Investigations on the rate performance of LiFePO₄/CeO₂ composite materials via polyol technique for rechargeable lithium batteries

Abstract

An attempt has been made to synthesize CeO₂ modified LiFePO₄ composite cathode materials via a polyol technique with a chemical combination route. The surface modified LiFePO₄ samples exhibit superior electrochemical performances compared to bare samples. CeO₂ may help to induce the fast lithium-ion diffusivity of LiFePO₄ cathode materials to promote high-rate stable cycling and a good coulombic efficiency. The complete coverage of a mild coating of CeO₂ under an optimized concentration on LiFePO₄ may limit the direct contact of the active material with the electrolyte, which improves the interface stability by preventing the dissolution of Fe ions in the electrolyte.

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1. Introduction

Olivine-structured LiFePO₄ (LFP) has been highlighted as one of the most promising cathode materials for rechargeable lithium-ion batteries (LIBs) because of its environmental compatibility, low cost, high capacity and thermal stability.^1,2 However, the inherent low ionic and electronic conductivities of LiFePO₄ significantly diminish the electrochemical performance; especially at a high rate. The surface coating technology plays an important role in the electrochemical performance of cathode materials. Surface coating has been divided into three different configurations such as rough coating,³ core shell structure coating^4,5 and ultra thin film coating.⁶ Surface coating has been proven to be effective for improving the capacity retention, rate capability and even thermal stability of cathode materials for lithium ion batteries.^7,8 In recent years, the surface coating/modification of LiFePO₄ with different materials such as carbon,^9,10 metal oxides,^11–13 fluorides¹⁴ and metal phosphates,¹⁵ etc. has been investigated. However, metal or non metal oxides significantly improved the electrochemical performance and most of the oxides are reported as electrochemically inactive oxides or semiconductor oxides, whereas the electronic conductivity is lower.

Cerium(IV) oxide and CeO₂-containing materials are intensively studied as catalysts in numerous three-way catalyst formulations. Yao et al.¹⁶ and Liu et al.⁷ reported that the electrochemical properties of pristine LiFePO₄ electrodes were improved when the LiFePO₄ electrodes were modified with CeO₂. They suggested that modification with CeO₂ is an effective way to improve the electrochemical properties of the LiFePO₄ cathode material. CeO₂ modification could enhance the ionic conductivity of LiFePO₄ and produce a good electrical contact between the oxides. Also, the resistance between the electrolyte and electrode has been reduced.^17–19 Quan et al.²⁰ introduced the effects of CePO₄ tailored LiFePO₄ in the form of LiFePO₄/C/CePO₄ composites. CeO₂ is a good oxygen storage material based on the reversible redox reaction. However, the LFP modification using CeO₂ via a polyol technique has been rarely carried out in the literature. Therefore, an attempt has been made to prepare the cerium metal oxide (CeO₂) tailored LiFePO₄ via a suitable economic energy-efficient polyol technique. The structural, morphological, chemical and electrochemical performance of the prepared materials were systematically studied in detail.

2. Experimental

2.1. Synthesis of cerium oxide coated LiFePO₄

LiFePO₄ cathode material was prepared via a conventional polyol technique.^11,21,22 The starting materials, iron(II) sulphate heptahydrate (FeSO₄·7H₂O, 99.9% of Alfa-Aesar) and lithium dihydrogen phosphate (LiH₂PO₄, 99.9% of Alfa-Aesar) were taken in stoichiometric molar ratio and dissolved in diethylene glycol (DEG, Aldrich), a polyol solvent. The mixed solution was heated close to the boiling point of the polyol solvent (245 °C) for 18 h under a refluxing process. After that, the reacted solution was washed several times with ethanol and acetone. The resulting particles were separated and dried in a vacuum oven at 150 °C for 48 h to obtain the uncoated-LiFePO₄ (bare LFP) sample. 1, 2 and 3 wt% of Ce(NO₃)₃·6H₂O and 0.3 N NH₄OH were dissolved in distilled water, in which the prepared LiFePO₄ was dispersed at room temperature. The mixture was slightly heated and maintained at a temperature of 50 °C while being stirred in order to evaporate the solvent. The resulting particles were heated at 500 °C for 1 h under an argon atmosphere. Finally, CeO₂-coated LiFePO₄ composite materials were obtained.

2.2. Structure, morphological and elemental characterizations

The powder X-ray diffraction (XRD) analyses of uncoated and coated LiFePO₄ samples were performed using a PANalytical X’-pert diffractometer with CuK_α radiation operated at 40 kV, 30 mA and the wavelength of λ = 1.54060 Å in the range 2θ = 10–70°. The functional group vibration was analysed using a Thermo Nicolet 380 FT-IR spectrophotometer using KBr pellets in the range 4000–400 cm⁻¹. The morphology of the prepared materials was studied by using field emission scanning electron microscopy (FESEM, LEO-1530, ZEISS, Germany) and energy dispersive spectrometry (EDS, X-MAX50, 30 kV). The nano particles and coating of CeO₂ were observed by high resolution transmission electron microscopy (HR-TEM) (Techni G2 S-TWIN, FEI, Netherlands) techniques. The chemical valence states of the elements were investigated by X-ray photoelectron spectroscopy (XPS, PHI model 5802).

2.3. Fabrication of coin cell and electrochemical studies

A Li-metal |LiPF₆(EC + DMC)| CeO₂ coated LiFePO₄ cell was used to investigate the electrochemical performances of the prepared composite cathodes using CR2032 type coin cells. The cathodes (positive electrodes) were prepared by the mixing of 80 wt% CeO₂/LiFePO₄ composite powder, 10 wt% super P, and 10 wt% poly-(vinylidene fluoride) (PVdF) in N-methylpyrrolidone (NMP) solvent to form a homogeneous slurry. Then, the mixed slurry was spread uniformly on thin aluminum foil and dried in vacuum at 120 °C for 6 h and then roll pressed; then the samples were punched into circular discs. A polypropylene separator (Celgard 2400, Hoechst Celanese Corp) was drenched in the electrolyte for 24 h prior to use. The coin cell assembling procedures were performed using an Ar-filled glove box by keeping both the oxygen and moisture levels less than 1 ppm. The galvanostatic charge–discharge analysis was performed using a BTS-55 Neware battery testing system between the potential 2.5 and 4.5 V (vs. Li/Li⁺) at ambient temperature with different C-rates. Electrochemical impedance (EIS) analysis was performed on a CHI 660D electrochemical analyser (CH Instruments) at room temperature in the frequency range 10⁶ to 0.01 Hz with AC signal amplitude of 5 mV.

3. Results and discussion

3.1. Structural studies

Fig. 1 shows the XRD patterns of bare and CeO₂-coated LiFePO₄ cathode materials with the standard data. The pristine and CeO₂-coated LiFePO₄ materials have been confirmed to possess a well-defined orthorhombic olivine structure with a space group of Pnma (JCPDS 83-2092). The diffraction peaks of CeO₂ not appearing in the 1 wt% based sample is due to the low content of CeO₂ in the precursor.²³ Fig. 1c and d show the existence of two minor peaks appearing at 2θ = 29 and 33° of (1 1 1) and (2 0 0) planes, corresponding to 2 and 3 wt% of the CeO₂ coated LiFePO₄ sample, respectively. The presence of impurity peaks suggests that the CeO₂ has formed a thin solid solution layer on the surface of the LiFePO₄ particles. CeO₂ has been confirmed by standard crystal diffraction data and literature reports (JCPDS 89-8436).^24,25 Hence, the CeO₂ coating does not change the structure of the bare LiFePO₄. These results indicate that cerium oxide is just coated on the surface of the LiFePO₄ particles. The poor crystallinity of CeO₂ is due to the fact that the precursor has been calcined at a low temperature for a short duration (500° for 1 h), which results in the disappearance of the face-centered cubic CeO₂ peaks in the CeO₂ coated samples. A similar phenomenon can be observed in the earlier reports of rare earth oxide modified samples after calcination at a low temperature (400 °C).^16,26,27

Fig. 1 XRD patterns of (i) (a) bare LiFePO₄, (b–d) 1 to 3 wt% of CeO₂ coated LiFePO₄ and (ii) enlarged patterns of selected 2θ range.

The crystallite size (D) has been calculated using Scherrer’s equation D = Kλ/b cos θ, where D is the crystallite size, K is the shape factor (0.9), λ is the X-ray wavelength, b is the full width at half maximum (FWHM) and θ is the Bragg’s angle. The calculated crystal lattice parameters and average crystallite sizes for all samples are shown in Table 1.

Table 1 The lattice parameters and crystallite sizes of uncoated and coated samples from XRD data

Structural parameters	Sample name
	JCPDS no. 83-2092	LiFePO4	1 wt% coated LiFePO4	2 wt% coated LiFePO4	3 wt% coated LiFePO4
Lattice constant values and volume	a = 10.33 Å	a = 10.3142 Å	a = 10.3141 Å	a = 10.3139 Å	a = 10.3137 Å
	b = 6.010 Å	b = 6.0022 Å	b = 6.0020 Å	b = 6.0019 Å	b = 6.0018 Å
	c = 4.693 Å	c = 4.6842 Å	c = 4.6842 Å	c = 4.6843 Å	c = 4.6843 Å
	V = 291.4 Å^3	V = 292.08 Å^3	V = 291.81 Å^3	V = 292.08 Å^3	V = 291.81 Å^3
Average crystallite sizes (nm)	—	51	52	53	59

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The slight change in the cell parameter values indicates that the presence of CeO₂ on the LiFePO₄ surface doesn’t affect the structure of bare LiFePO₄. It indicates that CeO₂ did not diffuse into the LiFePO₄ lattice; it is merely a coating on the surface of LiFePO₄ and could form a solid solution. Also, the average crystallite sizes are slightly increased with the increase of the coating content and do not change the phase structure of LiFePO₄, similar to other coating materials.^26,27 However, due to the fact that the coating procedure was carried out at 500 °C, a few amorphous particles may have been attached on the surface of the core material.

Fig. 2a–d shows the FT-IR spectra of the bare and CeO₂ coated LiFePO₄ samples. The band between 932 and 1134 cm⁻¹ corresponds to the symmetric and antisymmetric stretching mode of the P–O vibration peak of the PO₄ tetrahedron in defect-free LiFePO₄. Symmetric and antisymmetric O–P–O bending modes exist in the range of 460–633 cm⁻¹.²⁸ The bending and stretching modes of the bare and coated samples have been observed in the range 460–1134 cm⁻¹. In addition, no additional peaks are observed in the bare LiFePO₄ material.

Fig. 2 (a–d) FT-IR spectra of bare, 1, 2 and 3 wt% CeO₂ coated LiFePO₄ samples.

The presence of CeO₂ in the coated LiFePO₄ is confirmed through the band at 1381 cm⁻¹, which is the characteristic vibration mode of the CeO₂ stretching vibration.^29,30

The intensity of the vibrational peaks increases with the increase of CeO₂ content. The O–H bending and stretching vibrations are located at 1628 and 3436 cm⁻¹. This may be due to the lower calcination temperature or absorbed water molecules from air during the analysis, since ceria is slightly hygroscopic in nature.

3.2. Morphological and elemental analysis

The SEM images of the bare, 1, 2 and 3 wt% of CeO₂-coated LiFePO₄ materials are shown in Fig. 3a–d. The bare LiFePO₄ material has long rod-like particles with the average size 350 nm × 100 nm of length × width. The calcination results in the breaking of the lengthy rods into smaller rods and a few grains on its surface. It exhibits agglomeration between the grains, which are found less in the coated samples than the bare sample. The average particle sizes were measured as length × width (250 × 80), (128 × 56) and (180 × 65) nm respectively for the 1, 2 and 3 wt% of CeO₂ coated LiFePO₄ samples using the ‘measureIT’ software (Olympus soft imaging solution GMBH product). As the content of CeO₂ is increased, the solid solution or nano-sized CeO₂ particle growth is increased on the LFP surface. The surface modification on LiFePO₄ would affect the electrochemical properties.

Fig. 3 (a–d) SEM images of bare, 1, 2 and 3 wt% CeO₂ coated LiFePO₄ samples.

The energy dispersive X-ray (EDX) analysis was carried out to identify the elements present. Fig. 4a–d illustrates the EDX spectra of the bare, 1, 2 and 3 wt% CeO₂ coated LFP samples. According to the EDX results, a new phase containing Ce was detected in the coated samples. These results are in good agreement with the actual CeO₂ content used in the coated LFP materials. Also, from this analysis, we can calculate that the ratio of the elemental atoms such as Fe/P is almost equal to 1 and O/Fe or O/P is equal to 4, which gives an additional confirmation of LiFePO₄ formation.

Fig. 4 (a–d) EDX spectra of bare, 1, 2 and 3 wt% CeO₂ coated LiFePO₄ samples.

Fig. 5a–c clearly shows the TEM images of 1, 2 and 3 wt% of CeO₂ coated LFP particles. It is apparent from the TEM images (Fig. 5a–c) that there are two distinct morphologies; the dark part representing LiFePO₄ and the lighter part representing CeO₂ particles or the solid solution layer. Fig. 5b demonstrates that 2 wt% of CeO₂ has been coated as a thin layer on the surface of LiFePO₄ particles. Fig. 4c reveals the existence of a few particles and layers on the surface of the core particles. The related SAED pattern (Fig. 5d–f) exhibits a regular and clear diffraction spot array, which indicates that the particle is single-crystalline and it can be indexed to the orthorhombic phase of LiFePO₄. These results are in good agreement with the XRD results.

Fig. 5 (a–c) TEM images and (d–f) SAED patterns of 1, 2 and 3 wt% of CeO₂ coated LiFePO₄ samples and (g and h) higher magnification TEM images of 2 wt% CeO₂ coated LFP sample.

The surface composition and the oxidation state of the elements were analysed by X-ray photoelectron spectroscopy (XPS). Fig. 6a shows the wide range core spectra of 2 wt% CeO₂ coated LiFePO₄ composite and the results confirm the presence of all the elements in the prepared sample. Fig. 6b–f demonstrates the core spectra of Li 1s, Fe 2p, P 2p, O 1s and Ce 3d respectively. Fig. 6c (Fe 2p) has been split into two components, because of spin–orbit coupling, namely Fe 2p_3/2 and Fe 2p_1/2. LiFePO₄ shows Fe 2p_3/2 main peaks at 710 eV and 724 eV for Fe 2p_1/2, which are in good agreement with Fe²⁺ in LiFePO₄.³¹

Fig. 6 X-ray photoelectron spectra of 2 wt% CeO₂ coated LiFePO₄ composite: (a) wide range, (b) Li 1s, (c) Fe, (d) P, (e) O, and (f) cerium.

The presence of the carbon (C 1s) peak denotes the minor contribution of oxygenated carbon at the surface of the prepared material. The O 1s binding energy (BE) that has been observed at 530.46 eV corresponds to the lattice oxygen (O²⁻) of the orthorhombic structure.^32,33 The XPS spectrum of Ce is complex and split into Ce 3d_3/2 and Ce 3d_5/2 with multiple shake-up and shake-down satellites. The peaks between 875 and 895 eV belong to the Ce 3d_5/2, while peaks between 895 and 910 eV correspond to the Ce 3d_3/2 levels.^25,34 The peak at 916 eV is a characteristic satellite peak indicating the presence of CeO₂(IV).³⁵ Thus, the XPS analysis confirms the presence of elements in the as-prepared material.

3.3. Electrochemical studies

The electronic conductivity of the cathode material is very important for lithium intercalation/de-intercalation processes in the lithium ion cell. The electrical conductivities of the pristine and CeO₂ coated LFP samples are shown in Table 2. The electrical conductivity of the synthesized materials was determined by using a four-probe DC method. It is shown that the electrical conductivities of the coated samples are increased when compared to pristine LFP.³⁶ Electrical conductivity is a physical property reflecting the ability of a material to transfer electrical charge. The ionic mobility depends on many factors, but mostly on the size of ions and ionic bond strength. Therefore, an enhancement of conductivity is used to induce the EC performance of the LFP electrode material. The electronic conductivity of the active material has played a vital role in the transfer of charge from the current collector during the charging and discharging process. The actual lithium ionic conduction takes place between the electrodes through the electrolyte in the Li/electrolyte/CeO₂ coated LiFePO₄ cell couple. Normally, for bare LiFePO₄, the electronic conductivity is very low. Therefore, the charge transfer from the electrode material to the current collector was not up to the mark. But, after coating the CeO₂ on LiFePO₄ to an optimum level, the transfer of charge is excellent and exhibits appreciable improvement in performance over the bare material.³⁷

Table 2 Electronic conductivities of the pristine and CeO₂ coated LiFePO₄ samples at room temperature

Sample name	Electronic conductivity
Bare LiFePO4	2.88 × 10^−2 S cm^−1
1 wt% CeO2–LiFePO4	5.01 × 10^−2 S cm^−1
2 wt% CeO2–LiFePO4	7.03 × 10^−2 S cm^−1
3 wt% CeO2–LiFePO4	6.25 × 10^−2 S cm^−1

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Fig. 7a shows the initial charge/discharge curves of the pristine, 1, 2 and 3 wt% CeO₂-coated LiFePO₄ samples at 0.1C rate at room temperature. All the cells exhibited discharge voltage plateaus around 3.45 V, which is the main characteristic of the two-phase reaction of the lithium extraction and insertion between LiFePO₄ and FePO₄.²⁹ The initial charge/discharge capacity of pristine LiFePO₄ is 152/149 mA h g⁻¹, whereas for 1, 2 and 3 wt% CeO₂-coated LiFePO₄, it is 157/153, 164/163 and 160/159 mA h g⁻¹ at 0.1C rate, respectively. This value of coated LFP is higher when compared to the bare sample, which is due to the improvement of the electronic conductivity after the surface treatment. Obviously, the metal oxide particle coated cathode surface may limit the direct contact of the active material with the electrolyte, i.e., the metal oxide particles cover the core material. This improves the interface stability and prevents the dissolution of Fe-ions in the electrolyte.³⁸

Fig. 7 Charge–discharge performance of (a) bare and coated LiFePO₄ at 0.1C rate, (b) CeO₂ coated LiFePO₄ at 1C rate, (c) rate performance of CeO₂ coated LiFePO₄ at various rates, and (d–f) cyclic behaviour and coulombic efficiency of 1, 2 and 3 wt% of CeO₂ coated LiFePO₄ at 1C rate at room temperature.

Fig. 7b demonstrates the charge/discharge profile at 1C rate at room temperature for all coated LFP samples up to 50 cycles. Among the samples studied, the 2 wt% of CeO₂ coated on LiFePO₄ shows a better discharge capacity with cyclic stability and retention when compared with 1 and 3 wt% of CeO₂ coated LiFePO₄ composites. There are several factors affecting the electrochemical performances of the prepared materials: (i) the capacity increases upon increasing CeO₂ content until 2 wt%, which is due to the improvement of the electronic conductivity of the material. The coverage of the solid solution layer on its surface, which provides a better electrode–electrolyte contact and reduces the polarization of bare material (inset Fig. 7a) causes an enhancement in the electronic conductivity. (ii) The 2 wt% of CeO₂ coated LFP sample exhibits better capacity, retention and cyclic capability compared to the other samples studied. This is due to the complete coverage of a thin layer of CeO₂ solid solution, which may enhance the structural stability and improve the ionic/electronic conductivity of the bare LFP material.^24,26 (iii) Further increasing of the CeO₂ content causes the aggregation of the cerium oxide particles in a thick layer on the surface of the LiFePO₄, which leads to crystallite regions, and the CeO₂ particles tend to impede ionic movement by acting as mere insulators. An optimal content of CeO₂ addition would result in enhancing the electrochemical properties;²⁴ whereas beyond the optimal content of CeO₂, the electrochemical performance decreases since its higher content leads to an inactive or insulating nature. Similar observations have already been made by Ha et al.³⁹ and Liu et al.⁷

Recently, most of the high rate applications like EV (electric vehicles), HEV (heavy electric vehicles), HED (high energy density) batteries, etc. are occupying the current commercial market due to their appealing rate performance and cycling capability.³⁸ In the present study the high rate studies for the cells have been performed and are given in Fig. 7c. This represents the rate performances of 1, 2 and 3 wt% of CeO₂ coated LiFePO₄ between 0.1 and 30C rates at room temperature. The electrode discharge capacities are 151, 147, 126, 112, 65, 58 mA h g⁻¹ for 1 wt% of CeO₂, 163, 160, 152, 147, 116, 75 mA h g⁻¹ for 2 wt% of CeO₂ and 157, 156, 141, 134, 111, 68 mA h g⁻¹ for 3 wt% of CeO₂ coated samples respectively, up to 10 cycles at 0.1, 0.5, 1, 10, 20 and 30C rates. The aforementioned result of 2 wt% CeO₂ coated LFP electrodes exhibits remarkable improvement in discharge capacity compared to earlier reports viz., carbon, CeO₂ and other metal oxide coated LFP (Table 3).

Table 3 The CeO₂ coated LiFePO₄ samples compared with earlier reports of carbon, CNT, graphene coated and metal oxide modified LiFePO₄ samples

Material name	Observed discharge capacity (mA h g^−1) with current rate of 0.1C	Reference
LiFePO4 and LiFePO4–CNT	129 and 155 at 10 mA g^−1	Wu et al.^40
LiFePO4/C and graphene/LiFePO4/C	146.5 and 157.8	Wang et al.^41
C–LiFePO4 and graphene–LiFePO4	∼150 and ∼153	Kim et al.^42
LiFePO4/graphene	160.3	Wang et al.^43
LiFePO4/rGO	161	Nagaraju et al.^44
Sn-modified LiFePO4 (Sn/Fe ratio of 1 : 99, 3 : 97 and 5 : 95)	122, 122 and 114	Ziolkowska et al.^13
SiO2-coated LiFePO4	160	Li et al.^45
LiFePO4/C/CePO4	156	Quan et al.^20
2 wt% of CeO2-coated LiFePO4/C	153.8	Yao et al.^16
Pristine LiFePO4	149	In this work
1 wt% of CeO2–LFP	153
2 wt% of CeO2–LFP	163
3 wt% of CeO2–LFP	159

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Fig. 7d–f illustrate the 1, 2 and 3 wt% of CeO₂ coated LFP samples for 100 cycles of discharge capacity and coulombic efficiency at 1C rate at room temperature. The coulombic efficiency of CeO₂ coated LFP composites are almost the same (about 97–99%) at 1C rate for the coated samples. The 2 wt% of CeO₂ coated LFP sample has shown an excellent rate with an appealing cyclic performance and more stable coulombic efficiency than the rest. It is observed that the coulombic efficiency has been improved upon increasing the CeO₂ content until 2 wt%; further addition of CeO₂ affected this trend. This could be due to the fact that Ce⁴⁺ is not electrochemically active and, therefore, the presence of excess CeO₂ could lower the discharge capacity.³⁵ Consequently, the optimized amount of CeO₂ coating could improve the discharge capability and coulombic efficiency of the electrode materials.

The electrochemical kinetics of the pristine and CeO₂ coated LiFePO₄ composite has been studied by Electrochemical Impedance Spectroscopy (EIS). Fig. 8 shows the Nyquist impedance plots of the materials and the equivalent circuit (inset of Fig. 8). The Nyquist plots are composed of a semi-circle in the high frequency region and an inclined line in the low frequency region. The intercept of the high frequency region corresponds to the ohmic resistance (R_e), which represents the resistance of the electrolyte and electrode. The semicircle in the middle frequency range indicated the charge transfer resistance (R_ct). The inclined line corresponds to the diffusion of Li⁺ in the bulk electrode, namely the Warburg impedance (Z_w). The charge-transfer resistance of CeO₂ coated LiFePO₄ electrodes is much less than that of pristine LiFePO₄. This indicates that the coating is more favourable for the insertion and de-insertion of lithium ions during the charge and discharge process. Among the CeO₂ coated LiFePO₄ samples, the 2 wt% of CeO₂ coated LiFePO₄ shows the smallest R_ct of 230 Ω and Z_w, which is clearly seen in the plots (Fig. 8).

Fig. 8 Nyquist plots of the bare and CeO₂ coated LiFePO₄ composite electrodes.

The small resistance indicates that Li⁺ ion and electron transfer are more feasible on the electrode, which may be attributed to the decreasing electronic resistance of the composite material. The impedance plot of the bare and CeO₂ coated LFP electrodes are represented by a large depressed semicircle with an inductive line and a very small semicircle at room temperature, indicating a lower electronic/ionic conductivity of bare LFP than the values of CeO₂ coated LFP sample.

The impedance of the 2 wt% of the CeO₂ coated LiFePO₄ composite is significantly smaller than that of the other samples, which indicates an enhancement in both the ionic and electronic conductivity as well as lower interface impedance, relating to a superior rate performance.³⁷ CeO₂ can serve as protective layer and prevents the direct contact between LiFePO₄ and the electrolyte solution like a preformed SEI, which can reduce side reactions and improve the structure, cycle stability and decrease charge-transfer resistance.²⁴ The CeO₂ coating enhances the reversibility of the electrode reaction. However, when the amount of CeO₂ is increased to 3 wt%, the initial discharge capacity and cycling stability are decreased, which may be due to an excessive amount of CeO₂. The 2 wt% of CeO₂ coated on LiFePO₄ is found to be an optimum level of coating content which improves EC performance.⁴⁶

4. Conclusions

The olivine type orthorhombic structure of bare LiFePO₄ has been successfully prepared via the polyol technique with a low boiling point solvent (DEG), a simple binary precursor and also without post-heat treatment, any special environment or carbon materials. In addition, the CeO₂ coated LFP has been subjected to a one step short-time heat treatment without any change in the bare LFP structure. Due to the heat treatment, the particle size of the coated samples has been reduced. The 2 wt% of CeO₂ coated LFP surface was formed in a thin layer. It exhibits a superior electrochemical performance than other coated and bare LFP samples. It denotes that the appropriate content of CeO₂ coating has better ionic and electronic conduction, which enhances the ionic and electronic transport in the LiFePO₄ electrode. However, the higher content of the inactive materials may be impeding the mass and charge transfer and may reduce the reversible capacity. Therefore, an optimized content of CeO₂ protects the LiFePO₄ electrode from electrolyte corrosion and maintains the structural stability of the LiFePO₄. It is beneficial to use in high-rate battery applications. This is due to the excellent rate capability and cycle stability of CeO₂ tailored LiFePO₄.

Acknowledgements

The authors M. Sivakumar and R. Muruganantham gratefully acknowledge the financial support to carry out this work from the Department of Science and Technology (DST), New Delhi, Govt. of India under the DST-SERC major research project (SR/S2/CMP-0049/2008).

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