Electrolyte additive, electrolyte with electrolyte additive and secondary battery

Abstract

The invention discloses an electrolyte additive, an electrolyte with the electrolyte additive and a secondary battery, the electrolyte additive is diamino-terminated siloxane, the additive has relatively low oxidation film-forming potential, a stable CEI film with high ionic conductivity can be formed on the surface of a positive electrode, and the electrolyte additive can be applied to a lithium ion battery. And a flexible and compact solid electrolyte interface (SEI) film can be formed on the surface of the electrode, so that the stability of the electrode is improved, the electrolyte is protected from being decomposed, and the high-voltage tolerance is improved, thereby improving the specific capacity and cycle life of the battery.

Description

Technical Field

[0001] The invention relates to an electrolyte additive, an electrolyte having the electrolyte additive and a secondary battery, belonging to the technical field of secondary batteries. Background Art

[0002] With the development of society, people's demand for high-energy-density batteries is increasing. In order to meet the above needs, scientists have developed a variety of new high-voltage positive electrode materials, such as lithium-rich layered oxides, lithium cobalt oxide materials, nickel-cobalt-manganese materials, and lithium-rich manganese-based materials. However, the traditional electrolyte with lithium hexafluorophosphate as lithium salt and carbonate as solvent generally has a maximum withstand voltage below 4.3V. The use of high-voltage positive electrode materials will cause them to decompose severely, which will lead to problems such as increased battery impedance and decreased coulombic efficiency. In addition, since traditional carbonate electrolytes cannot form an effective passivation film on the positive electrode, the HF derived from lithium hexafluorophosphate will attack the oxide positive electrode, resulting in the dissolution of the transition metal.

[0003] Using efficient electrolyte additives to develop high-voltage resistant electrolytes is an effective strategy to stabilize the electrode / electrolyte interface, protect the electrode from corrosion, and prevent the electrolyte from oxidative decomposition. Studies have shown that most high-voltage electrolyte additives have a low oxidation potential and can decompose at the electrode / electrolyte interface before the main electrolyte solvent, forming a stable protective film. On the one hand, this film can protect the positive electrode from corrosion damage such as hydrofluoric acid in the electrolyte, and on the other hand, it can prevent the electrolyte from direct contact with the electrode and oxidative decomposition under high pressure.

[0004] Silicone is a multifunctional compound with a wide range of applications. In recent years, silicone has been introduced into the battery field. Studies have shown that amino groups have lithium affinity and multiple hydrogen bonding, which can improve the solvation structure of lithium ions, regulate the electrochemical window of the electrolyte, and improve the electrode / electrolyte interface film properties of the battery.

[0005] For example, Chinese patent application CN114824450A proposed that the acrylamide group in acrylamide silane has lithium affinity and multiple hydrogen bonding, which can regulate the interface problems of lithium-ion batteries, uniform lithium deposition / dissolution, and improve the cycle life of high-nickel ternary NCA lithium batteries. In addition, studies have shown that protective films containing nitrogen and silicon components have high stability and conductivity, and have certain potential in improving the performance of high-voltage batteries.

[0006] Patent EP2585817A1 introduces aminosilane derivatives (such as aminopropyltriethoxysilane) to improve the interface reaction between the electrode and the electrolyte, reduce the interface impedance, and thus improve the charge and discharge efficiency and cycle performance of the battery. In addition, aminosilane can react with hydrofluoric acid in the electrolyte to achieve Si-O-Si cross-linking film formation and acid and water removal, thereby protecting the electrode surface.

[0007] Chinese patent CN105552438B discloses a lithium-ion battery electrolyte and a preparation method thereof, wherein the additives include a first type of additive silazane compound or carbodiimide compound with the function of removing water and reducing acid, and a second type of additive thiocarboxylate compound or phosphite compound with the function of stabilizing the chromaticity change of the electrolyte, and at the same time, the water content, acidity and chromaticity of the electrolyte are controlled, the storage time of the electrolyte is extended, and the quality of the electrolyte is ensured. Bisamino-terminated siloxane has higher interfacial activity and hydrophilicity.

[0008] Chinese patent 202311252439.9 used diamino-terminated polydimethylsiloxane to directly modify the surface of the lithium negative electrode to protect the electrode surface and prevent the rupture of the SEI film.

[0009] However, the ion conductivity of the membrane layer constructed by the above surface modification is poor, especially under high voltage or high current charging and discharging conditions, the charging and discharging efficiency drops sharply. In addition, during high voltage charging and discharging, the non-in-situ film formation is difficult to withstand the huge volume change and is easy to fall off, resulting in corrosion of the electrode interface and oxidative decomposition of the electrolyte. Summary of the invention

[0010] The object of the present invention is to provide an electrolyte additive, an electrolyte having the same, and a secondary battery, wherein the additive has a low oxidation film-forming potential, can form a stable, high-ionic conductivity CEI film on the positive electrode surface, and can form a flexible and dense solid electrolyte interface (SEI) film on the electrode surface, improve electrode stability, and protect the electrolyte from being decomposed, improve its high-voltage tolerance, thereby improving the battery specific capacity and cycle life. Not only that, relying on the structural advantages of the additive itself, it can buffer the large volume change of the electrode under high pressure, while protecting the electrolyte body and the electrode surface from being damaged. Thereby improving the battery specific capacity and cycle life.

[0011] In order to achieve the above object, the technical solution adopted by the present invention is:

[0012] An electrolyte additive, the electrolyte additive is a bisamino-terminated siloxane, and its general structural formula is as follows:

[0013]

[0014] Among them, R 1and R 2 R is independently selected from hydrocarbon groups having 1 to 6 carbon atoms; 3 and R 4 Independently selected from hydrocarbon groups, hydrocarbonoxy groups, acyloxy groups, amine groups, amineoxy groups, amide groups or oxime groups having 1 to 4 carbon atoms; n is any integer between 1 and 5.

[0015] Preferably, R 1 and R 2 Independently selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, allyl, propargyl, phenyl, trifluoroethyl or pentafluoropropyl.

[0016] Preferably, the electrolyte additive is at least one of the following formulae:

[0017]

[0018] An electrolyte comprises an electrolyte salt, an organic solvent and any one of the above electrolyte additives, wherein the concentration of the electrolyte salt is 0.5-2 mol / L, and the amount of the electrolyte additive is 0.1-5 wt% of the total mass of the electrolyte.

[0019] Preferably, the electrolyte salt is at least one of lithium difluorophosphate, lithium hexafluorophosphate, lithium difluorooxalatoborate, lithium tetrafluoroborate, lithium bisfluorosulfonyl imide, lithium bisoxalatoborate, lithium bistrifluoromethylsulfonyl imide, lithium tetrafluorooxalatophosphate, and lithium difluorobisoxalatoborate.

[0020] Preferably, the organic solvent is at least one of methyl propyl carbonate, ethyl acetate, diethyl carbonate, propylene carbonate, dimethyl carbonate, ethyl propionate, ethylene glycol dimethyl ether, propyl propionate, ethyl methyl carbonate, ethylene carbonate, dioxolane and γ-butyrolactone.

[0021] A secondary battery comprises a positive electrode, a negative electrode and the above electrolyte.

[0022] Preferably, the material used for the positive electrode is at least one of lithium nickel manganese oxide, lithium-rich ternary oxide, lithium cobalt oxide, lithium iron phosphate, nickel cobalt manganese ternary oxide and layered nickel manganese oxide.

[0023] Preferably, the material used for the negative electrode is a material capable of inserting / extracting lithium, selected from at least one of a carbon material, metallic lithium and a silicon-based material.

[0024] The beneficial effects of the present invention are:

[0025] Using bisamino-terminated siloxane as an electrolyte additive has a unique molecular structure, high silicon-oxygen bond energy and long bond length. It can utilize its high HOMO energy level to form an in-situ film on the electrode surface before the solvent oxidative decomposition. The silicon-oxygen bond of the bisamino-terminated siloxane molecule has a high bond energy and bond length, a large bond angle and a high degree of rotational freedom of the bond, which makes the molecular chain have good flexibility and can be better bent and wound on the electrode surface, thereby forming a more flexible interface film. In addition, due to its high hydrophilicity and interfacial activity, bisamino-terminated siloxane can react faster with hydrofluoric acid in the electrolyte, and is more likely to be preferentially adsorbed on the electrode surface and arranged in a certain orientation. This orderly arrangement helps to form a tightly packed interface film structure and improve the density of the film. The bisamino-terminated siloxane electrolyte additive can also be preferentially adsorbed on the electrode surface in the electrolyte, arranged in a specific orientation, and deeply participate in the complex film-forming reaction, and finally form a flexible and high-strength solid electrolyte interface (SEI) film.

[0026] At the same time, the N and Si-containing interface film has good stability and high conductivity, can buffer the large volume change of the electrode under high pressure, and protect the electrolyte body and the electrode surface from being damaged. At the same time, bisamino-terminated siloxane has better acid and water removal effects than conventional aminosilane, so it can inhibit the occurrence of corrosion side reactions on the electrode surface. Therefore, the additive can improve the cycle life of the battery under high pressure. The amino group in bisamino-terminated siloxane has strong hydrophilicity and reactivity. When used for surface modification, the amino group can react with the groups on the surface of the membrane material, or through hydrogen bonds and other interactions, form channels or sites in the membrane layer that are conducive to ion transmission. These channels or sites can provide specific transmission paths for ions, making it easier for ions to move in the membrane layer, thereby improving the ion conductivity under high voltage or high current charge and discharge conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is the molecular structure formula of the diamino siloxane used in the examples. DETAILED DESCRIPTION

[0028] The molecular structures of the diamino siloxanes used in the following examples are as follows: Figure 1 As shown in III-1, 2, 3, 4, and 5.

[0029] Example 1

[0030] (1) Preparation of electrolyte

[0031] In an argon-filled glove box, LiPF 6Dissolved in a mixed solvent prepared by ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) in a volume ratio of 1:1:1 to prepare a blank electrolyte with a concentration of 1 mol / L. Then, 0.5 wt% of siloxane (III-1) containing a diamino group was added to 2 mL of the blank electrolyte, and the container was sealed and allowed to stand for 2 hours to complete the preparation of the electrolyte.

[0032] (2) Battery assembly

[0033] In an argon-filled glove box, 2032 button cells were assembled in the following order: negative electrode shell → spring → gasket → negative electrode → electrolyte → diaphragm → electrolyte → positive electrode → positive electrode shell. The electrolyte used was the solution prepared in step (1), and the injection volume was 90 μL. The positive electrode material was LiNi 0.5 Mn 1.5 O 4 The negative electrode material uses lithium sheet and the diaphragm uses polypropylene diaphragm (Celgard2400).

[0034] Example 2

[0035] (1) Preparation of electrolyte

[0036] The preparation of blank electrolyte was the same as in Example 1. 0.5 wt % of siloxane (III-2) containing diamino group was added into 2 mL of blank electrolyte, the solution was sealed and allowed to stand for 2 h.

[0037] (2) Battery assembly

[0038] Same as step 2 of Example 1.

[0039] Example 3

[0040] (1) Preparation of electrolyte

[0041] The preparation of blank electrolyte is the same as in Example 1. 0.5 wt % of siloxane (III-3) containing diamino group is added into 2 mL of blank electrolyte, the solution is sealed and allowed to stand for 2 h.

[0042] (2) Battery assembly

[0043] Same as step 2 of Example 1.

[0044] Example 4

[0045] (1) Preparation of electrolyte

[0046] The preparation of blank electrolyte is the same as in Example 1. 0.5 wt % of siloxane (III-4) containing diamino group is added into 2 mL of blank electrolyte, the solution is sealed and allowed to stand for 2 h.

[0047] (2) Battery assembly

[0048] Same as step 2 of Example 1.

[0049] Example 5

[0050] (1) Preparation of electrolyte

[0051] The preparation of blank electrolyte was the same as in Example 1. 0.5 wt % of siloxane (III-5) containing diamino group was added into 2 mL of blank electrolyte, the solution was sealed and allowed to stand for 2 h.

[0052] (2) Battery assembly

[0053] Same as step 2 of Example 1.

[0054] Comparative Example 1

[0055] The electrolyte contains no additives and is composed of only 1 mol / L LiPF 6 The battery is prepared by dissolving in a mixed solvent of EC, EMC and DMC in a volume ratio of 1:1:1; the battery assembly process is the same as step 2 in Example 1.

[0056] The electrochemical performance test results of the secondary batteries obtained in Examples 1-5 and Comparative Example 1 are as follows:

[0057] (1) Energy level theoretical calculation

[0058] Density functional theory (DFT) calculations were performed on the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the additives and solvents involved in Examples 1-5 and Comparative Example 1, and the energy level data obtained are shown in Table 1.

[0059] Table 1 Energy level calculation results of additives and solvents involved in Examples 1-5 and Comparative Example 1

[0060]

[0061]

[0062] According to the data in Table 1, in Examples 1 to 5, the HOMO energy level of the bisamino-terminated siloxane is higher than the HOMO energy level of the solvent EC, EMC and DMC, indicating that the additive exhibits a higher highest molecular orbital energy level (HOMO) and a lower oxidation film-forming potential, so that it can be preferentially oxidized in the electrolyte to form a nitrogen-containing, silicon-containing interface film (CEI film). This CEI film has high conductivity and good stability, combined with the excellent acid and water removal function of the bisamino-terminated siloxane, effectively protecting the positive electrode and electrolyte, thereby significantly improving the electrochemical performance of the battery. In addition, the additive contains both bisamino and siloxy groups, and has good lithium affinity, so it can regulate the solvation structure of lithium, thereby regulating the diffusion and deposition behavior of lithium; the above groups also have acid and water removal effects, which can inhibit the occurrence of corrosion side reactions on the electrode surface. Therefore, the additive can improve the cycle life of the battery under high pressure.

[0063] (2) Linear sweep voltammetry test

[0064] The electrolytes prepared in Control Example 1 and Examples 1-5 were subjected to oxidation potential tests in an electrolytic cell with platinum as the working electrode and metallic lithium as the reference electrode and the counter electrode, within a voltage range of 2-6 V and at a scan rate of 0.1 mv / s. The data are shown in Table 2.

[0065] Table 2 Oxidation potential test results of the electrolyte in Examples 1-5 and Comparative Example 1

[0066]

[0067] It can be seen from Table 2 that the film-forming potential of the electrolytes containing bisamino-terminated siloxane in Examples 1-5 is lower than the decomposition potential of Control Example 1, and the decomposition potential of the electrolyte is significantly increased, indicating that the additive can be oxidatively decomposed before the solvent, and after forming a stable CEI film on the surface of the positive electrode, the main body of the electrolyte is protected from decomposition, and at the same time, the electrode is protected from corrosion by HF in the electrolyte.

[0068] (3) Electrochemical cycle performance test

[0069] The button cells assembled in Control Example 1 and Examples 1-5 were left to stand for 2 h, activated for 2 cycles at 0.1C (1C=147 mA / g), and then subjected to charge and discharge cycle tests at 50°C, 1C, and a voltage range of 3.5-4.9 V. The first week specific capacity and the capacity retention rates after 100 and 200 weeks are shown in Table 3.

[0070] Table 3 Test results of the charge and discharge performance of the batteries in Examples 1-5 and Comparative Example 1

[0071]

[0072] As shown in Table 3, the first-week discharge specific capacity of the secondary batteries prepared in Examples 1-5 is close to that of Control Example 1. After 100 cycles, the capacity retention rate of Control Example 1 is 86.3%, while the capacity retention rates of Examples 1-5 are all above 88%; after 200 cycles, the capacity retention rates of Examples 1-5 are all above 80%, while that of Control Example 1 is only 65.30%. Therefore, using bisamino-terminated siloxane as an electrolyte additive can significantly extend the cycle life of high-voltage lithium batteries.

[0073] In summary, the electrolyte additive containing diamino-terminated siloxane provided in the present application can form a stable CEI film at the positive electrode, protect the electrode and broaden the electrochemical window of the electrolyte, thereby improving the cycle life of the lithium battery under high temperature and high pressure.

[0074] The above is only a preferred implementation of the patent of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the patent of the present invention. These improvements and modifications should also be regarded as the scope of protection of the patent of the present invention.

Claims

1. An electrolyte additive, characterized in that: The electrolyte additive is bisamino-terminated siloxane, and its general structural formula is as follows: Wherein, R1 and R2 are independently selected from hydrocarbon groups with carbon atoms of 1-6; R3 and R4 are independently selected from hydrocarbon groups, hydrocarbonoxy groups, acyloxy groups, amine groups, amineoxy groups, amide groups or oxime groups with carbon atoms of 1-4; and n is any integer between 1-5.

2. The electrolyte additive according to claim 1, characterized in that: The R1 and R2 are independently selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, allyl, propargyl, phenyl, trifluoroethyl or pentafluoropropyl.

3. The electrolyte additive according to claim 1, characterized in that: The electrolyte additive is at least one of the following formulas:

4. An electrolyte, characterized in that: The invention comprises an electrolyte salt, an organic solvent and the electrolyte additive according to any one of claims 1 to 3, wherein the concentration of the electrolyte salt is 0.5-2 mol / L, and the amount of the electrolyte additive is 0.1-5 wt % of the total mass of the electrolyte.

5. The electrolyte according to claim 4, characterized in that The electrolyte salt is at least one of lithium difluorophosphate, lithium hexafluorophosphate, lithium difluorooxalatoborate, lithium tetrafluoroborate, lithium bisfluorosulfonyl imide, lithium bisoxalatoborate, lithium bistrifluoromethylsulfonyl imide, lithium tetrafluorooxalatophosphate, and lithium difluorobisoxalatoborate.

6. The electrolyte according to claim 4, characterized in that The organic solvent is at least one of methyl propyl carbonate, ethyl acetate, diethyl carbonate, propylene carbonate, dimethyl carbonate, ethyl propionate, ethylene glycol dimethyl ether, propyl propionate, ethyl methyl carbonate, ethylene carbonate, dioxolane and γ-butyrolactone.

7. A secondary battery, characterized in that: The invention comprises a positive electrode, a negative electrode and the electrolyte according to claim 4.

8. The secondary battery according to claim 7, characterized in that: The material used for the positive electrode is at least one of lithium nickel manganese oxide, lithium-rich ternary oxide, lithium cobalt oxide, lithium iron phosphate, nickel cobalt manganese ternary oxide and layered nickel manganese oxide.

9. The secondary battery according to claim 7, characterized in that: The material used for the negative electrode is at least one of a carbon material, metallic lithium and a silicon-based material.

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