阅读说明：本技术 梯度含量正极材料及其制备方法 (Gradient-content positive electrode material and preparation method thereof ) 是由 黄玲 王英 李文超 郭宇 唐仁衡 肖方明 于 2020-06-16 设计创作，主要内容包括：本发明公开了一种梯度含量正极材料的制备方法,通过对多层镍钴铝氢氧化物包覆镍钴氢氧化物复合前驱体[Ni<Sub>x</Sub>Co<Sub>1-x</Sub>(OH)<Sub>2</Sub>]<Sub>y</Sub>[Ni<Sub>a</Sub>Co<Sub>b</Sub>Al<Sub>1-a-b</Sub>(OH)<Sub>2</Sub>]<Sub>1-y</Sub>进行组成结构改进和制备方法上的改进,得到外层镍钴铝元素扩散形成呈浓度梯度变化的正极材料,在保证较高的放电比容量下提高了NCA正极材料的循环稳定性能。(The invention discloses a preparation method of a gradient-content positive electrode material, which comprises the step of coating a nickel-cobalt hydroxide composite precursor [ Ni ] on a multilayer nickel-cobalt-aluminum hydroxide x Co 1‑x (OH) 2 ] y [Ni a Co b Al 1‑a‑b (OH) 2 ] 1‑y The improvement of the composition structure and the improvement of the preparation method are carried out, the anode material with the concentration gradient change formed by the diffusion of the nickel, cobalt and aluminum elements on the outer layer is obtained, and the cycling stability of the NCA anode material is improved under the condition of ensuring higher specific discharge capacity.)

1. The preparation method of the cathode material with gradient content is characterized in that the cathode material with gradient content is prepared from a composite precursor and LiOH & H₂O is uniformly mixed according to the mass ratio of (Ni + Co + Al)/Li of 1: 1.02-1.05 and is obtained by calcining in an oxygen atmosphere; the composite precursor is of a core-shell structure and is marked as [ Ni ]_xCo_1-x(OH)₂]_y[Ni_aCo_bAl_1-a-b(OH)₂]_1-yWherein x is more than or equal to 0.85 and less than or equal to 0.94, y is more than or equal to 0.50 and less than or equal to 0.89, a is more than or equal to 0.66 and less than or equal to 0.80, b is more than or equal to 0.15 and less than or equal to 0.24, the core layer is a nickel-cobalt hydroxide, the shell layer is a three-layer nickel-cobalt-aluminum hydroxide, each nickel-cobalt-aluminum hydroxide is uniformly composed, the aluminum content in each nickel-cobalt-aluminum hydroxide is sequentially increased from inside to outside, the nickel content is sequentially decreased, the cobalt content is sequentially decreased and is in gradient, the atomic ratio gradient change value of each nickel element is 0.02-0.05, the first layer_a1Co_b1Al_1-a1-b1(OH)₂A1 is more than or equal to 0.69 and less than or equal to 0.83, b1 is more than or equal to 0.15 and less than or equal to 0.26, and the mass percent of Ni is 52.42 to 44.78wt percent, Co is 16.36 to 9.86wt percent, and Al is 0.73 to 2.08wt percent; the second layer is Ni_a2Co_b2Al_1-a2-b2(OH)₂A2 is more than or equal to 0.65 and less than or equal to 0.81, b2 is more than or equal to 0.15 and less than or equal to 0.24, and according to mass percent, Ni accounts for 51.54 to 43.24wt percent, Co accounts for 15.79 to 9.70wt percent, and Al accounts for 1.46 to 3.96wt percent; the third layer is Ni_a3Co_b3Al_1-a3-b3(OH)₂A3 is more than or equal to 0.62 and less than or equal to 0.79, b3 is more than or equal to 0.14 and less than or equal to 0.23, Ni is 50.67-41.83 wt%, Co is 15.28-9.53 wt%, Al is 2.87-5.69 wt%, wherein a2 is more than or equal to a3 and less than or equal to a1, b3 is more than or equal to b2 and less than or equal to b1 by mass percent, and the nickel cobalt hydroxide is coated by the three-layer nickel cobalt aluminum hydroxide; the total content of nickel element in the composite precursor is 51.51 wt% -55.09 wt%, the total content of cobalt element is 7.27 wt% -9.70 wt%, and the total content of aluminum element is 0.21 wt% -1.49 wt%; the gradient content positive electrode materialThe material comprises LiNi_mCo_nAl_1-m-nO₂Wherein m is x × y + a × (1-y), n is (1-x) × y + b × (1-y), the content of aluminum elements is increased from the inner part to the surface, the content of nickel elements is decreased from the inner part to the surface, and the content of cobalt elements is increased and then decreased to form gradient change.

2. The method for preparing the cathode material with gradient content according to claim 1, wherein x is more than or equal to 0.85 and less than or equal to 0.92, y is more than or equal to 0.50 and less than or equal to 0.70, a is more than or equal to 0.68 and less than or equal to 0.75, b is more than or equal to 0.15 and less than or equal to 0.22, the shell layer is three-layer nickel-cobalt-aluminum hydroxide, the first layer is Ni from inside to outside, the first layer is Ni_a1Co_b1Al_1-a1-b1(OH)₂A1 is more than or equal to 0.71 and less than or equal to 0.80, b1 is more than or equal to 0.15 and less than or equal to 0.24, and according to mass percent, Ni accounts for 50.90 to 46.14wt percent, Co accounts for 15.00 to 10.22wt percent, and Al accounts for 1.54 to 2.08wt percent; the second layer is Ni_a2Co_b2Al_1-a2-b2(OH)₂A2 is more than or equal to 0.67 and less than or equal to 0.76, b2 is more than or equal to 0.15 and less than or equal to 0.23, and according to mass percent, Ni accounts for 49.15 to 44.55wt percent, Co accounts for 14.48 to 9.86wt percent, and Al accounts for 3.00 to 3.96wt percent; the third layer is Ni_a3Co_b3Al_1-a3-b3(OH)₂A3 is more than or equal to 0.64 and less than or equal to 0.72, b3 is more than or equal to 0.14 and less than or equal to 0.22, and according to mass percent, the Ni is 47.55 to 43.10 weight percent, the Co is 14.01 to 9.54 weight percent, the Al is 4.34 to 5.69 weight percent, and the three-layer nickel cobalt aluminum hydroxide coats the nickel cobalt hydroxide; the total content of nickel element in the composite precursor is 51.51 wt% -54.41 wt%, the total content of cobalt element is 7.27 wt% -9.70 wt%, and the total content of aluminum element is 0.87 wt% -1.49 wt%.

3. The method for preparing a cathode material with gradient content according to claim 1, wherein the composite precursor [ Ni ] is_xCo_1-x(OH)₂]_y[Ni_aCo_bAl_1-a-b(OH)₂]_1-yThe preparation method comprises the following steps:

(1) deposition (Ni)_xCo_1-x)(OH)₂Kernel precursor: preparing a nickel-cobalt salt solution A with the total cation concentration of 0.93-1.43 mol/L according to the molar ratio of Ni to Co of x: 1-x,preparing an aqueous alkali A with NaOH concentration of 1.5-3 mol/L and ammonia concentration of 0.8-1.4 mol/L, respectively injecting a nickel-cobalt salt solution A and the aqueous alkali A into a base solution A with ammonia concentration of 0.3-0.7 mol/L, wherein the injection speed of the nickel-cobalt salt solution A is 30-180 mL/h, adjusting the flow rate of the aqueous alkali A to control the pH of a reaction system to be 11.15-12.25, the temperature to be 40-60 ℃, the stirring speed to be 900r/min, the reaction time to be 12-20 h, obtaining a nickel-cobalt hydroxide slurry, aging for 2-24 h at 40-60 ℃, filtering, washing with deionized water to obtain an inner core precursor Ni_xCo_1-x(OH)₂；

(2) Depositing an outer layer of Ni_aCo_bAl_1-a-b(OH)₂: subjecting the kernel precursor Ni obtained in the step (1) to Ni_xCo_1-x(OH)₂Directly injecting a base solution B containing 0.3-0.6 mol/L of ammonia for even dispersion, and then respectively and synchronously injecting a nickel-cobalt salt solution B, an aluminum salt solution and an alkali solution B, wherein the nickel-cobalt salt solution B is injected at a constant speed of 30-150 mL/h, the aluminum salt solution is continuously injected in three sections at an accelerated speed, the injection rate of the first section is 30-90 mL/h, the injection rate of the second section is 60-180 mL/h, the injection rate of the third section is 90-270 mL/h, the injection time of each section is 80-6 h, then adjusting the flow rate of the alkali solution B to control the pH of a reaction system to be 9.95-10.85, the temperature to be 40-60 ℃, the stirring speed to be 900r/min, and the total reaction time to be 4-18 h; after the nickel cobalt salt solution B and the aluminum salt solution are injected simultaneously, the mixture is aged for 8-24 hours at the temperature of 40-60 ℃ and the stirring speed of 400r/min, then the mixture is filtered and washed to obtain a precipitate, and the precipitate is dried at the temperature of 80-100 ℃ to obtain three layers of composite precursors (Ni, Al, Ni and Co hydroxide) of the nickel cobalt hydroxide coated with the nickel cobalt aluminum hydroxide, wherein the concentration gradient changes according to the design of the target material_xCo_1-x(OH)₂]_y[Ni_aCo_bAl_1-a-b(OH)₂]_1-y(ii) a Wherein the nickel-cobalt salt solution B is a solution with the total cation concentration of 0.5-1.2 mol/L which is prepared according to the molar ratio of Ni to Co of a to B, and the aluminum salt solution is NaAlO₂And NaOH with the molar ratio of 1: 1-3, and then adding water to prepare a solution, wherein the molar ratio of Ni in the nickel-cobalt salt solution B to Co to Al in the aluminum salt solution is a: B: 1-a-B; the alkaline solution B is 0.8-2.0 mol/L of NaOH and ammoniaA mixed solution with a concentration of 0.9-1.5 mol/L.

4. The preparation method of the cathode material with gradient content according to claim 3, wherein the reaction time in the step (1) is 14-18 h.

5. The method for preparing a cathode material with a gradient content according to claim 3, wherein the total molar amount of nickel and cobalt in the nickel-cobalt salt solution A and the total molar amount of aluminum in the nickel-cobalt salt solution B are y: 1-y, and the composition ratio of the core layer and the outer layer and the thickness of the outer layer are controlled by controlling the ratio y: 1-y of the total molar amount of nickel and cobalt in the nickel-cobalt solution A and the total molar amount of nickel, cobalt and aluminum in the nickel-cobalt solution B and the aluminum salt solution; the concentration change of aluminum element in the reaction system is realized by controlling the flow rate and the reaction time of the aluminum salt solution, the change of the deposition amount is realized, and the injection concentration of the corresponding aluminum element is 0.024-0.112 g/h, 0.051-0.223 g/h and 0.076-0.336 g/h in the three-stage injection process of the aluminum salt solution.

The technical field is as follows:

the invention relates to an electrode material, in particular to a gradient-content positive electrode material and a preparation method thereof.

Background art:

at present, lithium ion batteries are widely used in electrical equipment such as mobile phones and computers, in particular in the fields of electric automobiles and energy storage. The performance of the lithium ion battery is directly influenced by the performance of the lithium ion battery anode material. Ternary positive electrode material lithium nickel cobalt aluminate (NCA, mainly LiNi)_0.80Co_0.15Al_0.05O₂Mainly) has the advantages of high capacity, high voltage, long service life and the like, is concerned by various fields and is widely researched, but the cycle and thermal stability performance of the lithium ion battery affects the industrial application of the lithium ion battery. While the nickel element in NCA is in positive trivalent state, the content thereof is increased, the specific capacity of the material is also increased, but the Ni is caused^3+/4+Is unstable and easy to be reduced into Ni²⁺，Ni²⁺Occupy Li⁺The battery capacity is reduced due to the position (cation mixed arrangement, different degrees of cation mixed arrangement exist in NCA materials), so that the specific capacity of the materials does not correspond to the content of nickel element one by one; during the charge and discharge of the NCA battery, the phenomenon can occur

The structural transformation between H2 and H3 leads to the volume contraction and expansion of crystals (the volume contraction is generated when H2 → H3 transforms), thereby causing lattice stress, so that the particles can be broken and cracked in the circulation process, and the phase structure transformation variable is increased along with the increase of the content of nickel element; the generation of cracks and the breakage of the particles cause the increase of the contact area between the particles and the electrolyte, the increase of the contact resistance between the particles and the formation of a rock salt phase NiO layer on the surfaces of the particles, which is the main reason for the deterioration of the cycle performance. Heat release associated with state of charge of NCA batteryDuring the reaction, especially the overcharge of the battery (the material is transformed from a layered structure to a disordered spinel and then to a NiO type rock salt structure), oxygen-containing substances are released, and the oxygen-containing substances have high activity, so that the reaction with flammable electrolyte is accelerated to cause thermal runaway and cause disasters. The phase transition temperature is related to the content of nickel element in the material, and the higher the content of nickel element is, the lower the phase transition temperature is. While the Al and Co elements in NCA occupy tetrahedral positions (low spin Ni)⁴⁺Unstable in tetrahedral position and convertible to high spin Ni³⁺Oxygen evolution) is stable, thereby increasing the temperature at which the phase transition to the rock salt structure, i.e., enhancing structural stability, but leading to a decrease in the capacity of the NCA. Patent CN110828804A proposes a gradient content cathode material and a preparation method thereof, wherein a high nickel NCA cathode material is prepared into a concentration gradient distribution NCA material, which can give consideration to both high capacity and good cycle performance, and the obtained gradient cathode material has a higher specific discharge capacity, but the cycle performance still cannot meet the industrial application requirements.

The invention content is as follows:

the invention aims to provide a gradient-content positive electrode material and a preparation method thereof_xCo_1-x(OH)₂]_y[Ni_aCo_bAl_1-a-b(OH)₂]_1-yThe composition structure is improved, and meanwhile, the anode material with the concentration gradient change formed by the diffusion of the nickel, cobalt and aluminum elements on the outer layer is obtained through the improvement on the preparation method, so that the cycling stability of the NCA anode material is improved under the condition of ensuring higher specific discharge capacity.

The invention is realized by the following technical scheme:

the preparation method of the gradient-content cathode material comprises a composite precursor and LiOH & H₂O is uniformly mixed according to the mass ratio of (Ni + Co + Al)/Li of 1: 1.02-1.05 and is obtained by calcining in an oxygen atmosphere; the composite precursor is of a core-shell structure and is marked as [ Ni ]_xCo_1-x(OH)₂]_y[Ni_aCo_bAl_1-a-b(OH)₂]_1-yWherein x is more than or equal to 0.85 and less than or equal to 0.94, y is more than or equal to 0.50 and less than or equal to 0.89, a is more than or equal to 0.66 and less than or equal to 0.80, b is more than or equal to 0.15 and less than or equal to 0.24, the core layer is a nickel-cobalt hydroxide, the shell layer is a three-layer nickel-cobalt-aluminum hydroxide, each nickel-cobalt-aluminum hydroxide is uniformly composed, the aluminum content in each nickel-cobalt-aluminum hydroxide is sequentially increased from inside to outside, the nickel content is sequentially decreased, the cobalt content is sequentially decreased and is in gradient, the atomic ratio gradient change value of each nickel element is 0.02-0.05, the first layer_a1Co_b1Al_1-a1-b1(OH)₂A1 is more than or equal to 0.69 and less than or equal to 0.83, b1 is more than or equal to 0.15 and less than or equal to 0.26, and the mass percent of Ni is 52.42 to 44.78wt percent, Co is 16.36 to 9.86wt percent, and Al is 0.73 to 2.08wt percent; the second layer is Ni_a2Co_b2Al_1-a2-b2(OH)₂A2 is more than or equal to 0.65 and less than or equal to 0.81, b2 is more than or equal to 0.15 and less than or equal to 0.24, and according to mass percent, Ni accounts for 51.54 to 43.24wt percent, Co accounts for 15.79 to 9.70wt percent, and Al accounts for 1.46 to 3.96wt percent; the third layer is Ni_a3Co_b3Al_1-a3-b3(OH)₂A3 is more than or equal to 0.62 and less than or equal to 0.79, b3 is more than or equal to 0.14 and less than or equal to 0.23, and according to mass percent, Ni accounts for 50.67 to 41.83 weight percent, Co accounts for 15.28 to 9.53 weight percent, and Al accounts for 2.87 to 5.69 weight percent; wherein a2 is more than or equal to a3 and less than or equal to a1, b3 is more than or equal to b2 and less than or equal to b1, the three-layer nickel-cobalt-aluminum hydroxide coats the nickel-cobalt hydroxide, the total content of nickel elements in the composite precursor is 51.51 wt% -55.09 wt%, the total content of cobalt elements is 7.27 wt% -9.70 wt%, and the total content of aluminum elements is 0.21 wt% -1.49 wt%; the gradient-content cathode material comprises LiNi_mCo_nAl_1-m-nO₂Wherein m is x × y + a × (1-y), n is (1-x) × y + b × (1-y), the content of aluminum elements is increased from the inner part to the surface, the content of nickel elements is decreased from the inner part to the surface, and the content of cobalt elements is increased and then decreased to form gradient change.

Preferably, x is more than or equal to 0.85 and less than or equal to 0.92, y is more than or equal to 0.50 and less than or equal to 0.7, a is more than or equal to 0.68 and less than or equal to 0.75, b is more than or equal to 0.15 and less than or equal to 0.22, the core layer is a nickel-cobalt hydroxide, the shell layer is a three-layer nickel-cobalt-aluminum hydroxide, the first layer is Ni from inside to outside_a1Co_b1Al_1-a1-b1(OH)₂，0.71≤a1≤0.80，0.15≤b1≤0.24，According to the mass percentage, Ni accounts for 50.90 wt% -46.14 wt%, Co accounts for 15.00 wt% -10.22 wt%, and Al accounts for 1.54 wt% -2.08 wt%; the second layer is Ni_a2Co_b2Al_1-a2-b2(OH)₂A2 is more than or equal to 0.67 and less than or equal to 0.76, b2 is more than or equal to 0.15 and less than or equal to 0.23, and according to mass percent, Ni accounts for 49.15 to 44.55wt percent, Co accounts for 14.48 to 9.86wt percent, and Al accounts for 3.00 to 3.96wt percent; the third layer is Ni_a3Co_b3Al_1-a3-b3(OH)₂A3 is more than or equal to 0.64 and less than or equal to 0.72, b3 is more than or equal to 0.14 and less than or equal to 0.22, and according to mass percent, the Ni is 47.55 to 43.10 weight percent, the Co is 14.01 to 9.54 weight percent, the Al is 4.34 to 5.69 weight percent, and the three-layer nickel cobalt aluminum hydroxide coats the nickel cobalt hydroxide; the total content of nickel element in the composite precursor is 51.51 wt% -54.41 wt%, the total content of cobalt element is 7.27 wt% -9.70 wt%, and the total content of aluminum element is 0.87 wt% -1.49 wt%.

The composite precursor [ Ni_xCo_1-x(OH)₂]_y[Ni_aCo_bAl_1-a-b(OH)₂]_1-yThe preparation method comprises the following steps:

(1) deposition (Ni)_xCo_1-x)(OH)₂Kernel precursor: preparing a nickel-cobalt salt solution A with a total cation concentration of 0.93-1.43 mol/L according to a molar ratio of Ni to Co of x: 1-x, preparing an alkali solution A with a NaOH concentration of 1.5-3 mol/L and an ammonia concentration of 0.8-1.4 mol/L, respectively injecting the nickel-cobalt salt solution A and the alkali solution A into a base solution A with an ammonia concentration of 0.3-0.7 mol/L, wherein the injection speed of the nickel-cobalt salt solution A is 30-180 mL/h, adjusting the flow speed of the alkali solution A to control the pH of a reaction system to be 11.15-12.25, the temperature to be 40-60 ℃, the stirring speed to be 900r/min, the reaction time to be 12-20 h, preferably 14-18 h, obtaining a nickel-cobalt hydroxide slurry, filtering, aging and washing with deionized water to obtain a precursor Ni core Ni_xCo_1-x(OH)₂；

(2) Depositing an outer layer of Ni_aCo_bAl_1-a-b(OH)₂: subjecting the kernel precursor Ni obtained in the step (1) to Ni_xCo_1-x(OH)₂Directly injecting the solution B containing ammonia with a concentration of 0.3-0.6 mol/LDispersing uniformly, and then respectively and synchronously injecting a nickel-cobalt salt solution B, an aluminum salt solution and an alkali solution B, wherein the nickel-cobalt salt solution B is injected at a constant speed of 30-150 mL/h, the aluminum salt solution is injected continuously and rapidly in three sections, the injection rate of the first section is 30-90 mL/h, the injection rate of the second section is 60-180 mL/h, the injection rate of the third section is 90-270 mL/h, the injection time of each section is 80 min-6 h, then the flow rate of the alkali solution B is adjusted to control the pH of a reaction system to be 9.95-10.85, the temperature to be 40-60 ℃, the stirring speed to be 900r/min, and the total reaction time to be 4-18 h; after the nickel cobalt salt solution B and the aluminum salt solution are injected simultaneously, the mixture is aged for 8-24 hours at the temperature of 40-60 ℃ and the stirring speed of 400r/min, then the mixture is filtered and washed to obtain a precipitate, and the precipitate is dried at the temperature of 80-100 ℃ to obtain three layers of composite precursors (Ni, Al, Ni and Co hydroxide) of the nickel cobalt hydroxide coated with the nickel cobalt aluminum hydroxide, wherein the concentration gradient changes according to the design of the target material_xCo_1-x(OH)₂]_y[Ni_aCo_bAl_1-a-b(OH)₂]_1-y(ii) a Wherein the nickel-cobalt salt solution B is a solution with the total cation concentration of 0.5-1.2 mol/L which is prepared according to the molar ratio of Ni to Co of a to B, and the aluminum salt solution is NaAlO₂And NaOH with the molar ratio of 1: 1-3, and then adding water to prepare a solution, wherein the molar ratio of Ni in the nickel-cobalt salt solution B to Co to Al in the aluminum salt solution is a: B: 1-a-B; the alkali solution B is a mixed solution with NaOH of 0.8-2.0 mol/L and ammonia concentration of 0.9-1.5 mol/L; in the three-stage injection process of the aluminum salt solution, a first layer of Ni is deposited during the first stage of injection_a1Co_b1Al_1-a1-b1(OH)₂Depositing to form a second Ni layer during the second stage injection_a2Co_b2Al_1-a2-b2(OH)₂Depositing a third layer of Ni during the third stage implantation_a3Co_b3Al_1-a3-b3(OH)₂。

The ratio of the total molar amount of nickel and cobalt elements in the nickel and cobalt solution A to the total molar amount of nickel and cobalt elements in the nickel and cobalt solution B and the aluminum salt solution, namely y to (1-y), is controlled to realize the regulation of the composition ratio of the core layer and the outer layer and the thickness of the outer layer; the concentration change of aluminum elements in a reaction system is realized by controlling the flow rate and the reaction time of an aluminum salt solution, and the change of the deposition amount is realized, namely in the three-stage injection process of the aluminum salt solution, the injection concentrations of the corresponding aluminum elements are respectively 0.024-0.112 g/h, 0.051-0.223 g/h and 0.076-0.336 g/h, so that the mass percentages of nickel, cobalt and aluminum elements are changed, and the three-layer nickel-cobalt-aluminum hydroxide coated composite precursor with the concentration gradient change designed according to a target material is obtained.

Compared with the prior art, the invention has the following advantages:

1) the invention coats a nickel-cobalt hydroxide composite precursor [ Ni ] by a multilayer nickel-cobalt-aluminum hydroxide_xCo_1-x(OH)₂]_y[Ni_aCo_bAl_1-a-b(OH)₂]_1-yThe composition structure is improved, particularly the composition of an outer layer and the total molar ratio (y: 1-y)) of metal elements of an inner core and the outer layer are improved, on one hand, the content of the metal elements in the composite precursor in the inner core and the outer layer is controlled, particularly the content of the nickel elements in the nickel-cobalt-aluminum hydroxide of the outer layer is reduced to increase the content of the cobalt elements and the content of the aluminum elements in a proper range, and the cycle stability of the NCA positive electrode material is improved under the condition of ensuring higher specific discharge capacity; on the other hand, the total molar ratio y of the metal elements of the inner core and the outer layer to (1-y) is reduced, the thickness of the outer layer is increased, and the thickness of the gradient layer is further increased, so that the cycle performance and the structural stability of the gradient NCA cathode material are improved.

2) The invention improves the cycle stability of the NCA anode material under the condition of ensuring higher specific discharge capacity through the improvement on the preparation method: firstly, increase kernel deposition time, can make the kernel granule grow up and particle size distribution is narrower, makes the kernel crystal develop more completely, obtains the more complete crystal of lamellar structure, is favorable to improving the electrochemical properties of material on the one hand, and nickel, aluminium element deposit and grow on the kernel granule surface when on the other hand is favorable to outer deposit, obtains the better compound precursor of uniformity, and the cladding is more even. Secondly, the outer layer deposition time is increased, so that the pH is slowly adjusted to a preset range for a long time, the deposition and growth of nickel, cobalt and aluminum elements on the surface of the inner core particle during the outer layer deposition are facilitated, and the phenomenon that the nickel, cobalt and aluminum elements independently nucleate or accumulate on the surface of the inner core particle or cause the proportion of metal elements in the outer layer to be disordered due to large and quick pH value fluctuation is avoided; the outer layer deposition time is increased, so that the outer layer crystal can be completely developed, and the electrochemical performance is improved. And thirdly, the calcining temperature and time are improved, the calcining temperature and time are the most critical factors directly influencing the concentration distribution of nickel, cobalt and aluminum elements in the gradient layer of the anode material, and the homogeneous anode material is obtained by fully diffusing the metal elements on the outer layer when the calcining temperature is too high or the calcining time is too long. For the composite precursor with a thin outer layer or the composite precursor with a small particle size, the homogeneous-phase anode material can be prevented from being generated by adopting a lower calcining temperature or a shorter calcining time; for the composite precursor with a thicker outer layer and larger particle size, the calcination temperature can be properly increased or the calcination time can be prolonged, and the electrochemical performance of the cathode material can be preserved.

3) In conclusion, the invention coats the nickel-cobalt hydroxide composite precursor [ Ni ] by the multilayer nickel-cobalt-aluminum hydroxide_xCo_1-x(OH)₂]_y[Ni_aCo_bAl_1-a-b(OH)₂]_1-yThe improvement of the composition structure, particularly the outer layer composition and the total molar ratio (y: 1-y) of the metal elements of the inner core and the outer layer is combined with the improvement of the preparation method, so that the cycling stability of the NCA cathode material is improved under the condition of ensuring higher specific discharge capacity.

Description of the drawings:

FIG. 1 is a schematic diagram of the structure of a composite precursor of the present invention; the composite precursor has 4 layers, 1-nuclear layer Ni_xCo_1-x(OH)₂2-outer first layer Ni_a1Co_b1Al_1-a1-b1(OH)₂3-outer second layer Ni_a2Co_b2Al_1-a2-b2(OH)₂4-outer third layer Ni_a3Co_b3Al_1-a3-b3(OH)₂Wherein x is more than or equal to 0.85 and less than or equal to 0.94, a2 is more than or equal to a3 and less than or equal to a1, b3 is more than or equal to b2 and less than or equal to b1, and 1-a1-b1 is more than or equal to 1-a2-b2 is more than or equal to 1-a3-b 3.

Fig. 2 is a charge-discharge cycle curve of the positive electrode materials of example 7, example 8, and comparative example 8.

Fig. 3 is a rate performance curve of the positive electrode materials of example 7, example 8, and comparative example 8.

FIG. 4 is a diagram of an electron probe of the cross-sectional composition of particles of the gradient cathode material of example 7.

FIG. 5 is a diagram of an electron probe of the cross-sectional composition of particles of the gradient cathode material of example 8.

Fig. 6 is a diagram of an electron probe of the cross-sectional composition of the gradient cathode material particles of example 9.

FIG. 7 shows the reaction between the precursor and LiOH. H in example 1₂And (3) a TG/DSC curve of O in an oxygen atmosphere after the O is uniformly mixed according to the mass ratio of (Ni + Co + Al)/Li of 1: 1.05.

Fig. 8 is an SEM image of the composite precursor and the gradient cathode material in example 1, wherein a-composite precursor, b-gradient cathode material.

Fig. 9 is an SEM image of the composite precursor and the gradient cathode material in comparative example 1, in which a-composite precursor, b-gradient cathode material.

Fig. 10 is XRD patterns of the positive electrode materials of example 7, example 8 and comparative example 8.

Fig. 11 is an SEM image of the positive electrode sheet after 100 weeks of charge and discharge cycles in example 5 and comparative example 7, in which a-the positive electrode sheet in example 5, and b-the positive electrode sheet in comparative example 7.

Fig. 12 is an SEM image of the positive electrode sheet after 200 weeks of charge and discharge cycles in example 7, example 8, and comparative example 8, in which a-the positive electrode sheet in example 7, b-the positive electrode sheet in example 8, and c-the positive electrode sheet in comparative example 8.

FIG. 13 is a graph of electrochemical impedance for example 7, example 8, and comparative example 8, where a is example 7, b is example 8, and c is comparative example 8.

The specific implementation mode is as follows:

the following is a further description of the invention and is not intended to be limiting.