Secondary battery and electric device

Abstract

Embodiments of this application provide a secondary battery and an electrical device. The secondary battery includes a positive electrode and an electrolyte. The positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer includes a positive active material, which may include a Prussian blue-based positive electrode material. The electrolyte includes a film-forming additive and an organometallic chelating agent; the organometallic chelating agent has a mass percentage of 0.1 wt% to 5 wt% in the electrolyte, and the film-forming additive has a mass percentage of 0.5 wt% to 10 wt% in the electrolyte. This suppresses the dissolution of transition metal ions, improving the stability and safety of the secondary battery.

Description

Technical Field

[0001] The exemplary embodiments of this application generally relate to the field of secondary batteries, and particularly to secondary batteries and electrical equipment. Background Technology

[0002] Sodium-ion batteries, as electrochemical energy storage devices that rely on the insertion and extraction of sodium ions between the positive and negative electrodes, are gradually becoming an important supplement to lithium-ion battery systems due to the abundance of sodium resources, low cost, high safety, and excellent low-temperature performance. In applications where energy density requirements are relatively low but economic efficiency and safety are particularly sensitive (such as low-speed electric vehicles, large-scale energy storage systems, and backup power for communication base stations), sodium-ion batteries demonstrate unique advantages and broad application prospects.

[0003] Prussian blue and its analogues as cathode materials (hereinafter referred to as Prussian blue-based cathode materials) are widely used in sodium-ion batteries due to their high theoretical capacity, suitable operating voltage, excellent rate performance, and good cycle stability. However, during charge and discharge, the Prussian blue-based cathode materials experience lattice expansion or contraction due to the insertion and extraction of transition metal ions. Repeated expansion-contraction cycles can easily lead to lattice distortion, particle pulverization, and even cause transition metal ions (such as Fe) to form on the surface of the Prussian blue-based cathode materials. 2+ Co 2+ Ni 2+ (etc.) dissolve, disrupting the reversibility of the reaction, ultimately leading to rapid capacity decay of the battery and increasing the risk of thermal runaway. Summary of the Invention

[0004] In a first aspect of this application, a secondary battery is provided. The secondary battery includes a positive electrode and an electrolyte. The positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer includes a positive active material, which may include a Prussian blue-based positive electrode material. The electrolyte includes a film-forming additive and an organometallic chelating agent. The organometallic chelating agent has a mass percentage of 0.1 wt% to 5 wt% in the electrolyte, and the film-forming additive has a mass percentage of 0.5 wt% to 10 wt% in the electrolyte.

[0005] The secondary battery provided in this application introduces an organometallic chelating agent into the electrolyte. This organometallic chelating agent can bind with dissolved Fe²⁺ through coordination. + Co² + Ni² +Transition metal ions combine to form stable chelates, and a uniform and dense positive electrode electrolyte interphase (CEI) film is constructed in situ on the surface of the positive electrode. This CEI film can effectively suppress the extraction of lattice water from Prussian blue-based positive electrode materials, while preventing transition metal ions from dissolving from the lattice. This reduces the risk of transition metal ions migrating to the negative electrode and catalyzing the continuous decomposition of the negative electrode solid electrolyte interphase (SEI) film from the source, thereby maintaining the high reversibility of ion insertion / extraction reactions, significantly improving the cycle stability of the secondary battery, reducing the risk of thermal runaway, and ensuring the long-term reliable operation of the battery.

[0006] Limiting the mass percentage of organometallic chelating agents in the electrolyte to 0.1wt%~5wt% ensures that the chelating effect is fully utilized, avoiding insufficient inhibition of transition metal ion dissolution due to excessively low organometallic chelating agent content. It also prevents excessively high organometallic chelating agent content from causing an increase in electrolyte viscosity and obstruction of sodium ion transport, ensuring a smooth and efficient sodium ion intercalation / deintercalation process, thereby effectively slowing down capacity decay during battery cycling.

[0007] During the first charge and discharge of a secondary battery, the film-forming additive preferentially undergoes reduction and decomposition, forming a dense, stable, and highly conductive solid electrolyte interphase (SEI) film on the surface of the negative electrode. As an interfacial barrier, the SEI film isolates the electrolyte from direct contact with the negative electrode active material, inhibits excessive reduction and decomposition of the electrolyte, and reduces side reactions at the negative electrode interface. The film-forming additive can also participate in interfacial modification on the positive electrode side, working with organometallic chelating agents to construct a stable positive electrode electrolyte interphase (CEI) film on the surface of the positive electrode and improve the density of the CEI film. Controlling the mass percentage of the film-forming additive in the electrolyte between 0.5wt% and 10wt% allows for the formation of a complete, continuous, and dense SEI film on the negative electrode surface and a stable and effective CEI film on the positive electrode. This not only blocks direct contact between the electrolyte and the electrode active material and inhibits interfacial side reactions but also ensures efficient ion transport across the membrane, significantly improving the battery's rate performance and charge / discharge efficiency.

[0008] Organometallic chelators and film-forming additives exhibit significant synergistic effects. Film-forming additives can construct stable CEI and SEI films on the surfaces of the positive and negative electrodes, respectively, during the first charge-discharge cycle. Organometallic chelators, through coordination, anchor themselves on the positive electrode surface, combining with transition metals and modifying the CEI film structure, filling defects, and making the CEI film denser, more uniform, and more resilient. The synergistic effect of both effectively inhibits lattice water extraction and transition metal dissolution, preventing transition metal ions from migrating to the negative electrode and damaging the SEI film. This reduces interfacial impedance, improves ion transport efficiency, significantly enhances battery cycle stability, rate performance, and safety reliability, and extends battery life.

[0009] In some embodiments, the organometallic chelating agent has a mass percentage of 1wt% to 3wt% in the electrolyte, and the film-forming additive has a mass percentage of 3wt% to 7wt% in the electrolyte.

[0010] In some embodiments, the mass percentage of the organometallic chelating agent in the electrolyte can be a value or a range formed by any two of the following: 0.1wt%, 0.5wt%, 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, 4.5wt%, and 5wt%.

[0011] In some embodiments, the mass percentage of the film-forming additive in the electrolyte may be a value or a range formed by any two of the following: 0.5wt%, 1.5wt%, 3wt%, 4.5wt%, 5wt%, 7wt%, 8.5wt%, and 10wt%.

[0012] In some embodiments, the organometallic chelating agent comprises a sodium salt derived from an organic acid and / or an organic amine.

[0013] Sodium salts derived from organic acids (such as sodium citrate and sodium tartrate) contain multiple carboxylate groups (-COO). - Sodium salts derived from organic amines (such as sodium ethylenediaminetetraacetate and sodium triethanolamine) contain active coordinating groups such as amino groups (-NH2) and phosphonic acid groups (-PO3H2). These active groups can react with transition metal ions (Fe2+) dissolved from Prussian blue-type cathode materials. 2 + Mn 2+ Ni 2+ These sodium salts form stable chelates that adhere to the CEI membrane. They also possess excellent water solubility and electrolyte compatibility, allowing for uniform dispersion in the electrolyte and rapid capture of dissolved transition metal ions, thus exhibiting highly efficient chelation. The chelating agents in the sodium salt system are highly compatible with the polarity and ionic environment of the sodium-ion battery electrolyte, ensuring excellent compatibility and preventing the introduction of foreign impurities that could interfere with the battery's electrochemical reactions.

[0014] In some embodiments, the organometallic chelating agent includes at least one of the following: sodium citrate, sodium tartrate, sodium ethylenediaminetetraacetate, sodium triethanolamine, sodium malonate, sodium succinate, sodium benzenesulfonate, sodium methanesulfonate, sodium malate, sodium polyacrylate, sodium aminotrimethylenephosphonate, and sodium ethylenediaminetetramethylenephosphonate.

[0015] In some embodiments, the organometallic chelating agent is sodium citrate.

[0016] Sodium citrate molecules contain three carboxylate groups (-COO). - Sodium citrate contains a hydroxyl group (-OH). This allows sodium citrate to react with Fe in Prussian blue-type cathode materials.2+ Mn 2+ Ni 2+ Co 2+ Transition metal ions form highly stable five- or six-membered ring chelates, which are then used to construct a uniform and dense CEI film on the cathode surface. This process offers unique advantages such as strong chelating ability, high film toughness, good electrolyte compatibility, electrochemical stability, and low cost. It effectively suppresses transition metal dissolution and lattice water extraction, significantly improving battery cycle stability and reliability.

[0017] In some embodiments, the electrolyte further includes a sodium electrolyte salt.

[0018] Sodium salts in the electrolyte can fully dissociate and release sodium ions in the electrolyte, providing a stable ion source for the sodium ion insertion / extraction process of Prussian blue cathode materials, improving the ionic conductivity of the electrolyte and reducing the internal resistance of the battery, while maintaining the internal charge balance and electrochemical environment stability of the battery. They are also highly compatible with organometallic chelating agents and film-forming additives, working together to ensure the stable formation of the interfacial film, thereby improving the battery's charge / discharge efficiency, rate performance and long-term cycle reliability.

[0019] In some embodiments, the concentration of the electrolyte sodium salt in the electrolyte is 0.5~2 mol / L.

[0020] In some embodiments, the concentration of the electrolyte sodium salt in the electrolyte can be a value of 0.5 mol / L, 1 mol / L, 1.5 mol / L, and 2 mol / L, or a range formed by any two of these values.

[0021] In some embodiments, the electrolyte sodium salt includes at least one of the following: sodium hexafluorophosphate, sodium tetrafluoroborate, sodium trifluoromethanesulfonate, sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide, sodium bis(oxalatoborate), and sodium perchlorate.

[0022] The aforementioned sodium electrolyte salts possess advantages such as complete dissociation, high ionic conductivity, wide electrochemical window, and good thermal stability, and can stably release Na+ in the electrolyte. + This provides a sufficient ion source for the sodium ion insertion / extraction of Prussian blue cathodes, significantly improving the electrolyte's ion conductivity and reducing the overall battery impedance. Simultaneously, this type of electrolyte sodium salt exhibits excellent compatibility with carbonate solvents, organometallic chelating agents, and film-forming additives, without damaging the CEI and SEI film structures. It effectively suppresses side reactions, enhances battery stability under different rate and long-term cycling conditions, and broadens the battery's operating temperature range, meeting the high reliability requirements of electrolytes in energy storage and power applications.

[0023] In some embodiments, the electrolyte further includes an organic solvent, including carbonate solvents.

[0024] Carbonate solvents possess the characteristics of moderate dielectric constant, high boiling point, wide electrochemical window, and excellent compatibility with electrode materials. They can fully dissolve electrolyte sodium salts, organometallic chelating agents, and film-forming additives, ensuring uniform dispersion and stable existence of each component. At the same time, they can provide a low-impedance transport medium for sodium ion migration, reduce electrolyte viscosity, improve ion transport efficiency, and are not prone to oxidative decomposition within the working potential range of Prussian blue cathodes. They are also compatible and stable with the positive and negative electrode interface films, which helps maintain the structural stability of CEI and SEI films.

[0025] In some embodiments, the organic solvent includes cyclic carbonates and chain carbonates, wherein the volume ratio of cyclic carbonates to chain carbonates is (1~2):(3~4).

[0026] Cyclic carbonates possess high dielectric constants, effectively dissociating sodium salts in the electrolyte and providing high ionic conductivity. They also exhibit good polarity matching with Prussian blue-based cathode materials, facilitating the formation and maintenance of a stable CEI film. Chain carbonates, on the other hand, have lower viscosity and excellent flowability, significantly reducing the overall viscosity of the electrolyte and accelerating the transport rate of sodium ions in the bulk phase and at the interface. Combining cyclic and chain carbonates at a volume ratio of (1-2):(3-4) synergistically achieves a balance between high ionic conductivity and excellent flowability in the electrolyte. Cyclic carbonates ensure high dissociation efficiency and interfacial compatibility, while chain carbonates reduce transport resistance, thereby accelerating sodium ion migration, optimizing electrode interface wettability, and promoting the formation of stable CEI and SEI films.

[0027] In some embodiments, the cyclic carbonate includes at least one of the following: ethylene carbonate, propylene carbonate, butene carbonate, and vinylene carbonate.

[0028] In some embodiments, the chain carbonate includes at least one of the following: dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dipropyl carbonate, diisopropyl carbonate, methyl propyl carbonate, and dibutyl carbonate.

[0029] In some embodiments, the film-forming additive includes at least one of the following: vinylene carbonate, fluorovinyl carbonate, ethyl acrylate, propylene oxide, hexamethyldisiloxane, diethoxyphosphoryl chloride, trimethyl phosphate, sodium difluorophosphate, succinic anhydride, sodium thiosulfate, methyl methacrylate, and ethylene nitrate.

[0030] All of the above film-forming additives have excellent interfacial reactivity and can preferentially undergo redox reactions on the electrode surface during the first charge and discharge process, synergistically constructing a stable CEI film and SEI film with good ion conductivity, suppressing electrolyte side reactions, reducing interfacial impedance and improving battery cycle stability.

[0031] In some embodiments, the film-forming additive is fluoroethylene carbonate.

[0032] Fluorinated ethylene carbonate (FEC), as a preferred film-forming additive, possesses advantages such as suitable film-forming potential, stable decomposition products, and outstanding interfacial compatibility. It can form a dense, flexible, and highly stable CEI film on the positive electrode surface, effectively inhibiting the extraction of lattice water and the dissolution of transition metals in Prussian blue-based positive electrode materials. Simultaneously, it forms a uniform and mechanically strong SEI film on the negative electrode surface, significantly improving the interfacial stability and preventing damage to the interfacial film by dissolved transition metal ions. The use of FEC further enhances the quality of the interfacial film, reduces interfacial impedance, minimizes side reaction gas generation and capacity decay, enabling the battery to maintain a higher capacity retention rate during long cycles, and improving rate performance, safety, reliability, and long-term storage stability.

[0033] In some embodiments, a positive electrode electrolyte interface film is formed on the surface of the positive electrode sheet, and the thickness of the positive electrode electrolyte interface film is 2~8nm.

[0034] By limiting the thickness of the positive electrode electrolyte interface film (CEI film), the CEI film can densely and continuously cover the surface of the positive electrode active material, effectively blocking direct contact between the electrolyte and the positive electrode active material, significantly inhibiting lattice water extraction, transition metal ion dissolution, and electrolyte side reaction decomposition; at the same time, the CEI film within the above-mentioned thickness range can also effectively control ion transport resistance and ensure Na+ + Rapid and stable insertion and deinsertion ensure the rate performance of the secondary battery.

[0035] In some embodiments, the molecular formula of Prussian blue-type cathode materials can be represented as Na. x M[M'(CN)6] y ·□ 1-y ·nH₂O. In the formula, □ represents a vacancy of M'(CN)₆, 0 < x ≤ 2, 0 < y ≤ 1. The transition metal M' is selected from any one of Fe, Mn, Ni, Co, and Cu, and the metal ion M is selected from any one of Fe, Mn, Ni, Co, and Cu.

[0036] In some embodiments, Prussian blue-based cathode active materials include one or more of Prussian blue, Prussian white, iron-based Prussian blue, manganese-based Prussian blue, iron-manganese Prussian blue, nickel-iron Prussian blue, copper-iron Prussian blue, zinc-iron Prussian blue, and derivatives that have been metal-doped, coated, or lattice-regulated.

[0037] The organometallic chelating agents and film-forming additives provided in this application can construct a cathode electrolyte interface film of suitable thickness on the surface of these cathode active materials, effectively suppressing the loss of lattice water and the dissolution of transition metal ions during charging and discharging, maintaining the integrity of the framework structure, and further improving the structural stability and long-cycle performance of the cathode material.

[0038] In some embodiments, the secondary battery further includes a negative electrode sheet. The negative electrode sheet includes a negative current collector and a negative active material layer. The negative active material layer includes a negative active material, which includes at least one of hard carbon, soft carbon, graphite, nitrogen-doped carbon material, and porous carbon material.

[0039] In a second aspect of this application, an electrical appliance is provided. The electrical appliance includes a secondary battery provided according to the first aspect of this application, the secondary battery serving as a power source for the electrical appliance.

[0040] In some embodiments, the electrical equipment can be application devices such as vehicles, mobile phones, portable devices, laptops, ships, spacecraft, electric toys, and power tools. Vehicles can be new energy vehicles, including pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles; spacecraft include airplanes, rockets, space shuttles, and spacecraft; electric toys include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys; 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. This application does not impose special limitations on the above-mentioned devices.

[0041] It should be understood that the content described in this content section is not intended to limit the key or essential features of the embodiments of this application, nor is it intended to restrict the scope of this application. Other features of this application will become readily apparent from the following description. Attached Figure Description

[0042] The above and other features, advantages, and aspects of the embodiments of this application will become more apparent from the accompanying drawings and the following detailed description. In the drawings, the same or similar reference numerals denote the same or similar elements, wherein: Figure 1 The image shows a transmission electron microscope photograph of the positive electrode sheet of the secondary battery prepared in Example 1 of this application after cycling. Figure 2 The diagram shows the cycling performance curves of Embodiment 1 and Comparative Example 1 at a current density of 1C. Detailed Implementation

[0043] The embodiments of this application will now be described in more detail. It should be understood that this application may be implemented in various forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of this application.

[0044] It should be noted that the headings of any section / subsection provided herein are not limiting. Various embodiments are described throughout this document, and embodiments of any type may be included under any section / subsection. Furthermore, embodiments described in any section / subsection may be combined in any way with any other embodiments described in the same section / subsection and / or different sections / subsections.

[0045] In the description of embodiments of this application, the term "comprising" and similar terms should be understood as open-ended inclusion, i.e., "including but not limited to". The term "based on" should be understood as "at least partially based on". The term "one embodiment" or "the embodiment" should be understood as "at least one embodiment". The term "some embodiments" should be understood as "at least some embodiments". Other explicit and implicit definitions may also be included below. The terms "first", "second", etc., may refer to different or the same objects. Other explicit and implicit definitions may also be included below.

[0046] As briefly mentioned earlier, Prussian blue and its analogues have attracted widespread attention due to their high theoretical capacity, suitable operating voltage, excellent rate performance, and good cycle stability. Their three-dimensional open framework not only provides a rapid migration channel for sodium ions, but also boasts abundant raw material sources, simple synthesis processes, and low cost, making them highly promising cathode materials for sodium-ion batteries. However, these materials still face bottlenecks in practical applications: Firstly, solution-synthesized Prussian blue and its analogues contain a certain amount of coordination water (crystallization water), which is crucial for maintaining framework integrity. Excessive extraction during cycling can easily lead to lattice collapse and loss of active sites. Furthermore, the extracted water reacts with the electrolyte, causing electrolyte decomposition and exacerbating gas production. Secondly, the transition metals composing the framework of Prussian blue and its analogues face the risk of dissolution during cycling. The dissolved transition metal ions (such as Fe)... 2+ Co 2+ Ni 2+ These particles (such as argon) can shuttle to the negative electrode side, catalyzing the decomposition of the SEI film and increasing the risk of thermal runaway. This poses a significant challenge to the commercial application of Prussian blue and its analogue cathode materials.

[0047] This application provides a secondary battery and an electrical device to solve or at least partially solve the aforementioned technical problems or other potential technical problems. According to the secondary battery provided in this application, by introducing an organometallic chelating agent into the electrolyte, it can react with dissolved Fe²⁺. + Co² + Ni² +Transition metal ions combine to form stable chelates, and in conjunction with film-forming additives, a uniform and effective positive electrode electrolyte interphase (CEI) film is formed on the surface of the positive electrode. Thus, the CEI film can prevent the lattice water from escaping from Prussian blue-based positive electrode active materials, while simultaneously hindering the dissolution of transition metals. This reduces the risk of transition metal ions migrating to the negative electrode and catalyzing the continuous decomposition of the negative electrode SEI film, thereby improving the stability and electrochemical performance of Prussian blue and its analogues in sodium-ion batteries. The secondary battery of this application embodiment will be further described below with reference to specific embodiments.

[0048] Preparation of secondary batteries Example 1 Preparation of electrolyte (1) In a glove box, propylene carbonate (PC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) are mixed in a volume ratio of 3:6:1 to obtain an organic solvent. The volume ratio k of cyclic carbonate to chain carbonate in the organic solvent is 3:7.

[0049] (2) The sodium electrolyte salt (sodium hexafluorophosphate 33.6g), the film-forming additive (ethylene fluorocarbonate 7.05g), and the organometallic chelating agent (sodium citrate 0.141g) were dissolved in the prepared organic solvent and brought to a final volume of 100ml to obtain the electrolyte. In the electrolyte, the mass percentage of the organometallic chelating agent sodium citrate was 2.5wt%, and the mass percentage of the film-forming additive ethylene fluorocarbonate was 5wt%.

[0050] Preparation of positive electrode sheet The positive current collector is a double-sided carbon-coated aluminum foil with a thickness of 12μm and a carbon coating thickness of 1μm on each side.

[0051] Iron-based Prussian blue (positive electrode active material), conductive carbon black (conductive agent), and polyvinylidene fluoride (PVDF) (binder) were mixed in a mass ratio of 97:2:1 and added to N-methylpyrrolidone (NMP) solvent. The mixture was stirred under vacuum until homogeneous to obtain a positive electrode slurry. The solid content of the positive electrode slurry was 70%. The positive electrode slurry was uniformly coated onto both sides of the positive electrode current collector aluminum foil and dried. After drying, the foil was rolled to obtain the positive electrode sheet. The coating weight of the positive electrode sheet was 150 g / m². 2 .

[0052] Preparation of negative electrode sheet The negative electrode current collector is aluminum foil with a thickness of 12μm.

[0053] Hard carbon (negative electrode active material), conductive carbon black (conductive agent), styrene-butadiene rubber (binder), and sodium carboxymethyl cellulose (thickener) were mixed in a mass ratio of 91:7:1:1 and dispersed in deionized water. The mixture was stirred under vacuum until homogeneous, yielding a negative electrode slurry. The solid content of the negative electrode slurry was 50%. The negative electrode slurry was uniformly coated onto both sides of the negative electrode current collector aluminum foil and dried. After drying, the foil was rolled to obtain the negative electrode sheet. The coating weight of the negative electrode sheet was 71 g / m². 2 .

[0054] The positive electrode sheet, PE separator, and negative electrode sheet prepared in this application are stacked in sequence, wound into corresponding cores, and then subjected to hot pressing, welding, assembly, baking, liquid injection, formation, and capacity testing to obtain a sodium-ion battery.

[0055] Examples 2-5 The difference from Example 1 is that the mass percentage of sodium citrate, an organometallic chelating agent, in the electrolyte is different.

[0056] Examples 6-9 The difference from Example 1 is that the mass percentage of the film-forming additive fluoroethylene carbonate in the electrolyte is different.

[0057] Examples 10-11 The difference from Example 1 is that the type of organometallic chelating agent in the electrolyte is different.

[0058] Examples 12-13 The difference from Example 1 is that the type of film-forming additive in the electrolyte is different.

[0059] Examples 14-15 The difference from Example 1 is that the concentration of the sodium salt of the electrolyte in the electrolyte is different.

[0060] Examples 16-17 The difference from Example 1 is that the type of sodium salt in the electrolyte is different.

[0061] Examples 18-21 The difference from Example 1 is that the type of organic solvent and the volume ratio k of cyclic carbonates to chain carbonates are different.

[0062] In Example 18, propylene carbonate (PC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 2:6:2 to obtain an organic solvent, in which the volume ratio k of cyclic carbonate to chain carbonate was 1:4.

[0063] In Example 19, propylene carbonate (PC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 4:5:1 to obtain an organic solvent, in which the volume ratio k of cyclic carbonate to chain carbonate was 2:3.

[0064] In Example 20, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 3:6:1 to obtain an organic solvent, in which the volume ratio k of cyclic carbonate to chain carbonate was 3:7.

[0065] In Example 21, ethylene carbonate (EC), dimethyl carbonate (DMC), and dipropyl carbonate (DPC) were mixed in a volume ratio of 3:6:1 to obtain an organic solvent, in which the volume ratio k of cyclic carbonate to chain carbonate was 3:7.

[0066] Comparative Example 1 The difference from Example 1 is that no organometallic chelating agent is added to the electrolyte.

[0067] Comparative Example 2 The difference from Example 1 is that the mass percentage of sodium citrate, an organometallic chelating agent, in the electrolyte is 6 wt%.

[0068] Comparative Example 3 The difference from Example 1 is that the mass percentage of sodium citrate, an organometallic chelating agent, in the electrolyte is 0.05 wt%.

[0069] Comparative Example 4 The difference from Example 1 is that the mass percentage of the film-forming additive fluoroethylene carbonate in the electrolyte is 12 wt%.

[0070] Comparative Example 5 The difference from Example 1 is that the mass percentage of the film-forming additive fluoroethylene carbonate in the electrolyte is 0.3 wt%.

[0071] Examples 1 to 21 are shown in Table 1.

[0072] Table 1

[0073] Note: In Table 1, " / " indicates that the component was not added or was not detected.

[0074] The abbreviations and official names of the organic solvents used in Table 1 are as follows: Propylene carbonate (PC), ethylene carbonate (EC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and dipropyl carbonate (DPC).

[0075] Performance testing The secondary batteries prepared in Examples 1-21 and Comparative Examples 1-5 were tested, and the test items included: 1) Thickness of the positive electrode electrolyte interface film on the surface of the positive electrode of a secondary battery First, fully discharge the secondary battery to the cutoff voltage. Disassemble the battery in a glove box and remove the positive electrode. Clean the surface of the electrode with ethyl methyl carbonate (EMC) to remove any residual electrolyte. After air drying, place the sample in a transmission electron microscope. Select at least five different observation areas and take high-resolution TEM images of each area. Using the microscope's built-in dimensional measuring tool, measure the thickness of the positive electrode electrolyte interphase (CEI) film in each area and calculate the average of the five measurements. This average value is the final thickness of the positive electrode electrolyte interphase film on the surface of the positive electrode.

[0076] 2) Transition metal content in the electrolyte of secondary batteries The residual electrolyte from the disassembled battery cell was collected and diluted in a mixed solution of ethanol, water, and methyl ethyl carbonate in a volume ratio of 4:5:1. The diluted solution was then subjected to inductively coupled plasma optical emission spectrometry (ICP-OES) to determine the transition metal content (ppm, w / w).

[0077] 3) Retention rate of secondary batteries after 100 cycles (%) In a 25℃ testing environment, the secondary battery was charged to 4V at a constant 1C rate and allowed to stand for 10 minutes. It was then discharged to 2V at a constant 1C rate and allowed to stand for 10 minutes. After 100 cycles, the battery's cycle capacity retention was calculated. Cycle capacity retention = Discharge capacity at 100th cycle / Discharge capacity at 1st cycle × 100%.

[0078] The detection data of Examples 1-21 and Comparative Examples 1-5 are shown in Table 2.

[0079] Table 2

[0080] As can be seen from Tables 1 and 2, the secondary batteries prepared in Examples 1-21 of this disclosure incorporate organometallic chelating agents in the electrolyte. These chelating agents can combine with dissolved transition metal ions through coordination to form stable chelates, thereby constructing a uniform and dense positive electrode electrolyte interphase (CEI) film in situ on the surface of the positive electrode. This effectively suppresses the extraction of lattice water from Prussian blue-based positive electrode materials and hinders the dissolution of transition metal ions from the lattice. This helps maintain the high reversibility of ion insertion / extraction reactions and significantly improves the cycle stability of the secondary battery. Film-forming additives can participate in the construction of the CEI film, improving its density and further enhancing the cycle performance of the secondary battery.

[0081] Figure 1 A transmission electron microscope image of the positive electrode of the secondary battery prepared according to Example 1 of this application is shown. Figure 1 As can be seen, a uniform and clear CEI film has formed on the surface of the positive electrode. Therefore, the CEI film can provide stable protection for the positive electrode active material of the positive electrode.

[0082] Referring to Examples 1-21 and Comparative Examples 1-3, when the mass percentage of the organometallic chelating agent in the electrolyte is in the range of 0.1wt% to 5wt%, and the mass percentage of the film-forming additive in the electrolyte is in the range of 0.5wt% to 10wt%, the thickness of the CEI film can be controlled within the range of 2nm to 8nm. This effectively inhibits the extraction of lattice water from Prussian blue-based cathode materials and simultaneously prevents transition metal ions from dissolving from the lattice. Comparative Example 1, due to the absence of an organometallic chelating agent, resulted in difficulty in forming the CEI film. Similarly, in Comparative Example 3, the low mass percentage of the organometallic chelating agent led to insufficient CEI film thickness (less than 2nm), limiting its protective ability against the cathode active material. Compared to Examples 1-21, the 100-cycle capacity retention rates of the secondary batteries in Comparative Examples 1 and 3 were reduced to varying degrees. In Comparative Example 2, the excessively high mass percentage of the organometallic chelating agent resulted in an excessively thick CEI film (greater than 8nm), which hindered sodium ion transport at the interface, causing a decrease in cycle capacity retention. The cycle capacity retention rate of Comparative Example 2 was lower than that of Examples 1-21.

[0083] Figure 2 The diagram shows the cycling performance curves of Embodiment 1 and Comparative Example 1 at a current density of 1C. (See diagram for reference.) Figure 2 As shown, the cycle capacity retention rate of Example 1 is significantly better than that of Comparative Example 1.

[0084] Referring to Examples 1-5, when the mass percentage of the organometallic chelating agent in the electrolyte is controlled within the range of 0.1 wt% to 5 wt%, the content (mass) of the transition metal dissolved in the electrolyte can be controlled below 46 ppm. The capacity retention rate of the secondary battery after 100 cycles can be above 93%. In Example 1, the secondary battery has a CEI film thickness of 3 nm on its positive electrode, and the content (mass) of the transition metal dissolved in the electrolyte is 35 ppm. The capacity retention rate of the secondary battery after 100 cycles is 95%.

[0085] Referring to Examples 1-21 and Comparative Examples 4 and 5, the film-forming additives have an auxiliary effect on the formation of the CEI film, and the change in their mass percentage affects the thickness of the CEI film to a certain extent. Furthermore, the addition of film-forming additives also improves the density of the CEI film, thereby improving its protective performance. However, the excessive amount of film-forming additives in Comparative Example 4 may result in an excessively thick solid electrolyte interphase (SEI) film at the negative electrode of the secondary battery, leading to obstructed ion transport at the negative electrode and affecting the cycle capacity retention rate. The 100-cycle capacity retention rates of the secondary batteries in Comparative Examples 4 and 5 are both lower than those of the secondary batteries in Examples 1-21.

[0086] Referring to Examples 1, 6-9, the mass percentage of the film-forming additive in the electrolyte is controlled within the range of 0.5wt% to 10wt%, and the content (mass) of the transition metal dissolved in the electrolyte can be controlled below 37ppm. The capacity retention rate of the secondary battery after 100 cycles can be above 93%.

[0087] Referring to Examples 1, 18-21, when the electrolyte uses a composite system of cyclic carbonate and chain carbonate in a volume ratio of (1-2):(3-4), it can synergistically balance the high ionic conductivity and excellent flow wettability of the electrolyte. Cyclic carbonate can improve the sodium salt dissociation efficiency and optimize the electrode interface compatibility; chain carbonate can reduce the electrolyte viscosity and ion transport resistance, accelerate the sodium ion migration rate, improve the overall electrode wettability, and synergistically promote the simultaneous formation of stable CEI and SEI films.

[0088] The various implementations of this application have been described above. The foregoing description is exemplary and not exhaustive, nor is it limited to the disclosed implementations. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described implementations. The terminology used herein is chosen to best explain the principles, practical applications, or improvements to technology in the market, or to enable others skilled in the art to understand the various implementations disclosed herein.

Claims

1. A secondary battery characterized by comprising: Including the positive electrode and the electrolyte, The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, the positive active material layer including a positive active material, the positive active material including a Prussian blue-based positive electrode material; The electrolyte includes a film-forming additive and an organometallic chelating agent; the organometallic chelating agent has a mass percentage of 0.1wt% to 5wt% in the electrolyte, and the film-forming additive has a mass percentage of 0.5wt% to 10wt% in the electrolyte.

2. The secondary battery according to claim 1, characterized in that, The organometallic chelating agent has a mass percentage of 1wt% to 3wt% in the electrolyte, and the film-forming additive has a mass percentage of 3wt% to 7wt% in the electrolyte.

3. The secondary battery according to claim 1, characterized in that, The organometallic chelating agent includes a sodium salt derived from an organic acid and / or an organic amine.

4. The secondary battery according to claim 3, characterized in that, The organometallic chelating agent includes at least one of the following: sodium citrate, sodium tartrate, sodium ethylenediaminetetraacetate, sodium triethanolamine, sodium malonate, sodium succinate, sodium benzenesulfonate, sodium methanesulfonate, sodium malate, sodium polyacrylate, sodium aminotrimethylenephosphonate, and sodium ethylenediaminetetramethylenephosphonate.

5. The secondary battery according to claim 4, characterized in that, The organometallic chelating agent is sodium citrate.

6. The secondary battery according to claim 1, characterized in that, The electrolyte also includes sodium electrolyte salt.

7. The secondary battery according to claim 1, characterized in that, The concentration of the sodium salt of the electrolyte in the electrolyte solution is 0.5~2 mol / L.

8. The secondary battery according to claim 7, characterized in that, The electrolyte sodium salt includes at least one of the following: sodium hexafluorophosphate, sodium tetrafluoroborate, sodium trifluoromethanesulfonate, sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide, sodium bis(oxalatoborate), and sodium perchlorate.

9. The secondary battery according to claim 1, characterized in that, The electrolyte also includes an organic solvent, which includes carbonate solvents.

10. The secondary battery according to claim 9, characterized in that, The organic solvent includes cyclic carbonates and chain carbonates, and the volume ratio of the cyclic carbonates to the chain carbonates is (1~2):(3~4).

11. The secondary battery according to claim 9, characterized in that, The cyclic carbonate includes at least one of the following: ethylene carbonate, propylene carbonate, butene carbonate, and vinylene carbonate, and / or The chain carbonates include at least one of the following: dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dipropyl carbonate, diisopropyl carbonate, methyl propyl carbonate, and dibutyl carbonate.

12. The secondary battery according to any one of claims 1 to 11, characterized in that, The film-forming additive includes at least one of the following: vinylene carbonate, fluorovinyl carbonate, ethyl acrylate, propylene oxide, hexamethyldisiloxane, diethoxyphosphoryl chloride, trimethyl phosphate, sodium difluorophosphate, succinic anhydride, sodium thiosulfate, methyl methacrylate, and ethylene nitrate.

13. The secondary battery according to claim 12, characterized in that, The film-forming additive is fluoroethylene carbonate.

14. The secondary battery according to claim 1, characterized in that, A positive electrode electrolyte interface film is formed on the surface of the positive electrode sheet, and the thickness of the positive electrode electrolyte interface film is 2~8nm.

15. An electrical appliance, characterized in that, include: The secondary battery according to any one of claims 1 to 14 serves as the power supply for the electrical equipment.

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