A composite positive electrode material and its preparation method and application

Abstract

The present invention discloses a composite cathode material, a preparation method thereof and an application. The composite cathode material includes: Mg-doped titanium pyrophosphate and magnesium titanium phosphate; the chemical general formula of the composite cathode material is Ti 1‑0.5x Mg x P2O7 / Mg 0.5 Ti2(PO4)3, where 0.01 ≤ x ≤ 0.015; the mass ratio of Mg-doped titanium pyrophosphate to magnesium titanium phosphate is [7-9]:[1-3]; the composite cathode material is prepared from a titanium element-containing material, a phosphorus element-containing material and a magnesium element-containing material by a solid-phase secondary sintering method, wherein the magnesium element-containing material is in excess by 2%; the mass percentage of magnesium element in the composite cathode material is 0.1%-0.5%; the peak intensity ratio of the strongest characteristic diffraction peak to the second-strongest characteristic diffraction peak in the X-ray diffraction pattern of the composite cathode material is 1.95.

Description

Technical Field

[0001] The present invention relates to the technical field of magnesium battery materials, and in particular to a composite positive electrode material and a preparation method and application thereof. Background Art

[0002] As one of the most abundant light metal elements on earth, magnesium is widely used in various fields due to its good physical, chemical, mechanical and other properties. The NASICON structure of magnesium titanium phosphate is relatively stable, and the interlayer spacing is large, which is conducive to the deintercalation of magnesium ions, but its capacity is relatively low, and there are few reports on it as a positive electrode material. Patent document CN102931403A, a method for applying magnesium titanium phosphate in a positive electrode material for a rechargeable magnesium battery, although magnesium titanium phosphate is used as a positive electrode material in a rechargeable magnesium battery, its capacity is relatively low. According to the patent embodiment 1 and the attached Figure 2 According to the records, the discharge capacity at the 27th cycle was only 48.6 mAh g -1 .

[0003] The chemical formula of titanium pyrophosphate is TiP 2 O 7 , is a polyanionic compound, which is composed of TiO 6 Octahedron and P 2 O 7 The double tetrahedrons are connected by vertex angles; this structure is very stable, which makes the titanium pyrophosphate material have excellent thermal stability. However, there are few reports on titanium pyrophosphate as a positive electrode material for magnesium secondary batteries, and its electrochemical performance is poor.

[0004] It is a common technical means to improve the electronic conductivity of materials and the electrochemical properties of materials by doping with elements, such as NASICON-type LiM 2 (PO 4 ) 3 (M is Ti, Ge, Zr) materials. Various methods to improve LiM have been reported in the literature. 2 (PO 4 ) 3 The methods to improve the performance mainly include coating with conductive materials, reducing the size of the material through appropriate synthesis methods, or element doping. Element doping can greatly increase the electronic conductivity and improve the electrochemical properties of the material.

[0005] Therefore, one of our current research directions is to find suitable doping elements to improve the performance of titanium pyrophosphate and to perform composite modification so that it can have excellent electrochemical properties when used in magnesium secondary batteries. Summary of the invention

[0006] A composite positive electrode material and a preparation method and application thereof provided by an embodiment of the present invention are obtained by subjecting a titanium-containing material, a phosphorus-containing material and an excess of a magnesium-containing material to a solid phase secondary sintering method to obtain a magnesium-doped titanium pyrophosphate and titanium magnesium phosphate composite positive electrode material, wherein, during the first sintering process, a trace amount of Mg can make the Mg-doped titanium pyrophosphate material have high conductivity, and the magnesium element replaces part of the titanium element and is doped into the titanium pyrophosphate without changing the crystal structure of the titanium pyrophosphate material, thereby improving the electrochemical properties of the titanium pyrophosphate material while having electrochemical stability; in addition, the titanium magnesium phosphate material generated by the secondary sintering has a NASICON structure with a sufficiently large interlayer spacing so that magnesium ions can be reversibly embedded and removed from the titanium magnesium phosphate, and even in the process of a large amount of magnesium ion embedding and de-embedding, the crystal structure of the titanium magnesium phosphate is still stable, which can improve the charge and discharge performance of the battery; the composite positive electrode material of the present invention, due to the composite of magnesium-doped titanium pyrophosphate and titanium magnesium phosphate materials, can play a synergistic role between the two, improve the initial capacity and cycle capacity of the battery, and improve the fast charging performance and discharge voltage of the battery.

[0007] The preparation method of the composite positive electrode material provided by the present invention is simple, suitable for batch production, low in cost, and can promote the wide application of titanium pyrophosphate materials and magnesium titanium phosphate materials in the field of battery materials.

[0008] In a first aspect, an embodiment of the present invention provides a composite positive electrode material, the composite positive electrode material comprising: Mg-doped titanium pyrophosphate and magnesium titanium phosphate; the chemical formula of the composite positive electrode material is Ti 1-0.5x Mg x P 2 O 7 / Mg 0.5 Ti 2 (PO 4 ) 3 , where 0.01≤x≤0.015;

[0009] The mass ratio of the Mg-doped titanium pyrophosphate and magnesium titanium phosphate is [1-3]: [7-9];

[0010] The composite positive electrode material is prepared by a solid phase secondary sintering method from a material containing titanium, a material containing phosphorus and a material containing magnesium, wherein the material containing magnesium is in excess of 2%.

[0011] The mass percentage of the magnesium element in the composite positive electrode material is 0.1%-0.5%;

[0012] The peak intensity ratio of the strongest characteristic diffraction peak to the second highest characteristic diffraction peak in the X-ray diffraction pattern of the composite positive electrode material is 1.95.

[0013] Preferably, the mass percentage of the titanium element in the composite positive electrode material is 20%-30%;

[0014] The mass percentage of the phosphorus element in the composite positive electrode material is 15%-30%;

[0015] The mass percentage of the oxygen element in the composite positive electrode material is 40%-55%.

[0016] Preferably, the particle size Dv50 of the composite positive electrode material is between 2 μm and 5 μm.

[0017] In a second aspect, an embodiment of the present invention provides a method for preparing the composite positive electrode material described in the first aspect, wherein the preparation method is a solid phase secondary sintering method, comprising:

[0018] Weigh a titanium-containing material, a phosphorus-containing material, and a magnesium-containing material according to a stoichiometric ratio, and place the materials in a mixer for mixing for 30 minutes to 2 hours to obtain a mixed material;

[0019] Placing the mixed material in a box-type high-temperature furnace and sintering it in an air atmosphere to obtain a precursor material;

[0020] The precursor material is subjected to secondary sintering in an air atmosphere to obtain a white block material;

[0021] The white block material is crushed and then sieved to obtain a composite positive electrode material;

[0022] The chemical formula of the composite positive electrode material is Ti 1-0.5x Mg x P 2 O 7 / Mg 0.5 Ti 2 (PO 4 ) 3 , wherein 0.01≤x≤0.015; the mass ratio of the Mg-doped titanium pyrophosphate and magnesium titanium phosphate is [1-3]:[7-9]; the mass percentage of the magnesium element in the composite positive electrode material is 0.1%-0.5%;

[0023] The material containing magnesium element is in excess of 2%.

[0024] Preferably, the titanium-containing material includes: titanium dioxide and / or titanium tetrachloride;

[0025] The phosphorus-containing material includes one or more of phosphoric acid, sodium phosphate, diammonium phosphate, and diammonium hydrogen phosphate;

[0026] The material containing magnesium element includes: one or more of magnesium oxide, magnesium carbonate, magnesium chloride and magnesium hydroxide.

[0027] Preferably, the sintering conditions include: a sintering temperature of 600°C-800°C, a heating rate of 2°C / min-5°C / min, and a holding time of 5 hours-10 hours;

[0028] The secondary sintering conditions include: the secondary sintering temperature is 300°C-500°C, the heating rate is 2°C / min-5°C / min, and the holding time is 2 hours-5 hours;

[0029] The crushing process is specifically as follows: the white block material is primarily crushed by a jaw crusher or a roller crusher to obtain white granular material; and the white granular material is then pulverized by a jet mill.

[0030] Preferably, the mass percentage of the titanium element in the composite positive electrode material is 20%-30%;

[0031] The mass percentage of the phosphorus element in the composite positive electrode material is 15%-30%;

[0032] The mass percentage of the oxygen element in the composite positive electrode material is 40%-55%;

[0033] The particle size Dv50 of the composite positive electrode material is between 2 μm and 5 μm.

[0034] In a third aspect, an embodiment of the present invention provides a positive electrode plate, wherein the positive electrode plate comprises the composite positive electrode material of the first aspect.

[0035] In a fourth aspect, an embodiment of the present invention provides a magnesium secondary battery, wherein the magnesium secondary battery comprises the positive electrode sheet described in the third aspect.

[0036] A composite positive electrode material and a preparation method and application thereof provided by an embodiment of the present invention are obtained by subjecting a titanium-containing material, a phosphorus-containing material and an excess of a magnesium-containing material to a solid phase secondary sintering method to obtain a magnesium-doped titanium pyrophosphate and titanium magnesium phosphate composite positive electrode material, wherein, during the first sintering process, a trace amount of Mg can make the Mg-doped titanium pyrophosphate material have high conductivity, and the magnesium element replaces part of the titanium element and is doped into the titanium pyrophosphate without changing the crystal structure of the titanium pyrophosphate material, thereby improving the electrochemical properties of the titanium pyrophosphate material while having electrochemical stability; in addition, the titanium magnesium phosphate material generated by the secondary sintering has a NASICON structure with a sufficiently large interlayer spacing so that magnesium ions can be reversibly embedded and removed from the titanium magnesium phosphate, and even in the process of a large amount of magnesium ion embedding and de-embedding, the crystal structure of the titanium magnesium phosphate is still stable, which can improve the charge and discharge performance of the battery; the composite positive electrode material of the present invention, due to the composite of magnesium-doped titanium pyrophosphate and titanium magnesium phosphate materials, can play a synergistic role between the two, improve the initial capacity and cycle capacity of the battery, and improve the fast charging performance and discharge voltage of the battery.

[0037] The preparation method of the composite positive electrode material provided by the present invention is simple, suitable for batch production, low in cost, and can promote the wide application of titanium pyrophosphate materials and magnesium titanium phosphate materials in the field of battery materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The technical solution of the embodiment of the present invention is further described in detail below through the drawings and examples.

[0039] Figure 1 is a flow chart of a method for preparing a composite positive electrode material provided by an embodiment of the present invention;

[0040] Figure 2 is the X-ray diffraction pattern (XRD) of the composite positive electrode material prepared in Example 1 of the present invention;

[0041] Figure 3 It is a discharge cycle curve diagram of a battery prepared with the composite positive electrode material provided in Example 1 and a battery prepared with the pyrophosphate carbon material of Comparative Example 1 and the titanium magnesium phosphate material of Comparative Example 2 as positive electrode materials. DETAILED DESCRIPTION

[0042] The present invention is further described in detail below through the accompanying drawings and specific embodiments, but it should be understood that these embodiments are only used for more detailed description and should not be understood as limiting the present invention in any form, that is, they are not intended to limit the protection scope of the present invention.

[0043] The embodiment of the present invention provides a composite positive electrode material, the composite positive electrode material comprises: Mg-doped titanium pyrophosphate and magnesium titanium phosphate; the chemical formula of the composite positive electrode material is Ti 1-0.5x Mgx P 2 O 7 / Mg 0.5 Ti 2 (PO 4 ) 3 , where 0.01≤x≤0.015; the mass ratio of Mg-doped titanium pyrophosphate and magnesium titanium phosphate is [1-3]:[7-9];

[0044] Among them, the mass percentage of magnesium element in the composite positive electrode material is 0.1%-0.5%; the mass percentage of titanium element in the composite positive electrode material is 20%-30%; the mass percentage of phosphorus element in the composite positive electrode material is 15%-30%; and the mass percentage of oxygen element in the composite positive electrode material is 40%-55%.

[0045] The particle size Dv50 of the composite positive electrode material is between 2 μm and 5 μm; the peak intensity ratio of the strongest characteristic diffraction peak to the second highest characteristic diffraction peak in the X-ray diffraction pattern of the composite positive electrode material is 1.95.

[0046] The embodiment of the present invention provides a method for preparing the composite positive electrode material of the first aspect, wherein the preparation method is a solid phase secondary sintering method, such as Figure 1 As shown, the specific steps include:

[0047] Step 110, weighing a titanium-containing material, a phosphorus-containing material, and a magnesium-containing material according to a stoichiometric ratio, and placing the materials in a mixer for mixing for a period of 30 minutes to 2 hours to obtain a mixed material;

[0048] Wherein, the material containing titanium element includes: titanium dioxide and / or titanium tetrachloride;

[0049] The material containing phosphorus element includes: one or more of phosphoric acid, sodium phosphate, diammonium phosphate and diammonium hydrogen phosphate;

[0050] The material containing magnesium element includes one or more of magnesium oxide, magnesium carbonate, magnesium chloride and magnesium hydroxide; the material containing magnesium element has an excess of 2%.

[0051] Step 120, placing the mixed material in a box-type high-temperature furnace and sintering it in an air atmosphere to obtain a precursor material;

[0052] Specifically, the sintering conditions include: a sintering temperature of 600° C.-800° C., a heating rate of 2° C. / min-5° C. / min, and a heat preservation time of 5 hours-10 hours.

[0053] Step 130, secondary sintering the precursor material in an air atmosphere to obtain a white block material;

[0054] Specifically, the conditions for the secondary sintering include: the secondary sintering temperature is 300° C.-500° C., the heating rate is 2° C. / min-5° C. / min, and the holding time is 2 hours-5 hours.

[0055] Step 140, crushing the white block material and then screening it to obtain a composite positive electrode material;

[0056] The specific process of the crushing treatment is as follows: the white block material is primarily crushed by a jaw crusher or a roller crusher to obtain white granular material; the white granular material is then crushed by a jet mill;

[0057] The chemical formula of the prepared composite cathode material is Ti 1-0.5x Mg x P 2 O 7 / Mg 0.5 Ti 2 (PO 4 ) 3 , where 0.01≤x≤0.015; the mass ratio of Mg-doped titanium pyrophosphate and magnesium titanium phosphate is [1-3]:[7-9]; the mass percentage of magnesium element in the composite positive electrode material is 0.1%-0.5%; the mass percentage of titanium element in the composite positive electrode material is 20%-30%; the mass percentage of phosphorus element in the composite positive electrode material is 15%-30%; the mass percentage of oxygen element in the composite positive electrode material is 40%-55%; the particle size Dv50 of the composite positive electrode material is between 2μm-5μm.

[0058] The composite positive electrode material provided in the embodiment of the present invention can be used in a positive electrode sheet as an active material of the positive electrode sheet, and the positive electrode sheet can be applied to a magnesium secondary battery.

[0059] In order to better understand the technical solution provided by the present invention, the preparation process and characteristics of the composite positive electrode material of the present invention are respectively described below with specific examples.

[0060] Example 1

[0061] This embodiment provides a process for preparing a composite positive electrode material, and the specific steps are as follows:

[0062] 1) According to the stoichiometric ratio, 431.1 g of titanium dioxide, 1341.2 g of ammonium dihydrogen phosphate and 28.37 g of magnesium chloride were weighed and placed in a mixer for mixing for 30 min to obtain a mixed material.

[0063] 2) The mixed material is placed in a box-type high-temperature furnace, heated to 600° C. at a heating rate of 2° C. / min in an air atmosphere, and kept at this temperature for 10 hours for sintering to obtain a precursor material.

[0064] 3) The precursor material is subjected to secondary sintering in an air atmosphere, the temperature is increased to 300° C. at a heating rate of 2° C. / min, and the temperature is kept for 5 hours, and sintering is performed to obtain a white block material.

[0065] 4) The white block material is placed in a jaw crusher for primary crushing to obtain white granular material, and then the white granular material is crushed by a jet mill and sieved to obtain a composite positive electrode material with a particle size Dv50 of 2 μm, the chemical formula of which is Ti 0.9925 Mg 0.015 P 2 O 7 / Mg 0.5 Ti 2 (PO 4 ) 3 .

[0066] The XRD pattern of the composite positive electrode material prepared in this embodiment is as follows: Figure 2 As shown in the figure, the ordinate is the diffraction peak intensity (Intensity), and it can be seen that the diffraction peak of the material is similar to that of titanium pyrophosphate (TiP 2 O 7 ) standard card PDF#38-1468, indicating that the magnesium-doped titanium pyrophosphate in the composite positive electrode material prepared in this embodiment has the same pure phase structure as titanium pyrophosphate, and the doping of Mg element has no effect on the purity and crystal form of the titanium pyrophosphate material; in addition, some diffraction peaks are consistent with magnesium titanium phosphate (Mg 0.5 Ti 2 (PO 4 ) 3 ) corresponds to the standard card PDF#82-0297, indicating that the composite positive electrode material prepared in this embodiment contains magnesium titanium phosphate material.

[0067] The composite positive electrode material prepared in this example was used to prepare button cells and tested:

[0068] The button cell can be prepared by the existing method. The composite positive electrode material prepared above is mixed with carbon black and polyvinylidene fluoride in a mass ratio of 8:1:1 to prepare a slurry, which is then coated on an aluminum foil current collector, dried, and cut into pieces to obtain a positive electrode sheet. Metal magnesium is used as the negative electrode, a single-layer polyethylene film is used as the separator, and 1 mol of LiClO 4 Ethyl acetate solution was used as the electrolyte and button half-cells were assembled in an argon-filled glove box.

[0069] The prepared button half-cell was tested in constant current charge and discharge mode using a charge and discharge instrument. The discharge cut-off voltage was 2.0 V, and the charge cut-off voltage was 4.0 V. The charge and discharge tests from the 1st week to the 100th week were all carried out at a current density of 1C. The initial capacity and the capacity data of 100 cycles are shown in Table 1.

[0070] The discharge cycle curve of the button-type half-cell prepared in this embodiment is as follows: Figure 3 As shown, the ordinate represents the special capacity, and the horizontal and vertical axes represent the number of cycles.

[0071] Example 2

[0072] This embodiment provides a process for preparing a composite positive electrode material, and the specific steps are as follows:

[0073] 1) 189.67 g of titanium tetrachloride, 816.4 g of phosphoric acid and 18.91 g of magnesium oxide were weighed according to the stoichiometric ratio, and placed in a mixer for mixing for 1 hour to obtain a mixed material.

[0074] 2) The mixed material is placed in a box-type high-temperature furnace, heated to 800° C. at a heating rate of 5° C. / min in an air atmosphere, and kept at this temperature for 5 hours for sintering to obtain a precursor material.

[0075] 3) The precursor material is subjected to secondary sintering in an air atmosphere, the temperature is increased to 500° C. at a heating rate of 5° C. / min, and the temperature is kept for 3 hours, and sintering is performed to obtain a white block material.

[0076] 4) The white block material is placed in a roller mill for primary crushing to obtain white granular material, and then the white granular material is crushed by a jet mill and sieved to obtain a composite positive electrode material with a particle size Dv50 of 4 μm, and the chemical formula is Ti 0.994 Mg 0.012 P 2 O 7 / Mg 0.5 Ti 2 (PO 4 ) 3 .

[0077] The composite positive electrode material prepared in this example was used to prepare button cells and conduct tests. The assembly method and test method were the same as those in Example 1. The test data are detailed in Table 1.

[0078] Example 3

[0079] This embodiment provides a process for preparing a composite positive electrode material, and the specific steps are as follows:

[0080] 1) According to the stoichiometric ratio, 625.95 g of titanium dioxide, 1190.62 g of diammonium hydrogen phosphate and 2.68 g of magnesium hydroxide were weighed and placed in a mixer for mixing for 2 hours to obtain a mixed material.

[0081] 2) The mixed material is placed in a box-type high-temperature furnace, heated to 700° C. at a heating rate of 3° C. / min in an air atmosphere, and kept at this temperature for 8 hours for sintering to obtain a precursor material.

[0082] 3) The precursor material is subjected to secondary sintering in an air atmosphere, the temperature is increased to 300° C. at a heating rate of 3° C. / min, and the temperature is kept for 4 hours to obtain a white block material.

[0083] 4) The white block material is placed in a roller mill for primary crushing to obtain white granular material, and then the white granular material is crushed by a jet mill and sieved to obtain a composite positive electrode material with a particle size Dv50 of 5 μm, and the chemical formula is Ti 0.995 Mg 0.01 P 2 O 7 / Mg 0.5 Ti 2 (PO 4 ) 3 .

[0084] The composite positive electrode material prepared in this example was used to prepare button cells and conduct tests. The assembly method and test method were the same as those in Example 1. The test data are detailed in Table 1.

[0085] In order to better illustrate the effect of the embodiment of the present invention, a comparative example is compared with the above embodiment.

[0086] Comparative Example 1

[0087] A button cell was prepared and tested using a pyrophosphate carbon material not doped with Mg as the positive electrode material. The preparation method and the test method were the same as those in Example 1. The test data are detailed in Table 1.

[0088] The discharge cycle curve of the button half-cell prepared in this comparative example is as follows: Figure 3 shown.

[0089] Comparative Example 2

[0090] Magnesium titanium phosphate material was used as the positive electrode material to prepare button cells and conduct tests. The preparation method and test method were the same as those in Example 1. The test data are detailed in Table 1.

[0091] The discharge cycle curve of the button half-cell prepared in this comparative example is as follows: Figure 3 As shown, through Figure 3 It can be seen that the discharge specific capacity of the button-type half-cell prepared by the composite positive electrode material of Example 1 of the present invention is much greater than the discharge specific capacity of Comparative Example 1 and Comparative Example 2.

[0092] Table 1 shows the initial capacity and capacity values ​​after 100 cycles of the button-type batteries prepared in Examples 1-3 and Comparative Example 1.

[0093]

[0094]

[0095] Table 1

[0096] According to the test results in Table 1, the initial capacity and cycle capacity of the button batteries prepared by the composite positive electrode materials in Examples 1-3 are better than those in the button batteries in Comparative Examples 1-2, indicating that the battery prepared by the composite positive electrode material of magnesium-doped titanium pyrophosphate and magnesium titanium phosphate provided in the embodiments of the present invention has better electrochemical performance. This is because the titanium pyrophosphate material doped with magnesium element improves the initial capacity and cycle capacity of the material, and improves the electrochemical performance of the titanium pyrophosphate material; at the same time, in the preparation process of the composite positive electrode material, the present invention adds an excess of magnesium-containing material, and generates a magnesium titanium phosphate material through secondary sintering. The NASICON structure of the magnesium titanium phosphate material has a sufficiently large interlayer spacing so that magnesium ions can be reversibly embedded and released in the magnesium titanium phosphate, and even in the process of a large number of magnesium ions being released, the crystal structure of the magnesium titanium phosphate is still stable, which can improve the charge and discharge performance of the battery; after the magnesium-doped titanium pyrophosphate and magnesium titanium phosphate materials are composited, the synergistic effect of the two can be exerted to improve the initial capacity and cycle capacity of the battery, and to improve the fast charging performance and discharge voltage of the battery.

[0097] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A composite positive electrode material, It is characterized in that The composite positive electrode material comprises: Mg-doped titanium pyrophosphate and magnesium titanium phosphate; the chemical formula of the composite positive electrode material is Ti 1-0.5x Mg x P 2 O 7 / Mg 0.5 Ti 2 (PO 4 ) 3 , where 0.01≤x≤0.015; The mass ratio of the Mg-doped titanium pyrophosphate and magnesium titanium phosphate is [1-3]: [7-9]; The composite positive electrode material is prepared by a solid phase secondary sintering method from a material containing titanium, a material containing phosphorus and a material containing magnesium, wherein the material containing magnesium is in excess of 2%. The mass percentage of the magnesium element in the composite positive electrode material is 0.1%-0.5%; The peak intensity ratio of the strongest characteristic diffraction peak to the second highest characteristic diffraction peak in the X-ray diffraction pattern of the composite positive electrode material is 1.95; The titanium-containing material includes: titanium dioxide and / or titanium tetrachloride; The phosphorus-containing material includes one or more of phosphoric acid, sodium phosphate, diammonium phosphate, and diammonium hydrogen phosphate; The material containing magnesium element includes: one or more of magnesium oxide, magnesium carbonate, magnesium chloride and magnesium hydroxide.

2. The composite cathode material according to claim 1, It is characterized in that The mass percentage of the titanium element in the composite positive electrode material is 20%-30%; The mass percentage of the phosphorus element in the composite positive electrode material is 15%-30%; The mass percentage of the oxygen element in the composite positive electrode material is 40%-55%.

3. The composite cathode material according to claim 1, It is characterized in that The particle size Dv50 of the composite positive electrode material is between 2 μm and 5 μm.

4. A method for preparing the composite positive electrode material according to any one of claims 1 to 3, It is characterized in that The preparation method is a solid phase secondary sintering method, comprising: Weigh a titanium-containing material, a phosphorus-containing material, and a magnesium-containing material according to a stoichiometric ratio, and place the materials in a mixer for mixing for 30 minutes to 2 hours to obtain a mixed material; Placing the mixed material in a box-type high-temperature furnace and sintering it in an air atmosphere to obtain a precursor material; The precursor material is subjected to secondary sintering in an air atmosphere to obtain a white block material; The white block material is crushed and then sieved to obtain a composite positive electrode material; The chemical general formula of the composite cathode material is Ti 1-0.5x Mg x P 2 O 7 / Mg 0.5 Ti 2 (PO 4 ) 3 , where 0.01 ≤ x ≤ 0.015; the mass ratio of the Mg-doped titanium pyrophosphate and magnesium titanium phosphate is [1-3]:[7-9]; the mass percentage of the magnesium element in the composite cathode material is 0.1%-0.5%; The material containing magnesium element is in excess of 2%.

5. The method for preparing the composite positive electrode material according to claim 4, It is characterized in that The titanium-containing material includes: titanium dioxide and / or titanium tetrachloride; The phosphorus-containing material includes one or more of phosphoric acid, sodium phosphate, diammonium phosphate, and diammonium hydrogen phosphate; The material containing magnesium element includes: one or more of magnesium oxide, magnesium carbonate, magnesium chloride and magnesium hydroxide.

6. The method for preparing the composite positive electrode material according to claim 4, It is characterized in that The sintering conditions include: a sintering temperature of 600°C-800°C, a heating rate of 2°C / min-5°C / min, and a holding time of 5 hours-10 hours; The secondary sintering conditions include: the secondary sintering temperature is 300°C-500°C, the heating rate is 2°C / min-5°C / min, and the holding time is 2 hours-5 hours; The crushing process is specifically as follows: the white block material is primarily crushed by a jaw crusher or a roller crusher to obtain white granular material; and the white granular material is then pulverized by a jet mill.

7. The method for preparing the composite positive electrode material according to claim 4, It is characterized in that The mass percentage of the titanium element in the composite positive electrode material is 20%-30%; The mass percentage of the phosphorus element in the composite positive electrode material is 15%-30%; The mass percentage of the oxygen element in the composite positive electrode material is 40%-55%; The particle size Dv50 of the composite positive electrode material is between 2 μm and 5 μm.

8. A positive electrode sheet, It is characterized in that The positive electrode sheet comprises the composite positive electrode material described in any one of claims 1 to 3.

9. A magnesium secondary battery, It is characterized in that The magnesium secondary battery comprises the positive electrode sheet according to claim 8.

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