Lithium ion battery fast charging electrolyte additive, electrolyte and battery

Abstract

The invention discloses an electrolyte additive, the structural general formula of which is as shown in formula I, and in the formula I, R1, R2, R3, R4, R5 and R6 are respectively and independently selected from at least one of nitrile groups or alkyl groups, and at least one nitrile group is provided. The invention also discloses an electrolyte containing the additive, an interfacial film with good thermal stability can be formed by adopting the electrolyte, and impurities such as acidic substances and H2O generated in the electrolyte can be removed in time, so that the high-temperature cycle life of the battery is remarkably prolonged.

Description

Technical Field

[0001] The present invention belongs to the field of lithium batteries, and in particular relates to a lithium ion battery fast-charging electrolyte additive, an electrolyte and a battery. Background Art

[0002] Compared with ternary batteries, lithium iron phosphate batteries are cheaper and have higher chemical and electrochemical stability, so their market share in power batteries has been increasing year by year. As the positive electrode material of the battery, lithium iron phosphate itself has low conductivity, and the problem of fast charging at low temperatures is more prominent. The electrolyte, another key component of the battery, is used to transmit Li + It is an important medium for fast charging and has a more significant impact on fast charging performance.

[0003] Li + The transmission process (taking the charging process as an example) can be divided into the following processes: (1) Li removal from the positive electrode material + ,(2)Li + After passing through the cathode interface CEI layer, it enters the electrolyte and is surrounded by the solvent to form a solvated structure. (3) The solvated Li + Diffusion and transport in the electrolyte, (4) desolvation near the negative electrode interface, (5) Li + The SEI film at the negative electrode interface enters the graphite layer structure to complete the Li + In this process, if the Li + If the intercalation process is not completed in time, it will lead to problems such as lithium plating, which will eventually trigger more side reactions and cause battery failure. Therefore, in the fast charging process, processes (3) to (5) have a decisive influence on the fast charging performance. By regulating the components of the electrolyte, the fast charging performance of lithium iron phosphate batteries can be effectively improved.

[0004] Carboxylate solvents, due to their low viscosity and high conductivity, are essential solvents for fast-charge electrolytes in lithium iron phosphate batteries. However, carboxylate esters have poor compatibility with graphite anodes, making it difficult to form a stable SEI film, resulting in poor capacity retention after high-temperature cycling and storage. Furthermore, carboxylate esters are chemically unstable and easily decompose during charge and discharge, producing various "harmful impurities," such as acidic substances, that can impair battery performance. Summary of the Invention

[0005] To address the above technical issues, the present invention provides a lithium-ion battery fast-charge electrolyte additive and electrolyte. The electrolyte containing the additive can form an interfacial film with excellent thermal stability and promptly remove impurities such as acidic substances and H2O generated in the electrolyte, significantly improving the high-temperature cycle life of the battery.

[0006] The specific scheme of the present invention is as follows:

[0007] One of the purposes of the present invention is to provide an electrolyte additive, the general structural formula of which is shown in Formula I:

[0008]

[0009] Wherein, R1, R2, R3, R4, R5 and R6 are independently selected from at least one of a nitrile group or an alkyl group, and there is at least one nitrile group.

[0010] Preferably, R1, R2, R3, R4, R5 and R6 are all nitrile groups.

[0011] Preferably, the preparation method of the electrolyte additive comprises:

[0012] S1, carrying out a nitrilation reaction of trichlorosilane and a nitrilation reactant in a solvent;

[0013] S2. After the nitrilization reaction is completed, the reaction system is added to ice water for hydrolysis reaction; after the hydrolysis reaction is completed, the organic phase is separated by extraction with an organic solvent, and then the solvent is removed to obtain trinitrile silanol;

[0014] S3. Using concentrated sulfuric acid as a dehydrating agent, trinitrile silanol is reacted with sulfuric acid to form a silicon-oxygen bond to obtain bis(trinitrile silyl) sulfate.

[0015] Preferably, in S1, the nitrilation reactant is sodium cyanide and the solvent is N,N-dimethylformamide.

[0016] Preferably, in S1, the nitrilation reaction is carried out under anhydrous and oxygen-free conditions, the reaction temperature is -10 to -30°C, and the reaction time is 2 to 5 hours.

[0017] Preferably, in S2, the organic solvent is dichloromethane.

[0018] A second object of the present invention is to provide a fast-charging electrolyte for a lithium-ion battery, comprising: a lithium salt, a solvent, and a first additive; the first additive is the electrolyte additive of the above structure.

[0019] Preferably, a second additive is further included; the second additive is selected from at least one of 1,3-propane sultone (PS), methylene methanedisulfonate (MMDS), lithium difluorooxalatoborate (LiODFB), vinylene carbonate (VC), fluoroethylene carbonate (FEC), vinyl sulfate (DTD) or lithium difluorophosphate (LiPO2F2).

[0020] Preferably, the second additive is a mixture of vinylene carbonate, vinyl sulfate and lithium difluorooxalatoborate.

[0021] In the present invention, the second additive can decompose into a more stable SEI film before the solvent, reducing the Li + The interfacial impedance of the transmission process helps the battery to cycle normally.

[0022] Preferably, the sum of the first additive and the second additive accounts for 0.1% to 5% of the total mass fraction of the electrolyte.

[0023] Preferably, the lithium salt is selected from one or more of lithium bis(trifluoromethyl)sulfonyl imide (LiTFSI), lithium bis(fluorosulfonyl imide) (LiFSI), lithium hexafluorophosphate (LiPF6), and lithium tetrafluoroborate (LiBF4). More preferably, the lithium salt is added in an amount of 10% to 15% of the total mass fraction of the electrolyte.

[0024] Preferably, the solvent is selected from carboxylate solvents and / or carbonate solvents; more preferably, the carbonate solvent is one or more of ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (GBL), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and diethyl carbonate (DEC); and the carboxylate is one or more of ethyl acetate (EA), ethyl propionate (EP), methyl propionate (MP), or methyl acetate (MA).

[0025] Preferably, the solvent accounts for 80% to 89% of the total mass fraction of the electrolyte.

[0026] A third object of the present invention is to provide a lithium-ion battery comprising the electrolyte described in any one of the above items.

[0027] Preferably, the lithium-ion battery is a lithium iron phosphate battery.

[0028] The beneficial effects of the present invention are:

[0029] The present invention provides an electrolyte additive with a new structure. The S element in the additive structure can form a SEI film with high ionic conductivity and high stability. On the other hand, the Si-O group in the structure can remove water and inhibit acid, and can also complex metal Fe 2+ Prevent it from depositing on the negative electrode, causing more serious interface side reactions, thereby improving the high-temperature capacity retention rate of lithium iron phosphate batteries.

[0030] In addition, the nitrile group (-C≡N) in the structure contains an unsaturated triple bond of an electron-rich group, which is easily absorbed by F on the positive electrode side during charging. - Ions attack the carbon nucleus and oxidize to -CF structure bond, which is easily attracted by electrophilic H on the negative electrode side. + The HF is attacked and reduced to -CH, which causes a chain addition reaction. This can further remove HF acidic substances in the electrolyte and further improve high-temperature performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is the H spectrum of the trichlorosilane HSiCl3 raw material in the electrolyte additive preparation method of Example 1;

[0032] Figure 2 This is the H spectrum of HSi(CN)3 in the electrolyte additive preparation method of Example 1;

[0033] Figure 3 This is the H spectrum of SiOH(CN)3 in the electrolyte additive preparation method of Example 1. DETAILED DESCRIPTION

[0034] Hereinafter, the technical solutions of the present invention will be described in detail through specific embodiments. However, it should be clearly stated that these embodiments are provided for illustration only and are not to be construed as limiting the scope of the present invention.

[0035] Example 1

[0036] An electrolyte additive, the structural formula of which is:

[0037] It is recorded as additive A1.

[0038] The preparation method of the additive A1 of the present invention is:

[0039] 1. Nitrilation reaction: Trichlorosilane HSiCl3 is slowly added dropwise to the nitrilation reactants sodium cyanide NaCN and N,N-dimethylformamide DMF reagent. The reaction is carried out in anhydrous and oxygen-free conditions at low temperature (-20°C) for 4 hours. The reaction process requires sufficient stirring.

[0040] The reaction formula is: HSiCl3+3NaCN→HSi(CN)3+3NaCl;

[0041] 2. Hydrolysis reaction: After the nitrilation reaction is completed, the reaction system is slowly poured into a large amount of ice water for hydrolysis (pay attention to control the dropwise addition rate to prevent the reaction from being too violent). After the hydrolysis is completed, the organic solvent dichloromethane (CH2Cl2) is used for extraction to separate the organic phase, and then dried over anhydrous sodium sulfate, filtered, and distilled under reduced pressure to remove the solvent and low-boiling impurities to obtain a crude trinitrile silanol product, which is further purified by column chromatography and other methods;

[0042] The reaction formula is: HSi(C≡N)3+3H2O→SiOH(CN)3+3H2;

[0043] 3. Dehydration reaction: Using concentrated sulfuric acid as a dehydrating agent, trinitrile silanol reacts with sulfuric acid to form a silicon-oxygen bond to obtain bis(trinitrile silyl) sulfate.

[0044] The reaction is as follows: 2SiOH(CN)3+H2SO4→SO2[O-Si(CN)3]2+2H2O.

[0045] A lithium ion battery fast-charging electrolyte comprising: a lithium salt, a solvent and an additive;

[0046] Wherein: the lithium salt is lithium hexafluorophosphate LiPF6 and lithium bis(fluorosulfonyl)imide LiFSI;

[0047] The solvent is a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and methyl acetate (MA);

[0048] The additives include a first additive and a second additive, the first additive is the additive of the above structure, and the second additive includes vinylene carbonate (VC), diethylene sulfate (DTD) and lithium difluorooxalatoborate (LiODFB).

[0049] The method for preparing the electrolyte described in this embodiment includes the following steps:

[0050] Take 100g sample as an example: in a glove box (H2O≤0.01ppm, O2≤0.01ppm), ethylene carbonate, ethyl methyl carbonate and methyl acetate were mixed in a mass ratio of 3:5:2, and 81.55g of the mixed solvent was weighed; then 11.25g of lithium hexafluorophosphate, 3g of lithium bis(fluoromethylsulfonyl)imide, 2g of vinylene carbonate, 1.5g of vinyl sulfate, 0.5g of lithium difluorobis(oxalato)borate, and 0.2g of Al additive were added to the mixture and mixed evenly to form an electrolyte.

[0051] The compositions and preparation process parameters of the electrolytes described in Examples 1 to 3 and Comparative Examples 1 to 3 are shown in Table 1 below:

[0052] Table 1. Composition of the electrolytes of Examples 1 to 3 and Comparative Examples 1 to 3 and the proportion of each component

[0053]

[0054] Comparative Example 4

[0055] A lithium ion battery fast charge electrolyte, the difference from Example 3 is that the first additive is replaced by additive A2 instead of additive A1. The structural formula of additive A2 is as follows

[0056] The other electrolyte components, contents, and preparation methods are the same as those in Example 3.

[0057] Comparative Example 5

[0058] A lithium ion battery fast charge electrolyte, the difference from Example 3 is that the first additive is replaced by additive A3 instead of additive A1. The structural formula of additive A3 is as follows

[0059] The other electrolyte components, contents, and preparation methods are the same as those in Example 3.

[0060] Comparative Example 6

[0061] A lithium ion battery fast charge electrolyte, the difference from Example 3 is that the first additive is replaced by additive A4 instead of additive A1. The structural formula of additive A4 is as follows

[0062] The other electrolyte components, contents, and preparation methods are the same as those in Example 3.

[0063] Comparative Example 7

[0064] A lithium ion battery fast charge electrolyte, the difference from Example 3 is that the first additive is replaced by additive A5 instead of additive A1. The structural formula of additive A5 is as follows

[0065] The other electrolyte components, contents, and preparation methods are the same as those in Example 3.

[0066] The electrolytes of Examples 1 to 3 and Comparative Examples 1 to 7 were used as electrolytes for lithium iron phosphate batteries to prepare lithium ion batteries. The specific methods are as follows:

[0067] (1) Preparation of positive electrode sheet: The positive electrode material LiFePO4, the binder PVDF, and the conductive agent acetylene black were mixed in a mass ratio of 94.25:3.15:2.6, and N-methylpyrrolidone solvent was added thereto and stirred into a uniform transparent state. The mixture was transferred to a vacuum mixer and stirred to obtain a positive electrode slurry, which was evenly coated on the current collector aluminum foil (thickness of 15 μm), and then transferred to a vacuum oven at 120 ° C for 2 hours. After that, it was cold pressed (compacted density of 2.5 g / cm 3 ), cut to obtain the positive electrode sheet.

[0068] (2) Preparation of negative electrode sheet: The negative electrode material graphite, thickener sodium carboxymethyl cellulose solution, and binder styrene butadiene rubber latex were mixed in a mass ratio of 96:2.5:1.5, deionized water solvent was added, and the negative electrode slurry was prepared by stirring with a vacuum mixer. The negative electrode slurry was then evenly coated on the current collector copper foil (thickness of 8 μm), dried at room temperature, and then transferred to a 120°C oven for drying for 2 h. After that, it was cold pressed (compacted density of 1.6 g / cm 3 ), cut to obtain the negative electrode sheet.

[0069] (3) Preparation of lithium-ion battery: The positive electrode sheet, negative electrode sheet and polypropylene separator are wound together, wrapped with aluminum plastic film, baked at 45℃ for 24h, and then the electrolyte is injected and sealed.

[0070] Among them, the electrolyte is injected twice. After the first injection, it is soaked, hot and cold pressed, and formed before the second injection. After the second injection, the volume is divided.

[0071] One injection of electrolyte is the electrolyte of Comparative Example 3, and the second injection of electrolyte is the electrolyte corresponding to each embodiment and comparative example in Table 1; the mass ratio of the first injection to the second injection of electrolyte is 4:1.

[0072] The lithium-ion batteries prepared in Examples 1 to 3 and Comparative Examples 1 to 7 were subjected to normal and high temperature (45° C.) cycle tests and 60° C. storage tests. The test methods and test results are shown below:

[0073] 1. Battery high temperature storage test:

[0074] The resulting battery was cycled five times at room temperature at a 1C rate and then fully charged. The room temperature 1C discharge capacity Q, battery internal resistance R0, and battery thickness T0 were recorded. The fully charged battery was then stored in a 60°C constant temperature incubator. After 14 days, the battery was removed and the 1C discharge capacity Q1, battery internal resistance R1, and thickness T1 were recorded. Following these measurements, the battery was cycled five times at a 1C rate, and the highest discharge capacity was recorded, designated Q2. Experimental data such as the battery's high-temperature storage capacity retention rate, capacity recovery rate, thickness change rate, and internal resistance change rate were calculated. The battery recording results are shown in Table 2.

[0075] in:

[0076] Capacity retention rate = Q1 / Q, capacity recovery rate = Q2 / Q

[0077] Internal resistance change rate = (R1-R0) / R0;

[0078] Thickness change rate = (T1-T0)T0;

[0079] 2. Battery normal high temperature cycle test:

[0080] The divided batteries were discharged at 1 / 3C to 2.5V, then charged at a constant current of 1C to a constant voltage of 3.65V, with a cutoff rate of 0.05C. They were then discharged at a constant current of 1C to 2.5V. After this charge / discharge cycle, the capacity retention rate after the target number of cycles was calculated. The capacity retention rate after N cycles is calculated as follows: Nth cycle capacity retention (%) = (Nth cycle discharge capacity / first cycle discharge capacity) × 100%. The test results are shown in Table 2.

[0081] Among them, the internal resistance change rate (%) = (T1-T0) / T0×100; the thickness change rate (%) = (H1-H0) / H0×100; the capacity retention rate (%) = Q1 / Q×100; the capacity recovery rate (%) = Q2 / Q×100.

[0082] 3. Electrolyte high temperature storage test:

[0083] The resulting battery was cycled five times at a 1C rate and then fully charged. The battery was weighed using the water displacement method, and the initial mass (V1) was recorded. The battery was then placed in a 60°C oven for 14 days, 28 days, and 56 days, and weighed using the water displacement method, with the mass (V2) recorded. After weighing, the battery was disassembled, and the electrolyte was removed and tested for acidity and color. The corresponding gas production and electrolyte composition were determined. The test results are shown in Table 3.

[0084] Where: Gas production = (volume after storage – volume before initialization) = V2 - V1

[0085] Table 2. Battery normal and high temperature cycle test results

[0086]

[0087] Table 3. Test results of electrolyte normal and high temperature storage performance

[0088]

[0089] Comparing the data from Comparative Examples 1-3, we can see that with only two additives, the additives are consumed early in the battery cycle, resulting in faster capacity decay later in the cycle. Because LiODFB consumes more slowly than DTD, the resulting membrane is rich in B and F elements, resulting in superior long-term cycling performance. The combined use of VC, DTD, and LiODFB significantly improves the electrolyte's cycling performance.

[0090] Comparing the data of Examples 1 to 3 with Comparative Example 3, it can be seen that Comparative Example 3 is used as the reference electrolyte, and different amounts of additive A1 are added thereto to improve the high-temperature cycle performance. The structure of additive A1 contains S element, which can help to form a SEI / CEI film with good high-temperature stability and good conductivity. In addition, it also contains Si-O element, which can directly react with HF / PF5 to generate Si-F and participate in film formation, effectively inhibiting the acidic substances in the electrolyte, which is beneficial to the high-temperature performance of the battery. In addition, A1 also contains a nitrile group (-C≡N) group, and -C≡N contains an unsaturated triple bond with an electron-rich group. During the charging process, it is easily oxidized by F on the positive electrode side. - Ions attack the carbon nucleus and oxidize to -CF structure bond, which is easily attracted by electrophilic H on the negative electrode side. +The nitrile group is attacked and reduced to -CH, which undergoes a chain addition reaction. This further removes HF acidic substances from the electrolyte, which is beneficial for high-temperature performance. However, the nitrile group is prone to chain addition reactions, which is detrimental to impedance and has limited effect on suppressing gas production.

[0091] By comparing the data of Example 3 with Comparative Examples 4 to 7, it can be seen that the alkyl functional groups have almost no water removal and acid inhibition effect, and therefore have almost no obvious effect on high-temperature cycling and high-temperature storage of the battery.

[0092] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. An electrolyte additive, characterized in that Its general structural formula is shown in Formula I: Wherein, R1, R2, R3, R4, R5 and R6 are independently selected from at least one of a nitrile group or an alkyl group, and there is at least one nitrile group.

2. The electrolyte additive according to claim 1, characterized in that R1, R2, R3, R4, R5 and R6 are all nitrile groups.

3. A lithium ion battery fast charge electrolyte, characterized in that: include: A lithium salt, a solvent and a first additive; the first additive is the electrolyte additive according to claim 1 or 2.

4. The lithium ion battery fast charge electrolyte according to claim 3, characterized in that The invention also includes a second additive; the second additive is selected from at least one of 1,3-propane sultone, methylene methanedisulfonate, lithium difluorooxalatoborate, vinylene carbonate, fluoroethylene carbonate, vinyl sulfate or lithium difluorophosphate.

5. The lithium ion battery fast charge electrolyte according to claim 3 or 4, characterized in that The second additive is a mixture of vinylene carbonate, vinyl sulfate, and lithium difluorooxalatoborate.

6. The lithium ion battery fast charge electrolyte according to claim 3 or 4, characterized in that The sum of the first additive and the second additive accounts for 0.1% to 5% of the total mass fraction of the electrolyte.

7. The lithium ion battery fast charge electrolyte according to claim 3 or 4, characterized in that The lithium salt is selected from one or more of lithium bis(trifluoromethyl)sulfonyl imide, lithium bis(fluorosulfonyl imide), lithium hexafluorophosphate, and lithium tetrafluoroborate; preferably, the amount of the lithium salt added is 10% to 15% of the total mass fraction of the electrolyte.

8. The lithium ion battery fast charge electrolyte according to claim 3 or 4, characterized in that The solvent is selected from carboxylate solvents and / or carbonate solvents; Preferably, the carbonate solvent is one or more of ethylene carbonate, propylene carbonate, γ-butyrolactone, ethyl methyl carbonate, dimethyl carbonate, and diethyl carbonate; and the carboxylate solvent is one or more of ethyl acetate, ethyl propionate, methyl propionate, or methyl acetate.

9. The lithium ion battery fast charge electrolyte according to claim 3 or 4, characterized in that The solvent accounts for 80% to 89% of the total mass fraction of the electrolyte.

10. A lithium ion battery, characterized in that: The electrolyte comprising the electrolyte according to any one of claims 3 to 9.

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