Electrolyte and secondary battery

Abstract

The invention provides an electrolyte and a secondary battery. The electrolyte comprises a lithium salt, an organic solvent, a first additive and a second additive, based on the total mass of the electrolyte, the mass percentage content of the first additive is W1%, the mass percentage content of the second additive is W2%, 0.01 < = W1 < = 3, and 0.01 < = W2 < = 3. When the first additive and the second additive are matched to be used as the electrolyte additive, on one hand, a layer of uniform and compact solid electrolyte interface film can be formed on the surface of an electrode, and the solid electrolyte interface film has relatively good mechanical strength and stability; solvent decomposition of electrolyte at high temperature and high pressure can be effectively prevented from damaging an electrode structure, the impedance of the electrode surface can be effectively reduced, and the rate capability, the cycling stability and the service life of the battery are improved; on the other hand, the first additive and the second additive have a synergistic effect, so that gas production can be effectively inhibited, and the safety of the battery is improved.

Description

Technical Field

[0001] This application relates to the field of electrochemical technology, and in particular to an electrolyte and a secondary battery. Background Technology

[0002] Secondary batteries (such as lithium-ion batteries) are widely used in consumer electronics and energy storage and power batteries due to their advantages such as high specific energy, long cycle life, and low self-discharge. With the rapid development of markets such as pure electric vehicles, smart homes, power tools, and intelligent transportation, consumers' performance requirements for batteries are constantly increasing. To improve the energy density of secondary batteries, the specific capacity of the positive electrode active material and the charging voltage of the battery are usually increased. However, when increasing the energy density of the battery through these methods, the cycle performance and high-temperature storage performance of the battery are severely compromised. This is because: firstly, the crystal structure of the positive electrode active material is more prone to structural collapse; secondly, the electrolyte is highly susceptible to oxidative decomposition or side reactions with the electrodes, affecting the cycle performance of the secondary battery; and thirdly, at high temperatures, the side reactions between the electrolyte and the electrodes are exacerbated, leading to a decrease in high-temperature cycle capacity and a larger DCR (discharge rate-delay ratio) growth rate. Therefore, developing an electrolyte with low impedance and high-temperature resistance is of great significance for expanding the application scenarios of secondary batteries. Summary of the Invention

[0003] The purpose of this application is to provide an electrolyte and a secondary battery to improve the battery's room temperature and high temperature cycle performance, rate performance, and gas generation suppression capability. The specific technical solution is as follows:

[0004] A first aspect of this application provides an electrolyte comprising a lithium salt, an organic solvent, a first additive, and a second additive; the first additive comprising a compound of formula I: ;

[0005] R1-R3 are each independently selected from H, F, C1-C4 alkyl, C1-C4 fluoroalkyl, C2-C4 alkenyl, C2-C4 alkynyl, C2-C4 fluoroalkynyl, C5-C7 cycloalkyl, unsubstituted or Ra-substituted phenyl, and unsubstituted or Ra-substituted benzyl.

[0006] The Ra is selected from F, C1-C4 alkyl, C2-C4 alkenyl, C2-C4 alkynyl, C1-C4 fluoroalkyl, C2-C4 fluoroalkenyl, and C2-C4 fluoroalkynyl.

[0007] The second additive includes at least one of compounds with the structural formulas II-IV: ;

[0008] Based on the total mass of the electrolyte, the mass percentage of the first additive is W1%, the mass percentage of the second additive is W2%, 0.01≤W1≤3, and 0.01≤W2≤3.

[0009] In one embodiment of this application, 0.1 ≤ W1 / W2 ≤ 10.

[0010] In one embodiment of this application, the electrolyte further includes a third additive, which is triphenyl phosphite; based on the total mass of the electrolyte, the mass percentage of the third additive is W3, where 1‰≤W3≤5‰.

[0011] In one embodiment of this application, the first additive is selected from compounds represented by the following formulas I-1 to I-22: .

[0012] In one embodiment of this application, the solvent includes at least one selected from ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-butyrolactone, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, ethyl propionate, propyl propionate, butyl propionate, and methyl trifluoroethyl carbonate.

[0013] In one embodiment of this application, the lithium salt includes at least one of lithium hexafluorophosphate, lithium bisfluorosulfonylimide, lithium tetrafluoroborate, and lithium bis(trifluoromethanesulfonylimide).

[0014] In one embodiment of this application, the molar concentration of lithium salt in the electrolyte is 0.4 mol / L to 2 mol / L.

[0015] A second aspect of this application provides a secondary battery comprising the electrolyte provided in the first aspect of this application.

[0016] In one embodiment of this application, the secondary battery includes a positive electrode, a negative electrode, and a separator. The positive electrode includes a positive electrode material layer, which includes a positive electrode active material selected from lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide, lithium manganese oxide, and Li. a Ni b Co c M1 d M2 e O f R gat least one of them, where 1 ≤ a ≤ 1.2, 0.6 < b < 1, 0 < c < 0.4, 0 < d < 0.4, 0 ≤ e ≤ 0.2, b + c + d + (m / 3)e = 1, m is the absolute value of the valence of M2, 1 ≤ f ≤ 2, 0 ≤ g ≤ 1, f + (n / 2)g = 2, n is the absolute value of the valence of R; M1 is selected from at least one of Mn and Al, M2 is selected from at least one of Zr, Zn, Cu, Cr, Mg, Fe, V, Ti, Sr, Sb, Y, W and Nb, and R is selected from at least one of N, F, S and Cl.

[0017] In one embodiment of the present application, the positive electrode active material is selected from Li a Ni b Co c M1 d M2 e O f R g ; the negative electrode sheet includes a negative electrode active material layer, the negative electrode active material layer includes a negative electrode active material, and the negative electrode active material is graphite.

[0018] In one embodiment of the present application, the secondary battery satisfies: W1×(1 - b)+W2×b = K, 0.5 ≤ K ≤ 2.0, preferably: 0.8 ≤ K ≤ 1.5.

[0019] Advantages of the present application:

[0020] The present application provides an electrolyte and a secondary battery. The electrolyte includes a lithium salt, an organic solvent, a first additive and a second additive; based on the total mass of the electrolyte, the mass percentage content of the first additive is W1%, and the mass percentage content of the second additive is W2%, 0.01 ≤ W1 ≤ 3, 0.01 ≤ W2 ≤ 3. When the first additive and the second additive of the present application are used as electrolyte additives in combination, on the one hand, a uniform and dense SEI film can be formed on the electrode surface. This SEI film has good mechanical strength and stability, can effectively inhibit the solvent decomposition of the electrolyte under high temperature and high pressure to damage the electrode structure, effectively reduce the impedance on the electrode surface, and improve the rate performance, cycle stability and service life of the battery; on the other hand, the first additive and the second additive act synergistically to effectively inhibit gas generation, so as to improve the safety of the battery.

[0021] Of course, implementing any product or method of the present application does not necessarily require achieving all the above advantages simultaneously. Detailed implementation manners

[0022] The technical solutions of this application will be clearly and completely described below with reference to the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0023] A first aspect of this application provides an electrolyte comprising a lithium salt, an organic solvent, a first additive, and a second additive; the first additive comprising a compound of formula I: ;

[0024] R1-R3 are each independently selected from H, F, C1-C4 alkyl, C1-C4 fluoroalkyl, C2-C4 alkenyl, C2-C4 alkynyl, C2-C4 fluoroalkynyl, C5-C7 cycloalkyl, unsubstituted or Ra-substituted phenyl, and unsubstituted or Ra-substituted benzyl.

[0025] The Ra is selected from F, C1-C4 alkyl, C2-C4 alkenyl, C2-C4 alkynyl, C1-C4 fluoroalkyl, C2-C4 fluoroalkenyl, and C2-C4 fluoroalkynyl.

[0026] The second additive includes at least one of compounds with the structural formulas II-IV: ;

[0027] Based on the total mass of the electrolyte, the mass percentage of the first additive is W1%, where 0.01 ≤ W1 ≤ 3. For example, W1% can be 0.01%, 0.05%, 0.1%, 0.4%, 0.6%, 0.8%, 1.0%, 1.3%, 1.5%, 1.7%, 2.0%, 2.5%, 3.0%, or a range of any two of these values. The mass percentage of the second additive is W2%, where 0.01 ≤ W2 ≤ 3. For example, W2% can be 0.01%, 0.05%, 0.1%, 0.4%, 0.6%, 0.8%, 1.0%, 1.3%, 1.5%, 1.7%, 2.0%, 2.5%, 3.0%, or a range of any two of these values.

[0028] The inventors discovered that the first additive can form LiF and Li3PO4 on the negative electrode surface. LiF possesses good chemical stability and ionic conductivity, forming a uniform and dense protective film on the electrode surface, effectively preventing further reactions between the electrolyte and electrode materials. It also reduces the impedance of the electrode surface, improving the battery's rate performance and cycle stability. Li3PO4 is also a stable inorganic compound that can enhance the mechanical strength and stability of the solid electrolyte interphase (SEI) film, improving battery safety and lifespan. However, when the first additive is used alone, gas production increases during high-temperature storage. This is because the Si-C bonds in the first additive are prone to breakage at high temperatures. Simultaneously, the first additive reacts with HF in the electrolyte to form a strong acid, further promoting the hydrolysis of LiPF6. This creates a self-reinforcing, acid-catalyzed vicious cycle. HF corrodes the SEI film, causing electrolyte decomposition at the negative electrode and increasing gas production. To address this issue, a second additive is introduced to work synergistically with it.

[0029] The second additive exhibits better thermodynamic stability compared to other lithium salt additives. It possesses a strongly electronegative CF bond (bond energy 485 kJ / mol) and an S=O conjugated structure, giving it a higher decomposition temperature (>300℃) than conventional LiPF6. The CF3 groups in its structure neutralize the O2 generated by electrolyte oxidation through an electron-trapping mechanism (LUMO level -1.8 eV). - Free radicals can block chain decomposition reactions, thereby inhibiting the solvent decomposition of the electrolyte under high temperature and pressure, which damages the electrode structure and improves the stability of the negative electrode interface film. Furthermore, CF3SO3... - The continuous decomposition of the second additive can dynamically replenish SEI film defects. On the other hand, because LiPF6 forms PF5 at high temperatures, PF5 readily reacts with H2O to form HF. The second additive preferentially reacts with HF before the first additive, converting it into a stable fluorosulfonic acid derivative rather than a strong acid. This blocks the chain reaction of HF corrosion on the electrode material and avoids the vicious cycle of LiPF6 hydrolysis caused by the strong acid produced from the reaction of the first additive with HF. Simultaneously, the oxygen atoms in the second additive can form complexes with PF5 through lone pair electrons, inhibiting the catalytic activity of PF5, reducing solvent decomposition and gas production, thereby inhibiting high-temperature disproportionation of LiPF6, reducing LiPF6 hydrolysis, and further reducing HF formation. Furthermore, the second additive can neutralize the O2 generated by electrolyte oxidation. - Free radicals reduce the probability of Si and P groups in the first additive being oxidized by the positive electrode lattice oxygen, thus inhibiting their decomposition and gas generation. However, when the second additive is used alone, the SO3F in the second additive... -Anions exhibit poor chemical stability under high voltage (>4.2V) and are prone to oxidation reactions, producing SO2 and F. - Byproducts include SO2 gas, which may cause an increase in internal battery pressure, and F... - It may corrode the aluminum current collector and damage the electrode structure. Simultaneously, oxidative decomposition consumes additives, reducing the electrochemical window of the electrolyte. Furthermore, the anions in the second additive have strong coordination ability, readily forming close contact ion pairs or even aggregates with lithium ions, thereby reducing the lithium-ion transference number and increasing electrolyte impedance. This leads to increased polarization and decreased rate performance at high rates. The phosphorus atoms (Lewis acidic center) in the first additive and the sulfonate oxygen atoms (Lewis basic center) in the second additive form a dynamic complex through reversible coordination. This complex reduces the Lewis acidity of TMS-DFP, suppressing its hydrolysis side reactions with the solvent; simultaneously, the sulfonate anion is "shielded," increasing the oxidative decomposition barrier. Furthermore, the formation of this complex alters the ion pair structure of the second additive, transforming it from tightly packed ion pairs to solvent-separated ion pairs. This is due to the weak coordination between the fluorine atom of the first additive and lithium ions, disrupting the original ion aggregation and increasing the lithium-ion transference number (from 0.3 to 0.5). Meanwhile, the overall electrolyte conductivity remains stable above 1 mS / cm, avoiding the impedance increase seen when using LiSO3F alone. The synergistic effect of the first and second additives reduces the DCR growth rate during battery cycling and storage, while also reducing gas generation during storage, thereby improving the battery's room temperature and high temperature cycling performance and rate performance. The mass percentages of the first and second additives within the scope of this application fully leverage their synergistic effect without negatively impacting other electrolyte properties.

[0030] In one embodiment of this application, 0.1 ≤ W1 / W2 ≤ 10. For example, W1 / W2 can be 0.1, 0.5, 1, 2, 5, 7, 10, or a range of any two values ​​therein. The mass ratio of the first additive to the second additive is within the range of this application, which can fully exert the synergistic effect of the two without negatively affecting other properties of the electrolyte.

[0031] In one embodiment of this application, the electrolyte further includes a third additive, which is triphenyl phosphite. Based on the total mass of the electrolyte, the mass percentage of the third additive is W3, where 1‰ ≤ W3 ≤ 5‰. For example, W3 can be 1‰, 1.5‰, 2‰, 2.5‰, 3‰, 3.5‰, 4‰, 4.5‰, 5‰, or a range of any two of these values. The third additive is a stabilizing additive, which can improve the storage stability of the electrolyte, helping it maintain stable properties during storage and transportation. Furthermore, a smaller amount added helps reduce the manufacturing cost of the electrolyte and broadens its application scenarios.

[0032] In one embodiment of this application, the first additive is selected from compounds represented by the following formulas I-1 to I-22: ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; The compound shown in Formula I-19 The compound shown in Formula I-20 The compound shown in Formula I-21 The compound shown in Formula I-22 .

[0033] In this application, compounds of formulas I-18 to I-22 can be prepared by the following synthetic method: Under anhydrous conditions, sodium difluorophosphate (50 mmol, moisture ≤ 20 ppm) is added to a reaction vessel, followed by ultra-dry acetonitrile solvent (550 mmol). The solid is stirred and dissolved at a controlled temperature of 20-60°C. Then, the starting material (50 mmol, moisture ≤ 20 ppm) is slowly added, maintaining a nitrogen atmosphere and stirring continuously at 20-60°C for about 8 hours. After the reaction is complete, the mixture is allowed to stand at room temperature for about 2 hours, then filtered to remove the solid precipitate. Finally, nitrogen is bubbled to remove the solvent and volatile byproducts, thus obtaining the desired silicon-based fluorophosphate product. The raw materials required for the synthesis of formula I-18 are trivinylchlorosilane (CAS No. 1871-21-2), the raw materials required for the synthesis of formula I-19 are dimethylynyl butylchlorosilane (CAS No. 2069196-19-4), the raw materials required for the synthesis of formula I-20 are dimethyl (trifluoropropenyl)chlorosilane (CAS No. 89705-02-2), the raw materials required for the synthesis of formula I-21 are tris(pentafluoroethyl)chlorosilane (CAS No. 1620665-21-5), and the raw materials required for the synthesis of formula I-22 are dimethyl (p-methylbenzyl)chlorosilane (CAS No. 1833-28-9).

[0034] In one embodiment of this application, the solvent includes at least one selected from ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-butyrolactone, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, ethyl propionate, propyl propionate, butyl propionate, and methyltrifluoroethyl carbonate. Using solvents within the scope of this application can improve the dispersion uniformity and ionic conductivity of the electrolyte, further improving the battery's cycle life, charge / discharge rate, high-temperature performance, and energy density.

[0035] In one embodiment of this application, the lithium salt includes at least one selected from lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium tetrafluoroborate, and lithium bis(trifluoromethanesulfonyl)imide. For example, the lithium salt can be lithium bis(fluorosulfonyl)imide, which helps reduce the generation of hydrogen fluoride in the electrolyte and lowers manufacturing costs.

[0036] In one embodiment of this application, the molar concentration of the lithium salt in the electrolyte is 0.4 mol / L to 2 mol / L. For example, the molar concentration of the lithium salt can be 0.4 mol / L, 0.9 mol / L, 1.0 mol / L, 1.2 mol / L, 1.4 mol / L, 1.6 mol / L, 1.8 mol / L, 1.9 mol / L, 2 mol / L, or a range of any two of these values. When the molar concentration of the lithium electrolyte in the electrolyte is within the range of this application, the lithium electrolyte can be fully dissolved in the solvent, and the electrolyte combines high ionic conductivity with low manufacturing cost.

[0037] A second aspect of the present application provides a secondary battery comprising the electrolyte provided by the first aspect of the present application.

[0038] In one embodiment of the present application, the secondary battery includes a positive electrode sheet, a negative electrode sheet, and a separator. The positive electrode sheet includes a positive electrode material layer, the positive electrode material layer includes a positive electrode active material, and the positive electrode active material is selected from at least one of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminate, lithium iron phosphate, lithium-rich manganese-based material, lithium cobalt oxide, lithium manganese oxide, and Li a Ni b Co c M1 d M2 e O f R g where at least one of them satisfies 1≤a≤1.2, 0.6 < b < 1, 0 < c < 0.4, 0 < d < 0.4, 0≤e≤0.2, b + c + d + (m / 3)e = 1, m is the absolute value of the valence of M2, 1≤f≤2, 0≤g≤1, f + (n / 2)g = 2, n is the absolute value of the valence of R; M1 is selected from at least one of Mn and Al, M2 is selected from at least one of Zr, Zn, Cu, Cr, Mg, Fe, V, Ti, Sr, Sb, Y, W, and Nb, and R is selected from at least one of N, F, S, and Cl.

[0039] In one embodiment of the present application, the positive electrode active material is selected from Li a Ni b Co c M1 d M2 e O f R g ; the negative electrode sheet includes a negative electrode active material layer, the negative electrode active material layer includes a negative electrode active material, and the negative electrode active material is graphite.

[0040] In one embodiment of this application, the secondary battery satisfies: W1 × (1-b) + W2 × b = K, 0.5 ≤ K ≤ 2.0, preferably: 0.8 ≤ K ≤ 1.5. The inventors have found that increasing the nickel content of the positive electrode active material can significantly improve the energy density of the battery. However, this is accompanied by an increase in irreversible phase transitions in the positive electrode active material during charge-discharge cycles, leading to problems such as transition metal dissolution and loss of active sites. During cycling, transition metals deposit on the graphite surface of the negative electrode, causing the SEI film of the graphite to continuously decompose and regenerate, continuously consuming active lithium. Due to the reduced amount of active lithium re-intercalated to the positive electrode, the amount of low-lithium-intercalated graphite in the negative electrode increases, increasing the degree of disorder on the graphite surface, which also promotes the SEI film regeneration reaction. Both the decomposition and regeneration of the SEI film promote the decomposition of the electrolyte, resulting in high-temperature cycle capacity decay and a larger increase in the rate of increase in DC resistance (DCR) of the high-nickel ternary battery. High-nickel cathode materials (b > 0.6) exhibit high oxidation activity, readily catalyzing the oxidative decomposition of the electrolyte, leading to gas generation and capacity decay. The first additive, a Lewis acid additive, effectively captures trace amounts of moisture and free acid in the electrolyte. The second additive provides sulfonate anions, which possess high oxidation stability and can form an electrostatic shielding layer on the cathode surface, inhibiting further electrolyte oxidation. When the addition of the first and second additives to the electrolyte matches the nickel content in the cathode active material as follows: 0.5 ≤ W1 × (1-b) + W2 × b ≤ 2.0, the ratio of the two additives can match the oxidation activity of the cathode, synergistically improving the overall oxidation stability of the electrolyte and reducing side reactions. W1 reduces corrosion by lowering interfacial acidity, while W2 promotes lithium-ion insertion / extraction kinetics through anion effects. Optimizing the ratio of W1 and W2 can regulate the ion migration behavior at the cathode-electrolyte interface, optimizing their balance, reducing interfacial impedance fluctuations caused by excessive amounts of a single additive, thereby maintaining low interfacial impedance and good lithium-ion transport efficiency. Furthermore, high-nickel cathodes easily trigger electrolyte decomposition and gas production. Optimizing the amounts of W1 and W2 can effectively reduce gas generation (such as CO2 and CO). By balancing the reduction and oxidation potentials of the additives, catalytic side reactions can be suppressed. The linear relationship in the formula reflects the positive correlation between nickel content and additive requirements. That is, the higher the value of b, the more W2 is needed to provide antioxidant protection, while W1 controls acidity. An imbalance in the ratio of the two will lead to insufficient protection. This application controls the addition of the first and second additives to the electrolyte to match the nickel content in the cathode active material with the following ratio: 0.5 ≤ W1 × (1-b) + W2 × b ≤ 2.0. The synergistic effect of the two additives can reduce the DCR growth rate of the battery during high-temperature storage, reduce gas production during storage, and improve the battery's room temperature and high-temperature cycle performance, as well as its room temperature rate performance.

[0041] In this application, the positive electrode sheet includes a positive current collector and a positive active material layer located on at least one surface of the positive electrode sheet. The aforementioned "positive material layer disposed on at least one surface of the positive current collector" means that the positive material layer can be disposed on one surface of the positive current collector along its thickness direction, or on two surfaces of the positive current collector along its thickness direction. It should be noted that the "surface" here can be the entire surface area of ​​the positive current collector, or only a portion of the surface area; this application has no particular limitation, as long as the purpose of this application is achieved. This application has no particular limitation on the positive current collector, as long as the purpose of this application is achieved; for example, the positive current collector can be an aluminum foil, an aluminum alloy foil, or a composite positive current collector. The aforementioned composite positive electrode current collector can be a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The material of the polymer material base layer can be, but is not limited to, at least one of polypropylene (PP), polyethylene terephthalate (PET), or polybutylene terephthalate (PBT). The material of the metal layer can be, but is not limited to, at least one of aluminum, aluminum alloy, nickel, or nickel alloy. This application does not impose any particular limitation on the thickness of the positive electrode material layer and the positive electrode current collector, as long as the purpose of this application can be achieved. For example, the thickness of the single-sided positive electrode material layer can be from 50 μm to 250 μm, and the thickness of the positive electrode current collector can be from 7 μm to 20 μm.

[0042] The positive electrode material layer may further include a positive electrode conductive agent and a positive electrode binder. This application does not impose any particular limitation on the types of positive electrode conductive agents and positive electrode binders, as long as they achieve the purpose of this application. For example, the positive electrode conductive agent may include, but is not limited to, at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon fiber. For example, the positive electrode binder may include, but is not limited to, at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), or a terpolymer of PVDF-PTFE-propylene. This application does not impose any particular limitation on the mass ratio of the positive electrode active material, positive electrode conductive agent, and positive electrode binder in the positive electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application is achieved.

[0043] In this application, the negative electrode sheet includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The phrase "the negative electrode material layer is disposed on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be disposed on one surface of the negative electrode current collector along its thickness direction, or on two surfaces of the negative electrode current collector along its thickness direction. It should be noted that the "surface" here can be the entire surface area of ​​the negative electrode current collector, or only a portion of the surface area; this application has no particular limitation, as long as the purpose of this application is achieved. This application has no particular limitation on the negative electrode current collector, as long as the purpose of this application is achieved; for example, the negative electrode current collector can be copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foamed nickel or foamed copper, aluminum foil, or a composite negative electrode current collector. The aforementioned composite negative electrode current collector can be a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The material of the polymer material base layer can be, but is not limited to, at least one of polypropylene (PP), polyethylene terephthalate (PET), or polybutylene terephthalate (PBT). The material of the metal layer can be, but is not limited to, at least one of copper, copper alloy, nickel, or nickel alloy. This application does not impose any particular limitation on the thickness of the negative electrode material layer and the negative electrode current collector, as long as the purpose of this application can be achieved. For example, the thickness of the single-sided negative electrode material layer can be from 50 μm to 180 μm, and the thickness of the negative electrode current collector can be from 3 μm to 10 μm.

[0044] In this application, the negative electrode material layer may further include a negative electrode conductive agent, a negative electrode binder, and a negative electrode thickener. This application does not impose any particular limitation on the types of negative electrode conductive agents, negative electrode binders, and negative electrode thickeners, as long as they achieve the purpose of this application. For example, the negative electrode conductive agent may include, but is not limited to, at least one of superconducting carbon, acetylene black, carbon black, and graphene. For example, the negative electrode binder may include, but is not limited to, at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), and polyacrylamide (PAM). For example, the negative electrode thickener may include, but is not limited to, sodium carboxymethyl cellulose (CMC-Na). This application does not impose any particular limitation on the mass ratio of the negative electrode active material, negative electrode conductive agent, and negative electrode binder in the negative electrode material layer; those skilled in the art can select them according to actual needs, as long as the purpose of this application is achieved.

[0045] In this application, the secondary battery also includes a separator. This application does not impose any particular limitations on the separator; a porous structure separator with good stability can be selected. For example, the separator material can be at least one of polyethylene separator, polypropylene separator, and PE ceramic-coated separator. In this application, the thickness of the separator is not particularly limited, as long as it achieves the purpose of this application; for example, the separator thickness can be from 4 μm to 20 μm.

[0046] In this application, the secondary battery also includes a casing for housing the positive electrode, separator, negative electrode, and electrolyte, as well as other components known in the field of secondary batteries. This application does not limit the scope of these other components. This application does not impose any particular limitation on the casing; it can be a casing known in the art, as long as it achieves the purpose of this application. For example, the casing can be a rigid casing or a flexible casing. The material of the rigid casing can be metal; this application does not limit the type of metal and can use known metal rigid casings, as long as they achieve the purpose of this application. The flexible casing can be a metal plastic film, such as aluminum-plastic film, steel-plastic film, etc.

[0047] In some embodiments of this application, the secondary battery may include, but is not limited to: lithium metal secondary battery, lithium-ion secondary battery (lithium-ion battery), lithium polymer secondary battery or lithium-ion polymer secondary battery, etc.

[0048] The fabrication process of the secondary battery described in this application is well known to those skilled in the art, and this application does not impose any particular limitations. For example, the fabrication process of the secondary battery may include, but is not limited to, the following steps: stacking the positive electrode, separator, and negative electrode in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain the secondary battery. Alternatively, stacking the positive electrode, separator, and negative electrode in sequence, and then fixing the four corners of the entire stacked structure with tape to obtain a stacked electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain the secondary battery. In addition, overcurrent protection elements, conductive plates, etc., may be placed in the housing as needed to prevent pressure rise and overcharging / discharging inside the secondary battery.

[0049] Example

[0050] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0051] Test methods and equipment:

[0052] 25℃ Cyclic Performance Test

[0053] The lithium-ion battery was placed in a 25°C environment and left to stand for 2 hours. It was then charged at a constant current of 1C to a voltage of 4.25V, and then charged at a constant voltage of 4.25V until a cutoff current of 0.05C was reached. Finally, it was discharged at a constant current of 1C to a voltage of 2.75V. This constitutes one charge-discharge cycle, and the initial discharge capacity is recorded as C1. This charge-discharge cycle was repeated for 800 cycles, and the discharge capacity C1 in the 800th cycle was recorded. 800 Room temperature cycling capacity retention rate = (C 800 / C1)×100%.

[0054] High-temperature cycling performance test

[0055] The lithium-ion battery was placed in a 45℃ environment and left to stand for 2 hours. It was then charged at a constant current of 1C until the voltage reached 4.25V, and then charged at a constant voltage at 4.25V until the cutoff current reached 0.05C. Finally, it was discharged at a constant current of 1C until the voltage reached 2.75V. This constitutes one charge-discharge cycle, and the initial discharge capacity was recorded as C1. This charge-discharge cycle was repeated for 400 cycles, and the discharge capacity C on the 400th cycle was recorded. 400 High-temperature cycling capacity retention rate = (C 400 / C1)×100%.

[0056] High-temperature storage expansion rate test

[0057] The lithium-ion battery was placed in a constant temperature environment of 25℃ and charged with a constant current of 1C to a voltage of 4.25V. The initial thickness of the lithium-ion battery was measured using a micrometer with an accuracy of ±1μm and recorded as d0. Then, it was stored at 65℃ for 60 days, and the thickness of the lithium-ion battery after storage was measured at 65℃ and recorded as d1. The expansion rate of the lithium-ion battery was calculated according to the formula: High-temperature storage expansion rate (%) = (d1 - d0) / d0 × 100%.

[0058] High-Temperature Storage DCR Growth Rate Test

[0059] The battery was placed in a 25°C environment and discharged at a constant current of 1C to a cutoff voltage of 2.75V. After resting for 5 minutes, it was charged at a constant current and constant voltage of 1C to the upper limit voltage of 4.25V, with a cutoff current of 0.05C. It was then discharged at a constant current of 1C for 30 minutes, and the battery was adjusted to 50% state of charge (SOC). The battery voltage value V0 was recorded. At 25°C, the battery was allowed to rest for 5 minutes, and then discharged at a constant current of 2C for 10 seconds. The discharge current during 2C discharge was I. 2C Record the voltage V1 after 10 seconds of discharge. The formula for calculating the DC internal resistance of discharge at 50% SOC is as follows: Initial DCR (mΩ) = (V0 - V1) / I 2C ×100%.

[0060] Next, the battery was stored in a high-temperature chamber at 60±2℃ for 60 days, maintaining a fully charged state and periodically checking temperature stability. On the 60th day of storage, the battery was removed from the high-temperature chamber, allowed to return to room temperature, and then discharged at a 2C discharge current for 10 seconds. The voltage change was recorded, and the DCR value after storage was calculated. The formula for calculating the DCR growth rate after 60 days of storage at 60℃ is as follows: High-temperature storage DCR growth rate = (DCR value after storage - Initial DCR value) / Initial DCR value × 100%.

[0061] Room temperature rate cycling performance test

[0062] The lithium-ion battery was placed in a constant temperature environment of 25°C and left to stand for 2 hours to allow it to reach a constant temperature. The battery was then charged at a constant current of 2C to a voltage of 4.25V, followed by constant voltage charging at 4.25V until the current was less than or equal to 0.05C. After standing for 5 minutes, it was discharged at a constant current of 1C to a voltage of 2.75V. This constitutes one charge-discharge cycle. This charge-discharge cycle was repeated 800 times. The discharge capacity of the first cycle was recorded as C1, and the discharge capacity of the 800th cycle was recorded as C800. The capacity retention rate at room temperature was calculated as (C800 / C1) × 100%.

[0063] Example 1-1

[0064] <Preparation of the positive electrode>

[0065] The positive electrode active material NCM811 (LiNi) 0.8 Mn 0.1 Co 0.1 O2), binder polyvinylidene fluoride (PVDF), conductive agent carbon black, and conductive agent carbon nanotubes are mixed evenly in a weight ratio of 95:3:1.3:0.7. N-methylpyrrolidone (NMP) is added, and the mixture is stirred under vacuum until a homogeneous and fluid positive electrode slurry is formed. The positive electrode slurry is then uniformly coated onto the positive electrode current collector aluminum foil with a coating amount of 35 g / m². 2 After drying at 85℃, the material is cold-pressed, trimmed, cut into pieces, and slit. After slitting, it is dried at 85℃ for 4 hours under vacuum conditions, and then the tabs are welded to obtain the positive electrode sheet.

[0066] <Preparation of Negative Electrode Sheets>

[0067] The negative electrode active material graphite, conductive agent carbon black, thickener sodium carboxymethyl cellulose (CMC-Na), and binder styrene-butadiene rubber were mixed in a weight ratio of 95:1.5:1:2.5, and deionized water was added. The mixture was stirred under vacuum to obtain a negative electrode slurry. The negative electrode slurry was then uniformly coated onto the negative electrode current collector copper foil with a coating amount of 20 g / m². 2 After drying at 85℃, the material is cold-pressed, trimmed, cut into pieces, and slit. After slitting, it is dried at 85℃ for 4 hours under vacuum conditions, and then the tabs are welded to obtain the negative electrode sheet.

[0068] <Preparation of Electrolyte>

[0069] In an argon-filled glove box (moisture <10ppm, oxygen <1ppm), ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate were mixed uniformly at a mass ratio of 3:5:2 to obtain a base solvent. Thoroughly dried lithium hexafluorophosphate was rapidly added to the base solvent to achieve a lithium hexafluorophosphate molar concentration of 1 mol / L in the electrolyte. Then, the first additive, trimethyl difluorophosphate (CAS No.: 4414-25-9), and the second additive, lithium trifluoromethanesulfonate, as shown in Formula I-2, were added and thoroughly mixed to obtain the electrolyte. The mass percentage of the first additive and the second additive were both 1.5% based on the mass of the electrolyte.

[0070] <Preparation of the diaphragm>

[0071] A polyethylene separator with a thickness of 8μm was selected.

[0072] <Preparation of Lithium-ion Batteries>

[0073] The positive electrode, negative electrode, and separator prepared according to the above process are stacked to form a lithium-ion battery with a thickness of 4.7 mm, a width of 55 mm, and a length of 60 mm. The battery is then vacuum-baked at 75°C for 10 hours and injected with the previously prepared electrolyte. After standing for 24 hours, the battery is placed in an environment of 45°C, subjected to a pressure of 3 kg, and charged to 4.0 V at 0.1C (160 mA). It is then left to stand for 2 days (to fully activate the battery) to obtain the final battery.

[0074] Examples 1-2 to Examples 1-36

[0075] Except for the adjustment of the type and mass percentage of the first additive and the type and mass percentage of the second additive according to Table 1 in the <Preparation of Electrolyte>, and the change of the mass percentage of the base solvent, the rest is the same as in Example 1-1.

[0076] Comparative Example 1

[0077] Except for the fact that in the <Preparation of Electrolyte>, no second additive is added and the mass percentage of the first additive is adjusted according to Table 1, and the mass percentage of the base solvent is changed accordingly, the rest is the same as in Example 1-1.

[0078] Comparative Example 2

[0079] Except for the fact that in the <Preparation of Electrolyte>, the first additive is not added and the mass percentage of the second additive is adjusted according to Table 1, and the mass percentage of the base solvent is changed accordingly, the rest is the same as in Example 1-1.

[0080] Comparative Examples 3 to 4

[0081] Except for the adjustment of the mass percentage of the first and second additives according to Table 1 in the <Preparation of Electrolyte>, and the change of the mass percentage of the base solvent accordingly, the rest is the same as in Example 1-1.

[0082] Examples 2-1 to 2-3

[0083] Except for the adjustment of the amounts of the first, second, and third additives according to Table 2 in the <Preparation of Electrolyte>, and the change in the mass percentage of the base solvent, the rest is the same as in Examples 1-1.

[0084] Examples 3-1 to 3-7

[0085] Except for adjusting the type of positive electrode active material and the amount of the first and second additives according to Table 3, the rest is the same as in Example 1-1.

[0086] The preparation and performance parameters of each embodiment and comparative example are shown in Tables 1 to 3.

[0087] Table 1

[0088] Table 2

[0089] Table 3

[0090] As can be seen from Examples 1-1 to 1-36 and Comparative Examples 1 to 4, the electrolyte includes the first additive and the second additive within the scope of this application, and the mass ratio of the first additive to the second additive is controlled within the scope of this application. The additives can play a good synergistic role, which can improve the room temperature and high temperature cycle capacity retention rate and the room temperature rate cycle capacity retention rate of the secondary battery, reduce the DCR growth rate of the battery during high temperature storage, reduce the high temperature storage expansion rate of the battery, and thus improve the battery safety performance.

[0091] As can be seen from Examples 1-1, 2-1, and 2-3, based on the electrolyte containing the first additive and the second additive with controlled dosages within the scope of this application, further adding a third additive with controlled dosages within the scope of this application can improve the room temperature and high temperature cycle capacity retention rate and the room temperature rate cycle capacity retention rate of the secondary battery, reduce the DCR growth rate during high temperature storage, reduce the high temperature storage expansion rate of the battery, and thus improve battery safety performance. The third additive is a stabilizing additive, which can improve the storage stability of the electrolyte, helping the electrolyte maintain stable properties during storage and transportation. At the same time, a smaller amount added helps reduce the manufacturing cost of the electrolyte and broadens the application scenarios of the electrolyte.

[0092] As can be seen from Examples 1-1, 3-1, and 3-7, in the high-nickel ternary battery system, the first additive and the second additive have a certain synergistic effect. The nickel content in the positive electrode active material and the additive content in the electrolyte are limited to the following: 0.5≤W1×(1-b) + W2×b≤2.0. This can improve the room temperature and high temperature cycle capacity retention rate and the room temperature rate cycle capacity retention rate of the secondary battery, reduce the DCR growth rate of the battery during high temperature storage, reduce the high temperature storage expansion rate of the battery, and thus improve the battery safety performance. This shows that the synergistic effect of the first additive and the second additive in the high-nickel ternary system is stable and effective.

[0093] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. An electrolyte, comprising a lithium salt, an organic solvent, a first additive, and a second additive; the first additive comprises a compound shown in formula I: ； wherein R1-R3are each independently selected from H, F, C1-C4alkyl, C1-C4fluoroalkyl, C2-C4alkenyl, C2-C4alkynyl, C2-C4fluoroalkynyl, C5-C7cycloalkyl, unsubstituted or substituted with Ra, phenyl, unsubstituted or substituted with Ra, benzyl; the Ra is selected from F, C1-C4alkyl, C2-C4alkenyl, C2-C4alkynyl, C1-C4fluoroalkyl, C2-C4fluoroalkenyl, C2-C4fluoroalkynyl; the second additive comprises any one of the compounds shown in formulae II-IV: ； the mass percentage content of the first additive is W1%, and the mass percentage content of the second additive is W2%, based on the total mass of the electrolyte, 0.01≤W1≤3, 0.01≤W2≤3.

2. The electrolyte of claim 1, wherein, 0.1≤W1 / W2≤10.

3. The electrolyte of claim 1, wherein, the electrolyte further comprises a third additive, the third additive is triphenyl phosphite; the mass percentage content of the third additive is W3, based on the total mass of the electrolyte, 1‰≤W3≤5‰.

4. The electrolyte of claim 1, wherein, the first additive is selected from the compounds shown in formulae I-1 to I-22: 。 5. The electrolyte of claim 1, wherein, the solvent comprises at least one of vinyl carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, ethyl propionate, propyl propionate, butyl propionate, and methyl trifluoroethyl carbonate.

6. The electrolyte of claim 1, wherein, the lithium salt comprises at least one of lithium hexafluorophosphate, lithium bisfluorosulfonimide, lithium tetrafluoroborate, and lithium bis-trifluoromethanesulfonimide.

7. The electrolyte of claim 1, wherein, the molar concentration of the lithium salt in the electrolyte is 0.4mol / L-2mol / L. 8.A secondary battery, comprising the electrolyte according to any one of claims 1 to 7.

9. The secondary battery according to claim 8, wherein The secondary battery includes a positive electrode sheet, a negative electrode sheet, and a separator, the positive electrode sheet includes a positive electrode material layer, the positive electrode material layer includes a positive electrode active material, the positive electrode active material is selected from at least one of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based material, lithium cobaltate, lithium manganate, and Li a Ni b Co c M1 d M2 e O f R g at least one of R, F, S, and Cl, wherein 1 M1 is selected from at least one of Mn and Al, M2 is selected from at least one of Zr, Zn, Cu, Cr, Mg, Fe, V, Ti, Sr, Sb, Y, W, and Nb, and R is selected from at least one of N, F, S, and Cl.

10. The secondary battery according to claim 9, wherein The positive active material is selected from Li a Ni b Co c M1 d M2 e O f R g The negative electrode tab includes a negative active material layer, the negative active material layer including a negative active material, the negative active material being graphite.

11. The secondary battery according to claim 10, wherein the secondary battery satisfies W1×(1-b)+W2×b=K, 0.5≤K≤2.0, preferably 0.8≤K≤1.5.

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