Secondary battery and electric device

Abstract

Provided in the present disclosure are a secondary battery and an electric device. The secondary battery comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte. The positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer located on at least one surface of the positive electrode current collector. The positive electrode film layer and / or the electrolyte comprises an additive. The secondary battery is charged with a constant current to a charging cut-off voltage, and is then continuously charged with the constant current for 1 hour to obtain an overcharge curve, wherein the constant current I satisfies the following relationship: I=Pre / Unom, where Pre is a rated charging power, and Unom is a nominal voltage. In the overcharge curve, the voltage of the secondary battery is less than 1.5×ΔU, where ΔU is the charging cut-off voltage of the secondary battery. The oxidation potential ΔE of the additive and ΔU satisfy the following relationship: 1≤ΔE / ΔU<1.5.

Description

Secondary batteries and electrical devices

[0001] Cross-reference to related applications

[0002] This disclosure is based on and claims priority to Chinese Patent Application No. 202411819567.1, filed on December 11, 2024, entitled "Secondary Battery and Electrical Device", the entire contents of which are incorporated herein by reference. Technical Field

[0003] This disclosure relates to the field of battery technology, and in particular to a secondary battery and an electrical device. Background Technology

[0004] In recent years, with the increasingly wide application of secondary batteries, they have been widely used in energy storage power systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, aerospace, and many other fields. Due to the significant development of secondary batteries, higher requirements have been placed on their safety.

[0005] Overcharging a secondary battery can cause it to release excessive heat, potentially leading to thermal runaway and posing a risk of combustion or explosion. Therefore, improving the safety of secondary batteries is a pressing technical challenge. Summary of the Invention

[0006] This disclosure is made in view of the above-mentioned problems, and its object is to provide a secondary battery, a method for manufacturing the same, and an electrical device comprising the secondary battery. This secondary battery has a low risk of thermal runaway during overcharge testing, thereby exhibiting high safety performance.

[0007] To achieve the above objectives, a first aspect of this disclosure provides a secondary battery comprising a positive electrode and an electrolyte; the positive electrode comprises a positive current collector and a positive electrode film layer located on at least one surface of the positive current collector, the positive electrode film layer and / or the electrolyte comprising additives; the secondary battery is charged to the charging cutoff voltage by a constant current I, and then charged for another hour by a constant current I to obtain an overcharge curve, the constant current I satisfying the following relationship, I = P re / U nom P re For rated charging power, U nom The nominal voltage is given. In the overcharge curve, the voltage of the secondary battery is <1.5×ΔU, where ΔU is the charging cutoff voltage of the secondary battery. The oxidation potential ΔE of the additive satisfies the following relationship with ΔU: 1≤ΔE / ΔU<1.5. This secondary battery has a low risk of thermal runaway, high safety, and meets the requirements of the national standard GBT 36276-2023 for energy storage batteries.

[0008] In some implementations, the charging cutoff voltage is 3.6V-4.2V. This secondary battery has a low risk of thermal runaway and high safety.

[0009] In some embodiments, the oxidation potential of the additive at 10°C to 60°C is above 3.6V and below 6.3V. This secondary battery has a low risk of thermal runaway and high safety.

[0010] In some implementations, a voltage plateau exists in the overcharge curve, where the difference between the maximum and minimum voltage of the plateau is less than or equal to 0.5V. Therefore, the battery voltage remains stable during overcharging, reducing the risk of thermal runaway.

[0011] In some implementations, the difference between the maximum and minimum voltage of the voltage plateau is 0.1V-0.35V. By keeping the difference between the maximum and minimum voltage of the voltage plateau within this range, the voltage fluctuations during overcharging are minimized, thereby helping to reduce the risk of thermal runaway in the battery.

[0012] In some embodiments, the duration of the voltage plateau is 30-60 minutes. In other embodiments, the duration of the voltage plateau is 30-57 minutes. A longer duration of the voltage plateau is more conducive to voltage stability during overcharging, further reducing the risk of thermal runaway.

[0013] In some embodiments, the positive electrode film layer includes a positive electrode active material, which includes a phosphate positive electrode active material. In some embodiments, the positive electrode active material includes at least one selected from lithium iron phosphate, lithium manganese iron phosphate, and sodium iron pyrophosphate. The above-mentioned positive electrode active materials exhibit high stability, which is beneficial for improving battery safety.

[0014] In some embodiments, the positive electrode active material includes lithium iron phosphate, the charging cut-off voltage is 3.6V-3.8V, and the oxidation potential of the additive at 10℃ to 60℃ is above 3.6V and below 5.7V. These settings help to further improve the safety performance of the battery.

[0015] In some embodiments, the positive electrode active material includes lithium manganese iron phosphate, the charging cut-off voltage is 4.0V-4.2V, and the oxidation potential of the additive at 10℃ to 60℃ is above 4.0V and below 6.3V. These settings help to further improve the safety performance of the battery.

[0016] In some embodiments, the positive electrode active material includes lithium iron phosphate and / or lithium manganese iron phosphate; the additive includes at least one of 2-trifluoromethyl-4,5-dicyanimidazolium lithium, 2-pentafluoroethyl-4,5-dicyanimidazolium lithium, 2-heptafluoropropyl-4,5-dicyanimidazolium lithium, and lithium oxalate. This is beneficial for further improving the safety performance of the battery.

[0017] In some embodiments, the positive electrode active material includes sodium iron pyrophosphate, the charging cut-off voltage is 3.6V-3.8V, and the oxidation potential of the additive at 10℃ to 60℃ is above 3.6V and below 5.7V. These settings help to further improve the safety performance of the battery.

[0018] In some embodiments, the positive electrode active material includes sodium iron pyrophosphate, and the additives include at least one selected from sodium 2-trifluoromethyl-4,5-dicyanimidazolium, sodium 2-pentafluoroethyl-4,5-dicyanimidazolium, sodium 2-heptafluoropropyl-4,5-dicyanimidazolium, and sodium oxalate. By combining the above-mentioned additives with the positive electrode active material, the additives can be used as the positive electrode active material, which is beneficial to increasing the proportion of the positive electrode active material and improving the energy density of the secondary battery.

[0019] In some embodiments, the oxidation potential ΔE of the additive and the charging cutoff voltage ΔU satisfy the following relationship: 1.15 ≤ ΔE / ΔU ≤ 1.35. This further reduces the impact of the additive on battery performance, more effectively suppresses the increase in battery voltage during overcharging, and reduces the risk of battery thermal runaway.

[0020] In some embodiments, the electrolyte includes an electrolyte salt, which in turn includes additives, with the additive content being 80%-100% relative to the total mass of the electrolyte salt. When the electrolyte includes additives, the additives can act as the electrolyte salt, which helps the secondary battery maintain a low voltage during the overcharge phase, greatly reduces the oxidation of the solvent in the electrolyte, and further reduces the heat generated by the battery.

[0021] In some embodiments, the molar concentration of the additive in the electrolyte is 0.5 mol / L to 1.5 mol / L. Maintaining the molar concentration of the additive in the electrolyte within this range is beneficial for improving battery safety performance.

[0022] In some embodiments, the mass percentage of the additive relative to the mass of the positive electrode film is 8.2% or less. In some embodiments, the mass percentage of the additive relative to the mass of the positive electrode film is 4.2% to 8.2%. In some embodiments, the mass percentage of the additive relative to the mass of the positive electrode active material is 10% or less. In some embodiments, the mass percentage of the additive relative to the mass of the positive electrode active material is 4.9% to 10%. By ensuring that the mass percentage of the additive in the positive electrode film is within the above range, it is beneficial to balance safety performance and energy density.

[0023] A second aspect of this disclosure provides an electrical device including a secondary battery as described in the first aspect of this disclosure. This electrical device offers excellent safety.

[0024] A third aspect of this disclosure provides an electrolyte comprising an electrolyte salt, the electrolyte salt including an additive, the additive having an oxidation potential of 3.6V or higher and less than 6.3V at 10°C to 60°C. When this electrolyte is used in a secondary battery, the secondary battery exhibits excellent safety.

[0025] In some embodiments, the additive content is 80%-100% relative to the total mass of the electrolyte salt. In some embodiments, the molar content of the additive in the electrolyte is 0.5 mol / L-1.5 mol / L. Maintaining the additive content within the above range is beneficial for improving battery safety performance.

[0026] This disclosure provides a positive electrode sheet, comprising a positive current collector and a positive electrode film layer located on at least one surface of the positive current collector. The positive electrode film layer includes an additive, the additive having an oxidation potential of 3.6V or higher and less than 6.3V at 10°C to 60°C. When this positive electrode sheet is applied to a secondary battery, the secondary battery exhibits excellent safety.

[0027] In some embodiments, the mass percentage of the additive in the positive electrode film is 4.2% to 8.2% relative to the mass of the positive electrode film. In some embodiments, the mass percentage of the additive is 4.9% to 10% relative to the mass of the positive electrode active material. By keeping the mass percentage of the additive in the positive electrode film within the above range, it is beneficial to balance safety performance and energy density. Attached Figure Description

[0028] Figure 1 is a schematic diagram of a battery cell according to an embodiment of the present disclosure.

[0029] Figure 2 is an exploded view of a battery cell according to an embodiment of the present disclosure shown in Figure 1.

[0030] Figure 3 is a schematic diagram of a battery module according to one embodiment of the present disclosure.

[0031] Figure 4 is a schematic diagram of a battery pack according to one embodiment of the present disclosure.

[0032] Figure 5 is an exploded view of a battery pack according to an embodiment of the present disclosure, as shown in Figure 4.

[0033] Figure 6 is a schematic diagram of an electrical device using a secondary battery as a power source according to an embodiment of the present disclosure.

[0034] Figure 7 shows the linear sweep voltammetry (LSV) curves of the experimental half-cell and the control half-cell.

[0035] Figure 8 shows the overcharge curves of the battery prepared in Example 1 of this disclosure and the battery prepared in Comparative Example 1 at a test temperature of 25°C.

[0036] Figure 9 shows the overcharge curves of the battery prepared in Example 1 and the battery prepared in Comparative Example 1 at a test temperature of 100°C.

[0037] Figure 10 shows the overcharge curve of the battery prepared in Comparative Example 2 of this disclosure at a test temperature of 25°C.

[0038] Figure 11 shows the overcharge curve of the battery prepared in Comparative Example 3 of this disclosure at a test temperature of 25°C.

[0039] Explanation of reference numerals in the attached diagram: 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Battery cell; 51 Housing; 52 Electrode assembly; 53 Top cover assembly. Detailed Implementation

[0040] The following detailed description, with appropriate reference to the accompanying drawings, provides a specific embodiment of the secondary battery, power-consuming device, electrolyte, and positive electrode of this disclosure. 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 to enable those skilled in the art to fully understand this disclosure and are not intended to limit the subject matter of the claims.

[0041] The "range" disclosed in this disclosure is defined by a lower limit and an upper limit, whereby a given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the 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 expected. Furthermore, if minimum range values ​​1 and 2 are listed, and if maximum range values ​​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 disclosure, 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.

[0042] Unless otherwise specified, all embodiments and optional embodiments of this disclosure can be combined to form new technical solutions.

[0043] Unless otherwise specified, all technical features and optional technical features of this disclosure can be combined to form new technical solutions.

[0044] Unless otherwise specified, the terminology used in this disclosure has the common meaning as commonly understood by those skilled in the art.

[0045] Unless otherwise specified, the values ​​of the parameters mentioned in this disclosure can be determined using various test methods commonly used in the art, for example, according to the test methods given in this disclosure.

[0046] The term "rechargeable battery" as used in this article refers to a single battery cell, battery module, or battery pack. Typically, a single rechargeable battery cell includes a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor of ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0047] For secondary batteries, especially energy storage batteries, overcharging is a common inducing factor for thermal runaway. During overcharging, as the charging current is continuously input, the battery voltage will gradually rise. The increase in voltage exacerbates the heat generation of the battery. As heat accumulates, thermal runaway will occur, eventually leading to battery combustion or explosion, which poses a significant safety problem and fails to meet the requirements of the national standard GBT 36276-2023.

[0048] To address the aforementioned issues, some reports have suggested improving thermal diffusion during overcharging by adding highly heat-absorbing materials (such as thermosensitive materials or phase change materials) to the surface of the membrane module. However, this approach only slows down the thermal runaway time and does not solve the heat generation problem during battery overcharging.

[0049] Based on this, the present disclosure proposes a secondary battery and an electrical device containing the secondary battery. The secondary battery has a low risk of thermal runaway during overcharging and high safety performance.

[0050] The present invention and its optional embodiments will be described in more detail below.

[0051] Secondary batteries

[0052] This disclosure provides a secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte; the positive electrode includes a positive current collector and a positive electrode film layer located on at least one surface of the positive current collector, the positive electrode film layer and / or the electrolyte including additives; after the secondary battery is charged to the charging cutoff voltage with a constant current I, it is further charged with a constant current I for 1 hour to obtain an overcharge curve, the constant current I satisfying the following relationship, I = P re / U nom P re For rated charging power, U nom The nominal voltage is 1.5 × ΔU in the overcharge curve, where ΔU is the charging cutoff voltage of the secondary battery. The oxidation potential ΔE of the additive satisfies the following relationship with ΔU: 1 ≤ ΔE / ΔU < 1.5.

[0053] In this disclosure, by including additives in the positive electrode film and / or electrolyte, these additives do not decompose during normal battery use (i.e., use under conditions below the charging cut-off voltage) and decompose stably during battery overcharging, thus suppressing the rise in battery voltage and ensuring that the secondary battery voltage in the overcharge curve is <1.5×ΔU. This reduces the risk of thermal runaway due to excessive voltage and improves the safety performance of the secondary battery. Furthermore, by using additives with oxidation potentials within the aforementioned range, the additives do not decompose during battery operation before the battery voltage reaches the charging cut-off voltage, thereby reducing the impact of the additives on battery performance. On the other hand, this allows the battery voltage to be kept below 1.5ΔU, reducing the risk of battery thermal runaway.

[0054] In the national standard GB / T 36276-2023, the overcharge performance is described as "with I = P re / U nom "Constant current charging until the voltage reaches 1.5 times its charging cutoff voltage or for 1 hour should not cause fire, explosion, or rupture outside of the explosion-proof valve or pressure relief point." This disclosure applies overcharge testing under more stringent conditions than the aforementioned national standard; specifically, the secondary battery is charged at I=P... re / U nom After constant current charging until the voltage reaches the battery's charging cutoff voltage (ΔU), continue charging at I=P. re / U nom The overcharge curve of the secondary battery was obtained by constant current charging for 1 hour. This disclosure, by containing a certain amount of the aforementioned additive, ensures that the voltage of the secondary battery remains below 1.5 × ΔU during the 1-hour overcharge process, preventing the battery from catching fire, exploding, or rupturing outside of the explosion-proof valve or pressure relief point. It exhibits excellent safety performance and meets the requirements of the national standard GB / T 36276-2023 for energy storage batteries.

[0055] In this disclosure, the term "charging cut-off voltage" is a conventional term in the art, referring to the upper limit of the battery's operating voltage. The maximum permissible voltage for a battery in its specifications is defined as the charging cut-off voltage.

[0056] In this disclosure, rated charging power is a conventional term in the art, referring to the maximum charging power that a charging device can continuously and stably output while meeting national safety and performance requirements.

[0057] In this disclosure, nominal voltage has a conventional meaning in the art and is used to reflect the voltage characteristics of a secondary battery, as detailed in GB / T 156-2007 Nominal Voltage.

[0058] In this disclosure, the oxidation potential ΔE of the additive can be tested using conventional methods in the art. For example, it can be determined as follows: Disassemble the battery to obtain the electrolyte. Assemble the electrolyte, the positive electrode (glassy carbon electrode), and the negative electrode (lithium metal) to obtain the half-cell to be tested (experimental half-cell). At 25°C, use a CHI660E electrochemical analyzer to scan from the open-circuit voltage to 7V at a scan rate of 0.2mV / s to obtain the voltammetric curve LSV, where the starting voltage of the voltammetric curve LSV is the oxidation potential ΔE of the additive.

[0059] In this disclosure, ΔE / ΔU can be 1.0, 1.1, 1.15, 1.20, 1.25, 1.30, 1.35, 1.40, 1.45, 1.49, or any value between two of these. Optionally, 1.15 ≤ ΔE / ΔU ≤ 1.35.

[0060] In some implementations, the charging cutoff voltage is 3.6V-4.2V. For example, it can be 3.6V, 3.65V, 3.7V, 3.8V, 3.9V, 4.0V, 4.1V, 4.2V, or any value between two of these. Optionally, the charging cutoff voltage is 3.6V-3.8V or 4.0V-4.2V. Therefore, the risk of thermal runaway in the aforementioned secondary battery is low, and its safety is high.

[0061] In some embodiments, the oxidation potential of the additive at 10°C to 60°C is 3.6V or higher and less than 6.3V. For example, it can be 3.6V, 3.65V, 3.7V, 3.8V, 3.9V, 4.0V, 4.1V, 4.2V, 4.4V, 4.6V, 4.8V, 5.0V, 5.2V, 5.4V, 5.6V, 5.8V, 6.0V, 6.15V, 6.2V, 6.3V, or any value between two of these. Optionally, the oxidation potential of the additive is 4.14V or higher and less than 5.67V. Secondary batteries using the above-mentioned additive have a low risk of thermal runaway and high safety.

[0062] In some implementations, the overcharge curve of the secondary battery contains a relatively stable voltage range, known as a voltage plateau. The difference between the maximum and minimum voltage of this plateau is less than or equal to 0.5V. For example, it can be 0.5V, 0.4V, 0.35V, 0.3V, 0.25V, 0.2V, 0.15V, 0.1V, 0.05V, or any value between these two. Optionally, the difference between the maximum and minimum voltage of the plateau is between 0.1V and 0.35V. A small difference between the maximum and minimum voltage of the plateau reflects small voltage fluctuations, indicating voltage stability during overcharging and a low risk of thermal runaway.

[0063] In some implementations, the duration of the voltage plateau is 30-60 minutes. For example, it can be 30 minutes, 35 minutes, 40 minutes, 45 minutes, 50 minutes, 55 minutes, 57 minutes, 60 minutes, or any value between two of these. Optionally, the duration of the voltage plateau is 30-57 minutes. A longer voltage plateau duration is more conducive to battery voltage stability during overcharging, further reducing the risk of battery thermal runaway.

[0064] In some embodiments, the additive includes at least one selected from lithium 2-trifluoromethyl-4,5-dicyanimidazolium, lithium 2-pentafluoroethyl-4,5-dicyanimidazolium, lithium 2-heptafluoropropyl-4,5-dicyanimidazolium, sodium 2-trifluoromethyl-4,5-dicyanimidazolium, sodium 2-pentafluoroethyl-4,5-dicyanimidazolium, sodium 2-heptafluoropropyl-4,5-dicyanimidazolium, lithium oxalate, and sodium oxalate. Using the above additives helps the secondary battery maintain a stable low voltage during the overcharge phase, reducing the oxidation of the solvent in the electrolyte and further reducing battery heat generation. Although the mechanism is not yet clear, the inventors believe that under overcharge conditions, the above additives are oxidized on the positive electrode side, and the oxidized products diffuse through the solvent to the negative electrode where they are reduced, thereby contributing to the formation of a stable voltage plateau in the battery.

[0065] Positive electrode sheet

[0066] 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.

[0067] In some embodiments, the positive electrode active material includes a phosphate positive electrode material. Phosphate materials have high thermal and structural stability, and by selecting a phosphate positive electrode material, it is beneficial to further reduce the risk of thermal runaway in the battery.

[0068] In some embodiments, the positive electrode active material includes at least one of lithium iron phosphate, lithium manganese iron phosphate, and sodium iron pyrophosphate. This is beneficial for improving the energy density of the secondary battery.

[0069] In some embodiments, the positive electrode active material includes lithium iron phosphate, and the battery's charging cutoff voltage (ΔU) is 3.6V-3.8V (e.g., 3.65V). In this case, the oxidation potential of the additives included in the battery at 10°C to 60°C is above 3.6V and below 5.7V. Optionally, the additives include at least one of 2-trifluoromethyl-4,5-dicyanimidazolium lithium, 2-pentafluoroethyl-4,5-dicyanimidazolium lithium, 2-heptafluoropropyl-4,5-dicyanimidazolium lithium, and lithium oxalate. When the positive electrode film layer includes additives, the additives can serve as the positive electrode active material, which is beneficial for increasing the proportion of the positive electrode active material, thereby improving the energy density of the lithium-ion battery. When the electrolyte includes additives, the additives can serve as the electrolyte salt, which is beneficial for stabilizing the lithium-ion battery at a low voltage during the overcharge phase, greatly reducing the oxidation of the solvent in the electrolyte, thereby further reducing the heat generation of the battery.

[0070] In some embodiments, the positive electrode active material includes lithium manganese iron phosphate, and the battery's charging cutoff voltage (ΔU) is 4.0V-4.2V (e.g., 4.1V). In this case, the oxidation potential of the additives included in the battery at 10°C to 60°C is above 4.0V and below 6.3V. Optionally, the additives include at least one of 2-trifluoromethyl-4,5-dicyanimidazolium lithium, 2-pentafluoroethyl-4,5-dicyanimidazolium lithium, 2-heptafluoropropyl-4,5-dicyanimidazolium lithium, and lithium oxalate. When the positive electrode film layer includes additives, the additives can serve as the positive electrode active material, which is beneficial for increasing the proportion of the positive electrode active material, thereby improving the energy density of the lithium-ion battery. When the electrolyte includes additives, the additives can serve as the electrolyte salt, which is beneficial for stabilizing the secondary battery at a low voltage during the overcharge phase, greatly reducing the oxidation of the solvent in the electrolyte, thereby further reducing the heat generation of the battery.

[0071] In some embodiments, the positive electrode active material comprises a lithium phosphate with an olivine structure. Examples of lithium phosphates include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4, also known as LFP), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium iron manganese phosphate, and lithium iron manganese phosphate and carbon composites. In some embodiments, the positive electrode active material comprises more than 90 wt% lithium iron phosphate, which is beneficial for maintaining energy density while preventing overcharge thermal runaway.

[0072] In some embodiments, the positive electrode active material includes sodium iron pyrophosphate, and the battery's charging cutoff voltage (ΔU) is 3.6V-3.8V (e.g., 3.65V). In this case, the oxidation potential of the additives included in the battery at 10°C to 60°C is above 3.6V and below 5.7V. Optionally, the additives include sodium 2-trifluoromethyl-4,5-dicyanoimidazolium, sodium 2-pentafluoroethyl-4,5-dicyanoimidazolium, sodium 2-heptafluoropropyl-4,5-dicyanoimidazolium, and sodium oxalate. When the positive electrode film layer includes the above-mentioned additives, the additives can serve as the positive electrode active material, which is beneficial for increasing the proportion of the positive electrode active material, thereby improving the energy density of the sodium-ion battery. When the electrolyte includes the additives, the additives can serve as the electrolyte salt, which is beneficial for the secondary battery to stabilize at a low voltage during the overcharge phase, greatly reducing the oxidation of the solvent in the electrolyte, thereby further reducing the heat generation of the battery.

[0073] In some embodiments, the positive electrode film layer includes an additive, the mass percentage of which is 8.2% or less relative to the mass of the positive electrode film layer, optionally between 4.2% and 8.2%. For example, it can be 8.2%, 7%, 6%, 5%, 4.2%, or any value between two of these. In some embodiments, the mass percentage of the additive in the positive electrode film layer is 10% or less relative to the mass of the positive electrode active material, optionally between 4.9% and 10%. For example, it can be 10%, 9%, 8%, 7%, 6%, 5%, 4.9%, or any value between two of these. By maintaining the mass percentage of the additive within the above range in the positive electrode film layer, it is beneficial to improve the safety performance of the battery. Furthermore, since the mass percentage of the positive electrode active material is increased, the energy density of the secondary battery can also be improved. Therefore, maintaining the mass percentage of the additive within the above range is beneficial for balancing safety performance and energy density.

[0074] In this disclosure, the mass of the additive in the positive electrode film can be determined according to the following method: The battery is disassembled to obtain the positive electrode sheet, and the mixture of positive electrode active material and additive is peeled off from the positive electrode current collector. The mass of nitrogen element in the mixture is determined according to GB / T 22386.1-2008, and then the mass of the additive is determined based on the mass of the nitrogen element. Finally, based on the mass of the additive, the mass ratio of the additive relative to the mass of the positive electrode active material / positive electrode film can be calculated.

[0075] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0076] 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.

[0077] 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.

[0078] In some embodiments, 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.

[0079] 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.).

[0080] electrolytes

[0081] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This disclosure does not impose specific limitations on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel-like, or entirely solid.

[0082] In some embodiments, the electrolyte comprises an electrolyte salt, which in turn comprises an additive, wherein the additive comprises 80%-100% by mass relative to the total mass of the electrolyte salt. Optionally, the additive comprises 90%-100% by mass. For example, the additive content is 80%, 85%, 90%, 95%, or 100%. In one embodiment, the additive content is 100% relative to the total mass of the electrolyte salt, meaning that the electrolyte salt is entirely composed of additives in the electrolyte. This is more conducive to improving battery safety performance.

[0083] In this disclosure, the mass percentage of the additive relative to the total mass of the electrolyte salt can be determined by the following method: The battery is disassembled to obtain the electrolyte, and then the solvent is evaporated to obtain the electrolyte salt. The mass of the electrolyte salt is weighed. The mass of nitrogen (N) is tested according to GB / T 22386.1-2008, and then the mass of the additive is determined based on the mass of the N element. Finally, based on the mass of the additive and the mass of the electrolyte salt, the mass percentage of the additive relative to the total mass of the electrolyte salt is calculated.

[0084] In some embodiments, in addition to the additives described above, the electrolyte salt may also include at least one selected from sodium hexafluorophosphate, sodium tetrafluoroborate, sodium perchlorate, sodium hexafluoroarsenate, sodium difluorosulfonamide, sodium ditrifluoromethanesulfonamide, sodium trifluoromethanesulfonate, sodium difluorophosphate, sodium difluorooxalate borate, sodium dioxalate borate, sodium difluorodioxalate phosphate, and sodium tetrafluorooxalate phosphate.

[0085] In some embodiments, the molar concentration of the additive in the electrolyte is 0.5 mol / L to 1.5 mol / L. For example, the molar concentration of the additive is 0.5 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, 1.3 mol / L, 1.4 mol / L, 1.5 mol / L, or any value between two of these. Optionally, the molar concentration of the additive is 0.8 mol / L to 1.2 mol / L. Maintaining the molar concentration of the additive within the above range is beneficial for improving battery safety performance and also helps reduce the release of gas or heat during the oxidative decomposition of the additive, thereby further reducing the risk of battery thermal runaway.

[0086] In this disclosure, the molar content of the additive can be determined by the following method: The battery is disassembled to obtain the electrolyte. The volume of the electrolyte is measured, and then the mass of nitrogen (N) is tested according to GB / T 22386.1-2008. Based on the mass of N, the mass of the additive is determined. Finally, the molar content of the additive is calculated based on the mass of the additive and the volume of the electrolyte.

[0087] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes a reducing agent.

[0088] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0089] In some embodiments, the solvent includes a mixture of ethylene carbonate, diethyl carbonate, and dimethyl carbonate.

[0090] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0091] Negative electrode sheet

[0092] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0093] 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.

[0094] 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 material substrate and a metal layer formed on at least one surface of the polymer material 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 material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0095] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. Silicon-based materials may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this disclosure is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0096] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from 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).

[0097] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0098] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0099] 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.

[0100] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0101] In some embodiments, the battery cell may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte described above.

[0102] In some embodiments, the outer packaging of the battery cell can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the battery cell can also be a flexible package, such as a pouch. The material of the flexible package can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0103] This disclosure does not impose any particular limitation on the shape of the battery cell; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 shows a square battery cell 5 as an example.

[0104] In some embodiments, referring to FIG2, the outer packaging may include a housing 51 and a top cover assembly 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 can cover the opening to close the receiving cavity. The positive electrode sheet, negative electrode sheet, and separator may be formed into an electrode assembly 52 by 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 number of electrode assemblies 52 contained in the battery cell 5 may be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0105] In some implementations, individual battery cells can be assembled into a battery module. The number of individual battery cells contained in a battery module can be one or more, and the specific number can be selected by those skilled in the art based on the application and capacity of the battery module.

[0106] Figure 3 shows a battery module 4 as an example. Referring to Figure 3, in the battery module 4, multiple battery cells 5 can be arranged sequentially along the length of the battery module 4. Of course, they can also be arranged in any other manner. Furthermore, the multiple battery cells 5 can be fixed in place using fasteners.

[0107] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0108] 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.

[0109] Figures 4 and 5 show a battery pack 1 as an example. Referring to Figures 4 and 5, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper box 2 and a lower box 3, with the upper box 2 covering the lower box 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.

[0110] This disclosure also provides a method for preparing the above-mentioned secondary battery. In this preparation method, the positive electrode, negative electrode, electrolyte, and separator can be prepared and assembled as described above. Additives are added to the positive electrode film and / or electrolyte, wherein the oxidation potential of the additives at 10°C to 60°C is greater than 3.6V and less than 6.3V.

[0111] Electrical appliances

[0112] In addition, this disclosure also provides an electrical device, which includes a secondary battery provided by this disclosure. The secondary battery can be used as a power source for the electrical device or as an energy storage unit of 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.

[0113] As an electrical device, you can choose individual battery cells, battery modules, or battery packs according to your usage requirements.

[0114] Figure 6 shows an example of an electrical device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0115] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a single battery cell as their power source.

[0116] Example

[0117] The following describes embodiments of this disclosure. The embodiments described below are exemplary and are only used to explain this disclosure, and should not be construed as limiting this disclosure. 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 the art or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0118] Example 1

[0119] Preparation of secondary batteries

[0120] (1) Preparation of the positive electrode sheet:

[0121] Lithium iron phosphate (LiFePO4), conductive carbon black, and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 90:5:5, and then N-methylpyrrolidone solvent was added and stirred until homogeneous to obtain a positive electrode slurry. The positive electrode slurry was coated onto both surfaces of the positive electrode current collector aluminum foil, and after drying and cold pressing, a positive electrode sheet was obtained.

[0122] (2) Preparation of negative electrode sheet:

[0123] Graphite, conductive carbon black, polystyrene-polybutadiene rubber, and carboxymethyl cellulose were mixed in a mass ratio of 97.5:0.7:1.1:0.7, then added to water and stirred until homogeneous to obtain a negative electrode slurry. The negative electrode slurry was coated onto both surfaces of the negative electrode current collector copper foil, and after drying and cold pressing, a negative electrode sheet was obtained.

[0124] (3) Separating membrane:

[0125] A 7μm polyethylene film was used as the separator.

[0126] (4) Preparation of electrolytes:

[0127] Ethylene carbonate, diethyl carbonate, and dimethyl carbonate were mixed in a volume ratio of 3:4:3 to obtain a mixed solvent. An additive (lithium 2-trifluoromethyl-4,5-dicyanimidazolium, LiTDI) was dissolved in this mixed solvent to obtain an electrolyte. The molar concentration of LiTDI in this electrolyte was 1 mol / L.

[0128] (5) Assembly of secondary batteries:

[0129] The negative electrode, separator, and positive electrode are arranged in sequence, with the separator positioned between the negative and positive electrodes to provide insulation. Then, electrolyte is injected for encapsulation and formation to obtain the battery. The resulting battery has a charging cut-off voltage (ΔU) of 3.65V.

[0130] Three identical batteries were prepared in accordance with the above method and used for LSV testing and overcharge testing at 25℃ / 100℃, respectively.

[0131] LSV testing

[0132] Ethylene carbonate, diethyl carbonate, and dimethyl carbonate were mixed in a volume ratio of 3:4:3 to obtain a mixed solvent. LiPF6 was then added to obtain an electrolyte with a LiPF6 molar concentration of 1 mol / L. A control half-cell was assembled using this electrolyte with a glassy carbon electrode as the positive electrode and lithium metal as the negative electrode. The voltammetric curve of the control half-cell was obtained at 25℃ using a CHI660E electrochemical analyzer at a scan rate of 0.2 mV / s from the open-circuit voltage to 7 V. The test results are shown by the dashed line in Figure 7.

[0133] The battery prepared in Example 1 was disassembled to obtain the electrolyte. A glassy carbon electrode was used as the positive electrode, and lithium metal was used as the negative electrode. The electrolyte and the electrolyte were then assembled to obtain an experimental half-cell. The voltammogram of the experimental half-cell was obtained at 25°C using a CHI660E electrochemical analyzer at a scan rate of 0.2 mV / s from the open-circuit voltage to 7 V. The test results are shown by the solid line in Figure 7. The initial peak voltage of the solid line is the oxidation potential of the additive. From the voltammogram in Figure 7, the oxidation potential of lithium 2-trifluoromethyl-4,5-dicyanimidazolium can be read as approximately 4.75 V.

[0134] Overcharge test

[0135] a) Overcharge test at 25°C:

[0136] ①At 25℃, let the battery stand for 10 minutes before connecting it to the charging and discharging device.

[0137] ② Charge the battery with a constant current (I = Pre / Unom) to the charging cutoff voltage ΔU. The battery in this example is an energy storage battery with a rated power Pre = 0.5P and I = Pre / Unom = 0.5C. It is charged to ΔU in 84 minutes with a constant current of 0.5C.

[0138] ③ Using the above constant current (P) re / U nom Continue charging for 1 hour or stop charging when the voltage reaches 1.5*ΔU (whichever comes first), and then observe for 1 hour.

[0139] The overcharge curve of the battery in Example 1 at 25°C is shown as the dashed line in Figure 8.

[0140] As can be seen from the dashed line in Figure 8, at 25°C, the maximum voltage of the battery in Example 1 after one hour of overcharging is 4.754V. A voltage plateau appears within the overcharge time range of 3-60 minutes, lasting for 57 minutes, with a voltage value distribution of (4.689±0.066V). The maximum voltage value of the plateau is 4.754V, and the minimum voltage value is 4.623V. The test results are recorded in Table 1-2.

[0141] b) Overcharge test at 100°C:

[0142] ① Let the battery stand at 100℃ for 10 minutes.

[0143] ② Charge the battery with a constant current until the charging cutoff voltage ΔU. The battery in this example is an energy storage battery with a rated power Pre = 0.5P and I = Pre / Unom = 0.5C. Charge it with a constant current of 0.5C for 84 minutes until it reaches ΔU. ③ Stop charging with a constant current for 1 hour or until the voltage reaches 1.5*ΔU (whichever comes first), and then observe for 1 hour.

[0144] The overcharge curve of the battery in Example 1 at 100°C is shown as the dashed line in Figure 9.

[0145] As can be seen from the dashed line in Figure 9, at 100℃, the maximum voltage of the battery in Example 1 was 4.328V after 1 hour of overcharging. A voltage plateau appeared within the overcharge time range of 7-60 minutes, lasting for 53 minutes, with a voltage value distribution of (4.263±0.065V). The maximum voltage of the plateau in the overcharge curve was 4.328V, and the minimum voltage was 4.197V. The test results are recorded in Tables 1-3.

[0146] evaluate

[0147] If the battery does not catch fire, explode, or rupture outside of the explosion-proof valve or pressure relief point during the overcharge test, the battery passes the overcharge test and is deemed qualified.

[0148] If a battery fails the overcharge test and is deemed unqualified, it will be deemed to have committed at least one of the following problems: fire, explosion, or rupture at a location other than the explosion-proof valve or pressure relief point.

[0149] Example 2

[0150] The secondary battery was prepared using the same preparation method as in Example 1, with the following differences:

[0151] When preparing the positive electrode slurry, LiTDI, lithium iron phosphate, conductive carbon black, and PVDF are mixed in a mass ratio of 8.2:81.8:5:5, and then N-methylpyrrolidone solvent is added and stirred evenly to obtain the positive electrode slurry.

[0152] The battery of Example 2 was subjected to overcharge testing and evaluation using the same method as in Example 1, and the results are recorded in Tables 1-2 and 1-3.

[0153] Example 3

[0154] The secondary battery was prepared using the same preparation method as in Example 1, with the following differences:

[0155] When preparing the electrolyte, LiTDI in the electrolyte is replaced with LiPF6.

[0156] When preparing the positive electrode slurry, LiTDI, lithium iron phosphate, conductive carbon black and PVDF are mixed in a mass ratio of 10:80:5:5, and then N-methylpyrrolidone solvent is added and stirred evenly to obtain the positive electrode slurry.

[0157] The battery of Example 3 was subjected to overcharge testing and evaluation using the same method as in Example 1, and the results are recorded in Tables 1-2 and 1-3.

[0158] Example 4

[0159] The secondary battery was prepared using the same preparation method as in Example 1, with the following differences:

[0160] When preparing the positive electrode slurry, lithium iron phosphate was replaced with lithium iron manganese phosphate (LiMnFePO4). The charging cut-off voltage of this secondary battery is 4.1V.

[0161] The battery of Example 4 was overcharged and evaluated using the same method as in Example 1 (constant current charging to the charging cutoff voltage of 4.1V), and the results are recorded in Tables 1-2 and 1-3.

[0162] Example 5

[0163] The secondary battery was prepared using the same preparation method as in Example 1, with the following differences:

[0164] When preparing the positive electrode slurry, lithium iron phosphate is replaced with sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7, NFPP).

[0165] When preparing the electrolyte, LiTDI was replaced with sodium 2-trifluoromethyl-4,5-dicyanimidazolium (NaTDI).

[0166] The battery of Example 5 was overcharged and evaluated using the same method as in Example 1, and the results are recorded in Tables 1-2 and 1-3.

[0167] Example 6

[0168] The secondary battery was prepared using the same preparation method as in Example 1, with the following differences:

[0169] When preparing the electrolyte, LiTDI is replaced with lithium oxalate.

[0170] The battery of Example 6 was overcharged and evaluated using the same method as in Example 1, and the results are recorded in Tables 1-2 and 1-3.

[0171] Example 7

[0172] The secondary battery was prepared using the same preparation method as in Example 1, with the following differences:

[0173] When preparing the electrolyte, LiTDI was replaced with lithium 2-pentafluoroethyl-4,5-dicyanimidazolium (LiPDI).

[0174] The battery of Example 7 was overcharged and evaluated using the same method as in Example 1, and the results are recorded in Tables 1-2 and 1-3.

[0175] Example 8

[0176] The secondary battery was prepared using the same preparation method as in Example 1, with the following differences:

[0177] When preparing the electrolyte, LiTDI was replaced with 2-heptafluoropropyl-4,5-dicyanimidazolium lithium (LiHDI).

[0178] The battery of Example 8 was subjected to overcharge testing and evaluation using the same method as in Example 1, and the results are recorded in Tables 1-2 and 1-3.

[0179] Comparative Example 1

[0180] The secondary battery was prepared using the same preparation method as in Example 1, with the following differences:

[0181] When preparing the electrolyte, LiTDI in the electrolyte is replaced with LiPF6.

[0182] The battery of Comparative Example 1 was subjected to an overcharge test using the same test method as in Example 1, and the test results are recorded in Tables 1-2 and 1-3.

[0183] The overcharge curve of the battery in Comparative Example 1 at 25°C is shown as the solid line in Figure 8. From the solid line in Figure 8, it can be seen that at 25°C, the overcharge curve of the battery in Comparative Example 1 did not show a voltage plateau. After 25 minutes of overcharging, the voltage reached 5.5V, and the battery caught fire and exploded.

[0184] The overcharge curve of Comparative Example 1's battery at 100℃ is shown as the solid line in Figure 9. From the solid line in Figure 9, it can be seen that the overcharge curve of Comparative Example 1's battery at 100℃ does not show a voltage plateau. After 18 minutes of overcharging, the voltage reached 5.475V, and the battery caught fire and exploded.

[0185] Table 1-1

[0186] Table 1-2

[0187] Table 1-3

[0188] As can be seen from Tables 1-1 to 1-3, compared with Comparative Example 1 (without additives), the batteries of Examples 1-8 did not catch fire or explode during the overcharge test, and there was no cracking at locations other than the explosion-proof valve or pressure relief point. They passed the overcharge test and have high safety performance.

[0189] Furthermore, the national standard GB / T 36276-2023 describes the overcharge test method as "I = P re / U nom "Constant current charging until the voltage reaches 1.5 times the battery's charging cutoff voltage or for 1 hour." In this disclosure, since the above embodiment did not reach the charging cutoff voltage after 1 hour of constant current charging with I = Pre / Unom, it cannot reach an overcharge state. Therefore, in the overcharge test, I = P... re / U nom The battery was charged at a constant current until the charging cutoff voltage was reached, and then overcharged for another hour to obtain the overcharge curve for evaluation. It is evident that the overcharge test in this specification was conducted under more stringent conditions than the national standard GB / T 36276-2023, yet the test results still meet the requirements of GB / T 36276-2023, indicating that the secondary battery of this invention has higher safety.

[0190] Example 9

[0191] The secondary battery was prepared using the same method as in Example 1, except that the following electrolyte was used: ethylene carbonate, diethyl carbonate, and dimethyl carbonate were mixed in a volume ratio of 3:4:3 to obtain a mixed solvent. LiPF6 and an additive (LiTDI) were dissolved in the mixed solvent to obtain the electrolyte. The mass ratio of LiPF6 to the additive (LiTDI) was 1:4, and the molar concentration of LiTDI in the electrolyte was 0.5 mol / L.

[0192] Examples 10 and 11

[0193] The secondary battery was prepared using the same preparation method as in Example 1, except that the amount of LiTDI added was adjusted as shown in Table 2-1 in the electrolyte preparation step to obtain the secondary battery.

[0194] The batteries from Examples 9-11 were subjected to overcharge tests using the same testing method as in Example 1. The test results at 25°C are recorded in Table 2-2. The test results at 100°C are recorded in Table 2-3.

[0195] Comparative Example 2

[0196] The secondary battery was prepared using the same preparation method as in Example 1, except that the amount of LiTDI added was adjusted as shown in Table 2-1 in the electrolyte preparation step to obtain the secondary battery.

[0197] Using the same test method as in Example 1, the battery of Comparative Example 2 was overcharged at 25°C, and the results are shown in Figure 10. As can be seen from Figure 10, although LiTDI was added, the amount added was insufficient, and the battery voltage reached 5.475V after 44 minutes of overcharging, which is unacceptable.

[0198] Table 2-1

[0199] Table 2-2

[0200] Table 2-3

[0201] As can be seen from the data in Tables 2-1 to 2-3, when the content of additives is 80%-100% relative to the total mass of electrolyte salts and the molar concentration of additives in the electrolyte is 0.5mol / L-1.5mol / L, the secondary battery passes the overcharge test and has high safety performance, meeting the requirements of national standard GBT 36276-2023.

[0202] Examples 12 and 13

[0203] The secondary battery was prepared using the same preparation method as in Example 3, except that the mass ratio of LiTDI, lithium iron phosphate, conductive carbon black, and polyvinylidene fluoride in the positive electrode slurry was adjusted as shown in Table 3-1 during the preparation of the positive electrode slurry.

[0204] Overcharge tests were performed on the batteries of Examples 12 and 13 using the same test method as in Example 1. The test results at 25°C are recorded in Table 3-2. The test results at 100°C are recorded in Table 3-3.

[0205] Comparative Example 3

[0206] The secondary battery was prepared using the same preparation method as in Example 3, except that the mass ratio of LiTDI, lithium iron phosphate, conductive carbon black, and polyvinylidene fluoride in the positive electrode slurry was adjusted as shown in Table 3-1 during the preparation of the positive electrode slurry.

[0207] Using the same test method as in Example 1, the battery of Comparative Example 3 was overcharged at 25°C, and the results are shown in Figure 11. As can be seen from Figure 11, although LiTDI was added to the positive electrode film layer, the amount added was insufficient, and the battery voltage reached 5.475V after 59 minutes of overcharging, which is unacceptable.

[0208] Table 3-1

[0209] Table 3-2

[0210] Table 3-3

[0211] As can be seen from Tables 3-1 to 3-3, the mass ratio of additives is 5%-10% relative to the mass of the positive electrode active material, and 4.2%-8.2% relative to the mass of the positive electrode film. The secondary batteries all passed the overcharge test, demonstrating high safety performance and meeting the requirements of the national standard GB / T 36276-2023.

[0212] It should be noted that this disclosure is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same essential structure and achieving the same effect as the technical concept within the scope of this disclosure are included in the technical scope of this disclosure. 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, are also included in the scope of this disclosure without departing from the spirit of this disclosure.

Claims

1. A secondary battery, comprising a positive electrode and an electrolyte; the positive electrode comprising a positive current collector and a positive electrode film layer located on at least one surface of the positive current collector, the positive electrode film layer and / or the electrolyte comprising additives; After the secondary battery is charged to the charging cutoff voltage using a constant current I, it is continued to be charged with the same constant current I for 1 hour to obtain an overcharge curve. The constant current I satisfies the following relationship: I = P re / U nom P re For rated charging power, U nom Nominal voltage, In the overcharge curve, the voltage of the secondary battery is <1.5×ΔU, where ΔU is the charging cutoff voltage of the secondary battery; The oxidation potential ΔE of the additive and the ΔU satisfy the following relationship: 1≤ΔE / ΔU<1.

5.

2. The secondary battery according to claim 1, wherein, The charging cutoff voltage is 3.6V-4.2V.

3. The secondary battery according to claim 1 or 2, wherein, The additive has an oxidation potential of 3.6V or higher and less than 6.3V at 10℃ to 60℃.

4. The secondary battery according to any one of claims 1 to 3, wherein, The overcharge curve contains a voltage plateau, in which the difference between the maximum and minimum voltages is less than or equal to 0.5V.

5. The secondary battery according to claim 4, wherein, The difference between the maximum and minimum voltages in the voltage platform is 0.1V-0.35V.

6. The secondary battery according to claim 4 or 5, wherein, The duration of the voltage plateau is 30-60 minutes.

7. The secondary battery according to any one of claims 4 to 6, wherein, The duration of the voltage plateau is 30 min to 57 min.

8. The secondary battery according to any one of claims 1 to 7, wherein, The positive electrode film layer includes a positive electrode active material, and the positive electrode active material includes a phosphate positive electrode active material.

9. The secondary battery according to claim 8, wherein, The positive electrode active material includes at least one of lithium iron phosphate, lithium manganese iron phosphate, and sodium iron pyrophosphate.

10. The secondary battery according to claim 9, wherein, The positive electrode active material includes lithium iron phosphate, the charging cutoff voltage is 3.6V-3.8V, and the oxidation potential of the additive at 10℃~60℃ is above 3.6V and less than 5.7V.

11. The secondary battery according to claim 9, wherein, The positive electrode active material includes lithium manganese iron phosphate, the charging cutoff voltage is 4.0V-4.2V, and the oxidation potential of the additive at 10℃~60℃ is above 4.0V and less than 6.3V.

12. The secondary battery according to claim 10 or 11, wherein, The additive includes at least one of 2-trifluoromethyl-4,5-dicyanoimidazolium lithium, 2-pentafluoroethyl-4,5-dicyanoimidazolium lithium, 2-heptafluoropropyl-4,5-dicyanoimidazolium lithium, and lithium oxalate.

13. The secondary battery according to claim 9, wherein, The positive electrode active material includes sodium iron pyrophosphate, the charging cutoff voltage is 3.6V-3.8V, and the oxidation potential of the additive at 10℃~60℃ is above 3.6V and less than 5.7V.

14. The secondary battery according to claim 13, wherein, The additive includes at least one of 2-trifluoromethyl-4,5-dicyanimidazolium sodium, 2-pentafluoroethyl-4,5-dicyanimidazolium sodium, 2-heptafluoropropyl-4,5-dicyanimidazolium sodium, and sodium oxalate.

15. The secondary battery according to any one of claims 1 to 14, wherein, The oxidation potential ΔE of the additive and the charging cutoff voltage ΔU satisfy the following relationship: 1.15≤ΔE / ΔU≤1.

35.

16. The secondary battery according to any one of claims 1 to 15, wherein, The electrolyte includes an electrolyte salt, and the electrolyte salt includes the additive. The content of the additive is 80%-100% relative to the total mass of the electrolyte salt.

17. The secondary battery according to claim 16, wherein, In the electrolyte, the molar content of the additive is 0.5 mol / L to 1.5 mol / L.

18. The secondary battery according to any one of claims 1 to 17, wherein, The mass percentage of the additive relative to the mass of the positive electrode film is less than 8.2%.

19. The secondary battery according to claim 18, wherein, The additive accounts for 4.2% to 8.2% of the mass of the positive electrode film.

20. The secondary battery according to claim 18, wherein, The mass percentage of the additive relative to the mass of the positive electrode active material is less than 10%.

21. The secondary battery according to claim 20, wherein, The additive accounts for 4.9% to 10% of the mass of the positive electrode active material.

22. An electrical device comprising a secondary battery as described in any one of claims 1 to 21.

23. An electrolyte comprising an electrolyte salt, said electrolyte salt comprising an additive, said additive having an oxidation potential of 3.6V or higher and less than 6.3V at 10°C to 60°C.

24. The electrolyte according to claim 23, wherein, The content of the additive is 80%-100% relative to the total mass of the electrolyte salt.

25. The electrolyte according to claim 23 or 24, wherein, In the electrolyte, the molar content of the additive is 0.5 mol / L to 1.5 mol / L.

26. A positive electrode sheet, comprising a positive current collector and a positive electrode film layer located on at least one surface of the positive current collector, the positive electrode film layer comprising an additive. The additive has an oxidation potential of 3.6V or higher and less than 6.3V at 10℃ to 60℃.

27. The positive electrode sheet according to claim 26, wherein, The additive accounts for 4.2% to 8.2% of the mass of the positive electrode film.

28. The positive electrode sheet according to claim 26 or 27, wherein, The additive accounts for 4.9% to 10% of the mass of the positive electrode active material.

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