An electrolyte additive composition, an electrolyte containing the same, and a preparation method and application thereof

Abstract

The application belongs to the technical field of lithium ion batteries, and provides an electrolyte additive composition, an electrolyte containing the same, and a preparation method and application thereof. The electrolyte additive composition comprises an alcohol aluminum compound and a phosphazene compound; the structure of the phosphazene compound is shown in formula I, wherein R1-R6 are each independently selected from fluorine, alkoxy or phenoxy. The electrolyte containing the additive composition has good thermal stability and flame retardancy at high voltage, greatly improving the safety of the battery; in addition, the electrolyte additive composition can also inhibit the hydrolysis of LiPF6, thereby reducing the generation of HF and the side effects caused thereby.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium ion batteries and provides an electrolyte additive composition, an electrolyte containing the electrolyte additive composition, and a preparation method and application thereof. Background Art

[0002] Lithium-ion secondary batteries, due to their high energy density, long cycle life, low self-discharge, environmental friendliness, and lack of memory effect, are widely used in portable electronic devices, power batteries, and large-scale energy storage. To further improve battery life and safety and alleviate consumer "battery anxiety," the development of high-energy-density, high-safety lithium-ion secondary batteries is a critical and pressing issue.

[0003] Increasing the battery's charge cut-off voltage can increase the battery's specific capacity and operating voltage platform, thereby increasing the battery's energy density. Currently, the charge cut-off voltage of the commonly used positive electrode material LiFePO4 for lithium-ion secondary batteries is 3.65V, while that of LiCoO2, LiNi x Co y Mn 1-x-y O2、LiNi x Co y Al 1-x-y The O2 charge cutoff voltage is 4.2V, and the electrolyte used is primarily a carbonate solution of lithium hexafluorophosphate (LiPF6). However, due to its low decomposition potential, flammability, and difficulty forming a uniform and stable electrode / electrolyte interface, higher voltages lead to rapid electrolyte decomposition, increased interfacial side reactions between the electrode and electrolyte, accelerated lithium dendrite formation, and structural degradation of the cathode material, leading to battery capacity decay and failure, and even serious safety accidents such as battery short circuits and fires. Therefore, the development of electrolytes with high voltage stability and non-flammability is crucial.

[0004] Electrolyte additives are the main means of regulating the electrolyte. The addition of additives can directly regulate the components of the electrolyte, thereby changing the properties of the electrolyte. At the same time, additives can also decompose to form an electrode / electrolyte interface layer containing specific components to improve the stability of the interface and ionic conductivity. However, the components of the electrode / electrolyte interface formed by simple chemical decomposition of additives are often unevenly distributed, resulting in uneven lithium ion flow and stress within the electrode. Uneven lithium ion concentration can cause uneven lithium deposition and the formation of lithium dendrites, which can easily pierce the diaphragm and increase the risk of internal short circuit and thermal runaway of the battery. Under high voltage, due to the deep delithiation of the positive electrode material, the lattice parameters change significantly, resulting in large mechanical stress. The uneven stress within the electrode will cause microcracks in the positive electrode material due to stress, increase the adverse side reactions between the positive electrode material and the electrolyte, accelerate the structural degradation and capacity decay of the positive electrode material, and significantly increase the interface impedance. In order to ensure uniform transmission of lithium ion flow and uniform distribution of stress in the electrode under high voltage, so that lithium ion secondary batteries can operate safely and stably at high voltage, new interactive additives are needed to improve the uniformity and stability of the electrode / electrolyte interface.

[0005] Cyclotriphosphazene and its derivatives are organic-inorganic hybrid compounds with alternating nitrogen and phosphorus backbones. They have been widely studied as flame retardants. For example, Chinese invention patent CN107302108B discloses a flame retardant, epoxytrifluorocyclotriphosphazene, which, as a flame retardant additive for lithium-ion battery electrolytes, significantly improves flame retardancy through the synergistic effect of four flame-retardant elements: nitrogen, phosphorus, fluorine, and silicon. However, research on the application of cyclotriphosphazene and its derivatives in lithium-ion battery electrolytes has primarily focused on their flame retardancy; their impact on battery performance remains under investigation.

[0006] Therefore, it is necessary to conduct in-depth research on the application of cyclotriphosphazene and its derivatives as electrolyte additives for lithium-ion batteries, especially electrolyte additives suitable for high voltage systems. Summary of the Invention

[0007] The present invention aims to solve the problems in the prior art and provides an electrolyte additive composition, an electrolyte containing the electrolyte additive composition and a preparation method thereof.

[0008] The technical solutions of the present invention are as follows:

[0009] The present invention provides an electrolyte additive composition comprising an aluminum alcoholate compound and a phosphazene compound; the phosphazene compound has the following structural formula:

[0010]

[0011] wherein R1-R6 are each independently selected from fluorine, alkoxy or phenoxy.

[0012] The mechanism of interaction between aluminum alkoxides and phosphazenes is that a nucleophile attacks the electrophilic center, resulting in a nucleophilic substitution reaction. This occurs when the electron-rich phosphazene acts as a nucleophile to attack the aluminum atom of the aluminum alkoxide. Due to its electron deficiency, the aluminum atom can accept electron pairs from other reagents containing lone pairs of electrons, resulting in the replacement of the alkoxy group of the aluminum alkoxide by the electron-rich group of the phosphazene, forming a new covalent bond between the two. Alternatively, the oxygen of the alkoxy group in the aluminum alkoxide initiates a nucleophilic attack on the phosphorus atom in the phosphazene, forming a new covalent bond. This nucleophilic substitution reaction between the two compounds results in a tight covalent bond between them.

[0013] In addition, aluminum alcohol compounds can act as crosslinkers, promoting the formation of a three-dimensional polymer network. This allows the inorganic products (such as Li3N, Li3P, etc.) generated by the additive during cycling to be evenly distributed in the solid electrolyte interface (SEI) and cathode electrolyte interface (CEI), thereby reducing the activation energy of lithium ion transport, ensuring the consistency of lithium ion transport, and reducing the internal stress within the electrode. This promotes the uniform deposition of lithium at high voltages and inhibits stress cracking and structural degradation of the cathode material at high voltages.

[0014] Preferably, the phosphazene compound is one or more of ethoxy(pentafluoro)cyclotriphosphazene (PEPN), pentafluoro(phenoxy)cyclotriphosphazene (FPPN), hexaphenoxycyclotriphosphazene (HPCTP) and hexafluorocyclotriphosphazene (HFPN).

[0015] Furthermore, the mass ratio of the aluminum alcohol compound to the phosphazene compound is 0.1-2:5-10.

[0016] Furthermore, the structural formula of the aluminum alcohol compound is as follows:

[0017]

[0018] Wherein, R7, R8, and R9 are each independently selected from an alkyl group of 1 to 8 carbon atoms.

[0019] Preferably, the aluminum alkoxide compound is selected from one or more of aluminum triethoxide, aluminum isopropoxide, aluminum n-propoxide, aluminum sec-butoxide and aluminum isopentanol.

[0020] The present invention also provides an electrolyte comprising an electrolyte solute, a non-aqueous solvent and any one of the above electrolyte additive compositions.

[0021] Furthermore, the electrolyte additive composition is used in an amount of 0.1-20% of the total mass of the electrolyte.

[0022] Preferably, the amount of the electrolyte additive composition is 5-15% of the total mass of the electrolyte.

[0023] Furthermore, the amount of the aluminum alcohol compound is 0.1-2% of the total mass of the electrolyte; and / or the amount of the phosphazene compound is 5-10% of the total mass of the electrolyte.

[0024] Preferably, the amount of the aluminum alcohol compound is 0.1-0.5% of the total mass of the electrolyte; and / or the amount of the phosphazene compound is 8-10% of the total mass of the electrolyte.

[0025] More preferably, the amount of the aluminum alcohol compound is 0.5% of the total mass of the electrolyte; and / or the amount of the phosphazene compound is 10% of the total mass of the electrolyte.

[0026] Furthermore, the electrolyte solute is selected from one or more of LiPF6, LiFSI, LiTFSI, LiBOB, LiDFOB, LiClO4, LiPO2F2, and LiBF4.

[0027] Furthermore, the non-aqueous solvent is selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC).

[0028] The present invention also provides a method for preparing an electrolyte, which specifically comprises: dissolving an electrolyte solute, an aluminum alcohol compound and a phosphazene compound in a non-aqueous solvent to obtain the electrolyte.

[0029] The present invention also provides the use of the electrolyte composition additive, the electrolyte or the electrolyte prepared by the preparation method in the preparation of lithium ion secondary batteries.

[0030] The present invention also provides a lithium-ion secondary battery, which is prepared from a positive electrode material, a negative electrode material and the above electrolyte.

[0031] Furthermore, the positive electrode material is selected from lithium nickel cobalt manganese oxide (LiNi x Co y Mn 1-x-y O2), lithium iron phosphate (LiFePO4), lithium nickel cobalt aluminum oxide (LiNixCoyAl 1-x-y O2), lithium cobalt oxide (LiCoO2), lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 O4) and lithium-rich manganese (Li 1+x [NiMnCo] 1-x O2) or more.

[0032] Furthermore, the negative electrode material is selected from any one of graphite, lithium metal, silicon carbon and silicon.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] (1) The lithium secondary battery prepared using the electrolyte containing the additive of the present invention can operate stably at a high voltage of more than 4.7V, far exceeding the charging cut-off voltage of 4.2V under the existing technology, greatly improving the specific capacity and energy density of the battery.

[0035] (2) The phosphorus atoms and nitrogen atoms of the phosphazene compound in the composite additive of the present invention have high electronegativity and can attract the surrounding electron cloud to form stable chemical bonds, thereby enhancing the thermal stability of the electrolyte and the SEI and CEI layers; at the same time, they can form new free radicals (such as RPO·) by breaking their own structures, thereby removing combustion free radicals (such as H·, HO·), etc., and interrupting the combustion chain reaction; at the same time, adding an alcohol aluminum compound on the basis of a specific phosphazene compound can further improve its flame retardancy and improve battery performance; the electrolyte containing the composite additive has good thermal stability and flame retardancy, cannot be ignited by open flames, and greatly improves the safety of the battery.

[0036] (3) In addition, the additive composed of an aluminum alcohol compound and a phosphazene compound of the present invention can remove trace water from the electrolyte, inhibit the hydrolysis of electrolyte solutes such as LiPF6, thereby reducing the generation of HF, alleviating HF corrosion on electrode materials, current collectors, etc., and reducing the generation of gas during the cycle. This is because the phosphazene can provide an electrophilic center, attracting the alkoxy group of the aluminum alcohol to initiate a nucleophilic attack, promoting the hydrolysis reaction of the aluminum alcohol, consuming trace water in the electrolyte, inhibiting the hydrolysis of LiPF6, and thus reducing the generation of HF and its side effects. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the embodiments described are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention. It is worth noting that the raw materials used in the present invention are all common commercially available products, and their sources are not specifically limited.

[0038] The aluminum alkoxide additive (aluminum triethoxide) used in the comparative example of the present invention was prepared according to the method disclosed in "Transformation of bulk alloys to oxide nanowires" (Lei, D. et al. Science. 355: 267-271 (2017)). The specific preparation process is as follows:

[0039] (1) First, lithium powder and aluminum powder were used as materials for synthesizing LiAl alloy. 0.085g lithium powder and 0.3g aluminum powder (atomic ratio Li:Al = 1:1, 10% excess Li) were added to a graphite crucible lined with graphite paper. After heating at 800℃ for 30min, the graphite crucible was removed from the muffle furnace and the graphite plunger was immediately pressed down to produce LiAl alloy.

[0040] (2) 0.115 g of LiAl alloy was placed in 20 mL of anhydrous ethanol and placed at 60 °C for 30 h. Uniform triethanolamine (in the form of nanowires with a diameter of 20-200 nm) was obtained through a chemical desalination process.

[0041] Preparation of positive electrode sheet: positive electrode active material LiN i0.8 Co 0.1 Mn 0.1 O2 (NCM811), conductive agent Super P, and binder polyvinylidene fluoride (PVDF) are added to N-methyl-2-pyrrolidone (NMP) in a mass ratio of 94:3:3, stirred evenly to form a positive electrode slurry, and the positive electrode slurry is evenly coated on the positive electrode current collector aluminum foil. After baking, rolling, and cutting, the positive electrode sheet is obtained.

[0042] The negative electrode was purchased from Tianjin Zhongneng Lithium Industry Co., Ltd.

[0043] The negative electrode shell, positive electrode shell, stainless steel sheet, gasket, and spring were purchased from Dongguan Kelude Experimental Equipment Technology Co., Ltd., model CR2032.

[0044] The diaphragm was purchased from Dongguan Kelude Experimental Equipment Technology Co., Ltd., model Celgard 2500.

[0045] Example 1

[0046] Preparation of the electrolyte: Ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1 as the electrolyte solvent; LiPF6 was added to a concentration of 1M and mixed evenly; 0.5% by weight of aluminum triethanolate and 10% by weight of ethoxy(pentafluoro)cyclotriphosphazene (PFPN) were added to the electrolyte and mixed evenly.

[0047] Example 2

[0048] The only difference from Example 1 is that the electrolyte additives used are 0.5% by weight of aluminum triethoxide and 10% by weight of pentafluoro(phenoxy)cyclotriphosphazene (FPPN).

[0049] Example 3

[0050] The difference from Example 1 is only that the electrolyte additive uses 0.5% triethanol aluminum and 10% hexaphenoxy cyclotriphosphazene (HPCTP) by mass of the electrolyte.

[0051] Example 4

[0052] The difference from Example 1 is only that the electrolyte additive uses 0.5% triethanol aluminum and 10% hexaphenoxy cyclotriphosphazene (HPCTP) by mass of the electrolyte.

[0053] Example 5

[0054] The difference from Example 1 is only that the electrolyte additive uses 0.5% triethanol aluminum and 10% hexaphenoxy cyclotriphosphazene (HPCTP) by mass of the electrolyte.

[0055] Example 6

[0056] The difference from Example 1 is only that the electrolyte additive uses 0.5% triethanol aluminum and 10% hexaphenoxy cyclotriphosphazene (HPCTP) by mass of the electrolyte.

[0057] Example 7

[0058] The difference from Example 1 is only that the electrolyte additive uses 0.5% triethanol aluminum and 10% hexaphenoxy cyclotriphosphazene (HPCTP) by mass of the electrolyte.

[0059] Comparative Example 1

[0060] The difference from Example 1 is only that no electrolyte additive is added.

[0061] Comparative Example 2

[0062] The difference from Example 1 is only that the electrolyte additive uses 0.5% triethanol aluminum by mass of the electrolyte.

[0063] Comparative Example 3

[0064] The difference from Example 1 is only that the electrolyte additive uses 0.5% triethanol aluminum and 10% hexaphenoxy cyclotriphosphazene (HPCTP) by mass of the electrolyte.

[0065] Comparative Example 4

[0066] The difference from Example 1 is only that the electrolyte additive uses 0.5% triethanol aluminum and 10% hexaphenoxy cyclotriphosphazene (HPCTP) by mass of the electrolyte.

[0067] Comparative Example 5

[0068] The difference from Example 1 is only that the electrolyte additive uses 0.5% triethanol aluminum and 10% hexaphenoxy cyclotriphosphazene (HPCTP) by mass of the electrolyte.

[0069] Comparative Example 6

[0070] The difference from Example 1 is only that the electrolyte additive uses 0.5% of aluminum triethoxide and 10% of hexachlorocyclotriphosphazene (HCPN) by mass percentage of electrolyte.

[0071] Test Example

[0072] Assembly and test of full battery Li||NCM811: Assemble the button cell in the order of negative electrode shell, negative electrode sheet, 15 μL of electrolyte, separator, 15 μL of electrolyte, positive electrode sheet, gasket, spring, positive electrode shell. At 30°C, the battery is placed for 8 hours, then charged to 4.7V (or 4.2V) at a current of 0.2C, discharged to 3.0V at a current of 0.5C, and the discharge capacity of the battery at 0.5C is obtained; the battery is cycled for 50 cycles under the above conditions, and the discharge capacity of the battery at the 50th cycle is obtained, and the capacity retention rate is calculated, capacity retention rate = discharge capacity at the 50th cycle / discharge capacity at the first cycle.

[0073] Assembly of Li||SS battery and test of electrolyte electrochemical window: Assemble the button cell in the order of negative electrode shell, lithium sheet, 30 μL of electrolyte, separator, 30 μL of electrolyte, stainless steel sheet, gasket, spring, positive electrode shell. At 30°C, the battery is placed for 8 hours, then scanned from open circuit voltage to 6V at a scan rate of 1mV / s by linear sweep voltammetry (LSV), and the current-potential curve is obtained, and the potential corresponding to the point where the curve slope changes significantly is taken as the decomposition potential of the electrolyte, and the electrochemical window of the electrolyte is determined.

[0074] Flame retardancy test of electrolyte: Take 1g of electrolyte in the battery shell, use an ignition device to ignite the electrolyte for 5s, then remove the ignition device, and record the time from removal of the ignition device to automatic extinguishing of the flame, and the self-extinguishing time (SET) of the electrolyte corresponding to the unit mass of electrolyte is recorded.

[0075] The test results of high-voltage full battery cycle performance, electrochemical window and flame retardancy of different electrolytes in the above examples are shown in Table 1.

[0076] Table 1 Test results of high-voltage full battery performance

[0077]

[0078] As can be seen from the results in Table 1, when no electrolyte additives are used, the battery capacity decays rapidly at high charge cutoff voltages, and the electrolyte will continue to burn after ignition until all the electrolyte is consumed. When only aluminum alcohol compounds are added to the electrolyte, the high-voltage cycling stability of the battery is improved to a certain extent, but the flame retardancy of the electrolyte is not improved, and it will continue to burn after ignition. When only phosphazene compounds are added, the flame retardancy of the electrolyte is improved, and it will not continue to burn. The high-voltage cycling stability of the battery is improved to a certain extent, but the capacity decay is still relatively rapid. By simultaneously adding an aluminum alcohol compound and a specific phosphazene compound to the electrolyte, the electrochemical window of the electrolyte is expanded to varying degrees. The battery's cycle stability at a high charge cut-off voltage of 4.7V is significantly improved compared to the control examples of no additives and a single additive. The specific capacity is also increased by 29.1% compared to 4.2V, and the flame retardant performance is further enhanced. In addition, when hexachlorocyclotriphosphazene (HCPN) is used in Comparative Example 6, only one cycle can be achieved. This indicates that the aluminum alcohol compound only has a synergistic effect with the specific phosphazene compound, effectively improving the high-voltage stability of the electrolyte. At the same time, the flame retardant performance of the electrolyte is also significantly improved, and it will not be ignited, which will greatly improve the safety of the battery.

[0079] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, rather than to limit the scope of protection of the present invention. Simple modifications or equivalent substitutions of the technical solution of the present invention by ordinary technicians in this field do not deviate from the essence and scope of the technical solution of the present invention.

Claims

1. An electrolyte, characterized in that: A composition comprising an electrolyte solute, a non-aqueous solvent, and an electrolyte additive; The electrolyte additive composition includes an aluminum alcohol compound and a phosphazene compound; The phosphazene compound is one or more of ethoxy (pentafluoro) cyclotriphosphazene, pentafluoro (phenoxy) cyclotriphosphazene, hexaphenoxy cyclotriphosphazene and hexafluoro cyclotriphosphazene; The structural formula of the aluminum alcohol compound is as follows: wherein R7, R8, and R9 are each independently selected from an alkyl group of 1 to 8 carbons; The amount of the aluminum alcohol compound is 0.1-2% of the total mass of the electrolyte; the amount of the phosphazene compound is 5-10% of the total mass of the electrolyte.

2. The electrolyte according to claim 1, characterized in that The aluminum alkoxide compound is specifically selected from one or more of aluminum triethoxide, aluminum isopropoxide, aluminum n-propoxide, aluminum sec-butoxide and aluminum isopentanol.

3. The electrolyte according to claim 1, characterized in that The amount of the aluminum alcohol compound is 0.1-0.5% of the total mass of the electrolyte; the amount of the phosphazene compound is 8-10% of the total mass of the electrolyte.

4. The electrolyte according to claim 3, characterized in that The amount of the aluminum alcohol compound used is 0.5% of the total mass of the electrolyte; the amount of the phosphazene compound used is 10% of the total mass of the electrolyte.

5. The electrolyte according to any one of claims 1 to 4, characterized in that The electrolyte solute is selected from one or more of LiPF6, LiFSI, LiTFSI, LiBOB, LiDFOB, LiClO4, LiPO2F2, and LiBF4; the non-aqueous solvent is selected from one or more of ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate.

6. The method for preparing an electrolyte according to any one of claims 1 to 5, wherein: The electrolyte solution is obtained by dissolving the electrolyte solute, the aluminum alcohol compound and the phosphazene compound in a non-aqueous solvent.

7. Use of the electrolyte according to any one of claims 1 to 5 or the electrolyte prepared by the preparation method according to claim 6 in the preparation of lithium-ion secondary batteries.

8. A lithium ion secondary battery, characterized in that: Prepared from a positive electrode material, a negative electrode material and an electrolyte, wherein the electrolyte is the electrolyte according to any one of claims 1 to 5 or the electrolyte prepared by the preparation method according to claim 6; The positive electrode material is selected from any one of lithium nickel cobalt manganese oxide, lithium iron phosphate, lithium nickel cobalt aluminum oxide, lithium cobalt oxide, lithium nickel manganese oxide and lithium-rich manganese oxide; The negative electrode material is selected from any one of graphite, lithium metal, silicon carbon and silicon.

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