Cathode material and preparation method thereof

Abstract

The present invention relates to a cathode material and a method for preparing the same. The cathode material of the present invention is a secondary particle having a core and a shell, wherein the porosity of the core is greater than that of the shell. The present invention forms larger inter-particle gaps in the core of the secondary particle, reduces stress concentration generated by volume change during lithium ion intercalation and deintercalation, and improves the stability, rate capability, safety, and cycle life of the material, thereby being particularly suitable for high-energy-density lithium-ion batteries.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a cathode material in the form of secondary particles with a core and a shell and a preparation method thereof. BACKGROUND

[0002] With the rapid development of electric vehicles, the requirements for its endurance mileage, service life and safety performance are becoming more and more stringent. Correspondingly, the battery as a key component in electric vehicles also faces great challenges. The performance and safety of electric vehicle batteries are closely related to the cathode material, which needs to be studied in key areas.

[0003] Ternary materials, such as LiNi x Co y Mn 1-x-y O2, are one of the widely used cathode materials. With the increasing demand for battery energy density, the content of nickel element in ternary materials is increasing.

[0004] High-nickel cathode materials have the problems of fast capacity decay and poor safety. The main reasons include: (1) when the high-nickel cathode material deintercalates Li, there are multiple phase transitions, which produces internal stress and strain of the particles, leading to particle rupture, increase of material decay points and structure collapse, etc.; (2) uneven distribution of lithium ions and Li / Ni mixing in the cathode material affect the lithium ion transport channel, thereby affecting the cycle life and safety of the material; (3) in the charged state of the high-nickel cathode material, the surface Ni 4+ oxidizes the electrolyte and produces gas, which has a great impact on the safety of the battery.

[0005] Various methods have been proposed in the prior art to improve the internal crystal structure and surface interface stability of high-nickel cathode materials, thereby improving the rate performance, cycle performance and safety performance of high-nickel cathode materials, including, for example: surface coating, bulk doping, and controlling particle size, etc.

[0006] Chinese patent CN108269970B prepared a cathode active material with gradient doping type, which has good surface interface stability and can balance high capacity and long cycle performance of the material. Chinese patent CN110422889B prepared a gradient-doped cathode active material, which has high discharge capacity and good cycle performance.

[0007] The electrical performance has been improved by modifying the composition of the cathode material in the prior art. However, there is still a need for alternative solutions to modify the cathode material to further improve its performance. SUMMARY

[0008] To meet the above needs, the present application provides a cathode material in the form of secondary particles having a core and a shell, and a method for preparing the same, wherein the cathode material is configured such that the porosity of the core is greater than the porosity of the shell. This configuration at least makes the interstices between primary particles in the core greater than the interstices between primary particles in the shell. The configuration of the present application can effectively improve the battery performance and safety of the cathode material, and thus is particularly suitable for high-energy-density lithium-ion batteries. Moreover, the preparation process of the cathode material is simple.

[0009] In one aspect, the present application provides a cathode material for a lithium-ion battery, which is a secondary particle having a core and a shell, wherein the porosity of the core is greater than the porosity of the shell.

[0010] In another aspect, the present application provides a method for preparing a cathode material for a lithium-ion battery, which is a secondary particle having a core and a shell, the method being characterized by comprising the following steps:

[0011] (1) performing first co-precipitation of a solution containing soluble salts of Ni, Co, M, and a dispersion containing D, to form a core of a secondary particle;

[0012] (2) in the presence of the core of step (1), performing second co-precipitation of a solution containing soluble salts of Ni, Co, M, and a dispersion containing G, to form a shell on the core, to obtain a secondary particle having a core and a shell;

[0013] wherein the pH value at the first co-precipitation is lower than the pH value at the second co-precipitation, and the temperature at the first co-precipitation is lower than the temperature at the second co-precipitation, and

[0014] wherein the porosity of the core is greater than the porosity of the shell; and

[0015] (3) converting the secondary particle obtained in step (2) into the cathode material, comprising the step of: obtaining the cathode material by contacting the secondary particle obtained in step (2) with a lithium source material at 300-900°C, wherein the lithium source material is one or more of Li2O, LiOH, LiOH•H2O, LiNO3, and Li2CO3,

[0016] wherein M is one or both of Mn and Al, D is one or more of Mo, Ti, Y, W, Ta, Nb, Cr, Sm, Sb, V, La, Ca, Hf, Zr, and G is one or more of Mo, Ta, Zn, Ti, Y, Zr, W, Nb, Cr, Sm, Al, V, La, Ca, Sb, Hf, Mg, B.

[0017] Compared with the prior art, the present application has the following beneficial effects:

[0018] (1) The porosity in the positive electrode material means that the material has a porous structure, which helps lithium ion transport and enhances the battery performance, such as rate capability.

[0019] (2) The porosity of the core is greater than that of the shell in the positive electrode material means that there is a different gap between the primary particles in the core and the shell, which reduces the possibility of cracks in the material due to stress and strain, achieving the purpose of stabilizing the material structure.

[0020] (3) The porosity of the core is greater than that of the shell in the positive electrode material also means that the shell is relatively dense, which enhances the structural stability of the material, and thus improves the cycle performance and safety of the material.

[0021] (4) According to the preferred scheme of the present application, the pore volume of part of the pores in the positive electrode material increases with increasing pressure under pressure. This can be due to the pressure causing the closed pores in the positive electrode material to be opened, thereby exposing new open pores. This means that the positive electrode material of the present application can alleviate the problems caused by the collapse of the positive electrode material due to the pressure generated during the operation of the battery, thereby improving the cycle performance of the battery. BRIEF DESCRIPTION OF DRAWINGS

[0022] The accompanying drawings are included to provide a further understanding of the present application, and constitute a part of the specification, and are used together with the following detailed description to explain the present application, but do not constitute a limitation of the present application. In the drawings:

[0023] Figure 1 SEM image of the positive electrode material of Example 1.

[0024] Figure 2 Partial cross-sectional SEM image of the positive electrode material of Example 1.

[0025] Figure 3 Partial cross-sectional SEM image of the positive electrode material of Example 1.

[0026] Figure 4 Cycle performance graph of button cells prepared using the positive electrode materials from Example 1, Comparative Example 1 and Comparative Example 2. DETAILED DESCRIPTION

[0027] Except in the Examples, all numerical values of parameters in this text should be understood, in all instances, to be modified in all cases by the term "about" whether or not "about" actually appears before the value.

[0028] In one embodiment, the present application relates to a positive electrode material for a lithium ion battery, which is a secondary particle having a core and a shell, wherein the porosity of the core is greater than that of the shell.

[0029] The term "secondary particle" as used herein refers to a particle composed of a plurality of primary particles agglomerated together. In one aspect, the secondary particles of the present application have a spherical or near-spherical shape, such as an ellipsoidal shape. In one variant, the secondary particles of the present application have a size in the micrometer range, such as 1-100 micrometers.

[0030] Typically, the nickel-containing positive electrode material is in the form of flakes, which are packed into primary particles. The primary particles have an elongated shape with a maximum dimension size of 300-500 nm.

[0031] The primary particles themselves can be porous. Also, the secondary particles composed of the primary particles can be porous. There are gaps between the primary particles that make up the secondary particles. As a whole, the positive electrode material is therefore porous. The porous structure includes macropores (50-500 nm in diameter), mesopores (20-50 nm), and micropores (2-20 nm). The porosity of the positive electrode material of the present application is mainly attributed to the macropores. These macropores are mainly the gaps between the primary particles.

[0032] The porous structure of the positive electrode material of the present application can be defined by "porosity". In the present application, porosity = (1 - p1 / p2) * 100%, where p1 is the tap density and p2 is the true density. The tap density refers to the density measurement of the positive electrode material when the gaps (or macropores) between the primary particles in the positive electrode material are filled by compression of the positive electrode material under an external force. The true density refers to the density measurement of the positive electrode material when all pores are excluded by completely compacting the positive electrode material. The tap density and the true density can be directly measured by a tap density meter and a true density meter, respectively.

[0033] In the present application, the positive electrode material is uniformly pressed for 30 min to 20 kN during the tap density test.

[0034] In one variant, the porosity of the core is 50-90%, preferably 60-80%, and the porosity of the shell is 10-50%, preferably 20-40%. Preferably, the ratio of the porosity of the core to the porosity of the shell is greater than 1.2, preferably greater than 1.5, such as 1.5-5.0, preferably 2.0-2.5.

[0035] In one variant, the core comprises macropores with a pore size of 50-500 nm, and the porosity of the macropores of the core is 60-80%, preferably 65-75%; and the shell comprises macropores with a pore size of 50-500 nm, and the porosity of the macropores of the shell is 20-40%, preferably 25-35%.

[0036] In one variant, the pore volume of the positive electrode material increases as the pressure applied to the positive electrode material increases. Preferably, the pore volume of the pores with a pore size of 50-110 nm in the positive electrode material is denoted as PVi where i = 0, 1, 2, 3, and 4, respectively, correspond to the pore volume of the pores when subjected to 0 ton, 1.5 ton, 2.5 ton, 3.5 ton and 4.5 ton pressure applied on the positive electrode material, wherein the pore volume under pressure satisfies the following formula: 1 < PV1 / PV0 < 1.5, 1 < PV2 / PV0 < 4.0, 1 < PV3 / PV0 < 6.0, and 1 < PV4 / PV0 < 8, preferably 1 < PV1 / PV0 < 1.4, 1 < PV2 / PV0 < 3.5, 1 < PV3 / PV0 < 5.5, and 1 < PV4 / PV0 < 7.5.

[0037] The present application does not particularly limit the composition of the positive electrode material. It can be a doped type of nickel-containing positive electrode material commonly used in the art. In one variant, the average composition of the positive electrode material is as follows: Li 0.6+δ [(Ni x1 Co y1 M 1-x1-y1 D η1 ) z ·(Ni x2 Co y2 M 1-x2-y2 G η2 ) 1-z ]O2, 0≤δ≤0.6, 0.6≤x1≤1, 0.6≤x2≤1, 0≤y1≤0.4, 0≤y2≤0.4, 0<z<1, 0≤η1≤0.1, 0≤η2≤0.1, M is one or both of Mn and Al, D is one or more of Mo, Ti, Y, W, Ta, Nb, Cr, Sm, Sb, V, La, Ca, Hf, Zr, and G is one or more of Mo, Ta, Zn, Ti, Y, Zr, W, Nb, Cr, Sm, Al, V, La, Ca, Sb, Hf, Mg, B. Preferably, in the positive electrode material, 0.7≤x1<1, 0.7≤x2<1, 0<y1≤0.3, 0<y2≤0.3, 0<η1≤0.05, 0<η2≤0.05, 0.4≤z≤0.5, and the value of 1-x1-y1-η1 is not 0, and the value of 1-x2-y2-η2 is not 0.

[0038] In one variant, D and G are distributed in concentration gradient in the core and shell of the positive electrode material, respectively. Preferably, in the positive electrode material, the concentration of D gradually decreases along the radial direction from the core center to the core surface, and the concentration of G gradually increases along the radial direction from the core surface to the shell surface. The concentrations of nickel, cobalt and M (i.e. manganese and / or aluminum) in the positive electrode material can be distributed in gradient, respectively. For example, the concentration of manganese gradually increases along the radial direction from the core center to the core surface, and the concentration of nickel gradually decreases along the radial direction from the core surface to the shell surface.

[0039] In yet another embodiment, the present application provides a method for preparing a cathode material for a lithium ion battery, the cathode material being a secondary particle having a core and a shell, the method being characterized by comprising the steps of:

[0040] (1) performing a first co-precipitation of a solution containing soluble salts of Ni, Co, M with a dispersion containing D to form a core of a secondary particle;

[0041] (2) performing a second co-precipitation of a solution containing soluble salts of Ni, Co, M with a dispersion containing G in the presence of the core of step (1) to form a shell on the core to obtain a secondary particle having a core and a shell;

[0042] wherein the pH value at the first co-precipitation is lower than the pH value at the second co-precipitation, and the temperature at the first co-precipitation is lower than the temperature at the second co-precipitation, and

[0043] wherein the porosity of the core is greater than the porosity of the shell; and

[0044] (3) converting the secondary particle obtained in step (2) into the cathode material, comprising the step of obtaining the cathode material by contacting the secondary particle obtained in step (2) with a lithium source material at 300-1000 °C, wherein the lithium source material is one or more of Li2O, LiOH, LiOH H2O, LiNO3 and Li2CO3,

[0045] wherein M is one or both of Mn and Al, D is one or more of Mo, Ti, Y, W, Ta, Nb, Cr, Sm, Sb, V, La, Ca, Hf, Zr, and G is one or more of Mo, Ta, Zn, Ti, Y, Zr, W, Nb, Cr, Sm, Al, V, La, Ca, Sb, Hf, Mg, B.

[0046] In one variant, the step (1) comprises performing the first co-precipitation at a pH of 9-12.5, preferably 10-12; and the step (2) comprises performing the second co-precipitation at a pH of 10-13, preferably 11-13. Preferably, the pH value used for the first co-precipitation is 0.5-2, preferably 0.5-1 lower than the pH value used for the second co-precipitation. Preferably, the step (1) comprises performing the first co-precipitation at a temperature of 45-65 °C, preferably 50-60 °C; and the step (2) comprises performing the second co-precipitation at a temperature of 50-70 °C, preferably 55-65 °C.

[0047] In one variant, the step (1) comprises: performing the first co-precipitation in the presence of a complexing agent; and the step (2) comprises: performing the second co-precipitation in the presence of a complexing agent. Preferably, the concentration of the complexing agent used in the first co-precipitation is 5-12 mol / L, preferably 7-10 mol / L; and the concentration of the complexing agent used in the second co-precipitation is 0.1-5 mol / L, preferably 0.5-4.5 mol / L. Preferably, the concentration of the complexing agent used in the first co-precipitation is higher than the concentration of the complexing agent used in the second co-precipitation, for example, higher by 3.0-9.0 mol / L. The concentration of the complexing agent used in the first co-precipitation is the same as or different from the concentration of the complexing agent used in the second co-precipitation. The complexing agent is selected from one or more of ammonia, ammonium sulfate, ammonium chloride or ammonium nitrate.

[0048] In one variant, the soluble salt of nickel is selected from one or more of nickel sulfate, nickel chloride, nickel nitrate, nickel acetate; the soluble salt of cobalt is selected from one or more of cobalt sulfate, cobalt chloride, cobalt nitrate, cobalt acetate; when M is manganese, the soluble salt of manganese is selected from one or more of manganese sulfate, manganese chloride, manganese nitrate, manganese acetate; and when M is aluminum, the soluble salt of aluminum is selected from one or more of sodium metaaluminate, aluminum oxide sol.

[0049] In one variant, the dispersion of D can be a sol containing elemental D, or a solution containing a soluble salt of D, for example, a solution selected from one or more of a sulfate, a chloride and a nitrate of D.

[0050] In one variant, the dispersion of G can be a sol containing elemental G, or a solution containing a soluble salt of G, for example, a solution selected from one or more of a sulfate, a chloride and a nitrate of G.

[0051] In one variant, the concentration or flow rate of the dispersion of D in step (1) is controlled so that the concentration of D gradually decreases along the radial direction from the center of the core to the surface of the core; and the concentration or flow rate of the dispersion of G in step (2) is controlled so that the concentration of G gradually increases along the radial direction from the surface of the core to the surface of the shell. Preferably, in step (1), the dispersion of D is added at a gradually decreasing concentration; and in step (2), the dispersion of G is added at a gradually increasing concentration. Preferably, in step (1), the dispersion of D is added at a gradually decreasing flow rate; and in step (2), the dispersion of G is added at a gradually increasing flow rate.

[0052] In one variation, one or two of a precipitant, an antioxidant, and a processing aid are added during the first and second co-deposition processes. The precipitant is selected from one or more of sodium hydroxide, potassium hydroxide, and lithium hydroxide. The antioxidant is selected from one or more of ascorbic acid, sodium bisulfite, and tea polyphenols. The processing aid is selected from one or more of polyvinylpyrrolidone, polyvinyl alcohol, and sodium dodecyl sulfate. The amounts of the precipitant, antioxidant, and processing aid added are well known to those skilled in the art.

[0053] In one variation, the first and second co-depositions are performed under a protective atmosphere. The protective atmosphere is selected from one or more of nitrogen and inert gases, such as argon.

[0054] In one variation, step (3), which converts the secondary particles obtained in step (2) into the cathode material, comprises sintering the secondary particles obtained in step (2) with a lithium source material at 300-1000°C, preferably 500-900°C, for 1-36 hours, preferably 6-24 hours. Preferably, the lithium source material is one or more of Li₂O, LiOH, LiOH•H₂O, LiNO₃, and Li₂CO₃, preferably LiOH or LiOH•H₂O. Preferably, the sintering is carried out in an oxygen-containing atmosphere, such as in the presence of air or oxygen.

[0055] The positive electrode material of this invention can be used in secondary batteries, such as lithium-ion batteries. A lithium-ion battery includes a negative electrode, a positive electrode, a separator, and an electrolyte.

[0056] The positive electrode includes a positive electrode material layer on a positive electrode current collector, the positive electrode material layer comprising the positive electrode material of the present invention. The positive electrode material layer may also include an adhesive and a conductive agent. The adhesive may be selected from resin materials such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylonitrile (PAN), styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC). The conductive agent may include carbon-based materials or conductive polymers. Carbon-based materials may include, for example, graphite, acetylene black, carbon fiber, nanotubes, graphene, and carbon black. Conductive polymers may include polyaniline, polythiophene, polyacetylene, polypyrrole, etc. The current collector may include at least one of stainless steel, aluminum, nickel, iron, titanium, copper, tin, or any other conductive material known to those skilled in the art. In some variations, the current collector may be pre-coated, such as carbon-coated aluminum foil.

[0057] The present invention does not particularly limit the specific composition of the negative electrode, and it can be a negative electrode commonly used in the art, such as a lithium metal sheet.

[0058] The present invention does not particularly limit the specific composition of the diaphragm, and it can be any diaphragm conventionally used in the art. For example, the diaphragm is a porous membrane made of polyolefins such as polytetrafluoroethylene, polypropylene, or polyethylene.

[0059] The electrolyte can be various conventional electrolytes, for example, a nonaqueous electrolyte. The nonaqueous electrolyte is a solution of an electrolytic lithium salt in a nonaqueous solvent, and a conventional nonaqueous electrolyte known to those skilled in the art can be used. For example, the electrolyte can be selected from at least one of lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), and lithium hexafluorosilicate (LiSiF6). The nonaqueous solvent can be selected from at least one of a chain acid ester, a cyclic acid ester, or a mixture thereof. The chain acid ester can be at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), methylpropyl carbonate (MPC), and dipropyl carbonate (DPC). The cyclic acid ester can be at least one of ethylene carbonate (EC), propylene carbonate (PC), and vinylene carbonate (VC). Examples

[0060] The features and advantages of the present application will become apparent from the following examples. The examples are intended to describe the application and are not meant to limit the application in any way.

[0061] Test Methods

[0062] 1. Scanning Electron Microscope Test

[0063] The morphology of the sample was characterized by scanning electron microscope. Specifically, a Smartlab tester from Hitachi High-Technologies Corporation was used, and the test was performed using a magnification of 3000 and 10000 times.

[0064] 2. Compacted Density Test

[0065] The compacted density of the sample was characterized by a compacted density tester. Specifically, a MCP-PD51 tester from Mitsubishi Chemical Corporation, Japan was used, in which the sample was uniformly pressed for 30 min to 20 kN, and the test result was read from the test software.

[0066] 3. True Density Test

[0067] The true density of the sample was characterized by a true density tester. Specifically, an AccuPyc 1340 tester from Micromeritics Corporation, USA was used, and the test result was read from the test software.

[0068] 4. Pore Size and Pore Volume Test

[0069] Adsorption curves of the samples were obtained using a Tri-star 3020 surface area analyzer from Micromeritics, USA. The sample pretreatment conditions included: heating temperature 300 ℃, degassing time 120 min. The pore diameter of the sample was calculated using the BJH method, and the pore volume of the sample was calculated using the BET method.

[0070] 5. Change of pore volume with pressure

[0071] 3 g of the sample was taken, and 0 ton, 1.5 ton, 2.5 ton, 3.5 ton and 4.5 ton pressure was applied respectively to obtain the samples after pressure application. The above-mentioned pore diameter and pore volume test was repeated using the samples after pressure application, and the pore volume data of the pores with a pore diameter of 90±10 nm were obtained, which were PV0, PV1, PV2, PV3 and PV4 respectively. The ratios of PV1, PV2, PV3 and PV4 to PV0 were calculated respectively to represent the change of the pore volume of the pores with a pore diameter of 90±10 nm with pressure.

[0072] 6. Battery performance

[0073] 6.1 Preparation of the battery sample:

[0074] The positive electrode active material sample 9.2 g, acetylene black 0.4 g and polyvinylidene fluoride (PVDF) containing 0.4 g were mixed to form a positive electrode slurry. It was coated on an aluminum foil and subjected to a drying treatment, and was punched into a 12 mm diameter, 120 μm thick positive electrode sheet under a pressure of 100 MPa, and then was dried in a vacuum drying oven at 120 ℃ for 12 h to obtain a positive electrode sheet.

[0075] The negative electrode used a Li metal sheet with a diameter of 17 mm and a thickness of 1 mm; the separator used a polyethylene porous film with a thickness of 25 μm; and a 1.0 mol / L LiPF6 solution was used as the electrolyte, in which an equal amount of a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) was used as the solvent.

[0076] The positive electrode sheet, the separator, the negative electrode sheet and the electrolyte were assembled into a 2025 type button cell.

[0077] 6.2 Performance evaluation of the battery sample:

[0078] (1) Initial discharge specific capacity:

[0079] The prepared button cell sample was placed for 24 h. The battery sample was charged at a current density of 20 mA / g until the cut-off voltage of 4.3 V. The constant voltage charging was carried out at 4.3 V for 30 min. Subsequently, the battery sample was discharged at a current density of 20 mA / g until the cut-off voltage of 3.0 V, and the discharge time was recorded. The initial discharge specific capacity of the battery sample = current density * discharge time.

[0080] (2) Cycling performance:

[0081] The battery sample was charged and discharged twice at a current density of 20 mA / g, with a cut-off voltage of 3.0-4.3 V, and the activation was completed. Using the activated battery sample, the specified number of charge and discharge cycles, for example, 80 times, were performed at a temperature of 45 °C, at a current density of 1 C, and in a voltage range of 3.0-4.3 V. As described above, the specific discharge capacity of each charge and discharge cycle was obtained by the current density and the discharge time of each cycle. The cycling performance of the battery sample was characterized by the high-temperature capacity retention rate, wherein the high-temperature capacity retention rate = specific discharge capacity at the specified cycle / initial specific discharge capacity * 100%.

[0082] Comparative Example 1

[0083] Nickel sulfate, cobalt sulfate, and manganese sulfate were mixed in a molar ratio of 0.88:0.05:0.07 to form a 1.5 mol / L aqueous solution of transition metal salts. Nano-ZrO2 was mixed with water to form a 0.5 mol / L Zr dopant solution. The aqueous solution of transition metal salts, the Zr dopant solution, 6.0 mol / L NaOH, and 3.0 mol / L NH3·H2O were slowly added to a stirred reaction kettle. Co-precipitation was carried out at a pH of 12.0 and a temperature of 55 °C under a N2 atmosphere. The reaction was carried out for 70 hours to obtain secondary particles having a total particle size of 14.2 μm.

[0084] The secondary particles were mixed with lithium hydroxide corresponding to Li / (Ni+Co+Mn)=1.04, and sintered at 735 °C for 15 h in an oxygen atmosphere to obtain a comparative cathode material CC-1 having a composition of Li 1.04 Ni 0.88 Co 0.05 Mn 0.06 Zr 0.01 O2.

[0085] The performance of the comparative cathode material CC-1 was tested as described above. The test results showed that the comparative cathode material CC-1 did not have a core-shell structure, and the porosity of the comparative cathode material CC-1 was 65%. The change in pore volume of the comparative cathode material CC-1 with pressure is shown below:

[0086] The pore volume corresponding to a pore size of 90±10 nm, wherein PV1 / PV0=0.1, PV2 / PV0=0.3, PV3 / PV0=0.6, and PV4 / PV0=0.7.

[0087] The cycling performance of the button cell battery prepared from the comparative cathode material CC-1 of Comparative Example 1 is shown in Table 2. Figure 4 Table 2

[0088] Comparative Example 2

[0089] Nickel sulfate, cobalt sulfate and manganese sulfate were mixed in a molar ratio of 0.88:0.05:0.07 to form a 1.5 mol / L aqueous solution of transition metal salts. Nano-Zr02was mixed with water to form a 0.5 mol / L Zr dopant solution. The aqueous solution of transition metal salts, the Zr dopant solution, 6.0 mol / L NaOH and 3.0 mol / L NH3H20 were slowly added to a stirred reaction vessel. The first co-precipitation was carried out at a pH of 12.5, a temperature of 60 °C, under N2atmosphere. After the first co-precipitation was carried out for 40 hours, the second co-precipitation was carried out at a pH of 11.0, 8.0 mol / L NH3H20, a temperature of 55 °C, under N2atmosphere. After the second co-precipitation was carried out for 30 hours, secondary particles were obtained, which had a total particle size of 14.1 μm.

[0090] The secondary particles were mixed with lithium hydroxide corresponding to Li / (Ni+Co+Mn) = 1.04, and sintered at 735 °C for 15 h in an oxygen atmosphere to obtain comparative cathode material CC-2, which had a composition of Li 1.04 Ni 0.88 Co 0.05 Mn 0.06 Zr 0.01 O2.

[0091] The performance of comparative cathode material CC-2 was tested as described above. The test results showed that comparative cathode material CC-2 had a core-shell structure, in which the core had a radius size of 1.5 μm, as shown by the scanning electron microscope test results. The pore size, porosity and pore volume test results showed that the porosity of the shell was 71%, and the porosity of the core was 30%. The change of the pore volume of comparative cathode material CC-1 with pressure was as follows:

[0092] The pore volume corresponding to the pore size of 90 ± 10 nm, in which PV1 / PV0= 0.05, PV2 / PV0= 0.1, PV3 / PV0= 0.6, and PV4 / PV0= 1.233.

[0093] The cycle performance of the button cell prepared from comparative cathode material CC-2 of Comparative Example 2 was as shown in Figure 4 .

[0094] Comparative Example 3

[0095] Nickel sulfate, cobalt sulfate and manganese sulfate were mixed in a molar ratio of 0.97:0.02:0.01 to form a 1.5 mol / L aqueous transition metal salt solution (1). Nickel sulfate, cobalt sulfate and manganese sulfate were mixed in a molar ratio of 0.94:0.02:0.04 to form a 1.5 mol / L aqueous transition metal salt solution (2). Nano-Nb2O5 was mixed with water to form a 0.5 mol / L Nb dopant solution (3) and a 0.01 mol / L Nb dopant solution (4), respectively. The aqueous transition metal salt solution (2) was added to the aqueous transition metal salt solution (1) at a flow rate of 0.2 L / h to form a mixed aqueous transition metal salt solution, and the Nb dopant solution (4) was added to the Nb dopant solution (3) at a flow rate of 0.1 L / h to form a mixed Nb dopant solution. At the same time, the mixed aqueous transition metal salt solution, the mixed Nb dopant solution, 6.0 mol / L NaOH and 3.0 mol / L NH3-H2O were slowly added to a reaction kettle under stirring. The first co-precipitation was carried out at a pH of 12.0 and a temperature of 55°C under a N2 atmosphere. The first co-precipitation was carried out for 40 hours.

[0096] Nickel sulfate, cobalt sulfate and manganese sulfate were mixed in a molar ratio of 0.86:0.05:0.09 to form a 1.5 mol / L aqueous transition metal salt solution (5). Nano-ZrO2 was mixed with water to form a 0.01 mol / L Zr dopant solution (6) and a 1.0 mol / L Zr dopant solution (7), respectively. The Zr dopant solution (7) was added to the Zr dopant solution (6) at a flow rate of 0.1 L / h to form a mixed Zr dopant solution. At the same time, the aqueous transition metal salt solution (2), the mixed Zr dopant solution, 6.0 mol / L NaOH and 3.0 mol / L NH3-H2O were slowly added to the reaction kettle in which the first co-precipitation had just been completed under stirring. The second co-precipitation was carried out at a pH of 12 and a temperature of 55°C under a N2 atmosphere. After the second co-precipitation was carried out for 30 hours, the addition of the aqueous transition metal salt solution (2) was stopped, and after 0.5 hours of reaction, secondary particles were obtained, which had a particle size of 14.0 μm.

[0097] The secondary particles were mixed with lithium hydroxide corresponding to Li:(Ni+Co+Mn)=1.04, and sintered at 735°C for 15 h in an oxygen atmosphere to obtain a comparative cathode material CC-3, which had a composition of Li[(Ni 0.95 Co 0.02 Mn 0.02 Nb 0.01 ) 0.014 ·(Ni 0.875 Co 0.05 Mn 0.07 Zr 0.005 ) 0.986O2. Wherein, in the core of the comparative cathode material CC-3, the content of Nb decreases from the center of the core to the surface of the core, the molar ratio decreases from 0.02 mol% to 0, at the same time, the content of Mn increases from the center of the core to the surface of the core, the molar ratio increases from 0.01 mol% to 0.03 mol%, and the total amount of Nb and Mn elements is kept at 0.03 mol%. In the shell of the comparative cathode material CC-3, the content of Zr increases from the surface of the core to the surface of the shell, the molar ratio increases from 0 to 0.01 mol%, at the same time, the content of Ni decreases from the surface of the core to the surface of the shell, the molar ratio decreases from 0.91 mol% to 0.85 mol%, and the content of Mn increases from the surface of the core to the surface of the shell, the molar ratio increases from 0.04 mol% to 0.08 mol%.

[0098] The performance of the comparative cathode material CC-3 was tested as described above. The test results show that the comparative cathode material CC-3 has a core-shell structure, wherein the radius size of the core is 1.7 pm, as shown by the scanning electron microscope test results. The pore size, porosity and pore volume test results show that the porosity of the shell is 33%, and the porosity of the core is 33%. The change of the pore volume of the comparative cathode material CC-3 with pressure is shown as follows:

[0099] Corresponding to the pore volume with a pore size of 90±10 nm, wherein PV1 / PV0=0.1, PV2 / PV0=0.3, PV3 / PV0=0.6, and PV4 / PV0=0.8.

[0100] The capacity retention rate of the button cell prepared from the comparative cathode material CC-3 of Comparative Example 3 is 91.5% after 80 cycles.

[0101] Comparative Example 4

[0102] Comparative Example 3 was repeated, except that the first co-precipitation was carried out at a pH value of 12.5, 3.0 mol / L of NH3·H2O, a temperature of 60°C, under N2atmosphere; and the second co-precipitation was carried out at a pH value of 11.5, 8.0 mol / L of NH3·H2O, a temperature of 55°C, under N2atmosphere. The comparative cathode material CC-4 was obtained, which has the same chemical composition as the comparative cathode material CC-3.

[0103] The performance of the comparative cathode material CC-4 was tested as described above. The test results show that the comparative cathode material CC-4 has a core-shell structure, wherein the size of the core is 3.4 pm, as shown by the scanning electron microscope test results. The pore size, porosity and pore volume test results show that the porosity of the shell is 73%, and the porosity of the core is 32%. The change of the pore volume of the comparative cathode material CC-3 with pressure is shown as follows:

[0104] corresponding to a pore volume of 90 ± 10 nm in pore diameter, wherein PV1 / PV0 = 0.05, PV2 / PV0 = 0.1, PV3 / PV0 = 0.6, and PV4 / PV0 = 1.2.

[0105] The capacity retention rate after 80 cycles of the coin cell using the comparative positive electrode material CC-4 of Comparative Example 4 was 89.2%.

[0106] Example 1

[0107] Nickel sulfate, cobalt sulfate and manganese sulfate were mixed in a molar ratio of 0.97:0.02:0.01 to form a 1.5 mol / L aqueous transition metal salt solution (1). Nickel sulfate, cobalt sulfate and manganese sulfate were mixed in a molar ratio of 0.94:0.02:0.04 to form a 1.5 mol / L aqueous transition metal salt solution (2). Nano-Nb2O5 was mixed with water to form a 0.5 mol / L Nb dopant solution (3) and a 0.1 mol / L Nb dopant solution (4), respectively. The aqueous transition metal salt solution (2) was added to the aqueous transition metal salt solution (1) at a flow rate of 0.2 L / h to form a mixed aqueous transition metal salt solution, and the Nb dopant solution (4) was added to the Nb dopant solution (3) at a flow rate of 0.1 L / h to form a mixed Nb dopant solution. At the same time, the mixed aqueous transition metal salt solution, the mixed Nb dopant solution, 6.0 mol / L NaOH and 8.0 mol / L NH3·H2O were slowly added to a reaction kettle under stirring. The first co-precipitation was carried out at a pH of 11.0 and a temperature of 53°C under N2atmosphere. The first co-precipitation was carried out for 60 hours.

[0108] Nickel sulfate, cobalt sulfate and manganese sulfate were mixed in a molar ratio of 0.86:0.05:0.09 to form a 1.5 mol / L aqueous transition metal salt solution (5). Nano-ZrO2 was mixed with water to form a 0.01 mol / L Zr dopant solution (6) and a 1.0 mol / L Zr dopant solution (7), respectively. The Zr dopant solution (7) was added to the Zr dopant solution (6) at a flow rate of 0.1 L / h to form a mixed Zr dopant solution. At the same time, the aqueous transition metal salt solution (5), the mixed Zr dopant solution, 6.0 mol / L NaOH and 3.0 mol / L NH3·H2O were slowly added to the reaction kettle just after the first co-precipitation under stirring. The second co-precipitation was carried out at a pH of 11.5 and a temperature of 58°C under N2atmosphere. After the second co-precipitation was carried out for 40 hours, the addition of the aqueous transition metal salt solution (5) was stopped, and after 0.5 hours of reaction, secondary particles having a total particle size of 14.0 μm were obtained.

[0109] Corresponding to Li: (Ni+Co+Mn)=1.04, the secondary particles were mixed with lithium hydroxide and sintered in an oxygen atmosphere at 735 °C for 15 h to obtain a positive electrode material C-1 with a composition of Li [(Ni 0.95 Co 0.02 Mn 0.02 Nb 0.01 ) 0.014 ·(Ni 0.875 Co 0.05 Mn 0.07 Zr 0.005 ) 0.986 ]O2. Among the core of the positive electrode material C-1, the content of Nb continuously decreases from the center of the core to the surface of the core, with the molar ratio decreasing from 0.02 mol% to 0, while the content of Mn continuously increases from the center of the core to the surface of the core, with the molar ratio increasing from 0.01 mol% to 0.03 mol%, and the total amount of Nb and Mn elements remains 0.03 mol%. In the shell of the positive electrode material C-1, the content of Zr continuously increases from the surface of the core to the surface of the shell, with the molar ratio increasing from 0 to 0.01 mol%, while the content of Ni continuously decreases from the surface of the core to the surface of the shell, with the molar ratio decreasing from 0.91 mol% to 0.85 mol%, and the content of Mn continuously increases from the surface of the core to the surface of the shell, with the molar ratio increasing from 0.04 mol% to 0.08 mol%.

[0110] The performance of the positive electrode material C-1 was tested as described above.

[0111] The scanning electron microscope test results are shown in Figures 1-3 . As shown in Figures 1-3 , the positive electrode material C-1 has a spherical particle with a core-shell structure, wherein the radius size of the core is 1.7 pm, and the outer radius of the shell is 5.3 pm. The core and the shell have different pores.

[0112] The pore size, porosity and pore volume test results show that the porosity of the core is 70%, and the porosity of the shell is 32%. The change of the pore volume of the positive electrode material C-1 with pressure is as follows:

[0113] Corresponding to the pore volume with a pore size of 90±10 nm, wherein PV1 / PV0=0.6, PV2 / PV0=2.7, PV3 / PV0=3.5, and PV4 / PV0=4.9.

[0114] The cycle performance of the button cell prepared from the positive electrode material C-1 of Example 1 is shown in Figure 4 . As shown in Figure 4It can be seen that the positive electrode material C-1 of Example 1 obtains a higher capacity retention rate after the same number of cycles compared to the comparative positive electrode materials of Comparative Examples 1 and 2. The capacity retention rate of the button cell prepared using the positive electrode material C-1 of Example 1 is 93.9% after 80 cycles.

[0115] Example 2

[0116] Nickel sulfate, cobalt sulfate and manganese sulfate were mixed in a molar ratio of 0.89:0.055:0.055 to form a 1.5 mol / L aqueous solution of transition metal salts (1). Nano-Zr02was mixed with water to form a 0.5 mol / L Zr dopant solution (2). Nano-Nb205was mixed with water to form a 0.5 mol / L Nb dopant solution (3). The aqueous solution of transition metal salts (1), the Zr dopant solution (2), the Nb dopant solution (3), 6.0 mol / L NaOH and 8.0 mol / L NH3-H20 were slowly added to a stirred reaction kettle. The first co-precipitation was carried out at a pH of 11.5, a temperature of 54°C and under N2atmosphere. After the first co-precipitation was carried out for 60 hours, the second co-precipitation was carried out at a pH of 12.5, 2.5 mol / L NH3-H20, a temperature of 58°C and under N2atmosphere. After the second co-precipitation was carried out for 40 hours, secondary particles were obtained, which had a total particle size of 14.1 μm.

[0117] Corresponding to Li:(Ni+Co+Mn)=1.04, the secondary particles were mixed with lithium hydroxide and sintered at 735°C for 15 h in an oxygen atmosphere to obtain a positive electrode material C-1, which had a composition of Li 1.05 [(Ni 0.95 Co 0.05 Mn 0.015 Nb 0.015 ) 0.10 ·(Ni 0.905 Co 0.05 Mn 0.035 Zr 0.01 ) 0.90 ]02.

[0118] The performance of the positive electrode material C-2 was tested as described above.

[0119] The scanning electron microscope test results showed that the positive electrode material C-2 had a core-shell structure, in which the core had a radius size of 1.5 μm and the shell had an outer radius of 5.6 μm. The core and the shell had different pores.

[0120] The pore size, porosity and pore volume test results showed that the porosity of the core was 65% and the porosity of the shell was 30%. The change of the pore volume of the positive electrode material C-2 with pressure was as follows:

[0121] corresponding to a pore volume of 90 ± 10 nm in pore diameter, wherein PV1 / PV0 = 0.4, PV2 / PV0 = 1.8, PV3 / PV0 = 2.7, and PV4 / PV0 = 3.7.

[0122] The capacity retention rate after 80 cycles of the coin cell prepared using the positive electrode material C-2 of Example 2 was 93.1%.

[0123] Example 3

[0124] Nickel sulfate, cobalt sulfate and manganese sulfate were mixed in a molar ratio of 0.97:0.02:0.01 to form a 1.5 mol / L aqueous transition metal salt solution (1). Nickel sulfate, cobalt sulfate and manganese sulfate were mixed in a molar ratio of 0.94:0.02:0.04 to form a 1.5 mol / L aqueous transition metal salt solution (2). Nano-Nb2O5 was mixed with water to form a 2 mol / L Nb dopant solution (3) and a 0.05 mol / L Nb dopant solution (4), respectively. The aqueous transition metal salt solution (2) was added to the aqueous transition metal salt solution (1) at a flow rate of 0.2 L / h to form a mixed aqueous transition metal salt solution, and the Nb dopant solution (4) was added to the Nb dopant solution (3) at a flow rate of 0.1 L / h to form a mixed Nb dopant solution. At the same time, the mixed aqueous transition metal salt solution, the mixed Nb dopant solution, 6.0 mol / L NaOH and 9.0 mol / L NH3-H2O were slowly added to a reaction kettle under stirring. The first co-precipitation was carried out at a pH of 10.0 and a temperature of 53°C under a N2 atmosphere. The first co-precipitation was carried out for 58 hours.

[0125] Nickel sulfate, cobalt sulfate and manganese sulfate were mixed in a molar ratio of 0.89:0.055:0.055 to form a 1.5 mol / L aqueous transition metal salt solution (5). Nano-ZrO2 was mixed with water to form a 0.01 mol / L Zr dopant solution (6) and a 2.0 mol / L Zr dopant solution (7), respectively. The Zr dopant solution (7) was added to the Zr dopant solution (6) at a flow rate of 0.1 L / h to form a mixed Zr dopant solution. At the same time, the aqueous transition metal salt solution (5), the mixed Zr dopant solution, 6.0 mol / L NaOH and 2.0 mol / L NH3-H2O were slowly added to the reaction kettle just after the first co-precipitation under stirring. The second co-precipitation was carried out at a pH of 11.5 and a temperature of 56°C under a N2 atmosphere. After the second co-precipitation was carried out for 44 hours, the addition of the aqueous transition metal salt solution (5) was stopped, and after 0.5 hours of reaction, secondary particles having a total particle size of 14.9 μm were obtained.

[0126] corresponding to Li: (Ni + Co + Mn) = 1.05, the secondary particles were mixed with lithium hydroxide and sintered in an oxygen atmosphere at 740 °C for 15 h to obtain a positive electrode material C-3, which has a composition of Li 1.05 [(Ni 0.95 Co 0.02 Mn 0.015 Nb 0.015 ) 0.10 ·(Ni 0.905 Co 0.05 Mn 0.035 Zr 0.01 ) 0.90 ]O2. In the core of the positive electrode material C-3, the content of Nb decreases from the center of the core to the surface of the core, with a molar ratio decreasing from 0.02 mol% to 0.01 mol%, while the content of Mn increases from the center of the core to the surface of the core, with a molar ratio increasing from 0.01 mol% to 0.02 mol%, and the total amount of Nb and Mn elements remains 0.015 mol%. In the shell of the positive electrode material C-3, the content of Zr increases from the surface of the core to the surface of the shell, with a molar ratio increasing from 0 to 0.02 mol%, while the content of Ni decreases from the surface of the core to the surface of the shell, with a molar ratio decreasing from 0.93 mol% to 0.88 mol%, and the content of Mn increases from the surface of the core to the surface of the shell, with a molar ratio increasing from 0.02 mol% to 0.05 mol%.

[0127] The performance of the positive electrode material C-3 was tested as described above.

[0128] The scanning electron microscope test results showed that the positive electrode material C-3 had a core-shell structure, in which the radius size of the core was 3.5 pm and the outer radius of the shell was 4.0 pm. The core and the shell had different pores.

[0129] The pore size, porosity and pore volume test results showed that the porosity of the core was 72% and the porosity of the shell was 32%. The change of the pore volume of the positive electrode material C-3 with pressure was as follows:

[0130] corresponding to the pore volume with a pore size of 90 ± 10 nm, in which PV1 / PV0 = 0.5, PV2 / PV0 = 2.3, PV3 / PV0 = 3.1, and PV4 / PV0 = 5.2.

[0131] The capacity retention rate of the coin cell prepared from the positive electrode material C-3 of Example 3 was 93.1% after 80 cycles.

[0132] Example 4

[0133] Nickel sulfate, cobalt sulfate and manganese sulfate were mixed in a molar ratio of 0.87:0.02:0.11 to form a 1.5 mol / L aqueous transition metal salt solution (1). Nickel sulfate, cobalt sulfate and manganese sulfate were mixed in a molar ratio of 0.84:0.02:0.14 to form a 1.5 mol / L aqueous transition metal salt solution (2). Nano-Nb2O5 was mixed with water to form a 2.0 mol / L Nb dopant solution (3) and a 0.05 mol / L Nb dopant solution (4), respectively. The aqueous transition metal salt solution (2) was added to the aqueous transition metal salt solution (1) at a flow rate of 0.2 L / h to form a mixed aqueous transition metal salt solution, and the Nb dopant solution (4) was added to the Nb dopant solution (3) at a flow rate of 0.1 L / h to form a mixed Nb dopant solution. At the same time, the mixed aqueous transition metal salt solution, the mixed Nb dopant solution, 6.0 mol / L NaOH and 10.0 mol / L NH3-H2O were slowly added to a stirred reaction kettle. The first co-precipitation was carried out at a pH of 10.8, a temperature of 52°C and under a N2 atmosphere. The first co-precipitation was carried out for 60 hours.

[0134] Nickel sulfate, cobalt sulfate and manganese sulfate were mixed in a molar ratio of 0.60:0.05:0.35 to form a 1.5 mol / L aqueous transition metal salt solution (5). ZrO2 was mixed with water to form a 0.01 mol / L Zr dopant solution (6) and a 2.0 mol / L Zr dopant solution (7), respectively. The Zr dopant solution (7) was added to the Zr dopant solution (6) at a flow rate of 0.1 L / h to form a mixed Zr dopant solution. At the same time, the aqueous transition metal salt solution (5), the mixed Zr dopant solution, 6.0 mol / L NaOH and 4.0 mol / L NH3-H2O were slowly added to the reaction kettle just after the first co-precipitation under stirring. The second co-precipitation was carried out at a pH of 11.5, a temperature of 58°C and under a N2 atmosphere. After the second co-precipitation was carried out for 60 hours, the addition of the aqueous transition metal salt solution (5) was stopped, and after 1.0 hours of reaction, secondary particles were obtained, which had a total particle size of 16.1 μm.

[0135] The secondary particles were mixed with lithium hydroxide corresponding to Li:(Ni+Co+Mn)=1.04, and sintered at 780°C for 15h in an oxygen atmosphere to obtain a positive electrode material C-4, which had a composition of Li 1.04 [(Ni 0.85 Co 0.02 Mn 0.115 Nb 0.015 ) 0.18 ·(Ni 0.675 Co 0.05 Mn 0.255 Zr 0.02 )0.82 ]O2. In the core of the positive electrode material C-3, the content of Nb decreases from the center of the core to the surface of the core, with the molar ratio decreasing from 0.04 mol% to 0, while the content of Mn increases from the center of the core to the surface of the core, with the molar ratio increasing from 0.06 mol% to 0.1 mol%, and the total amount of Nb and Mn elements remains 0.1 mol%. In the shell of the positive electrode material C-4, the content of Zr increases from the surface of the core to the surface of the shell, with the molar ratio increasing from 0 to 0.04 mol%, while the content of Ni decreases from the surface of the core to the surface of the shell, with the molar ratio decreasing from 0.85 mol% to 0.6 mol%, and the content of Mn increases from the surface of the core to the surface of the shell, with the molar ratio increasing from 0.1 mol% to 0.31 mol%.

[0136] The performance of the positive electrode material C-4 was tested as described above.

[0137] The scanning electron microscope test results show that the positive electrode material C-4 has a core-shell structure, with the radius of the core being 4.5 pm and the outer radius of the shell being 3.5 pm. The core and the shell have different pores.

[0138] The pore size, porosity and pore volume test results show that the porosity of the shell is 32% and the porosity of the core is 72%. The change in the pore volume of the positive electrode material C-4 with pressure is as follows:

[0139] The pore volume corresponding to the pore size of 90±10 nm, wherein PV1 / PV0=0.7, PV2 / PV0=2.9, PV3 / PV0=3.8 and PV4 / PV0=5.5.

[0140] The capacity retention rate of the button cell prepared from the positive electrode material C-4 of Example 4 after 80 cycles is 92.5%.

[0141] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, but not to limit it; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement to part or all of the technical features, and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A cathode material for a lithium-ion battery, which has a structure of secondary particles with a core and a shell, wherein the secondary particles are composed of primary particles of the cathode material, the primary particles have an elongated shape, the porosity of the core is greater than the porosity of the shell, and the average composition of the cathode material is as follows: Li 0.6+δ [(Ni x1 Co y1 M 1-x1-y1 D η1 ) z ·(Ni x2 Co y2 M 1-x2-y2 G η2 ) 1-z O2, 0 ≤ δ ≤ 0.6, 0.6 ≤ x1 ≤ 1, 0.6 ≤ x2 ≤ 1, 0 ≤ y1 ≤ 0.4, 0 ≤ y2 ≤ 0.4, 0 < z < 1, 0 ≤ η1 ≤ 0.1, 0 ≤ η2 ≤ 0.1, M is one or both of Mn and Al, D is one or more of Mo, Ti, Y, W, Ta, Nb, Cr, Sm, Sb, V, La, Ca, Hf, Zr, G is one or more of Mo, Ta, Zn, Ti, Y, Zr, W, Nb, Cr, Sm, Al, V, La, Ca, Sb, Hf, Mg, B, wherein the core contains macropores with a pore diameter of 50 - 500 nm, and the pressed pore volume of the pores with a pore diameter of 50 - 110 nm in the cathode material is denoted as PV i , where i = 0, 1, 2, 3, and 4, corresponding to the pore volume of the pores when the cathode material is subjected to 0 ton, 1.5 tons, 2.5 tons, 3.5 tons, and 4.5 tons of pressure respectively, and the pressed pore volume satisfies the following formula: 1 < PV1 / PV0 < 1.5, 1 < PV2 / PV0 < 4.0, 1 < PV3 / PV0 < 6.0, and 1 < PV4 / PV0 < 8.

0.

2. The positive electrode material of claim 1, wherein the porosity of the core is 50-90%, the porosity of the shell is 10-50%, and the ratio of the porosity of the core to the porosity of the shell is greater than 1.

2.

3. The positive electrode material of claim 1, wherein the porosity of the macropores of the core is 60-80%; and the shell comprises macropores with a pore size of 50-500 nm, and the porosity of the macropores of the shell is 20-40%.

4. The positive electrode material of claim 1, wherein the concentration of D gradually decreases along the radial direction from the center of the core to the surface of the core, and the concentration of G gradually increases along the radial direction from the surface of the core to the surface of the shell.

5. A method for preparing the positive electrode material for lithium ion batteries of any one of claims 1-4, the positive electrode material being a secondary particle having a core and a shell, the method comprising the following steps: (1) performing first co-precipitation of a solution containing soluble salts of Ni, Co, M with a dispersion containing D, to form a core of a secondary particle; (2) in the presence of the core of step (1), performing second co-precipitation of a solution containing soluble salts of Ni, Co, M with a dispersion containing G, to form a shell on the core, to obtain a secondary particle having a core and a shell; wherein the pH value at the first co-precipitation is lower than the pH value at the second co-precipitation, and the temperature at the first co-precipitation is lower than the temperature at the second co-precipitation, wherein the porosity of the core is greater than the porosity of the shell, (3) converting the secondary particle obtained in step (2) into the positive electrode material, comprising the step of: obtaining the positive electrode material by contacting the secondary particle obtained in step (2) with a lithium source material at 300-900°C, wherein the lithium source material is one or more of Li2O, LiOH, LiOH•H2O, LiNO3 and Li2CO3, wherein the average composition of the positive electrode material is as follows: Li 0.6+δ [(Ni x1 Co y1 M 1-x1-y1 D η1 ) z ·(Ni x2 Co y2 M 1-x2- y2 G η2 ) 1-z ]O2, 0≤δ≤0.6, 0.6≤x1≤1, 0.6≤x2≤1, 0≤y1≤0.4, 0≤y2≤0.4, 0<z<1, 0≤η1≤0.1, 0≤η2≤0.1, M is one or both of Mn and Al, D is one or more of Mo, Ti, Y, W, Ta, Nb, Cr, Sm, Sb, V, La, Ca, Hf, Zr, and G is one or more of Mo, Ta, Zn, Ti, Y, Zr, W, Nb, Cr, Sm, Al, V, La, Ca, Sb, Hf, Mg, B.

6. The method of claim 5, wherein in step (1), the concentration or flow rate of the dispersion of D is controlled so that the concentration of D gradually decreases along the radial direction from the center of the core to the surface of the core; and in step (2), the concentration or flow rate of the dispersion of G is controlled so that the concentration of G gradually increases along the radial direction from the surface of the core to the surface of the shell.

7. The method of claim 5, wherein the step (1) comprises: the first co-precipitation is performed in the presence of a complexing agent; and the step (2) comprises: performing the second co-precipitation in the presence of a complexing agent; the complexing agent is selected from one or more of aqueous ammonia, ammonium sulfate, ammonium chloride or ammonium nitrate.

8. The method of claim 7, wherein the step (1) comprises: the first co-precipitation is performed in the presence of 5-12 mol / L of the complexing agent; and the step (2) comprises: performing the second co-precipitation in the presence of 0.1-5 mol / L of the complexing agent.

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