Positive electrode material with stable interface, preparation method of positive electrode material and battery

Abstract

The invention discloses a positive electrode material with a stable interface, a preparation method thereof and a battery, and belongs to the technical field of lithium ion batteries. The positive electrode material comprises an inner core and a shell, the inner core is coated with the shell, the inner core is composed of a lithium-containing compound, the shell is composed of niobium difluorophosphate, and the positive electrode material provided by the invention is provided with a coating layer of niobium difluorophosphate. Phosphate radical has a regular tetrahedron structure and has relatively high structural stability, in addition, fluorine element has super-strong electronegativity, and niobium element has good chemical stability and electrochemical activity, so that niobium difluorophosphate has good structural stability, thermodynamic stability and chemical stability, and also has a good lithium ion transmission characteristic. And the excellent thermodynamic and chemical stability can ensure that the positive electrode material is prevented from being corroded by HF at high temperature, the dissolution of transition metal is inhibited, the negative electrode deposition and damage effects caused by the transition metal shuttle effect are reduced, and the high-temperature circulation and storage capacity retention rate of the battery can be effectively improved.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and more specifically, to an interface-stable cathode material, its preparation method, and a battery thereof. Background Technology

[0002] With the rapid development of lithium-ion batteries, high-energy-density batteries are ubiquitous, and the research and development of high-performance cathode materials is attracting increasing attention. Lithium spinel nickel manganese oxide, the most promising next-generation cathode material, is being explored and developed by more and more research institutions and companies due to its high average voltage (4.7V), excellent kinetic performance, low cost, and environmental friendliness.

[0003] Spinel nickel manganese oxide (LiMO) offers advantages in high energy density and low cost. Compared to LiMO where 25% of the manganese is replaced by nickel, its average voltage increases from 3.8V to 4.7V, and its capacity also improves somewhat. Furthermore, since manganese in LiMO exists in a tetravalent state, its Jan Taylor effect is significantly reduced, thereby minimizing transition metal dissolution and effectively improving the material's cycling stability.

[0004] However, like spinel lithium manganese oxide, high-temperature cycling and storage remain core issues that need to be addressed. Compared to lithium manganese oxide, lithium nickel manganese oxide operates at a higher voltage, leading to significant side reactions in the electrolyte and cathode material. This makes it highly susceptible to oxidative decomposition of the electrolyte and dissolution of transition metals, resulting in gas generation and capacity decay in the battery. This also hinders the industrial application of spinel lithium nickel manganese oxide.

[0005] In view of this, we propose an interface-stable cathode material, its preparation method, and a battery. Summary of the Invention

[0006] The purpose of this invention is to provide an interface-stable cathode material, its preparation method, and a battery to solve the problems mentioned in the background art. At high temperatures, the interface-coated NbPO4F2 can effectively suppress hydrofluoric acid corrosion of the electrode material and inhibit the dissolution of transition metals in the cathode. Simultaneously, NbPO4F2 remains stable under high voltage, effectively mitigating side reactions between the cathode material and the electrolyte under high voltage and improving material stability.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] An interface-stable cathode material includes a core and a shell, wherein the core is covered by the shell, the core is composed of a lithium-containing compound, and the shell is composed of niobium difluorophosphate.

[0009] Preferably, the mass ratio of niobium difluorophosphate to the lithium-containing compound is 0.05-5:100. 。

[0010] Preferably, the coating of the outer shell 2 has the chemical formula NbPO4F2;

[0011] Core 1 is a lithium-containing compound, spinel lithium nickel manganese oxide, with the chemical formula LiNi. 0.5-a Mn 1.5-b M c O 4-d , -0.1≤a≤0.1, -0.2≤b≤0.2, 0≤c≤0.1, -0.3≤d≤0.3, where M is selected from oxides containing one or more elements from Na, Sr, Co, Ru, Fe, Sb, Ce, Al, Zr, Y, Ta, Nb, W, Ti, Te, Sb, Rb, S, P, B, F and Si.

[0012] A method for preparing an interface-stable cathode material, comprising the following steps:

[0013] Step 1: Mix the weighed precursor, lithium salt and additives evenly to obtain a primary mixture powder;

[0014] Step 2: The mixture powder is sintered at high temperature in air atmosphere, followed by crushing and sieving to obtain the sintered material.

[0015] Step 3: Mix the primary sintering material with NbPO4F2 in a certain proportion to obtain a secondary mixture powder;

[0016] Step 4: Sinter the secondary mixture powder in an air atmosphere, then crush and sieve it to obtain the secondary sintered material, which is the cathode material.

[0017] Preferably, the additive is a compound of one or more elements selected from Na, Sr, Co, Ru, Fe, Sb, Ce, Al, Zr, Y, Ta, Nb, W, Ti, Te, Sb, Rb, S, P, B, F, and Si.

[0018] Preferably, in step two, the sintering temperature is 800℃~1000℃, the time is 6h~10h, and the D50 of the material sintered in one step is 2um~8um;

[0019] In step four, the heating and cooling rates of sintering are 0.33℃ / min to 5℃ / min, and the mixture is sintered at 400℃ to 800℃ for 4h to 8h. The D50 of the secondary sintered material is 2um to 8um.

[0020] Preferably, a lithium-ion secondary battery includes a separator, an electrolyte, a negative electrode, and a positive electrode, wherein the positive electrode material used to prepare the positive electrode is such as the interface-stable positive electrode material described above.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] (1) The interface-stable spinel lithium nickel manganese oxide cathode material provided by this invention has a niobium difluorophosphate coating layer. Due to the tetrahedral structure of phosphate, it has high structural stability. In addition, due to the strong electronegativity of fluorine, niobium has good chemical stability and electrochemical activity, which makes niobium difluorophosphate have good structural stability, thermodynamic and chemical stability, and good lithium-ion transport characteristics. Excellent thermodynamic and chemical stability can protect the cathode material from HF corrosion at high temperatures, inhibit the dissolution of transition metals, and reduce the negative electrode deposition and damage caused by the transition metal shuttle effect, which can effectively improve the high-temperature cycling and storage capacity retention of the battery. On the other hand, good structural stability can ensure that the material structure is not damaged under high voltage, and the fast lithium-ion transport characteristics can effectively improve the capacity and rate performance of the cathode material. This interface-stable modified lithium nickel manganese oxide cathode has extremely high capacity and rate performance, while taking into account the stability of high-temperature cycling and storage.

[0023] (2) The preparation method of the cathode material with good interfacial stability in this application is simple and convenient, and easy to mass-produce. At the same time, the cathode material coated with niobium difluorophosphate has the characteristics of high capacity, high rate and long cycle life, which provides a good application direction for the application of spinel lithium nickel manganese oxide cathode. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the cathode material structure of the present invention.

[0025] The labels in the diagram are as follows: 1. Kernel; 2. Shell. Detailed Implementation

[0026] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0027] Example:

[0028] Please see Figure 1 A cathode material with a stable interface includes a core 1 and a shell 2. The core 1 is covered by the shell 2. The core 1 is composed of a lithium-containing compound, and the shell 2 is composed of niobium difluorophosphate.

[0029] The mass ratio of niobium difluorophosphate to lithium-containing compounds is 0.05-5:100. 。

[0030] In this application, the chemical formula of the coating material of the outer shell 2 is NbPO4F2;

[0031] Core 1 is a lithium-containing compound, spinel lithium nickel manganese oxide, with the chemical formula LiNi. 0.5-a Mn 1.5-b M c O 4-d , -0.1≤a≤0.1, -0.2≤b≤0.2, 0≤c≤0.1, -0.3≤d≤0.3, where M is selected from oxides containing one or more elements from Na, Sr, Co, Ru, Fe, Sb, Ce, Al, Zr, Y, Ta, Nb, W, Ti, Te, Sb, Rb, S, P, B, F and Si.

[0032] In this application, the chemical formula of the cathode material is LiNi. 0.5-a Mn 1.5-b M c Nb x P y F z O 4-d The lithium-containing compound, wherein -0.1≤a≤0.1, -0.2≤b≤0.2, 0≤c≤0.1, -0.3≤d≤0.3, 0≤x≤0.1, 0≤y≤0.1, 0≤z≤0.1, and M is selected from oxides containing one or more elements selected from Na, Sr, Co, Ru, Fe, Sb, Ce, Al, Zr, Y, Ta, Nb, W, Ti, Te, Sb, Rb, S, P, B, F and Si.

[0033] The interface-stable cathode material provided in this application has a niobium difluorophosphate coating. Due to the tetrahedral structure of phosphate, it possesses high structural stability. Furthermore, due to the high electronegativity of fluorine, niobium exhibits good chemical stability and electrochemical activity, resulting in excellent structural, thermodynamic, and chemical stability of niobium difluorophosphate, along with good lithium-ion transport characteristics. Excellent thermodynamic and chemical stability protects the cathode material from HF corrosion at high temperatures, inhibits transition metal dissolution, and reduces the negative electrode deposition and damage caused by the transition metal shuttle effect, effectively improving the battery's high-temperature cycling and storage capacity retention. On the other hand, good structural stability ensures that the material structure remains intact under high voltage, and the rapid lithium-ion transport characteristic effectively improves the cathode material's capacity and rate performance. This interface-stable modified lithium nickel manganese oxide cathode exhibits extremely high capacity and rate performance while maintaining stability during high-temperature cycling and storage.

[0034] To further illustrate this point, the following comparative examples are provided:

[0035] Comparative Example 1

[0036] Weigh 1000g of nickel-manganese hydroxide precursor and 221.54g of lithium carbonate, put them into a small mixer, stir at 1000rpm for 30min to obtain a primary mixture powder; then, place the primary mixture powder in a box furnace and calcine at 940℃ for 10h, and after crushing and passing through a 325-mesh sieve, obtain the primary sintered material.

[0037] Comparative Example 2

[0038] Weigh 1000g of nickel-manganese hydroxide precursor, 221.54g of lithium carbonate and 5g of cobalt oxide, put them into a small mixer and stir at 1000rpm for 30min to obtain a primary mixture powder; then, place the primary mixture powder in a box furnace and calcine at 960℃ for 10h, and after crushing and passing through a 325-mesh sieve, obtain the primary sintered material.

[0039] Comparative Example 3

[0040] Weigh 1000g of nickel-manganese hydroxide precursor, 251.63g of lithium hydroxide monohydrate and 5g of tantalum oxide, put them into a small mixer and stir at 1000rpm for 30min to obtain a primary mixture powder; then, place the primary mixture powder in a box furnace and calcine at 920℃ for 6h, and after crushing and passing through a 325-mesh sieve, obtain the primary sintered material.

[0041] Comparative Example 4

[0042] Weigh 1000g of nickel-manganese hydroxide precursor, 251.63g of lithium hydroxide monohydrate and 5g of tungsten oxide, put them into a small mixer and stir at 1000rpm for 30min to obtain a primary mixture powder; then, place the primary mixture powder in a box furnace and calcine at 920℃ for 6h, and after crushing and passing through a 325-mesh sieve, obtain the primary sintered material.

[0043] Comparative Example 5

[0044] Weigh 1000g of nickel-manganese hydroxide precursor, 251.63g of lithium hydroxide monohydrate and 5g of ammonium dihydrogen phosphate, put them into a small mixer and stir at 1000rpm for 30min to obtain a primary mixture powder; then, place the primary mixture powder in a box furnace and calcine at 950℃ for 8h, and after crushing and passing through a 325-mesh sieve, obtain the primary sintered material.

[0045] Comparative Example 6

[0046] Weigh 1000g of nickel-manganese hydroxide precursor, 221.54g of lithium carbonate and 5g of niobium oxide, put them into a small mixer and stir at 1000rpm for 30min to obtain a primary mixture powder; then, place the primary mixture powder in a box furnace and calcine at 955℃ for 10h, and after crushing and passing through a 325-mesh sieve, obtain the primary sintered material.

[0047] Comparative Example 7

[0048] Weigh 1000g of nickel-manganese hydroxide precursor, 221.54g of lithium carbonate and 5g of alumina, put them into a small mixer and stir at 1000rpm for 30min to obtain a primary mixture powder; then, place the primary mixture powder in a box furnace and calcine at 985℃ for 9h, and after crushing and passing through a 325-mesh sieve, obtain the primary sintered material.

[0049] Comparative Example 8

[0050] Weigh 1000g of nickel-manganese hydroxide precursor, 221.54g of lithium carbonate, 2.5g of cerium fluoride, and 2.5g of ruthenium oxide, and put them into a small mixer. Stir at 1000rpm for 30min to obtain a primary mixture powder. Then, place the primary mixture powder in a box furnace and calcine at 990℃ for 12h. After crushing and passing through a 325-mesh sieve, obtain the primary sintered material.

[0051] Comparative Example 9

[0052] Weigh 1000g of nickel-manganese hydroxide precursor, 251.63g of lithium hydroxide monohydrate, 2.5g of antimony oxide, and 2.5g of ammonium dihydrogen phosphate. Put them into a small mixer and stir at 1000rpm for 30min to obtain a primary mixture powder. Then, place the primary mixture powder in a box furnace and calcine at 980℃ for 6h. After crushing and passing through a 325-mesh sieve, obtain the primary sintered material.

[0053] Comparative Example 10

[0054] Weigh 1000g of nickel-manganese hydroxide precursor, 251.63g of lithium hydroxide monohydrate, 1.67g of tellurium oxide, 1.67g of niobium oxide, and 1.67g of ammonium dihydrogen phosphate. Place them in a small mixer and stir at 1000rpm for 30min to obtain a primary mixture powder. Then, place the primary mixture powder in a box furnace and calcine at 955℃ for 10h. After crushing and passing through a 325-mesh sieve, obtain the primary sintered material.

[0055] In the comparative examples above, Comparative Example 1 contained no additives, while Comparative Examples 2-10 contained one or more of the following elements with good effects: Co, Ta, Al, W, Ti, Nb, Sb, Te, Rb, P, and F. To verify the effects of adding the corresponding compounds, physicochemical and battery tests were conducted on the primary sintered materials of each comparative example. Particle size, specific surface area, and pH value were tested according to industry standard methods. Button cells were subjected to 0.1C specific capacity and 1C 100-cycle high-temperature (45°C) tests, with an active material loading of 95%, areal density controlled at 8-10 mg / cm², and an electrochemical window of 3.5-4.95 V. The test data are shown in Table 1.

[0056] Table 1 shows the physicochemical and battery test data of the primary sintered materials in the comparative examples:

[0057] Comparative Example D50 / um BET / m2 / g pH 0.1C capacity First-efficacy / % 100-time retention rate 1 4.82 0.5323 9.56 131.96 86.92 92.18 2 5.12 0.5189 9.78 132.99 88.90 93.88 3 5.31 0.4799 9.88 132.57 87.82 94.15 4 4.38 0.4073 10.23 131.25 86.78 94.01 5 4.91 0.5113 9.97 133.77 89.75 95.15 6 4.42 0.4561 10.02 133.79 88.59 94.72 7 4.82 0.4924 10.16 133.87 88.20 93.91 8 4.49 0.5144 9.91 132.88 88.26 94.01 9 5.24 0.4981 9.85 133.87 90.26 95.03 10 5.08 0.4792 9.59 132.64 90.46 95.89

[0058] As shown in the table above, the addition of Co, Ta, Al, W, Ti, Nb, Sb, Te, Rb, P, and F improves the first-efficiency performance and high-temperature cycle retention of the material to varying degrees. This is mainly because the doping of metal oxides further widens the lithium-ion channels of high-voltage nickel-manganese lithium oxide, making lithium-ion transport easier; while the doping of non-metallic elements enhances bond energy, making the material structure more stable. Specifically, Nb and W effectively reduce BET and improve stability, while the fast-ion conductors formed by Co, Ta, Al, and P effectively improve lithium-ion transport efficiency, thus increasing the material's capacity and stability.

[0059] A method for preparing an interface-stable cathode material, comprising the following steps:

[0060] Step 1: Add the weighed precursor, lithium salt and additive to a high-speed mixer and mix evenly to obtain a primary mixture powder; wherein, the additive is a compound of one or more elements selected from Na, Sr, Co, Ru, Fe, Sb, Ce, Al, Zr, Y, Ta, Nb, W, Ti, Te, Sb, Rb, S, P, B, F and Si.

[0061] Step 2: The primary mixture powder is sintered at high temperature in air atmosphere, followed by crushing and sieving to obtain the primary sintered material; in Step 2, the sintering temperature is 800℃~1000℃, the time is 6h~10h, and the D50 of the primary sintered material is 2um~8um.

[0062] Step 3: Add the primary sintering material and NbPO4F2 to a high-speed mixer in a certain proportion and mix evenly to obtain a secondary mixture powder;

[0063] Step 4: Sinter the secondary mixture powder in an air atmosphere, followed by crushing and sieving to obtain the secondary sintered material, which is the cathode material. In Step 4, the sintering heating and cooling rate is 0.33℃ / min~5℃ / min, and the mixture is sintered at 400℃~800℃ for 4h~8h. The D50 of the secondary sintered material is 2um~8um.

[0064] The method for preparing the cathode material with improved interfacial stability described in this application is simple and convenient, and easy to mass-produce. Furthermore, the cathode material coated with niobium difluorophosphate exhibits high capacity, high rate capability, and long cycle life, providing a promising application direction for spinel lithium nickel manganese oxide cathodes.

[0065] To further illustrate the preparation method of the interface-stable cathode material of the present invention, the following specific embodiments are provided.

[0066] Example 1

[0067] Weigh 1000g of nickel-manganese hydroxide precursor, 221.54g of lithium carbonate and 5g of cobalt oxide, put them into a small mixer and stir at 1000rpm for 30min to obtain a primary mixture powder; then, place the primary mixture powder in a box furnace and calcine at 960℃ for 10h, and after crushing and passing through a 325-mesh sieve, obtain the primary sintered material.

[0068] 1000g of the primary sintering material was mixed evenly with 0.5g of niobium difluorophosphate in a high-speed mixer to obtain a secondary mixture powder. Under air atmosphere, the secondary mixture powder was placed in a box furnace for sintering at a heating / cooling rate of 2℃ / min, and sintered at 400℃ for 5 hours. Subsequently, it was crushed and sieved to obtain the secondary sintered material, which is the interface-stable spinel nickel-manganese oxide cathode material.

[0069] Example 2

[0070] Weigh 1000g of nickel-manganese hydroxide precursor, 221.54g of lithium carbonate and 5g of cobalt oxide, put them into a small mixer and stir at 1000rpm for 30min to obtain a primary mixture powder; then, place the primary mixture powder in a box furnace and calcine at 960℃ for 10h, and after crushing and passing through a 325-mesh sieve, obtain the primary sintered material.

[0071] 1000g of the primary sintering material was mixed evenly with 2g of niobium difluorophosphate in a high-speed mixer to obtain a secondary mixture powder. Under air atmosphere, the secondary mixture powder was placed in a box furnace for sintering at a heating / cooling rate of 2℃ / min, and sintered at 500℃ for 5 hours. Subsequently, it was crushed and sieved to obtain the secondary sintered material, which is the interface-stable spinel nickel-manganese oxide cathode material.

[0072] Example 3

[0073] Weigh 1000g of nickel-manganese hydroxide precursor, 221.54g of lithium carbonate and 5g of cobalt oxide, put them into a small mixer and stir at 1000rpm for 30min to obtain a primary mixture powder; then, place the primary mixture powder in a box furnace and calcine at 960℃ for 10h, and after crushing and passing through a 325-mesh sieve, obtain the primary sintered material.

[0074] 1000g of the primary sintering material was mixed evenly with 5g of niobium difluorophosphate in a high-speed mixer to obtain a secondary mixture powder. Under air atmosphere, the secondary mixture powder was placed in a box furnace for sintering at a heating / cooling rate of 2℃ / min, and sintered at 600℃ for 8 hours. Subsequently, it was crushed and sieved to obtain the secondary sintered material, which is the interface-stable spinel nickel-manganese oxide cathode material.

[0075] Example 4

[0076] Weigh 1000g of nickel-manganese hydroxide precursor, 221.54g of lithium carbonate and 5g of cobalt oxide, put them into a small mixer and stir at 1000rpm for 30min to obtain a primary mixture powder; then, place the primary mixture powder in a box furnace and calcine at 960℃ for 10h, and after crushing and passing through a 325-mesh sieve, obtain the primary sintered material.

[0077] 1000g of the primary sintering material was mixed evenly with 10g of niobium difluorophosphate in a high-speed mixer to obtain a secondary mixture powder. Under air atmosphere, the secondary mixture powder was placed in a box furnace for sintering at a heating / cooling rate of 1℃ / min, and sintered at 700℃ for 4 hours. Subsequently, it was crushed and sieved to obtain the secondary sintered material, which is the interface-stable spinel nickel-manganese oxide cathode material.

[0078] Example 5

[0079] Weigh 1000g of nickel-manganese hydroxide precursor, 221.54g of lithium carbonate and 5g of cobalt oxide, put them into a small mixer and stir at 1000rpm for 30min to obtain a primary mixture powder; then, place the primary mixture powder in a box furnace and calcine at 960℃ for 10h, and after crushing and passing through a 325-mesh sieve, obtain the primary sintered material.

[0080] 1000g of the primary sintering material was mixed evenly with 20g of niobium difluorophosphate in a high-speed mixer to obtain a secondary mixture powder. Under air atmosphere, the secondary mixture powder was placed in a box furnace for sintering at a heating / cooling rate of 1℃ / min, and sintered at 700℃ for 8 hours. Subsequently, it was crushed and sieved to obtain the secondary sintered material, which is the interface-stable spinel nickel-manganese oxide cathode material.

[0081] Example 6

[0082] Weigh 1000g of nickel-manganese hydroxide precursor, 221.54g of lithium carbonate and 5g of cobalt oxide, put them into a small mixer and stir at 1000rpm for 30min to obtain a primary mixture powder; then, place the primary mixture powder in a box furnace and calcine at 960℃ for 10h, and after crushing and passing through a 325-mesh sieve, obtain the primary sintered material.

[0083] 1000g of the primary sintering material was mixed evenly with 50g of niobium difluorophosphate in a high-speed mixer to obtain a secondary mixture powder. Under air atmosphere, the secondary mixture powder was placed in a box furnace for sintering at a heating / cooling rate of 1℃ / min, and sintered at 800℃ for 6 hours. Subsequently, it was crushed and sieved to obtain the secondary sintered material, which is the interface-stable spinel nickel-manganese oxide cathode material.

[0084] Example 7

[0085] Weigh 1000g of nickel-manganese hydroxide precursor, 251.63g of lithium hydroxide monohydrate, 1.67g of tellurium oxide, 1.67g of niobium oxide, and 1.67g of ammonium dihydrogen phosphate. Place them in a small mixer and stir at 1000rpm for 30min to obtain a primary mixture powder. Then, place the primary mixture powder in a box furnace and calcine at 955℃ for 10h. After crushing and passing through a 325-mesh sieve, obtain the primary sintered material.

[0086] 1000g of the primary sintering material was mixed evenly with 0.5g of niobium difluorophosphate in a high-speed mixer to obtain a secondary mixture powder. Under air atmosphere, the secondary mixture powder was placed in a box furnace for sintering at a heating / cooling rate of 2℃ / min, and sintered at 450℃ for 4 hours. Subsequently, it was crushed and sieved to obtain the secondary sintered material, which is the interface-stable spinel nickel-manganese oxide cathode material.

[0087] Example 8

[0088] Weigh 1000g of nickel-manganese hydroxide precursor, 251.63g of lithium hydroxide monohydrate, 1.67g of tellurium oxide, 1.67g of niobium oxide, and 1.67g of ammonium dihydrogen phosphate. Place them in a small mixer and stir at 1000rpm for 30min to obtain a primary mixture powder. Then, place the primary mixture powder in a box furnace and calcine at 955℃ for 10h. After crushing and passing through a 325-mesh sieve, obtain the primary sintered material.

[0089] 1000g of the primary sintering material was mixed evenly with 2g of niobium difluorophosphate in a high-speed mixer to obtain a secondary mixture powder. Under air atmosphere, the secondary mixture powder was placed in a box furnace for sintering at a heating / cooling rate of 2℃ / min, and sintered at 500℃ for 5 hours. Subsequently, it was crushed and sieved to obtain the secondary sintered material, which is the interface-stable spinel nickel-manganese oxide cathode material.

[0090] Example 9

[0091] Weigh 1000g of nickel-manganese hydroxide precursor, 251.63g of lithium hydroxide monohydrate, 1.67g of tellurium oxide, 1.67g of niobium oxide, and 1.67g of ammonium dihydrogen phosphate. Place them in a small mixer and stir at 1000rpm for 30min to obtain a primary mixture powder. Then, place the primary mixture powder in a box furnace and calcine at 955℃ for 10h. After crushing and passing through a 325-mesh sieve, obtain the primary sintered material.

[0092] 1000g of the primary sintering material was mixed evenly with 5g of niobium difluorophosphate in a high-speed mixer to obtain a secondary mixture powder. Under air atmosphere, the secondary mixture powder was placed in a box furnace for sintering at a heating / cooling rate of 2℃ / min, and sintered at 650℃ for 8 hours. Subsequently, it was crushed and sieved to obtain the secondary sintered material, which is the interface-stable spinel nickel-manganese oxide cathode material.

[0093] Example 10

[0094] Weigh 1000g of nickel-manganese hydroxide precursor, 251.63g of lithium hydroxide monohydrate, 1.67g of tellurium oxide, 1.67g of niobium oxide, and 1.67g of ammonium dihydrogen phosphate. Place them in a small mixer and stir at 1000rpm for 30min to obtain a primary mixture powder. Then, place the primary mixture powder in a box furnace and calcine at 955℃ for 10h. After crushing and passing through a 325-mesh sieve, obtain the primary sintered material.

[0095] 1000g of the primary sintering material was mixed evenly with 10g of niobium difluorophosphate in a high-speed mixer to obtain a secondary mixture powder. Under air atmosphere, the secondary mixture powder was placed in a box furnace for sintering at a heating / cooling rate of 1℃ / min, and sintered at 700℃ for 6 hours. Subsequently, it was crushed and sieved to obtain the secondary sintered material, which is the interface-stable spinel nickel-manganese oxide cathode material.

[0096] Example 11

[0097] Weigh 1000g of nickel-manganese hydroxide precursor, 251.63g of lithium hydroxide monohydrate, 1.67g of tellurium oxide, 1.67g of niobium oxide, and 1.67g of ammonium dihydrogen phosphate. Place them in a small mixer and stir at 1000rpm for 30min to obtain a primary mixture powder. Then, place the primary mixture powder in a box furnace and calcine at 955℃ for 10h. After crushing and passing through a 325-mesh sieve, obtain the primary sintered material.

[0098] 1000g of the primary sintering material was mixed evenly with 20g of niobium difluorophosphate in a high-speed mixer to obtain a secondary mixture powder. Under air atmosphere, the secondary mixture powder was placed in a box furnace for sintering at a heating / cooling rate of 1℃ / min, and sintered at 750℃ for 8 hours. Subsequently, it was crushed and sieved to obtain the secondary sintered material, which is the interface-stable spinel nickel-manganese oxide cathode material.

[0099] Example 12

[0100] Weigh 1000g of nickel-manganese hydroxide precursor, 251.63g of lithium hydroxide monohydrate, 1.67g of tellurium oxide, 1.67g of niobium oxide, and 1.67g of ammonium dihydrogen phosphate. Place them in a small mixer and stir at 1000rpm for 30min to obtain a primary mixture powder. Then, place the primary mixture powder in a box furnace and calcine at 955℃ for 10h. After crushing and passing through a 325-mesh sieve, obtain the primary sintered material.

[0101] 1000g of the primary sintering material was mixed evenly with 50g of niobium difluorophosphate in a high-speed mixer to obtain a secondary mixture powder. Under air atmosphere, the secondary mixture powder was placed in a box furnace for sintering at a heating / cooling rate of 1℃ / min, and sintered at 800℃ for 6 hours. Subsequently, it was crushed and sieved to obtain the secondary sintered material, which is the interface-stable spinel nickel-manganese oxide cathode material.

[0102] The high-voltage lithium nickel manganese oxide cathode material prepared in the above examples was subjected to physicochemical and battery tests. Particle size, specific surface area, and pH value were tested according to industry standard methods. A coin cell was subjected to 0.1C specific capacity and 1C 100-cycle 45℃ high-temperature cycling tests, and the transition metal content was determined after cycling. The active material loading was 95%, the areal density was controlled at 8-10 mg / cm², and the electrochemical window was 3.5-4.95 V. The results are shown in Table 2 below.

[0103] Table 2. Physicochemical and battery test data of the cathode materials prepared in the examples:

[0104] Example D50 / um BET / m2 / g pH 0.1C capacity First-efficacy / % 100-time retention rate 1 5.05 0.5255 9.71 133.18 90.41 93.95 2 5.11 0.5516 9.67 133.54 91.43 94.39 3 5.13 0.5428 9.57 133.99 91.56 94.66 4 5.09 0.5317 9.52 134.72 92.55 96.94 5 5.21 0.5119 9.31 132.18 91.29 95.17 6 5.19 0.5049 9.40 130.94 92.01 93.57 7 5.14 0.5281 9.60 132.99 92.52 95.88 8 5.02 0.5477 9.58 133.82 93.66 96.36 9 4.99 0.5548 9.62 134.82 94.29 97.04 10 5.10 0.5296 9.41 135.28 94.81 98.56 11 5.19 0.5171 9.52 133.19 93.17 96.42 12 5.22 0.5216 9.44 132.16 92.58 95.16

[0105] Table 3 Transition metal content of electrode samples after testing cycles in Comparative Example 10 and Examples 7-12:

[0106]

[0107]

[0108] As shown in Table 2, regardless of whether the cobalt-doped primary sintered sample or the tellurium, niobium, and phosphorus composite-doped primary sintered sample was used as the coating substrate, the capacity, initial efficiency, and high-temperature capacity retention of the cathode material were significantly improved after coating with niobium difluorophosphate. This is because niobium difluorophosphate has good ion conductivity, thus increasing its capacity and initial efficiency by about 2%. Table 3 shows that with the increase in niobium difluorophosphate coating amount, the interface of spinel lithium nickel manganese oxide became significantly more stable, and the dissolution of transition metals gradually decreased. This is because it is insoluble in hydrofluoric acid, effectively inhibiting the corrosive effect of hydrofluoric acid and effectively improving the capacity retention of the material. The decrease in capacity retention in Examples 11-12 was due to excessive coating material, leading to a significant increase in interfacial impedance and consequently, capacity decay.

[0109] Furthermore, the present invention also relates to a lithium-ion secondary battery, comprising a separator, an electrolyte, a negative electrode, and a positive electrode, wherein the positive electrode material for preparing the positive electrode is such as the aforementioned interface-stable positive electrode material.

[0110] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A cathode material with stable interface, characterized in that: It includes a core (1) and a shell (2), the core (1) being covered by the shell (2), the core (1) being composed of a lithium-containing compound, and the shell (2) being composed of niobium difluorophosphate.

2. The interface-stable cathode material according to claim 1, characterized in that: The mass ratio of niobium difluorophosphate to the lithium-containing compound is 0.05-5:

100.

3. The interface-stable cathode material according to claim 1 or 2, characterized in that: The chemical formula of the coating material of the outer shell (2) is NbPO4F2; The core (1) is a lithium-containing compound, spinel lithium nickel manganese oxide, with the chemical formula LiNi. 0.5-a Mn 1.5-b M c O 4-d , -0.1≤a≤0.1, -0.2≤b≤0.2, 0≤c≤0.1, -0.3≤d≤0.3, where M is selected from oxides containing one or more elements from Na, Sr, Co, Ru, Fe, Sb, Ce, Al, Zr, Y, Ta, Nb, W, Ti, Te, Sb, Rb, S, P, B, F and Si.

4. A method for preparing an interface-stable cathode material, used to prepare the interface-stable cathode material according to any one of claims 1-3, characterized in that, Includes the following steps: Step 1: Mix the weighed precursor, lithium salt and additives evenly to obtain a primary mixture powder; Step 2: The mixture powder is sintered at high temperature in air atmosphere, followed by crushing and sieving to obtain the sintered material. Step 3: Mix the primary sintering material with NbPO4F2 in a certain proportion to obtain a secondary mixture powder; Step 4: Sinter the secondary mixture powder in an air atmosphere, then crush and sieve it to obtain the secondary sintered material, which is the cathode material.

5. The method for preparing an interface-stable cathode material according to claim 4, characterized in that: The additive is a compound of one or more elements selected from Na, Sr, Co, Ru, Fe, Sb, Ce, Al, Zr, Y, Ta, Nb, W, Ti, Te, Sb, Rb, S, P, B, F, and Si.

6. A method for preparing an interface-stable cathode material according to claim 4 or 5, characterized in that: In step two, the sintering temperature is 800℃~1000℃, the time is 6h~10h, and the D50 of the material sintered once is 2um~8um; In step four, the heating and cooling rate of sintering is 0.33℃ / min to 5℃ / min, and the mixture is sintered at 400℃ to 800℃ for 4h to 8h. The D50 of the secondary sintered material is 2um to 8um.

7. A lithium-ion secondary battery, characterized in that: It includes a separator, an electrolyte, a negative electrode, and a positive electrode, wherein the positive electrode material used to prepare the positive electrode is an interface-stable positive electrode material as described in any one of claims 1-3.

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