Lithium-ion secondary battery

Abstract

A lithium-ion secondary battery comprising a positive electrode plate, a negative electrode plate and an electrolyte, wherein the electrolyte comprises a sodium ion additive; the positive electrode plate comprises a current collector and a layer of positive electrode material, and the negative electrode plate comprises a current collector and a layer of negative electrode material; the lithium-ion secondary battery meets: 0.1 ≤ X / Y ≤ 0.95, where X represents a characteristic peak area of ​​the element Na at an XPS etching depth of 30 nm on a side of the positive electrode material layer facing away from the current collector during the XPS test of the positive electrode plate; Y represents a characteristic peak area of ​​the element Na at an XPS etching depth of 30 nm on a side of the negative electrode material layer facing away from the current collector during the XPS test of the negative electrode plate.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of battery technology, in particular to a lithium-ion secondary battery. STATE OF THE ART

[0002] Lithium-ion secondary batteries generally include a compound containing a transition metal element (such as lithium iron phosphate, lithium manganese iron phosphate, lithium cobalt oxide, etc.) as the positive electrode material. During the cycling process of lithium-ion secondary batteries, especially under high-temperature conditions, the transition metal element in the positive electrode material dissolves in ionic form in the electrolyte, resulting in a significant loss of active lithium. Furthermore, it migrates to the interface of the negative electrode plate, increasing the interfacial impedance. On the other hand, these ions also reduce the stability of the electrolyte, causing chemical reactions that generate byproducts, which ultimately affect the battery's cycling performance. SUMMARY

[0003] The present application overcomes deficiencies in the prior art by providing a lithium-ion secondary battery. By introducing a sodium ion additive into the battery electrolyte and simultaneously controlling the ratio of the peak areas of the Na element in the surface of the active material layer of the positive electrode plate, measured by an XPS test at a depth of 30 nm, to those of the negative electrode plate within a specific range, the phenomenon of dissolution of transition metal elements in the positive electrode material under high-temperature conditions can be effectively optimized, the loss of active lithium can be reduced, the kinetic performance of the battery can be improved, the protective effect on the negative electrode plate can be enhanced, and ultimately the cycle performance of the battery can be improved.

[0004] To achieve the above object, in a first aspect of the present invention, there is provided a lithium ion secondary battery comprising a positive electrode plate, a negative electrode plate and an electrolyte; wherein the electrolyte comprises a sodium ion additive; the positive electrode plate comprises a current collector and a layer of positive electrode material, and the negative electrode plate comprises a current collector and a layer of negative electrode material; the lithium-ion secondary battery: 0.1 ≤ X / Y ≤ 0.95; where X represents a characteristic peak area of ​​the element Na at an XPS etching depth of 30 nm on a side of the positive electrode material layer facing away from the current collector during the XPS test; Y represents a characteristic peak area of ​​the element Na at an XPS etching depth of 30 nm on a side of the negative electrode material layer facing away from the current collector during the XPS test.

[0005] The advantageous effects of the present application are as follows: The present application provides a lithium-ion secondary battery in which, by introducing a sodium ion additive into the electrolyte of the battery and simultaneously controlling the ratio of peak intensities of the element Na measured by an XPS test at a depth of 30 nm in the surfaces of the respective layers of the active material of the positive electrode plate to the negative electrode plate within a specific range, the dissolution phenomenon of transition metal elements in the positive electrode material under high temperature conditions can be effectively optimized, the loss of active lithium can be reduced, the kinetic performance of the battery can be improved, the protective effect on the negative electrode plate can be enhanced, and ultimately the cycle performance of the battery can be improved. BRIEF DESCRIPTION OF THE DRAWINGS In Fig.1 shows the XPS spectrum as a result of measuring the characteristic peak area of ​​the element Na at an XPS etching depth of 30 nm on a side of the positive electrode material layer facing away from the current collector in an XPS test of the positive electrode plate of the lithium ion secondary battery in Embodiment 1 of the present application. In Fig. 2 shows the XPS spectrum as a result of measuring the characteristic peak area of ​​the element Na at an XPS etching depth of 30 nm on a side of the negative electrode material layer facing away from the current collector in an XPS test of the negative electrode plate of the lithium ion secondary battery in Embodiment 1 of the present application. DETAILED DESCRIPTION

[0006] To clarify the purpose, technical solutions, and advantages of the embodiments of the present application, the technical solutions in the embodiments of the present application are described clearly and completely below. Obviously, the described embodiments represent only a portion of the embodiments of the present application, not all embodiments. All further embodiments derivable by a person skilled in the art from the embodiments of the present application without creative effort fall within the scope of the present application.

[0007] In the present application, technical features described in an open manner include closed technical solutions composed of the listed features as well as open technical solutions containing the listed features.

[0008] In this application, with respect to numerical ranges, unless otherwise stated, the above numerical ranges are considered continuous and include the minimum and maximum values ​​of the range, as well as any value between those minimum and maximum values. Furthermore, when the range refers to integers, any integer between the minimum and maximum values ​​of the range is included. Additionally, when multiple ranges are provided to describe features or characteristics, those ranges may be combined. In other words, all ranges disclosed herein are to be understood as including any and all subranges included therein, unless otherwise stated.

[0009] The present application is further illustrated by specific embodiments below: A lithium-ion secondary battery comprises a positive electrode plate, a negative electrode plate and an electrolyte; the electrolyte includes a sodium ion additive; the positive electrode plate comprises a current collector and a layer of positive electrode material, and the negative electrode plate comprises a current collector and a layer of negative electrode material; the lithium-ion secondary battery meets: 0.1 ≤ X / Y ≤ 0.95; where X represents a characteristic peak area of ​​the element Na at an XPS etching depth of 30 nm on a side of the positive electrode material layer facing away from the current collector during the XPS test of the positive electrode plate; Y represents a characteristic peak area of ​​the element Na at an XPS etching depth of 30 nm on a side of the negative electrode material layer facing away from the current collector during the XPS test of the negative electrode plate.

[0010] In some specific embodiments, the coordinate for the characteristic peak area is 1070 to 1075 eV.

[0011] In lithium-ion secondary batteries, the transition metal element in the positive electrode material dissolves during battery cycling, especially under high-temperature conditions. This not only consumes active lithium, but also affects the interfacial impedance of the negative electrode plate and the stability of the electrode, ultimately weakening the battery's cycling performance. In the lithium-ion secondary battery of the present application, by adding a sodium ion additive to the electrolyte during battery cycling, this additive can first form an inorganic SEI film on the negative electrode plate, which reacts at a high temperature to form a highly stable organic / inorganic composite SEI film. On the other hand, the sodium ion additive also forms a high-density CEI film on the positive electrode plate during battery charging.The combined effect of these two films suppresses the leaching of the transition metal element from the positive electrode material and effectively protects the negative electrode plate. By controlling the ratio of the characteristic peak areas of the element sodium measured by XPS (X-ray photoelectron spectroscopy) at an etch depth of 30 nm on both the positive and negative electrode plates, the thickness relationship of the respective SEI / CEI films formed by the sodium ion additive on the respective electrode plates can be effectively controlled.As a result, the lithium-ion secondary battery can achieve both electrode stability and a lower rate of DCR increase during cycling under high-temperature conditions, thereby improving kinetic performance with good passivation of the negative electrode plate, low electrode interface impedance, a low degree of interfacial side reactions, high lithium ion transport efficiency, and excellent cycling capacity. In some embodiments, the lithium-ion secondary battery satisfies the following: X / Y is equal to one of 0.1, 0.2, 0.21, 0.23, 0.25, 0.3, 0.35, 0.38, 0.4, 0.5, 0.55, 0.6, 0.65, 0.7, 0.73, 0.75, 0.8, 0.9, 0.95, or falls within a range between any two of these values.

[0012] Controlling the ratio of XPS characteristic peak areas of the element Na at the same etching depth for both electrode plates directly affects the electrochemical performance of the lithium-ion secondary battery. If the ratio is too high, the kinetic performance of the lithium-ion secondary battery deteriorates, and the passivation protection provided by the SEI film on the negative electrode plate is insufficient, resulting in a sharp decrease in cycle capacity under high-temperature conditions. If the ratio is too low, the interfacial stability of the positive electrode material is insufficient, and the kinetic performance is correspondingly low, preventing the battery from achieving ideal electrochemical performance. Therefore, the ratio must be maintained within the above-mentioned range.

[0013] In some embodiments, the lithium-ion secondary battery satisfies the following: 0.21 ≤ X / Y ≤ 0.75.

[0014] More preferably, the lithium ion secondary battery satisfies the following: X / Y is equal to one of 0.21, 0.23, 0.25, 0.3, 0.35, 0.38, 0.4, 0.5, 0.55, 0.6, 0.65, 0.7, 0.73, 0.75 or falls within a range between any two of these values.

[0015] When the ratio of the characteristic XPS peak areas of the element Na under specific conditions for both electrode plates falls within the above preferred range, the overall ion / electron transport efficiency of the lithium-ion secondary battery is higher, thereby achieving better cycling performance under high temperature conditions.

[0016] In some embodiments, X = 1000 to 19000.

[0017] More preferably, X is equal to one of 1000, 2400, 2500, 3000, 3500, 4000, 5000, 5500, 6000, 6500, 6800, 6850, 7000, 8000, 9000, 10000, 11000, 12000, 13000, 14000, 14500, 15000, 16000, 17000, 19000 or falls within a range between any two of these values.

[0018] More preferred is X = 2000 to 18000.

[0019] In some embodiments, Y = 11000 to 25000.

[0020] More preferably, Y is equal to one of 11000, 12500, 13000, 13500, 14000, 15000, 16000, 16500, 17000, 18000, 18050, 18500, 19000, 20000, 21000, 21500, 22000, 23000, 25000 or falls within a range between any two of these values.

[0021] More preferred is Y = 12000 to 24000.

[0022] When the X value of the characteristic peak area of ​​Na element in XPS text changes under specific conditions for the positive electrode plate, it affects the degree and rate of dissolution of the transition metal element from the active material in the positive electrode plate, as well as the interfacial impedance and the degree of side reactions of the interface. On the other hand, the Y value of the characteristic peak area of ​​Na element under specific conditions for the negative electrode plate is related to the interfacial stability of the negative electrode plate. When both values ​​are preferably within the above ranges, the overall stability and interaction of both electrode plates are higher, the amount of effective transport and the efficiency of transport of lithium ions between the electrode plates are better, resulting in better cycling performance and better kinetic performance at high temperatures.

[0023] In the technical solution of the present application, the X and Y test method is as follows: the lithium-ion secondary battery is disassembled at zero state of charge. The resulting positive or negative electrode plate is soaked in dimethyl carbonate for 60 minutes at room temperature, removed, and dried. Then, the resulting electrode plate is fixed in the test mold with a conductive tape and tested using an XPS etching analyzer with a NEXSA-GA model. After the tests are completed, the X or Y test results can be obtained by integrating the area at corresponding coordinates from the XPS spectrum of the test sample according to the test data. The XPS test conditions are as follows: a 120 W monochromatic Al-Kα X-ray source with an energy resolution of ≤0.48 eV and a test beam spot of 400 μm is used. The instrument automatically provides the energy range of the test run according to the element under test.The etching conditions are as follows: Ar ions are used for etching, and the etching depth is controlled to 30 nm on either one side of the positive electrode material layer or one side of the negative electrode material layer by adjusting the etching rate or etching time. After the tests are completed, the X or Y test results can be read from the XPS spectrum of the test sample.

[0024] In some embodiments, the positive electrode material layer comprises a positive electrode material, and the lithium-ion secondary battery satisfies: 0.05 × 10 -3 ≤ W / X ≤ 7.5 × 10 -3 ; where W = (D''90 - D''10) / D''50, D v 90 represents the particle size corresponding to a case where a cumulative distribution of the percentage of the positive electrode material reaches 90%; D v50 represents the particle size corresponding to a case where a cumulative distribution of the percentage of the positive electrode material reaches 50%; D v 10 represents the particle size corresponding to a case where a cumulative distribution of the percentage of the positive electrode material reaches 10%.

[0025] More preferably, W / X is equal to one of 0.05 × 10 -3 , 0.1 × 10 -3 , 0.3 × 10 -3 , 0.6 × 10 -3 , 0.8 × 10 -3 , 0.9 × 10 -3 , 1 × 10 -3 , 1.2 × 10 -3 , 1.5 × 10 -3 , 1.8 × 10 -3 , 2 × 10 -3 , 2.5 × 10 -3 , 2.8 × 10 -3 , 3 × 10 -3 , 3.5 × 10 -3 , 4 × 10 -3 , 4.2 × 10 -3 , 4.36 × 10 -3 , 5 × 10 -3 , 6 × 10 -3 , 7 × 10 -3 , 7.5 × 10 -3 or falls within a range between any two of these values.

[0026] More preferably, the lithium-ion secondary battery meets the following requirements: 0.1×10−3≤W / X≤7×10−3.

[0027] In the present application, W represents the particle size distribution range coefficient obtained from the combination of D''90, D''50, and D''10 for the positive electrode material, which can effectively measure the particle size distribution range of the positive electrode material. W has certain relationships with the wettability of the positive electrode material in the electrolyte, the dissolution efficiency of transition metal elements, the quality of the CEI film formed by the electrolyte on the surface of the positive electrode plate, and the lithium ion transport efficiency.Provided that a sodium ion additive is introduced into the lithium-ion secondary battery and the ratio of X to Y is controlled, by adjusting the range of the particle size distribution of the positive electrode material and the characteristic peak area of ​​the Na element in an XPS test of the positive electrode plate under specific conditions within the above range of proportions, the possibility of additional side reactions occurring on the surface of the positive electrode plate after the electrolyte wets the positive electrode material can be further reduced, leaching of the transition metal element can be suppressed, and better electrochemical performance can be achieved.

[0028] In some embodiments, W = 0.6 to 16.

[0029] More preferably, W is equal to one of 0.6, 0.8, 1, 1.2, 1.25, 1.5, 1.8, 2, 2.05, 2.2, 3, 3.01, 3.5, 5, 5.5, 8, 10, 12, 15, 16 or falls within a range between any two of these values.

[0030] More preferred is W = 1 to 15.

[0031] More preferred is D v 90 = 2.5 to 25 µm, D v 50 = 0.5 to 5 µm, D''10 = 0.2 to 1 µm.

[0032] More preferred is D v 90 is equal to one of 2.5 µm, 3 µm, 5 µm, 8 µm, 10 µm, 12 µm, 15 µm, 18 µm, 20 µm, 22 µm, 25 µm or falls within a range between any two of these values; D v 50 is equal to one of 0.5 µm, 0.6 µm, 0.8 µm, 1 µm, 1.1 µm, 1.2 µm, 1.3 µm, 1.4 µm, 1.5 µm, 2 µm, 5 µm or falls within a range between any two of these values; D''10 is equal to one of 0.2 µm, 0.3 µm, 0.4 µm, 0.45 µm, 0.5 µm, 0.6 µm, 0.7 µm, 0.8 µm, 0.9 µm, 1 µm or falls within a range between any two of these values.

[0033] In the technical solution of the present application, D''90, D v 50 and D''10 for the positive electrode material were determined using the following test method: The lithium-ion secondary battery was disassembled, then the positive electrode plate was soaked in dimethyl carbonate (DMC) for 60 min, taken out, and dried. The layer of the positive electrode material was scraped off the positive electrode plate. The resulting powder was spread on a conductive adhesive for scanning electron microscopy observation at a magnification of 30,000 times. Then, particle size measurement software (e.g., Nano Measurer) was used to detect and measure 200 individual particles of the positive electrode material. Finally, D''90, D v50 and D''10 were determined by sorting according to the particle size distribution test results. Specifically, the particle size of the positive electrode material was measured in SEM images using MEARSURE NANO software, the particle sizes of the positive electrode material were acquired using the diagonal line method, and parameters related to the particle size of the positive electrode material were calculated by analyzing the particle size distribution after collecting more than 200 samples.

[0034] In some embodiments, the sodium ion additive comprises sodium difluoro(oxalato)borate, sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethylsulfonyl)imide, and / or sodium difluorophosphate.

[0035] In the electrolyte of the lithium-ion secondary battery of the present application, the sodium ion additive can be selected from the above types, but is not limited to them. During cycling of the lithium-ion battery at high temperatures, these sodium ion additives first react to form an inorganic SEI film layer when introduced into the surface of a negative electrode material layer, while some intermediate compounds continue to react at high temperature to form layers of an organic oligomeric SEI film. By combining these two types of composite film layers, the negative electrode material layer on the surface of the negative electrode plate is effectively and quickly passivated and protected.On the other hand, the sodium ion additive also forms CEI film layers on the surface of the positive electrode, thereby reducing the leaching of transition metal ions and thus reducing the loss of active lithium ions during cycling. Furthermore, if the ratio of the characteristic XPS peak areas of the element Na is controlled under specific conditions for both the negative and positive electrode plates within the range specified in this application, the lithium-ion secondary battery can achieve both high stability and high cycling activity under high-temperature conditions, thereby achieving high cycle retention and a low rate of DCR increase.

[0036] In some embodiments, the sodium ion additive has a mass percentage of 0.05 to 1.2% in the electrolyte.

[0037] More preferably, the sodium ion additive has a mass percentage of 0.5 to 0.8% in the electrolyte.

[0038] By adjusting the content of the sodium ion additive in the electrolyte, insufficient protection on the positive electrode plate can be avoided if the content is too low (because the sodium ion additive preferentially forms films on the negative electrode plate), which results in an increased concentration of dissolved transition metal ions from the positive electrode plate; or reduced kinetic performance of both electrode plates due to excessive addition can be avoided.

[0039] In some embodiments, the electrolyte further comprises a solvent and a lithium salt.

[0040] In some embodiments, the solvent comprises a carbonate solvent, a carboxylate solvent, an ether solvent, a sulfone solvent, a nitrile solvent, and / or a phosphate solvent.

[0041] For example, the carbonate solvent includes, but is not limited to, propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and / or ethyl methyl carbonate (EMC); the carboxylate solvent includes, but is not limited to, ethyl acetate, methyl formate, and / or γ-butyrolactone (or 1,4-dibutyl butyrate); the ether solvent includes dimethyltetrahydrofuran, tetrahydrofuran, and / or 1,2-dimethoxyethane; the sulfone solvent includes methyl sulfone and / or dimethyl sulfoxide; the nitrile solvent includes propionitrile, butyronitrile, 1-(2-cyanoethyl)pyrrole, and / or 1,3,6-hexanetrinitrile; the phosphate solvent includes trimethyl phosphate and / or triethyl phosphate.

[0042] In some embodiments, the lithium salt comprises lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium trifluoromethanesulfonate, lithium bis(fluorosulfony)sulfonimide, lithium bis(trifluoromethanesulfonyl)sulfonimide, lithium difluoro(dioxalato)phosphate and / or lithium tetrafluoro(oxalato)phosphate.

[0043] In some embodiments, the electrolyte further comprises a second additive, wherein the second additive includes, but is not limited to, vinyl carbonate.

[0044] In some embodiments, the positive electrode material comprises lithium manganese iron phosphate, lithium iron phosphate, doped lithium manganese iron phosphate, and / or doped lithium iron phosphate.

[0045] More preferably, the doping element in the doped lithium manganese iron phosphate and the doped lithium iron phosphate includes V, W, Ti and / or Mg.

[0046] For example, the doped lithium manganese iron phosphate is LiMn x Fe y M z n PO4, where x is greater than 0 and less than 1; y is greater than 0 and less than 1; z is greater than or equal to 0 and less than 1; M denotes the doping element, n denotes the valence of the doping element; the doped lithium manganese iron phosphate satisfies the following: 2(x + y) + n * z = 2.

[0047] More preferably, the positive electrode material further comprises a carbon material.

[0048] Carbon materials, especially graphite and graphene, with high conductivity and high adsorption efficiency can both improve the overall conductivity of the positive electrode material and suppress the leaching of transition metal elements. Those skilled in the art can add carbon materials with a specific content and configuration to the positive electrode material under actual conditions to form / formulate the lithium intercalation / deintercalation active material, as long as this does not affect the function of the sodium ion additive in the electrolyte and the control of the Na element characteristic peak areas of the electrode plates as described in this application.

[0049] In some embodiments, the positive electrode material layer in the positive electrode plate comprises a positive electrode material, a binder, and a conductive agent, and the mass percentage of the positive electrode material in the positive electrode material layer is 70 to 98%.

[0050] In some embodiments, the negative electrode material comprises natural graphite, artificial graphite, mesocarbon microspheres, hard carbon, soft carbon, elemental silicon, silicon monoxide, a silicon-carbon composite material, and / or lithium titanate.

[0051] In some embodiments, the negative electrode material layer in the negative electrode plate comprises a negative electrode material, a binder, and a conductive agent, and the mass percentage of the negative electrode material in the negative electrode material layer is 70 to 99%.

[0052] The present application is further illustrated by specific embodiments below, which, however, should not be construed as limiting the scope of protection sought by the present application: Embodiment 1

[0053] A lithium-ion secondary battery is provided, and the method for manufacturing the lithium-ion secondary battery includes the following steps: (1) Preparation of a positive electrode plate: The ground and sieved positive electrode material (ie LiMn 0,6 Fe 0,4PO4), a conductive agent (i.e., carbon black) and a binder (i.e., polyvinylidene fluoride) in N-methylpyrrolidone are dispersed in a mass ratio of 96:1.5:2.5, a slurry is prepared by stirring under reduced pressure, then applied to a current collector (i.e., aluminum foil) with a surface coating density of 400 g / m 2 applied after drying, cold pressing and cutting at a compression density of 2.2 g / cm 3 rolled to obtain the positive electrode plate; (2) Preparation of a negative electrode plate: The negative electrode material (ie artificial graphite), a conductive agent (ie carbon black), a thickener (ie sodium carboxymethyl cellulose) and a binder (ie styrene-butadiene rubber) are dispersed in water in a mass ratio of 96.5:1:1:1.5, a slurry is prepared by stirring under reduced pressure, then applied to a current collector (ie copper foil) with a surface coating density of 180 g / m 2 applied after drying, cold pressing and cutting at a compression density of 1.65 g / cm 3 rolled to obtain the negative electrode plate; (3) Preparation of an electrolyte: EC and EMC are combined in a mass ratio of 3:7 as solvents, then, based on the total mass of the electrolyte, 13.5 wt% of lithium hexafluorophosphate and sodium ion additive, 1 wt% of FEC, 2 wt% of MMDS, 1 wt% of TMSP, and 2 wt% of VC film-forming additives are added to obtain the electrolyte; the content of the sodium ion additive in the electrolyte is shown in Table 1. (4) The positive electrode plate, a commercial PP separator, and the negative electrode plate are stacked and wound in this sequence to manufacture the battery cell, the battery cell is placed in an outer packaging case, the electrolyte is injected after drying, and the lithium-ion secondary battery is obtained after packaging under negative pressure, standing, a formation step, and capacity adjustment.

[0054] The formation step includes the following: (i) The lithium-ion secondary battery is charged at a rate of 0.05 C with a constant current for 2 h, then the rate is reduced to 0.33 C, and charging is continued at a constant current for 2 h.

[0055] The parameters of the lithium-ion secondary battery are shown in Tables 1 and 2, where A (wt%) represents the mass percentage of the sodium ion additive in the electrolyte, D v 90 (µm) represents the particle size corresponding to the case where the cumulative distribution of the percentage of the positive electrode material reaches 90%; D v 50 (µm) represents the particle size corresponding to the case where the cumulative distribution of the percentage of the positive electrode material reaches 50%; D v10 (µm) represents the particle size corresponding to the case where the cumulative distribution of the percentage of the positive electrode material reaches 10%. X and Y are determined by the above test method, in particular, the XPS etching test spectrum of the positive electrode plate corresponding to X is Fig. 1, and the XPS etching test spectrum of the negative electrode plate corresponding to Y in Fig. 2 is shown. Embodiments 2 to 4

[0056] A lithium ion secondary battery is provided in which various parameters are different from Embodiment 1, wherein X and Y are controlled by the mass percentage of the sodium ion additive in the electrolyte, and W is controlled by the grinding time of the positive electrode material and the sieve for sieving particles. Embodiment 5

[0057] A lithium ion secondary battery is provided in which various parameters are different from Embodiment 1, wherein X and Y are controlled by the formation step and W is controlled by the grinding time of the positive electrode material and the sieve for sieving particles.

[0058] The formation step is as follows: (ii) The lithium-ion secondary battery is charged and discharged 3 times at a rate of 0.02 C and a working voltage between 2 V and 2.5 V, then charged at a rate of 0.05 C with a constant current for 2 h, then the rate is reduced to 0.33 C and charging is continued at a constant current for 2 h. Embodiment 6

[0059] A lithium ion secondary battery is provided in which various parameters are different from Embodiment 1, wherein X and Y are controlled by the mass percentage of the sodium ion additive in the electrolyte, and W is controlled by the grinding time of the positive electrode material and the sieve for sieving particles. Embodiment 7

[0060] A lithium ion secondary battery is provided in which various parameters are different from Embodiment 1, wherein X and Y are controlled by the mass percentage of the sodium ion additive in the electrolyte and the formation step, and W is controlled by the grinding time of the positive electrode material and the sieve for sieving particles;

[0061] The formation step is as follows: (iii) The lithium-ion secondary battery is charged at a rate of 0.05 C with a constant current for 2 h, then the rate is reduced to 0.33 C, and charging is continued for 2 h at a constant current, finally the rate is reduced to 0.1 C, and charging is continued for 1 h at a constant current. Embodiments 8 to 10

[0062] A lithium ion secondary battery is provided in which various parameters are different from Embodiment 1, wherein X and Y are controlled by the mass percentage of the sodium ion additive in the electrolyte, and W is controlled by the grinding time of the positive electrode material and the sieve for sieving particles. Embodiment 11

[0063] A lithium ion secondary battery is provided in which various parameters are different from Embodiment 1, wherein X and Y are controlled by the mass percentage of the sodium ion additive in the electrolyte and the formation step, and W is controlled by the grinding time of the positive electrode material and the sieve for sieving particles;

[0064] The formation step is as follows: (iii) The lithium-ion secondary battery is charged at a rate of 0.05 C with a constant current for 2 h, then the rate is reduced to 0.33 C, and charging is continued for 2 h at a constant current, finally the rate is reduced to 0.1 C, and charging is continued for 1 h at a constant current. Embodiment 12

[0065] A lithium ion secondary battery is provided in which various parameters are different from Embodiment 1, wherein X and Y are controlled by the mass percentage of the sodium ion additive in the electrolyte, and W is controlled by the grinding time of the positive electrode material and the sieve for sieving particles. Embodiment 13

[0066] A lithium ion secondary battery is provided in which various parameters are different from Embodiment 1, wherein X and Y are controlled by the mass percentage of the sodium ion additive in the electrolyte and the formation step, and W is controlled by the grinding time of the positive electrode material and the sieve for sieving particles;

[0067] The formation step is as follows: (iv) The lithium-ion secondary battery is charged at a rate of 0.02 C with a constant current for 2 h, then the rate is reduced to 0.33 C, and charging is continued for 2 h at a constant current, finally the rate is reduced to 0.1 C, and charging is continued for 1 h at a constant current. Embodiments 14 to 18

[0068] A lithium ion secondary battery is provided in which various parameters are different from Embodiment 1, wherein X and Y are controlled by the mass percentage of the sodium ion additive in the electrolyte, and W is controlled by the grinding time of the positive electrode material and the sieve for sieving particles. Embodiment 19

[0069] A lithium ion secondary battery is provided in which various parameters are different from Embodiment 1, wherein X and Y are controlled by the mass percentage of the sodium ion additive in the electrolyte and the formation step, and W is controlled by the grinding time of the positive electrode material and the sieve for sieving particles;

[0070] The formation step is as follows: (v) The lithium-ion secondary battery is charged at a rate of 0.08 C with a constant current for 2 h, then the rate is reduced to 0.33 C, and charging is continued for 2 h at a constant current, finally the rate is reduced to 0.1 C, and charging is continued for 1 h at a constant current. Embodiments 20 to 24

[0071] A lithium ion secondary battery is provided in which various parameters are different from Embodiment 1, wherein X and Y are controlled by the mass percentage of the sodium ion additive in the electrolyte, and W is controlled by the grinding time of the positive electrode material and the sieve for sieving particles. Embodiment 25

[0072] A lithium-ion secondary battery is provided in which various parameters are different from Embodiment 1; where X and Y are controlled by the mass percentage of the sodium ion additive in the electrolyte and the formation step, and W is controlled by the grinding time of the positive electrode material and the sieve for sieving particles;

[0073] The formation step is as follows: (ii) The lithium-ion secondary battery is charged and discharged 3 times at a rate of 0.02 C and a working voltage between 2 V and 2.5 V, then charged at a rate of 0.05 C with a constant current for 2 h, and then the rate is reduced to 0.33 C and charging is continued for 2 h with a constant current. Embodiments 26 to 27

[0074] A lithium ion secondary battery is provided in which various parameters are different from Embodiment 1, wherein X and Y are controlled by the mass percentage of the sodium ion additive in the electrolyte, and W is controlled by the grinding time of the positive electrode material and the sieve for sieving particles. Embodiment 28

[0075] A lithium ion secondary battery is provided in which various parameters are different from Embodiment 1, wherein X and Y are controlled by the mass percentage of the sodium ion additive in the electrolyte and the formation step, and W is controlled by the grinding time of the positive electrode material and the sieve for sieving particles;

[0076] The formation step is as follows: (v) The lithium-ion secondary battery is charged at a rate of 0.08 C with a constant current for 2 h, then the rate is reduced to 0.33 C, and charging is continued for 2 h at a constant current, finally the rate is reduced to 0.1 C, and charging is continued for 1 h at a constant current. Embodiment 29

[0077] A lithium ion secondary battery is provided which differs from Embodiment 1 in that the ground and particle size-screened LiFePO4 is used as the positive electrode material in the manufacture of the positive electrode plate. Comparison example 1

[0078] A lithium ion secondary battery is provided in which various parameters are different from Embodiment 1, wherein X and Y are controlled by the mass percentage of the sodium ion additive in the electrolyte, and W is controlled by the grinding time of the positive electrode material and the sieve for sieving particles. Comparison example 2

[0079] A lithium ion secondary battery is provided in which various parameters are different from Embodiment 1, wherein X and Y are controlled by the mass percentage of the sodium ion additive in the electrolyte and the formation step, and W is controlled by the grinding time of the positive electrode material and the sieve for sieving particles;

[0080] The formation step is as follows: (vi) The lithium-ion secondary battery is charged at a rate of 1 C to a voltage of 2.5 V, then charged at a rate of 0.33 C to a voltage of 4 V, finally charged and discharged 3 times at a rate of 0.05 C at a working voltage between 4 V and 4.3 V. Comparison example 3

[0081] A lithium ion secondary battery is provided which differs from Embodiment 1 in that the sodium ion additive is replaced by an equal mass of LiODFB. Comparison example 4

[0082] A lithium ion secondary battery is provided in which various parameters are different from Embodiment 1, wherein X and Y are controlled by the mass percentage of the sodium ion additive in the electrolyte and the formation step, and W is controlled by the grinding time of the positive electrode material and the sieve for sieving particles;

[0083] The formation step is as follows: (ii) The lithium-ion secondary battery is charged and discharged 3 times at a rate of 0.02 C at a working voltage between 2 V and 2.5 V, then charged at a rate of 0.05 C with a constant current for 2 h, then the rate is reduced to 0.33 C and charging is continued at a constant current for 2 h. Comparison example 5

[0084] A lithium ion secondary battery is provided in which various parameters are different from Embodiment 1, wherein X and Y are controlled by the mass percentage of the sodium ion additive in the electrolyte. Comparison example 6

[0085] A lithium ion secondary battery is provided in which various parameters are different from Embodiment 1, wherein X and Y are controlled by the mass percentage of the sodium ion additive in the electrolyte and the formation step, and W is controlled by the grinding time of the positive electrode material and the sieve for sieving particles;

[0086] The formation step is as follows: (ii) The lithium-ion secondary battery is charged and discharged 3 times at a rate of 0.02 C at a working voltage between 2 V and 2.5 V, then charged at a rate of 0.05 C with a constant current for 2 h, then the rate is reduced to 0.33 C and charging is continued at a constant current for 2 h. Comparison example 7

[0087] A lithium ion secondary battery is provided in which various parameters are different from Embodiment 1, wherein X and Y are controlled by the mass percentage of the sodium ion additive in the electrolyte. Table 1 parameter X Y X / Y Sodium ion additive A (wt%) Embodiment 1 9400 17000 0,553 NaODFB 0,3 Embodiment 2 2150 11850 0,181 NaODFB 0,58 Embodiment 3 2400 24000 0,100 NaODFB 0,4 Embodiment 4 11000 15000 0,733 NaODFB 0,6 Embodiment 5 5000 22000 0,227 NaODFB 0,2 Embodiment 6 6898 18040 0,382 NaODFB 0,5 Embodiment 7 10560 16000 0,660 NaODFB 0,6 Embodiment 8 1850 12350 0,150 NaODFB 0,075 Embodiment 9 4401 24450 0,180 NaODFB 1,1 Embodiment 10 18260 22280 0,820 NaODFB 1 Embodiment 11 17890 19880 0,900 NaODFB 1 Embodiment 12 9000 14516 0,620 NaFSI 0,25 Embodiment 13 9020 16365 0,551 NaFSI 0,25 Embodiment 14 4350 24166 0,180 NaFSI 0,17 Embodiment 15 18260 20520 0,890 NaFSI 0,95 Embodiment 16 8960 15448 0,580 NaTFSI 0,23 Embodiment 17 10500 15000 0,700 NaTFSI 0,55 Embodiment 18 1800 12050 0,149 NaTFSI 0,07 Embodiment 19 10890 11450 0,951 NaTFSI 0,55 Embodiment 20 9500 16000 0,594 NaODFB 0,33 Embodiment 21 9450 16700 0,566 NaODFB 0,31 Embodiment 22 9320 16068 0,580 NaODFB 0,28 Embodiment 23 9400 16785 0,560 NaODFB 0,29 Embodiment 24 9480 15160 0,625 NaODFB 0,32 Embodiment 25 11000 16899 0,651 NaODFB 0,12 Embodiment 26 6260 12000 0,522 NaODFB 0,1 Embodiment 27 2000 12010 0,167 NaODFB 0,27 Embodiment 28 2000 15050 0,133 NaODFB 0,275 Embodiment 29 9500 17000 0,559 NaODFB 0,3 Comparison example 1 628 12550 0,050 NaODFB 0,03 Comparison example 2 13005 8670 1,500 NaODFB 0,5 Comparison example 3 / / / / / Comparison example 4 2010 23550 0,085 NaODFB 0,1 Comparison example 5 17750 12050 1,473 NaODFB 0,92 Comparison example 6 2030 23550 0,086 NaFSI 0,1 Comparison example 7 16750 12000 1,396 NaFSI 0,91 Table 2 parameter D v 90 (µm) D,10 (µm) D,50 (µm) W W / X (× 10 -3 ) Embodiment 1 2,50 0,20 0,76 3,01 0,320 Embodiment 2 23,50 1,00 1,50 15,00 6,977 Embodiment 3 2,55 0,31 1,09 2,05 0,854 Embodiment 4 3,00 0,35 0,79 3,35 0,305 Embodiment 5 4,20 0,25 0,71 5,60 1,120 Embodiment 6 2,50 0,20 0,76 3,01 0,436 Embodiment 7 3,00 0,35 0,79 3,35 0,317 Embodiment 8 19,50 1,00 1,50 12,33 6,667 Embodiment 9 2,50 0,20 0,76 3,01 0,684 Embodiment 10 4,00 0,25 0,66 5,65 0,309 Embodiment 11 5,30 0,80 0,93 4,85 0,271 Embodiment 12 2,50 0,20 0,76 3,01 0,334 Embodiment 13 2,50 0,20 0,76 3,01 0,334 Embodiment 14 2,50 0,20 0,76 3,01 0,692 Embodiment 15 2,50 0,20 0,76 3,01 0,165 Embodiment 16 2,50 0,20 0,76 3,01 0,336 Embodiment 17 3,00 0,35 0,79 3,35 0,319 Embodiment 18 2,50 0,20 0,76 3,01 1,672 Embodiment 19 3,00 0,35 0,79 3,35 0,308 Embodiment 20 2,50 0,20 0,76 3,01 0,317 Embodiment 21 3,80 0,60 0,91 3,50 0,370 Embodiment 22 5,00 0,25 0,63 7,50 0,805 Embodiment 23 6,50 0,30 0,67 9,20 0,979 Embodiment 24 4,00 0,25 0,66 5,65 0,596 Embodiment 25 2,50 1,00 1,50 1,00 0,091 Embodiment 26 2,50 1,00 2,50 0,60 0,096 Embodiment 27 23,50 1,00 1,50 15,00 7,500 Embodiment 28 25,00 1 1,5 16 8 Embodiment 29 2,50 0,20 0,76 3,01 0,317 Comparison example 1 2,50 1,00 1,50 1,00 1,592 Comparison example 2 4,00 0,25 0,66 5,65 0,434 Comparison example 3 2,50 0,20 0,76 3,01 / Comparison example 4 4,00 0,25 0,66 5,65 2,811 Comparison example 5 4,00 0,25 0,66 5,65 0,318 Comparison example 6 4,00 0,25 0,66 5,65 2,783 Comparison example 7 4,00 0,25 0,66 5,65 0,337 Examples of the effect

[0088] The following tests were conducted on the sodium ion secondary batteries obtained from all embodiments and comparative examples: (1) Cycle capacity retention ratio test: (I) The lithium-ion secondary batteries obtained by the formation method of each embodiment and each comparative example are charged and discharged at 45°C at a rate of 0.33C for 3 cycles for capacity adjustment, using the third discharge capacity as the initial capacity C1; (II) It is charged at a rate of 1 C with a constant current up to an upper voltage limit U Höchst charged, then charged at a constant voltage to a rate ≤0.05 C; where U Höchst = 4.3 V batteries from embodiments 1 to 28 and comparative examples 1 to 7, which are charged with a constant current, and U Höchst = 3.65 V of the battery of embodiment 29 charged with a constant current; (III) It is left to rest for 5 minutes; (IV) It is discharged at a rate of 1 C to a voltage of 2.5 V; (V) It is left to rest for 5 minutes; (VI) Steps (II) to (V) are repeated for 200 cycles, the battery capacity C2 after cycling is recorded; (VII) The cycle capacity retention ratio in % = 100% × C2 / C1 is calculated. (2) Test of the rate of DCR increase: (I) A capacity adjustment of the batteries obtained after the formation step is carried out at a rate of 0.33 C, ie they are charged at 0.33 C with a constant current up to an upper voltage limit U HöchstCharged to a current of ≤0.05 C; then discharged at 0.33 C to a lower voltage limit of 2.5 V. The above steps are repeated 3 times, using the third discharge capacity as the battery discharge capacity. They are adjusted to an SOC of 50% by discharging at a rate of 0.33 C and resting for 2 hours. Discharged at 1 C for 18 s. The initial discharge voltage V0, the voltage V1 at the end of the 18-s discharge, the discharge current I1 are recorded, and the initial internal resistance of the battery is calculated, ie, DCR1 = |V o -V1| / I1; (II) The battery is fully discharged at 45 °C at a rate of 0.33 C to the lower voltage limit of 2.5 V, then it is discharged 200 times at the same temperature at a rate of 1 C to an upper voltage limit U Höchst of 2.5 V charged and discharged with 0.33 C with a constant current up to an upper voltage limit UHöchst and a current of ≤0.05 C; then discharged at 0.33 C to a lower voltage limit of 2.5 V, the above steps are repeated 3 times, taking the third discharge capacity as the battery discharge capacity, it is adjusted to an SOC of 50% by discharging at a rate of 0.33 C and resting for 2 hours, discharged at 1 C for 18 s, the initial discharge voltage V'0, the voltage V'1 at the end of the 18-s discharge, the discharge current I'1 are recorded, and the initial internal resistance of the battery is calculated, ie, DCR2 = |V'0 - V'1 / I'1; (III) The rate of DCR increase of the battery in % = 100% × (DCR2 - DCR1) / DCR1 is calculated;

[0089] Embodiments 1 to 28 and Comparative Examples 1 to 7 correspond to a U Höchst = 4.3 V for the lithium-ion secondary battery, and embodiment 29 corresponds to a U Höchst= 3.65 V for the lithium-ion secondary battery;

[0090] The test results are shown in Table 3. Table 3 Performance Cycle capacity retention ratio (%) Rate of DCR increase (%) Embodiment 1 95,6 4,8 Embodiment 2 94,1 11,7 Embodiment 3 92,3 5,3 Embodiment 4 94,2 10,9 Embodiment 5 93,2 5,2 Embodiment 6 95,2 5 Embodiment 7 93,9 11,1 Embodiment 8 84,6 13,2 Embodiment 9 88,5 15,7 Embodiment 10 89,7 16,1 Embodiment 11 89,3 14,8 Embodiment 12 94,9 5 Embodiment 13 95,2 4,9 Embodiment 14 87,8 15,9 Embodiment 15 88,8 16,4 Embodiment 16 94,5 5,1 Embodiment 17 93,7 11,1 Embodiment 18 83,7 13,7 Embodiment 19 89,7 14,7 Embodiment 20 94,3 5,4 Embodiment 21 94,7 5,3 Embodiment 22 95,4 5 Embodiment 23 95,1 5,2 Embodiment 24 94,9 5,5 Embodiment 25 88,4 13,6 Embodiment 26 86,5 14,2 Embodiment 27 91,2 12,1 Embodiment 28 89,6 12,9 Embodiment 29 96,2 4,7 Comparison example 1 70,4 35,4 Comparison example 2 86,4 40,7 Comparison example 3 67,3 42,6 Comparison example 4 76,8 24,5 Comparison example 5 80,8 35,6 Comparison example 6 75,7 26,5 Comparison example 7 78,4 34,2

[0091] According to Table 3, the following can be seen: (1) In the lithium-ion secondary battery of the present application, by introducing a sodium ion additive into the electrolyte and simultaneously controlling the ratio of the characteristic peak areas of the Na element on both the positive and negative electrode plates measured by an XPS test at a depth of 30 nm, the lithium-ion secondary battery achieves both electrode stability and kinetic performance during cycling under high-temperature conditions, with a good passivation effect of the negative electrode plate, a low electrode interface impedance, a low degree of interfacial side reactions, and high lithium ion transport efficiency. After 200 cycles at a rate of 1 °C at high temperature, the capacity retention ratio can reach more than 80%, and the DCR increase rate does not exceed 16.5%.In contrast, the lithium-ion secondary battery in Comparative Example 3, which uses a conventional lithium-ion additive instead of the sodium-ion additive in the electrolyte, exhibits serious leaching of transition metal elements from the positive electrode material at high temperature, resulting not only in loss of active lithium but also inability to quickly form a thick passivating SEI film on the surface of the negative electrode plate and a thin, dense CEI film on the surface of the positive electrode plate during cycling, ultimately failing to achieve good high-temperature cycling performance.The lithium-ion secondary batteries of Comparative Examples 1 to 2 and 4 to 5 contain a sodium ion additive in the electrolyte, but do not have proper control of the ratio of X to Y, which shows that they still cannot achieve a high cycle capacity retention ratio and a low rate of DCR increase at high temperature. (2) From Embodiments 1 to 11 and Embodiments 20 to 24, it can be seen that the values ​​of X and Y can be changed by adjusting the mass percentage of the sodium ion additive in the electrolyte and the formation steps, thereby changing the range of the X / Y ratio. Further, when the X / Y ratio is preferably in the range of 0.21 to 0.75, better effects can be achieved during cycling of the lithium-ion secondary battery containing the sodium ion additive. The SEI film layer on the surface of the negative electrode plate provides better protection for the negative electrode material, making it difficult for transition metal elements to erode the negative electrode material. On the other hand, the positive electrode plate has a lower impedance, and the kinetic performance between electrodes is excellent.After 200 cycles at a rate of 1 C at high temperature, the capacity retention ratio can reach more than 93%, with the rate of DCR increase not exceeding 11.5%. (3) From Embodiments 12 to 19 and Comparative Examples 6 to 7, it is clear that the X / Y control for the positive and negative electrode plates in this application is not limited to lithium-ion secondary batteries with a single range of sodium ion additive. This limitation applies to various ranges of sodium ion additive. If X / Y is not in the range of 0.1 to 0.95, the lithium-ion secondary battery cannot achieve good cycling stability and high-temperature conductivity. Further, if X / Y is preferably in the range of 0.21 to 0.75, the performance of the lithium-ion secondary battery is better. (4) From Embodiment 1 and Embodiments 20 to 28, it can be seen that the range W of the particle size of the positive electrode material has certain correlations with the wettability of the positive electrode material in the electrolyte and the dissolution efficiency of transition metal elements. When W / X is preferably in the range of 1 × 10 -3 up to 7 × 10 -3 The positive electrode material wetted with sodium ion additive can maintain high ion transport efficiency, reducing the dissolution frequency of transition metal elements at high temperature and reducing the probability of additional side reactions in the electrolyte, resulting in better electrochemical performance of the lithium-ion secondary battery.

Claims

[1] A lithium-ion secondary battery comprising a positive electrode plate, a negative electrode plate and an electrolyte, wherein the electrolyte comprises a sodium ion additive; the positive electrode plate comprises a current collector and a layer of positive electrode material, and the negative electrode plate comprises a current collector and a layer of negative electrode material; the lithium-ion secondary battery meets: 0.1 ≤ X / Y ≤ 0.95, where X represents a characteristic peak area of ​​the element Na at an XPS etching depth of 30 nm on a side of the positive electrode material layer facing away from the current collector during the XPS test of the positive electrode plate; Y represents a characteristic peak area of ​​the element Na at an XPS etching depth of 30 nm on a side of the negative electrode material layer facing away from the current collector during the XPS test of the negative electrode plate. [2] The lithium ion secondary battery according to claim 1, wherein the lithium ion secondary battery satisfies: 0.21 ≤ X / Y ≤ 0.

75. [3] A lithium ion secondary battery according to claim 1 or 2, wherein X = 1000 to 19000 and / or Y = 11000 to 25000. [4] The lithium ion secondary battery according to any one of claims 1 to 3, wherein the positive electrode material layer comprises a positive electrode material, the lithium ion secondary battery satisfying: 0.05 × 10 -3 ≤ W / X ≤ 7.5 × 10 -3 ; where W = (D''90 - D v 10) / D v 50, D v 90 represents a particle size corresponding to a case where the cumulative distribution of the percentage of the positive electrode material reaches 90%; D v 50 represents a particle size corresponding to a case where the cumulative distribution of the percentage of the positive electrode material reaches 50%; and D''10 represents a particle size corresponding to a case where the cumulative distribution of the percentage of the positive electrode material reaches 10%. [5] A lithium ion secondary battery according to claim 4, wherein W = 0.6 to 16. [6] The lithium ion secondary battery according to any one of claims 1 to 5, wherein the sodium ion additive comprises sodium difluoro(oxalato)borate, sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethylsulfonyl)imide and / or sodium difluorophosphate. [7] A lithium-ion secondary battery according to claim 6, wherein the sodium ion additive has a mass percentage of 0.05 to 1.2% in the electrolyte. [8] The lithium ion secondary battery according to any one of claims 1 to 7, wherein the electrolyte further comprises a solvent and a lithium salt; and the solvent comprises a carbonate solvent, a carboxylate solvent, an ether solvent, a sulfone solvent, a nitrile solvent and / or a phosphate solvent and / or the lithium salt comprises lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium trifluoromethanesulfonate, lithium bis(fluorosulfony)sulfonimide, lithium bis(trifluoromethanesulfonyl)imide, lithium difluoro(dioxalato)phosphate and / or lithium tetrafluoro(oxalato)phosphate. [9] A lithium-ion secondary battery according to any one of claims 1 to 8, wherein the positive electrode material comprises lithium manganese iron phosphate, lithium iron phosphate, doped lithium manganese iron phosphate and / or doped lithium iron phosphate and / or the negative electrode material comprises natural graphite, artificial graphite, mesocarbon microspheres, hard carbon, soft carbon, elemental silicon, silicon monoxide, a silicon-carbon composite material and / or lithium titanate. [10] An electrical device comprising the lithium ion secondary battery according to any one of claims 1 to 9, wherein the lithium ion secondary battery serves as a power source of the electrical device.

Images