Optimization of Electrochemical Performance of LiFePO 4 / C by Indium Doping and High Temperature Annealing

We have prepared nano-structured In-doped (1 mol %) LiFePO4/C samples by sol–gel method followed by a selective high temperature (600 and 700 °C) annealing in a reducing environment of flowing Ar/H2 atmosphere. The crystal structure, particle size, morphology, and magnetic properties of nano-composites were characterized by X-ray diffraction (XRD), scanning electron microsopy (SEM), transmission electron microscopy (TEM), and 57Fe Mössbauer spectroscopy. The Rietveld refinement of XRD patterns of the nano-composites were indexed to the olivine crystal structure of LiFePO4 with space group Pnma, showing minor impurities of Fe2P and Li3PO4 due to decomposition of LiFePO4. We found that the doping of In in LiFePO4/C nanocomposites affects the amount of decomposed products, when compared to the un-doped ones treated under similar conditions. An optimum amount of Fe2P present in the In-doped samples enhances the electronic conductivity to achieve a much improved electrochemical performance. The galvanostatic charge/discharge curves show a significant improvement in the electrochemical performance of 700 °C annealed In-doped-LiFePO4/C sample with a discharge capacity of 142 mAh·g−1 at 1 C rate, better rate capability (~128 mAh·g−1 at 10 C rate, ~75% of the theoretical capacity) and excellent cyclic stability (96% retention after 250 cycles) compared to other samples. This enhancement in electrochemical performance is consistent with the results of our electrochemical impedance spectroscopy measurements showing decreased charge-transfer resistance and high exchange current density.


Introduction
LiFePO 4 has become one of the most viable commercial cathode materials after the ground breaking work of Padhi et al. [1].This material has received an extensive attention due to its high thermal and electrochemical safety, lower cost compared to mixed oxide cathode materials, low toxicity, stable voltage range even at overcharge condition, and long cycle life.However, the poor electronic conductivity and slow diffusion of lithium ion in bulk LiFePO 4 have been major challenges requiring new electrode material engineering.To improve electronic conductivity and reduce lithium ion diffusion length, many approaches, such as reducing the particle size to nanoscale [2][3][4][5], coating the particles with conductive carbon [6][7][8][9][10][11][12], and doping LiFePO 4 with various cations [13][14][15][16][17][18][19][20] have been proposed.In addition, LiFePO 4 decomposes above 700 • C leading to in-situ formation of conductive iron phosphides (Fe 2 P, FeP, Fe 3 P), and compounds with superior lithium-ion diffusion coefficients, such as, Li 3 PO 4 and Li 2 FeP 2 O 7 [21][22][23][24][25][26].Although initial formation of conductive iron phosphides at the grain boundaries improves electrochemical performance, these phases are not electrochemically active and excessive decomposition of LiFePO 4 reduces the active material leading to reduced specific capacity of the sample.Therefore, careful annealing temperature and addition of proper amount of dopants that reduce the decomposition of active material is crucial for preparation of high performance LiFePO 4 cathode materials.In our previous work [27], we studied the formation Fe 2 P and Li 3 PO 4 by the decomposition of LiFePO 4 /C as a function of annealing temperature between 600-900 • C in a reducing environment and found that the amount of Fe 2 P increases very steeply from 5 to 38 wt % with the annealing temperature.Li 3 PO 4 may contribute to the high ionic conduction at the electrode/electrolyte interface when preferentially deposited on the surface at the grain boundaries [28].The presence of conductive Fe 2 P significantly improved the electronic conductivity of the samples which varied from 2 × 10 −3 S•cm −1 (600 • C) to 2 × 10 −1 S•cm −1 (900 • C).Of all the samples studied, the LiFePO 4 /C sample calcined at 700 • C which consists of 14 wt % of Fe 2 P exhibited a better electrochemical performance with a discharge capacity of ~136 mAh•g −1 at 1 C, ~121 mAh•g −1 at 10 C (70% of the theoretical capacity of LiFePO 4 ), and excellent cycleability.The observed steep decrease in the discharge capacity of samples annealed at higher temperatures was attributed to the increased amount of inactive decomposed products in the electrode.Hence, our previous work suggests that the synthesis environment can be controlled to optimize the amount of Fe 2 P to obtaining the best discharge capacity of LiFePO 4 /C nanocomposites.
In addition, cation doping at Li and Fe sites in LiFePO 4 have been investigated by several researchers [13][14][15][16][17][18][19][20] to improve the electrochemical properties of LiFePO 4 .Substitution of Mg, Al, Na at Li sites [14,15,19] have been shown to improve the overall electrochemical properties of LiFePO 4 .Theoretical calculations by Islam et al. [20] have suggested, on energetic grounds, that LiFePO 4 , is favorable for divalent dopants (e.g., Mg, Mn, Co), but not tolerant to aliovalent doping (e.g., Nd, La, In) on either Li (M1) or Fe (M2) sites.Nevertheless, a few experimental studies have investigated the effects of substituting aliovalent ions, such as Gd, Nd, La, at Fe sites in LiFePO 4 [16][17][18].For example, 1% La-doped LiFePO 4 /C sample showed the best electrochemical behavior with a discharge capacity of 156 mAh•g −1 at a rate of 0.2 C [18].However, there have been no experimental studies available in the literature to see the effect of In-doping in LiFePO 4 .There are multiple beneficial effects expected with In-doping.The redox potential of indium in nonaqueous electrolyte has shown that In remains in 3-oxidation state (In III ) at voltages above 1.5 V vs. lithium.In addition, indium oxide has superior electronic conductivity compared to the LiFePO 4 that may lead to an improved electronic conductivity, particularly when it resides on the surface of the sample.On the other hand, if some of the iron sites are occupied by indium ions, it may increase the concentration of charge carrier in the sample as indium has a high thermodynamic tendency to remain as In III cation, while Fe in the original LiFePO 4 material is at Fe II state.Furthermore, the indium ion is a more polarizable, softer and diffuse ion than the hard sphere Fe III .Therefore, Indium doping may reduce ionic lattice energy and energy barrier for Li-ion hopping between available sites.
In this work, we have studied the effect of In (1 mol %)-doping on the formation of Fe 2 P due to decomposition of LiFePO 4 /C nanocomposites when annealed at two different temperatures of 600 and 700 • C in a reducing environment.We find that the In-doped-LiFePO 4 /C sample annealed at 700 • C which consists of 11 wt % of Fe 2 P showed an improved discharge capacity (142 mAh•g −1 at 1 C rate), better rate capability at higher rates (~128 mAh•g −1 at 10 C rate, ~75% of the theoretical capacity) and excellent cyclic stability compared to that of un-doped sample annealed under similar conditions.By combining In-doping with high temperature (700 ± 50 • C) annealing, the electrochemical performance of LiFePO 4 /C can be further improved by optimizing the amount of Fe 2 P in the nanocomposites for good electronic conductivity without sacrificing the active material.

X-ray Diffraction
The In-doped-LiFePO 4 /C samples were analyzed by XRD to verify both the crystallinity and phase purity.Their XRD patterns (Figure 1) were indexed to an orthorhombic LiFePO 4 phase with space group Pnma, according to the standard pattern of JCPDF 83-2092, indicating that an olivine-type structure is well maintained upon doping with 1 mol % of In.We do notice the presence of minor impurity phases that are indexed to iron phosphide (Fe 2 P) and lithium phosphate (Li 3 PO 4 ), which are formed, particularly, in In-LFP-700 sample.We have performed Rietveld analysis of XRD patterns using GSAS (General Structure Analysis System, Los Alamos National Laboratory Report LAUR 86-748 (2000)), software implemented with EXPGUI interface, to estimate the amount of Fe 2 P and Li 3 PO 4 in In-LFP-600 and In-LFP-700 samples (Figure 1), and the estimated amounts are listed in Table 1.For comparison, we have also including the data for LFP-600 and LFP-700 samples from our previous study [27].The threshold temperature for forming Fe 2 P from the decomposition of LiFePO 4 /C appears to be around 700 • C for both un-doped and doped samples.It is interesting to note the amount of Fe 2 P formed is less in In-LFP-700 compared to LFP-700 sample.In-doping seems to reduce the formation of Fe 2 P or the rate of decomposition of LiFePO 4 /C at 700 • C. As Fe 2 P is conducting, it affects the conductivity of the In-doped-LiFePO 4 /C and hence its electrochemical properties.The reduction of Fe 2 P with in In-doping in In-LFP-700, as indicated by XRD Rietveld analysis, is also confirmed by Mössbauer spectroscopy measurement, as discussed in a later section.The effect of In-doping on the crystallite size was also investigated using Rietveld fitting (GSAS software package) of the XRD patterns.The In-doped samples seem to have slightly smaller crystallite size compared to un-doped samples.
Inorganics 2017, 5, 67 3 of 12 performance of LiFePO4/C can be further improved by optimizing the amount of Fe2P in the nanocomposites for good electronic conductivity without sacrificing the active material.

X-ray Diffraction
The In-doped-LiFePO4/C samples were analyzed by XRD to verify both the crystallinity and phase purity.Their XRD patterns (Figure 1) were indexed to an orthorhombic LiFePO4 phase with space group Pnma, according to the standard pattern of JCPDF 83-2092, indicating that an olivinetype structure is well maintained upon doping with 1 mol % of In.We do notice the presence of minor impurity phases that are indexed to iron phosphide (Fe2P) and lithium phosphate (Li3PO4), which are formed, particularly, in In-LFP-700 sample.We have performed Rietveld analysis of XRD patterns using GSAS (General Structure Analysis System, Los Alamos National Laboratory Report LAUR 86-748 (2000)), software implemented with EXPGUI interface, to estimate the amount of Fe2P and Li3PO4 in In-LFP-600 and In-LFP-700 samples (Figure 1), and the estimated amounts are listed in Table 1.For comparison, we have also including the data for LFP-600 and LFP-700 samples from our previous study [27].The threshold temperature for forming Fe2P from the decomposition of LiFePO4/C appears to be around 700 °C for both un-doped and doped samples.It is interesting to note the amount of Fe2P formed is less in In-LFP-700 compared to LFP-700 sample.In-doping seems to reduce the formation of Fe2P or the rate of decomposition of LiFePO4/C at 700 °C.As Fe2P is conducting, it affects the conductivity of the In-doped-LiFePO4/C and hence its electrochemical properties.The reduction of Fe2P with in In-doping in In-LFP-700, as indicated by XRD Rietveld analysis, is also confirmed by Mössbauer spectroscopy measurement, as discussed in a later section.The effect of In-doping on the crystallite size was also investigated using Rietveld fitting (GSAS software package) of the XRD patterns.The In-doped samples seem to have slightly smaller crystallite size compared to un-doped samples.

Electrical Conductivity
The room temperature electrical conductivity was measured for the samples using Van der Pauw method.The electronic conductivity for the LFP-600, In-LFP-600, LFP-700 and In-LFP-700 are 2 × 10 −3 , 8 × 10 −3 , 8 × 10 −2 and 1 × 10 −2 S•cm −1 , respectively.These results indicate that electronic conductivity of un-doped samples increases with the annealing temperature which is attributed to the formation of conductive Fe 2 P phase at higher temperatures.We will show from our Mössbauer data analysis that the electrical conductivity correlates with the amount of Fe 2 P (crystalline or sub-nanocrystalline) present in these samples [27].

Morphology and Microstructure
The morphology of the In-LFP-600 and In-LFP-700 samples was analyzed by SEM and they are shown in Figure 2a,b.The samples show a uniform distribution of nearly spherical particles with some agglomerated particles very similar to the un-doped samples [27], and 1 mol % In-doping does not affect the morphology significantly.This could be due to the fact that once the particles are carbon coated, the particle growth and the formation of aggregates are suppressed.In addition, the presence of carbon prevents the oxidation of Fe 2+ to Fe 3+ .Thus, the addition of the surfactant, lauric acid, is believed to play a crucial role in controlling the particle size and morphology of samples.We also investigated the particle size distribution using TEM as shown in Figure 2c,d for In-LFP-600 and In-LFP-700 samples.Again, the size distribution is very similar to the corresponding un-doped samples (~80-100 nm), with a rough morphology due to decomposition of LiFePO 4 into Fe 2 P and Li 3 PO 4 .Our previous work [27] also showed that the particle surface of un-doped samples (for example, LFP-700) reveals sub-nano (2-4 nm) regions of the decomposed products, which may not be detected by XRD.The TEM the results are consistent with the average particle size calculated using XRD patterns.

X-ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy allows us to access the local environment of atoms and their oxidation states.The technique, therefore, is used to differentiate between Fe 3+ and Fe 2+ .The Fe elemental XPS spectra of In-LFP-600 and In-LFP-700 samples are shown in Figure 3.All the spectra were fitted with three peaks, two at 710 and 714.5 eV are due to Fe 2+ ions in LiFePO4 and the third one at 712 eV due to Fe 3+ originate mainly from FePO4 and/or Fe2P in agreement with the literature values [29,30].The amount of ferric iron in In-LFP-600 was determined to be lower (~10%) compared

X-ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy allows us to access the local environment of atoms and their oxidation states.The technique, therefore, is used to differentiate between Fe 3+ and Fe 2+ .The Fe elemental XPS spectra of In-LFP-600 and In-LFP-700 samples are shown in Figure 3.All the spectra were fitted with three peaks, two at 710 and 714.5 eV are due to Fe 2+ ions in LiFePO 4 and the third one at 712 eV due to Fe 3+ originate mainly from FePO 4 and/or Fe 2 P in agreement with the literature values [29,30].The amount of ferric iron in In-LFP-600 was determined to be lower (~10%) compared to In-LFP-700 (~13%) sample, which is consistent with the Mössbauer spectroscopy measurements as described in Section 2.5.

X-ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy allows us to access the local environment of atoms and their oxidation states.The technique, therefore, is used to differentiate between Fe 3+ and Fe 2+ .The Fe elemental XPS spectra of In-LFP-600 and In-LFP-700 samples are shown in Figure 3.All the spectra were fitted with three peaks, two at 710 and 714.5 eV are due to Fe 2+ ions in LiFePO4 and the third one at 712 eV due to Fe 3+ originate mainly from FePO4 and/or Fe2P in agreement with the literature values [29,30].The amount of ferric iron in In-LFP-600 was determined to be lower (~10%) compared to In-LFP-700 (~13%) sample, which is consistent with the Mössbauer spectroscopy measurements as described in Section 2.5.

57 Fe Mossbauer Spectroscopy
The room temperature 57 Fe Mössbauer spectra for the In-LFP-600 and In-LFP-700 samples are shown in Figure 4 to confirm the presence of Fe 2 P. A summary of the Mössbauer parameters are given in Table 2.The 57 Fe Mössbauer spectrum of LiFePO 4 with Fe 2 P consists of three quadrupole doublets.The dominant symmetric doublet with an isomer shift (IS) of 1.22 mm/s and quadrupole splitting (QS) of 2.97 mm/s arises from the high spin Fe 2+ configuration of the 3d electrons and the distorted environment at the Fe atom in LiFePO 4 [31,32].The other two doublets in the Mössbauer spectrum arise from two different favorable sites for Fe 3+ , namely, tetrahedral with four nearest neighbor P atoms (3f site) and pyramidal with five nearest neighbor P atoms (3g site) in the structure of Fe 2 P [31].The second doublet with an IS of 0.61 mm/s and a QS of 0.43 mm/s is assigned to Fe 3+ occupying 3f site and the third doublet with an IS of 0.19 mm/s and a QS of 0.1 mm/s is assigned to Fe 3+ occupying 3g site in Fe 2 P in the samples annealed at 700 • C [32].The amount of Fe 2+ and Fe 3+ have been estimated using relative area under the corresponding peaks in the Mössbauer spectra.Table 2 lists the percentage of Fe 2+ and Fe 3+ phases and Table 3 lists the corresponding mol % and wt % of LiFePO 4 , Fe 2 P and Li 3 PO 4 calculated using Equation (1).We have also listed the data for un-doped samples, LFP-600 and LFP-700, for a comparison.

57 Fe Mossbauer Spectroscopy
The room temperature 57 Fe Mössbauer spectra for the In-LFP-600 and In-LFP-700 samples are shown in Figure 4 to confirm the presence of Fe2P.A summary of the Mössbauer parameters are given in Table 2.The 57 Fe Mössbauer spectrum of LiFePO4 with Fe2P consists of three quadrupole doublets.The dominant symmetric doublet with an isomer shift (IS) of 1.22 mm/s and quadrupole splitting (QS) of 2.97 mm/s arises from the high spin Fe 2+ configuration of the 3d electrons and the distorted environment at the Fe atom in LiFePO4 [31,32].The other two doublets in the Mössbauer spectrum arise from two different favorable sites for Fe 3+ , namely, tetrahedral with four nearest neighbor P atoms (3f site) and pyramidal with five nearest neighbor P atoms (3g site) in the structure of Fe2P [31].The second doublet with an IS of 0.61 mm/s and a QS of 0.43 mm/s is assigned to Fe 3+ occupying 3f site and the third doublet with an IS of 0.19 mm/s and a QS of 0.1 mm/s is assigned to Fe 3+ occupying 3g site in Fe2P in the samples annealed at 700 °C [32].The amount of Fe 2+ and Fe 3+ have been estimated using relative area under the corresponding peaks in the Mössbauer spectra.Table 2 lists the percentage of Fe 2+ and Fe 3+ phases and Table 3 lists the corresponding mol % and wt % of LiFePO4, Fe2P and Li3PO4 calculated using Equation (1).We have also listed the data for un-doped samples, LFP-600 and LFP-700, for a comparison.6LiFePO4 + 8C → 3Fe2P + 2 Li3PO4 + P↑ + 8CO2↑ (1)    We note that the total amount of Fe 2 P determined by Mössbauer spectroscopy do not agree with the estimated values by Rietveld refinement of the XRD data (Table 1).As discussed in our earlier work [27], this is due to presence of amorphous or sub-nanoregions of Fe 2 P and Li 3 PO 4 .

Electrochemical Measurements
Galvanostatic charge/discharge curves of the coin cells prepared with un-doped and indium doped LiFePO 4 /C cathodes were measured between 2.2-4.2V versus lithium at different rates.Charge/discharge curves for In-LFP-600 and In-LFP-700 samples at 1 C are depicted in Figure 5.Typical two-phase nature of the lithium extraction and insertion reactions between LiFePO 4 and FePO 4 is implied by the flat nature of the charge-discharge potential curves around ~3.4 V [33].The steep rise and fall in the profiles at the large specific capacity values refer to the charge transfer activation and concentration polarizations with contribution from limited miscibility between the LiFePO 4 and FePO 4 .The expected capacities calculated by taking into account the amount of observed Fe 2 P and Li 3 PO 4 masses from the Mössbauer measurements, and the measured capacities for the samples are listed in Table 3.The data clearly shows that the expected (~158 mAh•g −1 and 152 mAh•g −1 ) and measured capacities (~120 mAh•g −1 and ~136 mAh•g −1 ) for LFP-600 and In-LFP-600 differ significantly, although the latter sample shows significant improvement due to improved electronic conductivity.At higher annealing temperature of 700 • C, the expected and measured capacities are very close to each other, and the capacity of In-LFP-700 (142 mAh•g −1 ) is larger than the corresponding un-doped sample.We observe that at both the annealing temperatures, the measured capacity increases upon adding the indium dopant.

Electrochemical Measurements
Galvanostatic charge/discharge curves of the coin cells prepared with un-doped and indium doped LiFePO4/C cathodes were measured between 2.2-4.2V versus lithium at different rates.Charge/discharge curves for In-LFP-600 and In-LFP-700 samples at 1 C are depicted in Figure 5.Typical two-phase nature of the lithium extraction and insertion reactions between LiFePO4 and FePO4 is implied by the flat nature of the charge-discharge potential curves around ~3.4 V [33].The steep rise and fall in the profiles at the large specific capacity values refer to the charge transfer activation and concentration polarizations with contribution from limited miscibility between the LiFePO4 and FePO4.The expected capacities calculated by taking into account the amount of observed Fe2P and Li3PO4 masses from the Mössbauer measurements, and the measured capacities for the samples are listed in Table 3.The data clearly shows that the expected (~158 mAh•g −1 and 152 mAh•g −1 ) and measured capacities (~120 mAh•g −1 and ~136 mAh•g −1 ) for LFP-600 and In-LFP-600 differ significantly, although the latter sample shows significant improvement due to improved electronic conductivity.At higher annealing temperature of 700 °C, the expected and measured capacities are very close to each other, and the capacity of In-LFP-700 (142 mAh•g −1 ) is larger than the corresponding un-doped sample.We observe that at both the annealing temperatures, the measured capacity increases upon adding the indium dopant.The capacity of the samples at various charge/discharge rates are shown in Figure 6, including the data for un-doped LFP-600 and LFP-700 samples for a comparison.At higher rate, for example at 10 C, the supply of electrons from the interface electrochemical reaction becomes a problem leading to a lower specific capacity for un-doped sample annealed at 600 °C.However, addition of In increases its capacity because of its enhanced electronic conductivity.As seen in Figure 6, even at a high charge/discharge of 10 C the capacity of un-doped LiFePO4/C annealed at 600 °C increases from 84 mAh•g −1 to 114 mAh•g −1 upon doping with In.When the doped sample is annealed at 700 °C, the The capacity of the samples at various charge/discharge rates are shown in Figure 6, including the data for un-doped LFP-600 and LFP-700 samples for a comparison.At higher rate, for example at 10 C, the supply of electrons from the interface electrochemical reaction becomes a problem leading to a lower specific capacity for un-doped sample annealed at 600 • C.However, addition of In increases its capacity because of its enhanced electronic conductivity.As seen in Figure 6, even at a high charge/discharge of 10 C the capacity of un-doped LiFePO 4 /C annealed at 600 • C increases from 84 mAh•g −1 to 114 mAh•g −1 upon doping with In.When the doped sample is annealed at 700 • C, the performance of the In-LFP-700 improves slightly at all rates, and at 10C rate it approached 128 mAh•g −1 (75% of the theoretical capacity).The inset in Figure 6 shows the cycling performance of the doped and un-doped samples annealed at 700 • C for 250 cycles.Clearly, In-LFP-700 shows better charge/discharge stability compared to the corresponding un-doped sample, and even after 250 cycles at 10 C rate the sample retains 96% of its initial capacity.Our results suggest that a combination of In-doping and annealing at high temperatures (700 ± 50 • C), the electrochemical performance of LiFePO 4 /C can be further improved by optimizing the amount of Fe 2 P for good electronic conductivity without sacrificing the active material.charge/discharge stability compared to the corresponding un-doped sample, and even after 250 cycles at 10 C rate the sample retains 96% of its initial capacity.Our results suggest that a combination of In-doping and annealing at high temperatures (700 ± 50 °C), the electrochemical performance of LiFePO4/C can be further improved by optimizing the amount of Fe2P for good electronic conductivity without sacrificing the active material.Electrical impedance spectroscopy (EIS) measurements were also performed to understand the effects of In-doping on electrode impedance.The impedance spectra (Nyquist plots) of un-doped and In-doped LiFePO4 samples are shown in Figure 7a are characteristic of electrochemical cells.The initial intercept of the semi-circle at highest frequency indicates resistance (Rs) associated to the electrolyte.The intercept of the semicircle in the intermediate frequency region corresponds to the charge transfer resistance (Rct) in the bulk of electrode material, and the inclined line in the low frequency range represents the Warburg resistance (Rw), which is associated with lithium-ion diffusion.The data can be fitted to a Randles circuit (see insert in Figure 7a) in consisting of a constant phase element (CPE) representing the double layer capacitance and passivation film capacitance [34].It has been observed that Rs values for the cells are very close to each other because the same electrolyte (1M LiPF6 in EC/DMC 50:50 solvent) is used in all the cells.On the other hand, Rct is lower in case of In-doped samples compared to the pure samples.This lower impedance of the In-doped sample may help to overcome the kinetic activation over potential for the Fe 2+ /Fe 3+ redox reaction during the charge-discharge process, and improve the capacity and cycling performance of the material.
We have determined the diffusion coefficient of lithium ion (DLi) by using Z′ dependence on ω in the low frequency region, which is described by [35], where, σ is the Warburg coefficient, Rs and Rct are the solution and the charge transfer resistances.σ Electrical impedance spectroscopy (EIS) measurements were also performed to understand the effects of In-doping on electrode impedance.The impedance spectra (Nyquist plots) of un-doped and In-doped LiFePO 4 samples are shown in Figure 7a are characteristic of electrochemical cells.The initial intercept of the semi-circle at highest frequency indicates resistance (R s ) associated to the electrolyte.The intercept of the semicircle in the intermediate frequency region corresponds to the charge transfer resistance (R ct ) in the bulk of electrode material, and the inclined line in the low frequency range represents the Warburg resistance (R w ), which is associated with lithium-ion diffusion.The data can be fitted to a Randles circuit (see insert in Figure 7a) in consisting of a constant phase element (CPE) representing the double layer capacitance and passivation film capacitance [34].It has been observed that R s values for the cells are very close to each other because the same electrolyte (1M LiPF 6 in EC/DMC 50:50 solvent) is used in all the cells.On the other hand, R ct is lower in case of In-doped samples compared to the pure samples.This lower impedance of the In-doped sample may help to overcome the kinetic activation over potential for the Fe 2+ /Fe 3+ redox reaction during the charge-discharge process, and improve the capacity and cycling performance of the material.
We have determined the diffusion coefficient of lithium ion (D Li ) by using Z dependence on ω in the low frequency region, which is described by [35], where, σ is the Warburg coefficient, R s and R ct are the solution and the charge transfer resistances.σ is related to D Li by where, R is the gas constant, T is the temperature in Kelvins, n is the number of electrons per molecule during oxidation, A is the surface area of the cathode (0.28 cm 2 in our case), F is the Faraday constant, and C Li is the concentration of lithium ion (0.0228 mol/cm 3 in this case).As expected, a plot of Z vesus ω −1/2 shows (Figure 7b) a linear relationship which yields σ.An apparent exchange current density (I o ) [35], has been calculated to measure the enhanced reaction rate of electrodes, which is a measure of kinetics of an electrochemical reaction.
Inorganics 2017, 5, 67 9 of 12 ω −1/2 shows (Figure 7b) a linear relationship which yields σ.An apparent exchange current density (Io) [35], has been calculated to measure the enhanced reaction rate of electrodes, which is a measure of kinetics of an electrochemical reaction.The calculated charge transfer resistance, lithium diffusion cofficent and apparent exchange current density along with other relevant parameters for the samples are given in Table 4. Indium doped LiFePO4/C samples have a lower charge transfer resistance of 77 Ω and 32 Ω for the In-LFP-600 and In-LFP-700 samples compared to 158 Ω and 72 Ω for the un-doped LFP-600 and LFP-700 samples.There is no drastic improvement in the lithium diffusion coefficient with In doping.However, lithium diffusion coefficient for In-LFP-700 is about a factor of two higher than the In-LFP-600 sample thus implying that annealing at 700 °C is desirable to improve the electrochemical properties of this material.Overall, the electrochemical measurements show that In-LFP-700 has the least charge transfer resistance, relatively higher Li-ion diffusion coefficient, and large exchange current density, which are consistent with its superior electrochemical performance in terms capacity and cycleability.

Synthesis Procedure
In-doped LiFePO4/C samples were prepared by sol-gel technique, using CH3CO2Li•2H2O, FeCl2•4H2O, P2O5 and InCl3 as starting raw materials.These chemicals were mixed in stoichiometric ratio in dry ethanol and stirred for three hours, followed by the addition of 0.75 M lauric acid as carbon source to the mixture.After three hours of mixing the sol was dried under atmospheric conditions.The In-doped-LiFePO4/C sample was prepared by adding 1 mol % of InCl3 during the The calculated charge transfer resistance, lithium diffusion cofficent and apparent exchange current density along with other relevant parameters for the samples are given in Table 4. Indium doped LiFePO 4 /C samples have a lower charge transfer resistance of 77 Ω and 32 Ω for the In-LFP-600 and In-LFP-700 samples compared to 158 Ω and 72 Ω for the un-doped LFP-600 and LFP-700 samples.There is no drastic improvement in the lithium diffusion coefficient with In doping.However, lithium diffusion coefficient for In-LFP-700 is about a factor of two higher than the In-LFP-600 sample thus implying that annealing at 700 • C is desirable to improve the electrochemical properties of this material.Overall, the electrochemical measurements show that In-LFP-700 has the least charge transfer resistance, relatively higher Li-ion diffusion coefficient, and large exchange current density, which are consistent with its superior electrochemical performance in terms capacity and cycleability.

Synthesis Procedure
In-doped LiFePO 4 /C samples were prepared by sol-gel technique, using CH 3 CO 2 Li•2H 2 O, FeCl 2 •4H 2 O, P 2 O 5 and InCl 3 as starting raw materials.These chemicals were mixed in stoichiometric ratio in dry ethanol and stirred for three hours, followed by the addition of 0.75 M lauric acid as carbon source to the mixture.After three hours of mixing the sol was dried under atmospheric conditions.The In-doped-LiFePO 4 /C sample was prepared by adding 1 mol % of InCl 3 during the first step so that Fe:In ratio remains 99:1.The dried powders were ground and annealed under reduced environment of H 2 (10%) and Ar (90%) for 10 h.Two temperatures, 600 • C and 700 • C, were used to investigate the effects of annealing temperature with heating and cooling rate of 1 • C/min.In what follows, In-doped-LiFePO 4 /C samples annealed at 600 • C and 700 • C will be referred as In-LFP-600, and In-LFP-700.The un-doped LiFePO 4 samples annealed at 600 • C and 700 • C, investigated in our previous study [27] will be referred as LFP-600 and LFP-700.

Characterization
Powder X-ray diffraction (XRD) patterns were obtained with a Rigaku Minflex-600 diffractometer (Osaka, Japan) using Cu Kα (λ = 1.54 Å) X-rays.Carbon content of the samples was measured by CHN analyzer, and found to be ~4.5%.The room temperature conductivity of the sample pellets were measured using 4-probe Van der Pauw method.The morphology of the samples was investigated using a JSM-6510-LV-LGS scanning electron microscope (SEM) (Tokyo, Japan) and a JEOL 2010 transmission microscope (TEM) (Tokyo, Japan). 57Fe Mössbauer spectra were measured and fitted to obtain Mössbauer parameters using equipment and procedure described in our earlier work [36].XPS measurements were performed using a Perkin-Elmer XPS systems (Waltham, UK), equipped with a cylindrical mirror analyzer and a highly monochromatic Al Kα (1486.6 eV) X-ray source.The observed binding energies of each element were identified with Perkin-Elmer database and an internal carbon source.

Electrochemical Measurements
The electrochemical characterization of the samples was performed in a standard coin cell geometry, using a Gamry electrochemical measurement system, in the frequency range of 0.1 Hz-100 kHz with an ac amplitude of 10 mV, as described in Ref. [12].

Conclusions
In (1 mol %)-doped-C-LiFePO 4 samples were prepared successfully by sol-gel method using lauric acid as surfactant to coat the particles under high temperature (600-700 • C) annealing.The carbon supplied by the decomposition of fatty acid not only provides reducing environment for maintaining Fe 2+ in the LiFePO 4 , but also restricts the growth of particle size of LiFePO 4 .The XRD patterns of the samples indicates that In-doping does not affect the olivine crystal structure of LiFePO 4 /C nanocomposites, but affects the amount of minority impurity phases (Fe 2 P and Li 3 PO 4 ) formed due to decomposition LiFePO 4 at higher annealing temperatures under a reducing environment.The presence of Fe 2 P in the samples significantly enhances the electronic conductivity and hence affects its electrochemical properties.Of all the samples studied (doped and un-doped), we found that the In-doped-LiFePO 4 /C nanocomposite annealed at 700 • C, containing 11 wt % Fe 2 P showed the highest specific discharge capacity of ~142 mAh•g −1 at 1 C, ~128 mAh•g −1 at 10 C rate with a retention of 96% after 250 cycles of charging/discharging.However, our results demonstrate that by combining In-doping with high temperature (700 ± 50 • C) annealing, the electrochemical performance of LiFePO 4 /C can be further improved by optimizing the amount of Fe 2 P in the nanocomposites for good electronic conductivity without sacrificing the active material.It is important to note that carbon coating alone would not enhance the performance of LiFePO 4 , but simultaneous indium doping and carbon coating is a feasible way to improve electrochemical performance of LiFePO 4 for high power applications.

Figure 6 .
Figure 6.Rate capability curves of LFP-600, In-LFP-600, LFP-700, and In-LFP-700 samples during continuous cycling at different charging rates.The inset shows the capacity retention for LFP-700 and In-LFP-700 sample at 10 C rate.

Figure 6 .
Figure 6.Rate capability curves of LFP-600, In-LFP-600, LFP-700, and In-LFP-700 samples during continuous cycling at different charging rates.The inset shows the capacity retention for LFP-700 and In-LFP-700 sample at 10 C rate.

Table 1 .
Values of LiFePO 4 , Fe 2 P and Li 3 PO 4 estimated from Rietveld analysis of XRD patterns.

Table 2 .
Room temperature Mossbauer parameters of In-doped LiFePO4/C samples annealed at 600 °C and 700 °C.

Table 2 .
Room temperature Mossbauer parameters of In-doped LiFePO 4 /C samples annealed at 600 • C and 700 • C.

Table 3 .
Percentage of LiFePO 4 , Fe 2 P and Li 3 PO 4 in LFP and In-LFP samples annealed at 600 • C and 700 • C deduced from Mössbauer measurements.

Table 3 .
Percentage of LiFePO4, Fe2P and Li3PO4 in LFP and In-LFP samples annealed at 600 °C and 700 °C deduced from Mössbauer measurements.