CN109860586B

CN109860586B - Modified manganese dioxide, high-temperature lithium manganate material and preparation method thereof

Info

Publication number: CN109860586B
Application number: CN201910110431.6A
Authority: CN
Inventors: 江奇; 高艺珂; 卢晓英
Original assignee: Southwest Jiaotong University
Current assignee: Southwest Jiaotong University
Priority date: 2019-02-11
Filing date: 2019-02-11
Publication date: 2020-12-01
Anticipated expiration: 2039-02-11
Also published as: CN109860586A

Abstract

The invention discloses a modified manganese dioxide, a high-temperature lithium manganate material and a preparation method thereof, and relates to the technical field of electrode materials. The preparation method of the modified manganese dioxide comprises the following steps: mixing and reacting bicarbonate, manganese salt, aluminum powder, a solvent and graphene, filtering and precipitating, and carrying out heat treatment. The preparation method of the high-temperature lithium manganate material takes lithium salt and the modified manganese dioxide as raw materials, and adopts an ion permeation method to lead Li to be⁺Mixed with modified manganese dioxide and infiltrated into the solid, which is then calcined. The prepared material is blocky, and the lithium manganate is doped with aluminum and is coated by graphene; preferably, the lithium manganate is in an octahedral block shape, and adjacent particles are embedded into each other. The high-temperature lithium manganate has the advantages of uniform size and small particle size, and has excellent electrochemical performance.

Description

Modified manganese dioxide, high-temperature lithium manganate material and preparation method thereof

Technical Field

The invention relates to the technical field of electrode materials, in particular to a modified manganese dioxide, a high-temperature lithium manganate material and a preparation method thereof.

Background

Spinel-type LiMn₂O₄The lithium ion battery cathode material has the characteristics of simple synthesis process, low cost, excellent low-temperature performance, good rate performance and the like, and becomes a key electrode material of the current three-power lithium ion batteries.

However, since LiMn₂O₄The high temperature is not stable enough, which always restricts the popularization and application. In recent years, researchers have been working on spinel-type LiMn₂O₄The structural change in the process of lithium insertion and removal proposes two different high-temperature cycle decay mechanisms, namely oxygen defect and Jahn-Teller effect. Wherein the Jahn-Teller effect is mainly realized by changing the crystal structure of spinel to ensure that LiMn is added₂O₄The solubility in organic solvents can be greatly improved. But instead of the other end of the tubeThis increases the electrolyte couple LiMn during battery cycling₂O₄Corrosion of LiMn₂O₄The high temperature stability of (b) has an effect.

Disclosure of Invention

The invention aims to provide modified manganese dioxide and a preparation method thereof, and aims to enable the particle size of the prepared manganese dioxide to be smaller, so that the subsequent preparation of lithium manganate with smaller particle size is facilitated.

The invention also aims to provide a high-temperature lithium manganate material and a preparation method thereof, aiming at improving the high-temperature stability and electrochemical performance of lithium manganate.

The technical problem to be solved by the invention is realized by adopting the following technical scheme.

The invention provides modified manganese dioxide, wherein the manganese dioxide is doped with aluminum and is coated by graphene;

preferably, the particle size of the modified manganese dioxide is 2.2-2.4 μm;

preferably, the modified manganese dioxide is spheroidal.

The invention provides a preparation method of modified manganese dioxide, which comprises the following steps:

mixing and reacting bicarbonate, manganese salt, aluminum powder, a solvent and graphene, filtering and precipitating, and carrying out heat treatment.

The invention also provides a preparation method of the high-temperature lithium manganate material, which takes lithium salt and the modified manganese dioxide as raw materials and adopts an ion permeation method to lead Li to be Li⁺Mixing with modified manganese dioxide and infiltrating into the solid, and then calcining the solid;

the lithium salt is selected from one or more of lithium sulfate, lithium hydroxide, lithium nitrate and lithium carbonate, and is preferably lithium hydroxide.

The invention also provides a high-temperature lithium manganate material, wherein the lithium manganate is blocky, and is doped with aluminum and coated by graphene;

preferably, the lithium manganate is in an octahedral block shape, and adjacent particles are embedded into each other;

more preferably, the lithium manganate electrode material is prepared by the preparation method.

The embodiment of the invention provides modified manganese dioxide and a preparation method thereof, and the modified manganese dioxide has the beneficial effects that: aluminum powder and graphene are added in the preparation process of manganese dioxide, so that the prepared manganese dioxide has a smaller average particle size, and the preparation of the lithium manganate with better high-temperature stability is facilitated.

The invention also provides a high-temperature lithium manganate material and a preparation method thereof, the high-temperature lithium manganate material is prepared by the modified manganese dioxide through an ion permeation method, and the obtained lithium manganate has a blocky structure, is doped with aluminum and is coated by graphene. The high-temperature lithium manganate provided by the invention has excellent high-temperature stability and electrochemical performance.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the embodiments will be briefly described below, it should be understood that the following drawings only illustrate some embodiments of the present invention and therefore should not be considered as limiting the scope, and for those skilled in the art, other related drawings can be obtained according to the drawings without inventive efforts.

FIG. 1 is a particle size distribution plot of a prepared sample;

FIG. 2 is an XRD pattern of the prepared sample;

FIG. 3 is an SEM photograph of a prepared sample;

FIG. 4 is a plot of the particle size distribution of the prepared samples;

fig. 5 shows the electrochemical performance of the prepared sample.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. The examples, in which specific conditions are not specified, were conducted under conventional conditions or conditions recommended by the manufacturer. The reagents or instruments used are not indicated by the manufacturer, and are all conventional products available commercially.

The modified manganese dioxide, the high-temperature lithium manganate material and the preparation method thereof provided by the embodiments of the present invention are specifically described below.

The embodiment of the invention provides a preparation method of a high-temperature lithium manganate material, which comprises the following steps:

s1 preparation of modified manganese dioxide

Mixing and reacting bicarbonate, manganese salt, aluminum powder, a solvent and graphene (G), filtering and precipitating, and carrying out heat treatment. Manganese salt reacts with bicarbonate to obtain manganese carbonate microspheres under the condition of stirring, and the modified manganese dioxide is obtained after heat treatment. The particle size of the manganese carbonate microspheres obtained in the reaction is relatively uniform, and the particle size of the manganese dioxide obtained is relatively small, approximately 2.445 μm.

Specifically, the bicarbonate is selected from any one of sodium bicarbonate, potassium bicarbonate and ammonium bicarbonate, preferably ammonium bicarbonate. The manganese salt is preferably manganese sulfate monohydrate, and can also be manganese nitrate. The reaction of the manganese sulfate monohydrate and the ammonium bicarbonate is more sufficient, impurities are not easy to introduce, and the obtained modified manganese dioxide is small in particle size and approximately spherical.

Specifically, the molar ratio of the manganese salt to the aluminum powder to the graphene is 1:0.0090-0.099: 0.01-0.1; preferably 1:0.0095-0.099: 0.01-0.05. The use amounts of the manganese salt, the graphene and the aluminum powder are preferably controlled within the above range, and the deposition, agglomeration and growth of particles can be effectively inhibited within the above range, so that the spherical modified manganese dioxide is obtained.

In practical operation, the preparation of modified manganese dioxide may comprise the following steps: dissolving manganese salt to obtain a first solution, and mixing and dissolving aluminum powder, graphene and bicarbonate to obtain a second solution; and dropwise adding the second solution into the first solution, reacting for 1.5-4h, filtering to obtain an intermediate solid, washing and drying the intermediate solid, and performing heat treatment. The second solution is dripped into the first solution in a mixing mode, so that the manganese carbonate microspheres obtained by the reaction have better uniformity.

Preferably, the bicarbonate is used in an amount to control Mn²⁺:HCO^3-1:2-2.5, the present examples employ bicarbonateThe excess mode can improve the reaction yield.

Specifically, the heat treatment process is to carry out heat treatment for 4-8h under the temperature condition of 350-450 ℃; more preferably, the heat treatment is carried out in an air atmosphere, and the temperature increase rate is controlled to be 0.5 to 1.5 ℃/min.

Preferably, polyvinylpyrrolidone and an organic solvent are also added to the first solution. The organic solvent can be methanol, ethanol, Ethylene Glycol (EG), etc., and organic high molecular compound polyvinylpyrrolidone (PVP) is added to form micelle when NH is added₄HCO₃With MnSO₄When the reaction generates precipitation, the precipitates are separated from each other by utilizing the steric hindrance effect of the micelle, so that the accumulation of particles can be obviously reduced; the addition of aluminum powder and graphene can reduce further deposition and agglomeration growth of particles; the aggregation and growth of the particles is further limited by the synergistic effect of the high viscosity ethylene glycol. Therefore, in the embodiment of the invention, the particle size of the modified manganese dioxide prepared by the method is adjusted by the organic matter, the aluminum powder and the graphene, so that the particle size of the modified manganese dioxide is smaller.

In the modified manganese dioxide prepared in the embodiment of the invention, the manganese dioxide is doped with aluminum and is coated by graphene, and the manganese dioxide can reach a sphere-like shape and has a smaller particle size through the doping of the aluminum powder and the graphene.

S2 preparation of lithium manganate by ion permeation method

Lithium salt and the modified manganese dioxide are used as raw materials, and Li is made by adopting an ion permeation method⁺Mixed with modified manganese dioxide and infiltrated into the solid, which is then calcined. The ion permeation method is that one raw material is in a solid state, the other raw material is in an ionic state in a solution, and ions can be deposited on the surface of the solid raw material more uniformly after a solvent is evaporated.

Specifically, modified manganese dioxide, lithium salt, water and an organic solvent are mixed, stirred under the heating condition until the solution is evaporated to dryness to obtain a solid manganese dioxide with lithium deposited on the surface, and then the solid is calcined for 5-7 hours under the temperature condition of 700-800 ℃ after being dried. In the evaporation process of water and the organic solvent, lithium ions are deposited on the surface of the solid manganese dioxide, and the evaporation refers to that the solution is basically evaporated, and the solvent is further removed in the subsequent drying process.

Further, the process of drying the solid manganese dioxide with lithium deposited on the surface is drying for 10-14h under the temperature condition of 100-140 ℃. Preferably, the solid manganese dioxide with lithium deposited on the surface is ground after drying and before calcining, so that the particle size and uniformity of the lithium manganate obtained by calcining are more ideal.

The lithium salt is selected from one or more of lithium sulfate, lithium hydroxide, lithium nitrate and lithium carbonate, and is preferably lithium hydroxide. The use of the above lithium salts can be adapted to the ion permeation method proposed in the examples of the present invention to form lithium ions in solution.

Wherein the molar ratio of the modified manganese dioxide to the lithium hydroxide is 1.03-1.15: 2; preferably 1.03-1.07: 2. The lithium hydroxide is preferably used in a slight excess amount so as to further improve the yield of the lithium manganate.

According to the high-temperature lithium manganate material provided by the embodiment of the invention, lithium manganate is in a block shape (please refer to a corresponding test example), and is doped with aluminum and coated with graphene; preferably, the lithium manganate is in an octahedral block shape, and adjacent particles are embedded into each other. The high-temperature lithium manganate has ideal high-temperature stability and electrical properties, and is suitable for being used as an electrode material.

The features and properties of the present invention are described in further detail below with reference to examples.

Example 1

This example provides a method for preparing modified manganese dioxide, which includes the following steps:

2.704g of MnSO₄.H₂And mixing and dissolving O, 80mL of pure water and 20mL of ethylene glycol, adding 0.5g of polyvinylpyrrolidone, and magnetically stirring for 2 hours to obtain a first solution. 2.528g of NH₄HCO₃The graphene and aluminum powder are dissolved in 30mL of pure water, 0.00192g of graphene and 0.0039g of aluminum powder are added, and the mixture is stirred until the graphene and aluminum powder are dissolved to obtain a second solution. Dripping the second solution into the first solution to gradually generate milky precipitate, continuously stirring for 1.5 hr, filtering to obtain precipitate, and standing at 80 deg.CVacuum drying for 12h, and treating at 350 deg.C in muffle furnace for 7h (heating rate of 0.5 deg.C. min)^-1Air atmosphere).

Example 2

2.704g of MnSO₄.H₂And mixing and dissolving O, 80mL of pure water and 20mL of ethylene glycol, adding 0.5g of polyvinylpyrrolidone, and magnetically stirring for 2 hours to obtain a first solution. 3.16g of NH₄HCO₃Dissolving the graphene and the aluminum powder in 30mL of pure water, adding 0.0192g of graphene and 0.0043g of aluminum powder, and stirring until the graphene and the aluminum powder are dissolved to obtain a second solution. Dropwise adding the second solution into the first solution to gradually generate milky precipitate, continuously stirring for 4h, filtering to obtain precipitate, vacuum drying at 60 deg.C for 15h, and treating at 450 deg.C in muffle furnace for 4h (heating rate of 1.5 deg.C. min)^-1Air atmosphere).

Example 3

2.704g of MnSO₄.H₂And mixing and dissolving O, 80mL of pure water and 20mL of ethylene glycol, adding 0.5g of polyvinylpyrrolidone, and magnetically stirring for 2 hours to obtain a first solution. 2.528g of NH₄HCO₃Dissolving in 30mL of pure water, adding 0.0096g of graphene and 0.0041g of aluminum powder, and stirring until the graphene and the aluminum powder are dissolved to obtain a second solution. Dropwise adding the second solution into the first solution to gradually generate milky precipitate, continuously stirring for 2h, filtering to obtain precipitate, vacuum drying at 80 deg.C for 12h, and treating at 400 deg.C in a muffle furnace for 5h (heating rate of 1 deg.C. min)^-1Air atmosphere).

Example 4

This example provides a process for the preparation of modified manganese dioxide, substantially as described in example 3, except that: the first solution does not contain polyvinylpyrrolidone and ethylene glycol.

Example 5

The embodiment provides a preparation method of a high-temperature lithium manganate material, which takes the modified manganese dioxide prepared in embodiment 3 as a raw material, and includes the following steps:

according to a molar ratio (Li)⁺Mn 1.05:2) weighing lioh₂Dissolving O and modified manganese dioxide in 40ml of deionized water, adding 10ml of ethylene glycol as a dissolution initiator, and stirring in a water bath at 80 ℃ until the solution is basically evaporated to dryness. Drying at 120 deg.C in a muffle furnace for 12h, grinding, and calcining at 750 deg.C for 6 h.

Example 6

according to a molar ratio (Li)⁺1.03:2) weighing LiOH₂Dissolving O and modified manganese dioxide in 40ml of deionized water, adding 10ml of ethylene glycol as a dissolution initiator, and stirring in a water bath at 80 ℃ until the solution is basically evaporated to dryness. Drying in a muffle furnace at 100 deg.C for 14h, grinding, and calcining at 700 deg.C for 7 h.

Example 7

according to a molar ratio (Li)⁺Mn 1.15:2) weighing lioh₂Dissolving O and modified manganese dioxide in 40ml of deionized water, adding 10ml of ethylene glycol as a dissolution initiator, and stirring in a water bath at 80 ℃ until the solution is basically evaporated to dryness. Drying in a muffle furnace at 140 deg.C for 10h, grinding, and calcining at 800 deg.C for 5 h.

Example 8

This example provides a method for preparing a high-temperature lithium manganate material, which is substantially the same as that in example 7, except that: according to a molar ratio (Li)⁺1.07:2) weighing LiOH₂O。

Comparative example 1

This comparative example provides a process for the preparation of modified manganese dioxide, which comprises the same steps as in example 3, except that: aluminum powder is not added in the preparation process.

Comparative example 2

This comparative example provides a process for the preparation of modified manganese dioxide, which comprises the same steps as in example 3, except that: graphene and aluminum powder are not added in the preparation process.

Comparative example 3

The comparative example provides a preparation method of a high-temperature lithium manganate material, which is substantially the same as that in example 5, and is different from the following steps: aluminum powder and graphene are not added in the preparation process.

Comparative example 4

The comparative example provides a preparation method of a high-temperature lithium manganate material, which takes the modified manganese dioxide prepared in example 3 as a raw material, and comprises the following steps:

according to a molar ratio (Li)⁺Mn 1.05:2) weighing Li₂CO₃And the prepared modified manganese dioxide is put into an agate mortar for grinding for 0.5h, and then the ground material is put into a muffle furnace for heat preservation for 12h at 280 ℃. After cooling, the material was removed and ground again for about 20 min. Presintering the mixture in a muffle furnace at 450 ℃ for 6h, heating to 750 ℃ and calcining at high temperature for 20h, and cooling to obtain LiMn₂O₄And the positive electrode material is marked as G-Al-LMO.

Comparative example 5

The comparative example provides a preparation method of a high-temperature lithium manganate material, which takes the manganese dioxide prepared in the comparative example 3 as a raw material, and comprises the following steps:

according to a molar ratio (Li)⁺Mn 1.05:2) weighing Li₂CO₃And the prepared modified manganese dioxide is put into an agate mortar for grinding for 0.5h, and then the ground material is put into a muffle furnace for heat preservation for 12h at 280 ℃. After cooling, the material was removed and ground again for about 20 min. Presintering the mixture in a muffle furnace at 450 ℃ for 6h, heating to 750 ℃ and calcining at high temperature for 20h, and cooling to obtain LiMn₂O₄And the cathode material is marked as LMO.

Test example 1

The particle size distribution of the prepared samples was determined using a Malvern Mastersizer 3000 particle size distribution instrument. The modified manganese dioxide in example 3 and comparative examples 1-2 was G-Al-M, G-M and M in this order, the results of the tests are shown in FIG. 1, and the particle size distribution of the samples is shown in Table 1.

As can be seen from FIG. 1, MnO to which aluminum powder and graphene were not added in comparative example 2₂The curve follows a standard normal distribution. In comparative example 1, after addition of G, small peaks appear between 0.01-0.1 μm and 10-100 μm, indicating that addition of graphene generates new small and large particles. In example 3, graphene and aluminum powder were added to increase the number of small particles and large particles, indicating that both graphene and aluminum powder can cope with MnO₂There is a certain influence on the particle distribution. And, in general, MnO₂The different particle size distribution will directly influence the prepared LiMn₂O₄The particle size distribution of (1).

Table 1 particle size distribution data

As can be seen from Table 1, in MnO₂The medium G-Al-M had the smallest average particle diameter, which was approximately 2.445 μ M. The results show that: graphene cladding and aluminum bulk phase doping can result in MnO₂The average particle diameter of (3) is decreased.

Test example 2

The XRD patterns of the lithium manganate produced in example 5 and comparative examples 4 to 5 were measured by analyzing the phase structure of the sample using an X-ray diffractometer manufactured by Pasacaceae, the Netherlands, and the results of the measurement are shown in FIG. 2. Example 5 and comparative examples 4-5 are labeled LZST-LMO, G-Al-LMO, and LMO in that order.

Fig. 2a shows XRD contrast curves for the three materials, and fig. 2b shows a magnified partial view of 2 theta values from 16 to 19. As can be seen from FIG. 2a, the sample obtained was indeed spinel LiMn₂O₄(corresponding to PDF 88-0589), belonging to Fd-3m space group, face-centered cubic structure. As can be seen from fig. 2b, the XRD curve of the sample prepared by the ion permeation method is more stable and the peak shape is sharper. The associated unit cell parameters, as well as the Rwp values (refined by HighScore Plus) are shown in table 2:

table 2 XRD data of the samples obtained

As can be seen from Table 2, the Rwp values of the three materials prepared are all around 3, which shows that the data deviation of the three samples is small and the reliability is high. Three kinds of LiMn₂O₄The unit cell parameter a is less than PDF 88-0589 LiMn₂O₄The crystal shows that the unit cell volume of the prepared sample is reduced, the atoms in the crystal are arranged more tightly, and the phenomena of replacement and dislocation among atoms are correspondingly reduced, thereby being beneficial to improving the stability of the unit cell.

Test example 3

The sample was subjected to morphological structure characterization analysis using a scanning electron microscope manufactured by Hitachi, Japan. The scanning electron micrographs of the lithium manganate produced in example 5 and comparative examples 4 to 5 were measured, and the results are shown in FIG. 3.

As can be seen from fig. 3a and b, LMO prepared by this method has an octahedral block structure, and small particles and large particles are randomly distributed in each position of the sample. The morphology of the prepared G-Al-LMO is shown in figures 3c and d after the G and the aluminum powder are added. In this case, the material is formed by small particles combined into a secondary quasi-spherical structure, which will be Li⁺Provide more sites for de-intercalation and improve Li in the charge and discharge process⁺The diffusion rate of (c). The morphology of the sample prepared by the ion permeation method is shown in fig. 3 e, f. Due to MnO in the preparation process₂With Li⁺When in combination, the LiMn is diffused in liquid and mixed with each other more fully and uniformly, and the formed LiMn₂O₄The structure of the material is more complete and regular octahedral block structure, and the particles are mutually embedded to form a stable crystal structure, so that structural collapse caused by electrolyte corrosion is inhibited in the charging and discharging process, and the cycling stability of the material at high temperature is improved.

Test example 4

The particle size distributions of the samples prepared in example 5 and comparative examples 4-5 were measured using a Malvern Mastersizer 3000 particle size distribution instrument and the results are shown in FIG. 4.

As can be seen from FIG. 4, the LMO has a large overall particle size and a wide distribution range, and is mainly distributed in the range of 1-100 μm, and the particle size is poorly uniform. With the addition of G and aluminum powder, the overall particle size of G-Al-LMO decreased and the primary particle size distribution range decreased to 1-10 μm. At this time, the particle size of the material prepared by the ion permeation method is greatly reduced, and the volume density of the material reaches 97.5 percent at the position of 0.378 mu m, which shows that the ion permeation method enables Li⁺And MnO with MnO₂Fully and uniformly mixing to prepare the obtained LiMn₂O₄The uniformity of the particle size is good.

Test example 5

The electrochemical properties of the samples prepared in example 5 and comparative examples 4 to 5 were tested, and the test results are shown in fig. 5 and table 3.

The test method comprises the following steps: mixing the obtained material with acetylene black and an adhesive (2.5 wt% of LA132 solution) according to a mass ratio of 85:10:5, fully grinding and uniformly mixing in an agate mortar, then coating the slurry on an aluminum foil to prepare an electrode plate, standing and drying, and finally blanking into a positive electrode wafer with the diameter of 14 mm. And (4) vacuum drying the electrode plates for 12h at 80 ℃ and assembling the button cell. The electrolyte adopts 1.0mol.L^-1LiPF of₆(V_EC:V_DMC1:1), the diaphragm is a U.S. Celgard 2400 diaphragm, the metal lithium sheet is a cathode, and the whole process is carried out in a glove box in an anhydrous and oxygen-free argon atmosphere. A Xinwei CT-3008 full-automatic battery program-controlled tester is adopted to perform constant-current charge and discharge test on the button cell at 55 ℃, wherein the voltage interval is 3.2-4.5V, and the test multiplying power is 0.2-5C. The assembled button cell is subjected to cyclic voltammetry test by adopting an electrochemical workstation of Shanghai Chenghua (voltage range is 3.2-4.5V, sweep rate is 0.2mV. s)^-1(ii) a AC impedance test (the test voltage is open circuit voltage (about 3.0V), and the frequency range is 10⁵-10^-2Hz, amplitude of 5 mV).

EIS data for samples obtained in Table 3

FIG. 5a is a graph showing the sample temperature at 55 ℃ and 0.2C timesAccording to the first charge-discharge curve under the rate, the three materials have two obvious voltage platforms (about 4.15V and 3.95V), and the synthesized materials are spinel LiMn₂O₄. The first discharge specific capacities of the three materials are respectively as follows: 117.2, 123.4, 124.1mAh.g^-1The coulombic efficiency is as follows in sequence: 72.7%, 94.7% and 88.1%, indicating that the irreversible capacity loss of LMO is the most serious in the first charge and discharge, and an excessive SEI film is formed, so that the internal resistance of the battery is increased. The loss of irreversible capacity of G-Al-LMO prepared after the G and the aluminum powder are added is greatly reduced, the LZST-LMO prepared by the ion permeation method also has higher coulombic efficiency, the overgrowth of the SEI film is inhibited while the SEI film is formed, the electrolyte can be prevented from corroding the material in the circulation process by the generation of the SEI film, and the circulation life of the material is prolonged.

FIG. 5b is a graph showing the cycling profile of the resulting sample at 55 ℃ and 0.2C magnification. It can be known that the addition of G and aluminum powder can improve the high-temperature cycle performance of the matrix material, the high-temperature cycle performance of the sample prepared by the ion permeation method is further improved, and after 100 cycles, the specific discharge capacity of the LZST-LMO is 112.1mAh^-1The capacity retention rate can reach 90.3%. The specific discharge capacities of the LMO and the G-Al-LMO are respectively as follows after 100 cycles: 22.4 and 101.2mAh.g^-1The capacity retention rates were 19.2% and 81.1%, respectively. Shows that the ion permeation method can improve the LiMn₂O₄High temperature stability of (3).

FIG. 5c is a graph of the rate performance of the resulting sample at 55 ℃. It can be known that the LMO and the G-Al-LMO have faster discharge specific capacity attenuation along with the increase of the multiplying power, while the LZST-LMO prepared by the ion permeation method has slower discharge specific capacity attenuation along with the increase of the multiplying power, and the capacities under different multiplying powers are highest. When the multiplying power returns to 0.2C again, the capacity recovery rate can reach 96.9 percent, which shows that the ion permeation method can improve the LiMn at high temperature₂O₄The rate capability of (2).

FIG. 5d shows the resulting sample at 0.2mV. s^-1CV curve below. It can be seen that the three materials all have two obvious pairs of oxidation-reduction peaks corresponding to charge and dischargeTwo voltage platforms in the process. With the addition of G and aluminum powder, the reduction peak gradually moves to a high potential, the oxidation peak moves to a low potential, and the potential difference between the reduction peak and the oxidation peak is reduced, which shows that the polarization of the material is reduced, the reversibility is better and better, two pairs of oxidation-reduction peaks of G-Al-LMO are more obvious than that of LMO, Li⁺The de-intercalation speed is increased. The LZST-LMO oxidation peak prepared by the ion permeation method becomes sharper at this time, which shows that the oxidation reaction occurs more rapidly, and Li⁺The separation speed is further increased.

FIG. 5e is an EIS map of the resulting sample. The EIS maps of the three materials are composed of semicircles in a high-frequency area and straight lines in a low-frequency area. Wherein the ohmic internal resistance (R) is obtained at the intersection of the high frequency region and the horizontal axis_s) Correlation; charge transfer resistance (R) of semicircular and electrolyte/electrode interface in high frequency region_ct) Correlation; while the straight line in the low frequency region corresponds to Li⁺Diffused Warburg impedance (Z)_w). Z' -omega of FIG. 5 f^-1/2The slope of the curve represents the wobbe constant (σ ω). Li⁺Diffusion coefficient (D) of_Li+) Then by formula D_Li+＝R²T²/2A²F⁴n⁴C²σ²Obtaining where R is a gas constant, T is an absolute temperature, n is a charge transfer number, and C is Li⁺The results of the concentration are shown in Table 3. After the G and the aluminum powder are added, the prepared G-Al-LMO has the minimum Rct and the maximum D_Li+The reason for this may be Li during charging and discharging⁺Can be carried out on each surface of the primary small particles, and the de-intercalation speed is greatly improved compared with the blocky structure of the LMO. The prepared LZST-LMO has a blocky structure, but the conductivity of the prepared LZST-LMO is improved due to the addition of G, and Li is minimum due to the minimum particle size of the LZST-LMO⁺The number of particles which can be de-intercalated is large, thereby improving D_Li ⁺。

Therefore, as can be seen from fig. 5, the sample prepared by the ion permeation method not only has faster Li⁺The diffusion rate also has better high-temperature cycle performance and rate capability.

In summary, according to the modified manganese dioxide and the preparation method thereof provided by the invention, aluminum powder and graphene are added in the preparation process of manganese dioxide, so that the prepared manganese dioxide has a smaller average particle size, and the preparation method is favorable for further preparing lithium manganate with better high-temperature stability.

The high-temperature lithium manganate material is prepared by the modified manganese dioxide through an ion permeation method, and the obtained lithium manganate has a blocky structure, is doped with aluminum and is coated by graphene. The prepared LiMn provided by the invention₂O₄Has standard spinel octahedral morphology. The particles are tightly connected together in an embedded mode, and the LiMn in the electrolyte in the charge and discharge process is inhibited₂O₄The corrosion effect of the alloy improves the electrochemical performance of the alloy at high temperature. The specific discharge capacity of the material is 124.1mAh.g when the material is tested at the temperature of 55 DEG C^-1The capacity retention rate after 100 cycles can reach 90.3%, and compared with the traditional method of adding lithium salt by grinding, the capacity retention rate is improved by about 10%.

The embodiments described above are some, but not all embodiments of the invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the invention as claimed, but is merely representative of selected embodiments of the invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Claims

1. Modified manganese dioxide is used for preparing a high-temperature lithium manganate material and is characterized in that the manganese dioxide is doped with aluminum and is coated by graphene;

the particle size of the modified manganese dioxide is 2.2-2.4 mu m; the molar ratio of manganese dioxide, aluminum and graphene is 1:0.0090-0.099: 0.01-0.1;

the modified manganese dioxide is in a sphere-like shape.

2. The method of claim 1, comprising the steps of:

3. The method of claim 2, wherein the molar ratio of the manganese salt, the aluminum powder, and the graphene is 1:0.0095-0.099: 0.01-0.05.

4. The method for preparing modified manganese dioxide according to claim 2, wherein said bicarbonate is any one selected from the group consisting of sodium bicarbonate, potassium bicarbonate and ammonium bicarbonate.

5. The method of claim 4, wherein the bicarbonate is ammonium bicarbonate.

6. The method of claim 2, comprising the steps of:

dissolving the manganese salt to obtain a first solution, and mixing and dissolving the aluminum powder, the graphene and the bicarbonate to obtain a second solution;

and dropwise adding the second solution into the first solution, reacting for 1.5-4h, filtering to obtain an intermediate solid, washing and drying the intermediate solid, and performing heat treatment.

7. The method of claim 6, wherein the bicarbonate is controlled in an amount of Mn²⁺:HCO^3-The molar ratio of (A) to (B) is 1: 2-2.5.

8. The method for preparing modified manganese dioxide according to claim 6, wherein said heat treatment process is heat treatment at a temperature of 350-450 ℃ for 4-8 h.

9. The method of claim 8, wherein the heat treatment is performed in an air atmosphere, and the temperature increase rate is controlled to be 0.5-1.5 ℃/min.

10. The method of claim 6, wherein the first solution further comprises polyvinylpyrrolidone and an organic solvent.

11. A method for preparing a high-temperature lithium manganate material, characterized in that Li is made by ion permeation method using lithium salt and modified manganese dioxide prepared by the method of any one of claims 2 to 10 as raw materials⁺Mixing with said modified manganese dioxide and infiltrating the solids, and then calcining the solids;

wherein, the lithium salt is selected from any one or more of lithium sulfate, lithium hydroxide, lithium nitrate and lithium carbonate.

12. The method for preparing a high-temperature lithium manganate material according to claim 11, wherein said lithium salt is lithium hydroxide.

13. The method for preparing a high-temperature lithium manganate material of claim 11, wherein the molar ratio of said modified manganese dioxide to said lithium hydroxide is 1.03-1.15: 2.

14. The method for preparing a high-temperature lithium manganate material of claim 13, wherein the molar ratio of said modified manganese dioxide to said lithium hydroxide is 1.03-1.07: 2.

15. The method for preparing the high-temperature lithium manganate material according to claim 11, characterized by comprising the following steps:

mixing the modified manganese dioxide, lithium salt, water and an organic solvent, stirring under the heating condition until the solution is evaporated to dryness to obtain a solid manganese dioxide with lithium deposited on the surface, drying the solid, and calcining for 5-7h at the temperature of 700-800 ℃.

16. The method for preparing a high-temperature lithium manganate material as defined in claim 15, wherein the drying of said solid manganese dioxide with lithium deposited on the surface is carried out at a temperature of 100-140 ℃ for 10-14 h.

17. The method of claim 16, wherein the solid manganese dioxide with lithium deposited on the surface is ground after drying and before calcining.

18. A high-temperature lithium manganate material, characterized in that said lithium manganate electrode material is prepared by the preparation method of any of claims 11-17.