Electrolyte and secondary battery

Abstract

The invention provides an electrolyte and a secondary battery, the electrolyte comprises a lithium salt, a non-aqueous solvent, a first additive and a second additive, the first additive is selected from at least one of compounds represented by a formula I, and the second additive is selected from lithium oxalate. Through the arrangement, when the first additive and the second additive are matched to be used as the electrolyte additive, the formation of a stable solid electrolyte interface film on a negative electrode interface is facilitated, and meanwhile, the formation of a stable positive electrode electrolyte interface film on a positive electrode interface is also facilitated, so that the stability of the positive electrode and negative electrode interfaces is improved; the synergistic effect of the first additive and the second additive can reduce active lithium ions consumed in the film forming process of the first additive, reduce the amount of gas generated by decomposition of the second additive in a high-temperature environment, improve the high-temperature resistance of the secondary battery, and improve the first effect, capacity exertion and safety of the battery.

Description

Technical Field

[0001] This application relates to the field of electrochemical technologies, and particularly to an electrolyte solution and a secondary battery. Background Art

[0002] Secondary batteries (such as lithium-ion batteries) are widely used in the fields of power, energy storage, aerospace, digital, etc. due to their advantages of high voltage, large capacity, no memory effect, and long service life. The wide application of secondary batteries requires them to be able to adapt to different usage environments and maintain good performance under different harsh environments. With the improvement of people's living standards, higher requirements have been put forward for the capacity and cycle life of batteries.

[0003] However, factors such as the dissolution of positive metal ions, the occurrence of side reactions in the electrolyte solution, and the deterioration at the electrode-electrolyte interface under high-temperature conditions lead to serious capacity attenuation of secondary batteries, and it is difficult to improve the comprehensive performance of secondary batteries. Therefore, how to improve the interface stability of secondary batteries and improve the high-temperature cycle performance has become an urgent problem to be solved. Summary of the Invention

[0004] The purpose of this application is to provide a non-aqueous electrolyte solution and a secondary battery to improve the interface stability of lithium batteries and at the same time improve the high-temperature cycle performance and capacity utilization of secondary batteries. The specific technical solutions are as follows:

[0005] The first aspect of this application provides an electrolyte solution, which includes a lithium salt, a non-aqueous solvent, a first additive, and a second additive. The first additive is selected from at least one of the compounds shown in Formula I;

[0006]

[0007] wherein, n, m = 0 or 1, and n and m are not both 0; A 1 and A 2 each independently selected from an oxygen atom or -(CH 2 ) k - or -O-(CH 2 ) k -, and at least one of A 1 , A 2 is -(CH 2 ) k - or -O-(CH 2 ) k -, k is an integer from 1 to 3; the second additive is selected from lithium oxalate salts.

[0008] In some embodiments of this application, the lithium oxalate salt is selected from LiB(C 2 O 4 ) 2 , LiP(C 2O 4 ) 3 , LiBF 2 C 2 O 4 、LiPF 2 (C 2 O 4 ) 2 and LiPF 4 C 2 O 4 At least one of .

[0009] In some embodiments of the present application, the mass ratio of the first additive to the second additive is 0.06-15.

[0010] In some embodiments of the present application, based on the mass of the electrolyte, the mass percentage of the first additive is 0.1%-3%.

[0011] In some embodiments of the present application, based on the mass of the electrolyte, the mass percentage of the second additive is 0.1%-3.0%.

[0012] In some embodiments of the present application, the first additive is selected from at least one of the following compounds:

[0013]

[0014] In some embodiments of the present application, the non-aqueous solvent is selected from at least one of carbonate, carboxylate and ether.

[0015] In some embodiments of the present application, the lithium salt is selected from LiPF 6 、LiAsF 6 、LiClO 4 、LiSO 3 CF 3 , LiBF 4 、LiN(SO 2 F) 2 、LiN(SO 2 CF 3 ) 2 and LiPO 2 F 2 At least one of; based on the mass of the electrolyte, the mass percentage of the lithium salt is 8% to 20%.

[0016] The second aspect of the present application provides a secondary battery, which includes a positive electrode plate, a negative electrode plate, a separator and the electrolyte described in the first aspect of the present application.

[0017] In some embodiments of the present application, 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, LiMn 1-x Fe x PO 4 and lithium nickel manganese oxide, where 0 < x < 1.

[0018] In some embodiments of the present application, the negative electrode sheet includes a negative electrode material layer, the negative electrode material layer includes a negative electrode active material, and the negative electrode active material is selected from at least one of graphite, silicon-based material, and lithium titanate.

[0019] Advantages of the present application:

[0020] The present application provides a non-aqueous electrolyte and a secondary battery. The electrolyte includes a lithium salt, a non-aqueous solvent, a first additive, and a second additive. The first additive is selected from at least one of the compounds shown in Formula I, and the second additive is selected from lithium oxalate salts. When the first additive and the second additive of the present application are used as electrolyte additives in combination, it is beneficial to form a stable solid electrolyte interface film at the negative electrode interface, and at the same time, it is also beneficial to form a stable positive electrode electrolyte interface film at the positive electrode interface, improving the stability of the positive and negative electrode interfaces. The two work together to reduce the active lithium ions consumed during the film formation process of the first additive, reduce the amount of gas generated by the decomposition of the second additive at high temperatures, improve the high-temperature resistance performance of the secondary battery, and improve the initial efficiency, capacity utilization, and safety of the battery.

[0021] Of course, it is not necessary for any product or method implementing the present application to achieve all of the above advantages simultaneously. Detailed embodiments

[0022] The technical solutions in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. All other embodiments obtained by those skilled in the art based on the present application belong to the scope of protection of the present application.

[0023] It should be noted that in the specific embodiments of the present application, a lithium-ion battery is used as an example of the secondary battery to explain the present application, but the secondary battery of the present application is not limited to lithium-ion batteries.

[0024] The first aspect of the present application provides an electrolyte, which includes a lithium salt, a non-aqueous solvent, a first additive, and a second additive; the first additive is selected from at least one of the compounds shown in Formula I;

[0025]

[0026] wherein, n, m = 0 or 1, and n and m are not both 0 at the same time; A 1 and A 2 each independently selected from an oxygen atom or -(CH 2 ) k - or -O-(CH 2 ) k -, for example, in formula I, A 1 and A 2 each independently selected from a single bond, methylene, ethylene, propylene, methoxy, ethoxy, propoxy or an oxygen atom, and at least one of A 1 , A 2 is -(CH 2 ) k - or -O-(CH 2 ) k -, k is an integer from 1 to 3; the second additive is selected from lithium oxalate salts.

[0027] The inventors' research found that lithium salt additives containing oxalate groups can form an interfacial film with good lithium ion conduction on the surfaces of the positive and negative electrodes, reduce the decomposition of the electrolyte by the electrodes, and maintain a good cycle life of the battery. However, at the same time, in a high-temperature environment, the cyclic oxalate groups in these lithium salt additives containing oxalate groups are prone to lose one electron and open the ring, and further lose two electrons, then two molecules of carbon dioxide gas are released, and the generation of the gas has a negative impact on the safety of the battery. The large π bond on the benzene ring in the compound shown in formula I will affect the activity of the entire compound shown in formula I, reduce its energy barrier, make the ring-opening reaction of sultone easy, and it can polymerize with the second additive that has lost one electron, and finally form a multi-component solid electrolyte interface film (SEI film) composed of organic polymer components such as oxalate groups, benzene rings, and sulfonate groups, as well as inorganic sulfates, inorganic sulfites, lithium alkyl sulfonates, lithium alkyl sulfates, etc. at the surface of the electrode, enhancing the stability at the interface between the electrode and the electrolyte. The compound shown in formula I participates in film formation with oxalate groups, thereby inhibiting the process of oxalate groups losing the second electron, and further inhibiting the generation of CO 2 , reducing the gas generation amount of the battery in a high-temperature environment, and improving the high-temperature resistance performance of the lithium ion battery. However, under the conjugation of the large π bond of the benzene ring, the intermediate-state radicals after the ring-opening of the compound shown in formula I are also prone to combine with the decomposition products of carbonate solvents. Using the compound shown in formula I alone will form an SEI film with low conductivity while consuming a large amount of active lithium ions. The lithium salt additives containing oxalate groups can reduce the active lithium ions consumed during the film formation process of the additive of formula I, and improve the initial efficiency and capacity performance of the whole battery. The first additive and the second additive act synergistically to form a good interfacial film with low gas generation amount while reducing the risk of low lithium ion conductivity of the SEI film.

[0028] In some embodiments of the present application, the lithium oxalate salt is selected from LiB(C2 O 4 ) 2 、LiP(C 2 O 4 ) 3 , LiBF 2 C 2 O 4 、LiPF 2 (C 2 O 4 ) 2 and LiPF 4 C 2 O 4 At least one of .

[0029] In some embodiments of the present application, the mass ratio of the first additive to the second additive is 0.06 to 15. The first additive and the second additive work synergistically to form a good interface film with low gas production while reducing the risk of low lithium ion conductivity of the SEI film.

[0030] In some embodiments of the present application, based on the mass of the electrolyte, the mass percentage of the first additive is 0.1%-3%; the mass percentage of the first additive is within the scope of the present application, and can play a good synergistic role with the second additive, and the obtained lithium-ion battery has a higher high-temperature cycle capacity retention rate and a lower high-temperature cycle gas production change rate, indicating that the high-temperature cycle performance of the lithium-ion battery is further improved, that is, the gas production inhibition ability and safety of the lithium-ion battery are further improved.

[0031] In some embodiments of the present application, based on the mass of the electrolyte, the mass percentage of the second additive is 0.1%-3.0%. The mass percentage of the second additive within the scope of the present application can play a good synergistic role with the first additive, and can make the lithium-ion battery have a higher first effect, better capacity performance, and a higher high-temperature cycle capacity retention rate, indicating that the first effect, high-temperature cycle performance and safety performance of the lithium-ion battery are further improved, that is, the capacity performance and high-temperature cycle life of the lithium-ion battery are further improved.

[0032] In some embodiments of the present application, the compound represented by formula I is selected from at least one of the following compounds:

[0033]

[0034] The compound of formula I-1 preferentially breaks the SC bond in the battery. After one-step reduction, the ring-opening product is a very stable free radical with a non-localized unpaired electron, which can combine with Li in the electrolyte to form RSO 2 Li and ROSO 2Li compound (R is various fragmented organic components generated by the ring-opening reaction of the compound shown in Formula I, and the organic components can be substituted or unsubstituted alkyl, alkoxy, phenyl, etc.); in Formula I-2, the C-O bond is preferentially broken and ring-opened in the battery, resulting in the formation of very unstable aryl radicals, and these radicals can form anions through proton transfer to attack ethylene carbonate (EC) molecules in the solvent, thereby participating in the composition of the SEI film; in Formula I-3, the C-O bond is preferentially broken and ring-opened in the battery, which can react with EC to form the SEI film. At the same time, some molecules can break the S-C bond, and further two-step reduction reaction recombination can occur to form a dianion self-polymerization product, thereby forming a multi-component RSO 3 Li and ROSO 2 Li compound forms a more stable multi-component organic component SEI film, which has stronger ionic conductivity and chemical stability to cope with extreme conditions on the premise of having a certain mechanical strength.

[0035] In some embodiments of the present application, the non-aqueous solvent is selected from at least one of carbonates, carboxylates and ethers. Based on the mass of the electrolyte, the mass percentage content of the solvent is 70% to 91.8%. The electrolyte includes the solvent within the above range, and by adjusting the mass percentage content of the solvent within the range of the present application, the electrolyte can have appropriate viscosity, high ionic conductivity and good electrochemical stability, and can further improve the low-temperature cycle performance, high-temperature cycle performance and room-temperature cycle performance of the secondary battery.

[0036] In the present application, the carbonate can be selected from at least one of dimethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, methyl butyl carbonate, diethyl carbonate, dipropyl carbonate and dibutyl carbonate. The carboxylate can be selected from at least one of methyl formate, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate and butyl butyrate. The ether can be selected from at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran and tetrahydrofuran.

[0037] In some embodiments of the present application, the lithium salt is selected from LiPF 6 、LiAsF 6 、LiClO 4 、LiSO 3 CF 3 、LiBF 4 、LiN(SO 2 F) 2 、LiN(SO 2 CF3 ) 2 and LiPO 2 F 2 at least one of; based on the mass of the electrolyte, the mass percentage content of the lithium salt is 8% to 20%. For example, the mass percentage content of the lithium salt can be 8%, 10%, 12%, 14%, 16%, 18%, 20% or a range composed of any two of these values. The electrolyte includes the lithium salt within the above range, and adjusting the mass percentage content of the lithium salt within the scope of this application can make the electrolyte have a high ionic conductivity and good electrochemical stability, and can further improve the high-temperature cycling performance of the secondary battery.

[0038] The second aspect of this application provides a secondary battery, which includes a positive electrode sheet, a negative electrode sheet, a separator, and the electrolyte provided in the first aspect of this application.

[0039] In this application, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer located on at least one surface of the positive electrode sheet. The above "positive electrode material layer provided on at least one surface of the positive electrode current collector" means that the positive electrode material layer can be provided on one surface of the positive electrode current collector along its own thickness direction, or can be provided on two surfaces of the positive electrode current collector along its own thickness direction. It should be noted that the "surface" here can be the entire area of the positive electrode current collector surface or a partial area of the positive electrode current collector surface. This application has no special restrictions as long as the purpose of this application can be achieved. This application has no special restrictions on the positive electrode current collector as long as the purpose of this application can be achieved. For example, the positive electrode current collector can be aluminum foil, aluminum alloy foil, or a composite positive electrode current collector. The above 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 substrate. The material of the above polymer material base layer can include but is not limited to at least one of polypropylene (PP), polyethylene terephthalate (PET), or polybutylene terephthalate (PBT). The material of the above metal layer can include but is not limited to at least one of aluminum, aluminum alloy, nickel, or nickel alloy. This application has no special restrictions 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 is 50 μm to 250 μm, and the thickness of the positive electrode current collector is 7 μm to 20 μm.

[0040] In some embodiments of this application, 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, LiMn 1-x Fe x PO 4 and lithium nickel manganese oxide, where 0 < x < 1.

[0041] The positive electrode material layer may further include a positive electrode conductive agent and a positive electrode binder. The present application does not particularly limit the types of the positive electrode conductive agent and the positive electrode binder, as long as the object of the present application can be achieved. 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 vinylidene fluoride-tetrafluoroethylene-propylene terpolymer. The present application does not particularly limit the mass ratio of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder in the positive electrode material layer. Those skilled in the art can select according to actual needs, as long as the object of the present application can be achieved.

[0042] In the present application, the negative electrode sheet includes a negative electrode current collector and a negative electrode material layer provided on at least one surface of the negative electrode current collector. The above "the negative electrode material layer is provided on at least one surface of the negative electrode current collector" means that the negative electrode material layer may be provided on one surface of the negative electrode current collector along its thickness direction, or may be provided on two surfaces of the negative electrode current collector along its thickness direction. It should be noted that the "surface" here may be the entire area of the surface of the negative electrode current collector, or may be a partial area of the surface of the negative electrode current collector. The present application does not particularly limit it, as long as the object of the present application can be achieved. The present application does not particularly limit the negative electrode current collector, as long as the object of the present application can be achieved. For example, the negative electrode current collector may be a copper foil, a copper alloy foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam or a copper foam, an aluminum foil, or a composite negative electrode current collector. The above composite negative electrode current collector may be a polymer material base layer and a metal layer formed on at least one surface of the polymer material base. The material of the above polymer material base layer may include, but is not limited to, at least one of polypropylene (PP), polyethylene terephthalate (PET), or polybutylene terephthalate (PBT). The material of the above metal layer may include, but is not limited to, at least one of copper, copper alloy, nickel, or nickel alloy. The present application does not particularly limit the thicknesses of the negative electrode material layer and the negative electrode current collector, as long as the object of the present application can be achieved. For example, the thickness of the single-sided negative electrode material layer is 50 μm to 180 μm, and the thickness of the negative electrode current collector is 3 μm to 10 μm.

[0043] In some embodiments of the present application, the negative electrode material layer includes a negative electrode active material, and the negative electrode active material is selected from at least one of graphite, silicon-based materials, and lithium titanate. The negative electrode material layer may further include a negative electrode conductive agent, a negative electrode binder, and a negative electrode thickener. The present application does not particularly limit the types of the negative electrode conductive agent, the negative electrode binder, and the negative electrode thickener, as long as the objectives of the present application can be achieved. 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). The present application does not particularly limit the mass ratio of the negative electrode active material, the negative electrode conductive agent, and the negative electrode binder in the negative electrode material layer, and those skilled in the art can select according to actual needs as long as the objectives of the present application can be achieved.

[0044] In the present application, the secondary battery further includes a separator. The present application does not particularly limit the separator, and a porous structure separator with good stability can be selected. For example, the material of the separator may include, but is not limited to, at least one of a polyethylene separator, a polypropylene separator, and a PE ceramic-coated separator. In the present application, the thickness of the separator is not particularly limited as long as the objectives of the present application can be achieved. For example, the thickness of the separator may be 4 μm to 20 μm.

[0045] In the present application, the secondary battery further includes a housing for accommodating the positive electrode plate, the separator, the negative electrode plate, and the electrolyte, as well as other components known in the field of secondary batteries. The present application does not limit the above-mentioned other components. The present application does not particularly limit the housing, and it can be a housing well-known in the art as long as the objectives of the present application can be achieved. For example, the housing can be a hard shell housing or a flexible housing. The material of the hard shell housing can be metal, and the present application does not limit the type of metal, and a metal hard shell housing known in the art can be used as long as the objectives of the present application can be achieved. The flexible housing can be a metal plastic film, such as an aluminum plastic film, a steel plastic film, etc.

[0046] In some embodiments of the present application, the secondary battery of the present application may include, but is not limited to: a lithium metal secondary battery, a lithium-ion secondary battery (lithium-ion battery), a lithium polymer secondary battery, or a lithium-ion polymer secondary battery, etc.

[0047] The preparation process of the secondary battery of the present application is well-known to those skilled in the art, and there is no special limitation in the present application. For example, the preparation process of the secondary battery may include, but is not limited to, the following steps: stacking the positive electrode sheet, the separator, and the negative electrode sheet in sequence, and winding, folding, etc. as required to obtain a wound electrode assembly, placing the electrode assembly into a housing, injecting an electrolyte into the housing and sealing it to obtain a secondary battery. Or, stacking the positive electrode sheet, the separator, and the negative electrode sheet in sequence, and then fixing the four corners of the entire laminated structure with tape to obtain a laminated electrode assembly, placing the electrode assembly into a housing, injecting an electrolyte into the housing and sealing it to obtain a secondary battery. In addition, an overcurrent protection element, a guide plate, etc. can be placed in the housing as required to prevent the pressure inside the secondary battery from rising and overcharging / discharging.

[0048] Example

[0049] Hereinafter, examples and comparative examples are given to more specifically illustrate the embodiments of the present application. Various tests and evaluations are carried out according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0050] Example 1-1

[0051] <Preparation of electrolyte>

[0052] In a glove box filled with argon (moisture < 10 ppm, oxygen content < 1 ppm), ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) were mixed uniformly in a mass ratio of 20:4:20:40 to obtain a base solvent. Lithium hexafluorophosphate (LiPF 6 ), the first additive, the compound shown in Formula I-3, and the second additive LiBF 2 C 2 O 4 were added and mixed uniformly to obtain an electrolyte. Among them, based on the mass of the electrolyte, the mass percentage content of LiPF 6 was 13%, the mass percentage content of the compound shown in Formula I-3 was 0.05%, the mass percentage content of LiBF 2 C 2 O 4 was 0.5%, and the balance of the base solvent was 86.45%.

[0053] <Preparation of positive electrode sheet>

[0054] The positive electrode active material lithium iron phosphate (LiFePO 4) The conductive agent Super P, binder PVDF, and carbon nanotubes CNT are mixed evenly with the solvent NMP (N-methylpyrrolidone) in a mass ratio of 96.5∶1.5∶1∶1 to make a positive electrode paste with a certain viscosity. The paste is coated on both surfaces of the positive electrode current collector aluminum foil, dried at 85°C, and then cold-pressed. The coating amount is 250 g / m 2 (excluding the solvent), and the compaction density is 2.2 g / cm 3 ; Then, trimming, slitting, and striping are carried out. After striping, it is dried at 85°C for 4 h under vacuum conditions, and the electrode tabs are welded to make the positive electrode plate.

[0055] <Preparation of negative electrode plate>

[0056] Artificial graphite is mixed evenly with the conductive agent Super P, thickener CMC, and binder SBR (styrene-butadiene rubber latex) in a mass ratio of 95∶1.5∶1.0∶2.5 with the solvent water to make a negative electrode paste with a certain viscosity. After the negative electrode paste is coated on both sides of the negative electrode current collector copper foil, it is dried at 85°C and then cold-pressed. The coating amount is 154 g / m 2 (excluding the solvent), and the compaction density is 1.6 g / cm 3 ; Trimming, slitting, and striping are carried out. After striping, it is dried at 85°C for 4 h under vacuum conditions to obtain the negative electrode plate.

[0057] <Preparation of separator>

[0058] A 16-μm-thick polypropylene porous membrane is used as the separator.

[0059] <Preparation of lithium-ion battery>

[0060] The positive electrode plate, negative electrode plate, and separator prepared according to the above process are made into a lithium-ion battery with a thickness of 4.7 mm, width of 55 mm, and length of 60 mm through the stacking process. The capacity is 1300 mAh, baked at 85°C under vacuum for 48 h, injected with the electrolyte prepared above, and then obtained through processes such as vacuum packaging, standing, formation, shaping, and sorting.

[0061] Examples 1-2 to Examples 1-24

[0062] Except that in <Preparation of electrolyte>, the types and mass percentage contents of the first additive and the types and mass percentage contents of the second additive are adjusted according to Table 1, and the mass percentage content of the base solvent changes accordingly, the rest is the same as Example 1-1. In the table, LiB(C 2 O 4 ) 2 is lithium bis(oxalato)borate, LiBF 2 C 2 O 4 is lithium difluoro(oxalato)borate, LiPF2 (C 2 O 4 ) 2 is lithium difluorodioxalate phosphate, LiPF 4 C 2 O 4 is lithium tetrafluoromonoxalate phosphate, LiP(C 2 O 4 ) 3 is lithium trioxalate phosphate.

[0063] Comparative Example 1

[0064] Except that in <Preparation of electrolyte>, the second additive is not added and the mass percentage content of the first additive solvent is adjusted according to Table 1, and the mass percentage content of the base solvent changes accordingly, the rest is the same as in Example 1-1.

[0065] Comparative Examples 2 to 3

[0066] Except that in <Preparation of electrolyte>, the first additive is not added and the mass percentage content of the second additive solvent and the type of the second additive are adjusted according to Table 1, and the mass percentage content of the base solvent changes accordingly, the rest is the same as in Example 1-1.

[0067] Test method and device:

[0068] 1) First efficiency and capacity performance test

[0069] Taking a lithium-ion battery as an example, at 25 °C, it is charged at a constant current of 0.2C to 3.65V, and then charged at a constant voltage until the cut-off current is 0.05C. The charging capacity is denoted as A 0 , and then the battery is discharged at a constant current of 0.2C to 2.0V. The discharge capacity is denoted as A 1 , first efficiency = (A 1 / A 0 ) × 100%. At 25 °C, it is charged at a constant current of 1C to 3.65V, and then charged at a constant voltage until the cut-off current is 0.05C. Then the battery is discharged at a constant current of 1C to 2.0V. The charge-discharge steps are repeated for 3 weeks to obtain the discharge capacity B 0 of the 3rd week. The battery thickness is denoted as C 0 , capacity performance = B 0 .

[0070] 2) High-temperature cycle performance and gas generation rate test

[0071] Taking a lithium-ion battery as an example, at 45 °C, it is charged at a constant current of 1.0C to 3.65V, and then charged at a constant voltage until the cut-off current is 0.05C. Then the battery is discharged at a constant current of 1.0C to 2.0V. The discharge capacity is denoted as D 0, repeat the charge-discharge steps for 1000 cycles to obtain the discharge capacity D at the 1000th cycle 1000 , the battery thickness is denoted as C 1000 , capacity retention rate = (D 1000 / D 0 ) × 100%, gas generation change rate = (C 1000 / C 0 ) × 100%.

[0072] Table 1

[0073]

[0074]

[0075] Note: " / " in Table 1 indicates that there is no corresponding preparation parameter or substance.

[0076] The mass ratio of the first additive to the second additive affects the interaction between the first additive and the second additive, and further affects the quality of the interface film, the gas generation amount, and the lithium-ion conductivity of the SEI film. It can be seen from Examples 1-1 to 1-24 and Comparative Examples 1 to 3 that the electrolyte including the mass ratio of the first additive and the second additive within the scope of the present application plays a good synergistic role. While forming a good interface film and having a low gas generation amount, it also reduces the risk of low lithium-ion conductivity of the SEI film.

[0077] The type and mass percentage content of the first additive affect the stability at the interface between the negative electrode of the lithium-ion battery and the electrolyte, and affect the gas generation amount of the battery in a high-temperature environment. It can be seen from Examples 1-1 to 1-7, Examples 1-17 to 1-19, and Comparative Examples 2 to 3 that on the basis of the electrolyte including the second additive within the scope of the present application, introducing the first additive and regulating the mass percentage content of the first additive within the scope of the present application plays a good synergistic role. The obtained lithium-ion battery has a higher high-temperature cycle capacity retention rate and a lower high-temperature cycle gas generation change rate, indicating that the high-temperature cycle performance of the lithium-ion battery is further improved, that is, the gas generation inhibition ability and safety of the lithium-ion battery are further enhanced.

[0078] The type and mass percentage content of the second additive affect the initial efficiency, capacity performance, and cycle life of the lithium-ion battery. It can be seen from Examples 1-2, Examples 1-8 to Examples 1-14, Examples 1-20 to Examples 1-24, and Comparative Example 1 that on the basis of the electrolyte including the first additive within the scope of the present application, introducing the second additive and regulating the mass percentage content of the second additive within the scope of the present application plays a very good synergistic effect, enabling the lithium-ion battery to have a higher initial efficiency, better capacity performance, and higher high-temperature cycle capacity retention rate, indicating that the initial efficiency, high-temperature cycle performance, and safety performance of the lithium-ion battery are further improved, that is, the capacity performance and high-temperature cycle life of the lithium-ion battery are further increased.

[0079] It can be seen from Comparative Example 1 and Example 1-2 that only adding the first additive, although the obtained lithium-ion battery has a lower high-temperature cycle gas generation change rate, its initial efficiency, capacity performance, and high-temperature cycle capacity retention rate all decrease.

[0080] It can be seen from Comparative Example 2 and Example 1-2 that only adding the second additive, the high-temperature cycle capacity retention rate of the obtained lithium-ion battery decreases significantly, its high-temperature cycle gas generation rate increases significantly, and the high-temperature cycle life and safety performance of the lithium-ion battery become worse.

[0081] Example 2-1

[0082] <Preparation of electrolyte>

[0083] In a glove box filled with argon (moisture < 10 ppm, oxygen content < 1 ppm), the solvents ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) were mixed evenly according to a mass ratio of 20∶4∶20∶40 to obtain a base solvent. Lithium hexafluorophosphate (LiPF 6 )、the compound shown in Formula I-3 as the first additive and LiBF as the second additive 2 C 2 O 4 were mixed evenly to obtain the electrolyte. Among them, based on the mass of the electrolyte, the mass percentage content of LiPF 6 is 13%, the mass percentage content of the compound shown in Formula I-3 is 1%, the mass percentage content of LiBF 2 C 2 O 4 is 0.5%, and the balance of the base solvent is 85.5%.

[0084] <Preparation of positive electrode sheet>

[0085] Mix the cathode active material nickel cobalt manganese 523 (NCM523), conductive agent Super P, binder PVDF, and carbon nanotube CNT in a mass ratio of 96.5∶1.5∶1∶1 with the solvent NMP and mix evenly to make a cathode slurry with a certain viscosity. Coat it on both surfaces of the cathode current collector aluminum foil, dry it at 85 °C, and then perform cold pressing. The coating amount is 250 g / m 2 (excluding the solvent), and the compaction density is 2.2 g / cm 3 ; Then, perform edge trimming, slicing, and slitting. After slitting, dry it at 85 °C for 4 h under vacuum conditions, weld the tabs, and make the cathode electrode sheet.

[0086] <Preparation of the negative electrode sheet>

[0087] Mix artificial graphite, conductive agent Super P, thickener CMC, and binder SBR (styrene-butadiene rubber latex) in a mass ratio of 95∶1.5∶1.0∶2.5 with the solvent water and mix evenly to make a negative electrode slurry with a certain viscosity. After coating the negative electrode slurry on both sides of the negative electrode current collector copper foil, dry it at 85 °C and then perform cold pressing. The coating amount is 154 g / m 2 (excluding the solvent), and the compaction density is 1.6 g / cm 3 ; Perform edge trimming, slicing, and slitting. After slitting, dry it at 85 °C for 4 h under vacuum conditions to obtain the negative electrode sheet.

[0088] <Preparation of the separator>

[0089] Use a 16-μm-thick polypropylene porous membrane as the separator.

[0090] <Preparation of the lithium-ion battery>

[0091] Make a lithium-ion battery with a thickness of 4.7 mm, a width of 55 mm, and a length of 60 mm from the cathode electrode sheet, negative electrode sheet, and separator prepared according to the above process. The capacity is 1300 mAh. Bake it at 85 °C under vacuum for 48 h, inject the electrolyte prepared above, and obtain the lithium-ion battery through processes such as vacuum packaging, standing, formation, shaping, and sorting.

[0092] Comparative Example 4

[0093] Except that in <Preparation of the electrolyte>, the second additive is not added and the mass percentage content of the first additive solvent is adjusted according to Table 2, and the mass percentage content of the base solvent changes accordingly, the rest is the same as in Example 2-1.

[0094] Comparative Example 5

[0095] Except that in <Preparation of the electrolyte>, the first additive is not added and the mass percentage content of the second additive solvent is adjusted according to Table 2, and the mass percentage content of the base solvent changes accordingly, the rest is the same as in Example 2-1.

[0096] Test method and device:

[0097] 1) Initial efficiency and capacity performance test

[0098] Taking a lithium-ion battery as an example, at 25 °C, it is charged at a constant current of 0.2C to 4.4V, and then charged at a constant voltage until the cut-off current is 0.05C. The charging capacity is recorded as A 0 , and then the battery is discharged at a constant current of 0.2C to 2.75V. The discharge capacity is recorded as A 1 , initial efficiency = (A 1 / A 0 ) × 100%. At 25 °C, it is charged at a constant current of 1C to 4.4V, and then charged at a constant voltage until the cut-off current is 0.05C. Then the battery is discharged at a constant current of 1C to 2.75V. The charge-discharge steps are repeated for 3 weeks to obtain the discharge capacity B of the 3rd week 0 , the battery thickness is recorded as C 0 , capacity performance = B 0 .

[0099] 2) High-temperature cycle performance and gas generation change rate test

[0100] Taking a lithium-ion battery as an example, at 45 °C, it is charged at a constant current of 1.0C to 4.4V, and then charged at a constant voltage until the cut-off current is 0.05C. Then the battery is discharged at a constant current of 1.0C to 2.75V. The discharge capacity is recorded as D 0 , the charge-discharge steps are repeated for 1000 weeks to obtain the discharge capacity D of the 1000th week 1000 , the battery thickness is recorded as C 1000 , capacity retention rate = (D 1000 / D 0 ) × 100%, gas generation change rate = (C 1000 / C 0 ) × 100%.

[0101] Table 2

[0102]

[0103] Note: " / " in Table 2 indicates that there is no corresponding preparation parameter or substance.

[0104] Compared with the use of the first or second additive alone, under the combined action of the first additive and the second additive, the NCM523 battery maintains a high initial efficiency and capacity performance, and its high-temperature cycle performance is improved, and the gas generation amount is also maintained at a low level, indicating that there is a certain synergistic effect between the first additive and the second additive in the NCM system.

[0105] The foregoing are only the preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present application shall be included within the scope of protection of the present application.

Claims

1. An electrolyte comprising a lithium salt, a non-aqueous solvent, a first additive and a second additive; the first additive is selected from at least one of the compounds shown in Formula I; in, n, m = 0 or 1, and n and m are not 0 at the same time; A1 and A2 are each independently selected from an oxygen atom or -(CH2) k -or-O-(CH2) k -, and at least one of A1 and A2 is -(CH2) k -or-O-(CH2) k -, k is an integer from 1 to 3; The second additive is selected from lithium oxalate salts.

2. The electrolyte according to claim 1, wherein The lithium oxalate salt is selected from at least one of LiB(C2O4)2, LiP(C2O4)3, LiBF2C2O4, LiPF2(C2O4)2 and LiPF4C2O4.

3. The electrolyte according to claim 1, wherein The mass ratio of the first additive to the second additive is 0.06-15.

4. The electrolyte according to claim 1, wherein Based on the mass of the electrolyte, the mass percentage of the first additive is 0.1%-3.0%.

5. The electrolyte according to claim 1, wherein Based on the mass of the electrolyte, the mass percentage of the second additive is 0.1%-3.0%.

6. The electrolyte according to claim 1, wherein The first additive is selected from at least one of the following compounds:

7. The electrolyte according to claim 1, wherein The non-aqueous solvent is selected from at least one of carbonate esters, carboxylates and ethers.

8. The electrolyte according to claim 1, wherein The lithium salt is selected from at least one of LiPF6, LiAsF6, LiClO4, LiSO3CF3, LiBF4, LiN(SO2F)2, LiN(SO2CF3)2 and LiPO2F2; based on the mass of the electrolyte, the mass percentage of the lithium salt is 8% to 20%. 9 . A secondary battery comprising the electrolyte according to claim 1 .

10. The secondary battery according to claim 9, wherein the secondary battery comprises a positive electrode sheet, a negative electrode sheet and a separator, wherein the positive electrode sheet comprises a positive electrode material layer, wherein the positive electrode material layer comprises a positive electrode active material, wherein the positive electrode active material is selected from lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based material, lithium cobalt oxide, lithium manganese oxide, LiMn 1-x Fe x At least one of PO4 and lithium nickel manganese oxide, 0<x<1.

11. The secondary battery according to claim 10, wherein The negative electrode plate includes a negative electrode material layer, the negative electrode material layer includes a negative electrode active material, and the negative electrode active material is selected from at least one of graphite, silicon-based materials and lithium titanate.

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