Nghiên cứu quá trình tổng hợp vật liệu nano silic để chế tạo anode định hướng ứng dụng cho ắc quy li ion tt tiếng anh

MINISTRY OF EDUCATION AND TRAINING MINISTRY OF NATIONAL DEFENCE ACADEMY OF MILITARY SCIENCE AND TECHNOLOGY NGUYEN VAN THANG STUDY ON SYNTHESIS PROCESS OF SILICON NANOMATERIALS TO FABRICATE ANODE ORIENT APPLICATION FOR Li-ION BATTERIES Specialization: Theoretical chemistry and Physical chemistry Code: 44 01 19 SUMMARY OF DOCTORAL THESIS Hanoi - 2019 This thesis has been completed at: Academy of Military Science and Technology, Ministry of Defence Scientific supervisors: Dr Nguyen Tran Hung Assoc Prof Dr Nguyen Manh Tuong Reviewer 1: Prof Dr Vu Thi Thu Ha Vietnam Institute of Industrial Chemistry Reviewer 2: Assoc Prof Dr Le Minh Cam Hanoi National University of Education Reviewer 3: Assoc Prof Dr Tran Van Chung Academy of Military Science and Technology The thesis was defended in front of the Doctoral Evaluating Council at Academy level held at Academy of Military Science and Technology at …… , date … month… , 2019 The thesis can be found at: - Academy of Military Science and Technology Library - National Library of Vietnam INTRODUCTION The urgency of the thesis topic Global warming, rising pollution problems are serious challenges that promote the replacement of non-renewable fossil fuels with green energy sources such as solar, wind and nuclear energy… Most of the green energy sources often depend on the geographical location of each region, the weather and the season, so the equipment cannot operate continuously, the efficiency is not high, the nuclear energy source is at risk potential for radiation damage to human health and the environment Therefore, energy storage issues are becoming increasingly important today Li-ion battery (LIB) is an advanced battery generation with many advantages such as high specific energy density, fairly compact size, the high number of discharging cycles (about 400 - 600 times) should be applied widely used However, due to the increasing demands of practice, there are still many studies to increase the capacity and durability of LIB One of the studies concerned is the battery anode material Graphite materials are widely used because they are cheap and easy to manufacture but have low capacity, only in the range of 130 - 270 mAh /g in actual operation In materials that can be applied to the anode of LIB, silicon is a potential material, thanks to its very high specific capacity (up to 4200 mAh/g), corresponding to Li22Si5 compound In operation, the reaction of Si with Li can cause material cracking due to an increase in the volume of about 400%, reducing the capacity of the electrode as well as the battery capacity and reducing the durability of the electrode The situation of nano-silicon synthesis for the application of anode Liion battery: Complex silicon nanoparticles technology, requiring modern and expensive equipment; Sources of silicon in nature: in rice husk, SiO2 content accounts for a high proportion (over 20% of mass) Rice husk has a large yield, hardly used effectively Studies on the synthesis of silicon nanoparticles from rice husk with simple and easy-to-implement technology have not been paid much attention Stemming from the practical problems, the PhD student has proposed and implemented the thesis topic: “Study on synthesis process of silicon nanomaterials to fabricate anode orient application for Li-ion batteries” 2 Objectives of the thesis: Determining some factors affecting the synthesis of silicon nanoparticles from rice husk Study on the structure and electrochemical properties of anode electrodes manufactured on the basis of silicon nanoparticles, rGO and nano Si@rGO materials Study on fabricating LIB using anode on the basis of silicon nanoparticles, rGO and nano Si@rGO materials and surveying battery characteristics Subjects and scope of research of the thesis * Research subjects: Rice husk, silica, silicon nanoparticles and experimental conditions for the synthesis of silicon nano from rice husk; rGO, nano Si@rGO The anode of LIB is made on the basis of silicon nanoparticles, rGO, nano Si@rGO * Research scope: Research on synthesis of silicon nanoparticles, rGO, nano Si@rGO for manufacturing anode of LIB within the laboratory The research content of the thesis: Synthesis and investigation of morphology, the structure of silica and silicon nanoparticles from rice husk Study on some thermodynamic characteristics, the kinetics of synthesis of silicon nanoparticles from rice husk Fabrication and survey of morphology, structure, electrochemical properties of anode electrodes based on silicon nanoparticles, rGO and nano Si@rGO materials Fabrication of LIB with anode electrode on the basis of silicon nanoparticles, rGO, nano Si@rGO materials and survey of battery characteristics The scientific and practical significance of the thesis: The research results of the thesis contribute to the basic research direction of thermodynamic and kinetic properties of the synthesis process of silicon nanoparticles, used to manufacture anode of LIB with the aim of increasing the capacity and performance of anode electrode This is a meaningful research direction, if successful will contribute to solving the current problems of anode material sources, thereby improving the battery quality of devices using power from batteries The layout of the thesis: The thesis consists of 130 pages divided into the following sections: Introduction; Chapter 1: Overview; Chapter 2: Subjects and research methods; Chapter 3: Results and discussion; Conclusion; List of published scientific works; References Chapter OVERVIEW Overview of LIB, on anode electrode materials of LIB, on silicon nanoparticle synthesis methods and the current status of thermodynamics, kinetics of synthesis of silicon from rice husk, overview of graphene synthesis methods Since then, set the scientific basis and orientation for the implementation of the research content of the thesis Chapter SUBJECTS AND METHODS OF RESEARCH 2.1 Research subjects Rice husk, nano silica, silicon nanoparticles and experimental conditions for the synthesis of silicon nanoparticles from rice husk; rGO, nano Si@rGO The anode of LIB is made on the basis of silicon nanoparticles, rGO, nano Si@rGO 2.2 Methods of analysis TG / DTA, DSC thermal analysis method; Method of scanning electron microscopy (SEM); Energy dispersive spectral method (EDX); Fourier transform infrared spectrum (FT-IR); X-ray diffraction method (XRD); Methods of surveying the electrochemical properties of electrodes: Galvanostatic cyclations (GC) and Cyclic voltammetry (CV) 2.3 Research methods 2.3.1 Process of synthesizing nano silica from rice husk Figure 2.1 Process of synthesizing of silica nanoparticles from rice husk Rice husk is milled, washed several times with tap water to remove residue and soil and is dried in an oven at 100 oC, resulting in processed rice husk After that, the processed rice husk is treated with 10% HCl acid solution at 100 oC After hours of treatment, rice husk is washed several times with water to a neutral environment Rice husk samples after acid treatment are heated in air at 650 oC for hours The product obtained is silica nanoparticles Effect of factors such as acid treatment time, acid treatment temperature, rice husk/acid ratio, firing temperature, firing time, heating rate were studied 2.3.2 Process of synthesizing silicon nano from silica nano Mix SiO2 with Mg powder with specific mass ratios, add polyvinylic 3% solution, mix well and dry in an argon inert gas cabinet to obtain a gray mixture The post-dried mixture is compressed under cylindrical member pressure Transfer the mixture to the nickel reaction boat and into the reactor center, the reactor is blown with argon gas (air flow 250 cm3/min), heat the mixture for hours at a temperature of 600 - 800 oC, let cool mixture in the furnace, remove the mixture of product from the furnace, crush it, wash with a solution of HCl and HF to remove impurities, then wash the mixture with distilled water to neutral, centrifuge and decant the solids The resulting solids bring vacuum drying to dryness, grinding, and obtaining silicon nanopowder Figure 2.2 Process of synthesizing silicon nano from silica nano 2.3.3 rGO synthesis process from graphite 2.3.3.1 Oxidizing graphite into graphene oxide (GO) Add the mixture of graphite and KMnO4 (mass ratio 1: 6) to the mixture of concentrated H2SO4 and H3PO4 (volume ratio of 9: 1) in the flask, placed on the ice cube tray (the mixture temperature does not exceed 10 oC) The ratio of acid mixture: graphite volume is 100 mL: g graphite The mixture is continued to stir for 10 minutes, remove the ice tray, start heating, keep the mixture reacted at 80 oC for hours The reaction mixture is cooled to room temperature, diluted with distilled water in a L volumetric flask, centrifuged, filtered, washed and obtained GO in a dark brown gel 2.3.3.2 Reduce GO into rGO by thermal shock method Figure 2.3 rGO synthesis process from graphite Freeze dried GO in the furnace, quickly raise the temperature to 800 oC, keep this temperature for 20 minutes in an argon stream The product obtained after heat reduction is rGO 2.3.4 The synthesis process of nano Si@rGO Figure 2.4 The synthesis process of nano Si@rGO 2.3.5 The anode fabrication process of LIB Figure 2.5 The anode fabrication process of LIB 2.3.6 Kinetic and thermodynamic equations: - Arrhenius equation: - Flynn - Walls - Ozawa model: - Kissinger model: - Gibbs free energy variation: ∆G* = ∆H* - T∆S* Chapter RESULTS AND DICUSSION 3.1 Synthetic nano silica from rice husk Silica nanoparticles are synthesized by the method of rice husk pyrolysis in an air furnace Influence of factors such as acid treatment time, acid treatment temperature, rice husk/acid ratio, firing temperature, heating time, heating rate were studied Figure: Experiment: Mau trau C.min-1 Crucible:PT 100 µl 12/02/2016 Procedure: RT > 900C (3 C.min-1) (Zone 2) Labsys TG Atmosphere:Air Mass (mg): TG/% 10.14 d TG/% /min Peak :54.34 °C 60 Peak :475.19 °C 40 -3 20 Peak :303.45 °C -6 Mass variation: -7.62 % -20 Mass variation: -51.48 % -40 -9 -60 Mass variation: -32.61 % -80 -12 -100 100 200 300 400 500 600 700 Furnace temperature /°C Figure 3.1 TG/DTA curve of rice husk at the heating rate of oC/min The TGA curve of RHs in the air has an obvious mass loss process where percentage of mass loss increases gradually with the increasing of ramp rates and TGA curve seems to move toward to the right In details, the TGA plots of leached RHs show a typical three-stage mass loss in air: (i) mass loss below 100 °C, which corresponds to humidity loss; (ii) mass loss around 300 °C, which corresponds to cellulose/hemicellulose/lignin degradation; and (iii) mass loss between 350-550 °C, corresponding to the burning of carbonous residues Around 12 % of the mass remains as the SiO2 product 3.1.1 Investigate the effects of acid treatment 3.1.1.1 The effect of the acid treatment time Figure 3.2 SiO2 content in rice husk ash dependence on acid treatment time The SiO2 content increases when the HCl acid wash time increases, helping to remove metal impurities in rice husk During acid washing time from and hours, SiO2 content did not change much Because metal impurities combine organic and inorganic components in rice husk, the treatment process, acid cannot penetrate and completely dissolve impurity metals Maybe this is the limit of the process of dissolving metal impurities in rice husk, so we choose an acid treatment time of hours 3.1.1.2 The effect of the acid treatment temperature With an acid treatment time of hours, the SiO2 content in rice husk ash increases with increasing treatment temperature This is explained when the temperature increases, the rate of dissolution of metal impurities in rice husk increases, making the treatment of fast metal impurities balanced However, at 100 oC, the vapor evaporates strongly, leading to strong HCl vapor Therefore, we choose the acid treatment temperature at 90 oC Figure 3.3 SiO2 content in rice husk ash dependence on aci treatment temperature 3.1.1.3 The effect of the ratio RHs/HCl acid: The content of SiO2 in rice husk ash decreases as the ratio of rice husk/acid increases The ratio of rice husk/40 ml of acid is 2.5 or 3.0, the SiO2 content of rice husk ash is similar, this is because the ratio of rice husk is almost completely submerged in acid, making the process level dissolving metal impurities in rice husk, helping to process metal impurities completely Therefore, the ratio of 3.0 g rice husk/40 mL acid is suitable Figure 3.4 SiO2 content in rice husk ash dependence on ratio RHs/HCl acid 3.1.1.4 Investigate the possibility of post-treatment acid reuse The concentration of acid after each reduction is between 0.9 % and 1.3 % compared to the original 10 % concentration After times of acid reuse, SiO2 content in rice husk ash is still very high, proving the effective treatment of rice husk of good reuse Reuse cycle Content of SiO2 in rice husk ash, % weight 96,79 96,64 96,73 96,81 96,65 Remaining acid concentration, C % 8,7 9,1 8,9 9,0 8,8 Thus, the proper condition of the treatment of RHs by acid are: HCl acid 10 %, temperature 90 oC, time hours, ratio of RHs/acid: 3.0 g/40 mL 3.1.2 Investigate the effects of calcination mode 3.1.2.1 The effect of calcination temperature: Temperatures below 600 oC, low SiO2 content in rice husk ash, organic matter has not completely burned, ash is tarnish black At temperatures of 600 °C or more, other substances have almost burned, the ash has turned white, the SiO2 content has also increased When the temperature reaches 650 oC, SiO2 content in rice husk ash reaches over 97 % When increasing the calcination temperature to 700 oC, the SiO2 content in rice husk ash increased insignificantly, indicating that the burning process took place quite thoroughly Thus, an appropriate husk burning temperature is 650 oC Figure 3.5 Influence of calcination temperature on SiO2 content in rice husk ash 3.1.2.2 Effect of calcination time: The time of burning rice husk from to hours, the SiO2 content in rice husk ash increased from 96.72% to 98.78% For hours and 2.5 hours samples, the upper surface of the obtained ash is white, however, the ash layers below are still dark brown because the carbon is still not completely burned Therefore, the hours calcination time is appropriate 11 Figure 3.9: Plots of lg and 1/Tp of RHs of the F-W-O model Figure 3.10: Plots of ln(β/Tp2) and 1/Tp of RHs of Kissinger model From the results of DSC thermal analysis, data on the reaction rate constants and according to the formulas, calculations for determining thermodynamic parameters ∆H*, ∆S* and ∆G* are presented in the table: Thermodynamic parameters of the synthesis of nano silica from rice husk T (K) 298 ∆S*(J/mol.K) ∆H* (kJ/mol) ∆G* (kJ/mol) -61,5 120,1 138,4 373 -63,4 119,5 143,1 573 -66,9 117,8 156,1 723 -68,9 116,6 166,4 923 -70,9 114,9 180,3 ΔG˚> 0: not self-evolving; ΔS˚ 0: endothermic reaction 3.2 Synthesis of silicon nano from silica nano 3.2.1 Investigate the effects of the silicon nano synthesis process 3.2.1.1 Effect of molar ratio SiO2:Mg Figure 3.11 Dependence of Si content on molar ratio Mg / SiO2 In fact, the obtained silicon content is lower than the theory and the largest is the molar ratio of Mg/SiO2 of 2.1: (corresponding to an excess of % Mg compared to the amount needed to react) and decrease when the 12 ratio mol Mg/SiO2 increased Since any excess Mg will form Mg2Si, reduce the Si content, resulting in reduced Si synthesis efficiency Therefore, it can be seen that Si fusion does not achieve theoretical performance under actual conditions, which may be due to the kinetic limitations of the reaction 3.2.1.2 The effect of calcination temperature on nano Si content Figure 3.12 Dependence on Si content on calcination temperature The content of silicon increases with temperature and this is achieved by reducing the amount of Mg2Si formed This result is reasonable, at higher temperatures, the test results reach theoretical values, showing that the reaction of SiO2 by Mg occurs faster when the temperature increases 3.2.1.3 Effect of heating rate when calcination to Si nanoparticle size Figure 3.13 SEM images of Si nano samples obtained after heating at a temperature of 800 °C at and 15 °C/min, respectively The heating rate significantly affects the morphology and size of silicon nanoparticles At a heating speed of oC, the size of Si-R5 silicon nanoparticles is uniform and smaller than that of Si-R15 particle heating rate of 15 oC/min The different characteristics of Si are generated at different heating rates possibly by local heat accumulation Since the reduction of SiO2 reaction by Mg is an exothermic reaction, a large amount of heat is released at the locations where SiO2 is in direct contact with Mg With a fast heating rate, there is not enough time for heat or transfer to occur, so the local temperature rises at nearby reaction centers, adding to the 13 local temperature These high-temperature reaction centers cause the synthesis of Si products and the disappearance of small pores Figure 3.14 DSC curve of SiO2 reduction by Mg Figure 3.15 XRD pattern of RH-5 silicon nano sample DSC shows the reaction occurring around 330-350 oC, and the greater the heating rate, the greater the heat build up (Figure 3.14), leading to a strong reaction and pushing the reactants Therefore, to obtain nano-Si, a slow heating rate is required, about °C/min is appropriate The obtained nano-Si material has a crystal structure The mechanism of Si formation is the crystalline structure from amorphous SiO2 material due to the chemical reaction with Mg in liquid Mg-Si-O alloy state After removing MgO, the structure of the remaining Si will be crystallized from the liquid state Si nanoparticles tend to agglomerate into larger particles, so it is necessary to grind the material in the ceramic grinding device, where Si blocks can break into Si nanoparticles in the range of 30-50 nm 3.2.2 Kinetic characteristics of silicon nano synthesis from silica nano * DSC thermal analysis: The mixture of Mg and SiO2 with a molar ratio of Mg/SiO2 is 2.1: 1, calcined in an argon atmosphere of 650 oC inert gas with different heating speeds The DSC thermal analysis curve of the calcination process is shown in Figure 3.16: Figure 3.16: DSC curves of nano SiO2 reduction process with Mg corresponding to the heating rates 3, 6, 9, 12, 15oC / minute 14 Basic kinematic parameters according to the FWO model and Kissinger model in the reduction of SiO2 by Mg: β (K/min) 12 15 Tp (K) 603,5 609,3 611,0 614,2 1/Tp103 (K-1) 1,66 1,64 1,63 1,62 Figure 3.17 Plots of lg and 1/Tp of the reduction of SiO2 of the F-W-O model FWO, lgβ 0,70 0,95 1,10 1,18 Kissinger, ln (β/Tp2) -11,20 -10,63 -10,35 -10,13 Figure 3.18 Plots of ln(β/Tp2) and 1/Tp of the reduction of SiO2 of Kissinger model - Model FWO: Activated energy, E* = 308.34 (kJ/mol) - Kissinger model: Activated energy, E* = 314 (kJ / mol), A = 8.19.1026 + Reaction rate constant according to Arrhenius equation: 3.3 Synthesis of rGO and nano composite Si@rGO nanomaterials 3.3.1 Synthesis of rGO nanomaterials 3.3.1.1 Investigating graphite oxidation stage products into GO Figure 3.19 GO gel after acid washing Figure 3.20 GO gel after freeze-drying GO ranges in size from 0.2 to 10 microns, concentrates between 2.3 and 5.1 microns, accounting for 68.5 % Compared to raw material graphite, GO's average size decreased by ½ compared to raw graphite This suggests that oxidation does not disrupt many of graphite's structures 15 Figure 3.21 Graphite particle size distribution Figure 3.22 GO particle size distribution From GO FTIR spectra showed the formation of organic radicals in the obtained products, which proved that graphite was oxidized Figure 3.23 FTIR spectrum of graphite Figure 3.24 FTIR spectrum of GO - Survey of crystal structure and surface morphology of graphite oxide: Graphite clearly shows the structure of the crystallized material, GO has an unclear crystalline structure This result is due to the functional groups containing oxygen on the surface and boundary of GO plate Figure 3.25 XRD pattern of graphite Figure 3.26 XRD pattern of GO Figure 3.27 SEM image of graphite Figure 3.28 SEM image of GO 16 GO products have a clear layer structure, made up of small GO pieces This result shows the ability to self-assemble when drying GO This has practical implications for the purpose of the thesis when using graphene films wrapped around silicon nanoparticles 3.3.1.2 Results of rGO synthesis After drying, dehumidifying rGO retains graphite layer structure, the thickness has decreased many times, due to the oxidation process, the size of rGO plates has decreased compared to raw graphite Figure 3.29 SEM images of rGO at different resolutions Figure 3.30 XRD pattern of rGO Figure 3.31 EDX pattern of rGO XRD spectroscopy shows characteristic peaks of graphite structure at 2theta = 28o, proving that synthetic graphene only changes the thickness of the graphene sheet without changing the crystal structure After reduction, the content of oxygen in graphene is greatly reduced Content of C and O in GO samples were 56.96 % and 17.82 % respectively; The content of C and O in rGO samples is 75.2 % and 3.45 % The results are consistent with the above analysis 3.3.2 Surveying structure and composition of nano Si@rGO After being reduced, the content of Si, C and O in Si@rGO samples is 71.28 %, 20.85 % and 6.58 %, respectively The results are consistent with the above calculations and analyzes of the Si@rGO component above Thus, nano Si@rGO has a ratio of Si: rGO  0.7: 0.3 The diffraction pattern of the composite is in good agreement with the cubic Si The diffraction peaks at 2 = 28.4o, 47.3o and 56.1o can be 17 attributed to the Si (111), (220) and (311) structure, respectively The characteristic diffraction peak of graphite at 26.4o didn’t appear, revealing that Si nanoparticles deposited on the graphene surface could efficiently suppress the stacking of reduced graphene layers, thus, in this case, no graphite-like layered structure formed again Figure 3.32 EDX pattern of nano Si@rGO Figure 3.33 XRD pattern of nano Si@rGO Figure 3.34 SEM image of nano Si@rGO Figure 3.35 TEM image of nano Si@rGO From the SEM image, there was no clear morphological difference between the two silicon and rGO nanomaterials after GO reduction GO elimination does not affect the intrinsic properties of both materials From TEM images, the formed rGO plates were wrapped around silicon nanoparticles, not affecting the size of silicon nanoparticles Therefore, the Si@rGO nanocomposite mixture is used as a slurry mixture for lithium ion battery testing 3.4 Study on the applicability of rGO and Si@rGO nanomaterials used to fabricate anode of LIB 3.4.1 Experimental fabrication of anode material combination Figure 3.36 Anode of LIB before and after drying in vacuum 18 After the testing process, we selected the rate of 80 % electrode materials, 10 % PVDF and 10 % SUP-P carbon to produce electrodes With this ratio, the fabricated electrodes have a smooth surface, good adhesion to the surface of the copper plate Observe the SEM image of the combination of anode materials on the picture, the surface is quite smooth, the polymer adhesive material layer has covered a thin layer on inorganic material particles 3.4.2 Electrochemical performances of LIB 3.4.2.1 Survey the electrochemical performances of LIB with anode fabricated on rGO basis * Cyclic Voltammetry The sample of half cell of rGO (Li/rGO) was analyzed by CV method with a scanning speed of 0.1mV/s in the range of - V Electrochemical performances by CV method of half cell: Figure 3.37 Cyclic voltammetry curves of LIB Li/EC:DMC 1:1, LiPF6 1M/rGO at a scan rate of 0.1 mV/s in the voltage range of to 2.0 V Sample of rGO anode material can charge at V nearby, corresponding to published documents on this material type The peak at 1.6 V of the first cycle shows the formation of SEI layer on the electrode surface, which results from the reaction of metal Li with LiPF6 salts and organic components in the electrolyte * The charge-discharge characteristics of LIB: The charge-discharge characteristics of the battery are evaluated by galvanostatic analysis with constant current density at 0.1C (C = 372 mAh/g is the theoretical capacity of carbon materials) The results are shown in the figure With an anode on rGO base, the battery can only work 35 cycles with capacitance reduced from 320 mAh/g to 220 mAh/g From the 36th cycle, the specific capacitance of the material drops very rapidly, down to 30 mAh/g in the 100th cycle Especially in the first cycle, it shows very high 19 loading capacity, reaching 1875 mAh/g, however, the Discharge capacity is only 345 mAh/g, resulting in very low Coloumbic performance This clearly shows that during the loading of the first cycle, the formation of SEI layer is very clear on the surface of carbon materials, similar to the results of CV analysis with the characteristic peak at 1.6 V Coulombic performance of the remaining cycle is also very low, at about 75% until the 35th cycle This demonstrates the charge-charging process of anode materials with low reversibility The flow density was started at 0.1C (37.2 mA/g) with a sample of halfanalyzed CV cells, gradually increased after every 10 cycles, to 50C (18600mA/g), as shown in Figure 3.38 The results show that anode can work at very high current density, but the capacity drops very low At low current density (0.1C; 0.2C; 0.5C) the capacity of the anode is unstable, indicating that the charging and discharging process is not effective However, after charging-discharging at high current density, the anode is evaluated at 0.1C current density The 10-cycle nap-discharge result shows that the anode works fairly stable with unchanged capacity compared to before the evaluation This suggests that it is possible for rGO anode materials to be charged - discharge at high current density before being able to work stably at low current density Figure 3.38 Cycling performance of LIB Li/EC:DMC 1:1, LiPF6 1M/rGO under the different current rates in the voltage range of to 2.0 V 3.4.2.2 Survey the electrochemical performances of LIB with anode fabricated on nano Si basis * Cyclic Voltammetry Figure 3.39 Cyclic voltammetry curves of LIB Li/EC:DMC 1:1, 20 LiPF6 1M/Si at a scan rate of 0.1 mV/s in the voltage range of to 2.0 V Electrochemical characteristics through CV analysis show that in the first cycle the charge potential of the Si anode ~ 100 mV shows that Si materials have a crystal structure, due to crystalline Si reacting with Li+ at this voltage (while amorphous Si is ~ 200 mV) This result again demonstrates that nano Si have a crystal structure Finish the loading process (lithiation) with the result of forming a crystalline Li15Si4 compound The delithiation begins at a voltage of ~ 200 mV, indicating that Li+ starts to come out of the Li15Si4 compound The apparent peak discharge at 300 - 400 mV indicates the existence of two forms of material at this time: Li15Si4 amorphous crystalline and Si forms, as a result of the Li+ process leaving the compound Li15Si4 crystal form At the second cycle, the charge starts at ~ 500 mV and there are two distinct lithiation peaks at ~ 200 mV and ~ mV This result shows that after the first discharge process, Si material exists in amorphous form and after the recharge process in the second cycle, there is Li15Si4 crystalline compound, similar to cycle the first period The 2nd cycle discharge is similar to the first cycle, with peaks in the range of 300 - 400 mV The electrochemical properties of the Si electrode through CV analysis showed that the charging process in the first cycle is different from the subsequent cycles, due to the conversion from crystalline Si material to Li15Si4 compound From the first-cycle discharge, the electrochemical properties of the electrode remain unchanged * The charge-discharge characteristics of LIB: Figure 3.40 Cycling performance of LIB Li/EC:DMC 1:1, LiPF6 1M/Si under the different current rates in the voltage range of to 2.0 V The charge and discharge capacity of the Si nano anode are evaluated at a current density of 0.1C (1C = 4200 mA/g) The current density is calculated on the basis of the specific capacity of Si, which has a value of 4200 mAh/g according to the theory for Li15Si4 compound The lower the current density, the higher the capacity value of anode, close to the theoretical value With a current density of 420 mA/g, half cell Li/EC: 21 DMC 1: 1, LiPF6 1M/Si reaches the specific capacity of the adjacent anode 3000 mAh/g for the first 10 cycles From cycle 11 onwards the capacity decreases, to 2250 mAh/g in the 35th cycle After the 35th cycle, the capacity decreases sharply, indicating the battery's ability to work is not guaranteed This result corresponds to other claims about nano-Si materials It is worth noting here that the Coulombic efficiency is not high, reaching in the range of 93 – 97 %, indicating that the reversible charge-discharge process of the Si anode is not satisfactory This result can explain the working durability (number of charging and discharging cycles) of only 35 cycles 3.4.2.3 Survey the electrochemical performances of LIB with anode fabricated on nano Si@rGO basis * Cyclic Voltammetry The CV curve for the first cycles, showing the electrochemical properties of Si@rGO nano anode If compared with the CV characteristics of nano-Si anode, there will not be many other points With the first cycle, due to the presence of rGO, the charging process starts around 170 mV From 100 mV is the charging process of crystalline Si nanomaterials At the end of the charging process, the potential drops to 0mV, corresponding to the formation of Li15Si4 crystalline and C6Li compounds At the discharging process, peaks at about 330 mV and 490 mV This demonstrates the simultaneous existence of Li15Si4 amorphous Si crystalline and nano form At the end of the first cycle, the remaining nano-Si anode is amorphous and carbon (graphene) At the second cycle, the charging process starts earlier, at 500 mV nearby This is the formation of the SEI layer on the anode surface (mainly amorphous nano-Si) in parallel with the process of creating LixSiy compound When the potential decreases to mV, the amorphous Si nanotubes and graphene end up as Li15Si4 crystalline and C6Li compounds, similar to the first cycle The discharging process in the second cycle is similar to the first cycle, and the following cycles 0.6 1st cycle 2nd cycle 0.4 Current (mA) 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 0.0 0.5 1.0 1.5 2.0 + Potential (V vs Li/Li ) Figure 3.41: Cyclic voltammetry curves of LIB Li/EC:DMC 1:1, LiPF6 1M/Si@rGO at a scan rate of 0.1 mV/s in the voltage range of to 2.0 V 22 * The charge-discharge characteristics of LIB: Parallel to the results of analysis by characteristic CV, the half cell is evaluated by galvanostatic analysis for the first cycles The results show that the basic difference of cycles is at the charging process At the first cycle, the charge at the voltage is lower than that of the second cycle, because the first silicon nanomaterial is in crystal form At the end of the first cycle discharge process only exists amorphous nano-Si form Therefore the charging process will be the same from the second cycle The point here is that for the discharge characteristic line, it is shown that the flat section at the voltage is about 40 mV, corresponding to the CV characteristic when the compound exists Li15Si4 amorphous Si crystal and nano form At the 0.05C current density, the anode capacity is > 3000 mAh/g This is the common value for anode including nano-Si and rGO The first cycle, specific capacity reaches > 1800 mAh/g and Coulombic performance reaches 96 % These are very high values, comparable to Si nanowire materials in published studies Within the first 200 cycles, the specific capacity of the anode stays at values > 1200 mAh/g and stays at this value for 500 cycles This value can be compared to the number of discharge cycles of a commercial Li-ion battery using graphite anode material This result shows that the role of rGO material has high electrical conductivity, which helps stabilize the charging and discharging of the battery It can be explained by the nanocomposite structure, the graphene sheets are wrapped around and bonding Si nanoparticles, making Si nanoparticles interlinked, thus increasing the number of charge-discharge cycles The discharge result at 1.5C current density also shows that Coulombic efficiency reaches 98 % from the second cycle This result contributes to explain the anode's working durability Figure 3.42 Cycling performance of LIB Li/EC:DMC 1:1, LiPF6 1M/Si@rGO under the different current rates in the voltage range of to 2.0 V 23 Figure 3.43 shows the behavior over 50 cycles, when the current rate is progressively increased from C/20 to 5C, the latter rate corresponding to a charge-discharge current of 15 A/g Note that at 5C, the capacity is still ∼130 mAh/g After the 45 cycles with the increased current rates up to 5C, Figure 3.43 Rate capability at the current rates from 150 mA/g to 15 A/g the capacity at a C/10 rate is still ∼2600 mAh/g, see the last cycles at the extreme right - hand side of Figure 3.43 CONCLUSION * Research results of the thesis: Determined the appropriate conditions selected for the process of manufacturing synthetic silica nano from rice husk is the acidity of treatment is 10 % HCl acid, acid treatment at 90 °C, hours treatment time, The ratio of rice husk/10 % HCl acid is g/40 mL Rice husk is fired at 650 o C for hours at a heating rate of oC/min The silica nanoparticles obtained are 50 - 70 nm in size, amorphous phase structure, purity > 99 % Determined thermodynamic parameters, kinetics of the synthesis of silica nano from rice husk: activation energy of synthesis of silica nano from rice husk: E* = 126.14 (kJ/mol) (according to the FWO model); E* = 122.6 (kJ/mol) and the pre-exponential factor in the Arrhenius equation is A = 1.033.1010 (according to the Kissinger model), there by determining the reaction rate constant according to the Arrhenius equation Determination of thermodynamic parameters of the synthesis of silica nano from rice husk: ∆G* = 138.5  180.4 kJ/mol, ∆H* = 114.9  120.1 kJ/mol and ∆S* = -70.9  -61.5 J/mol.K Determined suitable conditions selected for the fabrication of silicon nanoparticles from silica nano with reducing agent Mg, molar ratio Mg: SiO2 is 2.1: 1, calcination temperature at 800 oC in hours at a heating rate 24 of oC/min Silicon nano obtained has a particle size of 30-50 nm, crystal phase structure, purity > 99 % Activation energy of silica reduction process with Mg: E* = 308.34 (kJ/mol) (according to F-W-O model); E* = 314.13 (kJ/mol) and the pre-exponential factor in Arrhenius equation is A = 8.19.1026 (according to Kissinger model) Has synthesized rGO, nano-Si@rGO and surveyed the structure and morphology of rGO and nano-Si@rGO materials Content C in rGO sample is 75.2 %; Si and C contents in Si@rGO nano samples were 69.96 % and 31.04 %, respectively Successfully fabricated LIB with anode based on rGO, nano-Si and nano Si@rGO Specific electrochemical characteristics are as follows: - LIB with anode on rGO base: maximum capacity of 372 mAh/g; maximum current density of 50C (18600 mAh/g); the maximum number of discharge cycles 100 cycles; Coulombic efficiency reaches 75 % - LIB with anode on nano-Si base: maximum capacity of 2250 mAh/g; maximum current density 1C (3800 mAh/g); maximum discharge cycle number of 35 cycles; Coulombic efficiency reaches 93 % - LIB with anode on nano Si@rGO base: maximum capacity 1800 mAh/g; Maximum current density 5C (13850 mAh/g); the maximum number of discharge cycles of 500 cycles; Coulombic efficiency reaches 98 % * New contributions of the thesis: Determined conditions for synthesizing silicon nanomaterials from rice husk Fabricated nano-Si anode for electrochemical characteristics such as high specific capacity, high current density, number of charge cycles, high Coulombic efficiency * Further research directions: Further research on thermodynamic and kinetic characteristics affecting the synthesis of silicon nano from silica nano Further study the process of fabricating anode on the basis of synthetic materials, which helps optimize the process of fabricating anode for LIB Investigate the electrochemical characteristics of anode manufactured from rGO, nano-Si and nanocomposite Si@rGO to get more data about these batteries 25 LIST OF SCIENTIFIC WORK PUBLISHED Nguyen Van Thang, Nguyen Manh Tuong, Nguyen Tran Hung (2016), “Thermodynamic evaluation of synthesis of nanosilica from the rice husk”, Proceeding of The 5th Asian materials data symposium, Hanoi 11/2016, pp 331-340 Nguyen Van Thang, Nguyen Manh Tuong, Nguyen Tran Hung (2017), “Silicon nanoparticles from the rice husk - thermodynamic evaluation and synthesis”, Vietnam journal of chemistry, 55(3e), pp 176-182 Nguyen Van Thang, Nguyen Manh Tuong, Nguyen Tran Hung (2018), “Characteristic of thermodynamics, kinetics of the process silicon nanoparticle synthesis from rice husk”, Journal of Military Science and Technology, Special Issue CBES2, pp 107114 Nguyen Van Thang, Nguyen Manh Tuong, Nguyen Tran Hung (2018), “Synthesis and investigate the electrochemical performance of Si/Graphene nanocomposite anode for Lithiumion batteries”, Vietnam journal of chemistry, 56(4e), pp 168-171 ... battery capacity and reducing the durability of the electrode The situation of nano- silicon synthesis for the application of anode Liion battery: Complex silicon nanoparticles technology, requiring... properties of anode electrodes manufactured on the basis of silicon nanoparticles, rGO and nano Si@rGO materials Study on fabricating LIB using anode on the basis of silicon nanoparticles, rGO and nano. .. rice husk; rGO, nano Si@rGO The anode of LIB is made on the basis of silicon nanoparticles, rGO, nano Si@rGO * Research scope: Research on synthesis of silicon nanoparticles, rGO, nano Si@rGO for