity environment; when the encapsulation layer thickness is greater than the aforementioned range, since the encapsulation layer does not contribute lithium ions, it will reduce the overall specific capacity of the lithium replenishment material.

[0019] In one possible implementation, the encapsulation layer accounts for 1%-10% of the mass of the lithium replenishment material. In the lithium replenishment material of this application embodiment, the mass ratio of the encapsulation layer and the core simultaneously ensures the added specific capacity and electronic conductivity environment. If the encapsulation layer content is too high, the overall specific capacity of the lithium replenishment additive will be reduced because the encapsulation material does not contribute lithium ions; if the encapsulation layer content is too low, it is not conducive to forming a complete and uniformly thick encapsulation layer on the outer surface of the core, resulting in incomplete coverage and hindering the construction of a good electronic conductivity environment.

[0020] Secondly, this application also provides a method for preparing a lithium replenishing material, comprising: providing a first lithium replenishing material, the first lithium replenishing material comprising a core containing a lithium-sulfur compound and a first encapsulation layer covering the outer surface of the core; performing a pore-forming treatment on the first lithium replenishing material to obtain a second lithium replenishing material, the second lithium replenishing material comprising a core containing a lithium-sulfur compound and a second encapsulation layer covering the outer surface of the core, the second encapsulation layer having channels, and the core communicating with the external space through the channels.

[0021] Thirdly, this application also provides a cathode material, including a cathode active material and a lithium supplementation material as described in any one of the first aspects.

[0022] Fourthly, this application also provides a positive electrode sheet, comprising the positive electrode material described in the third aspect.

[0023] Fifthly, this application also provides a secondary battery, including the positive electrode sheet described in the fourth aspect. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0025] Figure 1 This is a schematic cross-sectional view of the lithium replenishment material in one embodiment;

[0026] Figure 2 This is a cross-sectional schematic diagram of a lithium replenishment material including an isolation encapsulation layer and an ion conductor encapsulation layer in one embodiment;

[0027] Figure 3 This is a cross-sectional schematic diagram of a lithium replenishment material including an isolation encapsulation layer and an electronic conductor encapsulation layer in one embodiment;

[0028] Figure 4 This is a cross-sectional schematic diagram of a lithium replenishment material comprising an isolation encapsulation layer, an electronic conductor encapsulation layer, and an ion conductor encapsulation layer in one embodiment.

[0029] Figure 5 This is a flowchart of a method for preparing lithium-supplementing materials in one embodiment. Detailed Implementation

[0030] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0031] It should be noted that when a component is said to be "fixed" to another component, it can be directly on the other component or it can be in a middle component. When a component is said to be "connected" to another component, it can be directly connected to the other component or it may be in a middle component.

[0032] Unless otherwise defined, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in this application's specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The term "and / or" as used in this application includes any and all combinations of one or more of the associated listed items.

[0033] The following detailed description of some embodiments of this application is provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0034] Firstly, this application provides a lithium supplementation material, please refer to... Figure 1 The lithium supplement material includes a core 10 containing a lithium sulfur compound and an encapsulation layer 20 covering the outer surface of the core 10. The encapsulation layer 20 has channels 30, through which the core 10 communicates with the external space. The channels 30 are used to allow lithium polysulfide (Li2S) to pass through. n Pass, where 2≤n≤8.

[0035] Specifically, the core 10 is an amorphous lithium-containing compound, which has the advantages of lower polarization and a lower voltage plateau during charging. This effectively reduces the activation barrier of the lithium replenishment material and significantly improves the lithium-ion insertion / extraction efficiency, thereby achieving a higher specific capacity for lithium replenishment. The encapsulation layer 20 not only improves the stability of the lithium replenishment material but also effectively enhances the electronic and ionic conductivity of the lithium-containing compound in the core 10, increasing lithium extraction during charging. It also provides some protection against water and oxygen, further improving the stability of the lithium replenishment material and achieving a stable lithium replenishment effect.

[0036] In one possible implementation, the core 10 of the lithium-sulfur compound comprises the lithium-sulfur compound Li x S y Where 1≤x≤2, 1≤y≤6. Core 10 is the lithium source core used for lithium replenishment. Thus, the lithium source core is rich in lithium, ensuring that the lithium replenishment additive in this embodiment can provide abundant lithium. Added to the electrode as an additive, it acts as a "sacrificial agent" during the first charging cycle, releasing as many lithium ions as possible at once to replenish the irreversible lithium ions consumed in the formation of the SEI film at the negative electrode. This maintains a sufficient supply of lithium ions in the battery system, improving the battery's first-cycle efficiency and overall electrochemical performance. Compared to other lithium replenishment materials, lithium replenishment materials made from lithium-sulfur compounds have weaker binding forces between lithium ions and sulfur ions, lower polarization, and a lower voltage plateau required during charging. Therefore, they can effectively reduce the activation barrier of lithium-sulfur compounds, significantly improving the lithium ion insertion / extraction efficiency, resulting in a higher specific capacity for lithium replenishment. Furthermore, lithium-sulfur compounds do not produce oxygen during lithium replenishment, reducing the risk of gas formation in lithium batteries. Meanwhile, the lithium polysulfides formed after the lithium-containing sulfur compounds are delithiated can diffuse to the negative electrode to alleviate the lithium plating problem at the negative electrode.

[0037] In one possible implementation, the lithium-containing sulfur compound includes at least one of Li₂S, Li₂S₂, Li₂S₄, and Li₂S₆. The reaction equation between the high-valence lithium polysulfides (Li₂S₈, Li₂S₆) generated after delithiation of these lithium-containing compounds and the lithium metal deposited at the negative electrode is as follows:

[0038] Li2S8+2Li=2Li2S4, 2Li2S6+2Li=3Li2S4

[0039] In one possible implementation, the encapsulation layer 20 also has multiple channels 30, which connect the core 10 and the external space of the lithium replenishment material. This is because the encapsulation layer 20 also allows lithium ions in the core 10 to escape to the outside of the lithium replenishment material, enabling the lithium replenishment material to release enough lithium ions to diffuse to the negative electrode and then participate in the formation of the SEI film, thus playing a lithium replenishment role. Therefore, the purpose of designing channels 30 on the encapsulation layer 20 is to improve the lithium replenishment efficiency.

[0040] Meanwhile, when lithium-sulfur compounds are selected as the core 10, lithium polysulfides will form in the core 10 after lithium ions are extracted. By designing the pores 30, the lithium polysulfides can also be extracted to the outside of the lithium replenishment material, dissolved in the electrolyte, and then diffused to the negative electrode, thus eliminating lithium deposition. The specific principle is that lithium sulfides produce high-valence intermediate products such as lithium polysulfides (Li2S) during the delithiation process. n Without physical confinement or chemical adsorption, lithium polysulfides will dissolve into the electrolyte and diffuse to the negative electrode. If lithium plating occurs at the negative electrode (especially under fast charging or low-temperature conditions), the diffused lithium polysulfides will react with the plating and be reduced to lower-valence Li₂S. m These reduction products diffuse to the vicinity of the positive electrode due to the concentration gradient, and under the influence of the potential difference, they lose electrons and generate high-valence Li₂S. n This process can remove lithium metal.

[0041] This application mitigates the lithium desorption problem by providing appropriately sized channels 30 on the encapsulation layer 20, allowing lithium polysulfides generated from the lithium sulfide reaction to enter the electrolyte through these channels 30 and further diffuse to the negative electrode.

[0042] In one possible implementation, please refer to Figures 2 to 4The encapsulation layer 20 includes at least one of an isolation encapsulation layer 22, an ion conductor encapsulation layer 21, and an electronic conductor encapsulation layer 23. These encapsulation layers 20 effectively improve the electronic and ion conductivity of the lithium-replenishing material in the core, increasing lithium extraction during charging; they also provide some moisture isolation, improving the stability of the positive electrode lithium-replenishing additive and achieving a stable lithium-replenishing effect. Furthermore, they ensure the stability, uniform dispersion, and good processing performance of the positive electrode lithium-replenishing additive in the electrode active slurry and active layer.

[0043] In one possible implementation, when the encapsulation layer includes an isolation encapsulation layer, the material of the isolation encapsulation layer includes at least one of ceramic, polymer, or carbon materials. Specifically, when the material of the isolation encapsulation layer is a ceramic layer, a ceramic target can be sputtered onto the surface of the core using magnetron sputtering, but not limited to this method. The magnetron sputtering conditions are adjusted according to the specific target properties. For example, the ceramic may include at least one of Al2O3, SiO2, boehmite, Si3N4, SiC, and BN.

[0044] In other possible implementations, when the material of the isolation encapsulation layer is a polymer layer, the step of forming the polymer isolation encapsulation layer may be: dispersing the core in a solution containing the polymer, followed by vacuum drying to form a dense polymer-like encapsulation layer on the surface of the core. The solvent in the solution is a solvent capable of uniformly dispersing or dissolving the polymer, such as one or more of N-methylpyrrolidone, methanol, ethanol, isopropanol, acetone, tetrahydrofuran, and diethyl ether. For example, the polymer may include [C6H7O6Na]. n Organic polymers with the structure [C6H7O2(OH)2OCH2COONa] n Organic polymers with the structure [C3H4O2]n, organic polymers with the structure [C3H3O2Na]n n Organic polymers with the structure [C3H3N] n It is an organic polymer with a structure containing -[CH2-CF2]. nOrganic polymers containing the -[NHCO]- structure, organic polymers containing an imide ring -[CO-N-CO]- structure on the main chain, and polyvinylpyrrolidone are one or more of the following. In other possible embodiments, the polymer may also include one or more of polyvinylidene fluoride, sodium alginate, sodium carboxymethyl cellulose, polyacrylic acid, polyacrylate, polyacrylonitrile, polyamide, polyimide, polyvinylpyrrolidone, polyethylene oxide (PEO), polypyrrole (PPy), polytetrafluoroethylene (PTFE), and polyurethane (PU). In other possible embodiments, the polymer may also include one or more of sodium carboxymethyl cellulose and polyacrylic acid. Sodium carboxymethyl cellulose and polyacrylic acid are two-dimensional planar polymers with good adhesive properties, which can effectively encapsulate the lithium-rich material core, thereby preventing the lithium-rich material core from contacting air and improving the stability of the lithium supplementation additive. In other possible embodiments, the molecular weight of the polymer may be greater than or equal to 100,000. The molecular weight of the polymer may specifically be, but is not limited to, 100,000, 150,000, 200,000, 300,000, 500,000, or 1,000,000. The larger the molecular weight of the polymer, the higher the density and structural strength of the polymer layer, which is more conducive to protecting the core of lithium-rich materials.

[0045] In other possible implementations, when the material of the isolation encapsulation layer is a carbon material layer, the method for forming a carbon material encapsulation layer includes the following steps: dispersing the core in a solution containing a carbon source, drying it, and then carbonizing it to form a dense carbon isolation encapsulation layer on the surface of the core. For example, the core can be uniformly mixed with PEO, and the PEO can reach its melting point at 300°C, uniformly coating the surface of the core. The coated material is then sintered in an inert atmosphere at 600°C for 16 hours, resulting in a dense carbon layer. The carbon source can be, but is not limited to, PEO, and can also be other carbon sources. Any material capable of forming a carbon source coating layer on the surface of the core is suitable for this application. For example, at least one of graphene, carbon nanotubes, amorphous carbon, graphite, and carbon black.

[0046] In one possible implementation, when the encapsulation layer includes an ion conductor encapsulation layer, the material of the ion conductor encapsulation layer may include at least one of perovskite, NASICON, garnet, or polymer solid electrolytes; the method and conditions for forming the ion conductor encapsulation layer are specifically formed according to the method for forming perovskite, NASICON, garnet, or polymer solid electrolytes.

[0047] Specifically, perovskite type includes Li 3x La2 / 3-x TiO3(LLTO), specifically Li 0.5 La 0.5 TiO3, Li 0.33 La0.57 TiO3, Li 0.29 La 0.57 TiO3, Li 0.33 Ba 0.25 La 0.39 TiO3, (Li 0.33 La 0.56 ) 1.005 Ti 0.99 Al 0.01 O3, Li 0.5 La 0.5 Ti 0.95 Zr 0.05 At least one of O3, etc., NASICON type such as but not limited to Li 1.4 Al 0.4 Ti 1.6 (PO4)3(LATP), garnet type including Li7La3Zr2O 12 (LLZO), Li 6.4 La3Zr 1.4 Ta 0.6 O 12 Li 6.5 La3Zr 1.5 Ta 0.5 O 12 At least one of the following, polymer-type solid electrolytes include at least one of PEO / PPO / PVDF, etc., containing dissolved lithium salts. By adjusting the thickness and material of the ion conductor encapsulation layer, its ionic conductivity can be further improved. The ion conductor encapsulation layer enhances the ionic conductivity of the encapsulation layer, thereby enhancing the ionic conductivity of the lithium replenishment material, which is beneficial for the outward transport of lithium ions from the core. Simultaneously, after the core has released all lithium ions and is no longer useful, the ion conductor encapsulation layer can be reused to play an auxiliary role in enhancing ion transport within the electrode. Based on the function of the ion conductor encapsulation layer, it can be a dense structure or a loose structure, and can be fully encapsulated or non-fully encapsulated.

[0048] In one possible implementation, when the encapsulation layer includes an electronic conductor encapsulation layer, the material of the electronic conductor encapsulation layer includes at least one of carbon materials, conductive polymers, or conductive oxides. Specifically, the carbon materials include at least one of mesoporous carbon, carbon nanotubes, graphite, carbon black, and graphene; the conductive polymers may include, but are not limited to, the conductive polymers contained in the aforementioned isolation encapsulation layer; and the conductive oxides include at least one of In₂O₃, ZnO, and SnO₂. By adjusting the thickness of the electronic conductor encapsulation layer 23, its electronic conductivity can be further improved. The electronic conductor encapsulation layer can enhance the electronic conductivity of the encapsulation layer, thereby enhancing the electronic conductivity of the lithium replenishment material, which is beneficial for reducing the impedance inside the electrode. At the same time, after the core has released lithium ions as a "sacrifice" and has no further function, the electronic conductor encapsulation layer can be reused to act as an auxiliary conductive agent inside the electrode. Based on the function of the electronic conductor encapsulation layer, it can be a dense structure or a loose structure, and it can be a fully encapsulated or non-fully encapsulated layer structure.

[0049] In one possible implementation, please refer to Figures 2 to 4 Encapsulation layer 20 can simultaneously be two or three of the above three layers, and the encapsulation order is not limited. For example, encapsulation layer 20 includes an isolation encapsulation layer 22 and an ion conductor encapsulation layer 21, with the ion conductor encapsulation layer 21 encapsulating the core and the isolation encapsulation layer 22 encapsulating the ion conductor encapsulation layer 21. In another possible embodiment, encapsulation layer 20 includes an isolation encapsulation layer 22 and an electronic conductor encapsulation layer 23, with the isolation encapsulation layer 22 encapsulating the core and the electronic conductor encapsulation layer 23 encapsulating the isolation encapsulation layer 22. In yet another possible embodiment, encapsulation layer 20 includes an isolation encapsulation layer 22, an electronic conductor encapsulation layer 23, and an ion conductor encapsulation layer 21, with the ion conductor encapsulation layer 21 encapsulating the core, the isolation encapsulation layer 22 encapsulating the ion conductor encapsulation layer 21, and the electronic conductor encapsulation layer 23 encapsulating the isolation encapsulation layer 22.

[0050] In one possible implementation, the cross-sectional profile of the channel is circular, with a diameter d of 0.5 nm ≤ d ≤ 5 nm. Specifically, the encapsulation layer may include an inner surface and an outer surface, wherein the inner surface is connected to the core, and the outer surface faces away from the inner surface and is exposed to the external space. The channel extends from the inner surface in a meandering or straight line, and the internal shape of the channel can be cylindrical, i.e., the cross-sectional profile of the channel is circular. Of course, since the channel is a micro-nano structure on the lithium-supplementing material, due to different fabrication processes, the cross-section of the channel can also be other shapes, such as irregular polygons or curves, etc., without specific limitations. When the cross-section of the channel is circular, its diameter d is 0.5 nm-5 nm. The advantage of designing the channel diameter within this range is that it ensures that lithium polysulfides can pass through while controlling the extraction of lithium-containing sulfur compounds. Understandably, while lithium polysulfides can pass through, lithium sulfides that have not been de-ionized may also be extracted from the channel, thereby affecting the structural stability of the lithium-supplementing material, so it is important to reasonably control the channel size. When the pore diameter is smaller than the aforementioned range, the extraction efficiency of lithium polysulfides is too low, failing to effectively eliminate lithium plating. When the pore diameter is larger than the aforementioned range, water vapor in the air can more easily enter the core through the pores, reacting with the lithium sulfides in the core. Since lithium sulfides are extremely sensitive to water, hydrogen sulfide gas is generated, thus negating the protective function of the encapsulation layer. Furthermore, excessively large pores allow excess lithium polysulfides to migrate from the interior into the electrolyte and further to the negative electrode. The loss of a large amount of incompletely delithiated lithium polysulfides to the negative electrode results in a very low specific capacity of lithium sulfides as a lithium replenishment material, rendering it ineffective. Therefore, only a suitable pore size can simultaneously achieve the dual functions of lithium replenishment and prevention of lithium plating at the negative electrode while ensuring the interfacial stability of lithium sulfides.

[0051] In one possible implementation, the channel is a straight channel that penetrates the encapsulation layer. That is, as in the above implementation, it extends in a straight line from the outer surface to the inner surface. The advantage of a straight channel is that the extraction path of lithium polysulfides is the shortest, resulting in the highest extraction efficiency.

[0052] In one possible implementation, the volume of the channel occupies 0.01%-5% of the encapsulation layer volume. Understandably, the volume of the channel refers to the space occupied by the channel on the encapsulation layer. Maintaining a suitable channel volume percentage avoids excessive or overly dense channels, which could allow external moisture to enter the core through the channels.

[0053] In one possible implementation, the core particle size is 10 nm-2 μm. Understandably, smaller core particle sizes result in a larger active specific surface area, which is beneficial for lithium-ion insertion and extraction. When the core particle size is smaller than this range, the overall size of the lithium replenishment material is easily reduced, leading to significant particle agglomeration. When the core particle size is larger than this range, the specific surface area of ​​the core decreases, reducing the lithium-ion insertion and extraction efficiency, thus resulting in poor lithium replenishment performance.

[0054] In one possible implementation, the thickness of the encapsulation layer is 3nm-50nm. Understandably, the thickness of the encapsulation layer simultaneously ensures both the specific capacity and electronic conductivity environment of the lithium-ion-adding material. When the encapsulation layer thickness is less than the aforementioned range, the encapsulation layer does not completely cover the core, which is detrimental to building a good electronic conductivity environment; when the encapsulation layer thickness is greater than the aforementioned range, since the encapsulation layer does not contribute lithium ions, it will reduce the overall added specific capacity.

[0055] In one possible implementation, the encapsulation layer accounts for 1%-10% of the mass of the lithium replenishment material. Specifically, the mass fraction of the encapsulation layer can be 1%-10%, and the mass fraction of the core can be 90%-99%. In the lithium replenishment material of this application embodiment, the mass ratio of the encapsulation layer and the core simultaneously ensures the added specific capacity and electronic conductivity environment. If the encapsulation layer content is too high, the overall added specific capacity will be reduced because the encapsulation material does not contribute lithium ions; if the encapsulation layer content is too low, it is not conducive to forming a complete and uniformly thick encapsulation layer on the outer surface of the core, resulting in incomplete coverage and hindering the construction of a good electronic conductivity environment.

[0056] Secondly, this application also provides a method for preparing a lithium-supplementing material, please refer to... Figure 5 Specifically, it is used in the preparation of the lithium supplementation material in the first aspect. Its preparation method includes the following steps:

[0057] Step S10: Provide a first lithium replenishment material, which includes a core containing a lithium-sulfur compound and a first encapsulation layer covering the outer surface of the core.

[0058] Step S20: The first lithium replenishment material is subjected to a pore-forming process to obtain a second lithium replenishment material. The second lithium replenishment material includes a core containing a lithium-sulfur compound and a second encapsulation layer covering the outer surface of the core. The second encapsulation layer has channels, and the core communicates with the external space through the channels.

[0059] In this embodiment, the core in step S10 and the core in step S20 are made of the same material, and the first encapsulation layer in step S10 and the second encapsulation layer in step S20 are made of the same material. That is, the material composition of the first lithium replenishment material and the second lithium replenishment material does not change. For specific material types, please refer to the implementation method in the first aspect, which will not be repeated here.

[0060] This application does not specifically limit the method for preparing the core in step S10 above, as long as a lithium-sulfur compound can be obtained. For example, the method for preparing the core of the lithium-sulfur compound can be atomic layer deposition. In one possible embodiment, the steps for preparing the lithium-sulfur compound include:

[0061] Step S001: Dissolve elemental sulfur in an organic solvent to obtain a first reaction solution; wherein the end group of the organic solvent is amino. Step S002: Under reflux conditions, mix the first reaction solution with elemental lithium and separate to obtain lithium sulfide.

[0062] Meanwhile, this application does not specifically limit the preparation method of the first lithium replenishment material in step S10 above, as long as the first lithium replenishment material can be obtained.

[0063] In step S20, the specific pore-forming method can be plasma treatment. For example, the first lithium replenishment material can be placed in a plasma generator, evacuated to negative pressure, and then the machine can be started. The large number of ions, excited-state molecules, and free radicals excited in the plasma generator act on the wall surface of the first lithium replenishment material, resulting in multiple channels on the first encapsulation layer to obtain the second encapsulation layer. Oxygen plasma treatment can be used in this process.

[0064] In one possible implementation, the plasma generator has a power of 50-500W. Understandably, when the plasma generator power is outside this range, it may result in too few or too many pores on the second encapsulation layer, thereby reducing the performance of the lithium replenishment material.

[0065] In one possible implementation, the plasma treatment time is 1-10 minutes. Understandably, when the plasma treatment time is outside the above range, it may result in too few or too many pores on the second encapsulation layer, thereby reducing the performance of the lithium replenishment material.

[0066] In other embodiments, the aforementioned channels can also be fabricated using acid-base etching or a template method. For example, when fabricating the first encapsulation layer outside the core, an easily removable material can be added to the material used to fabricate the first encapsulation layer as a dopant. For instance, a material that can be etched by a weak acid or weak base. The first lithium-filling material is immersed in a weak acid or weak base, and the etched channels are formed by the acid-base interaction. Of course, other removable materials can also be used; there are no specific limitations.

[0067] In one possible implementation, the physical properties of the second lithium replenishing material should be those of the lithium replenishing material provided in the first aspect above. Specifically, the volume fraction of the pores in the second lithium replenishing material relative to the volume of the second encapsulation layer is 0.01%-5%. The core particle size of the second lithium replenishing material is 10nm-2μm, and the thickness of the second encapsulation layer is 3nm-50nm; the mass percentage of the second encapsulation layer in the second lithium replenishing material is 1%-10%.

[0068] Since plasma treatment also has an etching and thinning effect on the first lithium replenishment material, the physical properties of the first lithium replenishment material should differ somewhat from those of the second lithium replenishment material. For example, the core particle size of the first lithium replenishment material can be 10 nm-2 μm; due to the presence of the encapsulation layer, plasma treatment has a relatively small impact on the core. However, the thickness of the first encapsulation layer can be 4 nm-60 nm. Because plasma treatment of the first lithium replenishment material thins the first encapsulation layer, the thickness of the first encapsulation layer should be greater than 3 nm-50 nm to ensure that the processed second encapsulation layer falls within the range of the above embodiments. Simultaneously, the mass percentage of the first encapsulation layer in the first lithium replenishment material is 3%-20%.

[0069] Thirdly, this application also provides a cathode material. The cathode material includes a cathode active material and a lithium-supplementing material prepared by the method described in the first aspect or the preparation method described in the second aspect. Thus, the cathode material of the embodiments of this application has excellent lithium-supplementing performance and good processing performance, which can improve the quality of the cathode active material and impart electrochemical performance to the corresponding cathode sheet. The cathode active material can be a phosphate cathode active material or a ternary cathode active material. In specific embodiments, it includes one or more of lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadium oxide phosphate, lithium fluorinated vanadium phosphate, lithium titanate, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide.

[0070] In one possible implementation, the content of the lithium replenishing material in the cathode material can be controlled at 1%-6% of the mass of the cathode active material. This ratio can precisely compensate for the loss of active lithium during the first charge of the battery. If the amount of lithium replenishing material added to the cathode is too low, the lost active lithium in the cathode material cannot be fully replenished, which is detrimental to improving the energy density and capacity retention of the battery. If the amount of lithium replenishing material added to the cathode material is too high, it may cause severe lithium plating on the anode and increase costs. In some specific embodiments, the mass percentage of the lithium replenishing material in the cathode material can be 1%, 2%, 4%, 6%, etc.

[0071] Fourthly, this application also provides a positive electrode sheet, which includes the lithium replenishment material described in the first aspect, or the lithium replenishment material prepared by the preparation method in the second aspect, or the positive electrode material described in the third aspect. The positive electrode sheet provided by this application, because it includes the aforementioned lithium replenishment material, and this lithium replenishment material can provide compensation for the active lithium ions consumed during the first charge of the battery due to the formation of the SEI film, effectively maintains the specific capacity of the positive electrode sheet and improves the capacity retention rate of the positive electrode sheet; at the same time, the lithium replenishment material can release lithium polysulfides to eliminate lithium plating, which not only maintains the performance of the positive electrode sheet but also improves its service life.

[0072] In one possible embodiment, the positive electrode sheet further includes a positive current collector, on which a positive active layer is formed. The positive active layer comprises a positive electrode material, a conductive agent, a binder, and other components. This application does not specifically limit these materials; appropriate materials can be selected according to actual application requirements. The positive current collector includes, but is not limited to, any one of copper foil and aluminum foil. The conductive agent includes one or more of graphite, carbon black, acetylene black, graphene, carbon fiber, C60, and carbon nanotubes, and the content of the conductive agent in the positive active layer is 3wt%-5wt%. The binder includes one or more of polyvinylidene chloride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, methylcellulose, carboxymethyl cellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and chitosan derivatives, and the content of the binder in the positive active layer is 2wt%-4wt%.

[0073] Fifthly, this application also provides a secondary battery comprising the aforementioned positive electrode. The addition of the aforementioned lithium-replenishing material to the positive electrode effectively compensates for the active lithium ions consumed during the first charge due to the formation of the SEI film, effectively maintaining the specific capacity of the positive electrode and improving its capacity retention rate. Simultaneously, it also maintains the shape of the secondary battery, improving its lifespan. Furthermore, due to the elimination of lithium plating, it also increases the charging capacity of the secondary battery and reduces the possibility of spontaneous combustion.

[0074] The technical solution of the present invention will be described in detail below through specific embodiments.

[0075] Example 1

[0076] This embodiment provides a lithium replenishment material and its preparation method. The lithium replenishment material includes a core material Li2S and a carbon layer (encapsulation layer) covering the outer surface of the core material Li2S. The carbon layer has multiple channels with a diameter d of 0.5 nm.

[0077] The preparation method of this lithium supplement material includes the following steps:

[0078] (1) Lithium foil was placed in a tubular furnace and heated to 600°C until the lithium was molten. Then, CS2 gas (containing argon as a carrier gas) was introduced into the furnace at a flow rate of 40 sccm. A displacement reaction occurred between the two gases, generating Li2S nanoparticles. The generated carbon coated a graphene layer on the surface of the Li2S nanoparticles. The reaction equation is: 4Li(l) + CS2(g) → 2Li2S(s) + C(s)

[0079] (2) The lithium replenishing material prepared in step (1) is placed in a plasma generator and the environment is set to vacuum -0.1 MPa; the plasma generator is turned on and the power of the plasma generator is adjusted to 80W to generate plasma particles to impact the lithium replenishing material. The duration is 5 minutes and then stopped to obtain carbon-coated Li2S lithium replenishing material with a pore diameter of 0.5 nm.

[0080] Example 2

[0081] This embodiment provides a lithium replenishment material and its preparation method. The lithium replenishment material includes a core material Li2S and a carbon layer (encapsulation layer) covering the outer surface of the core material Li2S. The carbon layer has multiple channels with a diameter d of 1.5 nm.

[0082] The preparation method of this lithium supplement material includes the following steps:

[0083] (1) Same as step (1) in Example 1;

[0084] (2) The lithium replenishing material prepared in step (1) is placed in a plasma generator and the environment is set to vacuum -0.1 MPa; the plasma generator is turned on and the power of the plasma generator is adjusted to 150W to generate plasma particles to impact the lithium replenishing material. The duration is 5 minutes and then stopped to obtain carbon-coated Li2S lithium replenishing material with a pore diameter of 1.5 nm.

[0085] Example 3

[0086] This embodiment provides a lithium replenishment material and its preparation method. The lithium replenishment material includes a core material Li2S and a carbon layer (encapsulation layer) covering the outer surface of the core material Li2S. The carbon layer has multiple channels with a diameter d of 3 nm.

[0087] The preparation method of this lithium supplement material includes the following steps:

[0088] (1) Same as step (1) in Example 1;

[0089] (2) The lithium replenishing material prepared in step (1) is placed in a plasma generator and the environment is set to vacuum -0.1 MPa; the plasma generator is turned on and the power of the plasma generator is adjusted to 300W to generate plasma particles to impact the lithium replenishing material. The duration is 5 minutes and then stopped to obtain carbon-coated Li2S lithium replenishing material with a pore diameter of 3 nm.

[0090] Example 4

[0091] This embodiment provides a lithium replenishment material and its preparation method. The lithium replenishment material includes a core material Li2S and a carbon layer (encapsulation layer) covering the outer surface of the core material Li2S. The carbon layer has multiple channels with a diameter d of 5 nm.

[0092] The preparation method of this lithium supplement material includes the following steps:

[0093] (1) Same as step (1) in Example 1;

[0094] (2) The lithium replenishing material prepared in step (1) is placed in a plasma generator and the environment is set to vacuum -0.1 MPa; the plasma generator is turned on and the power of the plasma generator is adjusted to 400W to generate plasma particles to impact the lithium replenishing material. The duration is 5 minutes and then stopped to obtain a carbon-coated Li2S lithium replenishing material with a pore diameter of 5 nm.

[0095] Example 5

[0096] This embodiment provides a lithium replenishment material and its preparation method. The lithium replenishment material includes a core material Li2S2 and a carbon layer (encapsulation layer) covering the outer surface of the core material Li2S2. The carbon layer has multiple channels with a diameter d of 10 nm.

[0097] The preparation method of this lithium supplement material includes the following steps:

[0098] (1) Same as step (1) in Example 1;

[0099] (2) The lithium replenishing material prepared in step (1) is placed in a plasma generator and the environment is set to vacuum -0.1 MPa; the plasma generator is turned on and the power of the plasma generator is adjusted to 450W to generate plasma particles to impact the lithium replenishing material. The duration is 8 minutes and then stopped to obtain carbon-coated Li2S lithium replenishing material with a pore diameter of 10 nm.

[0100] Comparative Example 1

[0101] This comparative example provides a lithium replenishment material whose carbon layer does not contain porous channels obtained through plasma treatment, compared to Example 1.

[0102] The preparation method of this lithium replenishment material is the same as step (1) in Example 1.

[0103] The lithium replenishment materials provided in Examples 1 to 5 and the lithium replenishment material provided in Comparative Example 1 were assembled into positive electrode and lithium-ion battery respectively according to the following methods:

[0104] Positive electrode: The lithium supplement material and lithium iron phosphate are mixed at a mass ratio of 4:96 to obtain a mixture. The mixture is then mixed with polyvinylidene fluoride and SP-Li at a mass ratio of 93:3:4 and ball-milled to obtain a positive electrode slurry. The positive electrode slurry is coated on the surface of aluminum foil, vacuum dried at 110°C overnight, and rolled to obtain a positive electrode sheet.

[0105] Negative electrode: Graphite, carboxymethyl cellulose (CMC), SBR and SP are mixed and ball-milled in a mass ratio of 95.8:1.2:2:1 to obtain a negative electrode slurry. The negative electrode slurry is coated on the surface of copper foil and dried under vacuum at 110°C overnight to obtain a negative electrode sheet.

[0106] Electrolyte: Ethyl carbonate and ethyl methyl carbonate are mixed in a volume ratio of 3:7, and LiPF6 is added to form an electrolyte with a concentration of 1 mol / L.

[0107] Diaphragm: Polypropylene microporous diaphragm;

[0108] Lithium-ion battery assembly: Assemble button-type lithium-ion full cells in an inert atmosphere glove box according to the assembly sequence of graphite negative electrode sheet - separator - electrolyte - positive electrode sheet.

[0109] The electrochemical performance of each lithium-ion battery assembled in the above lithium-ion battery examples was tested as shown in Table 1. The test results are shown in Table 1 below:

[0110] Table 1

[0111]

[0112] As shown in Table 1, the test results of Examples 1-5 and Comparative Example 1 indicate that the lithium deposition rate (mass of deposited lithium / mass of the negative electrode) in Examples 1-5 is almost zero, while the lithium deposition rate in Comparative Example 1 is as high as 130 mg / g. This demonstrates that for lithium sulfide replenishment materials, a coating layer with certain micropores allows lithium polysulfides to diffuse to the negative electrode, solving the lithium deposition problem. However, if the pore diameter is too large, such as the 10 nm diameter in Example 5, excessive lithium polysulfides dissolve in the electrolyte, leading to a significant decrease in the initial charge capacity, which is only 161 mAh / g, hindering lithium replenishment. Furthermore, excessively large pores also affect the stability of the replenishment material in air. For example, in Example 5, after the electrode was placed in an environment with 1% humidity for 24 hours, the charge capacity decreased significantly. This was mainly because water vapor in the air entered the interior through the pores and reacted with the lithium sulfide, causing the replenishment material to lose its lithium replenishment function and resulting in a decrease in electrode capacity. In Examples 1-4, the pore diameter was not too large, and the specific capacity of the electrode after 24 hours of storage did not decrease significantly. In Comparative Example 1, there were no pores, and the specific capacity of the electrode after 24 hours of storage also did not decrease significantly.

[0113] In summary, by creating appropriate pores in the dense carbon coating layer, it is possible to achieve a high specific capacity while maintaining the stability of the lithium sulfide lithium replenishment material, and also to prevent lithium plating on the negative electrode.

[0114] In the description of the embodiments of this application, it should be noted that the orientation or positional relationship of the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer" and other indicators are based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and are not intended to indicate or imply that the device or component referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application.

[0115] The above-disclosed embodiments are merely preferred embodiments of this application and should not be construed as limiting the scope of this application. Those skilled in the art will understand that all or part of the processes for implementing the above embodiments and equivalent variations made in accordance with the claims of this application are still within the scope of this application.

Claims

1. A lithium supplementation material, characterized in that, The device includes a lithium-sulfur compound core and an encapsulation layer covering the outer surface of the core. The encapsulation layer has channels through which the core communicates with the external space. The diameter d of the channels satisfies: 0.5 nm ≤ d ≤ 5 nm. The channels are used to allow lithium polysulfide (Li₂S₂) to pass through. n Pass, where 2≤n≤8; The channel is a straight channel that penetrates the encapsulation layer through plasma impact, and the volume of the channel accounts for 0.01%-5% of the volume of the encapsulation layer.

2. The lithium replenishment material according to claim 1, characterized in that, The core formula of the lithium-sulfur compounds includes Li x S y , where 1≤x≤2, 1≤y≤6.

3. The lithium replenishment material according to claim 2, characterized in that, The lithium-sulfur compound includes at least one of Li2S, Li2S2, Li2S4, and Li2S6.

4. The lithium replenishment material according to claim 1, characterized in that, The encapsulation layer includes at least one of an isolation encapsulation layer, an ion conductor encapsulation layer, and an electronic conductor encapsulation layer.

5. The lithium replenishment material according to claim 1, characterized in that, The cross-sectional profile of the channel is circular.

6. The lithium replenishment material according to claim 1, characterized in that, The core has a particle size of 10nm-2µm; and / or the encapsulation layer has a thickness of 3nm-50nm; and / or the encapsulation layer accounts for 1%-10% of the mass of the lithium replenishment material.

7. A method for preparing a lithium-supplementing material, characterized in that, The preparation method is used to prepare the lithium supplementation material according to any one of claims 1-6, and the preparation method includes: A first lithium replenishment material is provided, the first lithium replenishment material comprising a core containing a lithium-sulfur compound and a first encapsulation layer covering the outer surface of the core; The first lithium replenishing material is subjected to a pore-forming process to obtain a second lithium replenishing material. The second lithium replenishing material includes a core containing a lithium-sulfur compound and a second encapsulation layer covering the outer surface of the core. The second encapsulation layer has channels, and the core communicates with the external space through the channels.

8. A positive electrode material, characterized in that, It includes positive electrode active materials and lithium supplementation materials as described in any one of claims 1-6.

9. A positive electrode plate, characterized in that, Including the cathode material as described in claim 8.

10. A secondary battery, characterized in that, Includes the positive electrode as described in claim 9.

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