CN113998744A

CN113998744A - High-capacity and high-multiplying-power LiNi0.6Co0.2Mn0.2O2Positive electrode material, preparation method and application

Info

Publication number: CN113998744A
Application number: CN202111275646.7A
Authority: CN
Inventors: 江奇; 杨潇潇; 卢晓英
Original assignee: Southwest Jiaotong University
Current assignee: Southwest Jiaotong University
Priority date: 2021-10-29
Filing date: 2021-10-29
Publication date: 2022-02-01

Abstract

The invention relates to the technical field of preparation of ternary cathode materials, in particular to high-capacity and high-multiplying-power LiNi_0.6Co_0.2Mn_0.2O₂The positive electrode material and the preparation method thereof comprise the following steps: s1, taking nickel salt, cobalt salt, manganese salt, organic matter and polyvinylpyrrolidone to react in a solvent, and fully drying after the reaction is finished to obtain black powder; s2, uniformly mixing the black powder with lithium hydroxide, and sintering to obtain the LiNi_0.6Co_0.2Mn_0.2O₂. According to the invention, a complex combustion method is utilized, organic matters such as citric acid and the like are used as complexing agents, PVP is added to assist in regulating and controlling the crystal morphology, and the high-temperature calcination condition is strictly controlled, so that a loose and porous precursor and an NCM622 material with excellent performance are prepared. The material shows excellent high discharge capacity, high rate and good cycle performance within the normal charge-discharge voltage range (2.8-4.3V).

Description

High-capacity and high-multiplying-power LiNi0.6Co0.2Mn0.2O2Positive electrode material, preparation method and application

Technical Field

The invention relates to the technical field of preparation of ternary cathode materials, in particular to high-capacity and high-multiplying-power LiNi_0.6Co_0.2Mn_0.2O₂A positive electrode material and a preparation method thereof.

Background

In recent years, ternary positive electrode materials have been widely used in the fields of portable electronic devices, hybrid vehicles, electric vehicles, and the like, because of their advantages of high specific discharge capacity, high operating voltage, and long service life. However, the electric vehicle is limited by cost, mileage ratio and charging time, and still cannot meet the customer requirements well. The pursuit of longer mileage, fast charging and high safety requirements on batteries have become a new direction for the development of lithium ion batteries. And in the ternary positive electrode material, LiNi_0.6Co_0.2Mn_0.2O₂(NCM622) materials compatible with LiNi_0.5Co_0.2Mn_0.3O₂High stability of (NCM523) and proximity to LiNi_0.8Co_0.1Mn_0.1O₂The high specific discharge capacity of (NCM811) has become one of the mainstream products of ternary cathode materials at present. However, in response to the demands of high-capacity and high-rate charging of current market users, the NCM622 material still faces the problem of how to improve its capacity performance and how to improve its high-rate performance. As is well known, the charging and discharging process of lithium ion batteries is carried out by Li⁺The positive electrode, the negative electrode, the electrolyte and the diaphragm are inserted back and forth, and the charge of an external circuit is transferred. Thus NCM622 for high rate performance relies primarily on Li⁺Rapid de-intercalation in the cell, and rapid transfer of charge; while higher capacities are more dependent on Li deintercalatable in NCM622⁺The number of the cells. At present, many researchers improve the discharge specific capacity and rate capability of the NCM622 material by means of doping, cladding, improving the working voltage, preparing single crystals and the like. In recent years, there has been a trend toward the combined use of various means, such as Yao et al (Yao L, Liang F, Jin J, et al. improved electrochemical property of Ni-rich LiNi)_0.6Co_0.2Mn_0.2O₂ cathode via in-situ ZrO₂coating for high energy density lithium ion batteries[J]Chemical Engineering journal.2020,389:124403.) the person used commercial NCM622 secondary particles to obtain NCM622 by two-step calcination, giving 197mah.g under increased operating voltage (2.8-4.5V)^-1The specific discharge capacity at (0.1C) and the specific discharge capacity at 10C were also 81.8mAh.g^-1. Then coating ZrO₂The specific discharge capacity of the coating under 10C is improved to 112.7mAh.g^-1. Wang et al (Wang R, Li Z, Yang Z, et al. synergistic effect of Ce⁴⁺ modification on the electrochemical performance of LiNi_0.6Co_0.2Mn_0.2O₂ cathode materials at high cut-off voltage[J]1268-^-1(0.1C) specific discharge capacity. The specific discharge capacity under 5C is 91.9mAh.g^-1. Then through Ce⁴⁺The discharge capacity is improved to 114.9mAh.g by doping^-1(5C) In that respect The combined use of multiple means can improve the capacity performance and the high rate performance of the NCM622 material to a certain extent. However, the improvement of the working voltage, the surface coating and the element doping are modification treatment of the electrode material, which belongs to external causes and inevitably causes cost increase and some performance reduction. If the electrode material can be prepared by strictly controlling the electrode material, the obtained electrode material body (internal cause) can obtain high capacity and high rate performance, and if the subsequent treatment method is further used, a more excellent electrode material is obtained.

In view of this, the chapter uses complexation reaction to complex the metal ions of nickel, cobalt, manganese with organic matter (citric acid, etc.), so as to avoid agglomeration. And regulating the crystal morphology by polyvinylpyrrolidone (PVP), and removing organic matters by a combustion method to obtain the nano-scale precursor material. And finally, obtaining the NCM622 material with high capacity performance and high rate performance under the normal working voltage range without material coating and element doping through accurate calcining conditions.

Disclosure of Invention

The invention aims to overcome the defects of the prior art and provide a high-capacity and high-rate LiNi_0.6Co_0.2Mn_0.2O₂A positive electrode material and a preparation method thereof.

The purpose of the invention is realized by the following technical scheme: LiNi_0.6Co_0.2Mn_0.2O₂The preparation method comprises the following steps:

s1, taking nickel salt, cobalt salt, manganese salt, organic matter and polyvinylpyrrolidone to react in a solvent, and fully drying to obtain black powder after the reaction is finished;

s2, uniformly mixing the black powder with lithium hydroxide, and sintering to obtain the LiNi_0.6Co_0.2Mn_0.2O₂。

Preferably, in S1, the nickel salt, the cobalt salt and the manganese salt are respectively nickel acetate, cobalt acetate and manganese acetate;

and/or the organic matter is citric acid, oxalic acid, tartaric acid, malic acid and maleic acid.

Preferably, in S1, the solvent is water or ethanol.

Preferably, in S1, the molar ratio of the nickel salt, the cobalt salt, the manganese salt and the organic matter is 0.6:0.2:0.2: 0.5-5.

And/or the mass ratio of the polyvinylpyrrolidone to the sum of the mass of the nickel salt, the mass of the cobalt salt and the mass of the manganese salt is 0.005-0.025: 1.

preferably, in S2, the black powder is uniformly mixed with an excess amount of the lithium hydroxide, and the ratio of the sum of the molar amounts of the metal elements in the black powder to the molar amount of the lithium hydroxide is 1:1 to 1.2.

Preferably, in S1, the method for sufficiently drying includes: firstly, evaporating a mixed solution obtained after the reaction is finished to dryness to obtain initial black powder, uniformly grinding the initial black powder, and drying again to obtain the black powder. The method of re-drying is not limited in the present invention, and operations such as low-temperature drying may be used.

Preferably, in S2, the sintering method is: sintering the uniformly mixed black powder and lithium hydroxide in an air atmosphere at 400-600 ℃ for 12-1h, cooling, grinding again, and sintering at 700-950 ℃ for 20-2h in an oxygen atmosphere to obtain the LiNi_0.6Co_0.2Mn_0.2O₂。

The invention also provides LiNi prepared by the preparation method_0.6Co_0.2Mn_0.2O₂A ternary positive electrode material.

The invention also provides a lithium ion battery which comprises a positive plate, wherein the positive material used by the positive plate is LiNi_0.6Co_0.2Mn_0.2O₂A ternary positive electrode material.

The invention also provides the LiNi_0.6Co_0.2Mn_0.2O₂The ternary anode material or the lithium electronic battery is applied to the fields of portable electronic equipment, hybrid electric vehicles and electric vehicles.

The invention has the beneficial effects that: according to the invention, a complexing combustion method is utilized, citric acid is used as a complexing agent, PVP is added to assist in regulating and controlling the crystal morphology, and the high-temperature calcination condition is strictly controlled, so that a loose and porous precursor and an NCM622 material with excellent performance are prepared. The material shows excellent high discharge capacity, high rate and good cycle performance within a normal charge-discharge voltage range (2.8-4.3V), and specifically comprises the following components: the specific discharge capacity under 0.2C can reach 209.10mAh^-1Even under 10C, 116.46mAh.g can be achieved^-1Specific discharge capacity of (2). After 100 cycles under the condition of a 0.5C half cell, the discharge capacity retention rate is 100.01 percent. Through structural representation, the prepared material has an excellent layered structure and low cation mixed arrangement. The excellent electrochemical performance of the lithium ion battery is theoretically verified by the small charge transfer resistance and the high lithium ion diffusion coefficient of the lithium ion battery.

Drawings

FIG. 1 is a schematic diagram of a material preparation process;

FIG. 2 shows XRD diffraction patterns (a) and partial enlargements (b) of 6 materials (622-T800, 622-T825, 622-T850, 622-T875, 622-T900, 622-T925);

FIG. 3 shows XRD diffraction pattern (a), XRD refinement result (b) and Raman spectrum (c) of 622-T850 material;

FIGS. 4-1 and 4-2 are SEM images of precursors, 6 materials and their corresponding particle size distribution maps;

FIG. 5 is a plot of electrochemical performance for 6 materials, where a is the rate performance curve; b is a cycle performance curve at 0.5C; c is cyclic voltammetry curve; d is a first charge-discharge curve at 0.1C;

FIG. 6 is a graph of electrochemical performance of 622-T850 materials, where a is the rate performance curve; b is a cycle performance curve at 0.5C; c is cyclic voltammetry curve; d is a first charge-discharge curve at 0.1C, and e is a capacity differential curve;

FIG. 7 shows EIS curves (e) and Z' -omega for 6 materials^-1/2Curve (f);

FIG. 8 shows EIS curves (f) and Z' -omega for 622-T850 material^-1/2The corresponding equivalent circuit diagram is shown in the inset of the curves (g), (f).

Detailed Description

The technical solutions of the present invention are further described in detail below with reference to the accompanying drawings, but the scope of the present invention is not limited to the following.

Example 1 preparation of the Material

Refer to the schematic preparation flow of fig. 1. The molar ratio of the massage is n_Ni：n_Co：n_Mn6: 2: 2, weighing a certain amount of Ni (CH)₃OO)₂·4H₂O，Co(CH₃OO)₂·4H₂O，Mn(CH₃OO)₂·4H₂O，C₆H₈O₇·H₂O(n_citricacid: n_Ni+Co+Mn1.5:1) and a certain amount of PVP (m)_pvp:m ₆₂₂1 wt%) in 60ml of deionized water, stirring on a magnetic stirrer for 0.5h and then sonicating for 1h, and transferring the mixed solution to a resistance furnace to evaporate to dryness to obtain an initial black powder. Grinding the powder uniformly, placing in a muffle furnace at 150 deg.C (heating rate at 5 deg.C for min)^-1) Sintering for 6h to completely remove the materialWater content in the material to obtain black powder. After cooling with the furnace, the mixture is added with 5mol percent of excess LiOH & H₂Grinding O thoroughly, mixing, and placing in muffle furnace at 450 deg.C (heating rate of 5 deg.C for min)^-1) And sintering for 6 h. Cooling, grinding, and placing into a tube furnace at 800 deg.C, 825 deg.C, 850 deg.C, 875 deg.C, 900 deg.C, 925 deg.C (heating rate of 2 deg.C. min.) in oxygen atmosphere^-1) Sintering for 10h to obtain LiNi_0.6Co_0.2Mn_0.2O₂. The temperatures are labeled 622-T800, 622-T825, 622-T850, 622-T875, 622-T900, 622-T925, respectively, in that order.

Example 2 structural characterization test

3.1X-ray diffractometer

The prepared material is subjected to phase-by-phase analysis by an X-ray diffractometer (XRD, X' Pert PRO), and unit cell parameters and the like are determined according to the position intensity and width of diffraction peaks. Thereby judging the cation mixed-arrangement degree of the material, whether the laminated structure is good or not, and the like. CuKa is used as a target source, the wavelength lambda of X-ray is 0.154nm, the working voltage is 40KV, the working current is 40mA, and the scanning range is 10-90 degrees.

3.2 Raman tester

The analysis of ion bond Raman vibration factor group was performed by using Raman tester (HORIBA XPLORA ONE).

3.3 scanning Electron microscope

The microstructure of the sample was analyzed by Scanning Electron Microscope (SEM, JEOL JSM-7001F). The difference of particle size, particle shape and the like of the material prepared by adopting different sintering temperatures and different raw materials can be seen visually.

3.4 dynamic light scattering particle size analyzer

The particle size was measured using a dynamic light scattering particle size analyzer (Malvern Mastersizer 3000). So that the size distribution range, the uniformity degree and the like of the material particles can be judged.

Example 3 electrochemical Performance testing

The positive plate is prepared from the prepared material, acetylene black and a binder according to the mass ratio of 8:1:1The components are added with a proper amount of N-methyl pyrrolidone (NMP), evenly mixed into smooth slurry, coated on the surface of a clean aluminum foil, and placed in a vacuum drying oven to be dried for 12 hours in vacuum at the temperature of 120 ℃. It was then cut into 15mm positive electrode disks. And then assembling the half-cell in a glove box filled with argon according to the sequence of the positive electrode shell, the positive electrode plate, the diaphragm, the lithium plate, the steel sheet, the spring piece and the negative electrode shell. Finally, the battery is placed into a sealing machine for packaging. Wherein the diaphragm adopts Celgard2400, and the electrolyte is 1 mol.L^-1LiPF of₆Solution (solvent EC: DMC ═ 1: 1).

The constant-current charge and discharge test is carried out on the battery by adopting a new power high-precision battery program-controlled tester, wherein the voltage range is 2.8-4.3V, and the test temperature is constant at 25 ℃. The cells were subjected to Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) using an Electrochemical workstation. The voltage range of the cyclic voltammetry test is 3.0-4.3V, and the scanning speed is 0.1mV^-1. In the AC impedance test, the amplitude of the test is 5mV, and the frequency range is 10⁵-0.01Hz。

Example 4 results and discussion

4.1XRD diffraction Pattern

Refer to the XRD diffraction pattern of fig. 2. The materials obtained by sintering at different temperatures all correspond to PDF #87-1562 and have typical hexagonal alpha-NaFeO₂Structure belonging to the R3m space group. The diffraction peak of each group of XRD is sharp and high in intensity, and no obvious impurity peak exists, so that the obtained six materials are high in purity and are single-phase NCM622 materials. The c/a values for all samples were greater than 4.9, indicating that these six materials had a better layered structure. The value of I (003)/I (004) of the material obtained by sintering at 800 ℃ is less than 1.2, which indicates that the cation mixed discharge is serious. And the cation mixed-discharging degree is improved along with the increase of the temperature, and the I (003)/I (004) values of the obtained material are all larger than 1.2. Furthermore, as the temperature increases, the I (003)/I (004) values increase and then decrease. The temperature is too low, the reaction is incomplete, and the crystallization performance of the material is poor. Excessive volatilization of lithium salt and oxygen loss are easily caused during the process of over-high temperature. The material is made of when the sintering temperature is 850 DEG CThe value of I (003)/I (004) is the largest, and the cation mixed-discharging degree is the smallest, so that the temperature is the optimal sintering temperature of the material. (Table 1 below is the unit cell parameters for the 6 materials)

TABLE 1

Referring to fig. 3, fig. 3 shows XRD diffraction pattern (a), XRD refinement result (b) and raman spectrum (c) of 622-T850 material. As can be seen from a and b in fig. 3, the splitting of the two sets of peaks (006)/(102) and (108)/(110) is quite apparent, indicating that the material has a highly ordered layered structure. In order to obtain accurate unit cell parameter data, XRD data of the material obtained by Rietveld fine modification through GSAS software is wRp which is less than 10%, which shows that the fitting degree is good, and the unit cell parameters are obtained as follows: the a axis is 0.28740nm, the c axis is 1.42083nm, and the value of c/a is 4.9437 and is more than 4.9, which indicates that the material has a good development of a laminated structure. I is₍₀₀₃₎/I₍₁₀₄₎The ratio of the peak intensities was 1.3761, indicating that the material had less cation shuffling. As can be seen from c in FIG. 3, the resulting material was found to be at 300-800cm^-1There are two distinct absorption peaks in the range. At 500cm^-1Left and right are E_gVibration absorption peak from O-M-O bending vibration, i.e. two oxygen atoms along the perpendicular to LiMO₂The c-axis of (a) rotationally vibrates. At 600cm^-1Left and right are A_1gVibration absorption peaks from symmetric stretching of M-O, i.e. two oxygen atoms from adjacent oxygen layers parallel to the LiMO₂C-axis of (A) in the opposite direction_1gHas a vibration intensity significantly greater than E_gAnd no other miscellaneous peak appears, which indicates that the material has high purity. (Table 2 below is the unit cell parameters for 622-T850 materials)

TABLE 2

4.2SEM image and corresponding particle size distribution diagram

Referring to fig. 4-1 and 4-2, fig. 4-1 and 4-2 are SEM images of the precursor, 6 materials and their corresponding particle size distribution maps. The precursor is black powder completely dehydrated, and as can be seen from the figure, the precursor is formed by agglomeration of small uniform and fine particles and is porous. Then, after two-step sintering, final sintering is performed at different sintering temperatures to obtain NCM materials at different sintering temperatures as shown in fig. 4-1 and 4-2. It can be seen that under the sintering condition of 800 ℃, the grain boundaries between the obtained material particles are not obvious, the particle size distribution is not uniform, the distribution range is 136-1681nm, the grain development is incomplete, and the crystallinity is not good. The reaction is incomplete due to the low temperature, amorphous particles are easy to generate, and the electrochemical performance of the material is influenced. With the increase of temperature, the crystal grains grow gradually and are uniform. When the sintering temperature is 900 ℃, the obtained material begins to have abnormal enlargement of partial particles, the particle size distribution range is in between, and in addition, some small particles which are not formed are also subjected to regrowth. The excessive temperature can enlarge the crystal grains and reduce the specific surface area, which is not beneficial to the insertion and extraction of lithium ions. When the temperature is 850 ℃, the crystallinity of the particles of the material is good and the particles are uniformly distributed, the particle size distribution range is 636-1121nm, the particle size distribution range is mainly concentrated to about 800-900nm, and the particles with uniform size are beneficial to the extraction of lithium ions, thereby indicating that the 622-T850 material obtained by sintering under the temperature condition has the best electrochemical performance.

Refer to SEM pictures of 622-T850 in FIGS. 4-1 and 4-2 and their corresponding particle size distribution plots. 622-T850 is porous material, and the porous structure enlarges the contact area of the material and the electrolyte, increases the reaction sites thereof, and enables Li⁺And is easier to be separated from the material, thereby improving the capacity of the material. Meanwhile, the porous structure is beneficial to the shuttle of lithium ions, and Li is realized⁺Thereby improving the rate capability of the material. The particle size distribution is relatively uniform, the particle size distribution range is 636-1121nm, the particle size distribution is normal, and the particle size distribution is concentrated around 900 nm. The surface of the particles is smooth, the grain boundary between the particles is obvious, and the surface has certain gaps, thereby being beneficial to the full contact of the electrolyte and the material and shortening the transmission of lithium ionsThe transport path is beneficial to the rapid migration of lithium ions, and thus the rate capability of the material is also expected to be excellent.

4.3 electrochemical Performance Curve

Referring to fig. 5, fig. 5 is a graph of electrochemical performance of 6 materials. In fig. 5, a is a rate performance curve of six materials, each rate is cycled for six times, and detailed data is shown in table 3. It can be seen that the specific discharge capacity of the material is gradually reduced as the rate is increased, which is due to the polarization phenomenon caused by the insertion and extraction of lithium ions into and from the interior under a large current. Along with the increase of the temperature, the discharge specific capacity of the material is gradually increased and then reduced, and the method is consistent with the analysis of the previous structural test. The crystallization performance of the material is poor due to the fact that the temperature is too low, the reaction is more and more complete along with the increase of the temperature, and crystal grains are gradually enlarged and uniform. When the temperature is too high, the production speed of the particles is increased, and primary particles are increased. Too high or too low a temperature is detrimental to the electrochemical performance of the material.

TABLE 3

Referring to fig. 6, a in fig. 6 is the electrochemical performance curve of 622-T850 material. When the calcining temperature is 850 ℃, the rate capability of the material is higher than that of other samples, and the method has obvious advantages. The excellent rate performance is shown even at 5C and 10C, which is mainly caused by the moderate particle size and the high lithium ion diffusion coefficient. With the increase of the magnification, the specific discharge capacity of the material gradually decreases due to polarization caused by insertion and extraction of lithium ions into and from the inside under a large current. The material still has high specific discharge capacity under high multiplying power, which is mainly due to the fact that the material is small in particle size and beneficial to full infiltration of electrolyte, a porous precursor structure is beneficial to forming of porous materials, and the porous structure provides a channel for lithium ion migration and is beneficial to rapid migration of lithium ions.

In fig. 5 b is the cycle performance curve at 0.5C for the six materials. After 100 cycles, the capacity retention rates of the six materials are 105.08%, 95.47%, 101.07%, 92.75%, 78.51% and 59.94%, respectively. As the temperature increases, the capacity retention decreases. Probably, when the temperature is too high, particles of the material are increased, the transmission path of lithium ions is increased, the lithium ion desorption is hindered, the internal resistance of the battery is increased, and the cycle performance is weakened; meanwhile, the larger particle size of the particles can increase the side reaction with the electrolyte, and destroy the stability of the material structure, thereby sharply attenuating the capacity of the material.

In FIG. 6 b is the cycle performance curve at 0.5C for 622-T850 material. As can be seen from the graph, the initial discharge capacity was about 176.60mAh.g^-1The discharge capacity increased with the progress of the cycle, and reached 195.06mAh.g after 37 cycles^-1Then maintained at this level, after 70 cycles, there was also 196.13mAh^-1After which the discharge capacity began to drop, and after 100 f the capacity was still 178.51mAh^-1. The reason for this is believed to be that the discharge capacity of the material increases continuously in the first 37 cycles because the material is electrochemically activated only 1 time at a rate of 0.1C, but the material is not sufficiently activated. After 37 cycles, the discharge capacity was stable until after 70 cycles no decay occurred. In conclusion, after the material is subjected to 100 cycles, the material discharges mAh^-1The capacity retention rate was 100.01%. The material still has higher specific discharge capacity and higher capacity retention rate after being cycled for 100 times, and shows good cycling stability. This is consistent with the previous structural analysis. Therefore, the damage to the surface structure of the material after the contact surface between the electrolyte and the material is expanded and the reaction is accelerated is very small, which shows that the material has very stable structure and excellent electrolyte corrosion resistance.

In fig. 5, C is the first charge-discharge curve of six materials at 0.1C. All six materials showed the same characteristic curve. The material obtained by calcining at 800 ℃ has the shortest discharge platform and the first discharge capacity of 113.7 mAh/g. The material calcined at 850 ℃ has the longest charge-discharge platform and the first discharge specific capacity of 204.7 mAh/g. A longer discharge plateau corresponds to a higher capacity, which corresponds to a high rate capability.

In FIG. 6, C is the first charge-discharge curve of 622-T850 material at 0.1C. As can be seen from the figure, the material has a longer charge-discharge platform, and the first discharge specific capacity reaches 204.75mAh^-1The coulombic efficiency was 81.97%. This is mainly due to the fact that part of Li is consumed during the first charge-discharge process⁺Forming a stable SEI film, thereby reducing coulombic efficiency; due to the small size of the material and the porous structure, more SEI films are formed, so that the coulombic efficiency is low, which is consistent with the SEM test results.

FIG. 5 d is a cyclic voltammogram of six materials, each of which shows a pair of redox peaks, representing Ni²⁺/Ni³⁺/Ni⁴⁺To convert between them. The oxidation-reduction peaks corresponding to the six materials respectively correspond to peak potentials of: 3.868V/3.672V, 3.853V/3.681V, 3.731/3.672V, 3.794/3.682V, 3.808/3.679V and 3.854/3.691V. In addition, the difference of the oxidation-reduction peaks is generally used as a measure of the degree of polarization of the electrode sheet. The peak difference values of the redox peaks of the six materials are respectively as follows: 0.196V, 0.172V, 0.059V, 0.112V, 0.129V, 0.163V. The smaller the peak difference value is, the smaller the polarization degree is, and the more excellent the electrochemical performance of the material is. Similarly, the material obtained by calcining at 850 ℃ has the smallest difference of oxidation reduction peaks and the smallest polarization degree, which indicates that the material has excellent stability. Meanwhile, the areas included in the CV curves of the six materials are integrated to obtain the areas of the six materials, which are 0.11774, 0.12499, 0.13531, 0.13418, 0.13202 and 0.12666 respectively. The larger the area integral corresponding to the CV curve, the larger the discharge specific capacity of the material is, the more excellent the corresponding material performance is. This also explains that the 622-T850 material has a high discharge capacity.

In FIG. 6, d is the cyclic voltammogram of 622-T850 material, and it can be seen that the material has a pair of redox peaks, representing Ni²⁺/Ni³⁺/Ni⁴⁺To convert between them. The oxidation reduction peak of the compound has the corresponding peak potentials as follows: 3.731/3.672V; the peak difference values are: 0.059V. The difference of oxidation reduction peak is usually used to measure the polarization degree of electrode slice, and the difference is compared withThe small peak difference shows that the polarization degree is small, which is beneficial to the expression of high-rate electrochemical performance.

In FIG. 6, e is the capacity differential curve for 622-T850 material cycling at 0.5C. The method is used for researching the phase change behavior of the material in the oxidation-reduction process, and the capacity difference-voltage calculation is carried out on the material in the charge-discharge process. As can be seen from the figure, the capacity differential curves of the materials all show double oxidation peaks, which respectively represent the transition from the hexagonal phase H1 phase to the monoclinic phase M phase and the transition from the phase M to the hexagonal phase H2. The oxidation peak of the material appears at a higher voltage during the initial cycling, and the oxidation peak position of the material drops and shifts to a lower potential as the cycling progresses, indicating that the material undergoes an irreversible electrochemical reaction during the initial cycling. In addition, the phase change of the material H2 to H3 is irreversible phase change, and the material has good reversibility due to the lack of the phase change of H2 to H3, and the structural stability of the material is reflected, which corresponds to the excellent cycle performance and high rate performance of the material.

4.4 alternating Current impedance test (EIS) curves and Z' -omega of six materials^-1/2Curve

Referring to FIG. 7, FIG. 7 shows EIS curves (e) and Z' - ω of 6 materials^-1/2Curve (f). All the EIS curves of the materials are composed of a semicircle and a diagonal line. The semi-circle of the high frequency region represents the charge transfer impedance Rct, which corresponds to the transmission impedance of lithium ions at the electrode surface. The intersection of the high-frequency region semicircle and the x-axis is Rs, which is the internal resistance of the battery, corresponding to the resistance of lithium ions diffusing and migrating through the SEI film. The diagonal lines in the low frequency region indicate the lithium ion diffusion resistance Zw, which is caused by diffusion of lithium ions in the electrode. Z' -omega^-1/2The slope of the straight line in the curve corresponds to the wobbe constant (σ).

Usually we use formulas

D_li ⁺＝R²T²/2A²F⁴n⁴C²σ²

To calculate the diffusion coefficient (D) of lithium ions_li ⁺). Wherein R is 8.314 J.K^-1·mol^-1Represents a gas constant, T298K, represents an absolute temperature, and n representsTable charge transfer number, C represents lithium ion concentration, F ═ 96500C/mol, represents faradic constant, and a is active area.

Impedance fitting was performed on the EIS curve and the resulting impedance data is shown in table 4 according to the calculation formula. As can be seen from the table, the charge transfer resistance of the material at 850 ℃ is the smallest and 30.71 omega, and the maximum ion diffusion coefficient is 1.093X 10^-13cm².S^-1. The material has smaller charge transfer resistance and larger ion diffusion coefficient, which shows that the lithium ions have higher speed in the migration process and are beneficial to the rate capability of the material.

TABLE 4

The foregoing is illustrative of the preferred embodiments of this invention, and it is to be understood that the invention is not limited to the precise form disclosed herein and that various other combinations, modifications, and environments may be resorted to, falling within the scope of the concept as disclosed herein, either as described above or as apparent to those skilled in the relevant art. And that modifications and variations may be effected by those skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. LiNi_0.6Co_0.2Mn_0.2O₂The preparation method is characterized by comprising the following steps:

s1, taking nickel salt, cobalt salt, manganese salt, organic matter and polyvinylpyrrolidone to react in a solvent, and fully drying after the reaction is finished to obtain black powder;

2. The method according to claim 1, wherein in S1, the nickel salt, the cobalt salt, and the manganese salt are respectively nickel acetate, cobalt acetate, and manganese acetate;

3. The method according to claim 1, wherein the solvent in S1 is water or ethanol.

4. The method according to claim 1, wherein in S1, the molar ratio of the nickel salt, the cobalt salt, the manganese salt and the organic matter is 0.6:0.2:0.2: 0.5-5;

5. the method according to claim 1, wherein in S2, the black powder is uniformly mixed with an excess amount of the lithium hydroxide, and the ratio of the sum of the molar amounts of the metal elements in the black powder to the molar amount of the lithium hydroxide is 1:1 to 1.2.

6. The method according to any one of claims 1 to 5, wherein in S1, the method for sufficiently drying is: firstly, evaporating a mixed solution obtained after the reaction is finished to dryness to obtain initial black powder, uniformly grinding the initial black powder, and drying again to obtain the black powder.

7. The method according to any one of claims 1 to 5, wherein in S2, the sintering is performed by: sintering the uniformly mixed black powder and lithium hydroxide in an air atmosphere at 400-600 ℃ for 12-1h, cooling, grinding again, and sintering at 700-950 ℃ for 20-2h in an oxygen atmosphere to obtain the LiNi_0.6Co_0.2Mn_0.2O₂。

8. The process according to any one of claims 1 to 7Obtaining LiNi_0.6Co_0.2Mn_0.2O₂A ternary positive electrode material.

9. A lithium ion battery comprising a positive electrode sheet, wherein the positive electrode material used in the positive electrode sheet is LiNi according to claim 8_0.6Co_0.2Mn_0.2O₂A ternary positive electrode material.

10. The LiNi of claim 8_0.6Co_0.2Mn_0.2O₂Use of a ternary positive electrode material or a lithium-ion battery according to claim 9 in the field of portable electronic devices, hybrid vehicles and electric vehicles.