Sodium ion battery monomer, battery device and electric equipment

Abstract

The invention discloses a sodium ion battery monomer, a battery device and electric equipment. Comprising a positive pole piece, a negative pole piece and electrolyte. The positive pole piece comprises a positive current collector and a positive film layer, and the thickness L of the positive film layer is more than or equal to 50 microns and less than or equal to 150 microns; the positive electrode film layer comprises a phosphate material, the phosphate material comprises ferric sodium pyrophosphate and ferric sodium phosphate, and based on the total mass of the phosphate material, the sum P1 of the mass content of the ferric sodium pyrophosphate and the mass content of the ferric sodium phosphate meets the condition that P1 is larger than or equal to 1% and smaller than or equal to 8%; the pore volume V1 of the pore structure with the pore diameter larger than 10 nm in the carbon-based material measured based on a nitrogen adsorption method and the total pore volume V of the carbon-based material meet the condition that V1 / V is larger than or equal to 20% and smaller than or equal to 40%; and the conductivity sigma of the electrolyte at the temperature of 25 DEG C + / -1 DEG C is more than or equal to 8mS / cm and less than or equal to 20mS / cm.

Description

Technical Field

[0001] This application relates to the field of battery technology, and more specifically, to a sodium-ion battery cell, battery device, and electrical equipment. Background Technology

[0002] Sodium-ion batteries are gaining popularity due to their low cost, good low-temperature performance, and abundant sodium resources. Particularly in the high-power sector, their cost advantage and low-temperature discharge performance make them competitive. They have wide applications in new energy vehicles, drones and robots, smart grids, data centers, renewable energy storage, large-scale energy storage systems, power starters, and power tools.

[0003] With the development of battery technology, the application of sodium-ion batteries in the high-power field is no longer solely focused on their high-power performance, but also on their overall performance while maintaining high power output. Therefore, how to provide a sodium-ion battery with good overall performance is a pressing technical problem that needs to be solved. Summary of the Invention

[0004] This application is made in view of the above-mentioned problems, and its purpose is to provide a sodium-ion battery cell that combines high power discharge and long service life.

[0005] To achieve the above objectives, this application provides a sodium-ion battery cell, a battery device, and an electrical device.

[0006] In a first aspect, a sodium-ion battery cell is provided, comprising: a positive electrode sheet, a negative electrode sheet, and an electrolyte. The positive electrode sheet includes a positive current collector and a positive electrode film layer. The positive electrode film layer is disposed on at least one surface of the positive current collector, and the thickness L of the positive electrode film layer satisfies: 50 μm ≤ L ≤ 150 μm. The positive electrode film layer includes a phosphate material, which includes sodium iron pyrophosphate and sodium iron phosphate. Based on the total mass of the phosphate material, the sum of the mass contents P1 of the sodium iron pyrophosphate and the sodium iron phosphate satisfies: 1% ≤ P1 ≤ 8%; the negative electrode sheet includes a negative current collector and a negative electrode film layer, the negative electrode film layer is disposed on at least one side surface of the negative current collector, the negative electrode film layer includes a carbon-based material, the carbon-based material includes a porous structure, and the pore volume V1 of the porous structure with a pore size greater than 10 nm in the carbon-based material, measured by nitrogen adsorption method, satisfies the following: 20% ≤ V1 / V ≤ 40%; the conductivity σ of the electrolyte at a temperature of 25℃ ± 1℃ satisfies the following: 8mS / cm ≤ σ ≤ 20mS / cm.

[0007] In this embodiment, the cathode phosphate material contains heterogeneous phases sodium iron pyrophosphate (NFPO) and sodium iron phosphate (NFP), which form a heterostructure with the main phase material of the phosphate material, sodium iron pyrophosphate (NFPP). This significantly enhances the diffusion ability of sodium ions from the interior to the surface of the phosphate material, thereby reducing charge transfer resistance and improving the power performance and cycle performance of the sodium-ion battery cell. However, since the voltage plateau of the NFPO heterogeneous phase is low, it affects the lithium-ion migration rate during battery charging and discharging. The NFP heterogeneous phase is an inert component that occupies active sites, reducing the electrochemical activity of the material. Therefore, it is necessary to control the proportion of the heterogeneous phases NFPO and NFP in the cathode phosphate material. In other words, including an appropriate proportion of NFPO and NFP heterogeneous phases in the cathode phosphate material is beneficial to improving the power performance and cycle performance of the sodium-ion battery cell. In addition, controlling the thickness of the cathode film within the above-mentioned range can increase the sodium-ion transport rate in the cathode film, allowing sodium ions to migrate rapidly between the interior and surface of the cathode, thereby improving the charge and discharge power of the battery cell. Furthermore, due to the poor conductivity of phosphate materials, this application also uses a highly conductive electrolyte to improve the migration rate of ions at the interface between the electrolyte and the positive electrode, reduce ion accumulation at the interface, improve charge distribution uniformity, reduce diffusion resistance, and thus improve electron conduction, thereby improving the power performance and cycle performance of the battery cell. Further, by controlling the volume ratio of large-pore structures (greater than 10 nm) in the carbon-based material, the proportion of large pores is reduced to shorten the migration path of sodium ions from the interior to the surface of the negative electrode active material, allowing them to escape more quickly and thus improving the discharge power of the battery cell. Moreover, reducing the proportion of large pores in the negative electrode carbon-based material can reduce side reactions at the interface between the negative electrode and the electrolyte, reducing the consumption of electrolyte and active ions, thereby improving the lifespan of the battery cell. Combining the above technical solutions, the transport rate of sodium ions between the positive and negative electrodes is improved, resulting in enhanced power performance. During the discharge of a sodium-ion battery cell, sodium ions released from the carbon-based material can reach the positive electrode interface more quickly to participate in the reaction, achieving high-rate discharge of the sodium-ion battery cell and improving discharge power performance. Simultaneously, during the migration of sodium ions between the positive and negative electrodes, the charge transfer resistance and diffusion resistance decrease, battery side reactions are reduced, and the power performance and cycle performance of the sodium-ion battery cell are improved. The technical solution of this application embodiment can balance the high power discharge performance and long service life of the sodium-ion battery cell, providing a sodium-ion battery cell with excellent overall performance.

[0008] In one possible implementation, 3% ≤ P1 ≤ 6%.

[0009] In one possible implementation, the phosphate material has the general structural formula Na. a Fe b M c M'd (Condensed polyanions) e (Anion) f Where M is one or more transition metals, M' is one or more non-transition metals, a>b, c>0, d≥0, e≥1, f>0, and the condensed polyanion comprises from P2O7 4- P3O9 5- and P4O 11 6- One or more groups selected from [the group].

[0010] In one possible implementation, the phosphate material has the general structural formula Na. x Fe a-y M y (NO4) z (P2O7) w Wherein, 2≤x≤4, 1≤a≤4, 1≤y≤4, 0≤z≤4, 0≤w≤1, at least one of z and w is greater than or equal to 1, M includes at least one of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and N includes at least one of Al, Si, P, S, Ti, V, W.

[0011] In one possible implementation, the change ΔP of the sum of the mass contents of sodium iron pyrophosphate and sodium iron phosphate in the sodium-ion battery cell after 100 cycles and before 100 cycles, based on the total mass of the phosphate material, satisfies: ΔP ≤ 3%.

[0012] In this embodiment, the heterostructure formed by the impurity material NFPO and the main phase material NFPP helps to stabilize the crystal structure of the phosphate material. During the cycling process of the sodium-ion battery cell, the phosphate material maintains structural stability. Therefore, the content of the impurity materials NFPO and NFP changes little during cycling, which is beneficial for the sodium-ion battery cell to maintain a higher capacity retention rate.

[0013] In one possible implementation, the pore volume V2 of the pore structure with a pore size of 5 nm to 10 nm in the carbon-based material, measured by nitrogen adsorption, satisfies the following condition: 5% ≤ V2 / V ≤ 15%.

[0014] By controlling the volume ratio of larger pores (5nm~10nm) in carbon-based materials, the proportion of macropores is reduced, thereby shortening the migration path of sodium ions from the interior to the surface of the negative electrode active material, allowing them to escape more quickly, and thus improving the discharge power of the battery cell.

[0015] In one possible implementation, the pore volume V3 of the pore structure with a pore size of 2nm to 5nm in the carbon-based material, measured by nitrogen adsorption, satisfies the following condition: 20% ≤ V3 / V ≤ 40%.

[0016] Sodium ions diffuse rapidly in pore structures with a pore size of 2nm to 5nm. Setting the pore volume ratio of pore structures with a pore size of 2nm to 5nm in carbon-based materials within this range is beneficial for increasing the diffusion rate of sodium ions in the negative electrode active material, leading to faster insertion and thus improving the charging power of the battery cell. Furthermore, setting the pore volume ratio of pore structures with a pore size of 2nm to 5nm within this range also improves the wettability of the electrolyte on the negative electrode sheet, thereby enhancing the battery's rate performance and cycle stability.

[0017] In one possible implementation, the pore volume V4 of the pore structure with a pore size of less than 2 nm in the carbon-based material, as measured by nitrogen adsorption, satisfies the following condition: 5% ≤ V4 / V ≤ 15%.

[0018] Sodium ions have a shorter migration path in pore structures with a pore size of less than 2 nm. By setting the pore volume ratio of pore structures with a pore size of less than 2 nm in carbon-based materials within the above range, the proportion of micropores (pore structures with a pore size of less than 2 nm) in carbon-based materials can be increased to shorten the migration path of sodium ions from the interior to the surface of the negative electrode active material, allowing them to escape more quickly and thus improve the discharge power of the battery cell.

[0019] In one possible implementation, the carbon-based material comprises a plurality of graphite-like sheets, with at least a portion of the graphite-like sheets having an interlayer spacing between two of them, wherein the interlayer spacing satisfies that the volume H1 of the space between 0.35 nm and 0.4 nm and the total volume H of the space formed by the plurality of graphite-like sheets satisfy: 20% ≤ H1 / H ≤ 60%.

[0020] Sodium ions can be rapidly extracted between graphite-like layers with an interlayer spacing of 0.35 nm to 0.4 nm. By setting the spatial volume ratio of graphite-like layers with an interlayer spacing of 0.35 nm to 0.4 nm within the above range, increasing the spatial proportion of graphite-like layers with larger interlayer spacing is beneficial for sodium ions to be rapidly extracted from the interior of carbon-based materials, thereby improving the discharge power of the battery cell.

[0021] In one possible implementation, the volume H2 of the space where the interlayer spacing is greater than 0.4 nm and the volume H of the total space formed by the plurality of graphite-like sheets satisfy: 20% ≤ H2 / H ≤ 50%.

[0022] Graphite-like sheets with an interlayer spacing greater than 0.4 nm have a large space volume between the sheets, which makes it easier for active ions to embed and adsorb onto the surface of the graphite-like sheets for energy storage. However, if the proportion of their space volume is too large, it will lead to an excessively high capacity in the slope section of the carbon-based material, thereby affecting the first-cycle coulombic efficiency of the sodium-ion battery cell. Therefore, the proportion of the space volume between graphite-like sheets with an interlayer spacing greater than 0.4 nm is set within the above range to achieve both the kinetic performance and the first-cycle coulombic efficiency of the sodium-ion battery cell.

[0023] In one possible implementation, the volume H3 of the space where the interlayer spacing is less than 0.35 nm and the volume H of the total space formed by the plurality of graphite-like sheets satisfy: 0 ≤ H3 / H ≤ 5%.

[0024] Setting the proportion of the space volume between graphite-like sheets with an interlayer spacing of less than 0.35 nm within the above range can reduce the risk of sodium deposition on the negative electrode, which is conducive to improving the capacity of the negative electrode and thus increasing the energy density of the battery cell.

[0025] In one possible implementation, the carbon-based material includes at least one of the following materials: hard carbon, a mixture of hard carbon and soft carbon, and a mixture of hard carbon and graphite.

[0026] In one possible implementation, the electrolyte comprises cyclic carbonates and chain esters.

[0027] In one possible implementation, the viscosity V of the chain ester at 25℃±1℃ satisfies: 0.2mPa·s≤V≤3mPa·s, and the mass content P3 of the chain ester, based on the total mass of the electrolyte, satisfies: 40%≤P3≤80%.

[0028] In this embodiment, the viscosity of the chain ester in the electrolyte is within the above-mentioned range, which is beneficial to improve the conductivity of the electrolyte, thereby increasing the migration rate of sodium ions in the electrolyte and reducing the liquid phase diffusion resistance. Furthermore, by setting the mass ratio of the chain ester in the electrolyte, the power performance and cycle performance of the sodium-ion battery cell can be improved.

[0029] In one possible implementation, the freezing point F of the cyclic carbonate satisfies: -60℃≤F≤-10℃, and the mass content P4 of the cyclic carbonate, based on the total mass of the electrolyte, satisfies: 10%≤P4≤50%.

[0030] In this embodiment, the electrolyte contains cyclic carbonates with a freezing point within the aforementioned range, which prevents the electrolyte from freezing at low temperatures (below zero). When used in combination with chain esters, the electrolyte still has high conductivity at low temperatures. Furthermore, by setting the mass percentage of cyclic carbonates in the electrolyte, the sodium-ion battery cells still have good power performance and cycle performance at low temperatures.

[0031] In one possible implementation, the cyclic carbonate includes at least one of propylene carbonate and ethylene carbonate.

[0032] In one possible implementation, the chain ester includes at least one of chain carbonates and chain carboxylic acid esters; the chain carbonate includes at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, methyl butyl carbonate, ethyl propyl carbonate, dipropyl carbonate, and dibutyl carbonate.

[0033] The chain carboxylic acid ester includes one of methyl acetate, ethyl acetate, ethyl propionate, methyl formate, ethyl formate, methyl propionate, propyl propionate, ethyl butyrate, and propyl acetate.

[0034] In one possible implementation, a carbon material is disposed on the surface of the phosphate material. This improves the electronic conductivity of the positive electrode active material, which is beneficial for improving the cycle stability of the battery cell.

[0035] In one possible implementation, the positive electrode film layer further includes a conductive agent, which includes at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0036] In one possible implementation, the compaction density T of the positive electrode sheet satisfies: 1.5 g / cm³. 3 ≤T≤2.3g / cm 3 .

[0037] In one possible implementation, the resistance R of the positive electrode plate satisfies: 0.01Ω≤R≤0.1Ω.

[0038] In one possible implementation, the ratio CB of the active material capacity of the negative electrode to the active material capacity of the positive electrode satisfies: 1.07 ≤ CB ≤ 1.2.

[0039] In one possible implementation, the positive current collector comprises aluminum foil.

[0040] In one possible implementation, the sodium-ion battery cell further includes: a housing, which is a hollow structure with an opening, wherein the electrolyte, the negative electrode, and the positive electrode are housed within the housing; and a top cover that closes the opening.

[0041] In one possible implementation, the material of the housing includes aluminum.

[0042] In one possible implementation, the sodium-ion battery cell further includes an electrode post disposed on the side of the top cover away from the hollow structure, the electrode post being made of aluminum.

[0043] In a second aspect, a battery device is provided, comprising a sodium-ion battery cell as described in the first aspect and any possible implementation thereof.

[0044] Thirdly, an electrical device is provided, comprising a sodium-ion battery cell as described in the first aspect and any possible implementation thereof, or a battery device as described in the second aspect, wherein the sodium-ion battery cell or the battery device serves as a backup power source for the electrical device. The electrical device is applicable to fields such as new energy vehicles, drones and robots, smart grids, data centers, renewable energy storage, large-scale energy storage systems, and power starters and power tools. Attached Figure Description

[0045] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments of this application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the drawings without creative effort.

[0046] Figure 1 This is a schematic diagram of the structure of a positive electrode sheet according to an embodiment of this application;

[0047] Figure 2 This is a schematic diagram of the structure of a negative electrode sheet according to an embodiment of this application;

[0048] Figure 3 This is a schematic diagram of a battery cell according to an embodiment of this application;

[0049] Figure 4 This is a schematic diagram of a battery device according to an embodiment of this application;

[0050] Figure 5 This is a schematic diagram of a vehicle according to an embodiment of this application. Detailed Implementation

[0051] Embodiments of the sodium-ion battery cell, battery device, and power supply of this application have been described in detail with reference to the accompanying drawings, but unnecessary details may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0052] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0053] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0054] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0055] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0056] Typically, a battery cell includes a positive electrode, a negative electrode, an electrolyte, and a separator. During the charging and discharging process of the battery cell, active ions move back and forth between the positive and negative electrodes, inserting and extracting. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, located between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing active ions to pass through; for example, the separator can be a membrane. In some embodiments, the above-described battery cell is also referred to as a secondary battery, and the battery cell can be the smallest possible battery unit.

[0057] During the charging process of a sodium-ion battery, sodium ions are released from the positive electrode active material, move and embed into the negative electrode material; while during the discharging process, sodium ions are released from the negative electrode material, move and embed into the positive electrode active material.

[0058] It should be understood that the “intercalation” process described in this application refers to the process in which sodium ions are intercalated in the positive electrode active material and the negative electrode material due to an electrochemical reaction, and the “deintercalation” and “deintercalation” processes described in this application refer to the process in which sodium ions are deintercalated in the positive electrode active material and the negative electrode active material due to an electrochemical reaction.

[0059] If mentioned, "graphite-like sheets" can refer to local layered structures composed of carbon atoms in carbon-based materials, or "graphite-like microcrystals," or "pseudo-graphitic domains," or disordered stacked carbon sheets, or symbiotic large-area graphite-like crystals, or "amorphous carbon" and / or "difficult-to-graphitize carbon" as defined in current national standards (such as GB / T 43114-2023 Hard Carbon).

[0060] Sodium-ion batteries have gradually come into view due to their advantages such as low cost, good low-temperature performance, and abundant sodium resources. Especially in the high-power field, the cost advantage and low-temperature discharge performance of sodium-ion batteries make them competitive in the high-power market.

[0061] In specific applications, such as new energy vehicles, drones and robots, smart grids, data centers, renewable energy storage, large-scale energy storage systems, power start-up, and power tools, the development of high-power sodium-ion battery technology requires consideration of multiple design factors. For example, it's necessary to balance high-power discharge performance with high energy density, cycle performance, and lifespan. Furthermore, depending on the vehicle application scenario, the performance requirements of sodium-ion batteries at low temperatures, such as low-temperature high-power discharge performance, also need to be considered.

[0062] The design of the positive electrode, negative electrode, and electrolyte in a battery cell is crucial to its performance. For example, the power performance of a battery cell is related to the migration rate of sodium ions between the positive and negative electrodes. This involves the migration of sodium ions in the positive electrode, the negative electrode, the electrolyte, and at the solid-liquid interface. With the development of battery technology, many solutions have emerged to improve the power performance of battery cells by modifying the positive electrode, negative electrode, or electrolyte. However, battery cells with high-power discharge often have a shorter lifespan. This is because high-current discharge generates Joule heat, causing the battery temperature to rise, accelerating electrode deformation and active material shedding, shortening cycle life, and also accelerating electrode corrosion and electrolyte decomposition, which also shortens cycle life.

[0063] Current technologies primarily focus on the interface between the electrode and the electrolyte, aiming to improve battery life by mitigating side reactions between them. The applicant recognizes that the main bottleneck in addressing these issues lies in the excessive charge transfer impedance during the solid-phase diffusion process of the positive electrode active material and the structural stability of sodium ions during long-term rapid intercalation in the early stages of sodium-ion battery cell cycling. In the later stages, repeated high-power discharges damage the structure of the negative electrode active material, and negative electrode side reactions lead to the consumption of electrolyte and active ions. Furthermore, the applicant has found that the selection of the negative electrode material significantly affects the polarization process on the negative electrode surface, and the electrolyte liquid-phase diffusion impedance also impacts the high-power discharge of sodium-ion battery cells. Especially under prolonged discharge conditions, the liquid-phase diffusion impedance accumulates during discharge, further exacerbating polarization within the battery and affecting the discharge power and lifespan of the sodium-ion battery cell. In summary, the bottleneck to the repeated high-rate discharge performance of sodium-ion battery cells lies in simultaneously reducing the positive electrode charge transfer impedance, negative electrode side reactions, and liquid-phase diffusion impedance. Only by addressing these bottlenecks can repeated high-rate discharge of sodium-ion battery cells be achieved.

[0064] In view of this, this application provides a sodium-ion battery cell, including a positive electrode, a negative electrode, and an electrolyte. Specifically, the thickness L of the positive electrode film layer of the positive electrode satisfies: 50μm≤L≤150μm; the positive electrode film layer includes a phosphate material, which includes sodium iron pyrophosphate and sodium iron phosphate, and the sum of the mass contents P1 of sodium iron pyrophosphate and sodium iron phosphate, based on the total mass of the phosphate material, satisfies: 1%≤P1≤8%; the negative electrode includes a negative current collector and a negative electrode film layer, the negative electrode film layer being disposed on at least one side surface of the negative current collector, the negative electrode film layer including a carbon-based material, the carbon-based material including a porous structure, the pore volume V1 of the pore structure with a pore size greater than 10nm in the carbon-based material, measured by nitrogen adsorption method, and the total pore volume V of the carbon-based material satisfy: 20%≤V1 / V≤40%; the conductivity σ of the electrolyte at a temperature of 25℃±1℃ satisfies: 8mS / cm≤σ≤20mS / cm. The sodium-ion battery cell provided in this application embodiment can balance high power discharge performance and long service life.

[0065] The applicant discovered that the selection of cathode phosphate materials significantly affects the polarization process of the cathode surface reaction, thereby influencing the charge transfer impedance of the cathode. Specifically, the NFPO impurity phase in the cathode phosphate material has a low voltage plateau, which affects the lithium-ion migration rate during battery charging and discharging and accelerates battery capacity decay; the NFP impurity phase is an inert component that occupies active sites, reducing the electrochemical activity of the material. Therefore, related technologies generally aim to minimize the content of NFPO and NFP impurities in the cathode phosphate material. However, the applicant found that an appropriate content of NFPO and NFP impurities can improve the power performance of the battery. Specifically, the NFPP material, the main phase of the phosphate material, has large ion channels and a low barrier for sodium ion migration. It forms a heterostructure with the impurity NFPO material, significantly enhancing the diffusion ability of sodium ions from the interior to the surface of the phosphate material; the NFP material can form a symbiotic structure with NFPP and undergo amorphization during the first charge cycle, activating its electrochemical activity. Therefore, controlling the proportion of NFPO and NFP materials in the phosphate material within an appropriate range can improve the cathode surface reaction polarization process and the stability of the material structure.

[0066] The applicant also discovered during the anode selection process that the microstructure of the carbon-based anode material significantly affects the degree of side reactions and structural damage to the active material. Specifically, macropores in the carbon-based anode material increase the migration path of sodium ions as they escape from the pore structure and also increase the contact area with the electrolyte, exacerbating side reactions between the anode and the electrolyte. Therefore, it is necessary to control the proportion of open pores in the carbon-based anode material.

[0067] Therefore, by controlling the proportion of impurity phases NFPO and NFP in the positive electrode phosphate material and matching it with an appropriate thickness of the positive electrode film, the migration rate of sodium ions between the inside and surface of the positive electrode can be increased, and the charge transfer impedance of the positive electrode can be reduced, thereby improving the discharge power and early cycle performance of the sodium-ion battery cell. Furthermore, due to the poor conductivity of phosphate materials, this application also uses a highly conductive electrolyte to increase the migration rate of sodium ions at the interface between the electrolyte and the positive electrode, reducing charge transfer impedance and liquid phase diffusion impedance, further contributing to improving the high-power lifespan of the sodium-ion battery cell. Furthermore, by controlling the pore design in the negative electrode carbon-based material, the ability of sodium ions to escape from the negative electrode is improved while reducing side reactions between the negative electrode and the electrolyte, thus improving the power performance and later cycle performance of the battery cell. In summary, the technical solution provided in this application can simultaneously improve the power performance and cycle performance of the battery cell.

[0068] As an example, the sodium-ion battery cell provided in this application can be used in automotive high-power discharge systems as a high-power discharge battery. Furthermore, the sodium-ion battery cell provided in this application combines high-power discharge and long service life. When used in automotive high-power discharge systems, it can repeatedly discharge at high rates, meeting automotive usage requirements and reducing the frequency of battery replacements.

[0069] Sodium-ion battery cell

[0070] This application provides a sodium-ion battery cell, including a positive electrode, a negative electrode, and an electrolyte.

[0071] Figure 1 This is a schematic diagram of the structure of a positive electrode sheet according to an embodiment of this application. For example, as shown... Figure 1 As shown, the positive electrode 1 includes a positive current collector 10 and a positive electrode film layer 11 disposed on at least one side surface of the positive current collector 10.

[0072] The positive current collector 10 has two opposing surfaces along its thickness direction. The positive electrode film layer 11 can be disposed on one surface of the positive current collector 10 or on both surfaces of the positive current collector 10. As an example, such as... Figure 1 As shown, the positive electrode film layer 11 is disposed on both sides of the positive electrode current collector 10.

[0073] In this embodiment, the thickness L of the positive electrode film 11 satisfies: 50μm≤L≤150μm.

[0074] For example, L can be 50μm, 60μm, 70μm, 80μm, 90μm, 100μm, 110μm, 120μm, 130μm, 140μm, 150μm, or a value within the range obtained by any combination of the above two values.

[0075] It should be noted that when positive electrode films are disposed on both sides of the positive electrode current collector, the thicknesses of the positive electrode films on both sides may be equal or unequal. Here, "equal" can be considered as approximately equal. For example, within a certain error range known to those skilled in the art, the thicknesses of two positive electrode films with different thickness values ​​are considered to be approximately equal.

[0076] Setting the thickness of the positive electrode film 11 within the above-mentioned range, i.e., the positive electrode sheet is thinly coated, can improve the sodium ion transport rate in the positive electrode film, so that the sodium ions released from the positive electrode active material can quickly reach the positive electrode interface to participate in the reaction, reduce interface polarization, thereby reducing interface impedance, which is beneficial to improving the discharge power and service life of the battery cell.

[0077] The positive electrode film layer 11 includes a phosphate material, which includes sodium iron pyrophosphate and sodium iron phosphate. Based on the total mass of the phosphate material, the sum of the mass contents of sodium iron pyrophosphate and sodium iron phosphate, P1, satisfies: 1% ≤ P1 ≤ 8%.

[0078] For example, P1 can be 1%, 2%, 3%, 3.5%, 4%, 5%, 5.5%, 6%, 8%, or a value within the range obtained by any combination of the above two values.

[0079] In the phase composition design of cathode phosphate materials, the NFPP main phase contains impurity phases NFPO and NFP. The NFPP main phase has large ion channels and low barriers to sodium ion migration. It forms a heterostructure with the impurity NFPO, significantly enhancing the diffusion capacity of sodium ions from the interior to the surface of the phosphate material. NFP can also form a symbiotic structure with NFPP, undergoing amorphization during the first charge cycle and activating its electrochemical activity. Simultaneously, the proportions of NFPO and NFP affect the cathode surface reaction polarization process and material structural stability. By controlling the content of NFPO and NFP in the cathode phosphate material, the cathode surface reaction polarization process and material structural stability can be improved, thereby reducing the cathode charge transfer impedance and improving the power performance and early cycle performance of sodium-ion battery cells. Furthermore, controlling the thickness of the cathode film can increase the sodium ion transport rate within the cathode film, enabling rapid migration of sodium ions between the cathode interior and surface, thus improving the charge and discharge power of the battery cell.

[0080] In addition, since phosphate materials have poor conductivity, this application also uses a highly conductive electrolyte. The conductivity σ of the electrolyte at a temperature of 25℃±1℃ satisfies: 8mS / cm≤σ≤20mS / cm.

[0081] For example, σ can be 8mS / cm, 9mS / cm, 10mS / cm, 15mS / cm, 17mS / cm, 20mS / cm, or its value can be within the range obtained by any combination of the above two values.

[0082] The combination of positive electrode phosphate material with the above-mentioned highly conductive electrolyte can improve the migration rate of ions at the interface between the electrolyte and the positive electrode, reduce ion accumulation at the interface, improve the uniformity of charge distribution, reduce diffusion resistance, and thus improve electron conduction, thereby improving the power performance and cycle performance of the battery cell.

[0083] Figure 2 This is a schematic diagram of the structure of a negative electrode sheet according to an embodiment of this application. For example, as shown... Figure 2 As shown, the negative electrode 2 includes a negative current collector 20 and a negative electrode film layer 21 disposed on at least one side surface of the negative current collector 20.

[0084] The negative electrode current collector 20 has two opposing surfaces along its thickness direction. The negative electrode film layer 21 can be disposed on one surface of the negative electrode current collector 20 or on both surfaces. As an example, such as... Figure 2 As shown, the negative electrode film layer 21 is disposed on both sides of the negative electrode current collector 20.

[0085] The negative electrode film layer includes a carbon-based material, which has a porous structure. The pore volume V1 of the pore structure with a pore size greater than 10 nm in the carbon-based material, measured by nitrogen adsorption method, satisfies the following condition: 20% ≤ V1 / V ≤ 40%.

[0086] For example, V1 / V can be 20%, 22%, 25%, 27%, 30%, 33%, 35%, 38%, 40%, or a value within the range obtained by any combination of the above two values.

[0087] In the pore structure design of carbon-based materials, macropores in the negative electrode carbon-based material increase the migration path of sodium ions as they escape from the pore structure, and also increase the contact area with the electrolyte, exacerbating the side reactions between the negative electrode and the electrolyte. Therefore, reducing the proportion of macropores in the negative electrode carbon-based material can shorten the migration path of sodium ions from the interior to the surface of the negative electrode active material, allowing them to escape more quickly, thereby increasing the discharge power of the battery cell. It can also reduce the side reactions at the interface between the negative electrode sheet and the electrolyte, reducing the consumption of electrolyte and active ions, thereby improving the service life of the battery cell.

[0088] Combining the above technical solutions, the transport rate of sodium ions between the positive and negative electrode plates is improved, resulting in enhanced power performance. During the discharge of a sodium-ion battery cell, sodium ions released from the carbon-based material can reach the positive electrode interface more quickly to participate in the reaction, achieving high-rate discharge and improving discharge power performance. Simultaneously, during the migration of sodium ions between the positive and negative electrode plates, charge transfer resistance and diffusion resistance are reduced, battery side reactions are decreased, and the power performance and cycle performance of the sodium-ion battery cell are improved. The technical solution of this application embodiment can balance the high-power discharge performance and long service life of sodium-ion battery cells, providing a sodium-ion battery cell with excellent overall performance.

[0089] In some embodiments, 3% ≤ P1 ≤ 6%.

[0090] In some embodiments, the change ΔP in the sum of the mass contents of sodium iron pyrophosphate and sodium iron phosphate, based on the total mass of phosphate materials, between 100 cycles and 100 cycles of a sodium-ion battery cell satisfies: ΔP ≤ 3%.

[0091] For example, ΔP can be 1%, 1.5%, 2%, 2.5%, 3%, or a value within the range obtained by any combination of the above two values.

[0092] In this embodiment, the heterostructure formed by the impurity material NFPO and the main phase material NFPP helps to stabilize the crystal structure of the phosphate material. During the cycling process of the sodium-ion battery cell, the phosphate material maintains structural stability. The content of the impurity materials NFPO and NFP changes little during cycling, which is beneficial for the sodium-ion battery cell to maintain a higher capacity retention rate.

[0093] In some embodiments, the general structural formula of the phosphate material is Na. a Fe b M c M' d (Condensed polyanions) e (Anion) f Where M is one or more transition metals, M' is one or more non-transition metals, a>b, c>0, d≥0, e≥1, f>0, and the condensed polyanion comprises from P2O7 4- P3O9 5- and P4O 11 6- One or more groups selected from [the group].

[0094] For example, the general structural formula of phosphate materials is Na x Fe a-y M y (NO4) z (P2O7)w Wherein, 2≤x≤4, 1≤a≤4, 1≤y≤4, 0≤z≤4, 0≤w≤1, at least one of z and w is greater than or equal to 1, M includes at least one of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and N includes at least one of Al, Si, P, S, Ti, V, W.

[0095] For example, x can be 2, 2.1, 2.3, 2.5, 2.7, 2.9, 3, 3.2, 3.5, 3.7, 4, or a value within the range obtained by any two combinations of the above values. a can be 1, 1.5, 1.7, 2, 2.3, 2.5, 3, 3.2, 3.5, 3.7, 4, or a value within the range obtained by any two combinations of the above values. y can be 1, 2, 3, 4, or a value within the range obtained by any two combinations of the above values. z can be 0, 0.1, 0.2, 0.5, 0.8, 0.9, 1, 2, 3, 4, or a value within the range obtained by any two combinations of the above values. w can be 0.1, 0.2, 0.5, 0.8, 0.9, 1, or a value within the range obtained by any two combinations of the above values.

[0096] In some embodiments, the carbon-based material includes at least one of the following materials: hard carbon, a mixture of hard carbon and soft carbon, and a mixture of hard carbon and graphite.

[0097] In some embodiments, the pore volume V2 of the pore structure with a pore size of 5 nm to 10 nm in the carbon-based material, as measured by nitrogen adsorption, satisfies the following condition: 5% ≤ V2 / V ≤ 15%.

[0098] For example, V2 / V can be 5%, 6%, 8%, 10%, 12%, 14%, 15%, or a value within the range obtained by any combination of the above two values.

[0099] By controlling the volume ratio of larger pores (5nm~10nm) in carbon-based materials, the proportion of macropores is reduced, thereby shortening the migration path of sodium ions from the interior to the surface of the negative electrode active material, allowing them to escape more quickly, and thus improving the discharge power of the battery cell.

[0100] In some embodiments, the pore volume V3 of the pore structure with a pore size of 2nm to 5nm in the carbon-based material, as measured by nitrogen adsorption, satisfies the following condition: 20% ≤ V3 / V ≤ 40%.

[0101] Specifically, V3 / V can be 20%, 25%, 27%, 30%, 32%, 35%, 38%, 40%, or a value within the range obtained by any combination of the above two values.

[0102] Sodium ions diffuse rapidly in pore structures with a pore size of 2nm to 5nm. Setting the pore volume ratio of pore structures with a pore size of 2nm to 5nm in carbon-based materials within this range is beneficial for increasing the diffusion rate of sodium ions in the negative electrode active material, leading to faster insertion and thus improving the charging power of the battery cell. Furthermore, setting the pore volume ratio of pore structures with a pore size of 2nm to 5nm within this range also improves the wettability of the electrolyte on the negative electrode sheet, thereby enhancing the battery's rate performance and cycle stability.

[0103] In some embodiments, the pore volume V4 of the pore structure with a pore size of less than 2 nm in the carbon-based material, as measured by nitrogen adsorption, satisfies the following condition: 5% ≤ V4 / V ≤ 15%.

[0104] Specifically, V4 / V can be 5%, 6%, 8%, 10%, 12%, 14%, 15%, or a value within the range obtained by any combination of the above two values.

[0105] Sodium ions have a shorter migration path in pore structures with a pore size of less than 2 nm. By setting the pore volume ratio of pore structures with a pore size of less than 2 nm in carbon-based materials within the above range, the proportion of micropores (pore structures with a pore size of less than 2 nm) in carbon-based materials can be increased to shorten the migration path of sodium ions from the interior to the surface of the negative electrode active material, allowing them to escape more quickly and thus improve the discharge power of the battery cell.

[0106] In some embodiments, the carbon-based material further includes graphite-like sheets, wherein the spatial volume H1 between graphite-like sheets with an interlayer spacing of 0.35 nm to 0.4 nm and the total spatial volume H between the layers in the graphite-like sheets satisfy the following condition: 20% ≤ H1 / H ≤ 60%.

[0107] Specifically, H1 / H can be 20%, 22%, 25%, 27%, 30%, 33%, 35%, 38%, 40%, 45%, 50%, 55%, 59%, 60%, or a value within the range obtained by any combination of the above two values.

[0108] Sodium ions can be rapidly extracted between graphite-like layers with an interlayer spacing of 0.35 nm to 0.4 nm. By setting the spatial volume ratio of graphite-like layers with an interlayer spacing of 0.35 nm to 0.4 nm within the above range, increasing the spatial proportion of graphite-like layers with larger interlayer spacing is beneficial for sodium ions to be rapidly extracted from the interior of carbon-based materials, thereby improving the discharge power of the battery cell.

[0109] In some embodiments, the spatial volume H2 between graphite-like sheets with an interlayer spacing greater than 0.4 nm and the total spatial volume H between the layers in the graphite-like sheets satisfy the following: 20% ≤ H2 / H ≤ 50%.

[0110] Specifically, H2 / H can be 20%, 25%, 27%, 30%, 32%, 35%, 38%, 40%, 45%, 50%, or a value within the range obtained by any combination of the above two values.

[0111] Graphite-like sheets with an interlayer spacing greater than 0.4 nm have a large space volume between the sheets, which makes it easier for active ions to embed and adsorb onto the surface of the graphite-like sheets for energy storage. However, if the proportion of their space volume is too large, it will lead to an excessively high capacity in the slope section of the carbon-based material, thereby affecting the first-cycle coulombic efficiency of the sodium-ion battery cell. Therefore, the proportion of the space volume between graphite-like sheets with an interlayer spacing greater than 0.4 nm is set within the above range to achieve both the kinetic performance and the first-cycle coulombic efficiency of the sodium-ion battery cell.

[0112] In some embodiments, the spatial volume H3 between graphite-like sheets with an interlayer spacing of less than 0.35 nm and the total spatial volume H between all layers in the graphite-like sheet satisfy: 0 ≤ H3 / H ≤ 5%.

[0113] Specifically, H3 / H can be 0, 2%, 4%, 5%, or its value can be within the range obtained by any combination of the two values ​​mentioned above.

[0114] Setting the proportion of the space volume between graphite-like sheets with an interlayer spacing of less than 0.35 nm within the above range can reduce the risk of sodium deposition on the negative electrode, which is conducive to improving the capacity of the negative electrode and thus increasing the energy density of the battery cell.

[0115] In this embodiment of the application, H1 / H, H2 / H, and H3 / H can be obtained by peak fitting of the XRD pattern of carbon-based materials.

[0116] Specifically, the XRD diffraction pattern of the carbon-based material was first tested using the following method: The XRD diffraction pattern of the carbon-based material can be tested using an X-ray diffractometer according to JIS K 0131-1996. The test conditions were as follows: the carbon-based material and silicon powder were uniformly mixed at a mass ratio of 5:1, and the sample was prepared using the plate sample preparation method. CuKα rays were used as the radiation source, and a copper target was used as the anode target. The wavelength λ of the copper target was... The scanning 2θ angle range is 10°–40°, and the scanning rate is 1° / min. A Bruker D8 Discover X-ray diffractometer can be used as the testing instrument.

[0117] Next, the XRD diffraction pattern of the carbon-based material was fitted using XPS peak software. The XRD pattern of the carbon-based material was fitted into three small peaks, namely the first fitted peak A, the second fitted peak B, and the third fitted peak C. The fitting criteria were: the 2θ angle of the first fitted peak A was less than 22.2°, the 2θ angle of the second fitted peak B was 22.2° to 24.7°, and the 2θ angle of the third fitted peak C was greater than 24.7°.

[0118] The interlayer spacing and the 2θ angle satisfy Bragg's law: 2dsinθ=kλ, where d is the interlayer spacing of the (002) crystal plane of the carbon-based material, θ is the diffraction angle, k is the reflection order, and λ is the wavelength of the copper target. In this disclosure, k is 1, and λ is... According to Bragg's formula, the 2θ angle corresponding to graphite-like sheets with an interlayer spacing greater than 0.4 nm is less than 22.2°, the 2θ angle corresponding to graphite-like sheets with an interlayer spacing of 0.35 nm to 0.4 nm is 22.2° to 24.7°, and the 2θ angle corresponding to graphite-like sheets with an interlayer spacing of <0.35 nm is greater than 24.7°.

[0119] Finally, the ratio of the area of ​​the first fitting peak to the total area of ​​the three fitting peaks is equal to the ratio of the spatial volume H2 between graphite-like sheets with an interlayer spacing greater than 0.4 nm to the total spatial volume H between the layers in the graphite-like sheets (H2 / H); the ratio of the area of ​​the second fitting peak to the total area of ​​the three fitting peaks is equal to the ratio of the spatial volume H1 between graphite-like sheets with an interlayer spacing of 0.35 nm to 0.4 nm to the total spatial volume H between the layers in the graphite-like sheets (H1 / H); and the ratio of the area of ​​the third fitting peak to the total area of ​​the three fitting peaks is equal to the ratio of the spatial volume H3 between graphite-like sheets with an interlayer spacing less than 0.35 nm to the total spatial volume H between the layers in the graphite-like sheets (H3 / H).

[0120] For example, H1 / H, H2 / H, and H3 / H can be calculated as follows: the area of ​​the graphite-like sheet is 15.24 mm². 2 In this case, H2 / H = 15.24 mm 2 *Sum of interlayer spacings greater than 0.4 nm in carbon-based materials / 15.24 mm 2 The total interlayer spacing in carbon-based materials is 15.24 mm. 2 *Area of ​​the first fitted peak / 15.24 mm 2 *(Area of ​​the first fitted peak + Area of ​​the second fitted peak + Area of ​​the third fitted peak) = Area of ​​the first fitted peak / (Area of ​​the first fitted peak + Area of ​​the second fitted peak + Area of ​​the third fitted peak) = Area of ​​the first fitted peak / Total area of ​​the three fitted peaks. H1 / H = 15.24 mm 2 *The sum of interlayer spacings from 0.35 nm to 0.4 nm in carbon-based materials / 15.24 mm 2 The total interlayer spacing in carbon-based materials is 15.24 mm. 2 *Area of ​​the second fitted peak / 15.24 mm 2*(Area of ​​the first fitted peak + Area of ​​the second fitted peak + Area of ​​the third fitted peak) = Area of ​​the second fitted peak / (Area of ​​the first fitted peak + Area of ​​the second fitted peak + Area of ​​the third fitted peak) = Area of ​​the second fitted peak / Total area of ​​the three fitted peaks. H3 / H = 15.24 mm 2 *Sum of interlayer spacings smaller than 0.35 nm in carbon-based materials / 15.24 mm 2 The total interlayer spacing in carbon-based materials is 15.24 mm. 2 *Area of ​​the third fitted peak / 15.24 mm 2 *(Area of ​​the first fitted peak + Area of ​​the second fitted peak + Area of ​​the third fitted peak) = Area of ​​the third fitted peak / (Area of ​​the first fitted peak + Area of ​​the second fitted peak + Area of ​​the third fitted peak) = Area of ​​the third fitted peak / Total area of ​​the three fitted peaks.

[0121] In some embodiments, the electrolyte comprises cyclic carbonates and chain esters.

[0122] In some embodiments, the viscosity V of the chain ester at 25℃±1℃ satisfies: 0.2mPa·s≤V≤3mPa·s, and the mass content P3 of the chain ester based on the total mass of the electrolyte satisfies: 40%≤P3≤80%.

[0123] For example, V can be 0.2 mPa·s, 0.5 mPa·s, 0.8 mPa·s, 1 mPa·s, 1.5 mPa·s, 1.7 mPa·s, 1.9 mPa·s, 2 mPa·s, 2.5 mPa·s, 2.7 mPa·s, 3 mPa·s, or a value within the range obtained by any combination of the above two values. P3 can be 40%, 50%, 60%, 70%, 80%, or a value within the range obtained by any combination of the above two values.

[0124] In this embodiment, the viscosity of the chain ester in the electrolyte is within the above-mentioned range, which is beneficial to improve the conductivity of the electrolyte, thereby increasing the migration rate of sodium ions in the electrolyte and reducing the liquid phase diffusion resistance. Furthermore, by setting the mass ratio of the chain ester in the electrolyte, the power performance and cycle performance of the sodium-ion battery cell can be improved.

[0125] In some embodiments, the freezing point F of the cyclic carbonate satisfies: -60℃≤F≤-10℃, and the mass content P4 of the cyclic carbonate, based on the total mass of the electrolyte, satisfies: 10%≤P4≤50%.

[0126] For example, F can be -60℃, -50℃, -40℃, -30℃, -20℃, -10℃, or a value within the range obtained by any combination of the above two values. P4 can be 10%, 20%, 30%, 40%, 50%, or a value within the range obtained by any combination of the above two values.

[0127] In this embodiment, the electrolyte contains cyclic carbonates with a freezing point within the aforementioned range, which prevents the electrolyte from freezing at low temperatures (below zero). When used in combination with chain esters, the electrolyte still has high conductivity at low temperatures. Furthermore, by setting the mass percentage of cyclic carbonates in the electrolyte, the sodium-ion battery cells still have good power performance and cycle performance at low temperatures.

[0128] For example, cyclic carbonates include at least one of propylene carbonate and butene carbonate.

[0129] For example, the chain ester includes at least one of chain carbonates and chain carboxylic esters.

[0130] Chain carbonates include at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, methyl butyl carbonate, ethyl propyl carbonate, dipropyl carbonate, and dibutyl carbonate.

[0131] Chain carboxylic esters include one of methyl acetate, ethyl acetate, ethyl propionate, methyl formate, ethyl formate, methyl propionate, propyl propionate, ethyl butyrate, and propyl acetate.

[0132] In some embodiments, a carbon material is disposed on the surface of the phosphate material to improve the electronic conductivity of the positive electrode active material, which is beneficial to improving the cycle stability of the battery cell.

[0133] In some embodiments, the positive electrode film layer further includes a conductive agent, which includes at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0134] Adding conductive agents to the positive electrode film can improve the electron transport rate of the positive electrode and reduce the interfacial contact resistance, which is beneficial to improving the charge-discharge performance and cycle stability of the battery.

[0135] In some embodiments, the compaction density T of the positive electrode sheet satisfies: 1.5 g / cm³ 3 ≤T≤2.3g / cm 3 .

[0136] For example, T can be 1.5 g / cm³ 3 1.6g / cm 3 1.9g / cm 3 2g / cm3 2.2g / cm 3 2.3g / cm 3 , or its value is within the range obtained by combining any two of the above values.

[0137] The following provides a method for testing the compaction density of a positive electrode sheet.

[0138] Sample Preparation: Sodium-ion battery cells at any stage of use were subjected to discharge treatment. Specifically, at 25°C, the battery cells were discharged at a constant current of 0.33C to 2.0V, then allowed to stand for 20 minutes, and then discharged at a constant voltage of 2.0V until the current reached 0%, ending the discharge. The state of the battery cell at this point was defined as 0% SOC. The battery cells were then disassembled in an argon atmosphere (H2O content <10ppm, O2 content <1ppm) glove box to obtain the positive electrode. The residual electrolyte on the electrode surface was cleaned with dimethyl carbonate (DMC) or acetone, and then allowed to air dry for 24 hours.

[0139] Test: Randomly select 30 unit areas on the positive electrode sheet to be tested. Take the electrode sheet of each unit area and weigh the mass m1 of the material (excluding the current collector) on that unit area electrode sheet. Measure the electrode thickness H1 and the current collector thickness H0. The compaction density of each unit area electrode sheet is calculated as m1 / (H1-H0). Sum the results of the 30 randomly selected unit areas on the electrode sheet to be tested and then divide by 30 to obtain the compaction density of the electrode sheet to be tested. The measurement deviation of the compaction density is within ±0.05 g / cm³. 3 Within the range.

[0140] In some embodiments, the resistance R of the positive electrode plate satisfies: 0.01Ω≤R≤0.1Ω.

[0141] For example, R can be 0.01Ω, 0.02Ω, 0.03Ω, 0.04Ω, 0.05Ω, 0.08Ω, 0.1Ω, or a value within the range obtained by any combination of the above two values.

[0142] Electrode resistance can be tested using methods known in the art. For example, the resistance of the positive electrode to be tested can be tested using the Yuaneng Technology Electrode Resistance Meter (BER2500). Before use, the resistance and pressure should be reset. The positive electrode is placed under the probe for testing, and the average value of the resistance measured at 12 different positions is recorded as the resistance value of the positive electrode.

[0143] In some embodiments, the ratio CB of the active material capacity of the negative electrode to the active material capacity of the positive electrode satisfies: 1.07≤CB≤1.2.

[0144] For example, CB can be 1.07, 1.08, 1.1, 1.12, 1.15, 1.2, or a value within the range obtained by any combination of the above two values.

[0145] In some embodiments, the positive current collector comprises aluminum foil.

[0146] In some embodiments, the sodium-ion battery cell further includes: a housing, which is a hollow structure with an opening, and a positive electrode sheet is housed within the housing; and a top cover that closes the opening.

[0147] In some embodiments, the housing is made of aluminum.

[0148] In some embodiments, the sodium-ion battery cell further includes an electrode post disposed on the side of the top cover away from the hollow structure, and the electrode post is made of aluminum.

[0149] The embodiments of this application do not impose any particular restrictions on the shape of the battery cell; it can be cylindrical, square, or any other arbitrary shape.

[0150] Figure 3 This is a schematic diagram of a battery cell according to an embodiment of this application. For example, such as... Figure 3 As shown, the battery cell 3 is a square battery cell. The battery cell 3 includes a housing 31, an end cap assembly 32, and an electrode assembly 33 disposed in the housing 31.

[0151] The electrode assembly 33 can be made from a positive electrode 1, a negative electrode 2 and a separator through a winding process or a stacking process.

[0152] End cap assembly 32 includes electrode terminals 322, such as Figure 3 As shown, the end cap assembly 32 includes two electrode terminals 322, one of which is a positive electrode terminal and the other is a negative electrode terminal.

[0153] The battery cell 3 also includes a current collector 34, which is used to connect the tab 332 and the electrode terminal 322 of the electrode assembly 33. For example, in the case of a negative electrode in this embodiment, one current collector 34 is used to connect the tab and the negative electrode terminal of the negative electrode, and another current collector 34 is used to connect the tab and the positive electrode terminal of the positive electrode.

[0154] In some embodiments, the battery cell 3 includes an electrode assembly 33, which includes an electrode assembly body 331 and a tab 332 extending from the electrode assembly body 331.

[0155] In some embodiments, individual battery cells can be assembled into a battery module. The number of individual battery cells contained in a battery module can be one or more, and the specific number can be selected by those skilled in the art based on the application and capacity of the battery module.

[0156] [Positive electrode plate]

[0157] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0158] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0159] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0160] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0161] During the charging and discharging process of a battery, sodium (Na) undergoes insertion / extraction and consumption, resulting in varying molar Na content at different discharge states. In the examples of cathode materials in this application, the molar Na content refers to the initial state of the material, i.e., the state before feeding. When the cathode material is applied to the battery system, the molar Na content changes after charge-discharge cycles.

[0162] In the examples of cathode materials in this application, the molar content of O is only a theoretical value. Oxygen release from the crystal lattice will cause changes in the molar content of oxygen, and the actual molar content of O will fluctuate.

[0163] [Negative electrode plate]

[0164] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0165] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0166] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0167] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na) or lithium carboxymethyl cellulose (CMC-Li)).

[0168] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0169] [Electrolytes]

[0170] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific limitations on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel-like, or entirely solid.

[0171] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0172] The electrolyte salt includes, but is not limited to, at least one of NaPF6, NaClO4, NaBCl4, NaSO3CF3, and Na(CH3)C6H4SO3. One of the above electrolyte salts may be used alone, or two or more may be used simultaneously.

[0173] Solvents include carbonate or ether solvents. Carbonate solvents include cyclic ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), and chain-like dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), etc.; ether solvents include ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, tetrahydrofuran, 1,3-dioxane, etc.

[0174] The electrolyte may also optionally include electrolyte additives. For example, electrolyte additives may include negative electrode film-forming additives, positive electrode film-forming additives, and performance additives that can improve certain battery performance, such as performance additives that improve battery overcharge performance, or improve battery high-temperature or low-temperature performance.

[0175] [Isolation Component]

[0176] The separator is used to isolate the positive electrode and the negative electrode. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0177] The material of the separator can be selected from one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film; there are no particular restrictions. When the separator is a multi-layer composite film, the materials of each layer can be the same or different; there are no particular restrictions.

[0178] Positive electrode, negative electrode and separator can be made into electrode assembly by winding process or stacking process.

[0179] In some embodiments, the above-mentioned positive electrode, negative electrode and separator can be fabricated into an electrode assembly by a winding process or a stacking process.

[0180] In some embodiments, the battery cell may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte described above.

[0181] In some embodiments, the outer packaging of the battery cell can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the battery cell can also be a flexible package, such as a pouch. The material of the flexible package can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0182] [Battery Device]

[0183] This application provides a battery device, including the battery cell described in the above embodiments. Figure 4 This is a schematic diagram of the structure of a battery device according to an embodiment of this application. Figure 4 As shown, the battery device 5 may include multiple battery cells 3 to meet different power usage requirements. The shape of the battery cell 3 in this embodiment can be set according to actual application. For example, the battery cell 3 may be as follows: Figure 3 The cuboid shown can also be different. Figure 3 The embodiments shown are cylindrical or other shapes, but are not limited to these.

[0184] It should be understood that, such as Figure 4 As shown, the battery device 5 in this embodiment may further include a housing 51, which can be used to accommodate multiple battery cells 3. The housing 51 in this embodiment has a hollow interior, and the multiple battery cells 3 are accommodated within the housing 51. The housing 51 may include two parts, referred to herein as a first housing portion 511 and a second housing portion 512, which are fastened together. The shapes of the first housing portion 511 and the second housing portion 512 can be determined according to the shape of the components housed inside, for example, according to the shape of the combination of the multiple battery cells 3 housed inside. At least one of the first housing portion 511 and the second housing portion 512 has an opening. For example, as... Figure 4 As shown, the first housing portion 511 and the second housing portion 512 can both be hollow cuboids with one open side. The openings of the first housing portion 511 and the second housing portion 512 are opposite to each other, and the first housing portion 511 and the second housing portion 512 are interlocked to form a housing 51 with a closed cavity, which can be used to accommodate multiple battery cells 3. The multiple battery cells 3 are connected in parallel, series, or mixed and placed inside the housing 51 formed by the interlocking of the first housing portion 511 and the second housing portion 512.

[0185] For example, unlike Figure 4 As shown, either the first housing portion 511 or the second housing portion 512 may have only one hollow cuboid with an opening, while the other is plate-shaped to cover the opening. Taking the second housing portion 512 as a hollow cuboid with one opening, and the first housing portion 511 as a plate-shaped example, then the first housing portion 511 covers the opening of the second housing portion 512 to form a housing 51 with a closed chamber, which can be used to accommodate multiple battery cells 3.

[0186] The battery cells 3 can be directly assembled into the battery device 5, or they can be first assembled into battery modules, and then multiple battery modules can be assembled into the battery device 5.

[0187] [Electrical Equipment]

[0188] This application provides an electrical device including the battery device described in the above embodiments.

[0189] The technical solutions described in the embodiments of this application are applicable to various electrical devices that use battery devices. For example, electrical devices can be used in fields such as new energy vehicles, drones and robots, smart grids, data centers, renewable energy storage, large-scale energy storage systems, power starters, and power tools.

[0190] Electrical equipment can include vehicles, mobile phones, portable devices, laptops, ships, spacecraft, electric toys, and power tools, etc. Vehicles can be gasoline-powered cars, natural gas-powered cars, or new energy vehicles; new energy vehicles can be pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles, etc. Spacecraft include airplanes, rockets, space shuttles, and spacecraft, etc. Electric toys include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc. Power tools include metal cutting power tools, grinding power tools, assembly power tools, and railway power tools, such as electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers, etc. This application does not impose any special limitations on the above-mentioned electrical equipment.

[0191] For ease of explanation, the following embodiments use a vehicle as an example of electrical equipment.

[0192] For example, such as Figure 5 The diagram shown is a structural schematic of a vehicle 7 according to one embodiment of this application. The vehicle 7 can be a gasoline-powered vehicle, a natural gas-powered vehicle, or a new energy vehicle. New energy vehicles can be pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles, etc. The vehicle 7 can have a motor 4, a controller 6, and a battery device 5 installed inside. The controller 6 controls the battery device 5 to supply power to the motor 4. For example, the battery device 5 can be installed at the bottom, front, or rear of the vehicle 7. The battery device 5 can be used to power the vehicle 7; for example, it can serve as the operating power source for the vehicle 7's electrical system, such as meeting the power requirements for starting, navigation, and operation. In another embodiment of this application, the battery device 5 can not only serve as the operating power source for the vehicle 7 but also as the driving power source, replacing or partially replacing gasoline or natural gas to provide driving power to the vehicle 7.

[0193] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0194] [Examples and Comparative Examples]

[0195] 1. Preparation of positive electrode sheet

[0196] Preparation of positive electrode active material: A mixture containing ferric oxide, sodium ferric pyrophosphate, ferric phosphate, glucose monohydrate and deionized water is spray-dried to obtain precursor powder. The precursor powder is sintered under inert gas protection to finally obtain phosphate positive electrode active material.

[0197] It should be noted that, in the preparation process of the positive electrode active material, the mass content of the main phase sodium pyrophosphate, as well as the impurity phases sodium phosphate and sodium pyrophosphate, in the phosphate positive electrode active material can be controlled by adjusting the mass ratio of ferric oxide, sodium pyrophosphate, and iron phosphate, as well as the sintering temperature and time.

[0198] Positive electrode 1: The phosphate material Na4Fe3(PO4)2P2O7, conductive agent acetylene black, and binder PVDF prepared above are mixed evenly at a mass ratio of 94:3:3 and dissolved in solvent N-methylpyrrolidone (NMP). After thorough mixing, a positive electrode slurry is obtained. The positive electrode slurry is uniformly coated on two opposite surfaces of a 13 μm thick positive electrode current collector aluminum foil. After drying, cold pressing, and slitting, positive electrode 1 is formed. In positive electrode 1, based on the total mass of the phosphate material, the mass content P1 of sodium iron phosphate and sodium iron pyrophosphate is 5%; the thickness L of the positive electrode film layer of positive electrode 1 is 50 μm.

[0199] Positive electrode plate 2-4

[0200] The preparation process of positive electrode 2-4 is similar to that of positive electrode 1, except that P1 in positive electrode 2-4 is 3%, 6%, and 6%, respectively.

[0201] Positive electrode plate 5-6

[0202] The preparation process of positive electrode 5-6 is similar to that of positive electrode 1, except that the thickness L of the positive electrode film layer of positive electrode 5-6 is 100μm and 150μm, respectively.

[0203] The preparation process of positive electrode 7 is similar to that of positive electrode 1, except that P1 in positive electrode 7 is 10%.

[0204] 2. Preparation of negative electrode sheet

[0205] Preparation of negative electrode active material: Under the conditions of introducing carrier gas nitrogen and dopant gas oxygen, the biomass carbon precursor coconut shell was placed in a box furnace and heated to 600℃ at a heating rate of 5℃ / min and held for 3h for pre-carbonization treatment. The volume ratio of oxygen to nitrogen was 0.05:1. The flow rate of the nitrogen and oxygen mixture was 10mL / min. Under the condition of introducing nitrogen, the mixture was heated to 1300℃ and held for 2h, and then cooled to room temperature. The sample obtained above was crushed, graded, sieved, and demagnetized to finally obtain hard carbon negative electrode active material.

[0206] It should be noted that in the preparation process of negative electrode active materials, by adjusting parameters such as carbonization temperature and temperature change rate, extending the high-temperature treatment time can increase the orderliness of the hard carbon structure, reduce porosity and defects, and thus optimize the interlayer spacing ratio.

[0207] Negative electrode 1: The hard carbon material, conductive agent acetylene black, binder SBR, and thickener CMC-Na prepared above are mixed evenly in a mass ratio of 90:4:4:2, dissolved in deionized water, and stirred thoroughly to obtain a negative electrode slurry. The negative electrode slurry is evenly coated on the two opposite surfaces of the negative electrode current collector copper foil. After drying, cold pressing, and slitting, negative electrode 1 is formed. In the negative electrode 1, the ratio of pore volume V1 of the hard carbon material with a pore size greater than 10 nm to the total pore volume V of the hard carbon material, measured by nitrogen adsorption, is 30%; the ratio of pore volume V2 of the carbon-based material with a pore size of 5 nm to 10 nm to the total pore volume V of the carbon-based material, measured by nitrogen adsorption, is 15%; the ratio of pore volume V3 of the carbon-based material with a pore size of 2 nm to 5 nm to the total pore volume V of the carbon-based material, measured by nitrogen adsorption, is 40%; and the ratio of pore volume V4 of the carbon-based material with a pore size less than 2 nm to the total pore volume V of the carbon-based material, measured by nitrogen adsorption, is 15%.

[0208] Negative electrode plate 2-3

[0209] The preparation process of negative electrode 2-3 is similar to that of negative electrode 1, except that the V1 / V ratios in negative electrode 2-3 are 20% and 40%, respectively.

[0210] The preparation process of negative electrode 4 is similar to that of negative electrode 1, except that the V1 / V ratio in negative electrode 4 is 53%.

[0211] 3. Preparation of electrolyte

[0212] In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), organic solvents propylene carbonate (PC) and ethyl methyl carbonate (EMC) are mixed evenly at a volume ratio of 25:75. 1 mol / L NaPF6 sodium salt is added and dispersed evenly. Then, fluoroethylene carbonate (FEC) is added and dissolved in the organic solvent. The mixture is stirred evenly to obtain the electrolyte. The volume percentage of FEC in the electrolyte is 5%.

[0213] Electrolyte 1: The conductivity σ of electrolyte 1 at 25℃ is 10mS / cm.

[0214] Electrolyte 2-3: The conductivity σ of electrolyte 2-3 at 25℃ is 8mS / cm and 20mS / cm, respectively.

[0215] Electrolyte 4: The conductivity σ of electrolyte 4 at 25℃ is 6.5mS / cm.

[0216] 4. Preparation of the separator membrane: The separator membrane is a conventional polyethylene membrane with a thickness of 7μm.

[0217] 5. Assembly of sodium-ion batteries

[0218] The positive electrode, separator, and negative electrode are stacked in sequence and then wound to obtain an electrode assembly. The electrode assembly is placed in a shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a sodium-ion battery is obtained.

[0219] [Examples 1-4]

[0220] In Examples 1-4, the same separator, negative electrode 1, and electrolyte 1 were used. The difference was that different positive electrode 1-4 were used respectively, as detailed in Table 1.

[0221] [Examples 5-6]

[0222] In Examples 5-6, the same separator, positive electrode 1, and electrolyte 1 were used. The difference was that different negative electrode 2 and 3 were used respectively, as detailed in Table 2.

[0223] [Examples 7-8]

[0224] In Examples 7-8, the same separator, negative electrode 1, and electrolyte 1 were used. The difference was that different positive electrode 5 and 6 were used respectively, as detailed in Table 2.

[0225] [Examples 9-10]

[0226] In Examples 9-10, the same separator, negative electrode 1, and positive electrode 1 were used. The difference was that different electrolytes were used, namely electrolytes 2 and 3, as detailed in Table 3.

[0227] [Comparative Example 1]

[0228] In Comparative Example 1, the same separator, negative electrode 1 and electrolyte 1 as in Example 1 were used. The difference was that a different positive electrode 7 was used in Comparative Example 1. See Table 1 for details.

[0229] [Comparative Example 2]

[0230] In Comparative Example 2, the same separator, positive electrode 1, and electrolyte 1 as in Example 5 were used. The difference was that a different negative electrode 4 was used in Comparative Example 2. See Table 2 for details.

[0231] [Comparative Example 3]

[0232] In Comparative Example 3, the same separator, positive electrode 1 and negative electrode 1 as in Example 9 were used. The difference was that a different electrolyte was used. Comparative Example 3 used electrolyte 4. See Table 3 for details.

[0233] In Table 1-3 below, P1 represents the sum of the mass contents of sodium iron phosphate and sodium iron pyrophosphate based on the total mass of phosphate materials in a fresh sodium-ion battery cell; P2 represents the sum of the mass contents of sodium iron phosphate and sodium iron pyrophosphate based on the total mass of phosphate materials after 100 cycles in a fresh sodium-ion battery cell; ΔP represents the difference between P2 and P1; L represents the thickness of the positive electrode film layer of the positive electrode sheet; V1 / V represents the ratio of the pore volume V1 of the pore structure with a pore size greater than 10 nm in the hard carbon material to the total pore volume V of the carbon-based material, as measured by the nitrogen adsorption method; σ represents the conductivity of the electrolyte at a temperature of 25℃±1℃.

[0234] Table 1. Product parameters and performance parameters of Examples 1-4 and Comparative Example 1

[0235]

[0236] Combining the results of Examples 1-4 and Comparative Example 1, it is evident that setting the sum of the contents of the impurity phases NFPO and NFP in the cathode phosphate material within the range specified in this application results in better discharge power performance and cycle performance for the sodium-ion battery cell. In Comparative Example 1, the sum of the contents of the impurity phases NFPO and NFP in the cathode phosphate material is relatively high, and the sum of the contents of NFPO and NFP increases significantly after 100 cycles of a fresh sodium-ion battery cell, resulting in poor discharge power performance and cycle performance. The reason for this is that the volume expansion rate of the impurity phase NFPO in the cathode phosphate material during sodium removal is relatively large, leading to increased cell distortion and greater damage to ion channels, thereby affecting ion conduction, increasing battery polarization loss, and ultimately causing a decrease in battery power performance and cycle performance. Therefore, when the sum of the contents of the impurity phases NFPO and NFP is high in Comparative Example 1, the discharge power performance and cycle performance of the sodium-ion battery cell are both poor. In Examples 1-4, the sum of the contents of the impure phases NFPO and NFP is set within a small and suitable range. It is speculated that the above-mentioned impure phase components can form a heterostructure with the main phase material NFPP of the phosphate material, which significantly improves the diffusion ability of sodium ions from the interior to the surface of the phosphate material, thereby reducing the charge transfer impedance and improving the power performance and cycle performance of sodium-ion battery cells.

[0237] Based on the results of Examples 3 and 4, it can be seen that when the sum of the contents of impurity phases NFPO and NFP in a fresh sodium-ion battery cell is the same at the initial value, the discharge power performance and cycle performance of the sodium-ion battery cell will deteriorate if the sum of the contents of impurity phases NFPO and NFP increases too much after 100 cycles.

[0238] Table 2. Product parameters and performance parameters of Examples 5-8 and Comparative Example 2

[0239]

[0240] Combining the results of Examples 5-6 and Comparative Example 2, it can be seen that, with the same positive electrode active material and electrolyte configuration, setting the volume ratio of the large-pore structure in the negative electrode carbon-based material within the range of this application results in better discharge power performance and cycle performance for the sodium-ion battery cell. The reason for this is that in Comparative Example 2, the volume ratio of the large-pore structure in the negative electrode carbon-based material is larger than that in Examples 5-6. This increased proportion of large-pore structure means that sodium ions have a longer migration path in the large pores and are extracted more slowly, thus reducing discharge power. Furthermore, the larger contact area between the large pores and the electrolyte exacerbates side reactions at the interface between the negative electrode and the electrolyte, leading to greater consumption of electrolyte and active ions, thereby reducing the cycle life of the battery cell.

[0241] Table 3. Product parameters and performance parameters of Examples 9-10 and Comparative Example 3

[0242]

[0243] Based on the results of Examples 9-10 and Comparative Example 3, it can be seen that when the positive and negative active materials are the same, the discharge power performance and cycle performance of sodium-ion battery cells are poor when the electrolyte conductivity is low. However, when the electrolyte conductivity is set within the range of this application (8 mS / cm to 20 mS / cm), the discharge power and cycle performance of sodium-ion battery cells are both good. Furthermore, when the electrolyte conductivity is within the range of this application, as the conductivity increases, the discharge power of the sodium-ion battery cell is better, but the cycle performance will decrease. Therefore, controlling the conductivity within a suitable range can balance the discharge power performance and cycle performance of sodium-ion battery cells. Furthermore, combining the cycle performance of Examples 9-10 and Comparative Example 3 at high temperature (45°C) and normal temperature (25°C), it can be seen that under high temperature conditions, the decrease in electrolyte conductivity has little impact on the cycle performance of sodium-ion battery cells. However, under normal temperature conditions, when the electrolyte conductivity decreases to outside the range of this application, the cycle performance of sodium-ion battery cells deteriorates significantly. Therefore, it is necessary to control the electrolyte conductivity within a suitable range to ensure the cycle life of sodium-ion battery cells when used in a wide temperature range.

[0244] The following is a brief description of the testing methods for the physicochemical and performance parameters involved in the embodiments of this application. It should be understood that the following testing methods are only examples, and other testing methods known in the art can also be used for testing.

[0245] 1. Test method for discharge power of sodium-ion batteries

[0246] At 25°C, the battery is charged at a constant current of 1 / 3C to 3.65V, then charged at a constant voltage of 3.65V to a current of 0.05C, rested for 5 minutes, and then discharged at 1 / 3C to 2V. The average capacity obtained after three cycles is recorded as the initial capacity D0. Then, the battery is charged under the same conditions and discharged at 3C, and the average capacity obtained after three cycles is recorded as the initial capacity D1. The capacity retention rate Q at 3C discharge is calculated using the following formula: Q = (D1 / D0) × 100%.

[0247] 2. Test methods for the cycle performance of sodium-ion batteries

[0248] (1) Cyclic performance at 25℃ / 45℃

[0249] At 25℃ / 45℃, the sodium-ion battery is charged at a constant current of 0.33C to 3.65V, then charged at a constant voltage of 3.65V to a current of 0.05C. After resting for 10 minutes, it is discharged at 0.33C to 2V. The resulting capacity is recorded as the initial capacity D0. The above steps are repeated, and the discharge capacity Dn of the battery after the nth cycle is recorded. The battery capacity retention rate (%) after n cycles is calculated as (Dn / D0) × 100%.

[0250] For example, the capacity retention rate (%) of a lithium-ion battery after 1000 cycles at 25°C = (discharge capacity of the 1000th cycle / discharge capacity of the first cycle) × 100%; the capacity retention rate (%) of a lithium-ion battery after 200 cycles at 25°C = (discharge capacity of the 200th cycle / discharge capacity of the first cycle) × 100%.

[0251] 3. Method for measuring the thickness of the positive electrode film

[0252] Sample Preparation: Sodium-ion battery cells at any stage of use were subjected to discharge treatment. Specifically, at 25°C, the battery cells were discharged at a constant current of 1C to 1.5V, then allowed to stand for 20 minutes, and then discharged at a constant voltage of 1.5V until the current reached 0%, ending the discharge. The state of the battery cell at this point was defined as 0% SOC. The battery cells were then disassembled in an argon atmosphere (H2O content <10ppm, O2 content <1ppm) glove box to obtain the positive electrode. The residual electrolyte on the electrode surface was cleaned with dimethyl carbonate (DMC) or acetone, and then allowed to air dry for 24 hours.

[0253] Take a 1m positive electrode sheet and use a micrometer to measure the thickness at 20 different points. Take the average value to obtain the thickness of the positive electrode film.

[0254] 4. Determination of phosphate materials

[0255] Sample Preparation: Sodium-ion battery cells at any stage of use were discharged. Specifically, at 25°C, the cell was discharged at a constant current of 1C to 1.5V, then allowed to stand for 20 minutes, followed by a constant voltage discharge of 1.5V until the current reached zero. This state of discharge was defined as 0% SOC. The cell was then disassembled in an argon atmosphere (H2O <10ppm, O2 <1ppm) glove box to obtain the positive electrode. The surface of the electrode was cleaned with dimethyl carbonate (DMC) or acetone to remove residual electrolyte, and then allowed to air dry for 24 hours. The positive electrode surface was then powdered for sample preparation. The powder sample needed to be ground uniformly to avoid interference from large particles. The positive electrode powder was mixed with N-methylpyrrolidone (NMP), stirred, filtered, and dried to obtain the sample to be tested.

[0256] Detection: The composition of the positive electrode active material was analyzed using X-ray diffraction (XRD). Specifically, the crystal structure of the sample was characterized using an XRD diffractometer (Rigaku Ultima IV). The testing range was 2θ = 5°–80°. The molecular structure and chemical composition of the sample were analyzed using Fourier transform infrared spectroscopy (Nicoleti S50). The physical properties of the sample were analyzed using Raman spectroscopy (LabRAM HR Evolution) at an excitation wavelength of 532 nm. The chemical valence states of the elements on the sample surface were analyzed using X-ray photoelectron spectroscopy (VGEscalab250xi). The morphology and microstructure of the sample were observed using scanning electron microscopy (FEI-Nova NanoSEM230) and transmission electron microscopy (FEI Titan G260–300). The thermal stability and carbon content of the sample were analyzed using thermogravimetric analysis (Netzsch STA449C).

[0257] 5. Methods for testing the pore volume of pore structures with different pore sizes in carbon-based materials

[0258] Sample Preparation: Sodium-ion battery cells at any stage of use were discharged. Specifically, at 25°C, the cell was discharged at a constant current of 1C to 1.5V, then allowed to stand for 20 minutes, followed by a constant voltage discharge of 1.5V until the current reached zero. This state of discharge was defined as 0% SOC. The cell was then disassembled in an argon atmosphere (H2O <10ppm, O2 <1ppm) glove box to obtain the negative electrode. The surface of the electrode was cleaned with dimethyl carbonate (DMC) or acetone to remove residual electrolyte, and then allowed to air dry for 24 hours. The negative electrode surface was then powdered for sample preparation. The powder sample needed to be ground uniformly to avoid interference from large particles. The negative electrode film powder was mixed with deionized water, stirred, filtered, and dried to obtain the sample to be tested.

[0259] Detection: (1) First, the sample is pretreated to remove moisture, impurities and other substances adsorbed on the surface of the hard carbon sample, so as to ensure that nitrogen adsorption only occurs on the clean material surface.

[0260] Specifically, first weigh an appropriate amount of hard carbon sample and perform vacuum degassing. Place the weighed sample into a sample tube and put it into a vacuum degassing device. The degassing temperature needs to be strictly controlled. Since hard carbon may oxidize at temperatures above 300℃ in air, the degassing temperature should be set between 120 and 300℃, with 150 to 250℃ being recommended. This ensures effective degassing while avoiding damage to the material structure. The degassing time should be 6 to 12 hours to ensure that impurities on the sample surface are fully removed.

[0261] (2) Then, isotherm tests are performed to obtain adsorption-desorption data of nitrogen on hard carbon materials, providing the original basis for subsequent pore structure analysis.

[0262] Specifically, the pretreated sample tube is first placed in the test position of the adsorption instrument and then placed in a liquid nitrogen bath at 77K to stabilize the test environment temperature at the liquid nitrogen temperature. The adsorption process is then tested. Nitrogen gas is introduced, and the relative pressure P / P0 (P refers to the partial pressure of nitrogen, and P0 refers to the saturated vapor pressure of the adsorbate gas at the adsorption temperature) is gradually increased from 0.01 to 0.995. The amount of nitrogen adsorbed by the hard carbon material is recorded at each pressure point. At low relative pressures (P / P0), nitrogen molecules first form a monolayer adsorption on the pore walls, gradually forming a multilayer as the pressure increases. When the pressure reaches a certain value, capillary condensation occurs in the mesopores, resulting in a sharp increase in adsorption. Micropores, due to their extremely small pore size, can be filled with nitrogen molecules even at very low P / P0, without a significant condensation step. After the adsorption process is complete, the desorption process is tested. The relative pressure is gradually decreased, and the amount of desorption at different pressures is recorded to obtain desorption branch data. The shape of the isotherms formed during the adsorption-desorption process (such as the hysteresis loop type) can reflect the structural characteristics of the pores.

[0263] (3) Data Calculation

[0264] Specifically, specialized software was used to process the adsorption-desorption isotherm data obtained from the tests, and the relevant parameters of the pore structure were calculated.

[0265] Calculate the total pore volume: Usually, the adsorption amount is taken when the relative pressure (P / P0) ≈ 0.99, and it is converted into the volume of liquid nitrogen. This volume is the total pore volume of the material, which represents the total volume of nitrogen that the material can hold.

[0266] Calculating micropore volume: For micropores with a diameter <2 nm, the t-plot method or the DR (Dubinin-Radushkevich) equation can be used for calculation. The t-plot method requires the selection of a suitable t-curve (such as the t-curve of standard carbon materials) to avoid baseline errors; the DR equation is suitable for hard carbon with relatively weak polarity.

[0267] Calculating mesopore volume: For hard carbon materials containing only micropores and mesopores, the mesopore volume is equal to the difference between the total pore volume and the micropore volume.

[0268] Analysis of pore size distribution: Pore size distribution reflects the proportion of pore volume within different pore size ranges and can be calculated using corresponding models. For mesopores (pore size of 2nm to 50nm), the BJH (Barret-Joyner-Halenda) model is used, and calculations are performed based on desorption branch data; the pore size distribution of micropores needs to be analyzed in conjunction with their corresponding calculation models.

[0269] (4) Results Analysis

[0270] Based on the obtained parameters such as total pore volume, micropore volume, mesopore volume, and pore size distribution, analysis is performed in conjunction with the shape of the adsorption-desorption isotherm. For example, the mesoporous structure can be determined by the type of hysteresis ring: H1 type hysteresis rings correspond to cylindrical pores with uniform pore size, while H2 type hysteresis rings correspond to pores with uneven pore size or "ink bottle" pores.

[0271] 6. Determination of the types and mass content of substances in the electrolyte.

[0272] Sample Preparation: Battery cells at any stage of use are discharged. Specifically, at 25°C, the battery cell is discharged at a constant current of 1C to 1.5V, then allowed to stand for 20 minutes, and then discharged at a constant voltage of 1.5V until the current reaches zero, ending the discharge. The state of the battery cell at this point is defined as 0% SOC. Then, in an argon atmosphere (H2O content <10ppm, O2 content <1ppm) glove box, the battery cell is disassembled to obtain the electrolyte. (If the electrolyte content in the battery cell is low, the electrolyte can be separated from the positive and negative electrode plates by centrifugation). After sample preparation, the sample is vacuum-sealed and transferred out of the glove box for subsequent testing.

[0273] Detection: Comprehensive qualitative and quantitative analysis of components such as organic solvents, lithium salts, and additives in electrolytes can be performed using various analytical techniques such as gas chromatography / mass spectrometry (GC / MS), liquid chromatography / mass spectrometry (LC / MS), and inductively coupled plasma mass spectrometry (ICP-MS).

[0274] For example, when using GC / MS to identify high levels of volatile components in the electrolyte, the sample is diluted 1000-fold with dichloromethane and analyzed in split mode. For the analysis of trace volatile organic additives, the electrolyte sample is directly injected into the GC / MS without split mode and dilution is not required.

[0275] 7. Electrolyte conductivity testing methods

[0276] The conductivity of the electrolyte is measured using a conductivity meter. The specific steps are as follows:

[0277] (1) Take an appropriate amount of electrolyte sample from a sodium-ion battery in a glove box with relative humidity ≤1%, avoiding moisture or contamination of the sample. Place the sample in an environment with a temperature of 25℃±1℃.

[0278] (2) Install the measuring electrode and temperature sensor. When the temperature reaches 25℃, insert the measuring electrode and temperature sensor into the sample to be tested. The temperature sensor is suspended and stationary. The measuring electrode is suspended and slowly stirs the liquid to be tested clockwise. After the test data stabilizes, record the experimental data to obtain the conductivity of the electrolyte. Alternatively, the average conductivity can be calculated as needed.

[0279] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A sodium-ion battery cell, characterized in that, It includes positive electrode, negative electrode, and electrolyte; among which The positive electrode includes a positive current collector and a positive electrode film layer. The positive electrode film layer is disposed on at least one surface of the positive current collector, and the thickness L of the positive electrode film layer satisfies: 50μm≤L≤150μm; The positive electrode film layer includes a phosphate material, which includes sodium iron pyrophosphate and sodium iron phosphate. Based on the total mass of the phosphate material, the sum of the mass contents of sodium iron pyrophosphate and sodium iron phosphate, P1, satisfies: 1% ≤ P1 ≤ 8%. The negative electrode sheet includes a negative current collector and a negative electrode film layer. The negative electrode film layer is disposed on at least one side surface of the negative current collector. The negative electrode film layer includes a carbon-based material. The carbon-based material includes a porous structure. Based on the nitrogen adsorption method, the pore volume V1 of the porous structure with a pore size greater than 10 nm in the carbon-based material and the total pore volume V of the carbon-based material satisfy the following: 20% ≤ V1 / V ≤ 40%. The conductivity σ of the electrolyte at a temperature of 25℃±1℃ satisfies: 8mS / cm≤σ≤20mS / cm.

2. The sodium-ion battery cell according to claim 1, characterized in that, 3％≤P1≤6％。 3. The sodium-ion battery cell according to claim 1 or 2, characterized in that, The general structural formula of the phosphate material is Na. a Fe b M c M' d (Condensed polyanions) e (Anion) f Where M is one or more transition metals, M' is one or more non-transition metals, a>b, c>0, d≥0, e≥1, f>0, and the condensed polyanion comprises from P2O7 4- P3O9 5- and P4O 11 6- One or more groups selected from [the group].

4. The sodium-ion battery cell according to claim 3, characterized in that, The general structural formula of the phosphate material is Na. x Fe a-y M y (NO4) z (P2O7) w Wherein, 2≤x≤4, 1≤a≤4, 1≤y≤4, 0≤z≤4, 0≤w≤1, at least one of z and w is greater than or equal to 1, M includes at least one of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and N includes at least one of Al, Si, P, S, Ti, V, W.

5. The sodium-ion battery cell according to any one of claims 1 to 4, characterized in that, The change ΔP in the sum of the mass content of sodium iron pyrophosphate and sodium iron phosphate, based on the total mass of the phosphate material, after 100 cycles and before 100 cycles, satisfies the following condition: ΔP ≤ 3%.

6. The sodium-ion battery cell according to any one of claims 1 to 5, characterized in that, The pore volume V2 of the pore structure with a pore size of 5nm to 10nm in the carbon-based material, as measured by nitrogen adsorption, satisfies the following condition: 5% ≤ V2 / V ≤ 15%.

7. The sodium-ion battery cell according to any one of claims 1 to 6, characterized in that, The pore volume V3 of the pore structure with a pore size of 2nm to 5nm in the carbon-based material, as measured by nitrogen adsorption, satisfies the following condition: 20% ≤ V3 / V ≤ 40%.

8. The sodium-ion battery cell according to any one of claims 1 to 7, characterized in that, The pore volume V4 of the pore structure with a pore size of less than 2 nm in the carbon-based material, as measured by nitrogen adsorption, satisfies the following condition: 5% ≤ V4 / V ≤ 15%.

9. The sodium-ion battery cell according to any one of claims 1 to 8, characterized in that, The carbon-based material comprises multiple graphite-like sheets, and at least some of the graphite-like sheets form an interlayer spacing between two of the graphite-like sheets. The interlayer spacing satisfies that the volume H1 of the space of 0.35nm to 0.4nm and the total volume H of the space formed by the multiple graphite-like sheets satisfy: 20% ≤ H1 / H ≤ 60%.

10. The sodium-ion battery cell according to claim 9, characterized in that, The volume H2 of the space where the interlayer spacing is greater than 0.4 nm and the volume H of the total space formed by the plurality of graphite-like sheets satisfy: 20% ≤ H2 / H ≤ 50%.

11. The sodium-ion battery cell according to claim 9 or 10, characterized in that, The volume H3 of the space where the interlayer spacing is less than 0.35 nm and the volume H of the total space formed by the multiple graphite-like sheets satisfy: 0 ≤ H3 / H ≤ 5%.

12. The sodium-ion battery cell according to any one of claims 1 to 11, characterized in that, The electrolyte comprises cyclic carbonates and chain esters.

13. The sodium-ion battery cell according to claim 12, characterized in that, The viscosity V of the chain ester at 25℃±1℃ satisfies: 0.2mPa·s≤V≤3mPa·s, and the mass content P3 of the chain ester based on the total mass of the electrolyte satisfies: 40%≤P3≤80%.

14. The sodium-ion battery cell according to claim 12 or 13, characterized in that, The freezing point F of the cyclic carbonate satisfies: -60℃≤F≤-10℃, and the mass content P4 of the cyclic carbonate, based on the total mass of the electrolyte, satisfies: 10%≤P450%.

15. The sodium-ion battery cell according to any one of claims 12 to 14, characterized in that, The cyclic carbonates include at least one of propylene carbonate and ethylene carbonate.

16. The sodium-ion battery cell according to any one of claims 12 to 15, characterized in that, The chain ester includes at least one of chain carbonates and chain carboxylic esters; The chain carbonate includes at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, methyl butyl carbonate, ethyl propyl carbonate, dipropyl carbonate, and dibutyl carbonate. The chain carboxylic acid ester includes one of methyl acetate, ethyl acetate, ethyl propionate, methyl formate, ethyl formate, methyl propionate, propyl propionate, ethyl butyrate, and propyl acetate.

17. The sodium-ion battery cell according to any one of claims 1 to 16, characterized in that, The compaction density T of the positive electrode sheet satisfies: 1.5 g / cm³ 3 ≤T≤2.3g / cm 3 .

18. The sodium-ion battery cell according to any one of claims 1 to 17, characterized in that, The resistance R of the positive electrode plate satisfies: 0.01Ω≤R≤0.1Ω.

19. The sodium-ion battery cell according to any one of claims 1 to 18, characterized in that, The ratio CB of the active material capacity of the negative electrode to the active material capacity of the positive electrode satisfies: 1.07≤CB≤1.

2.

20. The sodium-ion battery cell according to any one of claims 1 to 19, characterized in that, The positive current collector includes aluminum foil.

21. The sodium-ion battery cell according to any one of claims 1 to 20, characterized in that, The sodium-ion battery cell also includes: The housing is a hollow structure with an opening, and the electrolyte, the negative electrode plate, and the positive electrode plate are housed within the housing. A top cover that closes the opening.

22. The sodium-ion battery cell according to claim 21, characterized in that, The material of the casing includes aluminum.

23. The sodium-ion battery cell according to claim 21 or 22, characterized in that, The sodium-ion battery cell also includes an electrode post disposed on the side of the top cover away from the hollow structure, and the electrode post is made of aluminum.

24. A battery device, characterized in that, Includes sodium-ion battery cells according to any one of claims 1 to 23.

25. An electrical appliance, characterized in that, Includes a sodium-ion battery cell according to any one of claims 1 to 23, or a battery device according to claim 24, wherein the sodium-ion battery cell or the battery device is used as a backup power source for the electrical equipment.

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