Secondary battery, electric device, positive electrode active material and preparation method therefor, and positive electrode sheet

Abstract

A secondary battery, an electric device, a positive electrode active material and a preparation method therefor, and a positive electrode sheet, relating to the technical field of batteries. The secondary battery comprises a positive electrode sheet; the positive electrode sheet comprises lithium phosphate-containing positive electrode active particles, each lithium phosphate-containing positive electrode active particle comprises a central part and a surface part, and the surface part is continuously or discontinuously distributed on the surface of the central part; the thickness of the surface part is less than or equal to 10 nm, and the lithium content of the central part is greater than that of the surface part. The secondary battery has high cycle performance.

Description

Secondary battery, electrical equipment, positive electrode active material and preparation method thereof, and positive electrode sheet

[0001] Cross-references

[0002] This application claims priority to the Chinese invention patent application with application number 202311628117.X filed on November 29, 2023, and invention name “Positive electrode active material and preparation method thereof, positive electrode sheet, battery and electrical equipment”, the contents of which are incorporated herein by reference. Technical Field

[0003] The present application relates to the field of battery technology, and in particular to a secondary battery, an electrical device, a positive electrode active material, a preparation method thereof, and a positive electrode sheet. Background Art

[0004] In recent years, with the development of secondary battery technology, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and other fields. Currently, the cycle performance of secondary batteries needs to be further improved.

[0005] Summary of the Invention

[0006] In response to the deficiencies of the prior art, the present application provides a secondary battery, an electrical device, a positive electrode active material, a preparation method thereof, and a positive electrode plate, so that the capacity of the positive electrode active material can be fully utilized and has a higher electronic conductivity, and the secondary battery has a higher cycle performance.

[0007] In a first aspect, the present application provides a secondary battery, which includes a positive electrode plate, the positive electrode plate includes lithium phosphate positive electrode active particles, the lithium phosphate positive electrode active particles include a central portion and a surface portion, the surface portion is continuously or discontinuously distributed on the surface of the central portion; the thickness of the surface portion is ≤10nm; the lithium content of the central portion is greater than the lithium content of the surface portion.

[0008] In the positive electrode sheet of the secondary battery provided in the present application, the lithium content in the center of the lithium phosphate positive active particle is greater than the lithium content in the surface portion, and the thickness of the surface portion is ≤10nm, which can make the secondary battery have higher cycle performance.

[0009] In some embodiments, in the Raman spectrum of the lithium-containing phosphate positive active particles, the Raman shift of 200 cm -1 ~250cm -1 The position has an O-Fe-O stretching vibration peak; and / or, in the Raman spectrum of the lithium phosphate positive active particles, the Raman shift is 255cm -1 ~300cm -1The O-Fe-O bending vibration peak is located at the position of the lithium phosphate positive electrode active particles. This not only allows the capacity of the lithium phosphate positive electrode active particles to be fully utilized and the lithium phosphate positive electrode active particles to have higher electronic conductivity, but also allows the secondary battery prepared using the lithium phosphate positive electrode active particles to have higher cycle performance.

[0010] In some embodiments, the surface portion includes iron oxide. The surface portion of the lithium-containing phosphate positive electrode active particles contains iron oxide, which has high electronic conductivity. This not only allows the lithium-containing phosphate positive electrode active particles to have high electronic conductivity, but also allows the capacity of the lithium-containing phosphate positive electrode active particles to be fully utilized. Furthermore, compared to secondary batteries prepared using only lithium-containing phosphate as the positive electrode active material, the lithium-containing phosphate positive electrode active particles provided in this application that contain iron oxide on the surface can improve the cycle performance of the resulting secondary battery.

[0011] In some embodiments, the surface portion includes ferric oxide, which not only allows the capacity of the lithium-phosphate-containing positive electrode active particles to be fully utilized and the lithium-phosphate-containing positive electrode active particles to have higher electronic conductivity, but also allows the secondary battery prepared using the lithium-phosphate-containing positive electrode active particles to have higher cycle performance.

[0012] In some embodiments, the lithium phosphate positive active particles include Li 1+x M 1-y A y P 1-z R z O 4-t , M includes at least one of Fe, Co and Ni, A includes at least one of Mn, Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and R includes at least one of B, S, Si and N; -0.1≤x≤0.1, 0≤y≤0.1, 0≤z≤0.1, 0≤t≤0.1.

[0013] In some embodiments, the lithium phosphate positive active particles include Li 1+x1 Fe 1-y1 A y1 PO 4-t1 , A includes at least one of Mn, Al, Ti, V, Ni and Zn; 0≤x1≤0.05, 0≤y1≤0.05, 0≤t1≤0.02.

[0014] In some embodiments, the thickness of the surface portion is 1.5 nm to 4 nm. A thickness of the surface portion within the above range is beneficial for further enhancing the capacity of the lithium-containing phosphate positive electrode active particles and improving the electronic conductivity of the lithium-containing phosphate positive electrode active particles, and is also beneficial for further improving the cycle performance of a secondary battery prepared using the lithium-containing phosphate positive electrode active particles.

[0015] In some embodiments, the volume average particle size D of the lithium phosphate positive active particles is V 50 is 300nm~10.5μm.

[0016] In a second aspect, the present application provides an electrical device, which includes a battery provided in any one of the first aspects above.

[0017] In a third aspect, the present application provides a positive electrode active material, the positive electrode active material comprising: a substrate and an oxide layer located on the surface of the substrate; the substrate comprises Li 1+x M 1-y A y P 1-z R z O 4-t , wherein M includes at least one of Fe, Co and Ni, A includes at least one of Mn, Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and R includes at least one of B, S, Si and N; -0.1≤x≤0.1, 0≤y≤0.1, 0≤z≤0.1, 0≤t≤0.1; the oxide layer includes iron oxide.

[0018] In the positive electrode active material provided in the present application, the surface of the substrate has an oxide layer containing iron oxide, and the iron oxide has a high electronic conductivity, which not only makes the positive electrode active material have a high electronic conductivity, but also makes the capacity of the substrate fully utilized; in addition, compared with the secondary battery prepared by using only the substrate as the positive electrode active material, the use of the positive electrode active material provided in the present application can improve the cycle performance of the secondary battery prepared.

[0019] In some embodiments, the matrix includes Li 1+x1 Fe 1-y1 A y1 PO 4-t1 , A includes at least one of Mn, Al, Ti, V, Ni and Zn; 0≤x1≤0.05, 0≤y1≤0.05, 0≤t1≤0.02.

[0020] In some embodiments, in the Raman spectrum of the positive electrode active material, at a Raman shift of 200 cm -1 ~250cm -1The position has an O-Fe-O stretching vibration peak; and / or, in the Raman spectrum of the positive electrode active material, the Raman shift is 255cm -1 ~300cm -1 The position of the positive electrode active material has an O-Fe-O bending vibration peak. This not only allows the capacity of the matrix to be fully utilized and the positive electrode active material to have a higher electronic conductivity, but also allows the secondary battery prepared using the positive electrode active material to have a higher cycle performance.

[0021] In some embodiments, the oxide layer includes ferric oxide; this not only allows the capacity of the matrix to be fully utilized and the positive electrode active material to have a higher electronic conductivity, but also allows the secondary battery prepared using the positive electrode active material to have a higher cycle performance.

[0022] In some embodiments, the thickness of the oxide layer is ≤10 nm. When the thickness of the oxide layer is within the above range, not only can the capacity of the matrix be fully utilized and the positive electrode active material have a high electronic conductivity, but also the secondary battery prepared using the positive electrode active material can have a high cycle performance.

[0023] In some embodiments, the thickness of the oxide layer is 1.5 nm to 4 nm. The thickness of the oxide layer within the above range is conducive to further improving the capacity of the matrix and the electronic conductivity of the positive electrode active material, and is also conducive to further improving the cycle performance of the secondary battery prepared using the positive electrode active material.

[0024] In some embodiments, the volume average particle size D of the positive electrode active material is V 50 is 300nm~10.5μm.

[0025] In a fourth aspect, the present application provides a method for preparing a positive electrode active material, the method comprising: treating a substrate to form an oxide layer on the surface of the substrate; the substrate comprising Li 1+x M 1-y A y P 1-z R z O 4-t , wherein M includes at least one of Fe, Co and Ni, A includes at least one of Mn, Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and R includes at least one of B, S, Si and N; -0.1≤x≤0.1, 0≤y≤0.1, 0≤z≤0.1, 0.001≤t≤0.1; and the oxide layer includes iron oxide.

[0026] The present application forms an oxide layer containing iron oxide on the surface of the substrate. The iron oxide has a high electronic conductivity, which not only makes the positive electrode active material have a high electronic conductivity, but also allows the capacity of the substrate to be fully utilized. In addition, compared with a secondary battery prepared by using only the substrate as the positive electrode active material, the positive electrode active material provided by the present application can improve the cycle performance of the secondary battery prepared.

[0027] In some embodiments, the matrix includes Li 1+x1 Fe 1-y1 A y1 PO 4-t1 , A includes at least one of Mn, Al, Ti, V, Ni and Zn; 0≤x1≤0.05, 0≤y1≤0.05, 0≤t1≤0.02; the substrate is oxidized to form an oxide layer on the surface of the substrate. The present application performs an oxidation treatment on a substrate containing Fe elements, and an in-situ oxidation reaction can be performed on the surface of the substrate to form an oxide layer containing iron oxide through a one-step oxidation operation, which has the advantages of simple process and easy scalability; and the method of in-situ oxidation on the surface of the substrate containing Fe elements to form an oxide layer containing iron oxide can make the combination between the oxide layer and the substrate more tight, and the distribution of iron oxide on the surface of the substrate is more uniform, which is conducive to further fully utilizing the capacity of the substrate, and is conducive to improving the electronic conductivity of the positive electrode active material and improving the cycle performance of the secondary battery prepared using the positive electrode active material.

[0028] In some embodiments, the substrate is oxidized using an oxidizing gas. The oxidizing gas can react with the surface of the substrate containing the Fe element to form iron oxide, thereby forming an oxide layer containing iron oxide on the surface of the substrate. This allows the capacity of the substrate in the prepared positive electrode active material to be fully utilized, resulting in the positive electrode active material having higher electronic conductivity and a secondary battery prepared using the positive electrode active material having higher cycle performance.

[0029] In some embodiments, the oxidizing gas includes at least one of oxygen and ozone.

[0030] In some embodiments, the combined volume of oxygen and ozone in the oxidizing gas accounts for 10% to 100% of the total volume of the oxidizing gas. The above technical solution is conducive to increasing the rate of formation of the iron oxide-containing oxide layer formed on the surface of the substrate, and is conducive to improving the preparation efficiency of the positive electrode active material.

[0031] In some embodiments, the temperature of the oxidation treatment is ≥300° C. When an oxidizing gas is used to oxidize a substrate containing an Fe element, the oxidation treatment temperature is ≥300° C. This allows the oxidizing gas to undergo an oxidation reaction with the substrate containing the Fe element to form iron oxide, thereby forming an oxide layer containing iron oxide on the surface of the substrate. This allows the capacity of the substrate in the prepared positive electrode active material to be fully utilized, resulting in the positive electrode active material having higher electronic conductivity, and the secondary battery prepared using the positive electrode active material having higher cycle performance.

[0032] In some embodiments, the oxidation treatment temperature is 300° C. to 600° C. When the oxidation treatment temperature is within the above range, the generation rate of the iron oxide layer formed on the surface of the substrate containing the Fe element can be faster, which is beneficial to improving the preparation efficiency of the positive electrode active material.

[0033] In some embodiments, during the oxidation treatment, the flow rate of the oxidizing gas is 200 sccm to 500 sccm. The oxidizing gas flow rate within the above range is conducive to forming a relatively dense and uniform oxide layer on the surface of the substrate containing the Fe element, thereby further facilitating the full utilization of the substrate capacity and improving the electronic conductivity of the positive electrode active material, and enabling the secondary battery prepared using the positive electrode active material to have better cycle performance.

[0034] In some embodiments, the oxidation treatment time is 2 minutes to 60 minutes; when the oxidation treatment time is within the above range, the surface of the matrix containing the Fe element can fully react with the oxidizing gas, which is beneficial for the mass fraction of the formed oxide layer containing iron oxide in the entire positive electrode active material to be within a more appropriate range, which can not only fully utilize the capacity of the matrix in the positive electrode active material and make the positive electrode active material have a higher electronic conductivity, but also make the secondary battery prepared using the positive electrode active material have a higher cycle performance.

[0035] In a fifth aspect, the present application provides a positive electrode plate, which includes a positive electrode collector and a positive electrode active layer covering at least one surface of the positive electrode collector in the thickness direction; wherein the positive electrode active layer includes a first active material, and the first active material includes the positive electrode active material provided in any one of the third aspects above or the positive electrode active material prepared by the preparation method provided in any one of the fourth aspects above.

[0036] In some embodiments, the positive active layer further includes a second active material, and the second active material is different from the first active material.

[0037] In a sixth aspect, the present application provides a battery, which includes the positive electrode plate provided in the fifth aspect above.

[0038] In a seventh aspect, the present application provides an electrical device, which includes the battery provided in the sixth aspect.

[0039] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Various other advantages and benefits will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiment below. The accompanying drawings are for illustration purposes only and are not to be considered as limiting the present application. The same reference numerals are used throughout the drawings to represent the same components. In the drawings:

[0041] FIG1 is a schematic structural diagram of a vehicle provided in some embodiments of the present application;

[0042] FIG2 is a schematic diagram of the exploded structure of a battery provided in some embodiments of the present application;

[0043] FIG3 is a schematic structural diagram of a battery cell provided in some embodiments of the present application;

[0044] FIG4 is an exploded view of a battery cell provided in some embodiments of the present application;

[0045] FIG5 is a schematic structural diagram of an electrode assembly provided in some embodiments of the present application;

[0046] FIG6 is a TEM-EDS image of the first active material prepared in Example 6 of the present application;

[0047] FIG7 is a TEM-EDS image of the first active material provided in Comparative Example 1 of the present application;

[0048] FIG8 is a comparison chart of Raman spectra of the first active material prepared in Example 6 of the present application, the first active materials prepared in Examples 9 to 10, and the first active material provided in Comparative Example 1.

[0049] Icons: 1000-vehicle; 100-battery; 10-housing; 11-accommodation space; 12-first part; 13-second part; 20-battery cell; 21-housing; 211-opening; 22-end cover assembly; 221-end cover; 222-electrode terminal; 23-electrode assembly; 231-positive electrode sheet; 232-negative electrode sheet; 233-separation membrane; 24-current collecting member; 25-insulating protection member; 200-controller; 300-motor. DETAILED DESCRIPTION

[0050] The following embodiments of the technical solution of the present application will be described in detail with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application and are therefore only examples and are not intended to limit the scope of protection of the present application.

[0051] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" and any variations thereof in the specification and claims of this application and the above-mentioned figure descriptions are intended to cover non-exclusive inclusions.

[0052] In the description of the embodiments of this application, the technical terms "first" and "second" are used only to distinguish different objects and should not be understood to indicate or imply relative importance or implicitly specify the quantity, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, the meaning of "plurality" is more than two, unless otherwise clearly and specifically defined.

[0053] References herein to "embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiments may be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0054] In the description of the embodiments of this application, the term "and / or" is simply a description of the association relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent the following three situations: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in this document generally indicates that the associated objects are in an "or" relationship.

[0055] In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

[0056] In the description of the embodiments of the present application, the technical terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the embodiments of the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the embodiments of the present application.

[0057] In the description of the embodiments of the present application, unless otherwise expressly specified or limited, technical terms such as "installed," "connected," "connected," and "fixed" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integration; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; internal connections between two components or interactions between two components. Those skilled in the art can understand the specific meanings of the above terms in the embodiments of the present application based on specific circumstances.

[0058] Currently, market developments indicate that power batteries are becoming increasingly widely used. They are not only used in energy storage systems such as hydropower, thermal, wind, and solar power plants, but are also widely used in electric vehicles like electric bicycles, electric motorcycles, and electric vehicles, as well as in military equipment and aerospace. As power battery applications continue to expand, market demand is also growing.

[0059] Power batteries can be secondary batteries such as lithium-ion batteries. During the charging process of lithium-ion batteries, lithium ions are released from the positive electrode active material, transported through the electrolyte, passed through the separator, and embedded in the negative electrode active layer. The positive electrode active material is one of the key factors affecting the performance of lithium-ion batteries and other secondary batteries.

[0060] However, at present, there is still room for improvement in the capacity of positive electrode active materials, the improvement of electronic conductivity, and the improvement of the cycle performance of secondary batteries such as lithium-ion batteries.

[0061] Based on the above considerations, in order to make the secondary battery have higher cycle performance, the present application designs a secondary battery, the secondary battery includes a positive electrode plate, the positive electrode plate includes lithium phosphate positive electrode active particles, the lithium phosphate positive electrode active particles include a central portion and a surface portion, the surface portion is continuously or discontinuously distributed on the surface of the central portion; the thickness of the surface portion is ≤10nm; the lithium content of the central portion is greater than the lithium content of the surface portion.

[0062] In such a secondary battery, the lithium content in the center of the lithium-containing phosphate positive active particles in the positive electrode sheet is greater than the lithium content in the surface portion, and the thickness of the surface portion is ≤10nm, which can make the secondary battery have higher cycle performance.

[0063] The positive electrode sheets can be assembled into batteries, which can be battery cells, modules, battery packs, etc., and can be used in, but not limited to, electrical equipment such as vehicles, ships, or aircraft. The batteries disclosed herein can be used to form power supply systems for such electrical equipment, thereby improving the battery's cycle performance and service life at higher temperatures.

[0064] The embodiments of the present application provide an electrical device that uses a battery as a power source. The electrical device may be a vehicle, a mobile phone, a portable device, a laptop computer, a ship, a spacecraft, an electric toy, an electric tool, and the like. The vehicle may be a fuel vehicle, a gas vehicle, or a new energy vehicle. The new energy vehicle may be a pure electric vehicle, a hybrid vehicle, or an extended-range vehicle. The spacecraft may include an airplane, a rocket, a space shuttle, and a spacecraft. The electric toys may include fixed or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys. The electric tools may include metal cutting electric tools, grinding electric tools, assembly electric tools, and railway electric tools, such as electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers. The embodiments of the present application do not impose any special restrictions on the above-mentioned electrical devices.

[0065] For the convenience of description, the following embodiments are described by taking the electric device as a vehicle as an example.

[0066] Please refer to Figure 1, which is a schematic diagram of the structure of a vehicle 1000 provided in some embodiments of the present application. A battery 100 is disposed within vehicle 1000. Battery 100 can be located at the bottom, front, or rear of vehicle 1000. Battery 100 can be used to power vehicle 1000, for example, as an operating power source for vehicle 1000.

[0067] The vehicle 1000 may further include a controller 200 and a motor 300 . The controller 200 is used to control the battery 100 to supply power to the motor 300 , for example, to meet the power requirements of the vehicle 1000 during startup, navigation, and driving.

[0068] In some embodiments of the present application, the battery 100 can not only serve as the operating power source of the vehicle 1000, but also serve as the driving power source of the vehicle 1000, replacing or partially replacing fuel or natural gas to provide driving power for the vehicle 1000.

[0069] FIG2 is a schematic diagram of the exploded structure of a battery 100 provided in some embodiments of the present application. Referring to FIG2 , the battery 100 includes a housing 10 and a battery cell 20 , wherein the battery cell 20 is housed in the housing 10 .

[0070] The housing 10 is used to provide a storage space 11 for the battery cells 20. In some embodiments, the housing 10 may include a first portion 12 and a second portion 13, which overlap to define the storage space 11 for accommodating the battery cells 20. Of course, the connection between the first portion 12 and the second portion 13 can be sealed by a seal (not shown), such as a sealing ring, sealant, or the like.

[0071] The first portion 12 and the second portion 13 can have various shapes, such as a rectangular parallelepiped, a cylinder, etc. The first portion 12 can be a hollow structure with an opening on one side to form a receiving cavity for accommodating the battery cell 20. The second portion 13 can also be a hollow structure with an opening on one side to form a receiving cavity for accommodating the battery cell 20. The open side of the second portion 13 covers the open side of the first portion 12, thereby forming the box 10 with the receiving space 11. Of course, as shown in Figure 2, the first portion 12 can also be a hollow structure with an opening on one side, and the second portion 13 can be a plate-like structure. The second portion 13 covers the open side of the first portion 12, thereby forming the box 10 with the receiving space 11.

[0072] In the battery 100, there can be one or more battery cells 20. If there are multiple battery cells 20, the multiple battery cells 20 can be connected in series, in parallel, or in a mixed connection. Mixed connection means that the multiple battery cells 20 are connected in series and in parallel. The multiple battery cells 20 can be directly connected in series, in parallel, or in a mixed connection, and then the whole formed by the multiple battery cells 20 is accommodated in the box 10; of course, it is also possible that the multiple battery cells 20 are first connected in series, in parallel, or in a mixed connection to form a battery module, and the multiple battery modules are then connected in series, in parallel, or in a mixed connection to form a whole, and accommodated in the box 10. The battery cell 20 can be cylindrical, flat, rectangular, or in other shapes. Figure 2 exemplifies the case where the battery cell 20 is square.

[0073] In some embodiments, the battery 100 may further include a busbar component (not shown), and the multiple battery cells 20 may be electrically connected via the busbar component to achieve series connection, parallel connection, or mixed connection of the multiple battery cells 20 .

[0074] Figure 3 is a schematic diagram of the structure of a battery cell 20 provided in some embodiments of the present application, and Figure 4 is an exploded view of a battery cell 20 provided in some embodiments of the present application. Referring to Figures 3 and 4, the battery cell 20 may include a housing 21, an end cap assembly 22, and an electrode assembly 23. The housing 21 has an opening 211, and the electrode assembly 23 is accommodated within the housing 21. The end cap assembly 22 is used to seal the opening 211.

[0075] The shape of the housing 21 can be determined according to the specific shape of the electrode assembly 23. For example, if the electrode assembly 23 is a rectangular parallelepiped structure, the housing 21 can be a rectangular parallelepiped structure. Figures 3 and 4 exemplarily show the case where the housing 21 and the electrode assembly 23 are square.

[0076] The shell 21 may be made of a variety of materials, such as copper, iron, aluminum, stainless steel, aluminum alloy, etc., and the embodiment of the present application does not impose any special restrictions on this.

[0077] The end cap assembly 22 includes an end cap 221 and an electrode terminal 222. The end cap assembly 22 is used to seal the opening 211 of the outer shell 21 to form a sealed installation space (not shown), which is used to accommodate the electrode assembly 23. The installation space is also used to accommodate an electrolyte, such as an electrolyte. The end cap assembly 22 serves as a component for outputting the electrical energy of the electrode assembly 23. The electrode terminal 222 in the end cap assembly 22 is used to electrically connect to the electrode assembly 23, that is, the electrode terminal 222 is electrically connected to the tab of the electrode assembly 23. For example, the electrode terminal 222 is connected to the tab through the current collecting member 24 to achieve electrical connection between the electrode terminal 222 and the tab.

[0078] It should be noted that the number of openings 211 of the outer shell 21 can be one or two. If the number of openings 211 of the outer shell 21 is one, the number of end cap assembly 22 can also be one, and two electrode terminals 222 can be provided in the end cap assembly 22. The two electrode terminals 222 are respectively used to electrically connect to the positive electrode tab and the negative electrode tab of the electrode assembly 23. If the number of openings 211 of the outer shell 21 is two, for example, the two openings 211 are provided on opposite sides of the outer shell 21, the number of end cap assemblies 22 can also be two, and the two end cap assemblies 22 are respectively covered at the two openings 211 of the outer shell 21. In this case, the electrode terminal 222 in one end cap assembly 22 can be a positive electrode terminal, which is used to electrically connect to the positive electrode tab of the electrode assembly 23; and the electrode terminal 222 in the other end cap assembly 22 can be a negative electrode terminal, which is used to electrically connect to the negative electrode tab of the electrode assembly 23.

[0079] In some embodiments, as shown in FIG4 , the battery cell 20 may further include an insulating protective member 25 fixed to the periphery of the electrode assembly 23, and the insulating protective member 25 is used to insulate and isolate the electrode assembly 23 from the housing 21. Exemplarily, the insulating protective member 25 is a tape bonded to the periphery of the electrode assembly 23. In some embodiments, there are multiple electrode assemblies 23, and the insulating protective member 25 is arranged around the periphery of the multiple electrode assemblies 23, and the multiple electrode assemblies 23 are formed into an integral structure to maintain the structural stability of the electrode assembly 23. Among them, the electrode assembly 23 can be a wound electrode assembly or a laminated electrode assembly, and the embodiments of the present application are not limited thereto.

[0080] Figure 5 is a structural schematic diagram of the electrode assembly provided in some embodiments of the present application. Please refer to Figure 5. The electrode assembly 23 includes a positive electrode plate 231, a negative electrode plate 232 and an isolation membrane 233. The isolation membrane 233 is arranged between the positive electrode plate 231 and the negative electrode plate 232. The electrolyte is located in the installation space and is filled in the gap of the electrode assembly 23.

[0081] The present application has no particular limitation on the isolation membrane 233 , the negative electrode plate 232 and the electrolyte.

[0082] The isolation membrane 233 may be a PP (polypropylene) porous membrane, a PE (polyethylene) porous membrane, a polyimide porous membrane, or a porous membrane formed by a composite of multiple polymers.

[0083] The negative electrode sheet 232 includes a negative electrode current collector and a negative electrode active layer covering at least one surface of the negative electrode current collector in the thickness direction. The thickness of the negative electrode current collector and the negative electrode active layer are not particularly limited in this application, as long as they can achieve the purpose of this application. For example, the thickness of the negative electrode current collector is 4 μm to 12 μm, and the thickness of the negative electrode active layer on a single surface of the negative electrode current collector is 30 μm to 130 μm.

[0084] The material of the negative electrode current collector may include aluminum foil, copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam or a polymer substrate covered with a conductive metal, etc.; wherein the conductive metal includes but is not limited to copper, nickel or titanium, and the material of the polymer substrate includes but is not limited to at least one of polyethylene, polypropylene, ethylene propylene copolymer, polyethylene terephthalate, polyethylene naphthalate and poly(p-phenylene terephthalamide).

[0085] The negative electrode active material in the negative electrode active layer includes graphite, coke, etc., or the negative electrode active material in the negative electrode active layer includes lithium alone, an alloy formed by lithium and other metal elements or non-metal elements, wherein the metal elements include tin (Sn), zinc (Zn), aluminum (Al), magnesium (Mg), silver (Ag), gold (Au), gallium (Ga), indium (In), foil (Pt), etc., and the non-metal elements include boron (B), carbon (C), silicon (Si), etc.

[0086] The conductive agent in the negative electrode active layer may include but is not limited to carbon materials, metals or conductive polymers. The carbon material may include at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon nanofibers, natural graphite, artificial graphite, flake graphite, carbon dots or graphene. The metal may include metal powder or metal fibers of copper, iron, aluminum, etc. The conductive polymer may include at least one of polythiophene, polypyrrole, polyaniline, polyphenylene and polyphenylene ethylene.

[0087] The binder in the negative electrode active layer may include, but is not limited to, at least one of polypropylene glycol, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyamideimide, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl pyrrolidone, polyethylene, polypropylene, epoxy resin, nylon, styrene-butadiene rubber (SBR), polyvinyl alcohol (PVA), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl butyral, aqueous acrylic resin, carboxymethyl cellulose (CMC) or sodium carboxymethyl cellulose (CMC-Na).

[0088] The negative electrode sheet 232 can be prepared according to conventional methods in the art. For example, the negative electrode active material, conductive agent, and binder described above are dispersed in a solvent, such as N-methylpyrrolidone (NMP) or deionized water, to form a uniform negative electrode slurry. The negative electrode slurry is then coated on the negative electrode current collector. The negative electrode sheet 232 is obtained through drying and cold pressing.

[0089] For the electrolyte, the electrolyte includes a sodium salt and a non-aqueous solvent, or the electrolyte includes a lithium salt and a non-aqueous solvent; wherein the sodium salt may include at least one of NaPF6, NaClO4, NaBCl4, NaSO3CF3, or Na(CH3)C6H4SO3; the present application has no particular limitation on the concentration of the sodium salt in the electrolyte, as long as the purpose of the present application can be achieved. The lithium salt may include at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalatoborate) (LiBOB), or lithium difluoroborate; the present application has no particular limitation on the concentration of the lithium salt in the electrolyte, as long as the purpose of the present application can be achieved. The present application has no particular limitation on the above-mentioned non-aqueous solvent, as long as the purpose of the present application can be achieved, for example, it may include but is not limited to at least one of carbonate compounds, carboxylate compounds, ether compounds or other organic solvents; the above-mentioned carbonate compounds may include but are not limited to at least one of chain carbonate compounds, cyclic carbonate compounds or fluorocarbonate compounds; the above-mentioned chain carbonate compounds may include but are not limited to at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC) or methylethyl carbonate (MEC); the above-mentioned cyclic carbonates may include but are not limited to at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) or vinylethylene carbonate (VEC); the fluorocarbonate compounds may include but are not limited to fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2-tetrafluoroethylene carbonate At least one of fluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate or trifluoromethylethylene carbonate; the above-mentioned carboxylic acid ester compound may include but is not limited to at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanoic acid, valerolactone or caprolactone; the above-mentioned ether compound may include but is not limited to at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanoic acid, valerolactone or caprolactone; the above-mentioned ether compound may include but Not limited to at least one of dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran or tetrahydrofuran; the above-mentioned other organic solvents may include but are not limited to at least one of dimethyl sulfoxide, 1,2-dioxolane, cyclopentane, methylcyclopentane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate or trioctyl phosphate.

[0090] The positive electrode sheet 231 includes a positive electrode current collector and a positive electrode active layer covering at least one surface of the positive electrode current collector in the thickness direction. The positive electrode current collector may be made of aluminum foil, aluminum foam, an aluminum composite current collector (a current collector with a polymer support layer in the middle and aluminum metal layers on both surfaces of the support layer), nickel foil, nickel foam, etc. The binder in the positive electrode active layer is selected from at least one of vinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyacrylate, sodium carboxymethyl cellulose, styrene-butadiene rubber, polyurethane, ethylene-vinyl acetate copolymer, and ethylene-acrylic acid copolymer. The dispersant in the positive electrode active layer is selected from polyvinyl pyrrolidone, etc. The conductive agent in the positive electrode active layer is selected from at least one of conductive carbon black, acetylene black, Ketjen black, carbon fiber, carbon nanotubes, graphene, activated carbon, graphite flakes, graphite particles, and mesocarbon microbeads.

[0091] In the present application, the positive electrode plate includes lithium phosphate positive electrode active particles, which include a central portion and a surface portion, the surface portion is continuously or discontinuously distributed on the surface of the central portion; the thickness of the surface portion is ≤10nm; the lithium content of the central portion is greater than the lithium content of the surface portion.

[0092] As an example, the thickness of the surface portion may be any value among 10 nm, 8 nm, 7.5 nm, 6 nm, 5 nm, 4 nm, 3 nm, 2 nm, 1 nm and 0.1 nm, or a range of values ​​between any two of them.

[0093] In the positive electrode sheet of the secondary battery provided in the present application, the lithium content in the center of the lithium phosphate positive active particle is greater than the lithium content in the surface portion, and the thickness of the surface portion is ≤10nm, which can make the secondary battery have higher cycle performance.

[0094] In some embodiments, the lithium content of the surface of the lithium-containing phosphate positive electrode active particles is 0, which is beneficial to further improve the cycle performance of the secondary battery.

[0095] In some embodiments, in the Raman spectrum of the lithium-containing phosphate positive active particles, the Raman shift of 200 cm -1 ~250cm -1 The position has an O-Fe-O stretching vibration peak; and / or, in the Raman spectrum of the lithium phosphate positive active particles, the Raman shift is 255cm -1 ~300cm -1The lithium-phosphate positive electrode active particles have corresponding Raman characteristic peaks within the above wavenumber range, which not only fully utilize the capacity of the lithium-phosphate positive electrode active particles and provide them with high electronic conductivity, but also enable secondary batteries prepared using the lithium-phosphate positive electrode active particles to have high cycle performance.

[0096] In some embodiments, the surface portion includes iron oxide.

[0097] Among them, "iron oxide" refers to: iron oxide, a compound containing only iron and oxygen elements (Fe x O y , wherein 3≥x≥1, 4≥y≥1); for example, the iron oxide may be a substance containing only iron and oxygen elements, such as ferrous oxide (FeO), ferric oxide (Fe2O3) or / and ferrosoferric oxide (Fe3O4).

[0098] In the positive electrode sheet, the surface of the lithium phosphate positive electrode active particles contains iron oxide, which has a high electronic conductivity. It can not only make the lithium phosphate positive electrode active particles have a high electronic conductivity, but also make the capacity of the lithium phosphate positive electrode active particles fully utilized; in addition, compared with the secondary battery prepared by using only lithium phosphate as the positive electrode active material, the lithium phosphate positive electrode active particles with iron oxide on the surface provided by the present application can improve the cycle performance of the secondary battery prepared.

[0099] In some embodiments, the content of iron oxide in the surface portion gradually decreases along a direction from the outermost portion of the surface portion to the central portion.

[0100] In some embodiments, the surface portion includes ferric oxide (Fe2O3). This not only allows the capacity of the lithium-phosphate-containing positive electrode active particles to be fully utilized and the lithium-phosphate-containing positive electrode active particles to have higher electronic conductivity, but also allows the secondary battery prepared using the lithium-phosphate-containing positive electrode active particles to have higher cycle performance.

[0101] In some embodiments, the lithium phosphate positive active particles include Li 1+x M 1-y A y P 1-z R z O 4-t, M includes at least one of Fe, Co and Ni, A includes at least one of Mn, Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and R includes at least one of B, S, Si and N; -0.1≤x≤0.1, 0≤y≤0.1, 0≤z≤0.1, 0≤t≤0.1.

[0102] As an example, Li 1+x M 1-y A y P 1-z R z O 4-t In the formula, the value of x can be any point value among -0.1, -0.07, -0.05, -0.02, 0, 0.02, 0.05, 0.07 and 0.1, or a range of values ​​between any two of them; the value of y can be any point value among 0, 0.001, 0.01, 0.02, 0.05, 0.07, 0.09 and 0.1, or a range of values ​​between any two of them; the value of z can be any point value among 0, 0.01, 0.02, 0.05, 0.07, 0.09 and 0.1, or a range of values ​​between any two of them; the value of t can be any point value among 0, 0.001, 0.01, 0.02, 0.05, 0.07, 0.09 and 0.1, or a range of values ​​between any two of them.

[0103] In some embodiments, the lithium phosphate positive active particles include Li 1+x1 Fe 1-y1 A y1 PO 4-t1 , A includes at least one of Mn, Al, Ti, V, Ni and Zn; 0≤x1≤0.05, 0≤y1≤0.05, 0≤t1≤0.02.

[0104] As an example, Li 1+x1 Fe 1-y1 A y1 PO 4-t1 In the formula, the value of x1 can be any point value among 0, 0.01, 0.02, 0.03, 0.04 and 0.05, or a range of values ​​between any two of them; the value of y1 can be any point value among 0, 0.01, 0.02, 0.03, 0.04 and 0.05, or a range of values ​​between any two of them; the value of t1 can be any point value among 0, 0.001, 0.005, 0.01, 0.015, 0.017 and 0.02, or a range of values ​​between any two of them.

[0105] Further, in some embodiments, the matrix includes LiFePO4.

[0106] In some embodiments, the thickness of the surface portion is ≤8 nm, which is beneficial for further enhancing the capacity of the lithium-containing phosphate positive electrode active particles and improving the electronic conductivity of the lithium-containing phosphate positive electrode active particles, and is also beneficial for further improving the cycle performance of secondary batteries prepared using the lithium-containing phosphate positive electrode active particles.

[0107] In some embodiments, the thickness of the surface portion is 1 nm to 8 nm, which is beneficial for further enhancing the capacity of the lithium-containing phosphate positive electrode active particles and improving the electronic conductivity of the lithium-containing phosphate positive electrode active particles, and is also beneficial for further improving the cycle performance of secondary batteries prepared using the lithium-containing phosphate positive electrode active particles.

[0108] In some embodiments, the thickness of the surface portion is 1.5 nm to 4 nm. The surface portion within the above thickness range is beneficial for further enhancing the capacity of the lithium-containing phosphate positive electrode active particles and improving the electronic conductivity of the lithium-containing phosphate positive electrode active particles, and is also beneficial for further improving the cycle performance of secondary batteries prepared using the lithium-containing phosphate positive electrode active particles.

[0109] In some embodiments, the volume average particle size D of the lithium phosphate positive active particles is V 50 is 300nm~10.5μm.

[0110] As an example, the volume average particle size D of the lithium phosphate positive electrode active particles is V 50 can be any value among 300nm, 400nm, 500nm, 600nm, 700nm, 800nm, 900nm, 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm and 10.5μm, or a range between any two of them.

[0111] In the present application, the positive electrode active material in the positive electrode active layer includes a first active material, and the first active material includes: a substrate and an oxide layer located on the surface of the substrate; the substrate includes Li 1+x M 1-y A y P 1-z R z O 4-t , wherein M includes at least one of Fe, Co and Ni, A includes at least one of Mn, Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and R includes at least one of B, S, Si and N; -0.1≤x≤0.1, 0≤y≤0.1, 0≤z≤0.1, 0≤t≤0.1; the oxide layer includes iron oxide.

[0112] Here, “the oxide layer located on the surface of the substrate” means that the oxide layer at least partially covers the surface of the substrate. The entire surface of the substrate may be covered with the oxide layer, or only a part of the surface of the substrate may be covered with the oxide layer.

[0113] "Iron oxide" refers to: Iron oxide, a compound containing only iron and oxygen (Fe x O y , wherein 3≥x≥1, 4≥y≥1); for example, the iron oxide may be a substance containing only iron and oxygen elements, such as ferrous oxide (FeO), ferric oxide (Fe2O3) or / and ferrosoferric oxide (Fe3O4).

[0114] As an example, Li 1+x M 1-y A y P 1-z R z O 4-t In the formula, the value of x can be any point value among -0.1, -0.07, -0.05, -0.02, 0, 0.02, 0.05, 0.07 and 0.1, or a range of values ​​between any two of them; the value of y can be any point value among 0, 0.001, 0.01, 0.02, 0.05, 0.07, 0.09 and 0.1, or a range of values ​​between any two of them; the value of z can be any point value among 0, 0.01, 0.02, 0.05, 0.07, 0.09 and 0.1, or a range of values ​​between any two of them; the value of t can be any point value among 0, 0.001, 0.01, 0.02, 0.05, 0.07, 0.09 and 0.1, or a range of values ​​between any two of them.

[0115] In the first active material provided herein, the surface of the substrate has an oxide layer containing iron oxide. Iron oxide has high electronic conductivity, which can improve the electronic conductivity of the first active material and effectively utilize the capacity of the substrate in the first active material. Furthermore, compared to secondary batteries prepared using only the substrate as the positive electrode active material, the use of the first active material provided herein can improve the cycle performance of the resulting secondary battery.

[0116] In some embodiments, the lithium content of the matrix is ​​greater than the lithium content of the oxide layer; this can result in a secondary battery having higher cycle performance.

[0117] In some embodiments, the lithium content of the oxide layer is 0, which is beneficial to further improve the cycle performance of the secondary battery.

[0118] In some embodiments, the matrix includes Li 1+x1 Fe 1-y1 A y1 PO4-t1 , A includes at least one of Mn, Al, Ti, V, Ni and Zn; 0≤x1≤0.05, 0≤y1≤0.05, 0≤t1≤0.02.

[0119] As an example, Li 1+x1 Fe 1-y1 A y1 PO 4-t1 In the formula, the value of x1 can be any point value among 0, 0.01, 0.02, 0.03, 0.04 and 0.05, or a range of values ​​between any two of them; the value of y1 can be any point value among 0, 0.01, 0.02, 0.03, 0.04 and 0.05, or a range of values ​​between any two of them; the value of t1 can be any point value among 0, 0.001, 0.005, 0.01, 0.015, 0.017 and 0.02, or a range of values ​​between any two of them.

[0120] Further, in some embodiments, the matrix includes LiFePO4.

[0121] In some embodiments, the Raman spectrum of the first active material has a Raman shift of 200 cm -1 ~250cm -1 The position has an O-Fe-O stretching vibration peak; and / or, in the Raman spectrum of the first active material, the Raman shift is 255cm -1 ~300cm -1 The first active material has a corresponding Raman characteristic peak within the above wavenumber range, which not only allows the capacity of the matrix to be fully utilized and the first active material to have a high electronic conductivity, but also allows the secondary battery prepared using the first active material to have a high cycle performance.

[0122] In some embodiments, the oxide layer includes ferric oxide (Fe2O3), which not only allows the capacity of the matrix to be fully utilized and the first active material to have a higher electronic conductivity, but also allows the secondary battery prepared using the first active material to have a higher cycle performance.

[0123] In some embodiments, the content of iron oxide in the oxide layer gradually decreases along a direction from the outermost side of the oxide layer to the substrate.

[0124] In some embodiments, the thickness of the oxide layer is ≤10 nm; this not only allows the capacity of the matrix to be fully utilized and the first active material to have a higher electronic conductivity, but also allows the secondary battery prepared using the first active material to have a higher cycle performance.

[0125] As an example, the thickness of the oxide layer can be any value among 10 nm, 8 nm, 7.5 nm, 6 nm, 5 nm, 4 nm, 3 nm, 2 nm, 1 nm and 0.1 nm, or a range of values ​​between any two of them.

[0126] In some embodiments, the thickness of the oxide layer is ≤8 nm, which is beneficial for further improving the capacity of the matrix and the electronic conductivity of the first active material, and is also beneficial for further improving the cycle performance of the secondary battery prepared using the first active material.

[0127] In some embodiments, the thickness of the oxide layer is 1 nm to 8 nm, which not only allows the capacity of the matrix to be fully utilized and the first active material to have a high electronic conductivity, but also allows the secondary battery prepared using the first active material to have a high cycle performance.

[0128] As an example, the thickness of the oxide layer can be any value among 1 nm, 1.5 nm, 2 nm, 2.5 nm, 3 nm, 3.5 nm, 4 nm, 4.5 nm, 5 nm, 6 nm, 7.5 nm and 8 nm, or a range between any two values.

[0129] In some embodiments, the thickness of the oxide layer is 1.5 nm to 4 nm, which is beneficial for further improving the capacity of the matrix and improving the electronic conductivity of the first active material, and is also beneficial for further improving the cycle performance of the secondary battery prepared using the first active material.

[0130] In some embodiments, the volume average particle size D of the first active material is V 50 is 300nm~10.5μm.

[0131] As an example, the volume average particle size D of the first active material is V 50 can be any value among 300nm, 400nm, 500nm, 600nm, 700nm, 800nm, 900nm, 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm and 10.5μm, or a range between any two of them.

[0132] After the above introduction to the first active material, the preparation method of the first active material will be specifically introduced below.

[0133] In the present application, the substrate is treated to form an oxide layer on the surface of the substrate; the substrate includes Li 1+x M 1-y A y P 1-z R z O 4-t, wherein M includes at least one of Fe, Co and Ni, A includes at least one of Mn, Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and R includes at least one of B, S, Si and N; -0.1≤x≤0.1, 0≤y≤0.1, 0≤z≤0.1, 0.001≤t≤0.1; and the oxide layer includes iron oxide.

[0134] Here, “forming an oxide layer on the surface of the substrate” means that the oxide layer at least partially covers the surface of the substrate. The entire surface of the substrate may be covered with the oxide layer, or only a part of the surface of the substrate may be covered with the oxide layer.

[0135] Iron oxide refers to: Iron oxide, a compound containing only iron and oxygen elements (Fe x O y , wherein 3≥x≥1, 4≥y≥1); for example, the iron oxide may be a substance containing only iron and oxygen elements, such as ferrous oxide (FeO), ferric oxide (Fe2O3) or / and ferrosoferric oxide (Fe3O4).

[0136] As an example, in the matrix, the value of x can be any point value among -0.1, -0.07, -0.05, -0.02, 0, 0.02, 0.05, 0.07 and 0.1, or a range of values ​​between any two of them; the value of y can be any point value among 0, 0.01, 0.02, 0.05, 0.07, 0.09 and 0.1, or a range of values ​​between any two of them; the value of z can be any point value among 0, 0.01, 0.02, 0.05, 0.07, 0.09 and 0.1, or a range of values ​​between any two of them; the value of t can be any point value among 0, 0.001, 0.01, 0.02, 0.05, 0.07, 0.09 and 0.1, or a range of values ​​between any two of them.

[0137] The present application forms an oxide layer containing iron oxide on the surface of the substrate, and the iron oxide has a high electronic conductivity, which not only allows the first active material to have a high electronic conductivity, but also allows the capacity of the substrate to be fully utilized; in addition, compared with a secondary battery prepared by using only the substrate as the positive electrode active material, the use of the first active material provided by the present application can improve the cycle performance of the secondary battery prepared.

[0138] In some embodiments, the matrix includes Li 1+x1 Fe 1-y1 A y1 PO 4-t1, A includes at least one of Mn, Al, Ti, V, Ni, and Zn; 0≤x1≤0.05, 0≤y1≤0.05, 0≤t1≤0.02; the substrate is oxidized to form an oxide layer on the surface of the substrate. The present application performs an oxidation treatment on a substrate containing the Fe element, and through a one-step oxidation operation, an in-situ oxidation reaction is caused on the surface of the substrate containing the Fe element to form an oxide layer containing iron oxide on the surface of the substrate. That is, the oxide layer containing iron oxide is formed by in-situ oxidation of the surface of the substrate containing the Fe element.

[0139] It should be noted that, in other feasible embodiments, the step of forming an oxide layer on the surface of the substrate can also be prepared in the following two ways: Method 1: mixing a slurry containing iron oxide with the substrate, and then drying and grinding; Method 2: mixing an iron source with the substrate, and then oxidizing the mixed system. Compared with the above two methods, the method of "oxidizing a substrate containing Fe elements" provided in this application can cause an in-situ oxidation reaction on the surface of the substrate containing Fe elements to form an oxide layer containing iron oxides through a one-step oxidation operation, which has the advantages of simple process and easy scale-up; and the method of in-situ oxidation on the surface of the substrate containing Fe elements to form an oxide layer containing iron oxides can make the bonding between the oxide layer and the substrate more tight, and the distribution of iron oxides on the surface of the substrate is more uniform, which is conducive to further giving full play to the capacity of the substrate, and is conducive to improving the electronic conductivity of the first active material and improving the cycle performance of the secondary battery prepared using the first active material.

[0140] As an example, Li 1+x1 Fe 1-y1 A y1 PO 4-t1 In the formula, the value of x1 can be any point value among 0, 0.01, 0.02, 0.03, 0.04 and 0.05, or a range of values ​​between any two of them; the value of y1 can be any point value among 0, 0.01, 0.02, 0.03, 0.04 and 0.05, or a range of values ​​between any two of them; the value of t1 can be any point value among 0, 0.001, 0.005, 0.01, 0.015, 0.017 and 0.02, or a range of values ​​between any two of them.

[0141] Further, in some embodiments, the matrix is ​​LiFePO4.

[0142] In some embodiments, an oxidizing gas is used to oxidize the substrate; an oxidizing gas is used to oxidize the substrate containing the Fe element, and the oxidizing gas can react with the surface of the substrate containing the Fe element to form iron oxide, so that an oxide layer containing iron oxide is formed on the surface of the substrate, thereby fully utilizing the capacity of the substrate in the prepared first active material, so that the first active material has a higher electronic conductivity, and the secondary battery prepared using the first active material has a higher cycle performance.

[0143] In some embodiments, the oxidizing gas includes at least one of oxygen and ozone.

[0144] Here, "the oxidizing gas includes at least one of oxygen and ozone" means that the oxidizing gas can be only oxygen or only ozone, or the oxidizing gas can contain only oxygen and ozone, or the oxidizing gas can also contain other gases, for example, inert gases (including but not limited to nitrogen and / or argon, etc.); as an example, the oxidizing gas can be air.

[0145] In some embodiments, the combined volume of oxygen and ozone in the oxidizing gas accounts for 10% to 100% of the total volume of the oxidizing gas, which is beneficial for increasing the generation rate of the iron oxide layer formed on the surface of the substrate and improving the preparation efficiency of the first active material.

[0146] As an example, in the oxidizing gas, the volume fraction of the sum of the volumes of oxygen and ozone to the volume of the oxidizing gas can be any value among 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% and 100%, or a range of values ​​between any two of them.

[0147] In some embodiments, the temperature of the oxidation treatment is ≥300° C. This allows the matrix containing the Fe element to undergo an oxidation reaction to form iron oxide, so that an oxide layer containing iron oxide is formed on the surface of the matrix, thereby fully utilizing the capacity of the matrix in the prepared first active material, making the first active material have higher electronic conductivity, and making the secondary battery prepared using the first active material have higher cycle performance.

[0148] The inventors speculate that when the substrate includes LiFePO4, when the oxidation treatment temperature is ≥300°C, the reaction equation of the oxidizing gas such as oxygen and LiFePO4 is: 12LiFePO4+3O2→2Fe2O3+4Li3Fe2(PO4)3.

[0149] As an example, the temperature of the oxidation treatment may be 300° C., 400° C., 500° C., 600° C., or 700° C., etc.

[0150] In some embodiments, the temperature of the oxidation treatment is 300° C. to 600° C. This can accelerate the generation rate of the oxidation layer containing iron oxide formed on the surface of the substrate containing the Fe element, thereby improving the preparation efficiency of the first active material.

[0151] As an example, the temperature of the oxidation treatment may be any value among 300° C., 350° C., 400° C., 450° C., 500° C., 550° C. and 600° C., or a range of values ​​therebetween.

[0152] Furthermore, in some embodiments, the temperature of the oxidation treatment is 400° C. to 500° C. This is beneficial for further fully utilizing the capacity of the matrix in the prepared first active material, further improving the electronic conductivity of the first active material, and further improving the cycle performance of the secondary battery prepared using the first active material.

[0153] In some embodiments, when an oxidizing gas is used to oxidize a substrate containing an Fe element, the oxidizing gas flow rate is 200 sccm to 500 sccm. This facilitates the formation of a dense and uniform oxide layer on the surface of the substrate containing the Fe element, thereby further facilitating full utilization of the substrate's capacity and improving the electronic conductivity of the first active material, thereby enabling a secondary battery prepared using the first active material to have better cycle performance.

[0154] As an example, when an oxidizing gas is used to oxidize a substrate containing Fe elements, the flow rate of the oxidizing gas can be any value among 200sccm, 220sccm, 250sccm, 270sccm, 300sccm, 320sccm, 350sccm, 370sccm, 400sccm, 420sccm, 450sccm, 470sccm and 500sccm, or a range value between any two of them.

[0155] In some embodiments, when an oxidizing gas is used to oxidize a substrate containing an Fe element, the oxidation treatment time is 2 to 60 minutes. This allows the surface of the Fe element-containing substrate to fully react with the oxidizing gas, which helps ensure that the mass fraction of the formed iron oxide layer in the entire first active material is within a relatively suitable range. This not only fully utilizes the capacity of the substrate in the first active material and enables the first active material to have higher electronic conductivity, but also allows the secondary battery prepared using the first active material to have higher cycle performance.

[0156] As an example, when an oxidizing gas is used to oxidize a substrate containing an Fe element, the oxidation treatment time can be any value among 2 min, 5 min, 10 min, 15 min, 20 min, 25 min, 30 min, 35 min, 40 min, 45 min, 50 min, 55 min and 60 min, or a range value between any two of them.

[0157] Furthermore, in some embodiments, when an oxidizing gas is used to oxidize a matrix containing Fe elements, the oxidation treatment time is 10 minutes to 30 minutes. This is beneficial for further fully utilizing the capacity of the matrix in the prepared first active material, further improving the electronic conductivity of the first active material, and further improving the cycle performance of the secondary battery prepared using the first active material.

[0158] In some embodiments, the oxidation treatment is performed in a cyclone oxidation furnace. Passing an oxidizing gas into the cyclone oxidation furnace to oxidize the Fe-containing substrate allows the oxidizing gas to fully contact the surface of the Fe-containing substrate, thereby facilitating the formation of a dense and uniform oxide layer on the surface of the Fe-containing substrate. This further facilitates further utilizing the substrate's capacity and improving the electronic conductivity of the first active material, thereby enabling a secondary battery prepared using the first active material to have better cycle performance.

[0159] As an example, the steps of oxidizing lithium iron phosphate include: spreading the substrate containing Fe elements flat in a cyclone oxidation furnace, and placing the substrate containing Fe elements at intervals in the cyclone oxidation furnace, and introducing 200 sccm to 500 sccm of air into the furnace body at 300°C to 600°C for 2 minutes to 60 minutes.

[0160] It should be noted that the oxidation treatment can also be carried out in a tubular oxidation furnace.

[0161] In some embodiments, the positive electrode active material in the positive electrode active layer contains not only the first active material provided above, but also a second active material, which is different from the first active material.

[0162] In some embodiments, the second active material includes: a body and a carbon layer located on the surface of the body; the body includes Li 1+m C 1-n D n P 1-s E s O 4-q, wherein C includes at least one of Fe, Co and Ni, D includes at least one of Mn, Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, E includes at least one of B, S, Si and N, 0.1≤m≤0.1, 0≤n≤0.1, 0≤s≤0.1, 0≤q≤0.1.

[0163] Here, "the carbon layer located on the surface of the body" means that the surface of the body is covered with a carbon layer, and the carbon layer at least partially covers the surface of the body (the entire surface of the body may be covered with the carbon layer, or only a part of the surface of the body may be covered with the carbon layer).

[0164] Further, in some embodiments, the body of the second active material includes LiFePO 4 .

[0165] In some embodiments, the mass ratio of the first active material to the second active material in the positive electrode active layer is 1:(0.5-3). Within the above-mentioned mass ratio of the first active material to the second active material, the capacity of the positive electrode active material can be fully utilized, and the positive electrode active material has high electronic conductivity. Furthermore, the secondary battery prepared using the positive electrode active material can have good cycle performance.

[0166] As an example, in the positive electrode active layer, the mass ratio of the first active material to the second active material can be any value among 1:0.5, 1:0.7, 1:1, 1:1.5, 1:2, 1:2.5 and 1:3, or a range between any two values.

[0167] In some embodiments, in the positive electrode active material, the carbon layer in the second active material has a thickness of 0.5 nm to 10 nm.

[0168] As an example, the thickness of the carbon layer of the second active material can be any value among 0.5nm, 1nm, 1.5nm, 2nm, 3nm, 4nm, 5nm, 6nm, 7nm, 8nm, 9nm and 10nm, or a range between any two values.

[0169] In some embodiments, the volume average particle size D of the second active material is V 50 is 0.5μm~5μm.

[0170] As an example, the volume average particle size D of the second active material is V 50 can be any value among 0.5μm, 0.8μm, 1.0μm, 1.2μm, 1.4μm, 1.6μm, 1.8μm, 2μm, 3μm, 4μm and 5μm, or a range between any two of them.

[0171] In some embodiments, in the second active material, the mass fraction of the carbon layer in the second active material is 0.5% to 5%.

[0172] As an example, in the second active material, the mass fraction of the carbon layer in the second active material can be any value among 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, 1.8%, 2%, 2.5%, 3%, 4% and 5%, or a range between any two values.

[0173] In some embodiments, the material of the carbon layer in the second active material includes inorganic carbon and / or organic carbon. For example, the material of the carbon layer may include at least one of graphene, carbon black, conductive graphite, amorphous carbon, and carbon nanotubes.

[0174] It should be noted that the present application does not limit the preparation method of the second active material.

[0175] It should be noted that the second active material in the positive electrode active layer is not limited to the above substances. The second active material can be selected from one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide and olivine-structured lithium phosphate, for example, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium cobalt oxide, lithium manganese oxide, etc.

[0176] The above-mentioned positive electrode active materials can be used to prepare the positive electrode sheet 231. The positive electrode sheet 231 can be prepared according to conventional methods in the art. For example, the positive electrode active materials, conductive agent, and binder described above are dispersed in a solvent, which can be N-methylpyrrolidone (NMP) or deionized water, to form a uniform positive electrode slurry. The positive electrode slurry is then coated on the positive electrode current collector. The positive electrode sheet 231 is obtained through drying, cold pressing, and other processes.

[0177] The positive electrode sheet 231 can be used to prepare an electrode assembly 23 , and the electrode assembly can be used to prepare a battery 100 , which can be used as a power source for electrical equipment.

[0178] Next, one or more embodiments will be described in more detail with reference to the following examples. Of course, these examples do not limit the scope of one or more embodiments.

[0179] Experimental Example 1

[0180] (1) Preparation of the first active material:

[0181] Weigh 2g of carbon-free LiFePO4 and evenly spread it in a corundum crucible measuring 6cm x 3cm x 2cm. Before oxidation, heat the cyclone oxidation furnace to 400°C, the oxidation temperature, and introduce air into the furnace at a flow rate of 400sccm as the oxidizing gas. Once the furnace temperature stabilizes at the oxidation temperature, place the corundum crucible containing the carbon-free LiFePO4 into the furnace and perform oxidation for 10 minutes. After oxidation, remove the oxidized carbon-free LiFePO4 powder and allow it to cool naturally at room temperature. Once the temperature drops to room temperature, grind the resulting powder in a mortar for 10 minutes to obtain the first active material.

[0182] Among them, "carbon-free LiFePO4" refers to lithium iron phosphate without any coating treatment, D V 50 is 2.3μm.

[0183] (2) Preparation of positive electrode sheet:

[0184] The positive electrode active material (the positive electrode active material is a mixture of the first active material and the second active material, and the second active material is carbon-containing LiFePO4), the conductive agent acetylene black and the binder polyvinylidene fluoride (PVDF) with a mass ratio of 92:2.5:5.5 are mixed, and the solvent N-methylpyrrolidone (NMP) is added and stirred until the system becomes uniform. After grinding for 10 minutes, a positive electrode slurry with a solid content of 50wt% is prepared.

[0185] Among them, "carbon-containing LiFePO4" refers to LiFePO4 with only a carbon layer on the surface, and the volume average particle size of carbon-containing LiFePO4 is D V 50 is 1.6 μm, the average thickness of the coated carbon layer is 1.5 nm, and the mass fraction of the carbon layer in the entire carbon-containing LiFePO4 is 1.6 wt%; in the positive electrode active material, the mass ratio of the first active material to the second active material is 1:1.

[0186] The positive electrode slurry was evenly coated on both sides of the positive electrode current collector aluminum foil (thickness of 6 μm) and dried at 120°C for 4 hours, then cold pressed, and then trimmed and cut into pieces, and dried under vacuum conditions at 120°C for 4 hours to obtain a positive electrode sheet, wherein the thickness of the single-sided positive electrode active material layer was 200 μm.

[0187] (3) Preparation of button batteries:

[0188] A lithium sheet (0.5 mm thick) was used as the negative electrode, and a solution of 1 mol / L LiPF6 in ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) in a volume ratio of 1:1:1 was used as the electrolyte. Together with the positive electrode sheet prepared above, they were assembled into a CR2430 button battery in a glove box.

[0189] Example 2

[0190] This embodiment provides a button-type battery. The only difference between Embodiment 2 and Embodiment 1 is that the oxidation temperature during the preparation of the first active material is 300°C.

[0191] Example 3

[0192] This embodiment provides a button-type battery. The only difference between Embodiment 3 and Embodiment 1 is that the oxidation temperature during the preparation of the first active material is 500°C.

[0193] Example 4

[0194] This embodiment provides a button-type battery. The only difference between Embodiment 4 and Embodiment 1 is that the oxidation temperature during the preparation of the first active material is 600°C.

[0195] Example 5

[0196] This embodiment provides a button-type battery. The only difference between Example 5 and Example 1 is that the oxidation temperature during the preparation of the first active material is 280°C.

[0197] Example 6

[0198] This embodiment provides a button-type battery. The only difference between Example 6 and Example 1 is that the oxidation temperature during the preparation of the first active material is 650°C.

[0199] Example 7

[0200] This embodiment provides a button-type battery. The only difference between Example 7 and Example 1 is that the oxidation treatment time during the preparation of the first active material is 2 minutes.

[0201] Example 8

[0202] This embodiment provides a button-type battery. The only difference between Example 8 and Example 1 is that the oxidation treatment time during the preparation of the first active material is 60 minutes.

[0203] Example 9

[0204] This embodiment provides a button-type battery. The only difference between Example 9 and Example 1 is that the oxidation treatment time during the preparation of the first active material is 20 minutes.

[0205] Example 10

[0206] This embodiment provides a button-type battery. The only difference between Example 10 and Example 1 is that the oxidation treatment time during the preparation of the first active material is 30 minutes.

[0207] Example 11

[0208] This embodiment provides a button-type battery. The only difference between Example 11 and Example 1 is that the oxidation treatment time during the preparation of the first active material is 1.8 minutes.

[0209] Example 12

[0210] This embodiment provides a button-type battery. The only difference between Example 12 and Example 1 is that the oxidation treatment time during the preparation of the first active material is 65 minutes.

[0211] Example 13

[0212] This embodiment provides a button-type battery. The only difference between Example 13 and Example 1 is that the flow rate of the oxidizing gas during the preparation of the first active material is 200 sccm.

[0213] Example 14

[0214] This embodiment provides a button-type battery. The only difference between Example 14 and Example 1 is that the flow rate of the oxidizing gas during the preparation of the first active material is 500 sccm.

[0215] Example 15

[0216] This embodiment provides a button-type battery. The only difference between Example 15 and Example 1 is that the oxidizing gas used in the preparation process of the first active material is oxygen.

[0217] Example 16

[0218] This embodiment provides a button-type battery. The only difference between Example 16 and Example 1 is that the oxidizing gas in the preparation process of the first active material is a mixed gas of ozone and argon with a volume ratio of 1:4.

[0219] Example 17

[0220] This embodiment provides a button-type battery. The only difference between Example 17 and Example 1 is that in the positive electrode active material, the mass ratio of the first active material to the second active material is 1:0.5.

[0221] Example 18

[0222] This embodiment provides a button-type battery. The only difference between Example 18 and Example 1 is that in the positive electrode active material, the mass ratio of the first active material to the second active material is 1:3.

[0223] Example 19

[0224] This embodiment provides a button cell. The only difference between Example 19 and Example 1 is that the carbon-free LiFePO4 used to prepare the first active material is D V 50 is 0.25μm.

[0225] Example 20

[0226] This embodiment provides a button cell. The only difference between embodiment 20 and embodiment 1 is that the carbon-free LiFePO4 D used to prepare the first active material is V 50 is 8.2μm.

[0227] Comparative Example 1

[0228] This comparative example provides a button-type battery. The only difference between comparative example 1 and example 1 is that the first active material is D V 50 is 2.3 μm carbon-free LiFePO4, and the positive electrode active material is only the first active material.

[0229] Among them, "carbon-free LiFePO4" means lithium iron phosphate without any coating treatment.

[0230] Comparative Example 2

[0231] This comparative example provides a button-type battery. The only difference between comparative example 1 and example 1 is that the first active material is D V 50 is 2.3 μm carbon-free LiFePO4, and the second active material is carbon-containing LiFePO4.

[0232] Among them, "carbon-free LiFePO4" refers to lithium iron phosphate without any coating layer treatment; "carbon-containing LiFePO4" refers to LiFePO4 with only a carbon layer on the surface, and the volume average particle size of carbon-containing LiFePO4 is D V 50 is 1.6 μm, the average thickness of the coated carbon layer is 1.5 nm, and the mass fraction of the carbon layer in the entire carbon-containing LiFePO4 is 1.6 wt%.

[0233] Table 1 Preparation parameters of the first active material and parameters of the positive electrode active material of Examples 1 to 20 and Comparative Examples 1 to 2

[0234] In Table 1, “ / ” means that there is no corresponding parameter.

[0235] Testing the performance of the first active material and the button battery:

[0236] (1) TEM-EDS analysis of the first active material

[0237] A cross-section polisher (IB-09010CP argon ion cross-section polisher from JEOL Ltd.) is used to cut a smooth cross-section through the core of the first active material. EDS elemental analysis is then combined with TEM (such as the X-Max EDS from Oxford Instruments in the UK combined with the Thermo Scientific-Talos F200S G2 TEM from Thermo Fisher Scientific in the United States) to scan the cross-section of the first active material to obtain an element distribution map in the cross-section.

[0238] The thickness of the oxide layer is obtained according to the element distribution of the cross section; the thickness of the oxide layer at 10 different positions on the test cross section is averaged and recorded as the thickness of the oxide layer.

[0239] (2) Raman spectroscopy analysis of the first active material

[0240] The first active material was measured using a LabRAM HR Evolution laser micro-Raman spectrometer, wherein a solid laser with a wavelength of 523 nm was used as the light source, a beam diameter of 1.2 μm, and a power of 1 mW; a macro Raman measurement mode was used; and a CCD detector was used.

[0241] (3) Volume average particle size D of the first active material V 50 tests

[0242] Equipment model: Malvern 2000 (MasterSizer 2000) laser particle size analyzer, reference standard process: GB / T19077-2016 / ISO 13320:2009.

[0243] Specific test process: Take an appropriate amount of the first active material (the sample concentration is sufficient to ensure 8-12% light shielding), add 20mL of anhydrous ethanol, and ultrasonicate for 5min (53KHz / 120W) to completely disperse the first active material. Then, measure the volume average particle size D of the first active material according to GB / T19077-2016 / ISO 13320:2009 standard. V 50 measurements.

[0244] (4) Electronic conductivity test of the first active material

[0245] The resistance of the first active material was tested at 100 MPa using a powder resistivity meter (PRCD1100, manufactured by Yuanneng Technology Co., Ltd.). Conductivity = powder thickness / (resistance value×powder area).

[0246] (5) Specific capacity test of the first active material

[0247] At 25°C, the button cells prepared in Examples 1-20 and Comparative Examples 1-2 were first charged at a constant current of 0.1C to 4.3V, then charged at a constant voltage of 4.3V to a current of 0.01C, left for 5 minutes, and then discharged at 0.1C to 2.0V. The resulting discharge capacity was recorded as C1, and the capacity obtained after repeating the cycle a second time was recorded as C2. Three parallel samples of the button cells were prepared, and the average C2 of the three parallel samples was recorded as the average discharge capacity C0 (unit: mAh / g). The specific capacity of the first active material = (C0-Cm×A1) / A2, where Cm is the discharge capacity of the second active material, which is 150 mAh / g; A1 is the mass percentage of the second active material in the positive electrode active material (unit: %), and A2 is the mass percentage of the first active material in the positive electrode active material (unit: %).

[0248] (6) Cycling performance test of button batteries

[0249] At 25°C, the button batteries prepared in Examples 1-20 and Comparative Examples 1-2 were first charged to 4.3V at a constant current of 0.1C, then charged at a constant voltage of 4.3V to a current of 0.01C, left for 5 minutes, and then discharged at 0.1C to 2.0V. This constituted one charge-discharge cycle, and the discharge capacity was the discharge capacity of the first cycle. The button batteries were then subjected to 50 cycles of charge-discharge testing in this manner. The discharge capacity at the 50th cycle was measured, and the post-cycle capacity retention of the button batteries was calculated using the following formula.

[0250] Capacity retention rate (%) of the button cell after 50 cycles = [discharge capacity at the 50th cycle / discharge capacity at the 1st cycle] × 100%.

[0251] The properties of the first active material and the button battery are shown in Table 2:

[0252] Table 2 Performance of the first active material and button cell

[0253] In Table 2, “ / ” means that the corresponding parameter does not exist; “thickness of the oxide layer” means: the average thickness of the oxide layer; for Examples 1 to 20, the oxide layer refers to: the iron oxide layer on the surface of lithium iron phosphate; “overrange” means: the corresponding data is too small to be measured.

[0254] FIG6 is a TEM-EDS graph of the first active material prepared in Example 6 of the present application, and FIG7 is a TEM-EDS graph of the first active material provided in Comparative Example 1 of the present application.

[0255] As can be seen from Figure 6, the surface of the first active material prepared in Example 6 of the present application has an iron-enriched surface layer (the dotted line in Figure 6 represents: the interface between the iron-enriched surface layer of the first active material and the LiFePO4 matrix), and the surface layer thickness is about 8 nm; as can be seen from Figure 7, the first active material provided in Comparative Example 1 (i.e., carbon-free LiFePO4, LiFePO4 without any coating treatment) has uniform phosphorus and iron distribution throughout the particles and does not have an iron-enriched surface layer; this indicates that: after oxidation treatment, the first active material prepared in Example 6 of the present application forms an iron-enriched surface layer on the surface of the carbon-free LiFePO4.

[0256] FIG8 is a comparison chart of Raman spectra of the first active material prepared in Example 6 of the present application, the first active materials prepared in Examples 9 to 10, and the first active material provided in Comparative Example 1.

[0257] As can be seen from FIG8 , the Raman spectra of the first active materials obtained in Example 6 and Examples 9 to 10 of the present application after oxidation treatment for different times are as follows: -1 ~300cm -1 There are two significant Raman characteristic peaks in the range of 225 cm -1 and 290cm -1 Nearby, corresponding to the υ(O-Fe-O) stretching vibration peak and the δ(O-Fe-O) bending vibration peak, respectively, indicating that the first active materials prepared in Examples 6 and Examples 9-10 after different oxidation treatment times formed iron oxide (α-Fe2O3). The Raman spectrum of the first active material provided in Comparative Example 1 (i.e., carbon-free LiFePO4, LiFePO4 without any coating treatment) has a wave number of 200 cm -1 ~300cm -1 There is no Raman characteristic peak in the range.

[0258] 6 and 8 , it can be seen that after the carbon-free LiFePO 4 is oxidized in the present application, an oxide layer containing α-Fe 2 O 3 can be formed on the surface of the carbon-free LiFePO 4 .

[0259] As can be seen from Table 2, the electronic conductivity and specific capacity of the first active materials prepared in Examples 1 to 20 of the present application are higher than those of the first active material provided in Comparative Example 1 (i.e., carbon-free LiFePO4, LiFePO4 without any coating treatment); this indicates that the first active material with an oxide layer provided in the present application improves the shortcomings of low capacity and low electronic conductivity of LiFePO4 without a carbon layer coating.

[0260] It can also be seen from Table 2 that the cycle performance of the button batteries corresponding to Examples 1 to 20 of the present application is better than the cycle performance of the button batteries corresponding to Comparative Examples 1 to 2, indicating that the first active material with an oxide layer provided by the present application improves the shortcoming of poor cycle performance of LiFePO4 coated without a carbon layer, and has better cycle performance.

[0261] The embodiments described above are part of the embodiments of the present application, rather than all of the embodiments. The detailed description of the embodiments of the present application is not intended to limit the scope of the present application for protection, but merely represents selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

Claims

1. A secondary battery, wherein: The secondary battery includes a positive electrode sheet, the positive electrode sheet includes lithium phosphate positive electrode active particles, the lithium phosphate positive electrode active particles include a central portion and a surface portion, the surface portion is continuously or discontinuously distributed on the surface of the central portion; the thickness of the surface portion is ≤10nm; the lithium content of the central portion is greater than the lithium content of the surface portion.

2. The secondary battery according to claim 1, wherein In the Raman spectrum of the lithium-containing phosphate positive electrode active particles, at a Raman shift of 200 cm - 1 ~250cm -1 The position has an O-Fe-O stretching vibration peak; And / or, in the Raman spectrum of the lithium-containing phosphate positive electrode active particles, at the Raman shift of 255 cm -1 ~300cm -1 The position has an O-Fe-O bending vibration peak.

3. The secondary battery according to claim 1 or 2, wherein: The surface portion includes iron oxide.

4. The secondary battery according to any one of claims 1 to 3, wherein The surface portion includes ferric oxide.

5. The secondary battery according to any one of claims 1 to 4, wherein The lithium-containing phosphate positive electrode active particles include Li 1+x M 1-y A y P 1-z R z O 4-t , wherein M includes at least one of Fe, Co and Ni, A includes at least one of Mn, Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and R includes at least one of B, S, Si and N; -0.1≤x≤0.1, 0≤y≤0.1, 0≤z≤0.1, 0≤t≤0.

1.

6. The secondary battery according to claim 5, wherein The lithium-containing phosphate positive electrode active particles include Li 1+x1 Fe 1-y1 A y1 PO 4-t1 , A includes at least one of Mn, Al, Ti, V, Ni and Zn; 0≤x1≤0.05, 0≤y1≤0.05, 0≤t1≤0.

02.

7. The secondary battery according to any one of claims 1 to 6, wherein The thickness of the surface portion is 1.5 nm to 4 nm.

8. The secondary battery according to any one of claims 1 to 7, wherein The volume average particle size D of the lithium phosphate positive electrode active particles V 50 is 300nm~10.5μm.

9. An electrical device, wherein: The electrical equipment comprises the secondary battery according to any one of claims 1 to 8.

10. A positive electrode active material, wherein: The positive electrode active material comprises: a substrate and an oxide layer located on the surface of the substrate; The matrix includes Li 1+x M 1-y A y P 1-z R z O 4-t , wherein M includes at least one of Fe, Co and Ni, A includes at least one of Mn, Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and R includes at least one of B, S, Si and N; -0.1≤x≤0.1, 0≤y≤0.1, 0≤z≤0.1, 0≤t≤0.1; The oxide layer includes iron oxide.

11. The positive electrode active material according to claim 10, wherein The matrix includes Li 1+x1 Fe 1-y1 A y1 PO 4-t1 , A includes at least one of Mn, Al, Ti, V, Ni and Zn; 0≤x1≤0.05, 0≤y1≤0.05, 0≤t1≤0.

02.

12. The positive electrode active material according to claim 10 or 11, wherein In the Raman spectrum of the positive electrode active material, at the Raman shift of 200 cm - 1 ~250cm -1 The position has an O-Fe-O stretching vibration peak; And / or, in the Raman spectrum of the positive electrode active material, at the Raman shift of 255 cm -1 ~300cm -1 The position has an O-Fe-O bending vibration peak.

13. The positive electrode active material according to any one of claims 10 to 12, wherein The oxide layer includes ferric oxide.

14. The positive electrode active material according to any one of claims 10 to 13, wherein The thickness of the oxide layer is ≤10nm; Optionally, the oxide layer has a thickness of 1.5 nm to 4 nm.

15. The positive electrode active material according to any one of claims 10 to 14, wherein The volume average particle size D of the positive electrode active material V 50 is 300nm~10.5μm.

16. A method for preparing a positive electrode active material, wherein: include: Treating a substrate to form an oxide layer on a surface of the substrate; The matrix includes Li 1+x M 1-y A y P 1-z R z O 4-t , wherein M includes at least one of Fe, Co and Ni, A includes at least one of Mn, Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and R includes at least one of B, S, Si and N; -0.1≤x≤0.1, 0≤y≤0.1, 0≤z≤0.1, 0.001≤t≤0.1; The oxide layer includes iron oxide.

17. The preparation method according to claim 16, wherein: The matrix includes Li 1+x1 Fe 1-y1 A y1 PO 4-t1 , A includes at least one of Mn, Al, Ti, V, Ni and Zn; 0≤x1≤0.05, 0≤y1≤0.05, 0≤t1≤0.02; the substrate is oxidized to form the oxide layer on the surface of the substrate.

18. The preparation method according to claim 17, wherein: Using an oxidizing gas to oxidize the substrate; Optionally, the oxidizing gas includes at least one of oxygen and ozone; Optionally, in the oxidizing gas, the sum of the volumes of the oxygen and the ozone accounts for 10% to 100% of the total volume of the oxidizing gas.

19. The preparation method according to claim 17 or 18, wherein: The temperature of the oxidation treatment is ≥300°C; Optionally, the temperature of the oxidation treatment is 300°C to 600°C.

20. The preparation method according to claim 18, wherein: During the oxidation treatment, the flow rate of the oxidizing gas is 200 sccm to 500 sccm; Optionally, the oxidation treatment time is 2 min to 60 min.

21. A positive electrode sheet, wherein: The positive electrode sheet includes a positive electrode current collector and a positive electrode active layer covering at least one surface of the positive electrode current collector in a thickness direction; Wherein, the positive electrode active layer comprises a first active material, and the first active material comprises the positive electrode active material according to any one of claims 10 to 15 or the positive electrode active material prepared by the preparation method according to any one of claims 16 to 20.

22. The positive electrode sheet according to claim 21, wherein: The positive electrode active layer further includes a second active material, and the second active material is different from the first active material.

23. A secondary battery, wherein: The secondary battery comprises the positive electrode sheet according to claim 21 or 22.

24. An electrical device, wherein: The electric device comprises the secondary battery according to claim 23.

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