Negative electrode sheet and its preparation method and lithium secondary battery

Abstract

This application provides a negative electrode sheet, comprising a negative electrode current collector, a lithium metal active layer disposed on the negative electrode current collector, and a negative electrode modification layer. The negative electrode modification layer is disposed on the lithium metal active layer and includes a porous glass layer in which the mass percentage of oxygen element is 50%-60%. This porous glass layer can significantly improve the deposition morphology of lithium metal, suppress side reactions between lithium metal and the electrolyte, thereby suppressing the growth of lithium dendrites, suppressing negative electrode expansion, and improving battery cycle life and safety. This application also provides a method for preparing the negative electrode sheet and a lithium secondary battery.

Description

Technical Field

[0001] This application relates to the field of lithium-ion batteries, specifically to a negative electrode sheet and its preparation method, and a lithium secondary battery. Background Technology

[0002] With the widespread application of smart electronic devices, electric vehicles, and large-scale energy storage devices, there is an urgent need for lithium-ion batteries with higher energy density and cycle life. Lithium metal, due to its high theoretical specific capacity (3860 mAh / g) and low electrochemical potential (-3.040 V vs. standard hydrogen electrode), is considered an ideal next-generation lithium-ion battery anode material. Lithium-ion batteries using lithium metal as the anode can exhibit extremely high theoretical energy density.

[0003] However, lithium metal anodes still have some problems during battery cycling, such as the easy formation of lithium dendrites, drastic volume changes, and the formation of a solid electrolyte interphase (SEI) film, which leads to low coulombic efficiency, short circuits and battery failure, and fire risk.

[0004] Therefore, it is necessary to improve the lithium metal anode used in lithium secondary batteries to enhance the battery's cycle life and safety. Summary of the Invention

[0005] In view of this, this application provides a negative electrode sheet, including a negative electrode modification layer, wherein the negative electrode modification layer includes a porous glass layer. The porous glass layer enables uniform deposition of metallic lithium, which is beneficial for suppressing the growth of lithium dendrites, suppressing negative electrode expansion, and improving the cycle life and safety of the battery.

[0006] Specifically, in a first aspect, this application provides a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector, a lithium metal active layer disposed on the negative electrode current collector, and a negative electrode modification layer disposed on the lithium metal active layer; the negative electrode modification layer comprises a porous glass layer, wherein the mass percentage of oxygen element in the porous glass layer is 50%-60%.

[0007] In this embodiment of the application, the porous glass layer includes a silicon-oxygen network, wherein the Si element in the silicon-oxygen network has a +4 valence.

[0008] In this embodiment of the application, the porous glass layer further includes a boron-oxygen network, wherein the boron element in the boron-oxygen network has a +3 valence.

[0009] In this embodiment of the application, the porous glass layer has active sites at the pore edges, and these active sites can react with metallic lithium to form an alloy phase.

[0010] In this embodiment of the application, in the porous glass layer, the molar percentage of Si atoms in both Si atoms and B atoms is 95%-100%, and the molar percentage of B atoms in both Si atoms and B atoms is 0%-5%.

[0011] In this embodiment of the application, the pore size of the porous glass layer is 2nm-100nm.

[0012] In this embodiment of the application, the porosity of the porous glass layer is 1%-10%.

[0013] In this embodiment of the application, the thickness of the porous glass layer is 1μm-50μm.

[0014] The negative electrode sheet provided in the first aspect of this application enables uniform deposition of lithium metal on the negative electrode surface, significantly improving the deposition morphology of lithium metal, suppressing side reactions between lithium metal and electrolyte, and inhibiting the growth of lithium dendrites.

[0015] Secondly, this application provides a method for preparing a negative electrode sheet, comprising the following steps:

[0016] The silicon source is hydrolyzed to obtain a silicon source solution;

[0017] Dissolve the boron source to obtain a boron source solution;

[0018] The boron source solution and the silicon source solution are mixed and stirred to obtain a glass precursor solution;

[0019] The glass precursor solution is allowed to stand to obtain a glass sol, which is then coated onto a substrate and allowed to stand to obtain a glass gel.

[0020] The glass gel is dried to obtain a glass substrate;

[0021] The glass substrate is immersed in a dilute hydrochloric acid solution for acid etching to create pores, thereby obtaining a porous glass layer with a porous structure.

[0022] The porous glass layer is transferred onto the negative electrode current collector with a lithium metal active layer to obtain a negative electrode sheet.

[0023] In this embodiment of the application, based on the molar number of silicon atoms and boron atoms, the silicon source accounts for 90%-99% of the total molar percentage of the silicon source and the boron source, and the boron source accounts for 1%-10% of the total molar percentage of the silicon source and the boron source.

[0024] In this embodiment of the application, the concentration of the dilute hydrochloric acid solution is 0.01 mol / L-2 mol / L; the acid etching time is 1 h-24 h.

[0025] The method for preparing the negative electrode sheet provided in the second aspect of this application is simple in process, low in cost, and suitable for large-scale production.

[0026] Thirdly, this application provides a lithium secondary battery, which includes the negative electrode provided in the first aspect of this application.

[0027] The lithium secondary battery provided in the third aspect of this application has high cycle life and safety. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the negative electrode sheet in the embodiments of this application;

[0029] Figure 2 This is the atomic structure composition of the porous glass layer in one embodiment of this application;

[0030] Figure 3 This is the atomic structure composition of the porous glass layer in another embodiment of this application;

[0031] Figure 4 This describes the atomic structure composition of the glass substrate in the embodiments of this application. Detailed Implementation

[0032] 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 some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0033] See Figure 1 , Figure 1 This is a schematic diagram of the structure of the negative electrode 100 provided in the embodiments of this application. The negative electrode 100 provided in this application includes a negative electrode current collector 10, a lithium metal active layer 20 disposed on the negative electrode current collector 10, and a negative electrode modification layer 30. The negative electrode modification layer 30 is disposed on the lithium metal active layer 20 and includes a porous glass layer. In the porous glass layer, the mass ratio of O element is 50%-60%.

[0034] In this embodiment, the porous glass layer exhibits stable properties and high mechanical strength, maintaining good stability within the battery system. This effectively suppresses lithium dendrite formation, improving battery cycle life and safety. The porous glass layer in this embodiment possesses a uniformly distributed porous structure. This porous structure provides transport channels for lithium ions, ensuring a uniform lithium ion flow and guiding the uniform precipitation of metallic lithium, thereby suppressing lithium dendrite growth. Furthermore, the porous structure can alleviate negative electrode expansion to some extent, further enhancing battery cycle life and safety.

[0035] In some embodiments of this application, see Figure 2 The porous glass layer may be primarily composed of a silicon-oxygen network and does not contain a boron-oxygen network. See also other embodiments of this application. Figure 3 Porous glass layers can also be based on a silicon-oxygen network as the main structure, containing a small amount of boron-oxygen network. Figure 2 and Figure 3 The position A indicated by the dashed circle in the middle represents the location of the missing atom.

[0036] In this embodiment, the silicon element in the silicon-oxygen network is in its highest oxidation state, i.e., Si has a +4 valence. In this embodiment, the boron element in the boron-oxygen network is in its highest oxidation state, i.e., B has a +3 valence. In this embodiment, the silicon-oxygen network is formed by [SiO4] groups with tetrahedral structures and a small amount of [SiO3R] groups with tetrahedral structures, which are interconnected by sharing a common vertex O atom, where R is a C1-C4 alkyl group. The boron-oxygen network is formed by [BO3] groups with triangular structures, [BO4] groups with tetrahedral structures, a small amount of [BO2R] groups with triangular structures, and a small amount of [BO3R] groups with tetrahedral structures, which are interconnected by sharing a common vertex O atom, where R is a C1-C4 alkyl group. Among them, the R-containing groups are formed due to incomplete hydrolysis of the silicon or boron source used to prepare the porous glass layer. In the embodiments of this application, the content of the R-containing groups in the porous glass layer is small and insufficient to affect the mechanical strength of the porous glass layer. However, the silicon-oxygen bonds and boron-oxygen bonds in the silicon-oxygen network and the boron-oxygen network have high bond energies and strong stability, which is beneficial to enable the porous glass layer to obtain high mechanical strength, thereby inhibiting the growth of lithium dendrites and improving the cycle life and safety of the battery.

[0037] It should be noted that, Figures 2 to 4 In the diagram, the tetrahedral structures of the [SiO4], [SiO3R], [BO4], and [BO3R] groups contain one oxygen atom perpendicular to the page direction; this oxygen atom is not shown. The triangular structures of the [BO3] and [BO2R] groups do not contain an oxygen atom perpendicular to the page direction. The upright, unbold B represents tetracoordinated boron, i.e., boron in the [BO4] and [BO3R] groups. The italicized, bold B represents tricoordinated boron, i.e., boron in the [BO3] and [BO2R] groups.

[0038] In some embodiments of this application, the porous glass layer described above is obtained by etching some or all of the boron-oxygen network into a glass comprising a silicon-oxygen network and a boron-oxygen network, wherein the locations of the etched boron-oxygen network form a porous structure. Figure 4 The atomic composition at position A' is the atomic composition at that position before acid etching. Figure 2 and Figure 3The atoms at position A in the diagram are composed of Figure 4 The atoms at position A' are formed by acid etching, and are composed of... Figure 2 and Figure 3 It can be seen that position A represents a missing atom after acid etching. The degree of acid etching of the boron-oxygen network affects the molar proportions of Si and B atoms in the porous glass layer. Specifically, the higher the degree of acid etching, the greater the molar proportion of Si atoms and B atoms in the porous glass layer, and the smaller the molar proportion of B atoms. Conversely, the lower the degree of acid etching, the smaller the molar proportion of Si atoms and B atoms in the porous glass layer, and the greater the molar proportion of B atoms.

[0039] Based on different degrees of acid etching of the boron-oxygen network, the molar percentage of Si and B atoms in the porous glass layer can be 95%-100%, specifically 95%, 96%, 97%, 98%, 99%, 100%, etc.; the molar percentage of B atoms in the porous glass layer can be 0%-5%, specifically 0%, 1%, 2%, 3%, 4%, 5%, etc. In some embodiments of this application, the molar percentage of Si atoms in the porous glass layer is 100%, and the molar percentage of B atoms in the porous glass layer is 0%, that is, the boron-oxygen network is completely etched, and the glass network of the porous glass layer only contains a silicon-oxygen network and does not contain a boron-oxygen network. In some embodiments of this application, the molar percentage of Si atoms in the porous glass layer is greater than 95% and less than 100% of both Si and B atoms, and the molar percentage of B atoms in the porous glass layer is greater than 0% and less than 5% of both Si and B atoms. This means the boron-oxygen network is partially etched by acid, and the porous glass layer contains a silicon-oxygen network and a small amount of boron-oxygen network. The presence of a small amount of boron-oxygen network is beneficial for improving the thermal stability of the porous glass layer and reducing its expansion. Controlling the molar percentage of B atoms in the porous glass layer to a small content, i.e., controlling the total content of the boron-oxygen network to a small amount, helps ensure that the porous glass layer has high strength and strong corrosion resistance, thereby better adapting to various electrolyte systems and to the chemical and electrochemical reactions during long-term battery cycling.

[0040] In this embodiment, the mass percentage of oxygen in the porous glass layer can be 50%-60%, specifically 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, 60%, etc. A suitable oxygen content in the porous glass layer can enable it to form a more stable glass network structure.

[0041] In the embodiments of this application, the atomic arrangement in the porous glass layer exhibits a state of short-range order and long-range disorder.

[0042] In this embodiment, the porous glass layer has active sites at the pore edges. These active sites are formed by the breakage of the silicon-oxygen network and the boron-oxygen network in the glass network. These active sites can include silicon active sites and boron active sites. During battery cycling, these active sites can react with metallic lithium to form an alloy phase. This alloy phase can guide the deposition of metallic lithium, thereby achieving a denser deposition state, improving the lithium metal deposition morphology, suppressing lithium dendrite growth, and also helping to suppress negative electrode expansion, ultimately improving battery safety and cycle life. In this embodiment, the main components of the aforementioned alloy phase can be one or more of lithium-boron alloys and lithium-silicon alloys.

[0043] In this embodiment, both the pore size and porosity of the porous glass layer affect its performance. Appropriate pore size and porosity can, on the one hand, give the porous glass layer suitable interfacial resistance, thus facilitating the smooth migration of lithium ions; on the other hand, they can provide a suitable path for lithium ion migration, guiding the uniform distribution of lithium ion flow, thereby guiding the uniform deposition of metallic lithium and protecting the negative electrode. In this embodiment, the pore size in the porous glass layer can be 2nm-100nm, specifically 2nm, 5nm, 10nm, 15nm, 20nm, 30nm, 40nm, 50nm, 60nm, 70nm, 80nm, 90nm, 100nm, etc. The porosity of the porous glass layer can be 1%-10%, specifically 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, etc.

[0044] In some embodiments of this application, the porous glass layer further includes a lithium compound. The lithium compound may exist in the vacancies of the glass network and be connected to O atoms in the glass network (including silicon-oxygen networks and boron-oxygen networks) via ionic bonds. The lithium compound includes, but is not limited to, one or more of lithium ethoxide, lithium methanol, lithium oxalate, lithium carbonate, lithium acetate, lithium formate, and lithium citrate. This lithium compound can, on the one hand, supplement lithium during battery cycling, alleviating the cycle degradation problem caused by the consumption of active lithium in the negative electrode during the formation of the solid electrolyte interphase (SEI) film; on the other hand, it helps improve the formation conditions of the glass substrate, namely, it helps shorten the time for the glass gel to form the glass substrate through heat treatment drying and lowers the high-temperature treatment temperature for the glass gel to form the glass substrate. Controlling the content of the lithium compound in the porous glass layer within a suitable range is beneficial for better utilizing the beneficial effects of the lithium compound. In the embodiments of this application, the molar percentage of the lithium compound in the porous glass layer, measured by the number of lithium atoms, can be controlled to be 2%-8%, specifically 2%, 3%, 4%, 5%, 6%, 7%, 8%, etc. The molar percentage of lithium compounds in a porous glass layer refers to the percentage of the number of moles of lithium atoms relative to the total number of moles of silicon, boron, and lithium atoms.

[0045] In this embodiment, the thickness of the porous glass layer can be 1μm-50μm. A porous glass layer within this thickness range can better guide the deposition of metallic lithium, effectively suppress negative electrode expansion, and improve battery safety. Specifically, the thickness of the porous glass layer in this embodiment can be 1μm, 2μm, 5μm, 10μm, 20μm, 30μm, 40μm, 50μm, etc.

[0046] In this embodiment, the negative electrode current collector 10 can be a copper foil, and the lithium metal active layer 20 can be composed of elemental lithium and / or a lithium alloy. The lithium alloy can be one or more of lithium-silicon alloy, lithium-aluminum alloy, lithium-tin alloy, and lithium-indium alloy. The negative electrode modification layer 30 is in direct contact with the lithium metal active layer 20, meaning the negative electrode modification layer 30 is disposed on the side of the lithium metal active layer 20 away from the negative electrode current collector 10.

[0047] The negative electrode sheet provided in this application enables uniform deposition of lithium metal, significantly improves the deposition morphology of lithium metal, and suppresses side reactions between lithium metal and electrolyte, as well as the growth of lithium dendrites.

[0048] This application also provides a method for preparing a negative electrode, comprising the following steps:

[0049] (1) Hydrolyze the silicon source to obtain a silicon source solution;

[0050] (2) Dissolve the boron source to obtain a boron source solution;

[0051] (3) Mix the boron source solution and the silicon source solution and stir to obtain a glass precursor solution;

[0052] (4) The glass precursor solution was allowed to stand to obtain glass sol, and then the glass sol was coated on the substrate and allowed to stand to obtain glass gel.

[0053] (5) The glass gel is dried to obtain a glass substrate;

[0054] (6) Immerse the glass substrate in a dilute hydrochloric acid solution to perform acid etching and create pores, thereby obtaining a porous glass layer with a porous structure.

[0055] (7) The porous glass layer is transferred onto the active lithium metal layer to obtain the negative electrode.

[0056] In this embodiment, by selecting appropriate silicon and boron sources, silicon and boron sources are respectively prepared into silicon source solutions and boron source solutions, and then mixed in an appropriate molar ratio to prepare a glass precursor solution. The porous glass layer formed by the glass precursor solution after gelation, heat treatment drying and acid etching can have a better ability to guide the deposition of metallic lithium, thereby helping to suppress the growth of lithium dendrites and improve the cycle life and safety of the battery.

[0057] In this embodiment, the silicon source serves as the source of the silicon oxide component in the porous glass layer, and the silicon-oxygen network formed by the silicon source in the porous glass layer constitutes the main structural component of the porous glass layer. The silicon source can be selected from one or more of, including but not limited to, methyl orthosilicate, ethyl orthosilicate, trimethyl methylsilicate, triethyl methylsilicate, dimethyl dimethylsilicate, and diethyl dimethylsilicate. In this embodiment, the silicon source is added to water or ethanol and stirred to hydrolyze the silicon source, obtaining a silicon source solution. The stirring temperature can be room temperature, and the stirring time can be 1-10 hours, specifically 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, or 10 hours. In some embodiments of this application, a few drops of dilute hydrochloric acid with pH=2 can be added to the hydrolysis system to adjust the pH of the system to below 4, thereby accelerating the hydrolysis rate.

[0058] In this embodiment, the boron source is the source of the boron oxide component in the glass substrate formed after the glass precursor solution is gelled and dried by heat treatment. This boron oxide network forms a boron-oxygen network in the glass substrate, which is uniformly embedded in the silicon-oxygen network in an enriched state within the 2nm-200nm range. Since the boron oxide in the glass substrate is sensitive to acid and can be etched by acid, the boron oxide phase regions in the glass substrate can gradually form channels during the acid etching process, providing a porous structure for the porous glass layer. The aforementioned boron source includes, but is not limited to, one or more of boric acid, metaboric acid, methyl borate, ethyl borate, diethyl methylborate, propyl borate, dipropyl methylborate, isopropyl borate, diisopropyl methylborate, and trimethylcyclotriboroxane. In this embodiment, the boron source solution is obtained by fully dissolving the boron source in a solvent. The solvent can be one or more of water, ethanol, ethylene glycol, and dimethyl ethylene glycol ether.

[0059] In this embodiment, since the porous structure of the porous glass layer is obtained by acid etching of boron-oxygen phase regions embedded in the silicon-oxygen network, the ratio of silicon-oxygen phase to boron-oxygen phase in the glass substrate has a significant impact on the final porous structure formed in the porous glass layer. Specifically, an increase in the ratio of silicon-oxygen phase to boron-oxygen phase, i.e., an increase in the proportion of silicon-oxygen phase and a decrease in the proportion of boron-oxygen phase, may lead to a decrease in the pore size and porosity of the resulting porous glass layer, which in turn leads to an increase in the interfacial resistance of the porous glass layer, hindering the migration of lithium ions and affecting the electrochemical performance of the battery. Conversely, a decrease in the ratio of silicon-oxygen phase to boron-oxygen phase, i.e., a decrease in the proportion of silicon-oxygen phase and an increase in the proportion of boron-oxygen phase, may lead to an increase in the pore size and porosity of the resulting porous glass layer, affecting the guiding effect of the porous glass layer on the deposition of metallic lithium. In this embodiment, the silicon source solution and boron source solution prepared above are mixed in a specific molar ratio and stirred to obtain a glass precursor solution. This glass precursor, after gelation, heat treatment drying, and acid etching, can form a porous glass layer with a more suitable pore size and porosity.

[0060] In the glass precursor solution of this application embodiment, the molar percentage of silicon source in the total molar percentage of silicon source and boron source can be 90%-99%, specifically 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, etc.; the molar percentage of boron source in the total molar percentage of silicon source and boron source can be 1%-10%, specifically 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, etc.

[0061] In a glass substrate prepared from a glass precursor solution having the aforementioned proportions of silicon and boron sources, the molar percentage of Si atoms in the glass substrate, comprising both Si and B atoms, can be 90%-99%, specifically 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, etc., and the molar percentage of B atoms in the glass substrate, comprising both Si and B atoms, can be 1%-10%, specifically 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, etc.

[0062] In some embodiments of this application, lithium compounds can be added to the glass precursor solution. These lithium compounds can, on the one hand, supplement lithium during battery cycling, alleviating the cycle degradation problem caused by the consumption of active lithium in the negative electrode during the formation of the solid electrolyte interphase (SEI) film; on the other hand, they help improve the formation conditions of the glass substrate, namely, shortening the time for heat treatment drying of the glass gel to form the glass substrate and lowering the high-temperature treatment temperature for heat treatment drying of the glass gel to form the glass substrate. The aforementioned lithium compounds include, but are not limited to, one or more of lithium oxalate, lithium carbonate, lithium acetate, lithium formate, and lithium citrate. Controlling the content of lithium compounds in the porous glass layer within a suitable range is beneficial for better utilizing the beneficial effects of lithium compounds. In the embodiments of this application, the molar percentage of lithium compounds in the glass precursor solution is controlled at 2%-8%, specifically 2%, 3%, 4%, 5%, 6%, 7%, 8%, etc., measured by the molar number of lithium atoms. The molar percentage of lithium compounds in the glass precursor solution refers to the proportion of the molar number of lithium atoms in the glass precursor solution relative to the total molar number of silicon atoms, boron atoms, and lithium atoms.

[0063] In this embodiment, after obtaining the glass precursor solution, it is allowed to stand at room temperature for 1-12 hours to obtain a glass sol. The obtained glass sol is then coated onto a substrate, and allowed to stand at room temperature for another 1-12 hours to gel the coating on the substrate, resulting in a glass gel. In this embodiment, the standing time of the glass precursor solution and the standing time of the glass sol can specifically be 1 hour, 2 hours, 4 hours, 6 hours, 8 hours, 10 hours, 12 hours, etc. The substrate used for coating the glass sol can be, for example, copper foil. The coating method can be, for example, spin coating, blade coating, transfer coating, extrusion coating, etc.

[0064] In this embodiment, the glass gel on the substrate is dried in a vacuum environment to obtain a glass substrate. In some embodiments of this application, water is used as the solvent for the silicon source solution and the boron source solution. The glass gel on the substrate is placed in a vacuum environment and dried at 60°C-120°C for 1-30 days, allowing the wet glass gel with a high moisture content to slowly dry and shrink to form a dry glass gel. This dry glass gel is the glass substrate with a silicon-oxygen network and a boron-oxygen network as its main structural components. See also Figure 4 , Figure 4 The atomic structure of the glass substrate is shown. The drying temperatures can be 60℃, 70℃, 80℃, 90℃, 100℃, 110℃, 120℃, etc., and the drying times can be 1d, 2d, 3d, 4d, 5d, 6d, 7d, 8d, 9d, 10d, 12d, 15d, 18d, 30d, etc.

[0065] In other embodiments of this application, the solvents for the silicon and boron source solutions include organic solvents. The glass gel on the substrate is placed in a vacuum environment and first dried at 60°C-120°C for 1-30 days to obtain a dry glass gel. Then, the temperature is increased to 200°C-350°C at 5°C / h-10°C / h and held for 5-48 hours to allow the organic solvent components in the dry glass gel to fully evaporate, resulting in a glass substrate with a silicon-oxygen network and a boron-oxygen network as the main structural components. The drying temperature can specifically be 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, etc., and the drying time can specifically be 1 day, 2 days, 3 days, 4 days, 5 days, 6 days, 7 days, 8 days, 9 days, 10 days, 12 days, 15 days, 18 days, 30 days, etc. The heating rate can be 5℃ / h, 6℃ / h, 7℃ / h, 8℃ / h, 9℃ / h, 10℃ / h, etc., the maximum temperature reached can be 200℃, 250℃, 300℃, 350℃, etc., and the holding time can be 5h, 10h, 12h, 18h, 20h, 24h, 36h, 48h, etc.

[0066] In this embodiment, a porous glass layer with a porous structure is obtained by immersing a glass substrate in a dilute hydrochloric acid solution for acid etching to create pores. See also Figure 2 and Figure 3 , Figure 2 and Figure 3 As shown Figure 4The atomic structure of the porous glass layer formed after acid etching creates a porous structure in the glass substrate. During the acid etching process, as the glass substrate is immersed in dilute hydrochloric acid for an extended period, the degree of etching of the boron-oxygen phase in the glass substrate gradually increases, and the boron-oxygen phase regions in the glass substrate are gradually opened up to form pores. Subsequently, the pore size gradually increases until the boron-oxygen phase is completely etched. Therefore, if the acid etching time is too short, the etching of the boron-oxygen phase may be insufficient. On the one hand, this may result in insufficient opening of the boron-oxygen phase regions in the glass substrate, thus reducing the number of pores; on the other hand, it may result in the boron-oxygen phase not being completely etched, leaving boron-oxygen phase residues in the pores, thus reducing the pore size. The embodiments of this application control the acid etching time within a suitable range, which is beneficial for obtaining a porous glass layer with more suitable pore size and porosity. This is beneficial for the porous glass layer to better guide the uniform deposition of metallic lithium, thereby better inhibiting lithium dendrite growth, improving battery cycle life, and enhancing battery safety. In this embodiment, the concentration of the dilute hydrochloric acid solution can be between 0.01 mol / L and 2 mol / L, specifically 0.01 mol / L, 0.05 mol / L, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.5 mol / L, 1 mol / L, 1.5 mol / L, and 2 mol / L. In this embodiment, the etching time can be controlled within a suitable range of 1 h to 24 h, specifically 1 h, 2 h, 4 h, 6 h, 8 h, 10 h, 12 h, 14 h, 16 h, 18 h, 20 h, 22 h, and 24 h. By adjusting the concentration of the dilute hydrochloric acid and the etching time, the pore size of the porous pores and the porosity of the porous glass layer can be controlled.

[0067] In the porous glass layer prepared by the above method, the molar percentage of Si atoms in the total Si and B atoms can be 95%-100%, specifically 95%, 96%, 97%, 98%, 99%, 100%, etc., and the molar percentage of B atoms in the total Si and B atoms can be 0%-5%, specifically 0%, 1%, 2%, 3%, 4%, 5%, etc. The pore size of the porous glass layer can be 2nm-100nm, specifically 2nm, 5nm, 10nm, 15nm, 20nm, 30nm, 40nm, 50nm, 60nm, 70nm, 80nm, 90nm, 100nm, etc. The porosity of the porous glass layer can be 1%-10%, specifically 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, etc.

[0068] In this embodiment, the porous glass layer can be self-supported after being peeled off from the copper foil, or it can be directly transferred to the surface of the lithium metal anode by pressure transfer. The pressure transfer method can be, for example, rolling, flat pressing, or isostatic pressing. In this embodiment, the thickness of the porous glass layer can be 1μm-50μm, specifically 1μm, 2μm, 5μm, 10μm, 20μm, 30μm, 40μm, 50μm, etc.

[0069] The preparation method provided in this application is simple, low-cost, and suitable for large-scale production.

[0070] This application also provides a lithium secondary battery, including a positive electrode, the aforementioned negative electrode, a separator, an electrolyte or electrolyte solution, wherein the separator is located between the positive electrode and the negative electrode.

[0071] The positive electrode sheet includes a positive current collector and a positive active material, binder, and conductive agent loaded on the positive current collector. The positive current collector can be aluminum foil, and the positive active material can be various available positive active materials, including but not limited to LiFePO4, LiMnO2, LiNiO2, LiCoO2, LiVPO4F, LiFeO2, and ternary positive active materials.

[0072] The binder may be any binder known in the art, including but not limited to one or more of polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, and polyacrylate. The conductive agent may be any conductive agent known in the art, including but not limited to one or more of graphite, carbon fiber, carbon black, metal powder, metal oxide, and fiber.

[0073] The lithium secondary battery provided in this application has high cycle life and safety.

[0074] The implementation scheme of this application will be further explained below with several case studies.

[0075] Example 1

[0076] A method for preparing a negative electrode includes the following steps:

[0077] (1) Dissolve 95% tetraethyl orthosilicate in ethanol, add a few drops of dilute hydrochloric acid with pH=2, and stir magnetically for 1 hour at room temperature to obtain a clear ethanol solution of tetraethyl orthosilicate, i.e., the silicon source solution.

[0078] (2) Dissolve 5% boric acid in ethylene glycol dimethyl ether. After complete dissolution, a boron source solution is obtained.

[0079] (3) Add the boron source solution to the silicon source solution and mix and stir to obtain a glass precursor solution; based on the molar number of silicon atoms and boron atoms, the above 95% tetraethyl orthosilicate accounts for 95% of the total amount of silicon source and boron source; the above 5% boric acid refers to the molar percentage of boric acid in the total amount of silicon source and boron source as 5%;

[0080] (4) The glass precursor solution was left to stand at room temperature for 1 hour to obtain glass sol. The glass sol was then extruded and coated onto copper foil, and left to stand at room temperature for 2 hours to form glass gel.

[0081] (5) The gel coated on the copper foil was placed in a vacuum furnace at 80°C and dried slowly for 7 days. Then, the temperature was slowly increased to 300°C at a heating rate of 5°C / h and kept at that temperature for 24 hours to obtain a glass substrate coated on the copper foil.

[0082] (6) The copper foil coated with the glass substrate was immersed in dilute hydrochloric acid with a concentration of 0.1 mol / L for 12 h to obtain a porous glass layer with nanopores.

[0083] (7) The porous glass layer is transferred onto a copper foil with a lithium metal active layer, and the porous glass layer is in direct contact with the lithium metal active layer to obtain a negative electrode sheet.

[0084] Testing revealed that the median pore size of the porous glass layer on the negative electrode was 57 nm, the porosity was 3.5%, the molar percentage of Si atoms in the porous glass layer was 97.4% of the total Si and B atoms, the molar percentage of B atoms in the total Si and B atoms was 2.6%, and the mass fraction of oxygen accounted for 53.8% of the total atomic mass.

[0085] Example 2

[0086] A method for preparing a negative electrode sheet differs from Example 1 only in that the molar percentage of tetraethyl orthosilicate in the total amount of silicon and boron sources is 98%, and the molar percentage of boric acid in the total amount of silicon and boron sources is 2%.

[0087] Testing revealed that the median pore size of the porous glass layer on the negative electrode was 28 nm, the porosity was 1.8%, the molar percentage of Si atoms in the porous glass layer was 99.8% of the total Si and B atoms, the molar percentage of B atoms in the total Si and B atoms was 0.2%, and the mass fraction of oxygen accounted for 53.4% ​​of the total atomic mass.

[0088] Example 3

[0089] A method for preparing a negative electrode sheet differs from Example 1 only in that the molar percentage of tetraethyl orthosilicate in the total amount of silicon and boron sources is 90%, and the molar percentage of boric acid in the total amount of silicon and boron sources is 10%.

[0090] Testing revealed that the median pore size of the porous glass layer on the negative electrode was 98 nm, the porosity was 5.2%, the molar percentage of Si atoms in the porous glass layer was 96.0% of the total Si and B atoms, the molar percentage of B atoms in the total Si and B atoms was 4.0%, and the mass fraction of oxygen accounted for 54.1% of the total atomic mass.

[0091] Example 4

[0092] A method for preparing a negative electrode sheet, which differs from Example 1 only in that the acid etching time of the glass substrate in step (6) is 6 hours.

[0093] Testing revealed that the median pore size of the porous glass layer on the negative electrode was 40 nm, the porosity was 2.5%, the molar percentage of Si atoms in the porous glass layer was 98.0% of the total Si and B atoms, the molar percentage of B atoms in the total Si and B atoms was 2.0%, and the mass fraction of oxygen accounted for 53.7% of the total atomic mass.

[0094] Example 5

[0095] A method for preparing a negative electrode sheet, which differs from Example 1 only in that the acid etching time of the glass substrate in step (6) is 24 hours.

[0096] Testing revealed that the median pore size of the porous glass layer on the negative electrode was 68 nm, the porosity was 4.6%, the molar percentage of Si atoms in the porous glass layer was 95.5% of the total Si and B atoms, the molar percentage of B atoms in the total Si and B atoms was 4.5%, and the mass fraction of oxygen accounted for 54.1% of the total atomic mass.

[0097] Example 6

[0098] A method for preparing a negative electrode sheet differs from Example 1 only in that a copper foil coated with a glass substrate is immersed in a 1 mol / L dilute hydrochloric acid solution for 12 hours for acid etching.

[0099] Testing revealed that the porous glass layer on the negative electrode has a pore size of 110 nm and a porosity of 5.7%. The molar percentage of Si atoms in the porous glass layer is 99.3%, the molar percentage of B atoms in the porous glass layer is 0.7%, and the mass fraction of oxygen accounts for 53.5% of the total atomic mass.

[0100] Example 7

[0101] A method for preparing a negative electrode sheet differs from Example 1 only in that the molar percentage of tetraethyl orthosilicate in the total amount of silicon and boron sources is 85%, and the molar percentage of boric acid in the total amount of silicon and boron sources is 15%.

[0102] Testing revealed that the porous glass layer on the negative electrode has a pore size of 118 nm and a porosity of 10%. In the porous glass layer, Si atoms account for 95.5% of the total Si and B atoms, B atoms account for 4.5% of the total Si and B atoms, and oxygen accounts for 54.1% of the total atomic mass.

[0103] Example 8

[0104] A method for preparing a negative electrode sheet differs from Example 1 in two aspects: firstly, the molar percentage of tetraethyl orthosilicate in the total amount of silicon and boron sources is 80%, and the molar percentage of boric acid in the total amount of silicon and boron sources is 20%; secondly, the acid etching time is 8 hours.

[0105] Testing revealed that the porous glass layer on the negative electrode has a pore size of 60 nm and a porosity of 3.5%. The molar percentage of Si atoms in the porous glass layer is 83.3% of the total Si and B atoms, the molar percentage of B atoms is 16.7% of the total Si and B atoms, and the mass fraction of oxygen is 56.3% of the total atomic mass.

[0106] Example 9

[0107] A method for preparing a negative electrode sheet differs from Example 1 in two aspects: firstly, the molar percentage of tetraethyl orthosilicate in the total amount of silicon and boron sources is 96%, and the molar percentage of boric acid in the total amount of silicon and boron sources is 4%; secondly, the copper foil coated with a glass substrate is immersed in a 0.5 mol / L dilute hydrochloric acid solution for 18 hours for acid etching.

[0108] Testing revealed that the porous glass layer on the negative electrode has a pore size of 58 nm and a porosity of 3.6%. The molar percentage of Si atoms in the porous glass layer is 99.6% of the total Si and B atoms, the molar percentage of B atoms is 0.4% of the total Si and B atoms, and the mass fraction of oxygen is 53.4% ​​of the total atomic mass.

[0109] Example 10

[0110] A method for preparing a negative electrode sheet differs from Example 1 only in that 5% lithium ethanol is added as a lithium compound during the mixing of the boron source and silicon source solutions. The aforementioned 5% lithium ethanol refers to the fact that lithium ethanol accounts for 5% of the total molar ratio of the silicon source and boron source.

[0111] Testing revealed that the porous glass layer on the negative electrode has a pore size of 62 nm and a porosity of 4.1%. The molar percentage of Si atoms in the porous glass layer is 98.5% of the total Si and B atoms, the molar percentage of B atoms is 1.5% of the total Si and B atoms, and the mass fraction of oxygen is 54.9% of the total atomic mass.

[0112] Example 11

[0113] A method for preparing a negative electrode sheet differs from Example 1 only in that 2.5% lithium acetate is added as a lithium compound during the mixing of the boron source and silicon source solutions. The aforementioned 2.5% lithium acetate refers to the molar ratio of lithium acetate to the total amount of silicon and boron sources being 2.5%.

[0114] Testing revealed that the porous glass layer on the negative electrode has a pore size of 60 nm and a porosity of 4.0%. The molar percentage of Si atoms in the porous glass layer is 98.1% of the total Si and B atoms, the molar percentage of B atoms is 1.9% of the total Si and B atoms, and the mass fraction of oxygen is 54.8% of the total atomic mass.

[0115] To highlight the beneficial effects of this application, the following comparative examples are provided:

[0116] Comparative Example 1

[0117] The negative electrode sheet only includes the copper foil with a lithium metal active layer as in Example 1, and no porous glass layer is provided on the surface of the lithium metal active layer.

[0118] Comparative Example 2

[0119] A method for preparing a negative electrode sheet differs from Example 1 only in that, after preparing a porous glass layer rich in nanopores in step (6) of Example 1, it is placed in a hydrogen gas flow at 350°C for 2 hours to reduce the mass ratio of oxygen in the porous glass layer.

[0120] Experimental results show that the prepared porous glass layer has large-area cracks, weakened strength, and cannot be completely transferred to the Li metal surface. The test results show that in this state, the molar ratio of Si atoms in the porous glass layer is 97.7% of the total Si and B atoms, the molar ratio of B atoms in the total Si and B atoms is 2.3%, and the mass fraction of oxygen accounts for 27% of the total atomic mass.

[0121] To verify the beneficial effects of the technical solutions in this application, the negative electrode sheets of Examples 1-10 and Comparative Examples 1 and 2 were respectively used to form half-cells with lithium-plated copper foil for cycle testing. The electrode size was 22mm × 46mm, and the electrolyte was a 1mol / L LiPF6 EC:DEC (V / V = 1:1) electrolyte. During the cycle, the initial flow rate was 0.5mA / cm². 2 Current deposition of Li 1mAh / cm 2 Then, reverse the Li removal process until the voltage is 1.0V vs Li / Li. + The Coulomb efficiency of each cycle was recorded and compared, and the results are shown in Table 1.

[0122] Table 1

[0123] Number of cycles when Coulomb efficiency is 80% Example 1 174 Example 2 152 Example 3 156 Example 4 170 Example 5 166 Example 6 130 Example 7 133 Example 8 144 Example 9 185 Example 10 188 Example 11 180 Comparative Example 1 20 Comparative Example 2 25

[0124] As shown in Table 1, compared to Comparative Example 1, the batteries of Examples 1-10 of this application have a higher number of cycle times at a coulombic efficiency of 80%. This is because the negative electrode sheets of Examples 1-10 have a porous glass layer on the surface of the lithium metal active layer. The porous glass layer can guide the uniform deposition of lithium metal and suppress the formation of lithium dendrites, thereby improving the cycle performance of the lithium metal negative electrode and effectively improving the cycle performance of the battery. In contrast, Comparative Example 1, lacking the porous glass layer provided by this application, suffers from difficulty in uniformly depositing lithium metal, making it difficult to suppress the growth of lithium dendrites and the expansion of the negative electrode, resulting in a shorter cycle life and poorer safety.

[0125] The results of Examples 1-9 also show that by controlling the ratio of silicon and boron sources and the acid etching pore-forming parameters, the porous glass layer can have a more suitable porous structure with appropriate pore size and porosity. This is beneficial for promoting uniform lithium deposition, effectively suppressing lithium dendrite growth, and improving the cycle life of the battery. Compared to Examples 6 and 7, the porosity and / or median pore size of the negative electrode sheets in Examples 1-5 are within a more suitable range, resulting in better guidance for uniform lithium deposition and better suppression of lithium dendrite growth, thus improving the battery cycle performance compared to Examples 6-7. Examples 1 and 8 have comparable porosity and median pore size, but because the molar percentage of silicon atoms in Example 1 is higher than in Example 8, the strength and corrosion resistance of the porous glass layer are improved, resulting in better long-cycle performance than in Example 8. Example 9 has comparable porosity and median pore size to Example 1, but its molar percentage of silicon atoms is higher than in Example 1, resulting in improved cycle performance compared to Example 1. Compared with Example 1, Examples 10 and 11 introduce lithium compounds as a supplement to lithium, which alleviates the cycle degradation problem caused by the consumption of negative electrode active lithium in the process of forming solid electrolyte interphase (SEI) film and improves the cycle performance of the battery.

Claims

1. A negative electrode sheet characterized by comprising: The negative electrode sheet includes a negative electrode current collector, a lithium metal active layer disposed on the negative electrode current collector, and a negative electrode modification layer disposed on the lithium metal active layer; the negative electrode modification layer includes a porous glass layer, the pore size of the porous glass layer is 2nm-100nm; in the porous glass layer, the mass ratio of O element is 50%-60%.

2. The negative electrode sheet as described in claim 1, characterized in that, The porous glass layer comprises a silicon-oxygen network; the Si element in the silicon-oxygen network has a +4 valence.

3. The negative electrode sheet as described in claim 2, characterized in that, The porous glass layer also includes a boron-oxygen network; the boron element in the boron-oxygen network has a +3 valence.

4. The negative electrode sheet as described in claim 1, characterized in that, The porous glass layer has active sites at the pore edges, which can react with metallic lithium to form an alloy phase.

5. The negative electrode sheet according to any one of claims 1-3, characterized in that, In the porous glass layer, the molar percentage of Si atoms in both Si atoms and B atoms is 95%-100%, and the molar percentage of B atoms in both Si atoms and B atoms is 0%-5%.

6. The negative electrode sheet as described in claim 1, characterized in that, The porosity of the porous glass layer is 1%-10%.

7. The negative electrode sheet as described in claim 1, characterized in that, The porous glass layer further includes lithium compounds; the lithium compounds include one or more of lithium ethoxide, lithium methoxide, lithium oxalate, lithium carbonate, lithium acetate, lithium formate, and lithium citrate.

8. The negative electrode sheet as described in claim 1, characterized in that, The thickness of the porous glass layer is 1μm-50μm.

9. A method for preparing a negative electrode sheet, characterized in that, Includes the following steps: The silicon source is hydrolyzed to obtain a silicon source solution; Dissolve the boron source to obtain a boron source solution; The boron source solution and the silicon source solution are mixed and stirred to obtain a glass precursor solution; The glass precursor solution is allowed to stand to obtain a glass sol, which is then coated onto a substrate and allowed to stand to obtain a glass gel. The glass gel is dried to obtain a glass substrate; The glass substrate is immersed in a dilute hydrochloric acid solution for acid etching to create pores, thereby obtaining a porous glass layer with a porous structure. The porous glass layer is transferred onto the negative electrode current collector with a lithium metal active layer to obtain a negative electrode sheet.

10. The preparation method according to claim 9, characterized in that, Based on the molar number of silicon atoms and boron atoms, the silicon source accounts for 90%-99% of the total molar number of the silicon source and the boron source, and the boron source accounts for 1%-10% of the total molar number of the silicon source and the boron source.

11. The preparation method according to claim 9, characterized in that, The concentration of the dilute hydrochloric acid solution is 0.01 mol / L to 2 mol / L; the acid etching time is 1 h to 24 h.

12. A lithium secondary battery, characterized in that, The lithium secondary battery includes the negative electrode sheet as described in any one of claims 1-8.

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