Additive for lithium-ion battery electrolyte, electrolyte containing the same, and lithium-ion secondary battery

Abstract

The present invention provides an additive for lithium ion battery electrolyte, an electrolyte comprising the additive and a lithium ion secondary battery. Specifically, the present invention provides an additive for lithium ion battery electrolyte, having a structure of formula (1), wherein R1, R2 and R3 are each independently substituted by at least one fluorine atom and having a carbon number of 5-15 saturated or unsaturated hydrocarbon group, cyano group, amide group, pyridyl group, thienyl group or aryl group, and an electrolyte bag for lithium ion battery including the additive and a lithium ion battery containing the electrolyte. By the additive for lithium ion battery electrolyte, the electrolyte comprising the additive and the lithium ion secondary battery of the present invention, it is achieved to improve the high temperature cycle performance of lithium ion secondary battery, and suppress oxygen evolution and reduce the effect of gas production.

Description

Technical Field

[0001] The present invention relates to the field of lithium-ion secondary batteries, and in particular to an additive for lithium-ion battery electrolyte, an electrolyte containing the additive, and a lithium-ion secondary battery. Background Art

[0002] In recent years, with the continuous advancement of electronic technology, the demand for battery devices to power electronic devices has also increased. Batteries capable of storing large amounts of energy and delivering high power are now in demand. Traditional lead-acid and nickel-metal hydride batteries are no longer able to meet the demands of new electronic products. Consequently, lithium batteries have attracted widespread attention. The development of lithium batteries has significantly improved their capacity and performance.

[0003] In lithium-ion batteries, ternary cathode materials have become a research hotspot due to their high specific capacity. However, ternary materials suffer from poor cycling performance under high temperature and high pressure. In particular, they are prone to phase transitions at high voltages, leading to oxygen evolution in the lattice and causing side reactions at the electrode interface. Currently, widely used lithium-ion battery electrolytes are primarily composed of lithium salts and electrolyte solvents. However, these electrolytes still have many shortcomings, particularly at high voltages, resulting in poor performance of lithium-ion batteries, such as poor high-temperature cycling performance and poor high-temperature storage performance. Summary of the Invention

[0004] The main purpose of the present invention is to provide an additive for lithium-ion battery electrolyte, an electrolyte containing the additive, and a lithium-ion secondary battery, so as to solve the problem of poor electrochemical performance of lithium-ion batteries in the prior art, such as poor cycle performance under high temperature and high pressure.

[0005] In order to achieve the above object, according to one aspect of the present invention, there is provided an additive for lithium ion battery electrolyte having the structure of the following formula (1):

[0006]

[0007] wherein R1, R2 and R3 are each independently a saturated or unsaturated hydrocarbon group, a cyano group, an amide group, a pyridyl group, a thienyl group or an aryl group substituted by at least one fluorine atom and having 5 to 15 carbon atoms.

[0008] Furthermore, in the above additive, R1, R2 and R3 are each independently an alkenyl group, an alkynyl group, a cyano group, a pyridyl group or a thienyl group substituted with at least one fluorine atom and having 5 to 15 carbon atoms.

[0009] Furthermore, among the above-mentioned additives, the additive is selected from at least one of the following compounds:

[0010]

[0011]

[0012] According to another aspect of the present invention, an electrolyte for a lithium-ion battery is provided, characterized in that the electrolyte comprises the additives according to the above aspects, an organic solvent and a lithium salt.

[0013] Further, in the above electrolyte, the amount of the additive is in the range of 0.5 to 2 parts by weight based on 100 parts by weight of the organic solvent and the lithium salt.

[0014] Furthermore, in the above electrolyte, the organic solvent includes cyclic carbonate, linear carbonate, or any combination thereof.

[0015] Furthermore, in the above electrolyte, the organic solvent is selected from the group consisting of propylene carbonate, butylene carbonate, fluoroethylene carbonate (fluoroethylene carbonate), ethylene carbonate (ethylene carbonate), diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, dimethyl carbonate, or a combination thereof.

[0016] Furthermore, in the above electrolyte, the lithium salt is selected from the group consisting of LiCl, LiBr, LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, LiAlCl4, Li2SiF6, or a combination thereof.

[0017] According to another aspect of the present invention, a lithium-ion secondary battery is provided, comprising: a positive electrode sheet, a negative electrode sheet, a separator, and the electrolyte according to the above aspects of the present invention.

[0018] Furthermore, in the above lithium-ion secondary battery, the positive electrode active material of the positive electrode sheet is a high-nickel positive electrode material.

[0019] The additive for lithium ion battery electrolyte, the electrolyte containing the additive and the lithium ion secondary battery of the present invention can improve the electrochemical performance of the lithium ion battery, especially the cycle performance under high temperature and high pressure. DETAILED DESCRIPTION

[0020] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present invention will be described in detail below with reference to the embodiments.

[0021] As explained in the Background section, conventional lithium-ion battery electrolytes are primarily composed of lithium salts and electrolyte solvents. However, these electrolytes still have numerous drawbacks, particularly at high voltages, resulting in poor performance of lithium-ion batteries, such as poor high-temperature cycling and storage performance.

[0022] Phosphite additives are widely recognized as effective cathode film-forming additives in lithium-ion battery electrolytes. These additives inhibit the hydrolysis of LiPF6, remove HF, react with reactive oxygen compounds on the cathode surface, inhibit oxygen evolution, and improve the battery's high-temperature and high-voltage performance. However, existing single-function electrolyte additives often have limitations. This paper aims to develop an electrolyte additive that overcomes these challenges and improves the battery's high-temperature and high-voltage performance.

[0023] According to a typical embodiment of the present application, an additive for lithium-ion battery electrolyte is provided, having the structure of the following formula (1):

[0024]

[0025] wherein R1, R2 and R3 are each independently a saturated or unsaturated hydrocarbon group substituted by at least one fluorine atom and having 5 to 15 carbon atoms, a cyano group, an amide group, a pyridyl group, a thienyl group or an aryl group.

[0026] Research by the inventors has shown that fluorine atoms have strong electron-withdrawing properties. When fluorine atoms are substituted for additives, the electron cloud disperses, making it more difficult for the substance to lose electrons and thus possessing higher antioxidant capacity. Furthermore, fluorine substitution can enhance the antioxidant capacity and chemical stability of existing carbonate and carboxylate materials, further improving the battery's high-temperature performance and making it suitable for higher-voltage lithium battery systems.

[0027] Furthermore, the fluorine-substituted additive increases its wettability in the original solvent, increasing the migration rate of lithium ions and reducing the impedance at the electrolyte-electrode interface, thereby improving the battery's low-temperature performance and rate capability. Furthermore, the fluorine-substituted solvent reduces its flammability, enhancing the battery's flame retardancy and safety. Therefore, by fluorinating the phosphite additive, the present invention improves the electrolyte's oxidation resistance and thermal stability while stabilizing the positive electrode interface, thereby improving the battery's high-temperature performance.

[0028] The electrolyte system for lithium-ion batteries of the present invention is obtained by adding an additive according to an embodiment of the present invention to a conventional electrolyte. The addition of this additive effectively improves the high-temperature cycling and storage performance of the high-nickel cathode material for lithium-ion batteries, inhibits oxygen evolution, reduces gas production, and thus reduces volume growth during battery cycling.

[0029] As shown in formula (1), the additive of the present invention comprises a phosphite structure. The additive comprising the phosphite can decompose on the electrode surface to form a stable passivation film, thereby preventing the electrolyte from being oxidized and decomposed on the positive electrode surface and inhibiting the dissolution of positive electrode metal ions. In addition, the phosphite can react with the O 2- 、O2 2- The oxides react to form stable phosphate compounds, which inhibit oxygen evolution and reduce gas production, thereby improving the high-temperature performance of the battery.

[0030] While not wishing to be bound by theory, it is believed that the reaction mechanism of the phosphite additive of the present invention in combination with oxygen evolution on the surface of the cathode material in the battery is as follows:

[0031]

[0032] In some embodiments of the present application, the additive is selected from at least one of the following compounds:

[0033]

[0034] In the compound of formula (2), R1, R2, and R3 in formula (1) are each independently a fluoropyridine group, such as a 2-fluoro-4-pyridine group. The inventors have discovered that by introducing a fluoropyridine group, transition metal ions and hydrogen fluoride in the electrolyte can be complexed, inhibiting the dissolution of transition metal ions and their deposition at the negative electrode, thereby further protecting the positive and negative electrodes.

[0035] In the compound of formula (3), R1, R2, and R3 in formula (1) are each independently a trifluoromethylthiophene group, such as a 2-trifluoromethyl-5-thiophene group. The inventors have discovered that by introducing a trifluoromethylthiophene group, the thiophene group released after decomposition of the additive can preferentially polymerize on the surface of the positive electrode to form a film over the solvent, and the trifluoromethyl group can supplement LiF into the SEI film, thereby improving the high-voltage cycling performance of the battery.

[0036] Correspondingly, the above-mentioned fluoropyridine group also has the same effect.

[0037] In addition, for the compound of formula (3), the film formed on the positive electrode surface by the thiophene groups released after the decomposition of the additive has good conductivity, thereby suppressing the increase of battery impedance during cycling.

[0038] For the compounds of formula (4) and formula (5), R1, R2, and R3 in formula (1) are each independently an alkenyl group or an alkynyl group substituted with a fluorine atom, such as 3,4-difluoro-1-penten-5-yl and 3,4-difluoro-1-pentyn-5-yl. Due to the unsaturated bond, the additive easily polymerizes to form a film on the surface of the positive electrode, and because the unsaturated bond is in an electron-deficient state, the additive easily obtains electrons on the surface of the negative electrode to form an SEI film.

[0039] For the compound of formula (6), R1, R2, and R3 in formula (1) are each independently a cyano group substituted with a fluorine atom, such as 4-fluoro-1-pentancyano-5-yl. The cyano group in the additive can complex with transition metal ions in the electrolyte and remove acid from the electrolyte, thereby facilitating improved cycle retention of the battery.

[0040] In addition, when R1, R2, or R3 in formula (1) is an aromatic group, when the battery is overcharged, the additive can polymerize into a film on the electrode surface, increasing the internal resistance of the battery and causing the battery to lose power, thereby preventing the battery from overcharging. When R1, R2, or R3 in formula (1) is an amide group, since the nitrogen element in the amide group has a strong electronegativity and can bind to lithium ions, the additive can increase the conductivity of the electrolyte and improve the battery's rate performance.

[0041] According to another typical embodiment of the present invention, there is provided an electrolyte for a lithium-ion battery, comprising the additives of the above various aspects of the present invention, as well as an organic solvent and a lithium salt. Based on 100 parts by weight of the organic solvent and the lithium salt, the amount of the additive is in the range of 0.5 parts by weight to 2 parts by weight. As described in detail in the following embodiments, when the amount of the additive is lower than this range, it has an adverse effect on the cycle retention rate and the battery volume growth rate; when the amount of the additive is higher than this range, it has an adverse effect on the cycle retention rate and post-cycle impedance of the battery, while increasing the cost of the electrolyte. In particular, when the amount of the additive is outside the above range, it has an adverse effect on the high-temperature cycle performance of the battery.

[0042] According to a further embodiment of the present application, the organic solvent in the electrolyte for lithium-ion batteries of the present invention includes cyclic carbonate, linear carbonate, or any combination thereof.

[0043] In a specific embodiment, the organic solvent is selected from the group consisting of propylene carbonate, butylene carbonate, fluoroethylene carbonate, ethylene carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, dimethyl carbonate, or a combination thereof.

[0044] As described above, the additives of the present invention can enhance the antioxidant capacity and chemical stability of existing carbonate materials, further improving the battery's high-temperature performance and making it suitable for higher-voltage lithium battery systems. Furthermore, by adding the additives of the present invention to an electrolyte containing a cyclic carbonate solvent, a linear carbonate solvent, and a lithium salt, the high-temperature cycling and storage performance of high-nickel cathode active materials for lithium-ion batteries is effectively improved, oxygen evolution is suppressed, and gas production is reduced.

[0045] According to a specific embodiment, the lithium salt in the electrolyte for a lithium-ion battery of the present invention is selected from the group consisting of LiCl, LiBr, LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, LiAlCl4, Li2SiF6, or a combination thereof. As described above, the additive of the present invention can inhibit the hydrolysis of LiPF6, thereby improving the high temperature and high pressure performance of the battery.

[0046] According to another typical embodiment of the present invention, a lithium-ion secondary battery is provided, comprising: a positive electrode sheet, a negative electrode sheet, a separator, and the electrolyte according to the above aspects of the present invention.

[0047] In a specific embodiment of the present invention, the lithium ion secondary battery of the present invention is prepared by the following steps.

[0048] Preparation of the positive electrode sheet: The positive electrode active material, conductive agent, binder, and dispersant are mixed to obtain a positive electrode mixture, and the mixture is dispersed in a solvent to obtain a positive electrode mixture slurry. The positive electrode mixture slurry is then coated onto the positive electrode current collector, dried, and stamped to form a positive electrode sheet.

[0049] Preparation of negative electrode sheets: The negative electrode active material, conductive agent, binder, and water are mixed to prepare negative electrode slurry. The negative electrode slurry is then coated onto the negative electrode collector, dried, and stamped to form a negative electrode sheet.

[0050] Preparation of electrolyte: The organic solvent, lithium salt and additives mentioned above are mixed to prepare the electrolyte.

[0051] Battery assembly: The positive electrode sheet prepared in the above steps is used as the positive electrode, the negative electrode sheet is used as the negative electrode, and the electrolyte, separator and battery shell are assembled into a battery.

[0052] In a preferred embodiment of the present invention, the active material of the positive electrode sheet in the lithium ion secondary battery of the present invention is a high nickel positive electrode active material. Examples of high nickel positive electrode active materials include but are not limited to lithium nickel cobalt manganese oxide (e.g. LiNi 0.6 Co 0.2 Mn 0.2O2, or NCM622, or LiNi 0.8 Co 0.1 Mn 0.1 O2, or NCM811) and lithium nickel cobalt aluminum oxide (NCA, such as LiNi 0.8 Co 0.15 Al 0.05 O2).

[0053] The electrolyte of the present invention is particularly suitable for lithium-ion batteries with high nickel positive electrodes. As mentioned above, the addition of the additive of the present invention effectively improves the high-temperature cycle performance and storage performance of the high-nickel positive electrode material of lithium-ion batteries.

[0054] The present invention is further described in detail below with reference to specific examples. These examples should not be construed as limiting the scope of protection claimed in the present invention.

[0055] Comparative Example

[0056] The lithium ion battery used in Comparative Example was prepared by the following steps.

[0057] Preparation of positive electrode sheet: The positive electrode active material lithium nickel cobalt aluminum oxide NCA (specifically LiNi 0.8 Co 0.15 Al 0.05 A positive electrode mixture was prepared by mixing a positive electrode mixture of 0.1% MgO2 (95.5 parts by weight), conductive carbon black (2.5 parts by weight), a binder of polyvinylidene fluoride (1.9 parts by weight), and a dispersant of polyvinyl pyrrolidone (0.1 parts by weight), and the positive electrode mixture was dispersed in N-methylpyrrolidone to obtain a positive electrode mixture slurry. The positive electrode mixture slurry was then applied to a positive electrode collector made of aluminum foil, which was dried and stamped to form a positive electrode sheet.

[0058] Preparation of the negative electrode sheet: A mixture of 95.85 parts by weight of silicon dioxide (SiO) and graphite powder (with a weight ratio of 9:1) as the negative electrode active materials, conductive carbon black (1 part by weight), and the binders carboxymethyl cellulose and styrene-butadiene latex (3.15 parts by weight) were stirred with an appropriate amount of water to prepare a negative electrode mixture slurry. The negative electrode mixture slurry was then evenly coated onto a negative electrode collector made of copper foil, dried, and stamped to form a negative electrode sheet.

[0059] Preparation of the electrolyte: Ethylene carbonate (or ethylene carbonate) (20 parts by weight), dimethyl carbonate (62 parts by weight) and lithium hexafluorophosphate (18 parts by weight) were mixed to prepare a basic electrolyte.

[0060] Battery Assembly: CR2016 button cells were assembled in a dry laboratory. The positive electrode sheet prepared in the above steps served as the positive electrode, the negative electrode sheet served as the negative electrode, and the electrolyte, separator, and button cell casing were assembled. After assembly, the battery was aged for 12 hours to obtain an NCA button cell.

[0061] Examples 1-13

[0062] The lithium-ion batteries of Examples 1-13 were prepared using the same steps as in the comparative example above. The difference was that in Examples 1-13, in the electrolyte preparation step, ethylene carbonate (20 parts by weight), dimethyl carbonate (62 parts by weight), and lithium hexafluorophosphate (18 parts by weight) were mixed to prepare a basic electrolyte, and the additives listed in Table 1 below were added to 100 parts by weight of the prepared basic electrolyte, respectively. The amounts of the additives added to 100 parts by weight of the basic electrolyte were also as shown in Table 1.

[0063] Table 1

[0064]

[0065]

[0066] Battery performance testing

[0067] The batteries prepared in Comparative Example and Examples 1-13 were subjected to charge and discharge tests and impedance tests at room temperature and at 2.5-4.25 V.

[0068] First, a 0.1C cycle test was performed at 25 degrees Celsius for one cycle, and then a 5C cycle test was performed at 60 degrees Celsius for 100 cycles. The battery capacity was measured before and after the cycle test at 60 degrees Celsius to determine the battery's capacity retention rate at 60 degrees Celsius. The battery volume was measured before and after the cycle test to determine the battery's volume growth rate. The battery impedance value was also measured after the cycle test.

[0069] For the batteries prepared in Comparative Example and Examples 1-13, the results obtained in the above tests are shown in Table 2.

[0070] Table 2

[0071]

[0072]

[0073] As can be seen from Table 2 above, compared with the lithium ion battery made using the electrolyte without the additive of the present invention (Comparative Example), the lithium ion battery made using the electrolyte containing the additive of the present invention (Examples 1-13) shows improvement in at least one of the cycle retention rate, post-cycle impedance and battery volume growth rate at 60°C.

[0074] In addition, by comparing the results of Examples 1-3 and 7-8, and Examples 4-6 and 9-10, it can be seen that when the amount of the additive in 100 parts by weight of the basic electrolyte is in the range of 0.5 to 2 parts by weight, the corresponding lithium-ion battery, especially in terms of cycle retention at 60°C, exhibits better electrochemical performance. In particular, Examples 4-6 exhibit better performance in terms of cycle retention at 60°C, post-cycle impedance, and battery volume growth rate compared to the comparative example. Among them, although not wishing to be bound by theory, it is believed that the increase in post-cycle impedance shown in Examples 1-3 and 11-13 relative to the comparative example is due to the increase in resistance caused by the additive forming a film on the electrode surface, while the decrease in post-cycle impedance shown in Example 4-6 relative to the comparative example proves that the film formed by the thiophene group in the additive has good conductivity, and thus can suppress the increase in impedance after the cycle.

[0075] In addition, compared with the comparative example, the cycle retention rate and battery volume growth rate of Examples 11-13 at 60°C showed performance improvement.

[0076] The results of Examples 7-8 show that when the additive content is below the above range of the present invention, although the post-cycle impedance is improved compared to Examples 1-3, the cycle retention rate and battery volume growth rate at 60°C are both degraded compared to Examples 1-3. When the additive content is above the above range of the present invention, although the battery volume growth rate is improved compared to Examples 1-3, the cycle retention rate and post-cycle impedance at 60°C are both degraded compared to Examples 1-3.

[0077] The results of Examples 9-10 show that when the additive content is below the above-mentioned range of the present invention, the cycle retention rate at 60°C, the post-cycle impedance, and the battery volume growth rate all deteriorate compared to Examples 4-6. When the additive content is above the above-mentioned range of the present invention, although the battery volume growth rate is improved compared to Examples 4-6, the cycle retention rate at 60°C and the post-cycle impedance both deteriorate compared to Examples 4-6.

[0078] In summary, by adding the additive of the present invention to the electrolyte of a lithium-ion battery, especially a lithium-ion battery using a high-nickel positive electrode active material, and especially adding the additive of the present invention in the specific amount specified in the present invention, the high-temperature cycle performance of the lithium-ion battery can be effectively improved, and oxygen evolution can be suppressed and gas production can be reduced.

[0079] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. An electrolyte for a lithium ion battery, comprising an additive, an organic solvent and a lithium salt, characterized in that: The additive has the structure of the following formula (1): wherein R1, R2 and R3 are each independently a cyano group, a pyridyl group or a thienyl group having 5 to 15 carbon atoms and substituted by at least one fluorine atom.

2. The electrolyte according to claim 1, characterized in that The additive is selected from at least one of the following compounds:

3. The electrolyte according to claim 1 or 2, characterized in that The amount of the additive is in the range of 0.5 parts by weight to 2 parts by weight based on 100 parts by weight of the organic solvent and the lithium salt.

4. The electrolyte according to claim 1 or 2, characterized in that The organic solvent includes cyclic carbonate, linear carbonate, or any combination thereof.

5. The electrolyte according to claim 4, characterized in that The organic solvent is selected from the group consisting of propylene carbonate, butylene carbonate, fluoroethylene carbonate, ethylene carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, dimethyl carbonate, or a combination thereof.

6. The electrolyte according to claim 1 or 2, characterized in that The lithium salt is selected from the group consisting of LiCl, LiBr, LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, LiAlCl4, Li2SiF6, or a combination thereof.

7. A lithium ion secondary battery, characterized in that: include: Positive electrode, Negative electrode, diaphragm, and The electrolyte according to any one of claims 1 to 6.

8. The lithium-ion secondary battery according to claim 7, wherein The positive electrode active material of the positive electrode sheet is a high-nickel positive electrode material.

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