Positive electrode material and preparation method therefor, and lithium-ion battery

Abstract

The present invention provides a positive electrode material and a preparation method therefor, and a lithium-ion battery. The positive electrode material comprises components as shown in formula (I): NixCoyMn1-x-yDkLizO2 (I); in the positive electrode material, the value ranges of x, y, z and k are as follows: 0.6<x<1, 0<y<0.2, and x+y<1; 1≤z≤1.05, and 0≤k≤0.05, wherein D is a modified element, and the modified element comprises at least one of S, P, F, B, Al, Ti, Mg, Cr, Zr, V, Nb, Y, W, Ta, Co, Ce, W, and Zn. A three-electrode battery cell is prepared from the positive electrode material of the present invention, and in charge-discharge cycles of the three-electrode battery cell, the change rate a of the electrochemical active surface area (ECSA) of the positive electrode material is less than 5%. The positive electrode material provided by the present invention can effectively inhibit the increase in ECSA with the cycles, reduce interface side reactions, and improve the cycle stability.

Description

Positive electrode material, preparation method thereof, and lithium ion battery

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on June 15, 2023, with application number 202310705475.X and application name “A positive electrode material, its preparation method and lithium-ion battery”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present invention relates to the technical field of battery materials, and in particular to a positive electrode material and a preparation method thereof, and a lithium ion battery. Background Art

[0003] With the promotion and popularization of the concept of new energy, the importance of lithium-ion batteries in the fields of electric vehicles and energy storage has become increasingly prominent. As an important component of lithium-ion batteries, the properties of positive electrode materials directly affect the various performances of lithium-ion batteries. The electrochemically active surface area (ECSA) of the positive electrode material refers to the surface area where the positive electrode material is in direct contact with the electrolyte and can undergo charge transfer reactions. A larger ECSA is conducive to the rapid insertion and extraction of lithium ions, improving the charge and discharge rate performance of the positive electrode material, but it also leads to an accelerated rate of side reactions between the positive electrode material and the electrolyte, which damages the cycle stability of the material.

[0004] The ECSA of the positive electrode material is not fixed during the cycle. The particles of the positive electrode material will gradually break during the cycle, resulting in an increase in ECSA and accelerating interfacial side reactions.

[0005] Application Contents

[0006] In order to solve the above problems, the present invention provides a positive electrode material with a small ECSA change rate.

[0007] Specifically, the present invention provides a positive electrode material, the composition of which is shown in formula (I):

[0008] Ni x Co y Mn 1-x-y D k Li z O2 (Ⅰ);

[0009] In the positive electrode material, the value ranges of x, y, z, and k are as follows: 0.6<x<1, 0<y<0.2, and x+y<1; 1≤z≤1.05, 0≤k≤0.05; wherein D is a modifying element, and the modifying element includes at least one of S, P, F, B, Al, Ti, Mg, Cr, Zr, V, Nb, Y, W, Ta, Co, Ce, W, and Zn; the value range of the compacted density PD of the electrode sheet prepared from the positive electrode material is as follows: 3.0 g / cm 3 <PD<3.8g / cm 3 ; The particle size distribution of the positive electrode material is as follows: D10<8μm, 5μm<D50<15μm, 10μm<D90<30μm; the numerical range of the lithium-nickel mixing degree Li / Ni mixing of the positive electrode material is as follows: 0%<Li / Ni mixing<2.5%; A three-electrode battery cell is prepared using the positive electrode material, and during the charge and discharge cycle of the three-electrode battery cell, the electrochemical active surface area change rate a of the positive electrode material is less than 5%.

[0010] In this technical solution, the compaction density of the material is one of the reference indicators of the material energy density. The size of the compaction density has a certain relationship with the particle size and particle gradation in the positive electrode material. The compaction density of the electrode sheet prepared by the positive electrode material provided by the present invention is 3.0g / cm 3 <PD<3.8g / cm 3 , and its particle size distribution is uniform; therefore, the battery prepared using this positive electrode material has a higher capacity.

[0011] In the present application, the sum of the positive and negative valences of each element in the positive electrode material is 0.

[0012] The positive electrode material in the present invention is a material based on nickel, cobalt, and manganese as the ternary materials. Furthermore, it may contain a modifying element; wherein the modifying element can be a doping element or a coating element. Specifically, the doping element includes at least one of S, P, F, B, Al, Ti, Mg, Cr, Zr, V, Nb, Y, W, and Ta, and the coating element includes at least one of Co, P, F, B, Al, Ti, Mg, Cr, Zr, Ce, W, and Zn. The present invention defines the positive electrode material by its composition and the ratio of each element. Among them, cobalt, as one of the elements in the nickel-cobalt-manganese ternary material, can also be applied to the above-mentioned positive electrode material as a coating element; therefore, its content is limited from two perspectives, which is specially explained.

[0013] The positive electrode material provided by the present invention is a high-nickel polycrystalline material with excellent kinetic properties and high specific capacity. In this positive electrode material, the nickel content is higher; in terms of molar ratio, the nickel element is more than 60% of the sum of the three elements of nickel, cobalt and manganese; at the same time, the amount of lithium element exceeds the sum of the three elements of nickel, cobalt and manganese. Furthermore, in the positive electrode material, the higher the degree of lithium-nickel mixing, the more serious the lithium-nickel mixing, which is easy to aggravate the side reactions at the interface, and make the positive electrode material easy to collapse during the charge and discharge process, hindering the deintercalation of lithium ions, and thus reducing the electrochemical performance of the positive electrode material; the lithium-nickel mixing degree of the positive electrode material of the present invention is obtained by XRD refinement test, and the lithium-nickel mixing degree does not exceed 2.5%; therefore, its electrochemical performance is effectively guaranteed.

[0014] During the charge and discharge process, the unit cell parameters of the positive electrode material will change greatly. The unit cell parameters will first increase slightly with charging, and then decrease significantly. The discharge process is the opposite. This process will cause greater stress inside the material. In severe cases, it will cause cracks or particle breakage. If the particles of the positive electrode material are broken, the electrochemically active surface area (ECSA) of the positive electrode material will increase, the active surface in contact with the electrolyte will increase, and the side reactions will also intensify, resulting in a decrease in the battery cycle stability. Therefore, during the cycle process, the rate of change of the electrochemically active surface area is also an important measurement indicator. The electrochemically active surface area of ​​the positive electrode material provided by the present invention shows high stability, and the rate of change does not exceed 5% during the cycle process; thus, it has a higher cycle capacity retention rate and a lower DC internal resistance growth rate.

[0015] Furthermore, the calculation method of the electrochemical active surface area change rate a is shown in formula (II):

[0016] a=(S n -S0) / n×100% (Ⅱ);

[0017] Where n is the number of charge and discharge cycles of the three-electrode cell, S0 is the electrochemically active surface area of ​​the positive electrode material during the first cycle of the three-electrode cell; S n It is the electrochemically active surface area of ​​the positive electrode material after n charge and discharge cycles of the three-electrode battery cell.

[0018] In this technical solution, the rate of change of the electrochemically active surface area is the change value of the electrochemically active surface area of ​​the three-electrode cell before and after n charge and discharge cycles (S n -S0) to the number of charge and discharge cycles n. The electrochemically active surface area change rate a can accurately measure the degree of change in the electrochemically active surface area after each cycle. Among them, S0 is the electrochemically active surface area of ​​the positive electrode material during the first cycle of the three-electrode battery cell, which is defined as 1 in actual use. The calculation of the electrochemically active surface area change rate a can be simplified to a=(S n-1) / n×100%; during the test, the number of charge and discharge cycles n is generally set to within 50 cycles.

[0019] Furthermore, the electrochemically active surface area is obtained by the following method: testing the positive electrode overpotential response η caused by the excitation current I of the three-electrode battery cell; using the Buter-Volmer equation to perform least squares fitting on the excitation current I of different current sizes and the corresponding positive electrode overpotential response η to obtain the electrochemically active surface area of ​​the positive electrode material.

[0020] In this technical solution, the electrochemically active surface area of ​​the material is calculated by fitting the Butler-Volmer equation for different excitation currents I and the corresponding positive electrode overpotential responses η. This allows for dynamic analysis of battery cell performance and is applicable to cells with different material properties. Furthermore, the test results obtained using this method are more accurate, the fitting method is simple, does not involve complex mathematical derivations, and can be directly derived through model reasoning using the Butler-Volmer equation, making it more versatile.

[0021] Furthermore, the Butler-Volmer equation expresses the relationship between the excitation current I and the exchange current i0;

[0022] Among them, the exchange current i0 is proportional to the electrochemically active surface area; the exchange current i0 is the product of the electrochemically active surface area and the proportionality coefficient.

[0023] It should be noted that the prototype of the Butler-Volmer equation is shown in formula (IV), which is quoted from the textbook "Electrochemical Systems", 4th edition, published by John Wiley and Sons Ltd.

[0024] In this technical solution, i n is the net current; i0 is the exchange current; in the equilibrium state, the forward and reverse reaction rates are equal, the currents cancel each other out, and thus the net current is 0; at this time, the absolute value of the current of the forward / reverse reaction is the exchange current i0, which is affected by the concentration of redox substances CR and CO on the surface of the material. During the test, for the changing net current i nThe larger the electrochemically active surface area of ​​a material, the smaller the change in the redox species concentrations CR and CO on the material surface, and thus the smaller the change in the exchange current i0. This indicates that there is a negative correlation between the electrochemically active surface area of ​​the material and the rate of change of the exchange current i0. Furthermore, the normalized electrochemically active surface area A is used as a variable, and the remaining parameters can be represented by the proportionality coefficient N; a relationship between the two and the exchange current i0 is established. Simply put, the exchange current i0 in the Butler-Volmer equation is defined as the product of the electrochemically active surface area A and the proportionality coefficient N.

[0025] In the present invention, the excitation current I is also the net current i in the prototype of the Butler-Volmer equation. n By fitting multiple sets of different excitation currents I and the corresponding positive electrode overpotential responses η, the proportional coefficient N and the size of the electrochemically active surface area A can be obtained.

[0026] Furthermore, the excitation current I of different current magnitudes and the corresponding positive electrode overpotential response η are fitted with the Butler-Volmer equation by least squares fitting; the values ​​of the proportionality coefficient N and the electrochemically active surface area A can be obtained.

[0027] Furthermore, the Butler-Volmer equation is shown in formula (III):

[0028] Where I is the excitation current, N is the proportional coefficient obtained by fitting, and R S is the ohmic impedance, R SEI is the interface impedance, R CT is the charge transfer impedance, F is the Faraday constant, R is the gas constant, T is the temperature of the three-electrode cell during the test, A is the electrochemically active surface area obtained by fitting, and η is the positive electrode overpotential response.

[0029] In this technical solution, a specific expression of the Butler-Volmer equation is provided; wherein the ohmic impedance R S , interface impedance R SEI , charge transfer resistance R CT All can be obtained by testing and fitting. The Faraday constant F is the product of Avogadro's number NA and the elementary charge e; it is generally believed that its value is 96485.3383±0.0083C / mol; the gas constant R is 8.314J / (mol·K). The size of the proportionality coefficient N is related to the composition and specific surface area of ​​the material; for the positive electrode material provided by the present invention, when its specific surface area BET is 0.2m 2 / g<BET<3.5m 2 / g; the numerical range of the proportional coefficient N obtained by fitting is as follows: -0.03<N<-0.002.

[0030] The present invention also provides a method for preparing the above-mentioned positive electrode material, comprising the following steps: primary calcination: calcining a nickel-cobalt-manganese ternary precursor to obtain an oxide precursor P1; secondary calcination: mixing the oxide precursor P1 with a first lithium source and calcining to obtain an oxide precursor P2; and tertiary calcination: mixing the oxide precursor P2 with a second lithium source and calcining to obtain a positive electrode material.

[0031] In the synthesis process of high-nickel polycrystalline positive electrode materials, the calcination process is the most core part. The preparation method provided by the present invention is synthesized through a refined lithium mixing and multi-stage sintering process; specifically, the nickel-cobalt-manganese precursor is first calcined once, and the lithium required in the positive electrode material is added twice, and calcined twice and three times respectively. The purpose of multiple mixing of lithium sources for calcination is to change the size and stacking number of primary particles; thereby changing the ECSA change during the cycle of the positive electrode material. Generally speaking, positive electrode materials are spherical particles at the microscopic level, and a spherical particle is composed of many irregular small particles; irregular small particles are called "primary particles", and the spherical particles obtained by stacking primary particles are called secondary particles; that is, secondary particles are stacked by primary particles. In the conventional single-stage lithium mixing and sintering process, the primary particles obtained are relatively small, and each secondary particle requires a large number of primary particles. In the present invention, the primary particles of the positive electrode material obtained by mixing the lithium source and calcining twice are larger. If the secondary particles of the same size are stacked, fewer primary particles are required. Initially, the ECSA of the material particles is the surface area of ​​the secondary particles. However, as the secondary particles break due to the influence of unit cell stress during the cycle, the primary particles inside the secondary particles are gradually exposed to the electrolyte, contributing additional reaction surface area. The samples obtained by conventional single-stage mixed lithium sintering show a rapid increase in ECSA after crushing due to the small and numerous primary particles. In contrast, the samples obtained by multi-stage mixed lithium sintering in the present invention have large primary particles and a small number of them. Even if the secondary particles are crushed, they only expose a small additional reaction surface area, resulting in a smaller ECSA increase.

[0032] Furthermore, the amount M1 of the lithium element in the first lithium source and the sum M0 of the amounts of nickel, cobalt and manganese in the nickel-cobalt-manganese ternary precursor satisfy: 0<M1 / M0≤0.8; the amount M2 of the lithium element in the second lithium source and the amount M1 of the lithium element in the first lithium source, as well as the sum M0 of the amounts of nickel, cobalt and manganese in the nickel-cobalt-manganese ternary precursor satisfy: 1-M1 / M0≤M2 / M0≤1.05-M1 / M0.

[0033] In this technical solution, the total amount of lithium added is 1-1.05 times the sum of the amounts of nickel, cobalt and manganese substances; wherein, when lithium is added for the first time, it does not exceed 80% of the total amount of lithium; the particle size and stacking quantity of the positive electrode material obtained by sintering are thus better.

[0034] Furthermore, the atmosphere of the primary calcination is a reaction atmosphere containing oxygen; and / or the atmosphere of the secondary calcination is oxygen; and / or the atmosphere of the tertiary calcination is oxygen; and / or the first lithium source includes lithium hydroxide monohydrate; and / or the second lithium source includes lithium hydroxide monohydrate.

[0035] In this technical solution, calcination can be completed using a primary reaction atmosphere containing oxygen. Specifically, air or a mixture of other gases and oxygen can be used. However, during the secondary and tertiary calcinations, the secondary reaction atmosphere is oxygen, which results in a predominantly +3 nickel valence state in the material. Calcination in air would prevent the formation of a layered phase. Both the primary and secondary lithium sources are preferably lithium hydroxide monohydrate. Using lithium hydroxide monohydrate requires a lower sintering temperature, allowing for sufficient calcination and resulting in a more uniform particle size in the resulting positive electrode material.

[0036] Further, the first lithium source includes lithium hydroxide monohydrate and a lithium salt, and the lithium salt includes at least one of lithium carbonate, lithium nitrate, lithium phosphate, lithium formate, lithium acetate, and lithium ethoxide; and / or the second lithium source includes lithium hydroxide monohydrate and a lithium salt, and the lithium salt includes at least one of lithium carbonate, lithium nitrate, lithium phosphate, lithium formate, lithium acetate, and lithium ethoxide; and / or the process conditions for the first calcination include: a heating rate of 2-5°C / min, a calcination temperature of 300-500°C, and a calcination time of 1-5h; and / or the process conditions for the second calcination include: a heating rate of 2-5°C / min, a calcination temperature of 600-1000°C, a calcination time of 2-8h, and a cooling rate controlled at 2-5°C / min; and / or the process conditions for the third calcination include: a heating rate of 2-5°C / min, a calcination temperature of 600-1000°C, a calcination time of 6-20h, and a cooling rate controlled at 2-5°C / min.

[0037] In this technical solution, the first and second lithium sources comprise a mixture of other lithium salts and lithium hydroxide monohydrate, with the lithium salt being selected from at least one of lithium carbonate, lithium nitrate, lithium phosphate, lithium formate, lithium acetate, and lithium ethoxide. This combination of lithium salts and lithium hydroxide monohydrate provides multiple alternatives, and the cathode material provided by the present invention is sintered. After the primary calcination, the material can be cooled in the furnace. After the secondary and tertiary calcinations, the cooling rate must be controlled.

[0038] Furthermore, in the three calcination steps, the dopant is mixed with the oxide precursor P2 and the second lithium source and calcined; wherein the dopant includes at least one element of S, P, F, B, Al, Ti, Mg, Cr, Zr, V, Nb, Y, W, and Ta.

[0039] In this technical solution, the dopant, that is, the substance containing the doping element, is added during the three calcination stages to increase the structural stability of the bulk phase of the material and inhibit the migration and phase change of transition metals in the bulk phase of the material during long cycle processes.

[0040] Furthermore, the preparation method also includes modifying and sintering the positive electrode material once or multiple times; the modified sintering is to mix the positive electrode material with the coating agent and sinter them; the sintering temperature of the modified sintering does not exceed 600°C; wherein the coating agent contains at least one element of Co, P, F, B, Al, Ti, Mg, Cr, Zr, Ce, W, and Zn.

[0041] In this technical solution, the cathode material can also undergo a modified sintering process, wherein the coating agent is a substance containing a coating element. Excessively high sintering temperatures during the modified sintering process can damage the structure of the secondary particles in the cathode material; therefore, the modified sintering temperature must be controlled below 600°C. Specifically, the modified sintering temperature is 300-600°C, and the sintering time is 2-6 hours.

[0042] The present invention further provides a battery comprising the positive electrode material of any of the above technical solutions, thereby having all the beneficial effects of any of the above technical solutions, which will not be described in detail here. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the following description of the embodiments with reference to the accompanying drawings, in which:

[0044] FIG1 is a fitting diagram of charging current and overpotential of the positive electrode material H1 according to an embodiment of the present invention;

[0045] FIG2 is a graph showing the ECSA change rate of the positive electrode material H1 according to an embodiment of the present invention;

[0046] FIG3 is a fitting diagram of the charging current and overpotential of the positive electrode material Q1 according to an embodiment of the present invention;

[0047] FIG4 shows the ECSA change rate of the positive electrode material Q1 according to an embodiment of the present invention;

[0048] FIG5 is a comparison of cycle capacity retention rates of positive electrode materials H1 and Q1 according to an embodiment of the present invention;

[0049] FIG6 is a SEM image of the positive electrode material Q1 according to an embodiment of the present invention;

[0050] FIG. 7 is a SEM image of the positive electrode material H1 according to an embodiment of the present invention. DETAILED DESCRIPTION

[0051] To make the above-mentioned objects, features, and advantages of the present invention more clearly understood, the technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by persons of ordinary skill in the art based on the embodiments of the present invention without creative effort shall fall within the scope of protection of the present invention.

[0052] The embodiments of the present invention provide a method for preparing a positive electrode material, as well as the prepared positive electrode material and a battery prepared using the positive electrode material.

[0053] On the one hand, since the particles of the positive electrode material will be broken during the cycle, resulting in a large rate of change in the electrochemically active surface area, the preparation method provided in this embodiment adopts a refined lithium blending and multi-stage sintering process. The primary particles of the positive electrode material obtained by adding lithium source twice and sintering are larger. Therefore, when the secondary particles are broken during the cycle, the rate of change in the electrochemically active surface area is smaller.

[0054] On the other hand, there are many methods for measuring the electrochemically active surface area in the relevant technology; for example, ECSA is directly estimated by the gas adsorption BET specific surface area test method. This method uses the gas adsorption characteristics of the particle surface to estimate the surface area. The advantages are fast test speed and good test stability; but the surface area measured by the BET method is not completely equivalent to ECSA. The BET surface area measures the surface size related to gas adsorption, while the ECSA is the surface size in contact with the electrolyte. For porous electrode materials, there may be a large difference between the two. In addition, the BET test method is only applicable to the testing of initial electrode powder samples, and is not applicable to the testing of electrode materials in battery cells, because in most cases the electrode materials in the battery cells have been mixed with conductive carbon, binders, etc., and are not easy to separate. The BET test method cannot distinguish the surface area of ​​the electrode material alone.

[0055] For example, electrochemical impedance spectroscopy (EIS) testing and capacitive reactance fitting are also common methods for determining ECSA. Through EIS testing and fitting of battery cells, the resistance response and capacitance response of the charge transfer reaction can be obtained. The capacitance response is closely related to the ECSA of the electrode material and can represent the relative size of the ECSA. However, in actual situations, the surface of the electrode material is rough and porous, and its capacitance response does not appear in the form of a pure capacitance component, but rather as a certain constant phase element, making it difficult to obtain a capacitance value. Therefore, the EIS fitting method for determining ECSA has significant limitations and is only applicable to cases where the uncertainty in model construction leads to large errors and mistakes in the results.

[0056] Cyclic voltammetry (CV) can also be used as an ECSA determination method. ECSA can be fitted by CV tests at different scan rates. However, CV ECSA determination requires selecting a reasonable scan voltage or finding a suitable supporting electrolyte to avoid the Faradaic reaction range of the substance being measured. This requirement makes CV ECSA determination difficult to implement in electrode materials.

[0057] In this embodiment, the ECSA is determined by fitting the Butler-Volmer equation to different excitation currents I and the corresponding positive electrode overpotential responses η, and the electrochemically active surface area of ​​the material can be calculated.

[0058] Example 1

[0059] This embodiment provides a method for preparing a positive electrode material, comprising the following steps:

[0060] S10: One calcination: Obtain nickel-cobalt-manganese ternary precursor Ni 0.85 Co 0.1 Mn 0.05 (OH)2, and calcined in air to obtain the oxide precursor P1.

[0061] The process parameters of the primary calcination are as follows: heating rate 5°C / min, calcination temperature 500°C, and calcination time 2h.

[0062] S20: Secondary calcination: calcining the oxide precursor P1 and the first lithium source in oxygen to obtain the oxide precursor P2, wherein the first lithium source is lithium hydroxide monohydrate.

[0063] The amount M1 of lithium in the first lithium source and the sum M0 of the amounts of nickel, cobalt, and manganese in the nickel-cobalt-manganese ternary precursor satisfy: M1 / M0=0.4.

[0064] The process conditions of the secondary calcination include: a heating rate of 2°C / min, a calcination temperature of 600°C, a calcination time of 2h, and a cooling rate controlled at 2°C / min.

[0065] S30: calcining three times: calcining the oxide precursor P2 and the second lithium source in oxygen to obtain the positive electrode material H1. The second lithium source is a mixture of lithium hydroxide monohydrate and lithium nitrate (molar ratio 8:2).

[0066] In the second lithium source, the amount M2 of the lithium element and the sum M0 of the amounts of nickel, cobalt, and manganese in the nickel-cobalt-manganese ternary precursor satisfy: M2 / M0=0.6025.

[0067] The process conditions of the three calcinations include: a heating rate of 2°C / min, a calcination temperature of 750°C, a calcination time of 12h, and a cooling rate controlled at 2°C / min.

[0068] The ECSA change rate of the above-mentioned cathode materials was measured:

[0069] 1. Take 9.5g of positive electrode material H, prepare a slurry according to the ratio of positive electrode material: HVDF: SH = 95:2.5:2.5 and coat it on aluminum foil to form a pole piece. The pole piece surface density is controlled to 15mg / cm 2 The electrode sheets were punched into small discs with a diameter of 1.5 cm and assembled into two button-type three-electrode batteries, labeled as batteries H1-1 and H1-10, and lithium plating of the three electrodes was completed.

[0070] 2. After fully charging battery H1-1, let it stand for 1 hour, record the equilibrium voltage, perform EIS test and fit the ohmic impedance R S , interface impedance R SEI , charge transfer resistance R CT .

[0071] 3. Battery H1-1 was subjected to multiple 10s DC charging. After each charging, it was allowed to rest for 60s to return to the equilibrium state. The charging excitation current I was 0mA, 0.1mA, 0.2mA, 0.4mA, 0.6mA, 0.8mA, 1.2mA, 1.5mA, and 2mA, respectively. The positive electrode-reference voltage at the end of each charging step was recorded. The difference between this voltage and the equilibrium voltage was the positive electrode overpotential response η.

[0072] 4. The different excitation currents I of battery H1-1 and the corresponding positive electrode overpotential response η and the fitted ohmic impedance R S , interface impedance R SEI and charge transfer resistance R CT Substitute the data into the Butler-Volmer equation for data fitting. Set the ECSA of the H1-1 battery cathode material to 1 (i.e., A = 1), and the proportionality coefficient N = -0.059 is obtained from the fitting.

[0073] The Butler-Volmer equation has the following form:

[0074] Where I is the excitation current, N is the proportional coefficient obtained by fitting, and R S is the ohmic impedance, R SEI is the interface impedance, R CT is the charge transfer impedance, F is the Faraday constant, R is the gas constant, T is the temperature of the three-electrode cell during the test, A is the electrochemically active surface area obtained by fitting, and η is the positive electrode overpotential response.

[0075] 5. After the battery H1-10 is fully charged after 10 cycles at 45°C, the above steps 2 and 3 are performed. Similarly, the different excitation currents I and the corresponding positive electrode overpotential response η and the fitted ohmic impedance R are calculated.S , interface impedance R SEI and charge transfer resistance R CT Substituting the data into the above Butler-Volmer equation for fitting, assuming the proportionality factor to be the same as for the H1-1 battery, that is, N = -0.059, the fitting yielded an ECSA of 1.27 for the H1-10 battery's cathode material relative to that of the H1-1 battery. Therefore, the ECSA change rate, a, is (1.27 - 1) / 10 × 100% = 2.7%.

[0076] FIG1 is a fitting diagram of the positive electrode overpotential response η to the excitation current I of this embodiment, and FIG2 is a fitting diagram of the relative value and change rate a of the ECSA obtained by this embodiment.

[0077] Comparative Example 1

[0078] The conventional sintering process was used to synthesize the nickel-cobalt-manganese ternary precursor Ni 0.85 Co 0.1 Mn 0.05 (OH)2 and lithium hydroxide monohydrate were mixed and heated to 500℃ at 2℃ / min and kept at this temperature for 2h in an oxygen atmosphere, and then heated to 750℃ at 2℃ / min and kept at this temperature for 12h to obtain Li 1.025 Ni 0.85 Co 0.1 Mn 0.05 O2 (recorded as positive electrode material Q1).

[0079] The ECSA change rate of the positive electrode material Q was measured, and the specific steps were consistent with Example 1. 9.5 g of positive electrode material Q1 was prepared into a slurry to make pole pieces, which were punched out and assembled into two button-type three-electrode batteries, recorded as batteries Q1-1 and Q1-10, and lithium plating of the three electrodes was completed. Battery Q1-1 was fully charged and then allowed to stand for 1 hour for testing, and battery Q1-10 was fully charged and tested after 10 cycles at a high temperature of 45°C. According to the above method, the ECSA of the positive electrode material of battery Q1-10 relative to the positive electrode material of battery Q1-1 was obtained as 1.82, so the ECSA change rate was (1.82-1) / 10*100%=8.2%.

[0080] Figure 3 shows the fitted graph of the positive electrode overpotential response η for the excitation current I, and Figure 4 shows the relative ECSA values ​​and rate of change a obtained from the fitting. Since the ECSA rate of change for cathode material Q1 is greater than 5%, this indicates poor cycling stability. A comparison of the measured cycling performance of cathode materials H1 and Q1 is shown in Figure 5.

[0081] Table 1

[0082] The test parameters of positive electrode materials H1 and Q1 are shown in Table 1. Figure 7 is an electron micrograph of positive electrode material H1 in Example 1; Figure 6 is an electron micrograph of positive electrode material Q1 in Comparative Example 1. It can be seen that the primary particles of positive electrode material H1 provided in Example 1 have a larger particle size, resulting in a smaller rate of change in its ECSA.

[0083] Example 2

[0084] This embodiment provides a method for preparing a positive electrode material. The preparation steps are similar to those in Example 1, except that the nickel-cobalt-manganese ternary precursor is Ni 0.85 Co 0.06 Mn 0.11 (OH)2; the second calcination time is 4h; the calcination temperature of the third calcination is 770℃. The obtained positive electrode material is recorded as H2.

[0085] Comparative Example 2

[0086] The conventional sintering process was used to synthesize the nickel-cobalt-manganese ternary precursor Ni 0.85 Co 0.06 Mn 0.11 (OH)2 and lithium hydroxide monohydrate were mixed and heated to 500°C at a rate of 2°C / min and kept constant for 2 hours in an oxygen atmosphere, and then heated to 770°C at a rate of 2°C / min and kept constant for 12 hours. The obtained positive electrode material was recorded as Q2.

[0087] Example 3

[0088] This embodiment provides a method for preparing a positive electrode material. The preparation steps are similar to those in Example 1, except that the nickel-cobalt-manganese ternary precursor is Ni 0.9 Co 0.05 Mn 0.05 (OH)2; the second calcination time is 4h; the calcination temperature of the third calcination is 740℃. The obtained positive electrode material is recorded as H3.

[0089] Comparative Example 3

[0090] The conventional sintering process was used to synthesize the nickel-cobalt-manganese ternary precursor Ni 0.9 Co 0.05 Mn 0.05 (OH)2 and lithium hydroxide monohydrate were mixed and heated to 500°C at a rate of 2°C / min and kept constant for 2 hours in an oxygen atmosphere, and then heated to 740°C at a rate of 2°C / min and kept constant for 12 hours. The obtained positive electrode material was recorded as Q3.

[0091] Example 4

[0092] This embodiment provides a method for preparing a positive electrode material. The preparation steps are similar to those in Example 1, except that the nickel-cobalt-manganese ternary precursor is Ni 0.92 Co 0.03 Mn 0.05 (OH)2; the second calcination time is 4h; the calcination temperature of the third calcination is 740℃. The obtained positive electrode material is recorded as H4.

[0093] Comparative Example 4

[0094] The conventional sintering process was used to synthesize the nickel-cobalt-manganese ternary precursor Ni 0.92 Co 0.03 Mn 0.05 (OH)2 and lithium hydroxide monohydrate were mixed and heated to 500°C at a rate of 2°C / min and kept constant for 2 hours in an oxygen atmosphere, and then heated to 740°C at a rate of 2°C / min and kept constant for 12 hours. The obtained positive electrode material was recorded as Q4.

[0095] Example 5

[0096] This embodiment provides a method for preparing a positive electrode material. The preparation steps are similar to those in Example 1, except that the nickel-cobalt-manganese ternary precursor is Ni 0.96 Co 0.01 Mn 0.03 (OH)2; the second calcination time is 4h; the calcination temperature of the third calcination is 730℃. The obtained positive electrode material is recorded as H5.

[0097] Comparative Example 5

[0098] The conventional sintering process was used to synthesize the nickel-cobalt-manganese ternary precursor Ni 0.96 Co 0.01 Mn 0.03 (OH)2 and lithium hydroxide monohydrate were mixed and heated to 500°C at a rate of 2°C / min and kept constant for 2 hours in an oxygen atmosphere, and then heated to 730°C at a rate of 2°C / min and kept constant for 12 hours. The obtained positive electrode material was recorded as Q5.

[0099] Example 6

[0100] This embodiment provides a method for preparing a positive electrode material. The preparation steps are similar to those in Example 1, except that: the precursor is Ni 0.92 Mn 0.08 (OH)2. The obtained positive electrode material is recorded as H6.

[0101] Comparative Example 6

[0102] The conventional sintering process was used to synthesize the precursor Ni 0.92 Mn 0.08(OH)2 and lithium hydroxide monohydrate were mixed and sintered in an oxygen atmosphere at a temperature of 500°C at a rate of 2°C / min and kept constant for 2 hours, and then at a temperature of 750°C at a rate of 2°C / min and kept constant for 12 hours. The obtained positive electrode material was recorded as Q6.

[0103] Performance testing was performed on the positive electrode materials provided in Examples 2-6 and Comparative Examples 2-6. The test results are shown in Tables 2 and 3. The method for detecting the ECSA change rate is shown in Example 1. Comparison of various performance parameters shows that the preparation methods of the positive electrode materials provided in the examples of the present application can produce positive electrode materials with better performance; their capacity retention rate is greater during cycling, and their DCR growth rate is smaller.

[0104] Table 2

[0105] Table 3

[0106] Example 7

[0107] This embodiment provides a series of positive electrode materials and preparation methods thereof, wherein the preparation methods of the positive electrode materials are similar to those of Example 1, with the difference being the selection of various process parameters; the specific process parameters are shown in Table 4.

[0108] Table 4

[0109] In this embodiment, the performance parameters of the positive electrode material are shown in Table 5.

[0110] Table 5

[0111] Example 8

[0112] This embodiment provides a positive electrode material and a preparation method thereof, wherein the preparation method is similar to that of Example 1, except that in this embodiment, a dopant is mixed and calcined with an oxide precursor P2 and a second lithium source during the third sintering; the dopant is aluminum oxide.

[0113] Example 9

[0114] This embodiment provides a positive electrode material and a preparation method thereof, wherein the preparation method is similar to that of Example 1, except that in this embodiment, a dopant is mixed and calcined with the oxide precursor P2 and the second lithium source during the third sintering; the dopant is sulfur.

[0115] Example 10

[0116] This embodiment provides a positive electrode material and a preparation method thereof, wherein the preparation method is similar to that of Example 1, except that in this embodiment, the obtained positive electrode material is mixed with a coating agent for modification and sintering.

[0117] The coating agent is red phosphorus, the sintering temperature of the modified sintering is 500°C, and the sintering time is 4 hours.

[0118] Example 11

[0119] This embodiment provides a positive electrode material and a preparation method thereof, wherein the preparation method is similar to that of Example 1, except that in this embodiment, the obtained positive electrode material is mixed with a coating agent for modification and sintering.

[0120] The coating agent is magnesium hydroxide, the sintering temperature of the modified sintering is 400° C., and the sintering time is 5 h.

[0121] The performance parameters of the positive electrode materials provided in Examples 8-11 are shown in Table 6.

[0122] Table 6

[0123] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A positive electrode material, in, The composition of the positive electrode material is shown in formula (I): Ni x Co y Mn 1-x-y D k Li z O 2 (I); In the positive electrode material, the value ranges of x, y, z, and k are as follows: 0.6<x<1, 0<y<0.2, and x+y<1; 1≤z≤1.05,0≤k≤0.05; Wherein, D is a modifying element, and the modifying element includes at least one of S, P, F, B, Al, Ti, Mg, Cr, Zr, V, Nb, Y, W, Ta, Co, Ce, and Zn; The value range of the compaction density PD of the electrode sheet prepared by the positive electrode material is as follows: 3.0g / cm 3 <PD<3.8g / cm 3 ; The particle size distribution of the positive electrode material is as follows: D10<8μm, 5μm<D50<15μm, 10μm<D90<30μm; The lithium-nickel mixing degree Li / Ni mixing of the positive electrode material has a numerical range as follows: 0%<Li / Ni mixing<2.5%; A three-electrode battery cell is prepared by using the positive electrode material. During the charge and discharge cycle of the three-electrode battery cell, the electrochemical active surface area change rate a of the positive electrode material is less than 5%.

2. The positive electrode material according to claim 1, in, The calculation method of the electrochemical active surface area change rate a is shown in formula (II): a=(S n -S 0 ) / n×100% (Ⅱ); Wherein, n is the number of charge and discharge cycles of the three-electrode battery cell, S 0 is the electrochemically active surface area of ​​the positive electrode material during the first cycle of the three-electrode battery cell; S n It is the electrochemically active surface area of ​​the positive electrode material after the three-electrode battery cell is charged and discharged for n cycles.

3. The positive electrode material according to claim 1 or 2, in, The electrochemically active surface area is obtained by the following method: Testing the positive electrode overpotential response η caused by the excitation current I of the three-electrode battery cell; The excitation current I of different current magnitudes and the corresponding positive electrode overpotential response η are subjected to least squares fitting using the Butler-Volmer equation to obtain the electrochemically active surface area of ​​the positive electrode material.

4. The positive electrode material according to claim 3, in, The Butler-Volmer equation represents the excitation current I and the exchange current i 0 relationship; Wherein, the exchange current i 0 is proportional to the electrochemically active surface area; The exchange current i 0 It is the product of the electrochemical active surface area and the proportionality coefficient.

5. The positive electrode material according to claim 4, in, The Butler-Volmer equation is shown in formula (III): Where I is the excitation current, N is the proportional coefficient obtained by fitting, and R S is the ohmic impedance, R SEI is the interface impedance, R CT is the charge transfer impedance, F is the Faraday constant, R is the gas constant, T is the temperature of the three-electrode cell during the test, A is the electrochemically active surface area obtained by fitting, and η is the positive electrode overpotential response.

6. A method for preparing a positive electrode material, in, The preparation method is used to prepare the positive electrode material according to any one of claims 1 to 5, and the preparation method comprises the following steps: Primary calcination: calcining the nickel-cobalt-manganese ternary precursor to obtain the oxide precursor P 1 ; Secondary calcination: The oxide precursor P 1 Mixed with the first lithium source and calcined to obtain an oxide precursor P 2 ; Three calcinations: The oxide precursor P 2 The positive electrode material is mixed with a second lithium source and calcined to obtain the positive electrode material.

7. The preparation method according to claim 6, in, The amount of lithium in the first lithium source is M 1 The sum of the amounts of nickel, cobalt and manganese in the nickel-cobalt-manganese ternary precursor M 0 Satisfy: 0<M 1 / M 0 ≤0.8; The amount of lithium in the second lithium source is M 2 The amount M of the lithium element in the first lithium source 1 , and the sum of the amounts of nickel, cobalt and manganese in the nickel-cobalt-manganese ternary precursor M 0 Satisfies: 1-M 1 / M 0 ≤M 2 / M 0 ≤1.05-M 1 / M 0 .

8. The preparation method according to claim 6, in, The process conditions of the primary calcination include: a heating rate of 2-5°C / min, a calcination temperature of 300-500°C, and a calcination time of 1-5h.

9. The preparation method according to claim 6 or 8, in, The process conditions of the secondary calcination include: a heating rate of 2-5°C / min, a calcination temperature of 600-1000°C, a calcination time of 2-8h, and a cooling rate controlled at 2-5°C / min.

10. The preparation method according to any one of claims 6, 8 or 9, in, The process conditions of the three calcinations include: a heating rate of 2-5°C / min, a calcination temperature of 600-1000°C, a calcination time of 6-20h, and a cooling rate controlled at 2-5°C / min.

11. The preparation method according to any one of claims 6 to 10, in, In the three calcination steps, the dopant and the oxide precursor P 2 Mixing and calcining with the second lithium source; Wherein, the dopant includes at least one element of S, P, F, B, Al, Ti, Mg, Cr, Zr, V, Nb, Y, W, and Ta.

12. The preparation method according to any one of claims 6 to 10, in, The preparation method further comprises modifying and sintering the positive electrode material once or multiple times; The modified sintering is to mix and sinter the positive electrode material and the coating agent; the sintering temperature of the modified sintering does not exceed 600°C; Wherein, the coating agent contains at least one element of Co, P, F, B, Al, Ti, Mg, Cr, Zr, Ce, W, and Zn.

13. A battery, in, Comprising the positive electrode material as described in any one of claims 1 to 5.

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