Electrolyte and lithium ion battery

Abstract

The present application relates to the technical field of lithium ion battery, especially relates to an electrolyte and a lithium ion battery. The electrolyte comprises a lithium salt, an organic solvent and an additive, the additive comprises p-fluorobenzonitrile and a compound with a structure shown in formula I, wherein R1-R5 are each independently selected from any one of H, halogen, alkyl with 1-5 carbon atoms, alkyl with 1-5 carbon atoms containing sulfite, alkyl with 1-5 carbon atoms containing a silicon-oxygen bond, alkoxy with 1-5 carbon atoms and halogenated alkoxy with 1-5 carbon atoms. The mass of p-fluorobenzonitrile is 0.01%-3% of the total mass of the electrolyte, and the mass of the compound with the structure shown in formula I is 0.1%-5% of the total mass of the electrolyte. The electrolyte improves the high-temperature storage gas production performance and low-temperature cycle performance of the lithium ion battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to an electrolyte and a lithium ion battery. BACKGROUND

[0002] Lithium ion batteries with graphite negative electrodes have been widely used in portable electronic devices, electric tools, electric vehicles and other fields. However, due to the low working potential of the graphite negative electrode, when charging at low temperature (lithium ions are inserted between the layers of graphite), it is difficult for lithium ions to pass through the passivation film, and the increase in polarization will result in low lithium ion insertion capacity, and even lithium deposition on the surface of graphite, causing the battery to fail at low temperature. Therefore, accelerating the diffusion kinetics of lithium ions in the passivation film is of great significance to improve the low-temperature performance of graphite and its battery. At the same time, the nickel in high-nickel ternary positive electrode materials has a strong catalytic effect on the oxidative decomposition of the electrolyte, especially at high temperatures, the nickel deposited in the electrode material will exacerbate the oxidative decomposition of the electrolyte and exacerbate the gas production problem. Therefore, by using certain electrolyte additives, a stable passivation film with small impedance can be formed on the surface of the positive electrode and the negative electrode, which is of great significance to improve the performance of lithium ion batteries.

[0003] In view of this, the present application is proposed. SUMMARY

[0004] The first object of the present application is to provide an electrolyte which can form a stable passivation film with small impedance on the surface of the positive electrode and the negative electrode, reduce the polarization of the battery at low temperature, and thus improve the high-temperature storage gas production performance and low-temperature cycle performance of the lithium ion battery.

[0005] The second object of the present application is to provide a lithium ion battery with excellent low-temperature cycle capacity retention rate and less high-temperature storage gas production.

[0006] In order to achieve the above object of the present application, the following technical scheme is adopted:

[0007] The present application provides an electrolyte comprising a lithium salt, an organic solvent and an additive, the additive comprising p-fluorobenzonitrile and a compound having a structure as shown in Formula I:

[0008] ;

[0009] Formula I

[0010] wherein R1~R5 are each independently selected from any one of H, halogen, alkyl with carbon atom number of 1~5, sulfite-containing alkyl with carbon atom number of 1~5, siloxane bond-containing alkyl with carbon atom number of 1~5, alkoxy with carbon atom number of 1~5 and halogenated alkoxy with carbon atom number of 1~5;

[0011] The mass of the p-fluorobenzonitrile is 0.01% to 3% of the total mass of the electrolyte;

[0012] The mass of the compound with the structure shown in Formula I is 0.1% to 5% of the total mass of the electrolyte.

[0013] Furthermore, at least one of R1 to R5 possesses at least one of the following groups:

[0014] -H, -CH3, -F, -Cl, -Br, , and .

[0015] Furthermore, the compound with the structure shown in Formula I is selected from any one or more of the following structural formulas:

[0016] ; ; ; ; ; ; ; ; .

[0017] Furthermore, the additive also includes chlorosulfonyl isocyanate.

[0018] Furthermore, the electrolyte shall satisfy at least one of the following conditions:

[0019] (1) The mass of the chlorosulfonyl isocyanate is 0.01% to 2% of the total mass of the electrolyte;

[0020] (2) The mass ratio of the compound with the structure shown in Formula I, p-fluorobenzonitrile and chlorosulfonyl isocyanate is (1~40):(1~20):(1~10).

[0021] Furthermore, the lithium salt includes lithium hexafluorophosphate; the mass of the lithium hexafluorophosphate is 12% to 15% of the total mass of the electrolyte.

[0022] Furthermore, the lithium salt further includes a second lithium salt, which includes one or more of lithium tetrafluoroborate, lithium bis(oxalate)borate, lithium difluorooxalateborate, lithium difluorodi(oxalate)phosphate, lithium tetrafluorooxalate phosphate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethylsulfonyl)imide; the mass of the second lithium salt is 0.01% to 6% of the total mass of the electrolyte.

[0023] Further, the organic solvent is selected from one or more of propylene carbonate, ethylene carbonate, butene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, γ-butyrolactone, methyl acetate, ethyl acetate, dimethyl sulfoxide, and sulfolane.

[0024] The present invention also provides a lithium-ion battery, comprising a positive electrode active material, a negative electrode active material and an electrolyte as described above.

[0025] Furthermore, the chemical formula of the positive electrode active material includes Li a Ni x Co y Mn z M e O2, wherein 0.9≤a≤1.1, 0.6≤x≤0.94, 0<y≤0.2, 0<y≤0.2, 0≤e≤0.1, and x+y+z=1, and M contains at least one of Al, Zr, Sr, Ti, B, Mg, Sn, W, Y, Ba, Nb, Mo, Ta, Si, La, Er, Nd, Gd, Ce.

[0026] Furthermore, the negative electrode active material includes at least one of mesophase carbon microspheres, artificial graphite, natural graphite, graphene, and carbon nanotubes.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0028] The electrolyte additive of this invention includes p-fluorobenzonitrile and benzenesulfonyl isocyanate compounds. P-fluorobenzonitrile effectively inhibits electrolyte decomposition, thereby reducing battery gas generation. The benzenesulfonyl isocyanate compounds contain isocyanate groups and sulfur atoms, resulting in a passivation film with good performance and low impedance, protecting the electrodes while reducing polarization. The sulfur-containing compounds contribute to improving the ionic conductivity of the passivation film, thereby reducing battery polarization at low temperatures; the isocyanate groups facilitate the formation of stable passivation films on the positive and negative electrode surfaces, respectively, inhibiting electrolyte decomposition and suppressing the increase in interfacial impedance. The electrolyte of this invention improves the high-temperature storage gas generation performance and low-temperature cycling performance of lithium-ion batteries. Detailed Implementation

[0029] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0030] The following is a detailed description of an electrolyte and a lithium-ion battery according to an embodiment of the present invention.

[0031] In some embodiments of the present invention, an electrolyte is provided, comprising a lithium salt, an organic solvent, and an additive, the additive comprising p-fluorobenzonitrile and a compound having a structure as shown in Formula I: Formula I;

[0032] R1 to R5 are each independently selected from H, halogen, alkyl group with 1 to 5 carbon atoms, alkyl group containing sulfite with 1 to 5 carbon atoms, alkyl group containing silicon-oxygen bond with 1 to 5 carbon atoms, alkoxy group with 1 to 5 carbon atoms, and haloalkoxy group with 1 to 5 carbon atoms.

[0033] The mass of p-fluorobenzonitrile is 0.01% to 3% of the total mass of the electrolyte;

[0034] The mass of the compound with the structure shown in Formula I is 0.1% to 5% of the total mass of the electrolyte.

[0035] In the electrolyte additive of the present invention, p-fluorobenzonitrile (EI) can inhibit the decomposition of the electrolyte, thereby suppressing battery gas generation, while the compound with the structure shown in Formula I can effectively form a low-resistance passivation film at the positive and negative electrode interfaces, improving the ion conductivity of the battery, protecting the electrodes while reducing polarization. The electrolyte additive of the present invention improves the high-temperature storage gas generation performance and low-temperature cycling performance of lithium-ion batteries. Typically, but not limitingly, the mass of p-fluorobenzonitrile is 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3% of the total mass of the electrolyte, or any combination thereof. Typically, but not limitingly, the mass of the compound with the structure shown in Formula I is 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5% of the total mass of the electrolyte, or any combination thereof. Too little additive may result in a passivation film that is too thin on the positive and negative electrode surfaces and is easily broken. Too much additive may result in an SEI film that is too thick on the positive and negative electrode surfaces, increasing the battery impedance and polarization.

[0036] In some embodiments of the present invention, at least one of R1 to R5 comprises at least one of the following groups:

[0037] -H, -CH3, -F, -Cl, -Br, , and .

[0038] In some embodiments of the present invention, the compound with the structure shown in Formula I is selected from one or more of the following structural formulas:

[0039] (A1); (A2); (A3); (A4); (A5); (A6); (A7); (A8); (A9).

[0040] In some embodiments of the present invention, the additive further includes chlorosulfonyl isocyanate (FI). FI is a small molecule compound with low diffusion resistance in the electrolyte, thus it first reaches the electrode surface and decomposes on the electrode surface to form a nitrogen-containing heteroatom film, thereby effectively inhibiting the decomposition of the electrolyte solvent and improving the interfacial ion conductivity. FI is a nitrogen-containing sacrificial additive; after it forms a film on the electrode surface, the FI content in the system decreases rapidly. At this time, a dynamic process of dissolution-formation exists in the passivation film. The compound with the structure shown in Formula I will decompose on the electrode surface to form a film, thereby forming a more stable passivation film because its reduction product has a longer carbon chain and can be better adsorbed on the graphite anode surface.

[0041] In some embodiments of the present invention, the electrolyte satisfies at least one of the following conditions:

[0042] (1) The mass of chlorosulfonyl isocyanate is 0.01% to 2% of the total mass of the electrolyte;

[0043] (2) The mass ratio of the compound with the structure shown in Formula I, p-fluorobenzonitrile and chlorosulfonyl isocyanate is (1~40):(1~20):(1~10).

[0044] Typically, but not limitingly, the mass of chlorosulfonyl isocyanate is in the range of 0.1%, 0.5%, 1%, 1.5%, 2%, or any combination thereof, of the total mass of the electrolyte. Reasonably controlling the mass ratio of the compound with the structure shown in Formula I, p-fluorobenzonitrile, and chlorosulfonyl isocyanate is beneficial for promoting the full utilization of their respective advantages in the electrolyte, thereby improving the overall performance of the battery. Preferably, the mass ratio of the compound with the structure shown in Formula I, p-fluorobenzonitrile, and chlorosulfonyl isocyanate is (10~25):(8~15):(1~5).

[0045] In some embodiments of the present invention, the lithium salt includes lithium hexafluorophosphate (LiPF6).

[0046] Lithium hexafluorophosphate has moderate ion transference number, moderate dissociation constant, good oxidation resistance and good aluminum foil passivation ability in commonly used non-aqueous organic solvents, and can be matched with various positive and negative electrode materials, making it the most important lithium salt in lithium-ion batteries.

[0047] In some embodiments of the invention, the mass of lithium hexafluorophosphate is 12% to 15% of the total mass of the electrolyte; typically, but not limitingly, for example, the mass of lithium hexafluorophosphate is 12%, 13%, 14%, 15% of the total mass of the electrolyte or any combination thereof.

[0048] If the concentration of lithium hexafluorophosphate in the electrolyte is too low, it will affect the conductivity of the electrolyte; if the concentration is too high, it will increase the viscosity of the electrolyte, which will also affect the conductivity.

[0049] In some embodiments of the present invention, the lithium salt further includes a second lithium salt, which includes one or more of lithium tetrafluoroborate (LiBF4), lithium bis(oxalate-borate)borate (LiBOB), lithium difluorooxalate-borate (LiDFOB), lithium difluorodi(oxalate-phosphate) (LiDFOP), lithium tetrafluorooxalate-phosphate, lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI); preferably, the second lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI).

[0050] In some embodiments of the present invention, the mass of the second lithium salt is 0.01% to 6% of the total mass of the electrolyte; typically, but not limitingly, for example, the mass of the second lithium salt is 0.01%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6% or any combination thereof of the total mass of the electrolyte.

[0051] The second lithium salt, as an auxiliary lithium salt, plays a role in improving the stability of the electrolyte and the lithium ion transference number. If the concentration is too high, the viscosity of the electrolyte will increase, affecting the conductivity of the electrolyte.

[0052] In some embodiments of the present invention, the organic solvent is selected from one or more of propylene carbonate (PC), ethylene carbonate (EC), butenyl carbonate (BC), diethyl carbonate (DEC), dimethyl carbonate (DMC), methyl ethyl carbonate (EMC), methyl propyl carbonate (MPC), dioxane, 1,2-dimethoxyethane, tetrahydrofuran (THF), 2-methyltetrahydrofuran, γ-butyrolactone (γ-GBL), methyl acetate, ethyl acetate (EA), dimethyl sulfoxide, and sulfolane (TMS).

[0053] In some embodiments of the present invention, a method for preparing the above-mentioned electrolyte is also provided, comprising the following steps:

[0054] (A) At room temperature, lithium salt is added to the dehydrated organic solvent one by one under an inert atmosphere, and the mixture is stirred and cooled to obtain a colorless and transparent liquid.

[0055] (B) Adding additives to a colorless and transparent liquid yields an electrolyte.

[0056] In some embodiments of the present invention, the addition of lithium salt releases heat, causing the electrolyte temperature to rise and resulting in a certain degree of thermal decomposition of the lithium salt. Therefore, when adding lithium salt, dry ice is used to cool the electrolyte. Lithium salt can continue to be added when the electrolyte temperature rises by no more than 2°C.

[0057] In some embodiments of the present invention, a lithium-ion battery is also provided, comprising a positive electrode active material, a negative electrode active material, and an electrolyte as described above.

[0058] In some embodiments of the present invention, the chemical formula of the positive electrode active material includes Li a Ni x Co y Mn z M e O2, wherein 0.9≤a≤1.1, 0.6≤x≤0.94, 0<y≤0.2, 0<y≤0.2, 0≤e≤0.1, and x+y+z=1, and M contains at least one of Al, Zr, Sr, Ti, B, Mg, Sn, W, Y, Ba, Nb, Mo, Ta, Si, La, Er, Nd, Gd, Ce.

[0059] In some embodiments of the present invention, the negative electrode active material includes at least one of mesophase carbon microspheres, artificial graphite, natural graphite, graphene, and carbon nanotubes.

[0060] The present invention will now be described in detail with reference to specific embodiments.

[0061] The examples and comparative examples respectively provide an electrolyte and a lithium-ion battery containing the electrolyte. The composition of the electrolyte is shown in Table 1.

[0062] The preparation method of lithium-ion batteries includes the following steps:

[0063] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes. After winding, hot pressing and shaping, and welding of the tabs, a bare cell is obtained. The bare cell is placed in an outer aluminum-plastic film and baked in an oven at 85±10℃ for 24 hours. The electrolyte is injected into the dried battery, and the battery is allowed to stand, form, and undergo capacity testing to obtain a lithium-ion battery.

[0064] The preparation method of the positive electrode sheet includes the following steps:

[0065] The positive electrode active material Li(Ni) 0.8 Mn 0.1 Co 0.1O2 (NMC811), conductive agent acetylene black (Super P), and binder polyvinylidene fluoride (PVDF) are mixed evenly in a mass ratio of NMC811:Super P:PVDF=94:3:3, and then uniformly dispersed in 1-methyl-2-pyrrolidone (NMP) to form a uniform black slurry. The slurry is coated on both sides of aluminum foil, and after baking, rolling, and cutting, the positive electrode sheet is obtained.

[0066] The method for preparing the negative electrode sheet includes the following steps:

[0067] The negative electrode active material graphite, conductive agent acetylene black (Super P) and binder SBR are mixed evenly in a mass ratio of graphite:SuperP:SBR=94:3:3, and then evenly dispersed in deionized water to make a uniform black slurry. The slurry is coated on both sides of copper foil, and after baking, rolling, and cutting, the negative electrode sheet is obtained.

[0068] The method for preparing the electrolyte includes the following steps:

[0069] At room temperature, in a glove box filled with argon (H2O < 1 ppm, O2 < 1 ppm), after removing water from the organic solvent with a 4 Å molecular sieve, lithium salt is added to the organic solvent one by one while continuously stirring and cooling. When the electrolyte temperature rises by no more than 2 °C, lithium salt can be added again, and finally a colorless and transparent liquid is obtained. Additives are added and the mixture is stirred evenly to obtain the electrolyte.

[0070] The low-temperature cycling performance test method is as follows: At -30±2℃, the lithium-ion batteries prepared in the examples and comparative examples are subjected to charge-discharge cycle tests at a charge-discharge rate of 1C / 1C within the range of 2.8~4.4V, and the discharge specific capacity of the battery in the first cycle and the discharge specific capacity after 500 cycles are recorded. The low-temperature capacity retention rate after 500 cycles = discharge specific capacity after 500 cycles / discharge specific capacity in the first cycle * 100%.

[0071] The test method for room temperature cycling performance is as follows: At 25±2℃, the lithium-ion batteries obtained in the examples and comparative examples are subjected to charge-discharge cycle tests at a charge-discharge rate of 1C / 1C within the range of 3.5~4.8V, and the discharge specific capacity of the battery in the first cycle and the discharge specific capacity after 500 cycles are recorded. The capacity retention rate after 500 cycles = discharge specific capacity after 500 cycles / discharge specific capacity in the first cycle * 100%.

[0072] Test method for gas generation performance during high-temperature storage: The initial volume V0 is tested by the water displacement method. The fully charged core is placed in a high and low temperature chamber at 70℃±2℃ and left for 30 days. The volume V1 is tested after that, and the expansion rate A is calculated.

[0073] The calculation method for the volume expansion rate at 70℃ is: A = (V1 - V0) / V0

[0074] Table 1. Composition and content of substances in the electrolyte.

[0075]

[0076] Note: The amount of lithium salt and additives refers to their content in the total mass of the electrolyte, and the ratio of organic solvents refers to the mass ratio.

[0077] As can be seen from Examples 1-15 and Comparative Example 1 in Table 1, the room temperature cycle performance, low temperature cycle performance, and high temperature gas generation performance of lithium-ion batteries are all improved when p-fluorobenzonitrile, compounds with the structure shown in Formula I, and chlorosulfonyl isocyanate are present in the electrolyte. This is mainly because the compounds with the structure shown in Formula I are benzenesulfonyl isocyanate compounds, which contain sulfur atoms. The participation of sulfur atoms in film formation helps to improve the ionic conductivity of lithium ions in the passivation film, which is beneficial to the transport of lithium ions in the passivation film, thereby reducing the polarization of the battery at low temperatures and improving the low temperature cycle performance of lithium-ion batteries. Furthermore, the compounds with the structure shown in Formula I can form a stable passivation film on the surfaces of the positive and negative electrodes, inhibiting electrolyte decomposition and suppressing the increase in interfacial impedance, thus improving the high temperature storage gas generation performance of lithium-ion batteries.

[0078] As can be seen from Examples 1 to 6, as the content of the compound with the structure shown in Formula I increases, the low-temperature cycle performance and high-temperature storage gas generation performance of lithium-ion batteries are effectively improved. When the ratio of the compound with the structure shown in Formula I, p-fluorobenzonitrile, and chlorosulfonyl isocyanate is 20:10:1, the performance of lithium-ion batteries is better.

[0079] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An electrolyte, characterized in that, It includes lithium salts, organic solvents, and additives, said additives including p-fluorobenzonitrile, compounds having the structure shown in Formula I, and chlorosulfonyl isocyanates: ； Formula I R1 to R5 are each independently selected from H, halogen, alkyl group with 1 to 5 carbon atoms, alkyl group containing sulfite with 1 to 5 carbon atoms, alkyl group containing silicon-oxygen bond with 1 to 5 carbon atoms, alkoxy group with 1 to 5 carbon atoms, and haloalkoxy group with 1 to 5 carbon atoms. The mass of the p-fluorobenzonitrile is 0.01% to 3% of the total mass of the electrolyte; The mass of the compound with the structure shown in Formula I is 0.1% to 5% of the total mass of the electrolyte; The mass of the chlorosulfonyl isocyanate is 0.01% to 2% of the total mass of the electrolyte.

2. The electrolyte according to claim 1, characterized in that, At least one of R1 to R5 has at least one of the following groups: -H, -CH3, -F, -Cl, -Br, , and .

3. The electrolyte according to claim 1, characterized in that, The compound with the structure shown in Formula I is selected from any one or more of the following structural formulas: ； ； ； ； ； ； ； ； 。 4. The electrolyte according to claim 1, characterized in that, The mass ratio of the compound with the structure shown in Formula I, p-fluorobenzonitrile and chlorosulfonyl isocyanate is (1~40):(1~20):(1~10).

5. The electrolyte according to claim 1, characterized in that, The lithium salt includes lithium hexafluorophosphate; the mass of the lithium hexafluorophosphate is 12% to 15% of the total mass of the electrolyte.

6. The electrolyte according to claim 5, characterized in that, The lithium salt further includes a second lithium salt, which includes one or more of lithium tetrafluoroborate, lithium bis(oxalate)borate, lithium difluorooxalateborate, lithium difluorodi(oxalate)phosphate, lithium tetrafluorooxalate phosphate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethylsulfonyl)imide; the mass of the second lithium salt is 0.01% to 6% of the total mass of the electrolyte.

7. A lithium-ion battery, characterized in that, It includes positive electrode active material, negative electrode active material and electrolyte as described in any one of claims 1 to 6.

8. The lithium-ion battery according to claim 7, characterized in that, The chemical formula of the positive electrode active material includes Li a Ni x Co y Mn z M e O2, wherein 0.9≤a≤1.1, 0.6≤x≤0.94, 0<y≤0.2, 0<y≤0.2, 0≤e≤0.1, and x+y+z=1, and M contains at least one of Al, Zr, Sr, Ti, B, Mg, Sn, W, Y, Ba, Nb, Mo, Ta, Si, La, Er, Nd, Gd, Ce.

9. The lithium-ion battery according to claim 7, characterized in that, The negative electrode active material includes at least one of mesophase carbon microspheres, artificial graphite, natural graphite, graphene, and carbon nanotubes.

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