Electrolyte and Lithium-Ion Battery

Abstract

The present invention discloses an electrolyte and a lithium-ion battery. The electrolyte includes a lithium salt, an organic solvent, and an additive. The additive includes: compound A shown in structural formula I and / or compound B shown in structural formula II: wherein, R1 to R6 are each independently selected from a hydrogen atom, a substituted or unsubstituted C1-C6 alkyl group, and a substituted or unsubstituted C1-C6 unsaturated hydrocarbon group. Both compound A and compound B contain cyclic unsaturated double bonds, and are reduced to a relatively tough interfacial film at the positive electrode / electrolyte interface. Moreover, this film has a good lithium-ion conduction channel, and will not cause the collapse of the lithium-ion channel during cycling, thus improving the cycling and low-temperature performance. In particular, introducing symmetric carboxylic ester groups into the cyclic structure can improve the stability of the carboxylic ester-containing SEI, enrich the components of the positive electrode / electrolyte interfacial film, and further improve the structural stability of the interfacial film, thereby improving the high-temperature storage performance of the lithium-ion battery.

Description

Technical Field

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

[0002] Lithium-ion batteries are widely used in fields such as 3C digital products, power tools, aerospace, energy storage, and electric vehicles due to their advantages of high specific energy, no memory effect, and long cycle life. Nickel-cobalt-manganese ternary cathode materials (NCM materials) have become the preferred materials for the cathode active materials of lithium-ion batteries due to their good safety and low price. However, with the development and popularization of lithium-ion batteries with higher voltage systems, the requirements for the electrical performance of lithium-ion batteries are getting higher and higher.

[0003] At present, ternary cathode materials are prone to irreversible phase transitions from H2 to H3 at high voltages and high temperatures, resulting in the precipitation of oxygen, which causes instability at the electrolyte and electrode interfaces. The battery faces problems such as poor high-temperature storage and serious gas generation during cycling. At the same time, conventional carboxylic acid ester-containing electrolytes have high conductivity, but they will be oxidized and decomposed on the surface of the battery cathode at a high voltage of 4.4V. Especially at high temperatures, it will accelerate the oxidation and decomposition of the electrolyte and promote the deterioration reaction of the cathode material.

[0004] Therefore, it is urgent 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 deficiencies of the existing technology. Summary of the Invention

[0005] The purpose of the present invention is to provide an electrolyte that enables a lithium-ion battery to have good high-temperature storage and cycling performance at a high voltage of 4.4V and good low-temperature performance.

[0006] Another purpose of the present invention is to provide a lithium-ion battery that has good high-temperature storage and cycling performance at a high voltage of 4.4V and good low-temperature performance.

[0007] To achieve the above purposes, the present invention provides an electrolyte, which includes a lithium salt, an organic solvent, and an additive. The additive includes:

[0008] Compound A shown in Structural Formula I and / or Compound B shown in Structural Formula II:

[0009]

[0010] Wherein, R1 to R6 are each independently selected from a hydrogen atom, a substituted or unsubstituted C1-C6 alkyl group, or a substituted or unsubstituted C1-C6 unsaturated hydrocarbon group.

[0011] Compared with the prior art, the electrolyte of the present invention comprises compound A represented by structural formula I and / or compound B represented by structural formula II. Both compound A and compound B contain cyclic unsaturated double bonds and are reduced to a relatively tough interfacial film (SEI film) at the positive electrode / electrolyte interface. This film has good lithium-ion conduction channels and does not cause the collapse of lithium-ion channels during cycling, thus improving the cycling and low-temperature performance. In particular, introducing symmetric carboxylic ester groups into the cyclic structure can improve the stability of the SEI containing carboxylic esters, enrich the components of the positive electrode / electrolyte interfacial film, and further improve the structural stability of the interfacial film, thereby improving the high-temperature storage performance of the lithium-ion battery.

[0012] Among them, C1-C6 alkyl represents an alkyl group with 1-6 carbon atoms. The alkyl group can be a linear alkyl group or a cycloalkyl group. The hydrogen on the ring of the cycloalkyl group can be substituted by an alkyl group. Specific examples of the alkyl group include, but are not limited to, methyl, ethyl, propyl, butyl, pentyl, and cyclohexyl.

[0013] Among them, C1-C6 unsaturated hydrocarbon group represents a hydrocarbon group with 1-6 carbon atoms, which can be a cyclic alkenyl group or a linear alkenyl group. Further, the hydrocarbon group includes, but is not limited to, vinyl, propenyl, etc.

[0014] Preferably, the compound A is selected from at least one of compound 1 to compound 3:

[0015]

[0016] Preferably, the compound B is selected from at least one of compound 4 to compound 6:

[0017]

[0018] Preferably, by mass percentage, the mass of the additive accounts for 0.1-5% of the total mass of the electrolyte, and specifically may be, but is 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%, 5%.

[0019] Preferably, the lithium salt is selected from at least one of lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium methyl sulfonate (LiCH3SO3), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF3SO2)2), lithium bis(oxalato)borate (C4BLiO8), lithium difluoro(oxalato)borate (C2BF2LiO4), lithium difluorophosphate (LiPO2F2), lithium difluoro(bis(oxalato))phosphate (LiDFBP), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0020] Preferably, by mass percentage, the mass of the lithium salt accounts for 5-25% 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%, 25%, but not limited to the listed values, and other unlisted values within this range are also applicable. Further, the mass of the lithium salt accounts for 6-20% of the total mass of the electrolyte.

[0021] Preferably, the organic solvent is selected from at least one of chain carbonates, cyclic carbonates, carboxylic acid esters, 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), n-butyl acetate (n-Ba), γ-butyrolactone (γ-Bt), n-propyl propionate (n-PP), ethyl propionate (EP), and ethyl butyrate (Eb).

[0022] Preferably, the electrolyte of the present invention further includes an additive, and the additive is selected from at least one of vinylene carbonate (VC), vinyl ethylene carbonate (VEC), fluoroethylene carbonate (FEC), ethylene sulfite (ES), 1,3-propane sultone (PS), and divinyl sulfite (DTD). By mass percentage, the mass of the additive accounts for 0.1-6.0% 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%, 6.0%. The additive can further improve the cycle performance and high-temperature storage performance of the lithium-ion battery.

[0023] Correspondingly, the present invention also provides a lithium-ion battery, including a positive electrode, a negative electrode, and the above-mentioned electrolyte, and the positive electrode is made of a nickel cobalt manganese oxide material. When the lithium-ion battery uses the above electrolyte, it can still achieve good high and low temperature discharge performance at a maximum charging voltage of 4.4V, and the cycle life of the battery is significantly increased.

[0024] Preferably, the nickel-cobalt-manganese oxide material is a high-nickel cobalt-manganese oxide, which is LiNi x Co y Mn (1-x-y) M z O2, where 0.6 ≤ x < 0.9, x + y < 1, 0 ≤ z < 0.08, and M is at least one of Al, Mg, Zr, and Ti.

[0025] 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, where the mass ratio of carbon to silicon is 90:10. Detailed implementation manners

[0026] To better illustrate the purpose, technical solution, 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 described methods are further explanatory descriptions of the present invention and should not be regarded as limitations on the present invention.

[0027] Example 1

[0028] (1) Preparation of the electrolyte solution:

[0029] Ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) are mixed according to a mass ratio of EC:DEC:EMC = 1:1:1 to obtain 81 g of an organic solvent. After mixing, 12.5 g of lithium hexafluorophosphate (LiPF6) is added. After the lithium salt is completely dissolved, 1 g of vinylene carbonate (VC), 5 g of fluoroethylene carbonate (FEC), and 0.5 g of a positive electrode protection additive compound 1 are added.

[0030] (2) Preparation of the positive electrode sheet:

[0031] The ternary lithium nickel cobalt manganese oxide material LiNi 0.6 Co 0.2 Mn 0.2 Zr 0.03 O2, a conductive agent SuperP, a binder PVDF, and carbon nanotubes (CNT) are mixed uniformly according to a mass ratio of 97.5:1.5:1:1 to form a lithium-ion battery positive electrode slurry with a certain viscosity. After the mixed slurry is coated on both sides of the aluminum foil, it is dried and roll-pressed to obtain the positive electrode sheet.

[0032] (3) Preparation of the negative electrode sheet:

[0033] After mixing artificial graphite and silicon in a mass ratio of 90:10, a slurry is prepared with a conductive agent SuperP, a thickening agent CMC, and a binder SBR (styrene-butadiene rubber emulsion) in a mass ratio of 95:1.5:1.0:2.5. After mixing evenly, the prepared slurry is coated on both sides of a copper foil, dried, and rolled to obtain a negative electrode sheet, thus making a negative electrode sheet of a lithium-ion battery that meets the requirements.

[0034] (4) Preparation of lithium-ion battery:

[0035] The positive electrode, separator, and negative electrode are made into a square battery cell in a stacked manner, packaged with a polymer, filled with the non-aqueous electrolyte of the lithium-ion battery prepared above, and a lithium-ion battery with a capacity of 1000 mAh is made after processes such as formation and grading.

[0036] The electrolyte composition components of Examples 2 - 8 and Comparative Example 1 are shown in Table 1, and the steps for preparing the electrolyte and the lithium-ion battery are the same as those in Example 1.

[0037] Table 1 Electrolyte composition components of examples and comparative examples

[0038]

[0039] The lithium-ion batteries made in Examples 1 - 8 and Comparative Example 1 are respectively subjected to normal temperature cycle tests, high temperature cycle tests, and high temperature storage tests under the following test conditions, and the test results are shown in Table 2.

[0040] Normal temperature cycling test :

[0041] Under normal temperature (25 °C) conditions, a lithium-ion battery is charged and discharged at 1.0C / 1.0C once (the battery discharge capacity is C0), the upper limit voltage is 4.4V, and then it is charged and discharged at 1.0C / 1.0C for 500 cycles under normal temperature conditions (the battery discharge capacity is C1);

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

[0043] High temperature cycling test :

[0044] Under high temperature (45 °C) conditions, a lithium-ion battery is charged and discharged at 1.0C / 1.0C once (the battery discharge capacity is C0), the upper limit voltage is 4.4V, and then it is charged and discharged at 1.0C / 1.0C for 300 cycles under normal temperature conditions (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 once (the battery discharge capacity is recorded as C0), and the upper limit voltage is 4.4V; the battery is placed in an oven at 60 °C for 15 days, taken out, placed in an environment at 25 °C, and discharged at 0.3C, and the discharge capacity is recorded as C1; then the lithium-ion battery is charged and discharged at 0.3C / 0.3C once (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 once (the battery discharge capacity is recorded as C0), and the upper limit voltage is 4.4V; the battery is placed in an oven at -20 °C for 4h, discharged at 0.3C, and 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 in examples and comparative examples

[0054]

[0055] As can be seen from Table 2, Comparative Example 1 does not contain Compound A shown in Structural Formula I and Compound B shown in Structural Formula II, and its high-temperature storage performance, high-temperature cycling performance, and low-temperature discharge performance are not ideal. However, for the batteries of Examples 1-8, the high-temperature storage performance, high-temperature cycling performance, and low-temperature performance are relatively excellent. This is because both Compound A and Compound B contain cyclic unsaturated double bonds C=C, which can be reduced to a relatively tough interfacial film (SEI film) at the positive electrode / electrolyte interface, and this film has a good lithium-ion conduction channel, so that the collapse of the lithium-ion channel does not occur during the cycle, and the cycle and low-temperature performance are improved. In particular, introducing a symmetric carboxylic ester group into the cyclic structure can improve the stability of the SEI containing carboxylic ester, enrich the components of the positive electrode / electrolyte interfacial film, and further improve the structural stability of the interfacial film, thereby improving the high-temperature storage performance of the lithium-ion battery.

[0056] From the data of Examples 1-6, it can also be known that using Compounds 1-3 as additives has more advantages in low-temperature performance than Compounds 4-6.

[0057] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced 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 additive includes: Compound A represented by Structural Formula I and / or Compound B represented by Structural Formula II: Wherein, R1 to R6 are each independently selected from a hydrogen atom, a substituted or unsubstituted C1-C6 alkyl group, and a substituted or unsubstituted C1-C6 unsaturated hydrocarbon group.

2. The electrolyte according to claim 1, characterized in that, Compound A is selected from at least one of Compound 1 to Compound 3:

3. The electrolyte according to claim 1, wherein Compound B is selected from at least one of Compound 4 to Compound 6: 。 4. The electrolyte according to claim 1, wherein The mass of the additive accounts for 0.1-5% of the total mass of the electrolyte.

5. The electrolyte according to claim 1, characterized in that, The lithium salt is selected from at least one of lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium methylsulfonate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium difluorophosphate, lithium difluorobis(oxalato)phosphate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethylsulfonyl)imide.

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

7. The electrolyte according to claim 1, wherein The organic solvent is selected from at least one of chain carbonates, cyclic carbonates, carboxylic acid esters, ethers, and heterocyclic compounds.

8. The electrolyte according to claim 1, wherein It further includes an auxiliary agent, and the auxiliary agent is selected from at least one of vinylene carbonate, ethylene vinyl carbonate, fluorinated ethylene carbonate, ethylene sulfite, 1,3-propane sultone, and ethylene sulfate.

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

10. The lithium ion battery according to claim 9, wherein, The nickel cobalt manganese oxide material is LiNi x Co y Mn (1-x-y) M z O2, where 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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