Nickel hydroxide material, method of making and battery

Abstract

This invention relates to a nickel hydroxide material with a core-shell structure, comprising a nickel hydroxide matrix and an outer coating layer. The coating layer includes cobalt hydroxyl oxide and is doped with lithium, sodium, and potassium, all of which exist in a solid solution state within the coating layer. By specifically doping its coating layer, this nickel hydroxide material significantly improves its electrochemical performance and structural stability. This invention also relates to a method for preparing this nickel hydroxide material and a battery using this nickel hydroxide material.

Description

Technical Field

[0001] This invention relates to the field of positive electrode active materials for nickel-based secondary batteries, and in particular to a nickel hydroxide material and its preparation method, as well as a battery using the nickel hydroxide material as the positive electrode active material. Background Technology

[0002] This section provides background information relevant to this application, which does not necessarily constitute prior art.

[0003] Spherical nickel hydroxide is a positive electrode active material in various alkaline secondary batteries (such as Ni-Zn, Ni-Cd, Ni-Mn, Ni-MH, etc.). However, spherical nickel hydroxide is a low-conductivity P-type semiconductor with low charge-discharge efficiency and a low discharge voltage plateau. To improve the performance of spherical nickel hydroxide, existing technical solutions include: (1) The specific capacity, cycle stability and charge-discharge efficiency of spherical nickel hydroxide can be improved by element doping. For example, by doping with elements such as Co, Al, Zn, Mn and Mg, the electronic conductivity and structural stability of spherical nickel hydroxide can be effectively improved. Among them, Al doping can significantly improve the discharge platform and cycle life. (2) By modifying the surface of spherical nickel hydroxide, the problem of Ni(OH)2 generated during discharge accumulating at the electrode / solution interface and forming a high interfacial resistance that prevents further discharge can be solved. For example, cobalt-coated spherical nickel hydroxide can be prepared by chemically plating cobalt onto the surface of nickel hydroxide particles, which can effectively improve the electrochemical cycle stability and capacity retention of spherical nickel hydroxide.

[0004] However, the existing technical solutions have the following technical problems: (1) Intracrystalline doping of spherical nickel hydroxide can improve its intracrystalline electron and proton conductivity by increasing the crystal defects of nickel hydroxide and control the crystal transformation of active material during electrode reaction, but it does not significantly improve the conductivity between active material particles, between active material and conductive framework, or between active material and electrolyte. (2) Cobalt is coated on the surface of nickel hydroxide particles by chemical plating. The crystal bundles of this coating layer grow randomly and form a large number of heterogeneous nuclei, resulting in loose coating. This leads to blockage of the cobalt conductive network structure on the surface, which means that the high current charge and discharge performance, cycle life, and charge retention rate cannot meet the requirements of alkaline secondary batteries. Furthermore, if the coating layer cannot cover the surface of nickel hydroxide well, nickel hydroxide will still come into contact with the alkaline electrolyte, causing increased gas generation and affecting the life of alkaline secondary batteries.

[0005] The applicant gained the following knowledge through research: (1) Ni(OH)2 without Co(OH)2 coating is prone to local overcharging during charging and discharging, which leads to excessive oxidation of Ni(OH)2 to γ-NiOOH phase (Ni(OH)2→β-NiOOH→γ-NiOOH+O2↑), resulting in large volume expansion and ultimately electrode pulverization; (2) Co(OH)2 coated with Ni(OH)2 can effectively avoid local overcharging and improve cycle stability. The specific principle is: Ni(OH)2 + Co(OH)2 → (Ni,Co)OOH (β phase, stable) + H2O + e - The coating layer of Co(OH)2 will be converted into highly conductive CoOOH after oxidation. At the same time, the preferential oxidation of CoOOH will "buffer" the rise of the positive electrode potential and reduce the excessive oxidation of Ni(OH)2 to the γ-NiOOH phase. (3) Co(OH)2 is coated on the surface of Ni(OH)2. This coating layer generates a highly conductive CoOOH conductive network during charging and discharging. Due to Co 2+ / Co 3+ The electrode reaction is irreversible, so CoOOH can exist on the surface of nickel hydroxide for a long time and act as a conductive network. However, the process of Co(OH)2 being converted into CoOOH during the initial activation of the battery will cause irreversible capacity of the battery. Therefore, directly coating the surface with CoOOH can better improve the electrochemical capacity and performance of nickel hydroxide. (4) γ-CoOOH has better conductivity than β-CoOOH. γ-CoOOH has a lower reduction potential in alkaline electrolyte than β-CoOOH and is not easily reduced. The conductive layer formed is more stable. The valence state of cobalt in β-CoOOH is +3, while when γ-CoOOH is formed, the valence state of cobalt is higher than +3, reaching more than +3.2. (5) After coating the nickel hydroxide surface with cobalt hydroxide, the cobalt coating on the nickel hydroxide surface is oxidized to γ-cobalt hydroxide by using alkali metal hydroxide and oxygen or by using oxidants (such as hydrogen peroxide, sodium hypochlorite, etc.), so that the conductivity of the conductive network of the final γ-cobalt hydroxide (γ-CoOOH) coating layer is better. However, the charge retention rate, cycle stability, discharge voltage plateau and uniformity and integrity of the coating layer need to be further improved. Summary of the Invention

[0006] This invention aims to provide a nickel hydroxide material and its preparation method. By specifically doping its coating layer, the electrochemical performance and structural stability of the nickel hydroxide material are significantly improved, as are the uniformity and integrity of the coating layer. Batteries using this nickel hydroxide material as the positive electrode active material can simultaneously and significantly improve charge retention, charge-discharge cycle stability, and discharge voltage plateau over a wide temperature range.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: a nickel hydroxide material, wherein the nickel hydroxide material has a core-shell structure, comprising a nickel hydroxide matrix and an outer coating layer, the coating layer comprising cobalt hydroxyoxide (CoOOH), and the coating layer is doped with lithium (Li), sodium (Na), and potassium (K), wherein Li, Na, and K exist in a solid solution state within the coating layer. The coating layer effectively suppresses side reactions between the nickel hydroxide matrix and the electrolyte and improves conductivity, thereby effectively improving rate performance and cycle stability.

[0008] With the sum of the amounts of Co, Li, Na and K being 100%, the mole fraction of Li is 0.01%-0.2%, the mole fraction of K is 0.01%-0.3%, and the mole fraction of Na is 0.3%-3%.

[0009] In one or more embodiments, the coating layer comprises a layer having the general formula CoOOH (Li x K y Na z The material, wherein 0.01≤x≤0.2, 0.01≤y≤0.3, 0.3≤z≤3, has a powder resistivity of not more than 1.0 × 10⁻⁶ at 150 MPa. -1 When the nickel hydroxide material is subjected to X-ray diffraction using Cu-Kα rays, diffraction peaks are detected in the region of 18° to 20° and in the region of 44° to 45°.

[0010] In one or more embodiments, the molar fraction of Ni is 94%-96% based on the sum of the amounts of Ni, Co, Li, Na and K being 100%.

[0011] In one or more embodiments, the nickel hydroxide matrix is ​​spherical nickel hydroxide, and the nickel hydroxide in the nickel hydroxide matrix is ​​pure α-phase nickel hydroxide or pure β-phase nickel hydroxide.

[0012] In one or more embodiments, the nickel hydroxide matrix is ​​spherical nickel hydroxide, and the nickel hydroxide in the nickel hydroxide matrix is ​​a mixed form of α-Ni(OH)2 phase and β-Ni(OH)2 phase.

[0013] In one or more embodiments, the nickel hydroxide matrix is ​​doped with aluminum (Al) and magnesium (Mg), and the nickel hydroxide matrix comprises elements having the general formula Ni. 1-x (Al, Mg) x The material is (OH)2 and satisfies: 0.15≥x≥0.

[0014] In one or more embodiments, an intermediate layer is provided between the nickel hydroxide substrate and the coating layer, the intermediate layer comprising, from the inside out, a first intermediate layer and a second intermediate layer, the first intermediate layer comprising a substrate having the general formula Ni. 1-x1-y1 Al x1 Mg y1 The material (OH)2, the second intermediate layer includes Ni... 1-x2-y2 Al x2 Mg y2 The material is (OH)₂ and satisfies the following conditions: x₂>x₁≥0, x₂ is 0.01-0.1; y₂>y₁≥0, y₂ is 0.01-0.1. The content gradient of Al and Mg elements decreases from the second coating layer to the first coating layer. This "gradient decrease" can specifically mean "gradually decreasing." The second intermediate layer contains magnesium, while the first intermediate layer may not necessarily contain magnesium, but if the first intermediate layer contains magnesium, the amount of magnesium in the first intermediate layer should be less than the amount of magnesium in the second intermediate layer. The second intermediate layer contains aluminum, while the first intermediate layer may not necessarily contain aluminum, but if the first intermediate layer contains aluminum, the amount of aluminum in the first intermediate layer should be less than the amount of aluminum in the second intermediate layer.

[0015] Unexpectedly, it was discovered that when the nickel hydroxide material simultaneously satisfies the following conditions: Al and Mg are doped in the nickel hydroxide matrix; Li, Na, and K are doped in the cobalt-containing coating layer, the nickel hydroxide material can simultaneously achieve: significantly improved charge-discharge cycle stability; increased discharge voltage plateau; significantly improved rate performance; and significantly suppressed self-discharge in high-temperature environments (ambient temperature 40-80℃). The speculated reason is that the Al and Mg doped nickel hydroxide matrix and the Li, Na, and K doped cobalt-containing coating layer work together to form a composite structure with a synergistic effect. This structure not only improves the material's conductivity and structural stability but also significantly enhances electrochemical performance, such as cycle stability and rate performance. Through Mg and Al co-doping, the crystal structure can be stabilized, the γ-NiOOH phase transition can be suppressed, cation vacancies can be introduced, the electronic conductivity of Ni(OH)₂ can be enhanced, polarization can be reduced, and the discharge plateau can be increased. The conductive network formed by the coating layer further reduces the electrode internal resistance and improves charge-discharge efficiency.

[0016] In one or more embodiments, the average particle size D50 of the nickel hydroxide material is 2μm-20μm, which is beneficial for the uniform distribution of the nickel hydroxide material in the positive electrode.

[0017] In one or more embodiments, the thickness of the coating layer of the nickel hydroxide material is 20nm-300nm, so as to effectively suppress the side reactions between the nickel hydroxide matrix and the electrolyte while reducing material impedance, improving rate performance and cycle performance.

[0018] It should be noted that, without affecting the technical effects described in this invention, the coating layer and the nickel hydroxide matrix may be doped with other elements. For example, the other elements include, but are not limited to, Co, Mg, Ca, Sr, Ba, B, Al, Ga, In, Ti, Mn, Fe, Cu, Zn, Mo, Ce and Zr.

[0019] On the other hand, the present invention provides a method for preparing the above-mentioned nickel hydroxide material, comprising the following steps: The process of preparing a cobalt salt solution using a soluble salt containing cobalt (Co); The process of preparing an alkali metal salt solution containing sodium (Na), potassium (K) and lithium (Li); The process of preparing complexing agent solutions; The process of dispersing a nickel hydroxide matrix in deionized water to prepare a suspension; The process of adding a cobalt salt solution, an alkali metal salt solution, a complexing agent solution and a first alkali solution to a suspension under stirring to carry out a first reaction; The process of adding an oxidant and a second alkali solution to the mixture after the first reaction to carry out a second reaction; The mixture after the second reaction is completed undergoes a series of solid-liquid separation, washing, and drying processes.

[0020] The cobalt-containing soluble salt is a soluble salt containing divalent cobalt, such as cobalt-containing acetates, nitrates, sulfates, halides, sulfides, hydroxides, oxides, hydroxyoxides, or combinations thereof. Specifically, it may include at least one of cobalt sulfate (CoSO4), cobalt nitrate (Co(NO3)2), and cobalt chloride (CoCl2). Preferably, the cobalt content in the cobalt salt solution is 0.01-1.5 mol / L, more preferably, the cobalt content in the cobalt salt solution is 0.5-1 mol / L. The amount of cobalt salt solution added to the suspension is based on the amount of cobalt in the coating layer, that is, the amount of cobalt is controlled by controlling the amount of cobalt salt solution added. For the purpose of maintaining the integrity of the coating layer while preventing the coating layer from being too thick (affecting conductivity), preferably, the amount of cobalt-containing soluble salt added is controlled to be 0.2wt%-5wt% of the weight of the nickel hydroxide matrix.

[0021] The alkali metal salt solution comprises: alkali metal salts containing sodium, such as carboxylates, nitrates, sulfates, carbonates, halides, sulfides, hydroxides, oxides, hydroxyoxides, or combinations thereof, specifically including at least one of sodium sulfate (Na₂SO₄), sodium nitrate (NaNO₃), and sodium chloride (NaCl); and alkali metal salts containing potassium, such as carboxylates, nitrates, sulfates, carbonates, halides, sulfides, etc. The coating may consist of compounds, hydroxides, oxides, hydroxyoxides, or combinations thereof, specifically at least one of potassium sulfate (K₂SO₄), potassium nitrate (KNO₃), and potassium chloride (KCl); the alkali metal salt containing lithium may be at least one of lithium chloride (LiCl), lithium sulfate (Li₂SO₄), lithium nitrate (LiNO₃), lithium perchlorate (LiClO₄), lithium hydroxide (LiOH), lithium oxalate (Li₂C₂O₄), lithium citrate (Li₃C₆H₅O₇), and lithium lactate (C₃H₅LiO₃). The amount of alkali metal salt solution added to the suspension is based on the doping amounts of Li, Na, and K in the coating layer. The content of Na, K, and Li elements in the coating layer is controlled by adjusting the concentrations of Na, K, and Li in the alkali metal salt solution.

[0022] The alkali metal salt solution can be prepared by adding an alkali metal salt containing sodium, an alkali metal salt containing potassium, and an alkali metal salt containing lithium to a solvent, specifically water, or a mixture of an organic solvent (e.g., an alcohol) and water that is homogeneous with water. Alternatively, the alkali metal salt solution can be prepared by mixing an aqueous solution of an alkali metal salt containing sodium, an aqueous solution of an alkali metal salt containing potassium, and an aqueous solution of an alkali metal salt containing lithium. Preferably, the alkali metal salt solution contains: Na content of 0.1-8 mol / L, K content of 0.1-8 mol / L, and Li content of 0.1-10 mol / L.

[0023] Preferably, the nickel hydroxide matrix and deionized water are mixed at a mass ratio of 1:4-12 to prepare the suspension.

[0024] Preferably, the complexing agent is one or more of ammonia, EDTA, citric acid, or sodium citrate. Preferably, the content of the complexing agent in the complexing agent solution is 0.5-2 mol / L. By adding the complexing agent solution, the deposition rate of cobalt can be controlled, the agglomeration of the coating layer can be avoided, the uniformity and density of the coating layer can be further improved, and the uniformity of the doping elements (Li, Na, and K) in the coating layer can be achieved.

[0025] The first reaction is a hydrothermal reaction, which is carried out in a hydrothermal reactor. During the first reaction: the pH of the reaction system is controlled to be 11-12 using a first alkaline solution; the reaction temperature is 80-150℃, as excessively high temperatures will cause coarse crystals in the coating layer, affecting the uniformity of the coating layer. Preferably, the reaction temperature is 100-140℃; the reaction time is 12-48 hours, preferably 20-30 hours; the reaction is carried out under stirring; the first alkaline solution is an ammonia solution, preferably an ammonia solution with a concentration of 25% to 28%.

[0026] In the second reaction process: the pH of the reaction system is controlled at 12-13 using a second alkaline solution; the reaction temperature is 80-90℃; the reaction time is 2-10h; the reaction process is kept under stirring; the second alkaline solution is a 5-10 mol / L sodium hydroxide solution and / or a 5-10 mol / L potassium hydroxide solution.

[0027] The amount of oxidant added is 1wt%-5wt% of the weight of nickel hydroxide matrix, and the oxidant is sodium hypochlorite and / or hydrogen peroxide.

[0028] After the first reaction, a coating layer doped with Li, Na, and K is formed on the surface of the nickel hydroxide substrate, and the coating layer also includes Co(OH)₂. During the second reaction, the Li, Na, and K dopants in the coating layer promote the oxidation of the Co(OH)₂ in the coating layer to highly conductive γ-CoOOH. The highly conductive γ-CoOOH can form a conductive network, which can significantly reduce charge transfer impedance and improve rate performance. Unexpectedly, it was found that the doping of Li, Na, and K in the coating layer can significantly inhibit the excessive oxidation of Ni(OH)₂ to γ-NiOOH during the second reaction, thereby improving cycle stability.

[0029] Preferably, the stirring speed in the first reaction process and the stirring speed in the second reaction process are both 400-1000 r / min.

[0030] In one or more embodiments, the mixture after the second reaction is completed is subjected to solid-liquid separation, specifically by filtration or centrifugation to obtain a solid product. Before washing the collected solid product, it is first aged. The aging process involves aging the solid product for 5-10 hours with a 3-6 mol / L sodium hydroxide solution and / or a 3-6 mol / L potassium hydroxide solution. After aging, the solid product is washed with deionized water until the pH of the washed solution is not greater than 8. Then, the solid product is placed in an oven and dried at an ambient temperature of 80-120°C.

[0031] On the other hand, the present invention provides a battery in which the positive electrode material includes the nickel hydroxide material described above. The battery stably exhibits excellent discharge capacity, output characteristics and capacity retention. Therefore, the battery is suitable for: portable electronic devices such as mobile phones, laptops, and digital cameras; power tools; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs); and power storage systems, etc.

[0032] In one or more embodiments, the battery is a nickel-based secondary battery.

[0033] This invention provides a nickel hydroxide material and its preparation method. The cobalt-containing coating layer of the nickel hydroxide material, through doping with Li, Na, and K, significantly improves the structural stability of the nickel hydroxide matrix. Li doping in the coating layer stabilizes the crystal structure of the nickel hydroxide matrix, suppresses phase transitions during charge-discharge processes (such as the formation of γ-NiOOH), and reduces the expansion of the nickel hydroxide matrix during charge-discharge, thereby improving the material's cycle stability. Na and K doping in the coating layer enhances the material's conductivity, suppresses side reactions, stabilizes the α-Ni(OH)2 phase, suppresses phase transitions during charge-discharge, and further improves the material's cycle stability. These dopants not only improve the material's structural stability but also enhance electron hopping conduction, reduce polarization, and improve rate performance by introducing electron holes or defects. In particular, the simultaneous doping of Li, Na, and K has a synergistic effect, improving the proton diffusion rate and enhancing high-rate charge-discharge performance. The nickel hydroxide material provided by this invention significantly improves the electrochemical performance and structural stability of the material through a composite structure and a specific element doping strategy.

[0034] The following description is based on specific embodiments. Attached Figure Description

[0035] The accompanying drawings further illustrate the invention, but the embodiments in the drawings do not constitute any limitation on the invention.

[0036] Figure 1 The XRD patterns of nickel hydroxide materials provided in Examples 1, 2, and 6 are shown. Detailed Implementation

[0037] It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0038] To provide a clearer understanding of the technical features, objectives, and beneficial effects of the present invention, the present invention will now be described in detail below, but this should not be construed as limiting the scope of the invention.

[0039] It should be noted that, unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0040] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0041] It should be understood that the terms “comprising,” “including,” and / or “containing” as used herein specify the presence of the stated features, integers, steps, components, or combinations thereof, but do not exclude the presence or addition of one or more other features, integers, steps, components, or combinations thereof.

[0042] In this invention, the average particle size D50 refers to the particle size at 50% of the volumetric cumulative particle size distribution of the powdered material. D50 can be measured using laser diffraction. For example, after dispersing the powdered material in a dispersion medium, the resulting mixture is irradiated using a laser diffractometer to obtain a volumetric cumulative particle size distribution map, and then the average particle size D50 can be obtained by obtaining the particle size corresponding to 50% of the cumulative volume.

[0043] [Example 1] This embodiment provides a nickel hydroxide matrix, wherein the nickel hydroxide matrix is ​​spherical nickel hydroxide, the average particle size D50 of the spherical nickel hydroxide is about 10 μm, and the tap density is ≥2.0 g / cm³.

[0044] The preparation method of the nickel hydroxide matrix includes the following steps: First, add a 5 mol / L ammonia solution to the reactor as a base liquid, adjust the reactor temperature to 60℃, and set the stirring speed to 150 r / min. Continuously add a 5 mol / L nickel-containing solution (prepared by dissolving nickel sulfate in deionized water), a 3 mol / L sodium hydroxide solution, and a 2 mol / L ammonia solution to the reactor using a continuous overflow method. The flow rate of the nickel-containing solution is 10 L / min, the flow rate of the sodium hydroxide solution is 3 L / min, and the flow rate of the ammonia solution is 3 L / min. Control the pH of the system at approximately 12, and continue the reaction to obtain a solid product. Wash and dry the solid product to obtain the nickel hydroxide matrix.

[0045] [Example 2] This embodiment provides a nickel hydroxide matrix, wherein the nickel hydroxide matrix is ​​a spherical nickel hydroxide doped with Mg and Al, and the chemical formula of the nickel hydroxide matrix is ​​Ni. 0.9 Al 0.05 Mg 0.05 (OH)2, wherein the average particle size D50 of the nickel hydroxide matrix is ​​about 10 μm.

[0046] The preparation method of the nickel hydroxide matrix includes the following steps: 6g of nickel sulfate is dissolved in 50mL of deionized water, then aluminum sulfate and magnesium sulfate are added and stirred until dissolved to obtain solution A. 2g of sodium hydroxide and 0.03g of ammonium sulfate are dissolved in 50mL of deionized water to obtain solution B. Under magnetic stirring, solutions A and B are mixed and reacted in a reaction vessel. The pH of the reaction system is controlled to 11-11.5 using 1M sodium hydroxide solution. The reaction is carried out at 80℃ for 10 hours. The resulting reaction mixture is filtered, and the solid is then washed and dried to obtain the nickel hydroxide matrix. The content of aluminum and magnesium elements in the core material is controlled by adjusting the concentrations of aluminum sulfate and magnesium sulfate in solution A. [Example 3] This embodiment provides a nickel hydroxide matrix with an average particle size D50 of approximately 10 μm. The nickel hydroxide matrix is ​​spherical nickel hydroxide with a surface modification layer. The thickness of the modification layer is approximately 500 nm, and the chemical formula of the modification layer is Ni. 0.9 Al 0.05 Mg 0.05 (OH)2.

[0047] The preparation method of the nickel hydroxide matrix includes the following steps: 0.28 g of nickel sulfate is dissolved in 100 mL of deionized water, and then aluminum sulfate and magnesium sulfate are added and stirred to obtain a reaction solution (the concentration of aluminum sulfate in the reaction solution does not exceed 3.0 mol / L, and the concentration of magnesium sulfate does not exceed 3.0 mol / L). Under magnetic stirring, 3.59 g of the spherical nickel hydroxide provided in Example 1 is mixed with the reaction solution in a reaction vessel. The pH of the reaction system is controlled at 11-11.5 using 1M sodium hydroxide solution. The reaction is carried out at 50°C for 3 hours. The obtained reaction material is filtered, and then the solid is washed and dried to obtain the nickel hydroxide matrix. The content of aluminum and magnesium elements in the modified layer is controlled by adjusting the concentration of aluminum sulfate and magnesium sulfate in the reaction solution.

[0048] [Examples 4-6] Example 4 uses the nickel hydroxide matrix provided in Example 1 to prepare nickel hydroxide material.

[0049] Example 5 uses the nickel hydroxide matrix provided in Example 2 to prepare nickel hydroxide material.

[0050] Example 6 uses the nickel hydroxide matrix provided in Example 3 to prepare nickel hydroxide material.

[0051] The preparation methods of nickel hydroxide materials provided in Examples 4-6 all include the following steps: Step 1: Prepare a cobalt salt solution by dissolving cobalt sulfate in deionized water. The cobalt content in the cobalt salt solution is 0.5 mol / L. Step 2: Prepare an alkali metal salt solution by dissolving sodium sulfate, potassium nitrate, and lithium hydroxide in deionized water; Step 3: Prepare a complexing agent solution by dissolving citric acid in deionized water. The citric acid content in the complexing agent solution is 1 mol / L. Step 4: Disperse 100 g of nickel oxide matrix in 800 mL of deionized water to prepare a suspension; Step 5: Add the suspension to the hydrothermal reactor (the inner lining of the hydrothermal reactor is made of polytetrafluoroethylene PTFE). The magnetic stirring speed is 800 r / min. Add the alkali metal salt solution, 50 mL of cobalt salt solution, 30 mL of complexing agent solution and the first alkali solution to the hydrothermal reactor to carry out the first reaction (i.e., hydrothermal reaction). The first alkali solution is a 25% ammonia solution. The pH of the reaction system is controlled at 12 using the first alkali solution. The reaction temperature is 120℃ and the reaction time is 24 h. The reaction is carried out under stirring. Step 6: After the first reaction is completed, add the mixture to the reactor, adjust the reactor temperature to 85℃, the stirring speed to 600r / min, add 3g of sodium hypochlorite and the second alkali solution to the reactor, use the second alkali solution to control the pH of the reaction system to 13, the reaction time to 5h, keep the reaction under stirring, and the second alkali solution is an 8 mol / L sodium hydroxide solution. Step 7: After the second reaction, the mixture is centrifuged to obtain a solid product. The solid product is aged for 5 hours with a 5 mol / L sodium hydroxide solution. After aging, the solid product is washed with deionized water until the pH of the washed solution is not greater than 8. Then, the solid product is placed in an oven and dried at an ambient temperature of 80-120℃ for 4 hours to obtain nickel hydroxide material.

[0052] In the nickel hydroxide materials provided in Examples 4-6, the content of doped elements in the coating layer is controlled by adjusting the concentrations of Na, K, and Li in the alkali metal salt solution. The specific content differences are shown in Table 1.

[0053] [Comparative Examples 1-10] The only difference between the preparation method of nickel hydroxide material provided in Comparative Example 1 and the preparation method of nickel hydroxide material provided in Example 1 is that no alkali metal salt solution was used when preparing the nickel hydroxide material provided in Comparative Example 1.

[0054] The only difference between the preparation method of nickel hydroxide material provided in Comparative Example 2 and the preparation method of nickel hydroxide material provided in Example 1 is that potassium nitrate and lithium hydroxide were not used when preparing the nickel hydroxide material provided in Comparative Example 2.

[0055] The only difference between the preparation method of the nickel hydroxide material provided in Comparative Example 3 and the preparation method of the nickel hydroxide material provided in Example 1 is that sodium sulfate and lithium hydroxide were not used when preparing the nickel hydroxide material provided in Comparative Example 3.

[0056] The only difference between the preparation method of the nickel hydroxide material provided in Comparative Example 4 and the preparation method of the nickel hydroxide material provided in Example 1 is that sodium sulfate and potassium nitrate were not used when preparing the nickel hydroxide material provided in Comparative Example 4.

[0057] The only difference between the preparation method of the nickel hydroxide material provided in Comparative Example 5 and the preparation method of the nickel hydroxide material provided in Example 1 is that sodium sulfate was not used when preparing the nickel hydroxide material provided in Comparative Example 5.

[0058] The only difference between the preparation method of the nickel hydroxide material provided in Comparative Example 6 and the preparation method of the nickel hydroxide material provided in Example 1 is that potassium nitrate was not used when preparing the nickel hydroxide material provided in Comparative Example 6.

[0059] The only difference between the preparation method of the nickel hydroxide material provided in Comparative Example 7 and the preparation method of the nickel hydroxide material provided in Example 1 is that lithium hydroxide was not used when preparing the nickel hydroxide material provided in Comparative Example 7.

[0060] The preparation methods of the nickel hydroxide materials provided in Comparative Examples 8-10 are the same as those provided in Example 1. However, the contents of Na, K and Li elements in the coating layer are controlled by adjusting the concentrations of Na, K and Li in the alkali metal salt solution. The specific content differences are shown in Table 1.

[0061] [Comparative Test of Nickel Hydroxide Materials] The nickel hydroxide materials provided in Examples 1-3 and Comparative Examples 1-10 were tested as follows: (1) The thickness of the coating layer of nickel hydroxide material was tested by scanning electron microscopy. The results showed that the thickness of the coating layer was approximately 200 nm to approximately 210 nm. (2) The elemental content of the nickel hydroxide coating layer was obtained by scanning electron microscopy and EDS test. The results are shown in Table 1.

[0062] (3) The resistivity of nickel hydroxide powder was tested using a four-probe powder conductivity meter. The test conditions were: ambient temperature of 28℃, relative humidity of RH≤60%, and pressure of 150MPa. The results are shown in Table 1.

[0063] Table 1

[0064] As shown in Table 1, when Li, Na and K are doped simultaneously in the coating layer, the synergistic effect of the doping elements can significantly reduce the powder resistance of nickel hydroxide material.

[0065] X-ray diffraction analysis was performed on the nickel hydroxide materials provided in Example 1, Comparative Example 2, and Comparative Example 6 using an X-ray diffractometer. The results are as follows: Figure 1 As shown. Specifically, the test conditions included: Cu-Kα radiation, a voltage of 40 kV, a current of 40 mA, and a scan rate of 2... ° The X-ray diffraction pattern was obtained by measuring the 2θ value at a rate of 1 / min. This X-ray diffraction pattern revealed that when the coating layer was doped with Li, Na, and K (corresponding to Example 1), diffraction peaks were detected in the 18° to 20° region, and in the 44° to 45° region. Further analysis... Figure 1 The peak values ​​in the XRD patterns indicate that cobalt exists in the coating layer of the tested nickel hydroxide materials in the form of CoOOH. However, β-CoOOH is formed in the coating layers of the nickel hydroxide materials provided in Comparative Examples 2 and 6, while γ-CoOOH is formed in the coating layer of the nickel hydroxide material provided in Example 1. Unexpectedly, it was found that when Li, Na, and K are simultaneously doped in the coating layer, the dopant elements can synergistically promote the oxidation of cobalt to a valence state higher than +3 during the second reaction.

[0066] [Half-Battery Test] Half-cells were fabricated using the nickel hydroxide materials provided in Examples 1-3 and Comparative Examples 1-10, respectively. The half-cell test configurations allow for the testing of the electrochemical characteristics of the nickel hydroxide materials before they are used in full-cell applications. The specific steps are as follows: Step 1: Mix nickel hydroxide material and sodium carboxymethyl cellulose (CMC) in deionized water at a mass ratio of 16:1 to prepare a positive electrode slurry composition. The viscosity of the positive electrode slurry composition can be adjusted with deionized water. Coat the positive electrode slurry composition onto one surface of nickel foam and then vacuum dry it at 80°C for 1 hour to obtain the working electrode. Step 2: Using a standard three-electrode system, the working electrode, platinum plate counter electrode, and Hg / HgO reference electrode are immersed in an alkaline electrolyte (6M potassium hydroxide solution). Cyclic voltammetry is performed at 25°C using an electrochemical workstation with a scan rate of 5mV / s.

[0067] Half-cell test results: (1) Specific capacity of nickel hydroxide material (for comparison, the theoretical capacity of nickel hydroxide is about 289 mAh / g); (2) Open-circuit voltage (OCV) of nickel hydroxide material relative to Hg / HgO reference electrode; (3) Peak potential difference ΔEp = |Epa - Epc|, where Epa is the oxidation peak potential and Epc is the reduction peak potential. The smaller ΔEp is, the better the reversibility of the reaction. The larger ΔEp is, the greater the polarization of the material and the greater the kinetic resistance of the reaction.

[0068] The test results are shown in Table 2. As can be seen from Table 2, when Li, Na and K are doped simultaneously in the coating layer, the synergistic effect of the dopant elements increases the specific capacity of the nickel hydroxide material while reducing its polarization.

[0069] Table 2

[0070] [Full Battery Test] Full cells were fabricated using the nickel hydroxide materials provided in Examples 1-3 and Comparative Examples 1-10, respectively. The specific steps are as follows: Step 1: Prepare a composite slurry by mixing nickel hydroxide, yttrium oxide, and 24% PTFE emulsion at a mass ratio of 97:2:1. Step 2: The composite slurry is filled into the foamed nickel sheet that serves as the positive electrode substrate, and then dried, rolled and cut in sequence to obtain a nickel electrode with size AA.

[0071] Step 3: Take 100 parts by weight of hydrogen storage alloy powder (specifically (La) 0.32 Sm 0.55 ) 1.00 Mg 0.14 Ni 3.24 Al 0.15 Co 0.07 0.4 parts by weight of sodium polyacrylate, 0.1 parts by weight of carboxymethyl cellulose, 1.0 parts by weight of styrene-butadiene rubber (SBR) dispersion, 1.0 parts by weight of carbon black and 30 parts by weight of water are mixed to form a paste. Step 4: The paste is coated on both sides of the steel perforated plate, which serves as the negative electrode substrate, and the coating thickness is uniform. The thickness of the steel perforated plate is 60μm and the surface of the steel perforated plate is nickel plated. After coating, the plate is dried, rolled and cut in sequence to obtain a negative electrode with size AA. Step 5: After sequentially stacking the negative electrode, separator (the separator is made of sulfonated polypropylene fiber nonwoven fabric with a thickness of 0.11 mm and a density of 48 g / m²), and nickel electrode, wind them to obtain the battery cell. Place the battery cell into a bottomed cylindrical casing and inject a predetermined amount of prepared alkaline electrolyte (an aqueous solution containing potassium hydroxide, sodium hydroxide, and lithium hydroxide is used as the alkaline electrolyte; the mass ratio of potassium hydroxide, sodium hydroxide, and lithium hydroxide in the alkaline electrolyte is 1.3:6.4:0.85, and the concentration of hydroxide ions in the alkaline electrolyte is 5 mol / L). Then, seal the opening of the cylindrical casing with a cap and assemble an AA-size full battery (i.e., a nickel-metal hydride secondary battery) with a nominal capacity (nominal capacity refers to the discharge capacity when the battery is charged at 0.1C for 16 hours at 20°C and then discharged at 0.2C until the battery voltage reaches 1.0V) of 1450 mAh. The full battery was charged at 0.1C for 16 hours at 20°C, then discharged at 0.2C. This charge-discharge cycle was repeated five times until the battery voltage reached 1.0V, completing the activation process. The activated battery was then calibrated for its initial discharge capacity by charging at 0.1C for 10 hours at an ambient temperature of 20°C, followed by discharging at 0.1C until 1.0V was reached, yielding the initial discharge capacity.

[0072] The following tests were performed using the activated full cells: (1) Cyclic performance test: At an ambient temperature of 20°C, the system was charged at a constant current of 0.5C for 2 hours, and then discharged at a constant current of 0.5C until 1.0V. The charge-discharge cycle was repeated for a maximum of 500 cycles. The capacity retention rate (%) was calculated using the initial discharge capacity and the discharge capacity at the predetermined number of cycles.

[0073] Capacity retention rate (%) = Discharge capacity at the 500th charge-discharge cycle ÷ Initial discharge capacity × 100.

[0074] (2) Rate Cycling Performance Test: At an ambient temperature of 20°C, the system was charged at a constant current of 0.5C for 2 hours, and then discharged at a constant current of 3C until 1.0V. The charge-discharge cycle was repeated for a maximum of 100 cycles. The capacity retention rate (%) was calculated using the initial discharge capacity and the discharge capacity at the predetermined number of cycles.

[0075] Capacity retention rate (%) = Discharge capacity at the 100th charge-discharge cycle ÷ Initial discharge capacity × 100.

[0076] (3) Capacity retention test under high temperature environment: In an ambient temperature of 20℃, constant current charging is performed for 10 hours at a current value of 0.1C. Then, the fully charged battery is placed in an environment of 60℃ for 30 days, and then placed in an ambient temperature of 20℃ for 12 hours. Finally, constant current discharge is performed at a current value of 0.1C until 1.0V is obtained to obtain the discharge capacity. The capacity retention rate (%) is calculated by comparing the discharge capacity with the initial discharge capacity.

[0077] Capacity retention rate (%) = Discharge capacity at the 100th charge-discharge cycle ÷ Initial discharge capacity × 100.

[0078] The test results are shown in Table 3. Table 3

[0079] As shown in Table 3, when the coating layer is simultaneously doped with Li, Na, and K, the synergistic effect of the dopant elements improves the cycling performance and rate performance of the nickel hydroxide material, and significantly suppresses self-discharge at high temperatures, exhibiting excellent high-temperature lifetime and electrical resistance characteristics. Compared with the comparative examples, the cycle stability of the examples is comprehensively superior to that of the comparative examples at both low and high rates. This fully demonstrates that the nickel hydroxide material provided by this invention exhibits highly stable cycling performance and excellent rate performance. This is due to the fact that the cobalt-containing coating layer with multiple dopants reduces charge transfer impedance, and the synergistic effect of the dopant elements further stabilizes the structure and optimizes electron conduction, enabling the nickel hydroxide material to maintain structural integrity during rapid charge and discharge, thus exhibiting excellent capacity retention. In Comparative Examples 1-7, this stable synergistic structure could not be formed due to the lack of dopant elements. Furthermore, it was unexpectedly found that if the dopant content of the dopant elements is too high, it will have a negative impact on the cycling performance of the nickel hydroxide material.

[0080] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0081] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A nickel hydroxide material, said nickel hydroxide material having a core-shell structure, comprising a nickel hydroxide matrix and an outer coating layer, said coating layer comprising cobalt hydroxyl oxide, characterized in that, The coating layer is doped with lithium, sodium, and potassium, and the lithium, sodium, and potassium all exist in a solid solution state in the coating layer.

2. The nickel hydroxide material according to claim 1, characterized in that, The total molar fraction of cobalt, lithium, sodium, and potassium is 100%, with the molar fraction of lithium being 0.01%-0.2%, the molar fraction of potassium being 0.01%-0.3%, and the molar fraction of sodium being 0.3%-3%.

3. The nickel hydroxide material according to claim 2, characterized in that, The coating layer comprises a general formula CoOOH(Li) x K y Na z The material has the following properties: 0.01≤x≤0.2, 0.01≤y≤0.3, 0.3≤z≤3; the powder resistivity of the nickel hydroxide material at 150MPa is not greater than 1.0 × 10⁻⁶. -1 Ω·cm; When the nickel hydroxide material is subjected to X-ray diffraction using Cu-Kα rays, diffraction peaks are detected in the region of 18° to 20° and in the region of 44° to 45°.

4. The nickel hydroxide material according to claim 2, characterized in that, The nickel hydroxide matrix is ​​spherical nickel hydroxide, and the nickel hydroxide in the nickel hydroxide matrix is ​​pure α-phase nickel hydroxide and / or pure β-phase nickel hydroxide.

5. The nickel hydroxide material according to claim 2, characterized in that... The nickel hydroxide matrix is ​​doped with aluminum and magnesium elements, and the nickel hydroxide matrix comprises elements having the general formula Ni. 1-x (Al, Mg) x The material is (OH)2 and satisfies: 0.15≥x≥0.

6. The nickel hydroxide material according to claim 2, characterized in that... An intermediate layer exists between the nickel hydroxide substrate and the coating layer. This intermediate layer comprises, from the inside out, a first intermediate layer and a second intermediate layer. The first intermediate layer comprises components having the general formula Ni. 1-x1-y1 Al x1 Mg y1 The material (OH)2, the second intermediate layer includes Ni... 1-x2-y2 Al x2 Mg y2 The material is (OH)2 and satisfies the following conditions: x2>x1≥0, x2 is 0.01-0.1; y2>y1≥0, y2 is 0.01-0.1; the content gradient of Al and Mg elements decreases from the second coating layer to the first coating layer.

7. The method for preparing the nickel hydroxide material according to any one of claims 1-6, characterized in that, The preparation method includes the following steps: The process of preparing a cobalt salt solution using a soluble salt containing cobalt. The process of preparing an alkali metal salt solution containing sodium, potassium and lithium elements; The process of preparing complexing agent solutions; The process of dispersing a nickel hydroxide matrix in deionized water to prepare a suspension; The process of adding cobalt salt solution, alkali metal salt solution, complexing agent solution and first alkaline solution to a suspension under stirring to carry out a first reaction; The process of adding an oxidant and a second alkali solution to the mixture after the first reaction to carry out a second reaction; The mixture after the second reaction is completed is subjected to solid-liquid separation, washing and drying in sequence; The first reaction is a hydrothermal reaction, which is carried out in a hydrothermal reactor. During the first reaction, the pH of the reaction system is controlled at 11-12 using a first alkaline solution, the reaction temperature is 90-180℃, the reaction time is 12-48h, and the reaction is kept under stirring. The first alkaline solution is an ammonia solution.

8. The preparation method according to claim 7, characterized in that, The complexing agent is one or more of ammonia, EDTA, citric acid, or sodium citrate; the amount of oxidant added is 1wt%-5wt% of the weight of the nickel hydroxide matrix, and the oxidant is sodium hypochlorite and / or hydrogen peroxide.

9. The preparation method according to claim 8, characterized in that, In the second reaction process: the pH of the reaction system is controlled at 12-13 using a second alkaline solution, the reaction temperature is 80-90℃, the reaction time is 2-10h, and the reaction process is kept under stirring; the second alkaline solution is a 5-10 mol / L sodium hydroxide solution and / or a 5-10 mol / L potassium hydroxide solution.

10. A battery, characterized in that, The positive electrode material of the battery includes the nickel hydroxide material as described in any one of claims 1-6.

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