CN115498169B - A method for preparing a core-shell structure precursor and a positive electrode material - Google Patents

Abstract

The invention discloses a preparation method of a core-shell structure precursor and a positive electrode material, which is characterized in that the prepared core-shell structure precursor has a structural general formula [(Ni_aM_1‑a)_x(Ni_bM_1‑b)_y(Ni_cM_1‑c)_z(Ni_dM_1‑d)_m…](OH)₂,, wherein M is one or more of Co, mn, al, mg, ti, W, ta, nb, zr. Compared with the traditional intermittent preparation technology, the technical route of the invention can obviously reduce the production cost of the precursor with the core-shell structure and improve the stability of the production batch of the precursor with the core-shell structure. And the core-shell or gradient structure anode material can be directionally and selectively prepared by selecting the additive in the solid phase reaction. Meanwhile, the method has simple steps and easy control, and is suitable for large-scale industrialized production.

Description

A method for preparing a core-shell structure precursor and a positive electrode material

Technical Field

The present invention belongs to the technical field of lithium ion batteries, and in particular relates to a method for preparing a core-shell structure precursor and a positive electrode material.

Background Art

The information disclosed in this background technology section is only intended to increase some understanding of the overall background of the invention, and should not be necessarily regarded as an admission or any form of suggestion that the information constitutes the prior art already known to ordinary technicians in this field.

Due to the increasing scarcity of resources and the intensification of environmental pollution, the development of renewable clean energy has become the theme of current energy development and is also related to the sustainable development of society. Renewable clean energy such as solar energy, wind energy, and hydropower are the best choices to replace traditional fossil energy. However, the most important characteristics of these renewable energy sources are discontinuity and instability. Therefore, renewable clean energy needs to be stabilized by the energy storage system before it can be connected to the grid. The energy storage system is indispensable in the application of renewable energy. Batteries, especially rechargeable secondary batteries, can realize the mutual conversion between chemical energy and electrical energy and are an important medium for the effective use of renewable clean energy. Compared with the low energy density of lead-acid batteries and the pollution of lead heavy metals, lithium-ion batteries are considered to be the best energy storage technology at present due to their high energy density, excellent cycle life and low self-discharge.

At present, the positive and negative electrode materials of commercial lithium-ion batteries are mainly layered _LiCoO2 and graphite, among which the practical specific capacity of graphite negative electrode has exceeded 300mAh g ^-1 , and it is low-cost and has excellent structural stability. In contrast, when the amount of Li ⁺ released in Li1 _{- xCoO2} is x>0.5, an irreversible transition from the hexagonal phase to the monoclinic phase will occur, causing a sharp decay of the discharge capacity. Therefore, the practical specific capacity of the positive electrode material _LiCoO2 is only about 140mAh g ^-1 , corresponding to a reversible release/embedding of about 0.5Li ⁺ . In addition, the scarce cobalt resources lead to a high cost of _LiCoO2 materials. In short, the low capacity and high cost of _LiCoO2 positive electrodes have become the main factors limiting the performance improvement of lithium-ion batteries and their widespread application in large-capacity, high-power systems.

High nickel layered material LiNi _x Co _y Mn _1-xy O ₂ (x ≥ 0.6) has become one of the research hotspots of positive electrode materials due to its high specific capacity (≥ 180 mAh g ^-1 ), low cost and excellent rate performance, and has become one of the most promising positive electrode materials for lithium-ion batteries. LiNi _x Co _y Mn _1-xy O ₂ is a solid solution formed by LiCoO ₂ -LiNiO ₂ -LiMnO ₂ , which fully combines the advantages of LiCoO ₂ , LiNiO ₂ and LiMnO ₂ , and has obvious ternary synergistic effect. However, due to the high nickel content, it also faces the shortcomings of layered LiNiO _2. Therefore, how to further stabilize the structure of high nickel materials while ensuring high capacity is the focus of current research.

Recently, through the redesign and reasonable combination of high-nickel layered materials, a series of core-shell layered cathode materials LiNi _1-xy Co _x Mn _y O ₂ were prepared by co-precipitation route combined with high-temperature solid phase method. High-capacity, low-stability nickel-rich materials were selected as core components, and structurally stable manganese-rich materials were selected as shell components. Through the clever combination of core and shell components, the performance of core and shell materials was composited and complementary, and the cycle life and thermal stability of the materials were significantly improved. Core-shell structure precursors are the key to the preparation of core-shell structure materials. However, the preparation of core-shell structure precursors is mainly based on the co-precipitation technology route in patent CN102347483 B. This technology route cannot be continuously produced, resulting in poor batch stability and high cost of core-shell precursors, which restricts the large-scale application of high-performance core-shell and gradient structure cathode materials.

Summary of the invention

In order to improve the production efficiency and batch stability of core-shell structure precursors and meet the market demand for high-performance electrochemical performance of core-shell structure positive electrode materials, the inventors have proposed a method for continuous preparation of core-shell structure precursors based on multi-stage reactor series co-precipitation technology after extensive research and exploration, and can controllably prepare core-shell structure and gradient structure positive electrode materials through additive selection in solid phase reaction.

Specifically, the present invention adopts the following technical solutions:

In a first aspect of the present invention, a method for preparing a core-shell structure precursor is provided.

The core-shell structure precursor has a general structural formula of [(Ni _a M _1-a ) _x (Ni _b M _1-b ) _y (Ni _c M _1-c ) _z (Ni _d M _1-d ) _m ....](OH) ₂ (1.0≥a≥0.7, a＞b＞c＞d, 0.9≥x≥0.5, 0.15≥y≥0.03, 0.15≥z≥0.03, 0.15≥m≥0.03), wherein M is one or more of Co, Mn, Al, Mg, Ti, W, Ta, Nb, and Zr;

The preparation method comprises the following steps:

First, the component Ni _a M _1-a salt solution 1 is added to the reactor 1, and the coprecipitation conditions are controlled to synthesize the inner core material [Ni _a M _1-a ](OH) ₂ of the precursor. When the liquid in the reactor 1 exceeds its designed rated volume, part of the [Ni _a M _1-a ](OH) ₂ and the solution will continuously flow out through the overflow port of the reactor 1 and be injected into the reactor 2 connected in series therewith. At the same time, the component Ni _b M _1-b salt solution 2 is added to the reactor 2, and the coprecipitation conditions are controlled so that the precipitate continues to grow on the surface of the [Ni _a M _1-a ](OH) ₂ precursor to synthesize the single-layer shell structure precursor [(Ni _a M _1-a ) _x (Ni _b M _1-b ) _y ](OH) _2. When the liquid in the reactor 2 exceeds its designed rated volume, the synthesized precursor [(Ni _a M _1-a ) _x (Ni _b M _1-b ) _y ](OH) ₂ and solution will continuously flow out through the overflow port of reactor 2 and be injected into reactor 3 connected in series therewith. At the same time, component Ni _c M _1-c solution 3 is gradually added into reactor 3. The co-precipitation conditions are controlled so that the precipitate continues to grow on the surface of the [(Ni _a M _1-a ) _x (Ni _b M _1-b ) _y ](OH) ₂ precursor to synthesize a two-layer shell structure precursor [(Ni _a M _1-a ) _x (Ni _b M _1-b ) _y (Ni _c M _1-c ) _z ](OH) ₂ . Similarly, by changing the number of reactors and the components of the reaction solution, core-shell structure precursors [(Ni _a M _1-a ) _x (Ni _b M _1-b ) _y (Ni _c M _1-c ) _z (Ni _d M _1-d ) _m ....](OH) ₂ with different numbers of shells and components can be prepared.

In one or more embodiments of the present invention, the core-shell structure precursor is synthesized by a hydroxide co-precipitation route. The hydroxide co-precipitation method refers to uniformly precipitating some metal or transition metal ions by changing pH or adding hydroxide ions (OH ^- ) in an experiment.

In one or more embodiments of the present invention, the number of stages in the multi-stage reactor series is 2-8;

Preferably, the number of levels is 3-5.

M ²⁺ +nNH ₃ →[M(NH ₃ ) _n ] ²⁺ (1)

[M(NH ₃ ) _n ] ²⁺ +OH ^- →M(OH) ₂ ↓+nNH ₃ (2)

The synthesis of the target precursor by co-precipitation reaction involves two key steps: complexation reaction and precipitation reaction. Therefore, in order to ensure the continuous growth of the precursor and prepare the core-shell structure precursor, it is necessary to strictly control the precipitation process parameters in different reactors, especially the pH and complexing agent concentration.

Compared with other metal ions M ²⁺ , Ni ²⁺ and NH ₃ can form a more stable complex [Ni(NH ₃ ) _n ] ²⁺ ; at the same time, since the solubility product constant of precipitated Ni(OH) ₂ (K _sp = 2.0×10 ^-15 ) is relatively low. Therefore, the inner core with high nickel content (Ni≥80w/w%) requires relatively high pH (12.5≥pH≥11.5) and low ammonia concentration (0.1≤OH ^- /NH ₃ ≤0.3), while the outer shell structure with low nickel content (80w/w%＞Ni≥40w/w%) requires low pH (10.5≤pH＜11.5) and high ammonia concentration (0.5≥OH ^- /NH ₃ >0.3), so as to ensure the continuous growth of the precursor in different reactors, and finally the multi-shell core-shell structure precursor can be prepared in a controlled manner.

In a second aspect of the present invention, a core-shell structure precursor prepared by the method described in the first aspect is provided.

In a third aspect of the present invention, a method for preparing a core-shell structure cathode material or a gradient structure cathode material using the core-shell structure precursor described in the second aspect is provided, wherein the general structural formula of the core-shell structure cathode material is Li[(Ni _a M _1-a ) _x (Ni _b M _1-b ) _y (Ni _c M _1-c ) _z (Ni _d M _1-d ) _m ....]O ₂ (1.0≥a≥0.7, a＞b＞c＞d, 0.9≥x≥0.5, 0.15≥y≥0.03, 0.15≥z≥0.03, 0.15≥m≥0.03), wherein M is one or more of Co, Mn, Al, Mg, Ti, W, Ta, Nb, and Zr, and the preparation method comprises the following steps:

The core-shell structure precursor is uniformly mixed with a stoichiometric lithium source and an ion diffusion inhibitor, and a high-temperature solid phase reaction is performed under an oxygen atmosphere to inhibit the diffusion of Ni and M elements between shell layers to prepare a core-shell structure layered positive electrode material, wherein the ion diffusion inhibitor is a mixture of one or more oxides containing Ti, Mo, Ta, Nb, and Zr elements;

The general structural formula of the gradient structure positive electrode material is Li[Ni _δ M _1-δ ]O ₂ (0.9≥δ≥0.6), wherein the Ni content of the positive electrode material secondary particles decreases from the inside to the surface, and the M content increases, and M is one or more of Co, Mn, Al, Mg, Ti, W, Ta, Nb, and Zr. The preparation method comprises the following steps:

The core-shell structure precursor is evenly mixed with a stoichiometric lithium source and a flux (ion diffusion promoter), and a high-temperature solid-phase reaction is performed under an oxygen atmosphere to promote the diffusion of Ni and M elements between shell layers to prepare a gradient structure layered positive electrode material, wherein the flux is a mixture of one or more of cobalt cyclopentaneate, manganese cyclopentaneate, lithium dodecane sulfonate, lithium stearate, lithium hydroxide, and lithium carbonate.

In one or more embodiments of the present invention, the lithium source is one or more of lithium carbonate, lithium hydroxide, lithium fluoride, lithium nitrate, lithium acetate and lithium oxalate.

In one or more embodiments of the present invention, the conditions for high temperature solid phase reaction are calcination at 750-900° C. for 2-16 h;

Preferably, the high temperature sintering condition is calcining at 780-850° C. for 6-10 h.

In one or more embodiments of the present invention, the high-temperature solid-phase reaction atmosphere is oxygen, and the heating rate of the material is 2-6°C/min.

In a fourth aspect of the present invention, a core-shell structure positive electrode material or a gradient structure positive electrode material prepared by the method described in the third aspect is provided.

In a fifth aspect of the present invention, a lithium ion battery using the core-shell structure positive electrode material or the gradient structure positive electrode material described in the fourth aspect is provided.

Compared with the related art known to the inventor, one of the technical solutions of the present invention has the following beneficial effects:

The present invention proposes for the first time a method for continuous preparation of core-shell structure precursors based on co-precipitation in series of multi-stage reactors. Compared with the traditional intermittent preparation technology, the technical route of the present invention can significantly reduce the production cost of core-shell structure precursors, improve the production batch stability of core-shell structure precursors, and effectively reduce the content of impurity ions (Na ⁺ and SO ₄ ^2- ) in the core-shell structure precursors, and improve the electrochemical performance of subsequent positive electrode materials. In addition, by selecting additives in the solid phase reaction, core-shell or gradient structure positive electrode materials can be selectively prepared.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their description are used to explain the present invention and do not constitute improper limitations on the present invention.

Figure 1 SEM image of the core-shell structure precursor prepared in Example 1;

Figure 2 is a cross-sectional SEM image of a single particle of a core-shell structure precursor prepared in Example 1;

FIG3 shows the element distribution of a single particle cross section of the core-shell structure precursor prepared in Example 1;

FIG4 shows the element distribution of a single particle cross section of the core-shell structure positive electrode material prepared in Example 2;

FIG5 is the element distribution of a single particle cross section of the positive electrode material prepared in Example 7;

FIG6 shows the element distribution of a single particle cross section of the gradient structure positive electrode material prepared in Example 8;

FIG7 shows the element distribution of a single particle cross section of the core-shell structure positive electrode material prepared in Comparative Example 2;

FIG8 shows the element distribution of a single particle cross section of the concentration gradient structure positive electrode material prepared in Comparative Example 3;

FIG9 compares the cycle stability of the concentration gradient structure positive electrode materials prepared in Example 8 and Comparative Example 3.

DETAILED DESCRIPTION

It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and/or "including" are used in this specification, it indicates the presence of features, steps, operations and/or combinations thereof.

In order to enable those skilled in the art to more clearly understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below in conjunction with specific embodiments.

Comparative Example 1: Intermittent preparation of core-shell structure precursor

7.396Kg NiSO ₄ ·6H ₂ O, 1.124Kg CoSO ₄ ·7H ₂ O and 0.676Kg MnSO ₄ ·H ₂ O were weighed to prepare 14L of 2M salt solution, wherein the Ni-Co-Mn molar ratio was 8/1/1, and named as salt solution A. 0.736Kg NiSO ₄ ·6H ₂ O, 0.169Kg CoSO ₄ ·7H ₂ O and 0.101Kg MnSO ₄ ·H ₂ O were weighed to prepare 2L of 2M salt solution, wherein the Ni-Co-Mn molar ratio was 7/1.5/1.5, and named as salt solution B. 0.631Kg NiSO ₄ ·6H ₂ O, 0.225Kg CoSO ₄ ·7H ₂ O and 0.135Kg MnSO ₄ ·H ₂ O were weighed to prepare 2L of 2M salt solution, wherein the molar ratio of Ni-Co-Mn was 6/2/2, and named salt solution C. 0.526Kg NiSO ₄ ·6H ₂ O, 0.281Kg CoSO ₄ ·7H ₂ O and 0.169Kg MnSO ₄ ·H ₂ O were weighed to prepare 2L of 2M salt solution, wherein the molar ratio of Ni-Co-Mn was 5/2.5/2.5, and named salt solution D.

The above salt solutions A, B, C, and D were sequentially added dropwise into a continuously stirred reactor, and the pH value (11.5) in the reactor was controlled with a mixed solution of 10 M NaOH and 1.5 M ammonia water (OH ^- /NH ₃ = 0.15) until the above salt solutions were sequentially added into the reactor and finally the salt solutions were completely consumed. The produced precipitate was centrifuged, washed, and dried to obtain a core-shell structure precursor [(Ni _0.8 Co _0.1 Mn _0.1 ) _0.7 (Ni _0.7 Co _0.15 Mn _0.15 ) _0.1 (Ni _0.6 Co _0.2 Mn _0.2 ) _0.1 (Ni _0.5 Co _0.25 Mn _0.25 ) _0.1 ](OH) ₂ .

Example 1: Continuous preparation of core-shell structure precursor by connecting multiple reactors in series

10.566Kg NiSO ₄ ·6H ₂ O, 1.606Kg CoSO ₄ ·7H ₂ O and 0.966Kg MnSO ₄ ·H ₂ O were weighed to prepare 20L of 2M salt solution, wherein the Ni-Co-Mn molar ratio was 8/1/1, and named as salt solution A. 7.361Kg NiSO ₄ ·6H ₂ O, 1.691Kg CoSO ₄ ·7H ₂ O and 1.012Kg MnSO ₄ ·H ₂ O were weighed to prepare 20L of 2M salt solution, wherein the Ni-Co-Mn molar ratio was 7/1.5/1.5, and named as salt solution B. 6.311 kg NiSO ₄ ·6H ₂ O, 2.250 kg CoSO ₄ ·7H ₂ O and 1.351 kg MnSO ₄ ·H ₂ O were weighed to prepare 20 L of a 2M salt solution, wherein the molar ratio of Ni-Co-Mn was 6/2/2, and the solution was named salt solution C. 5.2601 kg NiSO ₄ ·6H ₂ O, 2.811 kg CoSO ₄ ·7H ₂ O and 1.691 kg MnSO ₄ ·H ₂ O were weighed to prepare 20 L of a 2M salt solution, wherein the molar ratio of Ni-Co-Mn was 5/2.5/2.5, and the solution was named salt solution D.

The salt solution A was added dropwise to the continuous stirring reactor A, and the pH value (11.5) in the reactor was controlled by a mixed solution of 10M NaOH and 1.5M ammonia (OH ^- /NH ₃ = 0.15) until the liquid in the reactor A exceeded 80% of the rated volume, and the liquid in the reactor A was continuously discharged through the overflow port and injected into the reactor B connected in series with the reactor A. The salt solution B was added dropwise to the continuous stirring reactor B, and the pH value (11.2) in the reactor was controlled by a mixed solution of 10M NaOH and 3.0M ammonia (OH ^- /NH ₃ = 0.3) until the liquid in the reactor B exceeded 80% of the rated volume, and the liquid in the reactor B was continuously discharged through the overflow port and injected into the reactor C connected in series with the reactor B. The salt solution C was added dropwise to the continuous stirring reactor C, and the pH value (11.2) in the reactor was controlled by a mixed solution of 10M NaOH and 3.2M ammonia (OH ^- /NH ₃ =0.32) is used to control the pH value (11.1) in the reactor until the liquid in reactor C exceeds 80% of the rated volume, the liquid in reactor C will be continuously discharged through the overflow port and injected into reactor D connected in series with reactor C. The above salt solution D is added dropwise to the continuously stirred reactor D. At the same time, a mixed solution of 10M NaOH and 3.4M ammonia water (OH ^- /NH ₃ =0.34) is used to control the pH value (11.0) in the reactor until the liquid in reactor D exceeds 80% of the rated volume, the liquid in reactor D will be continuously discharged through the overflow port. The precipitate was centrifuged, washed and dried to obtain a core-shell structure precursor [(Ni _0.8 Co _0.1 Mn _0.1 ) _0.7 (Ni _0.7 Co _0.15 Mn _0.15 ) _0.1 (Ni _0.6 Co _0.2 Mn _0.2 ) _0.1 (Ni _0.5 Co _0.25 Mn _0.25 ) _0.1 ](OH) ₂ . The micromorphology and element distribution of the precursor are shown in Figures 1 to 3.

Table 1 Particle size distribution statistics of the core-shell structure precursor synthesized four times in Example 1 and Comparative Example 1

It can be seen from Table 1 that, compared with Comparative Example 1, the particle size difference of the core-shell precursor structure synthesized four times according to the method of Example 1 is smaller, indicating that this method improves the production batch stability of the core-shell structure precursor.

Table 2 Impurity ion content of the core-shell structure precursor synthesized four times in Example 1 and Comparative Example 1

Table 3 Impurity ion content of the core-shell structure precursor synthesized four times in Example 1 and Comparative Example 1

It can be seen from Tables 2 and 3 that, compared with Comparative Example 1, the impurity ion (Na ⁺ and SO ₄ ^2- ) content in the core-shell structure precursor synthesized by repeating the method of Example 1 four times is lower, and the low impurity ion content in the subsequent lithium source sintering process has weaker damage to the lattice structure, indicating that this method improves the core-shell structure precursor to have more excellent physical and chemical properties.

Example 2: Preparation of core-shell structure cathode material under diffusion inhibitor conditions

91.5 g of the precursor prepared in Example 1 was weighed and evenly mixed with 36.9 g of battery-grade lithium carbonate and 0.8 g of TiO ₂ , and the mixture was placed in an oxygen atmosphere furnace and sintered at 820° C. for 12 h. The mixture was cooled naturally to prepare a core-shell structured positive electrode material Li[(Ni _0.8 Co _0.1 Mn _0.1 ) _0.7 (Ni _0.7 Co _0.15 Mn _0.15 ) _0.1 (Ni _0.6 Co _0.2 Mn _0.2 ) _0.1 (Ni _0.5 Co _0.25 Mn _0.25 ) _0.1 ]O ₂ . The element distribution of the cross section of a single particle is shown in FIG4 .

Example 3: Preparation of core-shell structure cathode material under diffusion inhibitor conditions

91.5 g of the precursor prepared in Example 1 was weighed and mixed evenly with 36.9 g of battery-grade lithium carbonate and 0.6 g of MoO ₃ , and the mixture was placed in an oxygen atmosphere furnace and sintered at 820° C. for 12 h. The mixture was cooled naturally to prepare a core-shell structured positive electrode material Li[(Ni _0.8 Co _0.1 Mn _0.1 ) _0.7 (Ni _0.7 Co _0.15 Mn _0.15 ) _0.1 (Ni _0.6 Co _0.2 Mn _0.2 ) _0.1 (Ni _0.5 Co _0.25 Mn _0.25 ) _0.1 ]O ₂ .

Example 4: Preparation of core-shell structure cathode material under diffusion inhibitor conditions

91.5 g of the precursor prepared in Example 1 was weighed and mixed evenly with 36.9 g of battery-grade lithium carbonate and 0.4 g of Ta ₂ O ₅ , and the mixture was placed in an oxygen atmosphere furnace and sintered at 820° C. for 12 h. The mixture was cooled naturally to prepare a core-shell structured positive electrode material Li[(Ni _0.8 Co _0.1 Mn _0.1 ) _0.7 (Ni _0.7 Co _0.15 Mn _0.15 ) _0.1 (Ni _0.6 Co _0.2 Mn _0.2 ) _0.1 (Ni _0.5 Co _0.25 Mn _0.25 ) _0.1 ]O ₂ .

Example 5: Preparation of core-shell structure cathode material under diffusion inhibitor conditions

91.5 g of the precursor prepared in Example 1 was weighed and mixed evenly with 36.9 g of battery-grade lithium carbonate and 0.8 g of Nb ₂ O ₅ , and the mixture was placed in an oxygen atmosphere furnace and sintered at 820° C. for 12 h. The mixture was cooled naturally to prepare a core-shell structured positive electrode material Li[(Ni _0.8 Co _0.1 Mn _0.1 ) _0.7 (Ni _0.7 Co _0.15 Mn _0.15 ) _0.1 (Ni _0.6 Co _0.2 Mn _0.2 ) _0.1 (Ni _0.5 Co _0.25 Mn _0.25 ) _0.1 ]O ₂ .

Example 6: Preparation of core-shell structure cathode material under diffusion inhibitor conditions

91.5 g of the precursor prepared in Example 1 was weighed and mixed evenly with 36.9 g of battery-grade lithium carbonate and 0.5 g of ZrO ₂ , and the mixture was placed in an oxygen atmosphere furnace and sintered at 820° C. for 12 h. The mixture was cooled naturally to prepare a core-shell structured positive electrode material Li[(Ni _0.8 Co _0.1 Mn _0.1 ) _0.7 (Ni _0.7 Co _0.15 Mn _0.15 ) _0.1 (Ni _0.6 Co _0.2 Mn _0.2 ) _0.1 (Ni _0.5 Co _0.25 Mn _0.25 ) _0.1 ]O ₂ .

Example 7: Preparation of positive electrode materials without adding diffusion inhibitors or solvents

91.5 g of the precursor prepared in Example 1 was weighed and mixed evenly with 36.9 g of battery-grade lithium carbonate, and sintered in an oxygen atmosphere furnace at 820° C. for 12 h. The cathode material Li[Ni _0.74 Co _0.13 Mn _0.13 ]O ₂ was prepared by natural cooling. The element distribution of the cross section of a single particle is shown in FIG5 , which illustrates that the failure to add an ion diffusion inhibitor and a co-solvent results in uncontrollable inter-shell ion diffusion in a high-temperature solid phase reaction.

Example 8: Preparation of gradient structure positive electrode material under co-solvent conditions

91.5 g of the precursor prepared in Example 1 was weighed and mixed with 36.9 g of battery-grade lithium carbonate and 2.723 g of lithium dodecane sulfonate, and placed in a muffle furnace for sintering at 820°C for 12 h. The gradient structure positive electrode material Li[Ni _0.74 Co _0.13 Mn _0.13 ]O ₂ was naturally cooled to prepare the element distribution of the cross section of a single particle. A lithium-ion full battery was assembled with the above-mentioned gradient structure material positive electrode, mesophase carbon microspheres as negative electrode and organic electrolyte (1M LiPF ₆ dissolved in EC and DMC solvents, volume ratio 3:7). The cycle stability of the above-mentioned lithium-ion full battery was tested in the voltage range of 2.7-4.5 V, current density of 100 mA/g, and room temperature (25°C).

Example 9: Preparation of gradient structure positive electrode material under co-solvent conditions

91.5 g of the precursor prepared in Example 1 was weighed and mixed evenly with 36.9 g of battery-grade lithium carbonate and 3.5 g of cobalt naphthenate, sintered at 820° C. for 12 h in a muffle furnace, and cooled naturally to prepare a gradient structure positive electrode material Li[Ni _0.74 Co _0.13 Mn _0.13 ]O ₂ .

Example 10: Preparation of gradient structure positive electrode material under co-solvent conditions

91.5 g of the precursor prepared in Example 1 was weighed and mixed evenly with 36.9 g of battery-grade lithium carbonate and 3.0 g of manganese naphthenate, sintered at 820° C. for 12 h in a muffle furnace, and cooled naturally to prepare a gradient structure positive electrode material Li[Ni _0.74 Co _0.13 Mn _0.13 ]O ₂ .

Example 11: Preparation of gradient structure positive electrode material under co-solvent conditions

91.5 g of the precursor prepared in Example 1 was weighed and mixed evenly with 36.9 g of battery-grade lithium carbonate and 2.2 g of lithium stearate, and sintered in a muffle furnace at 820° C. for 12 h. The gradient structure positive electrode material Li[Ni _0.74 Co _0.13 Mn _0.13 ]O ₂ was prepared by naturally cooling.

Example 12: Preparation of gradient structure positive electrode material under co-solvent conditions

91.5 g of the precursor prepared in Example 1 was weighed and mixed evenly with 36.9 g of battery-grade lithium carbonate and 1.8 g of lithium hydroxide, and sintered in a muffle furnace at 820° C. for 12 h. The temperature was naturally lowered to prepare a gradient structure positive electrode material Li[Ni _0.74 Co _0.13 Mn _0.13 ]O ₂ .

Example 13: Preparation of gradient structure positive electrode material under co-solvent conditions

91.5 g of the precursor prepared in Example 1 was weighed and mixed evenly with 40.0 g of battery-grade lithium carbonate and 1.8 g of lithium hydroxide, and sintered in a muffle furnace at 820° C. for 12 h. The temperature was naturally lowered to prepare a gradient structure positive electrode material Li[Ni _0.74 Co _0.13 Mn _0.13 ]O ₂ .

Comparative Example 2: Preparation of core-shell structure cathode material under diffusion inhibitor conditions

91.5 g of the precursor prepared in Comparative Example 1 was weighed and evenly mixed with 36.9 g of battery-grade lithium carbonate and 0.8 g of TiO ₂ , and the mixture was placed in an oxygen atmosphere furnace and sintered at 820° C. for 12 h. The mixture was cooled naturally to prepare a core-shell structured positive electrode material Li[(Ni _0.8 Co _0.1 Mn _0.1 ) _0.7 (Ni _0.7 Co _0.15 Mn _0.15 ) _0.1 (Ni _0.6 Co _0.2 Mn _0.2 ) _0.1 (Ni _0.5 Co _0.25 Mn _0.25 ) _0.1 ]O ₂ . The element distribution of the cross section of a single particle is shown in FIG7 .

Comparative Example 3: Preparation of gradient structure positive electrode materials under co-solvent conditions

91.5g of the precursor prepared in Comparative Example 1 was weighed and mixed with 36.9g of battery-grade lithium carbonate and 2.723g of lithium dodecane sulfonate, and placed in a muffle furnace for sintering at 820°C for 12h, and cooled naturally to prepare a gradient structure positive electrode material Li[Ni _0.74 Co _0.13 Mn _0.13 ]O ₂ , the element distribution of the cross section of a single particle of which is shown in Figure 8. A lithium-ion full battery was assembled with the above-mentioned gradient material positive electrode, mesophase carbon microspheres as the negative electrode and an organic electrolyte (1M LiPF ₆ dissolved in EC and DMC solvents, volume ratio 3:7). The cycle stability of the above-mentioned lithium-ion full battery was tested in the voltage range of 2.7-4.5V, the current density of 100mA/g, and room temperature (25°C).

Even if the core-shell structure precursor prepared in Comparative Example 1 is added with an ion diffusion inhibitor or promoter to cause irregular diffusion between shell layers, it is still impossible to controllably prepare a core-shell or gradient structured positive electrode material, as shown in FIGS. 7 and 8 .

As shown in FIG9 , after the lithium-ion full battery is cycled 50 times, the discharge specific capacity efficiency of Example 8 and Comparative Example 3 are 98.7% and 88.9%, respectively, and the difference between the two is significant. It can be seen that the cycle stability performance of the lithium-ion full battery composed of Example 8 is significantly better than that of Comparative Example 3.

The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims (8)

1. A preparation method of a positive electrode material with a gradient structure is characterized in that,

The structural general formula of the positive electrode material with the gradient structure is Li [ Ni _δM_1-δ]O₂ ], wherein delta is more than or equal to 0.9 and more than or equal to 0.6, the content of Ni element from the inside to the surface of the secondary particles of the positive electrode material is in a decreasing trend, the content of M element is in an increasing trend, and M is one or more of Co, mn, al, mg, ti, W, ta, nb, zr; the preparation method comprises the following steps:

uniformly mixing a core-shell structure precursor with a lithium source and a fluxing agent in stoichiometric ratio, and carrying out high-temperature solid phase reaction in an oxygen atmosphere to promote diffusion of Ni and M elements between shell layers to prepare a layered anode material with a gradient structure, wherein the fluxing agent is one or two of cobalt naphthenate and manganese naphthenate;

Wherein the structural general formula of the core-shell structure precursor is [(Ni_aM_1-a)_x(Ni_bM_1-b)_y(Ni_cM_1-c)_z(Ni_dM_1-d)_m....](OH)₂ ,, wherein 1.0 is more than or equal to 0.7, a is more than or equal to b is more than or equal to c and is more than or equal to d, 0.9 is more than or equal to x is more than or equal to 0.5, 0.15 is more than or equal to y is more than or equal to 0.03, 0.15 is more than or equal to z is more than or equal to 0.03, 0.15 is more than or equal to M is more than or equal to 0.03, and M is one or more of Co, mn, al, mg, ti, W, ta, nb, zr; the preparation method of the core-shell structure precursor comprises the following steps:

Firstly adding a component Ni _aM_1-a salt solution 1 into a reaction kettle 1, controlling coprecipitation conditions to synthesize a precursor inner layer core material [ Ni _aM_1-a](OH)₂, after the liquid in the reaction kettle 1 exceeds the designed rated volume number of the precursor inner layer core material, continuously flowing out part of [ Ni _aM_1-a](OH)₂ and solution through an overflow port of the reaction kettle 1, injecting the mixture into the reaction kettle 2 connected with the reaction kettle in series, simultaneously adding the component Ni _bM_1-b salt solution 2 into the reaction kettle 2, controlling coprecipitation conditions to enable precipitation to continue to grow on the surface of the [ Ni _aM_1-a](OH)₂ precursor to synthesize a single-layer shell structure precursor [ (Ni _aM_1-a)_x(Ni_bM_1-b)_y](OH)₂), continuously flowing out the synthesized precursor [ (Ni _aM_1-a)_x(Ni_bM_1-b)_y](OH)₂ and solution through an overflow port of the reaction kettle 2 after the liquid in the reaction kettle 2 exceeds the designed rated volume number of the precursor inner layer core material, simultaneously adding the component Ni _cM_1-c solution 3 into the reaction kettle 3 gradually, controlling the coprecipitation conditions to enable precipitation to continue to grow on the surface of the [ (Ni _aM_1-a)_x(Ni_bM_1-b)_y](OH)₂ precursor, synthesizing a two-layer shell structure precursor [(Ni_aM_1-a)_x(Ni_bM_1-b)_y(Ni_cM_1-c)_z](OH)₂,, and so on the basis, and preparing the precursor inner layer shell structure precursor through the same number and the precursor layers by the same number as that the precursor layers are prepared [(Ni_aM_1-a)_x(Ni_bM_1-b)_y(Ni_cM_1-c)_z(Ni_dM_1-d)_m....](OH)₂;

Synthesizing a precursor with a core-shell structure by adopting a hydroxide coprecipitation route;

The number of stages in the multistage reactor series is 3-5 stages.

2. The method of claim 1, wherein the inner core having a high nickel content requires pH and ammonia concentration conditions of: 12.5 The pH value is more than or equal to 11.5,0.1 and less than or equal to C [ OH ^-/NH₃ ] and less than or equal to 0.3, wherein Ni in the inner core with high nickel content is more than or equal to 80w/w%; the conditions of pH and ammonia concentration required for the outer shell structure with low nickel content are: the pH value is more than or equal to 10.5 and less than 11.5,0.5 and more than or equal to C (OH ^-/NH₃) and more than 0.3, wherein the content of Ni in the outer shell structure with low nickel content is more than 80w/w% and more than or equal to 40w/w%.

3. The method of claim 1, wherein the lithium source is one or more of lithium carbonate, lithium hydroxide, lithium fluoride, lithium nitrate, lithium acetate, and lithium oxalate.

4. The method of claim 1, wherein the high temperature solid phase reaction is performed at 750-900 ℃ for 2-16 hours.

5. The method according to claim 4, wherein the conditions of the high temperature solid phase reaction are 780-850 ℃ for calcination for 6-10 hours.

6. The method of claim 1, wherein the high temperature solid phase reaction has a ramp rate of 2 to 6 ℃/min.

7. A positive electrode material of gradient structure prepared by the method of any one of claims 1 to 6.

8. A lithium ion battery comprising the gradient structure positive electrode material of claim 7.