Electrolyte and electrochemical device

Abstract

The invention relates to the field of batteries, in particular to an electrolyte and an electrochemical device. The electrolyte comprises an organic solvent, an electrolyte and an electrolyte additive, and the electrolyte additive comprises a nitrile additive A and a nitrile additive B; the nitrile additive A comprises at least one of acetonitrile or butanedinitrile; the nitrile additive B comprises at least one of compounds shown as a formula I, # imgabs0, and R1 and R2 are independently and optionally selected from hydrogen, a straight chain or branched chain cyano group with the carbon atom number of 1-10, a straight chain or branched chain alkoxy group with the carbon atom number of 1-10, and a straight chain or branched chain halogenated hydrocarbon with the carbon atom number of 1-5. By combining the nitrile additive A and the nitrile additive B, the high-temperature storage performance and the cycle performance of the battery can be effectively improved at the same time.

Description

Technical Field

[0001] The present invention relates to the field of batteries, and in particular to an electrolyte and an electrochemical device. Background Art

[0002] Lithium-ion batteries are a commonly used secondary battery. Since their introduction, they have gradually dominated the markets of digital products, power batteries, and large-scale energy storage with their advantages such as long life and good cycle stability. With the rapid development of smart technology, electric transportation, and renewable energy, the demand for high performance of lithium-ion batteries has also increased sharply.

[0003] Lithium-ion batteries are mainly composed of positive electrodes, negative electrodes, separators, and electrolytes. Lithium ions are transported in the electrolyte to achieve intercalation and deintercalation between the positive and negative electrodes, thereby realizing the charge and discharge process of the battery. Therefore, the electrolyte is the most important part of lithium-ion batteries, and it is also closely related to the performance of lithium-ion batteries. At present, the electrolytes commonly used in lithium-ion batteries are usually composed of carbonate organic solvents and lithium hexafluorophosphate. This electrolyte will decompose and produce hydrofluoric acid (HF) during the high-temperature storage of lithium-ion batteries due to the presence of trace water. Hydrofluoric acid will destroy the positive electrode material, resulting in the deterioration of the high-temperature storage performance of lithium-ion batteries.

[0004] At present, the high temperature storage performance of lithium-ion batteries is usually improved by adding acetonitrile (CN) or succinonitrile (SN) to the electrolyte. However, when the amount of acetonitrile or succinonitrile added is too small, the improvement effect on the high temperature storage performance of the battery is not obvious, and when the amount of acetonitrile or succinonitrile added is too large, it will cause serious damage to the negative electrode, resulting in poor cycle performance of the battery. That is, this method has certain limitations for improving the high temperature storage performance of lithium-ion batteries.

[0005] Therefore, developing an electrolyte that can improve the high-temperature storage performance of the battery without deteriorating the battery's cycle performance remains an urgent problem to be solved. Summary of the invention

[0006] Based on this, it is necessary to provide an electrolyte that takes into account both the high temperature storage performance and the cycle performance of the battery.

[0007] Furthermore, an electrochemical device is provided, which has good high-temperature storage performance and cycle performance.

[0008] An electrolyte comprises an organic solvent, an electrolyte and an electrolyte additive, wherein the electrolyte additive comprises a nitrile additive A and a nitrile additive B;

[0009] The nitrile additive A includes at least one of acetonitrile or succinonitrile;

[0010] The nitrile additive B comprises at least one of the compounds shown in Formula I,

[0011]

[0012] Among them, R1 and R2 are each independently selected from hydrogen, a straight chain or branched chain cyano group having 1 to 10 carbon atoms, a straight chain or branched chain alkoxy group having 1 to 10 carbon atoms, and a straight chain or branched chain halogenated hydrocarbon having 1 to 5 carbon atoms.

[0013] In one embodiment, the nitrile additive B is selected from B1-B n At least one of the compounds shown:

[0014]

[0015] In one embodiment, the electrolyte additive further includes at least one of a carbonate additive, a metal salt additive and a sulfur-containing additive;

[0016] The carbonate additive includes at least one of vinylene carbonate and fluoroethylene carbonate;

[0017] The metal salt additive includes at least one of lithium bis(fluorosulfonyl)imide, lithium difluorophosphate, lithium bis(oxalatoborate), and lithium difluorobis(oxalatophosphate);

[0018] The sulfur-containing additive includes at least one of 1,3-propylene sultone, vinyl sulfate, and methylene methane disulfonate.

[0019] In one embodiment, based on the total mass of the electrolyte being 100%, the electrolyte additive includes the following components by mass fraction:

[0020] Nitrile additive A 0.2-1.5%, nitrile additive B 0.3-1%, carbonate additive 1-10%, metal salt additive 0.5-1.0%, sulfur-containing additive 0-2.0%.

[0021] In one embodiment, the concentration of electrolyte in the electrolyte is 1.1-1.5 mol / L.

[0022] In one embodiment, the electrolyte includes lithium hexafluorophosphate.

[0023] In one embodiment, the organic solvent includes two or three of ethylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0024] In one embodiment, based on the total volume of the organic solvent being 100%, the organic solvent comprises the following components by volume percentage:

[0025] 15% - 35% ethylene carbonate, 10% - 30% dimethyl carbonate, 0% - 20% diethyl carbonate, and 20% - 50% ethyl methyl carbonate.

[0026] An electrochemical device, which includes a positive electrode plate, a negative electrode plate, a separator disposed between the positive electrode plate and the negative electrode plate, and an electrolyte as described in any one of the above embodiments.

[0027] In one of the embodiments, the positive electrode plate includes a positive electrode current collector and a positive electrode film coated on the surface of the positive electrode current collector. The positive electrode film includes a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder. The positive electrode active material includes a nickel cobalt manganese ternary material (Li(Ni x Co y Mn z )O2) (where 0.5 ≤ x < 0.8, 0 < y ≤ 0.3, 0 < z ≤ 0.3 and x + y + z = 1).

[0028] In summary, the present invention has the following beneficial effects:

[0029] 1. For the electrolyte provided in the embodiments of the present invention, the cyano groups in nitrile additive A and nitrile additive B can both react with hydrofluoric acid and water, thereby enhancing the stability of the electrolyte itself. Moreover, nitrile additive A has low reactivity and a slow increase in high-temperature storage impedance, which can effectively improve the high-temperature storage performance of the battery. Nitrile additive B has at least two more carbon atoms than nitrile additive A, and its molecular spatial structure is curved. The cyano groups at both ends can complex with transition metal ions, so it has minimal damage to the negative electrode and can effectively improve the cycle stability of the battery. By combining nitrile additive A and nitrile additive B, the high-temperature storage performance of the battery can be effectively improved while ensuring its cycle performance, thus balancing the high-temperature storage performance and cycle performance of the battery.

[0030] 2. For the electrochemical device provided in the embodiments of the present invention, by using an electrolyte that can balance the high-temperature storage performance and cycle performance of the battery, the electrochemical device simultaneously has good high-temperature storage performance and cycle performance. Detailed Embodiments

[0031] The following embodiments are provided to better understand the present invention further. They are not limited to the best embodiment, and do not constitute a limitation to the content and protection scope of the present invention. Any product that is the same as or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with other prior art features falls within the protection scope of the present invention.

[0032] For those not specifying specific experimental procedures or conditions in the examples, operations or conditions of conventional experimental procedures described in the literature in this field can be followed. For reagents or instruments without indicating the manufacturer, they are all conventional reagent products that can be obtained through commercial purchase.

[0033] An embodiment of the present invention provides an electrochemical device that simultaneously has good high-temperature storage performance and cycling performance.

[0034] In one embodiment, the electrochemical device is a lithium-ion battery. In other embodiments, the electrochemical device can also be a metal-ion secondary battery such as a sodium-ion battery, an aluminum-ion battery, a magnesium-ion battery, or a zinc-ion battery.

[0035] In one embodiment, the electrochemical device includes a positive electrode plate, a negative electrode plate, a separator disposed between the positive and negative electrodes, and an electrolyte.

[0036] Among them, the positive electrode plate includes a positive current collector and a positive electrode film disposed on the surface of the positive current collector. The positive electrode film includes a positive active material, a positive conductive agent, and a positive binder.

[0037] In one embodiment, the positive current collector is aluminum foil, the positive active material is a nickel-cobalt-manganese ternary material (Li(Ni x Co y Mn z )O2) (where 0.5 ≤ x < 0.8, 0 < y ≤ 0.3, 0 < z ≤ 0.3 and x + y + z = 1), the positive conductive agent is acetylene black, and the positive binder is polyvinylidene fluoride (PVDF). In other embodiments, the positive current collector can also be selected from nickel foil, the positive active material can also be other metal oxides such as lithium cobaltate, lithium nickelate, or lithium manganate, the positive conductive agent can also be at least one of carbon black, graphene, or carbon fiber, and the positive adhesive can be polytetrafluoroethylene, polyvinyl fluoride, polyethylene, polypropylene, etc. That is, the positive current collector, the positive active material, the positive conductive agent, and the positive adhesive can all be selected from other commonly used materials in this field that meet the battery performance, and their specific materials are not limited.

[0038] In one embodiment, the negative electrode plate includes a negative current collector and a negative electrode film disposed on the surface of the negative current collector. The negative electrode film includes a negative active material, a negative conductive agent, and a negative binder.

[0039] In one embodiment, the negative electrode current collector is copper foil, the negative electrode active material is graphite, the negative electrode conductive agent is acetylene black, and the negative electrode adhesive is a mixture of sodium carboxymethyl cellulose (CMC) and polystyrene butadiene copolymer (SBR). In other embodiments, the negative electrode current collector may also be at least one of nickel foam or copper foam, the negative electrode active material may also be at least one of hard carbon or soft carbon, the negative electrode conductive agent may also be at least one of carbon black, acetylene black or carbon fiber, and the negative electrode adhesive may also be at least one of styrene butadiene rubber, polyvinyl chloride, and polyvinyl fluoride. It is understood that the negative electrode active material, the negative electrode conductive agent, and the negative electrode adhesive may all be selected from other materials commonly used in the art that meet the performance requirements, and the specific materials are not limited.

[0040] In one embodiment, the diaphragm is selected from a polypropylene diaphragm. In other embodiments, the diaphragm may also be a polyethylene diaphragm or a polyvinylidene fluoride diaphragm. The specific material is not limited, and the diaphragm material commonly used in the art may be selected.

[0041] In one embodiment, the electrolyte includes an organic solvent, an electrolyte, and an electrolyte additive.

[0042] Specifically, the electrolyte additive includes a nitrile additive A and a nitrile additive B; the nitrile additive A includes at least one of acetonitrile (AN) or succinonitrile (SN); the nitrile additive B includes at least one of the compounds shown in formula I,

[0043]

[0044] Among them, R1 and R2 are each independently selected from hydrogen, a straight chain or branched chain cyano group having 1 to 10 carbon atoms, a straight chain or branched chain alkoxy group having 1 to 10 carbon atoms, and a straight chain or branched chain halogenated hydrocarbon having 1 to 5 carbon atoms.

[0045] Nitrile additives A and nitrile additives B have both similarities and differences. On the one hand, both nitrile additives A and nitrile additives B have cyano groups, which can react with hydrofluoric acid and water to improve the stability of the electrolyte itself, and cyano groups can participate in the formation of the positive electrode CEI film, balance the metal charge on the positive electrode surface, and reduce the dissolution of the positive electrode transition metal, thereby improving the stability of the positive electrode material. On the other hand, the electron cloud distribution of the cyano group of nitrile additive B is denser than that of nitrile additive A, so the reaction activity is higher than that of nitrile additive A, so that the impedance increases faster than nitrile additive A during high-temperature storage, but at the same time, nitrile additive B has at least two more carbon atoms than nitrile additive A, and the molecular spatial structure is curved. The cyano groups at both ends can be complexed with transition metal ions, so the damage to the negative electrode is minimal, which can effectively improve the battery cycle stability. By combining nitrile additives A and nitrile additives B, the cycle performance of the battery can be guaranteed while ensuring the improvement of the high-temperature storage performance of the battery, and the high-temperature storage performance and cycle performance of the battery can be balanced.

[0046] In one embodiment, the nitrile additive B is selected from the compounds shown in B1:

[0047]

[0048] In other embodiments, the nitrile additive B may also be selected from at least one of the compounds shown in B1-B5:

[0049]

[0050] It should be noted that the performance characterization data of the compounds shown in B1-B5 are shown in Table 1.

[0051] Table 1. Performance characterization data of compounds shown in B1-B5

[0052]

[0053] Among them, the compound shown in B1 has 2-4 more carbon atoms than cyanocyanide or succinonitrile, and its spatial structure is curved. The cyano groups at both ends can be complexed with the positive electrode, so it is less destructive to the negative electrode. When used in combination with cyanocyanide or succinonitrile in a straight chain, it can reduce the destructiveness to the negative electrode while ensuring the improvement of the high-temperature storage performance of the battery. The compounds shown in B2, B4, and B5 have 6-9 carbon atoms, which can be complexed at both ends, and there are F atoms on the side chains, which can form a more stable interface film, thereby ensuring the cycle performance of the battery. The compound shown in B3 has three cyano groups, so it has a stronger ability to complex with the positive electrode. When it is used in combination with the nitrile additive A with small steric hindrance, the two can cross-complex with the positive electrode, so it can also ensure the improvement of the high-temperature storage performance of the battery while reducing the destructiveness to the negative electrode.

[0054] In one embodiment, the electrolyte additive further includes a carbonate additive, a metal salt additive and a sulfur-containing additive.

[0055] Among them, carbonate additives have the function of forming a dense solid electrolyte interface film (SEI film) at the negative electrode, with high film formation potential and low film formation impedance. Metal salt additives can increase the number of metal ion migration, participate in the film formation of the positive and negative electrodes, and enhance the stability of the interface film. Sulfur-containing additives can assist in film formation at the positive and negative electrodes, and have high thermal stability. By further adding carbonate additives, metal salt additives and sulfur-containing additives to the electrolyte additives, the generated SEI film can be made denser, while inhibiting the dissolution of positive electrode metal ions, weakening the damage of metal ions to the negative electrode SEI film, and improving the thermal stability of the formed SEI film, thereby improving the high temperature storage performance and cycle performance of the battery.

[0056] In other embodiments, the electrolyte additive may not include at least one of the carbonate additive, the metal salt additive and the sulfur-containing additive.

[0057] In one embodiment, the carbonate additive is selected from vinylene carbonate (VC). In other embodiments, the carbonate additive can also be selected from fluoroethylene carbonate (FEC), or VC and FEC can be selected at the same time. Among them, FEC has a good film-forming effect. When it is selected as a carbonate additive, a SEI film rich in lithium fluoride (LiF) can be generated on the silicon surface. The higher the inorganic content, the mechanical strength of the SEI film can be significantly improved, and the SEI film with good mechanical strength can effectively improve the cycle stability of the silicon negative electrode. At the same time, FEC will also enter the silicon particles and generate nano-scale LiF in the electrochemical process, which effectively hinders the crystalline Li 15 The formation of Si4 will generate a lot of amorphous Li 15 Si4, amorphous Li 15 Si4 has a small internal resistance and an insignificant volume effect, which reduces the "lithium eating" phenomenon inside the silicon particles and reduces the loss of active lithium.

[0058] In one embodiment, the metal salt additive is selected from lithium bis(fluorosulfonyl)imide (LiFSI). In other embodiments, the metal salt additive may also be selected from lithium difluorophosphate (LiPO2F2), lithium bis(oxalatoborate) L i BOB ), lithium difluorobis(oxalate) phosphate (LiODFP) or a mixture of the above salts.

[0059] In one embodiment, the sulfur-containing additive is selected from polystyrene (PS). In other embodiments, the sulfur-containing additive may also be selected from 1,3-propylene sultone (PST), dithiothreitol (DTD), methylene disulfonate (MMDS) or a mixture of the above sulfur-containing substances.

[0060] In one embodiment, based on the total mass of the electrolyte as 100%, the electrolyte additive includes the following components by mass fraction: nitrile additive A 0.2-1.5%, nitrile additive B 0.3-1%, carbonate additive 1-10%, metal salt additive 0.5-1.0%, sulfur-containing additive 0-2.0%.

[0061] By limiting the content of each additive in the electrolyte additive, the synergistic effect of various additives can be optimized, thereby further ensuring the effectiveness of the electrolyte.

[0062] In one embodiment, in the electrolyte solution, the concentration of the electrolyte is 1.1-1.5 mol / L.

[0063] In one embodiment, the electrolyte is selected from lithium hexafluorophosphate.

[0064] In one embodiment, the organic solvent includes ethylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. Taking the total volume of the organic solvent as 100%, the organic solvent includes the following components by volume percentage: 15% to 35% of ethylene carbonate, 10% to 30% of dimethyl carbonate, 0% to 20% of diethyl carbonate, and 20% to 50% of ethyl methyl carbonate.

[0065] In other embodiments, the organic solvent may also include only two or three of ethylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0066] In one embodiment, the electrolyte is prepared according to the following steps: under an argon atmosphere, a carbonate additive, a metal salt additive, a sulfur-containing additive, a nitrile additive A and a nitrile additive B are added to an organic solvent in a formulated amount, and then an electrolyte is added, and the mixture is stirred and mixed at 10° C. to obtain an electrolyte.

[0067] The electrochemical device involved in the embodiment of the present invention uses an electrolyte that can improve both the high-temperature storage performance and the cycle performance of the battery, thereby having both good high-temperature storage performance and good cycle performance.

[0068] Example 1

[0069] This embodiment provides an electrolyte, which includes an organic solvent, an electrolyte and an electrolyte additive, wherein the electrolyte additive includes a nitrile additive A, a nitrile additive B, a carbonate additive, a metal salt additive and a sulfur-containing additive.

[0070] The nitrile additive A is SN, and its mass fraction in the electrolyte is 0.3%;

[0071] The nitrile additive B is a compound represented by formula B1, and its mass fraction in the electrolyte is 0.3%.

[0072]

[0073] The carbonate additive is fluoroethylene carbonate FEC, with a mass fraction of 5% in the electrolyte;

[0074] The metal salt additive is lithium bis(oxalatoborate) LiBOB, with a mass fraction of 1.0% in the electrolyte;

[0075] The electrolyte is lithium hexafluorophosphate, and its concentration in the electrolyte is 1.2 mol / L;

[0076] The organic solvent comprises ethylene carbonate, diethyl carbonate and ethyl methyl carbonate. Taking the total volume of the organic solvent as 100%, the volume fraction of ethylene carbonate is 30%, the volume fraction of dimethyl carbonate is 50% and the volume fraction of ethyl methyl carbonate is 20%.

[0077] Example 2

[0078] This embodiment provides an electrolyte, which includes an organic solvent, an electrolyte and an electrolyte additive, wherein the electrolyte additive includes a nitrile additive A, a nitrile additive B, a carbonate additive, a metal salt additive and a sulfur-containing additive.

[0079] The nitrile additive A is AN, and its mass fraction in the electrolyte is 0.2%;

[0080] The nitrile additive B is a compound represented by formula B2, and its mass fraction in the electrolyte is 0.3%.

[0081]

[0082] The carbonate additive is fluoroethylene carbonate FEC, with a mass fraction of 5% in the electrolyte;

[0083] The metal salt additive is lithium bis(oxalatoborate) LiBOB, with a mass fraction of 1.0% in the electrolyte;

[0084] The electrolyte is lithium hexafluorophosphate, and its concentration in the electrolyte is 1.2 mol / L;

[0085] The organic solvent comprises ethylene carbonate, diethyl carbonate and ethyl methyl carbonate. Taking the total volume of the organic solvent as 100%, the volume fraction of ethylene carbonate is 30%, the volume fraction of dimethyl carbonate is 50% and the volume fraction of ethyl methyl carbonate is 20%.

[0086] Example 3

[0087] This embodiment provides an electrolyte, which includes an organic solvent, an electrolyte and an electrolyte additive, wherein the electrolyte additive includes a nitrile additive A, a nitrile additive B, a carbonate additive, a metal salt additive and a sulfur-containing additive.

[0088] The nitrile additive A is SN, and its mass fraction in the electrolyte is 0.3%;

[0089] The nitrile additive B is a compound represented by formula B2, and its mass fraction in the electrolyte is 0.3%.

[0090]

[0091] The carbonate additive is fluoroethylene carbonate FEC, with a mass fraction of 5% in the electrolyte;

[0092] The metal salt additive is lithium bis(oxalatoborate) LiBOB, with a mass fraction of 1.0% in the electrolyte;

[0093] The electrolyte is lithium hexafluorophosphate, and its concentration in the electrolyte is 1.2 mol / L;

[0094] The organic solvent comprises ethylene carbonate, diethyl carbonate and ethyl methyl carbonate. Taking the total volume of the organic solvent as 100%, the volume fraction of ethylene carbonate is 30%, the volume fraction of dimethyl carbonate is 50% and the volume fraction of ethyl methyl carbonate is 20%.

[0095] Example 4

[0096] This embodiment provides an electrolyte, which includes an organic solvent, an electrolyte and an electrolyte additive, wherein the electrolyte additive includes a nitrile additive A, a nitrile additive B, a carbonate additive, a metal salt additive and a sulfur-containing additive.

[0097] The nitrile additive A is SN, and its mass fraction in the electrolyte is 0.3%;

[0098] The nitrile additive B is a compound represented by formula B3, and its mass fraction in the electrolyte is 0.3%.

[0099]

[0100] The carbonate additive is fluoroethylene carbonate FEC, with a mass fraction of 6% in the electrolyte;

[0101] The metal salt additive is lithium bis(oxalatoborate) LiBOB, with a mass fraction of 1.0% in the electrolyte;

[0102] The electrolyte is lithium hexafluorophosphate, and its concentration in the electrolyte is 1.2 mol / L;

[0103] The organic solvent comprises ethylene carbonate, diethyl carbonate and ethyl methyl carbonate. Taking the total volume of the organic solvent as 100%, the volume fraction of ethylene carbonate is 30%, the volume fraction of dimethyl carbonate is 50% and the volume fraction of ethyl methyl carbonate is 20%.

[0104] Example 5

[0105] This embodiment provides an electrolyte, which includes an organic solvent, an electrolyte and an electrolyte additive, wherein the electrolyte additive includes a nitrile additive A, a nitrile additive B, a carbonate additive, a metal salt additive and a sulfur-containing additive.

[0106] The nitrile additive A is SN, and its mass fraction in the electrolyte is 0.3%;

[0107] The nitrile additive B is a compound represented by formula B4, and its mass fraction in the electrolyte is 0.3%.

[0108]

[0109] The carbonate additive is fluoroethylene carbonate FEC, with a mass fraction of 5% in the electrolyte;

[0110] The metal salt additive is lithium bis(oxalatoborate) LiBOB, with a mass fraction of 1.0% in the electrolyte;

[0111] The electrolyte is lithium hexafluorophosphate, and its concentration in the electrolyte is 1.2 mol / L;

[0112] The organic solvent comprises ethylene carbonate, diethyl carbonate and ethyl methyl carbonate. Taking the total volume of the organic solvent as 100%, the volume fraction of ethylene carbonate is 30%, the volume fraction of dimethyl carbonate is 50% and the volume fraction of ethyl methyl carbonate is 20%.

[0113] Example 6

[0114] This embodiment provides an electrolyte, which includes an organic solvent, an electrolyte and an electrolyte additive, wherein the electrolyte additive includes a nitrile additive A, a nitrile additive B, a carbonate additive, a metal salt additive and a sulfur-containing additive.

[0115] The nitrile additive A is SN, and its mass fraction in the electrolyte is 0.3%;

[0116] The nitrile additive B is a compound represented by formula B5, and its mass fraction in the electrolyte is 0.3%.

[0117]

[0118] The carbonate additive is fluoroethylene carbonate FEC, with a mass fraction of 5% in the electrolyte;

[0119] The metal salt additive is lithium bis(oxalatoborate) LiBOB, with a mass fraction of 1.0% in the electrolyte;

[0120] The electrolyte is lithium hexafluorophosphate, and its concentration in the electrolyte is 1.2 mol / L;

[0121] The organic solvent comprises ethylene carbonate, diethyl carbonate and ethyl methyl carbonate. Taking the total volume of the organic solvent as 100%, the volume fraction of ethylene carbonate is 30%, the volume fraction of dimethyl carbonate is 50% and the volume fraction of ethyl methyl carbonate is 20%.

[0122] Example 7

[0123] This embodiment provides an electrolyte, which includes an organic solvent, an electrolyte and an electrolyte additive, wherein the electrolyte additive includes a nitrile additive A, a nitrile additive B, a carbonate additive, a metal salt additive and a sulfur-containing additive.

[0124] The nitrile additive A is SN, and its mass fraction in the electrolyte is 0.3%;

[0125] The nitrile additive B is a mixture of a compound represented by formula B1 and a compound represented by formula B3, wherein the mass fraction of the compound represented by B1 in the electrolyte is 0.3%, and the mass fraction of the compound represented by B3 in the electrolyte is 0.2%.

[0126] The carbonate additive is fluoroethylene carbonate FEC, with a mass fraction of 5% in the electrolyte;

[0127] The metal salt additive is lithium bis(oxalatoborate) LiBOB, with a mass fraction of 1.0% in the electrolyte;

[0128] The electrolyte is lithium hexafluorophosphate, and its concentration in the electrolyte is 1.2 mol / L;

[0129] The organic solvent comprises ethylene carbonate, diethyl carbonate and ethyl methyl carbonate. Taking the total volume of the organic solvent as 100%, the volume fraction of ethylene carbonate is 30%, the volume fraction of dimethyl carbonate is 50% and the volume fraction of ethyl methyl carbonate is 20%.

[0130] Comparative Example 1

[0131] This comparative example provides an electrolyte, which is different from Example 1 only in that, in this comparative example, the electrolyte additives do not include nitrile additive A and nitrile additive B, and based on the total mass of the electrolyte, the mass percentage of lithium dihydrogen phosphate LiBOB is 1.3%, and the mass percentage of fluoroethylene carbonate FEC is 5.3%.

[0132] Comparative Example 2

[0133] This comparative example provides an electrolyte, which is different from Example 1 only in that, in this comparative example, the nitrile additive B is not included, and the mass percentage of lithium difluorophosphate LiBOB is adjusted to 1.3% based on the total mass of the electrolyte being 100%.

[0134] Test Method

[0135] The electrolytes provided in the embodiments and comparative examples were applied to lithium-ion batteries and the performance of the lithium-ion batteries was tested.

[0136] The lithium-ion battery used for the test is prepared according to the following specific steps: prepare the negative electrode material graphite, conductive agent acetylene black and binder CMC and SBR into a slurry in a mass percentage of 95:1:2:2, apply it on the copper foil current collector, vacuum dry it to obtain the negative electrode plate; prepare the positive electrode material NCM523, conductive agent acetylene black and binder PVDF into a slurry in a mass ratio of 94:4:2, apply it on the aluminum foil current collector, vacuum dry it to obtain the positive electrode plate. The positive electrode plate, negative electrode plate, diaphragm and electrolyte prepared in the embodiment or comparative example are assembled into 18650 batteries, and electrochemical tests are carried out using a Xinwei charge and discharge test cabinet, and the HF content of the electrolyte is determined by ice water titration. The specific test principle is as follows:

[0137] (1) Cycle performance test of lithium-ion batteries:

[0138] At 25 / 45℃, the lithium-ion battery is charged at a constant current of 1.0C (nominal capacity) to a voltage of 4.2V, then charged at a constant voltage of 4.2V to a current ≤0.05C, left for 10 minutes, and then discharged at a constant current of 1C to 2.5V. The above is one charge and discharge cycle. The lithium-ion battery is subjected to 600 charge and discharge cycles at 25 / 45℃ under the above conditions.

[0139] The capacity retention rate (%) of a lithium-ion battery after N cycles = (discharge capacity at the Nth cycle / initial discharge capacity) × 100%, where N is the number of cycles of the lithium-ion battery.

[0140] (2) Electrolyte HF content test:

[0141] The electrolyte was stored at 60°C, and the HF contents at 0d and 15d were measured by ice water titration, respectively, and recorded as HF-0d and HF-15d.

[0142] (3) High temperature storage performance test of lithium-ion batteries:

[0143] At 25°C, the lithium-ion battery was charged at a constant current of 1C to a voltage of 4.2V, and then charged at a constant voltage of 4.2V to a current of 0.05C, an initial capacity of C0, and an initial voltage of OCV1. The lithium-ion battery was then placed in a constant temperature box at 60°C and stored for 60 days. The voltage of the lithium-ion battery taken out for testing was OCV2, the capacity remained at C1, and the capacity recovered to C2.

[0144] Capacity retention rate of lithium-ion battery after 60 days storage at 60°C (%) = (C1 / C0) × 100%

[0145] Capacity recovery rate of lithium-ion battery after 60 days storage at 60°C (%) = (C2 / C0) × 100%

[0146] The batteries obtained in the above examples and comparative examples were tested as follows:

[0147] (a) High temperature storage experiment: The batteries obtained in the examples and comparative examples were subjected to three charge and discharge cycle tests at room temperature at a charge and discharge rate of 1C, and then charged to a fully charged state at a rate of 1C. The 1C capacity Q0 was recorded respectively, and the fully charged battery was stored at 60°C for 60 days, and the 1C discharge capacity Q1 of the battery was recorded. The battery was then charged and discharged at a rate of 1C for 3 weeks at room temperature, and the 1C discharge capacity Q2 was recorded. The experimental data such as the battery's high temperature storage capacity retention rate, capacity recovery rate, and OCV change were calculated, and the recorded results are shown in Table 2. The calculation formulas used are as follows: Capacity retention rate (%) = Q1 / Q0×100%; Capacity recovery rate (%) = Q2 / Q0×100%.

[0148] (b) Normal temperature cycle test: The battery was placed at 25°C, and the initial capacity was recorded as A1. The capacity after 3C / 5C cycle to 2000 cycles was recorded as A2. The capacity retention rate of the battery after 600 cycles of high temperature cycle was calculated by the following formula: Cycle capacity retention rate (%) = A2 / A1 × 100%. The recorded results are shown in Table 2.

[0149] (c) High temperature cycle test: The battery was placed at 45°C, and the initial capacity was recorded as A1, and the capacity after 3C / 5C cycle to 2000 cycles was recorded as A2. The capacity retention rate of the battery after 600 cycles of high temperature cycle was calculated by the following formula: Cycle capacity retention rate (%) = A2 / A1×100%. The recorded results are shown in Table 2.

[0150] Table 2. Test results of various embodiments and comparative examples

[0151]

[0152]

[0153] It can be seen from Table 2 that the combined use of nitrile additive A and nitrile additive B can effectively improve the high temperature storage performance of the battery while improving the battery cycle performance.

[0154] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived therefrom are still within the scope of protection of the invention.

Claims

1. An electrolyte, characterized in that: The method comprises an organic solvent, an electrolyte and an electrolyte additive, wherein the electrolyte additive comprises a nitrile additive A and a nitrile additive B; The nitrile additive A includes at least one of acetonitrile or succinonitrile; The nitrile additive B comprises at least one of the compounds shown in Formula I, Among them, R1 and R2 are each independently selected from hydrogen, a straight chain or branched chain cyano group having 1 to 10 carbon atoms, a straight chain or branched chain alkoxy group having 1 to 10 carbon atoms, and a straight chain or branched chain halogenated hydrocarbon having 1 to 5 carbon atoms.

2. The electrolyte according to claim 1, characterized in that The nitrile additive B is selected from B1-B n At least one of the compounds shown:

3. The electrolyte according to claim 1 or 2, characterized in that The electrolyte additive also includes at least one of a carbonate additive, a metal salt additive and a sulfur-containing additive; The carbonate additive includes at least one of vinylene carbonate and fluoroethylene carbonate; The metal salt additive includes at least one of lithium bis(fluorosulfonyl)imide, lithium difluorophosphate, lithium bis(oxalatoborate), and lithium difluorobis(oxalatophosphate); The sulfur-containing additive includes at least one of 1,3-propylene sultone, vinyl sulfate, and methylene methane disulfonate.

4. The electrolyte according to claim 3, characterized in that Taking the total mass of the electrolyte as 100%, the electrolyte additive includes the following components by mass fraction: Nitrile additive A 0.2-1.5%, nitrile additive B 0.3-1%, carbonate additive 1-10%, metal salt additive 0.5-1.0%, sulfur-containing additive 0-2.0%.

5. The electrolyte according to claim 1, characterized in that In the electrolyte, the concentration of the electrolyte is 1.1 to 1.5 mol / L.

6. The electrolyte according to claim 1 or 5, characterized in that The electrolyte includes lithium hexafluorophosphate.

7. The electrolyte according to claim 1, characterized in that The organic solvent includes two or three of ethylene carbonate, dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate.

8. The electrolyte according to claim 7, characterized in that Taking the total volume of the organic solvent as 100%, the organic solvent includes the following components by volume percentage: Ethylene carbonate 15%-35%, dimethyl carbonate 10%-30%, diethyl carbonate 0%-20% and ethyl methyl carbonate 20%-50%.

9. An electrochemical device, characterized in that: The electrochemical device comprises a positive electrode sheet, a negative electrode sheet, a separator disposed between the positive electrode sheet and the negative electrode sheet, and an electrolyte according to any one of claims 1 to 8.

10. The electrochemical device according to claim 9, characterized in that The positive electrode sheet includes a positive electrode current collector and a positive electrode film coated on the surface of the positive electrode current collector. The positive electrode film includes a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder. The positive electrode active material includes a nickel cobalt manganese ternary material (Li(Ni x Co y Mn z )O2) (where 0.5 ≤ x < 0.8, 0 < y ≤ 0.3, 0 < z ≤ 0.3 and x + y + z = 1).

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