Pyridyl Additive, Nitrile Electrolyte Containing Pyridyl Additive and Lithium Metal Secondary Battery

Abstract

The present invention relates to a pyridyl additive, a nitrile-based electrolyte containing the pyridyl additive, and a lithium metal secondary battery, belonging to the technical field of lithium metal secondary batteries. To solve the problems that the existing nitrile-based electrolytes are prone to increasing the desolvation energy and forming a mass transfer-inert interfacial film, the present invention provides a pyridyl additive obtained by substituting the hydroxyl group on the pyridine molecule with at least one strong electron-withdrawing group, and the strong electron-withdrawing group includes one or more of -F, -Cl, -Br, -SO3 or -NO2. The present invention uses a pyridine molecule substituted with a strong electron-withdrawing group. Through the electron-withdrawing inductive effect, the positive and negative charge centers of the pyridine molecule are separated, increasing the molecular dipole moment, thereby promoting the adsorption of the additive molecule on the lithium metal surface, reducing the interfacial nitrile molecule concentration, enhancing the electrode / electrolyte interface stability, reducing the interfacial lithium ion desolvation energy, enhancing the interfacial lithium ion conversion kinetics, and promoting the uniform deposition of lithium.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium metal secondary batteries, and particularly relates to a pyridyl additive, a nitrile-based electrolyte containing the pyridyl additive, and a lithium metal secondary battery. Background Art

[0002] Lithium-ion batteries are currently chemical power sources with relatively high energy density and play a crucial role in the development of the future energy field. The electrolyte is one of the core substances in lithium-ion batteries. The electrolyte is in direct contact with the electrodes, and its stability is directly related to the performance, safety, and cycle life of the battery. Nitrile-based electrolytes have excellent ion transport ability, a wide electrochemical window, and high thermal stability, making them an ideal choice for lithium-ion battery electrolytes.

[0003] However, in nitrile-based electrolytes, due to the strong coordination binding ability between nitrile molecules and lithium ions, nitrile molecules enter the inner part of the electric double layer on the lithium negative electrode side along with lithium ions. This results in that the desolvation process of lithium ions at the interface requires the removal of coordinated nitrile molecules, thereby increasing the desolvation energy. At the same time, nitrile molecules at the interface may also undergo a reduction polymerization reaction on the surface of the lithium negative electrode to form a mass transfer-inert interfacial film, thus hindering lithium ion transport. Summary of the Invention

[0004] To solve the problems that existing nitrile-based electrolytes are prone to increasing desolvation energy and forming a mass transfer-inert interfacial film, the present invention provides a pyridyl additive, a nitrile-based electrolyte containing the pyridyl additive, and a lithium metal secondary battery.

[0005] Technical solution of the present invention:

[0006] The pyridyl additive is obtained by substituting the hydroxyl group on the pyridine molecule with at least one strong electron-withdrawing group, and the strong electron-withdrawing group includes one or more of -F, -Cl, -Br, -SO3, or -NO2.

[0007] Further, it includes 2-fluoropyridine, 2,4-difluoropyridine, or 2-fluoro-5-nitropyridine.

[0008] The nitrile-based electrolyte containing the pyridyl additive provided by the present invention includes a nitrile compound, a lithium salt, and the pyridyl additive provided by the present invention; the addition amount of the pyridyl additive is 1 vt% to 20 vt%.

[0009] Further, the molar ratio of the nitrile compound to the lithium salt is 10:1, and the concentration of the lithium salt in the nitrile-based electrolyte is 0.5 mol·L -1 ~3 mol·L -1 .

[0010] Further, the nitrile compound includes one or more of acetonitrile, butyronitrile, valeronitrile, undecanenitrile, dodecanenitrile, succinonitrile, glutarodinitrile or adiponitrile; the lithium salt includes one or more of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(chlorosulfonyl)imide, lithium hexafluorophosphate or lithium tetrafluoroborate.

[0011] Further, a film-forming additive is further included, and the addition amount of the film-forming additive is 5 vt%.

[0012] Further, the film-forming additive is vinylene carbonate.

[0013] The lithium metal secondary battery containing the pyridyl additive provided by the present invention includes a positive electrode material, a negative electrode material, a separator and a nitrile electrolyte containing the pyridyl additive provided by the present invention.

[0014] Further, the nitrile electrolyte is the nitrile electrolyte containing the pyridyl additive provided by the present invention.

[0015] Further, the positive electrode material includes one or more of lithium iron phosphate, lithium cobaltate, nickel cobalt manganese ternary, nickel cobalt aluminum ternary or lithium vanadium phosphate; the negative electrode material includes metallic lithium; the separator includes one or more of an Al2O3-coated separator, a glass fiber separator, a PVDF separator, a PET / Al2O3 separator, a cellulose separator or an aramid separator.

[0016] Advantages of the present invention:

[0017] The pyridyl additive provided by the present invention uses a pyridine molecule substituted with a strong electron-withdrawing group. Through the electron-withdrawing inductive effect, the positive and negative charge centers of the pyridine molecule are separated, increasing the molecular dipole moment. Since the nitrile compound has a zero dipole moment, the charge-dipole interaction between the additive and the lithium negative electrode can promote the adsorption of the additive molecules on the lithium metal surface, reduce the interfacial nitrile molecule concentration, and improve the electrode / electrolyte interface stability. Since the electron-withdrawing group can reduce the pyridine-N charge density and the binding energy between the additive and lithium ions, the interfacial lithium ion desolvation energy is reduced, the interfacial lithium ion conversion kinetics is improved, and the uniform deposition of lithium is promoted. At the same time, the pyridyl additive substituted with multiple strong electron-withdrawing groups can also effectively improve the oxidation stability of the additive and the electrochemical window of the electrolyte. Description of the drawings

[0018] Figure 1 In-situ Raman spectrum of pyridine absorption peak during the discharge process of the lithium-copper battery assembled in Example 2;

[0019] Figure 2 In-situ Raman spectrum of nitrile absorption peak during the discharge process of the lithium-copper battery assembled in Example 2;

[0020] Figure 3The comparative diagram of the average Coulombic efficiency curves of the lithium-copper batteries assembled in Comparative Example 1 and Example 2, where a is Comparative Example 1 and b is Example 2;

[0021] Figure 4 The comparative diagram of the lithium deposition morphology on the copper surface after cycling of the lithium-copper batteries assembled in Comparative Example 1 and Example 2, where SN is Comparative Example 1 and SN-FPy is Example 2;

[0022] Figure 5 The cyclic stability curve diagram of the limited-lithium LiCoO2 batteries assembled in Comparative Example 2 and Example 3, where SNs is Comparative Example 2 and SN-FPy is Example 3;

[0023] Figure 6 The average Coulombic efficiency curve diagram of the lithium-copper battery assembled in Example 5;

[0024] Figure 7 The cyclic stability curve diagram of the LiCoO2 half-cells assembled in Comparative Example 3 and Example 6, where SNs is Comparative Example 2 and SN-DFPy is Example 6;

[0025] Figure 8 The comparative diagram of the positive electrode particle morphology after cycling of the LiCoO2 half-cells assembled in Comparative Example 3 and Example 6, where SNs is Comparative Example 2 and SN-DFPy is Example 6;

[0026] Figure 9 The average Coulombic efficiency curve diagram of the lithium-copper battery assembled in Example 8;

[0027] Figure 10 The LSV curves of the electrolytes prepared in Example 1, Example 7 and Comparative Example 1, where SNs is Comparative Example 1, SN-FPy is Example 1 and SN-FXPy is Example 7;

[0028] Figure 11 The cyclic stability curve diagram of the LiCoO2 half-cells assembled in Comparative Example 3 and Example 9, where SNs is Comparative Example 2 and SN-FXPy is Example 9. Detailed implementation manners

[0029] The technical solutions of the present invention will be further described below in conjunction with examples, but are not limited thereto. Any modification or equivalent replacement of the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention shall be covered within the protection scope of the present invention. For the process equipment or devices not specifically noted in the following examples, conventional equipment or devices in the art are used. Unless otherwise specified, the raw materials used in the embodiments of the present invention can be obtained commercially; unless otherwise specified, the technical means used in the embodiments of the present invention are conventional means well-known to those skilled in the art.

[0030] Example 1

[0031] This embodiment provides a pyridyl additive and a nitrile-based electrolyte.

[0032] This embodiment is obtained by substituting the hydroxyl group on the pyridine molecule with a strong electron-withdrawing group: -F, specifically 2-fluoropyridine.

[0033] This embodiment provides a nitrile-based electrolyte containing 2-fluoropyridine. The nitrile-based electrolyte contains a nitrile compound succinonitrile, a lithium salt lithium bis(trifluoromethanesulfonyl)imide, 2-fluoropyridine, and a film-forming additive vinylene carbonate; wherein the volume percentage content of 2-fluoropyridine in the nitrile-based electrolyte is 5 vt%, the molar ratio of succinonitrile to lithium bis(trifluoromethanesulfonyl)imide is 10:1, and the concentration of lithium bis(trifluoromethanesulfonyl)imide in the nitrile-based electrolyte is 1.23 mol·L -1 , and the volume percentage content of vinylene carbonate in the nitrile-based electrolyte is 5 vt%.

[0034] The nitrile-based electrolyte containing 2-fluoropyridine prepared in this embodiment is denoted as SN-FPy.

[0035] Example 2

[0036] This embodiment provides a lithium-copper battery containing 2-fluoropyridine.

[0037] The lithium-copper battery in this embodiment is assembled from a positive copper foil, a negative electrode material lithium sheet, a separator glass fiber separator, and the nitrile-based electrolyte containing 2-fluoropyridine prepared in Example 1.

[0038] Example 3

[0039] This embodiment provides a limited-lithium LiCoO2 battery containing 2-fluoropyridine.

[0040] The limited-lithium LiCoO2 battery in this embodiment is assembled from a positive electrode material LiCoO2, a negative electrode copper foil containing thin lithium (NP ratio is 2.5), a PP separator with a ceramic coating, and the nitrile-based electrolyte containing 2-fluoropyridine prepared in Example 1.

[0041] Example 4

[0042] This embodiment provides a pyridyl additive and a nitrile-based electrolyte.

[0043] This embodiment is obtained by substituting the hydroxyl group on the pyridine molecule with two strong electron-withdrawing groups: -F, specifically 2,4-difluoropyridine.

[0044] This embodiment provides a nitrile-based electrolyte containing 2,4-difluoropyridine. The nitrile-based electrolyte contains the nitrile compound succinonitrile, the lithium salt lithium bis(trifluoromethanesulfonyl)imide, 2,4-difluoropyridine, and the film-forming additive vinylene carbonate; wherein the volume percentage of 2,4-difluoropyridine in the nitrile-based electrolyte is 5 vt%, the molar ratio of succinonitrile to lithium bis(trifluoromethanesulfonyl)imide is 10:1, and the concentration of lithium bis(trifluoromethanesulfonyl)imide in the nitrile-based electrolyte is 1.23 mol·L -1 , and the volume percentage of vinylene carbonate in the nitrile-based electrolyte is 5 vt%.

[0045] The nitrile-based electrolyte containing 2,4-difluoropyridine prepared in this embodiment is denoted as SN-DFPy.

[0046] Example 5

[0047] This embodiment provides a lithium-copper battery containing 2,4-difluoropyridine.

[0048] The lithium-copper battery of this embodiment is assembled from a positive copper foil, a negative electrode material lithium sheet, a separator (PP separator with a ceramic coating), and the nitrile-based electrolyte containing 2,4-difluoropyridine prepared in Example 2.

[0049] Example 6

[0050] This embodiment provides a LiCoO₂ half-cell containing 2,4-difluoropyridine.

[0051] The LiCoO₂ half-cell of this embodiment is assembled from a positive electrode material LiCoO₂, a negative electrode material lithium sheet (200 μm), a PP separator with a ceramic coating, and the nitrile-based electrolyte containing 2,4-difluoropyridine prepared in Example 1.

[0052] Example 7

[0053] This embodiment provides a pyridyl additive and a nitrile-based electrolyte.

[0054] This embodiment is obtained by substituting the hydroxyl group on the pyridine molecule with two strong electron-withdrawing groups: -F and -NO₂, specifically 2-fluoro-5-nitropyridine.

[0055] This embodiment provides a nitrile-based electrolyte containing 2-fluoro-5-nitropyridine. The nitrile-based electrolyte contains the nitrile compound succinonitrile, the lithium salt lithium bis(trifluoromethanesulfonyl)imide, 2-fluoro-5-nitropyridine, and the film-forming additive vinylene carbonate; wherein the volume percentage of 2-fluoro-5-nitropyridine in the nitrile-based electrolyte is 5 vt%, the molar ratio of succinonitrile to lithium bis(trifluoromethanesulfonyl)imide is 10:1, and the concentration of lithium bis(trifluoromethanesulfonyl)imide in the nitrile-based electrolyte is 1.23 mol·L -1, the volume percentage content of vinylene carbonate in the nitrile-based electrolyte is 5 vt%.

[0056] The nitrile-based electrolyte containing 2-fluoro-5-nitropyridine prepared in this example is denoted as SN-FXPy.

[0057] Example 8

[0058] This example provides a lithium-copper battery containing 2-fluoro-5-nitropyridine.

[0059] The lithium-copper battery of this example is assembled from a positive copper foil, a negative electrode material lithium sheet, a PP separator with a ceramic coating, and the nitrile-based electrolyte containing 2-fluoro-5-nitropyridine prepared in Example 3.

[0060] Example 9

[0061] This example provides a LiCoO2 half-cell containing 2-fluoro-5-nitropyridine.

[0062] The LiCoO2 half-cell of this example is assembled from a positive electrode material LiCoO2, a negative electrode material lithium sheet, a PP separator with a ceramic coating, and the nitrile-based electrolyte containing 2-fluoro-5-nitropyridine prepared in Example 1.

[0063] Comparative Example 1

[0064] This comparative example provides a nitrile-based electrolyte and a lithium-copper battery without pyridyl additives.

[0065] The nitrile-based electrolyte in this comparative example contains a nitrile compound succinonitrile, a lithium salt lithium bis(trifluoromethanesulfonyl)imide, and a film-forming additive vinylene carbonate; the molar ratio of succinonitrile to lithium bis(trifluoromethanesulfonyl)imide is 10:1, and the concentration of lithium bis(trifluoromethanesulfonyl)imide in the nitrile-based electrolyte is 1.23 mol·L -1 , the volume percentage content of vinylene carbonate in the nitrile-based electrolyte is 5 vt%.

[0066] The nitrile-based electrolyte prepared in this comparative example is denoted as SNs.

[0067] The lithium-copper battery of this comparative example is assembled from a positive copper foil, a negative electrode material lithium sheet, a PP separator with a ceramic coating, and a nitrile-based electrolyte without pyridyl additives.

[0068] Comparative Example 2

[0069] This comparative example provides a nitrile-based electrolyte and a limited-lithium LiCoO2 battery without pyridyl additives.

[0070] In this comparative example, the nitrile-based electrolyte contains the nitrile compound succinonitrile, the lithium salt lithium bis(trifluoromethanesulfonyl)imide, and the film-forming additive vinylene carbonate; the molar ratio of succinonitrile to lithium bis(trifluoromethanesulfonyl)imide is 10:1, and the concentration of lithium bis(trifluoromethanesulfonyl)imide in the nitrile-based electrolyte is 1.23 mol·L -1 , and the volume percentage of vinylene carbonate in the nitrile-based electrolyte is 5 vt%.

[0071] The nitrile-based electrolyte prepared in this comparative example is denoted as SN or SNs.

[0072] The limited lithium LiCoO2 battery of this comparative example is assembled from the positive electrode material LiCoO2, the negative electrode being a copper foil containing thin lithium (NP ratio is 2.5), a PP separator containing a ceramic coating, and a nitrile-based electrolyte without a pyridyl additive.

[0073] Comparative Example 3

[0074] This comparative example provides a nitrile-based electrolyte without a pyridyl additive and a LiCoO2 half-cell.

[0075] In this comparative example, the nitrile-based electrolyte contains the nitrile compound succinonitrile, the lithium salt lithium bis(trifluoromethanesulfonyl)imide, and the film-forming additive vinylene carbonate; the molar ratio of succinonitrile to lithium bis(trifluoromethanesulfonyl)imide is 10:1, and the concentration of lithium bis(trifluoromethanesulfonyl)imide in the nitrile-based electrolyte is 1.23 mol·L -1 , and the volume percentage of vinylene carbonate in the nitrile-based electrolyte is 5 vt%.

[0076] The nitrile-based electrolyte prepared in this comparative example is denoted as SN or SNs.

[0077] The LiCoO2 half-cell of this comparative example is assembled from the positive electrode material LiCoO2, the negative electrode material lithium foil, a PP separator containing a ceramic coating, and a nitrile-based electrolyte without a pyridyl additive.

[0078] Figure 1 In-situ Raman spectrum of the pyridine absorption peak during the discharge process of the lithium-copper battery assembled in Example 2; Figure 2 In-situ Raman spectrum of the nitrile absorption peak during the discharge process of the lithium-copper battery assembled in Example 2; It can be seen from Figure 1 that after adding 2-fluoropyridine, during the discharge process, the absorption peak intensity of 2-fluoropyridine on the copper surface increases, indicating that 2-fluoropyridine is enriched on the electrode surface during the discharge process, which can effectively squeeze away the interfacial succinonitrile molecules. Therefore, Figure 2 the absorption peak intensity of the interfacial succinonitrile molecules shown gradually decreases.

[0079] Figure 3Figure for comparing the average Coulombic efficiency curves of the lithium-copper batteries assembled in Comparative Example 1 and Example 2. a is Comparative Example 1, and b is Example 2; from Figure 3 It can be seen that after adding the 2-fluoropyridine additive, due to the reduction of the number of succinonitrile molecules at the interface, the desolvation energy of lithium ions at the interface is reduced, the conversion and transport of lithium ions at the interface are accelerated, and the uniform deposition of lithium is promoted. Therefore, the average Coulombic efficiency of the lithium-copper battery after adding the additive is increased from 94.0% of the blank electrolyte to 99.1%.

[0080] Figure 4 Figure for comparing the lithium deposition morphologies on the copper surface after cycling of the lithium-copper batteries assembled in Comparative Example 1 and Example 2. SN is Comparative Example 1, from Figure 4 It can be seen that SN-FPy is Example 2; after adding the 2-fluoropyridine additive, the lithium deposition morphology is more uniform, so the cycling Coulombic efficiency of the lithium-copper battery is higher, while the uniformity of lithium deposition in the blank electrolyte is poor, there are a large number of dendrites, which are prone to generate dead lithium and lead to low Coulombic efficiency.

[0081] Figure 5 Figure for the cycling stability curves of the limited-lithium LiCoO2 batteries assembled in Comparative Example 2 and Example 3. SNs is Comparative Example 2, and SN-FPy is Example 3; from Figure 5 It can be seen that after adding the 2-fluoropyridine additive, the uniform deposition of lithium is promoted and the utilization efficiency of active lithium is improved. Therefore, the cycle life of the LiCoO2 battery in the limited-lithium state (NP ratio is 2.5) is increased from 120 times to more than 200 times.

[0082] Figure 6 Figure for the average Coulombic efficiency curve of the lithium-copper battery assembled in Example 5; from Figure 6 It can be seen that after adding the 2,4-difluoropyridine additive, the average Coulombic efficiency of the lithium-copper battery after the additive is increased from 94.0% of the blank electrolyte to 97.6%, effectively improving the utilization efficiency of active lithium.

[0083] Figure 7 Figure for the cycling stability curves of the LiCoO2 half-cells assembled in Comparative Example 3 and Example 6. SNs is Comparative Example 2, and SN-DFPy is Example 6; from Figure 7 It can be seen that after adding the 2,4-difluoropyridine additive, after the LiCoO2 half-cell is cycled 500 times at 1C, the capacity retention rate of the battery is increased from 16.7% of the blank electrolyte to 78.2%.

[0084] Figure 8 Figure for comparing the morphologies of the positive electrode particles after cycling stability of the LiCoO2 half-cells assembled in Comparative Example 3 and Example 6. SNs is Comparative Example 2, and SN-DFPy is Example 6; from Figure 8It can be seen that after adding 2,4-difluoropyridine additive, after 500 cycles at 1C of the LiCoO2 half-cell, the structure of the positive lithium cobaltate particles is complete and there is no obvious damage, while the particles are broken after cycling with the blank electrolyte, so its capacity retention rate is low.

[0085] Figure 9 Average Coulombic efficiency curve of the lithium-copper battery assembled for Example 8; from Figure 9 It can be seen that after adding 2-fluoro-5-nitropyridine additive, the average Coulombic efficiency of the lithium-copper battery is increased from 94.0% of the blank electrolyte to 95.9%, effectively improving the utilization efficiency of active lithium.

[0086] Figure 10 LSV curves of the electrolytes prepared for Example 1, Example 7 and Comparative Example 1, SNs is Comparative Example 1, SN-FPy is Example 1, SN-FXPy is Example 7; from Figure 10 It can be seen that after adding 2-fluoro-5-nitropyridine additive, the decomposition potential of the electrolyte increases, and the simultaneous substitution of the strong electron-withdrawing groups -F and -NO2 effectively improves the voltage window of the battery.

[0087] Figure 11 Cycle stability curves of the LiCoO2 half-cells assembled for Comparative Example 3 and Example 9, SNs is Comparative Example 2, SN-FXPy is Example 9, from Figure 11 It can be seen that after adding 2-fluoro-5-nitropyridine additive, after 500 cycles at 1C of the LiCoO2 half-cell, the battery capacity retention rate is increased from 16.7% of the blank electrolyte to 76.0%.

Claims

1. A nitrile-based electrolyte containing a pyridine-based additive, characterized in that: The invention comprises a nitrile compound, a lithium salt and a pyridine-based additive; the pyridine-based additive is 2-fluoropyridine, 2,4-difluoropyridine or 2-fluoro-5-nitropyridine, the addition amount of the pyridine-based additive is 1vt% to 20vt%, the molar ratio of the nitrile compound to the lithium salt is 10:1, and the concentration of the lithium salt in the nitrile-based electrolyte is 0.5mol·L -1 ~3mol·L -1 The nitrile compound includes one or more of acetonitrile, butyronitrile, n-valeronitrile, undecanenitrile, dodecanitrile, succinonitrile, glutaronitrile or adiponitrile; the lithium salt includes one or more of lithium bis(trifluoromethylsulfonyl imide), lithium bis(chlorosulfonyl imide), lithium hexafluorophosphate or lithium tetrafluoroborate.

2. The nitrile-based electrolyte containing a pyridine-based additive according to claim 1, characterized in that: A film-forming additive is also included, and the added amount of the film-forming additive is 5 vt%.

3. The nitrile-based electrolyte containing a pyridine-based additive according to claim 2, characterized in that: The film-forming additive is one or more of vinylene carbonate, fluoroethylene carbonate and lithium nitrate.

4. A lithium metal secondary battery containing a pyridine-based additive, characterized in that: The invention comprises a positive electrode material, a negative electrode material, a separator and a nitrile-based electrolyte containing a pyridine-based additive as claimed in claim 1.

5. The lithium metal secondary battery containing a pyridine-based additive according to claim 4, characterized in that: The positive electrode material includes one or more of lithium iron phosphate, lithium cobalt oxide, nickel cobalt manganese ternary, nickel cobalt aluminum ternary or lithium vanadium phosphate; the negative electrode material includes metallic lithium; the diaphragm includes one or more of Al2O3 coated diaphragm, glass fiber diaphragm, PVDF diaphragm, PET / Al2O3 diaphragm, cellulose diaphragm or aramid diaphragm.

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