Secondary battery and electronic device

Abstract

The invention provides a secondary battery and an electronic device, and belongs to the technical field of energy storage. The secondary battery comprises a positive electrode and an electrolyte, the positive electrode comprises a positive electrode current collector and a positive electrode material layer arranged on at least one surface of the positive electrode current collector, the positive electrode material layer comprises a positive electrode active material, and the positive electrode active material comprises lithium cobalt oxide; the lithium cobalt oxide comprises a titanium element and a magnesium element, based on the mass of the lithium cobalt oxide, the mass content of the titanium element is M%, the mass content of the magnesium element is N%, M is larger than or equal to 0.01 and smaller than or equal to 0.2, and N is larger than or equal to 0.01 and smaller than or equal to 0.5; the electrolyte comprises a compound shown as a formula I; based on the total mass of the electrolyte, the total mass content of the compound in the formula I is A%, and 0.5 < = A < = 20; the positive electrode active material includes a first element including at least one of a lanthanum element, a niobium element, or a yttrium element. By controlling the doping element and content of the lithium cobalt oxide and the composition of the electrolyte, the high-temperature cycle performance and thermal safety performance of the secondary battery can be improved. Compounds of Formula I.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of energy storage, and particularly relates to a secondary battery and an electronic device. BACKGROUND

[0002] As a rechargeable energy storage device, a secondary battery can realize the storage and release of electric energy through an electrochemical reaction, and is an indispensable key component of electronic devices, electric vehicles and energy storage systems. With the continuous iteration and upgrading of technology, lithium ion batteries are widely used due to their high energy density, long cycle life and environmental friendliness.

[0003] In the development process of lithium ion batteries, the selection of positive electrode materials is crucial. Among them, lithium cobalt oxide (LCO) is widely used in devices with strict space requirements (such as mobile phones, tablet computers, etc.) due to its high specific capacity and high compaction density. However, the working voltage of lithium cobalt oxide positive electrode material is relatively high, and the positive electrode / electrolyte interface reaction activity is high. Under high temperature conditions, metal ion dissolution, material structure destruction and discharge performance deterioration problems are prone to occur. In order to improve the stability of the bulk structure of lithium cobalt oxide, a method of doping with a heteroelement is proposed, which can inhibit the destruction of lithium cobalt oxide structure under high temperature and high voltage to a certain extent, but cannot provide effective interface protection. Therefore, providing an interface improvement scheme for lithium cobalt oxide batteries and improving the high-temperature cycle performance and thermal safety performance of secondary batteries have become a technical problem to be solved. SUMMARY

[0004] Therefore, the application provides a secondary battery and an electronic device, which improves the high-temperature cycle performance and thermal safety performance of the secondary battery by controlling the doping elements and content of lithium cobalt oxide (LCO) and the composition of the electrolyte.

[0005] In a first aspect, the application provides a secondary battery, which comprises a positive electrode and an electrolyte, the positive electrode comprises a positive electrode current collector and a positive electrode material layer arranged on at least one surface of the positive electrode current collector, the positive electrode material layer comprises a positive electrode active material, and the positive electrode active material comprises lithium cobalt oxide; The lithium cobalt oxide comprises titanium elements and magnesium elements, the mass content of the titanium elements is M%, 0.01≤M≤0.2, and the mass content of the magnesium elements is N%, 0.01≤N≤0.5, based on the mass of the lithium cobalt oxide; The electrolyte comprises a compound of formula I: The compound of formula I; wherein, in R1, R2, and R3, at least one is a fluorinated alkyl group having 1 to 4 carbon atoms, and the remaining groups are fluorine-substituted or unsubstituted alkyl groups having 1 to 4 carbon atoms, fluorine-substituted or unsubstituted alkenyl groups having 2 to 4 carbon atoms, or fluorine-substituted or unsubstituted alkynyl groups having 2 to 4 carbon atoms; the total mass content of the compound of formula I is A%, 0.5≤A≤20, based on the total mass of the electrolyte; the positive electrode active material comprises a first element, and the first element comprises at least one of a lanthanum element, a niobium element, or a yttrium element; and the total mass content of the first element is P%, 0.01≤P≤2, based on the mass of the positive electrode material layer. By controlling the doping elements and content of lithium cobalt oxide (LCO) and the composition of the electrolyte, the magnesium doping and titanium doping can inhibit lattice distortion, reduce lattice oxygen release and cobalt dissolution caused by phase transition and interlayer collapse during the cycle process, reduce the catalytic activity of the electrolyte, and thus reduce the heat and gas production of the side reaction, while the introduction of the compound of formula I into the electrolyte can decompose on the surface of the LCO to form a CEI layer rich in LiF and LiPO2F2, which has strong toughness, is more compact, and is more stable, can strengthen the interface protection while reducing the lithium ion transmission energy barrier of the interface, thereby improving the high-temperature cycle performance and thermal safety performance of the secondary battery. Further, by adjusting the doping amount of magnesium and titanium elements in the lithium cobalt oxide and the addition amount of the compound of formula I, the structure of the lithium cobalt oxide can be stabilized while the interface protection is strengthened, the particle cracking caused by the sudden change of the unit cell volume accompanying the lattice phase transition of the lithium cobalt oxide at high voltage is reduced, the volume stress is buffered, the micro-crack propagation is inhibited, the catalytic activity of the positive electrode to the electrolyte is reduced, the heat production of the side reaction is reduced, and the overall performance of the secondary battery is optimized.

[0006] The total mass content of the first element satisfies the above range, which can improve the structural stability of the lithium cobalt oxide positive electrode, inhibit the lattice distortion, phase transition, and lattice oxygen release caused by deep delithiation under high temperature and high voltage conditions; at the same time, the compound of formula I in the electrolyte forms a stable Li x PO y interface film on the surface of the positive electrode, effectively inhibits the side reaction of the electrolyte with the positive electrode at high temperature. The synergistic effect of the two can effectively alleviate the capacity decay and internal pressure rise of the battery during high-temperature cycle and storage, thereby further improving the high-temperature cycle performance and thermal safety performance of the secondary battery.

[0007] In some embodiments, the secondary battery satisfies at least one of the following conditions: (1) 0.03≤M≤0.15; (2) 0.05≤N≤0.3; (3) 1≤A≤15. The mass content of the titanium element and the magnesium element in the lithium cobalt oxide and the mass content of the compound of formula I in the electrolyte satisfy the above range, which can further improve the high-temperature cycle performance and thermal safety performance of the secondary battery.

[0008] In some embodiments, the secondary battery satisfies at least one of the following conditions: (1) 0.1≤M / N≤3; (2) 0.7≤A / (M+N)≤180. Regulating the mass content ratio of titanium element and magnesium element in lithium cobalt oxide, and the mass content relationship between the compound of formula I in the electrolyte and the titanium element and magnesium element in lithium cobalt oxide satisfying the above range can further improve the high-temperature cycle performance and thermal safety performance of the secondary battery.

[0009] In some embodiments, the first element further comprises an aluminum element or a zirconium element.

[0010] In some embodiments, the compound of formula I comprises at least one of the following compounds: Formula I-1, Formula I-2, Formula I-3, Formula I-4, Formula I-5, Formula I-6, Formula I-7, Formula I-8, Formula I-9, Formula I-10, Formula I-11, Formula I-12, Formula I-13, Formula I-14, Formula I-15, Formula I-16, Formula I-17, Formula I-18, Formula I-19, Formula I-20, Formula I-21, Formula I-22, Formula I-23, Formula I-24, Formula I-25. The selection of the compound of formula I in the present application can further improve the high-temperature cycle performance and thermal safety performance of the secondary battery.

[0011] In some embodiments, the electrolyte comprises a fluorinated linear ester, the fluorinated linear ester comprising a compound of formula II and / or a compound of formula III: Formula II compound, Formula III compound; wherein R 21 and R 22 are each independently selected from a fluorine-substituted or unsubstituted C1 to C10 alkyl group, and R 21 and R 22at least one of R and R is fluorine-substituted; R 31 and R 32 are each independently selected from the group consisting of fluorine-substituted or unsubstituted C1 to C10 alkyl, and R 31 and R 32 at least one of R and R is fluorine-substituted; the mass content of the fluorinated linear ester is B%, 20≤B≤60, based on the total mass of the electrolyte. The present application regulates the mass content of the fluorinated linear ester to meet the above range, which on the one hand can improve the oxidation resistance of the electrolyte and inhibit its oxidative decomposition at high voltage; on the other hand can synergize with the compound of formula I to form a stable interface film that is dense, uniform and rich in LiF on the surface of the lithium cobaltate positive electrode, further improving the stability of the lithium cobaltate-electrolyte interface at high temperature and inhibiting the interface side reaction, thereby further improving the high-temperature cycle performance and thermal safety performance of the secondary battery. At the same time, the moderate addition of the fluorinated linear ester avoids the problem of excessive interface film thickness or impedance rise caused by excessive addition.

[0012] In some embodiments, the compound of formula II includes at least one of the following compounds: formula II-1, formula II-2, formula II-3, formula II-4, formula II-5, formula II-6, formula II-7, formula II-8, formula II-9, formula II-10, formula II-11, formula II-12, formula II-13, formula II-14, formula II-15, formula II-16, formula II-17, formula II-18, formula II-19, formula II-20, formula II-21, formula II-22, formula II-23, formula II-24, formula II-25, formula II-26.

[0013] The compound of formula II of the present application can cooperate with the electrolyte system to further improve the high-temperature cycle performance and thermal safety performance of the secondary battery.

[0014] In some embodiments, the compound of Formula III includes at least one of the following compounds: Formula III-1, Formula III-2, Formula III-3, Formula III-4, Formula III-5, Formula III-6, Formula III-7, Formula III-8, Formula III-9, Formula III-10, Formula III-11, Formula III-12, Formula III-13, Formula III-14, Formula III-15, Formula III-16, Formula III-17, Formula III-18, Formula III-19, Formula III-20.

[0015] The compound of Formula III of the present application can be coordinated with the electrolyte system, further improving the high-temperature cycle performance and thermal safety performance of the secondary battery.

[0016] In some embodiments, the electrolyte includes a nitrile compound, the nitrile compound includes at least one of succinonitrile, glutaronitrile, methylglutaronitrile, adiponitrile, pimelonitrile, suberonitrile, azelonitrile, sebonitrile, 1,2-bis(cyanoethoxy)ethane, 1,2,3-tris(2-cyanoethoxy)propane, 1,3,5-pentanetristitrite, or 1,3,6-hexanetristitrite; the mass content of the nitrile compound is C%, 0.5≤C≤5, based on the total mass of the electrolyte. The nitrile compound is used in the present application to coordinate with the above-mentioned electrolyte system, and the mass content of the nitrile compound is controlled to meet the above-mentioned range, which can occur coordination with Co elements on the surface of the lithium cobaltate positive electrode, stabilize the interface transition metal active sites, inhibit Co dissolution and side reactions at high temperature; at the same time, the coordination also helps the compound of Formula I to form a more compact, flat and uniform interface film on the surface of the positive electrode, which not only effectively blocks the oxidation and decomposition of the electrolyte, but also does not hinder the intercalation and deintercalation of lithium ions, thereby effectively inhibiting gas production, heat production and capacity attenuation, further improving the high-temperature cycle performance and thermal safety performance of the secondary battery.

[0017] In some embodiments, the electrolyte comprises a first substance, the first substance comprising at least one of vinylene carbonate, fluoroethylene carbonate, ethylene sulfate, 1,3-propane sultone or propenyl-1,3-sultone; the mass content of the first substance being D%, 0.01≤D≤10, based on the total mass of the electrolyte. The present application uses the first substance in cooperation with the above-mentioned electrolyte system, and controls the mass content of the first substance to meet the above-mentioned range, which can synergistically react with the compound of Formula I on the surface of the lithium cobaltate positive electrode to generate a dense and uniform composite interface layer, which has excellent mechanical strength and high lithium ion conductivity, effectively improves the stability of the lithium cobaltate-electrolyte interface at high temperature and high voltage, suppresses the interface side reaction, and thus further improves the high-temperature cycle performance and thermal safety performance of the secondary battery.

[0018] In a second aspect, the present application provides an electronic device comprising any one of the above-mentioned secondary batteries. DETAILED DESCRIPTION

[0019] In order to make the purpose, technical scheme and advantages of the present application more clear, the present application is further described in detail below in combination with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the present application.

[0020] In order to solve the problems in the prior art, the present application provides a secondary battery in a first aspect, the secondary battery comprising a positive electrode and an electrolyte, the positive electrode comprising a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector, the positive electrode material layer comprising a positive electrode active material, the positive electrode active material comprising lithium cobaltate; The lithium cobaltate comprises titanium elements and magnesium elements, the mass content of the titanium elements being M%, 0.01≤M≤0.2, and the mass content of the magnesium elements being N%, 0.01≤N≤0.5, based on the mass of the lithium cobaltate; The electrolyte comprises a compound of Formula I: The compound of Formula I; wherein at least one of R1, R2 and R3 is a fluorinated alkyl group with 1 to 4 carbon atoms, and the remaining groups are fluorine-substituted or unsubstituted alkyl groups with 1 to 4 carbon atoms, fluorine-substituted or unsubstituted alkenyl groups with 2 to 4 carbon atoms, or fluorine-substituted or unsubstituted alkynyl groups with 2 to 4 carbon atoms; the total mass content of the compound of Formula I being A%, 0.5≤A≤20, based on the total mass of the electrolyte. The inventors have found that by controlling the doping elements and contents of the lithium cobaltate (LCO) and the composition of the electrolyte, Mg 2+ The doping utilizes its strong interaction with Co 3+The similar ion radius embeds the Co lattice site, strengthens the lattice skeleton rigidity by virtue of the strong Mg-O bond, suppresses the lattice distortion in the cycle process at high voltage, reduces the lattice oxygen release and cobalt dissolution caused by the phase change and interlayer collapse in the cycle process, and further reduces the side reaction heat and gas production. At the same time, the combination ability of Ti and O is stronger than that of Co, which further cooperatively suppresses the interlayer collapse of lithium cobalt oxide in the cycle process and maintains the integrity of the layered structure, so that the Mg-Ti co-doping can effectively improve the cycle stability of LCO at high voltage. However, the doping elements on the surface of LCO will form metal oxides with LCO through electrochemical reaction, making it difficult for Li ions to be removed. By introducing compound I to decompose on the surface of LCO to form a CEI layer rich in LiF and LiPO2F2, which has strong toughness, is more compact and more stable, the interface protection is strengthened while the lithium ion transfer energy barrier at the interface is reduced, thereby improving the high-temperature cycle performance and thermal safety performance of the secondary battery. Further, by adjusting the doping amount of magnesium and titanium elements in lithium cobalt oxide and the addition amount of compound I, the structure of lithium cobalt oxide can be stabilized while the interface protection is strengthened, the particle cracking caused by the cell volume mutation accompanying the lattice phase change of lithium cobalt oxide at high voltage is reduced, the volume stress is buffered, the micro-crack propagation is inhibited, and the side reaction heat is reduced, so that the overall performance of the secondary battery is better.

[0021] In some embodiments, the secondary battery satisfies at least one of the following conditions: (1) 0.03≤M≤0.15; (2) 0.05≤N≤0.3; (3) 1≤A≤15. By adjusting the mass content of titanium and magnesium elements in lithium cobalt oxide and the mass content of compound I in the electrolyte to satisfy the above range, on the one hand, the stability of the bulk phase structure of lithium cobalt oxide can be better improved, and on the other hand, the lithium cobalt oxide-electrolyte interface side reaction can be better inhibited, thereby further improving the high-temperature cycle performance and thermal safety performance of the secondary battery.

[0022] In some embodiments, 0.01≤M≤0.2, preferably 0.03≤M≤0.15, for example, the value of M can be 0.01, 0.02, 0.03, 0.05, 0.06, 0.08, 0.09, 0.1, 0.11, 0.13, 0.15, 0.18, 0.2 or a value within the range formed by any two of these values. By adjusting the mass content of titanium elements in lithium cobalt oxide to satisfy the above range, the interlayer collapse of lithium cobalt oxide in the cycle process can be suppressed, the integrity of the layered structure can be maintained, the lattice oxygen release and cobalt dissolution can be reduced, and the side reaction heat and gas production can be further reduced, thereby improving the high-temperature cycle performance and thermal safety performance of the secondary battery.

[0023] In some embodiments, 0.01≤N≤0.5, preferably 0.05≤N≤0.3, for example, the value of N can be 0.01, 0.02, 0.05, 0.07, 0.09, 0.1, 0.13, 0.15, 0.18, 0.2, 0.23, 0.3, 0.35, 0.4, 0.48, 0.5, or a value within a range consisting of any two of these values. The present application regulates the mass content of magnesium in lithium cobalt oxide to meet the above range, which can inhibit lattice distortion by enhancing the rigidity of the lattice framework, reduce lattice oxygen release and cobalt dissolution caused by phase transformation and interlayer collapse during the cycle process, and thus reduce the heat and gas production of side reactions, thereby further improving the high-temperature cycle performance and thermal safety performance of the secondary battery.

[0024] In some embodiments, 0.5≤A≤20, preferably 1≤A≤15, for example, the value of A can be 0.5, 1, 2, 5, 6, 8, 9, 11, 12, 14, 15, 18, 19, 20, or a value within a range consisting of any two of these values. The present application regulates the mass content of the compound of formula I in the electrolyte to meet the above range, which can form a CEI layer rich in LiF and LiPO2F2 on the surface of LCO, which has strong toughness, is more compact and more stable, can reduce the interface lithium ion transfer energy barrier while strengthening the interface protection, and can improve the interface ionic conductivity, thereby further improving the high-temperature cycle performance and thermal safety performance of the secondary battery.

[0025] In some embodiments, the secondary battery meets at least one of the following conditions: (1) 0.1≤M / N≤3; (2) 0.7≤A / (M+N)≤180. The present application regulates the mass content ratio of titanium and magnesium in lithium cobalt oxide, and the mass content relationship between the compound of formula I in the electrolyte and titanium and magnesium in lithium cobalt oxide meets the above range, which can better improve the stability of the bulk structure of lithium cobalt oxide on the one hand, and better inhibit the lithium cobalt oxide-electrolyte interface side reaction on the other hand, thereby synergistically further improving the high-temperature cycle performance and thermal safety performance of the secondary battery.

[0026] In some embodiments, 0.1≤M / N≤3, preferably 0.5≤M / N≤1, for example, the value of M / N can be 0.1, 0.2, 0.4, 0.5, 0.7, 0.8, 1, 1.2, 1.5, 1.7, 2, 2.3, 2.8, 3, or a value within a range consisting of any two of these values. The present application regulates the mass content ratio of titanium and magnesium in lithium cobalt oxide to meet the above range, which can better improve the stability of the bulk structure of lithium cobalt oxide and inhibit the lithium cobalt oxide-electrolyte interface side reaction, thereby further improving the high-temperature cycle performance and thermal safety performance of the secondary battery.

[0027] In some embodiments, 0.7≤A / (M+N)≤180, preferably 10≤A / (M+N)≤150, for example the value of A / (M+N) can be 0.7, 1, 5, 8, 10, 12, 26, 39, 44, 57, 64, 70, 86, 91, 100, 119, 120, 136, 145, 150, 168, 175, 180 or a value within a range consisting of any two of these values. The present application regulates the relationship between the mass content of the compound of formula I in the electrolyte and the titanium element and the magnesium element in the lithium cobalt oxide to meet the above range, which can stabilize the structure of the lithium cobalt oxide while strengthening the interface protection, reducing the particle cracking caused by the sudden change of unit cell volume accompanying the lattice phase transition of lithium cobalt oxide under high voltage, buffering the volume stress, inhibiting the expansion of micro-cracks, reducing the side reaction heat, and overall making the secondary battery performance more optimal.

[0028] In some embodiments, the positive electrode active material comprises a first element, the first element comprising at least one of a lanthanum element, an aluminum element, a niobium element, a yttrium element, or a zirconium element; the total mass content of the first element is P% based on the mass of the positive electrode material layer, 0.01≤P≤2, preferably 0.05≤P≤1.5, for example the value of P can be 0.01, 0.02, 0.05, 0.07, 0.08, 0.1, 0.2, 0.5, 0.6, 0.8, 1, 1.2, 1.5, 1.7, 1.8, 2 or a value within a range consisting of any two of these values. The present application regulates the total mass content of the first element to meet the above range, which can synergistically improve the stability of the layered structure of lithium cobalt oxide with the fixing effect of the oxygen layer of the strong Mg-O bond, inhibit the lattice distortion, phase transition and lattice oxygen release caused by deep delithiation under high temperature and high voltage conditions; at the same time, the compound of formula I in the electrolyte forms a stable Li x PO y interface film, effectively inhibiting the side reaction of the electrolyte with the positive electrode under high temperature. The synergistic effect of the two can effectively alleviate the capacity decay and internal pressure rise of the battery during high-temperature cycling and storage, thereby further improving the high-temperature cycling performance and thermal safety performance of the secondary battery.

[0029] In some embodiments, the compound of formula I comprises at least one of the following compounds: formula I-1, formula I-2, formula I-3, formula I-4, formula I-5, formula I-6, formula I-7, formula I-8, formula I-9, formula I-10, formula I-11, Formula I-12, Formula I-13, Formula I-14, Formula I-15, Formula I-16, Formula I-17, Formula I-18, Formula I-19, Formula I-20, Formula I-21, Formula I-22, Formula I-23, Formula I-24, Formula I-25.

[0030] The selection of the Formula I compound can preferentially decompose on the surface of the LCO to form a CEI layer rich in LiF and LiPO2F2, which has strong toughness, is more dense and more stable, can reduce the interface lithium ion transmission energy barrier while strengthening the interface protection; at the same time, the Formula I compound itself has flame retardant properties, which can improve the thermal runaway threshold of the electrolyte, thereby further improving the high-temperature cycle performance and thermal safety performance of the secondary battery.

[0031] In some embodiments, the electrolyte includes a fluoro linear ester, the fluoro linear ester including a Formula II compound and / or a Formula III compound: Formula II compound, Formula III compound; wherein R 21 and R 22 are each independently selected from a fluorine-substituted or unsubstituted C1 to C10 alkyl, and R 21 and R 22 are each independently selected from a fluorine-substituted or unsubstituted C1 to C10 alkyl, and R 31 and R 32 are each independently selected from a fluorine-substituted or unsubstituted C1 to C10 alkyl, and R 31 and R 32at least one of which is substituted with fluorine; the mass content of the fluorinated linear ester is B% based on the total mass of the electrolyte, 20≤B≤60, preferably 30≤B≤50, for example the value of B can be 20, 23, 25, 28, 30, 35, 37, 40, 43, 44, 46, 50, 52, 57, 60 or a value within a range consisting of any two of these values. The present application regulates the mass content of the fluorinated linear ester to meet the above range, which on the one hand can improve the oxidation resistance of the electrolyte and inhibit its oxidative decomposition at high voltage; on the other hand can synergize with the compound of formula I to form a stable interface film rich in LiF on the surface of the lithium cobaltate positive electrode, further improving the stability of the lithium cobaltate-electrolyte interface at high temperature and inhibiting the interface side reaction, thereby further improving the high-temperature cycle performance and thermal safety performance of the secondary battery. At the same time, the moderate addition of the fluorinated linear ester avoids the problem of excessive interface film thickness or impedance rise caused by excessive addition.

[0032] In some embodiments, the compound of formula II includes at least one of the following compounds: formula II-1, formula II-2, formula II-3, formula II-4, formula II-5, formula II-6, formula II-7, formula II-8, formula II-9, formula II-10, formula II-11, formula II-12, formula II-13, formula II-14, formula II-15, formula II-16, formula II-17, formula II-18, formula II-19, formula II-20, formula II-21, formula II-22, formula II-23, formula II-24, formula II-25, formula II-26.

[0033] The compound of formula II is matched with the electrolyte system, which can eliminate the influence of the high viscosity of the compound of formula I on the conductivity of the electrolyte, inhibit the interface side reaction of the positive electrode under high voltage and high temperature, improve the oxidation potential of the electrolyte and the ion migration rate in the electrolyte, and further improve the high-temperature cycle performance and thermal safety performance of the secondary battery.

[0034] In some embodiments, the compound of formula III includes at least one of the following compounds: Formula III-1, Formula III-2, Formula III-3, Formula III-4, Formula III-5, Formula III-6, Formula III-7, Formula III-8, Formula III-9, Formula III-10, Formula III-11, Formula III-12, Formula III-13, Formula III-14, Formula III-15, Formula III-16, Formula III-17, Formula III-18, Formula III-19, Formula III-20.

[0035] The compound of formula III is matched with the electrolyte system, which can further reduce the influence of the high viscosity of the compound of formula I on the conductivity of the electrolyte, inhibit the interface side reaction of the positive electrode under high voltage and high temperature, improve the oxidation potential of the electrolyte and the ion migration rate in the electrolyte, and further improve the high-temperature cycle performance and thermal safety performance of the secondary battery.

[0036] In some embodiments, the electrolyte comprises a nitrile compound, the nitrile compound comprising at least one of succinonitrile, glutaronitrile, methylglutaronitrile, adiponitrile, pimelonitrile, suberonitrile, azelonitrile, sebonitrile, 1,2-bis(cyanoethoxy)ethane, 1,2,3-tris(2-cyanoethoxy)propane, 1,3,5-pentanetrione or 1,3,6-hexanetrione; the mass content of the nitrile compound is C%, 0.5≤C≤5, preferably 1.4≤C≤3.8, for example C can be 0.5, 1.0, 1.4, 2.0, 2.3, 2.6, 3.2, 3.5, 3.8, 4.0, 4.6, 5 or a value within a range consisting of any two of these values, based on the total mass of the electrolyte. The nitrile compound is used in the electrolyte system described above, and the mass content of the nitrile compound is controlled to satisfy the range described above, which can form a complex with Co elements on the surface of the lithium cobalt oxide cathode, stabilizing the interface transition metal active site, inhibiting Co dissolution and side reactions at high temperatures; at the same time, the complexation also helps the compound of formula I to form a more compact, flat and uniform interface film on the surface of the cathode, which effectively blocks the oxidation and decomposition of the electrolyte, and does not hinder the intercalation and deintercalation of lithium ions, thereby effectively inhibiting gas production, heat production and capacity decay, and further improving the high-temperature cycle performance and thermal safety performance of the secondary battery.

[0037] In some embodiments, the electrolyte comprises a first substance, the first substance comprising at least one of vinylene carbonate, fluoroethylene carbonate, ethylene sulfate, 1,3-propane sultone or propenyl-1,3-sulfonic acid lactone; the mass content of the first substance is D%, 0.01≤D≤10, preferably 1.5≤D≤4, for example D can be 0.01, 1.5, 2.2, 2.6, 3.1, 3.5, 4.0, 4.3, 5.5, 6.9, 7.3, 8.0, 9.1, 10 or a value within a range consisting of any two of these values, based on the total mass of the electrolyte. The first substance is used in the electrolyte system described above, and the mass content of the first substance is controlled to satisfy the range described above, which can synergistically react with the compound of formula I on the surface of the lithium cobalt oxide cathode to generate a dense and uniform composite interface layer, which has excellent mechanical strength and high lithium ion conductivity, effectively improving the stability of the lithium cobalt oxide-electrolyte interface at high temperature and high voltage, inhibiting interface side reactions, and further improving the high-temperature cycle performance and thermal safety performance of the secondary battery.

[0038] In the present application, the electrolyte further includes a lithium salt and a nonaqueous solvent. The lithium salt can include, but is not limited to, at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis-trifluoromethanesulfonimide {LiN(CF3SO2)2, LiTFSI}, lithium bis(fluorosulfonyl)imide {Li(N(SO2F)2), LiFSI}, lithium bis(oxalato)borate {LiB(C2O4)2, LiBOB}, lithium difluoro(oxalato)borate {LiBF2(C2O4), LiDFOB}, LiNO3, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiC(SO2CF3)3, Li2SiF6, lithium bis-trifluoromethanesulfonimide (LiTFSI), or lithium difluoroborate. The present application does not limit the content of the lithium salt in the electrolyte, as long as the purpose of the present application can be achieved.

[0039] The non-aqueous solvent is not particularly limited in the present application, as long as it can function as a medium for moving ions involved in electrochemical reactions of the battery. For example, the non-aqueous solvent can include, but is not limited to, at least one of a carbonate compound, a carboxylate compound, an ether compound, or other organic solvents. The carbonate compound described above can include, but is not limited to, at least one of a chain carbonate compound and a cyclic carbonate compound. The chain carbonate compound described above can include, but is not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, or ethyl methyl carbonate (EMC). The cyclic carbonate described above can include, but is not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, or vinyl ethylene carbonate. The carboxylate compound described above can include, but is not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valerolactone, or hexanolactone. The ether compound described above can include, but is not limited to, at least one of 1,3-dioxolane (DOL), ethylene glycol dimethyl ether (1,2-dimethoxy ethane, DME), dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-diethoxy ethane, 1-ethoxy-1-methoxy ethane, 2-methyl tetrahydrofuran, or tetrahydrofuran. The other organic solvents described above can include, but are not limited to, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidinone, a ketone solvent such as cyclohexanone; an aromatic hydrocarbon solvent such as benzene, fluorobenzene; an alcohol solvent such as ethanol and isopropyl alcohol; a nitrile solvent (e.g., acetonitrile) such as R-CN (R is a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms, and can include a double-bonded aromatic ring or an ether bond); an amide solvent such as dimethylformamide; a dioxolane solvent such as 1,2-dioxolane, 1,3-dioxolane; or a sulfone solvent such as dimethyl sulfoxide, cyclobutyl sulfone, methyl cyclobutyl sulfone; or a phosphate ester solvent such as trimethyl phosphate, triethyl phosphate, trioctyl phosphate. In the above, the hydrocarbon group can be selected from one or more of an alkyl group, an alkenyl group, or an alkynyl group.

[0040] Positive electrode In the present application, the positive electrode is not particularly limited, as long as it can achieve the object of the present application. The positive electrode includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode current collector; the "positive electrode material layer provided on at least one surface of the positive electrode current collector" means that the positive electrode material layer can be located on one surface of the positive electrode current collector in the thickness direction of the positive electrode current collector, or can be located on both surfaces of the positive electrode current collector in the thickness direction of the positive electrode current collector. It should be noted that the "surface" here can be the entire area of the surface of the positive electrode current collector, or can be a partial area of the surface of the positive electrode current collector, and the present application is not particularly limited, as long as it can achieve the object of the present application.

[0041] The positive electrode current collector of the present application is not particularly limited as long as the object of the present application can be achieved. For example, the positive electrode current collector can include an aluminum foil, an aluminum alloy foil, or a composite current collector (e.g., an aluminum-carbon composite current collector), etc. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer substrate.

[0042] The positive electrode current collector of the present application can have an appropriate thickness as needed. Although not particularly limited, the positive electrode current collector can have a thickness in the range of 1 µm to 500 µm, or can have a thickness in the range of 1 µm to 300 µm, or can have a thickness in the range of 1 µm to 100 µm, or can have a thickness in the range of 1 µm to 50 µm, or can have a thickness in the range of 1 µm to 20 µm.

[0043] The terms thickness (or height), width, and length used in the present application mean average values, and can be measured by a measuring instrument that can measure the thickness (or height), width, and length, respectively, and according to the method in the art, unless otherwise specifically stated.

[0044] The positive electrode current collector can form fine irregularities on the surface, thereby further enhancing the adhesion with the positive electrode material layer. For example, the positive electrode current collector can be in the form of one or more selected from a film, a sheet, a foil, a mesh, a porous body, a foam, and a nonwoven fabric.

[0045] The positive electrode material layer of the present application includes a positive electrode active material, and the positive electrode active material of the present application can further include at least one of lithium nickel cobalt manganese oxide (LiNi 0.90 Co 0.05 Mn 0.05 O2 (NCM955), NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium-rich manganese-based material, lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, spinel-type lithium manganese oxide, spinel-type lithium nickel manganese oxide, and lithium titanate.

[0046] In the present application, the positive electrode material layer can further include a positive electrode binder and a positive electrode conductive agent. The present application does not particularly limit the type of the positive electrode binder in the positive electrode material layer as long as the object of the present application can be achieved, and for example, the positive electrode binder can include, but is not limited to, at least one of polyvinylidene fluoride, polytetrafluoroethylene, a polyolefin-based, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, modified polyvinylidene fluoride, modified styrene butadiene rubber (SBR), or polyurethane. In some embodiments, the polyolefin-based binder includes at least one of polyethylene, polypropylene, polyolefin ester, polyolefin alcohol, or polyacrylic acid.

[0047] The kind of the positive electrode conductive agent in the positive electrode material layer is not particularly limited in the present application, as long as the object of the present application can be achieved. In some embodiments, the positive electrode conductive agent includes a carbon-based material, such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, or carbon fiber; a metal-based material, such as metal powder or metal fiber of copper, nickel, aluminum, silver, or the like; a conductive polymer, such as a polyphenylene derivative; or a mixture thereof. The mass ratio of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder in the positive electrode material layer is not particularly limited in the present application, and can be selected by a person skilled in the art according to actual needs, as long as the object of the present application can be achieved. For example, the loading amount of the positive electrode active material in the positive electrode sheet is 4.0 mg / cm 2 to 10.0 mg / cm 2 .

[0048] In the present application, the positive electrode material layer can be formed by coating a positive electrode slurry on at least one side of the positive electrode current collector and drying, and calendering can be performed after drying as needed. The positive electrode slurry contains the positive electrode material and the positive electrode binder described above, and can further contain a conductive agent as needed. In addition, the positive electrode slurry can also contain a solvent, and the kind of the solvent is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the solvent can use an organic solvent such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, formamide, dimethylformamide, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolinone, propylene carbonate derivative, tetrahydrofuran derivative, methyl propionate, alcohol, or ethyl propionate, or an aqueous solvent such as water, or a mixed solvent composed of two or more kinds of the above-mentioned solvents, and the like.

[0049] The mass ratio of the positive electrode material, the positive electrode conductive agent, and the positive electrode binder in the positive electrode material layer is not particularly limited in the present application, and can be selected by a person skilled in the art according to actual needs, as long as the object of the present application can be achieved. These mass ratios can apply the known mass ratios.

[0050] Negative electrode The negative electrode is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the negative electrode includes a negative electrode current collector and a negative electrode material layer provided on at least one surface of the negative electrode current collector. The "negative electrode material layer provided on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be provided on one surface in the thickness direction of the negative electrode current collector, or can be provided on both surfaces in the thickness direction of the negative electrode current collector. Note that the "surface" herein can be the entire area of the negative electrode current collector, or can be a partial area of the negative electrode current collector. The present application is not particularly limited, as long as the object of the present application can be achieved.

[0051] The negative electrode current collector is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, it can include, but is not limited to, a copper foil, a copper alloy foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam, or a composite current collector (for example, a carbon-copper composite current collector, a nickel-copper composite current collector, a titanium-copper composite current collector, etc.), and the like.

[0052] The negative electrode current collector of the present application can have an appropriate thickness as needed. Although not particularly limited, the negative electrode current collector can have a thickness in the range of 1 μm to 500 μm, or can have a thickness in the range of 1 μm to 300 μm, or can have a thickness in the range of 1 μm to 100 μm, or can have a thickness in the range of 1 μm to 50 μm, or can have a thickness in the range of 1 μm to 20 μm, or can have a thickness in the range of 5 μm to 10 μm.

[0053] The negative electrode current collector can form fine irregularities on the surface, so that the adhesion to the negative electrode material layer can be further enhanced. For example, the negative electrode current collector can be in the form of one or more selected from a film, a sheet, a foil, a mesh, a porous body, a foam, and a non-woven fabric.

[0054] The negative electrode material layer of the present application includes a negative electrode active material, which can include, but is not limited to, graphite, mesocarbon microbeads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO x (0.5 < x < 1.6), Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel-structured lithium titanate Li4Ti5O 12 Li-Al alloy, and metallic lithium.

[0055] The negative electrode material layer in the present application can further include a negative electrode binder and a negative electrode conductive agent, or the negative electrode material layer can further include a negative electrode binder, a negative electrode conductive agent, and a thickening agent. The present application does not have a particular limitation on the type of the negative electrode binder in the negative electrode material layer as long as the purpose of the present application can be achieved, for example, the negative electrode binder can include, but is not limited to, at least one of polyvinylidene fluoride (PVDF), polyvinyl alcohol, styrene butadiene rubber (SBR), polyethylene oxide, carboxymethyl cellulose (CMC), hydroxypropyl cellulose, cellulose acetate, diacetyl cellulose, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, polymethyl methacrylate, polybutyl acrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyarylate, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymer, polyurethane, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, styrene butadiene rubber, acrylated styrene butadiene rubber, epoxy resin, or nylon.

[0056] The present application does not have a particular limitation on the type of the negative electrode conductive agent in the negative electrode material layer as long as the purpose of the present application can be achieved. In some embodiments, the negative electrode conductive agent includes a carbon-based material, such as graphite, e.g., natural graphite or artificial graphite, carbon black, e.g., acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, or the like, or carbon fiber; a metal-based material, such as metal powder or metal fiber of copper, nickel, aluminum, silver, or the like; a conductive polymer, such as a polyphenylene derivative; a conductive metal oxide, such as zinc oxide, titanium oxide, or the like; a conductive whisker, such as potassium titanate, or the like; or a mixture formed by any combination of these substances.

[0057] The present application does not have a particular limitation on the type of the thickening agent as long as the purpose of the present application can be achieved, for example, the thickening agent can include, but is not limited to, at least one of sodium carboxymethyl cellulose or carboxymethyl cellulose. The present application does not have a particular limitation on the mass ratio of the negative electrode active material, the negative electrode conductive agent, the negative electrode binder, and the thickening agent in the negative electrode material layer, and a person skilled in the art can select according to actual needs as long as the purpose of the present application can be achieved.

[0058] In the present application, the negative electrode material layer can be formed by coating a negative electrode slurry on at least one side of the negative electrode current collector and drying, and calendering can be performed after drying as necessary. The negative electrode slurry contains the negative electrode material and the negative electrode binder described above, and can further contain a negative electrode conductive agent as necessary. In addition, the negative electrode slurry can further contain a solvent, and the kind of the solvent is not particularly limited in the present application as long as the object of the present application can be achieved. For example, the solvent can use an organic solvent such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, formamide, dimethylformamide, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolinone, propylene carbonate derivatives, tetrahydrofuran derivatives, methyl propionate, alcohol, or ethyl propionate, or an aqueous solvent such as water, or a mixed solvent composed of two or more kinds of the above-described solvents, and the like.

[0059] Separator The separator of the present application refers to a film that prevents short circuiting between the positive electrode and the negative electrode while allowing the electron transport substance to pass through, and the separator is not particularly limited in the present application as long as the object of the present application can be achieved. For example, the material of the separator can include, but is not limited to, at least one of polyethylene (PE), polyolefin (PO) based on polypropylene (PP), polyester (for example, polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, aramid; and the type of the separator can include at least one of woven film, nonwoven film, microporous film, composite film, calendered film, and spunlaced film.

[0060] According to some embodiments of the present application, the separator can include a base material layer and a surface treatment layer. The base material layer can be a nonwoven fabric, a film, or a composite film having a porous structure, and the material of the base material layer can include at least one of polyethylene, polypropylene, polyethylene terephthalate, 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 can be used.

[0061] Optionally, a surface treatment layer is provided on at least one surface of the substrate layer, which can be a polymer layer or an inorganic layer, or a layer formed by mixing polymer and inorganic material. For example, the inorganic layer includes inorganic particles and a binder, which are not particularly limited in the present application, and can include at least one of alumina, silica, magnesia, titania, hafnia, tin oxide, ceria, nickel oxide, zinc oxide, calcium oxide, zirconia, yttria, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. The binder is not particularly limited in the present application, and can be at least one of the positive electrode binder or the negative electrode binder described above. The polymer layer includes a polymer, which is not particularly limited in the present application, and can include at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylic acid salt, polyvinylpyrrolidone, polyvinyl ether, or polyvinylidene fluoride, poly(vinylidene fluoride-hexafluoropropylene). In the present application, the thickness of the separator is not particularly limited, as long as the object of the present application can be achieved, and can be, for example, 5 μm to 500 μm.

[0062] The secondary battery of the present application further includes a packaging bag for containing the positive electrode, the negative electrode, the separator, and the electrolyte, and other components known in the art, which are not limited in the present application. The packaging bag is not particularly limited in the present application, and can be a packaging bag known in the art, as long as the object of the present application can be achieved.

[0063] The preparation process of the secondary battery of the present application is well known to those skilled in the art, and is not particularly limited in the present application, and can include, but is not limited to, the following steps: stacking the positive electrode sheet, the separator, and the negative electrode sheet in order, and winding, folding, or the like as needed to obtain an electrode assembly in a wound structure, placing the electrode assembly in a packaging bag, injecting electrolyte into the packaging bag and sealing it to obtain a secondary battery; or stacking the positive electrode sheet, the separator, and the negative electrode sheet in order, then fixing the four corners of the entire stack structure with a tape to obtain an electrode assembly in a stack structure, placing the electrode assembly in a packaging bag, injecting electrolyte into the packaging bag and sealing it to obtain a secondary battery. In addition, a current protection element, a guide plate, or the like can be placed in the packaging bag as needed to prevent the pressure inside the secondary battery from rising and overcharging or discharging.

[0064] In a second aspect, the present application provides an electronic device comprising any of the secondary batteries described above.

[0065] In some embodiments, the electronic device of the present application includes, but is not limited to, notebook computers, pen input computers, mobile computers, electronic book players, portable telephones, portable facsimile machines, portable copying machines, portable printers, head-mounted stereo headphones, video recorders, liquid crystal televisions, portable cleaners, portable CD players, mini-disc players, transceivers, electronic organizers, calculators, memory cards, portable audio recorders, radios, backup power supplies, motors, automobiles, motorcycles, power-assisted bicycles, bicycles, lighting appliances, toys, game machines, timepieces, power tools, flashlights, cameras, home-use large storage batteries, and lithium ion capacitors, etc.

[0066] The scheme of the present application will be described below with lithium ion batteries as examples in combination with the following specific embodiments. If no special description is given, the raw materials used in the following embodiments are all from ordinary commercial products, and the devices or equipment used are all purchased from conventional market sales channels. Various tests and evaluations are carried out according to the following methods. In addition, as long as no special description is given, "parts" and "%" are mass-based.

[0067] Test methods Element mass content test: The microwave digestion method and PE ICP-OES Optima 7000DV are used to test the mass proportion of metal elements.

[0068] High-temperature cycle performance test: The lithium ion battery is placed in a 45°C constant temperature environment and left for 30 min to make the lithium ion battery reach a constant temperature. It is charged at 0.5C constant current to 4.53V, then charged at 4.53V constant voltage to a current of 0.025C, left for 5 min, discharged at 0.5C constant current to 3.0V, and recorded as the initial discharge capacity C0; the above conditions are repeated for 600 cycles of charge and discharge, and the discharge capacity C1 after 600 cycles is recorded, the high-temperature cycle capacity retention rate is calculated, and used as an index for evaluating the high-temperature discharge performance of the lithium ion battery; High-temperature cycle capacity retention rate%=C1 / C0x100%.

[0069] Thermal safety performance test: The lithium ion battery is placed in a constant temperature environment of 25°C, and is left for 30 min to make the lithium ion battery reach constant temperature. After 1C constant current discharge to 2.8V, it is left for 10 min, and then 1C constant current charging to 4.53V, and then constant voltage charging to 0.05C at 4.53V, and left for 10 min for heat box test. Before testing, take a photo, measure the voltage resistance, and paste the temperature sensing line on the surface of the battery, and then put the sample into the heating furnace box, and heat to 130±2°C at a temperature rising speed of 5±2°C / min and keep for 60 min; after testing, take a photo, measure the voltage resistance. The battery does not catch fire or explode, and the parallel test of 20 samples is passed, and the heat box test pass rate is calculated and used as an index for evaluating the safety performance of the lithium ion battery; Heat box test pass rate%=number of samples passed / 20x100%.

[0070] Example 1-1 <Preparation of electrolyte> In an argon atmosphere glove box with water content less than 10 ppm, ethylene carbonate: propylene carbonate: propyl acetate: propyl propionate = 0.8: 1.1: 1.2: 1.5 (mass ratio) is used as a base solvent, then lithium hexafluorophosphate (LiPF6) and the compound of formula I are added, and stirred to obtain an electrolyte. Based on the total mass of the electrolyte, the mass content of lithium hexafluorophosphate is 12.5%, and the type and mass content of the compound of formula I are shown in Table 1 below, and the rest is the base solvent.

[0071] <Preparation of negative electrode sheet> Silicon-carbon particles and graphite particles are mixed in a mass ratio of 1:4 to obtain a negative electrode active material; the negative electrode active material, negative electrode conductive agent Super P, thickening agent sodium carboxymethyl cellulose (CMC), and negative electrode binder styrene-butadiene rubber (SBR) are mixed in a mass ratio of 95:2:1:2, then deionized water is added, and a negative electrode slurry with a solid content of 60wt% is prepared, the negative electrode slurry is uniformly coated on a negative electrode current collector copper foil with a thickness of 10µm, and then dried and cold-pressed to obtain a negative electrode sheet.

[0072] <Preparation of positive electrode active material lithium cobaltate> 286.7 g of Co(NO3)2·6H2O was weighed and dissolved in 1 L of deionized water to obtain a Co salt solution at room temperature by stirring. The concentration of NH4·HCO3 solution, the Co nitrate solution and the 40 g / L lithium hydroxide solution were 80 g / L, 80 g / L and 40 g / L respectively. The solutions were added to the reactor containing 5 L of deionized water by using a peristaltic pump in a co-current manner to carry out the co-precipitation reaction. The temperature of the reactor was controlled at 50°C, the feeding speed of each solution was adjusted, and the pH was controlled at 8±0.5. The feeding was continuously carried out for 24 hours. After the stirring was stopped, the precipitate was filtered and washed with deionized water until the pH of the filtrate was less than 7. The precipitate was dried at 90°C to obtain a precursor precipitate with a D50 of 4-5 μm and a spherical shape. The precursor precipitate was mixed with 79.0 g of lithium carbonate, 0.0161 g of titanium oxide and 0.800 g of magnesium oxide by high-speed ball milling. The mixture was sintered at 980°C for 10 hours in an air atmosphere to form a doped lithium cobalt oxide.

[0073] The mass content of the doping elements in the lithium cobalt oxide was controlled by adjusting the amounts of Co(NO3)2·6H2O, lithium carbonate, titanium oxide and magnesium oxide in the raw materials.

[0074] Preparation of the positive electrode sheet The positive electrode active material lithium cobalt oxide (LiCoO2), the positive electrode binder polyvinylidene fluoride (PVDF) and the positive electrode conductive agent Super P were mixed in a mass ratio of 97:1.6:1.4, N-methyl pyrrolidone (NMP) was added, and a slurry with a solid content of 75 wt% was prepared and stirred uniformly. The positive electrode slurry was uniformly coated on a positive electrode current collector aluminum foil with a thickness of 10 μm, dried and cold-pressed to obtain a positive electrode sheet.

[0075] Separator Polyethylene (PE) with a thickness of 7 μm and a porosity of 55% was used as a base film. PVDF slurry and inorganic particle (mass ratio of flaky boehmite and Al2O3 being 70:30) slurry were distributed and coated on both surfaces of the base film, and dried to obtain a separator. The coating thickness of each surface of the separator was 3 μm.

[0076] Preparation of the lithium ion battery The positive electrode sheet, the separator and the negative electrode sheet prepared above were stacked in order with the separator between the positive electrode sheet and the negative electrode sheet to play a barrier role, and then wound to obtain an electrode assembly. After the tab was welded, the electrode assembly was placed in an aluminum plastic film packaging shell, dried in a vacuum oven at 85°C for 12 hours to remove water, and then injected with the prepared electrolyte. After vacuum packaging, standing and formation, a lithium ion battery was obtained.

[0077] The main difference between each example and comparative example in Table 1 and Example 1-1 is that the parameters shown in Table 1 are adjusted.

[0078] Table 1

[0079] Note: The "M / N" values of Comparative Examples 1-4, Examples 1-10 to 1-13, Example 1-23, and Example 1-26 in Table 1 are kept to two decimal places. The "A / (M+N)" values in Table 1 are kept to one decimal place.

[0080] As can be seen from Table 1, by controlling the doping of titanium and magnesium elements in the lithium cobalt oxide and the electrolyte including the compound of Formula I, and controlling the mass content M% of the titanium element to satisfy 0.01≤M≤0.2, the mass content N% of the magnesium element to satisfy 0.01≤N≤0.5, and the total mass content A% of the compound of Formula I to satisfy 0.5≤A≤20, the high-temperature cycle performance and thermal safety performance of the secondary battery can be improved. Especially, when the respective mass contents are controlled to satisfy 0.03≤M≤0.15, 0.05≤N≤0.3, and / or 1≤A≤15, the high-temperature cycle performance and thermal safety performance of the secondary battery can be further improved.

[0081] And when the mass content ratio M / N of the titanium and magnesium elements in the lithium cobalt oxide is controlled to satisfy 0.1≤M / N≤3, and the mass content relationship of the compound of Formula I in the electrolyte and the titanium and magnesium elements in the lithium cobalt oxide satisfies 0.7≤A / (M+N)≤180, the high-temperature cycle performance and thermal safety performance of the secondary battery can be further improved. Especially, when 0.5≤M / N≤1 and 10≤A / (M+N)≤150 are controlled, the high-temperature cycle performance and thermal safety performance of the secondary battery can be further improved.

[0082] The main difference between each example in Table 2 and Example 1-19 is that the positive active material includes a first element, and the positive active material is prepared by a method including the following steps: <Preparation of positive active material lithium cobalt oxide> Example 2-1: 286.7 g of Co(NO3)2·6H2O was weighed and dissolved in 1 L of deionized water to obtain a Co salt solution at room temperature. The concentration of NH4·HCO3 solution, the cobalt nitrate solution and the lithium hydroxide solution were 80 g / L, 80 g / L and 40 g / L respectively. The solutions were added to the reactor containing 5 L of deionized water by peristaltic pump in a concurrent manner to carry out the co-precipitation reaction. The temperature of the reactor was controlled at 50°C, the pH was controlled at 8±0.5 by adjusting the feeding speed of each solution, and the continuous feeding was carried out for 24 hours. After stopping the stirring, the precipitate was washed with deionized water until the pH of the filtrate was less than 7, and then dried at 90°C to obtain a precursor precipitate with a D50 of 4-5 μm and a spherical shape. The precursor precipitate was mixed with 79.0 g of lithium carbonate, 0.0805 g of titanium oxide, 0.0800 g of magnesium oxide and 0.0113 g of lanthanum oxide by high-speed ball milling, and then sintered at 980°C for 10 hours in an air atmosphere to form a doped lithium cobalt oxide.

[0083] The mass content of the doping elements in the lithium cobalt oxide can be controlled by adjusting the amounts of Co(NO3)2·6H2O, lithium carbonate, titanium oxide, magnesium oxide and lanthanum oxide (or aluminum oxide, niobium oxide, yttrium oxide or zirconium oxide) in the raw materials.

[0084] Table 2

[0085] As shown in Table 2, by doping the first element in the positive active material and adjusting the total mass content P% of the first element to satisfy 0.01≤P≤2, preferably 0.05≤P≤1.5, the high-temperature cycle performance and thermal safety performance of the secondary battery can be further improved.

[0086] The main difference between each example in Table 3 and Example 2-3 is that the mass proportions of the fluorinated linear ester, the nitrile compound and the first substance in the electrolyte are adjusted according to Table 3. The specific adjustment parameters and performance test results are shown in Table 3. When the components or the amount of the components in the electrolyte are increased, the amount of the base solvent is reduced accordingly, and the mass ratio of the components in the base solvent is kept unchanged.

[0087] The electrolyte was prepared according to the method comprising the following steps: In an argon atmosphere glove box with water content less than 10 ppm, ethylene carbonate, propylene carbonate, propyl acetate, propyl propionate (mass ratio 0.8:1.1:1.2:1.5) were used as a base solvent, then lithium hexafluorophosphate (LiPF6), the compound of formula I, a fluorinated linear ester, a nitrile compound and a first substance were added, and stirred to obtain an electrolyte. Among them, the mass content of lithium hexafluorophosphate was 12.5% based on the total mass of the electrolyte, the type and mass content of the compound of formula I were the same as in Example 2-3, the types and mass contents of the fluorinated linear ester, the nitrile compound and the first substance were shown in Table 3 below, and the rest was the base solvent.

[0088] Table 3

[0089] Note: " / " in Table 3 indicates that the corresponding substance or parameter does not exist.

[0090] As can be seen from Table 3, by adding the fluorinated linear ester to the electrolyte and adjusting the mass content B% to satisfy 20≤B≤60, preferably 30≤B≤50, the high-temperature cycle performance and thermal safety performance of the secondary battery can be further improved.

[0091] On this basis, by adding the nitrile compound to the electrolyte and adjusting the mass content C% to satisfy 0.5≤C≤5, preferably 1.4≤C≤3.8, the high-temperature cycle performance and thermal safety performance of the secondary battery can be further improved.

[0092] By adding the first substance to the electrolyte and adjusting the mass content D% to satisfy 0.01≤D≤10, preferably 1.5≤D≤4, the high-temperature cycle performance and thermal safety performance of the secondary battery can be further improved.

[0093] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modifications, equivalent replacements and improvements made within the principles of the present application shall be included in the protection scope of the present application.

Claims

1. A secondary battery, characterized in that, The secondary battery includes a positive electrode and an electrolyte. The positive electrode includes a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector. The positive electrode material layer includes a positive electrode active material, which includes lithium cobalt oxide. The lithium cobalt oxide comprises titanium and magnesium. Based on the mass of the lithium cobalt oxide, the mass content of titanium is M%, 0.01≤M≤0.2%, and the mass content of magnesium is N%, 0.01≤N≤0.5%. The electrolyte comprises a compound of formula I: Compound of Formula I; In the presence of R1, R2, and R3, at least one is a C1 to C4 fluorinated alkyl group, and the remaining groups are fluorinated or unsubstituted C1 to C4 alkyl groups, fluorinated or unsubstituted C2 to C4 alkenyl groups, or fluorinated or unsubstituted C2 to C4 alkynyl groups; based on the total mass of the electrolyte, the total mass content of the compound of formula I is A%, 0.5 ≤ A ≤ 20; The positive electrode active material includes a first element, which includes at least one of lanthanum, niobium, or yttrium; based on the mass of the positive electrode material layer, the total mass content of the first element is P%, 0.01≤P≤2.

2. The secondary battery according to claim 1, characterized in that, The secondary battery satisfies at least one of the following conditions: （1）0.03≤M≤0.15； （2）0.05≤N≤0.3； （3）1≤A≤15； (4) 0.1≤M / N≤3; (5) 0.7≤A / (M+N)≤180.

3. The secondary battery according to claim 1, characterized in that, The first element also includes aluminum or zirconium.

4. The secondary battery according to any one of claims 1 to 3, characterized in that, The compound of formula I includes at least one of the following compounds: Formula I-1, Formula I-2, Formula I-3, Formula I-4, Formula I-5, Formula I-6, Formula I-7, Formula I-8, Formula I-9, Formula I-10, Formula I-11, Formula I-12, Formula I-13, Formula I-14, Formula I-15, Formula I-16, Formula I-17, Formula I-18, Formula I-19, Formula I-20, Formula I-21, Formula I-22, Formula I-23, Formula I-24, Formula I-25.

5. The secondary battery according to any one of claims 1 to 3, characterized in that, The electrolyte comprises a fluorinated linear ester, which includes compounds of formula II and / or formula III: Formula II compound, Formula III compound; Among them, R 21 and R 22 Each is independently selected from fluorinated or unsubstituted C1 to C10 alkyl groups, and R 21 and R 22 At least one of them is fluorinated; R 31 and R 32 Each is independently selected from fluorinated or unsubstituted C1 to C10 alkyl groups, and R 31 and R 32 At least one of them is fluorinated; Based on the total mass of the electrolyte, the total mass content of the fluorinated linear ester is B%, and 20 ≤ B ≤ 60%.

6. The secondary battery according to claim 5, characterized in that, The compound of formula II includes at least one of the following compounds: Formula II-1 Formula II-2 Formula II-3 Formula II-4 Formula II-5 Formula II-6 Formula II-7 Formula II-8 Formula II-9 Formula II-10 Formula II-11 Formula II-12 Formula II-13 Formula II-14 Formula II-15 Formula II-16 Formula II-17 Formula II-18 Formula II-19 Formula II-20 Formula II-21 Formula II-22 Formula II-23 Formula II-24 Formula II-25 Formula II-26.

7. The secondary battery according to claim 5, characterized in that, The compound of formula III includes at least one of the following compounds: Formula III-1 Formula III-2 Formula III-3 Formula III-4 Formula III-5 Formula III-6 Formula III-7 Formula III-8 Formula III-9 Formula III-10 Formula III-11 Formula III-12 Formula III-13 Formula III-14 Formula III-15 Formula III-16 Formula III-17 Formula III-18 Formula III-19 Formula III-20.

8. The secondary battery according to any one of claims 1 to 3, characterized in that, The electrolyte comprises nitrile compounds, including at least one selected from succinic anionyl nitrile, glutaronitrile, methylglutaronitrile, adiponitrile, heptacyanide, octanoic anionyl nitrile, nonadionitrile, sebaconitrile, 1,2-bis(cyanoethoxy)ethane, 1,2,3-tris(2-cyanoethoxy)propane, 1,3,5-pentanetrionitrile, or 1,3,6-hexanetrionitrile; Based on the total mass of the electrolyte, the mass content of the nitrile compound is C%, 0.5≤C≤5.

9. The secondary battery according to any one of claims 1 to 3, characterized in that, The electrolyte includes a first substance, which includes at least one of vinylene carbonate, fluorovinyl carbonate, ethylene sulfate, 1,3-propane sulpholactone, or propenyl-1,3-sulfonyl lactone. Based on the total mass of the electrolyte, the mass content of the first substance is D%, 0.01≤D≤10.

10. An electronic device, characterized in that, Includes the secondary battery as described in any one of claims 1 to 9.

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