Electrolyte solution free of ethylene carbonate, lithium battery

Abstract

The present application relates to a kind of electrolyte without ethylene carbonate, it includes: (I) lithium salt;(II) monomer;(III) non-water organic solvent;(IV) non-aqueous electrolyte additive.The electrolyte although without the addition of ethylene carbonate, but the electrolyte can interact with negative electrode surface to generate stable solid-state electrolyte interface film, and the lithium battery using the electrolyte can improve the storage stability of lithium battery, low temperature performance, rate performance, cycle life and restrain expansion performance.The present application also relates to a kind of lithium battery containing the electrolyte.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of chemical batteries, in particular to a non-ethylene carbonate electrolyte and a lithium battery containing the same. BACKGROUND

[0002] Lithium batteries with high energy density have been widely used in electronic devices such as mobile phones and computers, electric vehicles, etc. Lithium batteries are mainly composed of positive electrodes, negative electrodes, non-aqueous electrolytes and separators (or diaphragms). In the process of lithium ion insertion and extraction reaction at the negative and positive electrodes, due to the change of chemical potential of active materials, lithium batteries form electric energy. The electrolyte exists between the negative and positive electrodes and acts as an ion conductor to transport lithium ions between the positive and negative electrodes in lithium ion batteries. Selecting a suitable electrolyte is one of the many factors that improve the performance of lithium batteries, because the reaction between the electrode and the electrolyte will affect the performance of the lithium battery. At present, non-aqueous electrolyte is composed of lithium salt dissolved in non-aqueous solvents such as ethylene carbonate (EC), propylene carbonate, dimethyl carbonate, etc.

[0003] The existing electrolyte applied to lithium batteries has the following technical problems:

[0004] 1. Ethylene carbonate is the main component of the current commercial electrolyte, which mainly benefits from the good film-forming performance of ethylene carbonate on the surface of negative active materials (such as graphite). However, the high melting point (40℃) of ethylene carbonate reduces the liquid range of the electrolyte and inhibits the cycle performance of the battery under low temperature conditions. In addition, ethylene carbonate shows strong complexing ability with lithium ions (Li + ) in the electrolyte, which limits the migration of lithium ions and the improvement of the rate performance of the battery.

[0005] 2. The electrolyte uses an organic solvent with a low boiling point, which can cause the lithium battery to swell during high-temperature storage. As a result, the reliability and safety of the lithium battery are poor at high temperatures.

[0006] 3. To improve the durability of lithium batteries, an additive is added to the electrolyte, and the degradation caused by decomposition of the electrolyte on the surface of the active positive and negative electrodes is suppressed by the electrodeposition of the additive, thereby improving the durability of the lithium battery. For example, by adding oxalate to the electrolyte, the increase in internal resistance of the lithium battery and the degradation of the cycle characteristics are suppressed. For another example, by adding an unsaturated carbonic acid compound to the electrolyte, a passivation layer (e.g., SEI film) is formed on the electrode to improve the storage performance and safety of the lithium battery. However, the existing additives, by interacting with the negative active material during the initial charging and discharging of the lithium battery, can decompose or form an unstable film, resulting in poor ion migration in the electron and the generation of gas inside the lithium battery, which increases the internal pressure, thereby significantly degrading the storage safety, cycle life, and capacity performance of the lithium battery. SUMMARY

[0007] The present application aims to provide a non-ethylene carbonate electrolyte that, although not containing ethylene carbonate, can interact with the negative electrode surface to form a stable solid electrolyte interface film (i.e., SEI film), and a lithium battery using the electrolyte can improve the storage stability, low-temperature performance, rate performance, cycle life, and swelling suppression performance of the lithium battery.

[0008] The present application provides a non-ethylene carbonate electrolyte, comprising:

[0009] (I) a lithium salt;

[0010] (II) a monomer represented by the following general formula [1];

[0011] (III) a non-aqueous organic solvent;

[0012] (IV) an additive for non-aqueous electrolytes.

[0013]

[0014] The total mass of the monomer represented by the above general formula [1] present in the above electrolyte is 0.1-10% of the total mass of the electrolyte. It is believed that the monomer represented by the above general formula [1] constitutes part of the stable solid electrolyte interface film formed on the surface of the electrode during charging and discharging, improving the cycle characteristics. From the perspective of improving cycle characteristics, the content of (II) is preferably 0.1% or more relative to the total mass of the electrolyte. On the other hand, since there is a tendency for the rate characteristics to be impaired when the content of the monomer represented by the above general formula [1] is too high, the upper limit of the content of (II) is preferably 10%.

[0015] For (I) lithium salt, the lithium salt serves as an ion transport medium, and preferred lithium salts include, for example, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bisoxalate, lithium difluoro oxalate, lithium difluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium (fluorosulfonyl)(perfluoroalkylsulfonyl)imide, and the like. The lithium salt can be used alone or in combination of two or more. When the content of (I) is less than 5% relative to the total mass of the electrolyte solution, the cycle characteristics and output characteristics of the nonaqueous electrolyte secondary battery are decreased due to the decrease in ion conductivity. On the other hand, when the content of (I) is more than 30% relative to the total mass of the electrolyte solution, there is a concern that the cycle characteristics and output characteristics of the nonaqueous electrolyte secondary battery are decreased due to the increase in viscosity of the electrolyte solution, which in turn decreases the ion conductivity.

[0016] For (III) nonaqueous organic solvent, the nonaqueous solvent used in the nonaqueous electrolyte solution is generally referred to as a nonaqueous electrolyte solution, and as the nonaqueous organic solvent (III), any solvent can be used as long as it is aprotic and can dissolve (I), (II), and (IV) of the present application, except that it does not contain ethylene carbonate. For example, carbonates, esters, ethers, lactones, nitriles, imides, sulfones, and the like can be used. In addition, the solvent can be a single solvent or a mixture of two or more solvents. As specific examples, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, vinylene carbonate, propylene carbonate, fluoroethylene carbonate, methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, linear ethers, cyclic ethers, branched ethers, vinyl sulfite, propylene sulfite, dimethyl sulfite, methyl ethyl sulfite, 1,3-propylene sulfite, propylene sulfite, 1,4-butylene sulfite, fluoro-propyl sulfite, vinyl sulfate, 1,3-propyl sulfate, acetonitrile, propionitrile, butyronitrile, hexanedinitrile, butanedinitrile, and the like can be listed.

[0017] For (IV) nonaqueous electrolyte additive, the nonaqueous electrolyte additive includes, for example, a wetting property improving solvent and an SEI forming solvent, which improve the wetting property of the electrolyte solution to the separator (or the diaphragm). The wetting property improving solvent includes, for example, dibutyl carbonate, methyl butyl carbonate, ethyl butyl carbonate, and the like. The SEI forming solvent is a solvent that easily forms an SEI layer compared to cyclic carbonates and cyclic esters, and includes, for example, vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, fluoro-vinylene carbonate, vinyl ethylene carbonate, ethynyl vinyl ethylene carbonate, vinyl sulfite, fluoro-vinyl ethylene carbonate, and the like. The SEI forming solvent can be used alone or in combination of two or more. The nonaqueous electrolyte additive can further include a functional ionic compound, such as lithium difluoro oxalate, lithium oxalate, lithium bis(fluorosulfonyl)imide, ionic liquid, and the like. The content of (IV) is preferably 0.01 to 10% relative to the total mass of the electrolyte solution.

[0018] Further, an additive having an overcharge-preventing effect, a negative electrode coating film forming effect, and a positive electrode protecting effect, which is generally used in the nonaqueous electrolyte of the present application, can be added in any ratio without impairing the gist of the present application. For example, a fluoroether, a malononitrile, a sulfonate, and the like.

[0019] In addition, as in a nonaqueous electrolyte secondary battery called a polymer battery, the electrolyte provided by the present application can be used by being quasi-solidified by a gelling agent, a crosslinked polymer.

[0020] The present application also provides a lithium battery containing the electrolyte.

[0021] Other features and / or advantages of the present application will be apparent from the description of the specific embodiments. DETAILED DESCRIPTION

[0022] It should be understood that the specific embodiments described herein are merely illustrative of the present application and should not be used to limit the present application in any manner.

[0023] The present application provides an electrolyte not containing ethylene carbonate, and a lithium battery using the electrolyte provided by the present application can improve the storage stability, low-temperature performance, rate performance, cycle life, and swelling suppression performance of the lithium battery, as compared with a commercial electrolyte containing ethylene carbonate.

[0024] The present application provides an electrolyte not containing ethylene carbonate, and a lithium battery using the electrolyte provided by the present application can improve the storage stability, low-temperature performance, rate performance, cycle life, and swelling suppression performance of the lithium battery, as compared with a commercial electrolyte containing ethylene carbonate.

[0025] (I) a lithium salt;

[0026] (II) a monomer represented by the following general formula [1] (for convenience, hereinafter referred to as additive A);

[0027] (III) a nonaqueous organic solvent;

[0028] (IV) an additive for a nonaqueous electrolyte.

[0029]

[0030] The electrolyte provided by the present application can improve the swelling suppression performance of a lithium battery. The electrolyte provided by the present application can suppress the swelling of a lithium battery due to the generation of gas when the temperature is increased.

[0031] The initial charge-discharge efficiency of a lithium battery is related to the film layer formed on the surface of the electrode, particularly the negative electrode. The film layer determines most of the performance of the lithium battery, such as its initial charge-discharge efficiency. When an effective initial charge-discharge is obtained, the surface of the negative electrode in the charged state is uniform. However, when an ineffective initial charge-discharge is obtained, a large amount of lithium is deposited on the surface of the negative electrode. The additive A acts as a surface active agent between the electrolyte and the negative electrode, particularly a carbon-containing negative electrode, to reduce the electrical resistance at the surface of the negative electrode, thereby inhibiting the decomposition of the electrolyte at the surface of the electrode. The film layer formed on the negative electrode of the electrolyte containing the additive A inhibits the decomposition of the electrolyte on the negative electrode during the initial charging process, and the film layer is stably formed on the negative electrode, so that the decomposition of the electrolyte is inhibited after repeated charging / discharging cycles, and the battery can be prevented from swelling during storage at an elevated temperature due to the stability of the electrode.

[0032] The electrolyte provided by the present application can improve the cycle life performance of a lithium battery. Commercial electrolyte containing ethylene carbonate forms a dense and sticky electrolyte during the cycle charging and discharging of the battery, thereby reducing the mobility of lithium ions. This reduction in lithium ion mobility can cause poor battery performance at high efficiency and low temperature, and can reduce the cycle life performance. The inventors have unexpectedly found that the electrolyte containing the additive A can inhibit the increase in the viscosity of the electrolyte during the cycle charging and discharging of the battery, so that the lithium battery can obtain an effective improvement in electrochemical performance,

[0033] The present application also relates to a lithium battery using the electrolyte, which is a lithium ion secondary battery suitable for electronic devices, electric vehicles, hybrid vehicles, etc.

[0034] [Lithium battery]

[0035] The lithium battery includes a positive electrode having a lithium compound as a positive electrode active material, a negative electrode having a graphite-based carbon material as a negative electrode active material, a separator (or a separator membrane), an outer housing, and the electrolyte provided by the present application. The structure of the lithium battery is not particularly limited, and a button cell, a cylindrical battery, a square battery, a soft pack battery, etc. having a single-layer or multi-layer separator can be used.

[0036] The positive electrode is formed by mixing the positive electrode active material and the additives (a binder and a conductive additive) and coating them on a positive electrode current collector. The negative electrode is formed by mixing the negative electrode active material and the additives (a binder and a conductive additive) and coating them on a negative electrode current collector.

[0037] [Positive electrode active material]

[0038] The positive electrode active material of the positive electrode of the lithium battery is not particularly limited as long as it is a material capable of charge and discharge, and examples thereof include a material containing at least one selected from the group consisting of a lithium transition metal composite oxide having a layered structure containing at least one or more metals selected from the group consisting of nickel, manganese, and cobalt, a lithium manganese composite oxide having a spinel structure, a lithium-containing olivine-type phosphate, and a lithium-excess layered transition metal oxide having a layered rock-salt structure.

[0039] [Positive electrode current collector]

[0040] The positive electrode has a positive electrode current collector. As the positive electrode current collector, for example, aluminum, stainless steel, nickel, titanium, or an alloy thereof, or the like can be used.

[0041] [Positive electrode active material layer]

[0042] The positive electrode has a positive electrode active material layer formed on at least one surface of the positive electrode current collector, for example. The positive electrode active material layer is composed of, for example, the aforementioned positive electrode active material, a binder, and, if necessary, a conductive agent. As the binder, polytetrafluoroethylene, polyvinylidene fluoride, or a styrene butadiene rubber (SBR) resin, or the like can be used. As the conductive agent, for example, acetylene black, ketjen black, a carbon material such as carbon fiber or graphite (granular graphite, flaky graphite), or the like can be used. The positive electrode preferably uses acetylene black or ketjen black having low crystallinity.

[0043] [Negative electrode active material]

[0044] The negative electrode active material of the negative electrode of the lithium battery can be doped / dedoped with lithium ions, and examples thereof include at least one selected from the group consisting of a carbon material, an oxide of one or more metals selected from the group consisting of Si, Sn, and Al, an alloy of one or more metals selected from the group consisting of Si, Sn, and Al or containing these metals, or an alloy of these metals or alloy and lithium, and a lithium titanium oxide. These negative electrode active materials can be used alone or in combination with two or more.

[0045] [Negative electrode current collector]

[0046] The negative electrode has a negative electrode current collector. As the negative electrode current collector, for example, copper, stainless steel, nickel, titanium, or an alloy thereof, or the like can be used.

[0047] [Negative electrode active material layer]

[0048] The negative electrode has a negative electrode active material layer formed on at least one surface of the negative electrode current collector, for example. The negative electrode active material layer is composed of, for example, the aforementioned negative electrode active material, a binder, and, if necessary, a conductive agent. As the binder, polytetrafluoroethylene, polyvinylidene fluoride, or a styrene butadiene rubber (SBR) resin, or the like can be used. As the conductive agent, for example, acetylene black, ketjen black, a carbon material such as carbon fiber or graphite (granular graphite, flaky graphite), or the like can be used.

[0049] [Method for manufacturing electrode]

[0050] The electrode can be obtained, for example, by dispersing an active material, a binder, and a conductive agent as necessary in a solvent such as N-methyl-2-pyrrolidone (NMP), water, or the like at a prescribed compounding amount and kneading, coating the obtained slurry on a current collector, and drying to form an active material layer. The obtained electrode is preferably compressed using a method such as roll pressing to adjust the electrode to a proper density.

[0051] [Separator (or diaphragm)]

[0052] The separator (or diaphragm) of the lithium battery is used to prevent the positive electrode from contacting the negative electrode, and a nonwoven fabric, a porous sheet made of polyolefin such as polypropylene, polyethylene, fiber, paper, or glass fiber, or the like can be used. In order to allow the electrolyte to permeate and easily pass through ions, these films are preferably microporous.

[0053] As the polyolefin separator, for example, a microporous high molecular film such as a porous polyolefin film, a film that electrically insulates the positive electrode and the negative electrode and allows lithium ions to pass through, can be cited. As a specific example of the porous polyolefin film, for example, a porous polyethylene film alone or a porous polyethylene film and a porous polypropylene film stacked and used as a multilayer film can be cited. In addition, a film in which a porous polyethylene film and a polypropylene film are compounded, or the like can be cited.

[0054] [Outer housing]

[0055] The outer housing of the lithium battery can use, for example, a metal can such as a coin type, a cylindrical type, a square type, or the like, a laminated outer housing. As the metal can material, for example, a nickel-plated iron steel sheet, a stainless steel sheet, a nickel-plated stainless steel sheet, aluminum or an alloy thereof, nickel, titanium, or the like can be cited. As the laminated outer housing, for example, an aluminum laminated film, a SUS laminated film, a laminated film of polypropylene, polyethylene, or the like coated with silicon oxide, or the like can be used.

[0056] The present application is not particularly limited in the configuration of the lithium battery, and is configured to include an electrode element in which a positive electrode and a negative electrode are arranged opposite to each other, and the electrolyte provided by the present application. The shape of the lithium battery is not particularly limited, and the above-described elements can be assembled to form an electrochemical device in a coin shape, a cylindrical shape, a square shape, or an aluminum laminated sheet type.

[0057] Examples 1 to 4 each provide an electrolyte solution not containing ethylene carbonate, and Comparative Examples 1 to 3 each provide an electrolyte solution. The specific component parameters of the electrolyte solutions provided in Examples 1 to 4 and Comparative Examples 1 to 3 are shown in Table 1.

[0058] The electrolyte solutions provided in Examples 1 to 4 and Comparative Examples 1 to 3 were each used, and LiNi 1 / 3 Co 1 / 3Mn 1 / 3 A battery was produced using LiNi

[0059] Table 1

[0060]

[0061] A test battery was produced in the following manner.

[0062] LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, polyvinylidene fluoride, and acetylene black were mixed at a mass ratio of 90:5:5, and N-methylpyrrolidone was added to produce a slurry. The slurry was coated on an aluminum foil and dried to produce a test positive electrode body. In addition, graphite and polyvinylidene fluoride were mixed at a mass ratio of 9:1, and N-methylpyrrolidone was further added to produce a slurry. The slurry was coated on a copper foil and dried at 150°C for 12 hours to produce a test negative electrode body. Furthermore, a polyethylene separator was impregnated with an electrolyte, and a 50 mAh battery was assembled with an aluminum laminated outer package.

[0063] [Initial capacity]

[0064] The produced battery was used to perform a charge and discharge test at a current density of 0.35 mA / cm 2 After charging to 4.2 V, a discharge was performed to 3.0 V at a current density of 0.35 mA / cm 2 The initial discharge capacity at this time was taken as the initial capacity. Note that the measurement was performed at an ambient temperature of 25°C.

[0065] [High-temperature cycle characteristics]

[0066] A charge and discharge test was performed at an ambient temperature of 60°C using the above battery, and the cycle characteristics were evaluated. The charge and discharge were each performed at a current rate of 3C, and after charging to 4.2 V, the battery was maintained at 4.2 V for 1 hour, and then discharged to 3.0 V. The charge and discharge cycle was repeated. Furthermore, the deterioration of the battery was evaluated by the discharge capacity retention rate after 500 cycles. The discharge capacity retention rate after 500 cycles was:

[0067] Discharge capacity retention rate (%) = (discharge capacity after 500 cycles / initial discharge capacity) x 100

[0068] [Rate characteristics]

[0069] Using the above battery, a charge-discharge test at an ambient temperature of 25°C was performed to evaluate the high rate characteristics. Charging was performed at a constant current rate of 0.2C, and after reaching 4.2V, 4.2V was maintained for 1 hour, and discharging was performed to 3.0V, and the data at two points when the current rate was measured at 0.2C and at 5C, and the discharge capacity ratio at 0.2C discharge and at 5C discharge was:

[0070] High rate discharge capacity ratio (%) = (discharge capacity at 5C discharge / discharge capacity at 0.2C discharge) x 100

[0071] The results of the above test are shown in Table 2.

[0072] Table 2

[0073]

[0074] From the test results of Examples 1 to 4, it was found that the electrolyte according to the present application can exhibit the cycle characteristics and the rate characteristics in a balanced manner when used in a lithium battery. In contrast, in Comparative Example 1 in which an electrolyte not containing the additive A was used, it was confirmed that there was a tendency to have poor cycle characteristics and rate characteristics compared to Example 3. In addition, in Comparative Example 3 in which an electrolyte containing ethylene carbonate was used, it was also confirmed that there was a tendency to have poor cycle characteristics and rate characteristics compared to Example 3.

[0075] In addition, from Comparative Examples 2 and 3, it was found that when ethylene carbonate was used as the nonaqueous organic solvent, the cycle characteristics and the rate characteristics can be exhibited in a balanced manner by using the additive A.

[0076] In addition, from Examples 3 and 3, it was found that when the additive A was used, there was a tendency to have poor cycle characteristics and rate characteristics when ethylene carbonate was used as the nonaqueous organic solvent.

[0077] From Comparative Example 1, it was found that when an electrolyte not containing the additive A was used instead of fluoroethylene carbonate, there was a tendency to have poor cycle characteristics and rate characteristics compared to Example 3.

[0078] After the test batteries corresponding to Examples 3 and Comparative Examples 1 to 3 were left to stand at 85°C for 4 hours, the thickness of each battery was measured to determine the high temperature expansion suppression performance thereof. The results obtained are shown in Table 2. Among them, the thickness increase ratio is:

[0079] (Battery thickness after storage at 85°C - battery thickness after standard charging) / battery thickness after standard charging x 100

[0080] Table 3

[0081]

[0082] As shown in Table 3, the expansion-inhibiting performance of the lithium battery corresponding to Example 3 was significantly improved compared to the expansion-inhibiting performance of the lithium batteries of Comparative Examples 1 to 3.

[0083] The lithium battery containing the electrolyte according to the present application has significantly improved charge-discharge, cycle life performance and expansion-inhibiting performance compared to those containing conventional electrolytes.

[0084] The technical features of the above-described embodiments can be combined in any manner. For the sake of brevity, not all possible combinations of the technical features of the above-described embodiments are described, however, as long as the combination of the technical features does not result in a contradiction, it should be considered to be within the scope of the present disclosure.

[0085] The above-described embodiments only express several embodiments of the present application, and the description is specific and detailed, but it should not be construed as limiting the scope of the patent. It should be noted that for those skilled 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 scope of protection of the present patent should be subject to the appended claims.

Claims

1. An electrolyte that does not contain ethylene carbonate, characterized in that, It includes: (I) Lithium salts; (II) The monomers represented by the following general formula [1]; (III) Non-aqueous organic solvents; (IV) Improved wettability and solvent and SEI formation; [1]； The content of (II) is 0.1-10% relative to the total mass of the electrolyte; The content of (IV) is 0.01-10% relative to the total mass of the electrolyte; The wettability-improving solvent is selected from at least one of the following: dibutyl carbonate, methyl butyl carbonate, and ethyl butyl carbonate; The SEI forming solvent is selected from at least one of the following: vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, fluorovinylene carbonate, ethylene ethylene carbonate, ethynyl ethylene carbonate, vinyl sulfite, and fluorovinyl carbonate.

2. The electrolyte according to claim 1, characterized in that, The content of (I) is 5-30% relative to the total mass of the electrolyte.

3. The electrolyte according to claim 2, characterized in that, The (I) is selected from at least one of the following: lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(oxalate), lithium difluorooxalate, lithium difluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, and lithium (fluorosulfonyl)(perfluoroalkylsulfonyl)imide.

4. The electrolyte according to claim 1, characterized in that, The (III) is selected from at least one of the following: dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, vinylene carbonate, propyl carbonate, fluoroethylene carbonate, methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, straight-chain ether, cyclic ether, branched-chain ether, vinyl sulfite, propylene sulfite, dimethyl sulfite, methyl ethyl sulfite, 1,3-propylene sulfonate, propylene sulfonate, 1,4-butyl sulfonate, propyl fluorosulfonate, vinyl sulfate, 1,3-propyl sulfate, acetonitrile, propionitrile, butyronitrile, adiponitrile, butadiene nitrile.

5. A lithium battery, characterized in that, The lithium battery comprises at least: a positive electrode, a negative electrode, and an electrolyte as described in any one of claims 1-4.

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