Positive electrode additives, positive electrode sheets and their preparation methods, and electrochemical devices

Abstract

This application discloses a positive electrode additive, a positive electrode sheet, a preparation method thereof, and an electrochemical device. The positive electrode additive includes the structure shown in Formula I. When the positive electrode additive in this application is applied to the preparation of the positive electrode sheet, it can significantly improve the adhesion between the positive electrode active material layer and the positive electrode current collector, as well as the cohesive force between the positive electrode active materials. Furthermore, the positive electrode additive can effectively improve the electronic conductivity network of the positive electrode active material layer, reduce the film resistance, and improve the cycle life of the lithium-ion battery.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to a positive electrode additive, a positive electrode sheet, a method for preparing the same, and an electrochemical device. Background Technology

[0002] Lithium-ion batteries are widely used in 3C consumer products and power fields due to their long cycle life and high energy density. With the rapid development of mobile electronic devices and new energy vehicles, people have increasingly higher demands for the dynamic performance and cycle life of lithium-ion batteries.

[0003] Research on the positive electrode is crucial for improving the performance of lithium-ion batteries. Polyvinylidene fluoride (PVDF) is commonly used as a binder for the positive electrode of lithium-ion batteries to improve the adhesion of the positive electrode sheet. Summary of the Invention

[0004] The researchers in this application found that the bonding strength of the positive electrode sheet prepared using PVDF binder is still relatively weak, which adversely affects the cycle life of lithium-ion batteries. To avoid electrode powder shedding and film detachment problems, more PVDF can be used in the positive electrode sheet to improve its bonding strength. However, increasing the amount of PVDF added will reduce the energy density of the lithium-ion battery.

[0005] In view of this, this application provides a positive electrode additive, a positive electrode sheet, a method for preparing the same, and an electrochemical device. The positive electrode additive in this application can improve the adhesion between the active material layer and the current collector, increase the electronic conductivity of the electrode sheet, thereby reducing the film resistance of the electrode sheet, and simultaneously improve the cycle life of the lithium-ion battery.

[0006] In a first aspect, this application provides a positive electrode additive comprising at least one of the compounds shown in Formula I:

[0007]

[0008] In Formula I, R1 includes the groups shown in Formula a and / or Formula b, and R2 includes the groups shown in Formula c;

[0009]

[0010]

[0011] Wherein, * represents the connection end. The positive electrode additive in this application is a three-dimensional network macromolecular positive electrode additive containing benzene rings and triazine ring structures. This positive electrode additive has high adhesion, especially beneficial for forming covalent bonds with hydrated hydroxyl groups on the surface of aluminum foil substrate, thereby improving the adhesion between the active material layer and the current collector. At the same time, the positive electrode additive shown in Formula I in this application can effectively improve the electron transport network of the active material layer, reduce film resistance, and improve the cycle life of lithium-ion batteries.

[0012] In some embodiments, the number-average molecular weight of the cathode additive shown in Formula I is 600,000 to 800,000. This can further improve the adhesion between the active material layer and the current collector, as well as the electronic conductivity of the electrode, and reduce the film resistance of the electrode, thereby further improving the cycle life of the lithium-ion battery.

[0013] Secondly, this application provides a positive electrode sheet, which includes a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector. The positive active material layer includes a positive active material, substance A, and the aforementioned positive additive. The positive additive represented by Formula I in this application works synergistically with the positive active material and substance A as a binder to improve the cohesive force of the positive electrode sheet, alleviate the problem of significant rebound after cold pressing, and increase the compaction density (PD), thereby further improving the energy density and cycle life of the electrochemical device.

[0014] In some embodiments, substance A contains element F. Meeting the above conditions facilitates the formation of dense hydrogen bonds between the amide groups in the positive electrode additive shown in Formula I and the element F in the binder, resulting in synergistic effects that enhance the cohesion of the positive electrode active material layer. This helps reduce electrode rebound after cold pressing, further improving PD and cycle life. Preferably, substance A comprises polyvinylidene fluoride. More preferably, substance A is polyvinylidene fluoride.

[0015] In some embodiments, the positive electrode active material contains Co. The carbonyl group in the positive electrode additive structure of Formula I can form strong coordination and chelation with the Co element in the positive electrode active material, thereby further improving PD and cycle life. In some embodiments, the positive electrode active material contains T, which includes at least one of Ni, Mn, Al, Mg, Ti, Zr, La, Y, Mo, V, Cu, Zn, Ga, Nb, Cr, Ba, W, Ru, Ag, Sn, Au, Ce, Pr, Nd, Sm, Gd, or Ca. Satisfying the above conditions can further improve PD and cycle life. Preferably, the positive electrode active material includes lithium cobalt oxide and / or lithium nickel cobalt manganese oxide.

[0016] In some embodiments, the mass percentage of the positive electrode additive shown in Formula I is 0.05 wt% to 0.5 wt%, based on the mass of the positive electrode active material layer. Preferably, the mass percentage of the positive electrode additive shown in Formula I is 0.25 wt% to 0.5 wt%. By further adjusting the content of the positive electrode additive shown in Formula I in the positive electrode active material layer, the viscosity of the positive electrode slurry is controlled, resulting in a lower membrane resistance of the prepared positive electrode active material layer, which can further improve PD and cycle life.

[0017] In some implementations, based on the positive electrode active material, the molar amount of Co is denoted as m1, and the molar amount of T is denoted as m2, satisfying: 90% ≤ m1 / (m 1+ m2)≤100%. At this point, the synergistic effect between the positive electrode additive and the positive electrode active material shown in Formula I can be better utilized, which is conducive to the positive electrode active material layer obtaining higher cohesion, thereby improving PD and cycle life.

[0018] In some embodiments, the adhesion force between the positive electrode active material layer and the positive electrode current collector is 35 N / m to 65 N / m. At this point, the positive electrode exhibits good structural stability, which is beneficial for improving the cycle performance of the lithium-ion battery.

[0019] In some embodiments, the cohesive force of the positive electrode active material layer is 70 N / m to 100 N / m. This is advantageous for achieving high power density (PD), further improving the energy density of the lithium-ion battery.

[0020] In some embodiments, the film resistance of the positive electrode is 0.2Ω to 0.3Ω. The positive electrode additive shown in Formula I of this application can effectively improve the electronic conductivity of the electrode conductive network.

[0021] Thirdly, this application provides a method for preparing the above-mentioned positive electrode sheet, including the following steps:

[0022] The positive electrode active material, substance A, conductive agent, and positive electrode additive shown in Formula I are stirred and mixed in a non-aqueous solution to obtain a positive electrode slurry. The solid content of the positive electrode slurry is 65 wt% to 75 wt%, and the viscosity is 3000 mPa·s to 8000 mPa·s. Based on the solid content of the positive electrode slurry, the mass ratio of the positive electrode active material, substance A, conductive agent, and positive electrode additive is (95.5–96):(1.8–2.2):(1.8–2.2):(0.05–0.5). Meeting the above conditions can further improve the PD and cycle life.

[0023] Fourthly, this application provides an electrochemical device comprising the above-described positive electrode; and / or comprising a positive electrode prepared by the above-described preparation method.

[0024] The beneficial effects of the technical solutions provided by some embodiments of this application include at least the following: This application provides a positive electrode additive with the structure shown in Formula I. The special structure of the positive electrode additive gives it high adhesion. It can form covalent bonds with the hydrated hydroxyl groups on the surface of the aluminum foil substrate, thereby improving the adhesion between the active material layer and the current collector. Moreover, the addition of the positive electrode additive is beneficial to improving the electronic conductivity network of the positive electrode active material layer, reducing film resistance, increasing the energy density of the lithium-ion battery, and improving the cycle performance of the electrochemical device. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0026] Positive electrode additives

[0027] The cathode additive in this application includes at least one of the compounds shown in Formula I:

[0028]

[0029] In Formula I, R1 includes the groups shown in Formula a and / or Formula b, and R2 includes the groups shown in Formula c;

[0030]

[0031]

[0032] Where * represents the connection end.

[0033] In some embodiments, the number-average molecular weight of the cathode additive shown in Formula I is between 600,000 and 800,000. Exemplarily, the molecular weight of the cathode additive shown in Formula I is 600,000, 650,000, 700,000, 750,000, 800,000, or a range consisting of any two of the above values.

[0034] Positive electrode sheet

[0035] In some embodiments, the positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector. The positive active material layer includes a positive active material, substance A, and the aforementioned positive additive, wherein substance A is a binder.

[0036] For example, the positive current collector can be made of materials such as metal foil or porous metal plate, for example, foil or porous plate of metals or alloys of metals such as aluminum, copper, nickel, titanium or iron, such as Al (aluminum) foil.

[0037] In some embodiments, substance A contains element F. For example, substance A includes polyvinylidene fluoride.

[0038] In some embodiments, the positive electrode active material contains Co and optionally T, wherein T includes at least one selected from Ni, Mn, Al, Mg, Ti, Zr, La, Y, Mo, V, Cu, Zn, Ga, Nb, Cr, Ba, W, Ru, Ag, Sn, Au, Ce, Pr, Nd, Sm, Gd, or Ca. Exemplarily, the positive electrode active material includes one or more of lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and compounds obtained by adding other transition metals or non-transition metals to the above compounds.

[0039] In some embodiments, the positive electrode active material includes lithium cobalt oxide and / or lithium nickel cobalt manganese oxide. Specifically, in some embodiments, the positive electrode active material includes lithium cobalt oxide; in some embodiments, the positive electrode active material includes lithium nickel cobalt manganese oxide; and in some embodiments, the positive electrode active material includes both lithium cobalt oxide and lithium nickel cobalt manganese oxide.

[0040] In some embodiments, based on the mass of the positive electrode active material layer, the mass percentage content of the positive electrode additive shown in Formula I is from 0.05 wt% to 0.5 wt%. Exemplarily, the mass percentage content of the positive electrode additive shown in Formula I can be 0.05 wt%, 0.08 wt%, 0.1 wt%, 0.15 wt%, 0.2 wt%, 0.25 wt%, 0.3 wt%, 0.35 wt%, 0.4 wt%, 0.45 wt%, 0.5 wt%, or a range consisting of any two of the above values.

[0041] In some embodiments, based on the positive electrode active material, the molar amount of Co is denoted as m1, and the molar amount of T is denoted as m2, satisfying: 90% ≤ m1 / (m 1+ m2)≤100%.

[0042] In some embodiments, the adhesion force between the positive electrode active material layer and the positive electrode current collector is 35 N / m to 65 N / m. Exemplarily, the adhesion force between the positive electrode active material layer and the positive electrode current collector can be 35 N / m, 40 N / m, 45 N / m, 50 N / m, 55 N / m, 60 N / m, 65 N / m, or any range of two of the above values.

[0043] In some embodiments, the cohesive force of the positive electrode active material layer can be from 70 N / m to 100 N / m. Exemplarily, the cohesive force of the positive electrode active material layer is 70 N / m, 75 N / m, 80 N / m, 85 N / m, 90 N / m, 95 N / m, 100 N / m, or a range consisting of any two of the above values.

[0044] In some embodiments, the film resistance of the positive electrode is 0.2Ω to 0.3Ω. Exemplarily, the film resistance of the positive electrode active material layer can be 0.2Ω, 0.22Ω, 0.24Ω, 0.25Ω, 0.26Ω, 0.28Ω, 0.3Ω, or any combination of two of the above values.

[0045] Preparation method of positive electrode sheet

[0046] The preparation method of the positive electrode sheet includes at least the following steps:

[0047] The positive electrode active material, substance A, conductive agent and positive electrode additive shown in Formula I are dispersed in a non-aqueous solution and stirred to obtain a positive electrode slurry. The solid content of the positive electrode slurry is 65wt% to 75wt%, and the viscosity of the positive electrode slurry is 3000mPa.s to 8000mPa.s.

[0048] For example, the solid content of the positive electrode slurry can be 65wt%, 68wt%, 70wt%, 72wt%, 73wt%, 75wt%, or any combination of two of the above values.

[0049] For example, the viscosity of the positive electrode slurry can be 3000 mPa.s, 3500 mPa.s, 4000 mPa.s, 4500 mPa.s, 5000 mPa.s, 5500 mPa.s, 6000 mPa.s, 6500 mPa.s, 7000 mPa.s, 7500 mPa.s, 8000 mPa.s, or any two of the above values.

[0050] Based on the solid content in the positive electrode slurry, the mass ratio of the positive electrode active material, substance A, conductive agent and positive electrode additive shown in Formula I is (95.5~96):(1.8~2.2):(1.8~2.2):(0.05~0.5).

[0051] The above-mentioned positive electrode slurry is uniformly coated on one surface of the positive electrode current collector and dried to obtain a positive electrode sheet with one side coated with positive electrode active material. Then, the above operation steps are repeated on the other surface of the positive electrode current collector to obtain a positive electrode sheet with positive electrode active material coated on both sides.

[0052] other

[0053] The negative electrode sheet can be a lithium metal sheet, or it can include a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector.

[0054] The negative electrode active material layer typically includes the negative electrode active material and optional conductive agents and binders.

[0055] Exemplary examples include one or more of natural graphite, artificial graphite, mesophase microcarbon spheres (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composites, SiO, Li-Sn alloys, Li-Sn-O alloys, Li-Al alloys, or metallic lithium; conductive agents include one or more of conductive carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers; and binders include one or more of styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl butyral (PVB), water-based acrylic resin, or carboxymethyl cellulose (CMC). However, this application is not limited to these materials, and other materials that can be used as negative electrode active materials, conductive agents, and binders in lithium-ion batteries may also be used.

[0056] For example, the negative current collector can be made of materials such as metal foil or porous metal plate, for example, using foil or porous plate of metals or alloys of them such as copper, nickel, titanium or iron, such as copper foil.

[0057] The negative electrode sheet can be prepared according to conventional methods in the art. For example, the negative electrode active material and optional conductive agent and binder are dispersed in a solvent, which may be N-methylpyrrolidone (NMP) or deionized water, to form a uniform negative electrode slurry. The negative electrode slurry is coated on a negative electrode current collector, and the negative electrode sheet is obtained by processes such as drying and cold pressing.

[0058] There are no particular restrictions on the separator membrane. Any well-known porous separator membrane with electrochemical and chemical stability can be selected, such as one or more single-layer or multi-layer films made of glass fiber, non-woven fabric, polyethylene (PE), polypropylene (PP), and polyvinylidene fluoride (PVDF).

[0059] The electrolyte includes organic solvents, lithium electrolyte salts, and additives. This application does not impose specific limitations on its types; selection can be made according to actual needs.

[0060] For example, the organic solvents mentioned above include one or more of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butenyl carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), or diethyl sulfone (ESE), preferably two or more.

[0061] For example, the above-mentioned electrolyte lithium salt includes one or more of LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiFSI (lithium bisfluorosulfonylimide), LiTFSI (lithium bistrifluoromethanesulfonylimide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluorooxalate borate), LiBOB (lithium dioxalate borate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluorodioxalate phosphate), or LiTFOP (lithium tetrafluorooxalate phosphate).

[0062] The electrolyte may optionally include other additives, which can be any additive that can be used in lithium-ion secondary batteries. This invention does not impose specific limitations and can select additives according to actual needs. As an example, the additives may be one or more of the following: vinylene carbonate (VC), ethylene ethylene carbonate (VEC), succinate (SN), adiponitrile (ADN), 1,3-propenesulfonate lactone (PST), tris(trimethylsilane) phosphate (TMSP), or tris(trimethylsilane) borate (TMSB).

[0063] Electrochemical device

[0064] The electrochemical device of this application can include any device in which an electrochemical reaction occurs, and specific examples include all types of primary or secondary batteries. In particular, the electrochemical device is a lithium secondary battery, including lithium metal secondary batteries, lithium-ion secondary batteries, lithium polymer secondary batteries, or lithium-ion polymer secondary batteries.

[0065] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Unless otherwise stated, all parts, percentages and ratios listed below are by weight, and all raw materials used are commercially available or synthesized by conventional methods.

[0066] In some embodiments, the preparation method of the positive electrode additive shown in Formula I includes: mixing compound a and / or compound b with compound c (containing the group shown in Formula c), adding an organic solvent, placing the mixture in an oil bath environment, stirring at 550 rpm to 650 rpm for 68 h to 74 h at a temperature of 160 °C to 200 °C, washing, and drying to obtain the additive. The product obtained by reacting compound a with compound c has the group shown in Formula a, and the product obtained by reacting compound b with compound c has the group shown in Formula b. Specific parameters and the specific substances represented by compounds a, b, and c in each embodiment are detailed in Table 1 and their corresponding descriptions.

[0067] Example 1-1

[0068] Preparation of the positive electrode additive shown in Formula I

[0069] 1 mol of 1,3,5-benzenetricarboxylic acid (compound a) and 1 mol of 1,3,5-triaminobenzene (compound c) were added to a 2000 mL three-necked round-bottom flask, followed by the addition of 1000 mL of dimethyl sulfoxide. The mixture was placed in an oil bath at 180 °C and reacted with mechanical stirring at 600 rpm for 72 h. After the reaction, the mixture was washed sequentially with methanol and dimethyl sulfoxide. Finally, the resulting solid powder was dried in a vacuum drying oven at 60 °C for 24 h to obtain the positive electrode additive shown in Formula I.

[0070] Preparation of positive electrode sheet

[0071] LiCoO2 (positive electrode active material), PVDF (substance A), conductive carbon black (conductive agent), and the positive electrode additive prepared above were dispersed in NMP solvent and stirred until homogeneous to obtain a positive electrode slurry with a solid content of 70 wt% and a viscosity of 3000 mPa·s. The mass ratio of LiCoO2, PVDF, conductive carbon black, and positive electrode additive in the solid components was 95.95:2:2:0.05 (the sum of the mass percentages of the positive electrode active material and the positive electrode additive was 96%). The above positive electrode slurry was uniformly coated onto one surface of a 9 μm thick aluminum foil current collector and dried at 120°C to obtain a single-sided positive electrode sheet with a 75 μm thick positive electrode active material layer. The above steps were then repeated on the other surface of the same aluminum foil to obtain a double-sided positive electrode sheet. The coated positive electrode sheet was cold-pressed and then cut into sheets with a size of 70 mm × 800 mm for later use. Among them, the weight-average molecular weight Mw of PVDF is 80w to 110w, and the number-average molecular weight Mn is 40w to 60w.

[0072] Preparation of negative electrode sheet

[0073] Graphite, styrene-butadiene rubber, and sodium carboxymethyl cellulose (CMC) were mixed in a mass ratio of 97.6:1.1:1.3, and deionized water was added as a solvent. After stirring evenly, a negative electrode slurry with a solid content of 70 wt% was obtained. The negative electrode slurry was uniformly coated onto one surface of a 6 μm thick copper foil current collector and dried at 120°C to obtain a single-sided negative electrode sheet with a 110 μm thick negative electrode active material layer. The above steps were repeated on the other surface of the copper foil to obtain a double-sided negative electrode sheet with a negative electrode active material layer. The coated negative electrode sheet was cold-pressed and then cut into sheets with a size of 74 mm × 800 mm for later use.

[0074] Preparation of electrolyte

[0075] In a glove box filled with a dry argon atmosphere, organic solvents propylene carbonate (PC), diethyl carbonate (DEC), and ethylene carbonate (EC) were mixed in a mass ratio of 1:1:1. Then, lithium hexafluorophosphate (LiPF6) was added to the organic solvents to dissolve and mix thoroughly to obtain an electrolyte with a LiPF6 concentration of 1.15 mol / L.

[0076] Preparation of the separating membrane

[0077] A porous polyethylene film with a thickness of 16 μm was used as the separator.

[0078] Preparation of lithium-ion batteries

[0079] The prepared positive electrode, separator, and negative electrode are stacked sequentially and then wound to obtain an electrode assembly. After the tabs are welded, the electrode assembly is placed in an aluminum-plastic film and dried in a vacuum oven at 80°C for 12 hours to remove moisture. Then, the prepared electrolyte is injected, and the battery undergoes vacuum sealing, settling, formation (0.02C constant current charging to 3.5V, then 0.1C constant current charging to 3.9V), capacity testing (0.5C DC discharge to 3.0V), and shaping processes to obtain a lithium-ion battery. Unless otherwise specified, all preparation methods in this application can be referenced from conventional techniques in the field.

[0080] Test method:

[0081] (1) Viscosity test

[0082] The viscosity of the cathode slurry was tested using a digital rotational viscometer (Shanghai Jingtian Electronic Instruments Co., Ltd., LVDV1). A suitable rotor and rotation speed were selected based on the slurry being measured. The rotor was slowly inserted into the slurry for immersion. When the slurry level was in the center of the rotor groove, the rotational viscometer was started. After two minutes, when the reading remained unchanged, the reading was recorded as the viscosity of the prepared cathode slurry, in mPa·s.

[0083] (2) Adhesion test

[0084] Take the cold-pressed positive electrode sheet and punch it using a mold to obtain a test strip with a length of 100mm and a width of 20mm. Clean the surface of the steel plate with alcohol, and attach double-sided tape (NITTO.NO5000NS) with a length of 55mm to 70mm and a width of 20mm to the steel plate, ensuring no air bubbles are formed. Place the test strip centered on the double-sided tape, with the test side facing down. Use crepe tape (high-tack masking tape) to connect and fix a paper strip with a length of 50mm to 75mm and a width equal to the test strip to one end of the test strip. Manually push a 2kg rubber roller back and forth on the test strip 4 times to obtain the test sample. Test the sample using a tensile testing machine (Instron 3365). The test sample is fixed on the test stage, then the paper tape is folded upwards at 90° and secured with a clamp. The tensile testing machine then slowly pulls the paper tape at a speed of 10 mm / min until the positive electrode active material layer on the double-sided adhesive surface separates from the positive electrode current collector, ending the test. The average tensile force in the stable region is recorded as the adhesive force between the positive electrode active material layer and the positive electrode current collector, expressed in N / m.

[0085] (3) Cohesion test

[0086] Take the cold-pressed positive electrode sheet and punch it using a mold to obtain a test strip with a length of 80mm and a width of 20mm. Clean the surface of the steel plate with alcohol, and attach a 50mm to 60mm long and 20mm wide double-sided adhesive tape (NITTO.NO5000NS) to the steel plate, ensuring no air bubbles are formed. Attach the test strip to the double-sided adhesive tape, with the test side facing up. Attach a piece of cohesion-specific green adhesive (Todi adhesive paper, 20mm wide * 80mm long) centered on the test strip. Cut a 60mm long * 20mm wide paper strip and insert it into the gap between the test strip and the green adhesive, overlapping it by approximately 15mm. Push a 2kg rubber roller back and forth on the test strip 4 times by hand to obtain the test sample. Test the sample using a tensile testing machine (Instron 3365). The test sample is fixed on the test stage, then the paper strip is folded upwards 180° and secured with a clamp. The tensile testing machine then slowly pulls the paper strip at a speed of 10 mm / min until the green adhesive separates from the active material layer on the positive electrode surface, at which point the test ends. The average tensile force in the stable region is recorded as the cohesive force between the positive electrode active materials, expressed in N / m.

[0087] (4) PD calculation

[0088] After capacity testing, take the battery cell electrode sheets and use a stamping die to cut 12 small round pieces (1540.25 mm² in area). 2After zeroing the electronic scale, use tweezers to place the cut small round pieces onto the weighing platform and record the weight of each small round piece: m1, m2, m3, m4, m5, m6, m7, m8, m9, m 10 m 11 m 12 Use a micrometer to measure the thickness of each small disc at four positions: top, bottom, left, and right: h 11 h 12 h 13 h 14 h 21 h 22 h 23 h 24 h 31 h 32 h 33 h 34 h 41 h 42 h 43 h 44 h 51 h 52 h 53 h 54 h 61 h 62 h 63 h 64 h 71 h 72 h 73 h 74 h 81 h 82 h 83 h 84 h 91 h 92 h 93 h 94 h 101 h 102 h 103 h 104 h 111 h 112 h 113 h 114 h 121 h 122 h 123 h 124 The average thickness of each small circular piece is calculated as h1, h2, h3, h4, h5, h6, h7, h8, h9, h1. 10 h 11 h 12 The formula is used to calculate the value of each small disc. The average value of the 12 small circular PDs is recorded as the PD for each embodiment or comparative example.

[0089] (5) Diaphragm resistance test

[0090] The resistance of the electrode film after cold pressing was tested using the Yuaneng Technology electrode resistance meter (BER2500). Before use, the resistance and pressure were reset. The electrode was placed under the probe for testing, and the average value of the resistance measured at 12 different positions was recorded as the resistance value of the film.

[0091] (6) Cyclic performance test

[0092] Cycle performance was evaluated by the capacity retention rate of lithium-ion batteries. After formation, the lithium-ion batteries were placed in a constant-temperature environment at 25°C and charged at a constant current of 0.6C to 4.5V, then charged at a constant voltage to the cutoff current of 0.05C. After a 3-minute rest period following full charge, they were discharged at 0.5C to 3.0V, and the discharge capacity was recorded as D0. Cycling tests were then conducted using a 0.6C charge / 0.5C discharge cycle for 500 cycles. The discharge capacity after the 500th cycle was recorded as D1. The capacity retention rate (%) of the lithium-ion battery after 500 cycles at room temperature (25°C) is calculated as D1 / D0 × 100%.

[0093] Examples 1-2 to 1-25

[0094] Unlike Example 1-1, some parameters of the positive electrode additive and the positive electrode sheet in the preparation process were adjusted, as detailed in Table 1. The rest are the same as in Example 1-1.

[0095] Comparative Example 1

[0096] Unlike Example 1-1, no additives were used in the preparation of the positive electrode slurry, and the mass percentage of the positive electrode active material was 96%, while the rest was the same as in Example 1-1.

[0097] Comparative Example 2

[0098] Unlike Example 1-1, the additive used in the preparation of the positive electrode slurry is phenolic resin (molecular weight ~200,000), and the rest is the same as in Example 1-1.

[0099] The preparation and performance parameters of each embodiment and comparative example are shown in Table 1 and Table 2.

[0100] Table 1

[0101]

[0102] Table 2

[0103]

[0104] By comparing Examples 1-1 and Comparative Examples 1 and 2 with Tables 1 and 2, it can be seen that when the positive electrode sheet of the lithium-ion battery in this application contains the positive electrode additive described in this application, the adhesion between the positive electrode active material layer and the current collector is relatively large, reaching over 35 N / m. Furthermore, the cohesive force between the positive electrode active materials with the positive electrode additive shown in Formula I is relatively large, resulting in higher PD and cycle capacity retention. Examples 1-2 to 1-25 further verify that selecting the R1 group, the content of the positive electrode additive, and the molecular weight of the positive electrode additive within a suitable range in Formula I can achieve good results. In particular, Examples 1-10 show that the adhesion between the positive electrode active material layer and the current collector reaches as high as 65 N / m, the cohesive force between the positive electrode active materials reaches 100 N / m, the PD can reach 4.10 g / cc, and the cycle capacity retention rate after 500 cycles is 92.8%. It is evident that applying the positive electrode additive of this application to the positive electrode sheet can ensure good adhesion between the positive electrode active material layer and the current collector, as well as good cohesion between the positive electrode active materials. The lithium-ion battery made using the positive electrode sheet provided in this application has higher PD and better cycle performance.

[0105] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A positive electrode additive, characterized in that, The positive electrode additive includes at least one of the compounds shown in Formula I: Formula I; In Formula I, R1 is the group shown in Formula a and / or Formula b, and R2 is the group shown in Formula c; Formula a; Formula b; Formula c; Where * represents the connection end.

2. The positive electrode additive according to claim 1, characterized in that, The number-average molecular weight of the positive electrode additive is 600,000 to 800,000.

3. A positive electrode sheet, characterized in that, The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector; The positive electrode active material layer comprises a positive electrode active material, substance A, and a positive electrode additive as described in any one of claims 1 to 2, and satisfies at least one of the following conditions: (1) Substance A contains element F; (2) The positive electrode active material contains Co and optional T, wherein the T includes at least one of Ni, Mn, Al, Mg, Ti, Zr, La, Y, Mo, V, Cu, Zn, Ga, Nb, Cr, Ba, W, Ru, Ag, Sn, Au, Ce, Pr, Nd, Sm, Gd or Ca.

4. The positive electrode sheet according to claim 3, characterized in that, The positive electrode active material includes lithium cobalt oxide and / or lithium nickel cobalt manganese oxide; Substance A includes polyvinylidene fluoride.

5. The positive electrode sheet according to claim 3, characterized in that, Based on the mass of the positive electrode active material layer, the mass percentage of the positive electrode additive is 0.05wt% to 0.5wt%.

6. The positive electrode sheet according to claim 5, characterized in that, Based on the mass of the positive electrode active material layer, the mass percentage of the positive electrode additive is 0.25 wt% to 0.5 wt%.

7. The positive electrode sheet according to claim 3, characterized in that, Based on the aforementioned positive electrode active material, the molar amount of Co is denoted as m1, and the molar amount of T is denoted as m2, satisfying: 90% ≤ m1 / (m 1+ m2)≤100%.

8. The positive electrode sheet according to claim 3, characterized in that, The positive electrode sheet satisfies at least one of the following conditions: (1) The bonding force between the positive electrode active material layer and the positive electrode current collector is 35 N / m to 65 N / m; (2) The cohesive force of the positive electrode active material layer is 70 N / m to 100 N / m; (3) The film resistance of the positive electrode is 0.2Ω to 0.3Ω.

9. A method for preparing a positive electrode sheet as described in any one of claims 3 to 8, characterized in that, The preparation method includes the following steps: The positive electrode active material, substance A, conductive agent and positive electrode additive are stirred and mixed in a non-aqueous solution to obtain a positive electrode slurry; The positive electrode slurry has a solid content of 65wt% to 75wt% and a viscosity of 3000mPa·s to 8000mPa·s. Based on the solid content of the positive electrode slurry, the mass ratio of the positive electrode active material, substance A, the conductive agent, and the positive electrode additive is (95.5~96):(1.8~2.2):(1.8~2.2):(0.05~0.5).

10. An electrochemical device, characterized in that, The electrochemical device includes a positive electrode as described in any one of claims 3 to 8; and / or, The electrochemical device includes a positive electrode sheet prepared by the preparation method as described in claim 9.

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