Electrolyte and lithium ion battery

Abstract

The present application relates to the technical field of lithium ion battery, especially to an electrolyte and a lithium ion battery. The electrolyte provided by the present application comprises a lithium salt, a solvent, a diluent and an additive; the solvent comprises a compound containing at least one ester bond and / or at least one ether bond; the diluent comprises a fluorinated aromatic ether; and the additive comprises a compound containing a hexafluorophosphate group. The electrolyte of the present application uses a compound containing an ester bond and / or an ether bond as a solvent, so that it has excellent oxidation resistance and reduction capacity; the addition of a fluorinated aromatic ether makes it easier to form a SEI film rich in LiF on the lithium surface; the compound containing a hexafluorophosphate group can be preferentially decomposed on the surface of the positive and negative electrodes, thereby improving the interface stability, hindering the decomposition of the electrolyte, easily forming a passivation layer on the surface of the positive electrode, and inhibiting the corrosion effect of the lithium salt on the current collector at high voltage; thereby meeting the use requirements of the high-nickel-lithium metal system battery of the high-voltage system.

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] Developing lithium ion batteries with high energy density, longer cycle life and higher safety is the constant pursuit of competitive power sources for electric vehicles. The high energy density of lithium ion batteries depends largely on the positive and negative electrode materials used. Among them, the positive electrode material plays the most important role, because the properties of the positive electrode material are crucial to achieving high energy density and stability of lithium ion batteries. In recent years, nickel-rich layered oxides (Li-NMC) have attracted widespread attention as a highly potential positive electrode material due to their high capacity, high operating voltage and low cost. The use of high nickel content in the positive electrode can ensure the high capacity of the battery and reduce the cost of the material, such as LiNi x Mn y Co z O2and LiNi x Al y Co z O2(x+y+z=1,x≥8).

[0003] However, as the nickel content in the material increases, the material stability and stable operating voltage during cycling also decrease; during high-voltage discharge, the reduction of Ni releases oxygen that causes thermal runaway; the currently commonly used commercial carbonate electrolyte is prone to react with the carbonate solvent, leading to catastrophic failure of the battery, which will cause phase transition, capacity fading and voltage decay during long-term cycling, hindering the cycle performance. The root cause of these shortcomings is the continuous decomposition of the electrolyte due to the instability of the traditional carbonate electrolyte at high voltage, leading to undesirable surface reactions between the positive electrode material and the electrolyte, hindering the widespread application of high-nickel-based lithium ion batteries.

[0004] For high nickel-lithium metal system, the traditional carbonate solvent is not suitable for lithium metal battery because the traditional carbonate has poor compatibility with lithium metal, reacts with lithium to form alkyl lithium carbonate (ROCO2Li) organic SEI film, which cannot withstand the volume expansion of lithium deposition and thus continuously thickens, leading to rapid failure of lithium anode. Ether-based electrolyte has been used since the early stage of lithium metal battery research due to its good reduction stability, but it is not suitable for high nickel high voltage because the common ether solvent has poor oxidation stability and is difficult to meet the application requirements of high nickel high voltage lithium metal battery. In addition, it is well known that the safety performance of lithium metal battery is one of the problems limiting its commercialization. At present, the commonly used method to improve the safety of electrolyte is to use flame-retardant co-solvent or additive (nitrile, phosphate, fluorine reagent), but the flame-retardant solvent or additive is not friendly to the positive and negative electrode impedance, and is easy to make the SEI film and CEI film thicker and thicker under long cycle, the impedance increases, which seriously affects the rate performance of the battery.

[0005] In view of this, the present application is proposed. SUMMARY

[0006] The first object of the present application is to provide an electrolyte with excellent comprehensive performance, which can meet the use requirements of high nickel-lithium metal system battery.

[0007] The second object of the present application is to provide a lithium ion battery using the above-mentioned electrolyte, which is beneficial to improve the cycle performance, rate performance and safety performance of the battery.

[0008] In order to achieve the above-mentioned objects of the present application, the following technical solutions are adopted:

[0009] The present application provides an electrolyte comprising a lithium salt, a solvent, a diluent and an additive;

[0010] The solvent comprises a compound containing at least one ester bond and / or at least one ether bond;

[0011] The diluent comprises a fluorinated aromatic ether;

[0012] The additive comprises a compound containing a hexafluorophosphate group.

[0013] Further, the solvent comprises at least one of 2-((acetyloxy)methoxy)ethanol acetate, 2-oxa-1,4-butanediol diacetate, 2-(acetyloxymethoxy)ethoxymethyl acetate and methoxy methyl acetate.

[0014] Further, the diluent includes at least one selected from 3-fluoroanisole, 4-fluoroanisole, 2-fluoroanisole, pentafluoroanisole, m-fluoroanisole, 4-fluorobiphenyl ether, 2-fluorodiphenyl ether, bis(4-fluorophenyl) ether, 2,3-difluorophenylpropyl ether, 2,3-difluorophenylbutyl ether, 2,6-difluoroanisole, 2,4-difluoroanisole, 2,4-difluoroanisole, 2,3-difluoroanisole, 2,5-difluoroanisole, 2,3-difluoroanisole, and 2,6-difluoroanisole.

[0015] Further, the additive includes at least one of ethyl viologen hexafluorophosphate, 1-ethylpyridine hexafluorophosphate, 1-ethyl-2,3-dimethylimidazolium hexafluorophosphate, N,N-bis[4-(diethylamino)phenyl]-N',N'-diethyl-1,4-phenylenediamine hexafluorophosphate, (4-hydroxyphenyl)dimethylsulfonium hexafluorophosphate, (4-hydroxyphenyl)dimethylsulfonium hexafluorophosphate, tetraethylphosphonium hexafluorophosphate, triethylhexafluorophosphate, 1-vinyl-3-ethylimidazolium hexafluorophosphate, 1-methyl-1-ethylpyrrolidineonium hexafluorophosphate, trimethylaniline hexafluorophosphate, 1-hexylpyridine hexafluorophosphate, 1-butylpyridine hexafluorophosphate, and 1-vinyl-3-butylimidazolium hexafluorophosphate.

[0016] Furthermore, the lithium salt includes lithium bisfluorosulfonylimide and / or lithium bistrifluoromethylsulfonylimide.

[0017] Furthermore, it also includes a co-solvent; said co-solvent includes at least one of tris(2,2,2-trifluoroethane) phosphite, tris(2,2,2-trifluoroethyl) phosphate, and bis(2,2,2-trifluoroethyl) phosphite.

[0018] Furthermore, the content of the additive in the electrolyte is 0.001 wt% to 5 wt%.

[0019] And / or, in the electrolyte, the concentration of the lithium salt is 0.8 to 2.5 mol / L.

[0020] Further, the volume ratio of the solvent to the diluent is (0.5-5):(5-9.5).

[0021] Further, the volume ratio of the solvent, the co-solvent, and the diluent is (0.25-3):(0.25-2):(5-9.5).

[0022] The present invention also provides a lithium-ion battery comprising the electrolyte as described above.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] The electrolyte of this invention uses a compound containing at least one ester bond and / or at least one ether bond as a solvent, giving it excellent antioxidant and reducing properties. The addition of fluoroaromatic ethers makes it easier to form a LiF-rich SEI film on the lithium surface. Compounds containing hexafluorophosphate groups preferentially decompose on the positive and negative electrode surfaces, improving interface stability, hindering electrolyte decomposition, and easily forming a passivation layer on the positive electrode surface, inhibiting the corrosion of aluminum current collectors by lithium salts under high voltage. Thus, it can meet the requirements of high-voltage high-nickel-lithium metal system batteries, which is beneficial to improving the cycle performance, rate performance, and safety performance of the battery. Detailed Implementation

[0025] The technical solution of the present invention will be clearly and completely described below with specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, 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.

[0026] In some embodiments of the present invention, an electrolyte is provided, comprising a lithium salt, a solvent, a diluent, and an additive;

[0027] Solvents include compounds containing at least one ester bond and / or at least one ether bond;

[0028] Diluents include fluoroaromatic ethers;

[0029] Additives include compounds containing hexafluorophosphate groups.

[0030] This invention combines the advantages of ethers and esters by using a compound containing at least one ester bond (-COO-) and / or at least one ether bond (-O-) as a solvent. It possesses both excellent antioxidant properties and reducing ability, making it suitable for high-voltage high-nickel-lithium metal system batteries and relatively lithium-friendly.

[0031] This invention uses fluoroaromatic ethers as diluents. Benzene and its derivatives have low polarity and high symmetry, and fluorination of benzene can further improve its antioxidant properties. At the same time, because the symmetry of fluorobenzene is disrupted, it has a certain impact on the conjugated bonds of benzene. Therefore, the CF bonds on fluorobenzene are more reactive than ordinary CF bonds, and it is easier to form a LiF-rich SEI film on the lithium surface compared with ordinary fluoroethers.

[0032] This invention utilizes compounds containing hexafluorophosphate groups as additives, which are superior to solvents and diluents in their ability to decompose on the surfaces of the positive and negative electrodes, thus improving interfacial stability and hindering electrolyte decomposition. The phosphate groups readily form a passivation layer on the positive electrode surface, inhibiting the corrosion of the aluminum current collector by lithium salts under high voltage. The high phosphorus content enhances the electrolyte's cycle stability. Furthermore, the addition of a co-solvent to the electrolyte can suppress excessive decomposition of the co-solvent, preventing the adverse impedance effects of flame-retardant solvents on both positive and negative electrodes and ensuring unaffected rate performance.

[0033] In some embodiments of the present invention, the solvent includes at least one of 2-((acetoxy)methoxy)ethanol acetate (CAS59278-00-1), 2-oxa-1,4-butanediol diacetate (CAS 67429-12-3), 2-(acetoxymethoxy)ethoxymethyl acetate (CAS 90114-17-3), and methoxymethyl acetate (CAS 4382-76-7).

[0034] In some embodiments of the present invention, the diluent includes at least one selected from 3-fluoroanisole, 4-fluoroanisole, 2-fluoroanisole, pentafluoroanisole, m-fluoroanisole, 4-fluorobiphenyl ether, 2-fluorodiphenyl ether, bis(4-fluorophenyl) ether, 2,3-difluorophenylpropyl ether, 2,3-difluorophenylbutyl ether, 2,6-difluoroanisole, 2,4-difluoroanisole, 2,4-difluoroanisole, 2,3-difluoroanisole, 2,5-difluoroanisole, 2,3-difluoroanisole, and 2,6-difluoroanisole.

[0035] In some embodiments of the present invention, the additives include ethyl viologen dihexafluorophosphate (CAS 138926-07-5), 1-ethylpyridine hexafluorophosphate (CAS 103173-73-5), 1-ethyl-2,3-dimethylimidazolium hexafluorophosphate (CAS 292140-86-4), N,N-bis[4-(diethylamino)phenyl]-N',N'-diethyl-1,4-phenylenediamine hexafluorophosphate (CAS 199293-45-3), (4-hydroxyphenyl)dimethylsulfonium hexafluorophosphate (CAS 59626-68-5), and (4-hydroxyphenyl)dimethylsulfonium hexafluorophosphate (CAS 59626-68-5). The following are included: at least one of the following: 59626-68-5), tetraethylphosphonium hexafluorophosphate (CAS111928-07-5), triethylhexafluorophosphate (CAS17950-40-2), 1-vinyl-3-ethylimidazolium hexafluorophosphate (CAS 1034364-43-6), 1-methyl-1-ethylpyrrolidine onium hexafluorophosphate (CAS121057-90-7), trimethylaniline hexafluorophosphate (CAS2932-48-1), 1-hexylpyridine hexafluorophosphate (CAS797789-00-5), 1-butylpyridine hexafluorophosphate (CAS186088-50-6), and 1-vinyl-3-butylimidazolium hexafluorophosphate (CAS 915358-85-9).

[0036] The aforementioned additives preferentially decompose on the positive electrode surface compared to solvents, forming a passivation layer that inhibits electrolyte decomposition at the electrode-electrolyte interface. The negative electrode surface readily forms a layer rich in LiF and Li. x PO y F z SEI membrane.

[0037] In some embodiments of the present invention, the lithium salt includes lithium bis(fluorosulfonyl)imide and / or lithium bis(trifluoromethyl)sulfonyl)imide.

[0038] In some embodiments of the present invention, the electrolyte further includes a co-solvent; the co-solvent includes at least one of tris(2,2,2-trifluoroethane) phosphite, tris(2,2,2-trifluoroethyl) phosphate, and bis(2,2,2-trifluoroethyl) phosphite.

[0039] The addition of a co-solvent to the electrolyte of this invention helps to improve the flame retardancy of the electrolyte and enhance battery safety performance.

[0040] In some embodiments of the present invention, the content of the additive in the electrolyte is 0.001 wt% to 5 wt%; typically, but not limitingly, for example, the content of the additive in the electrolyte may be a range of 0.001 wt%, 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt%, 4 wt%, 4.5 wt%, 5 wt%, or any combination thereof.

[0041] In some embodiments of the present invention, the concentration of lithium salt in the electrolyte is 0.8 to 2.5 mol / L; typically, but not limitingly, for example, the concentration of lithium salt in the electrolyte can be a range of 0.8 mol / L, 1 mol / L, 1.3 mol / L, 1.5 mol / L, 2 mol / L, 2.3 mol / L, 2.5 mol / L, or any combination thereof.

[0042] In some embodiments of the present invention, when no co-solvent is added to the electrolyte, the volume ratio of solvent to diluent is (0.5 to 5):(5 to 9.5); typically, but not limitingly, for example, the volume ratio of solvent to diluent can be 0.5:9.5, 1:9, 1.5:8.5, 2:8, 2.5:7.5, 3:7, 3.5:6.5, 4:6, 4.5:5.5, 5:5, or a range of any two thereof.

[0043] In some embodiments of the present invention, when a co-solvent is added to the electrolyte, the volume ratio of solvent, co-solvent, and diluent is (0.25–3):(0.25–2):(5–9.5); typically, but not limitingly, for example, the volume ratio of solvent, co-solvent, and diluent can be 0.25:0.25:9.5, 0.5:0.5:9, 1:1:8, 1.5:1.5:7, 2:2:6, 3:2:5, or a range of any two thereof.

[0044] In some embodiments of the present invention, a method for preparing an electrolyte is also provided, comprising the following steps:

[0045] The components are mixed thoroughly to obtain the electrolyte.

[0046] In some embodiments of the present invention, a lithium-ion battery comprising the above-described electrolyte is also provided.

[0047] In some embodiments of the present invention, the lithium-ion battery further includes a positive electrode material, the positive electrode material including LiNi. x Mn y Co z O2 and / or LiNi x Al y Co z O2(x+y+z=1,x≥8).

[0048] The lithium-ion battery of the present invention belongs to the high-voltage, high-nickel-lithium metal system battery. Using the electrolyte of the present invention is beneficial to improving the cycle performance, rate performance and safety performance.

[0049] Examples 1-5

[0050] The electrolyte preparation method provided in this embodiment includes the following steps:

[0051] The components are mixed thoroughly to obtain the electrolyte.

[0052] The components and amounts of the electrolytes provided in each embodiment are shown in Table 1; the volume ratios in Table 1 are the volume ratios of solvent, co-solvent, and diluent.

[0053] Table 1

[0054]

[0055]

[0056] Comparative Examples 1-3

[0057] The method for preparing the electrolyte provided in this comparative example includes the following steps:

[0058] The components are mixed thoroughly to obtain the electrolyte.

[0059] The components and amounts of the electrolytes provided in each comparative example are shown in Table 2; the volume ratios in Table 2 are the volume ratios of solvent, co-solvent, and diluent.

[0060] Table 2

[0061]

[0062] Experimental Example 1

[0063] Preparation of the positive electrode sheet: LiNi9Co1Mn1O2, a ternary material of lithium nickel cobalt manganese oxide, conductive agent SuperP, binder PVDF and carbon nanotubes (CNT) are mixed evenly at a mass ratio of 96.5:2.5:1.0:0.5 to prepare a lithium-ion battery positive electrode slurry of a certain viscosity. The slurry is coated on aluminum foil for current collector, dried at 85°C and then cold-pressed. After trimming, cutting and slitting, the slurry is dried at 85°C under vacuum for 8 hours to prepare a lithium metal battery positive electrode sheet that meets the requirements.

[0064] The negative electrode sheet uses 8μm thick copper mesh foil purchased from the market and copper-lithium composite strips with lithium on both sides (lithium thickness is 10μm); it is made into lithium metal battery negative electrode sheets that meet the requirements through edge cutting, cutting and slitting.

[0065] Preparation of lithium metal pouch battery: The positive electrode, negative electrode and separator prepared according to the above process are stacked to form a lithium metal battery with six positive and seven negative electrodes and a capacity of 3000mAh. The electrolyte is then injected to complete the battery manufacturing.

[0066] Lithium metal pouch batteries were prepared using the electrolytes of Examples 1-5 and Comparative Examples 1-3 according to the above method. The electrochemical performance of each battery was tested, and the results are shown in Table 3.

[0067] Note: For lithium metal pouch batteries, if the capacity retention rate is below 80% and the coulombic efficiency is below 98% during cycle testing, the battery testing will be stopped.

[0068] Table 3

[0069]

[0070] As can be seen from Table 3, the electrolyte of the present invention is suitable for high-nickel-lithium metal system batteries, which is beneficial to improving the cycle performance and rate performance of the battery.

[0071] This description is intended 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, Includes lithium salts, solvents, diluents, and additives; The solvent includes compounds containing at least one ester bond and at least one ether bond; The diluent includes fluoroaromatic ethers; The additives include compounds containing hexafluorophosphate groups; The solvent includes at least one of 2-((acetoxy)methoxy)ethanol acetate, 2-oxa-1,4-butanediol diacetate, 2-(acetoxymethoxy)ethoxymethyl acetate and methoxymethyl acetate. The content of the additive is 0.001wt%~5wt%.

2. The electrolyte according to claim 1, characterized in that, The diluent includes at least one selected from 3-fluoroanisole, 4-fluoroanisole, 2-fluoroanisole, pentafluoroanisole, m-fluoroanisole, 4-fluorobiphenyl ether, 2-fluorodiphenyl ether, bis(4-fluorophenyl) ether, 2,3-difluorophenylpropyl ether, 2,3-difluorophenylbutyl ether, 2,6-difluoroanisole, 2,4-difluoroanisole, 2,4-difluoroanisole, 2,3-difluoroanisole, 2,5-difluoroanisole, 2,3-difluoroanisole, and 2,6-difluoroanisole.

3. The electrolyte according to claim 1, characterized in that, The additives include at least one of the following: ethyl viologen dihexafluorophosphate, 1-ethylpyridine hexafluorophosphate, 1-ethyl-2,3-dimethylimidazolium hexafluorophosphate, N,N-bis[4-(diethylamino)phenyl]-N',N'-diethyl-1,4-phenylenediamine hexafluorophosphate, (4-hydroxyphenyl)dimethylsulfonium hexafluorophosphate, (4-hydroxyphenyl)dimethylsulfonium hexafluorophosphate, tetraethylphosphonium hexafluorophosphate, triethylhexafluorophosphate, 1-vinyl-3-ethylimidazolium hexafluorophosphate, 1-methyl-1-ethylpyrrolidine onium hexafluorophosphate, trimethylaniline hexafluorophosphate, 1-hexylpyridine hexafluorophosphate, 1-butylpyridine hexafluorophosphate, and 1-vinyl-3-butylimidazolium hexafluorophosphate.

4. The electrolyte according to claim 1, characterized in that, The lithium salt includes lithium bis(fluorosulfonyl)imide and / or lithium bis(trifluoromethyl)sulfonyl)imide.

5. The electrolyte according to claim 1, characterized in that, It also includes a co-solvent; the co-solvent includes at least one of tris(2,2,2-trifluoroethane) phosphite, tris(2,2,2-trifluoroethyl) phosphate, and bis(2,2,2-trifluoroethyl) phosphite.

6. The electrolyte according to claim 1, characterized in that, In the electrolyte, the concentration of the lithium salt is 0.8~2.5 mol / L.

7. The electrolyte according to claim 1, characterized in that, The volume ratio of the solvent to the diluent is (0.5~5):(5~9.5).

8. The electrolyte according to claim 5, characterized in that, The volume ratio of the solvent, the co-solvent, and the diluent is (0.25~3):(0.25~2):(5~9.5).

9. A lithium-ion battery, characterized in that, Includes the electrolyte as described in any one of claims 1 to 8.

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