Electrolyte and secondary battery

Abstract

The invention provides an electrolyte and a secondary battery, the electrolyte comprises an additive, an electrolyte and a solvent, the additive comprises a first additive, the electrolyte comprises lithium bis (fluorosulfonyl) imide and lithium hexafluorophosphate, and the first additive is selected from at least one of compounds as shown in a formula I. The electrolyte comprises lithium bis (fluorosulfonyl) imide, lithium hexafluorophosphate and a compound as shown in the formula I, and the lithium bis (fluorosulfonyl) imide, the lithium hexafluorophosphate and the compound as shown in the formula I have a synergistic effect, so that the rate capability and the high-temperature performance of the secondary battery are improved, the thermal runaway is slowed down, the critical temperature of the thermal runaway is effectively increased, and the thermal safety performance of the secondary battery is improved. # imgabs0 #

Description

Technical Field

[0001] The present application relates to the field of electrochemistry technology, and particularly to an electrolyte and a secondary battery. Background Art

[0002] Secondary batteries, such as lithium-ion batteries, are widely used in fields such as consumer electronics, power batteries, and energy storage batteries due to their advantages of high energy density, long cycle life, and no memory effect. However, safety issues such as fires and explosions caused by secondary batteries have gradually attracted social attention. In particular, the development trend of secondary batteries with large capacities such as 280 Ah and large sizes has exacerbated the occurrence of safety accidents.

[0003] Thermal runaway is a common and critical inducement in the safety issues of secondary batteries. When the heat generation rate inside the secondary battery is greater than the heat dissipation rate, thermal runaway will be triggered, and side reactions will occur spontaneously to increase the temperature of the secondary battery. At relatively high temperatures, serious redox exothermic reactions may be triggered, generating a large amount of heat and getting out of control, leading to danger. Therefore, how to reduce the occurrence of exothermic reactions during the evolution of thermal runaway and improve the thermal safety performance of secondary batteries is an urgent problem to be solved. Summary of the Invention

[0004] The purpose of the present application is to provide an electrolyte and a secondary battery, which can improve the rate performance and high-temperature performance of the secondary battery, and at the same time, are beneficial to reducing the occurrence of exothermic reactions during the evolution of thermal runaway and improving the thermal safety performance of the secondary battery. The specific technical solutions are as follows:

[0005] The first aspect of the present application provides an electrolyte, which includes an additive, an electrolyte, and a solvent. The additive includes a first additive. The electrolyte includes lithium bis(fluorosulfonyl)imide and lithium hexafluorophosphate. The first additive is selected from at least one of the compounds shown in Formula I:

[0006]

[0007] Wherein, A1 and A2 are each independently selected from a single bond, an oxygen atom, a C1-C3 alkylene group, or a C1-C3 alkoxy group. n and m are each independently 0 or 1, and n and m are not both 0 at the same time.

[0008] In some embodiments of the present application, based on the mass of the electrolyte, the mass percentage content of lithium bis(fluorosulfonyl)imide is A, 1% ≤ A ≤ 10%, preferably, 3% ≤ A ≤ 7%. The mass percentage content of the first additive is B, 0.1% ≤ B ≤ 3%, preferably, 0.5% ≤ B ≤ 1%.

[0009] In some embodiments of the present application, A:B = (3-9):1.

[0010] In some embodiments of the present application, the compound represented by Formula I is selected from at least one of the following compounds:

[0011]

[0012] In some embodiments of the present application, based on the mass of the electrolyte, the mass percentage content of lithium hexafluorophosphate is D, where 8% ≤ D ≤ 15%.

[0013] In some embodiments of the present application, the solvent includes a cyclic organic solvent and / or a linear organic solvent. The cyclic organic solvent is selected from at least one of ethylene carbonate and propylene carbonate, and the linear organic solvent is selected from at least one of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and methyl propyl carbonate.

[0014] The second aspect of the present 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 the present application.

[0015] In some embodiments of the present application, the negative electrode sheet includes a negative electrode current collector and a negative electrode material layer located on at least one surface of the negative electrode current collector. The negative electrode material layer includes a negative electrode active material, and the negative electrode active material includes graphite.

[0016] In some embodiments of the present application, the positive electrode sheet includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode current collector. 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 lithium manganese iron phosphate.

[0017] Advantages of the present application:

[0018] The present application provides an electrolyte and a secondary battery. The electrolyte includes an additive, an electrolyte, and a solvent. The additive includes a first additive, the electrolyte includes lithium bis(fluorosulfonyl)imide and lithium hexafluorophosphate, and the first additive is selected from at least one of the compounds represented by Formula I. The electrolyte includes lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate, and the compound represented by Formula I. The three act synergistically, which is beneficial to improving the rate performance and high-temperature performance of the secondary battery. At the same time, it is also beneficial to slow down thermal runaway, effectively increasing the critical temperature at which thermal runaway occurs and improving the thermal safety performance of the secondary battery.

[0019] Of course, it is not necessary for any product or method implementing the present application to achieve all of the above advantages simultaneously. Description of the Drawings

[0020] To more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the accompanying drawings required for the description of the embodiments. Obviously, the accompanying drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, other embodiments can also be obtained based on these drawings.

[0021] Figure 1 It is the differential scanning calorimetry (DSC) exothermic test curve of Examples 1-4 of the present application;

[0022] Figure 2 It is the differential scanning calorimetry (DSC) exothermic test curve of Comparative Examples 1-4 of the present application. Specific embodiments

[0023] The following will clearly and completely describe the technical solutions in the present application in conjunction with the embodiments and accompanying drawings of the present application. Obviously, the described embodiments are only some embodiments of the present application, rather than all 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.

[0024] It should be noted that in the specific embodiments of the present application, a lithium-ion battery is used as an example of a secondary battery to explain the present application, but the secondary battery of the present application is not limited to lithium-ion batteries.

[0025] The first aspect of the present application provides an electrolyte, which includes an additive, an electrolyte, and a solvent. The additive includes a first additive, the electrolyte includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium hexafluorophosphate, and the first additive is selected from at least one of the compounds shown in Formula I:

[0026]

[0027] Wherein, A1 and A2 are each independently selected from a single bond, an oxygen atom, a C1-C3 alkylene group, or a C1-C3 alkoxy group, n and m are each independently 0 or 1, and n and m are not both 0 at the same time. For example, A1 and A2 can each independently be selected from a single bond, an oxygen atom, a methylene group, an ethylene group, a propylene group, a methoxy group, an ethoxy group, or a propoxy group.

[0028] The inventors have found through research that LiFSI itself has good thermal stability and high ionic conductivity, which is beneficial to improving the rate performance and high-temperature performance of secondary batteries. However, although LiFSI itself has good thermal stability, during the operation of secondary batteries, if extreme failure behavior occurs, at about 200 °C, LiFSI will react with the charged negative electrode material (such as lithiated graphite), releasing a large amount of heat, triggering thermal runaway, and posing a risk of violent explosion. The electrolyte of the present application also includes LiPF 6 ,LiPF6 When the battery exhibits extreme failure behavior, an endothermic reaction occurs due to decomposition at around 200 °C, thereby reducing the heat release during the thermal runaway evolution process, increasing the critical temperature at which thermal runaway occurs. Further, by introducing the compound represented by Formula I of the present application into the electrolyte, the large π bond on the benzene ring in the compound represented by Formula I affects the activity of the entire compound represented by Formula I, reducing its energy barrier and making the ring-opening reaction of sultone easier. As a result, a solid electrolyte interface film (SEI film) rich in inorganic sulfates, inorganic sulfites, lithium alkylsulfonates, or lithium alkylsulfates is formed on the surface of the negative electrode during the battery formation stage, enhancing the chemical stability at the interface between the negative electrode and the electrolyte. At the same time, the opening of the S-F bond and S=O bond in LiFSI can decompose to form LiF, Li 2 S, Li 2 O and other inorganic salt substances. These substances can cooperate with the products of the ring-opening reaction of sultone to construct a denser SEI layer, further enhancing the thermal stability of the SEI film, further improving the high-temperature performance of the secondary battery. Moreover, due to the thermally stable SEI film constructed by the products of the ring-opening reaction of LiFSI and sultone, it can effectively isolate the exothermic reaction between lithiated graphite and undecomposed LiFSI at around 200 °C, slow down thermal runaway, and effectively increase the critical temperature at which thermal runaway occurs, improving the thermal safety performance of the secondary battery.

[0029] On this basis, when the additive is the compound represented by Formula I-3, when the six-membered ring opens, the chain length of the molecules in the SEI film is increased, thereby improving the thermal stability of the SEI film. The presence of the O atom in the six-membered ring also increases the Li 2 O concentration in the SEI film components, and the Li 2 O-rich structure makes the SEI layer denser, thereby enhancing the thermal stability and effectively suppressing the exothermic reaction between lithiated graphite and undecomposed LiFSI, slowing down thermal runaway.

[0030] The electrolyte of the present application includes LiFSI, LiPF 6 and the compound represented by Formula I. The synergistic effect of the three is beneficial to improving the rate performance and high-temperature performance of the secondary battery, and at the same time is beneficial to slowing down thermal runaway and improving the thermal safety performance of the secondary battery. The "high temperature" referred to in the present application means a temperature greater than or equal to 45 °C.

[0031] In some embodiments of the present application, based on the mass of the electrolyte, the mass percentage content of lithium bis(fluorosulfonyl)imide is A, 1% ≤ A ≤ 10%, preferably, 3% ≤ A ≤ 7%, and the mass percentage content of the first additive is B, 0.1% ≤ B ≤ 3%, preferably, 0.5% ≤ B ≤ 1%. For example, the mass percentage content of lithium bis(fluorosulfonyl)imide can be 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10% or a range composed of any two of these values; the mass percentage content of the first additive can be 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.3%, 1.5%, 1.8%, 2%, 2.3%, 2.5%, 2.8%, 3% or a range composed of any two of these values. The electrolyte includes LiFSI, LiPF 6 and the compound shown in Formula I, and by controlling the mass percentage content of LiFSI and the compound shown in Formula I within the scope of the present application, the three act synergistically, which is beneficial to further improving the rate performance and high-temperature performance of the secondary battery, and at the same time, is beneficial to slowing down thermal runaway and improving the thermal safety performance of the secondary battery

[0032] In some embodiments of the present application, A:B = (3 to 9):1. For example, the value of A:B can be 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1. The electrolyte of the present application includes LiFSI, LiPF 6 and the compound shown in Formula I and by controlling the ratio of the mass percentage content of LiFSI to the compound shown in Formula I within the scope of the present application, it is beneficial to further improve the rate performance and high-temperature performance of the secondary battery, and at the same time, is beneficial to slowing down thermal runaway and improving the thermal safety performance of the secondary battery

[0033] In some embodiments of the present application, the compound shown in Formula I is selected from at least one of the following compounds:

[0034]

[0035] In some embodiments of the present application, based on the mass of the electrolyte, the mass percentage content of lithium hexafluorophosphate is D, 8% ≤ D ≤ 15%. For example, the mass percentage content of lithium hexafluorophosphate can be 8%, 8.5%, 9%, 9.5%, 10%, 10.5%, 11%, 11.5%, 12%, 12.5%, 13%, 13.5%, 14%, 14.5%, 15% or a range composed of any two of these values. The electrolyte includes lithium hexafluorophosphate, and by controlling the mass percentage content of lithium hexafluorophosphate within the scope of the present application, the secondary battery can have a relatively high ionic conductivity and good electrochemical stability. At the same time, LiPF 6When the battery undergoes extreme failure behavior, an endothermic reaction will occur due to decomposition at around 200°C, thereby reducing the heat release during the thermal runaway evolution process, delaying thermal runaway, increasing the critical temperature at which thermal runaway occurs, improving the cycle performance and rate performance of the secondary battery, and at the same time improving the thermal safety performance of the secondary battery.

[0036] In some embodiments of the present application, the solvent includes a cyclic organic solvent and / or a chain-like organic solvent. The cyclic organic solvent is selected from at least one of ethylene carbonate and propylene carbonate, and the chain-like organic solvent is selected from at least one of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and methyl propyl carbonate. The electrolyte includes a solvent within the above range, which can make the electrolyte have appropriate viscosity, high ionic conductivity, and good electrochemical stability, and can further improve the rate performance, high-temperature performance, and thermal stability of the secondary battery.

[0037] In some embodiments of the present application, the electrolyte includes lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate, a first additive, and the above solvent. Among them, based on the mass of the electrolyte, the mass percentage contents of lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate, and the first additive are as described above, and the mass percentage content of the solvent is 72% to 90.9%.

[0038] In some embodiments of the present application, the additive further includes vinylene carbonate (VC). Based on the mass of the electrolyte, the mass percentage content of vinylene carbonate is 0.5% to 5%, preferably 1% to 2%. For example, the mass percentage content of vinylene carbonate can be 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.2%, 4.5%, 4.8%, 5%, or any range composed of any two of these values. On the basis that the electrolyte includes LiFSI and the compound shown in Formula 1, further introducing VC and controlling the mass percentage content of VC within the scope of the present application, VC can first be reduced on the surface of the negative electrode to form a stable SEI film prior to other additives, slow down the occurrence of thermal failure problems in the initial stage of thermal runaway, further inhibit the exothermic reaction between LiFSI and lithiated graphite, and further improve the high-temperature performance and thermal safety performance of the secondary battery.

[0039] 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 provided in the first aspect of the present application.

[0040] In some embodiments of the present application, the negative electrode plate includes a negative electrode current collector and a negative electrode material layer located on at least one surface of the negative electrode current collector. The negative electrode material layer includes a negative electrode active material, and the negative electrode active material includes graphite.

[0041] In the present application, the negative electrode active material includes graphite, and the graphite is selected from at least one of natural graphite and artificial graphite. In some embodiments of the present application, the negative electrode active material is entirely selected from graphite. In some other embodiments of the present application, the negative electrode active material may further include, but is not limited to, silicon-based materials, soft carbon, hard carbon or mesophase microcarbon spheres, and the silicon-based materials may include, but are not limited to, at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites or silicon alloys.

[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-mentioned "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 own thickness direction, or may be provided on both surfaces of the negative electrode current collector along its own 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. There is no particular limitation in the present application, as long as the object of the present application can be achieved. There is no particular limitation on the negative electrode current collector in the present application, as long as the object of the present application can be achieved. For example, the negative electrode current collector may be copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam or copper foam, aluminum foil or a composite negative electrode current collector. The above-mentioned 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, and the material of the above-mentioned polymer material base layer may include, but is not limited to, at least one of polypropylene (PP), polyethylene terephthalate (PET) or polybutylene terephthalate (PBT), and the material of the above-mentioned metal layer may include, but is not limited to, at least one of copper, copper alloy, nickel or nickel alloy. There is no particular limitation on the thickness of the negative electrode material layer and the negative electrode current collector in the present application, 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 30 μm to 70 μm, and the thickness of the negative electrode current collector is 3 μm to 10 μm.

[0043] The negative electrode material layer further includes a negative electrode conductive agent and a negative electrode binder. There are no particular limitations on the types of the negative electrode conductive agent and the negative electrode binder in this application, as long as the objectives of this application can be achieved. For example, the negative electrode conductive agent may include, but is not limited to, at least one of Super P, acetylene black, Ketjen black, carbon nanotubes, graphene, or carbon fiber. The above-mentioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above-mentioned carbon fiber may include, but is not limited to, vapor-grown carbon fiber (VGCF) and / or nanofiber. 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), polymethacrylic acid (PMAA), or carboxymethyl chitosan (CMCS). There are no particular limitations on 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 in this application, and those skilled in the art can select according to actual needs as long as the objectives of this application can be achieved. In some embodiments of this application, the negative electrode material layer may further include a thickening agent, and the thickening agent may include, but is not limited to, sodium carboxymethyl cellulose (CMC), etc. There are no particular limitations on the mass ratio of the negative electrode active material, the negative electrode conductive agent, the negative electrode binder, and the thickening agent in the negative electrode material layer in this application, and those skilled in the art can select according to actual needs as long as the objectives of this application can be achieved.

[0044] In this application, there are no particular limitations on the method for preparing the negative electrode plate, as long as the objectives of this application can be achieved. For example, it can be prepared by the following method: adding the negative electrode active material, the negative electrode conductive agent, and the negative electrode binder to deionized water and stirring evenly to obtain a negative electrode slurry with a solid content of 45 wt% to 70 wt%. Coating the negative electrode slurry evenly on both surfaces of the negative electrode current collector, and drying to obtain a negative electrode plate with a double-sided coated negative electrode material layer. Then, it is cold-pressed and cut to obtain the negative electrode plate.

[0045] In some embodiments of this application, the positive electrode plate includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode current collector. 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 (such as NCM811, NCM712, NCM622, NCM523, NCM111), lithium nickel cobalt aluminate, lithium iron phosphate, lithium-rich manganese-based material, lithium cobalt oxide, lithium manganese oxide, lithium manganese iron phosphate, and preferably, the positive electrode active material is selected from lithium iron phosphate.

[0046] In this application, a lithium-ion battery includes a positive electrode sheet, which includes a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector. The above-mentioned "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 can be disposed 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 surface of the positive electrode current collector or a partial area of the surface of the positive electrode current collector. There is no special limitation in this application as long as the purpose of this application can be achieved. There is no special limitation on the positive electrode current collector in this application 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-mentioned composite positive electrode current collector can be a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The material of the above-mentioned polymer material substrate 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-mentioned metal layer can include, but is not limited to, at least one of aluminum, aluminum alloy, nickel or nickel alloy. There is no special limitation on the thickness of the positive electrode material layer and the positive electrode current collector in this application 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 15 μm.

[0047] The positive electrode material layer may further include a positive electrode conductive agent and a positive electrode binder. There is no special limitation on the types of the positive electrode conductive agent and the positive electrode binder in this application as long as the purpose of this application can be achieved. For example, the positive electrode conductive agent can include, but is not limited to, at least one of super conductive carbon black (Super P), acetylene black, Ketjen black, carbon nanotubes, graphene or carbon fiber. For example, the positive electrode binder can include, but is not limited to, at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer or fluorinated acrylate resin. There is no special limitation on 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 in this application, and those skilled in the art can select according to actual needs as long as the purpose of this application can be achieved.

[0048] In this application, there is no particular limitation on the method for preparing the positive electrode sheet, as long as the purpose of this application can be achieved. For example, it can be prepared by the following method: Mix the positive electrode active material, positive electrode conductive agent, and positive electrode binder, add N-methylpyrrolidone (NMP), and stir evenly to obtain a positive electrode slurry with a solid content of 50 wt% to 85 wt%. Coat the positive electrode slurry evenly on both surfaces of the positive electrode current collector, and after drying, obtain a positive electrode sheet with a double-sided coated positive electrode material layer. Then, through cold pressing and cutting, the positive electrode sheet is obtained.

[0049] The secondary battery of this application further includes a separator, which is used to separate the positive electrode sheet and the negative electrode sheet, prevent internal short circuit of the battery, allow electrolyte ions to pass through freely, and does not affect the progress of the electrochemical charge and discharge process. There is no particular limitation on the type of the separator in this application, and any porous structure separator with good chemical stability and mechanical stability can be selected. For example, the material of the separator can include at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The type of the separator can include but is not limited to at least one of woven film, non-woven film (non-woven fabric), microporous film, composite film, rolled film, or spun film, etc. The separator can be a single-layer film or a multi-layer composite film. In this application, there is no particular limitation on the thickness of the separator, as long as the purpose of this application can be achieved. For example, the thickness can be 5 μm to 20 μm.

[0050] In this application, the secondary battery further includes a housing, which is used to accommodate the positive electrode sheet, separator, negative electrode sheet, and electrolyte, as well as other components known in the field of lithium-ion batteries. This application does not limit the above-mentioned other components. There is no particular limitation on the housing in this application, and it can be a housing well-known in the art, as long as the purpose of this 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. This application does not limit the type of the metal, and a metal hard shell housing known in the art can be used, as long as the purpose of this application can be achieved. The flexible housing can be a metal plastic film, such as aluminum plastic film, steel plastic film, etc.

[0051] There is no particular limitation on the secondary battery of this application, and it can include any device that undergoes an electrochemical reaction. In one embodiment of this application, the secondary battery can include but is not limited to: lithium-ion secondary battery (lithium-ion battery), sodium-ion secondary battery (sodium-ion battery), etc.

[0052] 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 a lithium-ion battery may include, but is not limited to, the following steps: stacking a positive electrode sheet, a separator, and a negative electrode sheet in sequence, and winding, folding, etc. as needed 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. Alternatively, stack the positive electrode sheet, the separator, and the negative electrode sheet in sequence, and then fix the four corners of the entire laminated structure with tape to obtain a laminated electrode assembly, place the electrode assembly into the housing, inject the electrolyte into the housing and seal it to obtain a secondary battery. In addition, an overcurrent protection element, a guide plate, etc. can also be placed in the housing as needed to prevent the pressure inside the secondary battery from rising and overcharging and discharging.

[0053] The secondary battery of the present application can include a battery cell form, a battery module form, and a battery pack form. The battery cells can be assembled into a battery module. The number of battery cells contained in the battery module can be one or more, and those skilled in the art can select the specific number according to the application and capacity of the battery module. The battery module of the present application can also be assembled into a battery pack. The number of battery modules contained in the battery pack can be one or more, and those skilled in the art can select the specific number according to the application and capacity of the battery pack.

[0054] Examples

[0055] 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.

[0056] Testing method and device:

[0057] Rate performance test

[0058] Place the lithium-ion battery in a constant temperature test chamber at 25 °C and let it stand for 30 minutes to make the lithium-ion battery reach a constant temperature. Charge it at a constant current of 3C to 3.65V, then charge it at a constant voltage of 3.65V until the cut-off current is 0.05C, and discharge it at a constant current of 3C to 2V. Record the first discharge capacity as C0, and take this as one charge-discharge cycle. Repeat the above charge-discharge cycle, and record the number of cycles when the capacity decays to 80% of the capacity of the first cycle. The charge-discharge cycle test instrument is Neware BTS. The larger the number of cycles, the better the rate performance of the lithium-ion battery.

[0059] When performing the rate performance test on the lithium-ion batteries of Examples 1-21, Comparative Example 1-2, and Comparative Example 1-5, the upper limit voltage of 3.65 V in the above steps was adjusted to 4.4 V, and the lower limit voltage of 2 V was adjusted to 3 V; when performing the rate performance test on the lithium-ion batteries of Examples 1-22, Comparative Example 1-3, and Comparative Example 1-6, the upper limit voltage of 3.65 V in the above steps was adjusted to 4.45 V, and the lower limit voltage of 2 V was adjusted to 3 V; the remaining examples and comparative examples were all tested with an upper limit voltage of 3.65 V and a lower limit voltage of 2 V.

[0060] High-temperature cycle performance test

[0061] Place the lithium-ion battery in an incubator at 45 °C and let it stand for 4 h. Then, charge the lithium-ion battery at a constant current of 1 C until the voltage reaches 3.65 V, then charge it at a constant voltage of 3.65 V until the current reaches 0.05 C, and then discharge it at a constant current of 1 C until the voltage reaches 2.00 V. Record the initial discharge capacity as C1. This is one charge-discharge cycle. Repeat the above charge-discharge cycle 500 times, and record the discharge capacity after the 500th cycle as C2. The charge-discharge cycle test instrument is Neware BTS.

[0062] The capacity retention rate at 45 °C (%) = C2 / C1 × 100%; the high-temperature cycle performance of the lithium-ion battery is evaluated by the capacity retention rate at 45 °C. The larger the capacity retention rate, the better the high-temperature cycle performance of the lithium-ion battery.

[0063] When performing the high-temperature cycle performance test on the lithium-ion batteries of Examples 1-21, Comparative Example 1-2, and Comparative Example 1-5, the upper limit voltage of 3.65 V in the above steps was adjusted to 4.4 V, and the lower limit voltage of 2 V was adjusted to 3 V; when performing the high-temperature cycle performance test on the lithium-ion batteries of Examples 1-22, Comparative Example 1-3, and Comparative Example 1-6, the upper limit voltage of 3.65 V in the above steps was adjusted to 4.45 V, and the lower limit voltage of 2 V was adjusted to 3 V; the remaining examples and comparative examples were all tested with an upper limit voltage of 3.65 V and a lower limit voltage of 2 V.

[0064] High-temperature storage performance test

[0065] Place the lithium-ion battery in an environment at 25 °C and charge it at a constant current of 1 C to 3.65 V, then charge it at a constant voltage of 3.65 V until the current reaches 0.05 C. Measure the thickness of the lithium-ion battery and record it as D 1 , then place the lithium-ion battery in an explosion-proof oven at 60 °C and let it stand for 30 days. Take out the lithium-ion battery and measure the battery thickness again after it has completely cooled down and record it as D 2 , the thickness change rate = (D 2 - D 1 ) / D 1× 100%. The smaller the thickness change rate, the better the high-temperature storage performance of the lithium-ion battery.

[0066] When performing the high-temperature storage performance test on the lithium-ion batteries of Examples 1-21, Comparative Examples 1-2 and Comparative Examples 1-5, the upper limit voltage of 3.65 V in the above steps was adjusted to 4.4 V; when performing the high-temperature storage performance test on the lithium-ion batteries of Examples 1-22, Comparative Examples 1-3 and Comparative Examples 1-6, the upper limit voltage of 3.65 V in the above steps was adjusted to 4.45 V; the remaining Examples and Comparative Examples were all tested with an upper limit voltage of 3.65 V.

[0067] Heat release test

[0068] Place the lithium-ion battery in an environment of 25 °C and charge it at a constant current of 1C to 3.65 V, then charge it at a constant voltage of 3.65 V until the current is 0.05C to make the lithium-ion battery reach a fully charged state. Then, in a glove box filled with argon (moisture < 10 ppm, oxygen content < 1 ppm), disassemble the battery, take out the negative electrode sheet, cut the negative electrode sheet, weigh 3 mg of the negative electrode sheet and 5 μl of the corresponding electrolyte and place them in a DSC high-pressure crucible (model: 27 μl, manufacturer: Netzsch), and then use a press to seal the crucible to obtain a sample. After that, perform differential scanning calorimetry (DSC) test on the sample. The model of the test instrument is Netzsch differential scanning calorimeter DSC214. The temperature rising range is from 25 °C to 400 °C, and the temperature rising rate is 10 °C / min. Record the temperature of the main exothermic peak, and calculate the peak area from 150 °C to 350 °C to obtain the heat release from 150 °C to 350 °C. The larger the value of the temperature of the main exothermic peak, the higher the temperature point at which thermal runaway occurs, and the more lagged the corresponding point of thermal runaway, that is, the better the thermal stability.

[0069] When charging the lithium-ion batteries of Examples 1-21, Comparative Examples 1-2 and Comparative Examples 1-5 before the heat release test, the upper limit voltage of 3.65 V in the above steps was adjusted to 4.4 V; when charging the lithium-ion batteries of Examples 1-22, Comparative Examples 1-3 and Comparative Examples 1-6 before the heat release test, the upper limit voltage of 3.65 V in the above steps was adjusted to 4.45 V; the remaining Examples and Comparative Examples were all charged with an upper limit voltage of 3.65 V.

[0070] Example 1-1

[0071] <Preparation of electrolyte>

[0072] In a glove box filled with argon (moisture < 10 ppm, oxygen < 1 ppm), ethylene carbonate (EC), propylene carbonate (PC), and ethyl methyl carbonate (EMC) were mixed in a mass ratio of 32:4:45.7 to obtain a base solvent. Then, lithium bis(fluorosulfonyl)imide (LiFSI), lithium hexafluorophosphate (LiPF 6 ), the compound shown in Formula I-3, and vinylene carbonate were added to the base solvent and mixed evenly to obtain an electrolyte solution. Among them, based on the mass of the electrolyte solution, the mass percentage content of LiFSI was 1%, and the mass percentage content of LiPF 6 was 14%, the mass percentage content of the compound shown in Formula I-3 was 0.8%, the mass percentage content of vinylene carbonate was 2.5%, and the balance was the base solvent. Among them, the mass percentage content of EC was 32%, the mass percentage content of PC was 4%, and the mass percentage content of EMC was 45.7%.

[0073] <Preparation of negative electrode sheet>

[0074] The negative electrode active material artificial graphite, conductive agent Super P, binder sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 95:1.5:1.5:2, and deionized water was added as a solvent to prepare a slurry with a solid content of 49 wt%. After being stirred evenly by a vacuum mixer, a negative electrode slurry was obtained. The negative electrode slurry was evenly coated on both surfaces of a negative electrode current collector copper foil with a thickness of 9 μm, dried at 85°C and then cold-pressed to obtain a negative electrode sheet with a double-sided coated negative electrode material layer. Among them, the surface density of the single-sided negative electrode material layer was 8 mg / cm 2 , the thickness of the single-sided negative electrode material layer was 50 μm, and then trimming, slicing, and slitting were carried out. After slitting, it was dried at 85°C for 4 h under vacuum conditions, and the tab was welded to obtain a negative electrode sheet with a specification of 660 mm × 59 mm for use.

[0075] <Preparation of positive electrode sheet>

[0076] The positive electrode active material lithium iron phosphate (LiFePO 4 ), positive electrode conductive agent Super P, and positive electrode binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 96:2:2, and N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry with a solid content of 55 wt%. After being stirred evenly by a vacuum mixer, a positive electrode slurry was obtained. The positive electrode slurry was evenly coated on both surfaces of a positive electrode current collector aluminum foil with a thickness of 12 μm, dried at 85°C and then cold-pressed to obtain a positive electrode sheet with a double-sided coated positive electrode material layer. Among them, the surface density of the single-sided positive electrode material layer was 17.5 mg / cm 2, the thickness of the single-sided positive electrode material layer is 76.5 μm. Then, trimming, cutting, and slitting are carried out. After slitting, it is dried at 85 °C for 4 h under vacuum conditions, and the electrode tab is welded to obtain a positive electrode sheet with a specification of 540 mm × 55 mm for standby.

[0077] <Preparation of Separator>

[0078] A polypropylene (PP) porous membrane with a thickness of 16 μm (provided by Shenzhen Xingyuan Materials Technology Co., Ltd.) is used as the separator.

[0079] <Preparation of Lithium-Ion Battery>

[0080] The positive electrode sheet, separator, and negative electrode sheet prepared above are stacked in sequence, with the separator placed in the middle of the positive electrode sheet and the negative electrode sheet to play an isolation role, and the electrode assembly is obtained by winding. The electrode assembly is placed in an aluminum-plastic film packaging bag and vacuum baked at 85 °C for 48 h, and the electrolyte prepared above is injected, and a lithium-ion battery is obtained through processes such as vacuum packaging, standing, formation, and shaping.

[0081] Examples 1-2 to Examples 1-8

[0082] Except that in <Preparation of Electrolyte>, the mass percentage content of LiFSI and LiPF 6 is adjusted according to Table 1. Among them, when the total mass percentage content of LiFSI and LiPF 6 changes, the mass percentage content of the solvent EMC changes accordingly, and the mass percentage content of the remaining components remains unchanged. Otherwise, it is the same as Example 1-1.

[0083] Examples 1-9 to Examples 1-17

[0084] Except that in <Preparation of Electrolyte>, the type and mass percentage content of the compound shown in Formula I are adjusted according to Table 1. When the mass percentage content of the compound shown in Formula I changes, the mass percentage content of the solvent EMC changes accordingly, and the mass percentage content of the remaining components remains unchanged. Otherwise, it is the same as Example 1-4.

[0085] Examples 1-18 to Examples 1-20

[0086] Except that in <Preparation of Electrolyte>, the mass percentage content of LiPF 6 and LiFSI is adjusted according to Table 1. When the total mass percentage content of LiPF 6 and LiFSI changes, the mass percentage content of the solvent EMC changes accordingly, and the mass percentage content of the remaining components remains unchanged. Otherwise, it is the same as Example 1-4.

[0087] Example 1-21

[0088] Except that in the preparation of the positive electrode sheet, the positive electrode active material is replaced with LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811), and in the preparation of the electrolyte, vinylene carbonate is not added, and the mass percentage content of the solvent EMC changes accordingly, while the mass percentage contents of the other components remain unchanged. Otherwise, it is the same as in Examples 1-4.

[0089] Examples 1-22

[0090] Except that in the preparation of the positive electrode sheet, the positive electrode active material is replaced with LiCoO 2 , and in the preparation of the electrolyte, vinylene carbonate is not added, and the mass percentage content of the solvent EMC changes accordingly, while the mass percentage contents of the other components remain unchanged. Otherwise, it is the same as in Examples 1-4.

[0091] Examples 1-23 to Examples 1-24

[0092] Except that in the preparation of the electrolyte, the mass percentage content of the compound shown in Formula I is adjusted according to Table 1. When the mass percentage content of the compound shown in Formula I changes, the mass percentage content of the solvent EMC changes accordingly, while the mass percentage contents of the other components remain unchanged. Otherwise, it is the same as in Examples 1-4.

[0093] Examples 1-25 to Examples 1-26

[0094] Except that in the preparation of the electrolyte, the mass percentage content of LiFSI, the mass percentage content of the compound shown in Formula I, and LiPF 6 are adjusted according to Table 1, and the mass percentage content of the solvent EMC changes accordingly, while the mass percentage contents of the other components remain unchanged. Otherwise, it is the same as in Examples 1-1.

[0095] Comparative Example 1-1

[0096] Except that in the preparation of the electrolyte, LiFSI is not added according to Table 1, and the mass percentage content of LiPF 6 is adjusted. Otherwise, it is the same as in Examples 1-1.

[0097] Comparative Example 1-2

[0098] Except that in the preparation of the electrolyte, LiFSI is not added according to Table 1, and the mass percentage content of LiPF 6 is adjusted. Otherwise, it is the same as in Examples 1-21.

[0099] Comparative Example 1-3

[0100] Except in the <preparation of electrolyte>, LiFSI was not added according to Table 1, and the mass percentage of LiPF 6 was adjusted, and the rest was the same as in Examples 1-22.

[0101] Comparative Examples 1-4

[0102] Except in the <preparation of electrolyte>, the compound shown in Formula I was not added according to Table 1, and the mass percentage of the solvent EMC changed accordingly. Except that the mass percentages of the other components remained unchanged, the rest was the same as in Examples 1-4.

[0103] Comparative Examples 1-5

[0104] Except in the <preparation of electrolyte>, the compound shown in Formula I was not added according to Table 1, and the mass percentage of the solvent EMC changed accordingly. Except that the mass percentages of the other components remained unchanged, the rest was the same as in Examples 1-21.

[0105] Comparative Examples 1-6

[0106] Except in the <preparation of electrolyte>, the compound shown in Formula I was not added according to Table 1, and the mass percentage of the solvent EMC changed accordingly. Except that the mass percentages of the other components remained unchanged, the rest was the same as in Examples 1-22.

[0107] Comparative Examples 1-7

[0108] Except in the <preparation of electrolyte>, LiPF 6 was not added according to Table 1, the mass percentage of the solvent EMC changed accordingly, and the mass percentages of the other components remained unchanged. Except for this, the rest was the same as in Examples 1-4.

[0109] The relevant parameters and performance parameters of each example and comparative example are shown in Table 1.

[0110] Table 1

[0111]

[0112]

[0113] Note: " / " in Table 1 indicates that the corresponding substance or parameter does not exist.

[0114] As can be seen from Examples 1-1 to 1-26 and Comparative Examples 1-1 to 1-7, the electrolyte comprising lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate and the compound represented by Formula I within the scope of the present application can endow the lithium-ion battery with a high cycle number at 25 °C, a high capacity retention rate at 45 °C, a low thickness change rate, a high main exothermic peak temperature and a low heat release amount, indicating that the lithium-ion battery has good rate performance and high-temperature performance, and also has good thermal safety performance. The electrolytes of Comparative Examples 1-1 to 1-3 do not include lithium bis(fluorosulfonyl)imide, and their lithium-ion batteries have a low cycle number at 25 °C, a low capacity retention rate at 45 °C and a high thickness change rate, indicating that the rate performance and high-temperature performance of the lithium-ion battery are poor. Comparative Examples 1-4, 1-5 and 1-6 do not include the compound represented by Formula I, and their lithium-ion batteries have a low main exothermic peak temperature and a high heat release amount, indicating that the thermal safety of the lithium-ion battery is poor. From Figure 1 and Figure 2 it can be seen that the main exothermic peak temperature of Example 1-4 is 257.8 °C, and that of Comparative Example 1-4 is 220.9 °C. Comparative Example 1-7 does not include lithium hexafluorophosphate, and its lithium-ion battery has a low cycle number at 25 °C and a high heat release amount, indicating that the rate performance and thermal safety of the lithium-ion battery are poor. Thus, it shows that the electrolyte does not meet the scope of the present application, and the rate performance, high-temperature cycle performance and thermal safety performance of the lithium-ion battery cannot be taken into account simultaneously.

[0115] The mass percentage content of lithium bis(fluorosulfonyl)imide will affect the high-temperature performance, rate performance and thermal safety performance of the lithium-ion battery. As can be seen from Examples 1-1 to 1-8, by regulating the mass percentage content of lithium bis(fluorosulfonyl)imide within the scope of the present application, the lithium-ion battery can have a high cycle number at 25 °C, a high capacity retention rate at 45 °C, a low thickness change rate, a high main exothermic peak temperature and a low heat release amount, indicating that the lithium-ion battery has good rate performance and high-temperature performance, and also has good thermal safety performance.

[0116] The mass percentage content and type of the compound represented by Formula I will affect the high-temperature performance, rate performance and thermal safety performance of the lithium-ion battery. As can be seen from Examples 1-4, 1-9 to 1-17, by selecting the compound represented by Formula I within the scope of the present application and regulating the mass percentage content of the compound represented by Formula I within the scope of the present application, the lithium-ion battery can have a high cycle number at 25 °C, a high capacity retention rate at 45 °C, a low thickness change rate, a high main exothermic peak temperature and a low heat release amount, indicating that the lithium-ion battery has good rate performance and high-temperature performance, and also has good thermal safety performance.

[0117] The ratio of the mass percentage content A of lithium bis(fluorosulfonyl)imide to the mass percentage content B of the compound shown in Formula I will affect the high-temperature performance, rate performance, and thermal safety performance of lithium-ion batteries. It can be seen from Examples 1-1 to 1-26 that by adjusting the ratio of A to B within the scope of this application, the lithium-ion battery can have a higher number of cycles at 25 °C, a higher capacity retention rate at 45 °C, a lower thickness change rate, a higher main exothermic peak temperature, and a lower heat release, indicating that the lithium-ion battery has good rate performance and high-temperature performance, and at the same time has good thermal safety performance, and the comprehensive performance of the lithium-ion battery is improved.

[0118] The mass percentage content of lithium hexafluorophosphate will affect the high-temperature performance, rate performance, and thermal safety performance of lithium-ion batteries. It can be seen from Examples 1-1 to 1-8 and Examples 1-18 to 1-20 that by adjusting the mass percentage content of lithium hexafluorophosphate within the scope of this application, the lithium-ion battery can have a relatively high number of cycles at 25 °C, a relatively high capacity retention rate at 45 °C, a lower thickness change rate, a relatively high main exothermic peak temperature, and a lower heat release, indicating that the lithium-ion battery has good rate performance and high-temperature performance, and at the same time has good thermal safety performance.

[0119] The type of cathode active material will affect the high-temperature performance, rate performance, and thermal safety performance of lithium-ion batteries. It can be seen from Examples 1-4, Examples 1-21 to 1-22, and Comparative Examples 1-1 to 1-6 that by selecting the cathode active material within the scope of this application and adjusting the mass percentage contents of the compound shown in Formula I and LiFSI within the scope of this application, the lithium-ion battery can have a relatively high number of cycles at 25 °C, a relatively high capacity retention rate at 45 °C, a lower thickness change rate, a relatively high main exothermic peak temperature, and a lower heat release, indicating that the lithium-ion battery has good rate performance and high-temperature performance, and at the same time has good thermal safety performance.

[0120] The above are only the preferred embodiments of this application and are not intended to limit this application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of this application shall be included within the scope of protection of this application.

Claims

1. An electrolyte comprising an additive, an electrolyte and a solvent, wherein the additive comprises a first additive, the electrolyte comprises lithium bis(fluorosulfonyl)imide and lithium hexafluorophosphate, and the first additive is selected from at least one of the compounds shown in formula I: in, A1 and A2 are each independently selected from a single bond, an oxygen atom, a C1-C3 alkylene group or a C1-C3 alkyleneoxy group, n and m are each independently 0 or 1, and n and m are not 0 at the same time.

2. The electrolyte according to claim 1, based on the mass of the electrolyte, the mass percentage of the lithium bis(fluorosulfonyl)imide is A, 1%≤A≤10%, and the mass percentage of the first additive is B, 0.1%≤B≤3%.

3. The electrolyte according to claim 2, which satisfies at least one of the following conditions: (1)3％≤A≤7％； (2)0.5％≤B≤1％。 4. The electrolyte according to claim 2, characterized in that A:B=(3~9):

1.

5. The electrolyte according to claim 1, wherein The compound represented by formula I is selected from at least one of the following compounds:

6. The electrolyte according to any one of claims 1 to 5, wherein Based on the mass of the electrolyte, the mass percentage of the lithium hexafluorophosphate is D, 8%≤D≤15%.

7. The electrolyte according to any one of claims 1 to 5, wherein The solvent includes a cyclic organic solvent and / or a chain organic solvent, the cyclic organic solvent is selected from at least one of ethylene carbonate and propylene carbonate, and the chain organic solvent is selected from at least one of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate and methyl propyl carbonate. 8 . A secondary battery comprising a positive electrode sheet, a negative electrode sheet, a separator and the electrolyte according to claim 1 .

9. The secondary battery according to claim 8, wherein The negative electrode sheet includes a negative electrode current collector and a negative electrode material layer located on at least one surface of the negative electrode current collector. The negative electrode material layer includes a negative electrode active material, and the negative electrode active material includes graphite.

10. The secondary battery according to claim 8, wherein The positive electrode plate includes a positive electrode current collector and a positive electrode material layer arranged on at least one surface of the positive electrode current collector, 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 aluminum oxide, lithium iron phosphate, lithium-rich manganese-based material, lithium cobalt oxide, lithium manganese oxide, and lithium iron manganese phosphate.

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