Effect of the synthesis conditions on the electrochemical properties of LiFePO4 obtained from NH4FePO4

Materials Research Bulletin 48 (2013) 3438–3448

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Effect of the synthesis conditions on the electrochemical properties of LiFePO4 obtained from NH4FePO4 Pier Paolo Prosini *, Paola Gislon, Cinzia Cento, Maria Carewska, Amedeo Masci ENEA, Italian National Agency for New Technologies, Energy and Sustainable Economic Development, Casaccia Research Centre, Via Anguillarese 301, 00123 Santa Maria di Galeria, Rome, Italy

A R T I C L E I N F O

A B S T R A C T

Article history: Received 10 December 2012 Received in revised form 19 April 2013 Accepted 9 May 2013 Available online 18 May 2013

In this paper the morphological, structural and electrochemical properties of crystalline lithium iron phosphate (LiFePO4) obtained from ferrous ammonium phosphate (FAP) have been studied. The FAP was obtained following four different processes, namely: (1) homogeneous phase precipitation, (2) heterogeneous phase precipitation from stoichiometric sodium phosphate, (3) heterogeneous phase precipitation from stoichiometric ammonium phosphate, and (4) heterogeneous phase precipitation from over stoichiometric ammonium phosphate. Lithium iron phosphate was prepared by solid state reaction of FAP with lithium hydroxide. In order to evaluate the effect of reaction time and synthesis temperature the LiFePO4 was prepared varying the heating temperatures (550, 600 and 700 8C) and the reaction times (1 or 2 h). The morphology of the materials was evaluated by scanning electron microscopy while the chemical composition was determined by electron energy loss spectroscopy. X-ray diffraction was used to evaluate phase composition, crystal structure and crystallite size. The so obtained LiFePO4’s were fully electrochemical characterized and a correlation was found between the crystal size and the electrochemical performance. ß 2013 Elsevier Ltd. All rights reserved.

Keywords: A. Inorganic compounds B. Sol–gel chemistry C. Electron microscopy C. Thermogravimetric analysis (TGA) C. Electrochemical measurements

1. Introduction Among the materials suitable to be used as cathodes for lithium-ion batteries, lithium iron phosphate (LiFePO4) is emerging as one of the most promising, especially for large-scale applications. Compared to other electro active materials, LiFePO4 has the prerogative to contain iron, the cost of which is several times lower than that of other metals commonly used in lithiumion technology (such as cobalt, nickel or manganese). Several methods such as solid state, sol–gel, hydrothermal, microwave, spray pyrolysis, precipitation, drying and emulsion were employed for the synthesis of LiFePO4 [1–9]. Depending on the synthesis used for the preparation of the material, LiFePO4 samples with different morphology and grain size have been obtained. It was claimed that both particle size minimization and intimate carbon contact are necessary to optimize the electrochemical performance of the material. In 2001, Yamada et al. [10] proposed to reduce the particle size to overcome the low utilization of LiFePO4. Prosini et al. [11] prepared undoped nano-or sub-micron LiFePO4 with a particle size of 100–150 nm. The material was discharged at 510 A kg 1 with a specific capacity of 140 Ah kg 1. Masquelier et al. [12] found that the specific capacity at 850 A kg 1 discharge

* Corresponding author: Tel.: +39 06 3048 6768; fax: +39 06 3048 3109. E-mail address: [email protected] (P.P. Prosini). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.05.038

rate of un-doped, carbon uncoated lithium iron phosphate with particle size of about 140 nm is 147 Ah kg 1. Recently Chiang, that first proposed the niobium doping, has begun to focus on nanoparticles of lithium iron phosphate [13]. NTT and SONY groups have investigated the effect of the synthesis temperature on the electrochemical performance of LiFePO4 [14,15]. Both found that the reduction of the synthesis temperature decreases the particle size, increasing the specific surface area and enhancing the electrochemical performance. Yamada et al. [14] found a maximum in the specific capacity (160 Ah kg 1) for the sample prepared at 550 8C. The specific surface area was about 10 m2 g 1 with a grain-size ranging from 0.2 to 30 mm. To summarize, the literature results clearly show that both grain-size reduction and intimate contact with the conductive binder are necessary to optimize the electrochemical performance of LiFePO4 based electrodes. In 2006, Li et al. [16] proposed to synthesize the LiFePO4 from ammonium iron phosphates (FAP). That same year, Wang et al. [17] proposed a synthesis of LiFePO4 that, starting from Fe (III) and using the sulphite as a reducing agent, leaded to the formation of FAP, which was then decomposed with lithium acetate to give the final product. Li [18] and Wang [19] have also reported the synthesis of LiFePO4 from FAP via microwave. More recently, Gao et al. [20] reported the synthesis of LiFePO4 using aFe2O3, Fe3O4, FePO4 and NH4FePO4 as precursors. They found that the particle size of the precursors is strictly related to the electrochemical performance of the products. FAP and other

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ammonium metal phosphates are well known compound widely used in industry as pigments for protective finishes of metals and as flame retardants in paints and plastics. They are also used as fertilizers, since they can represent a source of macro and micronutrients (P, N, Mg, Fe, Zn, Mn, Cu, and Co). The NH4FePO4
H2O was proved useful for correcting iron deficiency (iron chlorosis) in plants grown in calcareous soils. The FAP is also used as a dietary supplement as a source of iron and phosphorus. Ammonium metal phosphates of general formula NH4Me(II)PO4
H2O (with Me = Mn, Fe, Co, Ni) were prepared by Bassett and Bedwell [21] by precipitation from an aqueous solution containing about 0.5 M of a metal salt, the sulphate in the case of Fe, the sulphate or chloride in the case of Mn and the chloride in the case of Co and Ni, added to a saturated solution of (NH4)2HPO4 in excess of about 10 M. The resulting precipitate was digested at 85 8C for 24– 48 h, after which the microcrystalline product was filtered, washed with deionised water and dried under vacuum. Other synthetic methods reported in literature include hydrothermal synthesis [22] and solid state reaction at low temperature [23]. In this work four different precipitation methods were used to prepare FAP with different morphology. The FAPs were transformed into LiFePO4 by solid state reaction with LiOH. To evaluate the effect of the reaction conditions on the morphology, crystal structure, and electrochemical properties of the materials the lithiation reaction was carried out at various temperatures and for different heating times. Three temperatures (550, 600 and 700 8C) and two reaction times (1 and 2 h) were tested. The precursors and the final products were morphologically characterized by means of scanning electron microscopy (SEM). The compositional analysis of the materials was performed by electron energy loss spectroscopy (EELS) while X-ray diffraction (XRD) was used to evaluate phase composition, crystal structure and crystallite size. Finally, the electrochemical active materials were characterized by galvanostatic charge/ discharge cycles to evaluate the specific capacity and the capacity retention as a function of the discharge current and cycle number. 2. Experimental De-ionized water (18 MV cm 1) produced by a Milli-Q water production system (Millipore, Bedford, MA) was used to prepare all the solutions. Ferrous sulphate heptahydrate (FeSO4
7H2O, Aldrich, ACS reagent, >99.0%), sodium dihydrogen phosphate (NaH2PO4, Aldrich, ReagentPlus, >99.0%), ammonium dihydrogen phosphate ((NH4)H2PO4, Aldrich, reagent grade, 98.0%), urea (NH2CONH2, Ashland Chemical Italian, reagent grade, >98%), phosphoric acid (H3PO4, Carlo Erba analytical grade), and lithium hydroxide monohydrate (99.9%, Fluka) were used as received. 32 wt.% ammonia solution (NH4OH, Carlo Erba, analytical grade) was also used. Four different precipitation methods have been carried out for the preparation of FAP: 1. Homogeneous phase precipitation. 2. Heterogeneous phase precipitation from stoichiometric sodium phosphate. 3. Heterogeneous phase precipitation from stoichiometric ammonium phosphate. 4. Heterogeneous phase precipitation from over stoichiometric ammonium phosphate. Details of material preparation can be found in the SI. LiFePO4 was prepared by solid state reaction using lithium hydroxide as lithiation agent. The lithium hydroxide monohydrate was ground in a mortar. Then the FAP was added in small portions and the mixture was well grinded after each addition. The powder was transferred into a crucible which was placed inside a quartz tube. The oxygen within the tube was removed by flowing a

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nitrogen/hydrogen mixture. After few minutes the flow was reduced to 100 ml min 1 and the tube was placed in an oven preheated at the desired temperature. After a fixed period of time the tube was removed from the oven and cooled. When the temperature reached the ambient temperature, the crucible was removed from the tube and the solid weighed to evaluate the yield of LiFePO4. The reaction was carried out by varying the heating temperature and the reaction time. Three different temperatures (550, 600 and 700 8C) and two reaction times (1 and 2 h) were tested. The morphology and the chemical composition of the materials were evaluated by a JEOL JSM-5510LV model SEM equipped with an IXRF EDS-2000 EELS. The EELS analysis was carried out to evaluate P and Fe atomic percentage both on single points and in areas by a surface scan. Phase identification was performed by XRD analysis using a Rigaku MiniFlex diffractometer with Cu Ka radiation. Thermal stabilities were verified in nitrogen using a simultaneous TG-DTA (Q600 SDT, TA Instruments) equipped with the Thermal Solution Software (version 1.4). The temperature was calibrated using the nickel Curie point as the reference. The mass was calibrated using ceramic standards provided with the instrument. High purity aluminum oxide was used as the reference material. During the experiments, to avoid contamination with the external atmosphere, high purity nitrogen was flowed at a relatively high rate (100 ml min 1). Open platinum crucibles (cross-section = 0.32 cm2) were used to contain the samples. The experiments were performed on 10–12 mg samples that were stored, handled and weighed in a dry-room (R.H. < 0.1% at 20 8C). The thermal stability was investigated by heating the samples from room temperature up to 800 8C at a rate of 10C min 1. The onset temperature was calculated by thermal analysis software (Universal Analysis version 2.5) as the intersection between the extrapolated baseline weight and the tangent through the inflection point of the weight vs. temperature curve. Composite cathode tapes were made by roll milling a mixture of 70 wt.% active material, 10 wt.% binder (Teflon, DuPont) and 20 wt.% carbon (Super P, MMM Carbon). Electrodes were punched in form of disc with a diameter of 10 mm. The electrode weight ranged from 5.5 to 12.0 mg corresponding to an active material mass loading of 5.0– 11.0 mg cm 2. The electrodes were assembled in sealed cells formed by a polypropylene T-type pipe connector with three cylindrical stainless steel (SS316) current collectors. A lithium foil was used both as a anode and a reference electrode and a glass fiber was used as a separator. The cells were filled with ethylene carbonate/diethyl carbonate 1:1 LiPF6 1 M electrolyte solution (Merck, battery grade). The cycling tests were carried out automatically by means of a battery cycler (Maccor 4000). Composite cathode preparation, cell assembly, test and storage were performed in a dry room (R.H. < 0.1% at 20 8C). 3. Results 3.1. FAP preparation and characterization 3.1.1. Precipitation in homogeneous phase Fig. 1 (top left) shows the picture of the sample obtained by homogeneous phase precipitation The material shows a regular surfaces formed by overlapping planes. The image shows three of these planes. The dimensions of the planes are of several microns. The Fe/P atomic ratio evaluated by EELS ranges between 0.56 and 0.71 with an average value of 0.67 indicating a over stoichiometric amount of phosphorus. 3.1.2. Precipitation in heterogeneous phase from stoichiometric sodium phosphate Fig. 1 (top right) shows the photomicrograph of the material obtained by precipitation in heterogeneous phase from

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Fig. 1. Photomicrographs of the samples prepared by precipitation in homogeneous phase (top left), precipitation in heterogeneous phase from stoichiometric sodium phosphate (top right), precipitation in heterogeneous phase from stoichiometric ammonium phosphate (bottom left), and precipitation in heterogeneous phase from over stoichiometric ammonium phosphate (bottom right).

stoichiometric sodium phosphate. The material is formed by micrometric aggregates that show a surprising graininess. The graininess is due to a very thin structure that cover the surface of the grains. Such a structure, observed at a magnification of 500 times, appears formed by thin lamellae. The height of the lamellae results very thin (less than 1 mm) while the lateral width dimension may reach 10 mm. The result of the EELS elementary analysis performed on six different points shows that the sample is not uniform in composition. Notwithstanding the large difference in the measured values, the average value is very close to unit. 3.1.3. Precipitation in heterogeneous phase from stoichiometric ammonium phosphate Fig. 1 (bottom left) shows the photomicrographs of the material obtained by precipitation in heterogeneous phase from stoichiometric ammonium phosphate. The material appears formed by two different types of formations: the first (formation of type I) is visible in the center while the second (formation of type II) is visible at the boundary. The formation of type I has a flat granular structure characterized by the presence of numerous particles while type II formation is constituted by several parallel overlapped layers. The formation of type I results from the aggregation of larger particles with a lateral dimension of the order of few microns located on a morphologically distinct surface formed by smaller particles. The larger particles have a diameter of about 10 mm and are bonded with the smaller particles. An identical analysis performed on the formations of type II reveals that they are formed by tiles with different sizes. The lateral dimensions of the tiles results of about 10–20 mm. The atomic percentages of iron and phosphorus are strongly different for the two structures. The Fe/P atomic ratio evaluated by EELS confirms that the type I

formations are rich in iron while the type II formations are rich in phosphorus. 3.1.4. Precipitation in heterogeneous phase from over stoichiometric ammonium phosphate The image of the material is shown in Fig. 1 (bottom right). The material appears very uniform. The material appears formed by overlapped tiles of small size (with a max surface of 2  4 square mm). Despite the various Fe/P ratios as evaluated by EELS deviate significantly from the unit, the average ratio is 1.12. The atomic percentage of sulphur in all the samples was lower than 1%. The sulphur content resulted particularly low in the samples 2 and 4 (less than 0.1%). 3.1.5. Thermal analysis The TGA and DTA curves of the four materials are shown in Fig. 2. The bibliographic data [23–25] show that, upon heating between room temperature and about 500 8C, the NH4FePO4
H2O decomposes following a two-stage process. The first stage is attributed to the loss of crystallization water while the second is related to the loss of structural water and ammonia. If the material is analyzed in nitrogen the final product is Fe2P2O7 [24]. In this conditions the theoretical weight loss is of 23.53 wt.%, which corresponds to a residue of 76.47 wt.%. The characteristics of the various decomposition stages depend on the morphology of the material. Yuan et al. [23], studying the DTA curve of a microcrystalline material, observed two endothermic effects with the maxima located at 105 8C and 218 8C, respectively. They also observed an exothermic effect (with a maximum at 555 8C) attributed to the phase transformation. On the other hand, the characteristic temperatures of the signal in macro-crystalline materials obtained by hydrothermal synthesis are shifted at

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Fig. 2. Thermogravimetry (solid line) and differential thermal analysis (dash line) for the sample obtained by precipitation in homogeneous phase (top left), precipitation in heterogeneous phase from stoichiometric sodium phosphate (top right), precipitation in heterogeneous phase from stoichiometric ammonium phosphate (bottom left), and precipitation in heterogeneous phase from over stoichiometric ammonium phosphate (bottom right).

260 8C, 370 8C and 410 8C [25]. The sample obtained by precipitation in homogeneous phase (Fig. 2, top left) has a final residue of 85.4% (about 10% higher than the theoretical one). The high value of the final residue could be justified by assuming the co-presence of impurity or other reaction products thermally stable up to 800 8C. In the DTA curve it is possible to observe two endothermic signals with the maxima at 204.18 8C and 245.48 8C and an exothermic signal with the maximum at 601.16 8C. The two endothermic signals are located at temperature values intermediate with respect to the values reported in literature. The TGA results for the sample obtained by precipitation in heterogeneous phase from stoichiometric sodium phosphate (Fig. 2, top right) are very similar to those reported in the literature [24]; in fact the fixed residue has a value very close to the theoretical one (76.26%) and also the trend of the DTA curve can be overlapped to that reported by Yuan et al. [23] for micro-crystalline FAP. There are three endothermic signals with the maxima at 135.6 8C, 263.8 8C and 483.3 8C and an exothermic signal with a maximum at 552.3 8C. The sample obtained by precipitation in heterogeneous phase from stoichiometric ammonium phosphate (Fig. 2, bottom left) has a fixed residue of 68.3%, lower than the theoretical one. Furthermore, the weight loss is observed up to 700 8C, while a stable weight is reported in literature for temperatures higher than 500 8C. This behavior might be caused by the presence of some residual compound thermally unstable above 500 8C. The DTA curve has a similar trend to that of the sample obtained by precipitation in heterogeneous phase from stoichiometric sodium phosphate (Fig. 2, top left), even if the signal relating

to thermal effects is less defined and the peaks appear wider. Even in the sample obtained by precipitation in heterogeneous phase from over stoichiometric ammonium phosphate (Fig. 2, bottom right) the fixed residue, equal to 76.19%, is close to the theoretical one and the DTA curve has a pattern similar to that of the sample obtained by precipitation in heterogeneous phase from stoichiometric sodium phosphate (Fig. 2, top right), but the peaks are larger and less defined. 3.1.6. XRD characterization Fig. 3 summarizes the results obtained from XRD analysis. The material obtained by precipitation in homogeneous phase as well as the material obtained by precipitation in heterogeneous phase from stoichiometric ammonium phosphate are amorphous (Fig. 3, patterns 1 and 2), whilst the materials obtained by precipitation in heterogeneous phase from over stoichiometric ammonium phosphate or from stoichiometric sodium phosphate are crystalline (Fig. 3, patterns 3 and 4). The two peaks located at 10.0328 and 31.6648 were identified belonging to crystalline NH4FePO4
H2O (JCPDS card number 45-0424). 3.2. LiFePO4 preparation and characterization 3.2.1. LiFePO4 obtained starting from FAP prepared by precipitation in homogeneous phase Fig. 4 (top left) shows the material obtained starting from FAP prepared by precipitation in homogeneous phase. It consists of agglomerates with a diameter of 50 mm or smaller (down to

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3.2.2. LiFePO4 obtained starting from FAP prepared by precipitation in heterogeneous phase from stoichiometric sodium phosphate The LiFePO4 produced from the precursor precipitated in heterogeneous phase by stoichiometric sodium phosphate is shown in Fig. 4 (top right). It appears very uniform with a well defined surface. The surface appears paved with small tiles ranging in size from 10 to 50 mm, separated by deep furrows. The tiles appear to be formed by flat flakes of irregular size equal to 2–5 mm. The EELS analysis showed that the sample is very homogeneous and rich in phosphorus with an average iron/phosphorus of 0.79.

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Fig. 3. Diffraction spectra of the materials: (1) precipitated in homogeneous phase, (2) precipitated in heterogeneous phase by stoichiometric ammonium phosphate, (3) precipitated in heterogeneous phase by stoichiometric sodium phosphate, and (4) precipitated in heterogeneous phase by over stoichiometric ammonium phosphate.

10 mm). The general appearance is thus very variegated even for the shape of the aggregates that, generally circular in shape (at the center in the picture), can also assume different form. The central grain appears as a superposition of circular overlapping tiles of about 50 mm in diameter. The surface of the tiles is extremely regular and constituted of particles with homogeneous dimensions. The EELS analysis showed that the sample is not chemically homogeneous. The average iron/phosphorus ratio turns out to be 1.08.

3.2.3. LiFePO4 obtained starting from FAP prepared by precipitation in heterogeneous phase from stoichiometric ammonium phosphate The LiFePO4 sample prepared in heterogeneous phase from stoichiometric ammonium phosphate (Fig. 4, bottom left) appears extremely smooth, with an even and compact surface, interspersed with small furrows. The smooth surface is due to the agglomeration of particles uniform in size and, at this magnification, is not possible to distinguish the microstructure and the sample appears very uniform. The EELS analysis evidenced that the sample is very homogeneous and rich in phosphorus (the average iron/phosphorus is 0.70). 3.2.4. LiFePO4 obtained starting from FAP prepared by precipitation in heterogeneous phase from over stoichiometric ammonium phosphate The sample obtained from the precursor prepared by precipitation in heterogeneous phase from over stoichiometric ammonium phosphate is shown in Fig. 4 (bottom right). The surface appears less uniform than in the previous sample and formed by differently oriented flakes. The flakes have a size of several microns while the width is extremely thin. From a chemical point of view,

Fig. 4. Photomicrographs of LiFePO4 obtained from the FAP prepared by precipitation in homogeneous phase (top left), precipitation in heterogeneous phase from stoichiometric sodium phosphate (top right), precipitation in heterogeneous phase from stoichiometric ammonium phosphate (bottom left), and precipitation in heterogeneous phase from over stoichiometric ammonium phosphate (bottom right).

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the sample appears very homogeneous and the average iron/ phosphorus turns out to be 1.10. 3.2.5. XRD characterization Fig. 5 shows the XRD spectra for the materials synthesized starting from the different precursors. In the same figure the pattern for the LiFePO4 (JCPDS card number 40-1499) is reported. Notwithstanding all the materials show the characteristic peaks of crystalline LiFePO4, their crystal structure is not well developed. The crystal structure of the materials improves passing from the materials obtained by precipitation in homogeneous phase (1) to the materials obtained by precipitation in heterogeneous phase from stoichiometric sodium phosphate (2). The peaks are short and broad, indicating that the crystal structure has a short range coherence. The materials obtained by precipitation in heterogeneous phase from stoichiometric (3) or over stoichiometric ammonium phosphate (4) are more crystalline with respect the

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Fig. 7. Voltage profiles for a cell cycled at various temperatures. The temperature values are reported in the figure. The LiFePO4 was prepared by heating the FAP coming from precipitation in heterogeneous phase by over stoichiometric ammonium phosphate and LiOH at 600 8C for 2 h.

above mentioned materials. From the comparison between the peaks it can be seen that, in all the spectra, the highest peak is located at 35.58. The Scherrer’s formula was used to evaluate the crystal size of the crystallite: d = k  l/B  cos u where d is the size of the crystallites, k is a constant that depends on the shape of the crystallites (in our case was assumed equal to 0.89), l is the wavelength of the radiation used (equal to 1.54 A˚ having used a copper lamp), B is the peak width at half height and u is the angle of diffraction of the peak. All the materials have a nanometric crystalline size ranging from 22.3 up to 34.9 nm. 3.2.6. Electrochemical characterization Fig. 6 shows the results of the electrochemical characterization performed at C/10 rate. As it can be observed, the materials with the higher specific capacity are those prepared by the method 3 and 4 which have a specific capacity of 0.096–0.1 Ah g 1 while the material prepared by the method 2 has a specific capacity of

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Fig. 6. Voltage profiles for four cells prepared with LiFePO4 coming from different precursors. (1) Precipitation in homogeneous phase, (2) precipitation in heterogeneous phase by stoichiometric sodium phosphate, (3) precipitation in heterogeneous phase by stoichiometric ammonium phosphate, and (4) precipitation in heterogeneous phase from over stoichiometric ammonium phosphate. The LiFePO4 was synthesized by solid state reaction by heating the FAP with LiOH at 600 8C for 2 h. The cells were cycled at C/10 rate.

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0.08 Ah g 1 and the material prepared by method 1 has a specific capacity just over 0.05 Ah g 1. Even in the best case the specific capacity falls below 60% of the theoretical one. Fig. 7 shows the voltage profile of the material obtained from FAP coming from precipitation in heterogeneous phase by over stoichiometric ammonium phosphate recorded at various temperatures. The capacity increases with temperature and it reaches 0.128 Ah g 1 at 80 8C. The average charge voltage is about 3.5 V while the average discharge voltage is about 3.3 V with an hysteresis of 0.2 V. The material reversibility upon cycling is shown in Fig. 8. The cell cycled 1000 times with a capacity retention at C/10 of 60% of the initial capacity. The voltage profiles for the materials prepared with the method 2, 3 and 4, cycled at various discharge rates, are shown in Fig. 9. For all the materials the specific capacity decreases by increasing the discharge current. The specific capacity at 5C rate of the materials prepared with the method 3 and 4 was about 10% of the capacity at C/10 rate while the specific capacity at 5C rate of the material prepared with the method 2 was about 20% of the capacity at C/10 rate.

is shown in Fig. 10 (bottom right). The grain surface appears formed by many small grains composed of tiles arranged in a very disordered way. The tiles have a regular shape and a surface dimension of several microns while their thickness is difficult to assess, since the tiles lay one on the other. Also in this case the sample appears very homogeneous with an average iron/phosphorus equal to 1.10. 3.3.2. XRD characterization All the materials show the characteristic peaks of crystalline LiFePO4. Obviously, the crystal structure of the materials enhances by increasing the crystallization time and temperature. The peaks of the material synthesized at 550 8C are short and broad, indicating that the crystal structure has a short range coherence while the materials obtained at higher temperatures are more crystalline with respect to the above mentioned material. Also in this case the highest peak is located at 35.58 and it is possible to note that the intensity of this peak increases by increasing the reaction time and temperature. This result could be explained by arguing the presence of a secondary crystalline phase in the samples. The amount of this secondary phase increases by increasing the heating temperature or the reaction time.

3.3. Effect of synthesis temperature and reaction time

3.3.3. Electrochemical characterization It was observed that the specific capacity of the samples increases with decreasing reaction times and synthesis temperatures. The material synthesized at 550 8C for 1 h has the higher specific capacity of 0.11 Ah g 1 corresponding to 65% of the theoretical one. The specific capacity of the materials synthetized at 600 8C for 1 h and 2 h show a slight decrease of the specific capacity while the voltage hysteresis is very similar to the previous one. The material synthesized at 7008 for 2 h shows a drastic reduction of the specific capacity that decreases at 0.08 Ah g 1. Furthermore the voltage hysteresis between charge and discharge increases. The decrease of the specific capacity with increasing temperatures can be related to a parasitic reaction occurring during the synthesis (i.e. the formation of secondary phases). By considering that the entity of this parasitic reaction increases with reaction time and temperature it could explain the reason why the specific capacity decreases by increasing the reaction time and temperature. The voltage profiles of the cell at various discharge rates are shown in Fig. 11. Obviously, for all the samples, the specific capacity decreases by increasing the discharge current, but the specific capacity recorded at high rate depends on the crystallization conditions. The material reversibility upon cycling for the sample synthesized at 550 8C is shown in Fig. 12. The cell cycled for 1000 times with a capacity retention at C/10 of 90% of the initial capacity. With

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3.3.1. Morphological characterization To evaluate the effect of the synthesis condition on the electrochemical performance, the sample obtained from the precursor prepared by precipitation in heterogeneous phase from over stoichiometric ammonium phosphate was heated at various temperatures and different reaction times. The picture of the sample prepared at 700 8C for 2 h is shown in Fig. 10 (top left). The sample shows a very smooth texture. The surface of the material appears as a globular structure with a grain size of about 10 mm. The globular structure is formed by numerous smaller globules, each less than 1 mm diameter, connected one to each other. The EELS analysis showed that the sample is very homogeneous and that the amount of phosphorus is larger than the amount of iron (the average iron/phosphorus atomic percentage was 0.87). The pictures of the material synthesized at 600 8C for 2 h is shown in Fig. 10 (top right). The grain surface is composed of flakes arranged in a very messy way with a width of several microns and extremely thin. Fig. 10 (bottom left) show the picture of the sample prepared at 600 8C for 1 h. Also in this case the surface of the sample is formed by small uniform particles, isolated or collapsed to form large agglomerates with different lateral dimensions. The dimension of the agglomerates is of about 5–8 mm. The agglomerates are surrounded by smaller particle not larger than few micron. By analyzing the EELS results it follows that the sample appears very homogeneous and rich in phosphorus (the average iron/phosphorus is 0.83). The picture of the material synthesized at 550 8C for 1 h

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Fig. 9. Voltage profiles for the LiFePO4 prepared from FAP coming from precipitation in heterogeneous phase by sodium phosphate (left), stoichiometric ammonium phosphate (center), and over stoichiometric ammonium phosphate (right) cycled at various discharge rate (C/10, C/5, C, 2C and 5C). The cells were galvanostatically charged at C rate up to 4.2 V followed by a potentiostatic charge at 4.2 V until the current dropped below C/10 rate.

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Fig. 10. Photomicrographs of LiFePO4 obtained from the FAP prepared by precipitation in heterogeneous phase from over stoichiometric ammonium phosphate and heated at 700 8C for 2 h (upper left), at 600 8C for 2 h (top right), at 600 8C for 1 h (bottom left), and at 550 8C for 1 h (lower right).

respect to the material prepared at 600 8C for 2 h an increase of the capacity retention of 50% is observed (i.e. the specific capacity after 1000 cycles increased from 60% for the sample prepared at 600 8C up to 90% for the sample prepared at 500 8C). 4. Discussion The term precipitation describes the phenomenon of the separation of a solid substance from a solution. This separation can take place as the result of a chemical reaction or as a variation of the physical conditions of the solution. Two steps, called nucleation and growth, can be observed during the precipitation. The nucleation consists in the formation of micro-crystals of solute, precisely the ‘‘cores’’ of crystallization. The growth consists instead in the increasing of the size of these crystals through the reaction with other solute surrounding the crystal. In homogeneous phase precipitation, the generation of the precipitating reagent is carried out by a chemical reaction which keeps its concentration at a very low level. This results in the formation of few nuclei that progressively become larger, giving rise to a precipitate with a consistent size. On the contrary in heterogeneous phase precipitation, the precipitating reagent is added to the solution very rapidly and this causes an oversaturation that leads to the formation of numerous nuclei of small dimensions. The characteristics of the materials were found strongly affected by the preparation method. The aggregation state of the materials progressively decreased by changing the precipitation method from the homogeneous to the heterogeneous phase precipitation. The morphology and the chemical composition were also found to be dependent on the precipitation conditions. Chemical analysis showed that the materials were not uniform in composition at microscopic level even if the average chemical composition confirmed a Fe/P ratio

close to the unity. Thermogravimetric analysis showed a thermal behavior very similar to those reported in the literature. The sample obtained by precipitation in heterogeneous phase from stoichiometric sodium phosphate as well as the sample obtained by precipitation in heterogeneous phase from over stoichiometric ammonium phosphate showed a weight loss very close to the theoretical one. Furthermore the crystal structure of these samples showed the presence of peaks belonging to crystalline hydrated ferrous ammonium phosphate. On the other hand the sample prepared in homogeneous phase as well as the sample obtained by precipitation in heterogeneous phase from stoichiometric ammonium phosphate has been found completely amorphous. LiFePO4 cathodes prepared from ammonium phosphate show specific capacities incontrovertibly better with respect to the materials prepared from sodium phosphate. The main reason can be attributed to the larger LiFePO4 content in the samples 3 and 4 with respect to the samples 1 and 2. The JCPDS card indicates that the highest peak, resulting from the overlapping of the reflections planes 2 0 0 and 1 2 1, should be located at 29.38. In the spectra the highest peak is located at 35.58. This result could be due to a preferential orientation of the crystallites within the material or to the presence of a second crystalline phase whit a peak located at 35.58. Among the various compounds containing iron the iron (III) oxide (Fe2O3, maghemite) presents the most intense diffraction peak (correspondent to the reflection plane 3 1 1) precisely localized to 35.58. Therefore it could be argued that the samples should contain Fe bounded in two different compounds, namely as LiFePO4 and Fe2O3. The LiFePO4 is predominant in samples 3 and 4 where the 35.58 peak is a little bit higher than the theoretical one, whilst in samples 1 and 2 it is much higher with respect to the other peaks belonging to LiFePO4. The presence of different iron compounds is also reflected in the different voltage discharge

P.P. Prosini et al. / Materials Research Bulletin 48 (2013) 3438–3448 4,5

4,5

4,0

4,0

Cell voltage / V

Cell voltage / V

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3,5

3,0

3,0

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2,0

2,0

0,00

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Specific capacity / Ah g

0,10

0,00

0,12

4,5

4,0

4,0

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2,0

2,0 0,04

0,06

0,08

0,08

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-1

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0,12

0,10

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-1

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0,06

3,5

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4,5

0,00

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-1

Cell volatge / V

Cell voltage / V

3,5

0,00

0,02

0,04

0,06

0,08

Specific capacity / Ah g

-1

Fig. 11. Voltage profile for the materials cycled at various discharge rate (C/10, C/5, C, 2C and 5C). Top left: material synthesized at 700 8C for 2 h. Top right material synthesized at 600 8C for 2 h. Bottom left: material synthesized at 600 8C for 1 h. Bottom right: material synthesized at 550 8C for 1 h. The cells were galvanostatically charged at C rate up to 4.2 V followed by a potentiostatic charge at 4.2 V until the current has dropped below C/10 rate.

profiles as observed in Fig. 6: samples 3 and 4 show, after the knee at 0.09 Ah g 1, a sharp voltage drop, while the voltage decline is slower for the samples 1 and 2. The slow voltage decline in samples 1 and 2 can be attributed to the reduction of iron oxide that takes 0,11 0,10

Specific capacity / Ah g

-1

0,09 0,08 0,07 0,06 0,05 0,04 0,03 0,02 0,01 0

200

400

600

800

1000

Cycle number Fig. 12. Change in the specific capacity as a function of the number of cycles for the material synthesized at 550 8C for 1 h. The cell was discharged at C/10 followed by 99 cycles at C rate. The cell was galvanostatically charged at C rate up to 4.2 V followed by a potentiostatic charge at 4.2 V until the current dropped below C/10 rate.

place at voltages lower than 3.5 V. The presence of different phases can be also evidenced by the samples morphologies: a larger LiFePO4 content induces a more uniform grain aggregation; this factor in turn positively affects the charge/discharge process. The fact that the materials are unable to reach the maximum theoretical capacity (0.17 Ah g 1) can be attributed, other than to the presence of impurities, to a slow diffusion of lithium ions inside the material. In fact, the low diffusion of lithium ion, limiting the amount of electrochemical active material, can reduce the overall capacity. To assess whether the missed ability is due to impurities contained in the sample or poor accessibility of the material, a cell was cycled at increasing temperatures. In this way the diffusion phenomena can be speeded up, increasing the cell capacity. It was observed that the specific capacity increased with the temperature and at 80 8C it reaches 0.128 Ah g 1. As a consequence it can be stated that the product is at least 75% pure and that the capacity loss due to diffusion phenomena account at least for a 15%. The presence of maghemite could explain the reason for which the material is not able to deliver the full theoretical capacity. The material showed good reversibility and cyclability. The charge discharge ratio at C/10 rate is very close to unity and the voltage hysteresis between charge and discharge is very low. Furthermore the charge recovery after 1000 cycles at C rate is quite good. About 60% of the initial capacity is kept after cyclation. This exceptionality low capacity fade can be related to the inherent structure of the material that is retained throughout the entire intercalation range. The fatigue and the stress that arise from the lithium insertion/release process are well tolerated by the

P.P. Prosini et al. / Materials Research Bulletin 48 (2013) 3438–3448

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Table 1 Specific capacity (at C/10 and 5C), capacity ratio, and average length of diffusion of the electrons in the grains (Da) as a function of firing temperature and reaction time. Reaction conditions 700 8C 600 8C 600 8C 550 8C

for for for for

2h 2h 1h 1h

Specific capacity @ C/10 0.081 Ah g 0.095 Ah g 0.100 Ah g 0.111 Ah g

1 1 1 1

material structure. In this way the structural changes of the electrode, that may cause contact loss and capacity fade upon cycling, are reduced and the cycle life of the cathode is enhanced. Nevertheless the dynamic behavior showed in Fig. 9 showed that the best samples reduce their maximum capacity more markedly with current: samples 3 and 4 capacities drop from 0.1 to 0.015 Ah g 1 when the charging current moves from C/10 to 5C, while sample 2 capacity reduces proportionally less, from 0.09 to 0.025. The best kinetic performance of the sample prepared by the method 2 can be attributed to the peculiar morphological structure obtained with this method. Probably the agglomerated in the form of large flat flakes can support larger currents than micrometersized round microstructures (as found in sample 3) or disoriented flat flakes (as observed in sample 4). XRD data allow us to correlate the crystalline structure with the electrochemical performance. In such kind of analysis the planes indicated as 0 1 1 and 2 1 1 are of particular interest because from these it is possible to derive the value of the average diffusion length of ions and electrons. Two limiting factors affect the electrochemical performance of the material namely the ionic and the electron conduction, respectively at low and high discharge (or charge) rate. The lithium ion diffuses in a one-dimension (1D) channel and can hardly pass to another channel in case the former 1D is blocked. If the 1D channel is very long, the diffusion of lithium ions in this channel will be difficult. In contrast, short 1D channels are useful as they allow the lithium-ion to reach the grain boundaries and to be delivered in the electrolyte [16,20]. Moreover, in a short 1D channel the probability of finding the channel blocked is lower. Furthermore, the smaller the particle size of the crystal, the more favorable is the diffusion of electrons and the greater is the electrochemical high-speed charge and discharge performance. Because the planes 1 0 0 and 0 0 1 refer to the weak peaks in the XRD spectra, according to Ref. [16] the planes 0 1 1 and 2 1 1 were selected in our calculation. The thickness of the planes 0 1 1 and 2 1 1 was used to calculate the approximate length of diffusion of the electrons. The electrons move in two directions (axes of the grains a- and c-), and this can be considered as a parallel circuit. Thus the average length of diffusion of the electrons in the grains, marked as Da, can be calculated as: 1/ Da = 1/D(0 1 1) + 1/D(2 1 1). At current density as low as C/10, the specific capacity of LiFePO4 depends on the number of lithium ions that can diffuse out from the grains. Then D(2 1 1), which refers to the length of the diffusion of the ions, mainly influences the electrochemical performance of the samples. At high current density as 5C, the specific capacity depends on how fast electrons can diffuse to grain boundaries. Therefore, Da, which is the average length of diffusion of electrons, influences the electrochemical performance of the samples. Table 1 shows the average length of diffusion of the electrons, the specific capacity at C/10 and 5C and their ratio as a function of the heat treatment. For temperatures higher than 550 8C, the average length of diffusion as well as the 5C over C/10 capacity ratio decreases by decreasing the reaction temperature or, for a fixed temperature, by decreasing the reaction time. The sample synthesized at 550 8C for 1 h has an average length of diffusion higher than the sample prepared at 600 8C for the same period of time. This behavior is difficult to explain. Anyway, we have to note that the 5C over C/10 capacity ratio for

Specific capacity @ 5C 0.0120 Ah g 0.0157 Ah g 0.0210 Ah g 0.0187 Ah g

1 1 1 1

Ratio

Da

0.148 0.165 0.210 0.168

22.50 19.21 13.27 15.89

the sample prepared at 550 8C also increases. So there is a direct correlation between the 5C over C/10 capacity ratio and the average length of diffusion. 5. Conclusions In this work it has been shown that FAP samples with different morphology, aggregation state and crystal structure can be obtained by varying the precipitation conditions. It was also observed that the purity of the obtained materials depends on the precipitation method. In fact, some materials showed an altered Fe/P ratio or a different final residue when compared to the theoretical value. The thermal analysis confirmed that the thermal properties are strongly influenced by the aggregation state of the material (micro or macro crystalline). The LiFePO4 obtained starting from the FAP prepared by heterogeneous phase precipitation from stoichiometric or over stoichiometric ammonium phosphate solutions showed superior specific capacity with respect to the materials obtained starting from the FAP prepared by homogeneous phase precipitation or by heterogeneous phase precipitation from sodium phosphate. Nevertheless, even in the best case, the specific capacity stood below 60% of the theoretical one. Cycling the cell at higher temperature allowed to state that the LiFePO4 prepared by precipitation from ammonium phosphate is at least 75% pure and that about 15% of the loss in capacity is due to diffusion phenomena. The material showed good capacity retention when cycled by increasing the discharge current or when submitted to prolonged cyclation. The material was able to deliver about 10% of the C/10 capacity when discharged at 5C rate and about 60% of the C/10 capacity after 1000 cycles. By increasing the reaction temperature or, for a fixed temperature, by increasing the reaction time the crystal structure of the samples increased. The specific capacity at C/10 increased by decreasing the firing temperature. The material synthesized at 550 8C for 1 h exhibited the higher specific capacity and capacity retention upon cycling. The first cycle specific capacity was 0.11 Ah g 1 while the capacity retention after 1000 cycles was almost 90% of the initial one. The capacity retention at high rate was seen to increase by decreasing the average length of diffusion. The sample synthesized at 600 8C for 1 h showed the lower average length of diffusion and the maximum specific capacity at high discharge rate, retaining 21% of the capacity exhibited at C/10 when discharged 50 times faster. Acknowledgements Part of this work was carried out within the activity ‘‘Ricerca Sistema Elettrico’’ under a program agreement signed between ENEA and the Italian Ministry for Economic Development. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.materresbull.2013. 05.038.

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