One of the possibilities to effectively combine Si and LTO, to keep their advantages and overcome disadvantages, is to apply nanomixing. One of the most suitable and promising methods of LTO synthesis is the spray pyrolysis technique as there are promising opportunities to establish a continuous preparation process. The structure and composition of the product were investigated by X-ray diffraction and elemental analysis.
The electrochemical performance of the material was improved when the materials were prepared with the addition of carbon sources. I would like to thank my advisor, Professor Zhumabay Bakenov, for introducing me to this research project and guiding me to the completion of the work.
Introduction
Motivation and Objectives
However, graphite can only allow the diffusion of one lithium atom with six carbon atoms (LiC6), resulting in a relatively low reversible capacity of 372 mAh g-1. In addition, the low diffusion rate of lithium into graphite results in a low power density of the battery. The electrodes in LIBs are separated by a membrane/separator, which is usually made of polypropylene or polyethylene.
The separator is filled (soaked) with an electrolyte solution of lithium salts in a mixture of alkyl carbonates. Therefore, the materials used in LIBs must maintain good ionic and electrical conductivity, long life (for longer battery life), high lithium diffusion rate into the electrode materials, reversible performance, and low cost.
Literature review on anode materials for anode of LIBs
- Carbon-based materials
- Titanium-based materials
- Alloy anodes
- Materials selection
In addition, the production of LIBs with CNTs is not commercially beneficial due to the high cost of the nanotubes. The values differ due to the reported modes of lithium ion adsorption: in the first case, the ions were absorbed by the inner and outer surfaces of graphene sheets (1 to 6 stoichiometric ratio), while the second one reported that lithium ions in a covalent is caught. bond in the benzene rings (1 to 2 stoichiometric ratio). One of the advantages of this compound is the stability of structure during the insertion of Li ions [16, 19].
In conclusion, it can be said that silicon is one of the best materials for the anode in LIBs, as it is a low-cost, abundant and environmentally friendly material with a large theoretical capacity of 4200 mAh g-1 for Li22Si5. However, it is impossible to use it only as an anode in LIBs due to the severe volume expansion during the lithiation process.
Literature review on synthesis techniques
The obtained precipitate was filtered and washed 3 times and then dried in a vacuum oven at 80 oC for 12-13 hours. As a result, the capacity of LTO reached its theoretical value and the life of the anode was calculated as 100 cycles. After the mixture had cooled, the resulting solids were ground to powder with a mortar and pestle.
The mixture is stirred for 3 hours and then dried at 65 oC to completely remove the liquid. The raw materials, titanium (IV) oxide and lithium carbonate, were mixed in a ratio of 5:2 and heated at 800 oC for 24 hours, respectively. Then, the crude LTO was transferred to a reaction tube for reaction with toluene vapor by nitrogen gas at a volumetric flow rate of 1 L min-1 at 700-900 oC for 2 h.
Polyethylene glycol was added at a ratio of 1:2 to the LTO solution and the resulting mixture was stirred for 12 hours. The obtained precipitate is washed with DI water and then with ethanol and placed in a vacuum oven for drying for 6 hours. Then the resulting mixture is heated to 100 oC to evaporate the water and calcined in air.
Prepared powders (collected in bag filter) were post-treated in the oven at 600-800 oC for 3 hours under air. The titanium precursor was TTIP dissolved in DI water; to eliminate the precipitate obtained, H2O2 was added to the solution and mixed with 12.5% ammonia solution (pH around 10). Therefore, it was decided to proceed with the preparation of LTO material using the above modified method.
Experimental procedures
- Materials preparation
- Apparatus set-up
- Cell configuration
- Structural characterization
- Electrochemical characterization
The aim was to study the effect of non-polymeric and polymeric carbon sources on the electrode performance and compare this with the performance of the material without any carbon additive. 2 below, consists of the droplet generator (nebulizer, OMRON), quartz tube furnace and filter introduced at the exit of the reactor. The Teflon filter is located at the exit of the tube to capture the flowing particles.
The temperature of the tube furnace was set at 800 oC and the temperature of the heating belt was 315 oC (which was the maximum technically set). First, in the early stages of the experiments, the precursor solution was fed into the nebulizer with a peristaltic pump at the rate of 1 rpm. For the experiments with carbon sources, the concentration of metal precursors was lowered twice: 0.5 M for Li+ and 0.25 M for Ti2+ to lower the viscosity of the precursor solution.
However, even at these concentrations, the ultrasonic nebulizer was unable to generate aerosols, as the sticky solution deposited on the walls of the nebulizer cup (Figure 2.3). In order to perform electrical testing, the batteries must be assembled. The above components were mixed in an agate grinder for 20-30 minutes until a homogeneous mixture with the following proportions of components was obtained: 80 wt. % of active substances, 15 wt. % AB and 0.5-0.6 mL of CMC solution.
The mass of active material is calculated based on the weight of active parts such as LTO, Si, AB and carbon sources. In detail, the mass of slurry on the electrode was obtained by weighing the electrode and subtracting the mass of copper foil of the same diameter from that mass. The mass fractions of LTO, Si and carbon components were determined based on XRD patterns.
Results and Discussion
Results of structural characterization
Nevertheless, as can be seen from the above samples, the presence of carbon sources was not revealed by XRD analysis. However, XRD of the resulting powder showed that most of the LTO was transformed into rutile TiO2, and the Si peaks also disappeared. A possible reason for this could be the low purity of the nitrogen gas (such as technical grade gas.
As can be seen from Table 3.1, the amount of carbon is very low in both samples. The possible reason could be the fact that most of the carbon burned at the inlet of the tube and the majority of it was converted to carbon monoxide and carbon dioxide due to the air intake through the compressor atomizer, which was discussed earlier. As can be seen, the original LTO prepared by gas-state method has spherical shape particles and the particle size distribution is between 100 nm to 1 µm.
When it was prepared with Si nanopowder, a nanomixture of beads can be observed. As can be seen in Figure 3.3b, the Si nanoparticles with a much smaller volume aggregate around the larger LTO particles. The pictures of the powder prepared with the addition of carbon sources are very different: the first presents sharp-angled agglomerates similar to those in fig.
To better understand the morphology of the last two powders, TEM images were taken (see Fig.3.4). However, TEM images show that Si not only covers LTO, but is also inserted into the structure of LTO, improving the electrochemical properties of the latter. It can be concluded that the gas-state method such as spray drying can provide very good mixing and linking of the components within the material.
Results of electrochemical characterization
A plateau in the first discharge curve in all three graphs in the Charge/Discharge profiles above corresponds to the transition of crystalline Si to the amorphous phase. The smaller plateau region at the 1.5 V point, which also exists in all three graphs, refers to the intercalation of Li ions into the spinel structure of LTO [51]. Comparing the results of the first sample with the other two, significant improvements can be observed.
Despite the fact that the first three cycles show a lower performance than the LTO/Si sample, the situation is the opposite with subsequent cycles. If the LTO/Si sample has almost zero capacity from the 10th cycle onwards, the other two samples have capacities between 200 and 300 mAh g-1. It can also be seen that the efficiency of the first sample fluctuates dramatically up to the 30th cycle, while the efficiency of the other two samples fluctuates.
The retention of about 59 and 62% from its initial capacity was shown by LTO/Si/Sucrose and LTO/Si/PEG, respectively, after 30 cycles at 0.2 C; the capacity retention of LTO/Si was only 1.2%. The carbon-containing samples were very similar to each other, although LTO/Si/PEG gave slightly better results in terms of capacity and efficiency. In general, it can be stated that electrochemical properties of LTO were successfully improved by Si nanopowder and presence of carbon source, even though the amount of the last one was small.
As it was shown by the TEM method, the Si nanoparticles were inserted into the structure of LTO which prevents Si from breaking due to the volume changes during cycling and improves the LTO capacity. From the battery test results, it can be seen that the characteristics of the batteries have been significantly improved.
Future work
Also, the proportions of the precursor solution can be varied to explore the possibilities of controlling the morphology and structure of the materials. The next step, which can be performed in the project, is to adjust the compressed nebulizer so that there is no air intake. It could possibly be located in the glove box with a very pure argon or nitrogen atmosphere, with insignificant air content.
This can provide different content of the material because the carbon will not be converted into carbon oxides.
Conclusion
Electrolyte reactions with the surface of LiNi0.5Mn1.5O4 high-voltage cathodes for lithium-ion batteries. Improved performance and performance of carbon nanotube-based anodes with titanium contacts for lithium-ion batteries. Flexible silicon-carbon nanowires sandwiched between sheets of reduced graphene oxide as lithium-ion battery anodes.
Investigation of different binders for nanocrystalline anatase TiO2 anodes and their application in a new green lithium-ion battery. Synthesis of phase-pure SnO2 nanosheets with different organized structures and their lithium storage properties. Synthesis of hollow spherical Li4Ti5O12 by macroemulsion method and its application in Li-ion batteries.
