Secondary battery and electrical apparatus

Abstract

A secondary battery and an electrical apparatus. The secondary battery comprises an electrolyte and a positive electrode sheet; the positive electrode sheet comprises a positive electrode active material, the positive electrode active material comprising a core made of lithium iron phosphate materials and a carbon shell layer at least partially coating the outer surface of the core; in a Raman spectrum of the positive electrode active material, the peak intensity at a wavenumber of 1360±50 cm-1 is Id, the peak intensity at a wavenumber of 1580±50 cm-1 is Ig, and the Id / Ig value of the positive electrode active material is 0.2-1.5; the electrolyte comprises a cyclic ester compound which accounts for no more than 25% of the total mass of an electrolyte solvent. The secondary battery has good charging capability, energy efficiency and cycle life.

Description

Secondary battery and power consuming device

[0001] Cross-reference to related applications

[0002] This application is based on the Chinese Patent Application No. 202411310454.9 entitled "Secondary battery and power consuming device" filed on September 19, 2024, which is incorporated by reference in its entirety. TECHNICAL FIELD

[0003] The present disclosure relates to the technical field of secondary batteries, and in particular to a secondary battery and a power consuming device. BACKGROUND

[0004] In recent years, secondary batteries are increasingly widely used in energy storage power supply systems such as hydroelectric, thermal, wind and solar power stations, and in many fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, etc. The market also puts forward higher requirements for the capacity of secondary batteries. Therefore, increasing the size of the battery assembly has become a development trend in the battery field.

[0005] The increase in the size of the battery assembly increases the distance from the tab to the pole, which increases the electron transmission path during the cycle process. At the same time, the height of the cell also increases, which makes the electrolyte wetting effect of the pole piece worse, resulting in an increase in the direct current resistance (DCR) of the cell, which deteriorates the energy efficiency and cycle life of the secondary battery. The larger the size of the battery assembly, the more prominent this problem is. SUMMARY

[0006] Based on the above status quo, the present application provides a secondary battery, comprising an electrolyte and a positive pole piece, the positive pole piece comprising a positive active material, the positive active material comprising a lithium iron phosphate material core and a carbon shell layer at least partially coated on the outer surface of the core, in the Raman spectrum of the positive active material, the peak intensity at a wave number of 1360±50 cm -1 -1 d , the peak intensity at a wave number of 1580±50 cm -1 -1 g , the I d / I g value of the positive active material is 0.2-1.5, and the electrolyte comprises a cyclic ester compound accounting for not more than 25% of the total mass of the electrolyte solvent.

[0007] The ratio of I d / I g in the Raman spectrum of the positive active material can represent the graphitization degree of the positive active material, and the lower the value, the closer the material is to the ideal graphite structure, which is more conducive to the formation of a continuous conductive network. The I d / I gThe ratio within a proper range helps to improve the electronic conductivity of the material, reduce the internal resistance of the battery cell, reduce the energy loss during charging and improve the energy efficiency of the battery. The cyclic ester compound can effectively improve the conductivity of the electrolyte due to its high dielectric constant, but it can also increase the viscosity of the electrolyte. Reducing the amount of cyclic ester compound in the electrolyte can reduce the viscosity of the electrolyte, improve the liquid absorption capacity and internal wettability of the battery cell, provide more ion channels and improve the active ion desolvation capacity, reduce the migration resistance of the active ion, thereby improving the cycle performance of the battery. d / I g The ratio and the amount of the cyclic ester compound in the electrolyte can make the positive active material fully wetted by the electrolyte, reduce the polarization of the positive active material, improve the electronic conductivity of the positive active material, and improve the cycle capacity retention rate of the battery while maintaining good electrolyte conductivity and battery charging capacity.

[0008] In any embodiment, the I d / I g value of the positive active material is 0.6-1.5. In any embodiment, the I d / I g value of the positive active material is 0.6-1.2. The electronic conductivity of the lithium iron phosphate material can be improved, the electronic transmission resistance can be reduced, the energy loss during charging can be reduced, and the contact between the positive active materials can be improved, so that the positive active materials have sufficient contact sites, and the energy efficiency of the secondary battery is improved.

[0009] In any embodiment, the mass content of the cyclic ester compound is 5%-25% based on the total mass of the electrolyte solvent. In any embodiment, the mass content of the cyclic ester compound is 5%-20% based on the total mass of the electrolyte solvent, which is beneficial to further reduce the viscosity of the electrolyte, improve the liquid absorption capacity and internal wettability of the electrolyte cell, provide more ion channels and improve the active ion desolvation capacity, reduce the migration resistance of the active ion, and thereby improve the cycle capacity retention rate of the battery.

[0010] In any embodiment, the cyclic carbonate compound includes one or more of vinyl carbonate, fluorinated vinyl carbonate, propylene carbonate, and gamma-butyrolactone. The cyclic ester compound has good stability and is not easy to decompose, and can well dissociate the electrolyte salt in the electrolyte, and also helps to reduce the polarization of the positive active material and improve the electronic conductivity of the positive active material.

[0011] The cyclic ester compound includes a cyclic carbonate compound, and the cyclic carbonate compound includes one or more of vinyl carbonate, fluorinated vinyl carbonate, and propylene carbonate.

[0012] In any of the embodiments, the cyclic carbonate compound includes ethylene carbonate and propylene carbonate in a mass ratio of (1-7): 1, and has good compounding performance.

[0013] In any of the embodiments, the electrolyte has an electrical conductivity of 9 mS / cm-11 mS / cm, which is conducive to the migration of active ions in the electrolyte.

[0014] In any of the embodiments, the lithium iron phosphate material includes a structural formula as shown in Formula I Li x Fe y M z PO4 Formula I

[0015] wherein 0.95

[0016] M includes one or more of Nb, Ti, V, W, and Mn, and the molar content of the M element in the positive electrode active material is 0.01%-0.2%.

[0017] The positive electrode active material has the advantages of high specific capacity, high safety, and long cycle life. The molar proportion of the M element doped in the positive electrode active material in a suitable range can effectively reduce the transport potential barrier of lithium ions, improve the diffusion rate of lithium ions, and improve the migration of lithium ions.

[0018] In any of the embodiments, the length of the secondary battery is L, the height is H, and the thickness is T, wherein 200mm≤L≤550mm, 150mm≤H≤250mm, 60mm≤T≤80mm, and 1≤L / H≤4, 2≤H / T≤4. This not only realizes the industrial manufacturability of the secondary battery, but also shortens the electron transport path, improves the non-uniformity of the current density, reduces the internal resistance of the secondary battery, and improves the battery energy efficiency and cycle capacity retention rate under the premise of improving the battery capacity.

[0019] In any of the embodiments, the single cell capacity of the secondary battery is greater than or equal to 300Ah and less than or equal to 1300Ah, which helps to store more electricity and provide a longer cycle life for the secondary battery.

[0020] The second aspect of the present disclosure provides a power utilization device including the secondary battery of the first aspect. BRIEF DESCRIPTION OF DRAWINGS

[0021] FIG. 1 is a schematic diagram of a secondary battery according to an embodiment of the present disclosure.

[0022] FIG. 2 is an exploded view of the secondary battery according to an embodiment of the present disclosure shown in FIG. 1.

[0023] FIG. 3 is a schematic diagram of a battery module according to an embodiment of the present disclosure.

[0024] Figure 4 is a schematic diagram of a battery pack according to one embodiment of the present disclosure.

[0025] Figure 5 is an exploded view of a battery pack according to an embodiment of the present disclosure, as shown in Figure 4.

[0026] 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.

[0027] Explanation of reference numerals in the attached diagram: 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Secondary battery; 51 Housing; 52 Electrode assembly; 53 Cover plate. Detailed Implementation

[0028] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the positive electrode active material, its manufacturing method, positive electrode sheet, secondary battery, battery module, battery pack, and electrical device 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.

[0029] The "range" disclosed in this disclosure 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 expected. 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 disclosure, unless otherwise stated, the numerical range "a–b" 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" and "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.

[0030] Unless otherwise specified, all embodiments and optional embodiments of this disclosure can be combined to form new technical solutions.

[0031] All the technical features and optional technical features of the present disclosure can be combined with each other to form new technical solutions, if not specifically stated.

[0032] All the steps of the present disclosure can be performed in sequence or randomly, preferably in sequence, if not specifically stated. For example, the method comprises steps (a) and (b), which means that the method can comprise steps (a) and (b) performed in sequence, or steps (b) and (a) performed in sequence. For example, the method can further comprise step (c), which means that step (c) can be added to the method in any sequence, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0033] The terms "comprise" and "include" mentioned in the present disclosure mean open type, and can also mean closed type, if not specifically stated. For example, the terms "comprise" and "include" can mean that other components not listed can also be included, or only the listed components can be included.

[0034] The term "or" in the present disclosure is inclusive, if not specifically stated. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any one of the following conditions satisfies the condition "A or B": 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 A and B are both true (or exist).

[0035] With the increasing demand for the capacity of secondary batteries in the market, the trend of the battery cell is towards larger size and larger capacity. After the size of the battery assembly increases, the distance from the pole piece to the pole lug will increase accordingly, which makes the electrons need to pass through a longer path to reach the external circuit, resulting in the increase of the direct current resistance of the battery and the uneven current, affecting the energy efficiency of the secondary battery, and further affecting the overall performance and life of the battery.

[0036] At the same time, the increase in the size of the battery assembly will also increase the height of the battery cell. The ability of the electrolyte to penetrate upward by capillary action is insufficient, and the electrolyte is difficult to be evenly distributed to the entire pole piece, especially in the top area of the battery cell, resulting in the increase of the resistance of the embedding and extraction process of the electrolyte active ion or the inability of ion transmission, which seriously reduces the energy efficiency of the battery and accelerates the capacity decay of the battery, shortening the cycle life of the battery.

[0037] [lithium secondary battery]

[0038] The first aspect of the present application provides a secondary battery, specifically, comprising a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator disposed between the positive electrode sheet and the negative electrode sheet. During the charging and discharging of the battery, active ions are inserted and de-inserted between the positive electrode sheet and the negative electrode sheet, the electrolyte serves to conduct ions between the positive electrode sheet and the negative electrode sheet, and the separator is disposed between the positive electrode sheet and the negative electrode sheet and mainly serves to prevent short circuiting between the positive electrode and the negative electrode while allowing ions to pass through. In some embodiments, the secondary battery is a lithium secondary battery.

[0039] The present application does not have specific limitations on the type of electrolyte, which can be selected as needed. For example, the electrolyte can be liquid, gel, or all-solid. In some embodiments, the electrolyte uses an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0040] In some embodiments, the positive electrode sheet includes 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 the positive electrode active material described above. As an example, the positive electrode current collector has two opposite surfaces in the thickness direction of the positive electrode current collector, and the positive electrode film layer is disposed on either one or both of the two opposite surfaces of the positive electrode current collector.

[0041] In some embodiments, the positive electrode active material includes a lithium iron phosphate-based material core and a carbon shell layer at least partially coated on the outer surface of the core, the I d / I g is 0.2-1.5, and the electrolyte solution includes a cyclic ester compound accounting for no more than 25% of the total mass of the electrolyte solution solvent.

[0042] In this context, the term "I d " refers to the peak intensity of the D peak.

[0043] In this context, the term "D peak" refers to a peak located at 1360±50 cm -1 (cm -1 ) in Raman spectrum analysis, which corresponds to a characteristic peak of lattice defects and disordered structures in the carbon shell layer coated on the surface of the positive electrode active material.

[0044] In this context, the term "I g " refers to the peak intensity of the G peak.

[0045] In this context, the term "G peak" refers to a peak located at 1580±50 cm -1 , which corresponds to a characteristic peak of graphite and graphite-like structures in the carbon shell layer coated on the surface of the positive electrode active material.

[0046] In this context, the term "I d / I gThe ratio of the intensity of the D peak to the intensity of the G peak is used to quantify the graphitization degree of the positive electrode active material.

[0047] In this document, the term "graphitization degree" refers to the degree of graphitization of the carbon component, reflecting the completeness of the graphite crystal structure in the carbon-coated lithium iron phosphate of the present disclosure, particularly in the carbon coating layer, i.e., the degree of regularity of the arrangement of carbon atoms in the graphite structure.

[0048] Graphite crystals have a layered structure, and electrons can move freely between the layers, making graphite have very high electrical conductivity. The higher the graphitization degree of the positive electrode active material, the closer the arrangement of carbon atoms on the surface of the material to the ideal graphite crystal structure, the more complete the layered structure in the material, the shorter the electron transport path, and the higher the electron mobility and electrical conductivity of the material. This helps to improve the electronic conductivity of the positive electrode active material, reduce the electron transport resistance and energy loss. Controlling the graphitization degree of the positive electrode active material d / I g within a suitable range helps to improve the electronic conductivity of the positive electrode active material, and can provide sufficient contact sites between positive electrode active material particles. With uniform distribution of the positive electrode active material in the positive electrode sheet, a continuous conductive network is formed within the positive electrode sheet, reducing the powder film resistance and the interface impedance, reducing the direct current resistance of the battery, reducing the energy loss during charging, and improving the energy efficiency of the battery.

[0049] In this document, the term "cyclic ester compound" refers to an organic compound containing an ester group (-C(=O)O-) that participates in forming a ring.

[0050] Cyclic ester compounds have relatively high dielectric constants, can improve the solubility of electrolyte salts and effectively stabilize and solvate electrolyte salt ions. It is generally believed that the presence of a high proportion of cyclic ester compounds in the electrolyte can improve the conductivity of the electrolyte and improve the liquid-phase transport capacity and charging capacity of the secondary battery. However, cyclic ester compounds have poor low-temperature performance. The present application found that by controlling the graphitization degree of the positive electrode active material and reducing the amount of cyclic ester compounds in the electrolyte to a mass ratio of not more than 25%, the secondary battery still has excellent liquid-phase transport capacity and charging capacity. Without being bound by any theory, this may be because the viscosity of cyclic ester compounds is relatively high, and under high content conditions, it can affect the electrolyte's wetting and liquid-phase transport of the electrode sheet, deteriorating the charging capacity; it can also reduce the desolvation capacity of electrolyte salt ions, affecting the charging capacity and cycle performance of the battery. Reducing the content of cyclic ester compounds in the electrolyte can fully wet the battery with the electrolyte, reduce the polarization of the positive electrode active material, and improve the electronic conductivity of the positive electrode active material; at the same time, it can help the battery to provide more ion channels and improve the desolvation capacity of electrolyte salt ions, reducing the migration resistance of active ions. Synergistically adjusting the I d / Ig The ratio can further improve the electron transport capability, comprehensively improve the liquid phase transport capability and charging capability of the secondary battery, and also help to improve the low-temperature charging performance of the secondary battery.

[0051] I d / I g The measurement method can use any known method in the art, for example, referring to GB / T 40219-2021, the positive electrode active material powder is pressed into a tablet, and a LabRAM HR Evolution type laser micro-Raman spectrometer is used to test the tablet. Three points on the tablet are randomly selected for testing, and three sets of measurement values are obtained and averaged. Among them, a solid laser with a wavelength of 523 nanometers (nm) is used as a light source, the beam diameter is 1.2 micrometers (μm), the power is 1 milliwatt (mW), the measurement mode uses macro-Raman, and a CCD detector is used.

[0052] In some embodiments, the I d / I g value of the positive electrode active material is 0.6-1.5. In some embodiments, the I d / I g value of the positive electrode active material is 0.6-1.2. In some embodiments, the I d / I g of the positive electrode active material is 0.9-1.2. In some embodiments, the I d / I g of the positive electrode active material is 0.2-1.4, 0.2-1.2, 0.2-1.0, 0.2-0.8, 0.2-0.6, 0.2-0.4, 0.4-1.5, 0.6-1.5, 0.8-1.5, 1.0-1.5, 0.9-1.0, 1.0-1.1, 1.1-1.2.

[0053] In some embodiments, the I d / I g of the positive electrode active material is 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, or any range or any value within a range between any two of the above values.

[0054] I d / I g Within the appropriate range, it is helpful to obtain a positive electrode active material with a high degree of graphitization, further improve the electron transport capability of the positive electrode active material, reduce the internal resistance of the battery, reduce the energy loss during charging, and have sufficient contact sites between positive electrode active material particles to build a good conductive network and improve the energy efficiency of the secondary battery.

[0055] In some embodiments, the lithium iron phosphate-based material comprises a structural formula as shown in Formula I Li x Fe y M z PO4 Formula I

[0056] wherein 0.95 < x < 1.05, 0.96 < y < 1, 0 < z < 0.04,

[0057] In some embodiments, M comprises one or more of Nb, Ti, V, W, and Mn.

[0058] In some embodiments, the M element is selected from Nb, Ti, V, or W.

[0059] In some embodiments, the molar content of the M element in the positive electrode active material is 0.01% to 0.2%.

[0060] The lithium iron phosphate-based material has the advantages of high specific capacity, high safety, and long cycle life, but its own electrical conductivity is relatively low. The molar proportion of the M element doped in a suitable range is beneficial to increasing the free electron concentration in the lithium iron phosphate lattice, reducing the transport potential barrier of lithium ions, improving the migration rate of lithium ions, and improving the lithium ion conduction.

[0061] The positive electrode active material can be prepared by a method commonly used in the art. In some embodiments, the method for preparing the positive electrode active material comprises the following steps: providing a lithium iron phosphate substrate, mixing a lithium source, an iron source, a phosphorus source, an M source, and a carbon source according to a certain proportion and ball milling, drying the ball-milled powder, and sintering, thereby obtaining a carbon-coated lithium iron phosphate positive electrode active material.

[0062] In some embodiments, the lithium source comprises one or more of Li2CO3, LiH2PO4, and Li3PO4.

[0063] In some embodiments, the iron source comprises one or more of FeSO4, FePO4, FeCl2, FeC2O4, and Fe2O3.

[0064] In some embodiments, the phosphorus source comprises one or more of NH4H2PO4 and H3PO4.

[0065] In some embodiments, the M source comprises one or more of a metal salt, a metal oxide, and a metal organic compound containing Nb, Ti, V, W, and Mn elements.

[0066] In some embodiments, the carbon source comprises one or more of glucose, sucrose, starch, polyethylene glycol, phenolic resin, carbon black, graphite, carbon nanotubes, graphene, tannic acid, and sodium dodecyl benzene sulfonate.

[0067] In some embodiments, the sintering temperature is 500 degrees Celsius (°C) - 900 °C, and the sintering time is 8 hours (h) - 20 h.

[0068] In some embodiments, the sintering temperature is 500 °C, 650 °C, 700 °C, 750 °C, 800 °C, 850 °C, 900 °C, or a range between any two of the above values or any value within the range.

[0069] In some embodiments, the sintering time is 8 h, 9 h, 10 h, 11 h, 12 h, 13 h, 14 h, 15 h, 16 h, 17 h, 18 h, 19 h, 20 h, or a range between any two of the above values or any value within the range.

[0070] In some embodiments, the Dv50 of the positive electrode active material is 300 μm - 3570 μm.

[0071] In some embodiments, the positive electrode current collector can be a metal foil or a composite current collector. For example, as a metal foil, an aluminum foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base material (e.g., a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0072] In some embodiments, the positive electrode film layer can further optionally include a binder. As an example, the binder can include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylic resin.

[0073] In some embodiments, the positive electrode film layer can further optionally include a conductive agent. As an example, the conductive agent can include at least one of super P, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0074] In some embodiments, the positive electrode tab can be prepared by dispersing the above-described components for preparing the positive electrode tab, such as the positive electrode active material, the conductive agent, the binder, and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry, coating the positive electrode slurry on a positive electrode current collector, and subjecting the same to a drying, cold-pressing, or the like process to obtain the positive electrode tab.

[0075] In some embodiments, the mass content of the cyclic ester compound is 5-25% based on the total mass of the electrolyte. In some embodiments, the mass content of the cyclic ester compound is 5-20% based on the total mass of the electrolyte. In some embodiments, the mass content of the cyclic ester compound is 8-20%, 10-20%, 12-20%, 14-20%, 16-20%, or 18-20% based on the total mass of the electrolyte.

[0076] In some embodiments, the mass content of the cyclic ester compound is 5%, 8%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, or any value or range between any of the aforementioned values based on the total mass of the electrolyte.

[0077] The content of the cyclic ester compound in the range is conducive to reducing the viscosity of the electrolyte, improving the liquid absorption capacity of the battery cell, allowing sufficient electrolyte to be soaked in the battery cell, reducing the polarization of the lithium iron phosphate active material, improving the ion path, and at the same time, is conducive to the complete dissociation of the electrolyte salt in the electrolyte, reducing the active salt ion migration resistance while having good conductivity, further improving the cycle capacity retention rate of the secondary battery while having good charging performance, and helping to improve the low-temperature charging performance of the secondary battery.

[0078] In some embodiments, the cyclic ester compound includes one or more of ethylene carbonate, fluoroethylene carbonate, propylene carbonate, and γ-butyrolactone.

[0079] The cyclic ester compound includes a cyclic carbonate compound, and the cyclic carbonate compound includes one or more of ethylene carbonate, fluoroethylene carbonate, and propylene carbonate.

[0080] The cyclic ester compound exhibits good chemical stability in the battery operating voltage range and is not prone to decomposition on the electrode surface, which helps to maintain the stability of the electrolyte and the long cycle life of the battery, and the cyclic ester compound has a high dielectric constant, which allows it to well dissociate the electrolyte salt in the electrolyte, while also helping to reduce the polarization of the positive active material and improve the electronic conduction and active ion transport of the positive active material.

[0081] In some embodiments, the cyclic ester compound includes vinyl carbonate and propylene carbonate in a volume ratio of (1-7): 1. In some embodiments, the cyclic ester compound includes vinyl carbonate and propylene carbonate in a volume ratio of (0.5-2): 1. As an example, the mass ratio of vinyl carbonate and propylene carbonate in the electrolyte can be 0.5: 1, 0.8: 1, 1: 1, 1.5: 1, 2: 1, 3: 1, 4: 1, 5: 1, 6: 1, 7: 1.

[0082] In some embodiments, the electrolyte further includes an electrolyte salt. In some embodiments, the electrolyte salt can be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonylimide, lithium bis-trifluoromethanesulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluoro oxalato borate, lithium di oxalato borate, lithium difluoro di oxalato phosphate, and lithium tetrafluoro oxalato phosphate.

[0083] In some embodiments, the electrolyte further includes a solvent other than the cyclic ester compound, for example, can include at least one selected from ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0084] In some embodiments, the electrolyte can further optionally include an additive. For example, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, and can further include an additive capable of improving certain performance of the battery, for example, an additive capable of improving overcharge performance of the battery, an additive capable of improving high-temperature or low-temperature performance of the battery, etc.

[0085] In some embodiments, the electrolyte has an electrical conductivity of 9 milli- Siemens per centimeter (mS / cm) - 11 mS / cm, for example, 9 mS / cm, 9.5 mS / cm, 10 mS / cm, 10.5 mS / cm, 11 mS / cm, which is advantageous for the migration of active ions in the electrolyte.

[0086] In some embodiments, the negative electrode tab includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, the negative electrode film layer including a negative electrode active material.

[0087] As an example, the negative electrode current collector has two surfaces opposite in the thickness direction thereof, and the negative electrode film layer is disposed on either one or both of the two opposite surfaces of the negative electrode current collector.

[0088] In some embodiments, the negative current collector can be a metal foil or a composite current collector. For example, as a metal foil, a copper foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base layer (e.g., a base layer of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0089] In some embodiments, the negative active material includes, but is not limited to, one or more of conventional natural graphite, other artificial graphite, soft carbon, hard carbon, silicon-based material, tin-based material, and lithium titanate. The silicon-based material can include one or more of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy material. The tin-based material can include one or more of elemental tin, tin oxide, and tin alloy material. However, the present disclosure is not limited to these materials, and other conventional materials that can be used as battery negative active materials can also be used. These negative active materials can be used alone or in combination with two or more.

[0090] In some embodiments, the negative film layer can further optionally include a binder. The binder can 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).

[0091] In some embodiments, the negative film layer can further optionally include a conductive agent. The conductive agent can be selected from at least one of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0092] In some embodiments, the negative film layer can further optionally include other auxiliary agents, such as thickening agents (e.g., sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0093] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as the negative active material, the conductive agent, the binder, and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry, coating the negative electrode slurry on a negative current collector, and then performing processes such as drying, cold pressing, and the like to obtain a negative electrode sheet including a negative film layer.

[0094] In some embodiments, the secondary battery further comprises a separator. The type of the separator is not particularly limited in the present disclosure, and any known porous separator having good chemical stability and mechanical stability can be used.

[0095] In some embodiments, the separator can be made of at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single layer film or a multi-layer composite film, and is not particularly limited. When the separator is a multi-layer composite film, the materials of the layers can be the same or different, and are not particularly limited.

[0096] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be used to make an electrode assembly by a winding process or a stacking process.

[0097] In some embodiments, the secondary battery can comprise an outer package. The outer package can be used to package the electrode assembly and the electrolyte described above.

[0098] In some embodiments, the outer package of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery can also be a soft package, such as a pouch-type soft package. The material of the soft package can be plastic, and as plastic, polypropylene, polybutylene terephthalate, and polybutylene succinate, etc. can be listed.

[0099] The shape of the secondary battery is not particularly limited in the present disclosure, and it can be cylindrical, square, or any other shape. In some embodiments, the secondary battery is square. For example, FIG. 1 is a secondary battery 5 of a square structure as an example, and the length of the secondary battery is L, the height is H, and the thickness is T.

[0100] In some embodiments, 200 millimeters (mm) ≤ L ≤ 550 mm, 150 mm ≤ H ≤ 250 mm, 60 mm ≤ T ≤ 80 mm, and 1 ≤ L / H ≤ 4, 2 ≤ H / T ≤ 4.

[0101] In some embodiments, the L of the secondary battery is 200 mm, 300 mm, 350 mm, 400 mm, 450 mm, 500 mm, 550 mm, or any value within a range between any two of the above values.

[0102] In some embodiments, the H of the secondary battery is 150 mm, 200 mm, 205 mm, 210 mm, 215 mm, 220 mm, 225 mm, 230 mm, 235 mm, 240 mm, 245 mm, 250 mm, or any value within a range between any two of the above values.

[0103] In some embodiments, the T of the secondary battery is 60 mm, 65 mm, 70 mm, 75 mm, 80 mm, or a range between or any value within a range of any two of the above.

[0104] In some embodiments, the L / H of the secondary battery is 1, 1.5, 2, 2.5, 3, 3.5, 4, or a range between or any value within a range of any two of the above.

[0105] In some embodiments, the H / T of the secondary battery is 2, 2.5, 3, 3.5, 4, or a range between or any value within a range of any two of the above.

[0106] The increased battery assembly size increases the transmission path of electrons in the tab to the tab, which further deteriorates the non-uniformity of the current density. Controlling the secondary battery monomer assembly size within a certain range can shorten the electron transmission path and the electrolyte wettability of the tab top end area, improve the non-uniformity of the current density, reduce the battery internal resistance, help to improve the energy efficiency, increase the battery capacity and cycle life, and at the same time, the size of the secondary battery also has industrial manufacturability.

[0107] In some embodiments, referring to FIG. 2, the battery outer package can include a shell 51 and a cover plate 53. The shell 51 can include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be provided on the opening to close the receiving cavity. The positive electrode tab, the negative electrode tab and the separator can form an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is packaged in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, which can be selected by those skilled in the art according to specific actual needs.

[0108] In some embodiments, the monomer cell capacity of the secondary battery is greater than or equal to 300 ampere hours (Ah) and less than or equal to 1300 Ah.

[0109] In some embodiments, the monomer cell capacity of the secondary battery is 300 Ah, 400 Ah, 450 Ah, 500 Ah, 550 Ah, 600 Ah, 650 Ah, 700 Ah, 750 Ah, 800 Ah, 850 Ah, 900 Ah, 950 Ah, 1000 Ah, 1050 Ah, 1100 Ah, 1150 Ah, 1200 Ah, 1250 Ah, 1300 Ah, or a range between or any value within a range of any two of the above.

[0110] In this paper, the term "monomer cell capacity" refers to the maximum amount of charge that can be stored in a single battery unit (cell).

[0111] The monomer cell capacity can be tested using devices and methods known in the art. Illustratively, the prepared secondary battery is cycled at a rate of 0.5P (i.e., both the charge rate and the discharge rate are 0.5P), with a charging voltage of 2.5 volts (V) to 3.65 V, as one cycle, and the value of the cycled discharge capacity is recorded as the monomer cell capacity of the secondary battery.

[0112] A secondary battery with a large monomer cell capacity can store more power and provide a longer service life. Controlling the monomer cell capacity of the secondary battery within an appropriate range helps to optimize the charge and discharge rate, reduce the chemical reaction loss inside the battery, and extend the cycle life of the battery.

[0113] The secondary battery provided herein can be applied to grid-side energy storage (e.g., large-scale energy storage devices) and user-side energy storage (e.g., industrial and commercial energy storage devices, household energy storage devices). As an example, the secondary battery can be used as a light storage charging station, a micro-grid, a backup power station for a factory, a shopping mall, a hospital, or a school. A secondary battery with a large monomer cell capacity can store more power, which helps to improve the cycle performance and service time of the secondary battery and reduce the cost of the power station, and can balance the capacity, performance, manufacturing cost, and use cost of the secondary battery.

[0114] In some embodiments, the secondary battery can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, and the specific number can be selected by a person skilled in the art according to the application and capacity of the battery module.

[0115] FIG. 3 is a battery module 4 as an example. Referring to FIG. 3, in the battery module 4, a plurality of secondary batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, other arbitrary arrangements can also be used. Further, the plurality of secondary batteries 5 can be fixed by fasteners.

[0116] Optionally, the battery module 4 can further include a housing having an accommodation space, and the plurality of secondary batteries 5 are accommodated in the accommodation space.

[0117] In some embodiments, the above battery module can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, and the specific number can be selected by a person skilled in the art according to the application and capacity of the battery pack.

[0118] FIGS. 4 and 5 are a battery pack 1 as an example. Referring to FIGS. 4 and 5, the battery pack 1 can include a battery case and a plurality of battery modules 4 disposed in the battery case. The battery case includes an upper case 2 and a lower case 3, and the upper case 2 is capable of being provided on the lower case 3 and forms an enclosed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery case in any manner.

[0119] In addition, the present disclosure also provides a power consuming device including at least one of the secondary battery, the battery module, or the battery pack provided by the present disclosure. The secondary battery, the battery module, or the battery pack can be used as a power source of the power consuming device, or can be used as an energy storage unit of the power consuming device. The power consuming device can include a mobile device (such as a mobile phone, a notebook computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto.

[0120] As the power consuming device, the secondary battery, the battery module, or the battery pack can be selected according to the use requirement thereof.

[0121] FIG. 6 is a power consuming device as an example. The power consuming device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the requirement of high power and high energy density of the secondary battery for the power consuming device, the battery pack or the battery module can be used.

[0122] As another example of the device, it can be a mobile phone, a tablet computer, a notebook computer, etc. The device usually requires thin and light, and the secondary battery can be used as a power source.

[0123] Embodiment

[0124] Hereinafter, the embodiments of the present disclosure will be described. The embodiments described below are exemplary and are only used to explain the present disclosure, and cannot be understood as a limitation of the present disclosure. If the specific technology or condition is not specified in the embodiments, it is performed according to the technology or condition described in the literature in the art or according to the product instruction. If the reagent or instrument used is not specified by the manufacturer, it is a conventional product that can be obtained by purchase.

[0125] I. Performance test method

[0126] 1. Positive active material graphitization degree I d / I g Test

[0127] Referring to GB / T 40219-2021, the positive active material powder is pressed into a tablet, and the tablet is tested by a LabRAM HR Evolution laser micro-Raman spectrometer. Three points on the tablet are randomly selected for testing, and three sets of measurement values are obtained and averaged. Among them, a solid laser with a wavelength of 523 nm is used as the light source, the beam diameter is 1.2 μm, the power is 1 mW, the measurement mode is macro-Raman, and a CCD detector is used.

[0128] 2. Cell capacity

[0129] At 25°C, the prepared secondary battery is charged at a cycle rate of 0.5P (i.e. the charge rate and discharge rate are both 0.5P), and the charging voltage is 2.5V to 3.65V. This is one charge and discharge process, and the value of the cycle discharge capacity is recorded as the cell capacity of the secondary battery.

[0130] 3. Test of electrolyte conductivity

[0131] Referring to the test method of conductivity specified in HG / T 4067-2015: about 100 mL of electrolyte is taken with a dry and clean corrosion-resistant sample bottle, and is placed in a constant temperature water bath at 25°C±0.5°C. Shake from time to time, when the temperature of the sample is constant, replace the sample bottle cap with a rubber plug inserted with a conductivity meter electrode. When the temperature is within 25°C±0.5°C, read the conductivity meter data, which is the conductivity of the sample being tested.

[0132] 4. Test of 0.5P energy efficiency of the battery

[0133] At 25°C and atmospheric pressure, the prepared secondary battery is discharged at 0.5P constant power to 2.5V, and is left for 30 minutes (min). Then it is charged at 0.5P constant power to 3.65V, and the charging energy E1 at this time is recorded. It is left for 30 min, and then it is discharged at 0.5P constant power to 2.5V, and the discharge energy E2 at this time is recorded. The energy efficiency of the cell at 0.5P rate = E2 / E1 x 100%.

[0134] 5. Test of cycle capacity retention rate of the battery

[0135] At 25°C, the prepared secondary battery is charged at a cycle rate of 0.5P (i.e. the charge rate and discharge rate are both 0.5P), and the charging voltage is 2.5V to 3.65V. This is one charge and discharge process, and the value of the cycle discharge capacity is recorded as the cell capacity of the secondary battery.

[0136] 6. Charge time test

[0137] The test battery is first charged and discharged at a current of 1C (i.e. the current value of the theoretical capacity fully discharged in 1h), specifically including: at 35°C, the battery is charged at a current of 1C to a voltage of 3.65V, then charged at a constant voltage to a current of ≤0.05C, rested for 5min, then discharged at a current of 0.33C to a voltage of 2.5V, and the actual capacity is recorded as C0. Then each battery is sequentially charged at a constant current of 0.5C0, 1.0C0, 1.3C0, 1.5C0, 1.8C0, 2.0C0, 2.3C0, 2.5C0, 3.0C0, 3.5C0, 4C0, 4.5C0, 5C0 to the full battery charging cutoff voltage of 3.65V or the negative electrode cutoff potential of 0V (whichever is reached first), and after each charging is completed, it needs to be discharged at 1C0 to the full battery discharge cutoff voltage of 2.5V, and the negative electrode potential corresponding to 10%, 20%, 30%, …, 80% SOC under different charging rates is recorded, and the charging rate-negative electrode potential curve under different SOC states is drawn, and the charging rate corresponding to the negative electrode potential of 0V under different SOC states is obtained by linear fitting, which is the charging window under the SOC state, and is respectively recorded as C(10% SOC), C(20% SOC), C(30% SOC), C(40% SOC), C(50% SOC), C(60% SOC), C(70% SOC), C(80% SOC). The charging time T of the battery from 0 to 80% SOC is calculated according to the formula (60 / C(10% SOC)+60 / C(20% SOC)+60 / C(30% SOC)+60 / C(40% SOC)+60 / C(50% SOC)+60 / C(60% SOC)+60 / C(70% SOC)+60 / (C80% SOC))x10%, and the unit is min. The shorter the charging time, the stronger the charging capacity.

[0138] II. Preparation of positive active material

[0139] Sucrose and polyethylene glycol (PEG) with a molecular weight of 3000 were used as carbon sources, and the mass ratio of sucrose and PEG was adjusted according to Table 1. Iron phosphate was used as the iron and phosphorus source, monoammonium phosphate was used as the phosphorus source, and lithium carbonate was used as the lithium source. The molar ratio of Li:Fe:P was 1.01:1:1.04, and the carbon content in the finished product was 1.20%. The above raw materials were uniformly mixed, ground in a sand mill, and then spray dried after the slurry particle size Dv50 was 0.38±0.01μm. The spray-dried material was sintered at 800°C under nitrogen atmosphere for 12h, and then naturally cooled and air-jet pulverized to a powder Dv50 of 1.4±0.2μm, obtaining lithium iron phosphate materials with different Id / Ig values.

[0140] Table 1

[0141] Preparation of a secondary battery

[0142] Example 1

[0143] 1) Preparation of a positive electrode sheet

[0144] The positive electrode active material prepared with Id / Ig of 1.01, the conductive agent Super P, and the binder polyvinylidene fluoride were mixed in a mass ratio of 96.5:1.5:2, a solvent N-methylpyrrolidone was added, and stirring was performed to obtain a positive electrode slurry; the positive electrode slurry was uniformly coated on a positive electrode current collector aluminum foil, and drying, cold pressing, and slitting were performed to obtain a positive electrode sheet. The single-sided coating amount of the positive electrode film layer was 21.7 mg / cm 2 .

[0145] 2) Preparation of a negative electrode sheet

[0146] The negative electrode active material graphite, the conductive agent Super P, the dispersant sodium carboxymethyl cellulose, and the binder styrene-butadiene rubber were mixed in a mass ratio of 96:0.7:1.0:2.3, a solvent deionized water was added, and stirring was performed to obtain a negative electrode slurry; the negative electrode slurry was uniformly coated on a negative electrode current collector copper foil, and drying, cold pressing, and slitting were performed to obtain a negative electrode sheet. The single-sided coating amount of the negative electrode film layer was 9.7 mg / cm 2 .

[0147] 3) Preparation of an electrolyte

[0148] In an argon atmosphere glove box with a water content of <10 ppm, ethylene carbonate (EC), dimethyl carbonate (DMC), methyl ethyl carbonate (EMC), and ethylene carbonate (VC) were mixed in a ratio of 15:53:30:2 to obtain an organic solvent, and then LiPF6 was dissolved in the organic solvent at a concentration of 1.1 mol / L to obtain an electrolyte with a conductivity of 10.2 mS / cm.

[0149] 4) Separation film

[0150] A polypropylene film was selected as the separation film.

[0151] 5) Preparation of a secondary battery

[0152] The above positive electrode sheet, the separation film, and the negative electrode sheet were sequentially stacked with the separation film between the positive electrode sheet and the negative electrode sheet to play a separation role, and then were wound to obtain an electrode assembly; the electrode assembly was placed in an outer package (length x height x thickness, 350 mm x 210 mm x 70 mm), and after drying, an electrolyte was injected, and vacuum packaging, standing, formation, shaping, and other processes were performed to obtain a lithium ion battery with a capacity of 587 Ah.

[0153] Examples 2-13

[0154] Examples 2-5 were prepared in a similar manner to Example 1, except that different Id / Ig positive active materials were used.

[0155] Examples 6-11 were prepared in a similar manner to Example 1, except that the type or ratio of the solvent of the electrolyte was adjusted, and the amount of DMC in the electrolyte was adjusted accordingly.

[0156] Examples 12-13 were prepared in a similar manner to Example 1, except that the capacity of the battery cell and the size of the battery were adjusted.

[0157] Comparative Examples 1-2

[0158] Comparative Example 1 was prepared in a similar manner to Example 1, except that different Id / Ig positive active materials were used.

[0159] Comparative Example 2 was prepared in a similar manner to Example 1, except that the content of ethylene carbonate in the electrolyte was adjusted to 40%, and the amount of DMC in the electrolyte was adjusted accordingly.

[0160] The specific preparation parameters of Examples 1-13 and Comparative Examples 1-2 are shown in Table 2.

[0161] Table 2

[0162] EC: ethylene carbonate; PC: propylene carbonate

[0163] The performance of the secondary batteries of Examples 1-13 and Comparative Examples 1-2 was tested using the methods under the "Performance Test Methods", and the results are shown in Table 3.

[0164] Table 3

[0165] As can be seen from the comparison of Examples 1-13 and Comparative Example 1, the Id / Ig of the positive active material of Examples 1-13 is higher than that of Comparative Example 1. d / I g In the range of 0.2-1.5, and the mass content of the cyclic ester compound in the electrolyte is in the range of 5%-25%, the charging time of the secondary battery is significantly reduced, and the energy efficiency and the 1000-cycle capacity retention rate are significantly improved. This indicates that the secondary battery has good charging performance and cycle stability, which is beneficial to improving the service life of the secondary battery.

[0166] As can be seen from the comparison of Examples 1-5 and Comparative Example 1, the Id / Ig of the positive active material of Examples 1-5 is higher than that of Comparative Example 1. d / I gIn the range of 0.6-1.5, the electronic conductivity of the positive active material is improved, the internal resistance of the battery is reduced, the secondary battery has good charging performance, and the energy efficiency is significantly improved, and accordingly, the 1000 cycle capacity retention rate is improved. The I d / I g is 0.6-1.2, and the secondary battery has excellent charging performance, energy efficiency and cycle retention rate.

[0167] As can be seen from the comparison of Examples 6-9 and Comparative Example 2, the charging time of the secondary battery is significantly reduced, the capacity retention rate is greatly improved, and the energy efficiency is improved. It shows that the secondary battery has good charging capacity and cycle capacity retention rate. The mass content of the cyclic ester compound in the electrolyte is 5%-20%, which can further improve the charging capacity and cycle capacity retention rate of the battery.

[0168] As can be seen from Examples 10 and 11, the use of propylene carbonate or the use of a mixture of ethylene carbonate and propylene carbonate in the electrolyte has excellent charging performance, energy efficiency and cycle capacity retention rate.

[0169] As can be seen from Examples 12-13, after adjusting the capacity of the secondary battery to 314-1132 Ah, the secondary battery still has excellent charging performance, energy efficiency and cycle capacity retention rate.

[0170] It should be noted that the present disclosure is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and embodiments having the same technical idea and substantially the same function and effect within the scope of the technical solutions of the present disclosure are all included in the technical scope of the present disclosure. In addition, within the scope of the main idea of the present disclosure, various modifications of the embodiments that can be thought of by those skilled in the art, and other ways constructed by combining part of the elements in the embodiments are also included in the scope of the present disclosure.

Claims

1. A secondary battery characterized by comprising: The device includes an electrolyte and a positive electrode. The positive electrode includes a positive active material, which comprises a lithium iron phosphate core and a carbon shell at least partially covering the outer surface of the core. The Raman spectrum of the positive active material is obtained at a wavenumber of 1360±50 cm⁻¹. -1 The peak intensity at I is d With a wavenumber of 1580±50cm -1 The peak intensity at I is g The positive electrode active material I d / I g The value is 0.2-1.

5. The electrolyte comprises a cyclic ester compound accounting for no more than 25% of the total mass of the electrolyte solvent.

2. The secondary battery according to claim 1, characterized by The I value of the positive electrode active material is 0.6-1.

5. d / g 0.6-1.

5.

3. The secondary battery according to claim 1 or 2, characterized by The I value of the positive electrode active material is 0.6-1.

2. d / g 0.6-1.

2.

4. The secondary battery according to any one of claims 1 to 3, characterized by The mass content of the cyclic ester compound is 5%-25% based on the total mass of the electrolyte solvent.

5. The secondary battery according to any one of claims 1 to 4, characterized by The mass content of the cyclic ester compound is 5%-20% based on the total mass of the electrolyte solvent.

6. The secondary battery according to any one of claims 1 to 5, characterized by The cyclic ester compound comprises one or more of ethylene carbonate, fluoroethylene carbonate, propylene carbonate, and gamma-butyrolactone.

7. The secondary battery according to any one of claims 1 to 6, characterized by The cyclic ester compound comprises a cyclic carbonate compound comprising one or more of ethylene carbonate, fluoroethylene carbonate, and propylene carbonate.

8. The secondary battery according to any one of claims 1 to 7, characterized by, The cyclic ester compound comprises ethylene carbonate and propylene carbonate in a mass ratio of (1-7):

1.

9. The secondary battery according to any one of claims 1 to 8, characterized by, The electrolyte has an electrical conductivity of 9 mS / cm-11 mS / cm.

10. The secondary battery according to any one of claims 1 to 9, characterized by The lithium iron phosphate material comprises a structural formula shown in Formula I, Li x Fe y M z PO4Formula I wherein 0.95 < x < 1.05, 0.96 < y < 1, and 0 < z < 0.04, M comprises one or more of Nb, Ti, V, W, and Mn, and the molar content of the M element in the positive electrode active material is 0.01%-0.2%.

11. The secondary battery according to any one of claims 1 to 10, characterized by The length of the secondary battery is L, the height is H, and the thickness is T. wherein 200 mm < L < 550 mm, 150 mm < H < 250 mm, 60 mm < T < 80 mm, 1 < L / H < 3, and 2 < H / T < 4.

12. The secondary battery according to any one of claims 1 to 11, characterized by The single cell capacity of the secondary battery is greater than or equal to 300 Ah and less than or equal to 1300 Ah.

13. An electrical device, characterized by The secondary battery comprises any one of claims 1-12. The secondary battery comprises any one of claims 1-12.

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