RU2587439C1 - Method of producing manganese dioxide nanorods - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: to obtain manganese dioxide nanorods aqueous solutions of potassium permanganate and sodium nitrite are mixed in molar ratio $$Mn{O}_4^{-}:N{O}_2^{-},$$

equal to 2:(1-5), to form homogeneous disperse phase in strong alkali solution. Then, at constant stirring inorganic acid is slowly dropped into until pH 2-0.5. Obtained suspension is placed into Teflon autoclave, which is installed in hydrothermally microwave apparatus for 5-25 minutes at 90-170 °C, pressure 1-20 atm and microwave heating power 150-1,000 W. Obtained precipitate is separated by decantation, washed with distilled water and dried in air at temperature not higher than 70 °C. Obtaining crystalline pyrolusite, particles of which have shape of rods with diameter more than 10 nm and length of up to 2 mcm. H₂SO₄ or HNO₃ is used as inorganic acid.

EFFECT: invention enables to obtain β-MnO₂ nanorods used in lithium current sources as cathode material with high output and high uniformity of fractions in rod diameter.

1 cl, 2 dwg, 1 tbl, 5 ex

Description

The invention relates to inorganic chemistry, specifically to the production of nanocrystalline manganese dioxide, polymorphic modifications of which, having various morphological forms, can be successfully used as part of catalysts, biosensors, adsorbents, and especially in current sources.

The main structural units of polymorphic modifications of manganese dioxide are MnO ₆ octahedra, the different interconnection of which leads to the formation _of layers and channels in the structure of MnO ₂ . It is the presence of channels that makes MnO ₂ interesting from the point of view of creating cathode materials. The smallest impurity content in MnO _{2 is} characteristic of pyrolyusite (β-MnO ₂ ) and ramsdelite (γ-MnO ₂ ). For other modifications of MnO ₂ (α, λ, ε, and δ), the presence of Na ⁺ and K ⁺ ions in their crystal structure is extremely characteristic.

Currently, the urgent task is the development of methods for producing β-modification MnO ₂ nanorods, which are of great practical interest, since their characteristic conductivity anisotropy can lead to the appearance of fundamentally new design solutions when creating battery cells [Xu M.-W. Bao S.-J. // Energy Storage in the Emerging Era of Smart Grids; In tech: 2011; V. 12, P. 251-278].

From [RU 2536649] it is known that when charging and discharging Li-ion batteries, topotactic reactions take place, they consist in the injection of an electron and the incorporation of the Li cation into a solid matrix without destroying the internal structure of the material. However, the intercalation of Li ions in the structure of the material can lead to significant changes in the structure of the material: the formation of a new phase, an increase in the volume of the crystalline cell, “swelling”, etc.

A material consisting of homogeneous nanorods is more suitable for intercalation of lithium ions, since it will not experience serious structural stresses when the Li ⁺ cation passes through channels in the MnO ₂ structure.

The known method [Xun Wang and Yadong Li // Synthesis and Formation Mechanism of Manganese Dioxide Nanowires / Nanorods, Chem. Eur. J: 2003, V. 9, No. 1, P. 19141-19147] for the production of manganese dioxide nanorods, namely, that the salts (NH ₄ ) ₂ S ₂ O ₈ and MnSO ₄ · H ₂ O are dissolved in distilled water at room temperature temperature and mix until a homogeneous solution is formed. Then it is transferred to an autoclave and subjected to hydrothermal treatment at a temperature of 140 ° C for 12 hours. The resulting product is filtered, washed with distilled water and dried in air. The formation of nanorods of α- and β-modifications of manganese dioxide passes through an intermediate stage of the formation of δ-modification of manganese dioxide with lamellar morphology.

The disadvantage of this method is that the resulting β-modification of manganese dioxide contains an admixture of δ-modification having a layered structure, which impairs the phase uniformity of the material required when used in electrochemical cells.

Another disadvantage of this method is the relatively high duration of the synthesis.

A known method of producing nanorods of manganese dioxide, described in [M. Wei, Y. Konishi, N. Zhou, N. Sugihara and N. Arakawa // Synthesis of single-crystal manganese dioxide nanowires by soft chemical process, Nanotechnology: 2005, V. 16, P. 245-249] (prototype), lies in the fact that commercial γ-MnO _{2 is} mixed with water, and the resulting suspension is subjected to hydrothermal treatment at a temperature of 140-200 ° C for 72 days. The resulting product is filtered, washed with water and dried at 60 ° C for 4 hours.

A significant drawback is the relatively low morphological homogeneity of the resulting product.

The disadvantage of the proposed method is too long synthesis time.

The technical problem is related to the fact that the commercial success of modern cathode materials largely depends on the method of their preparation, which should provide the ability to control the morphology and particle size.

The invention is aimed at finding a high-performance method for producing β-MnO ₂ nanorods for use in lithium current sources as a cathode material with a relatively high fraction uniformity in the diameter of the rods, which significantly improves the process of intercalation of Li ions into the structure of the cathode material.

, равном 2:1÷5, до образования однородной дисперсной фазы в сильнощелочном растворе, после чего к нему при постоянном перемешивании медленно прикапывают неорганическую кислоту до достижения значения pH от 2 до 0,5, полученную суспензию помещают в тефлоновый автоклав, который устанавливают в гидротермально-микроволновую установку на 5÷25 мин при 90÷170°C при давлении 1÷20 атм и мощности микроволнового нагрева 150÷1000 Вт, полученный осадок отделяют декантацией, промывают дистиллированной водой и высушивают на воздухе при температуре не выше 70°C, в результате получают кристаллический пиролюзит β-MnO₂, частицы которого имеют форму стержней диаметром свыше 10 нм и длиной до 2 мкм.The technical result is achieved by the fact that the proposed method for producing nanorods of manganese dioxide, which consists in mixing aqueous solutions of potassium permanganate and sodium nitrite in a molar ratio

equal to 2: 1 ÷ 5, until a homogeneous dispersed phase forms in a strongly alkaline solution, after which inorganic acid is slowly added dropwise to it with constant stirring until a pH value of 2 to 0.5 is reached, the resulting suspension is placed in a Teflon autoclave, which is placed in a hydrothermal microwave installation for 5 ÷ 25 min at 90 ÷ 170 ° C at a pressure of 1 ÷ 20 atm and a microwave heating power of 150 ÷ 1000 W, the precipitate obtained is separated by decantation, washed with distilled water and dried in air at a temperature not exceeding 70 ° C, res ultrathe receive crystalline β-MnO ₂ pyrolusite, particles of which have the form of rods with a diameter of more than 10 nm and a length of up to 2 microns.

It is advisable that either H ₂ SO ₄ or HNO _{3 be} used as the inorganic acid.

, равные 2:1÷5, выбирают из тех соображений, что при них образуется однородная дисперсная фаза.Molar ratios

equal to 2: 1 ÷ 5, choose from those considerations that they form a homogeneous dispersed phase.

The choice of the acidity range is due to the fact that in the medium with pH> 2 there is an extraneous admixture of δ-MnO ₂ nanoparticles in the final product, which are characterized by a spheroidal shape and rough surface, and at pH <0.5, the extraneous phase is also present in the final product.

The claimed time interval of 5–25 minutes is determined by the dynamics of the crystal formation process, which generally begins with 5 minutes and ends after 25 minutes, after which the linear dimensions of the nanorods do not change and the functional properties of the nanocrystals do not improve.

The claimed temperature range of the hydrothermal microwave treatment is determined experimentally and is optimal for obtaining a homogeneous phase of manganese dioxide nanorods containing crystals with a diameter of less than 100 nm. The minimum autoclave temperature is due to the fact that nanorods are not formed below 90 ° C. The upper temperature limit is due to the fact that at temperatures above 170 ° C in the final product obtained, there are no qualitative changes. The autoclave treatment is optimal for 8 minutes at 150 ° C, at which the uniformity of the 20–25 nm fraction in the final product is 90%.

As a hydrothermal-microwave installation, Berghof Speedwave MWS four equipment is used, characterized by a pressure of R20 atm and a microwave heating power of 150 ÷ 1000 watts.

The essence of the invention lies in the fact that varying the acidity of the medium, the duration and temperature of the synthesis allows you to get products with the given parameters of uniformity.

The invention is illustrated by the following micrographs.

FIG. 1. The results of scanning electron microscopy of a sample of manganese dioxide obtained by the proposed invention by hydrothermal microwave treatment (HTMV) for 8 min at 150 ° C from a reaction mixture with pH = 1 (example 1).

FIG. 2. The results of scanning electron microscopy of a sample of manganese dioxide obtained by the prototype.

The following are examples illustrating but not limiting the proposed method.

Example 1

составляло 2:3), после чего к смеси при постоянном перемешивании медленно прикапывали 0,5М H₂SO₄ до достижения рН=1. Полученную суспензию помещали в тефлоновый автоклав емкостью 100 мл (степень заполнения составляла 50%) и подвергали гидротермально-микроволновой обработке в установке Berghof Speedwave MWS four в течение 8 мин при 150°C. После завершения обработки автоклав извлекали и охлаждали на воздухе. Образовавшийся осадок отделяли декантацией, несколько раз промывали дистиллированной водой и сушили на воздухе при относительной влажности ~75% и температуре 60°C. Получали продукт с однородностью фракции 95% нм и диаметром наностержней 20÷25 нм.0.3 g of KMnO _{4 was} dissolved in 38 ml of distilled water, then 0.19 g of NaNO _{2 was} added to the resulting solution (molar ratio

amounted to 2: 3), after which 0.5 M H ₂ SO _{4 was} slowly added dropwise to the mixture with constant stirring until pH = 1 was reached. The resulting suspension was placed in a 100 ml Teflon autoclave (filling degree 50%) and subjected to hydrothermal microwave treatment in a Berghof Speedwave MWS four for 8 min at 150 ° C. After processing, the autoclave was recovered and cooled in air. The precipitate formed was separated by decantation, washed several times with distilled water, and dried in air at a relative humidity of ~ 75% and a temperature of 60 ° C. Received a product with a uniformity of the fraction of 95% nm and a diameter of nanorods 20 ÷ 25 nm.

Examples 2-5 were carried out according to Example 1, changing the molar ratio of the reactants, the acidity of the medium, the time and temperature of synthesis. The results are summarized in the Table: "The homogeneity index of the fraction of manganese dioxide nanorods synthesized by the proposed method."

The proposed method allows to obtain β-MnO ₂ nanorods with a sufficiently high productivity, as well as with relatively high homogeneity of the fraction along the diameter of the rods, which determines the suitability of their use in cathode materials.

Claims (2)

1. A method of producing manganese dioxide nanorods, which consists in mixing aqueous solutions of potassium permanganate and sodium nitrite in a molar ratio of MnO ₄ ^- : NO ₂ ^- equal to 2: (1 ÷ 5), until a uniform dispersed phase is formed in a strongly alkaline solution, after which inorganic acid is slowly added dropwise to it with constant stirring until a pH of 2 to 0.5 is reached, the resulting suspension is placed in a Teflon autoclave, which is installed in a hydrothermal-microwave unit for 5 ÷ 25 min at 90 ÷ 170 ° C at a pressure of 1 ÷ 20 atm and microwave heating oschnosti 150 ÷ 1000 W, the resulting precipitate was separated by decantation, washed with distilled water and dried in air at a temperature not higher than 70 ° C, to yield crystalline pyrolusite β-MnO _2, whose particles have the shape of rods than 10 nm in diameter and a length of 2 microns. 2. The method according to p. 1, characterized in that as the inorganic acid use either H ₂ SO ₄ or HNO ₃ .

Images