Low-temperature-resistant sodium-ion battery positive electrode sheet and preparation method thereof

Abstract

The application relates to the field of battery materials, and provides a low-temperature-resistant sodium ion battery positive plate and a preparation method thereof; the method comprises the following steps: S1, blending a positive active material, a conductive agent and a binder to obtain a first mixture; S2, adding a layered two-dimensional transition metal material and a low-temperature activator into the first mixture in proportion, and then mixing to obtain a second mixture; S3, drying the second mixture first, then adding a solvent to obtain a third mixture, and then performing step-by-step ball milling treatment on the third mixture to obtain a positive electrode slurry; and S4, coating the positive electrode slurry on the surface of an aluminum foil, and then sequentially performing drying and rolling to obtain the low-temperature-resistant sodium ion battery positive plate; the sodium ion battery positive plate prepared by the method exhibits a high capacity retention rate in a wide low-temperature temperature range, so that the working temperature range of the battery is significantly widened.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of battery materials, in particular to a low-temperature-resistant sodium-ion battery positive electrode sheet and a preparation method thereof. BACKGROUND

[0002] Sodium-ion batteries are considered as a key candidate technology for the next generation of large-scale energy storage and electric vehicles due to their abundant resources, low cost and high intrinsic safety, especially for applications in harsh environments such as high altitude and high latitude.

[0003] However, its industrialization faces the problem of rapid decline in low-temperature performance, that is, the available capacity and power output of the battery will decrease significantly below 0℃, especially in extreme environments of -20℃ or even lower, which seriously limits its actual application range. The root cause of this problem lies in the fact that low temperature significantly worsens two core kinetic processes inside the electrode: first, the transport of sodium ions inside the electrode solid-phase material and at the solid-liquid interface becomes extremely slow; second, the stability of the electrode-electrolyte interface (CEI) decreases, resulting in an exponential increase in interface impedance.

[0004] Based on the above-mentioned problems, the existing technology mainly adopts electrolyte optimization, that is, by using solvents with low freezing point and low viscosity, such as chain ethers, or high-concentration / local high-concentration electrolytes, aiming to reduce the freezing point of the electrolyte system and improve its low-temperature ionic conductivity; however, such methods can only extend the lower limit of the working temperature range of the battery to a certain extent, but often cause new contradictions:

[0005] (1) Ether solvents have poor oxidation stability at high-pressure positive electrodes, which easily causes continuous interface side reactions and gas production, seriously damaging the cycle life and safety.

[0006] (2) High-concentration electrolytes are accompanied by inherent disadvantages such as high cost, high viscosity, and poor wettability.

[0007] (3) Most importantly, electrolyte optimization can only improve the transport of ions in the bulk electrolyte, and has very limited effect on solving the slow diffusion of sodium ions inside the solid phase of the positive electrode active material and the high-energy barrier charge transfer process at the solid-liquid interface. SUMMARY

[0008] In view of the above deficiencies in the prior art, the core purpose of the present application is to solve the technical problem of how to improve the transmission dynamics of sodium ions at low temperatures and stabilize the electrode-electrolyte interface synchronously and cooperatively from the inside of the electrode, thereby significantly improving the low-temperature performance of sodium ion batteries in a wide temperature range; specifically, by creatively introducing a bifunctional composite additive system formed by an interface polarization inhibitor composed of an ion channel expander and a low-temperature activator of a layered two-dimensional transition metal material in the positive electrode slurry process, and combining a specific dispersion process, a composite functional layer integrating high-speed ion transmission and a solid low-temperature interface is constructed in situ on the surface of the positive electrode active material and between the particles.

[0009] The present application is realized by the following technical solutions:

[0010] The first object of the present application is to provide a preparation method of a low-temperature-resistant sodium ion battery positive electrode sheet, comprising the following steps:

[0011] S1, blending positive electrode active material, conductive agent and binder to obtain a first mixture;

[0012] S2, adding layered two-dimensional transition metal material and low-temperature activator to the first mixture in proportion, and then mixing to obtain a second mixture;

[0013] S3, drying the second mixture first, then adding a solvent to obtain a third mixture, and then performing a stepwise ball milling treatment on the third mixture to obtain a positive electrode slurry;

[0014] S4, coating the positive electrode slurry on the surface of an aluminum foil, and then sequentially drying and rolling to obtain a low-temperature-resistant sodium ion battery positive electrode sheet.

[0015] As a preferred, the positive electrode active material is one or more of sodium vanadium phosphate (Na3V2(PO4)3), sodium iron phosphate (NaFePO4), sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7), sodium iron sulfate (Na2Fe2(SO4)3), O3-type layered oxide (such as NaNi x Fe Mn z O2, wherein 0 0.8 Li 0.12 Ni 0.22 Mn 0.66 O2, Na 0.67 Cu 0.28 Fe 0.34 Mn 0.38 O2); the conductive agent is one or more of conductive carbon black and conductive graphite; and the binder is one or more of polyvinylidene fluoride, polyacrylic acid and polymethyl methacrylate.

[0016] Preferably, in step S1, the mass ratio of the positive electrode active material, the conductive agent and the binder is (87-91):(7-9):(2.5-3.5).

[0017] More preferably, in step S1, the mass ratio of the positive electrode active material, the conductive agent and the binder is 89:8:3.

[0018] Preferably, the layered two-dimensional transition metal material is one or more of WS2, MoS2, WSe2, MoSe2, WTe2, MoTe2, NbS2, TaS2 and VS2.

[0019] More preferably, the layered two-dimensional transition metal material is WS2 or MoS2.

[0020] Further preferably, the layered two-dimensional transition metal material is WS2 nanosheet or MoS2 nanosheet.

[0021] Preferably, the low-temperature activator is one or more of graphite-phase carbon nitride (g-C3N4), alpha-phase carbon nitride (a-C3N4), beta-phase carbon nitride (b-C3N4), cubic-phase carbon nitride (c-C3N4), wurtzite boron nitride (w-BN), rhombohedral / trigonal boron nitride (r-BN), hexagonal boron nitride (h-BN), cubic boron nitride (c-BN).

[0022] More preferably, the low-temperature activator is g-C3N4 or h-BN.

[0023] Preferably, in step S2, the added mass of the layered two-dimensional transition metal material is 0.1%-10% and the added mass of the low-temperature activator is 0.1%-5%, based on the total mass of the second mixture.

[0024] More preferably, in step S2, the added mass of the layered two-dimensional transition metal material is 2%-5% and the added mass of the low-temperature activator is 1%-3%, based on the total mass of the second mixture.

[0025] Preferably, in step S3, the solvent is N-methylpyrrolidone; the N-methylpyrrolidone is added to the dried second mixture to obtain the third mixture; wherein the solid content of the third mixture is 45%-60%.

[0026] More preferably, the solid content of the third mixture is 50%-55%.

[0027] Preferably, in step S3, the stepwise ball milling process comprises the following steps: first stage ball milling at a first rotating speed, and then second stage ball milling at a second rotating speed, wherein the first rotating speed is 100 rpm-300 rpm, and the second rotating speed is 400 rpm-600 rpm.

[0028] More preferably, the first rotating speed is 200 rpm-250 rpm, and the second rotating speed is 450 rpm-500 rpm.

[0029] Preferably, in step S3, the second mixture is dried at a temperature of 90℃-110℃ under vacuum or inert atmosphere protection for 2 h-6 h.

[0030] Preferably, in step S4, the drying comprises the following steps: first, air drying at 85℃-95℃ for 10 min-20 min to preliminarily solidify the surface layer of the slurry, and then drying in a vacuum drying oven at 100℃-120℃ for 4 h-8 h.

[0031] Preferably, in step S4, the line pressure of the rolling is 30 MPa-50 MPa.

[0032] The second object of the present application is to provide a low-temperature-resistant sodium-ion battery positive electrode sheet prepared by the above preparation method, comprising an aluminum foil and a positive electrode material layer combined on the surface of the aluminum foil; the positive electrode material layer comprises a positive electrode active material, a conductive agent, a binder, a layered two-dimensional transition metal material, and a low-temperature activator.

[0033] The inventive concept of the present application is that the present application creatively proposes to simultaneously introduce a layered two-dimensional transition metal material and a low-temperature activator in the positive electrode slurry process to form a dual-functional composite additive system; wherein the layered two-dimensional transition metal material acts as an ion channel expander, and the low-temperature activator acts as an interface polarization inhibitor, which synergistically optimizes the ion transport kinetics and interface stability in the electrode, and specifically:

[0034] Firstly, a three-dimensional porous framework with electronic conductivity and mechanical integrity is constructed by step S1, that is, the positive active material is the active center of the electrochemical reaction, and the physical contact resistance between the particles is high; and the conductive agent, such as conductive carbon black and graphite, fills the gap between the active particles to form a continuous conductive network, thereby greatly reducing the electronic transmission impedance of the electrode; further, the binder firmly combines the active material and the conductive agent particles through the physical entanglement and adhesion of the polymer chain, and forms a flexible framework after drying, which finally prevents the electrode from pulverizing during the subsequent rolling and charging and discharging processes; and the pore structure of the three-dimensional framework provides an initial space for the embedding of the subsequent functional materials, and the infiltration of the electrolyte and the liquid phase transmission of sodium ions in the application stage, which is the physical basis for realizing high-performance electrodes.

[0035] Further, the bifunctional synergistic system designed to improve low-temperature performance is creatively introduced by step S2, the core of which is to solve the two key problems of low-temperature ion transfer delay and interface impedance surge simultaneously from the inside of the electrode, namely: first, a layered two-dimensional transition metal material is used as an ion channel expander, on the one hand, its unique layered structure can be attached to the surface of the positive active material, its atomically smooth plane and abundant edge active sites can provide a significantly reduced energy barrier for the adsorption and deintercalation of sodium ions at the solid-liquid interface at low temperature, directly accelerating the low-temperature charge transfer process, effectively solving the problem of sodium ion diffusion delay in the electrode solid phase at low temperature which is difficult to overcome by simply relying on electrolyte optimization or material modification in the prior art; on the other hand, its inherent high electrical conductivity helps to improve the uniformity of the electric field inside the electrode in the case of increased low-temperature resistance, reducing the additional polarization and performance decay caused by excessive local current density. More importantly, a low-temperature activator is further introduced as an interface property modifier, taking g-C3N4 as an example, on the one hand, its nitrogen-rich structure can preferentially adsorb and catalyze the uniform and controllable decomposition of electrolyte components on the surface of the positive electrode, prompting the formation of a thin and dense, rigid inorganic component-rich, such as NaF, and elastic organic-inorganic hybrid CEI film at low temperature, which has higher ion conductivity and mechanical stability at low temperature, and can effectively inhibit the generation of interface cracks and impedance growth during low-temperature cycling; on the other hand, the robust CEI film constructed as if a low-temperature resistant armor covers the electrode surface, significantly improving the interface structure integrity of the positive electrode material during low-temperature and cycling; most importantly, the aforementioned two exhibit a key synergistic enhancement effect in a low-temperature environment: the layered two-dimensional material provides a high specific surface area dispersion carrier and modification site for g-C3N4, and the modification of g-C3N4 at its edges can further reduce the low-temperature ion transfer impedance; at the same time, the high-quality CEI film induced by g-C3N4 can effectively cover and protect the two-dimensional material, preventing side reactions and deactivation in the harsh interface environment at low temperature, and the two work together to construct a composite functional layer of high-speed ion transport and robust low-temperature interface on the surface of active particles and between particles, thereby essentially improving the low-temperature resistance of the electrode.

[0036] Further, by step S3, the dry ingredients are first contacted by drying, then the solvent is added to dissolve the binder, and the formed glue solution is used to wrap and lock the dry ingredients, effectively preventing the agglomeration of two-dimensional materials in the liquid phase; then a step ball mill is used, first low-speed depolymerization to soften the agglomerates, and then high-speed to realize the uniform distribution of each functional component on the surface of the active material.

[0037] Finally, the slurry is coated on the aluminum foil and dried by step S4, so that the binder is precipitated to form an interwoven polymer network, and all solid materials are bound and fixed; and the materials are pressed to produce closer contact and even plastic deformation, thereby significantly enhancing the connectivity of the electronic and ionic transport network of the electrode.

[0038] The technical scheme of the embodiment of the application has at least the following advantages and beneficial effects:

[0039] (1) The application constructs a bifunctional synergistic system of ion channel expander and interface polarization inhibitor to simultaneously improve the low-temperature ion transport rate and interface stability from the inside of the electrode, wherein the two-dimensional material expands the ion channel, significantly reduces the low-temperature charge transfer energy barrier, and accelerates the reaction dynamics; at the same time, the low-temperature activator induces the formation of a strong CEI film, stabilizes the electrode interface, and effectively inhibits the impedance growth.

[0040] (2) Through the special process of dry mixing and step-by-step ball milling, uniform dispersion and firm compounding of the nano additive on the surface of the active material are realized.

[0041] (3) It exhibits high capacity retention rate at-20℃ and-40℃, and significantly widens the working temperature range of the battery. BRIEF DESCRIPTION OF DRAWINGS

[0042] Figure 1 The SEM image of the sodium vanadate sample provided by the application (scale: 10 μm).

[0043] Figure 2 The SEM image of the sample of Example 1 (scale: 10 μm).

[0044] Figure 3 The SEM image of the sample of Example 1 at a higher magnification (scale: 5 μm).

[0045] Figure 4 The data statistical graph of Test Example 1.

[0046] Figure 5 The data statistical graph of Test Example 2. DETAILED DESCRIPTION

[0047] Example 1

[0048] The embodiment provides a preparation method of a low-temperature-resistant sodium ion battery positive electrode sheet, comprising the following steps:

[0049] S1, dry-blending sodium vanadate, conductive carbon black (type: Super P), conductive graphite (type: KS6) and polyvinylidene fluoride binder according to the mass ratio of 89:4:4:3 to obtain a first mixture.

[0050] S2, 5% of tungsten disulfide nanosheets by total mass and 2% of graphite phase carbon nitride by total mass were added to the first mixture, and uniform mixing was continued to obtain a second mixture.

[0051] S3, the second mixture was placed in a vacuum drying oven and dried at 100°C for 4 hours. Subsequently, N-methylpyrrolidone solvent was added to the dried material to prepare a slurry with a solid content of 50%, and a third mixture was obtained. The third mixture was placed in a planetary ball mill, first ball-milled at a speed of 220 rpm for 30 minutes, and then ball-milled at a speed of 480 rpm for 60 minutes to complete the stepwise ball-milling homogenization treatment, and a uniform positive electrode slurry was obtained.

[0052] S4, the obtained slurry was uniformly coated on a 16 μm thick aluminum foil, first dried at 90°C for 15 minutes to preliminarily solidify the surface layer, and then transferred to a 110°C vacuum drying oven for drying for 6 hours. Finally, the electrode sheet was rolled under a linear pressure of 40 MPa, and cut to the required size to prepare a low-temperature-resistant sodium-ion battery positive electrode sheet.

[0053] The positive electrode sheet prepared above was taken as a sample, and its SEM images under scales of 10 μm and 5 μm were tested, respectively, to obtain Figure 2 and Figure 3 ; similarly, under a scale of 10 μm, the SEM image of the sodium vanadium phosphate sample was obtained, as shown in Figure 1 .

[0054] From Figure 1 and Figure 2 and Figure 3 , it can be observed that the sample obtained in this embodiment has undergone significant changes in micro-morphology; the original sodium vanadium phosphate block particles are successfully coated and modified to form a uniform and interwoven composite functional layer composed of conductive agents, WS2 nanosheets and g-C3N4.

[0055] Example 2

[0056] The present embodiment provides a preparation method of a low-temperature-resistant sodium-ion battery positive electrode sheet, comprising the following steps:

[0057] S1, sodium iron phosphate, conductive carbon black (type: Super P), conductive graphite (type: KS15) and polyvinylidene fluoride binder were dry blended according to a mass ratio of 89:4:4:3 to obtain a first mixture.

[0058] S2, 3% of molybdenum disulfide nanosheets by total mass and 2% of hexagonal boron nitride by total mass were added to the first mixture, and uniform mixing was continued to obtain a second mixture.

[0059] S3, dry the second mixture at 95°C for 3 hours under argon protection. Then, N-methyl pyrrolidone solvent is added to prepare a slurry with a solid content of 55%, obtaining a third mixture. The third mixture is subjected to stepwise ball milling: first at 240 rpm for 20 minutes, and then at 520 rpm for 50 minutes, to obtain a uniform positive electrode slurry.

[0060] S4, the slurry is coated on an aluminum foil, dried at 85°C for 20 minutes by air blowing, dried at 100°C for 8 hours by vacuum drying, and then rolled under a linear pressure of 35 MPa, and cut to obtain a low-temperature-resistant positive electrode sheet.

[0061] Example 3

[0062] The embodiment provides a preparation method of a low-temperature-resistant positive electrode sheet of a sodium ion battery, comprising the following steps:

[0063] S1, sodium iron pyrophosphate, conductive carbon black (type: Super P), conductive graphite (type: KS6) and polyvinylidene fluoride binder are dry blended according to a mass ratio of 89:4:4:3 to obtain a first mixture.

[0064] S2, 3% of WS2 nanosheets and 1% of h-BN based on the total mass of the first mixture are added to the first mixture, and the mixture is uniformly mixed to obtain a second mixture.

[0065] S3, the second mixture is vacuum dried at 105°C for 2.5 hours. Then, N-methyl pyrrolidone solvent is added to prepare a slurry with a solid content of 48%, and stepwise ball milling is performed: first at 200 rpm for 40 minutes, and then at 500 rpm for 40 minutes.

[0066] S4, the slurry after homogenization is coated on an aluminum foil, dried at 95°C for 10 minutes by air blowing, dried at 115°C for 5 hours by vacuum drying, and then rolled under a linear pressure of 45 MPa, and cut to obtain a positive electrode sheet.

[0067] Example 4

[0068] The embodiment provides a preparation method of a low-temperature-resistant positive electrode sheet of a sodium ion battery, comprising the following steps:

[0069] S1, sodium iron sulfate, conductive carbon black (type: Super P), conductive graphite (type: KS15) and polyvinylidene fluoride binder are dry blended according to a mass ratio of 89:4:4:3 to obtain a first mixture.

[0070] S2, 2% of MoS2 nanosheets and 2% of g-C3N4 based on the total mass of the first mixture are added to the first mixture, and the mixture is uniformly mixed to obtain a second mixture.

[0071] S3, the second mixture was vacuum dried at 110°C for 2 hours. Subsequently, N-methyl pyrrolidone solvent was added to prepare a slurry with a solid content of 50%, and a step ball milling treatment was performed: first ball milling at 250 rpm for 25 minutes, and then ball milling at 450 rpm for 70 minutes.

[0072] S4, the obtained slurry was coated on an aluminum foil, dried by blowing at 88°C for 18 minutes, and then vacuum dried at 105°C for 7 hours. Finally, the roll was pressed under a linear pressure of 30 MPa and cut to obtain a low-temperature-resistant positive electrode sheet.

[0073] Example 5

[0074] The embodiment provides a preparation method of a low-temperature-resistant sodium ion battery positive electrode sheet, comprising the following steps:

[0075] S1, a sodium iron sulfate, conductive carbon black (type: Super P), conductive graphite (type: KS15) and polyvinylidene fluoride binder were dry blended in a mass ratio of 87:4.5:5:3.5 to obtain a first mixture. 0.3 Fe 0.3 Mn 0.4 O2, conductive carbon black (type: Super P), conductive graphite (type: KS15) and polyvinylidene fluoride binder were dry blended in a mass ratio of 89:4:4:3 to obtain a first mixture.

[0076] S2, 2% of WS2 nanosheets and 1% of g-C3N4 based on the total mass of the first mixture were added to the first mixture, and the mixture was uniformly mixed to obtain a second mixture.

[0077] S3, the second mixture was vacuum dried at 100°C for 5 hours. Subsequently, N-methyl pyrrolidone solvent was added to prepare a slurry with a solid content of 55%, and a step ball milling treatment was performed: first ball milling at 250 rpm for 30 minutes, and then ball milling at 500 rpm for 60 minutes.

[0078] S4, the slurry after homogenization was coated on an aluminum foil, dried by blowing at 92°C for 12 minutes, vacuum dried at 120°C for 4 hours, and then roll-pressed under a linear pressure of 50 MPa, and finally cut to obtain a low-temperature-resistant sodium ion battery positive electrode sheet.

[0079] Example 6

[0080] The embodiment provides a preparation method of a low-temperature-resistant sodium ion battery positive electrode sheet, comprising the following steps:

[0081] S1, a sodium iron sulfate, conductive carbon black (type: Super P), conductive graphite (type: KS15) and polyvinylidene fluoride binder were dry blended in a mass ratio of 87:4.5:5:3.5 to obtain a first mixture.

[0082] S2, 0.1% of tungsten disulfide nanosheets and 0.1% of graphite phase carbon nitride by total mass of the first mixture were added and uniformly mixed to obtain a second mixture.

[0083] S3, the second mixture was placed in a vacuum drying oven and dried at 95°C for 5 hours. Subsequently, N-methylpyrrolidone solvent was added to the dried material to prepare a slurry with a solid content of 45%, and a third mixture was obtained. The third mixture was placed in a planetary ball mill, first ball-milled at a speed of 200 rpm for 40 minutes, and then ball-milled at a speed of 450 rpm for 50 minutes to complete the stepwise ball-milling homogenization treatment, and a uniform positive electrode slurry was obtained.

[0084] S4, the obtained slurry was uniformly coated on an aluminum foil, first dried at 90°C for 15 minutes, and then transferred to a 110°C vacuum drying oven for 7 hours. Finally, the electrode sheet was rolled under a linear pressure of 35 MPa and cut to the required size to obtain a low-temperature-resistant sodium-ion battery positive electrode sheet.

[0085] Example 7

[0086] The present embodiment provides a preparation method of a low-temperature-resistant sodium-ion battery positive electrode sheet, comprising the following steps:

[0087] S1, sodium iron sulfate, conductive carbon black (type: Super P), conductive graphite (type: KS15) and polyvinylidene fluoride binder were dry blended according to a mass ratio of 91:3.5:3:2.5 to obtain a first mixture.

[0088] S2, 10% of molybdenum disulfide nanosheets and 5% of hexagonal boron nitride by total mass of the first mixture were added and uniformly mixed to obtain a second mixture.

[0089] S3, the second mixture was dried at 105°C under argon protection for 3 hours. Subsequently, N-methylpyrrolidone solvent was added to prepare a slurry with a solid content of 60% to obtain a third mixture. The third mixture was subjected to stepwise ball-milling treatment: first ball-milled at 220 rpm for 30 minutes, and then ball-milled at 480 rpm for 80 minutes to obtain a uniform positive electrode slurry.

[0090] S4, the slurry was coated on an aluminum foil, dried at 85°C for 20 minutes and 115°C for 6 hours in a vacuum drying oven, then rolled under a linear pressure of 40 MPa, and cut to obtain a low-temperature-resistant positive electrode sheet.

[0091] Test Example 1

[0092] The test purpose of this test example is to verify and optimize the addition amount ratio of the layered two-dimensional transition metal material (WS2) and the low-temperature activator (g-C3N4) described in Example 1, and to confirm the best range of the synergistic improvement of the two on the electrochemical performance of the positive plate in a-20℃ low-temperature environment.

[0093] Test method: According to the national standard “GB / T 44265-2024 Technical Specification for Sodium-ion Battery Power Storage Station” and the group standard “T / CNESA 1006-2021 General Specification for Sodium-ion Battery”, test the capacity retention rate, after standing for not less than 16h at an ambient temperature of-20℃±2℃, discharge at the same ambient temperature at 1.0I5 to the discharge termination voltage, and record the discharge capacity. The test results are as follows:

[0094] Table 1 -20℃ capacity retention rate test results

[0095]

[0096] According to Table 1 and Figure 4 Analysis shows that under the-20℃ test conditions, the composite addition of WS2 and g-C3N4 has a clear synergistic effect on the improvement of the-20℃ low-temperature performance of the positive plate, and there is an optimal ratio, which is specifically manifested as follows: when only WS2 or g-C3N4 is added, the capacity retention rate is improved to a certain extent, i.e. 87.2% when only 3.5% WS2 is added, and 85.3% when only 2% g-C3N4 is added, but after the two are compounded at a specific ratio, the performance is significantly improved; among them, when the WS2 addition amount is 3.5% and the g-C3N4 addition amount is 2%, the capacity retention rate reaches a peak value of 92.5%, which is significantly higher than the arithmetic mean of the best values of the two when added alone, confirming the unexpected effect of the synergistic enhancement of the two. Among them, the high-performance region with a capacity retention rate of >90% is concentrated in the range of WS2 dosage of 2% to 5% and g-C3N4 dosage of 1% to 3%, which can be used as the optimal ratio; it can be seen that the layered two-dimensional material and the low-temperature activator can synergistically construct an efficient ion transmission channel and a stable electrode interface when the ratio is optimized, thereby significantly improving the low-temperature capacity retention performance of the sodium-ion battery.

[0097] Test Example 2

[0098] The test purpose of this test example is to verify and optimize the addition amount ratio of the layered two-dimensional transition metal material (WS2) and the low-temperature activator (g-C3N4) described in Example 1, and to confirm the best range of the synergistic improvement of the two on the electrochemical performance of the positive plate in a-20℃ low-temperature environment.

[0099] Test method: According to the national standard "GB / T 44265-2024 Sodium-ion battery technical specification for power storage station" and the group standard "T / CNESA 1006-2021 General specification for sodium-ion battery", the capacity retention rate was tested. After being stored at an ambient temperature of -40℃±2℃ for not less than 16h, the battery was discharged at 1.0C to the end voltage under the same ambient temperature, and the discharge capacity was recorded. The test results are as follows:

[0100] Table 2 - Test results of capacity retention rate at -40℃

[0101]

[0102] According to Table 2 and Figure 5 Analysis shows that under more extreme low temperature conditions, i.e. -40℃ test conditions, the synergistic effect of WS2 and g-C3N4 is consistent with that at -20℃, and the optimal ratio is highly consistent, further proving the effectiveness of the dual-functional additive system in a wide temperature range. When the WS2 addition amount is 3.5% and the g-C3N4 addition amount is 2%, the capacity retention rate reaches a peak value of 81.2%, which is significantly higher than the performance of adding 3.5% WS2 alone or adding 2% g-C3N4 alone under the same conditions, further confirming the synergistic effect between the two. In addition, although the overall capacity retention rate decreases as the temperature decreases, the high performance region is still stably distributed in the range of WS2 dosage 2%~5% and g-C3N4 dosage 1%~3%, which is consistent with the conclusion of Test Example 1, indicating that this dosage range is universal. As can be seen, the composite additive scheme provided in the embodiment can continuously improve the low temperature resistance of the battery in a wide low temperature range of -20℃ to -40℃ through a stable synergistic mechanism.

Claims

1. A method for preparing a low-temperature-resistant sodium-ion battery cathode sheet, characterized in that, The method comprises the following steps: S1, blending a positive electrode active material, a conductive agent and a binder to obtain a first mixture; S2, adding a layered two-dimensional transition metal material and a low-temperature activator to the first mixture in proportion, and then mixing to obtain a second mixture; S3, drying the second mixture first, then adding a solvent to obtain a third mixture, and then performing a step-by-step ball milling treatment on the third mixture to obtain a positive electrode slurry; S4, coating the positive electrode slurry on the surface of an aluminum foil, and then sequentially drying and rolling to obtain a low-temperature-resistant sodium-ion battery positive electrode sheet; The layered two-dimensional transition metal material is one or more of WS2, MoS2, WSe2, MoSe2, WTe2, MoTe2, NbS2, TaS2 and VS2; The low-temperature activator is one or more of graphite-phase carbon nitride, alpha-phase carbon nitride, beta-phase carbon nitride, cubic-phase carbon nitride, wurtzite boron nitride, zinc-blende boron nitride, hexagonal boron nitride, cubic boron nitride.

2. The preparation method of the low-temperature-resistant sodium-ion battery positive electrode sheet according to claim 1, characterized in that, The positive electrode active material is one or more of sodium vanadium phosphate, sodium iron phosphate, pyrophosphate sodium iron phosphate, sodium iron sulfate, O3-type layered oxide and P2-type layered oxide; The conductive agent is one or more of conductive carbon black and conductive graphite; The binder is one or more of polyvinylidene fluoride, polyacrylic acid and polymethyl methacrylate.

3. The preparation method of the low-temperature-resistant sodium-ion battery positive electrode sheet according to claim 1, characterized in that, In step S1, the mass ratio of the positive electrode active material, the conductive agent and the binder is (87-91):(7-9):(2.5-3.5).

4. The preparation method of the low-temperature-resistant sodium-ion battery positive electrode sheet according to claim 1, characterized in that, In step S2, based on the total mass of the second mixture, the added mass of the layered two-dimensional transition metal material is 0.1%-10%, and the added mass of the low-temperature activator is 0.1%-5%.

5. The preparation method of the low-temperature-resistant sodium-ion battery positive electrode sheet according to claim 4, characterized in that, In step S2, based on the total mass of the second mixture, the added mass of the layered two-dimensional transition metal material is 2%-5%, and the added mass of the low-temperature activator is 1%-3%.

6. The preparation method of the low-temperature-resistant sodium-ion battery positive electrode sheet according to claim 1, characterized in that, In step S3, the solvent is N-methylpyrrolidone; the N-methylpyrrolidone is added to the dried second mixture to obtain the third mixture; wherein the solid content of the third mixture is 45%-60%.

7. The preparation method of the low-temperature-resistant sodium-ion battery positive electrode sheet according to claim 1, characterized in that, The step S3 includes the following steps: First, a first-stage ball milling is performed at a first rotational speed to disperse the third mixture; then the speed is accelerated to a second rotational speed for a second-stage ball milling to uniformly distribute the layered two-dimensional transition metal material, the low-temperature activator, the conductive agent and the binder on the surface of the positive electrode active material; The first rotational speed is 100 rpm-300 rpm; The second rotational speed is 400 rpm-600 rpm.

8. The low-temperature-resistant sodium-ion battery positive electrode sheet prepared by the preparation method according to any one of claims 1 to 7, characterized in that, The aluminum foil and the positive electrode material layer combined on the surface of the aluminum foil; the positive electrode material layer comprises a positive electrode active material, a conductive agent, a binder, a layered two-dimensional transition metal material and a low-temperature activator.

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