Electrolyte and lithium battery

Abstract

The invention discloses an electrolyte and a lithium battery, which are used for solving the technical problem that the cycle life of the lithium battery is worsened due to the fact that the negative electrode of the existing lithium battery is easy to expand in the charging and discharging process. The electrolyte comprises a lithium salt, a solvent and an additive, and the additive comprises a compound as shown in a structural formula 1. In the design, by adding the compound as shown in the structural formula 1, a stable interfacial film can be formed on the surface of the negative electrode, the consumption of electrolyte is inhibited, and meanwhile, the negative electrode has the effects of removing water and acid, reducing side reaction of an interface, effectively improving cycle thickness expansion and prolonging the cycle life of the lithium battery.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium battery design, and in particular to an electrolyte and a lithium battery. Background Art

[0002] Lithium-ion batteries have the advantages of high operating voltage, long cycle life, low self-discharge and no memory effect. They are widely used in portable electronic devices, electric vehicles and other products.

[0003] In the actual application of existing lithium-ion batteries, the negative electrode will expand to a certain extent during repeated charging and discharging, and the SEI film will be continuously repaired and destroyed during the cycle, thereby causing the cycle life of the lithium battery to deteriorate.

[0004] Therefore, finding a technical solution that can solve the above technical problems has become an important topic studied by technical personnel in this field. Summary of the invention

[0005] The invention discloses an electrolyte and a lithium battery, which are used to solve the technical problem that the negative electrode of the existing lithium battery is easy to expand during the charging and discharging process, thereby causing the cycle life of the lithium battery to deteriorate.

[0006] The present invention provides an electrolyte comprising a lithium salt, a solvent and an additive, wherein the additive comprises a compound represented by structural formula 1;

[0007] Structural formula 1:

[0008]

[0009] Wherein, R1 and R2 are each independently selected from a hydrogen atom, a halogen atom, a C1-C5 alkyl alkane or a halogenated alkane, a siloxane and a halogenated siloxane.

[0010] Optionally, the compound is selected from at least one of the following compounds; Compound 1:

[0011]

[0012] Compound 2:

[0013]

[0014] Compound 3:

[0015]

[0016] Compound 4:

[0017]

[0018] Compound 5:

[0019]

[0020] Optionally, the compound represented by structural formula 1 accounts for 0.01 wt% to 5 wt% of the electrolyte.

[0021] Optionally, the compound represented by structural formula 1 accounts for 0.1 wt% to 2 wt% of the electrolyte.

[0022] Optionally, the concentration of lithium salt in the electrolyte is 0.9M to 2M.

[0023] Optionally, the additive further includes at least one of a cyclic sulfate compound, a sultone compound, a cyclic carbonate compound, and a nitrile compound.

[0024] Optionally, the cyclic sulfate ester compound is selected from at least one of vinyl sulfate, propylene sulfate, methyl vinyl sulfate, divinyl sulfate, propylene sulfate, and butylene sulfate;

[0025] The sultone compound is selected from at least one of 1,3-propane sultone, 1,4-butane sultone, 1,3-propylene sultone or methylene disulfonate;

[0026] The nitrile compound is selected from at least two of succinonitrile, adiponitrile, 1,3,6-hexanetrinitrile, and ethylene glycol bis(propionitrile) ether;

[0027] The lithium salt includes one or more of lithium hexafluorophosphate (Li PF6), lithium difluorooxalatoborate (LiODFB), lithium difluorodioxalatophosphate (Li DFOP), lithium tetrafluoroborate (Li BF4), lithium bis(trifluoromethylsulfonyl)imide (Li TFS I), lithium bis(fluorosulfonyl)imide (L i FS I), and lithium difluorophosphate (Li POF2);

[0028] The solvent includes one or more of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethyl propionate (EP), and propyl propionate (PP).

[0029] The present invention provides a lithium battery, comprising a positive electrode sheet, a negative electrode sheet, a separator and the above-mentioned electrolyte.

[0030] Optionally, the positive electrode plate includes a positive electrode current collector and a positive electrode coating disposed on the positive electrode current collector, and the negative electrode plate includes a negative electrode current collector and a negative electrode coating disposed on the negative electrode current collector;

[0031] Wherein, the positive electrode coating comprises a positive electrode active material, a positive electrode conductive agent and a positive electrode binder, and the negative electrode coating comprises a negative electrode active material, a negative electrode conductive agent and a negative electrode binder;

[0032] The positive electrode active material includes one of lithium cobalt oxide, ternary material, and lithium iron phosphate;

[0033] The negative electrode active material includes one or more of graphite and silicon.

[0034] Optionally, the negative electrode plate is processed by laser scribing technology, and the weight loss rate is less than 2%, wherein the weight loss rate is preferably 0.5-2%;

[0035] The weight loss rate=(weight of the negative electrode coating before etching-weight of the negative electrode coating after etching) / weight of the negative electrode coating before etching.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] In the electrolyte of the present invention, by adding the compound shown in structural formula 1, the carbon-carbon double bond tends to obtain electrons, is reduced at the negative electrode, and induces EC ring opening, and participates in the polymerization process to build a stable interface film, inhibiting the consumption of the electrolyte. At the same time, the anhydride group has the effect of removing water and acid, reducing interfacial side reactions, effectively improving cycle thickness expansion, and increasing the cycle life of the lithium battery. DETAILED DESCRIPTION

[0038] The invention discloses an electrolyte and a lithium battery, which are used to solve the technical problem that the negative electrode of the existing lithium battery is easy to expand during the charging and discharging process, thereby causing the cycle life of the lithium battery to deteriorate.

[0039] In order to enable those skilled in the art to better understand the scheme of the present invention, the present invention is further described in detail below in conjunction with specific implementation methods. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present invention.

[0040] The present invention provides an electrolyte, a lithium salt, a solvent and an additive, wherein the additive includes a compound shown in structural formula 1;

[0041] Structural formula 1:

[0042]

[0043] Wherein, R1 and R2 are each independently selected from a hydrogen atom, a halogen atom, a C1-C5 alkyl alkane or a halogenated alkane, a siloxane and a halogenated siloxane.

[0044] In the electrolyte of the present invention, by adding the compound shown in structural formula 1, the carbon-carbon double bond tends to obtain electrons, is reduced at the negative electrode, and induces EC ring opening, and participates in the polymerization process to build a stable interface film, inhibiting the consumption of the electrolyte. At the same time, the anhydride group has the effect of removing water and acid, reducing interfacial side reactions, effectively improving cycle thickness expansion, and increasing the cycle life of the lithium battery.

[0045] Further, the compound of the present invention is selected from at least one of the following compounds;

[0046] Compound 1:

[0047]

[0048] Compound 2:

[0049]

[0050] Compound 3:

[0051]

[0052] Compound 4:

[0053]

[0054] Compound 5:

[0055]

[0056] It should be noted that designers can select compounds with different structures according to actual conditions, and the compounds of the present invention are not limited to the above five structures.

[0057] Furthermore, the compound represented by the structural formula 1 of the present invention accounts for 0.01% to 5wt% of the electrolyte. ， In some specific embodiments, the above ratio may be 0.02wt%, 0.05wt%, 0.12wt%, 0.16wt%, etc., which is not limited in this embodiment.

[0058] Preferably, the compound represented by structural formula 1 accounts for 0.1-2 wt% of the electrolyte. In some specific embodiments, the above ratio can be 0.2 wt%, 0.5 wt%, 1.2 wt%, 1.6 wt%, etc., which is not limited in this embodiment.

[0059] Furthermore, the concentration of lithium salt in the electrolyte of the present invention is 0.9M to 2M ， In some specific embodiments, the concentration of the lithium salt may be 0.95 M, 1.5 M, 1.8 M, etc., which is not limited in this embodiment.

[0060] Preferably, the concentration of the lithium salt is 1.0M to 1.3M. In some specific embodiments, the concentration of the lithium salt may be 1.1M, 1.2M, etc., which is not limited in this embodiment.

[0061] Specifically, the lithium salt in the present invention includes one or more of lithium hexafluorophosphate (LiPF6), lithium difluorooxalatoborate (LiODFB), lithium difluorobis(oxalatophosphate) (LiDFOP), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium difluorophosphate (LiPOF2).

[0062] Furthermore, the additive in the present invention also includes at least one of cyclic sulfate compounds, sultone compounds, cyclic carbonate compounds, and nitrile compounds.

[0063] Wherein, the cyclic sulfate ester compound is selected from at least one of vinyl sulfate, propylene sulfate, methyl vinyl sulfate, divinyl sulfate, propylene sulfate, and butylene sulfate;

[0064] The sultone compound is selected from at least one of 1,3-propane sultone, 1,4-butane sultone, 1,3-propylene sultone or methylene disulfonate;

[0065] The nitrile compound is selected from at least two of succinonitrile, adiponitrile, 1,3,6-hexanetrinitrile and ethylene glycol bis(propionitrile) ether.

[0066] It should be noted that designers can select the above additives according to actual needs, and this embodiment does not limit this.

[0067] Furthermore, the solvent of the present invention includes one or more of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethyl propionate (EP), and propyl propionate (PP).

[0068] It should be noted that designers can select the above additives according to actual needs, and this embodiment does not limit this.

[0069] The present invention provides a lithium battery, comprising a positive electrode sheet, a negative electrode sheet, a separator and the above-mentioned electrolyte.

[0070] It should be noted that the positive electrode sheet, the negative electrode sheet, and the separator in the present invention are wound or stacked into a battery cell, which is then placed in a shell and the above-mentioned electrolyte is injected into the shell to produce a lithium battery.

[0071] In the battery of the present invention, by adding the compound shown in structural formula 1 to the electrolyte, the carbon-carbon double bond tends to obtain electrons, is reduced at the negative electrode, and induces EC ring opening, and participates in the polymerization process to build a stable interface film, inhibiting the consumption of the electrolyte. At the same time, the anhydride group has the effect of removing water and acid, reducing the interface side reaction, effectively improving the cycle thickness expansion, and improving the cycle life of the lithium battery.

[0072] Furthermore, the positive electrode plate of the present invention includes a positive electrode current collector and a positive electrode coating disposed on the positive electrode current collector, and the negative electrode plate includes a negative electrode current collector and a negative electrode coating disposed on the negative electrode current collector;

[0073] Wherein, the positive electrode coating comprises a positive electrode active material, a positive electrode conductive agent and a positive electrode binder, and the negative electrode coating comprises a negative electrode active material, a negative electrode conductive agent and a negative electrode binder;

[0074] The positive electrode active material includes one of lithium cobalt oxide, ternary material and lithium iron phosphate.

[0075] The negative electrode active material includes one or more of graphite and silicon.

[0076] Furthermore, the negative electrode sheet in this embodiment is processed by laser scribing technology, and the weight loss rate is less than 2%, wherein the weight loss rate is preferably 0.5-2%. In some specific embodiments, the weight loss rate can be 0.5%, 1%, 1.5%, 2%, and this embodiment is not limited to this;

[0077] The weight loss rate=(the coating weight of the negative electrode coating before etching-the coating weight of the negative electrode coating after etching) / the coating weight of the negative electrode coating before etching.

[0078] It should be noted that processing the negative electrode plate through laser scribing technology can effectively increase the diffusion path of lithium ions in the electrode, thereby improving the fast charging performance. Combined with the compound shown in structural formula 1 in the above-mentioned electrolyte, the cycle thickness expansion can be improved together, thereby increasing the cycle life of the negative electrode battery.

[0079] The technical solution of the present invention is further described in detail below in conjunction with several preferred embodiments. This embodiment is implemented on the premise of the technical solution of the invention, and a detailed implementation method and specific operation process are given, but the protection scope of the present invention is not limited to the following embodiments.

[0080] Unless otherwise specified, the experimental materials used in the following examples can be purchased from conventional biochemical reagent companies.

[0081] Battery production

[0082] Preparation of positive electrode sheets: The positive electrode active material lithium cobalt oxide (Li CoO2), the conductive agent CNT, and the adhesive PVDF are fully stirred and mixed in NMP solvent at a weight ratio of 97:1.5:1.5. The slurry is coated on the positive electrode collector (aluminum foil). After drying, cold pressing, slitting, sheet making, welding, and lug gluing, a positive electrode sheet that meets the winding requirements is made.

[0083] Preparation of negative electrode sheet: The negative electrode active material silicon carbon negative electrode, conductive agent SP, thickener CMC, and adhesive SBR are mixed in a proper amount of deionized water solvent at a mass ratio of 96.3:1:1.2:1.5 to form a uniform negative electrode slurry. The slurry is coated on the negative electrode current collector Cu foil, and after drying, cold pressing, stripping, sheeting, welding, and lug gluing, negative electrode sheets that meet winding requirements are made.

[0084] The negative electrode sheet uses laser scribing technology, and the weight loss rate is controlled by adjusting the laser etching spacing and laser etching depth percentage.

[0085] Among them, the etching parameter comparison is shown in Table 1:

[0086]

[0087] Table 1

[0088] As the etching weight loss rate increases, the capacity loss increases, low-temperature discharge improves, which is related to the increase in contact area. High-temperature storage first improves and then deteriorates. It is speculated that excessive etching increases the contact area between the negative electrode active material and the electrolyte, and the side reactions increase at high temperatures, which deteriorates the high-temperature storage performance. Therefore, combined with capacity and performance, the negative electrode etching weight loss rate is less than 2%.

[0089] Preparation of electrolyte: The preparation steps of the electrolyte are as follows: EC: PC: DEC: EP: PP are mixed in a mass ratio of 15: 15: 15: 15: 40 as a solvent. After uniform mixing, 1.2 mol / L LiPF6 is slowly added to obtain a mixed solution with a LiPF6 concentration of 1.2 mol / L. After the lithium salt is completely dissolved, the corresponding additive is added to obtain the electrolyte.

[0090] Examples 1-11 and Comparative Examples 1-4: mainly composed of lithium hexafluorophosphate and solvent. Take the baked liquid-filled battery cells (water content <120ppm), and inject the electrolytes of Examples 1-8 and Comparative Examples 1-4 respectively. Then, according to the normal process of standing, forming, secondary sealing, and volume separation testing, the details are shown in Table 2:

[0091]

[0092]

[0093] Table 2

[0094] Lithium-ion battery performance test

[0095] Low temperature discharge performance test: At 25°C, let the lithium-ion battery stand for 30 minutes, then charge to 4.53V at 1.2C constant current and constant voltage, cut-off current 0.05C, stand for 5 minutes, then discharge to 3.0V at 25°C 0.2C constant current, record the discharge capacity C0. At 25°C, let the lithium-ion battery stand for 5 minutes, then charge to 4.53V at 1.2C constant current and constant voltage, cut-off current 0.05C, stand for 5 minutes, then at -20°C, leave for 3 hours, discharge to 3.0V at 0.2C constant current, record the discharge capacity C1. Calculate the -20°C discharge capacity retention rate of the battery: C1 / C0×100%.

[0096] High temperature storage test: At 25°C, the lithium-ion battery was left to stand for 30 minutes, then charged to 4.53V at 1.2C constant current and constant voltage, with a cut-off current of 0.05C. After standing for 5 minutes, the cell thickness h0 was measured. After storage at 60°C for 21 days, the battery thickness h1 was thermally measured, and the cell thickness expansion rate was calculated by the following formula: [(thickness after storage h1 - thickness before storage h0) / thickness before storage h0] × 100%.

[0097] Lithium-ion battery cycle performance test: At 25°C, test the thickness of the battery cell H0 at 30% SOC state; place the lithium-ion battery in a 45°C environment, let it stand for 60 minutes, then charge it to 4.53V at 1.2C constant current and constant voltage, cut off current 0.05C, let it stand for 5 minutes, discharge it to 3.0V at 0.5C constant current, and let it stand for 5 minutes. This is a charge and discharge cycle. The thickness of the battery cell Hn (n is the number of cycles) at 100% SOC state is measured every 100 cycles. With the capacity of the first discharge as 100%, charge and discharge are repeated. The capacity of the cycle to 400cl s divided by the capacity at the first discharge is recorded as the capacity retention rate of the cycle, and the thickness expansion is =Hn / H0-1.

[0098] The test results are shown in Table 3:

[0099]

[0100] Table 3

[0101] The test results of comparative examples 1 to 3 show that as the amount of laser etching of the negative electrode increases, it helps to improve the low-temperature discharge performance, and the appropriate etching amount helps to improve the high-temperature storage and cycle performance; excessive etching deteriorates high-temperature storage and cycling, which is presumably related to the interface surface of the negative electrode. The appropriate etching amount helps to increase the diffusion path of lithium ions in the electrode and improve the kinetics, but excessive etching will cause a larger specific surface area, increase the interface side reactions, and lead to deterioration of high-temperature storage and cycling.

[0102] From the comparative analysis of Example 1 and Example 4, it can be seen that the addition of Compound 4 improves the high-temperature storage and cycle, mainly because the substance can form a polymer film on the surface of the negative electrode, which can adapt to the high expansion of the negative electrode, and at the same time has the function of removing water and acid, reducing the interfacial side reactions, thereby improving the high-temperature performance.

[0103] Comparative analysis of Examples 1 to 5 and Comparative Example 2 shows that the compounds shown in Structural Formula 1 can improve cycle thickness expansion and capacity retention, but deteriorate low-temperature discharge.

[0104] The test results of Examples 6 to 11 show that an excessively high content of the compound shown in Structural Formula 1 will deteriorate the cycle capacity retention rate and thickness expansion, which is related to its high film resistance. Therefore, an appropriate amount of negative electrode laser etching (to improve kinetics) and the compound shown in Structural Formula 1 can improve high-temperature storage and cycle capacity retention rates, and increase the cycle life of the negative electrode battery.

[0105] The above is a detailed introduction to an electrolyte and a lithium battery provided by the present invention. For a person skilled in the art, according to the concept of the embodiments of the present invention, there may be changes in the specific implementation methods and application scopes. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. An electrolyte, characterized in that: It includes a lithium salt, a solvent and an additive, wherein the additive includes a compound shown in structural formula 1; Wherein, R1 and R2 are each independently selected from a hydrogen atom, a halogen atom, a C1-C5 alkyl alkane or a halogenated alkane, a siloxane and a halogenated siloxane.

2. The electrolyte according to claim 1, characterized in that The compound is selected from at least one of the following compounds; 3. The electrolyte according to claim 1, characterized in that The compound represented by the structural formula 1 accounts for 0.01 wt% to 5 wt% of the electrolyte.

4. The electrolyte according to claim 3, characterized in that The compound represented by the structural formula 1 accounts for 0.1 wt% to 2 wt% of the electrolyte.

5. The electrolyte according to claim 1, characterized in that The concentration of lithium salt in the electrolyte is 0.9M to 2M.

6. The electrolyte according to claim 1, characterized in that The additive further includes at least one of cyclic sulfate compounds, sultone compounds, cyclic carbonate compounds, and nitrile compounds.

7. The electrolyte according to claim 6, characterized in that The cyclic sulfate compound is selected from at least one of vinyl sulfate, propylene sulfate, methyl vinyl sulfate, divinyl sulfate, propylene sulfate, and butylene sulfate; The sultone compound is selected from at least one of 1,3-propane sultone, 1,4-butane sultone, 1,3-propylene sultone or methylene disulfonate; The nitrile compound is selected from at least two of succinonitrile, adiponitrile, 1,3,6-hexanetrinitrile, and ethylene glycol bis(propionitrile) ether; The lithium salt includes one or more of lithium hexafluorophosphate (LiPF6), lithium difluorooxalatoborate (LiODFB), lithium difluorobisoxalatophosphate (LiDFOP), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium difluorophosphate (LiPOF2); The solvent includes one or more of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethyl propionate (EP), and propyl propionate (PP).

8. A lithium battery, characterized in that: The invention comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte as claimed in any one of claims 1 to 7.

9. The lithium battery according to claim 8, characterized in that: The positive electrode plate includes a positive electrode current collector and a positive electrode coating disposed on the positive electrode current collector, and the negative electrode plate includes a negative electrode current collector and a negative electrode coating disposed on the negative electrode current collector; Wherein, the positive electrode coating comprises a positive electrode active material, a positive electrode conductive agent and a positive electrode binder, and the negative electrode coating comprises a negative electrode active material, a negative electrode conductive agent and a negative electrode binder; The positive electrode active material includes one of lithium cobalt oxide, ternary material, and lithium iron phosphate; The negative electrode active material includes one or more of graphite and silicon.

10. The lithium battery according to claim 8 or 9, characterized in that: The negative electrode sheet is processed by laser scribing technology, and the weight loss rate is less than 2% ， The weight loss rate is preferably 0.5 to 2%; The weight loss rate=(weight of the negative electrode coating before etching-weight of the negative electrode coating after etching) / weight of the negative electrode coating before etching.

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