MINISTRY OF EDUCATION AND TRAINING VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY --- LA THI HANG NANOCOMPOSITE AS CATHODE MATERIAL TO IMPR
Trang 1MINISTRY OF EDUCATION
AND TRAINING
VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY
GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY
-
LA THI HANG
NANOCOMPOSITE AS CATHODE MATERIAL TO IMPROVE ELECTROCHEMICAL PROPERTIES FOR
LITHIUM-ION BATTERIES
SUMMARY OF THESIS Major: Theoretical and Physical Chemistry
Code: 9440119
HCMC-2019
Trang 2The 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
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
Trang 3INTRODUCTION
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
Trang 4to international publications in the world as well as inheriting previous research based knowledge, this thesis
was chosen as its basis “Synthesis of LiFe x M 1-x PO 4 /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 LiFePO4(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
Trang 5enhancing 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
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
Trang 6Studies 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 capacityfor 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 LiFePO 4 , LiFe 1-x M x PO 4 , LiFe 1-x M x PO 4 /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
Trang 7for 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
Samples LiOH.H2O
(g)
FeSO4.7H2O (g)
C6H8O6 (g)
H3PO4(g)
C2H4(OH)2 (ml)
H2O (ml)
Tỉ lệ Li: Fe: P
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
(g)
FeSO4.7H2O (g)
M(NO3)n.6H2O (g)
C6H8O6 (g)
H3PO4(g) Khối lượng mol 41.96 278.01 - 176.12 97.99
Ni2+
STN1 1.2588 2.6970 0.0872 0.22015 1.1596 STN2 1.2588 2.6411 0.1454 0.22015 1.1596 STN3 1.2588 2.5029 0.2908 0.22015 1.1596
Ni(NO 3 ) 2 6H 2 O = 290,8; Mn(NO 3 ) 2 6H 2 O = 287,04; Y(NO 3 ) 3 6H 2 O = 383
Bảng 2.5 The mass/volume of precursor using for synthesis of LiFe1-x M x PO 4/Gr
(g)
FeSO4.7H2O (g)
M(NO3)n.6H2O (g)
C6H8O6 (g)
H3PO4(g)
Graphene (g)
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
Trang 82.2 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
oC, 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 (<10 nm) including Li metal, especially, determination of the percentage
of Fe2+ into structural materials was studied at AXIS-NOVA (XPS) X-ray photoelectric spectrometer (Kratos)
at DGIST University (Korea) with AlK filter (hυ = 1486.6 eV, 150 W, 2.6 × 10-9 Torr)
Trang 9The 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 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
Trang 10The 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
oC, this is a stable structure of material with less
none of negligible weight was lost Thereby
indicated the small amount of carbon free
non-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
oC and crystallization content accounted for 85% of
suitable reference [32, 109] (Fig.3.2)
Figure3.2: TG plot ST01 (1) and ST01 (2)
Trang 11
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
Trang 12Figure 3.4 XPS spectra of LFP Figure 3.5 The XPS of Fe2+ 2p 3/2 and Fe 3+ 2p 1/2 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 o C (a); 400 o C (b); 550 o C (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 LiFePO4and 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)
Trang 13Figure 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 ⁄ 𝑣1 2 ⁄ 𝐴𝐷1 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(cm 2 ), the potential scan rate 𝑣 (V s -1
Trang 14Figure 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
Figure 3.9 Nyquist plots of EIS of LFP
Successfully synthesized LFP by solvothermal method (EG/H 2 O = 4:1), ascorbic acid takes important
role an oxidizing preventive agency as well as reducing agent Fe 3+ to Fe 2+ and also changes different ratio
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/H 2 O 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
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
condition with 10-5 S/cm [52] To increase these
properties may add carbon as a nano film
covering material [54] published with LFP
(5.9.10-9 S.cm-1) and LFP/C (8.3.10-2 S.cm-1)