Electrolyte and lithium ion battery containing the same

Abstract

The application discloses an electrolyte, which comprises a lithium salt, an organic solvent and an additive, wherein the additive comprises at least one bridged tricyclic anhydride compound shown in structural formula I: wherein R1-R4 are each independently selected from a hydrogen atom, a sulfur atom, a nitrogen atom or a group containing 1-6 carbon atoms. The bridged tricyclic anhydride compound shown in structural formula I is used as an additive of the electrolyte, and when a lithium ion battery is in a formation stage, the additive can form an SEI film with excellent mechanical properties at a negative electrode material interface, so as to relieve the rupture of negative electrode particles caused by volume expansion in a charging process, thereby improving the normal-temperature cycle performance, high-temperature cycle performance and high-temperature storage performance of the battery.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and particularly relates to an electrolyte and a lithium-ion battery containing the electrolyte. Background Technology

[0002] In recent years, with the popularization of portable electronic devices and the development of power tools and electric vehicles, lithium-ion batteries, as a new generation of high-energy power sources, have received widespread attention. Currently, the main cathode materials for commercially available lithium-ion batteries include lithium manganese oxide, lithium cobalt oxide, ternary materials, and lithium iron phosphate. However, with the development and popularization of lithium-ion batteries with higher voltage systems, conventional electrolytes, in addition to their own oxidation and decomposition reactions, also undergo oxidation and decomposition on the surface of the cathode material. Cathode materials containing transition metal elements act as catalysts, thus promoting electrolyte oxidation. In particular, during normal operation, the negative electrode of a lithium-ion battery is prone to volume expansion leading to rupture, and the electrolyte undergoes a series of adverse reactions on the negative electrode material, continuously generating deposits, increasing the thickness of the solid electrolyte interface layer, ultimately leading to increased internal resistance, deterioration of cycle performance, and shortened battery life. Summary of the Invention

[0003] The purpose of this invention is to provide an electrolyte that can act on the negative electrode of a lithium-ion battery, enabling the lithium-ion battery to have good room temperature cycling performance, high temperature cycling performance, and high temperature storage performance at high voltages (e.g., 4.3V).

[0004] Another object of the present invention is to provide a lithium-ion battery containing the electrolyte, which has good room temperature cycling performance, high temperature cycling performance and high temperature storage performance at high voltage (e.g., 4.3V).

[0005] To achieve the above objectives, the present invention provides an electrolyte comprising a lithium salt, an organic solvent, and an additive, wherein the additive comprises at least one of the bridged tricyclic anhydride compounds represented by structural formula I:

[0006]

[0007] R1-R4 are each independently selected from hydrogen atoms, sulfur atoms, nitrogen atoms, or groups containing 1-6 carbon atoms.

[0008] In some embodiments, groups with 1-6 carbon atoms, for example, include substituted or unsubstituted C1-C6 alkyl groups, substituted or unsubstituted C1-C6 unsaturated hydrocarbon groups, phenyl groups, etc. C1-C6 alkyl groups refer to alkyl groups with 1-6 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 substituted by the alkyl group. Specific examples of alkyl groups include, but are not limited to, methyl, ethyl, propyl, butyl, pentyl, and cyclohexyl. C1-C6 unsaturated hydrocarbon groups refer to hydrocarbon groups with 1-6 carbon atoms. Preferably, they can be, but are not limited to, vinyl, propenyl, etc.

[0009] Preferably, the bridged tricyclic anhydride compound is selected from at least one of compounds 1 to 10:

[0010]

[0011]

[0012] Compound 7: 4,7-bridged methoxy-3a,7a-(bridged methoxy-bridged methylene)isobenzofuran-1,3,8,10(4H,

[0013] 7H)-Tetraketone

[0014]

[0015] Compound 10: Exo-2-methylthio-3-aminobicyclo[2.2.1]-5-heptene-2,3-dicarboxylic anhydride.

[0016] This invention also provides a preparation route for the bridged tricyclic anhydride compound shown in structural formula I. Taking compound 1 as an example, the preparation route for compound 1 (5-norbornene-2,3-dicarboxylic anhydride) is as follows:

[0017]

[0018] As a further aspect of the present invention, the specific preparation steps include using maleic anhydride and mixed C5 compounds as raw materials, reacting at a temperature of 0-5°C, whereby maleic anhydride absorbs cyclopentadiene to generate 5-norbornene-2,3-dicarboxylic anhydride, which is then crystallized to obtain pure 5-norbornene-2,3-dicarboxylic anhydride. The mixed C5 compounds consist of cyclopentadiene, isoprene, and isoprene. More specifically, maleic anhydride and ethyl acetate are added to a dry conical flask and heated in a water bath to completely dissolve them. Petroleum ether is then added, shaken well, and cooled in an ice-water bath. Freshly distilled cyclopentadiene is then added, and the reaction flask is shaken in an ice-water bath. When the reaction is no longer exothermic and white crystals have precipitated in the flask, the reaction mixture is heated in a water bath to redissolve the solid, and then allowed to cool slowly to obtain white needle-like crystals. The crystals are collected by filtration and dried to obtain the final compound 1 (5-norbornene-2,3-dicarboxylic anhydride).

[0019] Specifically, 3 g (0.03 mol) of maleic anhydride and 10 mL of ethyl acetate were added to a dry 100 mL Erlenmeyer flask. The mixture was heated in a water bath until completely dissolved. Then, 10 mL of petroleum ether (boiling point 60-90 °C) was added, and the mixture was shaken well and cooled in an ice-water bath. Next, 3 mL (2.4 g, 0.037 mol) of freshly distilled cyclopentadiene was added, and the reaction flask was shaken in an ice-water bath to prevent the loss of cyclopentadiene due to volatilization. When the reaction was no longer exothermic, white crystals precipitated in the flask. The reaction mixture was heated in a water bath to redissolve the solid, and then allowed to cool slowly to obtain white needle-like crystals of cis-5-norbornene-endo-2,3-dicarboxylic anhydride. The crystals were collected by suction filtration.

[0020] Based on the preparation route of compound 1, compounds 2 to 10 can be prepared, which will not be elaborated here.

[0021] In some embodiments, the additive accounts for 0.05-0.5% of the total mass of the electrolyte, specifically, but not limited to, 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, and 0.5%. Further, the additive accounts for 0.05-0.3% of the total mass of the electrolyte, more preferably 0.1%.

[0022] In some embodiments, the lithium salt is selected from one or more of lithium dioxaborate, lithium difluorooxaborate, lithium difluorophosphate, lithium difluorobis(oxaborate) phosphate, lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium methanesulfonate, lithium trifluoromethanesulfonate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide.

[0023] In some embodiments, 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%, and 25%, but is not limited to the listed values; other unlisted values ​​within this range also apply. Further, the lithium salt accounts for 6-20% of the total mass of the electrolyte.

[0024] In some embodiments, 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 may 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 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%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, etc.

[0025] In some embodiments, the electrolyte of the present invention further includes an additive selected from at least one of vinylene carbonate (VC), 1,3-propanesulfonate lactone (PS), 1,4-butanesulfonate lactone (BS), acrylate lactone (RPS), ethylene ethylene carbonate (VEC), fluoroethylene carbonate (FEC), and lithium difluorophosphate (LiPO2F2). The additive accounts for 0.1-8.0% 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%, 7%, and 8%. Further, the additive accounts for 0.1-6% of the total mass of the electrolyte. The addition of the additive can further improve the cycle performance and high-temperature storage performance of lithium-ion batteries.

[0026] Accordingly, the present invention also provides a lithium-ion battery, including a positive electrode, a negative electrode, and the electrolyte mentioned above. Using the electrolyte described above, this lithium-ion battery can still achieve good room-temperature cycling performance, high-temperature cycling performance, and high-temperature storage performance even at a maximum charging voltage of 4.3V.

[0027] In some embodiments, the active material of the positive electrode includes Li. (1+a) Ni 1-x-y-z Co x Mn y Al z O2+b , where -0.10≤a≤0.50, 0≤x≤1, 0≤y≤1, 0≤z≤1, -0.05≤b≤0.10.

[0028] In some embodiments, the negative electrode comprises at least one of artificial graphite, natural graphite, Si and its alloys, Sn and its alloys, lithium metal and its alloys, transition metal oxides, or lithium titanate. Lithium metal negative electrode is preferred, as it has a high theoretical specific capacity (3860 mAh / g), but it is prone to lithium dendrite formation during cycling, leading to decreased battery cycle stability. The additive of this invention, due to its structure combining anhydride groups, a three-membered ring structure, and a carbon-carbon double bond structure, has a low least unoccupied molecular orbital (LUMO) energy level, approximately -2.123 eV, and can preferentially participate in film formation on the lithium metal negative electrode, forming a robust three-dimensional network structure that protects the lithium metal negative electrode.

[0029] The beneficial effects of this invention are:

[0030] This invention uses a bridged tricyclic anhydride compound as shown in structural formula I as an additive for the electrolyte. During the formation stage of the lithium-ion battery, this additive can form an SEI film with excellent mechanical properties at the interface of the negative electrode material, so as to alleviate the rupture of the negative electrode particles due to volume expansion during charging, thereby improving the battery's room temperature cycle performance, high temperature cycle performance and high temperature storage performance. Detailed Implementation

[0031] 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.

[0032] Example 1

[0033] (1) Preparation of electrolyte:

[0034] 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 82.45 g of organic solvent. After mixing, 12.5 g of lithium hexafluorophosphate (LiPF6) was added. After the lithium salt was completely dissolved, 5 g of fluoroethylene carbonate (FEC) and 0.05 g of additive compound 1 were added.

[0035] (2) Manufacturing of lithium-ion batteries:

[0036] The electrolyte was injected into the ternary material LiNi 0.8 Co 0.1 Mn 0.1In a button lithium metal battery with O2 as the positive electrode and lithium metal as the negative electrode, it is charged three times with a constant current of 0.1C, then charged to 4.3V with a constant current of 0.5C, and discharged to 2.8V with a constant current of 0.5C.

[0037] The electrolyte composition of Examples 2-15 and Comparative Examples 1-7 is shown in Table 1. The steps for preparing the electrolyte and lithium-ion battery are the same as in Example 1.

[0038] Table 1. Electrolyte composition of the examples and comparative examples

[0039]

[0040]

[0041] In Comparative Example 7, a bridged bicyclic anhydride compound was used as an additive in the electrolyte, and its structural formula is as follows:

[0042]

[0043] The lithium-ion batteries prepared in Examples 1-15 and Comparative Examples 1-7 were subjected to room temperature cycling tests, high temperature cycling tests, and high temperature storage tests under the following test conditions. The test results are shown in Table 2. The test items are as follows:

[0044] ambient temperature cycling performance test :

[0045] The formed lithium-ion battery was charged to 4.3V under a constant current and constant voltage (0.5C) with a cutoff current of 0.02C, and then discharged to 2.8V under a constant current (0.5C). This cycle of 0.5C charge / discharge was repeated 200 times to evaluate cycle performance. Cycle performance was calculated using the capacity retention rate formula below:

[0046] Capacity retention rate (%) = (Discharge capacity of the 200th cycle / Initial discharge capacity) × Discharge capacity per cycle. High temperature cycle Ring performance test :

[0047] The lithium-ion battery was stored at 45°C, and then charged to 4.3V under constant current and constant voltage at 0.5C, with a cutoff current of 0.02C. It was then discharged to 2.8V under constant current at 0.5C, and this cycle of 0.5C charge / discharge was repeated 100 times to evaluate its cycle performance. The cycle performance was calculated using the capacity retention rate formula below.

[0048] Capacity retention rate (%) = (Discharge capacity of the 100th cycle / Initial discharge capacity) × Discharge capacity per cycle. High temperature storage Storage performance test :

[0049] The formed lithium-ion battery was charged to 4.3V under constant current and constant voltage at 0.5C, stored at 45℃ for 14 days, and then discharged to 2.8V under constant current at 0.5C. The capacity recovery rate of the battery was then measured.

[0050] Capacity recovery rate (%) = recovered capacity / initial capacity × initial capacity.

[0051] Table 2. Performance test results of lithium-ion batteries in the examples and comparative examples.

[0052]

[0053]

[0054] As shown in Table 2, compared with Comparative Examples 1-7, the lithium-ion battery prepared using the electrolyte containing the bridged tricyclic anhydride compound of the present invention exhibits significant improvements in room temperature cycling performance, high-temperature storage performance, and high-temperature cycling performance. This is because the bridged tricyclic anhydride compound shown in Formula I is used as an additive in the electrolyte. During the formation stage of the lithium-ion battery, this additive can form an SEI film with excellent mechanical properties at the interface of the negative electrode material, thereby mitigating the cracking of negative electrode particles due to volume expansion during charging, and thus improving the battery's room temperature cycling performance, high-temperature cycling performance, and high-temperature storage performance.

[0055] As can be seen from the data in Example 2, its high-temperature storage performance is excellent. This example uses Compound 1 as a functional additive. Compared with other functional additives with different structures mentioned in this invention, Compound 1 only has anhydride groups, a three-membered ring structure, and carbon-carbon double bonds. It can form a robust and low-resistance SEI film at the electrode / electrolyte interface, avoiding the formation of a high-resistance and loosely structured SEI film due to the additional large volume of aromatic groups or other functional groups that easily generate gas. Therefore, Example 2, which uses an appropriate amount of Compound 1, exhibits superior high-temperature storage performance.

[0056] The data from Example 13 also show that using Compound 10 as an electrolyte additive has advantages in high-temperature cycling performance. This is likely because Compound 10 has an S-containing group in its structure, resulting in an SEI film with lower impedance and higher stability. However, due to the presence of its amino structure, it is prone to gas generation. Therefore, in terms of high-temperature storage performance, Compound 10 is slightly inferior to Compound 1.

[0057] Data from Examples 1 and 14-15 show that the simultaneous addition of the bridged tricyclic anhydride compound, fluoroethylene carbonate (FEC), vinylene carbonate (VC), or 1,3-propane sulpholactone (PS) shown in Formula 1 to the non-aqueous electrolyte can further improve the battery's room temperature cycle performance, high temperature cycle performance, and high temperature storage performance.

[0058] As shown in Example 2 compared to Comparative Example 7, the use of a bridged tricyclic anhydride compound as an additive significantly improves the room-temperature cycling performance, high-temperature cycling performance, and high-temperature storage performance of lithium-ion batteries compared to a bridged dicyclic anhydride compound. This is because the bridged tricyclic anhydride compound of the present invention possesses anhydride groups, a three-membered ring structure, and a carbon-carbon double bond structure simultaneously. These three structures readily undergo electron gain and loss during battery charging and discharging. Therefore, they can form a robust three-dimensional network structure at the electrode / electrolyte interface together with other electrolyte components, constructing a solid electrolyte interphase (SEI) film. This inhibits a series of decomposition reactions of the electrolyte during battery cycling and protects the main structure of the electrode material, thereby improving room-temperature cycling performance, high-temperature cycling performance, and high-temperature storage performance.

[0059] 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 at least one of compound 2, compound 5, compound 6, compound 8, and compound 9: Compound 2 Compound 5 Compound 6, Compound 8, Compound 9.

2. The electrolyte as described in claim 1, characterized in that, The additive accounts for 0.05%-0.5% of the total mass of the electrolyte.

3. The electrolyte as described in claim 1, characterized in that, The lithium salt is selected from one or more of lithium dioxaborate, lithium difluorooxaborate, lithium difluorophosphate, lithium difluorobis(oxaborate) phosphate, lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium methanesulfonate, lithium trifluoromethanesulfonate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide.

4. 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.

5. The electrolyte as described in claim 4, characterized in that, The organic solvent is selected from at least one of ethylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, propylene carbonate, butyl acetate, γ-butyrolactone, propyl propionate, ethyl propionate, and ethyl butyrate.

6. The electrolyte as described in claim 1, characterized in that, It also includes an additive selected from at least one of vinylene carbonate, 1,3-propanesulfonate lactone, 1,4-butanesulfonate lactone, acrylate lactone, ethylene ethylene carbonate, and fluoroethylene carbonate.

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 1-6.

8. The lithium-ion battery as described in claim 7, characterized in that, The active material of the positive electrode includes Li （1+a） Ni 1-x-y-z Co x Mn y Al z O 2+b , where -0.10≤a≤0.50, 0≤x≤1, 0≤y≤1, 0≤z≤1, -0.05≤b≤0.

10.

9. The lithium-ion battery as described in claim 7, characterized in that, The negative electrode comprises at least one of artificial graphite, natural graphite, Si and its alloys, Sn and its alloys, lithium metal and its alloys, transition metal oxides or lithium titanate.

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