Rechargeable batteries and electronic devices containing said rechargeable batteries

Abstract

To provide a secondary battery that reduces a high-temperature internal resistance to improve cycle performance at high voltage, and an electronic apparatus including the secondary battery.SOLUTION: A secondary battery includes a positive electrode, a negative electrode, and an electrolyte. The electrolyte includes an organic solvent, and the organic solvent includes ethyl propionate and propyl propionate. When the mass content of ethyl propionate in the electrolyte is defined as m%, and the mass content of propyl propionate as n%, the difference between n and m (n-m) is 5-54. The negative electrode includes a negative electrode collector, and a negative electrode active material layer provided in the negative electrode collector. The negative electrode active material layer includes a negative electrode active material, the negative electrode active material includes graphite, and n and the Lc value of graphite satisfy that the ratio n / Lc is 1-2.14. The difference in the content between ethyl propionate and propyl propionate included in the electrolyte satisfies a specific request, and the ratio of the mass content n% of propyl propionate to the Lc of graphite (n / Lc) is 1-2.14.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to the field of battery technology, and particularly to secondary batteries and electronic devices including such secondary batteries.

Background Art

[0002] Secondary batteries such as lithium-ion batteries have advantages such as being environmentally friendly, having a high energy density, and a long cycle life, and thus are widely applied to mobile phones, personal computers, wearable devices, consumer drones, power tools, electric bicycles, electric vehicles or large-scale energy storage devices. As the market continues to expand, the requirements for battery characteristics are also continuously increasing. For example, the requirements for battery characteristics under high-temperature and low-temperature conditions are increasing day by day. Currently, the electrolytic solution of secondary batteries often selects a carbonic ester compound as the solvent system. However, such secondary batteries have relatively poor high-voltage stability, and the structure of the graphite material of the negative electrode is likely to become unstable, resulting in a decline in performance. Therefore, there is a need to further improve the performance of secondary batteries.

Summary of the Invention

[0003] In view of this, a first aspect of the present invention provides a secondary battery, which includes a positive electrode, a negative electrode, and an electrolytic solution. The electrolytic solution includes an organic solvent. The organic solvent includes ethyl propionate and propyl propionate. When the mass content ratio of ethyl propionate is m% and the mass content ratio of propyl propionate is n% with respect to the mass of the electrolytic solution, the difference n - m between n and m is 5 to 54. The negative electrode includes a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material. The negative electrode active material includes graphite with a mass content ratio of 90% or more, and the ratio of n to the Lc value of the graphite satisfies n / Lc being 1 to 2.14.

[0004] The secondary battery of the present invention satisfies the above specific requirements by setting the difference in the content of ethyl propionate and propyl propionate in the electrolyte. As a result, the relatively high content of propyl propionate exhibits low impedance characteristics at low temperatures, increasing the transmission rate of active ions. Furthermore, at high temperatures and high voltages, ethyl propionate and propyl propionate reduce corrosion due to oxidation of the graphite surface, decrease the consumption of active lithium, lower the internal resistance of the secondary battery, lower the high-temperature internal resistance, and improve the cycle performance at high voltages. Moreover, by satisfying the mass content n% of Lc in graphite and propyl propionate so that n / Lc is 1 to 2.14, a relatively short intercalation and release pathway for lithium ions can be provided, and the low-temperature rate performance of the secondary battery can be improved.

[0005] In some embodiments, the nm range is 18–54. This further improves the high-voltage cycle performance of the secondary battery and reduces the high-temperature internal resistance. In some embodiments, the nm range is 21–47.

[0006] In some examples, the sum of n and m, m+n, is between 42 and 70. By controlling the total mass content of both ethyl propionate and propyl propionate in the electrolyte to be within the above range, the internal impedance of the secondary battery can be further reduced, corrosion due to oxidation of the graphite surface can be reduced, and the high-voltage cycle performance and low-temperature rate performance of the secondary battery can be further improved. In some examples, m+n is between 53 and 69. In some examples, m+n is between 52 and 67.5.

[0007] In some embodiments, m is 8 to 24. This allows for a better balance between the viscosity of the electrolyte and the effect of reducing the internal impedance of the secondary battery, further improving the high-voltage cycle performance and low-temperature rate performance of the secondary battery. In some embodiments, m is 8 to 19. In some embodiments, m is 11 to 21.

[0008] In some embodiments, n is 29 to 62. This allows for a better balance between the viscosity of the electrolyte and the effect of reducing the internal impedance of the secondary battery, further improving the high-voltage cycle performance and low-temperature rate performance of the secondary battery. In some embodiments, n is 33 to 58. In some embodiments, n is 30 to 53.5.

[0009] In some examples, the electrolyte further contains lithium bis(fluorosulfonyl)imide, and when the mass content of lithium bis(fluorosulfonyl)imide is p% relative to the mass of the electrolyte, p is 0.07 to 4.3. Lithium bis(fluorosulfonyl)imide has relatively high stability at high temperature and high voltage, suppresses the decomposition of propyl propionate, and can form a protective film on the graphite surface that is low impedance, stable and resistant to oxidation, further reducing the internal impedance of the secondary battery and further improving the high-temperature characteristics of the secondary battery. In some examples, p is 0.7 to 2.4. In some examples, p is 1.3 to 3.5.

[0010] In some embodiments, the ratio of p to m, p / m, is 0.01 to 0.36. Controlling p / m to be within this range can increase the transmission rate of active ions, improve the oxidation resistance of the graphite surface, and further improve the performance of the secondary battery at high temperature and high voltage. In some embodiments, p / m is 0.07 to 0.17. In some embodiments, p / m is 0.09 to 0.32.

[0011] In some embodiments, the Lc value of graphite satisfies 25 ≤ Lc ≤ 38. By setting the Lc value of graphite, lithium ions have a relatively short intercalation and release pathway in the graphite, improving the low-temperature rate performance of secondary batteries. The Lc of the graphite material is achieved by methods such as controlling the particle size of the raw materials and the temperature of the graphitization treatment during the graphite preparation process. In some embodiments, the graphite includes at least one of artificial graphite and natural graphite.

[0012] In some examples, the electrolyte further contains lithium difluorophosphate, with a mass content of lithium difluorophosphate of 0.02% to 2.4% relative to the mass of the electrolyte. Adding lithium difluorophosphate to the electrolyte can improve the film formation stability of ethyl propionate on the graphite surface at low temperatures, improving performance at high temperatures and high voltages, and further improving the low-temperature rate performance of the secondary battery. In some examples, the mass content of lithium difluorophosphate is 0.2% to 1.6%.

[0013] In some examples, the electrolyte further contains 1,3-propanesultone, with a mass content of 1,3-propanesultone of 1.7% to 4.4% relative to the mass of the electrolyte. Adding 1,3-propanesultone to the electrolyte suppresses excessive film formation of ethyl propionate on the graphite surface, further reducing the internal impedance of the secondary battery and improving the high and low temperature characteristics of the secondary battery. In some examples, the mass content of 1,3-propanesultone is 1.7% to 2.9%. In some examples, the mass content of 1,3-propanesultone is 3.1% to 3.9%.

[0014] A second aspect of the present invention provides an electronic device, the electronic device including the above-mentioned secondary battery. By using the above-mentioned secondary battery as a power source for the electronic device, the performance of the secondary battery is stabilized, and power can be supplied to the electronic device stably.

[0015] Compared to conventional technology, the present invention has the following beneficial effects. The present invention provides a secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the electrolyte contains an organic solvent, the organic solvent contains ethyl propionate and propyl propionate, and when the mass content of ethyl propionate is m% and the mass content of propyl propionate is n% relative to the mass of the electrolyte, nm is 5 to 54, and in the negative electrode, the negative electrode active material contains 90% or more graphite by mass percentage, and the ratio of n to the Lc value of graphite is n / Lc = 1 to 2.14. By controlling the difference in the content of ethyl propionate and propyl propionate in the electrolyte to satisfy the above specific requirements, corrosion due to oxidation of the graphite surface can be reduced, the consumption of active lithium can be reduced, the internal resistance of the secondary battery can be reduced, the high-temperature internal resistance can be reduced, and the cycle performance at high voltage can be improved. Furthermore, a relatively short intercalation and release pathway can be provided for lithium ions, and the low-temperature rate performance of the secondary battery can be improved. [Modes for carrying out the invention]

[0016] The following describes embodiments of the present invention. Unless otherwise specified, the range values ​​of the data described in the present invention should include the endpoints.

[0017] According to a first aspect of the present invention, the present invention provides a secondary battery comprising a negative electrode, a positive electrode, an electrolyte, and a separator provided between the positive electrode and the negative electrode. The secondary battery may be a lithium-ion battery or any other suitable secondary battery.

[0018] The electrolyte contains an organic solvent and an electrolyte. The negative electrode includes a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector. The positive electrode includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector.

[0019] This invention aims to obtain a rechargeable battery with stable performance by making special improvements and limitations, mainly on the component content in the electrolyte and the negative electrode active material (graphite) of the negative electrode.

[0020] In this invention, the organic solvent of the electrolyte contains ethyl propionate and propyl propionate. In the electrolyte, the mass content of propyl propionate is greater than the mass content of ethyl propionate. In this invention, if the mass content of ethyl propionate is m% and the mass content of propyl propionate is n% relative to the mass of the electrolyte, the difference in nm between n and m is 5 to 54, that is, 5 ≤ nm ≤ 54. Preferably, the difference in nm between n and m is 18 to 54, and preferably 21 to 47. For example, the difference in nm between n and m may be 5, 11, 13, 17, 19, 23, 31, 34, 39, 47, 49, or 54, or a number within the range of any two of these values.

[0021] The negative electrode active material layer mainly contains the negative electrode active material. In this invention, the negative electrode active material mainly contains graphite, and the mass content of graphite relative to the mass of the negative electrode active material is 90% or more. In some examples, the negative electrode active material consists of graphite. In some other examples, the negative electrode active material is a mixture of graphite and silicon.

[0022] The Lc value of graphite is the crystallite size along the vertical axis of the graphite crystal material measured by X-ray diffraction, and its unit is nm. In this invention, the ratio of the n value in the mass content n% of propyl propionate in the above electrolyte to the Lc value of graphite satisfies the condition that n / Lc is between 1 and 2.14, i.e., 1 ≤ n / Lc ≤ 2.14. Preferably, n / Lc is between 1.10 and 2.00. For example, n / Lc may be 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.7, 1.9, 2.0, 2.1, or 2.14, or it may be a value within the range of any two of these values.

[0023] In secondary batteries, by setting the difference in the content of ethyl propionate and propyl propionate in the electrolyte to satisfy the above specific requirements, the relatively high content of propyl propionate exhibits low impedance characteristics at low temperatures, increasing the transmission rate of active ions. Furthermore, at high temperatures and voltages, ethyl propionate and propyl propionate reduce corrosion due to oxidation of the graphite surface, decrease the consumption of active lithium, lower the internal resistance of the secondary battery, reduce high-temperature internal resistance, and improve cycle performance at high voltages. Moreover, if the mass content n% of Lc in graphite and propyl propionate satisfies the condition that n / Lc is 1 to 2.14, it is possible to provide lithium ions with relatively short intercalation and release pathways and improve the low-temperature rate performance of the secondary battery.

[0024] In some embodiments, the Lc value of graphite is between 25 and 38, i.e., satisfying 25 ≤ Lc ≤ 38. For example, Lc may be 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, or 38, or any two of these values ​​within a given range. By setting the Lc value of graphite, lithium ions have a relatively short intercalation and release pathway in the graphite, improving the low-temperature rate performance of secondary batteries. The Lc of the graphite material is achieved by methods such as controlling the particle size of the raw materials and the temperature of the graphitization treatment during the graphite preparation process.

[0025] In some embodiments, the sum n + m of n and m is from 42 to 70, that is, 42 ≤ m + n ≤ 70 is satisfied. That is, with respect to the mass of the electrolyte solution, the total mass content ratio of both ethyl propionate and propyl propionate is 42% or more and 70% or less. Preferably, m + n is from 42 to 70, more preferably, m + n is from 53 to 69, and still more preferably, m + n is from 52 to 67.5. By way of example, the sum m + n of n and m is 42, 47, 51, 53, 57, 63, 64, 67, 69, or 70, etc., or may be a numerical value within the range composed of any two of these numerical values. When the total mass content ratio of both ethyl propionate and propyl propionate in the electrolyte solution is within the above range, the internal impedance of the secondary battery can be further reduced, the corrosion due to the oxidation of the graphite surface can be reduced, and the cycle performance and low-temperature rate performance of the secondary battery at high voltage can be further improved.

[0026] In some embodiments, m is from 8 to 24, that is, 8 ≤ m ≤ 24 is satisfied. With respect to the mass of the electrolyte solution, the mass content ratio of ethyl propionate is 8% or more and 24% or less. Preferably, m is from 8 to 24, more preferably, m is from 8 to 19, and still more preferably, m is from 11 to 21. By way of example, m is 8, 11, 13, 14, 16, 17, 19, 20, 21, 23, or 24, etc., or may be a numerical value within the range composed of any two of these numerical values. When the mass content ratio of ethyl propionate satisfies the above range, the balance between the viscosity of the electrolyte solution and the reduction effect of the internal impedance of the secondary battery can be made better, and the cycle performance and low-temperature rate performance of the secondary battery at high voltage can be further improved.

[0027] In some embodiments, n is from 29 to 62, that is, 29 ≤ n ≤ 62 is satisfied. With respect to the mass of the electrolyte, the mass content ratio of propyl propionate is 29% or more and 62% or less. Preferably, n is from 29 to 62, more preferably, n is from 33 to 58, and still more preferably, n is from 30 to 53.5. By way of example, n is 29, 31, 33, 37, 43, 47, 51, 53, 58, 61, or 62, etc., or a numerical value within the range composed of any two of these numerical values may also be used. When the mass content ratio of propyl propionate satisfies the above range, the balance between the viscosity of the electrolyte and the reduction effect of the internal impedance of the secondary battery can be made better, and the cycle performance and low temperature rate performance of the secondary battery at high voltage can be further improved.

[0028] In some embodiments, the electrolyte further contains lithium bis(fluorosulfonyl)imide (Li(N(SO2F)2, LiFSI)). When the mass content ratio of LiFSI is p% with respect to the mass of the electrolyte, p is from 0.07 to 4.3, that is, 0.07 ≤ p ≤ 4.3 is satisfied. Preferably, p is from 0.7 to 2.4, more preferably, p is from 1.3 to 3.5. LiFSI has relatively high stability at high temperature and high voltage, can suppress the decomposition of propyl propionate at high voltage, and can form a protective film with low impedance, stability, and low oxidation tendency on the graphite surface. When LiFSI within the above range of mass content ratio is added to the electrolyte, the internal impedance of the secondary battery can be further reduced, and the high temperature characteristics of the secondary battery can be further improved. By way of example, p is 0.07, 0.1, 0.3, 0.7, 1.3, 1.7, 2.3, 2.7, 3.1, 3.3, 3.7, 4.1, or 4.3, etc., or a numerical value within the range composed of any two of these numerical values may also be used.

[0029] In some embodiments, the ratio of p to m, p / m, is 0.01 to 0.36, i.e., satisfying 0.01 ≤ p / m ≤ 0.36. Preferably, p / m is 0.07 to 0.17, and more preferably, p / m is 0.09 to 0.32. Examples include p / m being 0.01, 0.07, 0.09, 0.13, 0.19, 0.23, 0.27, 0.31, 0.33, 0.35, or 0.36, or a value within the range of any two of these values. When the ratio p / m of the mass content of LiFSI p% to the mass content of ethyl propionate m% satisfies the above range, the transmission rate of active ions can be increased, the oxidation resistance of the graphite surface can be improved, and the performance of the secondary battery at high temperature and high voltage can be further improved.

[0030] In some embodiments, the electrolyte further contains lithium difluorophosphate. The mass content of lithium difluorophosphate relative to the mass of the electrolyte is 0.02% to 2.4%. Preferably, the mass content of lithium difluorophosphate is 0.2% to 1.6%. For example, the mass content of lithium difluorophosphate may be 0.02%, 0.2%, 0.7%, 0.9%, 1.1%, 1.7%, 1.9%, 2.1%, or 2.4%, or any two of these values. The addition of lithium difluorophosphate enhances the film formation stability of LiFSI and ethyl propionate on the graphite surface at low temperatures, improves performance at high temperatures and voltages, and further improves the low-temperature rate performance of the secondary battery.

[0031] In some embodiments, the electrolyte further contains 1,3-propanesultone. The mass content of 1,3-propanesultone is 1.7% to 4.4% relative to the mass of the electrolyte. Preferably, the mass content of 1,3-propanesultone is 1.7% to 2.9%, and more preferably, it is 3.1% to 3.9%. For example, the mass content of 1,3-propanesultone may be 1.7%, 1.9%, 2.1%, 2.3%, 2.7%, 2.9%, 3.1%, 3.3%, 3.7%, 3.9%, or 4.4%, or any value within the range of any two of these values. The addition of 1,3-propanesultone suppresses the formation of excessively thick films on the graphite surface by ethyl propionate and LiFSI, thereby further reducing the internal impedance of the secondary battery and improving its high and low temperature characteristics.

[0032] electrolyte The organic solvent in the electrolyte further comprises several other organic solvents in addition to ethyl propionate and propyl propionate as described above, the other organic solvents may be, but are not limited to, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), dimethyl carbonate (DMC), or propylene carbonate. In some examples, the other organic solvents include ether-based solvents, for example, at least one of 1,3-dioxolane (DOL) and ethylene glycol dimethyl ether (DME).

[0033] In the embodiments of the present invention, the electrolyte contains a lithium salt as the electrolyte. The lithium salt in the electrolyte is an inorganic lithium salt such as LiClO4, LiAsF6, LiPF6, LiBF4, LiSbF6, LiSO3F, LiN(FSO2)2; LiCF3SO3, LiN(FSO2)(CF3SO2), LiN(CF3SO2)2, LiN(C2F5SO2)2, cyclic 1,3-hexafluoropropanedisulfonimidolithium, cyclic 1,2-tetrafluoroethanedisulfonimidolithium, LiPF4(CF3)2, LiN(CF3SO2)(C4F9SO2), LiC(CF3SO2)3, LiPF4(CF This includes, but is not limited to, fluorine-containing organolithium salts such as 3SO2)2, LiPF4(C2F5)2, LiPF4(C2F5SO2)2, LiBF2(CF3)2, LiBF2(C2F5)2, LiBF2(CF3SO2)2, and LiBF2(C2F5SO2)2; and lithium salts containing dicarboxylic acid complexes such as lithium bis(oxalate)borate, lithium difluoro(oxalato)borate, lithium tris(oxalato)phosphate, lithium difluorobis(oxalato)phosphate, and lithium tetrafluoro(oxalato)phosphate. Note that one of the above lithium salts may be used alone, or two or more may be used in combination. For example, in some examples, the lithium salt includes a combination of LiPF6 and LiBF4. In some examples, the lithium salt is LiPF6. In some examples, the mass content of the lithium salt in the electrolyte is 10% to 20% of the total mass of the electrolyte.

[0034] The electrolyte further comprises several other additives. The additives in the electrolyte used in the embodiments of the present invention may be additives well known in the art that can be used to enhance the electrochemical properties of the battery. In some embodiments, the additives include, but are not limited to, at least one of polynitrile compounds, sulfur-containing additives, fluoroethylene carbonate (FEC), and 1,4-butanesultone.

[0035] The method for preparing the electrolyte in the embodiments of the present invention is not limited and can be prepared using the usual method for preparing electrolytes. In some embodiments, the electrolyte of the present invention is prepared by mixing the components.

[0036] negative electrode In some embodiments, the graphite comprises at least one of artificial graphite or natural graphite.

[0037] Negative electrode active material particles can be obtained by selecting petroleum coke with relatively low volatile content as a raw material, crushing it into single particles, converting it to graphite by heat treatment, and then performing surface modification treatment. Of course, the method of preparing negative electrode active particles is not limited to this, and they can also be prepared by other methods well known in this field.

[0038] In some embodiments, the negative electrode active material layer further comprises a binder. This binder may include a variety of binder polymers such as polyvinyl alcohol, carboxymethylcellulose, hydroxypropylcellulose, diacetylcellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxy-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylic acid (esterified) styrene-butadiene rubber, epoxy resin, and nylon.

[0039] In some embodiments, the negative electrode active material layer further contains a conductive agent to improve the electrical conductivity of the electrode. Any conductive material may be used as the conductive material, as long as it does not undergo chemical changes. Examples of conductive agents include, but are not limited to, carbon-based materials such as carbon black, acetylene black, Ketjen black, and carbon fibers; metallic materials such as metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0040] In some embodiments, the negative electrode current collector includes, but is not limited to, copper foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, foamed copper, polymer substrates coated with conductive metal, and any combination thereof. In some embodiments, the negative electrode current collector is copper foil.

[0041] In some embodiments, the structure of the negative electrode is a structure of a negative electrode well known in the art and used in electrochemical apparatuses.

[0042] In some embodiments, the method for preparing the negative electrode is a method for preparing a negative electrode that is well known in the art and used in electrochemical apparatus. Exemplarily, the negative electrode can be obtained by preparing a negative electrode active material slurry by mixing an active material, a conductive agent, a binder, and the above-mentioned negative electrode additives in a solvent, and adding a thickener as needed; coating the negative electrode active material slurry onto a current collector; drying and cold pressing to form a negative electrode active material layer. In some embodiments, the solvent may include, but is not limited to, water and N-methylpyrrolidone.

[0043] positive electrode The positive electrode includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector. The specific type of positive electrode active material is not limited and can be selected as needed.

[0044] In some examples, the positive electrode active material includes a lithium transition metal composite oxide. In some examples, the positive electrode active material is lithium cobalt oxide (LiCoO2), lithium nickel manganese cobalt ternary material (NCM), lithium manganese oxide (LiMn2O4), lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 It is at least one selected from the group consisting of O4 and lithium iron phosphate (LiFePO4).

[0045] In some embodiments, the positive electrode active material layer further comprises a binder and optionally a conductive material. The binder can enhance the bonding between positive electrode active material particles and enhance the bonding between the positive electrode active material and the positive electrode current collector. In some embodiments, the binder includes, but is not limited to, polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxy-containing polymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylic acid (esterified) styrene-butadiene rubber, epoxy resin, and nylon.

[0046] In some embodiments, the positive electrode active material layer further comprises a conductive material to impart conductivity to the electrode. This conductive material may include any conductive material, as long as it does not undergo chemical changes. Non-limiting examples of conductive materials include carbon-based materials (e.g., carbon black, acetylene black, Ketjen black, carbon fibers, etc.), metallic materials (e.g., metal powders, metal fibers, etc., including copper, nickel, aluminum, silver, etc.), conductive polymers (e.g., polyphenylene derivatives), and mixtures thereof.

[0047] In some embodiments, the positive electrode current collector is made of metal, which includes, but is not limited to, aluminum foil, for example.

[0048] In some embodiments, the structure of the positive electrode is the structure of a positive electrode well known in the art and used in secondary batteries.

[0049] In some embodiments, the method for preparing the positive electrode is a method for preparing a positive electrode that is well known in the art and used in secondary batteries. For example, the positive electrode can be obtained by preparing a positive electrode active material slurry by mixing an active material, a conductive material, a binder, and the above-mentioned positive electrode additives in a solvent, coating the positive electrode active material slurry onto a current collector, drying it, and cold pressing it to form a positive electrode active material layer. In some embodiments, the solvent may include, but is not limited to, water, N-methylpyrrolidone, etc.

[0050] Separator In some embodiments, a separator is provided between the positive and negative electrodes to prevent short circuits. The material and shape of the separator are not particularly limited and may be any technology disclosed in the prior art. In some embodiments, the separator includes a polymer or inorganic material made of a material stable with respect to the electrolyte of the present invention.

[0051] For example, in some embodiments, the separator includes a substrate layer. The substrate layer is a nonwoven fabric, film, or composite film having a porous structure. The material of the substrate layer may be at least one selected from the group consisting of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Specifically, the material of the substrate layer can be selected from polypropylene porous film, polyethylene porous film, polypropylene nonwoven fabric, polyethylene nonwoven fabric, and polypropylene-polyethylene-polypropylene porous composite film.

[0052] A surface treatment layer is provided on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer, an inorganic layer, or a layer formed by mixing a polymer and an inorganic material. Specifically, the inorganic layer includes inorganic particles and a binder. The inorganic particles can be selected from one or more combinations of alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. The binder may be at least one or more combinations selected from the group consisting of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene polymer, polyamide, polyacrylonitrile, polyacrylic acid ester, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene.

[0053] According to a second aspect of the present invention, the present invention provides an electronic device, the electronic device including the above-mentioned secondary battery.

[0054] The applications of the secondary battery of the present invention are not particularly limited and can be used in any electronic device known in the prior art. For example, such electronic devices include, but are not limited to, laptop computers, pen-input computers, mobile computers, e-book players, mobile phones, portable fax machines, portable copiers, portable printers, headphone stereos, video cameras, LCD televisions, handheld vacuum cleaners, portable CDs, MiniDiscs, transceivers, electronic organizers, calculators, memory cards, portable tape recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric assist bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, strobes, cameras, large household storage batteries, and lithium-ion capacitors. In addition to the electronic devices listed above, the secondary battery of the present invention can also be applied to energy storage power plants, maritime transport equipment, and air transport equipment. Air transport equipment includes air transport equipment within the atmosphere and air transport equipment outside the atmosphere.

[0055] The technical proposal of the embodiment of the present invention will be further explained below with reference to specific examples.

[0056] Example 1-1 1. Preparation of lithium-ion batteries: 1. Preparation of the negative electrode Preparation of graphite material for the negative electrode active material: Mesocarbon microbeads, which are artificial graphite, were crushed and sieved to control the particle size distribution, so that Dv90 < 25 μm and Dv50 9 μm were obtained to obtain primary particles. Bitumen, which is a binder, was added to the primary particles to bind them together, and the particle size was controlled by sieving and classifying so that Dv90 < 45 μm and Dv50 10.7 μm were obtained to obtain secondary particles. The primary and secondary particles were mixed in a number ratio of 3:7, graphitized at 2850°C for 10 hours, cooled, demagnetized, and sieved to obtain graphite material with a Dv50 of 10.2 μm. The Lc of the graphite material at the negative electrode can be controlled by the particle size of the raw material and the temperature of the graphitization process.

[0057] The graphite particles, which are the negative electrode active material prepared as described above, carbon black, which is the conductive agent, styrene-butadiene rubber (SBR), which is the binder, and sodium carboxymethylcellulose (CMC), which is the thickener, were placed in deionized water, which is the solvent, in a mass ratio of 95.7:1.5:1.8:1, and thoroughly stirred and mixed to form a uniform negative electrode slurry. The negative electrode slurry was uniformly applied to copper foil, which is the negative electrode current collector, dried, and cold-pressed to form a negative electrode active material layer. Further cutting and tab welding were performed to obtain the negative electrode.

[0058] 2. Preparation of the positive electrode Lithium cobalt oxide, the positive electrode active material, acetylene black, the conductive agent, and polyvinylidene fluoride (PVDF), the binder, were mixed in N-methylpyrrolidone (NMP), the solvent, in a mass ratio of 96.3:2.2:1.5. The mixture was thoroughly and uniformly stirred using a vacuum stirrer to obtain a positive electrode slurry. This positive electrode slurry was coated onto aluminum foil, which served as the positive electrode current collector, dried, and cold-pressed to form a positive electrode active material layer. Further cutting and tab welding were then performed to obtain the positive electrode.

[0059] 3. Preparation of the electrolyte In a dry argon-atmosphered glove box, propyl propionate (PP), ethyl propionate (EP), ethylene carbonate (EC), and propylene carbonate (PC) were uniformly mixed, dissolved, and thoroughly stirred. Then, the lithium salt LiPF6 was added and uniformly mixed to obtain an electrolyte. The mass content of the electrolyte was 12.5% ​​for the lithium salt LiPF6, 8% for EP, and 62% for PP. The remainder consisted of ethylene carbonate (EC) and propylene carbonate (PC), with a mass ratio of EC:PC = 2:1.

[0060] 4. Preparation of the separator A porous polyethylene (PE) polymer film was used as the separator.

[0061] 5. Preparation of lithium-ion batteries A separator was interposed between the positive and negative electrodes to act as an isolation layer. The resulting positive electrode, separator, and negative electrode were then stacked in sequence and wound up to obtain a bare cell. The bare cell was placed in an aluminum plastic film, which served as the outer foil, and an electrolyte solution was injected. After going through processes such as vacuum packaging, settling, and formation, a lithium-ion battery was obtained.

[0062] The preparation process for Examples 1-2 to 1-13 is almost the same as that for Example 1-1, with specific differences shown in Table 1. The graphitization treatment for Examples 1-11 to 1-13 is almost the same as that for Example 1-1, but there are some differences, which are as follows. In the graphitization treatment process for Example 1-11, the primary particle Dv50 was set to 7 μm and the secondary particle Dv50 to 9 μm, and the mixture was incubated at a graphitization temperature of 3100 °C for 12 hours. In the graphitization treatment process for Example 1-12, the primary particle Dv50 was set to 9.5 μm and the secondary particle Dv50 to 11.0 μm, and the mixture was incubated at a graphitization temperature of 2750 °C for 10 hours. In the graphitization process of Examples 1-13, the primary particles had a Dv50 of 10 μm, the secondary particles had a Dv50 of 11.5 μm, and the graphitization temperature was 2600°C for 12 hours. The preparation process for Comparative Examples 1-1 to 1-3 is almost the same as that of Example 1, and the specific differences are detailed in Table 1.

[0063] 2.Measurement method 1. Parameter measurement: The crystallite size Lc of the graphite material was measured by analyzing it along the vertical axis using an X-ray diffractometer. The Lc value was calculated from the X-ray diffraction measurement results using Scherrer's equation, which is Lc = Kλ / β(2θ)cosθ, where K is Scherrer's constant (K = 0.9), λ is the wavelength of the incident X-rays, θ is the angle at the maximum peak value, and β is the full width at half maximum.

[0064] 2. Characteristic testing of lithium-ion batteries 2.1 Low-temperature discharge rate performance Place the lithium-ion battery in a 25°C constant temperature bath, leave it for 5 minutes, discharge at 0.5C to a voltage of 2.5V, leave it for 30 minutes, charge at 0.2C to a voltage of 3.6V, charge at a constant voltage of 0.025C, leave it for 10 minutes, discharge at 2.0C to a voltage of 2.5V (discharge capacity at 25°C), leave it for 10 minutes, charge at 0.2C to a voltage of 3.6V, and 0.025 The battery was charged at a constant voltage up to C, left standing for 10 minutes, the temperature was adjusted to -10°C, left standing for 60 minutes, discharged at a constant current of 1.0C up to 2.5V, left standing for 30 minutes, charged at a constant current of 0.2C up to 3.6V, charged at a constant voltage down to 0.025C, left standing for 10 minutes, discharged at a constant current of 2.0C up to 2.5V (discharge capacity at -10°C), the temperature was adjusted to 25°C, and left standing for 60 minutes. Discharge ratio at -10°C = (Discharge capacity at -10°C / Discharge capacity at 25°C) × 100%. The discharge rate was 2C.

[0065] 2.2 High-voltage cycle performance At 45°C, a lithium-ion battery was charged with a constant current of 1C to a voltage of 4.4V, then charged with a constant voltage of 0.05C, and finally discharged with a constant current of 1C to a voltage of 3.0V. This constituted one charge-discharge cycle, and the discharge capacity of this cycle was defined as the discharge capacity of the first cycle. The lithium-ion battery underwent 300 charge / discharge tests using the above method, and the discharge capacity of the lithium-ion battery after 300 cycles was obtained. High-voltage cycle capacity retention rate (%) = (Discharge capacity of lithium-ion battery at 300 cycles / Discharge capacity of lithium-ion battery at the first cycle) × 100%.

[0066] 2.3 Measurement of High-Temperature DC Impedance A lithium-ion battery was placed in a 40°C constant temperature bath and left undisturbed for 2 hours. It was then charged with a constant current of 0.1C to a voltage of 4.46V, then charged with a constant voltage of 0.05C, and left undisturbed for 10 minutes. After that, it was discharged with a constant current of 0.1C to a voltage of 3.6V, and left undisturbed for 5 minutes. Then it was charged with a constant current of 0.1C to a voltage of 4.46V, then charged with a constant voltage of 0.03C, and left undisturbed for 10 minutes. Finally, it was discharged with a constant current of 0.1C for 2 hours, and the voltage V1 at this time was recorded. Next, it was discharged with a constant current of 1C for 1 second (sampling every 10ms, with the current calculated to correspond to the stated capacity of the lithium-ion battery), and the voltage V2 at this time was recorded. After that, the DC impedance corresponding to the 60% remaining capacity (SOC) state of the lithium-ion battery was calculated.

[0067] The formula for calculating high-temperature DC impedance is 60%SOC DCR = (V2 - V1) / 1C.

[0068] As can be seen from Table 1, the measurement results for Examples 1-1 to 1-13 are clearly superior to the measurement results for Comparative Examples 1-1 to 1-3. This indicates that, when the mass content of ethyl propionate in the electrolyte is m%, and the mass content of propyl propionate is n%, controlling the difference between n and m (nm) to 5-54 results in a lithium-ion battery where the Lc and n of the graphite contained in the negative electrode satisfy the condition that n / Lc is 1-2.14, exhibiting significantly improved low-temperature rate performance and high-temperature, high-voltage performance.

[0069] The preparation process for Examples 2-1 to 2-25 is almost the same as that for Example 1-2. The difference is that at least one of LiFSI, LiPO2F2, and 1,3-propanesultone (PS) is selectively added to the electrolyte. Details of the specific differences are shown in Tables 2-1 and 2-2.

[0070] As can be seen from Tables 2-1 and 2-2, the measurement results of Examples 2-1 to 2-25 are clearly superior to the measurement results of Example 1-2. Therefore, by further adding one or more of LiFSI, PS, and LiPO2F2 to the electrolyte, the low-temperature characteristics and high-temperature, high-voltage performance of lithium-ion batteries can be improved, and the high-temperature, high-voltage performance of secondary batteries can be improved when the p-to-m ratio p / m is between 0.01 and 0.36.

[0071] [Table 1] [Table 2-1] [Table 2-2]

[0072] The embodiments described above are for illustrative purposes only and are not limiting. While the application has been described in detail with reference to the preferred embodiments described above, those skilled in the art should understand that modifications or equivalent substitutions to the proposed invention of this application do not depart from the spirit and scope of the proposed invention.

Claims

1. A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, The aforementioned electrolyte contains an organic solvent. The aforementioned organic solvent comprises ethyl propionate and propyl propionate. When the mass content of ethyl propionate is m% and the mass content of propyl propionate is n% relative to the mass of the electrolyte, the difference between n and m, n-m, is 5 to 54. The negative electrode includes a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector. The aforementioned negative electrode active material layer contains a negative electrode active material. The aforementioned negative electrode active material contains graphite at a mass content of 90% or more. The ratio of n to the Lc value of the graphite is n / Lc = 1 to 2.

14. A secondary battery characterized in that the Lc value of the graphite is the crystallite size along the vertical axis of the graphite measured by X-ray diffraction, and the unit of the Lc value of the graphite is nm.

2. The aforementioned electrolyte is (1) n-m is between 18 and 54, (2) m+n is between 42 and 70, (3) m is between 8 and 24, (4) n is between 29 and 62, (5) n / Lc is between 1.10 and 2.00, The secondary battery according to claim 1, characterized in that it satisfies at least one of the following conditions.

3. The aforementioned electrolyte is (6) n-m is between 21 and 47, (7) m+n is between 53 and 69, (8) m is between 8 and 19, (9) n is between 33 and 58, The secondary battery according to claim 1, characterized in that it satisfies at least one of the following conditions.

4. The aforementioned electrolyte is (I) m+n is between 52 and 67.5, (II) m is between 11 and 21, (III) n is between 30 and 53.5, The secondary battery according to claim 1, characterized in that it satisfies at least one of the following conditions.

5. The electrolyte further contains lithium bis(fluorosulfonyl)imide, The secondary battery according to claim 1, characterized in that when the mass content of lithium bis(fluorosulfonyl)imide is p% relative to the mass of the electrolyte, p is 0.07 to 4.

3.

6. The secondary battery according to claim 5, characterized in that the ratio of p to m, p / m, is 0.01 to 0.

36.

7. The aforementioned electrolyte is (I) p is between 0.7 and 2.4 (II) The p / m is between 0.07 and 0.17, The secondary battery according to claim 5, characterized in that it satisfies at least one of the following conditions.

8. The aforementioned electrolyte is (III) p is between 1.3 and 3.5, (iv) The p / m is between 0.09 and 0.32, A secondary battery according to claim 5, satisfying at least one of the following conditions.

9. The secondary battery according to claim 1, characterized in that the Lc value of the graphite is 25 to 38.

10. The secondary battery according to claim 1, characterized in that the graphite comprises at least one of artificial graphite and natural graphite.

11. The aforementioned electrolyte is (a) The electrolyte further contains lithium difluorophosphate, and the mass content of lithium difluorophosphate is 0.02% to 2.4% relative to the mass of the electrolyte. (b) The electrolyte further contains 1,3-propanesultone, and the mass content of 1,3-propanesultone is 1.7% to 4.4% of the mass of the electrolyte. The secondary battery according to claim 1, characterized in that it satisfies at least one of the following conditions.

12. The aforementioned electrolyte is (c) The electrolyte further contains lithium difluorophosphate, and the mass content of lithium difluorophosphate is 0.2% to 1.6% of the mass of the electrolyte. (d) The electrolyte further contains 1,3-propanesultone, and the mass content of 1,3-propanesultone is 1.7% to 2.9% of the mass of the electrolyte. The secondary battery according to claim 1, characterized in that it satisfies at least one of the following conditions.

13. The secondary battery according to claim 11, characterized in that the electrolyte contains 1,3-propanesultone, and the mass content of the 1,3-propanesultone is 3.1% to 3.9%.

14. An electronic device characterized by including a secondary battery according to any one of claims 1 to 13.

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