Secondary battery and electric device

Abstract

The invention relates to a secondary battery and an apparatus. The secondary battery comprises a positive pole piece, a negative pole piece and an electrolyte, the positive pole piece comprises a positive active material, and the positive active material comprises lithium transition metal phosphate; the electrolyte includes a nitrogen-containing cyclic compound. The secondary battery has reduced impedance, improved first efficiency, rate characteristics, normal temperature and high temperature cycle performance and high temperature storage performance.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a secondary battery and an electrical device. Background Art

[0002] Lithium iron phosphate batteries have long been widely used in energy storage and power battery applications due to their low cost and long cycle life. However, with increasing demands for battery life, further improving the storage performance, cycle performance, safety, and dynamics of lithium-ion batteries at a lower cost remains crucial.

[0003] However, due to the characteristics of lithium iron phosphate cathode material itself, it undergoes Fe 2+ / Fe 3+ The redox reaction between the two 3+ It has a certain solubility in the electrolyte and will dissolve from the positive electrode and shuttle to the negative electrode surface to be reduced to Fe element and Fe 2+ , and Fe 2+ It can also shuttle to the cathode surface to be oxidized to generate Fe 3+ The whole process is repeated over and over again, leading to the occurrence of battery self-discharge, which ultimately causes the battery capacity to decrease and the cycle life to decline. The common solution is to add a certain amount of vinylene carbonate (VC) film-forming additive to the electrolyte to form a protective film on the electrode surface, but this will affect the battery's dynamic performance. To improve this problem, the secondary injection process after battery formation is currently adjusted to solve it, but this will also increase the manufacturing cost of the battery.

[0004] Therefore, it is still necessary to develop a low-cost lithium iron phosphate battery that can extend the battery cycle life without affecting the battery's kinetic performance. Summary of the Invention

[0005] In response to the problems existing in the prior art, the present invention aims to provide a secondary battery and an electrical device containing the same. The electrolyte of the secondary battery contains a nitrogen-containing cyclic compound, which can replace or partially replace the commonly used VC additive, forming a dense, nitrogen-rich positive electrode solid electrolyte interface (CEI) film on the positive electrode surface. This not only effectively inhibits the dissolution of transition metals from the positive electrode active material, but also provides more active sites in the CEI film, enhancing the transport of lithium ions and significantly improving the battery's cycling and kinetic performance. Furthermore, since a certain amount of the nitrogen-containing cyclic compound is added to the electrolyte, it can adsorb a small amount of water and HF in the complex electrolyte, reducing the decomposition of the lithium salt LiPF6 and improving the thermal stability of the lithium salt LiPF6. As a result, the high-temperature cycling and safety performance of the battery are also significantly improved.

[0006] A first aspect of the present application provides a secondary battery comprising a positive electrode sheet, a negative electrode sheet, and an electrolyte, wherein the positive electrode sheet comprises a positive electrode active material, the positive electrode active material comprises a lithium transition metal phosphate; the electrolyte comprises a nitrogen-containing cyclic compound, the nitrogen-containing cyclic compound comprises at least one of the compounds represented by Formula I and Formula II,

[0007]

[0008] In formula I and formula II, R1 is selected from O or R a R and R2 are each independently selected from substituted or unsubstituted C1-C4 alkyl groups, and R3 and R4 are each independently selected from hydrogen, fluorine, and fluorine-substituted or unsubstituted C1-C4 alkyl groups.

[0009] The inventors of this invention have discovered that when the VC additive content in the electrolyte of lithium iron phosphate batteries is increased to 2%, a thick CEI film forms on the surface of the lithium iron phosphate material particles, reducing the battery's kinetic performance and even leading to lithium plating at the negative electrode, which in turn worsens the battery's cycle life and safety performance. To address this shortcoming, the present invention replaces or partially replaces VC with a nitrogen-containing cyclic compound. This method inhibits Fe dissolution from the lithium iron phosphate cathode without affecting the battery's kinetic performance, and can also improve the battery's cycle performance and safety performance.

[0010] A second aspect of the present application provides an electrical device, which includes the secondary battery described in the first aspect.

[0011] The beneficial effects of this application are:

[0012] The secondary battery of the present application has a long cycle life and high safety performance. The nitrogen-containing cyclic compound added to the electrolyte can form a suitable interfacial passivation film (CEI) on the surface of the lithium transition metal phosphate particles, which can effectively inhibit the dissolution of the transition metal without affecting the kinetic performance of the battery. At the same time, it can greatly reduce the interfacial side reactions between the electrolyte and the positive electrode, thereby significantly improving the battery's cycle performance and high-temperature storage performance. DETAILED DESCRIPTION

[0013] For the sake of clarity, this application only specifically discloses certain numerical ranges. However, any lower limit may be combined with any upper limit to form an unspecified range; and any lower limit may be combined with other lower limits to form an unspecified range, and similarly, any upper limit may be combined with any other upper limit to form an unspecified range. In addition, each individually disclosed point or single value may itself serve as a lower limit or upper limit and be combined with any other point or single value, or with other lower limits or upper limits, to form an unspecified range.

[0014] Unless otherwise specified, the terms used in this application have the commonly understood meanings commonly understood by those skilled in the art. Unless otherwise specified, the numerical values ​​of the various parameters mentioned in this application can be measured using various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the examples of this application).

[0015] A list of items connected by the term "at least one of" or other similar terms can mean any combination of the listed items. For example, if items A and B are listed, the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A can contain a single component or multiple components. Item B can contain a single component or multiple components. Item C can contain a single component or multiple components.

[0016] The present application will be further described below in conjunction with specific embodiments. It should be understood that these specific embodiments are only used to illustrate the present application and are not used to limit the scope of the present application.

[0017] Primary and secondary batteries

[0018] The secondary battery provided in the present application includes a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the positive electrode sheet includes a positive electrode active material, and the positive electrode active material includes a lithium transition metal phosphate; the electrolyte includes a nitrogen-containing cyclic compound, and the nitrogen-containing cyclic compound includes at least one of the compounds represented by Formula I and Formula II.

[0019]

[0020] In formula I and formula II, R1 is selected from O or R a R and R2 are each independently selected from substituted or unsubstituted C1-C4 alkyl groups, and R3 and R4 are each independently selected from hydrogen, fluorine, and fluorine-substituted or unsubstituted C1-C4 alkyl groups.

[0021] In the present application, "*" indicates the attachment position of a group.

[0022] In this application, C 1-4 Alkyl groups include, but are not limited to, methyl, ethyl, n-propyl, isopropyl, cyclopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, and cyclobutyl.

[0023] In some embodiments, in Formula I and Formula II, R1 is selected from O or R aR and R2 are each independently selected from substituted or unsubstituted C1-C2 alkyl groups, and R3 and R4 are each independently selected from hydrogen, fluorine, and fluorine-substituted or unsubstituted C1-C2 alkyl groups.

[0024] In some embodiments, the nitrogen-containing cyclic compound includes at least one of the compounds represented by Formula I-1 to Formula I-6 and Formula II-1 to Formula II-2,

[0025]

[0026] In some embodiments, the nitrogen-containing compound includes a compound represented by formula I-1

[0027] In some embodiments, based on the total weight of the electrolyte, the content of the nitrogen-containing cyclic compound is 0.5 wt % to 10 wt %, for example, 0.5 wt %, 0.8 wt %, 1 wt %, 1.2 wt %, 1.5 wt %, 1.8 wt %, 2 wt %, 2.2 wt %, 2.5 wt %, 2.8 wt %, 3 wt %, 3.2 wt %, 3.5 wt %, 3.8 wt %, 4 wt %, 4.2 wt %, 4.5 wt %, 4.8 wt %, 5 wt %, 6 wt %, 7 wt %, 8 wt %, 9 wt %, 10 wt % or a range consisting of any two of these values. In some embodiments, based on the total weight of the electrolyte, the content of the nitrogen-containing cyclic compound is 0.5 wt % to 5 wt %. In some embodiments, based on the total weight of the electrolyte, the content of the nitrogen-containing cyclic compound is 1 wt % to 4 wt %.

[0028] In some embodiments, the electrolyte further comprises vinylene carbonate (VC), and the content of the vinylene carbonate is less than 2 wt %, for example, less than 1.8 wt %, based on the total weight of the electrolyte. Controlling the content of vinylene carbonate within the above range can reduce the deterioration of dynamic performance caused by vinylene carbonate and simplify the battery injection process.

[0029] In some embodiments, the weight ratio of the nitrogen-containing cyclic compound to the vinylene carbonate is between 5:1 and 1:2, for example, 5:1, 4:1, 3:1, 2:1, 1:1, 1:2, or a range consisting of two of these values. The weight ratio of the nitrogen-containing cyclic compound to the vinylene carbonate within the above range can further improve the overall performance of the battery. In some embodiments, the weight ratio of the nitrogen-containing cyclic compound to the vinylene carbonate is between 3:1 and 1:1.

[0030] In some embodiments, the electrolyte further comprises lithium hexafluorophosphate, and the content of the lithium hexafluorophosphate is 12% to 15% by weight based on the total weight of the electrolyte. In some embodiments, the weight ratio of the lithium hexafluorophosphate to the nitrogen-containing cyclic compound is between 2:1 and 20:1, for example, 2:1, 4:1, 6:1, 8:1, 10:1, 12:1, 14:1, 16:1, 18:1, 20:1, or a range consisting of any two of these values.

[0031] In some embodiments, the electrolyte does not include a lithium sulfonyl imide salt. Here, "does not include" means that the content of the lithium sulfonyl imide salt is less than 1% by weight, or even less than 0.1% by weight, based on the total weight of the electrolyte. The lithium sulfonyl imide salt includes but is not limited to lithium bis(fluorosulfonyl)imide salt.

[0032] In some embodiments, the electrolyte further includes an additive selected from fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), trifluoropropylene carbonate (TFPC), methylene disulfonate (MMDS), ethylene ethyl disulfonate, propylene disulfonate, 1,3-propane sultone (PS), 1-propylene-1,3-sultone (PST) and 1,4-butane sultone (BS), vinyl sulfate (DTD), 4-methylethylene sulfate (PCS), 4-ethylethylene sulfate (PES), 4-propylethylene sulfate (PEGLST), propylene sulfate (TS) and ethylene sulfite (DTO), based on the total weight of the electrolyte, The content of the additive is less than 5 weight%, for example, 0.2 weight% to 3 weight%.

[0033] In some embodiments, the electrolyte further comprises an additive selected from fluoroethylene carbonate, methylene disulfonate, 1,3-propane sultone, and 1-propylene-1,3-sultone, and the content of the additive is less than 5 wt %, for example, 0.2 wt % to 3 wt %, based on the total weight of the electrolyte.

[0034] In some embodiments, the electrolyte further comprises a solvent, and the solvent comprises at least one of a linear carbonate, a cyclic carbonate, and a carboxylate. In some embodiments, the linear carbonate comprises at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, and a fluorinated linear carbonate. In some embodiments, the cyclic carbonate comprises at least one of ethylene carbonate, propylene carbonate, and butylene carbonate. In some embodiments, the carboxylate comprises at least one of methyl formate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, gamma-butyrolactone, and a fluorinated carboxylate.

[0035] In some embodiments, the solvent includes a chain carbonate and / or a cyclic carbonate. Based on the mass of the solvent, the mass content of the chain carbonate and / or the cyclic carbonate is above 90%, for example, above 95% or above 98%. In some embodiments, the solvent does not include an ether.

[0036] In some embodiments, the electrolyte further includes lithium hexafluorophosphate, a cyclic carbonate, a chain carbonate, and an additive. Among them, based on the total weight of the electrolyte, the content of the nitrogen-containing cyclic compound is 0.5 wt% to 10 wt%, the content of lithium hexafluorophosphate is 12 wt% to 15 wt%, the content of the chain carbonate is 50 wt% to 65 wt%, the content of the cyclic carbonate is 15 wt% to 30 wt%, and the content of the additive is 0.2 wt% to 3 wt%.

[0037] In some embodiments, the lithium transition metal phosphate is at least one of the compounds represented by the formula Li x Fe y M (1-y) PO4, where -0.5 ≤ x ≤ 0.5, 0 < y ≤ 1, and M is selected from at least one of Mn, Ni, Co, Al, Mg, Ti, Zn, Ga, Cu, V, Nb, Zr, In, and Y. In some embodiments, x is -0.5, -0.4, -0.3, -0.2, -0.1, 0, 0.1, 0.2, 0.3, 0.4, 0.5, or a range formed by any two of these values. In some embodiments, y is 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, or a range formed by any two of these values.

[0038] In some embodiments, the lithium transition metal phosphate includes lithium iron phosphate, lithium manganese iron phosphate LiMn 0.6 Fe 0.4 PO4, or lithium manganese iron phosphate LiMn 0.8 Fe 0.2 PO4, or at least one of them.

[0039] In some embodiments, the positive electrode sheet further includes a binder and a conductive agent. In some embodiments, the binder includes, but is not limited to, polyvinylidene fluoride and polytetrafluoroethylene, and the conductive agent includes, but is not limited to, carbon nanotubes, carbon black, and acetylene black.

[0040] In some embodiments, the tap density of the positive electrode sheet is 2.5 g / cm 3 to 2.7 g / cm 3 , for example, 2.5 g / cm 3, 2.55g / cm 3 , 2.6g / cm 3 , 2.65g / cm 3 , 2.7g / cm 3 Or the range of any two of these values. The compaction density of the positive electrode sheet is increased to 2.5g / cm 3 After the above, in order to ensure the cycle performance of the battery, it is usually necessary to add a higher amount of VC, but this will cause the wettability of the electrolyte to the electrode to deteriorate, affecting the transmission of lithium ions. At the same time, the SEI film formed by the high content of VC is thicker, which will cause the internal resistance of the battery to increase, thereby deteriorating the electrochemical performance of the battery. Replacing or partially replacing VC with the nitrogen-containing cyclic compound of the present application can alleviate the deterioration of battery performance caused by high content of VC. In some embodiments, the compacted density of the positive electrode sheet is 2.55g / cm 3 to 2.7g / cm 3 .

[0041] In some embodiments, the negative electrode plate includes a negative electrode active material, wherein the negative electrode active material includes a carbon-based material, and the carbon-based material includes at least one of artificial graphite and natural graphite.

[0042] In some embodiments, the negative electrode plate further comprises a binder and a conductive agent. In some embodiments, the binder includes but is not limited to styrene-butadiene rubber and acrylated styrene-butadiene rubber, and the conductive agent includes but is not limited to carbon nanotubes, carbon black, and acetylene black.

[0043] In some embodiments, a separator is provided between the positive and negative electrode plates to prevent short circuits. The material and shape of the separator that can be used in the embodiments of the present application are not particularly limited and can be any material disclosed in the prior art. In some embodiments, the separator includes a polymer or inorganic material formed from a material that is stable to the electrolyte of the present application. For example, the separator may include a substrate layer and a surface treatment layer.

[0044] In some embodiments, the secondary battery preparation method includes providing an electrode assembly, injecting liquid, packaging, and forming. The secondary battery of the present application only requires one injection, not two injections, which reduces the manufacturing cost of the battery.

[0045] In some embodiments, the secondary battery has a filling coefficient of 3 g / Ah to 3.8 g / Ah, exemplified by 3 g / Ah, 3.1 g / Ah, 3.2 g / Ah, 3.3 g / Ah, 3.4 g / Ah, 3.5 g / Ah, 3.6 g / Ah, 3.7 g / Ah, 3.8 g / Ah, or a range consisting of any two of the foregoing values. If the filling coefficient of the secondary battery is too high, the energy density of the battery will be reduced, and if the filling coefficient is too low, the cycle performance of the battery will be degraded.

[0046] In some embodiments, the secondary battery may include an outer packaging, which may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery may also be a soft package, such as a pouch-type soft package. In some embodiments, the shape of the secondary battery is not particularly limited, and may be cylindrical, square, or any other shape.

[0047] In some embodiments, the present application also provides a battery module. The battery module includes the aforementioned secondary battery. The battery module of the present application utilizes the aforementioned secondary battery and therefore has at least the same advantages as the aforementioned secondary battery. The battery module of the present application may include multiple secondary batteries, the specific number of which can be adjusted based on the application and capacity of the battery module.

[0048] In some embodiments, the present application further provides a battery pack comprising the above-mentioned battery module. The number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0049] 2. Electrical devices

[0050] The present application also provides an electrical device, which includes at least one of the above-mentioned secondary battery, battery module or battery pack.

[0051] In some embodiments, the electrical device includes, but is not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, power storage systems, etc. To meet the device's requirements for high power and high energy density of secondary batteries, battery packs or battery modules may be used.

[0052] In other embodiments, the power-consuming device may be a mobile phone, a tablet computer, a laptop computer, etc. Such a device is usually required to be lightweight and thin, and may use a secondary battery as a power source.

[0053] Example

[0054] In this application, unless otherwise specified, all materials and reagents used are commercially available.

[0055] Example 1

[0056] The preparation steps of the positive electrode sheet are as follows: the positive electrode active material lithium iron phosphate (LiFePO4), the conductive agent carbon nanotube (CNT), the conductive agent acetylene black (Super-P) and the binder polyvinylidene fluoride (PVDF) are mixed in a weight ratio of 95:2.0:1.0:2, and then dispersed in N-methylpyrrolidone (NMP) solvent and fully homogenized. The resulting slurry is coated on the surface of a 12μm thick carbon-coated aluminum current collector, and after drying, rolling twice (rolling load 1000N / m), and stripping, a compaction density of 2.6g / cm 3 The positive electrode.

[0057] The steps for preparing the negative electrode sheet are as follows: the negative electrode active material artificial graphite, the conductive agent acetylene black, the binder styrene-butadiene rubber SBR, and the thickener sodium carboxymethyl cellulose (CMCNa) are mixed in a weight ratio of 95:2:2:1, then dispersed in a deionized water solvent, fully homogenized, and the resulting slurry is coated on the surface of an 8μm thick copper current collector. After drying, rolling, and striping, the negative electrode sheet is obtained.

[0058] Diaphragm: PP / PE / PP three-layer composite diaphragm.

[0059] Preparation of the electrolyte: In an argon-protected glove box (H2O<0.1ppm, O2<0.1ppm), solvents ethylene carbonate (EC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) were mixed in a weight ratio of 25:20:55, and lithium hexafluorophosphate (LiPF6) was added to prepare a solution with a lithium salt mass percentage of 13wt%, and then the nitrogen-containing cyclic compound of the structure of formula I-1 was added in an amount of 2wt% based on the total weight of the electrolyte as shown in Table 1. After stirring evenly, the lithium ion battery electrolyte of Example 1 was obtained.

[0060] Preparation of a lithium-ion battery: The prepared positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator positioned between the positive and negative electrodes, and then wound to form a bare cell. The bare cell is placed in an aluminum-plastic film outer packaging. After thorough drying, the prepared lithium-ion battery electrolyte is injected at an injection rate of 3.5g / Ah. After the battery is stored at 45°C, formed in a high-temperature fixture, and sealed again, conventional capacity separation is performed.

[0061] Examples 2-16 and Comparative Examples 1-6

[0062] Examples 2-16 and Comparative Examples 1-6 are achieved on the basis of Example 1 by adjusting the type and amount of nitrogen-containing cyclic compounds, the amount of VC, the type and amount of other additives in the electrolyte, the injection coefficient and the formation temperature. Specific adjustment measures and detailed data are shown in Table 1.

[0063] Table 1

[0064]

[0065]

[0066] a: The amount of additives is based on the total weight of the electrolyte.

[0067] The test results of the lithium ion batteries of Examples 1-16 and Comparative Examples 1-5 are shown in Table 2.

[0068] Test Method

[0069] 1. Determination of the compaction density of the positive electrode

[0070] The compaction density of the pole piece = the surface density of the pole piece (g / cm 2 ) / thickness of the active material layer (cm). Cut a piece of positive electrode and weigh its mass as M1 and thickness as H1. Then scrape off the active material layer on the electrode and weigh its mass as M2 and thickness as H2. Measure the area of ​​the electrode and record it as V. The surface density of the electrode is then calculated as (M1-M2) / V / (H1-H2). The mass can be measured using a standard balance, and the thickness can be measured using a micrometer.

[0071] 2. Determination of battery impedance

[0072] Before the cycle capacity retention test, the lithium-ion battery was discharged at 1C constant current to 2.5V at 25±2°C, then charged at 0.5C constant current to 3.8V, and then charged at 3.8V to 0.05C constant voltage. Then, it was discharged at 1C constant current to 50% SOC. After standing for 60 minutes, the voltage U1 after the standing period was recorded. Then, it was discharged at 2C constant current for 10 seconds, and the voltage U2 after the discharge was recorded. The 2C current was recorded as I, and it was stood for 60 minutes. The discharge DCR (impedance) of the battery at 50% SOC was calculated according to the formula DCR = (U1-U2) / I.

[0073] 3. Battery capacity retention rate test at room temperature

[0074] 25℃ 1C / 1C cycle 1000 cycles:

[0075] At 25°C, the prepared lithium-ion secondary battery was charged at a constant current rate of 2C to 3.8V, followed by constant voltage charging until the current was less than 0.05C. After standing for 5 minutes, the battery was discharged at a rate of 0.5C to 2.7V, and the initial discharge capacity was recorded. The lithium-ion secondary battery was cycled 1000 times using the above method, and the discharge capacity was recorded each time. The capacity retention rate of the lithium-ion secondary battery after 1000 cycles at 1C / 1C at 25°C = discharge capacity at the 1000th cycle / initial discharge capacity × 100%.

[0076] 25℃ 2C / 2C cycle 200 cycles:

[0077] At 25°C, the prepared lithium-ion secondary battery was charged at a constant current rate of 2C to 3.8V, followed by constant voltage charging until the current was less than 0.05C. After standing for 5 minutes, the battery was discharged at a rate of 2C to 2.7V, and the initial discharge capacity was recorded. The lithium-ion secondary battery was cycled 200 times using the above method, and the discharge capacity was recorded each time. The capacity retention rate of the lithium-ion secondary battery after 200 cycles of 2C / 2C at 25°C = discharge capacity at the 200th cycle / initial discharge capacity × 100%.

[0078] 4. Battery high temperature capacity retention test

[0079] 45℃ 1C / 1C cycle 500 cycles:

[0080] At 45°C, the prepared lithium-ion secondary battery was charged at a constant current rate of 1C to 3.8V, followed by constant voltage charging until the current was less than 0.05C. After standing for 5 minutes, the battery was discharged at a rate of 1C to 2.7V, and the initial discharge capacity was recorded. The lithium-ion secondary battery was cycled 500 times using the above method, and the discharge capacity was recorded each time. The capacity retention rate of the lithium-ion secondary battery after 500 cycles of 1C / 1C at 25°C = discharge capacity at the 500th cycle / initial discharge capacity × 100%.

[0081] 5. Determination of the thickness change rate of batteries stored at 50°C

[0082] A lithium-ion battery was discharged at a constant current of 1C to 2.7V at 25°C, then charged at a constant current of 1C to 3.8V. The thickness at this point was measured using a PPG soft-pack battery thickness gauge, and recorded as "a." The battery was then placed in an oven and stored at a constant voltage of 3.8V at 50°C for 21 days. The thickness after 21 days was recorded as "b." The thickness expansion ratio was calculated as: (b) / a × 100%.

[0083] 6. First effect

[0084] The lithium-ion battery is charged at 0.1C constant current to 3.8V at 25℃. The capacity at this time is the charge capacity C c ; Then discharge the lithium-ion battery at a constant current of 0.1C to 2.7V. At this time, the capacity is the charging capacity C d ; Full cell initial efficiency = C d / C c *100%.

[0085] Table 2

[0086]

[0087]

[0088] Although some exemplary embodiments of the present application have been illustrated and described, the present application is not limited to the disclosed embodiments. On the contrary, those skilled in the art will recognize that some modifications and changes may be made to the described embodiments without departing from the spirit and scope of the present application as described in the appended claims.

Claims

1. A secondary battery comprising a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein: The positive electrode plate includes a positive electrode active material, and the positive electrode active material includes a lithium transition metal phosphate; the electrolyte includes a nitrogen-containing cyclic compound, and the nitrogen-containing cyclic compound includes at least one of the compounds represented by Formula I and Formula II, In formula I and formula II, R1 is selected from O or R a R and R2 are each independently selected from substituted or unsubstituted C1-C4 alkyl groups, and R3 and R4 are each independently selected from hydrogen, fluorine, and fluorine-substituted or unsubstituted C1-C4 alkyl groups.

2. The secondary battery according to claim 1, wherein The nitrogen-containing cyclic compounds include the following formula 3. The secondary battery according to claim 1 or 2, characterized in that The content of the nitrogen-containing cyclic compound is 0.5 wt % to 10 wt % based on the total weight of the electrolyte; And / or, the electrolyte further comprises vinylene carbonate, and the content of the vinylene carbonate is less than 2 wt % based on the total weight of the electrolyte.

4. The secondary battery according to claim 1 or 2, characterized in that The compaction density of the positive electrode sheet is 2.5 g / cm 3 to 2.7g / cm 3 .

5. The secondary battery according to claim 1 or 2, characterized in that The positive electrode active material is lithium iron phosphate and / or lithium iron manganese phosphate; And / or, the negative electrode plate includes a negative electrode active material, and the negative electrode active material includes graphite.

6. The secondary battery according to claim 1 or 2, characterized in that The electrolyte further comprises lithium hexafluorophosphate, and the weight ratio of the lithium hexafluorophosphate to the nitrogen-containing cyclic compound is between 2:1 and 20:1; And / or, the electrolyte does not include lithium sulfonimide salt.

7. The secondary battery according to claim 1 or 2, characterized in that: Based on the total weight of the electrolyte, the content of the nitrogen-containing cyclic compound is 0.5 wt % to 5 wt %; And / or, the electrolyte further comprises vinylene carbonate, and the weight ratio of the nitrogen-containing cyclic compound to the vinylene carbonate is between 5:1 and 1:

2.

8. The secondary battery according to claim 1 or 2, characterized in that: The electrolyte further includes an additive selected from fluoroethylene carbonate, methylene disulfonate, 1,3-propane sultone, and 1-propylene-1,3-sultone, and the content of the additive is 0.2 wt % to 3 wt % based on the total weight of the electrolyte.

9. The electrolyte according to claim 1 or 2, characterized in that The electrolyte further comprises lithium hexafluorophosphate, a cyclic carbonate, a chain carbonate and an additive, wherein, based on the total weight of the electrolyte, the content of the nitrogen-containing cyclic compound is 0.5 wt % to 10 wt %, the content of the lithium hexafluorophosphate is 12 wt % to 15 wt %, the content of the chain carbonate is 50 wt % to 65 wt %, the content of the cyclic carbonate is 15 wt % to 30 wt %, and the content of the additive is 0.2 wt % to 3 wt %.

10. An electric device comprising the secondary battery according to any one of claims 1 to 9.

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