Electrolyte, high-voltage electrochemical device, and electronic apparatus

Abstract

The invention relates to an electrolyte, a high-voltage electrochemical device and electronic equipment. The electrolyte is applied to the high-voltage electrochemical device and comprises an organic solvent, lithium salt and additives, the additives comprise a multi-nitrile additive and a dinitrile additive, and the multi-nitrile additive comprises at least three cyano groups; the additive and the high-voltage electrochemical device meet the condition that (A + B) / X is greater than or equal to 0.5, the mass percent of the polynitrile additive in the electrolyte is A%, the mass percent of the dinitrile additive in the electrolyte is B%, and the full charge voltage of the high-voltage electrochemical device is XV. According to the scheme provided by the invention, the K value of the battery can be controlled within a reasonable range, and high-temperature cycle storage and high-temperature cycle performance of the battery under high voltage are considered.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to electrolytes and high-voltage electrochemical devices and electronic devices. Background Art

[0002] The popularity of portable electronic devices such as mobile phones, tablets, and laptops has promoted the development of high-energy-density secondary batteries. In order to obtain higher battery capacity, the charge cutoff voltage is sometimes set to 4.4V or even higher. In this high-voltage environment, the oxidation of the positive electrode material increases, and the electrolyte is easily oxidized and decomposed on the positive electrode side; at the same time, transition metal ions will dissolve, be reduced and deposited on the negative electrode side, and hinder the migration of lithium ions. Therefore, it is particularly important to protect the positive electrode surface of the battery under high voltage.

[0003] In the related art, by adding polynitrile compounds as additives into the electrolyte, the cobalt atoms in the positive electrode active material lithium cobalt oxide can be effectively complexed, thereby effectively improving the structural stability of the positive electrode active material under high voltage.

[0004] However, polynitrile compounds will complex free copper ions in the electrolyte and then reduce them to copper element on the surface of the negative electrode, resulting in abnormal K value of the battery cell and affecting the electrochemical performance of the battery. Summary of the invention

[0005] In order to solve or partially solve the problems existing in the related art, the present application provides an electrolyte, a high-voltage electrochemical device and an electronic device, which can control the K value of the battery within a reasonable range and take into account the high-temperature cycle storage and high-temperature cycle performance of the battery under high voltage.

[0006] In a first aspect, the present application provides an electrolyte for use in a high voltage electrochemical device, wherein the electrolyte comprises an organic solvent, a lithium salt and an additive, wherein the additive comprises a polynitrile additive and a dinitrile additive, wherein the polynitrile additive comprises at least three cyano groups; and the additive and the high voltage electrochemical device satisfy the following relationship:

[0007] (A+B) / X≥0.5

[0008] In the relationship, the mass percentage of the polynitrile additive in the electrolyte is A%, the mass percentage of the dinitrile additive in the electrolyte is B%, and the full charge voltage of the high voltage electrochemical device is XV.

[0009] As an optional embodiment, the additive and the high voltage electrochemical device satisfy: (A+B) / X≥0.556.

[0010] As an optional embodiment, 0.5≤A≤4.

[0011] As an optional embodiment, 0.5≤B≤4.

[0012] As an optional embodiment, 4≤X≤4.58.

[0013] As an optional embodiment, B / A≥0.5.

[0014] As an optional embodiment, the polynitrile additive includes at least one of 1,2,3,4,5-penta-oxo-(2-cyanoethyl) and 1,3,6-hexanetrinitrile.

[0015] As an optional embodiment, the dinitrile additive includes at least one of succinonitrile and adiponitrile.

[0016] As an optional embodiment, the additive further includes a sulfur-containing additive, and the structural formula of the sulfur-containing additive is as follows:

[0017]

[0018] In the structural formula 1, n is 0 or 1, and X is selected from or R1, R2 and R3 are each independently selected from H, At least one of the following, and R1, R2 and R3 are not selected from H at the same time, and at least one sulfur atom is contained in X, R1, R2 and R3.

[0019] As an optional embodiment, the mass percentage of the sulfur-containing additive in the electrolyte is C%, 0.5≤C≤4; and / or, C / (A+B)≥0.125.

[0020] As an optional embodiment, the sulfur-containing additive includes at least one of the following compounds:

[0021]

[0022] A second aspect of the present application provides a high voltage electrochemical device, comprising the aforementioned electrolyte.

[0023] A third aspect of the present application provides an electronic device, comprising the electrochemical device described above.

[0024] The technical solution provided by this application may have the following beneficial effects:

[0025] The additives in the present application include polynitrile additives and dinitrile additives. The polynitrile additive may refer to a compound having no less than 3 cyano functional groups, and the content of the additive and the full charge voltage of the high-voltage electrochemical device satisfy (A+B) / X≥0.5, which can control the K value of the battery within a reasonable range and take into account the high-temperature cycle storage and high-temperature cycle performance of the battery at high voltage.

[0026] It should be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present application. DETAILED DESCRIPTION

[0027] The embodiments of the present application will be described in more detail below. Although the embodiments of the present application are shown, it should be understood that the present application can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided to make the present application more thorough and complete, and to fully convey the scope of the present application to those skilled in the art.

[0028] The terms used in this application are for the purpose of describing specific embodiments only and are not intended to limit this application. The singular forms of "a", "said" and "the" used in this application and the appended claims are also intended to include plural forms unless the context clearly indicates other meanings. It should also be understood that the term "and / or" used herein refers to and includes any or all possible combinations of one or more associated listed items.

[0029] It should be understood that although the terms "first", "second", "third", etc. may be used in this application to describe various information, this information should not be limited to these terms. These terms are only used to distinguish the same type of information from each other. For example, without departing from the scope of this application, the first information may also be referred to as the second information, and similarly, the second information may also be referred to as the first information. Thus, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of this application, the meaning of "multiple" is two or more, unless otherwise clearly and specifically defined.

[0030] In the related art, by adding polynitrile compounds as additives into the electrolyte, the cobalt atoms in the positive electrode active material lithium cobalt oxide can be effectively complexed, thereby effectively improving the structural stability of the positive electrode active material under high voltage.

[0031] However, polynitrile compounds will complex free copper ions in the electrolyte and then reduce them to single copper on the surface of the negative electrode, resulting in abnormal K value of the battery cell and affecting the electrochemical performance of the battery.

[0032] In response to the above problems, an embodiment of the present application provides an electrolyte that can control the K value of the battery within a reasonable range, while taking into account the high-temperature cycle storage and high-temperature cycle performance of the battery under high voltage.

[0033] The present application provides an electrolyte for use in a high voltage electrochemical device. The electrolyte includes an organic solvent, a lithium salt, and an additive. The additive includes a polynitrile additive and a dinitrile additive. The polynitrile compound contains at least three cyano groups. The additive and the high voltage electrochemical device satisfy the following relationship:

[0034] (A+B) / X≥0.5

[0035] In the relationship, the mass percentage of the polynitrile additive in the electrolyte is A%, the mass percentage of the dinitrile additive in the electrolyte is B%, the mass percentage of the sulfur-containing additive in the electrolyte is C%, and the full charge voltage of the high voltage electrochemical device is XV.

[0036] In the embodiment of the present application, the lithium salt includes lithium hexafluorophosphate LiPF 6 、Lithium difluorooxalatoborate LiODFB、Lithium bis(oxalatoborate) LiBOB、Lithium difluorobis(oxalatophosphate) LiDFOP、Lithium tetrafluoroborate LiBF 4 、Lithium bis(trifluoromethylsulfonyl)imide LiTFSI、Lithium bis(fluorosulfonyl)imide LiFSI、Lithium difluorophosphate LiPOF 2 One or more of the above, the concentration of the electrolyte salt is 0.4 mol / L to 2.2 mol / L.

[0037] In an embodiment of the present application, the organic solvent includes at least one of ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, diphenyl carbonate, methyl acetate, methyl propionate, methyl butyrate, methyl acetate, ethyl acetate, propyl acetate, ethyl butyrate, propyl propionate, γ-butyrolactone, 1,3-dioxolane, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether.

[0038] In the embodiment of the present application, the full charge voltage of the high voltage electrochemical device is a high voltage (4V to 4.58V). The full charge voltage of the electrochemical device may refer to the voltage when the electrochemical device is fully charged, that is, the voltage value tested by a multimeter after the electrochemical device is charged to the rated capacity marked on the battery cell. The full charge voltage may be the voltage marked on the battery after the battery is manufactured, and will not change due to changes in additives.

[0039] In the embodiment of the present application, the additive includes a polynitrile additive and a dinitrile additive. The polynitrile additive may refer to a compound having not less than 3 cyano functional groups, and the content of the additive and the full charge voltage of the high voltage electrochemical device satisfy (A+B) / X≥0.5, which can control the K value of the battery within a reasonable range and take into account the high temperature cycle storage and high temperature cycle performance of the battery at high voltage. This is due to the following reasons:

[0040] ① Nitrile additives can improve the high voltage performance of batteries

[0041] The cyano functional group in the nitrile additive has a strong complexation with the transition metal on the surface of the positive electrode material, which can form a CEI film to protect the positive electrode active material and inhibit the dissolution of metal elements, effectively improving the structural stability of the positive electrode active material under high voltage. At the same time, the nitrile additive itself has good oxidation resistance, which can inhibit the oxidative decomposition of the electrolyte on the positive electrode side when the voltage is high. Therefore, the higher the voltage, the more nitrile additives are needed to improve the high voltage performance of the battery.

[0042] ②Nitrile additives can improve the high temperature storage performance of batteries

[0043] When the battery is stored at 85°C, the electrolyte is easily oxidized and decomposed to generate HF, which will further undergo side reactions at the electrode interface, resulting in the generation of a large amount of gas. The cyano functional group in nitrile additives has the function of removing water and acid, which can capture the HF generated under high-temperature storage and inhibit further side reactions of the electrolyte. Therefore, adding nitrile additives can effectively improve the high-temperature storage performance of the battery, especially multi-nitrile additives have the most significant effect in improving high-temperature storage.

[0044] ③Nitrile additives can synergistically control the battery K value within a reasonable range

[0045] Polynitrile compounds in nitrile additives, such as tetraether nitrile and HTCN, are prone to copper precipitation reaction with the electrolyte, resulting in an increase in the battery K value. The reaction mechanism is as follows:

[0046] R1: 1LiPF 6 +H 2 O→POF 3 +LiF+2HF;

[0047] R2: 2H++CuO x →H 2 O+Cu 2+ ;

[0048] Cu 2+ / Cu (~3.3V vs.Li+ / Li); Cu + / Cu (~3.5V vs.Li+ / Li);

[0049] R3:Cu n+ +2CN-R+nPF.→Cu(CN-R)PF. / Cu(CN-R)PF;

[0050] Cu n+ +nF→CuF (slightly soluble);

[0051] Cu n+ +nPF→Cu(PF);

[0052] R4:Cu n+ +ne - →Cu↓(reduction and precipitation on the anode surface).

[0053] The dinitrile additives such as SN and ADN in the nitrile additives can form a Cu(CN-R)·PF protective layer with Cu ions, inhibiting further corrosion of the copper foil and improving the antioxidant potential of Cu. Therefore, the combination of dinitrile additives and polynitrile additives has a synergistic effect and can control the battery K value within a reasonable range.

[0054] ④ The relationship between nitrile additives and the full charge voltage of the high voltage electrochemical device satisfies (A+B) / X≥0.5, which can take into account high temperature cycle performance at high voltage

[0055] Although nitrile additives can improve the high voltage performance of the battery, the film-forming impedance of nitrile additives is high at the positive electrode, which easily leads to increased polarization on the positive electrode side under high voltage, deteriorating the high temperature cycle performance. Therefore, the content of nitrile additives needs to maintain a certain proportional relationship with the full charge voltage of the battery, so as to better balance the nitrile additives on the basis of improving the high voltage performance of the battery, while taking into account the impedance on the positive electrode side and improving the high temperature cycle performance. The inventors have found that when (A+B) / X≥0.5, a better balancing effect can be achieved.

[0056] As an optional embodiment, the content of the additive and the full charge voltage of the high voltage electrochemical device satisfy: (A+B) / X≥0.556.

[0057] The inventors further discovered that A+B can represent the total content of polynitrile additives and dinitrile additives, that is, the total content of nitrile additives. When the total content of nitrile additives and the full charge voltage of the battery satisfy (A+B) / X≥0.556, the high voltage performance and high temperature cycle performance can be further balanced.

[0058] As an optional embodiment, 0.5≤A≤4.

[0059] In the embodiments of the present application, A can be 0.5, 1, 2, 3, 4 or any value within the above-defined range, and the present application does not impose any limitation on this.

[0060] If the A value is too low, the storage performance of the battery and the transition metal protection of the positive electrode will be insufficient, and the high-voltage and high-temperature cycle and high-temperature storage performance will be poor; if the A value is too large, the viscosity of the electrolyte will increase.

[0061] As an optional embodiment, 0.5≤B≤4.

[0062] In the embodiments of the present application, B can be 0.5, 1, 2, 3, 4 or any value within the above-defined range, and the present application does not impose any limitation on this.

[0063] Similarly, if the B value is too low, the k value of the battery will increase, and the high-voltage and high-temperature cycling and high-temperature storage performance will be poor; if the B value is too large, the viscosity of the electrolyte will increase.

[0064] As an optional embodiment, 4≤X≤4.58.

[0065] In the embodiment of the present application, X can be 4, 4.5, 4.58 or any value within the above-defined range, and the present application does not limit this. When the additives in the embodiment of the present application are applied to an electrochemical device with a full charge voltage within the above-defined range, better effects can be achieved.

[0066] As an optional embodiment, B / A≥0.5.

[0067] B / A represents the ratio of the mass content of the dinitrile additive to the polynitrile additive, for example, B / A can be 0.5, 1, 3, 10, 20, etc. Since the polynitrile additive is a deterioration factor for copper precipitation, the more the content, the more obvious the deterioration, and the dinitrile compound can form a Cu(CN-R)·PF protective layer with Cu ions, inhibiting further corrosion of the copper foil and improving the antioxidant potential of Cu. Therefore, when the content ratio of the dinitrile additive to the polynitrile additive is within the above range, the k value of the battery can be controlled within a reasonable range.

[0068] As an optional embodiment, the polynitrile additive includes at least one of 1,2,3,4,5-penta-oxo-(2-cyanoethyl) and 1,3,6-hexanetrinitrile.

[0069] The polynitrile additives selected in the embodiments of the present application have more cyano functional groups, which have better complexation effects on transition metals of the positive electrode and stronger oxidation resistance.

[0070] As an optional embodiment, the dinitrile additive includes at least one of succinonitrile and adiponitrile.

[0071] The dinitrile additives used in the embodiments of the present application are low in cost, relatively common, and have low chain length and molecular weight, so the viscosity and impedance are relatively controllable; at the same time, they have the effect of effectively inhibiting copper foil corrosion.

[0072] As an optional embodiment, the additive further includes a sulfur-containing additive, and the structural formula of the sulfur-containing additive is as follows:

[0073]

[0074] In the structural formula 1, n is 0 or 1, and X is selected from or R1, R2 and R3 are each independently selected from H, At least one of the following, and R1, R2 and R3 are not selected from H at the same time, and at least one sulfur atom is contained in X, R1, R2 and R3.

[0075] The sulfur-containing additive of structural formula 1 can form a film on both the positive and negative electrode sides, and the electrode interface film formed by it has good thermal stability, which can improve the high-temperature storage performance of the battery. At the same time, the CEI film formed by the sulfur-containing additive of structural formula 1 on the positive electrode side is rich in sulfur elements, which can reduce the impedance of the positive electrode side. Therefore, when the content of nitrile additives is high, adding sulfur-containing additives can reduce the positive electrode impedance, reduce polarization, and improve the high-voltage and high-temperature cycle performance of the battery.

[0076] As a preferred embodiment, the mass percentage of the sulfur-containing additive in the electrolyte is C%, and 0.5≤C≤4.

[0077] In the embodiments of the present application, C can be 0.5, 1, 2, 3, 4 or any value within the above-defined range, and the present application does not impose any limitation on this.

[0078] If the C value is too low, the impedance on the positive electrode side cannot be reduced, which is not conducive to improving the high-voltage and high-temperature cycle performance of the battery; if the C value is too large, the viscosity of the electrolyte will increase, deteriorating the battery performance.

[0079] As a preferred embodiment, C / (A+B)≥0.125

[0080] C / (A+B) represents the ratio of the content of sulfur-containing additives to nitrile additives. Since the sulfur-containing additives in the embodiments of the present application can form films on both the positive and negative electrode sides, and the electrode interface film formed by them has good thermal stability, it can improve the high-temperature storage performance of the battery. At the same time, the CEI film formed by the sulfur-containing additives on the positive electrode side is rich in sulfur elements, which can reduce the impedance of the positive electrode side. Therefore, when the content of nitrile additives is high, adding sulfur-containing additives can reduce the positive electrode impedance, reduce polarization, and improve the high-voltage and high-temperature cycle performance of the battery. Controlling the content of sulfur-containing additives and nitrile additives within the above range can effectively improve the high-voltage and high-temperature cycle performance.

[0081] As a preferred embodiment, the sulfur-containing additive includes at least one of the following compounds:

[0082]

[0083] The sulfur-containing additive of the embodiment of the present application includes a five-membered cyclic carbonate group, a five-membered cyclic sulfate group, a six-membered cyclic carbonate group or a six-membered cyclic sulfate group, and the sulfur-containing additive includes at least one cyclic sulfate group, and the sulfate group or carbonate group in the sulfur-containing additive can participate in the formation of the interface film and improve the stability of the interface film. When the sulfur-containing additive is selected from the above compounds, the sulfur-containing additive and the nitrile additive can better synergize to generate a CEI film with higher stability, thereby better alleviating the side reaction between the positive electrode additive and the positive electrode active material, and thus can further improve the high temperature cycle performance and high temperature storage performance of the battery at high voltage.

[0084] In addition, those skilled in the art, knowing the structural formula of the compound shown in Structural Formula 1, can know the preparation method of the above compound based on the common knowledge in the field of chemical synthesis.

[0085] Corresponding to the aforementioned application function realization method embodiment, the present application also provides a high voltage electrochemical device, an electronic device and corresponding embodiments.

[0086] The embodiment of the present application also provides a high voltage electrochemical device, comprising the aforementioned electrolyte, a positive electrode, a negative electrode and a separator.

[0087] As an optional embodiment, the positive electrode includes a positive electrode current collector and a positive electrode active material layer coated on the positive electrode current collector, the positive electrode active material layer includes a positive electrode active material, and the chemical formula of the positive electrode active material is LiNi a Co b Mn (1-a-b) M c O 2 , wherein 0.3≤a≤0.9, a+b<1, 0≤c<0.08, and M is at least one of Al, Mg, Zr and Ti.

[0088] In the embodiments of the present application, there is no particular restriction on the type of the positive electrode current collector, which can be any known material suitable for use as a positive electrode current collector. In one embodiment, the positive electrode current collector includes metal materials such as aluminum, stainless steel, nickel plating, titanium, tantalum, and carbon materials such as carbon cloth and carbon paper. Preferably, the positive electrode current collector is a metal material.

[0089] In one embodiment, the positive electrode active material layer further includes a conductive agent, a binder and a solvent.

[0090] In one embodiment, the conductive agent includes at least one of carbon materials such as natural graphite, artificial graphite, acetylene black, needle coke, carbon nanotubes, and graphene.

[0091] In one embodiment, the binder includes at least one of polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, and nitrocellulose.

[0092] In the embodiments of the present application, there is no particular limitation on the material and shape of the diaphragm, as long as the effect of the present application is not significantly impaired.

[0093] In one embodiment, the separator includes a porous sheet or non-woven fabric having excellent liquid retention. The materials of the resin or glass fiber separator include, but are not limited to, polyolefin, aromatic polyamide, polytetrafluoroethylene, polyethersulfone, and the like.

[0094] In some embodiments, the electrochemical device may include an outer package, which may be used to encapsulate the electrode assembly and the electrolyte.

[0095] In some embodiments, the outer packaging of the electrochemical device may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the electrochemical device may also be a soft package, such as a bag-type soft package. The material of the soft package may be plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0096] The present application has no particular limitation on the shape of the electrochemical device, which may be cylindrical, square, or any other shape.

[0097] The embodiments of the present application have no particular restrictions on the negative electrode current collector, as long as the purpose of the present application can be achieved, for example, it can be copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foam nickel, foam copper or a composite current collector.

[0098] An embodiment of the present application also provides an electronic device, comprising the aforementioned electrochemical device.

[0099] Exemplarily, the above-mentioned electrical devices may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto.

[0100] In order to further understand the present invention, the present application is described below in conjunction with embodiments. These embodiments are only used to illustrate the present application and are not used to limit the scope of the present application.

[0101] 1. Preparation of batteries

[0102] 1. Preparation of positive electrode sheet

[0103] The positive electrode active material lithium cobalt oxide, the conductive agent acetylene black SuperP and the polyvinylidene fluoride PVDF binder are mixed evenly in a mass ratio of 97:1.5:1.5, and evenly dispersed in 1-methyl-2-pyrrolidone NMP to form a uniform black slurry. The mixed slurry is coated on both sides of the aluminum foil current collector, and then baked, rolled and cut into pieces to obtain the positive electrode sheet.

[0104] 2. Preparation of negative electrode sheet

[0105] The negative electrode active material graphite, the conductive agent acetylene black Super P, the thickener CMC and the binder SBR are mixed evenly in a mass ratio of 94:2:1.2:2.8, and evenly dispersed in deionized water to form a uniform black slurry. The mixed slurry is coated on both sides of the aluminum foil current collector, and then baked, rolled and cut into pieces to obtain the negative electrode sheet.

[0106] 3. Preparation of electrolyte

[0107] a. Mix ethylene carbonate EC, propylene carbonate PC, propyl propionate PP and diethyl carbonate DEC in a mass ratio of 10:20:40:30 to form a mixed solvent, remove water with a molecular sieve, set aside, and add 1M LiPF 6 Mix well

[0108] b. Add additives to the colorless transparent liquid obtained in step a (the types and amounts of additives are shown in Table 1) to obtain the electrolytes of the embodiments and comparative examples.

[0109] 4. Battery production

[0110] The prepared positive electrode sheet, separator, and negative electrode sheet are stacked in order, so that the separator is in the middle of the positive and negative electrode sheets, and the bare battery cell is obtained by winding and welding the tabs, and the bare battery cell is placed in an aluminum-plastic film for packaging to obtain a battery. The positive electrode sheet, separator, and negative electrode sheet are stacked in order, so that the separator is in the middle of the positive and negative electrodes to play an isolation role, and then wound to obtain a bare battery cell. The bare battery cell is placed in an outer packaging bag, dried at high temperature until the water content of the electrode sheet is less than 100ppm, and the electrolyte prepared in each embodiment and comparative example is injected. After vacuum packaging, standing, formation, shaping and other processes, the preparation of the lithium-ion battery of each embodiment and comparative example is completed, and the full charge voltage of each embodiment and comparative example is set according to Table 1.

[0111] 2. Battery performance test

[0112] Each embodiment and comparative example was subjected to the following performance tests:

[0113] 1. 45℃ cycle test:

[0114] The test method is: in a constant temperature box at 45±2℃, the lithium-ion battery is charged at a constant current and constant voltage of 1C to the full charge voltage corresponding to each embodiment and comparative example, the cut-off current is 0.05C, and then discharged at 1C to 3V, and multiple charge and discharge cycles are performed under the above conditions, and the capacity retention rate of the battery after 600 cycles is calculated, with 5 batteries in each group.

[0115] Capacity retention rate (%) = discharge capacity corresponding to the number of cycles (mAh) / discharge capacity of the third cycle (mAh) * 100%.

[0116] The average value of the capacity retention rate of each group of 5 batteries after different cycles is recorded in Table 2.

[0117] 2. K value test:

[0118] The test method is: in a constant temperature box at 25±2℃, the lithium-ion battery is charged at a constant current and constant voltage of 1C to the full charge voltage corresponding to each embodiment and comparative example, the cut-off current is 0.05C, and the full charge voltage of the test battery is recorded as V1; the fully charged battery is left to stand at room temperature for 100h, and the test battery voltage is recorded as V2.

[0119] K = (V1-V2) / 100.

[0120] The average values ​​of 5 batteries in each group are recorded in Table 2.

[0121] 3. 85℃ storage performance test:

[0122] The test method is: charge the lithium-ion battery at 1C constant current and constant voltage in a 25±2℃ constant temperature box to the full charge voltage corresponding to each embodiment and comparative example, with a cut-off current of 0.05C, and test the thickness of the lithium-ion battery (this is the thickness before storage). Place the fully charged cell / battery at (85±2)℃ for 18 hours in an open circuit, store it for 18 hours, and place it in an open circuit for 2 hours at room temperature, test the thickness after storage, and calculate the thickness expansion rate of the lithium-ion battery:

[0123] Thickness expansion ratio = [(thickness after storage - thickness before storage) / thickness before storage] x 100%.

[0124] The average values ​​of 5 batteries in each group are recorded in Table 2.

[0125] Table 1 Formula and relationship

[0126]

[0127]

[0128]

[0129] Table 2 Test results

[0130]

[0131]

[0132] Combining the data of Table 1 and Table 2, it can be seen that by comparing the data of Examples 1 to 5, as well as Comparative Examples 3, 7 and 8, as the content of the polynitrile additive increases, the high temperature storage performance of the battery is significantly improved, but when the content of the polynitrile additive is too large, the K value of the battery increases, resulting in poor high temperature cycle performance. This is because pentaether nitrile is a deterioration factor for copper precipitation, and the more content, the more obvious the deterioration. When 0.5≤A≤4 is satisfied, the high temperature storage performance of the battery can be better improved.

[0133] By comparing the data of Example 3, Example 6 to Example 9, and Comparative Example 2, Comparative Example 9 and Comparative Example 10, it is found that the storage performance of the battery is slightly improved when the content of the dinitrile additive increases, and the high temperature storage performance of the battery can be improved when 0.5≤B≤4 is satisfied. Further combined with the data of Comparative Examples 22 to 24, it can be seen that when the content of the dinitrile compound is too low relative to the polynitrile compound, the K value increases abnormally, and the high temperature cycle deteriorates due to the increase in positive electrode impedance. When the content of the dinitrile additive relative to the polynitrile additive satisfies B / A≥0.5, the abnormal K value and high temperature storage performance of the battery can be improved.

[0134] By comparing the data of Example 3, Example 10 to Example 13, and Comparative Example 1, Comparative Example 11 and Comparative Example 12, it is found that when the sulfur-containing additive of structural formula 1 is added and the content of the sulfur-containing additive is increased, the high temperature cycle performance and storage performance of the battery are improved, but the content of the sulfur-containing additive should not be too high, otherwise the electrolyte viscosity will increase and the cycle performance will deteriorate. When 0.5≤C≤4 is met, the additive cost and battery performance can be better balanced. Further combined with the data of Comparative Examples 19 to 21, it can be seen that when the content of the sulfur-containing additive and the nitrile additive meets C / (A+B)≥0.125, the deterioration of the positive electrode impedance caused by the nitrile additive can be better improved, thereby improving the high temperature cycle performance.

[0135] By comparing the data of Example 13, Example 14 to Example 17, and Comparative Example 17 and Comparative Example 18, it is found that when the full charge voltage of the battery increases, the high temperature storage and high temperature cycle performance of the battery deteriorate. When the full charge voltage of the battery decreases, the content of polynitrile additives in the system is relatively excessive, affecting the K value of the battery and deteriorating the high temperature cycle performance. The higher the full charge voltage, the more nitrile additives are needed to improve the high voltage performance of the battery. Further combined with the data of Comparative Examples 24 to 30, it can be seen that nitrile additives have a higher film-forming impedance at the positive electrode, which easily leads to increased polarization on the positive electrode side at high voltage, deteriorating the high temperature cycle performance, so the content of nitrile additives needs to maintain a certain proportional relationship with the full charge voltage of the battery, in order to better balance the high temperature storage and cycle performance of nitrile additives. Further combined with Example 1 and Example 6, when (A+B) / X≥0.556 is satisfied, the above effect can be achieved.

[0136] Although the present application has been described with reference to preferred embodiments, it will be appreciated by those skilled in the art that various changes may be made and equivalents may be substituted for its elements without departing from the scope of the present application. In addition, many modifications may be made to adapt specific situations or materials to the teachings of the present application without departing from the substantive scope of the present application. Therefore, the present application is not intended to be limited to the specific embodiments disclosed as the best mode of implementing the present application contemplated, but the present application will include all embodiments falling within the scope of the appended claims.

[0137] All ranges disclosed in this application are inclusive of the endpoints, and the endpoints are combinable with each other.

[0138] The embodiments of the present application have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The selection of terms used herein is intended to best explain the principles of the embodiments, practical applications, or improvements to the technology in the market, or to enable other persons of ordinary skill in the art to understand the embodiments disclosed herein.

Claims

1. An electrolyte, characterized in that: Applied to a high voltage electrochemical device, the electrolyte comprises an organic solvent, a lithium salt and an additive, the additive comprises a polynitrile additive and a dinitrile additive, the polynitrile additive comprises at least 3 cyano groups; and the additive and the high voltage electrochemical device satisfy the following relationship: (A+B) / X≥0.5 In the relationship, the mass percentage of the polynitrile additive in the electrolyte is A%, the mass percentage of the dinitrile additive in the electrolyte is B%, and the full charge voltage of the high voltage electrochemical device is XV.

2. The electrolyte according to claim 1, characterized in that The additive and the high voltage electrochemical device satisfy the following condition: (A+B) / X≥0.

556.

3. The electrolyte according to claim 1 or 2, characterized in that 0.5≤A≤4; and / or, 0.5≤B≤4; and / or, 4≤X≤4.58; and / or, B / A≥0.

5.

4. The electrolyte according to any one of claims 1 or 2, characterized in that The polynitrile additive includes at least one of 1,2,3,4,5-penta-oxo-(2-cyanoethyl) and 1,3,6-hexanetrinitrile.

5. The electrolyte according to claim 1 or 2, characterized in that: The dinitrile additive includes at least one of succinonitrile and adiponitrile.

6. The electrolyte according to claim 1 or 2, characterized in that The additives also include sulfur-containing additives, and the structural formula of the sulfur-containing additives is as follows: In the structural formula 1, n is 0 or 1, and X is selected from R1, R2 and R3 are each independently selected from H, At least one of the following, and R1, R2 and R3 are not selected from H at the same time, and at least one sulfur atom is contained in X, R1, R2 and R3.

7. The electrolyte according to claim 6, characterized in that The mass percentage of the sulfur-containing additive in the electrolyte is C%, 0.5≤C≤4; and / or, C / (A+B)≥0.

125.

8. The electrolyte according to claim 6, characterized in that The sulfur-containing additive includes at least one of the following compounds:

9. A high voltage electrochemical device, characterized in that: The electrolyte comprising the electrolyte according to any one of claims 1 to 8.

10. An electronic device, characterized in that: An electrochemical device comprising the electrochemical device of claim 9.

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