Lithium ion battery electrolyte and lithium ion battery

Abstract

The present invention provides a lithium-ion battery electrolyte and a lithium-ion battery. By adding specific dimethyl sulfate, a compound represented by general formula (I), and 1-ethyl-3-methylimidazolium difluorophosphate to the electrolyte, a good SEI is formed synergistically, thereby improving initial resistance characteristics, high-temperature durability, cycle characteristics, and other performance, thereby meeting the battery requirements for application scenarios such as electronic equipment and automotive batteries.

Description

Technical Field

[0001] The present invention belongs to the field of lithium ion battery electrolyte, and in particular relates to a lithium ion battery electrolyte and a lithium ion battery. Background Art

[0002] In recent years, lithium-ion batteries have been widely used in electronic devices such as personal computers and communication equipment, and have attracted particular attention in electric vehicles, hybrid vehicles, fuel cell vehicles and power storage devices.

[0003] Lithium secondary batteries are primarily composed of a positive electrode and a negative electrode containing materials capable of absorbing and releasing lithium, as well as an electrolyte containing a lithium salt and a non-aqueous solvent. Positive electrode active materials commonly include lithium metal oxides, such as LiCoO2, LiNiO2, and LiMnO2. Negative electrode active materials are active substances capable of absorbing and releasing lithium, such as metallic lithium, lithium-containing alloys, coke, artificial graphite, and natural graphite. The electrolyte commonly includes a non-aqueous solvent mixed with a lithium electrolyte. Non-aqueous solvents include carbonate mixed solvents such as ethylene carbonate and methyl carbonate, and inorganic lithium electrolytes such as LiClO4, LiAsF6, LiPF6, LiBF4, LiAlF4, and LiN (SO2CF2CF3).

[0004] During the initial charge, lithium cations embed into the negative electrode. The negative electrode and lithium cations, or the negative electrode and the electrolyte solvent, react to form a film composed primarily of lithium carbonate and lithium oxide on the negative electrode surface, known as the solid electrolyte interface (SEI). The key to improving battery performance is to form a stable SEI with high lithium ion conductivity, low electron conductivity, and good durability. By adding a small amount of additives to the electrolyte to form a good SEI, performance such as high-temperature durability, initial resistance, and cycle characteristics can be improved to meet the battery requirements of applications such as electronic equipment and automotive batteries. Summary of the Invention

[0005] In order to overcome the problems existing in the prior art, the purpose of the present invention is to provide a lithium-ion battery electrolyte and a lithium-ion battery, which form a good SEI by synergistically adding specific dimethyl sulfate, a compound represented by general formula (I) and 1-ethyl-3-methylimidazolium difluorophosphate to the electrolyte, thereby improving the initial resistance characteristics, high-temperature durability, cycle characteristics and other performance, and meeting the requirements of batteries for application scenarios such as electronic equipment and automotive batteries.

[0006] The object of the present invention is achieved through the following technical solutions:

[0007] An electrolyte comprises a non-aqueous solvent, an electrolyte salt, dimethyl sulfate, a compound selected from the group consisting of compounds represented by general formula (I) and 1-ethyl-3-methylimidazolium difluorophosphate.

[0008] Wherein, the general formula (I) is R 1 R 2 N-SO3M 1 ;

[0009] R 1 and R 2 the same or different, being a linear or branched alkyl group having 1 to 6 carbon atoms, or a cycloalkyl group having 3 to 6 carbon atoms;

[0010] M 1 is one or more selected from Li, Na, K and Cs;

[0011] Preferably, the compound represented by general formula (I) is (CH3)2NSO3Li.

[0012] Preferably, the content of dimethyl sulfate in the electrolyte is preferably 0.01 to 10% by mass, more preferably 0.05 to 5% by mass, and particularly preferably 0.1 to 2% by mass, relative to the total amount of the electrolyte.

[0013] Preferably, the content of the compound selected from the group consisting of compounds represented by general formula (I) in the electrolyte is preferably 0.01 to 10% by mass, more preferably 0.05 to 5% by mass, and particularly preferably 0.1 to 2% by mass, relative to the total amount of the electrolyte.

[0014] Preferably, the content of 1-ethyl-3-methylimidazolium difluorophosphate in the electrolyte is preferably 0.01 to 10% by mass, more preferably 0.05 to 5% by mass, and particularly preferably 0.1 to 2% by mass, relative to the total amount of the electrolyte.

[0015] Preferably, the mass ratio of dimethyl sulfate, the compound selected from the group consisting of general formula (I) and 1-ethyl-3-methylimidazolium difluorophosphate is 1-5:1-5:1-5, more preferably 1:2-5:2-5, and even more preferably 1:3:3.

[0016] The non-aqueous solvents include cyclic non-aqueous solvents (to improve the flash point of the solvent and the safety of the battery) and chain non-aqueous solvents.

[0017] The cyclic non-aqueous solvent is one or more of a cyclic carbonate, a cyclic carboxylate, a cyclic sulfone, and a cyclic ether. The cyclic carbonate is preferably one or more of ethylene carbonate, propylene carbonate, and 2,3-butylene carbonate. The cyclic carboxylate is preferably one or more of γ-butyrolactone and δ-valerolactone. The cyclic sulfone is preferably one or more of sulfolane, dimethyl sulfone, and methyl ethyl sulfone. The cyclic ether is preferably dioxolane.

[0018] The mixing ratio of the cyclic aprotic solvent in the nonaqueous solvent is 10% to 100% by mass, more preferably 20% to 90% by mass, and particularly preferably 30% to 80% by mass. This ratio can improve the conductivity of the electrolyte solution, which is related to the charge and discharge characteristics of the battery.

[0019] The chain nonaqueous solvent is one or more of a chain carbonate, a chain carboxylate, a chain ether, and a chain phosphate. The chain carbonate is preferably one or more of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methylpropyl carbonate, dipropyl carbonate, dipentyl carbonate, dihexyl carbonate, ethyl octyl carbonate, and dioctyl carbonate. The chain carboxylate is preferably methyl pivalate. The chain ether is preferably dimethoxyethane. The chain phosphate is preferably trimethyl phosphate.

[0020] The mixing ratio of the chain aprotic solvent in the non-aqueous solvent is 10% by mass to 100% by mass, more preferably 20% by mass to 90% by mass, and particularly preferably 30% by mass to 80% by mass.

[0021] Preferably, the non-aqueous solvent is a combination of a cyclic carbonate and a chain carbonate, which can increase the conductivity of the electrolyte and improve the charge and discharge characteristics of the battery. The cyclic carbonate and the chain carbonate are represented by, and the mass ratio of the cyclic carbonate: chain carbonate is 5:95 to 80 to 20, more preferably 10:90 to 70:30, and particularly preferably 15:85 to 55:45. By becoming such a ratio, the viscosity increase of the electrolyte can be suppressed, the dissociation degree of the electrolyte can be increased, and the conductivity of the electrolyte related to the charge and discharge characteristics of the battery can be improved. In addition, the solubility of the electrolyte can be further improved. Accordingly, an electrolyte with excellent conductivity at room temperature or low temperature can be formed, so the load characteristics of the battery at room temperature to low temperature can be improved.

[0022] The electrolyte salt is a lithium salt, preferably one or more of LiPF6, LiBF4, LiClO4, and LiAsF6, more preferably LiPF6.

[0023] The concentration of the electrolyte salt in the electrolyte solution is 0.1 mol / L to 3 mol / L, preferably 0.5 mol / L to 2 mol / L.

[0024] The electrolyte of the present invention may further contain other additives, such as metal oxides, glass, etc., as needed.

[0025] The electrolyte solution of the present invention can be suitably used in electrochemical devices such as secondary batteries. An electrochemical device or secondary battery including the electrolyte solution of the present invention is also one aspect of the present invention.

[0026] The lithium secondary battery of the present invention comprises a positive electrode, a negative electrode, a separator and the above-mentioned electrolyte.

[0027] (positive electrode)

[0028] The positive electrode active material constituting the positive electrode can be any material capable of electrochemically occluding and releasing lithium ions. Specific examples include lithium-containing transition metal composite oxides and lithium-containing transition metal phosphates. Preferred are composite oxides containing lithium and transition metals, such as LiCoO2, LiMnO2, LiMn2O4, LiNiO2, LiNiXCo(1-X)O2〔0<X<1〕, and LiFePO4.

[0029] If the conductivity of the positive electrode active material is insufficient, it can be used together with a conductive additive to form a positive electrode. The conductive additive is one or more carbon materials such as carbon black, amorphous whiskers, and graphite.

[0030] (negative electrode)

[0031] The negative electrode consists of a negative electrode active material layer containing a negative electrode active material and a current collector. The negative electrode active material constituting the negative electrode can be any material capable of electrochemically occluding and releasing lithium ions. Specific examples include one or more of carbonaceous materials, metal oxide materials, lithium metal, lithium-containing alloys, and lithium-containing metal composite oxide materials.

[0032] Preferably, the carbon material is one or more of carbon black, activated carbon, artificial graphite, natural graphite, hard carbon, coke, mesocarbon microbeads (MCMB) sintered below 1500° C., and mesophase pitch carbon fiber (MCF).

[0033] The negative electrode active material layer further contains a binder, a thickener, and a conductive material.

[0034] The above-mentioned binder may be the same as the binder that can be used for the positive electrode. The ratio of the binder to the negative electrode active material is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, particularly preferably 0.6% by mass or more, and preferably 20% by mass or less, more preferably 15% by mass or less, and particularly preferably 8% by mass or less. When the ratio of the binder to the negative electrode active material exceeds the above range, the proportion of the binder that does not contribute to the battery capacity in the amount of binder increases, which sometimes leads to a decrease in battery capacity. In addition, when it is below the above range, it sometimes leads to a decrease in the strength of the negative electrode.

[0035] diaphragm

[0036] A separator is provided between the positive electrode and the negative electrode to electrically insulate the positive electrode from the negative electrode and allow lithium ions to pass therethrough. The separator is a porous film.

[0037] The separator is a porous sheet or nonwoven fabric, and is preferably one or more of a polyolefin porous membrane, a polyimide porous membrane, a polyvinylidene fluoride porous membrane, and a polyester porous membrane.

[0038] The polyolefin porous polymer film is preferably a porous polyethylene film, a porous polypropylene film, or a multilayer film of a porous polyethylene film and a polypropylene film.

[0039] The thickness of the separator is 1 μm to 50 μm, preferably 5 μm to 30 μm, and more preferably 8 μm to 30 μm.

[0040] The porosity of the separator is 20% to 95%, preferably 35% to 85% or higher, and more preferably 45% to 75%. If the porosity is too low compared to the above range, the membrane resistance increases, and the rate characteristics tend to deteriorate. If the porosity is too high compared to the above range, the mechanical strength of the separator tends to decrease, and the insulation properties tend to deteriorate.

[0041] The average pore size of the separator is 0.05 μm to 0.5 μm, preferably 0.05 μm to 0.2 μm. If the average pore size exceeds the above range, short circuiting is likely to occur. If it is below the above range, the membrane resistance may increase, and the rate characteristics may be reduced.

[0042] Battery composition

[0043] The positive plate and the negative plate are separated by the above-mentioned separator to form a laminated structure, or the positive plate and the negative plate are wound into a spiral structure with the above-mentioned separator to obtain an electrode group. The ratio of the volume of the electrode group to the internal volume of the battery (hereinafter referred to as the electrode group occupancy rate) is usually 40% or more, preferably 50% or more, and usually 90% or less, preferably 80% or less. The lithium secondary battery of the present invention can adopt various well-known shapes and can be formed into cylindrical, coin-shaped, square, membrane-shaped or other arbitrary shapes.

[0044] Beneficial effects:

[0045] By adding specific dimethyl sulfate, a compound represented by general formula (I), and 1-ethyl-3-methylimidazolium difluorophosphate to the electrolyte, a good SEI is formed synergistically, improving the initial resistance characteristics, high-temperature durability, cycle characteristics, and other performance, meeting the battery requirements for application scenarios such as electronic equipment and automotive batteries. DETAILED DESCRIPTION

[0046] The following will clearly and completely describe the technical solutions of the present invention in conjunction with the embodiments. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0047] The present invention will be further explained below with reference to specific embodiments.

[0048] Preparation Example 1

[0049] The electrolyte includes a non-aqueous solvent, an electrolyte salt, dimethyl sulfate, (CH3)2NSO3Li and 1-ethyl-3-methylimidazolium difluorophosphate. The preparation method is as follows:

[0050] Step 1: Ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed at a mass ratio of 55:45 to obtain a non-aqueous solvent.

[0051] Step 2: dissolving LiPF6 as an electrolyte salt in the non-aqueous solvent obtained in step 1 to obtain an electrolyte salt solution, wherein the electrolyte salt concentration is 1 mol / L.

[0052] Step 3: Dissolving dimethyl sulfate, (CH3)2NSO3Li, and 1-ethyl-3-methylimidazolium difluorophosphate as additives in the electrolyte salt solution obtained in step 2 to obtain an electrolyte salt solution, wherein the content of the additives is 0.5 wt % relative to the final electrolyte solution. The mass ratio of dimethyl sulfate, (CH3)2NSO3Li, and 1-ethyl-3-methylimidazolium difluorophosphate is 1:3:3.

[0053] Comparative Preparation Example 1

[0054] The electrolyte includes a non-aqueous solvent, an electrolyte salt, (CH3)2NSO3Li and 1-ethyl-3-methylimidazolium difluorophosphate. The preparation method is as follows:

[0055] Step 1: Ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed at a mass ratio of 55:45 to obtain a non-aqueous solvent.

[0056] Step 2: dissolving LiPF6 as an electrolyte salt in the non-aqueous solvent obtained in step 1 to obtain an electrolyte salt solution, wherein the electrolyte salt concentration is 1 mol / L.

[0057] In step 3, (CH3)2NSO3Li and 1-ethyl-3-methylimidazolium difluorophosphate are dissolved as additives in the electrolyte salt solution obtained in step 2 to obtain an electrolyte salt solution, wherein the additive content is 0.5 wt% relative to the final electrolyte solution. The mass ratio of (CH3)2NSO3Li to 1-ethyl-3-methylimidazolium difluorophosphate is 3:3.

[0058] Comparative Preparation Example 2

[0059] The electrolyte includes a non-aqueous solvent, an electrolyte salt, dimethyl sulfate, and 1-ethyl-3-methylimidazolium difluorophosphate. The preparation method thereof is as follows:

[0060] Step 1: Ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed at a mass ratio of 55:45 to obtain a non-aqueous solvent.

[0061] Step 2: dissolving LiPF6 as an electrolyte salt in the non-aqueous solvent obtained in step 1 to obtain an electrolyte salt solution, wherein the electrolyte salt concentration is 1 mol / L.

[0062] Step 3: Dissolve dimethyl sulfate and 1-ethyl-3-methylimidazolium difluorophosphate as additives in the electrolyte salt solution obtained in step 2 to obtain an electrolyte salt solution, wherein the content of the additives is 0.5 wt % relative to the final electrolyte solution. The mass ratio of dimethyl sulfate to 1-ethyl-3-methylimidazolium difluorophosphate is 1:3.

[0063] Comparative Preparation Example 3

[0064] The electrolyte includes a non-aqueous solvent, an electrolyte salt, dimethyl sulfate, and (CH3)2NSO3Li. The preparation method thereof is as follows:

[0065] Step 1: Ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed at a mass ratio of 55:45 to obtain a non-aqueous solvent.

[0066] Step 2: dissolving LiPF6 as an electrolyte salt in the non-aqueous solvent obtained in step 1 to obtain an electrolyte salt solution, wherein the electrolyte salt concentration is 1 mol / L.

[0067] Step 3: Dissolve dimethyl sulfate and (CH3)2NSO3Li as additives in the electrolyte salt solution obtained in step 2 to obtain an electrolyte salt solution, wherein the content of the additives is 0.5 wt % relative to the final electrolyte solution. The mass ratio of dimethyl sulfate to (CH3)2NSO3Li is 3:3.

[0068] Comparative Preparation Example 4

[0069] The electrolyte includes a non-aqueous solvent and an electrolyte salt. The preparation method thereof is as follows:

[0070] Step 1: Ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed at a mass ratio of 55:45 to obtain a non-aqueous solvent.

[0071] Step 2: dissolving LiPF6 as an electrolyte salt in the non-aqueous solvent obtained in step 1 to obtain an electrolyte salt solution, wherein the electrolyte salt concentration is 1 mol / L.

[0072] Example

[0073] A lithium secondary battery was manufactured through the following steps.

[0074] Step 1: Preparation of negative electrode

[0075] 97 parts by mass of artificial graphite, 1 part by mass of cellulose, and 2 parts by mass of SBR latex were weighed and dissolved in water. The mixture was kneaded to obtain a paste-like negative electrode active material layer slurry. The paste was applied to a 20 μm thick strip of copper foil as a negative electrode current collector, dried, and rolled to obtain a sheet-like negative electrode comprising the negative electrode current collector and the negative electrode active material layer. The coating density of the negative electrode active material layer at this time was 12 mg / cm 2 , the filling density is 1.3g / ml.

[0076] Step 2: Preparation of positive electrode

[0077] 92 parts by mass of LiMnO2, 4 parts by mass of acetylene black, and 4 parts by mass of PVDF were weighed and dissolved in NMP. The mixture was kneaded to obtain a paste-like positive electrode active material layer slurry. The paste was applied to a positive electrode current collector made of a 21 μm thick strip of aluminum foil, dried, and roll-pressed to obtain a sheet-like positive electrode containing the positive electrode current collector and the positive electrode active material layer. The coating density of the positive electrode active material layer at this time was 28 mg / cm 2 , the filling density is 2.3g / ml.

[0078] Step 3: Preparation of lithium-ion secondary battery

[0079] The negative electrode, positive electrode, and polyethylene separator prepared above were stacked in the order of negative electrode, separator, and positive electrode to produce a battery element.

[0080] The battery element was inserted into a bag consisting of a laminated film with both surfaces of an aluminum sheet (40 μm thick) coated with a resin layer, with the positive and negative terminals protruding. Thereafter, the electrolytes described in the preparation examples and comparative preparation examples were injected into the bag, respectively, and vacuum-sealed to produce a sheet-shaped lithium-ion secondary battery.

[0081] The performance of the prepared lithium-ion secondary battery was characterized:

[0082] 1. Initial characteristics of the battery: determination of initial resistance, initial discharge capacity and initial efficiency

[0083] The lithium ion secondary battery was charged at a constant current of 1 mA and a constant voltage of 4.2 V, and discharged at a constant current of 1 mA to 2.85 V, and this cycle was repeated 10 times. At this time, the initial charge and discharge efficiency (initial efficiency) was calculated from the charge capacity [mAh] of the first cycle and the discharge capacity [mAh] of the first cycle (initial discharge capacity) using the following formula.

[0084] Initial efficiency [%] = (discharge capacity of the first cycle [mAh] / charge capacity of the first cycle [mAh]) × 100 [%]

[0085] Based on the test results, the initial discharge capacity (relative value; %) of Example 1 and Comparative Examples 1-3 when the initial discharge capacity in Comparative Example 1 is set to 100%, and the initial efficiency (relative value; %) of Example 1 and Comparative Examples 1-3 when the initial efficiency in Comparative Example 4 is set to 100% are respectively calculated.

[0086] The lithium ion secondary battery was charged at a constant voltage of 4.0 V, then cooled to -20°C in a thermostatic chamber and discharged at a constant current of 0.2 mA at -20°C. The potential drop 10 seconds after the start of discharge was measured to measure the DC resistance [Ω] of the lithium ion secondary battery. The obtained value was set as the initial resistance value [Ω] (-20°C).

[0087] The initial resistance value [Ω] (-20°C) in Comparative Example 4 is set to 100%, and the initial resistance values ​​(relative value; %) in Example 1 and Comparative Examples 1-3 are: Initial battery resistance [%] = (Initial resistance value [Ω] (-20°C) in Example 1 / Initial resistance value [Ω] (-20°C) in Comparative Example 4) × 100 [%]

[0088] 2. Cycle characteristics

[0089] At 60°C, constant current-constant voltage charging (hereinafter referred to as CC / CV charging) (0.1C cut) was performed at a current equivalent to 0.2C to 4.9V, followed by discharge at a constant current of 0.2C to 3V. This was considered one cycle, and the initial discharge capacity was calculated from the discharge capacity of the third cycle. Here, 1C represents the current value that discharges the battery to its reference capacity in 1 hour, for example, 0.2C represents a current value of 1 / 5 of its reference capacity. After that, charge and discharge were performed under the above conditions, and the cycle was continued until the initial capacity reached 80%. The number of cycles required to reach 80% is recorded in the table.

[0090] Table 1 Lithium ion secondary battery performance

[0091]

[0092] According to the results shown in Table 1, it can be seen from the comparison between Example 1 and Comparative Examples 1-3 and Comparative Example 4 that the addition of dimethyl sulfate, (CH3)2NSO3Li and 1-ethyl-3-methylimidazolium difluorophosphate as additives to the electrolyte has a synergistic effect, which can significantly reduce the initial battery resistance of the lithium secondary battery while maintaining the initial efficiency and initial discharge capacity, improve the initial resistance characteristics, and at the same time improve the high temperature cycle characteristics.

[0093] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A lithium ion battery electrolyte comprising a non-aqueous solvent, an electrolyte salt, dimethyl sulfate, (CH3)2NSO3Li and 1-ethyl-3-methylimidazolium difluorophosphate; The content of dimethyl sulfate, (CH3)2NSO3Li and 1-ethyl-3-methylimidazolium difluorophosphate in the electrolyte is 0.5wt% relative to the total amount of the electrolyte; the mass ratio of dimethyl sulfate, (CH3)2NSO3Li and 1-ethyl-3-methylimidazolium difluorophosphate is 1:3:

3.

2. A lithium-ion battery electrolyte according to claim 1, characterized in that: The non-aqueous solvent is a cyclic non-aqueous solvent and a chain non-aqueous solvent.

3. A lithium-ion battery electrolyte according to claim 2, characterized in that: The cyclic non-aqueous solvent is one or more of a cyclic carbonate, a cyclic carboxylate, a cyclic sulfone, and a cyclic ether; wherein the cyclic carbonate is selected from one or more of ethylene carbonate, propylene carbonate, and 2,3-butylene carbonate; the cyclic carboxylate is selected from one or more of γ-butyrolactone and δ-valerolactone; the cyclic sulfone is selected from one or more of sulfolane, dimethyl sulfone, and methyl ethyl sulfone; and the cyclic ether is selected from dioxolane.

4. The lithium-ion battery electrolyte according to claim 3, characterized in that: The chain non-aqueous solvent is one or more of chain carbonate, chain carboxylate, chain ether, and chain phosphate; wherein the chain carbonate is selected from one or more of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, dipropyl carbonate, dipentyl carbonate, dihexyl carbonate, ethyl octyl carbonate, and dioctyl carbonate; the chain carboxylate is methyl pivalate, the chain ether is dimethoxyethane, and the chain phosphate is trimethyl phosphate.

5. A lithium-ion battery, characterized in that: The invention comprises a positive electrode, a negative electrode, a separator and the lithium ion battery electrolyte according to any one of claims 1 to 4.

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