Electrolyte and lithium-ion battery containing the same

Abstract

The present invention discloses an electrolyte and a lithium-ion battery containing the electrolyte. The electrolyte comprises a lithium salt, an organic solvent, and an additive, wherein the additive is selected from at least one of the compounds represented by structural formula I: wherein R1 to R4 are each independently selected from a hydrogen atom, a substituted or unsubstituted C1-C12 alkyl group, a substituted or unsubstituted C1-C12 unsaturated hydrocarbon group, a substituted or unsubstituted C2-C20 alkenyl group, a cyano group, or a group formed by replacing an aryl group having 6 to 18 carbon atoms with a carbonyl group, a cyano group, a halogen atom, a nitro group, a carboxyl group, or a sulfonic acid group, wherein the halogen atom is F, Cl, or Br. The electrolyte of the present invention can significantly improve the high-temperature cycling performance, high-temperature storage performance, and low-temperature performance of the battery.

Description

Technical Field

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

[0002] Lithium-ion batteries, due to their high specific energy, lack of memory effect, and long cycle life, are widely used in digital devices, power tools, aerospace, energy storage, and electric vehicles. Nickel-cobalt-manganese ternary cathode materials (NCM materials) are the preferred cathode active materials for lithium-ion batteries due to their safety and low cost. However, with the development and widespread adoption of higher-voltage lithium-ion batteries, the electrical performance requirements for lithium-ion batteries are becoming increasingly stringent.

[0003] Currently, ternary cathode materials are prone to irreversible H2-H3 phase transitions under high voltage and high temperature, leading to oxygen evolution and unstable electrolyte-electrode interfaces. This leads to poor high-temperature storage and severe cycle gassing. Furthermore, conventional carboxylate-containing electrolytes, while offering high conductivity, oxidize and decompose on the cathode surface at a high voltage of 4.4V. This, especially at high temperatures, accelerates the electrolyte's oxidative decomposition and worsens the cathode material.

[0004] Therefore, there is an urgent need to develop an electrolyte that can withstand a high voltage of 4.4V, so as to achieve excellent electrical performance of lithium-ion batteries and solve the shortcomings of existing technical problems. Summary of the Invention

[0005] The object of the present invention is to provide an electrolyte that can enable a lithium-ion battery to have good high-temperature storage and cycle performance at high voltage (such as 4.4V) and good low-temperature performance.

[0006] Another object of the present invention is to provide a lithium-ion battery containing the electrolyte, which has good high-temperature storage and cycle performance at high voltage (such as 4.4V) and good low-temperature performance.

[0007] To achieve the above objectives, the present invention provides an electrolyte comprising a lithium salt, an organic solvent, and an additive, wherein the additive is selected from at least one of the compounds represented by structural formula I:

[0008]

[0009] Among them, R1 to R4 are each independently selected from a hydrogen atom, a substituted or unsubstituted C1 to C12 alkyl group, a substituted or unsubstituted C1 to C12 unsaturated hydrocarbon group, a substituted or unsubstituted C2 to C20 alkenyl group, a cyano group, an aromatic group having 6 to 18 carbon atoms substituted by a carbonyl group, a cyano group, a halogen atom, a nitro group, a carboxyl group, or a sulfonic acid group, wherein the halogen atom is F, Cl, or Br, preferably F or Cl.

[0010] Compared to the prior art, the electrolyte of the present invention includes an additive selected from at least one of the compounds represented by Structural Formula I, which is a positive electrode protective additive. Specifically, the compound represented by Structural Formula I has a symmetrical structure and relatively high high-voltage stability. Furthermore, the C=C double bond in the structure is reduced to form a relatively tough interfacial film (SEI film) at the positive electrode / electrolyte interface. This film has good lithium ion conduction channels and does not collapse during cycling, significantly improving the battery's high-temperature cycling and low-temperature performance. In particular, the introduction of symmetrical carboxylate groups can enhance the stability of the carboxylate-containing SEI, further improving the structural stability of the interfacial film, and thus significantly improving the high-temperature storage performance of lithium-ion batteries.

[0011] Among them, C1~C12 alkyl represents an alkyl group with 1 to 12 carbon atoms. The alkyl group can be a chain alkyl group or a cycloalkyl group. The hydrogen on the ring of the cycloalkyl group can be replaced by an alkyl group. Preferably, the alkyl group is an alkyl group with 1-6 carbon atoms. The specific case of the alkyl group can be but not limited to methyl, ethyl, propyl, butyl, pentyl and cyclohexyl.

[0012] Here, the C1-C12 unsaturated hydrocarbon group refers to a hydrocarbon group having 1 to 12 carbon atoms.

[0013] The C2-C20 alkenyl group refers to an alkenyl group having 2 to 20 carbon atoms, and may be a cyclic alkenyl group or a chain alkenyl group. Furthermore, the alkenyl group may be an alkenyl group having 2 to 5 carbon atoms. Specifically, it may be, but is not limited to, ethenyl, propenyl, and the like.

[0014] The aryl group may be, but is not limited to, a phenyl group.

[0015] Preferably, the compound represented by structural formula I is at least one selected from Compound 1 to Compound 7:

[0016]

[0017] Preferably, the mass of the additive accounts for 0.1-5% of the total mass of the electrolyte, specifically but not limited to 0.1%, 0.3%, 0.5%, 0.8%, 1.2%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.2%, 4.5%, 4.8%, and 5%. Furthermore, the mass of the additive accounts for 0.2-3% of the total mass of the electrolyte.

[0018] Preferably, the lithium salt is selected from at least one of lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium methanesulfonate (LiCH3SO3), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(trifluoromethylsulfonyl)imide (LiN(CF3SO2)2), lithium bis(oxalatoborate) (C4BLiO8), lithium difluorooxalatoborate (C2BF2LiO4), lithium difluorophosphate (LiPO2F2), lithium difluorobis(oxalatophosphate) (LiDFBP), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI).

[0019] Preferably, the mass of the lithium salt accounts for 5% to 25% of the total mass of the electrolyte, specifically 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, and 25%, but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable. Further, the mass of the lithium salt accounts for 6% to 20% of the total mass of the electrolyte.

[0020] Preferably, the organic solvent is selected from at least one of linear carbonates, cyclic carbonates, carboxylates, ethers and heterocyclic compounds. More specifically, the organic solvent of the present invention can be selected from at least one of ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), propylene carbonate (PC), butyl acetate (n-Ba), γ-butyrolactone (γ-Bt), propyl propionate (n-PP), ethyl propionate (EP) and ethyl butyrate (Eb). Further, the mass of the organic solvent accounts for more than 60% of the total mass of the electrolyte, preferably more than 70%, more preferably more than 75%, such as but not limited to 78%, 80%, 82%, 85%, etc.

[0021] Preferably, the electrolyte of the present invention further includes an additive selected from at least one of vinylene carbonate (VC), vinylene carbonate (VEC), fluoroethylene carbonate (FEC), vinyl sulfite (ES), 1,3-propane sultone (PS), and diethylene sulfate (DTD). The mass of the additive accounts for 0.1 to 6.0% of the total mass of the electrolyte, specifically but not limited to 0.1%, 0.5%, 1.5%, 2%, 2.5%, 3%, 4%, 4.5%, 5%, 5.5%, and 6.0%. The addition of the additive can further improve the cycling performance and high-temperature storage performance of the lithium-ion battery.

[0022] Accordingly, the present invention also provides a lithium-ion battery comprising a positive electrode, a negative electrode, and the aforementioned electrolyte, wherein the positive electrode is made of a nickel-cobalt-manganese oxide material. The lithium-ion battery employing the aforementioned electrolyte achieves good high- and low-temperature discharge performance at a maximum charge voltage of 4.4V, and significantly increases the battery's cycle life.

[0023] Preferably, the nickel-cobalt-manganese oxide material is high nickel-cobalt-manganese oxide, which is LiNi x Co y Mn (1-x-y) M z O2, wherein 0.6≤x<0.9, x+y<1, 0≤z<0.08, and M is at least one of Al, Mg, Zr and Ti.

[0024] Preferably, the negative electrode of the present invention is made of a carbon negative electrode material, a silicon negative electrode material, or a silicon-carbon negative electrode material. The negative electrode is preferably a silicon-carbon negative electrode material, wherein the mass ratio of carbon to silicon is 90:10. DETAILED DESCRIPTION

[0025] In order to better illustrate the purpose, technical solutions and beneficial effects of the present invention, the present invention will be further described below in conjunction with specific embodiments. It should be noted that the following implementation method is a further explanation of the present invention and should not be used as a limitation of the present invention.

[0026] Example 1

[0027] (1) Preparation of electrolyte:

[0028] Ethylene carbonate (EC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) were mixed in a mass ratio of EC:DEC:EMC = 1:1:1 to prepare 81 g of an organic solvent. After mixing, 12.5 g of lithium hexafluorophosphate (LiPF6) was added. After the lithium salt was completely dissolved, 1 g of vinylene carbonate (VC), 5 g of fluoroethylene carbonate (FEC) and 0.5 g of the positive electrode protection additive compound 1 were added.

[0029] (2) Preparation of positive electrode sheet:

[0030] The lithium nickel cobalt manganese oxide ternary material LiNi 0.6 Co 0.2 Mn 0.2 Zr 0.03 O2, conductive agent SuperP, adhesive PVDF and carbon nanotubes (CNT) are evenly mixed in a mass ratio of 97.5:1.5:1:1 to form a lithium-ion battery positive electrode slurry with a certain viscosity. The mixed slurry is coated on both sides of an aluminum foil, dried and rolled to obtain a positive electrode sheet.

[0031] (3) Preparation of negative electrode sheet:

[0032] Artificial graphite and silicon are mixed in a mass ratio of 90:10, and then mixed with a conductive agent SuperP, a thickener CMC, and an adhesive SBR (styrene-butadiene rubber emulsion) in a mass ratio of 95:1.5:1.0:2.5 to make a slurry. The mixture is evenly mixed, and the mixed slurry is applied to both sides of the copper foil. The negative electrode sheet is obtained after drying and roller pressing to make a lithium-ion battery negative electrode sheet that meets the requirements.

[0033] (4) Preparation of lithium-ion batteries:

[0034] The positive electrode, separator and negative electrode are stacked to form a square battery cell, which is packaged with a polymer and filled with the prepared lithium-ion battery non-aqueous electrolyte. After the formation and capacity separation processes, a lithium-ion battery with a capacity of 1000mAh is produced.

[0035] The electrolyte compositions of Examples 2 to 9 and Comparative Example 1 are shown in Table 1. The steps for preparing the electrolyte and the lithium-ion battery are the same as those of Example 1.

[0036] Table 1 Electrolyte composition of Examples and Comparative Examples

[0037]

[0038]

[0039] The lithium ion batteries prepared in Examples 1-9 and Comparative Example 1 were subjected to normal temperature cycle test, high temperature cycle test, high temperature storage test and low temperature discharge test according to the following test conditions. The test results are shown in Table 2.

[0040] Normal temperature cycle test :

[0041] At room temperature (25°C), the lithium-ion battery is charged and discharged once at 1.0C / 1.0C (the battery discharge capacity is C0) with an upper limit voltage of 4.4V, and then charged and discharged at 1.0C / 1.0C for 500 cycles at room temperature (the battery discharge capacity is C1);

[0042] Capacity retention rate = (C1 / C0)*100%

[0043] High temperature cycle test :

[0044] At an excessively high temperature (45°C), the lithium-ion battery is charged and discharged at 1.0C / 1.0C once (the battery discharge capacity is C0) with an upper voltage of 4.4V, and then charged and discharged at 1.0C / 1.0C for 300 cycles at room temperature (the battery discharge capacity is C1);

[0045] Capacity retention rate = (C1 / C0)*100%

[0046] High temperature storage test :

[0047] At room temperature (25°C), the lithium-ion battery is charged and discharged at 0.3C / 0.3C (the battery discharge capacity is recorded as C0), with an upper limit voltage of 4.4V; the battery is placed in a 60°C oven for 15 days, the battery is taken out, and the battery is placed in a 25°C environment and discharged at 0.3C, with the discharge capacity recorded as C1; then the lithium-ion battery is charged and discharged at 0.3C / 0.3C (the battery discharge capacity is recorded as C2);

[0048] Capacity retention rate = (C1 / C0)*100%

[0049] Capacity recovery rate = (C2 / C0)*100%

[0050] Low temperature discharge test :

[0051] At room temperature (25°C), the lithium-ion battery is charged and discharged at 0.3C / 0.3C (the battery discharge capacity is recorded as C0), and the upper limit voltage is 4.4V; the battery is placed in a -20°C oven for 4 hours, and the battery is discharged at 0.3C, the discharge capacity is recorded as C1, and the cut-off voltage is 3.0V.

[0052] Capacity retention rate = (C1 / C0)*100%

[0053] Table 2 Performance test results of lithium ion batteries of Examples and Comparative Examples

[0054]

[0055] As can be seen from Table 2, compared with Comparative Example 1, the battery prepared using the electrolyte of the present invention has significantly improved room temperature cycling, high temperature storage performance, high temperature cycling performance, and low temperature discharge performance. This is because the compound shown in Structural Formula I is a symmetrical structure with relatively high high voltage stability. At the same time, the C=C double bond in the structure is reduced to a relatively tough interface film (SEI film) at the positive electrode / electrolyte interface, and the film has a good conductive lithium ion channel, which will not cause the collapse of the lithium ion channel during the cycle, and can greatly improve the high temperature cycle and low temperature performance of the battery. In particular, the introduction of symmetrical carboxylate groups can improve the stability of the carboxylate-containing SEI and further improve the structural stability of the interface film, thereby greatly improving the high temperature storage performance of the lithium ion battery.

[0056] From the data of Example 3, it can be seen that compound 3 has the best low-temperature discharge effect. This may be because the side chain of the C=C double bond has a symmetrical structure, which has a certain inhibitory effect on the polymerization of the C=C double bond, thereby regulating the growth of the SEI thickness. In addition, this side chain can support the structural stability of the SEI pores. At low temperatures, the SEI pores are not easy to close, thus greatly improving the low-temperature performance of the battery.

[0057] From the data of Example 7, it can also be seen that the use of compound 7 as an additive has more advantages in high temperature performance (high temperature storage performance and high temperature cycle performance) than compounds 1-6. This may be because compound 7 has a completely symmetrical structure, which has relatively good thermal stability. At the same time, the structure forms a relatively thick SEI at high voltage, which alleviates or supplements the decomposition of SEI under high temperature conditions, thereby effectively improving the high temperature performance of the battery.

[0058] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. An electrolyte comprising a lithium salt, an organic solvent and an additive, characterized in that: The mass of the additive accounts for 0.1% to 5% of the total mass of the electrolyte, and the additive is selected from at least one of Compound 5, Compound 6, Compound 7 and the compound represented by Structural Formula I: Wherein, R1 to R4 are each independently selected from a hydrogen atom, a substituted or unsubstituted C1 to C12 alkyl group, and a substituted or unsubstituted C2 to C20 alkenyl group.

2. The electrolyte according to claim 1, wherein The compound represented by structural formula I is selected from at least one of compound 1 to compound 4: .

3. The electrolyte according to claim 1, wherein The mass of the additive accounts for 0.2% to 3% of the total mass of the electrolyte.

4. The electrolyte according to claim 1, wherein The lithium salt is selected from at least one of lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium methanesulfonate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethylsulfonyl imide), lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium difluorophosphate, lithium difluorobis(oxalatophosphate), lithium bis(fluorosulfonyl imide), and lithium bis(trifluoromethylsulfonyl imide.

5. The electrolyte according to claim 1, wherein The mass of the lithium salt accounts for 5-25% of the total mass of the electrolyte.

6. The electrolyte according to claim 1, wherein The organic solvent is selected from at least one of chain carbonates, cyclic carbonates, carboxylates and ethers.

7. The electrolyte according to claim 1, wherein The invention further comprises an auxiliary agent, which is selected from at least one of vinylene carbonate, vinylene carbonate, fluoroethylene carbonate, vinyl sulfite, 1,3-propane sultone and vinyl sulfate.

8. A lithium-ion battery comprising a positive electrode and a negative electrode, characterized in that: It also includes the electrolyte according to any one of claims 1 to 7, wherein the positive electrode is made of nickel-cobalt-manganese oxide material.

9. The lithium-ion battery according to claim 8, wherein The nickel-cobalt-manganese oxide material is LiNi x Co y Mn (1-x-y) M z O2, wherein 0.6≤x<0.9, x+y<1, 0≤z<0.08, and M is at least one of Al, Mg, Zr and Ti.

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