Plate-Like LiFePO₄ Nanoparticles: Synthesis, Structure, Electrochemistry

Гидротермальным синтезом получены пластинчатые частицы литий-железного фосфата размерами 100–150 нм и толщиной до 10 нм. Целью было исследование влияния относительного содержания этиленгликоля и температуры реакционной среды на фазовый состав полученных материалов, их кристаллическую и магнитную мик...

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Published in:	Наносистеми, наноматеріали, нанотехнології
Date:	2017
Main Authors:	Kotsyubynsky, V.O., Ostafiychuk, B.K., Lisovsky, R.P., Moklyak, V.V., Hrubiak, A.B., Hryhoruk, I.I., Al-Saedi Abdul Halek Zamil
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Language:	English
Published:	Інститут металофізики ім. Г.В. Курдюмова НАН України 2017
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Cite this:	Plate-Like LiFePO₄ Nanoparticles: Synthesis, Structure, Electrochemistry / V.O. Kotsyubynsky, B.K. Ostafiychuk, R.P. Lisovsky, V.V. Moklyak, A.B. Hrubiak, I.I. Hryhoruk, Al-Saedi Abdul Halek Zamil // Наносистеми, наноматеріали, нанотехнології: Зб. наук. пр. — К.: РВВ ІМФ, 2017. — Т. 15, № 4. — С. 675-686. — Бібліогр.: 24 назв. — англ.

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spelling	Kotsyubynsky, V.O. Ostafiychuk, B.K. Lisovsky, R.P. Moklyak, V.V. Hrubiak, A.B. Hryhoruk, I.I. Al-Saedi Abdul Halek Zamil 2018-07-13T16:14:52Z 2018-07-13T16:14:52Z 2017 Plate-Like LiFePO₄ Nanoparticles: Synthesis, Structure, Electrochemistry / V.O. Kotsyubynsky, B.K. Ostafiychuk, R.P. Lisovsky, V.V. Moklyak, A.B. Hrubiak, I.I. Hryhoruk, Al-Saedi Abdul Halek Zamil // Наносистеми, наноматеріали, нанотехнології: Зб. наук. пр. — К.: РВВ ІМФ, 2017. — Т. 15, № 4. — С. 675-686. — Бібліогр.: 24 назв. — англ. 1816-5230 PACS: 61.05.cp, 61.05.Qr, 68.37.Lp, 81.07.-b, 81.16.-c, 82.45.Yz, 82.47.Aa https://nasplib.isofts.kiev.ua/handle/123456789/140669 Гидротермальным синтезом получены пластинчатые частицы литий-железного фосфата размерами 100–150 нм и толщиной до 10 нм. Целью было исследование влияния относительного содержания этиленгликоля и температуры реакционной среды на фазовый состав полученных материалов, их кристаллическую и магнитную микроструктуры, состояние поверхности и электрические свойства. Установлено, что имеется корреляция между морфологией материалов и их электрохимическими свойствами. Уменьшение размера частиц и степени агломерации приводит к увеличению удельной ёмкости литиевых источников энергии с катодами на основе синтезированных материалов. Гідротермічною синтезою одержано платівчасті частинки літій-залізного фосфату розмірами у 100–150 нм і товщиною до 10 нм. Метою було дослідження впливу відносного вмісту етиленгліколю та температури реакційного середовища на фазовий склад одержаних матеріялів, їхні кристалічну та магнетну мікроструктури, стан поверхні й електричні властивості. Визначено, що є кореляція між морфологією матеріялів та їхніми електрохемічними властивостями. Зменшення розміру частинок і ступеня аґломерації приводить до підвищення питомої місткости літійових джерел живлення з катодами на основі синтезованих матеріялів. Lithium iron phosphate plate-like particles of 100–150 nm sizes and to 10 nm thickness have been obtained by hydrothermal synthesis. It has been aim to investigate influence of ethylene glycol relative content and reaction medium temperature on the obtained-materials’ phase composition, crystalline and magnetic microstructure, surface condition and electrical properties. As determined, there is correlation between the materials’ morphology and their electrochemical properties. The reducing of a particle size and agglomeration degree leads to specific capacity growing for lithium power sources with cathodes based on synthesized materials. The publication contains the results of studies conducted under the President’s of Ukraine grant for competitive projects of the State Fund for Fundamental Research. en Інститут металофізики ім. Г.В. Курдюмова НАН України Наносистеми, наноматеріали, нанотехнології Plate-Like LiFePO₄ Nanoparticles: Synthesis, Structure, Electrochemistry Article published earlier
institution	Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection	DSpace DC
title	Plate-Like LiFePO₄ Nanoparticles: Synthesis, Structure, Electrochemistry
spellingShingle	Plate-Like LiFePO₄ Nanoparticles: Synthesis, Structure, Electrochemistry Kotsyubynsky, V.O. Ostafiychuk, B.K. Lisovsky, R.P. Moklyak, V.V. Hrubiak, A.B. Hryhoruk, I.I. Al-Saedi Abdul Halek Zamil
title_short	Plate-Like LiFePO₄ Nanoparticles: Synthesis, Structure, Electrochemistry
title_full	Plate-Like LiFePO₄ Nanoparticles: Synthesis, Structure, Electrochemistry
title_fullStr	Plate-Like LiFePO₄ Nanoparticles: Synthesis, Structure, Electrochemistry
title_full_unstemmed	Plate-Like LiFePO₄ Nanoparticles: Synthesis, Structure, Electrochemistry
title_sort	plate-like lifepo₄ nanoparticles: synthesis, structure, electrochemistry
author	Kotsyubynsky, V.O. Ostafiychuk, B.K. Lisovsky, R.P. Moklyak, V.V. Hrubiak, A.B. Hryhoruk, I.I. Al-Saedi Abdul Halek Zamil
author_facet	Kotsyubynsky, V.O. Ostafiychuk, B.K. Lisovsky, R.P. Moklyak, V.V. Hrubiak, A.B. Hryhoruk, I.I. Al-Saedi Abdul Halek Zamil
publishDate	2017
language	English
container_title	Наносистеми, наноматеріали, нанотехнології
publisher	Інститут металофізики ім. Г.В. Курдюмова НАН України
format	Article
description	Гидротермальным синтезом получены пластинчатые частицы литий-железного фосфата размерами 100–150 нм и толщиной до 10 нм. Целью было исследование влияния относительного содержания этиленгликоля и температуры реакционной среды на фазовый состав полученных материалов, их кристаллическую и магнитную микроструктуры, состояние поверхности и электрические свойства. Установлено, что имеется корреляция между морфологией материалов и их электрохимическими свойствами. Уменьшение размера частиц и степени агломерации приводит к увеличению удельной ёмкости литиевых источников энергии с катодами на основе синтезированных материалов. Гідротермічною синтезою одержано платівчасті частинки літій-залізного фосфату розмірами у 100–150 нм і товщиною до 10 нм. Метою було дослідження впливу відносного вмісту етиленгліколю та температури реакційного середовища на фазовий склад одержаних матеріялів, їхні кристалічну та магнетну мікроструктури, стан поверхні й електричні властивості. Визначено, що є кореляція між морфологією матеріялів та їхніми електрохемічними властивостями. Зменшення розміру частинок і ступеня аґломерації приводить до підвищення питомої місткости літійових джерел живлення з катодами на основі синтезованих матеріялів. Lithium iron phosphate plate-like particles of 100–150 nm sizes and to 10 nm thickness have been obtained by hydrothermal synthesis. It has been aim to investigate influence of ethylene glycol relative content and reaction medium temperature on the obtained-materials’ phase composition, crystalline and magnetic microstructure, surface condition and electrical properties. As determined, there is correlation between the materials’ morphology and their electrochemical properties. The reducing of a particle size and agglomeration degree leads to specific capacity growing for lithium power sources with cathodes based on synthesized materials. The publication contains the results of studies conducted under the President’s of Ukraine grant for competitive projects of the State Fund for Fundamental Research.
issn	1816-5230
url	https://nasplib.isofts.kiev.ua/handle/123456789/140669
citation_txt	Plate-Like LiFePO₄ Nanoparticles: Synthesis, Structure, Electrochemistry / V.O. Kotsyubynsky, B.K. Ostafiychuk, R.P. Lisovsky, V.V. Moklyak, A.B. Hrubiak, I.I. Hryhoruk, Al-Saedi Abdul Halek Zamil // Наносистеми, наноматеріали, нанотехнології: Зб. наук. пр. — К.: РВВ ІМФ, 2017. — Т. 15, № 4. — С. 675-686. — Бібліогр.: 24 назв. — англ.
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first_indexed	2025-11-26T20:25:48Z
last_indexed	2025-11-26T20:25:48Z
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fulltext	675 PACS numbers: 61.05.cp, 61.05.Qr, 68.37.Lp, 81.07.-b, 81.16.-c, 82.45.Yz, 82.47.Aa Plate-Like LiFePO4 Nanoparticles: Synthesis, Structure, Electrochemistry V. O. Kotsyubynsky1, B. K. Ostafiychuk2, R. P. Lisovsky1, V. V. Moklyak2, A. B. Hrubiak1, I. I. Hryhoruk1, and Al-Saedi Abdul Halek Zamil3 1Vasyl Stefanyk Precarpathian National University, 57 Shevchenko Str., 76018 Ivano-Frankivsk, Ukraine 2G. V. Kurdyumov Institute for Metal Physics, N.A.S. of Ukraine, 36 Academician Vernadsky Blvd., UA-03142 Kyiv, Ukraine 3South Ukrainian National Pedagogical University Named After K. D. Ushynsky, 26 Staroportofrankivs’ka Str., 65020 Odesa, Ukraine Lithium iron phosphate plate-like particles of 100–150 nm sizes and to 10 nm thickness have been obtained by hydrothermal synthesis. It has been aim to investigate influence of ethylene glycol relative content and reac- tion medium temperature on the obtained-materials’ phase composition, crystalline and magnetic microstructure, surface condition and electrical properties. As determined, there is correlation between the materials’ morphology and their electrochemical properties. The reducing of a parti- cle size and agglomeration degree leads to specific capacity growing for lithium power sources with cathodes based on synthesized materials. Гідротермічною синтезою одержано платівчасті частинки літій- залізного фосфату розмірами у 100–150 нм і товщиною до 10 нм. Ме- тою було дослідження впливу відносного вмісту етиленгліколю та тем- ператури реакційного середовища на фазовий склад одержаних матері- ялів, їхні кристалічну та магнетну мікроструктури, стан поверхні й електричні властивості. Визначено, що є кореляція між морфологією матеріялів та їхніми електрохемічними властивостями. Зменшення ро- зміру частинок і ступеня аґломерації приводить до підвищення питомої місткости літійових джерел живлення з катодами на основі синтезова- них матеріялів. Гидротермальным синтезом получены пластинчатые частицы литий- железного фосфата размерами 100–150 нм и толщиной до 10 нм. Це- Наносистеми, наноматеріали, нанотехнології Nanosistemi, Nanomateriali, Nanotehnologii 2017, т. 15, № 4, сс. 675–686  2017 ІÌÔ (Іíñòèòóò ìåòàëîôіçèêè іì. Ã. Â. Êóðäþìîâà ÍÀÍ Óêðàїíи) Надруковано в Óкраїні. Ôотокопіювання дозволено тільки відповідно до ліцензії http://en.wikipedia.org/wiki/G._V._Kurdyumov_Institute_for_Metal_Physics_of_the_National_Academy_of_Sciences_of_Ukraine 676 V. O. KOTSYUBYNSKY, B. K. OSTAFIYCHUK, R. P. LISOVSKY et al. лью было исследование влияния относительного содержания эти- ленгликоля и температуры реакционной среды на фазовый состав по- лученных материалов, их кристаллическую и магнитную микрострук- туры, состояние поверхности и электрические свойства. Óстановлено, что имеется корреляция между морфологией материалов и их электро- химическими свойствами. Óменьшение размера частиц и степени агло- мерации приводит к увеличению удельной ёмкости литиевых источни- ков энергии с катодами на основе синтезированных материалов. Key words: lithium iron phosphate, nanoparticles, morphology, conductiv- ity, cathode, lithium power sources. Ключові слова: літій-залізний фосфат, наночастинки, морфологія, про- відність, катода, літійові джерела живлення. Ключевые слова: литий-железный фосфат, наночастицы, морфология, проводимость, катод, литиевые источники питания. (Received 10 April, 2017) 1. INTRODUCTION Specific energy density of lithium iron phosphate (LiFePO4) is com- pared to best commercial compounds LiCoO2, LiNiO2, LiMn2O4. At the same time, the material is characterized by a slower rate of ca- pacity loss, high reversible capacity of about 170 Ah/kg at an open-circuit voltage of 3.45 V versus Li/Li  (Fe2/Fe3 redox couple) with long cycle ability (typically more than 2.000 charge–discharge cycles at 1C current rate with approximately 80% capacity saving) [1]. The other advantages are high charge–discharge current rates up to 10C, good safety owing to high thermal stability, flat charge/discharge curves (reversible LiFePO4/FePO4-phases’ trans- formation), low cost of production and utilization. The key point is a high performance of cathode material but low electronic conduc- tivity and one-dimensional diffusion of lithium ions require small particle sizes and shape control during the synthesis. The main im- perfection of LiFePO4 is low conductivity (about 10 9 S/cm in com- parison with 10 4 S/cm for LiCoO2 and 10 6 S/cm for LiCoO2), which complicates a theoretical capacity obtainment at high discharge cur- rent rates [2, 3]. Another problem is a one-dimensional slow diffu- sion of Li+ ions (10 13–10 14 cm2/s for LiFePO4 and about 10 16 cm2/s for FePO4). LiFePO4 particles’ carbon coating or non-stoichiometric material synthesis is usually used for conductivity increasing [2, 4– 6]. Another approach consists of particles sizes reduction for the lengths of ionic and electronic transport reducing [7]. Alongside with it, high specific surface area and low coordinated https://en.wikipedia.org/wiki/Open-circuit_voltage LiFePO4 NANOPARTICLES: SYNTHESIS, STRUCTURE, ELECTROCHEMISTRY 677 surface atoms may cause particles agglomeration and surface reac- tions. As a result, the properties of the LiFePO4 crucially depend on the synthesis route and the modification of it allows a predictable change of material morphological and electrical characteristics. There is a wide range of different approaches to LiFePO4 obtaining: solid-state methods, sol–gel route, microemulsion method [8–10]. A special place is taken by hydrothermal synthesis that allows to control additionally a reaction medium during the process and to influence actively the particles morphology [11]. The objective of this research is to improve the plate-like LiFePO4 nanoparticles obtaining method by hydrothermal synthesis and to find regularities between the materials’ electrical and morphological properties and their electrochemical performance. 2. MATERIALS AND METHODS Nanoparticles of LiFePO4 were prepared according to the following procedure [12]: 1 mole/L H3PO4 was mixed with ethylene glycol at a different ratio with the next 1 mole/L LiOH aqueous solution drop- wise addition under mechanical stirring. At the last stage, 3 mole/L FeSO47H2O aqueous solution was added to a white suspension formed after the neutralization reaction. Green coloured resulting suspension was sealed into a 0.5 L magnetic mixed autoclave. The autoclave was heated to a temperature of 200–240C at heating rate of 3–4C/min with the exposure for 1–5 hours under stirring. Thereafter, the autoclave was cooled to room temperature. The col- loidal suspension was collected via centrifuge and washed with dis- tilled water up to neutral pH. The obtained materials were dried under vacuum at 70–80C for 8 h. The two systems (S1 and S2) of materials were synthesized at those conditions. S1 series samples were differed by their relative content of ethylene glycol in the initial mixture and were obtained at 240C (Table). Samples series S2 were obtained at 67 vol.% eth- ylene glycol relative content after exposure at 2 different tempera- tures, 200 and 220C. For carbon coating formation, the S1-3 sam- ple were mixed with 17 wt.% of glucose and then sintered at 400C TABLE. S1 samples synthesis conditions. Sample Ethylene glycol relative content, vol. % pH of initial mixture S1-1 40 3.9 S1-2 57 4.4 S1-3 67 4.6 S1-4 77 5.0 678 V. O. KOTSYUBYNSKY, B. K. OSTAFIYCHUK, R. P. LISOVSKY et al. for 1 h under argon atmosphere with a heating rate of 5–6C/min. Diffraction patterns were obtained with diffractometer DRON-4- 07 (CuK radiation). Bragg–Brentano geometry type and a NiK- filter were applied. A quantitative analysis was done with a full pattern Rietveld refinement procedure using FullFrof Suite Pro- gram [12 XRD measurements were collected in a 2 range of 16– 65. High-resolution transmission electron microscopy (HRTEM) im- ages were obtained by a microscope FEI Tecnai Orisis TEM/STEM 80–200 at a 200 kV. The samples were ultrasonically mixed in iso- propyl alcohol and deposited on silica substrates. The Mössbauer spectra were measured with a MS-1104Еm spec- trometer using a 57Co -ray source and calibrated at room tempera- ture with -Fe as a standard (linewidth 0.29 mm/s). The isomer shifts () are relative to Fe metal. The model fitting was performed using Mosswin 3.0 software. The Brunauer–Emmett–Teller (BET) specific surface areas of the samples were measured by Nitrogen adsorption/desorption methods at 77 K with a Quantachrome NOVA 2200e analyser. Electrochemical impedance spectroscopy with Autolab PGSTAT 12 galvanostat/potentiostat (conventional four-electrode configura- tion) was used to explore the conductivity of the obtained samples over the frequency range 0.01 Hz–100 kHz. Electrochemical lithium intercalation/deintercalation tests were performed in Swagelok-type cells assembled in an argon-filled dry glow box. The negative electrode was a lithium metal foil. The elec- trolyte was 1 mole/L LiBF4 dissolved in -butyrolactone. Cathode mixture consisted of the obtained materials, black carbon and PVDF (mass ratio 85:10:5) in acetone was prepared in the paste form. The positive electrode was Ni grid coated by cathode mixture. The cell was charged and discharged at a rate of C/10. All electrochemical tests were carried out at room temperature. 3. RESULTS AND DISCUSSION According to XRD data (Fig. 1), S1-2 and S1-3 materials were a pure LiFePO4 [14]. A presence of Fe2P2O7 (more than 55 wt.%) impurity phase for S1-4 sample has been observed. The most interesting is a S1-1 sam- ple. The intensive peak at 217.37 on the XRD pattern for this sample was originally identified as a yield of LiFeP2O7 phase pres- ence. However, in LiFeP2O7 compound, the iron is in a Fe3+ state. As a result, it leads to the predicted differences between the contri- butions of LiFeP2O7 and LiFePO4 phases to the Mössbauer spectra integral intensity (Fig. 2). A doublet component presence with LiFePO4 NANOPARTICLES: SYNTHESIS, STRUCTURE, ELECTROCHEMISTRY 679 quadrupole splitting () in particular is less than 1 mm/s and could be considered as an evidence of LiFeP2O7 phase. At the same time, Mössbauer spectra of S1-1 sample consist of two doublet compo- nents. A dominating doublet component with integral intensity relative content of about 92% corresponds to LiFePO4 (2.93 mm/s, 1.21 mm/s). The obtained parameters of Mössbauer spectra are very close to the literature data for LiFePO4 [15]. The integral rela- tive intensity of the second doublet component (0.54 mm/s, Fig. 1. XRD patterns of S1 system samples. Fig. 2. Mössbauer spectra of S1 system samples. 680 V. O. KOTSYUBYNSKY, B. K. OSTAFIYCHUK, R. P. LISOVSKY et al. 0.42 mm/s) does not exceed 8%; so, the assumption about LiFeP2O7 phase availability has been neglected. Another working hypothesis is a sharp anisotropy of LiFePO4 particles’ shape that causes redistribution of diffraction peaks in- tensity. In the case of plate-like particles, the peak at 217.37 will correspond to (200) reflex of the LiFePO4 structure (result of XRD pattern modelling with the use of PowderCell software and Rietveld–Toraya model) [16]. This assumption is confirmed by TEM investigations (Fig. 3). The S1-1 sample is formed by separated plate-like particles of LiFePO4 with the sizes of 200–300 nm and of thickness to 20–30 nm with crystal orientation along the bc facet. The particles lamel- lar morphology also happens in S1-2 and S1-3 materials, but this form is not dominant, and the majority of particles are prismatic (Fig. 3). It can be assumed that the ethylene glycol molecules after absorption on the LiFePO4 nuclei prevent crystal growth. Similar results (preferred crystal orientation with a (200) texture) were ob- a b c Fig. 3. TEM picture of S1 system samples. Fig. 4. XRD patterns of S2 system samples. LiFePO4 NANOPARTICLES: SYNTHESIS, STRUCTURE, ELECTROCHEMISTRY 681 tained in [17] in a case of LiFePO4 hydrothermal synthesis. One of the biggest problems in LiFePO4 synthesis is to determine the conditions for a pure phase obtaining and to prevent oxidation of Fe2 to Fe3. The presence of Fe3+ is systematically detected by Mössbauer spectroscopy measurements of LiFePO4 nanoparticles at the absence of additional iron-containing phase [15]. One of the rea- sons for this phenomenon is the process of surface iron oxidation in the oxygen-containing medium [18]. Small contents of Fe3 ions in LiFePO4 could be caused by the Li  ions’ substitution for Fe2 sites with the next oxidation of some Fe2 to Fe3 for charge compensa- tion. Additional electron density on the lithium sites was fixed in [19]. The substitution does not cause significant structural changes because the ionic radii of Fe2 and Fe3 are close (92 and 79 pm, re- spectively) and phosphorus-oxygen [PO4] polyhedra are strongly bonded. Formation of Fe3 ions can be stimulated by the aqueous medium too [20]. Mössbauer spectroscopy is a good technique to investigate local electronic structure and coordination of Fe ions. The establishment of Fe3+ ions coordination type from the isomer shift calibration is possible. The following criteria can be used: Fe3 (tetrahedral coor- dinated)0.3–0.4 mm/sFe3 (octahedral coordinated) [15]. This value suggests that a minor doublet component is a result of Fe3+ ions presence in both coordination states. This estimation can be indirect evidence of the Fe3+ ions presence on disordered surface shell of the LiFePO4 particles. Mössbauer spectra of S1-2 and S1-3 materials have the composi- tion similar to S1-1 with the relative content of Fe3 ion of 11 and 13%, respectively. From this, it follows that LiFePO4 yield decreas- es with the increasing pH of initial precursor mixture. The result obtained is contrary to the data about an increase of the lithium iron phosphate yield with pH growth linked to LiFePO4 solubility enlarging in acidic condition under excessive pressure and at high temperature [17]. With XRD patterns of S2 system samples obtained at hydrother- mal treatment temperature of 200 and 220C at the same ethylene glycol contents (marked S2-1 and S2-2), no additional phases except LiFePO4 have been fixed in either case (Fig. 4). The average size of coherent scattering regions some decrease (from 16 to 14 nm) with the synthesis temperature elevation is possible. This result corre- lates to the adsorption porosimetry data: the specific surface areas of the S2-1 and S2-2 materials are 17 and 13 m2/g, respectively. The relative content of Fe3+ ions for S2-1 and S2-2 materials is unexpectedly high, i.e., 18 and 21%, respectively (Fig. 5). The iso- mer shift and quadruple splitting of minor doublet component for S2-1 and S2-2 are very close to the characteristic parameters of S1 682 V. O. KOTSYUBYNSKY, B. K. OSTAFIYCHUK, R. P. LISOVSKY et al. system samples. At the same time, some morphological differences have been found. For S2-1 sample, agglomerates with the sizes of 0.3–1.0 m consisting of ordered separate prismatic particles of 100–150 nm are typical. For S2-2 sample, such ordering has not been observed, and agglomerates with the close sizes are formed by primary particles with different sizes and shapes (Fig. 6). The increasing of Fe3 ions to 25% was observed after carbon coating procedure for S1-3 sample. Mössbauer spectra parameters for the minor doublet component were as follow: 0.84 mm/s and 0.43 mm/s (Fig. 7). Those parameters were close to Fe4P6O21 fer- ric pyrophosphate characteristics (0.80 mm/s and 0.42 mm/s), but that compound formation is hardly probable [21]. The frequency dependences of the complex conductivity for S1-3 and S1-3/carbon samples were obtained by the impedance spectros- copy method (Fig. 8). The curves for both samples can be divided into two regions: the linear frequency independent part and the sec- ond one where conductivity increases with frequency enlarging. The obtained dispersion curves were approximated by Jonscher’s power law [22]: ( ) 1 , s dc h               (1) where dc is the frequency independent part of conductivity, h is the hopping frequency of the charge carriers, and s is an exponent parameter characterizing the deviation of the system from the De- bye-type state (of s1). The parameter s is a measure of the interi- onic–environmental coupling strength and, for most cases, is in the range of 0s 1. Alongside with it, there is no physical reasons Fig. 5. Mössbauer spectra of S2 system samples. LiFePO4 NANOPARTICLES: SYNTHESIS, STRUCTURE, ELECTROCHEMISTRY 683 inability for the parameter s to take on values above 1 [23]. Jonscher’s law is performed for a wide range of the material types—from disordered semiconductors to conducting polymers and ion glasses. This qualitative characteristic of the universal response is relevant to material morphology and spatial structure of the con- duction network. In our case, the parameter s was 1.340.02 and 1.370.02 for S1-3 and S1-3/carbon materials, respectively. A model of electric-charge transfer in the disordered matter based on the distribution of the length of accessible conduction paths with power exponent s1 larger was proposed in [24]. For carbon-containing composite, dc-conductivity dc have been in- creased to (6.10.8)10 7 Smm 1 from (1.90.6)10 6 Smm 1 for initial S1-3 material. Average hopping frequency h of charge carri- ers was increased from (3.70.4)103 to (5.11.8)103 Hz. In possi- ble qualitative model, composite material is that of a grid of ran- dom oriented chains of various lengths that consists of LiFePO4 particles interconnected by carbon bridges with conformational dis- a b Fig. 6. TEM picture of S2 system samples. Fig. 7. Mössbauer spectra of S1-3/carbon sample. 684 V. O. KOTSYUBYNSKY, B. K. OSTAFIYCHUK, R. P. LISOVSKY et al. order. Electronic transport in olivine LiFePO4 is caused by polaron hopping. Thus, in carbon-containing composite, trapping was real- ized both along chain (intrachain transfer) and over cross-connected chains at the increasing of hops probability with grid density en- larging. Obtained discharge voltage profiles of the LiFePO4 cathode mate- rial appear to be a typical voltage plateau (at about 3.4 V vs. Li0/Li+) attributed to the coexistence of two isostructural similar phases—LiFePO4 and FePO4 (Fig. 9). The carbon-containing material exhibited a maximal capacity of about 140 mAh/g in the voltage range of 2.2–4.0 V. In comparison, the initial S1-3 material under the same conditions demonstrates a lower specific capacity of about 120 mAh/g. The plateau voltages for S1-3 and S1-3 /carbon are Fig. 8. Frequency dependency of S1-3 and S1-3/carbon samples electrical conductivity (solid lines represent fitting results). Fig. 9. Discharge voltage profiles using the LiFePO4/LiBF4–- butyrolactone/Li cell at room temperature for various cathode materials: (a) S1-3/carbon, (b) S1-3, and (c) S2-1. LiFePO4 NANOPARTICLES: SYNTHESIS, STRUCTURE, ELECTROCHEMISTRY 685 very close, but the width of the plateau for composite material is larger. For S2-1 material, a smaller plateau is characterized by some slope that is probably connected to diffuse distances increas- ing during Li intercalation with the particles (agglomerates) sizes growing. Indistinctive long ‘tails’ was observed on the voltage– capacity curves during the last discharge step. For all cases, we connect it with the relatively high inner resistance of the cathodes. 4. CONCLUSIONS Nanosize LiFePO4 particles have been prepared by hydrothermal route using ethylene glycol as morphology predicted surfactant. The plate-like LiFePO4 particles with a minimal Fe3 ions content have been obtained with 40 vol.% ethylene glycol relative content at 240C. Fe2P2O7 impurity phase formation at 77 vol.% ethylene gly- col relative content is fixed. As determined, the changes of the re- action medium temperature in a range of 200–240C have had no impact on a phase state of the material and on the average size of the primary prismatic particles (100–150 nm). The ordering charac- ter of the agglomerates’ formation is observed at the temperature of 200C. The frequencies dependences of conductivity for obtained materials and LiFePO4/carbon composite have been analysed with the using of electric-charge transfer in disordered matter formal- ism. 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Plate-Like LiFePO₄ Nanoparticles: Synthesis, Structure, Electrochemistry

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