Electrolyte and preparation method and application thereof

Abstract

The invention provides an electrolyte and a preparation method and application thereof, and relates to the technical field of lithium ion batteries. Specifically, the electrolyte comprises a lithium salt, a boric acid ester solvent, a carbonic ester solvent and a fluoro-ether diluent, wherein the boric acid ester solvent comprises at least one of a mixture of tripropargyl boric acid ester and a fluoro boric acid ester compound or triethyl borate. According to the invention, the technical defects of high viscosity, low solubility with a lithium salt, poor compatibility with a negative electrode and the like of a borate solvent can be effectively solved; by adopting the preparation process of two times of liquid injection and two times of formation, the prepared battery has good performance in the aspects of production efficiency, electrochemical performance, safety performance and the like, and has a good application prospect.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to an electrolyte and a preparation method and application thereof. Background Art

[0002] The existing lithium battery electrolyte solvent system mainly uses chain carbonates in combination with cyclic carbonates to balance the physical properties of the electrolyte, such as viscosity, conductivity, and saturated vapor pressure. However, carbonate solvents have the disadvantage of being flammable. When the battery experiences thermal runaway, the combustion of the carbonate solvent itself and the reaction of the carbonate solvent with the positive and negative electrode materials at high temperatures are the main sources of heat generated during thermal runaway.

[0003] Correspondingly, borate esters have similar physical and chemical properties to carbonates, rich structural types, and borate ester solvents can remove free radicals generated during battery thermal runaway, thereby blocking the battery thermal runaway chain reaction. Therefore, borate ester solvents have good flame retardancy and can be used as flame-retardant electrolyte solvents. Specifically, polymers decompose to form a small amount of hydrogen radicals and hydroxyl radicals during the combustion process. Borates and fluoroborates generate free radicals during thermal decomposition and capture hydrogen radicals and hydroxyl radicals, thereby preventing or slowing down the combustion chain reaction and enhancing the flame retardant effect. At the same time, borate ester compounds decompose at high temperatures to produce boron-containing flame retardant gases, which reduce the concentration of combustible gases and thus delay the spread of flames.

[0004] However, borate solvents have defects such as high viscosity, low solubility in lithium salts, poor compatibility with negative electrodes, and poor wettability, which to some extent reduce the injection efficiency of lithium-ion batteries, the wetting effect of electrolyte on the electrode, and the physical and chemical properties of the solid electrolyte interface (SEI) formed during formation. Therefore, borate solvents cannot be used as main solvents at present.

[0005] The problem of poor compatibility between borate solvents and negative electrodes can be solved by increasing the concentration of lithium salts. However, this method will use a large amount of lithium salts, resulting in a significant increase in the cost of the electrolyte. In addition, the viscosity of the high-concentration electrolyte increases and its wettability on the diaphragm and electrode interface is poor, which has a significant impact on the rate performance and manufacturing process of lithium-ion batteries.

[0006] For example, CN201310267745.X discloses a new borate solvent for lithium-ion secondary batteries. By using borate compounds and fluoroborates mixed compounds containing ether chains of specific structures, the water in the electrolyte can be automatically eliminated, reducing the impact of water on battery performance; in addition, the combustion product boron trioxide can adhere to the electrode surface and has solid flame retardant ability; and the solvent has little effect on the performance of lithium-ion batteries such as specific capacity and cyclability. Another example is CN202010418936.1 discloses an electrolyte containing a borate compound. By using a borate and fluoroborate mixed compound containing an S heterocycle, the electrode structure can be stabilized, the dissolution of transition metals can be inhibited, the side reactions at the interface between the positive electrode and the electrolyte can be reduced, and the electrochemical performance and safety performance of the battery can be improved. For example, CN202210352711.X discloses an electrolyte containing a fluorinated alkyl borate compound, which includes an electrolyte, an organic solvent, and a mixed compound of a fluorinated alkyl borate and a fluorinated borate having a specific structure. When used in a battery, the electrochemical properties of the resulting battery, such as charge and discharge performance and capacity retention rate, are improved.

[0007] However, the above existing processes cannot solve the problems of high viscosity of borate ester and fluoroborate mixed solvents, low solubility in lithium salts, and poor compatibility with negative electrodes. In addition, the battery rate performance of using borate ester and fluoroborate mixed solvents alone is poor, and the battery rate performance, cycle performance and high temperature performance cannot be improved. It cannot significantly improve the safety of the electrolyte and cannot effectively inhibit the thermal runaway reaction of the battery.

[0008] In view of this, the present invention is proposed. Summary of the invention

[0009] The first purpose of the present invention is to provide an electrolyte that solves the defects of high electrolyte viscosity, low solubility, and poor negative electrode compatibility under the premise of using a borate solvent as the main solvent, so that it has good comprehensive performance in terms of battery rate, preparation efficiency, high temperature performance and safety performance.

[0010] The second object of the present invention is to provide a method for preparing a battery containing the electrolyte, which is simple and easy to implement and can be promoted on a large scale; by adopting the methods of secondary liquid injection and secondary formation, the purpose of regulating the physical and chemical properties of the negative electrode and positive electrode film formation is achieved.

[0011] A third object of the present invention is to provide a lithium battery.

[0012] A fourth object of the present invention is to provide an electrical device.

[0013] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are particularly adopted:

[0014] An electrolyte comprising a lithium salt, a borate solvent, a carbonate solvent and a fluoroether diluent;

[0015] The borate solvent includes at least one of a mixture of tripropargyl borate and a fluoroborate compound or triethyl borate.

[0016] A method for preparing a battery, wherein the battery comprises the electrolyte, and the method comprises the following steps:

[0017] (1) preparing a first electrolyte; the first electrolyte comprising a lithium salt, a carbonate solvent and a fluoroether diluent; injecting the first electrolyte into a dry cell and performing a first formation treatment to obtain a battery precursor;

[0018] (2) preparing a second electrolyte; the second electrolyte comprising a lithium salt, a borate solvent and a fluoroether diluent; injecting the second electrolyte into the battery precursor and performing a second formation treatment to obtain a battery.

[0019] A lithium battery is prepared by the battery preparation method.

[0020] An electrical device comprises the lithium battery.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] (1) The present invention provides an electrolyte with high safety performance and high dynamic performance, which is mainly achieved by combining two types of mixed solvents, borate esters and borate esters, while ensuring the safety performance of the electrolyte, and realizing the high solubility of the electrolyte solvent to lithium salts and the high wetting effect on the electrode. Compared with borate esters or fluoroborates, the electrolyte of the present invention can effectively increase the infiltration speed of the electrolyte to the electrode, reduce the infiltration time after the battery is filled, greatly improve the production efficiency, reduce the generation of black spots and other unwetted dead areas during battery formation, and reduce the risk of lithium precipitation in the later cycle of the battery.

[0023] (2) The present invention provides an electrolyte with a specific composition and a specific preparation process thereof. By using a borate solvent with high safety performance and a carbonate solvent with high kinetic performance, and coordinating the partial injection and partial formation methods, an electrolyte with carbonate solvent as the main body is added in the first injection, and an electrolyte with borate solvent as the main body is added in the second injection; by partially injecting electrolytes of different solvent systems and combining the partial formation method, the wetting speed and wetting effect of the electrolyte on the electrode can be effectively improved, and the physical and chemical properties of the solid electrolyte interface film (SEI) can be improved, thereby improving the overall battery performance. Furthermore, by using the partial formation method and coordinating the electrolyte solvent and additives added in the injection scheme, the physical and chemical properties of the negative electrode SEI film and the positive electrode CEI film are optimized, achieving the goal of greatly improving the safety performance while ensuring the cycle performance and high temperature performance of the battery.

[0024] (3) By introducing fluoroether diluents into the electrolyte of the present invention, the total salt concentration can be reduced, while retaining the local coordination environment of the high-concentration salt-solvent clusters in the high-concentration electrolyte, so that the anions participate in the formation of the solid-liquid interface film, which greatly changes the physical and chemical properties of the solid-liquid interface film. In addition, the fluoroether diluent can also reduce the overall viscosity of the high-concentration electrolyte, effectively making up for the high cost, high viscosity and poor wettability of the lithium salt in the high-concentration electrolyte, so that the battery maintains good rate performance. DETAILED DESCRIPTION

[0025] The technical scheme of the present invention will be clearly and completely described below in conjunction with specific embodiments, but those skilled in the art will appreciate that the following described embodiments are part of embodiments of the present invention, rather than all embodiments, and are only used to illustrate the present invention, and should not be considered as limiting the scope of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present invention. Those who do not specify specific conditions in the embodiments are carried out according to conventional conditions or conditions recommended by the manufacturer. Those who do not specify the manufacturer for reagents or instruments used are conventional products that can be purchased commercially. In addition, the terms "first", "second" and "third" are only used for descriptive purposes and cannot be interpreted as indicating or implying relative importance.

[0026] The first aspect of the present invention is to provide an electrolyte. The electrolyte includes a lithium salt, a borate solvent, a carbonate solvent and a fluoroether diluent; wherein the borate solvent includes at least one of a mixture of tripropargyl borate and a fluoroborate compound or triethyl borate. The present invention can effectively solve the technical defects of borate or fluoroborate solvents such as high viscosity, low solubility in lithium salt, and poor compatibility with negative electrodes by coordinating carbonate solvents with borate solvents.

[0027] As a preferred embodiment, the fluoroborate compound includes at least one of tris(hexafluoroisopropyl)borate, tris(2,2,3,3-tetrafluoropropyl)borate or tris(pentafluorophenyl)borate.

[0028] As a preferred embodiment, when the borate solvent includes tripropargyl borate and fluoroborate compounds, the mass ratio of the tripropargyl borate and the fluoroborate compound is (1-10):(1-10), including but not limited to any one of 1:10, 2:9, 3:8, 4:7, 5:6, 1:1, 6:5, 7:4, 8:3, 9:2, 10:1 or a numerical range consisting of any two of them.

[0029] As a preferred embodiment, the lithium salt includes at least one of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium hexafluoroborate or lithium tetrafluoroborate.

[0030] As a preferred embodiment, the carbonate solvent includes at least one of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) or fluoroethylene carbonate (FEC).

[0031] As a preferred embodiment, the fluoroether diluent includes at least one of bis(2,2,2-trifluoroethyl) ether or 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0032] As a preferred embodiment, the electrolyte further includes an additive, and the additive includes at least one of 1,3-propane sultone (PS), diethyl sulfate (DTD) or vinylene carbonate (VC).

[0033] As a preferred embodiment, the mass ratio of the borate solvent to the carbonate solvent is 10% to 30%:70% to 90%.

[0034] The second aspect of the present invention is to provide a method for preparing a battery, wherein the battery includes the electrolyte. In the present invention, the electrolyte is injected and formed in parts. The first injection is to add an electrolyte mainly composed of carbonates to improve the solvent's high solubility for lithium salts and high wetting effect on the electrode sheet, thereby improving the battery production efficiency and battery performance; the second injection is to add a solvent mainly composed of borate esters, and optionally introduce additives that significantly improve the high temperature performance of the battery, and the physical and chemical properties of the positive electrode CEI film are improved by partial formation. Specifically, the preparation method mainly includes the following steps (1) to (2).

[0035] (1) preparing a first electrolyte; the first electrolyte comprising a lithium salt, a carbonate solvent and a fluoroether diluent; injecting the first electrolyte into a dry cell and performing a first formation treatment to obtain a battery precursor.

[0036] (2) preparing a second electrolyte; the second electrolyte comprising a lithium salt, a borate solvent and a fluoroether diluent; injecting the second electrolyte into the battery precursor and performing a second formation treatment to obtain a battery.

[0037] It can be understood that the dry cell refers to the precursor of a secondary battery that has not been injected with electrolyte; the dry cell should include at least a positive electrode, a negative electrode, an optional separator and necessary connecting or mounting elements, etc. The present invention does not impose any limitation on the type of positive or negative electrode.

[0038] As a preferred embodiment, the preparation method of the first electrolyte and / or the second electrolyte includes: fully mixing the raw material components; during the mixing, methods such as shaking, stirring, centrifugation, ultrasound, etc. can be optionally used to achieve uniform dispersion between the components.

[0039] As a preferred embodiment, when the electrolyte further includes an additive, the second electrolyte further includes the additive, while the first electrolyte does not include the additive.

[0040] As a more preferred embodiment, the mass ratio of the additive in the second electrolyte is 2% to 4%.

[0041] As a preferred embodiment, in the first electrolyte, the concentration of the lithium salt is 0.9 mol / L to 1.1 mol / L; in the second electrolyte, the concentration of the lithium salt is 0.1 mol / L to 0.2 mol / L.

[0042] As a preferred embodiment, after the first electrolyte is injected into the dry cell, the first electrolyte is allowed to stand until the electrode is fully soaked by the first electrolyte before the first formation treatment is performed; in some more preferred embodiments, the standing time is 10 min to 80 min.

[0043] As a preferred embodiment, the first formation treatment includes: first charging to 1.0V to 1.3V at a constant current of 0.02C to 0.05C, and then charging to 1.3V to 2.0V at a constant current of 0.05C to 0.1C.

[0044] As a preferred embodiment, the second chemical formation treatment comprises the following steps:

[0045] S1: Charge to 2.9V~3.4V at 0.1C~0.2C constant current;

[0046] S2: Charge to 4.3V~4.4V at a constant current of 0.33C~0.5C;

[0047] S3: Charge at a constant voltage of 4.25V to 4.4V until the cut-off current is 0.02C to 0.05C;

[0048] S4: Discharge at a constant current of 0.33C to 0.5C to 2.5V to 2.8V.

[0049] The third aspect of the present invention is to provide a lithium battery, which is prepared by the battery preparation method. Those skilled in the art can arbitrarily select or combine the structure and electrodes of the lithium battery according to their needs; as long as the production steps of the lithium battery include the battery preparation method of the present invention, the obtained lithium battery can be regarded as an embodiment of the present invention.

[0050] The fourth aspect of the present invention is to provide an electrical device, including the lithium battery. The electrical device can be any device or apparatus that relies on electric energy to work or operate, including but not limited to new energy vehicles, building electrical equipment, industrial electrical appliances, household and agricultural electrical appliances, etc. When the secondary battery is included, any electrical device equipped with the secondary battery can belong to an embodiment of the present invention.

[0051] Example 1

[0052] This embodiment uses borate ester and borate ester mixed-based high-safety electrolyte, which is composed of lithium salt, borate ester solvent, carbonate solvent, fluoroether diluent, and additives. The lithium salt is selected from lithium bis(fluorosulfonyl)imide (LFSI), the borate ester solvent is triethyl borate, the carbonate solvent includes ethylene carbonate (EC), ethyl methyl carbonate (EMC), fluoroethylene carbonate (FEC), the mass ratio is 2:1:1, the fluoroether diluent is bis(2,2,2-trifluoroethyl) ether, and the additives are 1,3-propane sultone (PS) and vinyl sulfate (DTD). The positive electrode material of the battery cell is Ni92 nickel cobalt lithium manganese oxide ternary material, and the negative electrode is a graphite-doped silicon system.

[0053] The mass ratio of borate solvents to carbonate solvents is 1:1.5. In the overall electrolyte, the added mass proportions of fluoroether diluents and two additives are 3%, 1.2% and 1.4% respectively, and the lithium salt concentration is 1.2 mol / L.

[0054] The preparation method of this embodiment is as follows:

[0055] (1) An electrolyte mainly composed of a carbonate solvent is added to the battery cell for the first injection, including a carbonate solvent, a lithium salt (accounting for 90 wt.% of the total lithium salt added), and a fluoroether diluent (accounting for 90 wt.% of the total fluoroether diluent added) to form a high-safety electrolyte system with a lithium salt concentration of 1.15 mol / L.

[0056] (2) After standing for 30 minutes, the first formation is carried out, first charging at a constant current of 0.05C to 1.3V, and then charging at a constant current of 0.1C to 2.0V.

[0057] (3) The second injection is to add an electrolyte with a borate solvent as the main component, including a borate solvent, an additive, a lithium salt (accounting for 10 wt.% of the total lithium salt addition), and a fluoroether diluent (accounting for 10 wt.% of the total fluoroether diluent addition) to obtain an overall electrolyte system.

[0058] (4) After standing for 60 minutes, the second formation was performed; first, the battery was charged to 3.4 V at a constant current of 0.15 C, then to 4.3 V at a constant current of 0.33 C, then charged to a cut-off current of 0.05 C at a constant voltage of 4.25 V, and finally discharged to 2.5 V at a constant current of 0.33 C. After sealing, the lithium ion battery of this embodiment was obtained.

[0059] Example 2

[0060] The method is basically the same as Example 1, except that the borate solvent is replaced by tripropargyl borate and tris(pentafluorophenyl)borate; and the mass ratio of the two is 3:1.

[0061] Example 3

[0062] It is basically the same as Example 1, except that the lithium salt is replaced by lithium hexafluoroborate or lithium tetrafluoroborate (mass ratio 1:1), and the fluoroether diluent is replaced by 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0063] Example 4

[0064] The method is basically the same as Example 1, except that the method for implementing the first chemical formation in step (2) of the preparation method is changed:

[0065] First charge at 0.02C constant current to 1.0V, then charge at 0.05C constant current to 1.3V.

[0066] Example 5

[0067] The method is basically the same as Example 1, except that the method for implementing the second chemical formation in step (4) of the preparation method is changed:

[0068] First, charge at a constant current of 0.2C to 2.9V, then charge at a constant current of 0.5C to 4.4V, then charge at a constant voltage of 4.4V to a cutoff current of 0.02C, and finally discharge at a constant current of 0.5C to 2.8V.

[0069] Comparative Example 1

[0070] The electrolyte and positive and negative electrodes are exactly the same as those in Example 1; the preparation method of this comparative example is as follows: the electrolyte is directly injected into the dry cell, and then the formation and packaging are performed. The formation step is consistent with step (4) of Example 1.

[0071] Comparative Example 2

[0072] A high-safety electrolyte is used, which is composed of lithium salt, carbonate solvent, and fluoroether diluent. The lithium salt is selected from lithium bis(fluorosulfonyl)imide (LFSI), and the carbonate solvent includes ethylene carbonate (EC), ethyl methyl carbonate (EMC), and fluoroethylene carbonate (FEC), with a mass ratio of 1:1:1. The additives are 1,3-propane sultone (PS) and vinyl sulfate (DTD), accounting for 1% and 1.5% of the total mass of the electrolyte, respectively, and the lithium salt concentration is 1.1 mol / L. The positive electrode material of the battery cell is Ni92 nickel cobalt lithium manganese oxide ternary material, and the negative electrode is a graphite-doped silicon system.

[0073] The preparation method of this comparative example is the same as that of comparative example 1.

[0074] Test example

[0075] 1) DCR, ACR: The AC internal resistance (ACR) of the battery prepared in each embodiment and comparative example was tested by an internal resistance meter, and the DC internal resistance (DCR) of the battery prepared in each embodiment and comparative example was tested by an electrochemical workstation. The DCR test conditions are to discharge at 1C current for 10s at 25°C environment and 50% SOC of the battery cell.

[0076] 2) Monthly self-discharge rate: Charge the battery to a certain full charge state and record the battery charge value at this time; place the battery in a constant temperature and humidity environment (25°C, relative humidity 30%) for 30 days, and then test the battery charge value again; record the ratio of the battery charge difference to the initial state battery charge value as a reference value for the monthly self-discharge rate.

[0077] Each embodiment and comparative example was subjected to the above test, and the test results are shown in Table 1.

[0078] Table 1

[0079] DCR / Ω ACR / Ω Monthly self-discharge rate / % Example 1 0.735 0.532 1.32％ Example 2 0.712 0.512 1.20％ Example 3 0.750 0.536 1.22％ Example 4 0.796 0.586 1.05％ Example 5 0.751 0.578 1.19％ Comparative Example 1 0.863 0.622 1.68％ Comparative Example 2 0.813 0.596 1.52％

[0080] Although the present invention has been illustrated and described with specific embodiments, it should be appreciated that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Those skilled in the art should understand that the technical solutions described in the above embodiments may be modified, or some or all of the technical features thereof may be replaced by equivalents without departing from the spirit and scope of the present invention. However, these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention. Therefore, this means that all such replacements and modifications within the scope of the present invention are included in the appended claims.

Claims

1. An electrolyte, characterized in that: The electrolyte includes lithium salt, borate solvent, carbonate solvent and fluoroether diluent; The borate solvent includes at least one of a combination of tripropargyl borate and a fluoroborate compound, or triethyl borate.

2. The electrolyte according to claim 1, characterized in that The fluoroboric acid ester compound includes at least one of tris(hexafluoroisopropyl)borate, tris(2,2,3,3-tetrafluoropropyl)borate or tris(pentafluorophenyl)borate.

3. The electrolyte according to claim 1, characterized in that When the borate solvent comprises a combination of tripropargyl borate and a fluoroborate compound, the mass ratio of the tripropargyl borate to the fluoroborate compound is (1-10):(1-10).

4. The electrolyte according to claim 1, characterized in that The lithium salt includes at least one of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium hexafluoroborate or lithium tetrafluoroborate; and / or, the carbonate solvent comprises at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate or fluoroethylene carbonate; And / or, the fluoroether diluent includes at least one of bis(2,2,2-trifluoroethyl) ether or 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

5. The electrolyte according to claim 1, characterized in that The electrolyte further includes an additive; The additive includes at least one of 1,3-propane sultone, vinyl sulfate or vinylene carbonate.

6. A method for preparing a battery, characterized in that: The battery comprises the electrolyte according to any one of claims 1 to 5, and the preparation method comprises the following steps: (1) preparing a first electrolyte; the first electrolyte comprising a lithium salt, a carbonate solvent and a fluoroether diluent; injecting the first electrolyte into a dry cell and performing a first formation treatment to obtain a battery precursor; (2) preparing a second electrolyte; the second electrolyte comprising a lithium salt, a borate ester solvent and a fluoroether diluent; The second electrolyte is injected into the battery precursor and then a second formation treatment is performed to obtain a battery.

7. The method for preparing an electrolyte according to claim 6, characterized in that: The first formation treatment comprises the following steps: First charge at a constant current of 0.02C to 0.05C to 1.0V to 1.3V, then charge at a constant current of 0.05C to 0.1C to 1.3V to 2.0V.

8. The method for preparing an electrolyte according to claim 6, characterized in that: The second formation treatment comprises the following steps performed in sequence: (S1) charging at a constant current of 0.1C to 0.2C to 2.9V to 3.4V; (S2) charging at a constant current of 0.33C to 0.5C to 4.3V to 4.4V; (S3) charging at a constant voltage of 4.25V to 4.4V until the cut-off current is 0.02C to 0.05C; (S4) Discharge at a constant current of 0.33C to 0.5C to 2.5V to 2.8V.

9. A lithium battery, characterized in that: The lithium battery is prepared by the method for preparing a battery as claimed in any one of claims 6 to 8.

10. An electrical device, characterized in that: The electrical device comprises the lithium battery as claimed in claim 9.