Lithium ion gel state electrolyte, lithium ion battery and preparation method therefor

Abstract

A lithium ion gel state electrolyte, a lithium ion battery and a preparation method therefor. The lithium ion gel state electrolyte comprises polymer monomers, an organic solvent, a lithium salt, an additive and an initiator, the polymer monomers at least comprising 1,3,5-triallyl isocyanurate and acrylonitrile, and the additive at least comprising FEC; the 1,3,5-triallyl isocyanurate accounts for 0.1-2% of the total mass of the lithium ion gel state electrolyte, and the acrylonitrile accounts for 2.5-7.5% of the total mass of the lithium ion gel state electrolyte. The synergistic effect of the various monomers and the additive of the gel electrolyte can achieve both the electrode / electrolyte interface stability and the high-temperature stability of lithium batteries, thus improving the use safety and the high-temperature storage capacity of batteries while optimizing the electrochemical stability of electrolyte / electrode interfaces.

Description

Lithium ion gel electrolyte, lithium ion battery and preparation method thereof Technical Field

[0001] The present invention belongs to the technical field of lithium batteries and relates to gel batteries, in particular to a lithium ion gel electrolyte, a lithium ion battery and a preparation method thereof. Background Art

[0002] Using high nickel ternary cathode to replace traditional lithium iron phosphate cathode is to increase the energy density of lithium battery to 300 Wh Kg -1 The above are effective approaches. However, the strong side reactions between high-voltage cathodes and traditional electrolytes can cause significant electrode / electrolyte interface impedance, affecting the practical use of the battery. To improve battery cycling stability, FEC is widely used as an additive in high-energy-density lithium batteries because it decomposes into a film on the electrode surface and inhibits interfacial side reactions.

[0003] On the other hand, the structural instability of the high-nickel ternary positive electrode determines that during the charge and discharge process, the transition metal elements (nickel, cobalt, and manganese) in its structure will dissolve in the electrolyte and be transferred from the positive electrode to the negative electrode surface during the charge and discharge process, seriously damaging the solid electrolyte intermediate layer. To address this problem, the in-situ polymer of polymer monomer + electrolyte + initiator is used to replace the traditional electrolyte, which can transform the electrolyte from a liquid state to a non-fluid gel state, thereby inhibiting the transfer of metal elements while maintaining the excellent interfacial wettability and ionic conductivity of the electrolyte. By using gel electrolyte and FEC additives, the cycle stability of high-nickel ternary gel batteries has been significantly improved compared to liquid batteries.

[0004] However, gel-state batteries require excellent high-temperature storage properties, driven by their intended use as power batteries and the high temperatures required for in-situ gel curing. However, FEC additives and traditional LiPF6 lithium salts, as components of gel electrolytes, are severely unstable at high temperatures. During high-temperature storage, they readily decompose to form hydrofluoric acid, which corrodes the cathode material and degrades battery performance.

[0005] In order to improve the high-temperature storage capacity of high-nickel ternary lithium batteries containing FEC additives, 1,3,5-triallyl isocyanurate is widely used as an electrolyte additive due to its low cost, low melting point and environmental friendliness. Its isocyanuric acid structure has the effect of removing water and controlling acid, and the nitrogen atom in the six-membered ring structure can react with the positive electrode transition metal to undergo a complexation reaction, thereby inhibiting the side reaction of the high-nickel ternary positive electrode with the electrolyte during the charge and discharge process. For example, Publication No. CN106450432A proposes a method of using 1,3,5-triallyl isocyanurate and FEC as additives at the same time, which effectively improves the high-temperature storage and cycle stability of the battery. Publication No. CN106654242A uses 1,3,5-triallyl isocyanurate, dinitrile compounds and FEC as electrolyte additives, which respectively play the role of protecting the positive electrode and the negative electrode, thereby improving the contact stability of the high-nickel ternary positive electrode and the silicon negative electrode with the electrolyte under high pressure and high temperature conditions.

[0006] However, due to the chemically unstable double bond in its structure, 1,3,5-triallyl isocyanurate is prone to polymerization on the negative electrode surface when used as an additive. Nitrile additives also have the disadvantage of unstable contact with the negative electrode, resulting in significant negative electrode interfacial impedance. Furthermore, the high viscosity of nitriles and isocyanuric acid can affect the lithium ion conduction of the electrolyte, affecting the ionic conductivity of the electrolyte. Summary of the Invention

[0007] To address the above issues, the present invention provides a lithium-ion gel electrolyte, a lithium-ion battery, and methods for preparing the same. The gel battery comprises a negative electrode, a gel electrolyte, and a high-nickel ternary positive electrode, and its preparation process is compatible with existing liquid batteries. Due to the presence of the gel electrolyte, the battery exhibits excellent electrochemical stability at the positive and negative electrode interfaces, as well as high-temperature storage performance.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] The present invention first provides a lithium ion gel electrolyte, comprising a polymer monomer, an organic solvent, a lithium salt, an additive and an initiator, wherein the polymer monomer comprises at least 1,3,5-triallyl isocyanurate and acrylonitrile, and the additive comprises at least FEC; the 1,3,5-triallyl isocyanurate accounts for 0.1-2% of the total mass of the lithium ion gel electrolyte, and the acrylonitrile accounts for 2.5-7.5% of the total mass of the lithium ion gel electrolyte.

[0010] As a preferred embodiment of the present invention, the polymer monomer further comprises one or more combinations of ethoxylated trimethylolpropane triacrylate, pentaerythritol tetraacrylate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, butyl acrylate, tert-butyl acrylate, isobutyl acrylate, 1,4-butylene glycol dimethacrylate, and vinylidene fluoride.

[0011] As a preferred embodiment of the present invention, the organic solvent includes a combination of two or more of ethyl propionate, propyl acetate, ethyl acetate, methyl acetate, dimethyl carbonate, diethyl carbonate, propyl propionate, ethylene carbonate, propylene carbonate, ethyl methyl carbonate, and dipropyl carbonate.

[0012] As a preferred embodiment of the present invention, the lithium salt includes one or more combinations of lithium hexafluorophosphate, lithium perchlorate, lithium difluorooxalatoborate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonyl imide), and lithium bis(fluorosulfonyl imide).

[0013] As a preferred embodiment of the present invention, FEC accounts for 5-20% of the total mass of the lithium ion gel electrolyte, and the additives further include one or more combinations of lithium difluorophosphate, lithium bis(oxalatoborate), vinyl sulfate, 1,3-propylene sultone, and N,N-dimethyltrifluoroacetamide.

[0014] As a preferred embodiment of the present invention, the initiator includes one or more combinations of azobisisobutyronitrile, azobisisoheptanenitrile, dibenzoyl peroxide, diisobutyl peroxydicarbonate, and dicyclohexyl peroxydicarbonate.

[0015] The present invention also provides a gel lithium ion battery comprising the above-mentioned lithium ion gel electrolyte, a negative electrode, a positive electrode and a separator.

[0016] As a preferred embodiment of the present invention, the positive electrode includes a positive electrode active material, a conductive agent, a binder and a current collector; wherein the positive electrode active material includes one or more combinations of lithium iron phosphate, lithium manganese iron phosphate, lithium nickelate, lithium manganate, lithium cobaltate, nickel-cobalt-manganese ternary material, and nickel-cobalt-aluminum ternary material.

[0017] As a preferred embodiment of the present invention, the negative electrode includes a negative electrode sheet containing lithium metal and a current collector; or a negative electrode sheet containing an active material, a conductive agent, a binder and a current collector; wherein the active material includes one or more combinations of graphite, hard carbon, soft carbon, silicon, silicon oxide, and silicon carbon.

[0018] Finally, the present invention provides a method for preparing the above-mentioned gel-state lithium-ion battery, comprising the following steps:

[0019] 1) Control the moisture content to ≤10ppm, mix the organic solvent evenly, and obtain an organic solvent component;

[0020] 2) Controlling the moisture content to ≤10 ppm, removing the polymerization inhibitor from the polymer monomers and mixing them uniformly to obtain polymer monomer components;

[0021] 3) controlling the moisture content to ≤10 ppm, mixing the polymer monomer component obtained in step 2), the organic solvent component obtained in step 1), additives, and lithium salt, adding an initiator, and stirring until the mixture is clear to obtain a precursor;

[0022] 4) Assemble the positive electrode shell, positive electrode sheet, separator, lithium sheet, nickel foam, and negative electrode sheet in order, inject the precursor obtained in step 3), and stamp and seal to obtain a gel lithium-ion battery.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] 1) The present invention achieves both electrode / electrolyte interface stability and high-temperature stability of lithium batteries through the synergistic effect of multiple monomers and additives in the gel electrolyte.

[0025] 2) Compared with conventional gel batteries, the gel lithium-ion battery of the present invention has low prices for its monomer components and does not require the addition of additional acid-controlling additives, thus having cost advantages.

[0026] 3) The gel-state lithium-ion battery of the present invention can improve the battery's safety and high-temperature storage capacity while optimizing the electrochemical stability of the electrolyte / electrode interface. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] FIG1 shows the activation discharge capacity and coulombic efficiency of Examples 1-4 and Comparative Examples 1-3.

[0028] Figure 2 is the EIS curve at room temperature. DETAILED DESCRIPTION

[0029] In order to make the technical means, creative features, purpose and efficacy of the present invention easy to understand, the present invention is further described below in conjunction with specific examples, but the following examples are only preferred embodiments of the present invention, not all. Based on the examples in the embodiments, other embodiments obtained by those skilled in the art without making creative work are within the scope of protection of the present invention. The experimental methods in the following examples, unless otherwise specified, are conventional methods, and the materials, reagents, etc. used in the following examples, unless otherwise specified, can be obtained from commercial channels.

[0030] The present invention provides a gel electrolyte and battery containing an FEC additive, a 1,3,5-triallyl isocyanurate crosslinker, and an acrylonitrile monomer. In the gel electrolyte, isocyanuric acid and acrylonitrile exist as polymer groups, significantly improving its electrochemical stability compared to the additives. 1,3,5-triallyl isocyanurate, as a crosslinker, primarily enhances the mechanical strength of the gel and controls acidity at high temperatures. Acrylonitrile, as a high-dielectric-constant monomer, complexes with transition elements at the positive electrode and enhances solid-liquid capacity. The FEC additive contributes to negative electrode film formation. The synergistic effect of the three effectively enhances the interfacial stability and high-temperature storage stability of lithium-ion batteries equipped with a high-nickel ternary positive electrode. Example

[0031] The gel-state battery used in this embodiment was prepared by the following method: the moisture content in the glove box was controlled to be no more than 10 ppm, ethylene carbonate, propylene carbonate, ethyl methyl carbonate, and dimethyl carbonate were thoroughly stirred and mixed in a mass ratio of 30 / 5 / 45 / 20, and then purified with molecular sieves to remove impurities and water to obtain a solvent component.

[0032] Under the same conditions, 1,3,5-triallyl isocyanurate, acrylonitrile, and butyl acrylate were mixed in a mass ratio of 2 / 9 / 9 using neutral alumina to remove the polymerization inhibitor, thereby obtaining a monomer component.

[0033] Under the same conditions, the monomer, solvent, FEC, and lithium hexafluorophosphate are mixed in a mass ratio of 15 / 67.5 / 5 / 12.5, and 0.5% of azobisisobutyronitrile by mass fraction of the monomer is added. The mixture is stirred until clear to obtain a precursor.

[0034] LiNi 0.83 Co 0.11 Mn 0.06 O2, conductive carbon fiber, and binder polyvinylidene fluoride are dispersed in an appropriate amount of N-methylpyrrolidone at a mass ratio of 96:2:2. The mixture is thoroughly stirred according to the slurrying process. The evenly dispersed positive electrode slurry is evenly coated on aluminum foil, and the positive electrode sheet is obtained through baking, rolling, slitting, and punching.

[0035] Inside a glove box, the positive electrode shell, positive electrode sheet, separator, lithium sheet, nickel foam, and negative electrode sheet were assembled in this order. The precursor from the example was injected, with 60 μL injected into each cell. After stamping and sealing, a lithium-ion button cell was produced. The button cell was placed in a 45°C oven and allowed to stand at high temperature for 48 hours to complete curing, resulting in a lithium-ion gel battery with a loading capacity of 2.04 mAh per cell. Example

[0036] Compared with Example 1, this comparative example is different in that the ratio of 1,3,5-triallyl isocyanurate, acrylonitrile and butyl acrylate is 4 / 13 / 13. Example

[0037] Compared with Example 1, this comparative example is different in that the ratio of 1,3,5-triallyl isocyanurate, acrylonitrile and butyl acrylate is 0.2 / 15 / 15.

[0038] Comparative Example 1

[0039] Compared with Example 1, the difference in this comparative example is that 1,3,5-triallyl isocyanurate in the monomer is replaced by 1,5-pentanediol diacrylate.

[0040] Comparative Example 2

[0041] Compared with Example 1, this comparative example is different in that the ratio of 1,3,5-triallyl isocyanurate, acrylonitrile and butyl acrylate is 1 / 0 / 9.

[0042] Comparative Example 3

[0043] Compared with Example 1, this comparative example is different in that the FEC component is replaced by an equal mass of solvent.

[0044] Comparative Example 4

[0045] Compared with Example 1, this comparative example is different in that the monomer in the precursor is replaced by an equal mass of solvent, and no initiator is added.

[0046] The lithium ion batteries of Examples 1-3 and Comparative Examples 1-4 were tested using the following method and the performance differences were compared.

[0047] Room temperature activation test:

[0048] The lithium-ion batteries of Examples 1 to 3 and Comparative Examples 1 to 4 were charged to 4.3 V at 25° C. using a constant current and constant voltage of 0.1 C, with a cut-off current of 0.05 C, and then discharged to 3 V at a constant current of 0.1 C. After repeating the charge and discharge three times, the discharge capacity and coulombic efficiency of the lithium-ion batteries are shown in FIG1 .

[0049] Comparative Examples 1 to 3 show that when the mass fraction of 1,3,5-triallyl isocyanurate in the gel is too high (2%), the gel ion conductivity will be suppressed, resulting in a decrease in discharge capacity; when the mass fraction is 1.5% and 0.1%, the battery coulomb efficiency reaches 99.5% and above, and the discharge capacity reaches more than 95% of the theoretical capacity, with relatively excellent performance. Comparative Example 1 and Comparative Examples 1 to 3 show that the presence of isocyanurate groups is an important reason for improving the battery coulomb efficiency. The presence of polar nitrile groups plays an important role in promoting the dissociation of lithium salts and improving the gel ion conductivity. FEC additives also help to inhibit interfacial reactions and promote the formation of a stable SEI film. The three have a synergistic effect in the gel system. Comparative Examples 1, 3 and Comparative Example 4 show that cross-linked gels with isocyanurate groups and cyanide groups can effectively improve the coulomb efficiency of lithium-ion batteries without significantly sacrificing ionic conductivity, which is of positive significance for improving battery cycle stability.

[0050] High temperature storage test:

[0051] The lithium ion batteries of Examples 1 and 3 and Comparative Examples 1 and 4 were charged to 4.3 V at a constant current and constant voltage of 0.1 C and a cut-off current of 0.05 C, and EIS tests were performed at room temperature. o The samples were stored in a constant temperature box at C, and EIS test was performed again at room temperature after 7 days. The EIS results before and after storage are shown in Figure 2.

[0052] In Figure 2, after charging to 4.3 V, the room temperature EIS curves of (a) Example 1, (b) Example 3, (c) Comparative Example 1, and (d) Comparative Example 4 before storage are shown; and the room temperature EIS curves of (e) Example 1, (f) Example 3, (g) Comparative Example 1, and (h) Comparative Example 4 after storage are shown.

[0053] As shown in Figure 2, when there are sufficient isocyanurate groups in the gel, the corresponding semi-solid-state battery has good high-temperature storage properties, which is manifested by the fact that the battery impedance does not change significantly before and after high-temperature storage. On the contrary, when the 1,3,5-triallyl isocyanurate content is low, or it is replaced by a common acrylate crosslinker, the cyano group and FEC cannot alone inhibit the reaction between the high-nickel ternary positive electrode and the electrolyte under high temperature and high pressure, resulting in poor high-temperature storage of the battery, which is manifested by a significant increase in the interfacial film impedance and charge transfer impedance after high-temperature storage. In the absence of acid-controlled gel, the hydrofluoric acid produced by the high-temperature decomposition of the electrolyte severely corrodes the SEI film and the positive electrode material, and the battery has the worst high-temperature storage capacity.

[0054] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form or substance. It should be pointed out that ordinary technicians in this technical field can make several improvements and supplements without departing from the method of the present invention. These improvements and supplements should also be regarded as the scope of protection of the present invention. Any equivalent changes, modifications and evolutions made by technicians familiar with this profession without departing from the spirit and scope of the present invention by using the technical content disclosed above are all equivalent embodiments of the present invention; at the same time, any equivalent changes, modifications and evolutions made to the above embodiments based on the essential technology of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A lithium ion gel electrolyte, characterized in that: The invention comprises a polymer monomer, an organic solvent, a lithium salt, an additive and an initiator, wherein the polymer monomer comprises at least 1,3,5-triallyl isocyanurate and acrylonitrile, and the additive comprises at least FEC; the 1,3,5-triallyl isocyanurate accounts for 0.1-2% of the total mass of the lithium ion gel electrolyte, and the acrylonitrile accounts for 2.5-7.5% of the total mass of the lithium ion gel electrolyte.

2. A lithium ion gel electrolyte according to claim 1, characterized in that: The polymer monomers further include one or more combinations of ethoxylated trimethylolpropane triacrylate, pentaerythritol tetraacrylate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, butyl acrylate, tert-butyl acrylate, isobutyl acrylate, 1,4-butylene glycol dimethacrylate, and vinylidene fluoride.

3. A lithium ion gel electrolyte according to claim 1, characterized in that: The organic solvent includes two or more of ethyl propionate, propyl acetate, ethyl acetate, methyl acetate, dimethyl carbonate, diethyl carbonate, propyl propionate, ethylene carbonate, propylene carbonate, ethyl methyl carbonate, and dipropyl carbonate.

4. A lithium ion gel electrolyte according to claim 1, characterized in that: The lithium salt includes one or more combinations of lithium hexafluorophosphate, lithium perchlorate, lithium difluorooxalatoborate, lithium trifluoromethylsulfonate, lithium bistrifluoromethylsulfonyl imide, and lithium bisfluorosulfonyl imide.

5. The lithium ion gel electrolyte according to claim 1, characterized in that: FEC accounts for 5-20% of the total mass of the lithium ion gel electrolyte, and the additives also include one or more combinations of lithium difluorophosphate, lithium bis(oxalatoborate), vinyl sulfate, 1,3-propylene sultone, and N,N-dimethyltrifluoroacetamide.

6. The lithium ion gel electrolyte according to claim 1, characterized in that: The initiator includes one or more combinations of azobisisobutyronitrile, azobisisoheptylnitrile, dibenzoyl peroxide, diisobutyl peroxydicarbonate, and dicyclohexyl peroxydicarbonate.

7. A gel lithium ion battery, characterized in that: The invention comprises the lithium ion gel electrolyte as claimed in any one of claims 1 to 6, a negative electrode, a positive electrode and a separator.

8. A gel lithium ion battery according to claim 7, characterized in that: The positive electrode includes a positive electrode active material, a conductive agent, a binder and a current collector; wherein the positive electrode active material includes one or more combinations of lithium iron phosphate, lithium iron manganese phosphate, lithium nickel oxide, lithium manganese oxide, lithium cobalt oxide, nickel-cobalt-manganese ternary material, and nickel-cobalt-aluminum ternary material.

9. A gel lithium ion battery according to claim 7, characterized in that: The negative electrode includes a negative electrode sheet containing lithium metal and a current collector; or a negative electrode sheet containing an active material, a conductive agent, a binder and a current collector; wherein the active material includes one or more combinations of graphite, hard carbon, soft carbon, silicon, silicon oxygen, and silicon carbon.

10. A method for preparing a gel lithium ion battery as claimed in claim 7, characterized in that: The following steps are involved: 1) Control the moisture content to ≤10ppm, mix the organic solvent evenly, and obtain an organic solvent component; 2) Control the moisture content to ≤10ppm, remove the polymerization inhibitor from the polymer monomers and mix them evenly to obtain polymer monomer components; 3) controlling the moisture content to ≤10ppm, mixing the polymer monomer component obtained in step 2), the organic solvent component obtained in step 1), the additive, and the lithium salt, adding the initiator, and stirring until clarified to obtain a precursor; 4) Assemble the positive electrode shell, positive electrode sheet, separator, lithium sheet, nickel foam, and negative electrode sheet in order, inject the precursor obtained in step 3), and obtain a gel lithium-ion battery after stamping and sealing.

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