Electrochemical and electronic devices

Abstract

The present invention provides an electrochemical device and an electronic device. The electrochemical device includes a positive electrode piece, a negative electrode piece, a separator and an electrolyte, the positive electrode piece includes a positive electrode active material, the positive electrode active material includes Co element, and the electrolyte includes a polynitrile compound. According to the content of Co element in the positive electrode active material, the content of polynitrile compound is adjusted, and further, through the coordination action of polynitrile compound with Co and the combination with other additives, the structural stability of the positive electrode material after the lithium is highly released is significantly improved, the dissolution of Co is inhibited, the oxidation of the positive electrode active material in the electrolyte is reduced, and the related side reactions are inhibited, thereby effectively improving the characteristics of the electrochemical device under high temperature.

Description

[Technical Field]

[0001] This application claims priority from a Chinese patent application filed with the China Patent Office on November 18, 2021, bearing application number 202111372370.4 and titled "Electrochemical Device and Electronic Device," the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to the field of energy storage, and in particular to electrochemical and electronic devices. [Background technology]

[0003] Currently, lithium-ion batteries are widely used in fields such as electric vehicles, consumer electronics, and energy storage devices, and are becoming the mainstream battery in these fields due to their advantages such as high energy density and no memory effect. Among the many well-developed cathode materials currently available, materials such as lithium cobalt oxide containing Co and lithium nickel cobalt manganese oxide have relatively high volumetric energy densities, making them the obvious choice for high-energy-density materials. As the end-of-charge voltage continues to increase in order to further improve energy density, the high-temperature performance of batteries faces severe challenges.

[0004] In view of the above-mentioned problems, further improving the high-temperature characteristics of lithium-ion batteries has become a top priority for lithium-ion battery manufacturers and related parties in the field of energy storage technology. Summary of the Invention

[0005] The present invention provides electrochemical and electronic devices for improving the performance of electrochemical devices at high temperatures.

[0006] The first aspect of the present invention provides an electrochemical device, which includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The positive electrode sheet contains a positive electrode active material, the positive electrode active material contains Co element, the electrolyte contains a polynitrile compound. When the mass fraction of the Co element is a with respect to the total mass of the positive electrode active material, and the mass fraction of the polynitrile compound is b% with respect to the total mass of the electrolyte, the mass fraction b% of the polynitrile compound and the mass fraction a of the Co element satisfy b = 9.7a - 0.07 + C, -1.5 < C ≤ 1.5, 0 < a ≤ 0.65. For example, C may be -1.4, -1.0, -0.5, 0, 0.5, 1.0, 1.5, or any range between them, and a may be 0.001, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, or any range between them. Through in-depth research, the inventors of the present invention have found that by controlling both the mass fraction of the Co element in the positive electrode active material and the mass fraction of the polynitrile compound within the above ranges, the protection of the polynitrile compound against the positive electrode active material can be effectively exerted, the destruction of the polynitrile compound against the negative electrode interface can be avoided, and the characteristics of the electrochemical device at high temperatures can be significantly improved.

[0007] In one embodiment of the present invention, the mass fraction b% of the polynitrile compound with respect to the total mass of the electrolyte satisfies 0 < b ≤ 7. For example, b may be 0.01, 0.5, 1, 2, 3, 4, 5, 6, 7, or any range between them. By controlling the mass fraction of the polynitrile compound within the above range, the stability of the positive electrode structure can be enhanced, and thereby, the characteristics of the electrochemical device at high temperatures can be improved.

[0008] In one embodiment of the present invention, the polynitrile compound includes a dinitrile compound and / or a trinitrile compound, and when the mass fraction of the dinitrile compound is b1% and the mass fraction of the trinitrile compound is b2% relative to the total mass of the electrolyte solution, b1 and b2 satisfy the relationship 0≦b1 / b2≦4, for example, b1 / b2 may be 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, or any range therebetween. Without being limited by any theory, the inventors have found that the high-temperature characteristics of an electrochemical device can be further improved by controlling the mass fractions of the dicarbonitrile compound and the tricarbonitrile compound so that the above relationship is satisfied.

[0009] In one embodiment of the present invention, the dinitrile compound includes at least one of butanedinitrile (SN), hexanedinitrile (AND), 1,2-bis(cyanoethoxy)ethane (DENE), and 1,4-dicyano-2-butene (HEDN), and the trinitrile compound includes at least one of 1,3,6-hexanetricarbonitrile (HTCN) and 1,2,3-tris(2-cyanoethoxy)propane (TCEP). Without being limited to any theory, the inventors have found that the high-temperature properties of electrochemical devices can be further improved by selecting the dicarbonitrile and tricarbonitrile compounds.

[0010] In one embodiment of the present invention, the electrolyte satisfies at least one of the following conditions: (1) The electrolytic solution further contains ethylene carbonate (EC), and when the mass fraction of the ethylene carbonate is c % with respect to the total mass of the electrolytic solution, c satisfies 0.5≦c / b≦20 and 3≦c≦30. For example, c / b may be 0.5, 1.5, 3.5, 5.5, 7.5, 9.5, 11.5, 13.5, 15.5, 17.5, 19.5, 20, or any range therebetween, and c may be 3, 5, 7, 9, 12, 15, 18, 21, 24, 27, 30, or any range therebetween. If the EC mass fraction is too low, the EC will not be able to form a good solid electrolyte interface (SEI) film on the negative electrode and will not be able to prevent the polynitrile compound from damaging the SEI film. If the EC mass fraction is too high, the mass fraction of the polynitrile compound will be too low and will not be able to effectively coordinate the active sites of the positive electrode active material. By controlling the mass fraction of ethylene carbonate in the electrolyte within the above range, the high-temperature characteristics of the electrochemical device can be improved. (2) The electrolyte solution contains lithium difluorophosphate, and when the mass fraction of the lithium difluorophosphate is d% with respect to the total mass of the electrolyte solution, d satisfies 0.01≦d≦1, and for example, d may be 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1.0, or any range therebetween. By controlling the mass fraction of lithium difluorophosphate in the electrolyte solution within the above range, a stable SEI film can be formed on the surface of the negative electrode, damage to the SEI film by Co eluted from the positive electrode can be prevented, and the characteristics of the electrochemical device at high temperatures can be improved. (3) The electrolyte solution contains lithium difluorophosphate, and when the mass fraction of the lithium difluorophosphate is d% with respect to the total mass of the electrolyte solution, d satisfies 0.015≦d / a. By controlling both the mass fractions of lithium difluorophosphate and Co element in the positive electrode active material so as to satisfy the above relational expression, a stable SEI film can be formed on the negative electrode surface, damage to the SEI film by Co eluted from the positive electrode can be prevented, and the characteristics of the electrochemical device at high temperatures can be improved. (4) The electrolyte solution contains at least one of fluoroethylene carbonate (FEC), vinylene carbonate (VC), 1,3-propylene sulfite (PS), ethylene sulfate (DTD), lithium difluoro(oxalato)borate (LiDFOB), and lithium bis(oxalato)borate (LiBOB). By selecting the additives, stable cathode-electrolyte interfaces (CEI) and SEI films are formed on the surfaces of the positive and negative electrodes, stabilizing the positive and negative electrodes and suppressing side reactions between the positive and negative electrodes and the electrolyte, thereby improving the high-temperature characteristics of the electrochemical device.

[0011] Without being limited to any theory, the inventors have found that by controlling the electrolyte solution to satisfy one, two, or a combination of two or more of the above conditions, the properties of the electrochemical device at high temperatures can be further improved.

[0012] In one embodiment of the present invention, the electrolyte solution includes a lithium salt, the lithium salt including at least one of an inorganic lithium salt and an organic lithium salt, and the mass fraction of the lithium salt is 7.5% to 25% relative to the total mass of the electrolyte solution, for example, the mass fraction of the lithium salt may be 7.5%, 10.0%, 12.5%, 15.0%, 17.5%, 20.0%, 22.5%, 25%, or any range therebetween. By selecting such a lithium salt and controlling the mass fraction of the lithium salt within the above range, the ionic conductivity of the electrolyte solution can be improved, thereby improving the high-temperature characteristics of the electrochemical device.

[0013] In one embodiment of the present invention, the lithium salt includes at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide. Without being limited to any theory, the present inventors have found that by selecting the above lithium salt, the properties of the electrochemical device at high temperatures can be further improved.

[0014] In one embodiment of the present invention, the electrolytic solution further includes at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, propylene carbonate, ethyl acetate, ethyl propionate, and propyl propionate. In the present invention, with respect to the total mass of the electrolytic solution, the mass fraction of the above non-aqueous solvent is 10% to 70%, for example, 10%, 20%, 30%, 40%, 50%, 60%, 70%, or any range therebetween. Without being limited to any theory, the inventors have found that by selecting the above solvents, the characteristics of the electrochemical device at high temperatures can be further improved.

[0015] In one embodiment of the present invention, the differential scanning calorimetry (DSC) curve of the positive electrode sheet includes at least one main exothermic peak. When at least one of the above main exothermic peaks appears in the DSC test of the positive electrode sheet, a high decomposition temperature (high stability) can be obtained by adjusting the electrolytic solution components, and the main exothermic peak is an exothermic peak with a heat generation amount > 2 mW / mg during the DSC test.

[0016] In one embodiment of the present invention, in the DSC curve, when the temperature of the main exothermic peak is T °C, T satisfies T = 10b + 267 + X, -20 ≤ X ≤ 20, 0 < b ≤ 7, 200 ≤ T ≤ 360. For example, T can be 200, 220, 240, 260, 280, 300, 320, 340, 360, or any range therebetween. When the main exothermic peak temperature and the mass fraction of the polynitrile compound satisfy the above relational expression, the polynitrile compound can achieve good coordination with the Co-containing positive electrode material, so that the surface structure stability of the positive electrode material can be improved, and furthermore, the characteristics of the electrochemical device at high temperatures can be improved.

[0017] In the present invention, the positive electrode current collector is not particularly limited as long as the object of the present invention can be achieved, and may include, but is not limited to, aluminum foil, aluminum alloy foil, or a composite current collector. In the present invention, the thickness of the positive electrode current collector is not particularly limited as long as the object of the present invention can be achieved, and may be, for example, 4 μm to 12 μm. In the present invention, the positive electrode material layer may be provided on one surface in the thickness direction of the positive electrode current collector, or on both surfaces in the thickness direction of the positive electrode current collector. Note that the "surface" here may refer to the entire region of the positive electrode current collector or a partial region of the positive electrode current collector, and is not particularly limited in the present invention as long as the object of the present invention can be achieved.

[0018] In the present invention, the positive electrode material layer includes the positive electrode active material according to any of the above embodiments of the present invention, and the positive electrode active material may include a composite oxide, and the composite oxide includes lithium and at least one element selected from cobalt, manganese, and nickel. The specific type of the positive electrode active material is not particularly limited as long as it can achieve the object of the present invention. Specifically, the positive electrode active material may be lithium cobalt oxide (LiCoO2), a lithium nickel manganese cobalt ternary material, lithium manganese oxide (LiMn2O4), lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 The positive electrode active material layer is at least one selected from the group consisting of lithium iron phosphate (LiFePO4), lithium iron phosphate (LiFePO4), and lithium iron phosphate (LiFePO4). In the present invention, the thickness of the positive electrode active material layer is not particularly limited as long as the object of the present invention can be achieved, and the thickness is, for example, 30 μm to 120 μm.

[0019] The positive electrode material layer may further include a binder. In the present invention, the binder is not particularly limited as long as it can achieve the object of the present invention, and may include, for example, at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, polyimide, polyamide-imide, styrene-butadiene rubber, and polyvinylidene fluoride, but is not limited thereto.

[0020] In the present invention, the positive electrode material layer may further include a conductive agent. The conductive agent is not particularly limited as long as it achieves the objectives of the present invention. For example, the conductive agent may include at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, Ketjen black, graphene, metal materials, and conductive polymers, but is not limited thereto. The carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or nanocarbon fibers. The metal material may include, but is not limited to, metal powder and / or metal fibers. Specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, and silver. The conductive polymer may include, but is not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, and polypyrrole.

[0021] Optionally, the positive electrode may further include a conductive layer located between the positive electrode current collector and the positive electrode material layer. In the present invention, the composition of the conductive layer is not particularly limited and may be a conductive layer commonly used in the art, for example, it may include the above-mentioned conductive agent and the above-mentioned binder, but is not limited thereto.

[0022] The negative electrode piece in the present invention is not particularly limited as long as it can achieve the object of the present invention. For example, the negative electrode piece typically includes a negative electrode current collector and a negative electrode material layer. In the present invention, the negative electrode material layer may be provided on one surface of the negative electrode current collector in the thickness direction, or may be provided on both surfaces of the negative electrode current collector in the thickness direction. Note that the "surface" here may be the entire area of ​​the negative electrode current collector or a partial area of ​​the negative electrode current collector. In the present invention, it is not particularly limited as long as it can achieve the object of the present invention.

[0023] In the present invention, the negative electrode current collector is not particularly limited as long as it can achieve the object of the present invention, and may include, but is not limited to, for example, copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or a composite current collector. In the present invention, the thickness of the negative electrode current collector is not particularly limited as long as it can achieve the object of the present invention, and is, for example, 4 μm to 12 μm.

[0024] In the present invention, the negative electrode layer contains a negative electrode active material. The negative electrode active material is not particularly limited as long as it can achieve the object of the present invention, and examples thereof include natural graphite, artificial graphite, mesocarbon microbeads, hard carbon, soft carbon, silicon, silicon-carbon composite, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, and lithiated TiO2-Li4Ti5O having a spinel structure. 12 and Li-Al alloys, but are not limited to these.

[0025] In the present invention, the negative electrode layer may further contain a conductive agent. In the present invention, the conductive agent is not particularly limited as long as it can achieve the object of the present invention, and may contain, for example, at least one of the conductive agents described above, but is not limited thereto.

[0026] In the present invention, the negative electrode material layer may further contain a binder. In the present invention, the binder is not particularly limited as long as the object of the present invention can be achieved, and may contain, for example, at least one of the binders described above, but is not limited thereto.

[0027] Optionally, the negative electrode may further include a conductive layer located between the negative electrode current collector and the negative electrode material layer. In the present invention, the composition of the conductive layer is not particularly limited and may be a conductive layer commonly used in the art, and the conductive layer may include, but is not limited to, the above-mentioned conductive agent and the above-mentioned binder.

[0028] In the present invention, the separator is not particularly limited as long as it can achieve the objectives of the present invention. For example, it may include at least one of polyethylene (PE), polypropylene (PP), polytetrafluoroethylene-based polyolefin (PO) separators, polyester films (e.g., polyethylene terephthalate (PET) films), cellulose films, polyimide films (PI), polyamide films (PA), spandex, aramid films, woven fabric films, nonwoven fabric films, microporous membranes, composite membranes, separator paper, laminate films, and spun films, but is not limited to these, and PP is preferred. The separator of the present invention may have a porous structure, and the pore size is not particularly limited as long as it can achieve the objectives of the present invention. For example, the pore size can be 0.01 μm to 1 μm. In the present invention, the thickness of the separator is not particularly limited as long as it can achieve the objectives of the present invention. For example, the thickness can be 5 μm to 500 μm.

[0029] For example, the separator may include a substrate layer and a surface treatment layer. The substrate layer may be a nonwoven fabric, a membrane, or a composite membrane having a porous structure. The material of the substrate layer may include, but is not limited to, at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Optionally, a polypropylene porous film, a polyethylene porous film, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite film may be used. Optionally, a surface treatment layer is provided on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or may be a layer formed by mixing a polymer and an inorganic material.

[0030] The inorganic layer may include, but is not limited to, inorganic particles and an inorganic layer binder. In the present invention, the inorganic particles are not particularly limited and may include, but are not limited to, at least one of alumina, silica, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium oxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. In the present invention, the inorganic layer binder is not particularly limited and may include, but is not limited to, at least one of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylonitrile, polyacrylic acid ester, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The polymer layer includes a polymer, and the polymer material may include, but is not limited to, at least one of polyamide, polyacrylonitrile, polymeric acrylic ester, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride, and poly(vinylidene fluoride-hexafluoropropylene).

[0031] The electrochemical device of the present invention is not particularly limited and may include any device in which an electrochemical reaction occurs. In some embodiments, the electrochemical device may include, but is not limited to, a lithium metal secondary battery, a lithium ion secondary battery (lithium ion battery), a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

[0032] The manufacturing process for an electrochemical device is well known to those skilled in the art and is not particularly limited in the present invention. For example, the manufacturing process may include, but is not limited to, stacking positive electrode pieces, separators, and negative electrode pieces in order, winding and folding them as necessary to obtain a wound electrode assembly, placing the electrode assembly in a packaging bag, injecting an electrolyte into the packaging bag and sealing it to obtain an electrochemical device; or stacking positive electrode pieces, separators, and negative electrode pieces in order, securing the four corners of the entire stack with tape to obtain a stacked electrode assembly, placing the electrode assembly in a packaging bag, injecting an electrolyte into the packaging bag and sealing it to obtain an electrochemical device. Furthermore, an overcurrent protection element, lead plates, etc. may be placed in the packaging bag as needed to prevent pressure buildup and overcharging and discharging within the electrochemical device.

[0033] A second aspect of the present invention provides an electronic device comprising the electrochemical device according to the first aspect of the present invention. The electrochemical device provided by the present invention has good high-temperature properties, and therefore the electronic device provided by the present invention has a long life and good properties.

[0034] The electronic device of the present invention is not particularly limited and may be used in any electronic device known in the prior art. In some embodiments, the electronic device may include, but is not limited to, a laptop computer, a pen-input computer, a mobile computer, an electronic book player, a mobile phone, a portable facsimile machine, a portable copier, a portable printer, a stereo headset, a video recorder, an LCD television, a portable vacuum cleaner, a portable CD player, a minidisc, a walkie-talkie, an electronic organizer, a calculator, a memory card, a portable tape recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, an electric bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flashlight, a camera, a large household battery, and a lithium ion capacitor.

[0035] The electrochemical device and electronic device provided by the present invention adjust the mass fraction of the polynitrile compound according to the mass fraction of Co element in the positive electrode active material, and further use the coordination effect of the polynitrile compound with Co and other additives to significantly improve the structural stability of the positive electrode material after a high degree of lithium release, inhibit Co leaching, reduce the oxidation of the active material in the electrolyte, and inhibit related side reactions, thereby effectively improving the high-temperature performance of the electrochemical device.

[0036] Of course, any product or method embodying the present invention need not necessarily achieve all of the above advantages simultaneously. DETAILED DESCRIPTION OF THE INVENTION

[0037] In order to clarify the objectives, technical solutions and advantages of the present invention, the present invention will be further described in detail below with reference to the following examples. Obviously, the described examples are only a part of the embodiments of the present invention, and are not all of the embodiments. All other technical solutions obtained by those skilled in the art based on the embodiments of the present invention fall within the protection scope of the present invention.

[0038] In the specific embodiment of the present invention, the present invention will be described using a lithium ion battery as an example of an electrochemical device, but the electrochemical device of the present invention is not limited to a lithium ion battery.

[0039] Test method and equipment: High temperature cycle characteristic test: The lithium-ion battery was placed in a 45°C incubator and left for 30 minutes to reach a constant temperature. Once the temperature had reached a constant value, the lithium-ion battery was charged at a constant current of 1C until the voltage reached 4.45V. It was then charged at a constant voltage of 4.45V until the current reached 0.05C, and then discharged at a constant current of 1C until the voltage reached 2.8V. This constituted one charge-discharge cycle. The initial discharge capacity was defined as 100%, and the charge-discharge cycle was repeated. The test was stopped when the discharge capacity had decayed to 80%. The number of cycles and the thickness expansion rate of the lithium-ion battery were recorded and used as indicators to evaluate the cycle characteristics of the lithium-ion battery. Thickness expansion rate = (thickness after cycle - initial thickness) / initial thickness x 100%.

[0040] High temperature storage characteristics test: The lithium-ion batteries were placed in a 25°C incubator and allowed to stand for 30 minutes to allow the temperature to stabilize. They were then charged at a constant current of 1C to 4.45V, then charged at a constant voltage until the current reached 0.05C. They were then discharged at a constant current of 1C to 2.8V. The discharge capacity was recorded and used as the initial capacity of the lithium-ion battery. They were then charged at a constant current of 0.5C to 4.45V and again at a constant voltage until the current reached 0.05C. The battery thickness was measured and recorded with a micrometer. The test lithium-ion batteries were then transferred to a 60°C incubator and stored for 90 days, during which time the battery thickness was measured and recorded every three days. After the 90-day storage period, the batteries were transferred to a 25°C incubator, left for 1 hour, and then discharged at a constant current of 1C to 2.8V. The discharge capacity was recorded and used as the remaining capacity of the lithium-ion battery. The battery was charged at a constant current of 1C up to 4.45V, then charged at a constant voltage until the current reached 0.05C, and then discharged at a constant current of 1C down to 2.8V, recording the discharge capacity as the recoverable capacity of the lithium-ion battery. The thickness of the battery (THK) was measured, and the storage thickness expansion rate of the lithium-ion battery was calculated, which was used as an index to evaluate the amount of gas generated by the lithium-ion battery when stored at high temperatures. Thickness expansion rate = [(24-hour storage thickness - initial thickness) / initial thickness] x 100%.

[0041] 0℃ DC Resistance (DCR) Test: The lithium-ion battery was placed in a low-temperature incubator at 0°C and allowed to stand for 4 hours to allow the battery to reach a constant temperature. It was then charged at a constant current of 0.1C to a voltage of 4.45V, then at a constant voltage until the current reached 0.05C, and allowed to stand for 10 minutes. It was then discharged at a constant current of 0.1C to 3.4V, and this capacity was recorded as the actual discharge capacity (D0). It was then allowed to stand for 5 minutes, then charged at a constant current of 0.1C to 4.45V, and then at a constant voltage until the current reached 0.05C (the current was calculated based on the capacity corresponding to D0). It was then allowed to stand for 10 minutes, and discharged at a constant current of 0.1C for 3 hours (the current was calculated based on the capacity corresponding to D0). The voltage (V1) was recorded. It was then discharged at a constant current of 1C for 1 second (points were taken every 10 ms, and the current was calculated based on the cell's nominal capacity), and the voltage (V2) was recorded. Next, calculate the DC resistance corresponding to the 70% state of charge (SOC) of the cell, using the following formula: 70%SOC DCR=(V2-V1) / 1C.

[0042] Overcharge test: The lithium-ion battery was discharged at 0.5C at 25°C to 2.8V, then charged at a constant current of 2C to 5V, and then charged at a constant voltage for another 3 hours, while monitoring the change in the cell surface temperature. The pass criterion was that the cell did not ignite, burn, or explode.

[0043] Calendar Life (ITC) Test: The lithium-ion battery was placed in a 45°C incubator and allowed to stand for 30 minutes to allow the battery to reach a constant temperature. After reaching a constant temperature, the lithium-ion battery was charged at a constant current of 1C until the voltage reached 4.45V, then charged at a constant voltage of 4.45V until the current reached 0.05C. The initial battery thickness was measured and recorded using a micrometer, and the battery was allowed to stand at 45°C for 24 hours. The battery was then discharged at a constant current of 1C until the voltage reached 2.8V. This constituted one charge-discharge cycle. The initial discharge capacity was defined as 100%, and the battery was repeatedly charged and discharged until the discharge capacity had decayed to 80%. Before stopping the test, the lithium-ion battery was placed in a 25°C incubator and allowed to stand for 30 minutes to allow the battery to reach a constant temperature. After the lithium-ion battery reached a constant temperature, it was charged at a constant current of 1 C until the voltage reached 4.45 V, and then at a constant voltage of 4.45 V until the current reached 0.05 C (the thickness of the battery after the cycle was measured and recorded with a micrometer), and the number of cycles was recorded and used as an index for evaluating the calendar life of the lithium-ion battery. Thickness expansion rate = [(thickness after cycle - initial thickness) / initial thickness] x 100%.

[0044] Floating charge characteristic test (CV test): The lithium-ion battery was placed in a 25°C incubator and left for 30 minutes to allow the temperature to stabilize. It was then charged at a constant current of 1C until the voltage reached 4.45V, and then again at a constant voltage until the current reached 0.05C. The initial battery thickness was measured and recorded using a micrometer. The test lithium-ion battery was then transferred to a 45°C incubator and charged at a current of 1C for 60 days. After charging, the battery was transferred to a 25°C incubator and the battery thickness was measured and recorded as the 60-day float charge thickness. The thickness expansion rate of the lithium-ion battery during the float charge test was calculated and used as an index to evaluate the float charge characteristics of the lithium-ion battery. Thickness expansion rate = [(60-day floating charge thickness - initial thickness) / initial thickness] x 100%.

[0045] Positive electrode thermal stability test (DSC test): The lithium-ion battery was placed in a 25°C incubator and left for 30 minutes to allow the battery to reach a constant temperature. It was then charged at a constant current of 1 C to 4.45 V, and then at a constant voltage until the current reached 0.05 C. The positive electrode piece was removed from the battery and immersed in dimethyl carbonate (DMC) for 24 hours. A DSC test was then performed using a differential scanning calorimeter, where the positive electrode piece was heated to 400°C at a rate of 2°C / min, and the main thermal reaction peak and the corresponding temperature, T, were recorded.

[0046] Example 1-1 (1) Preparation of positive electrode pieces The positive electrode active material (lithium-nickel-manganese-cobalt ternary material, NCM613), the conductive agent (Super P), and the binder (polyvinylidene fluoride) were mixed in a weight ratio of 97:1.4:1.6, followed by N-methylpyrrolidone (NMP). The mixture was stirred using a vacuum mixer until the mixture became uniform and transparent, resulting in a positive electrode slurry with a solids content of 72 wt%. The positive electrode slurry was uniformly coated onto a 12 μm-thick aluminum foil current collector. The aluminum foil was dried at 85°C and cold-rolled to obtain a positive electrode piece with a 100 μm-thick layer of positive electrode active material. The coating process was then repeated on the other surface of the positive electrode piece, resulting in a positive electrode piece coated on both sides with a positive electrode active material layer. The positive electrode piece was then cut to a size of 74 mm x 867 mm, tabs were welded, and it was ready for further use.

[0047] (2) Preparation of negative electrode pieces The negative electrode active material, artificial graphite, the conductive agent, Super P, sodium carboxymethylcellulose (CMC), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 96.4:1.5:0.5:1.6. Deionized water was then added as a solvent to prepare a slurry with a solids content of 54 wt%. The mixture was then uniformly stirred. The slurry was uniformly coated onto one surface of an 8 μm-thick copper foil, dried at 110°C, and cold-rolled to obtain a negative electrode piece coated with a negative electrode active material layer on one side with a thickness of 150 μm. The coating process was then repeated on the other surface of the negative electrode piece, resulting in a negative electrode piece coated with a negative electrode active material layer on both sides. The negative electrode piece was then cut to a size of (74 mm x 867 mm), tabs were welded, and the negative electrode piece was then prepared for further processing.

[0048] (3) Preparation of electrolyte Ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) were mixed uniformly in a mass ratio of EC:PC:EMC:DEC = 10:30:30 in an argon atmosphere glove box with a water content of <10 ppm to form a base solvent. Then, polynitrile compounds and lithium salt LiPF6 were added according to Table 1, dissolved, and stirred uniformly to obtain an electrolyte solution. The polynitrile compounds included hexanedinitrile (AND), 1,2-bis(cyanoethoxy)ethane (DENE), and 1,3,6-hexanetricarbonitrile (HTCN). The mass ratio of the polynitrile compounds was AND:DENE:HTCN = 1:1:1, and the mass fraction of LiPF6 was 12.5%.

[0049] (4) Separator A 7 μm thick polyethylene porous polymer film was used as the separator.

[0050] (5) Preparation of Lithium-ion Battery The positive electrode, separator, and negative electrode were stacked in this order, with a separator between the positive and negative electrodes to provide insulation, and then wound up to obtain a bare cell. After the tabs were welded, the bare cell was placed in an aluminum-plastic foil outer casing, the edges of the aluminum foil bag were sealed, and it was then placed in a vacuum oven at 85°C for 12 hours to remove moisture from the dried core. Finally, the above-prepared electrolyte was injected into the dried bare cell, which was then vacuum packaged, left to stand, formed, and molded to complete the preparation of a lithium-ion battery (3.3 mm thick, 39 mm wide, and 96 mm long).

[0051] Examples 1-2 to 1-13 The same as Example 1-1, except that the relevant preparation parameters and characteristic parameters were as shown in Table 1.

[0052] Comparative Examples 1-1 to 1-7 The same as Example 1-1, except that the relevant preparation parameters and characteristic parameters were as shown in Table 1.

[0053] [Table 1] From Examples 1-1 to 1-13 and Comparative Examples 1-1 to 1-7, it was found that the Co element and polynitrile compound in the positive electrode active material generally affect the high-temperature cycle characteristics and high-temperature storage characteristics of a lithium-ion battery, and that when a lithium-ion battery is selected in which the mass fraction a of the Co element in the positive electrode active material and the mass fraction b of the polynitrile compound are within the ranges of the present invention and b and a satisfy the relational expression of claim 1 of the present invention, the coordination action between the cyano group in the polynitrile compound and Co strengthens the structural stability, and the prepared lithium-ion battery has good ITC, high-temperature cycle, and high-temperature storage characteristics.

[0054] Examples 2-1 to 2-10 The same as Examples 1-5, except that the relevant preparation parameters and characteristic parameters were as shown in Table 2.

[0055] [Table 2] In Table 2, " / " indicates that the corresponding preparation parameter does not exist. Examples 2-1 to 2-10 show that the type and mass fraction of the polynitrile compound also affect the high-temperature characteristics of a typical lithium-ion battery. The mass fraction of the polynitrile compound and the ratio (b1 / b2) of the mass fraction of the dinitrile compound to the mass fraction of the trinitrile compound affect the high-temperature cycle characteristics and high-temperature storage characteristics of a lithium-ion battery. Examples 2-1 to 2-5 show that the high-temperature cycle characteristics and high-temperature storage characteristics of a lithium-ion battery are improved as the proportion of the trinitrile compound in the polynitrile compound increases.

[0056] Examples 3-1 to 3-8 The same as in Examples 1-3, except that the relevant preparation parameters and characteristic parameters were as shown in Table 3, and the sum of the mass fractions of EC and PC was maintained at 40% during the process of changing the mass fraction of EC.

[0057] Comparative Example 3-1 The same as Examples 1-3, except that the relevant preparation parameters and characteristic parameters were as shown in Table 3.

[0058] [Table 3] From Examples 3-1 to 3-8 and Comparative Example 3-1, it was found that polynitrile compounds and EC also affect the high-temperature cycle characteristics and high-temperature storage characteristics of ordinary lithium ion batteries, and that selecting a lithium ion battery that contains a polynitrile compound and EC and has a ratio (c / b) of the mass fraction of the polynitrile compound to the mass fraction of EC within the range of the present invention will result in better high-temperature cycle characteristics and high-temperature storage characteristics.

[0059] Examples 4-1 to 4-5 The same as Examples 1-4, except that the mass fraction of LiPO2F2 was adjusted according to Table 4.

[0060] [Table 4] From Examples 4-1 to 4-5, it was found that LiPO2F2 also affects the high-temperature cycle characteristics and high-temperature storage characteristics of normal lithium-ion batteries. With an increase in the mass fraction of LiPO2F2, LiPO2F2 forms a low-resistance cathode-electrolyte interface (CEI) on the surface of the positive electrode, inhibits the elution of Co, and forms a stable SEI film on the negative electrode, thereby effectively improving the high-temperature cycle and ITC characteristics of lithium-ion batteries. Furthermore, due to the strong water absorption of LiPO2F2, when used in combination with a polynitrile compound, the effect of the polynitrile compound can be better exerted, and the high-temperature storage and high-temperature cycle characteristics of lithium-ion batteries can be improved.

[0061] Furthermore, the lithium ion batteries prepared in Examples 1-4 to 1-7 were fully charged and disassembled to obtain positive electrode pieces, which were then subjected to a DSC test. Table 5 shows the obtained characteristic test results.

[0062] [Table 5] The DSC test results of the positive electrode pieces of Examples 1-4 to 1-7 showed that the coordination effect of the polynitrile compound to Co improved the stability of the positive electrode active material, and the temperature at which the positive electrode would break down due to the heat of lithium release increased significantly with the addition of the polynitrile compound.

[0063] Examples 5-1 to 5-7 The same procedures as in Examples 1-4 were carried out, except that fluoroethylene carbonate (FEC), vinylene carbonate (VC), 1,3-propylene sulfite (PS), ethylene sulfate (DTD), lithium difluoro(oxalato)borate (LiDFOB), or lithium bis(oxalato)borate (LiBOB) was added according to Table 6, and the mass fractions of the corresponding substances were adjusted.

[0064] [Table 6] In Table 6, " / " indicates that the corresponding preparation parameter does not exist. From Examples 5-1 to 5-7, it was found that when a lithium ion battery containing FEC, VC, PS, DTD, LiDFOB, and LiBOB as additives is selected, stable CEI and SEI films are formed on the surfaces of the positive and negative electrodes, and side reactions between the positive and negative electrodes and the electrolyte can be suppressed. In addition, due to the synergistic effect of the additive and the polynitrile compound, damage to the SEI film caused by reductive decomposition of the polynitrile compound at the negative electrode interface can be effectively suppressed. This can improve the high-temperature cycle characteristics, float charge characteristics, safety characteristics, and high-temperature storage characteristics of the lithium ion battery, and can also reduce the DC resistance of the lithium ion battery.

[0065] The above is only a preferred embodiment of the present invention, and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention are included in the scope of protection of the present invention.

Claims

1. 1. An electrochemical device comprising: a positive electrode piece, a negative electrode piece, a separator, and an electrolyte; The positive electrode piece includes a positive electrode active material, and the positive electrode active material includes a Co element; the electrolyte solution contains a polynitrile compound; When a is a mass fraction of the Co element with respect to the total mass of the positive electrode active material, and b % is a mass fraction of the polynitrile compound with respect to the total mass of the electrolyte solution, the mass fraction b % of the polynitrile compound and the mass fraction a of the Co element satisfy b = 9.7a - 0.07 + C, -1.5 < C ≦ 1.5, and 0 < a ≦ 0.65, the polynitrile compound includes a dinitrile compound and / or a trinitrile compound; the dinitrile compound comprises at least one of butanedinitrile, hexanedinitrile, 1,2-bis(cyanoethoxy)ethane, and 1,4-dicyano-2-butene; the trinitrile compound includes at least one of 1,3,6-hexanetricarbonitrile and 1,2,3-tris(2-cyanoethoxy)propane; the electrochemical device, wherein the electrolyte solution contains lithium difluorophosphate, and when a mass fraction of the lithium difluorophosphate is d % with respect to a total mass of the electrolyte solution, d satisfies 0.01≦d≦1 and 0.5 / 0.28≦d / a.

2. 2. The electrochemical device according to claim 1, wherein a mass fraction b% of the polynitrile compound relative to the total mass of the electrolyte solution satisfies 0<b≦7.

3. 2. The electrochemical device according to claim 1, wherein, when a mass fraction of the dinitrile compound is b1% and a mass fraction of the trinitrile compound is b2% with respect to the total mass of the electrolyte solution, b1 and b2 satisfy 0≦b1 / b2≦4.

4. The electrolyte solution is (1) The electrolytic solution further contains ethylene carbonate, and when a mass fraction of the ethylene carbonate is c% with respect to a total mass of the electrolytic solution, c satisfies 0.5≦c / b≦20 and 3≦c≦30; (2) The electrolyte solution contains at least one of fluoroethylene carbonate, vinylene carbonate, 1,3-propylene sulfite, ethylene sulfate, lithium difluoro(oxalato)borate, and lithium bis(oxalato)borate; The electrochemical device according to claim 1 , wherein at least one of the following is satisfied:

5. 2. The electrochemical device of claim 1, wherein the electrolyte solution contains a lithium salt, the lithium salt includes at least one of an inorganic lithium salt and an organic lithium salt, and a mass fraction of the lithium salt is 7.5% to 25% relative to the total mass of the electrolyte solution.

6. 6. The electrochemical device of claim 5, wherein the lithium salt comprises at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide.

7. 10. The electrochemical device of claim 1, wherein the electrolyte further comprises at least one of dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, propylene carbonate, ethyl acetate, ethyl propionate, and propyl propionate.

8. 10. The electrochemical device of claim 1, wherein the differential scanning calorimetry curve of the positive electrode strip includes at least one main exothermic peak.

9. 9. The electrochemical device according to claim 8, wherein, in the differential scanning calorimetry curve, when the temperature of the main exothermic peak is T ° C., T satisfies T = 10b + 267 + X, -20 ≦ X ≦ 20, 0 < b ≦ 7, and 200 ≦ T ≦ 360.

10. An electronic device comprising the electrochemical device according to any one of claims 1 to 9.

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