A positive electrode active material, a method for preparing the same, and a lithium ion battery including the same

Abstract

The application provides a positive electrode active material which is a particle formed by a core layer and a coating layer coating the core layer, wherein a superlattice structure gradually increasing in content from the center to the edge is contained. The application also provides a preparation method of the positive electrode active material and a positive electrode, a lithium ion battery prepared therefrom. The positive electrode active material provided by the application contains a superlattice structure in a 'gradient distribution' in content from the center to the edge and a coating layer containing elements such as P and S, can significantly improve the structural stability of the positive electrode material, and improve the interface stability of the positive electrode material and the electrolyte. The lithium ion battery prepared by using the positive electrode active material provided by the application can greatly increase the service life of the battery cell, significantly reduce the gas production rate, and obviously improve the overall performance of the battery, thereby further expanding the application field of the lithium ion battery. The positive electrode active material provided by the application has a simple preparation process, easy operation and control, and does not require high cost.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion batteries, specifically to a positive electrode active material, its preparation method, and the resulting positive electrode and lithium-ion battery. Background Technology

[0002] Since Sony used lithium cobalt oxide as the cathode material to make the first lithium-ion battery in the 1990s, lithium-ion batteries have been widely used in consumer electronics, pure electric vehicles, hybrid vehicles, smart grids and other fields as the most promising energy storage devices.

[0003] Lithium cobalt oxide, a key raw material limiting the energy density and cycle life of lithium-ion batteries, still has significant room for improvement in structural stability. Lithium manganese oxide, with its spinel structure, is more stable and possesses natural advantages such as a high discharge plateau, environmental friendliness, high safety, and abundant reserves, making it widely used in the 3C consumer electronics sector. Lithium nickel oxide has a low Fermi level binding energy with oxygen, resulting in high discharge capacity and a high discharge plateau, perfectly meeting the demands for high energy density; however, its highly unstable structure hinders its practical application.

[0004] Combining the advantages of the three materials mentioned above, NCM ternary cathode materials were developed. Based on the molar ratio of the three elements, they can be classified into types such as 333, 523, 622, and 811. All types possess an α-NaFeO2 structure. With increasing nickel content, the capacity of this material can be increased to 240 mAh / g or higher at the same cutoff voltage. Ternary cathode materials are characterized by low cost, ease of preparation, environmental friendliness, and high energy density, meeting the demand for high energy density. However, with the decrease in lithium content during charging, the expansion of the a / b axis and the expansion and collapse of the c-axis of the crystal lead to a phase transition of H1-M-H2-H3 in the crystal lattice structure. The drastic change in the c-axis inevitably causes deformation of the crystal volume. The expansion and contraction of multiple crystal lattices cause grain expansion and contraction problems, leading to cracks on the secondary sphere surface and even grain disintegration, further deteriorating the structural stability of the cathode material and reducing battery safety and lifespan.

[0005] Chinese patent CN 113611849A discloses a positive electrode active material with a superlattice structure and its preparation method. Although the superlattice structure is beneficial for the stability of layered structures, the non-gradient distribution of the superlattice structure cannot effectively release the crystal stress caused by phase transitions during charging and discharging. In addition, the material preparation process of this patent uses a gelatin solution containing colloids and sugars, which can cause incomplete decomposition of the gelatin during sintering, resulting in carbon residue, which is detrimental to the performance of the positive electrode material. Surface coating of the positive electrode material (e.g., Chinese patents CN 103840148A, CN 107895793B, CN112678878A) can reduce the ion transport resistance between the material surface and the electrolyte, but the coating material is easily dissolved into the electrolyte, causing the surface protective layer to disappear.

[0006] Therefore, it is evident that there is an urgent need to develop a positive electrode active material with excellent electrochemical performance and better safety, which is of great significance for the development and application of lithium-ion batteries. Summary of the Invention

[0007] To overcome the aforementioned deficiencies in the prior art, one objective of this invention is to provide a positive electrode active material that exhibits good structural stability and interfacial stability between the material and the electrolyte. Lithium-ion batteries prepared using this material can significantly improve cell lifespan and reduce gas generation, resulting in excellent battery performance.

[0008] Another object of the present invention is to provide a method for preparing the positive electrode active material.

[0009] Another object of the present invention is to provide a positive electrode made from the said positive electrode active material and a lithium-ion battery.

[0010] The positive electrode active material provided by the present invention is a particle formed by a core layer and a coating layer covering the core layer. The core layer is a multi-element positive electrode material doped with element D, wherein the element D is selected from one or more of elements Y, Ti, Ce, La, Mg, Al, W, Mo, Ge, Nb, Ba, Sb, and Ta, and the element D accounts for 0.1% to 5% of the positive electrode active material by weight. The coating layer is formed of lithium salt, wherein the lithium salt contains one or more of non-metallic elements from Group VA, Group VIIA, and Group VIIA excluding O. The particle contains a superlattice structure with a content that gradually increases from the center to the edge.

[0011] The first characteristic of the positive electrode active material provided by this invention is that it contains a superlattice structure with a content that gradually increases from the center to the edge, such as... Figure 1As shown in the schematic diagram, the multi-element cathode material is doped with D element, so the content distribution of D element can represent the distribution of the superlattice structure. Due to the difference in solid-phase diffusion of D element, the content of the superlattice structure shows a "gradient distribution" from the center to the edge. The cathode active material of the present invention has a higher concentration of superlattice structure at the edge, thereby forming a superlattice protective layer. The second feature of the cathode active material provided by the present invention is that it includes a coating layer containing non-metallic elements such as P, S, and F (hereinafter referred to as A element).

[0012] Through the synergistic effect of the superlattice protective layer and the coating layer, the positive electrode active material of this invention can significantly improve the structural stability of the positive electrode material. This alleviates the grain boundary stress concentration caused by different discharge depths at different locations within the positive electrode material, effectively solving problems such as volume shrinkage, interfacial impedance, and even particle pulverization and cracking. Using the positive electrode active material of this invention to prepare lithium-ion batteries can significantly extend battery life and improve the interfacial stability between the positive electrode and the electrolyte, thereby reducing side reactions such as electrolyte oxidation and decomposition, thus lowering the battery's gas production rate and reducing electrolyte consumption.

[0013] In some embodiments of the positive electrode active material according to the present invention, the D element, by weight, may be 0.1% to 5% of the positive electrode active material, including but not limited to weight percentages of about 0.1%, about 0.5%, about 1.0%, about 1.5%, about 2.0%, about 2.5%, about 3.0%, about 3.5%, about 4.0%, about 4.5%, about 5.0%, or any combination thereof. In some preferred embodiments, the D element, by weight, may be 0.4% to 4% of the positive electrode active material.

[0014] In some embodiments of the cathode active material according to the present invention, the multi-component cathode material can be a binary or ternary cathode material commonly used in the field of lithium-ion batteries, including but not limited to NCM, NCA, NM, NC, etc. In some preferred embodiments, the multi-component cathode material is selected from NCM.

[0015] In some embodiments of the positive electrode active material according to the present invention, the lithium salt may be selected from one or more of lithium phosphate, lithium sulfate, lithium fluoride, lithium iodide, lithium arsenate, lithium bismuthate, and lithium antimonyate. In some preferred embodiments, the lithium salt may be selected from lithium phosphate or lithium sulfate.

[0016] In some embodiments of the positive electrode active material according to the present invention, the D element may be selected from one or more of the elements W, Mo, Sb, and Ta, for example, one or two of them.

[0017] In some embodiments of the positive electrode active material according to the present invention, the molar ratio of the D-doped NCM to the lithium salt can be 0.95–0.999:0.05–0.001, including but not limited to molar ratios of about 0.95:0.05, about 0.96:0.04, about 0.97:0.03, about 0.98:0.02, about 0.985:0.015, about 0.99:0.01, about 0.995:0.005, about 0.999:0.001, or any combination thereof. In some preferred embodiments, the molar ratio of the D-doped NCM to the lithium salt can be 0.98–0.99:0.01–0.02.

[0018] In some embodiments of the positive electrode active material according to the present invention, the average particle size of the positive electrode active material can be 8 to 15 μm, including but not limited to particle size ranges of about 8 μm, about 9 μm, about 10 μm, about 11 μm, about 12 μm, about 13 μm, about 14 μm, about 15 μm, or any combination thereof. In some preferred embodiments, the average particle size of the positive electrode active material can be 8 to 13.5 μm.

[0019] In some embodiments of the positive electrode active material according to the present invention, the superlattice structure in the positive electrode active material has one or more of the following characteristics:

[0020] 1) The superlattice structure can be represented as (a×b)n×c, where n represents 2, 3 or 4.

[0021] 2) The distribution of the superlattice structure conforms to the following gradient distribution law:

[0022]

[0023] Where S represents the D element content at different positions in the particle, r represents the position radius of the superlattice structure in the particle, and R represents the average radius of the particle; D i This represents the solid-phase diffusion coefficient of element D.

[0024] 3) The superlattice structure follows the hexagonal close-packed α-NaFeO2 structure. The introduction of D element to form a superlattice does not cause any change in the main layered structure of the material.

[0025] The present invention also provides a method for preparing the positive electrode active material according to any one of the above technical solutions, comprising the following steps:

[0026] S1: The precursor of the multi-element cathode material is mixed with a salt containing element D in water or a solvent, the water or solvent is evaporated, and then mixed with the lithium source material. A first calcination is then performed to obtain the cathode active material intermediate.

[0027] S2: The positive electrode active material intermediate is mixed with the lithium salt that forms the coating layer, and the positive electrode active material is obtained by a second calcination.

[0028] In some embodiments of the preparation method according to the present invention, in step S1, the precursor of the multi-element cathode material can be any one of the aforementioned multi-element cathode materials NCM, NCA, NM, and NC. In some preferred embodiments, the precursor of the multi-element cathode material can be an NCM precursor with Ni ≥ 90 wt%, including but not limited to N... 0.9 C 0.05 M 0.05 (OH)2.

[0029] In some embodiments of the preparation method according to the present invention, the salt containing element D may be selected from one or more of acetate, nitrate, and ammonium salts containing element D, such as antimony acetate, ammonium molybdate, thallium nitrate, tungsten nitrate, etc.

[0030] In some embodiments of the preparation method according to the present invention, the solvent may be selected from one or more of methanol, ethanol, and acetone.

[0031] In some embodiments of the preparation method according to the present invention, the weight of the water or solvent may be 5 to 20% of the total weight of the precursor of the multi-element cathode material and the salt containing element D, including but not limited to a weight percentage range of about 5%, about 10%, about 15%, about 20%, or any combination thereof.

[0032] In some embodiments of the preparation method according to the present invention, the lithium source material may be lithium hydroxide monohydrate. In some preferred embodiments, the molar ratio of lithium hydroxide monohydrate to the precursor may be 1:1 to 2, for example, 1:1 to 1.5.

[0033] In some embodiments of the preparation method according to the present invention, the first calcination can be carried out in an air or oxygen atmosphere, at a calcination temperature of 600–900°C, and for a calcination time of 8–28 h. In some preferred embodiments, the calcination temperature is 700–800°C, and the calcination time is 8–18 h. In some more preferred embodiments, the calcination temperature is 750–800°C, and the calcination time is 10–15 h.

[0034] In some embodiments of the preparation method according to the present invention, the second calcination can be carried out in an air or oxygen atmosphere, at a calcination temperature of 300–800°C, and for a calcination time of 5–20 h. In some preferred embodiments, the calcination temperature is 400–500°C, and the calcination time is 5–12 h.

[0035] The present invention also provides a positive electrode for a lithium-ion battery, which is prepared using the positive electrode active material described in any of the above technical solutions.

[0036] The present invention also provides a lithium-ion battery, which is assembled from a positive electrode, a negative electrode, a separator and an electrolyte, wherein the positive electrode is the positive electrode described in any of the above technical solutions.

[0037] In some embodiments of the lithium-ion battery according to the present invention, the negative electrode may be made of graphite or graphite doped with 0.5 to 2 wt% silicon. For example, silicon suboxide may be used as the silicon source in the negative electrode.

[0038] In some embodiments of the lithium-ion battery according to the present invention, the separator may be a ceramic-coated separator.

[0039] In some embodiments of the lithium-ion battery according to the present invention, the electrolyte can be a carbonate-based electrolyte commonly used in the art. In some preferred embodiments, the electrolyte can be an electrolyte formed from ethylene carbonate and ethyl methyl carbonate in a mass ratio of 20–40:80–60.

[0040] In some embodiments of the lithium-ion battery according to the present invention, after the lithium-ion battery is assembled, it undergoes overcharge formation treatment at 40–80°C. This high-temperature overcharge formation treatment is beneficial for activating the superlattice structure, stabilizing the SEI film, and mitigating the deterioration of the acidic electrolyte environment, thereby extending battery life and reducing cell gas generation. In some preferred embodiments, the overcharge formation treatment is performed at 60–80°C. In some more preferred embodiments, for a 5Ah lithium-ion battery, activation can be achieved by overcharging to 4.3–4.8V.

[0041] The positive electrode active material provided by this invention comprises a superlattice structure with a gradient content distribution from the center to the edge, and a coating layer containing elements such as P and S. Through the combined effect of the superlattice protective layer and the coating layer, the structural stability of the positive electrode material and the interfacial stability between the positive electrode material and the electrolyte can be significantly improved. Lithium-ion batteries prepared using the positive electrode active material provided by this invention exhibit significantly increased cell lifespan, significantly reduced gas production rate, and markedly improved overall battery performance, thereby further expanding the application fields of lithium-ion batteries. The preparation process of the positive electrode active material provided by this invention is simple, easy to control, and requires no high cost.

[0042] In summary, the positive electrode active material provided by this invention has excellent performance, is easy to prepare, is suitable for large-scale production, and has significant economic and social value, thus showing great application potential. Attached Figure Description

[0043] Figure 1This is a schematic diagram of the superlattice structure distribution of the positive electrode active material described in this invention;

[0044] Figure 2a and Figure 2b The images shown are FIB-SEM and FIB-EDS spectra of the positive electrode active material prepared in Example 1.

[0045] Figure 3a and Figure 3b The images shown are FIB-SEM and FIB-EDS spectra of the positive electrode active material prepared in Example 1.

[0046] Figure 4a and Figure 4b HRTEM and SEAD images of different locations of the positive electrode active material prepared in Example 1 are shown respectively.

[0047] Figure 5 The graph shows a comparison of the full-cell cycle performance at 45°C for Examples 1-3 and Comparative Examples 1-2. Detailed Implementation

[0048] The technical solution of the present invention will be further described in detail below with reference to specific embodiments.

[0049] Unless otherwise specified, all raw materials or reagents used in the embodiments and comparative examples of this invention are commercially available products.

[0050] Unless otherwise specified, all percentages used in the embodiments and comparative examples of this invention are mass percentages.

[0051] Example 1

[0052] (1) Take 500g of N 0.9 C 0.05 M 0.05 The (OH)2 precursor (Huayou Cobalt, 9AM3 type Ni90 precursor) was poured into 100g of acetone solution containing antimony acetate (2.44g of Sb element). After being evaporated to dryness under reduced pressure, it was mixed with lithium hydroxide monohydrate at a molar ratio of 1.03:1 and calcined in a kiln at 800℃ for 12h to obtain the NCM cathode material intermediate, denoted as Sb-NCM.

[0053] (2) 98g of the cathode material intermediate and 1.1579g of Li3PO4 prepared in step (1) were added according to the molar ratio of Sb-NCM:Li3PO4=0.99:0.01. After mechanical mixing, the mixture was calcined at 500℃ for 10h, crushed and sieved to obtain a cathode active material containing a superlattice, denoted as Sb-NCM-PO10, with an average particle size of 9.5μm.

[0054] (3) A positive electrode slurry was prepared using the positive electrode active material, conductive agent Super P, and binder polyvinylidene fluoride (PVDF) at a mass ratio of 95:2:3. The positive electrode slurry was uniformly coated on the current collector aluminum foil at 90% of the negative electrode capacity, dried at 120°C, and then cold-pressed. After that, the slurry was trimmed, cut into pieces, and slit (for preparing 6.6×68×132 (H×D×L, mm) type soft-pack batteries). The slits were then dried under vacuum at 120°C for 2 hours to prepare the positive electrode of the lithium-ion battery.

[0055] (4) Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed uniformly at a mass ratio of EC:EMC = 30:70 to form the electrolyte. Graphite is used as the negative electrode active material, and the separator is a double-sided ceramic-coated separator. The battery positive electrode, negative electrode, separator, and electrolyte are assembled into a 5Ah soft-pack cell, which is then activated by overcharging to 4.4V under a constant temperature environment of 60℃.

[0056] Figure 2a , 2b The FIB-SEM and FIB-EDS characterization results of the positive electrode active material prepared in Example 1 are represented respectively, especially... Figure 2b The uniform distribution of Ni, Co, Mn, and O elements demonstrates the homogeneity of the material's main structure. Furthermore, Figure 3a , 3b The FIB-SEM and FIB-EDS characterization results demonstrate that the functional elements were successfully introduced into the cathode active material, and that D (Sb) was introduced into the ternary matrix material in a heterogeneous distribution. The D element exhibits a "valley" distribution with high concentration near the boundary and low concentration in the middle. The distribution of D element in different cross-sectional regions is shown in Table 2, and conforms to... The heterogeneous distribution pattern.

[0057] To further verify that the gradient distribution of doping elements causes structural distortion or the formation of a superlattice structure due to stacked dislocations, high-resolution transmission electron microscopy (TEM) and electron diffraction (SEAD) were used to characterize the boundary and bulk regions of the material, respectively. The results are as follows: Figure 4a and Figure 4b As shown. Figure 4aThe TEM results from different selected regions show that the layered structure and lithium-ion extraction channels in selected region 1 are perpendicular to the tangent direction of the crystal axis

[010] ; while the lithium-ion channels in selected region 2 are parallel to the normal direction of the crystal axis. This indicates that the introduction of dopants leads to crystal distortion or dislocations, resulting in superlattice structures with different orientations. To further explore the characteristics of the superlattice, SEAD characterization was performed on selected region 1. Based on the stacking arrangement of the superlattice structure in this characterization, the superlattice structure was divided into (a×b)n×c structures, where n = 2, 3, and 4 positive integers. The superlattice is periodically distributed at the interface and in the bulk phase of the material. The SEAD characterization of selected region 1 is as follows: Figure 4b As shown, the lattice parameters in the spectrum clearly exhibit a 3a×3b×c stacking structure, and the existence of this structure further proves the existence of superlattice materials.

[0058] Compare the TEM structure differences between selected region 1 and selected region 2, and combine Figure 2a and Figure 2b The gradient distribution phenomenon indirectly confirms that the superlattice structure was formed by the introduction of the dopant element Sb. The above characterization results corroborate each other, confirming that the introduction of high-valence D elements induced the bond energy difference between metal-oxygen bonds in the ternary material, causing partial lattice distortion, which further led to the lattice reconstruction of some layered structures, forming a superlattice structure.

[0059] Combining SEM-EDS characterization and elemental content analysis results in Table 2, the concentration distribution of dopant element D was determined to be as follows: If there is a pattern, then the superlattice structure will also exhibit a similar distribution pattern.

[0060] Figure 4a The ultrawide lattice fringes in selected region 1 confirm that the superlattice still exists in a layered structure, and its superlattice structure is as follows: Figure 4b As shown, in addition, Figure 4a The distinct ordered layered diffraction fringes in the selected area 2 confirm that the main structure of the material remains layered. The introduction of D element to form a superlattice did not cause any change in the main layered structure of the material, which still follows the hexagonal close-packed α-NaFeO2 structure.

[0061] Example 2

[0062] (1) Take 500g of N 0.9 C 0.05 M 0.05 The (OH)2 precursor was poured into a 70g aqueous solution containing ammonium molybdate (5.20g of Mo element). After being evaporated to dryness under reduced pressure, it was mixed with lithium hydroxide monohydrate at a molar ratio of 1.50:1 and calcined in a kiln at 750℃ for 12h to obtain the NCM cathode material intermediate, denoted as Mo-NCM.

[0063] (2) 98g of the cathode material and 1.65g of Li2SO4 prepared in step (1) were added according to the molar ratio of Mo-NCM:Li2SO4=0.985:0.015. After mechanical mixing, the mixture was calcined at 400℃ for 10h, crushed and sieved to obtain the cathode active material containing superlattice, denoted as Mo-NCM-P015, with an average particle size of 8μm.

[0064] (3) A positive electrode slurry was prepared using the positive electrode active material, conductive agent Super P, and binder PVDF at a mass ratio of 95:2:3. The positive electrode slurry was uniformly coated on the current collector aluminum foil at 90% of the negative electrode capacity, dried at 120°C, and then cold-pressed. The slurry was then trimmed, cut into pieces, and slit according to Example 1, and dried under vacuum at 120°C for 2 hours to produce a lithium-ion battery positive electrode.

[0065] (4) EC and EMC are mixed evenly at a mass ratio of EC:EMC = 30:70 to form the electrolyte. Graphite containing 1% silicon is used as the negative electrode active material, and the separator is a double-sided ceramic coated separator. The positive electrode, negative electrode, separator, and electrolyte are assembled into a 5Ah soft-pack cell and activated by overcharging to 4.5V in a constant temperature environment of 60℃.

[0066] Example 3

[0067] (1) Take 500g of N 0.9 C 0.05 M 0.05 The (OH)2 precursor was poured into a 60g aqueous solution containing Ta and W nitrates (Ta element weight is 4.53g, W element weight is 14.90g), evaporated under reduced pressure, and then mixed with lithium hydroxide monohydrate at a molar ratio of 1.06:1. The mixture was then calcined in a kiln at 790℃ for 12h to obtain the NCM cathode material intermediate, denoted as Ta / W-NCM.

[0068] (2) 98g of the cathode material intermediate and 2.21g of Li2SO4 prepared in step (1) were added according to the molar ratio of Ta / W-NCM:Li2SO4=0.98:0.02. After mechanical mixing, the mixture was calcined at 500℃ for 10h, crushed and sieved to obtain a cathode active material containing a superlattice, denoted as Ta / W-NCM-PO20, with an average particle size of 13.5μm.

[0069] (3) A positive electrode slurry was prepared using the positive electrode active material, conductive agent Super P, and binder PVDF at a mass ratio of 95:2:3. The positive electrode slurry was uniformly coated on the current collector aluminum foil at 90% of the negative electrode capacity, dried at 120°C, and then cold-pressed. The slurry was then trimmed, cut into pieces, and slit according to Example 1, and dried under vacuum at 120°C for 2 hours to produce a lithium-ion battery positive electrode.

[0070] (4) EC and EMC are mixed evenly at a mass ratio of EC:EMC = 30:70 to form the electrolyte. Graphite containing 0.5% silicon is used as the negative electrode active material, and the separator is a double-sided ceramic coated separator. The battery positive electrode, negative electrode, separator, and electrolyte are assembled into a 5Ah soft-pack cell and activated by overcharging to 4.3V in a constant temperature environment of 60℃.

[0071] Comparative Example 1

[0072] Except for the process without step (2), a 5Ah soft-pack battery cell was obtained and activated in the same manner as in Example 1.

[0073] Comparative Example 2

[0074] Except for using 1.433g ZrO2 instead of Sb in the acetate, a 5Ah pouch cell was obtained and activated in the same manner as in Example 1.

[0075] The elemental analysis results of the positive electrode active materials prepared in Examples 1-3 and Comparative Examples 1-2 are shown in Table 1.

[0076] Table 1. ICP-OES analysis of element content (wt%) in positive electrode active materials.

[0077]

[0078] Table 2 shows the content distribution of D and A elements at different positions in the particles of the positive electrode active materials prepared in Examples 1-3 and Comparative Examples 1-2.

[0079] Table 2 shows the D / A element distribution at different locations detected by EDS (expressed as a percentage of the total element count, in wt%).

[0080] serial number 0.01R 0.1R 0.3R 0.99R Example 1 - Sb / P 0.7 / 0.001 5.2 / 0.001 32.1 / 0.01 62.0 / 99.9 Example 2 - Mo / S 1.2 / 0.002 7.4 / 0.01 28.4 / 0.01 63.1 / 99.9 Example 3 - Ta / W / S 2.7 / 1.4 / 0.002 7.3 / 9.6 / 0.002 26.5 / 30.7 / 0.002 63.5 / 58.3 / 99.8 Comparative Example 1-Sb 0.7 5.2 32.1 62.0 Comparative Example 2-Zr / P 99.2 / 0.001 99.7 / 0.02 99.6 / 0.01 99.4 / 99.7

[0081] As shown in Table 2, for Examples 1-3, the distribution of D doped elements exhibits a gradient distribution, which indicates that the superlattice structure exhibits a gradient distribution, while A elements are mainly distributed on the surface of the material.

[0082] For Comparative Example 1, the distribution of Sb element content at different locations still shows a similar gradient distribution as in Example 1, but it does not contain a coating layer. For Comparative Example 2, the content of Zr element does not form a gradient distribution, indicating that the cathode material prepared in Comparative Example 2 does not form a gradient distribution superlattice structure, but it does contain a coating layer.

[0083] Test case

[0084] The battery cells prepared in Examples 1-3 and Comparative Examples 1-2 were subjected to a high-temperature gas generation test at 60°C and a 1C / 1C charge / discharge test at 45°C. The results are as follows: Figure 5 As shown in Table 3.

[0085] Table 3. Changes in volume and gas production rate after 90 days of storage at 60℃

[0086] Initial volume / mL Volume after storage / mL Gas production rate / % Example 1 35.0 40.0 14.3 Example 2 35.0 40.5 15.7 Example 3 35.0 39.8 13.7 Comparative Example 1 35.0 57.8 65.1 Comparative Example 2 35.0 47.8 36.6

[0087] The test results show that:

[0088] The cathode material prepared in Comparative Example 1 possesses a gradient-distributed superlattice structure, thus significantly extending the cycle life of the battery cell. However, the superlattice structure is continuously consumed during long-cycle operation, leading to a rapid decrease in capacity retention after 600 cycles. In terms of gas generation rate, it reached 65.1%, approximately five times that of Examples 1-3, and significantly higher.

[0089] The cathode material prepared in Comparative Example 2 lacked a gradient-distributed superlattice structure, resulting in a rapid decay of the cell capacity retention rate and significantly lower cycle performance compared to Examples 1-3 and Comparative Example 1. In terms of gas generation rate, it was lower than that of Comparative Example 1, but still significantly higher than that of Examples 1-3.

[0090] The battery cells prepared in Examples 1-3 retain over 93% of their capacity after more than 600 cycles, and the high-temperature gas generation rate can be reduced to about 15%, greatly improving the battery cell's service life and thermal safety. Therefore, the positive electrode active material provided by this invention introduces a gradient-distributed superlattice structure and a coating layer containing elements such as P and S for the first time. Battery cells prepared using this positive electrode active material can alleviate grain boundary stress during the charging and discharging process of the positive electrode material, enhance the stability of the battery cell, extend its service life, and also reduce the gas generation rate.

[0091] Unless otherwise specified, the terms used in this invention have the meanings commonly understood by those skilled in the art.

[0092] The embodiments described in this invention are for illustrative purposes only and are not intended to limit the scope of protection of this invention. Those skilled in the art can make various other substitutions, changes and improvements within the scope of this invention. Therefore, this invention is not limited to the above embodiments, but is only defined by the claims.

Claims

1. A positive electrode active material, characterized by, The positive electrode active material is a particle formed by a core layer and a coating layer coating the core layer, wherein the core layer is NCM doped with D elements selected from one or more of W, Mo, Sb, and Ta, and the D elements are 0.1-5% of the positive electrode active material by weight; the coating layer is formed by a lithium salt selected from lithium phosphate or lithium sulfate; the particle contains a superlattice structure with gradually increasing content from the center to the edge, the superlattice structure follows an α-NaFeO2 structure, and the distribution of the superlattice structure complies with the following gradient distribution rule: where S represents the D element content at different positions in the particle, r represents the position radius of the superlattice structure in the particle, and R represents the average radius of the particle; D i represents the solid-phase diffusion coefficient of the D element; The preparation of the positive electrode active material comprises the following steps: S1: uniformly mixing a precursor of NCM and a D-element-containing salt in water or a solvent, uniformly mixing the lithium source material after evaporating the water or the solvent, and obtaining a positive electrode active material intermediate through first calcination; and S2: uniformly mixing the positive electrode active material intermediate and a lithium salt forming the coating layer, and obtaining the positive electrode active material through second calcination. The precursor of the NCM is an NCM precursor with Ni≥90wt%.

2. The positive electrode active material according to claim 1, characterized by The molar ratio of the NCM doped with D elements to the lithium salt is 0.95-0.999:0.05-0.

001.

3. The positive electrode active material according to claim 2, characterized by The molar ratio of the NCM doped with D elements to the lithium salt is 0.98-0.99:0.01-0.

02.

4. The positive electrode active material according to claim 1, characterized by The average particle size of the positive electrode active material is 8-15μm.

5. The positive electrode active material according to any one of claims 1 to 4, characterized by, The superlattice structure is represented as (a×b)n×c, wherein n represents 2, 3, or 4.

6. The positive electrode active material according to claim 1, characterized by In the step S1, The D-element-containing salt is selected from one or more of D-element-containing acetate, nitrate, and ammonium salt; and / or The solvent is selected from one or more of methanol, ethanol, and acetone, and the weight of the water or the solvent is 5-20% of the total weight of the precursor of the NCM and the D-element-containing salt; and / or The lithium source material is lithium hydroxide monohydrate, and the molar ratio of the lithium source material to the precursor is 1:1-2.

7. The positive electrode active material according to claim 6, characterized by In the step S1, the lithium source material is lithium hydroxide monohydrate, and the molar ratio of the lithium source material to the precursor is 1:1-1.

5.

8. The positive electrode active material according to claim 1, characterized by The first calcination is performed in an air or oxygen atmosphere, the calcination temperature is 600-900℃, and the calcination time is 8-28h.

9. The positive electrode active material according to claim 8, characterized by The calcination temperature of the first calcination is 700-800℃, and the calcination time is 8-18h.

10. The positive electrode active material according to claim 1, characterized by The second calcination is performed in an air or oxygen atmosphere, the calcination temperature is 300-800℃, and the calcination time is 5-20h.

11. The positive electrode active material according to claim 10, characterized by The calcination temperature of the second calcination is 400-500℃, and the calcination time is 5-12h.

12. A positive electrode of a lithium-ion battery, characterized by comprising: The positive electrode is prepared using the positive electrode active material of any one of claims 1-11.

13. A lithium ion battery assembled from a positive electrode, a negative electrode, a separator and an electrolyte, characterized in that, The positive electrode is the positive electrode of claim 12.

14. The lithium-ion battery of claim 13, wherein, The material of the negative electrode is graphite or graphite doped with 0.5-2wt% of silicon; and / or The separator is a ceramic-coated separator; and / or The electrolyte is a carbonate electrolyte.

15. The lithium-ion battery of claim 14, wherein, The electrolyte is an electrolyte formed by vinyl carbonate and methyl ethyl carbonate in a mass ratio of 20-40:80-60.

16. The lithium-ion battery of claim 14 or 15, wherein, After the assembly of the lithium ion battery, overcharge formation treatment is performed at 40-80℃.

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