Synthesis, impedance and electrochemical studies of lithium iron fluorophosphate, LiFePO4F cathode

Electrochimica Acta 85 (2012) 572–578

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Synthesis, impedance and electrochemical studies of lithium iron fluorophosphate, LiFePO4 F cathode M. Prabu a,b , M.V. Reddy a,∗ , S. Selvasekarapandian b,∗ , G.V. Subba Rao a , B.V.R. Chowdari a,∗ a b

Department of Physics, National University of Singapore, Singapore 117542, Singapore Department of Nano Sciences and Technology, Karunya University, Karunya Nagar, Coimbatore 641 114, Tamil Nadu, India

a r t i c l e

i n f o

Article history: Received 5 April 2012 Received in revised form 19 August 2012 Accepted 20 August 2012 Available online 3 September 2012 Keywords: LiFePO4 F Tavorite-structure Cathode Impedance spectroscopy

a b s t r a c t Tavorite-structured LiFePO4 F is synthesized by solid-state reaction and characterized by Rietveld refined X-ray diffraction, scanning electron microscopy, BET surface area, IR and Raman spectroscopy techniques. The ionic conductivity ( ionic) of LiFePO4 F estimated using impedance analysis at 27 ◦ C and at 50 ◦ C are 0.6(±0.1) × 10−7 and 5.4(±0.1) × 10−7 S cm−1 , respectively, with an energy of activation (Ea ) = 0.75 eV. Its electrochemical behavior were examined by galvanostatic charge–discharge cycling up to 100 cycles, cyclic voltammetry (CV) and electrochemical impedance spectroscopy using Li-metal as the counter and reference electrode, at 0.1C rate in the voltage range, 1.5–4 V. LiFePO4 F delivers an initial discharge capacity of 115(±3) mAh g−1 which increases to 119(±3) mAh g−1 at the 20th cycle. The capacity degrades slowly thereafter over 100 cycles, with a capacity loss of 19%. CV data show a clear indication of the Fe3+/2+ redox couple at 3.3–2.6 V that involves a two-phase reaction. Electrochemical impedance spectra measured at various voltages at selected cycles were fitted to an equivalent circuit and the variation of impedance parameters interpreted. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, polyanionic three-dimensional structures built of XO4 (X = P, S) tetrahedra and MO6 (M = V, Fe, Co, Ni) octahedra have been extensively studied for their potential application as electrode material for lithium-ion batteries (LIBs). The olivine LiFe2+ PO4 compound, which is now used as one of the 3.5 V-commercial cathode for LIBs, has attractive properties like structural stability both in the virgin-state and Li-de-intercalated form, which is due to the presence of iron in two stable oxidation states (i.e., Fe2+ and Fe3+ ) and limits the likelihood of oxygen liberation and combustion upon heating in the de-intercalated state. Another family of polyanionic materials having fluorine as part of the network in the structural framework has also been studied as alternative cathode materials for LIBs. Well-known examples are LiVPO4 F [1–5] and LiFeSO4 F [6–8]. The higher electronegativity of F− compared to OH− serves to increase the thermal stability of LiFePO4 F compared to LiFePO4 (OH), and also raise the electrode potential from 2.40 V for LiFePO4 (OH), to 2.75 V for LiFePO4 F, as a consequence of the enhanced inductive effect [6,9]. As an example, the mineral, tavorite LiFePO4 ·OH exhibits a potential of 2.6 V vs.

∗ Corresponding authors. E-mail addresses: [email protected] (M.V. Reddy), [email protected] (S. Selvasekarapandian), [email protected] (B.V.R. Chowdari). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.08.073

Li/Li+ which is increased to 3.0 V in the isostructural phase, LiFePO4 F [10,11]. Recently, Ramesh et al. [12] and Ellis et al. [13] reported on the synthesis of LiFePO4 F, Li2 FePO4 F and electrochemical performance of LiFePO4 F as the 3 V-cathode for LIBs. The Li-M-phosphates, sulphates and fluorosulphates with the olivine and related structure are electronic insulators and also poor Li-ionic conductors. The major limitations of LiFePO4 , namely poor electrical conductivity and 1-D (one-dimensional) lithium ion diffusion. But tavorite-structured LiFePO4 F compounds are comprised of one-dimensional chains of metal octahedra interconnected by polyanion tetrahedra which allow 1-D electron transport, while intersecting channels that house the Li+ afford open pathways for 3-D (three-dimensional) ion transport [6]. For example, LiFePO4 shows at 300 K, electronic conductivity,  (electronic) ∼1 × 10−9 S cm−1 , and equally low ionic conductivity [14]. Recham et al. [8] found that the tavorite-structured LiFeSO4 F shows at 300 K  (electronic) 5.2 × 10−11 S cm−1 and  (ionic) 7 × 10−11 S cm−1 . Similarly, group of Tarascon et al. [6] performed the conductivity measurements on tavorite-structured LiFeSO4 F prepared by solid state reaction method shows  (electronic) 2.5 × 10−11 S cm−1 and  (ionic) 1.7 × 10−11 S cm−1 at 300 K. Increasing the  (electronic) and  (ionic) in the above compounds will greatly help in better utilization as cathode materials. For example, carboncoated LiFePO4 performs as the 3.5 V-cathode in comparison to uncoated-LiFePO4 . It will be of interest to examine the  (ionic) behavior of tavoriteLiFePO4 F as a function of temperature. Presently we report on the

M. Prabu et al. / Electrochimica Acta 85 (2012) 572–578

(0-21)

(hkl) lines Observed Calculated Difference (101)

(0-11)

10

20

(200) (120) (1-22)

(10-1)

The compound, LiFePO4 F was prepared by mixing stoichiometric amounts of FePO4 (4.266 g) and LiF (0.733 g, 2.5 wt% excess) (Merck, 99% purity) in the 1:1 mol ratio using a mechanical grinder for 1 h and heating the mixture at 575 ◦ C for 1 h in flowing argon gas in a tubular furnace (Carbolite, UK). The compound, FePO4 was synthesized by co-precipitation method using stoichiometric quantities of FeCl3 ·6H2 O (Aldrich, 99% purity) and Na3 PO4 ·12H2 O (Merck, 99% purity) followed by filteration, drying and heating at 750 ◦ C for 8 h in air. Characterization of the compound was carried out by X-ray diffraction (XRD) (Empyrean unit with Cu K␣ radiation, PANalytical) to identify the crystal structure. The XRD data were refined using a Rietveld refinement (TOPAS software version 2.1). The morphology of the powder was examined by means of scanning electron microscope (SEM) (JEOL JSM-67500F). The Brunauer, Emmett and Teller (BET) surface area of the powders was measured by Micromeritics Tristar 3000 (USA) unit. The infrared (IR) and Raman spectra were recorded by using Bruker Equinox 55 and Renishaw 2000 units, respectively, at ambient temperature. For ionic conductivity measurements, the powder was pressed (∼1.2 ton pressure) to form pellets (∼2 mm thick and 10 mm diameter) which were sintered at 500 ◦ C for 1 h in argon atmosphere and cooled to room temperature. Platinum metal, 1 ␮m thick (ion blocking electrode) was coated on both sides of the pellets using JOEL-JFC-1600 Auto fine coater. The pellet was mounted in to a Kiel-type four probe apparatus coupled with a variable temperature controller (Ionic Systems, model: KC 2.0, Germany) for conductivity measurements. The data were collected in the temperature range, 27–50 ◦ C using impedance analyzer (Solartron SI 1260, UK) in the frequency range of 10 MHz to 50 mHz with ac amplitude of 10 mV. For electrochemical Li-cycling studies, 0.4 g of LiFePO4 F was ball milled with super P carbon black (0.114 g) using Spex Ballmiller (8000D), USA and agate vial (volume of vial with weight of the ball ratio (2 ml: 0.33 g)) for 2 h. The composite electrodes were fabricated with the above active material and super P carbon mixture and the binder (Kynar 2801) in the weight ratio 70:20:10 using N-methyl-pyrrolidone (NMP) as solvent. Electrodes with thickness of ∼20 ␮m were prepared using an etched Al-foil as current collector by using the doctor-blade technique and then dried in an air oven at 70 ◦ C for 12 h. The coated composite material on Al-foil was pressed between twin rollers, cut into 16 mm diameter circular strips and finally dried in a vacuum oven at 70 ◦ C for 12 h, cooled and kept in a desiccator. The geometrical area of the electrode is 2.0 cm2 . The coin-type test cells (size 2016) were assembled using the composite electrode, 1 M solution of LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 by volume; Merck) as the electrolyte and glass microfiber filters (Whatman, Cat No. 1825-047) as a separator and Li-metal foil as the counter and reference electrode. The cells were assembled using coin cell crimper (Hosen, Japan) in argon-filled glove box (MBraun, Germany) and were aged for 12 h before the measurements. Charge/discharge cycling at constant current mode was carried out using a computer controlled Bitrode battery tester (Model SCN, Bitrode, USA) and cyclic voltammetry studies were carried out at room temperature using a computer controlled Mac-pile II system (Bio-logic, France). For ex situ XRD studies, the cells were dismantled in the glove box, the composite electrodes were recovered, washed in DEC solvent and dried in an oven. Impedance spectra of cells were measured with the Solartron impedance/gain-phase analyzer (model SI 1255) coupled to a potentiostat (SI 1268) at room temperature (RT).

Intensity / a.u.

2. Experimental

LiFePO4F

30

40 50 60 2 theta / degree

(301)

synthesis, characterization, impedance studies and electrochemical cycling behavior of LiFePO4 F.

573

70

80

Fig. 1. Rietveld refined X-ray diffraction pattern of triclinic tavorite-structured LiFePO4 F. The continuous line is fitted data and symbols are experimental data. The differences pattern is shown. Vertical lines are allowed (h k l) lines.

The frequency was varied from 0.35 MHz to 3 mHz with ac signal amplitude of 10 mV. Nyquist plots (Z vs. −Z ) were derived and analyzed using Z-plot and Z-view software (Version 2.2, Scribner Associates Inc., USA). 3. Results and discussion 3.1. Structure and morphology The phase, LiFePO4 F is brownish yellow in color and its synthesis critically depends on the temperature and time of heat-treatment. Higher or lower temperature than 575 ◦ C and times of <1 h or >1 h did not yield well-defined single phase material at solid state reaction method. The Rietveld refined XRD pattern of the compound LiFePO4 F is shown in Fig. 1. The XRD data have been subjected to Rietveld refinement on the basis of triclinic tavorite structure with space group, P−1. The obtained lattice parameters given in Table 1 are in good agreement with values reported by Ramesh et al. [12] and Ellis et al. [13] for LiFePO4 F. SEM photograph shows agglomeration of sub-micron particles with almost spherical morphology, similar to those reported by Ramesh et al. [12] for LiFePO4 F (Fig. 2). The measured BET surface area of LiFePO4 F is 1.5(±0.1) m2 g−1 , with pore diameter 29(±2) nm.

Fig. 2. SEM image of LiFePO4 F showing agglomerated sub-␮m size particles. Scale bar: 1 ␮m.

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Table 1 Lattice parameters of the tavorite-structured LiFePO4 F. The values derived from the virgin electrodes and from ex situ XRD of electrodes in the discharged-state and charged states are also given. LiFePO4 F

a (nm)

b (nm)

c (nm)

˛ (◦ )

ˇ (◦ )

 (◦ )

As-prepared powder As-prepared electrode Discharged to 1.5 V after 100 cycles Charged to 4 V after 100 cycles

0.5311(3) 0.5313(3) 0.5355(3) 0.5339(3)

0.7278(3) 0.7258(3) 0.7342(3) 0.7267(3)

0.5167(3) 0.5152(3) 0.5438(3) 0.5166(3)

107.83 108.41 109.71 108.04

98.64 98.18 94.99 97.46

107.12 107.33 108.65 107.36

Fig. 3a shows the FTIR absorption spectrum of LiFePO4 F. It is dominated by the internal vibrations (1 –4 ) of the (PO4 )3− units, which involve the displacement of oxygen atoms of the tetrahedral (PO4 )3− anions and present frequencies closely related to those of the free molecule. The strong band at 1045(±3) cm−1 corresponds to the stretching mode of the (PO4 )3− and the weak bands at 572 and 488(±3) cm−1 are assigned together to the (PO4 )3− bending mode of vibrations [9,12,15]. The spectrum bears good resemblance to that reported by Ramesh et al. [12] and Ellis et al. [13]. The Raman spectrum (Fig. 3b) shows bands in the region, namely, 1110 and 1003(±3) cm−1 , attributed to the (PO4 )3− stretching vibration. The bands at 612 and 454(±3) cm−1 can be assigned together to the (PO4 )3− bending vibrations. For LiFePO4 F, there are no peaks in the 3000–3600 cm−1 or 750–800 cm−1 regions of the IR spectrum and Raman spectrum indicating that the compound is free of hydroxide and is thus fully fluorinated sample [13]. 3.2. Ionic conductivity studies Fig. 4 shows the cole–cole plots (Z vs. −Z ) of LiFePO4 F in the temperature range, 27–50 ◦ C for both (a) heating and (b) cooling

Absorbance / a.u.

a) FTIR spectrum : LiFePO4 F

1045

cycle. The spectra show a single semicircle in higher frequency region followed by low frequency spike. High frequency semicircle can be fitted to a parallel combination of bulk resistance (Rb ) and bulk capacitance (Cb ) of the material (Fig. 4c). The capacitance value is of the order of 10–30 pF and thus confirmed the impedance to be due to the bulk of the material [16]. From the impedance plot, it can be seen that with the increase in temperature, the intercept of the low frequency arc on the real axis shifts towards the origin, i.e., the bulk resistance of the sample decreases with increase of temperature and thus conductivity increases. The low frequency tail of the impedance spectra suggests that the a.c. conductivity is mostly ionic [8]. The depressed semicircle is an indication of the non-ideal nature of the electrode material and hence a constant phase element (CPE) was used to replace capacitance in fitting the equivalent circuit of Fig. 4c. Bulk resistance values (Rb ) are obtained from the low frequency intercept of the semicircle on the real (Z )-axis using the program “EQ” developed by Boukamp [17] for the analysis of impedance data. The  (ionic) calculated using the equation:  (ionic) =

L Rb A

(1)

where L is the thickness (∼2 mm) and A is the geometrical area (∼0.785 cm2 ) of the pellet. The  (ionic) value of the LiFePO4 F is 0.6(±0.1) × 10−7 S cm−1 at 27 ◦ C and it increases to 5.4(±0.1) × 10−7 S cm−1 at 50 ◦ C. There is obvious different between the conductivity values for both heating and cooling cycle may be due to hysteresis effect of the material which exhibit

488 572

4000

3000

2000

1000

Wave number / cm-1

b)Raman spectrum : LiFePO4F Intensity / a.u.

1003

4000

454 612 1110

3000 2000 1000 -1 Raman shift / cm

Fig. 3. (a) FT-infrared (IR) spectrum and (b) Raman spectrum of LiFePO4 F. Numbers refer to wave numbers of the bands in cm−1 .

Fig. 4. Cole–cole plot (Z vs. −Z ) of LiFePO4 F at various temperatures in the range, 27–50 ◦ C. (a) heating (2nd) cycle and (b) cooling (2nd) cycle. Selected frequencies are shown. (c) Electrical circuit for fitting the spectra.

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0.8

heating cycle o 27 C (RT) o 30 C o 35 C o 40 C o 45 C o 50 C

-4 -5 -6 -7

0

2 4 6 log ( omega / Hz )

1 2 3

0.4 0.2

5

4

0.0 -0.2 (2.3V) (2.6V) -0.6 1.5 2.0 2.5 3.0 3.5 + Potential vs. (Li/ Li ) / V

8

4.0

b) Arrhenius plot of LiFePO4F Fig. 6. Cyclic voltammograms of LiFePO4 F in the potential range, 1.5–4.0 V vs. (Li/Li+ )/V Scan rate, 58 ␮V s−1 . Numbers refer to cycle number.

-3.8 -4.0

and  (ionic) 7 × 10−11 S cm−1 at 300 K with activation energy Ea = 0.77 eV and Ea = 0.99 eV. Thus, the  (ionic) of LiFePO4 F at 27 ◦ C and 50 ◦ C are one order of magnitude larger than LiFePO4 (and higher Ea ) and three orders of magnitude larger than LiFeSO4 F (and lower Ea ).

-4.2 -4.4 -4.6

3.3. Electrochemical properties

-1

(1000/T) / K

Fig. 5. (a) Conductance spectra of LiFePO4 F recorded in the temperature range, 27–50 ◦ C. (b) Arrhenius plot (log T vs. 1000/T) of LiFePO4 F for calculation of energy of activation, Ea .

discontinuous jumps in conductivities, indicated in Fig. 4. On cooling, the jump occurs at a somewhat lower temperature compared with the heating cycle. The exact mechanism is not clear at present, further studies are needed. Fig. 5a shows the frequency dependence of ac conductivity, (ω) at various temperatures during the heating cycle. For all temperatures, the conductivity is independent of frequency at low frequency (≤104 Hz) and is identical to dc conductivity  (ionic) of the material. The conductivity shows dispersion due to bulk relaxation phenomena and as the frequency is increased (>104 Hz), the conductivity shifts to higher frequencies with an increase in the temperature. The dispersion region is characterized by the random hopping of mobile Li-ions. The  (ionic) has been found to obey the Arrhenius relation,  (ionic) T = 0 exp

 −E  a

kT

(2)

where  0 is pre-exponential factor and k is the Boltzmann constant, T is the absolute temperature and Ea is the activation energy for the migration of Li ions (Fig. 5b). The Ea = 0.75(±0.02) eV in the temperature range 27–50 ◦ C. Molenda et al. [14] performed AC impedance measurements on polycrystalline LiFePO4 and they showed Ea = 0.63 eV and the  (ionic) at 27 ◦ C = 0.9 × 10−8 S cm−1 . As mentioned earlier, Recam et al. [8] measured the ionic conductivity of tavorite-structured LiFeSO4 F using impedance spectroscopy and they showed that Ea = 0.99(±0.01) eV with  (ionic) = 7 × 10−11 S cm−1 at 25 ◦ C. Group of Tarascon [6] performed the AC conductivity measurements on tavorite-structured LiFeSO4 F prepared by both solid state reaction and ionothermal method which shows  (ionic) 1.7 × 10−11 S cm−1

3.3.1. Cyclic voltammetry Cyclic voltammograms (CV) of LiFePO4 F with Li-metal as the counter and reference electrode, in the potential range 1.5–4 V at 24 ◦ C and scan rate of 0.058 mV s−1 are shown in Fig. 6 up to 5 cycles.

+

3.10 3.15 3.20 3.25 3.30 3.35

Potential vs. (Li/ Li ) / V

-4.8

4.0

a) LiFePO4F

100 1 2 50 25

3.5 3.0 2.5 2.0 1.5

100

0

20

40

60

80

25 50 2 1

100

120 -1

Specific Capacity / mAh g -1

-1

-3.6

(3.3V)

-0.4

-8 -2

log (sigma_ ionicT / Scm K)

Current / mA

-3

LiFePO4 F

0.6

Discharge capacity / mAh g

log (sigma / S cm-1)

-2 a)Conductivity vs. log : LiFePO4F

575

140 120

b)LiFePO4F

100 80 Voltage range : (1.5 - 4V) Current rate : 0.1C

60 40 20 0 0

10 20 30 40 50 60 70 80 90 100

Cycle number Fig. 7. (a) Galvanostatic charge–discharge profiles of LiFePO4 F. The numbers indicate cycle number. (b) Discharge capacity vs. cycle number. Voltage range, 1.5–4.0 V vs. (Li/Li+ )/V at a current of 15 mA g−1 (0.1C).

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Fig. 8. Nyquist plots (Z vs. −Z ) of LiFePO4 F at 0.1C and at various voltages. (a) 1st discharge cycle, (b) 1st charge cycle, (c) 20th discharge cycle, (d) 20th charge cycle, (e) 50th discharge cycle, (f) 50th charge cycle, (g) 100th discharge cycle, (h) 100th charge cycle. Li-metal was the counter electrode. Selected frequencies are indicated. Symbols represent experimental spectra and continuous lines represent fitted data using the equivalent electrical circuit shown in (i).

Significant extraction of Li from LiFePO4 F is not likely because the Fe3+/4+ potential lies too high for oxidation at accessible potentials. However, Li can be readily inserted in the structure to give Li2 FePO4 F as shown by Ramesh et al. [12] and Ellis et al. [13]. During the first cycle which started at the open circuit voltage (OCV) ∼3 V, the cathodic peak (Li-insertion) is seen at 2.6 V followed by low intensity peak at 2.3 V which could be due to a minor structural modification which is due to insufficient dispersion of the conductive carbon black, since this is not seen in subsequent cycles

similar to Ellis et al. [13]. The corresponding main anodic peak (Liextraction) is seen at 3.3 V. In the second and subsequent cycles, the high intensity anodic peaks shift to slightly higher potentials, and the corresponding cathodic peak shifts to slightly lower potentials which is due to electrode kinetics or electrode polarization. Slight differences in shapes of the cathodic and anodic peaks are due to formation cycles. A decrease in the area under the peaks with an increase in the cycle number indicates capacity-fading in LiFePO4 F.

M. Prabu et al. / Electrochimica Acta 85 (2012) 572–578

3.3.2. Galvanostatic cycling Discharge–charge cycling was carried out in the voltage window of 1.5–4.0 V vs. (Li/Li+ )/V at a current density of 15 mA g−1 (0.1C; assuming 1C = 150 mA g−1 ) up to 100 cycles at 24 ◦ C and the voltage vs. capacity profiles are shown in Fig. 7a. For clarity, only selected cycles are shown. During the first discharge, the voltage decreased steeply from the OCV ∼ 3 V to approximately 2.75 V, and shows voltage plateau at 2.7 V up to a capacity of ∼60 mAh g−1 . This is followed by a continuous decrease of the voltage up to the end of 1.5 V. The total first-discharge capacity is 115(±3) mAh g−1 . The first charge profile shows a voltage plateau at 2.85 V up to ∼60 mAh g−1 followed by a step increase of voltage up to 4.0 V. The total charge capacity is 101(±3) mAh g−1 . Thus, the irreversible capacity loss during the first cycle is 14(±3) mAh g−1 which corresponds to a coulombic efficiency of 88%. During subsequent discharge and charge cycles, up to 20 cycles, both charge and discharge capacity shows a slightly increasing trend due to ‘formation’ of the electrode which is due to nice contact between active material, conductive carbon and binder and current collector with a reversible capacity of 119(±3) mAh g−1 . Thereafter, both show a decreasing trend up to 100 cycles and also an increase in the electrode polarization, as is clear from the profiles of 25, 50 and 100th cycle (Fig. 7a). Similar effect of high polarization with low capacity was noticed by Ellis et al. [13] for as-prepared LiFePO4 F having large particle size which are not carbon coated. These shortcomings are eradicated by pulverizing the as-synthesized material for 3 h in a planetary ball mill. After milling, the particles have a spherical shape and range in size from 200 to 400 nm, and the carbon is better dispersed which shows high theoretical capacity. But from our SEM images and BET surface area results shows that the ball milled compound is in sub-micron range. The discharge capacity as a function of cycle number for the LiFePO4 F at 0.1C is shown in Fig. 7b. The initial discharge capacity of 115(±3) mAh g−1 , and it slowly increases to 119(±3) mAh g−1 up to 20th cycle. Thereafter, capacity-fading occurs, ∼19% between 20 and 100 cycles and a capacity of 96(±3) mAh g−1 is retained at the end of 100 cycles. The probable reason for capacity-fading is due to polarization of the electrode because of the two-phase reaction [5,13]. The Li-cycling behavior of the presently studied LiFePO4 F differs non-trivially from the results reported by Ramesh et al. [12] who found a single-phase Li-intercalation behavior up to x = 0.4 in Li1+x FePO4 F at a voltage of ∼3.1 V vs. Li. This is followed by a distinct and abrupt two-phase Li-intercalation at 2.8 V for x in the range, 0.4–0.8 that is characterized by very low electrode polarization. They realized a reversible capacity of 145 mAh g−1 (x = 0.96) which is close to the theoretical value of 152 mAh g−1 . They also found very good cycling stability up to 38 cycles when cycled both at ambient temperature and at 55 ◦ C at a current rate of 0.1C in the voltage range, 1.5–4.0 V, with only small capacity-fading. Ellis et al. [13] reported that by decreasing the particle size of the as-synthesized material to 200–400 nm and effectively dispersing the conductive carbon after milling for 1 h, the discharge capacity increased from 0.7 to 0.95 Li. The reason for the slightly different behavior of our LiFePO4 F is due to large particle size with sub-micron range after ball milling similar to that as-prepared LiFePO4 F (x > 0.1) by Ellis et al. [13]. 3.4. Ex situ XRD The ex situ XRD patterns of LiFePO4 F cycled electrodes in the discharged state (at 1.5 V) and in the charged state (at 4.0 V) after 100 cycles, along with that of the as-prepared electrode of LiFePO4 F are recorded (figures not shown). The patterns completely resemble that of Fig. 1 for the powder of LiFePO4 F. The calculated lattice parameter values are given in Table 1. As can be seen, the triclinic structure is retained after Li-insertion and the a, b, c, ˛ and  values

577

Table 2 Impedance parameters derived from experimental Nyquist plots of LiFePO4 F during 1st, 20th, 50th and 100th charge–discharge cycle at various voltages at 0.1C. Discharge cycle

1st cycle 20th cycle 50th cycle 100th cycle

Potential vs. (Li/Li+ )/V

3.5

3

2.5

2

1.5

R(sf+ct) (±3)  CPE(sf+dl) (±3) ␮F R(sf+ct) (±3)  CPE(sf+dl) (±3) ␮F R(sf+ct) (±3)  CPE(sf+dl) (±3) ␮F R(sf+ct) (±3)  CPE(sf+dl) (±3) ␮F

– – 157 74 220 59 268 62

251 85 160 67 252 80 288 79

291 121 170 66 257 87 320 84

242 144 174 85 340 87 333 84

247 194 313 249 459 297 391 239

Potential vs. (Li/Li+ )/V

2

2.5

3

3.5

4

R(sf+ct) (±3)  CPE(sf+dl) (±3) ␮F R(sf+ct) (±3)  CPE(sf+dl) (±3) ␮F R(sf+ct) (±3)  CPE(sf+dl) (±3) ␮F R(sf+ct) (±3)  CPE(sf+dl) (±3) ␮F

250 89 195 150 386 121 368 68

184 80 141 69 316 130 326 75

172 315 126 71 200 72 275 78

159 352 118 81 171 58 266 59

127 366 113 72 153 63 199 50

Charge cycle

1st cycle 20th cycle 50th cycle 100th cycle

show an increase (and a decrease in ˇ value) in the fully dischargedstate after 100 cycles in comparison to the values of the virgin electrode. As mentioned earlier, the observed reversible capacity at the end of 100 cycles is 96 mAh g−1 , corresponding to x = 0.63 in Li1+x FePO4 F. For x = 0.63, a corresponding amount of Fe3+ ions are reduced to Fe2+ ions. Since the latter ion has a larger ionic radius than Fe3+ ion, there will be a net increase in the lattice parameter values and expansion of the unit cell volume. A similar increase in the a, b, c, ˛ and  values (and a decrease in ˇ value) was observed by Ramesh et al. [12] after chemical Li-intercalation of LiFe3+ PO4 F using LiAlH4 to give Li2 Fe2+ PO4 F. In the fully charged-state, after 100 cycles, the lattice parameters decrease (and ˇ increases), as expected, in comparison to the values in the discharged-state. However, these values do not exactly match with those of the virgin electrode, possibly indicating some residual Li in the compound (x > 0 in Li1+x FePO4 F). 3.5. Electrochemical impedance spectroscopy Electrochemical impedance spectra at various voltages during cycling give information on the Li-ion kinetics and factors responsible for capacity-fading or otherwise of the oxide cathode materials [5]. The impedance spectra of LiFePO4 F with Li-metal as the counter electrode were measured at various voltages in the range, 1.5–4.0 V at a rate of 0.1C during the 1st, 20th, 50th and 100th cycle. At each voltage, the cells were equilibrated for 2 h and spectra were recorded. The corresponding Nyquist plots (Z vs. −Z ) are shown in Fig. 8. The impedance spectra were fitted to an equivalent circuit consisting of resistance (electrolyte, surface film (sf) and chargetransfer (ct)), a constant phase element (CPEi ) (sf + double layer (dl)), Warburg impedance (Ws ) and intercalation capacitance (Ci ). The circuit is shown in Fig. 8i. The CPE are used instead of capacitor due to the composite nature of the electrode, which is responsible for the observed depressed semicircle in the spectra. The extracted impedance parameters are listed in Table 2. A fresh cell (OCV ∼ 3 V) shows a single semicircle in the frequency range, 0.18 MHz to 15 Hz, followed by a Warburg region (Fig. 8a). In this case, there is no contribution from charge-transfer resistance. During both the charge and discharge cycles a single semicircle is observed and therefore the spectra are fitted using a combination of surface-film and charge-transfer resistance, R(sf+ct) and the corresponding CPE(sf+dl) . In most of the cases the resistance

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values during the discharge cycle show an increasing trend from 4 to 1.5 V and a decreasing trend during the charge cycle from 1.5 to 4 V. The R(sf+ct) values noted during the 20th cycle at various voltages are much lower when compared to the 1st cycle. These values show an increasing trend in the 50th and 100th cycle (Table 2). The CPE(sf+dl) values, on the other hand, show a decreasing trend with increasing cycle number, both in the charged-state and in the discharged-state. The observed trends in the impedance parameters indicate an increase in the polarization of the LiFePO4 F electrode that is also evident in CV and galvanostatic cycling data. 4. Conclusions Triclinic-structured LiFePO4 F was prepared by solid state reaction and characterized by XRD, Rietveld refinement, SEM, BET surface area, IR and Raman spectroscopy. Impedance spectral analysis shows the ionic conductivity,  (ionic) = 0.6(±0.1) × 10−7 S cm−1 at 27 ◦ C which increases to 5.4(±0.1) × 10−7 S cm−1 at 50 ◦ C with an energy of activation (Ea ) = 0.75 eV. The electrochemical Li-cycling behavior of LiFePO4 F has been examined at ambient temperature by galvanostatic discharge–charge cycling, cyclic voltammetry (CV), and impedance spectra in the range of 1.5–4.0 V vs. Li at 15 mA g−1 (0.1C rate) up to 100 cycles. Results show that a first-discharge capacity of 115(±3) mAh g−1 slowly increases to 119(±3) mAh g−1 up to 20 cycles. Thereafter, the reversible capacity shows a decreasing trend and after 100 cycles, a capacity of 96(±3) mAh g−1 (x = 0.63 in Li1+x FePO4 F) is observed corresponding to a capacity-fade of 19%. The CV data show a two-phase Liintercalation and de-intercalation mechanism, at potentials 2.6 V and 3.3 V, respectively. The results are compared with earlier studies on LiFePO4 F reported by Ramesh et al. [12]. Electrochemical impedance spectra at various voltages at selected cycles were fitted to an equivalent electrical circuit and the derived impedance parameters were interpreted.

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