A positive electrode material for a sodium ion battery, a preparation method thereof, a secondary battery, and an electrical device

Abstract

The present application provides a positive electrode material and a preparation method thereof, a secondary battery and an electrical device. The positive electrode material includes a sodium composite oxide, which contains sodium, iron, and at least one transition metal; the iron content of the surface area of ​​the positive electrode material is less than that of the internal area; the surface area encloses the internal area, and the radius of the internal area is 1 / 4 to 3 / 4 of the radius of the positive electrode material. Since the iron content of the surface area of ​​the positive electrode material is set to be less than that of the internal area, the present application can ensure that the positive electrode material is rich in iron while avoiding excessive iron content in the surface area of ​​the positive electrode material, thereby preventing excessive Fe 4+ ions are reduced to Fe in the charged state 3+ Therefore, while ensuring the conductivity and stability of the positive electrode material, it can prevent the iron in the positive electrode material from being converted into inactive ions, thereby avoiding affecting the battery cycle stability.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a positive electrode material for a sodium ion battery and a preparation method thereof, a secondary battery and an electrical device. Background Art

[0002] Secondary batteries, particularly sodium-ion secondary batteries, not only offer the advantages of abundant sodium resources, widespread distribution, low cost, no development bottlenecks, environmental friendliness, and compatibility with existing lithium-ion battery production equipment, but also boast superior power characteristics, wide temperature adaptability, safety, and freedom from overdischarge. Furthermore, sodium-ion batteries share a similar structure to lithium-ion batteries, allowing for large-scale production that leverages the same production and testing equipment, process technologies, and manufacturing methods as lithium-ion batteries.

[0003] However, the content of Fe in the positive electrode material of sodium ion battery is evenly distributed inside and on the surface of the positive electrode material particles. 3+ Oxidized to Fe by external electrode 4+ In the discharge state, sodium ions enter the positive electrode material from the electrolyte, and the Fe 4+ Needs to be reduced to Fe by external electrodes 3+ Due to Fe 4+ The activity is extremely high, and the Fe 4+ Easily reacts with the electrolyte and is reduced to Fe 3+ , the reduced Fe 3+ The loss of electrochemical activity leads to a loss of discharge capacity, which limits the cycle life of the battery. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the defect in the prior art that the iron in the positive electrode material is easily converted into inactive ions during the charge and discharge process, resulting in a short charge and discharge cycle life of the battery, thereby providing a positive electrode material and its preparation method, a secondary battery and an electrical device.

[0005] In view of this, the present application first provides a positive electrode material, which includes a sodium composite oxide, which contains sodium, iron, and at least one transition metal; the iron content in the surface area of ​​the positive electrode material is less than that in the internal area; wherein the surface area encloses the internal area, and the radius of the internal area is 1 / 4 to 3 / 4 of the radius of the positive electrode material.

[0006] Furthermore, the ratio of the iron content in the inner region to the iron content in the surface region is A, and A satisfies 1.2≤A≤3.0.

[0007] Furthermore, the positive electrode material includes primary particles and secondary particles formed by aggregation of the primary particles, wherein the Dv50 of the secondary particles is 8 to 20 μm, and the particle size of the primary particles is 0.5 to 2 μm; based on the weight of the positive electrode material, the mass content of the primary particles is 5% to 25%, and the mass content of the secondary particles is 75% to 95%.

[0008] Furthermore, the chemical formula of the positive electrode material includes Na x Fe a M 1-a O2, 0.8≤X≤1.2, 0.5≤a<1, M is the transition metal, including one or more of Ni, Mn, Li, Cu, Zn, Co and Ti.

[0009] Furthermore, the iron content gradually increases from the surface of the positive electrode material to the center of the positive electrode material, and the center refers to the geometric center or center of gravity of the positive electrode material.

[0010] Furthermore, the positive electrode material has a layered crystal structure.

[0011] Furthermore, the space group of the crystal structure of the positive electrode material is R-3m space group and / or P63 / mmc space group.

[0012] The present application also provides a method for preparing a positive electrode material, the preparation method comprising the following steps:

[0013] The iron source, the transition metal source and the alkaline solution are mixed and reacted to obtain a cathode material precursor;

[0014] The positive electrode material precursor is mixed with sodium salt and calcined at high temperature to prepare the positive electrode material.

[0015] The present application also provides a secondary battery, comprising a positive electrode plate, wherein the positive electrode plate comprises a current collector and a positive electrode active material layer, and the positive electrode active material layer comprises the above-mentioned positive electrode material.

[0016] The present application also provides an electrical device, comprising the above-mentioned secondary battery, wherein the secondary battery serves as a power supply for the electrical device.

[0017] The positive electrode material and preparation method thereof, the secondary battery and the electric device provided by the present application, because the iron content of the surface area of ​​the positive electrode material is set to be smaller than that of the internal area, while ensuring that the positive electrode material is rich in iron, the iron content of the surface area of ​​the positive electrode material can be avoided to be too high, thereby preventing excessive Fe 4+ Ions generate Fe in the charged state 3+ ions, thus ensuring the conductivity and stability of the positive electrode material while preventing the iron in the positive electrode material from being converted into inactive ions and affecting the battery cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0019] Figure 1 Schematic diagram of the structure of the positive electrode material of a sodium ion battery; wherein 1 represents the internal area of ​​the positive electrode material, 2 represents the surface area of ​​the positive electrode material, and 3 represents the surface of the positive electrode material;

[0020] Figure 2 Schematic diagram of the distribution of iron content in the positive electrode material from the center to the surface;

[0021] Figure 3 Schematic diagram of the relationship between battery capacity retention and cycle number at a charge and discharge rate of 1C. DETAILED DESCRIPTION

[0022] To make the purpose, technical solutions, and advantages of this application more clearly understood, the present application is further described in detail below with reference to the accompanying drawings and embodiments. Obviously, the embodiments described are only part of the embodiments of this application, not all of them.

[0023] Combine Figure 1 In one embodiment, the present application provides a positive electrode material, which includes a sodium composite oxide, wherein the sodium composite oxide contains sodium, iron, and at least one transition metal; the iron content in the surface area of ​​the positive electrode material is less than that in the internal area; wherein the surface area encloses the internal area, and the radius of the internal area is 1 / 4 to 3 / 4 of the radius of the positive electrode material.

[0024] In this embodiment, the surface area enclosing the internal area means that the surface area completely covers the internal area. Introducing iron and transition metal elements into the positive electrode material can improve the conductivity of the positive electrode material to a certain extent, and can also make the positive electrode material more stable under high and low temperature and cycle conditions. However, when the battery is in the charging state, the Fe 3+ ions are oxidized to Fe 4+ ions, due to Fe 4+ The ion activity is very high, resulting in Fe 4+ The ions react easily with the electrolyte and are reduced to Fe 3+ ions, reduced Fe 3+The ions lose their electrochemical activity, affecting the stability of the positive electrode material and the cycle life of the battery. However, the positive electrode material provided in this embodiment has an iron content in the surface area of ​​the positive electrode material that is smaller than that in the internal area. While ensuring that the positive electrode material is rich in iron, it can also avoid excessive iron content in the surface area of ​​the positive electrode material, thus preventing excessive Fe 4+ Ions are generated during charging, thus ensuring the conductivity and stability of the positive electrode material while preventing the iron in the positive electrode material from being converted into inactive ions, thereby avoiding affecting the battery cycle stability.

[0025] To further achieve better battery cycle stability, in another embodiment, the ratio of the iron content in the inner region to the iron content in the surface region is A, and A satisfies 1.2≤A≤3.0. The lower the iron content in the surface region of the positive electrode material, the better. If its content is too low, it will cause instability in the surface region of the positive electrode material and also cause low conductivity of the positive electrode material. Controlling the ratio of the iron content in the inner region to the iron content in the surface region within the range of 1.2 to 3.0 can achieve better cycle stability.

[0026] In another embodiment, the positive electrode material includes primary particles and secondary particles formed by aggregation of the primary particles, wherein the Dv50 of the secondary particles is 8 to 20 μm, and the particle size of the primary particles is 0.5 to 2 μm; based on the weight of the positive electrode material, the mass content of the primary particles is 5% to 25%, and the mass content of the secondary particles is 75% to 95%.

[0027] In this embodiment, the positive electrode material is mainly composed of primary particles and secondary particles. The primary particles can fill the gaps between the secondary particles, which can increase the compaction density of the positive electrode material and thus increase the battery's gram capacity. At the same time, due to the filling effect of the primary particles, the rate of sodium ion insertion and extraction can be increased, thereby improving the battery's cycle stability. The use of primary particles and secondary particles within the above-mentioned range of Dv50 can make the positive electrode material have a suitable specific surface area, increase the active sites of sodium ions during the charge and discharge cycle, reduce the impedance of the positive electrode material, and thus improve the battery's rate performance and cycle performance. The first particles and the second particles within the above-mentioned content range have a better effect of filling the gaps and obtain a better compaction density.

[0028] In some embodiments, the primary particles and secondary particles of the present application are both polycrystalline structures, and the space group of the crystal structure is the R-3m space group and / or the P63 / mmc space group. The R-3m space group and / or the P63 / mmc space group can improve the unit cell stability of the polycrystalline structure positive electrode material, reduce the lattice stress of the positive electrode material during the charge and discharge cycle, and also increase the migration rate of sodium ions in the positive electrode material. In another embodiment, the crystal structure of the positive electrode material is arranged in layers, that is, the positive electrode material has a layered crystal structure, and the layered crystal structure is more conducive to the deintercalation of sodium ions.

[0029] In some embodiments, the chemical formula of the positive electrode material includes Na x Fe a M 1-a O2, 0.8≤X≤1.2, 0.5≤a<1, M is the transition metal, including one or more of Ni, Mn, Li, Cu, Zn, Co and Ti. Doping with the above transition metals can further improve the specific capacity or stability of the positive electrode material.

[0030] In some embodiments, the iron content in the positive electrode material gradually increases from the surface of the positive electrode material to the center of the positive electrode material, where the center refers to the geometric center or center of gravity of the positive electrode material. This can reduce the iron content at the interface between the positive electrode material and the electrolyte, further avoiding excessive Fe 4+ The generated Fe 3+ In some embodiments, silver or a silver alloy is provided on part or all of the surface of the positive electrode material. Disposing the silver or silver alloy on the surface of the positive electrode material can improve the conductivity of the positive electrode material and improve the cycle performance of the battery. Moreover, since silver is relatively stable, it can also serve as a protective layer for the positive electrode material, reducing the contact area between the electrolyte and iron, further reducing the formation of inactive iron ions, and improving the stability of the positive electrode material and the cycle stability of the battery.

[0031] The present application also provides a method for preparing the above-mentioned positive electrode material, comprising the following steps:

[0032] S1, mixing an iron source, a transition metal source and an alkaline solution to react to obtain a cathode material precursor;

[0033] S2. Mixing the positive electrode material precursor with sodium salt, and calcining at high temperature to obtain the positive electrode material.

[0034] In step 1 above, the iron source includes ferrous sulfate, ferrous oxalate, or ferrous acetate, the transition metal source is a transition metal sulfate, and the alkaline solution includes a mixed solution of sodium hydroxide and ammonia. In this step, the iron ions and transition metal ions react with hydroxide and complex to form hydroxides. In this embodiment, the molar ratio of iron to transition metal in the iron source and the transition metal source can be adjusted to control the molar percentage of each metal element in the positive electrode material.

[0035] In step S2, the sodium salt may be sodium carbonate, sodium bicarbonate, or sodium citrate. The calcination temperature is 700-1000°C, and the calcination time is 10-15 hours. In some embodiments, the calcination includes a primary calcination and a secondary calcination. The primary calcination temperature is controlled at 400-500°C to pre-calculate the cathode precursor and remove organic impurities. The secondary calcination temperature is controlled at 700-1000°C to convert the cathode material precursor into a cathode material having an iron content concentration difference.

[0036] In another embodiment, the above step S1 includes:

[0037] S11, mixing an iron source and an alkaline solution to obtain a coprecipitation reaction solution;

[0038] S12. Add a transition metal source to the coprecipitation reaction solution to prepare a positive electrode material precursor.

[0039] In this embodiment, the iron in the iron source first reacts with hydroxide to form ferrous hydroxide particles, and then a transition metal hydroxide is formed on the surface of the ferric hydroxide particles to obtain a core-shell structured cathode material precursor.

[0040] The present application discloses a method for preparing a positive electrode material by preparing a positive electrode material precursor by a coprecipitation method and then calcining it at a high temperature. The process is simple and the conditions are easy to control. During the preparation process, positive electrode materials with different Fe concentration gradients can be obtained by controlling the molar ratio of Fe to transition metal and the reaction temperature and time.

[0041] The present application also provides a secondary battery comprising a positive electrode sheet, wherein the positive electrode sheet comprises a current collector and a positive electrode active material layer, wherein the positive electrode active material layer comprises the above-mentioned positive electrode material. In some embodiments, the surface density of the positive electrode active material layer is 3 mg / cm 2 ~30mg / cm 2 , compaction density 1g / cm3~4g / cm3.

[0042] The present application also provides an electrical device, which includes the above-mentioned secondary battery, and the secondary battery serves as a power supply for the electrical device.

[0043] Based on the positive electrode material and its preparation method, secondary battery and electrical equipment provided above, it is possible to ensure that the positive electrode material is rich in iron while avoiding excessive iron content in the surface area of ​​the positive electrode material, thereby preventing excessive Fe 4+ ions are reduced to Fe in the charged state 3+ Therefore, while ensuring the conductivity and stability of the positive electrode material, it can prevent the iron in the positive electrode material from being converted into inactive ions, thereby avoiding affecting the battery cycle stability.

[0044] The present application is further described in detail below with reference to specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in this application.

[0045] Example 1

[0046] 1) Preparation of positive electrode materials:

[0047] An alkaline solution with a volume ratio of 1:1 (the alkaline solution is a mixed solution of NaOH and NH3·H2O) is mixed with a 1 mol / L ferrous sulfate solution, and the mixture is stirred continuously at a temperature of 60-80°C and a pH of 8-10 to obtain a uniform mixture. After reacting for 2 hours, 1 mol / LMnSO4 and 1 mol / LNiSO4 solutions are slowly added to control the molar ratio of iron, manganese and nickel to 1:1:1, and the pH of the reaction solution is controlled at 7-10 by adding alkaline solution. The reaction is continued for 2 hours, and then the reaction product is filtered and the filter residue is transferred to a dryer for drying to obtain a positive electrode material precursor. The positive electrode material precursor is mixed with sodium carbonate, and the mixture is calcined at 900°C. After cooling, it is crushed and sieved to prepare a positive electrode material (chemical formula NaNi) with primary particles (Dv50 is 1.2μm) and secondary particles (Dv50 is 12μm). 0.33 Fe 0.33 Mn 0.33 O2), and the ratio of the iron content in the inner area of ​​the positive electrode material to the iron content in the surface area is 1.8, and the secondary particles account for 80% of the total weight of the positive electrode material.

[0048] 2) Preparation of positive electrode sheet:

[0049] The positive electrode material, conductive carbon black and binder PVDF prepared in step 1) are dispersed in solvent NMP and mixed evenly to obtain a positive electrode slurry (wherein the weight ratio of the positive electrode material, conductive carbon black and binder PVDF is 96:2:2); the positive electrode slurry is evenly coated on the positive electrode current collector aluminum foil, and after drying, cold pressing, slitting and cutting, a positive electrode sheet is obtained. The compaction density of the positive electrode sheet is 3g / cm 3 .

[0050] 3) Preparation of negative electrode sheets: The negative electrode active material graphite, conductive carbon black, thickener CMC and binder SBR are dispersed in a solvent deionized water in a weight ratio of 96:1:1:2 and mixed evenly to obtain a negative electrode slurry; the negative electrode slurry is evenly coated on the negative electrode current collector copper foil; after drying, cold pressing, slitting, and cutting, the negative electrode sheet is obtained.

[0051] 4) Preparation of sodium ion battery: The positive electrode sheet, the separator and the negative electrode sheet are stacked in sequence, and the separator adopts PP / PE / PP composite film. Then, the battery cell is wound into a battery cell and placed in a battery casing. After the top and side sealing and the injection of electrolyte (the preparation method of the electrolyte is: ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) are mixed in a mass ratio of 30:40:30 to obtain a non-aqueous organic solvent. 1 mol / L LiPF6 is dissolved in the above-mentioned non-aqueous organic solvent and mixed evenly to obtain an electrolyte), etc., a sodium ion battery is made.

[0052] Examples 2 to 5 are the same as Example 1, except that by controlling the reaction time of ferrous sulfate and alkaline solution, the reaction time of MnSO4 and NiSO4 and alkaline solution, and the calcination temperature, positive electrode materials with different ratios of iron content in the internal area and the iron content in the surface area are obtained.

[0053] Examples 6 to 10 are the same as Example 1, except that the type and molar ratio of the transition metal salt solution are adjusted to obtain positive electrode materials having different transition metal types (the chemical formulas of the positive electrode materials of each embodiment are shown in Table 1).

[0054] Examples 11 to 12 are the same as Example 1, except that the crushing and screening parameters are adjusted to obtain primary particles with different Dv50.

[0055] Examples 13 to 15 are the same as Example 1, except that the crushing and screening parameters are adjusted to obtain secondary particles with different Dv50.

[0056] Examples 16 to 19 are the same as Example 1, except that the sieving parameters are changed to obtain secondary particles with different mass percentages.

[0057] Comparative Example 1 is the same as Example 1, except that ferrous sulfate, manganese sulfate, nickel sulfate and alkaline solution are added to the reaction simultaneously, and the iron content of the obtained positive electrode material is uniform.

[0058] The relevant parameters of the positive electrode materials obtained in Examples 1 to 19 and Comparative Example 1 are recorded in Table 1. The sodium ion batteries prepared in Examples 1 to 19 and Comparative Example 1 were subjected to performance tests, and the test results are also recorded in Table 1.

[0059] The battery testing method is as follows:

[0060] 1) 100-cycle capacity retention test process:

[0061] Place the battery in a battery tester at room temperature, charge and discharge at a rate of 1C / 1C over a voltage range of 3.9V to 1.5V. Using the discharge capacity of the first cycle as the benchmark, the percentage of the discharge capacity of each subsequent cycle compared to the first cycle is the capacity retention rate for that cycle. The 100-cycle capacity retention rate is the percentage of the discharge capacity at the 100th cycle compared to the discharge capacity at the first cycle.

[0062] 2) Battery cycle life test method:

[0063] The battery is placed on a battery tester at room temperature, with a charge / discharge rate of 1C / 1C and a voltage range of 3.9V to 1.5V. The discharge capacity of the first cycle is used as the benchmark, and the percentage of the discharge capacity of each subsequent cycle compared to the first cycle is the capacity retention rate for that cycle. When the capacity retention rate reaches 80%, the number of cycles the battery has been cycled is the cycle life.

[0064] 3) Battery capacity test method:

[0065] Place the battery on a battery tester at room temperature, with a charge and discharge rate of 1C / 1C and a voltage range of 3.9V to 1.5V. Take the discharge capacity of the first cycle as the benchmark and divide it by the mass of the positive electrode material to obtain the battery's gram capacity.

[0066] Table 1

[0067]

[0068]

[0069]

[0070] According to the data of Examples 1 to 19 and Comparative Example 1, it can be seen that the arrangement of the positive electrode material in the surface area of ​​the positive electrode material is less than that in the inner area, which can significantly improve the battery cycle performance and battery life. The reason is that when the iron content in the surface area of ​​the positive electrode material is appropriately reduced, it is possible to ensure that the positive electrode material is rich in iron while avoiding excessive iron content in the surface area of ​​the positive electrode material, thereby preventing excessive Fe 4+ ions are reduced to Fe in the charged state 3+ Therefore, while ensuring the conductivity and stability of the positive electrode material, it can prevent the iron in the positive electrode material from being converted into inactive ions, thereby avoiding affecting the battery cycle stability.

[0071] The above description is merely a preferred embodiment of the present application and is not intended to limit the present application. Various modifications and variations are possible for those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

Claims

1. A positive electrode material, characterized in that The positive electrode material includes a sodium composite oxide, which contains sodium, iron, and at least one transition metal; the chemical formula of the positive electrode material includes Na x Fe a M 1-a O2, 0.8≤X≤1.2, 0.5≤a<1, M is the transition metal, including one or more of Ni, Mn, Li, Cu, Zn, Co and Ti; The iron content of the surface area of ​​the positive electrode material is less than that of the internal area; wherein, the surface area encloses the internal area, the radius of the internal area is 1 / 4 to 3 / 4 of the radius of the positive electrode material, the iron content gradually increases from the surface of the positive electrode material to the center of the positive electrode material, the center refers to the geometric center or center of gravity of the positive electrode material, and the ratio of the iron content of the internal area to the iron content of the surface area is A, and A satisfies 1.2≤A≤3.

0.

2. The positive electrode material according to claim 1, characterized in that The positive electrode material includes primary particles and secondary particles formed by aggregation of the primary particles, wherein the Dv50 of the secondary particles is 8 to 20 μm and the particle size of the primary particles is 0.5 to 2 μm; based on the weight of the positive electrode material, the mass content of the primary particles is 5% to 25%, and the mass content of the secondary particles is 75% to 95%.

3. The positive electrode material according to claim 1, characterized in that The positive electrode material has a layered crystal structure.

4. The positive electrode material according to claim 3, characterized in that The space group of the crystal structure of the positive electrode material is R-3m space group and / or P63 / mmc space group.

5. A method for preparing the positive electrode material according to any one of claims 1 to 4, characterized in that: The following steps are involved: Mixing an iron source with an alkaline solution to obtain a coprecipitation reaction solution, and adding a transition metal source to the coprecipitation reaction solution to prepare a positive electrode material precursor; The positive electrode material precursor is mixed with sodium salt, and then calcined at high temperature to prepare the positive electrode material, wherein the calcination temperature is 700-1000° C. and the calcination time is 10-15 hours.

6. A secondary battery, characterized in that: The invention comprises a positive electrode plate, wherein the positive electrode plate comprises a current collector and a positive electrode active material layer, and the positive electrode active material layer comprises the positive electrode material according to any one of claims 1 to 4.

7. An electrical device, characterized in that: The secondary battery according to claim 6 is included, and the secondary battery serves as a power supply for the electrical device.

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