Slow-release additive and preparation method thereof, electrolyte and lithium ion battery

Abstract

The invention belongs to the field of batteries, and particularly relates to a slow-release additive and a preparation method thereof, an electrolyte and a lithium ion battery. The slow-release additive comprises a core material and a wall material; the core material is a VC additive, and the wall material is a slow-release coating layer; according to the technical scheme, on the basis of an existing electrolyte, on one hand, a VC additive is improved from being directly dissolved in the electrolyte to being dissolved in the electrolyte in a slow-release additive mode, and when the content of VC in the electrolyte in the battery is reduced, VC slowly seeps out of a core-shell structure through continuous slow dissolution of a shell wall, so that the VC content in the electrolyte in the battery is reduced; therefore, the VC content in the electrolyte is supplemented, and the effect of reducing the cycle attenuation of the battery is achieved while the internal resistance is reduced. The slow release of VC reduces the internal resistance and improves the cycle stability. And on the other hand, secondary liquid injection is changed into primary liquid injection, so that the production efficiency is improved, and the production cost is reduced.

Description

Technical Field

[0001] The invention belongs to the field of batteries, and in particular relates to a slow-release additive and a preparation method thereof, an electrolyte and a lithium-ion battery. Background Art

[0002] In lithium-ion batteries, film-forming additives in the electrolyte promote the formation of solid electrolyte membrane (SEI) and improve the long-cycle performance of the battery. One of the most common film-forming additives is vinylene carbonate (VC). VC can be oxidized or reduced on the electrode to form a stable and dense SE I film composed of organic compounds and lithium salts. Therefore, adding VC to the electrolyte can improve the capacity retention rate of graphite-based lithium-ion batteries and improve the cycle performance. However, the secondary injection technology is currently commonly used on the market. A low content of VC is added for the first time, and a good SEI film is formed on the graphite surface after formation. Then a high content of VC is added in the second injection to continuously repair the SEI film during the later cycle.

[0003] Although VC additives extend the life of the battery, during the initial cycle, high concentrations of VC increase the interfacial resistance of the battery and reduce the rate performance of the battery. The secondary injection technology reduces production efficiency and increases production costs. Summary of the invention

[0004] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a slow-release additive and a preparation method thereof, an electrolyte and a lithium ion battery.

[0005] To achieve the above object, the technical solution adopted by the present invention is:

[0006] A sustained-release additive, comprising a core material and a wall material; the core material is a VC additive, and the wall material is a sustained-release coating layer;

[0007] The wall material is one of SiO2, Al2O3, MgO, ZrO2, TiO2 or a mixture thereof; preferably SiO2.

[0008] The thickness of the sustained-release coating layer is 0.1-10 nm, preferably 1-1.5 nm.

[0009] The present invention also includes a method for preparing the sustained-release additive, comprising the following steps: 1) adding a wall material monomer to the core material VC additive after stirring, 2) continuing stirring, then adding ammonia water to adjust the pH to alkaline, 3) continuing to react fully; 4) after the reaction is completed, centrifuging and drying to obtain an electrolyte additive with a sustained-release coating layer covering the core material VC; preferably, the wall material monomer is tetraethyl orthosilicate.

[0010] The mass ratio of the core material VC additive to the wall material monomer described in step 1) is (0.1:10)-(10:0.1); preferably 10:(0.1-0.5). When the core material wall is relatively large, due to excessive amount of core material, the sustained-release coating layer is not enough to completely cover the core material, or the formed wall material is thin, which is easy to break during the subsequent stirring process to cause the core material to leak, resulting in no VC sustained-release effect. When the core material wall is relatively small, the amount of wall material monomer is too high, making the sustained-release coating layer thicker, reducing the proportion of VC in the core-shell structure. Therefore, the mass ratio of the core material VC additive to the wall material monomer is preferably (0.1:10)-(10:0.1), specifically 0.1:10, 1:1 or 10:0.1; that is, the porosity is 20%-80%, which is determined by the VC seepage rate. Among them, the ratio of 10:(0.1-0.5) is more optimal, so it is used as a preferred embodiment for subsequent description.

[0011] Taking the case where the wall material monomer is ethyl orthosilicate and a SiO2 coating layer is formed as an example, excessive wall material monomers tend to self-polymerize to form a SiO2 empty shell.

[0012] Preferably, the stirring and reaction temperature is 40-80°C; preferably 60°C; the stirring speed is 50-150r / min; preferably 100r / min; the stirring time in step 1) is 10-30min; the stirring time in step 2) is 30-60min; the reaction time in step 3) is 5-10h; the centrifugal speed in step 4) is 3000r / min, and the drying conditions are drying at 45-60°C for 12-24h.

[0013] The present invention also includes an electrolyte, comprising a lithium salt, a solvent and an additive; the additive comprises the sustained-release additive according to any one of claims 1 to 3.

[0014] The added amount of the sustained-release additive is 1-5wt%, preferably 3-5wt%.

[0015] The lithium salt is Li PF6, and the mass content of the lithium salt in the electrolyte is 10-20%, preferably 13%.

[0016] The solvent is a mixture of DMC:EMC:EC; the mass ratio of DMC:EMC:EC is: (1-4):(2-4):(1-3); preferably 3:3:2.

[0017] The present invention also includes a lithium ion battery, including the electrolyte, and the lithium ion battery is prepared in the form of a single injection.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] The technical solution of the present application is based on the existing electrolyte. On the one hand, the VC additive is improved from being directly dissolved in the electrolyte to being dissolved in the electrolyte in the form of a slow-release additive. When the VC content in the electrolyte inside the battery decreases, VC slowly dissolves through the shell wall and slowly seeps out from the core-shell structure to replenish the VC content in the electrolyte, thereby reducing the internal resistance and slowing down the battery cycle decay. The slow release of VC reduces the internal resistance and improves the stability of the cycle. On the other hand, the change from secondary injection to single injection improves production efficiency and reduces production costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic diagram of the structure of a VC core-shell structure sustained-release additive according to an embodiment of the present invention;

[0021] Figure 2 A diagram showing the cycle results of a battery according to an embodiment of the present invention;

[0022] Figure 3 This is a DCIR test result diagram of an embodiment of the present invention. DETAILED DESCRIPTION

[0023] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and the best embodiments.

[0024] Embodiment 1:

[0025] 1. Preparation of VC core-shell structure sustained-release additive: Place a certain mass of VC liquid in a 60℃ oil bath, stir at 100r / min for 10min, then drop a certain amount of ethyl orthosilicate, continue stirring for 1h, then drop a certain amount of ammonia water, adjust the pH to alkaline, i.e. pH>7, and continue to fully react in a 60℃ oil bath for 5h. After the reaction, centrifuge at 3000r / min for 10min, dry at 60℃ for 12h, and obtain a core-shell structure additive with SiO2 shell wall covering the inner core VC (structure Figure 1 shown).

[0026] The mass ratio of the coating layer material ethyl orthosilicate to the VC additive is 0.1:10. The thickness of the shell wall is 1 nm.

[0027] 2. Preparation of electrolyte with VC core-shell structure slow-release additive:

[0028] The solvents are mixed in a mass ratio of DMC:EMC:EC=3:3:2, and then 5wt% VC core-shell structure additive and 13wt% lithium salt Li PF6 are added to the mixed solvent to obtain an electrolyte. Figure 1As shown, the electrolyte 1 contains a VC core-shell structure slow-release additive 2 and a slowly released free VC liquid 3 .

[0029] 3. Battery preparation:

[0030] The LFP positive electrode and graphite negative electrode powders are respectively prepared into positive and negative electrode sheets through electrode processes such as homogenization, coating, rolling, and slitting. The positive electrode sheets, negative electrode sheets, and separators (the base film is 9um, and ceramic and PVDF glue are coated on one side) are respectively made into semi-finished batteries through processes such as winding / stacking, hot pressing, and assembly. The electrolyte with a VC core-shell structure is injected, and the finished batteries are obtained after formation, aging, and capacity separation.

[0031] The specific process of injection is as follows: the total VC content in the electrolyte is 5% (the content of the wall material is negligible). Through one injection, the electrolyte containing 5% core-shell structure VC slow-release additive is injected into the uninjected battery, left to stand for a period of time, and then made into a finished battery after the processes of formation and aging.

[0032] Embodiment 2:

[0033] The difference between Example 2 and Example 1 is that the preparation method of the electrolyte with VC core-shell structure sustained-release additive is different, and specifically includes the following steps: mixing the solvents in a mass ratio of DMC:EMC:EC=3:3:2, and then adding 3wt% VC core-shell structure additive and 13wt% lithium salt LiPF6 to the mixed solvent to obtain an electrolyte after mixing.

[0034] Embodiment 3:

[0035] The difference between Example 3 and Example 1 is that the preparation method of the sustained-release additive with VC core-shell structure is different, specifically comprising the following steps: placing a certain mass of VC liquid in a 60°C oil bath, stirring at a speed of 100 r / min for 10 minutes, then dropping a certain amount of ethyl orthosilicate, continuing to stir for 1 hour, then dropping a certain amount of ammonia water, adjusting the pH to alkaline, that is, pH>7, and continuing to fully react in a 60°C oil bath for 5 hours. After the reaction is completed, centrifuge at 3000 r / min for 10 minutes, and dry at 60°C for 12 hours to obtain a core-shell structure additive with SiO2 shell wall covering the inner core VC.

[0036] The mass ratio of ethyl orthosilicate to VC additive in the coating layer is 0.5:10; the thickness of the shell wall is 1.5 nm.

[0037] Embodiment 4:

[0038] The difference between Example 4 and Example 1 is that the preparation method of the sustained-release additive with a VC core-shell structure is different, specifically comprising the following steps: placing a certain mass of VC liquid in a 60°C oil bath, stirring at a speed of 100 r / min for 10 minutes, then dropping a certain amount of aluminum chloride, continuing to stir for 1 hour, then dropping a certain amount of ammonia water, adjusting the pH to alkaline, that is, pH>7, and continuing to fully react in a 60°C oil bath for 5 hours. After the reaction is completed, centrifuge at 3000 r / min for 10 minutes, and dry at 60°C for 12 hours to obtain a core-shell structure additive with an Al2O3 shell wall covering an inner core VC.

[0039] The mass ratio of aluminum chloride to VC additive in the coating layer is 0.1:10. The thickness of the shell wall is 1 nm.

[0040] Comparative Example:

[0041] For square ultra-long cycle batteries, the total VC content in the electrolyte is generally 1-5%. The electrolyte containing 2% VC additive (mixing the solvent according to the mass ratio of DMC:EMC:EC=3:3:2, then adding 2wt% VC additive and 13wt% lithium salt Li PF6 to the mixed solvent to obtain the electrolyte) is injected into the unfilled battery, left to stand for a certain period of time, and formed to form a stable SE I film, and then the second injection is carried out, and the electrolyte containing 3% VC content is continuously injected (mixing the solvent according to the mass ratio of DMC:EMC:EC=3:3:2, then adding 3wt% VC additive and 13wt% lithium salt Li PF6 to the mixed solvent to obtain the electrolyte), and after aging, it is made into a finished battery.

[0042] Electrical performance test:

[0043] Cyclic test: Temperature: 45℃, 0.5CP charge to 3.65V, stand for 30min, 0.5CP discharge to 2.5V, stand for 30min.

[0044] DC internal resistance (DCI R) test: 25℃, charge to 3.65V at 0.5CP, leave for 30min, discharge at 1C for 30s, leave for 30min, capacitate to 90%SOC / 80%SOC...10%SOC, leave for 30min, discharge at 1C for 30s.

[0045] The batteries obtained in the examples and comparative examples were tested for internal resistance and subjected to cycle tests to verify the effect of the core-shell structure slow-release additive on the cycle performance. The internal resistance test is shown in Table 1. The cycle results are shown in Table 1. Figure 2 As shown, the DCI R test results are shown in Figure 3 .

[0046] Table 1 Comparison of AC internal resistance

[0047] Test items Example 1 Example 2 Example 3 Example 4 Comparative Example Internal resistance (ACIR) 0.153 0.15 0.156 0.160 0.172

[0048] The results show that by introducing the prepared VC core-shell structure slow-release additive into the electrolyte, the slow release of VC reduces the internal resistance and improves the cycle stability.

[0049] In summary, the technical solution of the present application is based on the existing electrolyte. On the one hand, the VC additive is improved from being directly dissolved in the electrolyte to being dissolved in the electrolyte in the form of a slow-release additive. When the VC content in the electrolyte inside the battery decreases, VC slowly seeps out from the core-shell structure through the continuous and slow dissolution of the outer shell wall to replenish the VC content in the electrolyte, thereby reducing the internal resistance and slowing down the battery cycle attenuation. The slow release of VC reduces the internal resistance and improves the stability of the cycle. On the other hand, the change from secondary injection to single injection improves production efficiency and reduces production costs.

[0050] The above is only a preferred embodiment 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 present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A sustained-release additive, characterized in that: It includes a core material and a wall material; the core material is a VC additive, and the wall material is a sustained-release coating layer.

2. The sustained-release additive according to claim 1, characterized in that: The wall material is one of SiO2, Al2O3, MgO, ZrO2, TiO2 or a mixture thereof; preferably SiO2.

3. The sustained-release additive according to claim 1, characterized in that The thickness of the sustained-release coating layer is 0.1-10 nm, preferably 1-1.5 nm.

4. A method for preparing the sustained-release additive according to any one of claims 1 to 3, characterized in that: The method comprises the following steps: 1) adding a core material VC additive dropwise to a wall material monomer after stirring; 2) continuing stirring, then adding ammonia water dropwise to adjust the pH to alkaline; 3) continuing to react fully; 4) after the reaction is completed, centrifuging and drying to obtain an electrolyte additive having a core material VC coated with a slow-release coating layer; preferably, the wall material monomer is tetraethyl orthosilicate.

5. The method for preparing the sustained-release additive according to claim 4, characterized in that: The mass ratio of the core material VC additive to the wall material monomer in step 1) is (0.1:10)-(10:0.1); preferably 10:(0.1-0.5).

6. The method for preparing the sustained-release additive according to claim 4, characterized in that: The stirring and reaction temperature is 40-80°C; preferably 60°C; the stirring speed is 50-150r / min; preferably 100r / min; preferably, the stirring time in step 1) is 10-30min; preferably, the stirring time in step 2) is 30-60min; preferably, the reaction time in step 3) is 5-10h; preferably, the centrifugal speed in step 4) is 3000r / min, and the drying condition is drying at 45-60°C for 12-24h.

7. An electrolyte, characterized in that: It comprises a lithium salt, a solvent and an additive; the additive comprises the sustained-release additive according to any one of claims 1 to 3.

8. The electrolyte according to claim 7, characterized in that The added amount of the sustained-release additive is 1-5wt%, preferably 3-5wt%.

9. The electrolyte according to claim 7, characterized in that The lithium salt is LiPF6, and the mass content of the lithium salt in the electrolyte is 10-20%, preferably 13%; Preferably, the solvent is a mixture of DMC:EMC:EC; the mass ratio of DMC:EMC:EC is: (1-4):(2-4):(1-3); preferably 3:3:

2.

10. A lithium ion battery, characterized in that: Comprising the electrolyte described in any one of claims 7 to 9; the lithium-ion battery is prepared in the form of a single injection.

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