Preparation method and application of carbon cage coated porous germanium composite material

Abstract

The application discloses a preparation method and application of a carbon cage coated porous germanium composite material, and specifically, the preparation method is as follows: magnesium germanate is heat treated in an oxygen atmosphere, then carbon coating is carried out by using acetylene decomposition, and finally, unique carbon cage coated porous germanium structure is obtained by acid pickling. The application provides a unique carbon cage and porous germanium composite structure, compared with a traditional porous germanium / carbon core-shell structure, the space provided by the pores in the porous germanium and between the carbon cage and the germanium for the volume expansion of the germanium, so that the stability of the carbon cage and the stability of the whole structure are more favorable. The carbon cage coated porous germanium composite material prepared as a negative electrode material is applied to a lithium ion battery, and the cycle stability of the lithium ion battery is significantly improved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of material science, and particularly relates to a preparation method and application of a carbon cage coated porous germanium composite material. BACKGROUND

[0002] Lithium ion battery is currently the most important rechargeable battery, which is widely used in consumer electronics such as mobile phones, notebook computers, pure electric vehicles and hybrid electric vehicles, and other various military and aerospace fields. Although lithium ion batteries have been widely used in these fields, with the rapid development of these industries, the current lithium ion batteries are increasingly difficult to meet their performance requirements. Especially in the field of electronic consumption, people are eager to have the emergence of batteries with smaller weight and volume, higher energy density, i.e. higher specific capacity.

[0003] In order to prepare a lithium ion battery with higher capacity, first of all, a positive and negative electrode material with higher capacity needs to be developed. Germanium, as a negative electrode material with high theoretical specific capacity, is one of the most potential materials for the next generation of lithium ion battery negative electrodes, because it has an ultra-high theoretical specific capacity (1384 mAh / g) and a relatively low lithium intercalation potential.

[0004] Although germanium has the above advantages, when used as a negative electrode material, it will swell by up to 270% after intercalating lithium ions, and the volume will shrink again after the lithium ions are removed. Therefore, after multiple cycles, due to the constant and severe volume change, the germanium active material will be broken, powdered and even fall off from the current collector, resulting in capacity degradation. Therefore, the most important problem hindering the application of germanium negative electrode is the rapid capacity decay caused by the huge volume change.

[0005] In order to solve the above problems, researchers have proposed many solutions. Among them, the composite with carbon is one of the most important solutions. This is because carbon has higher electronic conductivity than semiconductor germanium, and the composite with carbon can improve the overall conductivity of the system. Then, carbon has certain mechanical properties, which can buffer the volume expansion effect of germanium to a certain extent, reduce the internal stress caused by volume change, and reduce the possibility of breakage of germanium active material, but the existing technology usually first prepares porous germanium and then performs carbon coating to form a core-shell structure with the carbon layer closely attached to the porous germanium. SUMMARY

[0006] In view of the problems existing in the prior art, the purpose of the present application is to provide a preparation method of a carbon cage coated porous germanium composite material. The method obtains a mixture of magnesium oxide and germanium by first performing thermal oxidation on magnesium germanate, then performs carbon coating by using acetylene decomposition, and finally removes the magnesium oxide by acid washing to obtain a unique structure of carbon cage coated porous germanium.

[0007] The specific preparation method is as follows: germanium magnesium powder is heat treated at a high temperature of 500-800 DEG C, the heat treatment time is 5-30h, and the atmosphere is a mixed gas of 1-30% oxygen and argon by volume fraction. Then, the heat treated product is subjected to carbon coating treatment, carbon coating is carried out by using an acetylene cracking method, the carbon coating temperature is 550-800 DEG C, the time is 1-10h, and the atmosphere is a mixed gas of 1-20% acetylene and argon by volume fraction. Finally, the carbon coated product is subjected to acid washing by using excessive dilute hydrochloric acid, the concentration of the dilute hydrochloric acid is 1mol / L, after washing, the product is washed with deionized water for 5 times and then dried, thereby obtaining a carbon cage coated porous germanium composite material.

[0008] As preferred, the temperature of the magnesium germanide heat oxidation is 600-700 DEG C.

[0009] As preferred, the magnesium germanide heat oxidation time is 10-25h.

[0010] As preferred, the atmosphere of the magnesium germanide heat oxidation is 1-10% oxygen and argon mixed gas.

[0011] As preferred, the carbon coating heat treatment temperature is 600-750 DEG C.

[0012] As preferred, the carbon coating heat treatment time is 2-6h.

[0013] As preferred, the atmosphere of the carbon coating heat treatment is 5-15% acetylene and argon mixed gas.

[0014] In the present application, the method of first carbon coating and then acid washing is adopted to prepare the porous germanium, which is different from the traditional structure of first preparing the porous germanium and then carbon coating. In the traditional method, the carbon layer enters the pores of the porous germanium and is coated along the outline of the porous germanium, thereby forming a core-shell structure in which the carbon layer is closely attached to the porous germanium. In the present application, the self-supporting carbon cage is first formed by carbon coating and then the magnesium oxide template is removed by acid washing, thereby forming the pores inside the porous germanium and between the carbon cage and the germanium. It is a unique structure of carbon cage coated porous germanium.

[0015] The present application also discloses a carbon cage coated porous germanium composite material prepared by the above method and application of the carbon cage coated porous germanium composite material as a negative electrode material in a lithium ion battery.

[0016] Compared with the prior art, the present application has the following beneficial effects:

[0017] 1) The pores inside the porous germanium and between the carbon cage and the porous germanium provide space for the volume expansion of germanium, the volume expansion of germanium does not cause great pressure on the carbon cage, and the stability of the carbon cage and the overall structure is beneficially improved.

[0018] 2) The self-supporting carbon cage can seal the germanium to prevent the electrolyte from entering and reacting with the germanium;

[0019] 3) The contact of germanium with the carbon cage multi-point and the porous structure inside the germanium can promote the fast transmission of lithium ions and charges, and enhance the fast charge and discharge capacity of the electrode. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 Scanning electron microscope photos of the carbon cage coated porous germanium composite material prepared in Example 1;

[0021] Figure 2 Comparison of the capacity cycle performance of the materials prepared in Example 1 and Comparative Example as the negative electrode of lithium ion battery. DETAILED DESCRIPTION

[0022] The present application will be further described in detail below in combination with specific examples, so as to better understand the technical solutions.

[0023] Example 1

[0024] 1) The magnesium germanide was heat treated at a temperature of 650℃ for 20h, and the atmosphere was a mixed gas of 1% oxygen and argon by volume fraction;

[0025] 2) The product obtained in step 1) was subjected to carbon coating by acetylene decomposition. The carbon coating temperature was 650℃, the carbon coating time was 3h, and the atmosphere was a mixed gas of 10% acetylene and argon by volume fraction;

[0026] 3) The product obtained in step 2) was subjected to acid washing treatment. The acid washing was performed using an excess amount of dilute hydrochloric acid, and the concentration of the dilute hydrochloric acid was 1mol / L. After washing, the product was washed with deionized water for 5 times and then dried, thereby obtaining the carbon cage coated porous germanium composite material.

[0027] Figure 1 Scanning electron microscope photos of the carbon cage coated porous germanium composite material prepared in this example; from Figure 1 It can be seen that the carbon layer presents a complete and self-supporting cage structure, the germanium inside presents a porous structure, and the pores between the germanium inside and the carbon cage and the porous germanium can be clearly seen.

[0028] Example 2

[0029] 1) The magnesium germanide was heat treated at 600℃ for 25h, and the atmosphere was a mixed gas of 10% oxygen and argon by volume fraction;

[0030] 2) The product obtained in step 1) was subjected to carbon coating by acetylene decomposition. The carbon coating temperature was 700℃, the carbon coating time was 2h, and the atmosphere was a mixed gas of 5% acetylene and argon by volume fraction;

[0031] 3) The product obtained in step 2) is subjected to acid washing treatment. The acid washing is performed using excess dilute hydrochloric acid with a concentration of 1 mol / L. After washing, the product is washed with deionized water for 5 times and then dried to obtain the carbon cage coated porous germanium composite material.

[0032] Example 3

[0033] 1) The magnesium germanide is heat treated at 700℃ for 10h in an atmosphere of a mixed gas of 5% oxygen and argon by volume;

[0034] 2) The product obtained in step 1) is subjected to carbon coating by ethyne decomposition. The carbon coating temperature is 600℃, the carbon coating time is 5h, and the atmosphere is a mixed gas of 15% ethyne and argon by volume;

[0035] 3) The product obtained in step 2) is subjected to acid washing treatment. The acid washing is performed using excess dilute hydrochloric acid with a concentration of 1 mol / L. After washing, the product is washed with deionized water for 5 times and then dried to obtain the carbon cage coated porous germanium composite material.

[0036] Comparative Example

[0037] The comparative example is to prepare a traditional porous germanium / carbon core-shell structure composite material, which is prepared by a traditional method of first preparing porous germanium and then coating carbon. The specific steps are as follows:

[0038] 1) The magnesium germanide is heat treated at 650℃ for 20h in an atmosphere of a mixed gas of 1% oxygen and argon by volume;

[0039] 2) The product obtained in step 2) is subjected to acid washing treatment. The acid washing is performed using excess dilute hydrochloric acid with a concentration of 1 mol / L. After washing, the product is washed with deionized water for 5 times and then dried to obtain the first prepared porous germanium;

[0040] 3) The product obtained in step 2) is subjected to carbon coating by ethyne decomposition. The carbon coating temperature is 650℃, the carbon coating time is 3h, and the atmosphere is a mixed gas of 10% ethyne and argon by volume. Thus, the traditional porous germanium / carbon core-shell structure composite material can be obtained.

[0041] Performance Test

[0042] The lithium ion battery carbon cage coated porous germanium composite material of the present application is tested by using a half-cell test method. The slurry ratio used is: active material (porous germanium / carbon composite material prepared in Example 1): Super P (conductive agent): PVDF (binder) = 7:2:1.

[0043] The specific steps are as follows:

[0044] First, CMC was dissolved in deionized water to form a solution with a concentration of 5%, and then conductive agent (SP) and active material (carbon-coated porous germanium composite material prepared in Example 1) were added in sequence to form a slurry by stirring. Then the slurry was coated on a cleaned copper foil, vacuum dried for 12 h, and then punched to form a negative electrode sheet with a diameter of 12 mm. A lithium metal sheet was used as the counter electrode of the battery, and the electrolyte was a 1 M LiPF6 solution in a mixed solution of DMC and EC, wherein DMC: EC = 1:1 (volume ratio). The button cell was assembled in a glove box, and after standing for 12 h, the performance test was performed. The test voltage was 0.01-1.5 V, and the test current was 400 mA / g.

[0045] Figure 2 For Figure 2 The capacity cycle performance of the materials prepared in Example 1 and the comparative example as the negative electrode of lithium ion batteries was compared. It can be seen that the initial capacities of the two materials are similar, but after 10 cycles, the capacity of the material of Example 1 still remains at 1470 mA / g, and the capacity recovery rate is as high as 96%, while the capacity of the material prepared in the comparative example is only 1350 mAh / g after 10 cycles, and the capacity recovery rate is only 89%, which is much lower than that of Example 1. Therefore, it can be seen that the carbon-coated porous germanium composite material prepared by the preparation method of the present application has advantages in cycle performance.

Claims

1. A method for preparing a carbon cage-coated porous germanium composite material, characterized in that, The specific steps of the preparation method are as follows: magnesium germanide is heat-treated in an oxygen atmosphere, then the heat-treated product is carbon-coated under an acetylene atmosphere, and finally acid-washed to obtain carbon cage-coated porous germanium composite material. The heat treatment temperature is 600-700℃, and the heat treatment time is 10-25h; the oxygen atmosphere is a mixture of oxygen and argon, wherein the volume fraction of oxygen is 1-30%; the carbon coating temperature is 600-750℃, and the carbon coating time is 2-6h.

2. The method for preparing a carbon cage-coated porous germanium composite material as described in claim 1, characterized in that... The acetylene atmosphere conditions refer to a mixture of acetylene and argon, wherein the volume fraction of acetylene is 1-20%.

3. The carbon cage-coated porous germanium composite material prepared by any of the preparation methods described in claims 1-2.

4. The application of the carbon cage-coated porous germanium composite material according to claim 3 in the preparation of lithium-ion batteries.

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