Electrolyte, and lithium-ion battery and preparation method therefor

Abstract

An electrolyte, and a lithium-ion battery and a preparation method therefor. The electrolyte comprises: a lithium salt and a solvent. The solvent comprises, in parts by volume, 30-97.5 parts of a dinitrile compound, 2.5-20 parts of fluoroethylene carbonate, and 0.0001-10 parts of ethylene carbonate, wherein the volume ratio of the fluoroethylene carbonate to the ethylene carbonate is not less than 5:3. As can be seen from specific examples and test examples, when only a dinitrile compound and a fluorocarbonate are used together as a solvent, an excess of the dinitrile compound is present and will deteriorate the direct current impedance of a battery, resulting in the problem of a significant reduction in the high-temperature cycling performance of the battery; however, by means of the utilization of a combination of a dinitrile compound, a fluorocarbonate and a carbonate in an electrolyte, the problem is greatly ameliorated, and the high-temperature cycling performance of a lithium-ion battery in which the electrolyte is used is significantly improved, thereby broadening the use of a dinitrile compound as a solvent for an electrolyte.

Description

Electrolyte, lithium ion battery and preparation method thereof TECHNICAL FIELD

[0001] The present application relates to the field of secondary batteries, in particular to an electrolyte, a lithium ion battery and a preparation method thereof. BACKGROUND

[0002] Lithium ion batteries are a class of batteries that can be charged and repeatedly used. In addition to small consumer uses in electronic devices such as smartphones, laptops, tablets, and wearable devices, lithium ion batteries have recently been used for internal purposes in vehicles such as hybrid and electric vehicles, including power storage systems for uninterruptible power supplies (UPS) and base transceiver stations (BTS).

[0003] In the context of accelerating global decarbonization trends and reducing greenhouse gas emissions such as carbon dioxide (CO2), electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs) are becoming increasingly popular and demand is expected to increase.

[0004] Commercially available lithium ion battery packs have no safety problems under normal use, but if the protection circuit fails to work due to extreme misuse or manufacturing defects, the battery can catch fire, smoke, explode, or generate excessive heat. The most common cause of accidents is internal short circuit of the battery, and it is believed that the combination of heating, impact, overcharging, electrolyte leakage, and internal short circuit can cause fire or explosion.

[0005] There are many safety problems in the process of lithium ion battery materials, manufacturing and use, and safety improvement has become a major challenge for lithium battery manufacturers. The main solutions include improving the safety of electrolyte, improving the safety of electrode materials, and improving the safety protection design of the battery. In this context, the development of safer batteries is particularly important.

[0006] The CN bond of dinitrile compounds (e.g., succinonitrile) can coordinate with metal ions, weakening the oxidation of electrolyte by positive metal ions, so it is often used as an electrolyte additive to improve the safety of lithium ion batteries. However, dinitrile compounds in the electrolyte generally cannot be added too much, and excessive dinitrile compounds (generally more than 2.5 mass%) will worsen the direct current impedance of the battery, greatly reducing the high-temperature cycle performance of the battery, which undoubtedly greatly limits the use of dinitrile compounds as electrolyte solvents. SUMMARY

[0007] Therefore, it is necessary to provide an electrolyte that can solve the above problems.

[0008] In addition, it is also necessary to provide a lithium ion battery comprising the above electrolyte and a preparation method thereof.

[0009] An electrolyte, comprising: a lithium salt and a solvent;

[0010] The solvent comprises, in volume parts, 30-97.5 parts of a dinitrile compound, 2.5-20 parts of a fluorinated ethylene carbonate, and 0.0001-10 parts of an ethylene carbonate, the volume ratio of the fluorinated ethylene carbonate to the ethylene carbonate being not less than 5:3.

[0011] In one embodiment, the solvent comprises, in volume parts, 40-97.5 parts of the dinitrile compound, 5-20 parts of the fluorinated ethylene carbonate, and 0.0001-7.5 parts of the ethylene carbonate.

[0012] In one embodiment, the solvent comprises, in volume parts, 50-97.5 parts of the dinitrile compound, 7.5-20 parts of the fluorinated ethylene carbonate, and 0.0001-5 parts of the ethylene carbonate.

[0013] In one embodiment, the lithium salt comprises LiFSI and LiPF6, the concentration of the LiFSI being 2M-6M, and the concentration of the LiPF6 being 0.01M-0.2M.

[0014] In one embodiment, the dinitrile compound is selected from at least one of succinonitrile, adiponitrile, and hexanetritrile.

[0015] In one embodiment, the dinitrile compound comprises the succinonitrile, and the volume ratio of the succinonitrile in the dinitrile compound is not less than 50%.

[0016] In one embodiment, the electrolyte further comprises an additive, the mass ratio of the additive to the solvent being 0.1-5:100, the additive comprising at least one of a bisoxalate-containing compound and a sulfur-containing cyclic compound.

[0017] In one embodiment, the bisoxalate-containing compound is selected from at least one of lithium bisoxalate borate and lithium oxyfluoroborate;

[0018] The sulfur-containing cyclic compound is selected from at least one of 1,3-propane sulfite, prop-1-ene-1,3-sultone, and 1,3,2-dioxazolothiophene-2,2-dioxide.

[0019] In one embodiment, the mass ratio of the bisoxalate-containing compound to the sulfur-containing cyclic compound is 0.125-1.

[0020] A lithium ion battery, comprising a positive electrode, a negative electrode, a separator, and the electrolyte described above.

[0021] In one embodiment, the current collector of the positive electrode is an aluminum foil.

[0022] A method for manufacturing a lithium ion battery, comprising the steps of:

[0023] Assembling a positive electrode, a negative electrode and a separator into a semi-finished product;

[0024] Providing the electrolyte as described above, heating the electrolyte to melt it and injecting it into the semi-finished product;

[0025] Packaging the semi-finished product to obtain the desired lithium ion battery.

[0026] In one embodiment, the temperature of the heating to melt the electrolyte is 30-90℃.

[0027] The electrolyte of the present application comprises a lithium salt and a solvent, the solvent comprising 30-97.5 parts by volume of a dinitrile compound, 2.5-20 parts by volume of a fluorinated ethylene carbonate and 0.0001-10 parts by volume of an ethylene carbonate, the volume ratio of the fluorinated ethylene carbonate to the ethylene carbonate being not less than 5:3.

[0028] In combination with the specific examples and test examples, when only a dinitrile compound and a fluorinated ethylene carbonate are used together as the solvent, an excessive amount of the dinitrile compound (generally more than 2.5 mass%) can deteriorate the direct current impedance of the battery, resulting in a significant reduction in the high-temperature cycle performance of the battery. However, in the electrolyte of the present application, the use of a dinitrile compound, a fluorinated ethylene carbonate and an ethylene carbonate greatly improves this problem, and the high-temperature cycle performance of the lithium ion battery using the electrolyte of the present application is significantly improved, thereby broadening the use of dinitrile compounds as electrolyte solvents.

[0029] Preferably, the lithium salt comprises LiFSI and LiPF6, and the concentration of the LiFSI in the electrolyte is 2-6 M and the concentration of the LiPF6 is 0.01-0.2 M. In combination with the specific examples and test examples, the high-temperature cycle performance of the lithium ion battery using the electrolyte of the present application is further improved. That is, although the reason is unknown, the addition of a small amount of LiPF6 can further solve the problem of the deterioration of the direct current impedance of the battery by the dinitrile compound, thereby further broadening the use of dinitrile compounds as electrolyte solvents.

[0030] Preferably, the electrolyte further comprises a double oxalate-containing additive and a sulfur-containing cyclic compound. Preferably, the double oxalate-containing compound comprises at least one or more of lithium bis(oxalato)borate (LiBOB) and lithium oxyborofluoride (LiFOB).

[0031] Preferably, the sulfur-containing cyclic compound is selected from at least one of 1,3-propane sulfite, prop-1-ene-1,3-sulfite and 1,3,2-dioxathiolane-2,2-dioxide. Although one of the oxalic acid-containing compound and the sulfur-containing cyclic compound is sufficient, but the effect is better when both are used. The ratio of the oxalic acid-containing compound and the sulfur-containing cyclic compound is 0.5 mass% to 5 mass%. In combination with the specific examples and test examples, the high-temperature cycle performance of the lithium ion battery using the electrolyte of the present application is further improved. That is, due to the addition of the oxalic acid-containing additive and the sulfur-containing cyclic compound, the problem of the dinitrile compound deteriorating the direct current impedance of the battery can be further solved, thereby further expanding the use of the dinitrile compound as an electrolyte solvent.

[0032] In summary, in combination with the specific examples and test examples, the lithium ion battery using the electrolyte of the present application has the advantages of high capacity, good high-temperature cycle performance, high safety, and good application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0033] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained without creative labor on the basis of these drawings.

[0034] Among them:

[0035] FIG. 1 is a flowchart of the preparation method of the lithium ion battery of the embodiment. DETAILED DESCRIPTION

[0036] The technical solutions in the embodiments of the present application will be described clearly and completely below in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only some embodiments of the present application, not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0037] It should be noted that all directional indications (such as up, down, left, right, front, back, etc.) in the embodiments of the present application are only used to explain the relative positional relationship, motion condition, etc. between objects in a certain posture, and if the certain posture changes, the directional indications also change accordingly.

[0038] In addition, the descriptions related to "first", "second" and the like in the present application are only for the purpose of description, and cannot be understood as indicating or implying the relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first", "second" can explicitly or implicitly include at least one of the features. In addition, the technical solutions of various embodiments can be combined with each other, but it must be based on the realization of ordinary skilled in the art, when the combination of technical solutions appears contradictory or cannot be realized, it should be considered that the combination of technical solutions does not exist, nor within the protection scope required by the present application.

[0039] The electrolyte of the embodiment comprises: a lithium salt and a solvent.

[0040] The solvent comprises 30-97.5 parts by volume of the dinitrile compound, 2.5-20 parts by volume of the fluorinated ethylene carbonate, and 0.0001-10 parts by volume of the ethylene carbonate, and the volume ratio of the fluorinated ethylene carbonate to the ethylene carbonate is not less than 5:3.

[0041] The electrolyte of the embodiment comprises: a lithium salt and a solvent.

[0042] In combination with the specific embodiments and test examples, when only the dinitrile compound and the fluorinated carbonate are used together as the solvent, excessive dinitrile compound (generally more than 2.5 mass%) can deteriorate the direct current impedance of the battery, and the high-temperature cycle performance of the battery is greatly reduced. However, in the electrolyte of the present application, the dinitrile compound, the fluorinated cyclic ethylene carbonate and the non-fluorinated cyclic ethylene carbonate are used in a special proportion, which greatly improves this problem. The high-temperature cycle performance of the lithium ion battery using the electrolyte of the present application is significantly improved, thereby widening the use of dinitrile compound as an electrolyte solvent.

[0043] Preferably, in the embodiment, the solvent comprises 40-97.5 parts by volume of the dinitrile compound, 5-20 parts by volume of the fluorinated ethylene carbonate, and 0.0001-7.5 parts by volume of the ethylene carbonate.

[0044] More preferably, in the embodiment, the solvent comprises 50-97.5 parts by volume of the dinitrile compound, 7.5-20 parts by volume of the fluorinated ethylene carbonate, and 0.0001-5 parts by volume of the ethylene carbonate.

[0045] In addition, the electrolyte of the present application can also include other types of solvents in the art.

[0046] For example, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, γ-butyrolactone, γ-valerolactone, 1,2-dimethoxyethane, or tetrahydrofuran. 2-methyltetrahydrofuran, tetrahydropyran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, 1,3-dioxane or 1,4-dioxane, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, methyl butyrate, methyl isobutyrate, methyl trimethylacetate or ethyl trimethylacetate, acetonitrile, pentanitrile, adiponitrile, methoxyacetonitrile, 3-methoxypropionitrile, N,N-dimethylformamide, N-methylpyrrolidone, N-methyl oxazolidone, N,N'-dimethylimidazolidone, nitromethane, nitroethane, sulfolane, trimethyl phosphate, and dimethyl sulfoxide. Preferably, at least one or more of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate is used, and this can result in more excellent properties. At this time, a combination of a high viscosity (high dielectric constant) solvent (for example, relative dielectric constant ε ≥ 30) such as ethylene carbonate and a low viscosity solvent (for example, viscosity ≤ 1 mPa·s) such as dimethyl carbonate, ethyl methyl carbonate, or diethyl carbonate is more preferable. By using these combinations, the dissociation of the electrolyte salt and the mobility of the ions can be improved.

[0047] Preferably, in the present embodiment, the lithium salt includes LiFSI and LiPF6.

[0048] Specifically, in the electrolyte, the concentration of LiFSI is 2M to 6M, and the concentration of LiPF6 is 0.01M to 0.2M.

[0049] In combination with the specific examples and test examples, the high-temperature cycle performance of the lithium ion battery using the electrolyte of the present application is further improved. That is, although the reason is unknown, by adding a small amount of LiPF6, the problem of the dinitrile compound deteriorating the direct current impedance of the battery can be further solved, thereby further broadening the use of the dinitrile compound as an electrolyte solvent.

[0050] In combination with the specific examples and test examples, increasing the amount of LiPF6 can reduce the stability of the electrolyte, and in particular, can deteriorate the high-temperature cycle performance.

[0051] Therefore, in the present application, in the electrolyte, the concentration of LiFSI is 2M to 6M, and the concentration of LiPF6 is 0.01M to 0.2M.

[0052] In addition, in the electrolyte of the present application, other types of lithium salts in the art can also be included.

[0053] For example, compounds such as LiBF4, LiClO4, LiAsF6, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiAlCl4, Li2SiF6, LiTFSI, LiBESI, etc. can be listed.

[0054] Preferably, in the embodiment, the dinitrile compound is selected from at least one of succinonitrile (SN), adiponitrile (ADN) and hexanetritrile (HTCN).

[0055] Preferably, in the embodiment, the volume ratio of succinonitrile in the dinitrile compound is not less than 50%.

[0056] Preferably, in the embodiment, the electrolyte further comprises an additive, and the mass ratio of the additive to the solvent is 0.1-5:100, and the additive comprises at least one of a bisoxalate-containing compound and a sulfur-containing cyclic compound.

[0057] Preferably, the bisoxalate-containing compound is selected from at least one of lithium bis(oxalato)borate (LiBOB) and lithium oxyborofluoride.

[0058] Preferably, the sulfur-containing cyclic compound is selected from at least one of 1,3-propane sulfone (PS), prop-1-ene-1,3-sultone and 1,3,2-dioxazolothiophene-2,2-dioxide.

[0059] More preferably, the mass ratio of the bisoxalate-containing compound to the sulfur-containing cyclic compound is 0.125-1.

[0060] In combination with the specific examples and test examples, although one of the oxalate-containing compound and the sulfur-containing cyclic compound can be used, the effect is better when both are used.

[0061] Preferably, the ratio of the oxalate-containing compound to the sulfur-containing cyclic compound is 0.5 mass% to 5 mass%.

[0062] In combination with the specific examples and test examples, the high-temperature cycle performance of the lithium ion battery using the electrolyte of the application is further improved. That is, because the oxalate-containing additive and the sulfur-containing cyclic compound are added, the problem of the dinitrile compound deteriorating the direct current impedance of the battery can be further solved, thereby further widening the use of the dinitrile compound as an electrolyte solvent.

[0063] In addition, other types of additives in the art can also be included in the electrolyte of the application.

[0064] For example, the solvent contains a high-quality, unsaturated carbon bond-containing cyclic carbonate. Because the solvent contains the unsaturated carbon bond-containing cyclic carbonate, a stable protective film is formed on the surface of the negative electrode during charging and discharging, and thus the decomposition reaction of the electrolyte solution can be inhibited. The unsaturated carbon bond-containing cyclic carbonate is a cyclic carbonate having one or more unsaturated carbon bonds, such as vinylene carbonate or ethylene carbonate. The solvent preferably contains at least one of a high-quality, halogenated chain carbonate and a halogenated cyclic carbonate. Fluoroethylene carbonate is a compound that functions as such, and in addition thereto, 4,5-difluoro-1,3-dioxolan-2-one (DFEC = difluoroethylene carbonate) and the like can be used. Furthermore, an acid anhydride can be contained. By containing the acid anhydride in the solvent, the chemical stability of the electrolyte solution is further improved. The acid anhydride is, for example, a dicarboxylic acid anhydride, a disulfonic acid anhydride, or a carboxylic acid sulfonic acid anhydride. The dicarboxylic acid anhydride is, for example, succinic anhydride, glutaric anhydride, or maleic anhydride. The disulfonic acid anhydride is, for example, oxalic anhydride or malonic anhydride. The carboxylic acid sulfonic acid anhydride is, for example, sulfobenzoic anhydride, sulfopropionic anhydride, or sulfobutyric anhydride.

[0065] In summary, in combination with the specific examples and test examples, the lithium ion battery using the electrolyte of the present application has the advantages of high capacity, good high-temperature cycle performance, high safety, and the like, and has a good application prospect.

[0066] The present application also discloses a lithium ion battery of the embodiment, which comprises a positive electrode, a negative electrode, a separator, and the above-mentioned electrolyte.

[0067] The lithium ion battery of the present application comprises the above-mentioned electrolyte, and through the combined use of the dinitrile compound, fluoroethylene carbonate, and ethylene carbonate, the problem that excessive dinitrile compound (generally more than 2.5 mass%) deteriorates the direct current impedance of the battery and greatly reduces the high-temperature cycle performance of the battery is greatly improved, and the high-temperature cycle performance of the lithium ion battery of the present application is significantly improved.

[0068] Preferably, in the embodiment, the current collector of the positive electrode is an aluminum foil.

[0069] In other embodiments, the current collector of the positive electrode can also be titanium or gold, and the like, but in the potential region used by the positive electrode, the lithium ion material is not doped with other substances, and has high corrosion resistance and oxidation resistance. This is because the surface of aluminum is covered with a firm natural oxidation film.

[0070] In the embodiment, the active material of the positive electrode, the negative electrode active material, the separator, and the like can all be manufactured by selecting the materials commonly used in the field.

[0071]

Positive electrode active material

[0072] As the above positive electrode active material, there is no particular limitation as long as it is a substance capable of electrochemically occluding and releasing lithium ions, and for example, a substance containing lithium and at least one transition metal is preferred. Specifically, for example, lithium-containing transition metal complex oxides, lithium-containing transition metal phosphoric acid compounds, and the like are exemplified. Among the positive electrode active materials, particularly preferable are lithium-containing transition metal complex oxides capable of producing a high voltage.

[0073] The lithium-containing transition metal complex oxide is, for example:

[0074] Chemical formula: Li a Mn 2－b M1 b O4

[0075] LiMn2O4 represented by the chemical formula: Li

[0076] Chemical formula: LiNi 1－c M2 c O2

[0077] LiNiO2 represented by the chemical formula: Li

[0078] Chemical formula: LiCo 1－d M3 d O2

[0079] LiCoO2 represented by the chemical formula: Li

[0080] Among them, from the viewpoint of being able to provide a high energy density, high output alkali metal lithium ion secondary battery, high energy density complex oxides represented by LiCoO2, LiMnO2, LiNiO2, LiMn2O4, MLiNixCoyAlzO2 (x+y+z=1), and LiNixCoyMnzO2 (x+y+z=1) are preferred.

[0081] Other positive electrode active materials, such as LiFeM4PO4 (M4 is Co, Mn, Cu, Zn, Al, Sn, Cr, V, Ti, Mg, Ca, Sr, B, Ga, In, Si, and Ge), LiNi0.5 Mn 1.5 O2, LiV3O6, Li2MnO3, etc.

[0082]

Negative electrode

[0083] As the negative electrode active material, carbon materials that can absorb and release lithium ions in various thermal decomposition products of organic substances, artificial graphite, natural graphite, and the like; metal oxide materials that can absorb and release lithium ions in tin oxide, silicon oxide, and the like; lithium metal; various lithium alloys; lithium-containing metal composite oxide materials, and the like can be exemplified. Two or more of these negative electrode active materials can be mixed and used.

[0084]

Separator

[0085] The separator separates the positive electrode and the negative electrode, prevents short circuiting of electric current due to contact between the electrodes, and allows lithium ions to pass therethrough. The separator can be a porous film made of a synthetic resin or a ceramic, or a laminated film in which two or more porous films are laminated. The synthetic resin can be, for example, polytetrafluoroethylene, polypropylene, or polyethylene. In order to improve the heat resistance of the separator, a separator coated with inorganic particles or a high-heat-resistant polymer material can also be used.

[0086] In combination with FIG. 1, the present application also discloses a method for preparing a lithium ion battery according to an embodiment, comprising the following steps:

[0087] S10, assembling the positive electrode, the negative electrode, and the separator into a semi-finished product.

[0088] S20, providing the electrolyte described above, heating the electrolyte to melt it, and injecting it into the semi-finished product.

[0089] Preferably, in S20, the temperature of the heating to melt the electrolyte can be 30°C to 90°C.

[0090] More preferably, in S20, the temperature of the heating can be 40°C to 80°C.

[0091] Particularly preferably, in S20, the temperature of the heating can be 60°C to 80°C.

[0092] S30, packaging the semi-finished product to obtain the desired lithium ion battery.

[0093] The lithium ion battery prepared by the method for preparing a lithium ion battery of the present application includes the electrolyte described above, and through the combined use of the dinitrile substance, the fluoroethylene carbonate, and the ethylene carbonate, the problem that excessive dinitrile substance (generally more than 2.5 mass%) can deteriorate the direct current impedance of the battery and greatly reduce the high-temperature cycle performance of the battery is greatly improved, so that the high-temperature cycle performance of the lithium ion battery prepared by the method for preparing a lithium ion battery of the present application is significantly improved.

[0094] The following is a specific example.

[0095] Example 1

[0096] A mixture of 742 g of succinonitrile (SN) and 258 g of fluoroethylene carbonate (FEC) was weighed so that the volume ratio of the two was 80:20.

[0097] To 18.7 g (equivalent to 0.1 M) of lithium fluorosulfonimide (LiFSI) at 45°C on a hot plate, the above mixed solvent was added to adjust to 0.1 L to obtain a dissolved electrolyte.

[0098] Examples 2 to 29

[0099] The electrolytes were prepared according to the formulations of Table 1 below, referring to the method of Example 1.

[0100] Table 1

[0101] Test Example

[0102] 1) Preparation of lithium ion secondary battery

[0103] Lithium ion secondary batteries were prepared using the electrolytes prepared in Examples 1 to 28, respectively, and the specific preparation method was as follows:

[0104] LiNi 0.6 Mn 0.2 Co 0.2 O2 was used as the positive electrode active material, carbon black was used as the conductive material, and an N-methyl-2-pyrrolidone dispersion solution of polyvinylidene fluoride (PVdF) was used as the binder, and the active material, the conductive material, and the binder were mixed in a mass ratio of 92:3:5 to obtain a positive electrode mixture slurry. The obtained positive electrode mixture slurry was uniformly coated on an aluminum foil current collector having a thickness of 12 μm, and after drying, compression molding was performed using a press to obtain a positive electrode laminate.

[0105] Artificial graphite and amorphous silicon (SiO) were used as the negative electrode active material, a sodium carboxymethyl cellulose aqueous dispersion solution (sodium carboxymethyl cellulose concentration 1 mass%) was used as the thickening agent, and a styrene-butadiene rubber aqueous dispersion solution (styrene-butadiene rubber (concentration 50 mass%)) was used as the binder, and the artificial graphite, the amorphous silicon (SiO), the thickening agent, and the binder were mixed in a mass ratio of 93:4.6:1.2:1.2 to obtain a slurry-like negative electrode mixture slurry. The negative electrode mixture slurry was uniformly coated on a copper foil having a thickness of 20 μm and dried, and then the mixture was compression molded using a press to obtain a negative electrode.

[0106] The positive electrode and the negative electrode obtained above were arranged opposite to each other with a 20 μm-thick microporous polyethylene film (separator) interposed therebetween, and the electrolyte in a liquid state obtained above was injected at 60°C to coat the electrolyte in a liquid state on the separator, etc., and after the electrolyte in a liquid state was sufficiently permeated, the battery was sealed, pre-charged, aged, and a stacked lithium ion secondary battery having a capacity of about 1000 mAh was produced.

[0107] 2) Initial battery capacity, 200 cyc @ 60°C retention rate, and battery safety test

[0108] The initial battery capacity, 200 cyc @ 60°C retention rate (cycle capacity retention rate after 200 cycles), and battery safety of the lithium ion secondary battery produced were tested, and the test results are shown in Table 2 below.

[0109] The test method for the initial battery capacity and 200 cyc @ 60°C retention rate was as follows: the lithium ion secondary battery produced above was subjected to constant current-constant voltage charging (hereinafter referred to as "CC / CV charging") at 60°C at a current equivalent to 0.5C (0.1C cutoff), and the battery was discharged to 3V, and discharged at a constant current of 0.5C, which was regarded as one cycle, and the initial discharge capacity was determined from the discharge capacity of the first cycle. After CC / CV charging (0.1C cutoff) was performed again to 4.2V, charging and discharging were performed in the same manner, and the discharge capacity after 200 cycles was measured.

[0110] The ratio of the discharge capacity after 200 cycles to the initial discharge capacity was calculated based on the following formula, and was taken as the cycle capacity retention rate after 200 cycles. Cycle capacity retention rate after 200 cycles = (discharge capacity after 200 cycles) / (initial discharge capacity) x 100%.

[0111] Herein, 1C indicates a current value at which the standard capacity of the battery is discharged in one hour, and 0.5C indicates a current value of 1 / 2 of the value, for example.

[0112] The test method for battery safety was as follows:

[0113] The lithium ion secondary battery was subjected to constant current-constant voltage charging (hereinafter referred to as "CC / CV charging") at 60°C at a current equivalent to 0.5C (0.1C cutoff) in the same manner as above, and a heating test was performed in a state where the SOC was 100%. The battery voltage, the battery surface temperature, and the temperature of the constant temperature bath were measured while the temperature was increased from room temperature to 130°C at a temperature increase rate of 5°C / min. After 30 minutes of observation at 130°C, the temperature was increased to 150°C at a rate of 5°C / min.

[0114] Table 2

[0115] In the table, "◎" means 150℃ qualified, "〇" means 130℃ qualified, "△" means 30 minutes or less unqualified at 130℃, and "×" means unqualified below 130℃.

[0116] In combination with Table 1 and Table 2, it can be seen from Examples 1-4 that high concentration of SN deteriorates the direct current impedance of the battery, and the cycle performance of the prepared lithium ion battery is extremely poor.

[0117] In combination with Examples 5-7, it can be seen that the addition of LiPF6 and LiBOB greatly alleviates the deterioration of SN on the direct current impedance of the battery, and greatly improves the cycle performance of the prepared lithium ion battery.

[0118] In combination with Examples 12-15, it can be seen that by optimizing the ratio of FEC and EC, the deterioration of SN on the direct current impedance of the battery can be further alleviated, and the cycle performance of the prepared lithium ion battery can be improved. However, when FEC:EC is less than 5:3, the safety of the battery decreases sharply.

[0119] In combination with Examples 18-22, it can be seen that when FEC:EC is greater than 5:3, the prepared lithium ion battery can still ensure safety. Even without adding EC (FEC:EC=∞ when EC=0), the prepared lithium ion battery still has certain safety, but its cycle performance is poor. Although the specific reason is unknown, it is known that the heat generation of the battery will increase when EC is excessive. However, simply reducing EC will lead to a decrease in salt dissociation, thereby degrading its performance as an electrolyte. The present application finds a region that can highly satisfy the safety and electrochemical properties of the battery by regulating within a specific concentration range.

[0120] In combination with Examples 23-29, it can be seen that by adjusting the ratio of LiBOB and PS, the cycle performance of the prepared lithium ion battery can be improved to a certain extent.

[0121] The above-described examples only express several embodiments of the present application, and the description is more specific and detailed, but it should not be understood as limiting the scope of the application. It should be noted that for ordinary skilled persons in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which are within the scope of the present application. Therefore, the protection scope of the present application patent should be subject to the appended claims.

Claims

1. An electrolyte, characterized in that, include: Lithium salts and solvents; The solvent comprises, by volume, 30 to 97.5 parts of a dinitrile compound, 2.5 to 20 parts of fluoroethylene carbonate, and 0.0001 to 10 parts of ethylene carbonate, wherein the volume ratio of the fluoroethylene carbonate to the ethylene carbonate is not less than 5:

3.

2. The electrolyte according to claim 1, characterized in that, The solvent comprises, by volume, 40 to 97.5 parts of the dinitrile compound, 5 to 20 parts of the fluoroethylene carbonate, and 0.0001 to 7.5 parts of the ethylene carbonate.

3. The electrolyte according to claim 2, characterized in that, The solvent comprises, by volume, 50 to 97.5 parts of the dinitrile compound, 7.5 to 20 parts of the fluoroethylene carbonate, and 0.0001 to 5 parts of the ethylene carbonate.

4. The electrolyte according to any one of claims 1 to 3, characterized in that, The lithium salt includes LiFSI and LiPF6, wherein the concentration of LiFSI is 2M to 6M and the concentration of LiPF6 is 0.01M to 0.2M.

5. The electrolyte according to claim 4, characterized in that, The dinitrile compound is selected from at least one of succinic anhydride, adiponitrile, and hexanetrionitrile.

6. The electrolyte according to claim 5, characterized in that, The dinitrile compound includes succinic anionyl nitrile, and the volume ratio of succinic anionyl nitrile in the dinitrile compound is not less than 50%.

7. The electrolyte according to claim 4, characterized in that, The electrolyte also includes additives, wherein the mass ratio of the additives to the solvent is 0.1 to 5:100, and the additives include at least one of a compound containing bis(oxalic acid) and a sulfur-containing cyclic compound.

8. The electrolyte according to claim 7, characterized in that, The compound containing bis(oxalic acid) is selected from at least one of lithium bis(oxalic acid)borate and lithium boron oxyfluoride. The sulfur-containing cyclic compound is selected from at least one of 1,3-propanesulfonate lactone, prop-1-ene-1,3-sulfonate lactone, and 1,3,2-dioxazothiophene-2,2-dioxide.

9. The electrolyte according to claim 8, characterized in that, The mass ratio of the compound containing bis(oxalic acid) to the sulfur-containing cyclic compound is 0.125 to 1.

10. A lithium-ion battery, characterized in that, It includes a positive electrode, a negative electrode, a separator, and an electrolyte as described in any one of claims 1 to 9.

11. The lithium-ion battery according to claim 10, characterized in that, The current collector for the positive electrode is aluminum foil.

12. A method for preparing a lithium-ion battery, characterized in that, Includes the following steps: The positive electrode, negative electrode, and separator are assembled into a semi-finished product; Provide an electrolyte as described in any one of claims 1 to 9, heat the electrolyte to melt it, and inject it into the semi-finished product; The semi-finished product is packaged to obtain the desired lithium-ion battery.

13. The method for preparing a lithium-ion battery according to claim 12, characterized in that, In the operation of heating the electrolyte to melt it, the heating temperature is 30°C to 90°C.

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