A phosphazene derivative-containing cathode slurry composition, cathode and electrochemical device comprising the same

Abstract

The present invention provides a phosphazene derivative-containing cathode slurry composition, a cathode and an electrochemical device comprising the same, wherein the cathode slurry composition comprises a cathode active material, a flame retardant, a binder and a conductive additive, wherein the flame retardant comprises a phosphazene derivative of the formula (I):, wherein n, R1 and R2 are as defined herein. The present invention improves the safety characteristics of the electrochemical device by adding the aforementioned flame retardant to the cathode in the electrochemical device.

Description

[Technical Field]

[0001] This disclosure relates to a positive electrode slurry composition for use in an electrochemical device, and more particularly to a phosphazene derivative flame retardant used in a positive electrode slurry composition for use in an electrochemical device. [Previous Technology]

[0002] Nowadays, lithium batteries are widely used in electronic, energy storage, biomedical, and electric vehicle equipment and devices. Therefore, the batteries used in these devices are expected to have high energy density, high safety, and the ability to maintain high energy storage and high discharge capacity over long periods of use. As a result, lithium-ion batteries that meet these requirements have gradually gained attention from the academic and industrial communities.

[0003] Lithium-ion batteries have advantages such as light weight, high energy density, good cycle characteristics, and high power, and are increasingly used in high-power products such as power tools and electric vehicles. However, there are still frequent reports of fires caused by lithium-ion batteries during use, so improving safety issues such as thermal runaway is the key to the further development of lithium batteries.

[0004] During the charging and discharging process, especially at high temperatures, the positive electrode material of lithium-ion batteries is prone to generating oxygen, which poses a risk of battery swelling, positive electrode material disintegration, or combustion due to reaction with the electrolyte. Therefore, the mechanism of flame retardants is to block the release of oxygen before the combustion reaction occurs, thereby increasing the safety of lithium-ion batteries.

[0005] Phosphazene compounds are a class of flame retardants currently used in the market. The NP bond of phosphazene gives the compounds excellent thermal stability, and the phosphate radicals generated through the decomposition of phosphazene compounds can capture oxygen released from the cathode material, resulting in good flame retardant effects, which helps to improve the stability of the cathode and extend the battery cycle life. Phosphazene derivatives obtained by functional group modification of the phosphazene skeleton can give phosphazene compounds different properties, such as different thermal decomposition temperatures, which is one of the research directions for improving flame retardant effects.

[0006] However, phosphazene compounds typically have large molecular groups, and excessive addition of flame retardants may reduce conductivity and affect the rate performance of lithium-ion batteries. Therefore, how to provide a phosphazene derivative flame retardant that is highly stable, highly flame-retardant, and suitable for cathode slurry formulations is a problem that the industry urgently needs to solve. [Summary of the Invention]

[0007] In view of the various defects of the prior art, this disclosure provides a positive electrode containing a phosphazene derivative flame retardant without increasing the complexity of the manufacturing process. It has high electrochemical stability, requires a small amount of additive, and can improve the safety performance of lithium-ion batteries under good cycling conditions.

[0008] To achieve the above objectives, this disclosure provides a positive electrode slurry composition comprising a positive electrode active material, a flame retardant, a binder, and a conductive additive, wherein the flame retardant comprises a phosphazene derivative having the structure of formula (I):

[0009] wherein, n is an integer from 3 to 6, R1 and R2 are independently selected from the groups shown in formula (I-1) or formula (I-2), wherein A is -X, -R3-O-R4, -R3-CH3-m(OR4)m, -CH3-m(OR4)m or -R5-(OR6)k-OR7, wherein X is F, Cl or Br, and R3 is an unsubstituted or C1-C8 alkyl-substituted C1-C8 alkylene(ene) R4 is a C1-C8 alkyl group, R5 is a C1-C3 alkylene group, R6 is a C1-C3 alkylene group, R7 is a C1-C3 alkyl group, m is 2 or 3, k is 1 or 2, p is the number of carbons A bonded to the attached benzene ring, and p is an integer from 0 to 3; B is a C2-C8 alkenyl group, C1-C8 cyanoalkyl group, C1-C8 ester group, C1-C8 alkoxy group, or C1-C8 alkyl group, and q is an integer from 0 to 3; and 0 <p+q≦5。

[0010] In a specific embodiment of the positive electrode slurry composition disclosed herein, the phosphazene derivative having the structure of formula (I) has n as 3, R1 and R2 as groups as shown in (I-1), A as -X or -R3-O-R4, and p as 0 or 1; B as C2-C8 alkenyl, and q as 0 or 1; and 0 <p+q≦2。

[0011] In a specific embodiment of the positive electrode slurry composition disclosed herein, A is -X or -R3-O-R4, where X is F, R3 is an unsubstituted C1-C8 alkenyl group, R4 is a C1-C8 alkyl group, and p is 0 or 1; B is a C2-C8 alkenyl group, and q is 0 or 1; and 0 <p+q≦2。

[0012] In a specific embodiment of the positive electrode slurry composition disclosed herein, when R1 and R2 are selected from groups represented by formula (I-1), the phosphazene derivative is selected from one of the groups consisting of compounds (1-1) to (1-3): and

[0013]

[0014] In a specific embodiment of the cathode slurry composition disclosed herein, the cathode active material comprises at least one of the group consisting of lithium iron phosphate, lithium manganese phosphate, lithium iron manganese phosphate, lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium nickel manganese aluminum oxide, lithium nickel manganese oxide, nickel manganese cobalt oxide, nickel cobalt aluminum oxide, lithium nickel oxide, lithium manganese oxide, and lithium cobalt oxide.

[0015] In one specific embodiment of the positive electrode slurry composition disclosed herein, the positive electrode slurry composition further includes a solvent, and the solvent comprises at least one selected from the group consisting of N-methylpyrrolidone, N-ethylpyrrolidone, ethylene carbonate, propylene carbonate, butenyl carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dipropyl carbonate, γ-butyrolactone, ethyl propionate, propyl propionate, trimethyl phosphate, triethyl phosphate, dimethylformamide, dimethylacetamide, hexamethylphosphonic acid, dimethyl sulfoxide, cyclobutane, diethyl ether, acetonitrile, propionitrile, tetrahydrofuran, acetone, cyclohexanone, cyclopentanone, methanol, ethanol and isopropanol.

[0016] In one specific embodiment of the positive electrode slurry composition disclosed herein, the adhesive comprises at least one selected from the group consisting of polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl fluoride, polyvinyl chloride, polyvinyl chloride trifluoroethylene, polybutadiene, polyvinyl alcohol, polyacrylic acid, polyacrylonitrile, polyacrylate, vinylidene fluoride-hexafluoropropylene copolymer, fluorinated ethylene propylene copolymer and ethylene-tetrafluoroethylene copolymer.

[0017] In one specific embodiment of the positive electrode slurry composition disclosed herein, the conductive additive comprises at least one selected from the group consisting of carbon black, graphite, graphene, acetylene black, carbon fiber, carbon nanotubes, nitrogen-doped graphite, nitrogen-doped graphene and metal fibers.

[0018] In a specific embodiment of the positive electrode slurry composition disclosed herein, based on a total weight of 100 parts by weight of the positive electrode active material, adhesive, and conductive additive, the positive electrode active material, adhesive, and conductive additive comprise 80 to 98 parts by weight of the positive electrode active material, 1 to 10 parts by weight of the adhesive, and 1 to 10 parts by weight of the conductive additive.

[0019] In a specific embodiment of the positive electrode slurry composition disclosed herein, the content of the flame retardant in the positive electrode slurry composition is 0.01 to 10 parts by weight, relative to 100 parts by weight of the positive electrode active material, adhesive and conductive additive as a whole.

[0020] This disclosure also provides a positive electrode comprising an active material layer and a current collector, wherein the active material layer is formed by coating the current collector with the positive electrode slurry composition disclosed herein and evaporating the solvent.

[0021] In a specific embodiment of the positive electrode disclosed herein, based on a total weight of 100 parts by weight of the positive electrode active material, adhesive and conductive additive, the positive electrode active material, adhesive and conductive additive comprise 80 to 98 parts by weight of the positive electrode active material, 1 to 10 parts by weight of the adhesive and 1 to 10 parts by weight of the conductive additive.

[0022] In one specific embodiment of the positive electrode disclosed herein, the content of the flame retardant in the active material layer is 0.01 to 10 parts by weight, relative to 100 parts by weight of the positive electrode active material, adhesive and conductive additive as a whole.

[0023] This disclosure further provides an electrochemical device comprising the positive electrode, the negative electrode, an electrolyte filled between the positive electrode and the negative electrode, and a separating membrane disposed in the electrolyte.

[0024] In one specific embodiment of the electrochemical device disclosed herein, the electrochemical device is a lithium-ion secondary battery.

[0025] As described above, the positive electrode slurry composition for electrochemical devices disclosed herein contains a novel phosphazene derivative flame retardant. By using the flame retardant, the shortcomings of traditional lithium-ion batteries in terms of safety can be improved, the risk of thermal runaway can be reduced, and the addition of the flame retardant will not affect the electrical performance of the battery. [Simplified Explanation of the Diagram]

[0095] Figure 1 is a cross-sectional view of the electrochemical device disclosed herein.

[0096] Figure 2 is the NMR spectrum of compound 1-1.

[0097] Figure 3 shows the NMR spectra of compounds of formula 1-2.

[0098] Figure 4 shows the NMR spectra of compounds of formula 1-3.

[0099] Figure 5 shows the test results at 60°C for the positive electrode of the battery with no 1-1 compound, 0.5 parts by weight, 1.0 parts by weight, 2.0 parts by weight and 4.0 parts by weight of 1-1 compound.

Implementation Method

[0026] The embodiment of this disclosure is illustrated below by means of specific specific embodiments, the advantages and efficacy of this disclosure may be readily understood by persons familiar with the craft from what is revealed in this instruction manual. This disclosure may also be enforced or applied by means of other different embodiments, and various details in this instruction may be based on different views and applications, giving different modifications and changes without deviating from the spirit revealed by this disclosure. Furthermore, all ranges and values ​​in this paper are included and mergeable. Any numerical value or point that falls within the range described herein, such as any integer that can be used as a minimum or maximum value to derive a lower range.

[0027] In a specific embodiment, the disclosure provides a cathode slurry composition for an electrochemical device comprising a phosphazene-like derivative flame retardant as disclosed herein and, by virtue of the use of the flame retardant, an electrochemical device with better safety characteristics is provided. In one specific embodiment, the cathode slurry composition disclosed herein comprises a cathode active material, a flame retardant, an adhesive and a conductive additive, wherein the flame retardant comprises a phosphazene-like derivative with a structure of formula (I):

[0028]

[0029] Wherein, n is an integer from 3 to 6, R1 and R2 are independently selected from the groups shown in Equation (I-1) or Equation (I-2),

[0030]

[0031] Wherein, A is -X, -R3-O-R4, -R3-CH3-m(OR4)m, -CH3-m(OR4)m, or -R5-(OR6)k-OR7, wherein, -C8 elongated (sub)alkyl, R4 is C1-C8 alkyl, R5 is C1-C3 elongated (sub)alkyl, R6 is C1-C3 elongyl, R7 is C1-C3 alkyl, m is 2 or 3, k is 1 or 2, p is the number of A of carbons bonded to the linked benzene ring, and p is an integer from 0 to 3;

[0032] B is a C2-C8 alkenyl, a C1-C8 cyanoalkyl, a C1-C8 ester group, a C1-C8 alkoxy or C1-C8 alkyl, and q is an integer from 0 to 3; and 0 <p+q≦5。

[0033] In one specific embodiment, the range of carbon atoms disclosed herein may be extended from a lower limit value to an upper limit value, e.g., C1-C8 means that the number of carbon atoms may be 1, 2, 3, 4, 5, 6, 7, or 8.

[0034] In some specific embodiments, the compounds of phosphazene derivatives with the structure of formula (I) contained in the flame retardant of the positive electrode slurry composition disclosed herein are selected from Table 1, but are not limited thereto.

[0035]

[0036]

[0037] The positive electrode active material to which this disclosure applies is commonly used in the art. In a specific embodiment of the positive electrode slurry composition disclosed herein, the positive electrode active material may include at least one of the group consisting of lithium iron phosphate, lithium manganese phosphate, lithium iron manganese phosphate, lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, nickel cobalt manganese aluminum oxide, lithium nickel manganese oxide, nickel manganese cobalt oxide, nickel cobalt aluminum oxide, lithium nickel oxide, lithium manganese oxide, and lithium cobalt oxide.

[0038] There are no limitations on the positive electrode active material disclosed herein. For example, considering battery safety and manufacturing cost, lithium iron phosphate can be used as the positive electrode active material; considering battery energy density, ternary lithium ions containing cobalt, nickel, manganese or aluminum can be used as the positive electrode active material.

[0039] Because contact with moisture may degrade the performance of the positive electrode active material, the positive electrode slurry composition disclosed herein may also include a solvent. The applicable solvent is a non-aqueous organic solvent, which may be liquid or non-liquid, such as solid or gel, but is not limited thereto. In a specific embodiment of the positive electrode slurry composition disclosed herein, the solvent may include at least one selected from the group consisting of pyrrolidones, carbonates, esters, amides, sulfides, ethers, nitriles, furans, ketones, and alcohols.

[0040] In a specific embodiment of the positive electrode slurry composition disclosed herein, the solvent may include at least one selected from the group consisting of N-methylpyrrolidone, N-ethylpyrrolidone, ethylene carbonate, propylene carbonate, butenyl carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dipropyl carbonate, γ-butyrolactone, ethyl propionate, propyl propionate, trimethyl phosphate, triethyl phosphate, dimethylformamide, dimethylacetamide, hexamethylphosphonamide, dimethyl sulfoxide, cyclobutane, diethyl ether, acetonitrile, propionitrile, tetrahydrofuran, acetone, cyclohexanone, cyclopentanone, methanol, ethanol, and isopropanol.

[0041] The adhesives used in this disclosure are commonly used in the art. In a specific embodiment of the positive electrode slurry composition disclosed herein, the adhesive may comprise at least one of the group consisting of polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl fluoride, polyvinyl chloride, polychlorotrifluoroethylene, polybutadiene, polyvinyl alcohol, polyacrylic acid, polyacrylonitrile, polyacrylate, vinylidene fluoride-hexafluoropropylene copolymer, fluorinated ethylene propylene copolymer, and ethylene-tetrafluoroethylene copolymer.

[0042] Adhesives help to uniformly disperse and coat the positive electrode slurry composition, including the positive electrode active material, onto the electrode current collector. On the other hand, the use of adhesives can buffer the expansion and contraction of the material during battery charging and discharging, improving the stability of the electrode structure. Considering the compound's bonding properties, chemical and electrochemical stability, and solvent solubility, polyvinylidene fluoride (PVDF) is a high-performance adhesive material used in most lithium-ion battery positive electrodes.

[0043] The conductive additives used in this disclosure are commonly used in the art. In a specific embodiment of the positive electrode slurry composition disclosed herein, the conductive additive may include at least one selected from the group consisting of carbon black, graphite, graphene, acetylene black, carbon fiber, carbon nanotubes, nitrogen-doped graphite, nitrogen-doped graphene, and metal fibers.

[0044] Conductive additives can assist in connecting the active materials within the battery during charging and discharging, enhancing electron conductivity, thereby improving battery charging and discharging efficiency, cycle life, and capacity. The material, particle size, and flowability of the conductive additive can be selected according to the required conductivity and in combination with other electrode materials.

[0045] In one specific embodiment, the present disclosure further provides a positive electrode, which includes an active material layer and a current collector. The active material layer of the present disclosure can be obtained by mixing the powdered / solid components of the positive electrode slurry composition of the present disclosure with an organic solvent to form a slurry, then coating the slurry onto a current collector such as aluminum foil, and heating to evaporate the solvent to obtain a solid active material layer.

[0046] Since some embodiments of the positive electrode slurry composition disclosed herein contain solvents, the composition of the positive electrode is based on the total weight of the active material layer formed without solvent or after solvent evaporation. Furthermore, the flame retardant content disclosed herein is calculated based on the total weight of the positive electrode active material, adhesive, and conductive additives in the positive electrode slurry composition or active material layer, plus the amount of additional flame retardant added.

[0047] In a specific embodiment, based on 100 parts by weight of the total weight of the positive electrode active material, adhesive, and conductive additive, the positive electrode active material, adhesive, and conductive additive of the active material layer disclosed herein include approximately 80 to 98 parts by weight of the positive electrode active material, such as 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, or 98 parts by weight; approximately 1 to 10 parts by weight of the adhesive, such as 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 parts by weight; and approximately 1 to 10 parts by weight of the conductive additive, such as 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 parts by weight.

[0048] In a specific embodiment, the content of flame retardant in the active material layer disclosed herein is about 0.01 to 10 parts by weight, compared to 100 parts by weight of the positive electrode active material, adhesive and conductive additive as a whole. For example, if the positive electrode active material, adhesive and conductive additive as a whole is 100 parts by weight, the additional flame retardant content is 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 parts by weight.

[0049] The content of each component in the active material layer disclosed herein may be changed according to the actual application, but is not limited to the content described herein.

[0050] In another specific embodiment, the present disclosure applies the aforementioned components to an electrochemical device to obtain an electrochemical device that differs from the conventional one. That is, as shown in FIG1, the present disclosure also provides an electrochemical device 1, comprising the positive electrode 10, the negative electrode 12, the electrolyte 14 filled between the positive electrode 10 and the negative electrode 12, and the separator 16 disposed in the electrolyte.

[0051] The negative electrode, separator, and electrolyte materials applicable to this disclosure are all commonly used in the art. In one specific embodiment, the negative electrode disclosed herein may be a copper foil coated with materials such as artificial graphite, natural graphite, metastable spherical carbon, lithium titanate, silicon-based materials, or nitrides. In one specific embodiment, the separator disclosed herein may be a microporous membrane made of polypropylene, polyethylene, polypropylene / polyethylene / polypropylene composites, polytetrafluoroethylene, polyamide, polyvinyl chloride, etc. In one specific embodiment, the electrolyte disclosed herein may be a liquid electrolyte, such as an organic solvent (carbonate or ether solvent, etc.) containing lithium salts (lithium hexafluorophosphate, lithium fluoroborate, lithium trifluoromethanesulfonate, or lithium bis(trifluoromethanesulfonyl)imide); or a solid electrolyte, such as a sulfide solid electrolyte, an oxide solid electrolyte, or a polymer solid electrolyte.

[0052] In one specific embodiment, the electrochemical device disclosed herein is a lithium-ion secondary battery.

[0053] Example

[0054] The following detailed description of the various properties and effects of this disclosure is provided by way of examples. These detailed examples are only for illustrating the properties of this disclosure, and this disclosure is not limited to those exemplified in the specific examples.

[0055] Preparation of phosphazene derivatives:

[0056] The compound of formula 1-1 disclosed herein can be prepared by the following synthetic procedure 1. Specifically, hexachlorocyclotriphosphazene (1 g, 2.87 mmol, 1 eq) is mixed with acetone (20 mL) to form solution a; 4-(2-methoxyethyl)phenol (3.94 g, 25.9 mmol, 9 eq) is mixed with acetone (60 mL) to form solution b, and K2CO3 (3.58 g, 25.9 mmol, 9 eq) is added to solution b to form solution c. Solution c is poured into solution a to obtain a reaction mixture, which is then subjected to reflux in an oil bath at 70 °C for 4 days. After the reaction is complete, the solids in the reaction mixture are first filtered off, and then the solvent is removed by vacuum distillation to obtain a crude product. The crude product is repeatedly washed with methanol and water and then freeze-dried to obtain compound of formula 1-1 in powder form. The NMR spectrum of compound of formula 1-1 is shown in Figure 2.

[0057]

[0058] Compounds 1-2 and 1-3 were prepared using the same method as compound 1-1 in Preparation Example 1, except that 4-(2-methoxyethyl)phenol was replaced with 4-fluorophenol and 4-vinylphenol, respectively, to prepare compounds 1-2 and 1-3. The NMR spectra of compounds 1-2 and 1-3 are shown in Figures 3 and 4, respectively.

[0059] Preparation of the positive electrode slurry composition:

[0060] The phosphazene derivative of Formula 1-1 used in Example 1-1 (equivalent, about 0.02 g) and polyvinylidene fluoride (0.1 g, Solef® 5130) were dissolved in N-methylpyrrolidone (2.2 g) and stirred at 2000 rpm for 30 minutes using a defoaming mixer to form slurry a. A carbon conductive additive (0.1 g, Super-P) was added to slurry a, and the mixture was stirred at 2000 rpm for 30 minutes using a defoaming mixer to form slurry b. Lithium iron phosphate (0.8 g, Aleees, M121) was added to slurry b, and the mixture was stirred at 2000 rpm for 30 minutes using a defoaming mixer to form the positive electrode slurry composition. Similarly, the phosphazene derivatives of Formula 1-2 and 1-3 were used as the positive electrode slurry compositions in Examples 1-2 and 1-3, respectively, as shown in Table 2 below.

[0061] In addition, the positive electrode slurry composition used in Comparative Examples 1-1 to 1-3 was either not added with any phosphazene derivatives or was instead added with phosphazene derivatives of Formula 2-1 and Formula 2-2 as shown in Table 3, respectively. The specific components and contents are shown in Table 2 below.

[0062]

[0063]

[0064]

[0065] Preparation of the positive electrode:

[0066] The above-mentioned positive electrode slurry composition is coated onto aluminum foil at a speed of 15 mm / s to form a positive electrode slurry composition layer with a thickness of 115 μm. Then, the aluminum foil coated with the positive electrode slurry composition layer is placed in a vacuum oven at 100°C and heated for 24 hours to evaporate the solvent. The heating system is turned off, and the electrode is removed after the vacuum oven cools to room temperature. A rolling process is performed to press the electrode thickness to 40 μm, and a cutting process is performed to obtain a positive electrode with a diameter of 13 mm.

[0067] Preparation of the electrochemical device:

[0068] The above-mentioned positive electrode, negative electrode (lithium metal with a diameter of 15.6 mm), separator (Celgard® 2325) and electrolyte (1M lithium hexafluorophosphate in 1:1 ethylene carbonate: diethyl carbonate) are used to assemble a button battery.

[0069] A: Rate Discharge Test

[0070] Take the button battery assembled above and conduct discharge tests at different rates of 0.1C, 0.2C, 0.3C, 0.5C, 1C and 3C at an ambient temperature of 25°C (test instrument: Acutech Systems BAT-750B).

[0071] The test results of discharge specific capacity at different rates with the addition of different phosphazene derivatives are listed in Table 4 below. It can be seen that after adding 2 parts by weight of each of the phosphazene derivatives disclosed herein as flame retardants to the positive electrode slurry, Examples 1-1 to 1-3 can maintain high specific capacity at all rates, which is better than Comparative Example 1-0 without the addition of phosphazene derivatives. Furthermore, compared with the phosphazene derivatives of the comparative examples, the phosphazene derivatives of Examples 1-1 to 1-3 disclosed herein can still maintain specific capacity at high rates (3C) without a significant decrease.

[0072]

[0073] B: High-Temperature Cycle Life Test of Button Batteries

[0074] The positive electrode of the button batteries shown in Table 5 below was either untreated (Comparative Example 2, i.e., X=0 as the control group) or treated with 0.5 parts by weight, 1 part by weight, 2 parts by weight, and 4 parts by weight of compound 1-1. At a high temperature of 60°C, the batteries were charged to 4.0V at a constant current of 1C and then discharged to 2.5V at a constant current of 1C. The above steps (1) and (2) were repeated to charge and discharge the batteries, and the capacity decay in the first 250 cycles was recorded (charge and discharge test instrument: Acutech Systems BAT-750B). The test results are shown in Figure 5 and Table 6 below. It can be found that the batteries with compound 1-1 added as a flame retardant at the positive electrode have better specific capacity than the standard batteries in terms of high temperature cycle life, while the experimental group with 2 parts by weight of phosphazene derivative has the best stability and specific capacity retention rate.

[0075]

[0076]

[0077] C: Performance testing and nail penetration test of NMC811 ternary lithium battery

[0078] The pouch cell used in this test is a battery with a positive electrode of LiNi0.8Mn0.1Co0.1O2 and a negative electrode of artificial graphite (referred to as NMC811 ternary battery). The battery design is shown in Table 7 below. The battery electrode formulation design is shown in Table 8 below. One part by weight of compound of formula 1-1 as a flame retardant was added to the positive electrode of the pouch cell as the experimental group, and no flame retardant was added as the control group.

[0079]

[0080]

[0081] Battery performance tests (analysis equipment: Keysight 34972ALXI) and nail penetration tests were conducted on the experimental and control groups using a charge / discharge apparatus (CT-4008T-5V6A-S1, Xinwei). The nail penetration test equipment was custom-made; it consisted of a steel needle and controller installed inside a metal casing, with temperature and voltage monitors mounted externally and the probe circuitry extended into the equipment. The nail penetration test conformed to the specifications for needle diameter and penetration rate in standards such as IEC60086-4:2000, UL1642-2007, and UL2054. The battery performance test procedures and nail penetration test parameters are described below.

[0082] Battery performance testing steps:

[0083] Step 1: Charge at 1C constant current to 4.25V;

[0084] Step 2: Charge at 1C constant voltage until the current reaches 0.175A and then cut off;

[0085] Step 3: Let stand for 2 minutes;

[0086] Step 4: 1C constant current discharge to 2.8V; and

[0087] Step 5: Let stand for 2 minutes and repeat the above steps.

[0088] Acupuncture test parameters:

[0089] Needle diameter 3: mm;

[0090] Puncture speed: 4.2 cm / s;

[0091] Initial height: 24.5cm.

[0092] The battery performance test and nail penetration test results are shown in Table 9 below. In the nail penetration test, it was observed that the control group without any flame retardant produced a violent open flame and dense smoke, while the experimental group with compound 1-1 added as a flame retardant only showed a flash but quickly extinguished, and the highest temperature reached during nail penetration was significantly lower than that of the control group without added flame retardant. These results confirm that, compared to the cathode without any flame retardant, the cathode containing the disclosed compound 1-1 significantly improves battery safety.

[0093]

[0094] As can be seen from the above embodiments, compared with using phosphazene derivatives with other structures as flame retardants, adding the phosphazene derivatives with specific structures disclosed in this invention as flame retardants to the positive electrode of lithium-ion secondary batteries does not have a significant impact on the battery performance, and can significantly improve the safety of the battery when it is damaged by external forces, thus achieving a good flame retardant effect.

Claims

1. A positive electrode slurry composition comprising a positive electrode active material, a flame retardant, a binder, and a conductive additive, wherein, This flame retardant contains phosphazene derivatives with the structure of formula (I): Wherein, n is an integer from 3 to 6, R1 and R2 are independently selected from the groups shown in formula (I-1) or formula (I-2), wherein A is -R3-O-R4, -R3-CH3-m(OR4)m, -CH3-m(OR4)m or -R5-(OR6)k-OR7, wherein R3 is an unsubstituted or C1-C8 alkyl-substituted C1-C8 alkylene group, R4 is a C1-C8 alkyl group, R5 is a C1-C3 alkylene group, R6 is a C1-C3 alkylene group, R7 is a C1-C3 alkyl group, m is 2 or 3, k is 1 or 2, p is the number of carbons A bonded to the connected benzene ring, and p is an integer from 0 to 3; B is a C2-C8 alkenyl group, a C1-C8 cyanoalkyl group or a C1-C8 ester group, and q is an integer from 0 to 3; and 0 <p+q≦5。 2. The positive electrode slurry composition as described in claim 1, wherein, The groups shown in R1 and R2 systems (I-1) are: A is -R3-O-R4, and p is 0 or 1; B is a C2-C8 alkenyl group, and q is 0 or 1; and 0 <p+q≦2。 3. The positive electrode slurry composition as described in claim 2, wherein, R3 is an unsubstituted C1-C8 alkylene group, and R4 is a C1-C8 alkyl group.

4. The positive electrode slurry composition as described in claim 3, wherein, The phosphazene derivatives are selected from the following compounds (1-1) or (1-3):

5. The positive electrode slurry composition as described in claim 1, wherein, The positive electrode active material comprises at least one of the group consisting of lithium iron phosphate, lithium manganese phosphate, lithium iron manganese phosphate, lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, nickel cobalt manganese aluminum oxide, lithium nickel manganese oxide, nickel manganese cobalt oxide, nickel cobalt aluminum oxide, lithium nickel oxide, lithium manganese oxide, and lithium cobalt oxide.

6. The positive electrode slurry composition as claimed in claim 1 further includes a solvent, and the solvent comprises at least one selected from the group consisting of N-methylpyrrolidone, N-ethylpyrrolidone, ethylene carbonate, propylene carbonate, butenyl carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dipropyl carbonate, γ-butyrolactone, ethyl propionate, propyl propionate, trimethyl phosphate, triethyl phosphate, dimethylformamide, dimethylacetamide, hexamethylphosphonamide, dimethyl sulfoxide, cyclobutane, diethyl ether, acetonitrile, propionitrile, tetrahydrofuran, acetone, cyclohexanone, cyclopentanone, methanol, ethanol, and isopropanol.

7. The positive electrode slurry composition as described in claim 1, wherein, The adhesive comprises at least one of the group consisting of polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl fluoride, polyvinyl chloride, polychlorotrifluoroethylene, polybutadiene, polyvinyl alcohol, polyacrylic acid, polyacrylonitrile, polyacrylate, vinylidene fluoride-hexafluoropropylene copolymer, fluorinated ethylene-propylene copolymer and ethylene-tetrafluoroethylene copolymer.

8. The positive electrode slurry composition as described in claim 1, wherein, The conductive additive comprises at least one of the group consisting of carbon black, graphite, graphene, carbon fiber, carbon nanotubes, nitrogen-doped graphite, nitrogen-doped graphene, and metal fibers.

9. The positive electrode slurry composition as described in claim 1, wherein, Based on a total weight of 100 parts by weight of the positive electrode active material, adhesive, and conductive additive, the positive electrode active material, adhesive, and conductive additive comprise 80 to 98 parts by weight of the positive electrode active material, 1 to 10 parts by weight of the adhesive, and 1 to 10 parts by weight of the conductive additive.

10. The positive electrode slurry composition as described in claim 9, wherein, Compared to the positive electrode active material, adhesive and conductive additive totaling 100 parts by weight, the flame retardant content in the positive electrode slurry composition is 0.01 to 10 parts by weight.

11. A positive electrode comprising an active material layer and a current collector, wherein, The active material layer is formed by coating the current collector with the positive electrode slurry composition as described in claim 1 and evaporating the solvent.

12. The positive electrode as described in claim 11, wherein, Based on a total weight of 100 parts by weight of the positive electrode active material, adhesive, and conductive additive, the positive electrode active material, adhesive, and conductive additive comprise 80 to 98 parts by weight of the positive electrode active material, 1 to 10 parts by weight of the adhesive, and 1 to 10 parts by weight of the conductive additive.

13. The positive electrode as described in claim 12, wherein, Compared to the total of 100 parts by weight of the positive electrode active material, adhesive and conductive additive, the content of the flame retardant in the active material layer is 0.01 to 10 parts by weight.

14. An electrochemical device comprising a positive electrode, a negative electrode, an electrolyte filled between the positive and negative electrodes, and a separator disposed in the electrolyte, as described in claim 11.

15. The electrochemical device as described in claim 14, wherein, The electrochemical device is a lithium-ion secondary battery.

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