Diaphragm and sodium ion battery using same

Abstract

The invention relates to the technical field of sodium ion batteries, and discloses a diaphragm and a sodium ion battery using the diaphragm, the diaphragm comprises a porous base membrane and a functional coating coated on at least one surface of the porous base membrane; the functional coating is prepared from the following raw materials in parts by weight: 85-95 parts of a ferric sodium pyrophosphate active material, 4-10 parts of a binder and 1-5 parts of a conductive agent. In the sodium ion battery, one side of the diaphragm, which is provided with the functional coating, is arranged towards the positive pole piece, and the coating is in close contact with the positive pole through hot-pressing shaping. The high-proportion ferric sodium pyrophosphate is loaded on the diaphragm, and the sodium removal characteristic of the ferric sodium pyrophosphate during first-time charging is utilized, so that extra sodium ions are provided for the negative electrode to compensate for consumption formed by an SEI film, and the first-time coulombic efficiency and energy density of the battery are improved; meanwhile, high-temperature shrinkage of the base membrane is inhibited by utilizing a heat-resistant framework supporting effect of the coating, internal short circuit of the battery is prevented, and the thermal safety of the battery is enhanced.

Description

A separator and a sodium-ion battery using the separator Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, specifically to a separator and a sodium-ion battery using the separator. Background Technology

[0002] Sodium-ion batteries operate on a similar principle to lithium-ion batteries, primarily relying on the reversible insertion and extraction of sodium ions between the positive and negative electrodes to store and release energy. The separator, a key component of sodium-ion batteries, is located between the positive and negative electrodes. Its core function is to provide electronic insulation to prevent short circuits caused by contact between the positive and negative electrodes, while simultaneously providing microporous channels to allow sodium ions to pass through. The separator's permeability, thermal stability, and interfacial properties directly affect the battery's electrochemical performance and safety.

[0003] Currently, commercially available separators are mainly made of polyolefin materials such as polyethylene (PE) and polypropylene (PP). While these separators offer advantages such as low cost and good mechanical strength, their thermal stability is relatively insufficient. Polyethylene and polypropylene typically have low melting points. When the battery temperature rises due to internal short circuits or external heating, the polyolefin separator is prone to thermal shrinkage and deformation, leading to direct contact between the positive and negative electrodes and potentially causing thermal runaway. To improve the thermal stability of the separator, existing technologies typically employ coating the base membrane surface with inorganic ceramic particles such as alumina or boehmite, or heat-resistant polymers. These coatings maintain structural integrity at high temperatures, suppressing base membrane shrinkage through physical support.

[0004] However, existing separator modification technologies, while improving safety, introduce new problems. These ceramic or polymer coating materials typically lack electrochemical activity and are considered inactive components. Introducing these inactive substances into the battery increases its overall weight and volume, leading to a decrease in energy density. Simultaneously, some ceramic coating materials have low ionic conductivity and poor wettability with the electrolyte, increasing interfacial transport impedance and hindering the battery's rate performance.

[0005] Furthermore, sodium-ion batteries typically use hard carbon as the negative electrode material, which has a large specific surface area. During the first charge, the electrolyte decomposes on the surface of the hard carbon negative electrode to form a solid electrolyte interphase (SEI) film. This process consumes a large amount of active sodium ions from the positive electrode, resulting in significant irreversible capacity loss and a low initial coulombic efficiency. Existing inactive separator coatings cannot compensate for the loss of active sodium during this process.

[0006] Therefore, developing a multifunctional separator that can both improve the thermal stability of the separator and provide an additional active sodium source to compensate for the capacity loss in the first cycle is a technical problem that needs to be solved in the current sodium-ion battery field. Summary of the Invention

[0007] The technical problem solved by this invention is that, during the first charge of existing sodium-ion batteries, the formation of a solid electrolyte interface (SEI) film on the negative electrode surface consumes irreversible sodium ions, resulting in low initial coulombic efficiency and reversible capacity loss. At the same time, conventional polyolefin separators have poor thermal stability and are prone to shrinkage at high temperatures, thus posing safety hazards.

[0008] To solve the above problems, the present invention provides the following technical solution: In a first aspect, the present invention provides an electrochemically active composite membrane, which adopts the following technical solution: An electrochemically active composite membrane includes a porous base membrane and a functionalized coating coated on at least one surface of the porous base membrane; the functionalized coating is made of raw materials comprising the following parts by weight: sodium iron pyrophosphate active material: 85-95 parts; binder: 4-10 parts; conductive agent: 1-5 parts.

[0009] By adopting the above technical solution, and using a high proportion of sodium iron pyrophosphate as the main material of the functional coating, combined with a specific proportion of conductive agent and binder, the dual effects of active sodium source compensation and high-temperature thermal barrier can be achieved.

[0010] The specific mechanism and effects are described below: Electrochemical activity compensation: Sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7, NFPP), as a polyanionic cathode material, is coated on the separator surface at a high loading (85-95 parts) as an additional source of active sodium. When the battery undergoes its first charge, the NFPP in the coating undergoes an oxidation reaction with the assistance of a conductive network, releasing sodium ions. The released sodium ions migrate through the separator to the negative electrode side to replenish the sodium ions consumed in the formation of the SEI film on the negative electrode surface, thereby avoiding the consumption of the active sodium in the positive electrode itself and improving the battery's initial coulombic efficiency and energy density.

[0011] Improved thermal stability: Sodium iron pyrophosphate has a stable three-dimensional framework structure and PO bonds, resulting in high thermal stability. When the internal temperature of the battery rises, the functionalized coating on the surface of the base film forms a heat-resistant skeleton. Through the anchoring effect of the binder, it physically inhibits the thermal shrinkage of the polyolefin base film, prevents short circuits between the positive and negative electrodes, and improves the thermal safety of the battery.

[0012] Synergistic composition: 85-95 parts of active material ensure sufficient sodium replenishment capacity; 1-5 parts of conductive agent build electron transport channels on the membrane surface, enabling the active material to participate in electrochemical reactions through contact with the positive electrode current collector; 4-10 parts of binder ensure the integrity of the coating structure and prevent peeling or blockage of the base membrane pores.

[0013] Preferably, the binder is polyvinylidene fluoride; the conductive agent is selected from one or a combination of conductive carbon black, carbon nanotubes, and acetylene black.

[0014] By adopting the above technical solutions, polyvinylidene fluoride exhibits good chemical stability in the electrolyte, enhances the interfacial adhesion between the coating and the base film, and adapts to volume changes during cycling. Using conductive carbon black, carbon nanotubes, or acetylene black as conductive agents enables the construction of a synergistic long- and short-range conductive network, reduces coating resistance, and improves the utilization rate of active materials on the side furthest from the current collector.

[0015] Preferably, the thickness of the functionalized coating on one side is 3-8 μm.

[0016] By adopting the above technical solution, both ion transport performance and sodium replenishment capacity are balanced. With a thickness in the range of 3-8 μm, sufficient sodium source compensation can be provided while avoiding the extension of ion transport path and increase of battery internal resistance caused by excessive coating thickness.

[0017] Preferably, the median particle size D50 of the sodium iron pyrophosphate active material is 0.55-9.8 μm, the D90 is 2.1-28.5 μm, and the BET specific surface area is 5.2-14.8 m². 2 / g.

[0018] By adopting the above technical solution, the electrochemical kinetics are optimized by limiting the microstructure parameters of the active material: This improves reaction kinetics, resulting in a D50 of 0.55-9.8 μm and a specific surface area of ​​5.2-14.8 m². 2 The range of / g increases the contact area between the active particles and the electrolyte, shortens the sodium ion diffusion path, and enables the NFPP in the membrane coating to quickly remove sodium during the first charge cycle.

[0019] Ensuring coating quality: Limiting D90 to no more than 28.5μm can avoid coating defects or base film puncture risks caused by large particles, and ensure the smoothness and consistency of the coating surface.

[0020] Preferably, the sodium iron pyrophosphate active material is prepared by a method comprising the following steps: Step 1, preparation of precursor powder: iron source, phosphorus source, sodium source and reducing agent are weighed according to stoichiometric ratio, and carbon source is added. The mixture is ball-milled in a dispersion medium to obtain a precursor slurry; the precursor slurry is dried to obtain precursor powder; wherein, the ball-to-powder ratio of the ball-milling mixture is 5:1-10:1, the ball-milling speed is 300-500 rpm, and the ball-milling time is 4-20 hours; Step 2, sintering: the precursor powder is pre-calcined in an inert atmosphere, and then heated for crystallization sintering to obtain a sintered product; wherein, the crystallization sintering temperature is 600-750℃, and the holding time is 8-12 hours; Step 3, post-treatment: after cooling, the sintered product is pulverized and sieved to obtain the sodium iron pyrophosphate active material.

[0021] By adopting the above technical solution, the grain size and carbon coating of the material are controlled by mechanical ball milling and segmented sintering processes: raw material refinement and mixing: 300-500 rpm speed, 4-20 hours of ball milling time and 5:1-10:1 ball-to-material ratio, uniform mixing and refinement of raw materials are achieved through mechanical force, providing a basis for generating high-purity phases.

[0022] Crystal phase control: The crystallization sintering temperature range of 600-750℃ promotes the formation of highly crystallized NFPP phase, while avoiding grain coarsening and reduction of specific surface area due to excessive temperature.

[0023] Improved conductivity: The carbon source carbonizes during sintering to form a conductive coating layer, which improves the electronic conductivity of the material and enhances rate performance.

[0024] In a second aspect, the present invention provides a sodium-ion battery, which adopts the following technical solution: a sodium-ion battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator located between the positive electrode and the negative electrode; wherein, the side of the separator having a functionalized coating is disposed facing the positive electrode.

[0025] By adopting the above technical solution, the assembly direction of the composite membrane is defined, which is a necessary structural condition for sodium replenishment using functionalized coatings. Since sodium iron pyrophosphate needs to release sodium ions and undergo an oxidation reaction, aligning it towards the positive electrode allows for the establishment of an electron transport channel through the conductive network of the positive electrode and current collector, ensuring that the active material in the coating can gain electrons and release sodium ions. If aligned towards the negative electrode, the coating cannot perform its sodium replenishment function due to the insulated membrane substrate and the potential mismatch at the negative electrode, lacking an electron pathway. Furthermore, this arrangement avoids direct contact between the functionalized coating and the negative electrode, preventing the formation of an excessively thick solid electrolyte interphase (SEI) film on the coating surface and thus preventing membrane pore blockage.

[0026] Preferably, the positive electrode sheet comprises a layered oxide positive electrode material; the negative electrode sheet comprises a hard carbon negative electrode material; and the electrolyte comprises sodium hexafluorophosphate.

[0027] By adopting the above technical solution, layered oxides and hard carbon systems, combined with sodium hexafluorophosphate electrolyte, constitute the basic chemical unit of a sodium-ion battery. In this system, the hard carbon anode has a high specific surface area and suffers significant irreversible capacity loss during the first charge-discharge process. The sodium ions released by the separator coating can specifically replenish the sodium consumed in the film formation on the hard carbon surface, thereby improving the battery's initial coulombic efficiency.

[0028] This invention provides a method for preparing a sodium-ion battery, employing the following technical solution: A method for preparing a sodium-ion battery includes the following steps: S1, preparing a functionalized coating slurry: dissolving a binder in a solvent, adding a conductive agent and dispersing it evenly, then adding sodium iron pyrophosphate active material, adjusting the solid content and vacuum degassing to obtain a functionalized coating slurry, wherein the preferred viscosity is in the range of 1500-2800 mPa·s; S2, preparing a separator: coating the functionalized coating slurry onto the surface of a porous base membrane, baking and drying it at 65-75℃ to obtain a separator; S3, assembly and hot pressing: stacking or winding the positive electrode, separator, and negative electrode in sequence, ensuring that the coating side of the separator is tightly attached to the positive electrode to obtain a battery cell; hot pressing the battery cell at 70-80℃ to shape it; S4, electrolyte injection formation: injecting electrolyte into the hot-pressed and shaped battery cell and encapsulating it to form a sodium-ion battery.

[0029] By adopting the above technical solution, the interface performance is controlled by process parameters: slurry penetration control: the slurry viscosity is controlled in the range of 1500-2800 mPa·s. On the one hand, this ensures the leveling of the slurry on the surface of the base film to form a uniform coating. On the other hand, the high viscosity resistance prevents the slurry from excessively penetrating and clogging the micropores of the porous base film, thus maintaining the ion conductivity of the base film.

[0030] Reducing interfacial impedance: The 70-80℃ hot-pressing shaping process in step S3 utilizes the thermoplasticity of the binder at this temperature to physically bond the separator coating to the surface of the positive electrode, eliminating interlayer gaps and reducing interfacial contact resistance. This close physical contact facilitates electron transport between the positive electrode current collector and the active particles of the separator coating, improving the utilization rate of active materials and enhancing the overall rigidity of the cell structure.

[0031] Coating structure protection: The drying temperature of 65-75℃ in step S2 allows the solvent to evaporate smoothly, avoiding coating cracking or base film thermal shrinkage due to excessive temperature, and ensuring the dimensional stability of the diaphragm.

[0032] Preferably, in step S1, the solvent is N-methylpyrrolidone; the sodium iron pyrophosphate active material is pretreated by grinding and sieving before being added.

[0033] By adopting the above technical solution, the pretreatment step removes large agglomerated particles from the active material, preventing scratches or accumulation during the coating process, and ensuring the consistency of the coating thickness and the insulation safety of the diaphragm.

[0034] Preferably, in step S2, baking and drying cause the solvent to evaporate and control the thickness of the single-sided coating to be 3-8 μm.

[0035] By adopting the above technical solution, a balance is achieved between active material loading and ion transport impedance. The thickness of 3-8μm can provide effective sodium replenishment capacity without significantly increasing the battery's internal resistance.

[0036] This invention provides a separator and a sodium-ion battery using the separator. It offers the following advantages: 1. By coating the separator surface with a high content of sodium iron pyrophosphate active material and positioning the coated side towards the positive electrode, the invention utilizes sodium iron pyrophosphate as an additional source of active sodium. During the first charge of the battery, sodium iron pyrophosphate in the coating releases sodium ions with the assistance of a conductive network, migrating to the negative electrode side to compensate for the active sodium consumed by the formation of the solid electrolyte interphase (SEI) film. Without altering the original formulation of the positive electrode, this effectively compensates for the irreversible capacity loss of the negative electrode, thereby improving the overall energy density of the battery.

[0037] 2. This invention utilizes the excellent thermal stability of the sodium iron pyrophosphate polyanionic structure to construct a heat-resistant support framework on the surface of a polyolefin porous base membrane. When the internal temperature of the battery rises, the functionalized coating can suppress the thermal shrinkage behavior of the base membrane through physical support, maintaining the dimensional stability of the separator. This structure effectively prevents direct contact between the positive and negative electrodes caused by separator shrinkage at high temperatures, reducing the risk of internal short circuits and thermal runaway in the battery.

[0038] 3. This invention optimizes the interfacial bonding between the separator coating and the positive electrode by controlling the particle size of sodium iron pyrophosphate and combining it with a hot-pressing shaping process. The micron-sized particles shorten the ion diffusion path, and together with the conductive network constructed by the conductive agent, reduce electrochemical polarization. At the same time, the hot-pressing process eliminates the physical gap between the coating and the electrode, reduces the interfacial contact resistance, and ensures that the active material in the coating, which is far from the current collector, can also fully participate in the electrochemical reaction, thus guaranteeing the rate performance of the battery. Detailed Implementation

[0039] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the embodiments and comparative examples. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0040] Preparation Examples 1-3: Preparation Example 1: This preparation example provides a high specific surface area, small particle size sodium iron pyrophosphate (NFPP) material, including the following steps: accurately weigh ferrous oxalate, ammonium dihydrogen phosphate, and sodium carbonate according to the stoichiometric ratio (Na:Fe:P=4:3:4), add glucose accounting for 5% of the total mass of the precursor, and place in a planetary ball mill jar; add anhydrous ethanol as the dispersion medium and zirconia grinding balls, set the ball-to-material ratio to 10:1, and grind at 500 rpm for 20 hours to obtain a uniformly dispersed nanoscale precursor slurry; Uniformly dispersed nanoscale precursor slurry was dried under vacuum at 80℃ until the solvent was completely evaporated, then ground and passed through a 300-mesh sieve to obtain fine precursor powder. The fine precursor powder was placed in a tube furnace and, under argon atmosphere protection, heated to 350℃ at a rate of 3℃ / min and held for 4 hours, followed by further heating to 600℃ and holding for 8 hours for low-temperature restricted growth sintering. After natural cooling to room temperature in the furnace, the powder was dispersed using an air jet mill and sieved to obtain particles with a D50 of 0.55μm, a D90 of 2.1μm, and a BET specific surface area of ​​14.8m². 2 / g of the first NFPP active material.

[0041] Preparation Example 2: This preparation example provides a sodium iron pyrophosphate (NFPP) material with a conventional particle size, comprising the following steps: Ferrous oxalate, ammonium dihydrogen phosphate, sodium carbonate, and glucose (3% of the total precursor mass) are weighed according to stoichiometric ratio and placed in a planetary ball mill jar; anhydrous ethanol and zirconium oxide grinding balls are added, the ball-to-material ratio is set to 8:1, and the mixture is ground at 400 rpm for 10 hours to obtain a uniformly mixed precursor slurry; the uniformly mixed precursor slurry is spray-dried or vacuum-dried to remove the solvent and obtain a dried precursor powder; the dried precursor powder is placed in a tube furnace and pre-calcined at 350°C for 4 hours under a nitrogen atmosphere, followed by crystallization sintering at 650°C for 10 hours; after cooling, it is mechanically ground and sieved through a 400-mesh sieve to obtain a particle size D50 of 2.8 μm, a D90 of 12.5 μm, and a BET specific surface area of ​​9.5 m². 2 / g of the second NFPP active material.

[0042] Preparation Example 3: This preparation example provides a low specific surface area, large particle size sodium iron pyrophosphate (NFPP) material, comprising the following steps: ferrous oxalate, ammonium dihydrogen phosphate, sodium carbonate, and glucose accounting for 2% of the total precursor mass are weighed according to stoichiometric ratio, mixed evenly, and placed in a ball mill jar; an appropriate amount of ethanol and grinding balls are added, the ball-to-material ratio is set to 5:1, and the mixture is ground at a low speed of 300 rpm for 4 hours, only mixing without excessive crushing, to obtain a coarsely dispersed precursor slurry; the coarsely dispersed... The precursor slurry was dried and simply crushed to obtain coarse-grained precursor powder. The coarse-grained precursor powder was placed in a high-temperature furnace and heated to 350℃ at a rate of 5℃ / min under an inert atmosphere, held for 2 hours, and then heated to 750℃ and held for 12 hours to promote grain growth and fusion. After cooling, the powder was deagglomerated using a pulverizer and strictly passed through a 500-mesh standard sieve to remove oversized particles, yielding a particle size D50 of 9.8μm, a D90 of 28.5μm, and a BET specific surface area of ​​5.2m². 2 / g of the third NFPP active material.

[0043] Examples 1-3: Example 1: This example provides a separator and a sodium-ion battery using the separator, comprising the following steps: S1, dissolving 10 parts by mass of polyvinylidene fluoride (PVDF) binder in N-methylpyrrolidone (NMP), and after complete dissolution, adding 5 parts by mass of conductive agent (SuperP and carbon nanotubes mixed at an 8:2 ratio), dispersing evenly at high speed, and then adding 85 parts by mass of the first NFPP active material (D50≈0.55μm) prepared in Preparation Example 1, adjusting the solid content and vacuum degassing to obtain a functionalized coating slurry with a viscosity of 1500 mPa·s; S2, coating the functionalized coating slurry onto a polyethylene (PE) polycarbonate sheet with a thickness of 9μm using a microgravure coating machine. On one surface of the porous base membrane, the solvent is dried by baking at 65°C to evaporate, and the thickness of the coating on one side is controlled to be 3μm to obtain the separator; S3, the layered oxide positive electrode material, conductive carbon black and PVDF are mixed in a mass ratio of 90:5:5 and coated on aluminum foil to obtain the positive electrode sheet; the hard carbon, conductive carbon black, SBR and CMC are mixed in a mass ratio of 92:3:2.5:2.5 and coated on aluminum foil to obtain the negative electrode sheet; S4, the positive electrode sheet, separator and negative electrode sheet are stacked in sequence, wherein the side of the separator with coating must face the positive electrode sheet and the base membrane side faces the negative electrode sheet, hot-pressed and shaped at 70°C, and injected with sodium hexafluorophosphate electrolyte. After encapsulation and formation, the first sodium-ion battery is obtained.

[0044] Example 2: This example provides a separator and a sodium-ion battery using the separator, comprising the following steps: S1, dissolving 7 parts by mass of polyvinylidene fluoride (PVDF) binder in N-methylpyrrolidone (NMP), adding 3 parts by mass of conductive agent (SuperP), dispersing evenly at high speed, adding 90 parts by mass of the second NFPP active material (D50≈2.8μm) prepared in Preparation Example 2, adjusting the solid content and vacuum degassing to obtain a functionalized coating with a viscosity of 2000 mPa·s. S2. Coating the functionalized coating slurry onto one surface of a 9 μm thick polyethylene (PE) porous base membrane, baking and drying at 70°C, controlling the single-sided coating thickness to be 5 μm, to obtain a separator; S3. Preparing the positive electrode and negative electrode using the same process as in Example 1; S4. Winding the positive electrode, separator, and negative electrode in sequence, strictly controlling the coating side of the separator to be tightly attached to the positive electrode, hot-pressing and shaping at 75°C, injecting electrolyte and encapsulating to form a second sodium-ion battery.

[0045] Example 3: This example provides a separator and a sodium-ion battery using the separator, comprising the following steps: S1, dissolving 4 parts by mass of polyvinylidene fluoride (PVDF) binder in N-methylpyrrolidone (NMP), adding 1 part by mass of conductive agent (acetylene black), dispersing evenly at high speed, adding 95 parts by mass of the third NFPP active material (D50≈9.8μm) prepared in Preparation Example 3, adjusting the solid content and vacuum degassing to obtain a functionalized coating with a viscosity of 2800 mPa·s. S2. The functionalized coating slurry is coated on one surface of a 9 μm thick polyethylene (PE) porous base membrane and baked and dried at 75°C, with the single-sided coating thickness controlled to be 8 μm to obtain a separator; S3. The positive electrode and negative electrode are prepared using the same process as in Example 1; S4. The positive electrode, separator, and negative electrode are stacked in sequence, ensuring that the coating side of the separator faces the positive electrode, and hot-pressed and shaped at 80°C, injected with electrolyte and encapsulated to form a third sodium-ion battery.

[0046] Comparative Example 1: Compared with Example 2, the difference is that the second NFPP active material and conductive agent in the functionalized coating slurry were completely replaced by electrochemically inert alumina (Al2O3) powder (i.e., the slurry formulation was adjusted to: 93 parts by mass of Al2O3 powder and 7 parts by mass of PVDF binder), and a conventional ceramic membrane without electrochemical activity was prepared. The rest of the preparation process, base film parameters and battery assembly steps were the same, but the coating gap was adjusted during the coating process to control the thickness of the single-sided coating to 2 μm.

[0047] Test Examples 1-3: Test Example 1: Thermal stability test of materials; The second NFPP active material prepared in Preparation Example 2 was selected as the sample and tested using differential scanning calorimetry (DSC). An appropriate amount of NFPP powder was placed in a high-pressure resistant stainless steel crucible and compacted and sealed under an argon atmosphere. Another appropriate amount of NFPP powder was mixed evenly with an ester electrolyte (EC / DEC) and sealed in a crucible as a control group. The heating rate was set to 10℃ / min, and the test temperature range was 100℃ to 400℃. The heat flow changes during the heating process were recorded.

[0048] The test results are shown in Table 1.

[0049] Table 1. DSC thermal stability test data of sodium iron pyrophosphate active material and its mixture with electrolyte

[0050] Based on the data analysis in Table 1, the pure NFPP material exhibits good thermal stability in the temperature range of 100℃ to 300℃, and no obvious endothermic or exothermic reactions were detected. An exothermic onset signal only appears at 310℃, and no sharp decomposition peaks are observed within the test range, indicating that its crystal structure is stable.

[0051] In contrast, the NFPP-electrolyte mixture exhibited higher reactivity, with exothermic peaks detected at 234℃ and 263℃, respectively. This is mainly due to side reactions of the electrolyte itself or between the electrolyte and the material surface. However, the high thermal stability of the pure material (starting at 310℃) demonstrates its potential as a heat-resistant framework.

[0052] Sodium iron pyrophosphate (NFPP) is a polyanionic material with high PO bond energy in its crystal structure, which restricts the release of oxygen atoms. Under high-temperature conditions, this polyanionic framework is less prone to collapse and release lattice oxygen, exhibiting superior thermal stability compared to layered oxide materials. Compared to polyethylene (approximately 130°C) or polypropylene (approximately 165°C) base films with lower melting points, NFPP materials have a higher decomposition temperature. When coated onto the separator surface, the NFPP coating maintains structural integrity when the battery's internal temperature rises, causing the base film to melt and shrink. This physically blocks contact between the positive and negative electrodes, suppressing thermal runaway.

[0053] Test Example 2: Test of the physical properties of the diaphragm; The diaphragm prepared in Example 2 and the alumina-coated diaphragm prepared in Comparative Example 1 were selected as test samples.

[0054] The total thickness was measured at randomly selected points on the sample surface using a contact thickness gauge, and the average value was taken. The time required for 100 mL of air to pass through a fixed-area diaphragm under a pressure of 1.21 kPa was measured using a Gurley air permeability tester. The diaphragm was cut into 100 mm × 100 mm square samples, marked with longitudinal (MD) and transverse (TD) markings, laid flat between cardboard sheets, and placed in a forced-air constant-temperature chamber. The samples were incubated at 100℃, 130℃, and 160℃ for 1 hour each. After cooling to room temperature, the dimensional changes were measured, and the heat shrinkage rate was calculated.

[0055] The test data is shown in Table 2.

[0056] Table 2. Comparison of test data for the physical properties of the diaphragm

[0057] Analysis of the data in Table 2 shows that the composite membrane of Example 2, with a coating thickness of 5 μm, has a permeability value of 260.2 s; while Comparative Example 1, with a coating thickness of only 2 μm, has a permeability value of 280.3 s. Generally, increasing the coating thickness lengthens the gas transport path, leading to a higher permeability value. However, Example 2 exhibits a lower permeability value despite a thicker coating, indicating that the coating structure constructed from NFPP particles and the conductive agent has higher porosity and better pore connectivity, which is more conducive to electrolyte penetration and ion transport compared to a densely packed alumina coating.

[0058] Regarding heat shrinkage performance, 130°C is close to the melting point of the polyethylene base film, at which point the base film begins to soften. The shrinkage rate in the MD direction of Example 2 is 1.2%, lower than the 3.0% of Comparative Example 1. When the temperature rises to 160°C, far exceeding the melting point of the base film, the shrinkage rate of Example 2 is 6.0%, significantly lower than the 10.0% of Comparative Example 1. This result confirms the high-temperature support effect of NFPP material. Due to the high-temperature resistant polyanionic skeleton structure of NFPP, it forms a rigid support layer on the surface of the base film by being fixed with an adhesive. When the base film tends to melt due to heat, the NFPP coating can resist shrinkage stress and maintain the macroscopic dimensional stability of the separator. This characteristic can effectively delay or prevent large-area contact between the positive and negative electrodes in the early stages of battery thermal runaway, thereby improving battery safety.

[0059] Test Example 3: Electrochemical performance test; The second sodium-ion battery assembled in Example 2 and the control battery assembled in Comparative Example 1 were selected as test objects.

[0060] The separator to be tested was cut into circular pieces, sandwiched between two stainless steel electrodes, and assembled into a blocked symmetric cell by injecting electrolyte. An electrochemical workstation was used for electrochemical impedance spectroscopy (EIS) testing, with the frequency range set from 100 kHz to 0.01 Hz. The high-frequency intercept of the Nyquist plot was read as the bulk resistance, and the ionic conductivity was calculated. For full-cell performance, the assembled sodium-ion battery was charged and discharged using a battery testing system at a constant temperature of 25°C. The charge / discharge voltage range was set to 2.0V-4.0V, and constant current charge / discharge was performed at a rate of 0.1C. The first discharge specific capacity and first coulombic efficiency were recorded, and the energy density was calculated based on the total battery mass.

[0061] The test data are shown in Tables 3 and 4.

[0062] Table 3. Membrane ionic conductivity test data

[0063] Table 4. Electrochemical performance test data of sodium-ion batteries

[0064] Based on the data analysis in Tables 3 and 4, Example 2 is superior to Comparative Example 1 in terms of ionic conductivity and overall battery performance.

[0065] Regarding ion conduction, the membrane conductivity of Example 2 was 2.82 mS / cm, higher than that of Comparative Example 1 (2.35 mS / cm). This is attributed to the good wettability of the NFPP material with the organic electrolyte, and the porous structure formed by the accumulation of coating particles, which facilitates electrolyte adsorption and retention, reducing ion migration resistance.

[0066] Regarding battery capacity and efficiency, Example 2 exhibited a discharge specific capacity of 135 mAh / g and an initial coulombic efficiency of 89.5%, both higher than Comparative Example 1's 128 mAh / g and 86.2%. The difference stems from the electrochemical properties of the coating material. Comparative Example 1 used alumina, an electrochemically inert material that does not participate in charge storage, only increasing the battery's inactive mass. Example 2 employed NFPP, a sodium-rich polyanionic material, with the coating applied directly to the positive electrode side. During the first charge, sodium ions from the NFPP lattice were released into the electrolyte, participating in the electrochemical reaction. This additional sodium source compensated for the irreversible sodium loss caused by the formation of the solid electrolyte interphase (SEI) film on the negative electrode surface, thereby improving the initial coulombic efficiency. Simultaneously, NFPP, as an active material, contributed additional capacity, increasing the overall discharge capacity of the battery.

[0067] Regarding energy density, Example 2 achieved an energy density of 155 Wh / kg, an improvement of 10 Wh / kg compared to Comparative Example 1. This result confirms that by replacing the traditional inert ceramic coating with an active NFPP material, the energy density of sodium-ion batteries can be effectively improved without sacrificing safety, utilizing the separator coating as a sodium replenisher and auxiliary capacity source.

Claims

1. A diaphragm, characterized in that, The invention includes a porous base membrane and a functionalized coating coated on at least one surface of the porous base membrane; the functionalized coating is made from a raw material comprising the following parts by weight: sodium iron pyrophosphate active material: 85-95 parts; Adhesive: 4-10 parts; Conductive agent: 1-5 parts.

2. The diaphragm according to claim 1, characterized in that, The binder is polyvinylidene fluoride; the conductive agent is selected from one or a combination of conductive carbon black, carbon nanotubes, and acetylene black.

3. The diaphragm according to claim 1, characterized in that, The single-sided thickness of the functionalized coating is 3-8 μm.

4. A diaphragm according to claim 3, characterized in that, The median particle size D50 of the sodium iron pyrophosphate active material is 0.55-9.8 μm, the D90 is 2.1-28.5 μm, and the BET specific surface area is 5.2-14.8 m². 2 / g.

5. A diaphragm according to claim 1, characterized in that, The sodium iron pyrophosphate active material is prepared by a method comprising the following steps: Step 1, preparation of precursor powder: iron source, phosphorus source, sodium source and reducing agent are weighed according to stoichiometric ratio, and carbon source is added. The mixture is ball-milled in a dispersion medium to obtain a precursor slurry; the precursor slurry is dried to obtain precursor powder; wherein the ball-to-powder ratio of the ball-milling mixture is 5:1-10:1, the ball-milling speed is 300-500 rpm, and the ball-milling time is 4-20 hours; Step 2, sintering: the precursor powder is pre-calcined in an inert atmosphere, and then heated to crystallize and sinter to obtain a sintered product; wherein the crystallization and sintering temperature is 600-750℃, and the holding time is 8-12 hours; Step 3, post-treatment: after cooling, the sintered product is pulverized and sieved to obtain the sodium iron pyrophosphate active material.

6. A sodium-ion battery using the separator according to any one of claims 1-5, characterized in that, The sodium-ion battery includes a positive electrode, a negative electrode, an electrolyte, and a separator located between the positive electrode and the negative electrode; wherein the side of the separator with a functional coating is disposed facing the positive electrode.

7. The sodium-ion battery according to claim 6, characterized in that, The positive electrode sheet comprises a layered oxide positive electrode material; the negative electrode sheet comprises a hard carbon negative electrode material; and the electrolyte comprises sodium hexafluorophosphate.

8. The sodium-ion battery according to claim 6, characterized in that, The method for preparing the sodium-ion battery includes the following steps: S1, preparing a functionalized coating slurry: dissolving a binder in a solvent, adding a conductive agent and dispersing it evenly, then adding sodium iron pyrophosphate active material, adjusting the solid content and vacuum degassing to obtain a functionalized coating slurry; S2, preparing a separator: coating the functionalized coating slurry onto the surface of a porous base membrane, and baking and drying it at 65-75°C to obtain the separator; S3, assembly and hot pressing: stacking or winding the positive electrode, the separator, and the negative electrode in sequence, ensuring that the coating side of the separator is tightly attached to the positive electrode to obtain a cell; hot pressing the cell at 70-80°C to shape it; S4, electrolyte injection and formation: injecting the electrolyte into the hot-pressed and shaped cell and encapsulating it to form the sodium-ion battery.

9. The sodium-ion battery according to claim 8, characterized in that, In step S1, the solvent is N-methylpyrrolidone; the sodium iron pyrophosphate active material is pretreated by grinding and sieving before being added.

10. The sodium-ion battery according to claim 8, characterized in that, In step S2, the baking and drying process causes the solvent to evaporate and controls the thickness of the single-sided coating to be 3-8 μm.

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