Electrolyte and lithium-ion battery

Abstract

The present invention provides an electrolyte and a lithium-ion battery. The electrolyte comprises perfluoropolyether and fluoroether as electrolyte solvents, which are combined with lithium salts and additives to form the electrolyte. Compared with conventional carbonate electrolytes, the electrolyte can significantly increase the lithium ion migration number of the electrolyte and has better cycle performance under a 4.5V battery system. At the same time, since the solvent is non-flammable, the electrolyte can be made safer.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to lithium ion battery electrolyte and battery manufacturing. Background Art

[0002] Currently, commercial electrolytes mainly use carbonate organic solvents as electrolyte solvents. Carbonate solvents are easily oxidized and decomposed at a potential above 4.4V (Vs Li / Li+), causing gas bulging in the battery, less electrolyte decomposition affecting battery performance, and increasing decomposition products causing increased internal resistance of the battery, resulting in a decline in the performance of the entire lithium-ion battery system.

[0003] Perfluoropolyether (PFPE) is composed of multiple -CF2-O-CF2-ether bonds, has a low glass transition temperature, and a very wide range of temperatures from freezing point to boiling point. It has excellent heat resistance, chemical stability, oxidation stability, and is completely non-flammable. It is often used in lubricants. In addition, perfluoropolyether is an ether with good affinity for metallic lithium. Based on the above advantages, perfluoropolyether is feasible as an electrolyte solvent. Some experts and scholars have also made some attempts. Perfluoropolyether shows an extremely high lithium ion migration number when dissolving a small amount of lithium salt. However, perfluoropolyether has a large molecular weight, high viscosity, and weak ability to dissolve lithium salts. The electrolyte ion conductivity as a solvent is low, which limits its application in batteries.

[0004] Currently, many documents and patents describe perfluoropolyethers as additives and copolymers rather than direct electrolyte solvents. For example, CN112072169A, which relates to all-solid-state electrolytes, their compositions, electrodes, and all-solid-state lithium-ion batteries, uses perfluoropolyether as a copolymer, blended with PVDF-HFP and lithium salts to form a polymer to improve ionic conductivity and film mechanical strength. CN109802176A, which relates to electrolytes and lithium-ion batteries containing them, uses perfluoropolyether as an electrolyte additive to improve high-temperature storage performance, with the perfluoropolyether comprising less than 2% by weight of the total electrolyte. CN110444811A, which relates to a perfluoropolyether additive for lithium-ion battery anodes, pre-treats the perfluoropolyether additive onto the surface of the active material. During SEI layer formation, the PFPE additive reacts with the electrolyte to improve hydrophobicity and inhibit water diffusion and HF formation.

[0005] In the above technical solutions, perfluoropolyether is only used as an additive and copolymer, rather than as the main solvent of the electrolyte, and its characteristics of high lithium ion migration number, high oxidation potential and non-flammability in the electrolyte are not brought into play. Summary of the Invention

[0006] Traditional carbonate-based electrolytes are flammable and easily oxidized and decomposed at potentials above 4.4V (Vs Li / Li+). Against the backdrop of increasing battery energy density, it may be difficult to meet the operating voltage conditions of high-voltage battery systems (above 4.3V) and the needs for improved safety performance of electric vehicles in the future. In the era of popularization of electric vehicles, people's demand for improving the performance of electric vehicles is gradually increasing. Batteries need to meet higher rate discharge and faster charging capabilities, and it is even more urgent to increase the lithium ion migration number of the electrolyte.

[0007] The present invention uses perfluoropolyether and fluoroether as electrolyte solvents, and combines them with lithium salts and additives to form an electrolyte. Compared with conventional carbonate electrolytes, the electrolyte can increase the lithium ion migration number of the electrolyte, and the electrolyte is suitable for battery systems with an operating voltage of 4.4V or above. At the same time, because the solvent is non-flammable, the electrolyte can be made safer.

[0008] The present invention provides a lithium ion electrolyte comprising: perfluoropolyether, fluoroether, lithium salt and additives, wherein the perfluoropolyether has a molecular weight greater than 300 and has a chemical structure of formula (I):

[0009] R 1 -(CF2O) m -(CF2CF2O) n -(CF2) k -R 2 (Ⅰ)

[0010] In Formula I, n, k and m are independent integers ranging from 0 to 10, and R 1 and R 2 They can be methyl formate (-COOCH3), fluoromethyl (-CF3), or -OCF3 and fluorine atoms; preferably R 1 and R 2 At least one is a methyl formate group (-COOCH3), more preferably R 1 and R 2 All are methyl formate groups (-COOCH3).

[0011] Furthermore, the molecular weight of the perfluoropolyether is preferably 300 to 1800, and n, k and m in the structural formula are preferably n=1, m=1, k=1.

[0012] Preferably, the chemical structural formula of the fluoroether in the above-mentioned lithium ion electrolyte is formula (II):

[0013] C α F β H γ -OC x F y H z(Ⅱ)

[0014] In formula II, α, β, γ, x, y, and z are independent integers within the range of 0 to 12, and the molecular weight of the fluoroether is less than 500.

[0015] Furthermore, α and x are preferably in the range of 1 to 6 (i.e., the carbon chain length of the fluoroether is 2 to 12). β, γ, y, and z vary with α and x, and should satisfy 2*α+2=β+γ and 2*x=y+z. The molecular weight of the fluoroether is preferably 100 to 400. This is because a too high molecular weight will lead to a decrease in the amount of the fluoroether in the electrolyte (i.e., the number of molecules per mass is reduced). In addition, too long a carbon chain will lead to an increase in the viscosity of the fluoroether, while a shorter carbon chain will lower the boiling point of the fluoroether, limiting its scope of application and reducing its safety.

[0016] The lithium salt in the above-mentioned lithium ion electrolyte includes at least one of lithium hexafluorophosphate, lithium difluorooxalatoborate, lithium bisoxalatoborate, lithium tetrafluoroborate, lithium bis(trifluoromethanesulfonyl imide), lithium bis(fluorosulfonyl imide) and lithium trifluoroacetate;

[0017] The additive in the above-mentioned lithium ion electrolyte includes at least one of vinylene carbonate, 1,3-propane sultone, vinyl ethylene carbonate, ethylene sulfate, propylene sultone, succinonitrile, vinyl methyl sulfate, vinyl sulfite, methylene methanedisulfonate, tris(trimethylsilyl)phosphine, triethyl phosphate, hexafluorocyclotriphosphazene, 1,4-dicyano-2-butene, cyclopentane, and methyl ethyl sulfone;

[0018] The mass percentage of the perfluoropolyether in the above-mentioned lithium ion electrolyte is 0-60%, the mass percentage of the fluoroether in the electrolyte is 0-50%, the mass percentage of the lithium salt in the electrolyte is 0-30%, the mass percentage of the additive in the electrolyte is 0-10%, and the balance is a solvent, which can be a carbonate solvent, an ether solvent, a nitrile solvent, etc.

[0019] The present invention also relates to a battery prepared from the electrolyte.

[0020] The beneficial effects of the present invention are as follows: perfluoropolyether and fluoroether are used as electrolyte solvents, and are combined with lithium salts and additives to form an electrolyte. Compared with conventional carbonate electrolytes, the lithium ion migration number of the electrolyte can be increased, thereby enabling the battery to have a higher rate discharge and faster charging capacity; and it has better cycle performance under a 4.5V battery system. At the same time, the electrolyte can be made safer because the solvent is non-flammable. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is the first charge and discharge curve of Example 1;

[0022] Figure 2 The data are the cycle results of the embodiments and comparative examples;

[0023] Figure 3 The antioxidant test results of Example 1 and the comparative example are shown below:

[0024] Figure 4 This is the first charge and discharge curve of Example 2;

[0025] Figure 5 This is the first charge and discharge curve of Example 3;

[0026] Figure 6 This is the first charge and discharge curve of Example 4;

[0027] Figure 7 This is the first charge and discharge curve of the comparative example. DETAILED DESCRIPTION

[0028] The present invention will be further described below with reference to the embodiments, but the present invention is not limited thereto.

[0029] Example 1

[0030] Preparation of electrolyte: In a glove box filled with argon (oxygen content ≤ 0.5 ppm, water content ≤ 0.01 ppm), the components were mixed according to the following mass fractions to prepare the electrolyte: 59.09 wt% of perfluoropolyether-1#, molecular weight 336.03, with the structural formula: With 13.73wt% of dimethyl carbonate and 15.39wt% of 1,1,2,2-tetrafluoroethyl ethyl ether, the molecular weight is 146.08 and its structural formula is: Mixing uniformly, dissolving 8.78 wt% of lithium hexafluorophosphate in the mixed solvent and stirring uniformly, then adding 2% wt of vinylene carbonate and 1% wt of ethylene sulfate to the electrolyte and mixing uniformly to obtain an electrolyte;

[0031] Preparation of the positive electrode: Lithium cobalt oxide (LiCoO2) is mixed with conductive carbon black, carbon nanotubes, and polyvinylidene fluoride in a mass ratio of 95:1.5:0.5:2, along with N-methylpyrrolidone, and stirred to create a positive electrode slurry. This slurry is then coated onto aluminum foil, baked, rolled, and punched to form a 12mm diameter positive electrode disc. The disc is then dried to a moisture content of less than 100ppm.

[0032] Battery assembly: The prepared positive electrode disc and a 14 mm diameter lithium disc were assembled into a CR2032 button cell in a glove box with a 16 mm diameter double-sided ceramic diaphragm disc and the above electrolyte and allowed to stand for 10 h.

[0033] Battery cycle test: Tested using a BlueDian battery test system at 25°C. After standing for 30 minutes, the battery was charged at 1.6 mA / cm² to 4.55 V. After standing for 30 minutes, the battery was discharged at 1.6 mA / cm² to 3.0 V. The battery cycle was repeated 100 times.

[0034] Lithium ion migration number test: Use the prepared electrolyte, diaphragm, and lithium electrode plates of the working electrode and counter electrode to assemble into CR2032 button batteries. First, use the Princeton electrochemical workstation (P4000A) to perform AC impedance testing on the battery to obtain the bulk resistance RS and initial interface impedance RSEI. Then apply a polarization voltage V of 10mV. When the electrode decays to a relatively steady-state current Iss, stop applying the polarization voltage. Finally, perform AC impedance testing on the battery to obtain the bulk resistance RS2 and termination interface impedance RSEI2. The initial polarization current is calculated according to the formula Finally, through the formula: Obtain the lithium ion transfer number t(+);

[0035] Electrolyte oxidation resistance test: The positive electrode material prepared by the above positive electrode sheet was replaced with a lithium-rich manganese-based (Li 1.2 Mn 0.54 Ni 0.13 Co 0.13 O2), the other steps are the same as the positive electrode preparation steps; after completing the electrode preparation, the prepared electrolyte, diaphragm, the lithium-rich manganese-based positive electrode sheet prepared above and the lithium sheet are used to assemble a CR2032 button battery, which is tested using a blue battery test system. The battery is first charged and discharged once with a current of 1.6 mA / cm2 for activation, and then the oxidation resistance test is started. The battery is charged to 4.7 V with a current of 1.6 mA / cm2 and then maintained at a constant voltage for 10 hours. Then, the battery is charged to 4.8 V with a current of 1.6 mA / cm2 and then maintained at a constant voltage for 10 hours. Finally, the battery is charged to 4.9 V with a current of 1.6 mA / cm2 and then maintained at a constant voltage for 10 hours. The current size in the constant voltage stage is compared. A smaller current in the constant voltage stage corresponds to better oxidation resistance.

[0036] Flammability test: Use a 2+9+2 ceramic-coated PE diaphragm to immerse it in the prepared electrolyte for 5 minutes, then take out the diaphragm after absorbing the liquid and fix it with a clip. Ignite the diaphragm with absorbed electrolyte in air to observe whether it can burn;

[0037] See the attached first charge and discharge curve Figure 1 ;

[0038] See attached for the cycle results Figure 2 ;

[0039] The antioxidant results are shown in Figure 3 ;

[0040] The structure of lithium ion migration number is shown in Table I.

[0041] Attached from the loop results Figure 2 It can be seen that the perfluoropolyether with -CF3 substitution at both ends has a slightly worse cycle performance, which may be due to the weakening of the lithium salt dissociation ability caused by -CF3 substitution and the poor wetting effect with the diaphragm. Figure 3 It can be seen that the perfluoropolyether electrolyte system has a smaller reaction current under high-voltage float charge than the ordinary carbonate electrolyte system, which indicates that the perfluoropolyether system has better stability under high-voltage conditions.

[0042] Example 2

[0043] Preparation of electrolyte: In a glove box filled with argon (oxygen content ≤ 0.5 ppm, water content ≤ 0.01 ppm), the components were mixed according to the following mass fractions to prepare the electrolyte: 55.91 wt% of perfluoropolyether-2#, molecular weight 310.07, structural formula: With 14.89wt% dimethyl carbonate and 16.68wt% 1,1,2,2-tetrafluoroethyl ethyl ether, the molecular weight is 146.08 and its structural formula is:

[0044] Mixing uniformly, dissolving 9.52 wt% of lithium hexafluorophosphate in the mixed solvent and stirring uniformly, then adding 2% wt of vinylene carbonate and 1% wt of ethylene sulfate to the electrolyte and mixing uniformly to obtain an electrolyte;

[0045] The remaining steps are the same as in Example 1;

[0046] See the attached first charge and discharge curve Figure 4 ;

[0047] See attached for the cycle results Figure 2 ;

[0048] The structure of lithium ion migration number is shown in Table I.

[0049] Attached from the loop results Figure 2 It can be seen that the perfluoropolyether with -CF3 substituted at one end and -COOCH3 carbonate substituted at the other end has better cycle performance than the perfluoropolyether with -CF3 substituted at both ends, which may be because the -COOCH3 carbonate substitution can enhance the dissociation of lithium salt.

[0050] Example 3

[0051] Preparation of electrolyte: In a glove box filled with argon (oxygen content ≤ 0.5 ppm, water content ≤ 0.01 ppm), the components were mixed according to the following mass fractions to prepare the electrolyte: 55.25 wt% of perfluoropolyether-3#, molecular weight 350.12, structural formula: With 15.13wt% of dimethyl carbonate and 16.95wt% of 1,1,2,2-tetrafluoroethyl ethyl ether, the molecular weight is 146.08 and its structural formula is: Mixing uniformly, dissolving 9.67 wt% hexafluorophosphoric acid in the mixed solvent and stirring uniformly, then adding 2% wt vinylene carbonate and 1% wt ethylene sulfate to the electrolyte and mixing uniformly to obtain an electrolyte;

[0052] The remaining steps are the same as in Example 1;

[0053] See the attached first charge and discharge curve Figure 5 ;

[0054] See attached for the cycle results Figure 2 ;

[0055] The structure of lithium ion migration number is shown in Table I.

[0056] Attached from the loop results Figure 2 It can be seen that the perfluoropolyether with -COOCH3 carbonate groups substituted at both ends has the best cycle performance, which may be because the -COOCH3 carbonate groups substituted at both ends can enhance the solvation effect, and the perfluoropolyether has good high voltage stability.

[0057] Example 4

[0058] Preparation of electrolyte: In a glove box filled with argon (oxygen content ≤ 0.5 ppm, water content ≤ 0.01 ppm), the components were mixed according to the following mass fractions to prepare the electrolyte: 52.69 wt% of perfluoropolyether-1#, molecular weight 336.03, with the structural formula: With 14.42wt% of dimethyl carbonate and 20.67wt% of 1,1,2,2,3,3,4,4-octafluoro-5-(1,1,2,2-tetrafluoroethoxy)pentane, the molecular weight is 332.09 and its structural formula is: Mixing uniformly, dissolving 9.22 wt% hexafluorophosphoric acid in the mixed solvent and stirring uniformly, then adding 2% wt vinylene carbonate and 1% wt ethylene sulfate to the electrolyte and mixing uniformly to obtain an electrolyte;

[0059] The remaining steps are the same as in Example 1;

[0060] See the attached first charge and discharge curve Figure 6 ;

[0061] See attached for the cycle results Figure 2 ;

[0062] The structure of lithium ion migration number is shown in Table I.

[0063] Attached from the loop results Figure 2 It can be seen that replacing fluoroethers with high molecular weight and more fluorine substitutions leads to poor cycle performance, which may be because the introduction of high molecular weight fluoroethers reduces the solubility of lithium salts and increases the viscosity, resulting in a decrease in electrolyte performance.

[0064] Comparative Example

[0065] Preparation of the electrolyte: In an argon-filled glove box (oxygen content ≤ 0.5 ppm, water content ≤ 0.01 ppm), the components were prepared according to the following mass fractions: 33.70 wt% of ethylene carbonate, 27.25 wt% of dimethyl carbonate, and 24.49 wt% of diethyl carbonate, and 11.56 wt% of lithium hexafluorophosphate were dissolved in the mixed solvent and stirred uniformly, and then 2% wt of vinylene carbonate and 1% wt of ethylene sulfate were added to the electrolyte and uniformly mixed to obtain an electrolyte;

[0066] The remaining steps are the same as in Example 1;

[0067] See the attached first charge and discharge curve Figure 7 ;

[0068] The antioxidant results are shown in Figure 3 ;

[0069] See attached for the cycle results Figure 2 ;

[0070] The comparison of lithium ion migration number and flammability is shown in Table 1.

[0071] Attached from the loop results Figure 2 It can be seen that the cycle performance of ordinary carbonate electrolytes at high voltage is the worst, which may be due to the low oxidation potential of carbonate solvents leading to the decomposition of the electrolyte. Figure 3 It can be seen that carbonate electrolytes are not as stable as perfluoropolyether systems at high voltages, which also explains the poor cycle performance from another perspective.

[0072] Table 1 Comparison of lithium ion migration number and flammability

[0073] Group Lithium ion migration number Flammability Example 1 0.545 Non-flammable Example 2 0.504 Non-flammable Example 3 0.488 Non-flammable Example 4 0.493 Non-flammable Comparative Example 0.395 flammable

[0074] Traditional carbonate-based electrolytes are flammable and easily oxidized and decomposed at potentials above 4.4V (Vs Li / Li+), while fluorine-containing electrolyte solvent systems are non-flammable and have better antioxidant properties. In the future, with the popularization of electric vehicles, battery energy density continues to increase, and improving battery operating voltage and battery safety performance is becoming increasingly important. The present invention improves the lithium ion transport efficiency of the electrolyte by increasing the lithium ion migration number and reducing concentration polarization under high current. Figure 1-7 It can be clearly seen that the use of a combination of perfluoropolyether and fluoroether has the following advantages over conventional carbonate electrolytes: high lithium ion migration number, non-flammability, high cycle capacity retention rate under 4.55V conditions, and good oxidation resistance under high voltage float charge test.

[0075] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A lithium ion electrolyte, characterized in that perfluoropolyether and fluoroether are used as electrolyte solvents, and lithium salt and additives are combined to form the electrolyte; wherein the molecular weight of the perfluoropolyether is greater than 300, and the chemical structure of the perfluoropolyether is formula (I): R 1 -(CF2O) m -(CF2CF2O) n -(CF2) k -R 2 (Ⅰ) In Formula I, n, k and m are independent integers ranging from 1 to 10, and R 1 and R 2 are respectively selected from methyl formate (-COOCH3), or -CF3, or -OCF3, or fluorine atoms, and R 1 and R 2 At least one is a methyl formate group (-COOCH3); The chemical structure of fluoroether is formula (II): C α F β H γ -O-C x F y H z (Ⅱ) In formula II, the range of α and x is 1~6, and β, γ, y, and z change with α and x, and should satisfy and , the molecular weight of fluoroether is 100~400.

2. The lithium ion electrolyte according to claim 1, further characterized in that the lithium salt comprises at least one of lithium hexafluorophosphate, lithium difluorooxalatoborate, lithium bisoxalatoborate, lithium tetrafluoroborate, lithium bis(trifluoromethanesulfonyl imide), lithium bis(fluorosulfonyl imide) and lithium trifluoroacetate.

3. The lithium ion electrolyte according to claim 1 or 2, further characterized in that the additive comprises at least one of vinylene carbonate, 1,3-propane sultone, vinyl ethylene carbonate, ethylene sulfate, propene sultone, succinonitrile, vinyl methyl sulfate, vinyl sulfite, methylene methanedisulfonate, tris(trimethylsilyl)phosphine, triethyl phosphate, hexafluorocyclotriphosphazene, 1,4-dicyano-2-butene, cyclopentane, and methyl ethyl sulfone.

4. The lithium ion electrolyte according to claim 1, further characterized in that the molecular weight of the perfluoropolyether is 300-1800, and n, m, and k in the structural formula are n=1, m=1, and k=1, respectively.

5. The lithium ion electrolyte according to claim 1, 2 or 4, characterized in that: The mass percentage of perfluoropolyether in the electrolyte is 0-60%, the mass percentage of the fluoroether in the electrolyte is 0-50%, the mass percentage of the lithium salt in the electrolyte is 0-30%, the mass percentage of the additive in the electrolyte is 0-10%, and the balance is solvent.

6. The lithium ion electrolyte according to claim 3, wherein: The mass percentage of perfluoropolyether in the electrolyte is 0-60%, the mass percentage of the fluoroether in the electrolyte is 0-50%, the mass percentage of the lithium salt in the electrolyte is 0-30%, the mass percentage of the additive in the electrolyte is 0-10%, and the balance is solvent.

7. The lithium ion electrolyte according to claim 1, further characterized in that the chemical structure of perfluoropolyether is formula (I), R 1 and R 2 All are methyl formate groups (-COOCH3).

8. A battery prepared from the lithium ion electrolyte according to any one of claims 1 to 7.

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