Negative active material and preparation method thereof, negative pole piece and sodium ion battery

Abstract

The invention relates to a negative active material and a preparation method thereof, a negative pole piece and a sodium ion battery. The negative electrode active material comprises a core-shell structure metal phosphide, and the core-shell structure metal phosphide comprises: an inner core, which comprises a bimetal phosphide derived from a Prussian blue compound; the shell layer covers at least part of the surface of the inner core, and the shell layer comprises metal phosphide. According to the negative electrode active material provided by the invention, through the synergistic effect of the bimetal phosphide derived from the Prussian blue compound in the inner core and the metal phosphide of the shell layer, the cycle performance and the rate capability of the battery are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and particularly to a negative electrode active material, a preparation method thereof, a negative electrode sheet, and a sodium ion battery. Background Art

[0002] Secondary batteries represented by sodium ion batteries have been applied in many fields such as electric vehicles and energy storage power systems (such as hydroelectric, thermal power, wind power, and solar power stations, etc.). Compared with lithium ion batteries, element sodium and lithium are in the same main group, with similar physical and chemical properties, and sodium has low cost and rich reserves. The lower desolvation energy of sodium ions gives sodium ion batteries a wider temperature adaptability.

[0003] Among many electrode material systems of sodium ion batteries, transition metal phosphides (TMPs) have attracted much attention due to their high theoretical capacity, rich electron orbits, and relatively low intercalation potential.

[0004] However, during the charge and discharge process, the repeated deintercalation and intercalation of Na + cause the lattice of TMPs materials to change, resulting in volume expansion and contraction, thereby causing particle fragmentation and cracking, leading to rapid attenuation of capacity performance. In addition, the rate performance of TMPs materials also needs to be improved. Summary of the Invention

[0005] Based on this, it is necessary to provide a positive electrode active material, a preparation method thereof, and an application, to alleviate the volume strain generated by metal phosphide materials during the charge and discharge process, improve the structural stability and kinetic performance of the materials, increase the capacity of the materials, and the battery prepared therefrom can improve the cycle performance and rate performance.

[0006] The first aspect of the present application provides a negative electrode active material, which includes a core-shell structured metal phosphide. The core-shell structured metal phosphide includes: a core, including a double metal phosphide derived from a Prussian blue compound; and a shell layer, covering at least a part of the surface of the core, and the shell layer includes a metal phosphide.

[0007] In some embodiments, both the core and the shell layer include carbon nanotubes.

[0008] In some embodiments, the molar ratio of carbon nanotubes to the double metal phosphide is 1:(2 - 5).

[0009] In some embodiments, the negative electrode active material further includes: a conductive carbon layer, covering at least a part of the surface of the core-shell structured metal phosphide.

[0010] In some embodiments, the thickness of the conductive carbon layer is 1 nm - 8 nm.

[0011] In some embodiments, the core-shell structured metal phosphide has a porous structure.

[0012] In some embodiments, the bimetallic phosphide is copper-iron bimetallic phosphide, and the metal phosphide is cobalt phosphide.

[0013] The second aspect of the present application provides a method for preparing a negative electrode active material, which includes the following steps: coating a shell metal hydroxide sol on at least a part of the surface of a bimetallic Prussian blue compound to obtain a precursor material; performing a phosphating treatment on the precursor material to obtain a core-shell structured metal phosphide; wherein, the core-shell structured metal phosphide includes a core and a shell, the core includes a bimetallic phosphide derived from a Prussian blue compound, the shell is coated on at least a part of the surface of the core, and the shell includes a metal phosphide.

[0014] In some embodiments, before the step of coating a shell metal hydroxide sol on at least a part of the surface of a bimetallic Prussian blue compound to obtain a precursor material, the following steps are further included: adding a shell metal source and a bimetallic Prussian blue compound to an alkali solution to generate a shell metal hydroxide sol; wherein, the shell metal source is a cobalt source, and the alkali solution is ammonia water.

[0015] The third aspect of the present application provides a negative electrode sheet, which includes: a negative electrode current collector; and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, and the negative electrode active material layer includes the negative electrode active material provided in the first aspect above or the negative electrode active material prepared by using the preparation method provided in the second aspect above.

[0016] The fourth aspect of the present application provides a sodium ion battery, which includes the positive electrode sheet provided in the third aspect above.

[0017] Compared with the traditional technology, the present application has at least the following beneficial effects:

[0018] For the negative electrode active material provided in the present application, through the synergistic effect of the core and the shell in the core-shell structured metal phosphide, on the one hand, there is a heterostructure between the bimetallic phosphide derived from the Prussian blue compound in the core and the metal phosphide in the shell, and the core-shell structure improves the distribution uniformity of the heterostructure. An internal built-in electric field can be induced at the interface of the heterostructure, providing an additional driving force for the transfer of charges, so that the material has a lower sodium ion diffusion barrier and a higher charge state density, thereby the material exhibits higher conductivity, thus improving the rate performance of the battery; at the same time, the composite phosphide with a heterostructure has a higher sodium ion adsorption energy. During the repeated ion deintercalation process, the structure of the material can still remain stable, alleviating the volume strain generated by the metal phosphide material during charge and discharge, and showing more stable long-cycle performance.

[0019] On the other hand, since both the core material and the shell material are phosphides, the bonding properties of the bimetallic phosphide derived from the Prussian blue compound in the core and the metal phosphide in the shell are similar (both are M-P bonds). Therefore, an interlaced structure layer is easily formed at the interface between the core and the shell, thereby improving the structural stability of the material and further enhancing the cycling performance of the battery.

[0020] Therefore, the bimetallic phosphide derived from the Prussian blue compound in the core and the metal phosphide in the shell act together, and the battery prepared thereby has improved cycling performance and rate performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic structural diagram of the negative electrode active material in an embodiment of the present application.

[0022] Figure 2 It is a schematic process diagram of the preparation method of the negative electrode active material in an embodiment of the present application.

[0023] REFERENCE SIGNS:

[0024] 1. Negative electrode active material;

[0025] 10. Core-shell structured metal phosphide; 11. Core; 12. Shell; 13. Carbon nanotube;

[0026] 20. Conductive carbon layer. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0027] Reference will now be provided in detail to embodiments of the present application, one or more examples of which are described below. Each example is provided by way of explanation and not limitation of the present application. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made to the present application without departing from the scope or spirit of the present application. For example, features described or illustrated as part of one embodiment can be used in another embodiment to yield a further embodiment.

[0028] Accordingly, it is intended that the present application cover such modifications and variations that fall within the scope of the appended claims and their equivalents. Other objects, features, and aspects of the present application are disclosed in the following detailed description or are apparent therefrom. Those of ordinary skill in the art should understand that this discussion is only a description of exemplary embodiments and is not intended to limit the broader aspects of the present application.

[0029] In the present application, among the technical features described in an open-ended manner, a closed technical solution composed of the listed features is included, as well as an open technical solution including the listed features.

[0030] In this application, when it comes to numerical ranges, unless otherwise specified, the above numerical ranges are considered continuous and include the minimum and maximum values of the range, as well as each value between such minimum and maximum values. Further, when the range refers to integers, it includes each integer between the minimum and maximum values of the range. In addition, when multiple ranges are provided to describe features or characteristics, the ranges can be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all sub-ranges subsumed therein.

[0031] If there is no special instruction, all embodiments and optional embodiments of this application can be combined with each other to form new technical solutions.

[0032] If there is no special instruction, all technical features and optional technical features of this application can be combined with each other to form new technical solutions.

[0033] If there is no special instruction, all steps of this application can be carried out in sequence or randomly, and preferably in sequence. For example, the method includes steps (a) and (b), which means that the method can include steps (a) and (b) carried out in sequence, or can also include steps (b) and (a) carried out in sequence. For example, it is mentioned that the method may further include step (c), which means that step (c) can be added to the method in any order. For example, the method can include steps (a), (b) and (c), or can also include steps (a), (c) and (b), or can also include steps (c), (a) and (b), etc.

[0034] If there is no special instruction, the "including" and "comprising" mentioned in this application mean open-ended or can also be closed-ended. For example, the "including" and "comprising" can mean that other components not listed can also be included or comprised, or can only include or comprise the listed components.

[0035] If there is no special instruction, 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) while B is true (or exists); or both A and B are true (or exist).

[0036] The negative electrode active material 1 provided in the first aspect of this application, as Figure 1 shown, the negative electrode active material 1 includes a core-shell structured metal phosphide 10, and the core-shell structured metal phosphide 10 includes an inner core 11 and a shell layer 12. The inner core 11 includes a double metal phosphide derived from a Prussian blue compound. The shell layer 12 coats at least a part of the surface of the inner core 11, and the shell layer 12 includes a metal phosphide.

[0037] The negative electrode active material 1 provided in the present application has a synergistic effect between the core 11 and the shell 12 in the core-shell structured metal phosphide 10. On the one hand, the bimetallic phosphide derived from the Prussian blue compound in the core 11 and the metal phosphide in the shell 12 have a heterogeneous structure, and the core-shell structure improves the distribution uniformity of the heterogeneous structure, and can induce the generation of a built-in electric field at the interface of the heterogeneous structure, providing an additional driving force for the transfer of charges, so that the material has a lower sodium ion diffusion barrier and a higher charge state density, so that the material exhibits higher conductivity, thereby improving the rate performance of the battery; at the same time, the composite phosphide with a heterogeneous structure has a higher sodium ion adsorption energy, and the structure of the material can remain stable during repeated ion insertion and extraction, thereby alleviating the volume strain generated by the metal phosphide material during the charge and discharge process, and exhibiting a more stable long-cycle performance.

[0038] On the other hand, since the materials of the inner core 11 and the shell 12 are both phosphides, the bonding properties of the bimetallic phosphide derived from the Prussian blue compound in the inner core 11 and the metal phosphide in the shell 12 are similar (both are MP bonds), and therefore an interlaced structural layer is easily formed at the interface between the inner core 11 and the shell 12, thereby improving the structural stability of the material and further improving the cycle performance of the battery.

[0039] Therefore, the bimetallic phosphide derived from the Prussian blue compound in the core 11 works together with the metal phosphide in the shell 12, and the battery prepared thereby has improved cycle performance and rate performance.

[0040] It can be understood that the bimetallic phosphide derived from the Prussian blue compound in the inner core 11 has an open framework structure, provides abundant ion diffusion channels and active sites, reduces the ion transfer impedance, and improves the kinetic properties of the material, thereby improving the capacity performance and rate performance of the battery; the metal phosphide is used as the shell 12 to coat the inner core 11, and the metal phosphide of the shell 12 can not only provide additional capacity contribution, further improving the capacity performance of the battery, but also can play a synergistic role with the bimetallic phosphide derived from the Prussian blue compound in the inner core 11 to improve the cycle performance and rate performance of the battery.

[0041] In some embodiments, both the core 11 and the shell 12 include carbon nanotubes 13. In this way, the carbon nanotubes 13 are interlaced between the core 11 and the shell 12 to form a charge transfer channel, which improves the charge transfer ability of the material. In addition, the conductive channel structure improves the conductive uniformity of the material, reduces the resistance, improves the kinetic performance of the material, and thus improves the cycle stability of the battery.

[0042] In some embodiments, the molar ratio of carbon nanotubes 13 to bimetallic phosphide is 1:(2-5). For example, the molar ratio of carbon nanotubes 13 to bimetallic phosphide can be, but is not limited to, 1:2, 1:3, 1:4, 1:5. Controlling the molar ratio within a suitable range can ensure sufficient conductivity, promote rapid electron transmission, improve rate performance, and ensure a high active material content, thereby maintaining a high capacity.

[0043] In some embodiments, the negative electrode active material 1 further includes a conductive carbon layer 20. The conductive carbon layer 20 is coated on at least part of the surface of the core-shell structure metal phosphide 10. In this way, the conductive carbon layer 20 can act as a protective layer for the active material, which is beneficial to alleviate the volume expansion of the metal phosphide material during the charge and discharge process, further improve the structural stability of the material, reduce the crushing of the particle structure during the cycle, and thus improve the cycle performance of the battery; at the same time, the conductive carbon layer 20 is connected to the carbon nanotubes 13 staggered in the core-shell structure metal phosphide 10, and together form a high-speed conductive channel that is conducive to charge transfer, improve the conductive uniformity of the material, reduce the resistance, further improve the kinetic performance of the material, and thus improve the cycle stability of the battery.

[0044] In some embodiments, the conductive carbon layer 20 includes at least one of dopamine hydrochloride salt, pyrrole, chitosan, and graphene oxide.

[0045] In some embodiments, the thickness of the conductive carbon layer 20 is 1 nm to 8 nm. For example, the thickness of the conductive carbon layer 20 may be, but is not limited to, 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, and 8 nm. Controlling the thickness of the conductive carbon layer 20 within a suitable range can not only ensure the ion transmission rate, improve the specific capacity and power density of the material, but also effectively buffer the volume strain of the metal phosphide and improve the cycle stability of the battery.

[0046] In some of the embodiments, the core-shell structure metal phosphide 10 has a porous structure. In this way, the porous structure of the core-shell structure metal phosphide 10 can improve the wettability of the material to the electrolyte, provide more electrochemically active sites, and shorten the ion diffusion path, thereby improving the ion transfer rate and electrochemical reaction kinetics of the material. In addition, the porous structure of the core-shell structure metal phosphide 10 can also effectively alleviate the volume expansion of the material during the charge and discharge process, reduce structural stress, and improve the structural stability and cycle performance of the material. At the same time, the porous structure of the core-shell structure metal phosphide 10 can effectively reduce the density of the material and improve the specific capacity of the material.

[0047] Furthermore, the core-shell structured metal phosphide 10 has a microporous structure.

[0048] In some of these embodiments, in the core-shell structured metal phosphide 10, the mass ratio of the bimetallic phosphide to the metal phosphide is from 1:2 to 2:1. Exemplarily, in the core-shell structured metal phosphide 10, the mass ratio of the bimetallic phosphide to the metal phosphide can be, but is not limited to, 1:2, 2:3, 4:5, 1:1, 5:4, 3:2, 2:1.

[0049] In some of these embodiments, the bimetallic phosphide is a copper-iron bimetallic phosphide, and the metal phosphide is cobalt phosphide. Thus, the copper-iron bimetallic phosphide has a relatively high theoretical capacity and good electrochemical activity, and can provide a relatively high energy density, while cobalt phosphide has a relatively high electrical conductivity and good structural stability, and can promote electron transport and inhibit volume expansion; the two play a synergistic role, jointly improving the structural stability and kinetic performance of the anode active material 1, and further improving the cycle performance, rate performance and capacity performance of the battery.

[0050] The second aspect of the present application provides a method for preparing an anode active material, as Figure 2 shown, the preparation method includes the following steps:

[0051] S1. Coating a shell layer metal hydroxide sol on at least part of the surface of the bimetallic Prussian blue compound to obtain a precursor material.

[0052] S2. Performing a phosphating treatment on the precursor material to obtain a core-shell structured metal phosphide.

[0053] Wherein, the core-shell structured metal phosphide includes a core and a shell layer, the core includes a bimetallic phosphide derived from a Prussian blue compound, the shell layer is coated on at least part of the surface of the core, and the shell layer includes a metal phosphide.

[0054] The method for preparing the anode active material provided by the present application prepares a core-shell structured metal phosphide through sol coating and phosphating treatment, simplifies the preparation steps, reduces the cost, and is conducive to large-scale production preparation. Specifically, first coating the shell layer metal hydroxide sol on the surface of the bimetallic Prussian blue compound can achieve uniform coating of the core and form a core-shell structure with regular structure; subsequently, performing a phosphating treatment on the precursor material can not only convert the bimetallic Prussian blue compound into a bimetallic phosphide, but also convert the shell layer metal hydroxide into a metal phosphide, thereby realizing the phosphating of both the core and the shell layer simultaneously. The anode active material prepared by the preparation method of the present application has a core-shell structured metal phosphide, which improves the structural stability and kinetic performance of the material, and further improves the cycle performance, capacity performance and rate performance of the battery.

[0055] In some of these embodiments, the chemical general formula of the bimetallic Prussian blue compound is: A 2-x M 1[M 2 (CN) 6 1-y □ y ▪nH 2 O, where 0 < x < 2, 0 < y < 1, A is an alkali metal element, M 1 、M 2 represents a transition metal element, □ represents a vacancy. Further, M 2 is an Fe element. Exemplarily, the bimetallic Prussian blue compound can be: K 0.1 Cu[Fe(CN) 6 0.73 ·6H 2 O, Na 1.4 Cu 1.3 Fe(CN) 6 ·8H 2 O, etc.

[0056] In some embodiments, carbon nanotubes are present in the bimetallic Prussian blue compound. Thus, the carbon nanotubes are embedded in the open framework of the bimetallic Prussian blue compound. After the bimetallic Prussian blue compound is coated with a shell layer of metal hydroxide sol and phosphated, the carbon nanotubes are interlaced and connected between the inner core and the shell layer to form a charge transport channel, improving the electrical conductivity of the material and thus enhancing the rate performance of the battery.

[0057] In some embodiments, before step S1, the preparation method further includes the following steps:

[0058] S01. Add a shell layer metal source and a bimetallic Prussian blue compound to an alkali solution to generate a shell layer metal hydroxide sol.

[0059] Thus, a sol can be in-situ generated on the surface of the bimetallic Prussian blue compound, thereby achieving a more uniform and tighter coating effect and forming a more regular core-shell structure.

[0060] Further, in step S01, the shell layer metal source is a cobalt source and the alkali solution is ammonia water. Thus, the cobalt source can be first complexed by ammonia water and then precipitated to form a cobalt hydroxide shell layer. The cobalt hydroxide shell layer in-situ generates a sol on the surface of the bimetallic Prussian blue compound, thereby achieving a more uniform and tighter coating effect and forming a more regular core-shell structure, enhancing the interfacial bonding force between the shell layer and the inner core and improving the structural stability of the material. In addition, when the shell layer metal source is a cobalt source, the shell layer metal phosphide obtained after phosphating treatment is cobalt phosphide, which has a high electrical conductivity and good structural stability, and is particularly suitable as a shell layer material to promote electron transport and inhibit the volume expansion of the bimetallic phosphide material in the inner core.

[0061] ​​In some of these embodiments, in step S01, the mass ratio of the carbon source, the shell metal source, and the bimetallic Prussian blue compound is (5 to 10):(5 to 10). Exemplarily, the mass ratio of the carbon source, the shell metal source, and the bimetallic Prussian blue compound can be, but is not limited to, 5:10, 1:1, 10:5.

[0062] In some of these embodiments, in step S01, the alkali solution includes a dispersant.

[0063] In some specific embodiments, the dispersant includes at least one of polyvinylpyrrolidone (PVP) and cetyltrimethylammonium bromide (CTAB).

[0064] In some of these embodiments, before step S1, the preparation method further includes the following steps:

[0065] S02. Add the carbon source, the shell metal source, and the bimetallic Prussian blue compound to the alkali solution to generate a shell metal hydroxide sol, and form a conductive carbon layer on the surface of the shell metal hydroxide sol.

[0066] In some of these embodiments, in step S02, the mass ratio of the carbon source, the shell metal source, and the bimetallic Prussian blue compound is 1:(5 to 10):(5 to 10). Exemplarily, the mass ratio of the carbon source, the shell metal source, and the bimetallic Prussian blue compound can be, but is not limited to, 1:5:5, 1:5:10, 1:10:5, 1:10:10.

[0067] In some of these embodiments, in step S2, the phosphorus source for the phosphating treatment includes at least one of red phosphorus, sodium phosphate, sodium dihydrogen phosphate, and sodium hypophosphite.

[0068] In some of these embodiments, in step S2, the mass ratio of the phosphorus source for the phosphating treatment to the precursor material is (2 to 5):1. Exemplarily, the mass ratio of the phosphorus source for the phosphating treatment to the precursor material can be, but is not limited to, 2:1, 3:1, 4:1, 5:1. Controlling the mass ratio within a suitable range can not only ensure sufficient phosphating effect but also reduce impurity residues caused by excessive phosphorus source.

[0069] In some of these embodiments, in step S2, the temperature of the phosphating treatment is 200°C to 450°C. Exemplarily, the temperature of the phosphating treatment can be, but is not limited to, 200°C, 250°C, 300°C, 350°C, 400°C, 450°C.

[0070] In some of these embodiments, in step S2, the heating rate of the phosphating treatment is 0.5 °C / min to 3 °C / min. Exemplarily, the heating rate of the phosphating treatment can be, but is not limited to, 0.5 °C / min, 1 °C / min, 1.5 °C / min, 2 °C / min, 2.5 °C / min, 3 °C / min.

[0071] In some of these embodiments, in step S2, the holding time of the phosphating treatment is 2 h to 6 h. Exemplarily, the holding time of the phosphating treatment can be, but is not limited to, 2 h, 3 h, 4 h, 5 h, 6 h.

[0072] In some of these embodiments, in step S2, the phosphating treatment is carried out in a tube furnace, the phosphorus source is placed at the upper end of the tube furnace, and the precursor material is placed at the lower end of the tube furnace.

[0073] The third aspect of the present application provides a negative electrode plate, which includes a negative electrode current collector and a negative electrode active material layer. The negative electrode active material layer is disposed on at least one surface of the negative electrode current collector, and the negative electrode active material layer includes the negative electrode active material provided in the first aspect above or the negative electrode active material prepared by the preparation method provided in the second aspect above.

[0074] In some embodiments, the negative electrode current collector can be a metal foil or a composite current collector. As an example of the metal foil, copper foil can be used. The composite current collector can include a polymer material base layer and a metal material layer formed on at least one surface of the polymer material base layer. As an example, the metal material can be selected from at least one of copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy. As an example, the polymer material base layer can be selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0075] In some embodiments of the present application, the negative electrode active material layer further includes other negative electrode active materials. Exemplarily, the other negative electrode active materials can include one or more of carbon materials, alloy materials, transition metal oxides, transition metal sulfides, phosphorus-based materials, and titanate materials. In some specific embodiments, the other negative electrode active materials include carbon materials.

[0076] In some examples, the carbon materials can include one or more of natural graphite, artificial graphite, mesocarbon microbeads (MCMB), hard carbon, and soft carbon. The alloy materials can include one or more of alloy materials composed of multiple of Si, Ge, Sn, Pb, and Sb. The chemical formula of the transition metal oxide is, for example, M 1 u O v where M 1It may be selected from one or more of Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, and V, au = 2v, and a is the valence state of M. 1 The chemical formula of the transition metal sulfide is, for example, M 2 i S j , where M 2 may include one or more of Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, and V, bi = 2j, and b is the valence state of M. 2 The phosphorus-based material may include one or more of red phosphorus, white phosphorus, and black phosphorus. The titanate material may include Na 2 Ti 3 O 7 , Na 2 Ti 6 O 13 , Na 4 Ti 5 O 12 , Li 4 Ti 5 O 12 , NaTi 2 (PO 4 ) 3 or one or more of them.

[0077] In some embodiments of the present application, the negative electrode active material layer may further include a conductive agent and a binder. The types of the conductive agent and the binder in the negative electrode active material layer are not particularly limited in the embodiments of the present application and can be selected according to actual needs.

[0078] Exemplarily, the conductive agent may be, but is not limited to, one or more of graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers; the binder may be, but is not limited to, one or more of styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl butyral (PVB), sodium alginate, water-based acrylic resin, and carboxymethyl cellulose (CMC).

[0079] In the embodiments of the present application, the negative electrode active material, the binder, and the conductive agent are mixed in a certain mass ratio and sufficiently stirred in an appropriate amount of deionized water to form a uniform negative electrode slurry; the negative electrode slurry is coated on the surface of the negative electrode current collector, and after baking, drying, and cold pressing, a negative electrode plate is obtained.

[0080] The fourth aspect of the present application provides a sodium ion battery, which includes the negative electrode plate provided in the third aspect above.

[0081] It can be understood that a sodium-ion battery generally includes an electrode assembly and an electrolyte. The electrode assembly is composed of a positive electrode sheet, a negative electrode sheet, and a separator. The sodium-ion battery mainly operates by the movement of metal ions through the separator between the positive electrode sheet and the negative electrode sheet.

[0082] Positive electrode sheet:

[0083] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material.

[0084] In some of these embodiments, the positive electrode active material containing sodium ions includes one or more of Prussian blue compounds, sodium transition oxides, and polyanion compounds. The appropriate positive electrode active material can be flexibly selected, so that the sodium-ion battery has more selectivity and a wider application range.

[0085] The positive electrode active material may include, but is not limited to, one or more of sodium transition metal oxides, polyanion compounds, and Prussian blue compounds. However, the present application is not limited to the materials listed above, and other known materials that can be used as the positive electrode active material of the sodium-ion battery can also be used.

[0086] In some embodiments, the transition metal in the sodium transition metal oxide may be one or more of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. A non-limiting example of the sodium transition metal oxide may be Na x ZO 2 , where Z may be one or more of Ti, V, Mn, Co, Ni, Fe, Cr, and Cu, and 0 < x ≤ 1.

[0087] In some embodiments, the sodium transition metal oxide may be Na 1-x Cu h Fe k Mn l M 1 m O 2-y , where M 1 may be one or more of Li, Be, B, Mg, Al, K, Ca, Ti, Co, Ni, Zn, Ga, Sr, Y, Nb, Mo, In, Sn, and Ba, 0 < x ≤ 0.33, 0 < h ≤ 0.24, 0 ≤ k ≤ 0.32, 0 < l ≤ 0.68, 0 ≤ m < 0.1, h + k + l + m = 1, 0 ≤ y < 0.2.

[0088] In some embodiments, the sodium transition metal oxide may be Na 0.67 Mn 0.7 Ni q M2 0.3-z O 2 , where M 2 can be one or more of Li, Mg, Al, Ca, Ti, Fe, Cu, Zn, and Ba, and 0 < q ≤ 0.1.

[0089] In some embodiments, the sodium transition metal oxide can be Na a Li b Ni c Mn d Fe e O 2 , where 0.67 < a ≤ 1, 0 < b < 0.2, 0 < c < 0.3, 0.67 < d + e < 0.8, and b + c + d + e = 1.

[0090] In some embodiments, the polyanion compound can be a compound having sodium ions, transition metal ions, and a tetrahedral (YO 4 ) n- anion unit. The transition metal here can be one or more of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y can be one or more of P, S, and Si; n represents the valence state of (YO 4 ) n- .

[0091] In some embodiments, the polyanion compound can also be a compound having sodium ions, transition metal ions, a tetrahedral (YO 4 )n- anion unit, and a halogen anion. The transition metal here can be one or more of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y can be one or more of P, S, and Si, n represents the valence state of (YO 4 ) n- ; the halogen can be one or more of F, Cl, and Br.

[0092] In some embodiments, the polyanion compound can also be a compound having sodium ions, a tetrahedral (YO 4 ) n- anion unit, a polyhedral unit (ZO y ) m+ , and an optional halogen anion. Y can be one or more of P, S, and Si, n represents the valence state of (YO 4 ) n- ; Z represents a transition metal, which can be one or more of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce, and m represents (ZOy ) m+ valence state; the halogen may be one or more of F, Cl, and Br.

[0093] In some embodiments, the polyanion compound may be NaFePO 4 、Na 3 V 2 (PO 4 ) 3 、NaM’PO 4 F and Na 3 (VO y ) 2 (PO 4 ) 2 F 3- 2 y (0 ≤ y ≤ 1), or one or more of them; wherein, M’ in NaM’PO 4 F may be one or more of V, Fe, Mn, and Ni.

[0094] In some embodiments, the Prussian blue compound may be a class of compounds having sodium ions, transition metal ions, and cyanide ions (CN — ). The transition metal here may be one or more of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. Non-limiting examples of Prussian blue compounds may be Na a Me b Me’ c (CN) 6 , where Me and Me’ may each independently be one or more of Ni, Cu, Fe, Mn, Co, and Zn, 0 < a ≤ 2, 0 < b < 1, 0 < c < 1.

[0095] In some embodiments of the present application, the positive electrode current collector may be made of a metal foil or a porous metal plate, etc. Exemplarily, the material of the positive electrode current collector may be, but not limited to, a foil or a porous plate of a metal such as copper, nickel, titanium, or silver, or an alloy thereof. Further, in some specific embodiments of the present application, the positive electrode current collector is made of aluminum foil.

[0096] In some embodiments of the present application, the positive electrode active material layer may further include a conductive agent and a binder. The types of the conductive agent and the binder in the positive electrode active material layer in the embodiments of the present application are not particularly limited and can be selected according to actual needs.

[0097] Exemplarily, the conductive agent may, but is not limited to, be one or more of graphite, superconducting carbon, acetylene black, conductive carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers; the binder may, but is not limited to, be one or more of styrene-butadiene rubber (SBR), aqueous acrylic resin, carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl butyral (PVB), ethylene-vinyl acetate copolymer (EVA), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, fluorinated acrylic resin, and polyvinyl alcohol (PVA).

[0098] In an embodiment of the present application, the positive electrode active material, the conductive agent, and the binder are fully stirred and mixed in an appropriate amount of N-methylpyrrolidone (NMP) according to a certain mass ratio to form a uniform positive electrode slurry; the positive electrode slurry is coated on the surface of the positive electrode current collector aluminum foil, and after drying, cold pressing, and die cutting, a positive electrode sheet is obtained.

[0099] Electrolyte:

[0100] In the sodium ion battery provided by the present application, the electrolyte includes an organic solvent and a sodium salt electrolyte. The types of the sodium salt electrolyte and the organic solvent are not specifically limited and can be selected according to actual needs. The electrolyte may include an organic solvent and a sodium salt electrolyte.

[0101] In some embodiments of the present application, the organic solvent includes an ether-based organic solvent. For example, the ether-based organic solvent includes one or more of diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, or tetraethylene glycol dimethyl ether, and tetrahydrofuran.

[0102] In some embodiments of the present application, the molar concentration of the sodium salt electrolyte in the electrolyte is 0.5 mol / L - 3 mol / L, and the sodium salt electrolyte may include one or more of sodium hexafluorophosphate, sodium perchlorate, sodium fluoroborate, sodium bis(trifluoromethylsulfonyl)imide, sodium bis(fluorosulfonyl)imide, and sodium dioxalatoborate.

[0103] In some embodiments of the present application, the electrolyte may also optionally include an additive. For example, the additive may include a negative electrode film-forming additive, may also include a positive electrode film-forming additive, and may also include an additive that can improve certain properties of the sodium ion battery, such as an additive for improving the overcharge performance of the sodium ion battery, an additive for improving the high-temperature performance of the sodium ion battery, an additive for improving the low-temperature power performance of the sodium ion battery, etc.

[0104] Separator:

[0105] In the embodiments of the present application, there is no particular limitation on the material of the separator, and any well-known separator with good chemical stability and mechanical stability can be selected. For example, the separator may include one or more of a porous polyolefin-based resin film (such as one or more of polyethylene, polypropylene, and polyvinylidene fluoride), porous glass fiber, and porous non-woven fabric. The porous separator can be a single-layer film or a multi-layer composite film. When the porous separator is a multi-layer composite film, the materials of each layer can be the same or different.

[0106] The sodium-ion battery can be used as a power source for an electrical device or as an energy storage unit for an electrical device. The electrical device can be, but is not limited to, a mobile phone, a tablet, a laptop computer, an electric toy, an electric tool, an electric vehicle, an electric car, a ship, a spacecraft, an energy storage system, and the like. Among them, the electric toy can include a fixed or mobile electric toy, such as a game console, an electric vehicle toy, an electric ship toy, and an electric aircraft toy, etc. The spacecraft can include an airplane, a rocket, a space shuttle, a spaceship, and the like.

[0107] In some of these embodiments, the electrical device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc.

[0108] The present application will be further described below in conjunction with specific examples and comparative examples.

[0109] Example 1

[0110] Negative electrode active material:

[0111] S1. Dissolve carbon nanotubes and sodium citrate in 40 mL of deionized water, ultrasonicate for 1 h until in a dispersed state, and then add Cu(AC) 2 ·3H 2 O and stir for 2 h to obtain solution A; dissolve Na 4 Fe(CN) 6 ·10H 2 O in 50 ml of deionized water to obtain solution B; with continuous stirring, add solution B dropwise (drop rate of 0.2 mL / s) to solution A, age at room temperature for 12 h; perform centrifugal separation, wash the obtained solid 3 times with deionized water and ethanol, place the solid in an oven, and dry at 70 °C for 8 h to obtain a copper-iron Prussian blue compound with carbon nanotubes.

[0112] Among them, the molar ratio of carbon nanotubes, copper acetate trihydrate, and sodium ferrocyanide decahydrate is 1:5:5, and the mass of sodium citrate is 2% of the sum of the masses of the copper source and the iron source.

[0113] S2. Dissolve the cobalt source Co(AC) 2 ·3H 2O, dispersant PVP, and ammonia water were ultrasonically dissolved in 100 mL of deionized water for 1 h, then 100 μL of pyrrole monomer was added and stirred for 1 h; then 50 mL of a solution containing copper-iron Prussian blue compound with carbon nanotubes was added dropwise. After stirring for 2 h, the solid substance obtained by standing was washed 3 times with deionized water and ethanol respectively, collected by centrifugation, and the obtained solid substance was placed in a drying oven and dried at 70 °C for 8 h to form a shell layer Co(OH) coated on the surface of the copper-iron Prussian blue compound. 2 sol, and a conductive carbon layer was formed on the surface of the shell layer Co(OH) 2 sol to obtain a precursor material.

[0114] Among them, the mass ratio of pyrrole, cobalt acetate trihydrate, and copper-iron Prussian blue compound is 1:10:10, and the mass of PVP and ammonia water is 1.5% of the sum of the masses of the cobalt source and the copper-iron Prussian blue compound.

[0115] S3. The precursor material was ground and placed at the lower end of a tube furnace, and phosphorus powder was placed at the upper end of the tube furnace. The mass ratio of the precursor to the phosphorus powder is 1:2. Under the protection of inert gas Ar, it was heated to 350 °C at a heating rate of 0.5 °C / min and held for 3 h. After the calcination and cooling, the negative electrode active material was obtained.

[0116] Among them, in the negative electrode active material, the molar ratio of carbon nanotubes to bimetallic phosphide is 1:5; the thickness of the conductive carbon layer is 2 nm.

[0117] Negative electrode plate:

[0118] The above-mentioned negative electrode active material, conductive agent carbon black, and binder sodium alginate were uniformly mixed according to a mass ratio of 95:3:2, and deionized water was added to make a negative electrode slurry. Then it was coated on the surface of the negative electrode current collector copper foil and vacuum dried at 80 °C for 12 h to form a negative electrode active material layer. Finally, it was roll-pressed and blanked to obtain a negative electrode plate.

[0119] Sodium-ion battery:

[0120] The Na sheet used as the counter electrode and the above-mentioned negative electrode plate were assembled into a CR2032 type button battery in a glove box with a water oxygen content less than 0.01 ppm. The electrolyte used was 1M NaClO 4 and a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) with an equal volume ratio, and 5% fluoroethylene carbonate (FEC) was added as an additive.

[0121] Example 2

[0122] The preparation methods of the negative electrode active material, negative electrode plate, and sodium-ion battery provided in this example are basically the same as those in Example 1, except that:

[0123] In step S1, the molar ratio of carbon nanotubes, copper acetate trihydrate, and sodium ferrocyanide decahydrate is 1:2:2; the molar ratio of carbon nanotubes to bimetallic phosphide in the prepared negative electrode active material is 1:2; the thickness of the conductive carbon layer is 2 nm.

[0124] Example 3

[0125] The preparation methods of the negative electrode active material, negative electrode sheet, and sodium-ion battery provided in this example are basically the same as those in Example 1, except that:

[0126] In step S2, the mass ratio of pyrrole, cobalt acetate trihydrate, and copper-iron Prussian blue compound is 1:5:5; the molar ratio of carbon nanotubes to bimetallic phosphide in the prepared negative electrode active material is 1:5; the thickness of the conductive carbon layer is 4 nm.

[0127] Example 4

[0128] The preparation methods of the negative electrode active material, negative electrode sheet, and sodium-ion battery provided in this example are basically the same as those in Example 1, except that:

[0129] In step S2, the mass ratio of pyrrole, cobalt acetate trihydrate, and copper-iron Prussian blue compound is 1:10:5; the molar ratio of carbon nanotubes to bimetallic phosphide in the prepared negative electrode active material is 1:5; the thickness of the conductive carbon layer is 4 nm.

[0130] Example 5

[0131] The preparation methods of the negative electrode active material, negative electrode sheet, and sodium-ion battery provided in this example are basically the same as those in Example 1, except that:

[0132] In step S2, the mass ratio of pyrrole, cobalt acetate trihydrate, and copper-iron Prussian blue compound is 1:5:10; the molar ratio of carbon nanotubes to bimetallic phosphide in the prepared negative electrode active material is 1:5; the thickness of the conductive carbon layer is 2 nm.

[0133] Example 6

[0134] The preparation methods of the negative electrode active material, negative electrode sheet, and sodium-ion battery provided in this example are basically the same as those in Example 1, except that:

[0135] In step S2, cobalt acetate trihydrate is replaced by copper acetate trihydrate.

[0136] The molar ratio of carbon nanotubes to bimetallic phosphide in the prepared negative electrode active material is 1:5; the thickness of the conductive carbon layer is 2 nm.

[0137] Example 7

[0138] The preparation methods of the negative electrode active material, negative electrode sheet and sodium ion battery provided in this embodiment are basically the same as those in Embodiment 1, except that:

[0139] In step S1, carbon nanotubes are not added.

[0140] The thickness of the conductive carbon layer in the negative electrode active material prepared thus is 2 nm.

[0141] Embodiment 8

[0142] The preparation methods of the negative electrode active material, negative electrode sheet and sodium ion battery provided in this embodiment are basically the same as those in Embodiment 1, except that:

[0143] In step S2, pyrrole monomer is not added.

[0144] The molar ratio of carbon nanotubes to bimetallic phosphide in the negative electrode active material prepared thus is 1:5.

[0145] Embodiment 9

[0146] The preparation methods of the negative electrode active material, negative electrode sheet and sodium ion battery provided in this embodiment are basically the same as those in Embodiment 1, except that:

[0147] In step S2, ammonia water is replaced by sodium hydroxide.

[0148] The molar ratio of carbon nanotubes to bimetallic phosphide in the negative electrode active material prepared thus is 1:5; the thickness of the conductive carbon layer is 2 nm.

[0149] Comparative Example 1

[0150] Negative electrode active material:

[0151] S1. Dissolve carbon nanotubes and sodium citrate in 40 mL of deionized water, ultrasonic for 1 h until in a dispersed state, and then add Cu(AC) 2 ·3H 2 O and stir for 2 h to obtain solution A; dissolve Na 4 Fe(CN) 6 ·10H 2 O in 50 ml of deionized water to obtain solution B; under continuous stirring, add solution B drop by drop (drop rate is 0.2 mL / s) to solution A, age at room temperature for 12 h; perform centrifugal separation, wash the obtained solid 3 times with deionized water and ethanol, place the solid in an oven, and dry at 70 °C for 8 h to obtain a copper-iron Prussian blue compound with carbon nanotubes.

[0152] Among them, the molar ratio of carbon nanotubes, copper acetate trihydrate, and sodium ferrocyanide decahydrate is 1:5:5, and the mass of sodium citrate is 2% of the sum of the masses of the copper source and the iron source.

[0153] S2. Ultrasonically dissolve 100 μL of pyrrole monomer in 100 mL of deionized water for 1 h, stir for 1 h; then add dropwise 50 mL of a solution containing copper-iron Prussian blue compound with carbon nanotubes, stir for 2 h, let it stand, wash the obtained solid material 3 times with deionized water and ethanol respectively, collect by centrifugation, place the obtained solid material in an oven, dry at 70 °C for 8 h to form a conductive carbon layer on the surface of the copper-iron Prussian blue compound, and obtain a precursor material.

[0154] S3. Grind the precursor material and place it at the lower end of a tube furnace, place phosphorus powder at the upper end of the tube furnace, the mass ratio of the precursor to phosphorus powder is 1:2, under the protection of inert gas Ar, heat at a heating rate of 0.5 °C / min to 350 °C and hold for 3 h, after the calcination and cooling, obtain the negative electrode active material.

[0155] Among them, in the negative electrode active material, the molar ratio of carbon nanotubes to bimetallic phosphide is 1:5; the thickness of the conductive carbon layer of the negative electrode active material prepared thus is 2 nm.

[0156] Negative electrode plate:

[0157] Uniformly mix the above-mentioned negative electrode active material, conductive agent carbon black, and binder sodium alginate according to a mass ratio of 95:3:2, add deionized water to make a negative electrode slurry, then coat it on the surface of the negative electrode current collector copper foil, and vacuum dry at 80 °C for 12 h to form a negative electrode active material layer, and finally obtain a negative electrode plate through rolling and blanking.

[0158] Sodium ion battery:

[0159] Assemble a CR2032 type button battery with a Na sheet as the counter electrode and the above-mentioned negative electrode plate in a glove box with a water and oxygen content less than 0.01 ppm. The electrolyte used is 1M NaClO 4 And a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) with an equal volume ratio, and add 5% fluoroethylene carbonate (FEC) as an additive.

[0160] Comparative example 2

[0161] Negative electrode active material:

[0162] S1. Dissolve carbon nanotubes and sodium citrate in 40 mL of deionized water, ultrasonically disperse for 1 h, then add Cu(AC) 2 ·3H 2 O and stir for 2 h to obtain solution A; add Na 4 Fe(CN) 6 ·10H 2O was dissolved in 50 ml of deionized water to obtain solution B; under continuous stirring, solution B was added dropwise (drop rate: 0.2 mL / s) to solution A with the aid of a peristaltic pump, and aged at room temperature for 12 h; centrifuged, and the obtained solid was washed 3 times with deionized water and ethanol, and the solid was placed in an oven and dried at 70 °C for 8 h to obtain a copper-iron Prussian blue compound with carbon nanotubes.

[0163] Among them, the molar ratio of carbon nanotubes, copper acetate trihydrate, and sodium ferrocyanide decahydrate was 1:5:5, and the mass of sodium citrate was 2% of the sum of the masses of the copper source and the iron source.

[0164] S2. The copper-iron Prussian blue compound with carbon nanotubes was ground and placed at the lower end of a tubular furnace, and phosphorus powder was placed at the upper end of the tubular furnace. The mass ratio of the copper-iron Prussian blue compound with carbon nanotubes to the phosphorus powder was 1:1. Under the protection of inert gas Ar, it was heated to 350 °C at a heating rate of 0.5 °C / min and held for 3 h. After calcination and cooling, a copper-iron bimetallic phosphide was obtained.

[0165] S3. The copper-iron bimetallic phosphide was mixed with cobalt phosphide (Shanghai Macklin Biochemical Co., Ltd., CAS No. 12134-02-0) in a mass ratio of 1:1 to obtain a negative electrode active material.

[0166] Among them, in the negative electrode active material, the molar ratio of carbon nanotubes to the bimetallic phosphide was 1:5.

[0167] Negative electrode plate:

[0168] The above-mentioned negative electrode active material, conductive agent carbon black, and binder sodium alginate were uniformly mixed in a mass ratio of 95:3:2, and deionized water was added to make a negative electrode slurry, which was then coated on the surface of the negative electrode current collector copper foil and vacuum dried at 80 °C for 12 h to form a negative electrode active material layer. Finally, it was roll-pressed and punched to obtain a negative electrode plate.

[0169] Sodium ion battery:

[0170] The Na sheet used as the counter electrode and the above-mentioned negative electrode plate were assembled into a CR2032 type button battery in a glove box with a water and oxygen content less than 0.01 ppm. The electrolyte used was 1M NaClO 4 And a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) with an equal volume ratio, and 5% fluoroethylene carbonate (FEC) was added as an additive.

[0171] Comparative example 3

[0172] Negative electrode active material:

[0173] S1. Dissolve carbon nanotubes and sodium citrate in 40 mL of deionized water, and ultrasonicate for 1 h until a dispersed state is achieved. Subsequently, add Cu(AC) 2 ·3H 2 O and stir for 2 h to obtain solution A. Dissolve Na 4 Fe(CN) 6 ·10H 2 O in 50 ml of deionized water to obtain solution B. Under continuous stirring, use a peristaltic pump to gradually add solution B drop by drop (drop rate: 0.2 mL / s) to solution A, and age at room temperature for 12 h. Centrifuge and wash the obtained solid three times with deionized water and ethanol. Place the solid in an oven and dry at 70 °C for 8 h to obtain a copper-iron Prussian blue compound with carbon nanotubes.

[0174] Among them, the molar ratio of carbon nanotubes, copper acetate trihydrate, and sodium ferrocyanide decahydrate is 1:5:5, and the mass of sodium citrate is 2% of the sum of the masses of the copper source and the iron source.

[0175] S2. Grind the precursor material and place it at the lower end of a tube furnace. Place phosphorus powder at the upper end of the tube furnace. The mass ratio of the precursor to phosphorus powder is 1:2. Under the protection of inert gas Ar, heat at a heating rate of 0.5 °C / min to 350 °C and hold for 3 h. After cooling the calcination, obtain the negative electrode active material.

[0176] Among them, in the negative electrode active material, the molar ratio of carbon nanotubes to bimetallic phosphide is 1:5.

[0177] Negative electrode plate:

[0178] Uniformly mix the above-mentioned negative electrode active material, conductive agent carbon black, and binder sodium alginate according to a mass ratio of 95:3:2, add deionized water to make a negative electrode slurry, then coat it on the surface of the negative electrode current collector copper foil, and vacuum dry at 80 °C for 12 h to form a negative electrode active material layer. Finally, after rolling and punching, obtain the negative electrode plate.

[0179] Sodium-ion battery:

[0180] Assemble a CR2032-type coin cell with a Na sheet as the counter electrode and the above-mentioned negative electrode plate in a glove box with a water and oxygen content less than 0.01 ppm. The electrolyte used is a mixture of 1M NaClO 4 and an equal volume ratio of ethylene carbonate (EC) and dimethyl carbonate (DMC), and add 5% fluoroethylene carbonate (FEC) as an additive.

[0181] Performance test

[0182] Conduct the following tests on a BlueTron battery test system:

[0183] (1) Cycle performance test

[0184] At room temperature (25 °C ± 2 °C), in the first step, charge at a current of 0.1 A g -1 to the cut-off voltage of 2.5 V and let it stand for 30 min; in the second step, discharge at a current of 0.1 A g -1 to the cut-off voltage of 0.01 V and let it stand for 30 min; conduct cyclic tests, and take the average value of the reversible capacity in the first 5 weeks as C 0 . Test the cyclic performance of the sodium-ion batteries in the above-mentioned examples and comparative examples for 100 cycles. The reversible capacity in the 100th cycle is C 1 , and C 1 / C 0 is the capacity retention rate for 100 cycles. The test results are shown in Table 1.

[0185] (2) Rate performance test

[0186] At room temperature (25 °C ± 2 °C), conduct charge and discharge tests at a current of 0.1 A·g -1 in the voltage range of 0.01 - 2.5 V, and record the reversible capacity as C 0 ; similarly, conduct charge and discharge gradient tests at 0.3 A·g -1 , 0.5 A·g -1 , 1 A·g -1 , 2 A·g -1 currents, and record the reversible capacity at 2 A·g -1 as C 1 , and C 1 / C 0 is the capacity retention rate at 2 A·g -1 . The test results are shown in Table 1.

[0187] Table 1

[0188]

[0189] Combined with Table 1, by comparing Examples 1 - 9 and Comparative Examples 1 - 3, it can be seen that the negative electrode active material provided in this application improves the capacity performance, rate performance, and cyclic performance of the battery.

[0190] The technical features of the above-mentioned embodiments can be combined arbitrarily. For the sake of concise description, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope recorded in this specification.

[0191] The above-described embodiments merely represent several implementation manners of the present application. The description thereof is relatively specific and detailed, but it should not be construed as a limitation to the patent scope of the present application. It should be noted that for those of ordinary skill in the art, without departing from the technical concept of the present application, several modifications and improvements can still be made, and these all fall within the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the appended claims.

Claims

1. A negative electrode active material, characterized in that: It comprises a core-shell structure metal phosphide, wherein the core-shell structure metal phosphide comprises: a core comprising a bimetallic phosphide derived from a Prussian blue compound; and The shell layer covers at least a portion of the surface of the core, and the shell layer includes metal phosphide.

2. The negative electrode active material according to claim 1, characterized in that The core and the shell both include carbon nanotubes.

3. The negative electrode active material according to claim 2, characterized in that: The molar ratio of the carbon nanotubes to the bimetallic phosphide is 1:(2-5).

4. The negative electrode active material according to claim 2, characterized in that Also includes: The conductive carbon layer is coated on at least a portion of the surface of the core-shell structured metal phosphide.

5. The negative electrode active material according to claim 4, characterized in that The thickness of the conductive carbon layer is 1 nm to 8 nm.

6. The negative electrode active material according to any one of claims 1 to 5, characterized in that: The core-shell structured metal phosphide satisfies at least one of the following conditions: (1) The core-shell structured metal phosphide has a porous structure; (2) The bimetallic phosphide is copper-iron bimetallic phosphide, and the metal phosphide is cobalt phosphide.

7. A method for preparing a negative electrode active material, characterized in that: The following steps are involved: coating at least a portion of the surface of the bimetallic Prussian blue compound with a shell metal hydroxide sol to obtain a precursor material; Phosphating the precursor material to obtain a core-shell structured metal phosphide; The core-shell structured metal phosphide comprises a core and a shell, wherein the core comprises a bimetallic phosphide derived from a Prussian blue compound, and the shell is coated on at least a portion of the surface of the core, and the shell comprises a metal phosphide.

8. The preparation method according to claim 7, characterized in that: Before the step of coating at least a portion of the surface of the bimetallic Prussian blue compound with a shell metal hydroxide sol to obtain a precursor material, the step further includes: adding a shell metal source and the bimetallic Prussian blue compound into an alkaline solution to generate a shell metal hydroxide sol; Wherein, the shell metal source is a cobalt source, and the alkaline solution is ammonia water.

9. A negative electrode plate, characterized in that: include: Anode current collector; as well as A negative electrode active material layer is disposed on at least one side of the negative electrode current collector, wherein the negative electrode active material layer comprises the negative electrode active material according to any one of claims 1 to 6 or the negative electrode active material prepared by the preparation method according to claim 7 or 8.

10. A sodium ion battery, characterized in that: Including the negative electrode sheet as described in claim 9.

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