Summary of Chemistry doctoral thesis: Synthesis of LiFexM1-xPO4/graphene nanocomposite as cathode material to improve electrochemical properties for lithium-ion batteries

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Nanocomposite material LiFe1-xMxPO4/Gr was successfully synthesized achieving it with single phase. It’s response required electrochemical parameters of positive electrode materials for Li-ion batteries such as: improved diffusion coefficient and conductivity, higher capacity comparative with LFP in same conditional synthesis.

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Nội dung Text: Summary of Chemistry doctoral thesis: Synthesis of LiFexM1-xPO4/graphene nanocomposite as cathode material to improve electrochemical properties for lithium-ion batteries

MINISTRY OF EDUCATION VIETNAM ACADEMY OF AND TRAINING SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY ---------------------------------- LA THI HANG SYNTHESIS OF LiFe1-xMxPO4/GRAPHENE NANOCOMPOSITE AS CATHODE MATERIAL TO IMPROVE ELECTROCHEMICAL PROPERTIES FOR LITHIUM-ION BATTERIES SUMMARY OF THESIS Major: Theoretical and Physical Chemistry Code: 9440119 HCMC-2019
The Thesis has been performed at: Institute of Applied Materials Science (IAMS) – Graduate University Science and Technology – Vietnam Academy of Science and Technology (VAST) ……..….………… Advisor 1: Assoc. Prof. Nguyen Nhi Tru Advisor 2: Assoc. Prof. Le My Loan Phung Reviewer 1: … Reviewer 2: … Reviewer 3: …. The thesis was presented at National level Council of Doctoral Thesis Assessment held at Graduate University of Science and Technology – Vietnam Academy of Science and Technology at….on…… The Thesis can be stored at: - The Library of Graduate University of Science and Technology - -National Library of Vietnam
INTRODUCTION 1. The important role of thesis These days the seriousness surrounding and pertaining to the-renewable energy crisis can cause exploited demands to the surrounding ecosystems leading to depletion, pollution and climate change including the alarming negative effects such as eroded environments. For these reasons, renewable energy should be increasingly promoted and developed further development and research such as: solar, wind and tide… thereby reducing our reliance on fossil fuels and the depletion of crucial ecosystems. However, these resources have huge disadvantages that are intermittent and dependent on conditional factors related to the climate, so using them poses many challenges for its application. Therefore, solving these problems requires the creation of efficient energy storage devices for the development of renewable energy transferred from chemical energy to electrical energy. This is an important role and strategy toward the improvement of renewable energy for global solutions. Since the advent of these technologies the rechargeable of Li-ion batteries were born, it brings many achievements for application science to contribute enhancement of new technology and various type of batteries for modern electrical and electronic technology. Rechargeable batteries are devices which enable energy storage and release directly as electricity; by means of highly reversible electrochemical reactions. Today, this storage dominates market in Portable mobile electronics with more potentials than other devices consideration: Large battery power, high energy density, light weight, safe and stability, as well as design flexibility. With good characteristic, Li-ion batteries could replace traditional storage. In recent years, Li-ion batteries have been considered the main structure to investigate electrochemical properties, especially, material electrode. Actually, Commercial rechargeable LiCoO2 for Li-ion batteries with specific capacity: 248 mAh.g-1 has been the choice material in the predominate market with the electronic devices in this field since the commercialization by Sony Energitech in 1992. However, this compound using cathode material has met some limitations with respect to charge-discharge issue, additionally safe, unfriendly environments at expensive costs (rate of 0.001% Co, 1.5% Fe in the Earth). LiFePO4 structural olivine is excellent choice with prolonged it’s life time, deeply cycle stability, environmental friendliness, and advantageous low cost but olivine structure has only one channel of direction [010], the intrinsic electronic conductivity of LFP olivine structure. Actually, LiFePO4 (LFP) has one huge drawback shown due to its rigid orthorhombic and Li+ diffusion rate is considerably low to reach it’s full theoretical capacity during battery operation. Plainly, it’s oxidation properties from Fe2+ to Fe3+ make it a less suitable choice, these problems lead to poor performance with Li-ion batteries to meet some challenges due with commercialization. To solve this problem, researchers in the world focus on improvement of ionic conductivity (10-9-10-10 S.cm-1) and diffusion coefficient (10-12-10-14 cm2.S-1) ion Li+ in order to enhance higher capacity in a high rate charge-discharge by this method of expansion; instinct-LFP: doping metal as a new material LiFexM1-xPO4 (LFMP) or coating carbon (LFP/C) and reduced size. Recently, the publication of these materials using cathode to enhancement of electrochemical properties for Li ion batteries have related to doping metals or coating graphene as individuality instead of linking together in recent decades. Furthermore, the number of international publications on metal-doped LiFePO4 materials and coated and graphene has been limited with LiFexM1-xPO4/graphene composite nano materials less or none of researches studied deeply the differences with several kinds of electrochemical performance by evaluation of the kinetic element of Li+ ion diffusion process into olivine structure. According 1
to international publications in the world as well as inheriting previous research based knowledge, this thesis was chosen as its basis. “Synthesis of LiFexM1-xPO4/graphene nanocomposite as cathode material to improve electrochemical properties for lithium-ion batteries. In this study, therein, survey the influence various rate of metal-doped materials on diffusion kinetic process of ion Li+ to distinguish electrochemical properties before and after doping metals. Besides, graphene coated-LiFe1-xMxPO4 materials as nanocomposite LiFe1-xMxPO4/graphene was investigated scope of olivine structure, morphology and the role of their improvement of electrochemial properties. The research results offered considerable contributions of solving the urgent problems for LFP materials to offer new rechargeable batteries with at inexpensive price including safety and environmental friendliness. LFP cathode is also a promising material for use in rechargeable batteries in electric vehicles compared to the lithium oxide layer structure materials using Co, Ni due to low toxicity and abundant in nature. 2. The target of thesis Nanocomposite material LiFe1-xMxPO4/Gr was successfully synthesized achieving it with single phase. It’s response required electrochemical parameters of positive electrode materials for Li-ion batteries such as: improved diffusion coefficient and conductivity, higher capacity comparative with LFP in same conditional synthesis. There are the specific ideas. - Investigations relation of the process’s of synthesis in structural olivine LFP by the solvothermal method and investigation of different synthesized parameters such as: temperature, solvent and the ratio of precursors.… - Investigations in the process of structural olivine was synthesized by solvothermal method, analysis and appreciation of structure, morphology and the chemical components of compound. - The Study of kinetic process of lithium extraction/insertion after doped metal-LFP with various ratio of weight Mn+ (Ni, Mn, Y). We also evaluate the linkage between graphene thin film materials, LiFe1- xMxPO4/Gr structure as well as graphene's role in electrical conductivity and electrochemical properties of materials. - Investigations in the performance of LiFe1-xMxPO4/Gr materials for cathode Li-ion batteries (capacity, cyclic) in in the model CR2032. NEW CONTRIBUTIONS OF THE THESIS 1. LiFe1-xMxPO4/C nanocomposite was synthesized via solvothermal method by simultaneously doping various metals such as: Mn, Ni, Y, etc and carbon coating. The carbon coating is itself from two separate sources: organic carbon (in situ) and graphene (ex situ) powder. with LiFe1-xMxPO4/C material crystals of graphene honeycomb network based on the difference in electronegativity of the atoms creating the effect of charge attraction between graphene and LFMP hexagonal networks to create composite materials with structure sustainable structure and excellent electrochemical performance. In contrast, previously published research only involves either graphene coating on LiFePO 4 (LiFePO4/Gr) or doping M metals into the structure of material for LiFe1-xMxPO4 synthesis. 2. It was empirically shown that after doping with M metals, the structure of the resultant materials is linearly retained without alterations to the olivine structure, in accordance with Vegard’s Law. 3. With doping metal and coating graphene, the synthesized LiFe1-xMxPO4/C material exhibits behaviors with Li+ ions’ movements through the olivine structure is predominantly controlled by the diffusion Warburg mechanism. This is the theoretical basis for improving electrochemical efficiency through 2
enhancing the diffusion coefficient and conductivity (5,1.10-3 S.cm-1) increasing 104-105 times (in comparison with LiFePO4 theory), resulting in specific capacity reaching 155-165 mAh.g-1 at C/10 remaining 96% after 20 cycles. 4. Using solvothermal method, LiFe0.8Mn0.2PO4/5%Gr has been successfully fabricated achieving specific capacity approximately equivalent to theoretical value of LiFePO4. The weight ratio of 20% Mn and 5% graphene can therefore be used to prepare LiFe1-xMxPO4/C as cathode materials for rechargeable Li-ion batteries, opening up new opportunities for expanding the scope of applications at a higher capacity. PUBLICATIONS 1. Huynh Le Thanh Nguyen, Nguyen Thi My Anh, Tran Van Man, La Thi Hang, Tran Thu Trang, Tran Thi Thuy Dung, Electrode composite LiFePO4@carbon: structure and electrochemical performances, Journal of Nanomaterials, 2019, 1-10 (SCI-E). 2. La Thi Hang, Nguyen Thi My Anh, Nguyen Nhi Tru, Huynh Le Thanh Nguyen, Le My Loan Phung,- Modification of nano-sized LiFePO4 via nickel doping and graphene coating, International Journal of Nanotechnology, 2019, 914-924(SCI-E). 3. Dinh Duc Thanh, Nguyen Thi My Anh, Nguyen Nhi Tru, La Thi Hang, Le My Loan Phung, The impact of carbon additives on lithium ion diffusion kinetic of LiFePO4/C composites, The Science and Technology Development Journal, 22(1), 2019, 173 – 179. 4. La Thi Hang, Nguyen Nhi Tru, Le My Loan Phung, Olivine structured LiFexYyPO4/C composite synthesized via solvothermal route as cathode material for lithium batteries, Vietnam Journal of Chemistry, 56(6E2), 2018, 267-271. 5. Bui Thi Thao Nguyen, Doan Thi Kim Bong, La Thi Hang, Nguyen Nhi Tru, Hoang Xuan Tung, Nguyen Thi My Anh, -Modification of Ketjenblack EC-600JD carbon as filler in cathode material for lithium-ion battery, Vietnam Journal of Chemistry, 56(6E2), 2018, 262-266. 6. Nguyen Thi My Anh, Doan Luong Vu, Nguyen Thai Hoa, Le My Loan Phung, Nguyen Ba Tai, La Thi Hang, Nguyen Ngoc Trung, Nguyen Nhi Tru- Characterization of LiFePO4 nanostructures synthesized by solvothermal method. Journal of Science and Technology, Technical universities 118, 2017, 45-50. 7. La Thi Hang, Le My Loan Phung, Nguyen Thi My Anh, Hoang Xuan Tung, Doan Phuc Luan, NguyenNhi Tru- Enhancement of li–ion battery capacity using nickel doped LiFePO4 as cathode material. Journal of Science and Technology 55_1B, 2017, 267-283. 8. La Thi Hang, Nguyen Nhi Tru, Nguyen Thi My Anh, Le My Loan Phung, Doan Luong Vu, Doan Phuc Luan, –Microwave-assisted solvothermal synthesis of LiFePO4/C nanostructures for lithium ion batteries, Proceedings of the 5th Asian Materials Data Symposium. HaNoi 10, 2016, 343-352. CHAPTER 1. INTRODUCTION This chapter expresses a brief overview of the Li-ion rechargeable battery and the history of it’s development and research in countries around the world. This secondary rechargeable battery is of special interest with some advantages in electrochemical properties as well as durability and especially positive trends impacts on the environment as compared to other chemical traditional storage devices. 3
Studies surrounding the principles of operation the properties of fabrication of the battery. Herein, analyzing the advantages and disadvantages of the battery to overcome the limitations of energy storage device. Surveys show that recent studies focus on changing cathode material composition to increase it charging efficiency and durable battery. Statistics and collection of documents from various publications about various types of cathode electrode materials and improvement directions. Specifically, highly studied cathode materials in which the olivine LiFePO4 (LFP) is considered as the most famous candidate for the family of olivine-type lithium transition metal phosphates by a relative specific capacity for the cathodes lithium-ion batteries. High capacity (170 mAh.g-1), flat voltage (3.45 V vs Li+/Li), slight weight, inexpensive (18-20 USD/kg of powder material) due to the amount of Fe in Earth's crust and nontoxic lead to environmental friendness. However, commercializing this material is a major barrier because the element Fe is easily oxidized, the + Li ion is less flexible because it moves a single diffraction direction [010] in the olivine tunnel structure, resulting in poor electrical conductivity (10-14 S.cm-1) and low diffusion coefficients (10-12-10-14 cm2.s-1). The author has surveyed and collected documents from published works, to improve the disadvantages of LFP materials with various approaches as a key in the electrochemistry. The following consideratiions and mentions are applicable. Control particle size (i) nanometer (nm) with nano size (100-350 nm) and well-shape crystals are important for enhancing the electrochemical properties because of shortening the amplitude of redox potential. Besides, reducing particle size increases electron density, shortening the distance of Li+ ion diffusion can increase diffusion coefficients. Another improvement that is of interest is metal doping (ii) electronic density enrichment that displaces the potential and increases the electrical conductivity of the material. Doping is good way for stabilizing the lattice structure of this class of inorganic electroactive materials. Lastly, It is well-known that carbon as a reducing agent prevents the formation of Fe3+ impurity and the agglomeration of particles during the preparation of LFP, but it also can significantly improve the battery performance. Coated-carbon (iii) on the material is also appreciated because it conducts electricity well on large surface contact areas as a conductive bridge and preventage corrosion of the electrode and limits exposure to the atmosphere. CHAPTER 2. EXPERIMENT 2.1. The route of synthesis LiFePO4, LiFe1-xMxPO4, LiFe1-xMxPO4/Gr The chemical precursors using synthesis of LFP, LiFe1-xMxPO4, LiFe1-xMxPO4/Gr as: LiOH.H2O (98%, Fisher); FeSO4.7H2O (99,9%, Fisher); Mn(NO3)2.4H2O (99,98%, Merck); Y(NO3)2.6H2O (99,9%, Merck), H3PO4 (99,98%, Merck), ascorbic acid (99,87%, Merck); H3PO4 (99,98%, Fisher) ethylene glycol (99, 87%, Merck), graphene (Merck). The precursors was weighted and measured basing on the rate of chemical formula of the material synthesis by an analytical balance of 4 odd numbers based on the ratio of Li: Fe: P and the number of moles of Li was chosen random: 0.03, 0.027, 0.025 mol. The author was defined ratio of Li:Fe:P = 3:1:1 achieving a single-phase material with high quality crystallization; ascorbic acid as a agent prevented oxidation Fe2+ and a reducing agent transferring Fe3+ to Fe2+. This mixture was dissolved in solvent with ethylene glycol/water (ratio volume: 4:1 stirring under ultrasonication until clear solution) in nitrogen atmosphere. The solution was finally transferred into autoclave under the argon atmosphere to perform the solvothermal reaction in 180 °C 4
for 5 h. After reaction, the greyish precipitate was centrifuged, rinsed repeatedly with ethanol and dried in vacuum at 70 °C for 7-9 hours. After that, the powder product is stored in desiccator at room temperature to avoid the moisture in the environment. Lastly, the sample was finally calcined at 550-600 °C in nitrogen atmosphere for 5 h to remove impurities. The mass and volume of precursors used for LFP synthesis are shown in Table 2.3; to synthesize doped LFP Mn, Ni, Y and to synthesize doped-LFP and graphene-coated LFP materials respectively Table 2.4, 2.5 Table 2.3. The mass/ volume of precursor expenditure for synthesis of LFP LiOH.H2O FeSO4.7H2O C6H8O6 H3PO4 C2H4(OH)2 H2O Tỉ lệ Samples (g) (g) (g) (g) (ml) (ml) Li: Fe: P A 41.96 278.01 176.12 97.99 - - - ST01 1.2588 2.7801 0.2202 1.1596 80 20 3.0:1:1 ST02 1.1329 2.7801 0.2202 1.1596 80 20 2.7:1:1 ST03 1.0490 2.7801 0.2202 1.1596 80 20 2.5:1:1 ST00 1.2588 2.7801 - 1.1596 - 100 3.0:1:1 Table 2.4. The mass/ volume of precursor expenditure for synthesis of metal dopant -LFP Samples LiOH.H2O FeSO4.7H2O M(NO3)n.6H2O C6H8O6 H3PO4 (g) (g) (g) (g) (g) Khối lượng mol 41.96 278.01 - 176.12 97.99 STN1 1.2588 2.6970 0.0872 0.22015 1.1596 STN2 1.2588 2.6411 0.1454 0.22015 1.1596 Ni2+ STN3 1.2588 2.5029 0.2908 0.22015 1.1596 STM1 1.2588 2.2241 0.5740 0.22015 1.1596 Mn2+ STM2 1.2588 2.0851 0.4305 0.22015 1.1596 Mn2+, STM3 1.2588 2.2241 0.4305, 0.1454 0.22015 1.1596 Ni2+ STY1 1.2588 2.6970 0.1149 0.22015 1.1596 STY2 1.2588 2.7245 0.0766 0.22015 1.1596 Y3+ STY3 1.2588 2.7523 0.0383 0.22015 1.1596 Ni(NO3)2.6H2O = 290,8; Mn(NO3)2.6H2O = 287,04; Y(NO3)3.6H2O = 383 Bảng 2.5. The mass/volume of precursor using for synthesis of LiFe1-xMxPO4/Gr LiOH.H2O FeSO4.7H2O M(NO3)n.6H2O C6H8O6 H3PO4 Graphene Samples (g) (g) (g) (g) (g) (g) M 41.96 278.01 (*) 176.12 97.99 12 STN1-G1 1.2588 2.6411 0.1454 0.22015 1.1596 0.08605 STN2-G2 1.2588 2.6411 0.1454 0.22015 1.1596 0.1721 STM1-G1 1.2588 2.2241 0.5740 0.22015 1.1596 0.0859 STM2-G2 1.2588 2.2241 0.5740 0.22015 1.1596 0.1718 STY1-G1 1.2588 2.7245 0.0766 0.22015 1.1596 0.08633 STY2-G2 1.2588 2.7245 0.0766 0.22015 1.1596 0.17266 STM3-G1 1.2588 2.2241 0.4305, 0.1454 0.22015 1.1596 0.08453 5
2.2. The Analysis Method Iron contents in LFP and nickel doped LFP were analyzed by the volumetric titration method using KMnO4 in concentrated H2SO4 medium to fully oxidized-Fe2+ to Fe3+. The LFP and LFNP samples were stirred with 50 mL H2SO4 until the formation of a clear green solution was created. The solution was then titrated with 0.0125N KMnO4 The LFP, LiFe1-xMxPO4, LiFe1-xMxPO4/Gr crystalline structure, phase purity and the particles size were characterized using a Rigaku/max 2500Pc and D8 Brucker X-ray diffractometer (XRD) with Cu-Kα radiation (λ=1.5418 Å, 2: 0° to 90° at a scan rate 0.25 -1.00 °/s). Raman measurements were performed using Horiba Jobin Yvon LabRAM HR300 system with 514.5 nm laser radiation, and the resolution of the measurement system was 2 cm-1. Raman spectra ranges from 100- 3000 cm-1. With 1 µm penetration, the vibration of LFNP bonds was determined and its structure and thin nanographene was identified. The system uses two excitation lasers and 4 magnetic ways (600 lines / mm to 2400 lines/mm). The controled heat speed is completed by software with an error of ± 0.1oC, measuring materials in powder form. Thermogravimetric analysis (TGA) technique with the type of Seratam LABSYS Evo TG-DSC at a heating rate of 10 oK/min in argon environment was implemented to determine the impurities phase contents in the samples. The mass in a sample of 100-150 mg its decreases it’s following when heating from 100 -1200 o C, rate scan 5-10 oC/minutes, time for scan reaching 50 minutes was performed at HCM City University Department of Education. This method was used as a predication of thermal stability with the calculation of crystallization content with the sample. Flame Atomic Absorption Spectrometry (FAAS) on AA-6800 machine (Shimadzu, Japan) ionte in air-acetylene flame at about 2700 °C. This technique is typically used for determinations in the mg/ L range, and may be extended down to a few μg /L with radiation wavelength: 253.7 nm, sensitivity: 0.1 ppm, operation mode: CV-AAS at HCM city University of Science. Energy dispersive X-rays (EDX - Hitachi SU6600, pressure < 10 pA, time lines: 30- 40 s, resolution: 127 eV) analysis was conducted to identify the elements occurring in structure. And using EDS Edax Team software analyzed data. The composite morphology and particle size were characterized by Scanning Electron Microscopy (SEM– type of 4800 machine: 5 kV, 8.5 mm x 20.0 k, the SEM images was studied at National Institute of Hygiene and Epidemiology), Field Emission Scanning Electron Microscopy (FESEM-Hitachi SU6600 equipment with machine parameters: 10 kV, 9.8 mm x 20 k at Institute for Nanotechnology in HCM city) and Transmission Electron Microscope (TEM-JEM-1400 with resolution of 0.2-0.38 nm, capacity of 100 kW, magnification of 2-3 Å), High-Resolution Transmission Electron Microscopy (HRTEM-FEI T20 at Singapore). Those methods was deeply investigated determination of particle morphology as well as analyzing the graphene film coating structure on material particles. In addition, with high-resolution HR-TEM technique, it’s possible to identify graphene film image and thickness and number of layers (sheet). X-ray Photoelectron spectroscopy (XPS) is well-method was used to determine the percentage of elements on the thin layer of materials (
The materials was mixed with acetylene black and copolymer binder (PVdF–HFP) (weight ratio 80:10:10) in N–methyl pyrrolidone (NMP) to make cell electrode. The ink solution was pasted on the aluminum foil with 0.1 mm thickness and dried in vacuum for 24 h. Electrochemical properties were specifically determined by different routes. Cyclic voltammetry method is the best choice to investigate the process of kinetic ion Li+ de-intercalation/intercalation on the face of surface, this measurement is performed on an electrochemical meter MGP2 (Bio-logic, France) with a scanning speed of 0.1-100 mV.s-1 with a voltage range of 2.5 to 4.0 V. Using A charge/discharge cycling method was studied structural stabilities and capacity performance. A charge/discharge cycling test for Swagelok–type battery was carried out in liquid electrolyte LiPF6/EC–DMC (1:1) at room temperature. Cells were assembled in a glove box under argon atmosphere with < 2 ppm H2O. Electrochemical studies were carried out using a MPG2 Galvano/Potentiostat (Bio–Logic, France; Applied Physical Chemistry Laboratory, University of Science, VNUHCM) in the potential window of 3.0-4.2 V versus Li/Li+ in the galvanostatic mode at the C/10 regime. The Electrochemical Impedance Spectra (EIS) supports the determination of conductivity and diffusion coefficient. In addition, the method supports determine the effect of diffusion or charge transfer when the battery performs charging. In this method, the battery system is assembled as in the charging and discharging procedure and measured on VSP devices (Bio-Logic, France) between 2.7 and 4.2 V versus Li+/Li. CHAPTER 3. RESULTS AND DISCUSSION 3.1. LFP materials electrode for Li-ion batteries 3.1.1. Characterization of crystalline structure and phase composition LiFePO4 was prepared to synthesize by solvothermal with variety of the different condition (table 2.3). Among them, ST01 sample using the ratio of precursors of Li: Fe: P = 3: 1: 1 was dissolved in ethylene glycol solvents (volume ratio EG/W = 4: 1). Ascorbic acid was added into the solution as an agent prevents oxidation Fe2+ to Fe3+. Structure of material having orthorhombic olivine-type shape with Pnma space group (JPCDS 96-101- 1112) without impurity phase and calculation of lattice parameters (Å) shows a =10.334 Å, b = 6.010 Å, c = 4.693 Å. Specifically, the sharp peaks in the patterns of LFP indicate that the powders are well crystallized and intensity its characteristic main peaks at the diffraction angles 2θ: 36o, 30o, 26o, 15o correspondent with the crystal planes of {311};{211},{202};{111}{200}. In addition, the amount of diffraction peaks includes over 20 peaks with low baseline and without spotting (Fig.3.1) Figure 3.1. XRD pattern of synthesized LFP comparative with standard LFP 7
The elements impact the rate of chemical reaction, type of solvents and the temperature heating on LFP phase structure. For the effect of firing temperature, LFP materials after drying have formed olivine phase but can still be mixed with organic or complex water-based impurities (XRD results do not detect impurities with small amount of compound less than 5.wt% such as Fe3(PO4)2 which can be pyrolysis at 200-300 oC . To solve this problem that is elevate these impurities need to heat the material in gas atmosphere and perform thermal analysis to determine the amount of contaminant decomposing the decomposition temperature. Furthermore, the results of TGA analysis of ST01 (L1) and ST01 (L2) specimen also support determination of crystallization temperature and the ability stable phase. It was heated in the argon environment through 3 main steps. - The process heating range of 30-300 oC. In this stage, we can observe clearly the TG plot variations in temperature with less than 20 % of weight loss by release residual water and de-composite of organic compound to perform CO2 gas and H2O steam. - The second stage heating from 300-500oC, the weight has trend decreasing slightly with 2-3 wt% from SO42-, OH- (base) - Continuous increasingly heating between 550-900 o C, this is a stable structure of material with less none of negligible weight was lost. Thereby indicated the small amount of carbon free non- Figure3.2: TG plot ST01 (1) and ST01 (2) bonded to the LFP particle. Most of carbon modification is expected to be chemical linking to the surface of LFP particles. Hence, the content of crystallization of thermal stability materials 500-650 o C and crystallization content accounted for 85% of suitable reference [32, 109] (Fig.3.2) 8
3.1.2. Analysis of chemical elements composition LFP material In order to determine chemical components material using different methods to look for the accurately chemical elements into LFP material was supported by advancement of structural analysis techniques such as: Energy Dispersive X-rays (EDS), Atomic Absorption Spectrometry (AAS) and X-ray Photoelectron spectroscopy. Actually, the analytical results show that full presence of elements in the material with relative content in accordance with the theoretical value. For instance, the analytical results EDS of LFP: the elemental components in LFP (ST01) including: Fe, C, O, P. The ratio each element accounting for: O (38-42%); Fe (15-18%); P (15-18%); C (6-9%) comparative with reference [163] (Fig. 3.3). Hình 3.3. EDS chart of ST01 sample XPS measurements of powder samples was not only considerable determination of the oxidation states of iron Fe2+ and Fe3+ at the extreme surface of the electrode materials but also determinant the component of elements by binding energy peak position of element such as: C (1s) is 284.5 eV, indicating the presence of carbon on the surface of the material due to the reduction of ascorbic acid during LFP synthesis to create a surface-covering carbon. The connecting energy of Li element (1s) has a position of 55.0 eV, characteristic for the atomic Li+ ion in the LFP material structure. At peak 531.8 eV of element O (1s) proved the presence of O2- in the P-O bond of anion (PO4)3- in the structured olivine type LFP. The observation of only one P 2p doublet at this binding energy reveals the presence of only one environment for phosphorus, in good agreement with a (PO4)3- phosphate group. Element P (2p) peaked at 133.2 eV characterizing the P5+ ion of the orthogonal LFP (orthorhombic) and FePO4 hexagonal structure In addition, the spectra of Fe (2p) were split into two parts due to the coupling of spin - orbit corresponding to Fe (2p3/2) and Fe (2p1/2) with peak positions at 711.6 eV and 725.1 eV. This is the connecting energy of Fe (2p3/2) with the charge of Fe3+ and Fe (2p1/2) with the charge of Fe2+. The peak of Fe (2p3/2) can be divided into two positions 711.2 and 715.1 eV; Similar peak Fe (2p1/2) can be divided into two peaks at 724.1 and 726.9 eV, completely separated from the spectrum of Fe3+ in the form of Fe2p elemental absorption peak decay with the document [184] (Fig. 3.4). The amount of ferric iron was determined by XPS data analysis reached 15 % while the values up to 20% determined from chemical titration method. 9
Figure 3.4. XPS spectra of LFP Figure 3.5. The XPS of Fe2+ 2p3/2 and Fe3+ 2p1/2 of LFP 3.1.3. Morphology and particle size of LFP SEM images was shown the crystal shape morphology of LFP as nanorod. Almost crystal phase similarly size, agglomerate, however, more and more heating trend is the agglomerate size tend to reduce respectively 100-500 oC with particles size between 50-150 nm. Average size approximately 136 nm was measured ImageJ software. Figure 3.6. SEM images of LFP with different heating degree : 100 oC (a); 400 oC (b); 550 oC (c) 3.1.4. Electrochemical properties of LFP 3.1.4.1. Evaluation ability of Li intercalation/de-intercalation by CV method To evaluate ability of Li intercalation/de-intercalation in surface materials using Cyclic Voltametric (CV) with different scan rate of from 0-100 µV/s. The cell was cycled between 2.8 and 4.2V and charged at 0.1C. It is shown that indicating two-phase nature of lithium extraction/insertion reactions between LiFePO4 and FePO4 (Fe3+/Fe2+) with flat voltage plateaus at 3.4-3.5V range (versus Li+/Li). The peaks appearing on CV curves are of the reversible oxidation-reduction process of Fe3+/Fe2+ corresponding intercalation/de-intercalation of Li into the structure without interference of strange phase (Fig. 3.7) 10
(a) (b) Figure 3.7. Cyclic voltammograms of LFP (ST01) with different scan rate (a) and comparison CV curves between ST01and ST02 a scan rate of 80 µV/s (b) From CV curve, determine cathode current density parameters (Ipc), anode current density (Ipa), redox potential of materials at various rate. Based on Randles-Sevcik equation, it’s possible to build a linear relationship between the cathode current strength (Ipc) according to the square root of the potential scanning rate (v1/2) and thereby can define the diffusion coefficient of the ion Li+ (DLi+) in the structure of cathode electrode materials. Using the above equation, the Li+ diffusion coefficients of ST01 (7.5.10-13 cm2.s-1), ST00 (1.6.10-13 cm2.s-1). 𝑖𝑝 = (2,69 × 105 )𝑛3⁄2 𝑣 1⁄2 𝐴𝐷 1⁄2 𝐶 ∗ (3.1) Where peak current 𝑖𝑝 , the number of electrons involved in the redox process n, the concentration of lithium in the electroactive material C* (mol.cm-3) was calculated as 0.0228 mol.cm-3, the electrode area A(cm2), the potential scan rate 𝑣 (V s-1), diffusion coefficient D. 3.1.4.2. Characterization of charge-discharge To compare charge-discharge voltage profile between two samples ST01, ST02 and ST00 using the fixed current discharge method at scan rate of C/10 and charge-discharge voltage between 3.4 and 3.5V (Vs. Li+/Li). All the samples have similar charge-discharge curve with flat plateaus corresponding to the lithium intercalation/de-intercalationin/out the olivine structure. As a result, the significant increase of specific capacity was observed for ST01 samples. Specifically, ST01 capacity up to 120 mAh.g-1 is higher than the ST00 reaching 95 mAh.g-1. The discharge efficiency reaches 60% after 20 cycles (Fig.3.8). The reduced discharge efficiency may be due to the Fe2+ material being corroded by the electrode, leading to un-stable structure. 11
Figure 3.8. Charge-discharge curve after 20 cycles at C/10 of ST00 and ST01 samples. 3.1.4.3. Conductivity of materials In Fig.3.9 the Nyquist impedance plots are observed semicircle line of two LFP material samples (ST01 and ST00). LFP was investigated with two stages obtaining: Kinetic control process at high frequencies (notice: A, C) and Warburg diffusion control process at low frequencies (notice: B, D). Actually, the Warburg diffusion process was dominated in total process. By the EIS plot, the resulting impedance spectra obtained the resistances of the samples respectively ST01 (100  ) and sample ST00 (200  ). According to the Randles equation - Sevcik the resistances inversely proportional diffusion coefficient so the resistances becomes smaller the greater the diffusion coefficient Lastly, the ion conductivity () sample ST01 (2.6.10-3 S.cm-1) and ST00 (1.2.10-3 S.cm-1). This is very consistent with our purpose to improve conductivity by using EG/H2O solvents and ascorbic agents to limit oxidation of Fe2+. Furthermore, organic solvents make increasing viscosity is a good way to reduce a particle agglomeration lead to Li+ ions more flexible. The conductivity of LFP material was synthesized by our research has a higher conductivity than the other publication similar Figure 3.9. Nyquist plots of EIS of LFP condition with 10-5 S/cm [52]. To increase these properties may add carbon as a nano film Successfully covering material synthesized LFP by [54] published solvothermal with LFP method (EG/H2O = 4:1), ascorbic acid takes important role -9 S.cm-1) preventive an oxidizing (5.9.10 (8.3.10-2asS.cm and LFP/Cagency ). reducing agent Fe3+to Fe2+and also changes different ratio well-1as Li:Fe:P, results are as follows with the best ratio 3:1:1, X-ray has strong diffraction intensity with lower background. Using ascorbic acid and EG/H2O solvents can control nanoparticle nano size to improve the capacity from 95 mAh.g-1 ( ST00) growing to 120 mAh.g-1 (ST01), the charging discharge efficiency reaches 60% after 20 cycles. 12
Although the conductivity (2.9.10-3 S.cm-1) and diffusion coefficient (7.5.10-13cm2.s-1) were improved significant, the capacity is low. It need to improve increasing material durability. 3.2. Doped-LFP materials 3.2.1. Analysis of structural crystal and component phase Metal doping is the excellent way to directly affects the olivine structure of LFP. Analysis of X-ray diffraction of LiFe1-xMxPO4 figure out that the material has olivine structure, the lattice parameters tend to decrease and coincide with the standard spectral up to 20 diffraction peaks (JPCDS 96-400-1850) (Fig 3.18). Further, after doping, pattern of X-ray diffraction was analyzed to find out without impurities, peak intensity highly and evenly rank of 2  = 40-70o. Thus, the olivine structure of doped-LFP has not changed phase comparison with LFP, however, this shift is consistent with the rule of the theory of crystallography involved in the structure of the chemical element depending un-doped atomic radius which can make change. For Ni, Mn has a radius smaller than Fe, so the diffraction points are slightly shifted to the right (Fig.3.19b). Therefore, to determine accurate this issue consideration the scope of each diffraction. Through the X- ray diffraction data, most diffraction peaks clearly illustrate the olivine LFP structure, the lattice parameters after the doping are insignificant, indicating the limitation of network defects and good dispersion in the material according to Vergard's law: aLiFe1 x Mx PO4  xaLiFePO4  (1  x)aLiMPO4 From other side, it is possible to explain the change in the volume of lattice cells after doping through two factors: firstly, the atomic radius of the doped metal element, the second is the content of the metal element doping. Thus, doped- nickel, the volume of crystal lattice is most reduced, which is perfectly suitable for theory because Ni's radius is the smallest compared to Mn, Y, Fe. Although yttrium has the smallest radius but a large electronic number (Z = 89) and the amount of doping is quite small, it has little impact on the network cell volume. In theory, the volume is inversely proportional to the electrical conductivity because it increases the ion density and shortens the diffusion distance. Figure 3.19. Figure out of shift of doped creating LiFe1-xMxPO4 (Ni, Mn, Y) 13
Raman spectroscopy is an effective method to determine the characteristic vibrational molecular bonds or functional groups through characteristic vibration frequencies [154]. It should be mentioned that this separation is a guide to discussion only, because the vibrations may be coupled. The Fe-O bond corresponds two vibrational bands rank of 150-300 cm-1 and 1200-1300 cm-1, other bonds obtaining M–O Ni–O, Mn–O, Y–O with oscillation region 500-800 cm-1. The separation of region modes at 995 cm–1 and 1067 cm-1 was observed P–O bond. In Figure 3.21a, two modes intensity of bond Fe–O và P–O is the most strongly was clearly proved a huge number of Fe, O, P in sample [154, 155]. Moreover, from the characteristic vibrational regions of the material, it has been shown that the oscillation area Fe-O has a much more intense peak intensity than Ni-O, Mn-O and Y-O, this is consistent in accordance with the content of mass components of the materials. In addition, the presence of doped elements makes slight shift, increasing the density of vibrations compared to LFP materials, indicating that the charge density increases gradually after doping (Fig. 3.21b) in the frequency range of 0-1300 cm-1 without C-O bond area. Hình 3.21. Raman spectra of LiFe1-xMxPO4 material in vibrational bond 0-1000 cm-1. 3.2.2. The chemical component of LiFe1-xYxPO4 materials EDS and XPS methods support to determine the composition of elements and the relative percentage of the elements in the material. Chemical titration and XPS spectra can determine Fe2+ amount in structure phase. EDS –Mapping technology was studied the ability to distribute materials and elements present in the structure. The result of AAS analysis was approximately measured theoretical the ratio of doped-metal. However, AAS results do not clearly show the elements present in the structure so we also re-investigated EDS technique determination of the amount of chemical elements by EDS, especially, XPS. 14
Figure 3.22. The distribution of elements of LiFe1-xYxPO4 (STY2) using SEM-EDS –Mapping LiFe0.95Ni0.05PO4 (STN2) was used EDS analysis the result as Fig. 3.23. Comparison STN2 with STY2 (Fig. 3.23) through EDS spectrum analysis showed that the intensity of element C, O, P was almost unchanged. Fe content in two different samples is due to Fe in STY2 sample higher than STN2 sample. This is entirely consistent with the theory because the Y- element doped sample has lower Y content than Ni, so the amount of Fe in the STY2 sample is less replaced. Figure 3.23. The EDS spectra of the grain of Ni, Y doped LiFePO4. For STN2 samples, the binding energy of Ni 2p3 (850 -900 eV) has appeared with a content of 1%, which confirms that Ni has participated in the material structure [45]. Similar to the STM3 sample that defines the binding energy area, there is element Ni 2p3 (850 -900 eV), Mn 2p3 (850 -900 eV). In addition, based on other binding energies, the presence of elemental components including Li, Ni, O, Fe, Mn can be accurately determined [86]. It’s also shown that the binding energy 710.1 eV. It indicates the oxidation state of Fe is +2. 15
Figure 3.24. XPS spetra of LiFe1-xMxPO4 material Table 3.9b. The result analysis of elements mass (wt.%) Sample Method C O P Fe Ni Y Mn STM2 EDS 4,96 41,08 15,10 19,04 - - 6,10 XPS 5,21 41,25 15,02 10,50 - - 6,25 STN2 EDS 6,27 40,99 14,01 20,03 3,10 - - XPS 5,49 42,05 16,20 21,00 3,10 STY2 EDS 5,69 40,78 18,80 20,22 - 0,01 - 3.2.3. Morlopholy and size of LiFe1-xMxPO4 In three Ni doped samples, the particle size of the material is nanometer (70-100 nm), uniform, distinct like rod (nanorod) and the particles are more separate than the non-doped LFP material. This result is consistent with previous publications [52,142]. In particular, the STN2 sample has the best results. For Mn doping samples, the particle size is 20-150 nm and yttrium doping is 20-150 nm. SEM and FESEM images all showed a less uniform distribution of particle size in STN2 and STM2 samples compared to STN2 samples as shown in Fig .3.25. Figure 3.25. FeSEM of LiFe0.98Y0.02PO4 (STY2); LiFe0.95Ni0.05PO4 (STN2); LiFe0.8Mn0.2PO4 (STM2). 3.2.4. Electrochemical performances 3.2.4.1. The kinetic process of intercalation/de-intercalation Li+ ion Electrochemical properties of synthesized materials are essential role for application in lithium-ion batteries. Compared to pristine LFP, nickel doped LFP enhances the number of lithium ions intercalated into the structure, thus increases specific capacity of batteries. In fact, the cyclic Voltammetric (CV) with various rate from 20 to 200 V/s) at cycling test in the potential range 3.4 -3.5 V (vs Li+/Li) and the occurrence of oxidation peaks on the CV curve is the reversible oxidation reduction of Fe 3+/ Fe2+ corresponding to the 16
intercalation/de-intercalation of Li+ ions into the olivine structure. The results of peak oxidation increasing respectively STN2, STY2, STM2. This can be explained that STM2 is the most currently density all of these. According to Randles-Sevcik equation to calculate the diffusion coefficient of the samples respectively in ascending order: ST01
Figure3.34. Charge-discharge curves LiFe0.8Mn0.2PO4 (a), LiFe0.98Y0.2PO4 (b), LiFe0.95Ni0.5PO4 (c) and compatitive betwwen charging capacity LiFe1-xMxPO4 (d) over 20 cycles. 3.2.4.3. The conductivity of LFMP material The impedance spectrum method (EIS) supports the determination of resistance. Through the Nyquist graph, it is possible to establish an equivalent circuit consisting of a fluid resistance (Rs), a transfer resistance (Rct), a double layer electrical capacitance (CPE) and a Warburg diffusion resistor (Zw) [33]. Resistance reduced means ion conductivity increasing, Li ion diffuses well in the material. The diffusion coefficient is studied by the CV method and the electrochemical total electrolytic method [33]. Both methods was achieved to get the best diffusion coefficients being STM2 (7.66.10- 12 cm2.S-1) samples, which are higher than those without doping 5-10 times. Thus, after metal doping Mn, Ni, Y improved diffusion coefficient reaches (Tab.3.9). A comparison of the documents [45, 52] after doped-Mn and Ni, conductivity 10-2-10-6 S.cm- 1 [54] doped 2% wt. Y reached 2.7.10-2 S.cm-1. Figure 3.35a EIS plot of LiFexM1-xPO4 18