Lithium secondary batteries and electrical devices

Abstract

This application provides a lithium secondary battery and an electrical device. The lithium secondary battery includes an electrolyte comprising a chain carboxylic acid ester, a cyclic carbonate, and a sulfate ester. Based on the total mass of the electrolyte, the chain carboxylic acid ester has a mass content W1 of 15%–75%, the cyclic carbonate has a mass content W2 of 2%–40%, and the sulfate ester has a mass content W3 of 0.1%–2%. This lithium secondary battery exhibits excellent cycle performance.

Description

[0001] This application is a divisional application of the invention patent application filed on July 18, 2023, with application number 202310883089.X and title "Lithium Secondary Battery and Electrical Device". Technical Field

[0002] This application relates to the field of lithium battery technology, and in particular to a lithium secondary battery and an electrical device. Background Technology

[0003] In recent years, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power plants, as well as in many fields such as power tools, electric bicycles, electric motorcycles, electric cars, military equipment, and aerospace. Furthermore, the requirements for the electrical performance of secondary batteries are constantly increasing, and there are also higher and higher expectations for the charging rate of secondary batteries.

[0004] The performance of electrolytes has a crucial impact on the performance of secondary batteries. Electrolytes with suitable compositions can improve the electrical performance or charging rate of secondary batteries, meeting the diverse performance requirements of different fields. Therefore, it remains necessary to develop electrolytes to meet the application needs of next-generation electrochemical systems. Summary of the Invention

[0005] This application is made in view of the above-mentioned problems, and its purpose is to provide a lithium secondary battery with excellent fast charging performance and cycle performance.

[0006] To achieve the above objectives, a first aspect of this application provides a lithium secondary battery, including an electrolyte and a negative electrode sheet, wherein the electrolyte includes a chain-like carboxylic acid ester and a cyclic carbonate, and the negative electrode sheet includes a negative current collector and a negative electrode film layer located on at least one side of the negative current collector.

[0007] Based on the total mass of the electrolyte, the mass content of the chain carboxylic acid ester W1 and the mass content of the cyclic carbonate W2 satisfy the following condition: 0.1≤W2 / W1≤2. The mass content of the chain carboxylic acid ester W1 and the thickness H of the negative electrode film satisfy the following condition: 0.2≤H / 100W1≤20. The unit of H is μm.

[0008] The introduction of chain carboxylic esters into the electrolyte can significantly reduce the viscosity of the electrolyte and improve the wettability of the electrolyte to the negative electrode, thereby increasing the liquid phase transport rate of lithium ions and reducing the migration resistance of lithium ions, significantly improving the fast-charging performance of lithium secondary batteries. At the same time, controlling the mass content W1 of chain carboxylic esters to satisfy the above relationship with the mass content W2 of cyclic carbonates and the thickness H of the negative electrode film can both reduce the passivation layer formed by the cyclic carbonates on the surface of the negative electrode, thereby reducing the side reactions between metallic lithium and chain carboxylic esters on the negative electrode side during the cycling of lithium secondary batteries, and reduce the degree of lithium plating on the negative electrode due to insufficient electrolyte wetting, so that the lithium secondary battery can achieve both fast-charging performance and cycling performance.

[0009] In any embodiment, the electrolyte further includes a sulfate ester compound, and based on the total mass of the electrolyte, the mass content W1 of the chain carboxylic acid ester and the mass content W3 of the sulfate ester compound satisfy the following condition: 0.005 ≤ W3 / W1 ≤ 0.4.

[0010] Sulfate compounds can undergo oxidation reactions on the positive electrode side of lithium secondary batteries before chain carboxylic esters. The oxides are deposited at the positive electrode interface to form a CEI film, which can reduce the degree of oxidation reaction of chain carboxylic esters under high voltage on the positive electrode side, reduce the loss and gas production of chain carboxylic esters, and further improve the fast charging performance and storage performance of lithium secondary batteries.

[0011] In any embodiment, the chain carboxylic acid ester includes one or more of ethyl formate, ethyl acetate, methyl acetate, methyl propionate, ethyl propionate, propyl propionate, propyl acetate, butyl formate, and methyl butyrate, and optionally includes one or more of ethyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, and propyl propionate.

[0012] The aforementioned chain carboxylic acid esters have a lower viscosity due to having fewer than 5 carbon atoms in their main chain. This can significantly reduce the viscosity of the electrolyte, increase the liquid phase transport rate of lithium ions, and thus improve the conductivity of the electrolyte, which is beneficial for improving the fast-charging performance of lithium secondary batteries.

[0013] In any embodiment, the cyclic carbonate includes one or more of ethylene carbonate, propylene carbonate, and fluoroethylene carbonate, and optionally includes one or more of ethylene carbonate and fluoroethylene carbonate.

[0014] The aforementioned cyclic carbonates can all be reduced to form a passivation layer on the negative electrode side, reducing side reactions between the chain carboxylic acid esters and metallic lithium, and improving the cycle performance and storage performance of lithium secondary batteries.

[0015] In any embodiment, the sulfate ester compound includes compounds with structures shown in Formula I and Formula II, diethyl sulfate, dimethyl sulfate, and dihydro-1,3,2-di(II) sulfate. One or more of the following: thio-(1,3,2-dioxane)-2,2,5,5-tetraoxide

[0016]

[0017] Among them, R1 and R2 each independently include hydrogen atoms and C atoms. 1-6 alkyl groups, R3 includes C 1-6 Alkyl groups. Optionally, the sulfate ester compounds include 4,4'-ethylene bisulfate, vinyl sulfate, diethyl sulfate, dimethyl sulfate, and dihydro-1,3,2-diethyl sulfate. One or more of the following: thio-(1,3,2-dioxane)-2,2,5,5-tetraoxide.

[0018] The aforementioned sulfate ester compounds can all participate in the formation of the CEI film on the positive electrode side, reducing the degree of oxidation reaction of chain carboxylic esters and improving the fast charging performance and storage performance of the battery.

[0019] In any embodiment, based on the total mass of the electrolyte, the mass content W1 of the chain carboxylic acid ester is 5% to 75%, and optionally 20% to 75%.

[0020] Controlling the mass content of chain carboxylic acid esters within a suitable range can significantly reduce the viscosity of the electrolyte, increase the liquid phase transport rate of lithium ions, and thus improve the conductivity of the electrolyte, which is beneficial to improving the fast charging performance of lithium secondary batteries. It can also reduce the degree of aggravation of side reactions between chain carboxylic acid esters and metallic lithium on the negative electrode side.

[0021] In any embodiment, the mass content W2 of the cyclic carbonate is 5% to 40% based on the total mass of the electrolyte, and can be optionally 10% to 30%.

[0022] By controlling the mass content of cyclic carbonates within a suitable range, sufficient cyclic carbonates can be provided to facilitate the dissociation of lithium salts in the electrolyte and to participate in the formation of a passivation layer on the surface of the negative electrode, thereby improving the electrolyte conductivity. At the same time, the impact of introducing too much cyclic carbonate on the electrolyte viscosity can be reduced, thus comprehensively improving the fast-charging performance of lithium secondary batteries.

[0023] In any embodiment, based on the total mass of the electrolyte, the mass content W3 of the sulfate ester compound is 0.1% to 2%, optionally 0.1% to 1%.

[0024] Controlling the mass content of sulfate ester compounds within a suitable range can provide enough sulfate ester compounds to participate in the formation of CEI film on the positive electrode side, inhibit the oxidation reaction of carboxylic acid esters, and improve the fast charging performance of lithium secondary batteries, while avoiding the introduction of excessive sulfate ester compounds that would increase the production cost of lithium secondary batteries.

[0025] In any embodiment, the thickness of the negative electrode film is 10 μm to 100 μm, and can be selected as 20 μm to 70 μm.

[0026] By controlling the thickness of the negative electrode film within a suitable range, both the fast-charging performance and energy density of the battery can be balanced.

[0027] In any embodiment, the electrolyte further includes a lithium salt, which includes LiPF6 and LiFSI. In any embodiment, the molar concentration C1 of LiPF6 and the molar concentration C2 of LiFSI satisfy the following condition: 0.8 mol / L ≤ C1 + C2 ≤ 1.3 mol / L, or optionally 0.8 mol / L ≤ C1 + C2 ≤ 1.2 mol / L.

[0028] In any embodiment, the molar concentration C1 of LiPF6 and the molar concentration C2 of LiFSI satisfy the following condition: 1≤C1 / C2≤5, which can be optionally 1≤C1 / C2≤3.

[0029] LiFSI exhibits a high degree of dissociation in electrolytes, significantly improving the electrolyte's conductivity at room temperature. However, it also poses a risk of corrosion to the aluminum foil used as the negative electrode current collector. While LiPF6 has a lower degree of dissociation than LiFSI in electrolytes, it passivates the aluminum foil and inhibits corrosion. By controlling the molar concentrations of LiFSI and LiPF6 to a specific relationship, it is possible to improve electrolyte conductivity, thus enhancing the fast-charging performance of lithium-ion batteries, while simultaneously reducing the risk of aluminum foil corrosion and improving the safety of lithium-ion batteries.

[0030] In any embodiment, the molar concentration C1 of LiPF6 is 0.2 mol / L to 1.2 mol / L, and can be selected as 0.2 mol / L to 1.1 mol / L; in any embodiment, the molar concentration C2 of LiFSI is 0.1 mol / L to 1 mol / L, and can be selected as 0.2 mol / L to 1 mol / L.

[0031] Controlling the molar concentrations of LiFSI and LiPF6 within appropriate ranges helps to balance the fast-charging performance and safety of lithium secondary batteries.

[0032] In any embodiment, the electrolyte further includes additives, which include one or more of lithium difluorophosphate, lithium fluorosulfonate, trimethyl phosphite, vinylene carbonate, lithium tetrafluoroborate, lithium difluorobis(oxalato)phosphate, tris(trimethylsiloxy)borate, trimethylfluorosilane, adiponitrile, and succinic anion, and optionally include one or more of lithium difluorophosphate, lithium fluorosulfonate, lithium tetrafluoroborate, and lithium difluorooxalatoborate.

[0033] The above-mentioned additives can all participate in the formation of a CEI film on the positive electrode side or an SEI film on the negative electrode side, reducing the direct contact between the chain carboxylic esters and the positive electrode or metallic lithium, thereby reducing the consumption of active lithium and the reduction and decomposition of the electrolyte. Lithium difluorophosphate, lithium fluorosulfonate, trimethyl phosphite, vinylene carbonate, lithium tetrafluoroborate, or lithium difluorobis(oxalato)phosphate participate in the formation of a CEI film on the positive electrode side, while tris(trimethylsiloxy)borate, trimethylfluorosilane, adiponitrile, or succinate participate in the formation of an SEI film on the negative electrode side.

[0034] In any embodiment, the positive electrode sheet in the lithium secondary battery includes a positive electrode active material, and the positive electrode active material includes Li. d [Ni x Co y X1 z M1 1-x-y-z ]O 2-S One or more of LiMn2O4, Li2MnO3·(1-a)LiAO2, and LiM2X2O4,

[0035] Wherein, 0.1≤d≤1, 0≤S≤2, X1 includes Mn and / or Al, M1 includes one or more of Co, Ni, Mn, Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Zr, Sr, V, and Ti, 0≤x<1, 0≤y≤1, 0≤z≤1, x+y+z≤1; A includes one or more of Ni, Co, and Mn, 0<a<1; M2 includes one or more of Fe, Mn, Ni, and Co, X2O4 h- X2 includes one or more of S, P, As, V, Mo, and W, and h = 2 or 3.

[0036] All of the above-mentioned positive electrode active materials can be used in combination with electrolytes to achieve lithium secondary batteries with excellent fast charging performance and cycle performance.

[0037] In any embodiment, the negative electrode film layer includes a negative electrode active material, which includes one or more of artificial graphite, natural graphite, soft carbon, hard carbon, mesophase micro carbon spheres, silicon-based materials, and tin-based materials, and optionally includes one or more of artificial graphite and natural graphite.

[0038] The negative electrode film layers with the above-mentioned negative electrode active materials can be used in combination with electrolytes to achieve lithium secondary batteries with excellent fast charging performance and cycle performance.

[0039] In any embodiment, the compaction density of the negative electrode film is 0.9 g / cm³. 3 ~1.6g / cm 3 The option is 1.1g / cm³. 3 ~1.6g / cm 3 .

[0040] Controlling the compaction density of the negative electrode film within a suitable range can satisfy the requirement for sufficient contact between the negative electrode active materials, so that the battery has excellent electron transport performance and energy density, while reducing the risk that the pores between the negative electrode active materials will be crushed during the preparation process, which would be detrimental to lithium ion insertion and extraction.

[0041] A second aspect of this application provides an electrical device including a lithium secondary battery as described in the first aspect of this application. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of a lithium secondary battery according to one embodiment of this application.

[0043] Figure 2 yes Figure 1 An exploded view of a lithium secondary battery according to an embodiment of this application is shown.

[0044] Figure 3 This is a schematic diagram of a battery module according to one embodiment of this application.

[0045] Figure 4 This is a schematic diagram of a battery pack according to one embodiment of this application.

[0046] Figure 5 yes Figure 5 An exploded view of a battery pack according to one embodiment of this application is shown.

[0047] Figure 6 This is a schematic diagram of an electrical device that uses a lithium secondary battery as a power source according to an embodiment of this application.

[0048] Explanation of reference numerals in the attached figures:

[0049] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Lithium secondary battery; 51 Casing; 52 Electrode assembly; 53 Top cover assembly Detailed Implementation

[0050] The following detailed description, with appropriate reference to the accompanying drawings, specifically discloses embodiments of the lithium secondary battery and power-consuming device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0051] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0052] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0053] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0054] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0055] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0056] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0057] The conductivity of the electrolyte significantly affects battery performance. For example, in lithium-ion batteries, low conductivity can lead to adverse effects such as reduced fast-charging performance. Introducing low-viscosity solvents into the electrolyte can lower the viscosity of the electrolyte system, thereby increasing the lithium-ion transport rate and ultimately improving the electrolyte conductivity. However, the introduction of low-viscosity solvents can also bring other adverse effects. For instance, the low-viscosity solvent chain carboxylic esters can undergo side reactions with the deposited metallic lithium, consuming some of the metallic lithium and generating gas, severely impacting the battery's cycle and storage performance. Therefore, a new battery design is needed that can balance fast-charging and cycle performance.

[0058] [Lithium-ion rechargeable battery]

[0059] Based on this, this application proposes a lithium secondary battery, which includes an electrolyte and a negative electrode sheet. The electrolyte includes a chain-like carboxylic acid ester and a cyclic carbonate, and the negative electrode sheet includes a negative current collector and a negative electrode film layer located on at least one side of the negative current collector.

[0060] Among them, based on the total mass of the electrolyte, the mass content of the chain carboxylic acid ester W1 and the mass content of the cyclic carbonate W2 satisfy the following condition: 0.1≤W2 / W1≤2, and the mass content of the chain carboxylic acid ester W1 and the thickness H (μm) of the negative electrode film layer satisfy the following condition: 0.2≤H / 100W1≤20, where H is in μm.

[0061] In this document, the term "chain carboxylic acid ester" refers to a carboxylic acid ester with fewer than 5 carbon atoms in its main chain. Generally, the longest carbon chain in the chain-like organic compound is the main chain. Exemplary chain carboxylic acid esters in this document include one or more of ethyl formate, ethyl acetate, methyl acetate, methyl propionate, ethyl propionate, propyl propionate, propyl acetate, butyl formate, and methyl butyrate, and optionally one or more of ethyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, and propyl propionate.

[0062] In this article, "W2 / W1" refers to the ratio between W2 and W1.

[0063] In this article, "H / 100W1" refers to the ratio between thickness H and 100 times W1, where H is in μm.

[0064] In some embodiments, the cyclic carbonate includes one or more of ethylene carbonate, propylene carbonate, and fluoroethylene carbonate, and optionally includes one or more of ethylene carbonate and fluoroethylene carbonate.

[0065] In some embodiments, based on the total mass of the electrolyte, the ratio of the mass content of the chain carboxylic acid ester W1 to the mass content of the cyclic carbonate W2 can be selected as 0.1, 0.2, 0.4, 0.5, 0.6, 0.8, 1, 1.1, 1.2, 1.4, 1.5, 1.6, 1.8, 2, or a value within the range formed by any two of the above points.

[0066] In some embodiments, the mass content W1 of the chain carboxylic acid ester in the electrolyte and the thickness H of the negative electrode film, i.e., H / 100W1, can be selected as a value within the range of 0.2, 0.4, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, or any two of the above points.

[0067] Chain carboxylic esters have low viscosity and low melting point. Introducing chain carboxylic esters into the electrolyte can significantly reduce its viscosity, which is beneficial to improving the liquid phase transport rate of lithium ions, thereby improving the conductivity of the electrolyte and enhancing the fast charging performance of lithium secondary batteries. However, chain carboxylic esters are also prone to side reactions with metallic lithium deposited on the negative electrode side. This not only consumes the chain carboxylic esters and metallic lithium, affecting the cycle performance of the battery, but also generates gas, causing the lithium secondary battery to swell, which affects the transport of lithium ions and the storage performance of the lithium secondary battery.

[0068] The cyclic carbonates added to the electrolyte can reduce and form a passivation layer on the surface of the negative electrode, reducing the degree of side reactions between metallic lithium and the chain carboxylic acid esters on the negative electrode side, thus improving the cycle performance and storage performance of lithium secondary batteries. Controlling the ratio of chain carboxylic acid esters to cyclic carbonates can both improve the conductivity of the electrolyte and help suppress side reactions between the chain carboxylic acid esters and metallic lithium, enabling lithium secondary batteries to achieve good fast-charging performance, cycle performance, and storage performance.

[0069] Furthermore, to improve the energy density of lithium-ion rechargeable batteries, the coating thickness of the active material on the negative electrode gradually increases, leading to a gradual decrease in the wettability of the electrolyte. Poor electrolyte wetting can cause a "broken bridge" phenomenon in lithium-ion transport. During charging, the unwetted areas of the negative electrode are ineffective regions, while the wetted areas may experience lithium plating, resulting in a sharp deterioration in battery cycle performance and safety. However, introducing chain carboxylic esters into the electrolyte reduces its viscosity, which improves the wettability of the negative electrode, thereby reducing the migration resistance of lithium ions and improving the fast-charging performance of the lithium-ion rechargeable battery. The thickness of the negative electrode film needs to be controlled within a suitable range to balance the requirements of energy density and fast-charging performance. By controlling the mass content of chain carboxylic acid esters and the thickness of the negative electrode film in the negative electrode sheet to satisfy the above relationship, on the one hand, the electrolyte can quickly wet the negative electrode film layer, improve the migration rate of lithium ions, which can meet the requirements of fast charging performance of lithium secondary batteries, and at the same time reduce the degree of lithium plating on the negative electrode due to insufficient electrolyte wetting, and reduce the impact of lithium dendrite formation on the cycle performance and storage performance of lithium secondary batteries. On the other hand, it can also reduce the degree of side reaction aggravation caused by excessive chain carboxylic acid esters or meet the energy density requirements of lithium secondary batteries.

[0070] In this article, the term "fast charging performance" refers to the capacity retention rate of a lithium secondary battery under high-rate charging conditions.

[0071] In some embodiments, the electrolyte further includes sulfated ester compounds, and based on the total mass of the electrolyte, the mass content W1 of the chain carboxylic acid ester and the mass content W3 of the sulfated ester compound satisfy the following condition: 0.005 ≤ W3 / W1 ≤ 0.4. In some embodiments, based on the total mass of the electrolyte, the ratio W3 / W1 can be selected as 0.005, 0.006, 0.008, 0.01, 0.02, 0.04, 0.05, 0.06, 0.08, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, or a value within a range consisting of any two of the above points.

[0072] In some embodiments, the sulfate ester compounds include compounds of formula I, formula II, and the structures shown, diethyl sulfate, dimethyl sulfate, and dihydro-1,3,2-di(II) sulfate. One or more of the following: thio-(1,3,2-dioxane)-2,2,5,5-tetraoxide

[0073]

[0074] Among them, R1 and R2 each independently include hydrogen atoms and C atoms. 1-6 alkyl groups, R3 includes C1-6 alkyl group, the C 1-6 The alkyl group includes one or more of methyl, ethyl, propyl, isopropyl, and n-butyl.

[0075] Optionally, the sulfate compounds include 4,4'-ethylene bisulfate, vinyl sulfate, diethyl sulfate, dimethyl sulfate, and dihydro-1,3,2-diethyl sulfate. One or more of the following: thio-(1,3,2-dioxane)-2,2,5,5-tetraoxide.

[0076] In this article, dihydro-1,3,2-dihydrogen The CAS number for thio-(1,3,2-dioxane)-2,2,5,5-tetraoxide is 496-45-7.

[0077] In this paper, when R1 in formula I is selected from hydrogen atoms and R2 is selected from... The obtained substance is 4,4'-ethylene bisulfate.

[0078] Understandably, during lithium-ion battery charging, under high voltage, chain carboxylic esters readily undergo oxidation at the positive electrode interface, producing gas—a phenomenon known as "oxidation-gas production." This is because transition metal ions in the positive electrode have a catalytic effect, and the chain carboxylic esters readily oxidize under the catalytic action of these transition metal ions. However, the introduction of sulfated compounds can lead to their preferential oxidation at the positive electrode interface over chain carboxylic esters. The oxidation products deposit at the positive electrode interface, forming a CEI film. The CEI film isolates the chain carboxylic esters from direct contact with the positive electrode, reducing the degree of oxidation on the positive electrode side, minimizing ester loss and gas production, and further improving the fast-charging and storage performance of lithium-ion batteries.

[0079] In some embodiments, the mass content W1 of the chain carboxylic acid ester, based on the total mass of the electrolyte, is 5% to 75%, and optionally 20% to 75%. In some embodiments, the mass content W1 of the chain carboxylic acid ester, based on the total mass of the electrolyte, can be selected as 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, or a value within the range of any two of the above.

[0080] Controlling the mass content of chain carboxylic acid esters within a suitable range can significantly reduce the viscosity of the electrolyte, increase the liquid phase transport rate of lithium ions, and thus improve the conductivity of the electrolyte, which is beneficial to improving the fast charging performance of lithium secondary batteries. It can also reduce the degree of aggravation of side reactions between chain carboxylic acid esters and metallic lithium on the negative electrode side.

[0081] In some embodiments, the mass content W2 of the cyclic carbonate is 5% to 40% based on the total mass of the electrolyte, and can be selected as 10% to 30%. In some embodiments, the mass content W2 of the cyclic carbonate is selected as 5%, 8%, 10%, 12%, 15%, 17%, 20%, 25%, 28%, 30%, 33%, 35%, 38%, 40%, or a value within the range of any two of the above, based on the total mass of the electrolyte.

[0082] By controlling the mass content of cyclic carbonates within a suitable range, sufficient cyclic carbonates can be provided to facilitate the dissociation of lithium salts in the electrolyte and to participate in the formation of a passivation layer on the surface of the negative electrode, thereby improving the electrolyte conductivity. At the same time, the impact of introducing too much cyclic carbonate on the electrolyte viscosity can be reduced, thus comprehensively improving the fast-charging performance of lithium secondary batteries.

[0083] In some embodiments, the mass content W3 of the sulfate ester compound, based on the total mass of the electrolyte, is 0.1% to 2%, and optionally 0.1% to 1%. In some embodiments, the mass content W3 of the sulfate ester compound, based on the total mass of the electrolyte, can be 0.1%, 0.2%, 0.4%, 0.5%, 0.6%, 0.8%, 1%, 1.1%, 1.2%, 1.4%, 1.5%, 1.6%, 1.8%, 2%, or a value within the range of any two of the above.

[0084] Controlling the mass content of sulfate ester compounds within a suitable range can provide enough sulfate ester compounds to participate in the formation of CEI film on the positive electrode side, inhibit the oxidation reaction of carboxylic acid esters, and improve the fast charging performance of lithium secondary batteries, while avoiding the introduction of excessive sulfate ester compounds that would increase the production cost of lithium secondary batteries.

[0085] In some embodiments, the thickness of the negative electrode film is 10 μm to 100 μm, and can be selected as 20 μm to 70 μm. In some embodiments, the thickness of the negative electrode film can be selected as 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, or a value within the range formed by any two of the above points.

[0086] By controlling the thickness of the negative electrode film within a suitable range, both the fast-charging performance and energy density of the battery can be balanced.

[0087] In some embodiments, the electrolyte further includes linear carbonates, including at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate.

[0088] In some embodiments, the electrolyte further includes a lithium salt, including LiPF6 and / or LiFSI.

[0089] In some embodiments, the molar concentration C1 of LiPF6 and the molar concentration C2 of LiFSI satisfy the following condition: 0.8 mol / L ≤ C1 + C2 ≤ 1.3 mol / L, or optionally 0.8 mol / L ≤ C1 + C2 ≤ 1.2 mol / L.

[0090] In some embodiments, the molar concentration C1 of LiPF6 and the molar concentration C2 of LiFSI satisfy the condition that C1+C2 can be selected from 0.8mol / L, 0.85mol / L, 0.9mol / L, 0.95mol / L, 1.0mol / L, 1.05mol / L, 1.1mol / L, 1.15mol / L, 1.2mol / L, 1.25mol / L, 1.3mol / L, or any two of the above values.

[0091] In some embodiments, the molar concentration C1 of LiPF6 and the molar concentration C2 of LiFSI satisfy the condition: 1 ≤ C1 / C2 ≤ 5, which can be optionally 1 ≤ C1 / C2 ≤ 3. In some embodiments, the molar concentration C1 of LiPF6 and the molar concentration C2 of LiFSI satisfy the condition that C1 / C2 can be 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, or a value within the range formed by any two of the above points.

[0092] LiFSI exhibits a high degree of dissociation in electrolytes, significantly improving the electrolyte's conductivity at room temperature. However, LiFSI poses a risk of corroding aluminum foil. LiPF6, while exhibiting a lower degree of dissociation than LiFSI in electrolytes, possesses the ability to passivate aluminum foil and inhibit corrosion. By controlling the molar concentrations of LiFSI and LiPF6 to satisfy a specific relationship, it is possible to improve electrolyte conductivity, thereby enhancing the fast-charging performance of lithium-ion batteries, while simultaneously reducing the risk of aluminum foil corrosion and improving the safety of lithium-ion batteries.

[0093] In some embodiments, the molar concentration C1 of LiPF6 is 0.2 mol / L to 1.2 mol / L, and can be selected as 0.2 mol / L to 1.1 mol / L, and the molar concentration C2 of LiFSI is 0.1 mol / L to 1 mol / L, and can be selected as 0.2 mol / L to 1 mol / L. In some embodiments, the molar concentration C1 of LiPF6 can be selected as 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1.0 mol / L, 1.1 mol / L, 1.2 mol / L, or a value within the range formed by any two of the above points; and the molar concentration C2 of LiFSI can be selected as 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1 mol / L, or a value within the range formed by any two of the above points.

[0094] By controlling the molar concentrations of LiFSI and LiPF6 within appropriate ranges, the fast-charging performance and safety of lithium secondary batteries can be balanced.

[0095] In some embodiments, the electrolyte further includes additives, including one or more of lithium difluorophosphate, lithium fluorosulfonate, trimethyl phosphite, vinylene carbonate, lithium tetrafluoroborate, lithium difluorobis(oxalato)phosphate, tri(trimethylsiloxy)borate, trimethylfluorosilane, adiponitrile, and succinic anion, and optionally one or more of lithium difluorophosphate, lithium fluorosulfonate, lithium tetrafluoroborate, and lithium difluorobis(oxalato)borate.

[0096] The above-mentioned additives can all participate in the formation of a CEI film on the positive electrode side or an SEI film on the negative electrode side, reducing the direct contact between the chain carboxylic esters and the positive electrode or metallic lithium, thereby reducing the consumption of active lithium and the reduction and decomposition of the electrolyte. Lithium difluorophosphate, lithium fluorosulfonate, trimethyl phosphite, vinylene carbonate, lithium tetrafluoroborate, or lithium difluorobis(oxalato)phosphate participate in the formation of a CEI film on the positive electrode side, which helps reduce the oxidative decomposition of the electrolyte on the positive electrode side, thereby reducing the cathode interface impedance, facilitating rapid lithium ion extraction, and improving fast-charging performance; the reduced degree of oxidative decomposition also reduces cell gas expansion and active lithium consumption, improving cell lifespan. Tris(trimethylsiloxy)borate, trimethylfluorosilane, adiponitrile, or succinic anhydride participate in the formation of an SEI film on the negative electrode side, which helps reduce the reduction and decomposition of the electrolyte on the negative electrode side, reducing gas production and improving the battery's electrical performance.

[0097] In some embodiments, the electrolyte has a conductivity of 9 mS·cm to 15 mS·cm at 25°C. The conductivity of the electrolyte can be obtained by detection methods known in the art.

[0098] When the conductivity of the electrolyte is within a suitable range, it can balance the fast-charging performance and storage performance of lithium-ion batteries. If the electrolyte has too low conductivity, it will seriously affect the fast-charging performance of lithium-ion batteries; if the electrolyte has too high conductivity, it will lead to an increase in gas production, which is detrimental to the storage performance of lithium-ion batteries.

[0099] In some embodiments, the electrolyte filling coefficient of the lithium secondary battery is 1.5 g / Ah to 4.0 g / Ah. In some embodiments, the electrolyte filling coefficient of the lithium secondary battery can be selected as 1.5 g / Ah, 2 g / Ah, 2.5 g / Ah, 3 g / Ah, 3.5 g / Ah, 4.0 g / Ah, or a value within the range formed by any two of the above points.

[0100] In this paper, the term "electrolyte injection ratio" is the ratio of electrolyte mass (g) to cell capacity (Ah).

[0101] Controlling the electrolyte injection coefficient of a lithium secondary battery within a suitable range can provide enough electrolyte to meet the battery's requirements while also freeing up space to accommodate more positive / negative active materials, thereby increasing the battery's energy density. Alternatively, it can free up space for gas transfer, reducing the internal pressure of the battery casing and improving the battery's lifespan and safety.

[0102] [Positive electrode plate]

[0103] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including a positive electrode active material.

[0104] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0105] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0106] In some embodiments, the positive electrode active material may be a known battery positive electrode active material. As an example, the positive electrode active material may include at least one of the following: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more.

[0107] In some embodiments, the positive electrode active material includes Li d [Ni x Co y X1 z M1 1-x-y-z ]O 2-S One or more of LiMn2O4, Li2MnO3·(1-a)LiAO2, and LiM2X2O4,

[0108] Wherein, 0.1≤d≤1, 0≤S≤2, X1 includes Mn and / or Al, M1 includes one or more of Co, Ni, Mn, Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Zr, Sr, V, and Ti, 0≤x<1, 0≤y≤1, 0≤z≤1, x+y+z≤1; A includes one or more of Ni, Co, and Mn, 0<a<1; M2 includes one or more of Fe, Mn, Ni, and Co, X2O4 h- X2 includes one or more of S, P, As, V, Mo, and W, and h = 2 or 3.

[0109] All of the above-mentioned positive electrode active materials can be used in combination with electrolytes to achieve lithium secondary batteries with excellent fast charging performance and cycle performance.

[0110] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0111] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0112] In some embodiments, the compaction density of the positive electrode film layer of the positive electrode sheet is 3.3 g / cm³. 3 ~3.7g / cm 3 .

[0113] In this paper, the compaction density of the positive electrode film can be tested using any known testing method. As an example, the compaction density of the positive electrode film is calculated by taking a positive electrode with a unit area of ​​S and weighing it as M1, the mass of the aluminum foil under a unit area of ​​S as M2, measuring the thickness of the positive electrode as H1, and the thickness of the aluminum foil as H2. Then, the compaction density of the positive electrode film is calculated as (M1-M2) / ((H1-H2)×S).

[0114] When the compaction density of the positive electrode film is within a suitable range, both the DC resistance and energy density of the lithium secondary battery can be taken into account.

[0115] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0116] [Negative electrode plate]

[0117] The negative electrode includes a negative current collector and a negative electrode film layer located on at least one side of the negative current collector and containing at least a negative electrode active material.

[0118] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0119] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may 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 (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0120] In some embodiments, the negative electrode film layer includes a negative electrode active material, which includes one or more of artificial graphite, natural graphite, soft carbon, hard carbon, mesophase micro carbon spheres, silicon-based materials, and tin-based materials, and optionally includes one or more of artificial graphite and natural graphite.

[0121] The negative electrode films containing the aforementioned negative electrode active materials can all be used in conjunction with electrolytes to achieve lithium secondary batteries with excellent fast-charging and cycle performance. Furthermore, due to the large specific capacity of artificial or natural graphite, and the stable graphite structure with low expansion and contraction rates during lithium insertion and extraction, artificial or natural graphite has greater advantages as negative electrode active materials.

[0122] In some embodiments, the compaction density of the negative electrode film is 0.9 g / cm³. 3 ~1.6g / cm 3 The option is 1.1g / cm³. 3 ~1.6g / cm 3 In some embodiments, the compaction density of the negative electrode film can be selected as 0.9 g / cm³. 3 0.95g / cm 3 1.0g / cm 3 1.05g / cm 3 1.1g / cm 3 1.15g / cm 3 1.2g / cm 3 1.25g / cm 3 1.3g / cm 3 1.35g / cm 3 1.4g / cm 3 1.45g / cm 3 1.5g / cm 3 1.55g / cm 3 1.6g / cm 3 The value within the range formed by any two of the above points.

[0123] Controlling the compaction density of the negative electrode film within a suitable range can satisfy the requirement for sufficient contact between the negative electrode active materials, so that the battery has excellent electron transport performance and energy density, while reducing the risk that the pores between the negative electrode active materials will be crushed during the preparation process, which would be detrimental to lithium ion insertion and extraction.

[0124] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may include at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0125] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0126] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0127] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0128] [Isolation membrane]

[0129] In some embodiments, the lithium secondary battery also includes a separator. Any known porous separator with good chemical and mechanical stability can be selected.

[0130] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of each layer can be the same or different.

[0131] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0132] In some embodiments, the lithium secondary battery may include an outer packaging. This outer packaging can be used to encapsulate the aforementioned electrode assembly and electrolyte.

[0133] To alleviate the problem of expansion force in secondary battery cells, expansion space can be reserved for the cells in the design. Specifically, the group margin of the lithium secondary battery can be controlled at 82% to 95%, where the group margin is the percentage of the cell thickness to the thickness of the inner cavity of the casing.

[0134] In some embodiments, the group margin of the lithium secondary battery can be selected as 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, or a value within a range consisting of any two of the above points.

[0135] Controlling the mass margin of lithium secondary batteries within a suitable range means providing enough space to accommodate electrode components while also allowing sufficient space for gas transfer, thereby reducing the internal pressure of the lithium secondary battery casing and improving the lifespan and safety of the lithium secondary battery.

[0136] In some implementations, the outer packaging of the lithium secondary battery can be a hard shell, such as a hard plastic shell, aluminum shell, or steel shell. The outer packaging of the lithium secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0137] This application allows for lithium secondary batteries to be cylindrical, square, or any other arbitrary shape. For example, Figure 1 This is an example of a square-structured lithium secondary battery 5.

[0138] In some implementations, refer to Figure 2 The outer packaging may include a housing 51 and a cover 53. The housing 51 may include a bottom plate and side plates connected to the bottom plate, the bottom plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 can be placed over the opening to close the receiving cavity. Positive electrode sheets, negative electrode sheets, and a separator can be formed into an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The lithium secondary battery 5 may contain one or more electrode assemblies 52, which can be selected by those skilled in the art according to specific practical needs.

[0139] In some implementations, lithium secondary batteries can be assembled into battery modules, and the number of lithium secondary batteries contained in a battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0140] Figure 3 This is battery module 4, used as an example. (See reference...) Figure 3 In battery module 4, multiple lithium secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple lithium secondary batteries 5 can be fixed in place using fasteners.

[0141] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of lithium secondary batteries 5 are received.

[0142] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0143] Figure 4 and Figure 5 This is battery pack 1 as an example. (See reference...) Figure 4 and Figure 5The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0144] In addition, this application also provides an electrical device, which includes at least one of the lithium secondary battery, battery module, or battery pack provided in this application. The lithium secondary battery, battery module, or battery pack can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0145] As the electrical device, a lithium secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0146] Figure 6 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the lithium-ion battery for this device, a battery pack or battery module can be used.

[0147] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a lithium-ion battery as their power source.

[0148] Example

[0149] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0150] I. Preparation Method

[0151] Example 1

[0152] 1) Electrolyte

[0153] In an argon-atmospheric glove box (H2O content <10ppm, O2 content <1ppm), 5g of ethyl formate, 10g of ethylene carbonate, and 71.13g of methyl ethyl carbonate were mixed thoroughly to obtain a mixed solvent. Then, lithium hexafluorophosphate (LiPF6) and lithium bisfluorosulfonyl imide (LiFSI) were added to the mixed solvent to achieve molar concentrations of 0.7mol / L (8.75g) and 0.3mol / L (4.62g), respectively. The mixture was stirred thoroughly, followed by the addition of 0.5g of ethylene 4,4'-disulfate, and the mixture was stirred until homogeneous. The electrolyte was then prepared.

[0154] 2) Preparation of positive electrode sheet

[0155] LiNi, the positive electrode active material 0.5 Co 0.2 Mn 0.3 A positive electrode slurry is prepared in N-methylpyrrolidone (NMP) using O2, conductive agent Super P, and binder polyvinylidene fluoride (PVDF); the solid content of the positive electrode slurry is 50 wt%, and the solid component is LiNi. 0.5 Co 0.2 Mn 0.3 The mass ratio of O2, Super P, and PVDF is 8:1:1. The positive electrode slurry is coated onto the negative electrode current collector aluminum foil and dried at 85°C. After cold pressing, the foil is trimmed, cut, and slit. Then, it is dried under vacuum at 85°C for 4 hours to produce the positive electrode sheet.

[0156] 3) Preparation of negative electrode sheet

[0157] Graphite (as the negative electrode active material), conductive agent Super P, thickener carboxymethyl cellulose (CMC), and binder styrene-butadiene rubber (SBR) were uniformly mixed in deionized water to prepare a negative electrode slurry. The solid content of the negative electrode slurry was 30 wt%, and the mass ratio of graphite, Super P, CMC, and SBR in the solid components was 80:15:3:2. The negative electrode slurry was coated onto copper foil as the negative electrode current collector and dried at 85°C. Then, it was cold-pressed, trimmed, cut into sheets, and slit. Finally, it was dried under vacuum at 120°C for 12 hours to produce a negative electrode sheet with a film thickness of 50 μm.

[0158] 4) Separating membrane

[0159] A 16μm polyethylene film (PE) was used as the separator.

[0160] 5) Battery manufacturing

[0161] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to isolate them. The cells are then wound to obtain a bare cell. The tabs are welded on, and the bare cell is placed in an outer package. The electrolyte prepared above is injected into the dried cell. After processes such as encapsulation, settling, formation, shaping, and capacity testing, the lithium secondary battery product of Example 1 is obtained.

[0162] The secondary batteries of Examples 2-29 and Comparative Examples 1-6 are prepared in a similar manner to the secondary battery of Example 1, but the composition of the battery electrodes and the product parameters are adjusted. The different product parameters are detailed in Tables 1 and 2.

[0163] The sizes of W1, W2, and W3, as well as the mass content of additives, can be adjusted by adjusting the mass of methyl ethyl carbonate in the electrolyte.

[0164] II. Performance Testing

[0165] 1. Compacted density of the negative electrode film

[0166] The film compaction density of the negative electrode sheet is calculated. Take a negative electrode sheet with a unit area of ​​s and weigh it as m1. The mass of the aluminum foil under a unit area of ​​s is m2. Measure the thickness of the negative electrode sheet as h1 and the thickness of the aluminum foil as h2. The film compaction density of the negative electrode sheet = (m1-m2) / ((h1-h2)×s).

[0167] 2. Battery fast charging performance

[0168] The prepared battery was discharged at 1C to 2.8V, then left to stand for 5 minutes, and then charged at 0.5C to 4.25V. The charging capacity C0 was recorded. The battery was then discharged at 1C to 2.8V and left to stand for 5 minutes, and then charged at 2C to 4.25V. The charging capacity C1 was recorded. The battery charging capacity retention rate = C1 / C0 × 100%.

[0169] 3. Battery cycle performance

[0170] At 25°C, the prepared battery was charged to 4.25V with a constant current of 1C, then charged at a constant voltage of 4.25V until the current dropped to 0.05C. After resting for 10 minutes, it was discharged to 2.8V with a constant current of 1C. This is the first charge / discharge cycle of the battery, and the discharge capacity of this cycle is recorded as the discharge capacity (C0) of the battery in the first cycle. The above steps were repeated for the same battery. The process capacity (C1) of the battery after 300 cycles was recorded. The capacity retention rate after 300 cycles is calculated as C1 / C0 × 100%. The testing process for the comparative example and other embodiments is the same as above.

[0171] 4. Battery's gas generation performance during storage

[0172] At 25°C, the prepared full cell was left to stand for 30 minutes, then charged at a constant current rate of 0.1C to 4.2V, and then charged at a constant voltage of 4.2V to 0.05C. After standing for 5 minutes, the cell volume V1 was tested using the water displacement method. After storing at 60°C for 30 days, the cell was taken out, cooled to 25°C, charged at 0.1C to 4.2V, and then charged at a constant voltage to 0.05C. The cell volume V2 was tested using the water displacement method. The cell volume expansion rate = (V2-V1) / V1×100%.

[0173] III. Analysis of Test Results for Each Embodiment and Comparative Example

[0174] Batteries for each embodiment and comparative example were prepared according to the above method, and various performance parameters were measured. The results are shown in Tables 1 and 2 below.

[0175] Table 1

[0176]

[0177]

[0178] Table 2

[0179]

[0180]

[0181] The lithium secondary batteries in Examples 1 to 29 all include an electrolyte and a negative electrode. The electrolyte includes a chain carboxylic acid ester and a cyclic carbonate. The negative electrode includes a negative current collector and a negative electrode film layer located on at least one side of the negative current collector. Based on the total mass of the electrolyte, the mass content of the chain carboxylic acid ester W1 and the mass content of the cyclic carbonate W2 satisfy the following relationship: 0.1≤W2 / W1≤2. The mass content of the chain carboxylic acid ester W1 and the thickness H of the negative electrode film layer satisfy the following relationship: 0.2≤H / 100W1≤20.

[0182] A comparison of Examples 1-7, Examples 22-24, and Comparative Examples 1-4 shows that, based on the total mass of the electrolyte, controlling the mass content of the chain carboxylic acid ester W1 and the mass content of the cyclic carbonate W2 to satisfy the following conditions: 0.1 ≤ W2 / W1 ≤ 2, and controlling the mass content of the chain carboxylic acid ester W1 and the thickness H of the negative electrode film to satisfy the following condition: 0.2 ≤ H / 100W1 ≤ 20, is beneficial to improving the fast charging performance and cycle performance of lithium secondary batteries, as well as reducing the storage gas generation rate of lithium secondary batteries and improving their storage performance.

[0183] A comparison of Examples 2-6 and 23-24 with Examples 1, 7 and 23 shows that, based on the total mass of the electrolyte, further controlling the mass content of the chain carboxylic acid ester W1 and the mass content of the cyclic carbonate W2 to satisfy: 0.1≤W2 / W1≤1.5 is beneficial to further improve the storage performance of the battery.

[0184] As seen in Examples 1-4, controlling the mass content W1 of the chain carboxylic acid ester to 5%-75% based on the total mass of the electrolyte results in excellent fast-charging performance, cycle performance, and storage performance of the lithium secondary battery. A comparison of Examples 2-4 with Example 1 shows that further controlling the mass content W1 of the chain carboxylic acid ester to 20%-75% based on the total mass of the electrolyte is beneficial for further improving the fast-charging performance, cycle performance, and storage performance of the lithium secondary battery.

[0185] As seen in Examples 2, 5-7, controlling the mass content W2 of cyclic carbonate to 5%-40% based on the total mass of the electrolyte results in excellent fast-charging performance, cycle performance, and storage performance of the lithium secondary battery. A comparison of Examples 2, 6 with Examples 5, 7 shows that further controlling the mass content W2 of cyclic carbonate to 5%-30% based on the total mass of the electrolyte is beneficial for further improving the fast-charging performance, cycle performance, and storage performance of the lithium secondary battery.

[0186] As can be seen from Examples 2 and 22-24, the thickness of the negative electrode film is controlled to be 10 μm to 100 μm to enable the lithium secondary battery to have excellent fast charging performance, cycle performance and storage performance.

[0187] As seen in Examples 2, 8-11, controlling the molar concentration of LiPF6 C1 and the molar concentration of LiFSI C2 to satisfy the condition 1 ≤ C1 / C2 ≤ 5 results in excellent fast-charging performance, cycle performance, and storage performance of the lithium secondary battery. A comparison of Examples 2, 8, 10-11 with Example 9 shows that further controlling the molar concentration of LiPF6 C1 and the molar concentration of LiFSI C2 to satisfy the condition 1 ≤ C1 / C2 ≤ 3 is beneficial for further improving the fast-charging performance, cycle performance, and storage performance of the lithium secondary battery.

[0188] As seen in Examples 2, 5-7, and 12-14, the chain carboxylic ester can be selected as ethyl formate or methyl acetate, the cyclic carbonate can be selected as ethylene carbonate or fluoroethylene carbonate, and the sulfate compound can be selected as 4,4'-ethylene bisulfate or the substance shown in Formula I (R1=b, R2=b), all of which can give the lithium secondary battery excellent fast-charging performance, cycle performance, and storage performance. As also seen in Examples 2, 5-7, and 12-14, based on the total mass of the electrolyte, controlling the mass content W2 of the cyclic carbonate to 5%-40% can balance the fast-charging performance, cycle performance, and storage performance of the lithium secondary battery.

[0189] As can be seen from Examples 12-18, the chain carboxylic ester can be selected from methyl acetate, ethyl acetate, methyl propionate, ethyl propionate or propyl acetate, the cyclic carbonate can be selected from ethylene carbonate or fluoroethylene carbonate, and the sulfate ester compound can be selected from the substance shown in Formula I (R1=b, R2=b), which can also enable the lithium secondary battery to have excellent fast charging performance, cycle performance and storage performance.

[0190] As seen in Examples 1-4 and 19-21, based on the total mass of the electrolyte, controlling the mass content of the chain carboxylic acid ester W1 and the mass content of the sulfate ester compound W3 to satisfy the condition 0.005 ≤ W3 / W1 ≤ 0.4 can give the lithium secondary battery excellent fast-charging performance, cycle performance, and storage performance. A comparison of Examples 1-4 and 20-21 with Example 19 shows that, based on the total mass of the electrolyte, further controlling the mass content of the chain carboxylic acid ester W1 and the mass content of the sulfate ester compound W3 to satisfy the condition 0.005 ≤ W3 / W1 ≤ 0.25 is beneficial for improving the fast-charging performance, cycle performance, and storage performance of the lithium secondary battery.

[0191] As can be seen from Examples 1-4 and 19-21, based on the total mass of the electrolyte, controlling the mass content W3 of sulfate ester compounds to be 0.1%-2% can enable lithium secondary batteries to have excellent fast charging performance, cycle performance and storage performance.

[0192] A comparison of Examples 25-26 with Example 2 shows that introducing lithium difluorophosphate or lithium fluorosulfonate as additives into the electrolyte is beneficial to improving the battery's fast charging performance, cycle performance, and storage performance.

[0193] As can be seen from Examples 2, 27-29, the compaction density of the negative electrode film layer was controlled to be 0.9 g / cm³. 3 ~1.6g / cm 3 This is to ensure that the lithium secondary battery has excellent fast-charging performance, cycle performance, and storage performance. A comparison of Examples 2, 28-29 with Example 27 shows that further control of the negative electrode film compaction density to 1.1 g / cm³ is effective. 3 ~1.6g / cm 3 This will help to further improve the battery's fast charging performance, cycle performance, and storage performance.

[0194] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A lithium secondary battery, characterized by comprising: The electrolyte comprises a chain carboxylic acid ester, a cyclic carbonate and a sulfate compound, the mass content W1 of the chain carboxylic acid ester is 15% to 20%, the mass content W2 of the cyclic carbonate is 10% to 35%, and the mass content W3 of the sulfate compound is 0.5% to 1%, based on the total mass of the electrolyte, and the sulfate compound comprises a compound shown in Formula I: Formula I Among them, R1 and R2 each independently include hydrogen atoms and C atoms. 1-6 alkyl groups, (a) (b) or (c); R3 includes C 1-6 Alkyl groups; The electrolyte further comprises a lithium salt, and the lithium salt comprises LiPF6 and LiFSI. The molar concentration ratio of the LiPF6 to the LiFSI is 1 to 3.

2. The lithium secondary battery according to claim 1, characterized by The chain carboxylic acid ester comprises one or more of ethyl formate, ethyl acetate, methyl acetate, methyl propionate, ethyl propionate, propyl propionate, propyl acetate, butyl formate and methyl butyrate. The cyclic carbonate comprises one or more of vinyl carbonate, propylene carbonate and fluorinated vinyl carbonate.

3. The lithium secondary battery according to claim 1, characterized by The chain carboxylic acid ester comprises one or more of ethyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate and propyl propionate. The cyclic carbonate comprises vinyl carbonate and / or fluorinated vinyl carbonate.

4. The lithium secondary battery according to claim 1, characterized by The sulfate compound also includes a compound having a structure represented by Formula II Formula II.

5. The lithium secondary battery according to claim 1, characterized by The compounds of the structure of Formula I include 4,4' Vinyl disulfonic acid.

6. The lithium secondary battery according to any one of claims 1 to 5, characterized by, The lithium secondary battery comprises a negative electrode sheet, the negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer located on at least one side of the negative electrode current collector, and the thickness H of the negative electrode film layer is 10 µm to 100 µm.

7. The lithium secondary battery according to claim 6, characterized by The thickness H of the negative electrode film layer is 20 µm to 80 µm.

8. The lithium secondary battery according to claim 6, characterized by The thickness H of the negative electrode film layer is 20 µm to 70 µm.

9. The lithium secondary battery according to claim 6, characterized by The ratio W2 / W1 of the mass content W2 of the cyclic carbonate to the mass content W1 of the chain carboxylic acid ester is 0.5 to 1, based on the total mass of the electrolyte; or the ratio of the mass content W3 of the sulfate compound to the mass content W1 of the chain carboxylic acid ester is 0.025 to 0.0667.

10. The lithium secondary battery according to claim 9, characterized by The mass content W1 of the chain carboxylic acid ester and the thickness H of the negative electrode film layer satisfy the following relationship: 2.5 ≤ H / 100W1 ≤ 5, and the unit of H is µm.

11. The lithium secondary battery according to claim 1, wherein The sum of the molar concentrations of the LiPF6 and the LiFSI is 0.8 mol / L to 1.3 mol / L.

12. The lithium secondary battery according to claim 11, characterized by The sum of the molar concentrations of the LiPF6 and the LiFSI is 0.8 mol / L to 1.2 mol / L.

13. The lithium secondary battery according to claim 1, characterized by The molar concentration of the LiPF6 is 0.2 mol / L to 1.2 mol / L, and the molar concentration of the LiFSI is 0.1 mol / L to 1 mol / L.

14. The lithium secondary battery according to claim 13, characterized by The molar concentration of the LiPF6 is 0.2 mol / L to 1.1 mol / L, and the molar concentration of the LiFSI is 0.2 mol / L to 1 mol / L.

15. The lithium secondary battery according to claim 1, characterized by The electrolyte further comprises a linear carbonate, and the linear carbonate comprises at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl propyl carbonate and ethyl propyl carbonate.

16. The lithium secondary battery according to claim 15, characterized by The linear carbonate comprises methyl ethyl carbonate.

17. The lithium secondary battery according to claim 1, characterized by The electrolyte further comprises an additive, the additive comprising one or more of lithium difluorophosphate, lithium fluorosulfate, trimethyl phosphite, vinylene carbonate, lithium tetrafluoroborate, lithium difluorobisoxalate phosphate, tris(trimethylsiloxy)borate, trimethylfluorosilane, adiponitrile, butanedinitrile.

18. The lithium secondary battery according to claim 17, characterized by The additive comprises one or more of lithium difluorophosphate, lithium fluorosulfate, lithium tetrafluoroborate, lithium difluorobisoxalate borate.

19. The lithium secondary battery according to claim 6, characterized by The lithium secondary battery includes a positive electrode sheet including a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, the positive electrode film layer including a positive electrode active material including Li d [Ni x Co y X1 z M1 1-x-y-z ]O 2-S one or more of LiNiO2, LiCoO2, LiMn2O4, Li2MnO3·(1-a)LiAO2, LiM2X2O4, wherein 0.1≤d≤1, 0≤S≤2, X1comprises Mn and / or Al, M1comprises one or more of Co, Ni, Mn, Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Zr, Sr, V, Ti, 0≤x<1, 0≤y≤1, 0≤z≤1, x+y+z≤1; A comprises one or more of Ni, Co, Mn, 0 h- X2comprises one or more of S, P, As, V, Mo, W, h=2 or 3.

20. The lithium secondary battery according to claim 6, characterized by The negative electrode film layer comprises a negative electrode active material, the negative electrode active material comprising one or more of artificial graphite, natural graphite, soft carbon, hard carbon, mesophase carbon microbeads, silicon-based materials, tin-based materials.

21. The lithium secondary battery according to claim 20, characterized by The negative electrode active material comprises one or more of artificial graphite, natural graphite.

22. The lithium secondary battery according to claim 19, characterized by The lithium secondary battery satisfies at least one of the following: (1) the compaction density of the negative electrode film layer is 0.9 g / cm 3 1.6 g / cm 3 ; (2) the compacted density of the positive electrode film layer is 3.3 g / cm 3 3.7 g / cm 3 ; (3) the electrolyte has an electrical conductivity of 9 mS / cm to 15 mS / cm at 25°C; (4) the lithium secondary battery has a liquid injection coefficient of 1.5 g / Ah to 4.0 g / Ah; (5) the lithium secondary battery has a group margin of 82% to 95%.

23. The lithium secondary battery according to claim 19, characterized by The lithium secondary battery satisfies at least one of the following: (1) the compaction density of the negative electrode film layer is 1.1 g / cm 3 1.6 g / cm 3 ; (2) the lithium secondary battery has a liquid injection coefficient of 2 g / Ah to 4.0 g / Ah; (3) the lithium secondary battery has a group margin of 85% to 95%.

24. The lithium secondary battery according to claim 19, characterized by The lithium secondary battery comprises a separator film, wherein the positive electrode sheet, the negative electrode sheet, and the separator film form an electrode assembly through a winding process or a stacking process.

25. An electrical device, comprising: The lithium secondary battery comprises the lithium secondary battery according to any one of claims 1 to 24.

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