Secondary battery and electric device

Abstract

The present application provides a secondary battery and an electrical device. The present application makes the secondary battery meet the following conditions by rationally designing the composition of the electrolyte of the secondary battery and the positive electrode material: 0.00208≤≤14.096; wherein N is the molar ratio of the lithium salt LiPO2F2 and LiPF6 in the electrolyte, N=0.01‑11; W1 is the weight ratio of the cyclic carbonate to the chain carbonate; W2 is the weight percentage of the A element in the positive electrode active material; C is the weight percentage of the sulfonate additive in the additive in the electrolyte. The present application will be kept within a suitable range, which can ensure that the kinetic performance in the lithium-ion battery is excellent and the impedance is low, so that the migration of lithium ions in the battery is maintained in a good state, and the secondary battery can maintain good cycle performance at high voltage.

Description

Technical Field

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

[0002] As electric vehicles enter thousands of households, higher requirements are placed on power batteries. In order to meet the needs of electric vehicle range, in addition to high specific energy cathode materials, increasing the battery cutoff voltage is also an effective way to increase battery energy density; however, the increase in charge cutoff voltage is limited by the low oxidation stability of traditional carbonate electrolytes and their weak passivation ability on the cathode. In addition, the SEI layer formed in carbonate electrolytes with low ionic conductivity is not conducive to the cycle stability of the battery. Summary of the invention

[0003] The purpose of this application is to overcome the defects of existing secondary batteries that the energy density and cycle stability still need to be further improved, to reasonably design the composition of the electrolyte and the positive electrode material of the secondary battery, and to prepare a secondary battery with excellent comprehensive performance.

[0004] To achieve the above-mentioned object, in a first aspect of the present application, a secondary battery is provided, comprising a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, wherein the positive electrode active material in the positive electrode sheet comprises an A element, wherein the A element comprises at least one of Al, Ti, Sr and W, and the electrolyte comprises a cyclic carbonate, a chain carbonate, a lithium salt and a sulfur-containing additive; the lithium salt comprises LiPO2F2 and LiPF6;

[0005] The chain carbonate includes ethyl methyl carbonate (EMC);

[0006] Based on the total weight of the electrolyte, the mass percentage of the ethyl methyl carbonate is greater than 50%;

[0007] The secondary battery satisfies the following relationship:

[0008] 0.00208≤ ≤14.096

[0009] Wherein, N is the mass ratio of lithium salt LiPO2F2 and LiPF6 in the electrolyte, N=0.01-11;

[0010] W1 is the weight ratio of cyclic carbonate to chain carbonate;

[0011] W2 is the weight percentage of element A in the positive electrode active material;

[0012] C is the weight percentage of sulfur-containing additive in the electrolyte.

[0013] As an embodiment of the present application, the secondary battery satisfies:

[0014] 0.072≤ ≤0.142.

[0015] As an embodiment of the present application, the secondary battery satisfies:

[0016] 0.042≤ ≤0.108.

[0017] As an embodiment of the present application, the sulfur-containing additive includes at least one of 1,3-propane sultone (PS), dithiothreitol (DTD), methylene disulfonate (MMDS), vinyl sulfite (ES), 1,4-butane sultone (1-4BS), and 1,3-propylene sultone (PST).

[0018] As an embodiment of the present application, the sulfur-containing additive includes at least one of 1,3-propane sultone (PS), dithiothreitol (DTD) and 1,3-propene sultone (PST).

[0019] As an embodiment of the present application, the sulfur-containing additive includes 1,3-propane sultone (PS) and 1,3-propene sultone (PST).

[0020] As an embodiment of the present application, the cyclic carbonate includes ethylene carbonate, and the mass fraction of the ethylene carbonate is 15%-40% based on the total weight of the electrolyte.

[0021] As an embodiment of the present application, the secondary battery satisfies: W1 is 0.25-0.85.

[0022] As an implementation scheme of the present application, the secondary battery satisfies: W2 is 0.25%-0.75%.

[0023] As an implementation scheme of the present application, the secondary battery satisfies: C is 0.1%-5%.

[0024] As an embodiment of the present application, the electrolyte satisfies: based on the total weight of the electrolyte, the weight percentage of the lithium salt is 8%-20%.

[0025] As an embodiment of the present application, the lithium salt further includes at least one of lithium tetrafluoroborate, lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium difluorobis(oxalatophosphate), lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethylsulfonyl)imide.

[0026] As an embodiment of the present application, the cyclic carbonate further includes at least one of vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate, and difluoroethylene carbonate.

[0027] As an embodiment of the present application, the linear carbonate further includes at least one of dimethyl carbonate, diethyl carbonate, methylpropyl carbonate, and ethylpropyl carbonate.

[0028] A second aspect of the present application provides an electrical device, wherein the electrical device comprises the secondary battery described in the first aspect of the present application.

[0029] Compared with the prior art, the beneficial effects of this application are:

[0030] This application makes reasonable design on the composition of electrolyte and positive electrode material of secondary battery. Maintaining it within a suitable range can ensure that the kinetic performance of the lithium-ion battery is excellent and the impedance is low, so that the migration of lithium ions in the battery is maintained in a good state, and the secondary battery can maintain good cycle performance at high voltage. DETAILED DESCRIPTION

[0031] To better illustrate the purpose, technical solutions and advantages of the present application, the present application will be further described below in conjunction with specific examples, but the examples do not limit the present application in any form. Unless otherwise specified, the reagents, methods and equipment used in the present application are conventional reagents, methods and equipment in the art. Unless otherwise specified, the reagents and materials used in the present application are commercially available.

[0032] The embodiment of the present application provides a secondary battery, comprising a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, wherein the positive electrode active material in the positive electrode sheet comprises an A element, wherein the A element comprises at least one of Al, Ti, Sr and W, and the electrolyte comprises a cyclic carbonate, a chain carbonate, a lithium salt and a sulfur-containing additive, wherein the lithium salt comprises LiPO2F2 and LiPF6;

[0033] The chain carbonate includes ethyl methyl carbonate (EMC);

[0034] Based on the total weight of the electrolyte, the mass percentage of the ethyl methyl carbonate is greater than 50%;

[0035] The secondary battery satisfies the following relationship:

[0036] 0.00208≤ ≤14.096

[0037] Wherein, N is the mass ratio of lithium salt LiPO2F2 and LiPF6 in the electrolyte, N=0.01-11;

[0038] W1 is the weight ratio of cyclic carbonate to chain carbonate;

[0039] W2 is the weight percentage of element A in the positive electrode active material;

[0040] C is the weight percentage of sulfur-containing additive in the electrolyte.

[0041] The inventors of the present application have found that the ratio of cyclic carbonate to chain carbonate in the secondary battery, the content of sulfonate additives, the content of element A in the positive electrode material and the molar ratio of LiPO2F2 / LiPF6 have a significant correlation with the performance of the secondary battery. By reasonably setting the above parameters to satisfy specific relationships, the interface stability can be effectively improved and the interface impedance can be reduced.

[0042] The present application adds a double lithium salt to the electrolyte. The double salt system is beneficial to improve the interface stability and reduce the interface impedance. Among them, LiPO2F2 has excellent film-forming stability, which can reduce the initial internal resistance of the battery and the internal resistance growth during the cycle, and improve the capacity retention rate of the normal temperature cycle. However, the solubility in the electrolyte is poor, and adding too much LiPO2F2 may cause the conductivity to decrease and the impedance to increase significantly. The applicant of the present application further studied and found that when the ratio of cyclic carbonate to chain carbonate in the secondary battery, the content of sulfur-containing additives, the content of A element in the positive electrode material, and the molar ratio of LiPO2F2 / LiPF6 meet the above relationship, the secondary battery has better comprehensive performance and meets the needs of new power batteries.

[0043] The addition of ethyl methyl carbonate in the electrolyte helps to improve the electrochemical performance of the battery at low temperatures, but if the amount added is too high, the internal resistance will be too high, affecting the performance of the positive electrode capacity and further affecting the cycle performance of the battery; and in the later stage of the cycle, too much electrolyte may also cause bloating, affecting the sealing and safety of the battery. However, by reasonably designing the parameters of the present application to satisfy the above-mentioned specific relationship, the amount of ethyl methyl carbonate added in the electrolyte can be further increased.

[0044] In some embodiments, the secondary battery satisfies:

[0045] 0.072≤ ≤0.142.

[0046] When the relationship in the secondary battery falls within the above range, the cycle retention rate of the secondary battery is even better, and can reach more than 90% after 500 cycles.

[0047] In some embodiments, the secondary battery satisfies:

[0048] 0.072≤ ≤0.142.

[0049] When the relationship in the secondary battery falls within the above range, the cycle retention rate and DCR growth rate of the secondary battery both maintain excellent levels, especially the growth rate of DCR is slower. After 500 cycles, the DCR growth rate can be maintained within 20%.

[0050] In some embodiments, based on the total weight of the electrolyte, the mass percentage of the ethyl methyl carbonate is greater than 55%, for example, it can be any one of 56%, 58%, 60%, 62%, 64%, 65%, 67%, or a range value between any two values.

[0051] In some embodiments, based on the total weight of the electrolyte, the mass percentage of the ethyl methyl carbonate is less than 68%.

[0052] In some embodiments, in the secondary battery, the weight ratio W1 of the cyclic carbonate to the chain carbonate satisfies: W1 is 0.25-0.85, and W1 can be any one of 0.25, 0.43, 0.50, 0.65, 0.85 or a range between any two values. The cyclic carbonate solvent has a high dielectric constant and a high viscosity, while the chain carbonate solvent has a low dielectric constant and a low viscosity. Controlling the ratio of the cyclic carbonate solvent to the chain carbonate solvent within a suitable range helps to improve the kinetic performance of the secondary battery.

[0053] In some embodiments, in the positive electrode active material, the weight percentage W2 of the A element is 0.25%-0.75%, and the W2 can be any one of 0.25%, 0.4%, 0.5%, 0.6%, 0.75%, or a range between any two values. The A element in the positive electrode active material can effectively improve the room temperature cycle and reduce the DC internal resistance (DCR) growth of the battery; this is because the introduction of A inhibits the lattice distortion caused by the irreversible phase change, reduces the side reaction, and improves the Li + migration dynamics.

[0054] In some embodiments, the concentration C of the sulfur-containing additive in the electrolyte is 0.1%-5%, and more preferably 0.1%-3%. The C can be any one of 0.1%, 1%, 2%, 3%, 4%, 5%, or a range between any two values. The sulfur-containing additive can reduce the initial internal resistance of the battery and the internal resistance growth during storage, and improve the capacity retention rate of normal temperature cycle and high temperature storage. The main reason is that it can form sulfur-containing substances (Li2SO b ) to form a dense CEI interface film, improve interface stability and reduce interface impedance. The addition of a suitable amount of sulfur-containing additives can ensure that the electrolyte has a suitable viscosity, so that the migration of lithium ions in the battery remains in a good state, and the lithium-ion secondary battery can maintain good cycle performance at high voltage.

[0055] In some embodiments, the sulfur-containing additive includes at least one of 1,3-propane sultone (PS), dithiothreitol (DTD), methylene disulfonate (MMDS), vinyl sulfite (ES), 1,4-butane sultone (1-4BS), and 1,3-propene sultone (PST).

[0056] In some embodiments, the sulfur-containing additive includes 1,3-propane sultone (PS), and at least one of dithiothreitol sulfate (DTD) and 1,3-propene sultone (PST).

[0057] In some embodiments, the sulfur-containing additive includes 1,3-propane sultone (PS) and 1,3-propene sultone (PST).

[0058] In some embodiments, the weight ratio of 1,3-propane sultone to 1,3-propylene sultone is 0.1-20, and can be any one of 0.1, 0.5, 2.5, 5, 7.5, 10, 15, 20 or a range between any two values. The weight ratio of the sulfur-containing additive is limited to this range, and the sulfur-containing additive can be decomposed to generate Li2S e O f , promoting the formation of a more uniform and dense SEI film, thereby further improving the cycle stability and reducing the DCR.

[0059] In some embodiments, the electrolyte satisfies: based on the total weight of the electrolyte, the weight percentage of the lithium salt is 8%-20%, specifically any one of 8%, 12.6%, 14.96%, 18.2%, 20%, or a range value between any two values. The appropriate lithium salt concentration can ensure that the conductivity and viscosity are within an appropriate range, thereby ensuring the migration rate of active ions. Selecting a dual lithium salt system can also improve the stability of the electrolyte and the cycle stability of the secondary battery.

[0060] In some embodiments, the lithium salt further comprises at least one of lithium tetrafluoroborate, lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium difluorobis(oxalatophosphate), lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethylsulfonyl)imide.

[0061] In some embodiments, the cyclic carbonate includes ethylene carbonate, and the mass percentage of ethylene carbonate is 15%-40% based on the total weight of the electrolyte, and can be any one of 15%, 20%, 25%, 30% / 35%, 40% or a range value between any two values.

[0062] In some embodiments, the cyclic carbonate includes ethylene carbonate, and based on the total weight of the electrolyte, the sum of the mass percentages of the ethyl methyl carbonate and the ethylene carbonate is greater than 75%.

[0063] In some embodiments, the cyclic carbonate includes ethylene carbonate, and based on the total weight of the electrolyte, the sum of the mass percentages of the ethyl methyl carbonate and the ethylene carbonate is greater than 80%.

[0064] In some embodiments, the cyclic carbonate includes ethylene carbonate, and based on the total weight of the electrolyte, the sum of the content of the ethyl methyl carbonate and the ethylene carbonate is less than 90%.

[0065] Ethylene carbonate has a high dielectric constant and good compatibility with graphite negative electrode. The SEI film formed on the graphite negative electrode has a good passivation effect and can effectively inhibit the further reduction of the electrolyte by the anode. The ethylene carbonate solvent has a high dielectric constant and a high viscosity, while the ethyl methyl carbonate solvent has a low dielectric constant and a low viscosity. Controlling the content of the two within a suitable range will help improve the kinetic performance of secondary batteries.

[0066] In some embodiments, the cyclic carbonate further includes at least one of vinylene carbonate, vinyl carbonate, fluoroethylene carbonate, and difluoroethylene carbonate.

[0067] In some embodiments, the linear carbonate further comprises at least one of dimethyl carbonate, diethyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate. In some embodiments, the electrolyte further comprises a silicon-containing additive, and the silicon-containing additive comprises at least one of tris(trimethylsilyl)phosphite, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)borate, trimethylfluorosilane, and heptamethyldisilazane.

[0068] In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector and containing a positive electrode active material, wherein the positive electrode active material includes a chemical formula of Li a Ni x Co y Mn z A （1-x-y-z） O2 compound, A includes at least one of Al, Ti, Sr, W, 0.9≤a≤1.1, 0<x<1, 0<y<1, 0 <z<1,x+y+z<1。

[0069] In some embodiments, the positive electrode material may further include an M element, and the M element includes at least one of Zr, Zn, Cu, Cr, Mg, Fe, V, Y, Nb, In, La, Ce, Sb, and B.

[0070] Among them, the M element can be incorporated into the positive electrode active material by doping and / or surface coating, and the testing method of the M element in the present invention is: cut the positive electrode sheet into small discs with a diameter of 12±2 mm, take 20 small discs for ICP testing, and refer to EPA 6010D-2018 inductively coupled plasma atomic emission spectrometry.

[0071] In some embodiments, 0.5<x<0.8, and 0.04<y<0.17.

[0072] In some embodiments, 0.5<x<0.8, and 0.04<y<0.14.

[0073] In some embodiments, 0.5<x<0.8, and 0.2<z<0.4.

[0074] In some embodiments, 0.5<x<0.72, 0.25<z<0.4, 0.04<y<0.15.

[0075] In some embodiments, the negative electrode plate includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector and containing a negative electrode active material, the negative electrode active material includes a silicon-carbon composite material formed by a silicon-based material and a carbon-based material, the silicon-based material includes at least one of silicon element, silicon oxide material and silicon-carbon material; the carbon-based material includes graphite.

[0076] In a second aspect of the present application, there is also provided an electric device, which includes the secondary battery described in the first aspect of the present application; the electric device includes but is not limited to electric vehicles and energy storage equipment.

[0077] The following are specific embodiments of the present application, and the technical scheme of the present application is further described in conjunction with the embodiments, but the present application is not limited to these embodiments. The reagents, methods and equipment used in the present application, unless otherwise specified, are conventional reagents, methods and equipment in the art.

[0078] Example 1

[0079] This embodiment and comparative example provide a secondary battery, and the specific preparation process includes the following steps:

[0080] (1) Preparation of positive electrode sheet:

[0081] The Al-containing compound Al(NO3)3·9H2O was dissolved in anhydrous ethanol to obtain an Al-containing solution. The mixture of the Al-containing solution, the positive electrode NCM precursor and lithium hydroxide was further calcined at 300°C for 4 hours, and then calcined in an O2 atmosphere at 750°C for 6 hours to obtain the positive electrode active material Li(Ni 0.6 Mn 0.28 Co0.1 Al 0.02 )O2(NCM), Al element accounts for 0.5% of the mass of the positive electrode active material; then the positive electrode active material, the conductive agent acetylene black (Super P) and the binder polyvinylidene fluoride (PVDF) are mixed uniformly according to the mass ratio of NCM:SuperP:PVDF=94:3:3, and evenly dispersed in 1-methyl-2-pyrrolidone (NMP) to form a uniform black slurry, and the mixed black slurry is coated on both sides of the aluminum foil, and then baked, rolled and cut into pieces to obtain the positive electrode sheet;

[0082] (2) Preparation of negative electrode sheet:

[0083] The negative electrode active material graphite, the conductive agent acetylene black (Super P) and the binder SBR are mixed evenly in a mass ratio of graphite:SuperP:SBR=94:3:3, and evenly dispersed in deionized water to form a uniform black slurry. The mixed slurry is coated on both sides of the copper foil, and then baked, rolled and cut into pieces to obtain the negative electrode sheet;

[0084] (3) Preparation of electrolyte:

[0085] At room temperature, in a glove box filled with argon (H2O<1ppm, O2<1ppm), ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed evenly, and water was removed with a 4Å molecular sieve to obtain a mixed solvent, and mixed lithium salts were added to the obtained mixed solvent one by one, and the mixture was stirred continuously and cooled with dry ice to ensure that the electrolyte temperature did not rise by more than 2°C, and finally a colorless transparent liquid was obtained, and then a sulfur-containing additive was added and mixed evenly to obtain the electrolyte; the specific composition of the electrolyte is shown in Table 1;

[0086] Table 1

[0087]

[0088] (4) Production of lithium-ion batteries:

[0089] The prepared positive electrode sheet, separator (polyethylene), and negative electrode sheet are stacked in order, with the separator placed between the positive and negative electrode sheets. The bare battery cell is obtained by winding, hot pressing and shaping, and the tabs are welded. The bare battery cell is placed in an outer packaging aluminum-plastic film and baked in an oven at 85±10℃ for 24h. The prepared electrolyte is injected into the dried battery, allowed to stand, formed, and divided to complete the preparation of the lithium-ion secondary battery.

[0090] Examples 2-24, Comparative Examples 1-6

[0091] The preparation methods of the positive electrode sheet, the negative electrode sheet, the separator and the lithium-ion battery are basically the same as those in Example 1. The parameters that are changed are the composition parameters of the electrolyte and the positive electrode active material, as shown in Table 2 for details. It should be noted that in the embodiments listed in Table 2, unless otherwise specified, the variables are all single-factor variables. Taking Examples 1-3 as an example, the variable is the mass ratio of the lithium salt LiPO2F2 and LiPF6, but the total amount of lithium salt added in the electrolyte needs to be kept unchanged.

[0092] Table 2

[0093]

[0094] Embodiment 25

[0095] This embodiment provides a secondary battery, wherein the preparation method of the positive electrode plate, the negative electrode plate, the separator and the lithium ion battery is basically the same as that of Example 1, and the difference from Example 1 is that the total amount of the sulfur-containing additive in the electrolyte is kept unchanged, and the sulfur-containing additive also includes 1,3-propene sultone, wherein the weight ratio of 1,3-propane sultone to 1,3-propene sultone is 25.

[0096] Embodiment 26

[0097] This embodiment provides a secondary battery, wherein the preparation method of the positive electrode plate, the negative electrode plate, the separator and the lithium ion battery is basically the same as that of Example 1, and the difference from Example 1 is that the total amount of the sulfur-containing additive in the electrolyte is kept unchanged, and the sulfur-containing additive also includes 1,3-propene sultone, wherein the weight ratio of 1,3-propane sultone to 1,3-propene sultone is 10.

[0098] Embodiment 27

[0099] This embodiment provides a secondary battery, wherein the preparation method of the positive electrode plate, the negative electrode plate, the separator and the lithium ion battery is basically the same as that of Example 1, and the difference from Example 1 is that the total amount of the sulfur-containing additive in the electrolyte is kept unchanged, and the sulfur-containing additive also includes 1,3-propene sultone, wherein the weight ratio of 1,3-propane sultone to 1,3-propene sultone is 20.

[0100] Embodiment 28

[0101] This embodiment provides a secondary battery, wherein the preparation method of the positive electrode plate, the negative electrode plate, the separator and the lithium ion battery is basically the same as that of Example 1, and the difference from Example 1 is that the total amount of the sulfur-containing additive in the electrolyte is kept unchanged, and the sulfur-containing additive also includes 1,3-propene sultone, wherein the weight ratio of 1,3-propane sultone to 1,3-propene sultone is 0.1.

[0102] Embodiment 29

[0103] This embodiment provides a secondary battery, wherein the preparation methods of the positive electrode plate, the negative electrode plate, the separator and the lithium-ion battery are basically the same as those in Example 1, and the difference from Example 1 is that the sulfur-containing additive in the electrolyte is replaced by an equal mass of methylene disulfonate.

[0104] Embodiment 30

[0105] This embodiment provides a secondary battery, wherein the preparation methods of the positive electrode plate, the negative electrode plate, the separator and the lithium ion battery are basically the same as those in Example 1, and the difference from Example 1 is that the sulfur-containing additive in the electrolyte is replaced by an equal mass of 1,4-butanesulfonate.

[0106] Embodiment 31

[0107] This embodiment provides a secondary battery, wherein the preparation method of the positive electrode plate, the negative electrode plate, the separator and the lithium ion battery is basically the same as that of Example 1. The difference from Example 1 is that in the electrolyte, the total weight of the chain carbonate is kept unchanged, and ethyl methyl carbonate is replaced by an equal mass of mixed chain carbonate, and the mixed chain carbonate is composed of components in the following weight ratio: ethyl methyl carbonate: dimethyl carbonate = 20:1.

[0108] Embodiment 32

[0109] The present embodiment provides a secondary battery, wherein the preparation methods of the positive electrode plate, the negative electrode plate, the separator and the lithium ion battery are basically the same as those in Example 1. The difference from Example 1 is that in the electrolyte, the total weight of the chain carbonate is kept unchanged, and ethyl methyl carbonate is replaced by an equal mass of mixed chain carbonate, and the mixed chain carbonate is composed of components in the following weight ratio: ethyl methyl carbonate: dimethyl carbonate: diethyl carbonate: methyl propyl carbonate: ethyl propyl carbonate = 60:1:1:0.5:0.5.

[0110] Embodiment 33

[0111] This embodiment provides a secondary battery, wherein the preparation method of the positive electrode plate, the negative electrode plate, the separator and the lithium ion battery is basically the same as that of Example 1. The difference from Example 1 is that in the electrolyte, the total weight of the cyclic carbonate is kept unchanged, and the cyclic carbonate is composed of components in the following weight ratio: ethylene carbonate: difluoroethylene carbonate = 2:1.

[0112] Embodiment 34

[0113] This embodiment provides a secondary battery, wherein the preparation method of the positive electrode plate, the negative electrode plate, the separator and the lithium ion battery is basically the same as that of Example 1. The difference from Example 1 is that in the electrolyte, the total weight of the lithium salt is kept unchanged, and the composition of the lithium salt includes the following components in the following weight ratio: LiPO2F2: LiPF6: lithium difluorooxalatoborate: lithium bis(trifluoromethylsulfonyl)imide = 1:1:0.5:0.5.

[0114] Embodiment 35

[0115] This embodiment provides a secondary battery, wherein the preparation method of the positive electrode plate, the negative electrode plate, the separator and the lithium-ion battery is basically the same as that of Example 1. The difference from Example 1 is that in the electrolyte, the total weight of the lithium salt is kept unchanged, and the composition of the lithium salt includes the following components in the following weight ratio: LiPO2F2: LiPF6: lithium bis(oxalatoborate): lithium bis(fluorosulfonyl)imide = 1:1:1:1.

[0116] Embodiment 36

[0117] This embodiment provides a secondary battery, wherein the preparation method of the positive electrode plate, the negative electrode plate, the separator and the lithium ion battery is basically the same as that of Example 1. The difference from Example 1 is that in the electrolyte, the mass ratio of LiPO2F2 and LiPF6 in the lithium salt is kept unchanged, the addition amount of the lithium salt is changed to 14.96%, and the components in the chain carbonate and the cyclic carbonate are reduced in proportion to keep the total amount of the electrolyte at 100%.

[0118] Embodiment 37

[0119] This embodiment provides a secondary battery, wherein the preparation method of the positive electrode plate, the negative electrode plate, the separator and the lithium ion battery is basically the same as that of Example 1. The difference from Example 1 is that in the electrolyte, the mass ratio of LiPO2F2 and LiPF6 in the lithium salt is kept unchanged, the addition amount of the lithium salt is changed to 18.2%, and the components in the chain carbonate and the cyclic carbonate are reduced in proportion to keep the total amount of the electrolyte at 100%.

[0120] Embodiment 38

[0121] This embodiment provides a secondary battery, in which the preparation method of the positive electrode plate, the negative electrode plate, the separator and the lithium ion battery is basically the same as that of Example 1. The difference from Example 1 is that 0.5% of a silicon-containing additive, tris(trimethylsilyl) phosphate, is further added to the electrolyte, and each component in the chain carbonate and the cyclic carbonate is reduced in proportion to keep the total amount of the electrolyte at 100%.

[0122] Embodiment 39

[0123] This embodiment provides a secondary battery, wherein the preparation method of the positive electrode plate, the negative electrode plate, the separator, the electrolyte and the lithium ion battery is basically the same as that of the embodiment 1, and the difference from the embodiment 1 is that the preparation of the positive electrode active material is as follows: the W-containing compound WO3 is dissolved in ethanol to obtain a tungsten-containing solution, the tungsten-containing solution and the NCM precursor mixture are further calcined at 300°C for 4h, and then calcined in an O2 atmosphere at 750°C for 6h to obtain the positive electrode active material Li(Ni 0.6 Mn 0.2973 Co 0.1 W 0.0027 )O2, W element accounts for 0.5% of the mass of the positive electrode active material.

[0124] Embodiment 40

[0125] This embodiment provides a secondary battery, wherein the preparation method of the positive electrode plate, the negative electrode plate, the separator, the electrolyte and the lithium ion battery is basically the same as that of the embodiment 1, and the difference from the embodiment 1 is that the preparation of the positive electrode active material is as follows: tetramethyl titanate is dissolved in ethanol to obtain a titanium-containing solution, and the mixture of the titanium-containing solution, the NCM precursor and lithium hydroxide is further calcined at 300° C. for 4 hours, and then calcined in an O2 atmosphere at 750° C. for 6 hours to obtain the positive electrode active material Li(Ni 0.6 Mn 0.3 Co 0.1 Ti 0.0087 )O2, Ti element accounts for 1.5% of the mass of the positive electrode active material.

[0126] Embodiment 41

[0127] This embodiment provides a secondary battery, wherein the preparation method of the positive electrode plate, the negative electrode plate, the separator, the electrolyte and the lithium ion battery is basically the same as that of the embodiment 1, and the difference from the embodiment 1 is that the preparation of the positive electrode active material is as follows: tetramethyl titanate and WO3 are dissolved in ethanol to obtain a solution containing titanium and tungsten, and the mixture of the solution, the NCM precursor and lithium hydroxide is further calcined at 300°C for 4h, and then calcined in an O2 atmosphere at 750°C for 6h to obtain the positive electrode active material Li(Ni 0.6 Mn 0.3 Co 0.1 Ti 0.0044 W 0.0032 )O2; it is calculated that the mass proportion of Ti element in the positive electrode active material is 0.75%, and the mass proportion of W element in the positive electrode active material is 0.75%.

[0128] Comparative Example 7

[0129] This comparative example provides a secondary battery, and the preparation methods of the positive electrode plate, the negative electrode plate, the separator and the lithium-ion battery are basically the same as those in Example 1. The difference from Example 1 is that no aluminum solution is added to the black slurry for preparing the positive electrode plate.

[0130] Comparative Example 8

[0131] This comparative example provides a secondary battery. The preparation methods of the positive electrode sheet, the negative electrode sheet, the separator and the lithium-ion battery are basically the same as those in Example 1. The difference from Example 1 is that in the electrolyte, ethyl methyl carbonate is replaced by an equal mass of mixed carbonates, and the mixed carbonates are composed of components in the following weight ratio: dimethyl carbonate: diethyl carbonate = 1:1.

[0132] Performance Testing

[0133] The performance of the secondary batteries obtained in the above examples and comparative examples was tested, and the specific test items, test methods and results are as follows:

[0134] (1) Room temperature DCR test: At 25±2℃, the soft pack batteries obtained from each implementation case and comparative example were charged to 4.5V at 1C, then discharged at 1C capacity for 30min, adjusted to 50% SOC, and then discharged at 10C constant current pulse for 10s. Then, the SOC was adjusted to 50% according to the above SOC adjustment method, and then charged for 10s. The DCR was calculated as (voltage before pulse discharge - voltage after pulse discharge) / discharge current * 100%. After 500 cycles, the DCR was tested again, and the DCR change rate = (DCR of 500 cycles - initial DCR) / initial DCR * 100%.

[0135] (2) Normal temperature cycle performance test: At 25±2°C, the soft-pack batteries obtained from each implementation case and comparative example were subjected to charge and discharge cycle tests at a charge and discharge rate of 1C / 2C in the range of 2.8~4.5V, and the discharge specific capacity of the battery after 500 cycles was recorded. The capacity retention rate of 500 cycles = the discharge specific capacity of 500 cycles / the discharge specific capacity of the first week*100%. The recorded data are shown in Table 3.

[0136] Table 3 Performance test results of secondary batteries prepared in Examples and Comparative Examples

[0137]

[0138] From the above results we can see that:

[0139] The present invention rationally designs the composition of the electrolyte and the positive electrode material of the secondary battery. Maintaining it within a suitable range can ensure that the kinetic performance of the lithium-ion battery is excellent and the impedance is low, so that the migration of lithium ions in the battery is maintained in a good state, and the secondary battery can maintain good cycle performance at high voltage.

[0140] Specifically, the initial DCR of the obtained secondary battery is below 22mΩ, and can be as low as 14.7mΩ; the cycle retention rate is above 86%, and can be as high as 94.5%; the internal resistance growth rate is below 25.5%, and can be as low as 17.8%.

[0141] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present application rather than to limit the scope of protection of the present application. Although the present application has been described in detail with reference to the preferred embodiments, ordinary technicians in this field should understand that the technical solution of the present application can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present application.

Claims

1. A secondary battery, comprising a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, characterized in that: The positive electrode active material in the positive electrode plate includes element A, and the element A includes at least one of Al, Ti, Sr, and W. The electrolyte includes cyclic carbonate, chain carbonate, lithium salt, and sulfur-containing additives; the lithium salt includes LiPO2F2 and LiPF6; The chain carbonate includes ethyl methyl carbonate; the cyclic carbonate includes ethylene carbonate, and the mass percentage of the ethylene carbonate is 24.06-40% based on the total weight of the electrolyte; Based on the total weight of the electrolyte, the mass percentage of the ethyl methyl carbonate is greater than 50%; Based on the total weight of the electrolyte, the weight percentage of the lithium salt is 8%-20%; the secondary battery satisfies the following relationship: 0.072≤ ≤14.096 Wherein, N is the mass ratio of lithium salt LiPO2F2 and LiPF6 in the electrolyte, N=0.01-11; W1 is the weight ratio of cyclic carbonate to chain carbonate; W2 is the weight percentage of element A in the positive electrode active material, W2=0.25%-0.75%; C is the weight percentage of sulfur-containing additive in the electrolyte.

2. The secondary battery according to claim 1, characterized in that: The secondary battery meets the following requirements: 0.072≤ ≤0.142。 3. The secondary battery according to claim 1, characterized in that: The secondary battery meets the following requirements: 0.072≤ ≤0.108。 4. The secondary battery according to claim 1, characterized in that: The sulfur-containing additive includes at least one of 1,3-propane sultone, vinyl sulfate, methylene methane disulfonate, vinyl sulfite, 1,4-butane sultone, and 1,3-propylene sultone.

5. The secondary battery according to claim 1 or 4, characterized in that: The sulfur-containing additive includes at least one of 1,3-propane sultone, vinyl sulfate and 1,3-propene sultone.

6. The secondary battery according to claim 1 or 4, characterized in that: The sulfur-containing additives include 1,3-propane sultone and 1,3-propylene sultone.

7. The secondary battery according to claim 1, characterized in that: The secondary battery satisfies at least one of the following characteristics: W1 is 0.25-0.85; C is 0.1%-5%.

8. The secondary battery according to claim 1, characterized in that: The electrolyte satisfies at least one of the following characteristics: The lithium salt further comprises at least one of lithium tetrafluoroborate, lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium difluorobis(oxalatophosphate), lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethylsulfonyl)imide; The cyclic carbonate further comprises at least one of vinylene carbonate, vinyl carbonate, fluoroethylene carbonate, and difluoroethylene carbonate; The chain carbonate further comprises at least one of dimethyl carbonate, diethyl carbonate, methylpropyl carbonate and ethylpropyl carbonate.

9. An electrical device, characterized in that: A secondary battery comprising the secondary battery according to any one of claims 1 to 8.

Images