Electrochemical device and electrolyte

Abstract

The present disclosure relates to an electrochemical device and an electrolyte. The electrochemical device comprises an electrolyte, a positive electrode sheet, a negative electrode sheet, and a separator located between the positive electrode sheet and the negative electrode sheet. The negative electrode sheet comprises a negative current collector and a negative electrode film layer arranged on at least one surface of the negative current collector, wherein the negative electrode film layer comprises a negative electrode active material, the negative electrode material comprising graphite and hard carbon. The electrolyte comprises a first additive and an organic solvent, wherein the organic solvent comprises a carboxylate solvent; the reduction potential of the first additive against lithium is less than 1.3 V; and the mass fraction of the first additive in the electrolyte is 2.5% to 8%.

Description

An electrochemical device and electrolyte

[0001] Cross-reference to related applications

[0002] This application is based on and claims priority to Chinese Patent Application No. 202411855619.0, filed on December 16, 2024, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This disclosure belongs to the field of electrochemical technology, and particularly relates to an electrochemical device and an electrolyte. Background Technology

[0004] Electric vehicles have seen significant development in recent years as a crucial means of achieving global carbon emission reduction. Lithium-ion batteries, as a common electrochemical device, have become the most popular energy storage system due to their high operating voltage, long lifespan, and environmental friendliness, and are now widely used in pure electric vehicles, hybrid electric vehicles, smart grids, and other fields. Summary of the Invention

[0005] The main objective of this disclosure is to provide an electrochemical device and an electrolyte. This electrochemical device can achieve both fast charging performance and cycling performance.

[0006] A first aspect of this disclosure provides an electrochemical device comprising an electrolyte, a positive electrode, a negative electrode, and a separator between the positive and negative electrodes. The negative electrode comprises a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector. The negative electrode film layer comprises a negative electrode active material, including graphite and hard carbon. The electrolyte comprises a first additive and an organic solvent, including a carboxylic acid ester solvent. The first additive has a lithium reduction potential of <1.3V, and the mass fraction of the first additive in the electrolyte is 2.5% to 8%.

[0007] In some embodiments, the first additive includes one or more of vinylene carbonate, vinyl sulfate, and fluoroethylene carbonate.

[0008] In some embodiments, the compaction density of the negative electrode film is 1.6 g / cm³. 2 Up to 1.75 g / cm 2 .

[0009] In some embodiments, the carboxylic acid ester solvent includes one or more of ethyl acetate, ethyl propionate, propyl acetate, and propyl propionate.

[0010] In some embodiments, the carboxylic acid ester solvent has a volume fraction of 10% to 70% in the organic solvent.

[0011] In some embodiments, the volumetric particle size distribution (Dv50) of the hard carbon is from 1.5 μm to 6.0 μm.

[0012] In some embodiments, the mass fraction of hard carbon in the negative electrode active material is 1% to 5%.

[0013] In some embodiments, the mass fraction of the first additive in the electrolyte is a, the volume fraction of the carboxylic acid ester solvent in the organic solvent is b, and the compaction density of the negative electrode film is c, wherein a / (b / c) is 0.1 g / cm³. 2 Up to 0.7 g / cm 2 .

[0014] In some embodiments, the volumetric particle size distribution (Dv50) of graphite is 8 μm to 17 μm.

[0015] In some embodiments, the electrolyte further includes a second additive having a lithium reduction potential ≥1.3V.

[0016] In some embodiments, the second additive includes one or more of lithium difluorooxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0017] In some embodiments, the second additive has a mass fraction of 0.1% to 1.5% in the electrolyte.

[0018] In some embodiments, the positive electrode includes a positive active material, which includes lithium iron phosphate.

[0019] Another aspect of this disclosure provides an electrolyte comprising a first additive and an organic solvent, the organic solvent comprising a carboxylic acid ester solvent, the first additive having a lithium reduction potential of <1.3V, and the first additive having a mass fraction of 2.5% to 8% in the electrolyte.

[0020] In some embodiments, the first additive includes one or more of ethylene carbonate, ethylene sulfate, and fluoroethylene carbonate.

[0021] In some embodiments, the carboxylic acid ester solvent includes one or more of ethyl acetate, ethyl propionate, propyl acetate, and propyl propionate.

[0022] In some embodiments, the carboxylic acid ester solvent has a volume fraction of 10% to 70% in the organic solvent.

[0023] In some embodiments, the electrolyte also includes a second additive having a lithium reduction potential ≥1.3V.

[0024] In some embodiments, the second additive includes one or more of lithium difluorooxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0025] In some embodiments, the second additive has a mass fraction of 0.1% to 1.5% in the electrolyte.

[0026] The electrochemical device of this disclosure uses a negative electrode active material that includes both graphite and hard carbon. Hard carbon has a low lithium intercalation energy barrier, which can improve the fast charging performance of the electrochemical device. The carboxylic acid ester solvent in the electrolyte has low viscosity, which can improve the wetting ability of the electrolyte, thereby further improving the fast charging performance of the electrochemical device. At the same time, the first additive in the electrolyte can generate an SEI film at the interface between the negative electrode film layer and the electrolyte, thereby improving the cycle performance of the electrochemical device.

[0027] Additional aspects and advantages of this disclosure will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this disclosure. Attached Figure Description

[0028] The above and / or additional aspects and advantages of this disclosure will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0029] Figure 1 shows a schematic diagram of a negative electrode sheet with severe lithium plating;

[0030] Figure 2 shows a schematic diagram of the negative electrode sheet with edge lithium plating;

[0031] Figure 3 shows a schematic diagram of a negative electrode sheet that does not deposit lithium. Detailed Implementation

[0032] The technical solutions of this disclosure will be clearly and completely described below with reference to the accompanying drawings and exemplary embodiments. However, those skilled in the art will understand that the embodiments described below are only some embodiments of this disclosure, not all embodiments, and are only used to illustrate this disclosure, and should not be regarded as limiting the scope of this disclosure. Based on the embodiments in this disclosure, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this disclosure. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0033] Unless otherwise specified, the terms "comprising" and "including" as used in this disclosure can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0034] Unless otherwise specified, the term "or" is inclusive in this disclosure. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0035] Unless otherwise specified, in this disclosure, "one or more" or "at least one" refers to any one, two, or more of the listed items. "Several" refers to any two or more.

[0036] Electric vehicles have seen significant development in recent years as a crucial means of achieving global carbon emission reduction. Lithium-ion batteries, as a common electrochemical device, have become the most popular energy storage system due to their high operating voltage, long lifespan, and environmental friendliness, and are now widely used in pure electric vehicles, hybrid electric vehicles, smart grids, and other fields.

[0037] In recent years, the market has placed higher demands on electrochemical devices, such as requiring them to balance fast charging capabilities and cycle performance. Improving fast charging performance typically requires increasing the conductivity of materials, but this may sacrifice structural stability. During rapid charging and discharging, significant volume expansion and contraction occur within the electrochemical device; insufficient structural stability can lead to material fracture, thus affecting cycle performance. Therefore, balancing battery cycle performance and fast charging performance remains a challenging technical hurdle for those skilled in the art.

[0038] The first aspect of this disclosure provides an electrochemical device. This electrochemical device can be a lithium-ion battery, a sodium-ion battery, a lithium metal battery, or a sodium metal battery, and the embodiments of this disclosure are not limited thereto.

[0039] Typically, an electrochemical device includes a positive electrode, a negative electrode, a separator, and an electrolyte. During battery charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and extracting. The electrolyte acts as a conductor of ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0040] The electrochemical device disclosed herein includes an electrolyte, a positive electrode, a negative electrode, and a separator between the positive and negative electrodes. The negative electrode includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector. The negative electrode film layer includes a negative electrode active material, which includes graphite and hard carbon. The electrolyte includes a first additive and an organic solvent, which includes a carboxylic acid ester solvent. The first additive has a lithium reduction potential of <1.3V and a mass fraction of 2.5% to 8% in the electrolyte.

[0041] In the above-described technical solution, the negative electrode active material includes graphite and hard carbon. Compared to graphite, hard carbon has a lower lithium intercalation energy barrier. Therefore, adding hard carbon to the negative electrode active material can improve the fast charging performance of the electrochemical device. Simultaneously, adding a low-viscosity carboxylic acid ester solvent to the electrolyte can improve the wetting ability of the electrolyte, allowing it to cover the electrode surface more uniformly and quickly, thereby improving ion transport efficiency and further enhancing the fast charging performance of the electrochemical device. Furthermore, adding a first additive with a lithium reduction potential <1.3V in the aforementioned mass fraction range (2.5% to 8%) to the electrolyte can form a moderately thick and uniform SEI film at the interface between the negative electrode film and the electrolyte before lithium intercalation in the graphite. This SEI film not only provides a stable transport channel for active ions (such as lithium ions) but also prevents the electrolyte solvent from directly contacting the negative electrode active material, reducing unnecessary side reactions and thus improving the cycle stability of the electrochemical device. Therefore, the electrochemical device disclosed in this disclosure can balance fast charging performance and cycle performance.

[0042] In this disclosure, the lithium reduction potential of the first additive is less than 1.3V. In some embodiments, the lithium reduction potential of the first additive is 0.6V or more and less than 1.3V, for example, it can be any value within the range of 1.3V, 1.2V, 1.1V, 1.0V, 0.9V, 0.8V, 0.7V, 0.6V or any combination thereof.

[0043] In this disclosure, the mass fraction of the first additive in the electrolyte is any value within the range of 2.5%, 3%, 4%, 5%, 6%, 7%, 8%, or any combination thereof.

[0044] In this disclosure, the lithium reduction potential of the additive refers to the potential of the additive during the reduction reaction relative to a lithium standard electrode. The lithium reduction potential of the additive can be measured by cyclic voltammetry (CV), potentiostatic step method, electrochemical impedance spectroscopy (EIS), or potentiometric titration. The measurement steps may include: 1) assembling an electrolyte containing the additive with a lithium metal electrode and a reference electrode (typically a lithium reference electrode) into a test cell; 2) setting an appropriate test program (such as cyclic voltammetry or electrochemical impedance spectroscopy) on the battery testing system; 3) performing the test, recording the corresponding data, and determining the lithium reduction potential of the additive by analyzing the data.

[0045] The following is a further explanation of the components of the electrochemical device:

[0046] The negative electrode sheet may include a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, the negative electrode film layer including a negative electrode active material. In this disclosure, the negative electrode active material includes hard carbon and graphite.

[0047] In some embodiments, the volumetric particle size distribution (Dv50) of the hard carbon is from 1.5 μm to 6.0 μm. By setting the volumetric particle size distribution (Dv50) of the hard carbon within this range, the hard carbon can have a suitable specific surface area, resulting in a negative electrode film with abundant porosity when used as a negative electrode active material. This provides ample transport channels for lithium ions, thereby contributing to improved fast-charging performance of the electrochemical device. Exemplarily, the volumetric particle size distribution (Dv50) of the hard carbon can be any value within the range of 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, or any combination thereof.

[0048] In this disclosure, the term "Dv50" (volume distribution particle size) has a well-known meaning in the art, representing the particle size corresponding to a cumulative volume distribution percentage of 50%, which can be determined using instruments and methods known in the art. For example, it can be determined using a laser particle size analyzer, referring to GB / T 19077-2016. The testing instrument can be a Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK.

[0049] In some embodiments, the mass fraction of hard carbon in the negative electrode active material is 1% to 5%. By setting the mass fraction of hard carbon within the above range, the fast charging performance of the electrochemical device can be improved while simultaneously maintaining its capacity and cycle performance. Exemplarily, the mass fraction of hard carbon in the negative electrode active material can be any value within the range of 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or any combination thereof.

[0050] In some embodiments, the volumetric particle size distribution (Dv50) of graphite is between 8 μm and 17 μm. When the Dv50 of graphite is within this range, a reasonable particle size distribution can be formed between graphite and hard carbon, resulting in better porosity in the negative electrode film layer. This is beneficial for improving the transport performance of ions and electrons, and further improving fast charging performance. For example, the Dv50 of graphite can be any value within the range of 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, or any combination thereof.

[0051] In some implementations, the negative electrode current collector can be a metal foil or a composite current collector.

[0052] In some embodiments, the negative electrode film layer further includes a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0053] In some embodiments, the negative electrode film layer further includes a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0054] In some embodiments, the negative electrode film layer also includes other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0055] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0056] electrolytes

[0057] The electrolyte plays a role in conducting ions between the positive and negative electrode plates.

[0058] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0059] In some embodiments, the electrolyte salt includes one or more of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium difluorophosphate (LiPO2F2), lithium bis(fluorosulfonyl)imide (LiTFSI), lithium bis(trifluoromethylsulfonyl)imide, lithium tetrafluoroborate, lithium hexafluoroantimonyate, lithium hexafluoroarsenate, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(pentafluoroethylsulfonyl)imide, lithium tri(trifluoromethylsulfonyl)methyl, or lithium bis(trifluoromethylsulfonyl)imide.

[0060] In this disclosure, the electrolyte further includes a first additive having a lithium reduction potential of <1.3V. In some embodiments, the first additive includes one or more of vinylene carbonate, vinyl sulfate, and fluorovinyl carbonate. The aforementioned first additive can form an SEI film before graphite lithium intercalation. The SEI film exists at the interface between the negative electrode film and the electrolyte, preventing the electrolyte solvent from directly contacting the negative electrode active material, reducing unnecessary side reactions, and thereby improving the cycle performance of the electrochemical device.

[0061] In this disclosure, the electrolyte also includes an organic solvent. The organic solvent includes carbonate solvents. Carbonate solvents include one or more of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), and ethyl propyl carbonate (EPC).

[0062] In this disclosure, the organic solvent also includes a carboxylic acid ester solvent. In some embodiments, the carboxylic acid ester solvent includes one or more of ethyl acetate (EA), ethyl propionate (PP), propyl acetate (PA), and propyl propionate (PP). The aforementioned carboxylic acid ester solvents have low viscosity, which enhances the wetting ability of the electrolyte, allowing the electrolyte to cover the electrode surface more uniformly and quickly, thereby improving ion transport efficiency and thus enhancing the fast charging capability of the electrochemical device.

[0063] In some embodiments, the carboxylic acid ester solvent has a mass fraction of 10% to 70% in the electrolyte. By using the above-mentioned mass fraction of carboxylic acid ester solvent, on the one hand, the viscosity of the electrolyte can be effectively reduced, and the fluidity of the electrolyte inside the battery can be improved, thereby enhancing the fast charging capability of the electrochemical device; on the other hand, the stability of the electrolyte can be maintained, which is beneficial to the cycle performance of the electrochemical device. Exemplarily, the mass fraction of the carboxylic acid ester solvent in the electrolyte can be any value within the range of 10%, 20%, 30%, 40%, 50%, 60%, 70%, or any combination thereof.

[0064] In some embodiments, the electrolyte further includes a second additive having a lithium reduction potential ≥ 1.3V. The second additive with a lithium reduction potential greater than or equal to 1.3V forms an SEI film before lithium intercalation on the hard carbon. The SEI film adheres at least to the surface of the hard carbon, preventing the electrolyte from entering the hard carbon and thus preventing side reactions at the hard carbon / electrolyte interface at low reduction potentials, thereby improving the cycle stability of the electrochemical device. In some embodiments, the lithium reduction potential of the second additive is greater than 1.3V and less than 2V, such as 1.3V, 1.4V, 1.5V, 1.6V, 1.7V, 1.8V, 1.9V, or 2.0V.

[0065] In some embodiments, the second additive includes one or more of lithium difluorooxalate borate (LiDFOB), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP). Using these substances as the second additive is beneficial for further improving the cycle performance of the electrochemical device.

[0066] In some embodiments, the second additive has a mass fraction of 0.1% to 1.5% in the electrolyte. By adding a second additive with a mass fraction within the above range, an SEI film of moderate thickness can be formed, which can reduce side reactions between the electrolyte and the active material and improve the cycle performance of the electrochemical device. Exemplarily, the mass fraction of the second additive in the electrolyte is any value within the range of 0.1%, 0.3%, 0.5%, 0.7%, 0.9%, 1.1%, 1.3%, 1.5%, or any combination thereof.

[0067] In some embodiments, the compaction density of the negative electrode film is 1.6 g / cm³. 2 Up to 1.75 g / cm 2 The compaction density of the negative electrode film layer, within the aforementioned range, balances the battery's energy density and fast-charging capability. For example, the compaction density of the negative electrode film layer can be 1.6 g / cm³. 2 1.62g / cm 2 1.64 g / cm 2 1.66 g / cm 2 1.68g / cm 2 1.70g / cm 2 1.72g / cm 2 1.74 g / cm 2 1.75g / cm 2 Or any value within a range consisting of any two of them.

[0068] In some embodiments, the mass fraction of the first additive in the electrolyte is a, the volume fraction of the carboxylic acid ester solvent in the organic solvent is b, and the compaction density of the negative electrode film is c, wherein a / (b / c) is 0.1 g / cm³. 2 Up to 0.7 g / cm 2 Increasing the compaction density of the negative electrode sheet can improve the energy density of the electrochemical device. However, excessively high compaction density makes it difficult for the electrolyte to uniformly and fully wet the negative electrode sheet, reducing the ionic conductivity of the electrolyte and thus reducing the fast-charging capability of the electrochemical device. To address this, adding a low-viscosity carboxylic acid ester solvent to the electrolyte improves the wetting ability of the electrolyte, while using hard carbon as the negative electrode active material can enhance the fast-charging capability of the electrochemical device. However, hard carbon has many surface defects, which act as reaction sites, exacerbating side reactions between active ions and the electrolyte, thereby reducing the energy density of the electrochemical device. In this disclosure, by adding a first additive, a stable SEI film is formed on the surface of the negative electrode sheet, preventing the above-mentioned side reactions. Based on the above analysis, the relationship between the mass fraction a of the first additive in the electrolyte, the mass fraction b of the carboxylic acid ester solvent in the electrolyte, and the compaction density c of the negative electrode film is satisfied as a / (b / c) 0.1 g / cm³. 2 Up to 0.7 g / cm 2 In this way, the electrochemical device can balance fast charging performance, energy density, and cycle stability. For example, a / (b / c) can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, or 0.70 g / cm³. 2 Or any value within a range consisting of any two of them.

[0069] Positive electrode sheet

[0070] In this disclosure, the positive electrode sheet includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including the positive electrode active material of the first aspect of this disclosure.

[0071] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0072] In some implementations, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil.

[0073] When the battery cell is a lithium-ion battery, the positive electrode active material can be any positive electrode active material known in the art for lithium-ion batteries. As an example, the positive electrode active material may include at least one of the following materials: lithium-containing phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this disclosure is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more. Exemplarily, lithium transition metal oxides include LiNi. 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 At least one of the following.

[0074] In some embodiments, the positive electrode includes a positive active material, such as lithium iron phosphate. Lithium iron phosphate has high structural stability, thus providing excellent safety performance for the electrochemical device. Furthermore, the addition of a carboxylic acid ester solvent to the electrolyte utilizes the low viscosity of the carboxylic acid ester solvent to improve the fast-charging performance of the electrochemical device. This overcomes the shortcomings of lithium iron phosphate in fast-charging performance due to its lower electron-ion conductivity. Therefore, the electrochemical device disclosed in this disclosure balances both safety and fast-charging performance.

[0075] In some embodiments, the number-average particle size of lithium iron phosphate is from 0.2 μm to 0.4 μm. Exemplarily, the number-average particle size of lithium iron phosphate is any value within the range of 0.2 μm, 0.3 μm, 0.4 μm, or any combination thereof. In some embodiments, the positive electrode film layer further includes a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a terpolymer of PVDF-tetrafluoroethylene-propylene, a terpolymer of PVDF-hexafluoropropylene-tetrafluoroethylene, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorinated acrylate resin.

[0076] In some embodiments, the positive electrode film layer further includes a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0077] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0078] Separating membrane

[0079] In this disclosure, the electrochemical device also includes a separator. This disclosure does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0080] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0081] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0082] Example 1

[0083] The preparation method of the electrochemical device is as follows:

[0084] (1) Preparation of negative electrode sheet

[0085] A negative electrode active material (artificial graphite with a Dv50 of 12.6 μm and hard carbon with a Dv50 of 3.4 μm), conductive carbon (SP), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed in deionized water at a weight ratio of 96.4:1:1.2:1.4 and stirred until homogeneous to obtain a negative electrode slurry. The mass fraction of hard carbon in the negative electrode active material was 3%. Copper foil was used as the negative electrode current collector. The negative electrode slurry was coated on one side of the negative electrode current collector and baked at 120°C for 1 hour. Subsequently, it was cold-pressed, cut, and slit to prepare the negative electrode sheet. The thickness of the negative electrode active layer was 104 μm, and the compaction density of the negative electrode film was 1.6 g / cm³. 2 .

[0086] (2) Preparation of positive electrode sheet

[0087] Lithium iron phosphate (number average particle size of 0.2 μm to 0.4 μm), conductive carbon (SP), carbon nanotubes (CNTs), and polyvinylidene fluoride (PVDF) were mixed in N-methylpyrrolidone solvent at a weight ratio of 96.3:0.7:1:2 and stirred until homogeneous to obtain a positive electrode slurry. Aluminum foil was used as the positive electrode current collector, and the positive electrode slurry was coated onto one side of the current collector. The foil was baked at 85°C for 1 hour, followed by cold pressing, cutting, and slitting to prepare the positive electrode sheet, wherein the thickness of the positive electrode active layer was 136 μm.

[0088] (3) Preparation of electrolytes

[0089] In an argon atmosphere, ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and ethyl acetate (EA) were mixed in a volume ratio of 3:2:4:1 to serve as an organic solvent. Then, vinylene carbonate (VC, with a lithium reduction potential <1.3V, used as a first additive) and lithium hexafluorophosphate (LiPF6) were added to the organic solvent to obtain the electrolyte. The electrolyte contained 4% vinylene carbonate and 12.5% ​​lithium hexafluorophosphate by mass.

[0090] (4) Preparation of the separating membrane

[0091] A polyethylene film with a thickness of 7 μm is used as the substrate. A PVDF coating with a thickness of 5 μm is applied to both sides of the substrate. A boehmite ceramic layer with a thickness of 3 μm is applied to one of the PVDF coated surfaces to obtain the isolation membrane.

[0092] (5) Assembly of electrochemical devices

[0093] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The side of the separator coated with boehmite ceramic is aligned with the positive electrode. The stacked positive electrode, separator, and negative electrode are then placed in an aluminum-plastic film, dried at 80°C, and injected with the prepared electrolyte. After vacuum sealing, settling, formation, and shaping, the electrochemical device is obtained.

[0094] The performance of the obtained electrochemical device was tested using the following methods:

[0095] Fast charging performance test:

[0096] The electrochemical device was placed in a constant temperature chamber at 25°C and left to stand for 30 minutes. It was then charged at a constant charging rate of 4C to 3.65V, followed by constant voltage charging to a charging rate of 0.05C. After standing for 5 minutes, it was discharged at a constant discharge rate of 1.0C to 3.0V. This cycle was repeated 100 times, with each cycle followed by a 5-minute rest, charging at a constant charging rate of 4C to 3.65V, and then constant voltage charging to a charging rate of 0.05C. The electrochemical device was then disassembled to obtain the negative electrode, and the lithium deposition on the surface of the negative electrode was observed.

[0097] Basis for judgment:

[0098] As shown in Figure 1, the presence of metallic lithium deposition on a large surface of the negative electrode indicates severe lithium deposition on the negative electrode, resulting in poor fast charging performance of the battery.

[0099] As shown in Figure 2, lithium metal is deposited at the edge of the negative electrode, indicating that lithium deposition at the edge of the negative electrode improves the battery's fast charging performance.

[0100] As shown in Figure 3, no metallic lithium is deposited on the surface of the negative electrode, indicating that the negative electrode does not deposit lithium, and the fast charging performance of the battery is further improved.

[0101] In this embodiment of the disclosure, if the disassembled negative electrode sheet exhibits any of the conditions shown in Figure 1 or Figure 2, it is determined that the negative electrode sheet has undergone lithium plating. If the negative electrode sheet exhibits the condition shown in Figure 3, it is determined that the negative electrode sheet has not undergone lithium plating.

[0102] Cyclic performance test:

[0103] The electrochemical device was placed in a constant temperature chamber at 25°C and allowed to stand for 30 minutes. It was then charged at a constant charging rate of 4C to 3.65V, followed by constant voltage charging to a charging rate of 0.05C. After standing for 5 minutes, it was discharged at a constant discharge rate of 1.0C to 3.0V, and the capacity was recorded as D0. Cyclic testing was then performed according to the following steps.

[0104] 1) Let stand for 5 minutes;

[0105] 2) Charge at a constant charging rate of 4C to 3.65V; then charge at a constant voltage to a charging rate of 0.05C;

[0106] 3) Let stand for 5 minutes;

[0107] 4) Discharge to 3.0V at a constant discharge rate of 1.0C;

[0108] 5) Repeat steps 1) to 4) 1500 times; the recording capacity is D1;

[0109] Cyclic capacity retention (%) = (D1-D0) / D0 × 100%.

[0110] Example 2

[0111] The electrochemical device was prepared using the same method as in Example 1, except that in Example 2, the mass fraction of the first additive VC in the electrolyte was 2.5%.

[0112] Example 3

[0113] The electrochemical device was prepared using the same method as in Example 1, except that in Example 3, the mass fraction of the first additive VC in the electrolyte was 8%.

[0114] Comparative Example 1

[0115] The electrochemical device was prepared using the same method as in Example 1, except that, in Comparative Example 1, hard carbon was not included in the negative electrode active material.

[0116] Comparative Example 2

[0117] The electrochemical device was prepared using the same method as in Example 1, except that in Comparative Example 2, the organic solvent included EC:EMC:DMC in a volume ratio of 3:5:2, i.e., the organic solvent did not include carboxylic acid esters.

[0118] Comparative Example 3

[0119] The electrochemical device was prepared using the same method as in Example 1, except that in Comparative Example 3, the mass fraction of the first additive VC in the electrolyte was 1%.

[0120] Comparative Example 4

[0121] The electrochemical device was prepared using the same method as in Example 1, except that in Comparative Example 4, the mass fraction of the first additive VC in the electrolyte was 10%.

[0122] The electrochemical devices obtained in Examples 1 to 3 and Comparative Examples 1 to 4 were tested using the same test methods as in Example 1. The test results are shown in Table 1 below.

[0123] Table 1

[0124] As can be seen from the data in Table 1, the negative electrode active material includes hard carbon, the electrolyte includes a first additive with a mass fraction of 2.5% to 8%, and the organic solvent of the electrolyte includes carboxylic acid esters. The resulting electrochemical device can balance fast charging performance and cycle performance.

[0125] Example 4

[0126] The electrochemical device was prepared using the same method as in Example 1, except that in the electrolyte preparation step, vinyl sulfate (DTD, with a lithium reduction potential of <1.3V, as the first additive) was used instead of VC, and the mass fraction of DTD in the electrolyte was 2%.

[0127] Example 5

[0128] The electrochemical device was prepared using the same method as in Example 1, except that in Example 5, fluoroethylene carbonate (FEC, lithium reduction potential <1.3V, as the first additive) was used instead of VC in the electrolyte preparation step.

[0129] The electrochemical devices obtained in Examples 4 and 5 were tested using the same testing methods as in Example 1.

[0130] The test results of Examples 4 and 5 and related examples are shown in Table 2 below.

[0131] Table 2

[0132] As can be seen from the data in Table 2, when vinylene carbonate (VC), vinyl sulfate (DTD), or fluoroethylene carbonate (FEC) is used as the first additive, the chemical device can achieve both fast charging performance and cycle performance.

[0133] Example 6

[0134] The electrochemical device was prepared using the same method as in Example 1, except that in Example 6, during the negative electrode preparation step, the spacing of the cold pressing rollers was adjusted so that the compaction density of the negative electrode was 1.65 g / cm³. 2 The electrolyte is prepared by the following method:

[0135] Under an argon atmosphere, ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl acetate (EA) were mixed in a volume ratio of 3:1:6 to obtain an organic solvent. Then, a first additive, vinylene carbonate (VC), was added to the organic solvent, followed by the addition of lithium hexafluorophosphate (LiPF6) to obtain an electrolyte. The electrolyte contained 5% by mass of the first additive and 12.5% ​​by mass of lithium hexafluorophosphate.

[0136] Example 7

[0137] The electrochemical device was prepared using the same method as in Example 1, except that in Example 7, during the negative electrode preparation step, the spacing of the cold pressing rollers was adjusted so that the compaction density of the negative electrode was 1.7 g / cm³. 2 The electrolyte is prepared by the following method:

[0138] Under an argon atmosphere, ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and ethyl acetate (EA) were mixed in a volume ratio of 3:1:4:2 to obtain an organic solvent. Then, a first additive, vinylene carbonate (VC), was added to the organic solvent, followed by the addition of lithium hexafluorophosphate (LiPF6) to obtain an electrolyte. The mass fraction of the first additive in the electrolyte was 5%, and the mass fraction of lithium hexafluorophosphate was 12.5%.

[0139] Example 8

[0140] The electrochemical device was prepared using the same method as in Example 1, except that in Example 8, the mass fraction of hard carbon in the negative electrode active material was adjusted to 2% during the negative electrode preparation step, and the spacing of the cold pressing rollers was adjusted during the cold pressing process so that the compaction density of the negative electrode sheet was 1.75 g / cm³. 2 The electrolyte is prepared by the following method:

[0141] Under an argon atmosphere, an organic solvent was prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and ethyl acetate (EA) in a volume ratio of 3:4:3. Then, a first additive, vinylene carbonate (VC), was added to the organic solvent, followed by the addition of lithium hexafluorophosphate (LiPF6) to obtain an electrolyte. The electrolyte contained 4% by mass of the first additive and 12.5% ​​by mass of lithium hexafluorophosphate.

[0142] Example 9

[0143] The electrochemical device was prepared using the same method as in Example 1, except that in Example 9, the mass fraction of hard carbon in the negative electrode active material was adjusted to 5% in the negative electrode preparation step, and the electrolyte was prepared by the following method:

[0144] Under an argon atmosphere, ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and ethyl propionate (EP) were mixed in a volume ratio of 3:1:5:1 to obtain an organic solvent. Then, a first additive, vinylene carbonate (VC), was added to the organic solvent, followed by lithium hexafluorophosphate (LiPF6) to obtain an electrolyte. The mass fraction of the first additive in the electrolyte was 2.5%, and the mass fraction of lithium hexafluorophosphate was 12.5%.

[0145] Example 10

[0146] The electrochemical device was prepared using the same method as in Example 10, except that in the negative electrode preparation step, the mass fraction of hard carbon in the negative electrode active material was adjusted to 1%, and the spacing of the cold pressing rollers during the cold pressing process was adjusted so that the compaction density of the negative electrode was 1.75 g / cm³. 2 .

[0147] Example 11

[0148] The electrochemical device was prepared using the same method as in Example 1, except that in the negative electrode preparation step of Example 11, the spacing of the cold pressing rollers was adjusted during the cold pressing process to achieve a compaction density of 1.65 g / cm³. 2 The electrolyte is prepared by the following method:

[0149] An organic solvent was prepared by mixing ethylene carbonate (EC) and ethyl acetate (EA) at a volume ratio of 3:7 under an argon atmosphere. Then, a first additive, vinylene carbonate (VC), was added to the organic solvent, followed by lithium hexafluorophosphate (LiPF6) to obtain an electrolyte. In the electrolyte, the mass fraction of the first additive was 5%, and the mass fraction of lithium hexafluorophosphate was 12.5%.

[0150] The electrochemical devices obtained in Examples 6 to 11 were tested using the same test methods as in Example 1. The test results of Examples 6 to 11 and related examples are shown in Tables 3-2 below.

[0151] Table 3-1

[0152] Table 3-2

[0153] As can be seen from the data in Tables 3-1 and 3-2, the compaction density of the negative electrode film is 1.6 g / cm³. 2 Up to 1.75 g / cm 2 When the mass fraction of the carboxylic acid ester solvent in the electrolyte is between 10% and 70%, the resulting electrochemical device can achieve both fast charging performance and cycling performance.

[0154] Example 12

[0155] The electrochemical device was prepared using the same method as in Example 1, except that in Example 12, the electrolyte was prepared using the following method:

[0156] In an argon atmosphere, solvents ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl acetate (EA) were mixed in a volume ratio of 3:3:4. Then, the first additive, vinylene carbonate (VC), was added, followed by lithium hexafluorophosphate (LiPF6) to obtain the electrolyte. The electrolyte contained 4.5% by mass of the first additive and 12.5% ​​by mass of lithium hexafluorophosphate.

[0157] Example 13

[0158] The electrochemical device was prepared using the same method as in Example 1, except that in Example 13, the mass fraction of hard carbon in the negative electrode active material was 2%, and the electrolyte was prepared using the following method:

[0159] In an argon atmosphere, ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and ethyl acetate (EA) were mixed in a volume ratio of 3:1:4:2 to obtain an organic solvent. Then, a first additive, vinylene carbonate (VC), was added to the organic solvent, followed by the addition of lithium hexafluorophosphate (LiPF6) to obtain an electrolyte. The mass fraction of the first additive in the electrolyte was 3%, of which the mass fraction of lithium hexafluorophosphate was 12.5%.

[0160] Example 14

[0161] The electrochemical device was prepared using the same method as in Example 1, except that in Example 14, the electrolyte was prepared using the following method:

[0162] Under an argon atmosphere, ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and ethyl acetate (EA) were mixed in a volume ratio of 3:1:4:2 to obtain an organic solvent. Then, a first additive, vinylene carbonate (VC), was added to the organic solvent, followed by lithium hexafluorophosphate (LiPF6) to obtain an electrolyte. The mass fraction of the first additive in the electrolyte was 4%, and the mass fraction of lithium hexafluorophosphate was 12.5%.

[0163] The electrochemical devices obtained in Examples 12 to 14 were tested using the same test methods as in Example 1. The test results of Examples 12 to 14 and related examples are shown in Table 4 below.

[0164] Table 4

[0165] The data in Table 4 show that the relationship between the mass fraction a of the first additive in the electrolyte, the mass fraction b of the carboxylic acid ester solvent in the electrolyte, and the compaction density c of the negative electrode film satisfies a / (b / c) = 0.1 g / cm³. 2 Up to 0.7 g / cm 2 At the same time, the electrochemical device can balance fast charging performance and cycle stability.

[0166] Example 15

[0167] The electrochemical device was prepared using the same method as in Example 1, except that in Example 15, the electrolyte was prepared by the following method:

[0168] In an argon atmosphere, ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl acetate (EA) were mixed in a volume ratio of 3:4:3 to obtain an organic solvent. Then, a first additive, vinylene carbonate (VC), was added to the organic solvent, followed by the addition of lithium hexafluorophosphate (LiPF6) to obtain an electrolyte. The mass fraction of the first additive in the electrolyte was 4%, of which the mass fraction of lithium hexafluorophosphate was 12.5%.

[0169] Example 16

[0170] The electrochemical device was prepared using the same method as in Example 15, except that in Example 16, the Dv50 of the negative electrode active material, hard carbon, was 1.5 μm.

[0171] Example 17

[0172] The electrochemical device was prepared using the same method as in Example 15, except that in Example 17, the Dv50 of the negative electrode active material, hard carbon, was 6 μm.

[0173] Example 18

[0174] The electrochemical device was prepared using the same method as in Example 15, except that in Example 18, the Dv50 of the negative electrode active material, artificial graphite, was 8 μm and the Dv50 of hard carbon was 6 μm.

[0175] Example 19

[0176] The electrochemical device was prepared using the same method as in Example 15, except that in Example 19, the Dv50 of the negative electrode active material, artificial graphite, was 17 μm and the Dv50 of hard carbon was 6 μm.

[0177] The electrochemical devices obtained in Examples 15 to 19 were tested using the same test methods as in Example 1. The test results are shown in Table 5 below.

[0178] Table 5

[0179] As can be seen from the data in Table 5, when the Dv50 of hard carbon is in the range of 1.5 μm to 6 μm and the Dv50 of graphite is in the range of 8 μm to 17 μm, the electrochemical device can achieve both fast charging performance and cycling performance.

[0180] Example 20

[0181] The electrochemical device was prepared using the same method as in Example 15, except that in Example 20, the electrolyte further included a second additive, lithium difluorooxalatoborate (LiDFOB), at a mass fraction of 0.3%.

[0182] Example 21

[0183] The electrochemical device was prepared using the same method as in Example 15, except that in Example 21, the electrolyte further included a second additive, lithium difluorodioxalate phosphate (LiDFOP), at a mass fraction of 0.5%.

[0184] Example 22

[0185] The electrochemical device was prepared using the same method as in Example 15, except that in Example 22, the electrolyte further included a second additive, lithium tetrafluorooxalate phosphate (LiTFOP), at a mass fraction of 0.5%.

[0186] Example 23

[0187] The electrochemical device was prepared using the same method as in Example 15, except that in Example 23, the electrolyte further included 0.5% by mass of lithium difluorooxalatoborate (LiDFOB) and 0.5% by mass of lithium difluorodioxalatophosphate (LiDFOP).

[0188] Example 24

[0189] The electrochemical device was prepared using the same method as in Example 15, except that in Example 24, the electrolyte further included a second additive lithium difluorooxalatoborate (LiDFOB) at a mass fraction of 0.8% and a second additive lithium difluorodioxalatophosphate (LiDFOP) at a mass fraction of 0.5%.

[0190] Example 25

[0191] The electrochemical device was prepared using the same method as in Example 15, except that in Example 25, the electrolyte further included a second additive, lithium difluorodioxalate phosphate (LiDFOP), at a mass fraction of 0.1%.

[0192] Example 26

[0193] The electrochemical device was prepared using the same method as in Example 15, except that in Example 26, the electrolyte further included 0.8% by mass of lithium difluorooxalate borate (LiDFOB), 0.5% by mass of lithium difluorodioxalate phosphate (LiDFOP), and 0.2% by mass of lithium tetrafluorooxalate phosphate (LiTFOP).

[0194] The electrochemical devices obtained in Examples 20 to 26 were tested using the same test methods as in Example 1. The test results of Examples 20 to 26 and related examples are shown in Table 6 below.

[0195] Table 6

[0196] As can be seen from the data in Table 6, when a second additive with a mass fraction of 0.1% to 1.5% is further included in the electrolyte, the cycle performance of the electrochemical device can be further improved, while also taking into account the fast charging performance.

[0197] It should be noted that this disclosure is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same essential structure and achieving the same effect as the technical concept within the scope of this disclosure are included in the technical scope of this disclosure. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, are also included in the scope of this disclosure without departing from the spirit of this disclosure.

Claims

1. An electrochemical device, wherein, The electrochemical device includes an electrolyte, a positive electrode, a negative electrode, and a separator between the positive and negative electrode. The negative electrode includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector. The negative electrode film layer includes a negative electrode active material, which includes graphite and hard carbon. The electrolyte includes a first additive and an organic solvent, which includes a carboxylic acid ester solvent. The first additive has a lithium reduction potential of <1.3V and a mass fraction of 2.5% to 8% in the electrolyte.

2. The electrochemical device according to claim 1, wherein, The first additive includes one or more of vinylene carbonate, vinyl sulfate, and fluoroethylene carbonate.

3. The electrochemical device according to claim 1 or 2, wherein, The compaction density of the negative electrode film is 1.6 g / cm³. 2 Up to 1.75 g / cm 2 .

4. The electrochemical device according to any one of claims 1 to 3, wherein, The carboxylic acid ester solvent includes one or more of ethyl acetate, ethyl propionate, propyl acetate, and propyl propionate.

5. The electrochemical device according to any one of claims 1 to 4, wherein, The carboxylic acid ester solvent has a volume fraction of 10% to 70% in the organic solvent.

6. The electrochemical device according to any one of claims 1 to 5, wherein, The volumetric particle size Dv50 of the hard carbon is 1.5 μm to 6.0 μm.

7. The electrochemical device according to any one of claims 1 to 6, wherein, The hard carbon in the negative electrode active material has a mass fraction of 1% to 5%.

8. The electrochemical device according to any one of claims 1 to 7, wherein, The first additive has a mass fraction of 'a' in the electrolyte, the carboxylic acid ester solvent has a volume fraction of 'b' in the organic solvent, and the negative electrode film has a compaction density of 'c', wherein a / (b / c) is 0.1 g / cm³. 2 Up to 0.7 g / cm 2 .

9. The electrochemical device according to any one of claims 1 to 8, wherein, The volumetric particle size distribution (Dv50) of the graphite is 8 μm to 17 μm.

10. The electrochemical device according to any one of claims 1 to 9, wherein, The electrolyte also includes a second additive, the second additive having a lithium reduction potential ≥1.3V.

11. The electrochemical device according to claim 10, wherein, The second additive includes one or more of lithium difluorooxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

12. The electrochemical device according to claim 10 or 11, wherein, The second additive has a mass fraction of 0.1% to 1.5% in the electrolyte.

13. The electrochemical device according to any one of claims 1 to 12, wherein, The positive electrode sheet includes a positive active material, which includes lithium iron phosphate.

14. An electrolyte, wherein, The electrolyte comprises a first additive and an organic solvent, the organic solvent comprising a carboxylic acid ester solvent, the first additive having a lithium reduction potential of <1.3V, and the first additive having a mass fraction of 2.5% to 8% in the electrolyte.

15. The electrolyte according to claim 14, wherein, The first additive includes one or more of ethylene carbonate, ethylene sulfate, and fluoroethylene carbonate.

16. The electrolyte according to claim 14 or 15, wherein, The carboxylic acid ester solvent includes one or more of ethyl acetate, ethyl propionate, propyl acetate, and propyl propionate.

17. The electrolyte according to any one of claims 14 to 16, wherein, The carboxylic acid ester solvent has a volume fraction of 10% to 70% in the organic solvent.

18. The electrolyte according to any one of claims 14 to 17, wherein, The electrolyte also includes a second additive, the second additive having a lithium reduction potential ≥1.3V.

19. The electrolyte according to claim 18, wherein, The second additive includes one or more of lithium difluorooxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

20. The electrolyte according to claim 18 or 19, wherein, The second additive has a mass fraction of 0.1% to 1.5% in the electrolyte.

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