Electrolyte additive, electrolyte and lithium iron phosphate battery

Abstract

The invention discloses an electrolyte additive, the structural general formula of which is as shown in formula I, R1, R2, R3, R4, R5 and R6 are respectively and independently selected from at least one of isocyanate or alkyl, and at least one isocyanate 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 HF, Fe < 2 + > 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 electrolyte additive, an electrolyte and a lithium iron phosphate 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] Based on the above technical problems, the present invention provides an electrolyte additive, an electrolyte and a lithium iron phosphate battery. The electrolyte containing the additive can form an interface film with good thermal stability and promptly remove HF, Fe generated in the electrolyte. 2+ , H2O and other impurities, which significantly improves 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 isocyanate or alkyl, and there is at least one isocyanate.

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

[0011] The S element in the additive structure of the present invention 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.

[0012] In addition, the isocyanate (-N=C=O) structure has a strong conjugated effect, which causes the electron cloud of the electron-donating group to be biased towards this group, making the Si reaction more active and thus the acid removal efficiency higher. In addition, this group itself can react with H in H2O / HF. + The reaction forms a polymer in which the C atom of the C=N- bond acts as an electrophilic center due to its low electron density and is easily attacked by the lone pair electrons in the O / F atoms of H2O / HF, while the N atom, as a nucleophilic center, is easily attacked by H, thus being able to remove water efficiently and form a polymer protective layer at the positive and negative electrode interfaces. In addition, the electron-rich -N=C=O group can also effectively complex the Fe in the positive electrode material. 3+ , preventing it from being deposited on the negative electrode and undergoing reduction reaction, which is beneficial to the cycle performance of the battery.

[0013] Preferably, the preparation method of the above-mentioned electrolyte additive includes: S1, using trichlorosilane as a raw material to undergo a triisocyanate silane synthesis reaction with silver isocyanate to obtain HSi(NCO)3; S2, subjecting HSi(NCO)3 to an oxidative coupling sulfurization reaction with sulfuryl chloride to obtain the electrolyte additive.

[0014] Preferably, in S1, the triisocyanate silane synthesis reaction is carried out in the absence of water and oxygen, the reaction temperature is -5 to 5°C, and the reaction time is 8 to 15 hours; in S2, the oxidative coupling sulfurization reaction is carried out in the absence of water, the reaction temperature is -10 to -30°C, and the reaction time is 4 to 10 hours.

[0015] The reaction formula of the above step S1 is: HSiCl3+3AgNCO→HSi(NCO)3+3AgCl; the reaction formula of the step S2 is: 2HSi(NCO)3+SO2Cl2+O2→[(NCO)3Si]2SO4+2HCl.

[0016] 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.

[0017] 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).

[0018] Preferably, the second additive is a mixture of vinylene carbonate, vinyl sulfate and lithium difluorooxalatoborate.

[0019] 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 cycle normally. In the preferred embodiment, the combination of vinylene carbonate, vinyl sulfate and lithium difluorooxalatoborate can significantly improve the cycle performance of the electrolyte.

[0020] 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. More preferably, the first additive accounts for 0.2 to 1.0% of the total mass fraction of the electrolyte; and particularly preferably, the first additive accounts for 0.5% of the total mass fraction of the electrolyte.

[0021] 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.

[0022] Preferably, the solvent is selected from a carboxylate solvent and / or a carbonate solvent; preferably, the carboxylate solvent is selected from one or more of ethyl acetate (EA), ethyl propionate (EP), methyl propionate (MP) or methyl acetate (MA). 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) or diethyl carbonate (DEC).

[0023] Preferably, the solvent accounts for 80% to 89% of the total mass fraction of the electrolyte.

[0024] A third object of the present invention is to provide a lithium iron phosphate battery comprising the electrolyte described in any one of the above items.

[0025] Preferably, the lithium iron phosphate battery and its preparation method include the following steps: stacking or winding the lithium iron phosphate positive electrode sheet, separator, and negative electrode sheet in sequence to form a battery cell; then, placing the battery cell into a shell and injecting electrolyte to obtain a lithium iron phosphate battery.

[0026] Preferably, when injecting the electrolyte, the electrolyte is injected twice. After the first injection, the electrolyte is injected twice after infiltration, hot and cold pressing, and formation, and the volume is separated after the second injection. Among them, the first injection uses an electrolyte without the additives described in the present invention; the second injection uses an electrolyte containing the electrolyte additives described in the present invention.

[0027] Preferably, the mass ratio of the electrolyte of the first injection to the electrolyte of the second injection is 4:1.

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

[0029] The present invention provides an electrolyte additive with a new structure, which can be used as an electrolyte additive for lithium iron phosphate batteries to form a SEI film with high ionic conductivity and high stability, effectively remove acid and water, and avoid Fe 2+ Prevent it from depositing on the negative electrode, causing more serious interface side reactions, and significantly improve the high-temperature cycle life. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is the H spectrum of the trichlorosilane HSiCl3 raw material in the electrolyte additive preparation method of Example 1;

[0031] Figure 2 This is the H spectrum of HSi(NCO)3 in the electrolyte additive preparation method of Example 1;

[0032] Figure 3 This is the chromatographic confirmation diagram of [(NCO)3Si]2SO4 in the electrolyte additive preparation method of Example 1. DETAILED DESCRIPTION

[0033] 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.

[0034] Example 1

[0035] An electrolyte additive, the structural formula of which is:

[0036] It is recorded as additive A1.

[0037] The preparation method of the additive A1 of the present invention is:

[0038] 1. Triisocyanate silane synthesis reaction:

[0039] Use trichlorosilane HSiCl3 as the raw material, slowly add it dropwise to the acetonitrile reagent neutralized with silver isocyanate AgNCO, and react for 10 hours under anhydrous and oxygen-free conditions at low temperature (0°C). The reaction process needs to be fully stirred;

[0040] The reaction formula is: HSiCl3+3AgNCO→HSi(NCO)3+3AgCl;

[0041] 2. Oxidative coupling sulfurization reaction:

[0042] React HSi(NCO)3 with sulfuryl chloride SO2Cl2 in anhydrous conditions at -20°C for 6h;

[0043] The reaction formula is: 2HSi(NCO)3+SO2Cl2+O2→[(NCO)3Si]2SO4+2HCl.

[0044] A lithium ion battery fast-charging electrolyte comprising: a lithium salt, a solvent and an additive;

[0045] Wherein: the lithium salt is lithium hexafluorophosphate LiPF6 and lithium bis(fluorosulfonyl)imide LiFSI;

[0046] The solvent is a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and methyl acetate (MA);

[0047] 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).

[0048] The method for preparing the electrolyte described in this embodiment includes the following steps:

[0049] 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.

[0050] 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:

[0051] Table 1. Composition of the electrolytes of Examples 1 to 3 and Comparative Examples 1 to 3 and the proportion of each component

[0052]

[0053] Comparative Example 4

[0054] 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

[0055] The other electrolyte components, contents, and preparation methods are the same as those in Example 3.

[0056] Comparative Example 5

[0057] 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

[0058] The other electrolyte components, contents, and preparation methods are the same as those in Example 3.

[0059] Comparative Example 6

[0060] 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

[0061] The other electrolyte components, contents, and preparation methods are the same as those in Example 3.

[0062] Comparative Example 7

[0063] 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

[0064] The other electrolyte components, contents, and preparation methods are the same as those in Example 3.

[0065] 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:

[0066] (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.

[0067] (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.

[0068] (3) Preparation of lithium-ion batteries: The positive electrode sheet, negative electrode sheet and polypropylene separator are wound to form a battery cell, which is then placed in a casing, baked at 45°C for 24 hours, and then injected with electrolyte and sealed.

[0069] 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.

[0070] 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.

[0071] In order to avoid the risk of increased impedance due to excessive use of the additives described in the present invention, the electrolyte containing the additives is injected a second time after the first injection to prevent the additives from affecting the initial internal resistance.

[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 1-3 with Comparative Example 3 shows that, using Comparative Example 3 as the baseline electrolyte, the addition of various concentrations of Additive A1 improves high-temperature cycling performance. Additive A1 contains the element S, which helps form SEI / CEI films with high-temperature stability and good conductivity. Furthermore, it contains the element Si-O, which directly reacts with HF / PF5 to form Si-F, which participates in film formation, effectively suppressing acidic species in the electrolyte and benefiting the battery's high-temperature performance.

[0091] In addition, its isocyanate (-N=C=O) structure has a strong conjugated effect, which causes the electron cloud of the electron-donating group to be biased towards this group, making the Si reaction more active and thus the acid removal efficiency higher. In addition, this group itself can react with H in H2O / HF. + The reaction forms a polymer in which the C atom of the C=N-bond acts as an electrophilic center due to its low electron density and is easily attacked by the lone pair electrons in the O / F atoms of H2O / HF, while the N atom, as a nucleophilic center, is easily attacked by H, thus enabling efficient water removal and forming a polymer protective layer at the interface between the positive and negative electrodes. In addition, the electron-rich -N=C=O group can also effectively complex the Fe in the positive electrode material. 3+ , preventing it from being deposited on the negative electrode and undergoing reduction reaction, which is beneficial to the cycle performance of the battery.

[0092] 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.

[0093] 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 isocyanate or alkyl, and there is at least one isocyanate.

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

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 sum of the first additive and the second additive accounts for 0.1% to 5% of the total mass fraction of the electrolyte.

6. 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.

7. 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 solvent accounts for 80% to 89% of the total mass fraction of the electrolyte.

8. A lithium iron phosphate battery, characterized in that: The electrolyte comprising the electrolyte according to any one of claims 3 to 7.

9. The lithium iron phosphate battery according to claim 8, characterized in that: The preparation method includes the following steps: stacking or winding the lithium iron phosphate positive electrode sheet, the separator, and the negative electrode sheet in sequence to form a battery core; then, placing the battery core into a shell, injecting electrolyte, and obtaining a lithium iron phosphate battery.

10. The lithium iron phosphate battery according to claim 9, characterized in that: When injecting the electrolyte, the electrolyte is injected twice. After the first injection, the second injection is carried out after infiltration, hot and cold pressing, and formation, and the volume is separated after the second injection; wherein, the electrolyte of the first injection does not contain the additives described in claim 1 or 2; the electrolyte used for the second injection contains the additives described in claim 1 or 2; preferably, the mass ratio of the electrolyte of the first injection to the electrolyte of the second injection is 4:1.

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