Negative electrode sheet, method for producing negative electrode sheet, secondary battery, battery module, battery pack, and electric device

Abstract

The application provides a negative electrode sheet, a preparation method of the negative electrode sheet, a secondary battery, a battery module, a battery pack and a power utilization device. The negative electrode sheet comprises a lithium metal layer and a composite protective film on at least one surface of the lithium metal layer, wherein the composite protective film comprises a precipitated salt film layer and an oxide film layer, and at least a part of the oxide film layer is distributed in the precipitated salt film layer. The composite protective film in the negative electrode sheet provided by the application can achieve the purpose of protecting the lithium metal in the charging and discharging cycle process, and can induce uniform deposition of the lithium metal in the charging and discharging cycle process, reduce the growth of lithium dendrites and the generation of dead lithium, thereby being beneficial to improving the capacity, cycle performance and safety performance of the secondary battery.

Description

Technical Field

[0001] This application belongs to the field of electrochemical technology, specifically relating to a negative electrode sheet, a method for preparing the negative electrode sheet, a secondary battery, a battery module, a battery pack, and an electrical device. Background Technology

[0002] Lithium-ion batteries, as a representative of rechargeable batteries, are widely used in all aspects of modern life due to their advantages such as no memory effect, long cycle life, and environmental friendliness. In recent years, rechargeable batteries have seen rapid development in the fields of new energy vehicles and large-scale energy storage. Among the existing anode materials for rechargeable batteries, lithium metal anodes have become the best choice for next-generation high-energy-density rechargeable battery anode materials due to their high theoretical specific capacity (3860 mAh / g). However, due to their high reactivity, lithium metal anodes are prone to reacting with environmental media in rechargeable batteries, leading to a deterioration in the cycle performance of the batteries, which greatly limits their large-scale application in rechargeable batteries. Summary of the Invention

[0003] The purpose of this application is to provide a negative electrode sheet, a negative electrode sheet for use in a secondary battery, a secondary battery, and an electrical device, with the aim of improving the cycle performance and safety performance of the secondary battery.

[0004] To achieve the above-mentioned objectives, the first aspect of this application provides a negative electrode sheet, comprising: a lithium metal layer, and a composite protective film located on at least one surface of the lithium metal layer, wherein the composite protective film comprises a precipitated salt film layer and an oxide film layer, and at least a portion of the oxide film layer is distributed in the precipitated salt film layer.

[0005] The composite protective film formed by the precipitated salt film layer and the oxide film layer in this application is very dense. In the secondary battery, it can isolate the organic solvent in the electrolyte and the contact between lithium salt anions and lithium metal, reduce the irreversible loss of the electrolyte, and thus achieve the purpose of protecting lithium metal during charge and discharge cycles.

[0006] In any embodiment of this application, the precipitated salt film layer includes (M1). x1 (PO4) y1 (M1) x2 (CO3) y2 Or a combination thereof, wherein M1 is at least one of Li, Mn, Mg, Ba, Ca and Sr, and x1, y1 and x2, y2 make the net charge of the compound zero.

[0007] The precipitated salt film mainly consists of inorganic phosphates and carbonates, which have good lithium-ion conductivity and can isolate electrons. Therefore, it can provide a migration channel for lithium ions to induce uniform deposition of lithium ions, thereby reducing the formation of lithium dendrites and improving the safety performance of secondary batteries.

[0008] In any embodiment of this application, the oxide film layer includes (M1). m O n Wherein, M1 is Li, Mn, Mg, Ba, Ca, Sr or a combination thereof, and m and n make the net charge of the metal oxide zero.

[0009] In this application, the oxide film layer and the precipitated salt film layer are combined / mixed together to form a composite protective film for the lithium metal layer. This composite protective film is a solid electrolyte layer with high mechanical strength, which can play a strong role in isolating and protecting the lithium metal layer. At the same time, it can also induce uniform deposition of lithium metal during charge and discharge cycles, inhibit the growth of lithium dendrites, and improve the cycle performance and safety performance of the secondary battery.

[0010] In any embodiment of this application, the oxide film layer is located on the side of the composite protective film closer to the lithium metal layer, and the precipitated salt film layer is located on the side of the composite protective film farther from the lithium metal layer.

[0011] In any embodiment of this application, the thickness of the composite protective film is from 1 μm to 25 μm, and optionally from 3 μm to 20 μm.

[0012] In any embodiment of this application, the thickness of the oxide film is 50 nm to 200 nm, and optionally 70 nm to 160 nm.

[0013] When the thickness of the oxide film and the composite protective film is within a suitable range, it can provide a good migration channel for lithium ions, enabling them to have good conductivity in the composite protective film, thereby reducing interfacial impedance and inducing uniform deposition of lithium metal, reducing or avoiding the growth of lithium dendrites; it can also effectively isolate the contact between the organic solvent in the electrolyte and lithium salt anions and lithium metal, reducing irreversible losses of the electrolyte, thus achieving the purpose of protecting lithium metal during charge and discharge cycles.

[0014] In any embodiment of this application, the passivation range of the composite protective film is 110mV to 420mV.

[0015] The passivation range of the composite protective film is within the above range, indicating that the composite protective film in this application has good high voltage resistance and will not easily decompose even under high voltage. This is because it can provide long-lasting protection for the lithium metal layer, which is beneficial to improving the cycle performance of the secondary battery.

[0016] A second aspect of this application provides a method for preparing a negative electrode sheet, comprising: a film-forming treatment step, including reacting a lithium metal layer with a film-forming solution to form a composite protective film to obtain a negative electrode sheet, wherein the composite protective film comprises a precipitated salt film layer and an oxide film layer, at least a portion of the oxide film layer being distributed in the precipitated salt film layer; the film-forming solution comprises precipitated salt cations, precipitated salt anions, oxidizing oxyacid salts, complexing agents, and catalysts dissolved in an inert solvent.

[0017] By using an aqueous solvent that readily reacts with lithium metal and preparing a film-forming solution based on the deposition film reaction mechanism, an inorganic salt composite protective film with controllable thickness and high voltage resistance can be formed on the surface of the lithium metal layer through the reaction of the film-forming solution with the lithium metal layer. This composite protective film can effectively isolate the contact between the organic solvent in the electrolyte and the lithium salt anions and the lithium metal, reduce the irreversible loss of the electrolyte, and thus achieve the purpose of protecting the lithium metal layer during charge and discharge cycles.

[0018] In any embodiment of this application, the preparation method further includes a film-forming solution providing step, which includes: dissolving a first film-forming salt containing the precipitated salt cation, the complexing agent, and the catalyst in a first inert solvent to obtain a first solution; dissolving a second film-forming salt containing the precipitated salt anion and the oxidizing oxyacid salt in a second inert solvent to obtain a second solution; and mixing the first solution and the second solution to obtain the film-forming solution.

[0019] In any embodiment of this application, the first film-forming salt includes MnCl2, Mn(NO3)2, MnSO4·H2O, MgSO4, Ca(NO3)2, Ba(NO3)2, Sr(NO3)2, or a combination thereof.

[0020] In any embodiment of this application, the second film-forming salt includes a carbonate, a phosphate, or a combination thereof, and may be selected as (M2)2CO3 or (M2). x3 H y3 (PO4) z3 (M2) x4 H y4 (P2O7) z4 Or a combination thereof, wherein M2 is Li, Na, K or a combination thereof, and x3, y3, z3 and x4, y4, z4 make the net charge of the second film-forming salt zero.

[0021] In any embodiment of this application, the complexing agent includes ethylenediaminetetraacetic acid, disodium ethylenediaminetetraacetic acid, tetrasodium ethylenediaminetetraacetic acid, ethanolamine, or a combination thereof.

[0022] In any embodiment of this application, the oxidizing oxyacid salt includes nitrates and inorganic salts (M2). x5 (M3) y5 O z5 Alternatively, it may be selected from NaNO3, KNO3, Ca(NO3)2, KMnO4, Na2FeO4, K2Cr2O7, NaCoO2, NaVO2 or combinations thereof, wherein M2 is Li, Na, K or combinations thereof, M3 is V, Cr, Mn, Fe, Co, Ni or combinations thereof, z5 is 1 to 7, and x5, y5, z5 make the net charge of the oxidizing oxyacid salt zero.

[0023] In any embodiment of this application, the catalyst comprises an ammonium salt, which may be NH4NO3, (NH4)2SO4, NH4Cl or a combination thereof.

[0024] In any embodiment of this application, the concentrations of the first film-forming salt, the second film-forming salt, the complexing agent, and the oxidizing oxyacid salt in the film-forming solution are from 10 g / L to 50 g / L, and the concentration of the catalyst is from 10 g / L to 30 g / L.

[0025] By adjusting the composition of the film-forming solution and controlling the concentration of the corresponding components within a suitable range, a dense solid electrolyte composite protective film that conducts lithium ions but does not conduct electrons can be generated in situ on the lithium metal surface. This composite protective film has low interfacial impedance, strong interfacial bonding force, and high mechanical strength. It can isolate the reaction between the electrolyte and lithium metal, reduce or avoid irreversible loss of the electrolyte, and induce uniform deposition of lithium metal during cycling, inhibit the growth of lithium dendrites, and improve the cycle performance and safety performance of the secondary battery.

[0026] In any embodiment of this application, the pH value of the film-forming solution is 5 to 7. By adjusting the pH value of the film-forming solution, the acidity of the solution can be reduced and its alkalinity increased, thereby increasing the number of basic acid radicals in the solution, which is beneficial to promoting the formation of the precipitated salt film layer.

[0027] A third aspect of this application provides a secondary battery, including the negative electrode sheet described in the first aspect of this application or the negative electrode sheet prepared by the preparation method described in the second aspect of this application.

[0028] A fourth aspect of this application provides a battery module including the secondary battery described in the third aspect of this application.

[0029] A fifth aspect of this application provides a battery pack including the battery module described in the fourth aspect of this application.

[0030] The sixth aspect of this application provides an electrical device, including at least one of the secondary battery described in the third aspect of this application, the battery module described in the fourth aspect of this application, or the battery pack described in the fifth aspect of this application.

[0031] The battery module, battery pack, and power device of this application include the secondary battery provided in this application, and therefore have at least the same advantages as the secondary battery. Attached Figure Description

[0032] Figure 1 This is a surface morphology diagram of an embodiment of the composite protective film in the negative electrode sheet of this application.

[0033] Figure 2 This is a schematic diagram of one embodiment of the secondary battery of this application.

[0034] Figure 3 This is an exploded view of one embodiment of the secondary battery of this application.

[0035] Figure 4 This is a schematic diagram of one embodiment of the battery module of this application.

[0036] Figure 5 This is a schematic diagram of one embodiment of the battery pack of this application.

[0037] Figure 6 yes Figure 5 The exploded diagram.

[0038] Figure 7 This is a schematic diagram of one embodiment of the power-consuming device that uses a secondary battery as a power source according to this application. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions of this application will be clearly and completely described below in conjunction with embodiments. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. The embodiments described herein are illustrative in nature and are used to provide a basic understanding of this application. The embodiments of this application should not be construed as limiting this application.

[0040] For the sake of brevity, this article only discloses a few specific numerical ranges. However, any lower limit can be combined with any upper limit to form an unspecified range; and any lower limit can be combined with other lower limits to form an unspecified range, just as any upper limit can be combined with any other upper limit to form an unspecified range. Furthermore, each individually disclosed point or single value can itself serve as a lower or upper limit and be combined with any other point or single value or with other lower or upper limits to form an unspecified range.

[0041] In this description, unless otherwise stated, "above" and "below" include the stated number.

[0042] Unless otherwise stated, the terms used in this application have their common meanings as commonly understood by those skilled in the art. Unless otherwise stated, the values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (e.g., they can be tested according to the methods given in the embodiments of this application).

[0043] The list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms can mean any combination of the listed items. For example, if items A and B are listed, then the phrase "at least one of A and B" means only A; only B; or A and B. In another instance, if items A, B, and C are listed, then the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single component or multiple components. Item B may contain a single component or multiple components. Item C may contain a single component or multiple components.

[0044] Lithium metal anodes have an extremely high theoretical specific capacity (3860 mAh / g), making them the best choice for next-generation high-energy-density rechargeable battery anode materials. However, the inventors discovered during their research that lithium metal is highly reactive. On the one hand, it is extremely prone to oxidation-reduction reactions with the environmental medium during storage and transportation, leading to its failure. It is also prone to violent reactions with aqueous solutions, thus limiting the use of many inorganic salt film-forming solutions in the rechargeable battery preparation process. On the other hand, during the charge and discharge of rechargeable batteries, it readily reacts with the electrolyte to form an SEI film. As the cycle time increases, the SEI film gradually thickens, leading to increased interfacial impedance and the formation of dead lithium, which in turn causes capacity loss and affects the cycle performance of the rechargeable battery. At the same time, during the charge and discharge of rechargeable batteries, uneven lithium deposition can lead to the formation of lithium dendrites. The growth of lithium dendrites may puncture the separator, causing a short circuit or even an explosion in the battery.

[0045] Currently, to solve storage and transportation problems, existing technologies often employ the method of coating a layer of polymer on the surface of lithium metal. However, this method is complex and costly, and the polymer and lithium metal are mainly bonded by organic-inorganic bonds, resulting in poor adhesion. In addition, polymer protective films have many defects, providing poor protection for the lithium metal anode. Furthermore, polymer films are not resistant to high voltage and are prone to decomposition under high voltage, thus limiting their protective capabilities for lithium metal.

[0046] To address the aforementioned issues, the inventors, through extensive research, proposed a negative electrode sheet. By employing an aqueous solvent that readily reacts with lithium metal, and configuring a film-forming solution based on the deposition film reaction mechanism, the reaction rate between the lithium metal and the film-forming solution is controlled through spraying. This allows for the deposition of a composite protective film of controllable thickness and high voltage resistance on the lithium metal surface. This composite protective film can protect the lithium metal during the charge-discharge cycle of the secondary battery and promote the uniform deposition of lithium metal, thereby improving the cycle performance and safety performance of the secondary battery.

[0047] Negative electrode sheet

[0048] A first aspect of this application provides a negative electrode sheet, comprising: a lithium metal layer, and a composite protective film located on at least one surface of the lithium metal layer, wherein the composite protective film comprises a precipitated salt film layer and an oxide film layer, and at least a portion of the oxide film layer is distributed in the precipitated salt film layer.

[0049] Not intended to be limited by any theory, the inventors discovered that the composite protective film formed by the precipitated salt film layer and the oxide film layer in this application is very dense. In the secondary battery, it can isolate the organic solvent in the electrolyte and the contact between lithium salt anions and lithium metal, reducing the irreversible loss of the electrolyte, thereby achieving the purpose of protecting lithium metal during charge and discharge cycles. At the same time, the composite protective film can induce uniform deposition of lithium metal during charge and discharge cycles, reduce the growth of lithium dendrites and the generation of dead lithium, thereby helping to improve the capacity, cycle performance and safety performance of the secondary battery.

[0050] In some implementations, see Figure 1 The composite protective film contains a mixture of precipitated salt film layer and oxide film layer. The precipitated salt film layer is located on the side of the composite protective film away from the lithium metal layer, while the oxide film layer is located on the side of the composite protective film closer to the lithium metal layer. The composite protective film is mainly bonded to the lithium metal layer through chemical bonds, resulting in a strong bond and high mechanical strength. This provides strong protection for the lithium metal layer, effectively isolating it from the electrolyte during long-term charge-discharge cycles. This reduces the SEI film formed by the reaction between the lithium metal layer and the electrolyte, thereby lowering the interfacial impedance and improving the ion conduction capacity and cycle performance of the secondary battery.

[0051] In some embodiments, the precipitated salt film layer includes (M1). x1 (PO4) y1 (M1) x2 (CO3) y2Or a combination thereof, wherein M1 is at least one of Li, Mn, Mg, Ba, Ca, and Sr, and x1, y1, and x2, y2 make the net charge of the compound zero. For example, the precipitated salt film layer can be Li3PO4, LiCO3, Mn3(PO4)2, MnCO3, Mg3(PO4)2, MgCO3, Ba3(PO4)2, BaCO3, Ca3(PO4)2, CaCO3, Sr3(PO4)2, SrCO3, or a combination thereof.

[0052] The precipitated salt film mainly consists of inorganic phosphates and carbonates, which have good lithium-ion conductivity and can isolate electrons. Therefore, it can provide a migration channel for lithium ions to induce uniform deposition of lithium ions, thereby reducing the formation of lithium dendrites and improving the safety performance of secondary batteries.

[0053] In some embodiments, the oxide film layer includes (M1). m O n Wherein, M1 is Li, Mn, Mg, Ba, Ca, Sr or a combination thereof, and m and n make the net charge of the metal oxide zero. For example, the oxide film can be Li2O, MnO, MgO, BaO, CaO, SrO or a combination thereof.

[0054] In this application, the oxide film layer and the precipitated salt film layer are combined / mixed together to form a composite protective film for the lithium metal layer. This composite protective film is a solid electrolyte layer with high mechanical strength, which can play a strong role in isolating and protecting the lithium metal layer. At the same time, it can also induce uniform deposition of lithium metal during charge and discharge cycles, inhibit the growth of lithium dendrites, and improve the cycle performance and safety performance of the secondary battery.

[0055] In some embodiments, the thickness of the composite protective film is from 1 μm to 25 μm. For example, the thickness of the composite protective film is 3 μm, 5 μm, 7 μm, 9 μm, 11 μm, 13 μm, 15 μm, 17 μm, 19 μm, 21 μm, 23 μm, or within any range of the above values. Optionally, the thickness of the composite protective film is from 3 μm to 20 μm.

[0056] In some embodiments, the thickness of the oxide film is from 50 nm to 200 nm. For example, the thickness of the oxide film is 70 nm, 90 nm, 110 nm, 130 nm, 150 nm, 170 nm, 190 nm, or within any range of the above values. Optionally, the thickness of the oxide film is from 70 nm to 160 nm.

[0057] The thickness of the oxide film and composite protective film in this application has a meaning known in the art and can be tested using methods known in the art, such as using a micrometer (e.g., Mitutoyo 293-100 type, with an accuracy of 0.1 μm).

[0058] In this application, the thickness of the oxide film and the composite protective film is within a suitable range, which can provide a good migration channel for lithium ions, enabling lithium ions to have good conductivity in the composite protective film, thereby reducing interfacial impedance and inducing uniform deposition of lithium metal, reducing or avoiding the growth of lithium dendrites; at the same time, it can effectively isolate the contact between the organic solvent in the electrolyte and the lithium salt anions and the lithium metal, reducing the irreversible loss of the electrolyte, thereby achieving the purpose of protecting lithium metal during charge and discharge cycles.

[0059] In some embodiments, the passivation range of the composite protective film is 110mV to 420mV. For example, the passivation range of the composite protective film can be 130mV to 400mV, 150mV to 380mV, 170mV to 360mV, 190mV to 340mV, 210mV to 320mV, 230mV to 300mV, or 250mV to 280mV.

[0060] The passivation range of the composite protective film in this application has a meaning known in the art and can be tested using methods known in the art. For example, an electrochemical polarization curve can be measured using a three-electrode system, and the passivation range of the composite protective film can be obtained from the electrochemical polarization curve. The fact that the passivation range of the composite protective film is within the above-mentioned range indicates that the composite protective film in this application has good high-voltage resistance and will not easily decompose even under high voltage. Therefore, it can provide long-lasting protection for the lithium metal layer, which is beneficial to improving the cycle performance of the secondary battery.

[0061] A second aspect of the embodiments of this application provides a method for preparing a negative electrode sheet, comprising:

[0062] The film-forming process includes reacting a lithium metal layer with a film-forming solution to form a composite protective film to obtain a negative electrode sheet, wherein the composite protective film includes a precipitated salt film layer and an oxide film layer, and at least a portion of the oxide film layer is distributed in the precipitated salt film layer.

[0063] This application employs an aqueous solvent that readily reacts with lithium metal and prepares a film-forming solution based on the deposition film reaction mechanism. Then, through the reaction between the film-forming solution and the lithium metal layer, an inorganic salt composite protective film with controllable thickness and high voltage resistance can be formed on the surface of the lithium metal layer. This composite protective film can effectively isolate the contact between the organic solvent in the electrolyte and the lithium salt anions and the lithium metal, reducing the irreversible loss of the electrolyte, thereby achieving the purpose of protecting the lithium metal layer during charge-discharge cycles.

[0064] In some embodiments, the film-forming solution includes precipitated salt cations, precipitated salt anions, oxidizing oxyacid salts, complexing agents, and catalysts dissolved in an inert solvent.

[0065] In some embodiments, the preparation method further includes a film-forming solution providing step, which includes:

[0066] S10. Dissolve the first film-forming salt containing the precipitated salt cation, the complexing agent, and the catalyst in a first inert solvent to obtain a first solution;

[0067] S20. Dissolve the second film-forming salt containing the precipitated salt anion and the oxidizing oxyacid acid in a second inert solvent to obtain a second solution;

[0068] S30. Mix the first solution and the second solution to obtain the film-forming solution.

[0069] In some embodiments, the concentrations of the first film-forming salt, the second film-forming salt, the complexing agent, and the oxidizing oxyacid salt in the film-forming solution are from 10 g / L to 50 g / L, for example, they can be 10 g / L, 15 g / L, 20 g / L, 25 g / L, 30 g / L, 35 g / L, 40 g / L, 45 g / L, 50 g / L, or any range thereof. The concentration of the catalyst is from 10 g / L to 30 g / L, for example, it can be 10 g / L, 15 g / L, 20 g / L, 25 g / L, 30 g / L, or any range thereof.

[0070] This application achieves in-situ conversion of a dense, lithium-ion-conducting but non-electro-conducting solid electrolyte composite protective film on the lithium metal surface by adjusting the composition of the film-forming solution and controlling the concentration of the corresponding components within a suitable range. This composite protective film has low interfacial impedance, strong interfacial bonding, and high mechanical strength. It can isolate the reaction between the electrolyte and lithium metal, reduce or avoid irreversible electrolyte loss, and induce uniform deposition of lithium metal during cycling, inhibit the growth of lithium dendrites, and improve the cycle performance and safety performance of the secondary battery.

[0071] In some embodiments, the film-forming liquid can be sprayed onto the lithium metal layer to react and generate the composite protective film. The thickness of the composite protective film on the lithium metal layer can be controlled by controlling the spraying rate. Specifically, the film-forming liquid can be sprayed onto the lithium metal layer using an atomizer, and the thickness of the resulting composite protective film can be controlled within the suitable range described above by controlling the amount of liquid sprayed from the atomizer.

[0072] In some embodiments, the first film-forming salt includes MnCl2, Mn(NO3)2, MnSO4·H2O, MgSO4, Ca(NO3)2, Ba(NO3)2, Sr(NO3)2, or a combination thereof.

[0073] In some embodiments, the second film-forming salt includes a carbonate, a phosphate, or a combination thereof, and may be (M2)2CO3, (M2) x3 H y3 (PO4) z3 (M2) x4 H y4 (P2O7) z4 Or a combination thereof, wherein M2 is Li, Na, K or a combination thereof, and x3, y3, z3 and x4, y4, z4 such that the net charge of the second film-forming salt is zero. For example, the second film-forming salt may be Li2CO3, Na2CO3, K2CO3, Li2HPO4, LiH2PO4, Na2HPO4, NaH2PO4, K2HPO4, KH2PO4, Li3HP2O7, Li2H2P2O7, LiH3P2O7, Na3HP2O7, Na2H2P2O7, NaH3P2O7, K3HP2O7, K2H2P2O7, KH3P2O7 or a combination thereof.

[0074] In some embodiments, the complexing agent includes ethylenediaminetetraacetic acid, disodium ethylenediaminetetraacetic acid, tetrasodium ethylenediaminetetraacetic acid, ethanolamine, or combinations thereof.

[0075] In some embodiments, the oxidizing oxyacid salt includes nitrates and inorganic salts (M2). x5 (M3) y5 O z5 Alternatively, it may be selected from NaNO3, KNO3, Ca(NO3)2, KMnO4, Na2FeO4, K2Cr2O7, NaCoO2, NaVO2 or combinations thereof, wherein M2 is Li, Na, K or combinations thereof, M3 is V, Cr, Mn, Fe, Co, Ni or combinations thereof, z5 is 1 to 7, and x5, y5, z5 make the net charge of the oxidizing oxyacid salt zero.

[0076] In some embodiments, the catalyst comprises an ammonium salt, optionally NH4NO3, (NH4)2SO4, NH4Cl, or a combination thereof.

[0077] The film-forming solution provided in this application has stable components in solution. When it comes into contact with lithium metal and reacts, the pH at the interface increases. At this time, the sparingly soluble inorganic salt in the solution reaches the critical supersaturation and induces deposition, transforming into a dense (inorganic salt) solid electrolyte composite protective film on the lithium metal surface. This composite protective film is bonded to lithium metal through chemical bonds, and the bonding effect is strong. Under high voltage, it can provide long-lasting protection for the lithium metal layer.

[0078] As a specific example, the reaction process between the film-forming solution and lithium metal in this application can be represented by the following reaction formula:

[0079] Li-e→Li + (1)

[0080] 2Li+VO3 - →VO2+Li2O (2)

[0081] 2H + +e→H2 (3)

[0082] H2PO4 - →HPO4 2- +H + (4)

[0083] HPO4 2- →PO4 3- +H + (5)

[0084] H2O→H + +OH - (6)

[0085] Li + +OH - →LiOH (7)

[0086] 3Li + +PO4 3- →Li3PO4 (8)

[0087] Mn 2+ +HPO4 2- →MnHPO4 (9)

[0088] In this process, lithium metal reacts with soluble hydrogen phosphate, consuming hydrogen ions to generate a poorly soluble phosphate precipitate film with strong protective properties. The conversion and precipitation efficiency can be controlled by adjusting the solution composition, and the reaction rate and the thickness of the composite protective film can be controlled by controlling the amount of sprayed liquid.

[0089] As shown in equations (9) to (11) below, a portion of Mn can be converted into a film-forming solution by introducing a complexing agent (represented by A). 2+They complex to maintain the free Mn in the solution. 2+ The concentration of Mn at the interface during the reaction process is such that... 2+ This allows for timely replenishment and simultaneously reduces the amount of free Mn in the solution. 2+ The concentration of H2PO promotes 4- Ionization increases the PO4 content in the solution. 3- Concentration, causing H2PO4 in the solution to react with the interface. 4- The concentration gradient decreases, reducing the amount of H2PO consumed during film formation and replenishing the ionization and diffusion processes. 4- This reduces dependence on Mn and improves solution stability; furthermore, the introduction of a complexing agent allows it to bind a portion of Mn in the solution. 2+ Complexation, while releasing H + This promotes the dissolution of Li and increases the precipitation rate. The reaction formula is as follows (B represents the intermediate product):

[0090] A+Mn 2+ →Mn(B) 2+ +H + (10)

[0091] The precipitation reaction that occurs at the interface is as follows:

[0092] Mn(B) 2+ +HPO4 2- →MnHPO4+B (11)

[0093] B+H + →A (12)

[0094] As shown in equations (12) to (14) below, introducing a catalyst into the film-forming solution can reduce the activation energy required for the reaction, thereby promoting the reaction and increasing the chemical reaction rate. The catalyst itself does not change before and after the reaction. The reaction equations are as follows:

[0095] 6NH4 + +Mn 2+ →Mn(NH3)6 2+ +H + (13)

[0096] Mn(NH3)6 2+ +HPO4 2- →MnHPO4+6NH3(14)

[0097] NH3+H + →NH4 + (15)

[0098] From equations (12) to (14) above, it can be seen that the NH in the catalyst in solution... 4+It can be hydrolyzed to produce NH3·H2O and release H. + Mn 2+ Together, they can reduce free Mn 2+ The concentration of Mn is controlled to maintain solution stability; the interfacial reaction consumes Mn. 2+ It can be released and replenished in a timely manner. The ligand NH3 reacts with H... + Combine to generate NH 4+ NH 4+ At the interface and H + Combining promotes H2PO 4- The ionization of the substance can accelerate the precipitation rate, thus playing a catalytic role.

[0099] In the reaction process of the above film-forming solution with lithium metal, a complexing agent can be introduced to establish Mn 2+ The buffer pair prevents the reduction of effective components for film formation in the solution; an appropriate amount of catalyst can increase the nucleation density of MnHPO4, improve the precipitation rate of MnHPO4, and enhance the corrosion resistance of the formed composite protective film; an appropriate amount of oxidizing oxyacid salt can promote the formation of a dense passivating Li2O film layer, thereby improving the precipitation rate.

[0100] In some embodiments, the pH value of the film-forming solution is 5 to 7. By adjusting the pH value of the film-forming solution, the acidity of the film-forming solution can be reduced and its alkalinity increased, thereby increasing the number of basic acid radicals in the solution, which is beneficial to promoting the formation of the precipitated salt film layer.

[0101] In some embodiments, the negative electrode sheet further includes a negative current collector, wherein the lithium metal layer is coated onto at least one side of the negative current collector using a rolling process.

[0102] In some embodiments, the negative electrode current collector has two surfaces opposite each other in its thickness direction, and a lithium metal layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0103] The negative electrode current collector can be made of metal foil or porous metal plate, such as foil or porous plate of metals or alloys thereof, such as copper, nickel, titanium, iron, etc. As an example, the negative electrode current collector is copper foil.

[0104] The negative electrode sheet can be prepared using conventional methods in the art. For example, a lithium metal sheet is bonded to the surface of a copper foil to obtain the negative electrode sheet. The bonding can be, but is not limited to, achieved by rolling.

[0105] Secondary batteries

[0106] A third aspect of this application provides a secondary battery, including any device in which an electrochemical reaction occurs to interconvert chemical energy and electrical energy, specific examples of which include all types of lithium primary batteries or lithium secondary batteries. In particular, lithium secondary batteries include lithium metal secondary batteries, lithium-ion secondary batteries, lithium polymer secondary batteries, or lithium-ion polymer secondary batteries.

[0107] In some embodiments, the secondary battery of this application includes a positive electrode, a negative electrode, a separator, and an electrolyte.

[0108] [Negative electrode plate]

[0109] The negative electrode used in the secondary battery of this application is the negative electrode of the first aspect of the embodiments of this application or the negative electrode prepared by the preparation method of the second aspect of the embodiments of this application.

[0110] [Positive electrode plate]

[0111] The materials, composition, and manufacturing methods of the positive electrode used in the secondary battery of this application may include any techniques known in the prior art.

[0112] The positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector and comprising a positive active material. As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive active material layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0113] In some embodiments, the positive electrode active material layer includes a positive electrode active material, and the specific type of positive electrode active material is not specifically limited and can be selected according to needs. For example, the positive electrode active material may include one or more of lithium transition metal oxides, lithium-containing phosphates with olivine structures, and their respective modified compounds. In the secondary battery of this application, the modified compounds of the above-mentioned positive electrode active materials may be used to modify the positive electrode active material by doping, surface coating, or doping and surface coating.

[0114] As examples, lithium transition metal oxides may include one or more of lithium cobalt oxides, lithium nickel oxides, lithium manganese oxides, lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, lithium nickel cobalt manganese oxides, lithium nickel cobalt aluminum oxides, and their modified compounds. As examples, olivine-structured lithium phosphates may include one or more of lithium iron phosphate, lithium iron phosphate and carbon composites, lithium manganese phosphate, lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, lithium manganese iron phosphate and carbon composites, and their modified compounds. These positive electrode active materials may be used alone or in combination of two or more.

[0115] In some embodiments, the positive electrode active material layer may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0116] In some embodiments, the positive electrode active material layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0117] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. As an example of a metal foil, the positive electrode current collector may be aluminum foil. The composite current collector may include a polymeric material substrate and a metal material layer formed on at least one surface of the polymeric material substrate. As an example, the metal material may be selected from one or more of aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. As an example, the polymeric material substrate may be selected from polypropylene, polyethylene terephthalate, polybutylene terephthalate, polystyrene, polyethylene, etc.

[0118] The positive electrode sheet in this application can be prepared according to conventional methods in the art. For example, the positive electrode active material layer is typically formed by coating a positive electrode slurry onto a positive electrode current collector, followed by drying and cold pressing. The positive electrode slurry is typically formed by dispersing the positive electrode active material, optional conductive agent, optional binder, and any other components in a solvent and stirring until homogeneous. The solvent can be N-methylpyrrolidone (NMP), but is not limited to it.

[0119] The positive electrode sheet of this application does not exclude other additional functional layers besides the positive electrode active material layer. For example, in some embodiments, the positive electrode sheet of this application further includes a conductive undercoat (e.g., composed of a conductive agent and a binder) sandwiched between the positive electrode current collector and the positive electrode active material layer and disposed on the surface of the positive electrode current collector. In other embodiments, the positive electrode sheet of this application further includes a protective layer covering the surface of the positive electrode active material layer.

[0120] Electrolyte

[0121] The electrolyte acts as a conductor of active ions between the positive and negative electrodes. The electrolyte used in the secondary battery of this application can be any electrolyte known in the prior art.

[0122] In some embodiments, the electrolyte includes an organic solvent, a lithium salt, and optional additives. The types of organic solvent, lithium salt, and additives are not specifically limited and can be selected as needed.

[0123] In some embodiments, as examples, the lithium salt includes, but is not limited to, at least one of LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiFSI (lithium bis(fluorosulfonyl)imide), LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluorooxalate borate), LiBOB (lithium dicoxalate borate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluorodioxalate phosphate), and LiTFOP (lithium tetrafluorooxalate phosphate). One of the above lithium salts may be used alone, or two or more may be used simultaneously.

[0124] In some embodiments, as examples, the organic solvent includes, but is not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butyl carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE). One of the above organic solvents may be used alone, or two or more may be used simultaneously. Optionally, two or more of the above organic solvents may be used simultaneously.

[0125] In some embodiments, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0126] As an example, the additives include, but are not limited to, at least one of fluoroethylene carbonate (FEC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), vinyl sulfate (DTD), propylene sulfate, vinyl sulfite (ES), 1,3-propanesulfonate lactone (PS), 1,3-propenesulfonate lactone (PST), sulfonate cyclic quaternary ammonium salts, succinic anhydride, succinic anhydride (SN), adiponitrile (AND), tris(trimethylsilane) phosphate (TMSP), and tris(trimethylsilane) borate (TMSB).

[0127] The electrolyte can be prepared according to conventional methods in the art. For example, an organic solvent, a lithium salt, and optional additives can be mixed evenly to obtain an electrolyte. There are no particular restrictions on the order of addition of the materials; for example, the lithium salt and optional additives can be added to the organic solvent and mixed evenly to obtain an electrolyte; or, the lithium salt can be added to the organic solvent first, and then the optional additives can be added to the organic solvent and mixed evenly to obtain an electrolyte.

[0128] [Isolation membrane]

[0129] The separator is positioned between the positive and negative electrodes, primarily serving to prevent short circuits between the positive and negative electrodes while allowing active ions to pass through. This application does not impose any particular restriction on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0130] In some embodiments, the material of the separator can be selected from one or more of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride, but is not limited to these. The separator can be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of each layer may be the same or different. In some embodiments, a ceramic coating or a metal oxide coating may also be provided on the separator.

[0131] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0132] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0133] In some implementations, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, aluminum shell, or steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0134] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. Figure 2 This is an example of a square-structured secondary battery 5.

[0135] In some implementations, refer to Figure 3The outer packaging may include a housing 51 and a cover plate 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 is used to cover the opening to close the receiving cavity. Positive electrode sheets, negative electrode sheets, and a separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated in the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be adjusted according to requirements.

[0136] In some implementations, the secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be multiple, the specific number of which can be adjusted according to the application and capacity of the battery module.

[0137] Figure 4 This is battery module 4, used as an example. (See reference...) Figure 4 In battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0138] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0139] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0140] Figure 5 and Figure 6 This is battery pack 1 as an example. (See reference...) Figure 5 and Figure 6 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3. The upper body 2 covers the lower body 3, forming a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0141] Electrical appliances

[0142] A fifth aspect of this application provides an electrical device comprising at least one of the secondary battery, battery module, or battery pack described in this application. The secondary battery, battery module, or battery pack can be used as a power source for the device or as an energy storage unit for the device. The device can be, but is not limited to, mobile devices, electric vehicles, electric trains, ships and satellites, energy storage systems, etc. For example, it can be a laptop computer, pen input computer, mobile computer, e-book player, portable telephone, portable fax machine, portable copier, portable printer, stereo headset, video recorder, LCD TV, portable cleaner, portable CD player, mini CD-ROM, transceiver, electronic notebook, calculator, memory card, portable recorder, radio, backup power supply, motor, automobile, motorcycle, electric bicycle, bicycle, lighting fixture, toy, game console, clock, power tool, flashlight, camera, household large-capacity storage battery, and lithium-ion capacitor, etc.

[0143] The device can be configured to use a secondary battery, battery module, or battery pack, depending on its usage requirements.

[0144] Figure 7 This is an example device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the device's requirements for high power and high energy density, a battery pack or battery module can be used.

[0145] Example

[0146] The following examples describe the disclosure of this invention in more detail. These examples are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of this disclosure. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are based on mass, and all reagents used in the examples are commercially available or synthesized by conventional methods and can be used directly without further processing. The instruments used in the examples are also commercially available.

[0147] Example 1

[0148] Preparation of negative electrode sheet

[0149] 1) Pretreatment: In a dry room at 25°C and humidity below 2%, a 25μm thick lithium foil was placed in anhydrous ethanol for 10s pretreatment. The reaction was carried out to remove the surface contaminant layer by hydrogen evolution. Then, it was soaked and cleaned with DOL / DME to remove the reaction product with ethanol. It was then dried and placed in a desiccator for later use.

[0150] 2) Preparation of film-forming solution: Prepare 35 g / L manganese sulfate monohydrate, 24 g / L EDTA4Na and 1 g / L ammonium sulfate together, and prepare 35 g / L sodium dihydrogen phosphate, 10 g / L NaNO3 and 10 mL 0.001 mol / L NaOH together. After dissolving them separately, mix them well and make up to volume to obtain the film-forming solution.

[0151] 3) Preparation of the composite protective film: The film thickness was controlled by spraying the atomizer onto the lithium foil treated in step 1). The spray volume was adjusted to maintain the film thickness. The temperature was maintained at 10℃, the spray rate was 5 mL / min, the atomizer was 10 cm away from the lithium foil, and the lithium foil belt speed was 1 m / s. The lithium belt was sprayed three times during transport. The prepared sample was then washed with anhydrous ethanol and dried. The thickness of the composite protective film can be controlled by adjusting the lithium foil transport speed and the spray volume, and is recorded as H. After chemical conversion treatment, a glassy, ​​uniform, and dense crystalline film was formed. The lithium foil with the composite protective film was obtained and subsequently cleaned with ethanol and DOL / DME, and then dried.

[0152] 4) Preparation of negative electrode sheet: Lithium foil with composite protective film is bonded to the surface of copper foil by roll forming to obtain negative electrode sheet.

[0153] Preparation of positive electrode sheet

[0154] The positive electrode active material LiCoO2, conductive carbon black, and binder PVDF were mixed in a mass ratio of 96.7:1.7:1.6, and an appropriate amount of solvent NMP was added. The mixture was stirred in a vacuum mixer to obtain a positive electrode slurry. The positive electrode slurry was uniformly coated on both surfaces of the positive electrode current collector aluminum foil. After vacuum drying at 70°C for 12 hours, the positive electrode sheet was obtained by slitting and cutting into sheets.

[0155] Preparation of electrolyte

[0156] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent; LiPF6 was dissolved in the above organic solvent, and then fluoroethylene carbonate (FEC) was added and mixed evenly to obtain an electrolyte; wherein the concentration of LiPF6 was 1 mol / L.

[0157] Preparation of the separating membrane

[0158] PE porous film is used as the separator.

[0159] Preparation of lithium metal secondary batteries

[0160] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The electrode assembly is then wound up to form the electrode assembly. The electrode assembly is placed in an outer package, injected with the prepared electrolyte, and then sealed. After processes such as formation, degassing, and edge trimming, a lithium metal secondary battery is obtained.

[0161] Examples 2 to 14 and Comparative Examples 1 to 3

[0162] The preparation method of the lithium metal secondary battery is similar to that of Example 1, except that the negative electrode sheet and related parameters in the preparation process are adjusted. The specific parameters are detailed in Table 1. Among them, Comparative Example 1 does not contain a composite protective film, Comparative Example 2 uses an ordinary polymer protective film instead of the composite protective film in Comparative Example 1, and the thickness of the composite protective film in Comparative Example 3 is not within the scope of protection of this application. " / " indicates that the corresponding substance is not included.

[0163] Table 1

[0164]

[0165] Test section

[0166] (1) Thickness test of composite protective film

[0167] The CHY-CA benchtop thickness gauge was used to measure five evenly distributed points on the sample surface to observe whether the composite protective film was prepared uniformly and to record the average thickness.

[0168] (2) Testing of the passivation range of the composite protective film

[0169] A three-electrode system was used to measure a square sample (surface area 1 cm²). 2 Electrochemical polarization curves were scanned separately from OCP towards both sides at a scan rate of 0.333 mV / s. The cutoff potential of the cathodic polarization curve was -300 mV, the cutoff potential of the anodic polarization curve was 1.6 V, and the cutoff current was 10 mA. After fitting, the passivation range of the composite protective film was analyzed.

[0170] (3) Cycle performance test of lithium metal secondary batteries

[0171] The cycle test temperature is 25℃. The battery is charged at a constant current of 0.33C to 4.3V, then charged at a constant voltage of 0.025C. After resting for 5 minutes, it is discharged at 0.33C to 2.8V. The capacity obtained in this step is used as the initial capacity. The 0.33C charge / 0.33C discharge cycle test is carried out until the cycle capacity retention rate of the secondary battery is 80%, and the corresponding cycle life is recorded.

[0172] (4) Safety performance testing of lithium metal secondary batteries

[0173] The battery cells were placed in a graduated measuring cup for cycling. The battery was then immersed in a 60°C thermally conductive silicone oil bath. The battery was charged at a constant current of 0.33C to 4.3V, then charged at a constant voltage of 0.025C. After resting for 5 minutes, it was discharged at 0.33C to 2.8V. The capacity obtained in this step was used as the initial capacity. Cyclic tests were performed at 0.33C charge / discharge until the secondary battery's cycle capacity retention rate reached 10%. After the cycle was completed, the readings were taken to determine the volume of gas produced by the battery cells, and the corresponding cell state was recorded. Gas production volume below 10% was considered mild gas production, 10%-50% was considered moderate gas production, and above 50% was considered severe gas production.

[0174] Table 2 presents the performance test results of Examples 1 to 14 and Comparative Examples 1 to 3.

[0175] Table 2

[0176] Passivation range of composite protective film (mV) Number of cycles to achieve 80% capacity retention High-temperature cycling performance safety test Example 1 240 330 No abnormalities Example 2 160 200 No abnormalities Example 3 320 460 No abnormalities Example 4 210 240 No abnormalities Example 5 280 360 No abnormalities Example 6 220 300 No abnormalities Example 7 290 485 No abnormalities Example 8 110 135 Moderate gas production Example 9 120 140 Moderate gas production Example 10 135 150 Mild gas production Example 11 260 310 No abnormalities Example 12 270 336 No abnormalities Example 13 280 360 No abnormalities Example 14 420 The polarization is too large to cycle. The polarization is too large to cycle. Comparative Example 1 0 80 Smoke and fire Comparative Example 2 90 110 Severe gas production Comparative Example 3 100 120 Severe gas production

[0177] Comparing Examples 1 to 14 with Comparative Examples 1 to 2, it can be seen that the passivation range of the composite protective film in the negative electrode sheet of this application is significantly higher than that in Comparative Examples 1 and 2, indicating that the composite protective film can play a good protective role for the lithium metal layer. In addition, the number of cycles in which the capacity retention rate of the secondary battery in Comparative Examples 1 and 2 is 80% is significantly lower than that in Examples 1 to 14, and the high-temperature cycle performance of the lithium metal secondary battery in Comparative Examples 1 and 2 is also poor. This shows that the composite protective film can effectively improve the cycle performance and safety performance of the secondary battery. Comparing Examples 1 to 14 with Comparative Example 3, it can be seen that when the thickness of the composite protective film is not within the range selected in this application, the passivation range of the composite protective film and the cycle performance of the secondary battery are significantly reduced, indicating that the thickness of the composite protective film needs to be controlled within the range selected in this application.

[0178] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in this application, and these modifications or substitutions should all be covered within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A negative electrode sheet, characterized by, include: A lithium metal layer, and a composite protective film located on at least one surface of the lithium metal layer, wherein the composite protective film comprises a precipitated salt film layer and an oxide film layer, at least a portion of the oxide film layer is distributed in the precipitated salt film layer, and the precipitated salt film layer comprises at least one of MnCO3, Mn3(PO4)2, CaCO3 and Mn2P2O7; The oxide film layer is located on the side of the composite protective film closer to the lithium metal layer, and the precipitated salt film layer is located on the side of the composite protective film farther away from the lithium metal layer.

2. The negative electrode sheet according to claim 1, characterized by, The oxide film layer comprises (M1) m O n wherein M1 is Li, Mn, Mg, Ba, Ca, Sr, or a combination thereof, and m, n are such that the net charge of the metal oxide is zero.

3. The negative electrode sheet according to claim 2, characterized by The oxide film layer includes at least one of MnO and CaO.

4. The negative electrode sheet according to any one of claims 1 to 3, characterized by, The negative electrode sheet satisfies at least one of the following conditions: The thickness of the composite protective film is 1 μm to 25 μm; The thickness of the oxide film is 50 nm to 200 nm; The passivation range of the composite protective film is 110mV to 420mV.

5. The negative electrode sheet according to claim 4, characterized by The thickness of the composite protective film is 3 μm to 20 μm.

6. The negative electrode sheet according to claim 4, wherein The thickness of the oxide film is 70 nm to 160 nm.

7. A method for producing a negative electrode sheet, characterized by include: The film-forming process includes reacting a lithium metal layer with a film-forming solution to form a composite protective film to obtain a negative electrode sheet. The composite protective film includes a precipitated salt film layer and an oxide film layer. At least a portion of the oxide film layer is distributed in the precipitated salt film layer. The precipitated salt film layer includes at least one of MnCO3, Mn3(PO4)2, CaCO3, and Mn2P2O7. The film-forming solution includes precipitated salt cations, precipitated salt anions, oxidizing oxyacid salts, complexing agents, and catalysts dissolved in an inert solvent.

8. The production method according to claim 7, characterized by, It also includes a film-forming solution providing step, which includes: A first film-forming salt containing the precipitated salt cation, the complexing agent, and the catalyst are dissolved in a first inert solvent to obtain a first solution; The second film-forming salt containing the precipitated salt anion and the oxidizing oxyacid acid salt are dissolved in a second inert solvent to obtain a second solution; The first solution and the second solution are mixed to obtain the film-forming solution.

9. The production method according to claim 8, characterized by, The preparation method satisfies at least one of the following conditions: The first film-forming salt includes MnCl2, Mn(NO3)2, MnSO4•H2O, MgSO4, Ca(NO3)2, Ba(NO3)2, Sr(NO3)2 or a combination thereof; The second film-forming salt includes carbonates, phosphates, or combinations thereof. Wherein, M2 is Li, Na, K or a combination thereof, and x3, y3, z3 and x4, y4, z4 make the net charge of the second film-forming salt zero; The complexing agent includes ethylenediaminetetraacetic acid, disodium ethylenediaminetetraacetic acid, tetrasodium ethylenediaminetetraacetic acid, ethanolamine, or a combination thereof; The oxidizing salt of an oxygen-containing acid comprises a nitrate salt, an inorganic salt (M2) x5 (M3) y5 O z5 or a combination thereof, Wherein, M2 is Li, Na, K or a combination thereof, M3 is V, Cr, Mn, Fe, Co, Ni or a combination thereof, z5 is 1 to 7, and x5, y5, z5 make the net charge of the oxidizing oxyacid salt zero; The catalyst comprises an ammonium salt.

10. The preparation method according to claim 9, characterized in that, the carbonate, phosphate, or combination thereof is (M2)2C03, (M2) x3 H y3 (PO4) z3 , (M2) x4 H y4 (P2O7) z4 or combinations thereof.

11. The preparation method according to claim 9, characterized in that, the nitrate, inorganic salt (M2) x5 (M3) y5 O z5 or combinations thereof are NaNCb, KNO3, Ca(NOs)2, KMnO4, Na2FeO4, K2Cr2O7, NaCoO2, NaVO2, or combinations thereof.

12. The method of claim 7, wherein, The catalyst comprises an ammonium salt, wherein the ammonium salt is NH4NO3, (NH4)2SO4, NH4Cl or a combination thereof.

13. The production method according to claim 8 or 9, characterized by, In the film-forming solution, the concentrations of the first film-forming salt, the second film-forming salt, the complexing agent, and the oxidizing oxyacid salt are from 10 g / L to 50 g / L, and the concentration of the catalyst is from 10 g / L to 30 g / L.

14. The production method according to claim 7 or 8, characterized by, The pH value of the film-forming solution is 5 to 7.

15. A secondary battery characterized by comprising: It includes the negative electrode sheet according to any one of claims 1-6 or the negative electrode sheet prepared by any one of claims 7-14.

16. A battery module, characterized by Includes the secondary battery as described in claim 15.

17. A battery pack, characterized in that, Includes the battery module as described in claim 16.

18. An electrical appliance, characterized in that, It includes at least one selected from the secondary battery of claim 15, the battery module of claim 16, or the battery pack of claim 17.

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