Exploring the Peculiarities of LiFePO4 Hydrothermal Synthesis Using

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Exploring the peculiarities of LiFePO 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 Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01366 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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Figure 1. Heat absorption curves (100-170oC region) for the formation of LiFePO4 under hydrothermal conditions for the LFP10 (a), LFP20 (b), LFP35 (c) and LFP65 (d) samples 84x166mm (300 x 300 DPI)

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Figure 2. PXRD data for the LFP10-LFP65 samples. The peaks from the LiFePO4 ICDD PDF card (#40-1499) are shown at the bottom 84x58mm (300 x 300 DPI)


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Figure 3. SEM images for the samples LFP10 (a), LFP 20 (b), LFP35 (c) and LFP65 (d). 84x57mm (300 x 300 DPI)



Figure 4. Charge and discharge galvanostatic curves collected at C/5 rate for the LFP10-LFP65 samples. 84x56mm (300 x 300 DPI)


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Exploring the peculiarities of LiFePO4 hydrothermal synthesis using in situ Calvet calorimetry Felix Yu. Sharikov1, Oleg A. Drozhzhin2,3*, Vasiliy D. Sumanov2, Andrey N. Baranov2, Artem M. Abakumov3, Evgeny V. Antipov2. 1

Saint Petersburg Mining University, 199106, St. Petersburg, Russia

2

Department of Chemistry, Moscow State University, 119991, Moscow, Russia

3

Skolkovo Institute of Science and Technology, 143026, Moscow, Russia

*e-mail: [email protected]

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 Li-ion cell.


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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 them1. 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 is used for preparing LFP-based cathode materials2. However, since Whittingham and co-workers proposed and developed a hydrothermal synthesis method of LiFePO4 nanoparticles3-5, this direction became dominant. Hydrothermal route is a low-temperature, low-cost and easy-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 formation6,7, 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 synthesis8-11. In the case of LiFePO4, in situ timeresolved 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 reagents8,9. However, this technique can only be applied to crystalline 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


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the specific phases presented in the reaction system. Calvet calorimetry is proved to be one of the most suitable in situ methods, granting the real-time information about the processes inside the hydrothermal reactor – for their heat generation or absorption12. 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 microscopy13. To 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, LiOH*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). The simple and well-known synthetic procedure was applied4: LiOH solution was added dropwise to a mixture of iron sulphate, phosphoric acid and ascorbic acid under mechanical stirring. The suspension was precipitated directly in a calorimetric reaction vessel. The vessel was blown with N2 gas, sealed and heated from room temperature to 190oC in 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


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package (ver. 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°С·min-1 was applied.

2.3. Phase composition and particle morphology The phase composition of the obtained samples was characterized by powder X-ray diffraction (PXRD, Huber Guinier Camera 670), using СuKα1 radiation (λ = 1.5406 Å). The observation of particles 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 600oC 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 Nmethylpyrrolidone and spreading it on an aluminum foil by doctor blade technique. Dried electrodes were rolled, punched to round discs and dried at 110oC for 3 h under dynamic vacuum. Two-electrode coin-type cells were assembled in an Ar-filled glove box (MBraun). Lithium metal was used as the counter electrode, 1M solution of LiPF6 in a 1:1 mixture of EC:DEC was used as the electrolyte. Galvanostatic study was carried out using a Biologic VMP3 potentiostat-galvanostat (EC-Lab software).

3. Results and discussion


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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 1, a-d.

Figure 1. Heat absorption curves (100-170oC region) for the formation of LiFePO4 under hydrothermal conditions for the LFP10 (a), LFP20 (b), LFP35 (c) and LFP65 (d) samples


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The initial concentration of the reagents used for precipitation has a considerable influence on the reaction path of LiFePO4 formation (Fig. 1). In the 0.1M solution a single sharp endothermic peak is observed at ~146 oC. Increasing concentration causes this peak to shift to lower temperatures (~142, 130 and 125 oC in LFP20, LFP35 and LFP65 samples, respectively). At the same time, a second high temperature stage develops, which manifests itself by a weak endothermic peak at ~148 oC in LFP20 and broad peaks with the center of mass at ~145 oC 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 the second stages increases concomitantly with the increasing precursor concentration. The “starting point” of the phase transformation process is vivanite Fe3(PO4)2*8H2O, as it was revealed by ex situ XRD measurements performed in this and previously published works6,7,13,14. Earlier thermogravimetric studies showed that dehydration of the dry vivianite starts at 105oC15. 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: Fe3(PO4)2*8H2O → Fe3(PO4)2 + 8H2O 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 methods7. Its crystal structure is very similar to 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 reaction process using ex situ PXRD. The 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 = 90.94o, V = 298.6


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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’s 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 2Ɵ ≈14.7o, which is absent in the triphilite pattern. We have detected such reflection is some diffraction patterns. Our results indicate that depending on the initial precursor concentration, the formation of the “antisite iron phosphate” stage can be separated or combined with the following step – reaction with Li+ and final LiFePO4 crystalline phase ordering. Moreover, we found that the observed changes in the reaction mechanism and the onset temperatures of the reaction stages cause profound difference in the unit cell parameters of the triphilite phase, particle morphology and electrochemical performance of the obtained LiFePO4 samples. PXRD profiles for all samples after hydrothermal synthesis are presented in Figure 2.

Figure 2. PXRD data for the LFP10-LFP65 samples. The peaks from the LiFePO4 ICDD PDF card (#40-1499) are shown at the bottom


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Triphilite phase was obtained in all experiments. LFP10 and LFP20 samples also contain weak (~5% and 3% intensity) peaks of an admixture which can be defined as Fe3(PO4)2(OH)2. The molar heat effect measured for each concentration and unit cell parameters of LiFePO4 phase (s.g. Pnma) are presented in Table 1.

Table 1. Measured heat effect and unit cell parameters of the triphilite phase in the LFP10LFP65 samples. C (FeSO4), М

Heat effect, kJ/mol, ±1

LiFePO4 unit cell parameters and volume, Å and Å3

LFP10

0.10

41

a = 10.358(4), b = 6.000(3), c = 4.716(4), V = 293.1(4)

LFP20

0.20

36

a = 10.341(4), b = 5.999(3), c = 4.707(3), V = 292.0(4)

LFP35

0.35

33

a = 10.324(2), b = 5.987(2), c = 4.701(1), V = 290.5(2)

LFP65

0.65

21

a = 10.317(3), b = 5.976(2), c = 4.696(2), V = 289.5(2)

As it follows from Table 1, increasing concentration of the initial reagents leads to decrease in absorbed energy and reduction of the unit cell parameters of the final triphilite phase. First effect can be attributed to the different phase transformation paths and the duration of corresponding reaction stages. Besides this, in the case of concentrated solutions the composition of the initial vivianite intermediate phase may deviate from stoichiometric phosphate/water ratio thus reducing the dehydration energy. Variation of the unit cell parameters can indicate different amount of intrinsic defects in the LiFePO4 phase. The most common type of disorder in LiFePO4 are antisite point defects, where Li and Fe partially exchange at the M1 4a and M2 4c positions, which ideally should have been occupied by pure Li and pure Fe, respectively. Increasing degree of antisite disorder enlarges the unit cell parameters4,5,7. In our samples the unit cell parameters generally increase with


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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 (

Exploring the Peculiarities of LiFePO4 Hydrothermal Synthesis Using

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