Electrolyte suitable for dry-process electrode sheet, preparation method thereof, and lithium-ion battery

Abstract

The present invention provides an electrolyte compatible with dry-process electrode sheets, a preparation method thereof, and a lithium-ion battery. The electrolyte compatible with dry-process electrode sheets comprises an organic solvent and a lithium salt. Based on the total weight of the electrolyte compatible with dry-process electrode sheets being 100%, the electrolyte compatible with dry-process electrode sheets further comprises 1% to 5% of a first additive, 2% to 6% of a second additive, and 1% to 3% of a third additive. The lithium salt is LiPF6. The LUMO energy levels of the first additive, the second additive, the third additive, and LiPF6 are ranked as follows: first additive < second additive < LiPF6 < third additive. By precisely controlling the amounts of the three special additives and the LUMO energy levels, the present invention significantly suppresses side reactions caused by the dry-process electrodes in the battery system in which the electrolyte is used.

Description

Technical Field

[0001] The present invention relates to the field of lithium-ion batteries, and in particular to an electrolyte adapted for dry-process electrode sheets, a preparation method thereof, and a lithium-ion battery. Background Art

[0002] At present, there are two main methods for preparing electrodes for lithium-ion batteries: dry method and wet method. The wet method is to prepare electrode sheets by mixing the active material with a solvent, a conductive agent, a binder, etc. at room temperature, then extruding and rolling the mixture, and then drying it at high temperature. The wet method is currently the mainstream method for preparing electrodes. However, with the continuous improvement of traditional wet battery technology, the performance improvement in both the material and the electrode layer has basically reached the upper bottleneck, and it is difficult to make a major breakthrough. Faced with the ever-increasing requirements for voltage, capacity, etc., researchers have gradually shifted their attention to dry electrodes. Dry electrode technology is an electrode sheet preparation process in which the active material, conductive agent and binder are mixed under high-speed stirring to obtain a fiberized electrode powder, which is then formed by continuous hot rolling and then thermally compounded with the current collector to obtain an electrode sheet.

[0003] The advantages of the dry-process battery electrode manufacturing process include high electrode porosity, good conductivity, good electrochemical stability and safety, high energy density, and long service life. The dry-process electrode preparation process also offers low production costs, higher production efficiency, and the absence of organic solvents in the electrode preparation process makes it extremely environmentally friendly and can significantly reduce the cost of lithium-ion battery production. However, since binders are inevitably used in the dry-process battery electrode manufacturing process, commonly used binders, especially polytetrafluoroethylene (PTFE), can produce side reactions on the electrode, especially the negative electrode, thereby affecting the capacity and cycle performance of the lithium-ion battery.

[0004] Based on this, how to develop electrolytes for electrode sheets obtained by dry manufacturing so that they can better adapt to electrode sheets prepared by dry manufacturing, reduce the side reactions caused by binders in dry electrodes, and thus enable the corresponding lithium-ion batteries to exhibit higher first charge and discharge efficiency and cycle capacity retention rate, is one of the important technical problems that need to be solved in this field. Summary of the Invention

[0005] The main purpose of the present invention is to provide an electrolyte suitable for dry-process electrode sheets, a preparation method thereof, and a lithium-ion battery, so as to solve the problem of poor initial efficiency and cycle stability of lithium-ion batteries in lithium-ion batteries where dry-process electrode sheets are used in the prior art due to side reactions caused by the binder in the dry-process electrodes.

[0006] In order to achieve the above-mentioned objectives, the first aspect of the present invention provides an electrolyte suitable for dry-process electrode sheets, comprising an organic solvent and a lithium salt. Taking the total weight of the electrolyte suitable for dry-process electrode sheets as 100%, the electrolyte suitable for dry-process electrode sheets also includes 1% to 5% of a first additive, 2% to 6% of a second additive, and 1% to 3% of a third additive; the lithium salt is LiPF6; the LUMO energy levels of the first additive, the second additive, the third additive and LiPF6 are arranged as follows: first additive < second additive < LiPF6 < third additive.

[0007] Furthermore, the first additive is selected from one or more electrolyte additives and / or sulfonimide additives; the second additive is selected from one or more organic ester additives; and the third additive is selected from one or more electrolyte additives.

[0008] Furthermore, the weight ratio of the first additive, the second additive and the third additive is (0.5-2.5):2:1.

[0009] Furthermore, the first additive is selected from one or more of lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium diethyl oxalate, N-phenylbis(trifluoromethanesulfonyl)imide, N-methylbis(trifluoromethanesulfonyl)imide and N-ethylbis(trifluoromethanesulfonyl)imide.

[0010] Furthermore, the second additive is selected from one or more of vinylene carbonate, vinyl sulfate, fluoroethylene carbonate, di(alkynyl) oxalate and methylene methanedisulfonate.

[0011] Furthermore, the third additive is selected from one or more of lithium difluorophosphate, lithium chloride, lithium phosphate and lithium carbonate.

[0012] Furthermore, the organic solvent is selected from one or more of ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, propylene carbonate, diethyl carbonate and butylene carbonate.

[0013] The second aspect of the present invention provides a method for preparing the above-mentioned electrolyte suitable for dry-process electrode sheets, comprising: step S1, preparing an organic solvent and a lithium salt into a lithium salt solution; step S2, adding a first additive to the lithium salt solution, and obtaining a first mixed solution after a first stirring; step S3, adding a second additive and a third additive to the first mixed solution, and obtaining an electrolyte suitable for dry-process electrode sheets after a second stirring.

[0014] Furthermore, the first stirring speed is 100 rpm to 200 rpm, and the time is 20 min to 30 min; and / or the second stirring speed is 400 rpm to 500 rpm, and the time is 30 min to 40 min.

[0015] The third aspect of the present invention provides a lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the electrolyte is the electrolyte adapted for the above-mentioned dry-process electrode sheet, and both the positive electrode sheet and the negative electrode sheet are dry-process electrode sheets.

[0016] The technical solution of this invention significantly suppresses side reactions caused by dry-process electrodes in the battery system by precisely controlling the dosage and LUMO energy levels of three special additives. The synergistic effect of the first and second additives effectively promotes the formation of the SEI film, while the third additive helps maintain the chemical stability of the electrolyte, reduces the occurrence of side reactions, and ultimately significantly improves the initial efficiency and cycling stability of the corresponding lithium-ion battery. DETAILED DESCRIPTION

[0017] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present invention will be described in detail below with reference to the embodiments.

[0018] As described in the background art, in the lithium-ion battery where the battery electrode sheet obtained by dry manufacturing in the prior art (hereinafter referred to as dry electrode sheet) is located, there is a problem of poor first efficiency and cycle stability of the lithium-ion battery due to the side reaction caused by the binder in the dry electrode. In order to solve the above technical problems, the first aspect of the present invention provides an electrolyte suitable for dry electrode sheets, including an organic solvent and a lithium salt. Based on the total weight of the electrolyte suitable for the dry electrode sheet as 100%, the electrolyte suitable for the dry electrode sheet also includes 1% to 5% of a first additive, 2% to 6% of a second additive, and 1% to 3% of a third additive; the lithium salt is LiPF6; the LUMO energy levels of the first additive, the second additive, the third additive and LiPF6 are arranged as follows: first additive < second additive < LiPF6 < third additive.

[0019] By precisely controlling the LUMO energy level order and content of the three additives, the present invention not only effectively inhibits the side reactions of the binder in the dry-process electrode sheet and reduces the loss of active lithium during battery use, but also optimizes the formation process of the SEI film, ultimately significantly improving the initial charge and discharge efficiency and cycle performance of the lithium-ion battery. Specifically:

[0020] The selection and ratio of the first additive and the second additive, these two LUMO energy levels are lower than LiPF6, which ensure that the resulting electrolyte can decompose preferentially over the lithium salt LiPF6 to form a SEI film during the first charge and discharge process of the battery. Especially in the battery system where dry electrode sheets are present, the first additive, due to its lower LUMO energy level, preferentially forms a film at a lower potential, which can effectively wrap the binder in the dry electrode sheet, prevent it from producing side reactions with lithium ions, and reduce the loss of active lithium. The LUMO energy level of the second additive is higher than that of the first additive but lower than LiPF6. It can further modify and optimize the structure of the membrane after the SEI film is formed, improve the density and stability of the membrane, reduce the impedance of the SEI membrane, and play a key role in improving the battery cycle performance. The addition of the third additive with a LUMO energy level higher than LiPF6 means that it will not preferentially decompose to form a SEI film, but will participate in the subsequent modification and stabilization of the SEI membrane, which can effectively improve the elasticity of the SEI membrane, thereby helping to improve the battery cycle performance.

[0021] The precise control of the concentrations of the above three additives directly affects the formation and properties of the SEI film. When the concentration of the first additive is set at around 1% to 5%, a LiF-rich SEI film can be quickly formed, effectively inhibiting the side reactions of the binder in the dry electrode sheet and reducing the loss of active lithium. When the concentration of the second additive is set at 2% to 6%, it can be optimized after the SEI film is formed to generate a dense and stable SEI film, further improving the stability and ionic conductivity of the film. When the concentration of the third additive is set between 1% and 3%, it helps to improve the elasticity of the SEI film, ensure the appropriate thickness and structure of the film, and avoid excessively thick films that increase the internal resistance of the battery and affect battery performance. By adjusting the concentration of the additives, the best balance can be found between the quality of the SEI film and the overall performance of the battery, and ultimately a significant improvement in the first effect and cycle stability of the corresponding lithium-ion battery can be achieved.

[0022] Furthermore, with regard to the content of the above three additives, it is preferred that the total weight of the electrolyte suitable for the dry electrode sheet is 100%, the content of the first additive is 1.8%~2%, the content of the second additive is 3%~4%, and the content of the third additive is 1.5%~2%, so as to more effectively realize the effective control of the SEI film formation, optimization, and maturation process in the corresponding battery application process, while improving the initial charge and discharge efficiency of the battery where the dry electrode is located, and better taking into account the cycle stability, cost, production efficiency and environmental adaptability.

[0023] In several preferred embodiments, the first additive is selected from one or more electrolyte additives and / or sulfonimide additives; the second additive is selected from one or more organic ester additives; and the third additive is selected from one or more electrolyte additives. The present invention utilizes differences in LUMO energy levels to sequentially cause the first, second, and third additives to undergo film formation, SEI film optimization, and SEI film maturation in the electrolyte. Further optimizing the specific types of the three additives can promote a more ordered chemical reaction sequence, more effectively addressing the side reaction issues of dry-process electrode sheets in batteries, thereby significantly improving the stability, ionic conductivity, and chemical compatibility of the SEI film formed during application, ultimately significantly enhancing the initial charge and discharge efficiency and cycle stability of the corresponding battery.

[0024] To ensure that the first and second additives prioritize SEI film formation during the formation process of the lithium-ion battery containing the electrolyte, while the third additive plays a role in the stability and maturity of the SEI film, thereby achieving better battery performance, the preferred weight ratio of the first additive, the second additive, and the third additive is (0.5-2.5):2:1. Furthermore, to more effectively balance the formation speed and maturity of the SEI film, ensuring that the SEI film forms quickly to protect the electrodes while not becoming too thick prematurely and causing an increase in battery internal resistance, thereby achieving better initial charge and discharge efficiency and cycle stability, the preferred weight ratio of the first additive, the second additive, and the third additive is (1-1.2):2:1.

[0025] In several preferred embodiments, the first additive is selected from one or more of lithium bis(oxalatoborate) (LiBOB), lithium trifluoromethanesulfonate (LiTFSI), lithium diethyl oxalate, N-phenylbis(trifluoromethanesulfonyl)imide (PTFSI), N-methylbis(trifluoromethanesulfonyl)imide, and N-ethylbis(trifluoromethanesulfonyl)imide. Lithium bis(oxalatoborate) and / or N-phenylbis(trifluoromethanesulfonyl)imide are preferred because, compared to other electrolyte and sulfonyl imide additives, they can rapidly form a more stable SEI film at lower potentials, thereby more effectively suppressing binder side reactions in dry-process electrodes and reducing active lithium loss, thereby significantly improving the initial charge and discharge efficiency and cycle life of the resulting lithium-ion battery.

[0026] In a particularly preferred embodiment, the first additive is a mixture of lithium bis(oxalatoborate) and N-phenylbis(trifluoromethanesulfonyl)imide, with the weight ratio of lithium bis(oxalatoborate) to N-phenylbis(trifluoromethanesulfonyl)imide being 1:(0.8-1). Combining these two additives in a specific weight ratio as the first additive allows lithium bis(oxalatoborate) to more rapidly form a BO compound-rich SEI film on the electrode surface, providing initial protection and mechanical strength. N-phenylbis(trifluoromethanesulfonyl)imide supplements the formation of a LiF- and lithium sulfonate-rich SEI film, improving the film's ionic conductivity and thermal stability. At this dosage ratio, the synergistic effect of these two additives in SEI film formation is more pronounced, ultimately significantly improving the initial charge / discharge efficiency and cycling stability of the corresponding lithium-ion battery.

[0027] Furthermore, the second additive is preferably selected from one or more of vinylene carbonate (VC), vinyl sulfate (DTD), fluoroethylene carbonate (FEC), di(alkyne) oxalate (BPO), and methylene methanedisulfonate (MMDS) to facilitate preferential film formation at relatively low potentials. The second additive is preferably a mixture of vinylene carbonate and vinyl sulfate, with the weight ratio of vinylene carbonate to vinyl sulfate in the mixture being (2-3):1. This is because the combination of VC and DTD can produce a synergistic effect, with VC helping to form a strong and stable SEI film, while the addition of DTD can reduce SEI film resistance and improve SEI film ionic conductivity, thereby improving the overall efficiency and life of the battery. For comprehensive considerations of battery performance, the two are preferably mixed in the above ratio. At this mixing ratio, the properties of the SEI film are further optimized, not only improving the initial charge and discharge efficiency but also enhancing cycling stability.

[0028] In several preferred embodiments, the third additive is selected from one or more of lithium difluorophosphate (LiPO2F2), lithium chloride (LiCl), lithium phosphate (Li3PO4), and lithium carbonate (Li2CO3). More preferably, it is LiPO2F2 because LiPO2F2 and the VC in the first additive still have a synergistic effect, forming an inorganic-organic hybrid interfacial film rich in polyVC, Li2C2O4, LiF, Li3PO4, etc. on the electrode surface. This effectively enhances the elasticity of the film, thereby significantly improving the stability of the film, reducing the incidence of side reactions, and ultimately improving the cycling performance and energy density of the lithium-ion battery in which the electrolyte is located.

[0029] The organic solvent in the electrolyte system is preferably selected from one or more of ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, propylene carbonate, diethyl carbonate, and butylene carbonate. The organic solvent is preferably selected from at least two of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate because a two-by-two mixture of EC, EMC, and DMC can better balance the dielectric constant and viscosity of the electrolyte, optimize the lithium ion transmission efficiency, and form an electrolyte with better ionic conductivity and chemical stability. More preferably, the organic solvent is a mixed solvent of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate, and the volume ratio of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in the mixed solvent is (0.7-0.8):(0.7-0.8):1. Adjusting the above volume ratio can better leverage the higher dielectric constant of EC to promote lithium ion dissolution, as well as the lower viscosity and good lithium ion conductivity of EMC and DMC, thereby improving fluidity. In particular, it promotes the improvement of lithium ion transmission efficiency and the ability of the three additives to form SEI film, ultimately significantly improving the initial efficiency and cycle life of the battery in which they are used.

[0030] After a large number of experiments, the inventors found that in several particularly preferred embodiments, the electrolyte suitable for the dry-process electrode sheet preferably includes 1% lithium bis(oxalatoborate), 1% N-phenylbis(trifluoromethanesulfonyl)imide, 3% vinylene carbonate, 1% vinyl sulfate and 2% lithium difluorophosphate, based on the total weight of the electrolyte suitable for the dry-process electrode sheet as 100%; or, the electrolyte suitable for the dry-process electrode sheet preferably includes 1. 6% lithium bis(oxalatoborate), 0.4% N-phenylbis(trifluoromethanesulfonyl)imide, 3% vinylene carbonate, 1% vinyl sulfate, and 2% lithium difluorophosphate; or, based on the total weight of the electrolyte suitable for the dry electrode sheet as 100%, the electrolyte suitable for the dry electrode sheet includes 0.8% lithium bis(oxalatoborate), 1.2% N-phenylbis(trifluoromethanesulfonyl)imide, 3% vinylene carbonate, 1% vinyl sulfate, and 2% lithium difluorophosphate. Based on a large amount of experimental data analysis, when the types and concentrations of the above components are optimized, the problem of binder side reactions in dry electrode batteries can be particularly effectively solved, thereby significantly improving the overall performance of the lithium-ion battery in which the dry electrode is located, including an increase in the first coulombic efficiency and enhanced cycle stability.

[0031] The second aspect of the present invention provides a method for preparing the electrolyte adapted for the dry-process electrode sheet, comprising: step S1, preparing an organic solvent and a lithium salt into a lithium salt solution; step S2, adding a first additive to the lithium salt solution, and obtaining a first mixed solution after a first stirring; step S3, adding a second additive and a third additive to the first mixed solution, and obtaining an electrolyte adapted for the dry-process electrode sheet after a second stirring. Step S1 creates the basic framework of the electrolyte, providing a more compatible environment for the subsequent addition of the three additives. Step S2 enables the first additive to be more fully dissolved and evenly dispersed in the lithium salt solution in advance, avoiding local concentrations that are too high or too low and affect the performance of the resulting electrolyte. The second stirring in step S3 further promotes the full mixing of all additives, making the film formation in the subsequent application process more controllable, thereby more effectively improving the initial coulombic efficiency and cycle life of the lithium-ion battery during application.

[0032] Furthermore, in order to promote the uniform dissolution of the first additive, reduce bubbles and local overheating that may be caused by high-speed stirring, and reduce the mechanical damage of the additive, so that the first additive can be evenly dispersed while maintaining its chemical activity, the preferred first stirring speed is 100rpm~200rpm, and the time is 20min~30min. For the second stirring process, the preferred second stirring speed is 400rpm~500rpm, and the time is 30min~40min, so as to further refine the distribution of components in the electrolyte, promote the integration of all additives into the lithium salt solution, form a more uniform and stable electrolyte system, and reduce the loss of additives therein. Furthermore, the conditions for the formation of the SEI film in the subsequent battery formation process are better, and ultimately more effectively improve the various performances of the lithium-ion battery in which it is located.

[0033] Regarding the electrolysis preparation process, at low temperatures, the solubility of lithium salts decreases, and undissolved lithium salt particles lead to localized uneven electrolyte concentrations, reducing the electrolyte ion transfer rate and resulting in poor initial efficiency and cycle performance. At high temperatures, lithium salts easily decompose to form hydrofluoric acid (HF), which corrodes electrode materials and has a significant adverse effect on the battery. Therefore, in several typical embodiments, the first and second stirring steps are independently performed at 25°C to 30°C to provide more suitable thermodynamic conditions. This accelerates the dissolution of additives and improves electrolyte homogeneity, while reducing side reactions that may be caused by overheating, thereby more effectively maintaining the activity of the additives and the stability of the electrolyte.

[0034] In several more typical embodiments, steps S1, S2, and S3 are all performed under conditions where the moisture content is less than 0.1 ppm and the oxygen content is less than 0.1 ppm. Because moisture and oxygen are the main pollutants in electrolyte preparation, their presence can trigger chain side reactions, causing the hydrolysis of lithium salts and additives to generate HF, which corrodes the materials. The additives are oxidized to form an overly porous SEI film structure, which in turn degrades electrolyte and battery performance. By optimizing the environmental conditions during electrolyte preparation as described above, the purity of the electrolyte components, especially the additives therein, can be effectively improved, thereby enhancing the cycling stability and safety of the lithium-ion battery.

[0035] The third aspect of the present invention provides a lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet, and an electrolyte, wherein the electrolyte is the electrolyte adapted for the dry-process electrode sheet, and both the positive electrode sheet and the negative electrode sheet are dry-process electrode sheets. Due to the use of the above-mentioned electrolyte with excellent performance adapted for the dry-process electrode sheet, the low LUMO energy level additive therein can preferentially form a film during the battery formation process, wrapping the PTFE binder, reducing its direct contact with lithium ions, thereby inhibiting the side reaction of the binder and significantly improving the battery's initial charge and discharge efficiency. In the subsequent charging stage, the high LUMO energy level additive modifies and supplements the SEI film that can be formed at low potential, generating a strong and moderately thick SEI film, and ultimately comprehensively improving the battery's initial coulombic efficiency and cycle life.

[0036] In practical applications, the positive electrode sheet and the negative electrode sheet each independently include an electrode active layer and a current collector. The electrode active layer is prepared by mixing, dry forming and rolling an electrode active material, a conductive agent and a binder in sequence.

[0037] In particular, when polytetrafluoroethylene is used as the binder in both the positive and negative electrode sheets, the potential risk of side reactions in the entire battery system increases. However, by using the electrolyte provided by the present invention, side reactions caused by PTFE can be more effectively controlled without affecting other battery properties, significantly improving the battery's overall performance, including capacity retention, coulombic efficiency, and cycle life.

[0038] The present application is further described in detail below with reference to specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in this application.

[0039] Unless otherwise defined, all technical terms used hereinafter have the same meanings as those generally understood by those skilled in the art. The technical terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0040] Example 1

[0041] Preparation of an electrolyte suitable for dry-process electrode sheets:

[0042] The following steps were carried out in an argon-filled glove box.

[0043] Environmental conditions: moisture content <0.1ppm, oxygen content <0.1ppm.

[0044] Step (1): Ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) are mixed in a volume ratio of 3:3:4 for about 30 minutes to obtain an organic solvent, and then 1 mol / L lithium hexafluorophosphate (LiPF6) is slowly added to the organic solvent and stirred for 30 minutes to form a lithium salt solution.

[0045] Step (2), adding N-phenylbis(trifluoromethanesulfonyl)imide (PTFSI), i.e., a first additive, to the lithium salt solution, and stirring at 25° C. at a rotation speed of 200 rpm for 30 minutes to obtain a first mixed solution;

[0046] Step (3): adding vinylene carbonate (VC) and vinyl sulfate (DTD) as the second additive, and lithium difluorophosphate (LiPO2F2) as the third additive to the first mixed solution, and stirring the mixture at 25°C with a rotation speed of 400 rpm for 30 minutes to obtain an electrolyte suitable for dry electrode sheets.

[0047] In the obtained electrolyte, based on the total weight of the electrolyte being 100%, the content of each additive is shown in Table 1.

[0048] Preparation of a lithium-ion battery:

[0049] Dry preparation of positive electrode sheet:

[0050] The positive electrode active material lithium iron phosphate (LiFePO4), the conductive agent SP and the binder polytetrafluoroethylene (PTFE) are placed in a stirring tank according to a weight ratio of 96:2:2. The high-speed shear force generated by high-intensity stirring is used to evenly disperse the materials to obtain a fibrous powder. Then, a pole piece film with a thickness of 160μm is obtained by extrusion molding and rolling. After being compounded with an aluminum foil current collector, a dry-process positive electrode piece for a lithium-ion battery is obtained.

[0051] Dry preparation of negative electrode sheet:

[0052] The negative electrode active material graphite, the conductive agent carbon nanotube (CNT) and the binder polytetrafluoroethylene (PTFE) are placed in a stirring tank in a weight ratio of 97.7:0.8:1.5. The high-speed shear force generated by high-intensity stirring is used to evenly disperse the materials to obtain a fibrous powder. Then, a pole piece film with a thickness of 100 μm is obtained by extrusion molding and rolling. After being compounded with a copper foil current collector, a dry-process negative electrode piece for a lithium-ion battery is obtained.

[0053] Battery cell assembly:

[0054] The positive electrode sheet, diaphragm PE, and negative electrode sheet prepared above are stacked in order, with the diaphragm placed between the positive and negative electrode sheets, and the sheets are stacked to obtain a bare battery cell; the bare battery cell is placed in an aluminum-plastic film outer packaging to obtain a soft-pack battery sample to be filled with liquid; the electrolyte suitable for the dry-process electrode sheet obtained above is injected into it to obtain a lithium-ion battery sample with a capacity of 5Ah.

[0055] The only difference between Examples 2 to 12 and Comparative Examples 1 to 5 and Example 1 is the type and amount of additives, as shown in Table 2.

[0056] Example 13

[0057] Preparation of an electrolyte suitable for dry-process electrode sheets:

[0058] The only difference between this embodiment and embodiment 1 is that the volume ratio of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) in the organic solvent prepared in step (1) is changed to 1:1:2.

[0059] Table 1

[0060]

[0061] In Table 1, the LUMO energy levels of the three additives used and the lithium salt LiPF6 are ranked as follows:

[0062] The first additive (PTFSI, LiBOB) < the second additive (VC, DTD) < LiPF6 < the third additive (LiPO2F2).

[0063] Electrical performance test method for battery samples

[0064] First effect: At 25°C, charge the battery to 2.8V with a constant current of 0.05C, let it stand for 5 minutes, then charge it to 3.3V with a constant current of 0.2C and let it stand for 12 hours. This is the formation stage. Then, charge the battery that has been standing for 12 hours with a constant current and constant voltage of 0.2C to 3.65V, let it stand for 5 minutes, and then discharge it to 2.5V with a constant current of 0.2C to complete the battery capacity separation. The sum of the capacities of the formation and the first step of the capacity separation is the first charge capacity of the battery; the capacity of the capacity separation discharge step is the first discharge capacity of the whole battery. Therefore:

[0065] First efficiency = discharge capacity in the second step of capacity division / (charge capacity in formation + charge capacity in the first step of capacity division) × 100%.

[0066] Cycling stability: At 25°C, each battery sample was charged at a constant current of 0.5C to 3.65V, with a cutoff rate of 0.05C. The sample was then discharged at a constant current of 0.5C to 2.5V. After repeated charge / discharge cycles, the capacity retention after a target number of cycles was calculated to evaluate cycling performance. The target number of cycles was 200.

[0067] The above test results are shown in Table 2.

[0068] Table 2

[0069]

[0070] From the above description, it can be seen that the above-mentioned embodiments of the present invention achieve the preparation of an electrolyte solution with excellent performance for dry-process electrode sheets. The LUMO energy level ordering and content control of the three additives not only effectively suppress the side reactions of the binder in the dry-process electrode sheet and reduce the loss of active lithium during battery use, but also significantly improve the initial charge and discharge efficiency and cycle performance of the lithium-ion battery by optimizing the formation process of the SEI film.

[0071] Specifically, as shown in the performance results in the table:

[0072] According to the performance data comparison of each embodiment and comparative example 1, the introduction of low LUMO energy level additive (PTFSI / LiBOB) can effectively improve the first coulombic efficiency and cycle stability. This may be because PTFSI / LiBOB preferentially generates products rich in LiF, lithium oxalate (LiC2O4), boron oxide compounds (BO) and other products at low potential to wrap PTFE, reducing the bonding between the binder and Li + Direct contact with the battery can inhibit the loss of active lithium caused by the reduction and decomposition of the binder and improve the overall performance of the battery.

[0073] Among them, it can be seen from Comparative Examples 1 to 3 that the first additive improves battery performance, and the second additive can provide organic-inorganic products to further enrich the SEI film interface. Therefore, the battery performance of Comparative Example 3 is better than that of Comparative Example 2.

[0074] Comparative Example 4 is the extreme value of the second and third additives. Excessive content of the second and third additives will cause excessive reduction and decomposition of the battery during the formation stage, and the SEI film formed will be too thick, seriously affecting the battery's initial charge and discharge efficiency; Comparative Example 5 is the extreme value of the first additive. Its content is too high, causing the battery's internal resistance to increase, affecting the ion transfer rate, and increasing the battery polarization, significantly reducing the battery's initial efficiency and cycle performance.

[0075] The performance differences between the various examples reflect the impact of different additive types and dosages. Comparing Examples 1-6 with Examples 7-10, particularly Example 7, reveals that the synergistic effect of PTFSI and LiBOB can form a composite SEI. PTFSI rapidly reacts to generate products such as LiF and lithium sulfonate, providing high mechanical strength and ionic conductivity. LiBOB stabilizes the negative electrode SEI layer, enabling the formation of an SEI film of moderate thickness. These two synergistic effects further enhance battery performance, significantly outperforming single-additive electrolyte systems. Furthermore, excessive PTFSI decomposition products can increase the battery's internal resistance, impacting initial charge and discharge efficiency.

[0076] Comparing Examples 2, 5, 7, 9, and 10 with the remaining examples, it can be seen that when the weight ratio of the first additive, the second additive, and the third additive is preferably (1-1.2):2:1, the formation speed and maturity of the SEI film can be more effectively balanced, so that the SEI film can be formed quickly to protect the electrode, but will not be too thick too early to increase the internal resistance of the battery, thereby achieving better results in the initial charge and discharge efficiency and cycle stability.

[0077] Comparing Example 7 with Examples 9 and 10, it can be seen that when the dosage ratios of the three additives are the same, the preferred weight ratio of PTFSI to LiBOB in the first additive is 1:(0.8-1), which can achieve a more significant synergistic effect in the SEI film formation process and ultimately further comprehensively improve the initial charge and discharge efficiency and cycle stability of the corresponding lithium-ion battery.

[0078] Comparing Example 1 with Examples 11-12, it can be seen that reducing the DTD content of the second additive will lead to insufficient sulfide in SEI, and the SEI crack resistance will decrease in long-term cycles, while increasing the DTD content will lead to more Li + It is consumed to generate Li2SO3 / Li2S, the irreversible capacity of the first cycle increases, and the first efficiency decreases slightly.

[0079] Comparing Example 13 with Example 1, it can be seen that Example 13 changes the solvent ratio and reduces the EC content, which reduces the overall conductivity of the electrolyte and affects the ion transfer rate, thereby having a certain impact on the initial efficiency and cycle of the battery sample.

[0080] It should be noted that the terms "first," "second," and the like in the specification and claims of this application are used to distinguish similar objects, and are not necessarily used to describe a specific order or precedence. It should be understood that the terms used in this manner are interchangeable where appropriate, so that the embodiments of the application described herein can, for example, be implemented in an order other than that described herein.

[0081] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A lithium-ion battery comprising a positive electrode sheet, a negative electrode sheet, and an electrolyte, characterized in that: The electrolyte is an electrolyte adapted for dry-process electrode sheets, and both the positive electrode sheet and the negative electrode sheet are dry-process electrode sheets; The electrolyte adapted for the dry-process electrode sheet includes an organic solvent and a lithium salt. Based on the total weight of the electrolyte adapted for the dry-process electrode sheet being 100%, the electrolyte adapted for the dry-process electrode sheet further includes 1% to 5% of a first additive, 2% to 6% of a second additive, and 1% to 3% of a third additive. The lithium salt is LiPF6; The LUMO energy levels of the first additive, the second additive, the third additive, and the LiPF6 are arranged in the following order: the first additive < the second additive < the LiPF6 < the third additive; The first additive is a mixture of lithium bis(oxalatoborate) and N-phenylbis(trifluoromethanesulfonyl)imide, and in the mixture, the weight ratio of the lithium bis(oxalatoborate) to the N-phenylbis(trifluoromethanesulfonyl)imide is 1:(0.8-1); The second additive is a mixture of vinylene carbonate and vinyl sulfate, and in the mixture, the weight ratio of the vinylene carbonate to the vinyl sulfate is (2-3):1; The third additive is selected from one or more of lithium difluorophosphate, lithium chloride, lithium phosphate and lithium carbonate.

2. The lithium-ion battery according to claim 1, wherein The weight ratio of the first additive, the second additive and the third additive is (0.5-2.5):2:

1.

3. The lithium-ion battery according to claim 1 or 2, characterized in that The organic solvent is selected from one or more of ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, propylene carbonate, diethyl carbonate and butylene carbonate.

4. The lithium-ion battery according to claim 1 or 2, characterized in that The method for preparing the electrolyte suitable for the dry-process electrode sheet includes: Step S1, preparing a lithium salt solution by mixing the organic solvent and the lithium salt; Step S2, adding the first additive to the lithium salt solution, and obtaining a first mixed solution after a first stirring; Step S3, adding the second additive and the third additive to the first mixed solution, and obtaining the electrolyte suitable for the dry-process electrode sheet after a second stirring.

5. The lithium-ion battery according to claim 4, characterized in that The first stirring speed is 200 rpm to 300 rpm, and the stirring time is 20 min to 30 min; and / or, The second stirring speed is 400 rpm to 500 rpm, and the time is 30 min to 40 min.

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