Preparation of tungsten oxide and modification of tungsten oxide for lithium electricity

Graphite is the most mature and widely used anode material at present, but its theoretical specific capacity is only 372 MahG-1, so there is limited room for improvement.  At present, silicon-based anode is considered as the most promising next-generation anode material, but there are also some problems such as volume expansion and poor cyclic stability.  Transition metal oxide materials have high energy density, good cycling performance and stable physical and chemical properties, and there are abundant reserves of transition metal oxide materials on earth, low production cost and relatively simple preparation process, so they are also one of the hot materials in the field of cathode materials.  

Among many transition metal oxides, tungsten oxide materials have attracted researchers' attention because of their excellent semiconductor properties and potential applications.  With the development of nanotechnology, tungsten oxide has also been brought into the nano field.  Compared with massive tungsten oxide materials, nano tungsten oxide is endowed with the common advantages of nano materials: small size effect and quantum confinement effect.  In addition, nano tungsten oxide also has a variety of morphologies.  Nanosized tungsten oxide has been widely studied in the fields of photocatalysis, electrochromism, gas sensing and supercapacitors due to its many advantages.  Compared with carbon materials, nano-tungsten oxide has a higher theoretical specific capacity, so it is increasingly concerned in the field of lithium electricity.

1. Preparation of tungsten oxide  

For the preparation of tungsten oxide, there are three main methods, respectively liquid phase method, gas phase method, solid phase method.  

Liquid phase method  

Liquid phase method is in the liquid phase environment, the use of heating, stirring and other ways to make the chemical reaction of the solution, the product obtained by this method has high purity, uniform morphology, and the use of simple equipment, can be used on a large scale.  Therefore, this method is the most common and effective method to prepare WO3.  

In addition, the liquid phase method has the advantages of simple operation, simple equipment, wide operating temperature range and preparation of various morphologies of nanomaterials.  The liquid phase synthesis of WO3 can control the crystal structure and microstructure of the sample.  WOx nanoparticles, nanowires, nanorods, nanosheets and nanotubes are synthesized by sol-gel, hydrothermal and template methods.  

Solid phase method  

Solid phase method is a process for preparing nano powder by mixing raw materials in a certain proportion and then calcination or hot pressing to make solid particles have solid phase reaction.  Sometimes the required ultrafine nano powders are further prepared in conjunction with grinding and comminution techniques.  

The pulverization process of solid phase method generally includes two kinds: one is to divide massive materials into small particle size materials, including mechanical pulverization method, no material change in the process.  The second is the combination of small particle size materials, including spray pyrolysis, etc., in the process of material change.  

The solid-phase method differs from the liquid-phase and gas-phase methods in that there is no liquid-solid or gas-solid transition.  Although the solid-phase method is simple, the WO3 obtained is of low purity and easy to agglomerate.  

The gas phase method  

In the gas phase method, physical vapor deposition (PVD) can be used to directly use tungsten wire and tungsten powder as tungsten sources, and the size and morphology of the deposited WO3 nanoparticles can be controlled by controlling the experimental parameters of vapor deposition.  Most WO3 obtained by gas phase method is WO3-X.  

Compared with solid phase method, gas phase method has the advantages of small particle size, narrow distribution, good dispersion, no impurities and low energy consumption.  The WO3 product obtained by gas phase method has high purity, but it has high requirements on experimental equipment, so it is only suitable for laboratory and not suitable for industrial large-scale production.

2. Modification of tungsten oxide  

When tungsten oxide is used as lithium anode material, pure WO3 exhibits better lithium storage performance than carbon material, but it also has shortcomings.  In the first charge-discharge cycle, the discharge specific capacity of the material is considerable, basically exceeding the theoretical specific capacity of WO3 693MahG-1. However, in the subsequent cycle, most reports show that the capacity is not well maintained and the attenuation is obvious.  Therefore, the researchers improved its electrochemical performance by coating method and composite material.  

Using carbon nanotube film (CMF) as flexible substrate, Zhang et al. fixed tungsten oxide (WO3) and carbon source (citric acid) on THE CMF by spraying method to form carbon-coated tungsten oxide/carbon nanotube film (WO3@C/CMF) composites.  The freeze-drying carbon coated tungsten oxide/carbon nanotube films (F-WO3@C/CMF) and the hydrothermal carbon coated tungsten oxide/carbon nanotube films (H-WO3@C/CMF) were obtained by freeze-drying and hydrothermal methods.  The results showed that WO3 in H-WO3@C/CMF had good dispersion.  

The electrochemical performance of H-WO3@C/CMF (1:1) obtained when the ratio of tungsten and citric acid was 1:1 was studied. The discharge capacity of H-WO3@C/CMF was 1180Mahg-1 after 50 cycles, and the discharge capacity of H-WO3@C/CMF was still 589Mahg-1.  The results show that H-WO3@C/CMF as the anode of lithium ion battery is expected to improve its lithium storage performance.  

Wang et al. successfully prepared WO3/O2 composite film as anode material of lithium ion battery by micro-arc oxidation method, using titanium foil as matrix and tungstate as electrolyte.  The microstructure of the composite film was characterized by scanning electron microscope and X-ray diffractometer. The composite film was mainly composed of WO3 and TiO2.  

When the mass of sodium tungstate in the electrolyte was 70g, the electrochemical performance of the cell was the most stable, and the specific capacity was 605.684 mahg-1. After 200 cycles, the specific capacity remained at 141.466 mahg-1.  After coated with graphene, the initial specific capacity was increased to 662.3 MahG-1, and after 200 cycles, the specific capacity remained at 614.1 MahG-1, with a capacity retention rate of 92.7%, indicating better electrochemical performance.  

Yoon et al. prepared cauliflower WO3 by hydrothermal method and conducted carbon coating treatment on it. Cauliflower material is composed of short rods with a length of about 50nm and a diameter of 20nm.  When the current density is 50 mag-1, the specific discharge capacities of the carbon-coated and uncoated materials are 650Mahg-1 and 400Mahg-1 respectively after 50 cycles. With the continuous increase of cycles, the properties of the carbon-coated materials are improved rather than attenuated, while those of the uncoated materials are attenuated seriously.  

Kim et al. synthesized composite materials of WO3 nanoplates and graphene nanosheets by hydrothermal method, and measured the cycle performance curves of 5wt%, 10wt% and 20wt% graphene in pure phase and composite under the current density of 80mag-1.  The discharge capacity and cycle stability of different amounts of composite graphene were higher than those of pure WO3, and the performance was the best when the amount of composite graphene was 5wt%.  The performance improvement can be attributed to the addition of graphene to increase the electrical conductivity of the material, WO3 as the active substance, graphene as an excellent electronic conductor, good contact, resulting in improved performance of the electrode material.

Compounding WO3 with other oxides can also improve the electrochemical properties of the materials.  Gao et al. prepared WO3@SnO2 core-shell nanowire array by two-step hydrothermal method, and the capacity of the material remained at 1000Mahg-1 after 200 cycles at 0.28C current density.  The specific surface area of the composite is twice as much as that of the WO3 material before the composite. The superior electrochemical performance of the composite nanostructure is attributed to the reduction of the internal resistance of the battery, the improvement of the conductivity of the composite electrode, and the stability of the WO3 nanostructure.  

In a word, tungsten oxide has the advantages of good chemical stability, environmental protection, low price, high capacity, and is a potential anode material.  However, the low conductivity of tungsten oxide results in poor cyclic stability, which limits its application as a anode material.  With the rapid development of the new energy field, the research on tungsten oxide in the field of lithium electricity is also deepening, and the future research results will be more and more abundant. It is believed that tungsten oxide will also have a good development in the field of lithium electricity materials.