Electrolyte, secondary battery, and electric device

Abstract

This application belongs to the field of battery technology, specifically relating to an electrolyte, a secondary battery, and an electrical device. The electrolyte provided in this application comprises sodium salt and lithium salt. The deposition overpotential of lithium metal in the electrolyte is higher than that of sodium metal, and this electrolyte exhibits film-forming selectivity, which is beneficial for improving the cycle performance of the secondary battery. The secondary battery provided in this application includes this electrolyte and also includes a positive electrode. The active material in the positive electrode includes sodium-ion positive electrode active material and lithium-ion positive electrode active material. The charging plateau voltage of the sodium-ion positive electrode active material in the battery is lower than that of the lithium-ion positive electrode active material. The electrolyte and positive electrode in this secondary battery are compatible, further improving the storage performance of the secondary battery.

Description

[0001] This application is a divisional application of the invention patent application filed on August 16, 2023, with application number 202311033121.1 and invention title "Electrolyte, Secondary Battery and Electrical Device". Technical Field

[0002] This application relates to the field of batteries, specifically to an electrolyte, a secondary battery, and an electrical device. Background Technology

[0003] In recent years, with the development of secondary battery technology, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power plants, as well as in electronic equipment power supplies, power tools, electric bicycles, electric motorcycles, electric vehicles and many other fields.

[0004] With the development of science and technology and society, the performance of various products has been further improved, thus placing higher demands on the cycle performance and storage performance of secondary batteries. How to provide a secondary battery with good cycle performance and storage performance is one of the key areas of focus for those skilled in the art. Summary of the Invention

[0005] In view of the above problems, this application provides an electrolyte, a secondary battery, and an electrical device, which can improve the cycle performance of the secondary battery and also enhance its storage performance.

[0006] A first aspect of this application is to provide an electrolyte comprising a sodium salt and a lithium salt, wherein the deposition overpotential of lithium metal in the electrolyte is higher than that of sodium metal.

[0007] The deposition overpotential in this application reflects the deposition efficiency of the metal in the electrolyte system. Generally, the lower the deposition overpotential, the lower the deposition resistance of the metal, making it easier to deposit, but the electrolyte is less likely to form a film. The electrolyte provided in this application has film-forming selectivity. Specifically, the deposition overpotential of lithium metal in the electrolyte is higher than that of sodium metal, which is beneficial to the film formation on the lithium metal surface, while the sodium metal surface is less likely to form a film. Specifically, sodium metal does not easily grow an SEI film in this electrolyte, resulting in good deposition and improving the cycle life of the secondary battery. Lithium metal easily grows an SEI film in this electrolyte. In the secondary battery, if lithium metal is further deposited on the sodium metal surface and a stable SEI film is formed on the lithium metal surface, the storage performance of the secondary battery can be effectively improved in addition to improving the cycle life.

[0008] In some embodiments of this application, the electrolyte satisfies the following: The deposition overpotential of lithium metal in the electrolyte is more than 20 mV higher than that of sodium metal.

[0009] In some embodiments of this application, the electrolyte satisfies one or a combination of the following: (1.1) The overpotential for lithium metal deposition in the electrolyte is greater than 30 mV; preferably 32 mV to 70 mV; (1.2) The overpotential for sodium metal deposition in the electrolyte is less than 20mV; preferably 5mV~18mV.

[0010] In some embodiments of this application, the concentration of the lithium salt is lower than the concentration of the sodium salt.

[0011] In some embodiments of this application, the electrolyte satisfies one or a combination of the following: (2.1) The concentration of the lithium salt is not less than 0.1 mol / L, preferably 0.1 mol / L to 0.2 mol / L; (2.2) The concentration of the sodium salt is not less than 0.8 mol / L, preferably 0.8 mol / L to 1.5 mol / L.

[0012] In some embodiments of this application, the lithium salt comprises one or more of lithium bis(fluorosulfonyl)imide, lithium bis(oxalatoborate) and lithium bis(trifluoromethanesulfonyl)imide.

[0013] In some embodiments of this application, the sodium salt comprises one or more of sodium hexafluorophosphate, sodium hexafluoroarsenate, and sodium tetrafluoroborate.

[0014] In some embodiments of this application, the electrolyte comprises an ether solvent; In some embodiments of this application, the ether solvent comprises one or more of dimethoxymethane (DMM), dimethoxyethane (DME), and diethoxyethane (DEE).

[0015] A second aspect of this application is to provide a secondary battery comprising the electrolyte described in the first aspect.

[0016] In some embodiments of this application, the secondary battery further includes: Positive electrode sheet: comprising a positive current collector and a positive active material disposed on at least one side surface of the positive current collector; Negative electrode plate: includes negative current collector; Separating membrane; The positive electrode active material includes sodium ion positive electrode active material and lithium ion positive electrode active material, and the charging plateau voltage of the sodium ion positive electrode active material in the battery is lower than that of the lithium ion positive electrode active material in the battery.

[0017] To achieve the goal of depositing lithium metal on the surface of sodium metal, this application further improves the storage life of secondary batteries by controlling the difference between the charging platform of lithium-ion positive electrode active material and the charging platform of sodium-ion positive electrode active material, and using high-voltage charging to deposit lithium metal, which easily forms a stable SEI film, as a protective layer on the surface of sodium metal to isolate the continuous reaction between sodium metal and electrolyte.

[0018] The secondary battery in this application is charged at the charging plateau voltage of the sodium-ion positive electrode active material. A sodium metal layer is first formed on the surface of the negative electrode current collector, and sodium ions are repeatedly deposited and stripped on the surface of the negative electrode current collector, which enables the secondary battery to achieve long cycle performance. The secondary battery is then charged further to above the charging plateau voltage of the lithium-ion positive electrode active material. Lithium ions are deposited on the surface of the sodium metal layer to form a lithium metal coating. This lithium metal coating can react with the film-forming components in the electrolyte to generate a stable SEI film, which isolates the negative electrode from the electrolyte, thereby reducing the probability of reaction between the negative electrode and the electrolyte and improving the stability of the lithium-sodium metal negative electrode. Therefore, the secondary battery provided in this application improves both the cycle performance and storage performance of the battery.

[0019] In some embodiments of this application, the charging plateau voltage of the sodium-ion positive electrode active material in the battery is at least 0.1V lower than that of the lithium-ion positive electrode active material in the battery.

[0020] In some embodiments of this application, the charging plateau voltage of the sodium-ion positive electrode active material in the battery is 0.1V to 1V lower than that of the lithium-ion positive electrode active material in the battery.

[0021] In some embodiments of this application, the charging plateau voltage of the sodium-ion positive electrode active material in the battery is 0.2V to 0.6V lower than that of the lithium-ion positive electrode active material in the battery.

[0022] In some embodiments of this application, the charging platform voltage of the sodium ion positive electrode active material in the battery is 2.0V~3.6V.

[0023] In some embodiments of this application, the charging plateau voltage of the sodium ion positive electrode active material in the battery is 3.2V~3.4V.

[0024] In some embodiments of this application, the sodium ion positive electrode active material comprises one or more of polyanionic compounds or Prussian blue compounds. Preferably, the sodium ion positive electrode active material comprises one or more combinations of Na3V2(PO4)3, Na2Fe[Fe(CN)6], Na2Mn[Fe(CN)6], and Na2Mn[Mn(CN)6].

[0025] In some embodiments of this application, the charging platform voltage of the lithium-ion positive electrode active material in the battery is 3.0V~4.3V.

[0026] In some embodiments of this application, the charging platform voltage of the lithium-ion positive electrode active material in the battery is 3.4V~4.0V.

[0027] In some embodiments of this application, the lithium-ion positive electrode active material comprises one or more combinations of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium phosphates with an olivine structure. Preferably, the lithium-ion cathode active material comprises LiFePO4, LiMn2O4, LiCoO2, and LiNi. 0.8 CO 0.1 Mn 0.1 One or more combinations of O2.

[0028] In some embodiments of this application, the secondary battery is charged at the charging platform voltage of the sodium-ion positive electrode active material, and a sodium metal layer is formed on the surface of the negative electrode current collector; The secondary battery is charged at the charging platform voltage of the lithium-ion positive electrode active material, and a lithium metal plating layer is formed on the surface of the sodium metal layer; Preferably, the thickness of the lithium metal coating is 0.5 μm to 3.0 μm.

[0029] In some embodiments of this application, the negative electrode sheet is a negative current collector.

[0030] A third aspect of this application is to provide an electrical device comprising the electrolyte described in the first aspect or the secondary battery described in the second aspect.

[0031] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Detailed Implementation

[0032] 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 "a~b" 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.

[0033] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0034] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0035] 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 method may also include step (c), indicating 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.

[0036] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0037] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0038] Unless otherwise specified, in this application, the terms "first," "second," etc., are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features.

[0039] Unless otherwise specified, in this application, the term "multiple" means two or more (including two), similarly, "multiple sets" means two or more (including two sets), and "multiple pieces" means two or more (including two pieces).

[0040] The negative electrode active materials of lithium-ion batteries include carbonaceous materials such as graphite. However, graphite has limited specific capacity and very little room for improvement in volumetric capacity, severely restricting further increases in the gravimetric and volumetric energy densities of lithium-ion batteries. With the current development of consumer electronics and electric vehicle technologies, developing battery systems with higher energy densities has become a top priority.

[0041] Sodium metal possesses high gravimetric and volumetric energy densities, making it a common choice for the negative electrode in metal batteries. To further achieve higher cell energy densities, "negative electrodeless" metal batteries have been developed, where sodium is deposited in situ from the positive electrode material onto the negative electrode current collector. This eliminates the need for pre-coating / depositing highly active metals on the negative electrode side, significantly improving the feasibility and safety of cell manufacturing. However, negative electrodeless sodium batteries require higher overpotentials for deposition on the negative electrode current collector surface, which can easily lead to uneven metal deposition, exacerbating side reactions with the electrolyte, consuming a large amount of active sodium, and ultimately affecting the cell's cycle performance and storage life.

[0042] Currently, a balance between film formation and non-film formation is often used to overcome the above-mentioned defects. Specifically, a thin and dense SEI layer is formed on the surface of the negative electrode to maximize storage life while meeting cycle life requirements. However, these two methods have the following advantages and disadvantages: 1. Using an electrolyte system that readily forms an SEI film allows the SEI film to isolate the reaction between the electrolyte and sodium metal during storage, resulting in a longer storage life. However, during cycling, the electrolyte continuously reacts with freshly deposited sodium metal to form new SEI, causing losses of both sodium metal and electrolyte. Furthermore, as the SEI is a rate-controlling step, sodium metal is more prone to dendrite growth at defects in the SEI film. Dendrite growth creates a larger specific surface area, and dendrites are easily broken during cycling, ultimately leading to greater capacity loss and failing to meet the requirements for long cycling.

[0043] 2. Select an electrolyte system that is not prone to forming an SEI film. Although sodium metal can be deposited in bulk to reduce the specific surface area and significantly improve cycle life, sodium metal without SEI protection will continuously react with the electrolyte and be consumed, which cannot meet the long-term storage requirements of sodium metal secondary batteries without negative electrodes.

[0044] To address the aforementioned technical problems, this application has developed an electrolyte through experimental research. This electrolyte contains sodium and lithium salts, with the deposition overpotential of lithium metal being higher than that of sodium metal. The electrolyte provided by this application exhibits film-forming selectivity; the higher deposition overpotential of lithium metal compared to sodium metal facilitates film formation on the lithium metal surface, while sodium metal is less prone to film formation. Specifically, sodium metal does not readily form an SEI film in this electrolyte, resulting in good deposition and improved cycle life of the secondary battery. Lithium metal readily forms an SEI film in this electrolyte. Furthermore, if lithium metal is deposited on the sodium metal surface and a stable SEI film is formed on the lithium metal surface, the storage performance of the secondary battery can be effectively improved in addition to its cycle life.

[0045] This application provides a secondary battery in some embodiments. In addition to the electrolyte described above, the secondary battery also includes a positive electrode, a negative electrode, and a separator. The positive electrode includes a positive current collector and a positive active material disposed on at least one side of the positive current collector. The negative electrode includes a negative current collector. The positive active material includes sodium-ion positive active material and lithium-ion positive active material. The charging plateau voltage of the sodium-ion positive active material in the battery is lower than that of the lithium-ion positive active material in the battery.

[0046] To achieve the goal of depositing lithium metal on the surface of sodium metal, this application further improves the storage life of the secondary battery by controlling the difference between the charging platform of the lithium-ion positive electrode active material and the charging platform of the sodium-ion positive electrode active material. High-voltage charging is used to deposit lithium metal, which easily forms a stable SEI film, as a protective layer on the sodium metal surface to isolate the continuous reaction between the sodium metal and the electrolyte. Specifically, the secondary battery provided in this application is charged at the charging platform voltage of the sodium-ion positive electrode active material. A sodium metal layer first forms on the surface of the negative electrode current collector, and sodium ions are repeatedly deposited and stripped on the surface of the negative electrode current collector, achieving long cycle performance. The secondary battery is then charged further to above the charging platform voltage of the lithium-ion positive electrode active material. Lithium ions are deposited on the surface of the sodium metal layer to form a lithium metal coating. This lithium metal coating can react with the film-forming components in the electrolyte to generate a stable SEI film, isolating the negative electrode from the electrolyte and reducing the probability of reaction between the negative electrode and the electrolyte, thereby improving the stability of the lithium-sodium metal negative electrode. Therefore, the secondary battery provided in this application improves both the cycle performance and storage performance of the battery.

[0047] The secondary battery provided in this application can be used as a power source for an electrical device or as an energy storage unit for an electrical device. For example, it can be used in, but is not limited to, laptops, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, energy storage and capacitors, etc.

[0048] Electrolyte According to some embodiments of this application, the electrolyte is used to conduct sodium ions and lithium ions. It contains sodium salt and lithium salt as electrolyte salts, wherein the deposition overpotential of lithium metal in the electrolyte is higher than that of sodium metal.

[0049] The deposition overpotential in this application is used to measure the deposition efficiency of each metal in the electrolyte. Generally speaking, the lower the overpotential, the smaller the deposition resistance, and the easier it is for deposition to occur. The deposition overpotential of each metal in the electrolyte can be obtained by direct measurement, for example, by using a Xinwei charge-discharge machine to test the overpotential of a Na-Na symmetric battery at 1 mA / cm². -2 The current density was measured, and the system was charged for 1 hour and then discharged for 1 hour. Each charge-discharge cycle was counted as one period. After 100 cycles, the stability of the electrolyte system was evaluated by the potential fluctuations, and a suitable electrolyte salt was finally obtained.

[0050] In this application, because lithium has a low electrochemical potential, it reacts with the electrolyte more readily than sodium to form SEI in the same electrolyte system.

[0051] The electrolyte provided in this application exhibits film-forming selectivity. Specifically, the deposition overpotential of lithium metal in the electrolyte is higher than that of sodium metal, which is beneficial for film formation on the lithium metal surface, while sodium metal is less prone to film formation. In particular, sodium metal does not readily form an SEI film in this electrolyte, resulting in good deposition and improved cycle life of the secondary battery. Lithium metal readily forms an SEI film in this electrolyte. Furthermore, if lithium metal is deposited on the sodium metal surface and a stable SEI film is formed on the lithium metal surface, the lithium metal and the SEI film improve the storage life of the secondary battery by isolating the reaction between the sodium metal and the electrolyte.

[0052] According to some embodiments of this application, the deposition overpotential of lithium metal in the electrolyte is more than 20 mV higher than that of sodium metal.

[0053] In this application, the deposition overpotential of lithium metal is selected to be more than 20 mV higher than that of sodium metal, which is beneficial for screening suitable electrolyte salts to improve the cycle life and storage performance of secondary batteries.

[0054] According to some embodiments of this application, the deposition overpotential of lithium metal in the electrolyte is greater than 30mV.

[0055] The electrolyte in this application needs to meet the requirements of easy molding of lithium metal. The deposition overpotential of lithium metal in the electrolyte is greater than 30mV, which is beneficial to meet the requirements of easy molding of lithium metal and easy to screen for suitable lithium salts. At the same time, due to the limitations of lithium salt type, concentration, solvent composition and other factors, the deposition overpotential of lithium salt in the electrolyte cannot be changed indefinitely.

[0056] According to some embodiments of this application, the overpotential for lithium metal deposition in the electrolyte is 32mV~70mV.

[0057] In specific embodiments, this application mainly explores the types of lithium salts with a deposition overpotential of 32mV to 70mV in the electrolyte, for the purpose of explaining lithium salts, rather than limiting them.

[0058] According to some embodiments of this application, the deposition overpotential of sodium metal in the electrolyte is less than 20mV.

[0059] The electrolyte in this application needs to satisfy the non-molding property of sodium metal, and the deposition overpotential of sodium metal in the electrolyte is less than 20mV. This is beneficial for satisfying the non-molding property of sodium metal and for easily screening suitable sodium salts. At the same time, the deposition overpotential of sodium metal in the electrolyte is not equal to zero.

[0060] According to some embodiments of this application, the deposition overpotential of sodium metal in the electrolyte is 5mV~18mV.

[0061] In specific embodiments, this application mainly explores the types of sodium salts with a deposition overpotential of 5mV to 18mV in the electrolyte, for the purpose of explaining sodium salts, rather than limiting them.

[0062] According to some embodiments of this application, the concentration of lithium salt is lower than that of sodium salt.

[0063] In this application, as described above, sodium-ion positive electrode active material is used to achieve long-cycle operation of the secondary battery, while lithium-ion positive electrode active material is used to improve the battery's storage performance. To improve the overall performance of the secondary battery in this application, sodium-ion positive electrode active material is used as the main material, and lithium-ion positive electrode active material assists sodium-ion positive electrode active material to isolate side reactions caused by contact between sodium metal and the electrolyte. Judging from their respective usage amounts, ideally, the amount of sodium-ion positive electrode active material used should be greater than that of lithium-ion positive electrode active material, corresponding to a lower lithium salt concentration than sodium salt concentration in the electrolyte.

[0064] According to some embodiments of this application, the concentration of lithium salt is not less than 0.1 mol / L.

[0065] According to some embodiments of this application, the concentration of lithium salt is 0.1 mol / L to 0.2 mol / L.

[0066] According to some embodiments of this application, the concentration of sodium salt is not less than 0.8 mol / L.

[0067] According to some embodiments of this application, the concentration of sodium salt is 0.8 mol / L to 1.5 mol / L.

[0068] In this application, selecting the concentrations of lithium salt and sodium salt in the electrolyte to meet the above-mentioned numerical range is beneficial to the matching between the electrolyte and the positive electrode active material.

[0069] According to some embodiments of this application, the lithium salt comprises one or more combinations of lithium bisfluorosulfonylimide (LiFSI), lithium bis(oxalateborate) (LiBOB), and lithium bis(trifluoromethanesulfonylimide) (LiTFSI).

[0070] The lithium salt selected in this application meets the deposition overpotential in the electrolyte, which is beneficial for the formation of the SEI film.

[0071] According to some embodiments of this application, the sodium salt comprises one or more combinations of sodium hexafluorophosphate (NaPF6), sodium hexafluoroarsenate (NaAsF6), and sodium tetrafluoroborate.

[0072] The electrolyte system selected in this application meets its deposition overpotential in the electrolyte, making it less likely to form an SEI film.

[0073] According to some embodiments of this application, the electrolyte of this application contains a solvent, which includes ether solvents.

[0074] In this application, during the cycling process, metallic sodium is deposited on the surface of the negative electrode current collector to form a sodium metal layer. The sodium metal layer has higher stability in ether solvents than in other solvents such as carbonates.

[0075] According to some embodiments of this application, the ether solvent comprises one or more combinations of dimethoxymethane (DMM), dimethoxyethane (DME), and diethoxyethane (DEE).

[0076] The ether solvents selected in this application are beneficial to improving the solvation structure of sodium and lithium ions and further affecting the deposition morphology of the negative electrode, thereby ultimately improving the cycle stability of the secondary battery.

[0077] In this application, the negative electrode sheet is a negative current collector.

[0078] The negative electrode in this application serves as the negative current collector, thereby forming a negative electrode-free metal secondary battery. This negative electrode-free metal secondary battery utilizes the lithium-sodium potential difference. During charging, a low potential enables reversible deposition and stripping of sodium metal, resulting in low loss and long cycle life. For storage, a high potential is used for charging, depositing a layer of lithium metal onto the sodium metal surface. Lithium metal has high reactivity and readily forms an electrolyte interphase (SEI), which can isolate the side reactions between the sodium metal negative electrode and the electrolyte, thus achieving a long storage life.

[0079] The ether solvents selected in this application are beneficial to improving the solvation structure of sodium and lithium ions and further affecting the deposition morphology of the negative electrode, thereby ultimately improving the cycle stability of the secondary battery.

[0080] Secondary batteries According to some embodiments of this application, this application provides a secondary battery comprising a positive electrode, a negative electrode, a separator between the positive and negative electrode, and an electrolyte. The positive electrode comprises a positive current collector and a positive active material disposed on at least one surface of the positive current collector, and the negative electrode comprises a negative current collector. The positive active material comprises sodium-ion positive active material and lithium-ion positive active material, and the charging plateau voltage of the sodium-ion positive active material in the battery is lower than that of the lithium-ion positive active material in the battery.

[0081] In this application, the method for determining the charging plateau voltage of the positive electrode material in the battery includes any form conventional in the art. For example, a direct measurement method can be used. Specifically, a button battery is prepared by using sodium-ion positive electrode active material or lithium-ion positive electrode active material, and a charging test is performed on the button battery. A charging characteristic curve is plotted with capacity as the abscissa and voltage as the ordinate. The charging plateau voltage is determined based on the charging characteristic curve. In this application, the same test conditions and the same method are used to determine the charging plateau voltage of sodium-ion positive electrode active material and lithium-ion positive electrode active material.

[0082] To achieve the goal of depositing lithium metal on the surface of sodium metal, this application further improves the storage life of the secondary battery by controlling the difference between the charging platforms of the lithium-ion cathode active material and the sodium-ion cathode active material, and using high-voltage charging to deposit lithium metal, which easily forms a stable SEI film, as a protective layer on the sodium metal surface to isolate the continuous reaction between sodium metal and the electrolyte. Simultaneously, the electrolyte in the secondary battery does not readily form a film on sodium metal, and sodium metal does not easily grow an SEI film in this electrolyte, resulting in good deposition and improving the cycle life of the secondary battery.

[0083] The secondary battery in this application is charged at the charging plateau voltage of the sodium-ion positive electrode active material. A sodium metal layer is first formed on the surface of the negative electrode current collector, and sodium ions are repeatedly deposited and stripped on the surface of the negative electrode current collector, which enables the secondary battery to achieve long cycle performance. The secondary battery is then charged further to above the charging plateau voltage of the lithium-ion positive electrode active material. Lithium ions are deposited on the surface of the sodium metal layer to form a lithium metal coating. This lithium metal coating can react with the film-forming components in the electrolyte to generate a stable SEI film, which isolates the negative electrode from the electrolyte, thereby reducing the probability of reaction between the negative electrode and the electrolyte and improving the stability of the lithium-sodium metal negative electrode. Therefore, the secondary battery provided in this application improves both the cycle performance and storage performance of the battery.

[0084] [Cathode Material] According to some embodiments of this application, the charging plateau voltage of the sodium-ion positive electrode active material in the battery is at least 0.1V lower than that of the lithium-ion positive electrode active material in the battery.

[0085] In this application, by controlling the difference between the charging platform of lithium-ion positive electrode active material and the charging platform of sodium-ion positive electrode active material, high-voltage charging is used to deposit lithium metal, which easily forms a stable SEI film, as a protective layer on the surface of sodium metal to isolate the continuous reaction between sodium metal and electrolyte, thereby improving the storage life of secondary batteries.

[0086] According to some embodiments of this application, the charging plateau voltage of the sodium-ion positive electrode active material in the battery is 0.1V~1V lower than that of the lithium-ion positive electrode active material in the battery.

[0087] In this application, if the difference between the charging platform voltage of the sodium ion positive electrode active material and the charging platform voltage of the lithium ion positive electrode active material is greater than 1V, the selectivity requirements for the positive electrode active material are higher, which is not easy to achieve based on the existing types of positive electrode active materials.

[0088] According to some embodiments of this application, the charging plateau voltage of the sodium-ion positive electrode active material in the battery is 0.2V~0.6V lower than that of the lithium-ion positive electrode active material in the battery.

[0089] In this application, the difference between the charging platform voltage of the sodium-ion positive electrode active material and the charging platform voltage of the lithium-ion positive electrode active material is 0.2V~0.6V. This not only improves the cycle performance and storage performance of the secondary battery, but also makes the selection of positive electrode active materials easier to achieve, which is conducive to meeting the needs of industrial production.

[0090] According to some embodiments of this application, the charging platform voltage of the sodium ion positive electrode active material in the battery is 2.0V~3.6V.

[0091] According to some embodiments of this application, the charging platform voltage of the sodium ion positive electrode active material in the battery is 3.2V~3.4V.

[0092] In this application, the specific test method for the charging platform voltage of the sodium-ion positive electrode active material includes: preparing the sodium-ion positive electrode active material into a button cell, and testing the button cell at 1mA / cm. 2 Charging tests were conducted using the current density, with a cutoff voltage of 2.0V to 4.3V. A charging characteristic curve was plotted with capacity on the x-axis and voltage on the y-axis. The charging platform voltage was determined based on the charging characteristic curve. The charging platform voltage refers to the voltage value corresponding to the smallest voltage change and the largest capacity change, which can be obtained from the peak value of dQ / dV.

[0093] For positive electrode active materials with a relatively obvious charging platform voltage, the charging platform voltage is relatively easy to measure or calculate. For positive electrode active materials with a less obvious charging platform voltage, the average voltage can be used instead. The average voltage is the effective area of ​​the charging characteristic curve (battery charging energy) divided by the capacity.

[0094] According to some embodiments of this application, the sodium ion positive electrode active material comprises any one or more combinations of polyanionic compounds or Prussian blue compounds.

[0095] In this application, the polyanionic compounds include one or more of sodium vanadium trifluorophosphate (Na3V2(PO4)2F3), sodium vanadium fluorophosphate (NaVPO4F), sodium vanadium phosphate (Na3V2(PO4)3), Na4Fe3(PO4)2P2O7, and NaFePO4. The Prussian blue compounds are Na... x M1M2(CN)6, wherein M1 and M2 are one or more of Fe, Mn, Co, Ni, Cu, Zn, Cr, Ti, V, Zr, and Ce, and 0 < x ≤ 2.

[0096] According to some embodiments of this application, the sodium-ion positive electrode active material comprises one or more combinations of Na3V2(PO4)3, Na2Fe[Fe(CN)6], Na2Mn[Fe(CN)6], and Na2Mn[Mn(CN)6]. This application selects several specific sodium-ion positive electrode active materials in specific embodiments, mainly to illustrate that the difference between charging platform voltages can solve the technical problems of this application.

[0097] According to some embodiments of this application, the charging platform voltage of the lithium-ion positive electrode active material in the battery is 3.0V~4.3V.

[0098] According to some embodiments of this application, the charging platform voltage of the lithium-ion cathode active material in the battery is 3.4V~4.0V.

[0099] In this application, the specific test method for the charging platform voltage of the lithium-ion positive electrode active material includes: preparing the lithium-ion positive electrode active material into a button cell, and testing the button cell at 1mA / cm. 2 Charging tests were conducted using a current density ranging from 2.0V to 4.3V, with the cutoff voltage between these values. A charging characteristic curve was plotted with capacity on the x-axis and voltage on the y-axis. The charging plateau voltage was determined based on this curve. The charging plateau voltage is the voltage value corresponding to the point where the voltage change is minimal while the capacity change is significant; it can be obtained from the peak value of dQ / dV.

[0100] For positive electrode active materials with a relatively obvious charging platform voltage, such as lithium iron sulfate, the charging platform voltage is relatively easy to measure or calculate. For positive electrode active materials with a less obvious charging platform voltage, the average voltage can be used instead. The average voltage is the effective area of ​​the charging characteristic curve (battery charging energy) divided by the capacity.

[0101] According to some embodiments of this application, the lithium-ion cathode active material comprises one or more combinations of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium phosphates with an olivine structure. Exemplarily, the cathode active material comprises, but is not limited to, LiCoO2, LiNiO2, LiMnO2, LiMn2O4, and LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(NCM111), LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM523), LiNi 0.5 Co 0.3 Mn 0.2 O2(NCM532), LiNi 0.6 CO 0.2 Mn 0.2 O2(NCM622), LiNi 0.8 CO 0.1 Mn 0.1 O2(NCM811), LiNi 0.85 CO 0.15 Al 0.05 One or more of O2, LiFePO4 (LFP) and LiMnPO4.

[0102] According to some embodiments of this application, the structural formula of the lithium phosphate with an olivine structure is: LiFe 1-x-y Mn x M y PO4, 0≤x≤1, 0≤y<1, 0≤x+y≤1, M contains one or more of the transition metal elements or non-transition metal elements other than Fe and Mn, and M preferably contains one or more of the following: Cr, Mg, Ti, Al, Zn, W, Nb, Zr.

[0103] According to some embodiments of this application, the lithium-ion cathode active material includes LiFePO4, LiMn2O4, LiCoO2, and LiNi. 0.8 CO 0.1 Mn 0.1 One or more combinations of O2. In specific embodiments, this application selects several specific sodium-ion positive electrode active materials, mainly to illustrate that the difference between charging platform voltages can solve the technical problems of this application.

[0104] According to some embodiments of this application, the secondary battery is charged at the charging platform voltage of the sodium-ion positive electrode active material, and a sodium metal layer is formed on the surface of the negative electrode current collector; the secondary battery is charged at the charging platform voltage of the lithium-ion positive electrode active material, and a lithium metal plating layer is formed on the surface of the sodium metal layer.

[0105] In this application, sodium ions are deposited on the surface of the negative electrode current collector. Since the deposition voltage is the charging plateau voltage of the sodium ion positive electrode active material, it is beneficial to the uniformity of sodium metal deposition, thus enabling the reversible charging and discharging process. Furthermore, the amount of sodium ion positive electrode active material affects the charging and discharging process; therefore, this application does not impose specific requirements on the exact amount of sodium ion positive electrode active material. Simultaneously, the secondary battery is charged using the charging plateau voltage of the lithium ion positive electrode active material, and lithium ions are deposited on the surface of the sodium metal layer to form a lithium metal coating. This lithium metal coating includes a stable SEI film to improve the stability of the lithium metal coating. This lithium metal coating improves the storage life of the secondary battery by isolating the reaction between the sodium metal and the electrolyte.

[0106] According to some embodiments of this application, the thickness of the lithium metal coating is 0.5 μm to 3.0 μm.

[0107] In this application, the thickness of the lithium metal coating can be obtained through photography and calculation. For example, a secondary battery can be disassembled, the cathode material photographed, and then directly measured. Specifically, a scanning electron microscope (SEM, PHILIPS, XL-30FG) is used to observe the sample deposition morphology and the thickness of the lithium metal coating. Thickness cross-sectional samples can be prepared by liquid nitrogen embrittlement or ion beam polishing, and semi-quantitative analysis can be performed using the energy dispersive spectroscopy (EDS) system equipped with the SEM.

[0108] The thickness of the lithium metal coating in this application cannot be too thick or too thin. If it is too thick, it will affect the amount of sodium-ion positive electrode active material used during the battery charge and discharge process, which may ultimately affect the cycle life of the secondary battery. If the lithium metal coating is too thin, it cannot effectively isolate sodium metal from the electrolyte, which may affect the storage performance of the secondary battery. Furthermore, the thickness of the lithium metal coating is determined by the amount of lithium-ion positive electrode active material used, and conversely, the amount of lithium-ion positive electrode active material used can be calculated based on the thickness of the lithium metal coating. A specific calculation method may include converting lithium metal to 0.206 mAh / (μm·cm). 2 Therefore, if a lithium metal layer with a thickness of 0.1~8μm is required, the positive electrode surface capacity should meet the requirement of 0.0206~1.648mAh / cm². 2 Based on the areal capacity range and the specific capacity data (m mAh / g) of lithium-ion cathode active materials, the mass of lithium-ion cathode active material per unit area (0.0206~1.648) ÷ mg / cm³ can be obtained. 2In this application, selecting a lithium metal coating thickness of 0.5μm to 3.0μm can relatively effectively improve the cycle life and storage performance of the secondary battery.

[0109] The thickness of the lithium metal coating in this application can be one of 0.5μm, 0.8μm, 1.0μm, 1.2μm, 1.5μm, 1.7μm, 2.0μm, 2.5μm, 3.0μm or any value within this range.

[0110] According to some embodiments of this application, binders and / or conductive agents may also be added to the positive electrode active material. There are no restrictions on the types of binders and conductive agents; those skilled in the art can select them according to actual needs. For example, the binder may be one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), polyvinyl alcohol (PVA), and styrene-butadiene rubber (SBR), and the conductive agent may be one or more of graphite, superconducting carbon, acetylene black, carbon black, carbon nanotubes, graphene, and carbon nanofibers.

[0111] According to some embodiments of this application, the positive electrode current collector can be a metal foil or a composite current collector. The metal foil can be an aluminum foil, and the composite current collector can include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector can be formed by forming a metal material, such as aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, or silver alloy, on a polymer substrate such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), or polyethylene (PE).

[0112] [Negative electrode plate] According to some embodiments of this application, the negative electrode sheet is a negative current collector, which can be a negative current collector substrate or a negative current collector substrate and a modification layer disposed on at least one side surface of the negative current collector substrate. Meanwhile, the modification layer on the surface of the negative current collector substrate can be formed in any form conventional in the art, such as deposition, coating, etc.

[0113] In this application, the modification layer is used to reduce the deposition potential of the metal, which can induce metal ions to deposit evenly and smoothly on the negative electrode surface, forming a compact SEI film and inhibiting the formation of metal dendrites, thus directly improving the cycle stability of the secondary battery. Furthermore, the material of the modification layer includes, but is not limited to, any one or more combinations of inorganic metal elements, inorganic oxides, and organic materials. For example, inorganic metal elements include any one or more combinations of platinum, aluminum, magnesium, zinc, gold, silver, and tin. Inorganic oxides include any one or more combinations of alumina, silicon dioxide, and titanium dioxide. Organic materials include polydimethylsiloxane, graphene oxide, and silicon dioxide-polymethyl methacrylate, etc.

[0114] In this application, the negative electrode current collector substrate can be a metal foil or a composite current collector. The metal foil can be a copper foil, and the composite current collector can include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by forming a metal material, such as copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, on a polymer material substrate such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0115] [Isolation membrane] In this application, the separator located between the positive and negative electrode sheets can be any known porous separator with good chemical and mechanical stability. For example, the separator material may include, but is not limited to, one or more combinations of the following: glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0116] To make the objectives, technical solutions, and advantages of this application clearer, the sodium battery separator and sodium replenishment method protected by this application are described in detail below with reference to specific embodiments. All chemical substances and reagents used in the following embodiments are of any type and manufacturer conventional in the art.

[0117] [Preparation of positive electrode sheet] Example 1 A method for preparing a positive electrode sheet is provided, comprising the following preparation process: LiFePO4 (10wt%), Na2Fe[Fe(CN)6] (85.7wt%), conductive agent acetylene black, and binder PVDF were mixed in an N-methylpyrrolidone (NMP) solvent at a mass ratio of 95.7:2.3:2 to prepare a positive electrode slurry. The slurry was then coated onto an aluminum foil surface using an extrusion coating machine and dried. The coated electrode was then cold-pressed to obtain a positive electrode with an areal capacity of 3 mAh / cm². 2 .

[0118] Determination of relevant parameters of the positive electrode: (1) Testing of the charging plateau voltage of sodium-ion positive electrode active material: Na2Fe[Fe(CN)6] was used to prepare a button cell, and the button cell was tested at 1mA / cm 2 Charging tests were conducted using the current density, with a cutoff voltage of 2.0V to 4.3V. A charging characteristic curve was plotted with capacity on the x-axis and voltage on the y-axis. The charging platform voltage was determined based on the charging characteristic curve, and it was obtained from the peak value of dQ / dV.

[0119] (2) Testing of the charging plateau voltage of lithium-ion cathode active material: LiFePO4 was used to prepare a button cell, and the button cell was tested at 1mA / cm 2 Charging tests were conducted using the current density, with a cutoff voltage of 2.0V to 4.3V. A charging characteristic curve was plotted with capacity on the x-axis and voltage on the y-axis. The charging platform voltage was determined based on the charging characteristic curve, and it was obtained from the peak value of dQ / dV.

[0120] The relevant parameters of the positive electrode sheets provided in Examples 1 to 19 and Comparative Example 1 are shown in Table 1.

[0121] Table 1. Parameter list of positive electrode plate

[0122] In this application, for lithium-ion cathode active materials, olivine-structured lithium phosphates include, but are not limited to, LiFePO4; lithium manganese oxides include, but are not limited to, LiMn2O4; lithium cobalt oxides include, but are not limited to, LiCoO2; and lithium nickel cobalt manganese oxides include, but are not limited to, NCM811. Table 1 mainly discusses LiFePO4, LiMn2O4, LiCoO2, and LiNi... 0.8 CO 0.1 Mn 0.1 O2 is used as a lithium-ion positive electrode active material in combination with sodium-ion positive electrode active materials to form a charging platform voltage difference. Other lithium nickel manganese oxides and lithium nickel cobalt aluminum oxides that meet the conditions should also be protected within the scope of this application.

[0123] Similarly, in this application, for sodium-ion positive electrode active materials, the polyanionic compounds include, but are not limited to, Na3V2(PO4)3, and for Prussian blue compounds, they include, but are not limited to, Na2Fe[Fe(CN)6], Na2Mn[Fe(CN)6], and Na2Mn[Mn(CN)6]. In Table 1, this application mainly discusses Na3V2(PO4)3, Na2Fe[Fe(CN)6], Na2Mn[Fe(CN)6], and Na2Mn[Mn(CN)6] as sodium-ion positive electrode active materials combined with lithium-ion positive electrode active materials to form a charging platform voltage difference. Other sodium-ion positive electrode active materials that meet the conditions should also be within the scope of protection of this application.

[0124] [Preparation of negative electrode sheet] A copper foil with a thickness of 8 μm was selected as the negative electrode current collector to prepare the negative electrode.

[0125] [Preparation of Electrolyte] The electrolyte in this embodiment comprises an ether solvent and lithium and sodium salts. The ether solvent comprises one or more combinations of dimethoxymethane, dimethoxyethane, and diethoxyethane. The lithium salt comprises one or more combinations of lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(oxalato)borate (LiBOB), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The sodium salt comprises one or more combinations of sodium hexafluorophosphate (NaPF6), sodium hexafluoroarsenate (NaAsF6), and sodium tetrafluoroborate. The lithium salt concentration is 0.1 mol / L to 0.2 mol / L; the sodium salt concentration is 0.8 mol / L to 1.5 mol / L.

[0126] Determination of electrolyte-related parameters: (3) Overpotential test of deposition in electrolyte: The overpotential of the Li-Li symmetric cell was tested using a Xinwei charge-discharge machine (model); at 1mA / cm -2 The system was charged for 1 hour and then discharged for 1 hour at a given current density. Each charge-discharge cycle was counted as one period, and 100 cycles were used as the cutoff condition. An Et curve was plotted, and the stability of the system was evaluated by the potential fluctuations. An overpotential of approximately 50 mV or higher was considered a film-forming passivation system.

[0127] (4) Overpotential test of deposition in electrolyte: The Na-Na symmetric cell was tested using a Xinwei charge-discharge machine (model); at 1mA / cm -2 The system was charged for 1 hour and then discharged for 1 hour at a given current density. Each charge-discharge cycle was counted as one period, and 100 cycles were used as the cutoff condition. An Et curve was plotted, and the stability of the system was evaluated by the potential fluctuations. An overpotential of approximately 5 mV indicates a system that is not prone to film formation.

[0128] The relevant parameters of the electrolytes provided in Examples 1 to 19 and Comparative Example 1 are shown in Table 2.

[0129] Table 2. Electrolyte Parameter List

[0130] Table 2 of this application further explores the film-forming selectivity of the electrolyte based on different types and concentrations of electrolyte salts.

[0131] [Isolation membrane] Choose polypropylene film.

[0132] [Preparation of Secondary Batteries] According to some embodiments of this application, the prepared negative electrode sheet, separator, and positive electrode sheet are stacked in sequence, with the separator in the middle of the positive and negative electrode sheets to provide isolation. The cells are then wound to obtain a bare cell. After the bare cell is baked at 100°C to remove water, electrolyte is injected and the cell is sealed. After that, a secondary battery is obtained through processes such as standing, hot and cold pressing, formation, secondary electrolyte injection, aging, shaping, and capacity testing.

[0133] Comparative Example 1 A secondary battery is provided, which differs from Example 1 in that the positive electrode active material contains only Na2Fe[Fe(CN)6].

[0134] [Performance Testing of Secondary Batteries] (5) Cycle performance test of secondary batteries: The secondary metal battery was subjected to an induction charge of 1.5 mA·cm at 25°C. -2 It is charged with a constant current to 4.3 V, and then charged with a constant voltage of 4.3 V until the current drops to 0.3 mA·cm⁻¹. -2 Then at 1.5 mA·cm -2 Discharge the battery with a constant current to 2.8 V to obtain the first cycle discharge specific capacity (Cd1); repeat this charge-discharge cycle until the capacity decays to 80% of the cycle.

[0135] (6) Battery storage performance test: Perform a 0.1C low-current charge-discharge cycle on the secondary battery and record the discharge capacity C1. Fully charge the cell and place it in a 25℃ environment, monitoring the cell voltage change. Perform a low-current charge-discharge cycle every 20 hours and record Cn. When the measured irreversible capacity decays to 80% of the initial capacity, record the storage time at this point as the storage life.

[0136] The specific performance of the secondary battery is shown in Table 3. Table 3 Battery Performance List

[0137] In Examples 1 to 7, this application investigated the effects of different types of positive electrode active materials on the performance of secondary batteries, and in Examples 8 to 17, it investigated the effects of different electrolytes on the performance of secondary batteries. Meanwhile, the examples in this application investigated that the lithium metal coating thickness was 1 μm, and that the lithium metal coating thickness could be controlled between 0.5 μm and 3.0 μm by adjusting the content of each component in the positive electrode active material.

[0138] As shown in Table 3, the secondary battery designed in this application not only improves the cycle performance of the secondary battery, but also enhances its storage performance.

[0139] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application, and they should all be covered within the scope of the claims and specification of this application. In particular, as long as there is no structural conflict, the various technical features mentioned in the embodiments can be combined in any way. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A non-aqueous electrolyte secondary battery characterized by comprising: The electrolyte includes a sodium salt and a lithium salt, wherein the concentration of the lithium salt is lower than the concentration of the sodium salt, and the electrolyte also contains an ether solvent.

2. The metal-free secondary battery according to claim 1, characterized in that: The deposition overpotential of lithium metal in the electrolyte is higher than that of sodium metal.

3. The metal-free secondary battery according to claim 2, characterized in that: The deposition overpotential of lithium metal in the electrolyte is more than 20 mV higher than that of sodium metal.

4. The metal-free secondary battery according to claim 2, characterized by: The electrolyte satisfies one or two of the following combinations: (1.1) The overpotential for lithium metal deposition in the electrolyte is greater than 30mV; (1.2) The deposition overpotential of sodium metal in the electrolyte is less than 20mV.

5. The metal-free secondary battery according to claim 4, characterized in that: The electrolyte satisfies one or two of the following combinations: (1.1) The overpotential for lithium metal deposition in the electrolyte is 32mV to 70mV; (1.2) The overpotential for sodium metal deposition in the electrolyte is 5mV to 18mV.

6. The metal-free secondary battery according to any one of claims 1 to 2, characterized by: The electrolyte satisfies one or two of the following combinations: (2.1) The concentration of the lithium salt is not less than 0.1 mol / L; (2.2) The concentration of the sodium salt is not less than 0.8 mol / L.

7. The metal-free secondary battery according to claim 6, characterized in that: The electrolyte satisfies one or two of the following combinations: (2.1) The concentration of the lithium salt is 0.1 mol / L to 0.2 mol / L; (2.2) The concentration of the sodium salt is 0.8 mol / L to 1.5 mol / L.

8. The metal-free secondary battery of claim 1, wherein: The lithium salt comprises one or more of lithium bis(fluorosulfonyl)imide, lithium bis(oxalateborate), and lithium bis(trifluoromethanesulfonyl)imide.

9. The metal-free secondary battery of claim 1, wherein: The sodium salt comprises one or more of sodium hexafluorophosphate, sodium hexafluoroarsenate, and sodium tetrafluoroborate.

10. The metal-free secondary battery according to any one of claims 1 to 2, characterized by: The ether solvent comprises one or more of dimethoxymethane, dimethoxyethane, and diethoxyethane.

11. The metal-free secondary battery according to claim 1, wherein Also includes: Positive electrode sheet: comprising a positive current collector and a positive active material disposed on at least one side surface of the positive current collector; Negative electrode plate: includes negative current collector; Separating membrane; The positive electrode active material includes sodium ion positive electrode active material and lithium ion positive electrode active material, and the charging plateau voltage of the sodium ion positive electrode active material in the battery is lower than that of the lithium ion positive electrode active material in the battery.

12. The metal-free secondary battery according to claim 11, wherein The charging plateau voltage of the sodium-ion positive electrode active material in the battery is at least 0.1V lower than that of the lithium-ion positive electrode active material in the battery.

13. The metal-free secondary battery according to claim 12, wherein The charging plateau voltage of the sodium-ion positive electrode active material in the battery is at least 0.1V to 1V lower than that of the lithium-ion positive electrode active material in the battery.

14. The metal-free secondary battery according to claim 13, wherein The charging plateau voltage of the sodium-ion positive electrode active material in the battery is at least 0.2V to 0.6V lower than that of the lithium-ion positive electrode active material in the battery.

15. The metal-free secondary battery of claim 11, wherein The charging plateau voltage of the sodium ion positive electrode active material in the battery is 2.0V to 3.6V.

16. The metal-free secondary battery according to claim 15, wherein The charging plateau voltage of the sodium ion positive electrode active material in the battery is 3.2V to 3.4V.

17. The metal-free secondary battery of claim 11, wherein The sodium ion positive electrode active material comprises one or more of polyanionic compounds or Prussian blue compounds.

18. The metal-free secondary battery of claim 17, wherein The sodium ion positive electrode active material comprises one or more combinations of Na3V2(PO4)3, Na2Fe[Fe(CN)6], Na2Mn[Fe(CN)6], and Na2Mn[Mn(CN)6].

19. The metal-free secondary battery of claim 11, wherein The charging plateau voltage of the lithium-ion positive electrode active material in the battery is 3.0V to 4.3V.

20. The negative electrode-free metal secondary battery according to claim 19, characterized in that, The charging platform voltage of the lithium-ion positive electrode active material in the battery is 3.4V to 4.0V.

21. The negative electrode-free metal secondary battery according to claim 11, characterized in that, The lithium-ion cathode active material comprises one or more combinations of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium phosphates with an olivine structure.

22. The negative electrode-free metal secondary battery according to claim 21, characterized in that, The lithium ion cathode active material comprises one or more of LiFeP04, LiMn204, LiCo02, LiNi 0.8 CO 0.1 Mn 0.1 O2 in combination.

23. The negative electrode-free metal secondary battery according to claim 11, characterized in that, The negative electrode-free metal secondary battery is charged at the charging platform voltage of the sodium ion positive electrode active material, and a sodium metal layer is formed on the surface of the negative electrode current collector. The negative electrode-free metal secondary battery is charged at the charging platform voltage of the lithium-ion positive electrode active material, and a lithium metal coating is formed on the surface of the sodium metal layer.

24. The negative electrode-free metal secondary battery according to claim 23, characterized in that, The thickness of the lithium metal coating is 0.5 μm to 3.0 μm.

25. The negative electrode-free metal secondary battery according to claim 11, characterized in that, The negative electrode sheet is a negative current collector.

26. An electrical appliance, characterized in that, The non-negative electrode metal secondary battery comprising any one of claims 1 to 25.

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