Cyclophosphamide electrolyte additive, electrolyte and lithium ion battery

Abstract

The invention belongs to the technical field of lithium ion batteries, and particularly relates to a cyclophosphamide electrolyte additive, an electrolyte thereof and a lithium ion battery, the additive is represented by a structural formula (1) imgabs0 #, n in the structural formula (1) is equal to 1 or 2, R1 and R2 are independently selected from alkyl groups, and R3 and R4 are independently selected from one of hydrogen groups or alkyl groups. Cyclophosphamide is used as an electrolyte additive, a decomposition product of cyclophosphamide participates in construction of an SEI film, a cyclic structure improves the chemical structure stability of the SEI film, a phosphamide structure improves the thermal stability of the SEI film, meanwhile, the flame retardance of the electrolyte is improved, the cycle life of a battery is prolonged, and the safety performance of the battery is improved at the same time.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion batteries, and in particular relates to a cyclophosphamide electrolyte additive and an electrolyte thereof and a lithium ion battery. Background Art

[0002] Lithium-ion secondary batteries have the characteristics of high energy density, long cycle life, wide operating range, and low pollution, and are widely used in the battery industry. With the rapid development of intelligent electronic products, the issue of improving battery life and safety performance has become a matter of great concern in the industry. One of the main factors is the side reaction between the highly active lithium battery negative electrode and the organic electrolyte, which not only consumes active lithium but also increases the impedance of the battery, while also increasing the potential safety hazards of the battery. Therefore, researchers have added a variety of functional additives to the electrolyte to improve battery performance, such as SEI film-forming additives, flame retardant additives, HF capture agents, etc.

[0003] The dominant solid electrolyte interface formed in conventional carbonate electrolytes has low interfacial energy and high resistance with the lithium negative electrode, resulting in too low lithium stripping Coulomb efficiency. In order to improve the compatibility of electrolytes with lithium-ion batteries, the strategy of introducing film-forming additives is to have excellent compatibility with electrolyte components, form a stable SEI film, and design the additive structure and composition. Summary of the invention

[0004] One of the purposes of the present invention is to provide a cyclophosphamide electrolyte additive to address the deficiencies of the prior art, which is uniformly dispersed in the electrolyte system, does not increase the viscosity of the electrolyte, has good compatibility with the electrolyte components, and can form a stable SEI film, thereby effectively improving the cycle life of the battery.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A cyclophosphamide electrolyte additive of the present invention, the additive has the following structural formula (1):

[0007]

[0008] Wherein, in the structural formula (1), n=1 or 2, R1 and R2 are selected from independent alkyl groups, and R3 and R4 are selected from independent hydrogen groups or alkyl groups.

[0009] The second object of the present invention is to provide an electrolyte that can generate a SEI film with chemical stability and thermal stability and has a flame retardant effect;

[0010] The electrolyte of the present invention comprises an organic solvent, an electrolyte lithium salt, an auxiliary additive and the cyclophosphamide electrolyte additive mentioned above.

[0011] Preferably, the organic solvent is at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, vinylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, methyl formate, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl difluoroacetate, ethyl difluoroacetate, ethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, acetonitrile, malononitrile, glutaronitrile, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, sulfolane, and dimethyl sulfoxide.

[0012] Preferably, the electrolyte lithium salt is at least one of lithium bis(trifluoromethanesulfonyl imide), lithium bis(fluorosulfonyl imide), lithium bis(trifluorosulfonyl imide), lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium bis(oxalatoborate), and lithium trifluoromethanesulfonate.

[0013] Preferably, the concentration of the electrolyte lithium salt is 0.6-3.0 mol / L.

[0014] Preferably, the auxiliary additive is at least one of vinylene carbonate, vinyl ethylene carbonate, vinyl vinyl acetate, vinyl sulfite, propylene sulfite, vinyl sulfate, 1,3-propane sultone, propenyl-1,3-propane sultone, 1,4-butane sultone, methylene disulfonate, hexamethyldisilazane, and fluorobenzene.

[0015] Preferably, the mass content of the auxiliary additive accounts for 0.1-3.0wt% of the total mass of the electrolyte.

[0016] Preferably, the mass content of the cyclophosphamide electrolyte additive accounts for 0.1-2.0wt% of the total mass of the electrolyte.

[0017] The third object of the present invention is to provide a lithium ion battery. The lithium ion battery provided by the present invention comprises a negative electrode, a positive electrode, a separator arranged between the negative electrode and the positive electrode, and the electrolyte described in any one of the above.

[0018] The beneficial effect of the present invention is that: with a certain content of cyclophosphamide as an electrolyte additive, the decomposition products of the cyclophosphamide skeleton can improve the chemical stability and thermal stability of the SEI film, while improving the flame retardancy of the electrolyte, thereby effectively improving the cycle life and safety performance of the battery. DETAILED DESCRIPTION

[0019] In order to make the technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0020] According to a first aspect of the present invention, the present invention provides a cyclophosphamide electrolyte additive, the additive comprising the following structural formula (1):

[0021]

[0022] Wherein, n in the structural formula (1) is 1 or 2, R1 and R2 are independently selected from alkyl groups, R3 and R4 are independently selected from hydrogen groups or alkyl groups, and the decomposition products of cyclophosphamide improve the structural stability of the SEI film due to the ring structure, while the P element in the fragments improves the thermal stability of the SEI film.

[0023] According to a second aspect of the present invention, there is provided an electrolyte capable of generating a stable SEI film, wherein the phosphoramide structure can improve the flame retardancy of the electrolyte;

[0024] The electrolyte of the present invention comprises an organic solvent, an electrolyte lithium salt, an auxiliary additive and the cyclophosphamide electrolyte additive mentioned above.

[0025] In one embodiment of the present invention, the organic solvent is at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, vinylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, methyl formate, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl difluoroacetate, ethyl difluoroacetate, ethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, acetonitrile, malononitrile, glutaronitrile, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, cyclopentane, and dimethyl sulfoxide.

[0026] In one embodiment of the present invention, the electrolyte lithium salt is at least one of lithium bis(trifluoromethanesulfonyl imide), lithium bis(fluorosulfonyl imide), lithium bis(trifluorosulfonyl imide), lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium bis(oxalatoborate), and lithium trifluoromethanesulfonate.

[0027] In one embodiment of the present invention, the concentration of the electrolyte lithium salt is 0.6-3.0 mol / L, for example, it can be 0.6 mol / L, 0.8 mol / L, 0.9 mol / L, 1.0 mol / L, 1.3 mol / L, 1.6 mol / L, 1.8 mol / L, 2.3 mol / L, 2.5 mol / L, 2.8 mol / L, 3.0 mol / L.

[0028] In one embodiment of the present invention, the auxiliary additive is at least one of vinylene carbonate, vinyl ethylene carbonate, vinyl vinyl acetate, vinyl sulfite, propylene sulfite, vinyl sulfate, 1,3-propane sultone, propenyl-1,3-propane sultone, 1,4-butane sultone, methylene disulfonate, hexamethyldisilazane, and fluorobenzene, and the auxiliary additive plays a good role in forming a stable SEI film and enhancing the wettability of the electrolyte.

[0029] In one embodiment of the present invention, the mass content of the auxiliary additive accounts for 0.1-3.0wt% of the total mass of the electrolyte, for example, it can be 0.1wt%, 0.5wt%, 0.8wt%, 1.0wt%, 1.3wt%, 1.6wt%, 1.8wt%, 2.3wt%, 2.6wt%, 2.8wt%, 3.0wt%.

[0030] In one embodiment of the present invention, the mass content of cyclophosphamide electrolyte added is 0.1-2.0wt% of the total mass of the electrolyte, for example, it can be 0.1wt%, 0.5wt%, 0.8wt%, 1.0wt%, 1.3wt%, 1.6wt%, 1.8wt%, 2.0wt%. At this content, the viscosity of the electrolyte is moderate and the conductivity is improved.

[0031] According to a third aspect of the present invention, the present invention provides a lithium ion battery. The lithium ion battery provided by the present invention comprises a negative electrode, a positive electrode, a separator arranged between the negative electrode and the positive electrode, and the above-mentioned electrolyte.

[0032] The negative electrode includes a negative electrode current collector and a negative electrode active material layer coated on at least one surface of the negative electrode current collector. The negative electrode active material used in the negative electrode active material layer may include but is not limited to one or more of graphite, soft carbon, hard carbon, carbon fiber, mesophase carbon microspheres, silicon-based materials, tin-based materials, etc. The graphite may be selected from one or more of artificial graphite, natural graphite and modified graphite; the silicon-based material may be selected from one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, and silicon alloys; the tin-based material may be selected from one or more of elemental tin, tin oxide compounds, and tin alloys. The negative electrode current collector is preferably copper foil.

[0033] Among them, the positive electrode includes a positive electrode collector and a positive electrode active material layer coated on at least one surface of the positive electrode collector. The positive electrode active material used in the positive electrode active material layer can be one or more of lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, lithium nickel oxide, nickel cobalt manganese oxide, etc.

[0034] The separator can be selected from various separators used in lithium-ion batteries known to those skilled in the art, such as polypropylene microporous membrane, polyethylene felt, glass fiber felt or ultrafine glass fiber paper.

[0035] In order to make the technical solutions and advantages of the present invention more clear, the present invention will be further described below through specific embodiments.

[0036] Example 1

[0037] The present embodiment provides a lithium ion battery electrolyte, which is composed of a mixed solvent of ethylene carbonate and ethyl methyl carbonate in an organic solvent mass ratio of 3:7, and a cyclophosphamide additive whose mass content accounts for 0.5%wt of the total mass of the electrolyte, and has a structural formula of Wherein n=1, R1 and R2 are methyl groups, R3 and R4 are hydrogen groups, the lithium salt electrolyte is lithium hexafluorophosphate, the concentration of which is 1.0 mol / L, and the auxiliary additives are composed of vinylene carbonate and vinyl sulfate, each of which has a content of 1.0 wt%, wherein the mass content of the auxiliary additives accounts for 0.8 wt% of the total mass of the electrolyte;

[0038] The conventional negative electrode, positive electrode, separator arranged between the negative electrode and the positive electrode and the above-mentioned electrolyte are prepared into a lithium ion battery.

[0039] Comparative Example 1

[0040] The present embodiment provides a lithium ion battery electrolyte, which is composed of a mixed solvent of ethylene carbonate and ethyl methyl carbonate in an organic solvent mass ratio of 3:7, a lithium salt electrolyte lithium hexafluorophosphate, the concentration of which is 1.0 mol / L, and auxiliary additives of vinylene carbonate and vinyl sulfate in an amount of 1.0 wt% respectively;

[0041] A lithium-ion battery is prepared by using a conventional negative electrode, a positive electrode, a separator disposed between the negative electrode and the positive electrode, and an electrolyte.

[0042] Example 2

[0043] The present embodiment provides a lithium ion battery electrolyte, which is composed of a mixed solvent of dimethyl carbonate and methyl formate in an organic solvent mass ratio of 3:7, and a cyclophosphamide additive whose mass content accounts for 0.8%wt of the total mass of the electrolyte, and has a structural formula of Wherein n=1, R1 and R2 are methyl, R3 and R4 are hydrogen and methyl, respectively, the lithium salt electrolyte is lithium bistrifluorosulfonyl imide, the concentration of which is 0.8 mol / L, and the auxiliary additives are composed of vinyl ethylene carbonate and vinyl sulfate, each of which has a content of 1.5 wt%, wherein the mass content of the auxiliary additives accounts for 0.5 wt% of the total mass of the electrolyte;

[0044] The conventional negative electrode, positive electrode, separator arranged between the negative electrode and the positive electrode and the above-mentioned electrolyte are prepared into a lithium ion battery.

[0045] Example 3

[0046] The present embodiment provides a lithium ion battery electrolyte, which is composed of a mixed solvent of methyl difluoroacetate and propyl propionate in an organic solvent mass ratio of 3:7, and a sultone additive whose mass content accounts for 1.0%wt of the total mass of the electrolyte, and has a structural formula of Wherein n=2, R1 and R2 are methyl groups, R3 and R4 are hydrogen groups, the lithium salt electrolyte is lithium tetrafluoroborate, the concentration of which is 1.5 mol / L, and the auxiliary additives are composed of vinyl ethylene carbonate and vinyl sulfate, each of which has a content of 1.8 wt %, wherein the mass content of the auxiliary additives accounts for 1.0 wt % of the total mass of the electrolyte;

[0047] The conventional negative electrode, positive electrode, separator arranged between the negative electrode and the positive electrode and the above-mentioned electrolyte are prepared into a lithium ion battery.

[0048] Example 4

[0049] The present embodiment provides a lithium ion battery electrolyte, which is composed of a mixed solvent of propyl propionate and methyl difluoroacetate in an organic solvent mass ratio of 3:7, and a cyclophosphamide additive whose mass content accounts for 1.2%wt of the total mass of the electrolyte, and has a structural formula of Wherein n=1, R1 and R2 are ethyl and methyl, respectively, R3 and R4 are methyl and hydrogen, respectively, the lithium salt electrolyte is lithium perchlorate, the concentration of which is 1.0 mol / L, and the auxiliary additives are composed of vinyl ethylene carbonate and vinyl sulfate, each of which has a content of 1.3 wt%, wherein the mass content of the auxiliary additives accounts for 1.2 wt% of the total mass of the electrolyte;

[0050] The conventional negative electrode, positive electrode, separator arranged between the negative electrode and the positive electrode and the above-mentioned electrolyte are prepared into a lithium ion battery.

[0051] The lithium-ion battery obtained in the embodiment was subjected to cycle performance tests at different temperatures, and the experimental results are shown in Table 1.

[0052] Table 1 - Lithium-ion battery performance test table

[0053]

[0054]

[0055] It can be seen from Table 1 that the cycle capacity retention rate of the battery prepared in Comparative Example 1 at a low temperature of -20°C is only 56.4%. This is because the lithium ions have a strong binding force with the organic solvent components, and the lithium ion desolvation energy barrier is large, which is not conducive to the solvation and desolvation of lithium ions, resulting in slow lithium ion transmission; and since conventional carbonate solvents are used in Comparative Example 1, the SEI film generated is unstable at a high temperature of 50°C, which aggravates the interaction between the electrode and the electrolyte. Therefore, the cycle capacity retention rate of the battery at 50°C is only 56.2%.

[0056] The battery prepared by adding the cyclophosphamide additive in Example 1 showed high capacity retention rates of 82.3%, 96.0% and 74.3% at 50°C, 25°C and -20°C respectively. This is because the decomposed fragments of cyclophosphamide participate in the film formation, forming a stable SEI film, and the P element in the SEI component improves the thermal stability of the SEI film, thereby effectively improving the cycle life of the battery. Similarly, under the same cycle test conditions, the changes in the five-membered ring or six-membered ring and the substituents in Examples 2-4 have the same effect of improving the capacity retention rate and cycle life for the additive containing the cyclophosphamide skeleton.

[0057] According to the disclosure and teaching of the above description, those skilled in the art to which the present invention belongs can also change and modify the above embodiment. Therefore, the present invention is not limited to the above specific embodiment, and any obvious improvement, replacement or modification made by those skilled in the art on the basis of the present invention belongs to the protection scope of the present invention. In addition, although some specific terms are used in this specification, these terms are only for the convenience of description and do not constitute any limitation to the present invention.

Claims

1. A cyclophosphamide electrolyte additive, characterized in that: The additive is the following structural formula (1): Wherein, in the structural formula (1), n=1 or 2, R1 and R2 are selected from independent alkyl groups, and R3 and R4 are selected from independent hydrogen groups or alkyl groups.

2. An electrolyte, characterized in that: The electrolyte comprises an organic solvent, an electrolyte lithium salt, an auxiliary additive and the cyclophosphamide electrolyte additive according to claim 1.

3. An electrolyte according to claim 2, characterized in that: The organic solvent is at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, vinylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, methyl formate, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl difluoroacetate, ethyl difluoroacetate, ethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, acetonitrile, malononitrile, glutaronitrile, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, sulfolane, and dimethyl sulfoxide.

4. An electrolyte according to claim 2, characterized in that: The electrolyte lithium salt is at least one of lithium bis(trifluoromethanesulfonyl imide), lithium bis(fluorosulfonyl imide), lithium bis(trifluorosulfonyl imide), lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium bis(oxalatoborate), and lithium trifluoromethanesulfonate.

5. An electrolyte according to claim 2, characterized in that: The concentration of the electrolyte lithium salt is 0.6-3.0 mol / L.

6. An electrolyte according to claim 2, characterized in that: The auxiliary additive is at least one of vinylene carbonate, vinyl ethylene carbonate, vinyl vinyl acetate, vinyl sulfite, propylene sulfite, vinyl sulfate, 1,3-propane sultone, propenyl-1,3-propane sultone, 1,4-butane sultone, methylene disulfonate, hexamethyldisilazane, and fluorobenzene.

7. An electrolyte according to claim 2, characterized in that: The mass content of the auxiliary additive accounts for 0.1-3.0wt% of the total mass of the electrolyte.

8. An electrolyte according to claim 2, characterized in that: The mass content of the added cyclophosphamide electrolyte accounts for 0.1-2.0wt% of the total mass of the electrolyte.

9. A lithium ion battery, characterized in that: The lithium-ion battery comprises a negative electrode, a positive electrode, a separator arranged between the negative electrode and the positive electrode, and the electrolyte according to any one of claims 2 to 8.

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