A lithium-ion secondary battery

Abstract

The present application discloses a lithium-ion secondary battery, which belongs to the field of battery technology. The technical solution of the present application introduces a sodium ion additive into the electrolyte of the battery, and simultaneously regulates the Na element peak area ratio of the active material layer of each of the positive electrode plate and the negative electrode plate at a depth of 30nm in the surface layer tested by XPS within a specific range, thereby effectively optimizing the dissolution of transition metal elements in the positive electrode material under high temperature conditions, reducing the loss of active lithium, improving the battery kinetics, enhancing the protection effect on the negative electrode plate, and ultimately improving the battery cycle performance.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to a lithium-ion secondary battery. Background Technology

[0002] Lithium-ion rechargeable batteries typically use compounds containing transition metal elements (such as lithium iron phosphate, lithium manganese iron phosphate, and lithium cobalt oxide) as positive electrode materials. However, during cycling, especially under high-temperature conditions, the transition metal elements in the positive electrode material dissolve into the electrolyte in ionic form, resulting in a significant loss of active lithium. These ions then migrate to the negative electrode interface, increasing interfacial impedance. Furthermore, these ions also reduce the stability of the electrolyte, causing it to undergo chemical reactions and produce byproducts, ultimately affecting the battery's cycle performance. Summary of the Invention

[0003] The purpose of this application is to overcome the shortcomings of the existing technology and provide a lithium-ion secondary battery. By introducing sodium ion additives into the electrolyte of the battery, and simultaneously controlling the ratio of the Na element peak area at a depth of 30nm in the surface layer of the active material layers of the positive and negative electrodes to a specific range, the 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 battery dynamic performance can be improved, the protection effect on the negative electrode can be enhanced, and the cycle performance of the battery can be improved.

[0004] To achieve the above objectives, in a first aspect of this application, a lithium-ion secondary battery is provided, comprising a positive electrode, a negative electrode, and an electrolyte.

[0005] The electrolyte contains sodium ion additives;

[0006] The positive electrode includes a current collector and a positive electrode material layer, and the negative electrode includes a current collector and a negative electrode material layer;

[0007] The lithium-ion secondary battery satisfies: 0.1≤X / Y≤0.95;

[0008] Where X is the characteristic peak area of ​​Na element in the positive electrode sheet under XPS testing and with an XPS etching depth of 30 nm on the side of the positive electrode material layer away from the current collector; Y is the characteristic peak area of ​​Na element in the negative electrode sheet under XPS testing and with an XPS etching depth of 30 nm on the side of the negative electrode material layer away from the current collector.

[0009] The beneficial effects of this application are as follows:

[0010] This application provides a lithium-ion secondary battery. By introducing sodium ion additives into the electrolyte of the battery and simultaneously controlling the ratio of the Na element peak intensity at a depth of 30 nm in XPS tests of the active material layers of the positive and negative electrodes within a specific range, the 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 battery dynamic performance can be improved, the protection effect on the negative electrode can be enhanced, and the cycle performance of the battery can be improved. Attached Figure Description

[0011] Figure 1 The XPS spectrum results of the characteristic peak area of ​​Na element in the lithium-ion secondary battery described in Embodiment 1 of this application are obtained by testing the positive electrode sheet under XPS testing, with an XPS etching depth of 30 nm on the side of the positive electrode material layer away from the current collector.

[0012] Figure 2 The XPS spectrum results of the characteristic peak area of ​​Na element in the lithium-ion secondary battery described in Embodiment 1 of this application are obtained by testing the negative electrode sheet under XPS testing, with an XPS etching depth of 30 nm on the side of the negative electrode material layer away from the current collector. Detailed Implementation

[0013] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0014] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0015] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0016] The present application is further illustrated below with specific embodiments:

[0017] A lithium-ion secondary battery includes a positive electrode, a negative electrode, and an electrolyte;

[0018] The electrolyte contains sodium ion additives;

[0019] The positive electrode includes a current collector and a positive electrode material layer, and the negative electrode includes a current collector and a negative electrode material layer;

[0020] The lithium-ion secondary battery satisfies: 0.1≤X / Y≤0.95;

[0021] Where X is the characteristic peak area of ​​Na element in the positive electrode sheet under XPS testing and with an XPS etching depth of 30 nm on the side of the positive electrode material layer away from the current collector; Y is the characteristic peak area of ​​Na element in the negative electrode sheet under XPS testing and with an XPS etching depth of 30 nm on the side of the negative electrode material layer away from the current collector.

[0022] In some specific embodiments, the coordinates of the characteristic peak area are 1070–1075 eV.

[0023] In lithium-ion secondary batteries, the positive electrode material undergoes transition metal leaching during battery cycling, especially at high temperatures. This not only depletes active lithium but also affects the interfacial impedance of the negative electrode and the stability of the electrolyte, ultimately weakening the battery's cycle performance. In the lithium-ion secondary battery described in this application, a sodium ion additive is added to the electrolyte. This additive pre-forms an inorganic SEI film on the negative electrode during battery cycling, which then further reacts at high temperatures to form a highly stable organic / inorganic composite SEI film. Furthermore, during battery charging, this additive also forms a high-density CEI film on the positive electrode. The combined effect of these two film formations suppresses the leaching of transition metals from the positive electrode material and effectively protects the negative electrode. Additionally, this application also controls the etching of the positive and negative electrodes at 30nm using XPS (X-ray Photoelectron) etching. Spectroscopy testing of the characteristic peak area ratio of sodium can effectively control the thickness relationship of the SEI / CEI film layers generated by sodium ion additives on the corresponding electrodes. This allows the lithium-ion secondary battery to maintain electrode stability and a low DCR growth rate during high-temperature cycling, thus improving kinetic performance. It also results in good negative electrode passivation, low interfacial impedance, low degree of interfacial side reactions, high lithium-ion transport efficiency, and excellent cycle performance. In some embodiments, the lithium-ion secondary battery satisfies the following range: X / Y = 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 any two of these values.

[0024] The ratio of the area of ​​the Na element XPS characteristic peak at the same etching depth on the two electrodes can directly affect the electrochemical performance of lithium-ion secondary batteries. If the ratio is too large, the kinetic performance of the lithium-ion secondary battery will deteriorate, and the passivation protection of the SEI film on the negative electrode will be insufficient, resulting in severe capacity decay of the battery at high temperatures. If the ratio is too small, the interfacial stability in the positive electrode material will be insufficient, and the kinetic performance will also be low, and the battery will still not achieve the ideal electrochemical performance. Therefore, the ratio needs to be maintained within the above range.

[0025] In some embodiments, the lithium-ion secondary battery satisfies: 0.21 ≤ X / Y ≤ 0.75. More preferably, the lithium-ion secondary battery satisfies: X / Y = one or any two of the following values: 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.

[0026] When the ratio of the area of ​​the Na element XPS characteristic peak under specific conditions is within the above-mentioned preferred range, the overall ion / electron transport efficiency of the lithium-ion secondary battery is higher, and better cycle performance under high temperature environment can be achieved.

[0027] In some implementations, X = 1000 to 19000.

[0028] More preferably, X is a range of one or any two of the following: 1000, 2400, 2500, 3000, 3500, 4000, 5000, 5500, 6000, 6500, 6800, 6850, 7000, 8000, 9000, 10000, 11000, 12000, 13000, 14000, 14500, 15000, 16000, 17000, and 19000.

[0029] More preferably, X = 2000 to 18000.

[0030] In some implementations, Y = 11000 to 25000.

[0031] More preferably, Y is a range of one or any two of the following: 11000, 12500, 13000, 13500, 14000, 15000, 16000, 16500, 17000, 18000, 18050, 18500, 19000, 20000, 21000, 21500, 22000, 23000, and 25000.

[0032] More preferably, Y = 12000 to 24000.

[0033] When the area X of the characteristic Na peak in XPS testing of the positive electrode changes under specific conditions, the degree and rate of dissolution of transition metal elements in the active material of the positive electrode will differ, as will the impedance and degree of interfacial side reactions at the positive electrode interface. On the other hand, the area Y of the characteristic Na peak in XPS testing of the negative electrode under specific conditions is related to the interfacial stability of the negative electrode. When the values ​​of both are preferably within the above range, the overall stability and interaction of the two electrodes are higher, the effective transport amount and efficiency of lithium ions between the two electrodes are better, and the cycling and kinetic performance at high temperatures is better.

[0034] In this application's technical solution, the testing method for X and Y is as follows: The lithium-ion secondary battery is disassembled in an empty state. The resulting positive or negative electrode sheet is immersed in dimethyl carbonate at room temperature for 60 minutes, removed, dried, and then fixed in a test mold using conductive tape. A NEXSAGA XPS etching analyzer is used for testing. After the test is completed, the X or Y test result value can be obtained by area integration from the XPS spectrum of the test sample based on the test data at the corresponding coordinates. The XPS testing conditions are: a 120W monochromatic Al Kα X-ray source; energy resolution less than or equal to 0.48 eV; test beam spot size of 400 μm; and the instrument automatically supplements the test pass range according to the element to be measured. The etching conditions are: etching with Ar ions; by adjusting the etching rate or etching time, the etching surface is either the positive electrode layer or the negative electrode layer, and the etching depth is controlled to 30 nm. After the test is completed, the X or Y test result value can be read from the XPS spectrum of the test sample.

[0035] In some embodiments, the positive electrode material layer comprises a positive electrode material, and the lithium-ion secondary battery satisfies: 0.05 × 10⁻⁶ -3 ≤W / X≤7.5×10 -3 Where, W = (D v 90-D v 10) / D v 50, D v 90 represents the particle size corresponding to a cumulative distribution percentage of 90% for the positive electrode material; D v 50 represents the particle size corresponding to a cumulative percentage distribution of the positive electrode material reaching 50%; D v 10 represents the particle size corresponding to a cumulative percentage distribution of the positive electrode material reaching 10%.

[0036] More preferably, the W / X = 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 The range of one or any two of them.

[0037] More preferably, the lithium-ion secondary battery satisfies: 0.1 × 10⁻⁶ -3 ≤W / X≤7×10 -3

[0038] The W mentioned in this application is the positive electrode material D. v 90. D v 50 and D v The particle size distribution span coefficient obtained by combining 10 can effectively measure the breadth of the cathode material particle size distribution. It is related to the wettability of the cathode material in the electrolyte, the dissolution efficiency of transition metal elements, the quality of the CEI film formed by the electrolyte on the cathode electrode surface, and the lithium-ion transport efficiency. Under the premise of introducing sodium ion additives into lithium-ion secondary batteries and controlling the ratio of X to Y, by adjusting the breadth of the cathode material particle size distribution and the characteristic peak area of ​​Na element XPS test under specific conditions of the cathode electrode within the above ratio range, the probability of additional side reactions occurring on the surface of the cathode material after the electrolyte wets it can be further reduced, the dissolution of transition metal elements can be suppressed, and better electrochemical performance can be achieved.

[0039] In some implementations, W = 0.6 to 16.

[0040] More preferably, W is a range of one or any two of the following: 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.

[0041] More preferably, W = 1 to 15.

[0042] More preferably, the D v 90 = 2.5~25μm, D v 50 = 0.5~5μm, D v 10 = 0.2~1μm.

[0043] More preferably, the D v 90 = a value within the range of one or any two of the following: 2.5μm, 3μm, 5μm, 8μm, 10μm, 12μm, 15μm, 18μm, 20μm, 22μm, and 25μm; the D v 50 = a range of one or any two of the following: 0.5μm, 0.6μm, 0.8μm, 1μm, 1.1μm, 1.2μm, 1.3μm, 1.4μm, 1.5μm, 2μm, and 5μm; the D v 10 = a range of one or any two of the following: 0.2μm, 0.3μm, 0.4μm, 0.45μm, 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, and 1μm.

[0044] In the technical solution of this application, the D of the positive electrode material v 90. D v 50 and D v 10. Confirmation was achieved through the following testing method: The lithium-ion secondary battery was disassembled, and the positive electrode sheet was immersed in dimethyl carbonate (DMC) for 60 minutes. After removal and drying, the positive electrode material layer was scraped off, and the resulting powder was coated onto conductive adhesive for observation under a scanning electron microscope (SEM) at 30,000x magnification. Subsequently, 200 individual positive electrode material particles were collected using Nano Measurer software for particle size measurement, and the particle size distribution was sorted for final confirmation. Specifically, the size of the positive electrode material particles was measured using MEARSURE NANO software on SEM images. The diagonal line method was used to collect the particle size data. After collecting more than 200 samples, the particle size distribution was statistically analyzed, and the particle size-related parameters of the positive electrode material were calculated.

[0045] In some embodiments, the sodium ion additive includes at least one of sodium difluorooxalate borate, sodium difluorosulfonamide, sodium difluoromethylsulfonamide, and sodium difluorophosphate.

[0046] In the electrolyte of the lithium-ion secondary battery described in this application, the sodium ion additive can be selected from, but is not limited to, the types mentioned above. During the high-temperature cycling process of the lithium-ion battery, these sodium ion additives will embed into the surface of the negative electrode material and first react to form an inorganic SEI film layer. At the same time, some intermediate compounds will further react under high temperature to form an organic oligomer SEI film layer. Through the combination of these two types of composite film layers, the negative electrode material layer on the negative electrode sheet is effectively and quickly passivated and protected. On the other hand, the sodium ion additives will also form a CEI film layer on the positive electrode surface, reducing the dissolution of transition metal ions and thus reducing the loss of active lithium ions during cycling. When the ratio of the XPS characteristic peak area of ​​Na element under specific conditions of the positive and negative electrode sheets is further controlled within the range specified in this application, the lithium-ion secondary battery can achieve both high stability and high cycle activity under high temperature conditions, realizing high cycle retention and low DCR growth rate.

[0047] In some embodiments, the sodium ion additive has a mass percentage content of 0.05% to 1.2% in the electrolyte.

[0048] More preferably, the sodium ion additive has a mass percentage of 0.5% to 0.8% in the electrolyte.

[0049] By adjusting the content of sodium ion additive in the electrolyte, it is possible to avoid the situation where the content is too low and the film is preferentially formed on the negative electrode, thus reducing the protective effect on the positive electrode and increasing the concentration of transition metal ions dissolved on the positive electrode. Alternatively, excessive addition can lead to a decrease in the kinetic performance of both the positive and negative electrodes.

[0050] In some embodiments, the electrolyte further includes a solvent and a lithium salt.

[0051] In some embodiments, the solvent includes at least one of carbonate solvents, carboxylic acid ester solvents, ether solvents, sulfone solvents, nitrile solvents, and phosphate ester solvents.

[0052] Exemplary examples include, but are not limited to, at least one of propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC); carboxylic acid ester solvents include, but are not limited to, at least one of ethyl acetate, methyl formate, and 1,4-butyrolactone; ether solvents include, at least one of dimethyltetrahydrofuran, tetrahydrofuran, and 1,2-dimethoxyethane; sulfone solvents include, at least one of methyl sulfone and dimethyl sulfoxide; nitrile solvents include, at least one of propionitrile, butyronitrile, 1-(2-cyanoethyl)pyrrole, and 1,3,6-hexanetrionitrile; and phosphate ester solvents include, at least one of trimethyl triphosphate and triethyl phosphate.

[0053] In some embodiments, the lithium salt includes at least one of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetrafluoroborate, lithium dioxalate borate, lithium difluorooxalate borate, lithium trifluoromethanesulfonate, lithium difluoromethanesulfonylimide, lithium ditrifluoromethanesulfonylimide, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0054] In some embodiments, the electrolyte further includes a second additive, which includes, but is not limited to, vinylene carbonate.

[0055] In some embodiments, the cathode material includes at least one of lithium manganese iron phosphate, lithium iron phosphate, doped lithium manganese iron phosphate, and doped lithium iron phosphate.

[0056] More preferably, the doped lithium manganese iron phosphate and the doping element in the doped lithium iron phosphate include at least one of V, W, Ti, and Mg.

[0057] 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 refers to the doping element, and n refers to the valence of the doping element; the doped lithium manganese iron phosphate satisfies: 2(x+y)+n*z=2.

[0058] More preferably, the cathode material further includes a carbon material.

[0059] Carbon materials, especially graphite and graphene with high conductivity and high adsorption efficiency, can improve the overall conductivity of the cathode material and suppress the dissolution of transition metal elements. Those skilled in the art can add a certain amount and configuration of carbon materials to the cathode material to compound the lithium intercalation / deintercalation active material, as long as it does not affect the role of sodium ion additives in the electrolyte and the control of the characteristic peak area ratio of Na element in the electrode sheet.

[0060] In some embodiments, the positive electrode material layer in the positive electrode sheet includes 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 70-98%.

[0061] In some embodiments, the negative electrode material includes at least one of natural graphite, artificial graphite, mesophase carbon microspheres, hard carbon, soft carbon, elemental silicon, silicon suboxide, silicon-carbon composite material, and lithium titanate.

[0062] In some embodiments, the negative electrode material layer in the negative electrode sheet includes a negative electrode material, a binder, and a conductive agent, and the mass percentage of the negative electrode material in the positive electrode material layer is 70-99%.

[0063] The present invention is further illustrated below with specific embodiments, which should not be construed as limiting the scope of protection claimed by the present invention:

[0064] Example 1

[0065] A lithium-ion secondary battery, the preparation method comprising the following steps:

[0066] (1) Preparation of positive electrode sheet: The positive electrode material LiMn after grinding and particle screening is... 0.6 Fe 0.4 PO4, conductive carbon black, and binder polyvinylidene fluoride were dispersed in N-methylpyrrolidone at a mass ratio of 96:1.5:2.5, and a slurry was prepared by vacuum stirring. This slurry was then coated onto the current collector aluminum foil, with a coating density set at 400 g / m². 2 After drying, cold pressing, and slitting, it is priced at 2.2 g / cm³. 3 The positive electrode sheet is obtained by rolling the compaction density.

[0067] (2) Preparation of negative electrode sheet: The negative electrode material artificial graphite, conductive agent carbon black, thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber are dispersed in water at a mass ratio of 96.5:1:1:1.5, and a slurry is prepared by vacuum stirring. The slurry is then coated onto the current collector copper foil, and the coating density is set to 180 g / m². 2 After drying, cold pressing, and slitting, the product is priced at 1.65 g / cm³. 3 The negative electrode sheet is obtained by rolling the compaction density.

[0068] (3) Preparation of electrolyte: EC and EMC were mixed at a mass ratio of 3:7 as solvent. Then, based on the total mass of the electrolyte, 13.5 wt% lithium hexafluorophosphate, sodium ion additive, 1 wt% FEC, 2 wt% MMDS, 1 wt% TMSP, and 2 wt% VC film-forming additive were added and mixed to obtain the electrolyte. The content of sodium ion additive in the electrolyte is shown in Table 1.

[0069] (4) The positive electrode, commercially available PP separator and negative electrode are stacked, wound and assembled into a battery cell in sequence. The battery cell is placed in the outer packaging shell, dried and injected with electrolyte. After vacuum sealing, standing, formation and volume adjustment, the lithium-ion secondary battery is obtained.

[0070] The transformation process includes the following steps:

[0071] (i) Charge the lithium-ion secondary battery at a constant current rate of 0.05C for 2 hours, then reduce the rate to 0.33C and continue charging at a constant current rate for 2 hours.

[0072] The parameters of the lithium-ion secondary battery are shown in Tables 1 and 2, where A (wt%) represents the mass percentage of sodium ion additive in the electrolyte, and D... v 90 (μm) is the particle size corresponding to when the cumulative percentage of the positive electrode material reaches 90%; D v 50 (μm) represents the particle size corresponding to a cumulative percentage distribution of the positive electrode material reaching 50%; it represents D v 10 (μm) is the particle size corresponding to a cumulative distribution percentage of 10% for the positive electrode material. X and Y were confirmed using the above testing methods. Specifically, the XPS etching-test spectrum of the positive electrode sheet tested for X is shown below. Figure 1 As shown, the XPS etching-test spectrum of the negative electrode corresponding to Y is as follows: Figure 2 As shown.

[0073] Examples 2-4

[0074] A lithium-ion secondary battery differs from Example 1 in that the parameters are different, wherein X and Y are controlled by the mass percentage of sodium ion additive in the electrolyte, and W is controlled by the grinding time of the positive electrode material and the sieve used for particle screening.

[0075] Example 5

[0076] A lithium-ion secondary battery differs from Example 1 in that the parameters are different, wherein X and Y are controlled by the formation steps, and W is controlled by the grinding time of the positive electrode material and the sieve used for particle screening.

[0077] The formation step is as follows:

[0078] (ii) Cycle the lithium-ion secondary battery three times at a rate of 0.02C under a working voltage of 2 to 2.5V, then charge it at a constant current rate of 0.05C for 2 hours, then reduce the rate to 0.33C and continue charging at a constant current rate for 2 hours.

[0079] Example 6

[0080] A lithium-ion secondary battery differs from Example 1 in that the parameters are different, wherein X and Y are controlled by the mass percentage of sodium ion additive in the electrolyte, and W is controlled by the grinding time of the positive electrode material and the sieve used for particle screening.

[0081] Example 7

[0082] A lithium-ion secondary battery differs from Example 1 in that the parameters are different, wherein X and Y are controlled by the mass percentage of sodium ion additive in the electrolyte and the formation steps, and W is controlled by the grinding time of the cathode material and the sieve used for particle screening.

[0083] The formation step is as follows:

[0084] (iii) Charge the lithium-ion secondary battery at a constant current rate of 0.05C for 2 hours, then reduce the rate to 0.33C and continue charging at a constant current rate for 2 hours, and finally reduce the rate to 0.1C and continue charging at a constant current rate for 1 hour.

[0085] Examples 8-10

[0086] A lithium-ion secondary battery differs from Example 1 in that the parameters are different, wherein X and Y are controlled by the mass percentage of sodium ion additive in the electrolyte, and W is controlled by the grinding time of the positive electrode material and the sieve used for particle screening.

[0087] Example 11

[0088] A lithium-ion secondary battery differs from Example 1 in that its parameters are different. X and Y are controlled by the mass percentage of sodium ion additive in the electrolyte and the formation steps, while W is controlled by the grinding time of the cathode material and the sieve used for particle screening. The formation steps are as follows:

[0089] (iii) Charge the lithium-ion secondary battery at a constant current rate of 0.05C for 2 hours, then reduce the rate to 0.33C and continue charging at a constant current rate for 2 hours, and finally reduce the rate to 0.1C and continue charging at a constant current rate for 1 hour.

[0090] Example 12

[0091] A lithium-ion secondary battery differs from Example 1 in that the parameters are different, wherein X and Y are controlled by the mass percentage of sodium ion additive in the electrolyte, and W is controlled by the grinding time of the positive electrode material and the sieve used for particle screening.

[0092] Example 13

[0093] A lithium-ion secondary battery differs from Example 1 in that its parameters are different. X and Y are controlled by the mass percentage of sodium ion additive in the electrolyte and the formation steps, while W is controlled by the grinding time of the cathode material and the sieve used for particle screening. The formation steps are as follows:

[0094] (iv) Charge the lithium-ion secondary battery at a constant current rate of 0.02C for 2 hours, then reduce the rate to 0.33C and continue charging at a constant current rate for 2 hours, and finally reduce the rate to 0.1C and continue charging at a constant current rate for 1 hour.

[0095] Examples 14-18

[0096] A lithium-ion secondary battery differs from Example 1 in that the parameters are different, wherein X and Y are controlled by the mass percentage of sodium ion additive in the electrolyte, and W is controlled by the grinding time of the positive electrode material and the sieve used for particle screening.

[0097] Example 19

[0098] A lithium-ion secondary battery differs from Example 1 in that its parameters are different. X and Y are controlled by the mass percentage of sodium ion additive in the electrolyte and the formation steps, while W is controlled by the grinding time of the cathode material and the sieve used for particle screening. The formation steps are as follows:

[0099] (v) Charge the lithium-ion secondary battery at a constant current rate of 0.08C for 2 hours, then reduce the rate to 0.33C and continue charging at a constant current rate for 2 hours, and finally reduce the rate to 0.1C and continue charging at a constant current rate for 1 hour.

[0100] Examples 20-24

[0101] A lithium-ion secondary battery differs from Example 1 in that the parameters are different, wherein X and Y are controlled by the mass percentage of sodium ion additive in the electrolyte, and W is controlled by the grinding time of the positive electrode material and the sieve used for particle screening.

[0102] Example 25

[0103] A lithium-ion secondary battery differs from Example 1 in that the parameters are different; wherein X and Y are controlled by the mass percentage of sodium ion additive in the electrolyte and the formation steps, and W is controlled by the grinding time of the positive electrode material and the sieve used for particle screening.

[0104] The formation step is as follows:

[0105] (ii) Cycle the lithium-ion secondary battery three times at a rate of 0.02C under a working voltage of 2 to 2.5V, then charge it at a constant current rate of 0.05C for 2 hours, then reduce the rate to 0.33C and continue charging at a constant current rate for 2 hours.

[0106] Examples 26-27

[0107] A lithium-ion secondary battery differs from Example 1 in that the parameters are different, wherein X and Y are controlled by the mass percentage of sodium ion additive in the electrolyte, and W is controlled by the grinding time of the positive electrode material and the sieve used for particle screening.

[0108] Example 28

[0109] A lithium-ion secondary battery differs from Example 1 in that the parameters are different, wherein X and Y are controlled by the mass percentage of sodium ion additive in the electrolyte and the formation steps, and W is controlled by the grinding time of the cathode material and the sieve used for particle screening.

[0110] The formation step is as follows:

[0111] (v) Charge the lithium-ion secondary battery at a constant current rate of 0.08C for 2 hours, then reduce the rate to 0.33C and continue charging at a constant current rate for 2 hours, and finally reduce the rate to 0.1C and continue charging at a constant current rate for 1 hour.

[0112] Example 29

[0113] A lithium-ion secondary battery differs from Example 1 in that, during the preparation of the positive electrode sheet, the positive electrode is made of LiFePO4 after grinding and particle screening.

[0114] Comparative Example 1

[0115] A lithium-ion secondary battery differs from Example 1 in that the parameters are different, wherein X and Y are controlled by the mass percentage of sodium ion additive in the electrolyte, and W is controlled by the grinding time of the positive electrode material and the sieve used for particle screening.

[0116] Comparative Example 2

[0117] A lithium-ion secondary battery differs from Example 1 in that the parameters are different, wherein X and Y are controlled by the mass percentage of sodium ion additive in the electrolyte and the formation steps, and W is controlled by the grinding time of the cathode material and the sieve used for particle screening.

[0118] The formation step is as follows:

[0119] (vi) Charge the lithium-ion secondary battery at a 1C rate to 2.5V, then charge it at a 0.33C rate to 4V, and finally cycle it three times at a 0.05C rate under a working voltage of 4 to 4.3V.

[0120] Comparative Example 3

[0121] A lithium-ion secondary battery differs from Example 1 in that the sodium ion additive is replaced with an equal mass of LiODFB.

[0122] Comparative Example 4

[0123] A lithium-ion secondary battery differs from Example 1 in that the parameters are different, wherein X and Y are controlled by the mass percentage of sodium ion additive in the electrolyte and the formation steps, and W is controlled by the grinding time of the cathode material and the sieve used for particle screening.

[0124] The formation step is as follows:

[0125] (ii) Cycle the lithium-ion secondary battery three times at a rate of 0.02C under a working voltage of 2 to 2.5V, then charge it at a constant current rate of 0.05C for 2 hours, then reduce the rate to 0.33C and continue charging at a constant current rate for 2 hours.

[0126] Comparative Example 5

[0127] A lithium-ion secondary battery differs from Example 1 in that the parameters are different, wherein X and Y are controlled by the mass percentage of sodium ion additive in the electrolyte.

[0128] Comparative Example 6

[0129] A lithium-ion secondary battery differs from Example 1 in that the parameters are different, wherein X and Y are controlled by the mass percentage of sodium ion additive in the electrolyte and the formation steps, and W is controlled by the grinding time of the cathode material and the sieve used for particle screening.

[0130] The formation step is as follows:

[0131] (ii) Cycle the lithium-ion secondary battery three times at a rate of 0.02C under a working voltage of 2 to 2.5V, then charge it at a constant current rate of 0.05C for 2 hours, then reduce the rate to 0.33C and continue charging at a constant current rate for 2 hours.

[0132] Comparative Example 7

[0133] A lithium-ion secondary battery differs from Example 1 in that the parameters are different, wherein X and Y are controlled by the mass percentage of sodium ion additive in the electrolyte.

[0134] Table 1

[0135]

[0136]

[0137] Table 2

[0138]

[0139]

[0140] Example of effect

[0141] The sodium-ion secondary batteries obtained in each embodiment and comparative example were tested as follows:

[0142] (1) Cyclic capacity retention test:

[0143] (I) At 45°C, the lithium-ion secondary batteries obtained from each embodiment and comparative example were subjected to three constant-capacity cycles at a rate of 0.33C, and the third discharge capacity was used as the initial capacity C1.

[0144] (II) Charge sequentially at a constant current rate of 1C to the upper limit voltage U. 上限 Subsequently, constant voltage charging is performed until the rate is ≤0.05C; wherein, during constant current charging of the lithium-ion secondary batteries corresponding to Examples 1-28 and Comparative Examples 1-7, U 上限 =4.3V, in Example 29, during constant current charging of the lithium-ion secondary battery, U 上限 =3.65V;

[0145] (III) Let stand for 5 minutes;

[0146] (IV) Discharge at a rate of 1C to a voltage of 2.5V;

[0147] (V) Let stand for 5 minutes;

[0148] (VI) Repeat steps (II) to (V) for 200 charge-discharge cycles, and record the battery capacity C2 after the cycle.

[0149] (VII) Calculate the cycle capacity retention rate % = 100% × C2 / C1. (2) DCR growth rate test:

[0150] (I) The lithium-ion secondary batteries obtained in each embodiment and comparative example were subjected to constant-capacity charging at a rate of 0.33C, i.e., constant-current and constant-voltage charging at 0.33C to the upper limit voltage U. 上限 The current is less than or equal to 0.05C; then discharge at 0.33C to the lower limit voltage of 2.5V, repeat the above supplement 3 times, and take the discharge capacity of the third discharge as the battery discharge capacity. After adjusting the load to 50% SOC by discharging at a rate of 0.33C, let it stand for 2 hours, discharge at 1C for 18 seconds, record the starting discharge voltage as V0, the voltage V1 after the 18-second discharge, and the discharge current I1, and calculate the initial battery internal resistance DCR1=|V0-V1| / I1;

[0151] (II) The lithium-ion secondary battery was completely discharged at 45°C with a constant current of 0.33C to the lower limit voltage of 2.5V, and then discharged at the same temperature from 2.5V to the upper limit voltage U. 上限 After 200 charge-discharge cycles at a 1C rate, charge at a constant current and constant voltage of 0.33C to the upper limit voltage U. 上限The current is less than or equal to 0.05C; then discharge at 0.33C to the lower limit voltage of 2.5V, repeat the above supplement 3 times, and take the discharge capacity of the third discharge as the battery discharge capacity. After adjusting the load to 50% SOC by discharging at a rate of 0.33C, let it stand for 2 hours, discharge at 1C for 18 seconds, record the starting discharge voltage as V'0, the voltage V'1 after the 18-second discharge, and the discharge current I'1, calculate the battery internal resistance DCR2 after the cycle = |V'0-V'1| / I'1;

[0152] (III) Calculate the battery's DCR growth rate % = 100% × (DCR2 - DCR1) / DCR1;

[0153] Examples 1-28 and Comparative Examples 1-7 correspond to the U of lithium-ion secondary batteries. 上限 =4.3V, U corresponding to the lithium-ion secondary battery in Example 29 上限 =3.65V;

[0154] The test results are shown in Table 3.

[0155] Table 3

[0156]

[0157]

[0158] As can be seen from Table 3:

[0159] (1) The lithium-ion secondary battery described in this application introduces sodium ion additives into the electrolyte and simultaneously controls the ratio of the characteristic peak area of ​​sodium in the positive and negative electrode sheets under XPS testing at 30nm etching. This allows the lithium-ion secondary battery to have both electrode stability and kinetic performance during high-temperature cycling. The negative electrode sheet has good passivation effect, low electrode interface impedance, low degree of interface side reaction, and high lithium-ion transport efficiency. After 200 cycles at 1C rate in a high-temperature environment, the capacity retention rate can reach more than 80%, and the DCR growth rate is no higher than 16.5%. In contrast, the lithium-ion secondary battery obtained by replacing the electrolyte with conventional lithium ion additives without introducing sodium ion additives is affected by the severe dissolution of transition metal elements in the positive electrode material in the positive electrode sheet under high-temperature conditions. This not only leads to the loss of active lithium, but also prevents the rapid formation of a thick passivating SEI film on the surface of the negative electrode sheet and the formation of a thin and dense CEI film on the surface of the positive electrode sheet during cycling. Ultimately, it cannot achieve good high-temperature cycling performance. The lithium-ion secondary batteries described in Comparative Examples 1-2 and 4-5 had sodium ion additives added to the electrolyte, but the ratio of X to Y was not well controlled. It can be seen that they still could not achieve high cycle capacity retention and low DCR growth rate under high temperature environment.

[0160] (2) As can be seen from Examples 1 to 11 and Examples 20 to 24, by adjusting the mass percentage of sodium ion additive in the electrolyte and the formation steps, the values ​​of X and Y can be changed, and the range of the X / Y ratio can also be changed. When the X / Y ratio is further optimized to be in the range of 0.21 to 0.75, the sodium ion additive can achieve better results in the lithium-ion secondary battery cycle. The SEI film layer on the surface of the negative electrode sheet provides higher protection for the negative electrode material, making it more difficult for transition metal elements to corrode the negative electrode material. The impedance of the positive electrode sheet is low, and the dynamic performance between the electrodes is excellent. After 200 cycles at 1C rate under high temperature, the capacity retention rate can reach more than 93% and the DCR growth rate is not higher than 11.5%.

[0161] (3) As can be seen from Examples 12-19 and Comparative Examples 6-7, the control of X / Y of the positive and negative electrode plates described in this application is not limited to a lithium-ion secondary battery under a sodium-ion additive system. This limitation applies to different sodium-ion additive systems. If X / Y is not in the range of 0.1-0.95, the lithium-ion secondary battery cannot achieve good cycle stability and conductivity at high temperature. If X / Y is further preferably in the range of 0.21-0.75, the performance of the lithium-ion secondary battery is better.

[0162] (4) As can be seen from Examples 1 and 20-28, the particle size distribution W of the cathode material is related to the wettability of the cathode material in the electrolyte and the dissolution efficiency of transition metals. When W / X is preferably 1×10 -3 ~7×10 -3 Within a certain range, the cathode material impregnated with sodium ion additives can reduce the dissolution frequency of transition metal elements at high temperatures and decrease the probability of additional side reactions in the electrolyte while maintaining high ion transport efficiency, resulting in better electrochemical performance of lithium-ion secondary batteries.

Claims

1. A lithium-ion secondary battery, characterized in that, Includes positive electrode, negative electrode, and electrolyte; The electrolyte contains sodium ion additives, solvents, and lithium salts; The sodium ion additive includes at least one of sodium difluorooxalate borate, sodium difluorosulfonamide, sodium difluoromethylsulfonamide, and sodium difluorophosphate; the solvent includes at least one of carbonate solvents, carboxylic acid ester solvents, ether solvents, sulfone solvents, nitrile solvents, and phosphate ester solvents. The positive electrode includes a current collector and a positive electrode material layer, and the negative electrode includes a current collector and a negative electrode material layer; The positive electrode material in the positive electrode material layer includes at least one of lithium manganese iron phosphate, lithium iron phosphate, doped lithium manganese iron phosphate, and doped lithium iron phosphate, wherein the doping element in the doped lithium manganese iron phosphate and doped lithium iron phosphate includes at least one of V, W, Ti, and Mg; the negative electrode material in the negative electrode material layer includes at least one of natural graphite, artificial graphite, mesophase carbon microspheres, hard carbon, soft carbon, elemental silicon, silicon suboxide, silicon-carbon composite material, and lithium titanate. The lithium-ion secondary battery satisfies: 0.1≤X / Y≤0.95; Where X is the characteristic peak area of ​​Na element in the positive electrode sheet under XPS testing with an XPS etching depth of 30 nm on the side of the positive electrode material layer away from the current collector; Y is the characteristic peak area of ​​Na element in the negative electrode sheet under XPS testing with an XPS etching depth of 30 nm on the side of the negative electrode material layer away from the current collector; X = 1000~19000 CPS.eV, and Y = 11000~25000 CPS.eV.

2. The lithium-ion secondary battery as described in claim 1, characterized in that, The lithium-ion secondary battery satisfies the following condition: 0.21≤X / Y≤0.

75.

3. The lithium-ion secondary battery as described in claim 1, characterized in that, The positive electrode material layer contains a positive electrode material, and the lithium-ion secondary battery satisfies: 0.05 × 10⁻⁶ -3 ≤W / X≤7.5×10 -3 Where W = (D v 90-D v 10) / D v 50, D v 90 represents the particle size corresponding to a cumulative distribution percentage of 90% for the positive electrode material; D v 50 represents the particle size corresponding to a cumulative percentage distribution of the positive electrode material reaching 50%; D v 10 represents the particle size corresponding to a cumulative distribution percentage of 10% for the positive electrode material; the D of the positive electrode material v 90. D v 50 and D v 10. The following test method was used to confirm the following: The lithium-ion secondary battery was disassembled, and the positive electrode sheet was immersed in dimethyl carbonate for 60 minutes. After being removed and dried, the positive electrode material layer on the positive electrode sheet was scraped off. The resulting powder was coated on conductive adhesive and observed under a scanning electron microscope at a magnification of 30,000. The size of the positive electrode material particles was measured using MEARSURE NANO software. The size of the positive electrode material particles was collected using the diagonal line method. After collecting more than 200 samples, the particle size distribution was statistically analyzed, and the particle size-related parameters of the positive electrode material were calculated.

4. The lithium-ion secondary battery as described in claim 3, characterized in that, The value of W is 0.6~16.

5. The lithium-ion secondary battery as described in claim 1, characterized in that, The sodium ion additive has a mass percentage content of 0.05~1.2% in the electrolyte.

6. The lithium-ion secondary battery as described in claim 1, characterized in that, The lithium salt includes at least one of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetrafluoroborate, lithium dioxarate borate, lithium difluorooxarate borate, lithium trifluoromethanesulfonate, lithium difluoromethanesulfonylimide, lithium ditrifluoromethanesulfonylimide, lithium difluorodioxarate phosphate, and lithium tetrafluorooxarate phosphate.

7. An electrical device, characterized in that, The device includes the lithium-ion secondary battery as described in any one of claims 1 to 6, wherein the lithium-ion secondary battery serves as the power supply for the electrical device.

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