Electrolyte additives and resulting electrolytes, lithium ion batteries

Abstract

The application provides an electrolyte additive and an obtained electrolyte and a lithium ion battery, and belongs to the technical field of electrolytes of lithium ion batteries. The electrolyte additive provided by the application can accelerate the formation of a dense film on the surface of a positive electrode and a negative electrode material, prevent transition metal dissolution of the positive electrode material, stabilize the positive electrode, improve the energy density of the lithium ion battery, and effectively improve the cycle performance, safety performance and stability of the lithium ion battery under high temperature and high pressure.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery electrolyte technology, and particularly relates to an electrolyte additive and the resulting electrolyte, and a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries have advantages such as high energy density and long cycle life. Currently, the commercially available cathode materials for lithium-ion batteries mainly include four types: lithium cobalt oxide, ternary materials, lithium manganese oxide, and lithium iron phosphate. The charging cut-off voltage is generally below 4.2V. With the advancement of technology and the continuous development of the market, higher requirements have been placed on the energy density of lithium batteries.

[0003] Commercial lithium-ion battery electrolytes typically consist of a solvent and a lithium salt (lithium salt: LiPF6; solvent: propylene carbonate PC, ethylene carbonate EC, methyl ethyl carbonate EMC, diethyl carbonate DEC, dimethyl carbonate DMC, or a mixture of solvents). The electrode-electrolyte interface is closely related to the chemical / electrochemical instability of high-capacity, high-voltage electrodes and electrolytes, and still faces significant challenges. Electrolyte decomposition at the electrode surface is considered a key factor influencing battery capacity, lifespan, and interfacial chemistry. During the first charge-discharge cycle of a lithium-ion battery, the electrolyte near the positive and negative electrodes undergoes oxidation / reduction decomposition, forming a passivation layer covering the electrode material surface, namely the Solid Electrolyte Interface (SEI film) / Cathode Electrolyte Interphase (CEI film). This SEI / CEI film allows lithium ions to freely enter and exit, while solvent molecules cannot pass through, preventing solvent molecules from embedding into the electrode and causing damage, thereby improving electrode lifespan and lithium intercalation / deintercalation capacity. Therefore, the formation of a stable protective film on the positive and negative electrode surfaces during the first charge-discharge cycle is crucial. After the interfacial film is formed, the surfaces of the positive and negative electrodes will be protected, and the contact between the electrolyte and the active material of the electrode will be inhibited, reducing the decomposition of the electrolyte on the electrode surface.

[0004] Improving the energy density of lithium-ion batteries by increasing their operating voltage is a current research focus. High-nickel ternary materials have become one of the most promising materials for high-energy-density lithium-ion batteries. However, at high voltages, high-nickel ternary materials are unstable and have stronger catalytic activity, making the electrolyte more prone to oxidation and decomposition, leading to gas generation. Simultaneously, transition metal dissolution occurs in the cathode material, ultimately resulting in increased internal resistance and a sharp deterioration in battery performance. Lithium-ion batteries are unstable at high voltages, prone to thermal runaway, and pose significant safety concerns. Current technologies often address these issues by adding film-forming additives to the electrolyte. These additives form a protective layer on the active sites of the electrode active materials, preventing direct contact between the active sites and the electrolyte, thus suppressing side reactions. However, the use of film-forming additives often leads to a decrease in the rate performance of lithium-ion batteries. Therefore, it is necessary to develop a multifunctional electrolyte additive that allows the battery to maintain good cycle stability, capacity retention, and safety even at high voltages. Summary of the Invention

[0005] This invention provides an electrolyte additive and the resulting electrolyte, as well as a lithium-ion battery. The electrolyte additive can accelerate the formation of a dense film on the surface of the positive and negative electrode materials, prevent the dissolution of transition metals in the positive electrode material, stabilize the positive electrode, and improve the energy density of the lithium-ion battery. It can effectively improve the cycle performance, safety performance, and stability of the lithium-ion battery under high temperature and high pressure.

[0006] To achieve the above objectives, the present invention provides an electrolyte additive, the structure of which is shown in formula (I):

[0007]

[0008] R1, R2, R3, R6, and R7 are each independently selected from hydrogen atoms, cyano groups, or fluorophosphates, and R4 and R5 are each independently selected from alkyl groups or fluoroalkyl groups. Furthermore, at least one of R1, R2, and R3 is a fluorophosphate, and at least one of R6 and R7 is a cyano group.

[0009] In the above structure, the cyano groups can form a very strong coupling effect with the transition metal elements in the positive electrode material of lithium-ion batteries, forming an adsorption film on the positive electrode surface. This film can prevent various adverse side reactions caused by direct contact between the electrolyte and the positive electrode material, and can also prevent the dissolution of transition metals in the positive electrode material. In addition, the cyano groups can promote the formation of CEI and SEI films on the positive and negative electrode surfaces, and the film formation impedance is lower (cyano groups are electrophilic, forming a highly conductive interface film), thus improving the high-voltage performance of the battery.

[0010] In the above structure, the borate ester groups can form a dense interfacial film on the surfaces of the positive and negative electrodes, reducing the contact between the positive and negative electrodes and the electrolyte. This is beneficial for electrode stability and preventing electrolyte decomposition, thereby improving the cycle performance and stability of lithium-ion batteries under high voltage. Furthermore, the presence of boron can improve the composition of the interfacial film, significantly enhancing its stability.

[0011] In the above structure, the difluorophosphate group contains a phosphorus-oxygen double bond, which can react on the surface of the positive electrode material to form a protective film. Furthermore, the phosphate ester can be used as a flame retardant. The fluorine atom can promote film formation at the interface between the positive and negative electrodes, reduce intermolecular forces, improve the conductivity of the electrolyte, and enhance the high-temperature performance of the lithium-ion battery.

[0012] Therefore, based on the above-mentioned groups, the resulting electrolyte additive formula (I) can accelerate the formation of a dense film on the surface of the positive and negative electrode materials, prevent the dissolution of transition metals in the positive electrode material, and stabilize the positive electrode, thereby effectively improving the cycle performance, safety performance and stability of lithium-ion batteries under high temperature and high pressure.

[0013] Preferably, the fluorinated phosphate is a monofluorophosphate or a difluorophosphate.

[0014] Preferably, the electrolyte additive is selected from at least one of the following compounds:

[0015]

[0016] This invention provides a lithium-ion battery electrolyte, comprising a solvent, a lithium salt, and the electrolyte additives described in any of the above technical solutions.

[0017] Preferably, the lithium salt is selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bisfluorosulfonylimide, lithium dioxalate borate, lithium dioxalate borate, lithium difluorooxalate borate, lithium trifluoromethanesulfonate, lithium perchlorate, lithium hexafluoroarsenate, and lithium bistrifluoromethanesulfonylimide.

[0018] The solvent is selected from at least two of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate.

[0019] Preferably, the electrolyte additive is selected from at least one of compound 1 to compound 6.

[0020] Preferably, the electrolyte additive accounts for 0.1% to 15% of the total mass of the lithium-ion battery electrolyte, more preferably 0.1% to 5%.

[0021] The present invention provides a lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and the lithium-ion battery electrolyte described in any of the above technical solutions.

[0022] Preferably, the negative electrode sheet includes a negative electrode active material, a conductive agent, a binder, and a negative electrode current collector copper foil, and the positive electrode sheet includes a positive electrode active material, a conductive agent, a binder, and a positive electrode current collector aluminum foil.

[0023] Preferably, the negative electrode active material is selected from one of natural graphite, artificial graphite, soft carbon, hard carbon, lithium titanate, silicon, silicon-carbon alloy, and silicon-oxygen alloy.

[0024] The positive electrode active material is selected from LiCoO2, LiMnO4, LiFePO4, and LiNi. x Co y Mn z M 1-x-y-z One of the O2 components, LiNi x Co y Mn z M 1-x-y-z In O2: 0≤x≤1, 0≤y≤1, 0≤z≤1, and M is selected from at least one of Fe, Mg, Cu, Zn, Al, Sn, B, V, Ti, Cr, and Ga.

[0025] It is understood that, in the above scheme, for lithium-ion batteries, apart from the active materials of the specified positive electrode material, the active materials of the negative electrode material, and the electrolyte, the other components are all commonly used materials by those skilled in the art and are commercially available, and are not specifically limited here.

[0026] Compared with the prior art, the advantages and positive effects of the present invention are as follows:

[0027] 1. The electrolyte additive provided by this invention can generate a very strong coupling effect with the transition metal elements in the positive electrode material of lithium-ion batteries, preventing the dissolution of transition metals in the positive electrode material from affecting battery performance during battery operation and improving the high voltage performance of lithium-ion batteries.

[0028] 2. The electrolyte additive provided by this invention has a good film-forming effect on both positive and negative electrodes, which can effectively protect the positive and negative electrodes and avoid the adverse effects (electrolyte decomposition, electrode corrosion, etc.) caused by direct decomposition of electrolyte on the electrode surface.

[0029] 3. The electrolyte additive provided by this invention is a multifunctional additive (such as electrode stabilization, flame retardancy, film formation, etc.), which can comprehensively improve the cycle performance, safety performance and stability of lithium-ion batteries under high temperature and high pressure. Detailed Implementation

[0030] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] Electrolyte preparation

[0032] Example 1

[0033] The electrolyte includes lithium hexafluorophosphate (LiPF6), ethylene carbonate (EC), dimethyl carbonate (DMC), and electrolyte additive compound 1.

[0034] In an argon-protected glove box, organic solvents were mixed at a mass ratio of EC:DMC = 35:65 to obtain a mixed solvent. Lithium hexafluorophosphate was added to the mixed solvent so that the weight of lithium hexafluorophosphate accounted for 14.3% of the total mass of the electrolyte. After the electrolyte was thoroughly mixed and dissolved, compound 1, equivalent to 3% of the electrolyte mass, was finally added to obtain the electrolyte used in Example 1.

[0035] Example 2

[0036] The difference from Example 1 is that the electrolyte additive is compound 2, and the amount added is equivalent to 3% of the electrolyte mass.

[0037] Example 3

[0038] The difference from Example 1 is that the electrolyte additive is compound 3, and the amount added is equivalent to 3% of the electrolyte mass.

[0039] Example 4

[0040] The difference from Example 1 is that the electrolyte additive is compound 4, and the amount added is equivalent to 3% of the electrolyte mass.

[0041] Example 5

[0042] The difference from Example 1 is that the electrolyte additive is compound 5, and the amount added is equivalent to 3% of the electrolyte mass.

[0043] Example 6

[0044] The difference from Example 1 is that the electrolyte additive is compound 6, and the amount added is equivalent to 3% of the electrolyte mass.

[0045] Comparative Example 1

[0046] The difference from Example 1 is that the electrolyte additive is fluoroethylene carbonate (FEC), and the amount added is equivalent to 3% of the electrolyte mass.

[0047] Comparative Example 2

[0048] The difference from Example 1 is that the electrolyte additive is tris(trimethylsilane)borate (TMSB), and the amount added is equivalent to 1% of the electrolyte mass.

[0049] Battery assembly

[0050] The batteries used in the above embodiments and comparative examples are all pouch cells, and the positive electrode material is LiNi. 0.6 Co 0.2 Mn 0.2 O2 (NCM622), the negative electrode material is artificial graphite.

[0051] Preparation of positive electrode sheet: The conductive agent Super-P, the binder PVDF and the positive electrode active material are dissolved in the solvent N-methylpyrrolidone at a mass ratio of 2:2:96 and mixed evenly to form a positive electrode slurry. Then the positive electrode slurry is evenly coated on the current collector aluminum foil, and after drying, rolling, die cutting and drying, the positive electrode sheet is obtained.

[0052] Preparation of negative electrode sheet: Conductive agent Super-P, binder SBR, carboxymethyl cellulose (CMC) and artificial graphite are dissolved in deionized water at a mass ratio of 1.5:2.3:1.5:94.7 and mixed evenly to prepare negative electrode slurry. Then, the negative electrode slurry is evenly coated on the current collector copper foil, and after drying, rolling, die cutting and drying, the negative electrode sheet is obtained.

[0053] The above-mentioned positive electrode, negative electrode and separator are stacked, tabs are welded, pre-packaged and baked to obtain the cell to be injected with electrolyte. After the electrolyte is injected and packaged, it is left to stand for 24 hours, and then formed and capacity separated for use.

[0054] Battery performance test

[0055] (1) Battery room temperature cycle performance test

[0056] The lithium-ion battery was charged to 4.5V at a constant current and constant voltage of 1C, with a cutoff current of 0.02C, and then discharged to 3.0V at a constant current of 1C. This cycle was repeated.

[0057] (2) Battery high-temperature cycle performance test

[0058] At a temperature of 50℃, the lithium-ion secondary battery is first charged to 4.5V with a constant current of 1C, then charged to 0.05C with a constant voltage of 4.5V, and then discharged to 3.0V with a constant current of 1C. This charge-discharge cycle is repeated.

[0059] The performance test results of the batteries prepared based on the above embodiments and comparative examples are shown in Table 1.

[0060] Table 1 Battery Performance Test

[0061]

[0062] As shown in Table 1, under a high voltage of 4.5V, comparing the data from Examples 1-6 and Comparative Examples 1-2, the capacity retention rates of batteries containing compounds 1-6 in the electrolyte showed varying degrees of improvement after both room temperature and high temperature cycling, compared to batteries with added FEC and TMSB in the electrolyte. Compound 6 exhibited the highest capacity retention rates after both room temperature and high temperature cycling. The retention rates of compounds 1-6, from highest to lowest, were: Compound 6 > Compound 5 > Compound 3 > Compound 2 > Compound 4 > Compound 1. This indicates that these fluorinated cyanoborophosphate compounds can improve the room temperature and high temperature cycling performance of lithium-ion batteries. This may be because the fluorinated cyanoborophosphate compounds can form a dense protective film on the positive and negative electrode surfaces of the battery, blocking direct contact between the electrolyte and the battery electrodes, suppressing various adverse side reactions. Furthermore, they can also generate a very strong coupling effect with transition metal elements in the positive electrode material of lithium-ion batteries, preventing the dissolution of transition metals in the positive electrode material, thereby improving the high voltage and high temperature performance of the battery.

[0063] Materials Studio theoretical calculations

[0064] The theoretical HOMO and LUMO values ​​of the structure proposed in this invention were calculated using Materials Studio software and compared with those of commonly used solvents and additives in commercial lithium-ion battery electrolytes (solvents: EC-ethylene carbonate, DMC-dimethyl carbonate; additives: FEC-fluoroethylene carbonate, TMSB-tris(trimethylsilane)borate).

[0065] In this section, the structure used for theoretical calculations is compound 3 (boron dicyanophosphate, abbreviated as BPF). Two control structures were also designed simultaneously: compound 7 (boron dicyanophosphate, abbreviated as BP) which does not contain fluorine atoms, and compound 8 (boron phosphate, abbreviated as PF) which does not contain borate esters, to theoretically study whether BPF is suitable as an electrolyte additive.

[0066]

[0067] In theoretical calculations, HOMO represents the highest energy level orbital already occupied by electrons, which is an orbital that can lose electrons. The higher the energy, the easier it is to lose electrons and the stronger the reduction ability. LUMO represents the lowest energy level orbital not occupied by electrons, which is an orbital that can accept electrons. The lower the energy, the easier it is to gain electrons and the stronger the oxidation ability. Therefore, negative electrode film-forming additives need a lower LUMO energy level, while positive electrode film-forming additives need a higher HOMO energy level.

[0068] Table 2 Theoretical calculation results of solvents and additives for lithium-ion batteries

[0069] EC DMC FEC TMSB BPF BP PF LUMO(eV) 0.673 0.910 0.189 0.861 -3.768 -3.405 -3.165 HOMO(eV) -8.288 -8.030 -8.805 -7.765 -7.486 -7.075 -8.489

[0070] The theoretical calculation results are shown in Table 2. FEC has a lower LUMO value than the solvent, allowing it to preferentially form a film on the negative electrode. TMSB has a higher HOMO value than the solvent, allowing it to preferentially form a film on the positive electrode. These theoretical calculation results are consistent with previous reports. FEC and TMSB are currently considered mature additives for forming films on both positive and negative electrodes. PF has a lower LUMO value than both the solvent and FEC, indicating that it can be reduced first on the negative electrode surface to form an SEI film. However, because the PF structure contains fluorine atoms, it has good stability and is oxidation-resistant, so its HOMO value is lower than that of the solvent, preventing it from oxidizing on the positive electrode surface to form a CEI film. BP has a lower LUMO value than the solvent and a higher HOMO value, so this structure has the ability to form films on both positive and negative electrodes simultaneously. Adding F atoms to the BP structure yields BPF (or adding a borate ester structure to the PF structure yields BPF). Table 2 shows that BPF has a lower LUMO value than the solvent, while its HOMO value is higher, indicating its ability to form films on both positive and negative electrodes. Furthermore, the fluorinated groups in the BPF structure can reduce intermolecular forces, lower electrolyte viscosity, improve electrolyte conductivity, and provide good stability. Additionally, the borate ester center atom B in the BPF structure is in an electron-deficient state, which can complex with PF6. - or F - This improves the degree of lithium salt dissociation, especially by reducing the LiF content on the electrode surface and lowering the electrode interface impedance. This also demonstrates that BPF possesses excellent positive and negative electrode film-forming capabilities and flame-retardant properties, ensuring the cycle performance, safety, and stability of lithium-ion batteries under high temperature and high pressure.

Claims

1. A lithium-ion battery electrolyte, characterized in that, The lithium ion battery electrolyte comprises a solvent, a lithium salt and an electrolyte additive selected from at least one of the following compounds:

2. The electrolyte for lithium ion batteries according to claim 1, characterized in that, The lithium salt is selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bisfluorosulfonimide, lithium dioxalate borate, lithium bisoxalate borate, lithium difluoro oxalate borate, lithium trifluoromethylsulfonate, lithium perchlorate, lithium hexafluoroarsenate and lithium bis-trifluoromethylsulfonimide; The solvent is selected from a combination of at least two of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate and methyl ethyl carbonate.

3. The electrolyte for a lithium-ion battery according to claim 1 or 2, characterized in that, The mass of the electrolyte additive accounts for 0.1-15% of the total mass of the lithium ion battery electrolyte.

4. The electrolyte for lithium-ion batteries according to claim 3, characterized in that, The mass of the electrolyte additive accounts for 0.1-5% of the total mass of the lithium ion battery electrolyte.

5. A lithium-ion battery, characterized by, The lithium ion battery electrolyte comprises a positive electrode sheet, a negative electrode sheet, a separator and the lithium ion battery electrolyte according to any one of claims 1-4.

6. The lithium-ion battery of claim 5, wherein, The negative electrode sheet comprises a negative electrode active material, a conductive agent, a binder and a negative electrode current collector copper foil, and the positive electrode sheet comprises a positive electrode active material, a conductive agent, a binder and a positive electrode current collector aluminum foil.

7. The lithium-ion battery of claim 6, wherein, The negative electrode active material is selected from one of natural graphite, artificial graphite, soft carbon, hard carbon, lithium titanate, silicon, silicon-carbon alloy and silicon-oxygen alloy. The positive active material is selected from one of LiCoO2, LiMnO4, LiFePO4, LiNi x Co y Mn z M 1-x-y-z O2, LiNi x Co y Mn z M 1-x-y-z O2: 0≤x≤1, 0≤y≤1, 0≤z≤1, M is selected from at least one of Fe, Mg, Cu, Zn, Al, Sn, B, V, Ti, Cr, and Ga.

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