Secondary battery and electronic device

Abstract

The invention discloses a secondary battery and an electronic device, the secondary battery comprises a positive pole piece, a negative pole piece and an electrolyte, the negative pole piece comprises a negative pole current collector and a negative pole material layer arranged on at least one surface of the negative pole current collector, and the negative pole material layer comprises a negative pole active material and a solid electrolyte; wherein the solid electrolyte contains a lanthanum element, and based on the mass of the negative electrode material layer, the mass ratio of the lanthanum element is 0.042%-4%; a coating layer is arranged on the surface of the solid electrolyte and comprises at least one of a silane coupling agent and halogen para-aromatic acid. According to the present invention, the solid electrolyte contains the lanthanum element, and the surface of the solid electrolyte is provided with the coating layer, such that the cycle stability and the lithium precipitation safety of the lithium ion battery are significantly improved on the basis of ensuring the excellent dynamic performance of the secondary battery so as to achieve the collaborative optimization of the comprehensive performance of the battery;

Description

Technical Field

[0001] The present application belongs to the field of electrochemical technology, and specifically relates to a secondary battery and an electronic device. Background Art

[0002] In the current field of lithium-ion batteries with silicon negative electrodes, there are a series of key issues that restrict their performance improvement. During the negative electrode cycle, the appearance of silicon powdering, the continuous consumption of electrolyte, and the lithium precipitation caused by insufficient negative electrode dynamics make it very easy for the battery capacity retention rate to drop sharply in the later stage, and the cycle life is always difficult to be effectively improved. Summary of the invention

[0003] In view of this, the embodiments of the present application provide a secondary battery and an electronic device, which, on the basis of having excellent kinetic performance, significantly improve the cycle stability performance and lithium plating safety of lithium-ion batteries, thereby achieving coordinated optimization of the comprehensive performance of the battery.

[0004] In a first aspect, the present application provides a secondary battery, which includes a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the negative electrode sheet includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode material layer includes a negative electrode active material and a solid electrolyte;

[0005] The solid electrolyte contains lanthanum, and the mass of lanthanum accounts for 0.042% to 4% based on the mass of the negative electrode material layer;

[0006] The solid electrolyte surface has a monomolecular coating layer, and the monomolecular coating layer includes at least one of a silane coupling agent and a halogen para-aromatic acid;

[0007] The ionic conductivity of the solid electrolyte is greater than or equal to 2×10 -4 S / cm; the electronic conductivity of the solid electrolyte is less than or equal to 1×10 -8 S / cm.

[0008] Based on the secondary battery of the embodiment of the present application, the lanthanum-containing solid electrolyte with a specific content in the negative electrode material layer can significantly improve the dynamic performance of the negative electrode plate, improve the ionic conductivity, and thus effectively improve the cycle performance of the battery. In addition, the silane coupling agent or halogen para-aromatic acid monolayer coated on the surface of the solid electrolyte can reduce the contact reaction between the lanthanum-containing solid electrolyte and water and carbon dioxide in the air during the production process, on the one hand, suppress the lithium hydrogen exchange side reaction occurring on the surface of the solid electrolyte, maintain its high ionic conductivity, and make it continuously and stably play the role of ion conduction during the long cycle process; on the other hand, during the battery cycle, it can reduce the diffusion of the residual protons (H) inside the battery core to the inside of the battery core, thereby avoiding the deterioration of the battery core performance. At the same time, the coating layer can also delay the decline of the catalytic ability of the solid electrolyte, maintain the reaction level of the specific components (such as sulfite compounds, fluoroethylene carbonate, etc.) in its directional catalytic electrolyte to decompose and form an inorganic SEI film, thereby maintaining the stability of the negative electrode SEI film, suppressing the consumption of the electrolyte during the cycle, and significantly improving the cycle capacity retention rate of the secondary battery. Ultimately, by optimizing the negative electrode kinetics and the stability of the SEI film, the cycle stability and lithium plating safety of lithium-ion batteries were significantly improved, further achieving the coordinated optimization of the battery's comprehensive performance.

[0009] In some embodiments, the silane coupling agent includes at least one of vinyl triethoxysilane, γ-aminopropyl triethoxysilane, γ-aminopropyl trimethoxysilane, γ-glycidyloxypropyl trimethoxysilane, γ-methacryloxypropyl trimethoxysilane, and mercaptopropyl trimethoxysilane. By selecting the above-mentioned silane coupling agent, the present application can further reduce the contact between the solid electrolyte and water and carbon dioxide in the air during the preparation of the pole piece, thereby better inhibiting the lithium hydrogen exchange side reaction on the surface of the solid electrolyte, thereby maintaining the high ionic conductivity of the electrolyte, so that it can continuously and stably play the role of ion conduction during a long cycle, thereby improving the cycle capacity retention rate of the secondary battery.

[0010] In some embodiments, the halogen para-aromatic acid comprises at least one of the following compounds:

[0011]

[0012] Wherein, X is one of F element, Cl element and Br element, and A is one of -SH, -COOH, -SO3H and -PO3H2.

[0013] This application uses a specific halogen-substituted aromatic acid as a functional material, mainly based on the following synergistic mechanism: First, the hydrophobic properties of halogen elements and their acidic functional groups (such as -SH, -COOH, -SO3H, -PO3H2) work together to isolate the ambient humidity through the hydrophobic effect and anchor it to the solid electrolyte through chemical bonds, effectively preventing it from undergoing lithium-hydrogen exchange reactions in the air and maintaining the lithium conductivity and catalytic activity of the solid electrolyte. In addition, halogen elements can also participate in SEI film formation to form a highly lithium-conductive and stable LiX-type SEI component, enhance the structural stability of the SEI film, and delay the rupture and regeneration of the SEI on the surface of the silicon-based active material during the cycle, thereby effectively inhibiting the continuous decomposition and consumption of the electrolyte, and significantly improving the battery's cycle stability and lithium precipitation safety. Secondly, after the solid electrolyte enters the battery cell, since the electrolyte is an organic system with high affinity to aromatic rings, the electrolyte can pass through the coating layer. Therefore, during the cycle, the solid electrolyte can exert its ability to catalyze the decomposition of electrolyte components, and can react with the hydrofluoric acid generated by the electrolyte during the cycle to prevent its damage to the SEI, CEI and cathode material structure, thereby improving the battery cell cycle performance and further maintaining the high ionic conductivity and catalytic activity of the solid electrolyte.

[0014] In some embodiments, the coating layer is a monomolecular coating layer, and the thickness of the monomolecular coating layer is 0.5 nm to 3 nm.

[0015] By regulating the thickness of the single-molecule coating layer within this appropriate range, the present application can balance the protective effect of the solid electrolyte and the ion transmission efficiency, which can reduce the occurrence of lithium-hydrogen exchange side reactions to a certain extent without causing a major obstacle to lithium ion transmission, ensuring smooth ion passage, thereby improving the cycle capacity retention rate of the secondary battery.

[0016] In some embodiments, based on the mass of the negative electrode material layer, the mass of the solid electrolyte accounts for 0.1% to 8%, preferably 0.2% to 3%. The present application can improve the negative electrode sheet dynamics and take into account the stability of the negative electrode structure by regulating the mass of the solid electrolyte within a suitable range, thereby improving the cycle capacity retention rate of the secondary battery.

[0017] In some embodiments, the particle sizes of the solid electrolyte are Dv90, Dv50 and Dv10 satisfying: 2 / 5Dv90≤Dv50≤3 / 2Dv10, 320nm≤Dv90≤1860nm, 93nm≤Dv10≤500nm. The present application improves the negative electrode sheet dynamics and takes into account the negative electrode structure stability by regulating the particle sizes Dv90, Dv50 and Dv10 of the solid electrolyte within the above appropriate range, thereby improving the cycle capacity retention rate of the secondary battery.

[0018] In some embodiments, the negative electrode active material is at least one of graphite, hard carbon, silicon alone, silicon-carbon material, silicon-oxygen material, and modified materials thereof.

[0019] In some embodiments, the negative electrode active material is a silicon-carbon material, wherein the mass proportion of silicon is 5% to 20% based on the mass of the negative electrode material layer. The present application selects silicon-carbon material as the negative electrode active material and controls the mass proportion of silicon within the above range, so that the secondary battery can continuously and stably perform ion conduction during a long cycle, thereby alleviating the problem of battery cell diving during the cycle of the silicon-carbon negative electrode active material, thereby improving the cycle capacity retention rate of the secondary battery.

[0020] In some embodiments, after the secondary battery has been cycled for 300 to 350 cycles, the secondary battery is discharged to below 3.0V and disassembled to separate the negative electrode. The X-ray photoelectron spectroscopy analysis of the negative electrode shows that the test results at 80nm from the surface of the negative electrode include two characteristic peaks of phosphorus, namely peak A and peak B; the peak position of peak A is in the range of 135.1au to 140ev, and the peak intensity is 1. A The peak position of peak B is in the range of 130ev to 134.9ev, and the peak intensity is I B ; Satisfy 0.2≤I A / I B ≤1. The inventors speculate that the B peak in this application is LiPO x Fy, A peak is PF5, the above two characteristic peaks exist in the SEI on the surface of the negative electrode, and the peak intensity ratio meets the above range, indicating that the La element in the solid electrolyte effectively catalyzes the decomposition of the electrolyte and promotes the simultaneous generation of LiPO with excellent lithium conductivity in the SEI layer. x Fy (B peak) and PF5 (A peak) for enhanced mechanical stability. The present application maintains the high ionic conductivity of SEI (dominated by B peak) and improves the stability of the interface structure (assisted by A peak) by precisely controlling the ratio of the two. The negative electrode sheet that does not use the solid electrolyte of the present application cannot produce these two characteristic peaks at the same time or the intensity of A peak is insufficient. This directly verifies the unique advantages of the present application in optimizing interface dynamics and extending battery cycle life.

[0021] In some embodiments, the electrolyte further includes a sulfite compound, and the mass proportion of the sulfite compound is 0.5% to 3% based on the mass of the electrolyte. The present application adds 0.5%-3% of a sulfite compound to the electrolyte, and the component forms a flexible interface layer rich in organic sulfide on the outer layer of SEI under the catalytic action of the solid electrolyte, thereby effectively reducing the rupture of SEI during the cycle.

[0022] In some embodiments, the electrolyte also includes a first substance, the first substance includes at least one of fluoroethylene carbonate and vinylene carbonate, and the mass of the first substance accounts for 7% to 25% based on the mass of the electrolyte. The present application introduces at least one of fluoroethylene carbonate and vinylene carbonate, and these substances decompose under the catalytic action of the solid electrolyte, and form a composite SEI film rich in LiF, Li2O and polymer at the electrode interface. The SEI film provides high ionic conductivity, mechanical strength and flexibility through LiF and Li2O, thereby effectively adapting to the volume change of the electrode material during the cycle, and significantly improving the interface stability and battery cycle performance.

[0023] In some embodiments, after the secondary battery has been cycled for 300 to 350 cycles, the secondary battery is discharged to below 3.0V and disassembled to separate the negative electrode. The negative electrode is subjected to X-ray photoelectron spectroscopy analysis. Based on the total molar amount of elements on the surface of the negative electrode, the molar proportion of the F element is 8.5% to 18.5%, the molar proportion of the P element is 1.5% to 6%, and the molar proportion of the S element is 0.8% to 2.9%; the elements selected in the XPS test include B element, C element, F element, La element, Li element, N element, O element, P element, S element, Si element, and Ti element. Under the above test conditions, the content of each element obtained by the test of the negative electrode provided in this application meets the above range, which characterizes the chemical composition of the solid electrolyte of the embodiment of this application. When the fluorine element and phosphorus element in the solid electrolyte meet the above range, the negative electrode dynamics can be improved, while taking into account the stability of the negative electrode structure, thereby improving the cycle capacity retention rate of the secondary battery.

[0024] In some embodiments, after the secondary battery has been cycled for 300 to 350 times, the secondary battery is discharged to below 3.0V and disassembled to separate the negative electrode. X-ray photoelectron spectroscopy analysis of the negative electrode is based on the total molar amount of elements at 80nm from the surface of the negative electrode, where the molar proportion of the F element is 12% to 23%, and the molar proportion of the P element is 2.2% to 4%; the elements selected in the XPS test include B element, C element, F element, La element, Li element, N element, O element, P element, S element, Si element, and Ti element. Under the above test conditions, the content of each element obtained by the test of the negative electrode provided in this application meets the above range, which characterizes the chemical composition of the solid electrolyte provided in this application. When the fluorine element and phosphorus element in the solid electrolyte meet the above range, the negative electrode kinetics can be improved, taking into account the stability of the negative electrode structure, thereby improving the cycle capacity retention rate of the secondary battery.

[0025] In a second aspect, the present application provides an electronic device, including a secondary battery provided by any embodiment of the first aspect of the present application. The electronic device provided by the present application has excellent kinetic performance, and at the same time significantly improves the cycle stability performance and lithium plating safety of lithium-ion batteries, thereby achieving coordinated optimization of the comprehensive performance of the electronic device. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application, and those skilled in the art can also obtain other drawings based on these drawings.

[0027] Figure 1 This is the P1s spectrum of the XPS characterization of the negative electrode surface after cycling in the specific embodiment 1 of the present application;

[0028] Figure 2 This is the XPS characterization P1s spectrum of the negative electrode sheet etched 80nm after the cycle of Example 1;

[0029] Figure 3 This is the F1s spectrum of the XPS characterization of the negative electrode surface after the cycle of Example 1;

[0030] Figure 4 This is the XPS F1s spectrum of the negative electrode sheet etched 80nm after the cycle of Example 1;

[0031] Figure 5 This is the O1s spectrum of the negative electrode surface XPS characterization after the cycle of Example 1;

[0032] Figure 6 This is the XPS characterization of O1s spectrum at 80nm etching of the negative electrode after the cycle of Example 1;

[0033] Figure 7 This is the XPS characterization S1s spectrum of the negative electrode surface after cycling in Example 1;

[0034] Figure 8 This is the XPS characterization S1s spectrum of the negative electrode sheet etched 80nm after the cycle of Example 1;

[0035] Fig. 9 This is the percentage of F, P, and S elements on the surface of the negative electrode after cycling;

[0036] Fig.10 This is the percentage of F, P, and S elements at 80nm etching of the negative electrode after cycling;

[0037] Fig.11Comparison of the effects of negative electrode plates with and without a coating layer (the left side is a negative electrode plate without a coating layer, and the right side is a negative electrode plate with a coating layer). DETAILED DESCRIPTION

[0038] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0039] In the current field of lithium-ion batteries containing silicon negative electrodes, there are a series of key issues that restrict their performance improvement. During the negative electrode cycle process, the appearance of silicon powdering, the continuous consumption of electrolyte, and the lithium precipitation caused by insufficient negative electrode dynamics make it very easy for the battery capacity retention rate to drop sharply in the later stage, and the cycle life is always difficult to be effectively improved. The inventors of this application have found that the current conventional strategy is to add oxide solid electrolytes to the negative electrode to construct an ion path. However, the common oxide solid electrolyte LATP (Li 1+a Al a Ti 2-a (PO4)3, 0<a≤0.5) will produce a violent side reaction in the low potential range of the negative electrode, which will cause a significant loss of battery capacity. Other lanthanum-containing solid electrolytes, such as LLTO (Li 3b La (2 / 3-b) TiO3, 0.03≤b≤0.167), LLZO (Li7La3Zr2O 12 ) and LLZTO(Li 7-c Ln3Z 2-c Ta c O 12 , 0.375≤c≤1.5), etc., also face the problem of instability with water and air. When the negative electrode is slurried and coated and the formed pole piece is exposed to air, they are prone to lithium-hydrogen exchange side reactions. This side reaction will not only cause the solid electrolyte to lose the ability to conduct lithium ions and fail to effectively construct an ion path, but also after contacting the electrolyte, the hydrogen in the solid electrolyte may be replaced by the lithium ions in the electrolyte, and enter the electrolyte system in the form of active protons, thereby inducing electrolyte degradation and deterioration, and producing impurities such as HF, which is very likely to cause the performance of the battery cell to deteriorate. In addition, after the lithium-hydrogen exchange occurs, the lanthanum-containing solid electrolyte material will have a greatly weakened ability to directional catalyze the decomposition of specific components in the electrolyte to form a high-quality SEI (solid electrolyte interface membrane) rich in inorganic substances, further affecting the overall performance of the battery. In view of this, the embodiments of the present application provide a secondary battery and an electronic device, which, while achieving superior kinetic performance, takes into account the improvement of capacity retention performance and cycle performance.

[0040] <Negative electrode>

[0041] In a first aspect, the present application provides a secondary battery, which includes a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the negative electrode sheet includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode material layer includes a negative electrode active material and a solid electrolyte;

[0042] The solid electrolyte contains lanthanum, and the mass of lanthanum accounts for 0.042% to 4% based on the mass of the negative electrode material layer;

[0043] The surface of the solid electrolyte has a coating layer, which includes at least one of a silane coupling agent and a halogen para-aromatic acid;

[0044] The ionic conductivity of the solid electrolyte is greater than or equal to 2×10 -4 S / cm;

[0045] The electronic conductivity of the solid electrolyte is less than or equal to 1×10 -8 S / cm.

[0046] Exemplarily, the mass proportion of lanthanum element can be 0.042%, 1.2%, 2.0%, 2.5%, 3.0%, 3.2%, 3.8%, 4%, etc. or within the range consisting of any two of the above values. Based on the secondary battery of the embodiment of the present application, the solid electrolyte containing lanthanum at a specific content in the negative electrode material layer can significantly improve the kinetic properties of the negative electrode plate and improve the ionic conductivity, thereby effectively improving the cycle performance of the battery. In addition, the silane coupling agent or halogen para-aromatic acid monolayer coated on the surface of the solid electrolyte can reduce the contact reaction between the solid electrolyte and water and carbon dioxide in the air during the production process. On the one hand, it inhibits the lithium-hydrogen exchange side reaction on the surface of the solid electrolyte, maintains its high ionic conductivity, and enables it to continue to stably play an ion conduction role during a long cycle; on the other hand, during the battery cycle, it can reduce the diffusion of residual protons (H) inside it into the battery core, thereby avoiding the deterioration of the battery core performance. At the same time, the coating can also delay the decline of the catalytic ability of the solid electrolyte, maintain its directional catalytic electrolyte specific components (such as sulfite compounds, fluoroethylene carbonate, etc.) decompose to form an inorganic SEI film, thereby maintaining the stability of the negative electrode SEI film, inhibiting the consumption of electrolyte during the cycle, and significantly improving the cycle capacity retention rate of the secondary battery. Ultimately, by optimizing the negative electrode kinetics and the stability of the SEI film, the cycle stability and lithium precipitation safety of the lithium-ion battery are significantly improved, and the coordinated optimization of the battery's comprehensive performance is further achieved.

[0047] In some embodiments, the silane coupling agent includes at least one of vinyl triethoxysilane, γ-aminopropyl triethoxysilane, γ-aminopropyl trimethoxysilane, γ-glycidyloxypropyl trimethoxysilane, γ-methacryloxypropyl trimethoxysilane, and mercaptopropyl trimethoxysilane. By selecting the above-mentioned silane coupling agent, the present application can further reduce the contact between the solid electrolyte and water and carbon dioxide in the air during the preparation of the pole piece, thereby better inhibiting the lithium hydrogen exchange side reaction on the surface of the solid electrolyte, thereby maintaining the high ionic conductivity of the electrolyte, so that it can continuously and stably play the role of ion conduction during a long cycle, thereby improving the cycle capacity retention rate of the secondary battery.

[0048] In some embodiments, the halogen para-aromatic acid comprises at least one of the following compounds:

[0049]

[0050] Wherein, X is one of F element, Cl element, and Br element, and A is one of -SH, -COOH, -SO3H, and -PO3H2. This application selects aromatic acids substituted with specific halogens as functional materials, mainly based on the following synergistic mechanism: First, the hydrophobic properties of halogen elements and their acidic functional groups (such as -SH, -COOH, -SO3H, and -PO3H2) are used together to isolate the ambient humidity through the hydrophobic effect, and to anchor the solid electrolyte through chemical bonds, effectively preventing it from undergoing lithium-hydrogen exchange reactions in the air, and maintaining the lithium conductivity and catalytic activity of the solid electrolyte. Moreover, halogen elements can also participate in SEI film formation to form a highly lithium-conductive and stable LiX-type SEI component, enhance the structural stability of the SEI film, and delay the rupture and regeneration of the SEI on the surface of the silicon-based active material during the cycle, thereby effectively inhibiting the continuous decomposition and consumption of the electrolyte, and significantly improving the cycle stability of the battery and the safety of lithium precipitation. Secondly, after the solid electrolyte enters the battery cell, since the electrolyte is an organic system with high affinity to aromatic rings, the electrolyte can pass through the coating layer. Therefore, during the cycle, the solid electrolyte can exert its ability to catalyze the decomposition of electrolyte components, and can react with the hydrofluoric acid generated by the electrolyte during the cycle to prevent its damage to the SEI, CEI and cathode material structure, thereby improving the battery cell cycle performance and further maintaining the high ionic conductivity and catalytic activity of the solid electrolyte.

[0051] In some embodiments, the coating layer is a monomolecular coating layer, and the thickness of the monomolecular coating layer is 0.5 nm to 3 nm.

[0052] Exemplarily, the thickness of the monomolecular coating layer can be 0.5nm, 1mm, 1.5mm, 2mm, 2.5mm, 3nm, etc. or within a range consisting of any two of the above values. The present application can balance the protective effect of the solid electrolyte and the ion transmission efficiency by regulating the thickness of the monomolecular coating layer within this suitable range, which can reduce the occurrence of lithium-hydrogen exchange side reactions to a certain extent, and will not form a significant obstacle to lithium ion transmission, ensuring smooth ion passage, thereby improving the cycle capacity retention rate of the secondary battery.

[0053] In some embodiments, based on the mass of the negative electrode material layer, the mass proportion of the solid electrolyte is 0.1% to 8%, preferably 0.2% to 3%. Exemplarily, based on the mass of the negative electrode material layer, the mass proportion of the solid electrolyte is 0.1%, 2%, 3%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, etc. or within the range consisting of any two of the above values. The present application can improve the negative electrode sheet dynamics and take into account the stability of the negative electrode structure by regulating the mass of the solid electrolyte in a suitable range, thereby improving the cycle capacity retention rate of the secondary battery.

[0054] In some embodiments, the particle sizes of the solid electrolyte are Dv90, Dv50 and Dv10 satisfying: 2 / 5Dv90≤Dv50≤3 / 2Dv10, 320nm≤Dv90≤1860nm, 93nm≤Dv10≤500nm.

[0055] Exemplarily, Dv90 can be 320 nm, 500 mm, 700 mm, 900 mm, 1100 mm, 1200 mm, 1300 mm, 1450 mm, 1550 mm, 1600 mm, 1750 mm, 1860 mm, etc., or within a range consisting of any two of the above values.

[0056] Exemplarily, Dv10 can be 93nm, 100mm, 110mm, 120mm, 200mm, 220mm, 250mm, 280mm, 310mm, 350mm, 380mm, 400mm, 410mm, 430mm, 450mm, 480mm, 500mm, etc., or within a range consisting of any two of the above values.

[0057] The present application improves the negative electrode sheet dynamics while taking into account the negative electrode structure stability by regulating the particle sizes Dv90, Dv50 and Dv10 of the solid electrolyte within the above-mentioned appropriate ranges, thereby improving the cycle capacity retention rate of the secondary battery.

[0058] In some embodiments, the negative electrode active material is at least one of graphite, hard carbon, silicon alone, silicon-carbon material, silicon-oxygen material and modified materials thereof.

[0059] In some embodiments, the negative electrode active material is a silicon-carbon material, wherein the mass proportion of silicon element is 5% to 20% based on the thickness of the negative electrode material layer. Exemplarily, based on the thickness of the negative electrode material layer, the mass proportion of silicon element is 5%, 8%, 10%, 13%, 15%, 18%, 20%, etc. or within the range consisting of any two of the above values. The present application selects silicon-carbon material as the negative electrode active material and regulates the mass proportion of silicon element within the above range, so that the secondary battery can continuously and stably play the role of ion conduction during a long cycle, thereby alleviating the problem of battery cell diving that occurs during the cycle of silicon-carbon negative electrode active materials, thereby improving the cycle capacity retention rate of the secondary battery.

[0060] In some embodiments, after the secondary battery has been cycled for 300 to 350 cycles, the secondary battery is discharged to below 3.0V and disassembled to separate the negative electrode. The negative electrode is subjected to X-ray photoelectron spectroscopy analysis. The test results at 80nm from the surface of the negative electrode include two characteristic peaks of phosphorus, namely peak A and peak B; the peak position of peak A is in the range of 135.1au to 140ev, and the peak intensity is 1. A The peak position of peak B is in the range of 130ev to 134.9ev, and the peak intensity is I B ; Satisfy 0.2≤I A / I B ≤1. The inventors speculate that the B peak in this application is LiPO x Fy, A peak is PF5, the above two characteristic peaks exist in the SEI on the surface of the negative electrode, and the peak intensity ratio meets the above range, indicating that the La element in the solid electrolyte effectively catalyzes the decomposition of the electrolyte and promotes the simultaneous generation of LiPO with excellent lithium conductivity in the SEI layer. x Fy (B peak) and PF5 (A peak) for enhanced mechanical stability. The present application maintains the high ionic conductivity of SEI (dominated by B peak) and improves the stability of the interface structure (assisted by A peak) by precisely controlling the ratio of the two. The negative electrode sheet that does not use the solid electrolyte of the present application cannot produce these two characteristic peaks at the same time or the intensity of A peak is insufficient. This directly verifies the unique advantages of the present application in optimizing interface dynamics and extending battery cycle life.

[0061] In some embodiments, the electrolyte further includes a sulfite compound, and the mass proportion of the sulfite compound is 0.5% to 3% based on the mass of the electrolyte. Exemplarily, based on the mass of the electrolyte, the mass proportion of the sulfite compound can be 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, etc. or within a range consisting of any two of the above values. The present application adds 0.5%-3% of a sulfite compound to the electrolyte, and the component decomposes under the catalytic action of the solid electrolyte, forming a flexible interface layer rich in organic sulfides on the outer layer of the SEI, thereby inhibiting the rupture of the SEI during the cycle.

[0062] In some embodiments, the electrolyte also includes a first substance, the first substance includes at least one of fluoroethylene carbonate and vinylene carbonate, and the mass proportion of the first substance is 7% to 25% based on the mass of the electrolyte. Exemplarily, based on the mass of the electrolyte, the mass proportion of fluoroethylene carbonate can be 7%, 9%, 10%, 15%, 18%, 22%, 25%, etc. or within the range of any two of the above values. The present application introduces at least one of fluoroethylene carbonate and vinylene carbonate, which decomposes under the catalytic action of the solid electrolyte and forms a composite SEI film rich in LiF, Li2O and polymer at the electrode interface. The SEI film provides high ionic conductivity, mechanical strength and flexibility through LiF and Li2O, thereby effectively adapting to the volume change of the electrode material during the cycle, and significantly improving the interface stability and battery cycle performance.

[0063] In some embodiments, after the secondary battery has been cycled for 300 to 350 times, the secondary battery is discharged to below 3.0V and disassembled to separate the negative electrode. The negative electrode is subjected to X-ray photoelectron spectroscopy analysis, based on the total molar amount of elements on the surface of the negative electrode, wherein the molar proportion of the F element is 8.5% to 18.5%, the molar proportion of the P element is 1.5% to 6%, and the molar proportion of the S element is 0.8% to 2.9%; the elements selected in the XPS test include B element, C element, F element, La element, Li element, N element, O element, P element, S element, Si element, and Ti element. Exemplarily, based on the total molar amount of elements on the surface of the negative electrode, based on the total molar amount of the elements selected for this XPS test, the atomic percentage of the F element can be 8.5%, 10%, 11%, 13%, 15%, 17%, etc. or within the range consisting of any two of the above values. Exemplarily, based on the total molar amount of elements on the surface of the negative electrode plate, based on the total molar amount of elements selected for this XPS test, the atomic percentage of the P element is 1.5%, 2.5%, 3.5%, 4%, 4.5%, 5%, etc., or within the range consisting of any two of the above values. Exemplarily, based on the total molar amount of elements on the surface of the negative electrode plate, based on the total molar amount of elements selected for this XPS test, the atomic percentage of the S element is 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, etc., or within the range consisting of any two of the above values. Under the above test conditions, the contents of each element obtained by testing the negative electrode plate provided in this application meet the above ranges, which characterizes the chemical composition of the solid electrolyte of the embodiment of this application. When the fluorine element and the phosphorus element in the solid electrolyte meet the above ranges, the negative electrode plate dynamics can be improved, taking into account the stability of the negative electrode structure, thereby improving the cycle capacity retention rate of the secondary battery.

[0064] In some embodiments, after the secondary battery has been cycled for 300 to 350 times, the secondary battery is discharged to below 3.0V and disassembled to separate the negative electrode. X-ray photoelectron spectroscopy analysis of the negative electrode shows that based on the total molar amount of elements at 80nm from the surface of the negative electrode, the molar proportion of the F element is 12% to 23%, and the molar proportion of the P element is 2.2% to 4%; the elements selected in the XPS test include B element, C element, F element, La element, Li element, N element, O element, P element, S element, Si element, and Ti element. Exemplarily, based on the total molar amount of elements at 80nm from the surface of the negative electrode, based on the total molar amount of the elements selected for this XPS test, the atomic percentage of the F element can be 12%, 15%, 18%, 20%, 23%, etc. or within the range composed of any two of the above values. Exemplarily, based on the total molar amount of elements at 80nm from the surface of the negative electrode plate, based on the total molar amount of elements selected for this XPS test, the atomic percentage of the P element can be 2.2%, 3%, 3.3%, 3.5%, 3.8%, 4%, etc. or within the range consisting of any two of the above values. Under the above test conditions, the contents of each element tested for the negative electrode plate provided in this application meet the above ranges, which characterizes the chemical composition of the solid electrolyte provided in this application. When the fluorine element and the phosphorus element in the solid electrolyte meet the above ranges, the negative electrode plate dynamics can be improved, taking into account the stability of the negative electrode structure, thereby improving the cycle capacity retention rate of the secondary battery.

[0065] The negative electrode material layer also includes a negative electrode binder. The negative electrode binder can improve the bonding between the negative electrode active material particles and the bonding between the negative electrode active material and the current collector. The present application does not particularly limit the type of negative electrode binder, as long as it is a material that is stable to the solvent used in the manufacture of the electrolyte or electrode. In some embodiments, the negative electrode binder includes a resin binder. Examples of resin binders include, but are not limited to, fluororesins, polyacrylonitrile (PAN), polyimide resins, acrylic resins, polyolefin resins, and the like. When an aqueous solvent is used to prepare the negative electrode mixture slurry, the negative electrode binder includes, but is not limited to, carboxymethyl cellulose (CMC) or its salt, styrene-butadiene rubber (SBR), polyacrylic acid (PAA) or its salt, polyvinyl alcohol, and the like.

[0066] The negative electrode material layer also includes a conductive agent. The present application has no particular restrictions on the type of the negative electrode conductive agent, as long as the purpose of the present application can be achieved. For example, the negative electrode conductive agent can be at least one of acetylene black, Ketjen black, carbon nanotubes, carbon fibers, carbon dots or graphene, and the above-mentioned carbon nanotubes can include but are not limited to at least one of single-walled carbon nanotubes or multi-walled carbon nanotubes.

[0067] The present application has no particular restrictions on the negative electrode current collector, as long as the purpose of the present application can be achieved. For example, the negative electrode current collector may include copper foil, aluminum foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam or a polymer substrate covered with a conductive metal, etc. Among them, the conductive metal includes but is not limited to copper, nickel or titanium, and the material of the polymer substrate includes but is not limited to at least one of polyethylene, polypropylene, ethylene propylene copolymer, polyethylene terephthalate, polyethylene naphthalate or poly(p-phenylene terephthalamide). In the present application, there is no particular restriction on the thickness of the negative electrode current collector and the negative electrode material layer, as long as the purpose of the present application can be achieved. For example, the thickness of the negative electrode current collector is 4μm to 12μm, and the thickness of the single-sided negative electrode material layer is 30μm to 160μm. In the present application, the negative electrode mixture layer can be arranged on one surface in the thickness direction of the negative electrode current collector, or on two surfaces in the thickness direction of the negative electrode current collector. It should be noted that the "surface" here can be the entire area of ​​the negative electrode current collector or a partial area of ​​the negative electrode current collector. This application has no special restrictions as long as the purpose of this application can be achieved.

[0068] The present application has no particular restrictions on the compaction density of the negative electrode sheet, as long as the purpose of the present application can be achieved. For example, the compaction density of the negative electrode sheet can be 1.0 g / cm 3 Up to 1.85g / cm 3 The present application has no particular limitation on the cold pressing pressure of the negative electrode sheet, as long as the purpose of the present application can be achieved. For example, the cold pressing pressure of the negative electrode sheet can be 3 tons to 30 tons.

[0069] Optionally, the negative electrode plate may further include a conductive layer, which is located between the negative electrode current collector and the negative electrode material layer. The present application has no particular restrictions on the composition of the conductive layer, which may be a conductive layer commonly used in the art. The conductive layer includes a conductive agent and a binder. The present application has no particular restrictions on the conductive agent and the binder in the conductive layer, which may be at least one of the above conductive agent and the above binder. The present application has no particular restrictions on the mass ratio of the conductive agent and the binder in the conductive layer, and those skilled in the art can choose according to actual needs, as long as the purpose of the present application can be achieved. The present application has no particular restrictions on the thickness of the conductive layer, as long as the purpose of the present application can be achieved, for example, the thickness of the conductive layer is 1μm to 10μm.

[0070] positive electrode

[0071] The positive electrode includes a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector. The positive electrode material layer includes a lithium-containing transition metal composite oxide, and the lithium-containing transition metal composite oxide includes Li x Na z Co 1-y M yO2, wherein 0.6<x<0.95, 0≤y<0.15, 0<z≤0.03, M is at least one selected from the group consisting of Al, Mg, Ti, Mn, Fe, Ni, Zn, Cu, Nb, Cr and Zr. For example, the above subscripts may be 0.6<x<0.7, 0≤y<0.10, and 0<z<0.01; 0.7<x<0.8, 0.05<y<0.10, and 0<z<0.02; 0.8<x<0.9, 0.06<y<0.09, and 0<z≤0.03; 0.9<x<0.95, 0.07<y<0.08, and 0.01<z<0.02; 0.6<x<0.9, 0.08<y<0.15, and 0.01<z≤0.03. By adding the above-mentioned lithium-containing transition metal composite oxide to the positive electrode material layer in the electrochemical device, and controlling the content ratio of each metal atom in the lithium-containing transition metal composite oxide, especially controlling the content ratio of sodium atoms to meet the above-mentioned range, it is possible to improve both the high-temperature storage performance and the low-temperature discharge performance of the electrochemical device.

[0072] In some embodiments, the positive electrode material layer includes a positive electrode conductive material. There is no limitation on the type of positive electrode conductive material, and any known conductive material can be used. Examples of positive electrode conductive materials may include, but are not limited to, carbon black such as acetylene black and Super-P; amorphous carbon such as needle coke; carbon nanotubes; graphene, etc. The above positive electrode conductive materials may be used alone or in any combination.

[0073] In some embodiments, the positive electrode material layer includes a positive electrode binder. The type of the positive electrode binder is not particularly limited, and in the case of a coating method, any material that can be dissolved or dispersed in a liquid medium used in electrode manufacturing can be used. Examples of positive electrode binders may include, but are not limited to, one or more of the following: resin polymers such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, nitrocellulose, etc.; rubber polymers such as styrene-butadiene rubber, nitrile rubber, fluororubber, isoprene rubber, polybutadiene rubber, ethylene-propylene rubber, etc.; thermoplastic elastomer polymers such as styrene-butadiene-styrene block copolymer or its hydride, ethylene-propylene-diene terpolymer, styrene-ethylene-butadiene-ethylene copolymer, styrene-isoprene-styrene block copolymer or its hydride; soft resin polymers such as syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymer, propylene-α-olefin copolymer; fluorine polymers such as polyvinylidene fluoride, polytetrafluoroethylene, fluorinated polyvinylidene fluoride, polytetrafluoroethylene-ethylene copolymer; polymer compositions having ion conductivity of alkali metal ions, etc. The above positive electrode binders may be used alone or in any combination.

[0074] There is no restriction on the type of solvent used to form the positive electrode slurry, as long as it is a solvent that can dissolve or disperse the positive electrode active material, the conductive material, the positive electrode binder and the thickener used as needed. Examples of solvents used to form positive electrode slurries may include any of aqueous solvents and organic solvents. Examples of aqueous media may include, but are not limited to, mixed media of alcohol and water or water, etc. Examples of organic media may include, but are not limited to, aliphatic hydrocarbons such as hexane; aromatic hydrocarbons such as benzene, toluene, xylene, methylnaphthalene; heterocyclic compounds such as quinoline and pyridine; ketones such as acetone, methyl ethyl ketone, and cyclohexanone; esters such as methyl acetate and methyl acrylate; amines such as diethylenetriamine and N,N-dimethylaminopropylamine; ethers such as diethyl ether, propylene oxide, and tetrahydrofuran; amides such as N-methylpyrrolidone, dimethylformamide, and dimethylacetamide; non-protonic polar solvents such as hexamethylphosphoramide and dimethyl sulfoxide, etc.

[0075] Thickeners are usually used to adjust the viscosity of the slurry. In the case of using an aqueous medium, a thickener and a styrene-butadiene rubber emulsion can be used to slurry. The type of thickener is not particularly limited, and examples thereof may include, but are not limited to, carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein, and salts thereof. The above thickeners may be used alone or in any combination.

[0076] There is no particular limitation on the type of positive electrode current collector, which can be any known material suitable for use as a positive electrode current collector. Examples of positive electrode current collectors may include, but are not limited to, metal materials such as aluminum, stainless steel, nickel plating, titanium, tantalum, and materials such as carbon cloth and carbon paper. In some embodiments, the positive electrode current collector is a metal material. In some embodiments, the positive electrode current collector is aluminum.

[0077] In order to reduce the electronic contact resistance between the positive electrode current collector and the positive electrode material layer, the surface of the positive electrode current collector may include a conductive additive or a conductive coating. Examples of conductive additives may include, but are not limited to, carbon and precious metals such as gold, platinum, and silver. Examples of conductive coatings may include a mixture layer containing an inorganic oxide, a conductive agent, and a binder.

[0078] Electrolyte

[0079] The electrolyte used in the secondary battery of the present application includes a lithium salt and a non-aqueous solvent that dissolves the lithium salt.

[0080] The present application has no particular restrictions on the type of lithium salt, as long as the purpose of the present application can be achieved. For example, the lithium salt may include but is not limited to at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium trifluoromethanesulfonyl imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(oxalate borate) (LiBOB) or lithium difluorooxalate borate (LiDFOB). Based on the mass of the electrolyte, the mass percentage of the lithium salt may be 8% to 15%, for example, the mass percentage of the lithium salt may be 8%, 9%, 10%, 11%, 12.5%, 13%, 15% or a range consisting of any two of the values.

[0081] The present application has no particular restrictions on the types of the above-mentioned non-aqueous solvents, as long as the purpose of the present application can be achieved, for example, it may include but is not limited to at least one of carbonate compounds, carboxylate compounds, ether compounds or other organic solvents. The above-mentioned carbonate compounds may include but are not limited to at least one of chain carbonate compounds or cyclic carbonate compounds. The above-mentioned chain carbonate compounds may include but are not limited to at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate or methyl ethyl carbonate. The above-mentioned cyclic carbonate compounds may include but are not limited to at least one of ethylene carbonate, propylene carbonate, butylene carbonate or ethylene ethylene carbonate. The above-mentioned carboxylate compounds may include but are not limited to at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decalactone, valerolactone or caprolactone. The above-mentioned ether compound may include but is not limited to at least one of ethylene glycol dimethyl ether, dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran or tetrahydrofuran. The above-mentioned other organic solvents may include but are not limited to at least one of dimethyl sulfoxide, 1,2-dioxolane, cyclopentane, methyl cyclopentane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate or trioctyl phosphate.

[0082] Diaphragm

[0083] In the present application, a diaphragm is usually provided between the positive electrode and the negative electrode. The diaphragm is used to separate the positive electrode plate and the negative electrode plate, reduce the problem of internal short circuit of the secondary battery, allow electrolyte ions to pass freely, and do not affect the electrochemical charge and discharge process.

[0084] The present application has no particular restrictions on the separator, as long as the purpose of the present application can be achieved. For example, the material of the separator may include but is not limited to polyethylene (PE), polypropylene (PP)-based polyolefins (PO), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex or aramid; the type of the separator may include at least one of woven membrane, non-woven membrane, microporous membrane, composite membrane, rolled membrane or spun membrane.

[0085] In the present application, the diaphragm may include a substrate and a surface treatment layer. The substrate may be a non-woven fabric or a composite film having a porous structure, and the material of the substrate may include at least one of polyethylene, polypropylene, polyethylene terephthalate or polyimide. Optionally, a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric or a polypropylene-polyethylene-polypropylene porous composite film may be used. Optionally, a surface treatment layer is provided on at least one surface of the substrate, and the surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by a mixed polymer and an inorganic substance. For example, the inorganic layer includes inorganic particles and a binder, and the present application has no particular restrictions on the above-mentioned inorganic particles, for example, it may include at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide or barium sulfate. The present application has no particular restrictions on the above-mentioned binder, for example, it may be at least one of the aforementioned binders. The polymer layer contains a polymer, and the material of the polymer includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyvinyl pyrrolidone, polyvinyl ether, polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene).

[0086] The pore size of the diaphragm in the present application is 0.01 μm to 1 μm, and the thickness is 5 μm to 50 μm. In some embodiments, the thickness of the diaphragm is greater than 1 μm, greater than 5 μm, or greater than 8 μm. In some embodiments, the thickness of the diaphragm is less than 50 μm, less than 40 μm, or less than 30 μm. When the thickness of the diaphragm is within the above range, the insulation and mechanical strength can be ensured, and the rate characteristics and energy density of the secondary battery can be ensured.

[0087] The present application also provides an electronic device, which includes the secondary battery of the present application. The electronic device includes but is not limited to a laptop computer, a pen-input computer, a mobile computer, an electronic book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, an LCD TV, a portable cleaner, a portable CD player, a mini CD, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, a car, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, an electric tool, a flashlight, a camera, a large household battery and a lithium-ion capacitor, etc.

[0088] Test Method

[0089] (1) La element content test

[0090] Discharge the lithium-ion battery to 3.0V and disassemble it, separate the negative electrode, clean it, dry it and use it

[0091] The powder of the negative electrode active material layer was scraped off with a scraper, and the scraped powder was dissolved in a mixed solvent (for example, 0.4 g of the negative electrode sheet was dissolved in a mixed solvent of 5 ml of aqua regia and 5 ml of deionized water), the volume was fixed to 100 mL, and then the mass content of the La element in the solution was tested using an ICP analyzer.

[0092] (2) Ionic conductivity test:

[0093] Different solid electrolytes were prepared into high-density ceramic sheets (density 0.96) by cold pressing, and gold was sprayed on both ends of the ceramic sheets to make blocking electrodes. The prepared blocking electrodes were subjected to AC impedance testing with a frequency range of 1MHz to 10Hz and a disturbance amplitude of 10mV. The ionic conductivity σ of the solid electrolyte was calculated as follows: σ=d / Re×S.

[0094] Where d is the thickness of the sample under test (in cm); Re is the body impedance of the sample under test (in Ω), which can be obtained from the intersection of the semicircle arc and the oblique line in the Nyquist diagram of the electrochemical impedance spectroscopy; S is the effective area of ​​the electrode (in cm 2 ). The ionic conductivity of different solid electrolyte materials was tested three times, and the average value was taken to obtain the ionic conductivity test results: the ionic conductivity of LLZTO is 4.12*10 -3 S / cm; the ionic conductivity of LLZO is 1.23*10 -3 S / cm; the ionic conductivity of LLTO is 8.15*10 -3 S / cm.

[0095] (3) Electronic conductivity test

[0096] The electronic conductivity of the solid electrolyte was tested using a resistivity tester (Suzhou Jingge Electronics, model ST-2255A).

[0097] Take 5g of solid electrolyte powder and use an electronic press to compress the solid electrolyte powder into tablets. The electronic press is pressed to 5000kg and maintained for 20s to obtain a solid electrolyte sample. The solid electrolyte sample is placed between the electrodes of a resistivity tester for testing. The electronic conductivity of the solid electrolyte is δ = h / (S×R) / 1000.

[0098] Wherein, the unit of δ is S / m, h is the height of the solid electrolyte sample (in cm), R is the resistance of the solid electrolyte sample (in KΩ), S is the area of ​​the solid electrolyte sample, and S=3.14 cm2.

[0099] The electronic conductivity of different solid electrolytes was tested and the electronic conductivity was less than 1*10 -8 S / cm.

[0100] (3)XPS test

[0101] Sample preparation: The lithium-ion battery that has been cycled to a specified number of times was discharged to 3.0 V and disassembled to separate the negative electrode plate. The anode plate was rinsed 5 times with sufficient DMC and fully dried to obtain a clean sample.

[0102] Testing process: Ar plasma is used to etch the material surface, and the etching depth is controlled by adjusting the etching time and rate. The etched surface is tested for XPS to obtain XPS spectra at different depths from the surface. When testing XPS, the negative electrode material is tested using the ESCALAB Xi+ X-ray photoelectron spectrometer (XPS) of Thermo Scientific, and elements including B, C, F, La, Li, N, O, P, S, Si, and Ti are selected as analysis elements. The XPS total spectrum in the range of -8 to 1352 eV is tested at a scan rate of 20 eV / s, the XPS spectra of C 1s and O 1s are tested at a scan rate of 0.1 eV / s, and the XPS spectra of each element are tested at a scan rate of 0.067 eV / s to obtain the XPS test curve.

[0103] In the XPS test spectrum, the area of ​​the X-ray absorption peak of the element to be tested is integrated and calculated, recorded as the first area, and the sum of the absorption peak areas of each element is calculated and recorded as the second area. The ratio of the first area to the second area is taken as the molar ratio of the element to be tested in the material.

[0104] (4) Coating thickness test:

[0105] Sample preparation: The lithium-ion battery that has been cycled to a specified number of times is discharged to 3.0V and disassembled, and the negative electrode is separated. It is first cleaned with DMC and dried. The powder obtained by scraping the electrode is added to ethanol and ultrasonicated to obtain a uniformly dispersed suspension. The suspension is dropped on the TEM copper grid and sampled for testing.

[0106] TEM test coating thickness: Combine particle morphology and elements to determine the solid electrolyte, magnify the outer surface of the solid electrolyte, and observe the thickness of the amorphous layer on the surface of the solid electrolyte, which is the coating thickness.

[0107] (5) SEM test: The SEM characterization was recorded by a Philips XL-30 field emission scanning electron microscope at 10 kV, 10 mA. Silicon particles and graphite particles were distinguished and counted by backscattering mode (silicon particle areas are brighter and graphite particle areas are darker).

[0108] (6) Infrared test:

[0109] The lithium-ion battery that has been cycled to a specified number of times is discharged to 3.0V and disassembled to separate the negative electrode plate. Pre-test treatment is performed: a sample of length × width = 10mm × 10mm is cut, rinsed three times with DMC, and soaked in sufficient DMC for 2h before the plate is taken out and dried. The solid electrolyte material or the negative electrode plate after cycling is tested using an FTIR-1500 Fourier transform infrared spectrometer.

[0110] (7) Particle size test:

[0111] Add about 0.02 g of powder sample into a 50 ml clean beaker, add about 20 ml of deionized water, and then add a few drops of 1% surfactant to completely disperse the powder in the water. Ultrasonicate in a 120 W ultrasonic cleaner for 5 minutes, and use MasterSizer 2000 to test the particle size distribution.

[0112] (8) Lithium deposition test:

[0113] The lithium-ion battery was placed at 45°C for 30 minutes to allow the lithium-ion battery to reach a constant temperature state, and then charged at a constant current of 4C to a voltage of 4.5V, and then charged at a constant voltage to a current of 0.05C to allow the lithium-ion battery to reach a fully charged state, and then discharged at a constant current of 4C to a voltage of 3.0V, and the above steps were repeated 100 times. The lithium-ion battery was then placed at 0°C for 30 minutes to allow the lithium-ion battery to reach a constant temperature state, charged at a constant current of 4C to a voltage of 4.5V, and then charged at a constant voltage to a current of 0.05C to allow the lithium-ion battery to reach a fully charged state. The lithium-ion battery was disassembled to observe the severity of lithium deposition on the negative electrode surface, and the proportion α of the lithium deposition area of ​​the negative electrode plate to the area of ​​the negative electrode active layer was measured. α>20% is defined as severe lithium deposition (D grade), 10%<α≤20% is defined as moderate lithium deposition (C grade), 5%<α≤10% is defined as slight lithium deposition (B grade), and α≤5% is defined as no lithium deposition (A grade).

[0114] (9) Cyclic performance test

[0115] The test temperature is 25°C. The lithium ion batteries of the embodiments and comparative examples are charged to 4.53V at 1.5C constant current, charged to 0.05C at constant voltage, and discharged to 2.8V at 1.5C constant current after standing for 5 minutes. The capacity obtained in the third cycle is taken as the initial capacity, and the cycle test is performed at 0.5C charge / 0.5C discharge. The capacity at each step is compared with the initial capacity to obtain the capacity decay curve.

[0116] The cycle performance is characterized by the number of cycles corresponding to a capacity retention rate of 80%, and a greater number of cycles indicates better cycle performance.

[0117] Example

[0118] Hereinafter, taking lithium-ion batteries as an example, examples and comparative examples are given to more specifically describe the implementation of the secondary battery of the present application. Those skilled in the art will understand that the preparation method described in the present application is only an example, and any other suitable preparation method is within the scope of the present application. Various tests and evaluations are carried out according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0119] Example 1-1

[0120] <Solid electrolyte coating>:

[0121] The first step is pretreatment: first, the La-containing solid electrolyte (LLZTO) particles are heated to 500 degrees at 5 degrees / min and calcined in air for more than 4 hours. Dry gas (dry air or dry nitrogen, etc.) is required when cooling. This pretreatment can reverse the side reactions that have occurred on the surface of La-containing solid electrolyte nanoparticles and restore the activity of the material.

[0122] The second step is coating: using a single molecule layer coating: using isopropanol as a solvent, the pretreated powder and the protective molecule are dispersed therein, and the solid is separated after soaking for 5 hours and subjected to high temperature drying treatment. The single molecule used can be a silane coupling agent or an aromatic molecule with an acidic functional group (acidic functional groups include -COOH, -SO3H, -PO3H2, -SH), preferably with F / Cl / Br or a functional group containing these atoms in the position opposite to the acidic functional group.

[0123] Polymer coating: Use a polymer solution dissolved in an aprotic solvent (such as NMP solution of PVDF or PAN), add the pretreated solid electrolyte, fully disperse for 5 hours and then spray dry; or use a polymerizable monomer PEGDA dissolved in an aprotic solvent, add the pretreated solid electrolyte and initiator AIBN, fully mix and spray dry while the polymerization reaction occurs.

[0124] <Preparation of negative electrode sheet>

[0125] Preparation of silicon-carbon negative electrode sheet: The above-coated solid electrolyte is used as the modified solid electrolyte material, the mixed material of silicon and carbon (mass ratio of 50:50) is used as the negative electrode active material, Super P is used as the conductive agent, styrene-butadiene rubber is used as the binder, and sodium carboxymethyl cellulose (CMC, weight average molecular weight 90000) is used as the thickener. The modified solid electrolyte material, negative electrode active material, Super P, sodium carboxymethyl cellulose, and styrene-butadiene rubber are mixed in a weight ratio of 1:95:0.5:0.8:2.7, deionized water is added, and a negative electrode slurry is obtained under the action of a vacuum mixer, wherein the solid content of the negative electrode slurry is 45wt%; the active material layer slurry is evenly coated on one surface of a negative electrode current collector copper foil with a thickness of 8μm, and dried at 80°C to form a negative electrode active material layer, and then cold pressed and cut to obtain a negative electrode sheet.

[0126] <Preparation of positive electrode sheet>

[0127] Preparation of lithium cobalt oxide positive electrode material: positive electrode active material LiCoO2, conductive agent acetylene black, binder polyvinylidene fluoride (PVDF, weight average molecular weight 5×10 5 ) are mixed in a weight ratio of 94:3:3, added to N-methylpyrrolidone (NMP) solvent, and stirred evenly under the action of a vacuum stirrer to obtain a positive electrode slurry, wherein the solid content of the positive electrode slurry is 75wt%; the positive electrode slurry is evenly coated on the surface of the positive electrode current collector aluminum foil, and dried at 90°C to obtain a positive electrode sheet.

[0128] <Preparation of diaphragm>:

[0129] Preparation of the diaphragm: A polyethylene (PE) with a thickness of 8 μm and a porosity of 55% was selected as the base film, and a PVDF slurry and an inorganic particle (lamellar boehmite and Al2O3 with a mass ratio of 70:30) slurry were distributed and coated on at least one surface of the base film, and dried to obtain a separator. The coating thickness of the surface of the separator was 3 μm.

[0130] <Preparation of Electrolyte>

[0131] In a dry (water content <10ppm) argon atmosphere glove box, the solvents were mixed in a mass ratio of ethylene carbonate (EC): propylene carbonate (PC): diethyl carbonate (DEC) = 5:10:35 to obtain a presolvent. 12.5wt% LiPF6 based on the final electrolyte was added to dissolve and fully stirred, and the electrolyte was obtained after mixing evenly.

[0132] <Assembly of lithium-ion batteries>

[0133] The above electrode is used as the negative electrode, and the PE porous polymer film is used as the isolation film. The positive electrode, isolation film, and negative electrode are stacked in order, so that the isolation film is placed between the cathode and the negative electrode to play an isolating role, and then wound to obtain a bare cell. The bare cell is placed in an outer package, injected with the prepared electrolyte, and then packaged. After the formation, degassing, trimming and other process flows, a full cell is obtained.

[0134] Comparative Example 1

[0135] Preparation of silicon-carbon negative electrode sheet: A mixed material of silicon and carbon (mass ratio of 50:50) is used as the negative electrode active material, Super P is used as the conductive agent, styrene-butadiene rubber is used as the binder, and sodium carboxymethyl cellulose (CMC, weight average molecular weight 90,000) is used as the thickener. The negative electrode active material, Super P, sodium carboxymethyl cellulose, and styrene-butadiene rubber are mixed in a weight ratio of 96:0.5:0.8:2.7, deionized water is added, and a negative electrode slurry is obtained under the action of a vacuum mixer, wherein the solid content of the negative electrode slurry is 45wt%; the active material layer slurry is evenly coated on one surface of a negative electrode current collector copper foil with a thickness of 8μm, and dried at 80°C to form a negative electrode active material layer, and then cold pressed and cut to obtain a negative electrode sheet. The method for preparing a lithium-ion battery is the same as described in <Assembly of lithium-ion batteries.

[0136] The difference between Examples 1-2 to 1-26 and Comparative Examples 1 to 2 and Example 1-1 is that, except for adjusting the parameters according to Tables 1 to 2, the other conditions are the same as those of Example 1-1. See Table 1 for the differences and Table 2 for the experimental test results.

[0137] Table 1

[0138]

[0139]

[0140] Table 2

[0141]

[0142]

[0143] Result analysis: From the experimental test data in Table 2, it can be seen that a specific content of lanthanum-containing solid electrolyte in the negative electrode material layer can significantly improve the kinetic performance of the negative electrode sheet and increase the ionic conductivity, thereby effectively improving the cycle performance of the battery.

[0144] The difference between Example 2-1 to Example 2-8 and Example 1-14 is that, except for adjusting the parameters according to Table 3, the other conditions are the same as those of Example 2-14. See Table 3 for the differences, and see Table 4 for the experimental test results.

[0145] Table 3

[0146]

[0147] Table 4

[0148]

[0149]

[0150] Result analysis: From the experimental test data in Table 4, it can be seen that the addition of sulfite and the first substance can further significantly improve the kinetic performance of the negative electrode plate, increase the ionic conductivity, and thus effectively improve the cycle performance of the battery.

[0151] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the principles of the present application should be included in the protection scope of the present application.

Claims

1. A secondary battery comprising a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the negative electrode sheet comprises a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode material layer comprises a negative electrode active material and a solid electrolyte; in, The solid electrolyte contains lanthanum, and the mass proportion of the lanthanum is 0.042% to 4% based on the mass of the negative electrode material layer; The solid electrolyte has a coating layer on its surface, and the coating layer includes at least one of a silane coupling agent and a halogen para-aromatic acid; The ionic conductivity of the solid electrolyte is greater than or equal to 2×10 -4 S / cm; The electronic conductivity of the solid electrolyte is less than or equal to 1×10 -8 S / cm. 2 . The secondary battery according to claim 1 , wherein the silane coupling agent comprises at least one of vinyl triethoxysilane, γ-aminopropyl triethoxysilane, γ-aminopropyl trimethoxysilane, γ-glycidoxypropyl trimethoxysilane, γ-methacryloxypropyl trimethoxysilane, and mercaptopropyl trimethoxysilane.

3. The secondary battery according to claim 1, wherein the halogen para-aromatic acid comprises at least one of the following compounds: in, X is one of F element, Cl element and Br element, and A is one of -SH, -COOH, -SO3H and -PO3H2. 4 . The secondary battery according to claim 1 , wherein the coating layer has a thickness of 0.5 nm to 3 nm. 5 . The secondary battery according to claim 1 , wherein the solid electrolyte accounts for 0.1% to 8% by mass, preferably 0.2% to 3% by mass, based on the mass of the negative electrode material layer. 6 . The secondary battery according to claim 1 , wherein the particle sizes of the solid electrolyte Dv90, Dv50 and Dv10 satisfy: 2 / 5Dv90≤Dv50≤3 / 2Dv10, 320nm≤Dv90≤1860nm, 93nm≤Dv10≤500nm. 7 . The secondary battery according to claim 1 , wherein the negative electrode active material is at least one of graphite, hard carbon, silicon alone, silicon-carbon, silicon-oxygen materials and modified materials thereof.

8. The secondary battery according to claim 7, wherein the negative electrode active material comprises a silicon-carbon material, wherein: Based on the mass of the negative electrode material layer, the mass proportion of silicon element is 5% to 20%.

9. The secondary battery according to claim 1, after the secondary battery has been cycled 300 to 350 times, the negative electrode plate is subjected to X-ray photoelectron spectroscopy analysis, and the test result at 80 nm from the surface of the negative electrode plate includes peak A and peak B, the peak position of peak A is in the range of 135.1 eV to 140 eV, and the peak intensity is 1 A The peak position of the B peak is in the range of 130ev to 134.9ev, and the peak intensity is 1 B ; Satisfy 0.2≤I A / I B ≤1. 10 . The secondary battery according to claim 1 , wherein the electrolyte comprises a sulfite compound, and the mass percentage of the sulfite compound is 0.5% to 3% based on the mass of the electrolyte. 11 . The secondary battery according to claim 1 , wherein the electrolyte comprises a first substance, the first substance comprises at least one of fluoroethylene carbonate and vinylene carbonate, and the mass proportion of the first substance is 7% to 25% based on the mass of the electrolyte.

12. The secondary battery according to claim 1, wherein after the secondary battery has been cycled for 300 to 350 cycles, an X-ray photoelectron spectroscopy analysis is performed on the negative electrode plate, based on the total molar amount of elements on the surface of the negative electrode plate, wherein: The molar proportion of the F element is 8.5% to 18.5%, the molar proportion of the P element is 1.5% to 6%, and the molar proportion of the S element is 0.8% to 2.9%.

13. The secondary battery according to claim 1, after the secondary battery has been cycled for 300 to 350 cycles, the negative electrode piece is subjected to X-ray photoelectron spectroscopy analysis, based on the total molar amount of elements at 80 nm from the surface of the negative electrode piece, wherein: The molar proportion of the F element is 12% to 25%, and the molar proportion of the P element is 2.2% to 6.7%. 14 . An electronic device comprising the secondary battery according to claim 1 .

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