Electrolyte and lithium ion battery containing the same

Abstract

The application discloses an electrolyte and a lithium ion battery containing the electrolyte. The electrolyte comprises a lithium salt, an organic solvent and an additive. The additive is at least one compound shown in a structural formula I. In the formula, R1-R3 are independently selected from a hydrogen atom, a substituted or unsubstituted C1-C12 alkyl, a substituted or unsubstituted C1-C12 unsaturated hydrocarbon group and a halogen. The electrolyte can improve the high-temperature cycle performance, high-temperature storage performance and low-temperature performance of the battery.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion batteries, and particularly relates to an electrolyte and a lithium ion battery containing the electrolyte. BACKGROUND

[0002] Lithium ion batteries are widely used in 3C digital, power tools, aerospace, energy storage, electric vehicles and other fields due to their high specific energy, no memory effect, long cycle life and other advantages. Nickel-cobalt-manganese ternary positive electrode material (NCM material) is the preferred material for the positive active material of lithium ion batteries due to its good safety and low price, but as higher voltage system lithium ion batteries develop and popularize, the electrical performance requirements of lithium ion batteries are getting higher and higher.

[0003] Currently, the ternary positive electrode material is prone to H2-H3 irreversible phase transition at high voltage and high temperature, leading to the evolution of oxygen, thereby causing the instability of the electrolyte and the electrode interface, and the problems of poor high-temperature storage and serious cycle gas production of the battery. At the same time, the conventional carboxylate-containing electrolyte has high electrical conductivity, but it will be oxidized and decomposed on the surface of the positive electrode of the battery at a high voltage of 4.4V, especially under high temperature conditions, which will accelerate the oxidation and decomposition of the electrolyte and promote the deterioration reaction of the positive electrode material.

[0004] Therefore, it is urgent to develop an electrolyte that can withstand a high voltage of 4.4V, thereby realizing the excellent performance of lithium ion batteries and solving the problems of the prior art. SUMMARY

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

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

[0007] To achieve the above purposes, the present application provides an electrolyte comprising a lithium salt, an organic solvent and an additive, the additive being selected from at least one of the compounds shown in structural formula I:

[0008]

[0009] wherein R1-R3 are each independently selected from a hydrogen atom, a halogen, a substituted or unsubstituted C1-C12 alkyl group, and a substituted or unsubstituted C1-C12 unsaturated hydrocarbon group.

[0010] Compared with the prior art, the electrolyte of the present application contains at least one compound shown in structural formula I as an additive, which is a positive electrode protection additive. Specifically, the compound shown in structural formula I contains a cyclic unsaturated double bond, which is reduced to a relatively tough interface film (SEI film) at the positive electrode / electrolyte interface, and the film has a good lithium ion conduction channel and does not collapse during the cycle process, thereby improving the high-temperature cycle and low-temperature performance of the battery. In particular, at least two oxygen elements and C=O bonds are introduced into the cyclic structure, which enriches the electrode / electrolyte interface film components and further improves the lithium ion conduction performance of the interface film, thereby improving the low-temperature performance of the lithium ion battery.

[0011] wherein C1-C12 alkyl represents an alkyl group having 1-12 carbon atoms, which can be a linear alkyl group or a cyclic alkyl group, and the hydrogen on the ring of the cyclic alkyl group can be replaced by an alkyl group. Preferably, the alkyl group is selected from an alkyl group having 1-6 carbon atoms, and as specific examples of the alkyl group, there can be mentioned, but are not limited to, methyl, ethyl, propyl, butyl, pentyl and cyclohexyl.

[0012] wherein C1-C12 unsaturated alkyl represents an alkyl group having 1-12 carbon atoms. Preferably, it can be, but is not limited to, a vinyl group, a propenyl group, etc.

[0013] wherein halogen can be, but is not limited to, F, Cl, Br.

[0014] Preferably, the compound shown in structural formula I is selected from at least one of compound 1 to compound 6:

[0015]

[0016] Preferably, the mass of the additive accounts for 0.1-5% of the total mass of the electrolyte, and specifically can 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%. Further, the mass of the additive accounts for 0.2-3% of the total mass of the electrolyte.

[0017] Preferably, the lithium salt is selected from at least one of lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium methylsulfonate (LiCH3SO3), lithium trifluoromethylsulfonate (LiCF3SO3), lithium bis(trifluoromethylsulfonyl)imide (LiN(CF3SO2)2), lithium bis(oxalato)borate (C4BLiO8), lithium difluoro(oxalato)borate (C2BF2LiO4), lithium difluorophosphate (LiPO2F2), lithium difluoro(bisoxalato)phosphate (LiDFBP), lithium bisfluorosulfonylimide (LiFSI), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI).

[0018] Preferably, the mass of the lithium salt accounts for 5-25% of the total mass of the electrolyte, and specifically can be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, but is not limited to the listed values, and other values not listed in the range are also applicable. Further, the mass of the lithium salt accounts for 6-20% of the total mass of the electrolyte.

[0019] 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 application 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%, preferably more than 70%, more preferably more than 75%, of the total mass of the electrolyte, such as but not limited to 78%, 80%, 82%, 85%, etc.

[0020] Preferably, the electrolyte of the present application further comprises an additive selected from at least one of vinylene carbonate (VC), vinylene ethylene carbonate (VEC), fluoroethylene carbonate (FEC), ethylene sulfite (ES), 1,3 propanesulfonic acid lactone (PS) and ethylene sulfate (DTD). The mass of the additive accounts for 0.1-6.0% of the total mass of the electrolyte, and specifically can be but not limited to 0.1%, 0.5%, 1.5%, 2%, 2.5%, 3%, 4%, 4.5%, 5%, 5.5%, 6.0%. The addition of the additive can further improve the cycle performance and high-temperature storage performance of the lithium ion battery.

[0021] Correspondingly, the present application also provides a lithium ion battery comprising a positive electrode and a negative electrode, and further comprising the above-mentioned electrolyte, and the positive electrode is made of a nickel-cobalt-manganese oxide material. The lithium ion battery uses the above-mentioned electrolyte, and can still achieve good high and low temperature discharge performance when the maximum charging voltage is 4.4V, and the cycle life of the battery is significantly increased.

[0022] 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, 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.

[0023] Preferably, the negative electrode of the present invention is made of carbon negative electrode material, silicon negative electrode material, or 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 Implementation

[0024] To better illustrate the purpose, technical solution, and beneficial effects of this invention, the invention will be further described below with reference to specific embodiments. It should be noted that the methods described below are further explanations of this invention and should not be construed as limiting it.

[0025] Example 1

[0026] (1) Preparation of electrolyte:

[0027] Ethylene carbonate (EC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC) were mixed in a mass ratio of EC:DEC:EMC = 1:1:1 to prepare 81g of organic solvent. After mixing, 12.5g of lithium hexafluorophosphate (LiPF6) was added. After the lithium salt was completely dissolved, 1g of vinylene carbonate (VC), 5g of fluoroethylene carbonate (FEC), and 0.5g of positive electrode protection additive compound 1 were added.

[0028] (2) Preparation of the positive electrode:

[0029] LiNi nickel cobalt manganese ternary material LiNi 0.6 Co 0.2 Mn 0.2 Zr 0.03 O2, conductive agent SuperP, binder PVDF and carbon nanotubes (CNT) are mixed evenly in a mass ratio of 97.5:1.5:1:1 to prepare a lithium-ion battery positive electrode slurry of a certain viscosity. The mixed slurry is coated on both sides of aluminum foil, dried and rolled to obtain the positive electrode sheet.

[0030] (3) Preparation of negative electrode:

[0031] Artificial graphite and silicon are mixed at a mass ratio of 90:10, and then mixed with conductive agent SuperP, thickener CMC, and binder SBR (styrene-butadiene rubber latex) at a mass ratio of 95:1.5:1.0:2.5 to form a slurry. After being mixed evenly, the slurry is coated on both sides of copper foil, dried, and rolled to obtain a negative electrode sheet, thus producing a lithium-ion battery negative electrode sheet that meets the requirements.

[0032] (4) Preparation of lithium-ion batteries:

[0033] The positive electrode, separator, and negative electrode are stacked to form a square cell, which is then packaged with polymer and filled with the prepared non-aqueous electrolyte for lithium-ion batteries. After formation, capacity testing, and other processes, a lithium-ion battery with a capacity of 1000mAh is produced.

[0034] The electrolyte composition of Examples 2-8 and Comparative Example 1 is shown in Table 1. The steps for preparing the electrolyte and lithium-ion battery are the same as in Example 1.

[0035] Table 1. Electrolyte composition of the examples and comparative examples

[0036]

[0037]

[0038] The lithium-ion batteries prepared in Examples 1-8 and Comparative Example 1 were subjected to room temperature cycling test, high temperature cycling test, high temperature storage test and low temperature discharge test under the following test conditions. The test results are shown in Table 2.

[0039] room temperature cycling test :

[0040] Under normal temperature (25℃) conditions, the lithium-ion battery is charged and discharged once at 1.0C / 1.0C (battery discharge capacity is C0), with an upper limit voltage of 4.4V. Then, it is charged and discharged for 500 cycles at 1.0C / 1.0C under normal temperature conditions (battery discharge capacity is C1).

[0041] Capacity retention rate = (C1 / C0) * 100%

[0042] high temperature cycling test :

[0043] Under high temperature (45℃) conditions, the lithium-ion battery is charged and discharged once at 1.0C / 1.0C (battery discharge capacity is C0), with an upper limit voltage of 4.4V, and then charged and discharged at 1.0C / 1.0C for 300 cycles at room temperature (battery discharge capacity is C1).

[0044] Capacity retention rate = (C1 / C0) * 100%

[0045] high temperature storage test :

[0046] Under normal temperature (25℃) conditions, the lithium-ion battery was subjected to one 0.3C / 0.3C charge and discharge cycle (the battery discharge capacity is recorded as C0), with an upper limit voltage of 4.4V; the battery was placed in a 60℃ oven for 15 days, then removed and placed in a 25℃ environment for 0.3C discharge, with the discharge capacity recorded as C1; then the lithium-ion battery was subjected to one 0.3C / 0.3C charge and discharge cycle (the battery discharge capacity is recorded as C2);

[0047] Capacity retention rate = (C1 / C0) * 100%

[0048] Capacity recovery rate = (C2 / C0) * 100%

[0049] low temperature discharge test :

[0050] At room temperature (25℃), a lithium-ion battery was subjected to a single 0.3C / 0.3C charge and discharge cycle (the battery discharge capacity is recorded as C0), with an upper limit voltage of 4.4V. The battery was then placed in a -20℃ oven for 4 hours, followed by a 0.3C discharge, with the discharge capacity recorded as C1 and a cutoff voltage of 3.0V.

[0051] Capacity retention rate = (C1 / C0) * 100%

[0052] Table 2. Performance test results of lithium-ion batteries in the examples and comparative examples.

[0053]

[0054] As shown in Table 2, compared to Comparative Example 1, the battery prepared using the electrolyte of this invention exhibits significant improvements in 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 contains cyclic unsaturated double bonds, which are reduced to a relatively robust interface film (SEI film) at the positive electrode / electrolyte interface. This film possesses excellent lithium-ion conduction channels, preventing collapse of lithium-ion channels during cycling and thus significantly improving the battery's high-temperature cycling and low-temperature performance. In particular, the introduction of at least two oxygen elements and C=O bonds into the cyclic structure enriches the composition of the electrode / electrolyte interface film, further improving the lithium-ion conduction performance of the interface film and thereby enhancing the low-temperature performance of the lithium-ion battery.

[0055] As can be seen from the data in Example 1, its low-temperature discharge performance is excellent. This may be because the SEI formed by compound 1 containing the symmetrical side chain has a relatively high lithium-ion shuttle rate. At low temperatures, the SEI channels are not easy to shrink, thus improving the low-temperature performance of the battery.

[0056] The data from Example 3 also show that using Compound 3 as an additive has advantages in high-temperature performance. This may be because the two C=C double bonds of Compound 3 undergo stepwise polymerization to form a relatively complete SEI. This SEI effectively isolates the direct contact between the electrolyte and the electrode material, suppresses the further generation of side reactions in the electrolyte, and the SEI formed by the C=C double bonds has relatively better stability under high voltage and high temperature conditions. Therefore, it can effectively improve the high-temperature performance of the battery.

[0057] 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 modifications or equivalent substitutions can be made to the technical solutions of the present invention 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 additives, characterized in that, The additive is selected from compound 3, and the mass of the additive accounts for 0.1% to 5% of the total mass of the electrolyte. 。 2. The electrolyte as described in claim 1, characterized in that, The additive accounts for 0.2% to 3% of the total mass of the electrolyte.

3. The electrolyte as described in claim 1, characterized in that, The lithium salt is selected from at least one of lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium methanesulfonate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonyl)imide, lithium dioxalate borate, lithium difluorooxalate borate, lithium difluorophosphate, lithium difluorobis(oxalate) phosphate, and lithium bis(fluorosulfonyl)imide.

4. The electrolyte as described in claim 1, characterized in that, The lithium salt accounts for 5-25% of the total mass of the electrolyte.

5. The electrolyte as described in claim 1, characterized in that, The organic solvent is selected from at least one of chain carbonates, cyclic carbonates, carboxylic acid esters, and ethers.

6. The electrolyte as described in claim 1, characterized in that, It also includes an additive selected from at least one of vinylene carbonate, vinylene carbonate, fluorovinyl carbonate, vinyl sulfite, 1,3-propanesulfonate lactone, and vinyl sulfate.

7. A lithium-ion battery, comprising a positive electrode and a negative electrode, characterized in that, It also includes the electrolyte as described in any one of claims 1 to 6, wherein the positive electrode is made of a nickel-cobalt-manganese oxide material.

8. The lithium-ion battery as described in claim 7, characterized in that, 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.

Images