CN111900476A - Electrolyte solution and electrochemical device - Google Patents

Electrolyte solution and electrochemical device Download PDF

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CN111900476A
CN111900476A CN202010732744.8A CN202010732744A CN111900476A CN 111900476 A CN111900476 A CN 111900476A CN 202010732744 A CN202010732744 A CN 202010732744A CN 111900476 A CN111900476 A CN 111900476A
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electrolyte
lithium
additive
ethylene carbonate
carbonate
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何�轩
王振东
罗世康
褚春波
张耀
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Sunwoda Electric Vehicle Battery Co Ltd
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/056—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes
    • H01M10/0564—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of organic materials only
    • H01M10/0566—Liquid materials
    • H01M10/0567—Liquid materials characterised by the additives
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/056—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes
    • H01M10/0564—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of organic materials only
    • H01M10/0566—Liquid materials
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M2300/00—Electrolytes
    • H01M2300/0017—Non-aqueous electrolytes
    • H01M2300/0025—Organic electrolyte
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    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

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Abstract

The present invention relates to an electrolyte and an electrochemical device. An electrolyte comprising an organic solvent, a lithium salt dissolved in the organic solvent, and an additive comprising a fluorosilane, a cyclic ethylene carbonate, and a lithium additive, wherein the lithium additive is selected from at least one of lithium tetrafluoroborate and lithium difluorooxalate phosphate. The electrolyte additive comprises fluoro-silane, cyclic ethylene carbonate and a lithium additive, can effectively inhibit the increase of direct current impedance (DCR) in the circulation process, effectively reduces the direct current impedance of the battery cell, and obviously improves the power performance of the battery cell; meanwhile, the cyclic ethylene carbonate can form an SEI film mainly comprising polymers on the surfaces of the positive electrode and the negative electrode, the SEI film has good mechanical toughness, is not easy to break and crush due to volume change, and can continuously protect the positive electrode and the negative electrode in the circulating process, so that the circulating performance of the battery under large current can be effectively improved.

Description

Electrolyte solution and electrochemical device
Technical Field
The invention relates to the technical field of batteries, in particular to an electrolyte and an electrochemical device.
Background
Lithium ion batteries have been widely used in the field of consumer electronics due to their high operating voltage, small size, no pollution, long cycle life, and the like. Meanwhile, as an environment-friendly energy source, the lithium ion battery also begins to occupy larger and larger market share in the field of electric automobiles, and the application range of the lithium ion battery in the fields of aerospace, national defense and military industry is gradually expanded, so that higher requirements are put forward on the power performance of the lithium ion battery.
Generally speaking, the lithium ion battery forms polarization to reduce charging and discharging efficiency when charging and discharging under a large current, so that the effective charging and discharging capacity and energy of the lithium ion battery are obviously reduced, and the cycle performance is reduced; in addition, the direct current impedance of the current lithium ion battery is high, and it is difficult to suppress the increase in the direct current impedance.
Disclosure of Invention
Accordingly, it is necessary to provide an electrolyte solution capable of reducing dc resistance, suppressing increase in dc resistance, and improving cycle performance at a large current.
In addition, an electrochemical device is also provided
An electrolyte comprising an organic solvent, a lithium salt dissolved in the organic solvent, and an additive comprising a fluorosilane, a cyclic ethylene carbonate, and a lithium additive, wherein the lithium additive is selected from at least one of lithium tetrafluoroborate and lithium difluorooxalate phosphate.
The additive of the electrolyte comprises fluoro silane, cyclic ethylene carbonate and lithium additive. The addition of the cyclic vinyl carbonate additive and the lithium additive into the electrolyte can form a stable composite solid electrolyte interface film (SEI film) with excellent ion permeability on the surfaces of a positive electrode and a negative electrode, and the SEI film can effectively inhibit the increase of direct current impedance (DCR) in the circulation process, so that the battery cell can keep good power performance; meanwhile, the solvation of lithium ions is improved, so that the conductivity of lithium ions is improved, and the transmission rate of the lithium ions is higher, thereby reducing the direct-current impedance of the battery cell. The cyclic ethylene carbonate can form an SEI film which takes polymers as main materials on the surfaces of the positive electrode and the negative electrode, the SEI film has good mechanical toughness, is not easy to break and crush due to volume change, and can continuously protect the positive electrode and the negative electrode in the circulating process, so that the circulating performance of the battery under large current can be effectively improved.
In one embodiment, the mass percentage of the fluoro silane in the electrolyte is 0.1-4%, the mass percentage of the cyclic ethylene carbonate in the electrolyte is 0.1-4%, and the mass percentage of the lithium additive in the electrolyte is 0.1-4%.
In one embodiment, the fluorosilane is selected from one or more of the following compounds represented by structural formula I:
Figure BDA0002603795520000021
wherein R is1、R2、R3、R4Each independently selected from halogen, C1～10Alkyl, substituted C1～10Alkyl radical, C1～10Alkoxy, substituted C1～10Alkoxy radical, C2～10Alkenyl, substituted C2～10Alkenyl radical, C2～10Alkynyl, substituted C2～10One of an alkynyl group, a silicon-containing group and a substituted silicon-containing group, and R1、R2、R3、R4At least one of which is a fluorine atom.
In one embodiment, the fluorosilane is selected from one or more of the following compounds:
Figure BDA0002603795520000022
Figure BDA0002603795520000031
the use of the fluoro-silane additives can effectively inhibit DCR growth, maintain power performance and reduce cell DC impedance.
In one embodiment, the cyclic ethylene carbonate is selected from at least one of the compounds represented by the following structural formula II:
Figure BDA0002603795520000032
wherein R is1And R2Each independently selected from C1～3Alkyl, substituted C1～3Alkyl radical, C2～3Alkylene, substituted C2～3Alkylene and halogen.
In one embodiment, the cyclic ethylene carbonate is selected from one or more of vinylene carbonate, ethylene carbonate and fluoroethylene carbonate.
In one embodiment, the lithium additive is a mixture of lithium tetrafluoroborate and lithium difluorooxalate phosphate.
In one embodiment, the organic solvent comprises fluorobenzene and propylene carbonate.
In one embodiment, the organic solvent further comprises at least one carbonate compound.
An electrochemical device comprises a positive electrode, a negative electrode, a separation film and the electrolyte.
Detailed Description
The present invention may be embodied in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.
The following terms used herein have the meanings indicated below, unless explicitly indicated otherwise.
The term "about" is used to describe and illustrate minor variations. When used in conjunction with an event or circumstance, the terms can refer to instances where the event or circumstance occurs precisely as well as instances where the event or circumstance occurs in close proximity. For example, when used in connection with data, the terms may refer to a range of variation of less than or equal to ± 10% of the stated value, such as less than or equal to ± 5%, less than or equal to ± 4%, less than or equal to ± 3%, less than or equal to ± 2%, less than or equal to ± 1%, less than or equal to ± 0.5%, less than or equal to ± 0.1%, or less than or equal to ± 0.05%. It is to be understood that such range format is used for convenience and brevity, and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or ranges encompassed within that range as if each numerical value and subrange is explicitly recited.
The term "halogen" encompasses F, Cl, Br, I.
The term "hydrocarbyl" encompasses alkyl, alkenyl, alkynyl.
The term "alkyl" is intended to be a straight chain saturated hydrocarbon structure having from 1 to 20 carbon atoms. "alkyl" is also contemplated to be a branched or cyclic hydrocarbon structure having from 3 to 20 carbon atoms. When an alkyl group having a particular carbon number is specified, all geometric isomers having that carbon number are intended to be encompassed; thus, for example, "butyl" is meant to include n-butyl, sec-butyl, isobutyl, tert-butyl, and cyclobutyl; "propyl" includes n-propyl, isopropyl and cyclopropyl. Examples of alkyl groups include, but are not limited to, methyl, ethyl, n-propyl, isopropyl, cyclopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, cyclobutyl, n-pentyl, isopentyl, neopentyl, cyclopentyl, methylcyclopentyl, ethylcyclopentyl, n-ethyl, hexyl, cyclohexyl, n-heptyl, octyl, cyclopropyl, cyclobutyl, norbornyl, and the like. In addition, the alkyl group may be optionally substituted.
The term "alkenyl" refers to a monovalent unsaturated hydrocarbon group that can be branched or branched and has at least one and typically 1,2, or 3 carbon-carbon double bonds. Unless otherwise defined, the alkenyl groups typically contain 2 to 20 carbon atoms and include, for example, -C2-4Alkynyl, -C3-6Alkynyl and-C3-10Alkynyl. Representative alkynyl groups include, for example, ethynyl, prop-2-ynyl (n-propynyl), n-but-2-ynyl, n-hex-3-ynyl, and the like.
The term "alkylene" means a divalent saturated hydrocarbon group that may be straight-chain or branched. Unless otherwise defined, allThe alkylene group usually contains 2 to 10 carbon atoms and includes, for example, -C2-3Alkylene and-C2-6An alkylene group. Representative alkylene groups include, for example, methylene, ethane-1, 2-diyl ("ethylene"), propane-1, 2-diyl, butane-1, 4-diyl, pentane-1, 5-diyl, and the like.
The term "aryl" means a monovalent aromatic hydrocarbon having a single ring (e.g., phenyl) or a fused ring. Fused ring systems include those that are fully unsaturated (e.g., naphthalene) and those that are partially unsaturated (e.g., 1,2,3, 4-tetrahydronaphthalene). Unless otherwise defined, the aryl group typically contains 6 to 26 carbon ring atoms and includes, for example, -C6-10And (4) an aryl group. Representative aryl groups include, for example, phenyl, methylphenyl, propylphenyl, isopropylphenyl, benzyl, and naphthalen-1-yl, naphthalen-2-yl, and the like.
When the above substituents are substituted, the substituents may be selected from the group consisting of: halogen, aryl, nitro, cyano, carboxyl and sulfate.
In the detailed description and claims, a list of items connected by the term "one of can mean any of the listed phases. For example, if items a and B are listed, the phrase "one of a and B" means a alone or B alone. In another example, if items A, B and C are listed, the phrase "one of A, B and C" means only a; only B; or only C. Item a may comprise a single element or multiple elements. Item B may comprise a single element or multiple elements. Item C may comprise a single element or multiple elements.
In the detailed description and claims, a list of items joined by the term "at least one of may mean any combination of the listed items. For example, if items a and B are listed, the phrase "at least one of a and B" means a only; only B; or A and B. In another example two, if items A, B and C are listed, the phrase "one of A, B and C" means only a; only B; only C; a and B (excluding C); a and C (excluding B); b and C (excluding A); or A, B and C. Item a may comprise a single element or multiple elements. Item B may comprise a single element or multiple elements. Item C may comprise a single element or multiple elements.
An electrochemical device of an embodiment includes a positive electrode, a negative electrode, a separator, and an electrolyte.
Wherein the positive electrode contains a positive electrode active material. Further, the positive electrode active material is selected from at least one of lithium-containing transition metal oxides. Further, the Ni content in the lithium-containing transition metal oxide is 30% to 90%.
Specifically, the positive electrode active material includes LiNi0.3Co0.3Mn0.3O2、LiNi0.5Co0.2Mn0.3O2、LiNi0.6Co0.2Mn0.2O2、LiNi0.8Co0.1Mn0.1O2，LiNi0.8Co0.1Al0.1O2At least one of (1).
The electrolyte comprises an organic solvent, a lithium salt dissolved in the organic solvent and an additive.
Further, the organic solvent includes Fluorobenzene (FB). Furthermore, the mass percentage of the fluorobenzene in the electrolyte is 2-10%. Furthermore, the mass percentage of the fluorobenzene in the electrolyte is 3-6%.
Further, the organic solvent also comprises propylene phosphate. In one embodiment, the organic solvent comprises propylene carbonate and fluorobenzene. The viscosity of the electrolyte can be reduced by using fluorobenzene in a solvent, the surface tension of the solution can be reduced by using propylene carbonate, and the wettability of the electrolyte can be obviously improved and the direct current impedance can be reduced by combining the fluorobenzene and the propylene carbonate.
Further, the organic solvent also comprises at least one of carbonate compounds. Further, the organic solvent includes at least one of a cyclic carbonate and a chain carbonate. Specifically, the organic solvent includes cyclic carbonates and chain carbonates. More specifically, the weight ratio of the cyclic carbonate to the chain carbonate is 20:80 to 35: 65.
Specifically, the cyclic carbonate comprises one or a combination of ethylene carbonate, fluoroethylene carbonate and propylene carbonate. Specifically, the chain carbonate includes one or a combination of several of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate and dipropyl carbonate.
Further, the mass content of the lithium salt accounts for 11-20% of the total mass of the electrolyte by taking the total mass of the electrolyte as 100%. Furthermore, the mass content of the lithium salt accounts for 12.5-15.5% of the total mass of the electrolyte.
Specifically, the lithium salt is selected from LiPF6、LiBF4LiBOB (lithium bis (oxalato) borate), LiAsF6、LiPO2F2、LiN(CF3SO2)2、LiCF3SO3、LiClO4、LiN(CxF2x+1SO2)(CyF2y+1SO2) Wherein LiN (C) isxF2x+1SO2)(CyF2y+1SO2) X and y in (1) are natural numbers.
Specifically, the additives include fluoro silanes, cyclic ethylene carbonates, and lithium additives. Wherein the lithium additive is selected from at least one of lithium tetrafluoroborate and lithium difluorooxalate phosphate.
The addition of the cyclic vinyl carbonate additive and the lithium additive into the electrolyte can form a stable composite solid electrolyte interface film (SEI film) with excellent ion permeability on the surfaces of the positive electrode and the negative electrode, and the SEI film can effectively inhibit the increase of direct current impedance (DCR) in the circulation process, so that the battery cell can keep good power performance. Meanwhile, the solvation of lithium ions is improved, so that the conductivity of lithium ions is improved, and the transmission rate of the lithium ions is higher, so that the direct current impedance (DCR) of the battery cell is reduced. Therefore, the low-temperature lithium precipitation of the battery can be effectively inhibited. In the process of charging and discharging of the battery, the positive electrode and the negative electrode can generate volume change (expansion or contraction) due to lithium intercalation and lithium separation, the phenomenon is particularly obvious (the speed is higher) under large current, the cyclic ethylene carbonate can form an SEI film which takes polymers as main materials on the surfaces of the positive electrode and the negative electrode, the SEI film has good mechanical toughness, is not easy to break and break due to volume change, can continuously protect the positive electrode and the negative electrode in the cycle process, and therefore the cycle performance of the battery under large current can be effectively improved.
The internal impedance of the battery consists of electrolyte impedance and SEI film impedance: r ═ Rs + RCT, where Rs is the electrolyte resistance and RCT is the SEI film resistance.
However, since the cyclic ethylene carbonate and the lithium additive both participate in film formation at the positive electrode and the negative electrode, the battery using the additives has high direct current impedance, and the battery performance is easily influenced by high polarization in the process of large current circulation of the battery; the addition of the fluoro-silane can effectively improve the conductivity of the lithium ion and improve the transmission capability of the lithium ion in the electrolyte, thereby improving the rate capability of the battery; in addition, the resistance of the battery can be reduced by reducing Rs, so that the low-temperature lithium precipitation of the battery can be improved.
Furthermore, the mass percentage of the fluoro silane in the electrolyte is 0.1-4%. Furthermore, the mass percentage of the fluoro silane in the electrolyte is 0.3% -2%.
Specifically, the fluoro silane is selected from one or a combination of several of the compounds shown in the following structural formula I:
Figure BDA0002603795520000081
wherein R is1、R2、R3、R4Each independently selected from halogen, C1～10Alkyl, substituted C1～10Alkyl radical, C1～10Alkoxy, substituted C1～10Alkoxy radical, C2～10Alkenyl, substituted C2～10Alkenyl radical, C2～10Alkynyl, substituted C2～10One of an alkynyl group, a silicon-containing group and a substituted silicon-containing group, and R1、R2、R3、R4At least one of which is a fluorine atom.
More specifically, the fluoro silane is selected from one or a combination of several of the following compounds:
Figure BDA0002603795520000082
further, the content of the cyclic ethylene carbonate in the electrolyte is 0.1-4% by mass. Furthermore, the content of the cyclic ethylene carbonate in the electrolyte is 0.5-2% by weight.
Specifically, the cyclic ethylene carbonate is selected from at least one of a compound represented by the following structural formula II:
Figure BDA0002603795520000083
wherein R is1And R2Each independently selected from C1～3Alkyl, substituted C1～3Alkyl radical, C2～3Alkylene, substituted C2～3Alkylene and halogen.
More specifically, the cyclic ethylene carbonate is selected from one or more of vinylene carbonate, ethylene carbonate and fluoroethylene carbonate. The cyclic ethylene carbonate additive can effectively improve the cycle performance of the battery cell.
The lithium tetrafluoroborate and the lithium difluorooxalate phosphate can effectively inhibit side reactions between the charged anode and cathode and the electrolyte, thereby prolonging the cycle life of the battery. In addition, lithium tetrafluoroborate and lithium difluorooxalate phosphate can reduce the increase of battery interfacial Resistance (RCT) in the cycling process and control the increase of cycling DCR.
Further, the mass percentage of the lithium tetrafluoroborate in the electrolyte is 0.1-4%. Furthermore, the mass percentage of the lithium tetrafluoroborate in the electrolyte is 0.2-2%.
Further, the mass percentage of the lithium difluorooxalate phosphate in the electrolyte is 0.1-4.0%. Furthermore, the mass percentage of the lithium difluorooxalate phosphate in the electrolyte is 0.2-2.0%.
In one embodiment, the additive further comprises one or more of cyclic sulfate additives such as ethylene sulfate, 1, 3-propane sultone, 1, 4-butane sultone, 1, 3-propene sultone, 1-propene-1, 3-sultone (PST).
In one embodiment, the total weight of the additive is 5-10% of the total weight of the electrolyte.
The electrolyte at least has the following advantages:
1) the additive of the electrolyte comprises fluoro silane, cyclic ethylene carbonate and lithium additive. The addition of the cyclic vinyl carbonate additive and the lithium additive into the electrolyte can form a stable composite solid electrolyte interface film (SEI film) with excellent ion permeability on the surfaces of a positive electrode and a negative electrode, and the SEI film can effectively inhibit the increase of direct current impedance (DCR) in the circulation process, so that the battery cell can keep good power performance; meanwhile, the solvation of lithium ions is improved, so that the conductivity of lithium ions is improved, and the transmission rate of the lithium ions is higher, thereby reducing the direct-current impedance of the battery cell. The cyclic ethylene carbonate can form an SEI film which takes polymers as main materials on the surfaces of the positive electrode and the negative electrode, the SEI film has good mechanical toughness, is not easy to break and crush due to volume change, and can continuously protect the positive electrode and the negative electrode in the circulating process, so that the circulating performance of the battery under large current can be effectively improved.
2) Because the cyclic ethylene carbonate and the lithium additive both participate in film formation at the positive electrode and the negative electrode, the battery using the cyclic ethylene carbonate and the lithium additive only has higher direct current impedance, and the battery performance is easily influenced by high polarization in the process of large-current circulation of the battery; the addition of the fluoro-silane can effectively improve the conductivity of the lithium ion and improve the transmission capability of the lithium ion in the electrolyte, thereby improving the rate capability of the battery.
3) The mass percentage of the fluoro-silane in the electrolyte is 0.1-4%, the mass percentage of the cyclic ethylene carbonate in the electrolyte is 0.1-4%, and the mass percentage of the lithium additive in the electrolyte is 0.1-4%. The addition amount of the additive is within the range, so that the battery performance is obviously improved, and the battery has better cycle performance; the improvement effect by continuously increasing the amount of the above additives is not significant, and the cell manufacturing cost is increased, and adverse effects such as increase in impedance/deterioration in cycle performance may be caused.
The following are specific examples:
1. preparing an electrolyte: in an argon atmosphere glove box (H)2O<10ppm,O2<1ppm), uniformly mixing ethylene carbonate (simple EC), Ethyl Methyl Carbonate (EMC), Fluorobenzene (FB) and Propylene Carbonate (PC) according to the mass ratio of (30-y) to (70-x) x to y to obtain a non-aqueous solvent, wherein x is more than or equal to 0 and less than or equal to 10, and y is more than or equal to 0 and less than or equal to 10; then fully dried lithium salt LiPF6Dissolving in the non-aqueous solvent to prepare LiPF6And the concentration of the basic electrolyte is 1.0 mol/L.
As shown in table 1, a fluorosilane additive, a cyclic vinyl carbonate additive, lithium tetrafluoroborate, and lithium difluorooxalate phosphate were added to the base electrolyte.
Examples of the fluorosilane-based additive are: trimethylsilylfluoride (TMSF, a 1); triethylsilicofluoride (a 2); dimethyl disilicon fluoride (a 3); methyltrisifluosilane (a 4); 1-fluorosilane (A5).
Examples of additives of the cyclic vinyl carbonate type are: ethylene carbonate (VC, B1), fluoroethylene carbonate (FEC, B2);
TABLE 1 electrolyte additives and addition amounts for examples 1-21 and comparative examples 1-12
Figure BDA0002603795520000111
2. Preparing a lithium ion battery:
1) preparing a positive plate: mixing lithium nickel cobalt manganese (LiNi) as positive electrode active material0.8Co0.1Mn0.1O2) The conductive agent acetylene black and the binder polyvinylidene fluoride (PVDF) are fully stirred and mixed in a proper amount of N-methyl pyrrolidone (NMP) solvent according to the weight ratio of 97:2:1 to form uniform anode slurry; and coating the slurry on an aluminum foil of a positive current collector, and drying, rolling and cutting into pieces to obtain the positive plate.
2) Preparation of a negative electrode: fully stirring and mixing a negative active material graphite, a conductive agent acetylene black and a binder Styrene Butadiene Rubber (SBR) in a proper amount of deionized water solvent according to a weight ratio of 96:1:3 to form uniform negative slurry; and coating the slurry on a copper foil of a negative current collector, and drying, rolling and cutting into pieces to obtain the negative plate.
3) Preparing a lithium ion battery: stacking the positive plate, the diaphragm and the negative plate in sequence to enable the isolating film to be positioned between the positive plate and the negative plate to play an isolating role, and then winding, hot-pressing and shaping, and welding tabs to obtain a bare cell; and placing the bare cell in an outer packaging foil, injecting the prepared electrolyte into the dried battery, and performing vacuum packaging, standing, formation, shaping and other processes to complete the preparation of the lithium ion battery.
The electrolytes and lithium ion batteries of examples 1 to 21 and comparative examples 1 to 12 were prepared according to the above preparation methods; the additives in the electrolyte and the respective amounts added are shown in table 1.
The lithium ion batteries of the comparative examples and examples of the present application were tested for performance by experiment as follows.
Test one, DC impedance DCR test
The lithium ion batteries obtained by the preparation were subjected to the following tests, respectively:
the test was completed by charging the example and comparative batteries to 4.2V with CC-CV at 25 deg.c, with a cutoff current of 0.05C, discharging for 30min at 1C capacity, adjusting to 50% SOC, then leaving at 25 for 2h, performing a pulse program, discharging at 5C for 10s at constant current, and leaving for 10 min.
DCR ═ (pre-pulse discharge voltage-post-pulse discharge voltage)/discharge current 100%;
the results are reported in Table 2.
Test two, cycle experiment
The lithium ion batteries obtained by the preparation were subjected to the following tests, respectively:
under the condition of 25, carrying out charge-discharge cycle test with the charge-discharge rate of 3C/3C within the voltage range of 2.8-4.2V, respectively recording the first charge-discharge capacity of the battery and the discharge capacity after each cycle, cycling 1000 times, and calculating the capacity retention rate of each lithium battery, wherein the capacity retention rate is the discharge capacity per cycle/the first discharge capacity of the battery and is 100%.
The DCR increase rate before and after the cycle (post-cycle DCR-pre-cycle DCR) × 100%
The electrolyte selected for each lithium ion battery, and the data of capacity retention rate and cyclic DCR increase rate after 1000 cycles are shown in Table 2.
TABLE 2
Figure BDA0002603795520000131
Figure BDA0002603795520000141
The following conclusions can be drawn in conjunction with the data in tables 1-2:
1) comparative examples 1 to 12, which use only a part of the additive or a part of the solvent, have some disadvantages in the battery performance. The absence of a part of key additives makes the battery insufficient to form a composite solid electrolyte interface film (SEI film) stable at high potential, affecting the cycle performance of the battery; in addition, the impedance of part of the battery is large, and lithium ion transmission is suppressed.
2) Comparing examples 1-6 with comparative example 6, the results show that additive A has good synergistic effect with other additives. The additive A is added into the electrolyte, so that the low-temperature lithium ion conductivity can be effectively improved, and the transmission capability of lithium ions in the electrolyte (namely Rs is reduced), thereby reducing the battery impedance and improving the battery rate performance. Comparing example 3 with example 2, it is shown that the use of a large amount of additive A has no obvious effect on further improving the impedance of the battery, and greatly increases the cost of the electrolyte, so that the additive A needs to be used in a proper amount.
3) The results of comparing examples 1-6 with comparative example 7 all show that the comparative additive 1-fluorosilane (A5) has significantly less effect on improving the cell DCR and the cycle performance than the additives A1-A4. The reason for this is that the Si — H bond in 1-fluorosilane has extremely strong reducibility, making the substance less stable in the electrolyte and susceptible to unwanted chemical reactions with the respective electrolyte components. In contrast, the Si-CH3 bond in the structure of the additive A1-A4 is relatively stable, which is beneficial for the additive to have electrochemical reaction with specific components under specific potential and exert excellent film-forming efficacy.
4) Comparing examples 7 to 10 with comparative example 8, the results show that additive B has a good synergistic effect with other additives. The additive B is added into the electrolyte, so that a polymer SEI film with excellent mechanical properties can be formed together with other additives, and the battery can be effectively protected in the circulating process. The combination of the additive B can effectively improve the normal-temperature cycle performance of the battery. Comparing example 9 with example 8, it was shown that excessive use of additive B resulted in excessive cell resistance (DCR), and therefore additive B was used in a proper amount.
5) Comparing examples 11 to 13 with comparative example 9, the results show that the organic solvent FB has a good synergistic effect with other additives. The FB can reduce the viscosity of the electrolyte and the surface tension of the solution, thereby obviously improving the wettability of the electrolyte and reducing the direct current impedance. Comparing example 13 with example 12, it is shown that the use of a large amount of FB has no significant effect on further reducing the impedance of the battery and greatly increases the cost of the electrolyte, so that FB needs to be used in a proper amount.
6) Comparing examples 14-16 with comparative example 10, and examples 17-19 with comparative example 11, the results show that the additives lithium tetrafluoroborate and lithium difluorooxalate phosphate have good synergistic effects with other additives. Additives such as lithium tetrafluoroborate, lithium difluorooxalate phosphate and the like can effectively inhibit side reactions between the charged anode and cathode and the electrolyte, thereby prolonging the cycle life of the battery. In addition, additives such as lithium tetrafluoroborate and lithium difluorooxalate phosphate can reduce the increase of battery interfacial Resistance (RCT) in the circulating process and control the increase of circulating DCR. However, since cyclic ethylene carbonate, lithium tetrafluoroborate and lithium difluorooxalato phosphate all participate in film formation at the positive and negative electrodes, excessive use of the cyclic ethylene carbonate, lithium tetrafluoroborate and lithium difluorooxalato phosphate all causes excessive initial DCR of the cell (comparative example 15/16, example 18/19).
The example with the synergistic effect of the four additives has a clear advantage in three properties of cyclic performance, dc resistance and cyclic DCR increase compared to the comparative example.
In conclusion, the performance of the lithium ion battery using the electrolyte is obviously improved, and the lithium ion battery can be stored and used more safely at high multiplying power and low temperature.
The technical features of the embodiments described above may be arbitrarily combined, and for the sake of brevity, all possible combinations of the technical features in the embodiments described above are not described, but should be considered as being within the scope of the present specification as long as there is no contradiction between the combinations of the technical features.
The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.

Claims (10)

1. An electrolyte comprising an organic solvent, a lithium salt dissolved in the organic solvent, and an additive, wherein the additive comprises a fluorosilane, cyclic ethylene carbonate, and a lithium additive, wherein the lithium additive is selected from at least one of lithium tetrafluoroborate and lithium difluorooxalate phosphate.
2. The electrolyte of claim 1, wherein the fluorosilane is present in the electrolyte in an amount of 0.1 to 4% by mass, the cyclic ethylene carbonate is present in the electrolyte in an amount of 0.1 to 4% by mass, and the lithium additive is present in the electrolyte in an amount of 0.1 to 4% by mass.
3. The electrolyte of claim 1, wherein the fluorosilane is selected from one or more of the following compounds of formula I:
Figure FDA0002603795510000011
wherein R is1、R2、R3、R4Each independently selected from halogen, C1～10Alkyl, substituted C1～10Alkyl radical, C1～10Alkoxy, substituted C1～10Alkoxy radical, C2～10Alkenyl, substituted C2～10Alkenyl radical, C2～10Alkynyl, substituted C2～10One of an alkynyl group, a silicon-containing group and a substituted silicon-containing group, and R1、R2、R3、R4At least one of which is a fluorine atom.
4. The electrolyte of claim 1, wherein the fluorosilane is selected from one or a combination of several of the following compounds:
Figure FDA0002603795510000012
5. the electrolyte of claim 1, wherein the cyclic ethylene carbonate is selected from the group consisting of compounds and structural formulas represented by the following structural formula II
Figure FDA0002603795510000022
At least one of the compounds shown:
Figure FDA0002603795510000021
wherein R is1And R2Each independently selected from C1～3Alkyl, substituted C1～3Alkyl radical, C2～3Alkylene, substituted C2～3Alkylene and halogen.
6. The electrolyte of claim 1, wherein the cyclic ethylene carbonate is selected from one or more of vinylene carbonate, ethylene carbonate and fluoroethylene carbonate.
7. The electrolyte of claim 1, wherein the lithium additive is a mixture of lithium tetrafluoroborate and lithium difluorooxalate phosphate.
8. The electrolyte of claim 1, wherein the organic solvent comprises fluorobenzene and propylene carbonate.
9. The electrolyte of claim 8, wherein the organic solvent further comprises at least one of a carbonate based compound.
10. An electrochemical device comprising a positive electrode, a negative electrode, a separator and the electrolyte solution according to any one of claims 1 to 9.
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CN106450438A (en) * 2016-10-17 2017-02-22 广州天赐高新材料股份有限公司 Lithium-ion battery electrolyte and lithium ion battery with the same
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