Background
Lithium ion batteries have been widely used in aerospace, hybrid electric vehicles, portable electronic devices, and other fields because of their significant advantages of high specific capacity, light weight, long life, no memory, and the like. At present, the commercial graphite cathode has low energy density, and has huge potential safety hazard due to easy precipitation of lithium dendrite and action with electrolyte, thereby greatly limiting the development and application of the lithium dendrite.
Molybdenum trioxide, a transition metal oxide, has a layered structure and a framework in which extensive channels exist, and can be used as a flow channel and an intercalation site for ions. Theoretically every unit of molybdenum trioxide can be mixed with 6 units of Li+The conversion reaction occurs, so the method has higher theoretical specific capacity. However, in the process of charging and discharging, the molybdenum oxide has obvious volume effect, is easy to expand in volume, and has obvious capacity attenuation and low capacity retention rate.
In response to the above deficiency, researchers mainly studied the following aspects: (1) the shape of the material is regulated by different methods, and the material with a special structure is prepared to slow down the volume effect caused by the ion disintercalation process, such as the thermal evaporation method [ Lili Cai, Pratap M. Rao, Xiaoolin Zheng. Morphology-controlled flame synthesis of single, branched, and flow-like alpha-MoO3 nanobelt arrays[J]. Nano Letters, 2011, 11(2):872-7. ]Solvothermal method [ Sakaushi K, Thomas J, Kaskel S, et al. Aqueous solution process for the synthesis and analysis of nanostructured one-dimensional alpha-MoO3 electrode materials[J]. Chemistry of Materials, 2013, 25(12):2557-2563. ]Template-assisted method [ Zhiming Cui, Weiying Yuan, Chang Ming Li. Template-mediated growth of microspheres, microbelt and nano alpha-MoO3structures and their high pseudo-capacitances[J]. J. Mater. Chem:a, 2013, 1(41):12926-12931. ]. (2) Coating a structurally stable material, such as a single-walled carbon nanotube [ Mendoza-S ] n P S. Charge storage properties of a α-MoO3/carboxyl-functionalized single-walled carbon nanotube composite electrode in a Li ion electrolyte[J]. Electrochimica Acta, 2013, 98:294-302. ]。[ Wang Q, Sun J, Wang Q, et al. Electrochemical performance of α-MoO3–In2O3 core–shell nanorods as anode materials for lithium-ion batteries[J]. Journal of Materials Chemistry A, 2015, 3(9):5083-5091. ]。
However, the following problems are common to the above-mentioned various modification methods: (1) the synthesis process needs the auxiliary action of surfactant and the like [ dyeing Cui, wetting Yuan, Chang Ming Li. Template-modified growth of microsphere, microsphere and nano alpha-MoO3 structures and their high pseudo-capacitances[J]. J. Mater. Chem:a, 2013, 1(41):12926-12931. ](2) The cycling stability of the prepared pure-phase molybdenum oxide electrode material still needs to be improved, and after multiple cycles, the cycling stability is lower than 80% [ Wang Q, Sun J, Wang Q, et al, Electrochemical performance of alpha-MoO3–In2O3 core–shell nanorods as anode materials for lithium-ion batteries[J]. Journal of Materials Chemistry A, 2015, 3(9):5083-5091. ]。
The high-efficiency freeze drying-heat treatment method is adopted to prepare MoO with special morphology3@ NC (NC, i.e. nitrogen-doped carbon), MoO3The @ NC material has the characteristics of high capacity, good conductivity, high ion diffusivity and stable structure.
Disclosure of Invention
The invention aims to provide MoO with a special morphology3@ NC and application in preparation of lithium ion battery cathode materials. The MoO3The @ NC material has the characteristics of high capacity, good conductivity, high ion diffusivity and stable structure, and has great potential in the application of lithium ion battery electrode materials. The method is simple to operate, and has good guiding significance for regulating and controlling the shapes of other transition metal oxides.
The specific technical scheme is as follows: bundle-shaped MoO3@ NC lithium ion battery electrode materialThe preparation method comprises the following steps:
(1) weighing a certain amount of chitosan, dissolving the chitosan in 30 mL of acid solutions with different concentrations, and magnetically stirring for 30-60 min to obtain a solution A;
(2) weighing a certain amount of molybdenum source, dissolving the molybdenum source in 30 mL of deionized water, and magnetically stirring for 30-60 min to obtain a solution B;
(3) slowly adding the obtained solution B into the solution A, and continuously magnetically stirring for 1-4 hours to obtain a solution C;
(4) heating the solution C to evaporate the solvent until the volume of the solution is 0.01-0.1 time of the volume of the initial solution, so as to obtain a suspension D;
(5) freeze-drying the suspension D, and collecting a dried product as a precursor E;
(6) and (3) putting a certain amount of the precursor E into a crucible and placing the precursor E into a tube furnace, carrying out heat treatment in an inert atmosphere Ar, and simultaneously carrying water vapor through Ar to synchronously realize water vapor activation-heat treatment. The heat treatment temperature is 350-550 ℃, the time is 0.5-3 h, the heating rate is 3-20 ℃/min, the Ar flow rate is 50-200 sccm, and the target product MoO is obtained3@NC。
The amount of the chitosan in the step (1) is 0.1-1 g.
The types of the acid in the step (1) are acetic acid and hydrochloric acid.
The concentration range of the acid in the step (1) is 3-12 mol/L.
The molybdenum source in the step (2) is ammonium molybdate tetrahydrate ((NH)4)6Mo7O24·4H2O)。
0.1-1 g of the certain amount of molybdenum source in the step (2).
And (3) heating the mixed solution at 50-80 ℃ by using an electric heating sleeve to evaporate the solvent.
And (4) carrying out freeze drying at-50 to-40 ℃.
Weighing a certain amount of the freeze-dried product E in the step (5) to be 1-4 g.
Compared with the prior art, the invention can obtain the following beneficial effects:
(1) the invention adopts an efficient freeze drying-heat treatment method to prepare MoO with special morphology3@NC。
(2) MoO prepared by the invention3The @ NC material has the characteristics of high capacity, good conductivity, high ion diffusivity and stable structure, and the method has good guiding significance for regulating and controlling the shapes of other transition metal oxides.
Detailed Description
The present invention will be further illustrated with reference to the following examples, but the present invention is not limited to the following examples.
Example 1
(1) Weighing 0.5 g of chitosan, dissolving the chitosan in 30 mL of acetic acid solution with the concentration of 6 mol/L, and magnetically stirring for 30 min to obtain a solution A;
(2) 1 g of ammonium molybdate tetrahydrate ((NH) was weighed4)6Mo7O24·4H2O) is dissolved in 30 mL of deionized water, and the solution B is obtained after magnetic stirring for 30 min;
(3) slowly adding the obtained solution B into the solution A, and continuously magnetically stirring for 4 hours to obtain a solution C;
(4) heating the solution C at 60 ℃ to evaporate the solvent until the volume of the solution is 0.1 time of the volume of the initial solution, thereby obtaining a suspension D;
(5) freeze-drying the suspension D at-45 ℃, and collecting a dried product as a precursor E;
(6) 1 g of the precursor E is placed in a crucible and placed in a tube furnace, heat treatment is carried out in an inert atmosphere Ar, and meanwhile water vapor is loaded through Ar, so that water vapor activation-heat treatment is synchronously realized. The heat treatment temperature is 450 ℃, the time is 1h, the heating rate is 10 ℃/min, the Ar flow rate is 100 sccm, and the target product MoO is obtained3@NC。
FIG. 1 is an X-ray diffraction analysis chart of the product of this example. XRD characteristic peak and MoO of product in figure 13The characteristic peaks are consistent, which shows that the MoO is obtained by the molybdenum source of the invention through the water vapor activation heat treatment3. Since chitosan is a nitrogen-containing carbon source, nitrogen-doped carbon, i.e., NC, is obtained after heat treatment. The analysis shows that the invention successfully prepares the target product MoO3@NC。
FIG. 2 is an SEM image of the product of example 1 at 7.0K, and FIG. 3 is an SEM image of the product of example 1 at 40.0K. FIGS. 2 and 3 show that MoO prepared by the invention3@ NC, having a bundle-like regular morphology formed by the assembly of a plurality of nanofibrous structures.
Example 2
(1) Weighing 1 g of chitosan, dissolving the chitosan in 30 mL of 12 mol/L acetic acid solution, and magnetically stirring for 60 min to obtain a solution A;
(2) 0.1 g of ammonium molybdate tetrahydrate ((NH) was weighed4)6Mo7O24·4H2O) is dissolved in 30 mL of deionized water, and the solution B is obtained after magnetic stirring for 40 min;
(3) slowly adding the obtained solution B into the solution A, and continuously magnetically stirring for 2 hours to obtain a solution C;
(4) heating the solution C at 80 ℃ to evaporate the solvent until the volume of the solution is 0.05 times of the volume of the initial solution, thereby obtaining a suspension D;
(5) freeze-drying the suspension D at-40 ℃, and collecting a dried product as a precursor E;
(6) 4 g of the precursor E is placed in a crucible and placed in a tube furnace, heat treatment is carried out in an inert atmosphere Ar, and meanwhile water vapor is loaded through Ar, so that water vapor activation-heat treatment is synchronously realized. The heat treatment temperature is 550 ℃, the time is 0.5 h, the heating rate is 20 ℃/min, the Ar flow rate is 200 sccm, and the target product MoO is obtained3@NC。
Example 3
(1) Weighing 0.1 g of chitosan, dissolving the chitosan in 30 mL of hydrochloric acid solution with the concentration of 3 mol/L, and magnetically stirring for 40 min to obtain a solution A;
(2) 0.5 g of ammonium molybdate tetrahydrate ((NH) was weighed4)6Mo7O24·4H2O) is dissolved in 30 mL of deionized water, and the solution B is obtained after magnetic stirring for 50 min;
(3) slowly adding the obtained solution B into the solution A, and continuously magnetically stirring for 1h to obtain a solution C;
(4) heating the solution C at 50 ℃ to evaporate the solvent until the volume of the solution is 0.01 time of the volume of the initial solution, so as to obtain a suspension D;
(5) freeze-drying the suspension D at-45 ℃, and collecting a dried product as a precursor E;
(6) 2 g of the precursor E is placed in a crucible and placed in a tube furnace, heat treatment is carried out in an inert atmosphere Ar, and meanwhile water vapor is loaded through Ar, so that water vapor activation-heat treatment is synchronously realized. The heat treatment temperature is 500 ℃, the time is 1.5 h, the heating rate is 5 ℃/min, the Ar flow rate is 150 sccm, and the target product MoO is obtained3@NC。
Example 4
(1) Weighing 0.2 g of chitosan, dissolving the chitosan in 30 mL of hydrochloric acid solution with the concentration of 12 mol/L, and magnetically stirring for 50 min to obtain a solution A;
(2) 1 g of ammonium molybdate tetrahydrate ((NH) was weighed4)6Mo7O24·4H2O) is dissolved in 30 mL of deionized water, and the mixture is magnetically stirred for 60 min to obtain a solution B;
(3) slowly adding the obtained solution B into the solution A, and continuously magnetically stirring for 4 hours to obtain a solution C;
(4) heating the solution C at 60 ℃ to evaporate the solvent until the volume of the solution is 0.08 times of the volume of the initial solution, so as to obtain a suspension D;
(5) freeze-drying the suspension D at-45 ℃, and collecting a dried product as a precursor E;
(6) 3 g of the precursor E is placed in a crucible and placed in a tube furnace, heat treatment is carried out in an inert atmosphere Ar, and meanwhile water vapor is loaded through Ar, so that water vapor activation-heat treatment is synchronously realized. The heat treatment temperature is 350 DEG CThe time is 3 h, the heating rate is 3 ℃/min, the Ar flow rate is 50 sccm, and the target product MoO is obtained3@NC。
Example 5
(1) Weighing 0.8 g of chitosan, dissolving the chitosan in 30 mL of acetic acid solution with the concentration of 3 mol/L, and magnetically stirring for 45 min to obtain a solution A;
(2) 0.5 g of ammonium molybdate tetrahydrate ((NH) was weighed4)6Mo7O24·4H2O) is dissolved in 30 mL of deionized water, and the solution B is obtained after magnetic stirring for 30 min;
(3) slowly adding the obtained solution B into the solution A, and continuously magnetically stirring for 1h to obtain a solution C;
(4) heating the solution C at 60 ℃ to evaporate the solvent until the volume of the solution is 0.02 times of the volume of the initial solution, thereby obtaining a suspension D;
(5) freeze-drying the suspension D at-45 ℃, and collecting a dried product as a precursor E;
(6) 1 g of the precursor E is placed in a crucible and placed in a tube furnace, heat treatment is carried out in an inert atmosphere Ar, and meanwhile water vapor is loaded through Ar, so that water vapor activation-heat treatment is synchronously realized. The heat treatment temperature is 400 ℃, the time is 2h, the heating rate is 15 ℃/min, the Ar flow rate is 100 sccm, and the target product MoO is obtained3@NC。