Electrolyte and secondary battery

Abstract

The invention provides an electrolyte and a secondary battery, the electrolyte comprises a solvent, an electrolyte and additives, the additives comprise a first additive and a second additive, the first additive is selected from a compound shown in a formula I, the second additive is selected from a compound shown in a formula II, based on the mass of the electrolyte, the mass percentage content of the first additive is A, 0.1% < = A < = 5%, and the mass percentage content of the second additive is A; the mass percentage of the second additive is B, and 0.1% < = B < = 3%. The two additives can play a synergistic role, and a polymer network with uniform thickness and crosslinking degree distribution is formed on the surfaces of a positive electrode and a negative electrode, so that the high-temperature cycle performance and the normal-temperature cycle performance of the secondary battery can be 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] In recent years, secondary batteries (such as lithium-ion batteries) have been widely used in various portable electronic devices, new energy electric vehicles, and energy storage systems. However, considering that automobiles need to operate under complex road conditions and environmental conditions, or that some electronic devices need to be used in harsh environments, secondary batteries, as the power source for electric vehicles or electronic devices, need to adapt to these complex conditions. Moreover, in addition to considering these conditions, the lifespan and charge-discharge cycle performance of the secondary battery also need to be considered, especially when electric vehicles or electronic devices are in high-temperature environments, requiring secondary batteries to have excellent high-temperature resistance. Therefore, how to improve the room-temperature cycle performance and high-temperature cycle performance of secondary batteries has become an urgent technical problem to be solved. Summary of the Invention

[0003] The purpose of this application is to provide an electrolyte and a secondary battery to improve the room temperature cycling performance and high temperature cycling performance of the secondary battery. The specific technical solution is as follows:

[0004] A first aspect of this application provides an electrolyte comprising a solvent, an electrolyte, and an additive, wherein the additive comprises a first additive and a second additive, the first additive being selected from compounds shown in Formula I, and the second additive being selected from compounds shown in Formula II.

[0005] Based on the mass of the electrolyte, the mass percentage of the first additive is A, 0.1% ≤ A ≤ 5%, and the mass percentage of the second additive is B, 0.1% ≤ B ≤ 3%.

[0006] In some embodiments of this application, 0.5%≤A≤3% and 0.5%≤B≤2%.

[0007] In some embodiments of this application, 0.1 ≤ A / B ≤ 20.

[0008] In some embodiments of this application, 0.5 ≤ A / B ≤ 2.

[0009] In some embodiments of this application, the electrolyte is selected from at least one of lithium hexafluorophosphate, lithium difluorosulfonylimide, lithium trifluoromethanesulfonate, lithium trifluoromethanesulfinate, lithium difluorooxalateborate, and lithium tetrafluoroborate; based on the mass of the electrolyte, and / or, the mass percentage of the electrolyte is 6% to 25%.

[0010] In some embodiments of this application, the solvent is selected from at least one of ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, fluoroethylene carbonate, propyl butyrate, ethyl butyrate, propyl propionate, ethyl propionate, methyl propionate, propyl acetate, ethyl acetate, methyl acetate, propyl formate, ethyl formate, methyl formate, and γ-butyrolactone; and / or, based on the mass of the electrolyte, the solvent has a mass percentage content of 67% to 93%.

[0011] The second aspect of this application provides a secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte provided in the first aspect of this application.

[0012] In some embodiments of this application, the positive electrode includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector, the positive electrode material layer including a positive electrode active material; the positive electrode active material includes LiFe. 1-x’ M' x’ PO4, LiMn 2-y’ M y’ O4 and LiNi x Co y Mn z M'' 1-x-y-z At least one of O2, wherein M' is selected from at least one of Mn, Mg, Co, Ni, Cu, Zn, Al, Sn, B, Ga, Cr, Sr, V and Ti, and M and M'' are each independently selected from at least one of Fe, Co, Ni, Mg, Cu, Zn, Al, Sn, Ga, Cr, Sr, V and Ti, 0≤x'<1, 0≤y'≤1, 0≤y≤1, 0≤x≤1, 0≤z≤1, x+y+z≤1.

[0013] In some embodiments of this application, the positive electrode active material includes LiCoO2, LiFePO4, and LiFe 0.8 Mn 0.2 PO4, LiMn2O4, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.6 Co 0.1 Mn 0.3 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.5 Co 0.2 Mn 0.2 Al 0.1O2 and LiNi 0.5 Co 0.2 Al 0.3 At least one of O2.

[0014] The beneficial effects of this application are:

[0015] This application provides an electrolyte and a secondary battery. The electrolyte includes a solvent, an electrolyte, and additives. The additives include a first additive and a second additive. The first additive is selected from compounds shown in Formula I, and the second additive is selected from compounds shown in Formula II. Based on the mass of the electrolyte, the mass percentage of the first additive is A, 0.1% ≤ A ≤ 5%, and the mass percentage of the second additive is B, 0.1% ≤ B ≤ 3%. The electrolyte of this application includes the aforementioned first and second additives, and the mass percentages of the two additives are controlled within the above-mentioned ranges. The two additives can exert a synergistic effect, forming a polymer network with uniform thickness and cross-linking degree on the surfaces of the positive and negative electrodes, thereby improving the high-temperature cycle performance and room-temperature cycle performance of the secondary battery.

[0016] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Detailed Implementation

[0017] 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.

[0018] It should be noted that, in the specific embodiments of this application, lithium-ion batteries are used as an example to explain this application, but the batteries in this application are not limited to lithium-ion batteries.

[0019] A first aspect of this application provides an electrolyte comprising a solvent, an electrolyte, and an additive, wherein the additive comprises a first additive and a second additive, the first additive being selected from compounds shown in Formula I, and the second additive being selected from compounds shown in Formula II.

[0020] Based on the mass of the electrolyte, the mass percentage of the first additive is A, where 0.1% ≤ A ≤ 5%, and the mass percentage of the second additive is B, where 0.1% ≤ B ≤ 3%, preferably 0.5% ≤ A ≤ 3% and 0.5% ≤ B ≤ 2%. For example, the value of A can be 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 4%, 4.5%, 5%, or a range of any two values ​​therein; the value of B can be 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, or a range of any two values ​​therein.

[0021] The inventors discovered that when the electrolyte contains only the first additive, the molecule contains P=O and PF bonds. During the first charge of the battery, these active groups are easily reduced to generate insoluble lithium salts such as lithium fluoride (LiF). This inorganic component can enhance the ionic conductivity of the SEI film. However, the trimethylsilyl group in the molecule decomposes to generate an organic SEI film on the electrode surface. But the cross-linking degree of this organic SEI film is not high enough. The electrode material is prone to cracking or dissolving during the volume expansion of the charge and discharge process or when stored at high temperature, which reduces its passivation protection ability. The electrolyte reacts with the electrode material, consumes active lithium, increases the battery internal resistance, and leads to capacity decay. When the electrolyte contains only the second additive, during the initial charging and cycling of a lithium-ion battery, the second additive, due to its unsaturated double bonds, has a low LUMO energy level. Furthermore, the Si-O bonds it contains have high bond energy and low rotational energy, resulting in strong molecular chain flexibility. Therefore, the second additive readily undergoes free radical polymerization on the positive and negative electrode surfaces to form a cross-linked elastic network structure. This network can buffer electrode volume changes during subsequent charging and discharging, reducing microcrack formation and improving the battery's high-temperature cycling performance. However, this free radical polymerization reaction is relatively fast, easily leading to explosive cross-linking in localized areas. This makes the branching and cross-linking degree of the polymerized network uncontrollable, resulting in uneven distribution of the three-dimensional cross-linked network thickness and cross-linking degree. This creates locally strong insulating regions, increasing ion transport resistance, increasing lithium-ion battery polarization, and reducing the battery's room-temperature cycling performance. Further research by the inventors revealed that the first additive can reduce the cross-linking degree of the network structure formed by the second additive and improve its thickness uniformity. It also introduces inorganic components into the SEI film composition, thereby enabling the lithium-ion battery to possess both excellent high-temperature and room-temperature cycling performance. This is mainly because the fluorine atom in the first additive has strong electronegativity, and its electron-withdrawing effect makes the trimethylsilyl group in the structure easy to undergo a siloxane exchange reaction with the second additive to generate unbranched small molecule unsaturated siloxane, thereby reducing the branching growth and cross-linking degree, so that the thickness and cross-linking degree of the polymer network finally generated on the positive and negative electrode surfaces are uniformly distributed, thus improving the high-temperature cycle performance of lithium-ion batteries while also enabling them to have room-temperature cycle performance.

[0022] The inventors' research also revealed that when A and / or B are too small, for example, A less than 0.1% or B less than 0.1%, the two additives are difficult to exert a synergistic effect; when both A and B are less than 0.1%, it is difficult to form a stable polymer network. When A and / or B are too large, for example, A greater than 5% and / or B greater than 3%, the resulting polymer network has excessive cross-linking degree and thickness, which is detrimental to lithium-ion transport and ultimately affects the high-temperature and room-temperature cycle performance of the lithium-ion battery. In summary, the electrolyte of this application contains a first additive and a second additive, and the content of the first and second additives is controlled within the range of this application. The first and second additives can exert a synergistic effect, forming a polymer network with uniform thickness and cross-linking degree on the positive and negative electrode surfaces, thereby improving the high-temperature and room-temperature cycle performance of the lithium-ion battery. In this application, high temperature refers to a temperature greater than or equal to 45°C.

[0023] In this application, the compounds of Formula I and Formula II are both commercially available conventional substances, and there are no special restrictions on their sources, as long as they can achieve the purpose of this application.

[0024] In some embodiments of this application, 0.1 ≤ A / B ≤ 20, preferably 0.5 ≤ A / B ≤ 2. For example, the value of A / B can be 0.1, 0.5, 1, 2, 5, 8, 10, 13, 15, 20, or a range of any two of these values. Excessive use of the first additive may excessively reduce the cross-linking degree of the polymer network, thereby reducing the strength of the polymer network and affecting its buffering effect on electrode volume changes. Excessive use of the second additive may exacerbate localized burst cross-linking, resulting in uneven distribution of polymer network thickness and cross-linking degree. By controlling the value of A / B within the above range, the two additives can further exert a synergistic effect, forming a polymer network with a more uniform cross-linking degree and thickness distribution on the positive and negative electrode surfaces, thereby further improving the high-temperature cycle performance and room-temperature cycle performance of lithium-ion batteries.

[0025] In some embodiments of this application, the electrolyte is selected from at least one of lithium hexafluorophosphate, lithium difluorosulfonylimide, lithium trifluoromethanesulfonate, lithium trifluoromethanesulfinate, lithium difluorooxalateborate, and lithium tetrafluoroborate; based on the mass of the electrolyte, the mass percentage of the electrolyte is 6% to 25%. For example, the mass percentage of the electrolyte can be 6%, 10%, 12%, 15%, 18%, 21%, 25%, or a range of any two of these values. By controlling the type and content of the electrolyte within the above range, the electrolyte can have a suitable ionic conductivity to meet the ion transport requirements of lithium-ion batteries during charging and discharging, thereby improving the high-temperature cycle performance and room-temperature cycle performance of lithium-ion batteries.

[0026] In some embodiments of this application, the solvent is selected from at least one of ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, fluoroethylene carbonate, propyl butyrate, ethyl butyrate, propyl propionate, ethyl propionate, methyl propionate, propyl acetate, ethyl acetate, methyl acetate, propyl formate, ethyl formate, methyl formate, and γ-butyrolactone; based on the mass of the electrolyte, the mass percentage of the solvent is 67% to 93%. For example, the mass percentage of the solvent can be 67%, 74%, 78%, 82%, 85%, 86%, 93%, or a range of any two of these values. By controlling the type and content of the solvent within the above range, the electrolyte can have suitable viscosity, high ionic conductivity, and good electrochemical stability, thereby improving the high-temperature cycle performance and room-temperature cycle performance of lithium-ion batteries.

[0027] In some embodiments, the electrolyte further includes a third additive selected from at least one of vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethylene sulfate (DTD), and 1,3-propane sulpholactone (PS). Based on the mass of the electrolyte, the mass percentage of the third additive is 0.3% to 5%, preferably 0.3% to 3%. Adding the aforementioned third additive to the electrolyte containing the first and second additives, and controlling the mass percentage of the third additive within the aforementioned range, is beneficial for improving the stability of the polymer network on the negative electrode surface and adjusting the ionic conductivity of the electrolyte, thereby further improving the high-temperature and room-temperature cycle performance of the lithium-ion battery.

[0028] The second aspect of this application provides a secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte provided in the first aspect of this application.

[0029] In some embodiments of this application, the positive electrode includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector, wherein the positive electrode material layer includes a positive electrode active material; the positive electrode active material includes LiFe. 1-x’ M' x’ PO4, LiMn 2-y’ M y’ O4 and LiNi x Co y Mn z M'' 1-x-y-zAt least one of O2, wherein M' is selected from at least one of Mn, Mg, Co, Ni, Cu, Zn, Al, Sn, Ga, Cr, Sr, V, and Ti, and M and M'' are each independently selected from at least one of Fe, Co, Ni, Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Sr, V, and Ti, 0≤x'<1, 0≤y'≤1, 0≤y≤1, 0≤x≤1, 0≤z≤1, and x+y+z≤1. Preferably, the positive electrode active material includes LiCoO2, LiFePO4 (LFP), and LiFe 0.8 Mn 0.2 PO4, LiMn2O4, LiNi 0.5 Co 0.2 Mn 0.3 O2 (NCM523), LiNi 0.6 Co 0.2 Mn 0.2 O2 (NCM622), LiNi 0.6 Co 0.1 Mn 0.3 O2 (NCM613), LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811), LiNi 0.5 Co 0.2 Mn 0.2 Al 0.1 O2 and LiNi 0.5 Co 0.2 Al 0.3 At least one of O2.

[0030] In some implementations, the positive electrode active material may also include at least one of sulfides, selenides, and halides. Positive electrode active materials comprising these substances can improve the energy density of lithium-ion batteries and optimize the charge-discharge kinetics of lithium-ion batteries by regulating ionic conductivity.

[0031] In this application, the aforementioned "positive electrode material layer disposed on at least one surface of the positive electrode current collector" means that the positive electrode material layer can be disposed on one surface of the positive electrode current collector along its own thickness direction, or 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 surface area of ​​the positive electrode current collector, or only a portion thereof; 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 electrode current collector, as long as the purpose of this application is achieved. For example, the positive electrode current collector can be aluminum foil, aluminum alloy foil, or a composite positive electrode 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 aforementioned polymer material base layer can be, but is not limited to, at least one of polypropylene (PP), polyethylene terephthalate (PET), or polybutylene terephthalate (PBT), polystyrene, or polyethylene. The material of the aforementioned metal layer can be, but is not limited to, at least one of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, or silver 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 is 30 μm to 200 μm, and the thickness of the positive electrode current collector is 5 μm to 20 μm, preferably 6 μm to 18 μm.

[0032] 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 black (Super P), acetylene black, Ketjen black, carbon nanotubes, graphene, or carbon fiber. For example, the positive electrode binder may include, but is not limited to, at least one of polyvinyl chloride, polyacryl alcohol, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene-propylene terpolymer, ethylene-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, or fluorinated acrylate resin. 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.

[0033] 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, foamed copper, 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 30 μm to 150 μm, and the thickness of the negative electrode current collector can be from 4 μm to 16 μm.

[0034] The negative electrode material layer includes a negative electrode active material. This application does not impose any particular limitation on the negative electrode active material, as long as it can achieve the purpose of this application. For example, the negative electrode active material may include, but is not limited to, at least one of natural graphite, artificial graphite, mesophase microcarbon spheres, hard carbon, soft carbon, silicon-based materials, tin-based materials, or lithium titanate. The aforementioned silicon-based materials may include, but are not limited to, at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, or silicon alloys; the aforementioned tin-based materials may include at least one of elemental tin, tin oxide compounds, or tin alloys.

[0035] The negative electrode material layer may further include a negative electrode conductive agent and a negative electrode binder. This application does not impose any particular limitation on the types of negative electrode conductive agents and negative electrode binders, as long as they can 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 black (Super P), acetylene black, Ketjen black, carbon black, carbon dots, carbon nanotubes, graphene, or carbon fibers. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers. For example, the negative electrode binder may include, but is not limited to, at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethyl methacrylate (PMAA), or carboxymethyl chitosan (CMCS). Optionally, the negative electrode material layer includes a thickener, which may include, but is not limited to, sodium carboxymethyl cellulose (CMC-Na). This application does not impose any particular restrictions on the mass ratio of negative electrode active material, negative electrode conductive agent, negative electrode binder, and thickener in the negative electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application can be achieved.

[0036] In this application, the secondary battery also includes a separator to separate the positive and negative electrode plates, prevent internal short circuits, allow ions to pass freely, and not affect the electrochemical charging and discharging process. This application does not impose any particular limitation on the separator, as long as it achieves the purpose of this application. For example, the separator material may include, but is not limited to, at least one of polyolefins (e.g., polyethylene (PE), polypropylene (PP)), glass fiber, polyester (e.g., polyethylene terephthalate (PET) film), cellulose, aromatic polyamides (e.g., polyimide (PI), polyamide (PA)), polytetrafluoroethylene, or polyethersulfone. The separator type may include at least one of woven membranes, nonwoven fabrics, microporous membranes, composite membranes, rolled membranes, or spun membranes. In this application, the separator thickness is not particularly limited, as long as it achieves the purpose of this application; for example, the separator thickness may be 10 μm to 12 μm.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] The secondary battery of this application can be in the form of a single battery cell, a battery module, or a battery pack. A single battery cell can be assembled into a battery module, and a battery module can contain one or more battery cells; the specific number can be selected by those skilled in the art based on the application and capacity of the battery module. The battery modules of this application can also be assembled into a battery pack, and a battery pack can contain one or more battery modules; the specific number can be selected by those skilled in the art based on the application and capacity of the battery pack.

[0041] Example

[0042] 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.

[0043] Test methods and equipment:

[0044] High-temperature cycling performance test:

[0045] The lithium-ion battery was left to stand at 45℃ for 4 hours to reach a constant temperature. It was then charged to 4.4V using a constant current of 1C, followed by constant voltage charging to a cutoff current of 0.05C, and then discharged to 2.75V using a constant current of 1C. The discharge capacity was recorded as C1. This constituted one charge-discharge cycle. This process was repeated for 1000 cycles, and the discharge capacity on the 1000th cycle was recorded as C2. The high-temperature cycle capacity retention rate of the lithium-ion battery was calculated.

[0046] High-temperature cycling capacity retention rate = C2 / C1 × 100%.

[0047] When the positive electrode active material of the battery is lithium iron phosphate (LFP), the charge and discharge voltage for high-temperature cycling tests is 2V~3.65V.

[0048] When the positive electrode active material of the battery is a high-nickel ternary LiNi 0.8 Co 0.1 Mn 0.1 At O2, the charge / discharge voltage for high-temperature cycling tests is 2.75V~4.2V.

[0049] Room temperature cycling performance test:

[0050] At an ambient temperature of 25°C, the lithium-ion battery was left to stand for 4 hours to reach a constant temperature. It was then charged at a constant current of 1C to 4.4V, followed by constant voltage charging to a cutoff current of 0.05C, and then discharged at a constant current of 1C to 2.75V. The discharge capacity was recorded as Q1. This constituted one charge-discharge cycle. This process was repeated for 2000 cycles, and the discharge capacity in the 2000th cycle was recorded as Q2. The capacity retention rate of the lithium-ion battery at room temperature was calculated.

[0051] Room temperature cycling capacity retention rate = Q2 / Q1 × 100%.

[0052] When the positive electrode active material of the battery is lithium iron phosphate (LFP), the charge and discharge voltage of the cycle test at room temperature is 2V~3.65V.

[0053] When the positive electrode active material of the battery is a high-nickel ternary LiNi 0.8 Co 0.1 Mn 0.1 At O2, the charge / discharge voltage for room temperature cycling tests is 2.75V~4.2V.

[0054] Example 1

[0055] <Preparation of Electrolyte>

[0056] Under an inert atmosphere (moisture <10 ppm, oxygen <1 ppm), ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed uniformly at a mass ratio of 3:7 to obtain a solvent. Then, the first additive shown in Formula I and the second additive shown in Formula II, as well as the electrolyte salts lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI), are added to the solvent. Based on the mass of the electrolyte, the mass percentages of LiPF6, LiFSI, the first additive (A), and the second additive (B) are 1%, with the remainder being solvent.

[0057] <Preparation of the positive electrode>

[0058] The positive electrode active material NCM613, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) were added to the solvent N-methylpyrrolidone (NMP) at a mass ratio of 96:2:2 and mixed evenly to prepare a positive electrode slurry with a solid content of 60wt%. The positive electrode slurry was uniformly coated onto one surface of a 16μm thick aluminum foil current collector and dried at 85℃ to obtain a positive electrode sheet with a single-sided coating of positive electrode material. The above steps were repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive electrode material. After drying, the sheet was cold-pressed, trimmed, cut, and slit. It was then dried under vacuum at 85℃ for 10 hours, and tabs were welded to obtain a positive electrode sheet with a size of 70mm × 54mm for later use. The thickness of the single-sided positive electrode material layer was 90μm, and the compaction density was 3.4g / cm³. 3 .

[0059] <Preparation of Negative Electrode Sheets>

[0060] A negative electrode active material (graphite), a conductive agent (acetylene black), a thickener (sodium carboxymethyl cellulose (CMC-Na), and a binder (styrene-butadiene rubber (SBR, where the styrene:butadiene mass ratio is 1:1)) were added to deionized water in a mass ratio of 95:1.5:1.5:2 and mixed evenly to prepare a negative electrode slurry with a solid content of 50 wt%. This slurry was then uniformly coated onto one surface of a 9 μm thick copper foil current collector and dried at 85°C to obtain a negative electrode sheet with a single-sided coating of the negative electrode material. The above steps were repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coating of the negative electrode material. After drying, the sheet was cold-pressed, trimmed, cut, and slit. It was then dried under vacuum at 85°C for 12 hours, and tabs were welded to obtain a negative electrode sheet with dimensions of 74 mm × 58 mm for later use. The thickness of the single-sided negative electrode material layer was 109 μm, and the compaction density was 1.6 g / cm³. 3 .

[0061] <Preparation of the diaphragm>

[0062] A 10µm thick porous polyethylene (PE) membrane was used as the separator.

[0063] <Preparation of Lithium-ion Batteries>

[0064] The prepared positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes for isolation. They are then wound, with the positive tab connected to the positive electrode and the negative tab connected to the negative electrode, thus obtaining the electrode assembly. The theoretical capacity of the electrode assembly is 1600mAh. The electrode assembly is placed in an aluminum foil packaging bag, with the positive and negative tabs extended from the inside to the outside of the bag. After drying at 75℃ for 10 hours to remove moisture, it is heat-sealed to obtain the cell ready for electrolyte injection. The prepared electrolyte is then injected into the dried cell. Following vacuum sealing, settling, formation, aging, and capacity testing, a lithium-ion battery is obtained. The formation conditions are: 45℃, 3MPa pressure, 0.05C constant current charging for 120 minutes, followed by 0.2C constant current charging for 270 minutes.

[0065] Examples 2 to 20

[0066] Except for adjusting the relevant parameters according to Table 1, everything else is the same as in Example 1.

[0067] The mass percentage of solvent in the electrolyte varies with the mass percentage of electrolyte and the mass percentage of additives.

[0068] Comparative Examples 1 to 9

[0069] Except for adjusting the relevant parameters according to Table 1, everything else is the same as in Example 1.

[0070] The mass percentage of solvent in the electrolyte varies with the mass percentage of electrolyte and the mass percentage of additives.

[0071] Comparative Example 10

[0072] Except for the second additive being the following compound, everything else is the same as in Example 1.

[0073]

[0074] The preparation and performance parameters of each embodiment and comparative example are shown in Table 1.

[0075] Table 1

[0076] Note: " / " in Table 1 indicates that there are no relevant parameters.

[0077] As can be seen from Examples 1 to 20 and Comparative Examples 1 to 9, the electrolyte includes the first additive and the second additive within the scope of this application, and the mass percentage content A of the first additive and the mass percentage content B of the second additive are controlled within the scope of this application. The lithium-ion battery has a high room temperature cycle capacity retention rate and a high temperature cycle capacity retention rate, indicating that the lithium-ion battery has good room temperature cycle performance and high temperature cycle performance.

[0078] The mass percentage A of the first additive affects the cycle performance of the lithium-ion battery. As can be seen from Examples 1 to 6, Comparative Examples 6 and 7, when the mass percentage A of the first additive is controlled within the range of this application, the resulting lithium-ion battery has a high room temperature cycle capacity retention rate and a high temperature cycle capacity retention rate, indicating that the lithium-ion battery has good room temperature cycle performance and high temperature cycle performance.

[0079] The mass percentage B of the second additive affects the cycle performance of the lithium-ion battery. As can be seen from Examples 1, 7 to 10, Comparative Examples 8 and 9, when the mass percentage B of the second additive is controlled within the range of this application, the resulting lithium-ion battery has a high room temperature cycle capacity retention rate and a high temperature cycle capacity retention rate, indicating that the lithium-ion battery has good room temperature cycle performance and high temperature cycle performance.

[0080] The A / B ratio affects the cycle performance of lithium-ion batteries. As can be seen from Examples 1 to 13, adjusting the A / B ratio within the range of this application results in lithium-ion batteries with high room temperature cycle capacity retention and high temperature cycle capacity retention, indicating that lithium-ion batteries have good room temperature cycle performance and high temperature cycle performance.

[0081] The mass percentage and type of electrolyte affect the cycle performance of lithium-ion batteries. As can be seen from Examples 1, 14, 15 and 18, by controlling the mass percentage and type of electrolyte within the range of this application, the resulting lithium-ion batteries have high room temperature cycle capacity retention and high temperature cycle capacity retention, indicating that lithium-ion batteries have good room temperature cycle performance and high temperature cycle performance.

[0082] The type of positive electrode active material affects the cycle performance of lithium-ion batteries. As can be seen from Examples 1, 16 and 17, by controlling the type of positive electrode active material within the scope of this application, the resulting lithium-ion battery has a high room temperature cycle capacity retention rate and a high temperature cycle capacity retention rate, indicating that the lithium-ion battery has good room temperature cycle performance and high temperature cycle performance.

[0083] The type and mass percentage of the third additive affect the cycle performance of the lithium-ion battery. As can be seen from Examples 1, 19 and 20, based on the electrolyte including the first and second additives within the scope of this application, the introduction of a third additive and the adjustment of the mass percentage of the third additive within the scope of this application result in a lithium-ion battery with high room temperature cycle capacity retention and high temperature cycle capacity retention, indicating that the lithium-ion battery has good room temperature cycle performance and high temperature cycle performance.

[0084] The type of the second additive affects the cycle performance of the lithium-ion battery. As can be seen from Example 1 and Comparative Example 10, by controlling the type of the second additive within the scope of this application, the resulting lithium-ion battery has a high room temperature cycle capacity retention rate and a high temperature cycle capacity retention rate, indicating that the lithium-ion battery has good room temperature cycle performance and high temperature cycle performance.

[0085] 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 solvent, an electrolyte, and an additive, said additive comprising a first additive and a second additive, the first additive being selected from compounds of formula I, and the second additive being selected from compounds of formula II. Based on the mass of the electrolyte, the mass percentage of the first additive is A, 0.1% ≤ A ≤ 5%, and the mass percentage of the second additive is B, 0.1% ≤ B ≤ 3%.

2. The electrolyte according to claim 1, wherein, 0.5%≤A≤3%, 0.5%≤B≤2%.

3. The electrolyte according to claim 1, wherein, 0.1≤A / B≤20.

4. The electrolyte according to claim 3, wherein, 0.5≤A / B≤2.

5. The electrolyte according to any one of claims 1 to 4, wherein, The electrolyte is selected from at least one of lithium hexafluorophosphate, lithium difluorosulfonylimide, lithium trifluoromethanesulfonate, lithium trifluoromethanesulfinate, lithium difluorooxalateborate, and lithium tetrafluoroborate; and / or, based on the mass of the electrolyte, the mass percentage of the electrolyte is 6% to 25%.

6. The electrolyte according to any one of claims 1 to 4, wherein, The solvent is selected from at least one of ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, fluoroethylene carbonate, propyl butyrate, ethyl butyrate, propyl propionate, ethyl propionate, methyl propionate, propyl acetate, ethyl acetate, methyl acetate, propyl formate, ethyl formate, methyl formate, and γ-butyrolactone; and / or, based on the mass of the electrolyte, the solvent has a mass percentage content of 67% to 93%.

7. A secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte according to any one of claims 1 to 6.

8. The secondary battery according to claim 7, wherein, The positive electrode sheet includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector, wherein the positive electrode material layer includes a positive electrode active material; The positive electrode active material includes LiFe 1-x’ M' x’ PO4, LiMn 2-y’ M y’ O4 and LiNi x Co y Mn z M'' 1-x-y-z At least one of O2, wherein M' is selected from at least one of Mn, Mg, Co, Ni, Cu, Zn, Al, Sn, Ga, Cr, Sr, V and Ti, and M and M'' are each independently selected from at least one of Fe, Co, Ni, Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Sr, V and Ti, 0≤x'<1, 0≤y'≤1, 0≤y≤1, 0≤x≤1, 0≤z≤1, x+y+z≤1.

9. The secondary battery according to claim 8, wherein, The positive electrode active material includes LiCoO2, LiFePO4, and LiFe 0.8 Mn 0.2 PO4, LiMn2O4, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.6 Co 0.1 Mn 0.3 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.5 Co 0.2 Mn 0.2 Al 0.1 O2 and LiNi 0.5 Co 0.2 Al 0.3 At least one of O2.

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