PAni-coated LiFePO₄ Synthesized by a Low Temperature Solvothermal Method

Abstract

The composite LiFePO₄/polyaniline was prepared by chemical synthesis to promote the intensification of the electrochemical properties for use as cathodes in lithium ion batteries. The X-ray diffraction (XRD) of LiFePO₄ synthesized by solvothermal method were indexed to the orthorhombic structure, according to the JCPDS 40-1499. The spectra Raman and FTIR showed a high degree of ordering of LiFePO₄ with interaction between LiFePO₄ surface with structure conjugate of conducting polymers. The cyclic voltammogram of the composite synthesized chemically showed a significant reduction in the value of ΔE _p (ΔE _p = 0.20 V) when compared to LiFePO₄ (ΔE _p = 0.41 V), with lower charge transfer resistance values, indicating favoring electron transfer rate in the composite. Thus, the alternative synthesis route of the LiFePO₄ / PAni composite was easy to handle and allowed an increase in the electrochemical properties of the LiFePO₄, compared to the traditional methods that require additional thermal treatments.

Keywords
solvothermal synthesis; PAni coated LiFePO₄; composites; cathode lithium batteries

1. Introduction

Currently, the demand for energy sources that are renewable and environmentally clean has driven research and the use of alternative energy sources instead of fossil fuel use. One can cite, for example, solar, wind energy, among others¹.

Most of these sources of energy is uncontrollable and / or intermittent nature, therefore, is associated with these energy sources a relatively high cost. Within this context, the lithium ions batteries are considered as one of the short-term solutions because of their high energy density and the relatively simple reaction mechanism².

Among the secondary energy storage devices, lithium ion stand out because they have features including: high voltage (approximately 3 V), high energy density (volumetric and mass), low self-discharge rate (2 to 8% per month) and wide operating temperature range³. Due to these superior features, the lithium ions batteries are widely used in various household appliances devices such as cell phones, notebooks, tablets, among others^4,5.

The transition metal oxides have been the main subject of research in the area of cathode materials for lithium ion batteries⁶. Such materials exhibit good charge storage capacity, high specific energy and excellent cycle life⁷.

Classified by structure, the cathode materials include: lamellar compounds LiMO₂ (M = Co, Ni, Mn, etc.), spinel compounds LiM₂O₄ (M = Mn, etc.) and olivine compounds LiMPO₄ (M = Fe, Mn, Ni, Co, etc.)².

Although the lithium cobaltate (LiCoO₂) material has high charge capacity and high stability, increasing the battery life, the cobalt presents high cost, high toxicity and moderate reversible capacity. On the other hand, the LiMn₂O₄ presents a problem that decreases its loading capacity significantly as compared to the LiCoO₂. The Jahn-Teller effect occurs due to a structural anisotropic distortion in the d orbitals, from the Mn³⁺, changing of the compact cubic symmetry to a tetragonal symmetry⁸.

The LiFePO₄ has theoretical specific capacity (170 mA h g^-1) lower than that obtained for LiCoO₂, however it has a production cost significantly low because the iron ore reserves are relatively abundant. Regarding security, the LiFePO₄ does not present the problem of oxygen evolution during cycling, which is common in LiCoO₂ and LiMn₂O₄. This advantage is attributed to the strong covalent bond between phosphorus and oxygen, it also provides a high thermal stability material. Among the cited cathode materials, LiFePO₄ has lower toxicity, which, together with its low cost, enables its large-scale production^9,10.

Despite these superior characteristics, the LiFePO₄ has low ionic and electron conductivity at room temperature, about 10^-5 S cm^-1 and 10^-10 S cm^-1, respectively^11,12.

Several methodologies have been employed to overcome the inherent limitations of the LiFePO₄, among the main can be mentioned: coating with conductive agents and anionic and cationic doping^13,14.

The conductive coating agents have been extensively used to increase the electronic conductivity of LiFePO₄ and hence improve its electrochemical performance. A range of conductive agents such as inorganic conductors (graphene, carbon nanotubes, etc.), metals, conductive oxides (RuO₂, etc.), and organic conductive (conducting polymers, etc.)¹⁵.

Conducting polymers belong to the class of conjugated polymers. They are called conjugated chains due to the fact that they show a sequence of alternating double and single bonds. Among various conducting polymers, polyaniline (PAni) is one of the most interesting in terms of its use as active component of the cathodes of lithium batteries since it has low cost, high energy density, facile synthesis and environmental friendliness. The PAni has different states oxidation and, among them, the green colored emeraldine salt has the highest conductivity (⁓0.1-10² S cm^-1)¹⁶. Some papers report the improvement in the electrochemical performance of LiFePO₄ from the polyaniline coating, since PAni can mediate the polarity difference between the cathode and electrolyte particles, promote the permeation of electrolytes on the surface of the active particles, acting as a conducting network that connects the particles of LiFePO₄, diminishing, therefore, the electrical resistivity between them^17,18.

This work has as aim an alternative synthesis route of the LiFePO₄/PAni composite to promote an increase in the electrochemical properties of the LiFePO₄, favoring electron transport of the LiFePO₄ synthesized by solvothermal method. Then, the LiFePO₄/PAni composite was synthesized by chemical synthesis and its electrochemical properties as cathode material in lithium ion batteries were investigated.

2. Experimental Section

2.1 Synthesis of LiFePO₄ by solvothermal method

The LiFePO₄ was obtained by solvothermal synthesis using the following reagents: lithium hydroxide, ferrous sulfate heptahydrate, phosphoric acid and ethylene glycol. The molar ratio of the reactants was 3: 1: 1. Initially, 1.2 × 10^-2 mol H₃PO₄ (ALDRICH, PA) and 1.2 × 10^-2 mol FeSO₄•7H₂O (ALDRICH, PA) were dissolved in 40 mL of ethylene glycol. At the same time, 3.6 × 10^-2 mol LiOH•H₂O (ACROS, PA) were dissolved in 30 mL of ethylene glycol. Subsequently, the LiOH solution was added slowly to the solution of H₃PO₄ / FeSO₄•7H₂O under constant stirring for 6 min. At the end, the mixture was placed in a reactor and heated at temperatures of 170°C, 180 °C, 190 °C and 200 °C for 10 h. The obtained precipitates were washed several times with an alcoholic solution of 50% (V/V) and dried at 70 °C for 8 h¹⁹.

2.2 Chemical synthesis of the LiFePO₄/PAni composite

Initially, LiFePO₄ particles were dispersed in 50.0 mL of 1 mol L^-1 H₃PO₄, containing approximately 6.25 × 10^-3 mol L^-1 (0.144 g) of sodium dodecyl sulfonate (dispersing agent). Subsequently, the solution was placed in an ultrasound, in which it was left for 10 min for a better dispersion of the phosphate particles. 1 mol L^-1 H₃PO₄ solution in which were added 4.1076 g (0.25 mol L^-1) of ammonium persulfate (oxidizing agent). The solution was cooled up to -3 °C in an ice bath containing NaCl, for better control of the polymerization kinetics of the aniline. Thereafter, it was added 1.34 mL (0.20 mol L^-1) of the aniline solution in the beaker. Under stirring, the oxidizing agent was added slowly. After synthesis, the solution was allowed to stand and after 48 h the solution was filtered. The dark powder was placed to dry in an oven at 80 °C for 24 h.

2.3 Microstructural characterization

2.3.1 Microstructural characterization by XRD

The structural characterization was made by X-ray diffractometry using a Shimadzu diffractometer (Model 6000, radiation Cu Kα λ = 1.5406 Å with a voltage of 40 kV, current 30 mA, at 2θ min^-1, from 20° to 60°. The unit cell parameters were calculated using the Unit Cell Win program, with the adjustment of the diffractogram peaks previously made on the Peak Fit software.

2.3.2 Microstructural characterization by FT-IR spectroscopy

FT-IR spectroscopy measurements were performed using an infrared spectrophotometer (ABB BOMEM MB series) using pellets of the composite powder containing KBr (1:100) from 2000 to 500 cm^-1.

2.3.3 Microstrutural characterization by Raman spectroscopy

Raman spectroscopy measurements were performed using a Raman spectrometer (LabRAM HR Evolution). Raman scattering spectra were performed from 2000 to 200 cm^-1 at room temperature. The emission from a laser He-Ne ion with a wavelength of 633 nm was used. To obtain a high signal-to-noise ratio, each Raman spectrum is the average of 20 successive scans obtained at a spectral resolution of 1 cm^-1.

2.4 Thermal characterization by TGA

Thermogravimetric Analysis (TGA) was performed on a DTG-60H Shimadzu equipment Simultaneous DTA-TG apparatus. The heating routine occurred from room temperature (approximately 25 °C) up to 900 °C at 10 °C min^-1 in synthetic air.

2.5 Morphological characterization by FEG

The FEG images were obtained using a Hitachi electronic scanning microscope, Model 3000, enlarging the images 80,000 times with acceleration voltage of 10 kV.

2.6 Electrochemical characterization

The electrodes were made containing 85% in mass of synthesized materials, 10% of black acetylene VULCAN XC 72-GP 2800 (CABOT USA) and 5% of Polyvinylidene Fluorite (PVDF) (ALDRICH, PA) dispersed in cyclohexanone (ALDRICH, PA). The slurry mixture was sonicated for 30 min to form a homogeneous solution. Then it was spun (5000 rpm for 5 s) on a platinum plate and, in order to evaporate the solvent, the electrodes were dried at 120 ºC for 24 h in vacuum²⁰.

The conventional electrochemical cell was composed of three electrodes: working electrode, as described, graphite as counter electrode and Li/Li⁺ as the reference electrode. The electrolyte was a solution of 1.0 mol L^-1 LiClO₄ in a dimethyl carbonate/ethylene carbonate (1:1 v/v) solution. The electrodes were characterized by using cyclic voltammetry in the potential range of 2.94 to 3.94 V vs. Li/Li⁺ at ν= 0.1 mV s^-1. All electrochemical experiments were performed in a Glove box using an Autolab PGSTAT 20 FRA, interfaced with a microcomputer using software NOVA version 1.11.

Electrochemical impedance spectroscopy (EIS) spectra were recorded by applying of alternating current (AC) amplitude of 10 mV, and the data were collected in the frequency range from 10⁴ to 10^-1 Hz.

3. Results and Discussion

3.1 Microstructural characterization by XRD of the LiFePO₄

Typical XRD pattern shows similarity with the standard JCPDS pattern 40-1499 indexed on basis an ortorrombic structure. The XRD pattern of LiFePO₄ (Figure 1) shows the presence of well resolved and highly defined peaks, thus confirming the formation of highly crystalline LiFePO₄ without any impurities. In the synthesis by solvothermal method is not observed the presence of peaks attributable to second phases such as Li₃PO₄ and Fe₃(PO₄)₂ because the ethylene glycol acts as a weak reducing agent and minimizes the oxidation of Fe²⁺ to Fe³⁺ during synthesis of the LiFePO₄¹⁷. Besides, ethylene glycol (EG) reduced significantly the mobility rates of the different ions species (Li⁺, Fe²⁺) in this medium and it also acted as a soft template for facilitating the self-assembly of in situ grown nanoparticles through forming hydrogen bonds which control the crystal growth of LiFePO₄²¹.

Figure 1
X-ray patterns of LiFePO₄ synthesized at 170 ºC, 180 ºC, 190 ºC and 200 ºC for 10 h.

The unit cell parameters calculated to LiFePO₄ are showed in the Table 1.

Table 1
Unit cell parameters calculated for LiFePO₄.

LiFePO4 synthesized at	a (Å)	b(Å)	c(Å)	V(Å3)
170 ºC	10.2811	5.9626	4.6814	286.9796
180 ºC	10.3202	5.9902	4.6885	289.8434
190 ºC	10.3215	5.9905	4.6943	290.2530
200 ºC	10.3328	6.0115	4.6958	291.6826
JCPDS pattern 40-1499	10.3470	6.0189	4.7039	292.9474*

• ^*
  calculated from unit cell parameters a*b*c.

There is a gradual increase of the parameters a, b and c as a function of the synthesis temperature, indicating the intercalation of lithium ions in the crystalline structure. The sample synthesized at 200 ºC presented similar parameters to the JCPDS, suggesting a stoichiometric structure. The intensity ratio between the diffraction peaks I₍₂₁₁₎/I₍₁₁₁₎ for LiFePO₄ synthesized at 190 °C and 200 °C showed higher values than others samples, as shown in Figure 2. Besides, it was noted a higher ratio between the intensity of the diffraction peaks I₍₂₁₁₎/I₍₁₁₁₎ (1.210) in the sample synthesized at 200 °C than those obtained at different synthesis temperatures. This higher ratio can be attributed to a probable arrangement of the LiFePO₄ particles due to the growth of crystals aligned in the horizontal face¹⁹. Then, the LiFePO₄ obtained at 200 ºC was used to synthesize conducting composite.

Figure 2
Ratio of intensity between (211)/(111) for the LiFePO₄ synthesized at 170 ºC, 180 ºC, 190 ºC and 200 ºC for 10 h.

3.2 Electrochemical characterization of the LiFePO₄

The Figure 3 shows the stabilized cyclic voltammograms (5^th cycles) of the LiFePO₄ obtained at different temperatures (from 170 to 200 ºC) for 10 h by solvothermal method. The voltammetric profiles of the samples were similar to those obtained for stoichiometric LiFePO₄, showing that the redox processes occur in one stage, at corresponding to the deintercalation/intercalation of lithium ion in the Li_x FePO₄ structure (0 ≤ x ≤ 1) in the octahedral sites¹². In the voltammogram of the LiFePO₄ (at 200 ºC) is showed defined redox peaks and more intense than those obtained for the LiFePO₄ obtained at 190 ºC. In this last, it is observed a transition of Fe²⁺ to Fe³⁺ at 3.67 V vs. Li/Li⁺, and the reducing process of the Fe³⁺ to Fe²⁺ (3.27 V vs. Li/Li⁺). The LiFePO₄ synthesized at 170 and 180 ºC did not present electrochemistry activity.

Figure 3
Cyclic voltammograms recorded at 0.1 mV s^-1, in a 1.0 mol L^-1 LiClO₄ in EC:DMC 1:1 (V/V), of LiFePO₄ synthesized at 170 ºC, 180 ºC, 190 ºC and 200 ºC for 10 h. Cyclic voltammogram of LiFePO₄ synthesized at 170 °C shown as inset.

The difference in sample responses is associated with the fact that possibly in the samples synthesized at 190 °C and 200 °C, the preferred diffusion axis (010) has been decreased. This explanation can be based on the intensity of the diffraction peaks I ₍₂₁₁₎ / I ₍₁₁₁₎, being attributed to a probable arrangement of the LiFePO₄ particles due to the growth of crystals aligned in the horizontal face¹⁹, which may favor lithium ion transport.

3.3 Microstructural characterization by FTIR

In order to obtain information about molecular structure of materials, the LiFePO₄/PAni composite and its constituent materials were characterized by FTIR spectral range of 2000-500 cm^-1. In the FTIR spectra of the LiFePO₄ (Figure 4), bands were observed in the region of 1139-500 cm^-1 corresponding to the internal intramolecular vibration modes arising from the (PO₄)^3-. In the region of 647-500 cm^-1 is observed bands attributed to bending modes (ν ₂ and ν ₄) of asymmetric and symmetric O-P-O links as well as vibrations of lithium ions²². The bands present in the range of 945-1139 cm^-1 correspond to symmetric and asymmetric stretching modes (ν ₁ and ν ₃) PO bond. The band at 547 cm^-1 corresponds to the vibration of lithium ions in the octahedral sites of the PO₄ ^3-. Moreover, it was not observed any other vibrational bands that are associated with Li₃PO₄, P₂O₇ or P₃O₁₀, which reflects the absence of the secondary phases in the LiFePO₄.

In the FTIR spectrum obtained for PAni and LiFePO₄ / PAni can observe the presence of a band around 1130 cm^-1, which is known in the literature as an "electronic band" being associated, so the doped form of polyaniline, more specifically the vibration mode the structures -NH⁺=²³. Moreover, it is observed for the LiFePO₄ / PAni there is inverse relationship intensity in 1382-1354 cm^-1 (*1 and *2) compared with polyaniline, revealing that the polyaniline in the composite is richer in quinoid units than the PAni pure, probably because the chain of polyaniline deposited on the surface of the LiFePO₄ has a greater conjugation²⁴.

The band corresponding to C-H out of plane deformation due to the presence of mono-substituted benzene rings was observed, indicating the high degree of polymerization in the composite (Figure 4). This interpretation is also supported by the presence, in both materials, a band around 1300 cm^-1, which is often associated with the C-N stretch aromatic secondary amine groups²⁵. The band observed at around 1232 cm^-1 can be assigned to CN stretching vibration of benzoic species²⁶.

Figure 4
FTIR spectra of LiFePO₄, PAni and LiFePO₄/PAni.

3.4 Microstructural characterization by Raman spectroscopy

In order to obtain information about molecular structure of materials, the LiFePO₄/PAni composite and its constituent materials were characterized by Raman spectroscopy. The Figure 5 shows the Raman spectra of LiFePO₄, PAni and LiFePO₄/ PAni obtained in the spectral range from 2000 to 200 cm^-1.

Figure 5
Raman spectra of LiFePO₄, PAni and LiFePO₄/PAni.

The characteristic vibrations of LiFePO₄ are observed below 1200 cm^-1 and the bands of LiFePO₄ can be divided into two classes: internal and external optic modes. The internal modes are located above 400 cm⁻¹ and are originate from the intramolecular vibrations of the PO₄ ^3- anion. The external modes are primarily composed of translatory and vibrational motions of the PO₄ ^3- ions and translatory motion of the Fe²⁺ ions²⁷. Moreover, the intense symmetric stretching band at 949 cm^-1 can be identified as well as the asymmetric stretching bands at 995 and 1067 cm^-1 indicating non-distorted PO₄^3-27. Furthermore, no evidence of secondary phases such as iron oxide was seen in the Raman spectra, in accordance with the XRD data.

The Raman spectra of PAni and LiFePO₄/PAni (Figure 5) show the all characteristic bands of PAni. The bands near 1500 cm^-1 and 1600 cm^-1 are assigned mainly to the benzenoid C-C ring stretching vibration and the quinoid C-C stretching mode, respectively. The band located at 1338 is atributed to C-N⁺ vibrations, indicating that the polyaniline is in the doped state. The band around 1168 cm^-1 is attributable to the out-of-plane-C-H bending. Moreover, the band around 575 cm^-1 assigned to cross linking between PAni chains²⁸. It should be noted that the bands present small shifts in the LiFePO₄ / PAni spectrum when compared to the PAni spectrum, which should be associated to an chemical interaction of Pani with LiFePO₄.

3.5 Thermal characterization by TGA

The composite LiFePO₄/PAni and its constituent materials were characterized by Thermogravimetric analysis (TGA) in synthetic air in order to determine the percentage of polyaniline in the composite as well as verify the percentage of Fe²⁺ in LiFePO₄ and the mass loss levels of materials (Figure 6).

Figure 6
TGA of LiFePO₄, PAni and LiFePO₄/PAni,in synthetic air.

The TGA curve LiFePO₄ shows a small increase in mass at 300 °C which is attributed to the Fe²⁺ oxidation in air and it can be represented by Equation 1 ^29,30.

(1) ${\mathit{LiFePO}}_{4\left(\mathit{s}\right)}+\left(\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$4$}\right.\right){\mathit{O}}_{2\left(\mathit{g}\right)}\to \left(\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$3$}\right.\right){\mathit{Li}}_3{\mathit{Fe}}_2{\left({\mathit{PO}}_4\right)}_{3\left(\mathit{s}\right)}+\left(\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$6$}\right.\right){\mathit{Fe}}_2{\mathit{O}}_{3\left(\mathit{s}\right)}$

In the TGA curve of PAni was observed three stages of weight loss below 850 °C. The first stage weight loss in the range 33-118 °C can be attributed to water molecules strongly bound to the polymers and will probably not have been removed. The second mass loss stage ranges 160-270 °C, it was assigned to the elimination of dopant anions (dihidrogen phosphate present in the electrolyte) and the last stage 340-850 °C is attributed to the thermal decomposition of polymer chains³¹. At the end of the analysis there was not residue of the analyzed conductive polymer.

For the LiFePO₄/PAni composite, there was a weight loss in the range 40-200 °C and 215-290 °C attributed to evaporation of water and elimination of dopant anions, respectively. A weight loss at 330-520 °C corresponds to the thermal decomposition of the polyaniline chains and leaving a residue of approximately 44% weight, corresponding to the Li₃Fe₂(PO₄)₃ and Fe₂O₃ (Table 2).

Table 2
Temperatures values of thermal events for LiFePO₄, PAni and LiFePO₄/PAni.

Material	Event temperature (ºC)	Change mass (%)	Thermal event assignment
LiFePO4	300-800	+5	Oxidation of LiFePO4
PAni	33-118	-14	Removal of adsorbed water
	160-270	-12	Elimination of dopant anion
	350-850	-72	Thermal decomposition of polymer chains
LiFePO4/PAni	40-200	-12	Removal of adsorbed water
	215-290	-8	Elimination of dopant anion
	330-530	-36	Thermal decomposition of polymer chains

3.6 Morphological characterization by FEG

The FEG micrographs shown in the Figure 7 (a) and (b) were further analyzed with the Image J software, which allowed counting the amount and measuring the size (d) of the particles. The amount of particles considered for the counting was equal to 100; to ensure reliability, this counting was done on five different micrographs of each material. The thus obtained average particle size distributions are shown in Fig. 7 (c).

Figure 7
FEG micrographs of: (a) LiFePO₄ and (b) LiFePO₄/PAni; (c) Average (n = 5) particle size distribution for the LiFePO₄ and LiFePO₄/PAni.

It is observed that both LiFePO₄ and LiFePO₄ / PAni presented a morphology in the shape of plates which size varied from 0.2 to 1.2 µm (Fig. 7). The polyaniline, on the other hand, presents a porous morphology, covering the particles of LiFePO₄ and allowing the intercalation of lithium ions on the surface of LiFePO₄. Thus, we infer that both materials, LiFePO₄ and LiFePO₄/PAni composite, present particle sizes of nanometric scale, suggesting polymer thin film in the surface LiFePO₄.

3.7 Electrochemical characterization of LiFePO₄/PAni composite and their constituent materials

In order to obtain information on the electrochemical activity of the materials as well as to verify the influence of PAni on the electrochemical properties of the LiFePO₄, the LiFePO₄ / PAni composite and its constituent materials were characterized by cyclic voltammetry. The Figure 7 shows the cyclic voltammograms of LiFePO₄, PAni and LiFePO₄ / PAni. In the voltammogram of the LiFePO₄ (at 200 ºC) is showed defined redox peaks and more intense than those obtained for the LiFePO₄ obtained at 190 ºC. In this last, it is observed a transition of Fe²⁺ to Fe³⁺ at 3.67 V vs. Li/Li⁺, and the reducing process of the Fe³⁺ to Fe²⁺ (3.27 V vs. Li/Li⁺). The LiFePO₄ synthesized at 170 ºC did not present electrochemistry activity.

In the voltammogram of PAni the profile is predominantly capacitive. In voltammogram of the LiFePO₄/PAni composite was observed the same well defined redox peaks associated oxidation of Fe²⁺ to Fe³⁺ and reducing of Fe³⁺ to Fe²⁺ at 3.61 and 3.42 V, respectively (Figure 8). However, in this voltammogram a current density is higher than that obtained for pure PAni. Besides, it is noteworthy that the distance between the oxidation and reduction peaks (ΔE _p = 0.20 V) is significantly lower when compared to LiFePO₄ (ΔE _p = 0.41 V).

Figure 8
Cyclic voltammograms recorded at 0.1 mV s^-1, in a 1.0 mol L^-1 LiClO₄ in EC:DMC 1:1 (V/V), of LiFePO₄ synthesized at 200 °C for 10 h, of PAni and LiFePO₄/PAni.

This is due to the fact that the composite material has a more reversible redox process as compared to pure LiFePO₄, therefore the PAni contributes effectively to the electronic conductivity of the composite material. It was observed a significant improvement of the redox reversibility of the LiFePO₄ with the use of other conductive agents such as: carbon, graphene, etc;^19,32as shown in Table 3.

Table 3
Values of oxidation and reducing potential vs. Li/Li⁺ for LiFePO₄ and LiFePO₄/PAni.

Material	Eox. (V vs . Li/Li+)	Ered. (V vs . Li/Li+)	Δ(E ox.- E red.) (V vs. Li/Li+)	Reference
LiFePO4	3.69	3.28	0.41	This work
LiFePO4 (solid state reaction)	~3.90	~3.10	0.80	30
LiFePO4/PAni	3.61	3.42	0.19	This work
LiFePO4/C (solid state reaction)	~3.80	~3.40	0.40	30

3.8 Electrochemical impedance of LiFePO₄ /PAni composite and their constituent materials

It was evaluated the electrical resistance of the films as well as the influence of polyaniline on the electrical conductivity of the LiFePO₄ film.

In the Nyquist plots (Z" vs. Z') of the PAni (Figure 9) it is observed a semi-circle in the region of high frequencies and by extrapolating towards the real axis impedance obtained the charge transfer resistance values (R _ct) corresponding to the electrical resistance on the film / electrode interface. The region of intermediate frequencies at low a linear relationship with slope of about 45 degrees corresponding to the diffusion of ionic species in the electrolyte toward to the polymeric film.

Figure 9
Nyquist diagrams of LiFePO₄, PAni and LiFePO₄/PAni, in a 1.0 mol L^-1 LiClO₄ in EC:DMC 1:1 (V/V), in the range 10⁴ to 10^-1 Hz to open circuit potential. High frequency region in Nyquist diagram of LiFePO₄/PAni is shown as inset.

Although the Nyquist plot of LiFePO₄ / PAni shows a similar profile to the PAni plot, a reduction of the diameter of the semicircle is observed, which indicates a significant decreased charge transfer resistance. The R _ct values were 50 and 14 Ω cm² for PAni and LiFePO₄/PAni, respectively (Table 4).

Table 4
Electrochemical parameters of the PAni and LiFePO₄/PAni.

Material	Rct (Ω cm2)	Rs (Ω cm2)	Cdc (µF)
PAni	50.00	18.13	60.13
LiFePO4/PAni	14.00	19.00	47.72

In the Nyquist plot of LiFePO₄ observed predominance of the diffusion process that is attributed to the lithium insertion and of the ions in the structure of the material.

Obviously, the LiFePO₄/PAni has the smallest R _ct, indicating that the polyaniline coating improves the electron transfer kinetics as already explained, the polymer chains of polyaniline form conductive networks that interconnect the LiFePO₄ particles, increasing the electronic conductivity of the composite and it can significantly improve the charge transfer reaction^18,29.

4. Conclusions

The solvothermal method at low temperature was used to synthesize the LiFePO₄ with high crystallinity and free from secondary phases. The alternative synthesis route of the LiFePO₄/PAni composite was easy to handle and allowed an improvement in the electrochemical properties of the LiFePO₄, compared to the traditional methods that require additional thermal treatments. The ratio of the constituents in composite materials of LiFePO₄/PAni was 44% to the oxide and 56% to the polyaniline. Raman spectrum of the composite showed a possible chemical interaction between LiFePO₄ surface and structure conjugate of conducting polymers. The cyclic voltammogram of LiFePO₄/PAni composite presented a predominant faradaic profile and a lower value of ΔE_p compared to the LiFePO₄. The charge transfer resistance to the composite was lower than the obtained for its constituent materials, indicating favoring electron transfer rate in the composite. Therefore, the synthesis of LiFePO₄ by solvothermal method as well as the synthesis of the LiFePO₄/PAni composite were effective approaches to overcome the problem of low ionic and electronic conductivity of the LiFePO₄ and thus becoming a promising material as cathode in lithium ion batteries.

5. Acknowledgments

The authors acknowledge to FAPEMIG (APQ- 02279-10 and APQ- 02249-14 and APQ - 03219-14), CAPES, RQMG (Rede Mineira de Química) and PROAP.

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Publication Dates

• Publication in this collection
  03 Dec 2018
• Date of issue
  2019

History

• Received
  17 Aug 2018
• Reviewed
  21 Sept 2018
• Accepted
  08 Oct 2018