Electrolyte, secondary battery, and electric device

Abstract

The invention belongs to the technical field of batteries, and particularly relates to an electrolyte, a secondary battery and an electric device. The electrolyte provided by the invention contains the sodium salt and the lithium salt, the deposition overpotential of the lithium metal in the electrolyte is higher than that of the sodium metal, and the electrolyte has film-forming selectivity and is beneficial to improving the cyclicity of the secondary battery. The secondary battery provided by the invention comprises the electrolyte and also comprises a positive pole piece, an active material in the positive pole piece comprises a sodium ion positive pole active material and a lithium ion positive pole active material, and the charging platform voltage of the sodium ion positive pole active material in the battery is lower than that of the lithium ion positive pole active material in the battery. The electrolyte in the secondary battery is matched with the positive pole piece, so that the storage property of the secondary battery is further improved.

Description

Technical Field

[0001] The present application relates to the field of batteries, and in particular to an electrolyte, a secondary battery and an electrical device. Background Art

[0002] 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, wind and solar power stations, as well as electronic equipment power supplies, power tools, electric bicycles, electric motorcycles, electric vehicles and other fields.

[0003] With the development of science and technology and society, the performance of various products has been further improved, so higher requirements are also put forward for 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

[0004] In view of the above problems, the present application provides an electrolyte, a secondary battery and an electrical device, which can improve the cycle performance of the secondary battery while also improving the storage performance of the battery.

[0005] A first aspect of the present application is to provide an electrolyte, wherein the electrolyte comprises a sodium salt and a lithium salt, and the deposition overpotential of lithium metal in the electrolyte is higher than the deposition overpotential of sodium metal.

[0006] The deposition overpotential of the present application is used to reflect the deposition efficiency of the metal in the electrolyte system. Generally speaking, the lower the deposition overpotential, the smaller the deposition resistance of the metal, the easier it is to deposit, but the electrolyte is less likely to form a film. The electrolyte provided in the present application has film-forming selectivity, wherein the deposition overpotential of lithium metal in the electrolyte is higher than the deposition overpotential of sodium metal, which is beneficial to the film-forming property of the lithium metal surface and the difficulty of film-forming property of the sodium metal surface. Specifically, sodium metal is not easy to grow SEI film in the electrolyte, and the deposition is good, which is beneficial to improving the cycle life of the secondary battery; lithium metal is easy to grow SEI film in the electrolyte. In the secondary battery, if lithium metal is further deposited on the surface of sodium metal and a stable SEI film is formed on the surface of lithium metal, then on the basis of improving the cycle life of the secondary battery, the storage property of the secondary battery can also be effectively improved.

[0007] In some embodiments of the present application, the electrolyte satisfies:

[0008] The deposition overpotential of lithium metal in the electrolyte is higher than the deposition overpotential of sodium metal by more than 20 mV.

[0009] In some embodiments of the present application, the electrolyte satisfies one or a combination of the following:

[0010] (1.1) The deposition overpotential of lithium metal in the electrolyte is greater than 30 mV; preferably 32 mV to 70 mV;

[0011] (1.2) The deposition overpotential of sodium metal in the electrolyte is less than 20 mV; preferably 5 mV to 18 mV.

[0012] In some embodiments of the present application, the concentration of the lithium salt is lower than the concentration of the sodium salt.

[0013] In some embodiments of the present application, the electrolyte satisfies one or a combination of the following:

[0014] (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;

[0015] (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.

[0016] In some embodiments of the present application, the lithium salt comprises one or a combination of two or more of lithium bis(fluorosulfonyl)imide, lithium bis(oxalatoborate), and lithium bis(trifluoromethylsulfonyl)imide.

[0017] In some embodiments of the present application, the sodium salt comprises one or a combination of two or more of sodium hexafluorophosphate, sodium hexafluoroarsenate, and sodium tetrafluoroborate.

[0018] In some embodiments of the present application, the electrolyte comprises an ether solvent;

[0019] In some embodiments of the present application, the ether solvent comprises one or a combination of two or more of dimethoxymethane (DMM), dimethoxyethane (DME), and diethoxyethane (DEE).

[0020] The second aspect of the present application provides a secondary battery, comprising the electrolyte described in the first aspect.

[0021] In some embodiments of the present application, the secondary battery further comprises:

[0022] Positive electrode sheet: comprising a positive electrode current collector and a positive electrode active material disposed on at least one side of the positive electrode current collector;

[0023] Negative electrode sheet: contains negative electrode current collector;

[0024] Isolation film;

[0025] The positive electrode active material comprises a sodium ion positive electrode active material and a lithium ion positive electrode active material, and a charging platform voltage of the sodium ion positive electrode active material in a battery is lower than a charging platform voltage of the lithium ion positive electrode active material in a battery.

[0026] In order to achieve the purpose of depositing lithium metal on the surface of sodium metal, the present application further controls 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, and utilizes high voltage charging to plate the lithium metal that is easy to form a stable SEI film as a protective layer on the surface of the sodium metal to isolate the continuous reaction between the sodium metal and the electrolyte, thereby improving the storage life of the secondary battery.

[0027] The secondary battery in the present application is charged at the charging platform voltage of the sodium ion positive electrode active material, and a sodium metal layer is first formed on the surface of the negative electrode current collector, and the sodium ions are repeatedly deposited and stripped on the surface of the negative electrode current collector, which can achieve the long cycle performance of the secondary battery. Continue to charge the secondary battery to above the charging platform voltage of the lithium ion positive electrode active material, and the lithium ions will be deposited on the surface of the sodium metal layer to form a lithium metal coating, which can react with the film-forming components in the electrolyte to generate a stable SEI film to isolate the contact between the negative electrode and the electrolyte, thereby reducing the probability of the reaction between the negative electrode and the electrolyte, so as to improve the stability of the lithium sodium metal negative electrode. Therefore, the secondary battery provided in the present application can improve the storage performance of the battery while improving the cycle performance of the secondary battery.

[0028] In some embodiments of the present application, the charging platform voltage of the sodium ion positive electrode active material in the battery is at least 0.1 V lower than the charging platform voltage of the lithium ion positive electrode active material in the battery.

[0029] In some embodiments of the present application, the charging platform voltage of the sodium ion positive electrode active material in the battery is 0.1V to 1V lower than the charging platform voltage of the lithium ion positive electrode active material in the battery.

[0030] In some embodiments of the present application, the charging platform voltage of the sodium ion positive electrode active material in the battery is 0.2V to 0.6V lower than the charging platform voltage of the lithium ion positive electrode active material in the battery.

[0031] In some embodiments of the present application, the charging platform voltage of the sodium ion positive electrode active material in the battery is 2.0V to 3.6V.

[0032] In some embodiments of the present application, the charging platform voltage of the sodium ion positive electrode active material in the battery is 3.2V to 3.4V.

[0033] In some embodiments of the present application, the sodium ion positive electrode active material comprises one or a combination of two or more of a polyanionic compound or a Prussian blue compound;

[0034] Preferably, the sodium ion positive electrode active material comprises one or a combination of two or more of Na3V2(PO4)3, Na2Fe[Fe(CN)6], Na2Mn[Fe(CN)6], and Na2Mn[Mn(CN)6].

[0035] In some embodiments of the present application, the charging platform voltage of the lithium ion positive electrode active material in the battery is 3.0V to 4.3V.

[0036] In some embodiments of the present application, the charging platform voltage of the lithium ion positive electrode active material in the battery is 3.4V to 4.0V.

[0037] In some embodiments of the present application, the lithium ion positive electrode active material comprises one or a combination of two or more 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 phosphate containing an olivine structure;

[0038] Preferably, the lithium ion positive electrode active material comprises LiFePO4, LiMn2O4, LiCoO2, LiNi 0.8 CO 0.1 Mn 0.1 One or a combination of two or more of O2.

[0039] In some embodiments of the present application, the secondary battery is charged at a 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;

[0040] and the secondary battery is charged at a 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;

[0041] Preferably, the thickness of the lithium metal plating layer is 0.5 μm to 3.0 μm.

[0042] In some embodiments of the present application, the negative electrode plate is a negative electrode current collector.

[0043] The third aspect of the present application provides an electrical device, comprising the electrolyte described in the first aspect or the secondary battery described in the second aspect.

[0044] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. DETAILED DESCRIPTION

[0045] Below, the embodiments of the secondary battery and the electrical device of the present application are specifically disclosed in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0046] The "range" disclosed in the present application is defined in the form of a lower limit and an upper limit, and a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of the particular range. The range defined in this way can be inclusive or exclusive of the end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a range. For example, if a range of 60 to 120 and 80 to 110 is listed for a particular parameter, it is understood that a range of 60 to 110 and 80 to 120 is also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4 and 5 are listed, the following ranges can all be expected: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4 and 2 to 5. In the present application, unless otherwise specified, the numerical range "a to b" represents an abbreviation of any real number combination between a and b, where a and b are both real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0047] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0048] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0049] If there is no special explanation, all steps of the present application can be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0050] If there is no special explanation, the "include" and "comprising" mentioned in this application are open-ended or closed-ended. For example, the "include" and "comprising" may mean that other components not listed may also be included or only the listed components may be included or only the listed components may be included.

[0051] If not specifically stated, in this application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": 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).

[0052] If not otherwise specified, in this application, the terms "first", "second", etc. are only used to distinguish different objects and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity, specific order or primary and secondary relationship of the indicated technical features.

[0053] Unless otherwise specified, in this application, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple sheets" refers to more than two sheets (including two sheets).

[0054] The negative active material of lithium-ion batteries contains carbonaceous materials, such as graphite, but the mass specific capacity of graphite is limited, and there is very little room for increasing the volume specific capacity, which seriously limits the further improvement of the weight energy density and volume energy density of lithium-ion batteries. With the current development of consumer electronics and electric vehicle technology, it is imperative to develop a battery system with higher energy density.

[0055] Sodium metal has high mass energy density and volume energy density, so it is often used as the negative electrode of metal batteries. In order to further obtain higher battery cell energy density, "negative electrode-free" metal batteries have also been studied by de-sodiumizing the positive electrode material and depositing it in situ on the negative electrode current collector. At the same time, the fact that there is no need to pre-press / coat / deposit highly active metals on the negative electrode side also greatly improves the manufacturing feasibility and safety of the battery cell. However, negative electrode-free sodium batteries require a higher overpotential for deposition on the surface of the negative electrode current collector, which can also easily lead to uneven metal deposition, aggravate side reactions with the electrolyte, greatly consume active sodium, and ultimately affect the cycle performance and storage life of the battery cell.

[0056] At present, the balance between film formation and non-film formation is often used to overcome the above defects. Specifically, a thin and dense SEI layer is generated on the negative electrode surface as much as possible, in order to meet the cycle life and increase the storage life as much as possible. However, these two methods have the following advantages and disadvantages:

[0057] 1. Choose an electrolyte system that is easy to form SEI film. During storage, SEI film can isolate the reaction between electrolyte and sodium metal, and obtain a longer storage life. However, during the cycle, the electrolyte will continue to react with the deposited fresh sodium metal to form new SEI, resulting in loss of sodium metal and electrolyte. In addition, SEI is a rate-controlled step, and sodium metal is more likely to grow dendrites at the defects of SEI film. Dendrite growth will form a larger specific surface area, and dendrites are easy to break during the cycle, ultimately resulting in greater capacity loss, which cannot meet the needs of long cycles.

[0058] 2. Choose an electrolyte system that is not easy to form SEI film. Although sodium metal can be deposited in bulk, reducing the specific surface area and significantly improving the cycle life, sodium metal without SEI protection will continue to react with the electrolyte and cause consumption, which cannot meet the long-term storage needs of negative electrode-free sodium metal secondary batteries.

[0059] In order to solve the above technical problems, the present application has obtained an electrolyte through experimental research. The electrolyte contains sodium salt and lithium salt, and the deposition overpotential of lithium metal in the electrolyte is higher than the deposition overpotential of sodium metal. The electrolyte provided by the present application has film-forming selectivity, wherein the deposition overpotential of lithium metal in the electrolyte is higher than the deposition overpotential of sodium metal, which is beneficial to the film-forming property of the lithium metal surface and the difficulty of film-forming property of the sodium metal surface. Specifically, sodium metal is not easy to grow SEI film in the electrolyte, and the deposition is good, which is beneficial to improving the cycle life of the secondary battery; lithium metal is easy to grow SEI film in the electrolyte. In the secondary battery, if lithium metal is further deposited on the surface of sodium metal and a stable SEI film is formed on the surface of lithium metal, then on the basis of improving the cycle life of the secondary battery, the storage property of the secondary battery can also be effectively improved.

[0060] In some embodiments, the present application provides a secondary battery, which, in addition to the above-mentioned electrolyte, also includes a positive electrode plate, a negative electrode plate and a separator, the positive electrode plate includes a positive electrode collector and a positive electrode active material arranged on at least one side of the positive electrode collector; the negative electrode plate includes a negative electrode collector; and the positive electrode active material includes a sodium ion positive electrode active material and a lithium ion positive electrode active material, and the charging platform voltage of the sodium ion positive electrode active material in the battery is lower than the charging platform voltage of the lithium ion positive electrode active material in the battery.

[0061] In order to achieve the purpose of depositing lithium metal on the surface of sodium metal, the present application further controls 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, and uses high voltage charging to plate the lithium metal that is easy to form a stable SEI film on the surface of the sodium metal as a protective layer to isolate the continuous reaction of the sodium metal and the electrolyte to improve the storage life of the secondary battery. Specifically, the secondary battery provided by the present application is charged at the charging platform voltage of the sodium ion positive electrode active material, and a sodium metal layer is first formed on the surface of the negative electrode current collector, and the sodium ions are repeatedly deposited and stripped on the surface of the negative electrode current collector, which can achieve the long cycle performance of the secondary battery. Continue to charge the secondary battery to above the charging platform voltage of the lithium ion positive electrode active material, and the lithium ions will be deposited on the surface of the sodium metal layer to form a lithium metal coating, which can react with the film-forming components in the electrolyte to generate a stable SEI film to isolate the contact between the negative electrode and the electrolyte, thereby reducing the probability of the reaction between the negative electrode and the electrolyte, so as to improve the stability of the lithium sodium metal negative electrode. Therefore, the secondary battery provided by the present application can improve the storage performance of the battery while improving the cycle performance of the secondary battery.

[0062] The secondary battery provided in the present application can be used as a power source for an electrical device, and can also be used as an energy storage unit for an electrical device, such as but not limited to laptop computers, pen-input computers, mobile computers, e-book players, portable phones, portable fax machines, portable copiers, portable printers, head-mounted stereo headphones, video recorders, LCD televisions, portable cleaners, portable CD players, mini CDs, transceivers, electronic notepads, calculators, memory cards, portable recorders, radios, backup power supplies, motors, cars, motorcycles, power-assisted bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, energy storage and capacitors, etc.

[0063] [Electrolyte]

[0064] According to some embodiments of the present application, the electrolyte is used to conduct sodium ions and lithium ions, and contains sodium salt and lithium salt as electrolyte salts, wherein the deposition overpotential of lithium metal in the electrolyte is higher than the deposition overpotential of sodium metal.

[0065] 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 to deposit. The deposition overpotential of each metal in the electrolyte can be obtained by direct measurement. For example, a charge-discharge machine of the model Xinwei is used to test the overpotential of a Na-Na symmetrical battery at 1 mA / cm -2The current density was set at 1 h for charging and 1 h for discharging. Each charge and discharge was counted as one cycle, and 100 cycles were used as the cutoff condition. The stability of the electrolyte system was evaluated by potential fluctuation, and finally a suitable electrolyte salt was explored.

[0066] In the present application, due to the low electrochemical potential of lithium, in the same electrolyte system, lithium is more likely to react with the electrolyte to form SEI than sodium.

[0067] The electrolyte provided in the present application has film-forming selectivity, wherein the deposition overpotential of lithium metal in the electrolyte is higher than the deposition overpotential of sodium metal, which is beneficial to the film-forming property of the lithium metal surface and difficult to form a film on the sodium metal surface. Specifically, sodium metal is not easy to grow SEI film in the electrolyte, and the deposition is good, which is beneficial to improve the cycle life of the secondary battery; lithium metal is easy to grow SEI film in the electrolyte, and in the secondary battery, if lithium metal is further deposited on the surface of sodium metal and a stable SEI film is formed on the surface of lithium metal, the lithium metal and SEI film improve the storage life of the secondary battery by isolating the reaction between sodium metal and the electrolyte.

[0068] According to some embodiments of the present application, the deposition overpotential of lithium metal in the electrolyte is more than 20 mV higher than the deposition overpotential of sodium metal.

[0069] In the present application, selecting a deposition overpotential of lithium metal that is 20 mV higher than a deposition overpotential of sodium metal is beneficial for screening out a suitable electrolyte salt to improve the cycle life and storage property of the secondary battery.

[0070] According to some embodiments of the present application, the deposition overpotential of lithium metal in the electrolyte is greater than 30 mV.

[0071] The electrolyte in the present application needs to meet the moldability of lithium metal, and the deposition overpotential of lithium metal in the electrolyte is greater than 30mV, which is conducive to meeting the moldability of lithium metal and making it easy to screen suitable lithium salts. At the same time, due to the limitations of various factors such as the type, concentration, and solvent composition of the lithium salt, the deposition overpotential of the lithium salt in the electrolyte cannot change indefinitely.

[0072] According to some embodiments of the present application, the deposition overpotential of lithium metal in the electrolyte is 32 mV to 70 mV.

[0073] In the specific embodiments of the present application, the types of lithium salts with a deposition overpotential of lithium metal in an electrolyte of 32 mV to 70 mV are mainly explored, which is only for the purpose of explaining the lithium salts, rather than limiting the lithium salts.

[0074] According to some embodiments of the present application, the deposition overpotential of sodium metal in the electrolyte is less than 20 mV.

[0075] The electrolyte in the present application needs to meet the requirement of sodium metal being difficult to form a mold, and the deposition overpotential of sodium metal in the electrolyte is less than 20mV, which is conducive to meeting the requirement of sodium metal being difficult to form a mold and making it easy to screen out suitable sodium salts. At the same time, the deposition overpotential of sodium metal in the electrolyte is not equal to zero.

[0076] According to some embodiments of the present application, the deposition overpotential of sodium metal in the electrolyte is 5 mV to 18 mV.

[0077] In the specific embodiments of the present application, the types of sodium salts with a deposition overpotential of sodium metal in an electrolyte of 5 mV to 18 mV are mainly explored, which is only for the purpose of explaining the sodium salts, rather than limiting the sodium salts.

[0078] According to some embodiments of the present application, the concentration of the lithium salt is lower than that of the sodium salt.

[0079] In this application, according to the above description, the sodium ion positive electrode active material is used to realize the long cycle process of the secondary battery, and the lithium ion positive electrode active material is used to improve the storage property of the secondary battery. In order to improve the comprehensive performance of the secondary battery of this application, in the positive electrode active material, the sodium ion positive electrode active material is used as the main material, and the lithium ion positive electrode active material assists the sodium ion positive electrode active material to play the role of isolating the side reactions caused by the contact between the sodium metal and the electrolyte. Judging from the respective usage amounts, under ideal conditions, the usage amount of the sodium ion positive electrode active material is more than that of the lithium ion positive electrode active material, and the corresponding lithium salt concentration in the electrolyte is lower than the sodium salt concentration.

[0080] According to some embodiments of the present application, the concentration of the lithium salt is not less than 0.1 mol / L.

[0081] According to some embodiments of the present application, the concentration of the lithium salt is 0.1 mol / L to 0.2 mol / L.

[0082] According to some embodiments of the present application, the concentration of the sodium salt is not less than 0.8 mol / L.

[0083] According to some embodiments of the present application, the concentration of the sodium salt is 0.8 mol / L to 1.5 mol / L.

[0084] In the present application, the concentrations of lithium salt and sodium salt in the electrolyte are selected to meet the above numerical range, which is beneficial to the matching between the electrolyte and the positive electrode active material.

[0085] According to some embodiments of the present application, the lithium salt includes one or a combination of two or more of lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(oxalatoborate) (LiBOB), and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI).

[0086] The type of lithium salt selected in the present application satisfies its deposition overpotential in the electrolyte, which is conducive to the formation of the SEI film.

[0087] According to some embodiments of the present application, the sodium salt comprises one or a combination of two or more of sodium hexafluorophosphate (NaPF6), sodium hexafluoroarsenate (NaAsF6), and sodium tetrafluoroborate.

[0088] The electrolyte system selected in the present application satisfies its deposition overpotential in the electrolyte, making it difficult to form a SEI film.

[0089] According to some embodiments of the present application, the electrolyte of the present application contains a solvent, and the solvent contains an ether solvent.

[0090] In the present application, during the cycle process, metallic sodium is deposited on the surface of the negative electrode current collector to form a sodium metal layer, and the stability of the sodium metal layer in ether solvents is higher than that in other solvents such as carbonate solvents.

[0091] According to some embodiments of the present application, the ether solvent includes one or a combination of two or more of dimethoxymethane (DMM), dimethoxyethane (DME), and diethoxyethane (DEE).

[0092] The type of ether solvent selected in the present application is beneficial to improving the solvation structure of sodium ions and lithium ions and further affecting the deposition morphology of the negative electrode to ultimately improve the cycle stability of the secondary battery.

[0093] In the present application, the negative electrode plate is the negative electrode current collector.

[0094] The negative electrode sheet of the present application is a negative electrode current collector, and then a negative electrode metal-free secondary battery is prepared. The negative electrode metal-free secondary battery adopts the lithium-sodium potential difference. During the charging process, the sodium metal is reversibly deposited and stripped at a low potential, with low loss and long cycle. When it is desired to store, a layer of lithium metal is plated on the surface of the sodium metal by charging at a high potential. The lithium metal has high reactivity and is easy to generate SEI, which can isolate the side reaction of the sodium metal negative electrode and the electrolyte, thereby achieving a long storage life.

[0095] The type of ether solvent selected in the present application is beneficial to improving the solvation structure of sodium ions and lithium ions and further affecting the deposition morphology of the negative electrode to ultimately improve the cycle stability of the secondary battery.

[0096] Secondary battery

[0097] According to some embodiments of the present application, the present application provides a secondary battery, which comprises a positive electrode plate, a negative electrode plate, an isolation membrane located between the positive electrode plate and the negative electrode plate, and an electrolyte, wherein the positive electrode plate comprises a positive electrode collector and a positive electrode active material arranged on at least one side surface of the positive electrode collector, and the negative electrode plate comprises a negative electrode collector; the positive electrode active material comprises a sodium ion positive electrode active material and a lithium ion positive electrode active material, and the charging platform voltage of the sodium ion positive electrode active material in the battery is lower than the charging platform voltage of the lithium ion positive electrode active material in the battery.

[0098] In the present application, the method for determining the charging platform voltage of the positive electrode material in the battery includes any conventional form in the art, such as a direct measurement method. Specifically, a sodium ion positive electrode active material or a lithium ion positive electrode active material is prepared into a button battery, and the button battery is charged and tested. A charging characteristic curve is plotted with capacity as the horizontal axis and voltage as the vertical axis, and the charging platform voltage is determined according to the charging characteristic curve. In this application, the same test conditions and the same method are used to determine the charging platform voltage of the sodium ion positive electrode active material and the lithium ion positive electrode active material.

[0099] In order to achieve the purpose of depositing lithium metal on the surface of sodium metal, the present application further controls 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, and uses high voltage charging to plate the lithium metal that is easy to form a stable SEI film on the surface of the sodium metal as a protective layer to isolate the continuous reaction between the sodium metal and the electrolyte to improve the storage life of the secondary battery. At the same time, the electrolyte in the secondary battery is not easy to form a film on the sodium metal, and the sodium metal is not easy to grow an SEI film in the electrolyte, and good deposition is conducive to improving the cycle life of the secondary battery.

[0100] The secondary battery in the present application is charged at the charging platform voltage of the sodium ion positive electrode active material, and a sodium metal layer is first formed on the surface of the negative electrode current collector, and the sodium ions are repeatedly deposited and stripped on the surface of the negative electrode current collector, which can achieve the long cycle performance of the secondary battery. Continue to charge the secondary battery to above the charging platform voltage of the lithium ion positive electrode active material, and the lithium ions will be deposited on the surface of the sodium metal layer to form a lithium metal coating, which can react with the film-forming components in the electrolyte to generate a stable SEI film to isolate the contact between the negative electrode and the electrolyte, thereby reducing the probability of the reaction between the negative electrode and the electrolyte, so as to improve the stability of the lithium sodium metal negative electrode. Therefore, the secondary battery provided in the present application can improve the storage performance of the battery while improving the cycle performance of the secondary battery.

[0101] [Cathode material]

[0102] According to some embodiments of the present application, a charging platform voltage of a sodium ion positive electrode active material in a battery is at least 0.1 V lower than a charging platform voltage of a lithium ion positive electrode active material in a battery.

[0103] In the present application, 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 is controlled, and high-voltage charging is used to plate lithium metal, which is easy to form a stable SEI film, on the surface of the sodium metal as a protective layer to isolate the continuous reaction between the sodium metal and the electrolyte, thereby improving the storage life of the secondary battery.

[0104] According to some embodiments of the present application, the charging platform voltage of the sodium ion positive electrode active material in the battery is 0.1V to 1V lower than the charging platform voltage of the lithium ion positive electrode active material in the battery.

[0105] In the present 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, higher selectivity is required for the positive electrode active material, which is not easy to achieve based on the existing types of positive electrode active materials.

[0106] According to some embodiments of the present application, the charging platform voltage of the sodium ion positive electrode active material in the battery is 0.2V to 0.6V lower than the charging platform voltage of the lithium ion positive electrode active material in the battery.

[0107] In the present 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 selected to be 0.2V to 0.6V, which not only improves the cyclability and storage performance of the secondary battery, but also makes the selection of the positive electrode active material relatively easier to achieve, which is conducive to meeting the needs of industrial production.

[0108] According to some embodiments of the present application, the charging platform voltage of the sodium ion positive electrode active material in the battery is 2.0V to 3.6V.

[0109] According to some embodiments of the present application, the charging platform voltage of the sodium ion positive electrode active material in the battery is 3.2V to 3.4V.

[0110] In the present application, the specific test method for the charging platform voltage of the sodium ion positive electrode active material comprises: preparing the sodium ion positive electrode active material into a button battery, and conducting a 1 mA / cm 2 The charging test is carried out at a current density of 2.0V~4.3V, and the cut-off voltage is 2.0V~4.3V. The charging characteristic curve is plotted with capacity as the horizontal axis and voltage as the vertical axis. The charging platform voltage is determined according to 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 through the peak value of dQ / dV.

[0111] For positive electrode active materials with obvious charging platform voltage, the charging platform voltage is relatively easy to measure or calculate. For positive electrode active materials with less obvious charging platform voltage, the value of the charging platform voltage can be replaced by the average voltage, which is the effective area of ​​the charging characteristic curve (battery charging energy) divided by the capacity.

[0112] According to some embodiments of the present application, the sodium ion positive electrode active material includes any one or a combination of two or more of a polyanion compound or a Prussian blue compound.

[0113] In the present application, the polyanionic compound includes 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 compound is 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.

[0114] According to some embodiments of the present application, the sodium ion positive electrode active material includes one or a combination of two or more of Na3V2(PO4)3, Na2Fe[Fe(CN)6], Na2Mn[Fe(CN)6], and Na2Mn[Mn(CN)6]. The present application selects several specific sodium ion positive electrode active materials in the specific embodiments, mainly to illustrate that the difference between the charging platform voltages can solve the technical problem of the present application.

[0115] According to some embodiments of the present application, the charging platform voltage of the lithium ion positive electrode active material in the battery is 3.0V to 4.3V.

[0116] According to some embodiments of the present application, the charging platform voltage of the lithium-ion positive electrode active material in the battery is 3.4V to 4.0V.

[0117] In the present application, the specific test method for the charging platform voltage of the lithium ion positive electrode active material comprises: preparing the lithium ion positive electrode active material into a button battery, and conducting a 1 mA / cm 2 The charging test is carried out with a current density of 2.0V to 4.3V, and the cut-off voltage is 2.0V to 4.3V; the charging characteristic curve is drawn with capacity as the horizontal axis and voltage as the vertical axis, and the charging platform voltage is determined according to 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 by the peak value of dQ / dV.

[0118] For positive electrode active materials with 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 less obvious charging platform voltage, the value of the charging platform voltage can be replaced by the average voltage, which is the effective area of ​​the charging characteristic curve (battery charging energy) divided by the capacity.

[0119] According to some embodiments of the present application, the lithium ion positive electrode active material includes 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 phosphate containing an olivine structure. Exemplary positive electrode active materials include but are not limited to LiCoO2, LiNiO2, LiMnO2, LiMn2O4, 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.

[0120] According to some embodiments of the present application, the structural formula of the lithium-containing phosphate containing 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 a combination of two or more transition metal elements or non-transition metal elements other than Fe and Mn, and M preferably contains one or a combination of two or more of Cr, Mg, Ti, Al, Zn, W, Nb, and Zr.

[0121] According to some embodiments of the present application, the lithium ion positive electrode active material includes LiFePO4, LiMn2O4, LiCoO2, LiNi 0.8 CO 0.1 Mn 0.1One or a combination of two or more of O2. This application selects several specific sodium ion positive electrode active materials in the specific embodiments, mainly to illustrate that the difference between the charging platform voltages can solve the technical problem of this application.

[0122] According to some embodiments of the present 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 using 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.

[0123] In the present application, sodium ions are deposited on the surface of the negative electrode current collector. Since the voltage during deposition is the charging platform voltage of the sodium ion positive electrode active material, it is beneficial to the uniformity of the deposition of sodium metal to achieve the reversible charging and discharging process. The amount of sodium ion positive electrode active material affects the charging and discharging process. The present application does not limit the specific amount of sodium ion positive electrode active material. At the same time, the secondary battery continues to be charged with the charging platform 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. The lithium metal coating contains a stable SEI film to improve the stability of the lithium metal coating. The lithium metal coating improves the storage life of the secondary battery by isolating the reaction between the sodium metal and the electrolyte.

[0124] According to some embodiments of the present application, the thickness of the lithium metal plating layer is 0.5 μm to 3.0 μm.

[0125] In the present application, the thickness of the lithium metal coating can be obtained by taking pictures and calculating, for example, disassembling the secondary battery, taking pictures of the positive electrode material, and then directly measuring. 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. The thickness cross-section sample can be prepared by liquid nitrogen brittle fracture or by using an ion beam polisher, and semi-quantitative analysis can be performed by the energy spectrum analysis system equipped with the SEM.

[0126] 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 charging and discharging process, which may eventually affect the cyclability of the secondary battery. If the thickness of the lithium metal coating is relatively thin, it cannot effectively isolate the sodium metal from the electrolyte, which may affect the storage performance of the secondary battery. The thickness of the lithium metal coating is determined by the amount of lithium ion positive electrode active material used. Conversely, the amount of lithium ion positive electrode active material used can be calculated based on the thickness of the lithium metal coating. The specific calculation method may include: converting lithium metal to 0.206mAh / (μm·cm 2 ), so if a lithium metal layer with a thickness of 0.1 to 8 μm is required, the positive electrode surface capacity should meet the requirements of 0.0206 to 1.648 mAh / cm2 According to the surface capacity range and the gram capacity data of lithium-ion positive electrode active materials m mAh / g, the mass of lithium-ion positive electrode active materials per unit area can be obtained (0.0206~1.648) ÷ mg / cm 2 In the present application, when the thickness of the lithium metal plating layer is selected to be 0.5 μm to 3.0 μm, the cyclability and storage performance of the secondary battery can be relatively improved.

[0127] The thickness of the lithium metal plating layer of the present application may be one of 0.5 μm, 0.8 μm, 1.0 μm, 1.2 μm, 1.5 μm, 1.7 μm, 2.0 μm, 2.5 μm, 3.0 μm or any value within the range.

[0128] According to some embodiments of the present application, a binder and / or a conductive agent may be added to the positive electrode active material, and the types of the binder and the conductive agent are not limited, and those skilled in the art may 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.

[0129] According to some embodiments of the present application, the positive electrode current collector may be a metal foil or a composite current collector, wherein the metal foil may be an aluminum foil, and the composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material such as aluminum, aluminum 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.

[0130] [Negative electrode]

[0131] According to some embodiments of the present application, the negative electrode pole piece is a negative electrode current collector, which can be a negative electrode current collector substrate, or a negative electrode current collector substrate and a modified layer disposed on the surface of at least one side of the negative electrode current collector substrate. At the same time, the formation method of the modified layer on the surface of the negative electrode current collector substrate includes any conventional form in the art, such as deposition, coating, etc.

[0132] In the present application, the modification layer is used to reduce the deposition potential of the metal, which can induce the metal ions to deposit evenly and uniformly on the negative electrode surface, forming a compact SEI film, inhibiting the formation of metal dendrites, and having a direct effect on improving the cycle stability of the secondary battery. And the material of the modification layer includes but is not limited to any one or a combination of two or more of inorganic metal elements, inorganic oxides, and organic substances. For example, the inorganic metal element includes any one or a combination of two or more of platinum, aluminum, magnesium, zinc, gold, silver, and tin. Inorganic oxides include any one or a combination of two or more of aluminum oxide, silicon dioxide, and titanium dioxide. Organic substances include polydimethylsiloxane, graphene oxide, silicon dioxide-polymethyl methacrylate, etc.

[0133] In the present application, the negative electrode current collector matrix may be a metal foil or a composite current collector, the metal foil may be a copper foil, and the composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may 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.

[0134] [Isolation film]

[0135] In the present application, the separator between the positive electrode and the negative electrode can be any known porous structure separator with good chemical stability and mechanical stability. For example, the material of the separator may include but is not limited to one or a combination of two or more of the following groups: glass fiber, non-woven fabric, polyethylene, polypropylene, polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film without special restrictions. When the separator is a multi-layer composite film, the materials of each layer can be the same or different without special restrictions.

[0136] To make the purpose, technical solution and advantages of this application clearer, the sodium battery separator and sodium replenishment method protected by this application are described in detail below in conjunction with specific examples. The chemical substances, reagents, etc. used in the following examples are all conventional models and manufacturers in the field.

[0137] [Preparation of positive electrode sheet]

[0138] Example 1

[0139] A method for preparing a positive electrode sheet is provided, comprising the following preparation process:

[0140] LiFePO4 (10wt%) and Na2Fe[Fe(CN)6] (85.7wt%), conductive agent acetylene black, and binder PVDF were selected and mixed in N-methylpyrrolidone (NMP) solvent in a mass ratio of 95.7:2.3:2 to prepare positive electrode slurry. The positive electrode slurry was coated on the surface of aluminum foil by an extrusion coater and dried. The coated electrode was then pressed by a cold press to obtain a positive electrode with a surface capacity of 3mAh / cm 2 .

[0141] Determination of related parameters of positive electrode:

[0142] (1) Charging platform voltage test of sodium ion positive electrode active materials: Na2Fe[Fe(CN)6] was prepared into a button cell and the button cell was charged at 1 mA / cm 2 The charging test is carried out with a current density of 2.0V~4.3V, and the cut-off voltage is 2.0V~4.3V. The charging characteristic curve is drawn with capacity as the horizontal axis and voltage as the vertical axis. The charging platform voltage is determined according to the charging characteristic curve, and the charging platform voltage is obtained by the peak value of dQ / dV.

[0143] (2) Charging platform voltage test of lithium-ion positive electrode active materials: LiFePO4 was prepared into a button cell and the button cell was charged at 1 mA / cm 2 The charging test is carried out with a current density of 2.0V~4.3V, and the cut-off voltage is 2.0V~4.3V. The charging characteristic curve is drawn with capacity as the horizontal axis and voltage as the vertical axis. The charging platform voltage is determined according to the charging characteristic curve, and the charging platform voltage is obtained by the peak value of dQ / dV.

[0144] Among them, the relevant parameters of the positive electrode sheets provided in Examples 1 to 19 and Comparative Example 1 are shown in Table 1.

[0145] Table 1 Parameters of positive electrode

[0146]

[0147]

[0148] In this application, for lithium ion positive electrode active materials, olivine-structured lithium-containing 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. This application mainly discusses LiFePO4, LiMn2O4, LiCoO2, LiNi in Table 1. 0.8 CO 0.1 Mn 0.1O2 is used as a lithium ion positive electrode active material and is compounded with a sodium ion positive electrode active material 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 within the protection scope of this application.

[0149] Similarly, in the present 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, the present 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 and 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 the present application.

[0150] [Preparation of negative electrode sheet]

[0151] A copper foil with a thickness of 8 μm was selected as the negative electrode current collector to prepare the negative electrode.

[0152] [Preparation of electrolyte]

[0153] The electrolyte in the embodiment of the present application includes an ether solvent and a lithium salt and a sodium salt. The ether solvent includes one or more of dimethoxymethane, dimethoxyethane, and diethoxyethane. The lithium salt includes one or more of lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(oxalatoborate) (LiBOB), and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI). The sodium salt includes one or more of sodium hexafluorophosphate (NaPF6), sodium hexafluoroarsenate (NaAsF6), and sodium tetrafluoroborate. And the lithium salt concentration is 0.1mol / L to 0.2mol / L; the sodium salt concentration is 0.8mol / L to 1.5mol / L.

[0154] Determination of electrolyte related parameters:

[0155] (3) Deposition overpotential test in electrolyte: The overpotential test of Li-Li symmetrical battery was carried out using Xinwei charge and discharge machine (model); -2 The current density is 1h for charging and 1h for discharging. Each charge and discharge is counted as one cycle. 100 cycles are used as the cutoff condition. The Et curve is made and the stability of the system is evaluated by the potential fluctuation. The overpotential is about 50mV and above for film-forming passivation system.

[0156] (4) Deposition overpotential test in electrolyte: The overpotential test of Na-Na symmetrical battery was carried out using Xinwei charge-discharge machine (model);-2 The current density is 1 hour for charging and 1 hour for discharging. Each charge and discharge is counted as one cycle. 100 cycles are used as the cutoff condition. The Et curve is made and the stability of the system is evaluated by the potential fluctuation. The overpotential is about 5mV, which is a system that is not easy to form a film.

[0157] Among them, the relevant parameters of the electrolyte provided in Examples 1 to 19 and Comparative Example 1 are shown in Table 2.

[0158] Table 2 Parameters of electrolyte

[0159]

[0160]

[0161]

[0162] In Table 2, the present application further explores the film-forming selectivity of the electrolyte based on different types and concentrations of electrolyte salts.

[0163] [Isolation film]

[0164] Choose polypropylene film.

[0165] [Preparation of Secondary Battery]

[0166] According to some embodiments of the present application, the prepared negative electrode plate, the isolation membrane and the positive electrode plate are stacked in order, the isolation membrane is placed between the positive and negative electrode plates to play an isolating role, and the bare battery cells are wound up, the bare battery cells are baked at 100°C to remove water, and then the electrolyte is injected and sealed, and then the secondary battery is obtained through processes such as standing, hot and cold pressing, formation, secondary liquid injection, aging, shaping, and capacity testing.

[0167] Comparative Example 1

[0168] A secondary battery is provided which is different from Example 1 in that the positive electrode active material contains only Na2Fe[Fe(CN)6].

[0169] [Performance test of secondary batteries]

[0170] (5) Secondary battery cycle performance test:

[0171] The secondary battery was charged at 25°C at 1.5 mA cm -2 The battery was charged at a constant current of 4.3 V and then at a constant voltage of 4.3 V until the current dropped to 0.3 mA cm -2 , and then at 1.5 mA·cm -2 The battery is discharged at a constant current to 2.8 V to obtain the first cycle discharge specific capacity (Cd1); the charge and discharge are repeated until the capacity decays to 80% of the corresponding number of cycles.

[0172] (6) Battery storage performance test:

[0173] The secondary battery is charged and discharged once at a low current of 0.1C, and the discharge capacity C1 is recorded. The battery cell is fully charged and placed in a 25°C environment, and the battery cell voltage change is monitored. A low current charge and discharge is performed every 20 hours and Cn is recorded. When the measured irreversible capacity decays to 80% of the initial capacity, the storage time at this time is recorded as the storage life.

[0174] The specific performance of the secondary battery is shown in Table 3;

[0175] Table 3 Battery performance list

[0176]

[0177]

[0178] This application explores the effects of different types of positive electrode active materials on the performance of secondary batteries in Examples 1 to 7, and explores the effects of different electrolytes on the performance of secondary batteries in Examples 8 to 17. At the same time, the examples of this application explore that the thickness of the lithium metal plating layer is 1 μm, and the content of each component in the positive electrode active material can be adjusted to control the thickness of the lithium metal plating layer to be between 0.5 μm and 3.0 μm.

[0179] It can be seen from Table 3 that the secondary battery designed in the present application can improve the storage performance of the battery while improving the cycle performance of the secondary battery.

[0180] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them; although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application, and they should all be included in the scope of the claims and specification of the present application. In particular, as long as there is no structural conflict, the various technical features mentioned in the various embodiments can be combined in any way. The present application is not limited to the specific embodiments disclosed herein, but includes all technical solutions that fall within the scope of the claims.

Claims

1. An electrolyte, characterized in that: The electrolyte contains sodium salt and lithium salt, and the deposition overpotential of lithium metal in the electrolyte is higher than the deposition overpotential of sodium metal.

2. The electrolyte according to claim 1, characterized in that: The deposition overpotential of lithium metal in the electrolyte is higher than the deposition overpotential of sodium metal by more than 20 mV.

3. The electrolyte according to any one of claims 1 to 2, characterized in that: The electrolyte satisfies one or a combination of the following: (1.1) The deposition overpotential of lithium metal in the electrolyte is greater than 30 mV; preferably 32 mV to 70 mV; (1.2) The deposition overpotential of sodium metal in the electrolyte is less than 20 mV; preferably 5 mV to 18 mV.

4. The electrolyte according to any one of claims 1 to 2, characterized in that: The concentration of the lithium salt is lower than the concentration of the sodium salt.

5. The electrolyte according to any one of claims 1 to 2, characterized in that: 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.

6. The electrolyte according to any one of claims 1 to 5, characterized in that: The lithium salt comprises one or a combination of two or more of lithium bis(fluorosulfonyl)imide, lithium bis(oxalatoborate), and lithium bis(trifluoromethylsulfonyl)imide.

7. The electrolyte according to any one of claims 1 to 5, characterized in that: The sodium salt comprises one or a combination of two or more of sodium hexafluorophosphate, sodium hexafluoroarsenate, and sodium tetrafluoroborate.

8. The electrolyte according to any one of claims 1 to 2, characterized in that: The electrolyte contains an ether solvent; Preferably, the ether solvent comprises one or a combination of two or more of dimethoxymethane, dimethoxyethane and diethoxyethane.

9. A secondary battery, characterized in that: The electrolyte comprising the electrolyte according to any one of claims 1 to 8.

10. The secondary battery according to claim 9, characterized in that: Also includes: Positive electrode sheet: comprising a positive electrode current collector and a positive electrode active material disposed on at least one side of the positive electrode current collector; Negative electrode sheet: contains negative electrode current collector; Isolation film; The positive electrode active material comprises a sodium ion positive electrode active material and a lithium ion positive electrode active material, and a charging platform voltage of the sodium ion positive electrode active material in a battery is lower than a charging platform voltage of the lithium ion positive electrode active material in a battery.

11. The secondary battery according to any one of claims 9 to 10, characterized in that: The charging platform voltage of the sodium ion positive electrode active material in the battery is at least 0.1V lower than the charging platform voltage of the lithium ion positive electrode active material in the battery, preferably 0.1V to 1V, and more preferably 0.2V to 0.6V.

12. The secondary battery according to any one of claims 9 to 10, characterized in that: The charging platform voltage of the sodium ion positive electrode active material in the battery is 2.0V to 3.6V, preferably 3.2V to 3.4V.

13. The secondary battery according to any one of claims 9 to 10, characterized in that: The sodium ion positive electrode active material comprises one or a combination of two or more of a polyanion compound or a Prussian blue compound; Preferably, the sodium ion positive electrode active material comprises one or a combination of two or more of Na3V2(PO4)3, Na2Fe[Fe(CN)6], Na2Mn[Fe(CN)6], and Na2Mn[Mn(CN)6].

14. The secondary battery according to any one of claims 9 to 10, characterized in that: The charging platform voltage of the lithium ion positive electrode active material in the battery is 3.0V to 4.3V, preferably 3.4V to 4.0V.

15. The secondary battery according to any one of claims 9 to 10, characterized in that: The lithium ion positive electrode active material comprises one or a combination of two or more 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 phosphate containing an olivine structure; Preferably, the lithium ion positive electrode active material comprises LiFePO4, LiMn2O4, LiCoO2, LiNi 0.8 CO 0.1 Mn 0.1 One or a combination of two or more of O2.

16. The secondary battery according to any one of claims 9 to 10, characterized in that: The secondary battery is charged at a 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; and the secondary battery is charged at a 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 plating layer is 0.5 μm to 3.0 μm.

17. The secondary battery according to any one of claims 9 to 10, characterized in that: The negative electrode plate is a negative electrode current collector.

18. An electrical device, characterized in that: A secondary battery comprising the electrolyte solution according to any one of claims 1 to 8 or any one of claims 9 to 17.

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