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 method for preparing the same, and an electrochemical device. The positive electrode additive includes at least one compound as shown in Formula I, where the R group comprises a saturated alkyl chain containing an ethylene glycol group. The positive electrode additive in this application can improve the flexibility and ionic conductivity of the positive electrode sheet, reduce the charge transfer resistance, increase the electrode sheet compaction density (PD), and thus improve the energy density 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] Polyvinylidene fluoride (PVDF) is commonly used as a binder for the positive electrode in lithium-ion batteries. However, positive electrode sheets prepared using PVDF have poor ionic conductivity and are relatively hard and brittle, posing a risk of breakage during winding. Furthermore, the low compaction density (PD) of the positive electrode sheet significantly reduces the energy density of the lithium-ion battery.

[0003] Currently, to address the poor ionic conductivity of positive electrode sheets, existing technologies typically add solid electrolytes to improve ionic conductivity. To avoid the risk of brittle breakage during winding due to hardness and brittleness, existing technologies usually increase the residual amount of N-methylpyrrolidone (NMP) in the electrode sheet to solve the hardness and brittleness problem. To achieve higher energy density, existing technologies typically reduce the amount of inactive materials added to the electrode sheet. However, solid electrolytes have strict environmental requirements and are prone to failure during use. Excessive residual NMP in the electrode sheet can cause problems such as gas expansion and abnormal thickness in the cell system. Furthermore, reducing the amount of inactive materials added to the electrode sheet reduces the electronic conductivity of the electrode sheet and the adhesion between the active material and the substrate. Summary of the Invention

[0004] 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. When the positive electrode additive in this application is applied to the preparation of the positive electrode sheet, it can not only improve the ionic conductivity of the positive electrode sheet, but also significantly improve the flexibility of the positive electrode sheet, which is beneficial to obtaining a higher compaction density (PD), thereby improving the energy density of the lithium-ion battery.

[0005] In a first aspect, this application provides a positive electrode additive comprising at least one of the compounds shown in Formula I:

[0006]

[0007] In Formula I, the R group comprises a saturated alkyl chain containing an ethylene glycol group, wherein the saturated alkyl chain has at least 5 carbon atoms. The cathode additive in this application is a small molecule additive containing a saturated alkyl chain with an ethylene glycol group. The cathode electrode prepared using the cathode additive with the shown structure exhibits good ionic conductivity and excellent flexibility, which can reduce charge transfer resistance and avoid the risk of brittle breakage during winding due to the cathode electrode's hardness and brittleness, thus improving power dissipation (PD).

[0008] In some embodiments, in Formula I, the R group includes the group shown in Formula a (i.e., polyethylene glycol with n terminal methylpropionic acid groups):

[0009]

[0010] In equation a, the value of n ranges from 1 to n ≤ 20, and * represents the connection terminal. Satisfying the above conditions can further improve the ionic conductivity of the compound shown in equation I, thereby further reducing the charge transfer resistance and polarization.

[0011] In some embodiments, the number-average molecular weight of the cathode additive is between 700 and 3000. Meeting these conditions is beneficial for further enhancing the cathode additive's role in reducing membrane resistance, increasing power dissipation (PD), and improving low-temperature discharge capacity in electrochemical devices.

[0012] In some embodiments, the positive electrode additive has a boiling point in the range of 300°C to 600°C. Therefore, the positive electrode additive in this application has low volatility, which is beneficial for further improving PD and low-temperature discharge performance, and reducing membrane resistance.

[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 positive additive described in the first aspect, wherein substance A contains CF bonds. Meeting the above conditions is beneficial for further improving PD and low-temperature discharge performance and reducing film resistance.

[0014] In some embodiments, the mass percentage of the positive electrode additive is 0.05 wt% to 0.5 wt%, based on the mass of the positive electrode active material layer. This application, by controlling the content of the positive electrode additive in the positive electrode active material layer, benefits both by reducing the film resistance of the positive electrode active material layer, thereby improving the low-temperature discharge performance of the lithium-ion battery, and by controlling the viscosity of the positive electrode slurry within a suitable range, thus improving the power dissipation (PD). Simultaneously, the positive electrode additive can absorb moisture from the air, providing lubrication and moisture retention effects. During cold pressing, it facilitates the slippage of the main material particles, reducing damage to the aluminum foil and further improving the PD. Preferably, the mass percentage of the positive electrode additive is 0.3 wt% to 0.5 wt%.

[0015] In some embodiments, substance A includes polyvinylidene fluoride (PVDF). The saturated alkyl chains containing ethylene glycol groups in the cathode additive of this application have good ionic conductivity, which can reduce charge transfer resistance and polarization. Simultaneously, when used in conjunction with PVDF, the cathode additive of this application can insert into the molecular chains of PVDF. The polar polyethylene glycol groups interact with the CF bonds in PVDF, which helps to weaken the intermolecular forces, improves the mobility of the PVDF molecular chains, lowers the glass transition temperature of PVDF, and thus improves the flexibility of the electrode, solving the problem of electrode brittleness and improving PD (Power Generation).

[0016] In some embodiments, the number-average molecular weight of substance A is between 800,000 and 1,200,000. Preferably, the number-average molecular weight of substance A is 1,000,000. In this case, it is more advantageous for the positive electrode additive to insert between the PVDF molecular chains, so that the polar long-chain alkyl groups in the positive electrode additive are sandwiched between the PVDF molecular chains, shielding the polar CF bonds in PVDF, reducing the intermolecular forces of PVDF molecular chains, and further improving the flexibility of the electrode sheet.

[0017] In some embodiments, the adhesion between the positive electrode active material layer and the positive electrode current collector is 26.5 N / m to 28.5 N / m. The positive electrode additive in this application improves the flexibility of the positive electrode sheet without reducing the adhesion between the positive electrode active material layer and the positive electrode current collector.

[0018] In some embodiments, the film resistance of the positive electrode is 0.18Ω to 0.23Ω. The positive electrode additive in this application improves the flexibility of the positive electrode while also enhancing its electronic conductivity, which is beneficial for further improving PD and low-temperature discharge performance and reducing film resistance.

[0019] Thirdly, this application provides a method for preparing the positive electrode sheet as described in the second aspect above, comprising the following steps:

[0020] 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 solid content of the positive electrode slurry is 66wt% to 76wt%, and the viscosity of the positive electrode slurry is 3000mPa·s to 8000mPa·s.

[0021] In some embodiments, 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 (95.5–97):(1–2):(1–2):(0.05–0.5). Meeting the above conditions is beneficial for further improving PD and low-temperature discharge performance and reducing membrane resistance.

[0022] 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.

[0023] The beneficial effects of the technical solutions provided in some embodiments of this application include at least the following: when the positive electrode additive of this application is applied to the preparation of lithium-ion battery electrodes, it can significantly improve the ionic conductivity and flexibility of the electrodes. The saturated alkyl chains containing ethylene glycol groups in the positive electrode additive can reduce charge transfer resistance and polarization. At the same time, the small molecule compounds containing saturated alkyl chains with methyl propionic acid-terminated polyethylene glycol groups can interact with the PVDF binder. When the polar polyethylene glycol groups interact with the CF bonds in PVDF, they can weaken the intermolecular forces of PVDF. Furthermore, the polar long-chain alkyl groups in the positive electrode additive can be sandwiched between the molecular chains of PVDF, shielding the polar CF bonds in PVDF, further reducing the intermolecular forces of PVDF, thereby improving the flexibility of the electrode and improving the power dissipation (PD). Detailed Implementation

[0024] 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.

[0025] Positive electrode additives

[0026] The cathode additive in this application includes at least one of the compounds shown in Formula I:

[0027]

[0028] In Formula I, the R group includes a saturated alkyl chain containing an ethylene glycol group, and the saturated alkyl chain has not less than 5 carbon atoms.

[0029] In some embodiments, in Formula I, the R group includes the group shown in Formula a:

[0030]

[0031]

[0032] In equation a, the range of n is 1≤n≤20, and * is the connecting end.

[0033] In some embodiments, the number average molecular weight of the cathode additive is 700 to 3000. Exemplarily, the number average molecular weight of the cathode additive is 700, 800, 900, 1000, 1500, 1800, 2000, 2200, 2500, 2800, 3000 or a range of any two of the above values.

[0034] In some embodiments, the positive electrode additive has a boiling point in the range of 300°C to 600°C. Exemplarily, the boiling point of the positive electrode additive is 300°C, 350°C, 400°C, 450°C, 500°C, 550°C, 600°C, or a range consisting of any two of the above values.

[0035] Preparation method of positive electrode additive

[0036] For example:

[0037] 1,3,5-tris(4-vinylphenyl)benzene and methoxy polyethylene glycol acrylate were added to a round-bottom flask containing deionized water, and an appropriate amount of potassium persulfate was added. The mixture was reacted at 80℃~90℃ and 350rpm~450rpm under mechanical stirring for 6h~12h. After the reaction was completed, the mixture was filtered and dried to obtain the positive electrode additive.

[0038] Positive electrode sheet

[0039] The positive electrode includes a positive current collector and a positive active material layer disposed on at least one side of the surface of the positive current collector. The positive active material layer includes a positive active material, substance A, and the aforementioned positive additive. Substance A contains CF bonds.

[0040] In some embodiments, the mass percentage of the positive electrode additive is from 0.05 wt% to 0.5 wt%, based on the mass of the positive electrode active material layer. Exemplarily, the mass percentage of the positive electrode additive is 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, substance A comprises polyvinylidene fluoride. Preferably, substance A is polyvinylidene fluoride.

[0042] In some embodiments, the number-average molecular weight of substance A is between 800,000 and 1,200,000. Exemplarily, the number-average molecular weight of substance A is 800,000, 850,000, 900,000, 950,000, 1,000,000, 1,050,000, 1,100,000, 1,150,000, 1,200,000 or any combination of two of the above values.

[0043] In some embodiments, the positive electrode active material in the positive electrode active material layer includes one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, and compounds obtained by adding other transition metals or non-transition metals to the above compounds.

[0044] In some embodiments, 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 thereof such as aluminum, copper, nickel, titanium or iron, such as Al (aluminum) foil.

[0045] In some embodiments, the adhesion force between the positive electrode active material layer and the positive electrode current collector is 26.5 N / m to 28.5 N / m. Exemplarily, the adhesion force between the positive electrode active material layer and the positive electrode current collector is 26.5 N / m, 26.8 N / m, 27 N / m, 27.2 N / m, 27.5 N / m, 27.8 N / m, 28 N / m, 28.2 N / m, 28.5 N / m, or a range consisting of any two of the above values.

[0046] In some embodiments, the film resistance of the positive electrode is from 0.18Ω to 0.23Ω. Exemplarily, the film resistance of the positive electrode is 0.18Ω, 0.19Ω, 0.20Ω, 0.21Ω, 0.22Ω, 0.23Ω, or a range consisting of any two of the above values.

[0047] Preparation method of positive electrode sheet

[0048] The method for preparing the positive electrode sheet in this application includes at least the following steps:

[0049] The positive electrode active material, substance A, positive electrode additive and conductive agent are dispersed in a non-aqueous solution and stirred to obtain a positive electrode slurry. The solid content of the positive electrode slurry is 66wt% to 76wt%, and the viscosity of the positive electrode slurry is 3000mPa·s to 8000mPa·s.

[0050] For example, the solid content of the positive electrode slurry is 66wt%, 68wt%, 70wt%, 72wt%, 74wt%, 76wt%, or any combination of two of the above values.

[0051] For example, the viscosity of the positive electrode slurry is 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 combination of two of the above values.

[0052] Based on the solid content of the positive electrode slurry, the mass ratio of positive electrode active material, substance A, conductive agent and positive electrode additive is (95.5~97):(1~2):(1~2):(0.05~0.5).

[0053] 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.

[0054] other

[0055] 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.

[0056] The negative electrode active material layer typically includes the negative electrode active material and optional conductive agents and binders.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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).

[0061] 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.

[0062] 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.

[0063] For example, the above-mentioned electrolyte lithium salt includes one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalate phosphate (LiDFOP), or lithium tetrafluorooxalate phosphate (LiTFOP).

[0064] 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).

[0065] Electrochemical device

[0066] 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.

[0067] 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.

[0068] Example 1-1

[0069] Preparation of positive electrode additives

[0070] 10 ml of 1,3,5-tris(4-vinylphenyl)benzene and 30 ml of methoxy polyethylene glycol acrylate were added to a round-bottom flask containing 500 ml of deionized water. 0.5 g of potassium persulfate was added, and the mixture was reacted at 85 °C and 400 rpm under mechanical stirring for 8 h. After the reaction was completed, the mixture was filtered and dried to obtain the positive electrode additive.

[0071] Preparation of positive electrode sheet

[0072] LiCoO2 (positive electrode active material), PVDF (number average molecular weight of 100w), conductive carbon black (conductive agent), and positive electrode additives (see Table 1) were dispersed in NMP solvent and stirred until homogeneous to obtain a positive electrode slurry with a solid content of 70wt% and a viscosity of 3028mPa·s. The mass ratio of LiCoO2, PVDF, conductive carbon black, and positive electrode additives 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 additives was 96%). The above positive electrode slurry was uniformly coated onto one surface of a 9μm thick aluminum foil current collector and dried at 120°C to obtain a single-sided positive electrode sheet with a 75μm 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 with positive electrode active material coating. After coating, the positive electrode sheet is cold-pressed and then cut into sheets with a size of 70mm×800mm for later use.

[0073] Preparation of negative electrode sheet

[0074] 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.

[0075] Preparation of electrolyte

[0076] 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. Lithium hexafluorophosphate (LiPF6) was then added to the above mixed organic solvents to dissolve and mix evenly to obtain an electrolyte with a LiPF6 concentration of 1.15 mol / L.

[0077] Preparation of the separating membrane

[0078] A porous polyethylene film with a thickness of 16 μm was used as the separator.

[0079] Preparation of lithium-ion batteries

[0080] 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.

[0081] Test method:

[0082] (1) Viscosity test

[0083] The viscosity of the cathode slurry was tested using a digital rotational viscometer (Shanghai Jingtian Electronic Instruments Co., Ltd., LVDV-1). 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.

[0084] (2) Adhesion test

[0085] 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 is then used to slowly pull 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.

[0086] (3) PD calculation

[0087] 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). 2 After 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 ;

[0088] h 61 h 62 h 63 h64 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, h. 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] (4) 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] (5) Low-temperature discharge performance test

[0092] The charge transfer resistance of lithium-ion batteries was evaluated based on their low-temperature discharge performance. A lithium-ion battery fully charged at 25°C was placed in a constant-temperature environment at 25°C for 30 minutes, then discharged at 0.2C to 3.0V, and the discharge capacity was recorded as D0. The same lithium-ion battery was then placed in a constant-temperature environment at -20°C for 30 minutes, and then discharged at 0.2C to 3.0V, and the discharge capacity was recorded as D1. The discharge capacity ratio (%) of the lithium-ion battery at low temperature (-20°C) was calculated as D1 / D0 × 100%.

[0093] Examples 1-2 to Examples 1-18

[0094] Unlike Example 1-1, the degree of polymerization parameter (n value) of the positive electrode additive during the preparation process was adjusted, as detailed in Table 1; otherwise, it is the same as Example 1-1. It should be noted that the R groups in Examples 1-2 to 1-18 include 2-20 methylpropionic acid polyethylene glycols.

[0095] Examples 1-19

[0096] The difference between this example and Example 1-1 is that the preparation process of the positive electrode additive is different, but everything else is the same as in Example 1-1.

[0097] Preparation of the positive electrode additive: 10 ml of 1,3,5-tris(4-vinylphenyl)benzene and 30 ml of methoxy polyethylene glycol methacrylate were added to a round-bottom flask containing deionized water. 0.5 g of potassium persulfate was added, and the mixture was reacted at 85 °C and 400 rpm with mechanical stirring for 8 h. After the reaction was completed, the mixture was filtered and dried to obtain the positive electrode additive. The structure of the positive electrode additive is shown in Formula I-1.

[0098]

[0099] Comparative Example 1

[0100] Unlike Examples 1-1, Comparative Example 1 did not use positive electrode additives in the preparation of the positive electrode sheet, and the mass percentage of the positive electrode active material was 96%, while the rest was the same as in Examples 1-1.

[0101] Comparative Example 2

[0102] Unlike Examples 1-1, Comparative Example 2 used triphenyl phosphate as the positive electrode additive in the preparation of the positive electrode sheet, while the rest was the same as in Examples 1-1.

[0103] Comparative Example 3

[0104] Unlike Examples 1-1, the R group in Comparative Example 3 includes terminal methylpropionic acid with an ethylene glycol content of 0. Specifically, the structural formula of the positive electrode additive in Comparative Example 3 is as shown in Formula II:

[0105]

[0106] Comparative Example 4

[0107] Unlike Example 1-1, the positive electrode additive used in the preparation of the positive electrode sheet is the compound shown in Formula III, while the rest is the same as in Example 1-1.

[0108]

[0109] In formula III, the R1 group is the group shown in formula b:

[0110]

[0111] The preparation and performance parameters of each embodiment and comparative example are shown in Table 1 and Table 2.

[0112] Table 1

[0113]

[0114] Table 2

[0115] Example 1-1 0.18 26.8 3.98 78.5 Examples 1-2 0.18 27.2 3.98 78.5 Examples 1-3 0.18 27.4 3.98 78.6 Examples 1-4 0.19 27.6 3.98 78.6 Examples 1-5 0.19 27.7 3.98 78.6 Examples 1-6 0.20 27.8 3.98 78.7 Examples 1-7 0.21 28.0 3.99 78.7 Examples 1-8 0.21 28.2 3.99 78.7 Examples 1-9 0.22 28.3 3.99 78.8 Examples 1-10 0.22 28.4 3.99 78.9 Examples 1-11 0.23 28.5 4.00 78.9 Examples 1-12 0.22 28.5 4.01 79.0 Examples 1-13 0.21 28.5 4.02 79.2 Examples 1-14 0.20 28.4 4.03 79.3 Examples 1-15 0.20 28.4 4.04 79.5 Examples 1-16 0.19 28.3 4.05 79.6 Examples 1-17 0.18 28.3 4.08 79.9 Examples 1-18 0.18 28.2 4.10 80.2 Examples 1-19 0.19 26.7 3.93 77.6 Comparative Example 1 0.26 26.3 3.72 74.5 Comparative Example 2 0.27 26.0 3.80 73.2 Comparative Example 3 0.23 26.5 3.90 76.2 Comparative Example 4 0.28 26.2 3.85 74.6

[0116] Comparing Examples 1-1 to 1-19 with Comparative Examples 1 to 4, and referring to Tables 1 and 2, it can be seen that when the positive electrode of the lithium-ion battery in this application contains the positive electrode additive described in this application, the film resistance, power dissipation (PD), and -20°C discharge capacity ratio of the positive electrode are all improved. Simultaneously, the small molecule additive inserted between the PVDF molecular chains (i.e., the positive electrode additive described in this application) can weaken the intermolecular forces of PVDF molecules, improve molecular chain mobility, solve the problem of electrode brittleness, improve electrode flexibility, and significantly enhance the electrode PD, especially in Examples 1-18, where the PD reaches as high as 4.10 g / cc, thus improving the energy density of the lithium-ion battery.

[0117] 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 by comprising: The positive electrode additive includes at least one of the compounds shown in Formula I: Formula I; In formula I, the R group is the group shown in formula a: Formula a; In equation a, the range of n is 1 ≤ n ≤ 20, where * is the connecting end.

2. The positive electrode additive according to claim 1, characterized in that, The cathode additive satisfies at least one of the following conditions: (1) The number average molecular weight of the positive electrode additive is 700 to 3000; (2) The boiling point of the positive electrode additive is 300℃ to 600℃.

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 includes a positive electrode active material, substance A, and the positive electrode additive as described in any one of claims 1 to 2; The substance A contains CF bonds.

4. 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%.

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 0.3wt% to 0.5wt%.

6. The positive electrode sheet according to claim 3, characterized in that, The positive electrode sheet satisfies at least one of the following conditions: (1) Substance A includes polyvinylidene fluoride; (2) The number-average molecular weight of substance A is between 800,000 and 1,200,000; (3) The bonding force between the positive electrode active material layer and the positive electrode current collector is 26.5 N / m to 28.5 N / m; (4) The film resistance of the positive electrode is 0.18Ω to 0.23Ω.

7. A method for preparing a positive electrode sheet as described in any one of claims 3 to 6, 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 66wt% to 76wt% and a viscosity of 3000mPa·s to 8000mPa·s.

8. The preparation method according to claim 7, characterized in that, 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 (95.5~97):(1~2):(1~2):(0.05~0.5).

9. An electrochemical device, characterized in that, The electrochemical device includes a positive electrode as described in any one of claims 3 to 6; and / or, The electrochemical device includes a positive electrode sheet prepared by the preparation method according to any one of claims 7 to 8.

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