A single-crystal positive electrode active material for sodium-ion secondary batteries, and a preparation method and applications thereof

Abstract

The application provides a single-crystal positive electrode active material of a sodium-ion secondary battery and a preparation method and application thereof; the positive electrode active material is a regular single-crystal particle, has a near cuboid structure appearance, the particle regularity is 0.9-1.0, the surface regularity is 0.9-1.0, wherein the particle regularity satisfies: the particle regularity represents the deviation degree between the single-crystal particle and a cube with the width of the single-crystal particle as the side; the surface regularity satisfies: the single-crystal layered oxide sodium electric positive electrode prepared through the synergistic effect of various metal elements and specific process conditions is regularly arranged on the surface of the single-crystal particle, which can reduce the internal inter-particle porosity, improve the compaction density on one hand, and the regularly arranged single-crystal particle can reduce the corrosion of electrolyte and thus improve the cycle stability of the battery on the other hand.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of positive electrode materials, in particular to a sodium-ion secondary battery positive electrode active material and a preparation method and application thereof. BACKGROUND

[0002] As a kind of energy storage equipment, battery can store the electric energy converted from various energy and then use it in various environments. Therefore, battery becomes an indispensable energy storage equipment. Lithium battery is favored by people due to its high energy density and long cycle life. The storage of lithium in the earth's crust is only 0.0065%, and the distribution is uneven, and there are many problems such as high cost and difficult mining. The demand for batteries in the new energy battery industry is increasing year by year, so the price of lithium has been at a high level in the past two years. Sodium and lithium are in the same main group, and their physical and chemical properties are similar. The global abundance of sodium resources is 2.74%, which is much higher than that of lithium resources. Sodium is easy to obtain and has low cost. Sodium-ion battery is expected to become a supplement to lithium battery and alleviate the problem of resource shortage.

[0003] Sodium-ion batteries have similar working principles as lithium-ion batteries, and can also be called "rocking chair batteries". The positive electrode material is an important component of the battery and directly affects the electrochemical performance of the battery. Sodium-ion battery positive electrode materials mainly include layered, polyanion and prussian materials. Layered oxides are one of the current research hotspots of sodium-ion battery positive electrode materials due to their high specific capacity, voltage advantage and simple synthesis process. The structural general formula of the layered oxide positive electrode material of the sodium-ion battery is NaxTMO2, wherein TM represents a transition metal or an alkaline earth metal, and the transition metal can be Fe, Cu, Ni, Mn, Co, etc. In the prior art, the sodium-ion battery layered oxide positive electrode is mainly polycrystalline. The polycrystalline material has a high specific capacity and thus a high energy density. However, the polycrystalline material has problems of agglomeration of nanoparticles to form agglomerates and low compaction density. Therefore, in the charging and discharging process, the grain boundaries between the agglomerated particles are constantly torn, causing the side reaction to intensify and the electrochemical performance to deteriorate. Single-crystal materials are independent primary particles that can inhibit cracking, improve cycle stability and safety, and have a compaction density higher than that of polycrystalline materials. However, sodium-ion battery layered oxides are mainly polycrystalline materials. Small primary particles of the polycrystalline material are beneficial to ion transmission, and thus have a high capacity. However, the polycrystalline material is formed by agglomeration of primary particles to secondary spheres, has poor particle strength and low compaction density. In the long cycle process, internal stress causes the secondary spheres to crack, which accelerates the side reaction, causes the particles to break, and deteriorates the cycle and produces gas, accelerating the attenuation of the battery. Therefore, single-crystal materials as positive electrode materials of sodium-ion batteries play an important role in improving the electrochemical performance of sodium-ion secondary batteries. Chinese Invention Patent CN116230903A provides a sodium-ion positive electrode material, a preparation method thereof and a secondary battery, which discloses a sodium-ion positive electrode material having a core-shell coating structure. The inner core material is prepared by high-temperature calcination of a transition metal compound and a fast ion conductor sodium salt. However, the core-shell structure needs two high-temperature calcinations, has high energy consumption, increases the number of preparation processes of the positive electrode material, increases the material defect rate, and greatly increases the production cost.

[0004] Based on the prior art, the present application provides a single-crystal positive electrode active material for a sodium-ion secondary battery and a preparation method thereof. The positive electrode active material is directly obtained by high-temperature sintering of a transition metal precursor by a high-temperature solid-phase method. SUMMARY

[0005] To solve the above technical problems, the present application provides a single-crystal positive electrode active material for a sodium-ion secondary battery and a preparation method and application thereof. The positive electrode active material has a regular block structure and a high compaction density, thereby reducing the erosion of the electrolyte on the particles, significantly reducing the residual sodium content, and improving the conductivity of the positive electrode material and the cycle stability of the battery.

[0006] To achieve the above object, the present application provides a single crystal positive electrode active material for a sodium-ion secondary battery, a sodium-ion single crystal positive electrode material, the particle regularity is greater than 0.3≤Â≤1.8; the surface regularity is 0.6≤Ô≤0.9;

[0007] Â represents the deviation degree between the morphology of the single crystal particle and the cube with the width of the single crystal as the side; and the particle regularity satisfies:Â= ;

[0008] The surface regularity Ô represents the ratio of the specific surface area per unit mass to the sum of the specific surface area per unit mass of the particles; and the surface regularity Ô satisfies: Ô = S(abcρ) / 2(ab+ac+bc);

[0009] Wherein, a, b, c are the length, width and height of the single crystal particle respectively, and ρ is the true density of the positive electrode active material; S is the specific surface area of the single crystal particle per unit mass.

[0010] The detection method comprises: statistically obtaining the length, width and height of a plurality of (≥100) single crystal particles by SEM, to obtain the average length a, width b and height c of the approximate cuboid single crystal particles, wherein b satisfies 4μm≥b≥2μm and a≥b≥c. The volume of the single crystal particle is V1, the volume of the hexahedron with b as the side is V2, and =V1 / V2 to obtainÂ= Further satisfiesÂ= .

[0011] The morphology of the single crystal particle of the present application is an approximate cuboid structure, the approximate cuboid is the minimum circumscribed cuboid of the single particle, and each face of the cuboid is tangent to the particle. By controlling the regularity of the particle, the particle morphology can be controlled, and the best physical and chemical properties of the material can be achieved. The length, width and height of the cuboid can be regarded as three directions a, b and c of the crystal, wherein c represents the sodium ion diffusion direction. On the basis of satisfying the fast sodium ion diffusion rate and satisfying the compaction and particle strength, the control of makes it have the best performance.

[0012] The single crystal positive electrode material described above is a single crystal particle, each single crystal particle can be approximated as a cuboid, and the length, width and height are a, b and c respectively. Therefore, the surface area of each particle S0=2(ab+ac+bc), and 1 gram (m) of sample contains N particles, so the specific surface area of the single gram sample is S1=S0 N; the volume of a single particle V=abc, m=ρ V, and then N = m / (pabc), and then S1 = 2(ab+ac+bc) / (pabc), and then = S / S1, and then = S(abcp) / 2(ab+ac+bc), wherein S is the specific surface area of the single crystal sample actually tested, p is the true density, m is the sample mass (1 gram). 3 , m is the sample mass (1 gram).

[0013] The more regular the particle surface is, the more conducive to the generation of a dense and uniform CEI film on the surface, the less loss of sodium ions, and the less side reaction between surfaces, and the higher the cycle stability.

[0014] The test method for the content of residual sodium adopts titration, specifically including: taking 5g of single crystal active positive electrode material, adding to 45mL of ethylene glycol solvent, taking the supernatant and using a potential titration method to perform titration test with 0.05mol / L hydrochloric acid as a standard solution.

[0015] Further, the sodium ion positive electrode material has a chemical formula of Na x Ni y Mn z N p M (1-y-z-p) O, 1.02 >= x >= 0.6, 0.8 >= y >= 0, 0.8 >= z >= 0, 0.5 >= p >= 0, 0.3 >= 1-y-z-p >= 0, N is one or both of Fe and Cu, and M is one or more of Zr, Zn, Ca, Mg, Ti, Sr, Al, Sn, Co, Li, Nb, and W.

[0016] Further, the single crystal positive electrode material 2T provided in the above technical solution has a compaction density of >= 2.85 g / cm 3 , and more preferably a compaction density of 2.9-3.1 g / cm 3 ; and a total residual sodium content of <= 0.2wt%.

[0017] As another object, the application further provides a preparation method of the single crystal positive electrode active material, specifically including:

[0018] S1. providing the transition metal precursor;

[0019] S2. at least high-temperature sintering the transition metal precursor with sodium;

[0020] S3. after the high-temperature sintering is completed, crushing and sieving to obtain the single crystal positive electrode active material.

[0021] As one of the preferred embodiments, the transition metal precursor is prepared by a co-precipitation method, which specifically comprises: mixing soluble transition metal salts in a certain proportion, and then reacting under ammonia complexation and using sodium hydroxide as a precipitant at 50-60°C under continuous stirring to obtain a spherical transition metal precursor; the particle size D50 of the transition metal precursor is 2-7 microns.

[0022] The soluble transition metal salt comprises a nickel salt, a manganese salt, an iron salt, or a nickel salt, a manganese salt, an iron salt and a copper salt; preferably, the soluble transition metal salt is a sulfate, chlorate or nitrate of a transition metal.

[0023] The transition metal precursor prepared by the co-precipitation method comprises a carbonate or hydroxide of a transition metal.

[0024] As one of the preferred embodiments, the transition metal precursor can further comprise a transition metal oxide prepared by a spray pyrolysis method.

[0025] The transition metal precursor and sodium, or the transition metal precursor, sodium and a doping metal compound are uniformly mixed and sintered at 850-1000°C for 4-12h.

[0026] The sodium is one or more of sodium carbonate, sodium hydroxide, sodium bicarbonate and sodium phosphate.

[0027] The doping metal compound comprises one or more of nano-oxides, carbonates or phosphates of Zr, Zn, Ca, Mg, Ti, Sr, Al, Sn, Co, Li, Nb and W, but is not limited thereto.

[0028] As one of the preferred embodiments, when the transition metal precursor contains Cu, or the doping metal compound contains one or more of Nb, Sr and Zn, the sintering temperature is 850-950°C.

[0029] As one of the preferred embodiments, when the transition metal precursor does not contain Cu, or the doping element does not contain one or more of Nb, Sr and Zn, but contains one or more of Zr, Ca, Mg, Ti, Al, Sn, Co, Li and W, the sintering temperature is 900-1000°C.

[0030] Preferably, in S3, the crushed material is subjected to secondary sintering at a sintering temperature of 800-1000°C, preferably 850-950°C, and a sintering time of 8-20h, preferably 10-16h.

[0031] As a preferred embodiment, the primary sintering and the secondary sintering are both carried out in a sintering atmosphere, which is one of air, oxygen, nitrogen or argon.

[0032] The present application has the following beneficial technical effects:

[0033] 1. The technical scheme of the present application provides a sodium battery cathode material with a single-crystal layered oxide structure, which has a regular structure, a particle regularity of 0.3≤Â≤1.8, and a surface regularity of 0.6≤Ô≤0.9. The sodium battery cathode material can avoid the problem of crystal face cracking during the charging and discharging process, and the more regular the particle surface is, the more conducive it is to form a dense and uniform CEI film, reduce the loss of sodium ions, and reduce the side reactions between interfaces, thereby improving the cycle stability and coulombic capacity of the battery.

[0034] 2. The technical scheme of the present application obtains a single-crystal sodium battery layered oxide based on transition metals by a high-temperature solid-phase method, which is used as a sodium battery cathode material. The sodium battery cathode material not only has a regular morphology structure, but also has a low residual sodium content on the surface, which can significantly improve the cycle stability of the battery.

[0035] 3. The technical scheme of the present application combines the structure of single crystals with the preparation process, avoids the influence of sintering temperature on different metal elements by using a specific sintering temperature, and obtains a single-crystal particle structure that is closer to a cuboid structure, thereby improving the product performance of the cathode material. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 The scanning electron microscope image of the positive electrode active material prepared in Example 1 of the present application.

[0037] Figure 2 The first cycle charge-discharge curve of the positive electrode active material prepared in Example 1 of the present application.

[0038] Figure 3 The scanning electron microscope image of the positive electrode active material prepared in Comparative Example 1 of the present application.

[0039] Figure 4 The scanning electron microscope image of the positive electrode active material prepared in Comparative Example 4 of the present application.

[0040] Figure 5 The scanning electron microscope image of the positive electrode active material prepared in Example 2 of the present application.

[0041] Figure 6 The scanning electron microscope image of the positive electrode active material prepared in Example 3 of the present application. DETAILED DESCRIPTION

[0042] In view of the defects of the prior art, the present application provides a sodium ion secondary battery positive electrode active material, and a preparation method thereof is also provided. The specific technical solutions of the present application are described in detail below through specific embodiments. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. In order to describe concisely and briefly, all the features of the actual embodiments cannot be described in detail in the specification. Based on the embodiments in the present application, all the other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0043] It should be noted that, unless otherwise defined, the technical terms or scientific terms used in the specification should be known to those skilled in the art.

[0044] The present application provides a single crystal positive electrode material with certain particle regularity and surface regularity. The material has the characteristics of high capacity and high cycle stability.

[0045] Specifically, the positive electrode active material is a regular single crystal particle with a near cuboid structure, and the particle regularity is 0.3≤Â≤1.8 and the surface regularity is 0.6≤Ô≤0.9. Wherein,Ârepresents the degree of deviation between the single crystal particle and a cube with the width of the single crystal particle as the side; and the particle regularity satisfies:Â= ; and the surface regularity Ô satisfies:Ô =S(abcρ) / 2(ab+ac+bc).

[0046] More specifically, the single crystal positive electrode active material is a single crystal layered oxide with a chemical formula of Na x Ni y Mn z N p M (1-y-z-p) O, wherein 1.02≥x≥0.6, 0.8≥y≥0, 0.8≥z≥0, 0.5≥p≥0, 0.3≥1-y-z-p≥0, and the stoichiometric ratio of each element satisfies the principle of electrical neutrality; N is a transition metal Fe, or Fe and Cu; M is a doping metal, including one or more of Zr, Zn, Ca, Mg, Ti, Sr, Al, Sn, Co, Li, Nb or W.

[0047] The technical solutions of the present application will be further described in detail below in combination with several preferred embodiments and the accompanying drawings. The embodiments are implemented on the premise of the technical solutions of the present application, and detailed implementation manners and specific operation processes are given, but the protection scope of the present application is not limited to the following embodiments.

[0048] Embodiment 1

[0049] The embodiment provides a preparation method of a single crystal positive electrode material, and the specific steps comprise the following steps:

[0050] A Ni:Fe:Mn=1:1:1 spherical hydroxide precursor is prepared by a coprecipitation method. Nickel sulfate, manganese sulfate and iron sulfate are dissolved in water to form solution A. Ammonia is used as a complexing agent to form solution B. Sodium hydroxide is used as a precipitant to form solution C. A, B and C are continuously added into a reaction kettle through different pipes and continuously stirred until D50 is about 5 microns, and then the reaction is stopped to obtain a Ni 1 / 3 Fe 1 / 3 Mn 1 / 3 (OH)2 precursor.

[0051] 100g of the precursor, 61.5g of sodium carbonate and a three-dimensional mixer are weighed and uniformly mixed. The mixture is heated to 950 DEG C at a rate of 4 DEG C / min in an air atmosphere, and then heat-treated for 8h. The mixture is crushed and sieved. Then, the mixture is heated to 900 DEG C at a rate of 4 DEG C / min, and then heat-treated for 15h in an air atmosphere. The mixture is crushed and sieved to obtain a NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 single crystal positive electrode material.

[0052] The morphology of the single crystal positive electrode material obtained by the above method is observed by a scanning electron microscope, and particle regularity and surface regularity are obtained.

[0053] Particle regularity: indicates the deviation of the morphology of a single crystal particle from a cube with the width of the single crystal particle as the edge. The detection method comprises the following steps: the length, width and height of a plurality of single crystal particles (more than 100) are counted by SEM testing, and the average length a, width b and height c of the approximate cuboid single crystal particles are obtained, wherein b satisfies 4 microns >= b >= 2 microns and a >= b >= c. The volume of the single crystal particle is V1, the volume of the cube with the width as the edge is V2, and the particle regularity is calculated as follows: = V1 / V2. Further, =. .

[0054] The calculation formula of surface regularity is: = S(abc rho) / 2(ab+ac+bc): each single crystal particle can be approximated as a cuboid, and the length, width and height are a, b and c respectively. Therefore, the surface area of each particle is S0=2(ab+ac+bc), and N particles are contained in 1 gram (m) of sample, so the specific surface area of the single crystal sample is S1=S0 N; the volume of a single particle is V=abc, and m= rho V, and then N = m / (pabc), and then S1 = 2(ab+ac+bc) / (pabc), and then = S / S1, and then = S(abcp) / 2(ab+ac+bc), wherein S is the specific surface area of the single crystal sample actually tested, p is the true density, and m is the sample mass (1 gram).

[0055] Referring to Figure 1 The morphology of the single crystal positive electrode material prepared in this embodiment is shown in the figure. As can be seen from the figure, the single crystal particles of the positive electrode material are uniform block structures. According to the scanning result of the SEM, it is calculated that the particle regularity of the single crystal in this embodiment is 0.7, and the surface regularity is 0.64.

[0056] Electrochemical performance test:

[0057] The discharge-charge cycle test under the condition of 2.0-4.0V (at room temperature 25℃) is carried out. The first week of the charge-discharge test is as Figure 2 The test results are shown in Table 1. The 0.2C discharge capacity is 137mAh / g, the reversible specific capacity under 1C rate is 134mAh / g, and the cycle retention rate under 1C for 100 weeks is 94.2%.

[0058] Example 2

[0059] The embodiment provides a preparation method of a single crystal positive electrode material. The specific steps include:

[0060] 100g of the transition metal precursor prepared in Example 1 and 61.5g of sodium carbonate are weighed and uniformly mixed using a three-dimensional mixer. The temperature is raised to 950℃ at a rate of 4℃ / min in an air atmosphere, and then the temperature is kept for 6h. The material is crushed and sieved. Then the material is heated to 910℃ at a rate of 4℃ / min, and then the temperature is kept for 12h in an air atmosphere. The material is crushed and sieved to obtain NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 single crystal positive electrode material.

[0061] Referring to Figure 5 The SEM scanning photograph of the single crystal particles of the single crystal positive electrode material prepared in this embodiment is shown. According to the SEM scanning result, it is calculated that the particle regularity of the single crystal in this embodiment is 0.68, and the surface regularity is 0.8.

[0062] Electrochemical performance test:

[0063] The discharge-charge cycle test under the condition of 2.0-4.0V (at room temperature 25℃) is carried out. The test results are shown in Table 1. The 0.2C discharge capacity is 138.1mAh / g, the reversible specific capacity under 1C rate is 133.6mAh / g, and the cycle retention rate under 1C for 100 weeks is 95.8%.

[0064] Example 3

[0065] The present example provides a preparation method of a single crystal positive electrode material, and the specific steps include:

[0066] 100 g of the transition metal precursor prepared in Example 1, 61.5 g of sodium carbonate, and 2.7 g of ZrO were weighed and uniformly mixed using a three-dimensional mixer. The mixture was then heated to 960°C at a rate of 4°C / min under an air atmosphere, and held for 8 h. The mixture was then uniformly heated to 910°C at a rate of 4°C / min under an oxygen atmosphere, and held for 12 h to obtain NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 single crystal positive electrode material.

[0067] Referring to Figure 6 The SEM scanning image of the single crystal particles of the single crystal positive electrode material prepared in the present example is shown in FIG. 3. According to the SEM scanning results, the particle regularity of the single crystal in the present example was calculated to be 0.56, and the surface regularity was calculated to be 0.79.

[0068] Electrochemical performance test:

[0069] The charge-discharge cycle test was performed at 2.0-4.0 V (at room temperature 25°C). The test results are shown in Table 1. The discharge capacity at 0.2C was 141 mAh / g, the reversible specific capacity at 1C was 135 mAh / g, and the cycle retention rate at 1C for 100 cycles was 97.3%.

[0070] Example 4

[0071] The present example provides a preparation method of a single crystal positive electrode material, and the specific steps include:

[0072] 100 g of the transition metal precursor prepared in Example 1, 61.5 g of sodium carbonate, and 2.7 g of ZrO were weighed and uniformly mixed using a three-dimensional mixer. The mixture was then heated to 960°C at a rate of 4°C / min under an air atmosphere, and held for 8 h. The mixture was then uniformly heated to 910°C at a rate of 4°C / min under an oxygen atmosphere, and held for 12 h to obtain NaNi 0.99 Ni 0.33 Fe 0.33 Mn 0.32 Zr 0.02 O2 single crystal positive electrode material.

[0073] The single crystal particles of the single crystal positive electrode material prepared in the present example were obtained. According to the SEM scanning results, the particle regularity of the single crystal in the present example was calculated to be 0.51, and the surface regularity was calculated to be 0.68.

[0074] Electrochemical performance test:

[0075] The discharge-charge cycle test under the deduction of electricity (at room temperature 25℃) is carried out at 2.0-4.0V, and the test results are shown in Table 1. The 0.2C discharge capacity is 141.6mAh / g, the reversible specific capacity under 1C rate is 136.3mAh / g, and the cycle retention rate under 1C for 100 weeks is 94.3%. Figure 2 and Table 1, the 0.2C discharge capacity is 141.6mAh / g, the reversible specific capacity under 1C rate is 136.3mAh / g, and the cycle retention rate under 1C for 100 weeks is 94.3%.

[0076] Example 5

[0077] The embodiment provides a preparation method of a single crystal type positive electrode material, and specific steps include the following:

[0078] The Ni 0.33 Fe 0.30 Co 0.03 Mn 0.34 (OH)2 precursor is prepared by a coprecipitation method, 100g of the precursor is weighed, 61.5g of sodium carbonate is uniformly mixed by using a three-dimensional mixer, and then the mixture is uniformly heated to 920℃ at 4℃ / min in an air atmosphere, and then the mixture is crushed and sieved after being kept at 920℃ for 8h; then, the mixture is uniformly heated to 920℃ at 4℃ / min in an air atmosphere, and then the mixture is kept at 920℃ for 12h to obtain Na 0.99 Ni 0.33 Fe 0.30 Co 0.03 Mn 0.34 O2 single crystal positive electrode material.

[0079] The single crystal particle of the single crystal type positive electrode material prepared in the embodiment is calculated according to the SEM scanning result, and the particle regularity of the single crystal in the embodiment is 0.81, and the surface regularity is 0.85.

[0080] The discharge-charge cycle test under the deduction of electricity (at room temperature 25℃) is carried out at 2.0-4.0V, and the test results are shown in Table 1. The 0.2C discharge capacity is 141.6mAh / g, the reversible specific capacity under 1C rate is 136.3mAh / g, and the cycle retention rate under 1C for 100 weeks is 94.3%.

[0081] Example 6

[0082] The embodiment provides a preparation method of a single crystal type positive electrode material, and specific steps include the following:

[0083] The Ni 0.28 Zn 0.05 Fe 0.33 Mn 0.34 (OH)2 precursor is prepared by a coprecipitation method, 100g of the precursor is weighed, 61.5g of sodium carbonate is uniformly mixed by using a three-dimensional mixer, and then the mixture is uniformly heated to 920℃ at 4℃ / min in an air atmosphere, and then the mixture is crushed and sieved after being kept at 920℃ for 8h; then, the mixture is uniformly heated to 920℃ at 4℃ / min in an air atmosphere, and then the mixture is kept at 920℃ for 12h to obtain Na 0.99 Ni0.28 Zn 0.05 Fe 0.33 Mn 0.34 O2 single crystal positive electrode material.

[0084] The single crystal positive electrode material single crystal particles prepared in this example have a particle regularity of 0.55 and a surface regularity of 0.68, as calculated from the results of SEM scanning.

[0085] The test results are shown in Table 1. The 0.2C discharge capacity is 136.7 mAh / g, the reversible specific capacity at a 1C rate is 132.2 mAh / g, and the cycle retention rate at 1C for 100 cycles is 92.6%.

[0086] Example 7

[0087] This example provides a preparation method of a single crystal positive electrode material, and the specific steps include:

[0088] The Ni 0.29 Fe 0.33 Mn 0.33 Cu 0.05 (OH)2 precursor is prepared by a coprecipitation method, and the preparation method is shown in Example 1.

[0089] 100 g of the above precursor and 60.3 g of sodium carbonate are weighed, uniformly mixed by using a three-dimensional mixer, and then heated to 900°C at a rate of 4°C / min in an air atmosphere, and held for 6 h. After being crushed and sieved, the mixture is heated to 900°C at a rate of 4°C / min again, and held for 10 h to obtain Na 0.99 Ni 0.29 Fe 0.33 Mn 0.33 Cu 0.05 O2 single crystal positive electrode material.

[0090] The single crystal positive electrode material single crystal particles prepared in this example have a particle regularity of 1.14 and a surface regularity of 0.81, as calculated from the results of SEM scanning.

[0091] The test results are shown in Table 1. The 0.2C discharge capacity is 136.7 mAh / g, the reversible specific capacity at a 1C rate is 132.2 mAh / g, and the cycle retention rate at 1C for 100 cycles is 92.6%.

[0092] Example 8

[0093] This example provides a preparation method of a single crystal positive electrode material, and the specific steps include:

[0094] NaNi 0.32 Fe 0.33 Mn 0.34 Cu 0.02 (OH)2precursor, the preparation method is shown in Example 1.

[0095] 100 g of the above precursor, 62.6 g of sodium carbonate and 2.7 g of ZrO2 were weighed and uniformly mixed by using a three-dimensional mixer, and then heated to 920℃ at a rate of 4℃ / min in an air atmosphere, and kept for 8 h. After crushing and sieving, the mixture was heated to 910℃ at a rate of 4℃ / min, and kept for 12 h.

[0096] NaNi 0.32 Fe 0.33 Mn 0.32 Cu 0.02 Zr 0.02 O2single-crystal positive electrode material.

[0097] According to the SEM scanning results, the particle regularity of the single crystal in this example was 0.78, and the surface regularity was 0.86.

[0098] The charge-discharge cycle test was carried out at 2.0-4.0V (at room temperature 25℃), and the test results are shown in Table 1. The 0.2C discharge capacity was 137.8 mAh / g, the reversible specific capacity at 1C rate was 132.6 mAh / g, and the cycle retention rate at 1C for 100 cycles was 97.5%.

[0099] Comparative Example 1

[0100] NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 (OH)2precursor (the method is the same as that in Example 1).

[0101] 100 g of the above precursor, 61.5 g of sodium carbonate were weighed and uniformly mixed by using a three-dimensional mixer, and then heated to 980℃ at a rate of 4℃ / min in an air atmosphere, and kept for 20 h to obtain NaNi 0.33 Fe 0.33 Mn 0.34 O2single-crystal positive electrode material.

[0102] The SEM image of the single-crystal positive electrode material prepared in this example is shown in Figure 3 According to the SEM scanning results, the particle regularity of the single crystal in this example was 0.17, and the surface regularity was 0.45.

[0103] Electrochemical performance test:

[0104] The discharge capacity at 0.2C is 140 mAh / g, the reversible specific capacity at 1C is 135.5 mAh / g, and the cycle retention rate at 1C for 100 cycles is 83.3%.

[0105] Comparative Example 2

[0106] The Ni 1 / 3 Fe 1 / 3 Mn 1 / 3 (OH)2precursor was prepared by a coprecipitation method (the method is the same as in Example 1).

[0107] 100 g of the above precursor, 60.3 g of sodium carbonate, and 2.7 g of ZrO2 were weighed, uniformly mixed by using a three-dimensional mixer, and then heated to 1050°C at a rate of 4°C / min in an air atmosphere, and held for 16 h to obtain a Na 0.99 Ni 0.33 Fe 0.33 Mn 0.34 O2 single-crystal positive electrode material.

[0108] The single-crystal positive electrode material single-crystal particles prepared in this example.

[0109] Meanwhile, according to the results of SEM scanning, the particle regularity of the single crystals in this example was calculated to be 0.26, and the surface regularity was 0.62.

[0110] Electrochemical performance test:

[0111] The discharge capacity at 0.2C is 138.4 mAh / g, the reversible specific capacity at 1C is 133.8 mAh / g, and the cycle retention rate at 1C for 100 cycles is 87%.

[0112] Comparative Example 3

[0113] The Ni 1 / 3 Fe 1 / 3 Mn 1 / 3 (OH)2precursor was prepared by a coprecipitation method (the method is the same as in Example 1).

[0114] 100 g of the above precursor, 62.6 g of sodium carbonate, and 2.7 g of ZrO2 were weighed, uniformly mixed by using a three-dimensional mixer, and then heated to 1050°C at a rate of 4°C / min in an air atmosphere, and held for 16 h to obtain a Na 0.99 Ni 0.33 Fe 0.33 Mn 0.32 Zr 0.02 O2 single-crystal positive electrode material.

[0115] The single crystal particles of the single crystal positive material prepared in the embodiment have a particle regularity of 0.2 and a surface regularity of 0.58, as calculated from the SEM image.

[0116] Electrochemical performance test:

[0117] The charge-discharge cycle test of the battery under the condition of 2.0-4.0 V (at room temperature 25℃) was performed, and the test results are shown in Table 1. The discharge capacity at 0.2 C is 137.4 mAh / g, the reversible specific capacity at 1 C is 131.8 mAh / g, and the cycle retention rate at 1 C for 100 cycles is 82%.

[0118] Comparative Example 4

[0119] The Ni 0.29 Fe 0.33 Mn 0.33 Cu 0.05 (OH)2precursor was prepared by the coprecipitation method (the method is the same as that in Example 1).

[0120] 100 g of the precursor, 61.5 f of sodium carbonate, and 0.5 f of sodium fluoride were weighed, uniformly mixed by using a three-dimensional mixer, and then heated to 980℃ at a rate of 4℃ / min in an air atmosphere, and kept for 16 h. After being crushed and sieved, NaNi 0.29 Fe 0.33 Mn 0.33 Cu 0.05 O2single crystal positive material was obtained.

[0121] Referring to Figure 4 The SEM scanning image of the single crystal particles of the single crystal positive material prepared in the comparative example is shown in FIG. 6. The particle regularity of the single crystal in the embodiment is 2.13, and the surface regularity is 0.49, as calculated from the SEM image.

[0122] Electrochemical performance test:

[0123] The charge-discharge cycle test of the battery under the condition of 2.0-4.0 V (at room temperature 25℃) was performed, and the test results are shown in Table 1. The discharge capacity at 0.2 C is 137.4 mAh / g, the reversible specific capacity at 1 C is 131.8 mAh / g, and the cycle retention rate at 1 C for 100 cycles is 82%.

[0124] Comparative Example 5

[0125] The Ni 0.30 Fe 0.30 Mn 0.30 Cu 0.05 Mg 0.05 (OH)2precursor was prepared by the coprecipitation method (the method is the same as that in Example 1).

[0126] Take 100 g of the precursor, 61 g of sodium carbonate, mix them uniformly using a three-dimensional mixer, then heat them to 1000°C at a rate of 4°C / min in oxygen, keep the temperature for 16 h, crush and sieve to obtain NaNi 0.30 Fe 0.30 Mn 0.30 Cu 0.05 Mg 0.05 O2 single crystal positive electrode material.

[0127] The single crystal positive electrode material particles prepared in this example have a particle regularity of 0.25 and a surface regularity of 0.54, as shown in the SEM images. Figure 4 The particle regularity of the single crystal in this example is 0.25 and the surface regularity is 0.54, as calculated from the SEM images.

[0128] Electrochemical performance test:

[0129] The charge-discharge cycle test of the button cell was carried out at 2.0-4.0 V (at room temperature 25°C), and the test results are shown in Table 1. The 0.2C discharge capacity is 132.6 mAh / g, the reversible specific capacity at 1C rate is 127.4 mAh / g, and the cycle retention rate at 1C for 100 cycles is 86%.

[0130] Table 1 Performance comparison table of single crystal positive electrode active material of the examples and the comparative examples

[0131]

[0132] Referring to Table 1, the relationship between the electrochemical performance and the regularity of the examples and the comparative examples of the application is shown. The cycle performance and capacity of the sodium ion battery using the positive electrode active material with a particle regularity in the range of 0.3-1.8 (preferably 0.5-1.20) and / or a surface regularity of 0.6-0.9 in the above examples are significantly better than those of the comparative examples.

[0133] In Comparative Example 4, although it has a higher tap density and particle regularity (Â is 2.13) and a lower surface regularity (Ô is 0.49), it is indicated that the single crystal positive electrode material has a single crystal particle with a particle size that is too large, a surface that is more rough, and a shape that is more irregular. Due to the poor contact of the single crystal particle with the conductive agent and the binder due to the particle size that is too large, a good conductive network cannot be formed, the path of electron transmission is prolonged, the conductive performance of the positive electrode material is reduced, and thus the electrochemical performance of the sodium battery is reduced, and the cycle performance and capacity of the battery are both obviously reduced. Therefore, the particle regularity and the surface regularity have a significant limiting effect on the performance of the positive electrode active material. Further, the single crystal particle has a higher particle regularity and surface regularity, the surface of the single crystal particle is more regular and smooth, and the side reaction at the contact interface with the electrolyte can also be reduced. The more regular the particle surface is, the more conducive it is to form a dense and uniform CEI film, reduce the loss of sodium ions, and reduce the side reaction between the surfaces, and improve the cycle stability. The sharp parts of the irregular single crystal particle are easily corroded during use, thereby reducing the cycle stability of the battery.

[0134] In summary, the ordered arrangement of the single crystal particle with the particle regularity  and the surface regularity Ô can reduce the internal inter-particle porosity and thus improve the tap density of the material, and the surface regularity can reduce the corrosion of the electrolyte and improve the cycle stability.

[0135] Further, as can be seen from the results in Table 1, the use of the technical solution of the present application can significantly reduce the residual sodium content on the surface of the positive electrode material. The residual sodium content on the surface of the positive electrode material in the prior art is usually 0.2-0.5wt%. The residual sodium content has an important influence on the capacity and stability of the battery, especially for soft-pack batteries. On the one hand, the residual sodium in the single crystal positive electrode material easily forms sodium carbonate and sodium hydroxide and other alkaline substances, which reduces the conductivity, especially these alkaline substances, on the one hand, affect the contact between the material and the electrolyte, and on the other hand, easily cause cross-linking reaction of the binder glue, affect the homogenate coating process, and further cause poor cycle stability. Therefore, a high residual sodium content will affect the stability of the battery and the first coulomb efficiency of the battery. Therefore, reducing the residual sodium content in the sodium ion battery positive electrode material is an important way to improve the electrochemical performance and stability of the battery in the prior art. However, in the calcination process of the sodium battery positive electrode material, the Na-H exchange continuously occurs during the calcination process, and thus the sodium ion content increases, and thus the residual sodium on the surface of the treated sodium positive electrode material also continues to increase, and thus the problem of poor conductivity and low cycle stability caused by high residual sodium amount in the calcination process cannot be avoided. As can be seen from the results of the examples and comparative examples of the present application, the use of the technical solution of the present application can reduce the residual sodium content on the surface of the positive electrode material, thereby improving the cycle stability and capacity of the battery.

[0136] The above merely describes the preferred embodiments of the present application, which are not intended to limit the protection scope of the present application. The present application can have various changes and modifications for those skilled in the art. Any changes, modifications, replacements, integrations and parameter changes made to the embodiments within the spirit and principle of the present application by conventional substitutions or capable of realizing the same functions without departing from the principles and spirit of the present application all fall within the protection scope of the present application.

Claims

1. A single-crystal positive electrode active material for sodium-ion secondary batteries, characterized in that, The positive electrode active material consists of regular single-crystal particles with a near-cubic-piezoelectric structure. The particle regularity is 0.3 ≤ ∠A ≤ 1.8, and the surface regularity is 0.6 ≤ ∠A ≤ 0.

9. Wherein, Â represents the degree of deviation between the morphology of the single crystal particle and the cube with the width of the single crystal particle as its side; and the particle regularity satisfies: Â= ; The surface regularity Ô represents the ratio of the specific surface area per unit mass to the sum of the specific surface areas of particles per unit mass; and the surface regularity Ô satisfies: Ô = S(abcρ) / 2(ab+ac+bc). a, b, and c represent the length, width, and height of the single crystal particle, respectively; ρ represents the true density of the positive electrode active material; and S represents the specific surface area per unit mass of the single crystal particle.

2. The single-crystal positive electrode active material for sodium-ion secondary batteries according to claim 1, characterized in that, The single-crystal positive electrode active material is a single-crystal layered oxide with the chemical formula Na. x Ni y Mn z N p M (1-y-z-p) O, where 1.02≥x≥0.6, 0.8≥y≥0, 0.8≥z≥0, 0.5≥p≥0, 0.3≥1-yzp≥0, and the stoichiometric ratio of each element satisfies the principle of electroneutrality; N is the transition metal Fe, or Fe and Cu; M is the doped metal, including one or more of Zr, Zn, Ca, Mg, Ti, Sr, Al, Sn, Co, Li, Nb or W.

3. The single-crystal positive electrode active material for sodium-ion secondary batteries according to claim 1, characterized in that, The specific surface area of ​​the positive electrode active material is less than 0.5 m². 2 / g; 2T compacted density 2.85~3.10g / cm³ 3 The residual sodium content is less than 0.2 wt%.

4. The method for preparing a single-crystal positive electrode active material for sodium-ion secondary batteries according to any one of claims 1-3, characterized in that, This includes preparing the precursor by sintering a transition metal precursor and a sodium precursor, or a transition metal precursor, a doped metal precursor, and a sodium precursor using a high-temperature solid-state method. The transition metal precursor includes a mixture of carbonates, hydroxides, or oxides containing nickel-iron-manganese, nickel-manganese-copper, or nickel-iron-manganese-copper. The doped metal precursor includes one or more oxides, sulfides, and nitrides of the doped metal; The sodium source is one or more of sodium carbonate, sodium bicarbonate, and sodium hydroxide; The preparation method includes the following steps: S1. Provide the transition metal precursor; S2. The transition metal precursor is sintered with sodium precursor at least once, and the sintered material is obtained after crushing and sieving. S3. After the sintering material is sintered twice, it is crushed and sieved again to obtain the single crystal positive electrode active material. S2 also includes doped metal compounds; After the transition metal precursor, sodium precursor, and doped metal salt are mixed evenly, the mixture is heated to the first sintering temperature under a sintering atmosphere and held at that temperature before being pulverized to obtain the sintered material. The heating rate for the first sintering is 3~5℃ / min; the sintering time for the first sintering is 4~12h; When the transition metal precursor contains Cu, or the doping element contains one or more elements selected from Nb, Sr, and Zn, the temperature of the first sintering is 850~950℃. When the transition metal precursor does not contain Cu, or the doping element does not contain one or more of Nb, Sr, and Zn, but contains one or more of Zr, Ca, Mg, Ti, Al, Sn, Co, Li, and W, the temperature of the first sintering is 900~1000℃.

5. The method for preparing the single-crystal positive electrode active material according to claim 4, characterized in that, The secondary sintering temperature is 800~1000℃, and the heating rate is 3~5℃ / min; the secondary sintering time is 8~20h. Both the primary sintering and the secondary sintering are carried out in a sintering atmosphere, which includes one or more of air, oxygen, nitrogen, and argon.

6. The method for preparing the single-crystal positive electrode active material according to claim 4, characterized in that, The transition metal precursor includes transition metal carbonates or transition metal hydroxides prepared by co-precipitation, or transition metal oxides prepared by spray pyrolysis. The doped metal compound includes oxides, sulfides, nitrides, carbonates, or phosphates of the doped metal. The sodium source includes one or more of sodium carbonate, sodium bicarbonate, and sodium hydroxide.

7. A sodium-ion secondary battery, wherein the positive electrode comprises a single-crystal positive electrode active material as described in any one of claims 1-3.

Images