Secondary battery electrolyte and battery thereof

Abstract

A secondary battery electrolyte, comprising a composite electrolyte salt. The composite electrolyte salt comprises lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide and a thio-based lithium salt. The thio-based lithium salt comprises a substituted sulfate (R 1-SO 3-Li) and / or a lithium salt with a sulfate ester structure (R 2-O-SO 3-Li). LiPF 6, the thio-based lithium salt and LiFSI are matched and used together. LiFSI has better hydrolysis stability, relatively good thermal stability and higher lithium migration capability, and has an obvious effect on improving the dynamics and high-temperature performance of the battery. The lithium sulfate and lithium salt with a sulfate ester structure also have relatively high lithium migration capability, and also, the S, O-rich anionic groups, after film forming, have a relatively good effect on improving the components of the SEI film and reducing film impedance, so that the dynamic performance and the high-temperature performance of the battery are improved.

Description

Secondary battery electrolyte and battery thereof Technical Field

[0001] The present invention belongs to the field of batteries, and in particular relates to a secondary battery electrolyte and a battery thereof. Background Art

[0002] As the environmental pressures caused by the extensive use of fossil fuels grow, new energy technologies are receiving unprecedented attention and development. Secondary batteries, due to their high energy density, long lifespan, and lack of memory effect, are widely used in the new energy sector. Clients are placing higher demands on the long-term reliability and high energy density of batteries, leading to continuous improvement and upgrades in the material design of secondary batteries.

[0003] Therefore, to meet people's demand for improved comprehensive performance of secondary batteries, it is necessary to provide a secondary battery with good comprehensive performance. In the battery, the electrolyte is responsible for transporting lithium ions and is called the battery's blood. The effective combination of lithium salts in the electrolyte can greatly enhance the battery's dynamic capabilities and improve its high-temperature performance.

[0004] Summary of the Invention

[0005] The purpose of the present invention is to provide a secondary battery electrolyte and a battery thereof to address the problems existing in the prior art. The purpose of the present invention can be achieved by the following scheme:

[0006] The present invention provides a secondary battery electrolyte, comprising a composite electrolyte salt, wherein the composite electrolyte salt comprises lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI) and a sulfur-based lithium salt; the sulfur-based lithium salt comprises a substituted sulfate (R1-SO3-Li) and / or a sulfate ester structure lithium salt (R2-O-SO3-Li).

[0007] The structural formula of the substituted sulfate (R1-SO3-Li) is shown in Formula 1 below, where R1 includes one of an alkyl group, a halogenated alkyl group, a hydroxyl group, a carboxyl group, an aldehyde group, a hydroxyethyl group, a carbonyl group, an aryl group, a benzyl group, a pyrazolyl group, and an imidazolyl group.

[0008] The structural formula of the sulfate ester structure lithium salt (R2-O-SO3-Li) is shown in Formula 2 below, where R2 includes one of an alkyl group, an unsaturated ethyl to hexyl group, a halogenated alkyl group, a hydroxyl group, a carboxyl group, an aldehyde group, a hydroxyethyl group, a carbonyl group, an aryl group, a benzyl group, a pyrazolyl group, and an imidazole group.

[0009] As one embodiment of the present invention, in the structural formula of the substituted sulfate (R1-SO3-Li), the alkyl group includes methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl or decyl.

[0010] As one embodiment of the present invention, in the structural formula of the substituted sulfate (R1-SO3-Li), the halogenated alkyl group includes a fluoroalkyl group.

[0011] As one embodiment of the present invention, in the structural formula of the lithium salt of sulfate structure (R2-O-SO3-Li), the alkyl group includes methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl or decyl.

[0012] As one embodiment of the present invention, in the composite electrolyte salt, the mass ratio of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI) and sulfur-based lithium salt is 1:(0.05-150):(0.005-10), preferably 1:(0.1-10):(0.07-3).

[0013] As one embodiment of the present invention, the sulfur-based lithium salt is preferably a combination of a substituted sulfate and a sulfate ester structure lithium salt; in the sulfur-based lithium salt, the mass ratio of the substituted sulfate and the sulfate ester structure lithium salt is 1: (0.1 to 10), preferably 1: (0.15 to 5).

[0014] As an embodiment of the present invention, the molar ratio of S to P in the sulfur-based lithium salt is 0

[0015] As an embodiment of the present invention, the molar ratio of S to P in the composite electrolyte salt is 0

[0016] As one embodiment of the present invention, the secondary battery electrolyte further includes an organic solvent, and the organic solvent includes a cyclic carbonate solvent and a unequal-length chain carbonate solvent. The cyclic carbonate solvent includes one or more of ethylene carbonate (EC), propylene carbonate (PC), and bisfluoroethylene carbonate (DFEC); the unequal-length chain carbonate solvent includes one or more of ethyl methyl carbonate (EMC), n1-fluoroethyl methyl carbonate, and n2-chloroethyl methyl carbonate. Wherein, n1 is the number of fluorine substitutions in fluoroethyl methyl carbonate, and n2 is the number of chlorine substitutions in chloroethyl methyl carbonate.

[0017] ​​As an embodiment of the present invention, described organic solvent also comprises isolong chain carbonate solvent.Described isolong chain carbonate solvent comprises one or more in dimethyl carbonate (DMC), diethyl carbonate (DEC), n3 fluorodimethyl carbonate, n4 fluorodiethyl carbonate, n5 dimethyl chlorocarbonate, n6 diethyl chlorocarbonate.Wherein, n3 is the quantity that replaces fluorine in dimethyl fluorocarbonate, n4 is the quantity that replaces fluorine in diethyl fluorocarbonate, and n5 is the quantity that replaces chlorine in dimethyl chlorocarbonate, n6 is the quantity that replaces chlorine in diethyl chlorocarbonate.

[0018] As an embodiment of the present invention, the secondary battery electrolyte further includes additives, and the additives include key additives; the key additives include one or more of vinylene carbonate (VC), diethylene sulfate (DTD), and methylene methanedisulfonate (MMDS).

[0019] As one embodiment of the present invention, the additive further includes an auxiliary additive, and the auxiliary additive includes one or more of lithium difluorophosphate (LDFP), lithium difluorooxalatoborate (LiODFB), lithium difluorobisoxalatophosphate (LiODFP), lithium bisoxalatoborate (LiBOB), lithium perchlorate (LiClO4), lithium fluorosulfonate (LiSO3F), lithium nitrate (LiNO3), lithium tetrafluorooxalatophosphate (LiOTFP), tris(trimethylsilyl) phosphate (TMSP), tris(trimethylsilyl) borate (TMSB), 1,3-propane sultone (PS), propene sultone (PST), propylene sulfite (TMS), fluoroethylene carbonate (FEC), and tetraethylenesilane (TS).

[0020] As an embodiment of the present invention, the content of the components in the secondary battery electrolyte complies with the following calculation formula:

[0021] Among them, F is the key characteristic ratio of the electrolyte, W 环状 % represents the mass percentage of cyclic carbonate solvent in the electrolyte, W 不等长链状 % represents the mass percentage of different length chain carbonate solvents in the electrolyte, W 锂盐 % represents the mass percentage of the composite electrolyte salt in the electrolyte, W 关键添加剂 % indicates the mass percentage of key additives in the electrolyte.

[0022] The present invention also provides a battery comprising the secondary battery electrolyte.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] (1) The combination of LiPF6, sulfur-based lithium salts and LiFSI. As a traditional lithium salt, LiPF6 has the advantages of moderate stability and good cost, but it is not enough for the performance requirements of current batteries. LiFSI has better hydrolysis stability, better thermal stability and higher lithium migration capacity, which has obvious advantages in reducing battery impedance and improving room temperature cycle dynamics. Batteries with dense film formation and low impedance often have better room temperature cycle capacity. The sulfur-based lithium salts in the present invention include substituted sulfates and / or sulfate ester structure lithium salts, wherein lithium sulfate and sulfate ester structure lithium salts also have high lithium migration capacity. At the same time, their S and O-rich anion groups have a good effect on improving the composition of the solid electrolyte interface film (SEI) film and reducing the film impedance after film formation. Moreover, the composite components of sulfates, sulfites and organic SEI films with sulfate groups also have good thermal stability, which can increase the thermal decomposition temperature of the SEI film and improve battery safety. Therefore, the combination of LiPF6, sulfur-based lithium salts and LiFSI has a significant effect on improving battery dynamics and high temperature stability, thereby improving battery safety.

[0025] (2) Batteries using electrolytes within the F value range have better high-temperature storage performance; rapid and effective initial judgment can be made on electrolyte design. DETAILED DESCRIPTION

[0026] The present invention is described in detail below with reference to specific embodiments. The following examples are implemented under the premise of the technical solution of the present invention, provide detailed implementation methods and specific operating procedures, and will help those skilled in the art to further understand the present invention. It should be pointed out that the protection scope of the present invention is not limited to the following embodiments, and a number of adjustments and improvements made under the premise of the concept of the present invention all fall within the protection scope of the present invention.

[0027] The brand of polyvinylidene fluoride in the examples and comparative examples of the present invention is PVDF_5130.

[0028] The brand of polyvinylpyrrolidone in the examples and comparative examples of the present invention is PVP_30.

[0029] The brand of sodium carboxymethyl cellulose in the examples of the present invention and the comparative examples is CMC_500.

[0030] Example 1

[0031] In this embodiment, the battery electrochemical system used is a lithium iron phosphate battery. The electrolyte in this embodiment, based on the total mass of the electrolyte being 100 wt.%, includes:

[0032] The composite electrolyte salt is LiPF6+LiFSI+lithium nonafluoro-1-butanesulfonate (LiC4F9SO3)+lithium ethylene sulfate (LiCH3SO4), wherein the content of LiPF6 is 6wt.%, the content of LiFSI is 5wt.%, the content of LiC4F9SO3 is 1wt.%, and the content of LiCH3SO4 is 1wt.%. The mass percentage of the composite electrolyte salt in the electrolyte is 13wt.% (W 锂盐 %), the molar ratio of S to P in the composite electrolyte salt is S / P=1.65;

[0033] The cyclic carbonate solvent includes EC 13wt.%+PC13wt.%, and the mass percentage of the cyclic carbonate solvent in the electrolyte is 26wt.% (W 环状 %);

[0034] The EMC of the unequal chain solvent is 12 wt.% (W 不等长链状 %);

[0035] The key additive is 3.5wt.% (W 关键添加剂 %), wherein the key additives include VC (vinylene carbonate) and DTD in a mass ratio of 3:0.5;

[0036] The auxiliary additive is 1.3 wt.%, wherein the auxiliary additive includes LiODFB and FEC in a mass ratio of 0.5:0.8;

[0037] Equal-length chain carbonate solvent: the balance (44.2 wt.%), wherein the equal-length chain carbonate solvent comprises DMC and DEC in a mass ratio of 1:1.

[0038] The components and contents of the electrolyte in specific Example 1 are shown in Table 1, and the F value of Example 1 is 0.956.

[0039] The process of preparing the battery in this embodiment is as follows:

[0040] First, the slurry is mixed. The ratio of the positive electrode slurry is lithium iron phosphate: conductive carbon black (SuperP): binder (polyvinylidene fluoride PVDF_5130): dispersant (polyvinyl pyrrolidone PVP_30) = 100:2:2.5:0.5.

[0041] The negative electrode slurry ratio is graphite: conductive agent (Super P): binder (styrene-butadiene rubber (SBR)): binder (sodium carboxymethyl cellulose CMC_500): H2O=100:2:2:0.8:99.

[0042] The positive electrode slurry is then applied to aluminum foil and the negative electrode slurry to copper foil, followed by drying and winding. Rolling is then performed, followed by slitting. The core is then wound, using the prepared positive and negative electrode sheets and a separator made of PP (12μm base film + 2μm Al2O3 ceramic layer + 1μm PVDF adhesive layer). The wound core is hot-pressed, the top cover welded, and then assembled into a shell to create a dry cell.

[0043] After the dry cells are baked, they are injected with liquid, and the batteries after immersion are formed and divided into different capacities to obtain finished cells.

[0044] Example 2

[0045] This embodiment provides a secondary battery electrolyte, the components and their contents of which are shown in Table 1. The only difference from Example 1 is that the sulfur-based lithium salt includes only 2 wt.% LiC4F9SO3 and does not contain LiCH3SO4, and the molar ratio of S to P in the composite electrolyte salt is S / P = 1.52.

[0046] Example 3

[0047] This embodiment provides a secondary battery electrolyte, the components and their contents of which are shown in Table 1. The only difference from Example 1 is that the sulfur-based lithium salt includes only 2 wt.% LiCH3SO4 and does not contain LiC4F9SO3, and the molar ratio of S to P in the composite electrolyte salt is S / P = 1.78.

[0048] Example 4

[0049] The only difference between this embodiment and embodiment 1 is that the sulfate is replaced by lithium trifluoromethanesulfonate (LiCF3SO3), the sulfate ester structure lithium salt is lithium phenyl sulfate (LiC6H5SO4), and the LiPF6 content is 8wt.%, the LiFSI content is 8wt.%, the LiCF3SO3 content is 8wt.%, and the LiC6H5SO4 content is 2wt.%, and the mass percentage of the composite electrolyte salt in the electrolyte is 26wt.% (W 锂盐 %), and the content of the isolong chain carbonate solvent was 31.2 wt.%. The F value obtained in this embodiment was 0.478, and the molar ratio of S to P in the composite electrolyte salt was S / P=2.81.

[0050] Example 5

[0051] The only difference between this embodiment and embodiment 1 is that the sulfate is replaced by lithium benzyl sulfate (Li(C6H5)CH2SO3), the sulfate ester structure lithium salt is lithium pyrazole sulfate (LiC3H3N2SO4), and the LiPF6 content is 3wt.%, the LiFSI content is 5wt.%, the Li(C6H5)CH2SO3 content is 0.5wt.%, the LiC3H3N2SO4 content is 0.5wt.%, and the mass percentage of the composite electrolyte salt in the electrolyte is 9wt.% (W 锂盐 %), and the content of the isolong chain carbonate solvent is 48.2 wt.%. The F value obtained in this example is 1.381, and the molar ratio of S to P in the composite electrolyte salt is S / P=3.00.

[0052] Example 6

[0053] This comparative example provides a secondary battery electrolyte, the component selection and its content are shown in Table 1, and the difference from Example 1 is that: the content of LiPF6 is 2wt.%, the content of LiFSI is 3wt.%, the content of LiC4F9SO3 is 1wt.%, and the content of LiCH3SO4 is 1wt.%, and the mass percentage of the composite electrolyte salt in the electrolyte is 7wt.% (W 锂盐 %), and the content of the isolong chain carbonate solvent is 50.2 wt.%. The F value obtained in this example is 1.776, and the molar ratio of S to P in the composite electrolyte salt is S / P=3.33.

[0054] Comparative Example 1

[0055] This comparative example provides a secondary battery electrolyte, the component contents of which are shown in Table 1. The difference from Example 1 is that the sulfur-based lithium salt is replaced by an equal amount of lithium hexafluorophosphate, and the molar ratio of S to P in the composite electrolyte salt is S / P=1.02.

[0056] Comparative Example 2

[0057] This comparative example provides a secondary battery electrolyte, the component contents of which are shown in Table 1. The difference from Example 1 is that the sulfur-based lithium salt is replaced by an equal amount of lithium bis(fluorosulfonyl)imide, and the molar ratio of S to P in the composite electrolyte salt is S / P=1.89.

[0058] Comparative Example 3

[0059] This comparative example provides a secondary battery electrolyte, the component contents of which are shown in Table 1. The difference from Example 1 is that the sulfur-based lithium salt is replaced by an equal amount of lithium tetrafluoroborate (LiBF4), and the molar ratio of S to P in the composite electrolyte salt is S / P=1.35.

[0060] Table 1

[0061] The electrolytes of each embodiment and comparative example were assembled into soft-pack batteries according to the steps of Example 1, and the performance was tested. The normal temperature cycle test steps are as follows:

[0062] 1.0.5C CC charge to 4.25V;

[0063] 2. Set aside for 10 minutes;

[0064] 3.1C discharge to 2.8V;

[0065] 4. Set aside for 10 minutes;

[0066] 5. Repeat the above steps to test;

[0067] 6. Check the capacity retention rate after 500 cycles.

[0068] The high temperature storage steps test are as follows:

[0069] 7. Charge the battery to 4.25V at 0.5C CCCV, with a current cutoff of 0.05C.

[0070] 8. Store the battery in a 60°C oven for 14 days.

[0071] 9. Place the battery in a room temperature environment and let it cool down for 4 hours;

[0072] The battery was discharged at 1C to 2.8V, which was recorded as the residual capacity. The test data of each embodiment and comparative example are shown in Table 2.

[0073] Table 2

[0074] From Comparative Examples 1-3, it can be seen that when no sulfur-based lithium salt is added to the electrolyte, the capacity retention rate and high-temperature storage performance after 500 cycles are both reduced.

[0075] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various variations or modifications within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A secondary battery electrolyte, characterized in that: The secondary battery electrolyte includes a composite electrolyte salt; the composite electrolyte salt includes lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide and a sulfur-based lithium salt; the sulfur-based lithium salt includes a substituted sulfate and / or a sulfate ester structure lithium salt; The structural formula of the substituted sulfate is shown in Formula 1, wherein R1 includes one of an alkyl group, a halogenated alkyl group, a hydroxyl group, a carboxyl group, an aldehyde group, a hydroxyethyl group, a carbonyl group, an aryl group, a benzyl group, a pyrazolyl group, and an imidazolyl group; The structural formula of the lithium salt of sulfate ester structure is shown in Formula 2, and R2 includes one of alkyl, unsaturated ethyl-hexyl, halogenated alkyl, hydroxyl, carboxyl, aldehyde, hydroxyethyl, carbonyl, aryl, benzyl, pyrazolyl, and imidazole.

2. The secondary battery electrolyte according to claim 1, characterized in that: The sulfur-based lithium salt is a combination of substituted sulfate and sulfate ester structure lithium salt.

3. The secondary battery electrolyte according to claim 1, characterized in that: The molar ratio of the S element to the P element in the composite electrolyte salt is 0. The secondary battery electrolyte further includes an organic solvent, and the organic solvent includes a cyclic carbonate solvent and an unequal-length chain carbonate solvent.

4. The secondary battery electrolyte according to claim 1, characterized in that: The cyclic carbonate solvent includes one or more of ethylene carbonate, propylene carbonate, and difluoroethylene carbonate; the unequal-length chain carbonate solvent includes one or more of ethyl methyl carbonate, n1-fluoroethyl methyl carbonate, and n2-chloroethyl methyl carbonate.

5. The secondary battery electrolyte according to claim 4, characterized in that: The organic solvent also includes an equal-length chain carbonate solvent; the equal-length chain carbonate solvent includes one or more of dimethyl carbonate, diethyl carbonate, n3 fluorodimethyl carbonate, n4 fluorodiethyl carbonate, n5 chlorodimethyl carbonate, and n6 chlorodiethyl carbonate.

6. The secondary battery electrolyte according to claim 4, characterized in that: The secondary battery electrolyte further includes additives, wherein the additives include key additives; the key additives include one or more of vinylene carbonate, vinyl sulfate, and methylene methane disulfonate.

7. The secondary battery electrolyte according to claim 4, characterized in that: The additives also include auxiliary additives, which include one or more of lithium difluorophosphate, lithium difluorooxalatoborate, lithium difluorobisoxalatophosphate, lithium bisoxalatoborate, lithium perchlorate, lithium fluorosulfonate, lithium nitrate, lithium tetrafluorooxalatophosphate, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)borate, 1,3-propane sultone, propene sultone, propene sulfite, fluoroethylene carbonate, and tetravinylsilane.

8. The secondary battery electrolyte according to claim 7, characterized in that: The battery comprises the secondary battery electrolyte according to any one of claims 1 to 9.

9. The secondary battery electrolyte according to claim 7, characterized in that: The content of the components in the secondary battery electrolyte conforms to the following calculation formula: Among them, F is the key characteristic ratio of the electrolyte, W 环状 % represents the mass percentage of cyclic carbonate solvent in the electrolyte, W 不等长链状 % represents the mass percentage of carbonate solvents of different length chains in the electrolyte, W 锂盐 % represents the mass percentage of the composite electrolyte salt in the electrolyte, W 关键添加剂 % indicates the mass percentage of key additives in the electrolyte.

10. A battery, characterized in that: ​

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