CN113013489B - Electrolyte and lithium ion battery comprising same - Google Patents
Electrolyte and lithium ion battery comprising same Download PDFInfo
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- CN113013489B CN113013489B CN202110212763.2A CN202110212763A CN113013489B CN 113013489 B CN113013489 B CN 113013489B CN 202110212763 A CN202110212763 A CN 202110212763A CN 113013489 B CN113013489 B CN 113013489B
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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
- H01M10/052—Li-accumulators
- H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
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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
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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
- H01M10/42—Methods or arrangements for servicing or maintenance of secondary cells or secondary half-cells
- H01M10/4235—Safety or regulating additives or arrangements in electrodes, separators or electrolyte
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- 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
Abstract
The invention provides an electrolyte and a lithium ion battery comprising the same. The electrolyte adopted by the invention comprises a non-aqueous organic solvent, electrolyte lithium salt and an electrolyte functional additive, wherein the electrolyte functional additive contains a polynitrile compound and a bis-trimethylsilyl nitrogenous heterocyclic compound. The inventors of the present application have creatively found that the Si-N bond in the bistrimethylsilyl nitrogen-containing heterocyclic compound has a higher electronegativity than the C-N bond in the polynitrile compound, and is capable of preferentially binding the proton H generated by the oxidative decomposition of the electrolyte component + Thereby avoiding protons H + Compared with a single polynitrile compound, the coordination of the polynitrile compound and the polynitrile compound can better stabilize the interface property of the positive electrode terminal electrode/electrolyte, inhibit the dissolution of transition metal ions and the further oxidative decomposition of electrolyte components, so that the lithium ion battery using the electrolyte system has excellent long-cycle stability.
Description
Technical Field
The invention belongs to the technical field of lithium ion battery materials, and particularly relates to an electrolyte and a lithium ion battery comprising the same.
Background
In recent years, with the rapid development of portable intelligent electronic products, lithium ion batteries playing an important role therein have become the focus of attention; the development of lithium ion batteries is directly related to the development of intelligent electronic products to a certain extent: among them, the high endurance of the intelligent electronic product is the focus of the attention of consumers, and this is not closely related to the energy density of the lithium ion battery.
Increasing the energy density of lithium ion batteries has been an important topic in the industry. One of the important means for increasing the energy density is to increase the charge cut-off voltage of the battery, but the problem caused by increasing the charge cut-off voltage of the battery is very serious: on one hand, electrolyte components are oxidized and decomposed under high voltage, on the other hand, the positive electrode is excessively delithiated after the charge cut-off voltage is increased, so that the structural instability of an active material is caused, the dissolution of transition metal ions is caused, the latter catalyzes the decomposition reaction of the electrolyte components, the interface property of an electrode/electrolyte is deteriorated, and finally, the rapid attenuation of the performance of the battery is caused.
The polynitrile compound as electrolyte additive can stabilize the interface between electrode and electrolyte via complexing positive transition metal ion and improve the electrochemical performance of lithium ion battery effectively. However, after the battery is cycled for a long time, the complexing ability of the polynitrile compound to the transition metal ions can be obviously deteriorated, and the battery is in a state of lacking force in the later period. In addition, the stability of the positive terminal interface is insufficient, so that the battery is in a weak state during long-term circulation, and the performance in the later period cannot be guaranteed.
Disclosure of Invention
In order to overcome the defects in the prior art, the invention provides an electrolyte and a lithium ion battery comprising the electrolyte, and the invention further introduces a bis-trimethylsilyl nitrogenous heterocyclic compound as an electrolyte functional additive on the basis of the electrolyte containing a polynitrile compound, wherein a Si-N bond in the bis-trimethylsilyl nitrogenous heterocyclic compound has higher electronegativity than a C-N bond in the polynitrile compound, and can preferentially combine protons H generated by the oxidative decomposition of electrolyte components + Thereby avoiding protons H + Compared with a single polynitrile compound, the coordination of the polynitrile compound and the polynitrile compound can better stabilize the interface property of a positive electrode terminal electrode/electrolyte, inhibit the dissolution of transition metal ions and the further oxidative decomposition of electrolyte components, so that a lithium ion battery using the electrolyte system has excellent long-cycle stability.
The purpose of the invention is realized by the following technical scheme:
an electrolyte comprising a non-aqueous organic solvent, an electrolytic lithium salt, and an electrolyte functional additive; the electrolyte functional additive comprises a first additive and a second additive;
wherein the first additive is selected from at least one of bis-trimethylsilyl nitrogen-containing heterocyclic compounds, and the second additive is selected from at least one of polynitrile compounds.
According to the invention, the first additive is at least one selected from bis-trimethylsilyl nitrogen-containing heterocyclic compounds with the structure shown in formula I,
in the formula I, R is selected from carbonyl and substituted or unsubstituted C 1 -C 6 Alkylene, substituted or unsubstituted C 2 -C 6 Alkenylene, substituted or unsubstituted C 2 -C 6 An alkynylene group; the substituents being selected from halogen, cyano, C 1 -C 3 At least one of alkyl groups.
According to the invention, R is selected from carbonyl, substituted or unsubstituted C 1 -C 3 Alkylene groups (e.g., methylene, ethylene, n-propylene, isopropylene, etc.).
According to the invention, the first additive is selected from at least one of A-1 to A-3:
according to the invention, the first additive is added in an amount of 0.1 to 2.0 wt.%, for example 0.1 wt.%, 0.2 wt.%, 0.3 wt.%, 0.4 wt.%, 0.5 wt.%, 0.6 wt.%, 0.7 wt.%, 0.8 wt.%, 0.9 wt.%, 1.0 wt.%, 1.1 wt.%, 1.2 wt.%, 1.3 wt.%, 1.4 wt.%, 1.5 wt.%, 1.6 wt.%, 1.7 wt.%, 1.8 wt.%, 1.9 wt.%, 2 wt.%, based on the total mass of the electrolyte.
According to the invention, the polynitrile compound is at least one of dinitrile compounds shown in formula II-1, trinitrile compounds shown in formula II-2 and tetranitrile compounds shown in formula II-3,
wherein R is 21 Is a group having 1 to 10 carbon atoms and having at least 2 substitution positions; r is 22 Is a group having 1 to 10 carbon atoms and having at least 3 substitution positions; r is 23 Is a carbon atom having at least 4 substitution positionsRadicals with a submultiple of 1-10.
According to the invention, the group with 1 to 10 carbon atoms is selected from substituted or unsubstituted C 1 -C 10 Alkyl, substituted or unsubstituted C 5 -C 10 Heteroaryl, substituted or unsubstituted C 6 -C 10 Aryl and the substituent is halogen.
According to the invention, the dinitrile compound shown in the formula II-1 is selected from at least one of the following compounds: succinonitrile, glutaronitrile, adiponitrile, sebaconitrile, nonanedionitrile, dicyanobenzene, terephthalonitrile, pyridine-3, 4-dinitrile, 2, 5-dicyanopyridine, 3' - [1, 2-ethanediylbis (oxy) ] biscapronitrile, fumaronitrile, ethylene glycol biscapronitrile ether.
According to the invention, the trinitrile compound shown in the formula II-2 is selected from at least one of the following compounds: 1,3, 6-hexanetricarbonitrile, 1,3, 5-cyclohexanetricarbonitrile, 1,3, 5-benzenetricyanide, 1,2, 3-propanetricyanide, glycerol trinitrile.
According to the invention, the tetracyanonitrile compound shown in the formula II-3 is selected from at least one of the following compounds: 1, 3-propanetetracyanonitrile, 1,2, 3-tetracyanopropane, 1,2,4, 5-tetracyanobenzene, 2,3,5, 6-pyrazinetetranitrile, 7, 8-tetracyanoterephthalenediquinodimethane, tetracyanoethylene.
According to the invention, the second additive is added in an amount of 0.5 to 10.0wt%, for example 0.5wt%, 1.0wt%, 1.5wt%, 1.6wt%, 1.7wt%, 1.8wt%, 1.9wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, 4.5wt%, 5wt%, 5.5wt%, 6wt%, 6.5wt%, 7wt%, 7.5wt%, 8wt%, 8.5wt%, 9wt%, 9.5wt%, 10wt%, based on the total mass of the electrolyte.
According to the invention, the electrolyte functional additive further comprises a third additive selected from at least one of the following compounds: 1, 3-propane sultone, 1, 3-propene sultone, ethylene sulfite, ethylene sulfate, vinylene carbonate, fluoroethylene carbonate, lithium dioxalate borate, lithium difluorooxalate borate, lithium difluorodioxalate phosphate, lithium difluorophosphate, lithium bistrifluoromethanesulfonylimide, lithium difluorosulfonylimide.
According to the present invention, the electrolyte lithium salt is selected from at least one of lithium hexafluorophosphate and lithium perchlorate.
According to the invention, the electrolyte lithium salt is added in an amount of 13 to 20wt%, for example 13wt%, 14wt%, 15wt%, 16wt%, 17wt%, 18wt%, 19wt% or 20wt%, based on the total mass of the electrolyte.
According to the invention, the non-aqueous organic solvent is selected from one or more of the following compounds: ethylene carbonate, propylene carbonate, ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, ethyl acetate, propyl propionate, ethyl propionate, sulfolane, n-butyl sulfone.
The invention also provides a lithium ion battery, which comprises the electrolyte.
According to the invention, the charge cut-off voltage of the lithium ion battery is more than or equal to 4.45V.
According to the invention, the lithium ion battery also comprises a positive plate, a negative plate and a diaphragm.
According to the invention, the positive plate comprises a positive current collector and a mixed layer of a positive active material, a conductive agent and a binder coated on the positive current collector; the negative plate comprises a negative current collector and a mixed layer of a negative active material, a conductive agent and a binder coated on the negative current collector.
According to the invention, the positive active material is selected from one or more of layered lithium transition metal composite oxide, lithium manganate, lithium cobaltate and mixed ternary material; the chemical formula of the layered lithium transition metal composite oxide is Li 1+ x Ni y Co z M (1-y-z) Y 2 Wherein x is more than or equal to-0.1 and less than or equal to 1; y is more than or equal to 0 and less than or equal to 1, z is more than or equal to 0 and less than or equal to 1, and y + z is more than or equal to 0 and less than or equal to 1; wherein M is one or more of Mg, zn, ga, ba, al, fe, cr, sn, V, mn, sc, ti, nb, mo and Zr; y is one or more of O, F, P and S.
Preferably, the positive active material is lithium cobaltate LiCoO 2 。
According to the invention, the negative active material is selected from one or more of carbon-based materials, silicon-based materials, tin-based materials or alloy materials corresponding to the carbon-based materials, the silicon-based materials and the tin-based materials.
According to the invention, the membrane is, for example, a microporous membrane, which may be, for example, a polyethylene membrane, and may be, in particular, a polyethylene microporous membrane.
Terms and explanations:
the term "halogen" refers to F, cl, br and I. In other words, F, cl, br, and I may be described as "halogen" in the present specification.
The term "C 1 -C 6 Alkyl "is understood as preferably meaning a straight-chain or branched, saturated monovalent hydrocarbon radical having from 1 to 6 carbon atoms. ' C 1 -C 6 Alkyl "is understood to preferably mean a straight-chain or branched, saturated monovalent hydrocarbon radical having 1,2,3, 4,5 or 6 carbon atoms. The alkyl group is, for example, methyl, ethyl, propyl, butyl, pentyl, hexyl, isopropyl, isobutyl, sec-butyl, tert-butyl, isopentyl, 2-methylbutyl, 1-ethylpropyl, 1, 2-dimethylpropyl, neopentyl, 1-dimethylpropyl, 4-methylpentyl, 3-methylpentyl, 2-methylpentyl, 1-methylpentyl, 2-ethylbutyl, 1-ethylbutyl, 3-dimethylbutyl, 2-dimethylbutyl, 1-dimethylbutyl, 2, 3-dimethylbutyl, 1, 3-dimethylbutyl, or 1, 2-dimethylbutyl, etc., or isomers thereof. In particular, said groups are, for example, methyl, ethyl, propyl, butyl, isopropyl, isobutyl, sec-butyl, tert-butyl, more particularly said groups have 1,2 or 3 carbon atoms ("C) 1 -C 3 Alkyl groups) such as methyl, ethyl, n-propyl or isopropyl.
The term "C 2 -C 6 Alkenyl "is understood to preferably mean a straight-chain or branched, monovalent hydrocarbon radical which contains one or more double bonds and has 2,3, 4,5 or 6 carbon atoms, in particular 2 or 3 carbon atoms (" C) 2 -C 3 Alkenyl "), it being understood that in the case where the alkenyl group comprises more than one double bond, the double bonds may be separated from each other or conjugated. The alkenyl group is, for example, vinyl, allyl, (E) -2-methylvinyl, (Z) -2-methylvinyl, (E) -but-2-enyl, (Z) -but-2-eneA radical, (E) -but-1-enyl, (Z) -but-1-enyl, pent-4-enyl, (E) -pent-3-enyl, (Z) -pent-3-enyl, (E) -pent-2-enyl, (Z) -pent-2-enyl, (E) -pent-1-enyl, (Z) -pent-1-enyl, hex-5-enyl, (E) -hex-4-enyl, (Z) -hex-4-enyl, (E) -hex-3-enyl, (Z) -hex-3-enyl, (E) -hex-2-enyl, m (Z) -hex-2-enyl, (E) -hex-1-enyl, (Z) -hex-1-enyl, isopropenyl, 2-methylprop-2-enyl, 1-methylprop-2-enyl, 2-methylprop-1-enyl, (E) -1-methylprop-1-enyl, (Z) -1-methylprop-1-enyl, 3-methylbut-3-enyl, 2-methylbut-3-enyl, 1-methylbut-3-enyl, 3-methylbut-2-enyl, (E) -2-methylbut-2-enyl, (Z) -2-methylbut-2-enyl, (E) -1-methylbut-2-enyl, (Z) -1-methylbut-2-enyl, (E) -3-methylbut-1-enyl, (Z) -3-methylbut-1-enyl, (E) -2-methylbut-1-enyl, (Z) -2-methylbut-1-enyl, (E) -1-methylbut-1-enyl, (Z) -1-methylbut-1-enyl, 1-dimethylprop-2-enyl, 1-ethylprop-1-enyl, 1-propylvinyl, 1-isopropylvinyl.
The term "C 2 -C 6 Alkynyl "is understood to mean a straight-chain or branched, monovalent hydrocarbon radical which contains one or more triple bonds and has 2 to 6 carbon atoms, in particular 2 or 3 carbon atoms (" C) 2 -C 3 Alkynyl "). The alkynyl group is, for example, ethynyl, prop-1-ynyl, prop-2-ynyl, but-1-ynyl, but-2-ynyl, but-3-ynyl, pent-1-ynyl, pent-2-ynyl, pent-3-ynyl, pent-4-ynyl, hex-1-ynyl, hex-2-ynyl, hex-3-ynyl, hex-4-ynyl, hex-5-ynyl, 1-methylprop-2-ynyl, 2-methylbut-3-ynyl, 1-methylbut-2-ynyl, 3-methylbut-1-ynyl, 1-ethylprop-2-ynyl. In particular, the alkynyl group is ethynyl, prop-1-ynyl or prop-2-ynyl.
The term "a group having 1 to 10 carbon atoms in at least 2 substitution positions" is understood to mean a C containing a linear, branched and/or cyclic chain in 2 substitution positions 1 -C 10 The carbon atom group of (2) includes, but is not limited to, alkyl, alkenyl, alkynyl, aryl, and the like.
The term "a group having 1 to 10 carbon atoms at least at 3 substitution positions" is intended to meanIs solved to represent C of a linear, branched and/or cyclic chain containing 3 substitution positions 1 -C 10 The carbon atom group of (2) includes, but is not limited to, alkyl, alkenyl, alkynyl, aryl, and the like.
The term "a group having at least 4 substitution positions and having 1 to 10 carbon atoms" is understood to mean a C group containing a linear, branched and/or cyclic chain of 4 substitution positions 1 -C 10 The carbon atom group of (2) includes, but is not limited to, alkyl, alkenyl, alkynyl, aryl, and the like.
The term "C 6 -C 10 Aryl "is to be understood as preferably meaning a monocyclic, bicyclic or tricyclic hydrocarbon ring of monovalent or partial aromaticity having from 6 to 10 carbon atoms, preferably a monocyclic, bicyclic or tricyclic hydrocarbon ring of monovalent or partial aromaticity having 6, 7,8, 9 or 10 carbon atoms, in particular a ring having 6 carbon atoms (" C) 6 Aryl "), such as phenyl; or biphenyl, or is a ring having 9 carbon atoms ("C 9 Aryl), such as indanyl or indenyl, or a ring having 10 carbon atoms ("C) 10 Aryl), such as tetralinyl, dihydronaphthyl, or naphthyl.
The term "C 5 -C 10 Heteroaryl "is understood to include such monovalent monocyclic, bicyclic or tricyclic aromatic ring systems: having 5-10 ring atoms and containing 1-4 heteroatoms independently selected from N, O and S, e.g., "5-10 membered heteroaryl. Which have 5,6, 7,8, 9 or 10 ring atoms, in particular 5 or 6 or 9 or 10 carbon atoms, and which contain 1 to 5, preferably 1 to 3, heteroatoms each independently selected from N, O and S and, in addition, can be benzo-fused in each case. In particular, the heteroaryl group is selected from thienyl, furyl, pyrrolyl, oxazolyl, thiazolyl, imidazolyl, pyrazolyl, isoxazolyl, isothiazolyl, oxadiazolyl, triazolyl, thiadiazolyl, thia-4H-pyrazolyl and the like and their benzo derivatives.
The "alkylene" in the present invention is a group obtained by substituting one H with the "alkyl".
The "alkenylene" is a group obtained by substituting one H with the "alkenyl".
The "alkynylene" is a group obtained by substituting one H by the "alkynyl".
The invention has the beneficial effects that:
the invention provides an electrolyte and a lithium ion battery comprising the same. The electrolyte adopted by the invention comprises a non-aqueous organic solvent, electrolyte lithium salt and an electrolyte functional additive; the electrolyte functional additive comprises at least one of bis-trimethylsilyl nitrogen-containing heterocyclic compounds and at least one of polynitrile compounds. The inventors of the present application have surprisingly found that lithium ion batteries produce a large amount of protons H in the electrolyte after long cycling + The proton H + The electronegativity of the polynitrile compound is reduced, and the capability of complexing transition metal ions and the capability of stabilizing a positive interface are weakened; on the basis of an electrolyte containing a polynitrile compound, the inventor creatively introduces a bis-trimethylsilyl nitrogenous heterocyclic compound as an electrolyte functional additive, wherein Si-N bonds in the bis-trimethylsilyl nitrogenous heterocyclic compound have higher electronegativity than C-N bonds in the polynitrile compound and can preferentially combine protons H generated by oxidative decomposition of electrolyte components + Thereby avoiding protons H + Compared with a single polynitrile compound, the coordination of the polynitrile compound and the polynitrile compound can better stabilize the interface property of the positive electrode terminal electrode/electrolyte, inhibit the dissolution of transition metal ions and the further oxidative decomposition of electrolyte components, so that the lithium ion battery using the electrolyte system has excellent long-cycle stability.
Detailed Description
The present invention will be described in further detail with reference to specific examples. It is to be understood that the following examples are only illustrative and explanatory of the present invention and should not be construed as limiting the scope of the present invention. The technical solution of the present invention should be covered by the protection scope of the present invention, in which modifications or equivalent substitutions are made without departing from the spirit scope of the technical solution of the present invention.
In the following examples, lithium cobaltate was used as the positive electrode active material and graphite was used as the negative electrode active material, and the preparation of the lithium ion battery included the following steps:
(1) Preparation of positive plate
Mixing a positive electrode active material Lithium Cobaltate (LCO), a binder polyvinylidene fluoride (PVDF) and a conductive agent acetylene black according to a weight ratio of 97.5; uniformly coating the positive electrode slurry on an aluminum foil with the thickness of 10 mu m; baking the coated aluminum foil in 5 sections of baking ovens with different temperature gradients, drying the aluminum foil in a baking oven at 120 ℃ for 8 hours, and rolling and cutting to obtain the required positive plate.
(2) Preparation of negative plate
Mixing a negative electrode active material graphite, a thickening agent sodium carboxymethyl cellulose (CMC-Na), a binder styrene-butadiene rubber and a conductive agent acetylene black according to a weight ratio of 97 to 1; uniformly coating the negative electrode slurry on a high-strength carbon-coated copper foil with the thickness of 8 mu m to obtain a pole piece; and (3) airing the obtained pole piece at room temperature, transferring the pole piece to an oven at 80 ℃ for drying for 10 hours, and then carrying out cold pressing and slitting to obtain the negative pole piece.
(3) Preparation of electrolyte
In a glove box filled with inert gas (argon) (H) 2 O＜0.1ppm，O 2 Less than 0.1 ppm), ethylene carbonate, propylene carbonate, diethyl carbonate and propyl propionate are uniformly mixed according to the mass ratio of 15 percent to 10 percent to 65 percent, and then 1.25mol/L of fully dried lithium hexafluorophosphate (LiPF) is rapidly added into the mixture 6 ) Dissolving the electrolyte in a non-aqueous organic solvent, uniformly stirring, and obtaining the basic electrolyte after the water and free acid are detected to be qualified.
(4) Preparation of separator
The polyethylene microporous membrane with the thickness of 8 mu m is selected.
(5) Preparation of lithium ion battery
Stacking the prepared positive plate, the diaphragm and the prepared negative plate in sequence to ensure that the diaphragm is positioned between the positive plate and the negative plate to play a role in isolation, and then winding to obtain a naked battery cell without liquid injection; and placing the bare cell in an outer packaging foil, injecting the prepared corresponding electrolyte into the dried bare cell, and performing vacuum packaging, standing, formation, shaping, sorting and other processes to obtain the corresponding lithium ion battery.
The electrolyte composition information of comparative examples 1 to 9 and examples 1 to 11 is shown in table 1.
Testing the electrochemical performance of the lithium ion battery:
(1) Normal temperature cycle test at 25 ℃: the obtained battery is placed at the normal temperature of 25 ℃ to be subjected to charge and discharge tests at a rate of 0.7C/0.7C, the cut-off voltage range is 3.0V-4.45V, the charge and discharge cycles are performed for 1000 times, the cycle discharge capacity is recorded and divided by the discharge capacity of the first cycle, the normal-temperature cycle capacity retention rate is obtained, and the capacity retention rate results of the 500 th time, the 800 th time and the 1000 th time are recorded respectively as shown in Table 2.
(2) High temperature cycle test at 45 ℃: the obtained battery is placed at a high temperature of 45 ℃ to carry out charge and discharge tests at a rate of 0.7C/0.7C, the cut-off voltage range is 3.0V-4.45V, the charge and discharge cycles are carried out for 1000 times, the cycle discharge capacity is recorded and divided by the discharge capacity of the first cycle to obtain the high-temperature cycle capacity retention rate, and the capacity retention rate results of the 500 th time, the 800 th time and the 1000 th time are respectively recorded as shown in Table 2.
TABLE 1 electrolyte compositions for comparative examples 1-9 and examples 1-11
TABLE 2 comparison of the results of the cycle stability at normal temperature and high temperature for the batteries of comparative examples 1-9 and examples 1-11
Comparison of Battery Performance test results
Results of tests on long-cycle properties at normal and high temperatures of the batteries of comparative examples 1 to 9 and examples 1 to 11 as shown in table 2, it is emphasized that the design and the manufacturing process of the batteries of all the above comparative examples and examples are completely the same, except that the kinds and contents of the functional additives in the electrolyte are different, and it can be considered that the difference in the electrochemical performance of the batteries is only from the difference of the functional additives of the electrolyte, and has no other influence factors.
In comparative example 1, the cycling tendency experienced by the cell can be summarized as: after the 500 th cycle obtains a certain capacity retention rate, the capacity retention rate of the battery undergoes a certain speed of attenuation when the cycle is up to 800 times, but when the cycle is up to 800 times, the attenuation trend of the capacity retention rate of the battery at the stage is obviously more severe than that of the stage from 500 times to 800 times, the battery presents a 'diving' trend, only unsatisfactory capacity retention rates are obtained, and the cycle trends of normal-temperature and high-temperature cycles are similar. The inventors conclude that this is not to be separable from the degradation of the electrode/electrolyte interface by the continued decomposition of the electrolyte components late in the battery cycle. The results of comparative examples 2 to 4 show that the compound a-1, the compound a-2 and the compound a-3 do not significantly improve the long cycle at normal and high temperatures of the battery. The results of comparative examples 5 to 9 show that the polynitrile compound has obvious improvement effect on the normal-temperature and high-temperature cycles of the battery at the early stage, the capacity retention rate is obviously improved at the 500 th cycle and the 800 th cycle, but the battery cycle also experiences the phenomenon of water jump at the stage after the 800 th cycle, and the capacity retention rate is not much different from the reference group at the 1000 th cycle. The inventors speculate that the stability capability of the polynitrile compound towards the positive electrode/electrolyte interface suffers "clipping" later with battery cycling; on one hand, because the electrochemical stability of the polynitrile compound is not enough, certain consumption exists along with the circulation of the battery; on the other hand, it is because the ability of the cyano group on the polynitrile compound to complex the transition metal ion is "weakened". A further inference made by the inventors is that protons H formed during the oxidation reaction of the electrolyte components + Will surround the cyano group by the action of electric charge, and the cyano group in the polynitrile compound is combined with proton H + The electronegativity is reduced, the complexing ability to the transition metal ions is "weakened" therewith, and the stabilizing ability to the positive electrode/electrolyte interface is reduced, leading to further improvement of the electrolyte compositionStep (d) oxidative decomposition of protons H + The generation of poly-nitrile compounds is increased, and the complexing ability of the poly-nitrile compounds is further reduced, so that the battery performance jumps immediately under vicious cycles until the poly-nitrile compounds lose the ability to stabilize the interface.
The invention creatively discovers the defect of the polynitrile compound of the electrolyte additive and provides a method for improving the defect by introducing the bis-trimethylsilyl nitrogen-containing heterocyclic compound as the synergic electrolyte functional additive of the polynitrile compound, as shown in examples 1-9, the cooperation of the bis-trimethylsilyl nitrogen-containing heterocyclic compound and the polynitrile compound can ensure that the battery still has excellent capacity retention rate in long circulation and avoid the phenomenon of 'water jump' of the battery in later circulation. For this improvement mechanism, the inventors believe that this is closely related to the molecular properties of both: the polynitrile compound has strong electronegativity due to the high electron cloud density of nitrogen atoms on a cyano group with strong electron-withdrawing capability, and generates complexation with active high-valence transition metal ions exposed on the surface of a positive electrode due to charging and lithium removal, so that a positive electrode end electrode/electrolyte interface is stabilized, and metal ion dissolution and electrolyte component decomposition are inhibited; however, at a high charge cut-off voltage, the oxidation resistance of the electrolyte components is poor and an oxidative decomposition reaction occurs, the product of which contains proton H + ，H + The cyano group in the polynitrile compound is attacked, thereby weakening the complexation of the latter with transition metal ions. And the nitrogen atom on the Si-N bond in the bis-trimethylsilyl nitrogen-containing heterocyclic compound is more electronegative than the nitrogen atom on the cyano group in the polynitrile compound, and preferentially binds H in the electrolyte + Realization of H + The cleaning function of the composite material ensures the complexing ability of the polynitrile compound to transition metal ions, so that the positive electrode end electrode/electrolyte interface is more stable.
Examples 10 to 11 further introduced a third additive as a negative electrode film-forming additive, which further improved the cycle stability of the battery.
In conclusion, the lithium ion battery applying the scheme of the invention can realize excellent long-cycle stability and show extremely high application value. The above is a specific description of possible embodiments of the invention, but does not limit the scope of the invention.
Claims (7)
1. An electrolyte, wherein the electrolyte comprises a non-aqueous organic solvent, an electrolytic lithium salt, and an electrolyte functional additive; the electrolyte functional additive comprises a first additive and a second additive;
wherein the second additive is selected from at least one of polynitrile compounds;
the first additive is at least one of bis-trimethylsilyl nitrogen-containing heterocyclic compounds with a structure shown in formula I,
In the formula I, R is selected from C 1 -C 6 An alkylene group;
the adding amount of the first additive accounts for 0.1 to 2.0wt% of the total mass of the electrolyte;
the adding amount of the second additive accounts for 0.5 to 10.0wt% of the total mass of the electrolyte.
2. The electrolyte of claim 1, wherein the first additive is selected from a-1 and/or a-3:
3. the electrolyte solution according to claim 1, wherein the polynitrile compound is at least one selected from the group consisting of a dinitrile compound represented by formula II-1, a trinitrile compound represented by formula II-2, and a tetranitrile compound represented by formula II-3,
Wherein R is 21 Is a group having 1 to 10 carbon atoms and having at least 2 substitution positions; r 22 Is a group having 1 to 10 carbon atoms and having at least 3 substitution positions; r 23 Is a group having 1 to 10 carbon atoms and having at least 4 substitution positions.
4. The electrolyte of claim 3, wherein the dinitrile compound of formula II-1 is selected from at least one of the following compounds: succinonitrile, glutaronitrile, adiponitrile, sebaconitrile, nonanedionitrile, dicyanobenzene, terephthalonitrile, pyridine-3, 4-dinitrile, 2, 5-dicyanopyridine, 3' - [1, 2-ethanediylbis (oxy) ] biscapronitrile, fumaronitrile, ethylene glycol biscapronitrile ether; and/or the presence of a gas in the gas,
the trinitrile compound shown in the formula II-2 is selected from at least one of the following compounds: 1,3, 6-hexanetricarbonitrile, 1,3, 5-cyclohexanetricarbonitrile, 1,3, 5-benzenetricyanide, 1,2, 3-propanetricyanide, glycerol trinitrile; and/or the presence of a gas in the gas,
the tetracyanonitrile compound shown in the formula II-3 is selected from at least one of the following compounds: 1, 3-propanetetracyanonitrile, 1,2, 3-tetracyanopropane, 1,2,4, 5-tetracyanobenzene, 2,3,5, 6-pyrazinetetranitrile, 7, 8-tetracyanoterephthalenediquinodimethane, tetracyanoethylene.
5. The electrolyte of any of claims 1-4, wherein the electrolyte functional additive further comprises a third additive selected from at least one of the following compounds: 1, 3-propane sultone, 1, 3-propene sultone, ethylene sulfite, ethylene sulfate, vinylene carbonate, fluoroethylene carbonate, lithium dioxalate borate, lithium difluorooxalate borate, lithium difluorodioxalate phosphate, lithium difluorophosphate, lithium bistrifluoromethanesulfonylimide, lithium difluorosulfonylimide.
6. The electrolyte as claimed in any one of claims 1 to 4, wherein the electrolyte lithium salt is selected from at least one of lithium hexafluorophosphate and lithium perchlorate, and the electrolyte lithium salt is added in an amount of 13 to 20wt% based on the total mass of the electrolyte.
7. A lithium ion battery comprising the electrolyte of any of claims 1-6.
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