Article pubs.acs.org/crystal
Cite This: Cryst. Growth Des. 2018, 18, 879−882
Exploring the Peculiarities of LiFePO4 Hydrothermal Synthesis Using In Situ Calvet Calorimetry Felix Yu. Sharikov,† Oleg A. Drozhzhin,*,‡,§ Vasiliy D. Sumanov,‡ Andrey N. Baranov,‡ Artem M. Abakumov,§ and Evgeny V. Antipov‡ †
Saint Petersburg Mining University, 199106, St. Petersburg, Russia Department of Chemistry, Moscow State University, 119991, Moscow, Russia § Skolkovo Institute of Science and Technology, 143026, Moscow, Russia ‡
ABSTRACT: Peculiarities of the hydrothermal synthesis of the lithium iron phosphate cathode material are studied using in situ Calvet calorimetry. Staging and temperature intervals of the phase formation process are determined as a function of the concentration of the initial reagents. Obtained results revealed a clear correlation between observed heat absorption behavior and lattice parameters, morphology, and electrochemical performance of the obtained LiFePO4 materials. Lowering temperature of the precursor dehydration leads to better Li/ Fe ordering, smaller particle size of LiFePO4 samples, and the highest charge−discharge capacity measured in the Li-ion cell.
1. INTRODUCTION Phosphate-based cathode materials for Li-ion batteries (LIB) belong to a prospective branch of development; LiFePO4 with the olivine structure is the most studied among them.1 In spite of almost 10 years of successful commercialization, LiFePO4 is still an object of a great number of comprehensive studies, often playing the role of a “model” polyanionic compound. A large variety of synthesis methods are used for preparing LFP-based cathode materials.2 However, since Whittingham and co-workers proposed and developed a hydrothermal synthesis method of LiFePO4 nanoparticles,3−5 this direction has become dominant. Hydrothermal route is a low-temperature, low-cost, and easily scalable method. It allows taking control over the phase composition, particle size, and morphology of the samples by varying a large number of experimental parameters, such as temperature, pH, concentration of the initial solutions, presence of organic solvents, etc. Among the reports on the hydrothermal synthesis of LFP only a small fraction is devoted to the reaction mechanism studies. Ex situ experiments have been conducted to evaluate the mechanisms of the phase formation,6,7 but there is no assurance in the one-to-one correspondence of the obtained results to the real system behavior. In situ techniques are obviously more powerful to provide better understanding of the phase evolution during synthesis.8−11 In the case of LiFePO4, in situ time-resolved synchrotron X-ray powder diffraction during hydrothermal synthesis revealed such intermediate phases as Fe3(PO4)2·8H2O (vivianite) and FeC2O4·C2H6O2 for different sets of initial reagents.8,9 However, this technique can only be applied to crystalline © 2017 American Chemical Society
phases, whereas amorphous precursors or intermediates could not be detected. Calorimetric techniques in turn provide another type of experimental data which determine heat effects and temperature intervals of the phase transformations but cannot identify the specific phases presented in the reaction system. Calvet calorimetry is proven to be one of the most suitable in situ methods, granting real-time information about the processes inside the hydrothermal reactorfor their heat generation or absorption.12 Calvet calorimetry was applied to hydrothermal preparations of various nanodispersed oxide materials and the obtained data on heat generation were successfully combined with ex situ diffraction and scanning electron microscopy.13 To the best of our knowledge, this work is the first example of applying in situ Calvet calorimetry for studying the phase formation in hydrothermal synthesis of the LiFePO4 cathode material.
2. MATERIAL AND METHODS 2.1. Synthesis. H3PO4, LiO·H2O, FeSO4·7H2O, and ascorbic acid (molar ratio 1:3:1:1) were used as raw materials for the synthesis. Four different concentrations of the initial solutions were used for the experiment: 0.1 M (LFP10), 0.2 M (LFP20), 0.35 M (LFP35), and 0.65 M (LFP65). A simple and well-known synthetic procedure was applied:4 LiOH solution was added dropwise to a mixture of iron sulfate, phosphoric acid, and ascorbic acid under mechanical stirring. The suspension was precipitated directly in a calorimetric reaction Received: September 26, 2017 Revised: November 18, 2017 Published: December 15, 2017 879
DOI: 10.1021/acs.cgd.7b01366 Cryst. Growth Des. 2018, 18, 879−882
Crystal Growth & Design
Article
vessel. The vessel was blown with N2 gas, sealed, and heated from room temperature to 190 °C over the course of a calorimetric run. Total time of sintering was ∼3.5 h. 2.2. Heat Flux Calorimetry. Calorimetric measurements were performed using a C80 CS Evolution Calvet calorimeter (SETARAM Instrumentation) equipped with a 3D Tian−Calvet sensors. Calisto program package (v 1.086, AKTS AG) was applied for running the experiment, data collection, and initial processing. Linear heating mode with the heating rate β = 0.75 °C·min−1 was applied. 2.3. Phase Composition and Particle Morphology. The phase composition of the obtained samples was characterized by powder Xray diffraction (PXRD, Huber Guinier Camera 670), using Cu Kα1 radiation (λ = 1.5406 Å). The observation of particle size and morphology was carried out using a FEG scanning electron microscope (Carl Zeiss NVision 40-38-50). 2.4. Electrochemical Testing. Before preparing the electrodes, the samples were annealed with glucose at 600 °C for 5 h under Ar to obtain carbon coating of the particles. Electrode material was prepared by mixing 75 mass % of active compound, 12.5% of carbon black, and 12.5% of PVDF binder in N-methylpyrrolidone and spreading it on an aluminum foil by doctor blade technique. Dried electrodes were rolled, punched to round discs, and dried at 110 °C for 3 h under dynamic vacuum. Two-electrode coin-type cells were assembled in an Ar-filled glovebox (MBraun). Lithium metal was used as the counter electrode; 1 M solution of LiPF6 in a 1:1 mixture of EC:DEC was used as the electrolyte. Galvanostatic study was carried out using a Biologic VMP-3 potentiostat-galvanostat (EC-Lab software).
3. RESULTS AND DISCUSSION Four different concentrations of the initial solutions were used in our study, ranging from 0.1 M (LFP10) to 0.65 M (LFP65). The fragments of the corresponding heat absorption curves are depicted in Figure 1a−d. The initial concentration of reagents used for precipitation has a considerable influence on the reaction path of LiFePO4 formation (Figure 1). In the 0.1 M solution a single sharp endothermic peak is observed at ∼146 °C. Increasing concentration causes this peak to shift to lower temperatures (∼142, 130, and 125 °C in LFP20, LFP35, and LFP65 samples, respectively). At the same time, a second hightemperature stage develops, which manifests itself by a weak endothermic peak at ∼148 °C in LFP20 and broad peaks with the center of mass at ∼145 °C in LFP35 and LFP65. The complex shape of these peaks reflects that this second stage consists of several overlapping processes. The temperature difference between the first and second stages increases concomitantly with the increasing precursor concentration. The “starting point” of the phase transformation process is vivanite Fe3(PO4)2·8H2O, as revealed by ex situ XRD measurements performed in this and previously published works.6,7,13,14 Earlier thermogravimetric studies showed that dehydration of the dry vivianite starts at 105 °C.15 The dependence of the position of the first peak in our heat absorption curves on the water/precipitate ratio indicates that the first stage of the reaction is also dehydration of vivianite:
Figure 1. Heat absorption curves (100−170 °C region) for the formation of LiFePO4 under hydrothermal conditions for the LFP10 (a), LFP20 (b), LFP35 (c), and LFP65 (d) samples.
reaction process using ex situ PXRD. Most of the “intermediate” samples contained vivianite Fe3(PO4)2·8H2O and triphilite LiFePO4 phases in different crystallization degrees. Additional problem is that PXRD pattern of sarcopside (space group P21/c, a = 6.014 Å, b = 4.773 Å, c = 10.405 Å, β = 90.94°, V = 298.6 Å3) is very similar to that of triphilite (space group Pnma, a = 10.332 Å, b = 6.010 Å, c = 4.692 Å, V = 291.3 Å3), and it is rather hard to distinguish between them from PXRD of poorly crystallized products. The main difference between the PXRD patterns of Fe3(PO4)2 and LiFePO4 is the presence of 100 sarcopside peak at 2θ ≈ 14.7°, which is absent in the triphilite pattern. We have detected such reflection is some diffraction patterns.
Fe3(PO4 )2 ·8H 2O → Fe3(PO4 )2 + 8H 2O
Appearance of the intermediate amorphous phosphate with sarcopside structure16 during hydrothermal synthesis of LiFePO4 was shown by Paolella et al. using both experimental and theoretical methods.7 Its crystal structure is very similar to that of olivine, containing Fe2+ in both M1 and M2 sites. The formula may be written as Fe0.5FePO4. We have not observed sarcopside phase in the pure state at intermediate points of the 880
DOI: 10.1021/acs.cgd.7b01366 Cryst. Growth Des. 2018, 18, 879−882
Crystal Growth & Design
Article
positions, which ideally should have been occupied by pure Li and pure Fe, respectively. Increasing degree of anti-site disorder enlarges the unit cell parameters.4,5,7 In our samples the unit cell parameters generally increase with decreasing precursor concentration that might indicate progressing antisite disorder (Table 1). However, this feature previously was observed only for the samples obtained in hydrothermal media at relatively low (
